Role of steps in epitaxial growth

The role of steps in the epitaxial growth of quantum structures is discussed. We present experimental results and theoretical predictions of growth on stepped surfaces. Scanning tunneling microscopy (STM) images of molecular beam epitaxy grown GaAs(001) surfaces misoriented by 1° and 2° towards the (111) A direction show non-uniform terraces with a peak in the terrace width distribution at 40 Å. Simple models of atoms landing on a step and attaching at the ascending step edge, however, predict an equalization of terrace widths. A thermodynamic model which allows the steps to move freely with the constraint that it costs energy to form a kink predicts step bunching for high kink energies. Steps on vicinal surfaces have been utilized for growing quantum wire structures using a technique where fractional monolayers of different materials are deposited on a stepped surface, leading to the creation of a lateral superlattice (LSL). The terrace width uniformity is observed by STM to improve dramatically with the growth of an AlAs---GaAs LSL. Cross-sectional transmission electron microscopy of LSLs shows good segregation of the composite layers.     

The calculation of drag on nano‐cylinders

The study of molecular flows at low Knudsen numbers (～0.1–0.5), over nano‐scaled objects of 20–100 nm size is becoming an important area of research. The simulation of fluid–structure interaction at nano‐scale is important for understanding the adsorption and drag resistance characteristics of nano‐devices in the fields of drug delivery, surface cleaning and protein movement. A novel formulation has been proposed that calculates localised values for both the kinetic and configurational parts of the Irving–Kirkwood stress tensor at given fixed positions within the computational domain. Macroscopic properties, such as streaming velocity, pressure and drag coefficients, are predicted by modelling the fluid–structure interaction using a moving least‐squares method. The gravitation‐driven molecular flow is examined over three different cross‐sectional shapes—i.e. diamond‐, circular‐ and square‐shaped cylinders—confined within parallel walls and has been simulated for rough and smooth surfaces. The molecular dynamics formulation has allowed, for the first time, the calculation of localised drag forces over nano‐cylinders. The computational simulation has shown that existing methods, including continuum‐based approaches, significantly underestimate drag coefficients over nano‐cylinders. The proposed molecular dynamics formulation has been verified on simulation based tests, as experimental and analytical results are unavailable at this scale. Copyright © 2016 John Wiley & Sons, Ltd.     

Determination of (111) ordered domains on platinum electrodes by irreversible adsorption of bismuth

Irreversible adsorbed bismuth can be used to determine the fraction of (111) domains on a given platinum sample. On Pt(111) electrodes, the surface redox process of adsorbed bismuth takes place at 0.63 V in a well-defined peak. The behavior of this redox process on the Pt(111) vicinal surfaces indicates that the bismuth atoms involved in the redox process are only those deposited on the (111) terrace sites and that the charge under the peak at 0.63 V is directly proportional to the number of sites on (111) ordered domains (terraces). The good linear relationship obtained between the charge for the bismuth redox process and the number of (111) terrace sites on the vicinal surfaces allows construction of a calibration curve. This calibration curve has been used to directly estimate the amount of (111) ordered domain terrace sites on polycrystalline platinum samples with different surface ordered domains. The results agree with what we would expect from our knowledge of these surfaces.     

In‐Situ Confined Growth of Monodisperse Pt Nanoparticle@Graphene Nanobox Composites as Electrocatalytic Nanoreactors

Monodisperse Pt nanoparticles (NPs) studded in a three‐dimensional (3D) graphene nanobox are successfully synthesized through a simple in‐situ confined growth route for the first time. The nano‐zeolite A was used as a 3D substrate for in‐situ growth of tri‐layered graphenes on the crystal‐surfaces, meanwhile, the inner micropores of which can also be utilized for the confined growth of Pt nanoparticles. The graphene sheets are curved on the edges to form a 3D hollow box morphology, where the monodisperse Pt nanoparticles are homogeneously studded on the inner surfaces. Moreover, the Pt content can be regulated from ～8 to 50 wt%, and the particle size can be tuned from 2–5 nm by varying the pristine Pt‐ion loading amount and CVD temperature. The Pt NP@graphene nanoboxes possess not only large pore volumes to effectively accommodate large amounts of oxygen, but also supply excellent electrical conductivity for the fast transfer of electrons (～3.96 e^‐), resulting in a high efficiency (175 mA/mg Pt) and long‐term stability (above 1000 cycles) for the oxygen reduction reaction.     

