Fixation mechanisms of nanoparticles on substrates by electron beam irradiation.

For applications such as the fabrication of plasmonic waveguides we developed a patterning technique to fabricate an array of nanoparticles on a substrate using focused electron beams (Noriki, T.; Abe, S.;.Kajikawa, K.; Shimojo, M. Beilstein J. Nanotechnol.2015,6, 1010-1015). This technique consists of three steps: Firstly, nanoparticles are placed over the entire surface of a substrate. Secondly, the nanoparticles are fixed on the substrate by focused electron beam irradiation. The electron beam decomposes the organic molecules located around the particle into amorphous carbon. The amorphous carbon immobilizes the particle on the substrate. Finally, the unfixed nanoparticles are removed. However, in this original technique, the area in which the nanoparticles were fixed was wider than the electron-probe size of a few nanometers. To understand this widening mechanisms, the effects of accelerating voltage, particle size and substrate material are investigated by means of both experiments and simulation. It is demonstrated that the fixing area is greatly affected by the electrons back-scattered by the substrate. The back-scattering leads to an increase in line width and thus reduces the resolution of this patterning technique.

Biometamaterials: Black Ultrathin Gold Film Fabricated on Lotus Leaf.

We report on a black metamaterial of gold fabricated on a lotus leaf that was used as a template. In spite of the extremely thin gold coating (10-nm thick) on the lotus leaf, the surface shows reflectivity below 0.01 over the entire visible spectral range. Finite-difference time-domain (FDTD) calculations suggest that the low reflectivity stems from the secondary structures on the lotus leaf, where randomly oriented nanorods are distributed.

Patterning technique for gold nanoparticles on substrates using a focused electron beam.

We propose a novel patterning technique for gold nanoparticles on substrates that combines a chemical reaction with electron beam irradiation. First, gold nanoparticles are placed in a two-dimensional arrangement on the substrate. Then, particular nanoparticles are fixed on the substrate by irradiation with a focused electron beam to produce a desired pattern. Finally, the unfixed nanoparticles are removed. Using this technique, an array of gold nanoparticles, for example, in the form of a line or patterned over an area, are prepared on the substrate. This technique could contribute to the fabrication of plasmonic devices and other applications that require the controlled placement of gold nanoparticles on substrates.

Modulation of friction dynamics in water by changing the combination of the loop- and graft-type poly(ethylene glycol) surfaces.

A Velcro-like poly(ethylene glycol) (PEG) interface was prepared in order to control the friction dynamics of material surfaces. Graft- and loop-type PEGs were formed on mirror-polished Ti surfaces using an electrodeposition method with mono- and di-amine functionalized PEGs. The friction dynamics of various combinations of PEG surfaces (i.e., graft-on-graft, loop-on-loop, graft-on-loop, and loop-on-graft) were investigated by friction testing. Here, only the Velcro-like combinations (graft-on-loop and loop-on-graft) exhibited a reversible friction behavior (i.e., resetting the kinetic friction coefficient and the reappearance of the maximum static friction coefficient) during the friction tests. The same tendency was observed when the molecular weights of loop- and graft-type PEGs were tested at 1 k and 10 k, respectively. This indicates that a Velcro-like friction behavior could be induced by simply changing the conformation of PEGs, which suggests a novel concept of altering polymer surfaces for the effective control of friction dynamics.

Microstructural analysis and transport properties of MoO and MoC nanostructures prepared by focused electron beam-induced deposition.

By electron-beam-induced deposition, we have succeeded in the direct fabrication of nanowires of molybdenum oxide (MoOx) and molybdenum carbide (MoC) on a SiO2 substrate set in a scanning electron microscope. In order to prepare MoOx specimens of high purity, a precursor gas of molybdenum hexacarbonyl [Mo(CO)6] is used, mixed with oxygen gas. On the other hand, MoC is grown by mixing H2O gas with the precursor gas. The electrical transport properties of the nanowires are investigated by the DC four-terminal method. A highly resistive MoOx nanowire prepared from an as-deposited specimen by annealing in air shows nonlinear current-voltage characteristics and a high photoconductivity. The resistivity ρ of an as-deposited amorphous MoC (a-MoC) nanowire takes its maximum at a temperature T ≈ 10 K and decreases to ≈ 0 with decreasing temperature. This behavior of ρ(T) indicates the possible occurrence of superconductivity in a-MoC nanowires. The characteristic of ρ(T) below the superconducting transition temperature Tc ≈ 4 K can be well explained by the quantum phase-slip model with a coherence length ξ(0) ≈ 8 nm at T = 0.

