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Tuning electronic and magnetic properties of armchair InSe nanoribbons by hydrogenation
Superlattices and Microstructures 135 (2019) 106282
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Tuning electronic and magnetic properties of armchair InSe nanoribbons by hydrogenation Xu Zhao a, *, Binru Zhao a, Hui Zhang a, Tianxing Wang a, Congxin Xia a, Xianqi Dai a, Shuyi Wei a, Lin Yang b a b
School of Physics, Henan Normal University, Xinxiang, Henan, 453007, China School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan, 453007, China
A R T I C L E I N F O
A B S T R A C T
Keywords: InSe Edge hydrogenation Spin-polarization First-principles calculation
Using the first-principles computations based on the density functional theory, we investigated structural, electronic and magnetic properties of armchair InSe nanoribbons (AInSeNRs) via edge hydrogenation. The calculated formation energy showed that H atoms preferred to absorb to edge In atoms. Partial hydrogenation could induce to edge magnetism of AInSeNR and the total magnetic moment of AInSeNRs was regularly 1 μB or 2 μB coming from the p orbitals of edge Se or In atoms. Moreover, the majority of magnetic systems performed 100% spin-polarization, which was similar with graphene nanoribbon. Under the strain from 3% to 3%, AInSeNRs remained magnetic properties and the total magnetic moment was still 1 μB or 2 μB. Nonmagnetic systems of In50%-Se0%-2 and In100%-Se100% showed wider direct band gap under edge hydrogenation, and the maximum band gap came to 1.732 eV under full hydrogenation. Our work provided an effective way to improve electronic, optical and magnetic properties of AInSeNR, which has promising applications in spintronics and novel optional devices via edge hydrogenation.
1. Introduction Since the isolated single graphene have been realized experimentally [1,2], research interest has shifted to its unique electronic properties and potential for device applications, such as high carrier mobility at room temperature, high electrical conductivity and excellent mechanical strength [3–6]. Unfortunately, graphene has an obvious shortage that it couldn’t keep a wide band gap and preserve its extreme carrier mobility at the same time. Such a dilemma has stimulated the search for other 2D materials, such as MoS2 [7], oxides [8] and III-VI compounds (e.g., BN, GaS and GaSe) [9,10]. MoS2 monolayer is a typical transition metal sulfide. Differently from graphene, MoS2 is characterized by its band gap of around 1.2 eV [11], which can be further increased by changing the thickness [12], or applying the mechanical strain [13,14]. Gas sensors based on few-layered MoS2 field effect transistors have been realized due to the natural abundance and distinctive optical, electrical, and mechanical properties of MoS2. Few-layered GaS, GaSe, and InSe have been proven to be promising candidates in field such as optoelectronics, field-effect transistors (FETs), and solar energy conversion [15–18]. The 2D InSe nanosheet exhibits the characteristic of direct band gap semiconductor. And it has smaller electron effective mass (m* ¼ 0.143 m0) and higher electron effective mobility (~103 cm2V–1S 1 at room temperature) [19], which demonstrates InSe nanosheets have the promising application in high-mobility electronic devices. However, graphene and MoS2 armchair nanoribbons
* Corresponding author. E-mail address: [email protected] (X. Zhao). https://doi.org/10.1016/j.spmi.2019.106282 Received 21 May 2019; Received in revised form 29 August 2019; Accepted 26 September 2019 Available online 30 September 2019 0749-6036/© 2019 Elsevier Ltd. All rights reserved.
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are not best candidates for the related FET devices with high-transport and fast-response characteristics [20] due to the low carrier mobility ~102 cm2V 1s 1. Furthermore, layered InSe has a crossover transition from direct to indirect band gap when the layer thickness is reduced to �6 nm, and its narrower band gap with smaller excition reduced mass, leads to stronger quantum confinement, which is well above that of many other 2D semiconductors [21]. Numerical simulations of structural, electric, and magnetic properties of InSe monolayers and their modulation by doping, defect, and, adsorption have been carried out [22–30]. Besides, theoretical calculations have predicted the existence of piezoeletricity in InSe with piezoelectric coefficient on the same order of magnitude as the earlier discovered two-dimensional piezoelectric materials [31]. The 1D nanostructures, such as nanoribbons, nanowires, nanorobs and nanotubes, have been extensively studied both experi mentally and found to possess some unusual properties compared to their 2D counterparts due to the low dimensionality, edge effect and the enhanced quantum confinement effect [32]. ML-MoS2 could be tailored into quasi-1D nanoribbons by an electro chemical/chemical route [33,34]. As a kind of classical nanoribbons, many researches demonstrate MoS2 nanoribbons could be induced remarkable optical, mechanical, electronic and magnetic properties due to edge modification [35–40]. Ouyang et al. found that armchair MoS2 nanoribbons (AMoS2NRs) possessed localized ferromagnetic semiconductors edge states due to edge hydroge nation in certain configurations and become antiferromagnetic semiconductors or ferromagnetic semiconductors. Additionally, partial edge hydrogenation as well as external electric field could tune AMoS2NRs in small dimension from semiconductors to metals or half metals [41,42]. Accordingly, edge passivation is an effective way to influence the electronic and magnetic properties of nanoribbons. Inspired by prediction of Pan et al. [43] that different edge structures might have different impacts on one-dimension materials. In addition, to our best knowledge, there are few studies on electronic and magnetic properties of one-dimension nanoribbons of InSe monolayer up to now. Hence, in this paper, we performed first-principles calculations to investigate systematically the electronic and magnetic properties of hydrogenated InSe armchair nanoribbons. This paper was organized as following, we gave the introduction in Section 1. Our calculation method was described in detail in Section 2. In Section 3, we presented the structure model, then gave the results and discussions for the electronic and magnetic properties of the AInSeNR. Our work provided an effective way to improve the electronic, optical and magnetic properties of AInSeNRs via edge hydrogenation, which has promising applications in electronic devices [44,45], spintronics and novel optional devices via edge hydrogenation.
