[CCoE Notice] Thesis Defense: Thermal and Thermoelectric Transport Measurements of Silicon Nanomembranes, Transition Metal Dichalcogenides, and Carbon Nanotube Networks

[Grayson, Audrey A]

PhD Defense Student: Milad Yarali
Date: Friday, December 1, 2017
Time: 9:30 AM
Location: College of Social Work, Room SW 229
Dissertation Chair: Dr. Anastassios Mavrokefalos
Title:

Thermal and Thermoelectric Transport Measurements of Silicon Nanomembranes, Transition Metal Dichalcogenides, and Carbon Nanotube Networks
Deep understanding and manipulating of energy transport characteristics in nanoscale systems is of fundamental importance in realizing high-performance solid-state devices. As dimension of these devices progressively shrink, the size effect of nanostructures, their interfacial scattering and interactions with the surrounding environment are increasingly dominating the electron and phonon transport properties. The objective of the work presented in this dissertation is to further the current understanding of interplay between structure and thermal and thermoelectric properties in nanofilm materials through experimental investigations. This objective is accomplished by utilizing a micro-fabricated device to preform coupled electrical-thermal-structural characterizations on the same individual nanofilm suspended between two resistance thermometers.
First, an approach was developed to manage thermal conductivity (κ) of Si thin-film based nanoarchitectures through the formation of radial and planar Si/SiOx hybrid nanomembrane superlattices (HNMSLs). For the 24 nm thick one-winding tube at room temperature * = 7.64 W m-1 K-1 which is 20 times smaller than the value of bulk single-crystalline silicon. Interestingly, a continuous reduction in * with increasing number of windings was observed. Meanwhile, the planar Si/SiOx HNMSL shows * = 5.3 W m-1 K-1, being the smallest in-plane thermal conductivity among all the reported values for Si-based superlattices.
Next, the effect of metal doping and intrinsic structural defects on κ of monolayer undoped MoS2 and MoSe2, and doped Mo0.82W0.18Se2 grown by chemical vapor deposition were investigated. The results show the grain boundaries and vacancies are responsible for over 2 times reduction in the room temperature κ in our samples compared to their exfoliated counterpart, while κ remains intact upon isoelectronic substitution of W for Mo atoms. Also, boundary scattering dominates over defects and phonon-phonon scattering at low temperatures.
            Lastly, the effect of physisorbed vs chemisorbed oxygen on transport properties of aligned single walled carbon nanotubes (SWCNT) nanofilm was investigated. The physisorbed oxygen molecules on the SWCNTs surface make them initially p-type with metallic behavior. Vacuum annealing leads to desorb these molecules resulting in transition to n-type with semiconducting behavior while κ remains intact. On the other hand, SWCNTs with chemisorbed oxygen molecules exhibit purely p-type metallic behavior with lower κ.

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