Magnetic force microscopy signal of flux line above a semi-infinite type II-superconductor

We have examined a single flux line in the semi-infinite type-II superconductor. The stray magnetic field of the flux line has been calculated. We have found that the vertical force exerted on a magnetic force microscopy (MFM) tip from the flux line is measurable by currently existing MFM. Two types of magnetic tips were taken into consideration, solid and thin film tips. For example, with a Cobalt film of the thickness of 100 nm and 30 nm on a tip, we found a vertical force of 4*10^–10 N and 1.5*10^–10 N, respectively. The lateral force exerted on a tip by the flux line was also calculated. The lateral force must be small enough to prevent the flux line from becoming depinned.     

Tip-induced oxidative nano-machining of conducting diamond-like carbon (DLC)

Electrically conducting diamond-like carbon films have been nano-machined by local deposition of thermal energy at the tip-to-film point of contact; the process is implemented on an atomic force microscope platform. Features with linewidth resolution down to 20 nm have been demonstrated; lateral irregularities along edges were less than 5 nm, while the radius of curvature at the edges was less than 10 nm; and the slope of features was limited by the aspect ratio of the tip. The mechanism arises from prompt thermal oxidation by intermittent transfer of heat from an ohmically heated tip, where Fowler–Nordheim tunnelling is likely to play a role in completing an electrical circuit when the physical continuity of the thermal circuit is interrupted. When heat transfer is insufficient to ensure prompt oxidation, then formation of a metastable low-density carbon phase is found to take place. That phase will then, at room temperature and in the presence of oxygen, relax back to a normal higher density phase over a period of one hour. The many desirable physico-chemical properties of diamond-like carbon, in combination with the good spatial resolution of local probe methodology, suggest that the outcomes could have significant implications for next-generation nano-machine and nano-templating technologies. PACS 07.79.Lh; 81.16.Nd; 81.16.Pr; 81.16.Rf     

Fabricating high-aspect-ratio sub-diffraction-limit structures on silicon with two-photon photopolymerization and reactive ion etching

We fabricated sub-micrometer objects with feature sizes about one third of the exposure wavelength using two-photon photopolymerization in an epoxy-based photoresist SU-8 . Owing to the high mechanical strength of this photoresist, an aspect ratio as high as nine was achieved with a 200–300 nm lateral dimension. A simple equation was used to estimate the feature size from the laser parameters such as spot size, exposure time, pulse width, pulse repetition rate, and the material properties including the two-photon absorption coefficient and the exposure threshold dose. Patterns in SU-8 were transferred onto silicon using reactive ion etching, preserving both the feature size and aspect ratio. Vertical sidewalls of the transferred patterns were achieved using the black silicon method. PACS 42.82.Cr; 82.35.Ej; 85.40.Hp     

A Si nano–micro-wire array on a Si(111) substrate and field emission device applications

A large number of Si wires on Si(111) can be fabricated selectively by the vapor–liquid–solid growth method with a high aspect ratio greater than 100. The diameter of the wire can be controlled from less than a micron to a few hundred microns. We propose a novel smart field electron emission device using silicon nano-wires fabricated by this vapor–liquid–solid growth method, and demonstrate field electron emission with a quite low operation voltage from a gated silicon nano-wire. The threshold voltage is about 13 V, and the value is similar to those for gated carbon-nanotube field emitters. The emission current reaches 10 nA at 15 V gate voltage.     

Electroless Nickel Plating under Continuous Ultrasonic Irradiation to Fabricate a Near-Field Probe Whose Metal Coat Decreases in Thickness toward the Tip

This paper describes a size-dependent electroless plating method to fabricate a new type of probe with a locally decreasing thickness of metal and a tiny tip size for a combined high resolution shear-force and near-field optical microscope. In this method, the tip size and decreasing thickness profile, which affect the resolution capabilities of the microscopes, are controlled by adding a continuous ultrasonic wave with a frequency of 1 MHz to a nickel plating bath. The probe with a tip radius of curvature of 25 nm was successfully fabricated at an ultrasonic power density of 1.6 W cm⁻², its metal thickness gradually decreased from 850 to 20 nm toward the distal tip.     

Effects of size and interactions on the magnetic behaviour of elliptical (0 0 1)Fe nanoparticles

Arrays of elliptical particles with aspect ratio 1:3 and short axes 50, 100 and 150 nm were prepared by electron-beam lithography and ion-beam milling of epitaxial (0 0 1)Fe films of thicknesses 10 and 20 nm. The domain state of an individual particle imaged by magnetic force microscopy in zero field after demagnetization was observed to change from being bi-domain or multidomain (MD) to stable single domains (SD) as the lateral size and film thickness were decreased. The critical size for SD formation was found to be close to the actual lateral sizes of 100 nm×300 nm and 150 nm×450 nm for the thicknesses of 20 and 10 nm, respectively. Only in the 10 nm thick ellipses of lateral size 100 nm×300 nm, the magnetization reversal may take place through coherent rotation. For all other investigated samples, the experimental switching field is lower than what would be required for this process.     

