Electro-Thermal Transport in Non-Homogeneous Network
Nano-structured networks have drawn attention as potential replacements for bulk-material approaches in applications including transparent conducting electrodes (TCEs). As TCEs, these systems can provide relatively low sheet resistances in the high optical transmission regime along with excellent mechanical flexibility. While percolation models have been developed to describe the general trends in sheet resistance and transmittance on nanowire density, prior experiments have not yielded details such as distributions of junction resistances and current pathways through the networks. Most experimental studies on nanostructured TCE properties have focused on large area steady state exploration of electrical and optical properties or more microscopic studies of single/few junctions within the networks. A more detailed and microscopic understanding of the conduction pathways is necessary to more completely understand the percolating transport in these network systems. In this thesis, we fabricate high quality graphene-silver nanowire (NW) based hybrid TCE and use transient thermoreflectance (TR) imaging technique with high temporal (200 ns) and spatial resolution (~ 200–400 nm) that allows simultaneous characterization of time and spatial dependence of the local self-heating around NW-NW junctions. Hotspots arise from self-heating associated with applied bias and are spatially correlated with current pathways through the network. Moreover, these hotspots can potentially redistribute and/or turn off percolating conductive current pathways due to elevated temperatures and thus impose reliability concerns. The ability to image the formation of the microscopic hotspots due to local self-heating (i.e. associated with local current pathways through the microscopic regions) provides a means to semi-quantitatively infer current pathways in these percolating systems. First, we investigate time dependent temperature rise at hotspots in hybrid network at low spatial resolution (i.e. each hotspot comprises multiple NW-NW junctions) that allows analysis of multiple hotspots residing in between the electrical contacts. We quantitatively determine the thermal time constants of the hotspots and show their dependence on different spatial locations within the network. Collectively, we decouple the temperature rise into separate contributions from local self-heating and heat spreading from the electrical contacts. As we identify the time regime when local self-heating at the hotspots is predominant, we focus on local self-heating (rather than heat spreading from contacts) for subsequent electro-thermal studies. Next, using high-resolution TR imaging in case of a silver NW network, we study microscopic hotspots corresponding to individual NW-NW junctions and show the temporal and spatial evolution of the temperature profiles along two crossing NWs. We quantify the local power generated at a hotspot (i.e. an individual junction) at steady state and the fraction of this power propagating along each constituent NW. We also compare material/composition dependence of hotspots (each containing multiple junctions) characteristics in terms of their transient electro-thermal response, number, average temperature, and spatial distribution by considering two random 2-D networks where different transport mechanism prevails: silver NW network (percolation) and graphene-silver NW hybrid network (copercolation). Finally, we do an extended study of temperature distributions (which can be described by Weibull distributions) in silver NW network that shows distinctive signatures (i.e. evolution of shape parameter with time) of local self-heating vs heat spreading through network. The ability to resolve the local self-heating with high temporal and spatial resolution uniquely enables a comprehensive understanding of electro-thermal response and current pathways in the distributed conductors.
Janes, Purdue University.
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