Tip‐enhanced Raman spectroscopy using single‐crystalline Ag nanowire as tip

We report the surface‐enhanced Raman scattering (SERS) effect from the apex of single‐crystalline Ag nanowires (NWs). We also fabricated tip‐enhanced Raman spectroscopy (TERS) tips by attaching individual Ag NWs to W wires by using the alternating current dielectrophoresis (AC‐DEP) method. The single‐crystalline Ag NW tips could overcome many of the shortcomings of conventional TERS tips. Most importantly, the results obtained from TERS using single‐crystalline metal NWs are very reproducible, and the tips are also reusable. This development represents a significant progress in making TERS a reliable optical characterization technique with nanometer spatial resolution. Copyright © 2010 John Wiley & Sons, Ltd.     

Trapped charge‐induced pyroelectric‐like transient response in single high‐resistivity Sb2Se3 nanowires

Dynamic scanning photocurrent microscopy was applied to Sb₂Se₃ crystalline single nanowires (NWs) to analyze their transient photocurrent responses. These NWs exhibited switching behavior with rapid rise and decay times upon illumination by laser pulses. The estimated spectral responsivity and external quantum efficiency for a freshly‐prepared NW at a bias voltage of 0.3 V and excitation wavelength of 488 nm were ～16.9 mA/W and ～42.9%, respectively. A pyroelectric‐like current transient was observed with reduced spectral responsivity when nonpolar Sb₂Se₃ single‐crystalline NWs were excited by laser pulses. Because Sb₂Se₃ NWs were nonpyroelectric or ferroelectric, the pyroelectric‐like current could possibly be attributed to temperature dependent nonlinear space‐charge distributions. Defects produced by the external electrical bias generated and re‐distributed space charges in the NWs. As a result, the temperature dependent inhomogeneous electric field led to nonlinear expansions or contractions of the lattice (electrostriction) that can produce pyroelectric current. We obtained a lower bound of equivalent pyroelectric coefficient α ≥ 60.09 μC/m² K from these materials by fitting the electrical transients. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Surface‐enhanced Raman scattering from gold‐coated germanium oxide nanowires

We utilized bulk‐synthesized nanowires (NWs) of germanium dioxide as nanoscale structures that can be coated with noble metals to allow the excitation of surface plasmons over a broad frequency range. The NWs were synthesized on substrates of silicon using gold‐catalyst‐assisted vapor–liquid–solid (VLS) growth mechanism in a simple quartz tube furnace setup. The resulting NWs have diameters of ∼100–200 nm, with lengths averaging ∼10–40 µm and randomly distributed on the substrate. The NWs are subsequently coated with thin films of gold, which provide a surface‐plasmon‐active surface. Surface‐enhanced Raman scattering (SERS) studies with near‐infrared (NIR) excitation at 785 nm show significant enhancement (average enhancement > 10⁶) with good uniformity to detect submonolayer concentrations of 4‐methylbenzenethiol (4‐MBT), trans‐1,2‐bis(4‐pyridyl)ethylene (BPE), and 1,2‐benzendithiol (1,2‐BDT) probe molecules. We also observed an intense, broad continuum in the Raman spectrum of NWs after metal coating, which tended to diminish with the analyte monolayer formation. Copyright © 2008 John Wiley & Sons, Ltd.     

Matrix‐isolation IR spectroscopy of R‐(+)‐3‐methylcyclopentanone in para‐hydrogen crystal

High‐resolution infrared (IR) spectra of R‐(+)‐3‐methylcyclopentanone (R3MCP) in para‐hydrogen (pH₂) crystal were recorded and compared with the corresponding IR spectra of R3MCP in Argon (Ar) isolation matrix as well as the IR spectra of the neat crystalline R3MCP at low deposition temperature of 4 ± 0.05 K. Moreover, IR spectra of R3MCP, hosted in pH₂ crystal, were recorded using a high‐resolution Fourier transform IR spectrometer as a function of sample concentration and over the range 10–300 ppm. Furthermore, density functional theory calculations of simulated IR spectra for the optimized geometries of R3MCP equatorial‐methyl and axial‐methyl conformers are compared with experimental spectra for the purpose of investigating molecular conformation. Upon comparison between theoretical and experimental IR spectra, vibrational modes arising from equatorial and axial conformers have been successfully assigned and related to the individual conformer's structure. Copyright © 2015 John Wiley & Sons, Ltd.     

