- Email: [email protected]
Impact of side passivation on the electronic structures and optical properties of GeSe nanobelts
Accepted Manuscript Impact of Side Passivation on the Electronic Structures and Optical Properties of GeSe Nanobelts
Qiuyan Ma, Xi Zhang, Dingyu Yang, Gang Xiang PII:
S0749-6036(17)32917-8
DOI:
10.1016/j.spmi.2018.03.075
Reference:
YSPMI 5605
To appear in:
Superlattices and Microstructures
Received Date:
14 December 2017
Accepted Date:
28 March 2018
Please cite this article as: Qiuyan Ma, Xi Zhang, Dingyu Yang, Gang Xiang, Impact of Side Passivation on the Electronic Structures and Optical Properties of GeSe Nanobelts, Superlattices and Microstructures (2018), doi: 10.1016/j.spmi.2018.03.075
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Impact of Side Passivation on the Electronic Structures and Optical Properties of GeSe Nanobelts Qiuyan Ma1, Xi Zhang1*, Dingyu Yang2, Gang Xiang1,3,4* 1College
2College
of Physical Science and Technology, Sichuan University, Chengdu 610064, China of Optoelectronics Technology, Chengdu University of Information Technology,
Chengdu, 610225, China 3Key
Laboratory of High Energy Density Physics and Technology of Ministry of Education,
Sichuan University, Chengdu 610064, China 4Key
Laboratory for Radiation Physics and Technology of Ministry of Education, Sichuan
University, Chengdu 610064, China
Abstract The effects of side passivation with different functional groups (-H, -F and -OH) on the electronic and optical properties of GeSe nanobelts (NBs) are investigated using density functional theory. The optimized structures of GeSe NBs are calculated by full optimization method. It is found that with side passivation of the terminators, pristine metallic GeSe NBs can be tuned to be p-typed semiconductors. Further analysis shows that the side terminators causes the shift of electronic bands and the modification of density of states (DOSs) of band edges of GeSe NBs, resulting in different effective masses of charge carriers depending on the terminator types. The study of optical properties of GeSe NBs shows that the main absorption peaks are modulated by side passivation of the terminators, and quantum yields of side 1
ACCEPTED MANUSCRIPT passivated GeSe NBs are evidently bigger than those of pristine ones mainly because passivated ones have suitable band gap values. Our calculated results demonstrate that the electronic and optical properties of GeSe NBs can be effectively tuned by side passivation with functional groups. Key words: GeSe nanobelts; passivation; electronic structure; optical properties *Corresponding authors: [email protected], [email protected]
2
ACCEPTED MANUSCRIPT 1. Introduction Recently,
a
group
of
two-dimensional
(2D)
nanomaterials,
group-IV
monochalcogenides (SnS, SnSe, GeS and GeSe), have been attracting more and more attention because of their orthorhombic structures and anisotropic characteristics similar to those of phosphorene [1-3]. These 2D sheets are made of abundant and nontoxic elements, and have suitable band gap values (1.1–1.5 eV) which can be used for superior photovoltaic and thermoelectrical applications [4-6]. Among them, 2D GeSe sheets caught more attention, probably because GeSe bulk is the only direct band gap material among the group-IV monochalcogenide bulks. For instance, the tuning of structures and properties of the 2D GeSe sheets has been demonstrated by applied strain [6] or adsorption of atoms on the surface [7,8]. However, although GeSe nanobelts (NBs) have been synthesized experimentally using solution-based seedless growth by Vaughn et al. [9] and are important for nanoscale GeSe-based device design and fabrication, the study on the quasi one-dimensional GeSe NBs is rare. As we know, with reduction in dimension of materials, passivation becomes an inevitable issue for successful design and fabrication of low dimensional electronic and optoelectronic devices. In fact, passivation of nanostructured materials has been demonstrated to be an important technique to adjust the structures and properties of nanoscale materials [10-12]. Here in this work, we will systematically study the effect of side passivation on the electronic structures and optical properties of GeSe NBs. The typical functional groups, including hydrogen (-H), fluorine (-F) and hydroxyl (3
ACCEPTED MANUSCRIPT OH), has been chosen as the terminators of the side passivation on the GeSe NBs. 2. Calculation methods In this paper, the calculations have been performed by using first-principles within generalized gradient approximation (GGA) form, and using PW91 scheme within density functional theory (DFT) in the Vienna ab initio simulation package (VASP)[13,14]. The Brillouin zone (BZ) is performed using a 1 × 9 × 1 MonkhorstPack grid for relaxation calculation within project augmented wave potentials. All atomic positions are optimized by minimization of the total energy and atomic forces. For geometry optimization, a sufficiently wide vacuum space is used to guarantee that the distance of the outermost atoms of any 2 adjacent nanobelts was at least 24Å to avoid interactions. The kinetic cutoff energy for the plane waves 550 eV, respectively. And the convergence for force is set to be 0.01 eV/Å between atoms. The convergence for energy is chosen to be 10-5eV between two ionic steps. In order to obtain the most stable state, our first step is to optimize the coordinates of each atoms of the cell, namely structure optimization. Static calculation is followed to calculate self-consistent charge density, and then we can get the data of band structures, density of states (DOSs) and optical properties. 3. Results and discussion 3.1. Simulative models A GeSe monolayer is 2D layer which extends in an x-y plane. In this work, the 2D layer is terminated along the x direction by different dangling bonds, including -H, -F and -OH, and the terminated GeSe layer only grows along the y direction. In this 4
ACCEPTED MANUSCRIPT way a one-dimensional (1D) NB is made which is periodically repeated along the ydirection. To investigate the impact of side passivation on the electronic structures and optical properties of the GeSe NBs, x and z direction with lager enough vacuum layer for the unit cell and special k points along G-Y direction are generated for the calculations. GeSe NBs with different passivation bonds are shown in Fig.1 (a-c). Top and side views are on the upper and lower panels, respectively
（a）
(b)
(c)
Fig.1. GeSe NBs with different terminators. (a) -H, (b) -F and (c) -OH. Red, blue, green, yellow and white balls represent Ge, Se, F, O and H atoms, respectively.