Simulation of polycrystalline 3D film growth: An investigation of the evolution of grain size and texture in diamond films

An analytical method for simulating gas phase film growth has been developed and used to study the growth of diamond films during prolonged deposition, i.e. the film thickness is much larger than the lateral grain size. From a model system composed of 10⁴ grains, reliable results can be evaluated for the growth of diamond films by (111) and (001) deposition under different initial conditions and with varying growth parameters. It is demonstrated that the rate of structure evolution is sensitively influenced by the aspect ratio of diamond crystal. A near-linear proportionality between the average grain size and the thickness of films can be approximately yielded for a large film thickness which is about 10 times of the average distance of the nuclei. The proportionality constant varies for a statistical nucleation from 0.0056 to 0.43 by changing the aspect ratio. Furthermore, the orientational distribution is drastically narrowed down so that the probability of coalescence of grains with a slight orientational difference is considerably increased. Received: 28 September 2000 / Accepted: 19 February 2001 / Published online: 3 May 2001     

绝缘衬底上高密度均匀纳米硅量子点的形成与表面形貌

利用等离子体增强化学气相淀积技术，在绝缘氮化硅(SiNx)衬底上制备超薄非晶硅(aSi：H)薄膜，通过超短脉冲激光辐照与准静态常规热退火技术处理，制备出高密度、均匀纳米硅(ncSi)量子点.使用原子力显微镜对处理前后样品的表面形貌进行了研究，发现激光辐照能量密度增加的同时，所形成的ncSi尺寸也随之增加.在合适的能量密度范围内，可以得到面密度大于10.11cm^2、尺寸分布标准偏差小于20％的10 nm ncSi量子点薄膜，表明所制备的ncSi量子点具有较好的均匀性及较高的面密度.同时，对ncS i量子点 关键词： 纳米硅 激光诱导 尺寸分布     

Formation of embedded patterns in glasses using femtosecond irradiation

Channel formation in glasses is demonstrated and discussed. Photosensitive glass (Foturan) was microstructured using femtosecond irradiation at 800 nm and 400 nm wavelengths, with subsequent thermal annealing, and HF etching of a lithium-silicate phase formed by exposure-annealing. It was found that there is a significant difference in the volume of lithium-silicate when the exposure was made with and without optical breakdown. Channels with a minimum cross section of approximately 10 m were achieved.In silicate glasses, the optically induced dielectric breakdown was used for the recording of channel patterns. The highest wet etching rate in a HF based solution was observed when the irradiance corresponded to the 2.5–3.0 thresholds of dielectric breakdown and the adjacent pulses were overlapping by more than 50% of the diameter of the focussed beam. Under these conditions, the formation of a void inside the glass was confirmed by optical observation of single shot damage under a microscope. The mechanism of selective wet etching in silicate glasses is discussed in terms of the stress corrosion effect, which explains crack propagation in glasses via the reaction of stretched Si–O–Si bonds at the tip of a crack with water, resulting in SiOH formation. It is shown that intra-connection of voxels was the key factor to achieve etching of high aspect ratio patterns in silica glass. PACS 42.70.Ce; 81.05.Kf; 82.50.Pt; 87.80.Mj     

Fabrication of high-aspect-ratio silicon nanopillar arrays with the conventional reactive ion etching technique

We fabricate silicon nanopillar arrays with pillar diameters smaller than 200 nm by using the conventional reactive ion etching (RIE) technique and nickel masks. We use the ratio between the lateral and vertical etching rates as an estimate of the etching anisotropy. The dependence of this ratio on the rf power, the chamber pressure, and the gas mixture is investigated systematically to achieve the largest etching anisotropy. Using the optimized etching parameters in the RIE process, we demonstrate silicon pillars with smooth surface, vertical sidewalls, and aspect ratios higher than 20. In addition, we employ dilute aqua regia to treat the pillars and shrink the diameters to 70 nm. The pillar height remains ∼2500 nm after the treatment. PACS 52.77.Bn; 81.65.Cf; 85.40.Hp     

