Y. M. Chong

Vertically aligned boron nitride nanosheets: chemical vapor synthesis, ultraviolet light emission, and superhydrophobicity.

Boron nitride (BN) is a promising semiconductor with a wide band gap ( approximately 6 eV). Here, we report the synthesis of vertically aligned BN nanosheets (BNNSs) on silicon substrates by microwave plasma chemical vapor deposition from a gas mixture of BF(3)-N(2)-H(2). The size, shape, thickness, density, and alignment of the BNNSs were well-controlled by appropriately changing the growth conditions. With changing the gas flow rates of BF(3) and H(2) as well as their ratio, the BNNSs evolve from three-dimensional with branches to two-dimensional with smooth surface and their thickness changes from 20 to below 5 nm. The growth of the BNNSs rather than uniform granular films is attributed to the particular chemical properties of the gas system, mainly the strong etching effect of fluorine. The alignment of the BNNSs is possibly induced by the electrical field generated in plasma sheath. Strong UV light emission with a broad band ranging from 200 to 400 nm and superhydrophobicity with contact angles over 150 degrees were obtained for the vertically aligned BNNSs. The present BNNSs possess the properties complementary to carbon nanosheets such as intrinsically semiconducting, high temperature stability, and high chemical inertness and may find applications in ultraviolet nanoelectronics, catalyst supports, electron field emission, and self-cleaning coatings, etc., especially those working at high temperature and in harsh environments.

Fabrication of diamond nanopillars and their arrays

High-density, uniform diamond nanopillar arrays were fabricated by employing bias-assisted reactive ion etching in a hydrogen/argon plasma. Gold nanodots were employed as etching masks. The formation of nanopillar structure is associated with the directional physical etching/sputtering by ion bombardment and selective chemical etching of sp2 carbons by reactive hydrogen atoms and ions. The density and geometry of the nanopillars depend on the initial structure of diamond films and reactive ion etching conditions. The nanopillars with high aspect ratio and large surface area may have potential applications in high-efficiency and high-sensitivity diamond-based biomedical and chemical sensors and in mechanical and thermal management.High-density, uniform diamond nanopillar arrays were fabricated by employing bias-assisted reactive ion etching in a hydrogen/argon plasma. Gold nanodots were employed as etching masks. The formation of nanopillar structure is associated with the directional physical etching/sputtering by ion bombardment and selective chemical etching of sp2 carbons by reactive hydrogen atoms and ions. The density and geometry of the nanopillars depend on the initial structure of diamond films and reactive ion etching conditions. The nanopillars with high aspect ratio and large surface area may have potential applications in high-efficiency and high-sensitivity diamond-based biomedical and chemical sensors and in mechanical and thermal management.

193nm deep-ultraviolet solar-blind cubic boron nitride based photodetectors

Deep-ultraviolet (DUV) solar-blind photodetectors based on high-quality cubic boron nitride (cBN) films with a metal/semiconductor/metal configuration were fabricated. The design of interdigitated circular electrodes enables high homogeneity of electric field between pads. The DUV photodetectors present a peak responsivity at 180nm with a very sharp cutoff wavelength at 193nm and a visible rejection ratio (180 versus 250nm) of more than four orders of magnitude. The characteristics of the photodetectors present extremely low dark current, high breakdown voltage, and high responsivity, suggesting that cBN films are very promising for DUV sensing.

Electrical properties of Be-implanted polycrystalline cubic boron nitride films

P-type conductivity of polycrystalline cubic boron nitride (cBN) films was achieved by implantation of beryllium ions. The effects of implantation doses and annealing on the phase composition and electrical properties of cBN films were studied. A reduction in resistivity by seven orders of magnitude was observed. Hall measurement revealed a corresponding hole concentration of 6.1×1018cm−3 and mobility of 3cm2∕Vs. The activation energy was estimated to be 0.20±0.02eV from the temperature dependence of resistance. The electrical properties of Be-implanted films are comparable to that of Be-doped cBN single crystals synthesized by high-pressure and high-temperature method.

p-type conduction in beryllium-implanted hexagonal boron nitride films

p-type conduction in hexagonal boron nitride (hBN) films was achieved by beryllium implantation and subsequent rapid thermal annealing treatment. The dependence of phase composition and electrical properties of hBN films on the implantation fluence and annealing was studied. A maximum resistivity reduction by six orders of magnitude was demonstrated. Hall measurements revealed a corresponding hole concentration of 3×1019 cm−3 and mobility of 27 cm2/V s. The activation energy of Be ions was estimated to be 0.21 eV. It is suggested that hBN is a promising wide bandgap semiconductor for applications in high-temperature electronic devices and transparent conductive coatings.

