Gold-coated AFM Tips Research Articles

Controllable resistive switching in Au/Nb:SrTiO3 microscopic Schottky junctions

The reversible resistive switching effect at oxide interface shows promising applications in information storage and artificial intelligence. However, the microscopic switching mechanism is still elusive due to the difficulty of direct observation of the electrical and chemical behavior at the buried interface, which becomes a major barrier to design reliable, scalable, and reproducible devices. Here we used a gold-coated AFM tip as a removable electrode to investigate the resistive switching effect in a microscopic Au/Nb:SrTiO3 Schottky junction. We found that unlike the inhomogeneous random resistive switching in the macroscopic Schottky junctions, the high and low resistance states can be reversibly switched in a controllable way on the Nb-doped SrTiO3 surface by the conductive tip. The switching between the high and low resistance states in vacuum is accompanied by the reversible shift of the surface Fermi level. We indicate that the transfer of the interface oxygen ion in a double-well potential is responsible for the resistive switching in both macroscopic and microscopic Schottky junctions. Our findings provide a guide to optimize the key performance parameters of a resistive switching device such as operation voltage, switching speed, on/off ratio, and state retention time by proper electrode selection and fabrication strategy.

Tip Enhanced Raman Spectroscopy of Rhodamine 6G on nanostructured gold substrate

A new concept based setup for Tip Enhanced Raman Scattering measurement assisted by gold nanostructure is presented, that can provide a platform for gap-mode enhancement of the signal at the single molecule level conjugated with controlled spatial localization of the molecule under investigation and a method to determine the diffraction limit properties of the tip. In essence, this effect is obtained illuminating a gold coated AFM tip which is raster scanned over a nanostructured gold substrate, after chemisorption of a Raman active molecule. We expect that the near-field Raman enhancement would be given by the gap-mode effect of the two facing nano-features. Thanks to the nanostructured substrate, we verify that the resolution of the Raman mapping signal is well below the diffraction limit given by the combination of the optics geometry and the laser wavelength. We show that the gap-mode TERS can generate an estimated field- enhancement (g) of ~20 in localized areas of the sample and we demonstrate the ability to spatially define the molecule position (by Raman mapping) at the tens of nanometers scale.

Direct and quantitative AFM measurements of the concentration and temperature dependence of the hydrophobic force law at nanoscopic contacts

By virtue of its importance for self-organization of biological matter the hydrophobic force law and the range of hydrophobic interactions (HI) have been debated extensively over the last 40years. Here, we directly measure and quantify the hydrophobic force–distance law over large temperature and concentration ranges. In particular, we study the HI between molecularly smooth hydrophobic self-assembled monolayers, and similarly modified gold-coated AFM tips (radii∼8–50nm). We present quantitative and direct evidence that the hydrophobic force is both long-ranged and exponential down to distances of about 1–2nm. Therefore, we introduce a self-consistent radius-normalization for atomic force microscopy data. This approach allows quantitative data fitting of AFM-based experimental data to the recently proposed Hydra-model. With a statistical significance of r2⩾0.96 our fitting and data directly reveal an exponential HI decay length of 7.2±1.2Å that is independent of the salt concentration up to 750mM. As such, electrostatic screening does not have a significant influence on the HI in electrolyte concentrations ranging from 1mM to 750mM. In 1M solutions the observed instability during approach shifts to longer distances, indicating ion correlation/adsorption effects at high salt concentrations. With increasing temperature the magnitude of HI decreases monotonically, while the range increases slightly. We compare our results to the large body of available literature, and shed new light into range and magnitude of hydrophobic interactions at very close distances and over wide temperature and concentration regimes.

