Nanoscale Vacuum Gap Research Articles

InSitu Observation of Field-Induced Nanoprotrusion Growth on a Carbon-Coated Tungsten Nanotip.

Nanoprotrusion (NP) on metal surface and its inevitable contamination layer under high electric field is often considered as the primary precursor that leads to vacuum breakdown, which plays an extremely detrimental effect for high energy physics equipment and many other devices. Yet, the NP growth has never been experimentally observed. Here, we conduct field emission (FE) measurements along with insitu transmission electron microscopy (TEM) imaging of an amorphous-carbon (a-C) coated tungsten nanotip at various nanoscale vacuum gap distances. We find that under certain conditions, the FE current-voltage (I-V) curves switch abruptly into an enhanced-current state, implying the growth of an NP. We then run field emission simulations, demonstrating that the temporary enhanced-current I-V is perfectly consistent with the hypothesis that a NP has grown at the apex of the tip. This hypothesis is also confirmed by the repeatable insitu observation of such a nanoprotrusion and its continued growth during successive FE measurements in TEM. We tentatively attribute this phenomenon to field-induced biased diffusion of surface a-C atoms, after performing a finite element analysis that excludes the alternative possibility of field-induced plastic deformation.

Performance analysis of photon-enhanced thermionic emission systems mediated by quantum tunneling

Reducing the gap between the electrodes to the nanoscale and utilizing quantum effects are an effective way to enhance the performance of a thermionic energy device. In this work, we establish the model of a photon-enhanced thermionic emission system with a nanoscale vacuum gap, where the electron transport due to electron tunneling and the near-field radiation resulting from photon tunneling are introduced. Analytical expressions for the thermionic emission current, electron tunneling current, and heat flux due to the near-field radiation are provided. By using the energy and particle balance equations, the electron concentration and the temperature of the cathode are determined. The impacts of the voltage, electron affinity, and gap distance on the performance are further analyzed. Results show that the suggested system can achieve high efficiency at the low-temperature cathode. Up to 34.7% of solar-to-electricity efficiency is possible at a cathode temperature of 472.5 K. The proposed model provides a strategy for designing highly efficient thermionic emission devices operating at low temperatures.

Role of acoustic phonon transport in near- to asperity-contact heat transfer

Acoustic phonon transport is revealed as a potential radiation-to-conduction transition mechanism for single-digit nanometer vacuum gaps. To show this, we measure heat transfer from a feedback-controlled platinum nanoheater to a laterally oscillating silicon tip as the tip-nanoheater vacuum gap distance is precisely controlled from a single-digit nanometer down to bulk contact in a high-vacuum shear force microscope. The measured thermal conductance shows a gap dependence of ${d}^{\ensuremath{-}5.7\ifmmode\pm\else\textpm\fi{}1.1}$ in the near-contact regime, which is in good agreement with acoustic phonon transport modeling based on the atomistic Green's function framework. The obtained experimental and theoretical results suggest that acoustic phonon transport across a nanoscale vacuum gap can be the dominant heat transfer mechanism in the near- and asperity-contact regimes and can potentially be controlled by an external force stimuli.

Thermoradiative devices enabled by hyperbolic phonon polaritons at nanoscales

Theromoradiative (TR) devices, though proposed a decade ago, have seen little investigation due to their low-performance compared to other solid-state energy conversion technologies. Herein, we propose an InSb-hBN TR device with a nanoscale vacuum gap down to 10 nm that has the potential to achieve efficient waste heat recovery using solid-state technologies. By coupling the hyperbolic phonon polaritons of hBN with the interband transition of InSb, this TR device design can achieve an output power nearly 4 orders-of-magnitude higher than far-field scenarios and an upper bound efficiency up to 80% of the Carnot limit corresponding to only considering radiative loss. A detailed balance model combined with fluctuational electrodynamics for a multilayered anisotropic structure is developed to theoretically estimate the performance of TR devices from far- to near-field regime. Nonradiative losses are also investigated to elucidate their effects on performance. This work helps deepen the understanding of TR devices and provides a promising pathway to efficient low-grade (< 600 K) heat recovery.

