Extremely High Mobilities in Two-Dimensional Group-VA Binary

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Extremely High Mobilities in Two-Dimensional Group-VA Binary Compounds with Large Conversion Efficiency of Solar Cell Wangping Xu, Yuanjun Jin, Baobing Zheng, and Hu Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10652 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 9, 2018

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Extremely High Mobilities in Two-Dimensional Group-VA Binary Compounds with Large Conversion Efficiency of Solar Cell Wangping Xu1, YuanJun Jin1, Baobing Zheng1,2,3,*, Hu Xu1,* 1

Department of Physics, Southern University of Science and Technology, Shenzhen 518055, China 2 College of Physics and Optoelectronic Technology, Nonlinear Research Institute, Baoji University of Arts and Sciences, Baoji, 721016, China. 3 School of Physics and Technology, Wuhan University, Wuhan, 430072, China.

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

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Abstract: The quest for high mobility nanomaterials with direct band gap is motivated by not only the scientific interest but also the practical need for fabricating the nanoelectronic and optoelectronic devices. Using high-throughput first-principles calculations, we have determined the ground state structures of two-dimensional group-VA binary compounds, and explored two stable  phases of BiAs with unexpected direct band gaps. The proposed BiAs monolayer has extremely high mobility of 8.38×104 cm2V1s1 close to that of graphene, and furthermore they are promising donor materials for solar cell. Significantly, our results definitely show that spin-orbit coupling plays a crucial role in exploring the electronic properties of the group-VA binary compounds, as well as studying other compounds with heavy atoms. Our findings pave the way for realizing the nanoelectronic and optoelectronic applications of two-dimensional group-VA binary compounds.

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Introduction

Two-dimensional

(2D)

materials,

such

as

graphene

and

transition-metal

dichalcogenides (TMDs), have attracted considerable attention due to their potential applications in nanoelectronic devices,1-8 especially for high-performance field-effect transitors (FETs). A high-performance FET requires not only the high carrier mobility but also the moderate electronic band gap to support the excellent current on-off ratio. Despite possessing extremely high carrier mobility (~2×105 cm2V1s1),9 the gapless nature of graphene makes graphene-based FETs large off current and thus significantly lowers their current on-off ratio. On the contrary, TMDs, such as MoS2, present very high current on-off ratio due to their appreciable direct electronic band gap,10 but the carrier mobilities of TMDs are two orders of magnitude lower than that of graphene.4, 11 Recently, black phosphorene (-BP), a new member of 2D layered material mechanically exfoliated from black phosphorus ( phase), has emerged as a promising material in fabricating few-layer FETs which show large on-off current ratio (up to ~105 ) and relatively high field-effect mobility compared to 2D MoS2 (up to ~1000 cm2V1s1) at room temperature.11-18 Following the success of -BP, tremendous interests have been stimulated in finding group-VA 2D materials that combine the properties of high carrier mobility and proper electronic band gap.19-25 The theoretically predicted allotrope of -BP, called blue phosphorene (marked as -BP),26 has high carrier mobility, which is also favorable in fabricating few-layer FETs. In the subsequent years, arsenene,20,21 antimonene,22, 23 and bismuthene24-25 were gradually explored. Unlike phosphorene, it is found that  phases of arsenene, antimonene, and bismuthene are the energetically most stable structures, and all reported  phases of group-VA 2D materials host indirect band gaps, which severely hinder their applications in nanoelectronics and opoeletronics. A new route to design the group-VA 2D materials with intriguing properties is to focus on the group-VA binary compounds. Theoretically, a class of 2D group-VA binary semiconductors with -BP or -BP structures were proposed.27,28 It is found that the -phases of group-VA binary compounds have the direct band gap, while the indirect gap for phases. Aside from AsP, all of the group-VA binary compounds with phase is energetically favorable than those of phaseIn addition, Zhang et al.29 3

