Background: Noncontact atomic force microscopy (NC-AFM) now regularly produces atomic-resolution images on a wide range of surfaces, and has demonstrated the capability for atomic manipulation solely using chemical forces. Nonetheless, the role of the tip apex in both imaging and manipulation remains poorly understood and is an active area of research both experimentally and theoretically. Recent work employing specially functionalised tips has provided additional impetus to elucidating the role of the tip apex in the observed contrast.
Results: We present an analysis of the influence of the tip apex during imaging of the Si(100) substrate in ultra-high vacuum (UHV) at 5 K using a qPlus sensor for noncontact atomic force microscopy (NC-AFM). Data demonstrating stable imaging with a range of tip apexes, each with a characteristic imaging signature, have been acquired. By imaging at close to zero applied bias we eliminate the influence of tunnel current on the force between tip and surface, and also the tunnel-current-induced excitation of silicon dimers, which is a key issue in scanning probe studies of Si(100).
Conclusion: A wide range of novel imaging mechanisms are demonstrated on the Si(100) surface, which can only be explained by variations in the precise structural configuration at the apex of the tip. Such images provide a valuable resource for theoreticians working on the development of realistic tip structures for NC-AFM simulations. Force spectroscopy measurements show that the tip termination critically affects both the short-range force and dissipated energy.
force spectroscopy; image contrast; noncontact AFM; qPlus; Si(001); Si(100); tip (apex) structure
Measurements of the frequency shift versus distance in noncontact atomic force microscopy (NC-AFM) allow measurements of the force gradient between the oscillating tip and a surface (force-spectroscopy measurements). When nonconservative forces act between the tip apex and the surface the oscillation amplitude is damped. The dissipation is caused by bistabilities in the potential energy surface of the tip–sample system, and the process can be understood as a hysteresis of forces between approach and retraction of the tip. In this paper, we present the direct measurement of the whole hysteresis loop in force-spectroscopy curves at 77 K on the PTCDA/Ag/Si(111) √3 × √3 surface by means of a tuning-fork-based NC-AFM with an oscillation amplitude smaller than the distance range of the hysteresis loop. The hysteresis effect is caused by the making and breaking of a bond between PTCDA molecules on the surface and a PTCDA molecule at the tip. The corresponding energy loss was determined to be 0.57 eV by evaluation of the force–distance curves upon approach and retraction. Furthermore, a second dissipation process was identified through the damping of the oscillation while the molecule on the tip is in contact with the surface. This dissipation process occurs mainly during the retraction of the tip. It reaches a maximum value of about 0.22 eV/cycle.
atomic force microscopy; energy dissipation; force spectroscopy; hysteresis loop; PTCDA/Ag/Si(111) √3 × √3
Intermodulation atomic force microscopy (ImAFM) is a mode of dynamic atomic force microscopy that probes the nonlinear tip–surface force by measurement of the mixing of multiple modes in a frequency comb. A high-quality factor cantilever resonance and a suitable drive comb will result in tip motion described by a narrow-band frequency comb. We show, by a separation of time scales, that such motion is equivalent to rapid oscillations at the cantilever resonance with a slow amplitude and phase or frequency modulation. With this time-domain perspective, we analyze single oscillation cycles in ImAFM to extract the Fourier components of the tip–surface force that are in-phase with the tip motion (F
I) and quadrature to the motion (F
Q). Traditionally, these force components have been considered as a function of the static-probe height only. Here we show that F
I and F
Q actually depend on both static-probe height and oscillation amplitude. We demonstrate on simulated data how to reconstruct the amplitude dependence of F
I and F
Q from a single ImAFM measurement. Furthermore, we introduce ImAFM approach measurements with which we reconstruct the full amplitude and probe-height dependence of the force components F
I and F
Q, providing deeper insight into the tip–surface interaction. We demonstrate the capabilities of ImAFM approach measurements on a polystyrene polymer surface.
