Browsing by Subject "AFM"

Now showing 1 - 4 of 4
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    Characterization of Graphene Grown Directly on Crystalline Substrates
    (2015-09) Rothwell, Sara
    Graphene has become one of the most popular materials under research, particularly since the 2010 Nobel Prize in Physics. Many visions posit that graphene electronics will be some of the fastest and smallest circuitry physically feasible, however before this becomes reality the scientific community must gain a firm handle on the creation of semiconducting varieties of graphene. In addition, well understood epitaxial growth of graphene on insulating materials will add to the facility of fabricating all-carbon electronics. This thesis presents experimental work detailing the growth of pristine graphene grown on sapphire (GOS) through the thermal decomposition of acetylene, and the electronic characterization of graphene grown on nitrogen-seeded silicon carbide (NG), a semiconducting variety of graphene grown in collaboration with researchers at Georgia Institute of Technology and Rutgers University. GOS displays turbostratic stacking and characteristics of monolayer graphene as analyzed by Raman spectroscopy and atomic force microscopy. Scanning tunneling microscopy characterization of NG illustrates a topography of pleats from 0.5-2 nm tall, 1-4 nm thick, and 1-20 nm long, as well as atomically flat plateaus and other areas of intermixed features. Scanning tunneling spectroscopy measurements across NG features show peaks interpreted as Landau levels induced by strain. Analysis of these Landau levels in coordination with previous characterization concludes that a model employing a bandgap fits best.
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    Data supporting Holey Substrate-Directed Strain Pattering in Bilayer MoS2
    (2021-11-10) Zhang, Yichao; Choi, Moon-Ki; Haugstad, Greg; Tadmor, Ellad B; Flannigan, David J; flan0076@umn.edu; Flannigan, David J
    This data set contains transmission electron microscopy (TEM), atomic force microscopy (AFM), and atomistic simulation data supporting "Holey Substrate-Directed Strain Pattering in Bilayer MoS2" manuscript cited in referenced by.
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    Probe based multimodal and multi frequency methods for material characterization at nanoscale
    (2014-02) Saraswat, Govind
    This thesis aims to provide a methodology and paradigm to aid emerging research studying and manipulating different mechanical properties (like topography, etc) of material. This material research is immensely enhanced by different probe based microscopy enabling design and discovery at the atomic scale. The Atomic Force Microscope (AFM) is one of the foremost technique for such interrogation of material. AFM is a micro-cantilever based device capable of achieving sub Angstrom resolution. Recently, probe researchers have shifted the focus to interrogate physical properties other than topography, namely stiffness and dissipation. Current AFM methods to determine these properties are not suitable for soft samples. Thus there is a scarcity of real-time techniques to quantify the mechanical properties of soft-matter, that include polymers and bio-matter. A method is reported (REEP algorithm) which is able to provide estimates of sample stiffness and dissipation during intermittent contact mode operation of AFM. A systems approach is first applied to relate the material properties to the parameters of a time-varying linear system. Then a RLS algorithm is used to estimate these paramters. The method is verified using the averaging theory and shown to accurately provide the dissipation estimates when applied on different polymer samples. A FPGA based hardware implementing the REEP algorithm is developed to make use of the real-time capability of the algorithm. The REEP module is then used to study stiffness and dissipation properties of PBMA-PLMA polymer blend. A magnetic excitation hardware is also built to accurately resonate the cantilever in liquid. A clean ac response of the cantilever is thus achieved. REEP module is then used to delineate material properties of microtubules in buffer solution. It is shown that REEP module is able to accurately estimate the stiffness and dissipation properties of the bio-sample during under liquid operation of AFM. Higher eigenmodes of cantilever dynamics often come into play when AFM is operated in liquid (low Q operation). To obtain better estimation of the contribution of these modes, a receding horizon Kalman Filter is reported. Estimates are shown to have an order of magnitude improvement compared to current methods. A high bandwidth detection algorithm is also reported which is useful if just the presence of the higher modes is to be detected. Finally, a a new mode of imaging using the equivalent parameters of the time-varying system as a feedback is proposed. Simulation and experimental results show higher resolution for low Q operation. This thesis separately explores a new hypothesis to understand directed transport in cells. Motor proteins which are the work-horse of intracellular transport, walk on microtubules (tubulin polymers) to transfer cellular cargo from one place to another inside a cell. Almost absolute directed transport is achieved while transport takes place over a random arrangement of microtubules. It is shown via simulation and analytical results that if the motors have an ability to switch between different microtubules, a little bias in the microtubule network can lead to directed transport. This renders useless any kind of recognition factor to be needed for motors to know which microtubule is target ended.
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    Vibrational Sum Frequency Generation Spectroscopy (VSFG) Studies of Organic Electronic Molecules
    (2016-12) Sohrabpour, Zahra
    This dissertation describes the application of vibrational sum frequency generation (VSFG) spectroscopy to the study of molecules utilized in organic solar cells (OSC). Two known molecules were chosen for this study, sexithiophene (6T) and fullerene (C60). In the first study VSFG is employed to probe C60 on dielectric surfaces. The SFG activity of this centrosymmetric molecule (theoretically SFG silent) is hypothesized to be from the surface perturbation, resulting an interruption in the symmetry and causing SFG activity. This suggestion is confirmed by experimental VSFG and calculations as well. This study also offers a unique method for estimating surface charge. In the second study the orientation of 6T molecules at different thicknesses is investigated using a combination of VSFG spectroscopy and atomic force microscopy (AFM). The results confirm previous X-ray studies that, 6T has two very different thickness-dependent orientations. At submonolayer thickness, 6T has an edge-on orientation which changes to an end-on orientation with increase in thickness. Additionally, using VSFG and the thin film interference model it is demonstrated that the orientation of 6T molecules are not the same on the two interfaces, 6T/air and substrate/6T. This study highlights the capability of SFG as a surface probe technique to analyze submonolayer thicknesses. The last study showcases electrical measurements on organic photovoltaic devices. The effect of different donor and acceptor materials and donor/acceptor modification through positioning a layer of a modifier molecule at the interface is examined.