Ngo Thi Thanh Tam

Comparison of field-electron emission from different carbon nanotube array structures

The effect of macroscopic cathode structures, which consisted of an array of the identical vertically aligned carbon nanotube (VACNT) columns in a particular arrangement, on the field emission was investigated. The electric field simulation based on the FEMLAB code showed characteristics of edge-induced U-shaped field distribution on CNT column. It was found that the degree of the field screening was dependent on VACNT-column array patterns, and that the overall field distribution depended on a number of VACNT columns at the periphery for the finite array size. Three different types of VACNT-column array cathodes were fabricated and tested for the field-electron emission: square, hexagonal, and triangular pattern arrays. All these VACNT-column array cathodes showed excellent electron-emission characteristics and a general trend consistent with field distribution simulations.

Synthesis of individual ultra-long carbon nanotubes and transfer to other substrates

In this article, we report the synthesis of ultra-long carbon nanotubes (CNTs) by thermal chemical vapour deposition method. Ultra-long, individual and aligned CNTs were directly grown on a flat silicon substrate. The orientation of the nanotubes was found parallel to the gas flow direction. The ultra-long CNTs were grown with different transition metallic salts, such as nickel chloride, iron (III) chloride, cobalt acetate and ruthenium acetate, as the catalysts. The influence of the growth conditions, such as growth temperature, reactive gas flow on the length and alignment of the CNTs was studied in detail. By using different catalysts, ultra-long single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs) were successfully grown. These ultra-long CNTs were transferred to other substrates by two methods. (1) The first method is to use polydimethylsiloxane as a stamp. (2) The second method is to use KOH as an etching agent. The diameter and length of the CNTs were characterised by transmission electron microscope, scanning electron microscope, atomic force microscope and Raman spectroscopy. The results indicate that the length of the CNTs can reach up to 4 mm. The diameter of the SWCNTs is in the range of 0.7–2.1 nm and the diameter of the MWCNTs is approximately 150 nm.

Study on hydrogen reactivity with surface chemical species of nanocrystalline porous silicon

Abstract The nanocrystalline porous silicon was prepared by the electrochemical etching of Si in a HF solution. A semi-transparent palladium layer was thermally deposited on its surface. Micro-Raman spectra were recorded in air and in the presence of hydrogen. Different SiHX stretching and wagging vibrational modes, a SiH stretching mode modified by the presence of three oxygen atoms in the Si backbonds (O3SiH unit) and a SiOSi vibration were readily revealed. Their behavior under laser irradiation and towards the hydrogen atoms dissociated from hydrogen molecules by palladium is reported.

Investigation of vibrational and photoluminescence spectra of nanocrystalline silicon by micro‐Raman spectroscopy using various laser powers

The Raman spectra of silicon nanocrystals embedded in silicon oxide and in porous silicon were measured at various laser powers. It was found that the Si–Si stretching Raman peak shifts to lower wavenumbers and broadens when the laser power increases. The effect is significant and reversible, i.e. the peak returns to its former position when the laser power is decreased to the initial level. It was found that this reversible phenomenon is caused by an increase in bond length due to the heating effect of the laser. In addition, the appearance of weak bands on both sides of the main Si–Si stretching mode peak at 155, 332, 620, 940, 2087, 2114 and 2145 cm−1 was observed at high laser power. It was found that the first four bands could be attributed to one- and multi-phonon Raman scattering and the last three bands are the expected SiH2, SiH and SiH3 stretching vibration modes. The photoluminescent spectrum was measured with the use of 1.959 eV illumination from an 11 mW helium–neon laser. It was found that the photoluminescence spectrum consists of a broad line centered at 1.79 eV with a full width at half-maximum of 0.21 eV and a tail located in an energy region higher than the laser incident excitation energy. Based on the analysis of Raman scattering spectra and the assumption that the luminescence spectrum arises from the ensemble of various sized particles distributed in the material, it is suggested that the later band of photoluminescence should be excited by the Raman scattering light in silicon nanocrystals. Copyright

Field-electron emission from flexible carbon nanotube array cathodes

The authors report two approaches to fabricate flexible cold cathodes having vertically aligned carbon nanotubes (VACNTs) as active electron-emitting material. In the first approach, VACNT arrays were removed from substrates, transferred to flexible metal foils or plastic films, and secured by conductive epoxy. In the other approach, polydimethylsiloxane (PDMS) was used as a supporting matrix to fabricate more sturdy freestanding flexible cathodes. Controlled infiltration of PDMS to keep the top surfaces of VACNT columns from being buried underneath PDMS was the key of this approach. Both of these methods allowed fabrication of flexible cold cathode, and preliminary test results of fabricated flexible cold cathodes showed good field-emission characteristics.

