Argonne National Laboratory

2015 Colloquium Archive

Date Title
December 23, 2015 NO Colloquium

December 16, 2015

4:00 pm

Bldg. 440, A105-106

"The Status and Challenges of Lead Halide Perovskite Solar Cells", Yanfa Yan, University of Toledo, hosted by Jianguo J.G. Wen

Organic-inorganic methylammounium lead halide perovskites, CH3NH3PbX3 (X= Cl, Br, I), have revolutionized the field of thin-film solar cells. Within five years, the efficiency of lead halide perovskite-based thin-film solar cells have increased rapidly from 3.8% in 2009 to 20.1% for a planar CH3NH3PbI3-based thin-film solar cell in 2014. Such rapid progress has never been seen before in the history of solar cell development. In this talk, I will review the history and status of lead halide perovskite thin films solar cells. I will explain why lead halide perovskites exhibit superior photovoltaic properties that conventional solar cell materials such as Si, Cu(In,Ga)Se2, and CdTe do not. I will further discuss the intrinsic problems and challenges that lead halide perovskite solar cells are facing.  Finally, I will give an outlook for perovskite solar cells.

December 9, 2015

4:00 pm

Bldg. 440, A105-106

“Growing Nanowires, Spirals, and Helices in Solution”, Hongyu Chen, Nanyang Technological University, Singapore, hosted by Yugang Sun

In this talk, I will discuss the small modifications that lead to different growth of straight nanowires, spirals, single and double helices. Previously, we reported the synthesis of ultrathin (d = 5-7 nm) Au nanowires, which are directly grown from Au seeds anchored on a substrate in an aqueous solution at room temperature. These NWs are typically polycrystalline with random lattice orientation. The growth behavior is similar to the vapor-liquid-solid growth at high temperature, but the underlying mechanism is fundamentally different. The transition from the normal straight nanowires to the spiral nanowires is not triggered by the presence of a particular reactant, but by the ratio of reactant concentration. We propose that disorganized ligand-Au cluster may partially block the Au-substrate interface, inducing imbalanced growth of the nanowire and causing it to coil. Blocking a single corner of the active interface leads to spiral nanowires whereas inequivalently blocking two corners leads to helical nanowires. Most recently, we found that a special ligand is able to form precipitates of metal-ligand complexes, out of which ultralong Au nano-helices were formed under ambient conditions. The floccules of ligand complex provide a robust environment for the sustained growth the nano-helices, leading to their record length and consistency. As the seeds are embedded in and asymmetrically blocked by the floccules, the extrusion of nanowire follows a constant helicity but differs from other individuals in the same sample. The structural characteristics, handedness, and internal lattice structure of the helices are consistent with our hypothesis.

November 25, 2015 NO Colloquium

November 18, 2015

4:00 pm

Bldg. 440/A105-106

"Charge Transfer at the Nano Scale - Single Molecule and X-ray Photoemission Spectroscopy Measurements", Latha Venkataraman, Columbia University, hosted by Pierre Darancet

Understanding and controlling electron transfer across metal/organic interfaces is of critical importance to the field of organic electronics and photovoltaics. Single molecule devices offer an ideal test bed for probing charge transfer and mechanics at these interfaces, while x-ray photoemission measurements enable a probing of femtosecond dynamics of charge transfer across interfaces. In this talk, I will review the scanning tunneling microscope break-junction technique we use to measure conductance through single molecule junctions1 and illustrate how it can be used to determine key parameters that control transport, namely the alignment of the molecular levels to the metal electrode and their electronic coupling. I will then compare these results with those from resonant x-ray photoemission spectroscopy measurements to illustrate how the dynamics of charge transfer can provide insights into these transport parameters.

November 11, 2015

4:00 pm

Bldg. 440/A105-106

"The Spin-Dependent Surface Chemical Bond", Dan Dougherty, North Carolina State University, hosted by Jeff Guest

Organic materials are exciting material for spin-based electronics because their magnetic properties are extraordinarily tunable by synthetic chemistry [1].  Since about 2004, several device groups have observed spin dependent transport phenomena in organic materials [2] and suggested a decisive role for metal-organic interfaces in controlling spin injection [3].  We need to understand the magnetic properties of these interfaces to advance the fledgling field of organic spintronics.

