Physics & Astronomy Department
2401 Vanderbilt Place
Nashville, TN 37240-1807
Weidong He, Department of Physics and Astronomy, Vanderbilt University
Room Temperature Colloidal Synthesis of Superantiferromagnetic EuTe Nanoparticles and Nanospindles
Our synthesis applies only two inorganic precursors sodium telluride and europium dichloride, one solvent ethylene glycol and one stabilizing organic chemical triethanolamine as well as phenanthroline and renders a fast growth mechanism at room temperature. Because of the formation of intermediate chelate compound [Eu(TEA)n]Cl2, the synthesis has a balanced thermodynamic and kinetic control between nucleation and growth processes, which helps to produce nanoparticles and nanospindles with relatively small size deviation. Owning these facile feathers, our synthesis routine helps to address some drawbacks faced by traditional synthetic methods of europium chalcogenide nanostructures, such as multistep and high temperature in the syntheses of europium sulfide and europium selenide nanoparticles. The work also introduces the magnetic properties of europium telluride nanoparticles, suggesting that finite size effect of the material would draw a lot of attraction in magnetic and spintronic research field.
Host: J. Dickerson
Suseela Somarajan, Department of Physics and Astronomy, Vanderbilt University
Physical properties of Lead Europium Sulfide ((PbEuS) Nanocrystals
Advancements in nanoscale engineering may be realized with novel, alloyed nanocrystals, which allow for customized physical properties with variations in size and composition. Lead europium chalcogenides (Pb1-xEuxX, X = S, Se, Te) have been studied for many years, as these materials are useful for infrared diode lasers and spintronics devices due to their tunable optical and semi-magnetic properties. Among the Pb1-xEuxX ternary compounds, lead europium sulfide (Pb1-xEuxS) is likely to be most suitable for applications that call for dilute magnetic semiconductors because they can form a completely miscible alloy system with tunable energy band gaps over a wide range. Here we report the structural and composition dependent magnetic properties of nanocrystalline PbEuS materials. The alloy structure of PbEuS nanocrystals was affirmed by X-ray diffraction and vibrating sample magnetometry measurements. PbEuS nanocrystals exhibited paramagnetic properties down to 2K, whereas PbS was diamagnetic and EuS was ferromagnetic at these temperatures.
Host: J. Dickerson
Federico Rosei, INRS Energie, Materiaux et Telecommunications, Universite du Quebec
Exploring Molecular Assembly at Surfaces
The adsorption and self–assembly of organic molecules at surfaces has recently been investigated extensively, both because of the fundamental interest and for prospective applications in nanoelectronics (1,2). Molecule–molecule and molecule–substrate interactions can be tuned by appropriate choice of substrate material and symmetry. Upon molecular adsorption, surfaces typically do not behave as static templates, but often rearrange to accommodate different molecular species (3,4). We review recent experiments using Scanning Tunnelling Microscopy, providing new insight into fundamental properties such as molecular diffusion (5,6) and self–assembly via surface templating (7-9) and H bonding driven by coadsorption (10, 11, 12). Our approach is to modify surfaces providing suitable surface cues, that may guide the assembly of adsorbates. We jokingly call this approach Playing Tetris at the Nanoscale (13). Recent advances in using the substrate as catalyst for surface confined polymerization reactions will also be discussed (14, 15, 16). 1. F. Rosei et al., Prog. Surf. Science 71, 95 (2003). 2. R. Otero, F. Rosei, F. Besenbacher, Annu. Rev. Phys. Chem. 57, 497 (2006). 3. F. Rosei et al., Science 296, 328 (2002). 4. R. Otero, F. Rosei, et al., Nanoletters 4, 75 (2004). 5. M. Schunack, T.R. Linderoth, F. Rosei, et al., Phys. Rev. Lett. 88, 156102 (2002). 6. J. Miwa, S. Weigelt, H. Gersen, F. Besenbacher, F. Rosei, T.R. Linderoth, J. Am. Chem. Soc. 128, 3164 (2006). 