Single-crystalline Nanostructures Research Articles

Optical Properties of Low-Loss Ag Films and Nanostructures on Transparent Substrates

We demonstrate the fabrication of a low-loss single-crystalline Ag nanostructure deposited on transparent substrates. Our approach is based on an epitaxial growth technique in which a NaCl(001) substrate is used. The NaCl substrate is dissolved in water to allow the Ag film to be transferred onto the desired substrates. Focused ion beam milling is subsequently employed to pattern a nanoarray structure consisting of 200 nanorods. The epitaxial Ag films with nanoarray structures grown in the study exhibited very flat and smooth surfaces having excellent crystallinity and local misorientation of less than 1°. Further, spectroscopic ellipsometry measurements indicated that the imaginary part of the dielectric constant of the single-crystalline film was smaller than that of a conventional polycrystalline film. Moreover, we used the three-dimensional finite-difference time-domain method to analyze the plasmonic properties of the nanoarray structure by considering the actual processed structure. Characteristically, when the SiO2 substrate was etched by ion beam milling to a depth of 30 nm, the spectrum showed a spectral shape 20% sharper than that of the substrate with no etching (depth: 0 nm). The plasmonic performance of the single-crystalline Ag nanostructure was largely determined by its structural precision and the dielectric properties of the metal.

General Strategy for Rapid Production of Low-Dimensional All-Inorganic CsPbBr3 Perovskite Nanocrystals with Controlled Dimensionalities and Sizes.

Currently, all-inorganic CsPbX3 (X = Br, I, Cl) perovskite nanocrystals (NCs) are shining stars with exciting potential applications in optoelectronic devices such as solar cells, light-emitting diodes, lasers, and photodetectors, due to their superior performance in comparison to their organic-inorganic hybrid counterparts. In the present work, we report a general strategy based on a microwave technique for the rapid production of low-dimensional all-inorganic CsPbBr3 perovskite NCs with tunable morphologies within minutes. The effect of the key parameters such as the introduced ligands, solvents, and PbBr2 precursors and microwave powers as well as the irradiation times on the production of perovskite NCs was systematically investigated, which allowed their growth with tunable dimensionalities and sizes. As a proof of concept, the ratio of OA to OAm as well as the concentration of PbBr2 precursor played important roles in triggering the anisotropic growth of the perovskite NCs, favoring their growth into 1D/2D single-crystalline nanostructures. Meanwhile, their sizes could be tailored by controlling the microwave powers and irradiation times. The mechanism for the tunable growth of perovskite NCs is discussed.

Nanotransplantation Printing of Crystallographic-Orientation-Controlled Single-Crystalline Nanowire Arrays on Diverse Surfaces.

The fabrication of a highly ordered array of single-crystalline nanostructures prepared from solution-phase or vapor-phase synthesis methods is extremely challenging due to multiple difficulties of spatial arrangement and control of crystallographic orientation. Herein, we introduce a nanotransplantation printing (NTPP) technique for the reliable fabrication, transfer, and arrangement of single-crystalline Si nanowires (NWs) on diverse substrates. NTPP entails (1) formation of nanoscale etch mask patterns on conventional low-cost Si via nanotransfer printing, (2) two-step combinatorial plasma etching for defining Si NWs, and (3) detachment and transfer of the NWs onto various receiver substrates using an infiltration-type polymeric transfer medium and a solvent-assisted adhesion switching mechanism. Using this approach, high-quality, highly ordered Si NWs can be formed on almost any type of surface including flexible plastic substrates, biological surfaces, and deep-trench structures. Moreover, NTPP provides controllability of the crystallographic orientation of NWs, which is confirmed by the successful generation of (100)- and (110)-oriented Si NWs with different properties. The outstanding electrical properties of the NWs were confirmed by fabricating and characterizing Schottky junction field-effect transistors. Furthermore, exploiting the highly flexible nature of the NWs, a high-performance piezoresistive strain sensor, with a high gauge factor over 200 was realized.

