Jih-Shang Hwang

Selective-area growth of indium nitride nanowires on gold-patterned Si(100) substrates

This letter reports the synthesis of indium nitride (InN) nanowires on gold-patterned silicon substrates in a controlled manner using a method involving thermal evaporation of pure indium. The locations of these InN nanowires were controlled by depositing gold in desired areas on the substrates. Scanning electron microscopy and transmission electron microscopy investigations showed that the InN nanowires are single crystals with diameters ranging from 40 to 80 nm, and lengths up to 5 μm. Energy dispersive x-ray spectrometry showed that the ends of the nanowires are composed primarily of Au, and the rest of the nanowires were InN with no detectable Au incorporations. The Raman spectra showed peaks at 445, 489, and 579 cm−1, which are attributed to the A1(transverse optical), E2, and A1(longitudinal optical) phonon modes of the wurtzite InN structure, respectively. Photoluminescence spectra of the InN nanowires showed a strong broad emission peak at 1.85 eV.

Measurement of heat capacity by fitting the whole temperature response of a heat-pulse calorimeter

A new method that fits the whole temperature response of a heat-pulse calorimeter for heat capacity is developed. Analyzing the thermal response of a heat-pulse calorimeter on a model that was used by the relaxation method, the authors derived some useful relations and further utilized the numeric method of the general linear least squares to determine the heat capacity of a sample. Absolute accuracy of the proposed method was verified by determining the heat capacity of a 0.249 76 g copper sample (purity 99.999%) on a self-designed and fully automated calorimetric system from 4.5 to 80 K. Comparing the result with the literature, the deviation in average was 1.2% from 4.5 to 20 K and 2.0% from 20 to 80 K. It is found that the proposed method is capable of measuring heat capacity regardless if the sample is adiabatically or nonadiabatically isolated. The size of the specimen is not critical for the application of the method and the temperature range of measurement can be expanded. It also deals with the τ...

SiC-capped nanotip arrays for field emission with ultralow turn-on field

Silicon nanotips with tip diameter and height measuring 1 nm and 1 μm, respectively, and density in the range of 109–3×1011 cm−2, were fabricated monolithically from silicon wafers by electron cyclotron resonance plasma etching technique at a temperature of 200 °C. Field emission current densities of 3.0 mA/cm2 at an applied field of ∼1.0 V/μm was obtained from these silicon nanotips. High-resolution transmission electron microscope and Auger electron spectroscopy analyses concluded that the nanotips are composed of monolithic silicon and nanometer-size SiC cap at the top. A 0.35 V/μm turn-on field to draw a 10 μA/cm2 current density was demonstrated, which is much lower than other reported materials. The excellent field emission property demonstrated by these nanotips, which were fabricated by a process integrable to the existing silicon device technology at low temperatures, is a step forward in achieving low-power field emission displays and vacuum electronic devices.

Infrared lasing in InN nanobelts

Infrared lasing from single-crystalline InN nanobelts grown by metal organic chemical vapor deposition was demonstrated. Transmission electron microscopy studies revealed that the InN nanobelts of rectangular cross section grew along [110] direction and were enclosed by ±(001) and ±(11¯0) planes. The infrared lasing action was observed at 20K in the InN nanobelts grown on an amorphous silicon nitride coated silicon substrate by continuous wave laser pumping.

Beaded stream-like CoSe2 nanoneedle array for efficient hydrogen evolution electrocatalysis

The development of earth-abundant and efficient electrocatalysts for the hydrogen evolution reaction (HER) is one of the keys to success for future green energy systems using hydrogen fuel. Nanostructuring of electrocatalysts is a promising way to enhance their electrocatalytic performance in the HER. In this study, pure pyrite-type beaded stream-like cobalt diselenide (CoSe2) nanoneedles are directly formed on flexible titanium foils through treating a cobalt oxide (Co3O4) nanoneedle array template with selenium vapor. The beaded stream-like CoSe2 nanoneedle electrode can drive the HER at a current density of 20 mA cm−2 with a small overpotential of 125 mV. Moreover, the beaded stream-like CoSe2 nanoneedle electrode remains stable in an acidic electrolyte for 3000 cycles and continuously splits water over a period of 18 hours. The enhanced electrochemical activity is facilitated by the unique three-dimensional hierarchical structure, the highly accessible surface active sites, the improved charge transfer kinetics and the highly attractive force between water and the surface of the nanoneedles that exceeds the surface tension of water.

