Polycrystal Synthesis, Crystal Growth, Structure, and Optical

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Polycrystal Synthesis, Crystal Growth, Structure, and Optical Properties of AgGaGenS2(n+1) (n = 2, 3, 4, and 5) Single Crystals for Mid-IR Laser Applications Wei Huang, Zhiyu He,* Shifu Zhu, Beijun Zhao, Baojun Chen, and Sijia Zhu

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/12/19. For personal use only.

College of Materials Science and Engineering, Sichuan University, Chengdu 610064, China ABSTRACT: AgGaGenS2(n+1) crystal is a series of quaternary nonlinear optical materials for mid-IR laser applications of converting a 1.064 μm pump signal (Nd:YAG laser) to 4−11 μm laser output, but only AgGaGeS4 has attracted the most attention, remaining the other promising AgGaGenS2(n+1) crystal whose physicochemical properties can be modulated by n value. In this work, AgGaGenS2(n+1) (n = 2, 3, 4, and 5) polycrystals are synthesized by vapor transport and mechanical oscillation method with different cooling processes. High-resolution X-ray diffraction analysis and refinement have revealed that all the four compounds are crystallized in the noncentrosymmetric orthorhombic space group Fdd2, resulting in the excellent nonlinear optical property, and the distortion of tetrahedron with the variation of n value causes the discrepancy of physicochemical property. Besides, using the modified Bridgman method, AgGaGenS2(n+1) single crystals with 15 mm diameter and 20−40 mm length have been grown. We have discussed the structure and composition of AgGaGenS2(n+1) by XPS spectra and analyzed the three kinds of vibration modes of tetrahedral clusters by the Raman spectra. The Hall measurement indicates that the AgGaGenS2(n+1) single crystals are p-type semiconductor, and the carrier concentration decreases with the increasing n value. All the transmittances of as-grown AgGaGenS2(n+1) samples exceeds 60% in the transparent range, especially the transmittance of AgGaGe2S6, is up to 70% at 1064 nm, and the band gap of as-grown crystal increases from 2.85 eV for AgGaGe2S6 to 2.92 eV for AgGaGe5S12. After a thermal annealing treatment, the absorptions at 2.9, 4, and 10 μm have been eliminated, and the band gap changed into the range of 2.89−2.96 eV.

1. INTRODUCTION In recent years, the new mid-infrared nonlinear optical (NLO) crystals have been increasingly explored due to the growing importance of laser applications in the 2−12 μm spectral range,1 but only several NLO crystals perform the excellent capability and realize commercial value, which still suffer from various deficiency. For instance, the AgGaS2 crystal with chalcopyrite structure has glorious nonlinear optical properties. Nevertheless, it is plagued by a low laser damage threshold which limits its application severely on high-power and durable leaser output.2−4 For the selenide analog AgGaSe2, which possesses a relative higher damage threshold, the insufficient birefringence for phase matching a 1.064 μm pump source and the poor thermal conductivity have become stumbling blocks.2,5 Therefore, there is a extremely urgent requirement to develop a more outstanding and integrating NLO material for frequency-shifting 1.064 μm Nd:YAG laser into midinfrared. The new ternary and quaternary chalcogenides are gradually approved as promising materials for its NLO properties and the transparent region which can be modulated with the composition.6−14 AgGaGeS4 crystal is just one of the solid solution crystal AgGaS2−nGeS2 (n = 1) and becomes a popular research subject.15,16 It has a high nonlinear optical © XXXX American Chemical Society

coefficient (d31 = 15 pm/V), a wide transmission range (0.5− 11.5 μm), and a low residual absorption coefficient (0.05 cm−1 at 1.064 μm), which make it a promising material for frequency-shifting 1.064 μm Nd:YAG laser into the range of 4−11 μm.17−21 However, the other AgGaS2−nGeS2 (n = 2, 3, 4, and 5) crystals, i.e., AgGaGe2S6, AgGaGe3S8, AgGaGe4S10 and AgGaGe5S12, which are rarely reported, also have the comparable NLO properties. The nonlinear optical coefficients of AgGaGe3S8 and AgGaGe4S10 crystal are d31 = 13.65 and 13.8 pm/V,22 respectively, which are very close to that of AgGaGeS4. In addition, the birefringence increases with n, and reaches a maximum value when n = 5 for AgGaGe5S12, which provides a unique possibility for engineering the phasematching properties.23 Therefore, AgGaS2−nGeS2 (n = 2, 3, 4, and 5) crystals would attract more interesting attentions on mid-infrared laser frequency conversion. Solid solutions system AgGaS2−nGeS2, i.e., AgGaGenS2(n+1) (the chemical formula is AgxGaxGe1−xS2), which exist only in a limited amount for the parameter n value were first synthesized and investigated for n = 1, 2, 3, 4, and 5 in ref 24. They inherited excellent optical properties from AgGaS2 and GeS2 Received: January 24, 2019

