Tuning the optical, electronic and thermal properties of Cu3NbS4−xSex through chemical substitution

Erica M. Chena, Stanislav S. Stoykob, Jennifer A. Aitkenb and Pierre F. P. Poudeu*a
aLaboratory for Emerging Energy and Electronic Materials, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, 48109, USA. E-mail: ppoudeup@umich.edu; Fax: +1-734-763-4788; Tel: +1-734-763-8436
bDepartment of Chemistry and Biochemistry, Duquesne University, Pittsburgh, PA 15205, USA

Received 15th May 2017 , Accepted 14th June 2017

First published on 15th June 2017


Several compositions of the Cu3NbS4−xSex (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, and 4) series were synthesized through combination of the elements using conventional solid-state reactions. X-ray powder diffraction patterns suggest that the synthesized polycrystalline powder samples are isostructural to sulvanite (Cu3VS4). However, plotting the refined lattice parameters as well as the measured “true” density as a function of Se content (x) showed deviations from linearity, suggesting that the Cu3NbS4−xSex series is not a “true” solid solution. Differential scanning calorimetry data showed that all compounds incongruently melt at ∼1100 K and recrystallize with a minor impurity phase. Optical diffuse reflectance UV/Vis/NIR spectroscopy revealed a nearly linear contraction of the optical band gap with increasing Se content. The direct band gap decreases from 2.53 eV for x = 0.1 to 2.43 eV for x = 0.8. Electronic transport data indicate p-type semiconducting behavior for all samples. Thermal conductivity data showed a rather surprising trend, with the substituted compositions exhibiting higher thermal conductivity than the two end members.


Introduction

Sustainable materials made from earth-abundant elements with tunable band gap within the energy range from 1 eV to 3 eV are of great interest for various optoelectronic applications. Among candidate material systems under investigation, copper chalcogenides with sulvanite (Cu3VS4) structure have attracted considerable attention owing to the compositional flexibility,1 of the sulvanite structure type, which enables band gap engineering within a wide range (from 0.93 eV for Cu3VTe4 to 2.6 eV for Cu3TaS4)2 through careful manipulation of the chemical composition.3 This paves the way for the deployment of these materials in photovoltaic devices,4 solar water splitting cells5 and transparent conducting materials (TCMs) for use in light emitting diodes,6 laser diodes,7 solar cells and flat panel displays.8 Despite this potential use of sulvanite compounds, there has not been significant attention paid to the synthesis and elucidation of their crystal structures and functional properties. Among known sulvanite compounds, Cu3VS4 has garnered substantial attention as a host material for lithium intercalation9,10 and as the absorbing layer for photovoltaic11 applications. However, its isostructural homologues that can be obtained through chemical substitution of (1) V by Nb12 or Ta13 and (2) S by Se or Te, have received far less attention in comparison. Recently, theoretical work utilizing first-principles calculations has predicted some of their optical and electronic properties.1,6,14,15 Interestingly, these phases were thought to be p-type materials, which are generally lacking for optoelectronic applications in comparison to the development and availability of n-type materials. This work provides insight into the potential of Cu3NbSe4 and Cu3NbS4 for photovoltaic and p-type transparent semiconductor applications. It has been demonstrated that thin film deposition of these p-type semiconductors, like Cu3TaQ4 (Q = S or Se), is possible.16 Crystals of Cu3NbSe4 have been reported to have a red-orange color while Cu3NbS4 has been described as having either orange or greenish color.17 Furthermore, it was predicted that the direct gaps and indirect gaps are 2.31 eV and 1.82 eV for Cu3NbS4 and 1.95 eV and 1.64 eV for Cu3NbSe4.1,6,14,15 These band gaps are not suitable for p-type transparent semiconductor applications, however, there is potential for solid-solution series between the two compounds through partial substitution of S by Se in the structure of Cu3NbS4−xSex. Such substitution could enable tuning the band gap from 2.31 eV for Cu3NbS4 to 1.95 eV for Cu3NbSe4, therefore enabling the design and synthesis of optoelectronic materials for devices covering the green to yellow regions of the electromagnetic spectrum. In addition, such S to Se substitution will enable tuning the lattice parameter to match that of the GaAs substrate, which should facilitate epitaxial growth of single-crystal, thin films of Cu3NbS4−xSex on GaAs substrates for various optoelectronic devices.