Fig. 1. (a–p) Top and side views of the sixteen different hydrogenated configurations In25%-Se0%, In50%-Se0%-1, In50%-Se0%-2, In50%-Se0%-3, In75%-Se0%, In100%-Se0%, In0%-Se25%, In0%-Se50%-1, In0%-Se50%-2, In0%-Se50%-3, In0%-Se75%, In0%-Se100%, In100%-Se50%-1, In100%Se50%-2, In100%-Se50%-3, In100%-Se100%, respectively. Green, purple and brown balls refer to Se, In and H atoms, respectively. (For inter pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 2
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2. Model and method In this work, the first-principles calculation was performed within the framework of density functional theory as implemented in Vienna ab-initio simulation package (VASP) [46,47]. The electron exchange-correlation functional was treated within the generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) parameterization [48,49]. The projector-augmented plane wave (PAW) [50] method was employed to model the ion-electron interactions. A plane wave basis set with a cut-off value of 500 eV was used. For the geometry optimization, nine Monkhost-Pack special k points were used for integration over the first Brillouin zone. All atoms were fully optimized and the energy and force convergence standard were set to 10 5 eV per atom and 0.01 eV/Å for each atom, respectively. The energy cutoff and k-point settings were proved to be sufficient for achieving converged results. To aviod the interactions between two periodical slabs in unit cell, armchair InSe nanoribbons were separated from each other by a vacuum space of 20 Å in the nonperiodic directions, which were perpendicular to and along the plane of the nanoribbon, respectively. The valence electrons for the In, Se and H are respectively 13 (In: 4d105s25p1), 6 (Se: 4s24p4) and 1 (H: 1s1). Similar to the structure of GaSe, GaS, InS and InTe, InSe has the honeycomb structure where In and Se atoms are uniformly and periodically arranged along the edge. The model of InSeNR can be constructed by cutting two-dimensional InSe monolayer along armchair or zigzag ori entations, but in this work, only armchair InSe nanoribbons have been considered because the bare zigzag InSe nanoribbon was metallic. Besides [51], showed us that armchair InSe nanoribbon had three kinds of edge reconstruction but we only considered the ribbon width of 11 dimer chainwith ac-p reconstruction [52] due to its more stable energy and noticeable electronic and magnetic properties. 3. Results and discussion 3.1. Stability of armchair InSe nanoribbons We were focused on the structural and stable characteristics of armchair InSe nanoribbons (AInSeNRs). Each primitive supercell contained twenty-two In atoms and twenty-two Se atoms and each edge was composed of two In atoms and two Se atoms stacking uniformly and periodically in the sequence of In–Se. Based on the ratio of H atoms adsorbed to the edge In or Se atoms [53] and structural features of AInSeNR, we considered sixteen configurations of hydrogenated AInSeNRs, as shown in Fig. 1. For example, the only difference between In50%-Se0%-1 and In50%-Se0%-3 was the relative position between two H atoms locating on the different edges. To examine the stability and formation possibility of AInSeNRs via edge hydrogenation, we calculated the formation energy [54] by the following definition: Eform ¼
Epri þ mμH m
Ehyd
where Epri and Ehyd were the total energies of the pristine and hydrogenated InSe nanoribbons, respectively. The μH was the 1/2 chemical potential of H2 molecule. The letter m represented the number of hydrogen atoms used for passivating In or Se atoms at the edges of AInSeNR. According to the definition, the structure with the more negative value was energetically favorable. As listed in Table 1, the calculated formation energies of In0%-Se50%-2 and In0%-Se100% were 0.015 eV and 0.047 eV, respectively. The positive value indicated that the formation energies of these structures were endothermic and these systems were not stable. For In25%-Se0%, Table 1 The band gap Eg (in eV), the formation energy Efrom (in eV) for semiconductors, the difference in binding energy of FM and AFM states (ΔEFM-AFM) (in meV), the total magnetic moments (Mtot in μB) and the main characteristics of armchair InSe nanoribbons via edge hydrogenation in different concentrations. The ribbons can be nonmagnetic semiconductor (NS), magnetic semiconductor (MS). Structures
Eg
Eform
ΔEFM-AFM
Mtot
Characteristics
Pristine In25%-Se0% In50%-Se0%-1 In50%-Se0%-2 In50%-Se0%-3 In75%-Se0% In100%-Se0% In0%-Se25% In0%-Se50%-1 In0%-Se50%-2 In0%-Se50%-3 In0%-Se75% In0%-Se100% In100%-Se50%-1 In100%-Se50%-2 In100%-Se50%-3 In100%-Se100%
1.021 1.365 1.532 (direct) 1.365 (direct) 1.505 (direct) 1.444 (direct) 1.509 0.820 (direct) 1.669 1.331 1.505 1.446 1.613 1.352 (direct) 1.547 1.332 (direct) 1.732 (direct)
–
–
0 1 2 0 2 1 0 1 2 0 2 1 0 2 0 2 0
NS MS MS NS MS MS NS MS MS NS MS MS NS MS NS MS NS
0.577 0.572 0.215 0.570 0.334 0.243 0.202 0.198 0.015 0.205 0.016 0.047 0.243 0.087 0.243 0.021
0.35 2.01 – 3.35 0.92 – 0.85 8.87 – 3.22 11.66 – 4.76 – 0.44 –
3
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(caption on next page)
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Fig. 2. The spin-polarized band structures and the total density of states (TDOS) of six magnetic configurations with partial hydrogenation (a) In25%-Se0%, (b) In50%-Se0%-1, (c) In50%-Se0%-3, (d) In75%-Se0%, (e) In0%-Se25%, (f) In0%-Se50%-1, (g) In0%-Se50%-3, (h) In0%-Se75%, (i) In100%-Se50%-1, (j) In100%-Se50%-3. The red and blue solid lines represent the spin-up and spin-down components, respectively. The Fermi energy is described by black dash line. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
In50%-Se0%-1, In50%-Se0%-2, In50%-Se0%-3, In75%-Se0% and In100%-Se0%, the calculated formation energies were 0.577 eV, 0.572 eV, 0.215eV,-0.570 eV, 0.334 eV and 0.243 eV, respectively. Besides, the formation energies of In0%-Se25%, In0%Se50%-1, In0%-Se50%-3, and In0%-Se75% were 0.202 eV, 0.198 eV, 0.205 eV and 0.016 eV, respectively. We could know that H atoms preferred to absorb on edge In atoms rather than edge Se atoms, which was similar to ZWS2NRs [55]. The calculated formation energies were 0.243 eV, 0.087 eV and 0.243 eV for In100%-Se50%-1, In100%-Se50%-2 and In100%-Se50%-3, respectively. Remarkably, for hydrogenated nanoribbons, most of structures showed magnetic state because the formation energy of magnetic state was always less than that of nonmagnetic state, as shown in Table 1.
