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© 2019 Elsevier Inc. Neuronal types in the central nervous system differ dramatically in their resilience to injury or other insults. Here we studied the selective resilience of mouse retinal ganglion cells (RGCs) following optic nerve crush (ONC), which severs their axons and leads to death of ∼80% of RGCs within 2 weeks. To identify expression programs associated with differential resilience, we first used single-cell RNA-seq (scRNA-seq) to generate a comprehensive molecular atlas of 46 RGC types in adult retina. We then tracked their survival after ONC; characterized transcriptomic, physiological, and morphological changes that preceded degeneration; and identified genes selectively expressed by each type. Finally, using loss- and gain-of-function assays in vivo, we showed that manipulating some of these genes improved neuronal survival and axon regeneration following ONC. This study provides a systematic framework for parsing type-specific responses to injury and demonstrates that differential gene expression can be used to reveal molecular targets for intervention.

Neuronal types in the central nervous system differ dramatically in their resilience to injury or other insults. Here we studied the selective resilience of mouse retinal ganglion cells (RGCs) following optic nerve crush (ONC), which severs their axons and leads to death of ∼80% of RGCs within 2 weeks. To identify expression programs associated with differential resilience, we first used single-cell RNA-seq (scRNA-seq) to generate a comprehensive molecular atlas of 46 RGC types in adult retina. We then tracked their survival after ONC; characterized transcriptomic, physiological, and morphological changes that preceded degeneration; and identified genes selectively expressed by each type. Finally, using loss- and gain-of-function assays in vivo, we showed that manipulating some of these genes improved neuronal survival and axon regeneration following ONC. This study provides a systematic framework for parsing type-specific responses to injury and demonstrates that differential gene expression can be used to reveal molecular targets for intervention.

Constrained by large ohmic and radiation losses, plasmonic nanolasers operated at visible regime are usually achieved either with a high threshold (100-10,000 MW/sq cm) or at cryogenic temperatures (4-120 K). Particularly, the bending-back effect of surface plasmon (SP) dispersion at high energy makes the SP lasing below 450nm more challenging. Here we demonstrate the first strong room temperature ultraviolet (370 nm) SP polariton laser with an extremely low threshold (3.5MW/sq cm). We find that a closed-contact planar semiconductor-insulator-metal interface greatly lessens the scattering loss, and more importantly, efficiently promotes the exciton-SP energy transfer thus furnishes adequate optical gain to compensate the loss. An excitation polarization-dependent lasing action is observed and interpreted with a microscopic energy-transfer process from excitons to SPs. Our work advances the fundamental understanding of hybrid plasmonic waveguide laser and provides a solution of realizing room temperature UV nanolasers for biological applications and information technologies., Published in Nature Communications, 5:4953, 23 Sep 2014.

Microbial fuel cells (MFCs) have been the focus of substantial research interest due to their potential for long-term, renewable electrical power generation via the metabolism of a broad spectrum of organic substrates, although the low power densities have limited their applications to date. Here, we demonstrate the potential to improve the power extraction by exploiting biogenic inorganic nanoparticles to facilitate extracellular electron transfer in MFCs. Simultaneous short-circuit current recording and optical imaging on a nanotechnology-enabled platform showed substantial current increase from Shewanella PV-4 after the formation of cell/iron sulfide nanoparticle aggregates. Detailed characterization of the structure and composition of the cell/ nanoparticle interface revealed crystalline iron sulfide nanoparticles in intimate contact with and uniformly coating the cell membrane. In addition, studies designed to address the fundamental mechanisms of charge transport in this hybrid system showed that charge transport only occurred in the presence of live Shewanella, and moreover demonstrated that the enhanced current output can be attributed to improved electron transfer at cell/electrode interface and through the cellular networks. Our approach of interconnecting and electrically contacting bacterial cells through biogenic nanoparticles represents a unique and promising direction in MFC research and has the potential to not only advance our fundamental knowledge about electron transfer processes in these biological systems but also overcome a key limitation in MFCs by constructing an electrically connected, three-dimensional cell network from the bottom-up., Published in Nano Letters, v14 p6737-6742, 13 Oct 2014. Prepared in collaboration with the Department of Chemistry and Chemical Biology and Division of Engineering and Applied Sciences, Harvard University, Cambridge, Ma, and the CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing. Sponsored in part by AFOSR.

Relaxation and dephasing of hole spins are measured in a gate-defined Ge/Si nanowire double quantum dot using a fast pulsed-gate method and dispersive readout. An inhomogeneous dephasing time T2(*)≈ 0.18 μs exceeds corresponding measurements in III-V semiconductors by more than an order of magnitude, as expected for predominately nuclear-spin-free materials. Dephasing is observed to be exponential in time, indicating the presence of a broadband noise source, rather than Gaussian, previously seen in systems with nuclear-spin-dominated dephasing.

