Uphill Diffusion Induced Point Contact Reaction in Si Nanowires.

The events of repeating nucleation in point contact reactions between nanowires of Si and Ni or Co have been revisited here due to uphill diffusion as well as an extremely high supersaturation, over a factor of 1000, needed for the nucleation. Also what is the diameter of the point contact needs to be defined. The stepwise growth of nanoscale epitaxial silicide can occur because the repeating nucleation events are restricted in nanoscale wires.

Distinct Carrier Transport Properties Across Horizontally vs Vertically Oriented Heterostructures of 2D/3D Perovskites.

Two-dimensional-on-three-dimensional (2D/3D) halide perovskite heterostructures have been extensively utilized in optoelectronic devices. However, the labile nature of halide perovskites makes it difficult to form such heterostructures with well-defined compositions, orientations, and interfaces, which inhibits understanding of the carrier transfer properties across these heterostructures. Here, we report solution growth of both horizontally and vertically aligned 2D perovskite (PEA)2PbBr4 (PEA = phenylethylammonium) microplates onto 3D CsPbBr3 single crystal thin films, with well-defined heterojunctions. Time-resolved photoluminescence (TRPL) transients of the heterostructures exhibit the monomolecular and bimolecular dynamics expected from exciton annihilation, dissociation, and recombination, as well as evidence for carrier transfer in these heterostructures. Two kinetic models based on Type-I and Type-II band alignments at the interface of horizontal 2D/3D heterostructures are applied to reveal a shift in balance between carrier transfer and recombination: Type-I band alignment better describes the behaviors of heterostructures with thin 2D perovskite microplates but Type-II band alignment better describes those with thick 2D microplates (>150 nm). TRPL of vertically aligned 2D microplates is dominated by directly excited PL and is independent of the height above the 3D film. Electrical measurements reveal current rectification behaviors in both heterostructures with vertical heterostructures showing better electrical transport. As the first systematic study on comparing models of 2D/3D perovskite heterostructures with controlled orientations and compositions, this work provides insights on the charge transfer mechanisms in these perovskite heterostructures and guidelines for designing better optoelectronic devices.

Facet-Dependent Surface Trap States and Carrier Lifetimes of Silicon.

The facet-dependent electrical conductivity properties of silicon wafers result from significant band structure differences and variations in bond length, bond geometry, and frontier orbital electron distribution between the metal-like and semiconducting planes of silicon. To further understand the emergence of conductivity facet effects, electrochemical impedance measurements were conducted on intrinsic Si {100}, {110}, and {111} wafers. The attempt-to-escape frequency, obtained from temperature-dependent capacitance versus applied frequency curves, and other parameters derived from typical semiconductor property measurements were used to construct a diagram of the trap energy level (Et) and the amount of trap states Nt(Et). The trap states are located 0.61-0.72 eV above the silicon conduction band. Compared to {100} and {110} wafers, Si {111} wafer shows far less densities of trap states over the range of -0.2 to 2 V. Since these trap states inhibit direct electron excitation to the conduction band, the {111} wafer having much fewer trap states presents the best electrical conductivity property. Impedance data also provide facet-specific carrier lifetimes. The {111} surface gives consistently the lowest carrier lifetime, which reflects its high electrical conductivity. Lastly, ultraviolet photoelectron spectra and diffuse reflectance spectra were taken to obtain Schottky barriers between Ag and contacting Si wafers. The most conductive {111} surface presenting the largest Schottky barrier means the degrees of surface band bending used to explain facet-dependent electrical behaviors cannot be reliably attained this way.

In Situ Investigation of Defect-Free Copper Nanowire Growth.

The fabrication and placement of high purity nanometals, such as one-dimensional copper (Cu) nanowires, for interconnection in integrated devices have been among the most important technological developments in recent years. Structural stability and oxidation prevention have been the key issues, and the defect control in Cu nanowire growth has been found to be important. Here, we report the synthesis of defect-free single-crystalline Cu nanowires by controlling the surface-assisted heterogeneous nucleation of Cu atomic layering on the graphite-like loop of an amorphous carbon (a-C) lacey film surface. Without a metal-catalyst or induced defects, the high quality Cu nanowires formed with high aspect ratio and high growth rate of 578 nm/s. The dynamic study of the growth of heterogeneous nanowires was conducted in situ with a high-resolution transmission electron microscope. The study illuminates the new mechanism by heterogeneous nucleation control and laying the groundwork for better understanding of heterosurface-assisted nucleation of defect-free Cu nanowire on a-C lacey film.

