Doctor of Philosophy
Carbon-based materials, such as 1D single-walled carbon nanotubes (SWCNT) or 2D graphene, are promising materials for a variety of applications in energy storage, biosensors, and medical imaging applications. Similarly, beyond graphene, 2D transition metal dichalcogenides (TDM) that can have either a metallic or semiconducting character have gained interest for potential applications in electronics and photonics. Specifically, metallic TMDs such as vanadium disulfide (VS2) show potential applications in optoelectronics and lithium-ion batteries. On the other hand, semiconducting TDMs like tungsten disulfide (WS2) show a direct band gap, making them interesting for photovoltaic applications, transistors, or photodetectors. The characterization of the chemical and physical properties of such nanomaterials thus become necessary to understand their performance. Moreover, given that structural defects have a negative impact on their integration into devices understanding the formation of these defects is therefore critical. Raman spectroscopy is one of the fundamental techniques that can help to identify chemical properties of materials revealing functional groups at the surface or the presence of crystalline and structural defects. Tip-Enhanced Raman Spectroscopy (TERS) combines Raman spectroscopy and atomic force microscopy (AFM) to obtain spatial resolution that goes beyond the diffraction limit. TERS relies on the resonance of the local surface plasmon (LSPR) and a lightning-rod effect in the vicinity of the apex of a sharp metallic nanoscale tip, yielding to sub 20nm spatial resolution.
In this thesis, 1D and 2D carbon materials such as single walled carbon nanotubes and graphene, are characterized through TERS and artificial intelligence (AI) methods. Artificial neural networks (ANNs), a sub-field of machine learning are applied to analyze large amount of collected spectra and sort them efficiently based on their metallic or semiconductor character. 2D metallic VS2 and semiconductive WS2 are synthesized using chemical vapor deposition and analyzed though TERS discovering surface ripples and hidden layers. Furthermore, through the characterization of VS2, it was found that photo-oxidation process promoted the production of vanadium oxides. Other characterization techniques such as Kelvin probe microscopy (KPFM) and nanomechanical modes were applied to reveal the electronic properties of these materials.
Summary for Lay Audience
Nanotechnology offers numerous benefits for society including advanced materials for health, energy capture and storage, sensors, and green chemistry and chemical engineering. As technology advances and potential applications become reality, the importance of designing materials free of structural defects become of prime importance. These studies require the need of state-of-the-art instruments that facilitates the characterization of those materials with high spatial resolution.
TERS is a technique that complies with such high precision combining topography and vibrational characterization that provides precise physical geometry, structure, and crystallinity information. TERS utilizes a tip with atomic dimensions running over the surface of a sample, revealing topographical details, and simultaneously detecting molecule vibrations through localized Raman measurements. The high spatial resolution of this optical technique allows to determine the presence of defects on the material surface and their impact on the mechanical, chemical, and electronic properties.
Our study focuses on the characterization of one- and two-dimensional materials referred as 1D and 2D materials. Example of 1D nanomaterials are carbon nanotubes which have a diameter less than 1 nm and lengths exceeding hundreds of microns. Examples of 2D materials are graphene or transition metal dichalcogenides (TDM) that can offer a variety of electronic properties varying from metallic to semiconducting. Metallic TMDs, such as vanadium disulfide (VS2), show potential applications in optoelectronics and lithium-ion batteries, whereas semiconducting TDMs, like tungsten disulfide (WS2), with specific band gap characteristics are of interest for photovoltaic and photodetector applications. In this work, the properties of such nanomaterials were investigated using nanophotonics tools in conjunction with nanoscale conduction measurements to better understand the properties of these materials.
Avilés, María Olivia, "Characterization of 1D and 2D Materials with Tip-Enhanced Raman Spectroscopy" (2022). Electronic Thesis and Dissertation Repository. 8909.