atomic force microscopy; AFM; frequency combs; force spectroscopy; high-quality-factor resonators; intermodulation; multifrequency
We report an atomic force microscopy (AFM) method for assessing elastic and viscous properties of soft samples at acoustic frequencies under non-contact conditions. The method can be used to measure material properties via frequency modulation and is based on hydrodynamics theory of thin gaps we developed here. A cantilever with an attached microsphere is forced to oscillate tens of nanometers above a sample. The elastic modulus and viscosity of the sample are estimated by measuring the frequency-dependence of the phase lag between the oscillating microsphere and the driving piezo at various heights above the sample. This method features an effective area of pyramidal tips used in contact AFM but with only piconewton applied forces. Using this method, we analyzed polyacrylamide gels of different stiffness and assessed graded mechanical properties of guinea pig tectorial membrane. The technique enables the study of microrheology of biological tissues that produce or detect sound.
The measuring tip of an atomic force microscope (AFM) can be upgraded to a specific biosensor by attaching one or a few biomolecules to the apex of the tip. The biofunctionalized tip is then used to map cognate target molecules on a sample surface or to study biophysical parameters of interaction with the target molecules. The functionality of tip-bound sensor molecules is greatly enhanced if they are linked via a thin, flexible polymer chain. In a typical scheme of tip functionalization, reactive groups are first generated on the tip surface, a bifunctional cross-linker is then attached with one of its two reactive ends, and finally the probe molecule of interest is coupled to the free end of the cross-linker. Unfortunately, the most popular functional group generated on the tip surface is the amino group, while at the same time, the only useful coupling functions of many biomolecules (such as antibodies) are also NH2 groups. In the past, various tricks or detours were applied to minimize the undesired bivalent reaction of bifunctional linkers with adjacent NH2 groups on the tip surface. In the present study, an uncompromising solution to this problem was found with the help of a new cross-linker (“acetal-PEG-NHS”) which possesses one activated carboxyl group and one acetal-protected benzaldehyde function. The activated carboxyl ensures rapid unilateral attachment to the amino-functionalized tip, and only then is the terminal acetal group converted into the amino-reactive benzaldehyde function by mild treatment (1% citric acid, 1–10 min) which does not harm the AFM tip. As an exception, AFM tips with magnetic coating become demagnetized in 1% citric acid. This problem was solved by deprotecting the acetal group before coupling the PEG linker to the AFM tip. Bivalent binding of the corresponding linker (“aldehyde-PEG-NHS”) to adjacent NH2 groups on the tip was largely suppressed by high linker concentrations. In this way, magnetic AFM tips could be functionalized with an ethylene diamine derivative of ATP which showed specific interaction with mitochondrial uncoupling protein 1 (UCP1) that had been purified and reconstituted in a mica-supported planar lipid bilayer.
Accurate mechanical characterization by the atomic force microscope at the highest spatial resolution requires that topography is deconvoluted from indentation. The measured height of nanoscale features in the atomic force microscope (AFM) is almost always smaller than the true value, which is often explained away as sample deformation, the formation of salt deposits and/or dehydration. We show that the real height of nano-objects cannot be obtained directly: a result arising as a consequence of the local probe-sample geometry.
Methods and Findings
We have modeled the tip-surface-sample interaction as the sum of the interaction between the tip and the surface and the tip and the sample. We find that the dynamics of the AFM cannot differentiate between differences in force resulting from 1) the chemical and/or mechanical characteristics of the surface or 2) a step in topography due to the size of the sample; once the size of a feature becomes smaller than the effective area of interaction between the AFM tip and sample, the measured height is compromised. This general result is a major contributor to loss of height and can amount to up to ∼90% for nanoscale features. In particular, these very large values in height loss may occur even when there is no sample deformation, and, more generally, height loss does not correlate with sample deformation. DNA and IgG antibodies have been used as model samples where experimental height measurements are shown to closely match the predicted phenomena.
