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15 Mar 2007

Volume 101, Issue 6, Articles (06xxxx)

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Skeletal Si–O–Si network connectivity of self-assembled porous silica for low-k dielectrics depending on organoalkoxysilane concentration in precursor solutions

Syozo Takada, Nobuhiro Hata, Kikuko Hayamizu, Miwa Murakami, Kenzo Deguchi, Shinobu Ohki, Masataka Tansho, and Tadashi Shimizu

J. Appl. Phys. 101, 064301 (2007); http://dx.doi.org/10.1063/1.2512648 (5 pages) | Cited 2 times

Online Publication Date: 16 March 2007

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Self-assembled porous silica films prepared from the tetraethoxysilane (TEOS)-based precursor solutions containing dimethyldiethoxysilane (DMDEOS) are known to have higher mechanical strength than the films prepared from TEOS precursor solution not containing DMDEOS. To clarify the origin of their higher mechanical strength, 29Si magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy was carried out for a series of self-assembled porous silica samples prepared by varying the DMDEOS concentration in the precursor solution. The MAS NMR spectra obtained were analyzed to deduce concentrations of the methyl and hydroxyl functional groups and Si–O–Si bonds, from which the connectivity number m, or the average coordination number, was calculated to assess the skeletal mechanical strength. A maximum of m appeared at the concentration of DMDEOS/(TEOS+DMDEOS) = 0.17. It is concluded that the enhancement of Si–O–Si network formation by the added DMDEOS into the TEOS-based precursor solution is the origin of the improvement of the skeletal mechanical strength.
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68.60.Bs Mechanical and acoustical properties
77.55.-g Dielectric thin films
76.60.-k Nuclear magnetic resonance and relaxation

Thermal conductivity improvement in carbon nanoparticle doped PAO oil: An experimental study

S. Shaikh, K. Lafdi, and R. Ponnappan

J. Appl. Phys. 101, 064302 (2007); http://dx.doi.org/10.1063/1.2710337 (7 pages) | Cited 28 times

Online Publication Date: 16 March 2007

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The present work involves a study on the thermal conductivity of nanoparticle-oil suspensions for three types of nanoparticles, namely, carbon nanotubes (CNTs), exfoliated graphite (EXG), and heat treated nanofibers (HTT) with PAO oil as the base fluid. To accomplish the above task, an experimental analysis is performed using a modern light flash technique (LFA 447) for measuring the thermal conductivity of the three types of nanofluids, for different loading of nanoparticles. The experimental results show a similar trend as observed in literature for nanofluids with a maximum enhancement of approximately 161% obtained for the CNT-PAO oil suspension. The overall percent enhancements for different volume fractions of the nanoparticles are highest for the CNT-based nanofluid, followed by the EXG and the HTT. The findings from this study for the three different types of carbon nanoparticles can have great potential in the field of thermal management.
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66.25.+g Thermal conduction in nonmetallic liquids
82.70.Kj Emulsions and suspensions

Artifacts identification in apertureless near-field optical microscopy

P. G. Gucciardi, G. Bachelier, M. Allegrini, J. Ahn, M. Hong, S. Chang, W. Jhe, S.-C. Hong, and S. H. Baek

J. Appl. Phys. 101, 064303 (2007); http://dx.doi.org/10.1063/1.2696066 (8 pages) | Cited 7 times

Online Publication Date: 20 March 2007

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The aim of this paper is to provide criteria for optical artifacts recognition in reflection-mode apertureless scanning near-field optical microscopy, implementing demodulation techniques at higher harmonics. We show that optical images acquired at different harmonics, although totally uncorrelated from the topography, can be entirely due to far-field artifacts. Such observations are interpreted by developing the dipole-dipole model for the detection scheme at higher harmonics. The model, confirmed by the experiment, predicts a lack of correlation between the topography and optical images even for structures a few tens of nanometers high, due to the rectification effect introduced by the lock-in amplifier used for signal demodulation. Analytical formulas deduced for the far-field background permit to simulate and identify all the different fictitious patterns to be expected from metallic nanowires or nanoparticles of a given shape. In particular, the background dependence on the tip-oscillation amplitude is put forward as the cause of the error-signal artifacts, suggesting, at the same time, specific fine-tuning configurations for background-free imaging. Finally a careful analysis of the phase signal is carried out. In particular, our model correctly interprets the steplike dependence observed experimentally of the background phase signal versus the tip-sample distance, and suggests to look for smooth variations of the phase signal for unambiguous near-field imaging assessment.
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07.79.Fc Near-field scanning optical microscopes
42.30.Sy Pattern recognition
42.30.Lr Modulation and optical transfer functions