Room-temperature ferromagnetic Cr-doped Ge/GeOx core-shell nanowires.

The Cr-doped tunable thickness core-shell Ge/GeOx nanowires (NWs) were synthesized and characterized using x-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy and magnetization studies. The shell thickness increases with the increase in synthesis temperature. The presence of metallic Cr and Cr3+ in core-shell structure was confirmed from XPS study. The magnetic property is highly sensitive to the core-shell thickness and intriguing room temperature ferromagnetism is realized only in core-shell NWs. The magnetization decreases with an increase in shell thickness and practically ceases to exist when there is no core. These NWs show remarkably high Curie temperature (TC > 300 K) with the dominating values of its magnetic remanence (MR) and coercivity (HC) compared to germanium dilute magnetic semiconductor nanomaterials. We believe that our finding on these Cr-doped Ge/GeOX core-shell NWs has the potential to be used as a hard magnet for future spintronic devices, owing to their higher characteristic values of ferromagnetic ordering.

Germanium Wafers Possessing Facet-Dependent Electrical Conductivity Properties.

Electrical conductivity properties of Ge {100}, {110}, {111}, and {211} facets have been measured by breaking Ge (100) and (111) wafers to expose {110} and {211} surfaces and contacting the different facets with tungsten probes. Ge {111} and {211} faces are far more conductive than the already conductive Ge {100} and {110} faces, matching with recent density functional theory (DFT) predictions. Asymmetric I-V curves resembling those of p-n junctions have been collected for the {110}/{111} and {110}/{211} facet combinations. The current-rectifying effects stem from different degrees of surface band bending for the highly and less conductive faces and the direction of current flow. This work demonstrates that germanium wafers also possess facet-dependent electrical conductivity responses that can be utilized in the fabrication of novel fin field-effect transistors (finFET).

Efficiency Enhancement of Silicon Heterojunction Solar Cells via Photon Management Using Graphene Quantum Dot as Downconverters.

By employing graphene quantum dots (GQDs), we have achieved a high efficiency of 16.55% in n-type Si heterojunction solar cells. The efficiency enhancement is based on the photon downconversion phenomenon of GQDs to make more photons absorbed in the depletion region for effective carrier separation, leading to the enhanced photovoltaic effect. The short circuit current and the fill factor are increased from 35.31 to 37.47 mA/cm(2) and 70.29% to 72.51%, respectively. The work demonstrated here holds the promise for incorporating graphene-based materials in commercially available solar devices for developing ultrahigh efficiency photovoltaic cells in the future.

Silicon Wafers with Facet-Dependent Electrical Conductivity Properties.

By breaking intrinsic Si (100) and (111) wafers to expose sharp {111} and {112} facets, electrical conductivity measurements on single and different silicon crystal faces were performed through contacts with two tungsten probes. While Si {100} and {110} faces are barely conductive at low applied voltages, as expected, the Si {112} surface is highly conductive and Si {111} surface also shows good conductivity. Asymmetrical I-V curves have been recorded for the {111}/{112}, {111}/{110}, and {112}/{110} facet combinations because of different degrees of conduction band bending at these crystal surfaces presenting different barrier heights to current flow. In particular, the {111}/{110} and {112}/{110} facet combinations give I-V curves resembling those of p-n junctions, suggesting a novel field effect transistor design is possible capitalizing on the pronounced facet-dependent electrical conductivity properties of silicon.

Facet-dependent electrical conductivity properties of Cu2O crystals.

