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.     

Imaging surfaces of nano‐scale roughness by atomic force microscopy with carbon nanotubes as tips: a comparative study

Accurate knowledge of the nanoroughness of surfaces is crucial for many applications related to optics, electronics or tribology. Although atomic force microscopy (AFM) can image surfaces with a nanometre spatial resolution, the finite size of standard tips means that pores, pits or grooves with dimensions similar to or smaller than the tip apex will not be accurately imaged. Furthermore, standard tips are made of silicon or silicon nitride and are prone to wear. Mitigation may arise from the availability of AFM tips with a carbon nanotube (CNT) at their foremost end. This study compares the imaging performance of ultrasharp Si tips, CNT AFM tips prepared by a Langmuir‐Blodgett (LB) technique, and of CNT AFM tips prepared by a chemical vapour deposition (CVD) technique. The free length of the CNT AFM tips is in the range 80–200 and 600–750 nm, respectively. A polycrystalline niobium film surface is imaged that shows nanoroughness. The measurements demonstrate that CNT AFM tips allow excellent imaging if the scan parameters are adjusted very carefully. Nevertheless, in some cases distortions are found. The measured average grain diameter is 19.9 ± 3.6 nm in the case of a CNT AFM tip made by the LB technique, and 18.0 ± 3.3 nm in the case of a CNT AFM tip made by CVD. In addition to cross‐sections of topography images, also the power spectral density (PSD) is analyzed. An empirical approach for the readout of the characteristic length is suggested that involves the first derivative of the decadic logarithm of the PSD. Copyright © 2011 John Wiley & Sons, Ltd.     

Alternative method for fabricating chemically functionalized AFM tips: silane modification of HF-treated Si3N4 probes

An alternative method for fabricating functionalized, atomic force microscopy (AFM) tips is presented. This technique is simple and requires only minimal preparation and tip modification to generate chemically sensitive probes that have a robust organic monolayer of flexible terminal chemistry attached to the surface. Specifically, commercially microfabricated Si3N4 AFM tips were modified with self-assembled monolayers (SAMs) of octadecyltrichlorosilane and (11-bromoundecyl)trichlorosilane after removing the native silicon oxide surface layer with concentrated hydrofluoric acid. The structure of these SAM films on solid silicon nitride surfaces was studied using contact angle goniometry and Fourier transform infrared spectroscopy. Pull-off force measurements on various bare (mica, graphite, and silicon) and SAM-functionalized substrates confirm that mechanically robust, long-chain organic silane SAMs can be formed on HF-treated Si3N4 tips without the presence of an intervening oxide layer. Adhesion experiments show that the integrity of the organic film on the chemically modified tips is maintained over repeated measurements and that the functionalized tips can be used for chemical sensing experiments since strong discrimination between different surface chemistries is possible.     

Quantitative nanotribology by AFM: a novel universal calibration platform

The quantitative determination of friction forces by atomic force microscopy (AFM) in nanotribology requires the conversion of the output voltage signal of the sector area-sensitive photodiode to force using (a) the torsional spring constant of the cantilever and (b) the lateral sensitivity of the photodiode. Many existing methods provide calibration factors with large errors and suffer from poor reproducibility. We report on the fabrication, validation, and application of a new, universally applicable standard specimen that enables one to accurately calibrate all types of AFM cantilevers and tips for quantitative friction force measurements. The Si(100) calibration standard, which exhibits 30 and 50 mum wide notches with tilt angles theta between 20 degrees and 35 degrees with respect to the wafer surface, was fabricated by focused ion beam (FIB) milling. The quantification of friction forces obtained on this universal standard specimen using a direct method (the improved wedge calibration method, as introduced by Ogletree, Carpick, and Salmeron Rev. Sci. Instrum. 1996, 67, 3298-3306), which yields (a) and (b) simultaneously, was critically tested for various types of Si3N4 integrated cantilever-tip assemblies. The error in the calibration factors obtained was found to be ca. 5%, which is a significant improvement compared to errors of 30-50% observed for the often applied two-step calibration procedures of cantilever lateral force constant and photodiode sensitivity. As demonstrated for oxidized Si(100), thin films of poly(methyl methacrylate) (PMMA), and micropatterned self-assembled monolayers (SAMs) on gold, the calibration of various V-shaped and single beam cantilevers based on the application of the new universal standard in conjunction with the direct wedge method proposed allows one to conveniently perform quantitative nanotribological measurements for a wide range of materials and applications.     

