sensors Article Visualized Multiprobe Electrical Impedance Measurements with STM Tips Using Shear Force Feedback Control Luis Botaya, Xavier Coromina, Josep Samitier, Manel Puig-Vidal * and Jorge Otero * Department of Electronics, Universitat de Barcelona, Barcelona 08028, Spain; lbotaya@el.ub.edu (L.B.); xcoromina@el.ub.edu (X.C.); jsamitier@ub.edu (J.S.) * Correspondence: manel.puig@ub.edu (M.P.-V.); jotero@el.ub.edu (J.O.); Tel.: +34-93-403-9161 (M.P.-V.) Academic Editor: Stephane Evoy Received: 16 March 2016; Accepted: 21 May 2016; Published: 25 May 2016 Abstract: 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. Keywords: scanning tunneling microscope (STM) tip; scanning probe microscopy; multiprobe SPM; quartz tuning forks; impedance measurement 1. Introduction Since the invention of the scanning tunneling microscope (STM) in 1981 by Binning and Rohrer [1], the investigation of the local electrical properties of a wide range of samples at the nanoscale has been widely reported [2,3]. The main idea is to measure the electrical interactions between a sharp metallic tip (nanometric tip radius) and the surface to determine the local properties of the sample of study. The original microscopy technique was developed by using the exponential decay of the tunneling current with the tip-sample distance. Controlling this current, it is possible to keep the tip-sample distance constant while scanning the sample to acquire the surface topography. The limitation of working with conducting samples was overcome with the development of the atomic force microscope [4], where the nanometric tip is placed on a microfabricated cantilever. Then, the bending of this cantilever is measured to determine the atomic forces between the tip and the sample surface (which depend on the tip-sample distance and have an exponential decay as well [5]), thus allowing for the reconstruction of the surface topography if the sample is scanned while keeping the force interaction constant. Evolved from these, several techniques have appeared over the last several decades to measure the nanoelectrical properties of the sample, where we can clearly identify two different groups: wire-based and cantilever-based techniques. Wire-based techniques (STM [6], electrochemical STM [7], scanning electrochemical microscopy (SECM) [8], scanning ion-conductance microscopy (SICM) [9], etc.) are limited in that the tip-sample distance is controlled by electrical Sensors 2016, 16, 757; doi:10.3390/s16060757 www.mdpi.com/journal/sensors Sensors 2016, 16, 757 2 of 9 measurement meanings, thus introducing artifacts due to the coupling between the topography and electrical properties of the surface. On the other hand, cantilever-based techniques (conductive atomic force microscopy (C-AFM) [10], Kelvin probe microscopy (KPM) [11], electrostatic force microscopy (EFM) [12], etc.) overcome the previous limitation by measuring the interaction forces (and then the topography) independently from the electrical signals; nevertheless, the electrostatic forces that appear between the tip and the sample at nanometric distances make the cantilever bend and collapse towards the sample surface; thus, the techniques are limited to larger tip-sample distances or contact-mode measurements. Simultaneous force and conductance measurements have been taken with cantilevered tips taking into account those considerations [13]. Further, for working in liquid environments, wires are easier to isolate electrically than are microfabricated cantilevers. Finally, both solutions (wire- and cantilever-based) measure the tip-sample electrical properties in Z direction, but they cannot determine the X and Y components (two probes would be needed for that). Putting it all together, a system which overcomes the limitations of both solutions would be desirable. In this manuscript, we present a system where two sharp wires are used for the measurement by controlling the tip-sample distance independently from the electrical measurements. This is accomplished by mechanically coupling the wires to quartz tuning fork (QTF) sensors to measure the shear force between the tip and the sample. The two sensors are constantly visualized by an optical microscope and can be positioned relatively between them with submicron accuracy; in this way, 3D electrical measurements can be made (in X, Y, and Z directions), together with differential measurements, which ease the data analysis by compensating parasitic effects (mainly the parasitic currents which appear due to the wires and electrical connections of the sample). Additionally, it opens the door for further applications where the optical images can give important information [14]. The use of wires instead of cantilevers also eases working in liquid media by isolating the wires with some of the well-known techniques used in electrochemical scanning tunneling microscopy (EC-STM) [15,16]. 