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Uniform large-area flat-panel X-ray sources have potential applications in projection imaging. Achieving such a source has remained a challenge. A diagonal 4-in cold cathode flat-panel X-ray source device has been fabricated using ZnO nanowire field emitter arrays. A large-area uniform X-ray emission was produced, and projection imaging with a resolution of more than 5.0 lp/mm was realized. Its projection imaging resolution and its dependence on geometric imaging parameter settings were studied along with the imaging performance of the device. Simulations using the fast ray-tracing algorithm were performed, yielding results consistent with those from experiments. Optimal parameter settings for high-resolution projection imaging using the flat-panel X-ray source are presented. This study has significance in the application of this cold cathode flat-panel X-ray source in X-ray imaging.

In order to develop a field emission cold cathode for power vacuum electronic device applications, it is important to realize the properties of large-current and high current density. This requires the design and preparation of cold cathode materials with good crystallization, suitable geometric structure, and good contact interface. In this study, we report a pyramidal molybdenum nanostructure with single crystalline nature, which was self-assembly grown by a thermal evaporation method. We also report the optimization of the nanostructure, successfully sharpening its top end and reducing the thickness of the intermediate layer between the structure and the substrate (from 31.4 to 3.1 nm). By this way, the pyramidal molybdenum nanostructure exhibits high conductivity of about 1.8 × 105 Ω-1 cm-1. The cold cathode composed by these nanostructures shows a large-current field emission performance, with the largest emission current of 47.62 mA as well as the highest current density of 2.38 A cm-2, under a pulsed electric field as high as 28 V μm-1. The proposed pyramidal molybdenum nanostructures provide a candidate for the large-current cold cathode of the power electronic devices.

Addressable nanowire field-emitter arrays (FEAs) show potential for application in vacuum microelectronic devices, and a focusing structure is essential for modulation of the electron beam in FEAs. However, because of the complicated device structure induced by the focusing structure, nanowire integration in such a device is very difficult. To address this issue, a coaxis-gated ZnO nanowire FEA structure with an in-plane focusing gate electrode is proposed. The focusing gate electrode is located in the same plane as the cathode and control gate, and the addressing function is realized using a via structure. The coaxis-gated ZnO nanowire FEAs were fabricated using a four-mask microfabrication process, thus requiring one mask less than traditional focused FEAs. The characteristics of the fabricated ZnO nanowire FEAs were studied, and their addressing and focusing abilities were verified.

Large area flat panel detector is the key component of X-ray imaging system. In this study, Ga2O3 thin film photoconductor was prepared using electron-beam evaporation. A cold cathode X-ray flat panel detector was fabricated using Ga2O3 thin film photoconductor as the anode. X-ray response of the detector was measured and the results verified the possibility of using Ga2O3 thin film as the photoconductor for vacuum X-ray detector.

Light-driven electron emission plays an important role in modern electronic and optoelectronic devices. However, such a process usually requires a light field either with a high intensity, or with a high frequency, which is not in favor of its applications. To solve these issues, we propose and realize a plasmon-mediated cold-cathode (PMCC), which combines plasmonic nanostructures with nano-electron-emitters of low work function. In a PMCC, hot-electrons generated by plasmon resonances upon light excitation will enter into adjacent emitter, and subsequently emit into vacuum. PMCCs greatly benefit from strong energy coupling of plasmonic effect, and work based on a plasmon-mediated electron emission process. Electron emission of high efficiency can be obtained with light field of moderate intensities and visible wavelengths. This electron emission is of interest for applications including cold-cathode electron sources, advanced photocathodes, and micro- and nano-electronic devices relying on free electrons. The above findings will be presented with a review of progresses in the study of such a light-driven electron emission.

A gated carbon nanotube cold cathode has been fabricated for miniature X-ray source application. Randomly aligned CNTs were prepared using CVD. Gated CNT cold cathode was fabricated using a metal mesh gate. The current-voltage characteristics, gate transmission rate and the current stability were studied. Current fluctuation less than 1% was achieved.

Flat-panel X-ray source could realize compact imaging system with low dosage. In order to better control the size of X-ray emission spot from the flat panel X-ray source, we fabricated a flat panel X-ray source with micro-pattern transmission anode. The size of X-ray emission spot can be accurately controlled by modulating the size of the transmission anode pattern, which is expected to improve the imaging performance.

Cold cathode micro-focus X-ray source with a single focusing lens was designed. The focus spot size (FSS) of the electron beam was simulated using CST software. The simulation method is verified by experimental results using an electron gun consisting of gated carbon nanotube cold cathode and focusing electrode.

