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Atomically Thin p-n/p-n Nanodevices by Surface Charge Transfer Doping of Arsenene/Antimonene Heterostructures Lei Zhang, and WanZhen Liang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06563 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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ACS Applied Materials & Interfaces

Atomically Thin p-n/p-n Nanodevices by Surface Charge Transfer Doping of Arsenene/Antimonene Heterostructures Lei Zhang and WanZhen Liang∗ State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian Province, People’s Republic of China E-mail: [email protected]

Abstract Surface charge transfer doping (SCTD) is a promising technique to construct high-performance nanodevices because of its high reproducibility, high spatial selectivity and little harm to host semiconductor. Here we performed a first-principles theoretical investigation to assess the effects of SCTD on the properties of two-dimensional (2D) arsenene, antimonene and arsenene/antimonene van der Waals (VDWs) heterostructure as well. It was found that doping O or S on the surfaces of arsenene and antimonene could achieve efficient p-type doping while doping Cs2 CO3 on them could achieve n-type doping. Furthermore, when O and Cs2 CO3 were co-doped on the two sides of arsenene/antimonene heterostructure, a typical type-II energy band alignment can be formed in O-arsenene/Cs2 CO3 -antimonene heterostructure, which effectively extends the range of the light absorption into near-infrared region and facilitates ∗ To

whom correspondence should be addressed

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the spatial separation of photo-generated electron-hole pairs. O- or S-doped arsenene and antimonene have tunable banggaps varying from 1.20 to 0.54 eV due to the doping-induced change of the conduction band minima (CBMs), and Cs2 CO3 -doped arsenene and antimonene have the bandgaps of 2.02 and 1.36 eV, respectively, because of the changes of both VB maxima and CBMs. This work offers a way to design p-n junctions with tunable character, and the 2D p-n/p-n O-arsenene/Cs2 CO3 -antimonene heterostructure might be applied to electronic and optoelectronic nanodevices.

Keywords Arsenene/Antimonene, P-N Junctions, SCTD, Doping-induced changes, Nanodevices

Introduction Two-dimensional (2D) nanosheets such as phosphorene 1 and graphene, 2 hold great potential in many fields, such as field effect transistors, 3 photodetectors, 4 topological insulators spintronic devices, 5 cancer therapy, 6 light emitting devices, gas sensors, 7 thermoelectric and energy devices, et. al. They thus have attracted considerable scientific attention. As the analogs of 2D phosphorene, monolayer arsenene (As) and antimonene (Sb) have been synthesized by different methods, 8–11 and have been demonstrated to have high carrier mobilities, wide bandgaps etc. Besides, both 2D arsenene and antimonene were theoretically predicted to be stable. 12–14 Up to now, monolayer arsenene and antimonene have been intensively studied. For example, Xu et al. 15 systematically investigated the optical properties of monolayer arsenene and antimonene allotropes and found that the direct band gaps of α -arsenene and antimonene were much smaller than the indirect band gaps of their β -counterpart, respectively. Ding’s group 16 found that all armchair As/Sb nanoribbons keeped the indirect band gap feature, while the zigzag ones transfer to direct semiconductors. Moreover, it was found that biaxial strain could tune the electronic properties of the 2D As/Sb and h-BN/Sb vdW heterostructures, 17,18 which could result in the indirect-to-direct gap transition. A 2