A

DOI: 10.1021/acs.inorgchem.9b00191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

2.2. Thermal Annealing Treatments of AgGaGenS2(n+1). A thermal annealing treatment was performed in order to improve the optical quality of the as-grown AgGaGenS2(n+1) single crystal. Four polished AgGaGenS2(n+1) single crystal wafers were introduced respectively into four quartz ampules which were then evacuated to 1 × 10−4 Pa by a molecular pump. Then, we sealed the quartz ampules with oxyhydrogen flame and put the them into a single-zone furnace. The annealing treatments for the wafers were all carried out at 500 °C for 250 h. After this process, the wafers which appear different shades had a more consistent color. 2.3. Characterizations. The AgGaGenS2(n+1) polycrystalline ingot was ground into powder and analyzed by EMPYREAN (PANalytical B.V., Netherlands) operating at Cu Kα (1.540598 Å) wavelength with 0.013° step in the 10−130° range, and the results were refined by the Rietveld method with the Fullprof program. The XRD pattern for phase analysis was recorded by DX-2000 (Dandong, China) with Cu Kα (1.54182 Å) wavelength in the 10−90° range. Compositions were determined using JSM-7500F (JEOL, Japan) via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) which was performed at 15 kV. X-ray photoelectron spectroscopy (XPS) has been employed to study the electronic structure and valence state of AgGaGenS2(n+1) single crystals. The XPS valence-band and core-level spectra of the single crystals have been recorded by AXIS Ultra DLD (Kratos, United Kingdom), and the results were analyzed by the CasaXPS. The Raman scattering measurements were performed by the LabRAM HR (HORIBA, French) at ambient temperature. The light source is a 632 nm laser with a radiation power of 30 mw. No sample deterioration was observed. Electrical properties were carried out at a magnetic field of 0.554 T at room temperature by the Ecopia HMS300 Hall effect measurement system. The optical properties were studied using a Fourier-transforminfrared (FTIR) spectrometer in the 2−13 μm range and a UV− visible−NIR spectrometery (Shi-madzu, Japan) in the 0.4−1.1 μm range. The samples were polished before measurements.

due to their special structures. The chalcopyrite AgGaS2 with space group I4̅2d is a well-established and characterized nonlinear optical crystal and already has a mature preparation process.25−27 The structure of GeS2 was also determined to be Fdd2 as early as 1936,28 while the crystallographic structure of AgGaGenS2(n+1) was only studied in detail for AgGaGeS429 which was recently characterized as a birefringent nonlinear optical crystal.17 These quaternary compounds belong to the Fdd2 diamondlike space group, whose structure results from the substitution of Ge4+ by Ga3+ in the GeS2 cation sublattice and the compensation by Ag ions filling the tetrahedral vacancies.29 Such a structure with tetrahedral coordination determines their excellent nonlinear optical properties. However, the polycrystal synthesis AgGaGenS2(n+1) was plagued by high vapor pressure of S and GeS2 which resulted in an extremely violent explosion.30 Not only that, but AgGaGenS2(n+1) (n = 2, 3, 4, and 5) with different proportion of volatile composition GeS2 have certain discrepancy in physical and chemical properties, so that it is difficult to use the same synthetic process to obtain high quality crystals. In addition, the problems, such as large degree of supercooling, stoichiometric variation, second-phase precipitates and twin crystals, are still difficult to avoid in AgGaGenS2(n+1) crystal growth processes.31−33 In this study, AgGaGenS2(n+1) polycrystalline materials were synthesized by a vapor transport and mechanical oscillation methods with different cooling processes. The explosion problem has been solved by a specially designed two-zone rocking furnace and a carefully controlled heating process. The unique cooling process has solved the problem of composition segregation. Then, the AgGaGenS2(n+1) single crystals were grown by a modified Bridgman method.34,35 Using powder XRD structure refinement, XPS, and Raman spectra, we have analyzed the crystal structure in detail, and characterized the electrical and optical properties of AgGaGenS2(n+1) single crystals. Finally, the asgrown crystals have undergone a thermal annealing treatment, so the detrimental absorptions have been eliminated and the band gap widened.