2. Shear Force Control on Metallic Sharp Probes In recent years, quartz tuning forks have been used as nanoscopy sensors due to their advantages under certain conditions over standard microcantilever sensors [17,18]. Some of the disadvantages of these QTF sensors are being overcome, such as the lack of a confident value for the spring constant [19] and the difficulty in making the tip radius comparable in size with the ones in commercial probes [20]; therefore, in the present work, QTF sensors were used to control the tip-sample distance. In this way, the shift in the frequency of oscillation of the QTF and the electric current flowing through the electrode are measured independently so that the former can be used to maintain the tip-sample distance constant and the latter to measure the impedance of the sample. The QTF is electrically excited, instead of mechanically, which enables a better quantitative control of the response of the system. The tip is not glued onto the QTF, but the two are in tight mechanical contact, which allows the use of relatively long metallic tips. QTF devices are mostly used in electronics for precise oscillation circuits. The resonance frequency depends on the effective mass spring constant of the device [21]. We used commercial QTFs encapsulated in vacuum conditions (with a resonance frequency of 32,768 Hz). To use these QTFs as distance nanosensors, they must be removed from their metallic capsules, and the wire must be placed in mechanical contact with one of the prongs of the resonator, thus producing a variation in the resonant frequency of the QTF. The nanosensor is oscillated parallel to the sample surface. When a shear force appears, there is a shift in the resonant frequency of the nano-tool. Control of this shift in the resonance frequency allows us to keep the tip-sample force constant and, hence, the tip at a very short distance from the surface of the sample. It should be noted, however, that metallic tips are heavier than fiber tips (the most common kind of tip used in QTF sensors for scanning probe microscopies). When a metallic tip is glued to a QTF prong, its weight produces resonating modes that make it extremely difficult to use as a nanosensor. We have avoided these problems by fixing the tip to the QTF prong without glue. Sensors 757 Sensors 2016, 2016, 16, 16, 757 3 of 9 3. Multiprobe Experimental Setup 3. Multiprobe Experimental Setup In order to work cooperatively under the optical microscope, the tungsten wires needed to be orderconsiderations to work cooperatively under opticalonmicroscope, wires tothe be tilted.InSome were taken intothe account that aspect.the Astungsten can be seen in needed Figure 1, tilted. Some considerations were taken into account on that aspect. As can be seen in Figure 1, the wires needed to be tilted at a minimum β angle from the horizontal axis to avoid touching the surface wires needed be tilted at a they minimum β angle the horizontal avoid touching the surface with the cone.toFurthermore, were tilted at afrom minimum angle β’ axis fromtothe vertical axis to make the with the cone. Furthermore, they were tilted at a minimum angle β’ from the vertical axis to make the tip end visible through the upright optical microscope. If a is the cone angle of the tip, the minimum tip visible upright optical microscope. If a ismetallic the conetips angle of the tip, thecommercial minimum tilt end angle is a, through and thethe maximum tilt angle is 90°-a. The selected were ˝ tilt angle isSTM a, and the(TT-ECM10, maximum tiltfrom angleBruker is 90 -a. The probes), metallic tips selected commercial tungsten tips AFM which had were a cone angle of tungsten 32° ± 6° ˝ ˘ 6˝ (measured STM tips (TT-ECM10, from Bruker AFM probes), which had a cone angle of 32 (measured from 10 different probes); therefore, the wires should be tilted an angle between 38° and from 10 different probes); therefore, the wires should be tilted an angle between 38˝ and 52˝ from the 52° from the horizontal axis. horizontal axis. Figure 1. Schematic representation of the geometries of the two tips. The wires should be tilted at a Figure 1. Schematic representation of the geometries of the two tips. The wires should be tilted at minimum angle β from the horizontal axis to ensure that the tip end interacts with the sample. a minimum angle β from the horizontal axis to ensure that the tip end interacts with the sample. Additionally, the wires should be tilted a minimum angle β’ from the vertical axes to ensure the tip Additionally, the wires should be tilted a minimum angle β’ from the vertical axes to ensure the tip