Enhancement of monolayer SnSe light absorption by strain engineering: A DFT calculation

Chemical Physics 521 (2019) 5–13

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Enhancement of monolayer SnSe light absorption by strain engineering: A DFT calculation

T

Tuan V. Vua,b, Hien D. Tonga,b, Truong Khang Nguyena,b, Chuong V. Nguyenc, A.A. Lavrentyevd, O.Y. Khyzhune, B.V. Gabrelianf, Hai L. Luongg, Khang D. Phama,b, Phuc Toan Danga,b, ⁎ Dat D. Voa,b, a

Division of Computational Physics, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam Faculty of Electrical & Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam c Department of Materials Science and Engineering, Le Quy Don Technical University, Ha Noi, Vietnam d Department of Electrical Engineering and Electronics, Don State Technical University, 1 Gagarin Square, 344010 Rostov-on-Don, Russian Federation e Frantsevych Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanivsky Street, UA-03232 Kyiv, Ukraine f Department of Computational Technique and Automated System Software, Don State Technical University, 1 Gagarin Square, 344010 Rostov-on-Don, Russian Federation g Department of Physics, Ho Chi Minh City University of Education, Ho Chi Minh City, Vietnam b

ARTICLE INFO

ABSTRACT

Keywords: Monolayer SnSe Strain Electronic band structure Optical properties First-principles

Strain effects on the electronic and optical properties of monolayer SnSe is studied by APW + lo method in DFT framework. The applied strains cause direct-indirect transition of SnSe band gap which is mainly constructed by s / p hybridization. The armchair ac and zigzag zz reduce the unstrained band gap of 1.05 eV down to 0 eV at 12% compression, but at 12% tension, the band gap decreases to 0.726–0.804 eV. The band gap always increases under biaxial strain b at 12% compression to 12% tension. We observe an enhancement of real 1 ( ) and imaginary 2 ( ) parts of dielectric function by 14%–30% of magnitude, wider peak distribution to infrared and ultra-violet regions, and appearance of new peaks in the 1 ( ) and 2 ( ) spectrums. As a consequence, the light absorption ( ) is significantly enhanced in the ultra-violet region and the absorption even starts at lower energy at infrared region.

1. Introduction Over the past few decades efforts have been made to resolve the excessive energy consumption problem, as the result, many renewable energy sources are discovered, supplying an important part for the rapidly increasing energy demand. Solar energy, despite its many advantages, donates very little part to the renewable energy, mainly because of the high cost and low efficiency of traditional energyconversion materials. Therefore, developments are focused on new energy-conversion materials and compounds, including thermoelectric materials PbTe [1,2], PbSe [3], GeTe [4], AgSnmSbTem + 2 [5], and their alloys [6]. The most popular photovoltaic material is silicon-based compounds with efficiency of about 29% [7]. Although polycrystalline semiconductor CdTe can absorb 90% of solar spectrum, the conversion is as low as that of Si [8]. The durability is well enhanced with organic, hybrid, and composite materials [9,10]. The efficiency of a material to convert solar energy directly depends on the basic loss energy including

losses of sub-band gap and hot electrons. Therefore, the efficiency is limited under 40% for semiconductors and lower 43.9% for singlejunction Si solar cells [11]. Huge attention on the application of tin selenide (SnSe) in renewable energy raised since the work of Zhao et al. was published, in which the dimensionless figure of merit ZT was revealed to be 2.6 ± 0.3 at 923 K along the b axis and 2.3 along the c axis [12]. The nontoxic and earth-abundant components together with unique electronic and optical properties make SnSe useful in a wide range of application, including Na-Ion Batteries/Li-Ion Batteries [13–16], Phase-Change Memory Devices [17], ultra-thin Nano-sheet photodetector [18], optoelectronic and photovoltaic applications [19]. In searching for higher energy conversion efficiency of SnSe and SnSe-based compounds, researches have studied the effect of high pressure, temperature, substitution, doping, and strain on their electronic, magnetic, and thermoelectric properties [20–26]. The high charge carrier mobility of order 10 4 cm2 V 1 S 1 of SnSe is studied under biaxial strains [27–29].