Flexible carbon nanotube-array cathodes: Fabrication and bending effect on field-electron emission

The authors report the fabrication of freestanding field-electron emitters based on arrays of vertically aligned carbon nanotubes (VACNTs) and flexible polydimethylsiloxane (PDMS). Transplant of VACNT arrays from silicon substrates to flexible PDMS platforms through a press-and-curing process resulted in PDMS-supported VACNT-array electron emitters. Test of field-electron emission from the PDMS-supported VACNT columns in a diode configuration showed good field-emission results regardless of cathode geometry, either planar or convex shape cathodes. Furthermore, the repeated bending of the PDMS-supported VACNT-column cathodes up to a few hundred times showed no noticeable degradation in field emission. Numerical simulations of electric field distribution at various bending angles and anode-cathode distance show that the general trend in emission-current variations is consistent with the difference in maximum electric field strength at the cathode surface.

Synthesis of vertically aligned carbon nanotubes and diamond films on Cu substrates for use in high-power electronic devices

Currently, most of the vertically aligned carbon nanotubes (VA-CNTs) and diamond films are mainly synthesised on flat silicon (Si) substrate. However, to achieve thermal dissipation in high-power electronic devices (HPEDs), the VA-CNTs and diamond films need to be attached to thermal dissipation metal substrates (like Cu, Ag, Al, etc.). In this paper, the fabrication process of the VA-CNTs and diamond films on Cu substrate is reported in detail. The VA-CNTs were synthesised by the thermal chemical vapour deposition (CVD) method. The VA-CNTs on Cu substrates were fabricated by two different methods: directly growing the VA-CNTs using thin catalytic metal layers such as Fe/Al or Cr/Al as a catalyst; transferring the VA-CNTs film that was pre-grown on Si substrate to Cu substrate. The diamond films were also directly grown on the Cu substrate by microwave plasma chemical vapour deposition (MPCVD). The grown VA-CNTs and diamond films were tested as the thermal dissipation media on a 0.5W InGaN LED chip. The VA-CNTs and diamond films greatly increased input current of the LED by more than 500 mA and 350 mA without reaching saturation. This is higher compared with that of the device packaged using normal commercial silver thermal paste. Initial experiment results on the LED demonstrated that the VA-CNTs and diamond films greatly improve the lights output power and that they are optimal choices for the thermal dissipation of HPED.

Thermal Dissipation Efficiency in a Micro‐Processor Using Carbon Nanotubes Based Composite

Modern electronic and optoelectronic devices such as μ‐processor, light emitting diode, semiconductor laser issued a challenge in the thermal dissipation problem. Finding an effective way for thermal dissipation therefore becomes a very important issue. It is known that carbon nanotubes (CNTs) is one of the most valuable materials with high thermal conductivity (2000 W/m.K compared to thermal conductivity of Ag 419 W/m.K). This suggested an approach in applying the CNTs as an essential component for thermal dissipation media to improve the performance of computer processor and other high power electronic devices. In this work multi walled carbon nanotubes (MWCNTs) based composites were utilized as the thermal dissipation media in a micro processor of a personal computer. The MWCNTs of different concentrations were added into polyaniline, commercial silicon thermal paste and commercial silver thermal paste by mechanical methods. A personal computer with configuration: Intel Pentium IV 3.066 GHz, 512 MB of ...

Flexible CNT-array cathodes: Effect of bending on field electron emission

Recently, flexible field-emission emitters based on carbon nanotubes (CNTs) materials attract some interests because of their perspective for applications. Those devices inherit both superior field-emission properties of CNTs and high flexibility of polymers such as PDMS (poly-dimethylsiloxane), PET (polyethylene-terephthalate), and EPDM (ethylene-propylene-diene rubber) [1–3]. In a general trend, we were successful to make flexible CNTs-based emitters using PDMS as a flexible platform. Field emission measurements revealed a high current density of 72 mA/cm2 (a respective net current of 0.57 mA) [4]. However, effect of bending on field emission characteristics is not paid attention much in the published literatures. In this work, we report results showing a correlation between the curvature of PDMS-supported CNT-tip arrays and their emission properties.

Fe nanodot system fabricated by non-lithographic method and its structural properties

In this work, we study the magnetic structure and morphology of the Fe nanodot system fabricated by the non-lithographic method, using anodic aluminum oxide (AAO) membrane as a template. By the two-steps aluminum anodization, the AAO patterns with the hexagonal pore arrangement have been achieved. Using AAO pattern as a template, under suitable conditions we successfully deposited the iron metal in the pores by the thermal vacuum evaporation. By the exposure of the deposited system from the bottom of the AAO membrane, the hexagonal ordered Fe nanodot system has been obtained. The morphologies of the nanodot system were imaged by the Atomic Force Microscopy (AFM) and Field Emission Scanning Microscopy (FESEM) methods. The magnetic structures were investigated by the Energy Dispersive X-Ray Fluorescence Spectroscopy (EDS) and Magnetic Force Microscopy (MFM) methods. Experimental results confirmed that the MFM image of the fabricated Fe nanodot system is similar to their AFM image.