148 research outputs found
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Laser ablation of graphite in different buffer gases
The KrF-laser ablation of graphite into 300 Torr of He, Ne, Ar, and Xe has been studied by fast imaging of the plasma emission and post-deposition analyses of collected film deposits. In each case, the soot which was redeposited on the irradiated rod following ablation was highly fullerene-deficient compared to the material collected on the sample disk 1.5 cm from the rod, as determined by laser desorption Fourier Transform Mass Spectrometry (FTMS) Investigation of the plasma plume propagation using fast ICCD photography reveals three main phases to the expansion: (1) forward motion, deceleration and stopping of the leading edge of the plume, (2) an apparent reflected shock within the plume which propagates backward and reflects from the rod surface, (3) coalescence of these two components, resulting in continued expansion and dissipation of the plasma and the appearance of glowing ultrafine particles. For the laser plume propagating in 300 Torr of Xe the characteristic time intervals for these three phases are 0-300 ns, 300-1000 ns, and 1-1000 {mu}s for phases (1), (2), and (3) respectively. The possible explanation of the observed difference in fullerene content is discussed on the basis of different plasma phases resulting in soot deposition on the rod and sample disk. The measurement of ro-vibrational spectra of electronically excited C{sub 2} has been performed. Rotational and vibrational temperatures, T{sub R} = 3000 {+-} 300K and T{sub V} = 6000 {+-} 500K have been obtained from the comparison of measured and calculated C{sub 2} -Swan band emission
Electronic Transport Imaging in a Multiwire SnO2 ChemFET Device
The electronic transport and the sensing performance of an individual SnO2
crossed nanowires device in a three-terminal field effect configuration were
investigated using a combination of macroscopic transport measurements and
Scanning Surface Potential Microscopy (SSPM). The structure of the device was
determined using both Scanning Electron- and Atomic Force Microscopy data. The
SSPM images of two crossed 1D nanostructures, simulating a prototypical
nanowire network sensors, exhibit large dc potential drops at the crossed-wire
junction and at the contacts, identifying them as the primary electroactive
elements in the circuit. The gas sensitivity of this device was comparable to
those of sensors formed by individual homogeneous nanostructures of similar
dimensions. Under ambient conditions, the DC transport measurements were found
to be strongly affected by field-induced surface charges on the nanostructure
and the gate oxide. These charges result in a memory effect in transport
measurements and charge dynamics which are visualized by SSPM. Finally,
scanning probe microscopy is used to measure the current-voltage
characteristics of individual active circuit elements, paving the way to a
detailed understanding of chemical functionality at the level of an individual
electroactive element in an individual nanowire.Comment: 30 pages, 8 figures, accepted to J. Appl. Phy
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Highly doped p-ZnTe films and quantum well structures grown by nonequilibrium pulsed laser ablation
Highly p-doped ZnTe films have been grown on semi-insulating GaAs (001) and unintentionally doped (p-type) GaSb (001) substrates by pulsed KrF (248 nm) excimer laser ablation of a ZnTe target through an N{sub 2} ambient, without the use of any assisting (DC or AC) plasma source. Free hole concentrations in the mid-10{sup 19} cm{sup {minus}3} to > 10{sup 20} cm{sup {minus}3} range have been obtained. This appears to be the first time that any wide band gap (E{sub g} {ge} 2 eV) II-VI compound (or other) semiconductor has been impurity-doped from the gas phase by pulsed-laser ablation (PLA). The maximum carrier concentrations also may be the highest obtained for ZnTe by any method thus far. Because pulsed laser deposition is inherently digital, attractive deposition rates can be combined with precise control of layer thickness in epitaxial multilayered structures. Typical deposition conditions are < 0.5 {angstrom} per laser pulse, with crystalline quality governed by tradeoffs between substrate temperature, pulse repetition rate, and the focused pulsed laser energy density. PLA`s capability for growth of very thin epitaxial layers is being exploited and studied through growth of doped heteroepitaxial quantum well structures in the nearly lattice-matched ZnTe/CdSe//GaSb(substrate) system. Results obtained from growth and characterization of heterostructures in this system will be presented
Growth of Highly Doped P-Type Znte Films by Pulsed Laser ablation in Molecular Nitrogen