3.2. Electronic structures and optical properties Although GeSe monolayer was found to be a direct semiconductor with a band gap of 1.16 eV [6], GeSe NBs have characteristics of metallic band structures, as shown in Fig.2 (a). Interestingly, with side passivation of the different terminators, GeSe NBs change from a metal into indirect semiconductors. The electronic band structures of GeSe NBs with different terminators are shown in Fig.2 and summarized in Table I. 5
ACCEPTED MANUSCRIPT Table I. Band gap characteristics of –H, -F and OH-saturated GeSe NBs. GeSeNBs
Eg(ev)
Band Gap Type
pristine
0
N/A
-H
1.4848
Indirect
-F
1.1508
Indirect
-OH
1.1564
Indirect
Conductivity
metal p typed semiconductor p typed semiconductor p typed semiconductor
2
（ a（
1
（ b（
（ c（
（ d（
0
-1
-2
Fig.2. Band structures of (a) pristine GeSe NBs and side passivated with (b) -H, (c) F, (d) -OH. The Fermi-level is set to be zero and shown by the dotted line.
It is found that the side passivation can modulate the electronic band structures of GeSe NBs effectively. On one hand, the dangling bonds of the terminators can open the band gaps of the pristine metallic GeSe NBs and result in typical semiconducting band gaps. The band gap values of GeSe NBs with side passivation of –F and –OH are around 1.15 eV, very close to that of Si (1.17 eV), while side passivation of -H 6
ACCEPTED MANUSCRIPT opens the band gap of GeSe NB more widely, and results in a band gap value of 1.48 eV, which is between those of InP (1.42 eV) and GaAs (1.52 eV). On the other hand, the terminators also result in complex shifts of the energy bands. For H-passivated and OH-passivated GeSe NBs, the projections of valence band maximum (VBM) in the Brillouin zone are all located at the Γ point, but conduction band minimum (CBM) moves from Γ point to Y point nearly ten percent, which make H-passivated and OHpassivated GeSe NBs indirect semiconductors. For F-passivated GeSe NBs, the projections of VBM is located at the Y point instead of the Γ point, and CBM moves from Y point to Γ point nearly fifteen percent, which also presents indirect band gap characteristics. In order to clarify the contributions from different orbitals to the electronic structures around the Fermi level of GeSe NBs with the different dangling bonds, we have calculated the total and partial density of states (DOSs). Fig.3 (a), (d), (i), (n) are four total DOSs (TDOSs) and the Fermi-level is set to be 0. The DOSs of GeSe NBs are shown in the upper left panel (Fig.3 (a-c)). It is found that the valence band edge (VBE) and the conduction band edge (VBE) are both contributed by 4p orbits of Se and Ge. For the H-saturated case (Fig.3 (d)-(h)), the VBE is mainly contributed by 4p orbits of Ge and Se, while 4p orbits of Ge atoms contribute most to the CBE. Obviously, H atoms contribute slightly to band edges, and thus can be neglected. For the F-saturated case (Fig.3 (i)-(m)), we can see that the major component of VBE is 4p orbits of Ge atoms, and the CBE is mainly composed of 4p orbits of Se and 2p orbits of F which connects to Se. From the results of OH-saturated GeSe NBs shown 7
ACCEPTED MANUSCRIPT in the lower right panel (Figs.3 (n)–(r)), 4p orbits of Ge and Se contribute most to VBE, while CBE is mainly composed of 4p orbits of Se and 2p orbits of O which connects to Se. Therefore, in terms of the impact on the DOSs of the band edges, the incorporation of -F and -OH bonds have much bigger contributions than that of -H bonds. 16 (a)
0 0.8
0.0
TDOS 16 (d)
TDOS
0 0.8 (e)
Se4s Se4p Se3d
(b)
Se4s 0.0 Se4p 0.4 Se3d
0.0 0.1 (g)
(c)
0.4
Se4s Se4p 0.0 Se3d 0.2
0.0
0.0
TDOS 16 (n)
0 (j) 1.5
Se4s 0 (o) 0.6 Se4p Se3d 0.0 Ge4s (p) 0.5 Ge4p Ge3d 0.0 F2S 0.8 (q) F2P 0.0 F2S 1.1 (r) F2P 0.0
1.3
(k)
0.0 (l) 1.5 0.0 (m) 1.5 0.0
