Solution-Based Synthesis of Sulvanite Cu 3 TaS 4 and Cu 3 TaSe 4 Nanocrystals

: Sulvanites have the parent formula Cu 3 MCh 4 . The metal M belongs to group 5 and Ch is a chalcogen. The tantalum sulvanites Cu 3 TaS 4 and Cu 3 TaSe 4 are predicted to have wide band gaps and p-type conductivity and show promise in optoelectronic applications. Their potential as p-type transparent conductors or efﬁcient photocatalysts for visible-light water splitting is a valuable incentive to explore these materials in their nanoscale form, toward bottom-up processing opportunities. Reported herein are the ﬁrst syntheses of nanosized Cu 3 TaS 4 and Cu 3 TaSe 4 sulvanites, which preserve the parent cubic crystal structure but show that morphology at the nanoscale is dependent of the reaction conditions. The two solution-based methods for synthesizing the tantalum S and Se sulvanites result in Cu 3 TaS 4 or Cu 3 TaSe 4 nanocrystals (NCs) with prismatic morphology, or, in the case of Cu 3 TaSe 4 , could lead to core-shell spherical nanostructures. The Cu 3 TaS 4 NCs and Cu 3 TaSe 4 NCs have good absorption in the UV-Vis region, while the Cu 3 TaSe 4 core-shell NCs possess broad absorption bands not only in the UV-Vis but also in the near-infrared region. Photoluminescence measurements of Cu 3 TaS 4 and Cu 3 TaSe 4 reveal optical bandgaps of 2.54 and 2.32 eV, respectively, consistent with the values measured in bulk. Additionally, the current–voltage (I-V) curve of Cu 3 TaS 4 NCs proves its electrical conductivity.


Introduction
In the last few years, the tantalum members of the sulvanites family Cu 3 MCh 4 (M = V, Nb, Ta, Ch = S, Se) have been predicted to act as p-type transparent conductive materials (TCMs).
Defined as optically transparent and electrically conductive materials, TCMs have a wide variety of applications, from front-surface electrodes for solar cells and flat-panel displays, energy-conserving windows, rear-view mirrors with automatic dimming capability, and other optoelectrical devices to gas sensors and biosensors [1][2][3][4][5][6][7]. To date, the most common and readily accessible inorganic transparent conducting materials are transparent conducting oxides (TCOs), generally in the form of indium tin oxide (In 2 O 3 : Sn, ITO), fluorine-doped tin oxide (SnO 2 : F, FTO), and aluminum-doped zinc oxide (ZnO: Al), all of which have high transparency and simultaneously high n-type conductivity [1,2,8,9].
Contrary to these well-exploited n-type TCOs, the p-type TCO materials have been explored scarcely due to the difficulty in experimental preparation of p-type TCOs with both p-type doping and high hole mobilities, where the primary difficulty finally lies in the contradiction of the fundamental requirements of p-type TCOs [10]. In a standard p-type TCOs, it is desirable to provide high transparency of metal oxides while simultaneously providing high hole mobility. However, the 2p levels of the oxygen atom that dominate the valence band maximum (VBM) of the metal oxides lie far lower than the valence orbitals of the metallic atoms, resulting in the ionicity of metallic oxides, and in turn causing the strong localization of the hole at the valence-band edge, finally leading to a large hole effective The specific absorption spectra of synthesized Cu 3 TaS 4 and Cu 3 TaSe 4 were obtained by UV-Vis-NIR.
As proof of concept, the electrical conductivity of Cu 3 TaS 4 NCs was determined by I-V examination of printed Cu 3 TaS 4 thin-films.

Characterization
X-ray diffraction was performed with a Siemens Diffractometer D5000 (München, (Bavaria), Germany) (Cu Kα radiation, λ = 1.5405 Å) used to determine the crystal structures of synthesized sulvanites. Raman spectra collected on the Raman Renishaw microscope (West Dundee, IL, USA) with 633 nm laser were used to determine purity of the prepared products. Philips CM200 Transmission electron microscopy (TEM) (Hillsboro, OR, USA) was used to investigate the shape and size of the synthesized TaS 2 , TaSe 2 nanoflakes, Cu 3  The absorption spectra of the synthesized Cu 3 TaS 4 NCs and Cu 3 TaSe 4 NCs were collected using Agilent Cary 5000 UV-vis-NIR spectrophotometer (Santa Clara, CA, USA). An automated gold sputter coater (PELCO ® SC-7) was used to fabricate gold electrodes on glass with 100 nm thickness and 1 cm length. The PINE Research WaveNow Potentiostat (Durham, NC, USA) was used to determine the electrical conductivity behavior of the Cu 3 TaS 4 and Cu 3 TaSe 4 .

Preparation of TaS 2 Nanoflakes
TaS 2 nanoflakes were synthesized by a hot-injection method. In a glove box filled with argon gas, 1 mmol TaCl 2 (358.2 mg), 9 mL OLA, and 6 mL ODE were loaded to a 100 mL of two-neck round-bottom flask sealed with a rubber septum, immediately transferred it to a Schlenk line, and then evacuated at 125 • C for 30 min. At the same time, 2 mL of 1-DDT and 3 mL of OLA were added to a 25 mL two-neck flask and stirred at room temperature under vacuum for 30 min. Afterward, the temperature of Ta precursor solution was raised to 300 • C under an argon atmosphere, at which point the S precursor solution was injected dropwise into the Ta precursor. The S precursor solution was prepared by adding 0.5 mL CS 2 to a syringe containing a mixture of 2 mL degassed 1-DDT and 3 mL degassed OLA. The reaction was maintained at 300 • C for 2 h and then quenched by removing the heating source. The resulting product was washed with chloroform (CHCl 3 ) and ethanol (C 2 H 5 OH) in a 1:3 volume ratio. (V:V, 1:3) three times, followed by drying in a vacuum oven overnight.

Cascade Synthesis of Cu 3 TaS 4 NCs
Herein, the Cu 3 TaS 4 NCs were prepared by the incorporation of Cu cations in to formed TaS 2 . In a typical synthesis, a pre-prepared Cu precursor solution was injected into the TaS 2 suspension formed above at 300 • C under an argon atmosphere, and then the Crystals 2021, 11, 51 4 of 18 reaction was held at 300 • C for different times. The Cu precursor solution was made via dissolving 1.55 mmol CuCl 2 ·2H 2 O (264.3 mg) into 5 mL of OLA. Afterward, the reaction was cooled to room temperature by removing the heating source. The product was washed three times with a mixture of chloroform and ethanol (V:V, 1:3). The collected precipitates were dried overnight in a vacuum oven.

