Properties and potential applications of two-dimensional AlN

Journal Pre-proof Properties and potential applications of two-dimensional AlN Milena Beshkova, Rositsa Yakimova PII:

S0042-207X(20)30068-3

DOI:

https://doi.org/10.1016/j.vacuum.2020.109231

Reference:

VAC 109231

To appear in:

Vacuum

Received Date: 17 July 2019 Revised Date:

19 January 2020

Accepted Date: 23 January 2020

Please cite this article as: Beshkova M, Yakimova R, Properties and potential applications of twodimensional AlN, Vacuum, https://doi.org/10.1016/j.vacuum.2020.109231. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.

Dear Dr. Beshkova: Thank you for requesting permission to reproduce material from AIP Publishing publications. Material to be reproduced: Fig. 2 and Fig. 5 from: "Evidence for graphite-like hexagonal AlN nanosheets epitaxially grown on single crystals Ag(111)", Appl. Phys. Lett. 103 (2013) 251605, https://doi.org/10.1063/1.4851239

For use in the following manner: Reproduced in the review paper, "Properties and potential applications of two dimensional AlN-theoretical aspects," to be submitted in July 2019 in the Journal Vacuum. Permission is granted subject to these conditions: 1. AIP Publishing grants you non-exclusive world rights in all languages and media. This permission extends to all subsequent and future editions of the new work. 2. The following notice must appear with the material (please fill in the citation information): “Reproduced from [FULL CITATION], with the permission of AIP Publishing.” The notice may appear in the figure caption or in a footnote. In cases where the new publication is licensed under a Creative Commons license, the full notice as stated above must be used. 3. If the material is published in electronic format, we ask that a link be created pointing back to the abstract of the article on the journal website using the article’s DOI. 4. This permission does not apply to any materials credited to another source. 5. If you have not already done so, please attempt to obtain permission from at least one of the authors. The authors’ contact information can usually be obtained from the article. For future permission requests, we encourage you to use RightsLink, which is a tool that allows you to obtain permission quickly and easily online. To launch the RightsLink application, simply access the appropriate article on the journal site, click on the “Tools” link in the abstract, and select “Reprints & Permissions.” Please let us know if you have any questions. Sincerely, Susann Brailey

Manager, Rights & Permissions

AIP Publishing 1305 Walt Whitman Road │ Suite 300 | Melville NY 11747-4300 │ USA t +1.516.576.2268 [email protected] │ publishing.aip.org

Table 1. Formation energy (eV) for different defects incorporated in an infinite 2D h-AlN sheet from Ref. [62] copyright Springer Nature, used with permission.

Table 2. Calculated energy gap (eV) for AlN bulk (wurtzite structure) and nanosheet from Ref. [67] copyright Elsevier, used with permission.

Table 3. Dielectric constants ε(ω) and refractive index n(ω) of AlN wurtzite structure (bulk) and AlN nanosheet, respectively from Ref. [67] copyright Elsevier, used with permission.

Table 4. Dynamical stability of rectangular (rec.) and hexagonal (hex.) AlN nanosheets under different kinds of strains (comp. = compressive strain, tens. = tensile strain) from Ref. [74] copyright Elsevier, used with permission.

Table 5. The calculated formation energy (in eV) of hydrogen adsorbed on Al or N sites in fully hydrogenated and semihydrogenated AlN NSs from Ref. [83] copyright John Wiley and Sons, used with permission.

Table 6. The distance between the AlN sheet and the nearest atom of the gas molecule, the adsorption energy, the charge transfer from AlN sheet to various gas molecules and the band gap of different systems from Ref. [89] copyright Elsevier, used with permission.

Table 7. Calculated adsorption energy (Eads, kJ/mol), the nearest distance between the molecule and the AlN surface (D), charge transfer from the AlN sheet to the molecules (Q), HOMO (highest occupied molecular orbital) energies (EHOMO), LUMO (lowest unoccupied molecular orbital)

energies (ELUMO), energy of Fermi level (EFL),

HOMO/LUMO energy gap (Eg) in eV, and changes in Eg(∆Eg) of systems due to molecule adsorption from Ref. [96] copyright Elsevier, used with permission.

Table 8. The adsorption energy (Ead), binding distance (d) and C-O bond length for the different configurations of CO molecule adsorption on pristine h-AlN nanosheet from Ref. [104] copyright Elsevier, used with permission.

Table 9. The adsorption energy, band gap value, band gap change and charge transfer in different systems from Ref. [104] copyright Elsevier, used with permission.

Fig. 1 (A) Optical image of graphene and h-BN flakes (scale bar 10 µm), (B) optical image of a hall bar measurements device for graphene electron mobility (scale bar 1 µm), (C) electron mobility as a function of gate voltage, with a histogram of mobility seen in inset, and (D) electron mobility enhancement in graphene on h-BN at various temperature as compared to graphene on SiO2. Panels (A–C) from Ref. [17] copyright Elsevier used with permission. Panel (D) from Ref. [18] copyright AIP Publishing, used with permission.

ELSEVIER LICENSE TERMS AND CONDITIONS Oct 30, 2019

This Agreement between Milena Beshkova ("You") and Elsevier ("Elsevier") consists of your license details and the terms and conditions provided by Elsevier and Copyright Clearance Center. License Number 4698740825023 License date Oct 30, 2019 Licensed Content Publisher Elsevier Licensed Content Publication Solid State Communications Licensed Content Title Graphene based heterostructures C. Dean,A.F. Young,L. Wang,I. Meric,G.-H. Lee,K. Licensed Content Author Watanabe,T. Taniguchi,K. Shepard,P. Kim,J. Hone Licensed Content Date Aug 1, 2012 Licensed Content Volume 152 Licensed Content Issue 15 Licensed Content Pages 8 Start Page 1275 End Page 1282 Type of Use reuse in a journal/magazine Requestor type academic/educational institute Portion figures/tables/illustrations Number of 3 figures/tables/illustrations Format both print and electronic Are you the author of this Yes Elsevier article? Will you be translating? No Properties and potential applications of two dimensional Title of new article AlN –theoretical aspects Lead author Milena Beshkova Title of targeted journal Vacuum Publisher Elsevier Expected publication date Oct 2019 Portions Fig.2, Fig.6, Fig.7e Milena Beshkova Institute of Elektronics 72. Tzarigradsko chaussee Blvd. Requestor Location Sofia, 1784 Bulgaria Attn: Milena Beshkova Publisher Tax ID GB 494 6272 12 Total 0.00 EUR Terms and Conditions

Fig. 2. Portion of the unit cell of a perfect h-AlN sheet from Ref. [62] copyright Springer Nature, used with permission.

Fig. 3. The absorption coefficient of AlN nanosheet for the electric field parallel and perpendicular to the AlN layer from Ref. [67] copyright Elsevier, used with permission.

Fig. 4. The real part of optical conductivity of AlN nanosheet for the electric field parallel and perpendicular to the AlN layer from Ref. [67] copyright Elsevier, used with permission.

Fig. 5. Ball and stick models for atomic structure of AlN nanosheets in two different supercells: a) rectangular symmetry (x axis: zig-zag direction, y axis: arm-chair direction); b) hexagonal symmetry (both x and y axes: zig-zag direction): [blue balls: Al atom, yellow balls: N atom] from Ref. [74] copyright Elsevier, used with permission.

Fig. 6. Band gap versus strain for AlN nanosheet in (a) the rectangular supercell and (b) the hexagonal supersell. Filled and unfilled symbols represent the indirect and direct band gaps, respectively from Ref. [74] copyright Elsevier, used with permission.

Fig. 7. Relaxed infinite two-dimensional AlN sheet formed by alternated honeycomb strips joined by an extended line defects (8-4 defects) from Ref. [78] copyright Elsevier, used with permission.

Fig. 8. Geometry of AlN NSs with two single bilayers a) initial wurtzite structure and b) graphitic structure after optimizarion with n<5 [lilac balls: Al atom, yellow balls: N atom] from Ref. [83] copyright John Wiley and Sons, used with permission.

Fig. 9. Optimized geometry and band structure and DOS for fully hydrogenated a) 1-AlN-2H and b) 2-AlN-2H [lilac balls: Al atom, yellow balls: N atom, blue balls: H atom] from Ref. [83] copyright John Wiley and Sons, used with permission.

Fig. 10. The optimized atomic structures of monolayer AlN and possible adsorption sites [grey balls: Al, blue balls: N atoms] from Ref. [89] copyright Elsevier, used with permission.

Fig. 11.

a) Demonstration of the electrons of CO2/AlN and the charge density

difference of AlN. Charge density difference plots for b) CO2/AlN, c) H2/AlN, d) CO/AlN, e) O2/AlN, f) NO/AlN. The red and blue iso-surface with an isovalue of 0.001e/Å3 corresponds to charge accumulation and depletion respectively from Ref. [89] copyright Elsevier, used with permission.

Fig. 12. a) RHEED patterns of bare Ag(111) and AlN/Ag(111) structures along [1-10] azimuth of silver. The overall streaky pattern shows that AlN grows epitaxially on the Ag(111) substrate. b) RHEED and c) intensity structures along [11-2] azimuth of silver. For illustrations purpose, in Figs. 12 (b) and 12 (c) are shifted in order for the left first order streaks to be aligned. A slightly larger lattice constant is determined for the ultrathin layer which is characteristic of its graphitic structure from Ref. [111] copyright AIP Publishing, used with permission.

Fig. 13. Valence band (VB) spectra of the AlN(bulk)/Si(111) and AlN(12ML/Ag(111) samples. The smaller VMB (valence band maximum) of the 12 ML-AlN/Ag comparing to the bulk w-AlN/Si could be an indication of the smaller energy bandgap, which is expected for a graphite-like hexagonal AlN phase from. Ref. [111] copyright AIP Publishing, used with permission.

Fig. 14. (a) RHEED pattern of the (111) Si surface with the highly ordered SiN (8 × 8) structure (b) the intensity of the (03/8) fractional spot as a function of nitridation time from Ref. [114] copyright Elsevier, used with permission.

Fig. 15. (a) The evolution of the AlN (01) streak intensity at different temperature; (b) Arrhenius plot of the AlN formation rate from Ref. [114] copyright Elsevier, used with permission.

Fig. 16. Schematic structure for the formation of 2D AlN layers sandwiched between graphene and Si substrate from Ref. [119] copyright John Wiley and Sons, used with permission.

This Agreement between Milena Beshkova ("You") and Elsevier ("Elsevier") consists of your license details and the terms and conditions provided by Elsevier and Copyright Clearance Center. Your confirmation email will contain your order number for future reference. printable details License Number

4624210526724

License date

Jul 08, 2019

Licensed Content Publisher

Elsevier

Licensed Content Publication

Chemical Physics Letters

Licensed Content Title

Extended line defects in BN, GaN, and AlN semiconductor materials: Graphene-like structures

Licensed Content Author Dulce C. Camacho-Mojica,Florentino López-Urías Licensed Content Date

May 16, 2016

Licensed Content Volume

652

Licensed Content Issue

n/a

Licensed Content Pages

6

Type of Use

reuse in a journal/magazine

Requestor type

publisher

Intended publisher of new work

Elsevier

Portion

figures/tables/illustrations

Number of 1 figures/tables/illustrations Format

both print and electronic

Are you the author of this No Elsevier article? Will you be translating?

No

Original figure numbers Fig.1 Title of the article

Properties and potential applications of two dimensional AlN – theoretical aspects

Publication new article is Vacuum in Publisher of the new article

Elsevier

Author of new article

Milena Beshkova

Expected publication date Jul 2019 Estimated size of new

1

article (number of pages) Requestor Location

Milena Beshkova Institute of Elektronics 72. Tzarigradsko chaussee Blvd. Sofia, 1784 Bulgaria Attn: Milena Beshkova

Publisher Tax ID

GB 494 6272 12

Total

0.00 EUR

Fig. 17 a) Schematic and b) atomic models of AlN NT fabrication process, which can be devided into four stages: i) epitaxy of GaN NWs, ii) deposition of AlN to form GaN/AlN core/shell structure, iii) selective thermal evaporation of dispersed NWs, and iv) formation of hexagonal AlN NTs with an open end. All the cross-section views in (b) are aquired along the red arrow lines located at representative positions from Ref. [123] copyright John Wiley and Sons, used with permission.

Fig.18 a) High-resolution XPS spectra of the N 1s core-level photoelectric peak of the AlN template and NTs. b) STEM-EELS spectra collected with primary electron beam of 80 keV for individual AlN NT with different wall thickness shown in the top inset: i) 8 ML, ii) 4 ML, and iii) 2 ML. The EELS was performed along the red dashed line, i.e. perpendicular to the tube axis. The inclined (horizontal) red lines in (a) and (b) are linear fritting ones (background level) of the energy loss spectra from Ref. [123] copyright John Wiley and Sons, used with permission.

Fig. 19. Adsorption energy of 2D tetragonal III-V materials on (100) surfaces on common metal substrates, Cu, Pd and Pt with lattice mismatches less than 1 %. The filled and empty squares denote adsorption energies with and without the inclusions of van der Waals interactions, respectively. The substrates significantly enhance the stability of the 2D materials from Ref. [132] copyright American Physical Society, used with permission.

This Agreement between Milena Beshkova ("You") and John Wiley and Sons ("John Wiley and Sons") consists of your license details and the terms and conditions provided by John Wiley and Sons and Copyright Clearance Center. Your confirmation email will contain your order number for future reference. printable details License Number

4624261023811

License date

Jul 08, 2019

Licensed Content Publisher

John Wiley and Sons

Licensed Content Publication

Advanced Materials

Licensed Content Title

2D AlN Layers Sandwiched Between Graphene and Si Substrates

Licensed Content Author

Wenliang Wang, Yulin Zheng, Xiaochan Li, et al

Licensed Content Date

Nov 4, 2018

Licensed Content Volume

31

Licensed 2 Content Issue Licensed 9 Content Pages Type of use

Journal/Magazine

Requestor type Publisher (STM Signatory) STM publisher Elsevier name Is the reuse no sponsored by or associated with a pharmaceutical or medical products company? Format

Print and electronic

Portion

Figure/table

Number of 1 figures/tables Original Wiley Fig.4. figure/table number(s) Will you be translating?