Being able to measure the true height of single nanoscale features is paramount in many nanotechnology applications since phenomena and properties in the nanoscale critically depend on dimensions. Our approach allows accurate predictions for the true height of nanoscale objects and will lead to reliable mechanical characterization at the highest spatial resolution.
Instrumental drift in atomic force microscopy (AFM) remains a critical, largely unaddressed issue that limits tip–sample stability, registration, and the signal-to-noise ratio during imaging. By scattering a laser off the apex of a commercial AFM tip, we locally measured and thereby actively controlled its three-dimensional position above a sample surface to <40 pm (Δf = 0.01–10 Hz) in air at room temperature. With this enhanced stability, we overcame the traditional need to scan rapidly while imaging and achieved a 5-fold increase in the image signal-to-noise ratio. Finally, we demonstrated atomic-scale (~100 pm) tip–sample stability and registration over tens of minutes with a series of AFM images on transparent substrates. The stabilization technique requires low laser power (<1 mW), imparts a minimal perturbation upon the cantilever, and is independent of the tip–sample interaction. This work extends atomic-scale tip–sample control, previously restricted to cryogenic temperatures and ultrahigh vacuum, to a wide range of perturbative operating environments.
The recent achievement of atomic resolution with dynamic atomic force microscopy (dAFM) [Fukuma et al., Appl. Phys. Lett.
2005, 87, 034101], where quality factors of the oscillating probe are inherently low, challenges some accepted beliefs concerning sensitivity and resolution in dAFM imaging modes. Through analysis and experiment we study the performance metrics for high-resolution imaging with dAFM in liquid media with amplitude modulation (AM), frequency modulation (FM) and drive-amplitude modulation (DAM) imaging modes. We find that while the quality factors of dAFM probes may deviate by several orders of magnitude between vacuum and liquid media, their sensitivity to tip–sample forces can be remarkable similar. Furthermore, the reduction in noncontact forces and quality factors in liquids diminishes the role of feedback control in achieving high-resolution images. The theoretical findings are supported by atomic-resolution images of mica in water acquired with AM, FM and DAM under similar operating conditions.
atomic force microscopy; dAFM; high-resolution; liquids
Atomic force microscopy (AFM) in contact mode and tapping mode is employed for high resolution studies of soft organic molecules (fetal bovine serum proteins) on hard inorganic diamond substrates in solution and air. Various effects in morphology and phase measurements related to the cantilever spring constant, amplitude of tip oscillations, surface approach, tip shape and condition are demonstrated and discussed based on the proposed schematic models. We show that both diamond and proteins can be mechanically modified by Si AFM cantilever. We propose how to choose suitable cantilever type, optimize scanning parameters, recognize and minimize various artifacts, and obtain reliable AFM data both in solution and in air to reveal microscopic characteristics of protein-diamond interfaces. We also suggest that monocrystalline diamond is well defined substrate that can be applicable for fundamental studies of molecules on surfaces in general.
Information obtained by atomic force microscopy (AFM) depends strongly on the kind of probe or tip used; therefore, probe and tip effects have to be taken into account when verifying or interpreting the data acquired. In many papers, double-tip effects have been mentioned while other research was done; however, there are only a few special reports on double- or triple-tip effects, especially double-probe effects. In our paper, metaphase chromosomes of Chinese hamster ovary (CHO) cells, aggregates of pectin molecules, membrane surface of mouse embryonic stem cells, and R-phycoerythrin-conjugated immunoglobulin G complexes were imaged by AFM with high-quality probes, double-probe cantilever, and double-tip and triple-tip probes, respectively, in order to determine double-probe, double-tip, and triple-tip effects during AFM scanning. We found that the double-probe, double-tip, and triple-tip effects share the same principle, and that these effects correlate with distance and height differences between probes of double-probe cantilever or tips of double-tip or multiple-tip probes. Since many other factors influence double-probe or double-tip effects, more in-depth studies must be undertaken. However, this initial research will make all users of AFM techniques aware of double-probe and double-tip or triple-tip effects during AFM scanning and aid in verifying or interpreting the data acquired.
