An atomic force microscopy tip model for investigating the mechanical properties of materials at the nanoscale

Investigation of the mechanical properties of materials at the nanoscale is often performed by atomic force microscopy nanoindentation. However, substrates with large surface roughness and heterogeneity demand careful data analysis. This requirement is even more stringent when surface indentations with a typical depth of a few nanometers are produced to test material hardness. Accordingly, we developed a geometrical model of the nanoindenter, which was first validated by measurements on a reference gold sample. Then we used this technique to investigate the mechanical properties of a coating layer made of Balinit C, a commercially available alloy with superior anti-wear features deposited on steel. The reported results support the feasibility of reliable hardness measurements with truly nanosized indents.     

Investigations of local electrical properties using tunneling/atomic force microscope with a quartz tuning fork nearfield sensor

Ability to determine local electric surface properties with a high resolution is a key issue in many modern industrial applications. In this article, authors will describe low-cost and reliable methods for investigations of electrical surface properties with a nanoscale resolution using a homebuilt modular tunneling/atomic force microscope with a quartz tuning fork as a probe. We will present the architecture of the designed system and the calibration method of the applied sensor. In our work, the usage of the tunneling atomic force microscope in the high-resolution investigations of the surface topography and identification of local spots where the tunneling current is observed will be demonstrated. We will also present current-voltage (I-V) spectroscopy performed on a gold thin film sputtered on silicon substrate and a highly oriented pyrolitic graphite (HOPG) surface, which we obtained in air ambient and at room temperature.     

Determination of the elastic modulus of adherent cells using spherical atomic force microscope probe

When the conventional Hertz formula is used to extract the elastic modulus, E, of cells based on the compression test using atomic force microscope spherical probe, the inconsistency between the actual situation and the assumption of the formula will lead to a large error. Using the ABAQUS for finite element modeling and analysis, here, a modified Hertz formula was developed to reduce the effects of cell radius, cell thickness, probe radius and compression depth on the extracted E of cells. Experimentally, the insensitivity of the extracted E to the compression region of cell and probe radius reflects the validity of the modified formula. Owing to the poor resolution of spherical probes, it's unlikely to know the actual thickness of cell at the measured point, which can lead to a huge error. Based on the modified formula, we further proposed an approach to control the effect of the uncertainty of cell thickness and ensured that a 10% difference in cell thickness does not incur over 10% variation in the obtained elastic modulus.

     

Fabrication of nanostructure on a polymer film using atomic force microscope

SPM based lithographic techniques have been developed to pattern various substrates such as metals, semiconductors, and organic/polymer films due to its simplicity and high spatial precision nanostructure. Fabrication of nanostructure using polymeric materials is a key technique for the development of nanodevices. Here, we report the fabrication of nanostructures from polyacrylicacid (PAA) and polymethacrylicacid (PMAA) film on a silicon substrate using atomic force microscope (AFM). The formation of the nanopattern from the polymer film was studied using electrostatic nanolithography and the optimization of the conditions for nanopatterning of the polymer film was investigated with respect to the applied potential and translational speed of the AFM tip. The nanostructure of size 28 nm was created using the biased AFM tip on the PMAA film coated on Si(100) substrate and found that this method is a direct and reliable method to produce uniform nanostructures on a polymer film.     

Lorentz force actuation of a heated atomic force microscope cantilever

We report Lorentz force-induced actuation of a silicon microcantilever having an integrated resistive heater. Oscillating current through the cantilever interacts with the magnetic field around a NdFeB permanent magnet and induces a Lorentz force that deflects the cantilever. The same current induces cantilever heating. With AC currents as low as 0.2 mA, the cantilever can be oscillated as much as 80 nm at resonance with a DC temperature rise of less than 5 °C. By comparison, the AC temperature variation leads to a thermomechanical oscillation that is about 1000 times smaller than the Lorentz deflection at the cantilever resonance. The cantilever position in the nonuniform magnetic field affects the Lorentz force-induced deflection, with the magnetic field parallel to the cantilever having the largest effect on cantilever actuation. We demonstrate how the cantilever actuation can be used for imaging, and for measuring the local material softening temperature by sensing the contact resonance shift.     

Mechanical characterization of nickel nanowires by using a customized atomic force microscope

A new experimental method to characterize the mechanical properties of metallic nanowires is introduced. An accurate and fast mechanical characterization of nanowires requires simultaneous imaging and testing of the nanowires. However, existing mechanical characterization techniques fail to accomplish this goal due either to the lack of imaging capability of the mechanical test setup or the difficulty of individual alignment and manipulation of single nanowires for each test. In this study, nanowire specimens prepared by an electroplating technique are located on a silicon substrate with trenches. A customized atomic force microscope is located inside a scanning electron microscope (SEM) in order to establish the visibility of the nanowires, and the tip of the atomic force microscope cantilever is utilized to bend and break the nanowires. The ability to visualize the nanowires in an SEM improves the speed and accuracy of the tests. Experimentally obtained force versus bending displacement curves are fitted into existing analytical formulations to extract the mechanical properties. Experimental results reveal that nickel nanowires have significantly higher strengths than their bulk counterparts, although their elastic modulus values are comparable to bulk nickel modulus values.     

