“Monodisperse Carbon Nanomaterial Heterostructures,” by Mark Hersam, Northwestern University, hosted by Tijana Rajh

Abstract: Improvements in carbon nanomaterial monodispersity have yielded corresponding enhancements in the performance of electronic, optoelectronic, sensing, and energy technologies. However, in all of these cases, carbon nanomaterials are just one of many materials employed, suggesting that further device improvements can be achieved by focusing on the integration of disparate nanomaterials into heterostructures with well-defined interfaces. For example, organic self-assembled monolayers on graphene act as effective seeding layers for atomic layer deposited (ALD) dielectrics, resulting in metal-oxide-graphene capacitors with wafer-scale reliability and uniformity comparable to ALD dielectrics on silicon.

Similarly, the traditional trade-off between on/off ratio and mobility in semiconducting carbon nanotube (CNT) thin-film transistors (TFTs) is overcome by replacing conventional inorganic gate dielectrics with hybrid organic-inorganic self-assembled nanodielectrics, yielding on/off ratios approaching 106 while concurrently achieving mobilities of ~150 cm2/V-s. By using unconventional gate electrode materials (e.g., nickel), the threshold voltage of semiconducting CNT TFTs can be further tuned, thus enabling the realization of CNT CMOS logic gates with subnanowatt static power dissipation and full rail-to-rail voltage swing.

Finally, p-type semiconducting CNT thin films are integrated with n-type single-layer MoS₂ to form p-n heterojunction diodes. The atomically thin nature of single-layer MoS₂ implies that an applied gate bias can electrostatically modulate both sides of the p-n heterojunction concurrently, thereby providing five orders of magnitude gate-tunability over the diode rectification ratio in addition to unprecedented anti-ambipolar behavior when operated as a three-terminal device.

Overall, this work establishes that carbon nanomaterial applications can be substantially enhanced and diversified into new areas through precise integration into heterostructure devices.

December 4, 2013

"Efficient and Accurate Quantum Chemistry for Biological Systems," by Heather J. Kulik, Massachusetts Institute of Technology, hosted by Maria Chan

Abstract: In this talk, I will discuss our recent efforts in developing and employing both novel algorithms and novel hardware [i.e., graphical processing units (GPUs)] to enable the dramatic speedup of quantum chemical techniques. Using these approaches, simulations of thousands of atoms that normally require half a week on a standard computer now only take half an hour. I will discuss recent methods we have developed to further enhance this speedup by improving the flexibility of minimal basis sets to further reduce the overhead in quantum chemical simulations. Using GPU-accelerated quantum chemistry, we have observed for the first time how quantum mechanical structures of proteins differ from those more typically obtained by force fields and identified how subtle features of the electronic structure of residues remote from the active site contribute strongly to mechanistic features of enzyme catalysis. Time permitting, I will also introduce recent work in accurate mechanistic modeling metalloenzymes and draw some parallels to related work relevant in materials science.

October 23, 2013

"Spin-Polarized Scanning Tunneling Microscopy Investigations of Nanospintronic Systems," by Arthur Smith, Ohio University, hosted by Saw-Wai Hla

Abstract: Many interesting and unanswered questions within magnetic/spintronic material systems are ideally investigated by using a hybrid approach consisting of ultrahigh-voltage scanning tunneling microscopy (STM) and molecular beam epitaxy. Further, by adding spin functionality to the STM tip, it is possible to study spin-polarized (SP) structures via SP-STM down to atomic scales. I will illustrate the power of our approach via our recent work on magnetic spin pyramids, discuss new developments within my labs, and show where we are going next with our research.

October 9, 2013

"DNA: Not Merely the Secret of Life," by Nadrian (Ned) Seeman, New York University, hosted by Elena Rozhkova

Abstract: We build branched DNA species that can be joined by using Watson-Crick base pairing to produce N-connected objects and lattices. We have used ligation to construct DNA topological targets, such as knots, polyhedral catenanes, Borromean rings, and a Solomon's knot. Branched junctions with up to 12 arms have been made.

