Browsing by Author "Shi, SC"

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Comparison of the electronic structures of AIN nanotips grown on p- and n-type Si substrates
(2005-12-07)
Chiou, JW
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Tsai, HM
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Pao, CW
;
Dong, CL
;
Chang, CL
At and N K-edge x-ray absorption near-edge structure (XANES), scanning photoelectron microscopy (SPEM) and x-ray emission measurements were performed on AIN nanotips grown on p- and n-type Si substrates (p-AIN and n-AIN). Features and intensities in the Al and N K-edge XANES spectra of these AIN nanotips overall are similar. In contrast, the intensities of the valence-band SPEM spectra of p-AIN are apparently larger than those of n-AIN, which indicates that the valence-band density of states of p-AIN exceeds that of n-AIN. This result may be related to the observed enhancement of field-emission intensity of AIN nanotips grown on the p-type Si substrate.
Field emission properties of aligned carbon nanotubes grown on stainless steel using CH4/CO2 reactant gas
(2004-04-01)
Lin, CL
;
Chen, CF
;
Shi, SC
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Aligned carbon nanotubes (CNTs) were grown on stainless steel 304 by bias-enhanced microwave plasma chemical vapor deposition, using CH4/CO2 as the reactant gas. A bias was applied to the microwave plasma to grow the nanotubes over various periods. Experimental results show that well-aligned CNTs grow on stainless steel at a negative bias of - 300 V Energy dispersive spectrometer on TEM indicated that the metal catalyst on the top of the CNTs includes Fe and Ni, but not Cr. The field emission properties of the resultant CNTs were obtained at a negative bias of - 300 V: the emission current was 194 muA at 2.2 V/mum; and the turn-on voltage, which is the voltage needed to extract current density of 10 muA /cm(2), was 1.4 V/mum. (C) 2003 Elsevier B.V. All rights reserved.
Growth of chromium carbide capped-carbon nanotip using bias-assisted microwave plasma chemical vapor deposition
(2004-12-22)
Hsu, CH
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Shi, SC
;
Chen, CF
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Chromium carbide capped-carbon nanotips were synthesized using bias-assisted microwave plasma chemical vapor deposition. Such a material grew up to several hundreds of nanometer long and tens of nanometer in diameter. The applied bias voltage is a significant parameter in the growth process whereas the higher bias voltage is effective in increasing the growth rate. However, the higher bias voltage also contributes to a rapid formation of chromium carbide which leads to a shorter length of carbon nanotip at the same time. Higher ion energies also vary the tip diameter due to strong ion bombardment which is a competitor to the deposition process. Our investigations revealed that the growth of chromium carbide capped-carbon nanotips reaches a limit due to the full carburization of chromium. (C) 2004 Elsevier B.V. All rights reserved.
Growth of single-crystalline wurtzite aluminum nitride nanotips with a self-selective apex angle
(2005-05-01)
Shi, SC
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Chen, CF
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Chattopadhyay, S
;
Lan, ZH
;
Chen, KH
Single-crystal line, hexagonal aluminum nitride nanotips are fabricated using a vapor-transport and condensation process (VTCP) on silicon substrates with or without a catalyst layer. The resultant tips have very sharp nanoscale apexes (&SIM; 1 nm), while their bases and lengths are up to hundreds of nanometers wide and several micrometers long, respectively. It has been demonstrated that the thickness of the gold-catalyst layer plays a critical role in controlling the size of the tip; in addition, a catalyst-free growth mode has been observed, which results in lesser control over the nanotip morphology. Nevertheless, a remarkably narrow distribution of the apex angle of the nanotips, regardless of whether or not a catalyst was used in the VTCP, has been obtained. Compared with the commonly observed ridge and pyramid structures, the nanotips produced by VTCP have higher angles (&SIM; 81&DEG;) between the tilted (221) and the basal (001) planes that encase it. A mechanism for this self-selective apex angle in aluminum nitride nanotip growth is proposed.