Cu3NbSe4[thin space (1/6-em)]18 and Cu3NbS4[thin space (1/6-em)]19 crystallize in the primitive cubic space group P[4 with combining macron]3m (#215) with lattice parameters of 5.638(1) Å and 5.501(2) Å respectively. The crystal structure contains two tetrahedrally coordinated metal atoms, NbQ4 and CuQ4 (Q = S, Se). The Cu atom is located at the 3d position, whereas the Nb atom occupied the 1a position. In the three-dimensional structure, six crystallographically equivalent CuQ4 tetrahedra share corners to form an octahedral cluster around the NbQ4 tetrahedron, which in turn shares edge with each of the six CuQ4 tetrahedra. The resulting neutral [Nb@Cu6/2]Se4 cluster (Fig. 1b) can be regarded as the building block for the formation of the primitive cubic lattice. Interestingly, the lattice parameter of Cu3NbSe4 (a = 5.638 Å) is practically identical to that of GaAs (a = 5.653 Å), whereas the lattice parameters of Cu3NbS4 (a = 5.50 Å)19 is similar to that of Si (a = 5.431 Å). There is a 0.265% lattice mismatch between the structures of Cu3NbSe4 and GaAs, which satisfies the precondition for low residual strain (or even strain-free) epitaxial growth of Cu3NbSe4 films on GaAs substrates.20 One could reasonably expect a dislocation at approximately every 380 planes. The lattice mismatch between Cu3NbS4 and Si is slightly larger, 1.3%, but is also within a suitable range for epitaxial growth of strained Cu3NbS4 thin films on Si substrates. To date, there has not been significant insight into the measurement of the electronic, optical and thermal properties of these compounds. These measurements are necessary to probe their thermoelectric performance as well as confirming their predicted p-type semiconducting behavior. Here, we report of the optical, electronic and thermal properties of the Cu3NbS4−xSex series (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, and 4). We show through careful measurement of the Seebeck coefficient that these materials feature holes as the majority carrier for electronic conduction. The effects of sulfur substitution by selenium on the refined lattice parameters; thermal stability; optical band gaps; thermal conductivity; and electronic transport properties are discussed.


image file: c7qi00264e-f1.tif
Fig. 1 (a) Crystal structure of Cu3NbQ4 (Q = S, Se) highlighting the primitive cubic packing of the neutral [Nb@Cu6/2Q4] cluster. (b) Coordination environment of Cu and Nb and their connectivity within the [Nb@Cu6/2Q4] cluster. NbQ4 tetrahedra share all edges with CuQ4 tetrahedra, CuQ4 tetrahedra exclusively share corners with each other and NbQ4 tetrahedra are isolated from each other.

Experimental

Synthesis

Elemental powders of niobium (99.5%, Alfa Aesar), copper (99.5%, Alfa Aesar), and selenium (99.5%, Sigma Aldrich) and/or sulfur (99.5%, Alfa Aesar) were weighed in the appropriate stoichiometric ratios for the x values (x = 0, 0.1, 0.2, 0.4, 0.6 and 0.8) and then manually ground with an agate mortar and pestle until homogeneously mixed. These mixtures were then flame-sealed in evacuated (∼10−4 Torr) quartz tubes and placed in a tube furnace. The temperature profile is programmed for ramping to 573 K over 10 hours and held steady for 24 hours to allow the selenium and sulfur to react with the rest of the elements. Then the furnace temperature was increased to 823 K in 4 hours, held for 120 hours and finally cooled to room temperature over 24 hours. The sintered powder products were again manually ground with an agate mortar and pestle. After grinding, the polycrystalline powder samples of Cu3NbS4 and all intermediate compositions (varying x-values) exhibited a very fine consistency with a light olive green color whereas the Cu3NbSe4 powder yields a reddish-orange color. All manipulations were handled in an argon-filled glovebox and the synthesized materials were stored within the glovebox prior to structural characterization and material processing in order to avoid any oxidation.