No

Circulation

10

Title of new article

Properties and potential applications of two dimensional AlN –theoretical aspects

Publication the Vacuum new article is in Publisher of new article

Elsevier

Author of new Milena Beshkova article Expected publication date of new article

Jul 2019

Estimated size 1 of new article (pages) Requestor Location

Milena Beshkova Institute of Elektronics 72. Tzarigradsko chaussee Blvd. Sofia, 1784 Bulgaria Attn: Milena Beshkova

Publisher Tax EU826007151 ID Total

0.00 EUR

American Physical Society Reuse and Permissions License 08-Jul-2019 T his license agreement between the American Physical Society ("APS") and Milena Beshkova ("You") consists of your license details and the terms and conditions provided by the American Physical Society and SciPris.

Licensed Content Information Lice nse Numbe r:

RNP/19/JUL/016445

Lice nse date :

08-Jul-2019

DO I:

10.1103/PhysRevB.87.165415

Title :

Computational discovery of single-layer III-V materials

Author:

Houlong L. Zhuang, Arunima K. Singh, and Richard G. Hennig

Publication:

Physical Review B

Publishe r:

American Physical Society

C ost:

USD $ 0.00

Request Details Doe s your re use re quire significant modifications: No Spe cify inte nde d distribution locations:

Worldwide

Re use C ate gory:

Reuse in a journal/magazine

Re que stor Type :

Publisher, ST M Signatory

Se le cte d STM Signatory:

Elsevier

Ite ms for Re use :

Figures/Tables

Numbe r of Figure /Table s:

1

Figure /Table s De tails:

Fig.7.

Format for Re use :

Print and Electronic

Total numbe r of print copie s:

Up to 1000

Information about New Publication: Publishe r:

Elsevier

Publication:

Vacuum

Publication Date :

Jul. 2019

Article Title :

Properties and potential applications of two dimensional AlNtheoretical aspects

Author(s):

Milena Beshkova, Rositsa Yakimova

License Requestor Information Name :

Milena Beshkova

Affiliation:

Individual

Email Id:

[email protected]

C ountry:

Bulgaria

Page 1 of 3

Page 2 of 3

American Physical Society Reuse and Permissions License TERMS AND CONDITIONS T he American Physical Society (APS) is pleased to grant the Requestor of this license a non-exclusive, non-transferable permission, limited to Print and Electronic format, provided all criteria outlined below are followed. 1. You must also obtain permission from at least one of the lead authors for each separate work, if you haven’t done so already. T he author's name and affiliation can be found on the first page of the published Article. 2. For electronic format permissions, Requestor agrees to provide a hyperlink from the reprinted APS material using the source material’s DOI on the web page where the work appears. T he hyperlink should use the standard DOI resolution URL, http://dx.doi.org/{DOI}. T he hyperlink may be embedded in the copyright credit line. 3. For print format permissions, Requestor agrees to print the required copyright credit line on the first page where the material appears: "Reprinted (abstract/excerpt/figure) with permission from [(FULL REFERENCE CITAT ION) as follows: Author's Names, APS Journal T itle, Volume Number, Page Number and Year of Publication.] Copyright (YEAR) by the American Physical Society." 4. Permission granted in this license is for a one-time use and does not include permission for any future editions, updates, databases, formats or other matters. Permission must be sought for any additional use. 5. Use of the material does not and must not imply any endorsement by APS. 6. APS does not imply, purport or intend to grant permission to reuse materials to which it does not hold copyright. It is the requestor ’s sole responsibility to ensure the licensed material is original to APS and does not contain the copyright of another entity, and that the copyright notice of the figure, photograph, cover or table does not indicate it was reprinted by APS with permission from another source. 7. T he permission granted herein is personal to the Requestor for the use specified and is not transferable or assignable without express written permission of APS. T his license may not be amended except in writing by APS. 8. You may not alter, edit or modify the material in any manner. 9. You may translate the materials only when translation rights have been granted. 10. APS is not responsible for any errors or omissions due to translation. 11. You may not use the material for promotional, sales, advertising or marketing purposes. 12. T he foregoing license shall not take effect unless and until APS or its agent, Aptara, receives payment in full in accordance with Aptara Billing and Payment Terms and Conditions, which are incorporated herein by reference. 13. Should the terms of this license be violated at any time, APS or Aptara may revoke the license with no refund to you and seek relief to the fullest extent of the laws of the USA. Official written notice will be made using the contact information provided with the permission request. Failure to receive such notice will not nullify revocation of the permission. 14. APS reserves all rights not specifically granted herein. 15. T his document, including the Aptara Billing and Payment Terms and Conditions, shall be the entire agreement between the parties relating to the subject matter hereof.

Page 3 of 3

• • • • •

DFT (density functional theory) computations on the stability of 2D-AlN DFT analysis of optical properties of 2D-AlN DFT computations of electronic and magnetic properties of 2D-AlN DFT analysis for potential applications of 2D-AlN on gas sensing Growth strategies of 2D-AlN

Review Properties and potential applications of two-dimensional AlN Milena Beshkova1* and Rositsa Yakimova2 1

Institute of Electronics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd, 1784 Sofia,

Bulgaria, [email protected] 2

Department of Physics, Chemistry and Biology (IFM) , Linköping University, SE-581 83 Linköping,

Sweden, [email protected]

Abstract The success of Graphene has triggered the research interest in other stable, single and few-atom-thick layers of van der Waals materials, which can possess attractive and technologically useful properties. Other complex structures, such as boron nitride, Mxenes and metal chalcogenides have been successfully synthesized as layered materials showing advanced properties. Here, after an introduction briefing novel 2D materials, we focus on 2D AlN and present a review covering theoretical considerations on the stability of an infinite hexagonal AlN (h-AlN) sheet, differences that occur in the electronic structure between bulk AlN and single layer and discuss possible methods of tuning their electronic and magnetic properties by manipulating the surface and strain using DFT (density functional theory) computations. We address potential applications of 2D-AlN with an emphasis on gas sensing for CO2, CO, H2, O2, NO and NO2 in the presence of NH3. Further, we discuss some growth strategies of AlN single layer and few layers on different substrates. 2D AlN layers and nanotubes with ultrawide bandgap (9.20-9.60 eV) which shows a great potential to support innovative and front-end development of deep-ultraviolet optoelectronic devices are illustrated.

1

Keywords: 2D-AlN, DFT, synthesis, sensing

1. Introduction Two-dimensional (2D) materials have been of a vast interest in the material science area because of their unique physico-chemical properties and rare physical phenomena that arise when charge and heat transport is confined to a plane [1]. The first experimentally demonstrated material of this class is graphene. Because of the unique lattice symmetry graphene shows high stability and exceptional physical properties [2-6] such as high electron mobility even at room temperature, ambipolar effect, Klein tunneling, and anomalous half-integer quantum Hall effect. With further development of graphene materials, e.g. functionalization, intercalation, etc, new effects and potential applications are discovered, which suggest that graphene has a plethora of unexplored performance. On another hand, graphene has some limitations for electronic device applications such as the zero-band-gap semi-metallic character which results in low on/off ratios in case of switching devices and low reproducibility in large-scale production [7]. 2D materials beyond graphene may bring new solutions of the identified problems. Different from gapless graphene, some two-dimensional layered materials (2DLMs) are found to be insulators, for example h-BN [8-10], topological insulators e.g. Bi2S3, Bi2Te3

and

Sb2Te3

[11-12]

and

semiconductors,

namely

transition-metal

dichalcogenides (TMDs, e.g., MoS2 and WS2) [13-15]. 2D hexagonal BN (h-BN) is the insulating isostructural form to graphene built up of alternating boron and nitrogen atoms and having band gap ~ 5.9 eV. As a monolayer it

2

has been studied over a decade [16]. BN nanosheets can be obtained by exfoliation methods as well as CVD (chemical vapor deposition). Exfoliated BN has better structural quality than the one made by the chemical deposition technique. Atomically thin h-BN has the potential to be used in many applications such as different types of coatings, gate and substrate dielectrics for nano-electronic devices, high-

temperature resistive layers, and other advanced applications of 2D material systems. It has been shown that by using h-BN as an encapsulant the mobility of graphene can be enhanced and the charge neutrality point shifted to zero, see Fig. 1 [17,18]. BN nanotubes have been also investigated. They are kind of analogue to CNTs but possess partly polar bonding between B and N. They are believed to be promising for future high-performance composites, bio-, electromechanical, and medical devices. A comprehensive review of BN nanotubes and nanosheets can be found in Ref. [8]. Transition metal dichalcogenides (e.g. MoS2) [19] have semiconducting properties which can complement those of graphene and BN. Interesting materials are group 13 metal chalcogenides (GIIIAMCs) [20] that can be devided into the following two categories on the basis of their stoichiometric composition: one is MX (e.g, InS [21] InSe [22], InTe[23], GaS [24], GaSe [25] and GaTe [26] and the other is MaXb (e.g. In2S3 [27], In2Se3 [28], In3Se4 [29-31] and In4Se3 [32] and In2Te3 [33]. GIIIAMCs exhibit a variety of polymorphous structures. InSe is a semiconductor with a direct energy bandgap (Eg=1.26 eV) at room temperature and the band gap can be further widened by having quantum confinement at reduced sample thickness [21, 34-36]. Layered InSe of three polytypes (β, γ and ε) can be made by changing the stacking sequence of primitive layers [22, 37].

3

The mobilities of mechanically exofoliated β-InSe FETs (field-effect transistors) with PMMA (polymetyl metacrylat)/Al2O3 bilayer dielectric [38] are up to 1055 cm2V1 -1

s and the current on/off ratio is 1×108. 2D chalcogenides Cs-Ag-Bi-Q (Q=S, Se), where Cs ion can be exchanged with

other cations, such as Ag+, Cd2+, Co2+, Pb2+, and Zn2+, form new phases with tunable band gaps between 0.66 and 1.20 eV [39]. Typically, 2D chalcogenides, along with graphene (conductor) and h-BN (insulator) can be arranged in multilayered van der Waals stacks having different applicationslasers, LEDs, sensors, etc. Much attention has been given recently to the emerging class of 2D materials named Mxenes. These are 2D transition metal carbides and/or nitrides [40-42] that have attracted extensive theoretical and experimental research [43-45]. MXenes are obtained [40,46-47] from the atomically laminated parent Mn+1AXn (MAX) phases, where M is a transition metal, A is an A-group element, X is C and/or N, and n=1-3 [48,49]. MAX phases are particularly interesting with the nature of bonding, which is a mixture of metallic, covalent and ionic character [50]. Because of that they show a combination of metallic and ceramic properties, e.g., high hardness, high thermal decomposition temperature, and oxidation resistance in parallel with good electrical and thermal conductivity and good machinability [48]. During removing the A-element from the MAX phase for MXene preparation the new surfaces are terminated by species that are native to the etchant and represent a combination of –O, -OH, and –F [51-52]. MXenes are different from other 2D materials with their high conductivity, hydrophilicity, and chemical diversity. Due to the latter they are tunable in composition, e.g. Ti3C2Tx, Mo2CTx, V2CTx. Changing the surface

4

terminations may further change the materials characteristics [53-55, 52]. Combining the above-mentioned properties gives benefits for e.g. energy storage in MXenes based electrodes [56]. Furthermore, it has been demonstrated [57] that in two-dimensional Mo1.33C MXene with divacancy ordering the specific capacitance can be increased remarkably. During the last years an emerging interest has been observed towards twodimensional aluminum nitride (2D AlN). Structure stability as well as the electronic properties of 2D AlN are becoming a “hot” research topic, which can expand the range of possible applications of AlN and open new perspectives for miniaturization in engineering functional nano-devices and interconnects. In AlN nanostructures such as nanosheets (NSs), the bonds between the nearest Al and N atoms are formed from the bonding combination of Al-pz and N-pz orbitals. Due to the electronegative difference between Al and N atoms, electrons are transferred from Al to N. This results in partially ionic bonding and makes a difference from the purely covalent bonds in graphene [2-6, 58]. Therefore, the charge transfer from Al to N dominates several properties of 2D AlN including opening of a band gap. While Al and N atoms are four-coordinated in bulk AlN, in AlN sheets they are three-fold-coordinated with dangling bonds, which can serve as active sites for gas adsorption [59,60]. Furthermore, in contrast to the nonpolar bonds in graphene, owing to the charge transfer from Al to N, the Al-N bonds in 2D AlN have a dipole moment, which also plays an important role in the adsorption of gas molecules. Such features may be beneficial for sensor applications.

Synthesis of 2D materials with dedicated properties is a case specific task and differs considerably from material to material.

5

Deposition of materials from their vapor is an attractive versatile synthesis strategy. However, the development of a controlled synthesis method of 2D materials such as 2D AlN by vapour deposition requires better understanding of the fundamentals involved. Vapour deposition has been extensively used for the growth of thin films and nanostructures, such as AlN nanowires [61]. Knowledge obtained from these materials may not be directly applied to 2D AlN materials, but the growth experience can be further used to elaborate new recipes. The key to the synthesis of substrate supported 2D AlN single layers is to realise monolayer (ML) van der Waals epitaxy in which the associated forces have much smaller energies (40-70 meV), than covalent binding energies of 200-6000 meV [1]. This review presents considerations related to the stability of an infinite hexagonal 2D AlN (h-AlN) sheet within the framework of DFT (density functional theory) as well as novel optical and magnetic properties of 2D AlN nanosheets with tuneable band structure. Potential applications in spintronics, gas sensing and gas storage are addressed. Experimental approaches towards growth of substrate supported 2D AlN nanosheets and their physical characterization are discussed.

2. Basic properties of 2D AlN 2.1 Structural issues

De Almeida Junior et al. [62] carried out theoretical calculations on the stability of an infinite hexagonal AlN (h-AlN) sheet and its structural and electronic properties within the framework of density functional theory [63-65]. For perfect h-AlN they predict an Al-N bond length of 1.82 Å (Fig.2) which agrees with works by others [66], and an indirect band gap of 2.81 eV.

