hexagonal boron nitride with strong interlayer coupling

Chemical Physics Letters 749 (2020) 137430

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Research paper

Two-dimensional van der Waals heterostructure of indium selenide/ hexagonal boron nitride with strong interlayer coupling Nai-feng Shena,b,c, Xiao-dong Yanga,b, Xin-xin Wangd, Guang-hou Wanga,b, Jian-guo Wana,b,

T ⁎

a

School of Physics, Nanjing University, Nanjing 210093, China National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China c First-class Disciplinesans and High-level University Construction Office, Nanjing University of Posts and Telecommunications, Nanjing 210023, China d School of Physics and Engineering, Henan University of Science and Technology, Luoyang 471000, China b

HIGHLIGHTS

of InSe/hBN heterostructure is significantly reduced. • Work-function states result in a red-shift of optical absorption due to strong interlayer coupling. • Emerging • Electronic and optical properties exist stably to different InSe/hBN stacking type. ABSTRACT

Indium selenide (InSe) is a promising two-dimensional photodetector material. However, band-gap and work-function of low-dimension InSe are too large to act as photodetector devices. Here, we design InSe-based vdWs heterostructure, InSe/hBN (hexagonal boron nitride), using first-principles. InSe/hBN possesses rather low work-function (~3.0 eV) compared with that of isolated InSe monolayer due to strong interlayer coupling. Besides, strong interlayer coupling induces extra emerging states, markedly reducing band-gap of InSe/hBN. Meanwhile, these emerging states ensure red-shift of optical absorption, from infrared to ultraviolet light-harvesting. Moreover, different heterostructure stacking mode doesn’t affect optical absorption and electron emission, convenient to the application of optoelectronics.

1. Introduction Two-dimensional (2D) van der Waals (vdWs) semiconductors, as novel electronic and photoelectric materials [1,2], exhibit new physicochemical property and potential applications. As isolated graphene is successfully obtained [3], a great number research interests are stimulated in of 2D vdWs materials, such as transition metal dichalcogenides (TMDs) [4,5], boron nitride (BN) [6,7], black phosphorus (BP) [8,9], which have been extensive fabricated. Especially, among all the 2D vdWs layered semiconductor materials, atomically thin layers of indium selenide (InSe), successfully synthesized via mechanical exfoliation [10-14], have drawn much attention due to its high electron mobility, wide band-gap range and good metal contacts. All these intriguing properties make InSe a promising two-dimensional photodetector material. For example, the band gap of InSe can be freely tuned from 1.25 eV for bulk to 2.37 eV for monolayer by changing the thickness of the material due to quantum confinement effect [15,16]. Besides, few-layer InSe owns ultra-high electronic mobility exceeding 103 cm2·V−1·s−1 at room temperature [16]. Simultaneously, InSe-based device could obtain on/off ratio as high as 108 [17], and the contact ⁎

resistance of In-InSe is up to 1.9 × 103 μm−1 [18]. In summary of these superior characters, InSe have become ideal candidates for photoelectric devices. Although InSe is a promising semiconductor material, there are still some major obstacles to be solved. For example, the band gap of InSe monolayer is 2.37 eV, which is too large to achieve visible light-harvesting. Although bulk InSe has a 1.25 eV band gap, it is difficult to adapt to Moore's law and realize device miniaturization. Furthermore, the work-function of InSe is rather large (φInSe = 5.2 eV [19]). It is well known that photoemission is based on the photoelectric effect. Under the light irradiation, electrons in photosensitive materials will get enough energy and escape from the surface of materials into the external space. Thus, the space electric field is formed under the action of electric current. However, too large work-function of InSe will lead to difficulties in electron emission and reduce photoelectric efficiency. Fortunately, the formation of heterostructures by stacking 2D materials can effectively improve the electronic and photoelectric properties. BN is reported to be a wide-bandgap two-dimensional material with roomtemperature single-photon emission [20]. Besides, hexagonal boron nitride (hBN) exhibits unique electronic properties, such as low

Corresponding author. E-mail address: [email protected] (J.-g. Wan).

https://doi.org/10.1016/j.cplett.2020.137430 Received 19 January 2020; Received in revised form 11 March 2020; Accepted 30 March 2020 Available online 02 April 2020 0009-2614/ © 2020 Elsevier B.V. All rights reserved.