Analysis of vertical alignment and bending of crystalline α-Fe2O3 nanowires using normal and grazing incidence x-ray diffraction intensities

U. P. Deshpande, T. Shripathi, D. Jain, A. V. Narlikar, S. K. Deshpande, and Y. Y. Fu

J. Appl. Phys. 101, 064304 (2007); http://dx.doi.org/10.1063/1.2435076 (9 pages) | Cited 1 time

Online Publication Date: 21 March 2007

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Owing to the increasing importance of orderly placed nanostructures, the following study was conducted on oriented and disoriented nanowires. Use of grazing incidence x-ray diffraction (GIXRD) and normal x-ray diffraction (XRD) as possible tools to assess their alignment is discussed. The property of crystalline nanowires to grow in a preferred direction over a substrate is made use of, in which deviation in diffraction intensities is measured on a group of oriented and disoriented nanowires. We find that the difference is sufficiently large to predict the spread in orientations away from a reference direction. The observed intensity variation is explained using a modeled view of diffraction geometry with different nanowire orientations. An alignment index Anw has been calculated using (110) and (300) peak intensities in XRD and GIXRD measurements. The values were found to be related to the extent of vertical alignment as observed using scanning electron microscopy (SEM). The difference in diffraction geometry and penetration depth between the two techniques is found to give complementary information on the upper and lower portions of the nanowires, respectively. Analysis of the diffraction patterns also shows that bending of these nanowires occurs preferentially about the c axis as compared to bending in other directions. In the present study samples with crystalline α-Fe2O3 nanowires grown on pure Fe substrate by controlled thermal oxidation route have been investigated using GIXRD, XRD, SEM and atomic force microscopy. Morphology of the nanowires is discussed briefly.
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61.46.Hk Nanocrystals
68.65.La Quantum wires (patterned in quantum wells)
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
68.37.Ps Atomic force microscopy (AFM)

Modeling and simulation of axisymmetric coating growth on nanofibers

K. Moore, C. B. Clemons, K. L. Kreider, and G. W. Young

J. Appl. Phys. 101, 064305 (2007); http://dx.doi.org/10.1063/1.2710442 (12 pages) | Cited 3 times

Online Publication Date: 21 March 2007

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This work is a modeling and simulation extension of an integrated experimental/modeling investigation of a procedure to coat nanofibers and core-clad nanostructures with thin film materials using plasma enhanced physical vapor deposition. In the experimental effort, electrospun polymer nanofibers are coated with metallic materials under different operating conditions to observe changes in the coating morphology. The modeling effort focuses on linking simple models at the reactor level, nanofiber level, and atomic level to form a comprehensive model. The comprehensive model leads to the definition of an evolution equation for the coating free surface. This equation was previously derived and solved under a single-valued assumption in a polar geometry to determine the coating morphology as a function of operating conditions. The present work considers the axisymmetric geometry and solves the evolution equation without the single-valued assumption and under less restrictive assumptions on the concentration field than the previous work.
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81.15.Jj Ion and electron beam-assisted deposition; ion plating
61.46.-w Structure of nanoscale materials
81.07.Bc Nanocrystalline materials
68.55.-a Thin film structure and morphology
68.55.A- Nucleation and growth
52.77.Dq Plasma-based ion implantation and deposition

Self-organization of gold nanoclusters on hexagonal SiC and SiO2 surfaces

F. Ruffino, A. Canino, M. G. Grimaldi, F. Giannazzo, C. Bongiorno, F. Roccaforte, and V. Raineri

J. Appl. Phys. 101, 064306 (2007); http://dx.doi.org/10.1063/1.2711151 (7 pages) | Cited 12 times

Online Publication Date: 22 March 2007

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Very thin Au layers were deposited on SiC hexagonal and SiO2 substrates by sputtering. The Au surface diffusion, clustering, and self-organization of Au nanoclusters on these substrates, induced by thermal processes, were investigated by Rutherford backscattering spectrometry, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. On both types of substrates, clustering is shown to be a ripening process of three-dimensional structures controlled by surface diffusion and the application of the ripening theory allowed us to derive the surface diffusion coefficient and all other parameters necessary to describe the entire process. The system Au nanoclusters/SiC and Au nanoclusters/SiO2 are proposed as nanostructured materials for nanoelectronic and nanotechnology applications.
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81.16.Dn Self-assembly
68.35.Fx Diffusion; interface formation
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
81.40.Gh Other heat and thermomechanical treatments