Nanorobotics is a key area of application. We have made robust two- and three-state sequence-dependent devices and bipedal walkers. We have constructed a molecular assembly line using a DNA origami layer and three two-state devices, so that there are eight different states represented by their arrangements. We have demonstrated that all eight products can be built from this system.

A central goal of DNA nanotechnology is the self-assembly of periodic matter. We have constructed two-dimensional DNA arrays with designed patterns from many different motifs. We have used DNA scaffolding to organize active DNA components. We have used pairs of two-state devices to capture a variety of different DNA targets.

One of the key aims of DNA-based materials research is to construct complex material patterns that can be reproduced. We have built such a system from bent TX molecules, which can reach two generations of replication. This system represents a first step in self-reproducing materials.

Recently, we have self-assembled a three-dimensional crystalline array and have solved its crystal structure to 4-angstrom resolution, using unbiased crystallographic methods. We can use crystals with two molecules in the crystallographic repeat to control the color of the crystals. Thus, structural DNA nanotechnology has fulfilled its initial goal of controlling the structure of DNA in three dimensions. A new era in nanoscale control awaits us.

This research has been supported by the NIGMS, NSF, ARO, ONR and DOE.

September 25, 2013

"Chemical Analysis with Sub-Angstrom Resolution of Light Elements Using Aberration-Corrected STEM," by Robert Klie, University of Illinois at Chicago, hosted by Maria Chan and Yuzi Liu

Abstract: The last few years have seen a paradigm change in scanning transmission electron microscopy (STEM) with unprecedented improvements in both spatial and spectroscopic resolution being realized by aberration correctors, cold-field emission guns, and monochromators. The successful correction of lens aberrations has greatly advanced the ability of STEM to provide direct, real space imaging at atomic resolution. Very complementary to reciprocal space methods, this is especially advantageous for aperiodic systems, nanostructures, interfaces, and point defects. Aberration correction has also enabled the development of new imaging techniques, such as incoherent annular bright field imaging, which enables the direct visualization of light atoms, such as hydrogen.

While these instrumentation developments have brought notable successes in materials analysis, in particular for hetero-interfaces, catalysis and thin-film studies, they have also challenged the established experimental protocols and our fundamental understanding of both imaging and spectroscopy in STEM. Aberration correction also allows increased flexibility in choosing the appropriate electron energy to minimize beam induced damage while maintaining atomic-resolution (e.g., 60-keV electrons for studying graphene with 1.3-angstrom resolution).

Here, I will present the latest results from a new probe, the aberration-corrected cold-field emission JEOL JEM-ARM200CF at UIC, which allows in situ characterization with 78-pm spatial resolution and an energy resolution of 350 meV in the temperature range between 10 and 1300 K using a variety of in situ heating, cooling, tomography, and electrical feedback holders. The primary electron energy can be chosen within a range from 80 and 200 kV. I will show how low-energy imaging can now be used to characterize beam-sensitive materials without significant loss in spatial resolution and how such experiments enable direct correlation with other techniques, including atom-probe tomography and first-principles modeling.

September 11, 2013

"Interfacial Control of Lithiation sing Layered Intermetallic Architectures," by Timothy Fister, Chemical Sciences and Engineering, Argonne National Laboratory, hosted by Maria Chan

Abstract: Next-generation lithium battery materials will require a fundamental shift from intercalation materials to elements or compounds that alloy directly with lithium. Intercalation compounds, such as graphite and LiCoO₂, provide a stable crystal structure with open sites for mobile lithium ions but are intrinsically limited in their energy density by the weight and volume of the host material. Intermetallics can electrochemically alloy to Li4.4M (M = silicon, germanium, tin, etc.), providing order-of-magnitude increases in energy density. However, this process can lead to volume changes (up to 300%) that rapidly degrade the performance of the battery due to delamination between the active material and its underlying current collector.

