Tunable Thermal Radiative Properties of Nanotube and Nanowire Arrays

Abstract

Abstract : A multiscale multiphysics simulation tool, by integrating excited state ab initio calculations and FDTD simulations, has been developed. It has the capability of predicting thermal radiative properties of nanotube/nanowire arrays given their atomic structures. Benchmark work is first performed on semiconductor GaAs to demonstrate the effectiveness of our approach, showing that both infrared and visible band radiative properties agree well with experimental data. Ab initio calculations on single walled carbon nanotubes (SWCNT) show that the dielectric function is very sensitive to chirality, polarization, and intertube coupling. GW and BSE methods have been successfully used to account for quasi-particle and exciton effects. The dielectric function is used in finite difference time domain (FDTD) simulations to calculate the macroscopic radiative properties of nanotube(NT)/nanowire(NW) arrays. The results show that the conventional Maxwell-Garnett method is not accurate. The effects of array filling fraction, diameter, and length are investigated systematically. In particular, it is proposed that the optical absorption can be enhanced by using disordered vertical CNT and Si NW arrays. E-beam lithography has been used to synthesize patterned carbon nanotube arrays to achieve good periodicity. Processing of controlled variation (ALD, etching) has been achieved. Thermal reflectance has been characterized on NT/NW samples, and tunable thermal radiative properties in total reflectance and spectra have been observed. The conclusion of this project represents a major advance of multiscale multiphysics understanding of thermal radiative properties of nanotube/nanowire arrays that are of significant technical importance.