7. R. Otero, Y. Naitoh, F. Rosei et al., Angew. Chem. 43, 4092 (2004). 8. F. Cicoira, J. A. Miwa, M. Melucci, G. Barbarella, F. Rosei, Small, 2, 1366 (2006). 9. F. Cicoira, J.A. Miwa, D.F. Perepichka, F. Rosei, J. Phys. Chem. A 111, 12674 (2007). 10. K.G. Nath, O. Ivasenko, J. Miwa, H. Dang, J. Wuest, A. Nanci, D.F. Perepichka, F. Rosei, J. Am. Chem. Soc. 128, 4212 (2006). 11. K.G. Nath, O. Ivasenko, J.M. MacLeod, J.A. Miwa, J.D. Wuest, A. Nanci, D.F. Perepichka, F. Rosei, J. Phys. Chem. C 111, 16996 (2007). 12. J. MacLeod, O. Ivasenko, D.F. Perepichka, F. Rosei, Nanotechnology 18, 424031 (2007). 13. F. Cicoira, F. Rosei, Surface Science 600, 1 (2006). 14. D.F. Perepichka, F. Rosei, Science 322, 216 (2009). 15. J. Lipton-Duffin, O. Ivasenko, D.F. Perepichka, F. Rosei, Small 5, 592 (2009). 16. J. Lipton-Duffin, J.A. Miwa, M. Kondratenko, F. Cicoira, B.G. Sumpter, V. Meunier, D.F. Perepichka, F. Rosei, Proc. Nat. Acad. Sci. U.S.A. 107, 11200 (2010).
Host: K. Bolotin
Purushottam Chakraborty*, Saha Institute of Nuclear Physics 1/AF Bidhannagar, Kolkata 700064 India
Large third-order optical responses of metal nanocluster – glass composites studied by ARINS
Nanoscopic materials have been attracting increasing interest for their unique chemical/physical properties and potential technological applications. Materials exhibiting large third-order optical nonlinearities and fast response time have been proven to be quite advantageous for various photonic device applications. Composite materials formed by nanometer-sized metal particles embedded in silicate glasses have drawn great interest owing to the large values of fast optical Kerr susceptibility, χ(3). Metal nanocluster-glass composites have been synthesized by metal (Au, Ag, Cu, Ni) ion implantations in fused silica glasses. UV-Vis spectroscopy indicates the appearance of significant SPR bands even without thermal treatments. Z-scan and Anti-Resonant Interferometric Nonlinear Spectroscopy (ARINS) techniques have been employed for the measurement of the third-order optical susceptibility of these nanocomposites. Optical nonlinearity has been explained to be due to two-photon absorption in these nanocomposite glasses and is essentially of electronic origin. The ARINS technique utilizes the dressing of two unequal-intensity counter-propagating pulsed optical beams with differential nonlinear phases, which occurs upon traversing the sample. This difference in phase manifests itself in the intensity-dependent transmission, measurement of which enables us to extract the values of nonlinear refractive index (η2) and nonlinear absorption coefficient (β), finally yielding the real and imaginary parts of the third-order dielectric susceptibility (χ(3)). ARINS has significant advantages over the conventional Z-scan technique in the sense that the former is very sensitive to detect extremely small changes (less than 0.005%) in the laser amplitude and the measurements are not affected by any external disturbances. Furthermore, this technique has the unique capability of discriminating against different nonlinear optical processes based on their response times. *email@example.com, firstname.lastname@example.org
Host: R. Haglund
October Break, no seminar
Ben Schmidt (VINSE postdoctoral scholar), Vanderbilt University
Optical properties of VOx nanoscrolls
Materials exhibiting intensity-dependent optical transmission are well-suited for applications like protective eyewear. Carbon nanotubes and similar 1D structures have been shown to act as antennas to couple with the radiation field and achieve very fast switching with high contrast ratios. Vanadium oxide nanoscrolls are an alternative to CNTs, and this talk focuses on material fabrication through a hydrothermal synthesis technique and subsequent characterization. Preliminary optical characterization of nanoscrolls of CNTs in suspension at 1064 nm using a Nd:YAG laser will be discussed, along with future plans to study the optical limiting performance at 532 nm.
