Strong Infrared NLO Tellurides with Multifunction - ACS Publications

Apr 12, 2016 - Academy of Sciences, Fuzhou, Fujian 350002, People,s Republic of China. ‡ ... great interest for their possible synergy of different ...
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Strong Infrared NLO Tellurides with Multifunction: CsXII4In5Te12 (XII = Mn, Zn, Cd) Hua Lin,† Yi Liu,‡ Liu-Jiang Zhou,§ Hua-Jun Zhao,∥ and Ling Chen*,† †

Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China ‡ State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China § Bremen Center for Computational Materials Science, University of Bremen, Am Falturm 1, 28359 Bremen, Germany ∥ School of Chemistry and Chemical Engineering, Zunyi Normal College, Zunyi, Guizhou 563002, People’s Republic of China S Supporting Information *

ABSTRACT: Chalcogenides are the most promising midand far-infrared materials for nonlinear optical (NLO) applications. Yet, most of them are sulfides and selenides, and tellurides are still rare. Herein, we report three new KCd 4 Ga 5 S 1 2 -structure type NLO-active tellurides, CsXII4In5Te12 (XII = Mn, Zn, Cd), synthesized by solid-state reactions. The structure features a 3D diamond-like framework constructed by vertex-sharing asymmetric MTe4 tetrahedra that are stacked along the c-axis. CsCd4In5Te12 exhibits the strongest powder second-harmonic generation (SHG) intensity at 2050 nm (0.61 eV) among tellurides to date, 9 × benchmark AgGaS2 in the range of 46−74 μm particle size. The primary studies reveal the 1.42 eV direct band gap and high absorption coefficient in the visible spectral region for CsCd4In5Te12, suggesting it is a new potential solar cell absorber material. In addition, CsMn4In5Te12 also displays a spin-canted antiferromagnetic property below 50 K.

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containing compounds. They are CsAg2TeS6,11 Ba5In4S7Te4,12 {[In(en)3][In5Te9(en)2]·0.5en}n,13 and Na2Ge2Se4.55Te0.45,14 all of them showing only very weak SHG intensities. The reason might be the unique chemical characteristics of the Te atom, e.g., the large radius, the small electronegativity, and the more diffuse 5p orbitals. Therefore, the synthesis of noncentrosymmetic telluride remains a grand challenge. Herein we reported three new NLO-active tellurides, namely, CsMn 4 In 5 Te 12 (CMIT), CsZn 4 In 5 Te 12 (CZIT), and CsCd4In5Te12 (CCIT). CCIT shows the strongest powder SHG intensity (9 × AGS) among tellurides to date in the particle size range of 46−74 μm at 2050 nm (0.61 eV). CCIT also possesses direct band gap energy (Eg ≈ 1.42 eV) and a large absorption coefficient in the visible spectral region, suggesting it is a potential new solar cell absorber material. In addition, thermal stabilities, transmission spectroscopy, and magnetic properties as well as density functional theory (DFT) studies are discussed.

INTRODUCTION Multifunctional solid-state materials have recently attracted great interest for their possible synergy of different properties and new potential applications.1 Multifunctional second-order infrared (IR) nonlinear optical (NLO) materials exhibit not only fascinating structural chemistry but also diverse properties. For example, RE4GaSbS92 and RE4InSbS93 show the coexistence of NLO property and magnetic property. CsZrPS6,4 CsZrPSe6,5 and Ba3AGa5Se10Cl2 (A = K, Rb, Cs)6 exhibit NLO and photoluminescent properties. (Hg8As4)(Bi3Cl13)7 is NLO and piezoelectric active, and Ba4MnGa4Se10Cl2 shows NLO, photoluminescent, and magnetic properties.8 In addition, sulfides and selenides of AXII4XIII5Q12 (A = K−Cs; XII = Mn, Cd; XIII = Ga, In) exhibit strong NLO, magnetic, and thermochromatic properties.9 Typically, on going from sulfide to telluride, the secondharmonic generation (SHG) intensity should increase and the infrared absorption edges will red-shift by several micrometers. For example, the calculated NLO coefficient d36 increases from 18.2 pm/V for AgGaS2 (AGS) to 99.5 pm/V for AgGaTe2.10 In addition, their transparency ranges red-shift from 0.48−11.4 μm to 0.91−23 μm.10 The larger NLO coefficient will enhance the laser conversion efficiency, and the materials should be transparent in the applied spectral region. However, the typical IR NLO compounds are sulfides and selenides; tellurides are relatively rare. As we know, there are only four NLO-active Te© XXXX American Chemical Society