visibility from the optical microscope. visibility from the optical microscope. With the visibility from the top, it is possible to precisely position and coordinate the tips in X With the visibility the top, it is possible to precisely position and coordinateand the control tips in and Y directions using from the optical microscope information throughout the strategies X and Y directions using the optical information throughout the and strategies andZ-axis control algorithms previously developed in microscope [22]. To control the tip-sample distance thus the of algorithms previously developed in [22]. To control the tip-sample distance and thus the Z-axis of the measurement system, QTF sensors were used (AB38T model, Abracon Corp., Irvine, CA, USA). the measurement QTF sensors used (AB38T Corp., Irvine, CA, USA). A common way tosystem, use these sensors forwere tip-sample distancemodel, controlAbracon is to glue the tip to them with an A common way to use these sensors for tip-sample distance control is to glue the tip to them epoxy. However, in our application, larger wires than usual were needed, so the added mass towith the an epoxy. However, in our application, larger wires thandifficult usual were needed, so the the added mass to QTF resonator is heavier than usual. Therefore, it is very to drive electrically sensor with the is heavier thanthe usual. Therefore, it is which very difficult driveto electrically thewithout sensor thisQTF hugeresonator added mass because electrical energy, can betogiven the device with this huge added mass because the electrical energy, which can be given to the device without damaging it, is very low. On the other hand, electrical excitation of the device was desirable because damaging it,the is very low.and On the other for hand, excitation of measurements the device was (when desirable it simplifies design, it allows theelectrical quantification of the thebecause QTF is it simplifies the design, it allows the quantification of coupling the measurements (when theand QTFthe is driven mechanically by and using a ditherfor piezo, the mechanical between the dither driven mechanically by using a dither piezo, the mechanical coupling between the dither and the QTF QTF should be known for quantification). Following the idea presented in [23] for using a glue-free should known for quantification). the idea in [23] for using a glue-free solution solutionbefor developing the distanceFollowing sensor based on presented optical fibers, authors presented a glue-free for developing the distance sensor based on optical fibers, authors presented a glue-free solution for solution for metallic tips in a previous work [24]. Briefly, the metallic tip was placed in mechanical metallic tips in a previous work [24]. Briefly, the metallic tip was placed in mechanical contact with contact with the QTF resonator by using a specifically designed holder with a 3D micropositioning the QTF In resonator by using a specifically designed holderexperimentally with a 3D micropositioning In that system. that way, the best contact point was found by looking atsystem. the frequency way, the best contact point was found experimentally by looking at the frequency response and then response and then maintained by using a spring-based system. The designed head was adapted so maintained by using a spring-based system. The designed head was adapted so that working with that working with tilted wires in multitip applications was possible, as shown in Figure 2. tilted wires in multitip applications was possible, as shown in Figure 2. Sensors 2016, 16, 757 4 of 9 Sensors 2016, 16, 757 4 of 9 Sensors 2016, 16, 757 4 of 9 (a) (b) Figure 2. (a) 3D CAD design the head. (b) Photograph of the designed head the multitip system. (a) of (b) for Figure 2. (a) 3D CAD design of the head; (b) Photograph of the designed head for the multitip system. The wire and the QTF can be positioned independently and precisely by using micrometric screws. The wire and the3DQTF be positioned preciselyhead by using screws. Figure 2. (a) CADcan design of the head. independently (b) Photograph ofand the designed for themicrometric multitip system. Rubber strips are used to maintain the mechanical contact between the wire and the QTF without the Rubber strips are used to maintain the mechanical contact between the wire and the QTF without The wire and the QTF can be positioned independently and precisely by using micrometric screws. the need glue. need of of any any glue. Rubber strips are used to maintain the mechanical contact between the wire and the QTF without the need of any glue. Heads with the integrated tungsten wires and QTF sensors were mounted into a micro- and Heads with the integrated tungsten wires and QTF sensors were mounted