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Microstructure and its effect on field electron emission of grain-size-controlled nanocrystalline diamond films Ultrananocrystalline diamond ͑UNCD͒ films 0.1-2.4 m thick were conformally deposited on sharp single Si microtip emitters, using microwave CH 4 -Ar plasma-enhanced chemical vapor deposition in combination with a dielectrophoretic seeding process. Field-emission studies exhibited stable, extremely high ͑60-100 A/tip͒ emission current, with little variation in threshold fields as a function of film thickness or Si tip radius. The electron emission properties of high aspect ratio Si microtips, coated with diamond using the hot filament chemical vapor deposition ͑HFCVD͒ process were found to be very different from those of the UNCD-coated tips. For the HFCVD process, there is a strong dependence of the emission threshold on both the diamond coating thickness and Si tip radius. Quantum photoyield measurements of the UNCD films revealed that these films have an enhanced density of states within the bulk diamond band gap that is correlated with a reduction in the threshold field for electron emission. In addition, scanning tunneling microscopy studies indicate that the emission sites from UNCD films are related to minima or inflection points in the surface topography, and not to surface asperities. These data, in conjunction with tight binding pseudopotential calculations, indicate that grain boundaries play a critical role in the electron emission properties of UNCD films, such that these boundaries: ͑a͒ provide a conducting path from the substrate to the diamond-vacuum interface, ͑b͒ produce a geometric enhancement in the local electric field via internal structures, rather than surface topography, and ͑c͒ produce an enhancement in the local density of states within the bulk diamond band gap.

We performed studies of electron emission from ultrananocrystalline diamond ͑UNCD͒-coated, ungated silicon field emitters as a function of in situ exposure to various gases during current versus voltage and current versus time measurements. The emitter arrays were fabricated by a subtractive tip fabrication process and coated with UNCD films using microwave plasma chemical vapor deposition with a CH 4 /Ar plasma chemistry. The emission characteristics of the coated tip arrays were studied in the diode configuration; using a 2 mm diameter anode with rounded edges to suppress arcing. Significant enhancement of the electron emission was observed, increasing from 35% to 100%, after the emitting surface was exposed to H 2 at pressures in the 10 Ϫ5 and 10 Ϫ4 Torr range. Upon termination of the H 2 exposure, the current decreased to the initial value of 2 A. The emission current subsequently remained stable at 2 A upon continued evacuation down to the base pressure below 10 Ϫ9 Torr. The emission current variation is repeatable with ensuing hydrogen exposure, indicating that the enhancement is due to the hydrogen exposure. Negligible emission current variations are observed at pressures less than 10 Ϫ5 Torr. Exposure to either Ar or N 2 resulted in a reduction of the emission current for ambients of up to 10 Ϫ5 Torr. This effect is reversible. The effect of the investigated gases on the emission characteristics of UNCD-coated Si tip arrays is attributed to a modification of the effective work function at the localized sites from where electrons are being emitted.

Energy crisis has become an urgent problem in the world. Hydrogen is a continuous and clean energy, but traditional electrocatalytic materials for water splitting have the shortcomings including high cost and low hydrogen production efficiency. Herein, we prepare a biomass carbon-based electrocatalyst doped with iron and nitrogen. The overpotential of so-obtained CA (abbreviation of "carbon from amaranth") @Fe(1:3)-800 is only 265 mV at 10 mA cm −2 for oxygen evolution reaction (OER), as well as the Tafel slope is 76.7 mV dec −1 , which are much superior to the results of CA-800 (the overpotential is 514 mV and the Tafel slope is 160.2 mV dec −1 ). It shows the positive influences of dual-doping with iron and nitrogen in biomass carbon. Our work opens up a low-cost and green avenue for fabricating high-performance electrocatalyst in hydrogen production field.

Competition of nitrogen doping and graphitization effect for field electron emission from nanocrystalline diamond films J.Synthesis and electron field emission of nanocrystalline diamond thin films grown from N 2 /CH 4 microwave plasmasWe have investigated the field emission properties of diamond films grown under substrate bias conditions in a microwave plasma chemical vapor deposition system, with a substrate temperature of 800°C, microwave power of 600 W, and a total pressure of 11 Torr. One group of films was grown with a substrate bias of Ϫ100 V in gas mixtures of 1% N 2 and 1%-20% CH 4 in H 2 , while a second group of films was grown with a substrate bias ranging from ϩ100 to Ϫ150 V in a gas mixture of 1%N 2 -10%CH 4 -89%H 2 . The field emission performance in terms of threshold field and emission current improved considerably as a function of increasing CH 4 concentration and negative bias voltage. Ultraviolet Raman analysis showed that the field emission enhancement resulting from an increase in CH 4 concentration from 1% to 5% correlates with a decrease in the sp 3 bonding character in the diamond film. The dependence of field emission on negative bias voltage appears to be correlated with ion bombardment-induced damage in the film during growth. The scanning electron microscopy image of the film grown with Ϫ150 V bias showed: smaller surface topographic features as compared to films grown under 0 and ϩ100 V bias. The film grown with a bias of Ϫ150 V showed the lowest threshold field ͑ϳ2.0 V/m͒ corresponding to an emission current density of 12.7 A/cm 2 . J vs E 0 measurements across a length of 40 mm over the film showed a uniform threshold field ͑2.0Ϯ0.55 V/m͒. The film grown with a positive bias ͑ϩ100 V͒ showed a relatively poor field emission performance.