Surfactant-Free Vapor-Phase Synthesis of Single-Crystalline Gold Nanoplates for Optimally Bioactive Surfaces

We report the surfactant-free vapor-phase synthesis of atomically flat and ultraclean gold nanoplates. These gold nanoplates can offer optimally functional surfaces through the immobilization of molecules without unspecific adsorption and defect, which could be quite valuable for diverse applications including biomedical sensing, plasmonics, molecular electronics, electrochemistry, etc. The ultraflat, ultraclean, and single-crystalline nanostructures, including gold nanoparticles (NPs), gold nanowires (NWs), gold nanobelts, and gold nanoplates, are stereoepitaxially grown on a substrate with a specific orientation. Moreover, the nanostructures can be selectively synthesized by experimental conditions and locations of the substrate. The geometry and orientation of the nanostructures show strong correlations, suggesting a growth process of seed NPs → NWs → nanobelts → nanoplates. This synthetic process can be explained by the mechanism in which the height-to-width ratio of gold nanostructures is determined ...

Vapor-phase hydrothermal growth of single crystalline NiS2 nanostructure film on carbon fiber cloth for electrocatalytic oxidation of alcohols to ketones and simultaneous H2 evolution

Electrocatalytic synthesis of value-added chemicals is attracting significant research attention owing to its mild reaction conditions, environmental benignity, and potentially scalable application to organic synthetic chemistry. Herein, we report the preparation of a single-crystalline NiS2 nanostructure film of ∼ 50 nm thickness grown directly on a carbon fiber cloth (NiS2/CFC) by a facile vapor-phase hydrothermal (VPH) method. NiS2/CFC as an electrocatalyst exhibits activity for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in alkaline media. Furthermore, a series of alcohols (2-propanol, 2-butanol, 2-pentanol, and cyclohexanol) were electrocatalytically converted to the corresponding ketones with high selectivity, efficiency, and durability using the NiS2/CFC electrode in alkaline media. In the presence of 0.45 M alcohol, a remarkably decreased overpotential (∼ 150 mV, vs. RHE) at the NiS2/CFC anode compared with that for water oxidation to generate O2, i.e., the OER, in alkaline media leads to significantly improved H2 generation. For instance, the H2 generation rate in the presence of 0.45 M 2-propanol is almost 1.2-times of that obtained for pure water splitting, but in a system that employs an applied voltage at least 280 mV lower than that required for water splitting to achieve the same current density (20 mA·cm–2). Thus, our results demonstrate the applicability of our bifunctional non-precious-metal electrocatalyst for organic synthesis and simultaneous H2 production.

Highly tensile-strained Ge/InAlAs nanocomposites

Self-assembled nanocomposites have been extensively investigated due to the novel properties that can emerge when multiple material phases are combined. Growth of epitaxial nanocomposites using lattice-mismatched constituents also enables strain-engineering, which can be used to further enhance material properties. Here, we report self-assembled growth of highly tensile-strained Ge/In0.52Al0.48As (InAlAs) nanocomposites by using spontaneous phase separation. Transmission electron microscopy shows a high density of single-crystalline germanium nanostructures coherently embedded in InAlAs without extended defects, and Raman spectroscopy reveals a 3.8% biaxial tensile strain in the germanium nanostructures. We also show that the strain in the germanium nanostructures can be tuned to 5.3% by altering the lattice constant of the matrix material, illustrating the versatility of epitaxial nanocomposites for strain engineering. Photoluminescence and electroluminescence results are then discussed to illustrate the potential for realizing devices based on this nanocomposite material.

Nonequilibrium Synthesis of Highly Porous Single‐Crystalline Oxide Nanostructures

A novel synthesis route to the formation of vertically aligned single-crystalline oxide nanostructures is found by precisely controlling the nonequilibrium pulsed laser deposition process. The columnar nanostructures with deep crevices offering a large surface area are generated owing to the diffusion limited geometric shadowing effect.

Ti3+ Self-Doped Blue TiO2(B) Single-Crystalline Nanorods for Efficient Solar-Driven Photocatalytic Performance.

