Nanowire Polarization Research Articles

In situ analysis of the nucleation of O- and Zn-polar ZnO nanowires using synchrotron-based X-ray diffraction.

The selection of the polarity of ZnO nanowires grown by chemical bath deposition offers a great advantage for their integration into a wide variety of engineering devices. However, the nucleation process of ZnO nanowires and its dependence on their polarity is still unknown despite its importance for optimizing their morphology and properties and thus to enhance the related device performances. To tackle this major issue, we combine an in situ analysis of the nucleation process of O- and Zn-polar ZnO nanowires on O- and Zn-polar ZnO single crystals, respectively, using synchrotron radiation-based grazing incidence X-ray diffraction with ex situ transmission and scanning electron microscopy. We show that the formation of ZnO nanowires obeys three successive phases from the induction, through nucleation to growth phases. The characteristics of each phase, including the nucleation temperature, the shape and dimension of nuclei, as well as their radial and axial development are found to depend on the polarity of ZnO nanowires. A comprehensive description reporting the dominant physicochemical processes in each phase and their dependence on the polarity of ZnO nanowires is presented, revisiting their formation process step-by-step. These findings provide a deeper understanding of the phenomena at work during the growth of ZnO nanowires by chemical bath deposition and open the perspective to develop a more accurate control of their properties at each step of the formation process.

REARRANGEMENT OF THE CONFORMATIONAL STRUCTURE OF POLYAMPHOLYTES ON THE SURFACE OF A METAL NANOWIRE IN A TRANSVERSE MICROWAVE ELECTRIC FIELD

Molecular dynamics has been employed to study the rearrangement of the conformational structure of polyampholytes adsorbed on the surface of a gold nanowire with a periodic change in time of its polarity in the transverse direction at an ultrahigh frequency. The radial distributions of the atomic density of the polypeptide and its angular distributions on the nanowire surface have been calculated. At high temperatures, temporary fluctuations in the conformational structure of the adsorbed polyampholyte polypeptide were observed. In this case, for half the period of the nanowire polarity change, the macrochain conformation changed from dense enveloping of the nanowire to an elongated conformational structure along the dipole moment of the nanowire. At low temperature and the nanowire dipole moment, the swelling of the fringe of the adsorbed polyampholyte was observed with a displacement of most of its links to one side with respect to the plane perpendicular to the direction of the nanowire dipole moment and passing through its axis. At low temperature and high values of the nanowire dipole moment, the polyampholyte polypeptide was desorbed from the nanowire surface. An analytical model of conformational rearrangements of a polyampholyte Gaussian chain in the form of an external field perturbation theory is presented.

Influence of Growth Polarity Switching on the Optical and Electrical Properties of GaN/AlGaN Nanowire LEDs

For the development and application of GaN-based nanowire structures, it is crucial to understand their fundamental properties. In this work, we provide the nano-scale correlation of the morphological, electrical, and optical properties of GaN/AlGaN nanowire light emitting diodes (LEDs), observed using a combination of spatially and spectrally resolved cathodoluminescence spectroscopy and imaging, electron beam-induced current microscopy, the nano-probe technique, and scanning electron microscopy. To complement the results, the photo- and electro-luminescence were also studied. The interpretation of the experimental data was supported by the results of numerical simulations of the electronic band structure. We characterized two types of nanowire LEDs grown in one process, which exhibit top facets of different shapes and, as we proved, have opposite growth polarities. We show that switching the polarity of nanowires (NWs) from the N- to Ga-face has a significant impact on their optical and electrical properties. In particular, cathodoluminescence studies revealed quantum wells emissions at about 3.5 eV, which were much brighter in Ga-polar NWs than in N-polar NWs. Moreover, the electron beam-induced current mapping proved that the p–n junctions were not active in N-polar NWs. Our results clearly indicate that intentional polarity inversion between the n- and p-type parts of NWs is a potential path towards the development of efficient nanoLED NW structures.

