A. C. Dillon

The first true inorganic fullerenes

Boron nitride and materials of composition MX2, where M is molybdenum or tungsten and X is sulphur or selenium, can form fullerene-like structures such as nested polyhedra or nanotubes. However, the analogy to the carbon fullerene family falls short because no small preferred structure akin to C60(ref. 5) has been found. We have discovered nano-octahedra of MoS2of discrete sizes in soots that we prepared by laser ablation of pressed MoS2targets. These nano-octahedra are much larger than C60structures, having edge lengths of about 4.0 and 5.0 nanometres, and may represent the first ‘inorganic fullerenes’.

Quantum rotation of hydrogen in single-wall carbon nanotubes

We report inelastic neutron scattering results on hydrogen adsorbed onto samples containing single-wall carbon nanotubes. These materials have attracted considerable interest recently due to reports of high density hydrogen storage at room temperature. Inelastic neutron scattering clearly shows the ortho‐para conversion of physisorbed hydrogen in a nanotube containing soot loaded with hydrogen. From the rotational Ja 0! 1 transition, no indication of a significant barrier to quantum rotation is seen. ” 2000 Elsevier Science B.V. All rights reserved.

Controlling single-wall nanotube diameters with variation in laser pulse power

Abstract We demonstrate that laser peak pulse power can be employed to tune carbon single wall nanotube (SWNT) diameters. The production of SWNTs was investigated at room temperature and at 1200°C. The diameters were shifted to smaller sizes in both cases as the pulse power was increased. SWNT size distributions and yields were studied with Raman spectroscopy and transmission electron microscopy. The evolution of the material quality with laser energy parameters offers insight in to SWNT formation mechanisms. These studies should aid in the development of methods for the rational control of SWNT growth.

Diethylsilane on silicon surfaces: Adsorption and decomposition kinetics

The adsorption and decomposition kinetics of diethylsilane (DES), (CH3CH2)2SiH2, on silicon surfaces were studied using laser‐induced thermal desorption (LITD), temperature programmed desorption, and Fourier transform infrared (FTIR) spectroscopic techniques. LITD measurements determined that the initial reactive sticking coefficient of DES on Si(111) 7×7 decreased versus surface temperature from S0≊1.7×10−3 at 200 K to S0≊4×10−5 at 440 K. The temperature‐dependent sticking coefficients suggested a precursor‐mediated adsorption mechanism. FTIR studies on high surface area porous silicon surfaces indicated that DES adsorbs dissociatively at 300 K and produces SiH and SiC2H5 surface species. Annealing studies also revealed that the hydrogen coverage on porous silicon increased as the SiC2H5 surface species decomposed. CH2=CH2 and H2 were the observed desorption products at 700 and 810 K, respectively, following DES adsorption on Si(111) 7×7. The ethylene desorption and growth of hydrogen coverage during eth...

Ammonia decomposition on silicon surfaces studied using transmission Fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) transmission spectroscopy was used to monitor the decomposition of NH3 and ND3 on silicon surfaces. Experiments were performed in situ in an ultrahigh vacuum (UHV) chamber using high surface area porous silicon samples. The FTIR spectra revealed that NH3 (ND3) dissociatively adsorbs at 300 K to form SiH (SiD) and SiNH2 (SiND2) surface species. A comparison of the vibrational absorbances for the SiNH2 and Si2NH surface species indicated that the Si2NH species could account for ≤8% of the surface coverage at 300 K. The infrared absorbances of the SiN–H2 (SiN–D2 ) scissors mode at 1534 cm−1 (1157 cm−1 ), the Si–H (Si–D) stretch at 2077 cm−1 (1510 cm−1 ) and the Si3 –N vibrational modes at 930 and 750 cm−1 were employed to monitor the decomposition of the SiNH2 (SiND2) surface species. As the silicon surface was annealed to 700 K, the FTIR spectra revealed that the SiNH2 (SiND2) surface species gradually decomposed to produce Si3N species and additional SiH species. Above 680...

Template synthesis of carbon nanotubes

Abstract The template synthesis and characterization of carbon nanotubes (CNTs) formed in porous alumina membranes (PAM) by the thermal chemical vapor decomposition (CVD) of propylene (Pr) gas are described. We found that the graphitic character of CNTs improved as CVD temperature was increased from 500 to 800 °C. Samples showed progressive increases in metallic appearance and layered tube wall structure and decreases in electrical resistance. No further enhancement of graphitization was observed among samples formed at 800, 900 and 1000 °C. X-ray diffraction (XRD) indicated that long-range order was absent in all CNTs tested. Localized crystalline domains of graphitic carbon, however, were detected by Roman spectroscopy and seen in light and dark field transmission electron microscopy (TEM) images. CNTs formed in the presence of catalytic Fe and Co particles at 600 and 700 °C with a Pr/N 2 flow rate of 94 sccm (standard cubic centimeters per minute) showed slightly lower electrical resistance than CNTs formed in control experiments. The catalytic effects of Fe and Co were observed for samples made at 800 °C with a 50 sccm Pr/N 2 flow rate as “nanotubes within nanotubes” were formed. No major differences were found between catalyst-containing and control samples formed at temperatures greater than or equal to 800 °C at 94 sccm.

