Scanning probe microscopy

The authors describe a range of microscopy techniques which are based on scanning a sharp tip over a surface. With such techniques it is possible both to image surfaces to atomic resolution, and to perform local property measurements to a resolution determined by the nature of the tip-surface interaction. As well as producing surface images, applications include imaging the electronic structure of semiconductor surfaces, measuring local magnetic properties and determining tribological properties of surfaces such as adhesion, lubrication and the coefficient of friction. The scanned probe techniques described are available commercially and therefore account for the majority of applications in this rapidly growing field     

Scanning probe microscopy in catalysis

This review discusses the recent progress in the application of scanning probe microscopy (SPM) in catalysis. SPM proves to be an invaluable technique for imaging catalytic surfaces and interfaces. Most SPM research is related to the structural and morphological transformation associated with catalyst preparation and use. Real-time SPM observation of surface dynamics including adsorption, diffusion and reaction, provides invaluable insights to the mechanism of catalysis. SPM is also used to shape and manipulate surfaces and surface processes. Fabrication of nanostructured catalysts, direct manipulation of adsorbed atoms and molecules and tip-mediated reactions are some examples of new SPM approach in catalyst research.     

Scanning probe microscopy based on magnetoresistive sensing

Integrated sensors are essential for scanning probe microscopy (SPM) based systems that employ arrays of microcantilevers for high throughput. Common integrated sensors, such as piezoresistive, piezoelectric, capacitive and thermoelectric sensors, suffer from low bandwidth and/or low resolution. In this paper, a novel magnetoresistive-sensor-based scanning probe microscopy (MR-SPM) technique is presented. The principle of MR-SPM is first demonstrated using experiments with magnetic cantilevers and commercial MR sensors. A new cantilever design tailored to MR-SPM is then presented and micromagnetic simulations are employed to evaluate the achievable resolution. A remarkable resolution of 0.84?? over a bandwidth of 1?MHz is estimated, which would significantly outperform state-of-the-art optical deflection sensors. Due to its combination of high resolution at high bandwidth, and its amenability to integration in probe arrays, MR-SPM holds great promise for low-cost, high-throughput SPM.     

Scanning probe microscopy of intrafibrillar crystallites in calcified collagen

Vertebrate mineralized tissues are composite materials formed by the organized growth of carbonated apatite crystals within a matrix of collagen fibres. Calcified collagen from turkey tendon was investigated using scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). Samples were treated with hydrogen peroxide to enhance the mineralized phase by removing part of the collagen matrix and the results compared with the untreated material. Plate-like crystalline entities with dimensions 35 nm × 5–8 nm by 1.5 nm were seen. These dimensions are consistent with previous reports using transmission electron microscopy (TEM) of calcified tendon and topographic imaging of tendon crystals. The resolution of the images obtained using STM is better than the previously reported pictures obtained using TEM or scanning electron microscopy (SEM). The value of 35 nm is the same as the gap region in the structure of the collagen fibrils. Stacking of plates and plate-aggregates are a dominant feature in the scanning images. These results support the concept of organized intra-fibril mineral crystals within the organic collagen matrix. Electron diffraction and X-ray diffraction were undertaken on the samples and the patterns recorded match those previously reported for carbonated apatite.     

Scanning probe microscopy studies of isocyanide functionalized polyaniline thin films

The technique of scanning probe microscopy was used to study the nanometer-scale morphological changes in isocyanide functionalized polyaniline films due to protonation in aqueous HCl, as well as exposure to Ir⁺ cations in CH₂Cl₂ solution. Electropolymerized isocyanide functionalized aniline films exhibited a highly oriented fibrous structure, with individual strands averaging 25 Å in diameter. Upon protonation, the fibrous structure was lost, with the material reorganizing into oriented, elongated bundles of average diameter 200 Å. Ir⁺ exposed unprotonated films exhibited an oriented, interlocking bundle structure, resulting from Ir⁺ incorporation into the film matrix. The diameter of these bundles averaged 400 Å.     

Scanning probe microscopy of single Au ion implants in Si

We have studied 5 MeV Au²⁺ ion implantation with fluences between 7 × 10⁷ and 2 × 10⁸ cm^− 2 in Si by deep level transient spectroscopy (DLTS) and scanning capacitance microscopy (SCM). The DLTS measurements show formation of electrically active defects such as the two negative charge states of the divacancy (V₂(/–) and V₂(–/0)) and the vacancy–oxygen (VO) center. It is observed that the intensity of the V₂(/–) peak is lower compared to that of V₂(–/0) by a factor of 5. This has been attributed to a highly localized distribution of the defects along the ion tracks, which results in trapping of the carriers at V₂(–/0) and incomplete occupancy of V₂(/–). The SCM measurements obtained in a plan view show a random pattern of regions with a reduced SCM signal for the samples implanted with fluence above 2 × 10⁸ cm^− 2. The reduced SCM signal is attributed to extra charges associated with acceptor states, such as V₂(–/0), formed along the ion tracks in the bulk Si. Indeed, the electron emission rate from the V₂(–/0) state is in the range of 10 kHz at room temperature, which is well below the probing frequency of the SCM measurements, resulting in “freezing” of electrons at V₂(–/0).     