Direct observation of elemental segregation in InGaN nanowires by X‐ray nanoprobe

Using synchrotron radiation nanoprobe, this work reports on the elemental distribution in single In_x Ga_1–xN nanowires (NWs) grown by molecular beam epitaxy directly on Si(111) substrates. Single NWs dispersed on Al covered sapphire were characterized by nano‐X‐ray fluorescence, Raman scattering and photoluminescence spectroscopy. Both Ga and In maps reveal an inhomogeneous axial distribution inside sin‐ gle NWs. The analysis of NWs from the same sample but with different dimensions suggests a decrease of In segregation with the reduction of NW diameter, while Ga distribution seems to remain unaltered. Photoluminescence and Raman scattering measurements carried out on ensembles of NWs exhibit relevant signatures of the compositional disorder. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lift‐off protocols for thin films for use in EXAFS experiments

Lift‐off protocols for thin films for improved extended X‐ray absorption fine structure (EXAFS) measurements are presented. Using wet chemical etching of the substrate or the interlayer between the thin film and the substrate, stand‐alone high‐quality micrometer‐thin films are obtained. Protocols for the single‐crystalline semiconductors GeSi, InGaAs, InGaP, InP and GaAs, the amorphous semiconductors GaAs, GeSi and InP and the dielectric materials SiO₂ and Si₃N₄ are presented. The removal of the substrate and the ability to stack the thin films yield benefits for EXAFS experiments in transmission as well as in fluorescence mode. Several cases are presented where this improved sample preparation procedure results in higher‐quality EXAFS data compared with conventional sample preparation methods. This lift‐off procedure can also be advantageous for other experimental techniques (e.g. small‐angle X‐ray scattering) that benefit from removing undesired contributions from the substrate.     

Piezoelectricity in two‐dimensional group III–V buckled honeycomb monolayers

We studied the elastic and piezoelectric properties of buckled honeycomb group III–V monolayers (GaP, GaAs, GaSb, InP, InAs and InSb) by DFT calculations. Those buckled monolayers are ferroelectric and have nonzero e₁₁, e₃₁, d₁₁ and d₃₁ piezoelectric coefficients. Our calculations show that those monolayers are good piezoelectric materials and a pronounced periodic trend of the piezoelectric coefficients e₁₁, e₃₁, d₁₁ and d₃₁ was found. Group III–V monolayers are promising candidates for future atomically thin piezoelectric applications such as transducers, sensors, and energy harvesting devices.     

Green Template‐Free Synthesis of Mesoporous Ternary CoNi–Mn Oxide Nanowires Towards High‐Performance Electrochemical Capacitors

One‐dimensional (1D) mesoporous ternary Co–Ni–Mn oxide nanowires (CNMO NWs) have been controllably fabricated via a facile yet scalable template‐free strategy involving a green hydrothermal route coupled with the following calcination. The composition of the reaction solvent system, i.e., the volume ratio of the ethylene glycol and deionized water, plays a significant role in tuning the specific morphologies and microstructures of the final CNMO products. The as‐derived lD CNMO NWs are constructed with numerous nanoparticle subunits with the size of ≈5–10 nm, and possess lots of interparticle mesopores ranged from 2 to 6 nm. The well‐defined mesoporous CNMO NWs apparently have appealing structural advantages, such as fast and convenient electron and ion transport, sufficient redox electrochemical centers with multiple valences, and large electroactive surface area. As a result, the as‐fabricated mesoporous CNMO NWs manifest exceptional pesudocapacitance and excellent cycling stability at high rates when evaluated as a striking low‐cost electrode for next‐generation electrochemical capacitors.     