into a micro- and nanopositioning system under an tungsten upright wires optical microscope (Nikkon, custominto made development Heads with the integrated and QTF sensors were mounted a microand nanopositioning system under an upright optical microscope (Nikkon, custom made development with nanopositioning interchangeablesystem objectives 5× to optical 50× magnification), as shown in made Figure 3. Macro- and underfrom an upright microscope (Nikkon, custom development with interchangeable objectives from 5ˆ to 50ˆ magnification), as shown in Figure 3. Macro- and with interchangeable objectives fromwere 5× to accomplished 50× magnification), shown in Figure 3. Macroand micropositioning of the end-effectors withasmicrostepper motors (THORLABS micropositioning of the end-effectors were accomplished with microstepper motors (THORLABS MT3 micropositioning of the end-effectors were accomplished with microstepper motors (THORLABS MT3 and APT604 stages) with a resolution of 40 nm and a travel range of 12 mm. Nanopositioning and APT604 stages) with a resolution of 40 nm and travel rangerange of 12ofmm. Nanopositioning of the MT3 andwas APT604 stages) withby a resolution of 40 nma and a travel 12 mm. of the tips accomplished two piezoelectric positioning systems (PI Nanopositioning Nanocube) with a tips was accomplished by two piezoelectric positioning systems (PI Nanocube) with a resolution of the tips was accomplished by two piezoelectric positioning systems (PI Nanocube) with a of resolution of 1 nm and a travel range of 100 μm. A digital controller (Dulcinea controller from of range 1 nm and a travel of controller 100 μm. A(Dulcinea digital controller (Dulcinea controller from 1 nm resolution and a travel of 100 µm. Arange digital controller from Nanotec Electrónica) Nanotec Electrónica) with 4 PI controllers, 2 lock-in amplifiers, and 16 analog I/0 channels was used Nanotec Electrónica) with 4 PI controllers, 2 lock-in amplifiers, and 16 analog I/0 channels was used with 4 PI controllers, 2 lock-in amplifiers, and 16 analog I/0 channels was used to drive the actuators, to drive the actuators, measure the sensors, and execute thecontrol controlalgorithms. algorithms. to drive the actuators, measure the sensors, and execute the measure the sensors, and execute the control algorithms. Figure 3. The nanopositioning stationwith withthe the end-effectors end-effectors integrated. Microstepper motors are used Figure 3. The nanopositioning station integrated. Microstepper motors are used for coarse positioning and piezoelectric actuators for fine positioning. The sample and the endfor coarse positioning and piezoelectric actuators for fine positioning. The sample and the end-effectors Figure 3. The are nanopositioning station with the microscope. end-effectors integrated. Microstepper motors are used effectors byoptical an upright optical are visualized byvisualized an upright microscope. for coarse positioning and piezoelectric actuators for fine positioning. The sample and the endFor are the visualized measurement of upright the shearoptical force in the QTF sensors, a custom-made electronics [25] was effectors by an microscope. For measurement of the shear forceamplifiers in the QTF sensors, custom-made electronics [25] was usedthe together with the integrated lock-in in the digitalacontroller. Briefly, the electronics usedFor together with the integrated lock-in amplifiers in the digital controller. Briefly, the electronics integrate the parasitic capacitor compensation, which allows for control of the amplitude of vibration the measurement of the shear force in the QTF sensors, a custom-made electronics [25] was integrate thequality parasitic compensation, whichFor allows for control of the amplitude vibration and the of the sensors independently. the impedance measurement, a custom I–V used together with factor thecapacitor integrated lock-in amplifiers in the digital controller. Briefly, theofelectronics and the quality factor of the sensors independently. For the impedance measurement, a custom I–V converter was developed based on the AD549JH amplifier (Analog Devices, Norwood, MA, USA), integrate the parasitic capacitor compensation, which allows for control of the amplitude of vibration and a was lock-in amplifier (Anfatec eLockIn204/2) wasamplifier used. A complete scheme of the whole electronic converter developed based on the AD549JH (Analog Devices, Norwood, MA, USA), and the quality factor of the sensors independently. For the impedance measurement, a custom I–V presented (Anfatec in Figure 4. and asystem lock-inisamplifier eLockIn204/2) was used. A complete scheme of the whole electronic converter was developed based on the AD549JH amplifier (Analog Devices, Norwood, MA, USA), system is presented