We report the fabrication and the characterization of carbon fibre tips for their use in combined scanning tunnelling and force microscopy based on piezoelectric quartz tuning fork force sensors. We find that the use of carbon fibre tips results in a minimum impact on the dynamics of quartz tuning fork force sensors yielding a high quality factor and consequently a high force gradient sensitivity. This high force sensitivity in combination with high electrical conductivity and oxidation resistance of carbon fibre tips make them very convenient for combined and simultaneous scanning tunnelling microscopy and atomic force microscopy measurements. Interestingly, these tips are quite robust against occasionally occurring tip crashes. An electrochemical fabrication procedure to etch the tips is presented that produces a sub-100 nm apex radius in a reproducible way which can yield high resolution images.
We use a piezoelectric quartz tuning fork to calibrate the displacement of ceramic piezoelectric scanners which are widely employed in scanning probe microscopy. We measure the static piezoelectric response of a quartz tuning fork and find it to be highly linear, non-hysteretic and with negligible creep. These performance characteristics, close to those of an ideal transducer, make quartz transducers superior to ceramic piezoelectric actuators. Furthermore, quartz actuators in the form of a tuning fork have the advantage of yielding static displacements comparable to those of local probe micro…
We have performed an experimental characterization of the dynamics of oscillating quartz tuning forks which are being increasingly used in scanning probe microscopy as force sensors. We show that tuning forks can be described as a system of coupled oscillators. Nevertheless, this description requires the knowledge of the elastic coupling constant between the prongs of the tuning fork, which has not yet been measured. Therefore tuning forks have been usually described within the single oscillator or the weakly coupled oscillators approximation that neglects the coupling between the prongs. We p…
We have developed a low temperature, high resolution magnetic force microscope (MFM) using a quartz tuning fork that can operate in a magnetic field. A tuning fork with a spring constant of 1300 N/m mounted with a commercial MFM cantilever tip was used. We have obtained high resolution images of the stray magnetic fields exerted from grains with a spatial resolution of 15 nm and force resolution of 2 pN at 4.2 K. Tuning fork based magnetic force microscopes have the potential to be used at millikelvin temperatures due to their low power dissipation and high force sensitivity.
With the objective of integrating single clean, as-grown carbon nanotubes into complex circuits, we have developed a technique to grow nanotubes directly on commercially available quartz tuning forks using a high temperature CVD process. Multiple straight and aligned nanotubes bridge the >100um gap between the two tips. The nanotubes are then lowered onto contact electrodes, electronically characterized in situ, and subsequently cut loose from the tuning fork using a high current. First quantum transport measurements of the resulting devices at cryogenic temperatures display Coulomb blockade c…
Using an ultra-low temperature, high magnetic field scanning probe microscope, we have measured electric potentials of a deeply buried two dimensional electron gas (2DEG). Relying on the capacitive coupling between the 2DEG and a resonant tip/cantilever structure, we can extract electrostatic potential information of the 2DEG from the dynamics of the oscillator. We present measurements using a quartz tuning fork oscillator and a 2DEG with a cleaved edge overgrowth structure. The sensitivity of the quartz tuning fork as force sensor is demonstrated by observation of Shubnikov de Haas oscillatio…
We have studied the dynamics of quartz tuning fork resonators used in atomic force microscopy taking into account mechanical energy dissipation through the attachment of the tuning fork base. We find that the tuning fork resonator quality factor changes even for 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 the dissipation through the attachment of the resonator base. This effect may yield a measured dissipation signal that can be diff…
The deflection signal of a thermally excited force sensor of an atomic force microscope can be analyzed to gain important information about the detector noise and about the validity of the equipartion theorem of thermodynamics. Here, we measured the temperature dependence of the thermal amplitude of a tuning fork and compared it to the expected values based on the equipartition theorem. In doing so, we prove the validity of these assumptions in the temperature range from 140K to 300K. Furthermore, the application of the equipartition theorem to quartz tuning forks at liquid helium temperatures…
The force sensor is key to the performance of atomic force microscopy (AFM). Nowadays, most AFMs use micro-machined force sensors made from silicon, but piezoelectric quartz sensors are applied at an increasing rate, mainly in vacuum. These self sensing force sensors allow a relatively easy upgrade of a scanning tunneling microscope to a combined scanning tunneling/atomic force microscope. Two fundamentally different types of quartz sensors have achieved atomic resolution: the 'needle sensor' that is based on a length extensional resonator and the 'qPlus sensor' that is based on a tuning fork.…
We have developed a low-temperature scanning probe microscope using a quartz tuning fork operating at 4.2 K. A silicon tip from a commercial cantilever was attached to one prong of the tuning fork. With a metallic coating, a potential could be applied to the tip to sense the charge distribution in a sample, while with a magnetically coated tip, magnetic force imaging could be performed. For the coarse approach mechanism, we developed a reliable low-temperature walker with low material cost and simple machining. We have obtained Coulomb force images of boron nanowires at room temperature and ma…
We study the dynamic response of a mechanical quartz tuning fork in the temperature range from 9 K to 300 K. Since the quality factor Q of the resonance strongly depends on temperature, we implement a procedure to control the quality factor of the resonance. We show that we are able to dynamically change the quality factor and keep it constant over the whole temperature range. This procedure is suitable for applications in scanning probe microscopy.