The fabrication of cubic boron nitride nanocone and nanopillar arrays via reactive ion etching

High-density (2 x 10(9) cm(-2)) uniform arrays of cubic boron nitride (cBN) nanocones and nanopillars with a high aspect ratio were fabricated by employing sequential growth and bias-assisted reactive ion etching using gold nano-dots as an etching mask. The mechanism of formation of the nanopillar and nanocone morphologies was discussed in terms of the relative action of ion bombardment etching and chemical etching due to activated hydrogen plasma constituents. The presented method enabled nanostructuring of cBN surfaces over large areas with great uniformity and reproducibility with a controlled aspect ratio. The unique morphology of the nanostructures offers diverse application opportunities in microelectromechanical devices.

Surface functionalization of cubic boron nitride films for biological sensing applications

Surface functionalization and modification scheme of cubic boron nitride (cBN) films deposited by chemical vapor deposition was demonstrated. A homogeneous layer of amino groups was bonded covalently on the B and/or N atoms of cBN surface via a photochemical reaction with allylamine. X-ray photoelectron spectroscopy was carried out to verify comprehensively each stage of the surface modification process. Gold nanoparticles (AuNPs) were self-assembled on the amine-terminated cBN surface, and a dense and well-distributed AuNPs monolayer was obtained. Modification of amine-terminated cBN films with amine-modified DNA probes presents an example of applications as DNA biosensors.

A cubic boron nitride film-based fluorescent sensor for detecting Hg2+

Cubic boron nitride (cBN) film-based sensors for detecting Hg2+ ions were developed by surface functionalization with dansyl chloride. To immobilize dansyl chloride, 3-aminopropyltriethoxy silane was modified on hydroxylated cBN surfaces to form an amino-group-terminated self-assembled monolayer. The covalent attachment of the amino groups was confirmed by x-ray photoelectron spectroscopy. The selectivity and sensitivity of the sensors to detect diverse metal cations in ethanol solutions were studied by using fluorescence spectroscopy, revealing a great selectivity to Hg2+ ions. Significantly, the dansyl-chloride-functionalized cBN film sensors were recyclable after the sensing test.

Nanocubic boron nitride/nanodiamond multilayer structures

Nanocubic boron nitride/nanodiamond (N-cBN/ND) multilayer structures with each alternating layer being ∼100nm thick have been prepared by magnetron sputter and microwave plasma enhanced chemical vapor depositions. These multilayers exhibit remarkable properties, in particular, the mechanical properties. The multilayer structure is characteristic with (i) extreme hardness (82GPa) considerably surpassing the values of the individual materials from which the multilayer is composed, (ii) high surface smoothness, (iii) significantly reduced film stress when compared with a single cBN layer of equivalent thickness, and (iv) great chemical stability. The N-cBN/ND multilayers developed have therefore important implications in mechanical and chemically resistant applications.

Structural analysis of cubic boron nitride films by ultraviolet raman spectroscopy

Cubic boron nitride (BN) films with improved crystallinity are deposited by physical vapor deposition at an extremely low substrate bias (−35V). The films are characterized by UV Raman in association with Fourier transformed infrared (FTIR) spectroscopy. The influences of bias voltage and film thickness on the characterizations are investigated. UV Raman, in contrast to FTIR, is demonstrated to be a more powerful tool with high sensitivity for quantitative and/or qualitative evaluation of the phase purity and crystallinity, especially as the film thickness increases. Hexagonal BN inclusions (less than 1%), not evident in FTIR, are clearly revealed by UV Raman analysis.