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EXPERIMENTAL SECTION

Syntheses. Pure phases were prepared by the reactions of CsCl/ XII/In/Te = 5:4:5:12 (XII = Zn, Cd). The reactants were loaded into a fused-silica tube and sealed under high vacuum. The assembly was heated to 1223 K over 50 h, sustained at 1223 K for 4 days, and then Received: February 3, 2016

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structures were solved by direct methods and refined using the SHELX-97 software.19 Taking CsCd4In5Te12 as an example, the final refinement converged to R1 = 0.0313, wR2 = 0.0683, with a formula of CsCd4.0(3)In5.0(3)Te12 and a Flack parameter of 0.04(10). This was consistent with the EDX analysis. An attempt to solve the structure in the higher acentric space group R32 or R3m failed. Additionally, the strong SHG effect of CCIT confirmed the correct assignment to the noncentrosymmetric space group. The refinement data are listed in Tables 1, S2, and S3 (Supporting Information, SI).

slowly cooled to 473 K at a rate of 5 K/h, before the furnace was turned off. The synthesis of CMIT was similar, but differed in the loading ratio of x:4:5:12. A pure CMIT was obtained when 1 < x < 2; but when x > 2, a small amount of the byproduct CsMnInTe3 (dark red plates)15 was also seen. The raw products were washed with distilled water. The semiquantitative energy dispersive X-ray (EDX) analyses results (e.g., Cs1.1(2)Cd4.0(3)In5.1(2)Te12) agreed well with the single-crystal XRD refinement data (e.g., CsCd4.0(3)In5.0(3)Te12) (Figure 1a, Table S1). These compounds were stable in ambient atmospheres for more than 12 months. The PXRD patterns matched well with the simulated ones, as shown in Figure 1b.

Table 1. Crystallographic Data and Refinement Details for CsXII4In5Te12 (XII = Mn, Zn, Cd) formula fw cryst syst cryst color cryst descrip cryst size (mm) space group a (Å) c (Å) V (Å3) Z Dc (g/cm3) μ (mm−1) GOOF on F2 R1, wR2 (I > 2σ(I))a R1, wR2 (all data) largest diff peak and hole (e/Å3) absolute struct param a

CMIT

CZIT

CCIT

CsMn4In5Te12 2457.97 trigonal black block 0.10 × 0.09 × 0.08 R3 (no. 146) 15.770(3) 10.721(3) 2309.2(2) 3 5.303 17.556 1.020 0.0265, 0.0552 0.0282, 0.0559 1.833, −1.459

CsZn4In5Te12 2499.69 trigonal black block 0.10 × 0.08 × 0.06 R3 (no. 146) 15.585(4) 10.591(4) 2227.9(2) 3 5.590 19.727 1.025 0.0306, 0.0660 0.0320, 0.0665 3.648, −1.432

CsCd4In5Te12 2687.81 trigonal black block 0.12 × 0.09 × 0.08 R3 (no. 146) 15.8837(6) 10.7939(7) 2358.4(2) 3 5.677 18.288 1.007 0.0313, 0.0683 0.0351, 0.0696 1.662, −2.082

−0.08(11)

0.0(2)

0.04(10)

R1 = ∑|Fo| − |Fc|/∑|Fo|, wR2 =

[∑w(Fo2

−

Fc2)2/∑w(Fo2)2]1/2.

