Rapid and Low-Temperature Molecular Precursor Approach toward Ternary Layered Metal Chalcogenides and Oxides: Mo1-x W x S2 and Mo1-x W x O3 Alloys (0 ≤ x ≤ 1)

Controllable growth of wafer-scale monolayer transition metal dichalcogenides ternary alloys with tunable band gap

The tuning of band gap is very important for the application of two-dimensional (2D) materials in optoelectronic devices. Alloying of 2D transition metal dichalcogenides (TMDCs) is an important way to tune the wide band gap. In this study, we report a multi-step vapor deposition method to grow monolayer TMDC ternary alloy films with wafer scale, including Mo_1-xW_xS₂, Mo_1-xW_xSe₂and MoS_2xSe_2(1-x), which are accurately controllable in the elemental proportion (xis from 0 to 1). The band gap of the three 2D ternary alloy materials are continuously tuned for the whole range of metal and chalcogen compositions. The metal compositions are controlled by the as-deposited thickness. Raman, photoluminescence, elemental maps and TEM show the high spatial homogeneity in the compositions and optical properties across the whole wafer. The band gap can be continuously tuned from 1.86 to 1.99 eV for Mo_1-xW_xS₂, 1.56 to 1.65 eV for Mo_1-xW_xSe₂, 1.56 to 1.86 eV for MoS_2xSe_2(1-x). Electrical transport measurements indicate that Mo_1-xW_xS₂and MoS_2xSe_2(1-x)monolayers shown-type semiconductor behaviors, and the carrier types of Mo_1-xW_xSe₂can be tuned asn-type, bipolar andp-type. Moreover, this control process can be easily generalized to other 2D alloy films, even to quaternary or multi-element alloy materials. Our study presents a promising route for the preparation of large-scale homogeneous monolayer TMDC alloys and the application for future functional devices.

Synthesis of ternary copper antimony sulfide via solventless thermolysis or aerosol assisted chemical vapour deposition using metal dithiocarbamates

Copper antimony sulfide (Cu-Sb-S) has recently been proposed as an attractive alternative photovoltaic material due to the earth-abundant and non-toxic nature of the elements, high absorption coefficients and band gaps commensurate with efficient harvesting of solar photonic flux across multiple phases of Cu-Sb-S. These materials are therefore highly desirable and sustainable and scalable deposition techniques to produce them are of interest. In this paper, we demonstrate two facile, low-temperature and inexpensive techniques (solventless thermolysis and aerosol-assisted chemical vapor deposition (AACVD)) for the preparation of binary digenite (Cu_1.8S), chalcocite (Cu₂S) and stibnite (Sb₂S₃) and several phases of ternary copper-antimony-sulfide (Cu_2xSb_2(1−x)S_y, where 0 ≤ x ≤ 1). It was found that by utilising these different techniques and varying the ratio of Cu:Sb, pure phases of ternary chalcostibite (CuSbS₂), fematinite (Cu₃SbS₄) and tetrahedrite (Cu₁₂Sb₄S₁₃) can be achieved. Two single-source precursors were investigated for this purpose, namely the diethyldithiocarbamate (DTC) complexes of copper and antimony Cu(DTC)₂ and Sb(DTC)₃. These were decomposed both individually (to produce binary materials) and combined (to produce ternary materials) at different ratios. From the solventless thermolysis and AACVD methods, either particulate or thin film material was formed, respectively. These materials were then characterised by powder XRD, SEM, EDX and Raman spectroscopies to determine the crystalline phase, material morphology and uniformity of elemental composition. This analysis demonstrated that as the Cu-content increases, the phase of the ternary material changes from chalcostibite (CuSbS₂) and fematinite (Cu₃SbS₄) at a low Cu:Sb ratio to tetrahedrite (Cu₁₂Sb₄S₁₃) at a high Cu:Sb ratio.

Molecular Precursor Route to Bournonite (CuPbSbS3) Thin Films and Powders

Quaternary metal chalcogenides have attracted attention as candidates for absorber materials for inexpensive and sustainable solar energy generation. One of these materials, bournonite (orthorhombic CuPbSbS3), has attracted much interest of late for its properties commensurate with photovoltaic energy conversion. This paper outlines the synthesis of bournonite for the first time by a discrete molecular precursor strategy. The metal dithiocarbamate complexes bis(diethyldithiocarbamato)copper (II) (Cu(S2CNEt2)2, (1)), bis(diethyldithiocarbamato)lead (II) (Pb(S2CNEt2)2, (2)), and bis(diethyldithiocarbamato)antimony (III) (Sb(S2CNEt2)3, (3)) were prepared, characterized, and employed as molecular precursors for the synthesis of bournonite powders and the thin film by solvent-less pyrolysis and spray-coat-pyrolysis techniques, respectively. The polycrystalline powders and thin films were characterized by powder X-ray diffraction (p-XRD), which could be indexed to orthorhombic CuPbSbS3. The morphology of the powder at the microscale was studied using scanning electron microscopy (SEM). Energy-dispersive X-ray spectroscopy (EDX) was used to elucidate an approximately 1:1:1:3 Cu/Pb/Sb/S elemental ratio. An optical band gap energy of 1.55 eV was estimated from a Tauc plot, which is close to the theoretical value of 1.41 eV.

Testing the Efficacy of the Synthesis of Iron Antimony Sulfide Powders from Single Source Precursors

The antimony-iron sulfide system in general does not produce alloys below 540 °C from traditional solid-state methods. However, single source precursors have been known to produce unexpected products that arise from kinetically trapped polymorphs. In this paper, we test the efficacy of this approach toward the Fe-Sb-S system. Antimony and iron diethyldithiocarbamate complexes of the form Sb[S2CN(Et2)]3 (1) and Fe[S2CN(Et2)]3 (2) were synthesised, characterised, and used as single-source precursors for the preparation of Sb2S3, FexSy, and mixed iron antimony sulfide Sb2(1−x)Fe2xS3 (0 ≥ x ≥ 1) powders using the solvent-less thermolysis method at different temperatures ranging from 300 to 475 °C. The effect of different mole fractions of the iron precursor was evaluated on morphology, shape, and optical and magnetic properties of Sb2(1−x)Fe2xS3 (0 ≥ x ≥ 1). The obtained powders were characterized by X-ray diffraction (XRD), Raman spectroscopy scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, magnetometer measurement, and UV/vis/NIR spectroscopy. The results demonstrated that the crystalline structure, morphology, and elemental composition of the samples changed with the mole fraction of the precursor. There was significant phase separation between Sb and Fe sulfides noted from EDX spectroscopic mapping, yet an optoelectronic study monitoring the direct band gap energy of antimony sulfide shows that the band gap energy increases as a function of Fe content, which suggests limited alloying is possible from the single source route.