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investigated five typical honeycomb configurations of monolayer SbAs polymorphs. Meanwhile, based on an evolutionary algorithm, Nahas et al.30 predicted a series of AsP phases. However, the ground state structures of 2D group-VA binary compounds are still open questions up to now. Moreover, the 2D group-VA binary compounds with outstanding properties that combine the high mobility and the direct band gap are still lacking. Therefore, it is desirable and important to investigate 2D group-VA materials that not only have high carrier mobilities comparable to graphene but also are semiconductors, preferably possessing the direct gap. In this work, we report the ground states of group-VA binary monolayers (denoted as phase) which are confirmed by first-principles calculations. Importantly, the two proposed BiAs monolayers with the direct band gap have ultrahigh carrier mobilities comparable to graphene. In addition, we discussed the influence of spin-orbit coupling (SOC) on the group-VA 2D materials in detail. The possibilities of our predicted BiAs monolayers used for donor material of solar cell are investigated systematically. Methods

All the first-principles calculations were performed in the framework of density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package. 31,32

The projector augmented wave method was used to describe ion-electron

interactions.33 The energy cutoff for the plane-wave basis set was 450 eV, and the Monkhorst-Pack k point meshes with a grid of 0.03 Å1 were adopted. All structures were fully relaxed until the forces acting on each atom are less than 0.01 eV/Å. The Perdew-Burke-Ernzerhof

34

functional

approximation (denoted by G0W0)

35,36

combined

with

the

one-shot

GW

was used when the excitonic effects were

considered by using Bethe-Salpeter equation (BSE),

37

in which the included

conduction bands are ten times of the valence bands, and the energy cutoff for the response functions parameter was set to 200 eV and 16×16×1 k-point meshes were used. To obtain the quasi-particle (QP) band structures, the wavefunction was expanded using maximally localized Wannier functions basis.38 The QP band structures and optical absorption spectra were obtained using the vacuum spacing of 4

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22 Å between adjacent layers. Semi-empirical DFT-D3 correction was employed to evaluate the van der Waals (vdW) interactions.39 Results and discussions

To determine ground states of group-VA binary monolayers, we firstly fixed 2D lattice and enumerated all the possibility of structures. The extended distance matrix approach40 was employed to distinguish the atomic structures and improve the efficiency of calculation. After fully optimizing the obtained crystal structures and comparing their total energies, the ground state structures of group-VA binary compounds are determined. We find that the most energetically favorable AsP, SbP, and BiP monolayers crystalize in buckled orthogonal lattice structures (marked as phase), which are similar with phases of group-VA monolayers,27 but hosting different atomic arrangement. As shown in Fig. 1(a), each unit cell of phase contains eight atoms, and the same element tends to form armchair atomic chains. The calculated total energies of AsP, SbP, and BiP with phases are totally lower than those ofphase and phase (see Table S1 in Supporting Information), suggesting that phases are the ground state structures for AsP, SbP, and BiP monolayers. Based on the above discussions, we hence propose -phase of BiAs monolayer whose total energy slightly differs from that of α-phase BiAs by 5 meV/atom. It is noteworthy that we also find a new orthogonal phase of BiAs monolayer (marked as -BiAs, shown in Fig. 1(b)) whose total energy is almost degenerate with that of -BiAs. Unlike phase, the same element prefers to connect by the zigzag atomic chains for -BiAs. Compared with the original phase, the same element of our predicted andphases tends to form chain-like building-block rather than alternatively atomic arrangement. To realize their experimental syntheses, we establish the effective substrates for these group-VA binary monolayers, and the detailed results are provided in Supporting Information. Note that the calculated adsorption energies per BiAs pair of -BiAs and-BiAs are 188 and 186 meV lower than that of phase of BiAs monolayer on an Ag(111) substrate, respectively. Therefore, we can conclude that our predicted -BiAs and-BiAs monolayers are more likely to be prepared via epitaxial growth on the Ag(111) substrate. 5

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Figure 1. Top (upper) and side (bottom) views of (a) phase and (b) phase of group-VA binary monolayers. Violet and green spheres represent different group-VA elements.

To verify dynamical stabilities of these predicted -AsP, -SbP, -BiP, -BiAs, and -BiAs monolayers, we employed the frozen phonon method to calculate their phonon spectra. The corresponding phonon dispersion curves of -BiAs and -BiAs monolayers are shown in Fig. 2, and the others are summarized in Fig. S2. Basically, no imaginary frequencies are found in the whole Brillouin zone, which indicate that all the predicted group-VA binary monolayers are dynamically stable. As is well-known, the freestanding monolayer usually presents a weakly unstable phonon near the  point in the flexural acoustic mode,28, 41 which may be suppressed with the existence of substrate or defects. Here, the similar characters, which however do not present the instability, are also found near the  point in the phonon

spectra

of

these

group-VA

binary

monolayers.