Quantitative analysis of the phonon confinement effect in arbitrarily shaped Si nanocrystals decorated on Si nanowires and its correlation with the photoluminescence spectrum

Arrays of single‐crystalline Si nanowires (NWs) decorated with arbitrarily shaped Si nanocrystals (NCs) are grown by a metal‐assisted chemical etching process using silver (Ag) as the noble metal catalyst. The metal‐assisted chemical etching‐grown Si NWs exhibit strong photoluminescence (PL) emission in the visible and near infrared region at room temperature. Quantum confinement of carriers in the Si NCs is believed to be primarily responsible for the observed PL emission. Raman spectra of the Si NCs decorated on Si NWs exhibit a red shift and an asymmetric broadening of first‐order Raman peak as well as the other multi‐phonon modes when compared with that of the bulk Si. Quantitative analysis of confinement of phonons in the Si NCs is shown to account for the measured Raman peak shift and asymmetric broadening. To eliminate the laser heating effect on the phonon modes of the Si NWs/NCs, the Raman measurement was performed at extremely low laser power. Both the PL and Raman spectral analysis show a log‐normal distribution for the Si NCs, and our transmission electron microscopy results are fully consistent with the results of PL and Raman analyses. We calculate the size distribution of these Si NCs in terms of mean diameter (D₀) and skewness (σ) by correlating the PL spectra and Raman spectra of the as‐grown Si NCs decorated on Si NWs. Copyright © 2015 John Wiley & Sons, Ltd.     

Critical diameters and temperature domains for MBE growth of III–V nanowires on lattice mismatched substrates

We report on the growth properties of InAs, InP and GaAs nanowires (NWs) on different lattice mismatched substrates, in particular, on Si(111), during Au‐assisted molecular beam epitaxy (MBE). We show that the critical diameter for the epitaxial growth of dislocation‐free III–V NWs decreases as the lattice mismatch increases and equals 24 nm for InAs NWs on Si(111), 39 nm for InP NWs on Si(111), 44 nm for InAs NWs on GaAs(111)B, and 110 nm for GaAs NWs on Si(111). When the diameters exceed these critical values, the NWs are dislocated or do not grow at all. The corresponding temperature domains for NW growth extend from 320 °C to 340 °C for InAs NWs on Si(111), 330 °C to 360 °C for InP NWs on Si(111), 370 °C to 420 °C for InAs NWs on GaAs(111)B and 380 °C to 540 °C for GaAs NWs on Si(111). Experimental values for critical diameters are compared to the previous findings and are discussed within the frame of a theoretical model. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Unusual magnetic behaviors and electrical properties of Nd-doped BiFeO3 nanoparticles calcined at different temperatures

In the present work, we have investigated the effect of deposition time on the morphological, structural, and photoluminescence (PL) properties of β-Ga₂O₃ NWs grown by CVD technique. The diameter and length of the as-grown NWs varied for the deposition time of 1–4 h, from 50 to 100 nm and 5–15 μm, respectively. The crystalline quality of the NWs improved with increasing the deposition time. The detailed transmission electron microscopy (TEM) and fast Fourier Transformation (FFT) measurements revealed that the as-grown β-Ga₂O₃ NWs were single crystalline. Furthermore, we have studied the variation of PL spectra of the NWs with deposition time and provided an energy band diagram to give a plausible explanation of the origin of different emissions in the PL spectra. The PL spectra showed a broad strong UV-blue emission band and a weak red emission for 1 h deposited sample. We suggested that the UV and red emission from β-Ga₂O₃ NWs are related to oxygen vacancies and impurities such as nitrogen, respectively. It was observed that the intensity of UV emission decreased with deposition time and this reduction was attributed primarily to the reduction of oxygen vacancies in the NWs.     

Surface passivation of phosphorus‐diffused n+‐type emitters by plasma‐assisted atomic‐layer deposited Al2O3

In recent years Al₂O₃ has received tremendous interest in the photovoltaic community for the application as surface passivation layer for crystalline silicon. Especially p‐type c‐Si surfaces are very effectively passivated by Al₂O₃, including p‐type emitters, due to the high fixed negative charge in the Al₂O₃ film. In this Letter we show that Al₂O₃ prepared by plasma‐assisted atomic layer deposition (ALD) can actually provide a good level of surface passivation for highly doped n‐type emitters in the range of 10–100 Ω/sq with implied‐V_oc values up to 680 mV. For n‐type emitters in the range of 100–200 Ω/sq the implied‐V_oc drops to a value of 600 mV for a 200 Ω/sq emitter, indicating a decreased level of surface passivation. For even lighter doped n‐type surfaces the passivation quality increases again to implied‐V_oc values well above 700 mV. Hence, the results presented here indicate that within a certain doping range, highly doped n‐ and p‐type surfaces can be passivated simultaneously by Al₂O₃. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural studies and Raman spectroscopy of forbidden zone boundary phonons in Ni‐doped ZnO ceramics