3. RESULT AND DISCUSSION Using the same synthetic process with AgGaGeS4 reported in our previous work,36 the chemical reaction has already proceeded in an orderly manner. However, the compositions of AgGaGenS2(n+1) (n = 2, 3, 4, and 5) polycrystal change with the increase of the n value, especially the increasing volatile GeS2, so the technical essentials of chemical synthesis are slightly different, which is mainly reflected in the cooling stage. Adopting the rapid cooling process, AgGaGenS2(n+1) polycrystal with small n value, such as AgGaGe2S6, can be obtained with good crystallinity. Nevertheless, AgGaGenS2(n+1) polycrystal with large n value, AgGaGe4S10 and AgGaGe5S12, tend to divide into two portions. One portion with silver color attached on the quartz ampule mainly consists of GeS2 with the monoclinic structure (a × b × c = 11.445 × 16.09 × 6.709, ⟨90.0 × 90.93 × 90.0⟩) according to Powder Diffraction File no. 27-0238 (Joint Committee on Powder Diffraction Standards (JCPDS), [1973]), which can be recognized by the XRD result in Figure 1. The other portion with overall gray presents severe component segregation, which is demonstrated by the EDS results in the Table 1. This is mainly due to the lack of GeS2 in the melt, which largely exists in the synthetic ampule in the form of steam at high temperature, and there is not enough time for this gaseous GeS2 to mix into the melt, only condensing on the quartz ampule and the surface of the synthesis product. Besides, the larger n value, the greater component segregation. Therefore, it is demonstrated that this method with fast cooling rate is only suitable for the compounds with a lower content of GeS2, such as n = 1 and 2 (AgGaGeS4 and AgGaGe2S6). However, AgGaGenS2(n+1)

2. EXPERIMENTAL SECTION 2.1. Chemical Synthesis and Single Crystal Growth of AgGaGenS2(n+1). High-purity (6 N) gallium, silver, germanium and sulfur were weighed according to the stoichiometry of AgGaGenS2(n+1) (n = 2, 3, 4, and 5) and introduced into a double-walled quartz ampule which was evacuated to 1 × 104 Pa and sealed by hydrogen and oxygen flame. The polycrystal synthesis was carried out in a twozone furnace by vapor transport and mechanical oscillation method, which is almost consistent with the synthesis method of AgGaGeS4 reported in previous work.36 However, on account of the discrepancy of chemical composition, the synthesis process needs to be adjusted and optimized according to the variation of composition, especially the cooling processes. Therefore, different cooling processes were adopted in the cooling process, respectively, to obtain stoichiometric composition and high quality of AgGaGenS2(n+1) polycrystal. The single crystals were grown in a four-zone furnace, each zone of which having independent temperature control and being separated by a heat sink which can be adjusted in thickness to vary the temperature gradient at the solid−melt interface in a quite wide range to realize an ideal temperature field. Growth conditions and relative parameters, including melt point, temperature gradient, and growth rate, had been referred to AgGaGeS4. With such optimized growth conditions, the temperature gradient (10−35 °C/mm) and the growth rate (0.1−0.6 mm/h), high-quality AgGaGenS2(n+1) single crystals were grown by the modified Bridgman method. B

DOI: 10.1021/acs.inorgchem.9b00191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Photograph of AgGaG5S12 polycrystalline ingots and XRD pattern of the impurity phase (Powder Diffraction File no. 27-0238, Joint Committee on Powder Diffraction Standards (JCPDS), [1973]).

Figure 2. Photographs of polycrystalline ingots (a) AgGaGe2S6; (b) AgGaGe3S8; (c) AgGaGe4S10; (d) AgGaGe5S12.

Table 1. EDS Data for the AgGaGenS2(n+1) Crystals Obtained by Fast (a) and Slow (b) Cooling Rates

parameters of the AgGaGenS2(n+1) phases and the interatomic distances are listed in Table 2. Atom coordinates of the structures are given in Table 3. The crystallographic sites of the S atoms are generally fully occupied in the structures of the compositions, except that of AgGaGe5S12 with somewhat high temperature factor, being likely due to the deficiency and high mobility area, which could explain the slightly segregation. Besides, unlike the Parasyuk’s result,37 the cation atoms with high values of the temperature displacement parameters are Ga/Ge atoms at 8a instead of Ag atoms, which demonstrates that the GeS2 are more volatile than GeSe2, causing the instability of Ge and S atoms. The schematic diagrams of AgGaGenS2(n+1) crystal structure are displayed in Figure 4, which is mainly consisted by a series of tetrahedral frames and the Ag atoms filling in the tetrahedral interstices. The Ga or Ge atoms, which can displace each other, occupy the center of the tetrahedral structure, while the S atoms constitute the frame of the tetrahedral. Therefore, it can be regarded as that partial Ge atoms are substituted by Ga atoms in the GeS2 structure. Due to the valence deficiency between Ga and Ge and the canals forming by the tetrahedral interstices, the compensation of Ag atoms becomes indispensable. Besides, structurally speaking, the existence of Ag atoms in the canals also affect the frame structure, leading to the distortive transformations of the GeS 4 and GaS 4 tetrahedron. It is obvious that the Ge−S and Ga−S bonds have slight changes, and that the crystal cell volume becomes smaller with the gradually decreasing Ag atoms occupancy from AgGaGe2S6 to AgGaGe5S12 (from 0.5 to 0.25), which are more similar to the structure of the high-pressure GeS2. In addition, it is very interesting that almost all the lattice parameters decrease when the Ag atoms decrease, except the lattice parameter c, which is just in accordance with the direction of the canals, as shown in Figure 4. In this case, the canals have been compressed and the length become slight longer. Such a slight distortion in structure does not affect the main tetrahedron structural units also exhibiting attractive nonlinear optics, resulting in a very small discrepancy of the nonlinear optical coefficients. The nonlinear optical coefficients of AgGaGeS4, AgGaGe3S8, and AgGaGe4S10 crystal are d31 = 10.2 pm/V22 (15 pm/V),17 13.65 pm/V, and 13.8 pm/ V,22 respectively. However, it causes the considerable differ-

