Evaluations of single walled carbon nanotubes using resonance Raman spectroscopy

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2004.Includes bibliographical references (p. 77-81).This work reports the results of two studies which use resonance Raman scattering to evaluate the vibrational properties of single walled carbon nanotubes (SWNTs). In the first study, we report an evaluation of second-order combination and over-tone modes in highly ordered pyrolytic graphite (HOPG), in SWNT bundles, and in isolated SWNTs. We found both dispersive and non-dispersive Raman bands in the range 1650-2100 cm⁻¹, and we show that the appearance and frequency vs. laser energy E[laser] behavior of these features are in agreement with predictions from double resonance Raman theory. In the case of SWNTs, these second-order bands depend on the one-dimensional structure of SWNTs, and, at the single nanotube level, the spectra vary from tube to tube, depending on tube diameter and chirality, and on the energy of the van Hove singularity relative to E[laser]. In the second study, we present a theoretical method of predicting, to within a linear constant [beta], the frequency shift in the Raman features of a SWNT material as the Fermi level is changed by depletion or addition of electrons. This constant is then evaluated for different Raman modes in SWNTs by comparing theoretical predictions to experimental observations by Corio et al. , where the Fermi level of SWNT bundles is raised by electrochemical doping and Raman spectra are collected in situ. It is determined that for the G-band of SWNTs, the dependence of frequency on Fermi energy is /[beta][sub]G = 271cm⁻¹ per hole per C-atom for metallic SWNTs with d[sub]f [approximately]= 1.25 ± 0.20nm.by Victor W. Brar.S.B

Three-Omega Thermal-Conductivity Measurements with Curved Heater Geometries

The three-omega method, a powerful technique to measure the thermal conductivity of nanometer-thick films and the interfaces between them, has historically employed straight conductive wires to act as both heaters and thermometers. When investigating stochastically prepared samples such as two-dimensional materials and nanomembranes, residue and excess material can make it difficult to fit the required millimeter-long straight wire on the sample surface. There are currently no available criteria for how diverting three-omega heater wires around obstacles affects the validity of the thermal measurement. In this Letter, we quantify the effect of wire curvature by performing three-omega experiments with a wide range of frequencies using both curved and straight heater geometries on SiO$_2$/Si samples. When the heating wire is curved, we find that the measured Si substrate thermal conductivity changes by only 0.2%. Similarly, we find that wire curvature has no significant effect on the determination of the thermal resistance of a $\sim$65 nm SiO$_2$ layer, even for the sharpest corners considered here, for which the largest measured ratio of the thermal penetration depth of the applied thermal wave to radius of curvature of the heating wire is 4.3. This result provides useful design criteria for three-omega experiments by setting a lower bound for the maximum ratio of thermal penetration depth to wire radius of curvature.Comment: 4 pages, 3 figure

Experimental Demonstration of >230{\deg} Phase Modulation in Gate-Tunable Graphene-Gold Reconfigurable Mid-Infrared Metasurfaces

Metasurfaces offer significant potential to control far-field light propagation through the engineering of amplitude, polarization, and phase at an interface. We report here phase modulation of an electronically reconfigurable metasurface and demonstrate its utility for mid-infrared beam steering. Using a gate-tunable graphene-gold resonator geometry, we demonstrate highly tunable reflected phase at multiple wavelengths and show up to 237{\deg} phase modulation range at an operating wavelength of 8.50 {\mu}m. We observe a smooth monotonic modulation of phase with applied voltage from 0{\deg} to 206{\deg} at a wavelength of 8.70 {\mu}m. Based on these experimental data, we demonstrate with antenna array calculations an average beam steering efficiency of 50% for reflected light for angles up to 30{\deg}, relative to an ideal metasurface, confirming the suitability of this geometry for reconfigurable mid-infrared beam steering devices

Electronic modulation of infrared emissivity in graphene plasmonic resonators

Electronic control of blackbody emission from graphene plasmonic resonators on a silicon nitride substrate is demonstrated at temperatures up to 250 C. It is shown that the graphene resonators produce antenna-coupled blackbody radiation, manifest as narrow spectral emission peaks in the mid-IR. By continuously varying the nanoresonators carrier density, the frequency and intensity of these spectral features can be modulated via an electrostatic gate. We describe these phenomena as plasmonically enhanced radiative emission originating both from loss channels associated with plasmon decay in the graphene sheet and from vibrational modes in the SiNx.Comment: 17 pages, 6 figure

