Au Tip Research Articles

Enhancement of Radiative Plasmon Decay by Hot Electron Tunneling.

Here we demonstrate that photon emission induced by inelastic tunneling through a nanometer single gap between a sharp Au tip and an Au substrate can be significantly enhanced by the illumination of the junction with 634 nm laser light with an electric field component oriented parallel to the tip-axis, i.e., perpendicular to the sample. Analyzing photoluminescence (PL) spectra recorded as a function of bias voltage allows us to distinguish between PL from (1) the decay of electron-hole pairs created by the laser excited sp/d interband transition with a characteristic band at 690 nm and (2) the red-shifted radiative decay of characteristic plasmon modes formed by the gap. Since the electroluminescence spectra (without laser) already show the plasmonic gap modes, we conclude that the enhanced intensity induced by laser illumination originates from the radiative decay of hot electrons closely above the Fermi level via inelastic tunneling and photon emission into the plasmon modes. Since these processes can be independently controlled by laser illumination and the amplitude of the bias voltage, it is of great interest for designing new switchable photon emission plasmonic devices.

Shot noise from single atom contacts in a scanning tunneling microscope

The shot noise of the current through single-atom contacts has been measured in a scanning tunneling microscope at a temperature of 5K. Electrical measurements at frequencies up to 120kHz were performed on Au, Fe, and Co atoms on Au(111) using Au tips. The data from Fe and Co indicate spin polarized transmission through a single conductance channel. Optical measurements at sub-petahertz frequencies were carried out on Cu adatoms on Cu(111) using Cu tips. The data are consistent with previous observations from coinage metal contacts.

Two-step fabrication technique of gold tips for use in point-contact spectroscopy.

For a successful point-contact spectroscopy (PCS) measurement, metallic tips of proper shape and smoothness are essential to ensure the ballistic nature of a point-contact junction. Until recently, the fabrication of Au tips suitable for use in point-contact spectroscopy has remained more of an art involving a trial and error method rather than an automated scientific process. To address these issues, we have developed a technique with which one can prepare high quality Au tips reproducibly and systematically. It involves an electronic control of the driving voltages used for an electrochemical etching of a gold wire in a HCl-glycerol mixture or a HCl solution. We find that a stopping current, below which the circuit is set to shut off, is a single very important parameter to produce an Au tip of desired shape. We present detailed descriptions for a two-step etching process for Au tips and also test results from PCS measurements using them.

Interfacial scanning tunneling spectroscopy (STS) of chalcogenide/metal hybrid nanostructure

The electronic structure at the interface of chalcogenide/metal hybrid nanostructure (CdSe–Au tipped) had been studied by UHV scanning tunneling spectroscopy (STS) technique at room temperature. This nanostructure was synthesized by a phase transfer chemical method. The optical absorption of this hybrid nanostructure was recorded, and the application of the effective mass approximation (EMA) model gave dimensions that were confirmed by the direct measurements using the scanning tunneling microscopy (STM) as well as the high-resolution transmission electron microscope (HRTEM). The energy band gap obtained by STS agrees with the values obtained from the optical absorption. Moreover, the STS at the interface of CdSe–Au tipped hybrid nanostructure between CdSe of size about 4.1±0.19nm and Au tip of size about 3.5±0.29nm shows a band bending about 0.18±0.03eV in CdSe down in the direction of the interface. Such a result gives a direct observation of the electron accumulation at the interface of CdSe–Au tipped hybrid nanostructure, consistent with its energy band diagram. The presence of the electron accumulation at the interface of chalcogenides with metals has an important implication for hybrid nanoelectronic devices and the newly developed plasmon/chalcogenide photovoltaic solar energy conversion.

Shot noise as a probe of spin-polarized transport through single atoms.

Single atoms on Au(111) surfaces have been contacted with the Au tip of a low temperature scanning tunneling microscope. The shot noise of the current through these contacts has been measured up to frequencies of 120kHz and Fano factors have been determined to characterize the transport channels. The noise at Fe and Co atoms, the latter displaying a Kondo effect, indicates spin-polarized transport through a single channel. Transport calculations reproduce this observation.

Hollow octahedral and cuboctahedral nanocrystals of ternary Pt-Ni-Au alloys.

Hollow particles of Pt-Ni-Au alloys have been prepared through a two-step reaction with the synthesis of NiPt octahedral and cuboctahedral templates followed by a galvanic replacement reaction by Au(iii). Metal etching presents an efficient method to yield hollow particles and investigate the Au diffusion in the metallic Pt-Ni framework through macroscopic (X-ray diffraction and SQUID magnetic measurement) and microscopic (HRTEM and STEM) measurements. The hollow particles retain the shape of the original nanocrystals. The nucleation of Au is found to be induced preferentially on the tip of the polyhedral nanocrystals while the etching of Ni starts from the facets leaving hollow octahedral particles consisting of 2 nm thick edges. In the presence of oleylamine, the Au tip grows and yields a heterogeneous dimer hollow-NiPt/Au. Without oleylamine, the Au nucleation is followed by Au diffusion in the Ni/Pt framework to yield a hollow single crystal Pt-Ni-Au alloy. The Pt-Ni-Au alloyed particles display a superparamagnetic behavior at room temperature.

