Person:

Narayanamurti, Venkatesh

The United States and the world need a revolution in energy technology—a revolution that would improve the performance of our energy systems to face the challenges ahead. A dramatic increase in the pace of energy innovation is crucial to meet the challenges of: • Energy and national security, to address the dangers of undue reliance on dwindling supplies of oil increasingly concentrated in some of the most volatile regions of the world, and to limit the connection between nuclear energy and the spread of nuclear weapons; • Environmental sustainability, to reduce the wide range of environmental damages due to energy production and use, from fine particulate emissions at coal plants, to oil spills, to global climate disruption; and • Economic competitiveness, to seize a significant share of the multi-trillion-dollar clean energy technology market and improve the balance of payments by increasing exports, while reducing the hundreds of billions of dollars spent every year on importing oil. In an intensely competitive and interdependent global landscape, and in the face of large climate risks from ongoing U.S. reliance on a fossil-fuel based energy system, it is important to maintain and expand long-term investments in the energy future of the U.S. even at a time of budget stringency. It is equally necessary to think about how to improve the efficiency of those investments, through strengthening U.S. energy innovation institutions, providing expanded incentives for private-sector innovation, and seizing opportunities where international cooperation can accelerate innovation. The private sector role is key: in the United States the vast majority of the energy system is owned by private enterprises, whose innovation and technology deployment decisions drive much of the country’s overall energy systems. Efficiently utilizing government investments in energy innovation requires understanding the market incentives that drive private firms to invest in advanced energy technologies, including policy stability and predictability. The U.S. government has already launched new efforts to accelerate energy innovation. In particular, the U.S. Department of Energy is undertaking a Quadrennial Technology Review to identify the most promising opportunities and provide increased coherence and stability. Our report offers analysis and recommendations designed to accelerate the pace at which better energy technologies are discovered, developed, and deployed, and is focused in four key areas: • Designing an expanded portfolio of federal investments in energy research, development, demonstration (ERD&D), and complementary policies to catalyze the deployment of novel energy technologies; • Increasing incentives for private-sector innovation and strengthening federal-private energy innovation partnerships; • Improving the management of energy innovation institutions to maximize the results of federal investments; and • Expanding and coordinating international energy innovation cooperation to bring ideas and resources together across the globe to address these global challenges.

For more than three decades, research on tunneling through planar barriers has focused principally on processes that conserve momentum parallel to the barrier. Here we investigate transport in which scattering destroys lateral momentum conservation and greatly enhances the tunneling probability. We have measured its energy dependence using capacitance spectroscopy, and we show that for electrons confined in a quantum well, the scattering enhancement can be quenched in an applied magnetic field, enabling this mechanism to function as an external probe of the origin of the quantum Hall effect.

We present an investigation of optically active near-surface defects in sputtered Al-doped ZnO films using scanning tunneling microscope cathodoluminescence (STM-CL). STM-CL maps suggest that the optically active sites are distributed randomly across the surface and do not correlate with the granular topography. In stark contrast to photoluminescence results, STM-CL spectra show a series of sharp, discrete emissions that characterize the dominant optically active defect, which we propose is an oxygen vacancy. Our results highlight the ability of STM-CL to spectrally fingerprint individual defects and contribute to understanding the optical properties of near-surface defects in an important transparent conductor.

This paper describes a strategy for the fabrication of functional electronic components (transistors, capacitors, resistors, conductors, and logic gates but not, at present, inductors) that combines a single layer of lithography with angle-dependent physical vapor deposition; this approach is named topographically encoded microlithography (abbreviated as TEMIL). This strategy extends the simple concept of ‘shadow evaporation’ to reduce the number and complexity of the steps required to produce isolated devices and arrays of devices, and eliminates the need for registration (the sequential stacking of patterns with correct alignment) entirely. The defining advantage of this strategy is that it extracts information from the 3D topography of features in photoresist, and combines this information with the 3D information from the angle-dependent deposition (the angle and orientation used for deposition from a collimated source of material), to create ‘shadowed’ and ‘illuminated’ regions on the underlying substrate. It also takes advantage of the ability of replica molding techniques to produce 3D topography in polymeric resists. A single layer of patterned resist can thus direct the fabrication of a nearly unlimited number of possible shapes, composed of layers of any materials that can be deposited by vapor deposition. The sequential deposition of various shapes (by changing orientation and material source) makes it possible to fabricate complex structures—including interconnected transistors—using a single layer of topography. The complexity of structures that can be fabricated using simple lithographic features distinguishes this procedure from other techniques based on shadow evaporation.