Corresponding author at: Division of Computational Physics, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam. E-mail addresses: [email protected] (T.V. Vu), [email protected] (H.D. Tong), [email protected] (T.K. Nguyen), [email protected] (O.Y. Khyzhun), [email protected] (D.D. Vo). ⁎

https://doi.org/10.1016/j.chemphys.2019.01.017 Received 6 November 2018; Received in revised form 29 December 2018; Accepted 14 January 2019 Available online 17 January 2019 0301-0104/ © 2019 Elsevier B.V. All rights reserved.

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However, the light absorption enhancement by strains for SnSe has not been studied despite the fact that SnSe is considered for optical, electronic, and solar cell application in many studies [13–16,30]. More recently, tunable band-gap materials are shown to significantly widen the light absorption range of solar cell devices, especially the absorption of infrared light [31–33] by which the efficiency of energy conversion is improved. The successful synthesis of SnSe monolayer or ultra-thin film are reported [34,18] with the band-gap greater than that of the bulk structure [35], and for single layer SnSe, the band gap can be transformed from indirect to direct and vice versa [28,36]. The tunable band gap of thin film or monolayer SnSe [37–39] suggest perspective application in better sun light absorption for solar energy conversion. One of the most popular method to modify the band gap of a monolayer is by strain engineering [21,26,29,40–42]. In this work, we use DFT calculation to study the effect of biaxial and uniaxial strains on the photo-electronic properties of monolayer SnSe, especially on the light absorption of this material. 2. Computational methods

Fig. 2. Relationship between total energy of monolayer SnSe and applied strain.

The effect of strains on the electronic and optical properties of monolayer SnSe is studied by applying APW + lo method [43] in DFT framework [44–46] as implemented in the WIEN2K simulation package [47]. The exchange–correlation interaction is described by generalized gradient approximation functional of Perdew-Burke-Ernzerh (GGAPBE) [48]. The Brillouin zone is sampled with 1000 k-points using tetrahedron method of Blöchl et al. [49]. The convergence threshold of self-consistent process is based on the difference in total energy from two consecutive iterations, which must be less than 0.0001 Ry. This value can guarantee the best balance between accuracy and computational cost. According to the convergence threshold, the wave function of current system is expanded using plane wave function for the atomic sphere with the orbital quantum number l max = 10 , and for the interMT k max = 7 , where stitial region with the k-point number such that R min MT R min is the smallest radius of the muffin-tin sphere [50], and kmax is the largest k-vector. The Fourier expansion of the charge density is limited by largest vector Gmax = 12 (a.u.) 1.

from 4s24p4 to 4s24p6 and Sn electronic configuration from 4d105s25p2 to 4d105s25p0 [55]. The Sn atom covalently bonds to three neighbors Se atoms resulting in 2D material with zigzag (ZZ) and armchair (AC) directions. Monolayer SnSe has lower symmetry space group Pmn21 than the space group Pmna of phosphorene. This origins from different elements constructing the compounds, in phosphorene [51] the two sublayers are parallel to each other, while group IV–VI monolayers they are not. The equilibrium monolayer SnSe (Fig. 1) is then modified by changing the lattice constants a, b, or both a and b corresponding to zigzag strain zz , armchair strain ac , and biaxial strain b , respectively. The strain rate is defined as = (a a0)/ a0 × 100%, where a0 , and a are the lattice constant of equilibrium and strained structures, respectively. The monolayer SnSe is set up under strains of different rate, ranging from −12% to 12% (Fig. 2). Under strains, the distance between two atoms becomes shorter or longer leading to more or less overlap of orbitals from different atoms, respectively. Because electrons are fermions, the orbital overlap must lead to the increasing of the number of bands, and as the result the total energy of the system increases. The reverse process happens when the distance between two atoms increases. The overall respond of the monolayer SnSe to applied strain can be expressed by total energy of the system. It can be seen that zz and ac cause similar effect on the total energy of the system. The difference in total energy of the system under these strains of −4% is just about 0.002 Ry, which is in good agreement with previous works on monolayers of black phosphorous and MoS2 [56–58]. Meanwhile, the biaxial strain b causes remarkable changing in total energy of the system. Biaxial strain of −4% causes a total energy of about 0.0086 Ry