Due to their unique characteristics of instant responses and low-power consumption, vacuum electronic devices based on field emission cold-cathode have potentials for realizing compact and novel high frequency electromagnetic wave radiation sources. Herein, a cold-cathode radiation source operating in the frequency range from gigahertz to terahertz is developed, which is based on frequency multiplication by taking advantage of nonlinearity of field electron emission. The radiation source is realized by introducing a compact structure with two re-entrant cavity resonators. Based on such a structure, radiation sources of frequencies at 12.10 GHz and 0.1 THz were, respectively, designed and simulated numerically. Based on the simulation results, the 12.10-GHz radiation source using carbon nanotubes cold-cathode was fabricated and tested experimentally. The results showed that an electromagnetic wave output at 12.10 GHz with power of $98~\mu \text{W}$ can be achieved upon excitation by a 6.05 GHz input signal with power of 1.05 mW. Our study provides an approach for microwave and terahertz wave generation.

The stability of material structure is the key factor to determine its service capacity and lifetime. The structure evolution of vertical few-layer graphene (FLG) under high electric field was directly observed in situ TEM. The structure stability of FLG depends on its crystallinity and edge morphology. The vertical FLG without pinhole defects can sustain an electrostatic field beyond 58.5 V/nm. The vertical FLG with pinhole defects disintegrated from top to bottom due to field evaporation and the critical electrostatic field weaken to 17.4 V/nm. The vertical FLG with curly layer perpendicular to the electric field stretched and slid at the edge due to the layer sliding and the critical fracture electrostatic field of it was low to 7.5 V/nm which is an order of magnitudes small than that of the curly layer parallel to the electrostatic field of 24.7 V/nm. Those results were the first direct observation of the nano-structure evolution of FLG under high electric field. It helps to understand the deterioration mechanism of FLG under high electric field in free space and provides guidance for the high electric field applications such as field electron emitter and scanning tunneling microscopy.

The improving of tip-to-tip electron emission uniformity in an array is one of the essential issues for the vacuum microelectronic/nanoelectronic applications. Here, we report the achieving of quasi-saturated ultrahigh arsenic concentration ( $\sim 6.7\times 10^{{21}}$ /cm3) at the Si nanotip apex, in the mechanisms of dopant segregation during the thermal oxidation and the limitation of arsenic solubility in Si. The tips with quasi-saturated high-level arsenic concentration possess a well tip-to-tip uniformity in surface work function. The measured work function for the tips in an array with saturated dopant concentration is in a narrow range of 4.347–4.371 eV (~2.1% fluctuation). This merit not only resulted in high emission current density (~418.36 mA/cm2) from the devices at a relatively lower gate voltage but also brought well spatially uniform electron emission in an array. The work inspired vital insight into the interpretation for mechanisms in dopant density control and the improving of emission uniformity of the Si-tip array. It could be used as an essential approach to acquire high-performance Si-based vacuum electronic devices.

The vacuum breakdown critical condition of individual carbon nanotube (CNT) during field emission was investigated experimentally using the in-situ field emission combined scanning electron microscope system. The results showed that the critical breakdown electric field of CNT was related to its length and diameter. CNT which has a shorter length and larger diameter could bear higher electric field. The underlying mechanism was attributed to the heat dissipation ability of CNT. A smaller length and larger diameter CNT has better heat dissipation. The results met with the theoretical calculations and helped to understand the physics mechanism of vacuum breakdown of CNT during field emission in a visual perspective.

In pursuing high emission current density and high brightness, it is difficult to avoid vacuum breakdown in nanowire cold cathode systems, which will shorten the lifetime of the electron sources. Therefore, investigations on the vacuum breakdown process are important for semiconductor nanowire point electron sources. In this study, non-catastrophic breakdown phenomena that could extend the lifetime of the nanowire electron source were observed in the field emission processes of individual defective WO3−x nanowires. In this non-catastrophic breakdown process, only part of the nanowire emitter was destroyed. After the breakdown, the remaining part of the nanowire could still emit electrons, and due to the shortening of its length, the maximum field emission current density of the remaining nanowire was increased. These results are consistent with the prediction given by theoretical calculations. A defect-related electrical transport induced breakdown mechanism and Nottingham effect induced cooling effect were proposed to be the main causes of this phenomenon. Our work provides an approach for designing long lifetime semiconductor nanowire point electron sources.