Using in situ Xray reflectivity, we have studied the interface of a model bilayer of silicon and chromium that act as an anode material and current collector, respectively. Even before lithiation, we find that the amorphous bilayer intermixes immediately following growth and eventually forms a layered CrSi_x heterostructure. Lithiation of such a structure begins at the interface between each layer and eventually stabilizes into a multilayer consisting of alternating LiSi_x phases and more chromium-rich CrSi_x phases. Inspired by this purely vertical phase separation and the stability of its lithiation, we have recently grown larger scale silicon/chromium multilayers (e.g., repeating this bilayer structure 20-50 times). With higher silicon content, these multilayers reversibly show 3.3-fold expansion and contraction and maintain their layered structure, as seen by in operando X-ray reflectivity. The electrodes also give substantial improvement over pure-phase silicon thin films in both long-term cycling and high-power applications.

Multilayers using germanium and titanium provide similar reversibility and performance. Constraining the alloying reaction in such a layered architecture appears to have similar structural behavior to a more traditional intercalation compound and may be useful for higher voltage alloying reactions in metal oxides and sulfides.

August 28, 2013

"Charge Separation, Transport, and Recombination in Organic Photovoltaics," by Jenny Nelson, Imperial College, London, hosted by Seth Darling

Abstract: Organic semiconductor materials are attracting intense interest for photovoltaic applications because of the prospect of low-cost module production using high-throughput processing techniques. For OPV technology to become commercially viable, however, power conversion efficiencies and lifetimes need to be improved relative to their present levels. This will require the development of new materials and improved device or module structures, which in turn requires a thorough understanding of the operation of organic photovoltaic devices. In this presentation, we will address the fundamental optoelectronic processes involved in current generation in an organic photovoltaic device, of charge pair generation, charge transport and charge recombination. We will present results for a range of different material systems and address the influence of electronic state disorder, heterogeneity, and the molecular nature of the materials. We will review the experimental techniques and analytical methods that are being developed to study these fundamental processes in organic devices, and outline the main differences with respect to inorganic photovoltaic materials.

July 31, 2013

"Toward High-Efficiency Polymer-Nanoparticle Hybrid Solar Cells," by Wei-Fang Su, National Taiwan University, hosted by Seth Darling

Abstract: Hybrid materials made from conducting polymer nanoparticles are attractive for solar cells because of the prospect of light weight, low cost, high throughput, and high energy density resulting from using reel-to-reel or spray deposition on a flexible substrate. In this research, we investigatea thermal stable polymer-metal oxide hybrid material for solar cells. We are able to greatly improve the efficiency of the hybrid solar cell by fabricating highly ordered nanostructure hybrids, studying the morphology and interlayer characteristics of hybrids, and modifying metal oxide surface.

The device can be either in forward structure or invert structure. The inclusion of TiO₂ nanorods into conducting polymer increases the ordering of polymer. Its absorption spectrum is red shifted; the exciton life decreases to less than half that of the neat polymer. The efficiency of P3HT-TiO₂ solar cells can be increased by 2.5 times by inserting a TiO₂ nanorod layer between the hybrid active layer and the aluminum electrode because the interconnecting network between the hybrid and electrode is enlarged.

The effect of polymer molecular weight on nanoscale morphology related to the performance of P3HT-TiO₂ hybrid solar cells was studied by scanning near field optical microscopy, atomic force microscopy, and confocal Raman microscopy. The results correlate well with the carrier transport behavior of different molecular weight polymesr investigated by the time-of-flight technique. The solar cell fabricated from surface-modified TiO₂ nanoparticles with bandgap tuned linker and P3HT hybrid can have an order increase in efficiency. The efficiency of the device is further improved by using newly developed self-assembled highly ordered nan structure copolymers and low-bandgap conducting copolymers to power conversion efficiency more than 7.3%.

July 17, 2013

"Functional DNA Nanotechnology: Precise Spatial and Dynamic Controls of Nanomaterials Assembly and its Applications in Sensing and Medicine," by Yi Lu, University of Illinois at Urbana-Champaign, hosted by Richard Schaller

Abstract: Genetic control of the assembly of complex biological structures in response to internal chemical or biological stimuli has been one of the hallmarks of biology. DNA has been shown to consist of highly programmable molecules resulting in a number of two- and three-dimensional nanostructures. Despite the promise, functionalizing these structures has been challenging.

