UC Berkeley

Ternary alloys composed of layered gallium monochalcogenides—that is GaS, GaSe, and GaTe—possess improved and tunable optoelectronic properties compared to their constituent binary compounds. In particular, the compositional dependence of the bandgap has been shown to be linear for GaSe1-xSx and slightly bowed for GaSe1-xTex alloys. The difference in the bowing behavior between these two alloys arises from the mismatch in electronegativity, ionization energy, and atomic radius between the chalcogens in each system. Based on this trend, GaS1-xTex alloys are expected to exhibit a much larger degree of band gap bowing as a result of the higher mismatch between sulfur and tellurium. However, GaS1-xTex alloys have not been synthesized thus far because of the higher volatility of sulfur and tellurium—which present experimental challenges to attaining compositional control—and the lower miscibility between the more stable GaS and GaTe compounds.

In this dissertation, sulfur-rich, GaS1-xTex alloy nanostructures were synthesized for the first time using a gold-catalyzed, vapor transport method. The dominant growth mechanism for alloy nanowires is the vapor-liquid-solid growth mechanism as evidenced by the presence of catalyst droplets at the end of the nanowires and by the large ratio of the length to width of the alloy nanowires. The morphology of pure GaS nanostructures is comprised of straight, zigzag, and saw-tooth nanobelts while the morphology of the alloy nanostructures is predominantly straight nanowires with trapezoidal cross-section. Furthermore, the fast-growth direction of GaS nanobelts is in the direction of layer edges while the growth direction of sulfur-rich, alloy nanowires is parallel to the c-axis (i.e., normal to the van der Waals layers). Alloy, c-axis nanowires offer new and exciting opportunities for sensor technologies due to the dangling bonds decorating the nanowire.

In addition to the changes in growth direction, the alloy nanowires showed an initial red shift in the Raman-active modes from GaS Raman-active modes with tellurium incorporation. As the tellurium composition continued to increase, the Raman-active modes did not continue to red shift. This behavior may indicate a solubility limit of substitutional tellurium in sulfur-rich GaS1-xTex alloys. Sulfur-rich, alloy nanowires also showed strong luminescence. The room-temperature photoluminescence (PL) signal is characterized by strong peaks at 1.42 eV and 1.55 eV while the low-temperature (85 K) cathodoluminescence (CL) signal has a high-intensity peak at 2.23 eV. A band model was proposed to explain the origins of the luminescence observed. In this model, two types of transitions can occur. In the first type of transition, CL excites carriers across the direct band gap of the alloy nanowire. This transitions corresponds to the high-intensity CL peak located at 2.23 eV. The second type of transition that occurs is the indirect exciton transition. Each of the two high-intensity, PL peaks is attributed to an indirect exciton transition from a different polytype. These observations point to a decrease of the fundamental (indirect) band gap of GaS by approximately 1 eV by its dilute alloying with GaTe (i.e., x < 0.10 in GaS1-xTex). Overall, the environmental stability and strong luminescence in the near infra-red region displayed by the GaS1-xTex alloy nanowires makes this material system a promising candidate for optoelectronic applications, such as photodetector technology.

Finally, DFT calculations support a picture in which the GaS1-xTex alloy system displays strong band gap bowing in the sulfur-rich side of composition (i.e., x < 0.50). These results are consistent with PL and CL results. The bowing originates from the large differences in atomic size and electronegativity between sulfur and tellurium. Further, DFT calculations predict a miscibility gap as well as a metastable phase in the GaS1-xTex alloy system. Such immiscibility supports the attribution of the optical trends (Raman and PL) with increasing tellurium composition to a saturation of the substitutional incorporation of tellurium in sulfur-rich alloys. These calculations contribute to the foundation for studying highly mismatched, layered materials possessing different crystal structures and polytypes.