Due to their unique boundary conditions, nanowire heterostructures may exhibit defect-free interfaces even for systems with large lattice mismatch. Heteroepitaxial material integration is limited by lattice mismatches in planar systems, but we use a variational approach to show that nanowire heterostructures are more effective at relieving mismatch strain coherently. This is an equilibrium model based on the Matthews critical thickness in which the lattice mismatch strain is shared by the nanowire overlayer and underlayer, and could as well be partially accomodated by the introduction of a pair of misfit dislocations. The model is highly portable to other nanowire material systems and can be used to estimate critical feature sizes. We find that the critical radius of this system is roughly an order of magnitude larger than the critical thickness of the corresponding thin film/substrate material system. Finite element analysis is used to assess some aspects of the model; in particular, to show that the variational approach describes well the decay of the strain energy density away from the interface.

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The creation of a sustainable energy generation, storage, and distribution infrastructure represents a global grand challenge that requires massive transnational investments in the research and development of energy technologies that will provide the amount of energy needed on a sufficient scale and timeframe with minimal impact on the environment and have limited economic and societal disruption during implementation. In this opinion paper, we focus on an important set of solar, thermal, and electrochemical energy conversion, storage, and conservation technologies specifically related to recent and prospective advances in nanoscale science and technology that offer high potential in addressing the energy challenge. We approach this task from a two-fold perspective: analyzing the fundamental physicochemical principles and engineering aspects of these energy technologies and identifying unique opportunities enabled by nanoscale design of materials, processes, and systems in order to improve performance and reduce costs. Our principal goal is to establish a roadmap for research and development activities in nanoscale science and technology that would significantly advance and accelerate the implementation of renewable energy technologies. In all cases we make specific recommendations for research needs in the near-term (2-5 years), mid-term (5-10 years) and long-term (>10 years), as well as projecting a timeline for maturation of each technological solution. We also identify a number of priority themes in basic energy science that cut across the entire spectrum Broader contextA major scientific and societal challenge of the 21st century is the conversion from a fossil-fuel-based energy economy to one that is sustainable. The energy challenge before us differs in three ways from past large scale challenges: the first is the large magnitude and relatively short time scale of the transition (a predicted doubling of energy demand by mid-century and a tripling by the end of the century); the second is the need to develop CO 2 -neutral, renewable energy sources; and the third is the cost-competitive aspect of the transition (insofar as the cost of energy to the consumer must be competitive with the fossil fuel energy supply being replaced). What is clear is that the science and engineering research communities working with industry, and policy makers (government, economists, social scientists) will have to educate the citizenry and get them to function collaboratively and globally to enhance the quality of life and to preserve the environment of our planet for future generations. Our team has prepared a technical article on the role of nanotechnology in our energy future aimed at guiding both our own community of scientists and engineers and our policy makers who interface with the public. This journal is ª The Royal Society of Chemistry 2009Energy Environ. Sci., 2009, 2, 559-588 | 559 ANALYSIS www.rsc.org/ees | Energy & Environmental Science of energy conversion, storage, and conservation technologies. We anticipate t...