Second Harmonic Generation Estimation. SHG intensities were estimated on polycrystalline samples using 2050 nm laser radiation on a modified Kurtz-NLO system.20 Samples and reference AGS were ground and sieved in distinct particle size ranges (30−46, 46−74, 74−106, 106−150, and 150−210 μm) as detailed previously.9 Theoretical Studies on Electronic Structures and Optical Properties. The calculations were carried out based on DFT21a with the generalized gradient approximation (GGA)21b as implemented in the Vienna ab initio simulation package (VASP).21c The plane-wave basis with projector augmented wave (PAW)21d,e potentials was used for the core electrons. A plane-wave cutoff energy of 450 eV was adopted for all calculations. The tetrahedron method was taken for the κ integration in the Brillouin zone.21 The precision of the κ-point spacing was 0.02 Å−1 in the Brillouin zone. In the optical property calculations, the denser κ-point grids of 0.02 Å−1 were used. The scissors operators of 0.61, 0.53, and 1.79 eV were applied for CZIT, CCIT, and AGS, respectively. The complex dielectric function ε(ω) = ε1(ω) + iε2(ω) was calculated,22 and relative linear optical parameters were gained through the Kramers−Kroning transform.23 The second-order nonlinear susceptibility χabc (−2ω,ω,ω) was calculated by the means of the length-gauge formalism.24 The parameter settings and calculation model were set as given in ref 9.

Figure 1. (a) EDX result of CCIT. (b) Experimental and simulated PXRD patterns of title compounds. The magnified part shows the peaks from 35° to 45° for all compounds are the same but shifted from CCIT to CZIT due to the decrease in cell volume. Measurements. The purity of compounds was checked with the aid of a Rigaku DMAX 2500 powder X-ray diffractometer with Cu Kα radiation. The elemental analyses were performed on a JSM6700F scanning electron microscope equipped with an EDX detector. Magnetic susceptibility data were collected on a Quantum Design MPMS-XL magnetometer in the range 2−300 K, and the fielddependent magnetization was recorded between ±8 T at 2 K. The optical band gaps were measured using a PerkinElmer Lambda 900 UV−vis spectrophotometer. BaSO4 as a 100% reflectance standard and the absorption data were calculated using the Kubelka−Munk equation based on the obtained reflectance spectra as described elsewhere.16,17 The thermal stability analyses were measured on a Netzsch STA 449C simultaneous analyzer. Single-Crystal X-ray diffraction (XRD). Single-crystal XRD at 293 K was collected on a Rigaku Saturn 70 CCD diffractometer with Mo Kα radiation. The absorption correction was done,18 and

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RESULTS AND DISCUSSION Crystal Structure. Compounds CsXII4In5Te12 (XII = Mn, Zn, Cd) adopt the KCd4Ga5S12-structure type9,25 and crystallize in the R3 space group (Table 1). Each metal atom (M) is disordered by XII and In atoms, with occupancies of 25−60% Mn, 32−53% Zn, and 36−49% Cd (Table S2, SI). The 3D diamond-like framework is formed by the MTe4 tetrahedron B

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Optical and Magnetic Properties. The solid-state diffuse reflectance spectra on polycrystalline CMIT, CZIT, and CCIT samples gave band gaps of 1.48, 1.61, and 1.42 eV, respectively (see Figure 3, Figure S1, and Table 2). These values are

aligning perpendicular to the ab plane (Figure 2). The structure can also be viewed as a ccp-like packing of the Cs-centered-Te12

Figure 3. UV−vis diffuse-reflectance spectrum of CCIT (inset: direct band gap fit based on the theoretical calculations).

Table 2. Measured Optical Properties for CsXII4In5Te12 (XII = Mn, Zn, Cd) experimental CsXII4In5Te12

SHG intensitya

transmn region (μm)b

Eg (eV)

CMIT CZIT CCIT

1.7 4.3 9.2

1.06−25 0.82−25 0.92−25

1.48 1.61 1.42

Relative to commercial AgGaS2 with λincident = 2.05 μm at particle sizes of 46−74 μm. bMeasured on a polycrystalline sample. a