Microbial fuel cells (MFCs) have been the focus of substantial research interest due to their potential for long-term, renewable electrical power generation via the metabolism of a broad spectrum of organic substrates, although the low power densities have limited their applications to date. Here, we demonstrate the potential to improve the power extraction by exploiting biogenic inorganic nanoparticles to facilitate extracellular electron transfer in MFCs. Simultaneous short-circuit current recording and optical imaging on a nanotechnology-enabled platform showed substantial current increase from Shewanella PV-4 after the formation of cell/iron sulfide nanoparticle aggregates. Detailed characterization of the structure and composition of the cell/nanoparticle interface revealed crystalline iron sulfide nanoparticles in intimate contact with and uniformly coating the cell membrane. In addition, studies designed to address the fundamental mechanisms of charge transport in this hybrid system showed that charge transport only occurred in the presence of live Shewanella, and moreover demonstrated that the enhanced current output can be attributed to improved electron transfer at cell/electrode interface and through the cellular-networks. Our approach of interconnecting and electrically contacting bacterial cells through biogenic nanoparticles represents a unique and promising direction in MFC research and has the potential to not only advance our fundamental knowledge about electron transfer processes in these biological systems but also overcome a key limitation in MFCs by constructing an electrically connected, three-dimensional cell network from the bottom-up.

Microbial fuel cells (MFCs) have been the focus of substantial research interest due to their potential for long-term, renewable electrical power generation via the metabolism of a broad spectrum of organic substrates, although the low power densities have limited their applications to date. Here, we demonstrate the potential to improve the power extraction by exploiting biogenic inorganic nanoparticles to facilitate extracellular electron transfer in MFCs. Simultaneous short-circuit current recording and optical imaging on a nanotechnology-enabled platform showed substantial current increase from Shewanella PV-4 after the formation of cell/iron sulfide nanoparticle aggregates. Detailed characterization of the structure and composition of the cell/nanoparticle interface revealed crystalline iron sulfide nanoparticles in intimate contact with and uniformly coating the cell membrane. In addition, studies designed to address the fundamental mechanisms of charge transport in this hybrid system showed that charge transport only occurred in the presence of live Shewanella, and moreover demonstrated that the enhanced current output can be attributed to improved electron transfer at cell/electrode interface and through the cellular-networks. Our approach of interconnecting and electrically contacting bacterial cells through biogenic nanoparticles represents a unique and promising direction in MFC research and has the potential to not only advance our fundamental knowledge about electron transfer processes in these biological systems but also overcome a key limitation in MFCs by constructing an electrically connected, three-dimensional cell network from the bottom-up.

The physics and operating principles of hybrid superconductor-semiconductor devices rest ultimately on the magnetic properties of their elementary subgap excitations, usually called Andreev levels. Here we report a direct measurement of the Zeeman effect on the Andreev levels of a semiconductor quantum dot with large electron g-factor, strongly coupled to a conventional superconductor with a large critical magnetic field. This material combination allows spin degeneracy to be lifted without destroying superconductivity. We show that a spin-split Andreev level crossing the Fermi energy results in a quantum phase transition to a spin-polarized state, which implies a change in the fermionic parity of the system. This crossing manifests itself as a zero-bias conductance anomaly at finite magnetic field with properties that resemble those expected for Majorana modes in a topological superconductor. Although this resemblance is understood without evoking topological superconductivity, the observed parity transitions could be regarded as precursors of Majorana modes in the long-wire limit. © 2014 Macmillan Publishers Limited.

Microbial fuel cells, in which living microorganisms convert chemical energy into electricity, represent a potentially sustainable energy technology for the future. Here we report the single-bacterium level current measurements of Geobacter sulfurreducens DL-1 to elucidate the fundamental limits and factors determining maximum power output from a microbial fuel cell. Quantized stepwise current outputs of 92(+ or -33) and 196(+ or -20) fA are generated from microelectrode arrays confined in isolated wells. Simultaneous cell imaging/tracking and current recording reveals that the current steps are directly correlated with the contact of one or two cells with the electrodes. This work establishes the amount of current generated by an individual Geobacter cell in the absence of a biofilm and highlights the potential upper limit of microbial fuel cell performance for Geobacter in thin biofilms., Published in Nature Communications, 4:2751, 8 Nov 2013.

