DOI: 10.25549/usctheses-c127-631687
title: Thermal transport of carbon nanotubes and graphene under optical and electrical heating measured by Raman spectroscopy

description: This thesis presents systematic studies of thermal transport in individual single walled carbon nanotubes (SWCNTs) and graphene by optical and electrical approaches using Raman spectroscopy. In the work presented from Chapter 2 to Chapter 6, individual suspended CNTs are preferentially measured in order to explore their intrinsic thermal properties. Moreover, the Raman thermometry is developed to detect the temperature of the carbon nanotube (CNT). A parabolic temperature profile is observed in the suspended region of the CNT while a heating laser scans across it, providing a direct evidence of diffusive thermal transport in an individual suspended CNT. Based on the curvature of the temperature profile, we can solve for the ratio of thermal contact resistance to the thermal resistance of the CNT, which spans the range from 0.02 to 17.The influence of thermal contact resistance on the thermal transport in an individual suspended CNT is also studied. The Raman thermometry is carried out in the center of a CNT, while its contact length is successively shortened by an atomic force microscope (AFM) tip cutting technique. By investigating the dependence of the CNT temperature on its thermal contact length, the temperature of a CNT is found to increase dramatically as the contact length is made shorter. This work reveals the importance of manipulating the CNT thermal contact length when adopting CNT as a thermal management material. ❧ In using a focused laser to induce heating in a suspended CNT, one open question that remains unanswered is how many of the incident photons are absorbed by the CNT of interest. To address this question, micro-fabricated platinum thermometers, together with micro-Raman spectroscopy are used to quantify the optical absorption of an individual CNT. The absorbed power in the CNT is equal to the power detected by two thermometers at the end of the CNT. Our result shows that the optical absorption lies in the range between 0.03 to 0.44%. In addition, the temperature gradient from the heating location to both ends of the suspended CNT, together with absorbed power and diameter of the CNT can be used to solve for the thermal conductivity of the CNT, which is found to span the range from 0.51nW/K to 2.28nW/K. ❧ We also conduct Raman measurements to explore the effect of gas molecules on thermal transport in electrically- or optically-heated suspended carbon nanotubes. Under the same electric heating power, the temperature increase of the CNT in gaseous environments is found to be considerably less than in vacuum, indicating non-negligible heat dissipation to the surrounding gas molecules. The result shows the approximately 50 to 60% of the heat is taken away by the surrounding gas molecules in a 5 μm long electrically-heated CNT. Following the electrical heating technique, a two-laser, purely optical measurement technique is utilized to observe heat dissipation in an ultra-long suspended CNT. Exponentially decaying temperature profiles with the heat decay lengths shorter than 7 μm are found in all samples measured. These relatively short heat decay lengths are attributed to the strong thermal coupling of carbon nanotubes’ surface to air molecules. Moreover, the previously unmeasured heat transfer coefficient between CNT and air molecules is determined to be on the order of 104 W/m²⋅K. ❧ In Chapter 7, we show some preliminary results of Raman measurements on graphene devices. The spatially-varying power generation in an electrically-biased supported graphene is found to depend on the local potential. The Joule-heated graphene exhibits a linear temperature gradient from the drain to the source contact, where the hottest spot corresponds to the location of lowest carriers’ density. Moreover, nonequilibrium phonon temperatures, as determined by Raman spectroscopy, are investigated in electrically-biased suspended graphene devices. Since gas molecules are proposed to be good media for the heat dissipation and hot phonons’ relaxation in carbon nanotubes, we utilized the same techniques, as described in Chapter 5 and 6, to examine if the gas molecules can eliminate hot phonons and facilitate heat dissipation in graphene. The results show similar I-V characteristics of suspended graphene measured in different surrounding environments, indicating trivial interaction of gas molecules to graphene. Moreover, similar temperature increases in Joule-heated graphene are observed in vacuum and in different gaseous environments, revealing insignificant effects of gas molecules on the heat dissipation and the lateral heat conduction governs the energy dissipation. In using the two-laser measurement technique on another suspended graphene sample, linear temperature gradients from a local hot spot are observed both in vacuum and in air, confirming the heat is carried away by the conduction mechanism to contacts.