Sequential electrochemical oxidation and site-selective growth of nanoparticles onto AFM probes

In this work, we reported an approach for the site-selective growth of nanoparticle onto the tip apex of an atomic force microscopy (AFM) probe. The silicon AFM probe was first coated with a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS) through a chemical vapor deposition (CVD) method. Subsequently, COOH groups were selectively generated at the tip apex of silicon AFM probes by applying an appropriate bias voltage between the tip and a flat gold electrode. The transformation of methyl to carboxylic groups at the tip apex of the AFM probe was investigated through measuring the capillary force before and after electrochemical oxidation. To prepare the nanoparticle terminated AFM probe, the oxidized AFM probe was then immersed in an aqueous solution containing positive metal ions, for example, Ag+, to bind positive metal ions to the oxidized area (COOH terminated area), followed by chemical reduction with aqueous NaBH 4 and further development (if desired) to give a metal nanoparticle-modified AFM probe. The formation of a metal nanoparticle at the tip apex of the AFM probe was confirmed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA).     

AFM诱导正十八硫醇在金基底上的选择性生长

扫描探针显微镜（SCCnningPF0boMICCOSCOPy,SPM）由于其极高的空间分辨能力和高度的可控性,已成为纳米尺度加工的有力工具［‘·’j．自Schneir等［’j报道原子级平整金基底的制备和用装备An针尖的扫描隧道显微镜（ScanningTunnelingMicroscoPy,STM）在基底上制备金纳米点以来,有关在All和HOPG等基底上制备由金点构成的任意图案的方法及用导电原子力显微镜（AtomicForceM卜roscopy,AFM）在HOPG和St基底上制备金点阵的工作已有许多报道［‘·’‘．用导电AFM和TaPPingmodeAFM”,’‘对St进行直接氧化可在其表面加…     

Mass transfer limitations at crystallizing interfaces in an atomic force microscopy fluid cell: a finite element analysis

Although atomic force microscopy (AFM) has emerged as the preeminent experimental tool for real-time in situ measurements of crystal growth processes in solution, relatively little is known about the mass transfer limitations that may impact these measurements. We present a continuum analysis of flow and mass transfer in an atomic force microscope fluid cell during crystal growth, using data acquired from calcium oxalate monohydrate (COM) crystal growth measurements as a comparison. Steady-state flows and solute concentration fields are computed using a three-dimensional, finite element method implemented on a parallel supercomputer. Steady-state flow results are compared with flow visualization experiments to validate the model. Computations of the flow field demonstrate how nonlinear momentum transport alters the spatial structure of the flow with increasing flow volume, altering mass transport conditions near the AFM cantilever and tip. The simulations demonstrate that the combination of solute depletion from crystal growth and mass transfer resistance lowers the solute concentration in the region between the tip and the crystal compared with the solute concentration at the inlet of the AFM cell. For example, using experimentally measured growth rates for COM, the solute concentration in this region is 3.1% lower than the inlet value because the solute consumed by crystal growth beneath the AFM tip cannot be replenished fully due to mass transport limitations. The simulations also reveal that increasing the flow rate through the cell does not affect this difference significantly because of the inherent shielding by the AFM tip in proximity with the crystal surface. Models such as the one presented here, used in conjunction with AFM measurements, promise more precise interpretations of measurement data.     

Control of localized nanorod formation and patterns of semiconducting CuTCNQ phase I crystals by scanning electrochemical microscopy

Use of the technique of scanning electrochemical microscopy (SECM) enables the surface of single crystals of 7,7',8,8'-tetracyanoquinodimethane (TCNQ) to be modified in a controlled manner to produce highly dense and micrometer sized regions of semiconducting phase I CuTCNQ nanorod crystals by a nucleation and growth mechanism. This method involves the localized reduction of solid TCNQ to TCNQ- by aqueous phase V(aq)2+ reductant generated at a SECM ultramicroelectrode tip by reduction of V(aq)3+, coupled with the incorporation and reduction of Cu(aq)2+ ions also present in the aqueous electrolyte. SECM parameters can be systematically varied to control the extent of surface modification and the packing density of the CuTCNQ crystals. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images provide evidence that the TCNQ to CuTCNQ solid-solid transformation is accompanied by a drastic localized crystal volume and morphology change achieved by fragmentation of the TCNQ crystal surface. Patterns of semiconducting CuTCNQ (phase I) nanorod shaped crystals have been characterized by SEM, AFM, and infrared (IR) techniques. A reaction scheme has been proposed for the interaction between the electrogenerated mediator V(aq)2+, Cu(aq)2+, and the TCNQ crystal in the nucleation and growth stages of phase I CuTCNQ formation.     