A nontoxic solvent based sol–gel Cu2ZnSnS4 thin film for high efficiency and scalable low-cost photovoltaic cells

A non-toxic sol–gel spin coating approach is one of the attractive routes to achieve high atom economy, good quality Cu2ZnSnS4 (CZTS) thin films. In this paper, we introduce 1,3-dimethyl-2-imadazolidinone as a solvent for the preparation of highly viscous, homogeneous, nontoxic CZTS ink that eliminates the need for the use of additional binders or additives to disperse the precursors. In addition, we further report the annealing of the spin coated CZTS thin film in 6% diluted H2S gas with an externally supplied tin and sulfur environment to suppress the loss of tin from the thin-film surface and to enhance the device performance. CZTS grain sizes greater than 0.7 μm have been achieved with no detectable presence of carbon rich layers or layers containing fine grain sizes at the Mo/CZTS interface. An efficiency of 5.67% for the champion device fabricated here has been achieved with an open circuit voltage of 0.58 V, a short current density of 18.48 mA cm−2, and a fill factor of 53.14%.

Imaging layer number and stacking order through formulating Raman fingerprints obtained from hexagonal single crystals of few layer graphene

Quantitative mapping of layer number and stacking order for CVD-grown graphene layers is realized by formulating Raman fingerprints obtained on two stepwise stacked graphene single-crystal domains with AB Bernal and turbostratic stacking (with ~30°interlayer rotation), respectively. The integrated peak area ratio of the G band to the Si band, A(G)/A(Si), is proven to be a good fingerprint for layer number determination, while the area ratio of the 2D and G bands, A(2D)/A(G), is shown to differentiate effectively between the two different stacking orders. The two fingerprints are well formulated and resolve, quantitatively, the layer number and stacking type of various graphene domains that used to rely on tedious transmission electron microscopy for structural analysis. The approach is also noticeable in easy discrimination of the turbostratic graphene region (~30° rotation), the structure of which resembles the well known high-mobility graphene R30/R2(±) fault pairs found on the vacuum-annealed C-face SiC and suggests an electron mobility reaching 14,700 cm(3) V(-1) s(-1). The methodology may shed light on monitoring and control of high-quality graphene growth, and thereby facilitate future mass production of potential high-speed graphene applications.

Antiferromagnetism and mass-enhanced behavior in Ce2CuSi3

Temperature dependences of the specific heat, electrical resistivity and magnetic susceptibility of single-phased AlB2-type compound Ce2CuSi3 as well as its nonmagnetic counterpart La2CuSi3 are reported. La2CuSi3 is Pauli paramagnetic whereas Ce2CuSi3 shows complex magnetic properties. An antiferromagnetic ordering at 2.1 k is observed by low field (50 Oe) susceptibility measurement on Ce2CuSi3. Magnetic susceptibility measured under 2000 Oe, however, appears to be saturated below 2.1 K. Heat capacity measurement down to 4.5 K reveals an upturn in C(T) of Ce2CuSi3 which could be correlated with magnetic transition at 2.1 K. The electronic specific heat in Ce2CuSi3 is obtained, indicating an enhanced temperature-independent γ of 76mJK−2 mol−1 Ce. A negative contribution to the enhanced γ that could be due to spin fluctuation, is observed. The resistivity data of Ce2CuSi3 shows the typical behavior of a Kondo compound, which might be responsible for the enhanced effective mass.

The preparation of silver nanoparticle decorated silica nanowires on fused quartz as reusable versatile nanostructured surface-enhanced Raman scattering substrates

We introduce a platform, comprised of silver nanoparticle decorated silica nanowires (SiONWs) dispersed on fused quartz substrates, for high sensitivity surface-enhanced Raman scattering (SERS) measurements using both frontal (through the analytes) and back-face (through the transparent substrate) excitation. Quasi-quantitative SERS performances on the specialized substrate, vis-à-vis a silver deposited bare fused quartz plate, showed: (i) the suitability of the Ag modified SiONW substrate for frontal as well as back-face excitation; (ii) a wider detection range with high sensitivity to Rhodamine 6G; and (iii) good underwater metal-oxide adhesion of the specialized substrates. Capable of surviving ultrasonic cleaning, the substrate introduced is one of the few reusable low-cost Ag-based nanostructured SERS substrates, requiring only a simple silver reload process (the silver mirror reaction).

Reduced temperature-quenching of photoluminescence from indium nitride nanotips grown by metalorganic chemical vapor deposition

We report metalorganic chemical vapor deposition of indium nitride (InN) nanotips with apex angles of 10° and length and base diameter of around 1μm and 200 nm, respectively. The structure of the hexagonal InN nanotips growing along [002] was studied by electron microscopy and x-ray diffraction, and the optical properties were studied using temperature-dependent photoluminescence (PL) measurements. A narrow emission peak with a 18 meV full width at half maximum positioned at 0.77 eV was obtained with no visible emission. A PL quenching of only 14% was observed with a temperature scan of 15–320 K.