in Figure 4. eLockIn204/2) was used. A complete scheme of the whole electronic and a lock-in amplifier (Anfatec system is presented in Figure 4. Sensors 2016, 2016, 16, 16, 757 757 Sensors of 99 55 of Figure 4. and control system. The The shearshear forceforce between the tips and theand surface Figure 4. Scheme Schemeofofthe theelectronics electronics and control system. between the tips the are measured through custom-designed electronics, and the digital controller uses them to control surface are measured through custom-designed electronics, and the digital controller uses themthe to tip-sample distance (PIdistance control)(PI by control) adjusting Z-axis of theZ-axis piezoelectric actuators. The impedance control the tip-sample bythe adjusting the of the piezoelectric actuators. The between thebetween two tipsthe is measured by an I–V and amplifier. impedance two tips isindependently measured independently by aanlock-in I–V and a lock-in amplifier. 4. Results Resultsand andDiscussion Discussion 4. All the the experiments experimentspresented presentedininthis thiswork workwere werecarried carried ambient conditions, All outout in in ambient conditions, withwith an 7 7 an excitation frequency 5 kHz and using a fixed gain forthe theI–V I–Vconverter. converter.Tungsten Tungstenwires wires were were excitation frequency of 5ofkHz and using a fixed 1010gain for tilted at at approximately 45 degrees with respect to the X plane. tilted The experimentsconsisted consistedofof measuring impedance between the probes two probes they The experiments measuring thethe impedance between the two whenwhen they were were placed at nanometric distances sample surface (distanceswere werethe thesample sample impedance impedance placed at nanometric distances fromfrom the the sample surface (distances dominates, as as discussed discussed in in the the next next section). section). Complex Complex impedance impedance has has contributions contributions from from resistive, resistive, dominates, capacitive, and inductive inductive contributions. contributions. For For the the calibration calibration experiments, experiments, purely purely resistive resistive or or capacitive capacitive capacitive, samples were used. samples 4.1. Calibration Calibration Experiments Experiments 4.1. The first first experiment experiment to to test test the the ability ability of of the the system system to to measure measure the the sample’s sample’s impedance impedance was was to to The determine the optimal working setpoint (tip-sample distance). When the metallic tip approaches the determine the optimal working setpoint (tip-sample distance). When the metallic tip approaches the sample, the sample (which depends on on the the tip-sample distance and sample, the impedance impedancebetween betweenthe thetip tipand andthe the sample (which depends tip-sample distance the dielectric constant of the medium in between) can be measured. Thismeasured. impedanceThis has and the dielectric constant of the medium in increases between)and increases and can be a mainly non-linear capacitive behavior [26]. To determine the adequate tip-sample distance where the impedance has a mainly non-linear capacitive behavior [26]. To determine the adequate tip-sample sample impedance becameimpedance dominant, became a series of I–V curves were of recorded while the recorded tip was moved distance where the sample dominant, a series I–V curves were while toward the sample. The tip-sample distance was determined from an amplitude vs. Z-curve made the tip was moved toward the sample. The tip-sample distance was determined from an amplitude prior to the measurements (the tip is moved toward the sample surface while recording the changes in vs. Z-curve made prior to the measurements (the tip is moved toward the sample surface while the amplitude oscillation). this case, the sample was aIngold to awas resistor. Figure recording the of changes in theInamplitude of oscillation). thistrace case,connected the sample a gold trace5 shows how the non-linear behavior (due to the tip-sample capacitance) was progressively reduced by connected to a resistor. Figure 5 shows how the non-linear behavior (due to the tip-sample moving the tip toward the sample up to a certain distance (150 nm), where only the linear (resistive) capacitance) was progressively reduced by moving the tip toward the sample up to a certain distance behavior the sample was observed. Thisbehavior distanceof was for the measurements in (150 nm), of where only the linear (resistive) theused sample wassubsequent observed. This distance was this work. used for the subsequent measurements in this work. Once the the minimum minimum tip-sample tip-sample distance distance was was determined, determined, aa