Size-dependent electrical conductivity of indium zinc oxide deposited by RF magnetron sputtering

We investigated the size-dependent electrical conductivities of indium zinc oxide stripes with different widths from 50 nm to 4 microm and with the same thickness of 50 nm deposited by RF magnetron sputtering. The size of the indium zinc oxide stripes was controlled by e-beam lithography. The distance of the two Ti/Au Ohmic electrodes along the indium zinc oxide stripes was kept constant at 25 microm. The electrical conductivity decreased as the size of the indium zinc oxide stripes decreased below a critical width (80 nm). The activation energy, derived from the electric conductivity versus temperature measurement, was dependent on the dimensions of indium zinc oxide stripes. These results can be understood as stemming from surface charge trapping from the absorption of oxygen and/or water vapor, which leads to an increase in the energy difference between the conduction energy band and the Fermi energy.     

Surface Characteristics,Copper Release,and Toxicity of Nano‐ and Micrometer‐Sized Copper and Copper(II) Oxide Particles: A Cross‐Disciplinary Study

An interdisciplinary and multianalytical research effort is undertaken to assess the toxic aspects of thoroughly characterized nano‐ and micrometer‐sized particles of oxidized metallic copper and copper(II) oxide in contact with cultivated lung cells, as well as copper release in relevant media. All particles, except micrometer‐sized Cu, release more copper in serum‐containing cell medium (supplemented Dulbecco's minimal essential medium) compared to identical exposures in phosphate‐buffered saline. Sonication of particles for dispersion prior to exposure has a large effect on the initial copper release from Cu nanoparticles. A clear size‐dependent effect is observed from both a copper release and a toxicity perspective. In agreement with greater released amounts of copper per quantity of particles from the nanometer‐sized particles compared to the micrometer‐sized particles, the nanometer particles cause a higher degree of DNA damage (single‐strand breaks) and cause a significantly higher percentage of cell death compared to cytotoxicity induced by micrometer‐sized particles. Cytotoxic effects related to the released copper fraction are found to be significantly lower than the effects related to particles. No DNA damage is induced by the released copper fraction.     

Fabrication and Optical Properties of Periodic Ag Nano‐Pore and Nano‐Particle Arrays with Controlled Shape and Size over Macroscopic Length Scales

A facile and economical route to preparation of highly ordered sliver pore or particle arrays with controlled pore‐shape and size extended over cm² areas is described. The substrates are prepared at planar and curved surfaces via sphere‐imprinted polymer (PDMS) templating using polystyrene spheres with diameters of 820, 600, or 430 nm. Nano‐pore arrays are created by sputtering 80 nm of Ag directly onto the templates and nano‐particle arrays are prepared by electrode‐less deposition of Ag from Tollen's reagent. The shape of the nano‐pore or particles in the array conformed to that of the imprint of the sphere on the template. Stretching the flexible template enable creation of cuboid shaped nano‐voids and nano‐particles following Ag deposition. Diffuse reflectance from the spherical Ag nano‐cavity arrays showed absorbance maxima at wavelengths comparable similar to the diameter of the templating sphere, whereas reflectance from the cuboid arrays, showed little correlation with the sphere diameter. The cuboid nano‐particle arrays showed the most intense visible absorption which is red‐shifted compared to the spherical arrays. White light diffraction from the arrays, observed by rotating 1 cm² substrates relative to a fixed light source, reflected exactly the symmetry axes of the periodic nano‐features in the arrays demonstrating the remarkable macroscopic order of the periodic structures. Raman spectra of 1‐benzenethiol adsorbed at the arrays indicated SERS enhancements from the substrates are attributed mainly to surface nano‐roughness with only moderate contributions from the periodically corrugated structures. Despite excitation at the major resonance dip in the reflectance spectrum, a weak, localized rim dipole mode is found to elicit a small increase in the SERS enhancement factor for the 430 nm diameter spherical arrays. FDTD studies of nano‐void arrays provided insights into v arious factors affecting the SERS experiment and confirmed the array's plasmonic spectra are dominated by propagating plasmon modes under microscope excitation/collection angles.     

