UC Riverside

The paramount issues of today such as the gathering energy crisis and rise in levels of pollution have largely resulted from our increasingly accelerated industrialization due to technology developments. Ironically, those issues demand the further advanced technology to answer. In addition, the overwhelming rate of data production due to the rapid development of computer technologies require even more improved technologies to overcome our current limits. Nanotechnology has been extensively developed to approach these issues, since engineered materials at the nano-scale level have demonstrated enhanced desirable properties due to their small dimensions. Chalcogenide nanostructures have shown potential for use in thermoelectric and phase change memory devices with potentially high performance.

The overall objective of this work is to engineer nanostructured chalcogenides using various techniques such as electrodeposition and wet-chemical synthesis as cost-effective and simple approaches. The dimension, size, composition and crystal structure of the chalcogenide materials are controlled to enhance their properties for thermoelectric and phase change memory applications. The specific research investigations in this work were the following:

1) Electrochemical analysis of SbxTe1-x electrodeposits was implemented. Linear sweep voltammetry and quartz crystal microbalance techniques were utilized to study the properties of SbxTe1-x depending on agitation and temperature. The mass transport, reduction behavior, surface morphology and crystal structure of SbxTe1-x electrodeposits were investigated.

2) SbxTe1-x nanocomposites were produced by the separation of the Sb2Te3 phase and the γ-SbTe phase in the annealed SbxTe1-x electrodeposits. The thermoelectric and phase transition properties of SbxTe1-x electrodeposits were characterized as a function of temperature and composition. The generated nanocomposites demonstrated a drastically enhanced Seebeck coefficient.

3) AgxTe1-x thin films were synthesized by a topochemical transformation reaction of thermally evaporated Te thin films. The thermoelectric properties of the transformed AgxTe1-x thin films with tailored compositions were analyzed. In addition, the composition-dependent Seebeck coefficients showed the transition of carrier transport behavior from p-type to n-type.

4) Ternary chalcogenide AgxSb1-xTey materials were synthesized by a cost-effective and simple cation exchange reaction of thermally evaporated SbxTey thin films. The composition of ternary AgxSb1-xTey was controlled by the reaction time. Temperature- and composition-dependent thermoelectric and phase transition properties of the synthesized films were investigated.

5) Single crystal PbTe nanowires were electrodeposited by a template-direct method. An electrical transport behavior of the electrodeposited PbTe nanowires with modulated electrical contacts was estimated. The improvement of the electrical contact was achieved by a galvanic displacement reaction of Au nanoparticles with PbTe exposed in the opened contact region.

6) Scalable Te nanoribbons with Au electrodes were fabricated by a lithographically patterned nanowire electrodeposition (LPNE) technique. A pulsed potential was applied to control the reduced width of nanoribbons. The dimensions of the nanoribbons were determined by a pattern design, the thickness of e-beam evaporated Ni layer and an electrodeposition time. Ultra-long Te nanoribbons with a length of several cm, a height of 100 nm and a ranged width from about 50 to 200 nm were demonstrated. Electrical properties of Te nanoribbons with different widths were studied. Moreover, the effect of the size of the nanoribbons and the electrical contact types on the NH3 sensing properties of Te nanoribbons on NH3 (g) was analyzed.

7) A combined technique of a LPNE and a galvanic displacement reaction was utilized to synthesize ultra-long BixTe1-x nanoribbons of which the dimensions were controlled in a length of several cm, a height of 100 nm and a ranged width from about 400 nm to 2.5 μm. Composition-dependent electrical resistivities and FET properties of the BixTe1-x nanoribbons were analyzed. Temperature-dependent thermoelectric properties of a Bi31Te69 nanoribbon were investigated.

In this dissertation, antimony telluride (SbxTe1-x), bismuth telluride (BixTe1-x), lead telluride (PbxTe1-x), tellurium (Te), silver telluride (AgxTe1-x), silver antimony telluride (AgxSb1-xTey) were synthesized by various techniques such as potentiostatic deposition, pulse plating, topochemical transformation reaction, cation exchange reaction and galvanic displacement reaction. The material properties and electrical/thermoelectric behaviors depending on their dimension, size, composition and crystal structure were investigated.