Nanografting: modeling and simulation

We present a simple phenomenological model of the nanografting process with an emphasis on the formation of binary self-assembled monolayers. This model includes dynamical processes that are involved in natural growth experiments, including molecular deposition, surface diffusion, and the phase transition from physisorption to chemisorption, and we show that it predicts domain formation in ungrafted deposition that matches experiment. The one-order-of-magnitude faster kinetics that is found in the nanografting experiments compared to natural self-assembly (or unconstrained self-assembly) is described with a key assumption that the deposition rate is greatly enhanced in the small region confined between the back side of the AFM tip and the edge of the previously deposited self-assembled monolayer. Monte Carlo simulations based on this model reproduce experimental observations concerning the variation of SAM heterogeneity with AFM tip speed. Our simulations demonstrate that the faster the AFM tip displaces adsorbed molecules in a monolayer, the more heterogeneous are the monolayers formed behind the tip, as this allows space and time for the formation of phase-segregated domains.     

Microstructural investigation of nickel deposits obtained by pulsed current

Electrodeposited nickel coatings are applied in functional applications to modify or improve corrosion resistance, hardness, wear, magnetic, and other properties. In this work, Nickel deposits were produced by simple pulse electrodeposition using a free-additive Watts bath. It has been shown that pulse current deposition is able to produce coatings with excellent properties in morphology, particle distribution, structure, grain size, hardness and wear resistance. Pulsed techniques present a larger number of variables, so we propose to optimize the parameters which define the pulsed current to obtain deposits with improved properties. The optimization of the conditions of deposition was established and the influence of pulse parameters, namely, pulse on-time, off-time and average current density, on the grain size, surface morphology and crystal orientation was determined. The morphology of the coatings was characterized by observations in scanning electronic microscopy (SEM). X-ray diffraction in symmetric mode was also used to evaluate the structure and principal crystallographic orientations of the deposits. The study showed that pulse current results in better properties of deposits and significantly refined the crystal grain.     

Probe-induced native oxide decomposition and localized oxidation on 6H-SiC (0001) surface: an atomic force microscopy investigation

We report, for the first time, the native oxide decomposition/etching and direct local oxide growth on 6H-SiC (0001) surface induced by atomic force microscopy (AFM). Surface native oxide was decomposed and assembled into protruded lines when the negatively biased AFM tip was scanned over surface areas. The mechanism of decomposition was found to be governed by the Fowler-Nordheim emission current enhanced by the negatively biased AFM tip. Direct oxide growth on the SiC surface was achieved when the AFM tip was immobilized and longer bias duration applied. In particular, the aspect ratio of oxide grown on SiC was found to be several times higher than that on the Si surface. The improved aspect ratio on SiC was attributed to the anisotropic OH(-) diffusion involved in vertical and lateral oxidation along the polar and nonpolar directions such as [0001] and [110] axis in SiC crystal. The electron transport in the above AFM grown oxide on SiC was further investigated by I-V characteristics. The dielectrical strength of AFM oxide against degradation and breakdown under electrical stressing was evaluated.     

Electrodeposition of CuInSe2 (CIS) via electrochemical atomic layer deposition (E-ALD)

The growth of stoichiometric CuInSe(2) (CIS) on Au substrates using electrochemical atomic layer deposition (E-ALD) is reported here. Parameters for a ternary E-ALD cycle were investigated and included potentials, step sequence, solution compositions and timing. CIS was also grown by combining cycles for two binary compounds, InSe and Cu(2)Se, using a superlattice sequence. The formation, composition, and crystal structure of each are discussed. Stoichiometric CIS samples were formed using the superlattice sequence by performing 25 periods, each consisting of 3 cycles of InSe and 1 cycle of Cu(2)Se. The deposits were grown using 0.14, -0.7, and -0.65 V for Cu, In, and Se precursor solutions, respectively. XRD patterns displayed peaks consistent with the chalcopyrite phase of CIS, for the as-deposited samples, with the (112) reflection as the most prominent. AFM images of deposits suggested conformal deposition, when compared with corresponding image of the Au on glass substrate.     

