Shear-force Distance Control Research Articles

Visualized Multiprobe Electrical Impedance Measurements with STM Tips Using Shear Force Feedback Control.

Here we devise a multiprobe electrical measurement system based on quartz tuning forks (QTFs) and metallic tips capable of having full 3D control over the position of the probes. The system is based on the use of bent tungsten tips that are placed in mechanical contact (glue-free solution) with a QTF sensor. Shear forces acting in the probe are measured to control the tip-sample distance in the Z direction. Moreover, the tilting of the tip allows the visualization of the experiment under the optical microscope, allowing the coordination of the probes in X and Y directions. Meanwhile, the metallic tips are connected to a current–voltage amplifier circuit to measure the currents and thus the impedance of the studied samples. We discuss here the different aspects that must be addressed when conducting these multiprobe experiments, such as the amplitude of oscillation, shear force distance control, and wire tilting. Different results obtained in the measurement of calibration samples and microparticles are presented. They demonstrate the feasibility of the system to measure the impedance of the samples with a full 3D control on the position of the nanotips.

Combined detection of electrochemical reactions and topographical effects - imaging with scanning ohmic microscopy

The ohmic principle for the detection of local current densities with capillary-based microreference electrodes was used for reactivity imaging. Specifically, reactions of adlayers and ion-intercalation and ion deintercalation reactions were imaged that are not directly visible to other electrochemical imaging methods such as scanning electrochemical microscopy. The microreference electrodes were inclined by 45° vs. the surface normal. During the scanning process, cyclic voltammograms (CVs) were performed at each grid point of the image while the voltage between the two reference electrodes was recorded and processed by a lock-in-amplifier. Images of specific electrochemical reaction rates were generated by integration of the corresponding data segments within the CVs. The setup was coupled with a shear force distance control to correlate the electrochemical reactivity with topographical data. The results clearly show that processes like adsorption and intercalation reactions, invisible to SECM, can be imaged by scanning ohmic microscopy with a resolution in the low micrometer range.

Nanomechanical method to gauge emission quantum yield applied to nitrogen-vacancy centers in nanodiamond

We present a technique to nanomechanically vary the distance between a fluorescent source and a mirror, thereby varying the local density of optical states at the source position. Our method can, therefore, serve to measure the quantum efficiency of fluorophores. Application of our technique to nitrogen-vacancy defects in diamond nanocrystals shows that their quantum yield can significantly differ from unity. Relying on a lateral scanning mechanism with shear-force probe-sample distance control our technique is straightforwardly implemented in most state-of-the-art near-field microscopes.

Nanopositioning system for macroscopic components based on shear-force detection

We present and describe an alternative device for positioning two massive optical elements at distances smaller than 1μm to each other. The developed technique is derived from the conventional nonoptical shear-force distance control usually used in scanning near-field optical microscopy (SNOM). It is suitable whatever the nature of the involved elements (metal, semiconductor, or dielectric) and is well adapted to solid immersion microscopy and virtual tip near-field microscopy.

Shear force distance control in a scanning near-field optical microscope: in resonance excitation of the fiber probe versus out of resonance excitation

Shear force distance control in a scanning near-field optical microscope: in resonance excitation of the fiber probe versus out of resonance excitation

Shear force distance control in a scanning near-field optical microscope: in resonance excitation of the fiber probe versus out of resonance excitation

The experimental results of the direct measurement of the absolute value of interaction force between the fiber probe of a scanning near-field optical microscope (SNOM) operated in shear force mode and a sample, which were performed using combined SNOM-atomic force microscope setup, are discussed for the out-of-resonance fiber probe excitation mode. We demonstrate that the value of the tapping component of the total force for this mode at typical dither amplitudes is of the order of 10 nN and thus is quite comparable with the value of this force for in resonance fiber probe excitation mode. It is also shown that for all modes this force component is essentially smaller than the usually neglected static attraction force, which is of the order of 200 nN. The true contact nature of the tip–sample interaction during the out of resonance mode is proven. From this, we conclude that such a detection mode is very promising for operation in liquids, where other modes encounter great difficulties.

A Near-Field Microwave Probe for Quantitative Characterization of Dielectric Thin Films

ABSTRACTWe have developed a novel scanning near-field microwave probe capable of precise quantitative measurements of dielectric constant of thin dielectric films. The technique is noncontact and has a few-micron sampling spot-size. For dielectric films with k<7 and thickness down to 200 nm the probe provides precision and accuracy better than 1% and 5%, respectively. The probe is based on a balanced parallel-plate microwave transmission line operating at 4 GHz. Unlike the apertureless STM- or AFM-based schemes that have been previously employed, our “apertured” approach allows for truly quantitative measurements on a few-micron length scale with result that is insensitive to the material property outside this probing volume.We will present quantitative measurements on a variety of so-called low-k dielectric films, which are of great interest to the semiconductor industry as replacements for SiO2 in interconnect wiring. When the probe is placed in close proximity to the film under test its fringe capacitance is governed by the sample permittivity, the tip geometry, and the tip-sample separation. We measure this capacitance with a resolution down to 30 zF using a microwave resonator. Extraction of the film dielectric constant is based on an original approach providing for removal of the substrate contribution. Bulk Si and a set of variable thickness thermal oxide films are employed to calibrate the probe. There is no need to know the absolute value of the tip-sample separation for either measurement or calibration procedures; this separation must only be kept nominally the same for both measurements, which is achieved by a virtually material independent shear-force distance control.