3. Results and discussion 3.1. Effect of strain on the state of monolayer SnSe The monolayer structure of SnSe is initially set by a unit cell with vacuum region height over 20 Åto avoid spurious interaction. The lattice constants of the obtained unstrained structure are a = 4.298 Åand b = 4.372 Å, which show good agreement with previous studies [51–54]. Fig. 1 shows the orthorhombic crystal structure of monolayer SnSe, a member of IV–VI semiconductor group. In SnSe chalcogen, Se receives two electrons from Sn, turning the Se electronic configuration

Fig. 1. Top view (a) and side view (b) of the SnSe monolayer structure. Sn and Se atoms are denoted by grey and yellow balls respectively. 6

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Fig. 3. Electronic band structure (a), density of states (DOS) (b) of monolayer SnSe at equilibrium and Brillouin zone with high-symmetry points labeled (c).

higher than uniaxial strains. Its interesting that more change than the b -tension.

b -compression

The strain reduces the symmetry of monolayer SnSe leading to the change in the band structure of this system, this can be understood by considering the relationship between the symmetry operations and the crystal Hamiltonian [65]. Under the strain of different rates and directions, both CBM and VBM shift between Γ–X and Y–Γ paths. The value of the band gap also varies according to the fluctuation of the extrema in the valence and conduction bands as shown in form of DOS in Fig. 5. Fig. 6 shows the strain effect on these extrema by investigating the partial DOS of monolayer SnSe. The two extrema in the conduction band are mainly constructed by the hybridization of the Se s-state and the Sn p-state, in which the density of Se s-state remains almost unchanged under strain, while the density of Sn px / py / pz -states vary a lot under different kinds of strain. Meanwhile, the two extrema in the valence band are formed by major donation of Se p-state and Sn s-state. Strain affects not only the Se pstates but also Sn s-state, leading to the shift and energy variation of the valence band extrema. The compression from −4% to −8% causes the band gap to decrease from 0.641 to 0.174 eV zz , 0.839 to 0.359 eV ac , and 0.854 to 0.583 eV b , the tension from 4% to 8% leads to band gap decrease from 1.042 to 0.893 eV ( zz ), 0.890 to 0.781 eV ( ac ), but the biaxial tension b continues to increase the band gap from 1.211 to 1.303 eV. The monolayer tin selenide shows its asymmetry behavior under applied strain in Fig. 7. The band gap energy quickly drops from 1.05 eV of unstrained system to 0 eV under −12% zz and ac compressions, however, at 12% tension of ac and zz the gap just drops to 0.726 eV and 0.804 eV, respectively. It is interesting to notice that the ac only tends to narrow the band gap, meanwhile, the zz enables this gap to reach its maximum of 1.1 eV at 2% zigzag tension before descending slowlys. At the same time, the compression b also reduces the band gap but the tension b keeps enlarging the band gap when strain rate increases. The zero band gap at −12% compression indicates the semiconductor-metal transition phase of monolayer SnSe.