Hot-Injection Synthesis of Cu 3 TaS 4 NCs
In a typical experiment, in a glove box filled with argon, 1.5 mmol TaCl 5 (537.5 mg) were added to a 100 mL two-neck round-bottom flask containing a mixture of OLA (9 mL) and ODE (6 mL). The reaction flask was sealed with a rubber septum, transferred to a Schlenk line system, and evacuated at 125 • C for 30 min, whereby a colorless, transparent solution was formed. Simultaneously, 3 mmol CuCl 2 ·2H 2 O (511.5 mg) was dissolved in 5 mL OLA in a 25 mL flask and degassed at 100 • C for 30 min. 2 mL of 1-DDT and 3 mL of OLA in a 25 mL two-neck flask were degassed at room temperature for 30 min. After degassing, each of the three flasks was filled with argon. The formed Cu precursor solution was injected into the Ta precursor at 125 • C, and the reaction mixture was heated to 300 • C, where the formation of a brown, transparent solution was observed. When the Cu-Ta solution reached the target temperature, the sulfur source CS 2 (0.5 mL) was added to a syringe containing a mixture of 2 mL degassed 1-DDT and 3 mL degassed OLA, and then injected dropwise into the brown Cu-Ta solution which immediately turned black. The final reaction mixture was held at 300 • C for 1 h, followed by cooling down to room temperature through removing the heating mantle. The resulting yellow product was purified with chloroform and ethanol (V:V, 1:3) three times. The obtained precipitates were dried in a vacuum oven overnight for further analysis.

Preparation of TaSe 2 Nanoflakes
In a glove box, TaCl 5 (1 mmol, 358.2 mg), Se (2 mmol, 158 mg), 6 mL of ODE, and 9 mL of OLA were loaded to a 100 mL two-neck round-bottom flask under argon atmosphere. This reaction flask was sealed with a rubber septum and then connected to a Schlenk line, followed by stirring for 15 min under vacuum at room temperature. Afterward, the reaction was heated to 305 • C in 30 min and then maintained at 305 • C for 1 h. Finally, cooled down the reaction to room temperature by removing the heating source. The precipitates were purified with chloroform and ethanol (V:V, 1:3) for three times.

Cascade Synthesis of Cu 3 TaSe 4 NCs
Similar to the cascade synthesis of Cu 3 TaS 4 NCs, the Cu 3 TaSe 4 NCs were prepared using the pre-prepared TaSe 2 nanoflakes as the starting material. In a typical synthesis, the TaSe 2 nanoflakes were grown at 305 • C for 1 h, followed by injecting Cu precursor solution into the TaSe 2 suspension. The reaction was further maintained at 305 • C for 1 h. The Cu precursor solution was prepared by dissolving CuCl 2 ·2H 2 O (1.45 mmol, 248 mg) into 5 mL OLA. Afterward, the reaction was cooled down by removing the heating plate. The precipitates were purified with a mixture of chloroform (CHCl 3 ) and ethanol (C 2 H 5 OH) in a 1:3 volume ratio, three times. The collected product was dried in a vacuum oven overnight for further characterization.

Hot-Injection Synthesis of Cu 3 TaSe 4 NCs
In the hot-injection synthesis of Cu 3 TaSe 4 NCs, the elemental Se powder was used as the Se source while TaCl 5 and CuCl 2 2H 2 O served as Ta and Cu source, respectively. In a 100 mL two-neck round-bottom flask, 158 mg Se powder (2 mmol), 9 mL OLA, and 6 mL ODE were loaded to prepare the Se precursor. The Ta precursor was prepared in a 25 mL two-neck round-bottom flask by dissolving 1 mmol TaCl 5 (358.2 mg) into a mixture of 3 mL OLA and 2 mL ODE under argon atmosphere. Meanwhile, 1 mmol CuCl 2 ·2H 2 O (170.5 mg) was added to a 25 mL two-neck round-bottom flask containing 5 mL OLA to make the Cu precursor solution. Each of the three precursors was degassed at 120 • C for Crystals 2021, 11, 51 5 of 18 30 min and then flushed with argon. Next, the Cu precursor solution was injected into the Ta precursor and then maintained at 120 • C to form a brown Cu-Ta solution. In parallel, the Se precursor was heated to 300 • C, and the Cu-Ta solution was injected immediately into the Se solution. An immediate black color change was observed (within one second). The reaction mixture was held at 300 • C for 1 h. Afterward, the reaction was quenched by removing the heating source. The product was washed with a mixture of chloroform and ethanol (V:V, 1:3) three times, collected, and dried in a vacuum oven overnight.

Fabrication of Cu 3 TaS 4 NCs and Cu 3 TaSe 4 Inks
Cu 3 TaS 4 inks were prepared by dispersing 30 mg of Cu 3 TaS 4 NCs into 1 mL of ethanol using probe ultrasonication for 20 min. Cu 3 TaSe 4 inks were prepared by the same dispersion method. Each ink was prepared from NCs that were subjected first to ligand exchange, toward removing residual OLA remaining on the NCs surface from synthesis and replace them with S 2− . The ligand exchange of Cu 3 TaS 4 NCs and Cu 3 TaSe 4 NCs followed a previously reported process for Cu 3 VSe 4 NCs [29]. In a typical experiment, Cu 3 TaS 4 NCs were suspended in chloroform (8 mg/mL). Then, 10 mL of this suspension was transferred to a 50 mL tube containing 10 mL of Na 2 S solution in formamide (0.2 M). Next, the mixture was vigorously shaken for 1 min and further allowed to rest until ligand exchange completion, when Cu 3 TaS 4 NCs were fully transferred from the chloroform phase (lower) to the formamide phase (upper). Afterward, the clear chloroform phase was removed, and 5 mL distilled water and 20 mL of ethanol was added to the aqueous phase to precipitate the Cu 3 TaS 4 NCs. The precipitate was purified by washing it twice with a mixture of ethanol and distilled water (ethanol:water = 4:1) followed by washing with a mixture of toluene and ethanol (toluene:ethanol = 1:3). The resulting product was collected and dried in a vacuum oven overnight. The same process was followed for Cu 3 TaSe 4 NCs ligand exchange.

Fabrication of Cu 3 TaS 4 NCs and Cu 3 TaSe 4 NCs Thin Films and Devices
To evaluate the electrical conductivity of the Cu 3 TaS 4 NCs and Cu 3 TaSe 4 NCs, thin films were fabricated with each of the NCs on glass, as follows. First, the glass slides were washed with distilled water, acetone, and methanol in sequence for 5 min each solvent, using an ultrasonic bath. Then, gold was coated on a clean glass substrate by using an Au sputter coater to fabricate two gold electrodes with a thickness of 100 nm and a width of 1 cm, leaving a separation of 1 cm between two Au electrodes. Afterward, an amount of 10 µL of Cu 3 TaS 4 or Cu 3 TaSe 4 inks (30 mg/mL) was coated to the inter-electrode gap by bar-coating (as depicted in Figure 7a. The thin film was dried in air at 100 • C for 1 min using a hot plate. The coating and drying process was repeated twice to manufacture the Cu 3 TaS 4 NCs and Cu 3 TaSe 4 NCs-based devices.

