14 research outputs found
Excitation Energy Dependent Ultrafast Luminescence Behavior of CdS Nanostructures
Selected
semiconductor nanostructures provide extremely localized
coherent light sources. Here an ensemble of CdS nanostructures was
excited by UV/vis femtosecond laser pulses and their ultrafast luminescence
characteristics were investigated as functions of the pulse energy
fluence and the photon quantum energy. All optical Kerr gating enabled
studies of the emission dynamics with a time resolution of 150 fs
avoiding any influence on the CdS emission. The initially observed
emission built up after a delay of 0.1ā3 ps and decayed rapidly
in a biexponential way, strongly dependent on both the laser energy
fluence and the quantum energy. The central wavelength of the emission
spectrum revealed a significant blue-shift within the first few ps
followed by a transient red-shift relative to spontaneous excitonic
emission of CdS. All findings are mainly attributed to stimulated
radiative carrier recombination in the laser excited electronāhole
plasma after its thermalization with the CdS lattice
Anomalous Plastic Deformation and Sputtering of Ion Irradiated Silicon Nanowires
Silicon nanowires of various diameters
were irradiated with 100 keV and 300 keV Ar<sup>+</sup> ions on a
rotatable and heatable stage. Irradiation at elevated temperatures
above 300 Ā°C retains the geometry of the nanostructure and sputtering
can be gauged accurately. The diameter dependence of the sputtering
shows a maximum if the ion range matches the nanowire diameter, which
is in good agreement with Monte Carlo simulations based on binary
collisions. Nanowires irradiated at room temperature, however, amorphize
and deform plastically. So far, plastic deformation has not been observed
in bulk silicon at such low ion energies. The magnitude and direction
of the deformation is independent of the ion-beam direction and cannot
be explained with mass-transport in a binary collision cascade but
only by collective movement of atoms in the collision cascade with
the given boundary conditions of a high surface to volume ratio
Hot-Electron Injection in Au NanorodāZnO Nanowire Hybrid Device for Near-Infrared Photodetection
In this Letter, we present a new
class of near-infrared photodetectors
comprising Au nanorodsāZnO nanowire hybrid systems. Fabricated
hybrid FET devices showed a large photoresponse under radiation wavelengths
between 650 and 850 nm, accompanied by an āultrafastā
transient with a time scale of 250 ms, more than 1 order of magnitude
faster than the ZnO response under radiation above band gap. The generated
photocurrent is ascribed to plasmonic-mediated generation of hot electrons
at the metalāsemiconductor Schottky barrier. In the presented
architecture, Au-nanorod-localized surface plasmons were used as active
elements for generating and injecting hot electrons into the wide
band gap ZnO nanowire, functioning as a passive component for charge
collection. A detailed investigation of the hot electron generation
and injection processes is discussed to explain the improved and extended
performance of the hybrid device. The quantum efficiency measured
at 650 nm was calculated to be approximately 3%, more than 30 times
larger than values reported for equivalent metal/semiconductor planar
photodetectors. The presented work is extremely promising for further
development of novel miniaturized, tunable photodetectors and for
highly efficient plasmonic energy conversion devices
Local Ion Irradiation-Induced Resistive Threshold and Memory Switching in Nb<sub>2</sub>O<sub>5</sub>/NbO<sub><i>x</i></sub> Films
Resistive switching devices with a Nb<sub>2</sub>O<sub>5</sub>/NbO<sub><i>x</i></sub> bilayer stack combine threshold and memory
switching. Here we present a new fabrication method to form such devices.
Amorphous Nb<sub>2</sub>O<sub>5</sub> layers were treated by a krypton
irradiation. Two effects are found to turn the oxide partly into a
metallic NbO<sub><i>x</i></sub> layer: preferential sputtering
and interface mixing. Both effects take place at different locations
in the material stack of the device; preferential sputtering affects
the surface, while interface mixing appears at the bottom electrode.