6

For evaluating the stability of an AlN model system, E.F. de Almeida Junior used the cohesive energy per atom Ecoh/at defined as follows:

Ecoh/at = [Etot- (nNEN+nAlEAl]/(nN + nAl)

(1)

where Etot is the total energy of the model system, nN and nAl are the number of N and Al atoms per unit cell, and EN and EAl are the energies of the isolated N and Al species, respectively. The calculated value for the cohesive energy per atom (Eq. (1)) for an infinite h-AlN sheet is 5.03 eV which is only by 6% lower than that of the bulk (wurtzite) AlN. This result can be assumed as a qualitative indication of the synthesizability of individual h-AlN sheets. Besides the study of a perfect h-AlN sheet, the formation energy of the most typical defects in both Al-rich and N-rich environments are listed in Table 1. They are described as following: (i)

vacancies at an N(Al) position in the layers’ network (VN/VAl),

(ii)

an antisite defect, i.e when at an N place there is an Al atom (NAl), or viceversa (AlN)

(iii)

substitutional defect –when an atom of a chemical element different from both N and Al occupies an N or Al position in the network (e.g., C atom at Al position is referred to as “CAl”, a Si atom at the N position as SiN, respectively).

The formation energy can be perceived as the energy cost for stabilizing the corresponding defects and is shown below.

7

Table 1 shows that the anti-site defects, which introduce homo-nuclear bonds, are expectedly the least feasible ones, while the impurity defects are the most energetically favourable. This is especially so for the defects arising from Si impurity. Defects significantly change the band structure in the vicinity of the Fermi level in comparison to the band structure of the perfect h-AlN which can be used for deliberately tailoring the electronic properties of individual h-AlN-sheets.

2.2 Optical properties Sh. Valedbagi et al. [67] calculated the electronic structure and optical properties of AlN nanosheets by density functional theory in the generalised gradient approximation [18-20].

Table 2 presents the energy gap of AlN bulk (wurtzite structure) and

nanosheet. The results are consistent with previous works [62, 71-72, 84]. It is seen that changes of dimensionality from 3D to 2D leads to almost two-fold narrowing of the energy bandgap of AlN. Optical characteristics such as dielectric function, extinction index, reflectivity, absorption coefficient, and optical conductivity of AlN nanosheets are modelled for both parallel and perpendicular electric field polarizations. Dielectric function is a complex measure that labels the linear response of the system to an electromagnetic radiation. In Table 3 the dielectric constants ε(ω) and refractive index n(ω) of AlN bulk (wurtzite structure) and of AlN nanosheet are compared. The results are consistent with a previous work [73].

8

The absorption coefficient for AlN nanosheets is shown in Fig. 2. In (E || x) geometry, the first absorption peak occurs at an energy of 6.08 eV, and the highest absorption peak is at energies of 10.5-11.4 eV. In (E || z) geometry, the first absorption peak occurs at an energy of 6.5 eV and the highest absorption peak is at an energy of 10.25 eV. The real part of the optical conductivity for AlN nanosheet is shown in Fig.3. Optical conductivity in (E || x) and (E || z) starts with a gap of about 5 eV and 4.0 eV, respectively, which confirms that the AlN nanosheet has semiconductor properties.

2.3 Effect of strain on the electronic properties of 2D AlN Strain and defects can be very influential for carrier mobility, electronic and band structure in nanomaterials. Peng Liu et al. [74] studied by DFT the electronic structure of AlN nanosheets at various strain conditions [75-77]. Also, phonon dispersion calculations have been applied to assess the dynamical stability of AlN under different modes of strains. For a systematic study the AlN nanosheet is set in a planar hexagonal honeycomb lattice in supercell with 2 different symmetries - hexagonal and rectangular (Fig.5) Fig.5a shows the rectangular supercell with the zig-zag and arm-chair directions aligned along its x and y-axes, respectively. The hexagonal supercell is shown in Fig.5b in which both the x and y axes are equivalent and aligned along the zig-zag direction. Then tensile and compressive axial strains were applied by simulation in which the lattice constant of the nanosheet was scaled.

For the rectangular supercell three

different modes of strain were investigated: (a) uniaxial strain along the x axis (i.e. zigzag direction); (b) uniaxial strain along the y axis (i.e. arm-chair direction); (c)

9

homogeneous or symmetric biaxial strain along both x and y directions. Shear strain is also considered: (d) simultaneous application of equal magnitude of compressive strain along x-direction and tensile strain along y-direction and (e) simultaneous applications of equal magnitude of compressive strain along y-direction and tensile strain along xdirection. Fig.6 shows the variation of the band gap of AlN nanosheets under different kinds of strain. In case of large shear strain (≥7.5 %) and 10 % uniaxial strain along the zig-zag direction an indirect-to-direct transition in the bandgap is observed. Notably, the effect of strain on the two- dimensional AlN nanosheet is controlled by quantum confinement effects, unlike the response of its 3-dimensional bulk phase counterparts to pressure [75]. A summary of the results on the dynamical stability of AlN nanosheets is tabulated in Table 4. The AlN nanosheet in both hexagonal and rectangular supercell is found to be stable in the range of 2.5-10 % tensile, as well biaxial strain. The AlN nanosheet shows reasonable dynamical stability under shear strain along the two equivalent zig-zag directions, while it is not found to be stable when exposed to shear strain along its two non-equivalent directions. In the case of three dimensional heterostructures, lattice mismatch could generate topological defects, like linear dislocations [76, 77]. Dulche C. Camacho-Mojica et al [78] focused on the study of extended line defect (ELD) formed by 8-4 membered rings, Fig.7, into the honeycomb monolayer of AlN using first-principles density functional theory calculations [79,80].

10

This ELD separates two honeycomb regions representing a grain boundary in a twodimensional system. The extended line defects in AlN sheet structure showed to be stable, keeping the planarity, the calculated formation energy is -0.765 eV. Electronic density of states calculations demonstrated that the band gap is reduced with 8 % compared with pristine AlN monolayer when ELD is introduced into the honeycomb structures. In addition, the electronic density of states exhibits in-gap states in the valence band. In depth studies related to ELDs suggest that these types of defects could have applications in photonic acting as a waveguide, where the grain boundaries act as a reflector at some energy.

2.4 Tuning electronic and magnetic properties Chemical functionalization is an effective way to modify and control the electronic and magnetic properties of nanostructures. Interaction of hydrogen with 2D AlN is of technological significance. Comparing with graphene, where carbon atoms are chemically equivalent, in AlN sheets Al and N atoms have different reacting and bonding behaviour with hydrogen. Based on first-principles calculations [81, 82], the geometric, electronic and magnetic properties as well as relative stability of fully hydrogenated and semi hydrogenated AlN nanosheets have been investigated by Chanwen Zhang et al [83]. For plain AlN NSs with n<5, where n is the number of single bilayers (Al-N), the single bilayer transforms to a graphite-like structure after structural optimization, Fig.8. In this case, the Al-N bond contracts by 0.089 Å within each bilayer and the inter

11

bilayer distance expands by 0.305 Å. AlN sheet exhibits an indirect band gap of 2.93 eV, which is smaller than that one in bulk (4.2 eV). It seems that adsorbed hydrogen can lead to structural transformation from planar AlN NSs to initial wurtzite configurations. Fig.8 shows the relaxed geometric and electronic properties of fully hydrogenated NSs with a thickness of one and two bilayers. Al and N atoms are in sp3 hybridization with H atoms bonded to Al and N atoms in the polar surface where the optimized Al-H bond lengths are 1.58 and 1.59 Å, and N-H bond lengths are1.03 and 1.02 Å for 1-AlN-2H and 2-AlN-2H structures, respectively. Interestingly, the band gap of fully hydrogenated sheets progressively increases with increasing bilayer thickness n, although it is narrower than that of a bare AlN graphitic sheet, which is 2.93 eV. As shown in Fig.9, the band gap increases from 0.33 eV in 1AlN-2H to 1.61 eV in 2-AlN-2H. However, when n>4, the hydrogenated n-AlN -2H becomes a ferromagnetic (FM) metal with a magnetic moment of 0.44∼0.66 µB. Semi-hydrogenated sheets on the Al sites show very interesting properties. When the number of bilayers n equals to even, the material is magnetic semiconductor, whereas for odd bilayer numbers, half-metallic properties are observed. The possible reason is attributed to a quantum confinement due to the Al-H bond length contraction. For two and four bilayers AlN sheets show an indirect band gap of 2.96 eV and 4.1 eV in the spin-up channel, respectively, but when the thickness increases to six bilayers, a direct band gap occurs in the spin-up channel.

12

In the case of semi hydrogenation on the N sites (n=1), the system presents nearly half-metallic properties in the metastable states, whereas at n>2 weak magnetic metallic properties are observed. The stability of hydrogenated AlN NSs has been studied by Chan-wen Zhang et al [83] who perform calculations of the formation energies for both fully hydrogenated and semi-hydrogenated systems. The formation energy is defined as

Ef= E(H:AlN) - E(AlN) - ∆nHµH

(2)

where E(H:AlN) and E(AlN) are the total energies of the supercell with and without hydrogen, respectively. µ is the half of the binding energy of H2, and ∆n is the difference in concentrations of hydrogen atoms in fully hydrogenated and semihydrogenated systems. According to this definition, the nanosystem with the smaller formation energy is more favourable and should be preferred in experiments (Table 5). Analyses of the formation energies show that thicker NSs are more likely to be realisable in all cases. For the case of n=1, the adsorption of hydrogen on the top of Al sites is the most stable structure, while for the other configurations (n>2), AlN-nH structure is easily realized in experiments. In another work Chan-wen Zhang et al [84] performed first-principle calculations to investigate the electronic structures and magnetic properties of two-dimensional AlN nanosheets decorated with fluorine or co-decorated with H and F on each side. The authors studied how to control the band structure and magnetism when F decorated Al atoms in AlN nanosheets (F-AlN) due to the weak bonding between F and N atoms

13

[85]. It was found that ferromagnetic (FM) state is half-metallic, where the spin up channel remains insulating with a band gap of 5.5 eV, and the spin down channel shows conducting character. Using mean field theory [86], the Curie temperature is predicted to be above room temperature, which is of interest for applications in spintronics. To find out how to co-decorate AlN nanosheet with H and F atoms on both sides, the relative stability energy between the case of F-AlN-H and H-AlN-F structures are calculated. It is concluded that the former is lower with 1.51 eV in energy than the latter one. So, the study is focused on F-AlN-H structures. The spin polarized calculations show that the co-decorated F-AlN-H system is an indirect band gap semiconductor with a band gap of 3.58 eV, larger than that of pristine AlN nanosheets. Although bare AlN nanosheets is a 2D isotropic system, when co-decorated with F and H atoms on each site of the nanosheets, they show anisotropic semiconducting character due to the charge redistribution in different atoms, which facilitates selfassembly under an applied electric field. Recently, there is a growing interest in introducing anisotropy [87] to nanostructures for novel applications in molecular recognition, self-assembly, sensors, solar cells, etc. We expect that the co-decorated anisotropic F-Al-H nanosheet may have a potential in these fields.

2.5 Adsorption properties

Adsorption of species (atoms, molecules or clusters) on surfaces is a fundamental process which has a tremendous importance for sensing, catalysis, surface physics, etc

14

and has resulted in extensive research [88]. In general, solid surfaces are likely to attract gas molecules and some of them are trapped on the surface. Y. Wang et al [89] carried out DFT computations to explore the adsorption behaviour of common gas molecules including CO2, CO, H2, CH4, O2 and NO on AlN monolayers. Typically, there are four different adsorption sites on AlN monolayers: the hollow site, which is the hollow centre of the Al-N hexagon (H), the bridge of Al-N (B), the top of the Al atoms (T1) and the top of the N atoms (T2), Fig.10. For calculations, the gas molecule is initially placed with its center of mass exactly located at these sites. Typical orientations of the gas molecules with respect to the AlN surface are considered. For example, a CO2 molecule is initially placed either as parallel or vertical to the surface of the AlN sheet for all possible adsorption sites. For all chosen molecule/AlN systems, the adsorption energy is defined as Ea=Egas+EAlN - Egas/AlN

(3)

where Egas, EAlN and Egas/AlN are the energies of the isolated gas molecule, AlN sheet and the gas molecule/AlN system, respectively. By this definition, the larger Ea the stronger the interaction between the gas molecule and the AlN sheet. A negative value of Ea indicates that the adsorption is endothermic. The adsorption energy, the charge transfer for the adsorption of gas molecules on the AlN sheet, and the distance between the AlN sheet and the nearest atom of the gas molecule are summarized in Table 6. Adsorption occurs depending on the distance between the molecule of interest and the AlN sheet. As an example, the distance between N2(CH4) and AlN sheet is 3.279 Å

15

(3.212 Å), which is larger than 3 Å. This suggests that the N2 and CH4 molecules cannot be adsorbed on the AlN sheet. Table 6 shows that CO2 can be firmly adsorbed on the AlN sheet with the adsorption energy of 0.91 eV, while the adsorption energies of other gas molecules such as CO, H2, N2, CH4, O2 and NO are less than 0.5 eV. It has been identified that the adsorption energy should be greater than 0.5 eV for an effective capture of a gas molecule on a solid surface [90, 91]. Therefore, the capture of CO2 on the AlN sheet is decidedly favoured over other gas molecules. Compared with GGA+vdW (van der Waals), the adsorption energy is underestimated about 0.05-0.23 eV by using only GGA (Generalized Gradient Approximation). This suggests that vdW interaction plays an important role in the adsorption between gas molecules and the AlN sheets. Fig.11 gives the charge density differences for gas molecule/AlN systems, which are obtained from the following formula

∆ρ=ρ(gas/AlN)-[ρ(gas) + ρ(AlN)]

(4)

where ρ(gas/AlN) and ρ(AlN) denote the total electron densities of the relaxed AlN sheet with and without gas molecule, respectively, and ρ(gas) is the total electron density of different gas molecules. The red region in Fig. 11 shows the charge accumulation, while the blue region represents the charge depletion. In the case of pristine AlN sheets, due to charge transferring from Al atom to N atom, the Al-N bond is polarized with negative charges on N atoms and positive charges on the Al atoms, Fig.11a. Thus, the AlN sheet can adsorb gas molecules through electrostatic and polarization interactions [92, 93].