Fig. 3. (a–d) Band structures of In50%-Se0%-2, In100%-Se0%, In100%-Se50%-2, In100%-Se100%. 5
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3.2. Electronic and magnetic properties of armchair InSe nanoribbons 3.2.1. Electronic property We also investigated the effect of passivation on the electronic properties of AInSeNRs. In Fig. 2, the spin-resolved band structures of all stable configurations presented asymmetrical spin-up and spin-down bands, which indicated these systems were magnetic. The band gap of In25%-Se0%, In50%-Se0%-1, In50%-Se0%-3, In75%-Se0%, In0%-Se25%, In0%-Se50%-1, In0%-Se50%-3, In0%-Se75%, In100%-Se50%-1 and In100%-Se50%-3 was 1.365 eV, 1.532 eV, 1.505 eV, 1.444 eV, 0.820 eV, 1.669 eV, 1.505 eV, 1.446 eV, 1.352 eV and 1.332 eV, respectively, which was mostly larger than the band gap of the bare AInSeNR with 1.021 eV In25%-Se0% was a typical semiconductor with an indirect band gap which the conduction band minimum (CBM) and valence band maximum (VBM) located at the different points. However, most of the spin-split band structures presented the semiconducting characteristic with direct band gap because CBM and VBM located at the same high symmetry points, shown in Fig. 2. In Fig. 3, four configurations In50%-Se0%-2, In100%-Se0%, In100%-Se50%-2, In100%-Se100% also remained semiconductors with a wider band gap. (d) Showed that In100%-Se100% had a direct band gap of 1.732 eV (in Table 1) with CBM and VBM located at the Gamma point, demonstrating that full hydrogenation could improve the optical properties of AInSeNR. Band gap of the In50%Se0%-2 was direct with 1.365 eV because of CBM and VBM both locating at the Y point. In100%-Se0% and In100%-Se50%-2 possessed indirect band gap of 1.509 eV and 1.547 eV, respectively. Thus, edge hydrogenation was an effective way to tune the band gap of AInSeNRs and could change indirect band gap to direct band gap. The Bader charges of Se, In and H atoms were summarized in Table 2. For the configurations that edge In atoms were hydrogenated, Se and In atoms lost electrons and H atoms gained electrons. Interestingly, the charge transfer from Se atoms was monotonously increased as the increasing concentration of H atoms, demonstrating selenium atoms provided enough charge transfer to edge H atoms. For the configurations that edge Se atoms were hydrogenated, Se atoms lost electrons from 0.012 |e| (In0%-Se25%) to 0.034 |e| (In0%-Se75%) and In atoms obtained electrons from þ0.010 |e| (In0%-Se25%) to þ0.026 |e| (In0%-Se75%). The charge transfer revealed that H atoms perferred to absorbed edge In atoms, which was consistent with the calculated results of the formation energy [56]. 3.2.2. Magnetic properties In order to investigate magnetic behaviours of these hydrogenated configurations, the spin density and the density of states (DOS) were shown in Fig. 4 and Fig. 5, respectively. The calculated total magnetic moments of In25%-Se0%, In75%-Se0% were both 1 μB as shown in Table 1. For In25%-Se0%, the spin-unpaired electron states were mainly contributed by partially hydrogenated edge atoms and inner Se atoms in the first hexagonal ring. For In75%-Se0%, the magnetic states were mainly composed of the unpassivated edge In atom and same edge Se atoms. For In50%-Se0%-1 and In50%-Se0%-3, both of total magnetic moment was increased to 2 μB. As seen in Fig. 4(b),(c), the spin density mainly came from the edge In and Se atoms. Noticeably, the magnetic properties of AInSeNRs was strongly depended on edge state, and edge hydrogenation could effectively induce edge magnetism of AInSeNR, which was similar to the hydrogenated zigzag InSe nanoribbons [57]. In the case of only edge Se atoms were hydrogenated, total magnetic moment of In0%-Se25%, In0%-Se50%-1, In0%-Se50%-3, In0%-Se75% was 1 μB, 2 μB, 2 μB, 1 μB, respectively. As seen in Fig. 4(e)–(h), for con figurations In0%-Se25% and In0%-Se75%, the valence electrons mainly located around edge atoms where was partially hydrogenated. In100%-Se50%-1 and In100%-Se50%-3 had the stable magnetic state with 2 μB total magnetic moment, respectively. We found that AInSeNRs could perform edge magnetism when an unpassivated atom and a passivated same atom occurred at the same edge. We calculated the ferromagnetic energy and antiferromagnetic energy of magnetic systems and gave the energy differences between them in Table 1. According to the experience of studying magnetic semiconductor based on the first-principles computations, ferromagnetic magnetic coupling can be stable at room temperature when the energy difference between ferromagnetic and antiferromagnetic is less than 50meV. From Table 1, we can found that the energy differences between FM and AFM are more than 50meV, demonstratingall of hydrogenation systems are antiferromagnetic, which is similar to the results of spin densities in Fig. 4. We took the partial density of states (PDOS) of In50%-Se0%-1 and In25%-Se0% as examples which were given in Fig. 5. The spin-up and spin-down of PDOS were asymmetric and showed magnetic behaviours of system, which was similar to the band structure. There were strong hybridization effects between p orbitals of edge Se atoms and p orbitals of edge In atoms. Interestingly, for the PDOS of In50%-Se0%-1 in Fig. 5(a), Table 2 Calculated Bader charge transfer for Se, In and H atoms. System
