Atomic Force Microscope Cantilever Research Articles

Thermo-magnetic behaviour of AFM–MFM cantilevers

Atomic force microscopy (AFM) experiments were performed to study the behaviour of AFM cantilevers under an external magnetic field B and temperature field produced by a coil with an iron core. Four cantilever types were studied. Forces were measured for different B values and at various coil-to-cantilever separation distances. The results were analysed on the basis of a phenomenological model. This model contains the contribution of two terms, one monopole–monopole interaction at short distance, and one apparent paramagnetic interaction in at large distance, which represents the temperature effects. We observe a good agreement between the model and the experimental data.

Modeling and simulation of AFM cantilever with two piezoelectric layers submerged in liquid over rough surfaces

In this paper, the vibration of an atomic force microscope (AFM) cantilever in tapping mode with two whole piezoelectric layers submerged in liquid medium is investigated. In the performed modeling, the sample surface has been considered as rough, and to show these surface asperities, two models of Rumpf and Rabinovich have been employed for analyzing the attractive Van der Waals force. This paper has been organized in two sections. The first section deals with the functioning of cantilever over rough surfaces, which accompanies the changes of the attractive Van der Waals force, and the second section involves the changes in the Van der Waals force which lead to a change in the liquid medium. The cantilever is totally submerged in the liquid. To show the effect of liquid on cantilever, first, only the cantilever tip is immersed in the liquid and it is dynamically analyzed. Then, the cantilever is totally submerged and then taken out of the liquid, so that the additional mass and damping of the cantilever could be calculated. In these two manners of cantilever immersion in liquid, the effects of the added mass and damping on the cantilever can be measured. When a cantilever vibrates totally in liquid, since the mass and damping of the liquid that is present on the cantilever cannot be determined, first, the cantilever's natural frequency in liquid is estimated in the laboratory and then by using this frequency and the cantilever stiffness (which is not medium-dependent and is always considered as constant), the additional mass and damping of the cantilever are determined.

Parametric study of electroplating conditions on mismatch strain of thin film for self-positioning of MEMS

In this paper, we report a detailed investigation of mismatch strain profiles taking account of the effects of process conditions, specifically the electrodeposition current density and temperature. This concept is demonstrated using the standard AFM (atomic force microscope) optical method. First, the thin layer of nickel to be measured is electrodeposited onto the tip side of the AFM cantilever at various current densities and temperatures. Then the cantilever deflections are measured by detecting the position change of the laser on the detector shifts and converted into the quantitative mismatch strain by appropriate mechanics. For a given deposition thickness, the magnitude of the mismatch strain increases with an elevation of the current density or plating temperature. The maximum deflection rate and mismatch strain happen on the interface when two different materials are connected. Next, the determined mismatch strain is further assessed experimentally by comparing its predictions for the self-deformed shape and the measured deflections of the AFM cantilever.

Nanoscale Pulling of Type IV Pili Reveals Their Flexibility and Adhesion to Surfaces over Extended Lengths of the Pili

Type IV pili (T4P) are very thin protein filaments that extend from and retract into bacterial cells, allowing them to interact with and colonize a broad array of chemically diverse surfaces. The physical aspects that allow T4P to mediate adherence to many different surfaces remain unclear. Atomic force microscopy (AFM) nanoscale pulling experiments were used to measure the mechanical properties of T4P of a mutant strain of Pseudomonas aeruginosa PAO1 unable to retract its T4P. After adhering bacteria to the end of an AFM cantilever and approaching surfaces of mica, gold, or polystyrene, we observed adhesion of the T4P to all of the surfaces. Pulling of single and multiple T4P on retraction of the cantilever from the surfaces could be described using the worm-like chain (WLC) model. Distinct peaks in the measured distributions of the best-fit values of the persistence length Lp on two different surfaces provide strong evidence for close-packed bundling of very flexible T4P. In addition, we observed force plateaus indicating that adhesion of the T4P to both hydrophilic and hydrophobic surfaces occurs along extended lengths of the T4P. These data shed new light, to our knowledge, on T4P flexibility and support a low-affinity, high-avidity adhesion mechanism that mediates bacteria-surface interactions.

Scanning Resonator Microscopy: Integrating Whispering Gallery Mode Sensing with Atomic Force Microscopy

Scanning resonator microscopy (SRM) is developed to integrate whispering gallery mode (WGM) sensing with atomic force microscopy (AFM). The hybrid technique combines the exquisite refractive index sensing of whispering gallery mode resonators with the topography mapping capabilities of AFM. A 45 μm diameter barium titanate microsphere is attached to the end of a conventional AFM cantilever and acts as both a WGM resonator and stylus for mapping surface topography. Calibration plots, taken in contact-mode feedback, show that the WGM spectrum responds to changes in both solution and substrate refractive index. SRM imaging of a glass substrate reveals changes in surface refractive index that correspond to a small, 36 nm high feature measured simultaneously in the contact-mode topography image. Spectral measurements confirm that the contrast arises from refractive index changes and not coupling with sample topography, thus validating the approach. Additional measurements on thin polymer films and protein-coated surfaces are presented and discussed in terms of possible areas of application for SRM.

