Finding renewable sources of energy is becoming an increasingly important component of scientific research. Greater competition for existing sources of energy has strained the world’s supply and demand balance and has increased the prices of traditional sources of energy such as oil, coal, and natural gas. The experiment discussed in this paper is designed to identify and build an inexpensive and simple method for creating an effective organic solar cell.

Thin film transistors (TFTs) can be used to determine the bulk-like mobilities of amorphous semiconductors. Different organic hole transporters (HTs) are under investigation including spiro-TPD, 2TNATA, NPB and TPD which are commonly used in organic light-emitting diodes (OLEDs). In addition, we also measure the TFT hole mobilities of two iridium phosphors: Ir(ppy)3 and Ir(piq)3. These materials were grown on two different gate dielectric surfaces which were SiO2 and polystyrene (PS). On SiO2, the TFT mobilities are found to be 1-2 orders smaller than the bulk hole mobilities as evaluated independently by time-of-flight (TOF) technique. On the other hand, on PS gate dielectric layer, the TFT mobilities of these hole transporters are found to be in good agreement with TOF data. A thickness dependence measurement was carried out on TFT with PS. We found that only 10nm of organic semiconductor is sufficient for TFTs to achieve TOF mobilities. We further investigate why organic semiconductors on SiO2 have such huge reduction of mobilities. Temperature dependent mobility measurements were carried out and the data were analyzed by the Gaussian Disorder Model (GDM). We found that on SiO2 surface, when compared to the bulk values, the energetic disorders (σ) of the HTs increase and simultaneously, the high temperature limits (∞) of the carrier mobilities decrease. Both σ and ∞ contribute to the reduction of the carrier mobility. The increase in σ is related to the presence of randomly oriented polar Si-O bonds. The reduction of ∞ is topological in origin and is related to the orientations of the more planar molecules on SiO2. The more planar molecules tend to lie horizontally on the surface and such orientation is unfavorable for charge transport in TFT configuration. Hybrid organic/inorganic perovskites have emerged as an outstanding material for photovoltaic cells. In the second part of this work, we setup a repeatable perovskite recipe and optimized the system under different conditions. Under certain circumstances, a perovskite solar cell with power conversion efficiency ~9% can be achieved with PEDOT:PSS as hole transporting layer with the conventional structure.

I present a new approach to stable n-type doping of organic matrices using organic dopants. In order to circumvent stability limitations of strong organic donors, I produce the donor from a stable precursor compound in situ. As an example, the cationic dye pyronin B chloride is studied as a dopant in an 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) matrix. A field effect and conductivity study of a series of doped NTCDA samples is carried out. It proves the increase of the electron density with the doping concentration. Conductivities up to 1.9*10^-4 S/cm are obtained for doped NTCDA, which is two orders of magnitude higher than the conductivity of NTCDA doped with (bis(ethylenedithio)-tetrathiafulvalene (BEDT- TTF) as investigated previously [A. Nollau, M. Pfeiffer, T. Fritz, K. Leo, J. Appl. Phys. 87, 4340 (2000)], and four orders of magnitude higher than nominally undoped NTCDA films. The experimental trends in the field effect study are interpreted in terms of transport in disordered solids. Detailed mass spectroscopic investigation are carried out to investigate the sublimation behaviour of the organic salt pyronin B chloride. We conclude that mainly HCl and the leuco base of pyronin B is present in the gas phase. Optical absorption spectroscopy shows that the leuco base is transformed to the pyronin B cation in air. A similar reaction is observed for mixed thin films of pyronin B and tetracyano quinodimethane. This supports the image of the doping process being due to an electron transfer from the pyronin to the matrix molecules. The formation of the leuco base