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comprehensive and up-to-date reviews of both experimental and theoretical studies can be found in Ref. 18. Nowadays, it is a hot topic to develop atomically thin 2D semiconductors for applying in semiconductor (opto)electronics. To date, most 2D semiconductors possess the n-type characteristic, therefore, developing p-type or ambipolar 2D materials with high carrier mobility is highly desired in the (opto)electronics fields. To achieve the p-type character or obtain ambipolar 2D materials, 2D materials can be doped by adopting surface charge transfer doping(SCTD). For example, black phosphorus(BP) after STCD with NO2 possesses the conduction type of n, 19 and Cs2 CO3 -doping BP possesses characteristic of p-type. 20 Additionally, by combining these various and emergent 2D semiconductors, one can fabricate the 2D VDWs heterostructures with a range of properties that can be tailored by design. In two-dimensional optoelectronic technologies, the heterostructures are required to drive charge separation, in addition to transport the independent charge carriers to the electrodes, a donor-acceptor heterostructure is thus formed in that the interfacial charge separation and recombination at heterojunctions of monolayer semiconductors are of interest. Both arsenene and antimonene have buckled hexagonal lattices with nearly isotropic mechanical properties. Each atom is 3-fold coordinated forming silicene-like 2D structure with buckling at the surface. 21 Due to their special geometric structures, where sp2 -like bonds form a nonplanar honeycomb structure, both arsenene and antimonene have good ductilities, which are good for tuning electronic properties by strains. 17,22,23 Therefore, we have been wondering if we could combine monolayer arsenene and antimonene together to create a vertical heterostructures holding high carrier mobility and ambipolar characteristic, satisfying the requirements of (opto)electronics. To address the issues, herein, we preformed a first-principles theoretical investigation on the intrinsic electronic structures and the surface charge transfer doping (SCTD)-induced changes on the properties of monolayer arsenene and antimonene, and the VDWs heterostructure of arsenene/antimonene as well. SCTD is a promising technique to construct high-performance nanodevices based on low-dimensional nanostructures because of high reproducibility and high spatial

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selectivity. 24,25 Our calculations revealed that doping O or S on arsenene and antimonene could achieve efficient p-type doping where electrons accumulate on O and S atoms from arsenene and antimonene while doping Cs2 CO3 on them could achieve n-type doping where electrons transfer from Cs2 CO3 to arsenene or antimonene because of the relative low work function of Cs2 CO3 . By packing O-doping arsenene and Cs2 CO3 -doping antimonene together, we constructed a 2D VDWs heterostructure, O-arsenene/Cs2 CO3 -antimonene, which possesses the character of a typical type-II energy band assignment where the electron-hole pairs created by the photons can be effectively separated. Therefore, they might be applied in designing high-performance electronic and optoelectronic devices.

Computational details All first-principles calculations were performed by density functional theory (DFT) in conjunction with projector augmented wave (PAW) potentials, which were implemented in the Vienna Ab-initio Simulation Package (VASP 5.4). 26–28 An energy cutoff of 500 eV was employed and the atomic positions were optimized using a conjugate gradient scheme without any symmetric restrictions until the maximum force on each of them is less than 0.02 eV/Å. A 8 × 8 × 1 κ -point grid was used for all slab models, which had the same vacuum spaces of 12 Å to avoid interactions between neighboring slabs. All electronic structures were computed by DFT with HSE06 functional 29 and the spin-orbit coupling (SOC) effect was accounted for. Additionally, the zero damping DFT-D3 of Grimme 30 was adopted to account for the weak interactions between adjacent surfaces.

Results and discussion The geometries of monolayer arsenene and antimonene, shown in Figure S1 in the supporting information (SI), were fully relaxed. The optimized lattice parameters, a, for arsenene and antimonene were 3.603 Å and 4.118 Å respectively, which were in good agreement with previous theoretical results (3.607 Å for arsenene 22 and 4.12 Å for antimonene 31 ). The lattice constants of 4


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Table 1: The calculated and experimentally-measured (in parenthesis) lattice constants. a(Å)

γ (◦ )

c(Å)

V(Å3 )

