Nano Letters

Published by American Chemical Society
Online ISSN: 1530-6992
Print ISSN: 1530-6984
Publications
Structural degradation of Ni-rich cathode materials (LiNixM1-xO2; M = Mn, Co, and Al; x > 0.5) during cycling at both high voltage (>4.3 V) and high temperature (>50 °C) led to the continuous generation of microcracks in a secondary particle which consisted of aggregated micrometer-sized primary particles. These microcracks caused deterioration of the electrochemical properties by disconnecting the electrical pathway between the primary particles and creating thermal instability owing to oxygen evolution during phase transformation. Here, we report a new concept to overcome those problems of the Ni-rich cathode material via nanoscale surface treatment of the primary particles. The resultant primary particles' surfaces had a higher cobalt content and a cation-mixing phase (Fm3 ̅m) with nanoscale thickness in the LiNi0.6Co0.2Mn0.2O2 cathode, leading to mitigation of the microcracks by suppressing the structural change from a layered to rock-salt phase. Furthermore, the higher oxidation state of Mn4+ at the surface minimized the oxygen evolution at high temperatures. This approach resulted in improved structural and thermal stability in the severe cycling-test environment at 60 °C between 3.0 and 4.45 V and at elevated temperatures, showing a rate capability that was comparable to that of the pristine sample.
 
We report a voltage controlled reversible creation and annihilation of a-axis oriented ∼10 nm wide ferroelastic nanodomains without a concurrent ferroelectric 180° switching of the surrounding c-domain matrix in archetypal ferroelectric Pb(Zr0.2Ti0.8)O3 thin films by using the piezo-response force microscopy technique. In previous studies, the coupled nature of ferroelectric switching and ferroelastic rotation has made it difficult to differentiate the underlying physics of ferroelastic domain wall movement. Our observation of distinct thresholds for ferroelectric and ferroelastic switching allows us investigate the ferroelastic switching cleanly and demonstrate a new degree of nanoscale control over the ferroelastic domains.
 
Control of competing parameters such as thermoelectric (TE) power and electrical and thermal conductivities is essential for the high performance of thermoelectric materials. Bulk-nanocomposite materials have shown a promising improvement in the TE performance due to poor thermal conductivity and charge carrier filtering by interfaces and grain boundaries. Consequently, it has become pressingly important to understand the formation mechanisms, stability of interfaces and grain boundaries along with subsequent effects on the physical properties. We report here the effects of the thermodynamic environment during spark plasma sintering (SPS) on the TE performance of bulk-nanocomposites of chemically synthesized Bi(2)Te(2.7)Se(0.3) nanoplatelets. Four pellets of nanoplatelets powder synthesized in the same batch have been made by SPS at different temperatures of 230, 250, 280, and 350 °C. The X-ray diffraction, transmission electron microscopy, thermoelectric, and thermal transport measurements illustrate that the pellet sintered at 250 °C shows a minimum grain growth and an optimal number of interfaces for efficient TE figure of merit, ZT∼0.55. For the high temperature (350 °C) pelletized nanoplatelet composites, the concurrent rise in electrical and thermal conductivities with a deleterious decrease in thermoelectric power have been observed, which results because of the grain growth and rearrangements of the interfaces and grain boundaries. Cross section electron microscopy investigations indeed show significant grain growth. Our study highlights an optimized temperature range for the pelletization of the nanoplatelet composites for TE applications. The results provide a subtle understanding of the grain growth mechanism and the filtering of low energy electrons and phonons with thermoelectric interfaces.
 
Domain states in PbZr((0.42))Ti((0.58))O(3) single-crystal ferroelectric nanodots, formed on cooling through the Curie temperature, were imaged by transmission electron microscopy. In the majority of cases, 90° stripe domains were found to form into four distinct "bundles" or quadrants. Detailed analysis of the dipole orientations in the system was undertaken, using both dark-field imaging and an assumption that charged domain walls were energetically unfavorable in comparison to uncharged walls. On this basis, we conclude that the dipoles in these nanodots are arranged such that the resultant polarizations, associated with the four quadrant domain bundles, form into a closed loop. This "polarization closure" pattern is reminiscent of the flux-closure already commonly observed in soft ferromagnetic microdots but to date unseen in analogous ferroelectric dots.
 