  I’ll describe scanning tunneling microscopy and spectroscopy studies of model interfaces designed to probe the most important electronic interactions between magnetic substrates and organic molecules.   First, I’ll describe a unique case of indirect interactions between an organic semiconductor (PTCDA) and a d-derived surface state on Cr(001) that provides a (perhaps cautionary) illustration of the richness of interfacial interactions that can occur in these systems [4].  This indirect interaction will be contrasted with spin polarized STM studies of Alq3 and related molecules that show hybrid interface states near the Fermi level when adsorbed on Cr(001).

[1] Sanvito, Chem. Soc. Rev. 40 3336 (2011).

[2] Dediu et al., 8, Nat. Mater. 8, 707 (2009).

[3] Barraud et al., Nat. Phys. 6, 615 (2010).

[4] Wang and Dougherty, Phys. Rev. B 92, 161401R (2015).

November 4, 2015

4:00 pm

Bldg. 440/A105-106

"Transition Metal Dichalcogenides as Electrodes for Energy and Electronics", Manish Chhowalla, Rutgers University, hosted by Ani Sumant

Two-dimensional transition metal dichalcogenides (2D TMDs) — whose generalized formula is MX2, where M is a transition metal of groups 4–7 and X is a chalcogen — consist of over 40 compounds. Complex metal TMDs assume the 1T phase where the transition metal atom coordination is octahedral. The 2H phase is stable in semiconducting TMDs where the coordination of metal atoms is trigonal prismatic. High performance of electronic and opto-electronic devices have been demonstrated with semiconducting TMDs while interesting condensed matter effects such as charge density waves and superconductivity have been observed in bulk metallic 1T phase TMDs. However, stability issues have hampered the study of interesting phenomena in two-dimensional 1T phase TMDs. Recently there has been a surge of activity in developing methodology to reversibly convert 2D 2H phase TMDs to 1T phase. In contrast with typical phase transformation conditions involving pressure and temperature, phase conversion in TMDs involves transformation by chemistry at room temperature and pressure. Using this method, we are able to convert 2H phase 2D TMDs to the 1T phase or locally pattern the 1T phase on 2H phase 2D TMDs. The chemically converted 1T phase 2D TMDs exhibit interesting properties that are being exploited for catalysis for hydrogen evolution reaction, source and drain electrodes in high performance field effect transistors, and as electrodes for energy storage. In this contribution, I will summarize the key properties of 2D 1T phase TMDs and their applications as electrodes for energy and electronics.

October 28, 2015

4:00 pm

Bldg. 440/A105-106

"Transport in Adatom-Decorated Graphene", Erik Henriksen, Washington University, St. Louis, hosted by Chad Husko

That graphene hosts a 2D electronic system unprotected from the environment is often detrimental: while the electron mobility is high compared to most semiconductors, it is far lower than it could be due to scattering from extrinsic disorder. Perhaps these interactions between graphene's quasi-relativistic electrons and various surface contaminants can be turned to advantage. For instance, can the electronic structure of graphene be controllably altered? We are starting to explore the possibility of adatom-induced spin-orbit couplings in graphene in order to realize the original topological insulator predicted by Kane and Mele in 2005. Initial experiments on dilute coatings of indium atoms on graphene will be presented, as well as our current efforts with osmium.

October 14, 2015

4:00 pm

Bldg. 440/A105-106

“Design and Control of Interface and Structure of Electrodes for Energy Conversion and Storage”, Guozhong Cao, University of Washington, hosted by Yugang Sun

In this presentation, I will share our recent research results and better understanding of some old results on both solar cells and battery electrodes. For solar cell research, I will demonstrate and discuss how the micro- and nanostructures and surface chemistry of photoanode would affect the power conversion efficiency; similarly, the significant impacts of the insertion of extra quantum dot layers and plasmonic nanocrystals as well as the doping or alloying in quantum dots on photon capturing, charge transfer and interface recombination will be elaborated. In the second part of my seminar, the focus will be devoted to the better understanding how the controlling factors in electrode materials affect the lithium-ion storage capacity and electrochemical potential. Particularly we will have a little in-depth discussion on the reversible 1st order phase transition and impacts of crystal field on the lithium ion storage through intercalation reactions.