铜在HOPG上电沉积过程的现场ECSTM研究

用自制的电化学扫描隧道显微镜（ＥＣＳＴＭ）现场研究Ｃｕ在ＨＯＰＧ上的电沉积过程．结果表明Ｃｕ在ＨＯＰＧ上的电沉积为三维成核的过程．当电位较低或Ｃｕ２＋离子浓度较低时，铜在本体金属生长主要沿着台阶方向．过电位较高时，铜的成核数目增加，沉积层的晶粒有所细化．同时，非现场ＥＣＳＴＭ比较研究表明，ＳＴＭ针尖对针尖局部区域的电沉积起屏蔽作用，针尖所在区域Ｃｕ的沉积速度比其它区域明显减小     

化学力显微镜针尖修饰技术研究新进展

评述了化学力显微镜的新成果。对自组装单分子膜修饰扫描探针显微镜针尖,生物分子修饰原子力显微镜针尖,电化学方法修饰扫描隧道显微镜针尖,纳米碳管材料修饰原子力显微镜针尖等作了介绍。     

Nanotribology under electrochemical conditions: influence of a copper (sub)monolayer deposited on single crystal electrodes on friction forces studied with atomic force microscopy

Friction force measurements performed by means of an atomic force microscope (AFM) under electrochemical conditions on a pure Au(111) electrode surface and one modified with a foreign metal are presented; after deposition of a (sub)monolayer copper on a Au(111) single crystal electrode a large increase of the friction force is observed compared to the pure Au(111) electrode surface; the extent of the increase not only depends on the copper coverage, but also on the normal load and may be explained by a higher energy dissipation due to motion of the sulfate anions adsorbed on the copper atoms induced by the AFM tip.     

电化学沉积金纳米线结构及其电学特性

用电化学沉积方法，在有机介孔模板上制备出直径为90 nm的金纳米线.透射电子显微镜（TEM）分析结果表明，纳米线表面光滑并呈单晶结构.去除有机模板的金纳米线阵列用扫描电子显微镜（SEM）测试，纳米线顶端呈平台状，直径分布均一.我们利用原子力显微镜（AFM）测量了金纳米线阵列的微观结构，得到与SEM相一致的结果.在大气和室温条件下，用导电AFM针尖在接触模式下测量了单根纳米线的轴向I－V特性曲线，其结果为金属性.     

Time-resolved ultrathin cobalt film growth on a colloidal polymer template

Cobalt (Co) sputter deposition onto a colloidal polymer template is investigated using grazing incidence small-angle X-ray scattering (GISAXS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). SEM and AFM data picture the sample topography, GISAXS the surface and near-surface film structure. A two-phase model is proposed to describe the time evolution of the Co growth. The presence of the colloidal template results in the correlated deposition of an ultrathin Co film on the sample surface and thus in the creation of Co capped polystyrene (PS) colloids. Well below the percolation threshold, the radial growth is restricted and only height growth is observed.     

Initial stages of Ni–P electrodeposition: growth morphology and composition of deposits

The evolution of growth morphology and composition of deposits during the initial stages of Ni–P electrodeposition is studied using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Combined electrochemical and surface analytical measurements show that the deposition process starts at relatively low cathodic potentials by instantaneous formation and growth of hemispherical centres. The phosphorus content of deposits in the initial deposition stages is found to increase gradually with the deposition time. Additional electrochemical and XPS measurements, carried out on Ni substrates under same polarisation conditions in a Ni²⁺ ion free electrolyte solution, show the occurrence of a time dependent Ni–P surface alloy formation indicating a strong Ni–P interaction. It is suggested that the very early stages of Ni–P electrodeposition involve a primary instantaneous nucleation of Ni followed by a Ni–P alloy formation induced by the strong Ni–P interaction. AFM images show that in advanced deposition stages the coalescence of growing Ni–P centres leads to formation of larger growth mounds. The evolution of the resulting surface roughness is analysed on the basis of the so-called dynamic scaling concept. The estimated values for the roughness exponent and the growth exponent (α=1.07±0.05 and β=0.28±0.05) correspond to a model involving a smoothing of the growing surface driven by surface diffusion.     

In-situ atomic force microscopy (AFM) imaging: influence of AFM probe geometry on diffusion to microscopic surfaces

The effect of AFM probe geometry on diffusion to micrometer-scale reactive (electrode) interfaces is considered. A disk-shaped substrate electrode was held at a potential to reduce a species of interest (aqueous Ru(NH 3) 6 (3+)) at a diffusion-controlled rate and the current response during AFM imaging provided information on local mass transport to the interface. This approach reveals how the AFM probe influences diffusion to a reactive surface, which is of importance in more clearly delineating the conditions under which in-situ AFM can be treated as a noninvasive probe of surface processes involving mass transport (e.g., electrode reactions and crystal dissolution and growth). An assessment has been made of three types of probes: V-shaped silicon nitride contact mode probes; single beam silicon probes; and batch-fabricated scanning electrochemical-atomic force microscopy (SECM-AFM) probes. Two disk electrodes, (6.1 microm and 1.6 microm diameter) have been considered as substrates. The results indicate that conventional V-shaped contact mode probes are the most invasive and that the batch-fabricated SECM-AFM probes are the least invasive to diffusion at both of the substrates used herein. The experimental data are complemented by the development of simulations based on a simple 2D model of the AFM probe and active surface site. The importance of probe parameters such as the cantilever size, tip cone height, and cone angle is discussed, and the implications of the results for studies in other areas, such as growth and dissolution processes, are considered briefly.     

