Silicon Atomic Force Microscope Tip Research Articles

The investigation of nanotribology of UHMWPE in fluid using atomic force microscopy.

The fundamental understanding of the nanowear behavior of ultrahigh molecular weight polyethylene (UHMWPE) at a nanometer scale needs to be achieved to provide a better understanding of the initiating wear process and the potential causes of the wear particles generation of joint replacement. A nanotribology study was performed using atomic force microscope (AFM) tips sliding against UHMWPE surfaces in both water and bovine serum lubricants. Frictional properties of the nanocontact, and the geometry and mechanical features of the resulting scratches have been quantitatively characterized using AFM lateral force and PeakForce QNM modes. The results in this work indicated that the friction force and friction coefficient were smaller in serum lubricant than that in water. A normal load of 120 nN was the transition point for the plastic deformation of the material. The plastic deformation and material accumulation evolute with the increase of applied normal loads. Material pileup formed at the edges of the scratch, but they were not symmetrical due to the asymmetrical geometry of the silicon AFM tip. The height of the material pileup on the right side was approximately 40-70% of the pileup on the left side. The information may be useful for developing strategies for surface finishing techniques, which can control and minimize the production of asymmetric asperity and the resulting pileup with particular features. Furthermore, the moduli of the pileups were much larger than that of the fresh UHMWPE, which had the moduli greater than those of the inner scratch area. This suggested that stress concentration at these points could cause the pileup to be more susceptible to further wear processes, and eventually result in detaching from the bulk material.

Grafting a homogeneous transition metal catalyst onto a silicon AFM probe: a promising strategy for chemically constructive nanolithography

We report a novel approach to chemically selective lithography using an atomic force microscope (AFM) probe with immobilized homogeneous catalyst, potentially giving access to diverse nanoscale transformations of the surface-bound functional groups. This new concept was proven for the local epoxidation of an alkene-terminated self-assembled monolayer on silicon using H2O2 as an oxidant and a catalytic silicon AFM tip charged with manganese complexes with 1,3,7-triazacyclononane type ligands.

Effects of Atomic Force Microscope Silicon Tip Geometry on Large-Scale Nanomechanical Modification of the Polymer Surface

The shape of an atomic force microscope (AFM) silicon tip has a significant effect on the mechanical modification of the polymer surface, especially for a longer sliding distance of from several to several hundreds of millimeters. In this work, a pyramidal silicon tip was used to cut into the polymethyl methacrylate (PMMA) surface, forming nanogrooves with a linear sliding distance of about 80 mm and wear box structures with a total tip sliding distance of 1,024 mm. The effects of the tip edges and the tip radius on the form of the wear debris chips, wear depth, and debris transferred to the tip were investigated. The experimental results showed that four sides of the tip influenced the morphology of the removed material. Adhesion appeared to play a role in the tip wear mechanism by successive removal of SiO2 layers during transfer of adhered PMMA from the tip to the surface. The tip radius generally increased with sliding distance. Simultaneously, adhesion of the removed materials to the tip induced a larger tip radius and a sharper tip was revealed as dropping off of the materials during the test from time to time. Thus, with the same normal load the worn tip may induce failure of the machining process. The results presented in this study provide insight into long-term nanoscratch/wear and nanomechanical machining of glassy polymer surfaces with a silicon AFM tip.

Nanocrystalline diamond AFM tips for chemical force spectroscopy: fabrication and photochemical functionalization

The chemical modification of nanocrystalline diamond (NCD) atomic force microscope (AFM) tips was investigated and used for chemical force spectroscopy (CFS). In contrast to common chemical modification routes for gold or silicon AFM tips, this method creates stable C–C bonding to attach functional moieties to the NCD tip. There have been no previous studies reporting the chemical functionalization of NCD AFM tips. In this study, hydrogen-terminated NCDs (H-NCDs) were deposited on both silicon wafers and silicon AFM tips and subsequently subjected to a photochemical reaction with undecylenic acid (UA) to create UA attached to NCD surface (UA-NCD). The UA-NCD on wafers were used for surface analyses (water contact angle, attenuated total reflectance-Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) measurements) to confirm the chemical modification. The UA-NCD AFM tips were subjected to fluorescent labelling to confirm the existence of carboxylic acid on the tip and AFM adhesion measurements to assess their performance as a probe for the detection of chemical species on surfaces. These results indicate the promising ability of this method to serve as an ideal platform for CFS, which requires robust chemical functionalities on the AFM tip surfaces.

