Browsing by Author "Yang, Yang"

Nanometer-scale metallic structures have been widely and intensively studied over the last decade because of their remarkable plasmonic properties that can enhance local electromagnetic (EM) fields. However, most plasmonic applications are restricted to the visible and near infrared photon energies due to the limitations of the surface plasmon resonance energies of the most commonly used plasmonic metals: Au and Ag. Plasmonic applications in ultraviolet (UV) are of great interest because Raman scattering sections are larger and do not overlap fluorescence spectra. UV plasmonics also benefit from high spatial resolution and low penetration depth. However, an appropriate UV plasmonic material must be identified.

We proposed and demonstrated that gallium is a highly-promising and compelling material for UV nanoplasmonics through synthesis of size-controlled nanoparticle arrays, EM modeling of local field enhancement, ellipsometric and spatial characterization of the arrays, and analytical measurement of UV- enhanced Raman and fluorescence spectra. Self-assembled arrays of hemispherical gallium nanoparticles deposited by molecular beam epitaxy on a sapphire support are characterized with spatial and ellipsometric measurements. Spin-casting a thin film of crystal violet upon these nanoparticles permitted the demonstration of surface-enhanced Raman spectra, fluorescence, and molecular photodegradation following excitation by a HeCd laser operating at 325 nm (UV). Measured local Raman enhancement factors exceeding 107 demonstrated the potential of gallium nanoparticle arrays for plasmonically-enhanced ultraviolet detection and remediation.

The accurate description of ground and electronic excited states is an important and challenging topic in quantum chemistry. The pairing matrix fluctuation, as a counterpart of the density fluctuation, is applied to this topic. From the pairing matrix fluctuation, the exact electron correlation energy as well as two electron addition/removal energies can be extracted. Therefore, both ground state and excited states energies can be obtained and they are in principle exact with a complete knowledge of the pairing matrix fluctuation. In practice, considering the exact pairing matrix fluctuation is unknown, we adopt its simple approximation --- the particle-particle random phase approximation (pp-RPA) --- for ground and excited states calculations. The algorithms for accelerating the pp-RPA calculation, including spin separation, spin adaptation, as well as an iterative Davidson method, are developed. For ground states correlation descriptions, the results obtained from pp-RPA are usually comparable to and can be more accurate than those from traditional particle-hole random phase approximation (ph-RPA). For excited states, the pp-RPA is able to describe double, Rydberg, and charge transfer excitations, which are challenging for conventional time-dependent density functional theory (TDDFT). Although the pp-RPA intrinsically cannot describe those excitations excited from the orbitals below the highest occupied molecular orbital (HOMO), its performances on those single excitations that can be captured are comparable to TDDFT. The pp-RPA for excitation calculation is further applied to challenging diradical problems and is used to unveil the nature of the ground and electronic excited states of higher acenes. The pp-RPA and the corresponding Tamm-Dancoff approximation (pp-TDA) are also applied to conical intersections, an important concept in nonadiabatic dynamics. Their good description of the double-cone feature of conical intersections is in sharp contrast to the failure of TDDFT. All in all, the pairing matrix fluctuation opens up new channel of thinking for quantum chemistry, and the pp-RPA is a promising method in describing ground and electronic excited states.