Influence of Si Substrate Preparation Procedure on Polarity of Self-Assembled GaN Nanowires on Si(111): Kelvin Probe Force Microscopy Studies

The growth of GaN nanowires having a polar, wurtzite structure on nonpolar Si substrates raises the issue of GaN nanowire polarity. Depending on the growth procedure, coexistence of nanowires with different polarities inside one ensemble has been reported. Since polarity affects the optical and electronic properties of nanowires, reliable methods for its control are needed. In this work, we use Kelvin probe force microscopy to assess the polarity of GaN nanowires grown by plasma-assisted Molecular Beam Epitaxy on Si(111) substrates. We show that uniformity of the polarity of GaN nanowires critically depends on substrate processing prior to the growth. Nearly 18% of nanowires with reversed polarity (i.e., Ga-polar) were found on the HF-etched substrates with hydrogen surface passivation. Alternative Si substrate treatment steps (RCA etching, Ga-triggered deoxidation) were tested. However, the best results, i.e., purely N-polar ensemble of nanowires, were obtained on Si wafers thermally deoxidized in the growth chamber at ~1000 °C. Interestingly, no mixed polarity was found for GaN nanowires grown under similar conditions on Si(111) substrates with a thin AlOy buffer layer. Our results show that reversal of nanowires’ polarity can be prevented by growing them on a chemically uniform substrate surface, in our case on clean, in situ formed SiNx or ex situ deposited AlOy buffers.

Structural and luminescence imaging and characterisation of semiconductors in the scanning electron microscope

The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and cathodoluminescence (CL) hyperspectral imaging provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a material’s light emission properties. Here we describe the EBSD, ECCI and CL techniques and illustrate their use for investigating the structural and light emitting properties of UV-emitting nitride semiconductor structures. We discuss our investigations of the type, density and distribution of defects in GaN, AlN and AlGaN thin films and also discuss the determination of the polarity of GaN nanowires.

Maskless Fabrication of Film-Patterned-Retarder (FPR) Using Wedged Liquid Crystal Cell

Film-patterned-retarder (FPR) is an optical film used in the stereoscopic 3D display and is fabricated using an expensive photomask and nano-wire grid polarizer. This method is expensive and the pitch of FPR is fixed for a given photomask. Here, we introduce a maskless fabrication method for FPR using a wedged liquid crystal (LC) cell. The retardation of the wedged LC cell is continuously increased from one edge to the other. When the polarized monochromatic laser beam (450 nm) passes through the cell, periodically undulated polarization pattern of light is obtained, and can be used to produce FPR by combining with the photo-alignment method. The method does not require the expensive photomask and nano-wire grid polarizer, and the pitch of FPR can be simply controllable by changing the birefringence of LC material with temperature or using a light source of different wavelengths. Thus, this method can provide a simple and cost-effective method for FPR fabrication.

Light programmable micro/nanomotors with optically tunable in-phase electric polarization

To develop active nanomaterials that can instantly respond to external stimuli with designed mechanical motions is an important step towards the realization of nanorobots. Herein, we present our finding of a versatile working mechanism that allows instantaneous change of alignment direction and speed of semiconductor nanowires in an external electric field with simple visible-light exposure. The light induced alignment switch can be cycled over hundreds of times and programmed to express words in Morse code. With theoretical analysis and simulation, the working principle can be attributed to the optically tuned real-part (in-phase) electrical polarization of a semiconductor nanowire in aqueous suspension. The manipulation principle is exploited to create a new type of microscale stepper motor that can readily switch between in-phase and out-phase modes, and agilely operate independent of neighboring motors with patterned light. This work could inspire the development of new types of micro/nanomachines with individual and reconfigurable maneuverability for many applications.

Determining GaN Nanowire Polarity and its Influence on Light Emission in the Scanning Electron Microscope.