We have developed a novel method of using phosphorothioate DNA as an anchor and a bifunctional linker as a rigid molecular fastener that can connect nanoparticles to specific locations on the DNA backbone. Precise distance controls between two nanoparticles or proteins with nanometer resolution have been demonstrated.

Furthermore, discovery of the genetic code is one of the most important achievements in biology. Inspired by this pioneering work, we have reported discovery of DNA codes for fine control of the shape and morphology of nanomaterials. Rules of shape control by different DNAs and their combinations are summarized. These new DNA codes can play an important role in rational design and synthesis of novel nanomaterials with predictive shape control.

Finally, while much work has been devoted to nanoscale assembly on surfaces, selective reversible assembly of components in the nanoscale pattern at selective sites has received much less attention. By taking advantage of different binding affinities of biotin and desthiobiotin toward streptavidin, we have demonstrated selective and reversible decoration of DNA origami tiles with streptavidin, including revealing an encrypted Morse code, “NANO.” We expect this versatile conjugation technique to be widely applicable with different nanomaterials and templates.

In addition to precise spatial control, dynamic control of the assembly of nanomaterials in response to internal stimuli under ambient conditions is also important. To meet this challenge, we took advantage of a recent advance in biology [i.e., discovery of functional DNA, a new class of DNAs that can either bind to a target molecule (known as aptamers) or perform catalytic reactions (known as DNAzymes), that are very specific for a wide range of targets] and demonstrated the use of functional DNA for dynamic control of assembly of gold nanoparticles, iron oxide nanoparticles, quantum dots, and nanotubes, in response to a wide range of chemical and biological stimuli from small metal ions to large biomolecules, including cancer cell markers. Because these nanomaterials possess unique optical, electrical, magnetic, and catalytic properties, these systems have been converted into colorimetric, fluorescent, electrochemical sensors, and magnetic resonance imaging agents for detection of a broad range of analytes with high sensitivity and selectivity. The application of functional DNA nanotechnology has also been expanded to include targeted drug delivery.

June 19, 2013

"In Operando Characterization of the Structural Dynamics of Nanoscale Catalytic Materials," by Ralph G. Nuzzo, University of Illinois at Urbana-Champaign, hosted by Yugang Sun

Abstract: The electronic and atomic structural properties of nanoscale metal catalysts exhibit complex structural and dynamical influences with origins related to impacts due to particle size, metal-support interactions, and specific — and strongly condition-dependent — features of metal-adsorbate bonding. The experimental investigation of these factors, as well as the elucidation of the impacts they have on mechanisms in catalysis, are hindered by their interdependency in working catalysts. In this talk, I will discuss methods suitable for characterizing such features by using combined high-spatial and -energy resolution methods of electron microscopy and X-ray absorption spectroscopy methods — illustrating their application to both model systems and functional catalysts. I will explore the emerging understandings coming from recent collaborative studies that examine dynamical features that underpin both condition responsive bond-strains and perturbations of electronic structure in supported heterogeneous catalysts, and the complexities that arise due to the interplay of metal-support and metal-adsorbate bonding effects. The work extends insights into the fluxional structural dynamics that are manifested in these systems, a feature harboring significant consequences for understandings of both their properties and mechanisms of action.

June 5, 2013

"Surface Chemistry of Gold Nanorods: Wrapping, Stitching, Exchanging, and Coating," by Catherine J. Murphy, University of Illinois at Urbana-Champaign, hosted by Yugang Sun

Abstract: Gold nanorods have potential applications as chemical sensing, biological imaging, and photothermal therapeutics. Our laboratory has developed the syntheses of these materials, in controlled size and shape, over the last few years. Surface modification of these nanomaterials is a key step to enable applications. In this talk, I will describe our recent efforts to wrap up nanorods with layer-by-layer polyelectrolyte deposition in aqueous solution, in a way that allows for “capture coating” of small molecules at defined distances from the surface; how we can “fix” the surface of the nanomaterials by on-particle polymerization reactions; and how surface ligand exchange and overcoating affects biological properties of these materials.