TaS 2 Nanoflakes Characterization
The crystal structure of the synthesized TaS 2 was identified from the XRD pattern shown in Figure 1a, exhibiting two characteristic peaks at 30.6 and 54.6 degrees, which could be indexed to the hexagonal TaS 2 phase (PDF# 04-003-1715) with a space group of P3m1. It is worth noting that the peak intensity ratio of the synthesized TaS 2 is not the same as the indexed pattern which could be attributed to the preferential growth of the two-dimensional (2D) nanostructures in specific crystallographic directions with a high aspect ratio. Figure 1b illustrates the crystal structure of TaS 2 . A low-magnification TEM image of the synthesized TaS 2 is showed in Figure 1c. Notably, the TaS 2 exhibit thin nanoflake morphological features, with nanosheet size~150nm in diameter. A stacking of the nanoflakes is observed.

Characterization of Cu3TaS4 NCs
XRD patterns of the products prepared using either the cascade method or the hotinjection method ( Figure 2a) shows a single cubic Cu3TaS4 phase with a space group of P43m, however, the product colors are different: Cu3TaS4 NCs (inset) synthesized using hot-injection is yellow, while the Cu3TaS4 NCs synthesized using the cascade method, displays a mustard-green color. Through investigating Cu3TaS4 NCs prepared with different amounts of Cu incorporation, we speculated that the variety in the color of the products should be caused by different amounts of Cu insertion. In the hot-injection synthesis of Cu3TaS4 NCs, when we added the Cu, Ta, and S precursors with stoichiometric ratio, the obtained product showed dark green and a mixture of Cu2S and Cu3TaS4 phases, as shown in supporting information (Supplementary Materials Figure S1). A comparison of Raman spectrum of the Cu3TaS4 NCs synthesized using cascade method and hot-injection in Figure 2b shows two identical dominant Raman scattering peaks at around 264.1 cm -1 and 413 cm -1 , which correspond to the A1 mode of Cu-S bond and A1 vibration mode of Ta-S bond of Cu3TaS4, respectively, in good agreement with the previous reports [30]. Our recent report showed that a cascade synthesis of Cu3VSe4 from VSe2 nanosheets leads to a preservation of nanosheet morphology [31]. In this light, it was expected that Cu3TaS4 formed using the same method will facilitate the preservation of nanosheets. However, Cu3TaS4 was formed in cubic shape with an average size of around 20 nm (Figure 2c), without maintaining the 2D morphology of the TaS2 nanoflakes template. Earlier reports show such example of the transformation from nanosheets to nanodisks upon adding the third cation, in the case of CuInS2 [32]. Ultrathin γ-In2S3 nanosheets were turned into CuInS2 nanodisks when incorporating the Cu + ions. In this case it was hypothesized that the reason for the formation of CuInS2 nanodisks from γ-In2S3 nanosheets during colloidal synthesis is that the incorporation of Cu + caused additional strain, which will lead the folded nanosheets to tear into nanodisks [32].Thus, we posit that the insertion of Cu cations during the preparation of Cu3TaS4 NCs by cascade method caused additional stress which resulted in tearing of the folded TaS2 nanoflakes into cubic nanocrystals in the solution process. Similarly, the TEM image (Figure 2c) of the Cu3TaS4 NCs prepared by hotinjection presents cubic shape with a smaller particle size of ~15 nm. The SEM-EDS elemental mapping of the synthesized Cu3TaS4 using the cascade method ( Figure 3) shows a homogeneous distribution of Cu, Ta, and S elements.

Characterization of Cu 3 TaS 4 NCs
XRD patterns of the products prepared using either the cascade method or the hotinjection method ( Figure 2a) shows a single cubic Cu 3 TaS 4 phase with a space group of P43m, however, the product colors are different: Cu 3 TaS 4 NCs (inset) synthesized using hotinjection is yellow, while the Cu 3 TaS 4 NCs synthesized using the cascade method, displays a mustard-green color. Through investigating Cu 3 TaS 4 NCs prepared with different amounts of Cu incorporation, we speculated that the variety in the color of the products should be caused by different amounts of Cu insertion. In the hot-injection synthesis of Cu 3 TaS 4 NCs, when we added the Cu, Ta, and S precursors with stoichiometric ratio, the obtained product showed dark green and a mixture of Cu 2 S and Cu 3 TaS 4 phases, as shown in supporting information (Supplementary Materials Figure S1). A comparison of Raman spectrum of the Cu 3 TaS 4 NCs synthesized using cascade method and hot-injection in Figure 2b shows two identical dominant Raman scattering peaks at around 264.1 cm −1 and 413 cm −1 , which correspond to the A 1 mode of Cu-S bond and A 1 vibration mode of Ta-S bond of Cu 3 TaS 4 , respectively, in good agreement with the previous reports [30]. Our recent report showed that a cascade synthesis of Cu 3 VSe 4 from VSe 2 nanosheets leads to a preservation of nanosheet morphology [31]. In this light, it was expected that Cu 3 TaS 4 formed using the same method will facilitate the preservation of nanosheets. However, Cu 3 TaS 4 was formed in cubic shape with an average size of around 20 nm (Figure 2c), without maintaining the 2D morphology of the TaS 2 nanoflakes template. Earlier reports show such example of the transformation from nanosheets to nanodisks upon adding the third cation, in the case of CuInS 2 [32]. Ultrathin γ-In 2 S 3 nanosheets were turned into CuInS 2 nanodisks when incorporating the Cu + ions. In this case it was hypothesized that the reason for the formation of CuInS 2 nanodisks from γ-In 2 S 3 nanosheets during colloidal synthesis is that the incorporation of Cu + caused additional strain, which will lead the folded nanosheets to tear into nanodisks [32].Thus, we posit that the insertion of Cu cations during the preparation of Cu 3 TaS 4 NCs by cascade method caused additional stress which resulted in tearing of the folded TaS 2 nanoflakes into cubic nanocrystals in the solution process. Similarly, the TEM image (Figure 2c) of the Cu 3 TaS 4 NCs prepared by hot-injection presents cubic shape with a smaller particle size of~15 nm. The SEM-EDS elemental mapping of the synthesized Cu 3 TaS 4 using the cascade method ( Figure 3) shows a homogeneous distribution of Cu, Ta, and S elements.