available in PMC 2013 April 11., The development of three-dimensional (3D) synthetic biomaterials as structural and bioactive scaffolds is central to fields ranging from cellular biophysics to regenerative medicine. As of yet, these scaffolds cannot electrically probe the physicochemical and biological microenvironments throughout their 3D and macroporous interior, although this capability could have a marked impact in both electronics and biomaterials. Here, we address this challenge using macroporous, flexible and free-standing nanowire nanoelectronic scaffolds (nanoES), and their hybrids with synthetic or natural biomaterials. 3D macroporous nanoES mimic the structure of natural tissue scaffolds, and they were formed by self-organization of coplanar reticular networks with built-in strain and by manipulation of 2D mesh matrices. NanoES exhibited robust electronic properties and have been used alone or combined with other biomaterials as biocompatible extracellular scaffolds for 3D culture of neurons, cardiomyocytes and smooth muscle cells. Furthermore, we show the integrated sensory capability of the nanoES by real-time monitoring of the local electrical activity within 3D nanoES/cardiomyocyte constructs, the response of 3D-nanoES-based neural and cardiac tissue models to drugs, and distinct pH changes inside and outside tubular vascular smooth muscle constructs., National Institutes of Health (U.S.) (Director’s Pioneer award), McKnight Foundation (Technological Innovations in Neurosciences Award), Boston Children's Hospital (Biotechnology Research Endowment), National Institutes of Health (U.S.) (DE013023), National Institutes of Health (U.S.) (DE016516)

Branched nanostructures represent unique, 3D building blocks for the “bottom-up” paradigm of nanoscale science and technology. Here, we report a rational, multistep approach toward the general synthesis of 3D branched nanowire (NW) heterostructures. Single-crystalline semiconductor, including groups IV, III–V, and II–VI, and metal branches have been selectively grown on core or core/shell NW backbones, with the composition, morphology, and doping of core (core/shell) NWs and branch NWs well controlled during synthesis. Measurements made on the different composition branched NW structures demonstrate encoding of functional p-type/n-type diodes and light-emitting diodes (LEDs) as well as field effect transistors with device function localized at the branch/backbone NW junctions. In addition, multibranch/backbone NW structures were synthesized and used to demonstrate capability to create addressable nanoscale LED arrays, logic circuits, and biological sensors. Our work demonstrates a previously undescribed level of structural and functional complexity in NW materials, and more generally, highlights the potential of bottom-up synthesis to yield increasingly complex functional systems in the future., United States. Air Force Office of Scientific Research, United States. Air Force Office of Scientific Research (National Security Science and Engineering Faculty Fellowship)

Nanowire FETs (NWFETs) are promising building blocks for nanoscale bioelectronic interfaces with cells and tissue since they are known to exhibit exquisite sensitivity in the context of chemical and biological detection, and have the potential to form strongly coupled interfaces with cell membranes. We present a general scheme that can be used to assemble NWs with rationally designed composition and geometry on either planar inorganic or biocompatible flexible plastic surfaces. We demonstrate that these devices can be used to measure signals from neurons, cardiomyocytes, and heart tissue. Reported signals are in millivolts range, which are equal to or substantially greater than those recorded with either planar FETs or multielectrode arrays, and demonstrate one unique advantage of NW-based devices. Basic studies showing the effect of device sensitivity and cell/substrate junction quality on signal magnitude are presented. Finally, our demonstrated ability to design high-density arrays of NWFETs enables us to map signal at the subcellular level, a functionality not enabled by conventional microfabricated devices. These advances could have broad applications in high-throughput drug assays, fundamental biophysical studies of cellular function, and development of powerful prosthetics.

Nanowire FETs (NWFETs) are promising building blocks for nanoscale bioelectronic interfaces with cells and tissue since they are known to exhibit exquisite sensitivity in the context of chemical and biological detection, and have the potential to form strongly coupled interfaces with cell membranes. We present a general scheme that can be used to assemble NWs with rationally designed composition and geometry on either planar inorganic or biocompatible flexible plastic surfaces. We demonstrate that these devices can be used to measure signals from neurons, cardiomyocytes, and heart tissue. Reported signals are in millivolts range, which are equal to or substantially greater than those recorded with either planar FETs or multielectrode arrays, and demonstrate one unique advantage of NW-based devices. Basic studies showing the effect of device sensitivity and cell/substrate junction quality on signal magnitude are presented. Finally, our demonstrated ability to design high-density arrays of NWFETs enables us to map signal at the subcellular level, a functionality not enabled by conventional microfabricated devices. These advances could have broad applications in high-throughput drug assays, fundamental biophysical studies of cellular function, and development of powerful prosthetics.

Microbial fuel cells (MFCs) represent a promising approach for sustainable energy production as they generate electricity directly from metabolism of organic substrates without the need for catalysts. However, the mechanisms of electron transfer between microbes and electrodes, which could ultimately limit power extraction, remain controversial. Here we demonstrate optically transparent nanoelectrodes as a platform to investigate extracellular electron transfer in Shewanella oneidensis MR-1, where an array of nanoholes precludes or single window allows for direct microbeelectrode contacts. Following addition of cells, short-circuit current measurements showed similar amplitude and temporal response for both electrode configurations, while in situ optical imaging demonstrates that the measured currents were uncorrelated with the cell number on the electrodes. High-resolution imaging showed the presence of thin, 4- to 5-nm diameter filaments emanating from cell bodies, although these filaments do not appear correlated with current generation. Both types of electrodes yielded similar currents at longer times in dense cell layers and exhibited a rapid drop in current upon removal of diffusible mediators. Reintroduction of the original cell-free media yielded a rapid increase in current to ?80% of original level, whereas imaging showed that the positions of >70% of cells remained unchanged during solution exchange. Together, these measurements show that electron transfer occurs predominantly by mediated mechanism in this model system. Last, simultaneous measurements of current and cell positions showed that cell motility and electron transfer were inversely correlated. The ability to control and image cell/electrode interactions down to the single-cell level provide a powerful approach for advancing our fundamental understanding of MFCs., Sponsored in part by the Air Force Office of Scientific Research (AFOSR). Performed in collaboration with Harvard University, Cambridge, MA. Published in the Proceedings of the National Academy of Sciences (PNAS), Early Edition, p1-5, 2010.