Constant-distance mode scanning electrochemical microscopy (SECM)--Part I: Adaptation of a non-optical shear-force-based positioning mode for SECM tips.

A non-optical shear-force-based detection scheme for accurately controlling the tip-to-sample distance in scanning electrochemical microscopy (SECM) is presented. With this approach, the detection of the shear force is accomplished by mechanically attaching a set of two piezoelectric plates to the scanning probe. One of the plates is used to excite the SECM tip causing it to resonate, and the other acts as a piezoelectric detector of the amplitude of the tip oscillation. Increasing shear forces in close proximity to the sample surface lead to a damping of the vibration amplitude and a phase shift, effects that are registered by connecting the detecting piezoelectric plate to a dual-phase analogue lock-in amplifier. The shear force and hence distance-dependent signal of the lock-in amplifier is used to establish an efficient, computer-controlled closed feedback loop enabling SECM imaging in a constant-distance mode of operation. The details of the SECM setup with an integrated piezoelectric shear-force distance control are described, and approach curves are shown. The performance of the constant-distance mode SECM with a non-optical detection of shear forces is illustrated by imaging simultaneously the topography and conductivity of an array of Pt-band microelectrodes.

Shear-force distance control at megahertz frequencies for near-field scanning optical microscopy

We show that the use of fiber overtone resonance modes up to 3 MHz for shear-force distance control has two advantages for near-field microscopy. The higher dither frequency allows a shorter feedback loop time delay, which in turn allows scanning speeds of 100 μm/s on a sample with, for example, a 150 nm height variation over a 1 μm distance. Experiments on a hard semiconductor device and on a soft polymer sample demonstrate a factor of 5 improvement in scanning speed when the dither frequency is increased by a factor of 20. The second advantage is a reduction of the minimum lateral force required for height regulation, which is important for soft samples. Modeling the piezoelectric detection system as a harmonic oscillator indicates a factor of 33 increase in lateral force sensitivity when using the third overtone resonance of a typical fiber tip. This result is confirmed experimentally by immersing the tip into water.

Scanning nanolithography using a material-filled nanopipette

A scanning nanolithography is demonstrated by employing near-field optical microscopy with a pulled micropipette which is used for material transport as well as distance regulation. Delivering the photoresist through the small aperture (300 nm diameter) of the pulled pipette with the shear-force distance control, we have fabricated nanometric dots (300 nm diameter) on the gold-sputtered substrate. This scheme may be also useful in nanometric control of chemical reaction and repair of nanometric structures.

Scanning Hall probe microscopy with shear force distance control

We describe a new type of scanning Hall probe microscope operating at room temperature for quantitative and noninvasive measurements of magnetic stray fields. The probe-sample distance is controlled by piezoelectrical detection of the shear forces acting on an oscillating cantilever. The Hall probes are manufactured from prepatterned GaAs wafers overgrown with a GaAs/AlGaAs heterostructure containing a high-mobility two-dimensional electron gas a few ten nm below the surface. The active Hall area is defined by optical and electron-beam lithography with a junction width of 0.6 μm yielding in a resolution of approximately 0.4 μm. The Hall coefficient of the sensor at room temperature is 0.23 Ω/G with a noise level of ∼0.1 G/Hz1/2. We show measurements of the stray field pattern of bits written on a magnetic hard disk.

Shear force distance control in near-field optical microscopy: experimental evidence of the frictional probe–sample interaction

The properties of the probe–surface contact for a near-field optical microscope driven in the shear force mode have been studied applying lateral amplitudes of the probing fiber tip larger than 15 nm. Electric current measurements between a conductive tip and a conductive sample reveal a pulsed current behavior at the very beginning of the approach curve. In the upper part of the approach curve it turns to the quasiconstant current. From this observation a conclusion is drawn about the presence of permanent mechanical contact between the probe and the surface in the shear force mode. A shift of the approach curve along the z-axis as a function of dither amplitude was discovered. These results are in contradiction to the established conception of possible physical mechanisms of shear force interaction. To settle this issue the friction model is proposed according to which the damping of the probe vibrations is caused by the friction between the tip and the surface.