Powder X-ray diffraction

X-ray powder diffraction patterns of the as-synthesized powders were recorded on a Rigaku Miniflex 600 with a graphite monochromator operating with Cu-Kα (λ = 1.54056 Å) radiation at 40 kV and 15 mA in Bragg–Brentano geometry. The scan condition was from 10° to 70° 2θ at 2° per second. The resulting powder patterns were compared with patterns in the PDF-4+ database as well as theoretical powder pattern simulated using single crystal data18,19 from the Inorganic Crystal Structure Database (ICSD). Structural data from the reference files were used as the starting models for the Rietveld refinement of the lattice parameters of the synthesized compounds using the FULLPROF software package.21

Differential scanning calorimetry (DSC)

Approximately 30 mg of Cu3NbS4−xSex powder was sealed in a small quartz DSC tube under a residual pressure of ∼10−3 Torr and loaded into a Netzsch Pegasus 404 F1 Differential Scanning Calorimeter with a blank quartz tube of approximately the same mass. The DSC data were recorded on heating and cooling between room temperature and 1273 K with heating rate of 20 K min−1 under flowing (20 L min−1) nitrogen gas. The measurement was repeated on the same sample to check for thermal stability.

Diffuse reflectance UV-Vis-NIR spectroscopy

Optical diffuse reflectance spectra of the Cu3NbS4−xSex (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, and 4) samples were collected on a Varian Cary 5000 UV-vis-NIR spectrometer equipped with a Harrick Praying Mantis diffuse reflectance accessory. BaSO4 was used as the 100% reflectance standard.22 Scans were made from 200 nm to 2500 at a rate of 600 nm per min. The percent reflectance was converted to absorption using the Kubelka–Munk relationship and wavelength was converted to energy.23

Pycnometry

The true density of the synthesized Cu3NbS4−xSex (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, and 4) samples was measured using He gas pycnometry on a Quantachrome Ultrapyc 1200e MUPY-30 pycnometer. Samples for the measurement were prepared by the uniaxial hot press method and data were collected by purging and pressurization the samples to 15 psi using helium gas.

Thermal conductivity

The as-synthesized polycrystalline powder samples were consolidated into 10 mm diameter and 2.5–3 mm thick pellets using a 20 ton uniaxial hot press. The pressing conditions were 100 MPa and 723 K (for samples with x ≤ 0.8) or 923 K (for the Cu3NbSe4 sample) for 4 hours. Cu3NbSe4 failed to fully densify with the lower annealing temperature of 723 K. Full densification procedures are described elsewhere.24 After densification, the pellets were polished to a mirror-finished surface on both sides. A thin layer of graphite was sprayed on both sides of the pellets that were then loaded into the laser flash apparatus, Linseis LFA 1000. Thermal diffusivity data were collected from room temperature to 623 K under dynamic vacuum. The thermal conductivity was calculated from the thermal diffusivity data using the relationship κ = DCpd, where D is the measured thermal diffusivity, Cp is the heat capacity and d is the geometrical density of the pellet. A Pyroceram 9606 reference material was measured alongside each sample, which enabled extraction of the heat capacity used for the thermal conductivity calculations. The instrument precision for the thermal diffusivity data is ±3%.

Thermopower and electrical conductivity

Hot pressed pellets for each composition were cut using a wire saw with a SiC slurry to produce bar specimens of about 9 mm × 2 mm × 2.5 mm. The bars were used for simultaneous measurement of the thermopower and electrical resistivity using an ULVAC-RIKO ZEM-3 system. The electrical resistivity and thermopower data were recorded from room temperature to 598 K with ΔT increments of 5 K, 10 K and 15 K. The measured data were used for the calculation of the power factor (S2σ), where S is the thermopower, and σ is the electrical conductivity.

