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g-C2N van der Waals heterostructures
Accepted Manuscript Full Length Article Tunable Schottky Barrier and Electronic Properties in Borophene/g-C2N van der Waals Heterostructures J.W. Jiang, X.C. Wang, Y. Song, W.B. Mi PII: DOI: Reference:
S0169-4332(18)30152-1 https://doi.org/10.1016/j.apsusc.2018.01.140 APSUSC 38279
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
20 October 2017 3 January 2018 15 January 2018
Please cite this article as: J.W. Jiang, X.C. Wang, Y. Song, W.B. Mi, Tunable Schottky Barrier and Electronic Properties in Borophene/g-C2N van der Waals Heterostructures, Applied Surface Science (2018), doi: https://doi.org/ 10.1016/j.apsusc.2018.01.140
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Tunable Schottky Barrier and Electronic Properties in Borophene/g-C2N van der Waals Heterostructures
J. W. Jiang,a X. C. Wang,a,* Y. Songb, W. B. Mib,*
a
Tianjin Key Laboratory of Film Electronic & Communicate Devices, School of Electrical and
Electronic Engineering, Tianjin University of Technology, Tianjin 300384, China
b
Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparation Technology,
School of Science, Tianjin University, Tianjin 300354, China
*
Author to whom all correspondence should be addressed. E-mail: [email protected] (Xiaocha Wang) and [email protected] (Wenbo Mi)
1
ABSTRACT
By stacking different layers of two dimensional (2D) monolayer materials, the electronic properties of the 2D van der Waals (vdW) heterostructures can be tailored. However, the Schottky barrier formed between 2D semiconductor and metallic electrode has greatly limited the application of 2D semiconductor in nanoelectronic and optoelectronic devices. Herewith, we investigate the electronic properties of borophene/g-C2N vdW heterostructures by first-principles calculations. The results indicate that electronic structures of borophene and g-C2N are preserved in borophene/g-C2N vdW heterostructures. Meanwhile, upon the external electric field, a transition from the n-type Schottky contact to Ohmic contact is induced, and the carrier concentration between the borophene and g-C2N interfaces can be tuned. These results are expected to provide useful insight in the nanoelectronic and optoelectronic devices based on the borophene/g-C2N vdW heterostructures.
Keywords: Borophene/g-C2N, Schottky barrier, Electrical control
2
1. Introduction
Compared with traditional semiconductors, the two-dimensional (2D) materials have unique optical and electrical characteristics [1,2]. Due to the simple structure and sensitiveness to external fields, the allotrope of element can be considered as the model of a two-dimensional crystal system, which is conducive to systematic research and exploration [3]. Since the synthesis of graphene, the allotropes of element have drew much attention of researchers, such as silicene, germanene and so on [4,5]. However, these materials are easily oxidized and cannot be stably present in air, thus they are difficult to be applied in electronic devices. Very recently, a new allotrope (borophene) of boron has been successfully grown on Ag(111) surface, which includes β12 and χ3 phase [6]. For newly discovered 2D material, stability is an important aspect of experimental synthesis and mass production-scale. Combined with the scanning tunnelling microscope (STM) experiments on the oxidization of borophene, β12 and χ3 borophene is relatively inert to oxidation [6]. In addition, the free-standing β12 and χ3 borophene is dynamically, thermodynamically and mechanically stable [7-8]. Considering its excellent metallic character and low mass density, borophene has potential application value for Li-ion and Na-ion batteries [9]. In both structures of borophene, the superconducting transition temperatures are around 20 K, which is higher than observed 7.4 K superconductivity in grapheme [10]. One recent work [11], which suggests that β12 borophene could be decomposed into two triangular sublattices in a way similar to that for a honeycomb lattice, thereby hosting Dirac cones. By stacking different layers of monolayer semiconductors or metal materials, the electronic properties of the 2D van der Waals (vdW) heterostructures can be tuned [12,13]. However, the formation of Schottky barrier between 2D semiconductor and traditional metallic electrode has
3
greatly limited the application of 2D semiconductor in nanoelectronic and optoelectronic devices. The study on metal/semiconductor vdW heterostructures helps to alter this inherent defect [14-17]. Via the application of a perpendicular electric field, it is possible to tune the band structure of the metal/semiconductor vdW heterostructure [18]. Two recent works [19-20], which suggest that Schottky barrier presented by borophene/semiconductor vdW heterostructure can be tuned upon an external electric field. In this letter, based on the first-principles calculations, we simulate the metallic borophene as an electrode, investigating the electronic properties of borophene/semiconductor vdW heterostructure. We consider the 2D g-C2N as the semiconductor system, which has been recently synthesized via a simple wet-chemical reaction. Furthermore, a field-effect transistor device fabricated using this material exhibits an on/off ratio of 10 7, with experimental bandgap of approximately 1.96 eV [21]. Both the energy evolution and phonon spectra confirm that the g-C2N has excellent mechanical stability which can bear tensile strain of up to 12% [22,23]. Theoretical calculation shows that g-C2N can be induced to ferromagnetism by hole doping with a quite low critical concentration [24]. We find that the electronic structures of borophene and C2N are basically retained in the borophene/C2N (B/C2N) vdW heterostructure. The capability to control and induce Schottky barrier is essential for the design of Schottky devices based on vdW heterostructures. By the influence of external electric field, it realizes a transition from n-type Schottky contact to Ohmic contact, and the carrier concentration can be tuned in the B/C2N vdW heterostructure.
