Laser modification of graphene oxide layers

The effect of linearly polarized laser irradiation with various energy densities was successfully used for reduction of graphene oxide (GO). The ion beam analytical methods (RBS, ERDA) were used to follow the elemental composition which is expected as the consequence of GO reduction. The chemical composition analysis was accompanied by structural study showing changed functionalities in the irradiated GO foils using spectroscopy techniques including XPS, FTIR and Raman spectroscopy. The AFM was employed to identify the surface morphology and electric properties evolution were subsequently studied using standard two point method measurement. The used analytical methods report on reduction of irradiated graphene oxide on the surface and the decrease of surface resistivity as a growing function of the laser beam energy density

Cytotoxicity of group 5 transition metal ditellurides (MTe2; M=V, Nb, Ta)

Much research effort has been put in to study layered compounds with transition metal dichalcogenides (TMDs) being one of the most studied compounds. Due to their extraordinary properties such as excellent electrochemical properties, tuneable band gaps, and low shear resistance due to weak van der Waals interactions between layers, TMDs have been found to have wide applications such as electrocatalysts for hydrogen evolution reactions, supercapacitors, biosensors, field-effect transistors (FETs), photovoltaics, and lubricant additives. In very recent years, Group 5 transition metal ditellurides have received an immense amount of research attention. However to date, little has been known of the potential toxicities posed by these materials. As such, we conducted the cytotoxicity study by incubating various concentrations of the Group 5 transition metal ditellurides (MTe2 ; M=V, Nb, Ta) with human lung carcinoma epithelial A549 cells for 24 hours and the remaining cell viabilities after treatment was measured. Our findings indicate that VTe2 is highly toxic whereas NbTe2 and TaTe2 are deemed to exhibit mild toxicities. This study constitutes an exemplary first step towards the understanding of the Group 5 transition metal ditellurides' toxicity effects in preparation for their possible future commercialisation.MOE (Min. of Education, S’pore

Pristine basal- and edge-plane-oriented molybdenite MoS2 exhibiting highly anisotropic properties

The layered structure of molybdenum disulfide (MoS2) is structurally similar to that of graphite, with individual sheets strongly covalently bonded within but held together through weak van der Waals interactions. This results in two distinct surfaces of MoS2: basal and edge planes. The edge plane was theoretically predicted to be more electroactive than the basal plane, but evidence from direct experimental comparison is elusive. Herein, the first study comparing the two surfaces of MoS2 by using macroscopic crystals is presented. A careful investigation of the electrochemical properties of macroscopic MoS2 pristine crystals with precise control over the exposure of one plane surface, that is, basal plane or edge plane, was performed. These crystals were characterized thoroughly by AFM, Raman spectroscopy, X-ray photoelectron spectroscopy, voltammetry, digital simulation, and DFT calculations. In the Raman spectra, the basal and edge planes show anisotropy in the preferred excitation of E2g and A1g phonon modes, respectively. The edge plane exhibits a much larger heterogeneous electron transfer rate constant k0 of 4.96×10−5 and 1.1×10−3 cm s−1 for [Fe(CN)6]3−/4− and [Ru(NH3)6]3+/2+ redox probes, respectively, compared to the basal plane, which yielded k0 tending towards zero for [Fe(CN)6]3−/4− and about 9.3×10−4 cm s−1 for [Ru(NH3)6]3+/2+. The industrially important hydrogen evolution reaction follows the trend observed for [Fe(CN)6]3−/4− in that the basal plane is basically inactive. The experimental comparison of the edge and basal planes of MoS2 crystals is supported by DFT calculations

Nano-LED induced chemical reactions for structuring processes

We present a structuring technique based on the initialization of chemical reactions by an array of nano-LEDs which is used in the near-field as well as in the far-field regime. In the near-field regime, we demonstrate first results with the nano-LED array for lithography using the photoresist DiazoNaphthoQuinone-(DNQ)-sulfonate for the fabrication of holes in the resist down to ∼75 nanometres in diameter. In contrast, the nano-LEDs can also be employed in the far-field regime to expose thin films of the monomer bisphenol A-glycidyl methacrylate (Bis-GMA) and to initialize polymerization locally. Photosensitive films were patterned and spherical cone-shaped three dimensional objects with diameters ranging from ∼480 nm up to 20 micrometres were obtained. The modification in the material as a result of the photochemical reaction induced i.e. by polymerization was confirmed by Raman spectroscopy. This structuring maskless technique has the potential to induce substantial changes in photosensitive molecules and to produce the desired structures from the tens of microns down to the nanometre scale