Advances in materials and the understanding of the behavior of materials arc critical to future Air force and DoD technologies, ranging from sensing, imaging and threat detection to information processing and storage. In each of these critical areas. there is the potential to make quantum jumps forward in capabilities by developing and exploiting the broad area of science and technology known as nanotechnology. Hence, the development of new nanoscale structures with the unique properties and fabrication of hybrid structures and systems that exploit these unique capabilities and those of conventional technology could lead to systems that are essential to and create powerful new technology. This project has addressed key scientific issues underlying the development of nanowire building blocks and their development in a variety nanophotonic devices. including nanoseale detectors with ultra-high sensitivity and sub-wavelength spatial resolution. no\el nanoscale photovoltaic devices with high efficiencies and capabilily of powering nanoelectronic devices. and nanoseale light sources. including nanolasers., The original document contains color images.

Efficient compact power sources are critical to future mobile technologies, yet limitations with existing sources have restricted development. The objective of this research is to exploit advances in nanoscience to enable new capabilities in compact biofuel cells. The research program has focused on (i) controlled synthesis and characterization of nanowire building blocks that can function as probes of fundamental biofuel cell processes, (ii) development of novel methods for hierarchical assembly of these nanoscale structures to enable studies of power scaling, and (iii) development and fabrication of new nanoscale electrodes enabling studies of microbial fuel cells down to the level of single bacteria. First, methods for the predictable and controlled synthesis of branched nanowires consisting of single crystal silicon backbones with metal nanowire branches have been developed. Nanocluster catalyzed growth was used to control the diameter and dopant concentration of silicon nanowire backbones, and metal branches were prepared using a novel nanocluster seeded solution phase growth. Second, an approach for large area, uniformly aligned and controlled density nanowire and nanotube films that involves expanding a bubble from a homogeneous suspension of these materials was developed. The blown-bubble films allow the unique properties of nanowires and to be exploited in applications that require large surface areas. Third, nanofabrication was used to define electrode arrays in which the exposed electrode area of individual elements was designed to limit interactions specifically with one or more bacterial cell and fabrication of optically transparent electrode arrays was carried out on transparent substrates to enable in-situ imaging of individual cells during electrochemical measurements. Results advance significantly our fundamental knowledge of branched nanoscale building blocks., The original document contains color images.

The overall project aimed to develop gated nanotubes and nanowires to be used in the development of spin qubits arranged in a 1D array. Nanotube circuits with gate-confined single and double dots were realized, shell-core nanowire growth methods were developed, gated nanowire circuits were fabricated, and double quantum dots, appropriate for singlet-triplet qubits, were investigated experimentally. We also investigated light emission from nanowires, to be of potential use in electron-photon coupled qubits. Theoretical work spanned the range from investigations of charge dephasing due to 1/f noise, quantum versus classical coupling to nuclear spins, and band mixing in graphene. Dozens of papers were published, several patents were filed, and several students received PhD degrees as a result of this funding.

This project is focused on developing the underlying science and technology required to enable the rational design and synthesis of functional nanoscale assemblies and the hierarchical assembly of these local functional structures into multifunctional systems. To achieve these objectives we are pursuing a highly interdisciplinary program that addresses in parallel the major scientific hurdles of the project, including development of electronically and optically well-defined nanoscale building blocks required to create novel functional nanostructures, and development of flexible and scalable methods for the assembly of these building blocks into novel functional nanostructures and nanostructured multifunctional systems. Significant progress has been made during the past fiscal year in several areas of the project. First, we have developed and demonstrated powerful methods for creating the first axial nanowire superlattice structures in which both composition and doping can be varied; Second, we have demonstrated that the nanowire possess novel photonic behavior and have configured optical bar codes and intrawire nanoscale light-emitting diodes. Third, we have demonstrated that doping modulation can be used to design and synthesize active nanoelectronic devices. These nanowire super lattice materials open up many new opportunities in nanoelectronics and nanophotonics., The original document contains color images. All DTIC reproductions will be in black and white.