2. Calculation details
The first-principles calculations on borophene/semiconductor vdW heterostructure are
4
performed using the Vienna ab initio simulation package (VASP), with the generalized gradient approximation by Perdew, Burke and Ernzerhof (PBE) [25-27]. Kohn-Sham single-particle wave functions are expanded in the plane wave basis set with a kinetic energy cutoff at 500 eV. A 3×5×1 k-point grid centered at the Г point is used for the heterostructures. A vacuum layer of 20 Å is adopted to prevent artificial couplings between adjacent periodic images. The energy and force convergence criteria are 10 -5 eV and 0.01 eV/Å, respectively. Long-range correlation is included in evaluating vdW interaction between the both monolayers by DFT-D2 method [28]. The binding energy is defined as
-
where Eb is the binding energy;
, EB, and
(1)
are the total energy of the heterostructure,
borophene monolayer, and g-C2N monolayer; and NB is the number of boron atoms in the supercell. The band bending can be estimated by the Fermi level difference (
) between the B/C2N contact
and the free-standing g-C2N, which is expressed as
-
where W,
s
(2)
s
are the work functions of B/C2N and the free-standing g-C2N, respectively.
3. Results and discussion
We use a 3×3 β12
orophene supercell and a √3×1
5
2
supercell to com ine β12
borophene/C2N vdW heterostructure; and for χ3 B/C2N, they are 5×1 χ3 borophene supercell and a √3×1
2N
supercell, as shown in Fig. 1. Among them, the relaxed geometric lattice constants of β12
and χ3 borophene unit cells are a = 5.094 Å, b = 2.914 Å and a = 2.899 Å, b = 8.447 Å, respectively; for g-C2N, they are a = b = 8.330 Å, which are in good agreement with previous results [5,21]. The lattice mismatches are about 3.7% in a direction and .4% in
direction in β12 B/C2N; in χ3 B/C2N,
they are only 0.2% and 0.7% in a and b direction, respectively. Thus, the small mismatches have little effects on the electronic properties of B/C2N vdW heterostructure. The values of Eb are 44 meV/atom and 43 meV/atom for β12 and χ3 B/C2N, respectively. After relaxation, the distance etween β12 borophene and C2N interface is 3.00 Å; for χ3 borophene and C2N, it is 3.08 Å. The interactions between borophenes and g-C2N are stronger than those between borophenes and MoSe2 [19], graphene and g-C2N [29] with larger binding energy and smaller interfacial distances. We present the electronic structures of borophenes and g-C2N in Figs. 2(a) - (c), which clearly show that β12 and χ3 borophene are both metallic, while g-C2N is a semiconductor with a direct bandgap of 1.66 eV. Figs. 2(d) and 2(e) show the projected band structures of β12 and χ3 B/C2N, respectively. For both structures, there are several bands crossing Fermi level, which indicate the metallic character. Owing to the weak interaction between the borophene and C2N layers, the respective electronic structures are basically retained in B/C2N vdW heterostructure. Compared with the free-standing g-C2N, the bandgap of the g-C2N layer (1.59 and 1.63 eV) is almost unchanged upon contacting with β12 and χ3 borophenes. These results suggest that borophene can be a very promising material for the metal electrode, and it can enhance the contact performance while still maintaining the electronic properties of g-C2N. Besides, in B/C2N vdW heterostructurs, the interaction moves the Fermi level of borophene to the conduction band of g-C2N, eventually forming an n-type doping to g-C2N. Based on the Schottky-Mott approach, Schottky barrier height
6
(SBH) is defined as the difference between the energy levels of band edges in the semiconductor and Fermi level of the metal [30,31]. Theoretical calculation demonstrates that the value of n-type SBH (
) is 0.29 eV, which is much smaller than 1.30 eV of p-type SBH (
while in χ3 B/C2N,
= 0.34 eV and
) in β12 B/C2N;
= 1.29 eV, which indicates that it forms an n-type
