Hot exciton dissociation in polymer solar cells

The standard picture of photovoltaic conversion in all-organic bulk heterojunction solar cells predicts that the initial excitation dissociates at the donor/acceptor interface after thermalization. Accordingly, on above-gap excitation, the excess photon energy is quickly lost by internal dissipation. Here we directly target the interfacial physics of an efficient low-bandgap polymer/PC60BM system. Exciton splitting occurs within the first 50 fs, creating both interfacial charge transfer states (CTSs) and polaron species. On high-energy excitation, higher-lying singlet states convert into hot interfacial CTSs that effectively contribute to free-polaron generation. We rationalize these findings in terms of a higher degree of delocalization of the hot CTSs with respect to the relaxed ones, which enhances the probability of charge dissociation in the first 200 fs. Thus, the hot CTS dissociation produces an overall increase in the charge generation yield.

Doc.: http://www.nature.com/nmat/journal/v12/n1/pdf/nmat3502.pdf

Grancini, Maiuri, Fazzi, Petrozza, Egelhaaf, Brida, & Cerullo. (09 de 12 de 2012). Hot exciton dissociation in polymer solar cells. Nature Materials, 29-33.

Atomic-Scale Evidence for Potential Barriers and Strong Carrier Scattering at Graphene Grain Boundaries: Scanning Tunneling Microscopy Study

An important factor in the potential use of graphene in nanoelectronic circuitry is scattering at grain boundaries; such scattering greatly limits the electronic performance of CVD-grown graphene compared to defect-free graphene.  The figure shows an atomic resolution STM image of a grain boundary on CVD grown graphene that has been transferred from its copper growth surface to an oxidized silicon wafer.  This paper studies the atomic level detail of electron scattering at grain boundaries in graphene.

Doc.: http://pubs.acs.org/doi/abs/10.1021/nn302064p.

Koepke, J., D. Wood, J., Estrada, D., Yong Ong, T. He, D., Pop, E., & W. Lyding, j. (2013). Atomic-Scale Evidence for Potential Barriers and Strong Carrier Scattering at Graphene Grain Boundaries: A Scanning Tunneling Microscopy Study. ACS Nano, 75-86 

Direct Writing of Sub-5 nm Hafnium Diboride Metallic Nanostructures

In a technique called electron beam induced deposition (EBID), electrons from a scanning tunneling microscope (STM) tip can induce deposition of materials onto surfaces by ionization of gases that are near the tip surface region.  EBID has previously been used to write sub-10 nm features, but in many cases the deposits contain less than 50 at. % metal, with consequent degradation of properties.  It has been stated that “Perhaps the greatest single limitation of EBID is that metal-containing nanostructures deposited from organometallic precursors typically possess unacceptable levels of organic contamination.”

Doc.: http://pubs.acs.org/doi/abs/10.1021/nn1018522. 

Ye, W., Peña MArtin, P., Kumar, N., A. Rocket, A., R. Abelson, J., Gitolami, G., & W. Lyding, J. (2010). Direct Writing of Sub-5 nm Hafnium Diboride Metallic Nanostructures. ASC Nano, 6818-6824 

Field-Directed Sputter Sharpening for Tailored Probe Materials and Atomic-Scale Lithography

Atomic force microscopy (AFM) is a powerful technique, but limited by the fact that probe tips made of silicon are fragile and quickly break or become blunt.  There is a need to develop AFM probe tips that are simultaneously very sharp and very hard. Here is shown a depiction of the field-directed sputter sharpening (FDSS) process, a new method to sharpen AFM probe tips. Incoming ions are deflected by the inhomogeneous electric field of a biased probe, resulting in reduced sputtering at the sharpest part of the probe. This increases the sharpness, which increases the local field inhomogeneity, ultimately leading to a self-limited sharpness. This is demonstrated for a tungsten STM tip in the middle images, showing before and after FDSS TEM images and a final tip radius of ~1 nm. The bottom images show before and after TEM images of a hafnium diboride-coated AFM probe. These FDSS processed probes are now being marketed by Tiptek, LLC, which is a startup company co-founded by CNEM participants Gregory Girolami and Joe Lyding and Advisory Board member Scott Lockledge.

