Boosting the electron beam transmittance of field emission cathode using a self-charging gate.

The gate-type carbon nanotubes cathodes exhibit advantages in long-term stable emission owing to the uniformity of electrical field on the carbon nanotubes, but the gate inevitably reduces the transmittance of electron beam, posing challenges for system stabilities. In this work, we introduce electron beam focusing technique using the self-charging SiNx/Au/Si gate. The potential of SiNx is measured to be approximately -60 V quickly after the cathode turning on, the negative potential can be maintained as the emission goes on. The charged surface generates rebounding electrostatic forces on the following electrons, significantly focusing the electron beam on the center of gate hole and allowing them to pass through gate with minimal interceptions. An average transmittance of 96.17% is observed during 550 hours prototype test, the transmittance above 95% is recorded for the cathode current from 2.14 µA to 3.25 mA with the current density up to 17.54 mA cm-2.

Modified rigorous coupled-wave analysis for multi-layer deformable gratings with arbitrary profiles and materials.

In this paper, the rigorous coupled-wave analysis (RCWA) is extended for general multi-layer deformable gratings with arbitrary numbers of layers, surface profiles, layer offsets, and materials. The contribution from the offset between grating layers and/or due to the movement of the deformable grating layer is included in the expansion of the relative permittivity by the Fourier series, enabling the calculations of deformable gratings commonly used in many optical-based displacement sensing devices. The accuracy and efficiency of the extended RCWA are verified by a number of grating models. It is found that the numerical results are in excellent agreement with those from the finite element method, while the RCWA method costs only ∼1/10 in computation time when compared to its counterpart. Our approach can be used for fast calculation and optimization of multi-layer deformable gratings for optical displacement sensing applications.

A low-temperature operated in situ synthesis of TiC-modified carbon nanotubes with enhanced thermal stability and electrochemical properties.

Carbon nanotubes (CNTs) with superior thermal and electrochemical properties are desirable for a large variety of applications. Herein, an in situ synthesis carried out at 1050 °C is proposed for the realization of titanium carbide (TiC) modified CNTs (TiC@CNTs) via a carbothermal treatment of the TiO2-coated CNTs deposited by a TALD technology, preserving the structural morphologies of CNT samples. Crystalline and amorphous TiC layers/nanoparticles are observed around the walls of CNTs, serving as a thermal insulation layer to enhance the thermal stability of CNTs. The TiC@CNT sample exhibits a minimal mass loss of 3.1%, which is 20.9% and 82.3% for the TiO2@CNT and pristine-CNT samples, respectively. In addition, the TiC@CNT electrode shows good energy storage performances, with a specific capacitance of 2.83 mF cm-2 at 20 µA cm-2, which is about 3.5 times higher than that of the pristine-CNT electrode, showing the potential of TiC@CNTs as next-generation electrode materials.

On-chip integration of bulk micromachined three-dimensional Si/C/CNT@TiC micro-supercapacitors for alternating current line filtering.

Three-dimensional (3D) micro-supercapacitors (MSCs) with superior performances are desirable for miniaturized electronic devices. 3D interdigitated MSCs fabricated by bulk micromachining technologies have been demonstrated for silicon wafers. However, rational design and fabrication technologies of 3D architectures still need to be optimized within a limited footprint area to improve the electrochemical performances of MSCs. Herein, we report a 3D interdigitated MSC based on Si/C/CNT@TiC electrodes with high capacitive properties attributed to the excellent electronic/ionic conductivity of CNT@TiC core-shells with a high aspect ratio morphology. The symmetric MSC presents a maximum specific capacitance of 7.42 mF cm-2 (3.71 F g-1) at 5 mV s-1, and shows an 8 times areal capacitance increment after material coating at each step, fully exploiting the advantage of 3D interdigits with a high aspect ratio. The all-solid-state MSC delivers a high energy density of 0.45 µW h cm-2 (0.22 W h kg-1) at a power density of 10.03 µW h cm-2, and retains ∼98% capacity after 10 000 cycles. The MSC is further integrated on-chip in a low-pass filtering circuit, exhibiting a stable output voltage with a low ripple coefficient of 1.5%. It is believed that this work holds a great promise for metal-carbide-based 3D interdigitated MSCs for energy storage applications.

Low Temperature Hydrophilic SiC Wafer Level Direct Bonding for Ultrahigh-Voltage Device Applications.