Mechanism of Cu3TaS4 NCs Cascade Synthesis
To get an insight into the mechanism of Cu3TaS4 NCs formation through cascade synthesis, a series of experiments with different reaction times were carried out, and the XRD patterns of the obtained products ( Figure 4) were used to investigate the phase transformations during the process. Upon the addition of Cu cations to the formed TaS2 nanoflakes, a rapid phase transition from hexagonal TaS2 to cubic Cu3TaS4 happened, which was observed to occur within 1 min. Additionally, when extending the reaction time to 1 h, neither secondary phase nor other crystalline impurities were detected, however, the color of the products gradually changed from black to yellow, and the crystallinity and yield of products were enhanced. The color changes of the product with the reaction time are detailed in the Supplementary Figure S2a. The phase transformation identified by XRD patterns proved that there was a major rearrangement of atoms in the TaS2 nanoflakes when inserting Cu cations. Herein, the ultrathin template of TaS2 nanoflakes was considered to be the key to the rapid synthesis of Cu3TaS4 nanocrystals, since the large surface area and high aspect ratio of the 2D nanostructures provide more

Mechanism of Cu3TaS4 NCs Cascade Synthesis
To get an insight into the mechanism of Cu3TaS4 NCs formation through cascade synthesis, a series of experiments with different reaction times were carried out, and the XRD patterns of the obtained products ( Figure 4) were used to investigate the phase transformations during the process. Upon the addition of Cu cations to the formed TaS2 nanoflakes, a rapid phase transition from hexagonal TaS2 to cubic Cu3TaS4 happened, which was observed to occur within 1 min. Additionally, when extending the reaction time to 1 h, neither secondary phase nor other crystalline impurities were detected, however, the color of the products gradually changed from black to yellow, and the crystallinity and yield of products were enhanced. The color changes of the product with the reaction time are detailed in the Supplementary Figure S2a. The phase transformation identified by XRD patterns proved that there was a major rearrangement of atoms in the TaS2 nanoflakes when inserting Cu cations. Herein, the ultrathin template of TaS2 nanoflakes was considered to be the key to the rapid synthesis of Cu3TaS4 nanocrystals, since the large surface area and high aspect ratio of the 2D nanostructures provide more

Mechanism of Cu 3 TaS 4 NCs Cascade Synthesis
To get an insight into the mechanism of Cu 3 TaS 4 NCs formation through cascade synthesis, a series of experiments with different reaction times were carried out, and the XRD patterns of the obtained products ( Figure 4) were used to investigate the phase transformations during the process. Upon the addition of Cu cations to the formed TaS 2 nanoflakes, a rapid phase transition from hexagonal TaS 2 to cubic Cu 3 TaS 4 happened, which was observed to occur within 1 min. Additionally, when extending the reaction time to 1 h, neither secondary phase nor other crystalline impurities were detected, however, the color of the products gradually changed from black to yellow, and the crystallinity and yield of products were enhanced. The color changes of the product with the reaction time are detailed in the Supplementary Figure S2a. The phase transformation identified by XRD patterns proved that there was a major rearrangement of atoms in the TaS 2 nanoflakes when inserting Cu cations. Herein, the ultrathin template of TaS 2 nanoflakes was considered to be the key to the rapid synthesis of Cu 3 TaS 4 nanocrystals, since the large surface area and high aspect ratio of the 2D nanostructures provide more reaction sites, thereby promoting

Mechanism for Hot-injection of Cu3TaS4 NCs
Briefly, the hot-injection method involves the injection of a cold reagent into hot precursors to trigger rapid nucleation which is followed by a crystal growth step. Figure 5 compares the XRD patterns of the Cu3TaS4 NCs products obtained at different reaction times after adding the sulfur source. The color changes of the products are shown in the Supporting Information (Supplementary Materials Figure S2b). Given that the primary XRD peak positions of the Cu2S (PDF# 00-053-0522) are close to those of Cu3TaS4 (PDF# 04-003-9455), we considered the distinguished peak at 49.5° 2θ (marked by the red dotted line) to be the indicative of the presence of the Cu3TaS4. As shown in Figure 5, 1 min after adding the S source into the Cu-Ta precursors, the XRD of the product showed a Cu2S phase. As noted above, after 30 min, a characteristic peak at 49.5° 2θ appeared in the XRD pattern, indicating the formation of the cubic Cu3TaS4 phase. As the reaction progressed, the XRD peaks intensity of Cu3TaS4 gradually increased; at 1 h the intensity of peak at 16.04° grew relatively higher than that of 46.5°, where the peak intensity ratio of 16.04° to 46.5° is close to the indexed patten, indicating the formation of pure phase of Cu3TaS4. Thus, in the hot-injection synthesis of Cu3TaS4 NCs, the swift addition of the sulfur precursor into the Cu-Ta metal precursor at 300 °C triggered the rapid nucleation of Cu2S seeds, and then the formed Cu2S seeds subsequently reacted with Ta cations to form the Cu3TaS4 nanocrystals. Compared with the cascade synthesis of Cu3TaS4 NCs that uses TaS2 nanoflakes as the starting templates, the hot injection method which employs the formed Cu2S as the starting seeds took a longer time to form Cu3TaS4 NCs. We attribute this to the larger radius of Ta cation.

