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Tuesday, 09 July 2013

Single nanowires characterised by Tip-Enhanced Raman Scattering

Publishing in Nano Letters, ICN2 Prof Clivia M. Sotomayor Torres and colleagues demonstrate nanoscale imaging of structural fluctuations in single nanowires using TERS.

Tip-Enhanced Raman Scattering (TERS) combines the morphological data from Scanning Probe Microscopy with the optical information provided by Raman spectroscopy.

A team of researchers from Germany and Spain, including ICN2 researchers Dr Markus R. Wagner, Dr Sebastian Reparaz and ICREA Professor and ICN2 group leader Prof Clivia M. Sotomayor Torres, have just published a study in Nano Letters demonstrating the power of Tip-Enhanced Raman Scattering (TERS) for nanoscale investigations of single nanowires. The article, entitled "Nanoscale imaging of InN segregation and polymorphism in single vertically aligned InGaN/GaN multi quantum well nanorods by tip enhanced Raman scattering", establishes this near-field optical imaging technique as a high-performance, comprehensive, and non-destructive tool for the optical characterisation of nanostructures.On the micrometer scale, information about chemical composition, strain, and structural quality can be easily obtained by various techniques such as Micro-Raman Spectroscopy. However, the optical diffraction limit complicates application of this approach at the nanoscale. Nanoscale material properties are therefore only accessible by local and partly destructive techniques such as High-Resolution Transmission Electron Microscopy (HRTEM) imaging, which underlines the need for an alternative and non-destructive technique. A possibility to circumvent the spatial resolution limit for optical measurements is given by optical near-field methods such as TERS.TERS combines the morphological characterisation capabilities of Scanning Probe Microscopy (e.g. STM or AFM) with optical characterisation by Raman spectroscopy. The enhanced near-field signal at the STM tip provides the increased sensitivity to obtain the Raman signal from structures with extremely low scattering volume, such as ultra-thin quantum wells, interfaces and surfaces.In their work, Prof Sotomayor Torres and colleagues studied the variation of material properties on the nanoscale in vertically aligned InGaN/GaN nanorod arrays with a spatial resolution well below the diffraction limit. These InGaN based nanostructures constitute a promising material class for novel highly efficient, large-scale solid state lighting devices. By analysing TERS maps of a single nanorod, the researchers obtained detailed information on the chemical composition, indium clustering, crystallographic modifications, local strain variations and charge accumulations, with a spatial resolution of less than 35 nm. Cubic and hexagonal modifications near the InGaN quantum well region were identified solely based on the TERS data and were confirmed independently by HRTEM images. Additionally, the researchers were able to observe local fluctuations in the indium concentration, and variations in strain, on the nanoscale.The local fluctuations of chemical composition, crystal structure and strain can strongly reduce the luminescence efficacy of light-emitting devices, as they introduce non-radiative carrier recombination centres. Using TERS, the authors reported an alternative approach to identify and visualise these limiting factors, which should contribute to the improvement and optimisation of the growth process of these structures and pave the way for the next generation of nanostructure-based light emitters. Their results demonstrate the potential of this technique for comprehensive analysis of material properties in semiconductor nanostructures.

To read the article, "Nanoscale imaging of InN segregation and polymorphism in single vertically aligned InGaN/GaN multi quantum well nanorods by tip enhanced Raman scattering", click here.