Range separated hybrid density functional study of organic dye sensitizers on anatase TiO2_2 nanowires

The adsorption of organic molecules coumarin and the donor-$\pi$-acceptor type tetrahydroquinoline (C2-1) on anatase (101) and (001) nanowires have been investigated using screened Coulomb hybrid density functional theory calculations. While coumarin forms single bond with the nanowire surface, C2-1 additionally exhibits bidentate mode giving rise to much stronger adsorption energies. Nonlinear solvation effects on the binding characteristics of the dye chromophores on the nanowire facets have also been examined. These two dye sensitizers show different electronic charge distributions for the highest occupied and the lowest unoccupied molecular states. We studied the electronic structures in terms of the positions of the band edges and adsorbate related band gap states and their effect on the absorption spectra of the dye-nanowire combined systems. These findings were interpreted and discussed from the view point of better light harvesting and charge separation as well as in relation to more efficient charge carrier injection into the semiconductor nanowire.Comment: 8 pages, 4 figures, and 1 tabl

Theoretical and experimental investigation on nanostructures

Ph.DDOCTOR OF PHILOSOPH

Investigation of solar cells and phototransistors based on hybrid copper (I) thiocyanate: methanofullerene materials

The combination of organic and inorganic semiconductors offers a route to the development of solar cells and other optoelectronic devices that combine flexibility, high efficiency and high stability. In this thesis, we have investigated the hybrid organic: inorganic system, copper (I) thiocyanate (CuSCN): phenyl C71 butyric methyl acid ester (PC70BM). We have shown that ~100 nm long CuSCN nanowires can be grown within PC70BM when a layer of PC70BM containing dilute CuSCN is deposited on top of a CuSCN layer on a heated substrate. Photovoltaic devices made from these nanowire-containing layers perform significantly better than devices made from CuSCN/PC70BM bilayers because the nanowires improve charge collection efficiency. The initial device based on a nanowire-containing CuSCN:PC70BM structure, the device shows ~5% power conversion efficiency. We show that these CuSCN NWs form well when the substrate temperature is about 115oC. We then discovered that by spin-coating interlayers with a CuSCN rich precursor solution between the compact CuSCN layer and CuSCN:PC70BM photoactive mixed layer, the power conversion efficiency could be further improved to nearly 6% power conversion efficiency. Finally, we measured the carrier mobility of CuSCN:PC70BM layer using field-effect transistors. We find the carrier mobility values for holes and electrons to be on the order of 10-3 to 10-2 cm2/Vs, similar to values for typical organic semiconducting materials. We have also characterized the same device as a phototransistor by illuminating the active area with light in the visible range. This work has demonstrated the concept of the initial application of CuSCN nanowires, which has never been reported in any organic: inorganic mixed layer. CuSCN nanowires could be a new approach for optimizing charge separation in light-absorbing acceptor systems.Open Acces

Precision manipulation of organic and inorganic nanoentities for enhanced optical biodetection at deterministic positions

In the last decade, considerable research interests are focused on applying semiconductor quantum dots (QDs) for bioimaging, sensing, and therapeutic delivery. Compared to traditional organic dyes, semiconductor QDs exhibit higher fluorescent brightness, better resistance to photo-bleaching, tunable sizes/colors, wider absorption peak and larger stokes shifts. However, the applications of QDs as biosensors are still largely conducted in bulk colloidal suspensions, which present considerable difficulties in sensing a minute amount of bioanalyte. It is highly desirable if the QDs can be registered at designated locations for position-predicable optical analysis and sensing. Raman scattering spectroscopy has been utilized to unambiguously identify molecules based on their intrinsic vibrational "fingerprint" states. However, due to the relatively small Raman scattering cross-section, the intensity of Raman signal is usually 1/10⁶ of that of Rayleigh scattering. The recent discovery of Surface enhanced Raman scattering (SERS) dramatically improves the Raman signal and rejuvenates this field. An enhancement factor (EF) as high as 10¹² have been reported, which can readily detect various single molecules, essential for early-stage disease detection, warfare agent detection, environmental pollutant detection, and biomolecule detection. However, SERS substrates with such high EF usually suffer from reproducibility and uniformity issues. Moreover, SERS detection is still largely conducted in a seek-and-find manner which substantially limits the detection efficiency. Most SERS detections are carried out by drying analyte solutions on SERS substrates to force molecules to attach to hotspots before the detection. The employed drying methods can be different among individual research groups. Quantitative comparison of these results should be conducted carefully. It is highly desirable to directly detect molecules in suspension to accurately evaluate the performances of different SERS substrates. However, when directly measuring SERS signals of molecules in suspension, due to the inefficient diffusion based binding process, much less molecules can closely interact with hot spots compared to those on dried SERS samples. As a result, direct SERS detection from suspension can often be less sensitive by a few orders of magnitudes compares to those in dried condition. It is of great interest to investigate new mechanisms to detect analyte molecules directly from analyte solutions with high sensitivity. In this research, I rationally designed and synthesized various types of nanostructures, including ZnO, Si, and Au nanowires, ZnO nanosuperstructures, and hybrid nanocapsules. Such materials have unique optical/plasmonic properties and could be used in various applications, particularly in biochemical sensing. Two types of optical nanobiosensors have been designed, fabricated, characterized, and investigated. They are fluorescence-based QD-on-nanowire assemblies and SERS-photonic-crystal hybrid nanosensors. The QD-on-nanowire florescent nanosensors operated uniquely by focusing analyte molecules to the assembled QDs on tips of nanowires before detection via specific biochemical conjugation. Molecules, such as biotin, can be revealed unambiguously in a location deterministic manner with substantially enhanced sensitivity. In the development of SERS-photonic-crystal hybrid nanosensors, two enhancement mechanisms, including guided-mode resonance (GMR) and electrokinetic effect, were studied and applied in improving the sensitivity and efficiency of molecule detection, respectively. Such a hybrid device has been proposed and studied for the first time, which can readily improve the detection sensitivity by a robust 4-5 times in addition to the 10⁹-10¹⁰ SERS enhancement. This dissertation work, exploring innovative materials design, synthesis, and manipulation, has made an important forward step in the next-generation biochemical detection platform.Materials Science and Engineerin

Advanced Synchrotron Radiation Techniques for Nanostructured Materials

Nanostructured materials exploit physical phenomena and mechanisms that cannot be derived by simply scaling down the associated bulk structures and phenomena; furthermore, new quantum effects come into play in nanosystems. The exploitation of these emerging nanoscale interactions prompts the innovative design of nanomaterials. Understanding the behavior of materials on all length scales—from the nanostructure up to the macroscopic response—is a critical challenge for materials science. Modern analytical technologies based on synchrotron radiation (SR) allow for the non-destructive investigation of the chemical, electronic, and magnetic structure of materials in any environment. SR facilities have developed revolutionary new ideas and experimental setups for characterizing nanomaterials, involving spectroscopy, diffraction, scatterings, microscopy, tomography, and all kinds of highly sophisticated combinations of such investigation techniques. This book is a collection of contributions addressing several aspects of synchrotron radiation as applied to the investigation of chemical, electronic, and magnetic structure of nanostructured materials. The results reported here provide not only an interesting and multidisciplinary overview of the chemicophysical investigations of nanostructured materials carried out by state-of-the-art SR-induced techniques, but also an exciting glance into the future perspectives of nanomaterial characterization methods