series series of of purely purely capacitive capacitive samples samples Once were measured (gold traces connected to discrete capacitors of a known value). The tip-sample distance were measured (gold traces connected to discrete capacitors of a known value). The tip-sample was maintained below 150 nm so that the sample capacitance (linear) dominated over the tip-sample distance was maintained below 150 nm so that the sample capacitance (linear) dominated over the capacitancecapacitance (non-linear). The amplitude of the signal at 5signal kHz was a sweep ´5 V and−55 V V. tip-sample (non-linear). The amplitude of the at 5 kHz was between a sweep between Results for the positive sweep (negative amplitudes produced symmetrical results) are shown in and 5 V. Results for the positive sweep (negative amplitudes produced symmetrical results) are Figure 6. shown in Figure 6. Sensors 2016, 16, 757 Sensors Sensors 2016, 2016, 16, 16, 757 757 6 of 9 66 of of 99 Figure Series I–V curves the sample recorded while the Figure I–V curves overover the resistor sample recorded while changing the tip-sample distance. Figure 5.5. 5.Series Seriesofof of I–V curves over the resistor resistor sample recorded while changing changing the tip-sample tip-sample distance. The due to capacitance is for greater The non-linear behaviorbehavior due to the capacitance is observed for distances greater than distance. The non-linear non-linear behavior duetip-sample to the the tip-sample tip-sample capacitance is observed observed for distances distances greater than 150 nm. 150 nm. than 150 nm. 18 18 Measured Measured Value Value Error Error Linear Linear Fit Fit of of Measured Measured Value Value measured(pF) (pF) CCmeasured Error(%) (%) Error 16 16 14 14 12 12 10 10 88 66 44 22 22 (a) (a) 44 66 88 10 10 12 12 14 14 16 16 C Cvalue value (pF) (pF) (b) (b) Figure 6. Capacitors measurements. kHz over known-value capacitors by Figure 6. 6. Capacitors Capacitors measurements. measurements. (a) (a) Curves Curves obtained obtained at at 555 kHz kHz over over known-value known-value capacitors capacitors by by Figure (a) Curves obtained at varying the voltage amplitude and measuring the current. (b) Error in the measurements. The I–V varying the voltage amplitude and measuring the current. (b) Error in the measurements. The I–V varying the voltage amplitude and measuring the current. (b) Error in the measurements. The I–V curves display the expected linear behavior. The values are great with curvesdisplay displaythe the expected linear behavior. The measured measured are in inaccordance great accordance accordance with curves expected linear behavior. The measured valuesvalues are in great with nominal 2 nominal values, with errors lower than 6%. Linear fit equation is 0.07 + 1.045x with adjusted nominal values, errors than fit 6%. Linear is fit0.07 equation is with 0.07 adjusted + 1.045x R with adjusted R R2 2 of 0.9997. values, with errorswith lower than lower 6%. Linear equation + 1.045x of 0.9997. of 0.9997. Capacitors were measured precisely prior to the measurements with with an impedance analyzer, Capacitors Capacitors were were measured measured precisely precisely prior prior to to the the measurements measurements with an an impedance impedance analyzer, analyzer, which gave the values used as “reference” (2.2 pF, 3.6 pF, 5.3 pF, 6.8 pF, 8 pF, 10.5 pF, 12.3 pF, and which gave the values used as “reference” (2.2 pF, 3.6 pF, 5.3 pF, 6.8 pF, 8 pF, 10.5 pF, 12.3 which gave the values used as “reference” (2.2 pF, 3.6 pF, 5.3 pF, 6.8 pF, 8 pF, 10.5 pF, 12.3 pF, pF, and and 15.8 pF). Thus, results obtained with the developed system displayed aa maximum difference of 6% 15.8 pF). Thus, results obtained with the developed system displayed maximum difference 15.8 pF). Thus, results obtained with the developed system displayed a maximum difference of of 6% 6% with these previous “reference” values, so we think that the results demonstrated that the system is with with these these previous previous “reference” “reference” values, values, so so we we think think that that the the results results demonstrated demonstrated that that the the system system is is capable of measuring a purely capacitive sample with ample accuracy. capable of measuring a purely capacitive sample with ample accuracy. capable of measuring a purely capacitive sample with ample accuracy. 4.2. Microparticle Impedance Measurement 4.2. 