16

Calculations show that with increasing the charge transfer the adsorption energy becomes larger. According to Bader charge analysis [94] the gas molecules behave as either charge acceptors (0.66e, 0.15e, 0.17e, and 0.30e for CO2, CO, O2 and NO respectively) or donors (0.20e for H2). After CO2, CO, O2 and NO molecules adsorption, electron charges are transferred to the gas molecule, signifying a p-doping effect on the AlN. Provided AlN is used as a channel in a device, the channel resistance will increase in the case the adsorbed molecules are trapping electrons. While the adsorption of H2 induces n- type doping effect on the AlN sheet. In that case the channel resistance decreases and the current increases. From this point of view, the AlN sheet can be used as gas sensors for the above-mentioned gas molecules. The band gaps of different systems are given in Table 6. The band structures are not significantly changed when CO or CO2 are adsorbed on the AlN sheet. In the case of H2/AlN due to the CBM (conduction band minimum) and the VBM (valence band minimum) moving down compared with the bare monolayer AlN, the spin up band lies near the Fermi level and the spin down crosses the Fermi level at Γ, which indicates that metallic state is achieved. Interestingly, for O2/AlN and NO/AlN, spin polarization was found, probably indicating the properties of diluted magnetic semiconductor. Especially, knowledge of adsorption of toxic species aiming at monitoring and reducing pollutants in the environment is of excessive importance [95].

Highly

selective and fast gas sensors are needed for diverse applications such as providing precaution against toxic gases that human beings do not have opportune alertness to their presence. S.F. Rastegar et al [96] have studied the adsorption of NH3 and NO2 molecules on the surface of AlN sheets exploring its potential application as a gas sensor. Adsorption

17

energies (Eads), density of states (DOSs), structural parameters and net electron transfers are calculated. The Eads of the molecules on the surface of the sheet is defined by Eq.(5) Eads = Ecomplex - EM - EAlN + EBSSE

(5)

where Ecomplex, EM and EAlN are energies of AlN sheet with adsorbed molecules, isolated NH3 (or NO2) and the free AlN, respectively. The sensitivity of AlN towards NH3 and NO2 was probed by positioning the molecules at different adsorption sites with different orientations. Table 7. summarises equilibrium geometries, stabilities, and electronic properties of NH3 and NO2 adsorptions on the surface of an AlN sheet. The computed Eads for only one stable configurations of the NH3/AlN sheet complex suggests a strong interaction between the molecule and the AlN sheet. In the case of NO2, two stable configurations were obtained with considerable Eads values. NH3 has a closed shell structure and usually donates electrons to AlN sheet which acts as adsorbent while NO2 has a strong electron acceptor character. It is expected that the charge transfer between the sheet and NH3 or NO2 rearranges the electronic levels of the AlN sheet. The DOS of the optimized AlN sheet exhibits an Eg of 4,68 eV, indicating a relatively wide band gap semiconductor. While upon adsorption of NH3, the Eg of the AlN sheet slightly changes from 4.68 to 4.66 eV, indicating that the sheet is not sensitive to NH3. In contrast, upon strong adsorption of NO2 Eg is considerably decreased. The drastic changes in Eg (about 68%) are responsible for making p-type semiconductor by shifting the LUMO of isolated sheet toward lower energies in its complex form. From the following relation [97]:

18

− Eg    2 kT 

σ ∝ exp 

(6)

at a given temperature, a smaller Eg leads to a larger electric conductivity ( σ is electric conductivity and k is the Boltzmann’s constant). As a result, it is expected that adsorption of NO2 on the surface of AlN sheet may lead to a strong enhancement of the electronic conductance of the sheet which is favourable for sensing applications. Hence, the AlN sheet may selectively detect NO2 molecules in the presence of NH3 molecules. Carbon monoxide (CO) is a colourless and odourless toxic gas, which can make human suffocated because of its higher binding ratio with haemoglobin than that of oxygen. Thus, it is crucial to monitor its leakage. A lot of materials such as carbon nanotubes [98, 99], nanotube structures of BN, AlN, BP, and AlP [100] and doped or decorated graphene [101-103] have been considered to detect CO molecule. While, the ability of h-AlN nanoshets for detecting CO molecule has not been deeply explored yet. T. Ouyang et al [104] studied the adsorption of CO molecules on pristine and defected h-AlN nanosheets including C-doped ones, also N-vacancy containing monolayers, and reveal the underlying electronic structure origins behind the adsorption behaviour [105]. The adsorption energy (Ead) of CO on AlN nanosheets is defined as follows: Ead=EAlN+CO –EAlN -ECO

(7)

where EAlN+CO is the total energy of the system with CO adsorbed on various AlN nanosheets, while ECO and EAlN are the total energy of isolated CO molecule and AlN nanosheets, respectively. To find out the most stable configuration of a CO molecule adsorbed on pure AlN nanosheets eight configurations for adsorption have been considered [105]

19

The calculated adsorption energies and geometry parameters for these different configurations are shown in Table 8. The computed binding distance which is defined as the nearest distance between CO and the h-AlN nanosheet for each of configurations agrees with the corresponding moderate adsorption energies. Among all the optimized structures, the absolute value of CO adsorption energy (Ead) in case (b) is the largest one amounting -3.847 eV and the corresponding binding distance is about 2.240 Å. In this geometry the C atom is situated above the Al atom and the C-O bond is vertical to the AlN sheet. The energy band gap changes from 2.943 eV to 2.593 eV, which designates that the pure h-AlN nanosheet is not very sensitive to CO gas adsorption. It would be of interest to estimate the capability of C-doped h-AlN nanosheet for adsorbing CO molecule. For that one N atom of pure h-AlN nanosheet is substituted by a C atom. The CO adsorption energy of this C-doped system is -5.192 eV, which is 1.35 times larger than the value of pristine h-AlN. It has been found that the band gap of the system changes from 3.048 eV to 0.490 eV after CO adsorption. This big change of the band gap is significant for changing the electrical conductivity, which is related to the sensitivity of the signal in gas sensing. There are 0.367 e- charge transferring from the nanosheet to the CO molecule. To understand the effect of vacancy defects on CO adsorption, both N-vacancy and Al-vacancy are studied. The formation energy of N-vacancy has been calculated to be more beneficial than that of Al-vacancy defect. In the case of N-vacancy defected hAlN nanosheet after the adsorption of CO molecule the band gap of the system increases from 0.070 eV to 1.366 eV. It implies that the change of electronic

20

conductivity of the material can be pronounced and the N-vacancy in a h-AlN nanosheet may also be a good candidate for CO gas sensing. Summarizing the above discussed cases, from the list of parameters in Table 9, it can be seen that compared with the C-doped AlN nanosheet, the N-vacancy containing nanosheet is inferior in terms of the value of the band gap change in CO gas sensing, but it is still a good candidate relative to pure AlN monolayer.

3. Synthesis of two-dimensional AlN Van der Waals epitaxy of crystalline 2D materials on crystalline substrates is defined by strong bonding at the reactive edges of the single-crystal domains of the material and weak interlayer forces between the sheets. The key issue is that 2D graphitic materials could couple weakly between each other through Van der Waals forces maintaining the integrity of each layer and preserving their good physical properties near to ideal, normally obtained in their free-standing form. However synthetic growth of such materials is a challenge because of structure stability problems. Naturally, AlN grows in the wurtzite crystal structure with 4-fold atomic coordination and a sp3-like bonding resulting in an in-plane constant of 3.11 Å [106]. However, it has been predicted that AlN can adopt a graphite-like hexagonal crystal structure of up to about 22 monolayers thick [107, 108] or even thicker of up to 48 monolayers if a 5% tensile strain is applied [109]. The atoms are 3-fold coordinated with sp2 bonding and a slightly larger in-plane lattice constant of 3.12-3.16 Å [62, 83, 110]. P. Tsipas et al [111] report on the epitaxial growth of AlN films by plasma assisted MBE (molecular beam epitaxy) on a single crystal Ag (111) substrates. AlN was made

21

by evaporating Al metal from a cold tip effusion cell with pyrolytic boron nitride crucible in the presence or after exposure to reactive atomic nitrogen beam generated by the remote radio frequency (rf) plasma source. Various deposition temperatures from 136 to 650 °C, Al deposition rates, from 0.06 to 0.22 Å/s, as well as AlN thickness from sub-ML to 20 ML were applied to achieve the desired structure. RHEED measurements were performed and from the REED pattern along the [110] azimuth of Ag(111) prior and after AlN growth, Fig. 12(a), one can conclude that there is a perfect alignment between the two hexagonal unit cells in the reciprocal space. This is indicative of epitaxial growth. Fig.12b represents the RHEED patterns of 4MLAlN/Ag(111) and bulk 200 nm-AlN/Si(111) along the [11-2] direction on silver, respectively. Given the lattice constant of silver is 2.89 Å, the in-plane lattice constants of the two AlN materials can be calculated from the spacing between streaks in the RHEED pattern (intensity pattern in Fig. 12c). For the bulk wurtzite structure, the inplane lattice constant is ∼3.1Å, whereas for the ultrathin AlN a slightly larger value was determined (∼3.13Å). This divergence suggests that most probably the ultrathin AlN layers are stabilized in a hexahonal graphite-like structure and not in a wurtzite structure. Further the UPS (ultraviolet photoelectron spectroscopy) indicated a reduced bandgap as expected for graphite-like hexagonal AlN, Fig.13. The valence band maximum positions measured from the Fermi level were determined by linear extrapolation of the leading edge of VB spectra to the base line. From Fig. 12 a value of ∼5.17 eV has been estimated for the VBM of the bulk-like wAlN/Si(111) reference while in the case of the 12 ML-AlN on Ag (111), the VBM is

22

located at ∼2.70 eV, a value that is smaller compared to the 3.33 eV measured in the Ag/wurtzite-AlN bulk system [112]. Fig.13 supports the statements made above on the basis of structural analysis that thin AlN/Ag(111) adopts a hexagonal graphite-like lattice with different electronic valence band structure compared to bulk wurtzite structure. The combination of other graphene-like materials, e.g. Si (silicene) or Ge (germanene)-which are gapless semiconductors with linear energy dispersion near the K points –with graphene-like dielectric materials (AlN, BN, BeO, ZnO) could lead to a foremost technological advance [111, 113]. Moreover, it is theoretically foreseen that silicene is stable when encapsulated between two graphene-like AlN (hereafter denoted as g-AlN) layers. V. Mansurov et al [114] investigated the formation of graphene-like (g-AlN) on the (111) Si surface in ammonia molecular beam epitaxy (MBE) by the RHEED technique. A flat ultrathin layer (5-6 monolayers) of AlN was prepared using the following two stage procedure. Initially, the clean (1×1) Si surface was exposed to an ammonia flux (10 sccm) at a substrate temperature of 1050°C, and in the second stage the AlN layer was formed by deposition of Al, the ammonia flux being switched off and the background ammonia pressure of 1.33 10-7 – 10-8 mbar being attained. The transformation of the surface structure from (1 × 1) to highly ordered (8 × 8) structure was observed by RHEED during the ammonia treatment of the Si surface for about 5 s, Fig. 14a. The ammonia flux was switched off at the instant when the best (8 × 8) RHEED pattern with sharp and bright eightfold fractional spots was reached, this corresponding to the maximum of the curve in Fig. 14 b (marked as “highly ordered 8 × 8).

23

RHEED diffraction pattern evolution during the deposition of Al was studied. Under the Al flux the intensity of the (8 × 8) fractional spots decreases, while the intensity of the g-AlN fundamental lines increases. The kinetics of the normalized intensity of the (01) AlN streak measured at different substrate temperatures is displayed in Fig. 15a. Visibly the streak intensities saturated during 30-60 s. V. Mansurov et al [114] assume that this saturation is due to the depletion of the surface in respect to the chemisorbed chem

NH 2

which is consumed in the chemical reaction with aluminum resulting in the

formation of g-AlN

chem

Al+ NH 2

→ g-AlN + H2

(8)

Also, the figure clearly indicates that the increase of the substrate temperature causes a decrease of AlN development rate. The maximum value of the AlN formation rate (VAlN) vs. temperature is plotted in Fig. 15 b. V. Mansurov et al presume that the slope of the dependence of 2.1 eV relates to the energy of desorption of Al atoms (Edes) from the surface and with the decrease of the chem

surface concentration of the NH 2

species.

The g-AlN lattice constant increases from 3.08 Å to 3.09 Å under the Al flux owing to the interaction of g-AlN with an Al adsorbed layer but remains unchanged with the flux of ammonia. When III-nitride semiconductors are thinned to only a few atomic layers regarded as 2D materials, it would lead to an ultrawide bandgap (Eg) due to the quantum confinement effect [115, 116], which can be used in next-generation optoelectronic devices [118, 119].

24

Wenliang Wang et al [120] have successfully pioneered epitaxial growth of 2D AlN layers sandwiched between three-layer graphene and Si substrate by metal organic chemical vapor deposition (MOCVD). In Fig. 16 schematics of the formation mechanism of 2D AlN layers is presented. AlN is sandwiched between graphene and Si substrate (a). A three-layer graphene was transferred to Si(111) surfaces (b), on which binding with van der Waals forces is expected [120]. Then, the dangling bonds of the Si(111) surfaces were passivated by hydrogen and also the structure of graphene was probably disrupted (c), this producing surface defects corresponding to wrinkles. Later, trimethylaluminum and ammonia decomposition yielded Al and N atoms which were used as precursors on the graphene surface (d) and reached the interlayer via the pathway (e). Finally, active Al and N atoms reacted in the interlayer and synthesized into 2D AlN. The removal of graphene was achieved after 30 min ultrasonic cleaning in deionized water. The Eg of 2D AlN layers was theoretically predicted to be ≈ 9.63 eV by DFT and was experimentally determined to be 9.20-9.60 eV and 9.26 eV by ultraviolet photoelectron spectrometer (UPS) and UV-vis spectroscopic ellipsometer, respectively. These findings are innovative and may result in front-end developments of optoelectronics devices. Nanotubes (NTs) as an important ramification of 2D materials [121-122] provide an alternative pathway to exploring the intrinsic properties of ultrathin nitride layers. By combining epitaxy with selective thermal evaporation, P. Wang et al [123] designed a new roadmap to fabricate AlN NTs.

