Condensed Matter and Optics Seminar
Seminars are held on Fridays at 3pm in Stevenson Center 6333, unless otherwise noted.
August 24, 2017 | 3:00 PM | SC 6333
Travis Wade, MIT Lincoln Laboratory
Controlling the stability of diamond's surface conductivity
After many years of development, AlGaN/GaN 2-D electron-gas FETs have revolutionized the power RF field. With a similar development effort to harness its superior semiconductor properties, diamond is expected to enable devices with an order of magnitude increase in performance. Primary among the development challenges is a stable, high conductivity active FET channel. Hydrogen-terminated diamond, with the appropriate surface treatment, forms a 2-D hole gas at its surface. Electron acceptors (NO2, O3, and Cl2) increase the conductance while electron donors (NH3 and NH2C6H5) diminish the conductance. All these surface dopants are unstable in air with their properties diminishing in time. We present a different doping approach using UV-generated free radicals. We theorize that these radicals abstract hydrogen from the diamond surface and insert themselves at that site. The resulting surface has stability comparable to ALD-deposited Al2O3 and conductivity comparable to the most effective surface treatments reported to date. Surface conductivity was measured with a Hall – Van der Pauw system to quantify carrier type, density, and mobility. The enhanced conductivity is often observed to be the result of increased carrier mobility instead of increased carrier density. These results indicate that a model of diamond surface conductivity that only considers the dopants as negatively charged states to generate holes is incomplete. We have also explored the impact of variations in physical properties on surface conductivity. We will report on the relative impacts of surface roughness, sub-surface polishing damage, chemical purity, bulk stress, and growth method. This presentation will discuss our work on free-radical-doped diamond surfaces and efforts toward maximizing the conductivity and stability.
host: Jimmy Davidson
September 22, 2017 | 3:00 PM | SC 6333
Richard Haglund, Physics and Astronomy, Vanderbilt Univ.
Is the laser-induced phase transition in VO2 due to thermal or electronic effects?
In a 1959 paper cited more than 2500 times, F. J. Morin described vanadium dioxide (VO2) as one of several oxides exhibiting insulator-to-metal transitions at their Neél temperatures. In 2001, Andrea Cavalleri and his collaborators provided evidence that this phase transition could be switched on an ultrafast time scale by femtosecond pulses from an amplified Ti:sapphire laser. As interest in possible applications of such a phase-change material has burgeoned over the last two decades, a critical question has assumed increasing prominence: Under what circumstances is the optically induced phase transition simply due to transient heating of the material, or are purely electronic mechanisms potentially at work? In this tutorial-style presentation, I discuss recent results that help both to answer this scientific question and to evaluate realistic possibilities for deploying VO2 in silicon photonic technology
host: Andrey Baydin, Norman Tolk
October 20, 2017 | 3:00 PM | SC 6333
Halina Krzyżanowska, Research Assistant Professor, Physics and Astronomy, Vanderbilt Univ.
Towards a Si-based laser – efficient infrared emission from Er-doped SiO2/nc-Si multilayers
Silicon photonics was pioneered in the 90’s. Despite much research, highly efficient light sources based on silicon have not yet been realized. One of the most promising ways of achieving efficient electrically pumped Si light sources at the standard telecommunication wavelength (1535 nm) is to use the unique properties of Si nanostructures doped with rare earth ions. This talk covers optical and electrical studies of Er doped SiO2/nc-Si multilayers fabricated by rf magnetron sputtering. Understanding and optimization of energy transfer from the Si nanocrystals to Er were achieved via visible and infrared time resolved photoluminescence. Energy transfer is necessary to achieve efficient, infrared emission from Er doped multilayers. Ultrafast pump-probe spectroscopy, using a prism coupler to launch light into the multilayer waveguide, allowed us to study free carrier absorption (FCA) and carrier dynamics in Si nanostructures. Optical confinement in the low refractive index medium (Er - doped SiO2) for TM polarization was used to suppress FCA process in nm-thick layers containing Si nanocrystals. Carrier transport and infrared electroluminescence under a novel, lateral device geometry was studied as a function of various parameters. The major advantage of the proposed lateral carrier injection approach compared to vertical one, is that transport is much easier in the multilayers. It proceeds inside the Si layers instead of through an oxide matrix. The strong photoluminescence under off-resonance excitation and electroluminescence under forward bias are very promising for Si-based light sources - the missing link in an all-silicon on-chip optical interconnection system.
host: Andrey Baydin, Norman Tolk
October 27, 2017 | 3:00 PM | SC 6333
Thomas G. Folland, Postdoctoral Scholar, Mechanical Enginneering, Vanderbilt Univ.