Scanning probe microscopy with landmark referenced control for direct biological investigations

We report the successful use of continuous wavelet transforms applied to atomic force microscope data sets for landmark recognition of biological features. The data sets were images of mixed red and white blood cells. Contrast enhancement followed by continuous wavelet transform of the data was used to successfully distinguish erythrocytes from neutrophil and monocyte leukocytes within the mixed cell images. All of the above are spherical objects between 6 and 8 microns in diameter, which demonstrates the ability to sort similar biological objects into distinct classes. The implications for development of on-line scanning probe recognition microscopy are discussed.     

Scanning probe microscopy study of electronic properties in alkyl-substituted oligothiophene-based field-effect transitors

It appeared in the past decades that semi-conducting organic liquid crystals could replace inorganic semi-conductors to manufacture field-effect transistors (FET). Indeed, they can be easily processed by simple methods such as inkjet printing. These simple and cheap manufacturing methods pave the way to new applications for plastic electronics: electronic tags, biosensors, flexible screens, etc. The performance of these liquid crystal nanomaterials is due to their specific nanoscale structure. However, one limitation to the improvement of organic electronic devices is an incomplete understanding of their optoelectronic properties at the nanoscale. The organic semiconductor films often contain a combination of many ordered and disordered regions, grain boundaries and localized traps. These features impact charge transport and trapping at the sub-100 nm length scales [1]. Electrical SPM techniques such as STM, KPFM, EFM and CS-AFM have the potential to provide a direct correlation between the electronic properties and the local film structure and have already made important contributions to the field of organic electronics.Here we report on preliminary investigations of the structural and electronic properties of p-conductive organic field-effect transistors (OFET) based on alkyl-substituted oligothiophenes with bottom-contact structure. For this purpose, we used atomic force microscopy (AFM) and Kelvin-probe force microscopy (KPFM) in dual frequency mode under ambient conditions. This study helps to determine the local potential in the channel of active OFETs. On the other hand the molecular arrangements of these molecules on HOPG have been studied using scanning tunnelling microscopy (STM) at the liquid–solid interface.     

Scanning hall probe microscopy (SHPM) using quartz crystal AFM feedback

Scanning Hall Probe Microscopy (SHPM) is a quantitative and non-invasive technique for imaging localized surface magnetic field fluctuations such as ferromagnetic domains with high spatial and magnetic field resolution of approximately 50 nm and 7 mG/Hz(1/2) at room temperature. In the SHPM technique, scanning tunneling microscope (STM) or atomic force microscope (AFM) feedback is used to keep the Hall sensor in close proximity of the sample surface. However, STM tracking SHPM requires conductive samples; therefore the insulating substrates have to be coated with a thin layer of gold. This constraint can be eliminated with the AFM feedback using sophisticated Hall probes that are integrated with AFM cantilevers. However it is very difficult to micro fabricate these sensors. In this work, we have eliminated the difficulty in the cantilever-Hall probe integration process, just by gluing a Hall Probe chip to a quartz crystal tuning fork force sensor. The Hall sensor chip is simply glued at the end of a 32.768 kHz or 100 kHz Quartz crystal, which is used as force sensor. An LT-SHPM system is used to scan the samples. The sensor assembly is dithered at the resonance frequency using a digital Phase Locked Loop circuit and frequency shifts are used for AFM tracking. SHPM electronics is modified to detect AFM topography and the frequency shift, along with the magnetic field image. Magnetic domains and topography of an Iron Garnet thin film crystal, NdFeB demagnetised magnet and hard disk samples are presented at room temperature. The performance is found to be comparable with the SHPM using STM feedback.     

Scanning transmission x-ray microscopy as a novel tool to probe colloidal and photonic crystals

Photonic crystals consisting of nano- to micrometer-sized building blocks, such as multiple sorts of colloids, have recently received widespread attention. It remains a challenge, however, to adequately probe the internal crystal structure and the corresponding deformations that inhibit the proper functioning of such materials. It is shown that scanning transmission X-ray microscopy (STXM) can directly reveal the local structure, orientations, and even deformations in polystyrene and silica colloidal crystals with 30-nm spatial resolution. Moreover, STXM is capable of imaging a diverse range of crystals, including those that are dry and inverted, and provides novel insights complementary to information obtained by benchmark confocal fluorescence and scanning electron microscopy techniques.     

Scanning probe microscopy investigation of self-organized perylenetetracarboxdiimide nanostructures at surfaces: structural and electronic properties

A scanning probe microscopy investigation of the self-organization and local electronic properties of spin-coated ultrathin films of N-alkyl substituted perylenetetracarboxdiimide (PDI) is described. By carefully balancing the interplay between molecule-molecule and molecule-substrate interactions, PDI is able to form highly ordered supramolecular architectures on flat surfaces from solution. On an electrically insulating yet highly polar surface (mica) PDI forms strongly anisotropic architectures with needlelike structures with lengths of up to a few micrometers. On a conductive yet apolar surface (highly oriented pyrolytic graphite), the competition between the strong molecule-substrate interactions and the intermolecular forces leads to the generation of more disordered structures. The local electronic properties of these architectures are studied by Kelvin probe force microscopy by estimating their surface potential (SP). Quantitative measurements of the SP are obtained by analyzing the experimentally estimated SP data with a computational model, which discriminates between the intrinsic SP and the effect of long-range tip-surface interactions. The SP of PDI aggregates depends on the structural order at the supramolecular level. Narrow needles of constant width reveal identical SPs independent of length. Wider needles with a polydisperse width distribution exhibit a greater SP.