6301 Stevenson Center
VU Station B #351807
Nashville, TN 37235
Justin Gregory, Interdisciplinary Graduate Program in Materials Science, Vanderbilt University
Characterization of Ion Implantation-Induced Vacancy Defects and Graphitization in Diamond Lattices by Coherent Acoustic Phonon Spectroscopy
I will describe implantation damage experiments on single-crystal diamond samples and subsequent characterization using optical pump-probe techniques. Samples are irradiated using He+ ions with energies on the order of 1 MeV, generating lattice defects and buried layers of graphitic carbon under high pressure. Using the time-resolved optical technique known as coherent acoustic phonon spectroscopy, these samples are characterized in a depth-dependent manner. The resulting oscillation patterns reveal clues about the optical and electronic structure of the defects and the graphitic layers.
Isabel Gonzalo-Juan, Department of Physics and Astronomy, Vanderbilt University
*Controlled Monolayer Growth of Ultra-thin Films of TiO2 Nanocrystals by Electrophoretic Deposition
The work discussed in this presentation contributes to the production of continuous, thin films of densely-packed titanium oxide nanocrystals, synthesized by solvothermal synthesis and assembled by electrophoretic deposition using non polar solvents. TiO2 has attracted significant research interest due to its photochemical activity and optical properties. Colloidal nanocrystals (NCs) have increasingly been employed due to their attractive size-dependent optical properties. Precise control of the morphology and the size of these materials at nanometric scale is of a basic importance to fine-tuning of their physical properties, such as electrical conductivity or magnetic, optical and mechanical characteristics. Hence, to improve the preparation method of nanocrystalline TiO2 with a narrow size distribution is of great importance. Electrophoretic deposition (EPD) of particles from colloidal suspensions stands out as a powerful and versatile colloidal process to face inexpensive and mass production of films using low concentrated suspensions. Ceramic NCs can be easily deposited by EPD on any conductive substrate. Furthermore, EPD can facilitate much thinner films than can be produced by other techniques, such as tape casting. Interestingly self-ordering occurs during EPD in monolayers of certain colloidal nanocrystals, which may allow the production of highly ordered 3D NC structures The electrical parameters of the TiO2 nanocrystal suspensions were optimized to achieve the necessary NC deflocculated conditions and to produce homogeneous films of nanocrystals by SLED using non polar solvents. The associated physical properties, current density, potential, electric field and deposition time, were optimized to produce homogeneous self supporting films.
Alex Krejci, Department of Physics and Astronomy, Vanderbilt University
*The Evolution of Ordering in Iron Oxide Nanoparticle Monolayers Using Electrophoretic Deposition
Iron oxide nanoparticle monolayers and multilayers were assembled using dc electrophoretic deposition. The rate of deposition and the total particle deposition were controlled by varying the concentration of nanoparticles and the deposition time, respectively. Using scanning electron microscopy, we performed a time resolved study that demonstrated the growth of the monolayer from a single isolated nanoparticle to a nearly complete layer. We observed tight, hexagonal packing of the nanoparticles indicating strong particle particle interaction. Future research topics include generalizing the process for various types of nanoparticles and controlling the total particle deposition with current measurements.
AKM Newaz, Department of Physics and Astronomy, Vanderbilt University
*Probing charge scattering mechanisms in suspended graphene by varying its dielectric environment
Yunhao Cao, Department of Electrical Engineering and Computer Science, Vanderbilt University
*Controlling growth morphology of carbon nanotubes: from suspended bridges to upright forests
We have developed two strategies to produce carbon nanotubes (CNTs) from low-density surface growth to high-density forest growth. We have demonstrated that by introducing a C2H2 pulse at the beginning of the growth, where methane is still used as the main carbon feeding gas, the growth tendency of CNTs can be changed and the resulting growth morphology will vary from surface growth to forest growth. Similarly, the growth morphology can be changed when the growth temperature is risen. Furthermore, by appropriately adjusting the growth temperature, we have managed to produce both suspended CNT bridges and upright forests within a single growth procedure. The further characterization via Raman spectroscopy indicates that the increasing C2H2 pulse time will lead to the rise of D peak for as-grown CNTs, due to the formation of more multi-walled CNTs and the amorphous carbon contamination introduced by extra C2H2, while high growth temperature tends to produce high-quality CNTs and reduce the amorphous carbon contamination. The systematic analysis for presented strategies will be important to understand the growth mechanism, and also help with the process development of full CNT-based electronic architecture in future.