Schottky contact in the B/C2N vdW heterostructure. Via a perpendicular electric field, it is possible to tune the band structure and Schottky barrier of the metal/semiconductor vdW heterostructure, such as borophene/MoSe2, borophene/WSe2 [19], borophene/phosphorene [20] and graphene/g-C2N [29]. In order to investigate the influences of external electric field on electronic structures, we calculate the projected and structures of β12 and χ3 B/C2N with different electric fields in Fig. 3. The band structures of borophenes are almost unchanged, which means that borophenes still retain the metallicity, and the band structure of C2N is maintained. For β12 B/C2N, the Fermi level moves up to the conduction band of C2N with the electric field from -0.5 to 0.8 V/Å, eventually forming the Ohmic contact; while there is still a small n-type Schottky barrier (0.05 eV) at S point in χ3 B/C2N. The changes of Schottky barrier can be explained by the influence of carrier concentration under the electric field. To investigate the charge transfer mechanism between borophene and C 2N layers, we calculate work function and charge density differences in the B/C2N vdW heterostructure. Additional, it is worth noting that the electronegativities of boron, carbon and nitrogen are 2.04, 2.55 and 3.04 eV, respectively. Theoretically, g-C2N has more potential than free-standing borophene in withdrawing electrons. As shown in Fig. 4(a), although the difference is not obvious, C2N has a deeper potential than borophene in the electrostatic potential of the B/C2N vdW heterostructure. For β12 B/C2N, χ3 B/C2N and free-standing g-C2N, work functions are calculated to be 2.36, 2.58 and 3.81 eV, respectively. Here we qualitatively discuss a possible application of them in a 2D electronic device
7
model. We anticipate the borophene and g-C2N to be the traditional noble metal electrode and the channel material that is connected to a metal/semiconductor contact, as plotted Fig. 4(b). We find < 0 in both heterostructures, hence electrons transfer from the B/C2N contact to the free-standing g-C2N and the channel is n-type. From the charge density differences in Fig. 5, we find that positive electric field facilitates the transmission of electrons from the B/C2N contact to the g-C2N (0.35 and 0.31 e in β12 and χ3 B/C2N, respectively), which upshifts the energy level of borophene close to the conduction band of g-C2 N, resulting in a transition from n-type Schottky contact to Ohmic contact in the vdW heterostructure eventually. Finally, we would like to point out that the heterostructures fabricated by stacking monolayer borophene on top of other 2D materials have been proposed [19.20]. The capability to control and induce the transformation of Schottky barrier is essential for the design of Schottky devices based on vdW heterostructures. Compared with other heterostructures, it forms a transition from n-type Schottky contact to Ohmic contact with a strong positive electric field applied in the borophene/g-C2N vdW heterostructure. In view of the previous work [32-34], our simulation electric field is -0.5 to 0.8 V/Å, which may be realistic in 2D materials.
4. Conclusion
In summary, the electronic structures of borophene/g-C2N vdW heterostructure are investigated using first-principles calculations. The results indicate that electronic structures of borophene and C2N are well preserved in B/C2N vdW heterostructure upon their contact. Charge transfer takes place between the B/C2N contact and the g-C2N. In particular, upon the external electric field, carrier concentration and Schottky barrier can be tuned in the vdW heterostructure. By the influence
8
of strong positive electric field, more electrons have transferred from borophene to C2N layer, where the n-type Schottky barrier is transformed into Ohmic contact eventually. These results are expected to provide useful insight in the nanoelectronic and optoelectronic devices based on the B/C2N vdW heterostructures.