Exploiting poly(dimethylsiloxane)-modified tips to evaluate frictional behavior by friction force microscopy

With the aim of investigating the effect of the surface properties on the friction behavior of self-assembled monolayers, we have modified tipless atomic force microscopy (AFM) cantilevers with a poly(dimethylsiloxane) (PDMS) lens. The friction coefficient using the silicon tip is strongly influenced by the mechanical properties of the substrate monolayer because hard, sharp silicon tips penetrate the surface of organic monolayers. However, the friction coefficient obtained for the PDMS-modified AFM cantilever is mostly due to the surface properties of the monolayer functional end group, rather than the viscoelastic deformation of the monolayer. The use of the PDMS tip was demonstrated as a novel means to investigate the effect of surface properties on the frictional behavior of self-assembled monolayers with various functional groups with less mechanical deformation.     

Interaction of scanning probes with semiconductor nanocrystals; physical mechanism and basis for near-field optical imaging

We investigate the modification of photoluminescence (PL) from single semiconductor nanocrystal quantum dots (NCs) in the proximity of metal and semiconducting atomic force microscope (AFM) tips. The presence of the tip alters the radiative decay rate of an emitter via interference and opens efficient nonradiative decay channels via energy transfer to the tip material. These effects cause quenching (or enhancement) of the emitter's PL intensity as a function of its distance from the interacting tip. We take advantage of this highly distance-dependent effect to realize a contrast mechanism for high-resolution optical imaging. AFM tips are optimized as energy acceptors by chemical functionalization with InAs NCs to achieve optical resolution down to 30 nm. The presented experimental scheme offers high-resolution optical information while maintaining the benefits of traditional AFM imaging. We directly measure the PL intensity of single NCs as a function of the tip distance. Our results are in good agreement with calculations made by a classical theoretical model describing an oscillating dipole interacting with a planar mirror.     

Nanoscale tripodal 1,3,5,7-tetrasubstituted adamantanes for AFM applications

The synthesis of four novel nanoscale 1,3,5,7-tetrasubstituted adamantanes 22 and 25-27 designed for atomic force microscopy (AFM) applications is described. Each tetrahedrally shaped molecule incorporates a broad tripodal base made up of three identical legs that terminate with a sulfur-containing moiety, which is either a 4-acetylsulfanylmethylphenyl unit or else a (1,2,5-dithiazepan-1-yl)phenyl unit. The sulfur atoms are intended for eventual binding of the molecule multivalently to the apex of a gold-coated commercial AFM tip through formation of multiple S-Au bonds. In each molecule, the fourth terminus is a para-substituted benzoic acid methyl ester that is designed to scan the sample. We demonstrate that 27 is sufficiently large and rigid to be imaged by a conventional AFM tip. Adamantanes 22 and 25-27 may also find application as chemically well-defined nanoscale objects for calibration of AFM tips.     

Tapping mode imaging and measurements with an inverted atomic force microscope

This report demonstrates the successful use of the inverted atomic force microscope (i-AFM) for tapping mode AFM imaging of cantilever-supported samples. i-AFM is a mode of AFM operation in which a sample supported on a tipless cantilever is imaged by one of many tips in a microfabricated tip array. Tapping mode is an intermittent contact mode whereby the cantilever is oscillated at or near its resonance frequency, and the amplitude and/or phase are used to image the sample. In the process of demonstrating that tapping mode images could be obtained in the i-AFM design, it was observed that the amplitude of the cantilever oscillation decreased markedly as the cantilever and tip array were approached. The source of this damping of the cantilever oscillations was identified to be the well-known "squeeze film damping", and the extent of damping was a direct consequence of the relatively shorter tip heights for the tip arrays, as compared to those of commercially available tapping mode cantilevers with integrated tips. The functional form for the distance dependence of the damping coefficient is in excellent agreement with previously published models for squeeze film damping, and the values for the fitting parameters make physical sense. Although the severe damping reduces the cantilever free amplitude substantially, we found that we were still able to access the low-amplitude regime of oscillation necessary for attractive tapping mode imaging of fragile molecules.     