Nanoparticles: Well Shaped Mn3O4 Nano‐octahedra with Anomalous Magnetic Behavior and Enhanced Photodecomposition Properties (Small 4/2011)

Very uniform and well shaped Mn₃O₄ nano‐octahedra are synthesized using a simple hydrothermal method under the help of polyethylene glycol (PEG200) as a reductant and shape‐directing agent. The nano‐octahedra formation mechanism is monitored. The shape and crystal orientation of the nanoparticles is reconstructed by scanning electron microscopy and electron tomography, which reveals that the nano‐octahedra only selectively expose {101} facets at the external surfaces. The magnetic testing demonstrates that the Mn₃O₄ nano‐octahedra exhibit anomalous magnetic properties: the Mn₃O₄ nano‐octahedra around 150 nm show a similar Curie temperature and blocking temperature to Mn₃O₄ nanoparticles with 10 nm size because of the vertical axis of [001] plane and the exposed {101} facets. With these Mn₃O₄ nano‐octahedra as a catalyst, the photodecomposition of rhodamine B is evaluated and it is found that the photodecomposition activity of Mn₃O₄ nano‐octahedra is much superior to that of commercial Mn₃O₄ powders. The anomalous magnetic properties and high superior photodecomposition activity of well shaped Mn₃O₄ nano‐octahedra should be related to the special shape of the nanoparticles and the abundantly exposed {101} facets at the external surfaces. Therefore, the shape preference can largely broaden the application of the Mn₃O₄ nano‐octahedra.     

Nanostructure of pure iron anodically oxidized in borate buffer solution and annealed by infrared radiation

The behavior of oxide film on pure iron passivated in a borate buffer solution and subsequently radiated by infrared light (IR) was investigated in comparing to that by just IR annealing without passivation, and was evaluated by film structure, etc. The effect of thermal annealing over 250 degrees C was observed with gamma-Fe2O3 grain growth and sharp increase in surface roughness, film thickness and oxygen content. An ellipsometric parameter of tan psi was sensitively reflected by annealing effect, and tan psi curve had a shoulder at 150 degrees C for 5 min and a peak of tan psi was shifted from 350 nm to 450 nm in wavelength. This shift was also caused by the formation of gamma-Fe2O3, because the peak was also observed in tan psi of the bulk Fe2O3 family. Passivation effects at 800 mV prior to IR annealing on thickness and oxygen content changed at 150 degrees C, and decreased tan psi at 350 nm and excessive film growth over 250 degrees C, and increased oxygen content under 100 degrees C and surface roughness at 50-250 degrees C. The terrace width with atomic scale flatness was slightly increase by passivation prior to IR annealing at 50-250 degrees C, and the maximum terrace width reached larger than 10 nm by passivation and IR annealing at 100 degrees C for 30 min.     

