Archive: Seminars 2013

"Scanning probe characterization of energy nanomaterials and devices," by Liwei Chen, Suzhou Institute of Nanotech and Nanobionics, Chinese Academy of Sciences, hosted by Saw Wai Hla

Abstract: Energy is the biggest challenge in the next 50 years. Energy nanotechnology, which combines nanomaterials and nanoscale effects of special properties for specific applications in energy, has become the focus of a mainstream campaign of research in the last decade. In this talk, I will present a few examples of scanning force microscopy investigations on materials and devices, especially on interfaces in devices. Firstly, a new scanning dielectric force microscopy technique will be introduced and its application in characterization of charge transport properties will be demonstrated. We will then move on to the interfacial dipole measurement in organic photovoltaic device and finally we show force spectroscopy study of solid-electrolyte interphase in lithium-ion batteries.

"Controlling Gold Nanoparticles with Atomic Precision," Rongchao Jin, Carnegie Mellon University, hosted by Yugang Sun and Gary Wiedderrecht

Abstract: Controlling nanoparticles with atomic precision has long been a major goal in nanoscience research. Gold nanoparticles are particularly attractive because of their chemical stability and elegant optical properties. The synthesis of atomically precise gold nanoparticles, however, remained a major challenge in the past, which hampered the pursuit of fundamental science of such nanoparticles (e.g., surface structure, quantum size effect).

This talk will present a size-focusing methodology successfully developed for synthesizing a series of atomically precise gold nanoparticles protected by thiolates [denoted as Aun(SR)m, with n ranging from a few dozens to several hundreds, also called nanoclusters)]. Such ultrasmall nanoparticles (ca. 1-3 nm) exhibit distinct quantum size effects and interesting electronic/optical properties, which are fundamentally different from those of larger counterparts, such as plasmonic nanoparticles. New types of atom-packing structures have been discovered in Aun(SR)m nanoclusters through X-ray crystallographic analysis.

A few representative size-specific Aun(SR)m nanoclusters will be discussed in detail. These well-defined nanoclusters also hold potential in catalysis as new model catalysts, and atomic-level correlation of the catalytic properties of Aun(SR)m with crystallographic structures will ultimately offer fundamental understanding on nanogold catalysis.

"Exerting Mechanical Force on Single Protein Molecules," Robert Szoszkiewicz, Kansas State University, hosted by Xiao-Min Lin and Tijana Rajh

Abstract: Using AFM force spectroscopy, one can measure physiologically relevant pN forces between an AFM tip and a biomolecule with a mean displacement resolution of about 0.1 nm. The last 15 years have witnessed an explosion of interest in single-molecule force spectroscopy fueled by

• New possibilities to advance in protein folding,
• Possibilities to elucidate molecular mechanisms of various cellular processes, and diseases, and
• Efforts to understand the nanomechanical properties of proteins, polysaccharides and DNAs in order to design biomimetic and/or mechanically functional materials.

In this seminar, we will present several examples of our AFM force spectroscopy data. First, we will present the results of mechanical unfolding of on a recombinant protein comprising an NRR domain from mammalian Notch 1. Notch is a transmembrane cell signaling protein, and understanding its mechanical properties at the single-molecule level is expected to help elucidate Notch's role in processes relevant to embryonic development, tissue homeostasis, and some breast cancers. Second, we will concentrate on elucidating early folding events in a simple model protein from changes of molecular compliance and dissipation factors. Using such measurements, we hope to provide basic understanding of early-folding events. Time permitting, we will show how mechanical force can influence the rate and mechanisms of an enzymatic cleavage of a single disulfide bond embedded in a protein.