Direct observation of delithiated structures of LiCoO2 at atomic scale has been achieved using spherical aberration-corrected scanning transmission electron microscopy (STEM) with high-angle annular-dark-field (HAADF) and annular-bright-field (ABF) techniques. The ordered Li, Co and O columns for LiCoO2 nano-particles are clearly identified in ABF micrographs. Upon the Li ions extraction from LiCoO2, the Co-contained (003) planes distort from the bulk to the surface region and the c-axis is expanded significantly. Ordering of lithium ions and lithium vacancies has been observed directly and explained by first-principles simulation. Based on HAADF micrographs, it is found that the phase irreversibly changes from O3-type in pristine LiCoO2, to O1-type LixCoO2 (x ≈ 0.50) after the first electrochemical Li extraction and back to O2-type LixCoO2 (x ≈ 0.93) rather than to O3-stacking after the first electrochemical lithiation. This is the first report of finding O2-LixCoO2 in the phase diagram of O3-LiCoO2, through which the two previously separated LiCoO2 phases, i.e. O2 and O3 systems are connected. These new investigations shed new insight on the lithium storage mechanism in this important cathode material for Li-ion batteries.
 
A facile wet chemical synthesis method was used to prepare a range of single-crystal Na(Y1.5 Na0.5)F6 nanorods with controllable aspect ratios. Their novel multicolor upconversion (UC) fluorescence has been successfully realized by doping Yb3+/Er3+ (green) and Yb3+/Tm3+ (blue) ion pairs. When doped with Eu3+ and Tb3+ ions, the strong red and green downconversion (DC) fluorescence has also been observed, respectively. Being covered with oleic acids, these luminescent nanorods have been transparently dispersed in nonpolar solvent. For their unique luminescence and controllable morphology and surface properties, these nanorods may find great applications in the fields of color displays, biolabels, light-emitting diodes (LEDs), optical storage, optoelectronics, anticounterfeiting, and solid-state lasers.
 
Quantum point contacts (QPCs) have shown promise as nanoscale spin-selective components for spintronic applications and are of fundamental interest in the study of electron many-body effects such as the 0.7 × 2e(2)/h anomaly. We report on the dependence of the 1D Landé g-factor g* and 0.7 anomaly on electron density and confinement in QPCs with two different top-gate architectures. We obtain g* values up to 2.8 for the lowest 1D subband, significantly exceeding previous in-plane g-factor values in AlGaAs/GaAs QPCs and approaching that in InGaAs/InP QPCs. We show that g* is highly sensitive to confinement potential, particularly for the lowest 1D subband. This suggests careful management of the QPC's confinement potential may enable the high g* desirable for spintronic applications without resorting to narrow-gap materials such as InAs or InSb. The 0.7 anomaly and zero-bias peak are also highly sensitive to confining potential, explaining the conflicting density dependencies of the 0.7 anomaly in the literature.
 
We demonstrate the first successful growth of large-area (200 × 200 μm(2)) bilayer, Bernal stacked, epitaxial graphene (EG) on atomically flat, 4H-SiC (0001) step-free mesas (SFMs) . The use of SFMs for the growth of graphene resulted in the complete elimination of surface step-bunching typically found after EG growth on conventional nominally on-axis SiC (0001) substrates. As a result heights of EG surface features are reduced by at least a factor of 50 from the heights found on conventional substrates. Evaluation of the EG across the SFM using the Raman 2D mode indicates Bernal stacking with low and uniform compressive lattice strain of only 0.05%. The uniformity of this strain is significantly improved, which is about 13-fold decrease of strain found for EG grown on conventional nominally on-axis substrates. The magnitude of the strain approaches values for stress-free exfoliated graphene flakes. Hall transport measurements on large area bilayer samples taken as a function of temperature from 4.3 to 300 K revealed an n-type carrier mobility that increased from 1170 to 1730 cm(2) V(-1) s(-1), and a corresponding sheet carrier density that decreased from 5.0 × 10(12) cm(-2) to 3.26 × 10(12) cm(-2). The transport is believed to occur predominantly through the top EG layer with the bottom layer screening the top layer from the substrate. These results demonstrate that EG synthesized on large area, perfectly flat on-axis mesa surfaces can be used to produce Bernal-stacked bilayer EG having excellent uniformity and reduced strain and provides the perfect opportunity for significant advancement of epitaxial graphene electronics technology.
 
We developed an easy, upscalable process to prepare lateral spin-valve devices on epitaxially grown monolayer graphene on SiC(0001) and perform nonlocal spin transport measurements. We observe the longest spin relaxation times τ(S) in monolayer graphene, while the spin diffusion coefficient D(S) is strongly reduced compared to typical results on exfoliated graphene. The increase of τ(S) is probably related to the changed substrate, while the cause for the small value of D(S) remains an open question.
 
