EXPLORING THE SOLID STATE PHASE TRANSITION IN DL-NORVALINE WITH TERAHERTZ SPECTROSCOPY

DL-Norvaline is a molecular crystal at room temperature and it undergoes a phase transition when cooled below 190 \textit{K}. This phase transition is believed to be Martensitic. We investigate this phase transition by measuring its terahertz (THz) spectrum over a range of temperatures. Temperature-dependent THz time-domain spectroscopy (THz-TDS) measurements reveal that the transition temperature ($\textit{T}_{\beta \rightarrow \alpha}$) is 190 \textit{K}. The influence of nucleation seeds was analyzed by determining the $\textit{T}_{\beta \rightarrow \alpha}$ of molecular crystals with varying grain size. Grains of 5 $\mu$m or less result in a lower transition temperature ($\textit{T}_{\beta \rightarrow \alpha}$ = 180 \textit{K}) compared to larger grains of 125–250 $\mu$m ($\textit{T}_{\beta \rightarrow \alpha}$ = 190 \textit{K}). Additionally, we gain insight into the physical process of the phase transition \textit{via} temperature-dependent THz-TDS spectra of doped and mixed molecular crystals. The addition of molecular dopants, which differ from DL-norvaline only at the end of the side chain which resides in the hydrophobic layers of the crystal, decreases $\textit{T}_{\beta \rightarrow \alpha}$. This is consistent with a solid-solid phase transition in which the unit cell shifts along this hydrophobic layer, and it leads us to believe that the phase transition in DL-norvaline is Martensitic in nature

Coherent control of photocurrent in a strongly scattering photoelectrochemical system

A fundamental issue that limits the efficiency of many photoelectrochemical systems is that the photon absorption length is typically much longer than the electron diffusion length. Various photon management schemes have been developed to enhance light absorption; one simple approach is to use randomly scattering media to enable broadband and wide-angle enhancement. However, such systems are often opaque, making it difficult to probe photo-induced processes. Here we use wave interference effects to modify the spatial distribution of light inside a highly-scattering dye-sensitized solar cell to control photon absorption in a space-dependent manner. By shaping the incident wavefront of a laser beam, we enhance or suppress photocurrent by increasing or decreasing light concentration on the front side of the mesoporous photoanode where the collection efficiency of photoelectrons is maximal. Enhanced light absorption is achieved by reducing reflection through the open boundary of the photoanode via destructive interference, leading to a factor of two increase in photocurrent. This approach opens the door to probing and manipulating photoelectrochemical processes in specific regions inside nominally opaque media.Comment: 21 pages, 4 figures, in submission. The first two authors contributed equally to this paper, and should be regarded as co-first author

Solvent Dependence of Lateral Charge Transfer in a Porphyrin Monolayer

Lateral charge transport in a redox-active monolayer can be utilized for solar energy harvesting. A model porphyrin system was chosen to study the influence of the solvent on lateral hole hopping, which plays a crucial role in the charge-transfer kinetics. We examined the influence of water, acetonitrile, and propylene carbonate as solvents. Hole-hopping lifetimes varied by nearly three orders of magnitude among solvents, ranging from 3 ns in water to 2800 ns in propylene carbonate, and increased nonlinearly as a function of added acetonitrile in aqueous solvent mixtures. These results elucidate the important roles of solvation, molecular packing dynamics, and lateral charge-transfer mechanisms that have implications for all dye-sensitized photoelectrochemical device designs

The 2017 Terahertz Science and Technology Roadmap

Science and technologies based on terahertz frequency electromagnetic radiation (100GHz-30THz) have developed rapidly over the last 30 years. For most of the 20th century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to “real world” applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2016, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 17 sections that cover most of the key areas of THz Science and Technology. We hope that The 2016 Roadmap on THz Science and Technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies

EXPLORING THE SOLID STATE PHASE TRANSITION IN DL-NORVALINE WITH TERAHERTZ SPECTROSCOPY

DL-Norvaline is a molecular crystal at room temperature and it undergoes a phase transition when cooled below 190 \textit{K}. This phase transition is believed to be Martensitic. We investigate this phase transition by measuring its terahertz (THz) spectrum over a range of temperatures. Temperature-dependent THz time-domain spectroscopy (THz-TDS) measurements reveal that the transition temperature ($\textit{T}_{\beta \rightarrow \alpha}$) is 190 \textit{K}. The influence of nucleation seeds was analyzed by determining the $\textit{T}_{\beta \rightarrow \alpha}$ of molecular crystals with varying grain size. Grains of 5 $\mu$m or less result in a lower transition temperature ($\textit{T}_{\beta \rightarrow \alpha}$ = 180 \textit{K}) compared to larger grains of 125–250 $\mu$m ($\textit{T}_{\beta \rightarrow \alpha}$ = 190 \textit{K}). Additionally, we gain insight into the physical process of the phase transition \textit{via} temperature-dependent THz-TDS spectra of doped and mixed molecular crystals. The addition of molecular dopants, which differ from DL-norvaline only at the end of the side chain which resides in the hydrophobic layers of the crystal, decreases $\textit{T}_{\beta \rightarrow \alpha}$. This is consistent with a solid-solid phase transition in which the unit cell shifts along this hydrophobic layer, and it leads us to believe that the phase transition in DL-norvaline is Martensitic in nature

Rotational spectroscopy using a Fabry-Perot cavity pulsed Fourier transform microwave spectrometer

Thesis (B.S.) in Chemistry--University of Illinois at Urbana-Champaign, 1985.Bibliography: leaf 20.U of I OnlyTheses restricted to UIUC community onl