SiC direct bonding using O2 plasma activation is investigated in this work. SiC substrate and n- SiC epitaxy growth layer are activated with an optimized duration of 60s and power of the oxygen ion beam source at 20 W. After O2 plasma activation, both the SiC substrate and n- SiC epitaxy growth layer present a sufficient hydrophilic surface for bonding. The two 4-inch wafers are prebonded at room temperature followed by an annealing process in an atmospheric N2 ambient for 3 h at 300 °C. The scanning results obtained by C-mode scanning acoustic microscopy (C-SAM) shows a high bonding uniformity. The bonding strength of 1473 mJ/m2 is achieved. The bonding mechanisms are investigated through interface analysis by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). Oxygen is found between the two interfaces, which indicates Si-O and C-O are formed at the bonding interface. However, a C-rich area is also detected at the bonding interface, which reveals the formation of C-C bonds in the activated SiC surface layer. These results show the potential of low cost and efficient surface activation method for SiC direct bonding for ultrahigh-voltage devices applications.

Silicon-Based 3D All-Solid-State Micro-Supercapacitor with Superior Performance.

The large-scale fabrication of high-performance on-chip micro-supercapacitors (MSCs) is the footstone for the development of next-generation miniaturized electronic devices. In practical applications, however, MSCs may suffer from a low areal energy density as well as a complicated fabrication strategy that is incompatible with semiconductor processing technology. Herein, we propose a scalable fabrication strategy for the realization of a silicon-based three-dimensional (3D) all-solid-state MSC via the combination of semiconductor-based electrode processing, chemical vapor deposition, and hydrothermal growth. The individual Si/C/MnO2 electrode shows a maximum specific capacitance of 223.74 mF cm-2, while the symmetric electrodes present a maximum areal energy density of 5.01 µWh cm-2 at the scan rate of 1 mV s-1. The full 3D Si/C/MnO2 MSC delivers a high energy density of 2.62 µWh cm-2, at a power density of 117.82 µW cm-2, as well as a long cycle life with capacitance retention >92% after 4000 cycles. Our proposed method enables the fabrication of 3D MSCs based on a thick silicon interdigitated electrode array, holding a great promise for the development of 3D on-chip microscale energy storage devices.

Temperature Gradient Method for Alleviating Bonding-Induced Warpage in a High-Precision Capacitive MEMS Accelerometer.

Capacitive MEMS accelerometers with area-variable periodic-electrode displacementtransducers found wide applications in disaster monitoring, resource exploration and inertialnavigation. The bonding-induced warpage, due to the difference in the coefficients of thermalexpansion of the bonded slices, has a negative influence on the precise control of the interelectrodespacing that is essential to the sensitivity of accelerometers. In this work, we propose the theory,simulation and experiment of a method that can alleviate both the stress and the warpage byapplying different bonding temperature on the bonded slices. A quasi-zero warpage is achievedexperimentally, proving the feasibility of the method. As a benefit of the flat surface, the spacing ofthe capacitive displacement transducer can be precisely controlled, improving the self-noise of theaccelerometer to 6 ng/√Hz @0.07 Hz, which is about two times lower than that of the accelerometerusing a uniform-temperature bonding process.

Flexible Ultra-Wideband Terahertz Absorber Based on Vertically Aligned Carbon Nanotubes.

Ultra-wideband absorbers have been extensively used in wireless communications, energy harvesting, and stealth applications. Herein, with the combination of experimental and theoretical analyses, we develop a flexible ultra-wideband terahertz absorber based on vertically aligned carbon nanotubes (VACNTs). Measured results show that the proposed absorber is able to work efficiently within the entire THz region (e.g., 0.1-3.0 THz), with an average power absorptance of >98% at normal incidence. The absorption performance remains at a similar level over a wide range of incident angle up to 60°. More importantly, our devices can function normally, even after being bent up to 90° or after 300 bending cycles. The total thickness of the device is about 360 µm, which is only 1/8 of the wavelength for the lowest evaluated frequency of 0.1 THz. The new insight into the VACNT materials paves the way for applications such as radar cross-section reduction, electromagnetic interference shielding, and flexible sensing because of the simplicity, flexibility, ultra-wideband operation, and large-scale fabrication of the device.

In-situ Functionalization of Metal Electrodes for Advanced Asymmetric Supercapacitors.