Prospects for resolving chemical structure by atomic force microscopy: a first-principles study

In a recent paper, the chemical structure of a molecule was resolved by means of atomic force microscopy (AFM): using a metal tip terminated in a CO molecule, the authors could image the internal bonding arrangement of a pentacene molecule with remarkable spatial resolution (notably better than with other tip terminations), as verified by their first-principles calculations. Here we further explore with first-principles calculations the mechanisms, applicability, and capabilities of this approach for a wider range of situations, by varying the imaged molecule and the tip beyond the experimental cases. In our simulations, a high atomic resolution is found to be dominated by the electronic structure of the last two atoms on the tip apex which are set perpendicularly to the sample molecule. For example, tips terminated in CH(4) or pentacene itself (both having a C-H apex) yield similar images, while tips terminated in O(2) or CO give quite different images. While using a CO-terminated tip successfully resolves the chemical structure of pentacene and of other extended planar networks based on C(6) rings, this tip fails to resolve the structures of benzene (with its single C(6) ring) or nonplanar C(6) networks, such as C(60) or small-diameter carbon nanotubes. Defects (such as N substitution for a C-H group) were also found to significantly influence the image resolution. Our findings indicate that further application of this approach requires, for each sample, careful selection of a suitable "imaging" molecule as tip termination.     

Synthesis of well-defined tower-shaped 1,3,5-trisubstituted adamantanes incorporating a macrocyclic trilactam ring system

We describe the synthesis of two novel well-defined tower-shaped 1,3,5-trisubstituted adamantanes 30 and 33 that incorporate a macrocyclic trilactam ring system. Each nanoscale molecule has a broad tripodal base consisting of three identical sulfur-containing termini as the tripod feet, 4-acetylsulfanylmethylphenyl units in the case of 30 and 3,5-bis(acetylsulfanylmethyl)phenyl units in the case of 33. The sulfur atoms are designed to bind the molecules trivalently to the apex of a gold-coated commercial AFM tip through formation of three S-Au bonds. The rigid adamantane-derived head unit with a single hydrogen atom at the apex is designed to scan the sample. Molecules 30 and 33 are synthesized from 1,3,5-triethynyladamantane by a series of Sonogashira coupling reactions involving terminal alkynes and aryl iodides. A macrocyclic trilactam unit is included for added rigidity. We demonstrate that molecule 30 is sufficiently large and rigid to be visualized by a conventional AFM tip. These nanoscale molecules may also find application as chemically well-defined nanoscale objects for calibration of AFM tips.     

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).     

Attachment of nanoparticles to the AFM tips for direct measurements of interaction between a single nanoparticle and surfaces

Here we report a universal method of attachment/functionalization of tips for atomic force microscope (AFM) with nanoparticles. The particles of interest are glued to the AFM tip with epoxy. While the gluing of micron size particles with epoxy has been known, attachment of nanoparticles was a problem. The suggested method can be used for attachment of virtually any solid nanoparticles. Approximately every other tip prepared with this method has a single nanoparticle terminated apex. We demonstrate the force measurements between a single approximately 50 nm ceria nanoparticle and flat silica surface in aqueous media of different acidity (pH 4-9). Comparing forces measured with larger ceria particles ( approximately 500 nm), we show that the interaction with nanoparticles is qualitatively different from the interaction with larger particles.     