Disentangling viscosity and hysteretic dissipative components in dynamic nanoscale interactions

The mechanisms through which energy is dissipated in nanoscale dynamic interactions might involve tens or hundreds of atoms and might be diverse. Here, a method is presented that provides the means to disentangle, with the use of common experimental parameters, short and long range viscosity and hysteretic dissipative components. While the approach is general, the experimental study is directed to show the mechanisms of energy dissipation between a silicon atomic force microscope tip and a carbon nanotube and a quartz surface. By stabilizing the tip in situ, quantitative information is found in a reproducible manner where the magnitude of energy dissipated remains constant in experiments thus allowing comparative studies.

Contributions of chemical and mechanical surface properties and temperature effect on the adhesion at the nanoscale

The atomic force microscope (AFM) is a powerful tool to investigate surface properties of model systems at the nanoscale. However, to get semi-quantitative and reproducible data with the AFM, it is necessary to establish a rigorous experimental procedure. In particular, a systematic calibration procedure of AFM measurements is necessary before producing reliable semi-quantitative data. In this paper, we study the contributions of the chemical and mechanical surface properties or the temperature influence on the adhesion energy at a local scale. To reach this objective, two types of model systems were considered. The first one is composed of rigid substrates (silicon wafers or AFM tips covered with gold) which were chemically modified by molecular self-assembling monolayers to display different surface properties (methyl and hydroxyl functional groups). The second one consists of model polymer networks (cross-linked polydimethylsiloxane) of variable mechanical properties. The comparison of the force curves obtained from the two model systems shows that the viscoelastic contributions dominate for the adhesion with polymer substrates, whereas, chemical contributions dominate for the rigid substrates. The temperature effect on the adhesion energy is also reported. Finally, we propose a relation for the adhesion energy at the nanoscale. This relation relates the energy measured during the separation of the contact to the three parameters: the surface properties of the polymer, the energy dissipated within the contact zone and the temperature.

Atom Scale Characterization of the Near Apex Region of an Atomic Force Microscope Tip

Three-dimensional atom probe tomography (APT) is successfully used to analyze the near-apex regions of an atomic force microscope (AFM) tip. Atom scale material structure and chemistry from APT analysis for standard silicon AFM tips and silicon AFM tips coated with a thin film of Cu is presented. Comparison of the thin film data with that observed using transmission electron microscopy indicates that APT can be reliably used to investigate the material structure and chemistry of the apex of an AFM tip at near atomic scales.

Length Control and Sharpening of Carbon Nanotube Scanning Probe Microscope Tips Using Carbon Nanotube "Nanoknife"

Through nanomanipulation inside scanning electron microscope, a carbon nanotube scanning probe microscope tip was made by connecting a carbon nanotube with a silicon atomic force microscope tip. The carbon nanotube scanning probe microscope tip was then tailored to the desired length and end structure by a "nanoknife" which is a carbon nanotube adhered to a metal tip. Through mapping the same carbon nanotube on SiO2 substrate, it was found that the lateral resolution of the carbon nanotube tips can be improved significantly through sharpening the tip ends, and the sharpened carbon nanotube tips had better performance than commercial silicon tips.

Evaluation of the insertion efficiencies of tapered silicon nanoneedles and invasiveness of diamond nanoneedles in manipulations of living single cells

We have been developing a low invasive cell manipulation technology based on inserting an ultra-thin needle--"nanoneedle"--into a living cell by using an atomic force microscope (AFM). The nanoneedle, made from a silicon AFM tip by focused-ion-beam etching, has a diameter of several hundred nanometers and a length of about 10 microns. Successful insertion of the nanoneedle into the cell can be confirmed by the appearance of a steep relaxation of repulsive force in the force-distance curve as monitored by the AFM system. This technology, termed "cell surgery", can be applied for the detection of intracellular proteins in a living cell or for highly efficient gene transfer. The present study shows that the durability of a tapered nanoneedle is superior to that of a cylindrical nanoneedle, and that a proper aspect ratio for the tapered nanoneedle must be chosen to maintain sufficient insertion efficiency for a particular target cell: tapered nanoneedles of an aspect ratio over 20 showed high insertion efficiency for various kinds of mammalian cells. We then used diamond for the material of the nanoneedle because its specific properties, such as high stiffness, heat conductivity, and electrical conductivity capacitated by boron doping, were deemed useful for the analysis and manipulation of intracellular phenomena. We compared the capability of the diamond nanoneedle in cell manipulation with that of the silicon nanoneedle. Evaluation of the effect of the former on transcription efficiency and localization analysis of p53 expression revealed the low invasiveness for cell manipulation as was also the case for the silicon nanoneedle. We also succeeded in achieving highly efficient plasmid DNA delivery into a mouse fibroblast C3H10T1/2 using the diamond nanoneedle. The diamond nanoneedle is expected to contribute to the versatility of "cell surgery" technology.