Ti3+ self-doped blue TiO2(B) single-crystalline nanorods (b-TR) are fabricated via a simple sol-gelation method, cooperated with hydro-thermal treatment and subsequent in situ treatment method, and afterward annealed at 350 °C in Ar. The structures are characterized by X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (UV-vis), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The prepared b-TR with narrow band gap possesses single-crystalline TiO2(B) phase, Ti3+ self-doping, and one-dimensional (1D) rodlike nanostructure. In addition, the improved photocatalytic performance is studied by decomposition of Rhodamine B (RhB) and hydrogen evolution. The degradation rate of RhB by Ti3+ self-doped blue TiO2(B) single-crystalline nanorods is ∼6.9- and 2.1-times higher compared with the rates of titanium dioxide nanoparticles and pristine TiO2(B) nanorods under visible light illumination, respectively. The hydrogen evolution rate of b-TR is 26.6 times higher compared with that of titanium dioxide nanoparticles under AM 1.5 irradiation. The enhanced photocatalytic performances arise from the synergetic action of the special TiO2(B) phase, Ti3+ self-doping, and the 1D rod-shaped single-crystalline nanostructure, favoring the visible light utilization and the separation and transportation of photogenerated charge carriers.

Electrochemical synthesis and photoelectrochemical properties of grass-like nanostructured α-Fe2O3 photoanodes for use in solar water oxidation

Grass-like nanostructured α-Fe2O3 photoelectrodes were prepared for the first time through a simple cathodic electrodeposition method from an oxygenated aqueous solution of Fe3+ at room temperature without using surfactant, capping agents or any other additives. The α-Fe2O3 electrodeposits were characterized by X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–vis absorption and photoelectrochemical (PEC) techniques. The SEM and XRD results indicated that the as-deposited α-Fe2O3 are composed of single crystalline nanoleaves. The formation mechanisms of α-Fe2O3 have also been proposed based on a series of cyclic voltammetric and XPS studies. This new electrochemical method is expected to be a useful technique for the fabrication of single crystalline and photoactive α-Fe2O3 nanostructures directly onto the electrode surface, which is required in most applications, such as energy conversion and storage and sensors.

Surface roughening and top opening of single crystalline TiO2 nanowires for enhanced photocatalytic activity

One dimensional (1D) single crystalline nanostructures facilitate electron transport and reduce the charge recombination; whilst nanotubular structure and a roughened surface provide more active sites as well as more opportunities for target molecules to reach the photocatalyst. As a result, TiO2 thin films with such a structural design can be anticipated to possess excellent photocatalytic activity. In this study, top opened nanowires with nanoprotrusions on the surface, which are hierarchical yet single crystalline as a whole, were synthesized by a simple acid treatment at 80°C for 36h of hydrothermally synthesized titanate nanowires, followed by a final calcination at 450°C for 1h. The hierarchical TiO2 films exhibited significantly enhanced efficiency when utilized to assist the photodegradation of rhodamine B and phenol in water under UV light illumination.

Enhanced non-volatile resistive switching in suspended single-crystalline ZnO nanowire with controllable multiple states

Resistive switching nanostructures are a promising candidate for next-generation non-volatile memories. In this report, we investigate the switching behaviors of single-crystalline ZnO nanowires suspended in air. They exhibit significantly higher current density, lower switching voltage, and more pronounced multiple conductance states compared to nanowires in direct contact with substrate. We attribute the effect to enhanced Joule heating efficiency, reduced surface scattering, and more significantly, the positive feedback established between the current density and local temperature in the suspended nanowires. The proposed mechanism has been quantitatively examined by finite element simulations. We have also demonstrated an innovative approach to initiating the current–temperature mutual enhancement through illumination by ultraviolet light, which further confirmed our hypothesis and enabled even greater enhancement. Our work provides further insight into the resistive switching mechanism of single-crystalline one-dimensional nanostructures, and suggests an effective means of performance enhancement and device optimization.

Photophysical and electrical properties of organic waveguide nanorods of perylene-3,4,9,10-tetracarboxylic dianhydride

The single crystalline nanostructure of organic semiconductors provides a very promising class of materials for applications in modern optoelectronic devices. However, morphology control and optoelectronic property modulation of high quality single crystalline samples remain a challenge. Here, we report the morphology-controlled growth of single crystalline nanorod arrays of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). We demonstrate that, unlike PTCDA film, PTCDA nanorods exhibits optical waveguide features, enhanced absorption, and Frenkel excitation emission in the visible region. Additionally, we measured the electrical properties of PTCDA nanorods, including the conductivity along the growth direction of the nanorod, which is roughly 0.61 S·m–1 (much higher than that of pure crystalline PTCDA films).

