Bulletin of the American Physical Society
83rd Annual Meeting of the APS Southeastern Section
Volume 61, Number 19
Thursday–Saturday, November 10–12, 2016; Charlottesville, Virginia
Session B2: Nanoscale Systems |
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Chair: Shengfeng Cheng, Virginia Tech University Room: Salon C |
Thursday, November 10, 2016 10:45AM - 10:57AM |
B2.00001: Spin torque nano-oscillator achieved with point contact current injection Xueyuan She, Mehdi Kabir, Linqiang Luo, Mircea Stan Spin torque devices can be used to generate microwave signals without capacitors and resistors normally required by traditional circuits, making such devices ideal in nanoelectronic designs. The limited power output (<5nW) of a single spin torque device makes it necessary to connect multiple devices in parallel to achieve a desired power level. The size of each individual spin torque nano-oscillator (STNO) is therefore critical to many proposed applications that depend on using a large array of such device. Instead of directly fabricating devices at tens of nanometers in diameter, we successfully achieve the behavior of a 50 nm STNO using a current injected through a point contact to the top layer of a much larger (~500 nm), in-plane magnetized device. The point contact is achieved with a conductive AFM tip. This result provides evidence that patterning with mechanical indentation, which creates point contacts at a similar size, is a possible way to fabricate STNO arrays. [Preview Abstract] |
Thursday, November 10, 2016 10:57AM - 11:09AM |
B2.00002: Spin-torque switching in large size nano-magnet with perpendicular magnetic fields. Linqiang Luo, Mehdi Kabir, Stu Wolf, Mircea Stan, Jiwei Lu DC current induced magnetization reversal and magnetization oscillation was observed in 500 nm large size Co90Fe10/Cu/Ni80Fe20 pillars. A perpendicular external field enhanced the coercive field separation between the reference layer (Co90Fe10) and free layer (Ni80Fe20) in the pseudo spin valve, allowing a large window of external magnetic field for exploring the free-layer reversal. The magnetization precession was manifested in terms of the multiple peaks on the differential resistance curves. Depending on the bias current and applied field, the regions of magnetic switching and magnetization precession on a dynamical stability diagram has been discussed in details. The ability to manipulate spin-dynamics on large size devices could prove useful for increasing the output power of the spin-transfer nano-oscillators (STNO). [Preview Abstract] |
Thursday, November 10, 2016 11:09AM - 11:21AM |
B2.00003: Temperature Evolation Of Quasi-One Demensional C$_{\mathrm{60}}$ Nanostructures On Rippled Graphene Chuanhui Chen, Husong Zheng, Adam Mills, Randy Heflin, Chenggang Tao As two nanostructured allotropes of carbon, both graphene and fullerene exhibit fascinating physical properties and have numerous applications. A particularly interesting arrangement of C$_{\mathrm{60}}$ is the quasi-one dimensional (1D) structures that are excellent model systems and prototypes of 1D quantum confinement of electronic states. However, quasi-1D C$_{\mathrm{60}}$ nanostructures have been rarely realized experimentally due to their highly anisotropic configuration.Here we experimentally realized quasi-1D C$_{\mathrm{60}}$ nanostructures on rippled graphene by utilizing the linear periodic potential in graphene as a template. Through careful control of the subtle balance between the linear periodic potential of rippled graphene and the C$_{\mathrm{60}}$ surface mobility, we demonstrated that C$_{\mathrm{60}}$ molecules can be arranged into a novel 1D C$_{\mathrm{60}}$ chain structure with widths of two to three molecules. At a higher annealing temperature, the 1D chain structure transitions to a more compact hexagonal close packed quasi-1D stripe structure. This first experimental realization of 1D C$_{\mathrm{60}}$ structures on rippled graphene paves a way for fabricating new C$_{\mathrm{60}}$/graphene hybrid structures for future applications in electronics, spintronics and quantum information. [Preview Abstract] |
Thursday, November 10, 2016 11:21AM - 11:33AM |
B2.00004: Length scale dependence of the thermal conductivity accumulation in nanograined Si-Ge alloys Long Chen, Brian Donovan, Patrick Hopkins, Joseph Poon The manipulation of the lattice thermal conductivity without significantly effecting electronic mobility is a crucial part to optimize the thermoelectric figure of merit. In order to fully understand the contributions to the lattice thermal conductivity, a calculation of the lattice thermal conductivity based on a phonon frequency-dependent model, derived using the effective medium method, is presented. This model predicts the lattice thermal conductivity of the fully nanostructured systems, and helps to understand the dependence of lattice thermal conductivity on various length scales. The simulation results are validated with experimental results obtained via time-domain thermoreflectance. By varying the modulation frequency of the pump-probe technique, the thermal conductivity of Si and Si-Ge systems over a variety of thermal penetration depths is measured. The combination of modeling and experimental findings shows insight into length scale effect on phonon wavelength and mean free path, as well as the resulting impact on the thermal conductivity. [Preview Abstract] |
Thursday, November 10, 2016 11:33AM - 11:45AM |
