Juncong She

Oxygen density dependent band gap of reduced graphene oxide

We investigated the stability of reduced graphene oxide for oxygen density ranging from 6.25% to 50% with the density functional theory and found the most, the second most, and the third most stable oxygen configurations. The effect of relaxation of lattice on the electronic properties is found to be negligible for low O coverage and crucial for higher O coverage, respectively. The densities of states and the band gaps were calculated. The bandgap is found to be a non-monotonic function of oxygen density, with minima at O/C = 11.1% and 25%.

Improved field emission characteristics of large-area films of molybdenum trioxide microbelt

We study the field emission characteristics of large-area films of crystalline MoO3 microbelt grown on silicon substrate by thermal evaporation in air using a commercial infrared sintering furnace. It is found that their turn-on field, threshold field, resistance to microdischarge and field emission current stability are better than MoO3 nanowires, MoO3 nanobelts and MoO3 nanoflower. In addition, good uniformdistribution of field emission sites can be observed. The physical reasons are explained responsible for such improvements on field emission characteristics of MoO3 material. These results indicate that large-area MoO3 microbelts may be suitable for cold-cathode electron source application.

Room-Temperature Strong Light–Matter Interaction with Active Control in Single Plasmonic Nanorod Coupled with Two-Dimensional Atomic Crystals

Strong light-matter coupling manifested by Rabi splitting has attracted tremendous attention due to its fundamental importance in cavity quantum-electrodynamics research and great potentials in quantum information applications. A prerequisite for practical applications of the strong coupling in future optoelectronic devices is an all-solid-state system exhibiting room-temperature Rabi splitting with active control. Here we realized such a system in heterostructure consisted of monolayer WS2 and an individual plasmonic gold nanorod. By taking advantages of the small mode volume of the nanorod and large transition dipole moment of the WS2 exciton, giant Rabi splitting energies of 91-133 meV can be achieved at ambient conditions, which only involve a small number of excitons. The strong light-matter coupling can be dynamically tuned either by electrostatic gating or temperature scanning. These findings can pave the way toward active nanophotonic devices operating at room temperature.

Transmission type flat-panel X-ray source using ZnO nanowire field emitters

A transmission type flat-panel X-ray source in diode structure was fabricated. Large-scale patterned ZnO nanowires grown on a glass substrate by thermal oxidation were utilized as field emitters, and tungsten thin film coated on silica glass was used as the transmission anode. Uniform distribution of X-ray generation was achieved, which benefited from the uniform electron emission from ZnO nanowires. Self-ballasting effect induced by the intrinsic resistance of ZnO nanowire and decreasing of screening effect caused by patterned emitters account for the uniform emission. Characteristic X-ray peaks of W-L lines and bremsstrahlung X-rays have been observed under anode voltages at a range of 18–20 kV, the latter of which were the dominant X-ray signals. High-resolution X-ray images with spatial resolution less than 25 μm were obtained by the flat-panel X-ray source. The high resolution was attributed to the small divergence angle of the emitted X-rays from the transmission X-ray source.

A Fully-Sealed Carbon-Nanotube Cold-Cathode Terahertz Gyrotron.

Gigahertz to terahertz radiation sources based on cold-cathode vacuum electron technology are pursued, because its unique characteristics of instant switch-on and power saving are important to military and space applications. Gigahertz gyrotron was reported using carbon nanotube (CNT) cold-cathode. It is reported here in first time that a fully-sealed CNT cold-cathode 0.22 THz-gyrotron is realized, typically with output power of 500 mW. To achieve this, we have studied mechanisms responsible for CNTs growth on curved shape metal surface, field emission from the sidewall of a CNT, and crystallized interface junction between CNT and substrate material. We have obtained uniform growth of CNTs on and direct growth from cone-cylinder stainless-steel electrode surface, and field emission from both tips and sidewalls of CNTs. It is essential for the success of a CNT terahertz gyrotron to have such high quality, high emitting performance CNTs. Also, we have developed a magnetic injection electron gun using CNT cold-cathode to exploit the advantages of such a conventional gun design, so that a large area emitting surface is utilized to deliver large current for electron beam. The results indicate that higher output power and higher radiation frequency terahertz gyrotron may be made using CNT cold-cathode electron gun.