Contrast in atomically resolved EF-SCEM imaging.

Energy-filtered scanning confocal electron microscopy (EF-SCEM) is a technique that uses the reduced depth of field of an aberration-corrected transmission electron microscope to provide three-dimensional (3D) compositional information. Using a silicon sample in the <110> orientation, we show that EF-SCEM image data can be recorded that shows lattice resolution in the plane perpendicular to the incident beam direction. The confocal effect is demonstrated through the reduction of the mean intensity as the confocal plane is displaced from the sample mid-plane, unlike optical sectioning in high-angle annular dark-field scanning transmission electron microscopy (STEM). Simulations of the EF-SCEM data show agreement with the experimental data, and allow the interpretability of the data to be explored. The effects of channelling, absorption and delocalisation complicate the quantitative and qualitative interpretation of the data, highlighting the need for matching to simulations. Finally the effects of the finite detector pin-hole aperture size are explored, and we show that the EF-SCEM contrast in the plane perpendicular to the beam direction starts to resemble that of a STEM spectrum imaging experiment as the aperture size increases.

Direct acquisition of interferogram by stage scanning in electron interferometry.

We present an electron holography technique to acquire an interferogram, that is, cosine image of phase distribution. The interferogram is constructed by shifting the specimen in one direction with a stage-scanning system and acquiring line intensities of holograms. Taking line intensities eliminates the carrier fringes in the holograms and yields the interferogram. Under phase object approximation, the object phase can be readily obtained from the interferogram without any reconstruction procedure. The spatial resolution of phase is determined independently of the fringe spacing, overcoming the limitation of conventional techniques based on the Fourier transformation method.

Effect of cold rolling on the magnetic susceptibility of Zr-14Nb alloy.

The magnetic susceptibility of cold-rolled Zr-14Nb was evaluated to apply a new metallic medical device used for magnetic resonance imaging (MRI). The magnetic susceptibility of cold-rolled Zr-14Nb decreased up to the reduction ratio of 30%, then gradually decreased up to the ratio of 90%. Transmission electron microscopic observation revealed the strain-induced formation of ω phase after cold rolling at the reduction ratio of 5%, indicating that the initial decrease in magnetic susceptibility was caused by the formation of the ω phase. The ω phase was saturated at the reduction ratio of 30%. The formation of the ω phase could be explained on the basis of the increase in the Young's modulus and Vickers hardness of cold-rolled Zr-14Nb. The effect of texture formation on these properties was not obvious in the cold-rolled Zr-14Nb. Because of the strain-induced formation of the ω phase, the magnetic susceptibility of Zr-14Nb can be reduced by cold rolling to as low as that of as-cast Zr-9Nb, which is one-third that of Ti and Ti alloys. Therefore, cold-workable Zr-14Nb with low magnetic susceptibility could be a promising alloy for medical devices under MRI.

Improvement of depth resolution of ADF-SCEM by deconvolution: effects of electron energy loss and chromatic aberration on depth resolution.

Scanning confocal electron microscopy (SCEM) is a new imaging technique that is capable of depth sectioning with nanometer-scale depth resolution. However, the depth resolution in the optical axis direction (Z) is worse than might be expected on the basis of the vertical electron probe size calculated with the existence of spherical aberration. To investigate the origin of the degradation, the effects of electron energy loss and chromatic aberration on the depth resolution of annular dark-field SCEM were studied through both experiments and computational simulations. The simulation results obtained by taking these two factors into consideration coincided well with those obtained by experiments, which proved that electron energy loss and chromatic aberration cause blurs at the overfocus sides of the Z-direction intensity profiles rather than degrade the depth resolution much. In addition, a deconvolution method using a simulated point spread function, which combined two Gaussian functions, was adopted to process the XZ-slice images obtained both from experiments and simulations. As a result, the blurs induced by energy loss and chromatic aberration were successfully removed, and there was also about 30% improvement in the depth resolution in deconvoluting the experimental XZ-slice image.