Graphene films on SiC exhibit coherent transport properties that suggest the potential for novel carbon-based nanoelectronics applications. Recent studies suggest that the role of the interface between single layer graphene and silicon-terminated SiC can strongly influence the electronic properties of the graphene overlayer. In this study, we have exposed the graphitized SiC to atomic hydrogen in an effort to passivate dangling bonds at the interface, while investigating the results utilizing room temperature scanning tunneling microscopy.
 
The electronic structure of epitaxial monolayer, bilayer, and trilayer graphene on Ru(0001) was determined by selected-area angle-resolved photoelectron spectroscopy (micro-ARPES). Micro-ARPES band maps provide evidence for a strong electronic coupling between monolayer graphene and the adjacent metal, which causes the complete disruption of the graphene pi-bands near the Fermi energy. However, the perturbation by the metal decreases rapidly with the addition of further graphene sheets, and already an epitaxial graphene bilayer on Ru recovers the characteristic Dirac cones of isolated monolayer graphene. A graphene trilayer on Ru behaves like free-standing bilayer graphene. Density-functional theory based calculations show that this decoupling is due to the efficient passivation of metal d-states by the interfacial graphene layer.
 
We have grown well-ordered graphene adlayers on the lattice-matched Co(0001) surface. Low-temperature scanning tunneling microscopy measurements demonstrate an on-top registry of the carbon atoms with respect to the Co(0001) surface. The tunneling conductance spectrum shows that the electronic structure is substantially altered from that of isolated graphene, implying a strong coupling between graphene and cobalt states. Calculations using density functional theory confirm that structures with on-top registry have the lowest energy and provide clear evidence for strong electronic coupling between the graphene pi-states and Co d-states at the interface.
 
Grain boundaries in epitaxial graphene on the SiC(000-1) substrate are studied using scanning tunneling microscopy and spectroscopy. All investigated small-angle grain boundaries show pronounced out-of-plane buckling induced by the strain fields of constituent dislocations. The ensemble of observations allows to determine the critical misorientation angle of buckling transition θC=19±2°. Periodic structures are found among the flat large-angle grain boundaries. In particular, the observed θ=3±2° highly ordered grain boundary is assigned to the previously proposed lowest formation energy structural motif composed of a continuous chain of edge-sharing alternating pentagons and heptagons. This periodic grain boundary defect is predicted to exhibit strong valley filtering of charge carriers thus promising the practical realization of all-electric valleytronic devices.
 
This paper reports a facile synthesis of anatase TiO(2) nanocrystals with exposed, chemically active {001} facets. The nanocrystals were prepared by digesting electrospun nanofibers consisting of amorphous TiO(2) and poly(vinyl pyrrolidone) with an aqueous acetic acid solution (pH = 1.6), followed by hydrothermal treatment at 150 degrees C for 20 h. The as-obtained nanocrystals exhibited a truncated tetragonal bipyramidal shape with 9.6% of the surface being enclosed by {001} facets. The use of electrospinning is critical to the success of this synthesis as it allows for the generation of very small particles of amorphous TiO(2) to facilitate hydrothermal crystallization, an Ostwald ripening process. The morphology of the nanocrystals had a strong dependence on the pH value of the solution used for hydrothermal treatment. Low pH values tended to eliminate the {001} facets by forming sharp corners while high pH values favored the formation of a rodlike morphology through an oriented attachment mechanism. When acetic acid was replaced by inorganic acids, the TiO(2) nanocrystals further aggregated into larger structures with various morphologies.
 
The deposition of coronene molecules from scanning tunneling microscope (STM) tips onto a clean Si(001)-2x1 surface at 25 degrees C was investigated. The STM tips, contaminated with coronene, were found to deposit coronene molecules on the clean Si(001) surface, allowing patterns to be generated. Covalent Si-C chemical bonds, formed between the coronene molecules and the Si substrate, froze the flip-flop motion of the adjacent Si-Si dimers on the substrate. In most cases, the mode of coronene bonding to Si(001) is independent of whether deposition occurs from the gas phase or from the STM tip. Despite the covalent chemical bonds formed between the coronene molecule and the Si substrate, the STM tip can drag the coronene laterally on the Si substrate without inducing a chemical change in the molecule. Sharp spikes observed in the tunneling current during the coronene deposition reflect the abrupt decrease of the tip-substrate distance at the instant of transport of the molecule from tip to surface.
 
We determined the enthalpic and entropic contributions to the thermodynamics of coherently strained nanocrystals grown via deposition of pure Ge on Si(001) surfaces at 600 and 700 degrees C by analyzing their composition profile and local strain. We found that the free energy associated with the entropy of mixing, which drives GexSi1-x alloy formation, was significantly larger than the relaxation enthalpy that produces the islands. Thus, entropy plays a significant role in the evolution of the size and shape of the islands during growth through the strong thermodynamic drive to form an alloy.
 