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dielectric constant, high thermal conductivity and chemical inertness [21], emerged as a fundamental building block for van der Waals heterostructures [22]. InSe and hBN possibly have chance to form a good nanocomposite to promote the electron excitation in field-emitter devices [23,24]. In this work, we explore the potential of InSe/hBN heterostructure as optoelectronic material from the infrared to ultraviolet absorption based on first-principles calculations. Our calculations demonstrate that the intrinsic electronic properties of InSe and hBN can be preserved in nanocomposite. The work-function of InSe layer is effectively reduced after compounded with hBN. Furthermore, band gap of InSe/hBN heterostructure abnormally reduces due to emerging states (electronically hybridized interfacial states) in addition to a type-I band alignment of the heterojunctions. Therefore, the InSe/hBN heterostructure could harvest the entire visible region. Meanwhile, these superior properties can exist stably to different InSe/hBN stacking mode while do not influence the electronic structure which is conducive to the application of electronic and photoelectric devices. 2. Theoretical methods All the first-principles calculations were performed by employing the Vienna ab initio Simulation Package (VASP) [25,26] in the framework of the ab initio density-functional theory (DFT). The projector augmented wave (PAW) method [27] was used to describe the electronion core interaction. Owing to the well-known fact that the GGA-PBE functional underestimates the band gap of semiconductors, the HeydScuseria-Ernzerhof screen hybrid functional (HSE06) [28] was used to obtain accurate electronic and optical properties. The kinetic cutoff energy for the plane-waves was set as 500 eV which can provide sufficient computational accuracy based on our energy tests. The k-point sampling in the first Brillouin zone was implemented by using 10 × 10 × 1 k-mesh according to the Monkhorst-Pack scheme, and Gaussian smearing broadening was set to be 0.05 eV. A vacuum region was set as 15 Å along the z-direction to avoid artificial interactions between two neighboring films. The dipole correction has been considered in all the calculations from geometry optimization to computing electronic and optical properties, which could remove the possible errors in total energy, electrostatic potential, and atomic force [29]. In consideration of the interfacial interactions of heterostructures, dispersion correction PBE + D3 [30] was taken into consideration to achieve the accurate total-energy, force and lattice constants.

Fig. 1. Top and side views of (a) InSe and (c) hBN geometric structures. Band structures of (b) InSe and (d) hBN. The Fermi level is set as zero. The In, Se, B and N atoms are shown by light green, pink, dark green and blue balls, respectively.

interlayer distances seeing right panels in Fig. 2a-c. The interlayer distance of hollow stacking structure is 4.70 Å, whereas the interlayer distances are 4.80 and 4.60 Å for B-top and N-top stackings, respectively. To further demonstrate the favorable stacking type, we also calculate the binding energies for these stacking modes, defined as:

Eb = EInSe / hBN

EInSe

EhBN

(1)

where EInSe/hBN, EInSe and EhBN represent the total energies of InSe/hBN heterostructure, isolated InSe and hBN monolayer, respectively. The binding energies are −53.3, −53.2 and −38.6 meV per atom for hollow, B-top and N-top stacking modes, respectively. The hollow, Btop and N-top stacking modes are feasible experimentally. Based on the stable stacking configurations, it is necessary to investigate the electronic structures. As a good photoelectric heterostructure material, especially a photodetector, carrier extraction efficiency and response speed are strongly affected by the work-function (WF), which is defined as [34]:

3. Results and discussion We first address the structural stability and electronic properties of independent InSe and hBN monolayer, which are crucial for understanding the geometries and electronic properties of InSe/hBN heterostructures. Fig. 1(a), (c) show the atomic structures of 2D InSe and hBN, respectively. The band structures of InSe and β-Sb are also displayed in Fig. 1(b), (d). The band gaps are 2.35 and 5.61 eV for InSe and hBN, respectively, which are also in accordance with previous experimental and theoretical results (InSe: 2.37 eV [15] and hBN: 5.8 ± 0.2 eV [31]). Besides, InSe and hBN are both hexagonal lattice and the lattice parameters are 4.07 and 2.51 Å for InSe and hBN based on the geometric optimization which are consistent with previous experimental and theoretical results [10,32,33]. The lattice constant, a = 4.20 Å for the InSe/hBN heterostructure, is obtained, and the induced strains in both InSe(1 × 1) and hBN( 3 × 3 ) lattices are only 3%, implying a promising heterostructure. Based on the well-matched lattice parameters, the InSe/hBN heterostructure can be built using unit cell. There are three possible stacking structures according to symmetry as shown in left panels in Fig. 2a-c, namely hollow, B-top and N-top stacking structures, respectively. We optimize the InSe/hBN heterostructures at different

