Eigenvalue Veering in Quartz Tuning Fork Sensors and its Effect on Dynamic Atomic Force Microscopy

Polythiophene molecules adsorbed on a highly oriented pyrolytic graphite surface were studied by combined dynamic scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM) with a quartz tuning fork sensor operating in Qplus mode and equipped with a Pt/Ir tip. Upon completing a careful sub-angström oscillation amplitude calibration of the probe, experiments were conducted in an ultra high vacuum at room temperature. By selecting the tip/surface distance regulation parameter, one can select the type of simultaneous information obtained in an area. For distance regulation based on the mean tunneling current, dynamic STM images together with maps of tip/surface force gradient were obtained. FM-AFM images with maps of the tunneling current were also acquired when the distance regulation was based on the frequency shift. Comparison between these images reveals interesting features. For example the tip which operates in STM mode with ultra low current (<10 pA) generates different interaction forces above molecules or graphite. Changes in energy dissipation processes as small as tens of millielectronvolts per cycle were recorded when the tip oscillates above the polymer or on the graphite surface. Hence data demonstrates that a stiff piezoelectric tuning fork of several kilonewtons/meters working as an AFM/STM probe with sub-angström amplitude can characterize weakly adsorbed molecules.

Summary There are currently no technologies available to measure polymer solution viscosities at realistic downhole conditions in a well during enhanced oil recovery (EOR). In this paper, custom-made probes using quartz tuning fork (QTF) resonators are demonstrated for measurements of viscosity of polymer fluids in the laboratory. The electromechanical response of the resonators was calibrated in simple Newtonian fluids and in non-Newtonian polymer fluids at different concentrations. The responses were then used to measure field-collected samples of polymer injection fluids. In the polymer fluids, the measured viscosity values by tuning forks were lower than those measured by the conventional rheometer at 6.8 s−1, closer to the solvent viscosity values. However, the predicted rheometer viscosity vs. QTF-measured viscosity showed a distinct exponential correlation (R2=0.9997), allowing for an empirical calibration between the two viscometers for fluids having the same solvent and polymer compositions. The QTF sensors produced acceptable viscosity measurements of polymer fluids within the required polymer concentration ranges used in the field and predicted field sample viscosities with less than 1–2 cp (or 10–20%) error from the rheometer data. Results were validated based on separate independent tests where the devices were used to measure the viscosity of Newtonian fluids and non-Newtonian polymer fluids in a series of consecutive dip tests, simulating more realistic usage. These devices can be used to measure either the “relative” viscosity changes from a polymer solution prior and post-injection or to measure a “calibrated” viscosity via empirical exponential correlation. The compact QTF sensors developed in this study can be easily integrated into portable systems for laboratory or wellsite deployment as well as logging tools for downhole deployment. This work also demonstrates the ability of these QTF devices to make sensitive viscosity measurements at high-frequencies, opening opportunities for their use in high-frequency rheology studies of EOR fluids.

In this work, we introduce position-resolved surface characterization and nanofabrication using an optical microscope (OM) combined with a nanopipette-based quartz tuning fork atomic force microscope (nanopipette/QTF-AFM) system. This system is used to accurately determine substrate position and nanoscale phenomena under ambient conditions. Solutions consisting of 5 nm Au nanoparticles, nanowires, and polydimethylsiloxane (PDMS) are deposited onto the substrate through the nano/microaperture of a pulled pipette. Nano/microscale patterning is performed using a nanopipette/QTF-AFM, while position is resolved by monitoring the substrate with a custom OM. With this tool, one can perform surface characterization (force spectroscopy/microscopy) using the quartz tuning fork (QTF) sensor. Nanofabrication is achieved by accurately positioning target materials on the surface, and on-demand delivery and patterning of various solutions for molecular architecture.

We discuss the influence of external forces on the motion of the tip in dynamic atomic force microscopy (AFM). First, a compact solution for the steady-state problem is derived employing a Fourier approach. Founding on this solution, we present an analytical framework to describe the transient behavior of the tip after perturbations of tip–sample forces and the excitation signal. The static and transient solutions are then combined to obtain the baseband response of the tip, i.e., the deflection signal demodulated with respect to the excitation. The baseband response generalizes the amplitude and phase response of the tip, and we use it to find explicit formulas describing the amplitude and phase modulation following the influence of external forces on the tip. Finally, we apply our results to obtain an accurate dynamic model of the amplitude controller and phase-locked loop driving the cantilever in a frequency modulated AFM setup. A special emphasis is put on discussing the tip response in environments of high damping, such as ambient or liquid.