-4
0
H1s
(h)
16 (i)
0.0
Ge4s Ge4p Ge3d
(f)
H1s
TDOS Se4s Se4p Se3d Ge4s Ge4p Ge3d O2s O2p H1s O2s O2p H1s
-4
4
0
4
Fig.3. DOSs for (a-c) GeSe NBs and GeSe NBs with different terminators (d-h) –H, (i-m) –F and (n-r) –OH. ∗ The effective mass 𝑚 of charge carrier is a significant parameter which is
inextricably linked with transport properties of semiconductor devices. It can be ∗
2
2
2
computed through the equation: 1/𝑚 = （1/ħ ）（∂ 𝐸/∂𝑘 ）[13] where k is the coordinate vector in the reciprocal space, E is the energy band and ħ is the reduced Planck constant. Here we estimated the electron effective mass 𝑚𝑒 near the 8
ACCEPTED MANUSCRIPT conduction band minimum, and the hole effective mass 𝑚ℎ near the valence band maximum for the three semiconducting GeSe NBs with the side terminators. The results are shown in Fig.4 and summarized in Table II. For F-saturated NBs, the effective mass of electrons is larger than those of H-saturated and OH-saturated NBs, while the effective mass of holes is the smallest. The robust modulation of the effective masses of electrons and holes by the terminators demonstrates the possibility of designing the microelectronic devices of GeSe NBs via side passivation. Table II. The effective masses of charge carriers of –H, -F and OH-saturated NBs.
𝒎𝒆 (/m0)
𝒎𝒉(/m0)
-H
0.2211
0.6598
-F
1.8457
0.4062
-OH
0.2639
0.6361
GeSeNBs
effective mass(/m0)
1.6
0.8
mh me
0.8
0.4
0.0
0.0
H
F
OH
Fig.4. The effective masses (in units of the free electron mass ??0) of charge carriers near band edges of –H, -F and OH-saturated GeSe NBs. Next, we show optical absorption spectra of GeSe NBs with different terminators. 9
ACCEPTED MANUSCRIPT The optical dielectric function of medium is described by plural ε(ω) = ε1(ω)+ iε2(ω). The dielectric function consists of two parts: the real part ε1(ω) and the imaginary part ε2(ω). The imaginary part ε2 describes the optical absorption can be divided into two components ε2//(ω) and ε2⊥(ω). The two components (ε2//(ω), ε2⊥(ω)) correspond to the polarizations along the wire direction and perpendicular to the wire direction, respectively. In this paper, we employ random-phase approximation (RPA) to calculate the absorption spectrum, neglecting local field effects, excitonic effects and self-energy correction which have very small impact on the parallel polarization component of sorption spectra [14]. As shown in the Fig.4 (a), the absorption edge of GeSe NBs is sharp: the spectrum is peaked around 6.69 eV with a high intensity. Compared with pristine GeSe NBs, the main peak of H- saturated GeSe NBs and Fsaturated GeSe NBs red shifts to 6.57 eV and 6.50 eV, respectively. The peak intensity of H-GeSe NBs is slightly smaller than that of GeSe NBs, but that of F-GeSe NBs is reduced by a large margin. The main peak of OH-GeSe NBs blue shifts to 6.83 eV, and the intensity of the peak is also lower than that of GeSe NBs. We further estimate quantum yield (QY) of the GeSe NBs with different terminators. QY refers to the utilization rate of light quantum, which is an important parameter for light sensitivity of semiconductors and can be estimated as ∝ωS(ω)
2∥ [15], where ω is light quantum frequency, S(ω) is the curve of solar energy flow changing with the photon wavelengths, and 2∥ is the polarization parallel to the NB axis. Since the numerical value of ωS(ω) 2∥ is large, a proportional reduction method is used to get the relative value, as shown in the grey area in Fig.5. The calculation 10