Bulk-As

3.823(3.760) 10.251(10.441) 120(120) 129.7(127.8)

arsenene

3.603

Bulk-Sb

4.365(4.332) 11.193(11.372) 120(120) 184.7(184.8)

antimonene 4.118

13.631

13.899

120

120

153.3

204.1

bulk As and Sb were also calculated which are close to the experimental values, 32,33 indicating the rational of geometrical structures. Table 1 lists the calculated and experimentally-measured lattice constants in detail. Then we calculated bandstructures of arsenene and antimonene by DFT with different XC functionals, PBE and HSE06, with and without the inclusion of SOC effect. The results were shown in Figure S2 in SI and Table 2. It was found that PBE underestimates the bandgaps and SOC has a significant effect on band structures of monolayer arsenene and antimonene, especially on VBM. However, the SOC had a weaker influence on bulk As and Sb. HSE06+SOC produced the band gaps of 2.22 eV and 1.64 eV for monolayer arsenene and antimonene, which are in accord with previous theoretical values of 2.11 and 1.70 eV. 22,31 Therefore, all following electronic structure calculations were performed by HSE06+SOC. In order to clarify the contributions from different orbits to the band structures around the Fermi level, density of states (DOSs) and projected DOSs (PDOSs) of monolayer arsenene and antimonene were calculated. VBMs of both arsenene and antimonene possess the characteristics of p-orbital, whereas CBMs have the characteristics of mixed s-, p- and d-orbitals (see Figure S3 in SI).

SCTD via Cs2CO3 absorption on Arsenene and Antimonene The optimized geometrical structures of Cs2 CO3 -doping arsenene and antimonene were shown in Figure 1. Cs2 CO3 molecules were bonded to layered arsenene or antimonene with As-O or Sb-O bonds forming, respectively. It is noteworthy that there are two anchor sites on Cs2 CO3 /arsenene

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Table 2: Bandgaps (in eV) of monolayer and bulk As and Sb by different DFT XC functionals with or without SOC effect, respectively. As

XCs

Sb

Bulk

Monolayer

Bulk

Monolayer

PBE

0.34

1.59

0.12

1.27

PBE+SOC

0.33

1.46

0.25

1.03

HSE06+SOC

0.40

2.22

0.29

1.64

Figure 1: Optimized structures of Cs2 CO3 /arsenene and Cs2 CO3 /antimonene (front view and top view). Green, brown, gray and red balls denote As, Sb, C and O atoms, respectively.

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complex while only one anchor site in Cs2 CO3 /antimonene complex due to the different distances between the atoms in a six-member ring, indicating the different interactions between Cs2 CO3 and two semiconductors. The calculated binding energies between Cs2 CO3 and arsenene or antimonene were −1.14 eV or −1.50 eV, respectively, indicating that Cs2 CO3 /antimonene is more stable than Cs2 CO3 /arsenene complex. To visualize the charge transfer between Cs2 CO3 and arsenene or antimonene, at first we calculated the difference charge densities (∆ρ ) of Cs2 CO3 -modified arsenene and antimonene, shown in the insets of Figure 2(a). We observe that there is a strong electron accumulation around arsenene or antimonene layer, while the electron depletion appears around Cs2 CO3 , indicating that the adsorbed Cs2 CO3 molecules act as electron donor and the semiconductor (arsenene or antimonene) act as electronic acceptor. Here ∆ρ is defined 25 as ∆ρ =ρCs2CO3 −X - ρX - ρCs2CO3 , where

ρCs2CO3 −X , ρX and ρCs2CO3 are charge densities of the Cs2 CO3 -modified systems, the semiconductor (arsenene or antimonene) and the isolated Cs2 CO3 molecule, respectively. The regions of electron accumulation and depletion were represented with yellow and green colors, respectively. The electronic properties of arsenene and antimonene semiconductors have also been tailored by the absorption of Cs2 CO3 as shown in Figure 2. To have a close comparison, the DOS and band structures of pure arsenene and antimonene have also been shown there. The absorption of Cs2 CO3 on arsenene or antimonene results in the decrease of bandgaps. Cs2 CO3 -doping arsenene or antimonene have the banggaps of 2.02 eV and 1.36 eV while the pure ones have the bandgaps of 2.22 and 1.64 eV, respectively. Moreover, Cs2 CO3 absorption also makes the band structures of arsenene and antimonene change a lot with the high symmetry point shifted from the middle of G-M to the middle of K-G. Cs2 CO3 molecules mainly contribute to the VBM near the fermi level whereas hardly ever contribute to CBM, indicating that they could further donate electrons to semiconductors at excited by light illumination.