In this paper, the processes associated with the electrodeposition of bismuth telluride (Bi 2 Te 3 ), a thermoelectric material, are reported along with an analysis of the composition and crystallinity of the resulting films. The electrodeposition can be described by the general reaction 3HTeO 2 ϩ ϩ 2Bi 3ϩ ϩ 18e Ϫ ϩ 9H ϩ → Bi 2 Te 3 ϩ 6H 2 O. Cyclic voltammetry studies of Bi, Te, and Bi/Te dissolved in 1 M HNO 3 reveal two different underlying processes depending on the deposition potential. One process involves the reduction of HTeO 2 ϩ to Te 0 and a subsequent interaction between reduced Te 0 and Bi 3ϩ to form Bi 2 Te 3 . A second process at more negative reduction potentials involves reduction of HTeO 2 ϩ to H 2 Te followed by the chemical interaction with Bi 3ϩ . Both processes result in the production of crystalline Bi 2 Te 3 films in the potential range Ϫ0.1 Ͻ E Ͻ Ϫ0.52 V vs. Ag/AgCl ͑3 M NaCl͒ on Pt substrates as determined by powder X-ray diffraction ͑XRD͒. Electron probe microanalyses and XRD reveal that the films are bismuth-rich and less oriented for more negative deposition potentials.Solid-state thermoelectric devices convert thermal energy from a temperature gradient into electrical energy ͑the Seebeck effect͒ or electrical energy into a temperature gradient ͑the Peltier effect͒. Thermoelectric power generators are used most notably in spacecraft power generation systems ͑for example, in Voyager I and II͒ 1,2 and in thermocouples for temperature measurement, while thermoelectric coolers are largely used in charge coupled device ͑CCD͒ cameras, laser diodes, microprocessors, blood analyzers, and portable picnic coolers. 1,2 Thermoelectric coolers ͑also known as Peltier coolers͒ offer several advantages over conventional systems. As solid-state devices, they have no moving parts. They use no ozonedepleting chlorofluorocarbons, potentially offering a more environmentally responsible alternative to conventional refrigeration. Although some large-scale applications have been considered ͑on submarines and surface vessels͒, their efficiency is low compared to conventional refrigerators.Scientific and technological interest in the production of nanostructured thermoelectric materials has been driven by recent theoretical studies, which suggest that quantum confinement of electrons and holes could enhance the efficiency of these materials significantly above that of their bulk values. 3-5 This hypothesis has already been verified for thin multilayers of PbTe/Pb 1Ϫx Eu x Te. 6-11 Larger enhancements are predicted for one-dimensional ͑1-D͒ systems ͑nanowires͒ compared to 2-D systems ͑thin films͒. 12,13 These predictions have stimulated research into the preparation of nanowires of thermoelectric materials.Bismuth telluride (Bi 2 Te 3 ) and its doped derivative compounds are considered to be the best materials to date for near roomtemperature thermoelectric applications. 14,15 The maximum figure of merit ͑ZT͒ occurs for optimized doping levels 16 at approximately 70°C with an effective operating range of Ϫ100 to...

Titanium nitride (TiN) is a plasmonic material having optical properties resembling gold. Unlike gold, however, TiN is complementary metal oxide semiconductor-compatible, mechanically strong, and thermally stable at higher temperatures. Additionally, TiN exhibits low-index surfaces with surface energies that are lower than those of the noble metals which facilitates the growth of smooth, ultrathin crystalline films. Such films are crucial in constructing low-loss, high-performance plasmonic and metamaterial devices including hyperbolic metamaterials (HMMs). HMMs have been shown to exhibit exotic optical properties, including extremely high broadband photonic densities of states (PDOS), which are useful in quantum plasmonic applications. However, the extent to which the exotic properties of HMMs can be realized has been seriously limited by fabrication constraints and material properties. Here, we address these issues by realizing an epitaxial superlattice as an HMM. The superlattice consists of ultrasmooth layers as thin as 5 nm and exhibits sharp interfaces which are essential for highquality HMM devices. Our study reveals that such a TiN-based superlattice HMM provides a higher PDOS enhancement than goldor silver-based HMMs.refractory plasmonics | metal nitrides | ceramics M etamaterials are artificially created materials with subwavelength building blocks and unconventional electromagnetic properties that enable devices with unique functionalities (1, 2). For example, highly anisotropic metamaterials that consist of deeply subwavelength dielectric-metallic multilayers can effectively act as a material that is metallic in one or two directions and dielectric in the other (3, 4). In such metamaterials, light encounters extreme anisotropy, resulting in a hyperbolic dispersion relation (therefore these materials are referred to as "hyperbolic metamaterials," HMMs), which causes dramatic changes in the light's behavior (5-7). HMMs enable many exotic devices for subwavelength-resolution imaging (5, 8-10), ultracompact resonators (11), highly sensitive sensors (12), and could lead to breakthrough quantum technologies (7, 13). The recent discovery of the enhancement of the photonic density of states (PDOS) within a broad bandwidth in HMMs could revolutionize PDOS engineering (14-17), enabling light sources with dramatically increased photon extraction and ultimately leading to nonresonant single-photon sources (6). These HMMs can be combined with wide-spectrum, room-temperature quantum emitters [such as quantum dots and nitrogen-vacancy color centers in diamonds (18)] to provide greatly enhanced spontaneous emission rates (19). Additionally, HMMs can transform an isotropic spontaneous emission profile into a directional one, leading to new types of light sources (20). Rather than obeying Planck's law, an extreme PDOS enhancement enables nearfield thermal radiation arising from the HMM to be significantly enhanced compared with the near-field thermal radiation from a dielectric (21). Moreover, the thermal conductivity...

