Prabhat Verma

Tip-enhanced nano-Raman analytical imaging of locally induced strain distribution in carbon nanotubes

Tip-enhanced Raman scattering microscopy is a powerful technique for analysing nanomaterials at high spatial resolution far beyond the diffraction limit of light. However, imaging of intrinsic properties of materials such as individual molecules or local structures has not yet been achieved even with a tip-enhanced Raman scattering microscope. Here we demonstrate colour-coded tip-enhanced Raman scattering imaging of strain distribution along the length of a carbon nanotube. The strain is induced by dragging the nanotube with an atomic force microscope tip. A silver-coated nanotip is employed to enhance and detect Raman scattering from specific locations of the nanotube directly under the tip apex, representing deformation of its molecular alignment because of the existence of local strain. Our technique remarkably provides an insight into localized variations of structural properties in nanomaterials, which could prove useful for a variety of applications of carbon nanotubes and other nanomaterials as functional devices and materials.

Diameter-selective near-field Raman analysis and imaging of isolated carbon nanotube bundles

Tip-enhanced near-field Raman scattering has been utilized to demonstrate the measurement of the distribution of single-walled carbon nanotubes (SWNTs) with a spatial resolution far beyond the diffraction limit of the probing light. This was done by measuring the radial breathing mode (RBM) of SWNTs in the near-field Raman spectra, which corresponded to the diameters of various SWNTs in the immediate vicinity of the tip. Further, near-field Raman imaging of the RBM provided a super-resolved color mapping corresponding to the diameter distribution of SWNTs within a bundle, which is not possible to realize by conventional topographic imaging methods.

Confinement of enhanced field investigated by tip-sample gap regulation in tapping-mode tip-enhanced Raman microscopy

The authors developed a tip-enhanced near field Raman microscope that can precisely regulate longitudinal distance between a metallic tip and sample molecules. This was done by employing a time-gated photoncounting scheme that enabled us to observe exponentially decaying near field Raman intensity with the tip-sample distance. The exponential decay shows a characteristic of the enhanced field generated by the localization of the surface plasmon polaritons near the tip apex. This microscope was applied to evaluate metal-coated tips and also to investigate confinement of the field generated at a gap between two metal nanostructures from the decay curves.

Raman studies on GaAs1−xBix and InAs1−xBix

The lattice vibrational properties of new semiconductor alloys, GaAs1−xBix and InAs1−xBix, are reported. These alloys, which were grown by metalorganic vapor phase epitaxy technique, contain a small amount (1.2%–3.8%) of Bi. A detail Raman scattering study of these new alloys, which exhibit weak temperature dependence of the band gap with increasing amount of Bi, is reported here. Good crystalline quality and spatial homogeneity was confirmed using micro-Raman technique. The alloys show ternary compound behavior, confirming substitutional incorporation of Bi into the lattice site. New vibrational modes observed were assigned to GaBi-like and InBi-like modes. In addition, phonon-plasmon coupled modes and vibrational modes corresponding to Bi and As materials were also observed. Results are discussed to characterize these new alloys in detail.

Tip-Enhanced Raman Spectroscopy: Technique and Recent Advances

This review discusses a relatively new technique for optical nanoimaging at visible wavelength, known as tip-enhanced Raman spectroscopy (TERS). This technique relies on the enhancement and spatial confinement of light in the close vicinity of the apex of a plasmonic nanotip. The plasmonic nanotip can be positioned on the sample and controlled by a suitable scanning probe microscopy, such as atomic force microscopy or scanning tunneling microscopy. By raster scanning the nanotip, one can obtain nanoimages with high spatial resolution. While enhancement helps measuring weak phonon modes from a tiny volume of the sample, confinement delivers extremely high spatial resolution in nanoimaging. We will discuss the technique of TERS in more detail with several applications and review the recent advances.

Quantitative analysis of polarization-controlled tip-enhanced Raman imaging through the evaluation of the tip dipole.