ACCEPTED MANUSCRIPT shows that QYs of pristine GeSe NBs, H-GeSe NBs, F-GeSe NBs and OH-GeSe NBs are 1.227, 1.3223, 1.364 and 1.47, respectively. Obviously, these side passivated GeSe NBs have evidently bigger QYs than that of pristine ones, probably because the side passivated GeSe NBs are semiconductors with suitable band gaps for optical absorption while the pristine ones are metals which usually don’t have superb absorption properties. As a result, the side-passivated GeSe NBs may have potentials for design and application of optoelectronic nano-devices in the future. 2
（a）
2∥
2(arb.untis)
2
2(arb.untis)
2
1
0
5
10
Photon Energy/eV （c）
2∥
0.4 2
5
10
Photon Energy/eV
5
10
（d）
0.0 0 0 15
Photon Energy/eV
0.0 15
2 2∥
0.2 1
0
0.4
0.2
0.0 0 0 15
2
1
0
2∥
0.4 2
1
0
2
（b）
0.4
0.2
5
10
0.0 15
Photon Energy/eV
Fig.5. Imaginary dielectric function of GeSe NBs with different terminators: (a) pristine GeSe NBs, (b) H-GeSe NBs (c) F-GeSe NBs (d) OH-GeSe NBs. The red line represents the polarization perpendicular to the wire axis. and the blue line represents the polarization parallel to the wire axis. The grey area represents quantum yield. 4. Conclusion 11
ACCEPTED MANUSCRIPT In conclusion, we have systematically studied the electronic structure and optical properties of GeSe NBs side passivated with typical -H, -F and –OH terminators by using first-principle calculations. The results show that the terminators can open the band gap of pristine metallic GeSe NBs, shift the bands and affect the band edges, resulting in semiconducting NBs with different effective masses of carriers depending on the types of terminators. The study of optical absorption shows that the terminators can also modulate the optical properties evidently. Our results show at the side passivation can serve as an effective way to tune the electronic and optical properties of GeSe NBs, which may have potential applications in electrical and optoelectrical nano-devices. Acknowledgements This work was supported by the Natural Science Foundation of China (NSFC) through Grant No. 51672179.
References 12
ACCEPTED MANUSCRIPT [1] J. Qiao, X. Kong, Z. X. Hu, F. Yang and W. Ji, Nat. Commun. 5 (2014) 4475. [2] Lídia C. Gomes and A. Carvalho, Phys. Rev. B 92 (2015) 085406. [3] J. S. Lee, P. Ramasamy, D. Kwak, D. H. Lim and H. S. Ra, J. Mater. Chem. C 4 (2016) 479-485. [4] A. K. Singh and R. G. Hennig, Appl. Phys. Lett. 105 (2014) 042103. [5] Z. Ma, B. Wang, L. Ou, Y. Zhang, X. Zhang and Z. Zhou, Nanotechnology 27 (2016) 415203. [6] Y. H. Hu, S. L. Zhang, S. F. Sun, M. Q. Xie, B. Cai, and H. B Zeng, Appl. Phys. Lett. 107 (2015) 404-377. Y. Hu, S. Zhang, S. Sun, M. Xie, B. Cai and H. Zeng, Appl. Phys. Lett. 107 (2015) 122107. [7] Z. Li, M. Liu, Q. Chen, Y. Huang , C. Cao and Y. He, Superlattice. Microst. 109 (2017) 829. [8] Y. Ding and Y. Wang, Phys. Chem. Chem. Phys. 18 (2016) 23080. [9] I. D. Vaughn, D. Sun, S. M. Levin, A. J. Biacchi, T. S. Mayer and R. E. Schaak, Chem. Mater 24 (2012) 3643-3649. [10] M. Nolan, Sean O’Callaghan, G. Fagas and J. C. Greer, Nano Lett.7 (2007) 34. [11] K. Zhuo and M. Y. Chou, J. Phys.: Condens. Matter 25 (2013) 145501. [12] A. Sanchez-Soares, C. O'Donnell and J. C. Greer, Phys. Rev. B 94 (2016) 23. [13] M. Z. Gao, S. Y. You and Y. Wang, J. Appl. Phys. 47 (2008) 3303. [14] M. Bruno, M. Palummo, A. Marini, R. D. Sole, V. Olevano, A.N. Kholod and S. Ossicini, Phys. Rev. B 72 (2005) 153310. 13
ACCEPTED MANUSCRIPT [15] R. Pekoz, O. B. Malcoglu and J. Y. Raty, Phys. Rev. B 83 (2011) 035317.
14
ACCEPTED MANUSCRIPT
Highlights 1. Pristine metallic GeSe NBs can be designed to be p-typed semiconductors with side passivation of typical terminators (-H, -F and -OH). 2. Evident modulation of effective masses of charge carriers of GeSe NBs can be achieved by shift of electronic bands and modification of density of states (DOSs) of band edges caused by side passivation. 3. Side-passivated GeSe NBs have bigger quantum yields than pristine ones because passivated ones have suitable band gap values (1.1 – 1.5 eV).
