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Quartz tuning fork-based conductive atomic force microscope with glue-free solid metallic tips
Sensors and Actuators A 232 (2015) 259–266
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Quartz tuning fork-based conductive atomic force microscope with glue-free solid metallic tips Luis Botaya a , Jorge Otero a , Laura González a , Xavier Coromina a , Gabriel Gomila a,b , Manel Puig-Vidal a,∗ a b
Departament d’Electrònica, Universitat de Barcelona, Martí i Franqès, 1, 08028 Barcelona, Spain Institut de Bioenginyeria de Catalunya (IBEC), Balidiri i Reixac 15-21, 08028, Barcelona, Spain
a r t i c l e
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Article history: Received 24 October 2014 Received in revised form 11 June 2015 Accepted 11 June 2015 Available online 19 June 2015 Keywords: AFM Conductive AFM Quartz tuning fork
a b s t r a c t Here, we devise a conductive Atomic Force Microscope (C-AFM) based on quartz tuning forks (QTFs) and metallic tips capable of simultaneously imaging the topography and conductance of a sample with nanoscale spatial resolution. The system is based on a header design which allows the metallic tip to be placed in tight and stable mechanical contact with the QTF without the need to use any glue. This allows electrical measurements to be taken with an electrically excited QTF with the two prongs free. The amplitude oscillation of the QTF is used to control the tip-sample distance and to acquire the topographic images. Meanwhile, the metallic tip is connected to a current–voltage amplifier circuit to measure the tip-sample field emission/tunneling current and to produce the conductive images. This method allows decoupled electrical measurement of the topography and electrical properties of the sample. The results we obtain from calibration samples demonstrate the feasibility of this measurement method and the adequacy of the performance of the system. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Different applications and studies have indicated the importance of obtaining simultaneous topographical and conductive images of sample surfaces with nanoscale spatial resolution. To this end conductive Atomic Force Microscopy (C-AFM) was developed several years ago [1,2]. In C-AFM, a cantilever-type conductive probe connected to a current amplifier is used to record the local conductivity of the sample, while the topography is recorded via the mechanical deflection of the cantilever. Since its invention, CAFM has been successfully applied in the electrical characterization of a large variety of small-scale systems such as carbon nanotubes [3], semiconductor nanowires [4], self-assembled molecular monolayers [5] and organic layers [6], to cite just a few examples. One of the limitations of conventional C-AFM is the need for the tip to be in physical contact with the sample in order to ensure good electrical contact for the DC current flow. This limits its use with soft or fragile samples, such as biological samples or soft polymers, which can become irreversible damaged during the image acquisition. In order to overcome this limitation, special C-AFM imaging
∗ Corresponding author. E-mail address: [email protected] (M. Puig-Vidal). http://dx.doi.org/10.1016/j.sna.2015.06.006 0924-4247/© 2015 Elsevier B.V. All rights reserved.
modes which minimize lateral shear forces have been developed [7] and applied to bio-samples; but they are still unwieldy to use. An alternative way to overcome the limitation consists of using an AFM system based on a quartz tuning fork (QTF). In such systems, the tip-QTF (nano-tool) is mechanically or electrically excited at its resonance frequency and oscillates parallel to the surface of the sample (shear mode) in close proximity to it (<5 nm distance) but not in contact with it. When a short-range force, caused by the tip-sample distance, interacts with the tip, the frequency at which the nano-sensor is oscillating varies. These frequency variations can be used as a feedback signal to record the sample topography with a constant distance [8]. It has been shown that it is possible to image the sample surface with nanometric accuracy using a QTF as a nano-tool [9,10]; this is primarily due to the excellent quality factor of QTFs [11]. Since the tip of a QTF-based AFM is maintained in close proximity to the sample surface, but not in contact with it, the lateral shear forces on the sample are drastically reduced and this method can safely be used with soft samples [12,13]. In addition, the distance at which the tip is maintained is within the range of tunneling or field emission currents, so that potentially conduction images could also be obtained with this type of system by using a metallic tip and connecting it to a current amplifier. Some work on QFT-based C-AFM systems using metallic tips has been published recently. The studies cover the fabrication of the
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Fig. 1. Schematic representation of the C-AFM system showing the most important elements (the Dulcinea control unit, capacity compensation unit and STM unit) and the signal distribution.
tip [14] and the remaining parts of the setup [15,16], and include validation tests [17,18]. In the published systems, the QTF sensors usually work in a Qplus configuration: one of the QTF prongs is clamped while the QTF is excited mechanically. In contrast to shear mode operation, in this setup the QTF oscillates perpendicular to the sample surface, and senses via a sharp metallic tip that is glued to the surface of one of the QTF prongs and connected to its corresponding electrode (thus both signals, force and current, are obtained through the electrode on that prong) [19]. Such setups have been used to take measurements from an electronic nanocircuit at very low temperatures [20] or from molecules [21]. However, with this system it is hard to obtain topography and electrical images simultaneously, because both signals are registered using the same electrode, which increases the difficulty both of simultaneous signal acquisition and interpretation of the images obtained. Moreover, if one prong of the QTF is clamped, or the excitation is mechanical, quantification of the measurements is challenging because it is very difficult to calculate/calibrate the mechanical coupling between the dither (used to vibrate the device externally) and the QTF, as well as the losses due to clamping the prong. Previous work implemented a setup for shear mode operation [22] in which the tip was glued to one prong of the fork and was connected directly to an I–V preamplifier, independently of the QTF electrode; but the excitation of the QTF was still mechanical. Here, we present a system which has several improvements with respect to those developed previously. The QTF is electrically
excited, instead of mechanically, which enables a better quantitative control of the response of the system. In addition, 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 track the sample topography and the latter to construct the conduction image. Finally, the tip is not glued onto the QTF, but the two are in tight mechanical contact. This glue-free method allows us to use relatively long metallic tips, which could have advantages for future applications (e.g., taking electrical measurements in liquids by just dipping the end of the tip into the liquid). Evaluation of the capabilities of this system, throughout the spectroscopic (I–V curves) and imaging working modes. Evaluation of the capabilities of this system throughout I–V curves and simultaneus topographical and current images provides qualitative verification fo this QTF-based C-AFM system (obtaining topographical and current simultaneus images), provides qualitative verification of this QTF-based C-AFM system. 2. Materials and methods The experiments were carried out using a customized commercial AFM (Cervantes System, Nanotec Electronica, Madrid, Spain) in air at room temperature (25 ◦ C). The commercial system was adapted to work with a QTF as a nano-tool. We devised our custom setup to obtain C-AFM images. The Cervantes System AFM consisted of a piezo scanner (which consists of a large XY scan, 60 × 60
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Fig. 2. (a) Virtual CAD image of the C-AFM holder. (b) Schematic representation of the process for placing the metallic tip in contact with the QTF: the tip can be moved freely in 3 orthogonal directions to ensure good mechanical contact with the QTF. (c) Photograph of the header used. (d) Magnified image of the point of contact between the QTF prong and tip.