Force Spectroscopy of DNA-Ctab Aggregates

Aggregates of oligonucleotides and the surfactant, CTAB, have been studied with force spectroscopy using an atomic force microscope. Gold coated AFM tips were derivatized with long carbon chain thiols and thiols functionalized with carboxylic acid and amine head groups and used to measure the strength of interaction or adhesive force between the AFM tip and aggregate surface. Force curves were obtained at various locations on the aggregate surface showing consistently reproducible adhesion within a given spot and some variability in adhesion from one spot to another. Adhesive forces were also measured in the same way on a model system composed of self-assembled monolayers (SAMs) formed using the thiols mentioned above on gold coated silicon wafers for comparison. Forces of attraction between the derivatized tip and the model SAM surface were quite a bit lower compared to measurements made on aggregates. Some force curves obtained from aggregate surfaces showed broad retraction traces indicative of a strong adhesive force that may include a cascade of detachment steps. Currently, friction (lateral) force experiments are underway on both the aggregate surface and the model system to determine the surface free energy between the tip and surface and the extent of homogeneity on the aggregate.

Nanoscale Infrared Spectroscopy of Biological Systems

Infrared microspectroscopy is a popular technique for investigating biological structures. It is relatively simple to use, and considered to be a non-destructive technique. By combining atomic force microscopy and infrared spectroscopy (AFM-IR), it is possible to resolve chemical differences on the scale of ∼100 to 200 nm, which often reveals information that could not have been obtained with conventional infrared microspectroscopy. Currently, AFM-IR spectroscopy has the ability to collect IR spectroscopic information below the diffraction limit with lateral resolution of ∼ 100 nm. However, there are still some limitations that prevent its use on many important nanoscale systems. One of the main limitations is the thickness of the sample required for examination (> 100 nm). Overcoming these limitations has a dramatic impact by enabling widespread use of nanoscale IR spectroscopy for spatially resolved chemical characterization. The use of a quantum cascade laser (QCL) as the IR source significantly increases the sensitivity of AFM-IR. The QCL has repetition rates 1000 times higher than previous lasers used for AFM-IR. This allows the ability to pulse the laser at the resonant frequency of the AFM cantilever giving rise to a high IR sensitivity mode referred to as resonance enhanced infrared nanospectroscopy (REINS). Additional enhancement of the AFM-IR signal results when a gold-coated AFM tip is used, producing a “lightning-rod” effect which enhances the intensity of the exciting electric field at the tip. Furthermore, if the sample is deposited onto a gold substrate, the local electric field is further enhanced allowing for chemical identification of samples as thin as 25 nm. In this presentation we will demonstrate the effectiveness of the AFM-IR technique on several biological systems, including; IR spectroscopy of a monolayer film of Halobacterium salinarium on a gold substrate and IR chemical imaging of Streptomyces bacteria.

Photoswitching tripodal single molecular tip for noncontact AFM measurements: synthesis, immobilization, and reversible configurational change on gold surface

Tripodal molecules consisting of a tetrasubstituted adamantane with three phenylacetylene legs and a reversibly photoswitching apex were designed as "single molecular tips" for both chemical and topographical characterization of the substrate surface. By covalent attachment onto gold-coated AFM tips through three S-Au bonds, these rigid tripodal molecules are expected to act as sharp, robust, and stationary molecular tips whose configuration can be reversibly changed upon irradiation with UV or visible light. In this report, the full account of the syntheses of two photoswitching tripodal molecular tips, their immobilization onto Au(111) surfaces, and the detection of photoinduced configurational change on Au(111) surface by SPM measurements are documented.

Oriented epitaxial growth of amyloid fibrils of the N27C mutant β25–35 peptide

Amyloid fibrils are present in the extracellular space of various tissues in neurodegenerative and protein misfolding diseases. Amyloid fibrils may be used in nanotechnology applications, because of their self-assembly properties and stability, if their growth and orientation can be controlled. Recently, we have shown that amyloid beta 25-35 (A beta 25-35) forms a highly oriented, K(+)-dependent network on mica. Here, we analyzed the properties of A beta 25-35_N27C, the cysteine residue of which may be used for subsequent chemical modifications. We find that A beta 25-35_N27C forms epitaxially growing fibrils on mica, which evolve into a trigonally oriented branched network. The binding is apparently more sensitive to cation concentration than that of the wild-type peptide. By nanomanipulating A beta 25-35_N27C fibrils with a gold-coated AFM tip, we show that the sulfhydryl of Cys27 is reactive and accessible from the solution. The oriented network of A beta 25-35_N27C fibrils can therefore be specifically labeled and may be used for constructing nanobiotechnological devices.

