Research On The Near-field Thermal Radiation Of Nanostructures Comprised Of Anisotropic Materials

As objects are in a close proximity that is smaller than the characteristic wavelength of thermal radiation,their heat transfer would exceed blackbody limits due to the tunneling of evanescent waves.This is the so-called near-field thermal radiation(NFTR),and has received much attention in noncontact thermal management and heat-to-electricity conversion of nanoscale photoelectric devices.Recently,anisotropic materials are exploited in nanoscale photoelectric devices due to their prominent electric,photonic,and thermal properties.Nevertheless,few researches on the NFTR of nanostructures comprised of anisotropic materials are reported.This paper investigates the NFTR of plain plates and nanostructures consisting of anisotropic materials,i.e.,graphite and black phosphorus.The theoreties of NFTR of plain plates and nanostructures consisting of anisotropic materials are elaborated,followed by computations via MATLAB R2017 a and analyses of near-field radiative heat flux(NFRHF)of graphite and black phosphorus.For graphite plates,nonresonant hyperbolic modes are supported to dominate the NFTR for optical axis parallel to the heat transfer direction.Besides that,hyperbolic surface plasmon polaritons(SPPs)are excited for optical axis perpendicular to the heat transfer direction,and their magnitude increases with decreasing thicknesses,leading to the higher NFRHF than bulk counterparts.For black phosphorus plates,doped monolayer structure can support three-order-of-magnitude enhanced heat exchange over blackbodies,even surpassing optimized graphene by 18.5%.The dispersion relation of coupled SPPs of black phosphorus is theoretically derived and excellently coincides with computations,demonstrating the major contributions of coupled SPPs to the NFTR of monolayer black phosphorus.The NFRHF of multilayer black phosphorus decreases with increasing numbers of layer.For graphite gratings,the NFRHF for optical axis parallel to the heat transfer direction is higher than that for optical axis perpendicular to the heat transfer direction,and is seven-fold higher than that for bulk counterparts,exceeding blackbody limits by four orders of magnitude.The dispersion relation of biaxial materials' SPPs is theoretically derived and agrees with computations very well.Nanopatterning enables the excitation of anisotropic hyperbolic SPPs,thus enhancing the NFTR for grating structure.For phosphorene ribbons,low chemical potentials enable them to be transparent to lights,while moderate values can dramatically enhance their NFTR.Increasing chemical potentials and filling factors can reduce and blueshift the peak of spectral radiative heat flux.Nanopatterning changes the coupled SPPs of phosphorene from closed quasi-ellipse for plates to opened quasi-hyperbola for ribbons,thus leading to the 65% higher NFRHF of ribbon structure.