um, and a short Z scan, 10 m), a Dulcinea AFM electronic driver and a digital controller with WS × M open Software [23]. The complete electronic setup is represented in Fig. 1 which is a schematic representation and general overview of all our CAFM device and signal information. The QTF excitation signal (AFM) and bias voltage (C-AFM) are controlled by the Cervantes System Dulcinea Controller. A capacity compensation circuit is used to minimize parasitic capacitances and amplify the QTF output current via a trans-impedance amplifier [24]. Finally, the amplified current is fed back to the Dulcinea module. Meanwhile, the current unit acquires the electric current through the tip and amplifies it in the I–V circuit; the resultant voltage is introduced into the Dulcinea module via external input signal connectors. 2.1. Quartz tuning forks QTF devices are mostly used in precise oscillation circuits. The QTF resonance frequency depends on its effective mass and its spring constant [25]. We used commercial QTFs encapsulated in vacuum conditions (with a resonance frequency of 32,768 Hz). To use these QTFs as AFM nano-sensors they must be removed from
their metallic capsules and the sharpened tip (for the atomic interaction with the sample) must be placed in mechanical contact with one prong of the QTF. This physical contact produces a variation of the QTF resonance frequency due to the changes in the effective mass and spring constant of the device. When a force acts on the nano-sensor, formed by the QTF–metallic tip junction, 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. Since the tip is not a cantilever type, this distance can be less than a few nanometres without it collapsing; something that is not possible with cantilever-based AFM systems. In addition, local electrical information can be obtained if the tip is made of a conductive material and it is connected to a current amplifier [26]. It should be noted, however, that metallic tips are heavier than fibre 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 which make it extremely difficult to use it as a nano-sensor. We have avoided these problems fixing the tip to the QTF prong without glue.
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Fig. 3. (a) Working mode diagram of the C-AFM set-up. This schematic representation gives an overview of the different parts of the set-up. (b) C-AFM working mode of the QFT-based system. A difference in potential (Vbias) is applied between the tip and the sample. As soon as the tip and sample are close enough (a few nm), and if an adequate voltage is applied to the sample, electrons can jump the gap between the tip and sample due to tunnelling and/or field emission effects.
2.2. Header for C-AFM measurements We have chosen tungsten tips for the conductive measurements because they are one of the most suitable tip types used for this kind of measures. However, tungsten tips are heavier and more rigid tips than fibers ones (most commonly tips used with QTF nanoprobes). In addition, the system has been designed to work with large tips, and hence still heavier, because we want the system to be also compatible with liquid measurements avoiding the immersion of the quartz tuning fork. Furthermore, the conductive tip can be straightaway connected to the I–V amplifier. All these requirements, prevent the tungsten tip to be glued to the QTF prong. In order to overcome these difficulties, we devised a new header system, inspired by pervious work [27,28], for positioning the tip in physical contact with the QTF prong in a fixed and stable way. In
the system, the input/output electrical signals for AFM-QTF control and current acquisition are independent. Furthermore, in pursuance of avoiding the electrical contact between the QTF electrode and the conductive tip, we isolated the tungsten tip. Before placing the tip into the holder, the tip is previously covered with an epoxy on the regions to be in physical contact with the QTF electrode. The header we devised allowed us to attach the tip to the QTF prong using a system based on a 3DoF positioning configuration (Fig. 2a). This header incorporates 3-axis control, adjusted by means of three short-step (0.1 mm) screws (Fig. 2b). The suspended metallic tip was brought close to the QTF prong by adjusting the position of these screws (palmers). The whole header system is shown in Fig. 2c and b with a magnified image of the point of contact between the QTF prong and tip.
Fig. 4. (a)–(c) SEM images of the tungsten tips at different magnifications. The estimated tip radius is 250 nm.
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Fig. 5. Time variation of the QTF electrical response. The black line represents the variation in the quality factor over 24 h (1440 min). This variation could be considered null after one hour. The blue line shows the resonance frequency deviation over 24 h. It shows a practically constant resonance frequency over the whole 24 h.
2.3. Electronics We designed specific electronics to control the sample-probe distance via the frequency shift induced by the force of the interaction between them, and to measure the electrical current flowing through the tip (Fig. 3a). The general schema consists of: (a) shear force measurement – the QTF is electrically excited, it works as a shear force nano-sensor, and its oscillation is parallel to the sample surface; (b) a polarized sample – the surface is electrically polarized throughout the (Vbias ); and (c) electrical measurement of the tip-sample current – this is measured through the etched wire by a specific current amplifier. The QTF is electrically excited and the tip-sample distance is controlled in accordance with the measured frequency shift (by converting the piezo current into vibrational amplitude). Electrical excitation is the proper way to drive a QTF as a force nano-sensor if quantification of the QTF mechanical measurements
is needed [29]. A constant tip–sample distance is maintained throughout the QTF frequency shift measurement using the electronics developed and presented in our previous work [24]. The electronics for measuring the current consists of an I–V converter which acquires current through the tip and transforms it to a voltage. The circuit is based on an AD549JH Operational Amplifier: an ultralow input bias current operational amplifier frequently used in I–V circuits. The I–V gain has a multiple user-selectable resistor configuration. The multiple gain configuration allows the most suitable gain to be chosen for each experiment. For C-AFM measurements, the samples have to be polarized. When the sharpened tip is close enough to the polarized surface and its voltage is high enough, a current flow across the gap and through them due to field emission/tunnelling effects (Fig. 3b). The sample is polarized by a bias voltage controlled by the commercial Cervantes System.
Fig. 6. Spectroscopic mode image over an Au (111) surface. These 2D (a) and 3D (b) images show the current variation with tip-sample distance. The Y-axis represents the displacement in the Z-direction and the X-axis represents the voltage variation, from −1 V to 1 V. The current ranges between 10 nA and −9 nA.