We investigate the behavior of the island vertical pairing probability in multilayer systems of Ge island quantum dots (QDs) in Si(001). By combining a simple kinetic rate model with our previously reported atomistic simulation results on the nature of the stress field from buried shallow Ge islands having {105}-oriented sidewalls, we derive an analytical expression for correlation probability as a function of the parameters characterizing the multi-QD systems. The approach is based upon continuum mechanochemical potential model, which allows one to introduce necessary elements of the kinetics of island formation in a simple way. We compare the model predictions with available experimental data and find that the model provides a satisfactory description of the coupling probability. The correlation probability behavior as a function of capping layer thickness, Ge island size, interisland distance, and Ge adatom diffusion length is investigated within the framework of the developed model.
 
We find that optical second-harmonic generation (SHG) in reflection from a chemical-vapor-deposition graphene monolayer transferred onto a SiO2/Si(001) substrate is enhanced about 3 times by the flow of dc electric current in graphene. Measurements of rotational-anisotropy SHG revealed that the current-induced SHG from the current-biased graphene/SiO2/Si(001) structure undergoes a phase inversion as the measurement location on graphene is shifted laterally along the current flow direction. The enhancement is due to current-associated charge trapping at the graphene/SiO2 interface, which introduces a vertical electric field across the SiO2/Si interface that produces electric field-induced SHG. The phase inversion is due to the positive-to-negative polarity switch in the current direction of the trapped charges at the current-biased graphene/SiO2 interface.
 
The biconformational switching of single cyclooctadiene molecules chemisorbed on a Si(001) surface was explored by quantum chemical and quantum dynamical calculations and low-temperature scanning tunneling microscopy experiments. The calculations rationalize the experimentally observed switching driven by inelastic electron tunneling (IET) at 5 K. At higher temperatures, they predict a controllable crossover behavior between IET-driven and thermally activated switching, which is fully confirmed by experiment.
 
Ionic liquid gating of three terminal field effect transistor devices with channels formed from SrTiO3(001) single crystals induces a metallic state in the channel. We show that the metallization is strongly affected by the presence of oxygen gas introduced external to the device whereas argon and nitrogen have no effect. The suppression of the gating effect is consistent with electric field induced migration of oxygen that we model by oxygen induced carrier annihilation.
 
Catalyst-free growth of (In)GaN nanowires on (001) silicon substrate by plasma-assisted molecular beam epitaxy is demonstrated. The nanowires with diameter ranging from 10 to 50 nm have a density of 1-2 x 10(11) cm(-2). P- and n-type doping of the nanowires is achieved with Mg and Si dopant species, respectively. Structural characterization by high-resolution transmission electron microscopy (HRTEM) indicates that the nanowires are relatively defect-free. The peak emission wavelength of InGaN nanowires can be tuned from ultraviolet to red by varying the In composition in the alloy and "white" emission is obtained in nanowires where the In composition is varied continuously during growth. The internal quantum efficiency varies from 20-35%. Radiative and nonradiative lifetimes of 5.4 and 1.4 ns, respectively, are obtained from time-resolved photoluminescence measurements at room temperature for InGaN nanowires emitting at lambda = 490 nm. Green- and white-emitting planar LEDs have been fabricated and characterized. The electroluminescence from these devices exhibits negligible quantum confined Stark effect or band-tail filling effect.
 
Molecular beam epitaxy growth of merging InAs nanowire intersections, i.e., a first step towards the realization of a network of such nanowires, is reported. While InAs nanowires play already a leading role in the search for Majorana fermions, a network of these nanowires is expected to promote their exchange and allow for further development of this field. The structural properties of merged InAs nanowire intersections have been investigated using scanning and transmission electron microscope imaging. At the heart of the intersection a sharp change of the crystal structure from wurtzite to perfect zinc blende is observed. The performed low temperature conductance measurements demonstrate that the intersection does not impose an obstacle to current transport.
 
We have developed a process for fabricating monodisperse noble metal/rare earth disilicide core-shell nanoparticles and nanowires in regular arrays on Si(001) with a density of 5 x 10(10) / cm2, and over areas > 1 mm2. Pt deposited via physical vapor deposition on a self-assembled rare earth disilicide nanowire template combined with reactive ion etching produces arrays of nanostructures. SEM images demonstrate the ability to select nanowires or nanoparticles as a function of Pt coverage. Statistical analysis of images of Pt nanoparticle arrays yield a mean feature size of 8 nm with a size variation of +/- 0.9 nm and interparticle spacing of approximately 15 nm.
 