Nanoscale patterning of gold nanoparticles using an atomic force microscope

We report on the use of the tip of an atomic force microscope to remove selectively, and subsequently to deposit, nanoparticles of gold passivated with tri-n-octylphosphine oxide (TOPO)/octadecylamine. The study has revealed a minimum feature size of 50 nm in the removal experiment, while lines of 180 nm could be drawn with the gold nanoparticles, limited by the quality of the substrate surface.     

The direct measurement of linewidth using an atomic force microscope

A method for directly measuring linewidth with an atomic force microscope using the first derivative of the signal is presented. The method showed that it is possible to make a direct measurement of the size of the upper base of a trapezoidal protrusion down to 30 nm. __________ Translated from Izmeritel’naya Tekhnika, No. 5, pp. 10–12, May, 2008.     

A method to quantitatively measure the elastic modulus of materials in nanometer scale using atomic force microscopy

A method is proposed for quantitatively measuring the elastic modulus of materials using atomic force microscopy (AFM) nanoindentation. In this method, the cantilever deformation and the tip-sample interaction during the early loading portion are treated as two springs in series, and based on Sneddon's elastic contact solution, a new cantilever-tip property α is proposed which, together with the cantilever sensitivity A, can be measured from AFM tests on two reference materials with known elastic moduli. The measured α and A values specific to the tip and machine used can then be employed to accurately measure the elastic modulus of a third sample, assuming that the tip does not get significantly plastically deformed during the calibration procedure. AFM nanoindentation tests were performed on polypropylene (PP), fused quartz and acrylic samples to verify the validity of the proposed method. The cantilever-tip property and the cantilever sensitivity measured on PP and fused quartz were 0.514?GPa and 51.99?nm?nA(-1), respectively. Using these measured quantities, the elastic modulus of acrylic was measured to be 3.24?GPa, which agrees well with the value measured using conventional depth-sensing indentation in a commercial nanoindenter.     

Nanoscale operation of a living cell using an atomic force microscope with a nanoneedle

We have developed a tool for performing surgical operations on living cells at nanoscale resolution using atomic force microscopy (AFM) and a modified AFM tip. The AFM tips are sharpened to ultrathin needles of 200-300 nm in diameter using focused ion beam etching. Force-distance curves obtained by AFM using the needles indicated that the needles penetrated the cell membrane following indentation to a depth of 1-2 microm. The force increase during the indentation process was found to be consistent with application of the Hertz model. A three-dimensional image generated by laser scanning confocal microscopy directly revealed that the needle penetrated both the cellular and nuclear membranes to reach the nucleus. This technique enables the extended application of AFM to analyses and surgery of living cells.     

Direct visualization and identification of biofunctionalized nanoparticles using a magnetic atomic force microscope

Because of its outstanding ability to image and manipulate single molecules, atomic force microscopy (AFM) established itself as a fundamental technique in nanobiotechnology. (1) We present a new modality that distinguishes single nanoparticles by the surrounding magnetic field gradient. Diamagnetic gold and superparamagnetic iron oxide nanoparticles become discernible under ambient conditions. Images of proteins, magnetolabeled with nanoparticles, demonstrate the first steps toward a magnetic analogue to fluorescence microscopy, which combines nanoscale lateral resolution of AFM with unambiguous detection of magnetic markers.     

Determination of boron by using a quartz crystal resonator coated with N-methyl-D-glucamine-modified poly(epichlorohydrin)

A nongravimetric quartz crystal resonator for determination of boron was proposed. The key step is the preparation of a polymer that forms a complex with boron (from borate ion). The polymeric film is deposited on one face of an electrode-separated quartz crystal. The backbone of the polymer is poly(epichlorohydrin), which is modified to anchor N-methyl-D-glucamine. After reticulation and reduction, the film presents high stability and sensitivity to boron at pH 8.5. A carrier solution containing 50 mM EDTA ensures high conductivity and the elimination of several interfering metal ions. Boron is strongly retained by the film, and a positive shift of the oscillating frequency is proportional to its concentration. Boron is eluted with 1 mL of a 1 M mannitol solution. For a 0.160-mL sample loop and concentration up to 600 microM, the calibration sensitivity was 1.67 Hz/microM and the LOD was 2 microM. This limit could be lowered to 0.3 microM by using a 1.00-mL sample loop. In both cases, it was possible to detect 3 ng of boron. It was estimated that the nongravimetric sensor is at least 10 times more sensitive that a hypothetical gravimetric sensor.     