Host: R. Haglund
Bin Wang, Department of Physics and Astronomy, Vanderbilt University
Electronic structure of monolayer graphene and biomimetic molecules on metal surfaces
Perfectly ordered graphene overlayers can be easily obtained on Ru(0001) surface and display a commensurate superstructure called moiré pattern with large (around 3 nm) periodicities. Based on DFT calculations, we have unveiled that the graphene overlayer displays periodic ripples of 1.5 Å amplitude, alternating strong and weak interaction with Ru. Our findings have been further completed by various electronic structure calculations directly comparable to the experimental data such as STM imaging, STS spectra, ARPES spectra, work function change. The specific geometric and electronic structure result in several interesting phenomena, such as enhanced chemical activity at the region strongly binded to Ru(0001) and controllable second graphene layer growth. Other work is devoted to a biologically relevant organic molecule such as the Fe-phtalocyanine molecule. We have studied its electronic structure on Au(111) and the ligand interaction with various ligands like NH3, O2, NO, pyridine and found that the magnetic property of the embedded metal center can be selectively tuned. By analyzing the electronic configuration of the Fe cation in the different species, we further propose a rule of thumb to predict which type of ligands may switch or not the spin of pristine FePc.
Host: S. Pantelides
Heungman Park, Department of Physics and Astronomy, Vanderbilt University
Second harmonic generation in Si/SiO2 systems and characterization of boron induced charge traps near the Si/SiO2 interface
Boron-induced interfacial charge traps were characterized using second harmonic generation (SHG) in Si/SiO2 systems. It was proposed that B- and B+ ions in Si substrate and SiO2 are present respectively across the interface. A two color pump-probe SHG experiment was performed to determine the threshold photon energy for filling the B+ induced charge traps in the oxide. A threshold photon energy of 2.61 eV (λ = 475 nm) was found for single photon excitation of electrons from the Si valence band to fill B+ charge traps in SiO2.
Host: N. Tolk
Jed Ziegler, Department of Physics and Astronomy, Vanderbilt University
Nanospirals as model systems for complex plasmonics
We describe the effect of completely broken symmetry on the optical response of a plasmonic structure, using the Archimedean spiral as a model. This model system exhibits both unique spatial organization of the electric field enhancement and a high degree of tunability. The spiral structure illustrates that complex geometries produce plasmonic features with unexpected spatial organization, such as non-dipolar modes, and a well defined spectral region for each configuration of near-field enhancement. This makes it possible to understand the complex intra-particle interactions and help us to understand the nanoscale mechanisms that underlie the resulting modes.
Host: R. Haglund
Krishen Appavoo, Institute for Nanoscale Science and Engineering and Department of Physics and Astronomy, Vanderbilt University
Detection of Phase Transformation in VO2 by a Single Gold Nanoantenna
Modulation of the optical properties of plasmonic components using solid-solid phase transitions in nanoscale volumes is one of the most promising approaches to nanophotonic technology. However, in order to tailor such a hybrid nanocomposite for active plasmonics, a fundamental understanding of the nanoscale properties of the phase-changing component is vital. Here, we detect modulation in the optical response of a single gold nanoparticle “sitting” on a VO2 film by means of dark-field resonance spectroscopy. By mapping the resonance shift to temperature of the film, we show that the gold nanoantenna mirrors the hysteretic behavior of the phase transformation, even at the single-particle level. Moreover, at a fixed wavelength, the scat-tering intensity of the gold particle also shows a hysteretic behavior decorated with an overshoot before (after) the insulator-metal (metal-insulator) phase transition of the vanadium dioxide film, suggesting that the nanoantenna is also registering the local statistical variations in the phase-transition.