Axonic Au Tips Induced Enhancement in Raman Spectra and Biomolecular Sensing

Axonic Au stars with sharp crystalline tips were synthesized via reduction of HAuCl4 capped with cetyl trimethylammonium bromide (CTAB). A seed-mediated method was adopted in which Au seeds were used in the growth solution containing Au+ ions, for initiating the nucleation with CTAB and with ascorbic acid acting as a mild reducing agent. Transmission Electron Microscopy image clearly reveals the synthesis of gold stars. The gold star dispersions display a well-defined optical response as observed by the presence of a sharp surface plasmon resonance centered at 525 nm. The Au stars functionalized with 11-mercaptoundecanoic acid and immobilized with urease showed improved urea-sensing behavior in comparison to pure gold disk in attenuated total reflection surface plasmon resonance (ATR SPR) mode. Surface-enhanced Raman spectroscopy performed using the functionalized Au stars showed a significant enhancement in Raman signals for a wider range of urea concentration (2–20 mg/ml).

Gas-phase generation of noble metal-tipped NiO nanorods by rapid thermal oxidation

The thermal oxidation of alloy nanoparticles (NPs) composed of nickel and a noble metal was investigated by high-resolution electron microscopic observations of the NPs oxidized in a gas phase under different oxidation conditions. When Ni0.8Au0.2 NPs were heated with oxygen from room temperature, oxidation progressed to form Au–NiO core–shell structures, however, the Au core spilled out by breaking the NiO shell at high temperatures. In contrast, when the alloy NPs were subjected to rapid thermal oxidation, which was enabled by heating the NPs at high temperatures (≥500 °C) and then abruptly exposed to oxygen, oxidation advanced anisotropically such that a NiO island protruded and built up to form a NiO nanorod. This resulted in the formation of Au-tipped NiO nanorods in which a hemispherical Au tip bonded to a NiO nanorod via a Au {111}/NiO{100} interface. We found that the relative sizes of Au and NiO in Au-tipped NiO nanorods were easily and widely controlled by changing the Au mole fraction (0.05–0.8) of the alloy NPs. Similarly, rapid thermal oxidation of Ni–Pt NPs generated Pt-tipped NiO nanorods in which a spherical Pt tip was half-embedded in a NiO nanorod. The present gas-phase approach has great potential for fabricating functional asymmetric hybrid nanostructures in clean conditions.

Correlation between the electrical conductance and the mechanical deformation of a graphite surface

We investigate the correlation between the electrical conductance and the mechanical deformation of a graphite surface by using the scanning tunneling microscope break junction (STM-BJ) technique. During STM-BJ, the Au tip is retracted from contact with the graphite surface as the top layer is lifted away from its rest position, resulting in a decrease of the conductance. We observe a positive correlation between the conductance of the contact and the length to which the top layer of graphite is lifted. We also find that the top layer recovers elastically to its original form after detaching from the layer on the tip. Furthermore, the lift length of the top layer of graphite during tip retraction is shorter than that of a single layer of graphene on a silicon oxide surface. Such a characteristic is probably the result of a weaker attractive force of the graphene with Si oxide, such as the van der Waals force, than that of the interlayer interaction of graphite. Our results thus open a new path to basic research of mechano-electronics with carbon-based materials and their applications.

From Schottky to Ohmic graphene contacts to AlGaN/GaN heterostructures: Role of the AlGaN layer microstructure

The electrical behaviour of graphene (Gr) contacts to AlxGa1−xN/GaN heterostructures has been investigated, focusing, in particular, on the impact of the AlGaN microstructure on the current transport at Gr/AlGaN interface. Two Al0.25Ga0.75N/GaN heterostructures with very different quality in terms of surface roughness and defectivity, as evaluated by atomic force microscopy (AFM) and transmission electron microscopy, were compared in this study, i.e., a uniform and defect-free sample and a sample with a high density of typical V-defects, which locally cause a reduction of the AlGaN thickness. Nanoscale resolution current voltage (I-V) measurements by an Au coated conductive AFM tip were carried out at several positions both on the bare and Gr-coated AlGaN surfaces. Rectifying contacts were found onto both bare AlGaN surfaces, but with a more inhomogeneous and lower Schottky barrier height (ΦB ≈ 0.6 eV) for AlGaN with V-defects, with respect to the case of the uniform AlGaN (ΦB ≈ 0.9 eV). Instead, very different electrical behaviours were observed in the presence of the Gr interlayer between the Au tip and AlGaN, i.e., a Schottky contact with reduced barrier height (ΦB ≈ 0.4 eV) for the uniform AlGaN and an Ohmic contact for the AlGaN with V-defects. Interestingly, excellent lateral uniformity of the local I-V characteristics was found in both cases and can be ascribed to an averaging effect of the Gr electrode over the AlGaN interfacial inhomogeneities. Due to the locally reduced AlGaN layer thickness, V defect act as preferential current paths from Gr to the 2DEG and can account for the peculiar Ohmic behaviour of Gr contacts on defective AlGaN.