Results and discussion

Synthesis and characterization

Fig. 2 shows the X-ray powder diffraction patterns for the Cu3NbS4−xSex series. A comparison of the measured XRD patterns to the theoretical patterns of Cu3NbS4 and Cu3NbSe4 calculated from single crystal data indicates that the intermediate compositions are isostructural to the two end members. No diffraction peaks from impurity phases or unreacted elements could be observed in the XRD patterns, suggesting complete substitution of S by Se in the crystal structure of Cu3NbS4. All XRD patterns were indexed in the space group P[4 with combining macron]3m (#215) and the lattice parameters of selected compositions of the Cu3NbS4−xSex series were refined using the Rietveld methods (Table 1). The refined values of the lattice constant of Cu3NbSe4 and Cu3NbS4 were 5.655(6) Å and 5.501(1) Å, respectively. These values are comparable, within 0.3%, to the reported lattice parameter for Cu3NbSe4[thin space (1/6-em)]18 and Cu3NbS4[thin space (1/6-em)]19 (Table 1). The lattice parameter for members of intermediate compositions increases with increasing x values (increasing Se content), which is consistent with the substitution of small sulfur atoms by the slightly larger selenium atoms within the structure of Cu3NbS4. However, the plot of the lattice parameter as a function of selenium content (Fig. 2b) exhibits a small deviation from the linear relationship (Vegard's law) anticipated for a “true” solid solution. Furthermore, a much larger deviation from linearity is observed in the plot of the density versus selenium content, suggesting that the Cu3NbS4−xSex series is not a “true” solid solution. This means that the replacement of selenium for sulfur is not completely random within the crystal lattice. The observed deviation from solid-solution behavior may originate from partial incorporation of selenium at vacant sites with the Cu3NbS4 structure instead of direct substitution at the S site. The measured densities for the Cu3NbS4−xSex series are comparable, within the precision (±5%) of the pycnometer, to the calculated densities obtained using the refined lattice parameters.
image file: c7qi00264e-f2.tif
Fig. 2 (a) Powder X-ray diffraction patterns of the as-synthesized Cu3NbS4−xSex (x = 0, 0.1, 0.2, 0.4, 0.6, 0.8 and 4) samples. (b) Variation of the refined lattice parameter and measured “true” density of Cu3NbS4−xSex samples with the Se content (x).
Table 1 Lattice parameter (Å), density (g cm−3) and thermal conductivity at 323 K (W m−1 K−1) for selected members of the Cu3NbS4−xSex series along with the fitting parameters, Rf and χ2 obtained from the Rietveld refinement x(Se)
  apublished arefined ρcalc[thin space (1/6-em)]a ρmeas κ323K Rf χ2
a The calculated density for the intermediate compositions was obtained using the refined lattice parameter.
0 5.501(2)19 5.501(1) 4.11 4.030 2.71 6.27 2.59
0.1 5.509(1) 4.14 4.220 3.91 5.10 1.96
0.2 5.511(2) 4.18 4.255 3.57 7.18 3.50
0.4 5.520(2) 4.25 4.270 2.61 8.24 2.4
0.6 5.528(2) 4.33 4.328 2.93 9.86 1.53
0.8 5.534(2) 4.40 4.453 2.86 8.47 1.61
4 5.638(1)18 5.655(6) 5.55 5.569 2.91 7.63 2.43


Fig. 3a and b show the first heating and cooling differential scanning calorimetry (DSC) curves for the Cu3NbS4−xSex series. All samples showed a single endothermic peak of melting upon heating from room temperature to 1300 K indicating the formation of nearly single-phase samples. The sulfur-rich compositions melt incongruently at ∼1086 K, and recrystallize on cooling at 1059 K with a minor impurity phase. A much better thermal stability is observed for the selenium phase, which melts congruently at much higher temperature (1200 K) and recrystallizes at without decomposition at 1150 K.


image file: c7qi00264e-f3.tif
Fig. 3 Differential scanning calorimetry (DSC) heating (a) and cooling (b) curves for the synthesized Cu3NbS4−xSex samples.