Bader charge |e| qSe
In25%-Se0% In50%-Se0%-1 In50%-Se0%-3 In75%-Se0% In100%-Se50%-1 In100%-Se50%-3 In0%-Se25% In0%-Se50%-1 In0%-Se50%-3 In0%-Se75%
0.004 0.008 0.008 0.0004 0.031 0.107 0.012 0.024 0.026 0.034
6
qIn
qH
0.007 0.015 0.015 0.038 0.022 0.040 þ0.010 þ0.020 þ0.021 þ0.026
þ0.261 þ0.260 þ0.261 þ0.284 þ0.192 þ0.542 þ0.041 þ0.045 þ0.057 þ0.058
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Fig. 4. Spin densities for (a) In 25%-Se0%, (b) In50%-Se0%-1, (c) In50%-Se0%-3, (d) In75%-Se0%, (e) In0%-Se25%, (f) In0%-Se50%-1, (g) In0%Se50%-3, (h) In0%-Se75%, (i) In100%-Se50%-1, (j) In100%-Se50%-3. Isovalue is set as 0.01e/Å3.
VBM in the spin-up channel touched the fermi level and CBM in the spin-down channel removed the Fermi level. It implied that In50%-Se0%-1 exhibited 100% spin-polarized behaviours (i.e. P ¼ |D↑(EF)-D↓(EF)|/[D↑(EF)þD↓(EF)] in the terms of the density of states at EF for each spin state), which was similar to armchair graphene nanoribbons passivated by 3d transition-metal atoms [58]. The PDOS of In25%-Se0% in Fig. 5(b) indicated this system was not 100% spin-polarization. It was worthy to mention that the rest of magnetic systems also performed 100% spin-polarization according to our calculation results. To prove the stability of the magnetism [59], the compressive and tensile strain along Y direction was applied from 3% to 3% and the formation energy and magnetic moments were given in Table 3. The magnetic H-AInSeNRs under 3% compressive strain was the most stable because the lower Eform and the total magnetic moments was equal to that of H-AInSeNRs without strain. Meanwhiles, we found that the effect of strain on the structure stability and magnetic properties was slight for the H-AInSeNRs. Thus, the regular total magnetic moment of AInSeNRs was stable. 4. Conclusion In this work, systematical investigation on the electronic and magnetic properties of armchair InSe nanoribbons with different hydrogenation configurations was carried out by the implement of the first-principles calculations. The formation energy showed that 7
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Fig. 5. The partial density of states of (a) In50%-Se0%-1 and (b) In25%-Se0%, respectively. Table 3 The formation energy and the total magnetic moment of all magnetic configurations under the strain from Strain
3% Eform
In25%-Se0% In50%-Se0%-1 In50%-Se0%-3 In75%-Se0% In0%-Se25% In0%-Se50%-1 In0%-Se50%-3 In0%-Se75% In100%-Se50%-1 In100%-Se50%-3
1.00 1.57 1.57 1.42 0.63 0.85 0.84 0.48 1.88 1.89
2%
μB 1 2 2 1 1 2 2 1 2 2
Eform 0.82 1.39 1.39 1.24 0.46 0.66 0.66 0.29 1.70 1.71
1%
μB 1 2 2 1 1 2 2 1 2 2
1%
Eform 0.58 1.24 1.24 1.09 0.32 0.50 0.49 0.15 1.56 1.40
3% to 3%. 2%
3%
μB
Eform
μB
Eform
μB
Eform
μB
1 2 2 1 1 2 2 1 2 2
0.51 1.08 1.07 0.94 0.14 0.33 0.33 0.02 1.40 1.40
1 2 2 1 1 2 2 1 2 2
0.49 1.06 1.05 0.91 0.12 0.31 0.32 0.05 1.38 1.38
1 2 2 1 1 2 2 1 2 2
0.50 1.05 1.05 1.05 0.13 0.33 0.33 0.04 1.39 1.39
1 2 2 1 1 2 2 1 2 2
H atoms preferred to absorb up to In atoms. In50%-Se0%-2 and In100%-Se100% showed wider direct band gap under edge hydro genation, and the maximum band gap came to 1.732 eV under full hydrogenation, which demonstrated hydrogenation could effec tively change the photoelectric properties of AInSeNRs. Partial hydrogenation could induce to edge magnetism of AInSeNRs and the total magnetic moment was 1 μB or 2 μB contributed by p orbital of edge Se and In atoms. The magnetic systems of H-AInSeNRs were 100% spin-polarized, except for In25%-Se0%. We should point out that the electron transfer and the hybridization of atomic orbitals from the edge atoms would affect the electronic and magnetic properties of hydrogenation systems. Our work would offer an useful method to modulate the electronic and magnetic properties of nanostructures and illustrated armchair InSe nanoribbons were promising candidates in nanoelectronics, spintronics and novel photoelectric devices. Acknowledgements This work is supported by a Grant from the National Natural Science Foundation of China, China under the Grant No. 11504092, 61674053, 11674084, the 111 Project, China (Grant No. D17007), Henan Center for Outstanding Overseas Scientists (Grant No. GZS2018003), and Training plan of youth backbone teacher of institution of higher learning of Henan province and High Performance Computing Center of Henan Normal University. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2019.106282.