consistent with their crystal colors. These are smaller than those of some typical IR NLO compounds, e.g., AGS (2.56 eV, SI, Figure S3), AgGaSe2 (1.80 eV),28 and ZnGeP2 (1.75 eV).29 The band gap of CCIT agrees well with that calculated by the equation αE = A(E − Eg),2 for a direct band gap semiconductor (Figure 3).17 Such a band gap is very close to those of typical inorganic semiconductor absorber materials, such as GaAs and CdTe.30 According to the Shockley−Queisser theory,31,32 the Eg (1.4−1.5 eV) will give rise to the conversion efficiency of a solar cell (Figure 4). Meanwhile, the direct band gap characteristic of CCIT satisfies one of the most important requirements of a solar cell absorber, because it determines the absorption intensity of sunlight.33 Moreover, this compound has a high absorption coefficient α (>104 cm−1) in the visible spectrum range (Figures S2, S11). Therefore, these features distinguish CCIT as a potential new direct and wide band gap chalcogenide-based semiconductor solar cell absorber. The optical transparencies for the polycrystalline samples were checked by IR spectra and solid-state diffuse reflectance spectra. The broad transparent regions in the range 1.0−25 μm (SI, Figure S2) can cover atmospheric transparent windows I (3−5 μm) and II (8−14 μm).34 The transparency of CCIT is comparable to that of the top IR NLO material AGS (0.15 to 23 μm, Figure S3). Meanwhile, CMIT displays a spin-canted antiferromagnetic property below 50 K based on the χ−T and M−T curves

Figure 2. (a) Hexagonal 3D structure of CsXII4In5Te12 with interstitial Cs atoms. (b) Polyhedral packing showing the alignment of all the MTe4 tetrahedra perpendicular to the ab plane (M is disordered by XII and In atoms). (c) Cs centers in a Te12 cuboctahedron (dark green) linked by M atoms.

cuboctahedron that is linked via M atoms (Figure 2c). All metal atoms are tetrahedral coordinated by four Te atoms with normal distances of 2.742(2)−2.796(2), 2.700(2)−2.754(2), and 2.771(2)−2.832(2) Å for CMIT, CZIT, and CCIT, respectively (Table S3). The distances of Cs−Te, ranging from 4.114(3) to 4.199(2) Å, are comparable to those in CsMnInTe3 (3.812(2)−4.429(2) Å),15 CsLnZnTe3 (3.793(8)−4.267(6) Å),26 and CsLnCdTe3 (3.813(2)−4.201(2) Å).27 C

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Figure 4. Band gap (eV) versus maximum conversion efficiency (%) for a solar cell according to the Shockley−Queisser theory.

(Figure S5). Similar behavior is observed in CsMn4In5Se129b and some other Mn-containing compounds.35 Thermal Stabilities. CCIT shows no obvious weight loss up to 1095 K, and the endothermic peak indicates that the compound decomposed after this temperature, as shown in Figure 5. To check the thermal stability, the CCIT sample was

Figure 6. SHG measurements on CMIT, CZIT, CCIT, and AGS (reference): (a) SHG intensities depend on particle size; (b) SHG signals in the particle size range 46−74 μm.

Figure 5. TG (black) and DTA (blue) diagrams of CCIT.

Theoretical Studies. The ab initio calculations demonstrate the direct energy gap feature for both CZIT and CCIT (Figure S7). Their density of states (DOSs) are similar, as shown in Figure 7. The Cs atoms serve as electron donors to stabilize the structure, and their contributions near EF are negligible. The XII and In atoms make the main contribution around EF to the conduction band minimum (CBM), and the Te-5p, as the main component, constitutes the valence band maximum (VBM). Therefore, the band gap can be attributed to

annealed under vacuum for 12 h at 1023, 1073, and 1123 K, respectively. Samples annealed at 1023 and 1073 K were still pure-phased CCIT. However, the 1123 K annealed sample contained minor binary CdTe (detailed information is given in Figure S6). NLO Properties. The SHG properties were checked on a 2050 nm Q-switch laser based on the Kurtz and Perry method.20 As shown in Figure 6a, the results indicate that CMIT, CZIT, and CCIT are non-phase-matchable in the spectral region examined.20,36,37 Relative SHG intensities of these polycrystalline compounds are about 1.7, 4.3, and 9.2 times that of AGS at the 46−74 μm particle size (Figure 6b), among which, that of CCIT is the largest among tellurides. The known Te-containing compounds usually exhibit weak SHG, such as {[In(en)3][In5Te9(en)2]·0.5en}n (∼0.7 × AGSe, in the particle size range of >150 μm),13 CsAg2TeS6 (∼0.3 × AGSe, at 45−63 μm particle size),11 Na2Ge2Se4.55Te0.45 (∼6.0 × AGS, at 125−150 μm particle size),14 and Ba5In4S7Te4 (∼0.5 × AGS, at 74−106 μm particle size).12