content of elements, atom % composition

Ag

Ga

Ge

S

AgGaGe2S6 n = 2 a b AgGaGe3S8 n = 3 a upper layer b under layer AgGaGe4S10 n = 4 a upper layer b under layer AgGaGe5S12 n = 5 a upper layer b under layer

10.00 10.32 10.81 7.69 6.94 7.33 7.67 6.25 5.45 6.69 7.01 5.26 4.20 5.22 6.10

10.00 11.91 11.72 7.69 10.25 8.41 8.87 6.25 7.73 6.12 6.90 5.26 5.68 5.57 5.83

20.00 21.58 21.03 23.08 27.33 25.03 25.60 25.00 29.18 24.72 24.91 26.32 31.30 27.06 26.06

60.00 56.19 56.44 61.54 55.48 59.23 57.87 62.50 57.64 62.48 61.15 63.16 58.82 62.15 62.01

polycrystals with large n value still display a distinct stratification using the slow cooling process. The layer with light yellow appearance and loose texture distributes on the upper surface of the polycrystalline ingot, while the other layer with a darker color concentrates at the bottom of the ingot. This difference is also reflected in the EDS data, although admittedly it is not that obvious. Therefore, at last, we adjust the tilt angle of the furnace during the cooling process and obtain the polycrystalline material with more uniform composition. Figure 2a−d are respectively the photographs of AgGaGe2S6, AgGaGe3S8, AgGaGe4S10, and AgGaGe5S12 polycrystalline ingot with porous texture the different shades, and only AgGaGe5S12 polycrystalline ingot appears to have slight composition segregation. Using the AgGaGenSe2(n+1) structure37 as the basic model, we have carried out the computation and structure refinement for the AgGaGenS2(n+1) solid solutions. The computation and refinement are reasonable and practicable because the fitted curves show a good agreement with experimental data and the values of fit factors are satisfactory. In this case, the four groups of experimental, calculated, and differential X-ray diffraction patterns are all plotted in Figure 3. The values of lattice C

DOI: 10.1021/acs.inorgchem.9b00191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. Experimental and calculated diffraction patterns of the solid solutions AgGaGenS2(n+1) and their difference.

room temperature for 20 h. Figure 5 is the photographs of AgGaGenS2(n+1) single crystals with 15 mm in diameter and 20−40 mm in length. From this figure, we can find that the seeds are well-developed, and each crystal takes on a complete appearance and different shades of color. XPS spectra of the AgGaGenS2(n+1) single crystal wafers which have undergone the mechanical and chemical polishing are presented in Figure 6. It is evident that all the spectral features are assigned to constituent element core-level or Auger lines, except the C 1s and O 1s levels and the O KLL Auger line. The spectra have been made a correction by the C 1s core-level lines fixed at 284.6 eV, which is narrow without any shoulders on their higher binding energy sides related to carbonate formation. In addition, although the polished wafers are exposed to air for a long time; the XPS results does not display the signs of oxidation. Binding energy values of the constituent elements of the AgGaGenS2(n+1) single crystal surfaces are listed in Table 4. It is found that the situation of the S 2p binding energy is very complicated, which is shown in Figure 7. The S atoms, consisting of the tetrahedron frame, are affected not only by the center atoms but also the Ag atoms in the canals, resulting in three binding energies, at about 160, 164.4, and 162.5 eV. Besides, it is obvious that the S 2p binding energies almost monotonously increase with the sequence of AgGaGe2S6, AgGaGe3S8, AgGaGe4S10, and AgGaGe5S12, and the percentages of the different binding energies have also changed, indicating some decrease of the negative charge of S atoms. On the contrary, the Ga 2p and Ag 3d binding energies decrease. However, there is no noticeable change about the Ge element. Therefore, we can conclude that the charge states of Ag, Ga, and S atoms only change slightly in the AgGaGenS2(n+1) single crystals when the n value varies. The basic structural building blocks for AgGaGenS2(n+1) crystal consist of Ge- and Ga-centered tetrahedra, i.e., GeS4 and GaS4, and are essentially the same as for GeS2. Koblar Jackson subdivided the GeS2 structure into three smaller cluster units, including corner-sharing (CS), edge-sharing (ES), and ethanelike (ETH) cluster structure, and concluded