causes

3.2. Electronic property of monolayer SnSe under strain The electronic properties of SnSe at equilibrium is obtained and shown in (Fig. 3) for analyzing effect of strain on the electronic structure of this material. The band structure is calculated along the Γ–X–S–Y–Γ direction of the Brillouin zone which is shown in (Fig. 3c). The band structure of monolayer SnSe is characterized by one valence band maximum (VBM) on the Γ–X path and two conduction band minimums (CBM) on the Γ–X and Y–Γ paths (Fig. 3). This DFT result suggests that monolayer SnSe is a quasi-direct-band semiconductor with the band gap of about 0.9 eV, which is in good agreement with previous studies [18]. The shape of the band structure is interestingly like the “pudding mold” proposed by Arita and Kuroki [59] indicating high electrical conductivity of monolayer SnSe. The experimental band gap of SnSe sample with thickness 2–100 μm measured by b-polarization is 1.047 eV, and by a-polarization is 1.238 eV [60]. Meanwhile, the band gap obtained by GGA-PBE calculation for monolayer SnSe is 0.914 eV [61], 0.9 eV (indirect), 1.3 eV (direct) [62]. Our calculation, as previous GGA-PBE approximation, underestimates the band gap energy, however, for studying optical and electronic properties semiconductor the APW + lo methods show advantages and reliable results [52,63,64]. It can be seen on Fig. 3 that the Sn s-state is at low density in whole energy range of valence band. Through hybridization with Se p-state, Sn s-state reach its global peak at energy −2 to 0 eV. This hybridization forms the extrema in the valance band of the band gap structure in Fig. 4, including the VBM on the Γ–X path and the second highest extreme on the Y–Γ path. Fig. 3 also shows the main contribution of Sn pstate to the conduction band with the highest density peak at energy level from 2 to 4 eV. Meanwhile, the Se s-state is always at low concentration in the whole conduction band. However, Fig. 4 shows that the Sn p-state and Se s-state hybridization shifts the concentration of Sn p-state to lower energy level, at the same time, lifts the concentration of Se s-state to higher energy level in order to form two CBMs near 2 eV on the Γ–X and Y–Γ paths. It is necessary to notice that the band structure depends on the polarization of p-state, while z- and y-projects of p-state of both Sn and Se atoms are involved in the above mentioned extrema, the remained extrema are constructed by s-state and x-project of Sn/Se p-state. The variety of local extrema in the monolayer SnSe allows this material to respond in different way to many kinds of applied strain.

3.3. Optical properties of monolayer SnSe under strain The dielectric tensors and Kramers–Kronig relations [66] are used to achieve the real 1 ( ) and imaginary 2 ( ) parts of dielectric function whose spectra are shown in Fig. 7. The significant effects of axial and uniaxial strains on the electronic properties of monolayer SnSe inspire us to study the optical properties of this material under strains. We expect the real 1 ( ) and imaginary 2 ( ) components of the dielectric function ( ) to vary under strains as the narrower band gap may allow more inter-band transitions as well as the wider band gap prevents these transitions. 7

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Fig. 4. Weighted bands of monolayer SnSe at equilibrium.

= -8%

= -4%

= 4%

Fig. 5. Band structure of monolayer SnSe under ZZ-strain 8

zz

(a), AC-strain

ac

= 8%

(b), and biaxial strain

b

(c).

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Fig. 6. Total and partial density of states (DOS) of monolayer SnSe under −8% (a), −4% (b), 4% (c), and 8% (d).

1.6 1.4

Band gap (eV)

1.2

Under low rate 4% of zz , ac , and b strains, Fig. 8(b), the values of both real 1 ( ) and imaginary 2 ( ) components of dielectric function are slightly different from the ones of unstrained SnSe. For compression with high rate of −8% as shown in Fig. 8(a), the biaxial strain b causes a remarkable increment about 30% of 1 ( ) while ac causes a lift of 14% and zz tends to decrease this value. The respond of 2 ( ) to the same compression is different from 1 ( ), b and ac still increase the value of 2 ( ) but with the same rate of about 8%, lower value than 30% in the case of 1 ( ) . The maxima of 1 ( ) and 2 ( ) at the energy level of 2 eV are also shifted to lower energy levels. It is interesting to notice that both 1 ( ) and 2 ( ) are anisotropic to applied strains, in which the compression biaxial strain b results in more effect than the armchair or zigzag strains. At biaxial compression of −8%, the second highest peaks of 1 ( ) and 2 ( ) at 1.2 eV is raised by about 45% and shifted to lower energy level of about 0.6 eV. This motivates us to study the b effect in more detail as shown in Fig. 8(c), the results confirm that 1 ( ) is affected by compression strain more than 2 ( ) . We also observe that the tension lower the value of 2 ( ) more than the one of 1 ( ) which are about 15% and 9%, respectively. The absorption coefficient ( ) has close relationship with 2 ( ) which is affected by only strains of high rate, so we study the ( ) under −8% and 8% of strains. Under compression of −8% rate

ZZ-strain AC-strain Biaxial strain

1.0 0.8 0.6 0.4 0.2 0.0 -14 -12 -10 -8 -6 -4 -2

0

2

4

6

8 10 12 14

Strain (%) Fig. 7. Relationship between band gap of monolayer SnSe and the applied strain.