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 signa…
We demonstrate a scanning force microscope, based upon a quartz tuning fork, that operates below 100 mK and in magnetic fields up to 6 T. The microscope has a conducting tip for electrical probing of nanostructures of interest, and it incorporates a low noise cryogenic amplifier to measure both the vibrations of the tuning fork and the electrical signals from the nanostructures. At millikelvin temperatures the imaging resolution is below 1 um in a 22 um x 22 um range, and a coarse motion provides translations of a few mm. This scanned probe is useful for high bandwidth measurement of many high…
This article describes how a frequency modulation AFM using a hanging fiber force probe made from a quartz tuning fork provides local measurements on liquid-liquid interfaces. After detailing the manufacture and calibration of the force probe, we provide evidence that this AFM is suitable for quantitative measurements at the interface between two liquids. The repeatability of the measurements allows a poly-dimethylsiloxane / water moving interface to be monitored over several hours : the evaporation of a water droplet immersed in poly-dimethylsiloxane is observed, and its interfacial tension e…
We demonstrate a dynamic scanning capacitance microscope (DSCM) that operates at large bandwidths, cryogenic temperatures and high magnetic fields. The setup is based on a non-contact atomic force microscope (AFM) with a quartz tuning fork sensor with non-optical excitation and read-out for topography, force and dissipation measurements. The metallic AFM tip forms part of an rf resonator with a transmission characteristics modulated by the sample properties and the tip-sample capacitance. The tip motion gives rise to a modulation of the capacitance at the frequency of the AFM sensor and its ha…
Ambient operation poses a challenge to AFM because in contrast to operation in vacuum or liquid environments, the cantilever dynamics change dramatically from oscillating in air to oscillating in a hydration layer when probing the sample. We demonstrate atomic resolution by imaging of the KBr(001) surface in ambient conditions by frequency-modulation atomic force microscopy with a cantilever based on a quartz tuning fork (qPlus sensor) and analyze both long- and short-range contributions to the damping. The thickness of the hydration layer increases with relative humidity, thus varying humidit…
A new technique for the fabrication of highly sensitive qPlus sensor for atomic force microscopy (AFM) is described. Focused ion beam was used to cut then weld onto a bare quartz tuning fork a sharp micro-tip from an electrochemically etched tungsten wire. The resulting qPlus sensor exhibits high resonance frequency and quality factor allowing increased force gradient sensitivity. Its spring constant can be determined precisely which allows accurate quantitative AFM measurements. The sensor is shown to be very stable and could undergo usual UHV tip cleaning including e-beam and field evaporati…
Scanning probe microscopy using nitrogen vacancy (NV) centers in diamond has become a versatile tool with applications in physics, chemistry, life sciences and earth and planetary sciences. However, the fabrication of diamond scanning probes with high photon collection efficiency, NV centers with long coherence times and integrated radio frequency (RF) remains challenging due to the small physical dimensions of the probes and the complexity of the fabrication techniques. In this work, we present a simple and robust method to reliably fabricate probes that can be integrated with conventional qu…