Table 2. Lattice Parameters, Fit Factors, and Interatomic Distances in the Solid Solutions AgGaGenS2(n+1) a b c V Rp (%) Rwp (%) density (g/cm3) Ag−S2 Ag−S2 Ag−S3 Ag−S1 Ga1−S1 Ga1−S2 Ga2−S1 Ga2−S2 Ga2−S3 Ga2−S3

AgGaGe2S6

AgGaGe3S8

AgGaGe4S10

AgGaGe5S12

11.9339(3) 22.8013(5) 6.8735(1) 1870.33(7) 6.01 8.44 3.6594(6)

11.8876(3) 22.7225(6) 6.8758(1) 1857.27(7) 6.18 8.97 3.4971(7)

11.8545(3) 22.6660(5) 6.8765(1) 1847.66(7) 7.33 10.6 3.4020(7)

11.8338(3) 22.6314(5) 6.8766(1) 1841.67(6) 7.80 11.2 3.3373(6)

2.3271(4) 2.5615(4) 2.7942(4) 2.5636(5) 2.2674(4) 2.4683(4) 2.1065(4) 2.2032(4) 2.3564(4) 2.3873(5)

2.3262(4) 2.5891(4) 2.4879(4) 2.8232(4) 2.3023(4) 2.4655(4) 2.1274(4) 2.2783(4) 2.3852(4) 2.3805(4)

2.4117(4) 2.4554(4) 2.6823(4) 2.7880(4) 2.2740(4) 2.5033(4) 2.1121(4) 2.2375(4) 2.3250(4) 2.4029(4)

2.3115(4) 2.5681(4) 2.5215(4) 2.7821(4) 2.2857(4) 2.5468(4) 2.1194(4) 2.1812(4) 2.3135(4) 2.3646(4)

ence of physicochemical properties which can adapt to the different application environments. We have adopted the way of spontaneous nucleation and geometric elimination to obtain the AgGaGenS2(n+1) single crystal seed, so at the beginning of nucleation, it is hard to avoid the influence of large supercooling degree which is demonstrated 86 °C in AgGaGeS4 crystal31 and can cause explosive nucleation. Therefore, through adjusting the structure of the four-zone furnace, we have obtained a suitable temperature field for each composition of AgGaGenS2(n+1) and a temperature gradient which is large enough to overcome the large supercooling, ensuring the spontaneous nucleation seed crystal achieve mononuclear growth. Besides, the corresponding descent rates also keep the convex solid−liquid interface to reduce the point defect and dislocation. When the growth process come to the end, the furnace is cooled down to the D

DOI: 10.1021/acs.inorgchem.9b00191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 3. Atom Coordinates in the Structures of the Solid Solutions AgGaGenS2(n+1) atom

position

Ag Ge1 Ge2 S1 S2 S3

16b 8a 16b 16b 16b 16b

Ag Ge1 Ge2 S1 S2 S3

16b 8a 16b 16b 16b 16b

Ag Ge1 Ge2 S1 S2 S3

16b 8a 16b 16b 16b 16b

Ag Ge1 Ge2 S1 S2 S3

16b 8a 16b 16b 16b 16b

x/a

y/b

AgGaGe2S6 (AgxGaxGe1−xS2, x = 0.333) 0.0826(6) 0.2089(3) 0 0 0.1262(4) 0.11129(17) 0.1971(9) 0.1709(4) 0.1716(6) 0.0185(4) 0.1922(7) 0.1154(3) AgGaGe3S8 (AgxGaxGe1−xS2, x = 0.250) 0.0858(7) 0.2068(4) 0 0 0.1337(3) 0.11200(19) 0.1858(8) 0.1746(4) 0.1758(7) 0.0210(4) 0.1927(8) 0.1181(4) AgGaGe4S10 (AgxGaxGe1−xS2, x = 0.200) 0.0865(9) 0.2070(5) 0 0 0.1184(4) 0.36105(18) 0.0609(9) 0.4247(4) 0.0770(7) 0.2687(4) 0.0565(9) 0.3657(4) AgGaGe5S12 (AgxGaxGe1−xS2 x = 0.167) 0.0918(10) 0.2051(6) 0 0 0.1324(3) 0.11017(17) 0.1883(8) 0.1727(4) 0.1775(7) 0.0162(4) 0.1928(11) 0.1137(7)

z/c

occupation

0.7821(8) 0.0363(8) 0.3127(8) 0.1067(8) 0.2438(8) 0.6429(8)

0.500 1a 1a 1 1 1

0.7113(10) 0.0047(1) 0.2710(10) 0.0646(10) 0.1957(1) 0.5944(10)

