Search

The thermal expansion occurring when a laser beam is incident on the surface of a solid can be used to excite ultrasonic waves in the solid medium, without causing damage to the material. The resulting wavefield is characterized by a dominant shear wave and a weak compressional wave. This Letter demonstrates the possibility of generating a dominant compressional wavefield by coating the surface with a layer of clear ice. This is achieved by exploiting a minimum in the attenuation spectrum of light in ice, which occurs at around 500 nm and renders the coating transparent to green laser radiation. Ice coatings could, therefore, provide a path for the development of more sensitive laser-based nondestructive testing methods that have traditionally been affected by the poor excitability of compressional waves, especially along the direction orthogonal to the surface. [ABSTRACT FROM AUTHOR]

We calculate the electron excitation in cubic silicon carbide caused by the intense femtosecond laser double pulses using the time-dependent density functional theory (TDDFT). After the first pulse ends, excited electrons should be relaxed by collisional processes. Because TDDFT does not include scattering processes, thermalization is mimicked by following three assumptions. First, we assume no collisions and relaxation processes. Second, we assume the partially thermalized electronic state defined by two quasi-temperatures in the conduction and valence bands individually. Third, we assume the thermalized electron distribution, which is expressed by single electron temperature. Our results indicate that the plasma frequency (ωpl) formed by the first pulse is the key parameter in energy absorption in the second pulse. When the plasma frequency of the plasma formed by the first laser approaches the frequency of the laser, resonant excitation by the second pulse occurs. The lower electron temperature shows higher ωpl and higher efficient energy absorption because the effective mass of the electron becomes smaller. [ABSTRACT FROM AUTHOR]

We present a method to make Pt nanometer-spaced electrodes that are free of metallic particles and stable at ambient conditions. The nanogaps are fabricated using feedback-controlled electromigration to form few-atom contacts. When performing this procedure at elevated temperatures (>420 K), the Pt contacts undergo self-breaking so that nanometer separated electrode pairs are formed. Once cooled down to lower temperatures, the nanogaps stabilize and can be characterized in detail. We find that current-voltage characteristics can be well fitted to a Simmons model for tunneling and gap-size fluctuations at room temperature determined from these fits stay within 0.6 Å for at least 50 h. [ABSTRACT FROM AUTHOR]

Here, we report a drastically improved anticorrosive characteristic of magnesium alloy composites with the introduction of multiwalled carbon nanotubes. Highly depressed corrosion of nanotube-filled magnesium composite in salt water is due to the formation of stable oxide films along the grain boundaries of magnesium. Our results indicate that carbon nanotube acted as effective multifunctional filler to improve both mechanical and anticorrosive performances of magnesium alloy. [ABSTRACT FROM AUTHOR]

Single-electron tunneling through a quantum dot is detected by means of a radio-frequency single-electron transistor. Poisson statistics of single-electron tunneling events are observed from frequency domain measurements, and individual tunneling events are detected in the time-domain measurements. Counting tunneling events gives an accurate current measurement in the saturated current regime, where electrons tunnel into the dot only from one electrode and tunnel out of the dot only to the other electrode. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Field emission transmission electron microscopy has revealed the presence of atomic filaments extending from the open edge of a graphite cage formed in a glow-discharge plasma. The filaments are vapor grown, presenting complicated deformations such as curling, waving, and looping in the free space. The filaments correspond well to one carbon atom in diameter, strongly indicating that they are linear chains of carbon atoms, the presence of which was earlier suggested by Smalley et al. [Science 269, 1550 (1995)]. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the deposition of cobalt-platinum-carbon (CoPt-C) thin films with chemical composition of Co[sub 50]Pt[sub 15]C[sub 35]. Range of grain sizes in magnetic thin films; Comparison of the coercivity between Co-C and CoPt-C thin films on coercivity; Application of Co-C based materials for magnetic recording media.

Investigates the growth of III-V based magnetic(gallium manganese arsenide)/nonmagnetic(aluminum arsenide) semiconductor superlattices (SL) by low-temperature molecular beam epitaxy. Characterization of the SL; Evidence of quantum confinement effect; Formation of the subbands in the SL.

Examines the transient absorbance of doped poly(methyl methacrylate) with pyrene or biphenyl polymers. Calculation of excitation wavelength at laser ablation conditions; Changes in absorbance due to transient species and scattering by ablated fragments; Number of photons absorbed in doped polymer films.

Observes the phase transmission of C70 crystal by transmission electron microscopy at low temperature. Index of the spots appearing in the electron diffraction (ED); Measurement of the lattice constant a; Estimation of the ED of the low-temperature phase; Detection of an intermediate state in the temperature range.

A comparison of the microtexture and structure of the low temperature heat-treated polyparaphenylene (PPP)-based carbons which are promising for the Li ion battery has been performed using high-resolution transmission electron microscopy and electron energy loss spectroscopy. It is found that the PPP-based carbon prepared by Kovacic method remains in suitable microtexture for high Li storage over a wider temperature range relative to that obtained by Yamamoto method. This characteristic of the Kovacic PPP-based carbon could lead to a more controlled microtexture for obtaining a higher Li storage capacity for Li ion battery applications. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electrical properties of p--Si/n--SiC, p-Si/n--SiC, p+Si/n--SiC, and n+-Si/n--SiC heterojunctions fabricated by using surface-activated bonding are investigated. Their flat-band voltages obtained from capacitance-voltage (C-V) measurements are found to be ~0.92eV, which suggests that the Fermi level should be pinned at the bonding interface. An analysis by using the charge neutral level model reveals that the C-V characteristics are sensitive to the density of interface states. The measured C-V characteristics of p+Si/n--SiC and n+-Si/n--SiC junctions are in quantitative agreement with modeled ones obtained for the interface states density and the conduction-band discontinuity of 2.3×1013cm-2eV-1 and 0.3eV, respectively. [ABSTRACT FROM AUTHOR]

