Standard Atomic Force Microscopy Tip Research Articles

Terahertz Nanofocusing with Cantilevered Terahertz-Resonant Antenna Tips

We developed THz-resonant scanning probe tips, yielding strongly enhanced and nanoscale confined THz near fields at their tip apex. The tips with length in the order of the THz wavelength (λ = 96.5 μm) were fabricated by focused ion beam (FIB) machining and attached to standard atomic force microscopy (AFM) cantilevers. Measurements of the near-field intensity at the very tip apex (25 nm radius) as a function of tip length, via graphene-based (thermoelectric) near-field detection, indicate their first and second order geometrical antenna resonances for tip length of 33 and 78 μm, respectively. On resonance, we find that the near-field intensity is enhanced by one order of magnitude compared to tips of 17 μm length (standard AFM tip length), which is corroborated by numerical simulations that further predict remarkable intensity enhancements of about 107 relative to the incident field. Because of the strong field enhancement and standard AFM operation of our tips, we envision manifold and straightforward future application in scattering-type THz near-field nanoscopy and THz photocurrent nanoimaging, nanoscale nonlinear THz imaging, or nanoscale control and manipulation of matter employing ultrastrong and ultrashort THz pulses.

High Resolution Tips for Switching Magnetization MFM

Switching magnetization magnetic force microscopy (SM-MFM) is based on two-pass magnetic force microscopy with opposite orientation of tip magnetization between two scans. The sum of the scanned data with reversed tip magnetization depicts local van der Waals forces, and their di erence maps the local magnetic forces. Tip magnetization can be easily reversed in external magnetic eld during the scanning. The separation of the forces mapped enables scanning in close proximity of the sample (∼ 5 nm). Therefore, extremely high spatial resolution (10 nm) is achievable by the SM-MFM. Image phase resolution of the MFM method depends on various geometric parameters of the tip, such as tip length, its apex radius and taper angle. The parameters are determined by the evaporation process, within which the standard atomic force microscopy tips are coated with magnetic layer. In this work we show that the thickness of the coated layer is important for the SM-MFM spatial resolution.

Diamond-Modified AFM Probes: From Diamond Nanowires to Atomic Force Microscopy-Integrated Boron-Doped Diamond Electrodes

In atomic force microscopy (AFM), sharp and wear-resistant tips are a critical issue. Regarding scanning electrochemical microscopy (SECM), electrodes are required to be mechanically and chemically stable. Diamond is the perfect candidate for both AFM probes as well as for electrode materials if doped, due to diamond's unrivaled mechanical, chemical, and electrochemical properties. In this study, standard AFM tips were overgrown with typically 300 nm thick nanocrystalline diamond (NCD) layers and modified to obtain ultra sharp diamond nanowire-based AFM probes and probes that were used for combined AFM-SECM measurements based on integrated boron-doped conductive diamond electrodes. Analysis of the resonance properties of the diamond overgrown AFM cantilevers showed increasing resonance frequencies with increasing diamond coating thicknesses (i.e., from 160 to 260 kHz). The measured data were compared to performed simulations and show excellent correlation. A strong enhancement of the quality factor upon overgrowth was also observed (120 to 710). AFM tips with integrated diamond nanowires are shown to have apex radii as small as 5 nm and where fabricated by selectively etching diamond in a plasma etching process using self-organized metal nanomasks. These scanning tips showed superior imaging performance as compared to standard Si-tips or commercially available diamond-coated tips. The high imaging resolution and low tip wear are demonstrated using tapping and contact mode AFM measurements by imaging ultra hard substrates and DNA. Furthermore, AFM probes were coated with conductive boron-doped and insulating diamond layers to achieve bifunctional AFM-SECM probes. For this, focused ion beam (FIB) technology was used to expose the boron-doped diamond as a recessed electrode near the apex of the scanning tip. Such a modified probe was used to perform proof-of-concept AFM-SECM measurements. The results show that high-quality diamond probes can be fabricated, which are suitable for probing, manipulating, sculpting, and sensing at single digit nanoscale.

Sharp High-Aspect-Ratio AFM Tips Fabricated by a Combination of Deep Reactive Ion Etching and Focused Ion Beam Techniques

The shape and dimensions of an atomic force microscope tip are crucial factors to obtain high resolution images at the nanoscale. When measuring samples with narrow trenches, inclined sidewalls near 90 degrees or nanoscaled structures, standard silicon atomic force microscopy (AFM) tips do not provide satisfactory results. We have combined deep reactive ion etching (DRIE) and focused ion beam (FIB) lithography techniques in order to produce probes with sharp rocket-shaped silicon AFM tips for high resolution imaging. The cantilevers were shaped and the bulk micromachining was performed using the same DRIE equipment. To improve the tip aspect ratio we used FIB nanolithography technique. The tips were tested on narrow silicon trenches and over biological samples showing a better resolution when compared with standard AFM tips, which enables nanocharacterization and nanometrology of high-aspect-ratio structures and nanoscaled biological elements to be completed, and provides an alternative to commercial high aspect ratio AFM tips.