Efficient Plasmon-Induced Hot Electron Transfer and Photochemistry in Semiconductor-Au Nanorod Nanoheterostructures

Surface plasmon resonance in metal nanostructures have been used to enhance the efficiency of semiconductors and/or molecular chromophore based solar energy conversion devices by increasing the absorption or energy transfer rate through the enhanced local field strength. In more recent years, it has been shown that excitation of plasmons in metal nanostructures can lead to hot electron transfer into semiconductors and enhanced photochemistry. This novel plasmon-exciton interaction mechanism suggests that plasmonic nanostructures can potentially function as a new class of widely tunable and robust light harvesting materials for photo-detection or solar energy conversion. However, plasmon-induced hot electron injections from metal to semiconductor or molecules are still inefficient because of the competing ultrafast hot electron relaxation (via ultrafast electron-electron and electron-phonon scattering) processes within the metallic domain.In this paper we discuss a recent study on the plasmon-exciton interaction mechanisms in colloidal quantum-confined epitaxially-grown semiconductor-gold plexcitonic nanorod heterostructures. Using transient absorption spectroscopy, we show that optical excitation of plasmons in the Au tip leads to efficient hot electron injection into the semiconductor nanorod. In the presence of sacrificial electron donors, this plasmon induced hot electron transfer process can be utilized to drive photoreduction reactions under continuous illumination. Ongoing studies are examining how to further improve the plasmon induced hot electron injection efficiency through controlling the size and shape of the plasmonic and excitonic domains.

Tip-Enhanced Raman Spectroscopy of Graphene Nanoribbons on Au(111)

We report tip-enhanced Raman spectroscopy of graphene nanoribbons (GNRs) fabricated on Au(111) by the on-surface polymerization technique under ultrahigh-vacuum conditions. The 0.74 nm wide armchair GNRs are directly observed by scanning tunneling microscopy at room temperature, and the characteristic vibration modes of GNRs appear in both the far- and near-field (tip-enhanced) Raman spectra. The Raman scattering is enhanced by up to 4 × 105 in the near-field, while a strong intensity fluctuation (blinking) frequently emerges in the time series of the near-field spectra. From the STM observation of a stable adsorption structure of GNRs under the laser illumination and statistical analysis of the intensity fluctuation, we attribute the origin predominantly to thermal fluctuations of the effective radius of the Au tip apex that induces the localized plasmonic field. The intensity distribution is qualitatively reproduced with a simple theoretical model in which the tip apex is approximated by ideal metal sphere.

Photocatalysis from Fluorescence-Quenched CdSe/Au Nanoheterostructures: A Size-Dependent Study.

We report a systematic study of the electron-transfer process from CdSe quantum dots (QDs) to the Au tips as a function of the QD diameter and also the size of the Au-tip. For Au-tips smaller than ∼3 nm, that is, when they are still not metallic, a reduction in PL behavior is observed as the excited electrons are transferred from the QD-conduction band to Au, with quenching being higher for larger tips and smaller QDs. A combination of steady state and time-resolved studies establish the mechanism of charge transfer that is further confirmed by dye-degradation studies, showing the possibility of ambient day light photocatalysis.

Tunneling spectroscopy of superconducting MoN and NbTiN grown by atomic layer deposition

A tunneling spectroscopy study is presented of superconducting MoN and Nb0.8Ti0.2N thin films grown by atomic layer deposition (ALD). The films exhibited a superconducting gap of 2 meV and 2.4 meV, respectively, with a corresponding critical temperature of 11.5 K and 13.4 K, among the highest reported Tc values achieved by the ALD technique. Tunnel junctions were obtained using a mechanical contact method with a Au tip. While the native oxides of these films provided poor tunnel barriers, high quality tunnel junctions with low zero bias conductance (below ∼10%) were obtained using an artificial tunnel barrier of Al2O3 on the film's surface grown ex situ by ALD. We find a large critical current density on the order of 4 × 106 A/cm2 at T = 0.8Tc for a 60 nm MoN film and demonstrate conformal coating capabilities of ALD onto high aspect ratio geometries. These results suggest that the ALD technique offers significant promise for thin film superconducting device applications.