Mechanism for Hot-Injection of Cu 3 TaS 4 NCs
Briefly, the hot-injection method involves the injection of a cold reagent into hot precursors to trigger rapid nucleation which is followed by a crystal growth step. Figure 5 compares the XRD patterns of the Cu 3 TaS 4 NCs products obtained at different reaction times after adding the sulfur source. The color changes of the products are shown in the Supporting Information (Supplementary Materials Figure S2b). Given that the primary XRD peak positions of the Cu 2 S (PDF# 00-053-0522) are close to those of Cu 3 TaS 4 (PDF# 04-003-9455), we considered the distinguished peak at 49.5 • 2θ (marked by the red dotted line) to be the indicative of the presence of the Cu 3 TaS 4 . As shown in Figure 5, 1 min after adding the S source into the Cu-Ta precursors, the XRD of the product showed a Cu 2 S phase. As noted above, after 30 min, a characteristic peak at 49.5 • 2θ appeared in the XRD pattern, indicating the formation of the cubic Cu 3 TaS 4 phase. As the reaction progressed, the XRD peaks intensity of Cu 3 TaS 4 gradually increased; at 1 h the intensity of peak at 16.04 • grew relatively higher than that of 46.5 • , where the peak intensity ratio of 16.04 • to 46.5 • is close to the indexed patten, indicating the formation of pure phase of Cu 3 TaS 4 . Thus, in the hot-injection synthesis of Cu 3 TaS 4 NCs, the swift addition of the sulfur precursor into the Cu-Ta metal precursor at 300 • C triggered the rapid nucleation of Cu 2 S seeds, and then the formed Cu 2 S seeds subsequently reacted with Ta cations to form the Cu 3 TaS 4 nanocrystals. Compared with the cascade synthesis of Cu 3 TaS 4 NCs that uses TaS 2 nanoflakes as the starting templates, the hot injection method which employs the formed Cu 2 S as the starting seeds took a longer time to form Cu 3 TaS 4 NCs. We attribute this to the larger radius of Ta cation.  Figure 6b shows the photoluminescence (PL) spectrum of the synthesized Cu3TaS4 NCs in ethanol, when using 360 nm as the excitation wavelength, the characteristic emission peaks occurred at 486.3 and 531.4 nm, corresponding to 2.54 eV and 2.33 eV, respectively. The green emission at 531.4 nm indicated a mid-bandgap charge trap caused by Cu vacancies near the valence band maximum, in good agreement with a previous report by Hersh [21], where green photoemission at 548 nm was present for the Cu3TaS4 single crystal.   Figure 6b shows the photoluminescence (PL) spectrum of the synthesized Cu 3 TaS 4 NCs in ethanol, when using 360 nm as the excitation wavelength, the characteristic emission peaks occurred at 486.3 and 531.4 nm, corresponding to 2.54 eV and 2.33 eV, respectively. The green emission at 531.4 nm indicated a mid-band-gap charge trap caused by Cu vacancies near the valence band maximum, in good agreement with a previous report by Hersh [21], where green photoemission at 548 nm was present for the Cu 3 TaS 4 single crystal.
To evaluate the electrical conductivity of the Cu 3 TaS 4 NCs, a Cu 3 TaS 4 NCs-based device with a configuration of Au/Cu 3 TaS 4 NCs-glass/Au was subjected to an I-V measurement. The schematic of the device configuration for the I-V measurement is shown in Figure 7a. The I-V curve of the Au/Cu 3 TaS 4 NCs-glass/Au device is shown in Figure 7b blue line, where the current progressively increases along with the voltage bias towards positive, implying the electrons could flow through the Cu 3 TaS 4 NCs area. Generally, glass is a good insulator that does not allow the flow of electrons, and its conductivity is negligible as compared to the material of the thin films. Thus, it could be concluded that in the Au/Cu 3 TaS 4 NCs-film/Au device, the electrons flow through the Cu 3 TaS 4 NCs thin film rather than the glass substrate. To prove our hypothesis, we prepared a control device configured as Au/Glass/Au, where the two gold electrodes connected only by clean glass. The I-V curves of the control device are shown in Figure 7b red line, where the current is kept at around zero amperes when increasing the voltage, suggesting that no electrons flow through the glass substrate, which further evidence our hypothesis that Cu 3 TaS 4 material has electrical conductivity. The obtained I-V plot of the Au/Cu 3 TaS 4 NCs-glass/Au device allowed us to calculate the electrical conductivity of Cu 3 TaS 4 NCs. Using the equation R = V/I (R-Resistance in ohms(Ω); V-voltage in volts (V); I-current in amperes(A)), we could calculate the resistance of Cu 3 TaS 4 NCs, namely, the slope of the I-V curve, which is Crystals 2021, 11, 51 10 of 18 1.49 × 10 7 Ω. Generally, the electrical conductivity of a thin film (σ) could be determined using the following expression: where R is the film resistance between the two electrodes, t is the thickness of the sample, and A is the surface area of the sample [33][34][35][36][37][38][39]. Here, R is 1.49 × 10 7 Ω, t is 2.05 * 10 −6 m (determined from SEM cross-section of the film), and A is 10 −4 m 2 , thus, the calculated electrical conductivity of Cu 3 TaS 4 NCs is 1.38 × 10 −9 S·m −1 . Although the calculated electrical conductivity is low, this could be attributed to a poor quality of the thin film and poor connection between Au electrodes and Cu 3 TaS 4 NCs thin film. The optoelectronic properties of Cu 3 TaS 4 NCs suggests that the materials are a promising candidate for the p-type transparent conductor applications.  Figure 6a depicts the optical absorption spectrum of the synthesized Cu3TaS4 NCs in the ultraviolet, visible, and near-infrared regions, which shows three absorbance bands at around 267.1, 300.8 and 352.6 nm. Figure 6b shows the photoluminescence (PL) spectrum of the synthesized Cu3TaS4 NCs in ethanol, when using 360 nm as the excitation wavelength, the characteristic emission peaks occurred at 486.3 and 531.4 nm, corresponding to 2.54 eV and 2.33 eV, respectively. The green emission at 531.4 nm indicated a mid-bandgap charge trap caused by Cu vacancies near the valence band maximum, in good agreement with a previous report by Hersh [21], where green photoemission at 548 nm was present for the Cu3TaS4 single crystal.  To evaluate the electrical conductivity of the Cu3TaS4 NCs, a Cu3TaS4 NCs-based device with a configuration of Au/Cu3TaS4 NCs-glass/Au was subjected to an I-V measurement. The schematic of the device configuration for the I-V measurement is shown in Figure 7a. The I-V curve of the Au/Cu3TaS4 NCs-glass/Au device is shown in Figure 7b blue line, where the current progressively increases along with the voltage bias towards positive, implying the electrons could flow through the Cu3TaS4 NCs area. Generally, glass is a good insulator that does not allow the flow of electrons, and its conductivity is negligible as compared to the material of the thin films. Thus, it could be concluded that in the Au/Cu3TaS4 NCs-film/Au device, the electrons flow through the Cu3TaS4 NCs thin film rather than the glass substrate. To prove our hypothesis, we prepared a control device configured as Au/Glass/Au, where the two gold electrodes connected only by clean glass. The I-V curves of the control device are shown in Figure 7b red line, where the current is kept at around zero amperes when increasing the voltage, suggesting that no electrons flow through the glass substrate, which further evidence our hypothesis that Cu3TaS4 material has electrical conductivity. The obtained I-V plot of the Au/Cu3TaS4 NCs-glass/Au device allowed us to calculate the electrical conductivity of Cu3TaS4 NCs. Using the equation R = V/I (R-Resistance in ohms(Ω); V-voltage in volts (V); I-current in amperes(A)), we could calculate the resistance of Cu3TaS4 NCs, namely, the slope of the I-V curve, which is 1.49 ×10 7 Ω. Generally, the electrical conductivity of a thin film (σ) could be determined using the following expression:

Optoelectrical and Conductive Properties of Cu3TaS4 NCs
where R is the film resistance between the two electrodes, t is the thickness of the sample, and A is the surface area of the sample [33][34][35][36][37][38][39]. Here, R is 1.49 × 10 7 Ω, t is 2.05*10 −6 m (determined from SEM cross-section of the film), and A is 10 −4 m 2 , thus, the calculated electrical conductivity of Cu3TaS4 NCs is 1.38 × 10 −9 S·m −1 . Although the calculated electrical conductivity is low, this could be attributed to a poor quality of the thin film and poor connection between Au electrodes and Cu3TaS4 NCs thin film. The optoelectronic properties of Cu3TaS4 NCs suggests that the materials are a promising candidate for the p-type transparent conductor applications.

