Force Gradient Detection Research Articles

A Non-Perturbative, Low-Noise Surface Coating for Sensitive Force-Gradient Detection of Electron Spin Resonance in Thin Films.

The sensitivity of magnetic resonance force microscopy (MRFM) is limited by surface noise. Coating a thin-film polymer sample with metal has been shown to decrease, by orders of magnitude, sample-related force noise and frequency noise in MRFM experiments. Using both MRFM and inductively detected measurements of electron-spin resonance, we show that thermally evaporating a 12 nm gold layer on a 40 nm nitroxide-doped polystyrene film inactivates the nitroxide spin labels to a depth of 20 nm, making single-spin measurements difficult or impossible. We introduce a "laminated sample" protocol in which the gold layer is first evaporated on a sacrificial polymer. The sample is deposited on the room-temperature gold layer, removed using solvent lift-off, and placed manually on a coplanar waveguide. Electron spin resonance (ESR) of such a laminated sample was detected via MRFM at cryogenic temperatures using a high-compliance cantilever with an integrated 100-nm-scale cobalt tip. A 20-fold increase of spin signal was observed relative to a thin-film sample prepared instead with an evaporated metal coating. The observed signal is still somewhat smaller than expected, and we discuss possible remaining sources of signal loss.

Optomechanical atomic force microscope

In the scanning probe microscope system, the weak signal detection of cantilever vibration is one of the important factors affecting the sensor sensitivity. In our current work, we present a novel design concept for an atomic force microscope (AFM) combined with optomechanics with an ultra-high quality factor and a low thermal noise. The detection system consists of a fixed mirror placed on the cantilever of the AFM and pump-probe beams that is equivalent to a Fabry–Perot cavity. We realize that the AFM combined with an optical cavity can achieve ultra-sensitive detection of force gradients of 10−12 N m−1 in the case of high-vacuum and low effective temperature of 1 mK, which may open up new avenues for super-high resolution imaging and super-high precision force spectroscopy.

High-speed digitization of the amplitude and frequency in open-loop sideband frequency-modulation Kelvin probe force microscopy

A more inclusive and detailed measurement of various physical interactions is enabled by the advance of high-speed data digitization. For surface potential characterization, this was demonstrated recently in terms of open-loop amplitude modulation Kelvin probe force microscopy (OL AM-KPFM). Its counterpart, namely open-loop frequency modulation Kelvin probe force microscopy (OL FM-KPFM), is examined here across different materials and under various bias voltages in the form of OL sideband FM-KPFM. In this implementation the changes in the amplitude and resonance frequency of the cantilever were continuously tracked as a conductive AFM probe was modulated by a 2 kHz AC bias voltage around the first eigenmode frequency of the cantilever. The contact potential difference (CPD) between the AFM probe and sample was determined from the time series analysis of the high-speed 4 MHz digitized amplitude and frequency signals of the OL sideband FM-KPFM mode. This interpretation is demonstrated to be superior to the analysis of the parabolic bias dependent response, which is more commonly used to extract the CPD in OL KPFM modes. The measured OL sideband FM-KPFM amplitude and frequency responses are directly related to the electrostatic force and force-gradient between the AFM probe and sample, respectively. As a result, clear distinction was observed for the determined CPD in each of these cases across materials of different surface potentials, with far superior spatial resolution when the force-gradient detection was used. In addition, the CPD values obtained from OL sideband FM-KPFM amplitude and frequency measurements perfectly matched those determined from their closed-loop AM-KPFM and FM-KPFM counterparts, respectively.

Characterization of Dielectric Nanocomposites with Electrostatic Force Microscopy

Nanocomposites physical properties unexplainable by general mixture laws are usually supposed to be related to interphases, highly present at the nanoscale. The intrinsic dielectric constant of the interphase and its volume need to be considered in the prediction of the effective permittivity of nanodielectrics, for example. The electrostatic force microscope (EFM) constitutes a promising technique to probe interphases locally. This work reports theoretical finite-elements simulations and experimental measurements to interpret EFM signals in front of nanocomposites with the aim of detecting and characterizing interphases. According to simulations, we designed and synthesized appropriate samples to verify experimentally the ability of EFM to characterize a nanoshell covering nanoparticles, for different shell thicknesses. This type of samples constitutes a simplified electrostatic model of a nanodielectric. Experiments were conducted using either DC or AC-EFM polarization, with force gradient detection method. A comparison between our numerical model and experimental results was performed in order to validate our predictions for general EFM-interphase interactions.