4.2. Microparticle Microparticle Impedance Impedance Measurement Measurement For the sample preparation, aluminum microparticles (of sizes ranging from 10 to 100 µm, For For the the sample sample preparation, preparation, aluminum aluminum microparticles microparticles (of (of sizes sizes ranging ranging from from 10 10 to to 100 100 μm, μm, purchased from Pomenton Inc., Maerne, Italy) where diluted in ethanol. A drop of the solution was purchased purchased from from Pomenton Pomenton Inc., Inc., Maerne, Maerne, Italy) Italy) where where diluted diluted in in ethanol. ethanol. A A drop drop of of the the solution solution was was deposited onto freshly cleaved mica and allowed to evaporate naturally. Then, the two tungsten deposited onto freshly cleaved mica and allowed to evaporate naturally. Then, the two tungsten deposited onto freshly cleaved mica and allowed to evaporate naturally. Then, the two tungsten wires were place over the sample. The left tip was placed over a microparticle, and the right tip wires wires were were place place over over the the sample. sample. The The left left tip tip was was placed placed over over aa microparticle, microparticle, and and the the right right tip tip was was was then placed at three different positions: over the same microparticle (Figure 7a), over a different then placed at three different positions: over the same microparticle (Figure 7a), over a different then placed at three different positions: over the same microparticle (Figure 7a), over a different microparticle in contact with the first one (Figure 7b), and over a different microparticle that was not microparticle microparticle in in contact contact with with the the first first one one (Figure (Figure 7b), 7b), and and over over aa different different microparticle microparticle that that was was not not in contact with the first one (Figure 7c). in in contact contact with with the the first first one one (Figure (Figure 7c). 7c). Sensors 2016, 16, 757 Sensors 2016, 16, 757 Sensors 2016, 16, 757 7 of 9 7 of 9 7 of 9 (a) (a) (b) (b) (c) (c) Figure 7. Microparticle measurement experiment as visualized with the optical microscope at Figure Microparticle measurement experimentvisualized as visualized the with the microscope optical microscope at Figure 7.7.Microparticle experiment at distances distances of 22 μm, 45 measurement μm, and 56 μm betweenas the tips. (a) with Both tipsoptical over the same microparticle; distances of 22 μm, 45 μm, and 56 μm between the tips. (a) Both tips over the same microparticle; of µm, 45 µm, and 56 µminbetween Both(c) tips over themicroparticles same microparticle; (b)22 tips over microparticles contact the withtips. each(a) other; tips over that are(b) nottips in (b) tips over microparticles inwith contact with each other; (c)microparticles tips over microparticles that are not in over microparticles in contact each other; (c) tips over that are not in contact with contact with each other. contact with each other. each other. Current–voltage curves on the three measurements are shown in Figure 8. When both tips are Current–voltage curves on the three measurements are shown in Figure 8. When both tips are over Current–voltage the same microparticle, an the impedance of 30.4 MΩ is can assume is are the curves on three measurements aremeasured, shown in which Figure one 8. When both tips over the same microparticle, an impedance of 30.4 MΩ is measured, which one can assume is the impedance of microparticle, a single microparticle. When two MΩ microparticles contact over the same an impedance of 30.4 is measured,inwhich oneare canmeasured, assume is the impedance of a single microparticle. When two microparticles in contact are measured, the impedance of measured is higher (104.1 MΩ)two than it is for a single microparticle; impedance is more impedance a single microparticle. When microparticles in contact are measured, the impedance impedance measured is higher (104.1 MΩ) than it is for a single microparticle; impedance is more than double due to the effects of the pseudo-spherical geometry of the particles and the non-ideal measured is higher (104.1 MΩ) than it is for a single microparticle; impedance is more than double than double due to the effects of the pseudo-spherical geometry of the particles and the non-ideal ohmic contact between them. Finally, when the tipsofare overand two different particles are due to the effects of the pseudo-spherical geometry theplaced particles the non-ideal ohmic that contact ohmic contact between them. Finally, when the tips are placed over two different particles that are not in contact eachwhen other, the impedance almost one orderthat of magnitude (876.8 MΩ); between them.with Finally, the tips are placedincreases over twotodifferent particles are not in contact with not in contact with each other, the impedance increases to almost one order of magnitude (876.8 MΩ); thus,other, the capacitor formed by the to two particles and the dielectric (876.8 (mica MΩ); and air) between them each the impedance increases almost one order of magnitude thus, the capacitor thus, the capacitor formed by the two particles and the dielectric (mica and air) between them is measured. formed by the two particles and the dielectric (mica and air) between them is measured. is measured. Figure Current–voltage curves curves with with one one tip tip over over aa microparticle and the Figure 8. 