and subsequent oxidation to the pyronin B cation is supported by Fourier transform infrared (FTIR) absorption spectroscopy. Doping experiments with other matrices reveal that the doping effect is universal for all materials of moderate acceptor strength. Matrices such as perylene-3,4,9,10-tetracarboxylic dimethyl diimide (Me-PTCDI) and fullerene C60 can be successfully doped. These compounds are frequently employed electron transport materials in organic solar cells. Therefore, such devices can be improved by this new doping approach. / In dieser Arbeit wird ein neuer Ansatz zur n-Dotierung organischer Dünnschichten mit organischen Dotanden vorgestellt. Die bisher zur n-Dotierung benötigten starken Donormoleküle zeigen eine hohe Reaktivität. Dies erschwert die Synthese und weiterhin die Verwendung solcher Verbindungen als Dotanden. Zur Vermeidung dieser Stabilitätsprobleme wird in dieser Arbeit der reaktive Dotand in situ aus einer stabilen Vorläüuferverbindung erzeugt. Beispielhaft wird der kationische Farbstoff Pyronin B Chlorid als Dotand in einer 1,4,5,8-Naphthalen Tetracarbonsäure Dianhydrid (NTCDA)-Matrix untersucht. Feldeffekt- und Leitfähigkeitsuntersuchungen an einer Serie von dotierten Dünnschichten werden durchgeführt. Eine Erhöhung der Elektronendichte mit der Dotierkonzentration wird gefunden. Dies führt zu Leitfähigkeiten von bis zu 1.9*10^-4 S/cm, d.h. vier Größenordnungen höher als undotiertes NTCDA und zwei Größenordnungen höher als das früher untersuchte NTCDA, dotiert mit Bis(Ethylendithio)-Tetrathiafulvalen (BEDT- TTF) [A. Nollau, M. Pfeiffer, T. Fritz, K. Leo, J. Appl. Phys. 87, 4340 (2000)]. Die Abhängigkeiten der elektrischen Kenngrößen Leitfähigkeit, Beweglichkeit und Ladungsträgerdichte werden anhand eine Modells erklärt, das den Transport in ungeordneten Systemen beschreibt. Massenspektrometrische Untersuchungen werden zur Untersuchung des Verdampfungsverhaltens des organischen Salzes Pyronin B Chlorid eingesetzt. Es wird gefunden, daß Pyronin B Chlorid durch Bildung von HCl und der Leukobase des Pyronins in die Gassphase übergeht. In der optische Absorptionsspektroskopie ist die Transformation der Leukobase unter Lufteinfluß in das Pyronin B-Kation zu beobachten. Das gleiche Verhalten wird für Mischschichten aus Pyronin B und Tetracyano Chinodimethan gefunden. Dies bestätigt die Vorstellung des Dotierprozesses als Elektronentransfer vom Pyronin B zum Matrixmolekül. Die Bildung der Leukobase und die anschlie\ss ende Oxidierung zum Pyronin B-Kation ist auch in der Infrarotspektroskopie sichtbar. Der Dotiereffekt ist nicht auf NTCDA beschränkt, sondern wird auch für andere Matrizen mit genügend hoher Elektronenaffinität gefunden. Matrixmaterialien wie z.B. Perylen-3,4,9,10-Tetracarbonsäure Dimethyldiimid (Me-PTCDI) und Fulleren C60 werden erfolgreich dotiert. Sie werden üblicherweise in organischen Solarzellen eingesetzt. Durch den hier demonstrierten Ansatz können folglich solche Bauelemente verbessert werden.

M. Tech. Digital Technology. / This dissertation reports on an optical-based two-dimensional position sensor for use in Active Magnetic Bearings (AMB) to measure the position of the levitated rotor. The motivation for the deployment of optical technology is the well-known advantage of high precision contactless displacement measurement. The radial and axial edges of the rotor are illuminated by red and green laser beams respectively. The position of the rotor is determined from its image projected on a Charge Coupled Device (CCD) sensor. The measuring principle is demonstrated as a position sampler in the closed loop control of an active magnetic bearing model. The image representing the position is processed with a real-time algorithm on a Field Programmable Logic Gate Array. The principle of operation of a CCD as a position sensor is analysed in order to establish how the image captured by the CCD can be processed to determine the position of the rotor. A simple AMB is modelled in which the sensor acts as a feedback position device. The main objective of the model is to evaluate the accuracy of the system. The purpose of the overall sensing technique to be used is to achieve highly accurate and precise measurements with CCD-based optical metrology.

