Investigation of Extreme Near-Field Thermal Radiation at Sub-10-nm Vacuum Gap Distances

CBET - 1605584PI: Park， keunhan当物体之间的间隙小于热辐射的波长时，辐射传热的速率可以显著超过预期的速率。这一观察结果使创新技术应用成为可能，如近场热光伏发电、热整流和局部辐射冷却。该项目的目的是对近场热辐射进行实验验证，这是一个相对未开发的领域，特别是对于小于10纳米的间隙。本研究的结果将为热辐射能量的光谱分布及其在极端近场状态下的输运提供知识。该项目的成果将激发纳米级中红外光探测、成像和光谱学以及近场热光伏发电机的发展。该项目将通过本科生和研究生参与拟议的研究活动来促进培训和学习。此外，K-12外展将通过犹他州科学奥林匹克，工程日和犹他州纳米日进行。极端近场热辐射的实验和理论研究将通过：(1)在超高真空原子力显微镜（UHV-AFM）上测量加热尖端与表面之间的极端近场辐射热传递来完成；(2)通过尖端增强光电流产生和尖端散射热辐射测量近场热谱；(3)利用数值精确和近似方法模拟尖端-表面近场热相互作用。该项目的创新之处在于在定制的UHV-AFM中使用了尖端平面配置，该配置与反射光学元件相结合，可以测量尖端散射热辐射，并结合尖端与衬底的直接热和辐射相互作用。自制的电阻纳米温度计和定制的光伏中红外探测器将作为衬底，直接测量衬底端近场能量输运。将获得的实验数据与称为热离散偶极子近似的数值模型进行比较，以了解影响极端近场热辐射和辐射-传导转变的潜在机制和因素。这项研究将提供极端近场热辐射的定量测量，这将允许与理论模型相结合，确定基于波动电动力学的连续统理论的适用范围。此外，对近场热辐射进行广泛的光谱分析将有助于理解近场对热辐射光谱的影响，包括当尖端与衬底发生近场相互作用时的潜在光谱红移。所提出的理论工作将克服当前基于点偶极子的尖端平面近场热辐射模型的挑战。

CBET - 1605584PI: Park, KeunhanThe rate of radiative heat transfer can significantly exceed the expected rate when the gap between objects is smaller than the wavelength of the thermal radiation. This observation enables innovative technological applications such as near-field thermophotovoltaic power generation, thermal rectification and local radiative cooling. The objective of this project is the experimental validation of near-field thermal radiation, which is relatively unexplored, especially for gaps less than 10 nanometers. The outcome of this research will provide knowledge in the spectral distribution of thermal radiative energy and its transport in the extreme near-field regime. Results of the project will spark the development of detection, imaging and spectroscopy of mid-infrared light at the nanoscale as well as near-field thermophotovoltaic power generators. The project will promote training and learning through the involvement of undergraduate and graduate students in the proposed research activities. Additionally, K-12 outreach will be performed via the Utah Science Olympiad, Engineering Day and Utah Nano Days.Experimental and theoretical investigation of extreme near-field thermal radiation will be accomplished by: (1) measuring the extreme near-field radiative heat transport between a heated tip and a surface in an ultrahigh vacuum atomic force microscope (UHV-AFM); (2) measuring the near-field thermal spectrum via tip-enhanced photocurrent generation and tip-scattered thermal radiation; and (3) modeling tip-surface near-field thermal interactions via numerically exact and approximate methods. Innovative features of this project originate from using the tip-plane configuration in a customized UHV-AFM, which is interfaced with reflective optics that allows the measurement of tip-scattered thermal radiation in conjunction with direct tip-substrate thermal and radiative interactions. Home-built resistive nanothermometers and a customized photovoltaic mid-infrared detector will be implemented as a substrate to directly measure tip-substrate near-field energy transport. The obtained experimental data will be compared against the numerical model, called the thermal discrete dipole approximation, to understand the underlying mechanisms and factors affecting extreme near-field thermal radiation and radiation-to-conduction transition. This research will provide quantitative measurements of extreme near-field thermal radiation which will allow, in tandem with the theoretical models, the determination of the limit of applicability of the continuum theory based on fluctuational electrodynamics. In addition, an extensive spectroscopic analysis of near-field thermal radiation will shed light on understanding the near-field effects on the thermal radiation spectrum, including the potential spectral redshift when a tip is in near-field interactions with a substrate. The proposed theoretical work will overcome the challenge of the current point-dipole based tip-plane near-field thermal radiation model.