We present X‐ray diffraction and Raman spectroscopy studies of Ni‐doped ZnO (Zn_1−xNi_xO, x = 0.0, 0.03, 0.06, and 0.10) ceramics prepared by solid‐state reaction technique. The presence of the secondary phase along with the wurtzite phase is observed in Ni‐doped ZnO samples. The E₂(low) optical phonon mode is seen to be shifted to a lower wavenumber with Ni incorporation in ZnO and is explained on the basis of force‐constant variation of ZnO bond with Ni incorporation. A zone boundary phonon is observed in Ni‐doped samples at ∼130 cm⁻¹ which is normally forbidden in the first‐order Raman scattering of ZnO. Antiferromagnetic ordering between Ni atoms via spin‐orbit mechanism at low temperatures (100 K) is held responsible for the observed zone boundary phonon. Copyright © 2008 John Wiley & Sons, Ltd.     

Improved device performance in nonpolar a ‐plane GaN LEDs using a Ni/Al/Ni/Au n‐type ohmic contact

The authors report upon the increased light‐output power (P_out) via a reduction in the forward voltage (V_f) for nonpolar a ‐plane GaN LEDs using Ni/Al/Ni/Au n‐type ohmic contacts. The specific contact resistivity of the Ni/Al/Ni/Au contact is found to be as low as 5.6 × 10^–5 whereas that of a typical Ti/Al/Ni/Au contact is 6.8 × 10^–4 Ω cm², after annealing at 700 °C. The X‐ray photoelectron spectroscopy results show that the upward surface band bending is less pronounced for the Ni/Al contact compared to the Ti/Al contact, leading to a decrease in the effective Schottky barrier height (SBH). The V_f of the nonpolar LEDs decreases by 10% and P_out increases by 15% when the Ni/Al/Ni/Au scheme is used instead of the typical Ti/Al/Ni/Au metal scheme. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and properties of self‐catalyzed (In,Mn)As nanowires

Mn‐assisted molecular beam epitaxy is used for the growth of (In,Mn)As nanowires (NWs) on GaAs(111)B. The transmission electron microscopy measurements revealed that despite the relatively high growth temperature regime this technique can be used to obtain (In,Mn)As NWs with high crystalline quality without any crystal defects, such as dislocations, stacking faults or precipitates inside the investigated NWs or on their side‐walls, although the growth processes of NWs were accompanied by the formation of MnAs precipitates between the NWs at the interface of the wetting layer. The results obtained are of importance for the realization of new spintronic nanostructured materials. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

X‐ray absorption study of the local structure at the NiO/oxide interfaces

This work reports an X‐ray absorption near‐edge structure (XANES) spectroscopy study at the Ni K‐edge in the early stages of growth of NiO on non‐ordered SiO₂, Al₂O₃ and MgO thin films substrates. Two different coverages of NiO on the substrates have been studied. The analysis of the XANES region shows that for high coverages (80 Eq‐ML) the spectra are similar to that of bulk NiO, being identical for all substrates. In contrast, for low coverages (1 Eq‐ML) the spectra differ from that of large coverages indicating that the local order around Ni is limited to the first two coordination shells. In addition, the results also suggest the formation of cross‐linking bonds Ni—O—M (M = Si, Al, Mg) at the interface.     

High‐performance In–Zn–O thin‐film transistors with a soluble processed ZrO2 gate insulator

Spin‐coated zirconium oxide films were used as a gate dielectric for low‐voltage, high performance indium zinc oxide (IZO) thin‐film transistors (TFTs). The ZrO₂ films annealed at 400 °C showed a low gate leakage current density of 2 × 10^–8 A/cm² at an electric field of 2 MV/cm. This was attributed to the low impurity content and high crystalline quality. Therefore, the IZO TFTs with a soluble ZrO₂ gate insulator exhibited a high field effect mobility of 23.4 cm²/V s, excellent subthreshold gate swing of 70 mV/decade and a reasonable I_on/off ratio of ～10⁶. These TFTs operated at low voltages (～3.0 V) and showed high drain current drive capability, enabling oxide TFTs with a soluble processed high‐k dielectric for use in backplane electronics for low‐power mobile display applications. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation of silver nanoparticles on zinc oxide nanowires by photocatalytic reduction for use in surface‐enhanced Raman scattering measurements