1
Qiuyan Ma, Xi Zhang, Dingyu Yang, Gang Xiang PII:
S0749-6036(17)32917-8
DOI:
10.1016/j.spmi.2018.03.075
Reference:
YSPMI 5605
To appear in:
Superlattices and Microstructures
Received Date:
14 December 2017
Accepted Date:
28 March 2018
Please cite this article as: Qiuyan Ma, Xi Zhang, Dingyu Yang, Gang Xiang, Impact of Side Passivation on the Electronic Structures and Optical Properties of GeSe Nanobelts, Superlattices and Microstructures (2018), doi: 10.1016/j.spmi.2018.03.075
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Impact of Side Passivation on the Electronic Structures and Optical Properties of GeSe Nanobelts Qiuyan Ma1, Xi Zhang1*, Dingyu Yang2, Gang Xiang1,3,4* 1College
2College
of Physical Science and Technology, Sichuan University, Chengdu 610064, China of Optoelectronics Technology, Chengdu University of Information Technology,
Chengdu, 610225, China 3Key
Laboratory of High Energy Density Physics and Technology of Ministry of Education,
Sichuan University, Chengdu 610064, China 4Key
Laboratory for Radiation Physics and Technology of Ministry of Education, Sichuan
University, Chengdu 610064, China
Abstract The effects of side passivation with different functional groups (-H, -F and -OH) on the electronic and optical properties of GeSe nanobelts (NBs) are investigated using density functional theory. The optimized structures of GeSe NBs are calculated by full optimization method. It is found that with side passivation of the terminators, pristine metallic GeSe NBs can be tuned to be p-typed semiconductors. Further analysis shows that the side terminators causes the shift of electronic bands and the modification of density of states (DOSs) of band edges of GeSe NBs, resulting in different effective masses of charge carriers depending on the terminator types. The study of optical properties of GeSe NBs shows that the main absorption peaks are modulated by side passivation of the terminators, and quantum yields of side 1
ACCEPTED MANUSCRIPT passivated GeSe NBs are evidently bigger than those of pristine ones mainly because passivated ones have suitable band gap values. Our calculated results demonstrate that the electronic and optical properties of GeSe NBs can be effectively tuned by side passivation with functional groups. Key words: GeSe nanobelts; passivation; electronic structure; optical properties *Corresponding authors: [email protected], [email protected]
2
ACCEPTED MANUSCRIPT 1. Introduction Recently,
a
group
of
two-dimensional
(2D)
nanomaterials,
group-IV
monochalcogenides (SnS, SnSe, GeS and GeSe), have been attracting more and more attention because of their orthorhombic structures and anisotropic characteristics similar to those of phosphorene [1-3]. These 2D sheets are made of abundant and nontoxic elements, and have suitable band gap values (1.1–1.5 eV) which can be used for superior photovoltaic and thermoelectrical applications [4-6]. Among them, 2D GeSe sheets caught more attention, probably because GeSe bulk is the only direct band gap material among the group-IV monochalcogenide bulks. For instance, the tuning of structures and properties of the 2D GeSe sheets has been demonstrated by applied strain [6] or adsorption of atoms on the surface [7,8]. However, although GeSe nanobelts (NBs) have been synthesized experimentally using solution-based seedless growth by Vaughn et al. [9] and are important for nanoscale GeSe-based device design and fabrication, the study on the quasi one-dimensional GeSe NBs is rare. As we know, with reduction in dimension of materials, passivation becomes an inevitable issue for successful design and fabrication of low dimensional electronic and optoelectronic devices. In fact, passivation of nanostructured materials has been demonstrated to be an important technique to adjust the structures and properties of nanoscale materials [10-12]. Here in this work, we will systematically study the effect of side passivation on the electronic structures and optical properties of GeSe NBs. The typical functional groups, including hydrogen (-H), fluorine (-F) and hydroxyl (3
ACCEPTED MANUSCRIPT OH), has been chosen as the terminators of the side passivation on the GeSe NBs. 2. Calculation methods In this paper, the calculations have been performed by using first-principles within generalized gradient approximation (GGA) form, and using PW91 scheme within density functional theory (DFT) in the Vienna ab initio simulation package (VASP)[13,14]. The Brillouin zone (BZ) is performed using a 1 × 9 × 1 MonkhorstPack grid for relaxation calculation within project augmented wave potentials. All atomic positions are optimized by minimization of the total energy and atomic forces. For geometry optimization, a sufficiently wide vacuum space is used to guarantee that the distance of the outermost atoms of any 2 adjacent nanobelts was at least 24Å to avoid interactions. The kinetic cutoff energy for the plane waves 550 eV, respectively. And the convergence for force is set to be 0.01 eV/Å between atoms. The convergence for energy is chosen to be 10-5eV between two ionic steps. In order to obtain the most stable state, our first step is to optimize the coordinates of each atoms of the cell, namely structure optimization. Static calculation is followed to calculate self-consistent charge density, and then we can get the data of band structures, density of states (DOSs) and optical properties. 3. Results and discussion 3.1. Simulative models A GeSe monolayer is 2D layer which extends in an x-y plane. In this work, the 2D layer is terminated along the x direction by different dangling bonds, including -H, -F and -OH, and the terminated GeSe layer only grows along the y direction. In this 4
ACCEPTED MANUSCRIPT way a one-dimensional (1D) NB is made which is periodically repeated along the ydirection. To investigate the impact of side passivation on the electronic structures and optical properties of the GeSe NBs, x and z direction with lager enough vacuum layer for the unit cell and special k points along G-Y direction are generated for the calculations. GeSe NBs with different passivation bonds are shown in Fig.1 (a-c). Top and side views are on the upper and lower panels, respectively
（a）
(b)
(c)
Fig.1. GeSe NBs with different terminators. (a) -H, (b) -F and (c) -OH. Red, blue, green, yellow and white balls represent Ge, Se, F, O and H atoms, respectively.