Three-dimensional observation of SiO2 hollow spheres with a double-shell structure using aberration-corrected scanning confocal electron microscopy.

Optical sectioning using scanning confocal electron microscopy (SCEM) is a new three-dimensional (3D) imaging technique which promises improved depth resolution, particularly for laterally extended objects. Using a stage-scanning system to move the specimen in three dimensions, two-dimensional (2D) images sliced from any plane in XYZ space can be obtained in shorter acquisition times than those required for conventional electron tomography. In this paper, a double aberration-corrected SCEM used in annular dark-field mode was used to observe the 3D structure of SiO(2) hollow spheres fabricated by a carbon template method. The double-shell structure of the sample was clearly reflected in both XY- and XZ-sliced images. However, elongation along the optical axis was still evident in the XZ-sliced images even when double aberration correctors were used. Application of a deconvolution technique to the experimental XZ-sliced images reduced the elongated shell thicknesses of the SiO(2) sphere by 40-50% and the selectivity of information at a certain sample depth was also enhanced. Subsequently, 3D reconstruction by stacking the deconvoluted slice images restored the spherical surface of a SiO(2) sphere.

Experimental examination of the characteristics of bright-field scanning confocal electron microscopy images.

We experimentally examined the characteristics of bright-field (BF) scanning confocal electron microscopy (SCEM) images by changing the observation conditions and comparing the images with those obtained by BF transmission electron microscopy (TEM) and BF scanning TEM (STEM) modes. The observation of 5-nm-diameter Au nanoparticles demonstrated that BF-SCEM produces object elongation of more than 2000 nm along the optical axis, as do BF-TEM and BF-STEM. We demonstrated the relationship between elongation length and geometric effects such as convergence and collection angles of a probe and the lateral size of an object; the relationship is consistent with previous theoretical prediction. Further, we observed interesting features that are seen only in the BF-SCEM images; the film contrast was strongly enhanced, compared with that of BF-STEM. In addition, a bright contrast appeared around the object position in the elongated images. Using this characteristic, we could determine the object position and structure.

Bright-field scanning confocal electron microscopy using a double aberration-corrected transmission electron microscope.

Scanning confocal electron microscopy (SCEM) offers a mechanism for three-dimensional imaging of materials, which makes use of the reduced depth of field in an aberration-corrected transmission electron microscope. The simplest configuration of SCEM is the bright-field mode. In this paper we present experimental data and simulations showing the form of bright-field SCEM images. We show that the depth dependence of the three-dimensional image can be explained in terms of two-dimensional images formed in the detector plane. For a crystalline sample, this so-called probe image is shown to be similar to a conventional diffraction pattern. Experimental results and simulations show how the diffracted probes in this image are elongated in thicker crystals and the use of this elongation to estimate sample thickness is explored.

Nanoscale energy-filtered scanning confocal electron microscopy using a double-aberration-corrected transmission electron microscope.

We demonstrate that a transmission electron microscope fitted with two spherical-aberration correctors can be operated as an energy-filtered scanning confocal electron microscope. A method for establishing this mode is described and initial results showing 3D chemical mapping with nanoscale sensitivity to height and thickness changes in a carbon film are presented. Importantly, uncorrected chromatic aberration does not limit the depth resolution of this technique and moreover performs an energy-filtering role, which is explained in terms of a combined depth and energy-loss response function.

Three-dimensional optical sectioning by scanning confocal electron microscopy with a stage-scanning system.