October 7, 2015

4:00 pm

Bldg. 440/A105-106

“Computational Design and Discovery of Earth-Abundant Thermoelectrics”, Vidvuds Ozolins, University of California, Los Angeles, hosted by Maria Chan

Many known good thermoelectric materials are comprised of elements that are in low abundance and require complex doping and synthesis procedures. Thermoelectrics comprised of earth-abundant elements would pave the way to new low-cost energy generation opportunities. We have used first-principles calculations to identify cubic Cu12Sb4S13 as a promising thermoelectric; subsequent experiments showed dimensionless thermoelectric figure of merit near unity in doped materials. These compounds span the range of compositions of the natural mineral family of tetrahedrites, the most widespread sulfosalts on Earth. In related work, we have predicted ultra-low thermal conductivity in new rocksalt-based I-V-VI semiconductors, where the group I elements are Cu, Ag, Au, or alkali metals, the group V elements are P, As, or Bi, and the group VI elements are S, Se, or Te. Many of these materials are found to exhibit soft phonon modes and large Grüneisen parameters due to the strong hybridization and repulsion between the lone-pair electrons of the group V cations and the valence p orbitals of the group VI anions. It is shown experimentally that in many of these cases Umklapp scattering reduces lattice thermal conductivity to the amorphous limit.

October 2, 2015

11:00 am

Bldg. 440/A105-106

"Molecular Science and Engineering for Energy, Water, and Beyond", Seth Darling, NST/CNM

Driven by climate change, population growth, and unsustainable development, energy and water are the two greatest challenges we face as a society. Addressing these challenges will require new technologies, themselves underpinned by new scientific insights. Our multidisciplinary group targets these challenges with efforts spanning basic to applied science and engineering. We integrate novel materials synthesis and fabrication approaches with advanced structural, optical, and electronic characterization complemented by modeling and simulation. Specific research areas covered in this talk will be (1) our discovery of the presence and importance of hierarchical morphologies in organic photovoltaics enabled by synchrotron x-ray scattering, electron microscopy, and optoelectronic measurements; (2) unconventional membrane architectures derived from self-assembled block copolymers and functionalized with doped semiconductors to impart anti-fouling properties; and (3) polyurethane foams with tailored surface chemistry for fluid separation applications. I will also address a vision for sustenance and invigoration of NST enlightened by 13 years working at Argonne in a variety of roles.

September 30, 2015 No Colloquium
September 16, 2015 No Colloquim
September 2, 2015

"Partially-Oxidized Graphene Nanostructures: Speciation and Li Capacity Studies", Brad Fahlman, Central Michigan University, hosted by Yugang Sun

Graphene oxide (GO) was synthesized from expanded graphite (EG) and multi-walled carbon nanotubes (MWCNTs) by a modified Hummer’s method, and was post-reduced under different temperatures and hydrazine conditions. GOs and partially/fully reduced GOs were characterized by a variety of techniques such as FT-IR, Raman spectroscopy, TGA, SEM, XRD, XPS, and elemental analysis. These characterization methods showed that temperature had a much more significant effect on the performance of reduced-GOs as anode materials than the choice of the environment. The electrochemical performance of reduced-GOs was greatest when the temperature of reduction was 250 °C regardless of the chemical environment. Within this temperature range, reduced-GOs show a high first-cycle specific capacity over 2000 mAh/g and 1000 mAh/g reversible and irreversible, respectively at a relatively large current density of 500 mA/g. For reduced-GOs at 250 °C and under vacuum, the reversible capacity was maintained at 500 mAh/g during 100 cycles. This performance points to reduced GOs as an attractive alternative to graphite; we will further delineate the effect of surface functionalization on the Li capacity in these materials. Overall, these nanomaterials are capable of further improving the capacity and lifetime of Li-ion batteries, without increasing the costs associated with anode production - a distinct benefit relative to more expensive nanostructural anode options such as carbon nanotubes and graphene nanosheets.

