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Jonathan E. Allen

Bio: Jonathan E. Allen is an academic researcher from Northwestern University. The author has contributed to research in topics: Nanowire & Photocurrent. The author has an hindex of 8, co-authored 9 publications receiving 1547 citations.

Papers
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Journal ArticleDOI
TL;DR: Results advance the quantitative correlation of atomic-scale structure with the properties of nanomaterials and can provide essential guidance to the development of nanowire-based device technologies.
Abstract: The potential for the metal nanocatalyst to contaminate vapour–liquid–solid grown semiconductor nanowires has been a long-standing concern, because the most common catalyst material, Au, is highly detrimental to the performance of minority carrier electronic devices. We have detected single Au atoms in Si nanowires grown using Au nanocatalyst particles in a vapour–liquid–solid process. Using high-angle annular dark-field scanning transmission electron microscopy, Au atoms were observed in higher numbers than expected from a simple extrapolation of the bulk solubility to the low growth temperature. Direct measurements of the minority carrier diffusion length versus nanowire diameter, however, demonstrate that surface recombination controls minority carrier transport in as-grown n-type nanowires; the influence of Au is negligible. These results advance the quantitative correlation of atomic-scale structure with the properties of nanomaterials and can provide essential guidance to the development of nanowire-based device technologies.

615 citations

Journal ArticleDOI
TL;DR: In this article, a near-field scanning optical microscope was used to image the photocurrent induced by local illumination along the length of a metal-semiconductor-metal (MSM) photodetector made from an individual CdS nanowire.
Abstract: A near-field scanning optical microscope was used to image the photocurrent induced by local illumination along the length of a metal-semiconductor-metal (MSM) photodetector made from an individual CdS nanowire. Nanowire MSM photodetectors exhibited photocurrents ∼105 larger than the dark current (<2pA) under uniform monochromatic illumination; under local illumination, the photoresponse was localized to the near-contact regions. Analysis of the spatial variation and bias dependence of the local photocurrent allowed the mechanisms of photocarrier transport and collection to be identified, highlighting the importance of near-field scanning photocurrent microscopy to elucidating the operating principles of nanowire devices.

234 citations

Journal ArticleDOI
TL;DR: The three-dimensional composition mapping of a semiconductor nanowire with single-atom sensitivity and subnanometer spatial resolution using atom probe tomography is demonstrated.
Abstract: We demonstrate the three-dimensional composition mapping of a semiconductor nanowire with single-atom sensitivity and subnanometer spatial resolution using atom probe tomography. A new class of atom probe, the local electrode atom probe (LEAP) microscope, was used to map the position of single Au atoms in an InAs nanowire and to image the interface between a Au catalyst and InAs in three dimensions with 0.3-nm resolution. These results establish atom probe tomography as a uniquely powerful tool for analyzing the chemical composition of semiconductor nanostructures.

227 citations

Journal ArticleDOI
TL;DR: Self-assembly of a lamellar hybrid containing periodic and alternating 1-nm-thick sheets of polycrystalline ZnO separated by 2-3 nm layers of conjugated molecules is demonstrated directly onto an electrode, making them technologically competitive as low-power, wavelength-tunable, flexible and environmentally benign photoconductors.
Abstract: Highly ordered nanostructured organic/inorganic hybrids offer chemical tunability, novel functionalities and enhanced performance over their individual components. Hybrids of complementary p-type organic and n-type inorganic components have attracted interest in optoelectronics, where high-efficiency devices with minimal cost are desired. We demonstrate here self-assembly of a lamellar hybrid containing periodic and alternating 1-nm-thick sheets of polycrystalline ZnO separated by 2-3 nm layers of conjugated molecules, directly onto an electrode. Initially the electrodeposited inorganic is Zn(OH)(2), but pi-pi interactions among conjugated molecules stabilize synergistically the periodic nanostructure as it converts to ZnO at 150 degrees C. As photoconductors, normalized detectivities (D(*)) greater than 2x10(10) Jones, photocurrent gains of 120 at 1.2 V microm(-1) and dynamic ranges greater than 60 dB are observed on selective excitation of the organic. These are among the highest values measured for organic, hybrid and amorphous silicon, making them technologically competitive as low-power, wavelength-tunable, flexible and environmentally benign photoconductors.