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Figure 2: (a)The DOS and (b)band structures of Cs2 CO3 -modified and pure arsenene and antimonene calculated by HSE06+SOC, respectively. The insets in (a) denote the charge-density difference of Cs2 CO3 /arsenene (left) and Cs2 CO3 /antimonene (right). Isosurface value is 0.0012 e/Å3 .

8


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SCTD via O and S absorption on arsenene and antimonene Optimal structures of O-doping on arsenene(As8 -O1 ) and antimonene(Sb8 -O1 ) were obtained and both distances of As-As and Sb-Sb near the O-doping area decreased obviously as shown in Figure 3. The detailed lattice constants are shown in Table S1. Similarly, S-doping arsenene and antimonene were investigated as well. Because S-doping systems have the similar variation tendency with O-doping systems(see Figure S4-5), herein, the O-doping arsenene and antimonene were chosen for discussion in detailed as an example.

Figure 3: Optimized structures of As8 -O1 and Sb8 -O1 (front view and top view). Green, brown and red balls denote As, Sb and O atoms, respectively. To explore how O-doping affects on the properties of arsenene and antimonene, we therefore calculated DOSs and band structures of these O-doping arsenene and antimonene materials. Figure 4 represents the DOSs and band structures of O-doping arsenene and antimonene calculated by HSE06+SOC, respectively. O-doping had a significant influence on CBM but little effect on VBM, which is different from the results of Cs2 CO3 -modified semiconductors. From the data in Figure 4a, all band gaps of O-doping arsenene and antimonene compounds were decreased remarkably with respect to their pure ones. It should be noted that their CBMs were mainly composed of As 9



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Figure 4: (a) DOSs and (b) band structures of O-doped and pure As and Sb calculated by HSE06+SOC, respectively. The insets in (a) show the difference charge densities.

Figure 5: DOS of O-doping arsenene varied with O concentration.

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or Sb and little of O, which means that the energy levels of As or Sb atoms were pulled down due to the high electronegativity of O. To further explore the effect of O-doping on arsenene and antimonene, the difference charge densities were carried out as well. It is interesting that electrons accumulated from layered semiconductor (arsenene or antimonene ) to O atoms in all doping compounds, shown in Figure 4(a), indicating that semiconductor plays a role of donator in O-doping systems. The different behavior of O-doping semiconductors from those of Cs2 CO3 -doping ones mainly results from their relative work functions as adopted in previous works. 25 Turning our attention to band structures shown in Figure 4b, we found that all O-doping arsenene and antimonene were direct semiconductors with band gaps of 1.20 eV and 0.57 eV, respectively. The calculated binding energies for O on arsenene and antimonene were −4.09 eV and −3.86 eV, respectively, showing strong chemical interaction between O atom and the semiconductors. Additionally, we had studied the effects of O concentration on the electronic properties of Odoping arsenene, shown in the Figure 5. It is obviously that the conduction band edges shifted to the lower energies with the increases of O concentration with the bandgaps varying from 1.72(O1 ) to 1.63 eV(O4 ), which was accord with previous variation tendency of band gap of antimonene oxide with O concentration. 34

