Jamie J. Gengler

Growth, Structure, and Thermal Conductivity of Yttria-Stabilized Hafnia Thin Films

Yttria-stabilized hafnia (YSH) films of 90 nm thickness have been produced using sputter-deposition by varying the growth temperature (T(s)) from room-temperature (RT) to 400 °C. The effect of T(s) on the structure, morphology, and thermal conductivity of YSH films has been investigated. Structural studies indicate that YSH films crystallize in the cubic phase. The lattice constant decreases from 5.15 to 5.10 Å with increasing T(s). The average grain size (L) increases with increasing T(s); L-T(s) relationship indicates the thermally activated process of the crystallization of YSH films. The analyses indicate a critical temperature to promote nanocrystalline, cubic YSH films is 300 °C, which is higher compare to that of pure monoclinic HfO(2) films. Compared to pure nanocrystalline hafnia, the addition of yttria lowers the effective thermal conductivity. The effect of grain size on thermal conductivity is also explored.

Two-color time-domain thermoreflectance of various metal transducers with an optical parametric oscillator

Conventional single-color laser pump?probe methods for measuring thermal properties are limited by sample requirements that arise from considerations of surface roughness and compatible thermoreflectance transducers. Here, we describe a new experimental arrangement for performing two-color time-domain thermoreflectance (TDTR). The technique is a variation of traditional pump?probe spectroscopy that is based on a femtosecond Ti:sapphire oscillator of a fixed wavelength and an optical parametric oscillator, with the goal being to create an independently tunable probe wavelength. This method offers two advantages: (1) spectral filtering of diffusely scattered pump light (to prevent it from reaching the detector) and (2) improvement in the thermoreflectance signal from different metal thin films. The wavelength tunability of the system allows enhancement of TDTR signal generation for multiple thermoreflectance transducer materials. This wavelength-adjustable feature, in turn, facilitates the direct measurement of the thermal transport properties of various thin films and substrates, which would be difficult with single-color femtosecond pump?probe systems. Demonstrated results include optimization of the probe wavelength for different metals, measurement of metal?graphite interfacial conductances on relatively rough samples and a two-order-of-magnitude calibration of thermal conductivity measurements using copper as a thermoreflectance transducer.

Multifold Seebeck increase in RuO2 films by quantum-guided lanthanide dilute alloying

Ab initio predictions indicating that alloying RuO2 with La, Eu, or Lu can increase the Seebeck coefficient α manifold due to quantum confinement effects are validated in sputter-deposited La-alloyed RuO2 films showing fourfold α increase. Combinatorial screening reveals that α enhancement correlates with La-induced lattice distortion, which also decreases the thermal conductivity twentyfold, conducive for high thermoelectric figures of merit. These insights should facilitate the rational design of high efficiency oxide-based thermoelectrics through quantum-guided alloying.

Synthesis of zinc fulleride (ZnxC60) thin films with ultra-low thermal conductivity

The structure and physical properties of doped fullerene materials were investigated for their interesting thermal properties. The synthesis and thermal properties of ZnxC60 thin films are reported. Thin films of ZnxC60 were found to have an exceedingly low thermal conductivity of 0.13 Wm−1 K−1. Differential scanning calorimetry results suggested that a temperature of 357 °C is needed in order to fully intercalate the Zn with the C60. Both charge transfer and covalent bonding (between Zn and C60) should be considered when attempting to understand the Raman spectra observed. Moreover, the ZnxC60 thin films created represent an interesting class of materials that could find use in several thermal applications. Furthermore, in the present case, the exceptionally low thermal conductivity is accompanied by a substantial increase in the electrical conductivity, suggesting interesting thermal and electrical transport.

Laser-assisted field emission in single-walled carbon nanotubes

Carbon nanotubes (CNTs) have many uses in energy storage, electron emission, molecular electronics, and optoelectronics. Understanding their light-matter interactions is crucial to their development. Here, we study a film of single-walled CNTs with a thickness of 1.67 μm and a 2D orientational order parameter of 0.51, measured by polarized Raman spectroscopy. The film is expected to have a work function of about 5.1 eV. In this study, ~100-fs pulses with 1.5 (ℏω) and 3 eV (2ℏω) photon energy are used to pump the CNT film while observing its electron emission in vacuum. Ultrafast pulses produce nonlinear phenomena in enhanced field emission, as the CNTs absorb strongly enough that thermally excited carriers can tunnel through the potential barrier. Through curve fitting of the power dependence for each pump energy, we find that the light at ℏω is absorbed via 5-photon absorption, and the light at 2ℏω is absorbed via a combination of 2- and 3-photon absorption. Further study reveals a space-charge limited regime with low applied bias, a photoemission regime with moderate bias, and a laser-assisted field emission regime when the bias is high enough that the photon pump is no longer important. Cross-correlation pumping with the two colors simultaneously shows 4x enhancement of the emission, with a FWHM that suggests a lifetime of ~190 fs, similar to the dephasing time of electrons in CNTs. These studies help illuminate the properties of CNTs as a nonlinear optical material and go towards a more thorough understanding of their optoelectronic properties.