To increase the sensitivity in surface‐enhanced Raman scattering (SERS) measurements, the high surface area of zinc oxide nanowires (ZnO NWs) was used. ZnO NWs on silicon substrates were prepared and used as substrates for further growth of silver nanoparticles (AgNPs). Ultraviolet (UV) irradiation was used to reduce silver ions to AgNPs on the ZnO wires. With proper growth conditions for both ZnO NWs and AgNPs, the substrates exhibit SERS enhancement factors greater than 10⁶. To understand the influences of the morphologies of the ZnO NWs on the growth of AgNPs, the growing time and temperature were varied. The concentration of silver nitrate and irradiation time of UV radiation were also varied. The resulting AgNPs were probed with para‐nitrothiophenol to quantify the SERS enhancements obtained from the varying conditions. The results indicate that ZnO NWs could be grown at temperatures higher than 490 °C and higher growth temperatures result in smaller diameter of the formed ZnO NWs. Also, the morphologies of ZnO NWs did not significantly alter the SERS signals. The concentration of silver nitrate affects the SERS signals significantly and the optimal concentration was found to be in the range of 10–20 mM. With irradiation times longer than 90 s, the resulting AgNPs showed similar SERS intensities. With optimized conditions, the AgNPs/ZnO substrates are highly suitable for SERS measurements with a typical enhancement factor of higher than 10⁶. Copyright © 2010 John Wiley & Sons, Ltd.     

High‐Pressure Evaporation‐Based Nanoporous Black Sn for Enhanced Performance of Lithium‐Ion Battery Anodes

Increasing the surface area to improve chemical activity is an unending task from conventional catalysis to recently emerging electrochemical energy conversion and storage. Here, a simple, vacuum‐deposition‐based method to form nanoporous structures of metals is reported. By utilizing thermal evaporation at a high pressure, fractal‐like nanoporous structures of Sn with porosity exceeding 98% are synthesized. The obtained nanostructure consists of nanoparticle aggregates, and the morphology can be controlled by adjusting the working pressure. The formation of the nanoporous structure is explained by homogeneous nucleation and diffusion‐limited aggregation, where nanoparticles produced by the repeated collisions of evaporated atoms adhere to the substrate without diffusion, forming porous aggregates. Due to the easy oxidation of Sn, the constituent nanoparticles are covered with amorphous SnO_x and crystalline SnO phases. When the nanoporous Sn/SnO_x aggregates are applied to a lithium‐ion battery anode through direct deposition on a Cu foil current collector without binders or conducting additives, the nanoporous Sn/SnO_x anode shows greatly enhanced cyclability and exceptional rate performance compared to those of a dense Sn thin film anode. The approach investigated in this work is expected to provide a new platform to other fields that require highly porous structures.     

FT‐Raman spectroscopic spectra on 3‐phenylpropylamine inclusion compounds

Some new Hofmann‐3‐phenylpropylamine‐type clathrates with chemical formulae of M(3‐phenylpropylamine)₂ Ni(CN)₄. 2G (MNi or Co, G = 1,2‐dichlorobenzene or 1,3‐dichlorobenzene) have been prepared and their Fourier transform infrared(FT‐IR; 4000–400 cm⁻¹), far‐infrared (600–100 cm⁻¹) and FT‐Raman (4000–60 cm⁻¹) spectra are reported. The ligand molecule, guest molecules, polymeric sheet and metal‐ligand bands of the clathrates are assigned in detail. The compounds are also characterized by thermal gravimetric analysis (TGA), differential thermal analysis (DTA), elemental analysis and magnetic susceptibility measurements. From the results, the monodentate 3‐phenylpropylamine ligand molecule bonds to the metal atom of |M‐Ni(CN)₄ | _∞ polymeric layers in the trans‐gauche‐gauche (TGG) form, and 1,2‐dichlorobenzene or 1,3‐dichlorobenzene molecules are guested by this structure revealing the inclusion ability of the host complexes. Copyright © 2010 John Wiley & Sons, Ltd.