Molecular Recognition Forces between Immunoglobulin G and a Surface Protein Adhesin on Living Staphylococcus aureus

We report AFM measurements of binding events between immunoglobulin G (IgG) and protein A (PA) on the surface of live Staphylococcus aureus bacteria. The experiments were carried out with IgG molecules tethered via CM-amylose linkers to thiol SAMs on gold-coated AFM tips. For comparison, a model system consisting of protein A molecules tethered to thiol SAMs on gold-coated silicon substrates was also investigated. Histograms of binding forces for the PA-IgG bond showed comparable rupture forces of 59 and 64 pN for the model system and live bacteria, respectively. We suggest that linker molecules with a length comparable to the AFM tip radius should make it possible to detect specific binding events on the surface of live bacteria with a lateral resolution of a few tens of nanometers. Furthermore, because S. aureus is an important human pathogen, especially methicillin-resistant strains (MRSA), it is possible that additional virulence factors beyond PA can be probed using this technique.

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.

Gold-Coated Conducting-Atomic Force Microscopy Probes

Some aspects of the performance of gold-coated conductive probes used in conducting atomic force microscopy (C-AFM) technique are discussed. The resistance of the nanocontact between the gold-coated AFM tip and the graphite substrate has been monitored at various applied forces. For small forces (<50 nN), resistance on the order of a few kiloohms was observed. Minimal contact resistance was observed for forces in the range 100-150 nN, beyond which the tip seems to undergo plastic deformation. The resistance of the nanocontact increased when current on the order of 100 microA was allowed to pass through, finally resulting in melting of the gold coating.

Strength of Integration of Transmembrane α-Helical Peptides in Lipid Bilayers As Determined by Atomic Force Spectroscopy

In this study we address the stability of integration of proteins in membranes. Using dynamic atomic force spectroscopy, we measured the strength of incorporation of peptides in lipid bilayers. The peptides model the transmembrane parts of alpha-helical proteins and were studied in both ordered peptide-rich and unordered peptide-poor bilayers. Using gold-coated AFM tips and thiolated peptides, we were able to observe force events which are related to the removal of single peptide molecules out of the bilayer. The data demonstrate that the peptides are very stably integrated into the bilayer and that single barriers within the investigated region of loading rates resist their removal. The distance between the ground state and the barrier for peptide removal was found to be 0.75 +/- 0.15 nm in different systems. This distance falls within the thickness of the interfacial layer of the bilayer. We conclude that the bilayer interface region plays an important role in stably anchoring transmembrane proteins into membranes.

Catalytic probe lithography: catalyst-functionalized scanning probes as nanopens for nanofabrication on self-assembled monolayers.

This article describes the use of scanning catalytic probe lithography for nanofabrication of patterns on self-assembled monolayers (SAMs) of reactive adsorbates. Catalytic writing was carried out by scanning over bis(omega-tert-butyldimethyl-siloxyundecyl)disulfide SAMs using 2-mercapto-5-benzimidazole sulfonic acid-functionalized gold-coated AFM tips. The acidic tips induced local hydrolysis of the silyl ether moieties in the contacted areas, and thus patterned surfaces were created. Diffusion effects arising from the use of an ink were excluded in these type of experiments, and therefore structures with well-defined shapes and sizes were produced. The smallest lines drawn by this technique were about 25 nm wide, corresponding to the actual contact area of the tip. Lateral force microscopy studies performed on different SAMs helped to clarify the nature and cause of the friction contrasts observed by AFM. Dendritic wedges with thiol functions inserted into the catalytically written areas, thus enhancing the height contrast. The created patterns open possibilities to build 3D nanostructures.

Beta-cyclodextrin host-guest complexes probed under thermodynamic equilibrium: thermodynamics and AFM force spectroscopy.