25

Fig.17 illustrates the preparation scheme of AlN NTs (panel a) and the corresponding atomic structures (panel b); i) first GaN nanowires (NWs) are grown; ii) core-shell GaN/AlN NWs are prepared, iii) the GaN core from the bottom is open end is evaporated; and iv) AlN NTs are formed. The bond energies of GaN (2.20 eV) and AlN (2.88 eV) are different and the corresponding decomposition temperatures for GaN and AlN are 850 and 1040°C, respectively at pressure <10-6 mbar [124-126]. Therefore, a thermal evaporating temperature (TE), being higher than the decomposition temperature of GaN but lower than that of AlN in stage iii, i.e., 850°C< TE<1040°C, can completely remove the GaN core following the principle of thermodynamics. The precisely controlled epitaxy of GaN/AlN core/shell NWs and the complete evaporating of GaN core (due to the relatively low diffusion coefficient of Ga/Al atoms at the annealing temperature 1000°C) allow exploring the evolution behavior of the bandgap as a function of the wall thickness. According to the TEM (transmission electron microscopy) images the radial growth rate should be around 0.067ML/s. Three representative NTs with AlN wall thickness about 8, 4 and 2 ML, respectively) were prepared and characterized, regarding band gap changes, by XPS (xray photoelectron spectroscopy) and EELS (electron energy loss spectroscopy). XPS scan of the N 1s core –level photoelectric peak is presented in Fig. 18a and EELS spectra taken from NTs with different wall thickness, 2, 4 and 8 ML respectively, are shown in Fig. 18b

26

From the above date, the bandgap energy of the AlN film is determined as Eg=Eloss EN1s=6.5 ± 0.2eV, which is comparable to the reported bandgap energy of AlN (6.2 eV) [124]. The bandgap energies of the AlN NTs with wall thickness of 8, 4 and 2 ML are determined to be 7.9, 8.4 and 9.2 eV, respectively. The measurement locations of EELS are indicated in the inset of the figure. These results are in good agreement with the values derived from the XPS measurements. The observation of bandgap widening in nitride thin-layered structures offers important material parameters for both fundamental physics study as well as future 2D optoelectronic devices. Xitian Zhang et al [127] present synthesis of AlN nanosheets on Au coated Si (001) substrates by vapor-phase transport method, using Al powder and ammonia as source materials. The substrate was placed above the source and with its Au-coated side facing it, which is a pure AlN powder accommodated in quartz crucible. The Au thin film is made by sputtering to a thickness of about 2 nm. The process temperature was continuously increased to 1150°C and held there for 120 min. At these conditions, Al vapor reacts with NH3 to form AlN according to the following reaction: 2Al (s) + 2NH3 (g)→2AlN(s) + 3H2(g). At high temperature AlN is deposited on the Si substrate coated with the catalytic Au film as nanoclusters. With growth velocity V〈1 1 00〉 along the 〈1 1 00〉 direction slightly faster than that of V〈0001〉, and both much faster than V〈11 2 0〉, the nuclei grow into rectangular nanosheets bounded by {11 2 0} surfaces as estimated by TEM studies. It can be assumed that the formed nanosheets act as a template, providing favorable nucleation sites for the growth of the next sheet. Such a process repeatedly happens,

27

ultimately leading to piling up of the nanosheets that develop into a stack morphology [128]. Au is a key factor during the formation of AlN nanosheets, although the detailed mechanism is not clear and needs further studies. Metal organic chemical vapor deposition of AlN on graphene heterostructures offers new opportunities for the development of flexible optoelectronic devices and for the advancement of conceptually new 2D AlN. MOCVD of AlN is complicated by the fact that the typical precursorstrimethylaluminum, (CH3)3Al, and ammonia, NH3 form highly-reactive (CH3)3Al:NH3 adducts, subsequently triggering intricate precursor/precursor and precursor/surface reaction [129]. Regardless of its limitation for treating electron transfer during chemical reactions [130] density-functional ab initio molecular dynamics (AIMD) is a brilliant compromise between accuracy and reliability vs. computational cost for identifying chemical reaction paths and approximating the relative existence of competing reactions. AIMD simulations were carried out by Sangiovanni et al [131] to reveal how each of the individual precursors-ammonia NH3, and trimethylaluminum (CH3)3Al – and the adduct –derived species (CH3)2AlNH2 with direct Al-N bonding, evolve/react with a graphene sheet. The simulations show much greater affinity of the Al-containing species as well as of the methyl groups to graphene as compared with NH3. It should be noted that Al has an empty orbital, and thus likely to accept graphene π electrons, while the N atom has a filled electronic shell that resists the graphene π-cloud and thus avoids ammonia attachment on graphene.

28

Using density-functional perturbation theory [75, 81,] Houlung L. Zhuang et al [132] identified three different 2D structures that are dynamically stable in III-V materials: a planar honeycomb hexagonal structure, a buckled hexagonal structure, and a surprising low-energy tetragonal structure. Except for AlN, GaN, and InN that have lower energies in the hexagonal structure than the tetragonal one. The nitrides and AlP prefer planar structures, while all other III-V materials exhibit buckled structures. High quality graphene has been grown on metal substrates [88, 89]. The metal surfaces efficiently catalyze the chemical vapor deposition process and stabilize graphene. Similarly, to synthesize 2D III-V materials, predicted by Houlong L. Zhuang et al. [132], suitable substrates are required that reduce the formation energies and stabilize the structure. Since most of the 2D tetragonal III-V materials have already quite low formation energies, the authors of [132] searched the Inorganic Crystal Structure Database [135] for suitable lattice matched substrate materials and calculated the adsorption energies. Five combinations of (100) surfaces of metal substrates, specifically Cu, Pt and Pd, and 2D tetragonal III-V materials were found to have lattice mismatches less than 1 %. The authors [132] computed the adsorption energies for these materials. The 2D materials are strained suitably to match the lattices of the substrates. Fig.19 illustrates the adsorption energies of the 2D tetragonal materials on these lattice-matched metal substrates. The III-V materials are chemisorbed on the metal surfaces, unlike graphene which is weakly physisorbed on transition metal substrates [136-138]. This is due to the polar nature of the III-V materials. The substrates significantly reduce the formation energies of the 2D materials.

29

Van der Waals interactions between the tetragonal 2D materials [139] further increase the adsorption energy, effectively reducing the energy of 2D tetragonal materials below that of the 3D bulk phase. These results demonstrate that the discussed hypothetical tetragonal 2D AlN materials could be synthesized on lattice-matched substrates. Further, related to electronic device applications, 2D AlN is a perspective material to be used as dielectric, for example, as gate dielectric in transistors. The AlN crystals exhibit many desirable dielectric properties, including high-k, low current leakage, and high breakdown voltage. Moreover, due to the atomic flatness of the AlN crystals (root mean square roughness, rms ≈0.19 nm calculated from atomic force microscopy, AFM) and its weak van der Waals interaction with molecules, the ultrathin 2,6diphenylanthracene DPA crystals (thickness of 7.5 nm estimated from line profile of AFM image, which is corresponding to 4-5 molecular layers) were successfully deposited onto the AlN crystal via van der Waals epitaxy by Yang at al [140]. A principle problem in such applications is the existence of defects and traps at the semiconductor/dielectric interface and a relatively high surface roughness of the dielectric layer, which play an adverse role in the efficient charge transport. The most commonly used morphologies of crystalline AlN are 1D nanostructures and 3D bulk phases while 2D AlN single crystals as gate dielectric are in their infancy. The development of such material still suffers from nonideal 2D morphology, not sufficiently thin and laterally continuous dielectric [141-144, 61, 145-149]. Morphology evolution of crystals during growth is governed by the thermodynamis of the growth system/process. Temperature and supersaturation of the AlN vapor are the

30

main parameters that can be controlled by the grower and have the major effect on the 2D AlN crystal morphology. [150]. Yang at al [140] synthesized high-quality c-axis oriented hexagonal AlN dielectric crystal with a typical 2D morphology and thickness (200-400 nm) by physical vapor transport method. This method provides independent control of the substrate temperature and source temperature, which allows the preparation of AlN crystals under high temperature (source temperature 2100°C and substrate temperature 100-150°C lower) and high supersaturation conditions. Combining 2D AlN dielectric with DPA molecular crystals in a field effect transistor may establish a perfect all-crystalline platform with minimized defects and charge traps at the interface. This will result in improved transistor performance. These results are among the best performance of Organic FETS based on the employment of high-k dielectric layers [151-154]

4. Summary and outlook In this review we have presented recent progress and trends in understanding properties, synthesis and some applications of 2D AlN nanosheets/layers. Due to the lack of knowledges and experience about 2D AlN synthesis, structure stability and physical properties the review covers the most important and relevant theoretical predictions along with recent results on 2D AlN synthesis. It has been demonstrated using DFT calculations that a perfect h-AlN sheet is only 6% less stable than wz-AlN and has an indirect energy gap of 2.81 eV.

31

DFT analysis indicates that optical conductivity of AlN nanosheets in both parallel and perpendicular electric field starts with a gap which confirms that the AlN nanosheets has semiconductor properties. Strain is known to be very effective in manipulating the band structure of AlN nanosheets. The DFT studies show that the strain can manipulate not only the value of band gap but can also induce an indirect and direct transition in the band gap. Extended line defects (ELDs) into the honeycomb monolayer of AlN studied by first-principle density functional theory could have applications in photonic acting as a waveguide. Chemical functionalization is an active way to control the electronic and magnetic properties of AlN sheets, and interaction of hydrogen with AlN is of great technological interest. Fully hydrogenated AlN NS with a single bilayer is an indirect band gap semiconductor of 0.33 eV, whereas those with thicker bilayers (n>4) become a ferromagnetic (FM) metal with a magnetic moment of 0.44∼0.66 µ B. This suggests an efficient route to tune the electronic and magnetic properties and to realize potential applications in the fields of electronics and spintronics. Notably, bare AlN nanosheet is a 2D isotropic system. When AlN nanosheets are co-decorated with H and F on each side, they show anisotropic semiconducting character, which is helpful to take advantage of the asymmetric charge distribution to promote self-assembly under an applied electric field. Due to the charge transfer between gas molecules and an AlN sheet, the AlN sheet can be used as a gas sensor for CO2, CO, H2, O2 and NO. Moreover, the AlN sheet can have a potential application as a sorbent material for CO2 separation and capture from gas mixture.

32

Density functional theory calculations revealed that the electrical conductivity of AlN nanosheets may be increased upon NO2 adsorption, being insensitive toward NH3 adsorption. Thus, the AlN sheet may selectively detect NO2 molecules in the presence of NH3 molecules. Using DFT calculations it is suggested that the C-doped or N-vacancy defected hAlN nanosheets can be a promising candidate for adsorption or detection of toxic CO gas. We refer to experimental evidence for the growth of thin hexagonal AlN (submonolayer to 12 monolayers) nanosheets by plasma assisted molecular beam epitaxy on Ag(111) substrates. AlN adopts a graphite-like hexagonal lattice with larger lattice constant compared to bulk wurtzite AlN. This claim is further supported by ultraviolet photoelectron spectroscopy indicating a reduced bandgap as expected for graphite-like hexagonal AlN. Graphene-like AlN layers with lateral lattice constant of 3.08 Å were formed on highly ordered (8×8) silicon nitride surface prepared on atomically flat (111) Si substrate by ammonia molecular beam epitaxy (MBE). Epitaxial growth of single crystal 2D AlN layers sandwiched between graphene and Si substrate have been realized for the first time by metal organic vapor deposition. Moreover, the bandgap of as-grown 2D AlN layers is theoretically predicted to be ≈ 9.63 eV and is experimentally determined to be 9.20-9.60 eV which shows a great promise in application of deep-ultraviolet optoelectronic devices. Also, 2D AlN can be obtained by chemical vapour deposition method only when the Si substrates were coated with Au.

33

Density-functional molecular dynamics (AIMD) simulations have been employed for achieving understanding at atomistic level of the rich chemistry of reactions occurring during MOCVD of AlN on graphene. Finally, it is found that metal substrates stabilize as-yet hypothetical tetragonal 2DAlN. In the published results reviewed here, all theoretical and experimental works reveal that two-dimensional AlN nanosheets are challenging nanomaterials with a great application potential. Moreover depending on the crystal structure the nano dimensional AlN can have a large range bandgap, respectively properties that are of a significant interest for different electronic and optoelectronic devices.

Acknowledgements The work was supported by the Bulgarian National Science Fund under contract DN 18/6. RY would like to acknowledge SSF (Sweden) for financial support via contracts GMT 14-077, RMA 15-0024 and VR 2018-04962.