Flat panel X-ray source enables the possibility of adaptive X-ray imaging which could achieve close range imaging. In this study, a flat panel X-ray source module based on ZnO nanowire field emitter arrays was fabricated. High resolution projection imaging using the module was achieved, which demonstrated the feasibility of adaptive X-ray imaging using nanowire FEA flat panel X-ray source.

In this study, a gated ZnO nanowire field emitter arrays (FEAs) was fabricated for the application of pulsed flat panel X-ray source. The ZnO nanowires were grown using thermal oxidation method and the growth ZnO nanowires in the device structure was optimized by adjusting the oxidation atmosphere. The pulsed response of the FEAs was studied and corresponding X-ray emission was detected.

The cold cathode flat-panel X-ray source is a new type of X-ray source with advantage of smaller footprint and lower-dose imaging than current sources. In this paper, we optimized the thickness of the anode target of a flat-panel X-ray source to maximize the photon fluence of X-rays generated from the device. Utilizing EGSnrc-based Monte Carlo code, X-ray fluence was calculate from the simulated photons deposited in the virtual detector. These results revealed an optimal target thickness at a specific anode voltage to maximize the output of X-ray intensity. For experimental validation, tungsten thin film anodes with different thicknesses were prepared, and a flat-panel X-ray source was fabricated using a ZnO nanowire cold cathode. A figure of merit was introduced to characterize the output efficiency of the X-rays. The experimental results agree well with the simulation results, showing an optimal tungsten-target thickness of 1200 nm at an anode voltage of 40 kV. Using a flat-panel X-ray source with the optimized anode thickness, we took X-ray absorption images of both biological and nonbiological subjects, and high-resolution images were demonstrated.

Nanowire field emitters have great potential for use as large-area gated field emitter arrays (FEAs). However, the micrometer-scale cathode patterns in gated FEA devices will reduce regulation of the gate voltage and limit the field emission currents of these devices as a result of field-screening effect among the neighboring nanowires. In this article, a ring-shaped ZnO nanowire pad is proposed to overcome this problem. Diode measurements show that the prepared ring-shaped ZnO nanowire pad arrays shows uniform emission with a turn-on field of 5.9 V/µm and a field emission current density of 4.6 mA/cm2 under an applied field of 9 V/µm. The ZnO nanowire pad arrays were integrated into coplanar-gate FEAs and enhanced gate-controlled device characteristics were obtained. The gate-controlled capability was studied via microscopic in-situ measurements of the field emission from the ZnO nanowires in the coplanar-gate FEAs. Based on the results of both simulations and experiments, we attributed the enhanced gate-controlled device capabilities to more efficient emission of electrons from the ZnO nanowires as a result of the increase edge area by designing ring-shaped ZnO nanowire pad. The results are important to the realization of large-area gate-controlled FEAs based on nanowire emitters for use in vacuum electronic devices.

Transition-metal dichalcogenide (TMD) semiconductors have attracted interest as photoelectrochemical (PEC) electrodes due to their novel band-gap structures, optoelectronic properties, and photocatalytic activities. However, the photo-harvesting efficiency still requires improvement. In this study, A TMD stacked heterojunction structure was adopted to further enhance the performance of the PEC cathode. A P-type WSe2 and an N-type MoS2 monolayer were stacked layer-by-layer to build a ultrathin vertical heterojunction using a micro-fabrication method. In situ measurement was employed to characterize the intrinsic PEC performance on a single-sheet heterostructure. Benefitting from its built-in electric field and type II band alignment, the MoS2/WSe2 bilayer heterojunction exhibited exceptional photocatalytic activity and a high incident photo-to-current conversion efficiency (IPCE). Comparing with the monolayer WSe2 cathode, the PEC current and the IPCE of the bilayer heterojunction increased by a factor of 5.6 and enhanced 50%, respectively. The intriguing performance renders the MoS2/WSe2 heterojunction attractive for application in high-performance PEC water splitting.

Thermo-enhancement is an effective way to achieve high performance field electron emitters, and enables the individually tuning on the emission current by temperature and the electron energy by voltage. The field emission current from metal or n-doped semiconductor emitter at a relatively lower temperature (i.e., < 1000 K) is less temperature sensitive due to the weak dependence of free electron density on temperature, while that from p-doped semiconductor emitter is restricted by its limited free electron density. Here, we developed full array of uniform individual p-Si/ZnO nanoemitters and demonstrated the strong thermo-enhanced field emission. The mechanism of forming uniform nanoemitters with well Si/ZnO mechanical joint in the nanotemplates was elucidated. No current saturation was observed in the thermo-enhanced field emission measurements. The emission current density showed about ten-time enhancement (from 1.31 to 12.11 mA/cm2 at 60.6 MV/m) by increasing the temperature from 323 to 623 K. The distinctive performance did not agree with the interband excitation mechanism but well-fit to the band-to-band tunneling model. The strong thermo-enhancement was proposed to be benefit from the increase of band-to-band tunneling probability at the surface portion of the p-Si/ZnO nanojunction. This work provides promising cathode for portable X-ray tubes/panel, ionization vacuum gauges and low energy electron beam lithography, in where electron-dose control at a fixed energy is needed.