August 19, 2015

"Radio-Frequency Nanoelectromechanical Systems in Atomically-Thin Semiconducting Crystals", Philip Feng, Case Western Reserve University, hosted by Changyao Chen

Nanoscience today enables exciting emergences of low-dimensional nanostructures and new materials with previously inaccessible properties.  We explore these intriguing properties, coupled with mechanical degrees of freedom in designed and engineered nanostructures, to innovate new nanomachines and transducers, for sensing and information processing.  In particular, nanoscale electromechanical systems (NEMS) operating in their resonant modes can be exquisitely sensitive to various processes.  By engineering high-performance NEMS resonators in the radio frequency (RF) and microwave bands, especially those based on one-dimensional (1D) and two-dimensional (2D) nanostructures, we have demonstrated various ultrasensitive transducers.  In this presentation, I will focus on introducing 2D NEMS based on atomically-thin semiconducting crystals.  While graphene has been very well known as the hallmark of 2D crystals, other interesting 2D crystals with tunable bandgaps have emerged, such as layers from transition metal di-chalcogenides (TMDCs) and black phosphorus.  Atomically-thin structures derived from these materials possess a number of interesting electrical, optical, and mechanical properties, and are attractive for new nanodevices.  I will describe our recent experiments on demonstrating various high-frequency MoS2, black phosphorus and other 2D NEMS resonators.  By performing sensitive optical and electronic measurements, in combination with modeling, we quantify the performance of these 2D NEMS, and evaluate their potential and ultimate limits in detecting strain, charge, and other quantities of interest.  Challenges and advances in experimental techniques will also be discussed.  as the center of our galaxy and regions of newly forming stars.

August 5, 2015 No Colloquium
July 22, 2015 No Colloquium
July 8, 2015

"Strongly Enhanced Light-Matter Interactions using Plasmonic Nanocavities", Maiken Mikkelsen, Duke University, hosted by Gary Wiederrecht

Metal-dielectric nanocavities have the ability to tightly confine light in small mode volumes resulting in a strongly increased local density of states. Placing fluorescing molecules or semiconductor materials in this region enables wide control of radiative processes including absorption and spontaneous emission rates, quantum efficiency, and emission directionality. In this talk, I will describe our recent experiments utilizing a tunable plasmonic platform where emitters are sandwiched in a sub-10-nm gap between colloidally synthesized silver nanocubes and a metal film. By varying the size of the nanocubes, we tune the plasmon resonance by ~200 nm throughout the excitation, absorption, and emission spectra of embedded fluorophores and, for nanocavities resonant with the excitation wavelength, observe a 30,000-fold enhancement in fluorescence intensity [Rose et al. Nano Letters 14 , 4797 (2014)]. Utilizing emitters with an intrinsic long lifetime, and cavities resonant with the emission, reveals spontaneous emission rate enhancements exceeding a factor of 1,000 while maintaining directional emission and high quantum efficiency [Akselrod et al. Nature Photonics 8, 835 (2014)]. Incorporating semiconductor quantum dots into the nanocavities enables ultrafast spontaneous emission [Hoang et al., Nature Communications, accepted (2015)] and the demonstration of an efficient single photon source. Additionally, by also utilizing the second order mode of the cavity, optical processes at multiple energies can be optimized simultaneously. We demonstrate this by enhancing both the absorption and the quantum yield in monolayer MoS2 resulting in a 2,000-fold enhancement in the overall fluorescence [Akselrod et al., Nano Letters 15, 3578 (2015)]. Finally, the large field enhancement in these types of nanocavities is well-suited for control of nonlinear processes, and we show an increase in the third-harmonic generation in Al2O3 by nearly five orders of magnitude [Lassiter et al., ACS Photonics 1, 1212 (2014)]. These artificially structured hybrid nanomaterials hold the promise to enable a new class of ultrafast optoelectronic devices such as LEDs, enhance the performance of photovoltaics and bio sensors, and impact quantum-based technologies.

June 24, 2015

"Atomic Engineering of III-V Semiconductors for Quantum Devices from Deep UV (200 nm) to Terahertz (300 microns) at CQD/NU: Recent Advances and Future Trends", Manijeh Razeghi, Northwestern University, hosted by Rich Schaller

