The tuning fork as sensor for dynamic force distance control in scanning near-field optical microscopy

The dynamic force distance control for scanning near-field optical microscopy on the basis of a tuning fork as piezoelectric force sensor is remarkably sensitive. In order to gain a better understanding of this sensitivity the vibrational properties of the tuning fork are modelled within the framework of two coupled harmonic oscillators. As a result, the effective force constant of the tuning fork at resonance frequency is determined. Furthermore, the influence of the additional mass by the attachment of the near-field probe is investigated.     

Topography and near-field image measurement of soft biological samples in liquid by using a tuning fork based bent optical-fiber sensor

The fabrication of a tuning fork based bent optical-fiber sensor and its application for topography and near-field image measurement of soft biological samples in physiological solution are reported. By adopting the bent optical fiber and tuning fork feedback scheme, the possibility of signal interference with stray light is minimized, which is especially important for near-field applications. From the measured tuning fork amplitude and its calibration with the preamplifier output voltage, it was determined that the interaction force between the fiber tip and a soft sample in liquid needs to be controlled within approximately 10 nN level and that the image quality depends sensitively to the interaction force. The results of topography measurements of fixed COS-7 and MCF-7 cells in phosphate buffered saline and of the near-field imaging of red blood cell also in phosphate buffered saline with a resolution of about 100 nm are presented.     

Molecular recognition imaging using tuning fork-based transverse dynamic force microscopy

We demonstrate simultaneous transverse dynamic force microscopy and molecular recognition imaging using tuning forks as piezoelectric sensors. Tapered aluminum-coated glass fibers were chemically functionalized with biotin and anti-lysozyme molecules and attached to one of the prongs of a 32 kHz tuning fork. The lateral oscillation amplitude of the tuning fork was used as feedback signal for topographical imaging of avidin aggregates and lysozyme molecules on mica substrate. The phase difference between the excitation and detection signals of the tuning fork provided molecular recognition between avidin/biotin or lysozyme/anti-lysozyme. Aggregates of avidin and lysozyme molecules appeared as features with heights of 1–4 nm in the topographic images, consistent with single molecule atomic force microscopy imaging. Recognition events between avidin/biotin or lysozyme/anti-lysozyme were detected in the phase image at high signal-to-noise ratio with phase shifts of 1–2°. Because tapered glass fibers and shear-force microscopy based on tuning forks are commonly used for near-field scanning optical microscopy (NSOM), these results open the door to the exciting possibility of combining optical, topographic and biochemical recognition at the nanometer scale in a single measurement and in liquid conditions.     

Ultra stable tuning fork sensor for low-temperature near-field spectroscopy

We report on a distance control system for low-temperature scanning near-field optical microscopy, based on quartz tuning fork as shear force sensor. By means of a particular tuning fork-optical fiber configuration, the sensor is electrically dithered by an applied alternate voltage, without any supplementary driving piezo, as done so far. The sensitivity in the approach direction is 0.2nm, and quality factors up to 2850 have been reached. No electronic components are needed close to the sensor, allowing to employ it in a liquid He environment. The system is extremely compact and allows for several hours of stability at 5 K.     

Separating different contributions to the shear force in near-field microscopy

The shear force between a gold and a graphite sample and an approaching near-field optical probe using tuning fork detection is studied in detail. The adiabatic and dissipative contributions are clearly distinguished by monitoring the amplitude as well as the phase of the tip vibration when approaching the surfaces. Their relative strengths vary differently but characteristically with the distance. The interaction starts in case of graphite at a much larger distance. The adiabatic contribution is larger in the case of gold, whereas graphite shows mostly dissipative interaction. Measurements at various temperatures are performed using a gold sample, showing a dependence of the shear force on the temperature.     

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.     

An improved dynamic model of tuning fork probe for scanning probe microscopy

This paper presents a two coupled oscillators model to describe the dynamics of a tuning fork with a probe attached. The two coupled oscillators are unbalanced only in their effective masses and the damping ratios. By applying a frequency domain system identification approach in experimental investigation of various probe attachment cases, a good accuracy of the model is demonstrated. The effectiveness of the model is further demonstrated in quantitative analysis of the noise performance and the sensitivity of force sensing with a tuning fork probe. Compared with existing models, the proposed model can more accurately characterize the dynamics of a tuning fork probe.     