Modulated shear–force distance control in near-field scanning optical microscopy

The tip–sample distance in near-field scanning optical microscopy is typically controlled by the shear–force interaction between the laterally vibrating tip and sample. In this article, a mode of shear-force feedback is described in which an additional vertical modulation is introduced. Similar to the tapping mode applied in atomic force microscopy, the modulated shear–force technique deals with problem due to the snap to contact and therefore improves the mapping of soft and ductile materials, such as biological samples and soft polymers. The imaging properties of the modulated shear–force mode is demonstrated on structures of a soft polymer blend. Additionally, the modulated shear–force technique allows a simple comparison between effects in the optical far field and in the optical near field.

Influence of environmental conditions on shear–force distance control in near-field optical microscopy

In our experiments we show, that a contaminating water film is very important for the shear–force distance control in near-field optical microscopy. This is demonstrated at the transition between a hydrophilic glass surface and a hydrophobic Langmuir–Blodgett film of arachidic acid at different relative humidities. This contaminating water film is one, if not the important reason for the damping of an oscillating fiber during surface approach. It is further shown, that the bulk viscosity of water alone cannot be responsible for the observed damping effect. A thickness dependent viscosity of this water film is proposed. These observations can also explain, why the shear–force distance control works on nearly all surfaces at ambient conditions, but fails to work at very low temperatures.

Method for increasing sensitivity of shear-force distance control for scanning near-field microscopy

Scanning-near field optical microscopy requires a distance control mechanism. In most cases, it is based on the shear-force detection. In this paper we report how the performance of the shear-force detection based on the most common nonoptical approach, a Quartz tuning fork, can be improved. Our approach is based on exciting oscillations in just one arm of the fork, not two. This approach reduces the response time of the shear-force detection system. We also introduce an ultra-sensitive system with a long free fiber tip.

Electrochemical pattern formation in a scanning near-field optical microscope

solution and a sample which is electrically conductive and transparent in the visible region (ITO coated glass). We also present some details such as shear force distance control in liquids and light guiding in pulled and filled micropipettes. Macroscopic experiments demonstrate localized deposition and allow to estimate parameters for microscopic deposition. Diameters of first deposited dots are smaller than 1 μm.

Reduction of tip–sample interaction forces for scanning near-field optical microscopy in a liquid environment

Tip-to-sample shear force distance control of a scanning near-field optical microscope (SNOM) is more critical to implement in a liquid environment than in air. The higher viscosity of the medium increases the interaction forces between the tip and the sample and may even damage the sample in the case of soft materials. In this work we measure the decrease of the quality factor of a vibrating fiber when it is immersed in water. The main loss of quality is already observed for situations where the part of the fiber dipped in the water is only a few tens of microns long. We propose a new experimental set-up which maintains a small (<50 μm) and stable depth of the fiber in water, independent from the tip to sample distance. The quality factor of the fiber tip is then Q=52, only five times smaller than in air. Topographical and near-field optical images of a soft hydrogel are obtained without inducing any damage on the observed material. Such images are impossible to obtain with a SNOM tip totally immersed in water, due to the very soft surface of the hydrogel which contains 70% of water.

Piezoelectrical shear-force distance control in near-field optical microscopy for biological applications

We present a piezoelectrical shear-force distance control setup for scanning near-field optical microscopy. The setup is compact and tip exchange is easy. The topographical sensitivity is comparable to optical feedback systems. With an acceptable vibration amplitude of 5–10 nm we obtained a topographical resolution of 5 pm/ Hz . Because there is no laser necessary for tip position feedback, there is no extraneous light to interfere with spectroscopic and other low-light level experiments. Our technique permits measurements of soft biological samples in aqueous solution, which opens up many possible applications of near-field optical microscopy in biology and medicine.

Near-field optical microscopy with uncoated tips: Calibration, chemical contrast on organic crystals, and photolithography

Reflection-back-through-the-fiber scanning near-field optical microscopy (SNOM) measurements with uncoated tapered fiber tips, that remain cold, were performed under shear-force distance control for opaque and transparent rough samples. The feedback dynamics of the commercial (DME) RASTERSCOPE SNOM were tested with a 3000/500 nm gold grating at very high scan rates. Slow rates give rise to thermal broadening due to heating of the gold surface by partial light absorption. Enhancement of the light intensity at the base by reflection from steep walls on four sides is reported, the steepness contrast being less important. Doped gallium arsenide substrates give materials contrast. Extremely rough porous silicon has been probed. Organic chemical contrast is seen for the first time with SNOM on partly oxidized anthracene at large dislocations. Anthracene, anthraquinone, and their charge-transfer complex are clearly discriminated by different reflectivities. Irradiated α-cinnamic acid exhibits a chemically uniform surface and, thus, no SNOM contrast despite the anisotropic μm high features that are observed. Photolithography on light-absorbing 2,5-dibenzylidene-cyclopentanone crystals with the tip illuminated at an ≈10 nm distance provides an easy, inexpensive, and rapid performance test of single tips. Very long local irradiation creates features that widen and reach heights of 300 nm.