Relationship between scattered intensity and separation for particles in an evanescent field

We describe measurements of the scattering of visible light from an evanescent field by both spherical particles (R = 1-10 mum) that are glued to atomic force microscopy (AFM) cantilevers, and by sharp tips (R < 60 nm) that were incorporated onto the cantilevers during manufacture. The evanescent wave was generated at the interface between a flat plate and an aqueous solution, and an atomic force microscope was used to accurately control the separation, h, between the particle and the flat plate. We find that, for sharp tips, the intensity of scattered light decays exponentially with separation between the tip and the plate all the way down to h approximately 0. The measured decay length of scattered intensity, delta, is the same as the theoretical decay length of the evanescent intensity in the absence of the sharp tip. For borosilicate particles, (R = 1-10 mum), the scattering also decays exponentially with separation at large separations. However, when the separation is less than roughly 3delta, the measured scattering intensity is smaller in magnitude than that which would be predicted by extrapolating the exponential decay observed at large separations. For these particles, the scattering approximately fits the sum of two exponentials. The magnitude of the deviation from exponential at contact was roughly 10-15% for R = 1 mum particles and about 30% for larger particles and is larger for s-polarized light. Preliminary experiments on polystyrene particles shows that the scattering is also smaller than exponential at small separations but that the deviation from exponential is larger for p-polarized light. In evanescent wave AFM (EW-AFM) the scattering-separation can be calibrated for situations where the scattering is not exponential. We discuss possible errors that could be introduced by assuming that exponential decay of scattering continues down to h = 0.     

Chemical force titrations of functionalized Si(111) surfaces

Chemical force titrations-plots of the adhesive force between an atomic force microscope tip and sample as a function of pH-were acquired on alkyl monolayer-derivatized Si(111) surfaces. Gold-coated AFM tips modified with thioalkanoic acid self-assembled monolayers (SAM) were employed. Alkyl monolayer-derivatized Si(111) surfaces terminated with methyl, carboxyl, and amine groups were produced via hydrosilylation reactions between 1-alkene reagents and H-terminated silicon. The functionalized surfaces were characterized using standard surface science techniques (AFM, FTIR, and XPS). Titration of the methyl-terminated surface using the modified (carboxyl-terminated) atomic force microscope tip resulted in a small pH-independent hydrophobic interaction. Titration of the amine-terminated surface using the same tip resulted in the determination of a surface pKa of 5.8 for the amine from the pH value from the maximum in the force titration curve. A pK(1/2) of 4.3 was determined for the carboxyl-terminated Si(111) in a similar way. These results will be discussed in relation to the modified Si(111) surface chemistry and organic layer structure, as well as with respect to existing results on Au surfaces modified with SAMs bearing the same functional groups.     

Study on the applications of SiC thin films to MEMS techniques through a fabrication process of cantilevers

We have tried to find the most suitable conditions for the deposition process of silicon carbide thin films as a material for MEMS techniques. We have also studied its application to semiconductor processes. To do this, we have tried to fabricate several dimensions of cantilevers with these silicon carbide thin films. High quality silicon carbide thin films are grown by metal-organic chemical vapor deposition (MOCVD). This process employs single molecular precursors such as diethylmethylsilane (DEMS), 1,3-disilabutane (DSB) at a pressure of 1 × 10⁻³ Pa and a growth temperature in the range of 700–1000 °C. Two fabrication methods are tested for initial fabrication of cantilevers. First, deposit SiC thin films on Si based atomic force microscopy (AFM) cantilevers. Second, used the lift-off process. To get three-dimensional cantilever-shaped SiC thin films, moreover, we chemically etched silicon substrate with strong alkaline solution such as TMAH at 80 °C. In addition, a high resolution of probe tips on the cantilevers was achieved using electron-beam deposition in a carbon atmosphere.     