Near-field scanning optical microscopy using polymethylmethacrylate optical fiber probes

We report the first use of polymethylmethacrylate (PMMA) optical fiber-made probes for scanning near-field optical microscopy (SNOM). The sharp tips were prepared by chemical etching of the fibers in ethyl acetate, and the probes were prepared by proper gluing of sharpened fibers onto the tuning fork in the conditions of the double resonance (working frequency of a tuning fork coincides with the resonance frequency of dithering of the free-standing part of the fiber) reported earlier for the case of glass fibers. Quality factors of the probes in the range 2000–6000 were obtained, which enables the realization of an excellent topographical resolution including state-of-art imaging of single DNA molecules. Near-field optical performance of the microscope is illustrated by the Photon Scanning Tunneling Microscope images of fluorescent beads with a diameter of 100 nm. The preparation of these plastic fiber probes proved to be easy, needs no hazardous material and/or procedures, and typical lifetime of a probe essentially exceeds that characteristic for the glass fiber probe.     

A low temperature scanning tunneling microscope for electronic and force spectroscopy

In this article, we describe and test a novel way to extend a low temperature scanning tunneling microscope with the capability to measure forces. The tuning fork that we use for this is optimized to have a high quality factor and frequency resolution. Moreover, as this technique is fully compatible with the use of bulk tips, it is possible to combine the force measurements with the use of superconductive or magnetic tips, advantageous for electronic spectroscopy. It also allows us to calibrate both the amplitude and the spring constant of the tuning fork easily, in situ and with high precision.     

A near-field scanning microwave microscope for characterization of inhomogeneous photovoltaics

We present a near-field scanning microwave microscope (NSMM) that has been configured for imaging photovoltaic samples. Our system incorporates a Pt-Ir tip inserted into an open-ended coaxial cable to form a weakly coupled resonator, allowing the microwave reflection S(11) signal to be measured across a sample over a frequency range of 1 GHz - 5 GHz. A phase-tuning circuit increased impedance-measurement sensitivity by allowing for tuning of the S(11) minimum down to -78 dBm. A bias-T and preamplifier enabled simultaneous, non-contact measurement of the DC tip-sample current, and a tuning fork feedback system provided simultaneous topographic data. Light-free tuning fork feedback provided characterization of photovoltaic samples both in the dark and under illumination at 405 nm. NSMM measurements were obtained on an inhomogeneous, third-generation Cu(In,Ga)Se(2) (CIGS) sample. The S(11) and DC current features were found to spatially broaden around grain boundaries with the sample under illumination. The broadening is attributed to optically generated charge that becomes trapped and changes the local depletion of the grain boundaries, thereby modifying the local capacitance. Imaging provided by the NSMM offers a new RF methodology to resolve and characterize nanoscale electrical features in photovoltaic materials and devices.     

Improve performance of scanning probe microscopy by balancing tuning fork prongs

This paper presents an approach for improving the Q$Q$-factor of tuning fork probe used in scanning probe microscopes. The improvement is achieved by balancing the fork prongs with extra mass attachment. An analytical model is proposed to characterize the Q$Q$-factor of a tuning fork probe with respect to the attachment of extra mass on the tuning fork prongs, and based on the model, the Q$Q$-factors of the unbalanced and balanced tuning fork probes are derived and compared. Experimental results showed that the model fits well the experimental data and the approach can improve the Q$Q$-factor by more than a factor of three. The effectiveness of the approach is further demonstrated by applying the balanced probe on an atomic force microscope to obtain improved topographic images.     

Angled long tip to tuning fork probes for atomic force microscopy in various environments

We expand the range of applications of a tuning fork probe (TFP) in frequency-modulation atomic force microscopy (FM-AFM) by attaching a long metal tip at a certain angle. By the combined flexure of the metal tip and the tuning fork prong, this TFP can change the direction of the detectable force by switching the resonance frequency, which has not been realized with conventional TFPs with short tips. The oscillatory behavior of the tip apex of the TFP is predicted by computer simulations and is experimentally confirmed with scanning electron microscope. FM-AFM operations using this TFP are performed in various environments, i.e., in ultrahigh vacuum, air, and water. FM-AFM images obtained at an atomic step of highly oriented pyrolytic graphite in air show a clear difference depending on the excitation frequency. It is also revealed that the higher order flexural modes of this TFP are advantageous for FM-AFM in water due to the reduction in the degree of hydrodynamic damping.     

Implementation of a short-tip tapping-mode tuning fork near-field scanning optical microscope

We present the implementation of a short‐tip tapping‐mode tuning fork near‐field scanning optical microscope. Tapping frequency dependences of the piezoelectric signal amplitudes for a bare tuning fork fixed on the ceramic plate, a short‐tip tapping‐mode tuning fork scheme and an ordinary tapping‐mode tuning fork configuration with an 80‐cm optical fibre attached are demonstrated and compared. Our experimental results show that this new short‐tip tapping‐mode tuning fork scheme provides a stable and high Q factor at the tapping frequency of the tuning fork and will be very helpful when long optical fibre probes have to be used in an experiment. Both collection and excitation modes of short‐tip tapping‐mode tuning fork near‐field scanning optical microscope are applied to study the near‐field optical properties of a single‐mode telecommunication optical fibre and a green InGaN/GaN multiquantum well light‐emitting diode.     