Easy‐to‐clean Schichten – spezielle Anwendungen

Easy to clean surfaces – special applications Easy to clean surfaces can be made by wet‐chemical coating with subsequent heat‐treatment. Organically modified metal oxide films form the base reinforced by nano composite structures. The hydro‐ and oleophobic effect is obtained by perfluorinated organic molecule chains in the nano composite sol‐gel coatings. Application specific materials can be synthesized by the proper choice of suitable starting compounds and process parameters. The resulting coatings consist of a three‐dimensional cross‐linked inorganic part (such as a silica network) combined with an organic part. The organic material acts either as a surface modifier (example: alkyl, phenyl) or as crosslinker (example: acrylic, epoxy). The properties of such coating systems can be adjusted to obtain a wide range of glass‐ceramic or polymer‐like properties. The incorporation of nanoparticles into these materials significantly enhances the abrasion and the scratch resistance. Such coatings mainly on metal parts are used in diagnostics, analytical chemistry and medical technology.     

Electronic‐Reconstruction‐Enhanced Tunneling Conductance at Terrace Edges of Ultrathin Oxide Films

Quantum mechanical tunneling of electrons across ultrathin insulating oxide barriers has been studied extensively for decades due to its great potential in electronic‐device applications. In the few‐nanometers‐thick epitaxial oxide films, atomic‐scale structural imperfections, such as the ubiquitously existed one‐unit‐cell‐high terrace edges, can dramatically affect the tunneling probability and device performance. However, the underlying physics has not been investigated adequately. Here, taking ultrathin BaTiO₃ films as a model system, an intrinsic tunneling‐conductance enhancement is reported near the terrace edges. Scanning‐probe‐microscopy results demonstrate the existence of highly conductive regions (tens of nanometers wide) near the terrace edges. First‐principles calculations suggest that the terrace‐edge geometry can trigger an electronic reconstruction, which reduces the effective tunneling barrier width locally. Furthermore, such tunneling‐conductance enhancement can be discovered in other transition metal oxides and controlled by surface‐termination engineering. The controllable electronic reconstruction can facilitate the implementation of oxide electronic devices and discovery of exotic low‐dimensional quantum phases.     

Terrace width distribution during unstable homoepitaxial growth of GaAs(110): An experimental study

The temporal evolution of the step bunching instability formed during GaAs homoepitaxial growth on the GaAs(110) vicinal to (111)A has been studied by atomic force microscopy (AFM) and the step–step distribution has been quantified as a function of deposition time. Analysis of the AFM data has shown that neither the terrace width distribution (TWD) nor the terrace height distribution (THD) fit to a Gaussian function in the initial stages of growth, but both evolve with time as the bunching instability develops. After deposition of 500 ML of GaAs the TWD exhibits a clear Gaussian behavior while the THD is very well fitted to a Lorentzian distribution. The GaAs surface morphology initially shows a great dispersion in terrace height and width values with a clear anisotropy along the <001> tilt direction, but evidence of self-controlled growth is observed irrespective of layer thickness.     

Phosphorene/ZnO Nano‐Heterojunctions for Broadband Photonic Nonvolatile Memory Applications

High‐performance photonic nonvolatile memory combining photosensing and data storage with low power consumption ensures the energy efficiency of computer systems. This study first reports in situ derived phosphorene/ZnO hybrid heterojunction nanoparticles and their application in broadband‐response photonic nonvolatile memory. The photonic nonvolatile memory consistently exhibits broadband response from ultraviolet (380 nm) to near infrared (785 nm), with controllable shifts of the SET voltage. The broadband resistive switching is attributed to the enhanced photon harvesting, a fast exciton separation, as well as the formation of an oxygen vacancy filament in the nano‐heterojunction. In addition, the device exhibits an excellent stability under air exposure compared with reported pristine phosphorene‐based nonvolatile memory. The superior antioxidation capacity is believed to originate from the fast transfer of lone‐pair electrons of phosphorene. The unique assembly of phosphorene/ZnO nano‐heterojunctions paves the way toward multifunctional broadband‐response data‐storage techniques.     