Characterization of TaSe2 Nanoflakes
The crystal structure of the synthesized TaSe2 was analyzed by powder XRD pattern as shown in Figure 8a, which is in good agreement with the patterns reported by Jeong at. Al [40], where they assigned the product to the hexagonal TaSe2 phase (PDF# 04-019-1305)

Characterization of TaSe 2 Nanoflakes
The crystal structure of the synthesized TaSe 2 was analyzed by powder XRD pattern as shown in Figure 8a, which is in good agreement with the patterns reported by Jeong at. Al [40], where they assigned the product to the hexagonal TaSe 2 phase (PDF# 04-019-1305) with a space group of P 6 3 /mmc. The identified crystal structure is shown in Figure 8b.
The TEM image shown in Figure 8c indicates the folded nanoflakes characteristic of the solution synthesized TaSe 2 .
Crystals 2020, 10, x FOR PEER REVIEW 11 of 18 with a space group of P 63/mmc. The identified crystal structure is shown in Figure 8b. The TEM image shown in Figure 8c indicates the folded nanoflakes characteristic of the solution synthesized TaSe2.

Characterization of Cu3TaSe4 NCs
A comparison of XRD pattern for Cu3TaSe4 prepared by cascade and hot-injection method is shown in Figure 9a, where the characteristic peaks are the same and match to the cubic Cu3TaSe4 with a space group of P43m (215). Simultaneously, the Raman scattering peaks of both the cascade synthesized Cu3TaSe4 and the Cu3TaSe4 prepared by the hotinjection method locate at around 167.4 and 242.8 cm −1 , as shown in Figure 9b, which are associated with F1 b and A1 mode of Cu3TaSe4, respectively [30,41]. In the synthesis of Cu3TaSe4, when the Cu, Ta, and Se precursors are added with stoichiometric ratio, the obtained product showed both Cu7.16Se4 and Cu3TaSe4 phases, as shown in supporting information ( Figure S3). As noted in the case of cascade synthesized Cu3TaS4 NCs, the synthesized Cu3TaS4 NCs did not maintain the 2D morphology of the TaS2 nanoflakes because that the incorporation of Cu cations caused additional stress which leads to tearing of the folded TaS2 nanoflakes into cubic nanocrystals. Similarly, the Cu3TaSe4 NCs prepared using the cascade method did not maintain the nanoflake morphology of TaSe2 template, but instead transformed into cubic shapes with irregular sizes, as shown in Figure 9c.
In contrast, the TEM image of the synthesized Cu3TaSe4 by hot-injection presents a core-shell morphology, where the interplanar spacing of 2.9 Å in the core region matches to the lattice (200) of cubic Cu3TaSe4, whereas the d-spacing of 2.57 Å located in the shell field could be attributed to the plane (210). Figure 10 displays the SEM-EDS of the synthesized Cu3TaSe4 using cascade synthesis, where Cu, Ta, and Se elements spread homogeneously in the synthesized Cu3TaSe4 materials. This suggest that core and shell have the same crystal structure, with different orientation. Further characterization is ongoing.

Characterization of Cu 3 TaSe 4 NCs
A comparison of XRD pattern for Cu 3 TaSe 4 prepared by cascade and hot-injection method is shown in Figure 9a, where the characteristic peaks are the same and match to the cubic Cu 3 TaSe 4 with a space group of P43m (215). Simultaneously, the Raman scattering peaks of both the cascade synthesized Cu 3 TaSe 4 and the Cu 3 TaSe 4 prepared by the hotinjection method locate at around 167.4 and 242.8 cm −1 , as shown in Figure 9b, which are associated with F 1 b and A 1 mode of Cu 3 TaSe 4 , respectively [30,41]. In the synthesis of Cu 3 TaSe 4 , when the Cu, Ta, and Se precursors are added with stoichiometric ratio, the obtained product showed both Cu 7. 16 Se 4 and Cu 3 TaSe 4 phases, as shown in supporting information ( Figure S3). As noted in the case of cascade synthesized Cu 3 TaS 4 NCs, the synthesized Cu 3 TaS 4 NCs did not maintain the 2D morphology of the TaS 2 nanoflakes because that the incorporation of Cu cations caused additional stress which leads to tearing of the folded TaS 2 nanoflakes into cubic nanocrystals. Similarly, the Cu 3 TaSe 4 NCs prepared using the cascade method did not maintain the nanoflake morphology of TaSe 2 template, but instead transformed into cubic shapes with irregular sizes, as shown in Figure 9c.  In contrast, the TEM image of the synthesized Cu 3 TaSe 4 by hot-injection presents a core-shell morphology, where the interplanar spacing of 2.9 Å in the core region matches to the lattice (200) of cubic Cu 3 TaSe 4 , whereas the d-spacing of 2.57 Å located in the shell field could be attributed to the plane (210). Figure 10 displays the SEM-EDS of the synthesized Cu 3 TaSe 4 using cascade synthesis, where Cu, Ta, and Se elements spread homogeneously in the synthesized Cu 3 TaSe 4 materials. This suggest that core and shell have the same crystal structure, with different orientation. Further characterization is ongoing.

Mechanism for Cascade Synthesis of Cu3TaSe4 NCs
Phase structures of the products obtained at different times after injecting Cu cations were identified from XRD patterns shown in Figure 11. The suspension of formed TaSe2 nanoflakes in OLA-ODE was used as starting template. The characteristic peaks at 27.04°, 44.89°, and 53.2° 2θ that belong to the cubic Cu7.16Se4 were detected 1 min after injecting Cu 2+ ions into the TaSe2 suspension. Meanwhile, trace amounts of TaSe2 residues could also be detected. When the reaction time was extended to 10 min, the characteristic Cu3TaSe4 peak at 15.64° started to appear, indicating the formation of Cu3TaSe4. However, it can be noted that the characteristic peaks of Cu7.16Se4 at 27.04°, 44.89°, and 53.2° are close to the dominant peaks of Cu3TaSe4 at 27.26°, 45.26°, and 53.65°, which makes it difficult to determine the purity of the products. Herein, we take the peak intensity ratio of the peak at 15.64° (100) to 45.26° (220) as the criteria to demonstrate the purity of the synthesized Cu3TaSe4, where the intensity ratio of (100) to (220) for the standard Cu3TaSe4 (PDF# 01-084-5208) is 1.08. Thus, when reacting for 10 min after injecting Cu cations, there were three phases in the reaction flask, including Cu7.16Se4, TaSe2, and Cu3TaSe4. When the reaction time was prolonged to 20 min, the TaSe2 phase disappeared, and a mixed phase of Cu7.16Se4 and Cu3TaSe4 were present in the reaction mixture at this stage. When the reaction proceeded for 1 h, the peak intensity ratio of (100) to (220) converted to approximately