3.2. Electronic structures and optical properties Although GeSe monolayer was found to be a direct semiconductor with a band gap of 1.16 eV [6], GeSe NBs have characteristics of metallic band structures, as shown in Fig.2 (a). Interestingly, with side passivation of the different terminators, GeSe NBs change from a metal into indirect semiconductors. The electronic band structures of GeSe NBs with different terminators are shown in Fig.2 and summarized in Table I. 5
ACCEPTED MANUSCRIPT Table I. Band gap characteristics of –H, -F and OH-saturated GeSe NBs. GeSeNBs
Eg(ev)
Band Gap Type
pristine
0
N/A
-H
1.4848
Indirect
-F
1.1508
Indirect
-OH
1.1564
Indirect
Conductivity
metal p typed semiconductor p typed semiconductor p typed semiconductor
2
（ a（
1
（ b（
（ c（
（ d（
0
-1
-2
Fig.2. Band structures of (a) pristine GeSe NBs and side passivated with (b) -H, (c) F, (d) -OH. The Fermi-level is set to be zero and shown by the dotted line.
It is found that the side passivation can modulate the electronic band structures of GeSe NBs effectively. On one hand, the dangling bonds of the terminators can open the band gaps of the pristine metallic GeSe NBs and result in typical semiconducting band gaps. The band gap values of GeSe NBs with side passivation of –F and –OH are around 1.15 eV, very close to that of Si (1.17 eV), while side passivation of -H 6
ACCEPTED MANUSCRIPT opens the band gap of GeSe NB more widely, and results in a band gap value of 1.48 eV, which is between those of InP (1.42 eV) and GaAs (1.52 eV). On the other hand, the terminators also result in complex shifts of the energy bands. For H-passivated and OH-passivated GeSe NBs, the projections of valence band maximum (VBM) in the Brillouin zone are all located at the Γ point, but conduction band minimum (CBM) moves from Γ point to Y point nearly ten percent, which make H-passivated and OHpassivated GeSe NBs indirect semiconductors. For F-passivated GeSe NBs, the projections of VBM is located at the Y point instead of the Γ point, and CBM moves from Y point to Γ point nearly fifteen percent, which also presents indirect band gap characteristics. In order to clarify the contributions from different orbitals to the electronic structures around the Fermi level of GeSe NBs with the different dangling bonds, we have calculated the total and partial density of states (DOSs). Fig.3 (a), (d), (i), (n) are four total DOSs (TDOSs) and the Fermi-level is set to be 0. The DOSs of GeSe NBs are shown in the upper left panel (Fig.3 (a-c)). It is found that the valence band edge (VBE) and the conduction band edge (VBE) are both contributed by 4p orbits of Se and Ge. For the H-saturated case (Fig.3 (d)-(h)), the VBE is mainly contributed by 4p orbits of Ge and Se, while 4p orbits of Ge atoms contribute most to the CBE. Obviously, H atoms contribute slightly to band edges, and thus can be neglected. For the F-saturated case (Fig.3 (i)-(m)), we can see that the major component of VBE is 4p orbits of Ge atoms, and the CBE is mainly composed of 4p orbits of Se and 2p orbits of F which connects to Se. From the results of OH-saturated GeSe NBs shown 7
ACCEPTED MANUSCRIPT in the lower right panel (Figs.3 (n)–(r)), 4p orbits of Ge and Se contribute most to VBE, while CBE is mainly composed of 4p orbits of Se and 2p orbits of O which connects to Se. Therefore, in terms of the impact on the DOSs of the band edges, the incorporation of -F and -OH bonds have much bigger contributions than that of -H bonds. 16 (a)
0 0.8
0.0
TDOS 16 (d)
TDOS
0 0.8 (e)
Se4s Se4p Se3d
(b)
Se4s 0.0 Se4p 0.4 Se3d
0.0 0.1 (g)
(c)
0.4
Se4s Se4p 0.0 Se3d 0.2
0.0
0.0
TDOS 16 (n)
0 (j) 1.5
Se4s 0 (o) 0.6 Se4p Se3d 0.0 Ge4s (p) 0.5 Ge4p Ge3d 0.0 F2S 0.8 (q) F2P 0.0 F2S 1.1 (r) F2P 0.0
1.3
(k)
0.0 (l) 1.5 0.0 (m) 1.5 0.0
-4
0
H1s
(h)
16 (i)