A state-of-the-art, metal-organic chemical vapor deposition (MOCVD) system was acquired and successfully installed. The instrumentation is capable of growth of group-III nitride alloys with control down to the atomic level. The new system has been used to grow novel gallium nitride nanowires, including gallium nitride/indium-gallium nitride/p-type gallium nitride core/shell/shell heterostructures. Structural measurements demonstrate the successful growth of these new nanomaterials, and moreover, physical measurements demonstrate that these new nanowire heterostructures enable high-efficiency blue nanoscale light-emitting diodes., The original document contains color images. All DTIC reproductions will be in black and white.

Instrumentation was acquired from the DURIP grant to set up a system for the growth and characterization of metal nitride materials. This instrument system was installed in extensively renovated laboratory space to meet the specialized needs of the equipment and is now fully functional. The instrumentation has been tested and shown to produce high-quality thin film materials with excellent photonic properties, and has also been used to prepare and characterize metal nitride nanowire materials for nanophotonic studies. The capabilities opened up by this new instrumentation enhance dramatically the scope of the P.I.'s AFOSR-funded research program., The original document contains color images. All DTIC reproductions will be in black and white.

Antiferromagnetic manipulation was used to controllably stretch individual metallic single-walled carbon nanotubes (SWNT's). We have found that SWNT's can sustain elongations as great as 30% without breaking. Scanned gate microscopy and transport measurements were used to probe the effects of the mechanical strain on the SWNT electronic properties, which revealed a strain-induced increase in intra-tube electronic scattering above a threshold strain of ~5–10 %. These findings are consistent with theoretical calculations predicting the onset of plastic deformation and defect formation in carbon nanotubes.

Antiferromagnetic manipulation was used to controllably stretch individual metallic single-walled carbon nanotubes (SWNT's). We have found that SWNT's can sustain elongations as great as 30% without breaking. Scanned gate microscopy and transport measurements were used to probe the effects of the mechanical strain on the SWNT electronic properties, which revealed a strain-induced increase in intra-tube electronic scattering above a threshold strain of ~5–10 %. These findings are consistent with theoretical calculations predicting the onset of plastic deformation and defect formation in carbon nanotubes.

Carbon nanotubes have been developed as probes for molecular resolution atomic force microscopy imaging. Reproducible methods for the preparation of carbon nanotube probe tips by chemical vapor deposition were demonstrated, and moreover, approaches for the preparation of controlled diameter carbon nanotubes were defined. The utility of nanotubes probes was demonstrated with development of new method of nanofabrication, and with development of novel method for DNA sequence analysis. In addition, semiconductor nanowires have been developed as building blocks for nanophotonics. A new concept for a nanoscale photonic source using crossed nanowires was demonstrated, and the novel photonic properties of modulated nanowire heterostructures were shown for the first time., The original document contains color images. All DTIC reproductions will be in black and white.

We have implemented a method for multiplexed detection of polymorphic sites and direct determination of haplotypes in 10-kilobasesize DNA fragments using single-walled carbon nanotube (SWNT) atomic force microscopy (AFM) probes. Labeled oligonucleotides are hybridized specifically to complementary target sequences in template DNA, and the positions of the tagged sequences are detected by direct SWNT tip imaging. We demonstrated this concept by detecting streptavidin and IRD800 labels at two different sequences in M13mp18. Our approach also permits haplotype determination from simple visual inspection of AFM images of individual DNA molecules, which we have done on UGT1A7, a gene under study as a cancer risk factor. The haplotypes of individuals heterozygous at two critical loci, which together influence cancer risk, can be easily and directly distinguished from AFM images. The application of this technique to haplotyping in population-based genetic disease studies and other genomic screening problems is discussed.

We have used an atomic-force microscope tip to mechanically buckle single-walled carbon nanotubes. The resistance of the induced defects ranged from 10 to 100 k Omega and varied with the local Fermi level, as determined by scanned-gate microscopy. By forming two closely spaced defects on metallic nanotubes, we defined quantum dots less than 100 nm in length. These devices exhibited single-electron charging behavior at temperatures up to ~165 K.

We have used an atomic-force microscope tip to mechanically buckle single-walled carbon nanotubes. The resistance of the induced defects ranged from 10 to 100 k Omega and varied with the local Fermi level, as determined by scanned-gate microscopy. By forming two closely spaced defects on metallic nanotubes, we defined quantum dots less than 100 nm in length. These devices exhibited single-electron charging behavior at temperatures up to ~165 K.

Chromatin-remodeling complexes alter chromatin structure to facilitate, or in some cases repress, gene expression. Recent studies have suggested two potential pathways by which such regulation might occur. In the first, the remodeling complex repositions nucleosomes along DNA to open or occlude regulatory sites. In the second, the remodeling complex creates an altered dimeric form of the nucleosome that has altered accessibility to transcription factors. The extent of translational repositioning, the structure of the remodeled dimer, and the presence of dimers on remodeled polynucleosomes have been difficult to gauge by biochemical assays. To address these questions, ultrahigh-resolution carbon nanotube tip atomic force microscopy was used to examine the products of remodeling reactions carried out by the human SWI/SNF (hSWI/SNF) complex. We found that mononucleosome remodeling by hSWI/SNF resulted in a dimer of mononucleosomes in which ~60 bp of DNA is more weakly bound than in control nucleosomes. Arrays of evenly spaced nucleosomes that were positioned by 5S rRNA gene sequences were disorganized by hSWI/SNF, and this resulted in long stretches of bare DNA, as well as clusters of nucleosomes. The formation of structurally altered nucleosomes on the array is suggested by a significant increase in the fraction of closely abutting nucleosome pairs and by a general destabilization of nucleosomes on the array. These results suggest that both the repositioning and structural alteration of nucleosomes are important aspects of hSWI/SNF action on polynucleosomes.