0.375 1b 1b 1 1 1

0.7211(12) 0.0050(12) 0.5231(12) 0.3177(12) 0.4475(12) 0.8542(12)

0.300 1c 1c 1 1 1

0.7478(10) 0.0477(10) 0.3151(10) 0.1114(10) 0.2349(10) 0.6528(10)

0.250 1d 1d 1 1 1

a

0.333 Ga + 0.667 Ge. b0.250 Ga + 0.750 Ge. c0.200 Ga + 0.800 Ge. d0.167 Ga + 0.833 Ge.

the composition, which illustrates that this vibration mode is caused by S atoms in the cluster and the variation of the composition only makes the peak blue-shift. The broad feature from 380 to 420 cm−1 contains contributions from both the CS and ES clusters, and the peak at about 436 cm−1 is entirely due to the highest ES cluster vibration mode. In an earlier report, Yoji Kawamoto use four normal modes of vibrations A1, E, F2, and F2 of Td symmetry to analyze the GeS2 molecules vibration mode. He attributes peaks observed over the wavenumber range 340−450 cm−1 to the F2 mode, except 361 cm−1 in GeS2.39 The 250 cm−1 peak attributed to ETH cluster vibration mode in GeS2 depends sensitively on the Ge content of the sample,40,41 but it is not obvious in AgGaGenS2(n+1) crystal, except AgGaGe5S12 whose Ge content is closest to GeS2. The other ETH cluster vibration mode at 179 cm−1 in GeS2 is also hardly observed in AgGaGenS2(n+1) crystal. The structure of ETH cluster mainly exhibits Ge−Ge bonds in GeS2, just like ethane. However, in AgGaGenS2(n+1) structure, Ag atoms fill in the interspace of tetrahedrons, blocking the connection between the Ge or Ga atoms, and prevent the vibration of such clusters. The remaining low-frequency vibration modes at about 105 and 130 cm−1 are unique in AgGaGenS2(n+1) crystal, which may be attributed to affection of Ag atom (Ag−S bond). The electrical properties of AgGaGenS2(n+1) single crystals are reflected by the Hall measurement data at room temperature listed in the Table 6. The AgGaGenS2(n+1) single crystals are p-type semiconductor which may cause the cationic acceptor defect or anti-position acceptor defect. Besides, the hole concentration decreases with the increasing n value, which

that the observed Raman intensity is relative to the concentration of cluster.38 In GeS2 crystal, the vibration mode at 343 cm−1 is attributed to the CS cluster which owns the overwhelming concentration, resulting in the highest intensity. Whereas for AgGaGeS4, there is also only an observed peak position at about 325 cm−1 in the 300−350 cm−1 region. With the increasing of the n value, Ga atom are gradually substituted by Ge atom in CS structure, so that the peak at about 325 cm−1 splits into two peaks. One is at about 325 cm−1 with diminishing intensity, the other is at about 340 cm−1 which strengthens gradually and becomes the strongest peak, both accompanying by a slight blueshift. Therefore, it can be speculated that the intensities of 325 and 340 cm−1 vibration modes are respectively associated with the concentration of Ga and Ge atom. In addition, it displays only one Raman peak in the 300−350 cm−1 region of AgGaGeS4 crystal in which the proportion of Ge and Ga is 1:1, which indicates that the intensity is correspondence not only with the atomic concentration but also with the absolute intensity. Besides, there is another Raman peak at 357 cm−1 which is with high intensity and very close to 340 cm−1, so the peak at 340 cm−1 is submerged by two adjacent strong peaks. In that case, from AgGaGe2S6 to AgGaGe5S12, Ga content in the crystal decreased from 1:2 to 1:5, and the relative intensity of the two peaks also changed correspondingly showing in the Figure 8. The observed frequencies of Raman spectra recorded at room temperature for AgGaGenS2(n+1) single crystal are listed in Table 5. The next peak at about 370 cm−1 is contributed by the ES cluster vibration mode,38 and its intensity changes hardly with E

DOI: 10.1021/acs.inorgchem.9b00191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Schematic diagrams of AgGaGenS2(n+1) crystal structure.

Figure 5. photographs of AgGaGenS2(n+1) single crystals (a) AgGaGe2S6; (b) AgGaGe3S8; (c) AgGaGe4S10; (d) AgGaGe5S12 Figure 6. XPS spectra recorded for pristine surfaces of the AgGaGenS2(n+1) single crystals.

suggests it relates to the Ag atoms in the canals, instead of the tetrahedron center atoms. The resistivity varies from 102 to 103 Ω·cm, and the mobility varies from 4.05 to 102 cm2/(V s). We also have attempted to carry out the Hall measurement at 77 K, but the AgGaGenS2(n+1) single crystals always crack when we

have cooled the temperature to 77 K. It illustrates that these crystals are very sensitive to temperature change.42,43 The as-grown AgGaGenS2(n+1) crystal ingots have been cut into wafers with a size of 5 × 5 × 1 cm3, respectively, which is F