Exploration is certainly part of the human being character, and it has been a significant part of human history. It stated by land exploration; but human found a new one in space. Space, as a new frontier for exploration, has always been a driver of the modern human. Electromagnetic waves have been mainly used in space exploration from the very first day, even before human can fly. Once in space the information of the universe appears to man or his spacecraft in the form of electromagnetic radiation: light. Using light, one can see deep space and discover new objects and cosmic events. We can also look down to the earth and monitor the weather, gather information about natural resources, monitor the activity of other humans, and etc. For each of this application different types of light, such as terahertz (THz), infrared (IR), and ultraviolet (UV), are needed. For example, when looking at the earth, we can observe the complex and evolving weather patterns but for this we need the right detectors. The thermal or infrared images recorded by sensors called scanning radiometers enable a trained analyst to determine cloud heights and types, to calculate land and surface water temperatures, and to locate ocean surface features. Weather observation is typically made via different 'channels' of the infrared: 3.9 μm – 7.3 μm (Water Vapor), 8.7 μm, – 13.4 μm (Thermal imaging). In space, there are many regions which are hidden from optical telescopes because they are embedded in dense regions of gas and dust. However, infrared radiation, having wavelengths which are much longer than visible light, can pass through dusty regions of space without being scattered. This means that we can study objects hidden by gas and dust in the infrared, which we cannot see in visible light, such as the center of our galaxy and regions of newly forming stars.

June 10, 2015

"Advanced ab initio methods: Bridging electron excitation and H-bond structure in liquid water and ion solutions",  Xifan Wu, Temple University, hosted by Jianguo J.G. Wen

Density functional theory (DFT) is a powerful tool both in the fundamental understanding of materials and in the design of functional properties at the level of quantum mechanics. However, its accuracy is strongly limited by the adopted approximations of electron exchange correlation.  Hybrid functional by including a fraction of exact exchange (EXX) overcomes the limitations of (semi)local approximations of DFT. Moreover, EXX is a basic ingredient in modern approaches to compute excitation properties, such as GW. So far, however, the demanding computational cost has limited the applications of EXX in plane wave calculations for extended systems. We show that this difficulty can be overcome by performing a unitary transformation from Bloch to maximally localized Wannier functions in combination with an efficient technique to compute real space Coulomb integrals. The resulting scheme scales linearly with system size and, when used in ab initio molecular dynamics simulations, requires only a modest increase in computational cost compared to standard DFT. We validate the scheme by the accurately computed H-bond structures, the photoemission spectra, and the X-ray absorption spectra of H-bonded liquids.

May 27, 2015

"Mapping Optoelectronic Properties at the Native Length Scale in Lead Halide Perovskites and 2-D MoSe2", Alexander Weber-Bargioni, Lawrence Berkeley National Laboratory, hosted by Jeff Guest

Understanding and eventually controlling opto electronic processes at the native length scale, e.g. deliberately transporting excitons to predetermined sites where they perform work, will provide the access to a new parameter space for the development of next generation light harvesting materials.

Two fascinating material systems of interest to study local optoelectronic processes are perovskites and 2-D Transition Metal Dichalcogenites: Lead Halide perovskites based solar cell have recently reached 20% power conversion efficiency, albeit the lack of understanding properly the mechanism. Using Photo Conductive AFM in pin-point mode we map the local mobility, short circuit current and open circuit voltage and find substantial anisotropy within and amongst individual grains with specific grains showing 30% higher local PCE. Our analysis suggest that this heterogeneity originates on the crystal orientation and the way it grew.

MoSe2 are fascinating 2-D materials with direct band gap, hight exciton binding energy and an equivalent effective mass of hole and electron, opening an enormous application space from valley electronics to ultra thin photo voltaic elements. On the search of an excitonic signature in photo excited Scanning Tunneling Spectroscopy, we found by accident 1-D mirror grain boundaries that create 1-D metallic structures exhibiting high temperature charge density waves.

STS mapping show the various periodicities of the in gap states and CO-functionalize AFM tips provide an exact atomic picture of the 1-D defect structure

May 20, 2015

“Interface Energetics in Organic Metal Halide Perovskite Solar Cells”, Philip Schulz, National Renewable Energy Laboratory, hosted by Maria Chan

In my presentation I will talk about the most recent findings on the electronic structure of methylammonium lead tri-halide (MAPbX3) perovskite films and their interfaces to adjacent transport layers. Intricate knowledge of the electronic alignment at the contact interfaces in perovskite solar cells is essential for the understanding of the exact working principle as well as improving engineering design and thus performance of respective devices.