Inspection of nano-sized SNOM-tips by optical far-field evaluation

High resolution optical microscopy has many interesting applications in solid state physics, low temperature physics, biology and semiconductor technology. Unfortunately, the lateral resolution of conventional microscopes is limited by the Rayleigh-limit. “Scanning nearfield optical microscopy” (SNOM) seems to be a promising new approach to characterize the properties of materials optically with a high lateral resolution of 50–100 nm. The most important part of such a microscope is the scanning probe (a special glass fiber tip). However, the quality of the optical fiber tip is of decisive importance. Since the production process of pulled and coated glass fiber tips is still highly empirical and error-prone, a technique would be useful to determine the tips’ quality before they are shipped to the user or mounted in the microscope. The tips’ apertures are smaller than λ/2 and therefore they cannot be measured in a non-destructive way by conventional optical microscopy. This paper discusses an easy and fast method for the optical characterization of common glass fiber SNOM tips. The effective aperture of the tip is measured from the far-field distribution of the emitted intensity recorded by a CCD target. A numerical model is introduced to solve this inverse task and a simple optical setup is presented to detect light emitted by the tip at an angle of up to 90° from the optical axis. Experimental investigation, near/far-field calculations and scanning electron microscope investigations show the working principle of this measurement technique for the analysis and evaluation of a typical nanostructured object.     

Design and Fabrication of Microlens Array for Near-Field Vertical Cavity Surface Emitting Laser Parallel Optical Head

A GaP microlens for collecting laser light was developed in the tip of a near-field probe. It is important to realize a near-field optical probe head with high throughput and a small spot size. The design and fabrication results of the GaP microlens array are described. The most suitable GaP microlens with a probe was calculated as having a 10 μm radius using the two-dimensional finite difference time domain (2-D FDTD) method. The full width half maximum (FWHM) spot size variation and optical power density tolerance were calculated as 157 nm ± 5 nm and 7%, respectively. A spherical GaP microlens was fabricated with a radius of 10 μm by controlling the Cl₂/Ar gas mixture ratio. The difference between the theoretical spherical shape and the fabricated GaP microlens was evaluated as 40 nm at peak to valley. The FWHM spot size and optical throughput of the fabricated microlens were measured as 520 nm and 63%, respectively. The microlens was the same as a theoretical lens with a 10 μm radius. The micron-lens array fabrication process for a near-field optical head was demonstrated in this experiment.     

Synthesis and attachment of silver nanowires on atomic force microscopy cantilevers for tip‐enhanced Raman spectroscopy

Surface‐enhanced Raman spectroscopy is based on the absorption of light by nanometer‐sized metal particles, resulting in large enhancement of the Raman signal. By replacing the metal particles by a metallic nanotip, the enhancement can be localized. The resulting tip‐enhanced Raman spectroscopy is capable of measuring Raman spectra with high spatial resolution, effectively overcoming the diffraction limit. A successful tip‐enhanced Raman spectroscopy experiment depends heavily on the ability to fabricate tips of a definite metal with the appropriate shape and size, which is still a challenging process. We have prepared silver nanowires with a diameter of 200–300 nm by templated electrochemical deposition and attached them onto atomic force microscope cantilevers by focused electron beam induced deposition. We found that they produce a reproducible enhancement of the Raman signal intensity. Other metals and smaller nanostructures might also be produced, suggesting an interesting development potential for these novel nanoprobes. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of atomic force microscopy nanoindentation for the development of nanostructures

We have studied the fabrication of atomic force microscope (AFM) based nanotemplates using electrically controlled indentation (ECI) and a composite barrier (photoresist/alumina) that is resistant to the lithography process and presents good mechanical properties for indentation. The indentation process is affected by several factors such as the indentation speed, the trigger voltage and the barrier type. We have used the nanotemplate technique to fabricate small gold–gold nanocontacts (1–10 nm). In this limit, the size of the contacts that is obtained through the indentation process seems to be stochastic. However, low dimension, clean metallic contacts were achieved with high temporal stability and compatible with low temperature measurements. The fabricated nanotemplates are versatile and can be used in a wide range of applications, from nanojunctions to connecting a single nano-object. Small area metallic contacts can be used to study spin injection or ballistic transport.     

Multiform structures with silicon nanopillars by cesium chloride self-assembly and dry etching

We develop a novel method to fabricate multiform structures of Si nanopillars (diameters > 40 nm, aspect ratio > 10, coverage ratio > 35%) by dry etch with self-assembled cesium chloride (CsCl) nanoislands as mask. The pillars can cover structures of lateral size 1 μm and unpolished Si wafer, enabling uneven surface to be textured by nanopillars without complex process or expensive polishing. Planar micro-patterns and tridimensional localization of nanopillars have been easily realized, useful for integrating nanopillars to devices. By figuring out substrate influences, fast formation of CsCl islands within 1 min has been achieved for the first time, making CsCl process flow to be possibly controlled within 30 min. Based on the deliquescence of salt, CsCl self-assembly is simple, widely tunable and compatible, which endows the approaches great practical potential.     

Controlling aspect ratios of suspended nanorods fabricated by multi-photon polymerization

Nanorods are building blocks of three-dimensional photonic crystals and other nanostructures fabricated by multi-photon polymerization with femtosecond laser pulses. The aspect ratios of their cross sections are critical to the in-plane and the interlayer rod distances, which greatly affect the performance. Here we demonstrate the control of aspect ratios from ∼3 to 0.85. At a high scanning speed, aspect ratios can be smaller than unity with a lateral size of ∼150 nm. The results indicate that cylindrical nanorods can be polymerized by the commonly used transverse scanning method to improve the qualities of three-dimensional nanostructures.     