Low-temperature synthesis multiwalled carbon nanotubes by microwave plasma chemical vapor deposition using CH4-CO2 gas mixture
(2003-02-01)
Chen, M
;
Chen, CM
;
Shi, SC
;
Chen, CF
;
材料科學與工程學系
Vertically aligned carbon nanotubes (CNTs) with multiwalled structures were successfully grown on a Fe-deposited Si substrate at low temperatures below 300 degreesC by microwave plasma chemical vapor deposition using of a CH4-CO2 gas mixture. The low temperature would be beneficial for reducing the diameter of CNTs but it will also decrease the growth rate on the substrate. The low-temperature growth of CNTs was achieved by decreasing both the microwave power and the total gas pressure. The constituents of the gas mixture are also key factors in chemical vapor deposition. After deposition, the microstructure morphology of CNTs was observed by using scanning electron microscope and high-resolution transmission microscope. The results showed that even at low temperature a high yield of carbon nanotubes with a high emission current density and a small diameter is obtained. Vertically aligned multiwalled CNTs with a diameter about 12 nm were investigated by scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM).
Molecular sensing with ultrafine silver crystals on hexagonal aluminum nitride nanorod templates
(2005-03-09)
Chattopadhyay, S
;
Shi, SC
;
Lan, ZH
;
Chen, CF
;
Chen, KH
Purification of multi-walled carbon nanotubes by microwave digestion method
(2004-04-01)
Chen, CM
;
Chen, M
;
Leu, FC
;
Hsu, SY
;
Wang, SC
Here developed is an efficient method for purification of multi-walled carbon nanotubes synthesized by electron cyclotron resonance chemical vapor deposition (ECRCVD). Carbon nanotubes may find their limited use for some applications as they contain a small fraction of metal catalysts in the tubes and tend to have defects along the graphene tube wall. Defects within the multi-walled carbon nanotubes would reduce electrical and structure properties. A two-step acidic treatment in microwave digestion system is used to dissolve metal catalysts. HNO3 or HCl can rapidly absorb microwave heat and energy and completely dissolve metal, which reside in carbon nanotubes without damage. The processing time of the two-step microwave-assisted and acid-treated time to dissolve metal in the MWCNTs is below 1 h. After purification, the amount of residual catalyst metals in samples is estimated by using the thermogravimetric analysis (TGA). The results show that a high-yield of multi-walled carbon nanotubes with approximately 5% of metal is obtained. (C) 2003 Elsevier B.V. All rights reserved.
Reduced temperature-quenching of photoluminescence from indium nitride nanotips grown by metalorganic chemical vapor deposition
(2005-11-14)
Shi, SC
;
Chen, CF
;
Hsu, GM
;
Hwang, JS
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Chattopadhyay, S
We report metalorganic chemical vapor deposition of indium nitride (InN) nanotips with apex angles of 10 degrees and length and base diameter of around 1 mu m 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. (C) 2005 American Institute of Physics.
Structural evolution of AlN nano-structures: Nanotips and nanorods
(2006-01-25)
Shi, SC
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Chattopadhyay, S
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Chen, CF
;
Chen, KH
;
Chen, LC
Aluminum nitride (AIN) nanostructures were prepared using thermal chemical vapour deposition process. At growth temperatures of 950 degrees C, AIN nanotips with apex diameters of 10 nm, base diameters of similar to 100 nm, and length of similar to 2000 nm were obtained. Whereas when the growth temperature was 1200 degrees C, we obtained shorter and ihicker AIN nanorods. Compelling microscopic evidences were obtained to show that stacked AIN platelets of diminishing size formed the building blocks for the nanotips. A reducing Ehrlich-Schwoebel barrier introduced into a diffusion mediated growth model explains the formation of AIN nanorods at increasing growth temperatures. (c) 2005 Elsevier B.V. All rights reserved.