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Oxidation Mechanism and Protection Strategy of Ultrathin Indium Selenide: Insight from Theory Li Shi,† Qionghua Zhou,† Yinghe Zhao,† Yixin Ouyang,† Chongyi Ling,† Qiang Li,*,† and Jinlan Wang*,†,§ †

School of Physics, Southeast University, Nanjing 211189, China Synergetic Innovation Center for Quantum Effects and Applications (SICQEA), Hunan Normal University, Changsha 410081, China

§

S Supporting Information *

ABSTRACT: Ultrathin indium selenide (InSe), as a newly emerging twodimensional material with high carrier mobility and a broad absorption spectrum, has been the focus of current research. However, the long-term environmental instability of atomically thin InSe seriously limits its practical applications. To develop an effective strategy to protect InSe, it is crucial to reveal the degradation mechanism at the atomic level. By employing density functional theory and ab initio molecular dynamics simulations, we provide an in-depth understanding of the oxidation mechanism of InSe. The defect-free InSe presents excellent stability against oxidation. Nevertheless, the Se vacancies on the surface can react with water and oxygen in air directly and activate the neighboring In−Se bonds on the basal plane for further oxidation, leading to complete degradation of InSe into oxidation products of In2O3 and elemental Se. Furthermore, we propose an efficient strategy to repair the Se vacancies by thiol chemistry. In this way, the repaired surface can resist oxidation from oxygen and retain the original high electron mobility of pristine InSe simultaneously.

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reaction between SeVs and O 2 /H 2 O introduces local deformation in the vicinity of the vacancies sites, thus activating neighboring In−Se bonds for further and consecutive oxidation. Moreover, we develop a surface passivation strategy to repair the SeVs by thiol chemistry. The repaired InSe survives from ambient oxidation and holds high carrier mobility of pristine InSe as well. First, we investigate the interaction between the surface of perfect InSe and active ingredients in the air (O2/H2O) and find that O2/H2O are physisorbed on the surface of InSe, indicating that the InSe basal plane is quite inert and the pristine InSe can survive stably in ambient conditions (see Figure S1a,b in the Supporting Information). We further consider the light illumination effect on the surface of InSe, which is the inducing factor for the degradation of BP.15,16 Unlike BP, the conduction band minimum of the InSe monolayer is located below the redox potential of O2/O2−, indicating that the O2/O2− process can hardly occur for pristine InSe. In addition, even in the presence of O2−, the radical does not show affinity toward the perfect surface (see Figure S2), which further ascertains the excellent stability of perfect InSe. Unfortunately, imperfections in the single Se and In vacancies are inevitable during the exfoliation process and may significantly affect the stability of InSe. We therefore

ltrathin indium selenide (InSe), as one of the most promising two-dimensional (2D) semiconductors for electronic and optoelectronic device applications, has attracted tremendous interest recently.1−7 Strong quantum confinement effects, layer-dependent band gaps, high carrier mobility, and broad spectral response are the most distinguished virtues of InSe.1,8−11 However, it is also reported that ultrathin InSe has a strong limitation in that it is unstable to exposure to air.1,12,13 Unlike fast degradation of black phosphorus (BP),14−16 fewlayered InSe’s are demonstrated to be chemically stable under ambient conditions over a period of several days.1,12 Nevertheless, a long time of exposure of InSe up to 1 month showed a significant drop in the PL intensity,1 indicating a slow oxidation process of InSe in air. Moreover, thermal- and photoannealing can induce an oxidation of the InSe surface over a short period of time, converting a few surface layers of InSe into In2O3.17 The big question here, regarding both fundamental research interests and industrial application of InSe, is that the degradation mechanism in ambient condition is still far from clear. Herein, we provide a comprehensive understanding of the ambient oxidation of InSe at the atomic level based on density functional theory (DFT) calculations, which has proven to be one of the most accurate methods for computation of the electronic structure of solids18−20 and ab initio molecular dynamics (AIMD) simulations. Possible oxidation routes have been considered, and it is found that the Se vacancies (SeVs) inherent in InSe take major responsibility. Specifically, the © 2017 American Chemical Society

Received: August 5, 2017 Accepted: August 28, 2017 Published: August 28, 2017 4368