Purity assessment of multiwalled carbon nanotubes by Raman spectroscopy

Roberta A. DiLeo, Brian J. Landi, and Ryne P. Raffaelle

J. Appl. Phys. 101, 064307 (2007); http://dx.doi.org/10.1063/1.2712152 (5 pages) | Cited 53 times

Online Publication Date: 22 March 2007

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Carbonaceous purity assessment for chemical vapor deposition multiwalled carbon nanotubes (MWNTs) using Raman spectroscopy was investigated. Raman spectroscopy was performed on a reference sample set containing predetermined ratios of MWNTs and representative synthesis by-products. Changes in the characteristic Raman peak ratios (i.e., ID/IG, IG′/IG, and IG′/ID) as a function of MWNT content were measured. Calibration curves were generated from the reference samples and used to evaluate MWNTs synthesized under different conditions with varying purity. The efficacy of using Raman spectroscopy in conjunction with thermogravimetric analysis for quantitative MWNT purity assessment is discussed.
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61.46.Fg Nanotubes
78.67.Ch Nanotubes
78.30.Na Fullerenes and related materials
81.70.Pg Thermal analysis, differential thermal analysis (DTA), differential thermogravimetric analysis

Effects of pretreatment processes on improving the formation of ultrananocrystalline diamond

Li-Ju Chen, Nyan-Hwa Tai, Chi-Young Lee, and I-Nan. Lin

J. Appl. Phys. 101, 064308 (2007); http://dx.doi.org/10.1063/1.2434008 (6 pages) | Cited 10 times

Online Publication Date: 22 March 2007

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Effects of pretreatment on the nuclei formation of ultrananocrystalline diamond (UNCD) on Si substrates were studied. Either precoating a thin layer of titanium ( ∼ 400 nm) or ultrasonication pretreatment using diamond and titanium mixed powder (D&T process) enhances the nucleation process on Si substrates markedly, and the UNCD nuclei formed and fully covered the Si substrate, when deposition was processed using the microwave-plasma-enhanced chemical-vapor deposition process for 10 min. In contrast, during the same period, ultrasonication pretreatment using diamond powders (D process) can only form large UNCD clusters, which were scarcely distributed on Si substrates. The analyses using x-ray diffractometer, secondary ion mass spectroscopy, and electron spectroscopy for chemical analysis reveal that the titanium layer reacted with carbon species in the plasma, forming crystalline TiC phase, which facilitates the subsequent formation of UNCD nuclei. The beneficial effect of Ti layer on enhancing the nucleation of UNCD is presumably owing to high solubility and high diffusivity of carbon species in Ti materials, as compared with those of Si materials.
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81.07.Bc Nanocrystalline materials
68.55.A- Nucleation and growth
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.46.Hk Nanocrystals
66.30.Pa Diffusion in nanoscale solids
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
81.05.Cy Elemental semiconductors

Observation of localized charge transport in isolated microscopic mats of single-wall carbon nanotubes

Brian B. Glover and W. Lee Perry

J. Appl. Phys. 101, 064309 (2007); http://dx.doi.org/10.1063/1.2561850 (4 pages) | Cited 2 times

Online Publication Date: 22 March 2007

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We have formulated a composite consisting of a low dielectric permittivity, low loss granular organic crystal, and 0.1% single-walled carbon nanotubes (SWNTs). The composite morphology was electrically isolated “mats” of SWNTs distributed on the surface of individual crystals having a typical particle size of 20 μm. The real part of the composite permittivity (relative) was not significantly different from that of the matrix material ( ∼ 2.0), while the dielectric loss increased from 0.0035 to 0.3 at 2.45 GHz. We used Maxwell-Garnett mixing theory, which accounts for interfacial polarization, to calculate complex permittivity and conductivity of the SWNT mats. The results indicated weak electrical contact between SWNTs within the mats, implying that localized electron transport was the primary dielectric loss mechanism.
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73.63.-b Electronic transport in nanoscale materials and structures
61.46.Fg Nanotubes
77.22.Ch Permittivity (dielectric function)
77.22.Gm Dielectric loss and relaxation