9

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Fig. 8. Real 1 ( ) and imaginary 2 ( ) components of dielectric function ( ) of monolayer SnSe under zigzag, armchair and biaxial strain of −8% (a), 4% (b), and biaxial strain of several rates (c).

Fig. 9(a), light absorption of SnSe is best enhanced by ac with high peaks of ∼6 × 105 cm−1 concentrating in the visual light region 2–3 eV. On the contrary, the zz strain lower the light absorption in this region but it start enhancing this value in the ultra-violet region, especially at energy level of about 6.5 eV. Unlike the other two strains, the b increase the absorption ( ) for wide range of energy level with nearly equal and scattering peaks. The tension of 8% show opposite pattern Fig. 9(b), in which all strains tends to decrease the light absorption of SnSe, the enhancement starts at high energy level 7 eV. Fig. 9(c) shows unique effect of biaxial strain b , the light absorption is lowered by tension while the compression not only increase the absorption but also tends to equalize the peaks. Under compression of rate higher than −8%, SnSe can absorb a wider range of electro-magnetic 70

(a)

α(ω) 104cm-1

60

wave-length. It is well-known that a wide absorption range, especially absorption in the infrared region, can improve the efficiency of a material to convert the solar energy into renewable energy [31,67]. The strain-driven absorption coefficient ( ) has a straight re2 k ij ( )

lationship to the extinction coefficient k ( ) by formula ij ( ) = c where c is velocity of light in vacuum, while the k ( ) itself depends on 1

1

ij the dielectric function as k ij ( ) = 2 [ 1ij ( )2 + 2ij ( )2 1 ( )] 2 [68]. In the visual light region 2–4 eV in Fig. 10, the k ( ) reaches its global peak of 2.4 at 2.5 eV, indicating the short-path penetration and high absorption of the visual light by SnSe. However, beyond the visual region, short-length wave can propagate through SnSe easier with nearly constant k ( ) of about 0.7, except for one low peak at 6.5 eV in the case

(b)

(c)

Biaxial strain

50 40 30

Unstrained

20

εzz = -8 %

0

1

2

3

4

5

Energy (eV) Fig. 9. Absorption coefficient

εb = 0 %

εac = 8 %

εb = -8 % 0

εb = +4 %

εzz = 8 %

εac = -8 %

10

εb = +8 %

Unstrained

εb = -4 %

εb = 8 % 6

7

8 0

1

2

3

4

εb = -8 % 5

Energy (eV)

6

7

8 0

1

2

3

4

5

6

7

8

Energy (eV)

( ) of monolayer SnSe under zigzag, armchair and biaxial strain of −8% (a), 8% (b), and biaxial strain of several rates (c). 10

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2.5

(a)

2.0 1.5

k(ω)

(b)

ZZ-strain εzz = +8 % εzz = +4 % εzz = 0 % εzz = -4 % εzz = -8 %

1.0

(c)

AC-strain εac = +8 % εac = +4 % εac = 0 % εac = -4 % εac = -8 %

Biaxial strain εb = +8 % εb = +4 % εb = 0 % εb = -4 % εb = -8 %

0.5 0.0

0

1

2

3

4

5

6

7

8 0

1

2

Energy (eV)

3

4

5

6

7

8 0

1

2

Energy (eV)

3

4

5

6

7

8

Energy (eV)

Fig. 10. Extinction coefficient k ( ) of monolayer SnSe under zigzag (a), armchair (b) and biaxial (c) strain of several rates.