179 citations


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Journal ArticleDOI
26 Mar 2013-ACS Nano
TL;DR: The properties and advantages of single-, few-, and many-layer 2D materials in field-effect transistors, spin- and valley-tronics, thermoelectrics, and topological insulators, among many other applications are highlighted.
Abstract: Graphene’s success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in...

4,123 citations

Journal ArticleDOI
17 Feb 2012-Science
TL;DR: The specific features of supramolecular polymers that can lead to applications in a variety of fields are reviewed, including: materials—in which processability and self-healing properties are of interest; biomedicine— in which the concerns are dynamic functionality and biodegradability; and hierarchical assembly and electronic systems—with an interest in unidirectionality of charge flow.
Abstract: Supramolecular polymers can be random and entangled coils with the mechanical properties of plastics and elastomers, but with great capacity for processability, recycling, and self-healing due to their reversible monomer-to-polymer transitions. At the other extreme, supramolecular polymers can be formed by self-assembly among designed subunits to yield shape-persistent and highly ordered filaments. The use of strong and directional interactions among molecular subunits can achieve not only rich dynamic behavior but also high degrees of internal order that are not known in ordinary polymers. They can resemble, for example, the ordered and dynamic one-dimensional supramolecular assemblies of the cell cytoskeleton and possess useful biological and electronic functions.

2,777 citations

Journal ArticleDOI
31 Jan 2014-ACS Nano
TL;DR: By critically assessing and comparing the performance of these devices with competing technologies, the merits and shortcomings of this emerging class of electronic materials are identified, thereby providing a roadmap for future development.
Abstract: With advances in exfoliation and synthetic techniques, atomically thin films of semiconducting transition metal dichalcogenides have recently been isolated and characterized. Their two-dimensional structure, coupled with a direct band gap in the visible portion of the electromagnetic spectrum, suggests suitability for digital electronics and optoelectronics. Toward that end, several classes of high-performance devices have been reported along with significant progress in understanding their physical properties. Here, we present a review of the architecture, operating principles, and physics of electronic and optoelectronic devices based on ultrathin transition metal dichalcogenide semiconductors. By critically assessing and comparing the performance of these devices with competing technologies, the merits and shortcomings of this emerging class of electronic materials are identified, thereby providing a roadmap for future development.

2,219 citations

Journal ArticleDOI
TL;DR: Durability tests revealed that the Co single atoms exhibit outstanding chemical stability during electrocatalysis and thermal stability that resists sintering at 900 °C, which could facilitate new discoveries at the atomic scale in condensed materials.
Abstract: A new strategy for achieving stable Co single atoms (SAs) on nitrogen-doped porous carbon with high metal loading over 4 wt % is reported. The strategy is based on a pyrolysis process of predesigned bimetallic Zn/Co metal–organic frameworks, during which Co can be reduced by carbonization of the organic linker and Zn is selectively evaporated away at high temperatures above 800 °C. The spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurements both confirm the atomic dispersion of Co atoms stabilized by as-generated N-doped porous carbon. Surprisingly, the obtained Co-Nx single sites exhibit superior ORR performance with a half-wave potential (0.881 V) that is more positive than commercial Pt/C (0.811 V) and most reported non-precious metal catalysts. Durability tests revealed that the Co single atoms exhibit outstanding chemical stability during electrocatalysis and thermal stability that resists sintering at 900 °C. Our findings open up a new routine for general and practical synthesis of a variety of materials bearing single atoms, which could facilitate new discoveries at the atomic scale in condensed materials.

1,779 citations

Journal ArticleDOI
TL;DR: It is discovered that the thermoconductivity of the silicon nanowires can be significantly reduced due to phonon scattering, pointing to a very promising approach to design better thermoelectrical materials.
Abstract: Semiconductor nanowires represent an important class of nanostructure building block for photovoltaics as well as direct solar-to-fuel application because of their high surface area, tunable bandgap and efficient charge transport and collection. In this talk, I will highlight several recent examples in this lab using semiconductor nanowires and their heterostructures for the purpose of solar energy harvesting. In addition, we have also discovered that the thermoconductivity of the silicon nanowires can be significantly reduced due to phonon scattering, pointing to a very promising approach to design better thermoelectrical materials. It is important to note that the engines that generate most of the world's power typically operate at only 30–40 per cent efficiency, releasing roughly 15 terawatts of heat to the environment. If this “wasted heat” could be recycled, the impact globally would be enormous. Our silicon nanowire thermoelectric technology could have a significant impact in alternative energy generation.

1,306 citations