Intramolecular junctions in single-walled carbon nanotubes are potentially ideal structures for building robust, molecular-scale electronics but have only been studied theoretically at the atomic level. Scanning tunneling microscopy was used to determine the atomic structure and electronic properties of such junctions in single-walled nanotube samples. Metal-semiconductor junctions are found to exhibit an electronically sharp interface without localized junction states, whereas a more diffuse interface and low-energy states are found in metal-metal junctions. Tight-binding calculations for models based on observed atomic structures show good agreement with spectroscopy and provide insight into the topological defects forming intramolecular junctions. These studies have important implications for applications of present materials and provide a means for assessing efforts designed to tailor intramolecular junctions for nanoelectronics.

To understand and control friction and wear in both macroscopic and microscopic technologies requires a detailed understanding of material properties, chemical reactivity and intermolecular interactions on the nanometer length scale. The role of nanoscale defects on friction has been elucidated through scanning tunneling microscopy and atomic force microscopy studies of molybdenum disulfide. These investigations have shown that friction increases systematically with increasing defect density, and have demonstrated a novel load-independent friction regime due to sliding on constant area nanocrystals. The mechanical properties of finite size materials, which are important to micro and nanomechanical systems, have also been probed through studies of the bending of different thickness molybdenum oxide nanocrystals. Significantly, this work has shown that there is a substantial and systematic decrease in the modulus with decreasing thickness. This large drop in stiffness shows that materials will exhibit greater flexibility as their dimensions are reduced. Lastly, a new generation of molecular resolution tools has been developed. Carbon nanotubes were attached to conventional force microscopy tips and shown to provide large improvements in image resolution. Methods to localize molecules at the nanotube ends were also developed, and these modified probes were used to measure intermolecular forces and image with chemical sensitivity.

Single-walled carbon nanotube (SWNT) probe microscopy tips were grown by a surface growth chemical vapor deposition method. Tips consisting of individual SWNTs (1.5–4 nm in diameter) and SWNT bundles (4–12 nm in diameter) have been prepared by design through variations in the catalyst and growth conditions. In addition to high-resolution imaging, these tips have been used to fabricate SWNT nanostructures by spatially controlled deposition of specific length segments of the nanotube tips.

Carbon nanotubes represent ideal probes for high-resolution structural and chemical imaging of biomolecules with atomic force microscopy. Recent advances in fabrication of carbon nanotube probes with sub-nanometer radii promise to yield unique insights into the structure, dynamics and function of biological macromolecules and complexes.

Carbon nanotubes are potentially ideal atomic force microscopy probes because they can have diameters as small as one nanometer, have robust mechanical properties, and can be specifically functionalized with chemical and biological probes at the tip ends. This communication describes methods for the direct growth of carbon nanotube tips by chemical vapor deposition (CVD) using ethylene and iron catalysts deposited on commercial silicon-cantilever-tip assemblies. Scanning electron microscopy and transmission electron microscopy measurements demonstrate that multiwalled nanotube and single-walled nanotube tips can be grown by predictable variations in the CVD growth conditions. Force-displacement measurements made on the tips show that they buckle elastically and have very small (≤100 pN) nonspecific adhesion on mica surfaces in air. Analysis of images recorded on gold nanoparticle standards shows that these multi- and single-walled carbon nanotube tips have radii of curvature of 3–6 and 2–4 nm, respectively. Moreover, the nanotube tip radii determined from the nanoparticle images are consistent with those determined directly by transmission electron microscopy imaging of the nanotube ends. These molecular-scale CVD nanotube probes have been used to image isolated IgG and GroES proteins at high-resolution.

A concept for molecular electronics exploiting carbon nanotubes as both molecular device elements and molecular wires for reading and writing information was developed. Each device element is based on a suspended, crossed nanotube geometry that leads to bistable, electrostatically switchable ON/OFF states. The device elements are naturally addressable in large arrays by the carbon nanotube molecular wires making up the devices. These reversible, bistable device elements could be used to construct nonvolatile random access memory and logic function tables at an integration level approaching 1012 elements per square centimeter and an element operation frequency in excess of 100 gigahertz. The viability of this concept is demonstrated by detailed calculations and by the experimental realization of a reversible, bistable nanotube-based bit.