Laser modification of graphene oxide layers

The effect of linearly polarized laser irradiation with various energy densities was successfully used for reduction of graphene oxide (GO). The ion beam analytical methods (RBS, ERDA) were used to follow the elemental composition which is expected as the consequence of GO reduction. The chemical composition analysis was accompanied by structural study showing changed functionalities in the irradiated GO foils using spectroscopy techniques including XPS, FTIR and Raman spectroscopy. The AFM was employed to identify the surface morphology and electric properties evolution were subsequently studied using standard two point method measurement. The used analytical methods report on reduction of irradiated graphene oxide on the surface and the decrease of surface resistivity as a growing function of the laser beam energy density

Laser modification of graphene oxide layers

The effect of linearly polarized laser irradiation with various energy densities was successfully used for reduction of graphene oxide (GO). The ion beam analytical methods (RBS, ERDA) were used to follow the elemental composition which is expected as the consequence of GO reduction. The chemical composition analysis was accompanied by structural study showing changed functionalities in the irradiated GO foils using spectroscopy techniques including XPS, FTIR and Raman spectroscopy. The AFM was employed to identify the surface morphology and electric properties evolution were subsequently studied using standard two point method measurement. The used analytical methods report on reduction of irradiated graphene oxide on the surface and the decrease of surface resistivity as a growing function of the laser beam energy density

InGaN nano-LEDs for energy saving optoelectronics

Vertically integrated III-nitride nano-LEDs designed for operation in the telecommunication-wavelength range were fabricated and tested in the (p- GaN/InGaN/n-GaN/sapphire) material system. We found that the band edge luminescence energy of the nano-LEDs could be engineered by their size and by the strain interaction with the masked SiO2/GaN substrates; it depends linearly on the structure size. The results of reliability measurements prove that our technological process is perfectly suited for long-term operation of the LEDs without any indication of degradation effects. The presented technology shows strong potential for future low energy consumption optoelectronics

Growth and properties of GaN and AlN layers on silver substrates

We report on the preparation and properties of GaN and AlN layers grown by molecular-beam epitaxy on silver metal substrates. X-ray diffraction rocking curves show polycrystalline character of GaN with high preferential GaN 11-22 orientation. An intermetallic phase of Ga3Ag is found at the GaN/Ag interface. On the other hand, AlN layers exhibit a monocrystalline structure with a growth direction of 0002 . Schottky diodes prepared on GaN layers show good rectifying behavior and relatively low leakage current 10−3 A/cm2 . These results indicate that the III-nitride growth on metallic substrates might be used for low-cost and large-area electronic and photonic devices

Non-uniform distribution of induced strain in gate-recessed AlGaN/GaN structure evaluated by micro-PL measurements

Micro-photoluminescence (μ-PL) studies were performed on AlGaN/GaN heterostructure field effect transistors (HFETs) with different gate-recessing depths. It was found that μ-PL is the method of choice for detecting dry etching damage and simultaneously recording strain and stress in the HFET GaN layer. Lateral sub-μm resolved mapping shows that the strain in the GaN layer after recessing is partially relaxed and non-uniform

Novel Double-Level-T-Gate Technology

We developed a novel double-level-T-gate technology based on wet etching of a metal gate interlayer. With the help of this technological process we prepared T-gate feet with widths as small as 200 nm. The major advantage of our process is its use of only standard optical lithography. It allows the fabrication of 100 nanometer size T-gates for transistors. High electron mobility transistors (HEMTs) were fabricated on an AlGaN/GaN/sapphire material structure with an original gate length Lg of 2 μm. Their cutoff frequency of 6 GHz was improved to 60 GHz by etching the gate to a 200 nm length double T-gate contact