Several developing countries seek to build knowledge-based economies by attempting to expand scientific research capabilities. Characterizing the state and direction of progress in this arena is challenging but important. Here, we employ three metrics: a classical metric of productivity (publications per person), an adapted metric which we denote as Revealed Scientific Advantage (developed from work used to compare publications in scientific fields among countries) to characterize disciplinary specialty, and a new metric, scientific indigeneity (defined as the ratio of publications with domestic corresponding authors) to characterize the locus of scientific activity that also serves as a partial proxy for local absorptive capacity. These metrics—using population and publications data that are available for most countries–allow the characterization of some key features of national scientific enterprise. The trends in productivity and indigeneity when compared across other countries and regions can serve as indicators of strength or fragility in the national research ecosystems, and the trends in specialty can allow regional policy makers to assess the extent to which the areas of focus of research align (or not align) with regional priorities. We apply the metrics to study the Middle East and North Africa (MENA)—a region where science and technology capacity will play a key role in national economic diversification. We analyze 9.8 million publication records between 1981–2013 in 17 countries of MENA from Morocco to Iraq and compare it to selected countries throughout the world. The results show that international collaborators increasingly drove the scientific activity in MENA. The median indigeneity reached 52% in 2013 (indicating that almost half of the corresponding authors were located in foreign countries). Additionally, the regional disciplinary focus in chemical and petroleum engineering is waning with modest growth in the life sciences. We find repeated patterns of stagnation and contraction of scientific activity for several MENA countries contributing to a widening productivity gap on an international comparative yardstick. The results prompt questions about the strength of the developing scientific enterprise and highlight the need for consistent long-term policy for effectively addressing regional challenges with domestic research.

Single-walled carbon nanotubes have been grown on a variety of substrates by chemical vapor deposition using low-coverage vacuum-deposited iron as a catalyst. Ordered arrays of suspended nanotubes ranging from submicron to several micron lengths have been obtained on Si, SiO2,SiO2, Al2O3,Al2O3, and Si3N4Si3N4 substrates that were patterned on hundred nanometer length scales with a focused ion beam machine. Electric fields applied during nanotubegrowth allow the control of growth direction. Nanotube circuits have been constructed directly on contacting metal electrodes of Pt/Cr patterned with catalysts. Patterning with solid iron catalyst is compatible with modern semiconductor fabrication strategies and may contribute to the integration of nanotubes in complex device architectures.

We report observation of catalyst-free hydride vapor phase epitaxy growth of InN nanorods. Characterization of the nanorods with transmission electron microscopy, and X-ray diffraction show that the nanorods are stoichiometric 2H–InN single crystals growing in the [0001] orientation. The InN rods are uniform, showing very little variation in both diameter and length. Surprisingly, the rods show clear epitaxial relations with the c-plane sapphire substrate, despite about 29% of lattice mismatch. Comparing catalyst-free with Ni-catalyzed growth, the only difference observed is in the density of nucleation sites, suggesting that Ni does not work like the typical vapor–liquid–solid catalyst, but rather functions as a nucleation promoter by catalyzing the decomposition of ammonia. No conclusive photoluminescence was observed from single nanorods, while integrating over a large area showed weak wide emissions centered at 0.78 and at 1.9 eV.

We present a general approach to growing ZnO nanowires on arbitrary, high melting point (above 970 °C) substrates using the vapor–liquid–solid (VLS) growth mechanism. Our approach utilizes the melting point reduction of sufficiently small (5 nm diameter) Au particles to provide a liquid catalyst without substrate interaction. Using this size-dependent melting effect, we demonstrate catalytic VLS growth of ZnO nanowires on both Ti and Mo foil substrates with aspect ratios in excess of 1000:1. Transmission electron microscopy shows the nanowires to be single-crystalline, and photoluminescence spectra show high-quality optical properties. We believe this growth technique to be widely applicable to a variety of substrates and material systems.

We conduct a comprehensive investigation into the electronic and magnetotransport properties of ZnO nanoplates grown concurrently with ZnO nanowires by the vapor-liquid-solid method. We present magnetoresistance data showing weak localization in our nanoplates and probe its dependence on temperature and carrier concentration. We measure phase coherence lengths of 50–100 nm at 1.9 K and, because we do not observe spin-orbit scattering through antilocalization, suggest that ZnO nanostructures may be promising for further spintronic study. We then proceed to study the effect of weak localization on electron mobility using four-terminal van der Pauw resistivity and Hall measurements versus temperature and carrier concentration. We report an electron mobility of ∼100 cm2/V s at 275 K, comparable to what is observed in ZnO thin films. We compare Hall mobility to field-effect mobility, which is more commonly reported in studies on ZnO nanowires and find that field-effect mobility tends to overestimate Hall mobility by a factor of 2 in our devices. Finally, we comment on temperature-dependent hysteresis observed during transconductance measurements and its relationship to mobile, positively charged Zn interstitial impurities.

In a GaMnAs/AlGaAs resonant tunneling diode (RTD) structure, we observe that both the magnitude and polarity of magnetoresistance are bias dependent when tunneling from a three-dimensional GaMnAs layer through a two-dimensional GaMnAs quantum well. This magnetoresistance behavior results from a shift of negative differential resistance features to higher bias as the relative alignment of the GaMnAs layer magnetizations is changed from parallel to antiparallel. Our observations agree with recent predictions from a theoretical analysis of a similar n-type structure by Ertler and Fabian, and our results suggest that further investigation into ferromagnetic RTD structures may result in significantly enhanced magnetoresistance.