We experimentally investigate the mechanism of formation of self-assembled arrays of nano-islands surrounding dopant sources on the (001) surface of yttria-stabilized zirconia. Initially, we used lithographically defined thin-film patches of gadolinia-doped ceria (GDC) as dopant sources. During annealing at approximately one-half the melting temperature of zirconia, surface diffusion of dopants leads to the breakup of the surface around the source, creating arrays of epitaxial nano-islands with a characteristic size (~100 nm) and alignment along elastically compliant directions, <110>. The breakup relieves elastic strain energy at the expense of increasing surface energy. Based on understanding the mechanism of island formation, we introduce a simple and versatile powder-based doping process for spontaneous surface patterning. The new process bypasses lithography and conventional vapor-source doping, opening the door to spontaneous surface patterning of functional ceramics and other refractory materials. In addition to using GDC solid-solution powders, we demonstrate the effectiveness of the process in another system based on Eu2O3.
 
On-chip optical interconnects still miss a high-performance laser monolithically integrated on silicon. Here, we demonstrate a silicon-integrated InP nano-laser that operates at room temperature with a low threshold of 1.69 pJ, and a large spontaneous emission factor of 0.04. An epitaxial scheme to grow relatively thick InP nanowires on (001) silicon is developed. The zincblende/wurtzite crystal phase polytypism and the formed type II heterostructures are found to promote lasing over a wide wavelength range.
 
In this study, we demonstrate the epitaxial growth of <001 ̅> defect-free zinc-blende structured InAs nanowires on GaAs {111}B substrate using Au catalysts in molecular beam epitaxy. It has been found that the catalysts and their underlying <001 ̅> nanowires have the orientation relationship of {11 ̅03}C//{002 ̅}InAs and [3 ̅302]C//[11 ̅0]InAs due to their small in-plane lattice mismatches between their corresponding lattice spacings perpendicular to the {001 ̅} atomic planes of the nanowires, leading to the formation of the {001 ̅} catalyst/nanowire interfaces, and consequently the formation of <001 ̅> nanowires. This study provides a practical approach to manipulate the crystal structure and structural quality of III-V nanowires through carefully control the crystal phase of the catalysts.
 
The localization of the donor electron wave function can be of key importance in various silicon application, since for example it determines the interactions between neighbouring donors. Interestingly, the physical confinement of the electrons in quasi-one-dimensional nanostructures, like silicon nanowires, noticeably affects this property. Using fully ab-initio calculations, we show that the delocalization of the donor electron wave function along the axis of a nanowire is much greater in [011] oriented nanowires for phosphorus and selenium donors. We also demonstrate that its value can be controlled by applying a compressive or tensile uniaxial strain. Finally, we discuss the implications of these features from both an experimental and a theoretical point of view.
 
The relationship of the gas bubble size to the size distribution critically influences the effectiveness of electrochemical processes. Several optical and acoustical techniques have been used to characterize the size and emission frequency of bubbles. Here, we used zero-dimensional (0D) ion-sensitive field-effect transistors (ISFETs) buried under a microbath to detect the emission of individual bubbles electrically and to generate statistics on the bubble emission time. The bubble size was evaluated via a simple model of the electrolytic current. We suggest that energy lost during water electrolysis could be used to generate electric pulses at an optimal efficiency with an array of 0D ISFETs.
 
Single silicon nanowires (Si-NWs) prepared by electron-beam lithography and reactive-ion etching are investigated by imaging optical spectroscopy under variable temperatures and laser pumping intensities. Spectral images of individual Si-NWs reveal a large variability of photoluminescence (PL) along a single Si-NW. The weaker broad emission band asymmetrically extended to the high-energy side is interpreted to be due to recombination of quasi-free 1D excitons while the brighter localized emission features (with significantly variable peak position, width, and shape) are due to localization of electron-hole pairs in surface protrusions acting like quasi-0D centers or quantum dots (QDs). Correlated PL and scanning electron microscopy images indicate that the efficiently emitting QDs are located at the Si-NW interface with completely oxidized neck of the initial Si wall. Theoretical fitting of the delocalized PL emission band explains its broad asymmetrical band to be due to the Gaussian size distribution of the Si-NW diameter and reveals also the presence of recombination from the Si-NW excited state which can facilitate a fast capture of excitons into QD centers.
 
Controlling the Dirac point of graphene is essential for complementary circuits. Here, we describe the use of 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzoimidazole (o-MeO-DMBI) as a strong n-type dopant for chemical-vapor-deposition (CVD) grown graphene. The Dirac point of graphene can be tuned significantly by spin-coating o-MeO-DMBI solutions on the graphene sheets at different concentrations. The transport of graphene can be changed from p-type to ambipolar and finally n-type. The electron transfer between o-MeO-DMBI and graphene was additionally confirmed by Raman imaging and photoemission spectroscopy (PES) measurements. Finally, we fabricated a complementary inverter via inkjet printing patterning of o-MeO-DMBI solutions on graphene to demonstrate the potential of o-MeO-DMBI n-type doping on graphene for future applications in electrical devices.
 
