Second-order nonlinear optical properties of CuGaxIn1-xSe2 (x = 0.54): Experimental and theoretical investigations

Accepted Manuscript Second-order nonlinear optical properties of CuGaxIn1-xSe2 (x = 0.54): experimental and theoretical investigations

Zong-Dong Sun, Yang Chi, Sheng-Ping Guo PII:

S1293-2558(18)30822-7

DOI:

10.1016/j.solidstatesciences.2018.09.002

Reference:

SSSCIE 5751

To appear in:

Solid State Sciences

Received Date:

28 July 2018

Accepted Date:

04 September 2018

Please cite this article as: Zong-Dong Sun, Yang Chi, Sheng-Ping Guo, Second-order nonlinear optical properties of CuGaxIn1-xSe2 (x = 0.54): experimental and theoretical investigations, Solid

State Sciences (2018), doi: 10.1016/j.solidstatesciences.2018.09.002

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ACCEPTED MANUSCRIPT

Graphical Abstract Experimental and theoretical NLO studies of CuGa0.54In0.46Se2.

ACCEPTED MANUSCRIPT

Second-order nonlinear optical properties of CuGaxIn1-xSe2 (x = 0.54): experimental and theoretical investigations Zong-Dong Sun, Yang Chi and Sheng-Ping Guo* School of Chemistry & Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu 225002, P. R. China Corresponding author: [email protected]

Abstract

CuGaxIn1-xSe2 (x = 0.54, CGISe) has been obtained by a high temperature solid-state method. It crystallizes in the typical chalcopyrite structure. CGISe exhibits a second-harmonic generation intensity about 0.5 times that of AgGaS2 in the particle size range 75–100 μm. It`s band structure, density of state, and dielectric function are studied by theoretical calculations. The nonlinear optical properties of CGISe and several related chalcopyrite-type chalcogenides are discussed based on optical property and dipole moment calculations. Key words: CuGa0.54In0.46Se2, Solid-state reaction, Chalcopyrite structure, Second harmonic generation, Theory calculation

1. Introduction Recently, second-order nonlinear optical (NLO) materials attract more and more interests due to their application for laser frequency-conversion technology [1,2,3]. Currently, several commercial excellent NLO materials are mainly applied for near-infrared, visible, and ultraviolet regions, such as BBO, LBO, KDP, and KTP [4,5]. However, commercial NLO 1

ACCEPTED MANUSCRIPT materials in ultraviolet (DUV) and the mid and far-infrared (MFIR) regions are relatively rare [6,7], and the dominant ones are chalcopyrite structure AgGaQ2 (Q = S, Se) and ZnGeP2 [8,9,10]. They all exhibit good second harmonic generation (SHG) responses and wide IR transparency regions, however, their drawbacks limit their extensive applications, such as low laser induced damage thresholds (LIDTs), non-phase-matchability (NPM), or harmful twophoto absorption (TPA) [11,12]. Therefore, it is significant and challengeable to explore new MFIR NLO materials. As typical semiconductor materials, chalcopyrite structure compounds have been paid much attention. Such compounds are potential materials applied in the fields of NLO, photovoltaic solar cells, light emitting diodes (LED), photo-detectors (PDs) field, et al [13]. Ternary chalcopyrites crystallized in the space group I42d of 42m point group can be classified into two types: I-III-VI2 and II-IV-V2 (I = Li, Ag, Cu; II = Zn, Cd; III = Ga, In; IV = Si, Ge, Sn; V = P, As; VI = S, Se, Te) compounds. The representative NLO materials of I-III-VI2 structures are already commercial AgGaS2 (AGS) and AgGaSe2 (AGSe). In addition, LiMQ2 (M = Ga, In; Q = S, Se) and CuInQ2 (Q = S, Se, Te) also exhibit good SHG responses [14,15,16]. Commercial AGS exhibits large SHG response, wide IR transparency region, phase matchability, but too small band gap induces a low laser damage threshold. With the development of laser frequency conversion, application requirements impel the discovery of excellent NLO materials with large SHG coefficients, high LIDT, and phase matchability. Chalcopyrite-like compounds Li2BaMQ4 (M = Ge, Sn; Q = S, Se) are promising candidates [17]. Li2BaGeS4 and Li2BaSnS4 show large SHG intensities and high laser damage thresholds about 0.5 and 11, and 0.7 and 6.5 times those of AGS, respectively. What`s more, they are all phase matchability. A typical commercial NLO 2

