Ming-Tsung Hung

Heat conduction in graphite-nanoplatelet-reinforced polymer nanocomposites

Heat transport in polymer nanocomposites reinforced with graphite nanoplatelets (GNPs) is studied using high-precision thermal conductivity measurements. The resistance to heat conduction across interfaces between GNPs and the polymer matrix has a strong effect on energy transport in the nanocomposites. The thermal conductivity is observed to increase when GNPs are pretreated with nitric acid to improve interfacial bonding. The improvement in the thermal conductivity, however, is much smaller than the corresponding improvement in mechanical properties. The thermal interface resistance extracted from the present thermal conductivity data is comparable to that obtained from the previously reported data on carbon nanotube suspensions.

Nanoscale Heat Conduction Across Metal-Dielectric Interfaces

We report a theoretical study of heat conduction across metal-dielectric interfaces in devices and structures of practical interest. At cryogenic temperatures, the thermal interface resistance between electrodes and a substrate is responsible for substantial reduction in the maximum permissible peak power in Josephson junctions. The thermal interface resistance is much smaller at elevated temperatures but it still plays a critical role in nanoscale devices and structures, especially nanolaminates that consist of alternating metal and dielectric layers. A theoretical model is developed to elucidate the impact of spatial nonequilibrium between electrons and phonons on heat conduction across nanolaminates. The diffuse mismatch model is found to provide reasonable estimates of the intrinsic thermal interface resistance near room temperature as well as at cryogenic temperatures.

Matrix and quantum confinement effects on optical and thermal properties of Ge quantum dots

The influence of SiO_2 and Si_3N_4 dielectric matrices on the structural, phonon, luminescence and thermal properties of Ge quantum dots (QDs) has been experimentally investigated. Compared with the case of QDs in SiO_2 layers, Si_3N_4 matrix imposes large interfacial surface energy on QDs and enhances their Ostwald ripening rate, appearing to be conducive for an improvement in crystallinity and a morphology change to a more perfectly spherical shape of Ge QDs. Quantum confinement induced electronic structure modulation for Ge QDs is observed to be strongly influenced not only by the QD size but also by the embedded matrix. Both matrix and surface effects offer additional mechanisms to QD itself for controlling the optical and thermal properties of the QDs.

Nanoscale heat conduction across tunnel junctions

Nanoscale heat conduction across tunnel junctions prepared through natural oxidation of metal electrodes is experimentally studied. The thermal resistance of AlOx tunnel barriers increases linearly with barrier thickness, which is consistent with the prevailing theory of heat conduction in highly disordered materials. Heat conduction across tunnel junctions is strongly impeded by finite thermal resistance at interfaces between barrier and electrode layers, which can be exploited to create superior thermal barrier coatings. The thermal conductivity of nanolaminates consisting of a series of Ta∕TaOx tunnel junctions is determined to be well below the minimum thermal conductivity limit.

High quality multifold Ge/Si/Ge composite quantum dots for thermoelectric materials

We present an effective approach to grow high-quality thin film of composite quantum dots (CQDs) as a building block for thermoelectric materials, in which 3 times the usual Ge deposition can be incorporated within a 3-fold CQD. Selective chemical etching experiments reveal that a thin Si inserted layer in the CQDs modifies the growth mechanism through surface-mediated diffusion and SiGe alloying. Such thin-film-like CQD materials are demonstrated to exhibit reduced thermal conductivity κ⊥ with respect to the conventional QDs, perhaps as a consequence of enhanced diffusive phonon scattering from the high Si/Ge interface density and enhanced local alloying effect.

CMOS-Compatible Generation of Self-Organized 3-D Ge Quantum Dot Array for Photonic and Thermoelectric Applications

We demonstrate a CMOS-compatible scheme, selective oxidation of SiGe pillars, for creating well-organized 3-D Ge quantum dot (QD) array by guiding QDs migration along the oxidation path and thus placing them on targeted locations where the ultimate oxidation occurs. Stacked QDs exhibit tunable luminescence over the visible and possess low thermal conductivity, showing promise for nanophotonic and energy conversion devices.

Large reduction in thermal conductivity for Ge quantum dots embedded in SiO2 system

Thermal conductivity (k(T)) of Ge quantum dots (QDs) embedded in SiO2 was investigated at T = 100–400 K. The Ge QD/SiO2 system appears to have much lower k(T) than their counterparts of bulk Ge and SiO2, and the reduction factor increases with the surface-to-volume ratio of the QD in SiO2. Attendant to reduced magnitude includes delayed Umklapp decline and weaker dependence on temperature for k(T). Effective medium analysis suggests the reduction in k primarily comes from the decreased group velocity thanks to the QD inclusion that induces interfacial stress on SiO2, phonon confinement, and boundary scatterings.

Exploration of thermolithography for micro- and nanomanufacturing

Lithography is a critical enabling technology for manufacturing micro- and nanoscale devices and structures. The present work explores alternative lithography techniques that pattern photoresist layers through selective thermochemical cross-linking. Microfabricated thin-film heaters are used as precisely defined heat sources to determine the thermal transport properties of photoresist layers and study the kinetics of cross-linking reactions. The present work identifies heating temperature, heating duration, and UV exposure dose as independent control parameters in thermolithography and demonstrates its potential for three-dimensional micro- and nanomanufacturing.

Process dependence of the thermal conductivity of image reversal photoresist layers

Thermal transport in polymer layers is an important consideration in various lithography techniques, including immersion lithography and thermolithography. The in-plane thermal conductivity of commercially available photoresist AZ 5214E is determined at each stage of lithography processes using ultrathin (<100nm) freestanding membrane devices. The authors find that UV exposure does not lead to any appreciable change in thermal conductivity whereas cross-linking induced by postexposure bake results in a slight increase (∼5%). The thermal boundary resistance across interfaces between the resist layers and metal films/substrates is also found to be significant. The experimental techniques and data presented here will facilitate a systematic evaluation of thermal phenomena during lithography processes.

Experimental Study of Heat Conduction in Aqueous Suspensions of Aluminum Oxide Nanoparticles

We perform a systematic experimental study of heat conduction in aqueous suspensions of aluminum oxide nanoparticles at volume concentrations up to 10%. We develop a micro-hotwire device to reduce experimental errors resulting from spatial or temporal temperature inhomogeneity within a sample. The volume concentration dependence of the thermal conductivity can be explained using the effective medium model with a physically reasonable set of parameters. The average particle size as well as the thermal conductivity is measured as a function of sample sonication time and temperature. The size of particles/aggregates in our nanofluid samples is much greater than the nominal particle size reported by the manufacturers and does not change appreciably with sonication for up to 24 hours. Our data do not reveal any anomalous enhancement in the thermal conductivity or strong temperature dependence reported in other previous studies. The discrepancy may reflect subtle differences in nanopowders or nanofluid preparation procedures that result in drastic difference in the size or shape of suspended particles/aggregates.