The crystal polarity of noncentrosymmetric wurtzite GaN nanowires is determined nondestructively in the scanning electron microscope using electron backscatter diffraction (EBSD). The impact of the nanowire polarity on light emission is then investigated using cathodoluminescence (CL) spectroscopy. EBSD can determine polarity of noncentrosymmetric crystals by interrogating differences in the intensity distribution of bands of the EBSD pattern associated with semipolar planes. Experimental EBSD patterns from an array of GaN nanowires are compared with theoretical patterns produced using dynamical electron simulations to reveal whether they are Ga- or N-polar or, as in several cases, of mixed polarity. CL spectroscopy demonstrates the effect of the polarity on light emission, with spectra obtained from nanowires of known polarity revealing a small but measurable shift (≈28 meV) in the GaN near band edge emission energy between those with Ga and N polarity. We attributed this energy shift to a difference in impurity incorporation in nanowires of different crystal polarity. This approach can be employed to nondestructively identify polarity in a wide range of noncentrosymmetric nanoscale material systems and provide direct comparison with their luminescence.

Polarity Control in Growing Highly Ga-Doped ZnO Nanowires with the Vapor-Liquid-Solid Process.

Surface behavior modification by forming surface-transparent conductive nanowires (NWs) is an important technique for many applications, particularly when the polarities of the NWs can be controlled. The polarities of Ga-doped ZnO (GaZnO) NWs grown on templates of different polarities under different growth conditions are studied for exploring a polarity control growth technique. The NWs are formed on Ga- and N-face GaN through the vapor-liquid-solid (VLS) process using Ag nanoparticles as growth catalyst. The NWs grown on templates of different polarities under the Zn- (O-) rich conditions are always Zn (O) polar. During the early stage of NW growth, because the lattice sizes among different nucleation islands formed at the triple-phase line are quite different, high-density planar defects are produced when lateral growths from multiple nucleation islands form a GaZnO double bilayer. In this situation, frequent domain inversions occur, and GaZnO polarity is unstable. Under the Zn- (O-) rich conditions, because the lateral growth rate of GaZnO in the Zn- (O-) polar structure is higher due to more available dangling bonds, the growth of the Zn- (O-) polar structure dominates NW formation such that the NW eventually becomes Zn (O) polar irrespective of the polarity of the growth template. Therefore, the polarity of a doped-ZnO NW can be controlled simply by the relative supply rates of Zn and O during VLS growth.

Calibration method for division of focal plane polarimeters.

The division of focal plane polarimeters is fabricated out of nanowire polarization filters attached to the surface of a focal plane array. Due to limitations in the manufacturing process, the transmissivity of each nanowire polarization filter is different, resulting in a combination of inhomogeneities in transmittance and detector response, and which also yields the problem of polarization imaging uniformity, a more complicated method than traditional intensity imaging. This study presents a brand new nonuniformity correction method for solving the correction inaccuracies in the traditional method. The method corrected the polarization filter transmittance with detector gain based on a new method for calculating the degree of linear polarization (DoLP). This paper includes the theoretical derivation, experimental testing, and numerical simulation. In this simulation environment, the DoLP value was accurately calculated.

Polarization transmission mechanism analyzation of bi-layer nanowire polarizer

The polarization transmission of bi-layer nanowire polarizer is analyzed physically by using equivalent medium theory and Fabry–Pérot resonate theory. The principal of the two theories are explained briefly. The analyzation of grating period, duty cycle and metal height on polarization transmission can be also applied to other type of nanowire polarizers. The critical period of the grating is given. The effects of the grating parameters on polarization performance are analyzed systematically on physical level. The extraordinary optical transmission of TM and specially TE polarization light through bi-layer nanowire grating is analyzed. The analyzation of the polarization transmission mechanism will be helpful for the design and fabrication of the gratings with good polarization properties.