"Manipulative Scanning Tunneling Microscopy and Single-Molecule Spintronics," by Andrew DiLullo, Ohio University, hosted by Saw Wai Hla

Abstract: Scanning tunneling microscopy (STM), a real-space local probe of nanoscale topology and electromagnetic properties, is applied to further our understanding of surfaces and surface supported atomic and molecular systems. In addition, STM manipulation techniques are implemented for bond dissociations, lateral manipulations, and surface augmentations. Diverse applications of STM techniques will be presented, with the primary focus of characterizing surface supported spintronic molecular systems. The controlled creation of surface nano-cavities will be shown, along with a method for extracting local surface and probe work functions through judicious measurement and data analysis. Surface-catalyzed molecular chain formation will be demonstrated, resulting in linear, covalently coordinated networks of spin-centers (cobalt ions) that are antiferromagnetically linked and interact individually with the Au(111) substrate through Kondo interactions. The spin polarization of molecular orbitals is mapped by application of spin-polarized STM, and reversible probe-induced molecular conformation switching is demonstrated. Results are summarized as related to the primary goal of creating functional spintronic molecular systems, and a brief outlook for future measurements will be presented

"Atomic and Molecular Nanocontacts: Structure, Magnetism, and Kondo Anomalies from First Principles," Erio Tosatti, SISSA, Trieste, Italy, hosted by Daniel Lopez

The nature and properties of atomic and molecular metallic nanocontacts, break junctions, of tip-surface tunnel contacts are difficult to access geometrically. Yet, they are rich in phenomena connected with structure, electron transport, and magnetism. Structurally, the formation of magic nanowires in gold is a remarkable phenomenon, explained by minima of the string tension. Electronically, first-principles calculations account well for the ballistic conductance of metal nanocontacts, both in magnetic and nonmagnetic metals.

Magnetic impurities bridging nonmagnetic metal contacts yield zero bias anomalies in STS spectra and/or in ballistic conductance because of the Kondo effect — a remarkable example of magnetically controlled current. Conductance is in this case ruled by the specificities of the atomic and molecular states involved, accessible only through a first-principles electronic structure approach.

The unsolved difficulty in combining the Kondomany body physics with standard electronic structure poses a problem to the theorist. We address this problem by means of a recently devised joint density functional plus numerical renormalization group approach. I will illustrate applications to transition metal impurities on gold nanowires and on carbon nanotubes, to platinum nanocontacts, and to magnetic molecules on gold surfaces. The circumstances leading to an exotic ferromagnetic Kondo effect, as opposed to an ordinary antiferromagnetic Kondo effect, will also be outlined along with possible systems where that unusual situation could be realized.

"Atomic Structure of Carbon and Nitrogen on the Pt(111) Surface," Michael Trenary, University of Illinois at Chicago, hosted by Tijana Rajh

Abstract: The structure and reactivity of elemental carbon and nitrogen on transition metal surfaces are important to a variety of problems in heterogeneous catalysis. Many of the surface chemical properties of both carbon and nitrogen can be deduced through studies that employ techniques that average over monolayers, while scanning tunneling microscopy (STM) can provide direct information on the structure of surface layers, often with atomic resolution. The techniques of reflection adsorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), and low-energy electron diffraction (LEED) have been used to study the formation of carbon and nitrogen on Pt(111) through dehydrogenation reactions.

In the case of carbon, the dehydrogenation of acetylene and ethylene was found to first produce ethylidyne (CCH₃), which then decomposes to form C_xH_y clusters of various sizes as observed with room-temperature STM. At higher temperatures, these clusters would undergo further dehydrogenation to form graphene islands. Under conditions that resulted in complete coverage of the Pt(111) surface with graphene, various rotational domains of graphene were observed. The boundaries between graphene domains provide nucleation sites for the growth of Pt nanoclusters when Pt is deposited onto the graphene covered Pt(111) surface. In the case of nitrogen, it was found that reaction between ammonia and molecularly adsorbed O₂ would result in the formation of H₂O, which desorbs below 200K to leave behind a well-ordered p(2×2)-N layer on Pt(111). This N layer readily reacts with H₂ to form NH molecules on the surface, as observed with RAIRS. Through collaborative research with a group in Japan, a low-temperature STM operated at 5K was used to obtain atomically resolved images of the p-(2×2)-N layer and of its hydrogenation to NH.