DOI: 10.1021/acs.jpclett.7b02059 J. Phys. Chem. Lett. 2017, 8, 4368−4373

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The Journal of Physical Chemistry Letters

surface triggered by chemisorbed H2O. Specifically, the two H atoms of adsorbed H2O migrate onto the Se atoms around the SeV and desorb from the surface in the form of H2Se. In this way, SeVs are first filled with O atoms from water and new SeVs are created simultaneously. However, the energy of the above-described configuration, that is, the new SeV covered with H2Se absorbed on the surface (see Figure 2b), is 1.88 eV higher in energy than the initial chemisorbed H2O configuration. Apparently, the process of forming H2Se gas is impractical from a thermodynamics point of view. Migration and Aggregation of SeVs. The SeV is a key factor for InSe degradation. The bigger local defects that may form by the migration or aggregation of dispersive surface SeVs possibly further result in the degradation of InSe. The important thing is whether the SeVs can easily migrate and aggregate or not. We calculate the migrated or aggregated energy barriers, and they are 1.48 eV for migration and 1.27 eV for aggregation, respectively (see Figure 2c,d). The much higher energy barriers, compared with the reactions between SeVs and O2/H2O, indicate that the oxidation through SeV migration or aggregation is slim. Activation of In−Se Bonds. A discusseds above, SeVs can be filled with O atoms (O*SeV) in both cases of the reactions with O2 and H2O. Nevertheless, because the nuclei radius of the O atom is much smaller than that of the Se atom, the replacement of O will introduce a local deformation around the vacancy site. The In−Se bonds surrounding SeV are elongated by about 2% (Figure 3a), and such deformation weakens the bond strength of In−Se, which serves as a prerequisite for further oxidation. In this respect, we take three possible attacking positions around the O*SeV site from another O2 into consideration, In-top, Setop, and hollow sites as displayed in Figure 3b−d, respectively. It is found that the In-top product is the energetically most favorable configuration (Figure S6 and Table S1). Consequently, the energy barriers required for H2O/O2-attacked SeV on the In-top product are 1.35/1.09 eV (Figure 3e,f), comparable to those of the oxidation of metal Fe (0.97 eV) and Sc in an O2 environment (1.43 eV).23,24 This reflects that integration of O2 into the in-plane of InSe is possibly achieved. Subsequently, the substitution of Se by O atoms is expected due to larger electronegativity of O compared with that of Se. Therefore, AIMD simulations were carried out to investigate this process at room temperature. We initiated the simulation from the In-top product in which the In−Se bonds are intercalated with O atoms (In−O−Se bonds) and the Se atoms are pushed out 0.4 Å from the surface. On a time scale up to 5 ps (Figure 4 and Movie S1), we notice that two Se atoms from the broken In−Se bonds gradually deviate from the surface and are dimerized in a short time of around 1.5 ps. In the end, the two Se atoms are replaced by oxygen, and a Se−Se dimer adsorbs on the surface. The replacement by oxygen continues and activates surrounding In−Se bonds for further oxidation and degradation, eventually leading to the oxidation products of In2O3 and elemental Se. Now we can have a complete picture of the degradation of InSe. The pristine InSe has good stability in air; the existence of SeVs is the essential reason for oxidation, which leads to high reaction activity toward O2/H2O. The degradation of InSe is accomplished through the following three-step procedure. First, H2O/O2 straightforwardly attacks SeVs, and the vacancy sites are filled by O atoms. Second, O atoms in the vacancy sites attract surrounding In atoms due to much smaller nuclei radius and higher electronegativity of O than Se; thus, the adjacent

consider the interaction of H2O/O2 with InSe with a single In or Se vacancy (see Figure S1c−f). It is found that H2O can penetrate into the SeV directly, forming In−O bonds, and then is dissociated into a hydroxyl and a hydrogen adsorbing onto the In atoms of SeV (the other dissociated product of H2O is presented in Figure S3). Afterward, two hydrogen atoms gradually approach each other, forming H2, and one oxygen atom enters into the vacancy (H2O-attacked SeV), as shown in Figure 1a. In the case of O2, a weak interaction is observed in

Figure 1. Reaction pathway of interaction of H2O/O2 with SeVs: (a) H2O attacked; (b) O2 attacked. The inset images are part of the configurations of different states. TS-1, TS-2, and MS stand for the transition states and intermediate state of two different processes, respectively. The inset pictures represent the top view of the atomic structure of transition states. In, Se, O, and H atoms are labeled as pink, light green, red, and white, respectively.

