Routine femtogram-level chemical analyses using vibrational spectroscopy and self-cleaning scanning probe microscopy tips

Simultaneous structural and chemical characterization of materials at the nanoscale is both an immediate need and an ongoing challenge. This article reports a route to address this need, which can be rapidly adopted by practitioners, by combining the benefits of widely available scanning probe microscopy and vibrational microspectrometry. In an atomic force microscope (AFM), the probe tip can provide a nanoscale topographic image. Here, we use a temperature-controlled probe tip to selectively acquire an analyte from a specified location and determine its mass in a thermogravimetric manner. The tip is then analyzed via complementary Raman and Fourier transform infrared microspectrometers, providing a molecular characterization of samples down to the femtogram level in minutes. The probe can be self-cleaned and employed for repeated use by rapidly heating it to vaporize the analyte. By combining the established analytical modalities of AFM and vibrational spectrometry, a complete physical and molecular characterization of nanoscale domains is possible: mass determination is facile, thermal analyses can be integrated on the probe, and the obtained spectral data can be related to existing knowledge bases.     

Cold-atom scanning probe microscopy

Scanning probe microscopes are widely used to study surfaces with atomic resolution in many areas of nanoscience. Ultracold atomic gases trapped in electromagnetic potentials can be used to study electromagnetic interactions between the atoms and nearby surfaces in chip-based systems. Here we demonstrate a new type of scanning probe microscope that combines these two areas of research by using an ultracold gas as the tip in a scanning probe microscope. This cold-atom scanning probe microscope offers a large scanning volume, an ultrasoft tip of well-defined shape and high purity, and sensitivity to electromagnetic forces (including dispersion forces near nanostructured surfaces). We use the cold-atom scanning probe microscope to non-destructively measure the position and height of carbon nanotube structures and individual free-standing nanotubes. Cooling the atoms in the gas to form a Bose-Einstein condensate increases the resolution of the device.     

Waveguide analysis of heat-drawn and chemically etched probe tips for scanning near-field optical microscopy

We analyze two basic aspects of a scanning near-field optical microscope (SNOM) probe's operation: (i) spot-size evolution of the electric field along the probe with and without a metal layer, and (ii) a modal analysis of the SNOM probe, particularly in close proximity to the aperture. A slab waveguide model is utilized to minimize the analytical complexity, yet provides useful quantitative results--including losses associated with the metal coating--which can then be used as design rules.     

Quantitative modelling in scanning probe microscopy

Significant progress has been made in the theoretical modelling of scanning probe microscopy. The models available now are sufficiently refined to provide information not only about the surface, but also the probe tip, and the physical changes occurring during the scanning process. This has significantly improved the quantitative analysis of experimental and theoretical results. Scanning probe microscopes can now be reliably used to analyse events on the level of single atoms and single electrons.     

Hydrothermally-grown ZnO nanowire tips for scanning tunnelling microscopy

The probe tip is pivotal in determining the resolution and nature of features observed in the Scanning Tunnelling Microscope (STM). We have augmented a conventional Pt/Ir metallic tip with a hydrothermally grown ZnO nanowire (NW). Atomic resolution imaging of graphite is attained. Current-voltage (IV) characteristics demonstrate an asymmetry stemming from the unintentional n-type doping of the ZnO NW, whereas the expected Schottky barrier at the ZnO-Pt/Ir interface is shown to have negligible effect. Moreover the photoconductivity of the system is investigated, paving the way towards a photodetector capable of atomic resolution.     

Nanoscale microstructural analysis of metallic materials by atom probe field ion microscopy

This paper reviews recent progress in atom probe field ion microscopy (APFIM) and its applications to nanoscale microstructural studies of metallic materials. By employing a three-dimensional atom probe (3DAP), elemental distributions can be mapped out in a nanometric volume with a near-atomic resolution. This technique has been shown to be suitable for characterizing nanocrystalline and nanocomposite materials as well as nanometric precipitates in various commercial metallic materials. This review highlights recent APFIM and 3DAP investigations conducted in the author's group, which have contributed to our understanding of the microstructure–property relationships in novel nanocrystalline and nanocomposite alloys, as well as conventional metallic materials containing nanoscale precipitates.     

