Single Semiconductor Nanowires Research Articles

Geometric Symmetry Breaking and Nonlinearity Can Increase Thermoelectric Power.

Direct thermal-to-electric energy converters typically operate in the linear regime, where the ratio of actual maximum power relative to the ideal maximum power, the so-called fill factor (FF), is 0.25. Here, we show, based on fundamental symmetry considerations, that the leading order nonlinear terms that can increase the FF require devices with broken spatial symmetry. Studying nonlinear, thermoelectric transport across an asymmetric energy barrier defined in a single semiconductor nanowire, we find in both experiment and theory that we can increase the FF as well as maximum power. Geometric symmetry breaking combined with the design of nonlinear behavior thus represents a strategy for increasing the performance of thermoelectric or hot-carrier devices.

Enhancement of the magnetic anisotropy in single semiconductor nanowires via surface doping and adatom deposition

The magnetic anisotropy of single semiconductor (ZnO and GaN) nanowires incorporating both a transition metal (Co and Mn, respectively) as a substitutional surface dopant and a heavy metal (Au, Bi, or Pt) adatom is studied by performing density-functional supercell calculations with the Hubbard U correction. It is found that a substantial enhancement in the magnetic anisotropy energy is obtained through the deposition of Bi; the deposition of Au and Pt leads to significant variation in other magnetic properties, but not in the magnetic anisotropy energy. An analysis within a band description shows that the coexistence of Bi adatom and a surface dopant with large spin moment activates a mechanism involving reorientation and readjustment of the spin moments of electrons in occupied bands in response to the change of magnetization direction, which promotes giant magnetic anisotropy. Our results for adsorption energetics indicate that the accommodation of Bi in the neighborhood of the surface dopant is more likely in GaN nanowires, because the Bi adatom does (not) tend to be closer to the Mn (Co) dopant on the surface of GaN (ZnO) nanowire. The stability of GaN nanowire with giant magnetic anisotropy owing to the incorporation of both Mn and Bi is demonstrated by performing ab initio molecular dynamics simulations at temperatures considerably higher than room temperature. These results suggest that adatom deposition and surface doping can be used complementarily to develop single nanowire-based spintronic devices.

Easy-plane magnetic anisotropy of a single dopant in a single semiconductor nanowire

The magnetic anisotropy of an isolated substitutional Co impurity in a single ZnO nanowire is studied by performing density-functional supercell calculations with the Hubbard U correction. The variation of the magnetic anisotropy energy with the location of the Co atom in different regions of the nanowire is explored. Contrary to usual expectation, it is found that single Co-doped ZnO nanowire possess an easy plane (as opposed to easy axis) of magnetization, which is neither parallel nor perpendicular to the nanowire axis; the orientation of the easy plane is determined by the local geometry around the dopant. Our results also show that the magnetic anisotropy energy is enhanced by almost an order of magnitude when the Co impurity moves to a surface site from an interior site. This enhancement is elucidated on the basis of local bonding characteristics, because a pronounced variation of the orbital moment of Co with the magnetization direction is observed only when Co is located at the nanowire surface. The complementary analysis of these findings shows that the magnetic anisotropy of a single dopant in a single nonmagnetic semiconductor nanowire can be directionally manipulated and substantially enhanced by controlling the local chemical environment of the dopant in the absence of magnetic coupling. Our findings thus imply that the directional control of magnetic anisotropy needs to be achieved for applications of magnetic ion doped semiconductor nanowires.

High-Dimensional Acousto-optoelectric Correlation Spectroscopy Reveals Coupled Carrier Dynamics in Polytypic Nanowires

The authors combine acousto-optoelectric and multi-channel photon correlation spectroscopy to probe spatio-temporal carrier dynamics induced by a piezoelectric surface acoustic wave (SAW). The technique is implemented by combining phase-locked optical micro-photoluminescence spectroscopy and simultaneous three-channel time resolved detection. From the recorded time correlated single photon counting data the time transients of individual channels and the second and third order correlation functions are obtained with sub-nanosecond resolution. The method is validated by probing the correlations SAW-driven carrier dynamics between three decay channels of a single polytypic semiconductor nanowire on a conventional LiNbO$_\mathrm{3}$ SAW delay line chip. The method can be readily applied to other types of nanosystems and probe SAW-regulated charge state preparation in quantum dots, charge transfer processes in van der Waals heterostructures or other types of hybrid nanoarchitectures.