Doc.: http://www.nature.com/ncomms/journal/v3/n7/full/ncomms1907.html

Schmucker, S., Kumar, N., Abelson, J., Daly, S., Girolamy , Bishof, . . . Lyding. (2012). Atomic force microscopy (AFM) is a powerful technique, but limited by the fact that probe tips made of silicon are fragile and quickly break or become blunt. Field-directed sputter sharpening for tailored probe materials and atomic-scale lithography. Nature Communications.

Halide Anions as Shape-Directing Agents for Obtaining High-Quality Anisotropic Au Nanostructures

Control of the shapes during growth of nanostructures will be critical for their application in devices.  The fundamental role of halide anions in the seed-mediated synthesis of anisotropic noble metal nanostructures has been a subject of debate within the nanomaterials community. Herein, we systematically investigate the roles of chloride, bromide and iodide anions in mediating the growth of anisotropic Au nanostructures. A high-purity surfactant solution of hexadecyltrimethylammonium bromide (CTABr) is used to reliably probe the role of each halide anion without interference from impurities. Our investigation reveals that bromide anions are required for the formation of Au nanorods, while the controlled combination of both bromide and iodide anions are necessary for the production of high-quality Au nanoprisms. Chloride anions, however, are ineffective at promoting anisotropic architectures and are detrimental to nanorod and/or nanoprism growth at high concentrations. We examine the seed structure and propose a growth model based on facet-selective adsorption on low-index Au facets to rationalize the nanostructures obtained by these methods. Our approach provides a facile synthesis of anisotropic Au nanostructures by way of a single growth solution and yields the desired morphologies with high purity. These results demonstrate that appropriate combinations of halide anions provide a versatile paradigm for manipulating the morphological distribution of Au nanostructures.

Doc.:  http://pubs.acs.org/doi/abs/10.1021/cm3020397

Joseph S. DuChene, W. N. (16 de juio de 2012). Halide Anions as Shape-Directing Agents for Obtaining High-Quality Anisotropic Au Nanostructures. ACS Publications/Chemistry of Materials, 1392-1399. 

Reducing electrical resistance in single-walled carbon nanotube networks: effect of the location of metal contacts and low-temperature annealing

Density control during the formation of 2-D networks of unsorted single-walled carbon nanotubes (SWNTs) allows their macroscopic electrical properties to be tuned from semiconductive to metallic conduction, even though they are composed of 1/3 metallic and 2/3 semiconductive nanotubes.  This allows their use in numerous new materials applications.  However, the resistance of such thin-films is generally high, dominated by the effects of inter-SWNT tunneling junctions, metal/SWNT contacts, sidewall defects, and the presence of residual dopants.  Initial studies have provided insight into the relative contributions of each of these contributors to the overall performance of SWNT networks in field-effect transistors.  Additionally, the effect of the structure of the metal/SWNT contact, and annealing temperature were investigated.  It was found that depositing the network on top of prefabricated metal contacts allowed up to a 13-fold reduction in resistance, much greater reproducibility in inter-network conductivity, and up to a 2-fold increase in on/off ratio.