= Evac

EF

(2)

where Evac and EF are the energies of vacuum level and Fermi level, respectively. Due to electronic state could not exist at the band gap region from VBM to CBM, WF φ can be used to describe the minimum energy needed to remove an electron to vacuum. Fig. 3 exhibits the electrostatic potential distribution of InSe/hBN heterostructure (with hollow stacking mode), isolated InSe and hBN monolayer, respectively. It is found that WF of InSe/hBN heterostructure is only 3.03 eV, much lower than that of InSe and hBN monolayers, implying that electrons could be promoted into vacuum and the heterostructure could be used as high efficiency photodetector. Similarly, the WFs of InSe/hBN heterostructures with B-top and N-top stacking mode are 3.05 and 3.06 eV, respectively. The reduction of the work-function results from the coupling between InSe and hBN layers, as discussed below. 2

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Fig. 4. Band structures of InSe/hBN heterostructures: (a) hollow, (b) B-top and (c) N-top stacking structures, respectively. (d) Band structure of InSe bilayer. The blue and red lines in the band structures denote the contributions from InSe and hBN, respectively.

from the Fermi level due to the large band-gap of pristine hBM monolayer (labeled in red). The large difference between band-gaps of InSe and hBN leads to a type-I band alignments, which can be confirmed by the calculated projected density of states in Fig. S1. However, the valence band maximum (VBM) and conduction band minimum (CBM) of InSe/hBN heterostructure are not distributed by InSe, either. Atom-decomposed band structures explicitly show several emerging states around the Fermi level in addition to a type-I band alignment of the heterojunctions. These emerging states result in a significant reduction of band-gap. This phenomenon is also not attributed to quantum confined states because the band structure of InSe bilayer is plotted in Fig. 4(d) for comparison. It is noted that the band-gap of InSe bilayer is as large as 1.63 eV with thickness of 13.65 Å, while InSe/hBN

Fig. 2. Top views of optimized structures (left panels) and total energies as functions of interlayer distances (right panels) of (a) hollow, (b) B-top and (c) N-top stacking structures, respectively. The In, Se, B and N atoms are shown by light green, pink, dark green and blue balls, respectively.

In order to gain a deep insight into the interlayer coupling, we further plot the band structures of InSe/hBN heterostructures. As shown in Fig. 4a-c, the contributions of electronic states of hBN are far away

Electrostatic Potential (eV)

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hBN Z axis direction

Fig. 3. Electrostatic potential distributions of (a) hollow InSe/hBN heterostructure, isolated (b) InSe and (c) hBN monolayer, respectively. The Fermi energy is shifted to the energy zero and indicated using a red line. 3

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heterostructure is only 9.97–10.17 Å thick and has the smaller bandgap of 1.16–1.17 eV. Thus, these bands are regarded as electronically hybridized interfacial states, which result from the frustrated strong interlayer coupling between InSe and hBN monolayers. These hybridized interfacial states, emerging within the original bandgap of vdW heterojunctions, are of particular interest particular interest in terms of exciton dynamics in the heterojunction and this phenomenon also occurs in other heterostructures [35]. Besides, redshift of band-gap while maintaining very thin thickness makes it a potential visible light absorber. These hybridized interfacial states are also widely observed and play important roles in spintronics [36], nanoelectronics [37], optoelectronics [38] and so on. Indeed, the reduction of both band gap and work function is important to many photodetector devices and transistors, which will enable improved photodetection and photoresponse time compared to bulk InSe when reaching high doping regime on such smaller band gap materials [39]. Besides, recent researches [40,41] also demonstrated that electrostatic contact engineer should dramatically improve on/off ratio on these heterostructrues compared to the bulk material. It is noted that all the band structures in Fig. 4 are indirect band gaps, however, this phenomenon could not affect the transition of electrons in InSe/hBN heterostructures. The calculated sum of the squares of TDMs (transition dipole moments) between CBM and VBM of InSe/hBN heterostructures are plotted in Fig. 5, which reveal the transition probabilities between two energy levels [42]. The most likely point for electron transition is at the Γ point. Thus, visible light could be absorbed effectively by InSe/hBN heterostructures. Furthermore, phonon dispersion and phonon DOS of InSe/hBN also demonstrate an excellent photoelectric performance resulting from the inelastic phonon activated process [43] (see Section S3 for details). Moreover, for different stacking structures, the band structures of InSe/hBN heterostructures do not obvious change. Emerging states and transition probabilities almost remain the same and are not affected by stacking type, convenient to application. Based on the good electronic structure stability, it can greatly improve the performance of electronic and photoelectric devices. Compared with other heterostructures, the InSe/hBN heterostructure exhibits many advantages. For example, recently reported InSe-based heterostructures are type-II band offsets. Band gap of InSe/ BP heterostructure is 1.39 eV [44] and GaSe/n-InSe van der Waals heterostructure has a band gap of 1.16 eV [45]. The small band gaps of these heterostructures stem from staggered band alignment. Our previous study [46] also demonstrated the type-II InSe/β-Sb has a small band gap of 0.96 eV. However, there exists no emerging state in these

