Surface enhanced Raman scattering of carbon nanotubes decorated by individual fluorescent gold particles

Confocal optical microscopy was employed to study the effect of surface-enhanced Raman scattering on individual single-walled carbon nanotubes covered with isolated gold particles. The gold particles with diameters between 10 and 120 nm were deposited in low densities on the tubes' sidewalls by an electrochemical method. In the spectra, Raman peaks associated with the nanotubes were found to be superimposed on a broad luminescence background originating from the metal particles. With increasing particle size, both the luminescence intensity as well as the Raman enhancement increased at ion er wavelengths. This finding is consistent with a size-dependent broadening of the gold plasmon frequency and a corresponding extension of the energetic range for local field enhancement on the particle surface. In addition, wavelength-dependent experiments revealed a maximum Raman intensity when both nanotube and metal particle were in optical resonance.

Carbon nanotubes functionalized with dye molecules and metal nanoparticles

Tilman Assmus

Single-walled carbon nanotubes are highly interesting quasi one-dimensional nanostructures. They possess a multitude of fascinating optical, mechanical and electrical properties, which can be even further expanded through appropriate functionalization strategies. This thesis covers a number of functionalization strategies and their effects on the nanotube's optical and electrical properties. The first part deals with the functionalization of carbon nanotubes with dye molecules and the characterization of the functionalized system. Both covalent and noncovalent approaches were investigated with the goal of modifying the nanotubes' photoelectrical properties. Upon excitation with light of the appropriate wavelength, a clear electrical response could be observed on some functionalized individual nanotubes. Depending on the attached dye molecule, the conductivity change is either induced by a light-induced conformational change or by a charge transfer between the nanotube and the excited dye molecule. The perspectives of constructing an optoelectronic nanoswitch based on these effects are discussed. The second part of the thesis addresses carbon nanotubes functionalized by a low density of electrodeposited gold nanoparticles with sizes in the range between 10 and 100nm. In this case, the functionalization is targeted on amplifying the nanotube's Raman response (SERS effect). The novel sample preparation technique developed in this thesis allowed for an investigation of SERS on the level of individual molecule / nanoparticle hybrids. The optical emission spectra of the nanotube-nanoparticle hybrid structure disclosed Raman peaks associated with the nanotubes, which are superimposed on a broad luminescence background originating from the metal particles. Wavelength dependent experiments revealed maximum Raman intensity when both the nanotube and the metal particle are in optical resonance. In well-prepared samples the Raman signals collected over isolated particles exhibited an intensity enhancement by at least one order of magnitude, as compared to bare sections on the same tube, without appreciably interfering with polarization dependent Raman measurements.

EPFL2008

Optical properties of small metal clusters

Chongqi Yu

This thesis is devoted to the study of the optical properties of small metal clusters and metal cluster organic compounds. For this purpose a ultra-high vacuum (UHV) experimental apparatus has been built which allows for the deposition of mass selected neutral clusters in a size range up to 16,000 amu which corresponds to Ag150 or Au80. Cluster cations are produced in a gas aggregation cluster source, steered in an ion optics device and co-deposited with electrons and excess rare gas on a cryogenic deposition head. Neutral mono dispersed clusters are analyzed by optical absorption, fluorescence and excitation spectroscopy in the UV-visible energy range. A novel custom-made conical octupole combined to an organic molecule injector has been built, serving as a generator of metal cluster-organic compounds. These are atomically precise model systems which permit to study surface-enhanced Raman scattering (SERS) in order to test the theoretical models and better understand the mechanism of SERS. Optical absorption on size-selected silver clusters of Agn (n= 20 â 120) embedded in neon solid matrix was investigated. The spectra show plasmon-like absorption profiles around a photon energy of 4.0 eV which match in energy molecular like absorption spectra for the smallest Ag clusters but are sensitively higher than what would be expected for the dipolar absorption for bulk silver of about 3.3 eV. The central absorption peak, i.e. the plasmon energy as a function of cluster size is presented. The data fall in a transition between a blueshift of the plasmon resonance with decreasing size to a superimposed red shift for the very smallest sizes. Superimposed, we observe a distinct structure where clusters with atom number of 8,18,34,58,92 show a localized maximum value of the plasmon energy. These numbers correspond perfectly to the fully filled states of 1p, 1d, 1f, 1g and 1h according to the electronic shell model, manifesting a clear sub-shell effect. Optical absorption, fluorescence and excitation spectroscopy on size-selected small gold clusters of Aum (m= 19,20,21) embedded in solid neon were measured. These are the first optical data on ligand free neutral atomically precise Au clusters. Au20, the famous tetraedic structure, shows a rich structured absorption spectrum, superimposed on a continuous background increasing to the UV part of the spectrum. Au21 and Au19 show the same tendency, however without any superimposed structure. Very interestingly, all three Au clusters show strong fluorescence centered at around 1.85 eV, here again Au20 being the prominent species. Comparison to ligand protected Au clusters clearly proves that the fluorescence comes from the Au cluster itself and is only weakly influenced by the ligands. For the first time, size-selected small metal-organic compounds of silver-pyridine AgaPyb (a= 1,3; b= 1,2) which are prototypes for surface enhanced Raman scattering (SERS) have been produced and isolated in a solid Ne matrix. Optical absorption and fluorescence spectra have been measured. Raman scattering cross sections with an enhancement factor of 103 have been found in fair agreement with recent high level time-dependent density functional theory (TDDFT) calculations.