DOI: 10.1021/acs.inorgchem.9b00191 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 4. Binding Energy Values of Constituent Element Core-Level Electrons and Auger Lines of the AgGaGenS2(n+1) Single Crystal Surfaces S 2p AgGaGe2S6 AgGaGe3S8 AgGaGe4S10 AgGaGe5S12

159.97 160.07 160.06 160.39

(8.45%) (9.06%) (8.37%) (6.33%)

161.34 161.36 161.40 161.41

(60.24%) (60.23%) (61.43%) (63.62%)

162.49 162.53 162.53 162.56

(31.31%) (30.07%) (30.20%) (30.05%)

Ga 2p3/2

Ga 2p1/2

Ge 3d

Ag 3d5/2

Ag 3d3/2

1117.35 1117.13 1117.07 1117.05

1144.15 1144.03 1143.97 1143.95

30.88 30.88 30.88 30.87

367.45 367.43 367.42 367.38

373.46 373.45 373.42 373.36

Figure 7. S 2p XPS spectra recorded for pristine surfaces of the AgGaGenS2(n+1) single crystals.

annealing. The transmittance spectra in the 0.4−13 μm range shown in Figure 10 indicate high transparency of as-grown AgGaGenS2(n+1) samples whose transmittance all exceed 60%, especially the transmittance of AgGaGe2S6 which is up to 70% at 1064 nm. The relatively low transmittance of AgGaGe3S8 and AgGaGe5S12 in the range from visible to near-infrared are suggested the causes of different defects. Therefore, after annealing, the absorption of AgGaGe2S6 and AgGaGe3S8, which is mainly caused by microscopic scattering centers, such as point defects and dislocation, have an obvious improvement through the atom thermal diffusion and dislocation climb. On the contrary, macroscopic defects remain in AgGaGe4S10 and AgGaGe5S12 so that the transmittance cannot be further improved. Nevertheless, this problem would be solved by optimizing the growth process in the further work. Besides, the transmittance in the middle infrared bands are all about 70%, and the absorptions at 2.9, 4, and 10 μm, especially in AgGaGe2S6 crystal, almost become the symbol of AgGaGenS2(n+1) single crystal and seriously affect the application of these crystals. The reason for these absorptions is not clear and

Figure 8. Raman spectra of as-grown AgGaGenS2(n+1) single crystals recorded at room temperature.

shown in Figure 9. Thermal annealing is then carried out under vacuum condition at a temperature of 500 °C for 250 h in a single zone furnace. The AgGaGenS2(n+1) crystal wafers with different shades tend to be almost the same color after G

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Inorganic Chemistry Table 5. Experimental Data of Phonon Frequencies of AgGaGenS2(n+1) Crystal wave number (cm−1) ETH GeS2exp GeS2th AgGaGeS4 AgGaGe2S6 AgGaGe3S8 AgGaGe4S10 AgGaGe5S12

CS

250 254 106 108 103 104 104

126 131 135 131 134

171 172 174 180

212

343 335 323 327 328 329 330

240

338 339 340 343

ES

ES

374 373 357 366 368 369 373

437 436 421 438 437 439 445

397

409 409 409 409

Table 6. Hall Measurement Data at Room Temperature of the AgGaGenS2(n+1) Single Crystals average Hall coefficient (cm3/C) AgGaGe2S6 AgGaGe3S8 AgGaGe4S10 AgGaGe5S12

−1.96 −9.99 −1.26 −1.21

× × × ×

bulk concentration (cm−3)

4

10 103 104 104

−1.45 −7.02 −5.13 −2.50

× × × ×

15

10 1014 1014 1014

resistivity (Ω cm) 6.98 6.72 8.63 1.97

× × × ×

2

10 103 102 103

mobility (cm2/(V s)) 4.06 4.05 1.71 1.66

× × × ×

101 100 101 102

Figure 9. Photographs of AgGaGenS2(n+1) single crystal wafers and absorption coefficient vs photon energy spectra of AgGaGenS2(n+1) single crystals.

may be attributed to water contaminations (H−S and −OH bonds).31 Through the thermal annealing treatment, the absorptions at 2.9, 4, and 10 μm are all eliminated, and the optical quality has been effectively improved in the midinfrared. Absorption coefficients (α) were estimated from the transmission spectra, and Figure 9 shows the curves of (αhν)2 value versus photon energy44,45 of as-grown and annealed AgGaGenS2(n+1) single crystals. The band gap of asgrown crystal changes along with composition variation of AgGaGenS2(n+1) single crystals, increasing from 2.85 eV for AgGaGe2S6 to 2.92 eV for AgGaGe5S12, which is slightly larger than the reported value varying from 2.78 eV for AgGaGeS4 to 2.81 eV for AgGaGe5S12.22 This weak discrepancy has been supposed to demonstrate evidently in the appearance of color, and AgGaGe5S12 is meant to be with the lightest color. However, the absorption results in the opposite. It also can be

observed that the tangent line at the absorption coefficient edge has a higher slope for AgGaGe2S6 single crystals. The absorption of scattering center in AgGaGe5S12 results in the decrease in the sharpness of the fall of the absorption coefficients. In addition, the band gap of all AgGaGenS2(n+1) single crystals increases after annealing, which also falls into procession in the same sequence, changing in the range of 2.89−2.96 eV, and all the slopes of absorption coefficients are sharper than before, which also reflects the significant improvement in optical properties.