Forced unfolding of single-chain polymeric nanoparticles.

Atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS) is applied to single-chain polymeric nanoparticles (SCPNs) to acquire information about the internal folding structure of SCPNs and inherent kinetic parameters of supramolecular self-assembling motifs embedded into the SCPNs. The SCPNs used here are polyacrylate-based polymers carrying 2-ureido-4-[1H]-pyrimidinone (UPy) or benzene-1,3,5-tricarboxamide (BTA) pendants that induce an intramolecular chain collapse into nanoparticles consisting of one polymer chain only via internal supramolecular cross-linking. The SCPN is stretched by an AFM cantilever to unfold mechanically, which allows measuring of force-extension profiles of the SCPNs. Consecutive peaks observed in the force profiles are attributed to rupture events of self-assembled UPy/BTA units in the SCPNs. The force profiles have been analyzed statistically for a series of polymers with different UPy/BTA incorporation densities. The results provide insights into the internal conformation of SCPNs, where the folding structure can be changed with the incorporation density of UPy/BTA. In addition, dynamic loading rate analysis allows the determination of kinetic parameters of BTA self-assembly, which has not been accessible by any other method. This study offers a rational tool for understanding the folding structure, kinetics, and pathway of two series of SCPNs.

Nano-Workbench: A Combined Hollow AFM Cantilever and Robotic Manipulator

To manipulate liquid matter at the nanometer scale, we have developed a robotic assembly equipped with a hollow atomic force microscope (AFM) cantilever that can handle femtolitre volumes of liquid. The assembly consists of four independent robots, each sugar cube sized with four degrees of freedom. All robots are placed on a single platform around the sample forming a nano-workbench (NWB). Each robot can travel the entire platform and has a minimum position resolution of 5 nm both in-plane and out-of-plane. The cantilever chip was glued to the robotic arm. Dispensing was done by the capillarity between the substrate and the cantilever tip, and was monitored visually through a microscope. To evaluate the performance of the NWB, we have performed three experiments: clamping of graphene with epoxy, mixing of femtolitre volume droplets to synthesize gold nanoparticles and accurately dispense electrolyte liquid for a nanobattery.

A study of long term operation and reliability of heated atomic force microscope cantilevers

This paper presents a detailed investigation of the long term operation and reliability of heated atomic force microscope (AFM) cantilevers. The electrical and thermal operation of these heated AFM cantilevers depends upon the distribution of dopants within the silicon cantilever. Over long term operation, or for operation at very high temperatures, the cantilevers’ electro-thermal properties can change due to dopant diffusion at the junction between the cantilever heater and the cantilever leg. We analyze this change in cantilever behavior at high heating temperatures, up to 1100 °C, and long heating times, up to 108 h. At extreme heating conditions, the cantilevers’ electro-thermal properties change by as much as 24%, resulting in significant errors in cantilever calibration. The main failure mechanism of these heated AFM cantilevers is the melting of heater-leg junction at high cantilever temperatures, as high junction resistances induced the localized heating. Understanding and accounting for long term operational reliability is crucial for consistent and repeatable uses of heated cantilevers, especially for applications in material property measurements and nanomanufacturing.

The multi-position calibration of the stiffness for atomic-force microscope cantilevers based on vibration

Calibration of the stiffness of atomic force microscope (AFM) cantilevers is critical for industry and academic research. The multi-position calibration method for AFM cantilevers based on vibration is investigated. The position providing minimum uncertainty is deduced. The validity of the multi-position approach is shown via theoretical and experimental means. We applied it to the recently developed vibration method using an AFM cantilever with a normal stiffness of 0.1 N m−1. The standard deviation of the measured stiffness is 0.002 N m−1 with a mean value of 0.189 N m−1 and the relative combined uncertainty is approximately 7%, which is better than the approach using the single position at the tip of the cantilever.