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Tuning electronic and magnetic properties of armchair InSe nanoribbons by hydrogenation Xu Zhao a, *, Binru Zhao a, Hui Zhang a, Tianxing Wang a, Congxin Xia a, Xianqi Dai a, Shuyi Wei a, Lin Yang b a b
School of Physics, Henan Normal University, Xinxiang, Henan, 453007, China School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan, 453007, China
A R T I C L E I N F O
A B S T R A C T
Keywords: InSe Edge hydrogenation Spin-polarization First-principles calculation
Using the first-principles computations based on the density functional theory, we investigated structural, electronic and magnetic properties of armchair InSe nanoribbons (AInSeNRs) via edge hydrogenation. The calculated formation energy showed that H atoms preferred to absorb to edge In atoms. Partial hydrogenation could induce to edge magnetism of AInSeNR and the total magnetic moment of AInSeNRs was regularly 1 μB or 2 μB coming from the p orbitals of edge Se or In atoms. Moreover, the majority of magnetic systems performed 100% spin-polarization, which was similar with graphene nanoribbon. Under the strain from 3% to 3%, AInSeNRs remained magnetic properties and the total magnetic moment was still 1 μB or 2 μB. Nonmagnetic systems of In50%-Se0%-2 and In100%-Se100% showed wider direct band gap under edge hydrogenation, and the maximum band gap came to 1.732 eV under full hydrogenation. Our work provided an effective way to improve electronic, optical and magnetic properties of AInSeNR, which has promising applications in spintronics and novel optional devices via edge hydrogenation.
1. Introduction Since the isolated single graphene have been realized experimentally [1,2], research interest has shifted to its unique electronic properties and potential for device applications, such as high carrier mobility at room temperature, high electrical conductivity and excellent mechanical strength [3–6]. Unfortunately, graphene has an obvious shortage that it couldn’t keep a wide band gap and preserve its extreme carrier mobility at the same time. Such a dilemma has stimulated the search for other 2D materials, such as MoS2 [7], oxides [8] and III-VI compounds (e.g., BN, GaS and GaSe) [9,10]. MoS2 monolayer is a typical transition metal sulfide. Differently from graphene, MoS2 is characterized by its band gap of around 1.2 eV [11], which can be further increased by changing the thickness [12], or applying the mechanical strain [13,14]. Gas sensors based on few-layered MoS2 field effect transistors have been realized due to the natural abundance and distinctive optical, electrical, and mechanical properties of MoS2. Few-layered GaS, GaSe, and InSe have been proven to be promising candidates in field such as optoelectronics, field-effect transistors (FETs), and solar energy conversion [15–18]. The 2D InSe nanosheet exhibits the characteristic of direct band gap semiconductor. And it has smaller electron effective mass (m* ¼ 0.143 m0) and higher electron effective mobility (~103 cm2V–1S 1 at room temperature) [19], which demonstrates InSe nanosheets have the promising application in high-mobility electronic devices. However, graphene and MoS2 armchair nanoribbons
* Corresponding author. E-mail address: [email protected] (X. Zhao). https://doi.org/10.1016/j.spmi.2019.106282 Received 21 May 2019; Received in revised form 29 August 2019; Accepted 26 September 2019 Available online 30 September 2019 0749-6036/© 2019 Elsevier Ltd. All rights reserved.
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are not best candidates for the related FET devices with high-transport and fast-response characteristics [20] due to the low carrier mobility ~102 cm2V 1s 1. Furthermore, layered InSe has a crossover transition from direct to indirect band gap when the layer thickness is reduced to �6 nm, and its narrower band gap with smaller excition reduced mass, leads to stronger quantum confinement, which is well above that of many other 2D semiconductors [21]. Numerical simulations of structural, electric, and magnetic properties of InSe monolayers and their modulation by doping, defect, and, adsorption have been carried out [22–30]. Besides, theoretical calculations have predicted the existence of piezoeletricity in InSe with piezoelectric coefficient on the same order of magnitude as the earlier discovered two-dimensional piezoelectric materials [31]. The 1D nanostructures, such as nanoribbons, nanowires, nanorobs and nanotubes, have been extensively studied both experi mentally and found to possess some unusual properties compared to their 2D counterparts due to the low dimensionality, edge effect and the enhanced quantum confinement effect [32]. ML-MoS2 could be tailored into quasi-1D nanoribbons by an electro chemical/chemical route [33,34]. As a kind of classical nanoribbons, many researches demonstrate MoS2 nanoribbons could be induced remarkable optical, mechanical, electronic and magnetic properties due to edge modification [35–40]. Ouyang et al. found that armchair MoS2 nanoribbons (AMoS2NRs) possessed localized ferromagnetic semiconductors edge states due to edge hydroge nation in certain configurations and become antiferromagnetic semiconductors or ferromagnetic semiconductors. Additionally, partial edge hydrogenation as well as external electric field could tune AMoS2NRs in small dimension from semiconductors to metals or half metals [41,42]. Accordingly, edge passivation is an effective way to influence the electronic and magnetic properties of nanoribbons. Inspired by prediction of Pan et al. [43] that different edge structures might have different impacts on one-dimension materials. In addition, to our best knowledge, there are few studies on electronic and magnetic properties of one-dimension nanoribbons of InSe monolayer up to now. Hence, in this paper, we performed first-principles calculations to investigate systematically the electronic and magnetic properties of hydrogenated InSe armchair nanoribbons. This paper was organized as following, we gave the introduction in Section 1. Our calculation method was described in detail in Section 2. In Section 3, we presented the structure model, then gave the results and discussions for the electronic and magnetic properties of the AInSeNR. Our work provided an effective way to improve the electronic, optical and magnetic properties of AInSeNRs via edge hydrogenation, which has promising applications in electronic devices [44,45], spintronics and novel optional devices via edge hydrogenation.