ACCEPTED MANUSCRIPT material of II-IV-V2 is ZnGeP2. Recently, a lot of quaternary diamond-like (DL) chalcogenides are obtained, such as Li2CdMS4 (M = Ge, Sn) [18], Li2MnSnSe4 [19], Ag2ZnSiS4 [20], Cu2CdSnS4, and Cu2ZnSiS4 [21], which can be considered as the derivatives of I-III-VI2 compounds. Most of them exhibit nice SHG responses. In our previous work, some novel chalcogenides being SHG-active have been obtained [22,23,24,25,26,27,28,29]. These progresses push us continuously to discover novel chalcogenide-type NLO materials. In order to explore more DL-type chalcogenides, we accidentally obtained the CuGa0.54In0.46Se2 (CGISe) crystals. Because CGISe crystallizes in the NCS space group and hasn’t been studied for its NLO application, its NLO property is extensively explored here. The NLO performances’ differences between CGISe and several other highly related chalcopyrite-type compounds also are discussed by means of theoretical calculation.

2. Experimental

2.1 Synthesis and analyses

Single crystals of CGISe were obtained by a high temperature solid-state reaction with KI (98 %) as the flux. All the reaction reagents are purchased without further purification, which include elementary substance Cu (99.999 %), Ga (99.99 %), In (99.99 %), and Se (99.95 %). The reagents are mixed by a stoichiometric ratio. The total mass of reagents is 500 mg with additional 400 mg KI. The mixture was further ground into fine powder by using agate mortar and pressed into a pellet. Then the pellet was loaded into a quartz tube, which was evacuated to be 1× 10–4 torr and sealed using oxy-hydrogen flame. The tube was put into a muffle furnace, 3

ACCEPTED MANUSCRIPT then set the heating profile as follows. The temperature was heated to 1223 K with the speed of 60 K/h and several interim homogenization processes, then held 5 days at 1223 K, finally cooled down slowly to 573 K in 5 days. After washed with hot water and ethanol under ultrasonic wave, black block crystals of CGISe were obtained, which were air and moisture stable.

Fig. 1. Powder X-ray diffraction pattern of CGISe.

A semi-quantitative microscope elemental analysis of CGISe crystals was performed using a filed-emission scanning electron microscope (FESEM, Zeiss-Supra55) equipped with an energy dispersive X-ray spectroscope (EDS, Bruker, Quantax). The EDS result indicates the existence of Cu, Ga, In, and Se with almost a composition of CuGa0.5In0.5Se. The powder Xray diffraction (PXRD) pattern was collected with a Bruker D8 Advance diffractometer at 40 kV and 100 mA for Cu-Kα radiation (λ = 1.5406 Å) with a scan speed of 5°/min at room temperature. The experimental and simulated PXRD patterns match well (Fig. 1), indicating that the obtained sample is pure CGISe. 4

ACCEPTED MANUSCRIPT 2.2 Structure determination

The crystallography data of CGISe was collected by a Bruker D8 QUEST X-ray diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structure was solved by Direct Methods and refined by full-matrix least-squares techniques on F2 with anisotropic displacement parameters for all atmos. All the calculations were operated via the Shextl-2014 crystallographic software package [30]. The final refinement included a secondary extinction correction. The crystallographic parameter data is listed in Table 1. The atomic coordinates and equivalent isotropic displacement parameters are summarized in Table 2. The CIF document also has been deposited with Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen,

Germany

[fax:

(49)

7247-808-666;

[email protected]] with depository number CSD-434510.

Table 1.

Crystal data and structure refinement parameters for CGISe.