Photonic p-n junction: An ideal near-field heat flux modulator

Using a pair of p- and n-type semiconductors separated by a nanoscale vacuum gap, we introduce an optoelectronics element prototype, "photonic p-n junction", as an analogue of the electronic p-n junction, which is demonstrated to serve as an ideal near-field heat flux modulator. The high modulation performance relies on the switch among three fundamental photon-carrier interaction modes (i.e., surface plasmon polaritons, symmetric depleted internal plasmon polaritons and symmetric accumulated surface plasmon polaritons) caused by the changes in hole and electron densities near the surfaces under a tunable bias. This prototype offers new thoughts not only for contactless thermal management at nanoscale but also for design of optoelectronics devices processing information carried by plasmon polaritons.

Thermophotovoltaic Energy Conversion in Far-to-Near-Field Transition Regime

Recent experimental studies on near-field thermophotovoltaic (TPV) energy conversion have mainly focused on enhancing performance via photon tunneling of evanescent waves. In the sub-micron gap, however, there exist peculiar phenomena caused by the interference of propagating waves, which is seldom observed due to the dramatic increase of the radiation by evanescent waves in full spectrum range. Here, we experimentally demonstrate the oscillatory nature of near-field TPV energy conversion in the far-to-near-field transition regime (250-2600 nm), where evanescent and propagating modes are comparable due to the selective spectral response by the PV cell. Noticeably, it was possible to produce the same amount of photocurrent at different vacuum gaps of 870 and 322 nm, which is 10\% larger than the far-field value. Considering the great challenges in maintaining nanoscale vacuum gap in practical devices, this study suggests an alternative approach to the design of a TPV system that will outperform conventional far-field counterparts.

Spatial profiles of photon chemical potential in near-field thermophotovoltaic cells

Emitted photons stemming from the radiative recombination of electron–hole pairs carry chemical potential in radiative energy converters. This luminescent effect can substantially alter the local net photogeneration in near-field thermophotovoltaic cells. Several assumptions involving the luminescent effect are commonly made in modeling photovoltaic devices; in particular, the photon chemical potential is assumed to be zero or a constant prescribed by the bias voltage. The significance of photon chemical potential depends upon the emitter temperature, the semiconductor properties, and the injection level. Hence, these assumptions are questionable in near-field thermophotovoltaic devices with nanoscale separation distances between the emitter and the cell. In the present work, an iterative solver that combines fluctuational electrodynamics with the drift-diffusion model is developed to tackle the coupled photon and charge transport problem, enabling the determination of the spatial profile of photon chemical potential beyond the detailed balance approach. The difference between the results obtained by allowing the photon chemical potential to vary spatially and by assuming a constant value demonstrates the limitations of the conventional approaches. This study is critically important for performance evaluation of near-field thermophotovoltaic systems with nanoscale vacuum gaps.

Quasi-Casimir coupling induced phonon heat transfer across a vacuum gap

In vacuum, thermal energy is transported by photons (thermal radiation) but not phonons. Recent studies, however, indicated that phonon heat transfer across a vacuum gap is mediated by the quantum fluctuation of electromagnetic fields. Specifically, in the heat exchange between two objects separated by a nanoscale vacuum gap, phonons carry thermal energy more efficiently than photons. However, it remains unclear if phonons can propagate without electromagnetic fields. Here, we demonstrate that phonon transmission across a sub-nanometer vacuum gap can be induced by quasi-Casimir force subjected to the Lennard–Jones atoms using classical molecular dynamics simulation. The net heat flux across the vacuum gap increases exponentially as the gap distance decreases, owing to acoustic phonon transmission. The local heat flux, evaluated using the Irving–Kirkwood method, increases singularly at the interfacial layers, while that at the inner layers agrees well with the net heat flux. These findings provide evidence of the strong thermal resonance induced by quasi-Casimir coupling between the interfacial layers. Thus, we conclude that the quasi-Casimir coupling induced by intermolecular interaction is a heat transfer mode for phonon heat transfer across a vacuum gap in nanoscale.