Highly p-doped ZnTe films have been grown on semi-insulating GaAs (001) substrates by pulsed-laser ablation (PLA) of a stoichiometric ZnTe target in a high-purity N{sub 2} ambient without the use of any assisting (DC or AC) plasma source. Free hole concentrations in the mid-10{sup 19} cm{sup {minus}3} to > 10{sup 20} cm{sup {minus}3} range were obtained for a range of nitrogen pressures The maximum hole concentration equals the highest hole doping reported to date for any wide band gap II-VI compound. The highest hole mobilities were attained for nitrogen pressures of 50--100 mTorr ({approximately}6.5-13 Pa). Unlike recent experiments in which atomic nitrogen beams, extracted from RF and DC plasma sources, were used to produce p-type doping during molecular beam epitaxy deposition, spectroscopic measurements carried out during PLA of ZnTe in N{sub 2} do not reveal the presence of atomic nitrogen. This suggests that the high hole concentrations in laser ablated ZnTe are produced by a new and different mechanism, possibly energetic beam-induced reactions with excited molecular nitrogen adsorbed on the growing film surface, or transient formation of Zn-N complexes in the energetic ablation plume. This appears to be the first time that any wide band gap (Eg > 2 eV) II-VI compound (or other) semiconductor has been impurity-doped from the gas phase by laser ablation. In combination with the recent discovery that epitaxial ZnSe{sub l-x}S{sub x} films and heterostructures with continuously variable composition can be grown by ablation from a single target of fixed composition, these results appear to open the way to explore PLA growth and doping of compound semiconductors as a possible alternative to molecular beam epitaxy
Recent advances in pulsed-laser deposition of complex-oxides
Pulsed-laser deposition (PLD) is one of the most promising techniques for the
formation of complex-oxide heterostructures, superlattices, and well-controlled
interfaces. The first part of this paper presents a review of several useful
modifications of the process, including methods inspired by combinatorial
approaches. We then discuss detailed growth kinetics results, which illustrate
that 'true' layer-by-layer (LBL) growth can only be approached, but not fully
met, even though many characterization techniques reveal interfaces with
unexpected sharpness. Time-resolved surface x-ray diffraction measurements show
that crystallization and the majority of interlayer mass transport occur on
time scales that are comparable to those of the plume/substrate interaction,
providing direct experimental evidence that a growth regime exists in which
non-thermal processes dominate PLD. This understanding shows how kinetic growth
manipulation can bring PLD closer to ideal LBL than any other growth method
available today.Comment: 37 pages, 9 figures. Revie
Nanoscale Processing by Adaptive Laser Pulses
We theoretically demonstrate that atomically-precise ``nanoscale processing"
can be reproducibly performed by adaptive laser pulses. We present the new
approach on the controlled welding of crossed carbon nanotubes, giving various
metastable junctions of interest. Adaptive laser pulses could be also used in
preparation of other hybrid nanostructures.Comment: 4 pages, 4 Postscript figure
Nonlinear magnetic susceptibility and aging phenomena in reentrant ferromagnet: CuCoCl-FeCl graphite bi-intercalation compound
Linear and nonlinear dynamic properties of a reentrant ferromagnet
CuCoCl-FeCl graphite bi-intercalation compound are
studied using AC and DC magnetic susceptibility. This compound undergoes
successive phase transitions at the transition temperatures (= 16 K),
(= 9.7 K), and (= 3.5 K). The static and dynamic behaviors of
the reentrant spin glass phase below are characterized by those of
normal spin glass phase with critical exponent = 0.57 0.10, a
dynamic critical exponent = 8.5 1.8, and an exponent (= 1.55
0.13) for the de Almeida -Thouless line. A prominent nonlinear
susceptibility is observed between and and around ,
suggesting a chaotic nature of the ferromagnetic phase () and the helical spin ordered phase (). The
aging phenomena are observed both in the RSG and FM phases, with the same
qualitative features as in normal spin glasses. The aging of zero-field cooled
magnetization indicates a drastic change of relaxation mechanism below and
above . The time dependence of the absorption
is described by a power law form () in the
ferromagnetic phase, where at =
0.05 Hz and = 7 K. No -scaling law for
[] is observed.Comment: 14 pages, 16 figures, and 2 table
Nanofabrication with Pulsed Lasers
An overview of pulsed laser-assisted methods for nanofabrication, which are currently developed in our Institute (LP3), is presented. The methods compass a variety of possibilities for material nanostructuring offered by laser–matter interactions and imply either the nanostructuring of the laser-illuminated surface itself, as in cases of direct laser ablation or laser plasma-assisted treatment of semiconductors to form light-absorbing and light-emitting nano-architectures, as well as periodic nanoarrays, or laser-assisted production of nanoclusters and their controlled growth in gaseous or liquid medium to form nanostructured films or colloidal nanoparticles. Nanomaterials synthesized by laser-assisted methods have a variety of unique properties, not reproducible by any other route, and are of importance for photovoltaics, optoelectronics, biological sensing, imaging and therapeutics
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