Electronic and magnetic properties of dopant atoms in SnSe monolayer: a first-principles study
Qingxia Wang, Weiyang Yu, Xiaonan Fu, Chong Qiao, Congxin Xia, Yu Jia
aa r X i v : . [ phy s i c s . c o m p - ph ] D ec Electronic and magnetic properties of dopant atoms in SnSe monolayer: afirst-principles study
Qingxia Wang, Weiyang Yu,
1, 2
Xiaonan Fu, Chong Qiao, Congxin Xia, ∗ and Yu Jia † International Laboratory for Quantum Functional Materials of Henan,and School of Physics and Engineering, Zhengzhou University, Zhengzhou, 450001, China School of Physics and Chemistry, Henan Polytechnic University, Jiaozuo, 454000, China Department of Physics and School of Science, Henan University of Technology, Zhengzhou 450001, China Department of Physics,Henan Normal University, Xinxiang, 453000, China (Dated: December 7, 2015)
Abstract:
SnSe monolayer with orthorhombic
Pnma
GeS structure is an important two-dimensional (2D) indirect band gap material at room temperature. Based on first-principles densityfunctional theory calculations, we present systematic studies on the electronic and magnetic prop-erties of X (X = Ga, In, As, Sb) atoms doped SnSe monolayer. The calculated electronic structuresshow that Ga-doped system maintains semiconducting property while In-doped SnSe monolayer ishalf-metal. The As- and Sb- doped SnSe systems present the characteristics of n -type semiconduc-tor. Moreover, all considered substitutional doping cases induce magnetic ground states with themagnetic moment of 1 µ B . In addition, the calculated formation energies also show that four typesof doped systems are thermodynamic stable. These results provide a new route for the potentialapplications of doped SnSe monolayer in 2D photoelectronic and magnetic semiconductor devices. Keywords:
SnSe monolayer, substitutional doping, electronic properties, magnetism
PACS numbers: 73.20.At, 75.50.Pp, 75.75.+a
1. Introduction
Along with the discovery of graphene, many new two-dimensional(2D) atomic layered systems have attractedspecial attention for future electronics applications.
Recently, more and more researchers devote themselveson 2D atomic-layer materials, such as silicene, ger-manane, stanene, phosphorene, and so on.
Just asthe scope of group IV semiconductors such as grapheneand silicene have been broadened significantly by in-troducing isoelectronic III-V compounds, phosphorenehas been broadened by introducing isoelectronic IV-VIcompounds. IV-VI group semiconductors have layeredstructures which are similar to phosphorene.
For bulkTin selenium (SnSe), it has two most common crystalstructures: one is cubic NaCl (rock salt) and the other isorthorhombic GeS structure with an orthorhombic
Pnma space group at room temperature.
Furthermore, bulkSnSe is an important double layered binary IV-VI semi-conductor with structural orthorhombic symmetry andweak van der Waals force between double layers. In ad-dition, bulk SnSe has both a direct band gap of 1.30 eVand an indirect band gap of 0.90 eV, just falling into theoptimum band gap for solar cells. Therefore, bulk SnSeis also considered as the most promising candidates forsolar cells.
Especially, with the successfully synthe-sizing of 2D SnSe experimentally, SnSe monolayer hasattracted more and more attention in monolayer IV-VIsemiconductor. SnSe monolayer is comprised of zigzagdouble-layered planes and exhibits strong anisotropicproperties.
As material of earth-abundance, envi-ronment friendly and good chemical stability, SnSe canbe applied in a wide range of potential applications, e.g.optical devices, memory switching devices, infrared op-toelectronic devices and anode materials for rechargeable lithium batteries.
SnSe monolayer has the similar structure to phospho-rene. Sn and Se atoms present a puckered surface due tothe sp hybridization. Each Sn atom forms three cova-lent bonds with Se, and the same as Se atom. The synthe-sis and properties of SnSe monolayer have attracted muchattention.
As we know, 2D material have many spe-cial properties due to their unique dimension-dependentproperties comparing with 3D material. Whereas thereis few literature about 2D SnSe monolayer, the elec-tronic and magnetic properties of substitutional dopingof SnSe monolayer have not been done. In this work, us-ing first-principles density functional theory (DFT) cal-culations, we present a systematic study of the electronicand magnetic properties of Ga-, In-, As-, and Sb- dopedSnSe monolayer. Firstly, we calculate the electronic andmagnetic properties of the doped systems. And then,we calculate the formation energy E f and find that theformation energy of Ga-doped system is lowest than theother three systems,indicating that Ga-doped system isapt to be realized in experiment.