In our studies we employ ultra-violet, X-ray and inverse photoemission spectroscopy (UPS, XPS, IPES) to directly determine valence and conduction band offsets. In this way we are able to report a direct measurement of the electronic band gap as well as ionization energy and electron affinity found for perovskite surfaces. Furthermore, our findings indicate that the electronic energy level alignment of adjacent organic hole transport layers, such as spiro-MeOTAD, can limit the maximum attainable open circuit voltage (Voc) in solar cells if the highest occupied molecular orbital of the hole transport material is not at the same position as the valence band maximum of the perovskite layer. By choosing the better suited hole transporter CBP values for Voc larger than 1.5 V could be achieved in the case of MAPbBr3 based devices.

More recently, inverted perovskite solar cells based on nickel oxide covered bottom anodes have been reported to yield viable power conversion efficiencies and stability. We find that the interface between the p-doped NiO surface and the MAPbI3 layer on top lead to de facto p-type perovskite film while the same material deposited on TiO2 in the conventional cell geometry turns out to be n-type. A further investigation of a C60 layer deposited on top of p-type perovskite films reveals an ideal alignment between the lowest unoccupied molecular orbital of the organic electron transport materials and the conduction band minimum of the perovskite film underneath. These results explain why the inverted solar cell structure could achieve similar successes as the conventional structure and highlight the versatility of perovksite sub-cells in potential tandem cell architectures.

May 13, 2015 NO Colloquium due to User Meeting
April 29, 2015

"Spin and Pseudospins in 2D Semiconductors", Xiaodong Xu, University of Washington, hosted by Anand Bhattacharya

Electronic valleys are extrema of Bloch energy bands in momentum space. Having multiple valleys gives the electron states pseudospin degrees of freedom in addition to their real spin. In this talk, I will discuss our experimental progress on the investigation of spins and pseudospins using atomically thin semiconductors, which are either single or bilayer group VIB transition metal dichalcogenides. I will first show that these new 2D semiconductors behave as remarkable excitonic systems, providing an ideal laboratory for optical manipulation and electrical control of valley degrees of freedom. I will then discuss strong coupling effects between spin, valley, and layer pseudo-spins in bilayers, which lead to spin polarization in electronic bands in the presence of bulk inversion symmetry and allow electrical control of spin states. I will conclude the talk with the discussion of spatially indirect exciton properties in monolayer semiconducting heterostructures.

April 15, 2015

"Structures, Devices, and Architectures for Nanoscale Solutions in Electrical Energy Storage", Gary Rubloff, University of Maryland, hosted by Maria Chan and Paul Fenter (CSE)

Nano science and technology promise enhancement to batteries and capacitors through higher power at given energy, accompanied by new possibilities for better capacity retention and safety.  Among the challenges to realize this promise are (1) the design of higher performance electrode and electrolyte materials and (2) the rational design of structures in which these materials are arranged.  In Nanostructures for Electrical Energy Storage (NEES, www.efrc.umd.edu), we have focused on the latter, seeking to understand how the structure of components and the architecture in which they are arranged determine the multifunctionality required for electrical energy storage: ion transport and storage, electron transport, and structural stability during charge/discharge cycling.

 Precision multistep synthesis has enabled the creation of heterogeneous nanostructures, involving multiple materials to confer the needed multifunctionality and to understand how design influences electrochemical behavior at the nanoscale and storage performance of nanostructures.   This is illustrated by several such structures which address fundamental phenomena important at the nanoscale and the mesoscale, including: (1) nanowire and nanotube structures with integrated electron transport components that achieve robust Li cycling despite large volume changes; (2) nanopore battery configurations to assess fundamental limits on ion transport in highly confined environments; (3) solid state electrolyte and battery configurations for scaling safe materials to the nanoscale; and (4) 3D nanostructure forests, both regular and pseudo-random, to analyze mesoscale architectures and new scientific challenges emerging at the mesoscale. These experimental advances have been accompanied by significant modeling and simulation insights, from DFT to continuum levels, and at the same time they pose formidable, important challenges for theory, particularly at the mesoscale.