The

ab

initio molecular dynamics (AIMD) simulations can provide the fundamental evaluation in the thermodynamical stability of given materials, especially for 2D materials. Therefore, we performed AIMD simulations at the temperature of 300 K to further confirm the stabilities of these predicted group-VA binary monolayers. The structural snapshots of the p(5×5) supercell at 5 ps, shown in Fig. 2 and Fig. S2, reveal that all of these monolayer structures have high thermodynamic stabilities, implying the possibility of successfully fabricating these 2D freestanding crystals at 6

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room temperature.

Figure 2. Phonon Spectra of (a) -BiAs and (b) -BiAs monolayers. The total energies of (c) -BiAs and (d) -BiAs monolayers as a function of time at 300K, and the corresponding insets are the snapshots of p(5×5) supercell of the -BiAs and -BiAs monolayers at 5 ps, respectively.

Because standard DFT calculations are known to underestimate the electronic band gap of the material due to the electron self-interaction error.42 Therefore, we employed the GW method to calculate the electronic band structures of the group-VA binary monolayers. The calculated QP band structures of -AsP, -SbP and -BiP monolayers show that they are all indirect band gap semiconductor. It is well-known that the original  phases of group-VA monolayers are indirect band gap semiconductors, but our predicted -BiAs and -BiAs monolayers similar to the  phase have the direct band gap, suggesting that the  phase is changed into direct band gap semiconductor by the rearrangement of the atoms. Thus, we mainly focus on the electronic properties of -BiAs and -BiAs monolayers in the following discussions, and the electronic propertiesof the other monolayers are also summarized in the Supporting Information. Contrary to -AsP, -SbP and -BiP, we find that the

valence band maximums (VBMs) and conduction band minimums

(CBMs) of -BiAs and -BiAs are both located at the Γ point with the QP band gaps 7

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of 1.64 eV and 1.43 eV without considering SOC (see Fig. 3), respectively. The projected density of states (PDOS) suggest the CBMs of -BiAs and -BiAs are mainly contributed by pz orbital of Bi atoms. The VBM of -BiAs consists of py orbitals of As and Bi atoms, and the main contribution of the VBM for -BiAs originates from the px and py orbitals of Bi atoms. Physically, the heavy atoms generally have strong SOC effects, which may significantly influence the electronic structure of group-VA binary monolayers. Therefore, we also discuss the SOC effects in details for -BiAs and -BiAs. As shown in Fig. 3, the electronic band structures of -BiAs and -BiAs are distinctly changed in the presence of SOC, and the corresponding QP band gaps of -BiAs and -BiAs are drastically reduced to 0.94 eV and 1.02 eV, respectively, suggesting the considerable SOC effects for -BiAs and -BiAs due to the heavy mass of Bi and As atoms.

Figure 3. The G0W0 band structures and PDOSs with SOC of (a) -BiAs and (b) -BiAs, and the charge distributions of the VBMs and CBMs are plotted as insets in (a) and (b) with the isosurface values of 0.03 and 0.06 e/Å3, respectively. The corresponding BSE optical spectra of (c) -BiAs and (d) -BiAs.

2D direct band gap semiconductors have great potentials in photoelectric devices, in which exciton plays an important role in understanding their optical properties, such as light absorption, interband photoluminescence. Therefore, we calculated the exciton binding energies (EBEs, defined by the difference between 8

