Abstract: Extreme near-field heat transfer between metallic surfaces is a subject of debate as the state-of-the-art theory and experiments are in disagreement on the energy carriers driving heat transport. In an effort to elucidate the physics of extreme near-field heat transfer between metallic surfaces, this paper presents a comprehensive model combining radiation, acoustic phonon, and electron transport across sub-10-nm vacuum gaps. The results obtained for gold surfaces show that in the absence of bias voltage, acoustic phonon transport is dominant for vacuum gaps smaller than ∼ 2 nm. The application of a bias voltage significantly affects the dominant energy carriers as it increases the phonon contribution mediated by the long-range Coulomb force and the electron contribution due to a reduced potential barrier. For a bias voltage of 0.6 V, acoustic phonon transport becomes dominant at a vacuum gap of 5 nm, whereas electron tunneling dominates at sub-nm vacuum gaps. The comparison of the theory against experimental data from the literature suggests that well-controlled measurements between metallic surfaces are needed to quantify the contributions of acoustic phonon and electron as a function of the bias voltage.

Abstract: The present article reports a comprehensive energy balance analysis of a photon-enhanced thermionic emission (PETE) device when it is used for concentrated solar power (CSP) generation. To this end, we consider a realistic PETE device composed of a boron-doped silicon emitter on glass and a phosphorus-doped diamond collector on tungsten separated by the interelectrode vacuum gap. Depth-dependent spectral solar absorption and its photovoltaic and photothermal energy conversion processes are rigorously calculated to predict the PETE power output and energy conversion efficiency. Our calculation predicts that when optimized, the power output of the considered PETE device can reach 1.6 W/cm2 with the energy conversion efficiency of ∼ 18% for 100× solar concentration, which is substantially lower than those predicted in previous works under ideal conditions. In addition, the photon enhancement ratio is lower than 10 and decreases with the increasing solar concentration due to the photothermal heating of the emitter assembly, suggesting that PETE should be more suitable for low-to-medium CSP below ∼ 100× concentration. These observations signify the importance of a rigorous energy balance analysis based on spectral and spatial solar absorption distribution for the accurate prediction of PETE power generation.

Abstract: This paper presents the comprehensive performance analysis of thermionic power generation when the interelectrode vacuum gap shrinks to the submicron range. Although reducing the vacuum gap has been suggested as an effective approach to mitigate space-charge accumulation in thermionic-energy conversion (TEC) devices, previous theoretical works have predicted the optimal gap distance in the single-digit micrometer range. However, we demonstrate that nanoscale charge and thermal interactions between thermionic electrodes, such as Schottky barrier lowering due to image charge perturbation and near-field enhanced radiative heat transfer, significantly affects the TEC performance within the submicron vacuum gap. Carefully conducted energy-balance analysis reveals that submicron-gap TEC at d ≈ 700 nm can produce an approximately fourfold increase in power output with a higher energy conversion efficiency than micron-gap TEC under the same operating condition. In addition, significant thermionic and nearfield radiative heating of the collector in the submicron-gap TEC system can be beneficially used to further enhance the power output and efficiency by combining with a bottom-cycle heat engine. We believe that the present work provides a theoretical framework for submicron-gap thermionic power generation as a promising energy recycling scheme for high-quality heat sources.

Abstract: This paper presents an in-depth review of ongoing experimental research efforts to fundamentally understand the strong near-field enhancement of radiative heat transfer and make use of the underlying physics for various novel applications. Compared to theoretical studies on near-field radiative heat transfer (NFRHT), its experimental demonstration has not been explored as much until recently due to technical challenges in precision gap control and heat transfer measurement. However, recent advances in micro-/nanofabrication and nanoscale instrumentation/control techniques as well as unprecedented growth in materials science and engineering have created remarkable opportunities to overcome the existing challenges in the measurement and engineering of NFRHT. Beginning with the pioneering works in 1960s, this paper tracks the past and current experimental efforts of NFRHT in three different configurations (i.e., sphere-plane, plane-plane, and tip-plane). In addition, future remarks on how to address current challenges in the experimental research of NFRHT are briefly discussed.

Abstract: Due to its superb imaging spatial resolution and spectroscopic viability, scattering-type scanning near-field optical microscopy (s-SNOM) has proven to be widely applicable for nanoscale surface imaging and characterization. However, limited works have investigated the sensitivity of the s-SNOM signal to sample temperature. This paper reports the sample temperature effect on the non-interferometric (self-homodyne) s-SNOM scheme at a visible wavelength (λ = 638 nm). Our s-SNOM measurements for an arrayed vanadium/quartz sample demonstrate a monotonic decrease in signal intensity as sample temperature increases. As a result, s-SNOM imaging cannot distinguish quartz or vanadium when the sample is heated to ∼309 K: all signals are close to the root- mean-square noise of the detection scheme used for this study (i.e., 19 μV-rms). While further studies are required to better understand the underlying physics of such temperature dependence, the obtained results suggest that s-SNOM measurements should be carefully conducted to meet a constant sample temperature condition, particularly when a visible-spectrum laser is to be used as the light source.