Quartz Tuning Fork Force Sensors Research Articles

Multiscale rheology from bulk to nano using a quartz tuning fork-atomic force microscope.

Rheological characteristics exhibit significant variations at nanoscale confinement or near interfaces, compared to bulk rheological properties. To bridge the gap between nano- and bulk-scale rheology, allowing for a better and holistic understanding of rheology, developing a single experimental platform that provides rheological measurements across different scales, from nano to bulk, is desirable. Here, we present the novel methodology for multiscale rheology using a highly sensitive atomic force microscope based on a quartz tuning fork (QTF) force sensor. We employ microscale and nanoscale shear probes attached to the QTF, oscillating parallel to a substrate surface for rheological measurements as a function of the tip-substrate distance with sub-nanometer resolution. Silicone oils with viscosities ranging from 5 cSt to 10 000 cSt are used as calibration samples, and we have successfully derived the bulk rheological moduli. Furthermore, an increase in modulus is observed within the regime of tribo-nanorheology at distances less than 50nm from the surface. Through such multiscale measurements, it is confirmed that this increase is due to the formation of a layered structure of silicone oil polymers on the solid surface. These results provide a comprehensive understanding of the tribo-rheological properties of complex fluids across different scales.

Enhancement of Q factor in quartz tuning fork force sensors for atomic force microscopy through counter piezo excitation

Enhancement of Q factor in quartz tuning fork force sensors for atomic force microscopy through counter piezo excitation

Fabrication of pyramid-shaped gold tip for adiabatic nanofocusing of surface plasmon polaritons

Scattering-type scanning near-field optical microscopy based on adiabatic nanofocusing of surface plasmon polaritons (SPPs) is a powerful technique that can achieve free-background nanoscale optical resolution of material. However, the performance of the propagated confined modes strongly depends on the characteristic structure of the probe. Although the metallic pyramid structure provides excellent tightly confined modes, however, it is challenging to realize the required pyramid geometry. Here, we propose a simple method for fabricating a reproducible and controllable gold pyramid-shaped tip. The produced pyramid-shaped tips were made by electrochemical etching and by applying a pulse wave to the system. From a systematic study, we found that the key factor of fabrication of desired tip geometry is based on the platinum (Pt) wire shapes. Traditional circular-shaped platinum ring electrodes are used for gold tip fabrication in an electrochemical etching. In our method, we bent the Pt wire into a triangular shape as the electrode for the etching process. The influence of the geometrical ring shapes on the fabrication of the Au tip structure is investigated. The gold tip structure was optimized by controlling the Pt ring shape, and the desired pyramid-shaped gold tip was achieved with a yield of 70%. The obtained etched pyramid-shaped tips were then mounted along the side of one of the arms of a quartz tuning fork force sensor to test their performance for shear-force topographical image and for guiding SPPs along the pyramid wedge based on adiabatic nanofocusing microscopy. The result shows topographical images of indium tin oxide with a spatial resolution smaller than 20 nm. Furthermore, we experimentally demonstrate the generation of the SPPs that propagated adiabatically along the wedge of an appropriate fabricated pyramid-shaped tip toward a nanometer-size spot at the tip apex. The demonstration of this method strongly suggests that the obtained pyramid-shaped tip will enable new experiments probing the dynamics of optical excitations of individual metallic, semiconducting, and magnetic nanostructures.

Electrochemical etching of lightweight nanotips for high quality‐factor quartz tuning fork force sensor: atomic force microscopy applications

Commercially available quartz tuning forks (QTFs) can be transformed into self-sensing and actuating force sensors by micro-assembling a sharp tip on the apex of a tine. Mass of the tip is critical in determining the quality (Q)-factor of the sensor, therefore, fabrication of the lightweight nanotips is a precondition for high Q-factor QTF sensors. The work reports fabrication of very lightweight tungsten nanotips with a two-step electrochemical etching technique which can be used to develop high Q-factor QTF force sensor. First, a tungsten wire with protective coating at one end (1–2 mm) is etched with a trapezoidal waveform to form a lengthy (∼2–5 mm) and slender (diameter ∼10–40 μm) micro-needle. In the second step, sharp tip apex is fabricated with a direct current etching. High Q-factor (6600–8000) QTF force sensors have been developed with the fabricated nanotips. Atomic force microscope scanning of nano-grating and a triblock copolymer surface validates the scanning performance of the developed sensors.

Atomic Force Microscopy Sidewall Imaging with a Quartz Tuning Fork Force Sensor.