References: [1] S. Z. Butler, S. M. Hollen, L. Cao, Y. Cui, J. A. Gupta, H. R. Gutiérrez, T. F. Heinz, S. S. Hong, J. Huang, A. F. Ismach, E. Johnston-Halperin, M. Kuno, V. V. Plashnitsa, R. D. Robinson, R. S. Ruoff, S. Salahuddin, J. Shan, L. Shi, M. G. Spencer, M. Terrones, W. Windl, and J. E. Goldberger, Progress, challenges, and opportunities in two-dimensional materials beyond graphene, Acs Nano 7(4)

(2013) 2898–2926.

https://pubs.acs.org/doi/10.1021/nn400280c

34

[2] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. Katsnelson, I. V. Grigorieva, S. V. Dubonos, A. A. Firsov, Two-dimensional gas of massless Dirac fermions in graphene Nature 438 (2005) 197-200. https://doi.org/10.1038/nature04233 [3] C. Itzykson, J.-B. Zuber, Quantum Field Theory; Dove: New York, 2006 [4] A. Calogeracos, N. Dombey, History and physics of the Klein paradox, Contemp. Phys. 40 (1999) 313-321. https://doi.org/10.1080/001075199181387 [5] C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A. N. Marchenkov, E. H. Conrad, P. N. First, W. A. de Heer, Electronic confinement and coherence in patterned epitaxial graphene, Science 312 (2006) 1191-1196. https://doi.org/10.1126/science.1125925 [6] Y. Zhang, Y-W. Tan, H. L. Stormer, P. Kim, Experimental observation of the quantum Hall effect and Berry's phase in graphene, Nature 438 (2005) 201-204. https://doi.org/10.1038/nature04235 [7] A. Cresti, N. Nemec, B. Biel, G. Niebler, F. Triozon, G. Cuniberti, S. Roche, Charge transport in disordered graphene-based low dimensional materials, Nano Res. 1 (2008) 361-394. https://doi.org/10.1007/s12274-008-8043-2 [8] D. Golberg, Y. Bando, Y. Huang, T. Terao, M. Mitome, C. C.Tang and C. Y. Zhi, Boron Nitride Nanotubes and Nanosheets, ACS Nano 4 (2010) 2979–2993. https://doi.org/10.1021/nn1006495 [9] A. Rubio, J.L. Corkill, M. Cohen, Theory of graphitic boron nitride nanotubes, Phys. Rev. B 49 (1994) 5081–5084. https://doi.org/10.1103/PhysRevB.49.5081 [10] X. Blase, A. Rubio, S.G. Louie, M.L. Cohen, Stability and band-gap constancy of boron-nitride

nanotubes,

Europhys.

Lett.

28

(1994)

335–340.

https://doi.org/10.1209/0295-5075/28/5/007

35

[11] H. Zhang, C.-X. Liu, X.-L. Qi, X. Dai, Z. Fang and S.-C. Zhang, Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface Nat. Phys. 5 (2009) 438–442. https://doi.org/10.1038/nphys1270 [12] H. Tang, D. Liang, R. L. J. Qiu and X. P. A. Gao, Two-dimensional transportinduced linear magneto-resistance in topological insulator Bi2Se3 nanoribbons, ACS Nano 5 (2011) 7510–7516. https://doi.org/10.1021/nn2024607 [13] J. A. Wilson and A. D. Yoffe, The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties Adv. Phys. 18 (1969) 193–335. https://doi.org/10.1080/00018736900101307 [14] C. Tan and H. Zhang, Two-dimensional transition metal dichalcogenide nanosheetbased

composites,

Chem.

Soc.

Rev.

44

(2015)

2713–2731.

https://doi.org/10.1039/C4CS00182F [15] X. Huang, C. Tan, Z. Yin and H. Zhang, 25th Anniversary Article: Hybrid nanostructures based on two‐dimensional nanomaterials, Adv. Mater. 26

(2014)

2185–2204. https://doi.org/10.1002/adma.201304964 [16] G.R.Bhimanapati, N.R.Glavin, J.A.Robinson, 2D Boron Nitride: Synthesis and Applications, Ch 3, Semiconductors and Semimetals 95 (2016) 101-147 Edited by F.Iacopi, J. J. Boeckl, C.Jagadish [17] C. Dean, A.F.Young, L.Wang, I.Meric, G. H.Lee, K.Watanabe, T.Taniguchi, K.Shepard, P.Kim, J.Hone, Graphene based heterostructures, Solid State Commun. 152 (2012) 1275–1282, https://doi.org/10.1016/j.ssc.2012.04.021

36

[18] W. Gannett, W. Regan, K Watanabe, T Taniguchi, M.F. Crommie, A. Zettl, Boron nitride substrates for high mobility chemical vapor deposited graphene. Appl. Phys. Lett. 98 (2011) 242105. https://doi.org/10.1063/1.3599708 [19] M. Osada and T. Sasaki, Two‐dimensional dielectric nanosheets: novel nanoelectronics from nanocrystal building blocks, Adv. Mater., 24 (2012), 210–228. https://doi.org/10.1002/adma.201103241 [20] W. Huang, L. Gan, H. Li, Y. Ma and T. Zhai, 2D layered group 13 metal chalcogenides: synthesis, properties and applications in electronics and optoelectronics, Cryst. Eng. Comm. 18 (2016) 3968-3984. https://doi.org/10.1039/C5CE01986A [21] V. Zólyomi, N. D. Drummond and V. I. Fal'ko, Electrons and phonons in single layers of hexagonal indium chalcogenides from ab initio calculations, Phys. Rev. B: Condens.

Matter

Mater.

Phys.

89

(2014)

205416.

https://doi.org/10.1103/PhysRevB.89.205416 [22] T. Ikari, S. Shigetomi and K. Hashimoto, Crystal Structure and Raman Spectra of InSe,

Phys.

Status

Solidi

B

111

(1982)

477–481.

https://doi.org/10.1002/pssb.2221110208 [23] S. Pal and D. N. Bose, Growth, characterisation and electrical anisotropy in layered chalcogenides GaTe and InTe Solid State Commun.

(1996)

97 725–729.

https://doi.org/10.1016/0038-1098(95)00608-7 [24] P. A. Hu, L. F. Wang, M. Yoon, J. Zhang, W. Feng, X. N. Wang, Z. Z. Wen, J. C. Idrobo, Y. Miyamoto, D. B. Geohegan and K. Xiao, Highly responsive ultrathin GaS nanosheet photodetectors on rigid and flexible substrates, Nano Lett. 13 (2013) 1649– 1654. https://doi.org/10.1021/nl400107k

37

[25] P. A. Hu, Z. Z. Wen, L. F. Wang, P. H. Tan and K. Xiao, Synthesis of few-layer GaSe nanosheets for high performance photodetectors, ACS Nano 6 (2012) 5988– 5994. https://doi.org/10.1021/nn300889c [26] D. N. Bose and S. Pal, Photoconductivity, low-temperature conductivity, and magnetoresistance studies on the layered semiconductor GaTe, Phys. Rev. B: Condens. Matter Mater. Phys. 63 (2001) 235321. https://doi.org/10.1103/PhysRevB.63.235321 [27] C. H. Ho and Y. P. Wang, NIR and UV enhanced photon detector made by diindium

trichalcogenides,

Opt.

Mater.

Express

3

(2013)

1420–1427.

https://doi.org/10.1364/OME.3.001420 [28] M. Ishikawa and T. Nakayama, Stacking and Optical Properties of Layered In 2Se 3, Jpn. J. Appl. Phys. 37 (1998) L1122. https://doi.org/10.1143/JJAP.37.L1122 [29] G. Han, Z. G. Chen, C. H. Sun, L. Yang, L. N. Cheng, Z. F. Li, W. Lu, Z. M. Gibbs, G. J. Snyder, K. Jack, J. Drennan and J. Zou, A new crystal: layer-structured rhombohedral

In3Se4,

Cryst.

Eng.

Comm.

16

(2014)

393–398.

https://doi.org/10.1039/C3CE41815D [30] G. Han, Z. G. Chen, L. Yang, L. N. Cheng, K. Jack, J. Drennan and J. Zou, Thermal stability and oxidation of layer-structured rhombohedral In3Se4 nanostructures, Appl. Phys. Lett. 103 (2014) 263105. https://doi.org/10.1063/1.4857655 [31] G. Han, Z. G. Chen, L. Yang, L. N. Cheng, J. Drennan and J. Zou, Phase control and

formation

mechanism

of

new-phase

layer-structured

rhombohedral

In3Se4 hierarchical nanostructures, Cryst. Growth Des. 13 (2013) 5092–5099. https://doi.org/10.1021/cg401269p

38

[32] J. Liu, Y. B. Losovyj, T. Komesu, P. A. Dowben, L. Makinistian, E. A. Albanesi, A. G. Petukhov, P. Galiy and Y. Fiyala, The bulk band structure and inner potential of layered

In4Se3,

Appl.

Surf.

Sci.

254

(2008)

4322–4325.

https://doi.org/10.1016/j.apsusc.2008.01.061

[33] Z. X. Wang, M. Safdar, C. Jiang and J. He, High-Performance UV–Visible–NIR Broad Spectral Photodetectors Based on One-Dimensional In2Te3 Nanostructures, Nano Lett. 12 (2012) 4715–4721. https://doi.org/10.1021/nl302142g

[34] G.W. Mudd, S. A. Svatek, T. Ren, A. Patanè, O. Makarovsky, L. Eaves, P. H. Beton, Z. D. Kovalyuk, G. V. Lashkarev, Z. R. Kudrynskyi and A. I. Dmitriev, Tuning the bandgap of exfoliated InSe nanosheets by quantum confinement Adv. Mater.25 (2013) 5714–5718. https://doi.org/10.1002/adma.201302616

[35] J. Sánchez-Royo, G. Muñoz-Matutano, M. Brotons-Gisbert, J. Martínez-Pastor, A. Segura, A. Cantarero, R. Mata, J. Canet-Ferrer, G. Tobias, E. Canadell, J. MarquésHueso and B. Gerardot, Electronic structure, optical properties, and lattice dynamics in atomically thin

indium

selenide

flakes

Nano

Res.7

(2014)

1556–1568.

https://doi.org/10.1007/s12274-014-0516-x [36] S. D. Lei, L. H. Ge, S. Najmaei, A. George, R. Kappera, J.Lou, M. Chhowalla, H. Yamaguchi, G. Gupta, R. Vajtai, A. D. Mohite and P. M. Ajayan, ACS Nano 8 (2014) 1263–1272. [37] A. M. Mancini, G. Micocci and A. Rizzo, New materials for optoelectronic devices: Growth and characterization of indium and gallium chalcogenide layer

39

compounds, Mater. Chem. Phys. 9 (1983) 29–54. https://doi.org/10.1016/02540584(82)90006-2 [38] W. Feng, W. Zheng, W. W. Cao and P. A. Hu, Back Gated Multilayer InSe transistors with enhanced carrier mobilities via the suppression of carrier scattering from

a

dielectric

interface,

Adv.

Mater.

26

(2014)

6587–6593.

https://doi.org/10.1002/adma.201402427 [39] J. Zhao, S. M. Islam, S. Hao, G. Tan, C. C. Stoumpos, C. Wolverton, H. Chen, Z. Luo, R. Li, and M. G. Kanatzidis, Homologous series of 2D chalcogenides Cs−Ag−Bi−Q (Q = S, Se) with ion-exchange properties, J. Am. Chem. Soc. 139 (2017) 12601−12609 https://doi.org/10.1021/jacs.7b06373 [40] M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi and M. W. Barsoum, Two‐Dimensional nanocrystals produced by exfoliation of Ti3AlC2, Adv. Mater. 23 (2011) 4248–4253. https://doi.org/10.1002/adma.201102306 [41] M. Naguib, O. Mashtalir, J. Carle, V. Presser, J. Lu, L. Hultman, Y. Gogotsi and M. W. Barsoum, Two-dimensional transition metal carbides, ACS Nano 6 (2012) 1322–1331. https://doi.org/10.1021/nn204153h [42] P. Urbankowski, B. Anasori, T. Makaryan, D. Er, S. Kota, P. L. Walsh, M. Zhao, V. B. Shenoy, M. W. Barsoum and Y. Gogotsi, Synthesis of two-dimensional titanium nitride

Ti4N3 (MXene),

Nanoscale

8

(2016)

11385–11391.

https://doi.org/10.1039/C6NR02253G [43] Q. Tao, J. Lu, M. Dahlqvist, A. Mockute, S. Calder, A. Petruhins, R. Meshkian, O. Rivin, D. Potashnikov, E. Caspi, H. Shaked, A. Hoser, C. Opagiste, R.-M. Galera, R. Salikhov, U.Wiedwald, C. Ritter, A. R. Wildes, B. Johansson, L. Hultman, M. Farle, M.

40

W. Barsoum, J.Rosen, Atomically layered and ordered rare-earth i-MAX Phases: A new class of magnetic quaternary compounds, Chem. Mater. 317 (2019) 2476-2485. https://doi.org/10.1021/acs.chemmater.8b05298 [44] L. Cheng, X. Li, H. Zhang and Q. Xiang, Two-dimensional transition metal MXene-based photocatalysts for solar fuel generation, J. Phys. Chem. Lett. 1012 (2019) 3488-3494. https://doi.org/10.1021/acs.jpclett.9b00736 [45] Q. Zhao, Q. Zhu, J. Miao, P. Zhang and B. Xu, 2D MXene nanosheets enable small-sulfur electrodes to be flexible for lithium–sulfur batteries, Nanoscale 11( 2019) 8442-8448. https://doi.org/10.1039/C8NR09653H [46] R. Meshkian, L.-Å. Näslund, J. Halim, J. Lu, M. W. Barsoum and J. Rosen, Synthesis of two-dimensional molybdenum carbide, Mo2C, from the gallium based atomic

laminate

Mo2Ga2C,

Scr.

Mater.,

2015,

108,

147–150.

https://doi.org/10.1016/j.scriptamat.2015.07.003 [47] M. Alhabeb, K. Maleski, T. S. Mathis, A. Sarycheva, C. B. Hatter, S. Uzun, A. Levitt and Y. Gogotsi, Selective Etching of Silicon from Ti3SiC2 (MAX) To Obtain 2D Titanium

Carbide

(MXene)

Angew

Chem.130

(2018)

5444–5448.

https://doi.org/10.1002/anie.201802232 [48] J. Thörnberg, J. Halim, J. Lu, R. Meshkian, J. Palisaitis, L. Hultman, P. O. Å. Persson and J. Rosen, Synthesis of (V2/3Sc1/3)2AlC i-MAX phase and V2−xC MXene scrolls, Nanoscale 11(2019) 14720-14726. https://doi:10.1039/c9nr02354b [49] M. W. Barsoum, MAX phases: properties of machinable ternary carbides and nitrides, John Wiley & Sons, 2013.

41

[50] M. Radovic and M. W. Barsoum, MAX phases: Bridging the gap between metals and ceramics Am. Ceram. Soc. Bull., 92 (2013) 20–27

[51] J. Halim, K. M. Cook, M. Naguib, P. Eklund, Y. Gogotsi,J. Rosen and M. W. Barsoum, X-ray photoelectron spectroscopy of select multi-layered transition metal carbides

(MXenes),

Appl.

Surf.

Sci.