Tu Hong, Department of Electrical Engineering and Computer Science, Vanderbilt University
*Carbon nanotube-mediated siRNA delivery for gene silencing in cancer cells
Small interfering RNA (siRNA) is potentially a promising tool in influencing gene expression with a high degree of target specificity. However, its poor intracellular uptake, instability in vivo, and non-specific immune stimulations impeded its effect in clinical applications. In this study, carbon nanotubes (CNTs) functionalized with phospholipid-polyethylene glycol (PEG) have shown capabilities to stabilize siRNA in cell culture medium during the transfection and efficiently deliver siRNA into neuroblastoma and breast cancer cells. Moreover, the intrinsic optical properties of CNTs have been investigated through absorption and fluorescence measurements. We have found that the directly-functionalized groups play an important role on the fluorescence imaging of functionalized CNTs. The unique fluorescence imaging and high delivery efficiency make CNTs a promising material to deliver drugs and evaluate the treatment effect simultaneously.
Terry Musho, Department of Mechanical Engineering, Vanderbilt University
NEGF Quantum Simulation of Nanotip Field and Thermionic Emitters for Direct Energy Conversion
Wide band-gap diamond nanotip field emission devices have been experimentally shown to have superior performance and lifetime. However, theoretical studies of the electronic emission from these devices using standard Fowler-Nordheim (FN) theory does not fully capture the physics as a result of the fitting parameters inherent to the FN approximation. The following research computationally models wide band-gap nanotip field emission devices from a quantum point of view, using a novel non-equilibrium Green's function (NEGF) approach previously applied to modeling the transport in solid-state electronic devices. In this research the IV characteristics of a single square tip diamond emitter and diamond films are investigated under several bias conditions. Those bias conditions include both field emission and thermionic emission regimes with comparison to availible experimental values. The NEGF model calculates the ballistic transport using a self-consistent Schr√∂dinger-Poisson solver that calculates the transmission at discrete energy levels which is used in conjuction with a Landauer formalism to determine the total current. Ultimately, this model allows the inherent quantum mechanical transport to be captured at sub-micron device sizes without need for fitting parameters. Findings from this research have confirmed non-linearities in the FN curve and have demonstrated the experimental transport trends. Additionally, thermionic emission trends suggest that select material parameters are target for enhanced emission.
Hadley Lawler, Department of Physics and Astronomy, Vanderbilt University
*First-principles methods for evaluation of the light-matter interaction in insulators and semiconductors
Polarization currents and their description within density-functional theory and many-body theory are introduced. Calculations of dielectric spectra for several simple systems demonstrate the essential physics in these approaches. For far-infrared spectra, relevant quantities discussed are Born-effective charges, optical phonon self-energies, and the electrical anharmonicity. For optical and UV spectra, various levels of interaction will be introduced, from single-particle transitions, to the random-phase approximation, to treatment of electron-hole interactions with the Bethe-Salpeter equation.
Hiram Conley, Department of Physics and Astronomy, Vanderbilt University
*Strained Graphene: Suppressing Flexural Phonons and Exploring the Psuedo Quantum Hall Effect
The interplay between the mechanical and electrical properties of materials is interesting and technologically relevant. Exploring this interplay between the mechanical and electrical properties in graphene may enable tuning graphene's electrical properties as well as opening up new exotic electronic states. We have found several promising methods to engineer strain in graphene and will discuss future experiments to explore the connection between strain and graphene's electrical properties.
Shuren Hu, Department of Physics and Astronomy, Vanderbilt University
*Creating Defects in Suspended and Supported Graphene FET Devices
Creating Defects in Suspended and Supported Graphene FET Devices Defects (such as dislocations or vacancies) in graphene can cause strong electron scattering. Therefore, to achieve high quality graphene-based electronic devices it is important to elucidate the mechanisms of defect formation. In this talk, we present our recent results on controllable creation of vacancy defects in graphene and measuring the impact of these defect on electron transport. Specifically, we focus on understanding the effect of the substrate under graphene on formation of defects. The vacancy defects were created by irradiating both suspended and supported-on-SiO2 graphene devices with high energy Ga ion beams. Electron transport was measured in these devices in situ in high vacuum as a function of irradiation dose. We find that the number of defect created in supported devices is dramatically higher in comparison with suspended devices. We identify formation of defects by secondary ions in the case of supported devices as a likely explanation of these results. These results may be important in designing devices for high radiation working environment, such as space electronics.
Dhiraj Prasai, Department of Physics and Astronomy, Vanderbilt University
*Interaction between photoexcited quantum dots and graphene
A quantum dot in its excited state can decay via two different pathways. First, it can fluoresce and relieve its energy by emitting a photon. Second, if a metal is placed near the dot, the excitation can be non-radiatevely transferred into that metal via the so-called Foster Resonance Transfer (FRET). In that second case the fluorescence is quenched. Here, we study fluorescence of quantum dots placed in the vicinity of graphene. By electrostatic gating we expect to tune the Fermi energy of graphene and hence control the efficiency of FRET. We thus expect to be able to electrically switch the fluorescence of the quantum dot between "on" and "off" states. In addition, by studying the detailed magnitude of the fluorescence quenching we expect to extract the spectrum of plasmons in graphene.