Length-dependent thermal transport in one-dimensional self-assembly of planar π-conjugated molecules.

This work reports a thermal transport study in quasi-one-dimensional organic nanostructures self-assembled from conjugated planar molecules via π-π interactions. Thermal resistances of single crystalline copper phthalocyanine (CuPc) and perylenetetracarboxylic diimide (PTCDI) nanoribbons are measured via a suspended thermal bridge method. We experimentally observed the deviation from the linear length dependence for the thermal resistance of single crystalline β-phase CuPc nanoribbons, indicating possible subdiffusion thermal transport. Interestingly, a gradual transition to the linear length dependence is observed with the increase of the lateral dimensions of CuPc nanoribbons. The measured thermal resistance of single crystalline CuPc nanoribbons shows an increasing trend with temperature. However, the trend of temperature dependence of thermal resistance is reversed after electron irradiation, i.e., decreasing with temperature, indicating that the single crystalline CuPc nanoribbons become 'amorphous'. Similar behavior is also observed for PTCDI nanoribbons after electron irradiation, proving that the electron beam can induce amorphization of single crystalline self-assembled nanostructures of planar π-conjugated molecules. The measured thermal resistance of the 'amorphous' CuPc nanoribbon demonstrates a roughly linear dependence on the nanoribbon length, suggesting that normal diffusion dominates thermal transport.

MOVPE growth of in situ Ga catalyzed AlGaAs nanowires on sapphire substrate

Growth of single crystalline AlGaAs nanostructures was carried out on highly lattice mismatched sapphire substrate by metal organic vapor phase epitaxy technique without using any external catalyst. In situ deposited Ga droplets were used as catalyst for the growth of AlGaAs nanostructures. The effects of growth temperature and V/III ratio were studied in detail. The growth of nanowires required a careful optimization of the growth conditions. The formation of well-faceted nanostructures and nanowires with hexagonal cross-section were found to be influenced by the growth parameters. The growth of nanostructures proceeds via VS growth mechanism after the consumption of initial Ga droplets depending on the growth conditions.

Effect of the Thermal Conductivity on Resistive Switching in GeTe and Ge2Sb2Te5 Nanowires.

The thermal conduction characteristics of GeTe and Ge2Sb2Te5(GST) nanowires were investigated using an optical method to determine the local temperature by Raman spectroscopy. Since the localization of surface charge in a single-crystalline nanostructure can enhance charge-phonon scattering, the thermal conductivity value (κ) of single crystalline GeTe and GST nanowires was decreased significantly to 1.44 Wm(-1) K(-1) for GeTe and 1.13 Wm(-1) K(-1) for GST, compared to reported values for polycrystalline structures. The SET-to-RESET state in single-crystalline GeTe and GST nanowires are characteristic of a memory device. Unlike previous reports using GeTe and GST nanowires, the SET-to-RESET characteristics showed a bipolar switching shape and no unipolar switching. In addition, after multiple cycles of operation, a significant change in morphology and composition was observed without any structural phase transition, indicating that atoms migrate toward the cathode or anode, depending on their electronegativities. This change caused by a field effect indicates that the structural phase transition does not occur in the case of GeTe and GST nanowires with a significantly lowered thermal conductivity and stable crystalline structure. Finally, the formation of voids and hillocks as the result of the electromigration critically degrades device reliability.

Fabrication of silver thin film for single-crystalline nanopillar: effects of thickness and grain size

A single-crystalline metallic nanostructure with a controlled crystallographic texture is expected to reduce optical losses. We have attempted to fabricate a single-crystalline metallic nanostructure using a physical technique only. The grain growth mechanism in silver films deposited on a highly transparent SiO2 substrate was elucidated by electron backscatter diffraction (EBSD) methods with nanoscale resolution. EBSD analysis suggested that the silver films deposited on SiO2 at lower thicknesses have a strong (111) texture. Additionally, we performed a grain size analysis using the EBSD crystal orientation map. When the film thickness increased from 154 to 1017 nm, most of the grains grew to a size of approximately 800 nm. Moreover, we successfully fabricated a conical nanopillar (undersurface diameter 450 nm) in a (111)-oriented large growing grain at the film thickness of 1017 nm by focused ion beam milling. A single-crystalline silver nanopillar was directly fabricated in controlled silver films using only physical processing.