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the QP band gap and the optical gap) of our predicted group-VA binary monolayers. Because of the significant SOC effect discussed above, the utilized values of the QP band gaps and the optical gaps to calculate the EBEs are taken from the results including SOC. As shown in the optical spectra of Fig. 3, the optical gaps are 0.69 eV for -BiAs and 0.74 eV for -BiAs. We thus obtain that the EBEs of -BiAs and -BiAs are 0.25 eV and 0.28 eV, respectively. Clearly, our calculated results show an excellent consistency with the recently reported linear scaling law between the QP band gap (Eg) and the EBE (Eb), namely Eb =Eg/4.43,44 Likewise, the EBEs of -AsP, -SbP and -BiP also satisfy this linear scaling law, which can be derived from the Supporting Information. This fact further demonstrates that it is essential to consider the SOC effect for the compound with heavy atoms. Based on the band structures with SOC, we studied the carrier mobilities of these group-VA binary monolayers. From the viewpoint of band structure, the sharply dispersive VBM and CBM generally associate with small effective mass of carriers, and thus result in large carrier mobility. Therefore, we mainly investigate the -BiAs and -BiAs due to their strong dispersions near VBM and CBM. Theoretically, it is known that the carrier mobility can be calculated according to deformation potential method.11 Thus, we employed this approach to determine the carrier mobility, and the detailed calculation procedure can be found in the Supporting Information. The calculated results of -BiAs and -BiAs at room temperature (300 K) are summarized in Table 1, together with those of -AsP, -SbP and -BiP monolayers for comparison. We find that the fitted carrier effective masses along x and y directions are only a few percent of the electron mass (m0), which are at least one order of magnitude smaller than those of -BP and -BP,11, 45 implying that the ultrahigh carrier mobilities of -BiAs and -BiAs. Our predicted carrier mobilities for -BiAs and -BiAs are extremely large, which are greater than 104 cm2V1s1 along both x and y directions. Compared to the typical -BP and -BP (several thousand cm2V1s1), the carrier mobilities of -BiAs and -BiAs are ten times as large. Moreover, the carrier mobilities of -BiAs and -BiAs are also much higher than the highest reported mobilities of group-VA binary monolayers (~104 cm2V1s1).46,47 More importantly, 9

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-BiAs presents dramatically large mobility of 8.38×104 cm2V1s1 along the x direction, which is not only significantly higher than those of other group-VA monolayers, but also is close to that of graphene. Combined with their direct band gap behavior, we believe that -BiAs and -BiAs are promising candidates for fabricating high-performance FETs considering their ultrahigh carrier mobilities. Table 1. The carrier mobilities (µ) at 300 K for electrons and holes in -(AsP, SbP and BiP) and ( and )-BiAs (x=zigzag, y=armchair) with and without SOC. Carrier typea e

Structure

SOC

b

effective masses *

mx /m0

*

my /m0

deformation potential (eV) E1x E1y

W 0.05 0.04 5.53 WO 0.08 0.06 4.62 W 0.05 0.05 4.72 β-BiAs WO 0.07 0.07 4.96 h W 0.07 0.04 2.45 β-BiAs WO 0.44 0.06 1.31 W 0.06 0.05 1.88 β-BiAs WO 0.13 0.10 1.67 W 0.26 0.14 4.36 β-AsP WO 0.46 0.11 2.70 W 0.29 0.11 1.91 β-SbP WO 0.40 0.10 1.65 W 0.16 0.07 2.26 β-BiP WO 0.39 0.08 2.09 a b e, electron; h, hole. W, with SOC; WO, without SOC. β-BiAs

5.27 5.21 4.96 5.20 3.59 5.26 3.83 4.91 3.94 3.80 2.46 4.18 4.45 5.13

2D elastic

mobilities 2

modulus (Jm ) Cx_2D Cy_2D 40.96 44.81 45.69 46.37 40.96 44.81 45.69 46.37 69.84 70.41 59.14 59.39 51.57 54.58

38.71 43.40 37.93 44.04 38.71 43.40 37.93 44.04 68.89 69.23 54.83 55.57 44.91 49.56

(104 cm2V1s1) μx_2D μy_2D 1.28 0.81 1.75 0.82 3.92 0.78 8.38 2.39 0.16 0.20 0.67 0.58 1.27 0.39

1.66 0.82 1.31 0.71 3.02 0.34 2.01 0.34 0.35 0.41 0.98 0.34 0.65 0.28

From the above discussions, it needs to point out that SOC dramatically changes the sizes of band gaps and line shapes of band structures, particularly for those BiAs monolayers, and thus essentially influences the electronic properties of the group-VA binary compounds. As shown in Fig. 3(a), we can clearly find that the VBMs and CBMs disperse sharply in the presence of SOC, consequently leading to the extremely large carrier mobilities of -BiAs and -BiAs. From the Table 1, the effective masses of our predicted group-VA binary monolayers decrease remarkably when SOC is included. For example, the hole effective masses of β-BiAs decrease by more than 53% along the x direction and 50% along the y direction, leading to the largest hole 10

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mobility of β-BiAs. Compared the mobilities with and without SOC, we find that hole mobilities of β-BiAs with SOC become 3 times and 5 times larger than those without SOC along the x and y directions, respectively. Moreover, we also find that our calculated EBEs meet the linear scale law of Eb =Eg/4 once SOC is considered. These facts suggest that SOC plays a crucial role in studying the electronic properties of compounds, especially for studying the carrier mobilites of compounds including heavy atoms. Therefore, we believe that SOC should be considered thoroughly to ensure the accuracy of the calculation results, although this point has been totally ignored in the previous studies.