Epitaxial ScN(001) thin films were grown on MgO(001) substrates by dc reactive magnetron sputtering. The deposition was performed in an Ar/N 2 atmosphere at 2 Â 10 À3 Torr at a substrate temperature of 850 C in a high vacuum chamber with a base pressure of 10 À8 Torr. In spite of oxygen contamination of 1.6 6 1 at. %, the electrical resistivity, electron mobility, and carrier concentration obtained from a typical film grown under these conditions by room temperature Hall measurements are 0.22 mX cm, 106 cm 2 V À1 s À1 , and 2.5 Â 10 20 cm À3 , respectively. These films exhibit remarkable thermoelectric power factors of 3.3-3.5 Â 10 À3 W/mK 2 in the temperature range of 600 K to 840 K. The cross-plane thermal conductivity is 8.3 W/mK at 800 K yielding an estimated ZT of 0.3. Theoretical modeling of the thermoelectric properties of ScN calculated using a meanfree-path of 23 nm at 300 K is in very good agreement with the experiment. These results also demonstrate that further optimization of the power factor of ScN is possible. First-principles density functional theory combined with the site occupancy disorder technique was used to investigate the effect of oxygen contamination on the electronic structure and thermoelectric properties of ScN. The computational results suggest that oxygen atoms in ScN mix uniformly on the N site forming a homogeneous solid solution alloy. Behaving as an n-type donor, oxygen causes a shift of the Fermi level in ScN into the conduction band without altering the band structure and the density of states. V C 2013 AIP Publishing LLC [http://dx.

Arrays of bismuth telluride (Bi2Te3) nanowires with diameters of ∼25, ∼50, and ∼75 nm have been produced by electrochemical deposition into porous anodic alumina templates. Scanning electron microscopy confirms that the nanowire arrays are dense with a narrow distribution of nanowire diameters. The structure of the nanowires was assessed immediately after deposition, after annealing to ∼80% of the melting point, and after melting/recrystallization. As determined by XRD analysis, there is strong fiber texture in the arrays that depends on both the nanowire diameter and the postdeposition processing conditions. Bright-field/dark-field imaging and diffraction in the transmission electron microscope reveal that the as-deposited nanowires are polycrystalline with a bamboo-type grain structure that does not change significantly upon annealing, and a similar grain structure is obtained after melting and resolidification.

With a motivation to understand microscopic aspects of ScN, ZrN, and HfN relevant to the thermoelectric properties of nitride metal/semiconductor superlattices, we determine their electronic structure, vibrational spectra and thermal properties using first-principles calculations based on density functional theory with a generalized gradient approximation of the exchange correlation energy. We find a large energy gap in the phonon dispersions of metallic ZrN and HfN, but a gapless phonon spectrum for ScN spanning the same energy range, this suggests that a reduced thermal conductivity, suitable for thermoelectric applications, should arise in superlattices made with ScN and ZrN or ScN and HfN. To obtain an electronic energy band gap of ScN comparable to experiment, we use a Hubbard correction with a parameter U ͑=3.5 eV͒. Anomalies in the acoustic branches of the phonon dispersion of ZrN and HfN, manifested as dips in the bands, can be understood through the nesting of Fermi surface determined from our calculations. To connect with transport properties, we have determined effective masses of ScN and determined their dependence on the U parameter. Using the relaxation time approximation in the Boltzmann transport theory, we estimate the temperature dependence of the lattice thermal conductivity and discuss the chemical trends among these nitrides.