Figure 7. DOSs of (a) CZIT and (b) CCIT. D

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Inorganic Chemistry Table 3. Property Comparison between Selected CZIT, CCIT, Commercial AGS, and AgGaTe2 SHG coeff (pm/V) at 2.05 μm

a

absorp edge (μm) transmn range (μm) Eg (eV) static birefringencea average refractive index at 2.05 μma average static dielectric constanta a

CZIT

CCIT

AGS

AgGaTe210

d11 = 95.1 d15 = 86.2 d22 = 72.7 d33 = 149.8 0.82(obs)b 0.75(calc)a 0.82−25b 1.61(obs)b 0.99(calc)a 0.022 2.98 8.51

d11 = 101.8 d15 = 106.7 d22 = 89.1 d33 = 184.3 0.92(obs)b 0.86(calc)a 0.92−25b 1.42(obs)b 0.89(calc)a 0.013 2.97 8.35

d36 = 18.2

d36 = 99.5

0.60(obs)b 0.46(calc)a 0.60−23b 2.56(obs)b 2.702 0.039 2.49 6.15

N/A 0.91−23 1.31(obs) 0.75(calc) 0.012 N/A N/A

Calculated value. See more details in the SI. bMeasured on ground crystals.

region, suggesting CCIT is a promising light absorber in thinfilm solar cells. However, some properties remain to be examined, and further investigations are worthwhile.

the electron transition from Te-5p to Zn-3d/Cd-4d, Zn-4s/Cd5p, In-5s, and In-5p orbitals. Figure S8 depicts the variation of the frequency-dependent dielectric function. In addition, more details on the spectral features of the optical susceptibilities are presented in Figures S9−13 and listed in Table 3. The static Δn values of CZIT and CCIT are 0.022 and 0.013, respectively, both smaller than that of AGS (0.039) (Figure S10). The relatively small birefringence (Δn) values in the IR range suggest a non-phase-matching feature, in general agreement with the experimental results. As shown in Figure 8, the calculated d33 are 149.8 and 184.3 pm/V

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00254. Crystallographic data, solid-state diffuse reflectance spectra, reflection and FT-IR spectra, TG-DTA diagrams, spectral features of the optical calculation results, and additional figures and tables (PDF) Crystallographic data file (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (011)86-591-63173131. Notes

The authors declare no competing financial interest.

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Figure 8. Static SHG coefficients of (a) CZIT and (b) CCIT as a function of the cutoff energy. Dashed line: EF; dotted line: different regions in valence bands (VB) and conduction bands (CB).

ACKNOWLEDGMENTS This research was supported by the NSF of China (Nos. 21225104, 21233009, 21301175, 91422303, 21571020, and 21171168) and the NSF of Fujian Province (No. 2015J01071). We thank Prof. Yong-Fan Zhang at Fuzhou University for helping with the DFT calculations and Prof. Ning Ye at FJIRSM for helping with the SHG measurements.

for CZIT and CCIT at 2050 nm (calc 0.61 eV), respectively, larger than d36 (18.2 pm/V) of AGS, in accordance with the experimental observation. The calculated values of AgGaTe2 are also listed for comparison.10 The origin of the SHG has been discussed by means of the cutoff-energy-dependent SHG coefficient,38 based on the length-gauge formalism.24 The SHG response originates dominantly from VB-1 and CB-3 zones, i.e., from the occupied Te-5p, Zn-4p/Cd-5p, Zn-3d/Cd4d, and In-5p orbitals to the empty Te-5p, Zn-4p/Cd-5p, and In-5p orbitals (Figure 8).

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REFERENCES

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CONCLUSION In conclusion, three new NLO-active tellurides with KCd4Ga5S12-structure type, CsXII4In5Te12 (XII = Mn, Zn, Cd), were synthesized using solid-state reactions. The polycrystalline samples show strong SHG intensities at 2050 nm laser radiation; among them, CCIT shows the strongest powder signal among tellurides to date (9 × AGS) in the range 46−74 μm. Besides, CCIT possesses a direct energy gap of 1.42 eV and a large absorption coefficient in the visible spectral E

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DOI: 10.1021/acs.inorgchem.6b00254 Inorg. Chem. XXXX, XXX, XXX−XXX