Polaritons in 2D materials for tuneable infrared light sources; A tale of two cities
Developing narrowband light sources in the mid and far-infrared spectral regions has long been a major scientific and engineering challenge. Solving this challenge ultimately requires new material approaches to emitter design, as semiconductor heterostructures cannot provide a complete solution. Two dimensional materials (2D materials) have been the subject of intense scrutiny for their optoelectronic properties – such as polarized excitons, tuneable bandgap absorption and polariton behaviour. Indeed, there has been extensive research into making inter-band emitters and lasers from 2D materials, but realizing these at mid and far infrared frequencies has proven a more significant challenge. This is mainly because achieving efficient emission requires careful design of both the optical mode and electronic states in the 2D material device. Here we will discuss an alternative approach; using polaritons within two dimensional materials to control an existing radiative process. This work is a tale of two cities combing my doctoral work, conducted at The University of Manchester in the UK, as well as the initial work and characterisation capabilities of the Caldwell Lab at Vanderbilt University. I will discuss how graphene can be coupled into a metal grating in the far infrared to create an electrically tuneable optical response from a terahertz laser. Second, I will introduce infrared characterisation and 2D material fabrication techniques which we will be exploiting in the Caldwell lab to perform experiments on polariton emitters in hexagonal boron nitride, transition metal dichalcogenides, and other material systems.
host: Andrey Baydin, Norman Tolk
November 3, 2017 | 3:00 PM | SC 6333
Deyu Li, Mechanical Enginneering, Vanderbilt Univ.
Understanding Nanoscale Transport Phenomena for Engineering Applications
Nanoscale energy, charge and mass transport plays critical roles in various phenomena in nature and has important engineering implications. In this talk, I will discuss some new understandings we obtained in nanoscale transport and some engineering applications. For energy transport, I will present a few interesting observations on phonon transport through individual nanostructures and their contacts. More specifically, I will show how kinks could affect the thermal conductivity of boron carbide nanowires, which provides new insights into tuning thermal properties of nanowires. I will also introduce the intriguing diameter dependence of contact thermal conductance between individual multi-walled carbon nanotubes, which comes from complex interplay between phonons and boundaries. Moreover, I will demonstrate two separate specularity parameters at interfaces for transmitted and reflected phonons through measurements of single and double nanoribbons. In terms of charge and mass transport, I will present a new approach taking advantage of the superior electronic transport properties of graphene to probe the electrical activities of individual dendritic spines and synapses of central nervous system neurons cultured in microfluidic platforms.
November 10, 2017 | 3:00 PM | SC 6333
Yun-Peng Wang, Postdoctoral Scholar, Physics and Astronomy, Vanderbilt Univ.
First-Principles Investigation of Electric Field Effects on Two-dimensional Materials
Two-dimensional (2D) materials have gained intense attention due to their emerging electronic structures and the potential for electronic device applications. The electric field effect is a valuable tool for studying electronic structure of 2D materials and for controlling electronic transport properties. The inadequacy of Maxwell’s macroscopic equations in atomic-scale 2D materials calls for a microscopic first-principles description. We adopted the density functional theory (DFT) method in combination with the effective screening medium (ESM) approach. Two applications will be discussed. I will firstly discuss the electric field effect on the band structure of trilayer graphene and the dependence on the stacking relation. Then I will discuss a magnetic phase transition in 2D metal-organic materials induced by electric fields.
host: Sokrates Pantelides
November 17, 2017 | 3:00 PM | SC 6333
Chris Stanton, Department of Physics, University of Florida
Tailoring and Manipulating Spin Polarization and Optically Pumped NMR in Semiconductor Nanostructures
There has been much interest in III-V and II-VI dilute magnetic semiconductors in which incorporation of magnetic impurities, such as manganese, is used to tailor the electronic, magnetic, and magneto-optical properties. Optically-pumped nuclear magnetic resonance (OPNMR) spectroscopy is an emerging technique to probe electronic and nuclear spin properties in bulk and quantum well semiconductors. In OPNMR, one uses optical pumping with circularly polarized light to create spin-polarized electrons in a semiconductor. The electron spin can be transferred to the nuclear spin bath through the Fermi contact hyperfine interaction which can then be detected by conventional NMR. The resulting NMR signal can be enhanced four to five orders of magnitude or more over the thermal equilibrium signal. We report on our OPNMR and magneto-optical studies in semiconductor nanostructures such as GaAs and InMnSb quantum wells. We focus on the theoretical calculations for the average electron spin polarization at different photon energies for different values of external magnetic field in both unstrained and strained quantum wells. The calculations are based on the 8- band Pidgeon-Brown model generalized to include the effects of the quantum confinement potential as well as pseudomorphic strain at the interfaces. Optical properties are calculated within the golden rule approximation. Detailed comparison to experiment allows one to accurately determine material properties such as valence band spin splitting including the effects of strain and suggest ways of controlling and manipulating both nuclear and electronic spin polarization in materials.
host: Norman Tolk
December 1, 2017 | 3:00 PM | SC 6333
Joy C. Garnett, Academic Pathways Postdoctoral Fellow, Department of Life and Physical Sciences, Fisk Univ.
