Raul Garcia-Sanchez

Thermal Effects Associated with the Raman Spectroscopy of WO3 Gas-Sensor Materials

Metal oxides are suitable for detecting, through conductive measurements, a variety of reducing and oxidizing gases in environmental and sensing applications. Metal-oxide gas sensors can be developed with the goal of sensing gases under specific conditions and, as a whole, are heavily dependent on the manufacturing process. Tungsten oxide (WO3) is a promising metal-oxide material for gas-sensing applications. The purpose of this paper is to determine the existence of a correlation between thermal effects and the changes in the Raman spectra for multiple WO3 structures. We have obtained results utilizing Raman spectroscopy for three different structures of WO3 (monoclinic WO3 on Si substrate, nanopowder, and nanowires) that have been subjected to temperatures in the range of 30-160 °C. The major vibrational modes of the WO3:Si and the nanopowder samples, located at ~807, ~716, and ~271 cm(-1), correspond to the stretching of O-W-O bonds, the stretching of W-O, and the bending of O-W-O, respectively; these are consistent with a monoclinic WO3 structure. However in the nanowires sample only asymmetric stretching of the W-O bonds occurs, resulting in a 750 cm(-1) band, and the bending of the O-W-O mode (271 cm(-1)) is a stretching mode (239 cm(-1)) instead, suggesting the nanowires are not strictly monoclinic. The most notable effect of increasing the temperature of the samples is the appearance of the bending mode of W-OH bonds in the approximate range of 1550-1150 cm(-1), which is related to O-H bonding caused by humidity effects. In addition, features such as those at 750 cm(-1) for nanowires and at 492 and 670 cm(-1) for WO3:Si disappear as the temperature increases. A deeper understanding of the effect that temperature has on the Raman spectral characteristics of a metal oxide such as WO3 has helped to extend our knowledge regarding the behavior of metal oxide-gas interactions for sensing applications. This, in turn, will help to develop theoretical models for the identification of specific metal oxide-gas relationships.

Thermal Characterization of Single-Walled Carbon Nanotubes and Tungsten Oxide-Based Nanomaterials via Raman Spectroscopy

The thermal characterization of single-walled carbon nanotubes (SWCNTs) and tungsten oxide (WO3)-based nanomaterials through the use of Raman spectroscopy is the primary aim of this study, and is focused mainly on the applications of SWCNTs for energy storage and WO3for toxic gas sensing, respectively. In the case of SWCNTs, the properties relevant to their performance obtained via resonant Raman spectroscopy were thermal expansion and thermal conductivity through the exploitation of the latter property’s relationship to the thermal behavior of the Raman G+-band of SWCNTs. In the case of the tungsten oxide-based nanomaterials, the responses of the various Raman signature peaks to different external stimuli, such as temperature variation, humidity changes, and toxic gas exposure, under controlled conditions were investigated.

Raman Spectroscopy, Modeling and Simulation Studies of Carbon Nanotubes

This chapter focuses on two types of carbon nanotubes (CNTs): single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). CNTs are cylindrically-shaped carbon allotropes. They consist of a single layer of sp2-hybridized carbon atoms, giving it a hollow cylindrical shape. The majority of SWCNT samples have diameters on the order of ~1 nm and lengths on the order of microns to centimeters. MWCNTs are composed of concentric layers of SWCNTs nested inside one another, giving it a layered cylindrical shape. In the present chapter, we will provide a historical overview of CNTs and examine specifically their thermal properties as it relates to their applications to the semiconductor industry and nanoelectronics. The understanding of CNT chirality through the visualization of rolled-up graphene sheets will provide insight into the versatility and myriad thermo-mechanical and electrical properties of CNTs. We will focus on the use of Raman spectroscopy and Molecular Dynamics (MD) simulations to characterize and investigate the thermal characteristics of SWCNTs.

Laser Optogalvanic Spectroscopy and Collisional State Dynamics Associated with Hollow Cathode Discharge Plasmas

In this chapter we will discuss the laser optogalvanic effect in a discharge plasma environment, specifically associated with an iron-neon (Fe–Ne) hollow cathode lamp. The history of the optogalvanic effect will serve as an introduction to the importance of the phenomena. The theoretical model behind the optogalvanic effect will provide insight into the importance of laser optogalvanic spectroscopy as a tool for spectral characterization of the plasma processes and enhanced understanding of the collisional state dynamics associated with the discharge species in hollow cathode lamps. The present chapter will focus on transition states of neon in the Fe–Ne hollow cathode lamp. The results presented here will use, for illustrative purposes, the waveforms associated with the laser-excited optogalvanic transitions of neon: 1s4–2p3 (607.4 nm), 1s5–2p7 (621.7 nm), 1s3–2p5 (626.6 nm), 1s5–2p8 (633.4 nm) and 1s5–2p9 (640.2 nm). A comparison between the experimentally recorded optogalvanic signal waveforms and the Monte Carlo fitting routine, along with a discussion related to the variation of the (ai and bj) fitting coefficients as a function of the discharge current, will illustrate the success of our theoretical model. We will also briefly touch upon the potential applications of the optogalvanic effect at the nanoscale in fields such as graphene-based nanoelectronics and nanoplasmonics.

Raman spectroscopy and molecular simulation studies of graphitic nanomaterials

Graphitic nanomaterials, such as graphene and carbon nanotubes, have been of particular interest in the development of applications such as supercapacitors, nanoprobes, drug delivery, biochemical sensors, and storage materials. This chapter will discuss the importance of researching these graphitic nanomaterials through Raman spectroscopy and molecular dynamics simulations. This chapter will also discuss results relating to thermal analysis of purified single-walled carbon nanotubes and the effects of increasing temperature on the Raman features of these materials. Finally, this chapter discusses the results of molecular dynamics simulations based on the Raman spectra thermal analysis results.

Raman Spectroscopy and Molecular Dynamics Simulation Studies of Carbon Nanotubes

Carbon Nanotubes (CNTs) are honey-combed lattices rolled up into cylinders with nanometer-sized diameters and lengths on the order of microns. Actively studied for over thirty years, and with now greater availability, single-walled nanotubes are predicted to significantly impact semi-conductor physics, owing to their unique electronic properties and reduced dimensionality. Some of the semi-conductor technologies in which CNTs are expected to hold significant promise are in super-capacitors, hydrogen storage materials, nanoprobes, and bio-chemical sensors. Necessary to many future CNT applications is a clear understanding of their thermal properties, as nano-devices based on single-walled and/or multi-walled nanotubes may have to experience high temperatures during the manufacturing process while being operated. This, in turn, affects the reliability due to thermal expansion and the ensuing strain in the electronic devices. The coefficient of thermal expansion (CTE) of CNTs is a key property for nano-electronic applications. In this paper we will present Raman Spectroscopy measurements of single-walled carbon nanotubes as a function of temperature in the range 25–200 °C, as well as Molecular Dynamics (MD) simulations that incorporate current state-of-the-art models of Carbon–Carbon interactions associated with the thermal expansion of carbon nanotubes.