O-Arsenene / Cs2CO3 -Antimonene VDWs heterostructure

Inspired by the results mentioned above, we constructed the O-arsenene/Cs2 CO3 -antimonene heterostructure(O As32 /Cs2 CO3 -Sb24 ) to identify the energy band assignment. The optimized geometrical structures of O4 -As32 /Cs2 CO3 -Sb24 are shown in Figure 6. As a reference, the optimized structure of arsenene/antimonene is shown in Figure S6. Local structure near the adsorption sites in O4 As32 /Cs2 CO3 -Sb24 -antimonene heterostructure changed obviously due to the layer’s interaction. Moreover, we observed that the distances of Sb-Sb were stretched because of the presence of Cs2 CO3 molecules, whereas those of As-As were compressed clearly due to the existence of As-O bonds. To further explore its intrinsic properties, DOS of O4 -As32 /Cs2 CO3 -Sb24 and the difference 11



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Figure 6: The front(left) and top views(right) of optimized O4 -As32 /Cs2 CO3 -Sb24 heterostructure. charge densities of O4 -As32 /Cs2 CO3 -Sb24 were calculated as shown in Figure 7. It is noteworthy that a novel p-n/p-n junction with parallel arrangement was formed, where electrons gathered on the oxygens atoms and holes accumulated on the Cs2 CO3 molecules. In addition, the electrons transferred to the area near the absorption sites from the other regions in-plane antimonene layer. The O and Cs2 CO3 co-doping change the bandstructures remarkably. Compared with undoped heterostructure of arsenene/antimonene, the novel p-n/p-n O4 -As32 /Cs2 CO3 -Sb24 heterostructure had a narrow bandgap of 0.91 eV. The typical type-II band alignment was formed obviously in O4 -As32 /Cs2 CO3 -Sb24 . Its VBM was mainly composed of Sb atoms and As atoms primarily contributed to the CBM, indicating that electrons will transfer from antimonene layer to arsenene layer after illumination and holes will stay on the antimonene, which are in accord with the results obtained from the DOS analysis. Meanwhile, due to the existence of p-n junctions, the electrons will further accumulate at O atoms in arsenene layer and holes gather at Cs2 CO3 molecules in antimonene layer, which could be applied to nanodevices with high performance. We further calculated the adsorption spectra of O4 As32 /Cs2 CO3 -Sb24 heterostructure and its components. The results were shown in Figure S7. It was found that an additional low-energy absorption peak appeared in the energy range of 1.0 − 1.5 eV in absorption spectrum of O4 -As32 /Cs2 CO3 -Sb24 , which indicates that the interlayer’s charge transfer takes place. To quantitatively demarcate the band alignment in the heterostructures according to the intrinsic energy-level alignments of monolayer arsenene and antimonene, we had carried out the com12


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putational analysis on the O4 -As32 /Cs2 CO3 -Sb24 heterostructure, and the corresponding arsenene and antimonene components, respectively. Figure 8 shows the relative energy levels and bandoffset schematic illustration in O4 -As32 /Cs2 CO3 -Sb24 heterostructure. It was found that VBM and CBM of O-doping arsenene lie 0.32 and 0.05 eV, above those of Cs2 CO3 -modified antimonene, respectively, which means that the effective carriers spatial separation could be achieved in the heterostructure. However, for arsenene/antimonene heterostructure, both As and Sb atoms contributed to the VBM and CBM without classical type-II band alignment, which could be further proven by the charge distributions of CBM and VBM shown in Figure S8 in SI, indicating the poor performance in practical applications.

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Figure 8: Upper panel: the partial charges of VBM and CBM of O4 -As32 /Cs2 CO3 -Sb24 heterostructure, respectively. Lower panel: bandoffset schematic illustration of O4 -As32 /Cs2 CO3 Sb24 .

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Page 14 of 22

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/Sb

32

-Cs CO

24

2

3

O -As 1

/Sb

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-Cs CO

24

2

3

O -As 2

/Sb

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Energy(eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

POS1

0

POS2

-1

-2

-3

K

G M K

G M K

G M K

G M

Figure 9: Calculated band structures of O-arsenene/Cs2 CO3 -antimonene heterostructures with different O concentrations. edges kept unchanged because the CBM was composed of As and O atoms while the VBM was attributed to Sb atoms. In addition, it was found that the relative positions between O and Cs2 CO3 molecules would change electronic properties of O-arsenene/Cs2 CO3 -antimonene heterostructures but little on both bandgaps, as shown in Figure 9. The detailed geometrical structures for POS1 and POS2 were shown in Figure S9.