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3. Results 3.1. System performance tests
Fig. 7. C-AFM (a) topographical and (b) current images acquired simultaneously over a silicon oxide grid on a highly doped silicon substrate. Both images were taken with 1024 points, at 0.291 lines per second, and with a 35 × 35 mm scan area. The QTF worked at a resonance frequency of 33,627.1 kHz with a Q factor of 301.6. The sample was biased at −10 V.
2.4. Metallic tips The metallic tips selected were commercial tungsten STM tips (TT-ECM10, from Bruker AFM probes); we chose them because previous work [30,31] has shown the importance of using such W tips, and the commercial tungsten tips offer us the possibility for working with a comparable tip radius. The tungsten wires are 0.25 mm in diameter and 14 mm in length; Fig. 4 shows SEM images of them.
In order to test the stability of the system, we recorded different resonance curves of the tip in contact with the QTF over 24 h using our custom made header. Fig. 5 shows the quality factor and the resonant frequency of the device over this long period (24 h). The quality factor decreases by 22% in the initial stages, during the first 30 min, and then remains almost stable. After this first 30 min, there is no significant variation; the resonance frequency increases less than 1%. The graph shows that there is a small deviation from the initial position but after an hour it could be considered that the system remains stable. From one hour until the last measured value (1440 min later: at 24 h), the quality factor and resonance frequency remain practically stable. These variations allow us to take good quality images after the first hour after initial positioning.
3.2. C-AFM Experiments 3.2.1. C-AFM Spectroscopic mode experiments In the spectroscopic mode, the tip is fixed in a specific position. At the same time, current is measured while voltage is ramped
Fig. 8. (a) and (b) are topographical and current images acquired simultaneously over a linear oxide grid with a 20 × 20 um scan area, with the same parameter configuration as in Fig. 3c and d show the corresponding cross-sectional profiles taken along the lines shown in the images. An oxide thickness of around 50 nm is observed, while a current level of around 4.5 nA is recorded over the silicon substrate. The profiles clearly show the difference between the conductive and non-conductive parts of the sample.
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from an initial voltage to a final voltage (I–V curves). Once one I–V curve has been plotted (one line), the tip is moved closer to the sample by a fixed step. All the lines together form the final image which corresponds to I–V traces at different distances between the sample and the tip: each line in the final image corresponds to one step. In the first experiment we acquired a spectroscopic C-AFM image over an Au (1 1 1) structure. The tip was moved 350 nm towards the Au (1 1 1) surface over 256 steps (image of 256 × 256 points). During this stepwise movement, an I–V curve corresponding to a voltage variation from −1 V to 1 V was recorded for each step. The resulting image is shown in Fig. 6a, together with a 3D reconstruction for a better visualization in Fig. 6b From the images it can be clearly seen that when the tip is far from the sample surface (>200 nm) the current is almost zero; as the tip approached the Au (1 1 1) surface an electric current is recorded at shorter distances, the value of the current depends on the applied voltage, until the saturation current of the amplifier is reached (in the present case, 10 nA). The low voltage slope systematically increases as the tip approaches the sample, which is consistent with an electric current generated by tunneling and field emission, and the reduction of the tip-sample gap. The I–V curves qualitatively reflect the physics of tunneling/field emission phenomena, although a quantitative interpretation was not possible, probably due to the effects of contamination in the tip or sample that could affect the curves obtained. So, the spectroscopic mode allowed us to demonstrate that the metallic tip can be maintained at a sufficiently close distance from the sample to observe current conduction by tunneling or field emission effects, and hence, to be able to record conductive images of samples when not in contact with them.
3.3. C-AFM imaging mode experiments In the imaging mode [32], the tip scans the surface at a constant distance (constant tip-sample force throughout the shifting frequency), while the sample is polarized by a constant voltage, which produces a current. In this way, both topographical and electric current images can be obtained. A silicon oxide/Si++ grid was used to test the conductive imaging capabilities of the system. This grid consists of a highly doped silicon substrate onto which micrometric silicon oxide strips have been micro-manufactured. The strips are 7 wide and 50 nm thick, and are 13 apart. The objective of this study was to test whether the QTF C-AFM system could discriminate between conductive and non-conductive parts in the same sample, and simultaneously produce coherent topographical and current images. Fig. 7 shows one of the simultaneous topography and electric current images obtained. As can be seen in Fig. 7a, the sample topography is clearly recorded; while in Fig. 7b the distinct conduction properties of the substrate and of the silicon oxide strips is clearly represented, showing almost no conduction of the silicon oxide parts, as expected. So the images demonstrate the possibility of differentiating conductive and non-conductive parts of the surface of the sample with the QFT-based C-AFM system developed. We note that since the silicon substrate has a natural oxide layer, a relatively high voltage of −10 V was necessary for electrons to cross the barrier. Magnified images are shown in Fig. 8((a) topography and (b) electric current), together with cross-section profiles taken along the lines shown in the images. In this case, the current increased from 0 nA over the non-conductive part, to around 4.5 nA, when tip was measuring the conductive parts. This experiment allows us to visualize electrical responses from the sample while a topographical image is acquired.