ZZ-strain εzz = +8 % εzz = +4 % εzz = 0 % εzz = -4 % εzz = -8 %

4

n(ω)

3

2

AC-strain εac = +8 % εac = +4 % εac = 0 % εac = -4 % εac = -8 %

Biaxial strain εb = +8 % εb = +4 % εb = 0 % εb = -4 % εb = -8 %

1

0

(a) 0

1

(b) 2

3

4

5

6

7

8 0

1

(c) 2

Energy (eV)

3

4

5

6

Energy (eV)

7

8 0

1

2

3

4

5

6

7

8

Energy (eV)

Fig. 11. Refractive index n ( ) of monolayer SnSe under zigzag (a), armchair (b) and biaxial (c) strain of several rates.

of zigzag compression of −8% rate. Under all three strains, the extinction coefficient will be lower with tension, while compressions will slightly lift this value. It can be seen that −8% compression of b causes largest increase in the extinction coefficient of 12.5% at 2.5 eV. This raised interest of studying how light propagate in SnSe via the re-

the slightly lift by strains. Opposite to the case of absorption and reflectance coefficients, the energy loss mainly happens in the ultra-violet region, in which the armchair strain effect become less important than the zigzag and biaxial strain. The highest peaks in L ( ) spectra play important role in optical application as they relate to the plasma resonance, based on which we can also estimate the plasma frequency [69]. As we can see in Fig. 12(a), the zz tension of 8% enhance the L ( ) peak at 5.5 eV by about 50%, while the compression of −8% lower this peak by nearly 40%. Fig. 12(b) shows the same energy loss behavior of SnSe under b strain but the magnitude of the energy loss is tuned with lower rate. In both cases, we also observe the shift of energy loss to lower energy level with tension and to higher energy level with compression.

1

1

fractive index nij ( ) = 2 [ 1ij ( ) 2 + 2ij ( )2 + 1ij ( )] 2 [68], the result of which is shown in Fig. 11. The zz strain only results in narrow shift of the refractive index values from the static value about 2.3 of the unstrained SnSe monolayer as shown in Fig. 11(a). However, the static refractive index n (0) can be lowered by tension or increased by compression of ac and b strains, Fig. 11(a) and (c) respectively. Beyond the energy level of 1.5 eV, strains cause minor effect on the n ( ) of current material. Attention must be paid to the b strain effect in Fig. 11(c), at energy level lower than 1.5 eV, the compression −8% causes two n ( ) peaks of 3.8 and 4.0 at 0.8 eV and 1.4 eV, respectively. This indicates that the b strain leads to more interaction between the propagating photon and electron of SnSe at 0.8–1.4 eV. The efficiency of monolayer SnSe in absorbing light is also expressed via the reflectance Rij ( ) = energy loss Lij ( ) =

Im (

1)ij

=

(nij (nij

1)2 + k ij 1)2

ij 2( ) ij 2 + ij ( )2 ( ) 1 2

k

2

ij2

=

ij 1 +i ij 1 +i

2 ij 1 2 , ij 2 +1

4. Conclusion In conclusion, the APW + lo method is applied to study monolayer SnSe, the cut-off parameters are carefully chosen to reproduce reliable atomic structure in comparison with experimental data, based on which further study of the strain effects on the electronic and optical properties of this material is performed. Strains mainly affect Sn s /p states and Se p-state leading to the fluctuation of extrema on the valence and conduction bands. Monolayer SnSe shows anisotropic behavior to the applied strains, the band gap energy quickly drops from 1.05 eV of unstrained structure to 0 eV under zz and ac compressions of −12%, however, at 12% tension the gap just drop to about 0.7 eV. At the same time, the compression b also reduces the band gap but the tension b keeps enlarging the band gap when

and

which are shown in

Figs. 11(c), and 12(b), respectively. It’s important to notice that under compression of −8%, Fig. 9, the light absorption of monolayer SnSe is significantly improved, it reaches nearly the same value of ∼6 × 105 cm−1 for wide energy range from visual to ultra-violet. However, the reflectance mainly happens in visual light region despite 11

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Fig. 12. Reflectance R ( ) (a) and energy loss L ( ) (b) of monolayer SnSe under zigzag, armchair and biaxial strain of several rates.

