Surface Enhanced Vibrational Spectroscopy and Electrical Characterization on Nanojunctions
Doctor of Philosophy
Molecular junctions are excellent systems for studying electronic and vibrational properties at the single molecule level. Molecular junctions bring together a wide variety of disciplines including quantum physics, nanofabrication and chemistry. Applications include single electron transistors and other novel electronic components. Each molecule possesses its own unique vibrational and electronic energy levels. These unique quantum states provide a fingerprint for identifying the molecule. Furthermore, the quantum states of a molecule can be coupled to nearby photons, atoms, molecules, surfaces and bulk materials yielding a new set of modified energy levels. Studying these highly sensitive and complex interactions requires advanced experimental techniques such as fabricating nanojunctions with sub 10 nm gap sizes. Small gap sizes are not alone enough to guarantee insight into molecular properties. The junctions must be able to magnify and focus photons of hundreds of nanometers in wavelength down into sub 10 nm scales. The corresponding enhancement of the local electromagnetic field is the essential ingredient to probing molecular quantum states. The best way to provide such strong enhancement of electromagnetic fields into sub wavelength regimes is through plasmonics. This thesis discusses the fabrication of plasmonic molecular junctions. These molecular junctions will be shown to demonstrate large electromagnetic fields sufficient for Surface Enhanced Raman Spectroscopy (SERS). These molecular junctions are also used to study simultaneous electronic transport through the molecule. This concurrent study of transport and optical vibrational spectroscopy opens an unprecedented realm into the study of quantum interactions of molecules, electrons, phonons and photons. Specific molecules which will be studied include C60, PCBM and BPE. We demonstrate large, reversible, voltage-driven shifts of vibrational mode energies of C60 and PCBM molecules in gold junctions. C60 mode energies are found to vary approximately quadratically with bias, but in a manner inconsistent with a simple vibrational Stark effect. PCBM mode energies showed a combination of linear and quadratic shifts in vibrational energies with voltage. In addition to facilitating SERS, metallic nanogap junctions also demonstrate the ability to enable Surface Enhance Infrared Absorption (SEIRA) spectroscopy. We demonstrated our nanogap stuctures have the sufficient high field enhancement of detecting the local dielectric composition of nm thick. This can open up opportunities of investigation of heat generation in metallic nanostructures and the surrounding dielectric environment. Throughout this thesis, experimental work will be shown to be in outstanding agreement with theoretical studies based on density functional theory(DFT) and finite difference time domain (FDTD) method.
vibrational spectroscopy; molecular junction; Surface enhanced Raman spectroscopy; Surface enhanced infrared absorption; electrical characterization