Host: R. Haglund
Thanksgiving Holidays, no seminar
Vladimir Dobrokhotov, Department of Physics and Astronomy, Western Kentucky University
Electronic Nose Technology for Remote Selective Trace Detection of Improvised Explosive Devices
At present, fabrication of conductometric sensors (chemiresistors) is probably one of the most promising applications of nanomaterials. Chemiresistor is an electronic device that changes electrical conductivity thanks to the adsorbed chemicals; hence, it converts the concentration of chemicals into a measurable electrical signal. Significant advantages of nanomaterials-based chemiresistors arise from their extremely high surface-to-volume ratio, which makes their electromechanical and thermal properties strongly dependent on surface phenomena. The condition of the surface of the nanostructure is determined by the parameters of the surrounding medium, which allows us to establish a direct correspondence between the properties of the nanostructure and the parameters of the medium in which this nanostructure is immersed. This basic principle drives the sensing mechanisms of most nanomaterials. In this talk I discuss the potential application of nanospring mats as chemiresistors. Nanosprings are potentially superior in sensing capabilities to all the presently existing nanostructures because of their extremely large surface area. Thanks to the spiral structure, a nanospring has up to 10,000 times more surface area than the footprint of its root on the substrate, which is 10-20 times larger comparing with carbon nanotubes or nanowires. Another advantage of nanosprings is the mats can be patterned prior to growth in order to achieve select geometric patterns: area of coverage can be precisely determined by the shape of the catalyst islands. Nanosprings can be formed at low temperatures (~325 °C), and they can therefore be incorporated into devices that contain certain soft polymeric materials. Our preliminary results on using the nanosprings as basic elements for electronic noses are very promising. Exposure to the explosive vapor pulses in the hundreds of ppm range of concentrations causes dramatic changes in conductivity of the nanospring mat. This change is different for different chemicals, which makes a chemiresistor of this kind very sensitive and selective. Moreover, a nanospring-based chemiresistors are self-refreshable and don’t need a reset mechanism. In addition to that we found that the functionalization of the nanosprings with the variety of different metallic nanoparticles (Au, Pt, Pd, Ag) causes a dramatic change in the chemiresistor response, sometimes even inverts the characteristic from increasing resistance to decreasing and sufficiently affects the adsorption-desorption time. All these experimentally confirmed effects will allow us to create a wide variety of highly-sensitive chemiresistors with specific responses to different chemicals. These chemiresistors will be used as building blocks for electronic noses: biologically inspired devices that identify and analyze chemical compounds in gaseous environments.
Host: K. Varga
Jason Valentine, Mechanical Engineering Department, Vanderbilt University
Transformation Optics: Cloaking, Photonic Black Holes and Beyond
Transformation optics is a new optical design methodology that allows unprecedented freedom in the manipulation of light propagation. In this methodology, optical space is designed through a virtual coordinate transformation which is then translated into a physical system via spatially varying optical properties. This methodology, combined with metamaterials, has allowed a number of novel optical devices to be conceptualized including the invisibility cloak, photonic black holes, worm holes, and space time manipulators. In this talk I will review current progress in the field including some of the most recent experimental realizations. I will also discuss some of the more exotic theoretical proposals and their prospects for realization.
Host: N. Tolk
Hiram Conley, Department of Physics and Astronomy, Vanderbilt University
*Exploring the Mechanical Properties of Graphene with Bimetallic Cantilevers
The study of graphene’s mechanical properties has been dominated by mechanical resonators made from graphene and AFM force distance spectroscopy. We demonstrate bimetallic cantilevers as a system to study the material properties of graphene. Using bimetallic cantilevers we measure strain, friction of graphene on a substrate, and the coefficient of thermal expansion of graphene.
Eric Van Stryland, CREOL, College of Optics and Photonics, University of Central Florida
Nonlinear Absorption Spectroscopy - and how to detect IR with wide-gap semiconductors
We have been developing nonlinear spectrophotometers to measure nonlinear absorption spectra, e.g 2-photon absorption, and the dispersion of the nonlinear refractive index in materials from semiconductors to organic dyes. We are close to having an automated system utilizing a white-light continuum, WLC, in conjunction with our Z-scan technique, i.e. “WLC Z-scan”. However, useful additional information on the physical processes can be obtained from pump-probe experiments which can measure frequency nondegenerate nonlinearities. In semiconductors the nondegenerate 2-photon absorption is greatly enhance wrt its degenerate counterpart. I will go over the reasons for this and demonstrate the results of using a GaN detector to measure fs 5.6 micron pulses by first 'dressing' the system with 390nm pulses. The preliminary results show better detection than obtained using a liquid nitrogen cooled HgCdTe detector.