Mechanism for Cascade Synthesis of Cu 3 TaSe 4 NCs
Phase structures of the products obtained at different times after injecting Cu cations were identified from XRD patterns shown in Figure 11. The suspension of formed TaSe 2 nanoflakes in OLA-ODE was used as starting template. The characteristic peaks at 27.04 • , 44.89 • , and 53.2 • 2θ that belong to the cubic Cu 7. 16 Se 4 were detected 1 min after injecting Cu 2+ ions into the TaSe 2 suspension. Meanwhile, trace amounts of TaSe 2 residues could also be detected. When the reaction time was extended to 10 min, the characteristic Cu 3 TaSe 4 peak at 15.64 • started to appear, indicating the formation of Cu 3 TaSe 4 . However, it can be noted that the characteristic peaks of Cu 7.16 Se 4 at 27.04 • , 44.89 • , and 53.2 • are close to the dominant peaks of Cu 3 TaSe 4 at 27.26 • , 45.26 • , and 53.65 • , which makes it difficult to determine the purity of the products. Herein, we take the peak intensity ratio of the peak at 15.64 • (100) to 45.26 • (220) as the criteria to demonstrate the purity of the synthesized Cu 3 TaSe 4 , where the intensity ratio of (100) to (220) for the standard Cu 3 TaSe 4 (PDF# 01-084-5208) is 1.08. Thus, when reacting for 10 min after injecting Cu cations, there were three phases in the reaction flask, including Cu 7. 16 Se 4 , TaSe 2 , and Cu 3 TaSe 4 . When the reaction time was prolonged to 20 min, the TaSe 2 phase disappeared, and a mixed phase of Cu 7. 16 Se 4 and Cu 3 TaSe 4 were present in the reaction mixture at this stage. When the reaction proceeded for 1 h, the peak intensity ratio of (100) to (220) converted to approximately 1.09, implying that the pure Cu 3 TaSe 4 was obtained. These XRD observations suggested that the incorporation of Cu cations triggered the rapid nucleation of Cu 7. 16 Se 4 , and then the formed Cu 7. 16 Se 4 consumed the folded TaSe 2 nanoflakes to form Cu 3 TaSe 4 NCs. The overall color change of the reaction with reaction time is shown in Supporting Information ( Figure S4a).

Mechanism for Hot-Injection Formation of Cu 3 TaSe 4 NCs
To assess the mechanism of Cu 3 TaSe 4 NCs formation using the hot-injection method, we investigated the XRD patterns and color change of products at different reaction times, as shown in Figure 12 and Figure S4b, respectively. Once injecting the Cu-Ta precursor into the hot selenium solution, the first primary phase of Cu 7. 16 Se 4 was observed in the XRD patterns (Figure 12), indicating that the Cu 7. 16 Se 4 formed rapidly within 1 min, which was consistent with the cascade synthesis of Cu 3 TaSe 4 NCs. After 5 min, the presence of the XRD peak at 15.64 • highlights the formation of the cubic Cu 3 TaSe 4 phase. As the reaction proceeded, the peak intensity ratio of the characteristic peak at 15.64 • to 45.26 increased gradually, indicating the progressive conversion from Cu 7. 16 Se 4 to cubic Cu 3 TaSe 4 . Therefore, we hypothesize that the formation of Cu 3 TaSe 4 NCs could follow the path below.
Cu − Ta + Se → Cu 7. 16  1.09, implying that the pure Cu3TaSe4 was obtained. These XRD observations suggested that the incorporation of Cu cations triggered the rapid nucleation of Cu7.16Se4, and then the formed Cu7.16Se4 consumed the folded TaSe2 nanoflakes to form Cu3TaSe4 NCs. The overall color change of the reaction with reaction time is shown in Supporting Information (Figure S 4a).

Mechanism for Hot-Injection Formation of Cu3TaSe4 NCs
To assess the mechanism of Cu3TaSe4 NCs formation using the hot-injection method, we investigated the XRD patterns and color change of products at different reaction times, as shown in Figure 12 and Figure S4b, respectively. Once injecting the Cu-Ta precursor into the hot selenium solution, the first primary phase of Cu7.16Se4 was observed in the XRD patterns (Figure 12), indicating that the Cu7.16Se4 formed rapidly within 1 min, which was consistent with the cascade synthesis of Cu3TaSe4 NCs. After 5 min, the presence of the XRD peak at 15.64° highlights the formation of the cubic Cu3TaSe4 phase. As the reaction proceeded, the peak intensity ratio of the characteristic peak at 15.64° to 45.26 increased gradually, indicating the progressive conversion from Cu7.16Se4 to cubic Cu3TaSe4. Therefore, we hypothesize that the formation of Cu3TaSe4 NCs could follow the path below.

Optoelectrical and Conductivity Properties of Cu 3 TaSe 4 NCs
Optical spectra of synthesized Cu 3 TaSe 4 NCs are shown in Figure 13a. The UV-Vis-NIR spectrum of the Cu 3 TaSe 4 NCs prepared by the cascade method shows three primary peaks at around 282.6, 339.6, and 393.5 nm, respectively, whereas the spectrum of the Cu 3 TaSe 4 core-shells exhibits a broad absorbing band at around 420.3 nm with two small characteristic peaks at 297.8 and 361.5 nm. Compared with the Cu 3 TaSe 4 NCs, the UV-Vis-NIR spectrum of Cu 3 TaSe 4 core-shells showed an additional absorption band at around 800-1000nm, which is related to the core-shell structured properties of Cu 3 TaSe 4 . Photoluminescence spectra of Cu 3 TaSe 4 prepared by either the cascade method or hot-injection method (Figure 13b) showed a characteristic peak at around 535 nm, suggesting that the optical bandgap of Cu 3 TaSe 4 was 2.32 eV, which is in good agreement with the previous reports [5,21,28,42].The I-V curve of fabricated Au/Cu 3 TaSe 4 NCsglass/Au device in Figure S5 shows a weak resistance. The electrical conductivity of Au/Cu 3 TaSe 4 NCs-glass/Au device is very low compared to that of Cu 3 TaS 4 NCs based device. Theoretically, the conductivity of semiconductor is proportional to the product of the mobility and the density of carriers. Herein, as p-type semiconductor, the majority carrier in Cu 3 TaS 4 and Cu 3 TaSe 4 semiconductors is the hole, thereby, their conductivity varies depending on the hole mobility and the density of holes. As described in the introduction, the calculated hole effective mass of Cu 3 TaS 4 and Cu 3 TaSe 4 are 0.944 and 0.831 m h *, Cu 3 TaSe 4 has a lighter hole effective mass and should be more likely to display good p-type mobility. However, the defect concentration and the hole concentration could also affect the conductivity of devices. Speculating about the reason for a less conductivity of Au/Cu 3 TaSe 4 NCs-glass/Au device, we could imagine that the fabricated devices would have a high defect density, which would trap the charge carriers or a low hole density of Cu 3 TaSe 4 NCs. Further investigations in future studies are necessary to resolve the reason for the low conductivity, such as improving the thin-film quality.