Local temperature measurements on nanoscale materials using a movable nanothermocouple assembled in a transmission electron microscope

A nanoscale thermocouple consisting of merged Cu and Cu-Ni tips is developed for local temperature measurements on advanced nanomaterials by using a probing technique in a high-resolution transmission electron microscope (TEM) equipped with a double probe scanning tunneling microcopy (STM) unit. The fabricated nanothermocouple works as the so-called T-type thermocouple and displays a quick response and high spatial and thermal resolutions. A generated thermoelectromotive force which reflects rapid temperature changes controlled by electron beam intensity alternations on a metal nanoelectrode proves the technique's usefulness for high-precision local temperature measurements. The developed method demonstrates the effectiveness while also measuring temperature changes in Joule heated multi-walled carbon nanotubes (CNTs) and in a modeled electrical conductive composite nanosystem.     

The determination of the elastic modulus of microcantilever beams using atomic force microscopy

An investigation into the determination of the micromechanical properties of thin film materials has been performed. Thin metal and ceramic films are used extensively in the computer microprocessor industry and in the field of micro-electromechanical systems (MEMS). The demand for miniaturization and increased performance has resulted in the use of materials without a clear understanding of their mechanical properties on this scale. Micromechanical properties are difficult to obtain due to the lack of adequate testing equipment. The atomic force microscope (AFM), most commonly used as an imaging tool, lends itself to mechanical interaction with the sample surface utilizing a cantilever probe. An array of aluminum microcantilever beams were fabricated using standard IC processing techniques. The microbeams were deflected by the AFM cantilever probe and from this, the micromechanical properties of stiffness and elastic modulus were determined. Initial results indicate that this technique reliably determines the micromechanical properties of thin films.     

The calibration of an atomic force microscope along three coordinates using a single certified dimension

A method of calibrating an atomic force microscope with respect to three coordinates using a single certified dimension of a test object is proposed. The method corresponds completely to the State Standards, which ensure the unity of measurements in nanotechnologies. __________ Translated from Izmeritel’naya Tekhnika, No. 5, pp. 13–15, May, 2008.     

Variations in properties of atomic force microscope cantilevers fashioned from the same wafer

Variations in the mechanical properties of nominally identical V-shaped atomic force microscope (AFM) cantilevers sourced from the same silicon nitride wafer have been quantified by measuring the spring constants, resonant frequencies and quality factors of 101 specimens as received from the manufacturer using the thermal spectrum method of Hutter and Bechhoefer. The addition of thin gold coatings always lowers the resonant frequency but the corresponding spring constant can either increase or decrease as a result. The observed broad spread of spring constant values and the lack of correlations between the resonant frequency and spring constant can be attributed in part to the non-uniformity of composition and material properties in the thinnest dimension of such cantilevers which arise from the manufacturing process. The effects of coatings are dictated by the competing influence of differences in mass density and Young's modulus between the silicon nitride and the gold coating. An implication of this study is that cantilever calibration methods based on the assumption of uniformity of material properties of the cantilever in the thinnest dimension are unlikely to be applicable for such cantilevers.     

Double-resonance quartz crystal oscillator and excitation of a resonator immersed in liquid media

In this work we propose a novel circuit design: a double-resonance oscillator. Its oscillation shows two oscillation modes: frequency locking to the quartz crystal resonance and LC resonance oscillation. Transition of the oscillation mode and the strength of oscillation are analyzed and reviewed for the fundamental mode in comparison with a Colpitts oscillator. The experimental results support the estimates of negative resistance for the double-resonance oscillator compared with the LC oscillator.     

Enhanced formation of a confined nano-water meniscus using a 780 nm laser with a quartz tuning fork-atomic force microscope

Demonstrated herein is the optical-field-induced enhancement of the formation of a confined nanowater meniscus using a distance-regulated quartz tuning fork-atomic force microscope (QTF-AFM) with a 780 nm laser. While a pulled optical fiber tip approaches the surface, the laser is suddenly turned on and focuses on the front spot of the tip by the shape of the pulled optical fiber, which plays the role of an objective lens and induces the gathering effect of the water molecules directed to the electromagnetic-field gradient in air. This phenomenon facilitates a new boundary condition to form a long-range confined nano-scale liquid bridge between the tip and the surface. After the pulling of the optical fiber, 20-nm-thick gold was sputtered on the apex (diameter: approximately 100 nm) of the tip to guide and focus the beam on the spot. The critical power of the laser to overcome the barrier for the formation of a new boundary is 100 microW at the distance of 22 nm from the substrate.