We implement a method to study transport in a basis of many-body molecular states using the nonequilibrium Hubbard Green's function technique. A well-studied system, a junction consisting of benzene-dithiol on gold, is the focus of our consideration. Electronic structure calculations are carried out at the Hartree-Fock (HF), density functional theory (DFT), and coupled-cluster singles and doubles (CCSD) levels, and multiple molecular states are included in the transport calculation. The conductance calculation yields new information about the transport mechanism in BDT junctions.
 
Selective photoluminescence enhancement of the specific Nd(3+) Stark transition for which laser gain has been obtained in Nd(3+):LiNbO3, is demonstrated by means of a plasmonic resonances with the appropriate symmetry configuration. By using the nonpolar Y-cut of a PPLN crystal as platform for photoreduction of metallic nanostructures, periodically distributed chains of Ag NPs oriented parallel to the ferroelectric c-axis are obtained. This alternative metallic nanostructure configuration supports the resonance between the localized surface plasmon and exclusively the π polarized Stark laser line of Nd(3+) ions at 1.08 μm, while maintaining the remaining crystal field transitions unchanged. The work provides the experimental proof on how plasmonic-based optical antennas can be used to influence selectively rare earth optical transitions to improve the performance of crystalline gain media.
 
Herein, we present the first use of a gallium oxide tunnelling layer to significantly reduce electron recombination in dye-sensitized solar cells (DSC). The subnanometer coating is achieved using atomic layer deposition (ALD) and leading to a new DSC record open-circuit potential of 1.1 V with state-of-the-art organic D-π-A sensitizer and cobalt redox mediator. After ALD of only a few angstroms of Ga(2)O(3), the electron back reaction is reduced by more than an order of magnitude, while charge collection efficiency and fill factor are increased by 30% and 15%, respectively. The photogenerated exciton separation processes of electron injection into the TiO(2) conduction band and the hole injection into the electrolyte are characterized in detail.
 
The compounds, ([SnSe]1.15)1(VSe2)n with n = 1, 2, 3, and 4, were prepared using designed precursors in order to investigate the influence of the thickness of the VSe2 constituent on the charge density wave transition. The structure of each of the compounds was determined using X-ray diffraction and scanning-transmission electron microscopy. The charge density wave transition observed in the resistivity of ([SnSe]1.15)1(VSe2)1 was confirmed. The electrical properties of the n = 2 and 3 compounds are distinctly different. The magnitude of the resistivity change at the transition temperature is dramatically lowered and the temperature of the resistivity minimum systematically increases from 118 K (n = 1) to 172 K (n = 3). For n = 1 this temperature correlates with the onset of the charge density wave transition. The Hall-coefficient changes sign when n is greater than 1, and the temperature dependence of the Hall coefficient of the n = 2 and 3 compounds are very similar to the bulk, slowly decreasing as temperature is decreased, while for the n = 1 compound the Hall coefficient increases dramatically starting at the onset of the charge density wave. The transport properties suggest an abrupt change in electronic properties on increasing the thickness of the VSe2 layer beyond a single layer.
 
Microbial fuel cells (MFCs) are an environmentally friendly method for water purification and self-sustained electricity generation using microorganisms. Microsized MFCs can also be a useful power source for lab-on-a-chip and similar integrated devices. We fabricated a 1.25 μL microsized MFC containing an anode of vertically aligned, forest type multiwalled carbon nanotubes (MWCNTs) with a nickel silicide (NiSi) contact area that produced 197 mA/m(2) of current density and 392 mW/m(3) of power density. The MWCNTs increased the anode surface-to-volume ratio, which improved the ability of the microorganisms to couple and transfer electrons to the anode. The use of nickel silicide also helped to boost the output current by providing a low resistance contact area to more efficiently shuttle electrons from the anode out of the device.
 
Obtaining high power density at low operating temperatures has been an ongoing challenge in solid oxide fuel cells (SOFC), which are efficient engines to generate electrical energy from fuels. Here we report successful demonstration of a thin-film 3-dimensional SOFC architecture achieving a peak power density of 1.3 W/cm2 obtained at 450 °C. This is made possible by nanostructuring of the ultra-thin (60 nm) electrolyte interposed with a nano-granular catalytic interlayer at the cathode/electrolyte interface. We attribute the superior cell performance to significant reduction in both the ohmic and polarization losses due to the combined effects of employing an ultra thin film electrolyte, enhancement of effective area by 3-D architecture and superior catalytic activity by the ceria-based interlayer at the cathode. These insights will help design high-efficiency SOFCs that operate at low temperatures with power densities that are of practical significance.
 