A comparative investigation of thickness measurements of ultra-thin water films by scanning probe techniques

A. Opitz, M. Scherge, S. I.-U. Ahmed, and J. A. Schaefer

J. Appl. Phys. 101, 064310 (2007); http://dx.doi.org/10.1063/1.2712155 (5 pages) | Cited 5 times

Online Publication Date: 23 March 2007

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The reliable operation of micro- and nanomechanical devices necessitates a precise knowledge of the water film thickness present on the surfaces of these devices with accuracy in the nanometer range. In this work, the thickness of an ultra-thin water film was measured by distance tunneling spectroscopy and distance dynamic force spectroscopy during desorption in an ultra-high vacuum system, from about 2.5 nm up to complete desorption at 10−8 mbar. The tunneling current and the amplitude of vibration and the normal force were detected as a function of the probe-sample distance. In these experiments, a direct comparison of both methods was possible. It was determined that dynamic force spectroscopy provides the most accurate values. The previously reported tunneling spectroscopy, which requires the application of significantly high voltages generally leads to values that are 25 times higher than values determined by dynamic force spectroscopy.
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07.79.Cz Scanning tunneling microscopes
07.79.Lh Atomic force microscopes
06.30.Bp Spatial dimensions (e.g., position, lengths, volume, angles, and displacements)
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Ps Atomic force microscopy (AFM)
68.15.+e Liquid thin films

Subband gap impact ionization and excitation in carbon nanotube transistors

Jing Guo, Muhammad A. Alam, and Yijian Ouyang

J. Appl. Phys. 101, 064311 (2007); http://dx.doi.org/10.1063/1.2435821 (5 pages)

Online Publication Date: 23 March 2007

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Impact excitation (IE) and impact ionization (II) play important roles in carbon nanotube (CNT) optoelectronics and device reliability. The Boltzmann transport equation (BTE) in both the real and k spaces is solved to study subband gap II and IE in a CNT metal-oxide-semiconductor field-effect transistor (MOSFET). We show that even when the band bending is smaller than the CNT band gap, considerable II or IE can occur. The subband gap II rate varies exponentially with the applied drain voltage, but the current varies linearly due to a small amount of excess carriers. In contrast, solving the BTE by assuming a constant electric field indicates that both the II rate and current varies exponentially. Subband gap II or IE explains why considerable light emission was observed even when the potential drop is smaller in a recent experiment on bright CNT light emitters. The observed exponential variation of light intensity versus the linear variation of current alone, however, is not sufficient to distinguish between subband gap II and subband gap IE. The results also indicate that the bias condition for the maximum hot carrier degradation in CNT MOSFETs is the same as in Si MOSFETs.
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85.30.Tv Field effect devices
85.35.Kt Nanotube devices

Multiwalled carbon nanotubes for flow-induced voltage generation

Jianwei Liu, Liming Dai, and Jeff W. Baur

J. Appl. Phys. 101, 064312 (2007); http://dx.doi.org/10.1063/1.2710776 (6 pages) | Cited 9 times

Online Publication Date: 26 March 2007

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Recently it has been reported that voltage can be generated by passing fluids over single-walled carbon nanotube (SWCNT) arrays with potential application to flow sensors with a large dynamic range. The present work investigates voltage generation properties of multiwalled carbon nanotubes (MWCNTs) as a function of the relative orientation of the nanotube array with respect to the flow direction, flow velocity, and solution ionic strength. It was found that the flow-induced voltage can be significantly enhanced by aligning the nanotubes along the flow direction, increasing the flow velocity and/or the ionic strength of the flowing liquid. A flow-induced voltage of ∼ 30 mV has been generated from our perpendicularly-aligned MWCNT in an aqueous solution of 1M NaCl at a relatively low flow velocity of 0.0005 m/s, which is 15 times higher than the highest voltage reported for single-walled carbon nanotubes. The results are generally consistent with the pulsating asymmetric ratcheting mechanism proposed for SWCNT arrays, in which an asymmetrical spatial distributed strain forms from interactions with the polar and ionic species at the tube surface and is driven along the tube by the fluid flow.
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47.85.Np Fluidics
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.61.Fg Flows in micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS)
81.07.De Nanotubes
81.16.Rf Micro- and nanoscale pattern formation

