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Optical Spectroscopy at the Nanoscale


Recent advances in material science and fabrication techniques enabled development of nanoscale applications and devices with superior performances and high degree of integration. Exotic physics also emerges at nanoscale where confinement of electrons and phonons leads to drastically different behavior from those in the bulk materials. It is therefore rewarding and interesting to investigate and understand material properties at the nanoscale. Optical spectroscopy, one of the most versatile techniques for studying material properties and light-matter interactions, can provide new insights into the nanomaterials. In this thesis, I explore advanced laser spectroscopic techniques to probe a variety of different nanoscale phenomena.

A powerful tool in nanoscience and engineering is scanning tunneling microscopy (STM). Its capability in atomic resolution imaging and spectroscopy unveiled the mystical quantum world of atoms and molecules. However identification of molecular species under investigation is one of the limiting functionalities of the STM. To address this need, we take advantage of the molecular `fingerprints' - vibrational spectroscopy, by combining an infrared light sources with scanning tunneling microscopy. In order to map out sharp molecular resonances, an infrared continuous wave broadly tunable optical parametric oscillator was developed with mode-hop free fine tuning capabilities. We then combine this laser with STM by shooting the beam onto the STM substrate with sub-monolayer diamondoids deposition. Thermal expansion of the substrate is detected by the ultrasensitive tunneling current when infrared frequency is tuned across the molecular vibrational range. Molecular vibrational spectroscopy could be obtained by recording the thermal expansion as a function of the excitation wavelength.

Another interesting field of the nanoscience is carbon nanotube, an ideal model of one dimensional physics and applications. Due to the small light absorption with nanometer size, individual carbon nanotube is not visible under any conventional microscopy and characterization of individual nanotube becomes a focused research interest. Although electron microscopies and optical spectroscopies are developed previously to study carbon nanotubes, none of them permitted versatile imaging and spectroscopy of individual nanotube in a non-invasive, high throughput and ambient way. In this thesis a new polarization-based optical microscopy and spectroscopy is developed with exceedingly better contrast for one dimensional nano-materials and capability of individual carbon nanotube imaging and spectroscopy. This development provides a reliable way to measure the absolute absorption cross-section of individual chirality-defined carbon nanotubes. It also enables fast profiling for growth optimization and in situ characterization for functioning carbon nanotube devices.

Two dimensional systems constitute another important family of nanomaterials, ranging from semi-metal (graphene), semiconductors (transition metal dichalcogenides) to insulators (h-BN). Despite of their scientific significance, they present a complete set of 2D building blocks for two dimensional electronics and optoelectronics. Heterostructures purely made of 2D thin films hold great promises due to functionality, scalability and ultrathin nature. Understanding the properties of the coupled heterolayers will be important and intriguing for these applications. With the advanced ultrafast laser spectroscopy, we study the dynamics of charge transfer process in two dimensional atomically thin semiconductors heterostructures. An extremely efficient charge transfer process is identified in atomically thin MoS2/WS2 system, which is expected to form a type-II heterojunction. Our discovery would greatly facilitate further studies of 2D materials as a photovoltaic device.

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