Mechanism analyzation of the effects of grating parameters on polarization transmission of single-layer nanowire gratings

Using equivalent medium theory, surface plasma resonance theory and Fabry–Pérot resonance theory, transmission mechanism of single-layer nanowire polarizer is systematically analyzed. Concept of skin depth is used to explain the effect of grating material on polarization transmission. Two transmission minimum points in the curve of grating period versus transverse magnetic transmission are analyzed quantitatively. Effects of grating duty cycle, height and type on polarization transmission of single-layer nanowire polarizer are also analyzed. The requirements of designing single-layer sub-wavelength metal gratings with good polarization properties are given. The analysis of the transmission mechanism of sub-wavelength metal gratings will be helpful for the design and fabrication of the gratings with good polarization properties.

Polarity Control of Heteroepitaxial GaN Nanowires on Diamond.

Group III-nitride materials such as GaN nanowires are characterized by a spontaneous polarization within the crystal. The sign of the resulting sheet charge at the top and bottom facet of a GaN nanowire is determined by the orientation of the wurtzite bilayer of the different atomic species, called N and Ga polarity. We investigate the polarity distribution of heteroepitaxial GaN nanowires on different substrates and demonstrate polarity control of GaN nanowires on diamond. Kelvin Probe Force Microscopy is used to determine the polarity of individual selective area-grown and self-assembled nanowires over a large scale. At standard growth conditions, mixed polarity occurs for selective GaN nanowires on various substrates, namely on silicon, on sapphire and on diamond. To obtain control over the growth orientation on diamond, the substrate surface is modified by nitrogen and oxygen plasma exposure prior to growth, and the growth parameters are adjusted simultaneously. We find that the surface chemistry and the substrate temperature are the decisive factors for obtaining control of up to 93% for both polarity types, whereas the growth mode, namely selective area or self-assembled growth, does not influence the polarity distribution significantly. The experimental results are discussed by a model based on the interfacial bonds between the GaN nanowires, the termination layer, and the substrate.

Comparison of convergent beam electron diffraction and annular bright field atomic imaging for GaN polarity determination.

A comparison of two electron microscopy techniques used to determine the polarity of GaN nanowires is presented. The techniques are convergent beam electron diffraction (CBED) in TEM mode and annular bright field (ABF) imaging in aberration corrected STEM mode. Both measurements were made at nominally the same locations on a variety of GaN nanowires. In all cases the two techniques gave the same polarity result. An important aspect of the study was the calibration of the CBED pattern rotation relative to the TEM image. Three different microscopes were used for CBED measurements. For all three instruments there was a substantial rotation of the diffraction pattern (120 or 180°) relative to the image, which, if unaccounted for, would have resulted in incorrect polarity determination. The study also shows that structural defects such as inversion domains can be readily identified by ABF imaging, but may escape identification by CBED. The relative advantages of the two techniques are discussed.

Single-Layer Nanowire Polarizer Integrated With Photodetector and Its Application for Polarization Navigation

We report a method of integrating the single-layer nanowire polarizers with photodetectors, and the fabricated device is used as the polarization navigation sensor. The fabrication processes mainly based on nanoimprint lithography and dry etching. The fabrication method can fabricate nanogratings with multi-orientation simultaneously, so angular alignment error is eliminated. The external quantum efficiency of the integrated device is tested, and a peak external quantum efficiency of 23% at around 440 nm is achieved. The indoor angle output experiment of a navigation sensor is investigated, and the angle output error is tested to be within ±0.1°. The outdoor angle output experiment has also been tested, and the errors are less than ±1.5°. The measurement results show that the fabricated device is suitable for polarization navigation.

260 frames-per-second 648x488 resolution division-of-focal-plane polarimeter with structural dynamics and tracking applications.

We have designed an image sensor that can capture the first three Stokes parameters at 648 by 488 spatial resolution at 260 frames per second. The sensor consists of a CCD image sensor monolithically integrated with pixel pitch-matched aluminum nanowire polarization filters. The sensor demonstrates a Malus law response over all pixels, and has a relatively uniform diattenuation over the visible spectrum. We demonstrate two potential applications for the sensor. The first uses circular polarization in transmission mode to observe high-speed stress failure in polycarbonate. The second uses polarization in reflected mode to track high speed automobile traffic.