Na to tailor the band gap and morphology of ZnO nanograins

Sodium doped and undoped zinc oxide compounds are synthesized using sol–gel auto combustion method. The doping has five different concentrations of sodium in zinc oxide. They are characterized for phase and crystal structure using X-ray diffractrometer. It is confirmed that the doping of the impurity atoms show lattice changes. The average particle size of samples are found to be in the range between 32 and 62 nm. Williamson–Hall plots show strain induced in crystallites. Scanning electron microscope micrographs show the morphology of the Na doped samples as nano pebbles, nano platelets and nano rods having considerable width and length; while small thickness. Fourier transform infra-red spectrophotometer and FT Raman studies show the persistent peaks corresponding to characteristic Zn–O bonds. Ultraviolet–visible–near infra-red spectrophotometer studies reveal the decrease in absorbance and red shift in band gap with increasing Na content. The fluorescence spectral analysis of the samples demystifies the existence of oxygen deficient sites. The varying of oxygen vacancy helps band gap tailoring while enhancing Na doping in ZnO.     

Super‐Resolution Nonlinear Photothermal Microscopy

Super‐resolution fluorescence microscopy enables imaging of fluorescent structures beyond the diffraction limit. However, this technique cannot be applied to weakly fluorescent cellular components or labels. As an alternative, photothermal microscopy based on nonradiative transformation of absorbed energy into heat has demonstrated imaging of nonfluorescent structures including single molecules and ~1‐nm gold nanoparticles. However, previously photothermal imaging has been performed with a diffraction‐limited resolution only. Herein, super‐resolution, far‐field photothermal microscopy based on nonlinear signal dependence on the laser energy is introduced. Among various nonlinear phenomena, including absorption saturation, multiphoton absorption, and signal temperature dependence, signal amplification by laser‐induced nanobubbles around overheated nano‐objects is explored. A Gaussian laser beam profile is used to demonstrate the image spatial sharpening for calibrated 260‐nm metal strips, resolving of a plasmonic nanoassembly, visualization of 10‐nm gold nanoparticles in graphene, and hemoglobin nanoclusters in live erythrocytes with resolution down to 50 nm. These nonlinear phenomena can be used for 3D imaging with improved lateral and axial resolution in most photothermal methods, including photoacoustic microscopy.     

晶核生长法制备纳米铜粒子

使用晶核控制生长的方法制备纳米铜粒子,用NaBH4还原出小粒径的纳米铜作为晶核,用抗坏血酸还原Cu2+在晶核上快速生长,制备出纳米铜粒子。经SEM观察纳米粒子直径为80～90nm,纳米粒子粒径均匀。     

Micro‐ and Nanopatterning of Functional Organic Monolayers on Oxide‐Free Silicon by Laser‐Induced Photothermal Desorption

The photothermal laser patterning of functional organic monolayers, prepared on oxide‐free hydrogen‐terminated silicon, and subsequent backfilling of the laser‐written lines with a second organic monolayer that differs in its terminal functionality, is described. Since the thermal monolayer decomposition process is highly nonlinear in the applied laser power density, subwavelength patterning of the organic monolayers is feasible. After photothermal laser patterning of hexadecenyl monolayers, the lines freed up by the laser are backfilled with functional acid fluoride monolayers. Coupling of cysteamine to the acid fluoride groups and subsequent attachment of Au nanoparticles allows easy characterization of the functional lines by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Depending on the laser power and writing speed, functional lines with widths between 1.1 μm and 250 nm can be created. In addition, trifluoroethyl‐terminated (TFE) monolayers are also patterned. Subsequently, the decomposed lines are backfilled with a nonfunctional hexadecenyl monolayer, the TFE stripes are converted into thiol stripes, and then finally covered with Au nanoparticles. By reducing the lateral distance between the laser lines, Au‐nanoparticle stripes with widths close to 100 nm are obtained. Finally, in view of the great potential of this type of monolayer in the field of biosensing, the ease of fabricating biofunctional patterns is demonstrated by covalent binding of fluorescently labeled oligo‐DNA to acid‐fluoride‐backfilled laser lines, which—as shown by fluorescence microscopy—is accessible for hybridization.     