Fig. 1. (a–p) Top and side views of the sixteen different hydrogenated configurations In25%-Se0%, In50%-Se0%-1, In50%-Se0%-2, In50%-Se0%-3, In75%-Se0%, In100%-Se0%, In0%-Se25%, In0%-Se50%-1, In0%-Se50%-2, In0%-Se50%-3, In0%-Se75%, In0%-Se100%, In100%-Se50%-1, In100%Se50%-2, In100%-Se50%-3, In100%-Se100%, respectively. Green, purple and brown balls refer to Se, In and H atoms, respectively. (For inter pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 2
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2. Model and method In this work, the first-principles calculation was performed within the framework of density functional theory as implemented in Vienna ab-initio simulation package (VASP) [46,47]. The electron exchange-correlation functional was treated within the generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) parameterization [48,49]. The projector-augmented plane wave (PAW) [50] method was employed to model the ion-electron interactions. A plane wave basis set with a cut-off value of 500 eV was used. For the geometry optimization, nine Monkhost-Pack special k points were used for integration over the first Brillouin zone. All atoms were fully optimized and the energy and force convergence standard were set to 10 5 eV per atom and 0.01 eV/Å for each atom, respectively. The energy cutoff and k-point settings were proved to be sufficient for achieving converged results. To aviod the interactions between two periodical slabs in unit cell, armchair InSe nanoribbons were separated from each other by a vacuum space of 20 Å in the nonperiodic directions, which were perpendicular to and along the plane of the nanoribbon, respectively. The valence electrons for the In, Se and H are respectively 13 (In: 4d105s25p1), 6 (Se: 4s24p4) and 1 (H: 1s1). Similar to the structure of GaSe, GaS, InS and InTe, InSe has the honeycomb structure where In and Se atoms are uniformly and periodically arranged along the edge. The model of InSeNR can be constructed by cutting two-dimensional InSe monolayer along armchair or zigzag ori entations, but in this work, only armchair InSe nanoribbons have been considered because the bare zigzag InSe nanoribbon was metallic. Besides [51], showed us that armchair InSe nanoribbon had three kinds of edge reconstruction but we only considered the ribbon width of 11 dimer chainwith ac-p reconstruction [52] due to its more stable energy and noticeable electronic and magnetic properties. 3. Results and discussion 3.1. Stability of armchair InSe nanoribbons We were focused on the structural and stable characteristics of armchair InSe nanoribbons (AInSeNRs). Each primitive supercell contained twenty-two In atoms and twenty-two Se atoms and each edge was composed of two In atoms and two Se atoms stacking uniformly and periodically in the sequence of In–Se. Based on the ratio of H atoms adsorbed to the edge In or Se atoms [53] and structural features of AInSeNR, we considered sixteen configurations of hydrogenated AInSeNRs, as shown in Fig. 1. For example, the only difference between In50%-Se0%-1 and In50%-Se0%-3 was the relative position between two H atoms locating on the different edges. To examine the stability and formation possibility of AInSeNRs via edge hydrogenation, we calculated the formation energy [54] by the following definition: Eform ¼
Epri þ mμH m
Ehyd
where Epri and Ehyd were the total energies of the pristine and hydrogenated InSe nanoribbons, respectively. The μH was the 1/2 chemical potential of H2 molecule. The letter m represented the number of hydrogen atoms used for passivating In or Se atoms at the edges of AInSeNR. According to the definition, the structure with the more negative value was energetically favorable. As listed in Table 1, the calculated formation energies of In0%-Se50%-2 and In0%-Se100% were 0.015 eV and 0.047 eV, respectively. The positive value indicated that the formation energies of these structures were endothermic and these systems were not stable. For In25%-Se0%, Table 1 The band gap Eg (in eV), the formation energy Efrom (in eV) for semiconductors, the difference in binding energy of FM and AFM states (ΔEFM-AFM) (in meV), the total magnetic moments (Mtot in μB) and the main characteristics of armchair InSe nanoribbons via edge hydrogenation in different concentrations. The ribbons can be nonmagnetic semiconductor (NS), magnetic semiconductor (MS). Structures
Eg
Eform
ΔEFM-AFM
Mtot
Characteristics
Pristine In25%-Se0% In50%-Se0%-1 In50%-Se0%-2 In50%-Se0%-3 In75%-Se0% In100%-Se0% In0%-Se25% In0%-Se50%-1 In0%-Se50%-2 In0%-Se50%-3 In0%-Se75% In0%-Se100% In100%-Se50%-1 In100%-Se50%-2 In100%-Se50%-3 In100%-Se100%
1.021 1.365 1.532 (direct) 1.365 (direct) 1.505 (direct) 1.444 (direct) 1.509 0.820 (direct) 1.669 1.331 1.505 1.446 1.613 1.352 (direct) 1.547 1.332 (direct) 1.732 (direct)
–
–
0 1 2 0 2 1 0 1 2 0 2 1 0 2 0 2 0
NS MS MS NS MS MS NS MS MS NS MS MS NS MS NS MS NS
0.577 0.572 0.215 0.570 0.334 0.243 0.202 0.198 0.015 0.205 0.016 0.047 0.243 0.087 0.243 0.021
0.35 2.01 – 3.35 0.92 – 0.85 8.87 – 3.22 11.66 – 4.76 – 0.44 –
3
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Fig. 2. The spin-polarized band structures and the total density of states (TDOS) of six magnetic configurations with partial hydrogenation (a) In25%-Se0%, (b) In50%-Se0%-1, (c) In50%-Se0%-3, (d) In75%-Se0%, (e) In0%-Se25%, (f) In0%-Se50%-1, (g) In0%-Se50%-3, (h) In0%-Se75%, (i) In100%-Se50%-1, (j) In100%-Se50%-3. The red and blue solid lines represent the spin-up and spin-down components, respectively. The Fermi energy is described by black dash line. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
In50%-Se0%-1, In50%-Se0%-2, In50%-Se0%-3, In75%-Se0% and In100%-Se0%, the calculated formation energies were 0.577 eV, 0.572 eV, 0.215eV,-0.570 eV, 0.334 eV and 0.243 eV, respectively. Besides, the formation energies of In0%-Se25%, In0%Se50%-1, In0%-Se50%-3, and In0%-Se75% were 0.202 eV, 0.198 eV, 0.205 eV and 0.016 eV, respectively. We could know that H atoms preferred to absorb on edge In atoms rather than edge Se atoms, which was similar to ZWS2NRs [55]. The calculated formation energies were 0.243 eV, 0.087 eV and 0.243 eV for In100%-Se50%-1, In100%-Se50%-2 and In100%-Se50%-3, respectively. Remarkably, for hydrogenated nanoribbons, most of structures showed magnetic state because the formation energy of magnetic state was always less than that of nonmagnetic state, as shown in Table 1.
