Mohd Ambri Mohamed

Synthesis-condition dependence of carbon nanotube growth by alcohol catalytic chemical vapor deposition method

Abstract We report the dependence of growth yield of single-walled carbon nanotubes (SWNTs) on heat-treatment time and catalyst film thickness by the alcohol catalytic chemical vapor deposition method. Three types of heat-treatment, synthesis of 30 min, synthesis of 30 min after annealing of 30 min, and synthesis of 60 min, were investigated. Thickness of Co catalyst film was varied from 1 to 10 nm. In the case of thinner Co film less than 3 nm, long synthesis time of 60 min is favorable for the effective SWNT growth, because of the small amount of Co catalyst. In the case of thicker Co film more than 3 nm, an amount of grown SWNTs by 30 min synthesis after 30 min annealing and by 60 min synthesis was much higher than that by 30 min synthesis without annealing, showing that total heat-treatment time of 60 min is important for the SWNT growth. Results suggest that the conversion from the thicker film of Co to nano-particle which acts as catalyst takes place during the first 30 min.

Fabrication of spintronics device by direct synthesis of single-walled carbon nanotubes from ferromagnetic electrodes

Abstract We describe an alternative method for realizing a carbon nanotube spin field-effect transistor device by the direct synthesis of single-walled carbon nanotubes (SWNTs) on substrates by alcohol catalytic chemical vapor deposition. We observed hysteretic magnetoresistance (MR) at low temperatures due to spin-dependent transport. In these devices, the maximum ratio in resistance variation of MR was found to be 1.8%.

Metamaterial absorber comprising chromium–gold nanorods-based columnar thin films

Abstract. The absorption characteristics of complex medium structures having metasurfaces comprised of columnar nanorods of gold were investigated. In this stream, a periodically arranged assembly of vertical gold nanorods of circular and elliptical cross sections, backed by chromium nanorods of the same cross-sectional size and shape, was considered to be the metasurface, and the comparative features of the absorption characteristics were emphasized. The results exhibit very high absorption corresponding to certain wavelengths in the visible span, and the absorber having elliptical gold nanorods yields a better performance than the one with circular nanorods in terms of the magnitude/smoothness of the absorption peaks.

Towards high performance graphene nanoribbon transistors (GNR-FETs)

We fabricated trilayer graphene nanoribbon (GNR) Field Effect Transistors (FETs) by means of mechanical exfoliation method and investigate the characteristics of the device. The device shows ambipolar operation with a good Ohmic contact between the graphene and electrodes. Arrhenius plots of temperature dependence of conductance are consistence with thermal activated conduction above 273 K, and variable range hopping conduction at low temperature. GNR charge carrier provides high electron mobility of 6198 cm2V-1s-1 at 273 K, and lower activation energy of 0.592 meV. This device possesses high performance and requires ultra-low power compare to other reported graphene-based transistors.

Thermal Degradation of Single-Walled Carbon Nanotubes during Alcohol Catalytic Chemical Vapor Deposition Process

We have grown single-walled carbon nanotubes (SWCNTs) from Co catalyst thin films using the alcohol catalytic chemical vapor deposition (CVD) method for different CVD processing times. The structural properties of the SWCNTs grown have been investigated by Raman spectroscopy with various laser excitations. The growth progressed up to t ¼ 20 min, but the Raman intensity of SWCNTs decreased with increasing CVD processing time. The quality of as-grown SWCNTs also decreased with increasing CVD processing times, similar to the trend for Raman intensity. Raman intensity analysis in the radial breathing mode region shows a relatively wide distribution for SWCNTs grown for all CVD processing times, owing to the variations in growth and burning rates with SWCNT diameter. In this paper, we suggest that Co catalyst poisoning and SWCNT burning occur when the CVD processing time is more than 20 min. # 2010 The Japan Society of Applied Physics

Device characteristics of carbon nanotube transistor fabricated by direct growth method

We have fabricated carbon nanotube (CNT) field-effect transistors (FETs) by direct growth of single-wall CNTs between the source and drain electrodes, and investigated their device characteristics. The FETs show ambipolar operation. The temperature and bias voltage dependence of device characteristics are consistent with device operation of the Schottky-type FET. The carrier injection barrier heights for both the electron and hole carriers show small values of 17–74meV, without any additional specific treatment after device fabrication.

