Polarized Raman Confocal Microscopy of Single Gallium Nitride Nanowires

Confocal Raman microscope with versatile dual polarization snapshot acquisition

In this paper we propose a new strategy towards simultaneous co- and cross-polarized measurements of Raman spectra in a confocal microscope. One of the advantages of this strategy is that it is immune to polarization-dependent efficiency of diffraction gratings. It is shown via linear angle-resolved and circular polarization measurements that the accuracy of these snapshot polarization measurements on solid and liquid samples are in good agreement with available models and data. The interest of simultaneous acquisition of the total Raman response and the degree of polarization is discussed as well.

Raman Submicron Spatial Mapping of Individual Mn-doped ZnO Nanorods

ZnO nanorods (NRs) arrays doped with a large concentration of Mn synthesized by aqueous chemical growth and were characterized by SEM, photoluminescence, Raman scattering, magnetic force microscopy (MFM). By comparison of spectra taken on pure and Mn-doped ZnO NRs, a few new Raman impurity-related phonon modes, resulting from the presence of Mn in the investigated samples. We also present a vibrational and magnetic characterization of individual lying nanorods using Raman and MFM imaging. Confocal scanning micro-Raman mapping of the spatial distribution of intensity and frequency of phonon modes in single Mn-doped ZnO NRs nanorods is presented and analyzed for the first time. Mn-related local vibrational modes are also registered in Raman spectra of the single nanorod, confirming the incorporation of Mn into the ZnO host matrix. At higher Mn concentration the structural transformation toward the spinel phase ZnMn₂O₄ and Mn3O4 is observed mainly in 2D bottom layers. MFM images of Mn-doped ZnO NR arrays and single nanorod were studied in nanoscale at room temperature and demonstrate magnetic behavior. The circular domain magnetic pattern on top of single nanorod originated to superposition of some separate domains inside rod. This demonstrates that long-range ferromagnetic order is present at room temperature. Aligned Mn-doped ZnO NRs demonstrates that long-range ferromagnetic order and may be applied to future spintronic applications.

Optical-Based Analysis of Soft Tissue Structures

Fibrous structures are an integral and dynamic feature of soft biological tissues that are directly related to the tissues' condition and function. A greater understanding of mechanical tissue behavior can be gained through quantitative analyses of structure alone, as well as its integration into computational models of soft tissue function. Histology and other nonoptical techniques have traditionally dominated the field of tissue imaging, but they are limited by their invasiveness, inability to provide resolution on the micrometer scale, and dynamic information. Recent advances in optical modalities can provide higher resolution, less invasive imaging capabilities, and more quantitative measurements. Here we describe contemporary optical imaging techniques with respect to their suitability in the imaging of tissue structure, with a focus on characterization and implementation into subsequent modeling efforts. We outline the applications and limitations of each modality and discuss the overall shortcomings and future directions for optical imaging of soft tissue structure.

Effects of La-doping on charge separation behavior of ZnO:GaN for its enhanced photocatalytic performance

In this work, lanthanum (La) has been proven as an effective space charge layer modifier to promote efficient photogenerated charge carrier separation for ZnO:GaN solid solution photocatalysts with enhanced photocatalytic water-splitting performance.

Tip-enhanced Raman imaging and nano spectroscopy of etched silicon nanowires

Tip-enhanced Raman spectroscopy (TERS) is used to investigate the influence of strains in isolated and overlapping silicon nanowires prepared by chemical etching of a (100) silicon wafer. An atomic force microscopy tip made of nanocrystalline diamond coated with a thin layer of silver is used in conjunction with an excitation wavelength of 532 nm in order to probe the first order optical phonon mode of the [100] silicon nanowires. The frequency shift and the broadening of the silicon first order phonon are analyzed and compared to the topographical measurements for distinct configuration of nanowires that are disposed in straight, bent or overlapping configuration over a microscope coverslip. The TERS spatial resolution is close to the topography provided by the nanocrystalline diamond tip and subtle spectral changes are observed for different nanowire configurations.

Diameter and polarization-dependent Raman scattering intensities of semiconductor nanowires

Diameter-dependent Raman scattering in single tapered silicon nanowires is measured and quantitatively reproduced by modeling with finite-difference time-domain simulations. Single crystal tapered silicon nanowires were produced by homoepitaxial radial growth concurrent with vapor-liquid-solid axial growth. Multiple electromagnetic resonances along the nanowire induce broad band light absorption and scattering. Observed Raman scattering intensities for multiple polarization configurations are reproduced by a model that accounts for the internal electromagnetic mode structure of both the exciting and scattered light. Consequences for the application of Stokes to anti-Stokes intensity ratio for the estimation of lattice temperature are discussed.

Composition and bandgap-graded semiconductor alloy nanowires

Semiconductor alloy nanowires with spatially graded compositions (and bandgaps) provide a new material platform for many new multifunctional optoelectronic devices, such as broadly tunable lasers, multispectral photodetectors, broad-band light emitting diodes (LEDs) and high-efficiency solar cells. In this review, we will summarize the recent progress on composition graded semiconductor alloy nanowires with bandgaps graded in a wide range. Depending on different growth methods and material systems, two typical nanowire composition grading approaches will be presented in detail, including composition graded alloy nanowires along a single substrate and those along single nanowires. Furthermore, selected examples of applications of these composition graded semiconductor nanowires will be presented and discussed, including tunable nanolasers, multi-terminal on-nanowire photodetectors, full-spectrum solar cells, and white-light LEDs. Finally, we will make some concluding remarks with future perspectives including opportunities and challenges in this research area.

