Fabrication of nanostructures on tips a proposed design for SNOM/TERS probes

Abstract

Scanning near-field optical microscopy (SNOM) opens new frontiers in optical microscopy and spectroscopy measurements beyond the diffraction limit. The evolution of the nanotechnology field of devices with new functionality, integration of new materials to form multifunctional devices, and the understanding of chemical interactions within complex biological systems such as DNA, bacteria, cells demand new tools to understand the physics and chemistry of the materials on the molecular level. SNOM arises as a pioneer tool for optical imaging in the sub-wavelength dimension [1, 2]. In the SNOM measurement, a probe with a sharp tip is illuminated with light all through its raster scan over the sample surface in the locale of the sample nearfield. Hence, the measurement in principle is limited by the volume of interaction of the nearfield with a size on the order of the probe tip radius and the sample surface [3]. SNOM emerged in 1984 as an optical counterpart of scanning probe microscopy. The idea had been introduced by E.H. Synge in 1928 in [4]. After decades it had been recognized as a visionary way to circumvent the optical microscopy classical diffraction limit [5]. SNOM complementary information to other scanning probe microscopy methods can be gained without pre-treatment of the samples and in a non-destructive way. Further, SNOM measurements can be performed for a broad range of samples and in native environments. SNOM has been successfully accompanied with spectroscopy measurements, such as tip-enhanced uorescence spectroscopy and/or tip-enhanced Raman spectroscopy (TERS) [6], since the visible optical wavelengths have energies that fall in the energy range of the electronic and vibrational transitions in molecules. Hence, simultaneous measurements were performed to characterize the samples' morphology and chemical properties. TERS measurements record the Raman scattering of the samples in nanoscale dimensions. They reveal valuable information about chemical interactions, since Raman scattering is considered as the fingerprint of the compounds and elements inside the material down to single molecules [7-9]. A recent development in the SNOM setups has been reported, in which the SNOM probe can scan the near field of the samples in the z-direction in addition to the x and y direction [10, 11]. Hence, SNOM field mapping in the 3D space has been recorded for different samples such as Cu nanoparticles [12] and Boron nitride nanotubes [13]. The resolution of the SNOM and TERS instruments is defined by the probes. Consequently, the fabrication of high quality and reproducible probes is at the core of developing the SNOM and TERS. There are a variety of probes types, where the plasmonic nanostructures serving as an antenna are the most successful ones in recording a high resolution and field enhancement. Outline Motivated by the theory and experimental results on the field enhancement and the high resolution of the nanocone probe, we aim to modify the fabrication of nanocones on non-planar substrates to have a good reproducibility and control over the cone geometry. In this work, we propose the fabrication of high quality SNOM/TERS probes, where the fabrication method has a good throughput and reproducibility. This work is divided into five chapters presenting the theory of optics and the physics of the fabrication techniques, then the fabrication method and the experimental results. Chapter 1 is a survey on the electrodynamics of plasmonics, then the SNOM/TERS principles and the state of art of the probes, finally the plasmonic nanocone optical properties, fabrication and application. Chapter 2 presents the theory of the fabrication techniques employed to engineer the nanocone. Chapter 3 shows the steps of the fabrication method of the nanocone on cantilever and optical fiber. Chapter 4 presents the optimization and experimental results of the fabricated probes made of a single gold nanocone on an AFM-cantilever. Chapter 5 presents the fabrication of gold nanocones on optical fibers and optimization of the probe's quality.