Organic light-emitting diodes (OLEDs) have drawn much attention in the last two decades. In recent years, the power efficiency of OLEDs has been increased to exceed the efficiency of fluorescent light bulbs. However, such high-efficiency devices are typically based on small molecules that have to be evaporated in vacuum. A much higher fabrication throughput and therefore lowered costs are expected if high-efficiency OLEDs were processed from solution. This thesis shows how solution-processed electrophosphorescent multilayer OLEDs can be achieved by starting with an evaporated three-layer device structure and replacing layer by layer with a solution-processed layer. First, the hole-transport layer was replaced by a polymer and high efficiencies were observed when using a hole-transport polymer with a high ionization potential and a low hole mobility. Then, the emissive layer was replaced by a copolymer consisting of hole-transport groups and emissive complexes in its side-chains. OLEDs with four different colors are shown where the orange devices showed the highest efficiency. The orange copolymer was further optimized by making changes to the chemical nature of the polymer, such as different molecular weight, different concentrations of the emissive complex and different linkers between the side-chains and the polymer backbone. Finally, a three-layer solution-processed OLED was fabricated by crosslinking the hole-transport and the emissive layer, and by spin-coating an electron-transport polymer on top. Moreover, using the photocrosslinking properties of the emissive layer, solution-processed multilayer OLEDs of two different colors were patterned using photolithography to fabricate a white-light source with a tunable emission spectrum. Furthermore, with more and more organic semiconductors being integrated into the circuitry of commercial products, good electrical models are needed for a circuit design with predictive capabilities. Therefore, a model for the example of an organic single-layer diode is introduced in the last chapter of this thesis. The model has been implemented into SPICE and consists of an equivalent circuit that is mostly based on intrinsic material properties, which can be measured in independent experiments. The model has been tested on four different organic materials, and good agreement between model and experimental results is shown.

We are using organic small molecules as absorbing material to investigate coherent perfect absorption in layered thin-film structures. Therefore we realize strongly asymmetric resonator structures with a high optical quality dielectric distributed Bragg reflector and thermally evaporated wedged organic materials on top. We investigate the optical properties of these structures systematically by selective optical pumping and probing of the structure. By shifting the samples along the wedge, we demonstrate how relations of phase and amplitude of all waves can be tuned to achieve coherent perfect absorption. Thus almost all incident radiation dissipates in the thin organic absorbing layer. Furthermore, we show how these wedged structures on a high-quality reflective dielectric mirror can be used to determine optical dispersion relations of absorbing materials in a broad spectral range. This novel approach does not require any specific a priori knowledge on the absorbing film.

I present a new approach to stable n-type doping of organic matrices using organic dopants. In order to circumvent stability limitations of strong organic donors, I produce the donor from a stable precursor compound in situ. As an example, the cationic dye pyronin B chloride is studied as a dopant in an 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) matrix. A field effect and conductivity study of a series of doped NTCDA samples is carried out. It proves the increase of the electron density with the doping concentration. Conductivities up to 1.9*10^-4 S/cm are obtained for doped NTCDA, which is two