Size-Dependent Resonant Frequency and Flexural Sensitivity of Atomic Force Microscope Microcantilevers Based on the Modified Strain Gradient Theory

In the present study, the resonant frequency and flexural sensitivity of atomic force microscope (AFM) microcantilevers are predicted incorporating size effects. To this end, the modified strain gradient elasticity theory is applied to the classical Euler-Bernoulli beam theory to develop a non-classical beam model which has the capability to capture size-dependent behavior of microcantilevers. On the basis of Hamilton's principle, the size-dependent analytical expressions corresponding to the frequency response and sensitivity of AFM cantilevers are derived. It is observed that by increasing the contact stiffness, the resonant frequencies of AFM cantilevers firstly increase and then tend to remain constant at an especial value. Moreover, the resonant frequencies of AFM cantilevers obtained via the developed non-classical model is higher than those of the classical beam theory, especially for the values of beam thickness close to the internal material length scale parameter.

Formation of High Aspect Ratio Nanostructures Using Focused Ion Beam Induced Deposition of Carbon

This paper presents results of the formation of high aspect nanostructures by local deposition of carbon, stimulated by focused ion beam (FIB). The structures used in the modification of the probe sensors were cantilevers for atomic force microscopy (AFM). The FIB structure of 5 mm length and 50 mm radius of curvature formed on the surface of the cantilever tip has shown to improve the accuracy of measurement by AFM. The outcome of this study is useful for the development of manufacturing processes and modification of the probe sensor-cantilever AFM structures of field-electron emitters as well as in the studies of micro- and nanosystems technology.

Note: A silicon-on-insulator microelectromechanical systems probe scanner for on-chip atomic force microscopy.

A new microelectromechanical systems-based 2-degree-of-freedom (DoF) scanner with an integrated cantilever for on-chip atomic force microscopy (AFM) is presented. The silicon cantilever features a layer of piezoelectric material to facilitate its use for tapping mode AFM and enable simultaneous deflection sensing. Electrostatic actuators and electrothermal sensors are used to accurately position the cantilever within the x-y plane. Experimental testing shows that the cantilever is able to be scanned over a 10 μm × 10 μm window and that the cantilever achieves a peak-to-peak deflection greater than 400 nm when excited at its resonance frequency of approximately 62 kHz.

Fabrication of a plasmonic nanocone on top of an AFM cantilever

A novel technique for the nanofabrication of gold nanocones on top of atomic force microscopy cantilever tips is demonstrated. The modified tips are intended as near-field scanning optical microscopy probes. The fabrication process is composed of cutting the probe apex with a focused gallium ion beam to create a platform for the nanocone fabrication. Three thin films are deposited on the cantilever, where gold as the active plasmonic material is sandwiched between a titanium adhesion layer and an Al2O3 thin film. At the top of the cantilever the deposited thin films are partially structured by a ring shaped focused ion beam pattern, leaving behind a cylindrical pillar. Finally, the gold nanocone is produced by argon ion etching with the top thin film on the pillar acting as a hard mask.

Topographical and mechanical characterization of living eukaryotic cells on opaque substrates: development of a general procedure and its application to the study of non-adherent lymphocytes

The mechanical behavior of living murine T-lymphocytes was assessed by atomic force microscopy (AFM). A robust experimental procedure was developed to overcome some features of lymphocytes, in particular their spherical shape and non-adherent character. The procedure included the immobilization of the lymphocytes on amine-functionalized substrates, the use of hydrodynamic effects on the deflection of the AFM cantilever to monitor the approaching, and the use of the jumping mode for obtaining the images. Indentation curves were analyzed according to Hertz’s model for contact mechanics. The calculated values of the elastic modulus are consistent both when considering the results obtained from a single lymphocyte and when comparing the curves recorded from cells of different specimens.

Revealing energy level structure of individual quantum dots by tunneling rate measured by single-electron sensitive electrostatic force spectroscopy.

We present theoretical and experimental studies of the effect of the density of states of a quantum dot (QD) on the rate of single-electron tunneling that can be directly measured by electrostatic force microscopy (e-EFM) experiments. In e-EFM, the motion of a biased atomic force microscope cantilever tip modulates the charge state of a QD in the Coulomb blockade regime. The charge dynamics of the dot, which is detected through its back-action on the capacitavely coupled cantilever, depends on the tunneling rate of the QD to a back-electrode. The density of states of the QD can therefore be measured through its effect on the energy dependence of tunneling rate. We present experimental data on individual 5 nm colloidal gold nanoparticles that exhibit a near continuous density of state at 77 K. In contrast, our analysis of already published data on self-assembled InAs QDs at 4 K clearly reveals discrete degenerate energy levels.

Fabrication of 10-nm-scale nanoconstrictions in graphene using atomic force microscopy-based local anodic oxidation lithography

We performed local anodic oxidation (LAO) lithography on monolayer graphene and highly oriented pyrolytic graphite (HOPG) using atomic force microscope (AFM). Auger electron spectroscopic measurements in the oxidized area formed on the HOPG revealed that the number of oxygen atoms systematically increased with the bias voltage applied to the AFM cantilever |Vtip|, which demonstrates the tunability of the extent of oxidation with |Vtip|. By optimizing the LAO conditions, we fabricated monolayer graphene nanoconstrictions with a channel width as small as 10 nm, which is the smallest graphene nanoconstriction so far achieved by utilizing AFM lithography techniques.