double-probe effects; double-tip effects; triple-tip effects; atomic force microscopy; tip artifacts; chromosome; pectin; phycoerythrin conjugated immunoglobulin G
We describe a technique for probing the elastic properties of biological membranes by using an atomic force microscope (AFM) tip to press the biological material into a groove in a solid surface. A simple model is developed to relate the applied force and observed depression distance to the elastic modulus of the material. A measurement on the proteinaceous sheath of the archaebacterium Methanospirillum hungatei GP1 gave a Young's modulus of 2 x 10(10) to 4 x 10(10) N/m2. The measurements suggested that the maximum sustainable tension in the sheath was 3.5 to 5 N/m. This finding implied a maximum possible internal pressure for the bacterium of between 300 and 400 atm. Since the cell membrane and S-layer (wall) which surround each cell should be freely permeable to methane and since we demonstrate that the sheath undergoes creep (expansion) with pressure increase, it is possible that the sheath acts as a pressure regulator by stretching, allowing the gas to escape only after a certain pressure is reached. This creep would increase the permeability of the sheath to diffusible substances.
In this study, we have mapped the surface charge of wool fibers using chemically specific high-resolution force spectroscopy in order to better understand the dispersion of amino acids in relation to fiber morphology. The inter-surface forces between standard atomic force microscopy (AFM) probe tips (tip radius ~ 50 nm) functionalized with COOH and NH3 terminated alkanethiol self assembling monolayers and the wool surface were used to estimate the surface charge per unit area using linear Poisson-Boltzmann-based electrostatic double layer theory. The positional measurement of nano-scale surface charge showed a correlation between the surface charge and fiber morphology, indicated that basic amino acids are located near the scale edges.
Interaction between P-factor, a peptide pheromone composed of 23 amino acid residues, and its pheromone receptor, Mam2, on the cell surface of the fission yeast Schizosaccharomyces pombe was examined by an atomic force microscope (AFM). An AFM tip was modified with P-factor derivatives to perform force curve measurements. The specific interaction force between P-factor and Mam2 was calculated to be around 120 pN at a probe speed of 1.74 μm/s. When the AFM tip was modified with truncated P-factor derivative lacking C-terminal Leu, the specific interaction between the tip and the cell surface was not observed. These results were also confirmed with an assay system using a green fluorescent protein (GFP) reporter gene to monitor the activation level of signal transduction following the interaction of Mam2 with P-factor.
Atomic force microscopy (AFM) has emerged as the only technique capable of real-time imaging of the surface of a living cell at nano-resolution. Since AFM provides the advantage of directly observing living biological cells in their native environment, this technique has found many applications in pharmacology, biotechnology, microbiology, structural and molecular biology, genetics and other biology-related fields. AFM has also proved to be a valuable tool for reproductive biologists. An exhaustive review on the various applications of AFM to sperm cells is presented. AFM has been extensively applied for determining the structural and topological features of spermatozoa. Unstained, unfixed spermatozoa in their natural physiological surroundings can be imaged by this technique which provides valuable information about the morphological and pathological defects in sperm cells as three-dimensional images with precise topographical details. Sperm head defects and the acrosome at the tip of the head responsible for fertilization, can be examined and correlated with the lack of functional integrity of the cell. Considerable amount of work is reported on the structural details of the highly condensed chromatin in sperm head using AFM. Detailed information on 3D topographical images of spermatozoa acquired by AFM is expected to provide a better understanding of various reproductive pathways which, in turn, can facilitate improved infertility management and/or contraceptive development.