Catalyst-free Growth of Single Crystalline Bi2Se3 Nanostructures for Quantum Transport Studies

In this work we report on the growth of single crystalline Bi2Se3 nanostructures (nanoribbons, nanoflakes, and nanowires) by catalyst-free decomposition sublimation in sealed silica ampules. The nanostructures directly grow on Si/SiO2 substrates by a vapor–solid growth mechanism and show high degree of crystallinity with dimensions of >10 μm in length and simultaneously <10 nm in height (nanoribbons). In order to optimize the growth process in a reproducible way, thermodynamic calculations were realized. The high quality of as-grown nanostructures was confirmed by transmission electron microscopy including selected area electron diffraction as well as electrical transport measurements. Our electrical transport data are evidence, on the one hand, of the high crystal quality and efficiency of the synthesis by decomposition sublimation. On the other hand, the catalyst-free approach offers the chance to investigate crystals with high purity and to measure surface state properties.

Effect of Grain-Boundaries in NiO Nanosheet Layers Room-Temperature Sensing Mechanism under NO2

The discovery of gas sensing properties of single-crystalline nanostructures comparable or even better than their polycrystalline counterparts has triggered the attention of the gas sensor research community. To eleborate the sensing mechanisms of single-crystalline and polycrystalline nanostructures, the single-crystalline (SC) NiO hexagonal nanosheet and the nanoparticle self-assembled polycrystalline (PC) NiO nanosheet were prepared for room-temperature NO2 sensing studies. The gas sensing studies revealed that the SC NiO exhibited a remarkably higher response to NO2 at room temperaure than the PC NiO. The correlation between the structural features of NiO and the room temperature NO2 sensing performances was discussed in detail via the grain boundary scattering theory. It was found that the scattering potential played a vital role in the sensing process. Because the absence of grain boundary in the SC NiO reduced the NO2 chemisorption-induced carrier scattering at interfaces, the SC NiO performed the ...

Green synthesis of the reduced graphene oxide–CuI quasi-shell–core nanocomposite: A highly efficient and stable solar-light-induced catalyst for organic dye degradation in water

Surfactant-free, reduced graphene oxide (RGO)–CuI quasi-shell−core nanocomposites were successfully synthesized using ultra-sonication assisted chemical method at room temperature. The morphologies, structures and optical properties of the CuI and CuI–RGO nanocomposites were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transformed infrared spectroscopy (FTIR), UV–visible absorption spectroscopy, and photoluminescence (PL) spectroscopy. Morphological and structural analyses indicated that the CuI–RGO core–shell nanocomposites comprise single-crystalline face-centered cubic phase CuI nanostructures, coated with a thin RGO quasi-shell. Photocatalysis experiments revealed that the as-synthesized CuI–RGO nanocomposites exhibit remarkably enhanced photocatalytic activities and stabilities for photo degradation of Rhodamine-B (RhB) organic dye under simulated solar light irradiation. The photo degradation ability is strongly affected by the concentration of RGO in the nanocomposites; the highest photodegradation rate was obtained at a graphene loading content of 2mgmL−1 nanocomposite. The remarkable photocatalytic performance of the CuI–RGO nanocomposites mainly originates from their unique adsorption and electron-accepting and electron-transporting properties of RGO. The present work provides a novel green synthetic route to producing CuI–RGO nanocomposites without toxic solvents or reducing agents, thereby providing highly efficient and stable solar light-induced RGO–CuI quasi-shell−core nanocomposites for organic dye photo degradation in water.

Facile Facet-Controllable Synthesis of BiOCL Single-Crystalline Nanostructure with Enhanced Photocatalytic Activity

Facile Facet-Controllable Synthesis of BiOCL Single-Crystalline Nanostructure with Enhanced Photocatalytic Activity