We present the results of simultaneous scanning-tunneling and frequency-modulated dynamic atomic force microscopy measurements with a qPlus setup. The qPlus sensor is a purely electrical sensor based on a quartz tuning fork. If both the tunneling current and the force signal are to be measured at the tip, a cross-talk of the tunneling current with the force signal can easily occur. The origin and general features of the capacitive cross-talk will be discussed in detail in this contribution. Furthermore, we describe an experimental setup that improves the level of decoupling between the tunneling-current and the deflection signal. The efficiency of this experimental setup is demonstrated through topography and site-specific force/tunneling-spectroscopy measurements on the Si(111) 7×7 surface. The results show an excellent agreement with previously reported data measured by optical interferometric deflection.

In order to improve the performances of dynamic atomic force microscopy (AFM), a higher-order resonant cantilever which is driven at its second order or even higher-order resonant frequency instead of its first order resonant frequency is proposed. Due to the increase in the frequency and quality factor of the higher-order resonant cantilever, the response time, air damping coefficient are reduced and the detecting sensitivity of the cantilever is improved. Meanwhile, because the angular deflection of the cantilever under higher-order resonant vibration is larger than that under the first order resonant vibration, the optical magnification level of the higher-order resonant cantilever is several times larger than that of the first order when the optical lever method is adopted to detect the variation of the cantilever. Theoretical analysis and experimental results show that the scanning method done by a higher-order cantilever is effective and feasible, and the scanning characteristics of dynamic AFM with the higher-order resonant cantilever are promoted obviously compared with that of the AFM operated in the first-order.

In this paper, we investigate the potential of using quartz tuning fork (QTF) sensing system for detection of specific metal ions in aqueous solutions with sample volume of $1~\mu \text{L}$ . Functionalized self-assembled monolayers (SAMs) of L-glutathione (LG) on gold-coated QTFs were used to sense specific analytes of CaCl2 and HgCl2, whereas functionalized L-cysteine based SAMs used to sense PbCl2. The resonance frequency undergoes a shift in response to the interaction between functionalized QTF and target analyte. X-ray photoelectron spectroscopy was used to study the interaction of thiol groups between the functionalized SAM and gold coated on QTF surface. Our analysis revealed that active QTFs functionalized with LG are more sensitive to detection of CaCl2 compared with HgCl2. The SAM-based QTF sensors were found to be capable of detecting low concentrations ( $10^{- {12}}$ M) of divalent analyte solutes with high sensitivity.

Advances in quantification of nanoscale forces are mainly focused on repulsive forces, the sample’s stiffness, and related parameters such as deformation. The reactivity and chemistry of the surface however depends on longer range forces that are prevalent in ambient conditions and control functional surfaces. Suitable models and appropriate parametrization of this range, however, are still emerging. Here, we define and employ metrics to parametrize long-range forces and experimental observables in dynamic atomic force microscopy and force measurements in a general way even when suitable physical models are lacking. We find that plateaus in the attractive part of the force that can be of more than 1 nm in range are a general characteristic of surfaces exposed to the ambient environment. These plateaus can be identified directly from force measurements or indirectly from computationally low-cost parameters that we define here. We further demonstrate that the defined metrics can be employed for rapid discri...

In this article, we explore methods that enable estimation of material properties with the dynamic mode atomic force microscopy suitable for soft matter investigation. The article presents the viewpoint of casting the system, comprising of a flexure probe interacting with the sample, as an equivalent cantilever system and compares a steady-state analysis based method with a recursive estimation technique for determining the parameters of the equivalent cantilever system in real time. The steady-state analysis of the equivalent cantilever model, which has been implicitly assumed in studies on material property determination, is validated analytically and experimentally. We show that the steady-state based technique yields results that quantitatively agree with the recursive method in the domain of its validity. The steady-state technique is considerably simpler to implement, however, slower compared to the recursive technique. The parameters of the equivalent system are utilized to interpret storage and dissipative properties of the sample. Finally, the article identifies key pitfalls that need to be avoided toward the quantitative estimation of material properties.