Assessment of Polarity in GaN Self-Assembled Nanowires by Electrical Force Microscopy.

In this work, we demonstrate the capabilities of atomic force microscopies (AFMs) for the nondestructive determination of the polarity of GaN nanowires (NWs). Three complementary AFMs are analyzed here: Kelvin probe force microscopy (KPFM), light-assisted KPFM, and piezo-force microscopy (PFM). These techniques allow us to assess the polarity of individual NWs over an area of tens of μm(2) and provide statistics on the polarity of the ensemble with an accuracy hardly reachable by other methods. The precise quantitative analysis of the tip-sample interaction by multidimensional spectroscopic measurements, combined with advanced data analysis, has allowed the separate characterization of electrostatic and van der Waals forces as a function of tip-sample distance. Besides their polarity, the net surface charge density of individual NWs was estimated.

Antimony Induced {112}A Faceted Triangular GaAs1−xSbx/InP Core/Shell Nanowires and Their Enhanced Optical Quality

Mid‐infrared GaAs1−xSbx/InP core/shell nanowires are grown coherently with perfectly twin‐free zinc blende crystal structure. An unusual triangular InP shell with predominantly {112}A facets instead of {112}B facets is reported. It is found that this polarity preference is due to the surfactant role of Sb, which inhibits InP shell growth rate in the 〈112〉A directions. This behavior reveals a new degree of control and tunability allowed in manipulating nanowire facet geometry and polarity in radial heterostructures by a simple means. Tuning the Sb composition in the core yields controllable intense photoluminescence emission in both the 1.3 and 1.5 μm optical telecommunication windows, up to room temperature for single nanowires. The internal quantum efficiency of the core/shell nanowires is experimentally determined to be as high as 56% at room temperature. Transient Rayleigh scattering analysis brings complementary information, revealing the photoexcited carrier lifetime in the core/shell nanowire to be ≈100 ps at 300 K and ≈800 ps at 10 K. In comparison, the carrier lifetime of core‐only nanowire is below the detection limit of the system (25 ps). The demonstrated superior optical quality of the core/shell nanowires and their ideal emission wavelength range makes them highly relevant candidates for near‐infrared optoelectronic applications.

A Robust Adaptive Angle Algorithm for Imaging Bionic Polarization Navigation Sensor

This paper presents a precise angular algorithm for the imaging bionic polarization navigation sensor. The algorithm is targeted for a imaging sensor with bilayer nanowire polarizer which is multi-orientated of 0°, 60°, 120° and 90°, 150°, 210°. The algorithm is considered to extract intensity values of pixels in homogeneous regions, eliminate noise contained in image and compensate errors occured in the fabricaiton process of nanowire polarization filters. And the occlusion problem has to be compensated by neighboring pixels. The algorithm considered in this paper is able to solve overdetermined system of linear equations, and give a set of least square solutions of the Stokes parameters and the angle of polarization as output. The performance of the algorithm along with the experimental results of the angle measurement are also presented. Based on the comparison results, the algorithm achieves a best performance for the angle of polarization.

Integrated polarization-dependent sensor for autonomous navigation

Based on the navigation strategy of insects utilizing the polarized skylight, an integrated polarization-dependent sensor for autonomous navigation is presented. The navigation sensor has the features of compact structure, high precision, strong robustness, and a simple manufacture technique. The sensor is composed by integrating a complementary-metal-oxide-semiconductor sensor with a multiorientation nanowire grid polarizer. By nanoimprint lithography, the multiorientation nanowire polarizer is fabricated in one step and the alignment error is eliminated. The statistical theory is added to the interval-division algorithm to calculate the polarization angle of the incident light. The laboratory and outdoor tests for the navigation sensor are implemented and the errors of the measured angle are ±0.02 deg and ±1.3 deg, respectively. The results show that the proposed sensor has potential for application in autonomous navigation.