Field-Induced Crystalline-to-Amorphous Phase Transformation on the Si Nano-Apex and the Achieving of Highly Reliable Si Nano-Cathodes

Nano-scale vacuum channel transistors possess merits of higher cutoff frequency and greater gain power as compared with the conventional solid-state transistors. The improvement in cathode reliability is one of the major challenges to obtain high performance vacuum channel transistors. We report the experimental findings and the physical insight into the field induced crystalline-to-amorphous phase transformation on the surface of the Si nano-cathode. The crystalline Si tip apex deformed to amorphous structure at a low macroscopic field (0.6~1.65 V/nm) with an ultra-low emission current (1~10 pA). First-principle calculation suggests that the strong electrostatic force exerting on the electrons in the surface lattices would take the account for the field-induced atomic migration that result in an amorphization. The arsenic-dopant in the Si surface lattice would increase the inner stress as well as the electron density, leading to a lower amorphization field. Highly reliable Si nano-cathodes were obtained by employing diamond like carbon coating to enhance the electron emission and thus decrease the surface charge accumulation. The findings are crucial for developing highly reliable Si-based nano-scale vacuum channel transistors and have the significance for future Si nano-electronic devices with narrow separation.

Tailoring of electromagnetic field localizations by two-dimensional graphene nanostructures

Graphene has great potential for enhancing light−matter interactions in a two-dimensional regime due to surface plasmons with low loss and strong light confinement. Further utilization of graphene in nanophotonics relies on the precise control of light localization properties. Here, we demonstrate the tailoring of electromagnetic field localizations in the mid-infrared region by precisely shaping the graphene into nanostructures with different geometries. We generalize the phenomenological cavity model and employ nanoimaging techniques to quantitatively calculate and experimentally visualize the two-dimensional electromagnetic field distributions within the nanostructures, which indicate that the electromagnetic field can be shaped into specific patterns depending on the shapes and sizes of the nanostructures. Furthermore, we show that the light localization performance can be further improved by reducing the sizes of the nanostructures, where a lateral confinement of λ0/180 of the incidence light can be achieved. The electromagnetic field localizations within a nanostructure with a specific geometry can also be modulated by chemical doping. Our strategies can, in principle, be generalized to other two-dimensional materials, therefore providing new degrees of freedom for designing nanophotonic components capable of tailoring two-dimensional light confinement over a broad wavelength range.

Field Emission From a MOSFET-Controlled ZnO-Nanowire Cold Cathode

Field emission from a MOSFET-controlled ZnO nanowire (NW) cathode is reported. The emission current was modulated in four orders of magnitude (from 0.2 nA to 1.15 μA ) by tuning the MOSFET gate voltage from 0.8385 to 1.5255 V. The modulation of the emission current showed an exponential relationship with the MOSFET gate voltage. The MOSFET control can dramatically reduce the emission current fluctuation, i.e., from 61.4% to 3.2%, at a current level of ~ 650 nA. The related mechanisms responsible for the improvement on field-emission current stability, the correlation between the vacuum breakdown of the NWs, and the failure of the MOSFET were discussed. The technique combines cold-cathode nanomaterials with mature solid-state technology to produce a reliable cathode in an on-chip form, which shows promising potential for modern vacuum micro-/nanosized electronic device applications.

Large piezoresistance of single silicon nano-needles induced by non-uniaxial strain

Silicon (Si) nanostructures may play an important role in future nanoelectronics. Here, arrays of single vertically aligned Si nano-needles (SiNNs) are fabricated and found to exhibit a significant piezoresistance effect under large non-uniaxial strain. The experiments were performed by in situ manipulating of the SiNNs in a scanning electron microscope. The method enables us to determine the piezoresistance effect under the combined action of compressive and tensile strains, which is different from reported works on the piezoresistance effect of Si nanostructures under uniaxial/biaxial strains. This has given rise to an opportunity to further explore the physical origins responsible for the piezoresistance of Si. The relative change in resistivity under the non-uniaxial strain was calculated from the experimental I-V data. The results show that a greater piezoresistance effect can be achieved as compared with that of Si nanowires under uniaxial strain. We propose that the suppression of inter-valley scat...

Zinc Oxide Nanowire Lateral Field Emission Devices and its Application as Display Pixel Structures

The rational design and fabrication of zinc oxide (ZnO) nanowire (NW) lateral field electron emission device and the possible application as a display pixel structure are reported. In the device, the cathode and anode are ranked side-by-side on the same panel. The NW-clusters were controlled to locally grow on the edges of the electrodes with different tilted status, i.e., in angle range of 75°-110°, 0°-110°, and 0°-57°, respectively. The devices with NWs at different tilt-angle showed distinct field electron emission properties. The device with 0°-57° tilted NWs possess the best performance, i.e., an emission current of 9.3 μA (current density: 6.22 A/cm2) was obtained at a low cathode-anode (50 μm in separation) bias of 477 V. Stable cathodoluminescence was observed from the indium titanic oxide anode, suggests a possibility for display application. Mechanisms responsible for the enhanced field electron emission and the related device physics are proposed. Significantly, the low temperature (~ 80°C) solution-phase growth of ZnO NWs enables the fabrication of the devices on flexible polyimide substrate, which has also been demonstrated here. This paper opens up possibilities on developing NW-based lateral field electron emission device for vacuum micro/nanoelectronics applications.