Sidewall roughness measurement is becoming increasingly important in the micro-electromechanical systems and nanoelectronics devices. Atomic force microscopy (AFM) is an emerging technique for sidewall scanning and roughness measurement due to its high resolution, three-dimensional imaging capability and high accuracy. We report an AFM sidewall imaging method with a quartz tuning fork (QTF) force sensor. A self sensing and actuating force sensor is fabricated by microassembling a commercial AFM cantilever (tip apex radius ≤10 nm) to a QTF. The attached lightweight cantilever allows high-sensitivity force detection (7.4% Q factor reduction) and sidewall imaging with high lateral resolution. Owing to its unique configuration, the tip of the sensor can detect sidewall surface orthogonally during imaging, which reduces lateral friction. In experiments, sidewalls of a micro-electro-mechanical system (MEMS) structure fabricated by deep reactive ion etching process and a standard step grating are scanned and the sidewall roughness, line edge roughness and sidewall angles are measured.

Optimizing the Quality Factor of Quartz Tuning Fork Force Sensor for Atomic Force Microscopy: Impact of Additional Mass and Mass Rebalance

A force sensor in the heart of an atomic force microscope (AFM) plays a key role in the AFM measurements. Quartz tuning fork (QTF) based force sensor is attracting huge attention due to its peculiar traits such as self-actuating and sensing capability, high quality factor and high force sensitivity. Unfortunately, mounting a tip on a tine of the QTF degrades its quality (Q)-factor and sensitivity. Attaching an equivalent counter mass on the opposite tine (mass rebalance) can improve the Q-factor. We investigate the impact of the attached mass and counter mass on different traits of the QTF such as Q-factor, inherent relationship between excitation voltage and output as well as shift in the resonance frequency. We propose straight forward strategies to rebalance the QTF force sensor. Experimental results demonstrate that by attaching a counter mass (at the parallel position) on the opposite tine, the tip mass can be rebalanced. Q-factor is significantly improved after mass rebalance. The increase in the Q-factor depends on the mass of the tip, counter mass and position of the counter mass relative to the position of the tip (r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> ).

Dynamics of quartz tuning fork force sensors used in standoff photoacoustic detection

In this paper, a two-degrees-of-freedom model with two coupled oscillators is established to study the dynamics of quartz tuning fork force sensors. Air squeeze-film damping is considered in this model. When the laser power is 40 mW and the distance between the tuning fork and detected objects is approximately 0.5 m, the resonance amplitude of the tuning fork under the electromagnetic radiation pressure of the laser can reach 0.22 pm. Electromagnetic radiation pressure and resonance amplitude have the tendency to exponentially decay along with the distance between the tuning fork and detected objects. The influence of laser power and distance between the tuning fork and detected objects on electromagnetic radiation pressure is also considered. Lastly, an experimental device is set up to verify the calculation result of the model. Analysis shows that the experimental data are in good agreement with the theoretical calculation results.

Reverse electrochemical etching method for fabricating ultra-sharp platinum/iridium tips for combined scanning tunneling microscope/atomic force microscope based on a quartz tuning fork

Sharp Pt/Ir tips have been reproducibly etched by an electrochemical process using an inverse geometry of an electrochemical cell and a dedicated electronic device which allows us to control the applied voltages waveform and the intensity of the etching current. Conductive tips with a radius smaller than 10 nm were routinely produced as shown by field emission measurements through Fowler–Nordheim plots. These etched tips were then fixed on a quartz tuning fork force sensor working in a qPlus configuration to check their performances for both scanning tunneling microscopy (STM) and atomic force microscopy (AFM) imaging. Their sharpness and conductivity are evidenced by the resolution achieved in STM and AFM images obtained of epitaxial graphene on 6H–SiC(0001) surface. The structure of an epitaxial graphene layer thermally grown on the 6H–SiC(0001) (63×63) R30° reconstructed surface, was successfully imaged at room temperature with STM, dynamic STM and by frequency modulated AFM.

Mechanical and electrical characterization of quartz tuning fork force sensors

The results of a systematic experimental characterization of the mechanical and electrical properties of quartz tuning forks (QTFs) are presented. Actuation efficiency and detection sensitivity are introduced as parameters for characterization of the QTF with a focus on their use as a force sensor. The spring constants, quality (Q) factors, actuation efficiencies and detection sensitivities of twelve QTFs were measured by combining heterodyne interferometry with an electrical excitation/detection set-up and found to be consistent for all twelve QTFs. Spring constants were determined using geometrical and thermal methods and were found to agree within 3%. The Q-factor, actuation efficiency and detection sensitivity of the QTFs were measured for the first two vibrational modes. The Q-factor and detection sensitivity were higher in the second vibrational mode while the actuation efficiency was lower. We show that the QTF displacement amplitude can be determined from the detection sensitivity and the measured current through the QTF. The performance of a QTF as a force sensor is discussed based on a comparison of the mechanical and electrical parameters measured for the first and second vibrational modes. The results are relevant for QTFs used in sensing applications, especially as force sensors in scanning probe microscopes.

