Nanometric Protrusion Research Articles

Highly Localized Optical Field Enhancement at Neon Ion Sputtered Tungsten Nanotips.

We present laser-driven rescattering of electrons at a nanometric protrusion (nanotip), which is fabricated with an in situ neon ion sputtering technique applied to a tungsten needle tip. Electron energy spectra obtained before and after the sputtering show rescattering features, such as a plateau and high-energy cutoff. Extracting the optical near-field enhancement in both cases, we observe a strong increase of more than 2-fold for the nanotip. Accompanying finite-difference time-domain (FDTD) simulations show a good match with the experimentally extracted near-field strengths. Additionally, high electric field localization for the nanotip is found. The combination of transmission electron microscope imaging of such nanotips and the determination of the near-field enhancement by electron rescattering represent a full characterization of the electric near-field of these intriguing electron emitters. Ultimately, nanotips as small as single nanometers can be produced, which is of utmost interest for electron diffraction experiments and low-emittance electron sources.

A Facile Approach towards Fabrication of GdB6‐ZnO Heteroarchitecture as High Current Density Cold Cathode

AbstractThree dimensional (3D) GdB6‐ZnO heteroarchitecture comprised of thin coating of GdB6 on self aligned ZnO urchins with pointed apex has been synthesized employing optimized Pulsed Laser Deposition (PLD) technique. The self aligned ZnO urchins on Zn substrate were obtained using hydrothermal route. The as‐synthesized GdB6‐ZnO heteroarchitecture was characterized using XRD, SEM, TEM XPS, and UPS in order to reveal its structural, morphological, chemical, and electronic properties. Interestingly, the GdB6‐ZnO heteroarchitecture exhibits superior field emission (FE) behviour in contrast to the pristine ZnO urchins envisaged by extraction of very high emission current density of ∼ 4.6 mA/cm2 at an applied field of ∼ 4.5 V/μm, against ∼1.5 mA/cm2 at an applied field of ∼ 5.6 V/μm from the pristine ZnO urchins emitter. Furthermore, the GdB6‐ZnOemitter exhibits good emission stability at pre‐set value of ∼5 μA over duration of more than 3 hours. The superior FE behaviour of the GdB6‐ZnO is attributed to low workfunction of GdB6 and presence of nanometric protrusions on the emitter surface, further enhancing the aspect ratio provided by the ZnO urchins. The present results demonstrate a facile approach towards fabrication of high current density cold cathodes due to rare earth hexaborides via designing heteroarchitectures comprised of their well adherent ultrathin coating on high aspect ratio metal oxide nanostructures.

Microscopic surface processes observed during the oxidative dissolution of sphalerite

Sphalerite cleavage surface dissolution in acidic (HC1) and oxygen-saturated solutions was investigated by liquid-cell Atomic Force Microscopy (AFM). The sphalerite surface was cleaved in the laboratory and then mounted in the liquid cell at 298 K with a fast continuous renewal of the interacting solution. AFM data indicate that unreacted (110) surfaces are characterised by flat surface terraces delimited by step edges aligned along [110] crystallographic directions. AFM imaging allowed us to investigate removal of matter only at pH = 0 (HC1). Under these conditions, etch pits develop that are delimited by 1–3-nm-high step edges. However, surface terraces are covered by nanometric protrusions, while the step edges are microrough. Ex-situ solution chemistry measurements performed in flow-through-reactor indicates strong undersaturation with respect to both zinc sulphide and zinc sulphate. The reactivity of the dissolving (110) surface decreases significantly during the 24 hours of run time. Such a decrease suggests a change in the mechanism governing the overall dissolution process. We interpret nanometric protrusions as due to oxidative reactions at the interface that result in a reorganisation of the surface at the nanometric scale. The mechanism limiting the rate of sphalerite dissolution would be the process of protrusion formation and dissolution. A similar phenomenon was observed in an AFM study of the galena surface. Finally, we propose that the process of protrusion formation could be general in the oxidative dissolution of metal sulphides.

Comparison between experimental and computer simulations of current–voltage ( I– V) characteristics of dielectric-coated photon-stimulated field emitters

For the purpose of simulating photon-stimulated field emission by taking account of three-dimensional aspects, a transfer-matrix formulation of electronic scattering was combined with a Floquet expansion of the wave function for taking account of quanta exchanges between the electrons and the external radiation. With specific techniques to preserve numerical stability, this transfer-matrix formalism is well suited to compute the transmission of the field-emitted/photon-stimulated electrons between two electrodes. This theory is applied to the computation of Fowler–Nordheim curves describing the photon-stimulated field emission of a tungsten plane emitter (described by z⩽0), which supports a nanometric protrusion and a dielectric coating. The extraction bias ranges from 12 to 24 V, for an inter-electrode distance of 4 nm. The electromagnetic radiation has a wavelength of 0.67 μm and a power flux density ranging from 5.96×10 10 to 5.96×10 12 W/m 2. The effects due to the protrusion and the dielectric coating are studied. These theoretical results are compared with the experimental data.