Chemical formula

CuGa0.54In0.46Se2

crystal size (mm3)

0.03×0.02×0.02

Fw

311.93

T (K)

293(2)

crystal system,

tetragonal

space group

I42d

Z

4

unit cell

a = b = 5.6977(4)

parameters (Å)

c = 11.343(2) 5

e-mail:

ACCEPTED MANUSCRIPT V(Å3)

368.24(8)

Dcalcd (g cm–3)

5.626

 (mm–1)

32.103

F(000)

545.0

2 range (º)

7.184 to 54.74

indep. reflns/Rint

210/0.0521

GOF on F2

1.057

R1a (I > 2(I))

0.0406

wR2b (all data)

0.1032

aR1

= ||Fo| - |Fc||/|Fo|. bwR2 = [w(Fo2 - Fc2)2]/[w(Fo2)2]1/2.

Table 2. Atomic coordinates (× 104) and equivalent isotropic displacement parameters (Ueqa, Å2 × 103) for CGISe. Atom

x

y

z

Ueq/Å2

In(1)

0

5000

2500

13.2(8)

Ga(1)

0

5000

2500

13.2(8)

Cu(1)

5000

10000

2500

24.1(10)

Se(1)

2500

7598(2)

3750

15.9(11)

2.3 Infrared and UV-Vis-NIR diffuse reflectance spectroscopies

The infrared spectrum was recorded using a TENSOR27 FT-IR spectrophotometer in the 6

ACCEPTED MANUSCRIPT range of 4000–400 cm–1. The powdery sample was pressed into a pellet with KBr. The diffuse reflectance spectrum was recorded with a computer-controlled Varian Cary 5000 UV-Vis-NIR spectrometer in the wavelength range of 200–1700 nm. A BaSO4 plate was used as a reference, on which the finely ground powdery sample was coated.

2.4 Second harmonic generation (SHG) measurement

Powder SHG measurement of CGISe was evaluated by a modified Kurtz-NLO system using 2.10 μm Q-switch laser radiation. Polycrystalline CGISe samples were ground and sieved into different size ranges (25–45, 45–62, 62–75, 75–100, 100–150, and 150–210 μm). AGS crystals with a uniform particle size range served as the standard. The samples were loaded into a glass microscope cover slide, surrounded by a 1 mm thick silicone insole with a 7 mm diameter hole. Then they were put into little-tight boxes and a pulsed infrared beam from a Q-switched laser with a wavelength of 2.09 μm radiated the crystals. The SHG signals were collected by the detector and showed the peaks on the oscilloscope.

2.5 Calculation details

The theoretical calculations include band structure, density of states (DOS), and optical properties. The calculations were performed using the CASTEP mode in Material Studio software [31]. The exchange-correlation functional was Perdew-Burke-Emzerhoff (PBE) functional within the generalized gradient approximation (GGA). A plane-wave cutoff energy of 880 eV and a threshold of 10–5 eV were set for the self-consistent field convergence of the total electronic energy. The interactions between the ionic cores and electrons are described by 7

ACCEPTED MANUSCRIPT the norm-conserving pseudopotential. The electronic configurations for Cu, Ga, In, and Se were 3d and 4s, 4s and 4p, 5s and 5p, 4s and 4p orbits, respectively. The numerical integration of the Brillouin zones was performed using 5 × 5 × 6 Monkhorst-Pack k-point meshes, and the Fermi level (Ef = 0 eV ) was chosen as the reference. The number of empty bands is three times that of valence bands in the calculations to ensure the SHG coefficients’ convergence. Calculated band gaps of CGISe is 0.54 eV, which are relatively smaller than the experimental 1.23 eV resulting from discontinuity of exchange-correlation energy functional. These differences will be corrected using a so-called scissors energy shift when evaluating the optical properties based on DFT band structure calculation results [32]. The optical property of CGISe was calculated and described based on the complex dielectric function ε(ω) = ε1(ω) + iε2(ω), in which ε1(ω) and ε2(ω) denote the real and imaginary parts of the dielectric function, respectively [33]. The first-order nonresonant susceptibility in the low-frequency region is given by x(1) (ω) = ε1(ω) 1 and the second-order susceptibilities can be expressed in terms of the first-order susceptibilities as follows: 𝑚𝑎

(2)

𝜒 ijk (𝜔3,𝜔1,𝜔2) =

(1)

