Scanning Tunneling Microscope (STM) based Tip-enhanced Raman Spectroscopy (TERS) was used to map Single-walled
Carbon Nanotubes (SWCNTs) dispersed on silicon surfaces. A software program developed with Labview
platform was used to perform the mapping. STM tips made of gold (Au) were fabricated by electrochemical etching and
employed in our TERS system to realize nanoscale spatial resolutions and obtain enhanced signals. Mapping of the
SWCNTs was also performed using a micro-Raman system. It was found that the SWCNTs could be well resolved by
the TERS system but could not be resolved by the micro-Raman system. Further analysis shows that, the ultimate
resolution of the TERS system can reach around 30 nm, while the micro-Raman system shows a resolution around 5 μm.
Controlled growth of self-aligned single-walled carbon nanotubes (SWNTs) was realized using optical near-field effects
in a laser-assisted chemical vapor deposition (LCVD) process. Electronic devices containing ultrashort suspended
SWNT channels were successfully fabricated at relatively low substrate temperatures. According to the numerical
simulations using High Frequency Structure Simulator (HFSS), significant local-heating enhancement occurred at
electrode tip apexes under laser irradiation, which was about ten times higher than the rest part of the electrodes.
Experimental results revealed that the localized heating enhancement at the electrode tip apexes significantly stimulates
the growth of SWNTs at a significantly reduced substrate temperature compared with the conventional LCVD process. The near-field enhancement dependence on metallic film thickness and laser polarization was investigated through numerical simulation using HFSS, which provided a guideline for further optimization of maximum near-field enhancement. This technique suggests a viable laser-based strategy for fabricating SWNT-based devices at relatively low substrate temperatures in a precisely controlled manner using the nanoscale optical near-field effects, which paves the way for the mass production of SWNT-based devices using expanded laser beams.
Raman spectroscopy (RS) is a key tool to characterize residual stress in silicon devices because the vibrational
frequencies of a silicon substrate change with its stress. However, due to the intrinsic optical diffraction limit,
conventional micro-Raman spectroscopy can only have a probe resolution of around 1 μm2, which is not sufficient for
nanotechnology-oriented electronic industry. Low sensitivity is another problem to be solved to maximize the potential
of this technique. In this study, a novel Raman spectrometer, which can overcome the optical diffraction limit, was built
with the attempt to improve the resolution as well as the detection sensitivity. This approach instrument, which is based
upon tip-enhanced near-field effects, has a nanoscale resolution by deploying a silver-coated tungsten tip mounted on a
scanning tunneling microscope (STM) with side illumination optics. It features fast and reliable optical alignment,
versatile sample adaptability and effective far-field signal suppression. The performance was evaluated by observing the
enhancement effects on silicon substrates and single-walled carbon nanotubes (SWCNTs). It was found that apparent
enhancement as high as 120% on silicon substrates could be achieved using the depolarization technique. It is believed
that this technique is promising for future diagnosis of semiconductor materials and devices at nanoscales, especially for
stress mapping of semiconductor devices.
Currently, enhancement of Raman scattering for nanoscale characterization is mostly based on tip- or surface-enhanced
methods. However, both approaches have some dilemmas which impede their wide applications. In this study, we
investigated a novel approach to enhance Raman scattering using closely-packed micro and submicro silica spherical
particles. The enhancement phenomena haven been demonstrated by the silicon phonon mode of crystalline silicon (c-Si)
substrates as well as the vibration modes of single-walled carbon nanotubes (SWCNTs) covered with microparticles. The
studies show that the enhancement effects strongly depend on the particle size. Specifically, when the particle size is
close to the beam waist of the incident laser, the strongest enhancement occurs. Numerical simulations are performed to
calculate electric field distribution inside and outside the dielectric particles using the OptiwaveTM software which is
based on the finite difference time domain (FDTD) algorithm under the perfectly matched layer (PML) boundary
conditions. The simulated results reveal the existence of photonic nanojects in the vicinity outside the particles along
with the light traveling direction. The nanojets outside of the particles with a length of 100 nm and a waist of 120 nm are
believed to be the base for Raman scattering enhancement. This technique has potential applications in many areas such
as surface science, biology, and microelectronics.
We report an apparatus designed to characterize two-dimensional (2D) surfaces of carbon films based on the principle of inelastic light scattering (Raman scattering). The design and construction details are presented. The system with a backscattering configuration, is constructed using a high power argon ion laser with a wavelength of 514.5 nm, an XYZ motorized stage with a step resolution of 3.175 μm, a microscope objective lens, a confocal spatial filter and a holographic notch filter, to achieve extremely low crosstalk and maximum resolution in spectroscopy. The radial resolution for film surface is much enhanced by confocal spatial filter due to its stray light suppression capability. A large depth of sampling field is achieved using an objective lens with a middle NA of 0.55 and a long working distance of 8 mm, thus the requirement of using auto-focusing can be avoided. A specific algorithm is designed to decide the film boundaries as well as the outline of surface structures from pre-defined spectral windows. Control software on LabviewTM platform has been developed for controlling movement of the sample stage, spectral acquisition and data visualization. Single-walled carbon nanotubes (SWCNTs) and patterned silicon were used to evaluate the sensitivity, 1D profile and 2D mapping functionality of the designed system. Diamond-like amorphous carbon (DLC) films prepared by pulsed-laser deposition (PLD) were studied using the developed instrument. The results from this approach are compared with those using general scanning tunneling microscope (STM). This comparable low-cost system with high performance is suitable to characterize semiconductors and other materials both for industrial applications and academic research.