Carbon-fiber tips for scanning probe microscopes and molecular electronics experiments

We fabricate and characterize carbon-fiber tips for their use in combined scanning tunneling and force microscopy based on piezoelectric quartz tuning fork force sensors. An electrochemical fabrication procedure to etch the tips is used to yield reproducible sub-100-nm apex. We also study electron transport through single-molecule junctions formed by a single octanethiol molecule bonded by the thiol anchoring group to a gold electrode and linked to a carbon tip by the methyl group. We observe the presence of conductance plateaus during the stretching of the molecular bridge, which is the signature of the formation of a molecular junction.

Real-time measurements of sliding friction and elastic properties of ZnO nanowires inside a scanning electron microscope

A real-time nanomanipulation technique inside a scanning electron microscope (SEM) has been used to investigate the elastic and frictional (tribological) properties of zinc oxide nanowires (NWs). A NW was translated over a surface of an oxidised silicon wafer using a nanomanipulator with a glued atomic-force microscopic tip. The shape of the NW elastically deformed during the translation was used to determine the distributed kinetic friction force. The same NW was then positioned half-suspended on edges of trenches cut by a focused ion beam through a silicon wafer. In order to measure Young’s modulus, the NW was bent by pushing it at the free end with the tip, and the interaction force corresponding to the visually observed bending angle was measured with a quartz tuning fork force sensor.

Force-gradient-induced mechanical dissipation of quartz tuning fork force sensors used in atomic force microscopy

We have studied the dynamics of quartz tuning fork resonators used in atomic force microscopy taking into account the mechanical energy dissipation through the attachment of the tuning fork base. We find that the tuning fork resonator quality factor changes even in the case of a purely elastic sensor–sample interaction. This is due to the effective mechanical imbalance of the tuning fork prongs induced by the sensor–sample force gradient, which in turn has an impact on dissipation through the attachment of the resonator base. This effect may yield a measured dissipation signal that can be different from the one exclusively related to the dissipation between the sensor and the sample. We also find that there is a second-order term in addition to the linear relationship between the sensor–sample force gradient and the resonance frequency shift of the tuning fork that is significant even for force gradients usually present in atomic force microscopy, which are in the range of tens of N/m.

Carbon fibre tips for scanning probe microscopy based on quartz tuning fork forcesensors

We report the fabrication and the characterization of carbon fibre tips for use incombined scanning tunnelling and force microscopy based on piezoelectric quartztuning fork force sensors. We find that the use of carbon fibre tips results in aminimum impact on the dynamics of quartz tuning fork force sensors, yielding ahigh quality factor and, consequently, a high force gradient sensitivity. This highforce sensitivity, in combination with high electrical conductivity and oxidationresistance of carbon fibre tips, make them very convenient for combined andsimultaneous scanning tunnelling microscopy and atomic force microscopy measurements.Interestingly, these tips are quite robust against occasionally occurring tip crashes. Anelectrochemical fabrication procedure to etch the tips is presented that produces asub-100-nm apex radius in a reproducible way which can yield high resolution images.

Dynamics of quartz tuning fork force sensors used in scanning probe microscopy

We have performed an experimental characterization of the dynamics of oscillating quartztuning forks which are being increasingly used in scanning probe microscopy asforce sensors. We show that tuning forks can be described as a system of coupledoscillators. Nevertheless, this description requires knowledge of the elastic couplingconstant between the prongs of the tuning fork, which has not yet been measured.Therefore, tuning forks have usually been described within the single oscillator or theweakly coupled oscillators approximation that neglects the coupling between theprongs. We propose three different procedures to measure the elastic couplingconstant: an opto-mechanical method, a variation of the Cleveland method anda thermal noise based method. We find that the coupling between the quartztuning fork prongs has a strong influence on the dynamics and the measuredmotion is in remarkable agreement with a simple model of coupled harmonicoscillators. The precise determination of the elastic coupling between the prongs of atuning fork allows us to obtain a quantitative relation between the resonancefrequency shift and the force gradient acting at the free end of a tuning fork prong.

A low noise transimpedance amplifier for cryogenically cooled quartz tuning fork force sensors

We have designed and built a low noise, broadband transimpedance amplifier that allows for sensing both the amplitude and phase of an oscillating quartz tuning fork for a cryogenic atomic force microscope. The circuit uses a global feedback scheme, where the input stage, located next to the quartz tuning fork at low temperature, is followed by an operational amplifier stage operated at room temperature. At 4.2 K, with a 1 MΩ metal film feedback resistor, the amplifier yields an output noise floor of 2×10−7 V/Hz and a bandwidth of 200 kHz. When it is used with a commercial 32 kHz quartz tuning fork, the calibrated sensitivity at 4.2 K is determined to be 30.8 μV/pm.