orders of magnitude higher than the conductivity of NTCDA doped with (bis(ethylenedithio)-tetrathiafulvalene (BEDT- TTF) as investigated previously [A. Nollau, M. Pfeiffer, T. Fritz, K. Leo, J. Appl. Phys. 87, 4340 (2000)], and four orders of magnitude higher than nominally undoped NTCDA films. The experimental trends in the field effect study are interpreted in terms of transport in disordered solids. Detailed mass spectroscopic investigation are carried out to investigate the sublimation behaviour of the organic salt pyronin B chloride. We conclude that mainly HCl and the leuco base of pyronin B is present in the gas phase. Optical absorption spectroscopy shows that the leuco base is transformed to the pyronin B cation in air. A similar reaction is observed for mixed thin films of pyronin B and tetracyano quinodimethane. This supports the image of the doping process being due to an electron transfer from the pyronin to the matrix molecules. The formation of the leuco base and subsequent oxidation to the pyronin B cation is supported by Fourier transform infrared (FTIR) absorption spectroscopy. Doping experiments with other matrices reveal that the doping effect is universal for all materials of moderate acceptor strength. Matrices such as perylene-3,4,9,10-tetracarboxylic dimethyl diimide (Me-PTCDI) and fullerene C60 can be successfully doped. These compounds are frequently employed electron transport materials in organic solar cells. Therefore, such devices can be improved by this new doping approach. / In dieser Arbeit wird ein neuer Ansatz zur n-Dotierung organischer Dünnschichten mit organischen Dotanden vorgestellt. Die bisher zur n-Dotierung benötigten starken Donormoleküle zeigen eine hohe Reaktivität. Dies erschwert die Synthese und weiterhin die Verwendung solcher Verbindungen als Dotanden. Zur Vermeidung dieser Stabilitätsprobleme wird in dieser Arbeit der reaktive Dotand in situ aus einer stabilen Vorläüuferverbindung erzeugt. Beispielhaft wird der kationische Farbstoff Pyronin B Chlorid als Dotand in einer 1,4,5,8-Naphthalen Tetracarbonsäure Dianhydrid (NTCDA)-Matrix untersucht. Feldeffekt- und Leitfähigkeitsuntersuchungen an einer Serie von dotierten Dünnschichten werden durchgeführt. Eine Erhöhung der Elektronendichte mit der Dotierkonzentration wird gefunden. Dies führt zu Leitfähigkeiten von bis zu 1.9*10^-4 S/cm, d.h. vier Größenordnungen höher als undotiertes NTCDA und zwei Größenordnungen höher als das früher untersuchte NTCDA, dotiert mit Bis(Ethylendithio)-Tetrathiafulvalen (BEDT- TTF) [A. Nollau, M. Pfeiffer, T. Fritz, K. Leo, J. Appl. Phys. 87, 4340 (2000)]. Die Abhängigkeiten der elektrischen Kenngrößen Leitfähigkeit, Beweglichkeit und Ladungsträgerdichte werden anhand eine Modells erklärt, das den Transport in ungeordneten Systemen beschreibt. Massenspektrometrische Untersuchungen werden zur Untersuchung des Verdampfungsverhaltens des organischen Salzes Pyronin B Chlorid eingesetzt. Es wird gefunden, daß Pyronin B Chlorid durch Bildung von HCl und der Leukobase des Pyronins in die Gassphase übergeht. In der optische Absorptionsspektroskopie ist die Transformation der Leukobase unter Lufteinfluß in das Pyronin B-Kation zu beobachten. Das gleiche Verhalten wird für Mischschichten aus Pyronin B und Tetracyano Chinodimethan gefunden. Dies bestätigt die Vorstellung des Dotierprozesses als Elektronentransfer vom Pyronin B zum Matrixmolekül. Die Bildung der Leukobase und die anschlie\ss ende Oxidierung zum Pyronin B-Kation ist auch in der Infrarotspektroskopie sichtbar. Der Dotiereffekt ist nicht auf NTCDA beschränkt, sondern wird auch für andere Matrizen mit genügend hoher Elektronenaffinität gefunden. Matrixmaterialien wie z.B. Perylen-3,4,9,10-Tetracarbonsäure Dimethyldiimid (Me-PTCDI) und Fulleren C60 werden erfolgreich dotiert. Sie werden üblicherweise in organischen Solarzellen eingesetzt. Durch den hier demonstrierten Ansatz können folglich solche Bauelemente verbessert werden.

No description available.