Polypyrrole-modified tips for functional group recognition in scanning tunneling microscopy

Tailored chemical modification of scanning tunneling microscopy (STM) tips is a promising method for the recognition of specific chemical species and functional groups in STM images. The present study shows for the first time that tips modified with polypyrrole can be used to measure STM images with molecular resolution. A high conductivity of the polypyrrole film was found to be important for the observation of STM images, while the thickness of the polymer film did not affect the images significantly. Furthermore, it was shown that recognition of functional groups in STM images is possible with tips coated with conductive polypyrroles. 1-Octadecanol and 1-octadecanoic acid monolayers with polypyrrole-modified tips gave high-resolution STM images in which aligned OH and COOH residues were represented by easily recognizable elevated bands. These selective contrast enhancements resemble those observed by us previously with gold tips modified with self-assembled monolayers (SAMs) and seem to be due to hydrogen bond interactions between functional groups of the tip-modifying molecules and the sample. The reproducibility of contrast enhancements in this study was significantly higher than for SAM-modified tips, suggesting that polymer modification of STM tips is particularly promising for specific functional group recognition with chemically modified STM tips.     

Energy dissipation measurements in frequency-modulated scanning probe microscopy

Local dissipation measurements by scanning probe microscopy have attracted increasing interest as a method for probing energy losses and hysteretic phenomena due to magnetic, electrical, and structural transformations at the tip-surface junction. One challenge of this technique is the lack of a standard for ensuring quantification of the dissipation signal. In the following, we explored magnetic dissipation imaging of an yttrium-iron garnet (YIG) sample, using a number of similar but not identical cantilever probes. Typical frequency-dependent dispersion of the actuator-probe assembly commonly approached ± 1 part in 10(3) Hz(-1), much larger than the minimum detectable level of ± 1 part in 10(5) Hz(-1). This cantilever-dependent behavior results in a strong crosstalk between the conservative (frequency) and dissipative channels. This crosstalk was very apparent in the YIG dissipation images and in fact should be an inherent feature of single-frequency heterodyne detection schemes. It may also be a common effect in other dissipation imaging, even down to the atomic level, and in particular may be a significant issue when there are correlations between the conservative and dissipative components. On the other hand, we present a simple method for correcting for this effect. This correction technique resulted in self-consistent results for the YIG dissipation measurements and would presumably be effective for other systems as well.     

Tapping-mode scanning force microscopy: Metallic tips and samples

The operation of a tapping-mode scanning force microscope using a metallic tip and metallic sample, with a bias voltage applied between the two, is modelled as a driven nonlinear oscillator, where metal-metal adhesion and electric forces are taken into account. The model, which applies to the case where the sample indentation by the tip is minimal, shows that one can obtain a good estimate of the tip-sample contact time from the tip-sample current.     

Cathodic electropaint insulated tips for electrochemical scanning tunneling microscopy

A method of preparing tungsten tips insulated for in situ scanning tunneling microscopy (STM) work is presented. Tips were electrocoated at low applied voltages using an organic solution of the cathodic electropaint rather than the more frequently utilized electropaint emulsions. The insulated tips were then characterized using cyclic voltammetry and electrochemical STM (EC-STM). They displayed low Faradaic leakage currents under electrochemical conditions and provided for high-resolution STM imaging in electrolyte solution. Flat terraces on a monocrystalline Au surface and adlayers of 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II) were imaged under potential control in dilute HClO4 using the fabricated tips, and atomic or molecular resolution images were obtained in both cases. The developed procedure allows for the fast, reproducible generation of large numbers of electrically insulated tungsten tips suitable for EC-STM imaging experiments.     

Scanning electrochemical microscopy: approach curves for sphere-cap scanning electrochemical microscopy tips

The parameters of functions used to predict diffusion-controlled scanning electrochemical microscopy approach curves under positive and negative (hindered diffusion) feedback for sphere-cap tips are reported. These functions were obtained by fitting approach curves simulated with an error-bounded adaptive finite element algorithm. Several geometries corresponding to different sphere-cap dimensions were considered including the effect of the tip insulating sheath. The simulated approach curves were successfully compared with experimental ones obtained with mercury sphere caps electrodeposited onto platinum microdisk electrodes.     

Microfluidic push-pull probe for scanning electrochemical microscopy

This paper presents a microfluidic push-pull probe for scanning electrochemical microscopy (SECM) consisting of a working microelectrode, an integrated counter/reference electrode and two microchannels for pushing and pulling an electrolyte solution to and away from a substrate. With such a configuration, a droplet of a permanently renewed redox mediator solution is maintained just at the probe tip to carry out SECM measurements on initially dry substrates or in microenvironments. For SECM imaging purposes, the probe fabricated in a soft polymer material is used in a contact regime. SECM images of various gold-on-glass samples demonstrate the proof-of-concept of a push-pull probe for local surface activity characterization with high spatial resolution even on vertically oriented substrates. Finite element computations were performed to guide the improvement of the probe sensitivity.     