2. Computational Details
The present first-principles DFT calculations wereperformed using Perdew-Burke-Ernzerh (PBE) in theprojector augmented wave (PAW) within Vienna abinitio Simulation Package (VASP) . The reciprocalspace was sampled with a fine grid of 3 × × k -pointsin the Brillouin zone of the primitive unit cell for bulkSnSe, and 1 × × k -points for the SnSe monolayer andthe doped systems. The wave functions were expandedin a plane-wave basis with an energy cutoff of 500 eV.The maximum force of each atom was less than 0.01eV/˚A.The geometrical structure was employed with a3 × FIG. 1: (a) The geometric and (b) electronic structures ofbulk SnSe. tration of 2.78%. A vacuum spacing perpendicular tothe plane was employed to be at least 15˚A in the unitcell to avoid the coupling between neighboring cells. Thespin-polarized was considered to analyze the magnetismof the doped systems.
3. Results and discussion
Fig. 1 presents the geometrical structure and elec-tronic band structure of bulk SnSe. The calculatedoptimal lattice parameters of bulk SnSe are a=11.50˚Ab=4.16˚A, and c=4.45˚A, which are in agreement with theexperiment values and previous theoretical values .The bond lengths of Sn-Se are d =2.81˚A and d =2.78˚A,which are coincided with the previous reported values, as seen in Fig. 1 (a). The calculated band gap is 0.50eV, which is smaller than experimental value and the pre-vious theoretical calculation, on account of the DFT-PBE method which underestimates the band gap of semi-conductor. Even though the fundamental band gapsare typically underestimated in DFT-PBE approach,the prediction of the indirect semiconductor system iscorrect. In Fig. 1 (b), we can see that the valenceband maximum (VBM) of bulk SnSe is at M point, whileconduction band minimum (CBM) is located at the mid-dle of Γ-M, indicating an indirect band gap. Moreover,the bulk SnSe has no magnetism according to our calcu-lation.The structure of SnSe monolayer is displayed in Fig. 2,along with the similar structure of phosphorene. As seenin Fig. 2 (a) and (b), Sn and Se atoms form three covalentbonds with each other in a puckered surface, which is sim-ilar to that of phosphorene, as shown in Fig. 2 (c) and(d). However, it is obvious to perceive that the structureof phosphorene is of higher symmetry than SnSe mono-layer, so they appear some different properties, e.g. SnSemonolayer is an indirect band gap semiconductor, whilephosphorene has a direct band gap. To investigate the electronic and magnetic proper-ties of substitutional doping in SnSe monolayer, theband structure, total density of states (DOS) and par-tial density of states (PDOS) of the doping systems
FIG. 2: (a) Top and (b) side views of SnSe monolayer with3 × Sn Se X Sn (X=Ga, In, As, Sb) are calculated, alongwith the pure SnSe monolayer for comparison.Firstly, band structures, DOS and PDOS of SnSemonolayer are calculated in Fig. 3. As shown in Fig. 3(a), the indirect band gap is 0.78 eV, which is bigger thanthat of bulk SnSe ( E g =0.50 eV). From the PDOS of SnSemonolayer , as shown in Fig. 3 (b), we can clearly see thatit is 5 s -orbitals of Sn and 4 p -orbitals of Se that contributemostly to the total DOS below Fermi level from -1.50 eVto -0.11 eV. While above Fermi level, 5 p -orbitals of Snand 4 s -orbitals of Se contribute mostly to the total DOS.In detail, it is the hybridization of 5 s -orbital of Sn and 4 p -orbital of Se that forms the valence band, and 5 p -orbitalof Sn and 4 s -orbital of Se hybridize to comprise the con-duction band. The spin-polarized calculations show thatpristine SnSe monolayer is of non-magnetism.Now we turn to investigate the doping effects on elec-tronic structures of SnSe monolayer. As we know that theelectronic structure of SnSe monolayer can be tuned in awidely range by substitutional doping with Sn atom re- FIG. 4: Calculated band structures of (a) Ga- and (b) In-doped systems, along Γ-M-N-Γ direction. The energy zero representsthe Fermi level.FIG. 5: DOS and PDOS of (a) Ga-doped and (b) In-doped systems. The energy zero represents the Fermi level. placed by other atoms because the outer electron valenceconfiguration of Sn is 4 d s p . There are two 5 p elec-trons of Sn which join in bonding, and two 5 s electronsleft to form a lone pair. As a result, Sn atom can forms p bonding with a lone pair of valence electrons. Inthe doped system Sn Se X Sn (X=Ga, In, As, Sb), Xdopant atom is neighborhood with Sn in the periodic ta-ble of elements, which can avoid of lattice distortion, giv-ing rise to the impurity states always appear either above valence band or below conduction band, or the middle ofband gap. To explore the underlying electronic and mag-netic properties of the doped monolayer Sn Se X Sn (X=Ga, In, As, Sb). We divide the related figures intotwo parts: spin up and spin down, which makes us tohave a clear insight to the substitutional doping systems.Fig. 4 (a) presents the split band structure of Ga-doped system. The impurity state is split into spin-upand spin-down cases, and the impurity energy level ap- FIG. 6: Spin