A microfluidic AFM cantilever based dispensing and aspiration platform

We present the development of a microfluidic AFM (atomic force microscope) cantilever-based platform to enable the local dispensing and aspiration of liquid with volumes in the pico-to-femtoliter range. The platform consists of a basic AFM measurement system, microfluidic AFM chip, fluidic interface, automated substrate alignment, external pressure control system and controlled climate near the dispensing area. The microfluidic AFM chip has a hollow silicon dioxide (SiO2) cantilever connected to an on-chip fluid reservoir at one end and a slicon nitride (Si3N4) tip with an aperture on the other end. A 3D printed plastic fluidic interface glued over the on-chip reservoir was used to connect microfluidics and macrofluidics. The fluidics is connected to an external pressure control system ranging from −0.8 bar to 5 bar with 0.1 bar resolution. This pressure range allows dispensing and aspiration of liquids through the cantilever tip aperture. The controlled climate with a temperature control range between 25°C – 40°C and humidity up to 95% near the dispensing area keeps the droplets for sufficiently long time before they evaporate. An array of droplets can be programmed to be dispensed automatically and access them again with a position accuracy of 1 micron. Experiments were performed with two types of cantilevers with different geometrical configurations. A minimum flow rate control of 50 fL/s was obtained and also frequency shift was monitored as the cantilever was filled with liquid. This platform will be used for various chemical and biological applications.

Accurate calibration and uncertainty estimation of the normal spring constant of various AFM cantilevers.

Measurement of force on a micro- or nano-Newton scale is important when exploring the mechanical properties of materials in the biophysics and nanomechanical fields. The atomic force microscope (AFM) is widely used in microforce measurement. The cantilever probe works as an AFM force sensor, and the spring constant of the cantilever is of great significance to the accuracy of the measurement results. This paper presents a normal spring constant calibration method with the combined use of an electromagnetic balance and a homemade AFM head. When the cantilever presses the balance, its deflection is detected through an optical lever integrated in the AFM head. Meanwhile, the corresponding bending force is recorded by the balance. Then the spring constant can be simply calculated using Hooke’s law. During the calibration, a feedback loop is applied to control the deflection of the cantilever. Errors that may affect the stability of the cantilever could be compensated rapidly. Five types of commercial cantilevers with different shapes, stiffness, and operating modes were chosen to evaluate the performance of our system. Based on the uncertainty analysis, the expanded relative standard uncertainties of the normal spring constant of most measured cantilevers are believed to be better than 2%.

Study of the sensitivity and resonant frequency of the torsional modes of an AFM cantilever with a sidewall probe based on a nonlocal elasticity theory

A relationship based on a nonlocal elasticity theory is developed to investigate the torsional sensitivity and resonant frequency of an atomic force microscope (AFM) with assembled cantilever probe (ACP). This ACP comprises a horizontal cantilever and a vertical extension, and a tip located at the free end of the extension, which makes the AFM capable of topography at sidewalls of microstructures. First, the governing differential equations of motion and boundary conditions for dynamic analysis are obtained by a combination of the basic equations of nonlocal elasticity theory and Hamilton's principle. Afterward, a closed-form expression for the sensitivity of vibration modes has been obtained using the relationship between the resonant frequency and contact stiffness of cantilever and sample. These analysis accounts for a better representation of the torsional behavior of an AFM with sidewall probe where the small-scale effect are significant. The results of the proposed model are compared with those of classical beam theory. The results show that the sensitivities and resonant frequencies of ACP predicted by the nonlocal elasticity theory are smaller than those obtained by the classical beam theory.

Nanosensors for cancer detection.

Cancer is a major burden in today's society and one of the leading causes of death in industrialised countries. Various avenues for the detection of cancer exist, most of which rely on standard methods, such as histology, ELISA, and PCR. Here we put the focus on nanomechanical biosensors derived from atomic force microscopy cantilevers. The versatility of this novel technology has been demonstrated in different applications and in some ways surpasses current technologies, such as microarray, quartz crystal microbalance and surface plasmon resonance. The technology enables label free biomarker detection without the necessity of target amplification in a total cellular background, such as BRAF mutation analysis in malignant melanoma. A unique application of the cantilever array format is the analysis of conformational dynamics of membrane proteins associated to surface stress changes. Another development is characterisation of exhaled breath which allows assessment of a patient's condition in a non-invasive manner.