Direct exchange interaction of localized spins associated with unpaired sp electrons in Be-doped low-temperature-grown GaAs layers

Beryllium-doped GaAs layers grown at low temperatures by molecular-beam epitaxy contain localized spins associated with unpaired sp electrons of AsGa+ ions. Interactions of these localized spins are investigated by measuring the magnetization with a superconducting quantum interference device and the peak-to-peak width of electron paramagnetic resonance (EPR) spectra for samples with different spin concentrations ranging from 3 × 1018 to 2.0 × 1019 cm−3. The results show that localized spins in this material antiferromagnetically interact on each other via direct exchange. From the analysis of the temperature dependence and field dependence of the magnetization on the basis of the Curie–Weiss law and the molecular-field approximation, exchange energy of each sample was derived. The dependence of the exchange energy on the concentration of localized spins is reasonably explained by a model of direct exchange, which results from the overlapping of wave functions of unpaired electrons at AsGa+ ions. The peak...

Cooperative transition of electronic states of antisite As defects in Be-doped low-temperature-grown GaAs layers

Magnetic properties resulting from localized spins associated with antisite arsenic ions AsGa+ in Be-doped low-temperature-grown GaAs (LT-GaAs) layers were studied by measuring the magnetization of lift-off samples. With fast cooling, the magnetization of samples at 1.8 K becomes significantly lower than that expected from Curie-type paramagnetism in the range of the applied field to 7 T, and a transition from low magnetization to the magnetization of paramagnetism occurs upon the heating of samples to 4.5 K. With slow cooling, on the other hand, samples have a paramagnetic temperature dependence throughout the measurement-temperature range. The magnetization was found to decrease monotonically when a sample was kept at a fixed low temperature. These observations are explained by the cooperative transition of electron states of AsGa defects, which is closely related to the normal-metastable state transition of EL2 defects in semi-insulating GaAs. The results of the magnetization measurements in the presen...

Fabrication and characterization of carbon nanotube field-effect transistors using ferromagnetic electrodes with different coercivities

We have succeeded in fabricating source and drain structures of carbon nanotube field-effect transistors (FETs) using ferromagnetic electrodes with different coercive fields. The electrodes were successfully bridged with single-walled carbon nanotubes (SWNTs) by a direct growth method. We investigated the magnetic properties of electrodes and FET characteristics. The magnetic properties of the electrodes survived the chemical vapor deposition process at up to 800 °C, and were found to be qualitatively preserved even at growth times of 20 and 30 min. In addition, the devices showed good field-effect modulation in conductivity. This device structure could be applied to carbon nanotube spintronics devices fabricated by a direct growth method.

FDTD analysis of structured metallic nanohole films for LSPR-based biosensor

We numerically investigated 3-dimensional (3D) sub-wavelength structured metallic nanohole films with various thicknesses using wavelength interrogation technique. The reflectivity and full-width at half maximum (FWHM) of the localized surface plasmon resonance (LSPR) spectra was calculated using finite-difference time domain (FDTD) method. Results showed that a 100nm-thick silver nanohole gave higher reflectivity of 92% at the resonance wavelength of 644nm. Silver, copper and aluminum structured thin films showed only a small difference in the reflectivity spectra for various metallic film thicknesses whereas gold thin films showed a reflectivity decrease as the film thickness was increased. However, all four types of metallic nanohole films exhibited increment in FWHM (broader curve) and the resonance wavelength was red-shifted as the film thicknesses were decreased.