ZnO nanowire lasers

The pathway towards the realization of optical solid-state lasers was gradual and slow. After Einstein's paper on absorption and stimulated emission of light in 1917 it took until 1960 for the first solid state laser device to see the light. Not much later, the first semiconductor laser was demonstrated and lasing in the near UV spectral range from ZnO was reported as early as 1966. The research on the optical properties of ZnO showed a remarkable revival since 1995 with the demonstration of room temperature lasing, which was further enhanced by the first report of lasing by a single nanowire in 2001. Since then, the research focussed increasingly on one-dimensional nanowires of ZnO. We start this review with a brief description of the opto-electronic properties of ZnO that are related to the wurtzite crystal structure. How these properties are modified by the nanowire geometry is discussed in the subsequent sections, in which we present the confined photon and/or polariton modes and how these can be investigated experimentally. Next, we review experimental studies of laser emission from single ZnO nanowires under different experimental conditions. We emphasize the special features resulting from the sub-wavelength dimensions by presenting our results on single ZnO nanowires lying on a substrate. At present, the mechanism of lasing in ZnO (nanowires) is the subject of a strong debate that is considered at the end of this review.

Polarized Rayleigh back-scattering from individual semiconductor nanowires

A complete understanding of the interaction between electromagnetic radiation and semiconductor nanowires (NWs) is required in order to further develop a new generation of opto-electronic and photonic devices based on these nanosystems. The reduced dimensionality and high aspect ratio of nanofilaments can induce strong polarization dependence of the light absorption, emission and scattering, leading in some cases to the observation of optical antenna effects. In this work we present the first systematic study of polarized Rayleigh back-scattering from individual crystalline semiconductor NWs with known crystalline structure, orientation and diameters. To explain our experimental Rayleigh polar patterns, we propose a simple theory that relies on a secondary calculation of the volume-averaged internal electromagnetic fields inside the NW. These results revealed that the internal and emitted field can be enhanced depending on the polarization with respect to the NW axis; we also show that this effect strongly depends on the NW diameter.

Probing strain in bent semiconductor nanowires with Raman spectroscopy

We present a noninvasive optical method to determine the local strain in individual semiconductor nanowires. InP nanowires were intentionally bent with an atomic force microscope and variations in the optical phonon frequency along the wires were mapped using Raman spectroscopy. Sections of the nanowires with a high curvature showed significantly broadened phonon lines. These observations together with deformation potential theory show that compressive and tensile strain inside the nanowires is the physical origin of the observed phonon energy variations.

Nondestructive in situ identification of crystal orientation of anisotropic ZnO nanostructures

We present a novel method for direct, fast, nonambiguous, and nondestructive identification of the growth direction and orientation of individual ZnO nanostructures in the device-ready environment by exploiting high-resolution confocal Raman mapping. Various features of the Raman spectrum of ZnO nanostructures, vapor deposition grown nanobelts and peptide-assisted vertical nanorods, were found to be sensitive to the relative orientation of the crystal plane. Furthermore, we discovered that the waveguiding property of the ZnO nanobelt is also orientation dependent and results in either apparent enhancement or suppression of Raman scattering from the underlying substrate. We demonstrate that various features of Raman spectrum of ZnO and the modulation of the substrate signal can be employed for the rapid and nondestructive identification of the crystal growth direction and orientation of these nanostructures even after integration into devices, which is impossible with current electron microscopy and diffraction techniques. We believe that the general features observed here are equally applicable to other wurtzite nanostructures (ZnS, GaN) which are critical in optoelectronics, lasing, and piezotronic applications.

Optical antenna effect in semiconducting nanowires

We report on investigations of the interaction of light with nanoscale antennae made from crystalline GaP nanowires (NWs). Using Raman scattering, we have observed strong optical antenna effects which we identify with internal standing wave photon modes of the wire. The antenna effects were probed in individual NWs whose diameters are in the range 40 < d < 300 nm. The data and our calculations show that the nature of the backscattered light is critically dependent on the interplay between a photon confinement effect and bulk Raman scattering. At small diameter, d < 65 nm, the NWs are found to act like a nearly perfect dipole antenna and the bulk Raman selection rules are masked leading to a polarized scattering intensity function I R approximately cos4 theta. Underscoring the importance of this work is the realization that a fundamental understanding of the "optical antenna effect" in semiconducting NWs is essential to the analysis of all electro-optic effects in small diameter filaments.

Pushing the limit of confocal polarized Raman microscopy

Raman microscopy has emerged as a powerful technique to characterize anisotropic materials with sub micro meter resolution. The use of polarized light allows one to obtain precise information about the local organization of the relevant molecular groups through the determination of the most probable distribution function. Such polarization analysis can be conducted under a confocal microscope, but caution must be exercised because of the use of objectives of high numerical value. The molecular orientation can be effectively correlated with the topography of the sample when atomic force microscopy experiments are conducted on the same object. In the present review paper, we present Raman imaging results that have been conducted on mesostructured polymer surfaces and on a single isolated semiconductor nanowire.Key words: Raman spectroscopy, confocal microscopy, orientation parameters, azopolymers, nanowires.

Resolving Radial Composition Gradients in Polarized Confocal Raman Spectra of Individual 3C-SiC Nanowires

Silicon carbide nanowires are being actively pursued as components for nanoelectromechanical sensors, nanocatalytic elements, and nano-optical circuits able to operate in harsh environment, high temperature, and high power applications. The effect of geometric confinement and polarization anisotropy in confocal Raman spectroscopy has been employed to detect axial and radial composition information in individual nanowires. Polarization anisotropy causes a significant increase in signal from the surface layer (relative to bulk), and combined with the increased surface-to-volume ratio at the nanoscale, it allows for the direct characterization of bulk and surface defects.