charge density ( ρ up - ρ down ) of (a) Ga- and (b)In-doped system (dark blue ball denotes Ga atom, light pinkball denotes In atom),respectively. The value of isosurfaces is0.03 e/˚ A . pears either above valence band (spin-up) or below con-duction bands (spin-down). Furthermore, the band gapis 0.66 eV for spin-up and 0.43 eV for spin-down, indicat-ing semiconductor properties. The Eg values of spin-upand spin-down are both smaller than that of pure SnSemonolayer. Since Ga is the acceptor, leading to the bandgap decreasing both for spin-up and spin-down. WhenGa atom is doped, the doped system has a red shift,giving rise to a relatively wide application in electrondevices. However, for In-dpoed system, it is exceptivethat the VBM rises for spin-up, while the CBM goesdown cross the Fermi level for spin-down, which lead toa half-metal property, as shown in Fig. 4 (b). For theGa- and In-doped systems, because Ga and In belongto the same group IIIA, they present different proper-ties. The intriguing phenomenon can be explained bythe nearly equal electronegativity (Ga ∼ ∼ ∼ s -, 4 p -orbital of Gaand 4 p -orbital of Se play an important role for the VBMand CBM, and make the main contribution to the totalDOS. Similarly, the In-doped system presents the almostsame phenomenon: 5 s -, 5 p -orbital of In and 4 p -orbitalof Se play an important role for VBM, CBM, and thetotal DOS. Comparing to the semiconductor property ofGa-doped system, it is obvious that the 5 p -orbits of Inatom presents hybridization at the Fermi level. Conse-quently, the doping system presents a half-metallic prop-erty, which is consistent with the band structure. More-over, we know that the total DOS is up and down asym-metry nearby the Fermi level, indicating that Ga- and In-doped system present magnetism with the values about1.00 µ B and 0.99 µ B , respectively, which are close to theirfree atoms (Ga ∼ µ B ; In ∼ µ B ). Furthermore,from Fig. 5 (a), it is obvious that the majority of mag-netism is induced by Ga atom while Se atom contributesminority to the magnetism. But in In-doped system, the magnetism is mainly induced by the In dopant, as seenin Fig. 5 (b).In order to get a clear explanation of magnetism, thespin charge density of Ga- and In- doped systems arecalculated in Fig. 6. From Fig. 6 (a) and (b) we can ob-viously see that the magnetism is induced by the dopantGa atom and In atom, respectively. The phenomena arecoincide well with the results of DOS and PDOS. Andin the Ga-doped system, the majority of magnetism dis-tributes around the Ga atom while the minority scattersaround Se atom. Comparing to Ga-doped system, themagnetism is mainly induced by the dopant atom in In-doped system. The reason of the magnetism is that whenGa and In replaced Sn in SnSe monolayer, the system willexist a hole because of the formed covalent band. Hence,the hole will induce the magnetism. As a result, both Gaand In dopant atoms lead to an acceptor level which givesrise to p -type doping systems with band gap decreasing.In addition, we calculate the electronic band structureof As- and Sb-doped systems in Fig. 7. From Fig. 7we can see that the impurity level appears in valencebands leading to the VBM rise for spin-up, while in con-duction bands resulting in the CBM declining for spin-down. However, the impurity level does not cross theFermi level, so the system maintains the semiconductorproperty for As- and Sb-doped systems. In detail, forthe As-doped system, as seen in Fig. 7 (a), the bandgap is divided into two parts: 0.48 eV for spin-up and0.88 eV for spin-down. At the same time the band gapof Sb-doped system is 0.26 eV for spin-up and 0.84 eVfor spin-down, as seen in Fig. 7 (b). Otherwise, com-paring to the band gap of SnSe monolayer, we find thatthe value of Sb-doped system has a less change than thatof As-doped system. This phenomenon can be explainedby electronegativity (As ∼ ∼ ∼ p -orbitals ofAs makes the contribution to not only VBM and CBM,but also the total DOS. In other words, the main con-tribution is from 4 p -orbitals of the dopant As, which iscoincide with the explanation by band structure. Fur-thermore, there is a flat-level for the band structure inspin-up, leading to a large density of states in that region,as shown in PDOS of As. In addition, we find that thesystem is not symmetry at the Fermi level in total DOS,which exhibits the system is of magnetism. Obviously,the magnetism is mainly induced by As atom. Then wecalculated the magnetic value (1.00 µ B ), which is smallerthan its free atom (As ∼ µ B ). On the other hand, forSb-doped system, as seen in Fig. 8 (b), it shows up thesimilar properties to As-doped system that 4 p -orbitalsof Sb play an important role for VBM, CBM and totalDOS. The total DOS appears asymmetry at the Fermilevel, indicating the Sb-doped system is of magnetism. FIG. 7: Calculated band structures of (a) As- and (b) Sb-doped systems, along Γ-M-N-Γ direction. The energy zero representsthe Fermi level.FIG. 8: DOS and PDOS of (a) As- and (b) Sb-doped systems. The energy zero represents the Fermi level.