the beginning (Figure S1d), but the physisorption of O2 on the InSe surface can slide over an energy barrier of 0.07 eV and transform to chemisorption (Figure 1b). Thereafter, by overcoming a small energy barrier of 0.15 eV, the OO bond of the chemisorbed O2 breaks; one oxygen atom occupies the vacancy site, and the other O inserts into the In−In bond (the other dissociated products of O2 are shown in Figure S4) (O2-attacked SeV). Under these circumstances, the degradation of InSe in the ambient most likely originates from surface SeVs. Possible degradation routes are then considered, and three of them are discussed in the main text (others are given in Figure S5). Deselenization. Because hydrodesulfuration is a widely used technique to obtain defective MoS2 for the hydrogen evolution reaction,21,22 continuous formation of SeVs, nucleated from deselenization on the surface, may lead to the degradation of InSe. Here we consider the deselenization process on the InSe 4369


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Figure 2. (a) Schematic of the degradation mechanism for InSe. (b) Energy difference of the initial (H2O adsorbing on the SeV) and final state (H2Se adsorbing on the SeV). Reaction pathway of migration (c) and aggregation (d) of the SeV. The black line segments indicate the energy levels of the initial state, transition state, and final state, respectively, with each corresponding atomic structure (top views) next to the energy level. The blue dashed curve stands for the SeV.

Figure 3. (a) Structure of local deformation after an O atom fills the SeV. Different reaction products of the attacking In−Se bond: (b) In-top, (c) Se-top, and (d) hollow. Reaction pathway for a physisorbed O2 to dissociate and insert into the activated In−Se bond of (e) H2O- and (f) O2attacked SeV. (g) Rate constants of InSe containing an oxygen atom reacting with oxygen, in comparison with the oxidation of 3d transition metals.25

oxidation and then turns back into the formation of In−O−Se bonds. Therefore, the step-by-step model of breaking In−Se bonds by oxygen plays a dominant role in the whole oxidation

In−Se bonds are activated and result in the intercalation of O2 into In−Se bonds. Third, the replacement of Se atoms by oxygen activates adjacent In−Se bonds for a next cycle of 4370


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5a, the reaction process of a SeV and CH3SH contains two steps. In the first step, CH3SH chemically absorbs onto a SeV through the S atom, and then it cleaves the S−H bond and forms a thiolate intermediate, where the dissociated H atom is bonded to a neighboring Se atom. The second step involves cleavage of the S−C bond and hydrogenation of the thiolate intermediate to form the final product methane (CH4). The whole reaction needs to overcome two small energy barriers (0.35 and 0.06 eV), suggesting that the reaction can be easily realized under mild conditions. With a sulfur atom-filled SeV (S*SeV), the structure of InSe is well kept, and O2/H2O are physisorbed over the S-repaired surface (see Figure S7 and Table S2). Moreover, the band structure of InSe with a SeV shows that the presence of SeV introduces defect states inside of the band gap, which are detrimental to the transport properties of materials.31,32 After thiol treatment, the defect states disappear and the band structure is similar to that of the vacancy-free InSe. Therefore, the repaired InSe via thiol chemistry prevents the oxidation of InSe by limited local deformation around the vacancy sites and preserves the transport property of pristine InSe simultaneously. In summary, we have explored the ambient degradation mechanism of InSe by combining DFT calculations and AIMD simulations. Our study revealed that defect-free InSe presents excellent stability against oxidation from O2/H2O/O2−, and the reported degradation originates from SeVs, which shows high reaction activity toward O2/H2O in air. Subsequently, the occupation of SeVs with oxygen atoms induces local deformation around the vacancy sites and activates adjacent In−Se bonds for further oxidation. The Se atoms in InSe are continually replaced by oxygen atoms, and In2O3 is formed eventually, in line with thermal oxidation experiments. Moreover, the step-by-step intercalation of O2 into In−Se bonds needs to overcome a relatively high energy barrier, and this corresponds to a slow oxidation process observed from experiments. We further demonstrate that InSe can be protected by repairing SeVs by thiol chemistry. The repaired InSe shows high stability against O2/H2O and preserves the electronic properties of pristine InSe by eliminating the localized defect states. Our study provides a profound understanding of the degradation mechanism of InSe via inair oxidation and opens a possible route to stabilize InSe against

Figure 4. Snapshots of the formation of elemental Se from AIMD simulations. The broken In−Se bonds gradually deviate from the surface in about 1.5 ps, and Se atoms are replaced by oxygen atoms in about 2.5 ps.