We demonstrated the effectiveness of deep-level photoluminescence (PL) mapping for nondestructive detection of dislocations and micropipes in high-resistivity 6H–SiC wafers. PL spectra of the wafers at room temperature were dominated by a broad band with a peak at 1.3 eV, which was traceable to the Si vacancy-related V1, V2, and V3 lines at 4.2 K. The intensity-mapping pattern agreed closely with the etch-pit pattern both on a wafer scale and on a microscopic scale. Large dark spots with one or two bright cores, small dark spots, and dark lines corresponded to micropipes, threading screw dislocations, and edge dislocations forming small angle grain boundaries, respectively. The intensity reduction around dislocations and micropipes was attributed to a decrease of the radiative centers for the 1.3 eV band, which occurred as a result either of the interaction between vacancies and dislocations or of the gettering effect of vacancy-related defects. [ABSTRACT FROM AUTHOR]

We investigate rf transport through an AlGaAs/GaAs single-electron transistor (SET). The presented rf-SET scheme provides a transmission coefficient proportional to the admittance of the device, which is desirable for impedance analysis as well as for high-sensitivity charge detection. The impedance of a SET, including the small tunneling capacitance, is successfully analyzed at the high frequency of 643 MHz, and is compared with a simple model. The ability to measure the impedance of a SET would expand the measurable regime of single-electron tunneling behavior. [ABSTRACT FROM AUTHOR]

Straight long carbon nanofibers with a large hollow core obtained by a floating reactant method show a stacking morphology of truncated conical graphene layers, which in turn exhibit a large portion of open edges on the outer surface and also in the inner channels. Through a judicious choice of oxidation conditions, nanofibers with increased active edge sites are obtained without disrupting the fiber's morphology. A graphitization process induces a morphological change from a tubular type to a reversing saw-toothed type and the formation of loops along the inner channel of the nanofibers, accompanied by a decrease in interlayer spacing. [ABSTRACT FROM AUTHOR]

Schottky photodiodes based on Au-ZnMgO/sapphire are demonstrated covering the spectral region from 3.35 to 3.48 eV, with UV/VIS rejection ratios up to ∼105 and responsivities as high as 185 A/W. Both the rejection ratio and the responsivity are shown to be largely enhanced by the presence of an internal gain mechanism, by which the compensated films become highly conductive as a result of illumination. This causes a large increase in the tunnel current through the Schottky barrier, yielding internal gains that are a function of the incident photon flux. [ABSTRACT FROM AUTHOR]

Pristine and molybdenum filled double walled carbon nanotubes (DWNTs) suspended in D2O show excellent ultrafast optical switching properties investigated through femtosecond Z-scan and degenerate pump-probe method using 50 fs pulses with central photon energy of 1.57 eV. For pristine-DWNT, the two photon absorption coefficient, β and nonlinear refraction coefficient, n2 are 4.9×10-8 cm/W, and 9.5×10-11 cm2/W, respectively, which yield one photon figure of merit, W=133 and two photon figure of merit, T=0.4. The degenerate pump-probe measurements show strong photoinduced bleaching with biexponential decay with time constants ∼150 and 600 fs. [ABSTRACT FROM AUTHOR]

A systematic analysis of the deep level spectrum in the lower half of the bandgap of Au–Zn1-xMgxO (0.056

The authors report a significant enhancement in the thermal conductivity of a conventional carbon fiber/phenolic resin composite system when adding highly crystalline multiwalled carbon nanotubes. They demonstrate that 7 wt % of carbon nanotubes dispersed homogeneously in a phenolic resin acted as an effective thermal bridge between adjacent carbon fibers and resulted in an enhancement of the thermal conductivity (e.g., from 250 to 393 W/m K). These results indicate that highly crystalline carbon nanotubes can be used as a multifunctional filler to enhance simultaneously the mechanical and thermal properties of the carbon fiber/phenolic resin composites. [ABSTRACT FROM AUTHOR]

Investigates the epitaxial buckminsterfullerene (C[sub 60]) films grown by molecular beam deposition in cadmium fluoride surfaces. Formation of stacked hexagonal layer overgrowth in C[sub 60]; Observation of structural phase transitions in fullerenes; Influence of surface steps on epitaxial growth.

Very different types of straight and long multiwalled carbon nanotubes, ranging from semi-crystalline to highly crystalline, are obtained using a floating reactant method, by merely changing the flow rate of the carrier gas. Graphitization behaviors of these nanotubes are studied comparatively in terms of their microstructure and morphology, showing a large change in structure for the semi-crystalline nanotubes, but little change in the highly crystalline nanotubes with heat treatment. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Arrays of aligned nanotubes of large diameter (100-250 nm) are synthesized by pyrolyzing a jet (spray) solution of Fe(C[sub 5]H[sub 5])[sub 2] and C[sub 6]H[sub 6] in an Ar atmosphere at relatively low temperatures (850 °C). The tubular structures consist of highly crystalline nested graphene cylinders (<200 concentric tubes) with tips that are usually open. Raman studies confirm the high degree of perfection of these "thick" structures. Tube diameter, degree of alignment, and crystallinity can be controlled by varying the Ar flow rate and the Fe:C ratio within the precursor solution. Based on these observations a possible growth mechanism is suggested. This low cost route for the synthesis of carbon nanotubes is advantageous due to the absence of H[sub 2] as a carrier gas and the low pyrolytic temperature. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]