(2016)

362,

406–417.

https://doi.org/10.1016/j.apsusc.2015.11.089

[52] I. Persson, L.-Å. Näslund, J. Halim, M. W. Barsoum,V. Darakchieva, J. Palisaitis, J. Rosen and P. O. Å. Persson, 2D Mater.5 (2017) 015002.

[53] B. Anasori, M. R. Lukatskaya and Y. Gogotsi, 2D metal carbides and nitrides (MXenes)

for

energy

storage,

Nat.

Rev.

Mater.2

(2017)

16098.

https://doi.org/10.1038/natrevmats.2016.98 [54] F. Shahzad, M. Alhabeb, C. B. Hatter, B. Anasori,S. M. Hong, C. M. Koo and Y. Gogotsi, Electromagnetic interference shielding with 2D transition metal carbides (MXenes) Science 353 (2016) 1137–1140. https://doi.org/10.1126/science.aag2421 [55] Q. Peng, J. Guo, Q. Zhang, J. Xiang, B. Liu, A. Zhou, R. Liu and Y. Tian, Unique lead adsorption behavior of activated hydroxyl group in two-dimensional titanium carbide, J. Am. Chem. Soc.136 (2014) 4113–4116. https://doi.org/10.1021/ja500506k [56] Q. Jiang N. Kurra , M. Alhabeb Y.Gogotsi H. N. Alshareef, All Pseudocapacitive MXene-RuO2 Asymmetric Supercapacitors, Adv. Energy Mater. 8 (2018) 1703043 https://doi.org/10.1002/aenm.201703043 [57] Q.Tao, M. Dahlqvist, J. Lu, S. Kota, R. Meshkian, J. Halim, J.Palisaitis, L. Hultman, M. W. Barsoum, Per O.Å. Persson and J. Rosen, Two-dimensional Mo1.33C MXene with divacancy ordering prepared from parent 3D laminate with in-plane

42

chemical

ordering,

Nature

Commun.

8

(2017)

14949

https://doi.org/10.1038/ncomms14949 [58] N. D. Mermin, Crystalline order in two dimensions, Phys. Rev. 176 (1968) 250 https://doi.org/10.1103/PhysRev.176.250 [59] X. Zhou, M.M. Wu, J. Zhou, Q. Sun, Hydrogen storage in AlN cage based nanostructures,

Appl.

Phys.

Lett.

94

(10)

(2009)

103105.

https://doi.org/10.1063/1.3098076 [60] Y. Li, Z. Zhou, P. Shen, S.B. Zhang, Z. Chen, Computational studies on hydrogen storage in aluminum nitride nanowires/tubes, Nanotechnology 20 (21) (2009) 215701. https://doi.org/10.1088/0957-4484/20/21/215701 [61] G. R. Yazdi, P. O. Å. Persson, D. Gogova, R. Fornari, L. Hultman, M. Syväjärvi and R. Yakimova, Aligned AlN nanowires by self-organized vapor-solid growth, Nanotechnology 20 (49) (2009) 495304. https://doi.org/10.1088/09574484/20/49/495304 [62] E.F. de Almeida Jr, F. de B. Mota, C.M.C. de Castilho, A. Kakanakova-Georgieva, and G.K. Gueorguiev, Defects in hexagonal-AlN sheets by first-principles calculations, Eur. Phys. J. B (2012) 85: 48. https://doi.org/10.1140/epjb/e2011-20538-6 [63] D. Sánchez-Portal, P. Ordejón, E. Artacho, J.M. Soler, Density‐functional method for very large systems with LCAO basis sets, Int. J. Quantum Chem. 65 (1997) 453461. https://doi.org/10.1002/(SICI)1097-461X(1997)65:5<453::AID-QUA9>3.0.CO;2V [64] N. Troullier, J.L. Martins, Efficient pseudopotentials for plane-wave calculations, Phys. Rev. B 43 (1991) 1993-2006. https://doi.org/10.1103/PhysRevB.43.1993

43

[65] L. Kleinman, D.M. Bylander, Efficacious form for model pseudopotentials, Phys. Rev. Lett. 48 (1982) 1425. https://doi.org/10.1103/PhysRevLett.48.1425 [66] Y. Wang, S. Shi, Structural and electronic properties of monolayer hydrogenated honeycomb III–V sheets from first-principles, Solid State Commun. 150 (2010) 14731478. https://doi.org/10.1016/j.ssc.2010.05.031 [67] Sh. Valedbagi , A.Fathalian, S.Mohamma Elahi, Electronic and optical properties of AlN nanosheet: An ab initio study, Opt. Commun. 309 (2013) 153–157. https://doi.org/10.1016/j.optcom.2013.06.061 [68] J. Zhang, J.-M. Zhang, K.-W. Xu, First-principles study on structural and electronic properties of AlNSix heterosheet, Physica B 407 (2012) 2301-2305. h ttps://doi.org/10.1016/j.physb.2012.03.018 [69] F.-L. Zheng, J.-M. Zhang, Y. Zhang, V. Ji, First-principles study of the perfect and vacancy

defect

AlN

nanoribbon,

Physica

B

405

(2010)

3775-3781.

https://doi.org/10.1016/j.physb.2010.05.085 [70] J. Zhang, Y. Zhang, J.-M. Zhang, K.-W. Xu, The effects of the dangling bond on the electronic and magnetic properties of AlN nanoribbon, Computational and Theoretical

Chemistry

967

(2011)

113-119.

https://doi.org/10.1016/j.comptc.2011.04.001 [71] R. Ahmed, H. Akbarzadeh, Fazal-e-Aleem, A first principle study of band structure of

III-nitride

compounds,

Physica

B

370

(2005)

52-60.

https://doi.org/10.1016/j.physb.2005.08.044 [72] C. Stampfl, C.G.Van de Walle, Density-functional calculations for III-V nitrides using the local-density approximation and the generalized gradient approximation Phys. Rev. B 59 (1998) 5521-5535. https://doi.org/10.1103/PhysRevB.59.5521

44

[73] Z.-Y. Jiao, S.-H. Ma, J.-F. Yang, A comparison of the electronic and optical properties of zinc-blende, rocksalt and wurtzite AlN: A DFT study, Solid State Sciences 13 (2011) 331-336. https://doi.org/10.1016/j.solidstatesciences.2010.11.030 [74] P. Liu , A. De Sarkar , R. Ahuja, Shear strain induced indirect to direct transition in band gap in AlN monolayer nanosheet,

Comput. Mater. Sci. 86 (2014) 206–210.

https://doi.org/10.1016/j.commatsci.2014.01.026 [75] F. Litimein, B. Bouhafs, Z. Dridi, P. Ruterana, The electronic structure of wurtzite and zincblende AlN: an ab initio comparative study, New J. Phys. 4 (2002) 64.1-64.12 https://doi.org/10.1088/1367-2630/4/1/364 [76] J. Narayan, K. Dovidenko, A.K. Sharma, S. Oktyabrsky, Defects and interfaces in epitaxial ZnO/alpha-Al2O3 and AlN/ZnO/alpha-Al2O3, heterostructures, J. Appl. Phys. 84 (1998) 2597. https://doi.org/10.1063/1.368440 [77] L.C. Chuang, M. Meowe, C. Chase, N.P. Kobayashi, C. Chang-Hasnain, Critical diameter for III–V nanowires grown on latticemismatched substrates, Appl. Phys. Lett. 90 (2007) 043115. https://doi.org/10.1063/1.2436655 [78] D.C. Camacho-Mojica, F. López-Urías, Extended line defects in BN, GaN, and AlN semiconductor materials: Graphene-like structures, Chem. Phys. Lett. 652 (2016) 73–78. https://doi.org/10.1016/j.cplett.2016.04.045 [79] P. Hohenberg, W. Kohn, Inhomogeneous electron gas, Phys. Rev. B 136 (1964) B864-871. https://doi.org/10.1103/PhysRev.136.B864 [80] W. Kohn, L.J. Sham, Self-consistent equations including exchange and correlation effects,

Phys.

Rev.

140

(1965)

A1133-1138.

https://doi.org/10.1103/PhysRev.140.A1133

45

[81] G. Kresse, J. Joubert, From ultrasoft pseudopotentials to the projector augmentedwave method, Phys. Rev. B 59 (1999) 1758-1175. https://doi.org/10.1103/PhysRevB.59.1758 [82] H. J. Monkhorst, J. D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B 13 (1976) 5188-5192. https://doi.org/10.1103/PhysRevB.13.5188 [83] Chang-wen Zhang and Fu-bao Zheng, First-principles prediction on electronic and magnetic properties of hydrogenated AlN nanosheets, J. Comput. Chem. 32 (14) (2011) 3122-3128. https://doi.org/10.1002/jcc.21902 [84] C.-w Zhang, P.-j. Wang, Tuning electronic and magnetic properties of AlN nanosheets with hydrogen and fluorine: First-principles prediction, Phys. Lett. A 375 (2011) 3583–3587. https://doi.org/10.1016/j.physleta.2011.07.039 [85] J. Zhou, Q. Wang, Q. Sun, P. Jena, Electronic and magnetic properties of a BN sheet decorated with hydrogen and fluorine, Phys. Rev. B 81 (2010) 085442. https://doi.org/10.1103/PhysRevB.81.085442 [86] J. Kudrnovský, I. Turek, V. Drchal, F. Máca, P. Weinberger, P. Bruno, Exchange interactions in III-V and group-IV diluted magnetic semiconductors, Phys. Rev.B 69 (2004) 115208. https://doi.org/10.1103/PhysRevB.69.115208 [87] Q. Sun, Q. Wang, P. Jena, Y. Kawazoe, Design of Janus nanoparticles with atomic precision: tungsten-doped gold nanostructures, ACS Nano 2 (2008) 341-347. https://pubs.acs.org/doi/10.1021/nn7002647 [88] A. Rochefort, J.D. Wuest, Interaction of substituted aromatic compounds with graphene, Langmuir 25 (2009) 210–215. https://doi.org/10.1021/la802284j

46

[89] Y. Wang, N. Song, X. Song, T. Zhang, D.Yang, M. Li, A first-principles study of gas

adsorption

on

monolayer

AlN

sheet,

Vac.

147

(2018)

18-23.

https://doi.org/10.1016/j.vacuum.2017.10.014 [90] X.-f. Yu, Y.-c. Li, J.-b. Cheng, Z.-b. Liu, Q.-z. Li, W.-z. Li, X. Yang, B. Xiao, Monolayer Ti2CO2: a promising candidate for NH3 sensor or capturer with high sensitivity and selectivity, ACS Appl. Mater. Interfaces 7 (24) (2015) 13707-13713. https://doi.org/10.1021/acsami.5b03737 [91] H. Guo, W. Zhang, N. Lu, Z. Zhuo, X.C. Zeng, X. Wu, J. Yang, CO2 capture on hBN sheet with high selectivity controlled by external electric field, J. Phys. Chem. C 119 (12) (2015) 6912-6917 https://doi.org/10.1021/acs.jpcc.5b00681 [92] Y.S. Wang, P. F. Yuan, M. Li, W. F. Jiang, Q. Sun, Y. Jia, Calcium-decorated graphyne nanotubes as promising hydrogen storage media: a first-principles study, J. Solid State Chem. 197 (2013) 323-328. https://doi.org/10.1016/j.jssc.2012.09.004 [93] S. Er, G.A.de Wijs, G. Brocks, Hydrogen storage by polylithiated molecules and nanostructures,

J.

Phys.

Chem.

C

113

(2009)

8997-9002.

https://doi.org/10.1021/jp901305h [94] G. Henkelman, A. Arnaldsson, H. Jónsson, A fast and robust algorithm for Bader decomposition of charge density, Comput. Mater. Sci. 36 (3) (2006) 354-360. https://doi.org/10.1016/j.commatsci.2005.04.010 [95] A. Ramraj, I.H. Hillier, Binding of pollutant aromatics on carbon nanotubes and graphite, J. Chem. Inf. Model. 50 (2010) 585–588. https://doi.org/10.1021/ci1000604 [96] S. F. Rastegar, A. A. Peyghan, H.R. Ghenaatianb, N. L. Hadipour, NO2 detection by nanosized AlN sheet in the presence of NH3:DFT studies, Appl. Surf. Sci. 274 (2013) 217–220. https://doi.org/10.1016/j.apsusc.2013.03.019

47

[97] S.S. Li, Semiconductor Physical Electronics, 2nd ed., Springer Science+Business Media, New York, 2006 [98] K. Azizi, S.M. Hashemianzadeh, S. Bahramifar, Density functional theory study of carbon monoxide adsorption on the inside and outside of the armchair single-walled carbon nanotubes, Curr. Appl. Phys. 11 (2011) 776–782. https://doi.org/10.1016/j.cap.2010.11.071 [99] L.B. da Silva, S.B. Fagan, R. Mota, Ab initio study of deformed carbon nanotube sensors

for

carbon

monoxide

molecules,

Nano

Lett.

4

(2005)

65–67.

https://doi.org/10.1021/nl034873d [100] J. Beheshtian, M.T. Baei, A.A. Peyghan, Theoretical study of CO adsorption on the surface surface of BN, AlN, BP and AlP nanotubes, Surf. Sci. 606 (2012) 981–985. https://doi.org/10.1016/j.susc.2012.02.019 [101] A.S. Rad, S.S. Shabestari, S.A. Jafari, M.R. Zardoost, A. Mirabi, N-doped graphene as ananostructure adsorbent for carbon monoxide: DFT calculations, Mol. Phys. 114 (2016) 1756–1762. https://doi.org/10.1080/00268976.2016.1145748 [102] F. Nasehnia, M. Seifi, Adsorption of molecular oxygen on VIIIB transition metaldoped graphene: a DFT study, Mod. Phys. Lett. B 28 (2014) 1450237. https://doi.org/10.1142/S0217984914502376 [103] I. Carrillo, L.F. Magana, Adsorption of carbon monoxide on graphene with high titanium coverage, Rev. Mex. Fis. 55 (2009) 409–411. [104] T. Ouyang, Z. Qian, R. Ahuja, X. Liu, First-principles investigation of CO adsorption on pristine, C-doped and N-vacancy defected hexagonal AlN nanosheets, Appl. Surf. Sci. 439 (2018) 196–201. https://doi.org/10.1016/j.apsusc.2018.01.040

48

[105] S. Grimme, Semiempirical GGA-type density functional constructed with a longrange

dispersion

correction,

J.