Host: N. Tolk
Keith Warnick, Department of Physics and Astronomy, Vanderbilt University
*Electric-field-activated diffusion and degradation in AlGaN HEMTs at room temperature
Diffusive phenomena do not generally occur in semiconductors at room temperature (RT) because of high activation energies. However, recent observations of electric-field-driven plastic deformation in AlGaN epilayers on GaN in High Electron Mobility Transistors (HEMTs) have been attributed to diffusive processes, possibly triggered by a critical strain induced by the inverse piezoelectric field. Here we report first-principles calculations showing that strain has little effect, but a unique set of conditions lead to RT diffusive phenomena: near-zero formation energy of triply-negative cation vacancies and a concomitant electric-field-induced lowering of migration energy.
Host: S. Pantelides
Chris Kang, Interdisciplinary Graduate Program in Materials Science, Vanderbilt University
*Optimizing photonic crystal slab cavities for small-molecule index sensing
Photonic crystal (PhC) slabs with defects are a promising platform for refractive index-change sensors due to their high field concentration, which allows reduced analyte volumes, and sharp spectral resonances that enable the detection of small refractive index perturbations in the defects. The sensitivity of photonic crystal and other sensor platforms is characterized by the wavelength shift of a distinct spectral feature as a function of the refractive index change of analyte exposed to the sensor. In this work, we show that adding surface area to the defect region of PhCs, by directly placing holes with diameters much smaller than the lattice constant within silicon PhC slab microcavities (“multiple hole defects”, MHDs), improves the refractive index-change sensitivity.
Host: S. Weiss
Joseph Driscoll, Department of Physics and Astronomy, Vanderbilt University
First-principles calculations of electron field emission in nanostructures
Field emission (FE) of electrons from nanostructures is the subject of intense experimental and theoretical research. The aim of these studies is to explore the properties of nanoscale materials in electric fields and exploit these properties for technological applications. Emission from carbon nanotubes (CNs) is particularly important as CNs are candidates for next-generation displays, electron sources, and high-resolution electron beam instruments. In this work, field emission is studied by propagating the electronic density in real space and time using time-dependent density functional theory. Several aspects are studied including adsorbate effects, spin polarization, laser-enhanced emission, and varying composition of nanotubes.
Yogesh Vohra, Department of Physics, Center for Nanoscale Materials and Biointegration (CNMB), University of Alabama-Birmingham
Single Crystal and Nanocrystalline Diamond in High Pressure and Biomedical Research
The diamond anvil cell devices are extensively utilized in high pressure research with applications in materials science, spectroscopy, planetary sciences, and nuclear stockpile stewardship program where knowledge of materials under extreme conditions is required. In conventional diamond anvil cells, diamond merely acts as a passive element and is used for generating and sustaining several million atmosphere pressures. The “designer diamond” technology developed by combining chemical vapor deposited diamond and lithography techniques allows for active recording of electrical resistance, magnetic susceptibility, and melting data by embedded sensors in diamond anvils. Some recent studies with the designer diamonds will be presented on iron-based superconductors and rare earth metals under high pressures and temperature to 10 K. Novel superconducting phases and electronic transitions have been documented at high pressure with the use of designer diamond anvils. In a parallel effort, growth of nanocrystalline diamond films using chemical vapor deposition has been studied on a variety of metals including titanium alloys and cobalt-chrome alloys. The applications of nanocrystalline diamond with a surface roughness of few nanometers as a wear-resistant coating in orthopaedic and dental implants will be briefly discussed.
Host: J. Davidson
Jonathan Jarvis, Department of Physics and Astronomy, Vanderbilt University
*Resonant tunneling and extreme brightness from carbon nanotubes and diamond field emitters
We report recent results from field emission microscopy studies of multiwall carbon nanotubes and from energy spectrum measurements of beams from diamond field emitters. In both systems, we find that resonant tunneling through adsorbed species on the emitter surface is an important and sometimes dominant effect. For diamond emitters our observations include order-of-magnitude emission enhancement without spectral broadening, complex spectral structure, and sensitivity of that structure to the applied electric field. For carbon nanotubes we have observed electron beams from individual adsorbates which are estimated to approach the maximum beam brightness allowed by Pauli exclusion.