Energy release characteristics of the nanoscale aluminum-tungsten oxide hydrate metastable intermolecular composite

W. Lee Perry, Bryce C. Tappan, Bettina L. Reardon, Victor E. Sanders, and Steven F. Son

J. Appl. Phys. 101, 064313 (2007); http://dx.doi.org/10.1063/1.2435797 (5 pages) | Cited 2 times

Online Publication Date: 26 March 2007

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Tungsten oxides are of interest as an oxidant for metals in metastable intermolecular composites (MICs), a reactive nanoscale powder useful for such applications as electric matches and gun primers. Smaller particles typically lead to fast reaction rates in this class of energetic material, and we have synthesized nanoscale WO3∙H2O using wet chemistry. Analysis by electron microscopy and small angle x-ray scattering revealed an approximately 100-nm-wide by 7-nm-thick platelet morphology. X-ray diffraction verified the orthorhombic structure and composition of the hydrate. A MIC material was formulated using 44 nm Al as the fuel. Performance was measured using a pressure cell where total enthalpy change and energy release rate was measured. This report includes the thermodynamic analysis of the pressure cell (calorimetry) that allows the determination of these metrics. Accuracy of the technique is discussed. Performance of the hydrate was found to significantly exceed that of MIC formulated with dehydrated tungsten oxide for one formulation, having an energy release of approximately 1.8 MJ/kg at a rate of approximately 215 GW/m2, compared to around 1.1 MJ/kg at a rate of around 130 GW/m2 for the dehydrated formulation. The data show that the enhanced behavior of the hydrated MIC formulation resulted from the reaction of aluminum with the interstitially bound water, which had additional energy release and generated hydrogen gas.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
68.37.-d Microscopy of surfaces, interfaces, and thin films
78.70.Ck X-ray scattering
65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems

Electrical conductivity of cluster-assembled carbon/titania nanocomposite films irradiated by highly focused vacuum ultraviolet photon beams

M. Amati, C. Lenardi, R. G. Agostino, T. Caruso, C. Ducati, S. La Rosa, G. Bongiorno, V. Cassina, P. Podestà, L. Ravagnan, P. Piseri, and P. Milani

J. Appl. Phys. 101, 064314 (2007); http://dx.doi.org/10.1063/1.2437658 (7 pages) | Cited 1 time

Online Publication Date: 26 March 2007

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We investigated the electrical transport properties of nanostructured carbon and carbon/titanium oxide nanocomposite films produced by supersonic cluster beam deposition and irradiated by highly focused vacuum UV photon beam. We have observed a relevant increase of the density of states at Fermi level, suggesting that the films acquire a “metallic” character. This is confirmed by the increment of the conductivity of four orders of magnitude for pure nanostructured carbon films and at least eight orders of magnitude for films containing 9 at. % of titanium. A partial reversibility of the process is observed by exposing the modified films to molecular oxygen or directly to air. We demonstrate the capability of writing micrometric conductive strips (2–3 μm width and 60 μm length) and controlling the variation of the conductivity as a function of the titanium concentration.
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73.61.-r Electrical properties of specific thin films
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
73.20.At Surface states, band structure, electron density of states

Photoexcited Fano interaction in laser-etched silicon nanostructures

Rajesh Kumar, H. S. Mavi, A. K. Shukla, and V. D. Vankar

J. Appl. Phys. 101, 064315 (2007); http://dx.doi.org/10.1063/1.2713367 (6 pages) | Cited 10 times

Online Publication Date: 27 March 2007

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Photoexcitation dependent Raman studies on the optical phonon mode in silicon nanostructures (Si NS) prepared by laser-induced etching are done here. The increase in the asymmetry of the Raman spectra on the increasing laser power density is attributed to Fano interference between discrete optical phonons and continuum of electronic excitations in the few nanometer size nanoparticles made by laser-induced etching. No such changes are observed for the same laser power density in the crystalline silicon sample or ion-implanted silicon sample followed by laser annealing. A broad photoluminescence spectrum from Si NS contains multiple peak behavior, which reveals the presence of continuum of electronic states in the Si NS.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.30.Am Elemental semiconductors and insulators
78.55.Ap Elemental semiconductors
81.65.Cf Surface cleaning, etching, patterning
61.72.Cc Kinetics of defect formation and annealing
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Hyperelastic behavior of single wall carbon nanotubes