Tip voltage controlled local modification of hydrogenated diamond surface with an atomic force microscope

AbstractThe influence of the voltage applied to the tip of an atomic force microscope (AFM) on the local modification of the surface of CVD‐grown diamond films is studied. By applying a negative voltage to the conductive (highly doped) silicon AFM tip, two kinds of patterns can be created. In the voltage range –6 V to –10 V structures that are elevated with respect to the surface (creation of bumps) are obtained, while at higher voltages (–12 V to –15 V) the patterns are scribed into the diamond surface (creation of pits). The origin of the observed modifications is discussed in terms of an electrochemical process that is enhanced by local heating occurring due to severe current crowding. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Biomimetic study of a polymeric composite material for joint repair applications

A longer lifespan is still being sought for biomaterials used for joint repair. We developed a new nanocomposite material of polytrimethylene carbonate (PTMC), hydroxyapatite (HAP), and multiwalled carbon nanotubes (MWNT) to mimic real cartilage. Experimental results were compared with those of natural cartilage and the conventional joint replacement material ultrahigh-molecular-weight polyethylene (UHMWPE). Friction experiments showed that our developed composite material had a coefficient of friction close to that of articular cartilage. Nanoindentation experiments indicated that the surface elastic behavior was similar to that of cartilage. The surface attraction forces on a silicon atomic force microscope tip were much higher for cartilage than those for the other two materials. These results hold promise for this artificial cartilage composite material’s performance in vivo, following further experimental investigations and chemical modifications.

Near-field second harmonic generation by using uncoated silicon tips

The authors present a concept of a scanning near-field optical microscope for second harmonic imaging. The microscope is based on uncoated silicon atomic force microscope tips, which provide high intensity transmission in the midinfrared spectral range. An approximately three times larger contrast was found for the second harmonic compared to the linear signal. By using a lithographically designed sample of an array of gold nanoparticles the capability of imaging structures much smaller than the fundamental and second harmonic wavelength is demonstrated.

Tip-enhanced fluorescence microscopy of high-density samples

High-density samples of fluorescent quantum dots (QDs) were imaged using an apertureless near-field optical microscopy technique. QD fluorescence was modulated by oscillating a silicon atomic force microscope tip above an illuminated sample and a lock-in amplifier was used to suppress background from the excitation laser. Spatial resolution near 10nm and a peak signal-to-noise ratio (SNR) of ∼60 were achieved. Individual QDs within high-density ensembles were still easily resolved (SNR>5) at a density of 14QDs∕μm2. These results have favorable implications for the eventual nanoscale imaging of viable biological systems, such as cellular membranes.

Multiregional Hybrid Method and Its Application: Formation of an Atomic Protrusion at an Atomic Force Microscope Tip Apex

We present a multiregional hybrid scheme which incorporates first-principles (FP), tight-binding (TB), and molecular mechanical calculations. The key to this hybrid scheme is to find an explicit description of the FP-TB region coupling. We apply it to the atomic structure of a clean silicon atomic force microscope tip, and find the formation of a distinct atomic protrusion at the tip apex. The present study gives the reason why the atomic protrusion exits at the apex despite the fact that the atomic geometry of the very end of the tip is practically uncontrollable in the tip preparation.

Contact mechanics and tip shape in AFM-based nanomechanical measurements

Stiffness–load curves obtained in quantitative atomic force acoustic microscopy (AFAM) measurements depend on both the elastic properties of the sample and the geometry of the atomic force microscope (AFM) tip. The geometry of silicon AFM tips changes when used in contact mode, affecting measurement accuracy. To study the influence of tip geometry, we subjected ten AFM tips to the same series of AFAM measurements. Changes in tip shape were observed in the scanning electron microscope (SEM) between individual AFAM tests. Because all of the AFAM measurements were performed on the same sample, variations in AFAM stiffness–load curves were attributed to differences in tip geometry. Contact-mechanics models that assumed simple tip geometries were used to analyze the AFAM data, but the calculated values for tip dimensions did not agree with those provided by SEM images. Therefore, we used a power-law approach that allows for a nonspherical tip geometry. We found that after several AFAM measurements, the geometry of the tips at the very end is intermediate between those of a flat punch and a hemisphere. These results indicate that the nanoscale tip-sample contact cannot easily be described in terms of simple, ideal geometries.