Figure 4. (a) Schematic of donor-acceptor band alignments between -BiAs and MoS2 monolayers, and between -BiAs and WS2 monolayers. (b) Power conversion efficiency as a function of the donor band gap and conduction band offset, and the black and white dots denote the PECs of -BiAs/ MoS2 and -BiAs/ WS2 systems, respectively.

The direct band gap behaviors of -BiAs and -BiAs, with the values of 0.94 eV and 1.02 eV, suggest that the -BiAs and -BiAs monolayers are highly favorable to be applied as solar cell donor materials. To realize the possible applications of -BiAs and -BiAs for solar cell, we should first find out the suitable acceptor material whose CBM and VBM are lower than those of donor materials for facilitating the conduction of electron, and consequently improve the cell efficiency. Our G0W0 calculations with band-gap-center approximation48 indicate that the vacuum level corrected CBM and VBM of -BiAs are 3.57 eV and 4.52 eV, 11

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while 3.44 eV and 4.46 eV for -BiAs, respectively. After careful examination of various 2D materials, we have found the optimal acceptor materials are monolayer MoS2 for -BiAs with CBM of3.72 eV and VBM of 6.45 eV, and monolayer WS2 for -BiAs with CBM of3.51 eV and VBM of 6.38 eV, which match well with those of -BiAs and -BiAs monolayers. Clearly, both -BiAs and -BiAs have type-II heterojunction alignments, as shown in Fig. 4(a). We obtain the power conversion efficiencies (PCEs) of the -BiAs/MoS2 and -BiAs/WS2, as shown in Fig. 4(b), according to the scheme49-50 described in the Supporting Information. We find the upper limit PCE for -BiAs/MoS2 heterojunctions is 16.7%, comparable of the phosphorene/MoS2 trilayer system.51 More importantly, the -BiAs/WS2 system has maximum PCE of 19.6%, which is close to the highly efficient edge-modified phosphorene nanoflake heterojunctions solar cells.52 Thus, we can conclude that our proposed -BiAs/ MoS2 and -BiAs/ WS2 systems are competitive for the solar cell.

Conclusions

In summary, we determined the ground state of 2D group-VA binary monolayers, and proposed two stable BiAs monolayers, which are verified by the phonon calculations and AIMD simulations. The successful establishment of substrates imply that these group-VA binary monolayers can be prepared on suitable metal substrates. We find that -BiAs and -BiAs monolayers with the direct band gaps have extremely high hole mobilities that are comparable to graphene, suggesting that the -BiAs and -BiAs are promising for fabricating nanoelectronic devices. The SOC effect considerably changes electronic properties of -BiAs and -BiAs, which significantly influences the effective masses of carriers, resulting in the improved carrier mobility. Thus, we can conclude that the SOC should be included as we study the carrier mobilities of compounds with heavy atoms. In addition, we find that -BiAs and -BiAs present moderately high maximum PCEs when they are applied as solar cell donor materials.

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Supporting Information The monolayers structures adsorbed on metal substrates, phonon spectra and AIMD results of -AsP, -SbP, and -BiP, QP band structures and BSE optical spectra of -AsP, -SbP, and -BiP, total energies of different AsP, SbP, and BiP monolayers, the discussion on the effective substrates for these group-VA binary monolayers, and the procedures of carrier mobility and PCE calculation.

Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC, Grant Nos. 11674148, 11334004 and 11404159), the Guangdong Natural Science Funds for Distinguished Young Scholars (No. 2017B030306008), the Basic Research Program of Science, Technology, and Innovation Commission of Shenzhen Municipality (Grant Nos. JCYJ20160531190054083, JCYJ20170412154426330), the Natural Science Basic Research plan in Shaanxi Province of China (Grant No. 2018JQ1083), and the Scientific Research Program Funded by Shaanxi Provincial Education Department (Grant No. 17JK0041).

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