Key developments in NC-AFM have generally involved atomically flat crystalline surfaces. However, many surfaces of technological interest are not atomically flat. We discuss the experimental difficulties in obtaining high-resolution images of rough surfaces, with amorphous SiO2 as a specific case. We develop a quasi-1-D minimal model for noncontact atomic force microscopy, based on van der Waals interactions between a spherical tip and the surface, explicitly accounting for the corrugated substrate (modeled as a sinusoid). The model results show an attenuation of the topographic contours by ~30% for tip distances within 5 Å of the surface. Results also indicate a deviation from the Hamaker force law for a sphere interacting with a flat surface.
graphene; model; noncontact atomic force microscopy; SiO2; van der Waals
Bacterial biofilms impair the operation of many industrial processes. Deinococcus geothermalis is efficient primary biofilm former in paper machine water, functioning as an adhesion platform for secondary biofilm bacteria. It produces thick biofilms on various abiotic surfaces, but the mechanism of attachment is not known. High-resolution field-emission scanning electron microscopy and atomic force microscopy (AFM) showed peritrichous adhesion threads mediating the attachment of D. geothermalis E50051 to stainless steel and glass surfaces and cell-to-cell attachment, irrespective of the growth medium. Extensive slime matrix was absent from the D. geothermalis E50051 biofilms. AFM of the attached cells revealed regions on the cell surface with different topography, viscoelasticity, and adhesiveness, possibly representing different surface layers that were patchily exposed. We used oscillating probe techniques to keep the tip-biofilm interactions as small as possible. In spite of this, AFM imaging of living D. geothermalis E50051 biofilms in water resulted in repositioning but not in detachment of the surface-attached cells. The irreversibly attached cells did not detach when pushed with a glass capillary but escaped the mechanical force by sliding along the surface. Air drying eliminated the flexibility of attachment, but it resumed after reimmersion in water. Biofilms were evaluated for their strength of attachment. D. geothermalis E50051 persisted 1 h of washing with 0.2% NaOH or 0.5% sodium dodecyl sulfate, in contrast to biofilms of Burkholderia cepacia F28L1 or the well-characterized biofilm former Staphylococcus epidermidis O-47. Deinococcus radiodurans strain DSM 20539T also formed tenacious biofilms. This paper shows that D. geothermalis has firm but laterally slippery attachment not reported before for a nonmotile species.
We report on the use of three different atomic force spectroscopy modalities to determine the nanomechanical properties of amyloid fibrils of the human α-synuclein protein. α-Synuclein forms fibrillar nanostructures of approximately 10 nm diameter and lengths ranging from 100 nm to several microns, which have been associated with Parkinson's disease. Atomic force microscopy (AFM) has been used to image the morphology of these protein fibrils deposited on a flat surface. For nanomechanical measurements, we used single-point nanoindentation, in which the AFM tip as the indenter is moved vertically to the fibril surface and back while the force is being recorded. We also used two recently developed AFM surface property mapping techniques: Harmonic force microscopy (HarmoniX) and Peakforce QNM. These modalities allow extraction of mechanical parameters of the surface with a lateral resolution and speed comparable to tapping-mode AFM imaging. Based on this phenomenological study, the elastic moduli of the α-synuclein fibrils determined using these three different modalities are within the range 1.3-2.1 GPa. We discuss the relative merits of these three methods for the determination of the elastic properties of protein fibrils, particularly considering the differences and difficulties of each method.
Micrometer resolution placement and immobilization of probe molecules is an important step in the preparation of biochips and a wide range of lab-on-chip systems. Most known methods for such a deposition of several different substances are costly and only suitable for a limited number of probes. In this article we present a flexible procedure for simultaneous spatially controlled immobilization of functional biomolecules by molecular ink lithography.
For the bottom-up fabrication of surface bound nanostructures a universal method is presented that allows the immobilization of different types of biomolecules with micrometer resolution. A supporting surface is biotinylated and streptavidin molecules are deposited with an AFM (atomic force microscope) tip at distinct positions. Subsequent incubation with a biotinylated molecule species leads to binding only at these positions. After washing streptavidin is deposited a second time with the same AFM tip and then a second biotinylated molecule species is coupled by incubation. This procedure can be repeated several times. Here we show how to immobilize different types of biomolecules in an arbitrary arrangement whereas most common methods can deposit only one type of molecules. The presented method works on transparent as well as on opaque substrates. The spatial resolution is better than 400 nm and is limited only by the AFM's positional accuracy after repeated z-cycles since all steps are performed in situ without moving the supporting surface. The principle is demonstrated by hybridization to different immobilized DNA oligomers and was validated by fluorescence microscopy.