The rupture forces of individual host-guest complexes between beta-cyclodextrin (beta-CD) heptathioether monolayers on Au(111) and several surface-confined guests were measured in aqueous medium by single molecule force spectroscopy using an atomic force microscope. Anilyl, toluidyl, tert-butylphenyl, and adamantylthiols (0.2-1%) were immobilized in mixed monolayers with 2-mercaptoethanol on gold-coated AFM tips. For all guests and for all surface coverages, the force-displacement curves measured between the functionalized tips and monolayers of beta-CD exhibited single, as well as multiple, pull-off events. The histograms of the pull-off forces showed several maxima at equidistant forces, with force quanta characteristic for each guest of 39 +/- 15, 45 +/- 15, 89 +/- 15, and 102 +/- 15 pN, respectively. These force quanta were independent of the loading rate, indicating that, because of the fast complexation/decomplexation kinetics, the rupture forces were probed under thermodynamic equilibrium. The force values followed the same trend as the free binding energy Delta G degrees measured for model guest compounds in solution or on beta-CD monolayers, as determined by microcalorimetry and surface plasmon resonance measurements, respectively. A descriptive model was developed to correlate quantitatively the pull-off force values with the Delta G degrees of the complexes, based on the evaluation of the energy potential landscape of tip-surface interaction.

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.

Comprehensive study of pulsed UV-laser modified polyamide fibers

Polyamide fibers (nylon 6) are modified by UV-radiation using a pulsed excimer laser with different treatment parameters. The treated samples are characterized by using a scanning electron microscopy (SEM), tapping mode atomic force microscopy (TM-AFM), x-ray photoelectron spectroscopy (XPS), and chemical force microscopy (CFM). Depending on the conditions during the irradiation, different surface modifications are obtained which can generally be distinguished as high-fluence (above the ablation threshold) and low-fluence (below the ablation threshold). Topographical results indicate that ripple-like structures of micrometer size are developed under high-fluence laser irradiation. On the contrary, smaller sub-micron structures are formed by low-fluence treatment. XPS results show that bond scission occurs on the polymer surface under the action of high-fluence while low-fluence modifies the sample by an oxidative process. Changes in surface chemical properties of the laser-irradiated polyamide are supported by chemical force microscopy experiments with gold-coated AFM tips modified with -COOH terminated self-assembled alkanethiol monolayers (SAMs).

Study on the surface chemical properties of UV excimer laser irradiated polyamide by XPS, ToF-SIMS and CFM

Polyamide (nylon 6) was irradiated by a pulsed ultraviolet (UV) excimer laser with a fluence below its ablation threshold. Chemical modifications on laser treated nylon were studied by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (Tof-SIMS) and chemical force microscopy (CFM). XPS study provides information about changes in chemical composition and the chemical-state of atom types on the fiber surface. The high sensitivity of ToF-SIMS to the topmost layers was used to detect crosslinking after the laser treatment. Gold-coated AFM tips modified with COOH terminated self-assembled alkanethiol monolayers (SAMs) were used to measure adhesion forces on the untreated and laser treated samples. XPS results revealed that the irradiated samples have higher oxygen content than prior to laser irradiation. Tof-SIMS analysis illustrated that carbonyl groups in nylon 6 decrease significantly but hydroxyl groups increase after low-fluence laser irradiation. The adhesion force measurements by CFM showed spatial distribution of hydroxyl groups on nylon 6 after the laser treatment.

Nanoscale 1,3,5,7-tetrasubstituted adamantanes and p-substituted tetraphenyl-methanes for AFM applications.

[structure: see text] Tetrahedrally shaped nanoscale molecules 18-20 were synthesized from the corresponding tetraiodide by a series of Sonogashira coupling reactions. Three of the sulfur-containing termini are intended for eventual binding to a gold-coated conventional AFM tip, while the fourth terminus scans the sample. AFM images of 19 demonstrate that the molecule is sufficiently large and rigid to be imaged by a conventional AFM tip.