Acknowledgements
This work is supported by the Key Project of the Natural Science Foundation of Tianjin City (14JCZDJC37800).
9
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Figure captions
Fig. 1. Top (a) and side (b) view of the β12 B/C2N vdW heterostructure. Top (c) and side (d) view of the χ3 B/C2N vdW heterostructure. The pink, grey and blue balls are for boron, carbon and nitrogen atoms, respectively.
Fig. 2. Band structures of β12 borophene (a), χ3 borophene (b), g-C2N (c), β12 B/C2N vdW heterostructure (d) and χ3 B/C2N vdW heterostructure (e). The grey and red symbols represent the contributions of borophene and C2N, respectively. Fermi level is set to zero. (f) The brillouin zone and the high symmetry points.
Fig. 3. Band structures of β12 B/C2N (a) and χ3 B/C2N (b) vdW heterostructures with different electric field. The grey and red symbols represent the contributions of borophene and C2N, respectively. Fermi level is set to zero.
Fig. 4. (a) Plane averaged electrostatic potential of B/C2N vdW heterostructure. (b) Schematic plot of a 2D electronic device model where the B/C2N contact is connected to the g-C2N channel extending to the right. W and Ws denote work functions of the B/C2N contact and the g-C2N, respectively; Evac denotes the vacuum level; CBM and VBM denote conduction band minimum and valence band maximum, respectively; The black lines qualitatively indicate the band bending.
Fig. 5. Plane-averaged charge density differences of β12 B/C2N (a) and χ3 B/C2N (b) with different
13
electric field. The yellow and blue zones represent gain and loss of electrons, respectively.
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19
Highlights
The electronic properties of borophene/semiconductor vdW heterostructure are predicted. Electronic structures of borophene and g-C2N are preserved in their vdW heterostructure. Upon electric field, a transition from the n-type Schottky contact to Ohmic contact is induced. The carrier concentration between the borophene and g-C2N interfaces can be tuned.
20
Graphical Abstract
21
S0169-4332(18)30152-1 https://doi.org/10.1016/j.apsusc.2018.01.140 APSUSC 38279
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
20 October 2017 3 January 2018 15 January 2018
Please cite this article as: J.W. Jiang, X.C. Wang, Y. Song, W.B. Mi, Tunable Schottky Barrier and Electronic Properties in Borophene/g-C2N van der Waals Heterostructures, Applied Surface Science (2018), doi: https://doi.org/ 10.1016/j.apsusc.2018.01.140
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Tunable Schottky Barrier and Electronic Properties in Borophene/g-C2N van der Waals Heterostructures
J. W. Jiang,a X. C. Wang,a,* Y. Songb, W. B. Mib,*
a
Tianjin Key Laboratory of Film Electronic & Communicate Devices, School of Electrical and
Electronic Engineering, Tianjin University of Technology, Tianjin 300384, China
b
Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparation Technology,
School of Science, Tianjin University, Tianjin 300354, China
*
Author to whom all correspondence should be addressed. E-mail: [email protected] (Xiaocha Wang) and [email protected] (Wenbo Mi)
1
ABSTRACT
By stacking different layers of two dimensional (2D) monolayer materials, the electronic properties of the 2D van der Waals (vdW) heterostructures can be tailored. However, the Schottky barrier formed between 2D semiconductor and metallic electrode has greatly limited the application of 2D semiconductor in nanoelectronic and optoelectronic devices. Herewith, we investigate the electronic properties of borophene/g-C2N vdW heterostructures by first-principles calculations. The results indicate that electronic structures of borophene and g-C2N are preserved in borophene/g-C2N vdW heterostructures. Meanwhile, upon the external electric field, a transition from the n-type Schottky contact to Ohmic contact is induced, and the carrier concentration between the borophene and g-C2N interfaces can be tuned. These results are expected to provide useful insight in the nanoelectronic and optoelectronic devices based on the borophene/g-C2N vdW heterostructures.