Material sensitive scanning probe microscopy of subsurface semiconductor nanostructures via beam exit Ar ion polishing

Whereas scanning probe microscopy (SPM) is highly appreciated for its nanometre scale resolution and sensitivity to surface properties, it generally cannot image solid state nanostructures under the immediate sample surface. Existing methods of cross-sectioning (focused ion beam milling and mechanical and Ar ion polishing) are either prohibitively slow or cannot provide a required surface quality. In this paper we present a novel method of Ar ion beam cross-section polishing via a beam exiting the sample. In this approach, a sample is tilted at a small angle with respect to the polishing beam that enters from underneath the surface of interest and exits at a glancing angle. This creates an almost perfect nanometre scale flat cross-section with close to open angle prismatic shape of the polished and pristine sample surfaces ideal for SPM imaging. Using the new method and material sensitive ultrasonic force microscopy we mapped the internal structure of an InSb/InAs quantum dot superlattice of 18 nm layer periodicity with the depth resolution of the order of 5 nm. We also report using this method to reveal details of interfaces in VLSI (very large scale of integration) low k dielectric interconnects, as well as discussing the performance of the new approach for SPM as well as for scanning electron microscopy studies of nanostructured materials and devices.     

Surface analysis algorithms for scanning probe microscopy

A single data set in scanning probe microscopy is large, typically in the megabyte range. As interpretation is accomplished by displaying the data in image form for visualization, image processing methods are used to both convert to visual images and to modify the images in order to clarify features of interest. Although an impressive number of image-processing algorithms are available on most commercial probe microscopes, many potentially very interesting ones are not. In addition, the special character of scanning probe data sets calls for development of new algorithms specially suited to this kind of problem. The work here analyzes images produced using atomic force microscope data sets. Algorithms are shown and discussed using images of oxide surfaces. The following algorithms are applied: tilt correction, scattering noise removal, surface smoothing, surface compression, probability density function analysis, correlation, and power spectrum analysis. Such algorithms and others serve to remove spurious surface spikes, enhance visualization of long-range surface features in the presence of short-range surface variations, remove line-to-line scanning artifacts, etc.     

Carbon nanotube scanning tunneling microscopy tips for chemically selective imaging

Carboxyl-terminated single-walled carbon nanotubes (SWNTs) were successfully immobilized from solution phases onto the apexes of gold tips for scanning tunneling microscopy (STM). Gold STM tips were first modified with self-assembled monolayers of 4-mercaptobenzoic acid, and its carboxyl groups were used to anchor carboxylated SWNTs through Zn2+ ion-bridged coordination. These SWNT tips gave high-resolution STM images of a diether monolayer formed on the graphite surface. In addition and more importantly, the ether oxygens of the sample molecules were selectively observed as bright spots with the SWNT tips with significantly high reproducibility, which is due to the facilitation of electron tunneling through hydrogen bond interactions between the ether oxygens and carboxyl groups at the end of the SWNT tips.     

Anisotropy in wear processes measured by scanning probe microscopy

Wear of smooth amorphous carbon overcoats was performed in continuous sliding contact in the range of 10⁴–10⁷ cycles by a diamond or diamond-like carbon counterbody. The overcoats on superpolished hard disks were examined using friction and topographic scanning probe microscopy. An optimal Fourier (Wiener) filter was designed which preferentially filtered noise out of images of the smooth films. Second derivatives of the z-height were calculated parallel and perpendicular to the wear direction. The crosscorrelation of a friction force image and the corresponding z-height image shows that the correlation length is also extended in the wear direction. The local friction is independent of the z-height before wear, but increases with z-height upon wear. A model for the z-height and friction force evolution in contact sliding is discussed and compared with data.     

Fabrication of nanometer-sized electrodes and tips for scanning electrochemical microscopy.

A novel method for fabrication of nanometer-sized electrodes and tips suitable for scanning electrochemical microscopy (SECM) is reported. A fine etched Pt wire is coated with polyimide, which was produced by polymerization on the Pt surface initiated by heat. This method can prepare electrodes with effective radii varying from a few to hundreds of nanometers. Scanning electron microscopy, cyclic voltammetry, and SECM were used to characterize these electrodes. Well-defined steady-state voltammograms could be obtained in aqueous or in 1,2-dichloroethane solutions. This method produced the nanoelectrodes with exposed Pt on the apex, and they can also be employed as the nanotips for SECM investigations. Different sizes of Pt nanotips made by this method were employed to evaluate the kinetics of the redox reaction of Ru(NH3)6(3+) on the surface of a large Pt electrode by SECM, and the standard rate constant kappa0 of this system was calculated from the best fit of the SECM approach curve. This result is similar to the values obtained by analysis of the obtained voltammetric data.