Xianwu Ling and S. N. Atluri

J. Appl. Phys. 101, 064316 (2007); http://dx.doi.org/10.1063/1.2409646 (4 pages)

Online Publication Date: 28 March 2007

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Single wall carbon nanotubes (SWNTs) are shown to obey a hyperelastic constitutive model at moderate strains and temperatures. The finite temperature is considered via the local harmonic approach. The equilibrium configurations were obtained by minimizing the Helmholtz free energy of a representative atom in an atom-based cell model. While the concept of strain-dependent tangent modulus using linear elasticity was considered in prior literature, a constant μ for Ogden’s hyperelastic model [R. W. Ogden, Nonlinear Elastic Deformation (Horwood, England, 1984)] is found in the current work for large tubes subjected to moderately large strains up to 900 K.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Shortening multiwalled carbon nanotube on atomic force microscope tip: Experiments and two possible mechanisms

Dal-Hyun Kim, Jinho Choi, Yung Ho Kahng, Sang Jung Ahn, Byong Chon Park, Joon Lyou, and Ki Young Jung

J. Appl. Phys. 101, 064317 (2007); http://dx.doi.org/10.1063/1.2490489 (8 pages) | Cited 2 times

Online Publication Date: 28 March 2007

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We demonstrate a technique to cut a multiwalled carbon nanotube (MWNT) attached on an atomic force microscope tip by flowing direct current through the nanotube as a method to precisely control nanotube tip length. The cutting process consists of two steps: (1) making a mechanical contact between a sharp metal wire and a target cutting position on a MWNT attached to a silicon mother tip, and (2) applying voltage between the MWNT and the metal wire until the current flow cuts the tube. To cut the MWNT without discharging, a firm mechanical and electrical contact was made between the tube and the wire. Nanotubes were reproducibly cut at the contact point, and we achieved a 30 nm cutting resolution and a 100 nm shortest tube protrusion. We simultaneously monitored the current flow through the nanotube during the cutting process and evaluated the current size of each MWNT tip at the moment of cutting. We found that the cutting process happened in two current ranges: higher than 100 μA and smaller than 1 μA. The mechanism of cutting at high current cutting process is attributed to unimolecular decomposition reaction, and we propose that at small current the cutting process may be done by chemical etching, which is activated by the current.
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73.63.Fg Nanotubes
73.40.Cg Contact resistance, contact potential
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Studies on the room temperature growth of nanoanatase phase TiO2 thin films by pulsed dc magnetron with oxygen as sputter gas

A. Karuppasamy and A. Subrahmanyam

J. Appl. Phys. 101, 064318 (2007); http://dx.doi.org/10.1063/1.2714770 (7 pages) | Cited 22 times

Online Publication Date: 28 March 2007

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The anatase phase titanium dioxide (TiO2) thin films were deposited at room temperature by pulsed dc magnetron sputtering using pure oxygen as sputter gas. The structural, optical, electrical, and electrochromic properties of the films have been studied as a function of oxygen pressure in the chamber. The x-ray diffraction results indicate that the films grown above 4.5×10−2 mbar are nanocrystalline (grain size of 28–43 nm) with anatase phase. The films deposited at the chamber pressure of 7.2×10−2 mbar are found to be highly crystalline with a direct optical band gap of 3.40 eV, refractive index of 2.54 (at λ = 400 nm), and work function of 4.77 eV (determined by the Kelvin probe measurements). From the optical emission spectra of the plasma and transport of ions in matter calculations, we find that the crystallization of TiO2 at room temperature is due to the impingement of electrons and ions on the growing films. Particularly, the negative oxygen ions reflected from the target by “negative ion effects” and the enhanced density of TiO, TiO+, TiO2+, and O2+ particles in the plasma are found to improve the crystallization even at a relatively low temperature. From an application point of view, the film grown at 7.2×10−2 mbar was studied for its electrochromic properties by protonic intercalation. It showed good electrochromic behavior with an optical modulation of ∼ 45%, coloration efficiency of 14.7 cm2 C−1, and switching time (tc) of 50 s for a 2×2 cm2 device at λ = 633 nm.
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68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
81.15.Cd Deposition by sputtering
78.66.Li Other semiconductors
73.61.Le Other inorganic semiconductors
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Orbiting atoms and C60 fullerenes inside carbon nanotori