0.0
Ge4s Ge4p Ge3d
(f)
H1s
TDOS Se4s Se4p Se3d Ge4s Ge4p Ge3d O2s O2p H1s O2s O2p H1s
-4
4
0
4
Fig.3. DOSs for (a-c) GeSe NBs and GeSe NBs with different terminators (d-h) –H, (i-m) –F and (n-r) –OH. ∗ The effective mass 𝑚 of charge carrier is a significant parameter which is
inextricably linked with transport properties of semiconductor devices. It can be ∗
2
2
2
computed through the equation: 1/𝑚 = （1/ħ ）（∂ 𝐸/∂𝑘 ）[13] where k is the coordinate vector in the reciprocal space, E is the energy band and ħ is the reduced Planck constant. Here we estimated the electron effective mass 𝑚𝑒 near the 8
ACCEPTED MANUSCRIPT conduction band minimum, and the hole effective mass 𝑚ℎ near the valence band maximum for the three semiconducting GeSe NBs with the side terminators. The results are shown in Fig.4 and summarized in Table II. For F-saturated NBs, the effective mass of electrons is larger than those of H-saturated and OH-saturated NBs, while the effective mass of holes is the smallest. The robust modulation of the effective masses of electrons and holes by the terminators demonstrates the possibility of designing the microelectronic devices of GeSe NBs via side passivation. Table II. The effective masses of charge carriers of –H, -F and OH-saturated NBs.
𝒎𝒆 (/m0)
𝒎𝒉(/m0)
-H
0.2211
0.6598
-F
1.8457
0.4062
-OH
0.2639
0.6361
GeSeNBs
effective mass(/m0)
1.6
0.8
mh me
0.8
0.4
0.0
0.0
H
F
OH
Fig.4. The effective masses (in units of the free electron mass ??0) of charge carriers near band edges of –H, -F and OH-saturated GeSe NBs. Next, we show optical absorption spectra of GeSe NBs with different terminators. 9
ACCEPTED MANUSCRIPT The optical dielectric function of medium is described by plural ε(ω) = ε1(ω)+ iε2(ω). The dielectric function consists of two parts: the real part ε1(ω) and the imaginary part ε2(ω). The imaginary part ε2 describes the optical absorption can be divided into two components ε2//(ω) and ε2⊥(ω). The two components (ε2//(ω), ε2⊥(ω)) correspond to the polarizations along the wire direction and perpendicular to the wire direction, respectively. In this paper, we employ random-phase approximation (RPA) to calculate the absorption spectrum, neglecting local field effects, excitonic effects and self-energy correction which have very small impact on the parallel polarization component of sorption spectra [14]. As shown in the Fig.4 (a), the absorption edge of GeSe NBs is sharp: the spectrum is peaked around 6.69 eV with a high intensity. Compared with pristine GeSe NBs, the main peak of H- saturated GeSe NBs and Fsaturated GeSe NBs red shifts to 6.57 eV and 6.50 eV, respectively. The peak intensity of H-GeSe NBs is slightly smaller than that of GeSe NBs, but that of F-GeSe NBs is reduced by a large margin. The main peak of OH-GeSe NBs blue shifts to 6.83 eV, and the intensity of the peak is also lower than that of GeSe NBs. We further estimate quantum yield (QY) of the GeSe NBs with different terminators. QY refers to the utilization rate of light quantum, which is an important parameter for light sensitivity of semiconductors and can be estimated as ∝ωS(ω)
2∥ [15], where ω is light quantum frequency, S(ω) is the curve of solar energy flow changing with the photon wavelengths, and 2∥ is the polarization parallel to the NB axis. Since the numerical value of ωS(ω) 2∥ is large, a proportional reduction method is used to get the relative value, as shown in the grey area in Fig.5. The calculation 10
ACCEPTED MANUSCRIPT shows that QYs of pristine GeSe NBs, H-GeSe NBs, F-GeSe NBs and OH-GeSe NBs are 1.227, 1.3223, 1.364 and 1.47, respectively. Obviously, these side passivated GeSe NBs have evidently bigger QYs than that of pristine ones, probably because the side passivated GeSe NBs are semiconductors with suitable band gaps for optical absorption while the pristine ones are metals which usually don’t have superb absorption properties. As a result, the side-passivated GeSe NBs may have potentials for design and application of optoelectronic nano-devices in the future. 2
