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 0.1–3 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 electron–hole 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⁺ ions on a rotatable and heatable stage. Irradiation at elevated temperatures above 300 °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 Nanorod–ZnO Nanowire Hybrid Device for Near-Infrared Photodetection

In this Letter, we present a new class of near-infrared photodetectors comprising Au nanorods–ZnO nanowire hybrid systems. Fabricated hybrid FET devices showed a large photoresponse under radiation wavelengths between 650 and 850 nm, accompanied by an “ultrafast” 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 metal–semiconductor 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 Nb₂O₅/NbO_<i>x</i> Films

Resistive switching devices with a Nb₂O₅/NbO_<i>x</i> bilayer stack combine threshold and memory switching. Here we present a new fabrication method to form such devices. Amorphous Nb₂O₅ layers were treated by a krypton irradiation. Two effects are found to turn the oxide partly into a metallic NbO_<i>x</i> 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 p–n Junctions

Axial GaAs nanowire p–n diodes, possibly one of the core elements of future nanowire solar cells and light emitters, were grown via the Au-assisted vapor–liquid–solid 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 p–n 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 p–n junction, which is in essential agreement with the model of delayed dopant incorporation in the Au-assisted vapor–liquid–solid 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⁺-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 donor–acceptor 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 electron–hole 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 II–VI 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