Obviously, the magnetism is mainly derived from the Sbatom, and the value of calculated magnetism is 1.00 µ B ,which is also smaller than its free atom (Sb ∼ µ B ).From Fig. 8, we obtain the same explanation with thatof the band structures. Finally, in order to explain themagnetic mechanism, we calculated the spin charge den-sity of As- and Sb-doped systems as shown in Fig. 9.From Fig. 9 (a) and (b), we can distinctly see thatthe magnetism is mainly induced by As atom and Sb atom for As- and Sb-doped systems, respectively. Forthe As-doped system, the magnetism mainly gathersaround the As atom, and minority scatters around Seatoms. Similarly, the magnetism is mainly induced bythe dopant atom in Sb-doped system. The phenomenaare in line with the results of DOS and PDOS. The reasonof the magnetism is that when As and Sb replaced Sn inSnSe monolayer, the system will exist an extra electron.Hence, the extra electron will induce the magnetism. As FIG. 9: Spin charge density( ρ up - ρ down ) of (a) As- and (b) Sb-doped system (red ball denotes As atom, brown ball denotesSb atom), respectively. The value of isosurface is 0.03 e/˚ A .TABLE I: Calculated structural and magnetic properties forsingle atoms doping on SnSe monolayer. Formation energy( E f ); the magnetic moments( µ ); minimum dopants-Se (D-Se)distance( d ).Dopant E f (eV/atom) µ ( µ B ) d (D-Se) (˚A)SnSe -4.13 0 2.81Ga -2.57 1.00 2.55In -2.54 0.99 2.79As -1.87 1.00 3.08Sb -1.91 1.00 3.17 a result, both As and Sb dopant atoms lead to an donorlevel resulting in n -type doped systems.Generally, the X-doped (X=Ga, In, As, Sb) systemswith either half-metal or semiconductor properties canbe explained by the sp bonding character of Sn atomswith a lone pair of valence electrons and their stronghybridizations with the sp orbitals of dopants. We canconclude that the doped systems Sn Se X Sn (X=Ga,In, As, Sb) are ferromagnetic states from the spin chargedensity. Considering our work, we are aware that theelectronic properties can be tuned by substitutional dop-ing.Finally, we calculate the formation energy of the sub-stitutional system in order to verify the stability of X-doped SnSe monolayer. The formation energy ( E f ) isdefined as E f = E dh - E d - E h , where E dh , E d , and E h are the total energies of the doped system Sn Se X Sn (X=Ga, In, As, Sb), the energy of monolayer Sn Se and single Sn atom and isolated X atoms, respectively.From our definition, the negative value of E f shows thata system is stable. The large absolute value of E f energymeans strong interaction between dopants and Sn. Asummary of the results is shown in Table 1. From Table1, we find that the formation energy of Ga-doped systemis minimum, which demonstrates Ga atom is easy to bedoped and the Ga-doped system is the most stable thanother three systems. In general, the doped systems canbe realized in experiment.
4. Conclusions
In summary, using first-principles calculations, wepresent the geometrical structure, electronic and mag-netic properties of monolayer SnSe substitutionallydoped by Ga, In, As, and Sb atoms, respectively. Wefind that the electron structure can be tuned by sub-stitutional doping. When Ga or In is doped, the sys-tem presents semiconductor and half-metal properties,respectively; while As or Sb substitute Sn, the systemsdisplay semiconductor properties. In Ga-doped system,due to the band gap declining, the system exhibits redshift, which makes the system to have a wide applica-tion in optoelectronic devises. When Sn atom is replacedby Ga atom or In atom, inducing a hole, so the systemsbelong to p -type doping; on the other hand, if As orSb replace Sn, the system has an unpaired electron, re-sulting n -type doping. No matter whether Ga and In asdopant atoms, or As and Sb, there is an unpaired electronwhich can induce about 1 µ B magnetism. Finally, com-paring to the formation energy of Sn Se X Sn (X=Ga,In, As, Sb), we find that Ga is easy to be doped and theGa-doped system of is the most stable, whereas the fourtypes of doped systems are thermodynamic stable. Theresults of substitutional doping make the application ofSnSe monolayer more extensive.
5. Acknowledgements
This work is supported by financial support fromthe National Basic Research Program of China(No.2012CB921300) and the National Natural ScienceFoundation of China (Grant Nos.11274280). ∗ e-mail address:[email protected] † e-mail address:[email protected] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y.Zhang, S.V. Dubonos, I.V. Grigorieva and A.A. Firsov,
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