Host: C. Brau
Jay Dickerson, Department of Physics and Astronomy, Vanderbilt University
*Physical Characteristics of PbxEuyS and EuTe Nanocrystals
Binary europium chalcogenides and ternary lead europium chalcogenides (PbxEuyX) have been studied for many years for their potential employment in diverse applications, usch as optical isolators, infrared diode lasers and spintronics devices, due to their tunable optical and semi-magnetic properties. Among the PbxEuyX ternary compounds, lead europium sulfide (PbxEuyS) is likely to be most suitable for applications that call for dilute magnetic semiconductors because they can form a completely miscible alloy system with tunable energy band gaps over a wide range. Europium telluride provides an intriguing system to explore size-dependent effects within antiferromagnetic materials. We report the structural and composition dependent magnetic properties of PbxEuyS and EuTe nanomaterials. The crystallinity of the nanocrystals was affirmed by X-ray diffraction and transmission electron microscopy, whereas the magnetic characteristics were probed by vibrating sample magnetometry. PbxEuyS nanocrystals exhibited paramagnetic properties down to 2K, whereas comparably-sized PbS was diamagnetic and EuS was ferromagnetic at these temperatures. For EuTe, we observed finite size effects that induced the formation of a superantiferromagnetic state.
Host: N. Tolk
David Hilton, Department of Physics, University of Alabama-Birmingham
Ultrafast THz Magnetospectroscopy in High Mobility Two-Dimensional Systems
Two-dimensional systems offer a rich array of physical phenomena that include the integer and fractional quantum Hall effects, both of which have been observed in multiple materials systems to date (gallium arsenide and graphene). These effects are related to the unique topology of 2D systems that are unavailable in 1D and 3D systems and provide a high quality system to study the limits of solid state physics and fundamental quantum phenomena. The mitigation and control of coherence in quantum states in 2D systems is an area of great current interest that is critical for the development of the next generation of solid state electronics based on quantum phenomena. In our experiments, we investigate the terahertz frequency properties of a high mobility (μ ≥ 102 cm2 V-1 s-1) gallium arsenide two-dimensional electron gas (2DEG) at cyclotron resonance in a perpendicular magnetic field. We use a picosecond ultrafast terahertz pulse to create a coherent superposition between the highest filled and lowest unfilled Landau level and monitor the dephasing of the cyclotron ensemble as a function of temperature and time. By using phase-sensitive ultrafast terahertz measurement techniques, we can overcome traditional limitations that have prevented accurate spectroscopic studies in high-mobility samples (i.e. overcome the saturation effect). This has been a critical limitation since these high-mobility samples are expected to have the long decoherence lifetimes, τ, that would be needed for device applications. Our experiments reveal a strong increase in the decoherence at low temperatures and a power law dependence, τ ~ T-0.29 from T = 0.4- 2 K, to the decoherence time. In the second part of this talk, I will discuss our recent work demonstrating how ultrafast THz pulses can be used to both create and control coherent states in these 2DEG’s. In this experiment, an initial THz pulse generates a coherent superposition state; we employ a second, time-delayed THz pulse to address this superposition and coherently control its wavefunction. By using two pulses, we can manipulate the phase of the coherent superposition to demonstrate coherent control, very important for any future quantum computation scheme based on 2DEG’s and not easily accessible through alternate techniques (at least on this picosecond time scale).
Host: N. Tolk
Charles Brau, Department of Physics and Astronomy, Vanderbilt University
What can you do with the world’s brightest electron beams?
Following up on the recent seminar by Jonathan Jarvis, I will discuss the scientific possibilities offered by electron beams of exquisite brightness. Already we have produced electron beams with emittance at the Heisenberg limit and brightness that approaches the limit of quantum degeneracy. Detecting and exploiting quantum degeneracy is of scientific interest by itself, but the applications of beams of very high brightness are equally interesting. They include high-brilliance hard X-rays from channeling radiation, a scanning electron microscope on a chip, point-projection images of nano-sized samples, and 3-D electron holograms of biomolecules.