Keywords: Borophene/g-C2N, Schottky barrier, Electrical control
2
1. Introduction
Compared with traditional semiconductors, the two-dimensional (2D) materials have unique optical and electrical characteristics [1,2]. Due to the simple structure and sensitiveness to external fields, the allotrope of element can be considered as the model of a two-dimensional crystal system, which is conducive to systematic research and exploration [3]. Since the synthesis of graphene, the allotropes of element have drew much attention of researchers, such as silicene, germanene and so on [4,5]. However, these materials are easily oxidized and cannot be stably present in air, thus they are difficult to be applied in electronic devices. Very recently, a new allotrope (borophene) of boron has been successfully grown on Ag(111) surface, which includes β12 and χ3 phase [6]. For newly discovered 2D material, stability is an important aspect of experimental synthesis and mass production-scale. Combined with the scanning tunnelling microscope (STM) experiments on the oxidization of borophene, β12 and χ3 borophene is relatively inert to oxidation [6]. In addition, the free-standing β12 and χ3 borophene is dynamically, thermodynamically and mechanically stable [7-8]. Considering its excellent metallic character and low mass density, borophene has potential application value for Li-ion and Na-ion batteries [9]. In both structures of borophene, the superconducting transition temperatures are around 20 K, which is higher than observed 7.4 K superconductivity in grapheme [10]. One recent work [11], which suggests that β12 borophene could be decomposed into two triangular sublattices in a way similar to that for a honeycomb lattice, thereby hosting Dirac cones. By stacking different layers of monolayer semiconductors or metal materials, the electronic properties of the 2D van der Waals (vdW) heterostructures can be tuned [12,13]. However, the formation of Schottky barrier between 2D semiconductor and traditional metallic electrode has
3
greatly limited the application of 2D semiconductor in nanoelectronic and optoelectronic devices. The study on metal/semiconductor vdW heterostructures helps to alter this inherent defect [14-17]. Via the application of a perpendicular electric field, it is possible to tune the band structure of the metal/semiconductor vdW heterostructure [18]. Two recent works [19-20], which suggest that Schottky barrier presented by borophene/semiconductor vdW heterostructure can be tuned upon an external electric field. In this letter, based on the first-principles calculations, we simulate the metallic borophene as an electrode, investigating the electronic properties of borophene/semiconductor vdW heterostructure. We consider the 2D g-C2N as the semiconductor system, which has been recently synthesized via a simple wet-chemical reaction. Furthermore, a field-effect transistor device fabricated using this material exhibits an on/off ratio of 10 7, with experimental bandgap of approximately 1.96 eV [21]. Both the energy evolution and phonon spectra confirm that the g-C2N has excellent mechanical stability which can bear tensile strain of up to 12% [22,23]. Theoretical calculation shows that g-C2N can be induced to ferromagnetism by hole doping with a quite low critical concentration [24]. We find that the electronic structures of borophene and C2N are basically retained in the borophene/C2N (B/C2N) vdW heterostructure. The capability to control and induce Schottky barrier is essential for the design of Schottky devices based on vdW heterostructures. By the influence of external electric field, it realizes a transition from n-type Schottky contact to Ohmic contact, and the carrier concentration can be tuned in the B/C2N vdW heterostructure.
2. Calculation details
The first-principles calculations on borophene/semiconductor vdW heterostructure are
4
performed using the Vienna ab initio simulation package (VASP), with the generalized gradient approximation by Perdew, Burke and Ernzerhof (PBE) [25-27]. Kohn-Sham single-particle wave functions are expanded in the plane wave basis set with a kinetic energy cutoff at 500 eV. A 3×5×1 k-point grid centered at the Г point is used for the heterostructures. A vacuum layer of 20 Å is adopted to prevent artificial couplings between adjacent periodic images. The energy and force convergence criteria are 10 -5 eV and 0.01 eV/Å, respectively. Long-range correlation is included in evaluating vdW interaction between the both monolayers by DFT-D2 method [28]. The binding energy is defined as
-
where Eb is the binding energy;
, EB, and
(1)
are the total energy of the heterostructure,
borophene monolayer, and g-C2N monolayer; and NB is the number of boron atoms in the supercell. The band bending can be estimated by the Fermi level difference (
) between the B/C2N contact
and the free-standing g-C2N, which is expressed as
-
where W,
s
(2)
s
are the work functions of B/C2N and the free-standing g-C2N, respectively.