"3D Composition Profiling at the Nanoscale: Doping Limits in Semiconducting Nanowires," by Justin Connell, Northwestern University, Vanderbilt University, hosted by Amanda Petford-Long

Abstract: The vapor-liquid-solid (VLS) mechanism of semiconductor nanowire growth provides a means to fabricate one-dimensional structures with control over doping and aspect ratio provided in situ during growth. Developing deep understanding and precise control of the structure and chemical composition of VLS-grown nanowires is crucial, as any unintended gradients in dopant concentration can severely degrade the ultimate device performance. This is particularly important for optoelectronic applications such as solar cells and LEDs, where broadened axial and/or radial doping junctions lower efficiencies.

Contrary to the traditional model of VLS growth, where dopant species are assumed to incorporate uniformly across a planar liquid-solid interface, we demonstrate that VLS-mediated doping is highly radially anisotropic, with dopant concentration variations across the nanowire diameter of as much as two orders of magnitude. Finite-element modeling of the doping process, coupled with recent in situ TEM observations reported in the literature, suggest that this radially inhomogeneous dopant distribution is a direct consequence of growth from a faceted liquid-solid interface, rather than the commonly assumed planar interface.

These observations suggest that this doping inhomogeneity is general to all nanowire systems, motivating the search for novel catalysts for nanowire growth that can alleviate or eliminate this side faceting behavior. Using both aqueous solution and e-beam lithographic techniques, we are able to fabricate composition-controlled Au-Cu alloy catalysts for nanowire growth, providing a platform on which to study the limits to which varying catalyst phase and chemistry can be used to control doping in VLS nanowire growth.

"Hybrid Plasmonic Phase-Changing Nanostructures: Active Reconfigurable Devices to Ultrafast Dynamics," by Kannatassen Appavoo, Vanderbilt University, hosted by Matt Pelton

Abstract: Ultrafast photo-induced phase transitions in quantum materials could revolutionize data storage and telecommunications technologies by modulating transport in integrated nanocircuits at terahertz speeds. In phase-changing (PC) materials, microscopic charge, orbital and lattice degrees of freedom interact cooperatively to modify macroscopic electrical and optical properties. Although these interactions are well documented for bulk single crystals and thin films, little is known when such PC materials are nanostructured and implemented in nanoscale switching configurations.

This talk presents a generalizable concept of incorporating a quantum material — vanadium dioxide (VO₂) — to create functionality in plasmonics, a new device technology that interfaces electronic and photonic components in a single chip. By designing, simulating, and fabricating hybrid plasmonic/PC nanostructures, we demonstrate at the single nanostructure level how signal modulation can be achieved when the VO₂ component undergoes its characteristic insulator-to-metal transition. Furthermore, a subwavelength hybrid nanomodulator is demonstrated that is both thermodynamically and wavelength tunable. Reconfigurability is enabled by spatially confining electromagnetic fields to nanoscale volumes by using a metallic nanostructure while simultaneously tailoring its near-field environment with a PC nanostructure.

By providing the first ultrafast optical studies of a hybrid nanomaterial, we also report a novel all-optical technique to trigger VO₂ PT on a timescale faster than its single-phonon cycle, accompanied by a decrease in switching threshold. The mechanism is based on ballistic hot electrons created by ultrafast optical excitation of gold nanoparticles, which are injected through the gold/VO₂-nanostructure interface. Density functional calculations show that the injected electrons cause the catastrophic collapse of the 6-THz optical phonon mode, associated with the structural phase transition of VO₂.

Most importantly, the hybrid nanostructures discussed here combine generic plasmonic (gold) and PC (VO₂) components. Therefore, this work aims to be generalizable, serving as a platform for designing other hybrid nanostructures operating at nanometer length scale and on femtosecond timescale for next-generation all-optical nanophotonic devices. Key scientific issues regarding the viability of such hybrid nanomodulators are also addressed, such as interfacial effects, intrinsic size-dependent switching of VO₂ and the potential for coherent control of the structural dynamics in VO₂.