Fluor‐Kohlenstoff‐Nanoschichten zur gezielten Oberflächenfunktionalisierung. Fluorine‐Carbon Nano Coatings for Specific Surface Functionalization

This article concerns some aspects of the research and development work, which is done within a project of the German Federal Ministry of Education and Research (BMBF) entitled: “nano functionalization of interfaces for data‐, textile‐, building‐, medicine‐, bio‐, and aerospace‐ technology”. In the following the broad field of applications of a surface modification on a nanometer scale is discussed. Also some scientific methods to characterize surface modifications of this kind are discussed. By means of low pressure plasma technology it is possible to functionalize surfaces and thus to well adjust their properties with respect to their application. This is done without changing the bulk material characteristics. The surfaces of the treated workpieces are covered by an ultrathin, i.e. only a few nanometer thick, fluorine‐carbon polymer layer by a plasma process. The physical and chemical surface properties, such as surface energy, roughness (on nanometer scale), dynamic wetting behaviour, or the adhesion properties against other materials, can be simple changed by varying the plasma process parameters. It is shown, that in future this surface modification will meet a broad field of applications.     

Cross Section Investigations of Compositions and Sub‐Structures of Tips Obtained by Focused Electron Beam Induced Deposition

The spatial evolution of compositions and sub‐structures inside focused‐electron‐beam‐deposited tips from dicobalt‐octacarbonyl Co₂(CO)₈ precursor at 25 keV and varying beam current (20 pA – 3 μA) is extensively studied for the first time by means of energy dispersive X‐ray spectroscopy, transmission electron microscopy, back‐scattered electron imaging, and ion‐induced secondary electron imaging. Transverse and longitudinal tip cross sections and lamellae were prepared by focused ion beam milling. Two sub‐structure types can be distinguished: a nano‐composite sub‐structure is grown during the initial deposition stage (small‐aspect‐ratio tips). It consists of cobalt nano‐crystals embedded in a carbonaceous matrix. A second distinct cobalt‐grain‐rich sub‐structure develops in high‐aspect ratio tips. Both sub‐structures vary in appearance and composition with increasing beam current: the initial nano‐composite sub‐structure increases in cobalt content and nano‐crystal size, and the cobalt‐grain sub‐structure develops polycrystal‐, texture‐, whisker‐, or platelet‐like habits. The directed precursor flux from a micro‐tube prevents a radial symmetry of the sub‐structures with respect to the impinging focused electron beam, at medium to high beam current. Homogeneous nano‐composite high‐resolution tips with small diameter and length were obtained at low beam current. Observations suggest an additional contribution to pure electron induced precursor molecule decomposition. The influence of electron beam heating and related chemical reactions is discussed.     

Plasma‐Induced Hetero‐Nanostructures on a Polymer with Selective Metal Co‐Deposition

Plasma‐induced pattern formation is explored on polyethylene terephthalate (PET) using an oxygen plasma glow discharge. The nanostructures on PET are formed through preferential etching directed by the co‐deposition of metallic elements, such as Cr or Fe, sputtered from a stainless‐steel cathode. The local islands formed by metal co‐deposition have significantly slower etching rates than those of the pristine regions on PET, generating anisotropic nanostructures in pillar‐ or hair‐like form during plasma etching. By covering the cathode with the appropriate material, the desired metallic or polymeric elements can be co‐deposited onto the target surfaces. When the cathode is covered by a relatively soft material composed of only carbon and hydrogen, such as polystyrene, nanostructures typically induced by preferential etching are not observed on the PET surface, and the surfaces are uniformly etched. A variety of metals, such as Ag, Cu, Pt, or Si, can be successfully co‐deposited onto the PET surfaces by simply using a cathode covered in the desired metal; high‐aspect‐ratio nanostructures coated with the co‐deposited metal are subsequently formed. Therefore this simple single‐step method for forming hetero‐nanostructures—that is, nanoscale hair‐like polymer structures decorated with metals—can be used to produce nanostructures for various applications, such as catalysts, sensors, or energy devices.