It is interesting to examine facet-dependent electrical properties of single Cu2O crystals, because such study greatly advances our understanding of various facet effects exhibited by semiconductors. We show a Cu2O octahedron is highly conductive, a cube is moderately conductive, and a rhombic dodecahedron is nonconductive. The conductivity differences are ascribed to the presence of a thin surface layer having different degrees of band bending. When electrical connection was made on two different facets of a rhombicuboctahedron, a diode-like response was obtained, demonstrating the potential of using single polyhedral nanocrystals as functional electronic components. Density of state (DOS) plots for three layers of Cu2O (111), (100), and (110) planes show respective metallic, semimetal, and semiconducting band structures. By examining DOS plots for varying number of planes, the surface layer thicknesses responsible for the facet-dependent electrical properties of Cu2O crystals have been determined to be below 1.5 nm for these facets.

Surfactant-directed fabrication of supercrystals from the assembly of polyhedral Au-Pd core-shell nanocrystals and their electrical and optical properties.

Au-Pd core-shell nanocrystals with cubic, truncated cubic, cuboctahedral, truncated octahedral, and octahedral structures have been employed to form micrometer-sized polyhedral supercrystals by both the droplet evaporation method and novel surfactant diffusion methods. Observation of cross-sectional samples indicates shape preservation of interior nanocrystals within a supercrystal. Low-angle X-ray diffraction techniques and electron microscopy have been used to confirm the presence of surfactant between contacting nanocrystals. By diluting the nanocrystal concentration or increasing the solution temperature, supercrystal size can be tuned gradually to well below 1 µm using the surfactant diffusion method. Rectangular supercrystal microbars were obtained by increasing the amounts of cubic nanocrystals and surfactant used. Au-Ag core-shell cubes and PbS cubes with sizes of 30-40 nm have also been fabricated into supercrystals, showing the generality of the surfactant diffusion approach to form supercrystals with diverse composition. Electrical conductivity measurements on single Au-Pd supercrystals reveal loss of metallic conductivity due to the presence of insulating surfactant. Cubic Au-Pd supercrystals show infrared absorption at 3.2 µm due to extensive plasmon coupling. Mie-type resonances centered at 9.8 µm for the Au-Pd supercrystals disappear once the Pd shells are converted into PdH after hydrogen absorption.

Direct Observation of Sublimation Behaviors in One-Dimensional In2Se3/In2O3 Nanoheterostructures.

Recently, in situ transmission electron microscopy (TEM) has provided a route to analyze structural characterization and chemical evolution with its powerful and unique applications. In this paper, we disclose the detailed phenomenon of sublimation on the atomic scale. In2Se3/In2O3 nanowires were synthesized via the vapor-liquid-solid mechanism and studied in an ultra-high-vacuum (UHV) TEM at high temperature in real time. During in situ observation of the sublimation process of the nanowires, the evolution and reconstruction of the exposed In2Se3 surface progressed in different manners with time. The surface structure was decomposed by mass-desorption and stepwise-migration processes, which are also energetically favored processes in the ab initio calculation. This study developed a new concept and will be essential in the development of atomic kinetics.

Complete replacement of metal in metal oxide nanowires via atomic diffusion: In/ZnO case study.

Atomic diffusion is a fundamental process that dictates material science and engineering. Direct visualization of atomic diffusion process in ultrahigh vacuum in situ TEM could comprehend the fundamental information about metal-semiconductor interface dynamics, phase transitions, and different nanostructure growth phenomenon. Here, we demonstrate the in situ TEM observations of the complete replacement of ZnO nanowire by indium with different growth directions. In situ TEM analyses reveal that the diffusion processes strongly depend and are dominated by the interface dynamics between indium and ZnO. The diffusion exhibited a distinct ledge migration by surface diffusion at [001]-ZnO while continuous migration with slight/no ledges by inner diffusion at [100]-ZnO. The process is explained based on thermodynamic evaluation and growth kinetics. The results present the potential possibilities to completely replace metal-oxide semiconductors with metal nanowires without oxidation and form crystalline metal nanowires with precise epitaxial metal-semiconductor atomic interface. Formation of such single crystalline metal nanowire without oxidation by diffusion to the metal oxide is unique and is crucial in nanodevice performances, which is rather challenging from a manufacturing perspective of 1D nanodevices.

Electric-field control of ferromagnetism in Mn-doped ZnO nanowires.