To separate both effects, devices were irradiated at different energies
(4, 10, and 35 keV). Structural changes caused by ion irradiation
are studied in detail. After successful electroforming, the devices
exhibit the desired threshold switching. In addition, the choice of
the current compliance defines whether a memory effect adds to the
device. Findings from electrical characterization disclose a model
of the layer modification during irradiation
Direct Determination of Minority Carrier Diffusion Lengths at Axial GaAs Nanowire pān Junctions
Axial GaAs nanowire pān diodes, possibly one of
the core elements of future nanowire solar cells and light emitters,
were grown via the Au-assisted vaporāliquidāsolid mode,
contacted by electron beam lithography, and investigated using electron
beam induced current measurements. The minority carrier diffusion
lengths and dynamics of both, electrons and holes, were determined
directly at the vicinity of the pān junction. The generated
photocurrent shows an exponential decay on both sides of the junction
and the extracted diffusion lengths are about 1 order of magnitude
lower compared to bulk material due to surface recombination. Moreover,
the observed strong diameter-dependence is well in line with the surface-to-volume
ratio of semiconductor nanowires. Estimating the surface recombination
velocities clearly indicates a nonabrupt pān junction, which
is in essential agreement with the model of delayed dopant incorporation
in the Au-assisted vaporāliquidāsolid mechanism. Surface
passivation using ammonium sulfide effectively reduces the surface
recombination and thus leads to higher minority carrier diffusion
lengths
Mode Switching and Filtering in Nanowire Lasers
Coherent
light sources confining the light below the vacuum wavelength barrier
will drive future concepts of nanosensing, nanospectroscopy, and photonic
circuits. Here, we directly image the angular emission of such a light
source based on single semiconductor nanowire lasers. It is confirmed
that the lasing switches from the fundamental mode in a thin ZnO nanowire
to an admixture of several transverse modes in thicker nanowires approximately
at the multimode cutoff. The mode competition with higher order modes
substantially slows down the laser dynamics. We show that efficient
photonic mode filtering in tapered nanowires selects the desired fundamental
mode for lasing with improved performance including power, efficiency,
and directionality important for an optimal coupling between adjacent
nanophotonic waveguides
Hopping Conduction in Mn Ion-Implanted GaAs Nanowires
We report on temperature-dependent charge transport in
heavily
doped Mn<sup>+</sup>-implanted GaAs nanowires. The results clearly
demonstrate that the transport is governed by temperature-dependent
hopping processes, with a crossover between nearest neighbor hopping
and Mott variable range hopping at about 180 K. From detailed analysis,
we have extracted characteristic hopping energies and corresponding
hopping lengths. At low temperatures, a strongly nonlinear conductivity
is observed which reflects a modified hopping process driven by the
high electric field at large bias
Dynamical Tuning of Nanowire Lasing Spectra
Realizing
visionary concepts of integrated photonic circuits, nanospectroscopy,
and nanosensing will tremendously benefit from dynamically tunable
coherent light sources with lateral dimensions on the subwavelength
scale. Therefore, we demonstrate an individual nanowire laser based
device which can be gradually tuned by reversible length changes of
the nanowire such that uniaxial tensile stress is applied to the respective
semiconductor gain material. By straining the device, the spontaneous
excitonic emission of the nanowire shifts to lower energies caused
by the bandgap reduction of the semiconductor. Moreover, the optical
gain spectrum of the nanolaser can be precisely strain-tuned in the
high excitation regime. The tuning of the emission does not affect
the laser threshold of the device, which is very beneficial for practical
applications. The applied length change furthermore adjusts the laser
resonances inducing a redshift of the longitudinal modes. Thus, this
concept of gradually and dynamically tunable nanolasers enables controlling
and modulating the coherent emission on the nanoscale without changing
macroscopic ambient conditions. This concept holds therefore huge
impact on nanophotonic switches and photonic circuit technology
Continuous Wave Nanowire Lasing
Tin-doped cadmium sulfide nanowires
reveal donorāacceptor
pair transitions at low-temperature photoluminescence and furthermore
exhibit ideal resonator morphology appropriate for lasing at continuous
wave pumping. The continuous wave lasing mode is proven by the evolution
of the emitted power and spectrum with increasing pump intensity.
The high temperature stability up to 120 K at given pumping power
is determined by the decreasing optical gain necessary for lasing
in an electronāhole plasma
Amphoteric Nature of Sn in CdS Nanowires
High-quality CdS nanowires with uniform
Sn doping were synthesized
using a Sn-catalyzed chemical vapor deposition method. X-ray diffraction
and transmission electron microscopy demonstrate the single crystalline
wurtzite structure of the CdS/Sn nanowires. Both donor and acceptor
levels, which originate from the amphoteric nature of Sn in IIāVI
semiconductors, are identified using low-temperature microphotoluminescence.
This self-compensation effect was cross examined by gate modulation
and temperature-dependent electrical transport measurement. They show
an overall n-type behavior with relatively low carrier concentration
and low carrier mobilities. Moreover, two different donor levels due
to intrinsic and extrinsic doping could be distinguished. They agree
well with both the electrical and optical data