Browsing by Subject "Chemical Physics"

Now showing 1 - 2 of 2
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    The electrostatically embedded many-body method for the efficient computation of properties of atmospherically relevant nanoparticles
    (2012-09) Leverentz, Hannah R.
    Condensed-phased particles suspended in the earth's atmosphere are called aerosols. These particles have important effects on climate and human health, but the mechanisms by which many of these particles form is not well understood. Experimental techniques for studying the early stages of the formation of these particles currently are not available, so computational methods that are capable of accurately handling property calculations on tens of thousands of configurations of the molecular clusters that serve as precursors to aerosols are needed in order to make predictions about the mechanisms of aerosol formation in the atmosphere. Fragment-based computational methods are a promising avenue for the affordable and accurate calculation of properties of large molecular clusters. Fragment-based methods use linear combinations of property calculations done on fragments of the system to obtain an approximation of those properties for the entire system, rather than attempting a highly demanding computation of those properties based on considering all of the molecules in the system at once. Many successful fragment-based methods have been presented in the literature, but this thesis focuses on one fragment-based method that is particularly straightforward and easy to implement: the electrostatically embedded many-body (EE MB) method. The thesis demonstrates the ability of the EE MB method to make accurate predictions of properties of atmospherically relevant clusters and explores a few variations of the EE MB approximation that were developed specifically for Monte Carlo simulations of atmospheric nucleation.
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    In-situ optical spectroscopy of the organic semiconductor/electrolyte dielectric interface.
    (2009-08) Kaake, Loren G.
    The implementation of organic thin film transistors into microelectronic devices hinges upon the development of organic semiconductors and gate dielectric materials. In a working device, the place where the two materials meet is critical to device performance. This buried interface between the organic semiconductor and the gate dielectric is notoriously difficult to characterize. One way to probe this interface is through the use of attenuated total internal reflection Fourier transform infrared and near infrared spectroscopy (ATR-FTIR). This method allows one to do optical spectroscopy on a working device and gain insight into the physical processes which occur at the semiconductor/dielectric interface during the application of voltage. One example of such a process is the induction of charge in the organic semiconductor layer. Charging of an organic semiconductor gives rise to distinct spectroscopic signatures which can be used to characterize properties intrinsic to the semiconductor. For example, high charge carrier density can give rise to unique spectroscopic signatures which may be related to the Mott insulator to metal transition. Crystallinity affects the spectral signatures of charge carriers, and these effects can give insight into sources of energetic disorder in the solid. Examining semiconductor charging also gives insight into the operating mechanisms of the dielectric material responsible for inducing charge carriers. Dielectric materials using mobile ions have become attractive for use in organic thin film transistors because they allow low voltage transistor operation. The physical mechanisms for charge induction are distinguishable when in-situ optical spectroscopy is applied. For example, the mobile ions in a dielectric material can penetrate the bulk of the semiconductor film or stop at the semiconductor/dielectric interface depending on the size of the ion. The rate of charging can be analyzed and used to estimate a material specific maximum operating speed of an electrochemical transistor. Using optical spectroscopy to examine the organic semiconductor/electrolyte dielectric interface gives insight into many aspects of device operation, many of which are critical to making organic thin film transistors a viable technology.