Nanostructured metal-based compound electrodes with excellent electrochemical activity and electrical conductivity are promising for high-performance energy storage applications. In this paper, we report an asymmetric supercapacitor based on Ti and Cu coated vertical-aligned carbon nanotube electrodes on carbon cloth. The active material is achieved by in-situ functionalization using a high-temperature annealing process. Scanning and transmission electron microscopy and Raman spectroscopy confirm the detailed nanostructures and composition of the electrodes. The TiC@VCC and CuxS@VCC electrodes show a high specific capacity of 200.89 F g-1 and 228.37 F g-1, respectively, and good capacitive characteristics at different scan speeds. The excellent performance can be attributed to a large surface area to volume ratio and high electrical conductivity of the electrodes. Furthermore, an asymmetric supercapacitor is assembled with TiC@VCC as anode and CuxS@VCC as cathode. The full device can operate within the 0-1.4 V range, and shows a maximum energy density of 9.12 Wh kg-1 at a power density of 46.88 W kg-1. These findings suggest that the metal-based asymmetric electrodes have a great potential for supercapacitor applications.

An Ultra-Wideband THz/IR Metamaterial Absorber Based on Doped Silicon.

Metamaterial-based absorbers have been extensively investigated in the terahertz (THz) range with ever increasing performances. In this paper, we propose an all-dielectric THz absorber based on doped silicon. The unit cell consists of a silicon cross resonator with an internal cross-shaped air cavity. Numerical results suggest that the proposed absorber can operate from THz to far-infrared regimes, having an average power absorption of ∼95% between 0.6 and 10 THz. Experimental results using THz time-domain spectroscopy show a good agreement with simulations. The underlying mechanisms for broadband absorption are attributed to the combined effects of multiple cavities modes formed by silicon resonators and bulk absorption in the doped silicon substrate, as confirmed by simulated field patterns and calculated diffraction efficiency. This ultra-wideband absorption is polarization insensitive and can operate across a wide range of the incident angle. The proposed absorber can be readily integrated into silicon-based photonic platforms and used for sensing, imaging, energy harvesting and wireless communications applications in the THz/IR range.

Scale Factor Calibration for a Rotating Accelerometer Gravity Gradiometer.

Rotating Accelerometer Gravity Gradiometers (RAGGs) play a significant role in applications such as resource exploration and gravity aided navigation. Scale factor calibration is an essential procedure for RAGG instruments before being used. In this paper, we propose a calibration system for a gravity gradiometer to obtain the scale factor effectively, even when there are mass disturbance surroundings. In this system, four metal spring-based accelerometers with a good consistency are orthogonally assembled onto a rotary table to measure the spatial variation of the gravity gradient. By changing the approaching pattern of the reference gravity gradient excitation object, the calibration results are generated. Experimental results show that the proposed method can efficiently and repetitively detect a gravity gradient excitation mass weighing 260 kg within a range of 1.6 m and the scale factor of RAGG can be obtained as (5.4 ± 0.2) E/µV, which is consistent with the theoretical simulation. Error analyses reveal that the performance of the proposed calibration scheme is mainly limited by positioning error of the excitation and can be improved by applying higher accuracy position rails. Furthermore, the RAGG is expected to perform more efficiently and reliably in field tests in the future.

Secure thermal infrared communications using engineered blackbody radiation.

The thermal (emitted) infrared frequency bands, from 20-40 THz and 60-100 THz, are best known for applications in thermography. This underused and unregulated part of the spectral range offers opportunities for the development of secure communications. The 'THz Torch' concept was recently presented by the authors. This technology fundamentally exploits engineered blackbody radiation, by partitioning thermally-generated spectral noise power into pre-defined frequency channels; the energy in each channel is then independently pulsed modulated and multiplexing schemes are introduced to create a robust form of short-range secure communications in the far/mid infrared. To date, octave bandwidth (25-50 THz) single-channel links have been demonstrated with 380 bps speeds. Multi-channel 'THz Torch' frequency division multiplexing (FDM) and frequency-hopping spread-spectrum (FHSS) schemes have been proposed, but only a slow 40 bps FDM scheme has been demonstrated experimentally. Here, we report a much faster 1,280 bps FDM implementation. In addition, an experimental proof-of-concept FHSS scheme is demonstrated for the first time, having a 320 bps data rate. With both 4-channel multiplexing schemes, measured bit error rates (BERs) of < 10(-6) are achieved over a distance of 2.5 cm. Our approach represents a new paradigm in the way niche secure communications can be established over short links.