EPFL2015

Metasurfaces for label-free biosensing mid-infrared optics and active photonic devices

Aleksandrs Leitis

Novel two-dimensional metamaterials, known as metasurfaces, have emerged as a breakthrough platform for controlling electromagnetic wave properties at the nanoscale. These metasurfaces consist of subwavelength nanoantennas or so-called meta-atoms, which can be engineered at will to obtain the desired optical functionalities. The doctoral thesis aims to advance the current state-of-the-art metasurface technology for mid-infrared (mid-IR) applications. The first part of the thesis focuses on surface-enhanced infrared absorption spectroscopy with metasurfaces supporting high-quality (high-Q) resonances. Specifically, it introduces an imaging-based nanophotonic method for detecting mid-infrared molecular fingerprints and its' implementation in chemical identification and compositional analysis of surface-bound analytes. This technique features a two-dimensional pixelated dielectric metasurface with a range of spectrally selective resonances, each tuned to a discrete frequency. The method enables a molecular absorption signature read-out at multiple spectral points and the translation of resulting information into a barcode-like spatial absorption map. Furthermore, the thesis demonstrates high-Q angle-multiplexed metasurfaces, which deliver a large number of on-demand resonances in the mid-IR. This method combines chemically specific broadband IR detection with device-level simplicity and spectrometer-less operation of angle-scanning refractometry. Strikingly, these novel metasurface-based chemical detection methods are capable of resolving absorption fingerprints without the need for spectrometry, thereby paving the way toward sensitive and versatile miniaturized mid-IR spectroscopy devices. Yet another major contribution of the thesis includes a universal method for large-scale nanofabrication of various mid-IR metasurfaces. The core of the approach is based on CMOS-compatible processes where the metasurfaces are fabricated on free-standing metal-oxide membranes. To demonstrate the versatility of our method, we realized metasurfaces for a diverse range of applications in the mid-IR, ranging from highly efficient optical wavefront and polarization control to plasmonic metasurfaces for label-free biochemical detection in aqueous solutions. The membrane-based metasurface concept overcomes the limitations of currently used materials in the mid-IR, therefore enabling mass-production of diverse photonic devices with applications in key areas such as biosensing, optical communications, thermal imaging and spectroscopy. The last chapter of the doctoral thesis shows programmable all-dielectric Huygens' metasurfaces consisting of multi-layer Ge disk meta-units with strategically incorporated nonvolatile phase change material Ge3Sb2Te6. Switching the phase-change material between its' amorphous and crystalline structural states enables nearly full dynamic light phase control with high transmittance in the mid-IR. The versatility of the method is demonstrated by optically programming the spatial light phase distribution of the metasurface with single meta-unit precision and retrieving high-resolution phase-encoded images using hyperspectral measurements. The programmable metasurface concept overcomes the static limitations of previous dielectric metasurfaces and moves the metasurface-based technology one step closer to ultra-compact active optical elements encompassing tunable lenses, dynamic holograms, and solid-state spatial light modulators.

EPFL2021