Session M50: Nanostructures and Metamaterials

Single-molecule Raman mapping with sub-nm resolution

Visualizing individual molecules with chemical recognition is a longstanding target in catalysis, bio-imaging, molecular nanotechnology, and material science. Molecular vibrations provide a valuable ``fingerprint'' for this identification. The spectroscopy based on tip-enhanced Raman scattering (TERS) has opened a path to obtain enhanced vibrational signals thanks to the strong localized plasmonic field originated at the tip apex. However, the best spatial resolution of the TERS imaging reported to date is still limited to a few nm, obviously not adequate for resolving a single molecule chemically. Here we demonstrate unprecedented sub-molecular Raman spectroscopic mapping with spatial resolution below 1 nm, resolving even the inner structure of a single molecule and its configuration on the surface [1]. This is achieved by creating a double-resonance nonlinear process via spectral matching, particularly by matching the resonance of the nanocavity plasmon to the downward molecular vibronic transitions [2]. Such exquisite tuning capability is provided by a combination of low-temperature ultrahigh-vacuum scanning tunneling microscopy with ultrasensitive optical detection. Our nonlinear TERS technique features the use of only a continuous wave laser rather than two pulse lasers. Our finding of Raman spectromicroscopy going intra-molecular and sub-nanometer may open up a new avenue to probe surface chemical identification, optical processes and photochemistry at the single-molecule scale. \\[4pt] [1] R. Zhang, et al., Nature 498, 82 (2013).\\[0pt] [2] Z.C. Dong, et al., Nature Photonics 4, 50 (2010).   

Effect of dielectric spacer layers on SERS with Au nanoparticle arrays on silicon substrates

The optical response of a plasmonic nanostructure is often highly dependent on the nature of the substrate supporting it. To study the effect of the substrate on surface enhanced Raman scattering (SERS), we have fabricated a series of SERS substrates consisting of a hexagonal array of Au nanoparticles self assembled on block copolymer films, a silicon oxide (dielectric) layer and a silicon substrate. The inter-particle distance and the dielectric layer thickness were controlled. The SERS Enhancement Factors (EF) were calculated by comparing the Raman spectra of 4-aminothiophenol adsorbed on the surface of the Au nanoparticles and in a standard solution. The SERS EF were found to be strongly affected by the inter-particle distance and silicon oxide thickness. Changing the inter-particle spacing induced a 10$^{\mathrm{2}}$ variation in the EF while changing the oxide thickness increased the range of EF values by an additional factor of 10. Maximal enhancement factors were found with oxide layer thicknesses between 50 nm and 100 nm beneath the 30 nm polymer film. This geometry both improved the resonance condition with the probe laser and reduced the absorption by the substrate. This work illustrates that optimization of plasmonic-based sensors should consider both the metallic and the surrounding structures.   

Mechanisms of surface-enhanced Raman scattering on metal oxide nanowires

For several decades, surface-enhanced Raman scattering (SERS) from various analytes adsorbed on metal has been studied and utilized for optoelectronic and biochemical devices. There are two well accepted mechanisms of SERS on metal: One is the electromagnetic enhancement and the other is the charge transfer enhancement. On the other hand, another mechanism depending on the geometry, for example diameter, aspect ratio, etc., of the sample has been considered recently to explain SERS from molecules adsorbed on dielectric nanostructures where far less free charges exist. In this study, we would like to explain the mechanism of SERS on metal oxide nanostructures. To study enhancement effects, we measured Raman scattering signal from molecules adsorbed on metal oxide nanowires and nanocones excited by lasers with three different wavelengths. We observed that the Raman signal was enhanced regardless of excitation wavelengths, even though the enhancement factor showed slight wavelength dependence. Importantly, we observed that the enhancement was always larger when the analytes were adsorbed on nanocones. From our understanding, we can suggest a way to systematically create, or control ``hot spots'' for enhancement of light field using one dimensional metal oxide nanostructures.   