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4. Conclusions Here, we report the architecture, design, and validation of a novel customized C-AFM system based on a QTF with a glue-free metallic tip working in shear mode. The tip works simultaneously as a force and current nano-sensor, acquiring topography and electric current images independently. Our system is applied successfully in different experiments that validate its adequate performance. Two different C-AFM modes are demonstrated: spectroscopic and imaging C-AFM modes. We obtain a different range of current values at different distances in the C-AFM spectroscopic mode; and finally, we can distinguish different conductive and nonconductive parts of a sample in the C-AFM imaging mode. Our QTF C-AFM instrument demonstrates that, in contrast to standard cantilever-based scanning probe microscopes, there is no significant influence of the electrostatic force on tip-surface distance control, as expected [33]. This is a consequence of the high spring constant of the tuning fork and also because the interaction is detected in shear mode. The system presented here overcomes most of the limitations of the existing solutions offered in the literature. Although this research offers qualitative verification, the fact that the QTF is electrically excited and resonates with both prongs free, offers us the possibility of quantifying when the experimental essays required it. This would not be compatible with gluing the tip to the QTF device. The results presented in this work show that it is feasible to overcome this limitation with a glue-free solution, if the considerations discussed are taken into account. Acknowledgment This work was supported in part by the Spanish Ministerio de Economía y Competitividad through project TEC2009-10114 and TEC2010-16844. References [1] F. Houzé, R. Meyer, O. Schneegans, L. Boyer, Imaging the local electrical properties of metal surfaces by atomic force microscopy with conducting probes, Appl. Phys. Lett. 69 (1996) 1975. [2] S.J. O’Shea, R.M.M. Atta, P. Murrell, M.E. Welland, Conducting atomic force microscopy study of silicon dioxide breakdown, J. Vac. Sci. Technol. B 13 (1995) 1945. [3] H. Dai, E.W. Wong, Ch. M. Lieber, Probing electrical transport in nanomaterials: conductivity of individual carbon nanotubes, Science 272 (1996) 523. [4] G. Cheng, S. Wang, K. Cheng, X. Jiang, L. Wang, L. Li, Z. Du, G. Zou, The current image of a single CuO nanowire studied by conductive atomic force microscopy, Appl. Phys. Lett. 92 (2008) 223116. [5] D.L. Klein, P.L. McEuen, Conducting atomic force microscopy of alkane layers on graphite, Appl. Phys. Lett. 66 (1995) 2478. [6] T.W. Kelley, E.L. Granstrom, C. Daniel Frisbie, Conducting probe atomic force microscopy: a characterization tool for molecular electronics, Adv. Mater. 11 (1999) 261. [7] I. Casuso, L. Fumagalli, J. Samitier, E. Padros, L. Reggiani, V. Akimov, G. Gomila, Electron transport through supported biomembranes at the nanoscale by conductive atomic force microscopy, Nanotech. 18 (2007) 465503. [8] P. Günther, U.C. Fischer, K. Dransfeld, Scanning near-field acoustic microscopy, Appl. Phys. B 48 (1989) 89–92. [9] R. Göttlich, J.D. Stark, Noncontact scanning force microscopy based on a modification of a tuning fork sensor, Rev. Sci Instrum. 71 (2000) 3104–3107. [10] H.P. Rust, M. Heyde, H.J. Freund, Signal electronics for an atomic force microscope equipped with a double quartz tuning fork sensor, Rev. Sci Instrum. 77 (2006) 43710. [11] J. Otero, G. González, M. Puig-Vidal, Multitool platform for morphology and nanomechanical characterization of biological samples with coordinated self-sensing probes, Mechatron. IEEE/ASME Trans. 18 (2013) 1152–1160. [12] G.Y. Shang, W.H. Qiao, F.H. Lei, J.F. Angiboust, M. Troyon, M. Manfait, Development of a shear force scanning near-field fluorescence microscope for biological applications, Ultramicroscopy 105 (2005) 324–329. [13] M. Hofer, S. Adamsmaier, T.S. van Zanten, L.A. Chtcheglova, C. Manzo, M. Duman, B. Mayer, A. Ebner, M. Moertelmaier, G. Kada, M.F. Garcia-Parajo, P. Hinterdorfer, F. Kienberger, Molecular recognition imaging using tuning fork-based transverse dynamic force microscopy, Ultramicroscopy 110 (2010) 605–611.
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diagnosis medical devices, microrobotics and AFM single-cell nanobiocharacterization for biomedical applications. Dr. G. Gomila obtained his Ph.D. in Physics in 1997. From 1999 to 2001 he was post-doctoral researcher at the three European Universities, and from 2001 to 2005 he was Ramon y Cajal Fellow at the University of Barcelona (Spain). Since 2005 he is Associate Professor at the Department of Electronics of the University of Barcelona (Spain), and since 2007 he is also Group Leader at the Institute for Bioengineering of Catalonia (Spain). His research interests are focused on the development of novel nanoscale electrical measuring techniques for Material Science and Biology, with an emphasis on scanning probe microscopy methods.
Dr. Jorge Otero Díaz was born in Barcelona in 1981. He got his degree in Computer Science from the Politechnical University of Catalonia in 2003, his M.Sc. in Electronics Engineering from the University of Barcelona in 2006 and his M.Sc.in Biomedical Engineering from both in 2008. From 2007 to 2011 he was researcher granted by the Spanish Minsterio de Educación y Ciencia (FPU grant) in the Department of Electronics at UB, and he received his Ph.D. degree in Biomedicine in 2011. Since then, Dr. Otero is assistant professor in the Physics Faculty of this University. Dr. Otero has participated in different European projects in the field of nanosensors for biomedical applications. His research work concerns the development of new sensor technologies for cell and molecular biology
Dr. Laura González received the degree in electrical engineering from the Polytechnic University of Catalonia, Barcelona, Spain, in 2008. She received the M.Sc. degree in biomedical engineering and the Ph.D. degree in Biomedicine from the University of Barcelona (UB), Barcelona, Spain, in 2010 and 2014 respectively. Her research interests include scanning probe microscopies and biosensors development and their associated interface electronics for molecular and cellular biological studies.