In situ investigation of galena dissolution in oxygen saturated solution: evolution of surface features and kinetic rate

In this work, an original Atomic Force Microscopy (AFM) investigation was carried out combining in situ microtopography measurements with solution chemistry. The dissolution of galena (PbS) surface in acidic (HCl, pH = 1) and oxygen saturated solution was investigated in an AFM liquid cell at room temperature over 45 h. In this apparatus, solution circulates into the flow-through reactor at high-flow rate and is periodically sampled for chemical analysis. In the first hours of microtopography measurement square etch pits delimited by steps of minimal depth of 1 nm nucleate and growth attaining a maximal depth of 80 nm. After this short stage, surface features evolve by formation of widespread protrusions 1 to 3 nm high and homogeneously distributed, while nucleation and growth of new etch pits is inhibited, and large rough terraces delimited by macrosteps 50 to 100 nm high are formed by dissolution. The solution interacting with galena surface is strongly undersaturated with respect to both galena and anglesite (PbSO 4), namely solution saturation state for these minerals decreases during the experiment by more than two order of magnitude, and dissolution rate was then derived from lead concentration in solution. During the duration of the experiment, dissolution rates decrease by more than one order of magnitude attaining a final value of 4.5 ± 0.5 × 10 −8 mol m −2 sec −1. Other dissolution experiments performed using different mineral grains confirmed the reproducibility of the galena surface evolution process. However, protrusion observed at lower hydrogen ion activity (pH = 3) showed a rounded shape. From thermodynamic and XPS data in the literature, we predicted that nanometric protrusions are composed of native sulphur, an intermediate and slowly dissolving phase that, in turn, react with oxygen to dissolve and migrate towards bulk solution in a more oxidised state (eg. as SO 4 2−, S 2 O 3 2−, etc.). We interpreted that the protrusion dissolution reaction represents the step limiting the rate of the overall dissolution reaction, and the evolution of the rate regime is a transitory stage due to the continuous increase in the thickness of the altered layer that coats the surface. Finally, this nanoscale detailed description of galena dissolution process is relevant in terms of both reaction mechanisms and computational modelling of metal sulphide oxidation processes.

Fabrication of a pencil-shaped fiber probe with a nanometric protrusion from a metal film for near-field optical microscopy

We propose a method to fabricate a probe with a nanometric protrusion for near-field optical microscopy. The method involves tapering process based on selective etching of an GeO2 doped fiber in buffered hydrogen fluoride solution and metallizing process by vacuum evaporation and chemical polishing. We obtained the tapered probe having a protrusion emerged from a metal film. The protrusion has an apex diameter less than 10nm and a foot diameter less than 20nm. Employing the probe, we succeeded in obtaining a highly spatially resolved image of 20nm sized gold particles.

Reproducible fabrication of a fiber probe with a nanometric protrusion for near-field optics

We propose a new type of fiber probe with a nanometric protruding tip emerging from a metal film and describe a novel method, called the selective resin-coating method, for fabricating such probes. It is a reproducible etching process consisting of four steps and can be applied to silica fibers sharpened by selective chemical etching. With this method, we obtained tips with the apex diameter and the foot diameter of the protrusion being less than 10 and 30 nm, respectively, when the gold film was ~120 nm thick.

Imaging of biological samples by a collection-mode photon scanning tunneling microscope with an apertured probe

We report on high resolution imaging by a collection-mode photon scanning tunneling microscope (c-mode PSTM). In our PSTM system, we have used a novel probe with a nanometric protrusion formed from a metal coated sharpened fiber. By using this probe, flagellar filaments of salmonella of diameter 25 nm could be imaged to have a full width at half maximum of 50 nm. Obtained images strongly depended on the separation of the sample to the probe, the diameter of the aperture, and polarization of the irradiated light. Comments on the origins of these dependencies are given.

Fabrication of a fiber probe with a nanometric protrusion for near-field optical microscopy by a novel technique of three-dimensional nanophotolithography

A new technique based on photolithography has been developed to fabricate a fiber probe with a nanometric protruding tip for near field optical microscopy. As a first step, an optical fiber is sharpened by chemical etching and coated with metallic film. Next, it is coated with photoresist and its apex region is selectively exposed to an evanescent wave. Finally, the metallic film at the apex region is etched away. The foot diameter of the protrusion fabricated by this method is about 30 nm.