Modulating Near-Field Radiative Heat Transfer through Thin Dirac Semimetal Films

ABSTRACT We propose a thermal modulation structure made of two identical SiO2 slabs coated by Dirac semimetal (DSM) films and separated by a nanoscale vacuum gap. The energy transmission probability reveals that the coupled surface plasmon polaritons (SPPs) between the two DSM films, and the surface phonon polaritons (SPhPs) supported by the SiO2 substrate can vary sensitively with the Fermi level, the degenerate factor of 3D Dirac points and the thickness of the DSM film, thus providing the possibilities for modulating the radiative heat transfer by tuning these parameters. Based on Maxwell’s equations incorporating fluctuational electrodynamics, the effects of these parameters on the heat transfer coefficient and the thermal modulation contrast are numerically analyzed. Under proper parameters, higher modulation contrasts are obtained by continuously tuning the Fermi level from 0.05 eV to 0.3 eV. The obtained results might be helpful in designing a DSM-based thermal modulator with higher modulation contrasts.

Impact of DC bias on weak optical-field-driven electron emission in nano-vacuum-gap detectors

In this work, we investigate multiphoton and optical field tunneling emission from metallic surfaces with nanoscale vacuum gaps. Using time-dependent Schrödinger equation (TDSE) simulations, we find that the properties of the emitted photocurrent in such systems can be greatly altered by the application of only a few-volt direct current (DC) bias. We find that when coupled with expected plasmonic enhancements within the nanometer-scale metallic gaps, the application of this DC bias significantly reduces the threshold for the transition to optical-field-driven tunneling from the metal surface, and could sufficiently enhance the emitted photocurrents, to make it feasible to electronically tag fJ ultrafast pulses at room temperature. Given the petahertz-scale instantaneous response of the photocurrents, and the low effective capacitance of thin-film nanoantenna devices that enables &lt; 1 f s response time, detectors that exploit this bias-enhanced surface emission from nanoscale vacuum gaps could prove to be useful for communication, petahertz electronics, and ultrafast optical-field-resolved metrology.

Extending the Thermal Near Field Through Compensation in Hyperbolic Waveguides

A promising method to leverage near-field power densities without the use of nanoscale vacuum gaps is through hyperbolic metamaterial (HMM) waveguides. When placed between a hot and cold reservoir, an ideal HMM can transmit surface waves across several microns, enabling an extension of near-field enhancements. However, when accounting for transmission loss due to realistic levels of absorption within the waveguide, previous studies have shown that the enhancements are significantly curtailed at wide separations. In our study, we investigate the role of internal sources within nonisothermal HMMs. We demonstrate that, in some cases, the emission from the HMM accounts for over 90% of the total radiative heat transfer to the receiver, and that these additional sources can largely compensate for optical losses associated with decreased transmission from the emitter to the receiver. Lastly, we investigate the spectral transport in a realistic three-body system that has mismatching optical properties between the boundaries (emitter and receiver) and the waveguide (HMM). Our model shows that the near-field thermal transport remains spectrally selective to the boundaries, even as major radiative contributions come from the waveguide. This work may enable the design of nonisothermal emitter-waveguide-receiver systems that transmit near-field power levels over wider separations.

Thermal transport across nanometre gaps: Phonon transmission vs. air conduction

Heat transfer between two surfaces separated by a nanometre gap is important for a number of applications ranging from spaced head disk systems, scanning thermal microscopy and thermal transport in aerogels. At these separation distances, near field radiative heat transfer competes with heat transfer mediated by phonons. Here we quantity the contribution of phonon assisted heat transfer between apolar solids using lattice dynamics combined with ab-initio calculations. We clearly demonstrate that phonons dominate heat transfer for subnanometre gaps. Strikingly, we conclude that even in the situation where the gap is filled with air molecules, phonons provide the dominant energy channel between the two solids nearly in contact. Our results predict orders of magnitude enhanced phonon heat transfer compared to previous works and bring forward a methodology to analyse phonon transmission across nanoscale vacuum gaps between apolar materials.