Properties of thin film silicon nitride deposited by hot wire chemical vapor deposition using silane, ammonia, and hydrogen gas mixtures

The structure of thin film SiN, deposited by the hot wire chemical vapor deposition (HWCVD) technique using SiH4 and NH3 gas mixtures, is examined as H2 dilution is added to the gas flow mixture. For NH3/SiH4 gas flow ratios greater than 1/2, all films are a-SiN:H for H2/SiH4 gas dilution ratios as high as 20/1. While H2 dilution does not change the basic film structure, it does increase the efficiency of NH3 dissociation in the gas phase, and causes a further reduction in the already small amount of N-H bonding in a-SiN:H films deposited by HWCVD. Differences in local N bonding sites are observed when the nitrogen source gas is changed from NH3 to N2. For NH3/SiH4 gas ratios less than 1/2 and with high H2 dilution, deposition of μc-SiN by HWCVD is demonstrated. X-ray diffraction measurements show that the structure of these films consists of silicon crystallites embedded in an a-SiN:H matrix. An upper limit for N incorporation with the preservation of microcrystallinity is found, beyond which the films a...

Adsorption and decomposition of trichlorosilane and trichlorogermane on porous silicon and Si(100)2×1 surfaces

The adsorption and decomposition of trichlorosilane (SiHCl3) and trichlorogermane (GeHCl3) on silicon surfaces were studied using Fourier transform infrared (FTIR)spectroscopy and temperature programmed desorption investigations. The FTIR transmission spectroscopy experiments were performed in situ in an ultrahigh vacuum chamber on high surface area porous silicon samples. The FTIR spectra revealed that SiHCl3 dissociatively adsorbed at 200 K and formed SiH, SiCl x , ClSiH, and Cl2SiH surface species. The presence of Cl x SiH species indicated incomplete decomposition of SiHCl3 upon adsorption at 200 K. GeHCl3 also dissociatively adsorbed at 200 K and formed SiH and SiCl x species. The absence of an infrared absorption in the Ge–H stretching region suggests complete transfer of hydrogen from Ge to surface Si atoms at 200 K. The thermal stabilities of the surface species were then studied with annealing experiments. The Cl x SiH species deposited by SiHCl3 exposure at 200 K decomposed between 200 and 590 K and formed additional SiH and SiCl species. The SiCl x (x=2 or 3) species deposited by either GeHCl3 or SiHCl3 dissociative adsorption converted to silicon monochloride species between 200 and 600 K. The SiH surface species subsequently decreased at temperatures between 680 and 780 K as H2 desorbed from the silicon surface. FTIRspectroscopy was also utilized to monitor the adsorption of SiHCl3 and GeHCl3 on porous silicon at different surface temperatures. These adsorption studies were in agreement with the thermal annealing experiments. Temperature programmed desorption(TPD) studies were performed following saturation SiHCl3 and GeHCl3 exposures on Si(100)2×1. After SiHCl3adsorption, the only desorption species were H2, HCl, and SiCl2, and they were observed at 810, 850, and 970 K, respectively. Following GeHCl3adsorption, the TPD experiments monitored H2, HCl, SiCl2, and Ge at 810, 850, 950, and 1200 K, respectively. The adsorption kinetics of SiHCl3 and GeHCl3 were also measured on Si(100)2×1 and initial reactive sticking coefficients of S 0≊.019 for SiHCl3 and S 0≊1 for GeHCl3 were determined at 500 K. These experiments provide insight into the surface chemistry of chlorosilanes and chlorogermanes during Si, Ge, and Si1−x Ge x chemical vapor deposition on silicon surfaces.

Hot wire chemical vapor deposition of isolated carbon single-walled nanotubes

Hot wire chemical vapor deposition (HWCVD) has been employed for the continuous generation of carbon single-walled nanotube (SWNT) materials. Interestingly, transmission electron microscopy analyses revealed only the presence of isolated SWNTs, rather than nanotubes existing in bundles. An analysis of the growth mechanism explaining the production of isolated SWNTs is provided. Also, the Raman radial breathing modes (RBMs) of the isolated HWCVD-generated nanotubes are compared to the RBMs of small bundles of nanotubes deposited by a conventional CVD technique having a similar diameter distribution.

Hot-wire chemical vapor deposition of carbon nanotubes

Abstract Hot-wire chemical vapor deposition (HWCVD) has been employed for the continuous gas-phase generation of both carbon multi-wall and single-wall nanotube (MWNT and SWNT) materials. Graphitic MWNTs were produced at a very high density at a synthesis temperature of ∼600 °C. SWNTs were deposited at a much lower density on a glass substrate held at 450 °C. SWNTs are typically observed in large bundles that are stabilized by tube–tube van der Waals’ interactions. However, transmission electron microscopy analyses revealed only the presence of isolated SWNTs in these HWCVD-generated materials.