The immobilization of different types of biomolecules in high-density microarrays is a challenging task for biotechnology. The method presented here not only allows for the deposition of DNA at submicrometer resolution but also for proteins and other molecules of biological relevance that can be coupled to biotin.
Drug resistance is a challenge that can be addressed using nanotechnology. We focused on the resistance of the bacteria Pseudomonas aeruginosa and investigated, using Atomic Force Microscopy (AFM), the behavior of a reference strain and of a multidrug resistant clinical strain, submitted to two antibiotics and to an innovative antibacterial drug (CX1). We measured the morphology, surface roughness and elasticity of the bacteria under physiological conditions and exposed to the antibacterial molecules. To go further in the molecules action mechanism, we explored the bacterial cell wall nanoscale organization using functionalized AFM tips. We have demonstrated that affected cells have a molecularly disorganized cell wall; surprisingly long molecules being pulled off from the cell wall by a lectin probe. Finally, we have elucidated the mechanism of action of CX1: it destroys the outer membrane of the bacteria as demonstrated by the results on artificial phospholipidic membranes and on the resistant strain.
Atomic force microscopy (AFM) has emerged as a powerful technique for mapping the surface morphology of biological specimens, including bacterial cells. Besides creating topographic images, AFM enables us to probe both physicochemical and mechanical properties of bacterial cell surfaces on a nanometer scale. For AFM, bacterial cells need to be firmly anchored to a substratum surface in order to withstand the friction forces from the silicon nitride tip. Different strategies for the immobilization of bacteria have been described in the literature. This paper compares AFM interaction forces obtained between Klebsiella terrigena and silicon nitride for three commonly used immobilization methods, i.e., mechanical trapping of bacteria in membrane filters, physical adsorption of negatively charged bacteria to a positively charged surface, and glutaraldehyde fixation of bacteria to the tip of the microscope. We have shown that different sample preparation techniques give rise to dissimilar interaction forces. Indeed, the physical adsorption of bacterial cells on modified substrata may promote structural rearrangements in bacterial cell surface structures, while glutaraldehyde treatment was shown to induce physicochemical and mechanical changes on bacterial cell surface properties. In general, mechanical trapping of single bacterial cells in filters appears to be the most reliable method for immobilization.
Semiconducting CrSi2 nanocrystallites (NCs) were grown by reactive deposition epitaxy of Cr onto n-type silicon and covered with a 50-nm epitaxial silicon cap. Two types of samples were investigated: in one of them, the NCs were localized near the deposition depth, and in the other they migrated near the surface. The electrical characteristics were investigated in Schottky junctions by current-voltage and capacitance-voltage measurements. Atomic force microscopy (AFM), conductive AFM and scanning probe capacitance microscopy (SCM) were applied to reveal morphology and local electrical properties. The scanning probe methods yielded specific information, and tapping-mode AFM has shown up to 13-nm-high large-area protrusions not seen in the contact-mode AFM. The electrical interaction of the vibrating scanning tip results in virtual deformation of the surface. SCM has revealed NCs deep below the surface not seen by AFM. The electrically active probe yielded significantly better spatial resolution than AFM. The conductive AFM measurements have shown that the Cr-related point defects near the surface are responsible for the leakage of the macroscopic Schottky junctions, and also that NCs near the surface are sensitive to the mechanical and electrical stress induced by the scanning probe.