In this Letter, the electric-field control of ferromagnetism was demonstrated in a back-gated Mn-doped ZnO (Mn-ZnO) nanowire (NW) field-effect transistor (FET). The ZnO NWs were synthesized by a thermal evaporation method, and the Mn doping of 1 atom % was subsequently carried out in a MBE system using a gas-phase surface diffusion process. Detailed structural analysis confirmed the single crystallinity of Mn-ZnO NWs and excluded the presence of any precipitates or secondary phases. For the transistor, the field-effect mobility and n-type carrier concentration were estimated to be 0.65 cm(2)/V·s and 6.82 × 10(18) cm(-3), respectively. The magnetic hysteresis curves measured under different temperatures (T = 10-350 K) clearly demonstrate the presence of ferromagnetism above room temperature. It suggests that the effect of quantum confinements in NWs improves Tc, and meanwhile minimizes crystalline defects. The magnetoresistace (MR) of a single Mn-ZnO NW was observed up to 50 K. Most importantly, the gate modulation of the MR ratio was up to 2.5 % at 1.9 K, which implies the electric-field control of ferromagnetism in a single Mn-ZnO NW.

All-color plasmonic nanolasers with ultralow thresholds: autotuning mechanism for single-mode lasing.

We report on the first demonstration of broadband tunable, single-mode plasmonic nanolasers (spasers) emitting in the full visible spectrum. These nanolasers are based on a single metal-oxide-semiconductor nanostructure platform comprising of InGaN/GaN semiconductor nanorods supported on an Al2O3-capped epitaxial Ag film. In particular, all-color lasing in subdiffraction plasmonic resonators is achieved via a novel mechanism based on a property of weak size dependence inherent in spasers. Moreover, we have successfully reduced the continuous-wave (CW) lasing thresholds to ultrasmall values for all three primary colors and have clearly demonstrated the possibility of "thresholdless" lasing for the blue plasmonic nanolaser.

Electron field emission enhancement of vertically aligned ultrananocrystalline diamond-coated ZnO core-shell heterostructured nanorods.

Enhanced electron field emission (EFE) behavior of a core-shell heterostructure, where ZnO nanorods (ZNRs) form the core and ultrananocrystalline diamond needles (UNCDNs) form the shell, is reported. EFE properties of ZNR-UNCDN core-shell heterostructures show a high emission current density of 5.5 mA cm(-2) at an applied field of 4.25 V µm(-1) , and a low turn-on field of 2.08 V µm(-1) compared to the 1.67 mA cm(-2) emission current density (at an applied field of 28.7 V µm(-1) ) and 16.6 V µm(-1) turn-on field for bare ZNRs. Such an enhancement in the field emission originates from the unique materials combination, resulting in good electron transport from ZNRs to UNCDNs and efficient field emission of electrons from the UNCDNs. The potential application of these materials is demonstrated by the plasma illumination measurements that lowering the threshold voltage by 160 V confirms the role of ZNR-UNCDN core-shell heterostructures in the enhancement of electron emission.

Au nanocrystal array/silicon nanoantennas as wavelength-selective photoswitches.

Au nanocrystal array/silicon nanoantennas exhibiting wavelength-selective photocurrent enhancement were successfully fabricated by a facile and inexpensive method combining colloidal lithography (CL) and a metal-assisted chemical etching (MaCE) process. The localized surface plasmon resonance (LSPR) response and wavelength-selective photocurrent enhancement characteristics were achieved by tuning the depth of immersion of Au nanocrystal arrays in silicon through a MaCE process. The wavelength selectivity of photocurrent enhancement contributed by LSPR induced local field amplification was confirmed by simulated near-field distribution. In addition, it can be integrated to well-developed Si-based manufacturing process. These characteristics make Au nanocrystal array/Si nanoantennas promising as low power-consumption photoswitches and nano-optoelectronic and photonic communication devices.

Dynamic evolution of conducting nanofilament in resistive switching memories.