Near-field radiative thermal modulation between Weyl semimetal slabs

We propose a thermal modulation structure composed of two identical Weyl semimetal (WSM) slabs separated by a nanoscale vacuum gap. Following Maxwell's equations and the dielectric tensor of the WSM, we derive the reflection matrix at the WSM/vacuum interface. Based on fluctuation electrodynamics, the effects of the Fermi level, the number of Weyl nodes, the separation between Weyl nodes and the thickness of vacuum gap on the heat transfer coefficient and the thermal modulation contrast are numerically discussed. It is found that the WSM possesses stronger near-field radiative ability in the low frequency region, where the coupled surface plasmon polaritons between the WSM slabs play a dominant role. By continuously tuning the Fermi level from 0.01 to 0.15 eV, a large modulation contrast as high as 58.5 is obtained at T = 300 K, while in the low temperature interval [50,160] K, the minimum modulation contrast can exceed 198 for a vacuum gap of 20 nm. The obtained results might be helpful in designing a WSM-based thermal modulator with giant modulation contrasts.

Development of TiN Vacuum-Gap Capacitor Lumped-Element Kinetic Inductance Detectors

We report on the implementation of a vacuum parallel-plate capacitor manufactured using a straightforward process that allows to accurately set nanoscale vacuum gaps in order to reach fairly large capacitances with small footprints. Vacuum gaps between 150 and 300 nm have been achieved. To prevent native oxides and therefore two-level systems, both electrodes are made of TiN-based layers. We implemented the vacuum parallel-plate capacitors into lumped-element kinetic inductance detectors and compared their performances to standard interdigitated capacitor-based detectors. The vacuum capacitor detectors achieved higher quality factors (up to 1.1 × 106) and lower frequency noise (2.8 ± 0.5 Hz2/Hz at 10 Hz). Furthermore, we demonstrate that the noise generated within the meander of the lumped-element kinetic inductance detector can be as important as the one generated in the capacitance.

Near-field radiative thermal rectification assisted by black phosphorus sheets

We suggest a thermal rectification structure where a semi-infinite InSb plate and a semi-infinite 3C–SiC plate coated by black phosphorus (BP) sheets are separated by a nanoscale vacuum gap. To obtain high and tunable thermal rectification efficiency, we employ the temperature-dependent dielectric properties of both InSb and 3C–SiC, and introduce BP sheets with in-plane anisotropy into the structure. Based on fluctuational electrodynamics, the near-field radiative heat transfer and the rectification efficiency are calculated. It is found that the rectification efficiency varies very sensitively with the emitter's temperature, the chemical potential and the number of BP layers, thus opening up possibilities to control the rectification efficiency through these parameters. Higher rectification efficiencies are obtained under some certain conditions, and the origins of the thermal rectification effect are elucidated in some detail. The obtained results might be helpful in designing a BP-based high-efficiency thermal diode.

Near-field radiative thermal modulation enabled by static magnetic fields

We propose a thermal modulation structure composed of two identical graphene-coated NaBr plates separated by a nanoscale vacuum gap (d). A static magnetic field perpendicular to the graphene sheet induces TE–TM hydrid surface plasmon polaritons between graphene and vacuum gap (called the hybrid GSPPs) and enhances the coupling of the hybrid surface plasmon polaritons in graphene to the surface phonon polaritons in the reststrahlen band of NaBr (called the coupled SPh–SPPs). The hybrid GSPPs and coupled SPh–SPPs play a key role in the thermal modulation. The heat flux is modulated by changing the static magnetic field, the Fermi level (EF) and carrier mobility (μ) in graphene. When the magnetic field varies from 0 to 30 T, the modulation contrast as high as 3.42, 3.25 and 4.48 is acquired at (d, EF, μ)=(60.64 nm, 0.2 eV, 1000 cm2/V · s), (100 nm, 0.21 eV, 1000 cm2/V · s) and (100 nm, 0.2 eV, 2500 cm2/V · s), respectively. The work opens new routes to magnetically modulate near-field radiative heat flux in graphene-based structures, and provides some guidance for applications in thermal management of nano/microscaled devices.