The gate-controllability of the Fermi-edge onset of interband absorption in graphene can be utilized to modulate near-infrared radiation in the telecommunication band. However, a high modulation efficiency has not been demonstrated to date, because of the small amount of light absorption in graphene. Here, we demonstrate a ~40% amplitude modulation of 1.55 µm radiation with gated single-layer graphene that is coupled with a silicon micro-ring resonator. Both the quality factor and resonance wavelength of the silicon micro-ring resonator were strongly modulated through gate tuning of the Fermi level in graphene. These results promise an efficient electro-optic modulator, ideal for applications in large-scale on-chip optical interconnects that are compatible with complementary metal-oxide-semiconductor technology.
 
Energy harvesting technologies that are engineered to miniature sizes, while still increasing the power delivered to wireless electronics, (1, 2) portable devices, stretchable electronics, (3) and implantable biosensors, (4, 5) are strongly desired. Piezoelectric nanowire- and nanofiber-based generators have potential uses for powering such devices through a conversion of mechanical energy into electrical energy. (6) However, the piezoelectric voltage constant of the semiconductor piezoelectric nanowires in the recently reported piezoelectric nanogenerators (7-12) is lower than that of lead zirconate titanate (PZT) nanomaterials. Here we report a piezoelectric nanogenerator based on PZT nanofibers. The PZT nanofibers, with a diameter and length of approximately 60 nm and 500 microm, were aligned on interdigitated electrodes of platinum fine wires and packaged using a soft polymer on a silicon substrate. The measured output voltage and power under periodic stress application to the soft polymer was 1.63 V and 0.03 microW, respectively.
 
We present the wafer-scale fabrication of self-catalyzed p-n homojunction 1.7-eV GaAsP core-shell nanowire photocathodes grown on silicon substrates by molecular beam epitaxy with the incorporation of Pt nanoparticles as hydrogen evolution co-catalysts. Under AM 1.5G illumination, the GaAsP nanowire photocathode yielded a photocurrent density of 4.5 mA/cm2 at 0 V versus reversible hydrogen electrode and a solar-to-hydrogen conversion efficiency of 0.5%, which are much higher than the values previously reported for wafer-scale III-V nanowire photocathode. In addition, GaAsP has been found to be more resistant to photocorrosion than InGaP. These results open up a new approach to develop efficient tandem photoelectrochemical devices via fabricating GaAsP nanowires on silicon platform.
 
Graphene-like two-dimensional (2D) materials, not only are interesting for their exotic electronic structure and fundamental electronic transport or optical properties but also, hold promises for device miniaturization down to atomic thickness. As one material belonging to this category, InSe, a III-VI semiconductor, is not only a promising candidate for optoelectronic devices but also has potential for ultrathin field effect transistor (FET) with high mobility transport. In this work, various substrates such as PMMA, bare silicon oxide, passivated silicon oxide, and silicon nitride were used to fabricate multi-layer InSe FET devices. Through back gating and Hall measurement in four-probe configuration, the devices' field effect mobility and intrinsic Hall mobility were extracted at various temperatures to study the material's intrinsic transport behavior and the effect of dielectric substrate. The sample's field effect and Hall mobilities over the range of 20-300K fall in the range of 0.1-2.0×10^3 cm^2/Vs, which are comparable or better than the state of the art FETs made of widely studied 2D transition metal-dichalcogenides.
 
Here we report the formation of high-performance and high-capacity lithium-ion battery anodes from high density germanium nanowire arrays grown directly from the current collector. The anodes retain capacities of ∼ 900 mAh/g after 1100 cycles with excellent rate performance characteristics, even at very high discharge rates of 20-100C. We show by an ex-situ HRTEM and HRSEM study that this performance can be attributed to the complete restructuring of the nanowires that occurs within the first 100 cycles to form a continuous porous network that is mechanically robust. This restructured anode, once formed, retains a remarkably stable capacity with a drop of only 0.01% per cycle thereafter. As this approach encompasses a low energy processing method where all the material is electrochemically active and binder free, the extended cycle life and rate performance characteristics demonstrated makes these anodes highly attractive for the most demanding lithium-ion applications such as long range battery electric vehicles.
 