Doc.: http://link.springer.com/content/pdf/10.1007%2Fs10853-011-6161-9.pdf

Zhang, Q., Vivhvhulada, P., B Shivareddy, S., & D. Lay, M. (2012). Density control during the formation of 2-D networks of unsorted single-walled carbon nanotubes (SWNTs) allows their macroscopic electrical properties to be tuned from semiconductive to metallic conduction, even though they are composed of 1/3 metallic an. J Mater Sci, 3233-3240

Photoemission Electron Microscopy of a Plasmonic Silver Nanoparticle Trimer

In order to exploit the surface plasmon mediated growth process described above, a fundamental understanding of the plasmonic properties of the nanostructured substrates will be required.  We present a combined experimental and theoretical study to investigate the spatial distribution of photoelectrons emitted from silver-coated polystyrene nanoparticles. We use two-photon photoemission electron microscopy (2P-PEEM) to image electron emission from a silver-capped aggregate trimer. Finite difference time domain (FDTD) simulations are performed to model the intensity distributions of the electromagnetic near- fields resulting from femtosecond (fs) laser excitation of localized surface plasmon oscillations in the trimer structure. We demonstrate that the predicted FDTD near-field intensity distribution reproduces the 2P-PEEM photoemission pattern.

Doc.: http://link.springer.com/article/10.1007/s00339-012-7316-5/fulltext.html.

Samuel J Peppernick, A. G. (2012). Photoemission electron microscopy of a plasmonic nanoparticle trimer. Applied Physics A.

InxGa1–xAs Nanowire Growth on Graphene: van der Waals Epitaxy Induced Phase Segregation

This paper leverages the nascent 2D materials growth effort in CNEM to utilize graphene and MoS₂ as substrates for for In_xGa_1-xAs nanowire growth.  On graphene, there is a phase separation wherein the nanowires grow with an InAs core and an InGaAs shell. On MoS₂, only homogeneous InGaAs nanowires grow, similarly to VLS growth. The phase segregation of nanowires on graphene is a form of van der Waals epitaxy in which the InAs nanowire core is lattice matched with the graphene growth template.

Doc.: http://pubs.acs.org/doi/abs/10.1021/nl304569d

k. Mohnseny, P., Behnam, A., D. Wood, J., D. English , C., W. Lyding, J., Pop, E., & Li, X. (2013). InxGa1–xAs Nanowire Growth on Graphene: van der Waals Epitaxy Induced Phase Segregation. Nano Letters, 1553-1161 http://pubs.acs.org/doi/abs/10.1021/nl304569d

Surface Plasmon Mediated Chemical Solution Deposition of Gold Nanoparticles on a Nanostructured Silver Surface at Room Temperature

This paper provides proof of concept for a new method of nanoscale fabrication:  exploitation of the optical properties of a nanostructured substrate to induce nanoparticle growth.  Sub-15 nm Au nanoparticles have been fabricated on a nanostructured Ag surface at room temperature via a liquid-phase chemical deposition upon excitation of the localized surface plasmon resonance (SPR) of the substrate.  Measurement of the SPR-mediated photothermal local heating of the substrate surface by molecular thermometry indicated the temperature to be above 230 °C, which is sufficient to induce molecules of CH₃AuPPh₃ to form Au nanoparticles on the Ag surface. Particle sizes are tunable between 3 and 10 nm by adjusting the deposition time and the nanoparticle surfaces are surfactant-free.  The deposition kinetics have been measured, and are consistent with a surface-limited growth model.

Doc.: http://pubs.acs.org/doi/ipdf/10.1021/ja309392x

Jingjing Qiu †, Yung-Chien Wu †, Yi-Chung Wang †,Mark H. Engelhard ‡, Lisa McElwee-White *†, andWei David Wei *† Department of Chemistry and Center for Nanostructured Electronic Materials, University of Florida, Gainesville, Florida 32611, United States. Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States. J. Am. Chem. Soc., 2013, 135 (1), pp 38–41. Publication Date (Web): December 16, 2012. American Chemical Society.

Faster Than Silicon: Redesigned Material Could Lead to Lighter, Faster Electronics

The same material that formed the first primitive transistors more than 60 years ago can be modified in a new way to advance future electronics, according to a new study.

Chemists at The Ohio State University have developed the technology for making a one-atom-thick sheet of germanium, and found that it conducts electrons more than ten times faster than silicon and five times faster than conventional germanium.