碳纳米管原子力显微镜针尖在结构生物学研究中的应用

碳纳米管以其较小的半径、较高的纵横比和高的柔韧性成为原子力显微镜对结构生物学进行研究的理想针尖。新的制备方法获得了亚纳米级半径的碳纳米管针尖,运用它们得到了生物大分子及其复合物的生物结构,并获得了新的生物信息,这用传统的原子力显微镜针尖是无法实现的。     

Optical modulation and selective recovery of Cy5 fluorescence

Fluorescence modulation offers the opportunity to detect low-concentration fluorophore signals within high background. Applicable from the single-molecule to bulk levels, we demonstrate long-wavelength optical depopulation of dark states that otherwise limit Cy5 fluorescence intensity. By modulated excitation of a long-wavelength Cy5 transient absorption, we dynamically modulate Cy5 emission. The frequency dependence enables specification of the dark-state timescales enabling optical-demodulation-based signal recovery from high background. These dual-laser illumination schemes for high-sensitivity fluorescence-signal recovery easily improve signal-to-noise ratios by well over an order of magnitude, largely by discrimination against background. Previously limited to very specialized dyes, our utilization of long-lived dark states in Cy5 enables selective detection of this very common single-molecule and bulk fluorophore. Although, in principle, the "dark state" can arise from any photoinduced process, we demonstrate that cis-trans photoisomerization, with its unique transient absorption and lifetime enables this sensitivity boosting, long-wavelength modulation to occur in Cy5. Such studies underscore the need for transient absorption studies on common fluorophores to extend the impact of fluorescence modulation for high-sensitivity fluorescence imaging in a much wider array of applications.     

Positioning isolation and biochemical analysis of single DNA molecules based on nanomanipulation and single-molecule PCR

Recently, the isolation and biochemical analysis of DNA at the single-molecule level has been recognized as very important for genetic research and clinical analysis. A unique technique for the positioning, dissection, and isolation of single DNA molecules using atomic force microscopy (AFM) has been demonstrated. Full-length genome DNA molecules were first deposited and stretched by a modified "molecular combing" technique onto a 3-aminopropyl triethoxysilane-coated mica substrate. A single DNA fragment was dissected from one of those genome DNA strands with the AFM tip at the desired position, and then isolated (or picked up) after a special operation called "kneading". All the operations including imaging, dissection, and isolation could be carried out with one tip. The isolated DNA fragment on the AFM tip could be successfully amplified by single-molecule PCR.     

Real-Time Infrared Overtone Laser Control of Temperature in Picoliter H(2)O Samples: "Nanobathtubs" for Single Molecule Microscopy

An approach for high spatiotemporal control of aqueous sample temperatures in confocal microscopy is reported. This technique exploits near-IR diode-laser illumination to locally heat picoliter volumes of water via first-overtone excitation in the OH-stretch manifold. A thin water cell after the objective resonantly removes any residual IR light from the detection system, allowing for continuous observation of single-molecule fluorescence throughout the heating event. This technique is tested quantitatively by reproducing single-molecule RNA folding results obtained from "bulk" stage heating measurements. Calibration of sample temperatures is obtained from time-correlated single-photon counting studies of Rhodamine B fluorescence decay. We obtain an upper limit to the heating response time (τ(heat) < 20 ms) consistent with even faster estimates (τ(heat) ≈ 0.25 ms) based on laser spot size, H(2)O heat capacit,y and absorption cross section. This combination of fast, noncontact heating of picoliter volumes provides new opportunities for real-time thermodynamic/kinetic studies at the single-molecule level.     

分子间相互作用力的直接测量

结合我们近期的研究工作,着重介绍如何将分子组装与单分子力谱相结合,从单分子水平直接研究分子间相互作用力,包括π-π相互作用、多价作用及嵌入作用.在实验中,将一个相互作用单元通过高分子间隔基共价连接到原子力显微镜针尖上,并将另一相互作用单元共价修饰到基底上,通过压电陶瓷管的移动获得力与拉伸长度的曲线.高分子柔性间隔基团的引入既可用来作为判别单链拉伸的"内标",又可避免非特异相互作用对待测的特异相互作用的影响.研究表明,结合静态和动态力学谱,不仅能够实现分子间相互作用力的直接测量,而且还可获得解离速率和相互作用的距离等参数.