ZnO nanowire field-emitter arrays (FEAs) show great application potentials in large-area vacuum microelectronics devices. However, the electron divergence is an issue which should be addressed. In this paper, ZnO nanowire FEAs with co-planar focus electrode were designed. A four-mask fabrication process and lift-off process were adopted to achieve the integration of focus electrode. The device demonstrated line-addressing and focusing capability with a spacing of 5 mm between anode and cathode, and the linewidth of field-emission images was reduced from 2.25 to 1.35 mm when the focus voltage changed from 50 to −50 V. Besides, line-shaped emission patterns were observed during focusing process, which was attributed to the emission pattern from the ZnO nanowire and the asymmetrical focusing electrical field.

This paper reports for the first time the theoretical analysis, design, and realization of a microwave (MW) frequency multiplier based on a field emission from carbon nanotube (CNT) cold-cathode. The nonlinear characteristic of field emission from CNT cold-cathode is utilized for generating field emission current with the harmonics of input signal and achieving frequency multiplication. We demonstrated both theoretically and experimentally that an MW electric field is capable of inducting current with harmonics from CNT cold-cathode and that a direct-current (dc) electric field can effectively increase the amplitudes of the harmonics. A reentrant resonant cavity structure was designed and fabricated to deliver the desired combined MW and dc electric fields onto CNT cold-cathode and realize the MW frequency multiplier. The device has the target second harmonic at a frequency of 1.868 GHz and a third harmonic at 2.802 GHz with a driving signal at frequency of 0.934 GHz. By simply increasing dc bias, a 12.43-dB increase of the amplitude of target second harmonic is successfully obtained. With such a device, directly modulated electron beam with MW frequency harmonics is obtained. Both the principle and the design can find applications in frequency tunable vacuum electron devices.

A cold cathode has many applications in high frequency and high power electronic devices, X-ray source, vacuum microelectronic devices and vacuum nanoelectronic devices. After decades of exploration on the cold cathode materials, ZnO nanowire has been regarded as one of the most promising candidates, in particular for large area field emitter arrays (FEAs). Numerous works on the fundamental field emission properties of ZnO nanowire, as well as demonstrations of varieties of large area vacuum microelectronic applications, have been reported. Moreover, techniques such as modifying the geometrical structure, surface decoration and element doping were also proposed for optimizing the field emissions. This paper aims to provide a comprehensive review on recent progress on the ZnO nanowire cold cathode and its applications. We will begin with a brief introduction on the synthesis methods and discuss their advantages/disadvantages for cold cathode applications. After that, the field emission properties, mechanism and optimization will be introduced in detail. Then, the development for applications of large-area ZnO nanowire FEAs will also be covered. Finally, some future perspectives are provided.

Addressable flat-panel x-ray source has important applications in next-generation low-dose x-ray imaging technology. In this study, a fully vacuum-sealed addressable flat-panel x-ray source has been fabricated using a ZnO nanowire cold cathode. The x-ray source was formed by assembling gated ZnO nanowire field emitters and a molybdenum thin film transmission anode target. Pulsed and addressable x-ray emission was achieved by applying voltages to the extraction gate. Radiation dose rates of 235 nGy/s were achieved when the device was operated at 21 kV anode voltage and 140 V gate voltage. Projection x-ray images of objects were obtained by the flat-panel x-ray source. Our results demonstrated the feasibility of a fully addressable cold cathode flat-panel x-ray source.

We develop a simple one-step growth method to obtain a novel kind of tree-like carbon nano composite structure on a 4″ stainless steel substrate, which is a nano graphene-carbon nanotube tree (GCT). The structures are synthesized by using a plasma enhanced chemical vapor deposition method. The GCT consists of few-layer graphenes (FLG) epitaxially growing from a carbon nanotube (CNT) and they can grow uniformly on a 4″ substrate. Analysis of chemical bonding between FLG and CNT is given. In addition, electron micrographs are presented to show how a tree grows. Finally, we also develop techniques to characterize both experimentally and theoretically the electrical conductivity, field electron emission, heat conduction and heat radiation of a GCT. The electrical conductivity of a GCT is compatible to high quality CNT, the current performance of field electron emission is significantly high compared to the other individual single nanostructure, and very outstanding heat radiation effect is found. We believe that this large-area GCTs film can find potential applications such as super capacitor, terahertz wave manipulation, heat dissipation and field electron emission.