April 10, 2013

"Enabling and Disrupting Impacts of Interfaces in Energy Systems," by Dawn Bonnell, The University of Pennsylvania, hosted by Chris Fry

Abstract: Recent advances allow us to manipulate, control, and measure local phenomena at nanometer scales. Size dependent behavior of solids has become the hallmark of nanoscale science and nanotechnology. As systems decrease in size, the influence of surfaces and interfaces can dominate the properties. In fact, properties of surfaces and interfaces dictate the behavior of devices ranging from biosensors to solar cells to computer processors. This is particularly true as the size of the constituents decreases.

This talk will present three model examples of behavior at interfaces that have dramatic consequences to energy systems and that also illustrate novel measurements of local properties:

• The first involves size-dependent electronic properties of metal-oxide interfaces, an important issue in device miniaturization.
• The second demonstrates how engineered interfaces in nanoparticle arrays yield new plasmonic interactions useful in energy harvesting.
• The final example illustrates how mechanisms of mass and energy exchange at interfaces in fuel cells can be determined from in situ local probes.

"Playing with broken symmetries in oxides," by Anand Bhattacharya, Argonne National Laboratory

Abstract: In recent years, we have synthesized and explored heterostructures of complex oxides where we tailor and manipulate broken symmetries. I will present our work on PMN-PT/Fe3O4 heterostructures where we have carried out reversible ferroelastic gating of resistivity and the Verwey metal-insulator transition. I will also present results on how we use synthesis with single atomic layer control to turn a non-polar material into a polar material in a class of layered perovskites. I will discuss the consequences this has for structural and electronic properties.

"Mind the gap: quantum effects and optical magnetism in plasmonic particle junctions," by Jennifer Dionne, Stanford University, hosted by Elena Shevchenko

Abstract: Electrons and photons can coexist as a single entity called a surface plasmon — an elementary excitation found at the interface between a conductor and an insulator. Plasmons are evident in the vivid hues of rose windows, which derive their color from small metallic nanoparticles embedded in the glass. They also provide the basis for color-changing biosensors, photo-thermal cancer treatments, improved photovoltaic cell efficiencies, and nano-optical tweezers.

While most applications have relied on classical plasmonic effects, quantum phenomena can also strongly influence the plasmonic properties of nanometer-scale systems. In this presentation, I'll describe my group's efforts to probe plasmon modes spanning both classical and quantum domains. We first explore the plasmon resonances of individual nanoparticles as they transition from a classical to a quantum-influenced regime. Then, using real-time manipulation of plasmonic particles, we investigate plasmonic coupling between pairs of particles separated by nanometer- and Angstrom-scale gaps.

For sufficiently small separations, we observe the effects of quantum tunneling between particles on the plasmonic resonances. Finally, using the properties of coupled metallic nanoparticles, we demonstrate the colloidal synthesis of an isotropic metafluid or "metamaterial paint" that exhibits a strong magnetic response at visible frequencies. By combining the electric and magnetic resonances of nonmagnetic nanoparticles, this metamaterial can achieve negative permeabilities and refractive indices. The ability to assemble, probe, and control both classical and quantum plasmonic junctions with electric and magnetic resonances may enable new opportunities in fields ranging from catalysis to molecular opto-electronics.

"Next-Generation Nanocrystals for Cellular Imaging," by Bruce Cohen, Lawrence Berkeley National Laboratory, hostged by Elena Shevchenko

Abstract: Nanocrystals that have unusual or exceptional optical properties have shown promise as transformative probes for biological imaging. Phosphorescent upconverting nanoparticles (UCNPs) have proven to be especially promising as biological labels, and single-particle studies of UCNPs have shown that they exhibit nearly ideal properties as single-molecule imaging probes. UCNPs absorb two or more photons in the near infrared (nIR) and emit one at shorter wavelengths in the visible or nIR, an unusual characteristic that distinguishes them from all luminescent chemicals in the cell, and one that suggests background-free cellular imaging.