B2.00005: High Excitation Density Effects in Plasmonic GaAs-AlGaAs-GaAs Core-Shell Nanowires. Masoud Kaveh-Baghbadorani, Qiang Gao, Chaennupati Jagadish, Hans-Peter Wagner We investigate the near-band emission of highly exited hybrid plasmonic GaAs-AlGaAs-GaAs core-shell nanowire (NW) heterostructures using time integrated (TI) photoluminescence (PL) measurements. The plasmonic structures are composed of 130 nm diameter zincblende NWs, either as bare NWs lying on an Au coated glass substrate or as Au coated NWs lying on a bare glass substrate. Intensity-dependent PL measurements on bare and plasmonic NW samples at high excitation densities reveal electron-hole-plasma (EHP) recombination. The EHP band shows a super-linear increase with increasing excitation intensity suggesting amplified spontaneous emission (ASE) at a threshold power density of around 60 microJ/cm2. Plasmonic NW samples excited above the threshold fluence reveal a weakly resolved sub-structure within the broad EHP band. The emerging sub-bands have a bandwidth which is by a factor of around 3 smaller than the width of the EHP background and are tentatively attributed to plasmonic lasing modes. This interpretation is supported by the fact that photonic lasing from 130 nm diameter thin uncoated GaAs NWs is theoretically not possible and that no sub-structure in the EHP band has been observed on bare nanowires. [Preview Abstract] |
Thursday, November 10, 2016 11:45AM - 11:57AM |
B2.00006: Self-assembly of Polyelectrolyte-coated Nanoparticles Chengyuan Wen, Shengfeng Cheng Polyelectrolyte adsorption and the resulting electrostatic repulsion have been widely used in the stabilization of colloids. In recent years, electrostatic self-assembly has attracted significant attention as a viable route towards novel nanostructures. We use molecular dynamics simulations to study the interactions and self-assembly of polyanion and polycation-coated nanoparticles. The nanoparticle is modeled as a layer of sites uniformly distributed on a spherical surface with a certain fraction of sites serving as the tether points of the grafted polymer brush. The polymer is modeled as bead-spring chains with a certain fraction of beads carrying charges. The solvent is treated as a uniform dielectric background but counterions are included explicitly as mobile beads. We vary the grafting density of the polyelectrolyte chains on the nanoparticles, the chain length, and the charge fraction and study their effects on the mutual interaction and the resulting assembly of nanoparticles. Our preliminary results have revealed possible routes towards supracrystals with NaCl or CsCl structure and other novel structures. [Preview Abstract] |
Thursday, November 10, 2016 11:57AM - 12:09PM |
B2.00007: Electrospray-assisted carbon nano tube deposition on aluminum without a binding agent Subhodip Maulik, Srismrita Basu, Theda Daniels-Race Carbon nanotubes (CNTs) are known for their exceptional properties of electrical conductivity, thermal tolerance, and tensile strength. Observation of CNT-based electronic phenomena often begins with a pre-treated or otherwise functionalized metal or semiconductor surface. For example, although successful deposition of CNTs onto aluminum (Al) has been reported in the literature, the steps usually involve some form of chemical vapor deposition (CVD) and pretreating the Al with a thin film catalyst (e.g., nickel), which can be costly and labor-intensive. In our work, we have investigated electrospraying as a method of CNT deposition that replaces CVD and does not require a surface binding agent to produce high quality CNT coverage. The electrospray is created when droplets of isopropyl alcohol saturated with CNTs are subjected to voltages in the 5-8 kV range while being deposited upon a non-functionalized Al surface. The enhanced charge density overcomes droplet surface tension to distribute CNTs via the liquid in the shape of a Taylor cone that is characteristic of electrospraying. Raman spectra verify deposition of multi-wall CNTs while examination via optical microscope shows appreciably uniform coverage using our electrospray-assisted method. [Preview Abstract] |
Thursday, November 10, 2016 12:09PM - 12:21PM |
B2.00008: Molecular Dynamics Modeling of Carbon Nanotube Composite Fracture using ReaxFF Benjamin Jensen, Kristopher Wise, Gregory Odegard Carbon nanotube (CNT) fiber reinforced composites with specific tensile strengths and moduli approaching those of aerospace grade carbon fiber composites have recently been reported. This achievement was enabled by the emerging availability of high N/tex yarns in kilometer-scale quantities. While the production of this yarn is an impressive advance, its strength is still much lower than that of the individual CNTs comprising the yarn. Closing this gap requires understanding load transfer between CNTs at the nanometer dimensional scale. This work uses reactive molecular dynamics simulations to gain an understanding at the nanometer scale of the key factors that determine CNT nanocomposite mechanical performance, and to place more realistic upper bounds on the target properties. [Preview Abstract] |
Thursday, November 10, 2016 12:21PM - 12:33PM |
B2.00009: Effect of Carbon Nanomaterials Embedded in a Cementitious Matrix Clarissa Roe In the current study, we have carried out an investigation on monoliths of novel cementitious composites with embedded carbon nanomaterials (CNMs). Cementitious samples were prepared using the carbon allotropes C$_{\mathrm{60}}$, Carbon Nanotubes, and Graphene Oxide. Due to their varying geometry, the steric effect on strength of the resulting monoliths was measured against the varying CNM concentrations. Results provide evidence that due to the dissimilar geometry and amount of CNM added, the CNMs disperse within the cement in different ways. Real-space imaging was obtained using the Large Chamber Scanning Electron Microscope (LC-SEM) at the WKU Nondestructive Analysis (NOVA) Center using Backscattered Electrons. The resulting strength of these cementitious composites was investigated using a uniaxial load frame. Comparing these results with the LC-SEM imaging provides a more comprehensive picture into the development of materials that are strong and durable relative to the abovementioned differences in CNM geometry and mass. Future studies will include the use of small-angle neutron scattering to provide independent data confirming measurement of the size and distribution of the CNM's within the cementitious matrix. The results of this study will be compared to future tests performed aboard the International Space Station to determine the effect of gravity on the CNM dispersion and resulting cement monolithic strengths. [Preview Abstract] |
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