Single-walled carbon nanotubes are ideal systems for investigating fundamental properties and applications of one-dimensional electronic systems. The interaction of magnetic impurities with electrons confined in one dimension has been studied by spatially resolving the local electronic density of states of small cobalt clusters on metallic single-walled nanotubes with a low-temperature scanning tunneling microscope. Spectroscopic measurements performed on and near these clusters exhibit a narrow peak near the Fermi level that has been identified as a Kondo resonance. Using the scanning tunneling microscope to fabricate ultra-small magnetic nanostructures consisting of small cobalt clusters on short nanotube pieces, spectroscopic studies of this quantum box structure exhibited features characteristic of the bulk Kondo resonance, but also new features due to finite size.

Carbon nanotubes, which have intrinsically small diameters and high aspect ratios and which buckle reversibly, make potentially ideal structures for use as tips in scanning probe microscopies, such as atomic force microscopy (AFM)1, 2, 3, 4. However, the present method of mechanically attaching nanotube bundles for tip fabrication is time consuming and selects against the smallest nanotubes, limiting the quality of tips. We have developed a technique for growing individual carbon nanotube probe tips directly, with control over the orientation, by chemical vapor deposition (CVD) from the ends of silicon tips. Tips grown in this way may become widely used in high-resolution probe microscopy imaging.

Significant effort has been placed on several areas. First, we have carried out systematic studies of the growth of carbon nitride materials as a function of carbon reactant energetics, nitrogen flux and growth temperature and have quantitatively analyzed the structure and local bonding in these materials. The emphasis of these studies has been to determine unambiguously conditions that can produce sp3-bonding since this is required for a superhard coating. Significantly, we have show recently that there is a nitrogen-driven sp3 to sp2 structural transformation in the carbon nitride materials as nitrogen content is increased about 15 atomic percent. With these limits worked out we have also prepared films with optimal C-N ratios for hardness, and investigated their nanotribological properties by force microscopy. In addition, we have carried out theoretical cluster calculations to understand the origin of this sp3 to sp2 transition. Significantly, these calculations demonstrate that the transition is an intrinsic property of carbon nitride system since (1) the sp2 bonded structure becomes thermodynamically favored for 15% nitrogen and (2) the barrier between sp3 and sp2 structures also reduces significantly for 15% nitrogen.

The overall goal of this project is to develop rational approaches for controlling the nanostructure of high-temperature superconductors (HTSs) and other complex solids. Our emphasis on the nanometer scale is motivated by the recognition that control of structure in this size regime leads in general to materials with enhanced and/or novel electrical, thermal, mechanical, optical and magnetic properties. In this regard, our main objective has been to control the nanometer scale defect structure in HTSs to enhance critical currents. The intrinsic problem of thermally-activated flux flow, which limits critical currents in all HTS materials, can be reduced significantly by creating nanometer diameter columnar defect structure in the HTSs. Specific objectives that have been pursued during the past year include (1) the design of a large scale synthesis of MgO nanorods that function as columnar defects in HTSs, (2) elucidation of factors critical to the creation of a well-defined nanorod/HTS nanostructure in bulk materials, and (3) the development of general approaches to the synthesis of nanowires of other materials.

The overall objectives of the AASERT project are to develop an approach for the controlled synthesis of single crystal, one-dimensional (1D) nanowires of the Bi2Te3 family of thermoelectric materials, and to characterize the potentially unique electronic properties of these 1D systems. To achieve these goals we will (1) develop synthetic procedures, which were initiated as part of our current ONR project on nanorod- superconductor composites, to prepare Bi2Te3 nanowires and (2) exploit existing scanning tunneling microscopy facilities and expertise to characterize the nanowire density of electronic states. The combination of systematic synthesis with high-resolution structural and electronic characterization will elucidate the basic factors that control the diameter, aspect ratio, and crystallographic growth direction of Bi2Te3 nanowires, and define the variation and potential enhancements in valence and conduction band edge density of states with nanowire diameter. These studies will thus provide a critical assessment of the possibility of obtaining enhanced thermoelectric figures of merit through material dimensional control.

Carbon nanotubes combine a range of properties that make them well suited for use as probe tips in applications such as atomic force microscopy (AFM)1, 2, 3. Their high aspect ratio, for example, opens up the possibility of probing the deep crevices4 that occur in microelectronic circuits, and the small effective radius of nanotube tips significantly improves the lateral resolution beyond what can be achieved using commercial silicon tips5. Another characteristic feature of nanotubes is their ability to buckle elastically4, 6, which makes them very robust while limiting the maximum force that is applied to delicate organic and biological samples. Earlier investigations into the performance of nanotubes as scanning probe microscopy tips have focused on topographical imaging, but a potentially more significant issue is the question of whether nanotubes can be modified to create probes that can sense and manipulate matter at the molecular level7. Here we demonstrate that nanotube tips with the capability of chemical and biological discrimination can be created with acidic functionality and by coupling basic or hydrophobic functionalities or biomolecular probes to the carboxyl groups that are present at the open tip ends. We have used these modified nanotubes as AFM tips to titrate the acid and base groups, to image patterned samples based on molecular interactions, and to measure the binding force between single protein–ligand pairs. As carboxyl groups are readily derivatized by a variety of reactions8, the preparation of a wide range of functionalized nanotube tips should be possible, thus creating molecular probes with potential applications in many areas of chemistry and biology.