process. However, relatively high energies are needed to conquer the barriers for dissociation of In−Se and OO bonds; therein, the reaction rates of the oxidation process are expected to be considerably low according to both the Arrhenius equation and Transition State Theory. To give a qualitative comparison, we choose several oxidation processes of 3d transition metals, Fe, Co, Ni and Cu.25 As shown in Figure 3g, the oxidation of In−Se bonds from O2*SeV and H2O*SeV at room temperature is comparable to the slow oxidation process of 3d transition metals. The reaction rates of the oxidation of In−Se bonds from O2*SeV and oxidation of the Fe process are quite close in the range of 300−1000 K, while oxidation of In−Se bonds from H2O*SeV is lower due to the larger activation energy of 1.35 eV. In fact, these conclusions are in line with the experimental observations that InSe flakes can be chemically stable up to a couple of days (or even up to 1 month) under ambient conditions.1,12 Our results suggest that the oxidation of InSe eventually results in In2O3, which is also supported by a recent thermal oxidation experiment, while the elemental Se is volatile for detection.26,27 From the above discussion, the SeVs are responsible for the degradation of InSe. Therefore, it is essential to passivate or repair the SeVs by means of chemical approaches, and we apply thiol chemistry here as inspired from previous studies on sulfur vacancy repairing in MoS2.28−30 Besides, the nuclei radius of sulfur is closer to that of Se with respect to O. We choose a simple case of thiol chemistry, that is, methanethiol (CH3SH), to investigate the feasibility of the method. As shown in Figure

Figure 5. (a) Kinetic and transient states of the reaction between a single SeV and CH3SH. (b) Electronic band structure of vacancy-free, SeV, and Srepaired InSe, respectively. The defect states are marked by blue color. A 3 × 2 supercell is used for all systems. 4371


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(BRA2016353), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1721) in China. The authors acknowledge computational resources from the SEU and National Supercomputing Center in Tianjin.

ambient. We expect that this detailed study also provides ideas for the universal degradation problem existing in 2D materials.

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COMPUTATIONAL METHODS All DFT calculations were performed by using the projectoraugmented wave (PAW)33 pseudopotential in conjunction with the Perdew−Burke−Ernzerhof generalized gradient approximation (PBE-GGA)34,35 and Heyd−Scuseria−Ernzerhof (HSE)36 as implemented in the Vienna ab initio simulation package.37,38 The effect of van der Waals (vdW) interactions was considered by using the DFT-D2 scheme.39 An ionic PAW pseudopotential approach40,41 was implemented to simulate the molecular anion. The ionic pseudopotential was generated by exciting inner core electrons to the valence shell. To obtain an ionic pseudopotential for a superoxide anion, we used the configuration 1s1.52s2 2p4.5 in which “half” electron from 1s shell is put into the valence shell 2p for both oxygen atoms. The climbing-image nudged elastic band (CI-NEB) method42 incorporated with spin-polarized DFT was used to locate the minimum-energy path. The intermediate images of each CINEB simulation were relaxed until the perpendicular forces were smaller than 0.05 eV/Å. The mixed Gaussian and planewave basis set code CP2K/QUICKSTEP43 were used for the AIMD simulations. The energy cutoff for the plane-wave basis set was set to be 400 eV. The convergence threshold was 10−4 eV for energy and 0.02 eV/Å for force. A vacuum at least 20 Å in the z-direction was introduced to avoid the interaction between two periodic units. The Brillouin zone was sampled by using a Monkhorst−Pack k-mesh of a 3 × 3 × 1 grid for a supercell