Polarization analysis in tip-enhanced Raman spectroscopy (TERS) is of tremendous advantage, as it allows one to study highly directional intrinsic properties of a sample at the nanoscale. However, neither evaluation nor control of the polarization properties of near-field light in TERS is as straightforward as in usual far-field illumination, because of the random metallic nanostructure attached to the tip apex. In this study, we have developed a method to successfully analyze the polarization of near-field light in TERS from the scattering pattern produced by the induced dipole in the metallic tip. Under dipole approximation, we measured the image of the dipole at a plane away from the focal plane, where the information about the direction of the dipole oscillation was intact. The direction of the dipole oscillation was determined from the defocused pattern, and then the polarization of near-field light was evaluated from the oscillation direction by calculating the intensity distribution of near-field light through Greens function. After evaluating the polarization of some fabricated tips, we used those tips to measure TERS images from single-walled carbon nanotubes and confirmed that the contrast of the TERS image depended on the oscillation direction of the dipole, which were also found in excellent agreement with the calculated TERS images, verifying that the polarization of the near-field was quantitatively estimated by our technique. Our technique would lead to better quantitative analysis in TERS imaging with consideration of polarization impact, giving a better understanding of the behavior of nanomaterials.

Nanoanalysis of crystalline properties of GaN thin film using tip-enhanced Raman spectroscopy

Nanoscaled variation in crystalline properties was demonstrated through tip-enhanced Raman scattering (TERS) for a thin GaN sample, which apparently showed good and uniform crystalline properties at microscale in conventional micro-Raman scattering. The observation was attributed to the field enhancement and the super-resolution associated with the near-field technique. The enhancement factor of the TERS intensity was assessed to be larger than 2.8×104. Variations in crystalline properties were observed at spatial resolution beyond the diffraction limit of the probing light. The authors propose TERS to be an efficient technique for nanoscale characterization of crystalline properties for the nitride-based semiconductor devices.

Temperature dependence of optical phonon lifetimes in ZnSe

Temperature-dependent Raman scattering experiments were performed on the (1 1 1) face of crystalline ZnSe to estimate the phonon lifetimes for the LO and TO phonons for temperatures between 20 and 300 K. The lifetimes at higher temperatures are mainly due to the anharmonic decay of optical phonons into low-energy phonons. The temperature-independent contributions from inherent crystal defects and from boundary scattering become comparable to the phonon scattering contribution at lower temperatures. The temperature dependence of the population of the acoustic mode (2TA) is found to follow Bose-Einstein statistics.

Optical antennas for tunable enhancement in tip-enhanced Raman spectroscopy imaging

The use of optical antennas in tip-enhanced Raman spectroscopy (TERS) makes it a powerful optical analysis and imaging technique at the nanoscale. Optical antennas can work as nano-light sources in the visible region. The plasmonic resonance of an antenna depends on its length; thus, by varying the length, one can control the enhancement in TERS. In this study, we demonstrated a fabrication method based on focused ion beam milling to realize optical antennas with desired lengths. We then measured the resonances of these fabricated antennas and performed TERS imaging of carbon nanotubes to demonstrate the antenna length dependence on plasmonic resonance.

Confinement effects on the electronic and vibronic properties of CdS0.65Se0.35 nanoparticles grown by thermal annealing

CdS0.65Se0.35 nanoparticles are grown in a glass matrix by thermally annealing a base glass material, in which Cd, S, and Se were introduced by diffusion. The starting base material contains no crystalline structure. A comparative study of the confinement effects on the annealed samples using photoluminescence, low-frequency Raman, and optical Raman scattering experiments is presented. Growth of nanoparticles is observed with the three independent experimental techniques as the annealing temperature is varied from 550 to 800 °C. Radii of the thermally grown nanoparticles calculated from the three independent techniques are found to be in good agreement. In addition, surface phonon modes are observed in the optical spectral range, the frequencies of which agree well with those calculated theoretically. As expected from the theory, the positions of the surface phonons are found to be independent of the particle size.