Fig. 3. (a–d) Band structures of In50%-Se0%-2, In100%-Se0%, In100%-Se50%-2, In100%-Se100%. 5
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3.2. Electronic and magnetic properties of armchair InSe nanoribbons 3.2.1. Electronic property We also investigated the effect of passivation on the electronic properties of AInSeNRs. In Fig. 2, the spin-resolved band structures of all stable configurations presented asymmetrical spin-up and spin-down bands, which indicated these systems were magnetic. The band gap of In25%-Se0%, In50%-Se0%-1, In50%-Se0%-3, In75%-Se0%, In0%-Se25%, In0%-Se50%-1, In0%-Se50%-3, In0%-Se75%, In100%-Se50%-1 and In100%-Se50%-3 was 1.365 eV, 1.532 eV, 1.505 eV, 1.444 eV, 0.820 eV, 1.669 eV, 1.505 eV, 1.446 eV, 1.352 eV and 1.332 eV, respectively, which was mostly larger than the band gap of the bare AInSeNR with 1.021 eV In25%-Se0% was a typical semiconductor with an indirect band gap which the conduction band minimum (CBM) and valence band maximum (VBM) located at the different points. However, most of the spin-split band structures presented the semiconducting characteristic with direct band gap because CBM and VBM located at the same high symmetry points, shown in Fig. 2. In Fig. 3, four configurations In50%-Se0%-2, In100%-Se0%, In100%-Se50%-2, In100%-Se100% also remained semiconductors with a wider band gap. (d) Showed that In100%-Se100% had a direct band gap of 1.732 eV (in Table 1) with CBM and VBM located at the Gamma point, demonstrating that full hydrogenation could improve the optical properties of AInSeNR. Band gap of the In50%Se0%-2 was direct with 1.365 eV because of CBM and VBM both locating at the Y point. In100%-Se0% and In100%-Se50%-2 possessed indirect band gap of 1.509 eV and 1.547 eV, respectively. Thus, edge hydrogenation was an effective way to tune the band gap of AInSeNRs and could change indirect band gap to direct band gap. The Bader charges of Se, In and H atoms were summarized in Table 2. For the configurations that edge In atoms were hydrogenated, Se and In atoms lost electrons and H atoms gained electrons. Interestingly, the charge transfer from Se atoms was monotonously increased as the increasing concentration of H atoms, demonstrating selenium atoms provided enough charge transfer to edge H atoms. For the configurations that edge Se atoms were hydrogenated, Se atoms lost electrons from 0.012 |e| (In0%-Se25%) to 0.034 |e| (In0%-Se75%) and In atoms obtained electrons from þ0.010 |e| (In0%-Se25%) to þ0.026 |e| (In0%-Se75%). The charge transfer revealed that H atoms perferred to absorbed edge In atoms, which was consistent with the calculated results of the formation energy [56]. 3.2.2. Magnetic properties In order to investigate magnetic behaviours of these hydrogenated configurations, the spin density and the density of states (DOS) were shown in Fig. 4 and Fig. 5, respectively. The calculated total magnetic moments of In25%-Se0%, In75%-Se0% were both 1 μB as shown in Table 1. For In25%-Se0%, the spin-unpaired electron states were mainly contributed by partially hydrogenated edge atoms and inner Se atoms in the first hexagonal ring. For In75%-Se0%, the magnetic states were mainly composed of the unpassivated edge In atom and same edge Se atoms. For In50%-Se0%-1 and In50%-Se0%-3, both of total magnetic moment was increased to 2 μB. As seen in Fig. 4(b),(c), the spin density mainly came from the edge In and Se atoms. Noticeably, the magnetic properties of AInSeNRs was strongly depended on edge state, and edge hydrogenation could effectively induce edge magnetism of AInSeNR, which was similar to the hydrogenated zigzag InSe nanoribbons [57]. In the case of only edge Se atoms were hydrogenated, total magnetic moment of In0%-Se25%, In0%-Se50%-1, In0%-Se50%-3, In0%-Se75% was 1 μB, 2 μB, 2 μB, 1 μB, respectively. As seen in Fig. 4(e)–(h), for con figurations In0%-Se25% and In0%-Se75%, the valence electrons mainly located around edge atoms where was partially hydrogenated. In100%-Se50%-1 and In100%-Se50%-3 had the stable magnetic state with 2 μB total magnetic moment, respectively. We found that AInSeNRs could perform edge magnetism when an unpassivated atom and a passivated same atom occurred at the same edge. We calculated the ferromagnetic energy and antiferromagnetic energy of magnetic systems and gave the energy differences between them in Table 1. According to the experience of studying magnetic semiconductor based on the first-principles computations, ferromagnetic magnetic coupling can be stable at room temperature when the energy difference between ferromagnetic and antiferromagnetic is less than 50meV. From Table 1, we can found that the energy differences between FM and AFM are more than 50meV, demonstratingall of hydrogenation systems are antiferromagnetic, which is similar to the results of spin densities in Fig. 4. We took the partial density of states (PDOS) of In50%-Se0%-1 and In25%-Se0% as examples which were given in Fig. 5. The spin-up and spin-down of PDOS were asymmetric and showed magnetic behaviours of system, which was similar to the band structure. There were strong hybridization effects between p orbitals of edge Se atoms and p orbitals of edge In atoms. Interestingly, for the PDOS of In50%-Se0%-1 in Fig. 5(a), Table 2 Calculated Bader charge transfer for Se, In and H atoms. System
Bader charge |e| qSe
In25%-Se0% In50%-Se0%-1 In50%-Se0%-3 In75%-Se0% In100%-Se50%-1 In100%-Se50%-3 In0%-Se25% In0%-Se50%-1 In0%-Se50%-3 In0%-Se75%
0.004 0.008 0.008 0.0004 0.031 0.107 0.012 0.024 0.026 0.034
6
qIn
qH
0.007 0.015 0.015 0.038 0.022 0.040 þ0.010 þ0.020 þ0.021 þ0.026
þ0.261 þ0.260 þ0.261 þ0.284 þ0.192 þ0.542 þ0.041 þ0.045 þ0.057 þ0.058
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Fig. 4. Spin densities for (a) In 25%-Se0%, (b) In50%-Se0%-1, (c) In50%-Se0%-3, (d) In75%-Se0%, (e) In0%-Se25%, (f) In0%-Se50%-1, (g) In0%Se50%-3, (h) In0%-Se75%, (i) In100%-Se50%-1, (j) In100%-Se50%-3. Isovalue is set as 0.01e/Å3.