We evaluated the depth resolution of annular dark-field (ADF) scanning confocal electron microscopy (SCEM) with a stage-scanning system by observation of nanoparticles. ADF-SCEM is a three-dimensional (3D) imaging technique that we recently proposed. An ADF-SCEM instrument involves a pinhole aperture before a detector for rejecting electrons from the out-of-focal plane in a specimen and an annular aperture under the specimen for collecting only scattered electrons. The stage-scanning system enables us to directly obtain optical slice images perpendicular and parallel to an optical axis at a desired position. In particular, the parallel slices visualize the elongation of nanoparticles along the optical axis, which depends on the depth resolution. ADF-SCEM effectively reduced the elongation length of the nanoparticles sufficiently to demonstrate depth sectioning, in comparison with scanning transmission electron microscopy and bright-field SCEM. The experimentally obtained length was nearly equal to the theoretically estimated one from the probe size considering the experimental conditions. Furthermore, we applied this ADF-SCEM technique to analysis of the 3D position of catalytic nanoparticles on carbon nanostructures.

Development of a stage-scanning system for high-resolution confocal STEM.

A stage-scanning system is composed of a specially designed transmission electron microscopy specimen holder equipped with a piezo-driven specimen stage, power supplier and control software. This system enables the specimen to be scanned three-dimensionally, and therefore confocal scanning transmission electron microscopy (STEM) can be performed with a fixed electron-optics configuration. It is demonstrated that stage-scanning confocal STEM images can be obtained with the lateral atomic resolution and the specimen can be moved three-dimensionally with high precision.

Controlled growth of Zn nano-dots on a Si(111)-7x7 surface saturated with C2H5OH.

Metal atoms bonded with Si adatoms on the Si(111)-(7x7) surface undergo migration by hopping adjacent Si-rest atoms with dangling bond. By saturated adsorption of Si(111)-(7x7) surface with C(2)H(5)OH, the whole Si-rest atoms and a half of Si adatoms are occupied with Si-H and Si-OC(2)H(5), so that the Zn atoms adsorbed on this surface cannot migrate by hopping. When Zn atoms were deposited on this surface, ca. 5 nm Zn dots were grown in the hexagonal spacing of ca. 5.4 nm width around the corner holes, which work as a mold. This is quite different from the growth of honeycomb layers composed of Zn(3) clusters on the clean Si(111)-(7x7) surface. The dots grow up to nine (1.97 nm) to 13 layers (2.64 nm) by keeping their size, which implies a layer-by-layer growth of dots in the mold, where the growth is controlled by the kinetics instead of energetic feasibility.

Second-harmonic spectroscopy of surface immobilized gold nanospheres above a gold surface supported by self-assembled monolayers.

We have investigated linear and nonlinear optical properties of surface immobilized gold nanospheres (SIGNs) above a gold surface with a gap distance of a few nanometers. The nanogap was supported by amine or merocyanine terminated self-assembled monolayers (SAMs) of alkanethiolates. A large second-harmonic generation (SHG) was observed from the SIGN systems at localized surface plasmon resonance condition. The maximum enhancement factor of SHG intensity was found to be 3 x 10(5) for the SIGN system of nanospheres 100 nm in diameter with a gap distance of 0.8 nm. The corresponding susceptibility was estimated to be chi((2))=750 pmV (1.8 x 10(-6) esu). In the SIGN system supported with the merocyanine terminated SAMs, the SHG response was also resonant to the merocyanine in the nanogap. It was found that the SHG response of the SIGN systems is strongly frequency dependent. This leads us to conclude that the large chi((2)) is caused by enhanced electric fields at the localized surface plasmon resonance condition and is not due to an increase of the surface susceptibility following from the presence of the gold nanospheres. The observed SHG was consistent with the theoretical calculations involving Fresnel correction factors, based on the quasistatic approximation.

TEM sample preparation using a new nanofabrication technique combining electron-beam-induced deposition and low-energy ion milling.

A new TEM sample preparation technique using electron-beam-induced deposition combined with low-energy ion milling was used to fabricate for two different shapes of sample, conical and plate. High-quality HREM images can be obtained from samples prepared by this technique. A desired sample position can be obtained with high accuracy, and the total sample preparation time can be much less than conventional techniques. Because the gas deposition system used can easily be integrated in a conventional SEM, the method can be performed in any laboratory equipped with a SEM and an ion milling machine.