Xavier Coromina was born in Olot, Spain, in 1983. He obtained his grade in telecommunications engineering in telematics in Barcelona in 2005 and his grade in Electronic engineering in Barcelona in 2012. Actually he is working as a technician in the group of Dr. Manel Puig at University of Barcelona. In 2014 he began his activity as an associated teacher at University of Barcelona
Biographies
Dr. Manel Puig-Vidal was born in Igualada (Barcelona) Spain, in 1965. He received his Master’s Science degree in Physics from the Barcelona University, in 1988. From 1989 to 1993 he was research fellow in the Laboratoire d’Automatique et d’Analyse des Systèmes (LAAS) in Toulouse, France, working in Latch-up free Smart-Power technology for automotive applications. He received his Ph.D. degree in Microelectronics from the University Paul Sabatier in Toulouse in 1993. In 1995 he has obtained the position of Professor Titular at the University of Barcelona. His academic activity is focused in Electronic Engineering, Informatics Engineering and Biomedical Engineering careers. Dr. Manel Puig-Vidal is the head of the Bioelectronics research group associated to the Institute of Bioengineering of Catalunya (IBEC), Barcelona (Spain). Dr. Manel Puig-Vidal has participated in different European projects in the field of microrobotics for biomedical applications. His research work concerns bioelectronics systems design, portable electronics for in-vitro
Luis Botaya was born in Zaragoza, Spain, in 2984. He obtained his grade in Electronic engineering in Zaragoza, in 2007 and his Mater’s in Biomedical engineering in 2011. Actually he is research fellow in advance technologies applied to biology and biomedical devices. In 2014 he began his activity as an associated teacher at University of Barcelona. His research interests focus in biosensor devices and their development.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Quartz tuning fork-based conductive atomic force microscope with glue-free solid metallic tips Luis Botaya a , Jorge Otero a , Laura González a , Xavier Coromina a , Gabriel Gomila a,b , Manel Puig-Vidal a,∗ a b
Departament d’Electrònica, Universitat de Barcelona, Martí i Franqès, 1, 08028 Barcelona, Spain Institut de Bioenginyeria de Catalunya (IBEC), Balidiri i Reixac 15-21, 08028, Barcelona, Spain
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Article history: Received 24 October 2014 Received in revised form 11 June 2015 Accepted 11 June 2015 Available online 19 June 2015 Keywords: AFM Conductive AFM Quartz tuning fork
a b s t r a c t Here, we devise a conductive Atomic Force Microscope (C-AFM) based on quartz tuning forks (QTFs) and metallic tips capable of simultaneously imaging the topography and conductance of a sample with nanoscale spatial resolution. The system is based on a header design which allows the metallic tip to be placed in tight and stable mechanical contact with the QTF without the need to use any glue. This allows electrical measurements to be taken with an electrically excited QTF with the two prongs free. The amplitude oscillation of the QTF is used to control the tip-sample distance and to acquire the topographic images. Meanwhile, the metallic tip is connected to a current–voltage amplifier circuit to measure the tip-sample field emission/tunneling current and to produce the conductive images. This method allows decoupled electrical measurement of the topography and electrical properties of the sample. The results we obtain from calibration samples demonstrate the feasibility of this measurement method and the adequacy of the performance of the system. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Different applications and studies have indicated the importance of obtaining simultaneous topographical and conductive images of sample surfaces with nanoscale spatial resolution. To this end conductive Atomic Force Microscopy (C-AFM) was developed several years ago [1,2]. In C-AFM, a cantilever-type conductive probe connected to a current amplifier is used to record the local conductivity of the sample, while the topography is recorded via the mechanical deflection of the cantilever. Since its invention, CAFM has been successfully applied in the electrical characterization of a large variety of small-scale systems such as carbon nanotubes [3], semiconductor nanowires [4], self-assembled molecular monolayers [5] and organic layers [6], to cite just a few examples. One of the limitations of conventional C-AFM is the need for the tip to be in physical contact with the sample in order to ensure good electrical contact for the DC current flow. This limits its use with soft or fragile samples, such as biological samples or soft polymers, which can become irreversible damaged during the image acquisition. In order to overcome this limitation, special C-AFM imaging
∗ Corresponding author. E-mail address: [email protected] (M. Puig-Vidal). http://dx.doi.org/10.1016/j.sna.2015.06.006 0924-4247/© 2015 Elsevier B.V. All rights reserved.
modes which minimize lateral shear forces have been developed [7] and applied to bio-samples; but they are still unwieldy to use. An alternative way to overcome the limitation consists of using an AFM system based on a quartz tuning fork (QTF). In such systems, the tip-QTF (nano-tool) is mechanically or electrically excited at its resonance frequency and oscillates parallel to the surface of the sample (shear mode) in close proximity to it (<5 nm distance) but not in contact with it. When a short-range force, caused by the tip-sample distance, interacts with the tip, the frequency at which the nano-sensor is oscillating varies. These frequency variations can be used as a feedback signal to record the sample topography with a constant distance [8]. It has been shown that it is possible to image the sample surface with nanometric accuracy using a QTF as a nano-tool [9,10]; this is primarily due to the excellent quality factor of QTFs [11]. Since the tip of a QTF-based AFM is maintained in close proximity to the sample surface, but not in contact with it, the lateral shear forces on the sample are drastically reduced and this method can safely be used with soft samples [12,13]. In addition, the distance at which the tip is maintained is within the range of tunneling or field emission currents, so that potentially conduction images could also be obtained with this type of system by using a metallic tip and connecting it to a current amplifier. Some work on QFT-based C-AFM systems using metallic tips has been published recently. The studies cover the fabrication of the
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Fig. 1. Schematic representation of the C-AFM system showing the most important elements (the Dulcinea control unit, capacity compensation unit and STM unit) and the signal distribution.
tip [14] and the remaining parts of the setup [15,16], and include validation tests [17,18]. In the published systems, the QTF sensors usually work in a Qplus configuration: one of the QTF prongs is clamped while the QTF is excited mechanically. In contrast to shear mode operation, in this setup the QTF oscillates perpendicular to the sample surface, and senses via a sharp metallic tip that is glued to the surface of one of the QTF prongs and connected to its corresponding electrode (thus both signals, force and current, are obtained through the electrode on that prong) [19]. Such setups have been used to take measurements from an electronic nanocircuit at very low temperatures [20] or from molecules [21]. However, with this system it is hard to obtain topography and electrical images simultaneously, because both signals are registered using the same electrode, which increases the difficulty both of simultaneous signal acquisition and interpretation of the images obtained. Moreover, if one prong of the QTF is clamped, or the excitation is mechanical, quantification of the measurements is challenging because it is very difficult to calculate/calibrate the mechanical coupling between the dither (used to vibrate the device externally) and the QTF, as well as the losses due to clamping the prong. Previous work implemented a setup for shear mode operation [22] in which the tip was glued to one prong of the fork and was connected directly to an I–V preamplifier, independently of the QTF electrode; but the excitation of the QTF was still mechanical. Here, we present a system which has several improvements with respect to those developed previously. The QTF is electrically
excited, instead of mechanically, which enables a better quantitative control of the response of the system. In addition, 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 track the sample topography and the latter to construct the conduction image. Finally, the tip is not glued onto the QTF, but the two are in tight mechanical contact. This glue-free method allows us to use relatively long metallic tips, which could have advantages for future applications (e.g., taking electrical measurements in liquids by just dipping the end of the tip into the liquid). Evaluation of the capabilities of this system, throughout the spectroscopic (I–V curves) and imaging working modes. Evaluation of the capabilities of this system throughout I–V curves and simultaneus topographical and current images provides qualitative verification fo this QTF-based C-AFM system (obtaining topographical and current simultaneus images), provides qualitative verification of this QTF-based C-AFM system. 2. Materials and methods The experiments were carried out using a customized commercial AFM (Cervantes System, Nanotec Electronica, Madrid, Spain) in air at room temperature (25 ◦ C). The commercial system was adapted to work with a QTF as a nano-tool. We devised our custom setup to obtain C-AFM images. The Cervantes System AFM consisted of a piezo scanner (which consists of a large XY scan, 60 × 60
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Fig. 2. (a) Virtual CAD image of the C-AFM holder. (b) Schematic representation of the process for placing the metallic tip in contact with the QTF: the tip can be moved freely in 3 orthogonal directions to ensure good mechanical contact with the QTF. (c) Photograph of the header used. (d) Magnified image of the point of contact between the QTF prong and tip.