Optical properties

Fig. 4a depicts the UV-Vis portion of the optical diffuse reflectance data that was converted to absorption spectra of the synthesized compositions in the Cu3NbS4−xSex series. All samples show a sharp increase in absorbance in the energy range from 2.1 eV to 2.5 eV, suggesting that the band gaps for various compositions in this series lies somewhere in this energy range. The peak from the first derivative of the absorbance versus energy curve and the band gap determination from Tauc analysis are compiled in Table 2. It can be observed from Table 2 that the band gap energy obtained from the peak in the first derivative of the absorbance versus energy curve matches well with the band gap values extracted from a Tauc analysis for direct allowed transitions. This suggests that the optical band gaps obtained from the optical measurement are direct gaps. This result is consistent with the calculated direct band gap of 2.4 eV reported for Cu3NbS4.25 Although the conduction band minimum (VBM) and valence band maximum (VBM) in the electronic band structure of Cu3NbS4 are dominated by Nb d-states and Cu d-states, respectively, a significant fraction of VBM and CBM states at are of S p-character. Therefore, one can speculate from the observed trend in the band gap of Cu3NbS4−xSex that the substitution of S by Se affects both the CBM and VBM, presumably shifting both bands to closer proximity.26
image file: c7qi00264e-f4.tif
Fig. 4 (a) UV/Vis optical absorption spectra for selected compositions of the Cu3NbS4−xSex series; (b) correlation between the optical band gap and lattice parameters of the Cu3NbS4−xSex series.
Table 2 Summary of the optical band gaps for Cu3NbS4−xSex samples extracted from Tauc analysis (right-hand column) compared to those obtained from the first derivative of the absorbance versus energy plot (middle column)
Composition Energy for peak in dA/dE(E) Direct allowed (r = 1/2)
Cu3NbS4 2.6 eV 2.55 eV
Cu3NbS3.9Se0.1 2.58 eV 2.53 eV
Cu3NbS3.8Se0.2 2.56 eV 2.50 eV
Cu3NbS3.6Se0.4 2.52 eV 2.46 eV
Cu3NbS3.4Se0.6 2.50 eV 2.44 eV
Cu3NbS3.2Se0.8 2.48 eV 2.43 eV
Cu3NbSe4 2.2 eV 2.17 eV


It is interesting to note that the band gap of 2.6 eV for Cu3NbS4 corresponds well with a photon energy in the lower bound of blue and upper bound of green, whereas the band gap of 2.2 eV for Cu3NbSe4 is in the upper bound of yellow and lower bound of green. Therefore, the substitution between S and Se in the structure of both Cu3NbS4 and Cu3NbSe4 enables tuning the direct band gap of intermediate compositions between 2.2 eV and 2.6 eV, which covers the full range of the green portion of the electromagnetic spectrum (Fig. 5).27–41 This paves the way for further exploration of optoelectronic devices operating within this energy range. For example, selected compositions from this series can be used as the p-type transparent conducting layer (electrode) in solar cell devices based on thin-film silicon, GaAs, chalcopyrite, organic, perovskite, etc. Additional potential use of such transparent electrodes includes organic and inorganic LEDs (OLEDs, LEDs), smart windows, and liquid-crystal displays (LCDs). One can also envision optoelectronic devices with optical emission in the blue and green regions as an alternative to using gallium phosphide (Fig. 5).27–41


image file: c7qi00264e-f5.tif
Fig. 5 Band gap versus lattice constant for common optoelectronic materials. The substitution of sulfur by selenium within the Cu3NbS4−xSex series enables variation in the energy band gap covering the full energy range of the green portion of the electromagnetic spectrum.27–41

Remarkably, the large change in the optical band gap in the Cu3NbS4−xSex series is associated with only a small variation in the lattice parameter (Fig. 5). For example, increasing the lattice parameter by only 0.6% results in a 0.13 eV drop in the optical band gap (Fig. 4b). This implies that one can engineer an optoelectronic device, which requires a series of band gaps, while maintaining an excellent lattice coherency at the interfaces between successive electrodes. The optical band gap of each of the electrodes within such device can be precisely tuned to produce the desired photon wavelength, by controlling the degree of selenium to sulfur substitution within the Cu3NbS4−xSex series.

