(Invited) Novel Strategies for the Bottom-up Assembly of Nanowires into Highly Efficient Bulk Thermoelectrics
Authors: Vaddiraju Sreeram 
Published: February 2020

The predicted exponential increase in the United States’ energy demand over the foreseeable future could be met by supplementing existing sources of energy with new renewable and/or non-renewable energy sources. Alternately, increasing the energy efficiencies of the existing processes and systems through waste heat scavenging could be employed to offset, at least partially, the projected increase in the energy demand. The importance of this alternate scenario could be easily inferred from the data supplied by the Energy Information Administration (EIA), which indicates that 56-57% of all energy produced each year in the US is rejected as waste heat. In the case of automobiles the importance of waste heat recovery becomes more apparent, where approximately 70% of energy generated by burning gasoline is lost either in the engine coolant or through the exhaust. Use of solid-state thermoelectric modules is a minimally invasive method for scavenging waste heat from systems and processes. The lack of any moving parts, portability, very long operational lifetimes, and minimal maintenance make thermoelectrics attractive for waste heat recovery.
The lack of the requisite materials for achieving a good $/watt heat-to-electricity conversion metric is currently preventing the large-scale deployment and use of thermoelectrics in terrestrial applications, and limits their use to niche applications (powering up deep space probes). A factor responsible for this bottleneck is the low efficiencies of the current thermoelectric materials, which in turn could be attributed to the lack of a pathway for precisely and independently tuning the thermal and electrical transport through them. Recent theoretical and experimental studies have demonstrated that nanostructuring of materials serves as a route for independently tuning the thermal and electrical transport. Most of these demonstrations were made using the following types of devices: suspended individual nanowire devices, thin-film superlattice devices, and bulk devices that have endotaxial nanostructures embedded within a host material. From these studies it could be concluded that although it appears that nanostructuring of materials for selectively reducing their thermal conductivities is the only essential requirement for enhancing their heat-to-electricity conversion efficiencies, the existence of an epitaxial relationship between the nanostructures after their assembly is also equally important for efficiency enhancement. Experimental methods employed for obtaining epitaxially-oriented bulk nanostructure assemblies require either the use of tedious process (e.g., atomic layer deposition, ALD) or the use of a specific scientific phenomenon (e.g., spinodal decomposition). These requirements not only make it impossible to mass produce thermoelectrics (ALD is a slow and non-scalable process), but also limit the applicability of these strategies to a few material systems (spinodal decomposition is currently only observed in a few materials systems composed of Pb, Bi, Sb, Te and Sr). Finally, the progress made in experimentally demonstrating enhanced thermoelectric performances of large-scale nanowire assemblies is minimal at best.
In this context, the aim of our work is the rational design and fabrication of next-generation, highly-efficient thermoelectrics through the development of strategies for mass producing nanowires and assembling them in an interface-engineered manner (1-3). The aim of this presentation is to showcase the progress that has been made in this research area by our group. The strategies useful for the mass production of nanowires and their in-situ functionalization with organic molecules (1), together with the strategies useful for assembling them in an interface-engineered manner on a large-scale into functional thermoelectric devices (2,3), will be presented. Demonstrations of how these strategies afford precise thermal and electrical transport control in bulk nanowire assemblies will be made using interface-engineered Zn3P2 and ZnO nanowire assemblies as illustrative examples (2-4). In addition, strategies for welding nanowires during assembly so as to ensure that the welds between the nanowires have the same chemical composition as those of the nanowires and that the nanowires assemblies have a single-crystalline path for electrical conduction will also be presented (5,6). Illustration of this concept will be made by demonstrating enhanced thermoelectric performance in Mg2Si nanowire assemblies obtained by welding pre-synthesized Si nanowires (6).
ACKNOWLEDGEMENTS
Partial financial support from NSF/DOE thermoelectrics Partnership (NSF CBET # 1048702) is gratefully acknowledged.
REFERENCES

L. Brockway, M. Van Laer, Y. Kang, S. Vaddiraju, Physical Chemistry Chemical Physics, 15, 6260-6267, (2013).

L. Brockway, V. Vasiraju, S. Vaddiraju, Nanotechnology, 25, 125402 (2014).

L. Brockway, V. Vasiraju, H. Asayesh-Ardakani, R. Shahbazian-Yassar, S. Vaddiraju, Nanotechnology, 25, 145401 (2014).

L. Brockway, V. Vasiraju, M. K. Sunkara, S. Vaddiraju, ACS Applied Materials & Interfaces
6, 14923-14930 (2014).

Y. Kang, L. Brockway, S. Vaddiraju, Materials Letters, 100, 106-110 (2013).

Y. Kang, S. Vaddiraju, Chemistry of Materials, 26, 2814-2819 (2014).