um, and a short Z scan, 10 m), a Dulcinea AFM electronic driver and a digital controller with WS × M open Software [23]. The complete electronic setup is represented in Fig. 1 which is a schematic representation and general overview of all our CAFM device and signal information. The QTF excitation signal (AFM) and bias voltage (C-AFM) are controlled by the Cervantes System Dulcinea Controller. A capacity compensation circuit is used to minimize parasitic capacitances and amplify the QTF output current via a trans-impedance amplifier [24]. Finally, the amplified current is fed back to the Dulcinea module. Meanwhile, the current unit acquires the electric current through the tip and amplifies it in the I–V circuit; the resultant voltage is introduced into the Dulcinea module via external input signal connectors. 2.1. Quartz tuning forks QTF devices are mostly used in precise oscillation circuits. The QTF resonance frequency depends on its effective mass and its spring constant [25]. We used commercial QTFs encapsulated in vacuum conditions (with a resonance frequency of 32,768 Hz). To use these QTFs as AFM nano-sensors they must be removed from
their metallic capsules and the sharpened tip (for the atomic interaction with the sample) must be placed in mechanical contact with one prong of the QTF. This physical contact produces a variation of the QTF resonance frequency due to the changes in the effective mass and spring constant of the device. When a force acts on the nano-sensor, formed by the QTF–metallic tip junction, 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. Since the tip is not a cantilever type, this distance can be less than a few nanometres without it collapsing; something that is not possible with cantilever-based AFM systems. In addition, local electrical information can be obtained if the tip is made of a conductive material and it is connected to a current amplifier [26]. It should be noted, however, that metallic tips are heavier than fibre 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 which make it extremely difficult to use it as a nano-sensor. We have avoided these problems fixing the tip to the QTF prong without glue.
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Fig. 3. (a) Working mode diagram of the C-AFM set-up. This schematic representation gives an overview of the different parts of the set-up. (b) C-AFM working mode of the QFT-based system. A difference in potential (Vbias) is applied between the tip and the sample. As soon as the tip and sample are close enough (a few nm), and if an adequate voltage is applied to the sample, electrons can jump the gap between the tip and sample due to tunnelling and/or field emission effects.
2.2. Header for C-AFM measurements We have chosen tungsten tips for the conductive measurements because they are one of the most suitable tip types used for this kind of measures. However, tungsten tips are heavier and more rigid tips than fibers ones (most commonly tips used with QTF nanoprobes). In addition, the system has been designed to work with large tips, and hence still heavier, because we want the system to be also compatible with liquid measurements avoiding the immersion of the quartz tuning fork. Furthermore, the conductive tip can be straightaway connected to the I–V amplifier. All these requirements, prevent the tungsten tip to be glued to the QTF prong. In order to overcome these difficulties, we devised a new header system, inspired by pervious work [27,28], for positioning the tip in physical contact with the QTF prong in a fixed and stable way. In
the system, the input/output electrical signals for AFM-QTF control and current acquisition are independent. Furthermore, in pursuance of avoiding the electrical contact between the QTF electrode and the conductive tip, we isolated the tungsten tip. Before placing the tip into the holder, the tip is previously covered with an epoxy on the regions to be in physical contact with the QTF electrode. The header we devised allowed us to attach the tip to the QTF prong using a system based on a 3DoF positioning configuration (Fig. 2a). This header incorporates 3-axis control, adjusted by means of three short-step (0.1 mm) screws (Fig. 2b). The suspended metallic tip was brought close to the QTF prong by adjusting the position of these screws (palmers). The whole header system is shown in Fig. 2c and b with a magnified image of the point of contact between the QTF prong and tip.
Fig. 4. (a)–(c) SEM images of the tungsten tips at different magnifications. The estimated tip radius is 250 nm.
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Fig. 5. Time variation of the QTF electrical response. The black line represents the variation in the quality factor over 24 h (1440 min). This variation could be considered null after one hour. The blue line shows the resonance frequency deviation over 24 h. It shows a practically constant resonance frequency over the whole 24 h.
2.3. Electronics We designed specific electronics to control the sample-probe distance via the frequency shift induced by the force of the interaction between them, and to measure the electrical current flowing through the tip (Fig. 3a). The general schema consists of: (a) shear force measurement – the QTF is electrically excited, it works as a shear force nano-sensor, and its oscillation is parallel to the sample surface; (b) a polarized sample – the surface is electrically polarized throughout the (Vbias ); and (c) electrical measurement of the tip-sample current – this is measured through the etched wire by a specific current amplifier. The QTF is electrically excited and the tip-sample distance is controlled in accordance with the measured frequency shift (by converting the piezo current into vibrational amplitude). Electrical excitation is the proper way to drive a QTF as a force nano-sensor if quantification of the QTF mechanical measurements
is needed [29]. A constant tip–sample distance is maintained throughout the QTF frequency shift measurement using the electronics developed and presented in our previous work [24]. The electronics for measuring the current consists of an I–V converter which acquires current through the tip and transforms it to a voltage. The circuit is based on an AD549JH Operational Amplifier: an ultralow input bias current operational amplifier frequently used in I–V circuits. The I–V gain has a multiple user-selectable resistor configuration. The multiple gain configuration allows the most suitable gain to be chosen for each experiment. For C-AFM measurements, the samples have to be polarized. When the sharpened tip is close enough to the polarized surface and its voltage is high enough, a current flow across the gap and through them due to field emission/tunnelling effects (Fig. 3b). The sample is polarized by a bias voltage controlled by the commercial Cervantes System.