The noise of the frequency-shift signal Δf in noncontact atomic force microscopy (NC-AFM) consists of cantilever thermal noise, tip–surface-interaction noise and instrumental noise from the detection and signal processing systems. We investigate how the displacement-noise spectral density d
z at the input of the frequency demodulator propagates to the frequency-shift-noise spectral density d
f at the demodulator output in dependence of cantilever properties and settings of the signal processing electronics in the limit of a negligible tip–surface interaction and a measurement under ultrahigh-vacuum conditions. For a quantification of the noise figures, we calibrate the cantilever displacement signal and determine the transfer function of the signal-processing electronics. From the transfer function and the measured d
z, we predict d
f for specific filter settings, a given level of detection-system noise spectral density d
ds and the cantilever-thermal-noise spectral density d
th. We find an excellent agreement between the calculated and measured values for d
f. Furthermore, we demonstrate that thermal noise in d
f, defining the ultimate limit in NC-AFM signal detection, can be kept low by a proper choice of the cantilever whereby its Q-factor should be given most attention. A system with a low-noise signal detection and a suitable cantilever, operated with appropriate filter and feedback-loop settings allows room temperature NC-AFM measurements at a low thermal-noise limit with a significant bandwidth.
Cantilever; feedback loop; filter; noncontact atomic force microscopy (NC-AFM); noise
The asymmetry of a bevel-tip needle results in the needle naturally bending when it is inserted into soft tissue. As a first step toward modeling the mechanics of deflection of the needle, we determine the forces at the bevel tip. In order to find the forces acting at the needle tip, we measure rupture toughness and nonlinear material elasticity parameters of several soft tissue simulant gels and chicken tissue. We incorporate these physical parameters into a finite element model that includes both contact and cohesive zone models to simulate tissue cleavage. We investigate the sensitivity of the tip forces to tissue rupture toughness, linear and nonlinear tissue elasticity, and needle tip bevel angle. The model shows that the tip forces are sensitive to the rupture toughness. The results from these studies contribute to a mechanics-based model of bevel-tip needle steering, extending previous work on kinematic models.
Controllers for scanning probe instruments can be programmed for automated lithography to generate desired surface arrangements of nanopatterns of organic thin films, such as n-alkanethiol self-assembled monolayers (SAMs). In this report, atomic force microscopy (AFM) methods of lithography known as nanoshaving and nanografting are used to write nanopatterns within organic thin films. Commercial instruments provide software to control the length, direction, speed, and applied force of the scanning motion of the tip. For nanoshaving, higher forces are applied to an AFM tip to selectively remove regions of the matrix monolayer, exposing bare areas of the gold substrate. Nanografting is accomplished by force-induced displacement of molecules of a matrix SAM, followed immediately by the surface self-assembly of n-alkanethiol molecules from solution. Advancements in AFM automation enable rapid protocols for nanolithography, which can be accomplished within the tight time restraints of undergraduate laboratories. Example experiments with scanning probe lithography (SPL) will be described in this report that were accomplished by undergraduate students during laboratory course activities and research internships in the chemistry department of Louisiana State University. Students were introduced to principles of surface analysis and gained “hands-on” experience with nanoscale chemistry.
Single-molecule force spectroscopy, as implemented in an atomic force microscope, provides a rarely-used method by which to monitor dynamic processes that occur near surfaces. Here, a methodology is presented and characterized that facilitates the study of polymer bridging across nanometer-sized gaps. The model system employed is that of DNA-based reversible polymers, and an automated procedure is introduced that allows the AFM tip-surface contact point to be automatically determined, and the distance d between opposing surfaces to be actively controlled. Using this methodology, the importance of several experimental parameters was systematically studied, e.g. the frequency of repeated tip/surface contacts, the area of the substrate surface sampled by the AFM, and the use of multiple AFM tips and substrates. Experiments revealed the surfaces to be robust throughout pulling experiments, so that multiple touches and pulls could be carried out on a single spot with no measurable affect on the results. Differences in observed bridging probabilities were observed, both on different spots on the same surface and, more dramatically, from one day to another. Data normalization via a reference measurement allows data from multiple days to be directly compared.
Single-molecule force spectroscopy; reversible polymer; atomic force microscope; DNA; supramolecular polymer; bridging probability