In contrast to the conventional planar p-i-n photodiode and a metal-semiconductor–metal photodetector, the 3-D dual-gate photosensitive thin-film transistor (TFT) architectures presented here attain excellent photoresponse characteristics. Operating the device in the subthreshold regime further boosts the photoconductive gain as a result of light-induced decrease in the threshold voltage. This paper presents design considerations along with a performance comparison between 3-D photosensitive TFTs that have $\pi $ - and FIN-shaped channels and conventional TFT with a planar channel. Our paper shows that the $\pi $ -shaped structure tends to have a higher sensitivity while the FIN-shaped counterpart is more responsive with wider dynamic range. For both structures, a measured photoconductive gain of $10^{\textsf {4}} \sim 10^{\textsf {6}}$ % is obtained with spectral responsivity ranging from near UV to near IR, and the photoresponse time in the range of tens of milliseconds. The 3-D dual-gate photosensitive TFT architecture appears to be very promising for large-area, low-level UV, visible, and IR detection applications.

The atomic edge, excellent conductivity, stable chemical and mechanical properties make graphene a good candidate for the scanning tunneling microscopy (STM) tip. Here, a one-step growth method is used to fabricate graphene STM tips. The graphene was directly grown on a sharp tungsten tip vertically by using microwave plasma enhanced chemical vapor deposition method without any catalyst. The high resolution TEM image demonstrated that inert W2C layer was formed at the surface of tungsten during growth which can protect the oxidation of tungsten tip and promote the electric conductivity of graphene STM tip of 4000 times than the oxidative tungsten tip. The in situ TEM field evaporation experiments revealed that the graphene has a good stability under high electric field of 24.7 V/nm. Using the graphene STM tip, atomic resolution images of highly oriented pyrolytic graphite and tungsten disulfide were obtained. Our experiment results demonstrated the graphene STM tip was available to get atomic resolution image for at least one week. It is a promising approach to fabricate STM tips for high resolution imaging in a fast and economical way.

Zinc oxide (ZnO) nanowires are prepared for application in large area gated field emitter arrays (FEAs). By oxidizing Al-coated Zn films, the population density of the ZnO nanowires was tuned precisely by varying the thickness of the Al film. The nanowire density decreased linearly as the thickness of the Al film increased. Optimal field emission properties with a turn-on field of 6.21 V μm–1 and current fluctuations less than 1% are obtained. This can be explained by the minimized screening effect and good electrical conductivity of the back-contact layer. The mechanism responsible for the linear variation in the nanowire density is investigated in detail. Addressable FEAs using the optimal ZnO nanowire cathodes were fabricated and applied in a display device. Good gate-controlled characteristics and the display of video images are realized. The results indicate that ZnO nanowires could be applied in large area FEAs.

Lanthanum hexaboride (LaB6) nanostructures have attracted much attention in recent years because they exhibit high electrical conductivity and thermal conductivity, low work function and high chemical stability, and can be expected to be an ideal cold cathode electron source for power device applications. Although some groups have developed means to grow LaB6 nanostructures and investigated their emission properties, the moderate synthesis of the LaB6 nanostructure cathode with high-performance is still a challenging issue. Here we developed a simple one-step chemical vapour deposition (CVD) method to prepare the LaB6 nanowire cold cathode film. The LaB6 nanowires have a mean length of tens of micrometres and their average diameter is about 100 nm. The formation of the nanowires is attributed to the synergy of the vapour–liquid–solid (VLS) and vapour–solid (VS) mechanisms. The LaB6 nanowires are found to have a low turn-on (2.2 V μm−1) and threshold field (2.9 V μm−1) as well as nice field emission (FE) stability with a current fluctuation of only 1.7%. And their emission current can reach 5.6 mA (16.7 mA cm−2) at 4.1 V μm−1, which is large enough for the high-current requirements of cathodes used in power devices. Moreover, the LaB6 nanowires still retain excellent performance even if the operation temperature is raised up to 773 K. It is noted that the LaB6 nanowire film exhibits quite different emission behaviours during a temperature cycling between room temperature and 773 K. The adsorption and desorption of oxygen onto and from the nanowire's surface is suggested to explain the discrepancy of such emission properties based on a series of our designed experiments. Most importantly, the LaB6 nanowire cathode film can almost recover to the original excellent FE performances after detachment of the surface oxygen molecules, which suggests that they should be ideal cathode nanomaterials for power device applications.