Resistive random access memory (ReRAM) has been considered the most promising next-generation nonvolatile memory. In recent years, the switching behavior has been widely reported, and understanding the switching mechanism can improve the stability and scalability of devices. We designed an innovative sample structure for in situ transmission electron microscopy (TEM) to observe the formation of conductive filaments in the Pt/ZnO/Pt structure in real time. The corresponding current-voltage measurements help us to understand the switching mechanism of ZnO film. In addition, high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) have been used to identify the atomic structure and components of the filament/disrupted region, determining that the conducting paths are caused by the conglomeration of zinc atoms. The behavior of resistive switching is due to the migration of oxygen ions, leading to transformation between Zn-dominated ZnO(1-x) and ZnO.

Electrical spin injection and detection in Mn5Ge3/Ge/Mn5Ge3 nanowire transistors.

In this Letter, we report the electrical spin injection and detection in Ge nanowire transistors with single-crystalline ferromagnetic Mn5Ge3 as source/drain contacts formed by thermal reactions. Degenerate indium dopants were successfully incorporated into as-grown Ge nanowires as p-type doping to alleviate the conductivity mismatch between Ge and Mn5Ge3. The magnetoresistance (MR) of the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor was found to be largely affected by the applied bias. Specifically, negative and hysteretic MR curves were observed under a large current bias in the temperature range from T = 2 K up to T = 50 K, which clearly indicated the electrical spin injection from ferromagnetic Mn5Ge3 contacts into Ge nanowires. In addition to the bias effect, the MR amplitude was found to exponentially decay with the Ge nanowire channel length; this fact was explained by the dominated Elliot-Yafet spin-relaxation mechanism. The fitting of MR further revealed a spin diffusion length of lsf = 480 ± 13 nm and a spin lifetime exceeding 244 ps at T = 10 K in p-type Ge nanowires, and they showed a weak temperature dependence between 2 and 50 K. Ge nanowires showed a significant enhancement in the measured spin diffusion length and spin lifetime compared with those reported for bulk p-type Ge. Our study of the spin transport in the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor points to a possible realization of spin-based transistors; it may also open up new opportunities to create novel Ge nanowire-based spintronic devices. Furthermore, the simple fabrication process promises a compatible integration into standard Si technology in the future.

Influence of catalyst choices on transport behaviors of InAs NWs for high-performance nanoscale transistors.

The influence of the catalyst materials on the electron transport behaviors of InAs nanowires (NWs) grown by a conventional vapor transport technique is investigated. Utilizing the NW field-effect transistor (FET) device structure, ~20% and ~80% of Au-catalyzed InAs NWs exhibit strong and weak gate dependence characteristics, respectively. In contrast, ~98% of Ni-catalyzed InAs NWs demonstrate a uniform n-type behavior with strong gate dependence, resulting in an average OFF current of ~10(-10) A and a high I(ON)/I(OFF) ratio of >10(4). The non-uniform device performance of Au-catalyzed NWs is mainly attributed to the non-stoichiometric composition of the NWs grown from a different segregation behavior as compared to the Ni case, which is further supported by the in situ TEM studies. These distinct electrical characteristics associated with different catalysts were further investigated by the first principles calculation. Moreover, top-gated and large-scale parallel-array FETs were fabricated with Ni-catalyzed NWs by contact printing and channel metallization techniques, which yield excellent electrical performance. The results shed light on the direct correlation of the device performance with the catalyst choice.

Kinetic competition model and size-dependent phase selection in 1-D nanostructures.

The first phase selection and the phase formation sequence between metal and silicon (Si) couples are indispensably significant to microelectronics. With increasing scaling of device dimension to nano regime, established thermodynamic and kinetic models in bulk and thin film fail to apply in 1-D nanostructures. Herein, we present an unique size-dependent first phase formation sequence in 1-D nanostructures, with Ni-Si as the model system. Interfacial-limited phase which forms the last in thin film, NiSi(2), appears as the dominant first phase at 300-800 °C due to the elimination of continuous grain boundaries in 1-D silicides. On the other hand, θ-Ni(2)Si, the most competitive diffusion-limited phase takes over NiSi(2) and wins out as the first phase in small diameter nanowires at 800 °C. Kinetic parameters extracted from in situ transmission electron microscope studies and a modified kinetic growth competition model quantitatively explain this observation. An estimated critical diameter from the model agrees reasonably well with observations.