Thermoelectric properties

Fig. 6 shows the thermal conductivity of the Cu3NbS4−xSex series calculated from the thermal diffusivity data. The thermal conductivity values at 323 K for Cu3NbS4 and Cu3NbSe4 are 2.91 W m−1 K−1 and 2.71 W m−1 K−1 respectively and decrease drastically with increasing temperature. The observed decrease in the thermal conductivity is associated with an increase in phonon scattering, due to an increase in lattice vibrations with temperature. The magnitude of the drop in the thermal conductivity diminishes at temperatures above 473 K. At 673 K, the thermal conductivity value is 1.16 W m−1 K−1 for both compositions. The electronic contribution to the thermal conductivity (κelec) estimated from the electrical conductivity using Wiedemann-Franz law, κelec = σLT, where L = 2.44 × 10−8 W Ω K−2 is the Lorenz number,42 is negligible given the very low electrical conductivity of both Cu3NbS4 and Cu3NbSe4 phases (Fig. 7). Therefore, the total thermal conductivity in these compounds is dominated by the lattice contribution (κlatt). Interestingly, the thermal conductivity values of the intermediate compositions are comparable or higher than those of the two end members. For instance, the substitution of 2.5% sulfur by selenium (x = 0.1) drastically increases the total thermal conductivity from 2.6 to 3.9 W m−1 K−1 at 327 K.
image file: c7qi00264e-f6.tif
Fig. 6 Temperature dependence of the thermal conductivity of the synthesized Cu3NbS4−xSex samples.

image file: c7qi00264e-f7.tif
Fig. 7 Temperature dependence of (a) electrical conductivity, (b) thermopower, (c) power factor and (d) ZT for the Cu3NbS4−xSex series.

This initial large increase in the total thermal conductivity of Cu3NbS4−xSex is quite surprising and suggests a greater stability Cu3NbS4 crystal lattice upon substitution of a small fraction of S by Se. Indeed, this sample maintains the largest thermal conductivity at high temperatures. As discussed above, such lattice stabilization may originate from the incorporation of a small fraction of Se at vacant crystallographically sites within the structure of Cu3NbS4. Further increasing the selenium contents to 5% (x = 0.2) result in a sharp drop in the thermal conductivity at high temperatures indicating effective mass fluctuation phonon scattering due to the intermixing of sulfur and selenium within the crystal lattice. The effect of mass fluctuation phonon scattering increases with the incorporation of 10% selenium (x = 0.4) within the Cu3NbS4 crystal lattice. The lowest lattice thermal conductivity, ∼1 W m−1 K−1 at 775 K, was observed for the composition with x = 0.8 and is attributed to a stronger contribution of mass fluctuation to phonon scattering in this sample.

Fig. 7a through d show the temperature dependent electrical conductivity, Seebeck coefficient, power factor and ZT for selected compositions of the Cu3NbS4−xSex series. The sample with x = 0 (Cu3NbS4) exhibits the lowest values of the electrical conductivity, starting from 14 S m−1 at 306 K and slightly decreasing to 3.4 S m−1 at 598 K. The selenium counterpart (x = 4) showed larger values of the electrical conductivity starting from 190 S m−1 at 306 K, which is over 10 times higher than that observed for Cu3NbS4, and increasing to 202 S m−1 at 323 K. Further increase in the temperature results in a gradual drop in the electrical conductivity down to 130 S m−1 at 598 K. The observed drop in the electrical conductivity is consistent with heavily doped semiconductor behavior, where the mobility of thermally excited carriers is severely diminished by electron – electron and acoustic phonon scattering. Regardless of the temperature, the electrical conductivity initially increases with increasing selenium content, reaching the highest values for the composition with x = 0.4 (10%). Further increase in the selenium content resulted in a drop in the electrical conductivity (Fig. 7a). The electrical conductivity of all intermediate compositions initially increases with rising temperature, reaching maximum values at ∼400 K. Further increase in temperature resulted in a drop in the electrical conductivity, which is consistent with heavily doped semiconductor behavior. The observed initial increase of the electrical conductivity below 400 K suggests thermal excitation of electrons from the valence band to acceptor impurity states within the band gap leading to heavily doped semiconducting behavior at temperatures above 400 K. This analysis is consistent with the nearly linear increase in the thermopower with increasing temperature for all composition of the Cu3NbS4−xSex series (Fig. 7b). All samples showed positive values for thermopower in the whole temperature range indicating p-type semiconductor behavior. The thermopower for Cu3NbS4 and Cu3NbSe4 are 26 μV K−1 and 17 μV K−1 at 309 K and increase with temperature to 60 μV K−1 and 40 μV K−1 at 600 K, respectively. Interestingly, the intermediate compositions showed larger thermopower values compared to the two end members. The largest thermopower values are observed for the composition with x = 0.4. At 309 K; the thermopower for this composition is 40 μV K−1 and increases linearly with temperature to ∼104 μV K−1 at 700 K.