Prediction of light yield of scintillators from composition using machine learning methods
We describe the development of machine learning methods for the prediction of optical properties of halide scintillators. Scintillating materials are of interest in detecting radiation for medical imaging, national security, and space research applications. The materials studied here include A2BX6 halide scintillators, where A is a monovalent ion such as Cs, K, Li, Rb, and, Na; B is a tetravalent ion such as Ti, Zr, Hf, Sn, Se, and Te; and X is a halide ion such as Cl, Br, and I. A2BX6 materials such as Cs2HfCl6 are the subject of a concerted effort in the search for new materials that can be incorporated into efficient, bright, cost-effective radiation detection and imaging devices. These are investigated based on their dielectric, ionic, and x-ray attenuation properties. Several machine learning models are used to utilize elemental data from the literature and are trained to learn the composition-property relationships of 550+ known scintillators compiled in the Lawrence Berkeley Scintillator Database. The trained models then use compositional correlations to predict light yield of various scintillators. The results of the model show that machine learning models can produce relatively accurate prediction of the properties of known scintillators and can be used to assess the performance of novel scintillators for various applications.
host: Andrey Baydin, Norman Tolk
December 8, 2017 | 3:00 PM | SC 6333
John Brehm, Postdoctoral Scholar, Physics and Astronomy, Vanderbilt Univ.
Electron beam-induced synthesis of hexagonal 1H-MoSe2 from square β-FeSe decorated with Mo adatoms
Two-dimensional (2D) materials have generated interest in the scientific community because of the advanced electronic applications they might offer. Powerful electron beam microscopes have been used not only to evaluate the structures of these materials, but to manipulate them as well, by forming vacancies and nano-fragments or joining nano-islands together. In this presentation I will show a movie in which the electron beam in a scanning transmission electron microscope (STEM) can be used in yet another way: to mediate the synthesis of 2D 1H-MoSe2 from Mo-decorated 2D β-FeSe and simultaneously image the process on the atomic scale. Prior to committing microscope and materials resources to this project, the choice of reactants was first evaluated as a reasonable system for this proof of concept synthesis via quantum mechanical calculations. These calculations will be described in detail and include a methodology for finding a reaction path to forming a stable 1H-MoSe2 nucleation kernel within pure β-FeSe in which the pertinent energy barriers are smaller than the energy that can be supplied by a STEM electron beam.
host: Sokrates Pantelides
December 15, 2017 | 3:00 PM | SC 6333
Joshua Caldwell, Mechanical/Electrical Engineering, Vanderbilt Univ.
Mid-IR to Thz Polaritons: Realizing Novel Materials for Nanophotonics
The field of nanophotonics is based on the ability to confine light to sub-diffractional dimensions. Up until recently, research in this field has been primarily focused on the use of plasmonic metals. However, the high optical losses inherent in such metal-based surface plasmon materials has led to an ever-expanding effort to identify, low-loss alternative materials capable of supporting sub-diffractional confinement. Beyond this, the limited availability of high efficiency optical sources, refractive and compact optics in the mid-infrared to THz spectral regions make nanophotonic advancements imperative. One highly promising alternative are polar dielectric crystals whereby sub-diffraction confinement of light can be achieved through the stimulation of surface phonon polaritons within an all-dielectric, and thus low loss material system. Due to the wide array of high quality crystalline species and varied crystal structures, a wealth of unanticipated optical properties have recently been reported. This talk will discuss recent advancements from our group including the realization of localized phonon polariton modes, the observation and exploitation of the natural hyperbolic response of hexagonal boron nitride. Beyond this, methods to improve the material lifetime, realize active modulation and to induce additional functionality through isotopic enrichment and hybridization of optical modes will also be presented.
host: Norman Tolk
January 12, 2017 | 3:00 PM | SC 6333
Hunter Sims, Postdoctoral Scholar, Vanderbilt Univ., Oak Ridge National Lab, MSTD
Atomic-scale investigation of the interface between monolayer FeSe and SrTiO3
It was recently demonstrated that monolayer FeSe on a SrTiO3 (STO) substrate is a superconductor with Tc between 60 and 100 K, compared to 8 K in bulk FeSe. Recent experiments have also revealed that an additional layer of titanium oxide lies between the STO substrate and the FeSe monolayer, but the detailed structure of this interface has remained elusive. Here we use a combination of quantum mechanical calculations and scanning transmission electron microscopy to definitively determine the atomic structure of the monolayer. We find that this monolayer is bonded to both the FeSe film and the substrate by van der Waals interactions and that magnetic interactions between this monolayer and the FeSe film generate distortions that are favorable for superconductivity. Finally, we note that a similar interface exists between an epitaxial monoclinic VO2 film grown on a STO sub, which suggests that such van der Waals bonded monolayers may be present at other complex-oxide interfaces.
January 19, 2018 | 3:00 PM | SC 6333
Jian Liu, Postdoctoral Scholar, Physics and Astronomy, Vanderbilt Univ.
January 26, 2018 | 3:00 PM | SC 6333
Ranganathan Parthasarathy, Postdoctoral Researcher, TSU Nanomaterials Research Lab
February 2, 2017 | 3:00 PM | SC 6333
Tianli Feng, Postdoctoral Scholar, Physics and Astronomy, Vanderbilt Univ.
February 9, 2017 | 3:00 PM | SC 6333
Norman Tolk, Physics and Astronomy, Vanderbilt Univ.
February 23, 2017 | 3:00 PM | SC 6333
Summayya Kouser, Postdoctoral Scholar, Physics and Astronomy, Vanderbilt Univ.
host: Sokrates Pantelides