Tamsyn A. Hilder and James M. Hill

J. Appl. Phys. 101, 064319 (2007); http://dx.doi.org/10.1063/1.2511490 (13 pages) | Cited 17 times

Online Publication Date: 29 March 2007

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The discovery of carbon nanostructures, such as carbon nanotubes and C60 fullerenes, has generated considerable interest for potential nanoelectronic applications. One such device is the high frequency nanoscale gigahertz oscillator. Several studies investigating these oscillators demonstrate that sliding an inner-shell inside an outer-shell of a multiwalled carbon nanotube generates oscillatory frequencies in the gigahertz range. Research has shown that the oscillation is sensitive to the diameter and the helicity of the tube and that the inner tube length can be used to tune the frequency, such that the smaller the inner tube length the higher the frequency of oscillation, suggesting that a C60 fullerene might provide the ultimate core. Recently, researchers have observed single continuous toroidal nanotubes with no beginning or end, effectively a single-walled carbon nanotube closed around onto itself so that the two open ends fuse together, stabilized by van der Waals forces alone, to form a perfect “nanotorus.” The question arises as to whether it is possible to create a C60- nanotorus oscillator or orbiter, comprising a C60 fullerene orbiting around the inside of a nanotorus. The C60- nanotorus orbiter has yet to be constructed and the aim here is to assess its feasibility by examining the dominant mechanics of this potential nanoscale device. As in previous studies, the Lennard-Jones potential is used to calculate the interatomic forces acting on the fullerene due to the nonbonded interactions. Furthermore, other relevant forces are examined. Initially, we investigate the dynamics of an orbiting single atom followed by the corresponding analysis for an orbiting C60 fullerene. The equilibrium position depends on the radius of the nanotorus tube for both the atom and the C60 fullerene. Gravity is shown to be negligible, while the centrifugal forces are shown to move the orbiting body further from the center of the nanotorus. The theory also predicts that by changing the orbital position, the resulting frequencies, which are in the gigahertz range, may vary to as much as four times those obtained for the C60-nanotube oscillator.
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36.40.Sx Diffusion and dynamics of clusters
34.20.Cf Interatomic potentials and forces
34.20.Gj Intermolecular and atom-molecule potentials and forces

Characterization of the morphology and optical properties of InAs/AlAs quantum dots with a GaAs insertion layer

Cheong Hyun Roh, Hong Joo Song, Dong Ho Kim, Joon Soo Park, Yeon-Shik Choi, Hoon Kim, and Cheol-Koo Hahn

J. Appl. Phys. 101, 064320 (2007); http://dx.doi.org/10.1063/1.2714689 (7 pages) | Cited 4 times

Online Publication Date: 29 March 2007

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InAs self-assembled quantum dots by utilizing a thin GaAs insertion layer (IL) on a 1 nm thick AlAs seed layer were grown on GaAs(100) substrates by using a molecular beam epitaxy technique. InAs quantum dots (QDs) were formed by varying the thickness of the GaAs IL from 1 to 9 ML (monolayer), and their morphological and optical properties were characterized by atomic force microscopy and photoluminescence (PL). As a result, when the GaAs IL was thicker than 5 ML, normal InAs QDs with an average diameter of 30 nm and a density of 2×1010/cm2 were formed, because the enhanced surface roughness due to the AlAs layer was leveled by the GaAs IL. However, when the thickness of the GaAs IL was decreased from 5 to 3 ML, the formed InAs QDs showed a bimodal size distribution, i.e., large dots with a lateral size of about 30 nm and small dots with that of about 20 nm. When the GaAs IL was below 1 ML, InAs QDs with an average diameter of less than 15 nm and a high density of 1.5×1011/cm2 were grown. Consequently, it was verified that the thickness parameter of the GaAs IL had an effect on the size distribution of InAs QDs. Furthermore, although the AlAs layer was used for the purpose of improving the density of the QDs, their PL intensity was comparable to that of the normal InAs QDs.
Show PACS
81.07.Ta Quantum dots
81.16.Dn Self-assembly
61.46.-w Structure of nanoscale materials
78.67.Hc Quantum dots
78.55.Cr III-V semiconductors
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