Comp.

Chem.

27

(2006)

1787–1799.

https://doi.org/10.1002/jcc.20495 [106] H. Schulz and K. H. Thiemann, Crystal structure refinement of AlN and GaN, Solid State Commun. 23 (1977) 815-819. https://doi.org/10.1016/0038-1098(77)909590 [107] C. L. Freeman, F. Claeyssens, N. L. Allan, and J. H. Harding, Graphitic nanofilms as precursors to wurtzite films: Theory, Phys. Rev. Lett. 96 (2006) 066102. https://doi.org/10.1103/PhysRevLett.96.066102 [108] H. Şahin, S. Cahangirov, M. Topsakal, E. Bekaroglu, E. Akturk, R. T. Senger, and S. Ciraci, Monolayer honeycomb structures of group-IV elements and III-V binary compounds: First-principles calculations, Phys. Rev. B 80 (2009) 155453. https://doi.org/10.1103/PhysRevB.80.155453 [109] D. Wu, M. G. Lagally, and F. Liu, Stabilizing graphitic thin films of wurtzite materials

by

epitaxial

strain,

Phys.

Rev.

Lett.

107

(2011)

236101.

https://doi.org/10.1103/PhysRevLett.107.236101 [110] Q. Peng, X.-J. Chen, S. Liu, and S. De, Mechanical stabilities and properties of graphene-like aluminum nitride predicted from first-principles calculations, RSC Adv. 3 (2013) 7083-7092. https://doi.org/10.1039/c3ra40841h [111] P. Tsipas, S. Kassavetis, D. Tsoutsou, E. Xenogiannopoulou, E. Golias, S. A. Giamini, C. Grazianetti, D. Chiappe, A. Molle, M. Fanciulli, and A. Dimoulas, Evidence for graphite-like hexagonal AlN nanosheets epitaxially grown on single crystal

Ag(111),

Appl.

Phys.

Lett.

103

(2013)

251605.

https://doi.org/10.1063/1.4851239

49

[112] C. H. Chen, Ph.D. thesis, Technischen Universität Berlin, 2003 [113] M. Houssa, G. Pourtois, V.V.Afanas′ev, A. Stesmans, Can silicon behave like graphene?

A

first-principles

study,

Appl.

Phys.

Lett.

97

(2010)

112106.

https://doi.org/10.1063/1.3489937 [114] V. Mansurov, T.Malin, Yu.Galitsyn, K.Zhuravlev, Graphene-like AlN layer formation on (111)Si surface by ammonia molecular beam epitaxy, J. Cryst. Growth 428 (2015) 93–97. https://doi.org/10.1016/j.jcrysgro.2015.07.030 [115] L. E. Brus, Electron–electron and electron‐hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state, J. Chem. Phys. 80 (1984) 4403-4409. https://doi.org/10.1063/1.447218 [116] Z. Y. Al Balushi, K. Wang, R. K. Ghosh, R. A. Vilá, S. M. Eichfeld, J. D. Caldwell, X. Qin, Y.-C. Lin, P. A. DeSario, G. Stone, S. Subramanian, D. F. Paul, R. M. Wallace, S. Datta, J. M. Redwing,, J. A. Robinson, Two-dimensional gallium nitride realized

via

graphene encapsulation

Nat.

Mater.15

(2016)

1166-1171.

https://doi.org/10.1038/nmat4742 [117] W. Tian, C. Zhang, T. Zhai, S.-L. Li, X. Wang, J. Liu, X. Jie, D. Liu, M. Liao, Y. Koide, D. Golberg, Y. Bando, Flexible ultraviolet photodetectors with broad photoresponse based on branched ZnS‐ZnO heterostructure nanofilms, Adv. Mater. 26 (2014) 3088-3093. https://doi.org/10.1002/adma.201305457 [118] G. Shin, H.-Y. Kim, J. Kim, Deep-ultraviolet photodetector based on exfoliated n-type β-Ga2O3 nanobelt/p-Si substrate heterojunction, Korean J. Chem. Eng. 35 (2018) 574-578. https://doi.org/10.1007/s11814-017-0279-7

50

[119] W. Wang, Y. Zheng, X. Li, Y. Li, H. Zhao, L. Huang, Z. Yang, X. Zhang, and G. Li, 2D AlN layers sandwiched between graphene and Si substrates, Adv. Mater. 31 (2018) 1803448. https://doi.org/10.1002/adma.201803448 [120] P. Ajayan, P. Kim, K. Banerjee, Two-dimensional van der Waals materials, Phys. Today 69 (2016) 38-44. https://doi.org/10.1063/PT.3.3297 [121] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56-58. https://doi.org/10.1038/354056a0 [122] L. Jiao, L. Zhang, X. Wang, G. Diankov, H. Dai, Narrow graphene nanoribbons from carbon nanotubes, Nature 458 (2009) 877-880. https://doi.org/10.1038/nature07919 [123] P. Wang, T. Wang, H.Wang, X. Sun, P. Huang, B. Sheng, X. Rong, X. Zheng, Z. Chen, Y. Wang, D. Wang, H. Liu, F. Liu, L.Yang, D. Li, L. Chen, X. Yang, F. Xu, Z. Qin, J. Shi, T. Yu, W. Ge, B. Shen, and X. Wang, Experimental evidence of large bandgap energy in atomically thin AlN, Adv. Funct. Mater. 29 (2019) 1902608. https://doi.org/10.1002/adfm.201902608 [124] J. Wu, When group-III nitrides go infrared: New properties and perspectives J. Appl. Phys. 106 (2009) 011101. https://doi.org/10.1063/1.3155798 [125] O. Ambacher, M. Brandt, R. Dimitrov, T. Metzger, M. Stutzmann, R. Fischer, A. Miehr, A. Bergmaier, G. Dollinger, Thermal stability and desorption of Group III nitrides prepared by metal organic chemical vapor deposition J. Vac. Sci. Technol. 14 (1996) 3532. https://doi.org/10.1116/1.588793 [126] O. Ambacher, Growth and applications of Group III-nitrides, J. Phys. D: Appl. Phys.31 (1998) 2653. https://doi.org/10.1088/0022-3727/31/20/001

51

[127] X. Zhang, Z. Liu, S. Hark, Synthesis and optical characterization of singlecrystalline

AlN

nanosheets,

Solid

State

Commun.

143

(2007)

317–320.

https://doi.org/10.1016/j.ssc.2007.05.039 [128] Y. Liang, X. Zhang, L. Qin, E. Zhang, H. Gao, Z. Zhang, Ga-assisted synthesis and optical properties of ZnO submicron- and nanotowers, J. Phys. Chem. B 110 (2006) 21593-21595. https://doi.org/10.1021/jp064316q [129] T. G. Mihopoulos, V. Gupta and K. F. Jensen, A reaction-transport model for AlGaN MOVPE growth, J. Cryst. Growth 195 (1998) 733-739. https://doi.org/10.1016/S0022-0248(98)00649-6 [130] A. J. Cohen, P. Mori-Sánchez and W. Yang, Insights into current limitations of density

functional

theory,

Science

321

(2008)

792-794.

https://doi.org/10.1126/science.1158722 [131] D. G. Sangiovanni, G. K. Gueorguiev and A. Kakanakova-Georgieva, Ab initio molecular dynamics of atomic-scale surface reactions: insights into metal organic chemical vapor deposition of AlN on graphene, Phys.Chem.Chem.Phys. 20 (2018) 17751-17761. https://doi.org/10.1039/C8CP02786B [132] H. L. Zhuang, A. K. Singh, and R. G. Hennig, Computational discovery of single-layer

III-V

materials,

Phys.

Rev.

B87

(2013)

165415.

https://doi.org/10.1103/PhysRevB.87.165415 [133] K. S. Novoselov, V. I. Falko, L. Colombo, P. R. Gellert, M. G. Schwab, and K. Kim, A roadmap for graphene, Nature 490 (2012) 192-200. https://doi.org/10.1038/nature11458 [134] C. Soldano, A. Mahmood, and E. Dujardin, Production, properties and potential of graphene, Carbon 48(2010) 2127-2150. https://doi.org/10.1016/j.carbon.2010.01.058

52

[135] G. Bergerhoff and I. D. Brown, Crystallographic Databases (International Union of Crystallography, Chester, 1987) [136] G. Giovannetti, P. A. Khomyakov, G. Brocks, V. M. Karpan, J. van den Brink, and P. J. Kelly, Doping graphene with metal contacts, Phys. Rev. Lett. 101 (2008) 026803. https://doi.org/10.1103/PhysRevLett.101.026803 [137] M. Fuentes-Cabrera, M. I. Baskes, A. V. Melechko, and M. L. Simpson, Bridge structure for the graphene/Ni(111) system: A first principles study, Phys. Rev. B 77 (2008) 035405. https://doi.org/10.1103/PhysRevB.77.035405 [138] M. Vanin, J. J. Mortensen, A. K. Kelkkanen, J. M. Garcia-Lastra, K. S. Thygesen, and K. W. Jacobsen, Graphene on metals: A van der Waals density functional study Phys. Rev. B 81 (2010) 081408. https://doi.org/10.1103/PhysRevB.81.081408 [139] J. Klimeš, D. R. Bowler, and A. Michaelides, Van der Waals density functionals applied

to

solids,

Phys.

Rev.

B

83

(2011)

195131.

https://doi.org/10.1103/PhysRevB.83.195131 [140] F. Yang, L. Jin, L. Sun, X. Ren, X. Duan, H. Cheng, Y. Xu, X. Zhang, Z. Lai, W. Chen, H. Dong, and W. Hu, Free-standing 2D hexagonal aluminum nitride dielectric crystals for high-performance organic field-effect transistors, Adv. Mater. 30 (2018) 1801891. https://doi.org/10.1002/adma.201801891 [141] G.R. Yazdi, R. Vasiliauskas, M. Syväjärvi, R. Yakimova, Fabrication of freestanding AlN crystals by controlled microrod growth, Journal of Crystal Growth 310 (2008) 935–939. https://doi.org/10.1016/j.jcrysgro.2007.11.124 [142] G. R. Yazdi, M. Syväjärvi, and R. Yakimova, Aligned AlN nanowires and microrods

by

self-patterning,

Appl.

Phys.

Lett.

90

(2007)

123103

https://doi.org/10.1063/1.2715129

53

[143] G. R. Yazdi, M. Syväjärvi, and R. Yakimova, Formation of needle-like and columnar structures of AlN,

Journal of Crystal Growth 300 (2007) 130–135.

https://doi.org/10.1016/j.jcrysgro.2006.11.005 [144] M.Beshkova, Z.Zakhariev, J.Birch, A.Kakanakova and R.Yakimova, Properties of AlN layers grown by sublimation epitaxy, Materials Science Forum Vols. 433-436 (2003) 995-998. https://doi.org/10.4028/www.scientific.net/MSF.433-436.995 [145] M.Beshkova, Z.Zakhariev, J.Birch, A.Kakanakova, R.Yakimova, Sublimation epitaxy of AlN layers on 4H-SiC depending on the type of crucible, Materials

Science:

Materials

in

Electronics

14

(2003)

Journal of 767-768.

https://doi.org/10.1023/A:1026184600220 [146] M.Beshkova, Z.Zakhariev, M.V. Abrashev, J.Birch, A.Kakanakova, R.Yakimova , Low-pressure sublimation epitaxy of AlN films-growth and characterization, Vacuum 76 (2004) 143-146. https://doi.org/10.1016/j.vacuum.2004.07.052 [147] M.Beshkova, Z.Zakhariev, M.V. Abrashev, J.Birch and R.Yakimova, Properties of AlN layers grown by sublimation epitaxy in different gas ambient, Materials Science & Engineering B, 129(1-3) (2006) 228-231. [148] M. Beshkova, K.G.Grigorov, Z. Zakhariev, M. Abrashev, M. Massi and R. Yakimova “Sublimation Epitaxy of AlN layers grown by different conditions on 4HSiC substrates”, Journal of Optoelectronics and Advance Materials, 9 (2007) (1) 213216. [149] M. Beshkova, Z. Zakhariev, G. Spasov and R. Yakimova, Method of producing aluminum nitride layers, patent for the invention, BG 66077 B1 (2011)

54

[150] M. Bickermann, B. M. Epelbaum, O. Filip, B. Tautz, P. Heimann, A. Winnacker, Faceting in AlN bulk crystal growth and its impact on optical properties of the crystals, Phys. Status Solidi C 9 (2012) 449. https://doi.org/10.1002/pssc.201100345 [151] H. Jinno, T. Yokota, N. Matsuhisa, M. Kaltenbrunner, Y. Tachibana, T. Someya, Low operating voltage organic transistors and circuits with anodic titanium oxide and phosphonic acid self-assembled monolayer dielectrics Org. Electron. 40 (2017) 58-64. https://doi.org/10.1016/j.orgel.2016.10.034 [152] T. Yokota, T. Kajitani, R. Shidachi, T. Tokuhara, M. Kaltenbrunner, Y. Shoji, F. Ishiwari, T. Sekitani,T. Fukushima, T. Someya, A few-layer molecular film on polymer substrates to enhance the performance of organic devices, Nat. Nanotechnol. 13 (2018) 139-144. https://doi.org/10.1038/s41565-017-0018-6 [153] O. Acton, M. Dubey, T. Weidner, K. M. O’Malley, T.-W. Kim, G. G. Ting, D. Hutchins, J. E. Baio, T. C. Lovejoy, A. H. Gage, D. G. Castner, H. Ma, A. K. Y. Jen, Adv. Funct. Mater. 21 (2011) 1476-1488. https://doi.org/10.1002/adfm.201002035 [154]Y. X. Ma, C. Y. Han, W. M. Tang, P. T. Lai, High-performance pentacene OTFT by incorporating Ti in LaON gate dielectric, Appl. Phys. Lett. 111 (2017) 023501. https://doi.org/10.1063/1.4993157

55

The authors declare that there is no conflict of interests.