Arrays of plasmonic nanocavities with very low volumes, down to λ(3)/1000, have been fabricated by soft UV nanoimprint lithography. Nearly perfect omnidirectional absorption (3-70°) is demonstrated for the fundamental mode of the cavity (λ ≃ 1.15 μm). The second-order mode exhibits a sharper resonance with strong angular dependence and total optical absorption when the critical coupling condition is fulfilled (45-50°, λ ≃ 750 nm). It leads to high refractive index sensitivity (405 nm/RIU) and figure of merit (∼21) and offers new perspectives for efficient biosensing experiments in ultralow volumes.
 
Relaxation is a most basic structural behavior of free surfaces, however, direct observation of surface relaxation remains challenging in atomic-scale. Herein, single-crystalline nanoislands formed in situ on ZnO nanowires and nanobelts are characterized using aberration-corrected transmission electron microscopy combined with ab initio calculations. For the first time, displacements of both Zn and O atoms in the fresh (10 ̅10) facets are quantified to accuracies of several picometers and the under-surface distributions of contractions and rotations of Zn-O bonds are directly measured, which unambiguously verify the theoretically predicted relaxation of ZnO (10 ̅10) free surfaces. Finally, the surface relaxation is directly correlated with the size effects of electromechanical properties (e.g., elastic modulus and spontaneous polarization) in ZnO nanowires.
 
We report detailed ab initio calculations of poly(3-hexylthiophene) (P3HT) on top of a ZnO (1010) surface. We studied different absorption sites and orientations. We found that the P3HT chain prefers to lay along the dimer row direction of the ZnO surface. We also found strong coupling between the P3HT molecule and the ZnO substrate in the conduction band states, while minimum coupling in the valence band states.
 
Hierarchical structures consisting of micropyramids and nanowires are demonstrated in Si/PEDOT:PSS hybrid solar cells to achieve a power conversion efficiency up to 11.50% with excellent omnidirectionality. The structure provides a combined concepts of superior light trapping ability, significant increase of p-n junction areas, and short carrier diffusion distance, improving the photovoltaic characteristics including JSC, FF, and PCE. The enhancement of power generation is up to 253.8% at high incident angles, showing the outstanding omnidirectional operation ability of hybrid cells with hierarchical Si surfaces. This properly designed hierarchical-structured device paves a promising way for developing low cost, high efficiency, and practical solar applications.
 
Palladium nanoparticles supported on rutile TiO(2)(110)-1 x 1 have been studied using the complementary techniques of scanning tunneling microscopy and X-ray photoemission electron microscopy. Two distinct types of palladium nanoparticles are observed, namely long nanowires up to 1000 nm long, and smaller dotlike features with diameters ranging from 80-160 nm. X-ray photoemission electron microscopy reveals that the nanoparticles are composed of metallic palladium, separated by the bare TiO(2)(110) surface.
 
Dicarboxystilbene, a molecule that becomes chiral in the adsorbed state through the loss of its improper axis of rotation, forms long-range "handed" structures when adsorbed on Cu(110) as revealed by scanning tunnelling microscopy. We show that these structures are created from chiral "adsorption complex" building blocks, giving rise to a complete set of racemic and enantiomerically pure structural assemblies. We interpret the formation of these structures in terms of a balance between hydrogen bond mediated intermolecular interactions and the adsorbate-surface structural relationship and discuss the reasons for temperature-induced conversion from the metastable enantiomerically pure to the racemic structure.
 
The metal-assisted etching direction of Si(110) substrates was found to be dependent upon the morphology of the deposited metal catalyst. The etching direction of a Si(110) substrate was found to be one of the two crystallographically preferred 100 directions in the case of isolated metal particles or a small area metal mesh with nanoholes. In contrast, the etching proceeded in the vertical [110] direction, when the lateral size of the catalytic metal mesh was sufficiently large. Therefore, the direction of etching and the resulting nanostructures obtained by metal-assisted etching can be easily controlled by an appropriate choice of the morphology of the deposited metal catalyst. On the basis of this finding, a generic method was developed for the fabrication of wafer-scale vertically aligned arrays of epitaxial [110] Si nanowires on a Si(110) substrate. The method utilized a thin metal film with an extended array of pores as an etching catalyst based on an ultrathin porous anodic alumina mask, while a prepatterning of the substrate prior to the metal depostion is not necessary. The diameter of Si nanowires can be easily controlled by a combination of the pore diameter of the porous alumina film and varying the thickness of the deposited metal film.
 
Top-cited authors
Yi Cui
  • Stanford University
Zhong Lin Wang
  • Georgia Institute of Technology
Jing Kong
  • Massachusetts Institute of Technology
Rodney Ruoff
  • Ulsan National Institute of Science and Technology
Naomi J Halas
  • Rice University