VBM in the spin-up channel touched the fermi level and CBM in the spin-down channel removed the Fermi level. It implied that In50%-Se0%-1 exhibited 100% spin-polarized behaviours (i.e. P ¼ |D↑(EF)-D↓(EF)|/[D↑(EF)þD↓(EF)] in the terms of the density of states at EF for each spin state), which was similar to armchair graphene nanoribbons passivated by 3d transition-metal atoms [58]. The PDOS of In25%-Se0% in Fig. 5(b) indicated this system was not 100% spin-polarization. It was worthy to mention that the rest of magnetic systems also performed 100% spin-polarization according to our calculation results. To prove the stability of the magnetism [59], the compressive and tensile strain along Y direction was applied from 3% to 3% and the formation energy and magnetic moments were given in Table 3. The magnetic H-AInSeNRs under 3% compressive strain was the most stable because the lower Eform and the total magnetic moments was equal to that of H-AInSeNRs without strain. Meanwhiles, we found that the effect of strain on the structure stability and magnetic properties was slight for the H-AInSeNRs. Thus, the regular total magnetic moment of AInSeNRs was stable. 4. Conclusion In this work, systematical investigation on the electronic and magnetic properties of armchair InSe nanoribbons with different hydrogenation configurations was carried out by the implement of the first-principles calculations. The formation energy showed that 7
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Fig. 5. The partial density of states of (a) In50%-Se0%-1 and (b) In25%-Se0%, respectively. Table 3 The formation energy and the total magnetic moment of all magnetic configurations under the strain from Strain
3% Eform
In25%-Se0% In50%-Se0%-1 In50%-Se0%-3 In75%-Se0% In0%-Se25% In0%-Se50%-1 In0%-Se50%-3 In0%-Se75% In100%-Se50%-1 In100%-Se50%-3
1.00 1.57 1.57 1.42 0.63 0.85 0.84 0.48 1.88 1.89
2%
μB 1 2 2 1 1 2 2 1 2 2
Eform 0.82 1.39 1.39 1.24 0.46 0.66 0.66 0.29 1.70 1.71
1%
μB 1 2 2 1 1 2 2 1 2 2
1%
Eform 0.58 1.24 1.24 1.09 0.32 0.50 0.49 0.15 1.56 1.40
3% to 3%. 2%
3%
μB
Eform
μB
Eform
μB
Eform
μB
1 2 2 1 1 2 2 1 2 2
0.51 1.08 1.07 0.94 0.14 0.33 0.33 0.02 1.40 1.40
1 2 2 1 1 2 2 1 2 2
0.49 1.06 1.05 0.91 0.12 0.31 0.32 0.05 1.38 1.38
1 2 2 1 1 2 2 1 2 2
0.50 1.05 1.05 1.05 0.13 0.33 0.33 0.04 1.39 1.39
1 2 2 1 1 2 2 1 2 2
H atoms preferred to absorb up to In atoms. In50%-Se0%-2 and In100%-Se100% showed wider direct band gap under edge hydro genation, and the maximum band gap came to 1.732 eV under full hydrogenation, which demonstrated hydrogenation could effec tively change the photoelectric properties of AInSeNRs. Partial hydrogenation could induce to edge magnetism of AInSeNRs and the total magnetic moment was 1 μB or 2 μB contributed by p orbital of edge Se and In atoms. The magnetic systems of H-AInSeNRs were 100% spin-polarized, except for In25%-Se0%. We should point out that the electron transfer and the hybridization of atomic orbitals from the edge atoms would affect the electronic and magnetic properties of hydrogenation systems. Our work would offer an useful method to modulate the electronic and magnetic properties of nanostructures and illustrated armchair InSe nanoribbons were promising candidates in nanoelectronics, spintronics and novel photoelectric devices. Acknowledgements This work is supported by a Grant from the National Natural Science Foundation of China, China under the Grant No. 11504092, 61674053, 11674084, the 111 Project, China (Grant No. D17007), Henan Center for Outstanding Overseas Scientists (Grant No. GZS2018003), and Training plan of youth backbone teacher of institution of higher learning of Henan province and High Performance Computing Center of Henan Normal University. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.spmi.2019.106282.
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