Nanoscale surface charge detection in epoxy resin materials using electrostatic force spectroscopy

Electrostatic force spectroscopy (EFS) operated in a conventional force gradient detection method allows determining local surface charges in epoxy samples. This is made possible through a detailed analysis of gradient versus DC voltage curves. The parabolic dependence of these curves is closely related to the charge density. Both maximum and origin-ordinate are key data from which it is possible to extract quantitative information on the detected charge. The study is based on the combined use of numerical and analytical simulations of the probe sample interaction. Excellent sensitivities to very low surface charge densities are reported.

Quantitative 3D-KPFM imaging with simultaneous electrostatic force and force gradient detection

Kelvin probe force microscopy (KPFM) is a powerful characterization technique for imaging local electrochemical and electrostatic potential distributions and has been applied across a broad range of materials and devices. Proper interpretation of the local KPFM data can be complicated, however, by convolution of the true surface potential under the tip with additional contributions due to long range capacitive coupling between the probe (e.g. cantilever, cone, tip apex) and the sample under test. In this work, band excitation (BE)-KPFM is used to negate such effects. In contrast to traditional single frequency KPFM, multifrequency BE-KPFM is shown to afford dual sensitivity to both the electrostatic force and the force gradient detection, analogous to simultaneous amplitude modulated and frequency modulated KPFM imaging. BE-KPFM is demonstrated on a Pt/Au/SiOx test structure and electrostatic force gradient detection is found to lead to an improved lateral resolution compared to electrostatic force detection. Finally, a 3D-KPFM imaging technique is developed. Force volume (FV) BE-KPFM allows the tip–sample distance dependence of the electrostatic interactions (force and force gradient) to be recorded at each point across the sample surface. As such, FVBE-KPFM provides a much needed pathway towards complete tip–sample capacitive de-convolution in KPFM measurements and will enable quantitative surface potential measurements with nanoscale resolution.

Long-lived frequency shifts observed in a magnetic resonance force microscope experiment following microwave irradiation of a nitroxide spin probe

We introduce a spin-modulation protocol for force-gradient detection of magnetic resonance that enables the real-time readout of longitudinal magnetization in an electron spin resonance experiment involving fast-relaxing spins. We applied this method to observe a prompt change in longitudinal magnetization following the microwave irradiation of a nitroxide-doped perdeuterated polystyrene film having an electron spin-lattice relaxation time of [Formula: see text]. The protocol allowed us to discover a large, long-lived cantilever frequency shift. Based on its magnitude, lifetime, and field dependence, we tentatively attribute this persistent signal to deuteron spin magnetization created via transfer of polarization from nitroxide spins.

Single-shot nuclear magnetization recovery curves with force-gradient detection.

We measure the spin-lattice relaxation time as a function of sample temperature in GaAs in a real-time single-shot inversion recovery experiment using spin force gradients acting on a magnetic tipped cantilever. After inverting 69Ga spins localized near the magnet with a single 20 ms adiabatic rapid passage sweep, the spins' magnetization recovery was passively tracked by recording the cantilever's frequency change, which is proportional to the longitudinal component of the spins' magnetization. The cantilever's frequency was recorded for a time 3*T1 for sample temperatures ranging from 4.8 to 25 K. The temperature dependence was observed for the 69Ga quadrupolar relaxation interaction.

Reliability Based Design Optimization for Selective Excitation of the Vibration Modes of a Cantilever Spring

This paper is devoted to procedures for the reliability-based optimization methods of engineering structures combining measurement and sensitivity technique, for the purpose of the better sensitivity in force-gradient detection. In the experiment part of this study, the mica muscovite cantilever beam clamped-free is used. The excitation of a cantilever beam with several small sheets of piezoelectric polymer adequately glued to it selects one high-frequency vibration mode of the cantilever. The proposed strategy is design into a framework that allows the solution of optimization problems involving a several number of design parameters that characterizes the systems, including dimensional tolerance, material properties, boundary conditions, loads, and model predictions, considered to be uncertainties or variables. The proposed methodology directly supports quality engineering aspects enabling to specify the manufacturing tolerances normally required to achieve desired product reliability. Within this context, the robust design obtained is optimal over the range of variable conditions because it considers uncertainties during the optimization process. The large number of exact evaluations of problem, combined with the typically high dimensions of FE models of industrial structures, makes reliability-based optimization procedures very costly, sometimes unfeasible. Those difficulties motivate the study reported in this paper, in which a strategy is proposed consisting in the use of reliability-based optimization strategy combined with measurement and sensitivity technique specially adapted to the structures of industrial interested.