Self-Assembled Monolayers on Gold of Ferrocene-Terminated Thiols and Hydroxyalkanethiols

In this paper, a study on the adsorption of mixed self-assembled monolayers (SAMs) for two different combinations of thiols (Fc(CH2)6SH/HO(CH2)2SH and Fc(CH2)16SH/HO(CH2)11SH (Fc = ferrocene)) is presented, to obtain surfaces with single isolated ferrocenylalkanethiols embedded in shorter hydroxyalkanethiols. These hydrophilic substrates with very low surface concentration of ferrocene moieties are required to perform force spectroscopy experiments on host−guest supramolecular complexes, by using SAMs of β-cyclodextrin heptathioether adsorbed on gold-coated AFM tips. Several SAMs have been prepared on polycrystalline gold electrodes from 1 mM thiol solutions, changing the ferrocenylalkanethiols/hydroxyalkanethiols ratio. The amount of electroactive component immobilized on the electrode was determined by cyclic voltammetry, and it has been related to the solution composition. The general trend is that the longer chain component is preferentially adsorbed, suggesting a thermodynamic control of the adsorption. However, relevant differences in the layer formation and composition can be observed for the two systems on the basis of a different balance of the driving forces that govern the adsorption process. Fc(CH2)16SH is adsorbed to a larger extent, compared to the Fc(CH2)6SH for the same solution composition, as shown by higher charge densities values. Furthermore for the Fc(CH2)16SH/HO(CH2)11SH system upon increase of the percentage of ferrocenylalkanethiols in solution, a shift in the redox peak position to more positive values is observed, indicating that phase segregation occurs. The differences between the two systems can be related to chain length effects from which arise favorable enthalpic contributions to the adsorption of the longer chain component.

Chemical Force Microscopic Study of UV Excimer Laser Irradiated Polyamide

ABSTRACTRecently, there has been great interest in physico-chemical treatment for modifying polymer surfaces. UV Excimer laser irradiation is of particular interest in morphological and chemical modifications. Depending on the laser energies used, effects of laser treatment fall into two groups: above the ablation threshold the ripple structures of micrometer size form (high-fluence), and below the ablation threshold the sub-micron structures emerge (low-fluence).Traditional methods of studying chemical properties are Fourier-transform infrared spectroscopy, x-ray photoelectron spectroscopy, and mass spectroscopy. The ability of a relatively new technique involving chemical force microscopy (CFM) can be used to image and discriminate the areas exposing different functional groups on polymers. Gold-coated AFM tip modified with carboxylic acid (-COOH) terminated self-assembled alkanethiol monolayers (SAMs) was used to measure the adhesive forces between the tip and the laser treated samples in a water or hexane medium.The CFM results showed that high-fluence laser treated polyamide has the highest adhesive force with the modified tip in a water medium when compared with the control and low-fluence ones. The adhesive force is due to electrostatic attraction between the negatively charged tips terminating in COO- groups and the positively charged sample terminating in NH+ groups. This indicates that the high-fluence laser treated sample results in the formation of many amine end-groups on the polymer surface. In comparison, only low-fluence treated surface shows adhesive force with the modified tip in a hexane medium. This implies that the low-fluence laser treated polyamide has substantially more hydrophilic groups than the untreated and high-fluence laser treated samples. The adhesion force measurements by CFM allow one to have better understanding of surface chemical modifications induced by UV excimer laser.

Study of microcontact printed patterns by chemical force microscopy

Patterned self-assembled monolayers (SAMs) on sputtered gold films prepared by microcontact printing (μCP) were studied by mapping adhesive forces with pulsed-force-mode atomic force microscopy. A stamp for μCP was fabricated by pouring polydimethylsiloxane (PDMS) over a photolithographically prepared master. The patterned SAMs were prepared by two methods. One is called the wet-inking method, in which inking was done by placing a thiol ethanol solution for 30 s on the stamp and then removing the excess ink solution under a stream of nitrogen. The other is called the contact-inking method, in which a pad made of PDMS was dipped overnight in a thiol ethanol solution and then the stamp was placed on the inker pad impregnated with the thiol ethanol solution. The second step for pattern formation was the same for both of the two different μCP methods. Namely, the gold surfaces stamped with alkanethiols were further reacted with a thiol terminating in COOH in ethanol. The resulting patterns with CH 3- and COOH-terminated regions were analyzed by imaging the adhesive forces with the chemically modified gold coated AFM tips with a SAM of CH 3 or COOH terminal functional groups.