The material's structure is closely related to that of graphene -- a much-touted two-dimensional material composed of single layers of carbon atoms. As such, graphene shows unique properties compared to its more common multilayered counterpart, graphite. Graphene has yet to be used commercially, but experts have suggested that it could one day form faster computer chips, and maybe even function as a superconductor, so many labs are working to develop it.

Joshua Goldberger, assistant professor of chemistry at Ohio State, decided to take a different direction and focus on more traditional materials.

"Most people think of graphene as the electronic material of the future," Goldberger said. "But silicon and germanium are still the materials of the present. Sixty years' worth of brainpower has gone into developing techniques to make chips out of them. So we've been searching for unique forms of silicon and germanium with advantageous properties, to get the benefits of a new material but with less cost and using existing technology."

In a paper published online in the journal ACS Nano, he and his colleagues describe how they were able to create a stable, single layer of germanium atoms. In this form, the crystalline material is called germanane.

Researchers have tried to create germanane before. This is the first time anyone has succeeded at growing sufficient quantities of it to measure the material's properties in detail, and demonstrate that it is stable when exposed to air and water.

In nature, germanium tends to form multilayered crystals in which each atomic layer is bonded together; the single-atom layer is normally unstable. To get around this problem, Goldberger's team created multi-layered germanium crystals with calcium atoms wedged between the layers. Then they dissolved away the calcium with water, and plugged the empty chemical bonds that were left behind with hydrogen. The result: they were able to peel off individual layers of germanane.

Studded with hydrogen atoms, germanane is even more chemically stable than traditional silicon. It won't oxidize in air and water, as silicon does. That makes germanane easy to work with using conventional chip manufacturing techniques.

The primary thing that makes germanane desirable for optoelectronics is that it has what scientists call a "direct band gap," meaning that light is easily absorbed or emitted. Materials such as conventional silicon and germanium have indirect band gaps, meaning that it is much more difficult for the material to absorb or emit light.

"When you try to use a material with an indirect band gap on a solar cell, you have to make it pretty thick if you want enough energy to pass through it to be useful. A material with a direct band gap can do the same job with a piece of material 100 times thinner," Goldberger said.

The first-ever transistors were crafted from germanium in the late 1940s, and they were about the size of a thumbnail. Though transistors have grown microscopic since then -- with millions of them packed into every computer chip -- germanium still holds potential to advance electronics, the study showed.

According to the researchers' calculations, electrons can move through germanane ten times faster through silicon, and five times faster than through conventional germanium. The speed measurement is called electron mobility.

With its high mobility, germanane could thus carry the increased load in future high-powered computer chips.

"Mobility is important, because faster computer chips can only be made with faster mobility materials," Golberger said. "When you shrink transistors down to small scales, you need to use higher mobility materials or the transistors will just not work," Goldberger explained.

Next, the team is going to explore how to tune the properties of germanane by changing the configuration of the atoms in the single layer.

Lead author of the paper was Ohio State undergraduate chemistry student Elizabeth Bianco, who recently won the first place award for this research at the nationwide nanotechnology competition NDConnect, hosted by the University of Notre Dame. Other co-authors included Sheneve Butler and Shishi Jiang of the Department of Chemistry and Biochemistry, and Oscar Restrepo and Wolfgang Windl of the Department of Materials Science and Engineering.

The research was supported in part by an allocation of computing time from the Ohio Supercomputing Center, with instrumentation provided by the Analytical Surface Facility in the Department of Chemistry and Biochemistry and the Ohio State University Undergraduate Instrumental Analysis Program. Funding was provided by the National Science Foundation, the Army Research Office, the Center for Emergent Materials at Ohio State, and the university's Materials Research Seed Grant Program

Elisabeth Bianco, Sheneve Butler, Shishi Jiang,Oscar D. Restrepo,Wolfgang Windl, and Joshua E. Goldberger.

Doc.:http://pubs.acs.org/doi/pdf/10.1021/nn4009406