The electron beam of an oscillating electric-field direct-driven cold-cathode electron gun in resonant structure is studied. Such electron gun is capable of providing modulated electron beam and has potential for novel vacuum electronic devices applications. However, the problem of electron beam divergence in the electron gun needs to be solved. By designing the position of cold-cathode and improving the electric field distribution on the cold-cathode surface, the divergence of the electron beam is reduced and the transmission ratio of the electron gun is effectively improved from 75.90% to 94.78%. Further, by applying an axial magnetic field, the transmission ratio is optimized to 97.73%. The method for electron beam bunching optimization is simply and effective, it could find applications in microwave and terahertz vacuum electronic devices.

A large area flat panel detector using ZnO nanowire cold cathode and ZnS photoconductor was fabricated. The effect of film thickness of ZnS photoconductor on the photoresponse of the device was studied. It found that the maximum current gain increased initially and decreased gradually as the ZnS films thickness increased. A maximum gain of 6.49×104was obtained at the thickness of 3.75 μm at an applied voltage of 730V. The results were explained using an equivalent circuit model, which considered EBIPC mechanism. Our results show that the cold cathode flat panel photodetector has great potential in indirect-conversion X-ray imaging.

Instability of amorphous InGaZnO thin-film transistors (a-IGZO TFTs) remains an obstacle for commercialization. Here, we systematically discuss the effect of nitrogen incorporation on a-IGZO TFT stability and developed Ar/O2/N2 atmosphere to improve the stability under stressing in different conditions. Based on X-ray photoelectron spectrometer results, it is revealed that the positive gate bias stress (PGBS) stability is significantly improved due to microscopically passivated metal–oxygen bonds. Yet, the negative gate bias and light stress (NBLS) stability is seriously deteriorated with heavily nitrogen incorporation probably due to the bandgap narrowing effect. By optimizing a mixed O2/N2 atmosphere, the subgap states are finely tuned to afford optimal performance and stability. The developed IGZO TFTs exhibit mobility (12.67 cm2/Vs), small shift of threshold voltage under PGBS (reduced by 64% as compared with the pristine a-IGZO TFTs), and good negative gate bias stability and with NBLS stability as well.

Amorphous silicon thin-film transistors (TFTs) have been widely used to build flat-panel X-ray detectors for medical imaging. To meet the growing demand for low dose and high sensitivity, attention has been paid to active pixel sensor (APS) with high signal-to-noise-ratio (SNR). However, most APS architectures generally require multiple TFTs in one pixel, resulting in low fill factor, low spatial resolution, and discounted SNR. We proposed a smart pixel architecture based on a dual-gate photosensitive TFT that combines sensor, switch, and storage into one TFT intended for high fill factor, high spatial resolution, and high SNR. This letter aims to investigate the sensitivity and noise performance of the smart pixel along with the evaluation of the first $4\times 4$ smart pixel sensor array.

This paper reports on an integrative approach to a fingerprint sensor for an active matrix liquid crystal display (AMLCD). By redesigning the current driver hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) in the display pixel and making it a dual-gate photosensitive TFT with a $\pi $ -shape channel, a fingerprint-sensor-integrated display pixel can be enabled with a potentially high sensitivity through properly biasing the TFT into the subthreshold region. We designed, fabricated and characterized the single-pixel TFT followed by the demonstration of a ${\hbox{4}} \times {\hbox{4}}$ sensor array along with capturing the first images.

The effects of nitrogen functionalization on the field emission properties of vertically aligned few-layer graphene (FLG) nanowall thin films are investigated. Lowered work function and enhanced electron emission property are observed after ammonia plasma treatment. The turn-on field (defined at 10 μAcm −2 ) is found to be decreased from 6.5 to 4.6 Vμm −1 . Additionally, more stable emission with fluctuation less than 5% is obtained after surface N-functionalization. The electron configuration of nitrogen in FLG edge is proposed to be amine (C–NH 2 ) terminal configuration. Nitrogen doping could significantly increases the π density states near the Fermi level. The maximum upward shift of E F is determined to be ∼0.23 eV by low-energy XPS measurements, which mainly accounts for the lower turn-on electric field. Meanwhile, nitrogen functionalization would minimize surface desorption and accounts for more uniform work function distribution. Finally, the nonlinear characteristics of Fowler–Nordheim plots could be understood by thermionic-field emission process.