3. Results and discussion
We use a 3×3 β12
orophene supercell and a √3×1
5
2
supercell to com ine β12
borophene/C2N vdW heterostructure; and for χ3 B/C2N, they are 5×1 χ3 borophene supercell and a √3×1
2N
supercell, as shown in Fig. 1. Among them, the relaxed geometric lattice constants of β12
and χ3 borophene unit cells are a = 5.094 Å, b = 2.914 Å and a = 2.899 Å, b = 8.447 Å, respectively; for g-C2N, they are a = b = 8.330 Å, which are in good agreement with previous results [5,21]. The lattice mismatches are about 3.7% in a direction and .4% in
direction in β12 B/C2N; in χ3 B/C2N,
they are only 0.2% and 0.7% in a and b direction, respectively. Thus, the small mismatches have little effects on the electronic properties of B/C2N vdW heterostructure. The values of Eb are 44 meV/atom and 43 meV/atom for β12 and χ3 B/C2N, respectively. After relaxation, the distance etween β12 borophene and C2N interface is 3.00 Å; for χ3 borophene and C2N, it is 3.08 Å. The interactions between borophenes and g-C2N are stronger than those between borophenes and MoSe2 [19], graphene and g-C2N [29] with larger binding energy and smaller interfacial distances. We present the electronic structures of borophenes and g-C2N in Figs. 2(a) - (c), which clearly show that β12 and χ3 borophene are both metallic, while g-C2N is a semiconductor with a direct bandgap of 1.66 eV. Figs. 2(d) and 2(e) show the projected band structures of β12 and χ3 B/C2N, respectively. For both structures, there are several bands crossing Fermi level, which indicate the metallic character. Owing to the weak interaction between the borophene and C2N layers, the respective electronic structures are basically retained in B/C2N vdW heterostructure. Compared with the free-standing g-C2N, the bandgap of the g-C2N layer (1.59 and 1.63 eV) is almost unchanged upon contacting with β12 and χ3 borophenes. These results suggest that borophene can be a very promising material for the metal electrode, and it can enhance the contact performance while still maintaining the electronic properties of g-C2N. Besides, in B/C2N vdW heterostructurs, the interaction moves the Fermi level of borophene to the conduction band of g-C2N, eventually forming an n-type doping to g-C2N. Based on the Schottky-Mott approach, Schottky barrier height
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(SBH) is defined as the difference between the energy levels of band edges in the semiconductor and Fermi level of the metal [30,31]. Theoretical calculation demonstrates that the value of n-type SBH (
) is 0.29 eV, which is much smaller than 1.30 eV of p-type SBH (
while in χ3 B/C2N,
= 0.34 eV and
) in β12 B/C2N;
= 1.29 eV, which indicates that it forms an n-type
Schottky contact in the B/C2N vdW heterostructure. Via a perpendicular electric field, it is possible to tune the band structure and Schottky barrier of the metal/semiconductor vdW heterostructure, such as borophene/MoSe2, borophene/WSe2 [19], borophene/phosphorene [20] and graphene/g-C2N [29]. In order to investigate the influences of external electric field on electronic structures, we calculate the projected and structures of β12 and χ3 B/C2N with different electric fields in Fig. 3. The band structures of borophenes are almost unchanged, which means that borophenes still retain the metallicity, and the band structure of C2N is maintained. For β12 B/C2N, the Fermi level moves up to the conduction band of C2N with the electric field from -0.5 to 0.8 V/Å, eventually forming the Ohmic contact; while there is still a small n-type Schottky barrier (0.05 eV) at S point in χ3 B/C2N. The changes of Schottky barrier can be explained by the influence of carrier concentration under the electric field. To investigate the charge transfer mechanism between borophene and C 2N layers, we calculate work function and charge density differences in the B/C2N vdW heterostructure. Additional, it is worth noting that the electronegativities of boron, carbon and nitrogen are 2.04, 2.55 and 3.04 eV, respectively. Theoretically, g-C2N has more potential than free-standing borophene in withdrawing electrons. As shown in Fig. 4(a), although the difference is not obvious, C2N has a deeper potential than borophene in the electrostatic potential of the B/C2N vdW heterostructure. For β12 B/C2N, χ3 B/C2N and free-standing g-C2N, work functions are calculated to be 2.36, 2.58 and 3.81 eV, respectively. Here we qualitatively discuss a possible application of them in a 2D electronic device