Quantitative measurement of indentation hardness and modulus of compliant materials by atomic force microscopy

An atomic force microscopy (AFM) based technique is proposed for the characterization of both indentation modulus and hardness of compliant materials. A standard AFM tip is used as an indenter to record force versus indentation curves analogous to those obtained in standard indentation tests. In order to overcome the lack of information about the apex geometry, the proposed technique requires calibration using a set of reference samples whose mechanical properties have been previously characterized by means of an independent technique, such as standard indentation. Due to the selected reference samples, the technique has been demonstrated to allow reliable measurements of indentation modulus and hardness in the range of 0.3-4.0 GPa and 15-250 MPa, respectively.

The structure and functionality of contractile forisome protein aggregates

Although they have been discovered decades ago, only in the last years forisome protein aggregates received broader attention due to their ability to convert chemical into mechanical energy. In contrast to most other motor proteins, these proteins from Fabaceae plants are independent of high-energy chemical compounds, like e.g. ATP, but undergo an anisotropic shape transition (longitudinally expanded to contracted) in response to ion concentration changes (Ca 2+, H +, etc.), instead. We present morphological and functional data on forisomes obtained using atomic force microscopy (AFM). High-aspect ratio AFM tips allow the detailed elucidation of structural characteristics that are inaccessible with standard AFM tips. Microindentation measurements were employed to calculate the elasticity of the forisome material. Young's moduli were found to be ∼32.7 kPa in the expanded state and ∼2.748 kPa in the contracted state of the polymer. These results are compared to investigations where a tipless AFM cantilever was utilized to exert a load against the shape transition. In the latter experiments, an energy conversion of ∼2.29 pJ per stroke was detected. Energetical considerations support the hypothesis that the switching process is accompanied by a change in cross-linking of the constituent subunits and allow estimating the extent of cooperativity during the pH-induced transition. Finally, useful parameters were identified and characterized that are crucial for the application of forisomes as functional elements in microfluidic chips.

Field-Assisted Nanopatterning

A new atomic force microscopy (AFM)-based lithography method called field-assisted nanopatterning (FAN) has been demonstrated. Through the use of a conventional atomic force microscope with no alterations, FAN controllably patterns solid or liquid organic and inorganic molecules in the air under ambient conditions. In this manner, patterns can be produced with feature sizes that range from tens of microns to sub-20 nm. Examples include the high-resolution FAN of [60]fullerene, N-methylpyrrole, naphthalene, poly-3-octylthiophene, polyaniline, meso-tetraphenylporphyrin, and gold. These molecules have been patterned onto highly ordered pyrolytic graphite, indium−tin oxide, Au, and passivated Au. The molecules are first coated on a standard AFM tip and then are deposited onto the substrate when a threshold tip bias is achieved. The deposition process is a field-assisted transfer of the molecules from the tip to the substrate. Patterning is turned on or off by controlling tip bias, and the same tip is used for...

Force Calibration in Lateral Force Microscopy

An analysis of the contact mechanics and the forces of interaction in lateral force microscopy measurements is presented. This analysis allows for a new method of interpretation of the frictional forces, the lateral contact stiffness, and the contact shear strength. The technique was developed for the interpretation of frictional data obtained with colloidal probes, although results are presented which illustrate its ability to interpret measurements recorded with both colloidal probes and standard atomic force microscopy tips. The technique is found to compensate for the variations in the contact geometry, giving repeatable results for probes of different sizes. A critical review of other techniques which have been employed to interpret the frictional force in lateral force microscopy is also presented.

An investigation into the use of micro-thermal analysis for the solid state characterisation of an HPMC tablet formulation

The use of micro-thermal analysis (μTA™) as a novel means of differentiating between components in a model tablet formulation is described. This technique involves a modification of atomic force microscopy (AFM) such that the standard AFM tip is replaced with a Wollaston wire, thereby allowing the probe to act as a thermistor and temperature probe. Consequently it is possible to map not only the topology but also the thermal conductivity of the sample. Furthermore, it is possible to apply a heating signal to the material and thereby to perform thermal analysis on highly localised regions of the sample. Compacts were prepared comprising ibuprofen, HPMC E4M prem and 1:1 mixes of the two components and analysed using a μTA™ micro-thermal analyser. The surface topology and conductivity images of the three systems are reported. In addition, the ability of the technique to perform thermal analysis on highly specific regions of the sample is described. The method was able to differentiate between the components of the sample on the basis of micro-thermomechanical experiments. The implications of the use of the technique for the study of pharmaceutical tablets is discussed.

Extending the Possibilities of Near-Field Scanning Optical Microscopy for Simultaneous Topographical and Chemical Force Imaging

AbstractThe Near-field Scanning Optical Microscope (NSOM) is an innovative new form of surface microscopy, which can be used to obtain local spectroscopic information about surfaces, enabling the characterization of nanometer-sized regions. The most important component of this instrument is the scanning probe tip. In this paper, we discuss the production of a novel fiber optic probe that can be used in local spectroscopy with an NSOM, but also for simultaneous imaging of topography and chemical forces. The probe consists of a bent, tapered silicon dioxide optical fiber. We have determined the rates of selective wet chemical etching of germanium dioxide doped pure silica optical fibers and used this information to optimize the probe etching process. A systematic approach for the development and testing of such probes is presented. The performance of the optical probes was characterized using surfaces prepared by the technique of microcontact printing. Phase and friction images of these surfaces were obtained using both standard atomic force microscopy tips and the optical fiber probe. The new optical probe was capable of distinguishing between different chemical regions on the patterned surface.