We have shown that UCNPs do not blink on and off as most other probes do, and they possess remarkable photostability, resisting photobleaching under continuous irradiation long after organic dyes, proteins, and even quantum dots are extinguished. We have recently developed synthetic methods for control of UCNP size and completed a combinatorial lanthanide scan in order to tune emission wavelengths for multicolor upconverted imaging. We have also developed methods for studying single nanocrystal lifetimes and emission spectra, which has allowed us to understand lanthanide-lanthanide communication within the nanocrystal.

We have also developed luminescent nanocrysta-based thermometers able to detect sub-°C variations within live cells. Temperature is a key parameter in all physiological processes, and probes able to detect small changes in local temperature are necessary for accurate physical descriptions of cellular events. We have conjugated aqueous CdSe-CdS quantum dot-quantum rods conjugated to far-red cyanine dyes, and these probes exhibit a ratiometric 2.4% change per °C over physiological temperatures in aqueous buffers, with a precision of at least 0.2 °C. Within cells, these nanothermometers showed an unexpected enhancement in their temperature response and sensitivity, highlighting the need to calibrate novel probes within the cell.

"Solution-Grown Silicon and Germanium Nanowires," by Brian A. Korgel, The University of Texas at Austin, hosted by Xiao-Min Lin

Abstract: Synthetic methods have been developed for a wide variety of nanocrystal and nanowire materials. Among these materials, silicon (Si) and germanium (Ge) have been some of the most challenging to synthesize in solution, but now chemical methods now exist for producing significant quantities of colloidal Si and Ge nanocrystals, nanorods and nanowires. This presentation will provide examples of sterically stabilized colloidal Si nanocrystals with narrow size distributions and good size control from 2 to >12 nm diameter and light emission that can be tuned to the bulk band edge of Si. It is now possible to form superlattices with Si nanocrystals by self-assembly. Transmission electron microscopy of Si nanocrystals on graphene enables the direct visualization of organic capping ligands. Accurate size-dependent Raman spectroscopy data of Si nanocrystals without the influence of stress from an embedding matrix will also be presented and compared with model calculations. Some latest results on Si and Ge nanowires produced by solution-based methods in our group will also be presented. Nonwoven fabric of Si and Ge nanowires can be formed with extremely high optical densities. Si and Ge nanowires can be combined with polymer and elastomeric hosts to create new semiconductor-based materials with unique combinations of optical, electronic and mechanical properties. And in high-capacity lithium-ion batteries, Si and Ge nanowire anodes can increase the storage capacity of the anode by more than five times its current capacity.

January 16, 2013

"Environmental Transmission Electron Microscopy for Catalysis Research: The Example of Carbon Nanotubes," by Eric Stach, Brookhaven National Laboratory, hosted by Yuzi Liu

Abstract: Crucial to the application of nanostructured materials is control over their nucleation and growth as these aspects determine their structure and thus properties. I will describe how we can exploit the unique capabilities of in situ environmental cell transmission electron microscopy (ETEM) to observe multiple aspects of these processes. With this approach, we can directly visualize how the catalysts that mediate nanotube growth respond to various changes in the growth environment, and correlate these changes with the resulting nanotube structures.

In the first part of the presentation, we will investigate how dynamic changes in the catalyst morphology are correlated with the termination of growth in vertically aligned SWNT arrays. In particular, we have investigate how the processes of catalyst coarsening, Ostwald ripening, and diffusion into the catalyst support can lead to growth termination, and we will describe how changes in the growth feedstock — in particular the incorporation of controlled amounts of water vapor — can alter the catalyst evolution.

In the second portion of the presentation, we will describe how altering other aspects of the growth feedstock — in this case, the carrier gas in combination with the water vapor content — can not only affect catalyst morphological evolution, but can also significantly bias the chiral distribution of the resulting nanotubes. We will correlate the changes in growth ambient with a faceting/defaceting transition, as well as a resulting change in the rate of Ostwald ripening.

Finally, ongoing developments of the ETEM technique will be presented, focusing on control of gas streams, improvements in data acquisition and correlative studies with X-ray absorption spectroscopy. Extension of the observations of morphological changes in carbon nanotube growth to broader studies in catalysis will be outlined.