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model. We anticipate the borophene and g-C2N to be the traditional noble metal electrode and the channel material that is connected to a metal/semiconductor contact, as plotted Fig. 4(b). We find < 0 in both heterostructures, hence electrons transfer from the B/C2N contact to the free-standing g-C2N and the channel is n-type. From the charge density differences in Fig. 5, we find that positive electric field facilitates the transmission of electrons from the B/C2N contact to the g-C2N (0.35 and 0.31 e in β12 and χ3 B/C2N, respectively), which upshifts the energy level of borophene close to the conduction band of g-C2 N, resulting in a transition from n-type Schottky contact to Ohmic contact in the vdW heterostructure eventually. Finally, we would like to point out that the heterostructures fabricated by stacking monolayer borophene on top of other 2D materials have been proposed [19.20]. The capability to control and induce the transformation of Schottky barrier is essential for the design of Schottky devices based on vdW heterostructures. Compared with other heterostructures, it forms a transition from n-type Schottky contact to Ohmic contact with a strong positive electric field applied in the borophene/g-C2N vdW heterostructure. In view of the previous work [32-34], our simulation electric field is -0.5 to 0.8 V/Å, which may be realistic in 2D materials.
4. Conclusion
In summary, the electronic structures of borophene/g-C2N vdW heterostructure are investigated using first-principles calculations. The results indicate that electronic structures of borophene and C2N are well preserved in B/C2N vdW heterostructure upon their contact. Charge transfer takes place between the B/C2N contact and the g-C2N. In particular, upon the external electric field, carrier concentration and Schottky barrier can be tuned in the vdW heterostructure. By the influence
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of strong positive electric field, more electrons have transferred from borophene to C2N layer, where the n-type Schottky barrier is transformed into Ohmic contact eventually. These results are expected to provide useful insight in the nanoelectronic and optoelectronic devices based on the B/C2N vdW heterostructures.
Acknowledgements
This work is supported by the Key Project of the Natural Science Foundation of Tianjin City (14JCZDJC37800).
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Figure captions
Fig. 1. Top (a) and side (b) view of the β12 B/C2N vdW heterostructure. Top (c) and side (d) view of the χ3 B/C2N vdW heterostructure. The pink, grey and blue balls are for boron, carbon and nitrogen atoms, respectively.
Fig. 2. Band structures of β12 borophene (a), χ3 borophene (b), g-C2N (c), β12 B/C2N vdW heterostructure (d) and χ3 B/C2N vdW heterostructure (e). The grey and red symbols represent the contributions of borophene and C2N, respectively. Fermi level is set to zero. (f) The brillouin zone and the high symmetry points.
Fig. 3. Band structures of β12 B/C2N (a) and χ3 B/C2N (b) vdW heterostructures with different electric field. The grey and red symbols represent the contributions of borophene and C2N, respectively. Fermi level is set to zero.
Fig. 4. (a) Plane averaged electrostatic potential of B/C2N vdW heterostructure. (b) Schematic plot of a 2D electronic device model where the B/C2N contact is connected to the g-C2N channel extending to the right. W and Ws denote work functions of the B/C2N contact and the g-C2N, respectively; Evac denotes the vacuum level; CBM and VBM denote conduction band minimum and valence band maximum, respectively; The black lines qualitatively indicate the band bending.
Fig. 5. Plane-averaged charge density differences of β12 B/C2N (a) and χ3 B/C2N (b) with different
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electric field. The yellow and blue zones represent gain and loss of electrons, respectively.
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Highlights
The electronic properties of borophene/semiconductor vdW heterostructure are predicted. Electronic structures of borophene and g-C2N are preserved in their vdW heterostructure. Upon electric field, a transition from the n-type Schottky contact to Ohmic contact is induced. The carrier concentration between the borophene and g-C2N interfaces can be tuned.
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Graphical Abstract
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