The threshold voltage shift ( $\Delta V_{\textrm {th}}$ ) in amorphous InZnSnO thin-film transistors (a-IZTO TFTs) during negative gate-bias stress (NGBS) is significantly improved by nitrogen doping. Numerous N–In bonds eliminate donorlike subgap states near the Fermi level, which improve stability during stress but degrade electron mobility. We developed tandem TFTs with an a-IZTO:N layer on top of an a-IZTO layer, in which mobility reaches 31.76 ± 0.81 cm2/Vs and the reliability is improved. Especially, $\Delta V_{\textrm {th}}$ in NGBS is reduced by 80% for pristine a-IZTO devices. This simple but an effective method achieves fast and reliable operation in the a-IZTO TFTs.

Graphene and its composites are widely investigated as supercapacitor electrodes due to their large specific surface area. However, the severe aggregation and disordered alignment of graphene sheets hamper the maximum utilization of its surface area. Here we report an optimized structure for supercapacitor electrode, i.e., the vertical graphene sheets, which have a vertical structure and open architecture for ion transport pathway. The effect of morphology and orientation of vertical graphene on the performance of supercapacitor is examined using a combination of model calculation and experimental study. Both results consistently demonstrate that the vertical graphene electrode has a much superior performance than that of lateral graphene electrode. Typically, the areal capacitances of a vertical graphene electrode reach 8.4 mF/cm(2) at scan rate of 100 mV/s; this is about 38% higher than that of a lateral graphene electrode and about 6 times higher than that of graphite paper. To further improve its performance, a MnO2 nanoflake layer is coated on the surface of graphene to provide a high pseudocapacitive contribution to the overall areal capacitance which increases to 500 mF/cm(2) at scan rate of 5 mV/s. The reasons for these significant improvements are studied in detail and are attributed to the fast ion diffusion and enhanced charge storage capacity. The microscopic manipulation of graphene electrode configuration could greatly improve its specific capacitance, and furthermore, boost the energy density of supercapacitor. Our results demonstrate that the vertical graphene electrode is more efficient and practical for the high performance energy storage device with high power and energy densities.

One of the most fascinating and important merits of graphene plasmonics is their tunability over a wide range. While chemical doping has proven to be a facile and effective way to create graphene plasmons, most of the previous studies focused on the macroscopic behaviors of the plasmons in chemically-doped graphene and little was known about their nanoscale responses and related mechanisms. Here, to the best of our knowledge, we present the first experimental near-field optical study on chemically-doped graphene with improved surface plasmon characteristics. By using a scattering-type scanning near-field optical microscope (s-SNOM), we managed to show that the graphene plasmons can be tuned and improved using a facile chemical doping method. Specifically, the plasmon interference patterns near the edge of the monolayer graphene were substantially enhanced via nitric acid (HNO3) exposure. The plasmon-related characteristics can be deduced by analyzing such plasmonic fringes, which exhibited a longer plasmon wavelength and reduced plasmon damping rate. In addition, the local carrier density and therefore the Fermi energy level (EF) of graphene can be obtained from the plasmonic nano-imaging, which indicated that the enhanced plasmon oscillation originated from the injection of free holes into graphene by HNO3. These findings were further corroborated by theoretical calculations using density functional theory (DFT). We believe that our findings provide a clear nanoscale picture on improving graphene plasmonics by chemical doping, which will be helpful for optimizing graphene plasmonics and for elucidating the mechanisms of two-dimensional light confinement by atomically thick materials.

The carbon nanotube (CNT) cold cathode can provide a high current and current density for microwave devices, which normally work under pulse mode. In this work, we studied the electron emission characteristics and strucutre stability of a CNT cold cathode in millisecond pulse mode. Peak current and average current are both key parameters for the performance of cold cathode. Increasing the duty ratio proved an effective way to increase the average current and average power of the cold cathode. The pulsing field emission characteristics of CNT cold cathode was optimized by modulating the pulse width, pulse interval time, and duty ratio. The average current in millisecond pulse mode was three orders of magnitude greater than that when using microsecond pulses. The average current also linearly increased with the duty ratio. The morphology of CNT films after pulsing and DC field emission was investigated to reveal the stability and the vacuum breakdown mechanism. The thermal equilibrium of the CNT was analyzed to reveal its underlying mechanism. This work provides a practical strategy for the application of CNT cold cathodes in high-power microwave devices.

Interface control is an effective way to improve the field emission performance for an existing cold cathode. In this study, a series of pre- and post-treatments have been carried out to reduce the thickness of mid-layer, from over 30 nm to 3 nm, for the single-crystalline molybdenum nanopyramid emitters. Consequently, enhanced field emission properties have been achieved.