Ionic self-assembled multilayers (ISAMs) can be formed by alternately dipping a substrate in anionic and cationic polyelectrolytes. Each immersion deposits a monolayer via electrostatic attraction, allowing for nanometer-scale control over film thickness. Additionally, ISAM films can be applied to arbitrary substrate geometries and can easily incorporate a variety of polymers and nanoscale organic or inorganic inclusions. The ISAM technique was used to tune and functionalize a rapid, sensitive fiber optic biosensor for textit{Brucella}, a family of bacteria that are detrimental to livestock and can also infect humans. The sensor was based on a turn-around point long period fiber grating (TAP-LPG). Unlike conventional LPGs, in which the attenuation peaks shift wavelength in response to environmental changes, TAP-LPGs have a highly sensitive single wavelength peak with variable attenuation. ISAMs were applied to a TAP-LPG to tune it to maximum sensitivity and to facilitate cross-linking of receptor molecules. Biotin and streptavidin were used to attach biotinylated hybridization probes specific to distinct species of textit{Brucella}. The sensor was then exposed to lysed cell cultures and tissue samples in order to evaluate its performance. The best results were obtained when using samples from textit{Brucella} infected mice, which produced a transmission change of 6.0 ± 1.4% for positive controls and 0.5 ± 2.0% for negative controls. While the sensor was able to distinguish between positive and negative samples, the relatively short dynamic range of the available fiber limited its performance. Attempts to fabricate new TAP-LPGs using a CO2 laser were unsuccessful due to poor laser stability. A second application of the ISAM technique was as a source of second-harmonic generation (SHG). SHG is a nonlinear optical process in which light is instantaneously converted to half its wavelength in the presence of intense electric fields. Localized surface plasmons (LSPs) in metal nanoparticles produce strong electric field enhancements, especially at sharp tips and edges, that can be used to increase SHG. Colloidally grown silver nanoprisms were deposited onto nonlinear ISAM films and conversion of 1064 nm Nd:YAG radiation to its 532 nm second-harmonic was observed. Little enhancement was observed when using nanoprisms with LSP resonance near 1064 nm due to their large size and low concentration. When using shorter wavelength nanoprisms, enhancements of up to 35 times were observed when they were applied by immersion, and up to 1380 times when concentrated nanoprisms were applied via dropcasting at high enough densities to broaden their extinction peak towards the excitation wavelength. A maximum enhancement of 2368 times was obtained when concentrated silver nanoprisms with LSP resonance around 900 nm were spincast with an additional layer of PCBS. / Doctor of Philosophy / Polyelectrolytes are long molecules composed of chains of charged monomers. When a substrate with a net surface charge is dipped into an oppositely charged polyelectrolyte solution, a single layer of molecules will be electrostatically deposited onto the substrate. Because the surface charge now appears to match the charge of the solution, no further deposition occurs. However, the process can be repeated by rinsing the substrate and immersing in a solution with the opposite charge. This technique forms ionic self-assembled multilayers (ISAMs), which can be assembled with nanometer-level control over thickness. The flexibility of polymer chemistry allows ISAMs to be formed from polyelectrolytes with a wide variety of properties. Additionally, the technique can easily incorporate other nanoscale materials, such as nanoparticles, clay platelets, and biological molecules, and has been investigated for applications ranging from dye-sensitized organic solar cells to drug delivery and medical implant coatings. This dissertation presents two applications of ISAM films. In one, ISAM films were used to tune and functionalize an optical biosensor for Brucella. Brucellosis primarily infects livestock, in which it causes significant reproductive problems leading to economic losses, but can also cause flu-like symptoms and more serious complications in humans. A rapid, sensitive test for Brucella is required to monitor herds and adjacent wild carriers, such as elk and bison. Optical biosensors, which operate by detecting changes due to the interaction between light and the stimulus, could satisfy this need. Long period fiber gratings (LPGs) are periodic modulations induced in the core of an optical fiber that cause transmitted light to be scattered at a resonant wavelength, resulting in attenuation. Conventional LPGs respond to changes in strain, temperature, or external refractive index by shifting their resonant wavelength. When special conditions are met, an LPG may exhibit a turn-around point (TAP), where dual peaks coalesce into a single peak with a constant wavelength but variable attenuation depth. TAP-LPGs are more sensitive than ordinary LPGs, and could be developed into inexpensive sensors with single-wavelength light sources and detectors. In this work, ISAMs were deposited onto an LPG to tune it near its TAP. Segments of single-stranded DNA, called hybridization probes, that were specific to individual species of Brucella were attached to the ISAM film before the sensor was exposed to lysed bacterial cultures. It was found that the sensor could distinguish between Brucella and other types of bacteria, but was less successful at distinguishing between Brucella species. The project was limited by the available TAP-LPGs, which had less dynamic range than those used in prior work by this group. Attempts were made to establish a new supply of TAP-LPGs by fabrication with a CO2 laser, but these efforts were unsuccessful due to poor laser stability. The second project discussed in this dissertation investigated ISAM films as a source of second-harmonic generation (SHG), a nonlinear optical process in which light is converted to half its fundamental wavelength in the presence of intense electric fields. Nonlinear ISAMs were constructed by choosing a polyelectrolyte with a hyperpolarizable side group in which SHG can occur. The SHG efficiency was increased by factors of several hundred to several thousand by the addition of silver nanoprisms. Metal nanoparticles can produce strong electric field enhancements, especially at their tips and edges, when incident light causes resonant collective oscillations in their electrons called localized surface plasmons (LSPs). It was found that while silver nanoprisms whose LSP resonant wavelength matched the fundamental wavelength were too dilute to produce noticeable enhancement, better results could be obtained by depositing shorter wavelength nanoprisms at sufficient density to broaden their extinction peak via interparticle interactions. The best enhancement observed was for a sample where concentrated silver nanoprisms with LSP resonance around 900 nm were dropcast onto an ISAM film and coated with an additional polymer layer, resulting in 2368 times more SHG than the plain ISAM film.