Single Nanowire Gas Sensor Able to Distinguish Fish and Meat and Evaluate Their Degree of Freshness

A non-invasive, small, and fast device is needed for food freshness monitoring, as current techniques do not meet these criteria. In this study, a resistive sensor composed of a single semiconductor nanowire was used at different temperatures, combining the responses and processing them with multivariate statistical analysis techniques. The sensor, very sensitive to ammonia and total volatile basic nitrogen, proved to be able to distinguish samples of fish (marble trout, Salmo trutta marmoratus) and meat (pork, Sus scrofa domesticus), both stored at room temperature and 4 °C in the refrigerator. Once separated, the fish and meat samples were classified by the degree of freshness/degradation with two different classifiers. The sensor classified the samples (trout and pork) correctly in 95.2% of cases. The degree of freshness was correctly assessed in 90.5% of cases. Considering only the errors with repercussions (when a fresh sample was evaluated as degraded, or a degraded sample was evaluated as edible) the accuracy increased to 95.2%. Considering the size (less than a square millimeter) and the speed (less than a minute), this type of sensor could be used to monitor food production and distribution chains.

Effective Separation of Photogenerated Electron–Hole Pairs by Radial Field Facilitates Ultrahigh Photoresponse in Single Semiconductor Nanowire Photodetectors

We report an investigation on the observation of ultrahigh photoresponse (photogain, G_Pc>106) in single nanowire photodetectors of diameter < 100 nm. The investigation which is a combination of experimental observations and a theoretical analysis of the ultrahigh optical response of semiconductor nanowires, has been carried out with emphasis on Ge nanowires. Semiconductor nanowire photodetectors show a signature of photogating where G_Pc rolls-off with increasing illumination intensity. We show that surface band bending due to depleted surface layers in nanowires induces a strong radial field (~ 108 V/m at the nanowire surface) which causes physical separation of photogenerated electron-hole pairs. This was established quantitatively through a self-consistent theoretical model based on coupled Schrodinger and Poisson Equations. It shows that carrier separation slows down the surface recombination velocity to a low value (< 1 cm/s) thus reducing the carrier recombination rate and extending the recombination lifetime by few orders of magnitude. An important outcome of the model is the prediction of G_Pc ~ 106 in a single Ge nanowire (with diameter 60 nm), which matches well with our experimental observation. The model also shows an inverse dependence of G_Pc on the diameter that has been observed experimentally. Though carried out in context of Ge nanowires, the physical model developed has general applicability in other semiconductor nanowires as well.

Beam damage of single semiconductor nanowires during X-ray nanobeam diffraction experiments.

Nanoprobe X-ray diffraction (nXRD) using focused synchrotron radiation is a powerful technique to study the structural properties of individual semiconductor nanowires. However, when performing the experiment under ambient conditions, the required high X-ray dose and prolonged exposure times can lead to radiation damage. To unveil the origin of radiation damage, a comparison is made of nXRD experiments carried out on individual semiconductor nanowires in their as-grown geometry both under ambient conditions and under He atmosphere at the microfocus station of the P08 beamline at the third-generation source PETRA III. Using an incident X-ray beam energy of 9 keV and photon flux of 1010 s-1, the axial lattice parameter and tilt of individual GaAs/In0.2Ga0.8As/GaAs core-shell nanowires were monitored by continuously recording reciprocal-space maps of the 111 Bragg reflection at a fixed spatial position over several hours. In addition, the emission properties of the (In,Ga)As quantum well, the atomic composition of the exposed nanowires and the nanowire morphology were studied by cathodoluminescence spectroscopy, energy-dispersive X-ray spectroscopy and scanning electron microscopy, respectively, both prior to and after nXRD exposure. Nanowires exposed under ambient conditions show severe optical and morphological damage, which was reduced for nanowires exposed under He atmosphere. The observed damage can be largely attributed to an oxidation process from X-ray-induced ozone reactions in air. Due to the lower heat-transfer coefficient compared with GaAs, this oxide shell limits the heat transfer through the nanowire side facets, which isconsidered as the main channel of heat dissipation for nanowires in the as-grown geometry.

Tailoring the longitudinal electric fields of high-order laser beams and their direct verification in three dimensions

We demonstrate the tailoring of longitudinal electric fields that arise from tight focusing of spatially phase-shaped high-order laser beams, and verify their spatial distribution in three dimensions. To tailor the three-dimensional (3D) spatial distribution of longitudinal electric fields, we precisely control the relative phase delay between the two lobes of a HG10laser beam using a spatial light modulator (SLM) before tight focusing. To verify the longitudinal electric fields in the focal volume, we employ 3D microscopic mapping of second-harmonic generation (SHG) from a single semiconductor nanowire that is extremely sensitive to the orientation of the longitudinal electric field. The technique is direct, general and expected to be useful in harnessing and in probing longitudinal electric fields with arbitrary spatial intensity distributions in the 3D focal volume.