strain rate increases. The real 1 ( ) and imaginary 2 ( ) parts of dielectric function show anisotropic behavior under compression or tension higher 8% rate. The maxima of 1 ( ) and 2 ( ) at the energy level of 0.7–1.5 eV and 1.5–2 eV respectively, are raised by 14% to 30% by zigzag and biaxial strains, respectively, these peaks are also shifted to lower energy levels, while the armchair strain results in not significant change in the dielectric function. The biaxial compression of −8% also induce the second highest peaks of 1 ( ) and 2 ( ) at 1.2 eV to increase by about 45% and shifted to lower energy level of about 0.6 eV. The tension of all three strains tends to decrease the absorption ability of SnSe. However, the compression of rate higher than 8% enhance the absorption, in which ac results in remarkable effect in the visual region 2–3 eV with ( ) of ∼6 × 105 cm−1, the zz strain causes a significant peak at 6.5 eV in the ultra-violet region, meanwhile the biaxial strain induces high peaks in ( ) spectra in both visual and ultra-violet regions. This result can lead to better photocurrent in solar cell devices. The higher absorption coefficient for wider range of wavelength of SnSe by strains can be considered in further studies concerning energy harvesting materials. The extinction coefficient and refraction index are affected by armchair and biaxial strains more than the zigzag strain. At energy from 0 to 2 eV, the refraction index is remarkably increased or decreased by different strains, indicating more photon electron interaction in this region. The reflectance is high in the energy region 1.5 to 4 eV and becomes significantly lower beyond 5 eV regardless to small change caused by the strains. On the contrary, the loss energy is low in the visual light, but it reaches the highest peak at about 5 eV in the ultra-violet region. The zz tension of 8% enhances the L ( ) peak at 5.5 eV by about 50%, while the compression of −8% lower this peak by nearly 40%. This opens potential application in optics and electronic because the highest

peaks in L ( ) spectra relate to plasma resonance and plasma frequency. Due to the fact that thin film SnSe is considered to be applied in many optical, electronic, and thermoelectric devices, there is high possibility of physical effects on the SnSe thin film. So detailed understanding on the relationship between strain and electronic/optical properties is important for avoiding unwanted effects, maintaining and developing the performance of the device. References [1] J.P. Heremans, V. Jovovic, E.S. Toberer, A. Saramat, K. Kurosaki, A. Charoenphakdee, S. Yamanaka, G.J. Snyder, Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states, Science 321 (5888) (2008) 554–557. [2] J. Androulakis, I. Todorov, D.-Y. Chung, S. Ballikaya, G. Wang, C. Uher, M. Kanatzidis, Thermoelectric enhancement in PbTe with K or Na codoping from tuning the interaction of the light- and heavy-hole valence bands, Phys. Rev. B 82 (11) (2010) 115209 . [3] D. Parker, D.J. Singh, High-temperature thermoelectric performance of heavily doped PbSe, Phys. Rev. B 82 (3) (2010) 035204 . [4] Y. Gelbstein, B. Dado, O. Ben-Yehuda, Y. Sadia, Z. Dashevsky, M.P. Dariel, Highly efficient Ge-Rich Ge_xPb_1-xTe thermoelectric alloys, J. Electron. Mater. 39 (9) (2010) 2049–2052. [5] M.-K. Han, J. Androulakis, S.-J. Kim, M.G. Kanatzidis, Lead-free thermoelectrics: High figure of merit in p-type AgSn_mSbTe_m+2, Adv. Energy Mater. 2 (1) (2011) 157–161. [6] T.C. Harman, P.J. Taylor, M.P. Walsh, B.E. LaForge, Quantum dot superlattice thermoelectric materials and devices, Science 297 (5590) (2002) 2229. [7] R. Swanson, Approaching the 29% limit efficiency of silicon solar cells, Thirty-First IEEE Photovoltaic Specialists Conference, 2005, pp. 889–894. [8] O. Mah, Fundamentals of Photovoltaic Materials, National Solar Power Research Institute Inc, 1998. [9] J. Chandrasekaran, D. Nithyaprakash, K.B. Ajjan, S. Maruthamuthu, D. Manoharan, S. Kumar, Hybrid solar cell based on blending of organic and inorganic materials – an overview, Renew. Sustain. Energy Rev. 15 (2) (2011) 1228–1238. [10] C.C. Sorrell, S. Sugihara, J. Nowotny, Materials for Energy Conversion Devices, Woodhead Publishing, 2005. [11] W. Shockley, H.J. Queisser, Detailed balance limit of efficiency of p-n junction solar

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