Evidence for Negative Electron Affinity in Laser Irradiated ZnTe Thin Films

Local transport properties of laser irradiated ZnTe thin films are reported. By rastering the laser beam (of 532 nm wavelength) appropriately, ZnTe decomposes into n-type ZnTe and Te and a grating like struct ure with micro-stripesof ZnTe separated by grooves of Te is obtained. Conductive atomic force microscopy studies and local I -V measurements made on thestripesand grooves show that the film properties are mainly determined by chemical composition, ratherthan by the topography of the film. When the tip is positively biased, the current images closely match the topography images. In contrast, when the tip is negatively biased the current and topography images are very different and a large negative curren t was also observed in the grooves. This is attributed to the variation in charge separation (interface capacitance) caused by the rough surface. It is shown that Te forms ohmic contact with Au tip, but the junction exhibits Schottky diode behavior under l ow biasing voltages. The large current at both high positive and negative tip biasing may arise due to semi-metallic properties of Te. The I -V characteristics measurement reveals formation of Schottky barrier between ZnTe-Au junctions with a very low valu e (32.4 �78.3 meV) of barrier which indicates the presence of negative electron affinity . OriginalResearch Article

Promoting photocatalytic multiple-electron reduction in aerobic solutions using Au-tipped CdSe nanorod clusters

Clusters of Au-tipped CdSe nanorods have been synthesized through nanorod polymerization driven by the controlled welding of the Au tips. The efficient collection of energetic electrons in the Au nanoparticles leads to a significant enhancement in catalysing multiple-electron reduction reactions by using methylene blue as the redox indicator.

In vitro study of the interaction of heregulin-functionalized magnetic-optical nanorods with MCF7 and MDA-MB-231 cells.

Multifunctional nanoparticles that actively target specific cells are promising tools for cancer diagnosis and therapy. In this article we review the synthesis and surface chemistry of Fe-Au nanorods and their characterization using microscopy. The diameter of the rods used in this study was selected to be 150-200 nm so that they did not enter the cells. The 80 nm-long Au tips of the nanorods were functionalized with heregulin (HRG), and the micron-long Fe portion was coated with a poly(ethylene glycol) monolayer to minimize non-specific interactions. Nanorods functionalized with HRG were found to preferentially bind to MCF7 cells that express high levels of the receptor tyrosine-protein kinase ErbB2/3. Magnetic tweezers measurements were used to characterize the kinetic properties of the bond between the HRG on the rods and ErbB2/3 on the surface of the cells. The strong magnetization of Fe-Au nanorods makes them excellent candidates for in-vitro and in-vivo imaging, and magnetic therapeutic applications targeting cancer cells in circulation.

Preferential growth of Au on CdSe quantum dots using Langmuir–Blodgett technique

Oleyl amine capped CdSe quantum dots are allowed to form monolayer on aqueous HAuCl4 subphase and Langmuir–Blodgett technique is used judicially to grow Au tips in a directed fashion on CdSe quantum dots.

Charge Transfer between STM Tip and Au(100) in Dry, H2O, and D2O Atmospheres

Charge transfer between STM tip and Au(100) has been investigated by using a Scanning Tunneling Microscopy (STM) technique in dry, , and atmospheres. Dry atmosphere was indicated by humidity as low as 5 % and high humidity as high as 98% was managed by injecting and to the chamber. The current decayed more slowly in high humidity than in dry atmosphere. The plateau currents were found to appear at separations larger than ca. where the current decay stopped depending on applied bias voltages. The polarity dependence was observed at the STM junction between Pt-Ir tip and the gold. On the contrary, little dependence was seen at the one between Au tip and the substrate electrode.

Detection of the Sn(III) Intermediate and the Mechanism of the Sn(IV)/Sn(II) Electroreduction Reaction in Bromide Media by Cyclic Voltammetry and Scanning Electrochemical Microscopy

Fast-scan cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) were used to investigate the reduction of Sn(IV) as the hexabromo complex ion in a 2 M HBr-4 M NaBr medium. CV at scan rates to 100 V/s and SECM indicated the reaction pathway involves ligand-coupled electron transfer via an ECEC-DISP process: (1) one-electron reduction of Sn(IV)Br6(2-) to Sn(III)Br6(3-); (2) bromide dissociation of the reduced Sn(III)Br6(3-) to Sn(III)Br5(2-); (3) disproportionation of the reduced 2Sn(III)Br5(2-) to Sn(IV)Br5(-) and Sn(II)Br5(3-); (4) one-electron reduction of Sn(III)Br5(2-) to Sn(II)Br5(3-); (5) bromide dissociation from Sn(II)Br5 to Sn(II)Br4(2-). The intermediate Sn(III) species was confirmed by SECM(3-), where the Sn(III) generated at the Au tip was collected on a Au substrate in the tip generation/substrate collection mode when the distance between the tip and substrate was a few hundred nanometers.