Sergiy Bubin, Department of Physics and Astronomy, Vanderbilt University
*Modeling Coulomb explosions of small hydrocarbons driven by intense femtosecond laser pulses
In the framework of real-time real-space time-dependent density functional theory (TDDFT) complemented with classical molecular dynamics for ions, we have studied strong field ionization and subsequent molecular fragmentation (Coulomb explosion) in small hydrocarbon molecules - methane and 1,3-butadiene. The mechanism of the process has been investigated and the spectra of the ejected protons have been computed. The results of the TDDFT simulations are compared with some recent experimental data.
Travis Wade, Interdisciplinary Graduate Program in Materials Science, Vanderbilt University
*Nanostructure analysis of diamond cold cathode field emitters
Diamond field emitter arrays have demonstrated extraordinary brightness and durability, but face fabrication yield issues before being adopted commercially. Transmission electron microscopy performed in this study provides new insight into tip structure and composition with implications for field emission and diamond growth. I will discuss the development of a process for TEM analysis of surface features without graphitization and how we have examined sub-surface cross-sections to offer feedback for growth parameter conditions.
Thanksgiving Holidays, no seminar
Davon Ferrara, Department of Physics and Astronomy, Vanderbilt University
*Correlated electron—Plasmon Coupling in Gold::Vanadium Dioxide Nanocomposites
The localized surface plasmon resonance (LSPR) frequency of gold nanoparticle (NP) arrays embedded in vanadium dioxide (VO2) is modulated by the semiconductor-to-metal phase transition (SMT) of the film. Since the 180-nm diameter particles are on the order of the VO2 grain size, the NPs serve as sensitive nanoantennas for measuring the changing VO2 dielectric function. As the split vanadium d-bands come together to form the metallic conduction band, the interaction between the plasmon and correlated electrons of the VO2 film decreases the Au LSPR dephasing time by 30% during the transition.
Krishen Appavoo, Department of Physics and Astronomy, Vanderbilt University
*Role of defects in phase transition of VO2 nanoparticles probed by plasmon resonance spectroscopy
Defects are known to affect nanoscale phase transitions, but their specific role in the metal-to-insulator transition in VO2 has remained elusive. By combining plasmon resonance nanospectroscopy with density-functional calculations, we correlate decrease in phase-transition energy with oxygen vacancies created by strain at grain boundaries. By measuring the degree of metallization in the lithographically-defined VO2 nanoparticles we find that hysteresis width narrows with increasing size, thus illustrating the potential for domain boundary engineering in phase-changing nanostructures.
Charles Adams, Department of Physics and Astronomy, Vanderbilt University
*Measuring the Nonlinear Refractive Index of VOx Nanotubes
I will discuss using the Z-scan method to measure the nonlinear index of VOx Nanotubes as well as preliminary results and comparison with results from measurements on carbon nanotubes.
Robert Marvel, Department of Physics and Astronomy, Vanderbilt University
*Deposition of vanadium dioxide films by electron beam evaporation
Thin films of vanadium dioxide with good uniformity and low surface roughness are desired for many optical applications. In addition, the ability to deposit VO2 on silicon substrates is of critical importance for integration with current silicon based electronics. Films deposited on silicon by Pulsed Laser Deposition (PLD) have been limited to 100nm minimum thickness, in contrast, good quality 30nm thick films have been deposited by electron beam evaporation. In addition, electron beam evaporation is significantly cheaper than PLD and is capable of much higher deposition rates. Switching contrast and hysteresis width of the VO2 films deposited by electron beam evaporation has also been observed to be much less affected by conditions during post-deposition annealing when compared with PLD films.
Daniel Mayo, Department of Physics and Astronomy, Vanderbilt University
*Metal Oxide Nanoparticle Film Deposition Using Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE)
The successful development of flexible, high performance solar cells that are competitive with silicon-based technology will likely depend on the ability to fabricate hybrid materials that incorporate nanomaterials, glasses, ceramics, polymers, and thin films. Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE) is an ideal method for depositing organic materials and nanoparticles with minimal photochemical or photothermal damage to the deposited material. Furthermore, there are many non-hazardous solvents containing chemical functional groups with infrared absorptions accessible using IR lasers. We report here the results from recent work using RIR-MAPLE to deposit films of TiO2 nanoparticles on Si substrates. Using an Er:YAG laser (λ = 2.94 μm), we investigated a variety of MAPLE solvents containing an –OH moiety, including all four isomers of butyl alcohol. We compare the effects of varying concentration and laser fluence. Results of our analysis of films of TiO2 nanoparticles using scanning electron microscopy, surface metrology, and energy dispersive X-ray spectroscopy will be presented.
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