Underwater and water-assisted laser processing: Part 2—Etching, cutting and rarely used methods

In the second part of the article, the subtractive processes—laser etching and cutting—in the presence of liquid water will be reviewed; but the rarely used methods of water assisted/underwater laser processing, such as welding, silicon wafer breaking, surface modification of polymers, pulsed laser deposition, particle formation and water mask defined microstructures fabrication, will also be described. Etching and cutting under water provide better tolerances and smaller heat-affected zone widths and avoid the re-deposition of debris. Irradiation under water results in increased wetting of fluoropolymers, and laser ablation in water vapor provides deposition of highly crystalline hydroxyapatite coatings. Laser irradiation of solid targets in water has been used to fabricate Ag, Au, Ni, Cu and carbon nanoparticles. The results of an original study on the formation of free-standing high aspect ratio Pb(Zr_xTi_1−x)O₃ microplates fabricated by laser irradiation of Pb(Zr_xTi_1−x)O₃ ceramics in water are also reported. The platelets were up to 60 μm in diameter and 50–160 nm in thickness. The use of neutral liquids other than water and some medical applications of underwater/water-assisted laser light driven processes will also be briefly reviewed.     

A novel noncontacting waveguide backshort for submillimeter wave frequencies

A novel noncontacting waveguide backshort has been developed for millimeter wave and submillimeter wave frequencies. It employs a metallic bar with rectangular or circular holes. The size and spacing of the holes are adjusted to provide a periodic variation of the guide impedance on the correct length scale to give a large reflection of rf power. This design is mechanically rugged and can be readily fabricated for high submillimeter wave frequencies where conventional backshorts are difficult or impossible to fabricate. Model experiments have been performed at 4 GHz – 6 GHz to empirically optimize the design parameters. Values of reflected power greater than 95% over a 30% bandwidth have been achieved. A specific design is presented which has also been successfully scaled to WR-10 band (75 GHz – 110 GHz). A theoretical analysis is compared to the experiments and found to agree well with the measured data.     

Migration-coalescence of Nanoparticles During Deposition of Au,Ag, Cu,and GaAs on Amorphous SiO2

Nanoparticles formed during the initial period of film growth can migrate, coalesce, and may also melt. Nanoparticles of Au, Ag, Cu, and GaAs ranging from 1 to 15nm in diameter were sputter-deposited on amorphous SiO₂ (a-SiO₂). Transmission electron microscopy was used to analyze the time-dependent change of the dispersion of particles on a thin film. The number density of nanoparticles was nearly constant during the deposition of Ag. For Au, Cu, and GaAs, however, the number density decreased with time during the early deposition period. For example, for Au the number density decreased from 2.8×10¹⁶m^–2 (surface coverage ratio of 0.08) to 1.8×10¹⁶m^–2 (surface coverage ratio of 0.14). The surface coverage increased because the particle size increased as the number density decreased. This decrease suggests that migration followed by coalescence occurred. For Au, although we found evidence of migration of 2-nm particles at 500°C, the migration rate was too slow to account for the results from the deposition experiments. These observations indicate an autocatalytic mechanism that migration followed by coalescence liberates energy by the formation of chemical bonds, heats the coalesced particles, and enhances further migration. The strong dependence of the structure of nanoparticle dispersions on the deposition rate is a direct consequence of the deposition mechanism, which is a nonlinear, kinetically-controlled process.     

Seed layer-free synthesis and characterization of vertically grown ZnO nanorod array via the stepwise solution route

A novel stepwise method was developed for the deposition of ZnO nanorod array (NRA) from the simple inorganic aqueous solution. Different from the traditional one-pot synthesis route, merely a thin liquid precursor layer adsorbed on the substrate instead of the bulk solution underwent the reaction at elevated temperature in a typical deposition cycle. Sparse and vertically grown wurtzite ZnO NRA was deposited on seed layer-free glass substrate after 20 cycles (in typically 20 min). Each individual ZnO rod possessed the well-defined hexagonal facet, the side length of about 150 nm, the aspect ratio of 2:3, and the small size dispersity. Also the overall ZnO NRA exhibited high ultraviolet photoluminescence and weak blue emission, indicating its good optical properties. Mechanism analysis indicated that, the decrease of the supersaturation degree in solution after the climax in the reaction period of each deposition cycle is the root cause of the sparse nucleation and the vertical growth of ZnO nanorods. The work has opened up a novel stepwise approach toward high quality ZnO NRA, being valuable for extending the synthetic methods of semiconductor nanostructures in mild solutions.