April 1, 2015 ----------
March 4, 2015
Bldg. 440
Room A105-106

"Colloidal Superparticles: A New Frontier of Nanomaterials," Y. Charles Cao, University of Florida, hosted by Yugang Sun

Colloidal superparticles are nanoparticle assemblies in the form of colloidal particles.  Assembling nanoscopic objects into meso/macroscopic complex architectures allows bottom-up fabrication of functional materials, which is essential for many nanomaterial-based technological applications.  In this colloquium, we will discuss the formation of superparticles with supercrystalline structures made from the self-assembly of nanoparticles with a verity of chemical compositions and with well-defined size and shapes.   We will show that the self-assembly of nanospheres, nanorods, and nanocubes, mediated by shape and structural anisotropy, produces mesoscopic colloidal superparticles having multiple well-defined supercrystalline domains.  Further, functionality-based anisotropic interactions between these CdSe/CdS core/shell semiconductor nanorods can be kinetically introduced during the self-assembly and in turn yield single-domain, needle-like superparticles having parallel alignment of constituent nanorods.  Unidirectional patterning of these mesoscopic needle-like superparticles gives rise to the lateral alignment of CdSe/CdS nanorods into macroscopic, uniform, freestanding polymer films that exhibit strong photoluminescence with a striking anisotropy, enabling their use as down-conversion phosphors to create polarized light-emitting diodes.

Feb. 4, 2015

"Photons in Flatland: Manipulating Light and Matter in Two-Dimensional Nanomaterials," Nathaniel Stern, Northwestern University, hosted by Il Woong Jung

Hybrid quantum systems integrating light with matter offer a highly-controllable landscape for understanding the interface between disparate physical entities. The emergence of monolayer materials of atomic-scale thickness suggests a new two-dimensional landscape in which to play with the coupling between light and low-dimensional materials.  Exemplifying the interest of this new regime, the crystal symmetry of monolayer two-dimensional (2D) semiconductors can exhibit degenerate, yet distinct, valleys in momentum space that can be separately addressed by polarized light.  In my talk, I will describe how this polarization-selectivity of optical transitions in 2D semiconductors can lead to observations of transverse motion of optically-polarized carriers through the valley Hall effect.

Despite these rich correlations between spin, momentum, and light, the ability to enhance light-matter interactions using engineered optical environments has not been actively applied to 2D semiconductors. Drawing on the developments in cavity quantum electrodynamics of the last several decades, I will discuss extensions of this approach to integrate 2D semiconductor materials with photonic devices.  I will use this approach to illustrate the potential for exploring new hybrid regimes of light-matter coupling based on engineering quantum interactions with nanoscale materials.

Jan. 21, 2015

"Design of Nanostructures for Electrochemical Energy Conversion Applications," Hong Yang, University of Illinois at Urbana-Champaign, hosted by Xiao-Min Lin

Preparation of precisely controlled metal and its alloy nanoparticles becomes an important research area in order to meet the increasingly stringent structural requirements for advanced performance, such as catalysts and electrocatalysts with high activity, stability and selectivity.  New approaches have been developed in recent years to meet these challenges.  At the same time, such endeavors also result in a major push for better fundamental understanding of nucleation and growth in non-aqueous solutions.  In this talk, I will present our work in the following topical areas: 1) recent advances in the solution processing of nanoparticles, especially metal nano-miscibility, carbon monoxide-based gas reducing agent in liquid solution method for the preparation of shape- and composition-controlled multimetallic nanostructures; 2) ligand chemistry in the design and controlled synthesis of metallic nanostructures; 3) applications of nanoparticles as electrocatalysts for oxygen reduction and evolution reactions, and 4) non-PGM electrocatalysts, such as oxygen deficient perovskite.  The use of density functional theory in understanding the growth will also be discussed.

Jan. 7, 2015

"Nanomaterials for Integrated Optics," Andrea Armani, University of Southern California, hosted by Carmen Lilley

Abstract: Integrated photonics offers a potential alternative to integrated electronics, with reduced heating and faster data rates.  However, to achieve many of the desired performance metrics, it is necessary to combine disparate material systems which is extremely difficult due to a wide variety of reasons often including different lattice constants, thermal expansion coefficients, and refractive indices.  Therefore, new materials and material systems are desired.  One approach is to combine the optical materials conventionally used in telecommunications, such as silica, silicon and lithium niobate, with polymeric materials and metallic materials.  These hybrid systems offer optical and mechanical properties which are not attainable with conventional material systems, such as athermal performance.  This talk will present an overview of the integrated hybrid photonic device research in the Armani Lab, including athermal resonant cavities with quality factors in excess of 10 million and plasmonic lasers.