Multifrequency Atomic Force Microscopy: Compositional Imaging with Electrostatic Force Measurements

We demonstrate that single-pass Kelvin force microscopy (KFM) and dC/dz measurements in different environments expand the compositional imaging with atomic force microscopy. The KFM and dC/dz studies were performed in the intermittent contact mode with force gradient detection of tip-sample electrostatic interactions. Both factors contribute to sensitive measurements of the surface potential and capacitance gradient with nanometer-scale spatial resolution as it was verified on a broad range of materials: metal alloys, polymers, organic layers, and liquid-like objects. For many samples the surface potential and dC/dz variations complement each other in identification of individual components of heterogeneous materials. In situ imaging in different humidity or vapors of various organic solvents further facilitate recognition of the constituents of multicomponent polymer samples due to selective swelling of components.

Single-pass Kelvin force microscopy and dC/dZ measurements in the intermittent contact: applications to polymer materials

We demonstrate that single-pass Kelvin force microscopy (KFM) and capacitance gradient (dC/dZ) measurements with force gradient detection of tip–sample electrostatic interactions can be performed in the intermittent contact regime in different environments. Such combination provides sensitive detection of the surface potential and capacitance gradient with nanometer-scale spatial resolution as it was verified on self-assemblies of fluoroalkanes and a metal alloy. The KFM and dC/dZ applications to several heterogeneous polymer materials demonstrate the compositional mapping of these samples in dry and humid air as well as in organic vapors. In situ imaging in different environments facilitates recognition of the constituents of multi-component polymer systems due to selective swelling of components.

Scanned-probe detection of electron spin resonance from a nitroxide spin probe

We report an approach that extends the applicability of ultrasensitive force-gradient detection of magnetic resonance to samples with spin-lattice relaxation times (T (1)) as short as a single cantilever period. To demonstrate the generality of the approach, which relies on detecting either cantilever frequency or phase, we used it to detect electron spin resonance from a T (1) = 1 ms nitroxide spin probe in a thin film at 4.2 K and 0.6 T. By using a custom-fabricated cantilever with a 4 microm-diameter nickel tip, we achieve a magnetic resonance sensitivity of 400 Bohr magnetons in a 1 Hz bandwidth. A theory is presented that quantitatively predicts both the lineshape and the magnitude of the observed cantilever frequency shift as a function of field and cantilever-sample separation. Good agreement was found between nitroxide T (1) 's measured mechanically and inductively, indicating that the cantilever magnet is not an appreciable source of spin-lattice relaxation here. We suggest that the new approach has a number of advantages that make it well suited to push magnetic resonance detection and imaging of nitroxide spin labels in an individual macromolecule to single-spin sensitivity.

Determination of the nanoscale dielectric constant by means of a double pass method using electrostatic force microscopy

We present a method to determine the local dielectric permittivity of thin insulating layers. The measurement is based on the detection of force gradients in electric force microscopy by means of a double pass method. The proposed experimental protocol is simple to implement and does not need any modification of standard commercial devices. Numerical simulations based on the equivalent charge method make it possible to carry out quantification whatever the thickness of film, the radius of the tip, and the tip-sample distance. This method has been validated on a thin SiO2 sample for which the dielectric permittivity at the nanoscale has been characterized in the literature. We also show how we can quantitatively measure the local dielectric permittivity for ultrathin polymer film of poly(vinyl acetate) and polystyrene.

Low Amplitude High Frequency Atomic Force Microscopy

Heterodyne laser Doppler interferometry was used to implement a high frequency atomic force microscope (AFM) that can accommodate a cantilever with a natural frequency of up to 200 MHz. Since Doppler interferometry measures velocity, the signal level increases proportionally with frequency for a given amplitude of oscillation, or the noise floor in terms of displacement decreases with 1/f. A noise level of 0.5 fm/√Hz was attained around 2 MHz, giving a large signal to noise margin for high frequency cantilevers and higher vibration mode detection. We have so far confirmed the following: (i) atomic resolution of Si(111) 7×7 with an amplitude of drive as low as 0.03 nm at 1.6 MHz, (ii) true atomic resolution dynamic lateral force microscopy, (iii) lateral force gradient detection at the atomic level, (iv) manipulation of Si atoms at room temperature, (v) vibration measurement of Si nanowires, tungsten oxide whiskers and graphene cantilevers above 100 MHz, (vi) atomic resolution imaging with the 2nd and 3rd mode of deflection and (v) true atomic resolution imaging in water using deflection or torsion with a few 10 pm amplitude of drive.