Electronically tunable extraordinary optical transmission in graphene plasmonic ribbons coupled to subwavelength metallic slit arrays

Subwavelength metallic slit arrays have been shown to exhibit extraordinary optical transmission, whereby tunnelling surface plasmonic waves constructively interfere to create large forward light propagation. The intricate balancing needed for this interference to occur allows for resonant transmission to be highly sensitive to changes in the environment. Here we demonstrate that extraordinary optical transmission resonance can be coupled to electrostatically tunable graphene plasmonic ribbons to create electrostatic modulation of mid-infrared light. Absorption in graphene plasmonic ribbons situated inside metallic slits can efficiently block the coupling channel for resonant transmission, leading to a suppression of transmission. Full-wave simulations predict a transmission modulation of 95.7% via this mechanism. Experimental measurements reveal a modulation efficiency of 28.6% in transmission at 1,397 cm^(−1), corresponding to a 2.67-fold improvement over transmission without a metallic slit array. This work paves the way for enhancing light modulation in graphene plasmonics by employing noble metal plasmonic structures

Electronically Tunable Perfect Absorption in Graphene

The demand for dynamically tunable light modulation in flat optics applications has grown in recent years. Graphene nanostructures have been extensively studied as means of creating large effective index tunability, motivated by theoretical predictions of the potential for unity absorption in resonantly excited graphene nanostructures. However, the poor radiative coupling to graphene plasmonic nanoresonators and low graphene carrier mobilities from imperfections in processed graphene samples have led to low modulation depths in experimental attempts at creating tunable absorption in graphene devices. Here we demonstrate electronically tunable perfect absorption in graphene, covering less than 10% of the surface area, by incorporating multiscale nanophotonic structures composed of a low-permittivity substrate and subwavelength noble metal plasmonic antennas to enhance the radiative coupling to deep subwavelength graphene nanoresonators. To design the structures, we devised a graphical method based on effective surface admittance, elucidating the origin of perfect absorption arising from critical coupling between radiation and graphene plasmonic modes. Experimental measurements reveal 96.9% absorption in the graphene plasmonic nanostructure at 1389 cm–1, with an on/off modulation efficiency of 95.9% in reflection

Highly Confined Tunable Mid-Infrared Plasmonics in Graphene Nanoresonators

Single-layer graphene has been shown to have intriguing prospects as a plasmonic material, as modes having plasmon wavelengths 20 times smaller than free space (λ_p ~ λ_0/20) have been observed in the 2–6 THz range, and active graphene plasmonic devices operating in that regime have been explored. However there is great interest in understanding the properties of graphene plasmons across the infrared spectrum, especially at energies exceeding the graphene optical phonon energy. We use infrared microscopy to observe the modes of tunable plasmonic graphene nanoresonator arrays as small as 15 nm. We map the wavevector-dependent dispersion relations for graphene plasmons at mid-infrared energies from measurements of resonant frequency changes with nanoresonator width. By tuning resonator width and charge density, we probe graphene plasmons with λ_p ≤ λ_0/100 and plasmon resonances as high as 310 meV (2500 cm^–1) for 15 nm nanoresonators. Electromagnetic calculations suggest that the confined plasmonic modes have a local density of optical states more than 10^6 larger than free space and thus could strongly increase light–matter interactions at infrared energies

Controlling and creating plasmonic absorption processes in graphene nanostructures

In this presentation, it will be shown that the plasmonic absorption of a graphene sheet can be enhanced and perturbed in controllable ways by controlling the thickness and permittivity of the supporting substrate. We will show the results of recent experiments where 25% absorption is achieved in the plasmonic modes of a graphene sheet by carefully selecting the properties of an underlying silicon nitride substrate. We also demonstrate how additional absorption pathways can be created by modifying the surrounding dielectric environment to have optical resonances that can couple to the graphene plasmons