Local surface patterning by chitosan-stabilized gold nanoparticles using the direct mode of scanning electrochemical microscopy (SECM)

An approach for patterning surfaces with prepared nanoparticles is described. Chitosan-stabilized gold nanoparticles (Au/chitosan NPs) were locally deposited on stainless steel (StSt), indium tin oxide (ITO), and highly-ordered pyrolytic graphite (HOPG). Deposition was driven by local pH gradient formed between a surface and a scanning electrochemical microscopy tip set in the direct mode. The pH at the substrate was increased upon biasing the surface by negative potentials, which caused the reduction of water. As the pH on the surface exceeded that of $ {\mathrm{pK}}_{{\mathrm{chitosanH}}^{+}}\sim 6.3 $ deprotonation of the amino groups of chitosan caused the irreversible deposition of the chitosan/AuNPs. The effect of different parameters, such as tip–surface distance and time, on deposition was studied. While the potential duration showed no clear influence, smaller tip–substrate distance and more negative potentials applied to the surface caused larger deposits. The overpotential needed for the deposition of nanoparticles on HOPG was the highest while that for StSt was the lowest. On the former, the sluggish kinetics caused the deposition of ring-shaped structures while disk-shaped deposits were formed on the other surfaces.     

碳纳米管原子力显微镜针尖对脱氧核糖核酸高分辨率成像研究

运用自制的碳纳米管原子力显微镜针尖,在液体中观察了脱氧核糖核酸（DNA）分子的精细结构。结果表明,运用碳纳米管针尖获得的DNA分子的高度与电子显微镜的结果非常接近,且没有造成样品的变形损伤;碳纳米管针尖得到的DNA分子的宽度与真实值相近,减小了原子力显微镜成像的增宽效应,这是用传统的硅针尖无法获得的。DNA分子精细结构的高分辨率图像的获得为研究其功能提供了有价值的信息。     

Atomic force microscopy assisted immobilization of lipid vesicles

We report on a new approach to direct the immobilization of unilamellar lipid vesicles on substrate-supported lipid bilayers in a spatially confined manner. The adsorption of vesicles from solution is limited to areas of disorder in the bilayers, which is induced by scanning a pattern in situ with an atomic force microscopy (AFM) tip using high imaging forces. Lines of vesicles with a length exceeding 25 microm and a width corresponding to that of a single surface-immobilized vesicle have been fabricated. The adsorbed vesicles are effectively immobilized and do not desorb spontaneously. However, AFM with forces of several nanoNewtons allows one to displace vesicles selectively. The novel methodology described, which may serve as a platform for research on proteins incorporated in the lipid bilayers comprising the vesicles, does not require chemical labeling of the vesicles to guide their deposition.     

Characterization of atomic force microscopy (AFM) tip shapes by scanning hydrothermally deposited ZnO thin films

Direct observation of tip shapes by atomic force microscopy (AFM) has been achieved using spike-like crystallites in ZnO thin films deposited on microscope glass slides by the hydrothermal deposition technique. Three types of AFM tips, e.g. standard Si(3)N(4) tips, a broken silicon supertip and a noncontact silicon tip were examined and the acquired images for these tips show that ZnO crystallites are good samples to image commonly used AFM tips. The most obvious characteristic of this method is that it is easy for every chemical laboratory to access.     

Influence of molecular weight on nanoscale modification of poly(methyl methacrylate) due to simultaneous mechanical and chemical stimulation

We report observations of poly(methyl methacrylate) films modified by the synergistic effect of solvent exposure and mechanical stress applied by the tip of an atomic force microscope (AFM). We show that these modifications are sensitive to polymer molecular weight as well as solvent strength and the force applied by the tip. Small-area scanning often produces localized patches of raised material as well as depressed areas. The volume change associated with the depressed areas generally increases with increasing solvent strength, increasing applied normal force, and decreasing polymer molecular weight. In contrast, the volume change associated with the raised patches is greatest for 25-145K Mw films in 60 and 100% ethanol solutions. In each case, the normal force applied by the AFM tip must exceed a threshold to significantly modify the surface; this threshold is associated with an increase in lateral force applied by the AFM tip during small-area scanning. We attribute the raised patches to mechanically enhanced swelling due to diffusion of solvent into near-surface material. Permanent net volume loss, when observed, is attributed to localized polymer dissolution.