Nuclear magnetic resonance imaging with 90-nm resolution

Magnetic resonance imaging (MRI) is a powerful imaging technique that typically operates on the scale of millimetres to micrometres. Conventional MRI is based on the manipulation of nuclear spins with radio-frequency fields, and the subsequent detection of spins with induction-based techniques. An alternative approach, magnetic resonance force microscopy (MRFM), uses force detection to overcome the sensitivity limitations of conventional MRI. Here, we show that the two-dimensional imaging of nuclear spins can be extended to a spatial resolution better than 100 nm using MRFM. The imaging of 19F nuclei in a patterned CaF(2) test object was enabled by a detection sensitivity of roughly 1,200 nuclear spins at a temperature of 600 mK. To achieve this sensitivity, we developed high-moment magnetic tips that produced field gradients up to 1.4 x 10(6) T m(-1), and implemented a measurement protocol based on force-gradient detection of naturally occurring spin fluctuations. The resulting detection volume was less than 650 zeptolitres. This is 60,000 times smaller than the previous smallest volume for nuclear magnetic resonance microscopy, and demonstrates the feasibility of pushing MRI into the nanoscale regime.

Force gradient detection under vacuum on the basis of a double pass method

The feasibility of detecting electrostatic gradients in the linear regime is shown under vacuum by combining intermittent contact atomic force microscopy and a double pass method. To achieve our goal, different flexure mode orders were employed. We show that the sensitivity of the frequency or phase shifts to a given gradient was reduced when the order was increased. This behavior is theoretically explained in quantitative agreement with the experiments. Thus, on the basis of different flexure mode orders, gradient detection can now be extended to other forces plus various environments, i.e., under vacuum or controlled atmosphere.

Inverse magnetic force microscopy of superconducting thin films

The recovery of the London penetration depth $\ensuremath{\lambda}$ from magnetic force microscopy (MFM) data is described in the case of finite-thickness superconductors. The thickness of the superconductor b can either be treated as available data or as an additional unknown. Specifically, we show that the problem of recovering the pair $(\ensuremath{\lambda},b)$ from experimental data is well posed and we give proof of the uniqueness. No assumption is made on the symmetry of the stray field and problems with spatially extended tips of arbitrary magnetization patterns can be treated. With the inclusion of a complex penetration depth the theory is extended to force gradient detection modes, in which the MFM tip is oscillated at a drive frequency ${\ensuremath{\omega}}_{\mathrm{d}}.$ For such cases, the customary methods of analysis have been revised, with the inclusion of energy transfer between the sample and the tip. We show that both the penetration depth $\ensuremath{\lambda}$ and the normal fluid conductivity ${\ensuremath{\sigma}}_{\mathrm{nf}}$ can be recovered.

Selective excitation of the vibration modes of a cantilever spring

We describe a method to increase the sensitivity of a resonant force sensor. The excitation of a cantilever beam with several small sheets of piezoelectric polymer adequately glued to it selects one high-frequency vibration mode of the cantilever. A higher resonance frequency leads to better sensitivity in force-gradient detection as examplified by the measurement of the force gradient between two magnets.

Simultaneous optical detection techniques, interferometry, and optical beam deflection for dynamic mode control of scanning force microscopy

In dynamic mode control of scanning force microscopy (SFM), optical beam deflection and interferometry are the techniques most used for detection of force gradients by means of a tip and a microcantilever that usually vibrates at the first resonant mode. In order to increase the sensitivity of these kinds of microscopes, one possible means is to investigate the potential of the highest resonance modes which allow an increase in the operating frequencies. For these two detection techniques, according to the local displacement slope, care must be taken in the choice of an appropriate microcantilever point where the laser beam is focused. In this article, an original technique based on simultaneous detection, interferometry, and the beam deflection method is introduced. These techniques are able to characterize within two degrees of freedom, normal displacement and angular deflection, thus resonating the SFM cantilever.

A high performance magnetic force microscope

We have constructed a high performance magnetic force microscope optimized for operation in the force gradient detection mode. The instrument incorporates a novel differential heterodyne interferometer that has a high immunity to microphonics and interference. The detection limit of the interferometer is limited by the intrinsic thermal noise of the cantilever sensor. Positioning and scanning of the sample is undertaken by a monolithic flexure stage driven by piezoelectric actuators. The stage incorporates capacitance sensors which are used to linearize the motion of the stage via a closed loop control system. The linearity achieved is better than 1% (full scale) with a positioning stability of 1 nm (rms). Recorded bit patterns in longitudinal media and domains in an AC-demagnetized Co/Pt multilayer have been studied.