(a)

( )=

2

c

)+

2 2(

)

1(

)]1/2

(3)

Since vdW interaction not only fails to destroy the intrinsic electronic property of sublayer, as expected, InSe/hBN heterostructure exhibits similar absorption range and intensity with the aggregation of InSe and hBN sublayer, but also leads to an obvious red-shift in visible light absorption. As presented in Fig. 6, the heterostructure can absorb light with energy smaller than 2 eV, covering from infrared to visible to ultraviolet light, which conduces to the application of photoelectric devices. Moreover, the reflectances of the InSe/hBN heterostructures are rather low (< 10%) at infrared and visible region, implying a strong light absorption property (see Section S4 for details). 4. Conclusions Base on the systemically DFT calculations, we have explored the possibility of InSe/hBN heterostructure as an efficient electronic and photoelectric device. Strong interlayer coupling results in a significant reduction of work-function after InSe integrated with hBN. Besides, due to the strong interlayer coupling, extra emerging states decrease the band-gap of InSe/hBN heterostructure. Simultaneously, the good optical absorption is also observed in InSe/hBN heterostructure which ensures harvesting light from infrared to ultraviolet region. Furthermore, these well properties of heterostructure are unaffected by different InSe/hBN stacking modes, convenient to the application of electronic and photoelectric devices.

(b)

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[

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InSe-based heterostructures means their interlayer couplings are rather weak. The strong interlayer coupling of InSe/hBN heterostructure not only induces extra emerging states, which reduce the band gap, but also results in rather low work-function compared with other InSe-based heterostructures [47]. The bandgap reduction in 2D InSe/hBN heterostructure benefits the IR and visible applications in photoelectric devices. Therefore, a good photo-absorption property is also expected. Hence, the absorption coefficient is calculated to examine the light absorption property. The optical absorption property is explored by calculating the imaginary part ε2(ω) of dielectric constant, which is determined by a summation over pairs of occupied and empty states [48]. The absorption coefficient originates from the complex dielectric function ε(ω) = ε1(ω) + iε2(ω), which depends on frequency ω. ε1(ω) andε2(ω) can be obtained using the Kramers-Kronig relation. The optical absorption coefficient α(ω) is derived by the following formula [49]:

0

Γ

M

K

Γ

0

Γ

M

K

Γ

0

Γ

M

K

Γ

Fig. 5. The calculated sum of the squares of TDMs between CBM and VBM of (a) hollow, (b) B-top and (c) N-top stacking structures, respectively. 4

Chemical Physics Letters 749 (2020) 137430

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(a)

InSe

(b)

hBN

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α//-xx in-plane

α//-zz out-of-plane

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Energy (eV) Fig. 6. Absorption properties of (a) InSe, (b) hBN sublayer and (c) InSe/hBN heterostructure along in-plane (α//-xx) and out-of-plane (α⊥-zz) polarization directions.

CRediT authorship contribution statement Nai-feng Shen: Data curation, Writing - original draft. Xiao-dong Yang: Visualization, Investigation. Xin-xin Wang: Data curation, Validation. Guang-hou Wang: Supervision, Project administration. Jian-guo Wan: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Key Research Programme of China (Grant No. 2016YFA0201004), the National Natural Science Foundation of China (Grant Nos. 91961101, 11464038, 11764034). The numerical calculations in this work have been done using the computing facilities of the High Performance Computing Center (HPCC) in Nanjing University. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2020.137430. References [1] A. Castellanos-Gomez, Nat. Photonics 10 (2016) 202–204. [2] Y. Liu, X. Duan, Y. Huang, X. Duan, Chem. Soc. Rev. 47 (2018) 6388–6409. [3] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666–669. [4] J. Zhou, J. Lin, X. Huang, Y. Zhou, Y. Chen, J. Xia, H. Wang, Y. Xie, H. Yu, J. Lei, D. Wu, F. Liu, Q. Fu, Q. Zeng, C.H. Hsu, C. Yang, L. Lu, T. Yu, Z. Shen, H. Lin, B.I. Yakobson, Q. Liu, K. Suenaga, G. Liu, Z. Liu, Nature 556 (2018) 355–359.

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