Bottom-up Patterning of Semiconductor Nanostructures for Large-Area Electronics

We demonstrate a versatile and programmable route to mask, with nanoscale precision, the surface of semiconductor nanostructures in an entirely bottom-up fashion. This capability promises to dramatically increase the manufacturing rate of high performance (i.e., > GHz, < 1 V) semiconductor devices for large-area, flexible electronics. Our approach – Selective CoAxial Lithography via Etching of Surfaces (SCALES) – involves three main steps: (1) Bottom-up synthesis of single crystalline semiconductor nanowires with an axially-encoded composition profile, (2) blanket surface-initiated polymerization of a masking material, and (3) selective removal of the masking material only from regions whose underlying surface is susceptible to an etchant. We show, among other examples, the selective patterning of dopant-modulated Si nanowires with a polymethylmethacrylate (PMMA) mask by leveraging the dependence of KOH etch rate on doping level. We will describe the role of polymerization parameters and post-polymerization etching on the patterning process with a suite of spectroscopy and microscopy techniques. Nanoscale masks produced via the SCALES process can subsequently direct the deposition of the oxides and metals needed to construct complete, fully-functional devices.

Beam damage of single semiconductor nanowires during X-ray nano-beam diffraction experiments

Beam damage of single semiconductor nanowires during X-ray nano-beam diffraction experiments

Self-powered single semiconductor nanowire photodetector

Self-powered photodetectors have been fabricated from a single germanium nanowire (NW) in the metal–semiconductor–metal (MSM) device configuration. The self-powered devices show a high photoresponse (responsivity ∼ 103–105 A W−1) in the wavelength range 300–1100 nm. It has been established from I–V characteristics that asymmetry exists in the Schottky barrier height (SBH) at the two MS contacts. We have used simulation to establish that the asymmetric SBH at the metal contacts in an MSM device is a major cause for the ‘built-in’ axial field that leads to separation of a light generated electron–hole pair in the absence of an applied bias. Thus, even in the absence of external bias, the photogenerated carriers can be separated, which then diffuse to the appropriate electrodes driven by the ‘built-in’ axial field. We also point out the physical origins that can lead to unequal barrier heights in seemingly identical NW/metal junctions in a MSM device.

Asymmetric photon transport in organic semiconductor nanowires through electrically controlled exciton diffusion.

The ability to steer the flow of light toward desired propagation directions is critically important for the realization of key functionalities in optical communication and information processing. Although various schemes have been proposed for this purpose, the lack of capability to incorporate an external electric field to effectively tune the light propagation has severely limited the on-chip integration of photonics and electronics. Because of the noninteractive nature of photons, it is only possible to electrically control the flow of light by modifying the refractive index of materials through the electro-optic effect. However, the weak optical effects need to be strongly amplified for practical applications in high-density photonic integrations. We show a new strategy that takes advantage of the strong exciton-photon coupling in active waveguides to effectively manipulate photon transport by controlling the interaction between excitons and the external electric field. Single-crystal organic semiconductor nanowires were used to generate highly stable Frenkel exciton polaritons with strong binding and diffusion abilities. By making use of directional exciton diffusion in an external electric field, we have realized an electrically driven asymmetric photon transport and thus directional light propagation in a single nanowire. With this new concept, we constructed a dual-output single wire-based device to build an electrically controlled single-pole double-throw optical switch with fast temporal response and high switching frequency. Our findings may lead to the innovation of concepts and device architectures for optical information processing.

In operando x-ray imaging of nanoscale devices: Composition, valence, and internal electrical fields.

We introduce a method for directly imaging depletion layers in operando with elemental specificity and chemical speciation at sub-100 nm spatial resolution applicable to today's three-dimensional electronic architectures. These typically contain complex, multicomponent designs consisting of epitaxial heterostructures, buried domains, or nanostructures with different shapes and sizes. Although the variety of devices is immense, they commonly rely on carrier separation in a built-in potential induced by composition or strain gradients. To image these, we scanned a focused synchrotron x-ray nanobeam over a single semiconductor nanowire heterostructure and simultaneously measured the current through the device and the emitted characteristic x-rays as a function of the incoming hard x-ray energy. With these results, it is possible to identify the compositional and molecular structure as well as localize the electrical fields present under typical working conditions. This information allows us to draw an energy band diagram consistent with the elemental distribution and a high-resolution chemical speciation map.