Electrostatic gating and single-molecule Raman spectroscopy

Simultaneous electronic and optical measurement on molecular junctions provide rich microscopic information about electronic and vibrational energy distributions at the atomic and molecular scale. We fabricate nanoscale gold bowtie structures as surface enhanced Raman (SERS) substrates. Following electromigration, these nanostructure with nanoscale interelectrode gaps support highly localized surface plasmon resonances, resulting in single-molecule sensitivity due to the high electromagnetic enhancement. In prior electronic transport studies, these structures have proven to be suitable tools to examine electronic and vibrational properties of single molecules, in which the underlying substrate is used as a gate electrode to capacitively shift the molecular level relative to the Fermi levels of the source and drain, enabling the studies in the nonresonant, resonant and Coulomb blockade regime. We will present preliminary results on the effect of gate modulation on the SERS and electrical properties of molecules in such junctions.   

Raman spectroscopic investigation of lithium niobate nanoparticles

Recently there has been a large interest in synthesizing nanoscale structures from ferroelectric materials. Due to the tendency of the nanoscale structures to form aggregates, characterizing the properties of isolated nanostructures can be challenging. Through combining Raman spectroscopy with an optical trap, we investigated the properties of lithium niobate nanoparticles synthesized by the sol-gel method. Analysis of the Raman spectrum shows that the stoichiometry of the nanoparticles is dependent on the starting stoichiometric ratio of lithium to niobium in the synthesis step. We also demonstrate the power of this technique to determine the orientation of ferroelectric nanoparticles in an external applied field.   

Crystal Structure Anisotropy Explains Anomalous Elastic Properties of Metal Nanorods

It is demonstrated that the frequency of the extensional vibrational mode of a nanorod made of an elastically anisotropic crystalline material deviates widely from the predictions of the theories based on the analysis of the long-wavelength limit. The dispersion relation for the fundamental extensional mode of a gold rod grown in the $[100]$ direction is calculated and found to be in an excellent agreement with experimental data obtained from the transient optical absorption measurements on gold nanorods.\footnote{H. Petrova, J. Perez-Juste, Zh. Zhang, J. Zhang, T. Kosel, and G.V. Hartland, J. Mater. Chem. {\bf 16}, 3957 (2006)} This explains an anomaly in the elastic properties of nanorods which was previously attributed to a 26\% decrease in Young's modulus for nanorods compared to its bulk value.   

A New Model with Internal and External Traps explaining Fluorescence Intermittency in a Quantum Dot Studied with Scanning Tunneling Spectroscopy

Recent studies on fluorescence intermittency, or called blinking, in quantum dots (QDs) show that complete control of this phenomenon is near at hand. Although a number of models deal with the transitions between on/off states in the intermittency, they do not consider the spatial and energy distribution of traps in a single QD. In this study, we measured the spatial and energy distribution of traps using scanning tunneling microscopy and spectroscopy. The trap states of CdSe/ZnS QD exhibit two distinct energy states and intensities in the tunneling spectra according to their residing positions (inside or surface of QD). We were able to simulate trapping dynamics of the fluorescence intermittency from the measured energy and spatial distribution. We used Monte Carlo method to render transitions between the trap states in this model. We can successfully explain the power-law distribution of on/off time, which is a characteristic feature of the blinking. The dependence is a consequence of a two-step trapping process through inner and surface traps. The simulation also predicts the suppression of the long tail in the power-law distribution by reducing the surface traps. This result is in good agreement with a recent fluorescence lifetime-intensity distribution measurement.   

Exploring the charge/energy transfer process at the graphene/giant nanocrystal quantum dots interfaces

Due to its transparency in wide spectral range and high charge mobilities, graphene has been considered to utilize as transparent electrode for nanocrystal based photo-voltaic and light emitting diodes.~ A detail understanding on charge/energy transfer (CT/ET) processes between zero dimensional quantum dots and 2D graphene layer hold the key in optimizing the performance of these devices.~ To attain this understanding, we conduct a systematic study on CT and ET processes between a graphene layer and~ CdSe/CdS giant nanocrystal quantum dots (g-NQD) as the function of CdS shell thickness.~ In addition to analyzing PL quenching and change of PL decay dynamic, we also perform 2$^{nd}$ order photon correlation spectroscopy studies to investigate the effect of graphene layer on dynamic and emission efficiency of g-NQDs' multi-exciton states.~ In case of g-NQDs over coated with a thick 16 ML CdS shell, we observed a surprising increase of multi-exciton emission efficiency.