4. CONCLUSIONS AgGaGenS2(n+1) (n = 2, 3, 4, and 5) polycrystal synthesis process by vapor transport with mechanical oscillation method with different cooling processes and single crystal growth by the modified Bridgman method have been described in detail H

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Figure 10. Transmittance of AgGaGenS2(n+1) single crystal wafers in the 0.4−13 μm range (a) AgGaGe2S6; (b) AgGaGe3S8; (c) AgGaGe4S10; (d) AgGaGe5S12. Generation in ZnGeP2 and AgGaS2. Advances in Optical Materials 2011, AIThB18. (4) Niwa, E.; Haidar, S.; Masumoto, K.; Ito, H. Growth of TwinFree AgGaS2 Single Crystals by a Self-Seeding Vertical Gradient Freezing Method and Difference Frequency Generation Using These Crystals. Jpn. J. Appl. Phys. 2000, 39 (S1), 46. (5) Zhu, S.; Liu, J.; Zhao, B.; Jiang, H.; Li, Z. Crystal Growth and Differential Thermal Analysis of AgGaSe2. Cryst. Res. Technol. 1995, 30 (8), 1165−1168. (6) Kim, Y.; Seo, I.; Martin, S. W.; Baek, J.; Shiv Halasyamani, P.; Arumugam, N.; Steinfink, H. Characterization of New Infrared Nonlinear Optical Material with High Laser Damage Threshold, Li2Ga2GeS6. Chem. Mater. 2008, 20 (19), 6048−6052. (7) Mei, D.; Zhang, S.; Liang, F.; Zhao, S.; Jiang, J.; Zhong, J.; Lin, Z.; Wu, Y. LiGaGe2S6: A Chalcogenide with Good Infrared Nonlinear Optical Performance and Low Melting Point. Inorg. Chem. 2017, 56 (21), 13267−13273. (8) Badikov, V. V.; Badikov, D. V.; Laptev, V. B.; Mitin, K. V.; Shevyrdyaeva, G. S.; Shchebetova, N. I.; Petrov, V. Crystal growth and characterization of new quaternary chalcogenide nonlinear crystals for the mid-IR: BaGa2GeS6 and BaGa2GeSe6. Opt. Mater. Express 2016, 6 (9), 2933. (9) Lin, X.; Guo, Y.; Ye, N. BaGa2GeX6(X = S, Se): New mid-IR nonlinear optical crystals with large band gaps. J. Solid State Chem. 2012, 195, 172−177. (10) Piasecki, M.; Myronchuk, G. L.; Parasyuk, O. V.; Khyzhun, O. Y.; Fedorchuk, A. O.; Pavlyuk, V. V.; Kozer, V. R.; Sachanyuk, V. P.; El-Naggar, A. M.; Albassam, A. A.; Jedryka, J.; Kityk, I. V. Synthesis, structural, electronic and linear electro-optical features of new quaternary Ag2Ga2SiS6 compound. J. Solid State Chem. 2017, 246, 363−371. (11) Huang, W.; Zhao, B.; Zhu, S.; He, Z.; Chen, B.; Wu, L.; Zhen, Z.; Pu, Y.; Sha, M. Temperature behavior of thermal expansion anisotropy, Grü neisen parameters and thermal conductivity of chalcopyrite AgGa 0.7 In 0.3 Se 2 crystal. J. Alloys Compd. 2016, 688, 173−179.

in this study. The noncentrosymmetric structure results in the excellent nonlinear and linear optical property, while the distortion of tetrahedron causes the discrepancy of physicochemical properties, such as binding energy, vibrational frequency, carrier concentration, and band gap, which make them suitable for different applications. Finally, annealing treatments are carried out in order to improve the quality of these crystals. The absorptions at 2.9, 4, and 10 μm have been eliminated, and the band gap changed into the range of 2.90− 2.97 eV.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 28 85412745. E-mail: [email protected]. ORCID

Wei Huang: 0000-0002-7329-8106 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Project supported by the National Natural Science Foundation of China (Grant No. 51702222) and Postdoctoral Research and Development Fund of Sichuan University (Grant No. 2017SCU12002).

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REFERENCES

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