Browsing by Subject "Nanoscale characterization"
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Item Infrared nano-spectroscopy via molecular expansion force detection(2015-12) Lu, Ph. D., Feng; Belkin, Mikhail A.; Yu, Edward; Dodabalapur, Ananth; Shvets, Gennady; Ellison, ChristopherMid-infrared absorption spectroscopy in the “molecular fingerprint” region (λ = 2.5–15 μm) is widely used for in situ analysis of chemical and biological samples. Due to the diffraction limit, traditional far-field techniques such as Fourier-transform infrared spectroscopy cannot take sample spectra with nanometer spatial resolution. To conduct nanoscale infrared measurement, in photoexpansion nano-spectroscopy, an atomic force microscope cantilever is used as a light absorption detector, in the way that the cantilever is deflected proportionally by the localized sample heating and expansion induced by infrared pulses. Previous studies of this new opto-mechanical technique demonstrated its powerfulness and simplicity, but relied on using high-power laser pulses to produce detectable cantilever deflection signal and it was difficult to measure ultra-thin samples below ~100 nm. In addition, the spatial resolution, though improved, is limited by the thermal diffusion length inside samples. This dissertation presents a set of experiments which have substantially improved photoexpansion nano-spectroscopy in terms of sensitivity and spatial resolution, and have explored other aspects of this technique. For the first time, high-quality photoexpansion spectra have been obtained from molecular monolayers using low-power infrared pulses from a tunable quantum cascade laser. The orders of magnitude improvement in sensitivity is due to the two methods we implemented: mechanical enhancement by the cantilever resonance, and optical enhancement by the metalized cantilever tip. The spatial resolution is also improved and only determined by the locally enhanced field below the tip. After that, the dissertation shows the spectral background signal, which comes from infrared absorption by the substrate and tip, can be suppressed using a second laser. We have also investigated the nonlinearity of tip-sample interaction, and are able to detect sample photoexpansion force at heterodyne frequency. In the last part of this dissertation, we use our technique to image local optical energy distribution and ohmic heat dissipation of the metal nanoantennas.Item Nanoscale electronic and thermal transport properties in III-V/RE-V nanostructures(2013-12) Park, Keun Woo; Yu, Edward T.The incorporation of rare earth-V (RE-V) semimetallic nanoparticles embedded in III-V compound semiconductors is of great interest for applications in solid-state devices including multijunction tandem solar cells, thermoelectric devices, and fast photoconductors for terahertz radiation sources and receivers. With regard to those nanoparticle roles in device applications and material itself, electrical and thermal properties of embedded RE-V nanoparticles, including nanoscale morphology, electronic structure, and electrical and thermal conductivity of such nanoparticles are essential to be understood to engineer their properties to optimize their influence on device performance. To understand embedded RE-V semimetallic nanostructures in III-V compound semiconductors, nanoscale characterization tools are essential for analysis their properties incorporated in compound semiconductors. In this dissertation, we used atomic force microscopy (AFM) with other secondary detection tools to investigate nanoscale material properties of semimetallic RE-V and GaAs heterostructures, grown by molecular beam epitaxy. We used scanning capacitance microscopy and conductive AFM techniques to understand electronic and electrical properties of ErAs/GaAs heterostructures. For the electrical properties, this thesis investigates details of statistical analysis of scanning capacitance and local conductivity images contrast to provide insights into (i) nanoparticle structure at length scales smaller than the nominal spatial resolution of the scanned probe measurement, and (ii) both lateral and vertical nanoparticle morphology at nanometer to atomic length scales, and their influence on electrical conductivity. To understand thermal properties of ErAs nanoparticles, in-plane and cross-sectional plane of ErAs/GaAs superlattice structure were investigated with a scanning probe microscopy technique implemented with 3[omega] method for thermal measurement. By performing detailed numerical modeling of thermal transport between thermal probe tip and employed samples, and estimation of additional phonon scattering induced by ErAs nanoparticles, we could understand influences of ErAs nanoparticles on the host GaAs thermal conductivity. Investigation of ErAs semimetallic nanostructure embedded in GaAs matrix with scanned probe microscopy provided detailed understanding of their electronic, electrical and thermal properties. In addition, this dissertation also demonstrates that an atomic force microscope with secondary detection techniques is promising apparatus to understand and investigate intrinsic properties of nanostructure materials, nanoscale charge transports, when the system is combined with detailed modeling and simulations.