Nanostructured materials play a vital role in almost all aspects of science and technology in the 21st century. The materials include nanoparticles, nanofilms, biological membranes etc. whose physicochemical properties are size-dependent. Thin films have wide range of applications in various branches of science. One of the efficient methods to form miniaturized structures for device applications is to fabricate nanostructured films on different substrates. Surfactant assembly on metallic and non-metallic surfaces based on self assembly and Langmuir-Blodgett technique offers a unique way to form thin films at molecular levels. The process of formation of unimolecular assemblies gives the flexibility of tuning the properties of underlying substrates for various applications including wetting characteristics, lubrication, passivation, mimicking biological phenomena etc. Towards this direction, self assembled monolayers (SAMs) of alkanethiols on gold and silver surfaces have been studied comprehensively for the past two decades. The reported literature on short chain length thiol-based monolayers is however, limited since the formation using conventional methods yield poor quality monolayers. The short chain length monolayers are useful in various applications like tribology, layer-by-layer assemblies, biosensors etc. Hence, it is essential to reproducibly form SAMs of various chain lengths and understand their properties. The present study is related to the formation of SAMs of alkanethiols and diselenides on gold and silver surfaces to form ordered and well-oriented monolayers. Monolayers of varying chain lengths (CH3(CH2)nSH where n = 3, 5, 7, 9, 11, 15) have been formed on gold and silver surfaces using different methods, (1) adsorption from neat thiols; (2) adsorption under electrochemical control and (3) adsorption from alcoholic solutions of the thiols. The characteristics features of the SAMs have been followed based on three different aspects, (i) structure and stability of the methylene groups (ii) interfacial characteristics involving the end group and the solvent and (iii) metal-head group interactions. The structure and stability of the monolayers have been followed based on vibrational spectroscopy and electrochemistry under different environment including thermal perturbations. The stability of the SAMs at different temperatures and subsequent changes associated with the orientation / packing has been monitored both in the dry state using reflection absorption infrared spectroscopy (RAIRS) and under electrochemical conditions using cyclic voltammetry and impedance analysis. Monolayers adsorbed from neat thiols show superior quality in terms of stability and structural arrangement. Short chain thiols with n = 3, 5, 7 show substantial stability when the adsorption is carried out from neat thiols. Figure 1 shows the RAIR spectra of hexanethiol SAM on gold adsorbed by three different procedures. Monolayers adsorbed under potential control behave very similar to the monolayers adsorbed from neat thiol as for as stability and structural orientation are concerned. Monolayers prepared using conventional methods of adsorption from alcoholic solutions are of inferior quality in terms of stability and arrangement especially for the short chain lengths. This is likely to be due to the fact that monolayers prepared using conventional methods may have intercalated solvent molecules within the monolayer assembly that degrade the integrity of the SAM leading to poor quality. The blocking characteristics of the monolayers for diffusing redox couple have been followed by determining the heterogeneous electron transfer rate constant using electrochemical techniques. The spectroscopic data and the electrochemical data follow similar trend indicating the superior quality of monolayer adsorbed from neat thiol in terms of stability as compared to conventionally prepared monolayers. Figure 1. RAIR spectra of hexanethiol-SAMs on Au(111) surface at 25C. The monolayers are formed by adsorption (A) from neat thiol (B) under potential control and (C) from alcoholic solution of the thiol. Wavenumber (cm-1) The interfacial characteristics of the monolayers (effect of end group functionality on the solvent properties) have been monitored on the basis of capacitance, contact angle and atomic force microscopy- measurements. Well-organized monolayers behave like good capacitors with relatively low values of double layer capacitance in presence of a liquid electrolyte as compared to the expected values based on known thickness and dielectric constant of the SAMs. This behavior can be explained by invoking the depletion of water density at the methyl terminated SAM-water interface where the solvent properties are different from that of bulk. Variation of one such property, dielectric constant, has been