Thermionic-enhanced near-field thermophotovoltaics

Solid-state heat-to-electrical power converters are thermodynamic engines that use fundamental particles, such as electrons or photons, as working fluids. Virtually all commercially available devices are thermoelectric generators, in which electrons flow through a solid driven by a temperature difference. Thermophotovoltaics and thermionics are highly efficient alternatives relying on the direct emission of photons and electrons. However, the low energy flux carried by the emitted particles significantly limits their generated electrical power density potential. Creating nanoscale vacuum gaps between the emitter and the receiver in thermionic and thermophotovoltaic devices enables a significant enhancement of the electron and photon energy fluxes, respectively, which in turn results in an increase of the generated electrical power density. Here we propose a thermionic-enhanced near-field thermophotovoltaic device that exploits the simultaneous emission of photons and electrons through nanoscale vacuum gaps. We present the theoretical analysis of a device in which photons and electrons travel from a hot LaB6-coated tungsten emitter to a closely spaced BaF2-coated InGaAs photovoltaic cell. Photon tunnelling and space charge removal across the nanoscale vacuum gap produce a drastic increase in flux of electrons and photons, and subsequently, of the generated electrical power density. We show that conversion efficiencies and electrical power densities of ∼ 30% and ∼ 70 W/cm2 are achievable at 2000 K for a practicable gap distance of 100 nm, and thus greatly enhance the performances of stand-alone near-field thermophotovoltaic devices (∼10% and ∼10 W/cm2). A key practical advantage of this nanoscale energy conversion device is the use of grid-less cell designs, eliminating the issue of series resistance and shadowing losses, which are unavoidable in conventional near-field thermophotovoltaic devices.

Surface plasmon-enhanced near-field thermal rectification in graphene-based structures

We propose a thermal rectification structure composed of InSb and graphene-coated 3C-SiC separated by a nanoscale vacuum gap. To obtain an obvious thermal rectification effect, the permittivities of these materials are all considered to be temperature-dependent. Numerical calculations based on fluctuation electrodynamics reveal that the introduction of graphene into the structure enhances significantly near-field radiative heat flux and thermal rectification efficiency owing to the strong coupling of surface plasmon-polaritons between InSb and graphene. In general, the rectification efficiency above 60% can be maintained for the vacuum gap less than 70 nm. The rectification efficiency exceeding 95% is realized for a vacuum gap of 10 nm and a chemical potential of 0.1 eV. Increasing the emitter’s temperature leads to the drastic increase of the rectification efficiency in a wider temperature range. A lower chemical potential seems more favorable to raising rapidly the rectification efficiency. The above results might be helpful in designing a thermal diode with higher efficiency and wider vacuum gap.

Highly efficient near-field thermal rectification between InSb and graphene-coated SiO2

We report a highly efficient thermal rectification model based on the near-field thermal radiation between InSb and graphene-coated SiO2, separated by nanoscale vacuum gaps. The results show that the introduction of graphene sheet enhances significantly near-field radiative heat flux and thermal rectification efficiency owing to the strong coupling of surface plasmon–polaritons between InSb and graphene. Specifically, under the same temperature bias, a rectification efficiency exceeding 80% is obtained at vacuum gaps varying from 10 nm to 100 nm for the graphene-coated SiO2 case, while such an efficiency requires a narrower gap between 10 nm to 20 nm for the bare SiO2 case. In addition, the introduction of graphene can lower greatly the emitter's temperature TH, e.g., an efficiency of 60% requires TH = 550 K in the bare SiO2 case, while it requires only the temperature around 400 K in the coated SiO2 case. The above results might be helpful in designing a highly efficient thermal diode with a wider vacuum gap and a lower operating temperature.

Precision Measurement of Phonon-Polaritonic Near-Field Energy Transfer between Macroscale Planar Structures Under Large Thermal Gradients.

Despite its strong potentials in emerging energy applications, near-field thermal radiation between large planar structures has not been fully explored in experiments. Particularly, it is extremely challenging to control a subwavelength gap distance with good parallelism under large thermal gradients. This article reports the precision measurement of near-field radiative energy transfer between two macroscale single-crystalline quartz plates that support surface phonon polaritons. Our measurement scheme allows the precise control of a gap distance down to 200nm in a highly reproducible manner for a surface area of 5×5 mm^{2}. We have measured near-field thermal radiation as a function of the gap distance for a broad range of thermal gradients up to ∼156 K, observing more than 40 times enhancement of thermal radiation compared to the blackbody limit. By comparing with theoretical prediction based on fluctuational electrodynamics, we demonstrate that such remarkable enhancement is owing to phonon-polaritonic energy transfer across a nanoscale vacuum gap.