The overall goal of this project is to develop rational approaches for controlling the structure and interfaces of high-temperature superconductors (HTSs) and other complex solids at the nanometer scale. Our emphasis on nanometer scale is motivated by the recognition that control of structure in this size regime leads in general to materials with enhanced and/or novel electrical, mechanical, optical and magnetic properties. In this regard, our main objective has been to control the nanometer scale defect structure in HTSs since this will enable the intrinsic problem of thermally-activated flux flow, which limits critical currents in all HTS materials, to be significantly reduced. Specific objectives that have been pursued during the past year include (1) the design and synthesis of one-dimensional nanostructures (nanorods) that are chemically and structurally compatible with HTS materials, (2) the synthesis and structural characterization of nanorod/HTS nanocomposites, and (3) the elucidation of critical current behavior of nanorod/FiTS materials as a function of temperature and magnetic field.

Scanning tunneling and atomic force microscopy studies of metal dichalcogenide materials demonstrate that wear proceeds via surface oxidation, and that wear rates are directly related to the oxidative stability of the surfaces. Surface oxidation was also shown to produce oriented MoO3 nanocrystals on MoS2 surfaces. This system was shown to be uniquely suited for nanotribology studies because the interface structure and contact area are atomically deformed and the MoO3 nanocrystals can be moved controllably with lateral force microscope tip. Highly anisotropic friction has been observed whereby MoO3 nanocrystals moved along only specific directions of the MoS2 surface lattice. An atomic model of the interface was developed to explain these In addition, chemical force microscopy has been used to measure adhesion and friction forces between probe tips and substrates covalently modified with self-assembled monolayers (SAMs) that terminate in distinct functional groups such as COOH, CH3,and NH2 in ethanol and water solvents. The measured adhesive forces were found to agree well with predictions of the Johnson, Kendall, and Roberts (JKR) theory of adhesive contact, and thus show that the observed adhesion forces correlate with the surface free energy. Electrostatic contributions to adhesive forces have also been characterized using COOH/NH2 functionalized tip/surface that exists as COO(-)/NH3(+) in aqueous solution. The dependence of friction forces on the tip and sample functionality has also been shown to be the basis for chemical force microscopy in which lateral force images are interpreted in terms of the strength of adhesive interactions between functional groups. Chemically sensitive imaging of photopatterned monolayers using probe tips modified with different functional groups has been demonstrated.

We report the first experimental observation of a semiconductor/liquid junction whose open circuit voltage Voc is controlled by bulk diffusion/recombination processes. Variation in temperature, minority-carrier diffusion length, and/or in majority-carrier concentration produces changes in the Voc of the n-Si/CH3OH interface in accord with bulk recombination/diffusion theory. Under AM2 irradiation conditions, the extrapolated intercept at 0 K of Voc vs T plots yields activation energies for the dominant recombination process of 1.1–1.2 eV, in accord with the 1.12-eV band gap of Si. A crucial factor in achieving optimum performance of the n-Si/CH3OH interface is assigned to photoelectrochemical oxide formation, which passivates surface recombination sites at the n-Si/CH3OH interface and minimizes deleterious effects of pinning of the Fermi level at the Si/CH3OH junction. Controlled Si oxide growth, combined with optimization of bulk crystal parameters in accord with diffusion theory, is found to yield improved photoelectrode output parameters, with 12.0±1.5% AM2 efficiencies and AM1 Voc values of 632–640 mV for 0.2-Ω cm Si materials.

We report the first experimental observation of a semiconductor/liquid junction whose open circuit voltage Voc is controlled by bulk diffusion/recombination processes. Variation in temperature, minority-carrier diffusion length, and/or in majority-carrier concentration produces changes in the Voc of the n-Si/CH3OH interface in accord with bulk recombination/diffusion theory. Under AM2 irradiation conditions, the extrapolated intercept at 0 K of Voc vs T plots yields activation energies for the dominant recombination process of 1.1–1.2 eV, in accord with the 1.12-eV band gap of Si. A crucial factor in achieving optimum performance of the n-Si/CH3OH interface is assigned to photoelectrochemical oxide formation, which passivates surface recombination sites at the n-Si/CH3OH interface and minimizes deleterious effects of pinning of the Fermi level at the Si/CH3OH junction. Controlled Si oxide growth, combined with optimization of bulk crystal parameters in accord with diffusion theory, is found to yield improved photoelectrode output parameters, with 12.0±1.5% AM2 efficiencies and AM1 Voc values of 632–640 mV for 0.2-Ω cm Si materials.