mapped using force measurement based on AFM. Dielectric constant of water changes from the bulk value of 78 to a low value as given in figure 2. This cross-over occurs within a span of 1-3 nm depending on the chain length of the thiol. Of the three procedures used, the ones based on the use of neat thiol and electrochemical adsorption result in well-oriented alkyl chains followed by highly oriented methyl terminal groups. This is responsible for the high hydrophobic nature of the interface and the subsequent observation of interfacial water properties. The SAMs prepared from ethanol fail to show the hydrophobic effects. Hydrophilic monolayers (NH2 terminated monolayers) fail to show depletion of water density at the interface indicating the importance of end group functionality in altering the interfacial characteristics of the monolayer. Figure 2. Spatial variation of dielectric permittivity of water at the hexanethiol SAM - water interface. The SAM is formed on gold (111) surface; (a) from ethanolic solution of the thiol (b) under electrochemical control (c) from neat thiol. The origin on the x-axis is the position of the methyl groups of SAM and the direction towards right side is in to the bulk water. The well-oriented SAMs have been used to follow the adsorption of a biopolymer. Zein protein is a prolamine of maize and is projected to be a biocompatible coating for food products and food containers. Hence, it is essential to prepare impermeable coatings of zein with different surface wetting properties. The adsorption of zein on highly ordered SAMs with hydrophobic or hydrophilic end group functionality has been studied and the orientation of the protein followed using spectroscopy, microscopy and electrochemistry. It is observed that zein shows higher affinity towards hydrophilic than hydrophobic surfaces with small foot print size on the Figure 3. Orientation of zein protein on hydrophilic and hydrophobic SAM as deciphered from the experimental data. hydrophilic surface resulting in large surface coverage. Figure 3 shows the schematics of zein deposits on hydrophilic and hydrophobic SAM surfaces determined based on spectroscopy, quartz crystal microbalance and electrochemical studies. The AFM shows cylindrical, rod-like and disc-like features of zein on hydrophilic surfaces that form the base units for the growth of cylindrical structures of zein. The published literature on the studies on SAMs on silver surfaces reveals that there is no consensus on the structure of the monolayers on silver. This may be due to the difficulty in getting pristine oxide-free surfaces in the case of silver and this is likely to affect the monolayer quality. Hence, it is decided to prepare SAMs of alkanethiols on silver and study their characteristics. Subtle differences between the monolayers adsorbed from neat thiol and from alcoholic solutions of thiols have been observed in terms of stability and permeability. Atomic force microscopic studies illustrate the presence of depletion of water at the SAM-aqueous interface. Diselenide-based monolayers have been formed on gold to understand the head group-substrate interactions on the monolayer properties. The disorder observed on short chain diselenide-based monolayers formed from alcoholic solutions can be eliminated by adsorption from neat compounds as described for the thiols. A preliminary account on the stability of SAMs under hydrodynamic conditions has been given based on rotating disc electrode voltammetry. It is observed that the SAMs get well-ordered when the electrode is rotated at a fast rate leading to the hypothesis that the monolayer assembly gets annealed as a function of the rotation rate. The thesis is planned as follows: Chapter 1 gives general introduction about organic thin films with particular emphasis on self-assembled monolayers on gold and silver, their characteristics in terms of stability, interfacial properties and adsorption behaviour. Chapter 2 deals with the experimental methodologies and schematics used for the preparation and characterization of the monolayers. Chapter 3 is on the contribution of alkyl spacer to the stability of the monolayers studied using spectroscopy and electrochemistry. Chapter 4 deals with the interfacial properties of the SAMs in presence of aqueous medium. In order to emphasize the importance of the terminal functional groups, adsorption of zein has been demonstrated on surfaces of controlled wettablity. Chapter 5 explains the formation and stability of monolayers of short and long chain alkyl diselenides on gold surfaces. Chapter 6 gives the structural and interfacial characteristics of alkanethiol monolayers on silver surfaces. The stability and subsequent changes of alkanethiol monolayers under hydrodynamic conditions has been discussed in the appendix section.(For fig pl refer pdf file.)