Near-field thermal radiation which can exceed blackbody radiation by several orders of magnitude has potential applications in energy conversion devices, nanofabrication, and near-field imaging. The present dissertation provides a comprehensive and thorough investigation of near-field heat transfer between parallel plates at nanometer distances.;The first part of this dissertation focuses on the fundamentals of nanoscale thermal radiation through a systematic study on the near-field heat transfer between doped Si plates. In order to calculate the near-field heat transfer, it is important to accurately predict the dielectric function of doped Si. The dielectric function of doped Si which is described by the Drude model is a function of carrier concentration and mobility. Hence, accurate ionization and carrier mobility models for both p- and n-type Si are identified after a careful review of the available literature. The radiative properties calculated using the improved dielectric function agrees to a good extent with measurements performed using a FTIR. The near-field heat transfer between doped Si plates at varying doping levels is then calculated using the improved dielectric functions. Several important and characteristic features of near-field radiation are revealed in the analysis. An interesting issue regarding the maximum achievable nanoscale thermal radiation arises out of the study on near-field heat transfer in doped Si.;The second part of this dissertation investigates the maximum achievable nearfield thermal radiation between two plates at finite vacuum gaps. Initially, both the emitter and the receiver are assumed to have identical frequency-independent dielectric functions and a cut off in the order of the lattice spacing is set on the upper limit of the wavevector. The energy transfer is maximum when the real part of dielectric function is around -1 due to surface waves. On the other hand, there is a strong relationship between the imaginary part of the dielectric function and the vacuum gap. While the study using frequency independent dielectric function is not realistic, it lays down the guidelines for the parametric optimization of dielectric functions of real materials for achieving maximum near-field heat transfer. A parametric study of the different adjustable parameters in the Drude and Lorentz model is performed in order to analyze their effect on the near-field heat transfer. It is seen that the optimized Drude model always results in greater near-field heat transfer compared to the Lorentz model and the maximum achievable near-field heat transfer is nearly 1 order greater than that between real materials.;In the third part of this dissertation, the unusual penetration depth and the energy streamlines in near-field thermal radiation are studied. It is seen that unlike far-field radiation, the penetration depth in near-field heat transfer is dependent on the vacuum gap. This unusual feature results in a 10 nm thick SiC film behaving as completely opaque when the vacuum gap is around 10 nm. The energy streamlines inside the emitter, receiver, and the vacuum gap are calculated using fluctuation electrodynamics and errors generated due to thin film optics are pointed out. It is seen that the lateral shift of the streamlines inside the emitter can be greater than that in the vacuum gap for SiC. However, for doped Si, the lateral shift is comparable in the different media. While the study on the penetration depth determines the thickness of the emitter, the streamlines determine the lateral dimension.
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