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Phone/Fax: þ86 10 623 32743Though carbon nanotube (CNT) arrays have tremendous potential due to their attractive mechanical, electrical, and thermal properties, the growth kinetics of CNTs are still not fully understood. Thus, we report on the effect of synthesis parameters, such as growth temperature, on the resulting arrays. In this work, CNT arrays were synthesized using catalytic chemical vapor deposition (CCVD) with furnace temperatures varying from 680 to 900 8C. Microscopy was used to investigate the effect of growth temperature on the structural properties, such as tube diameter, array length, and the amount of amorphous carbon produced at the top of the canopy as a growth by-product. Additionally, Raman spectroscopy was used to elucidate the effect growth temperature has on the resulting purity of the CNTs. It was then revealed that crystalline inhomogeneity exists along the length of the tubes with respect to crystallinity. Transmission electron microscopy (TEM) further determines the degree of tube crystallinity as well as the thickness of amorphous carbon coating around the nanotubes. Through both microscopy and spectroscopy, we found two distinct temperature regimes within the range of 680-900 8C. Below 800 8C, the growth of tube length and diameter remained relatively stagnant followed by a rapid growth rate above 800 8C with the highest tube crystallinity obtained within the regime of 800-840 8C. This indicates the presence of an important transitional temperature for CNT CCVD growth. Additionally, growth temperature was determined to play an important role in the amount of the resulting amorphous carbon by-product.

Pump-probe thermoreflectance techniques have been utilized extensively to measure the thermal properties; such as, thermal conductivity and interfacial conductances of thin film systems. While there have been many implementations of transient measurements, the most commonly operated systems utilize a near-IR femtosecond pulsed system as the fundamental laser source. The repetition rate of these femtosecond pulsed systems can vary from kHz to MHz. Thus, we perform an in-depth comparison of the implementation and analysis of low-repetition rate and high-repetition rate time-domain thermoreflectance (TDTR), demonstrating that despite varying the repetitionrate, the measured thermal properties are consistent. To illustrate the universality of these systems, we selected a range of materials to study on both TDTR setups , including the thermal conductivity of a SiC film, a SiO 2 film, and the interfacial conductances of Al/SiO 2 and Al/SiC systems.

The emergence of wide and ultrawide bandgap semiconductors has revolutionized the advancement of next-generation power, radio frequency, and opto- electronics, paving the way for chargers, renewable energy inverters, 5G base stations, satellite communications, radars, and light-emitting diodes. However, the thermal boundary resistance at semiconductor interfaces accounts for a large portion of the near-junction thermal resistance, impeding heat dissipation and becoming a bottleneck in the devices’ development. Over the past two decades, many new ultrahigh thermal conductivity materials have emerged as potential substrates, and numerous novel growth, integration, and characterization techniques have emerged to improve the TBC, holding great promise for efficient cooling. At the same time, numerous simulation methods have been developed to advance the understanding and prediction of TBC. Despite these advancements, the existing literature reports are widely dispersed, presenting varying TBC results even on the same heterostructure, and there is a large gap between experiments and simulations. Herein, we comprehensively review the various experimental and simulation works that reported TBCs of wide and ultrawide bandgap semiconductor heterostructures, aiming to build a structure–property relationship between TBCs and interfacial nanostructures and to further boost the TBCs. The advantages and disadvantages of various experimental and theoretical methods are summarized. Future directions for experimental and theoretical research are proposed.