Broadband Tunable Hybrid Photonic Crystal-Nanowire Light Emitter

We integrate about 100 single cadmium selenide semiconductor nanowires in silicon nitride photonic crystal cavities in a single processing run. Room temperature measurements reveal a single and narrow emission linewidth, corresponding to a Q-factor as large as 5000. By varying the structural parameters of the photonic crystal, the peak wavelength is changed, thereby covering the entire emission spectral range of the active material. A very large spectral range could be covered by heterogeneous integration of different active materials.

Toward a metrological calibration of the conversion efficiency in GaAs nanowire-based photodetectors

Abstract

Gate-controlled heat generation in ZnO nanowire FETs.

Nanoscale heating production using nanowires has been shown to be particularly attractive for a number of applications including nanostructure growth, localized doping, transparent heating and sensing. However, all proof-of-concept devices proposed so far relied on the use of highly conductive nanomaterials, typically metals or highly doped semiconductors. In this article, we demonstrate a novel nanoheater architecture based on a single semiconductor nanowire field-effect transistor (NW-FET). Nominally undoped ZnO nanowires were incorporated into three-terminal devices whereby control of the nanowire temperature at a given source-drain bias was achieved by additional charge carriers capacitatively induced via the third gate electrode. Joule-heating selective ablation of poly(methyl methacrylate) deposited on ZnO nanowires was shown, demonstrating the ability of the proposed NW-FET configuration to enhance by more than one order of magnitude the temperature of a ZnO nanowire, compared to traditional two-terminal configurations. These findings demonstrate the potential of field-effect architectures to improve Joule heating power in nanowires, thus vastly expanding the range of suitable materials and applications for nanowire-based nanoheaters.

Super-resolution imaging of light–matter interactions near single semiconductor nanowires

Nanophotonics is becoming invaluable for an expanding range of applications, from controlling the spontaneous emission rate and the directionality of quantum emitters, to reducing material requirements of solar cells by an order of magnitude. These effects are highly dependent on the near field of the nanostructure, which constitutes the evanescent fields from propagating and resonant localized modes. Although the interactions between quantum emitters and nanophotonic structures are increasingly well understood theoretically, directly imaging these interactions experimentally remains challenging. Here we demonstrate a photoactivated localization microscopy-based technique to image emitter-nanostructure interactions. For a 75 nm diameter silicon nanowire, we directly observe a confluence of emission rate enhancement, directivity modification and guided mode excitation, with strong interaction at scales up to 13 times the nanowire diameter. Furthermore, through analytical modelling we distinguish the relative contribution of these effects, as well as their dependence on emitter orientation.

Frequency-resolved optical gating measurement of ultrashort pulses by using single nanowire

The use of ultrashort pulses for fundamental studies and applications has been increasing rapidly in the past decades. Along with the development of ultrashort lasers, exploring new pulse diagnositic approaches with higher signal-to-noise ratio have attracted great scientific and technological interests. In this work, we demonstrate a simple technique of ultrashort pulses characterization with a single semiconductor nanowire. By performing a frequency-resolved optical gating method with a ZnO nanowire coupled to tapered optical microfibers, the phase and amplitude of a pulse series are extracted. The generated signals from the transverse frequency conversion process can be spatially distinguished from the input, so the signal-to-noise ratio is improved and permits lower energy pulses to be identified. Besides, since the nanometer scale of the nonlinear medium provides relaxed phase-matching constraints, a measurement of 300-nm-wide supercontinuum pulses is achieved. This system is highly compatible with standard optical fiber systems, and shows a great potential for applications such as on-chip optical communication.

Design of asymmetric nanovoid resonator for silicon-based single-nanowire solar absorbers

Single semiconductor nanowires offer an attractive option for low-cost and high-efficiency with diverse applications in sensing, photodetecting, and light-harvesting. One of the crucial goals for the single-nanowire-based devices is to find the most effective strategies to realize broadband and high optical absorption. We demonstrate in this study that integrating a nanovoid resonator into the single nanowire can substantially enhance the absorption to sunlight by exciting highly strengthened optical resonances in the nanovoid-deformed nanowire cavity. Without considering the factor of the reduced photoactive material, the photocurrent density of the single-nanowire solar absorber under the asymmetrical nanovoid design can be improved by ~37.5% over that based on the conventional solid nanowire devices. Moreover, the high absorption can be well preserved by either filling the nanovoid with low-index dielectric or introducing the dielectric substrate. This study advances our understanding on how the sunlight can be effectively manipulated by a nanovoid in high-index semiconductor nanostructures and provides an inspiring strategy to realize highly efficient photovoltaic devices.

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.