Characteristics of fracture during the approach process and wear mechanism of a silicon AFM tip

The wear of an atomic force microscope (AFM) tip is one of the crucial issues in AFM as well as in other probe-based applications. In this work, wear tests under extremely low normal load using an AFM were conducted. Also, in order to understand the nature of silicon tip wear, the wear characteristics of crystal silicon and amorphous silicon oxide layer were investigated by a high-resolution transmission electron microscope (HRTEM). It was found that fracture of the tip readily occurred due to impact during the approach process. Experimental results showed that the impact should be below 0.1 nN s to avoid significant fracture of the tip. Also, it was observed that wear of the amorphous layer, formed at the end of the tip, occurred at the initial stage of the silicon tip damage process. Based on Archard's wear law, the wear coefficient of the amorphous layer was in the range of 0.009–0.014. As for the wear characteristics of the silicon tip, it was shown that wear occurred gradually under light normal load and the wear rate decreased with increase in the sliding distance. As for the wear mechanism of the silicon tip, oxidation wear was identified to be the most significant. It was shown that the degree of oxidation was higher under high normal load and in a nitrogen environment, oxidation of the silicon tip was reduced.

Mechanical sensing of the penetration of various nanoneedles into a living cell using atomic force microscopy

Mechanical responses during insertion of a silicon nanoneedle into a living melanocyte were observed by using an atomic force microscope (AFM). In order to study the dependence of the mechanical response on the shape of the nanoneedle, we prepared various shapes of silicon AFM tips by focused-ion beam (FIB) etching. The force curves showed increases up to 0.65–1.9 nN after contact on the cell surface, and then the force dropped corresponding with the penetration of the needle through the cell membrane. The force required for penetration was significantly smaller than that using a normal pyramidal tip. The force curves with a cylindrical tip showed a shorter indenting distance before penetration than that with the cone-shaped tip. It is considered that the information about the geometry of penetrating material leads to the development of more suitable micro- and nano-materials to insert into a living cell for cell surgery.

Probing microplatform for the study of biological adhesion forces

A tool for the study of biological adhesion forces with the atomic force microscope (AFM) is introduced. The tool, a “microplatform,” can be functionalized with variety of specimens such as bacterial cells and used to study adhesion between the specimen and a surface. This tool is easily created using commercially available silicon AFM tips and an AFM, and can be customized in size to fit specific applications. Two custom fabricated microplatforms, ∼1 and ∼2.5 μm were tested. The method of microplatform fabrication, as well as adhesion force data between E. coli bacteria and a nanofiltration membrane is presented.

Adhesion Forces between Glass and Silicon Surfaces in Air Studied by AFM: Effects of Relative Humidity, Particle Size, Roughness, and Surface Treatment

Using the atomic force microscope (AFM), the pull-off forces between flat glass or silicon surfaces and silicon AFM tips or glass microspheres of different sizes have been extensively studied as a function of relative humidity (RH) in the range 5−90%, as model systems for the behavior of cohesive powders. The glass and silicon substrates were treated to render them either hydrophobic or hydrophilic. All the hydrophilic surfaces gave simple force curves and pull-off forces increasing uniformly with RH. Small contacts (R ∼ 20 nm) gave pull-off forces close to values predicted by simple Laplace−Kelvin theory (∼20 nN), but the values with microspheres (R ∼ 20 μm) fell well below predictions for sphere−flat or sphere−sphere geometry, due to roughness and asperity contacts. The hydrophobic silicon surfaces also exhibited simple behavior, with no significant RH dependence. The pull-off force again fell well below predicted values (Johnson−Kendall−Roberts contact mechanics theory) for the larger contacts. Hydrophobic glass gave similar adhesion to silicon over most of the RH range, but against both silicon tips and glass microspheres, there was an anomalously large adhesion in the RH range 20−40%, accompanied by a long-range noncontact force. The adhesion on fully hydrophilic surfaces and its RH dependence can be mostly explained by current theories of capillary bridges, but the interpretation is complicated by the sensitivity of theoretical predictions to contact geometry (and hence to roughness effects) and by uncertainties in the thickness of adsorbed water layers. The anomalous behavior on hydrophobic glass surfaces at intermediate values of RH is not fully understood, but possible causes are (1) dipole layers in the partially formed water film, giving rise to patch charges and long-range forces, or (2) fixed charges at a reactive glass surface, involving specific bonding reactions. The results may be useful in explaining the behavior of cohesive powders with different coatings or those which show a large humidity dependence (e.g., zeolites) or show electrostatic charging effects (e.g., silica aerogels).

Single electron tunneling detected by electrostatic force

Single electron tunneling events between a specially fabricated scanning probe and a conducting surface are demonstrated. The probe is an oxidized silicon atomic force microscope tip with an electrically isolated metallic dot at its apex. A voltage applied to the silicon tip produces an electrostatic force on the probe, which depends upon the charge on the metallic dot. Single electron tunneling events are observed in both the electrostatic force amplitude and phase signal. Electrostatic modeling of the probe response to single tunneling events is in good agreement with measured results.