- EaStCHEM Colloquia
- Physics and Astronomy Colloquia
- Irvine Lectures
- Photonics Seminar
- Special Seminars
- Synthesis Seminars
- Cond Mat Seminars
- Organic Semiconductor Centre
- Theoretical Physics Discussion Group
- ScotCHEM Colloquia
- History of Mathematics
- RSC Award Lectures
- Toy List
- Special Mini-Symposium - Structural Chemistry at Central Facilities
Viewing upcoming talks in: Physics and Astronomy Colloquia
Physics and Astronomy Colloquia
Speaker: Dr. Laura Fabris (Rutgers Materials Science and Engineering)
Near field techniques, such as surface enhanced Raman spectroscopy (SERS), rely on the ability of plasmonic nanoparticles to induce localized electromagnetic field enhancements in close proximity to the metallic surface. The possibility of achieving SERS signal enhancements high enough to enable sensitive identification of analytes down to the single molecule level depends on the presence of the so-called “hot spots”, which can be located at the vertices, edges, or crevices in isolated nanoparticles or at narrow junctions between assembled nanoparticles. In turn, the presence of finely tunable hot spots correlates to the possibility of applying SERS as a reliable spectroscopic technique in the analytical and biomedical fields. Our group has worked for several years on the implementation of SERS sensing substrates and imaging tags, in which gold nanostars have demonstrated to be excellent substrates. We have also shown that when these nanostructures are conformally coated with semiconductors such as TiO2 they can efficiently photocatalyze the evolution of hydrogen from water via near IR induced generation of hot electrons. However, for the realization of more quantitative approaches, and for a more reliable E-field manipulation, improved plasmonic platforms are necessary. For this reason, we have established a combined experimental and computational approach that has led us to synthesize by design highly monodispersed gold nanostars with localized surface plasmon resonances tunable between 600 and 2000 nm. We have measured their plasmonic response both at the single particle level (via EELS) and in ensemble averaged samples (UV-Vis and FT-IR spectroscopies), with excellent agreement with the theoretical predictions obtained with 3D finite element simulations, underscoring their importance as testbeds for the design and development of 3D colloidal antennas.
On: February 22, 2019 From: 10h00 To: 11h00View talk
Physics and Astronomy Colloquia
Speaker: Prof. Russell Holmes (University of Minnesota )
Organic semiconductors are conjugated molecular materials whose facile thin film processing and tunable optoelectronic properties have made them of interest for applications in light-emission, detection, and solar photoconversion. In addition, devices made from organic semiconductors can be integrated with a broad range of substrates allowing them to be mechanically flexible, enabling novel form and functionality. In contrast to conventional inorganic semiconductors, the excited state in these materials is a tightly-bound electron-hole pair termed an exciton. In organic semiconductor devices, exciton migration and recombination strongly dictate device design and performance. In an organic photovoltaic cell (OPV), excitons must be efficiently transported to a dissociating heterojunction in order to realize efficient photocurrent generation. In the simplest OPVs, performance is limited by an unfavorable trade-off between a short exciton diffusion length (LD) and the optical absorption length. Consequently, state-of-the-art devices rely on a morphology-optimized mixture of the active materials to reduce the distance an exciton must migrate. Frequently, fluorescence quenching methods are used to experimentally probe LD for potential active materials. Unfortunately, many promising active materials are non-luminescent, or rely on the formation of non-radiative spin-triplet excited states. These systems are not amenable to fluorescence-based methods and hence, values of LD are less frequently reported. While photocurrent spectroscopy can be used to probe optically dark states, such methods often require assumptions to be made about unknown charge carrier recombination losses.
In the first part of this talk, a device-based measurement technique will be described that is capable of yielding the intrinsic material LD despite the presence of unknown recombination losses. The method is first applied to extract LD for luminescent materials to demonstrate agreement with conventional fluorescence-based measurements. We also examine a series of dark small molecule and polymer semiconductors, as well as active materials that exhibit singlet fission to form dark triplet excitons. The broad usefulness of the method is further demonstrated by also extracting LD for a thin film of CdSe quantum dots. The second part of the talk will examine methods to directly engineer LD, including variations in molecular structure, thin film crystallinity, and the population of long lived spin-triplet states.
On: March 8, 2019 From: 10h00 To: 11h00View talk