（a）
2∥
2(arb.untis)
2
2(arb.untis)
2
1
0
5
10
Photon Energy/eV （c）
2∥
0.4 2
5
10
Photon Energy/eV
5
10
（d）
0.0 0 0 15
Photon Energy/eV
0.0 15
2 2∥
0.2 1
0
0.4
0.2
0.0 0 0 15
2
1
0
2∥
0.4 2
1
0
2
（b）
0.4
0.2
5
10
0.0 15
Photon Energy/eV
Fig.5. Imaginary dielectric function of GeSe NBs with different terminators: (a) pristine GeSe NBs, (b) H-GeSe NBs (c) F-GeSe NBs (d) OH-GeSe NBs. The red line represents the polarization perpendicular to the wire axis. and the blue line represents the polarization parallel to the wire axis. The grey area represents quantum yield. 4. Conclusion 11
ACCEPTED MANUSCRIPT In conclusion, we have systematically studied the electronic structure and optical properties of GeSe NBs side passivated with typical -H, -F and –OH terminators by using first-principle calculations. The results show that the terminators can open the band gap of pristine metallic GeSe NBs, shift the bands and affect the band edges, resulting in semiconducting NBs with different effective masses of carriers depending on the types of terminators. The study of optical absorption shows that the terminators can also modulate the optical properties evidently. Our results show at the side passivation can serve as an effective way to tune the electronic and optical properties of GeSe NBs, which may have potential applications in electrical and optoelectrical nano-devices. Acknowledgements This work was supported by the Natural Science Foundation of China (NSFC) through Grant No. 51672179.
References 12
ACCEPTED MANUSCRIPT [1] J. Qiao, X. Kong, Z. X. Hu, F. Yang and W. Ji, Nat. Commun. 5 (2014) 4475. [2] Lídia C. Gomes and A. Carvalho, Phys. Rev. B 92 (2015) 085406. [3] J. S. Lee, P. Ramasamy, D. Kwak, D. H. Lim and H. S. Ra, J. Mater. Chem. C 4 (2016) 479-485. [4] A. K. Singh and R. G. Hennig, Appl. Phys. Lett. 105 (2014) 042103. [5] Z. Ma, B. Wang, L. Ou, Y. Zhang, X. Zhang and Z. Zhou, Nanotechnology 27 (2016) 415203. [6] Y. H. Hu, S. L. Zhang, S. F. Sun, M. Q. Xie, B. Cai, and H. B Zeng, Appl. Phys. Lett. 107 (2015) 404-377. Y. Hu, S. Zhang, S. Sun, M. Xie, B. Cai and H. Zeng, Appl. Phys. Lett. 107 (2015) 122107. [7] Z. Li, M. Liu, Q. Chen, Y. Huang , C. Cao and Y. He, Superlattice. Microst. 109 (2017) 829. [8] Y. Ding and Y. Wang, Phys. Chem. Chem. Phys. 18 (2016) 23080. [9] I. D. Vaughn, D. Sun, S. M. Levin, A. J. Biacchi, T. S. Mayer and R. E. Schaak, Chem. Mater 24 (2012) 3643-3649. [10] M. Nolan, Sean O’Callaghan, G. Fagas and J. C. Greer, Nano Lett.7 (2007) 34. [11] K. Zhuo and M. Y. Chou, J. Phys.: Condens. Matter 25 (2013) 145501. [12] A. Sanchez-Soares, C. O'Donnell and J. C. Greer, Phys. Rev. B 94 (2016) 23. [13] M. Z. Gao, S. Y. You and Y. Wang, J. Appl. Phys. 47 (2008) 3303. [14] M. Bruno, M. Palummo, A. Marini, R. D. Sole, V. Olevano, A.N. Kholod and S. Ossicini, Phys. Rev. B 72 (2005) 153310. 13
ACCEPTED MANUSCRIPT [15] R. Pekoz, O. B. Malcoglu and J. Y. Raty, Phys. Rev. B 83 (2011) 035317.
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ACCEPTED MANUSCRIPT
Highlights 1. Pristine metallic GeSe NBs can be designed to be p-typed semiconductors with side passivation of typical terminators (-H, -F and -OH). 2. Evident modulation of effective masses of charge carriers of GeSe NBs can be achieved by shift of electronic bands and modification of density of states (DOSs) of band edges caused by side passivation. 3. Side-passivated GeSe NBs have bigger quantum yields than pristine ones because passivated ones have suitable band gap values (1.1 – 1.5 eV).
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