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(1) Bandurin, D. A.; Tyurnina, A. V.; Yu, G. L.; Mishchenko, A.; Zólyomi, V.; Morozov, S. V.; Kumar, R. K.; Gorbachev, R. V.; Kudrynskyi, Z. R.; Pezzini, S.; et al. High Electron Mobility, Quantum Hall Effect and Anomalous Optical Response in Atomically Thin InSe. Nat. Nanotechnol. 2016, 12, 3760−3766. (2) Lei, S.; Ge, L.; Najmaei, S.; George, A.; Kappera, R.; Lou, J.; Chhowalla, M.; Yamaguchi, H.; Gupta, G.; Vajtai, R.; et al. Evolution of the Electronic Band Structure and Efficient Photo-Detection in Atomic Layers of InSe. ACS Nano 2014, 8, 1263−1272. (3) Feng, W.; Zheng, W.; Cao, W.; Hu, P. Back Gated Multilayer InSe Transistors with Enhanced Carrier Mobilities via the Suppression of Carrier Scattering from a Dielectric Interface. Adv. Mater. 2014, 26, 6587−6593. (4) Lei, S.; Wen, F.; Li, B.; Wang, Q.; Huang, Y.; Gong, Y.; He, Y.; Dong, P.; Bellah, J.; George, A.; et al. Optoelectronic Memory Using Two-Dimensional Materials. Nano Lett. 2015, 15, 259−265. (5) Debbichi, L.; Eriksson, O.; Lebègue, S. Two-Dimensional Indium Selenides Compounds: An Ab Initio Study. J. Phys. Chem. Lett. 2015, 6, 3098−3103. (6) Xiao, K. J.; Carvalho, A.; Castro Neto, A. H. Defects and Oxidation Resilience in InSe. Phys. Rev. B: Condens. Matter Mater. Phys. 2017, 96, 054112. (7) Zólyomi, V.; Drummond, N. D.; Fal’ko, V. I. Electrons and Phonons in Single Layers of Hexagonal Indium Chalcogenides from Ab Initio Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 205416. (8) Mudd, G. W.; Svatek, S. A.; Hague, L.; Makarovsky, O.; Kudrynskyi, Z. R.; Mellor, C. J.; Beton, P. H.; Eaves, L.; Novoselov, K. S.; Kovalyuk, Z. D.; et al. High Broad-Band Photoresponsivity of Mechanically Formed InSe-Graphene Van der Waals Heterostructures. Adv. Mater. 2015, 27, 3760−3766. (9) Lei, S.; Wang, X.; Li, B.; Kang, J.; He, Y.; George, A.; Ge, L.; Gong, Y.; Dong, P.; Jin, Z.; et al. Surface Functionalization of TwoDimensional Metal Chalcogenides by Lewis Acid−base Chemistry. Nat. Nanotechnol. 2016, 11, 465−471. (10) Brotons-Gisbert, M.; Andres-Penares, D.; Suh, J.; Hidalgo, F.; Abargues, R.; Rodríguez-Cantó, P. J.; Segura, A.; Cros, A.; Tobias, G.; Canadell, E.; et al. Nanotexturing To Enhance Photoluminescent Response of Atomically Thin Indium Selenide with Highly Tunable Band Gap. Nano Lett. 2016, 16, 3221−3229. (11) Sucharitakul, S.; Goble, N. J.; Kumar, U. R.; Sankar, R.; Bogorad, Z. A.; Chou, F.-C.; Chen, Y.-T.; Gao, X. P. A. Intrinsic Electron Mobility Exceeding 103 cm2 /(V S) in Multilayer InSe FETs. Nano Lett. 2015, 15, 3815−3819. (12) Chen, Z.; Biscaras, J.; Shukla, A. A High Performance Graphene/Few-Layer InSe Photo-Detector. Nanoscale 2015, 7, 5981−5986. (13) Politano, A.; Chiarello, G.; Samnakay, R.; Liu, G.; Gürbulak, B.; Duman, S.; Balandin, A. A.; Boukhvalov, D. W. The Influence of Chemical Reactivity of Surface Defects on Ambient-Stable InSe-based Nanodevices. Nanoscale 2016, 8, 8474−8479. (14) Zhou, Q.; Chen, Q.; Tong, Y.; Wang, J. Light-Induced Ambient Degradation of Few-Layer Black Phosphorus: Mechanism and Protection. Angew. Chem. 2016, 128, 11609−11613. (15) Favron, A.; Gaufrès, E.; Fossard, F.; Phaneuf-L’Heureux, A.-L.; Tang, N. Y.-W.; Lévesque, P. L.; Loiseau, A.; Leonelli, R.; Francoeur, S.; Martel, R. Photooxidation and Quantum Confinement Effects in Exfoliated Black Phosphorus. Nat. Mater. 2015, 14, 826−832. (16) Walia, S.; Balendhran, S.; Ahmed, T.; Singh, M.; El-Badawi, C.; Brennan, M. D.; Weerathunge, P.; Karim, M. N.; Rahman, F.; Rassell, A.; Duckworth, J.; Ramanathan, R.; Collis, G. E.; Lobo, C. J.; Toth, M.; Kotsakidis, J. C.; Weber, B.; Fuhrer, M.; Dominguez-Vera, J. M.;

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b02059. Adsorbed structures of H2O and O2 on defect-free basal, HSE band for monolayer InSe/BP, different products of H2O/O2 splitting, other reaction pathways of H2O attacking SeV, different reaction products of attacking the In−Se bond, structure of H2O/O2 adsorbed on the S-repaired surface, and methods of calculated reaction rates (PDF) Dynamic behaviors of the formation of elemental Se (AVI) Structure of InSe (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.W.). *E-mail: [email protected] (Q.L.). ORCID

Yinghe Zhao: 0000-0002-2331-8973 Jinlan Wang: 0000-0002-4529-874X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Ministry of Science and Technology (2017YFA0204803), the National Natural Science Funds for Distinguished Young Scholars (21525311), NSFC (21373045, 21773027, 21703032), Jiangsu 333 project 4372


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