Rapid standoff spectroscopic characterization of plastic waste using quartz tuning fork

In this paper, an ultrahigh sensitive light‐induced thermoelastic spectroscopy (LITES) sensor based on a trilayer ultrathin perfect absorber coated T‐head quartz tuning fork (QTF) is reported for the first time. The absorber is designed according to the streamlined theoretical framework based on admittance matching principles. The incorporation of Cr as the primary absorber and the dual‐layer anti‐reflection coating of SiO2 and Si in the tri‐layer (Si‐SiO2‐Cr) ultra‐thin perfect absorber enables high absorption across a wide spectral range. It results in a 93% absorbance of the 1530 nm laser by the QTF. This feature enhances the QTF's photothermal‐elastic‐electric conversion efficiency and detection sensitivity. The self‐designed T‐head QTF with a low resonant frequency of 8.78 kHz is beneficial to increase the energy accumulation time. The signal‐to‐noise ratio of the T‐head QTF increases by 3.35‐fold after coating the tri‐layer absorber, and it is 6.90 times greater than that of the standard QTF. To achieve ultra‐high sensitive acetylene (C2H2) detection, a 40 m fiber‐coupled multi‐pass cell and an erbium‐doped fiber amplifier are integrated into the sensing system to augment the target gas's absorption of laser and boost laser power. Finally, a minimum detecting limit (MDL) of 209.59 ppt is obtained, indicating the ultrahigh detection sensitivity and the best MDL among all the reported C2H2‐LITES sensors. This work paves an effective new way to enhance the detection sensitivity of the LITES sensor, and can provide broader absorption enhancement of the QTF to detect additional gases with optimized coating architecture.

Dynamic force microscopy (DFM) with the self-oscillator (SO)method allows reasonably high scanning rates even with highQ-factors of the resonant force sensor, typical of cantilevers in ultra-high vacuum and ofquartz tuning forks. However, due to simpler interpretation of force spectroscopymeasurements, small oscillation amplitudes (sub-nm level) are generally preferred. Inapplications like ‘apertureless’ scanning near-field optical microscopy (SNOM), oscillationamplitudes of the order of 5–10 nm are needed to increase optical sensitivity and toapply standard optical artefact suppression methods. This motivates the studyof the behaviour of tuning forks driven at such high amplitudes, as comparedto usual air-operated cantilevers. Both constant-excitation-amplitude (CE) andconstant-oscillation-amplitude (CA) modes of SO-DFM are analysed, since the CA mode ismore convenient for SNOM applications, denoting remarkable differences. In particular,possible instability effects, previously found in CE mode, are not anticipated for CAmode. It is shown how resonance and approach (‘isophase’) curves in both modescan be conveniently described in terms of the usual ‘normalized frequency shift’γ and of a‘normalized gain’ η, defined as a measurement of surface dissipation.

In this paper the performances of two spectrophones for quartz-enhanced photoacoustic spectroscopy (QEPAS)-based ethane gas sensing were tested and compared. Each spectrophone contains a quartz tuning fork (QTF) acoustically coupled with a pair of micro-resonator tubes and having a fundamental mode resonance frequency of 32.7 kHz (standard QTF) and 12.4 kHz (custom QTF), respectively. The spectrophones were implemented into a QEPAS acoustic detection module (ADM) together with a preamplifier having a gain bandwidth optimized for the respective QTF resonance frequency. Each ADM was tested for ethane QEPAS sensing, employing a custom pigtailed laser diode emitting at ~1684 nm as the exciting light source. By flowing 1% ethane at atmospheric pressure, a signal-to-noise ratio of 453.2 was measured by implementing the 12.4 kHz QTF-based ADM, ~3.3 times greater than the value obtained using a standard QTF. The minimum ethane concentration detectable using a 100 ms lock-in integration time achieving the 12.4 kHz custom QTF was 22 ppm.

Quartz crystal is an elastomer with a natural oscillation frequency which can be served as a temperature sensor based on the resonance and thermal sensitivity subject to temperature when the forced vibration frequency is equal to its natural frequency. As a new type of temperature sensor, quartz crystal temperature sensor possesses high resolution, exact linearity and remarkable stability which could be used to measure temperature applied to moderate and low temperature environments. It is widely adopted in temperature measuring instruments under the circumstance of high-precision environments due to its availability of converting temperature signals to digital frequency signals. The oscillation frequency of the quartz fork resonator is extremely sensitive to the mass and stiffness of the quartz fork arm. Because of the electrode trench which can reflect the ambient temperature changes, it would be of special interest to design a temperature-sensitive quartz fork resonator for the study. The overall goal of this research is to study the surface electrode and quartz tuning fork temperature sensor in light of proposing an approach of temperature sensor design methodology, completing the sensor characteristic simulation, and designing the corresponding size of the sensor structure. Specific structure size: fork thickness 0.13mm, fork arm length 3mm, fork arm width 0.27mm, total length 5mm, total width 0.8mm.

High-speed dynamic atomic force microscopy by using a Q-controlled cantilever eigenmode as an actuator.