A single silicon Carbide(SiC) nanowire was fixed on the tungsten tip by using an electron beam induced deposition technique. The field emission properties of single SiC nanowire driven by DC and pulse have been characterized separately. The results show that the pulse-driven field emission performance of single SiC nanowire is better than the one driven by DC, including the turn-on voltage and the maximum current. An inference is proposed to qualitatively explain the improvement of field emission performance, which assumes the changes of work function on the top of single SiC nanowire due to the different stretching effect of the electrostatic field by two driving mode.

The structure stability of graphene field emitter is important for the performance of field emission devices. In this work, high dose electron beam illumination and high current field emission was applied on the graphene field emitter and its structure evolution and field emission characteristics were investigated using an in-situ TEM measurement system. The relation between structure and performance of graphene emitter was demonstrated. The interaction of electron and graphene was also discussed.

We fabricated full array of uniform individual pSi/ZnO nano-emitters and demonstrated the strong thermoenhanced field emission. The emission current density showed about ten-time enhancement (from 1.31 to 12.11 mA/cm2 at 60.6 MV/m) by increasing the temperature from 323 K to 623 K. The strong thermo-enhancement was proposed to be benefit from the increase of band-to-band tunneling probability at the surface portion of the p-Si/ZnO nano-junction. This work provides promising cathode for portable x-ray tubes/panel, ionization vacuum gauges and low energy electron beam lithography, in where electron-dose control at a fixed energy is needed.

An 8-inch diagonal ZnO nanowire cold cathode panel was fabricated. The morphologies and field emission properties of the ZnO nanowire array grown on the panel was characterized. The results demonstrated that the ZnO nanowires have a relatively uniform distribution and good emission properties with an emission current of 14.5 mA, showing promising application for diode-structure flat-panel X-ray source.

ZnO nanowires as field emitters have important applications in flat panel display and x-ray source. Understanding the intrinsic field emission mechanism is crucial for further improving the performance of ZnO nanowire field emitters. In this article, the temperature dependent field emission from individual ZnO nanowires was investigated by an in situ measurement in ultra-high vacuum. The divergent temperature-dependent Fowler-Nordheim plots is found in the low field region. A field-induced hot electrons emission model, that takes into account penetration length, is proposed to explain the results. The carrier density and temperature dependence of the field-induced hot electrons emission current are derived theoretically. The obtained results are consistent with the experimental results, which could be attributed to the variation of effective electron temperature. All of these are important for a better understanding on the field emission process of semiconductor nanostructures.

A hybrid active pixel optical sensor for high-resolution and high-sensitivity imaging is proposed and experimentally demonstrated. The sensor vertically integrates an amorphous silicon p-i-n photodiode and a low-temperature polycrystalline silicon readout thin-film transistor (TFT). The vertical integration results in a high fill factor (>70%) and an enlarged photosensing area in the pixel. In the photodiode-gated TFT structure of the sensor, the output current is amplified by operating the TFT in the sub-threshold regime. A weak wavelength-dependent photoconductive gain >10 is obtained at visible wavelengths, enabling large area low-level light detection.

A facile all-chemical vapor deposition approach is designed, which allows both sequentially grown Gr and monolayer MoS2 in the same growth process, thus allowing the direct construction of MoS2 /Gr vertical heterostructures on Au foils. A weak n-doping effect and an intrinsic bandgap of MoS2 are obtained from MoS2 /Gr/Au via scanning tunneling microscopy and spectroscopy characterization. The exciton binding energy is accurately deduced by combining photoluminescence measurements.

Large-scale WO3 nanowire patterns have been successfully fabricated on a 3.5 inch glass substrate at atmospheric pressure by a simple no-catalyst method. The nanowires were observed to have a mean length of about 40 μm, and their aspect ratio reaches 200. The nanowires were proven to be single crystalline WO3 with a monoclinic structure. It is found that the growth region and growth density of the WO3 nanowires differ with the interval or the width of the W stripes. By combination of designing a series of experiments and analyzing the growth kinetics theory, a novel self-supported vapor–solid (SVS) mechanism is proposed to be responsible for the formation of WO3 nanowires. Field emission (FE) measurements show that the WO3 nanowire patterns have excellent FE performance, which have a low turn-on field of 2.9 V μm−1 and good field emission uniformity of over 85%. Moreover, this SVS method may provide a helpful reference on low-temperature and no-catalyst growth of other metal oxide nanostructure arrays at atmospheric pressure.