A method for manufacturing a probe for a combined scanning tunneling and atomic-force microscope on the basis of a quartz tuning fork with a supersharp metal tip

A method for manufacturing a probe for a combined scanning tunneling and atomic-force microscope on the basis of a quartz tuning fork with a metal tip, which is equipped with an independent conductor, is described. When the probe is manufactured, the billet for a tip has the form of a rather small (in order not to change the frequency and quality factor of the quartz tuning fork) metal cone, which is glued to the end of the beam of the quartz resonator-tuning fork together with a carbon fiber as a conductor. A spark is used to form a melted ball at the vertex of the cone. The thickness of the cone near the ball is reduced to a diameter of <0.5 μm by the electrochemical technique, and the ball is then mechanically detached. The main advantage of this method is that it allows manufacturing a high-quality-factor force detector with a single super sharp and clean tip, which is made of platinum (or platinum alloys) and tungsten, with a yield of ≥80%.     

A tuning fork based wide range mechanical characterization tool with nanorobotic manipulators inside a scanning electron microscope

This study proposes a tuning fork probe based nanomanipulation robotic system for mechanical characterization of ultraflexible nanostructures under scanning electron microscope. The force gradient is measured via the frequency modulation of a quartz tuning fork and two nanomanipulators are used for manipulation of the nanostructures. Two techniques are proposed for attaching the nanostructure to the tip of the tuning fork probe. The first technique involves gluing the nanostructure for full range characterization whereas the second technique uses van der Waals and electrostatic forces in order to avoid destroying the nanostructure. Helical nanobelts (HNB) are proposed for the demonstration of the setup. The nonlinear stiffness behavior of HNBs during their full range tensile studies is clearly revealed for the first time. Using the first technique, this was between 0.009 N/m for rest position and 0.297 N/m before breaking of the HNB with a resolution of 0.0031 N/m. For the second experiment, this was between 0.014 N/m for rest position and 0.378 N/m before detaching of the HNB with a resolution of 0.0006 N/m. This shows the wide range sensing of the system for potential applications in mechanical property characterization of ultraflexible nanostructures.     

石英音叉扫描探针显微镜

石英音叉是一种谐振频率稳定、品质因数高的时基器件,其音叉臂的谐振参数(谐振振幅和谐振频率)对微力极其敏感。利用石英音叉对外力的敏感性,与钨探针结合,构成一种新型的表面形貌扫描测头。该测头与xyz压电工作台结合,利用测头音叉臂谐振频率对扫描微力的敏感性,研制基于相位反馈控制的扫描探针显微镜。首先介绍石英音叉测头的构成、工作原理和特性测试,以及由该测头构建的扫描探针显微镜的结构和测试、分析。通过对测头和系统的测试结果分析,系统达到1.2 nm的垂直分辨率,并通过对一维栅的测量,给出扫描获得的试样表面微观形貌图以及相位图,证明系统的有效性。另外,由于采用大长径比的钨探针,该系统具有测量大深宽比微器件表面轮廓的能力。     

Angle dependence of the interaction distance in the shear force technique

We study the interaction distance in the lateral force detection, using a standard quartz tuning fork as a force transducer. That is the distance at which the interaction sample-probe starts to be detected. We study in particular the dependence on the approaching angle. For angles smaller than 0.366 radians, we found an exponential behavior of the interaction distance as a function of the approaching angle. We show an equation that adjusts well with the experimental data, and discuss the possible phenomena.     

Note: In situ cleavage of crystallographic oriented tips for scanning probe microscopy

We report an in situ method of preparing tips for scanning probe microscopy (SPM). Oriented single-crystal nickel oxide (NiO) rods were diced, using a wafer saw, to prepare artificial breaking points. Two geometries, a single rod and a two-sided cut rod were fabricated. The cleavable tips were mounted to a force sensor based on a quartz tuning fork and cleaved using the coarse approach of the SPM. Atomically resolved force microscopy images of NiO (001) were taken with these NiO tips.     

Tunneling/shear force microscopy using piezoelectric tuning forks for characterization of topography and local electric surface properties

Characterization of novel nanoelectronic structures and materials requires advanced and high-resolution diagnostic methods. In this article new approach for high sensitivity measurements of electric surface properties using scanning probe microscopy is presented. In this procedure topography and tunneling current flowing between the metallic tip and the surface are observed simultaneously. In our design piezoelectric tuning fork equipped with metallic tip in shear force microscope is used.