Fig. 6. Spectroscopic mode image over an Au (111) surface. These 2D (a) and 3D (b) images show the current variation with tip-sample distance. The Y-axis represents the displacement in the Z-direction and the X-axis represents the voltage variation, from −1 V to 1 V. The current ranges between 10 nA and −9 nA.
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3. Results 3.1. System performance tests
Fig. 7. C-AFM (a) topographical and (b) current images acquired simultaneously over a silicon oxide grid on a highly doped silicon substrate. Both images were taken with 1024 points, at 0.291 lines per second, and with a 35 × 35 mm scan area. The QTF worked at a resonance frequency of 33,627.1 kHz with a Q factor of 301.6. The sample was biased at −10 V.
2.4. Metallic tips The metallic tips selected were commercial tungsten STM tips (TT-ECM10, from Bruker AFM probes); we chose them because previous work [30,31] has shown the importance of using such W tips, and the commercial tungsten tips offer us the possibility for working with a comparable tip radius. The tungsten wires are 0.25 mm in diameter and 14 mm in length; Fig. 4 shows SEM images of them.
In order to test the stability of the system, we recorded different resonance curves of the tip in contact with the QTF over 24 h using our custom made header. Fig. 5 shows the quality factor and the resonant frequency of the device over this long period (24 h). The quality factor decreases by 22% in the initial stages, during the first 30 min, and then remains almost stable. After this first 30 min, there is no significant variation; the resonance frequency increases less than 1%. The graph shows that there is a small deviation from the initial position but after an hour it could be considered that the system remains stable. From one hour until the last measured value (1440 min later: at 24 h), the quality factor and resonance frequency remain practically stable. These variations allow us to take good quality images after the first hour after initial positioning.
3.2. C-AFM Experiments 3.2.1. C-AFM Spectroscopic mode experiments In the spectroscopic mode, the tip is fixed in a specific position. At the same time, current is measured while voltage is ramped
Fig. 8. (a) and (b) are topographical and current images acquired simultaneously over a linear oxide grid with a 20 × 20 um scan area, with the same parameter configuration as in Fig. 3c and d show the corresponding cross-sectional profiles taken along the lines shown in the images. An oxide thickness of around 50 nm is observed, while a current level of around 4.5 nA is recorded over the silicon substrate. The profiles clearly show the difference between the conductive and non-conductive parts of the sample.
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from an initial voltage to a final voltage (I–V curves). Once one I–V curve has been plotted (one line), the tip is moved closer to the sample by a fixed step. All the lines together form the final image which corresponds to I–V traces at different distances between the sample and the tip: each line in the final image corresponds to one step. In the first experiment we acquired a spectroscopic C-AFM image over an Au (1 1 1) structure. The tip was moved 350 nm towards the Au (1 1 1) surface over 256 steps (image of 256 × 256 points). During this stepwise movement, an I–V curve corresponding to a voltage variation from −1 V to 1 V was recorded for each step. The resulting image is shown in Fig. 6a, together with a 3D reconstruction for a better visualization in Fig. 6b From the images it can be clearly seen that when the tip is far from the sample surface (>200 nm) the current is almost zero; as the tip approached the Au (1 1 1) surface an electric current is recorded at shorter distances, the value of the current depends on the applied voltage, until the saturation current of the amplifier is reached (in the present case, 10 nA). The low voltage slope systematically increases as the tip approaches the sample, which is consistent with an electric current generated by tunneling and field emission, and the reduction of the tip-sample gap. The I–V curves qualitatively reflect the physics of tunneling/field emission phenomena, although a quantitative interpretation was not possible, probably due to the effects of contamination in the tip or sample that could affect the curves obtained. So, the spectroscopic mode allowed us to demonstrate that the metallic tip can be maintained at a sufficiently close distance from the sample to observe current conduction by tunneling or field emission effects, and hence, to be able to record conductive images of samples when not in contact with them.
3.3. C-AFM imaging mode experiments In the imaging mode [32], the tip scans the surface at a constant distance (constant tip-sample force throughout the shifting frequency), while the sample is polarized by a constant voltage, which produces a current. In this way, both topographical and electric current images can be obtained. A silicon oxide/Si++ grid was used to test the conductive imaging capabilities of the system. This grid consists of a highly doped silicon substrate onto which micrometric silicon oxide strips have been micro-manufactured. The strips are 7 wide and 50 nm thick, and are 13 apart. The objective of this study was to test whether the QTF C-AFM system could discriminate between conductive and non-conductive parts in the same sample, and simultaneously produce coherent topographical and current images. Fig. 7 shows one of the simultaneous topography and electric current images obtained. As can be seen in Fig. 7a, the sample topography is clearly recorded; while in Fig. 7b the distinct conduction properties of the substrate and of the silicon oxide strips is clearly represented, showing almost no conduction of the silicon oxide parts, as expected. So the images demonstrate the possibility of differentiating conductive and non-conductive parts of the surface of the sample with the QFT-based C-AFM system developed. We note that since the silicon substrate has a natural oxide layer, a relatively high voltage of −10 V was necessary for electrons to cross the barrier. Magnified images are shown in Fig. 8((a) topography and (b) electric current), together with cross-section profiles taken along the lines shown in the images. In this case, the current increased from 0 nA over the non-conductive part, to around 4.5 nA, when tip was measuring the conductive parts. This experiment allows us to visualize electrical responses from the sample while a topographical image is acquired.