Sergiy Bubin, Department of Physics and Astronomy, Vanderbilt University
*First principles simulations of molecules and nanostructures subjected to ion irradiation
In the framework of real-time real-space time-dependent density functional theory complemented with classical molecular dynamics for ions, we have studied the behavior of small molecules and nanostructures, such as graphene fragments, irradiated by charged energetic particles. In particular, we have investigated the importance of electronic excitations and examined the regime when bond breaking (or defect formation) occurs. Based on the microscopic description of these processes, several quantities that are of interest for ion beam physics have been determined, such as the amount of energy transferred to the target system and the distribution of this energy between electronic excitations and vibrational motion.
Brandon Cook, Department of Physics and Astronomy, Vanderbilt University
*Charge Transport in Kinked and Multi terminal Nanostructures From First Principles
Host: K. Varga
Travis Wade, Materials Science and Engineering Program, Vanderbilt University
*Nanostructure analysis of diamond cold cathode field emitters
Diamond cold cathode devices have demonstrated significant potential as electron emitters. Ultra-sharp diamond pyramidal tips (~5nm tip radius) have been fabricated and the turn-on fields and current yields are an order of magnitude better than comparable silicon tips characterized in recent literature. However, the structure of these complex diamond field emitters are not well understood. Transmission electron microscopy performed at Oak Ridge National Labs provides new insight into tip structure and composition. An understanding of the conduction paths and emission mechanics is expected to yield new optimizations for future emitter tip fabrication.
Host: J. Davidson
Jeremy W. Mares, Electric Engineering and Computer Science Department, Vanderbilt University
*Wide bandgap transition metal-oxides for diverse optoelectronic applications
Transition metal-oxide (TMO) compounds comprise an impressively diverse group of materials in terms of their optical and electronic properties. One subcategory of growing interest within this family is that of the wide bandgap binary and ternary alloy oxide semiconductors. While many of these materials have classically been designated as insulators, the expansion of optoelectronic technologies into the ultraviolet and deep-ultraviolet (UV, DUV) spectral ranges makes research into their development increasingly important. Several of these compounds have seen an explosion in research in recent decades owing not only to their categorically important electronic transition energy, but also to the array of useful electrical and optoelectronic properties they exhibit. Two noteworthy examples of the versatility of TMOs are ZnO and NiO. Historically, ZnO has been well known for its piezoelectric properties. However, modern research has sought to exploit ZnO’s large direct bandgap (3.37 eV) as well as its exceptionally large exciton binding energy (60 meV) which enables highly efficient excitonic recombination, even at elevated temperatures. NiO (Eg ≈ 3.5 eV) is also a direct bandgap material but is most frequently touted for its antiferromagnetic ordering. Nickel oxide is also electrochromic, however, and has emerged as a very useful material for gas detection. Interestingly, both of these compounds can function quite well as transparent conducting oxides (TCOs) when grown with the appropriate doping and degree of crystallinity. Furthermore, both of these materials are popular candidates for resistively switched electronic devices (one such device being the highly publicized “memristor”), which exploit competition between ionic and electronic conduction mechanisms. This presentation will briefly present solid-phase metal oxides as an important and somewhat distinct field of research within optoelectronics, pointing to widely diverse oxides such as ZnO, NiO, TiO2, WO3 and VO2 as important examples. The talk will then discuss the growth and characterization of NiO and ZnO by molecular beam epitaxy (MBE) as well as the possibility of their juxtaposition for p-n or p-i-n optoelectronic heterostructures. Finally, commentary on the important physical principles which give rise to the unique attributes of these materials will be presented.
