K. J. Yi

Enhanced Raman scattering by self-assembled silica spherical microparticles

A technique was developed to achieve enhanced Raman scattering of the silicon photon modes using closely packed micro- and submicron silica spherical particles. Investigation on the particle-size dependence of Raman enhancement revealed that the strongest enhancement occurs when the particle diameter is equal to the spot size of the incident laser beam. Calculations using the OPTIWAVE™ software based on the finite difference time domain algorithm under the perfectly matched layer boundary conditions were carried out. The results showed that photonic nanojets are formed in the vicinity outside the particles along the propagation direction of incident light. It was found that the nanojets are confined to a length of 100nm with a waist of 120nm. The presence of the strongly localized electromagnetic fields within the nanojets accounts for the enhanced Raman scattering. This technique has potential applications both in modern and traditional areas of surface science such as surface oxidation, adhesion, corros...

Direct synthesis of single-walled carbon nanotubes bridging metal electrodes by laser-assisted chemical vapor deposition

Direct synthesis of single-walled carbon nanotubes (SWNTs) bridging prepatterned Mo electrodes has been achieved using laser-assisted chemical vapor deposition (LCVD). The synthesized SWNTs are found predominantly semiconducting. By controlling the spot size of the focused laser beam, synthesis of SWNTs can be achieved in a localized manner, which is governed by the thermal and optical properties of materials as well as the laser parameters. The synthesis process is fast and can be achieved in both far- and near-infrared laser wavelength regions. LCVD method provides a potential approach to in situ remove SWNTs with specific chiralities during the growth.

Tip-enhanced near-field Raman spectroscopy with a scanning tunneling microscope and side-illumination optics

Conventional Raman spectroscopy (RS) suffers from low spatial resolution and low detection sensitivity due to the optical diffraction limit and small interaction cross sections. It has been reported that a highly localized and significantly enhanced electromagnetic field could be generated in the proximity of a metallic tip illuminated by a laser beam. In this study, a tip-enhanced RS system was developed to both improve the resolution and enhance the detection sensitivity using the tip-enhanced near-field effects. This instrument, by combining RS with a scanning tunneling microscope and side-illumination optics, demonstrated significant enhancement on both optical sensitivity and spatial resolution using either silver (Ag)-coated tungsten (W) tips or gold (Au) tips. The sensitivity improvement was verified by observing the enhancement effects on silicon (Si) substrates. Lateral resolution was verified to be below 100 nm by mapping Ag nanostructures. By deploying the depolarization technique, an apparent enhancement of 175% on Si substrates was achieved. Furthermore, the developed instrument features fast and reliable optical alignment, versatile sample adaptability, and effective suppression of far-field signals.

Fabrication of nanostructures with high electrical conductivity on silicon surfaces using a laser-assisted scanning tunneling microscope

Nanostructures with high electrical conductivity were fabricated on silicon surfaces using a laser-assisted scanning tunneling microscope (LA-STM). The nanostructures, dots and lines, were fabricated on H-passivated p-doped silicon (110) surfaces. By precisely controlling the experimental conditions such as pulse energy and tip-surface gap distance, feature sizes (dot diameters and line widths) and heights of the fabricated nanostructures could be controlled. For instance, a dot with a diameter of 30nm and a line with a width of 30nm were obtained. In addition, scanning tunneling microscopy investigation of the structures revealed that their band gaps were changed during the LA-STM process. As a consequence, the local conductivity (more precisely the tunneling probability) was enhanced. Numerical simulations based upon the finite-difference-time-domain algorithm provide detailed insight into the spatial distribution of the enhanced optical field underneath the STM tip and associated physical phenomena. Po...

Controlled-growth of single-walled carbon nanotubes using optical near-field effects

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.

Self-aligned growth of single-walled carbon nanotubes using optical near-field effects

By applying optical near-field effects in a laser-assisted chemical vapor deposition (LCVD) process, self-aligned growth of ultra-short single-walled carbon nanotubes (SWNTs) was realized in a well controlled manner at a relatively low substrate temperature due to the nanoscale heating enhancement induced by the optical near-field effects. Bridge structures containing single suspending SWNT channels were successfully fabricated. Ultra-sharp tip-shaped metallic electrodes were used as optical antennas in localizing and enhancing the optical fields. Numerical simulations using High Frequency Structure Simulator (HFSS) reveal significant enhancement of electrical fields at the metallic electrode tips under laser irradiation, which induces localized heating at the tips. Numerical simulations were carried out to optimize SWNT growth conditions, such as electrode tip sharpness and film thickness, for maximal enhancement of electrical near field and localized heating.

Two-dimensional surface characterization of laser-deposited carbon films using Raman scattering

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.

Mapping of individual single-walled carbon nanotubes using nano-Raman spectroscopy

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.

Simultaneous growth of single-walled carbon-nanotube bridge structures using optical near-field effects

Single-walled carbon nanotubes (SWNT) are regarded as one of the most promising materials for next-generation nano-electronics. However, there are still several challenges limiting its wide applications, including the inability in controlled growth of SWNT connections. In this study, we developed a laser-based in-situ growth approach to simultaneously fabricate SWNT-bridge arrays on a single silicon substrate with precise control. Localized thermal enhancement induced by optical near-field effects and an external electric field enabled the SWNT growth with precise control of growth sites and directions. Furthermore, laser polarization also shows significant influence on the control of growth site for SWNTs. Simultaneous growth of SWNT-bridge arrays in various patterns was achieved. Raman spectroscopy and I-V analysis demonstrated the successful growth of SWNT bridge structures. The laser-based growth method suggests a promising solution for the fabrication of SWNT-based systems in nano-electronics.Single-walled carbon nanotubes (SWNT) are regarded as one of the most promising materials for next-generation nano-electronics. However, there are still several challenges limiting its wide applications, including the inability in controlled growth of SWNT connections. In this study, we developed a laser-based in-situ growth approach to simultaneously fabricate SWNT-bridge arrays on a single silicon substrate with precise control. Localized thermal enhancement induced by optical near-field effects and an external electric field enabled the SWNT growth with precise control of growth sites and directions. Furthermore, laser polarization also shows significant influence on the control of growth site for SWNTs. Simultaneous growth of SWNT-bridge arrays in various patterns was achieved. Raman spectroscopy and I-V analysis demonstrated the successful growth of SWNT bridge structures. The laser-based growth method suggests a promising solution for the fabrication of SWNT-based systems in nano-electronics.

Tip-enhanced near-field Raman spectroscopy using a scanning tunneling microscope with side illumination optics

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.