Optoelectrical and Conductivity Properties of Cu3TaSe4 NCs
Optical spectra of synthesized Cu3TaSe4 NCs are shown in Figure 13a. The UV-Vis-NIR spectrum of the Cu3TaSe4 NCs prepared by the cascade method shows three primary peaks at around 282.6, 339.6, and 393.5 nm, respectively, whereas the spectrum of the Cu3TaSe4 core-shells exhibits a broad absorbing band at around 420.3 nm with two small characteristic peaks at 297.8 and 361.5 nm. Compared with the Cu3TaSe4 NCs, the UV-Vis-NIR spectrum of Cu3TaSe4 core-shells showed an additional absorption band at around 800-1000nm, which is related to the core-shell structured properties of Cu3TaSe4. Photoluminescence spectra of Cu3TaSe4 prepared by either the cascade method or hot-injection method (Figure 13b) showed a characteristic peak at around 535 nm, suggesting that the optical bandgap of Cu3TaSe4 was 2.32 eV, which is in good agreement with the previous reports [5,21,28,42].The I-V curve of fabricated Au/Cu3TaSe4 NCs-glass/Au device in Figure S5 shows a weak resistance. The electrical conductivity of Au/Cu3TaSe4 NCs-glass/Au device is very low compared to that of Cu3TaS4 NCs based device. Theoretically, the conductivity of semiconductor is proportional to the product of the mobility and the density of carriers. Herein, as p-type semiconductor, the majority carrier in Cu3TaS4 and Cu3TaSe4 semiconductors is the hole, thereby, their conductivity varies depending on the hole mobility and the density of holes. As described in the introduction, the calculated hole effective mass of Cu3TaS4 and Cu3TaSe4 are 0.944 and 0.831 mh*, Cu3TaSe4 has a lighter hole effective mass and should be more likely to display good p-type mobility. However, the  Table 1 displays the morphology, size, and performance of the Cu 3 TaS 4 and Cu 3 TaSe 4 materials prepared under different experimental conditions. It is known from the table that the reaction temperature for preparing the Cu 3 TaS 4 and Cu 3 TaSe 4 materials by the solution-phase method is relatively lower than that of the solid-state method. Thereby, comparing the products prepared by the solid-state method and the solution-process, it is apparent that the solution-phase method is more controllable, facile, and time cost-effective.
which would trap the charge carriers or a low hole density of Cu3TaSe4 NCs. Further investigations in future studies are necessary to resolve the reason for the low conductivity, such as improving the thin-film quality.  Table 1 displays the morphology, size, and performance of the Cu3TaS4 and Cu3TaSe4 materials prepared under different experimental conditions. It is known from the table that the reaction temperature for preparing the Cu3TaS4 and Cu3TaSe4 materials by the solution-phase method is relatively lower than that of the solid-state method. Thereby, comparing the products prepared by the solid-state method and the solution-process, it is apparent that the solution-phase method is more controllable, facile, and time cost-effective.

Conclusions
We demonstrated and evaluated the methodology dependence of nanostructural geometries in the tantalum sulvanites. Using a cascade synthesis method, cubic green-yellow Cu3TaS4 NCs and brown Cu3TaSe4 NCs were successfully prepared. The hot-injection method leads to green Cu3TaS4 NCs and brown Cu3TaSe4 NC with core-shell nanostructure. XRD patterns analysis of the products prepared with different reaction times showed

Conclusions
We demonstrated and evaluated the methodology dependence of nanostructural geometries in the tantalum sulvanites. Using a cascade synthesis method, cubic greenyellow Cu 3 TaS 4 NCs and brown Cu 3 TaSe 4 NCs were successfully prepared. The hotinjection method leads to green Cu 3 TaS 4 NCs and brown Cu 3 TaSe 4 NC with core-shell nanostructure. XRD patterns analysis of the products prepared with different reaction times showed that Cu 3 TaS 4 NCs could be formed in very short time (1 min) after the addition of Cu cations when using the cascade method, whereas the hot-injection method requires 30 min to form Cu 3 TaS 4 NCs. During the formation of Cu 3 TaSe 4 using the cascade method, the incorporation of Cu cations triggered rapid nucleation of Cu 7. 16 Se 4 and then the formed Cu 7. 16 Se 4 consumed the folded TaSe 2 nanosheets to form Cu 3 TaSe 4 NCs. While using the hot-injection method, the Cu 3 TaSe 4 core-shells originated from Cu 7. 16 Se 4 seeds which rapidly formed within 1 min after adding the Cu-Ta precursors. The UV-Vis-NIR and photoluminescence measurements of the obtained Cu 3 TaS 4 presented three characteristic absorbance peaks in the UV-Visible regions and unique emission peaks at 486.3 and 526.1nm. The green emission of Cu 3 TaS 4 at 526.1 nm suggests the presence of a mid-band-gap charge trap caused by Cu vacancies near the valence band maximum. The absorption spectrum of Cu 3 TaSe 4 NCs prepared by the cascade method shows three primary peaks at around 282.6, 339.6, and 393.5 nm. Surprisingly, the Cu 3 TaSe 4 coreshells synthesized by the hot-injection method exhibit not only three primary peaks in the UV-Visible regions but also a unique absorption band at around 800-1000 nm; the phenomenon is currently investigated. Photoluminescence measurements of Cu 3 TaSe 4 prepared by either cascade method or hot-injection shows an optical bandgap of 2.32 eV. In addition, the I-V curve of Au/Cu 3 TaS 4 NCs-glass/Au device suggests the Cu 3 TaS 4 NCs possess nonnegligible conductivity properties. The preliminary optoelectronic study show potential of Cu 3 TaS 4 NCs as future transparent p-type conductors. Furthermore, the solution-based synthetic approaches of Cu 3 TaS 4 NCs and Cu 3 TaSe 4 NCs provide grounds for future applications of these promising semiconductors in the p-type TCM realm.