The observed increase in both thermopower and electrical conductivity for the intermediate compositions resulted in a large increase in the power factor when compared to the two end members. The power factor for Cu3NbS4 increases slightly with temperature from 9.7 × 10−3 μW m−1 K−2 at 306 K to 4.1 × 10−2 μW m−1 K−2 at 523 K (Fig. 7c). Slightly larger values of the power factor were observed for the selenium analog, Cu3NbSe4. At 309 K the power factor for this composition is 6 × 10−2 μW m−1 K−2 and increases almost linearly to 0.2 μW m−1 K−2 at 600 K. Partial substitution of sulfur by selenium in the Cu3NbS4−xSex series increases the power factor in the whole temperature range, when compared to Cu3NbS4. The largest power factor is observed for the composition with x = 0.4, starting with a value of 0.5 μW m−1 K−2 at 309 K, which rapidly increases with rising temperature to a peak value of 2.2 μW m−1 K−2 at 650 K. The observed increase in the electrical conductivity and thermopower of Cu3NbS4−xSex samples upon substitution of sulfur by selenium, while maintaining the thermal conductivity close to that of the pristine Cu3NbS4 sample resulted in improved thermoelectric figure of merit, ZT, when compared to the two end members. The largest enhancement in ZT is obtained for the composition with x = 0.4, which displays a ZT value of 10−3 at 650 K.

Conclusion

We have investigated the effects of sulfur to selenium substitution on the structure, optical, electronic and thermal properties of the Cu3NbS4−xSex series. Single-phase polycrystalline powders of selected compositions were synthesized through solid-state reaction of the elements at high temperature. X-ray powder diffraction on polycrystalline powders revealed that the synthesized materials are isostructural to the end members. However, a plot of the refined lattice parameter and measured density as a function of increasing selenium content (x values) show a slight deviation for linearity suggesting that the Cu3NbS4−xSex series is not a “true” solid-solution, meaning that the substitution of selenium for sulfur is not truly random throughout the crystal lattice. Electronic transport data showed p-type, heavily doped semiconductor behavior for all samples. Interestingly, the optical direct band gap of various Cu3NbS4−xSex compositions can be tuned within the range from 2.6 eV for the sulfide end member and 2.2 eV for the selenide end member with marginal increase in the lattice parameter. These band gaps represent photon energies that cover the lower bound of blue all the way to the upper bound of yellow. In addition, the lattice parameters ranging from 5.655(6) Å for Cu3NbSe4 to 5.501(1) Å for Cu3NbS4, are comparable to that of GaAs (a = 5.652 Å).34 These features could enable the design of fully tunable, green band gap optoelectronic devices, as can be seen in the map of band gap versus lattice parameter for several common optical materials shown in Fig. 5. The lattice compatibility with several commercial semiconductors allows for many device-engineering permutations.

Acknowledgements

This work was supported by the National Science Foundation under Award DMR-1561008. E. C. and P. F. P. P. gratefully acknowledge financial support for electrical conductivity and thermopower measurements from the Department of Energy, Office of Basic Energy Sciences under Award # DE-SC-0008574. J. A. Aitken and S. S. Stoyko acknowledge funding from the National Science Foundation under grant DMR-1611198.

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Footnote

In honor of Mercouri G. Kanatzidis for his contributions to Inorganic Chemistry for over 30 years.

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