8. Current–voltage microparticle and the second second tip tip at at different different Figure 8. Current–voltage curves with increased one tip over a microparticle and the the second tip atasdifferent distances. As expected, the impedance with the distance between tips, and distances. As expected, the impedance increased with the distance between the tips, and as the the tips tips distances. As expected, the impedance increased with the distance between the tips,with and each as theother, tips contacted the same microparticle (a distance of 22 µm), two microparticles in contact contacted the same microparticle (a distance of 22 μm), two microparticles in contact with each other, contacted the 45 same microparticle (a distance of 22inμm), two microparticles in contact withof each other, (a (a distance distance of of 45 µm) μm) and and two two microparticles microparticles not not incontact contactwith witheach eachother other(a (adistance distance56 56 ofµm). μm). (a distance of 45 μm) and two microparticles not in contact with each other (a distance 56 of μm). 5. Conclusions Conclusions 5. Conclusions Here we report the the architecture, architecture, design, design, and and validation validation of aa novel, novel, customized customized multitip multitip system system Here we report the architecture, design, and validation of a novel, customized multitip system based on a QTF with a glue-free metallic tip working in shear mode for impedance measurements. measurements. based on a QTF with a glue-free metallic tip working in shear mode for impedance measurements. Both tips as as force andand current nanosensors. The considerations on theon wire tipswork worksimultaneously simultaneously force current nanosensors. The considerations thetilting wire Both tips work simultaneously as force and current nanosensors. The considerations on the wire and tip-sample distance have beenhave presented, well as calibration experiments purely resistive tilting and tip-sample distance been as presented, as well as calibrationover experiments over tilting and tip-sample distance have been presented, as well as calibration experiments over and capacitive Finally, samples. an experiment with microparticles has microparticles been presented.has purely resistivesamples. and capacitive Finally, analuminum experiment with aluminum purely resistive and capacitive samples. Finally, an experiment with aluminum microparticles has system presented here overcomes most of the limitations of the existing solutions offered in beenThe presented. been presented. the literature. The tip-sample distance can be controlled independently from thesolutions measured current, The system presented here overcomes most of the limitations of the existing offered in The system presented here overcomes most of the limitations of the existing solutions offered in and differential measurements can be taken to the two-sensors configuration. Additionally, the the literature. The tip-sample distance can bedue controlled independently from the measured current, the literature. The tip-sample distance can be controlled independently from the measured current, use wires instead of cantilevers, which are easy to electrically isolate, opens theAdditionally, door for future and of differential measurements can be taken due to the two-sensors configuration. the and differential measurements can be taken due to the two-sensors configuration. Additionally, the applications in liquid media. use of wires instead of cantilevers, which are easy to electrically isolate, opens the door for future use of wires instead of cantilevers, which are easy to electrically isolate, opens the door for future applications in liquid media. applications in liquid media. Sensors 2016, 16, 757 8 of 9 Acknowledgments: Authors want to acknowledge Dra. Elena Xuriguera for assistance in the preparation of the sample of microparticles and Ángel María Benito for assistance in the holder design. This work was supported in part by the Spanish Ministerio de Economía y Competitividad through projects TEC2009-10114 and TEC2010-16844. Author Contributions: J.O. and M.P. conceived the idea and designed the experiments presented in this manuscript. L.B. and X.C. developed the hardware and software parts of the system. L.B. performed the experiments. J.O. and L.B. wrote the paper. J.S. provided advice on the revisions of the manuscript. 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