𝜒 𝑖𝑖 (𝜔3) 𝜒

2 3

𝑁 𝑒

(1) (1) jj (𝜔1)𝜒 kk (𝜔2)

(1)

Which is derived from a classical anharmonic oscillator model. m, e, and N are the electron mass, electron charge, and number density of the atoms, respectively, and the parameter a characterizing the nonlinearity of the response can be obtained by the experimental or theoretical assessment. Refractive index n was calculated based on the followed formula. n(ω) =

1

[(𝜀 (𝜔)

2

1

2

2 1/2

+ 𝜀2(𝜔) )

3 Results and discussion 8

]

+ 𝜀1(𝜔)

1/2

(2)

ACCEPTED MANUSCRIPT 3.1 Crystal structure

It has to be mentioned that there are several members of CuGaxIn1-xSe2 compounds have been reported [34,35,36,37], and the title compound has a different Ga/In ratio. CGISe crystallized in the tetragonal space group I42d belongs to the chalcopyrite structure, similar with AGS. As chalcopyrite structures have been extensively discussed before, here only a very simple description is given. There are one Cu, one Ga(In) and one Se atoms in the crystallographically independent unit and the Cu–Se and Ga(In)–Se bond lengths are 2.4315 (8) and 2.4961(8) Å, respectively. Both Cu and Ga (In) atoms are coordinated with four Se atoms to build CuSe4 and Ga(In)Se4 tetrahedra (Fig. 2). As shown in Fig. 3, CuSe4 and Ga(In)Se4 tetrahedra connect alternately via sharing corners to construct the 3D structure. Each CuSe4 or Ga(In)Se4 tetrahedron has eight Ga(In)Se4 and four CuSe4, or eight CuSe4 and four Ga(In)Se4 tetrahedra, respectively.

Fig. 2. Coordination geometry of CGISe.

9

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Fig. 3. 3D crystal structure of CGISe.

3.2 Optical properties

The IR spectrum of CGISe shows no obvious absorption peaks between 400~4000 cm–1 (2.5~25 μm), showing a wide IR transparent region (Fig. 4a). The diffuse-reflectance UV-VisNIR spectrum exhibits that the optical band gap of CGISe is about 1.23 eV (Fig. 4b), which is consistent with its black color.

(b)

(a)

Fig. 4. Diffuse reflection (a) and infrared (b) spectra for CGISe. 10

ACCEPTED MANUSCRIPT 3.3 NLO data

(a)

(b)

Fig. 5. (a) Size-dependent SHG intensity of CGISe; (b) Oscilloscope peaks of SHG signals for CGISe and AGS at a particle size of 75~100 μm.

As CGISe crystallizes with a NCS space group (I42d), it is possible that it is SHG-active. In order to investigate its NLO data systemically, the crystals were sieved to several different particle sizes, and the SHG intensities were compared with those of the benchmark material AGS with the same sizes under a 2.1 μm Q-switch laser radiation. As Fig. 5 shown, CGISe is non-phase-matchable as the SHG response firstly increases and then decreases. The SHG intensity of CGISe is about 0.5 times that of AGS at the particle size of 42~65 μm. It is proposed that the ideal Δn value for type-I phase matchable NLO material is in the range 0.03~0.1 [38]. The calculated birefringence Δn of CGISe is around 0.023, which is reasonable for its nonphase-matchable behavior.

3.4 Theory calculation

11

ACCEPTED MANUSCRIPT The band structures, density of states (DOS), and optical properties of CGISe, CuGaSe2 (CGSe), and CuInSe2 (CISe) were calculated using the Materials Studio software. The band gaps of CGISe, CGSe, and CISe were calculated to be 0.54, 0.72, and 0.61 eV, respectively (Fig. 6). It can be seen from the band structures that all the three lowest conduction bands and the three highest valence bands are situated at the G point, indicating that all of them are direct band gap semiconductors. The total and partial DOS of CGISe are shown in Fig. 7. The highest valence band is mainly composed by the Se-4p, Se-4s, and Ga-4s orbits. The lowest conduction band is primarily constituted by Cu-3d orbit. Obviously, the optical absorption of CGISe is mainly ascribed to the charge transition from the Se-4p to Cu-3d orbits.