Host: S. Weiss
Members of the DTRA Sensor Group (Appavoo, Casey, Gulka, Warnick and Haglund), Vanderbilt University
A novel autocatalytic sensor incorporating optical detection of a phase transition
Detection of explosive, chemical and biological agents remains a high priority for national security. Under DTRA sponsorship,* we are developing a novel sensor concept based on optical detection of the semiconductor-to-metal transition (SMT) in vanadium dioxide (VO2) nanodisks. The detection scheme involves autocatalytic decomposition of chemical and explosive agents at the interface between nanoporous gold and an array of VO2 disks of order 100 nm or less in diameter. Specificity is achieved by the use of molecular recognition agents tethered by thiol linkages to the nanoporous gold layer atop the nanodisks. Density functional calculations suggest that the energy released through autocatalytic decomposition should be sufficient to initiate the SMT in the VO2 nanodisks. This phase transition, in turn, can be detected optically by commercial low-power diode lasers in the near infrared. We will discuss the progress achieved in demonstrating molecular specificity and sensitivity, evaluate possible risks in this detection scheme, and discuss the potential for extending this strategy to detection of biological agents. *Supported under grant HDTRA1-1-10-0047 from the Defense Threat-Reduction Agency.
Host: R. Haglund
Drew Steigerwald, Department of Physics and Astronomy, Vanderbilt University
Novel applications of coherent acoustic phonon spectroscopy
Here we compare coherent acoustic phonon, channeling measurements, and simulation results to quantitatively determine depth profiles of lattice disorder in GaAs arising from ion irradiation. Our optoacoustic measurements are shown to be 2-3 orders of magnitude more sensitive in defect concentration than channeling techniques. Our measurements establish a quantitative dependence between the change in optical response and defect concentration between 1018-1021 defects/cm3. Further, we demonstrate the entire range over which the coherent acoustic phonon technique is applicable in defect studies, and show results ranging from no noticeable change in optical response to complete damping of the phonon wave. Here we also discuss the electronic nature of the CAP response, which can provide insight into the interplay between lattice disorder and electronic structure.
Host: N. Tolk
Marco Liscidini, Department of Physics, University of Pavia, Italy
Bloch surface waves with applications in sensing and fundamental condensed matter physics
Bloch surface waves (BSWs) are propagating photonic modes that exist at the interface between a photonic crystal and a homogeneous medium. These modes are characterized by field confinement at the photonic crystal surface. Another main features of a BSW is the sensitivity of its dispersion relation to a change in the cladding refractive index. All these properties can be exploited in a number of applications that range from optical sensing to the study of more fundamental condensed matter problem, e.g. strong light-matter coupling. After reviewing the main features of BSW, I will present two possible applications: (1) the use of BSW in optical sensors based on either diffraction or fluorescence; (2) a study of guided exciton-polaritons arising from the strong coupling between a single quantum-well and a guided BSW.
Host: S. Weiss
Shweta Bhandaru, Interdisciplinary program in Material Science, Vanderbilt University
*X-ray induced acceleration of silicon oxidation
In the past few decades, studies have been conducted to investigate photon assisted oxidation of silicon substrates. Most of these efforts have focused on understanding and modeling the oxide growth mechanism using photon energies spanning the visible (1.55 eV – 3.0 eV) to the UV range (3.0 eV – 6.5 eV). In this work, we study the influence of higher energy x-rays (10 keV) on silicon oxidation. We found that x-ray irradiation of silicon substrates, performed at ambient temperature and atmospheric pressure conditions, can significantly affect the formation of silicon oxide. The oxide formation is influenced by the dose rate and total dose of x-ray irradiation, as well as the initial silicon surface preparation. Initial XPS analysis showed that the oxide layers on the irradiated and control samples were chemically different, suggesting that differences observed in the ellipsometry analyses may be due, in part, to differences in the optical properties of the oxide layers and not purely due to a change in thickness. We will present the results of the oxidation study in addition to proposing a mechanism to explain the experimental observations based on ozone concentration measurements performed during irradiation. The possible generation of atomic oxygen, due to dissociation of molecular oxygen, by the high energy x-rays is suggested as a key factor in the observed x-ray irradiation induced silicon oxidation. Acknowledgement: This work was supported in part by the DTRA Basic Research Program (Grant No. HDTRA1-10-0041).
Host: S. Weiss
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