Concluding remarks Based on the first-principles DFT calculations, we systematically investigated the effects of SCTD on electronic and geometric structures of isolated arsenene and antimonene, and arsenene/antimonene heterostructures. It was found that doping with O, S and Cs2 CO3 on the surfaces of arsenene and antimonene changed the bonds near the absorbing sites, leading to the decrease of bandgaps compared with the pure ones. Specifically, As-As or Sb-Sb bond lengths of O, S-doped arsenene or antimonene decreased obviously while they increased in Cs2 CO3 -modified arsenene or antimonene systems. Because of the high electronegativity of O and S atoms, the energy levels of As and Sb orbitals in CB regions were pulled down in O- and S-doped arsenene and antimonene, resulting in narrow 15



Page 16 of 22

bandgaps varying from 1.20 to 0.54 eV. Besides, all O-,S-doped arsenene or antimonene are direct semiconductors, which are good for Visible-NIR absorption. As for Cs2 CO3 -doping arsenene and antimonene, VBMs change obviously and their bandgap semiconductors with bandgaps of 2.02 and 1.36 eV, respectively, which are better for visible light absorption. What’s more, it is pretty interesting that absorbing O or S on arsenene and antimonene could achieve efficient p-type doping, where electrons accumulated on O,S atoms from arsenene or antimonene layer. Contrarily, adsorbing Cs2 CO3 on arsenene or antimonene could achieve n-type doping, where electrons transferred from Cs2 CO3 into arsenene or antimonene because of the relative low work function of Cs2 CO3 . When O and Cs2 CO3 were co-doped into arsenene and antimonene, respectively, a heterostructure O-arsenene/Cs2 CO3 -antimonene was formed, which possesses a typical type-II energy band assignment, effectively extends the range of the light absorption into near-infrared region and facilitates the spatial separation of photo-generated electron-hole pairs. Meantime, due to the existences of p-n junction caused by O and Cs2 CO3 , the electrons would further gathered on regions near the O atoms while holes accumulated on Cs2 CO3 , which could achieve a higher efficient separation of electron-hole pairs. This work offers a way to design p-n junctions with different characters and the 2D p-n/p-n O-arsenene/Cs2 CO3 -antimonene heterostructure might be applied in the design of optoelectronic nanodevices.

ACKNOWLEDGMENTS Financial supports from National Natural Science Foundation of China (Grant No. 21573177) is gratefully acknowledged.

SUPPORTING INFORMATION The front and top views of the atomic structure of bulk As and Sb, and monolayer arsenene and antimonene. The calculated band structures of layered arsenene(left) and antimonene(right) and corresponding bulk As and Sb materials by different DFT XC functionals with or without 16


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SOC, respectively. Calculated DOSs of monolayer arsenene (over) and antimonene (under) by HSE06+SOC, respectively. The front(left) and top views(right) of optimized arsenene/antimonene heterostructure. Calculated lattice constants of O,S-doping arsenene and antimonene. The difference charge densities of As8 -S1 and Sb8 -S1 calculated by HSE06+SOC, respectively. DOSs and band structures of As8 -O1 , Sb8 -O1 , As8 -S1 and Sb8 -S1 calculated by HSE06+SOC. The front(left) and top views(right) of optimized arsenene/antimonene heterostructure. The adsorption spectra for Sb24 -Cs2 CO3 , O4 -As32 and O4 -As32 /Cs2 CO3 -Sb24 heterostructure, respectively. The partial charge densities of VBM and CBM of arsenene/antimonene heterostructure. The detailed geometrical structures for POS1:O1 -As32 /Sb24 -Cs2 CO3 and POS2:As32 /Sb24 -Cs2 CO3 , respectively.

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