Fig. 6. The calculated band structures of CGISe, CGSe, and CISe, the Fermi level is chosen at 0 eV. 12

ACCEPTED MANUSCRIPT

Fig. 7. DOS of CGISe, the Fermi level is chosen at 0 eV.

13

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Fig. 8. The calculated real parts (left) and imaginary parts (right) of optical dielectric functions of CGISe, CuGaSe2, and CuInSe2 in different directions.

To better understand the NLO properties of CGISe, the optical properties of CGISe, CGSe, and CISe are compared and discussed. These three compounds crystallize with the same structures, so the comparison makes sense. The real and imaginary parts of optical dielectric functions along the x, y, and z directions are shown in Fig. 8, the 𝜀𝑎𝑣𝑒𝑟 is the average value of εx + εy + εz. The strongest absorptions of CGISe, CGSe, and CISe are calculated to be 6.84, 5.41, and 5.87 eV, respectively (Fig. 8). Their strongest absorptions can be contributed to the electronic interband transitions from the Cu-3d and Se-4p to Ga-4p or In-5p states. They have two SHG tensors d14 and d36 under the restriction of Kleinman’s symmetry, due to chalcopyrite structures belong to the 42𝑚 point group. The d14 and d36 values of CGISe, CGSe, and CISe are shown in Fig. 9, both of d14 and d36 values of CGISe are 29.00 pm/V, and which are 58.21 and 57.46 pm/V for CGSe, and 36.00 and 36.12 pm/V for and CISe at 2.1 μm wavelength (0.59 eV). The calculated energy dependence of birefringence Δn values of CGISe, CGSe, and CISe are 0.023, 0.012 and 0.021, respectively, indicating that these Cu-based chalcogenides have little opportunity to be type-I phase-matchable. According to literature [39], CuMQ2 (M = Ga, 14

ACCEPTED MANUSCRIPT In; Q = S, Se, Te) chalcopyrite compounds exhibit higher χ(2) current from Ga to In, and S to Se to Te, especially, and anion plays a leading role. It is supposed that band gap transitions have predominate influences on the NLO responses, in other words, band gap and NLO response are negative-correlated. Based on the calculation results, the following sequences can be obtained: din (CuGaSe2) > din (CuInSe2) > din (CGISe) at 0~0.88 eV, din (CuGaSe2) > din (CGISe) > din (CuInSe2) at 0.88~1.85 eV, and din (CGISe) > din (CuGaSe2) > din (CuInSe2) at 1.85~2.5 eV. The experimental band gaps for CGSe, CISe, and CGISe are 1.72, 1.03 [40], and 1.23 eV, respectively, so the din sequence for these three compounds should be din (CGSe) > din (CGISe) > din (CISe), perfectly in accordance with the supposition that smaller band gap and higher χ(2) go from Ga to In.

(b)

(a)

Fig. 9 Calculated frequency-dependent SHG coefficients of CuGaSe2, CuInSe2, and CGISe (a), calculated energy dependence of birefringence Δn of CuGaSe2, CuInSe2, and CGISe (b) along different directions.

Conclusions

The experimental and theoretical investigations of CuGa0.54In0.46Se2 (CGISe) indicate that it 15

ACCEPTED MANUSCRIPT is SHG-active, but not phase-matchable, and the SHG intensity is not satisfactory, which is a normal phenomenon for Cu-based chalcopyrite chalcogenides. The discussion between the NLO properties of CGISe, CGSe, and CISe demonstrates that replacing In by Ga can increase the band gap, but decrease the NLO coefficient. It is supposed that these results can make some contribution to understand the structure-NLO property relationship as MQ4 (M = Ga, In; Q = S, Se) tetrahedra are important functional moieties for NLO applications [1,41,42,43].

Acknowledgement

We gratefully acknowledge the financial support by the NSF of China (21771159), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and Topnotch Academic Programs Project of Jiangsu Higher Education Institutions.

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Highlights 1. Single-crystal structure of CuGa0.54In0.46Se2 is firstly studied. 2. The NLO property of CuGa0.54In0.46Se2 is experimentally investigated. 3. Theoretical calculations are performed to explain the NLO property.