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4. Conclusions Here, we report the architecture, design, and validation of a novel customized C-AFM system based on a QTF with a glue-free metallic tip working in shear mode. The tip works simultaneously as a force and current nano-sensor, acquiring topography and electric current images independently. Our system is applied successfully in different experiments that validate its adequate performance. Two different C-AFM modes are demonstrated: spectroscopic and imaging C-AFM modes. We obtain a different range of current values at different distances in the C-AFM spectroscopic mode; and finally, we can distinguish different conductive and nonconductive parts of a sample in the C-AFM imaging mode. Our QTF C-AFM instrument demonstrates that, in contrast to standard cantilever-based scanning probe microscopes, there is no significant influence of the electrostatic force on tip-surface distance control, as expected [33]. This is a consequence of the high spring constant of the tuning fork and also because the interaction is detected in shear mode. The system presented here overcomes most of the limitations of the existing solutions offered in the literature. Although this research offers qualitative verification, the fact that the QTF is electrically excited and resonates with both prongs free, offers us the possibility of quantifying when the experimental essays required it. This would not be compatible with gluing the tip to the QTF device. The results presented in this work show that it is feasible to overcome this limitation with a glue-free solution, if the considerations discussed are taken into account. Acknowledgment This work was supported in part by the Spanish Ministerio de Economía y Competitividad through project TEC2009-10114 and TEC2010-16844. References [1] F. Houzé, R. Meyer, O. Schneegans, L. Boyer, Imaging the local electrical properties of metal surfaces by atomic force microscopy with conducting probes, Appl. Phys. Lett. 69 (1996) 1975. [2] S.J. O’Shea, R.M.M. Atta, P. Murrell, M.E. Welland, Conducting atomic force microscopy study of silicon dioxide breakdown, J. Vac. Sci. Technol. B 13 (1995) 1945. [3] H. Dai, E.W. Wong, Ch. M. Lieber, Probing electrical transport in nanomaterials: conductivity of individual carbon nanotubes, Science 272 (1996) 523. [4] G. Cheng, S. Wang, K. Cheng, X. Jiang, L. Wang, L. Li, Z. Du, G. Zou, The current image of a single CuO nanowire studied by conductive atomic force microscopy, Appl. Phys. Lett. 92 (2008) 223116. [5] D.L. Klein, P.L. McEuen, Conducting atomic force microscopy of alkane layers on graphite, Appl. Phys. Lett. 66 (1995) 2478. [6] T.W. Kelley, E.L. Granstrom, C. Daniel Frisbie, Conducting probe atomic force microscopy: a characterization tool for molecular electronics, Adv. Mater. 11 (1999) 261. [7] I. Casuso, L. Fumagalli, J. Samitier, E. Padros, L. Reggiani, V. Akimov, G. Gomila, Electron transport through supported biomembranes at the nanoscale by conductive atomic force microscopy, Nanotech. 18 (2007) 465503. [8] P. Günther, U.C. Fischer, K. Dransfeld, Scanning near-field acoustic microscopy, Appl. Phys. B 48 (1989) 89–92. [9] R. Göttlich, J.D. Stark, Noncontact scanning force microscopy based on a modification of a tuning fork sensor, Rev. Sci Instrum. 71 (2000) 3104–3107. [10] H.P. Rust, M. Heyde, H.J. Freund, Signal electronics for an atomic force microscope equipped with a double quartz tuning fork sensor, Rev. Sci Instrum. 77 (2006) 43710. [11] J. Otero, G. González, M. Puig-Vidal, Multitool platform for morphology and nanomechanical characterization of biological samples with coordinated self-sensing probes, Mechatron. IEEE/ASME Trans. 18 (2013) 1152–1160. [12] G.Y. Shang, W.H. Qiao, F.H. Lei, J.F. Angiboust, M. Troyon, M. Manfait, Development of a shear force scanning near-field fluorescence microscope for biological applications, Ultramicroscopy 105 (2005) 324–329. [13] M. Hofer, S. Adamsmaier, T.S. van Zanten, L.A. Chtcheglova, C. Manzo, M. Duman, B. Mayer, A. Ebner, M. Moertelmaier, G. Kada, M.F. Garcia-Parajo, P. Hinterdorfer, F. Kienberger, Molecular recognition imaging using tuning fork-based transverse dynamic force microscopy, Ultramicroscopy 110 (2010) 605–611.
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diagnosis medical devices, microrobotics and AFM single-cell nanobiocharacterization for biomedical applications. Dr. G. Gomila obtained his Ph.D. in Physics in 1997. From 1999 to 2001 he was post-doctoral researcher at the three European Universities, and from 2001 to 2005 he was Ramon y Cajal Fellow at the University of Barcelona (Spain). Since 2005 he is Associate Professor at the Department of Electronics of the University of Barcelona (Spain), and since 2007 he is also Group Leader at the Institute for Bioengineering of Catalonia (Spain). His research interests are focused on the development of novel nanoscale electrical measuring techniques for Material Science and Biology, with an emphasis on scanning probe microscopy methods.
Dr. Jorge Otero Díaz was born in Barcelona in 1981. He got his degree in Computer Science from the Politechnical University of Catalonia in 2003, his M.Sc. in Electronics Engineering from the University of Barcelona in 2006 and his M.Sc.in Biomedical Engineering from both in 2008. From 2007 to 2011 he was researcher granted by the Spanish Minsterio de Educación y Ciencia (FPU grant) in the Department of Electronics at UB, and he received his Ph.D. degree in Biomedicine in 2011. Since then, Dr. Otero is assistant professor in the Physics Faculty of this University. Dr. Otero has participated in different European projects in the field of nanosensors for biomedical applications. His research work concerns the development of new sensor technologies for cell and molecular biology
Dr. Laura González received the degree in electrical engineering from the Polytechnic University of Catalonia, Barcelona, Spain, in 2008. She received the M.Sc. degree in biomedical engineering and the Ph.D. degree in Biomedicine from the University of Barcelona (UB), Barcelona, Spain, in 2010 and 2014 respectively. Her research interests include scanning probe microscopies and biosensors development and their associated interface electronics for molecular and cellular biological studies.
Xavier Coromina was born in Olot, Spain, in 1983. He obtained his grade in telecommunications engineering in telematics in Barcelona in 2005 and his grade in Electronic engineering in Barcelona in 2012. Actually he is working as a technician in the group of Dr. Manel Puig at University of Barcelona. In 2014 he began his activity as an associated teacher at University of Barcelona
Biographies
Dr. Manel Puig-Vidal was born in Igualada (Barcelona) Spain, in 1965. He received his Master’s Science degree in Physics from the Barcelona University, in 1988. From 1989 to 1993 he was research fellow in the Laboratoire d’Automatique et d’Analyse des Systèmes (LAAS) in Toulouse, France, working in Latch-up free Smart-Power technology for automotive applications. He received his Ph.D. degree in Microelectronics from the University Paul Sabatier in Toulouse in 1993. In 1995 he has obtained the position of Professor Titular at the University of Barcelona. His academic activity is focused in Electronic Engineering, Informatics Engineering and Biomedical Engineering careers. Dr. Manel Puig-Vidal is the head of the Bioelectronics research group associated to the Institute of Bioengineering of Catalunya (IBEC), Barcelona (Spain). Dr. Manel Puig-Vidal has participated in different European projects in the field of microrobotics for biomedical applications. His research work concerns bioelectronics systems design, portable electronics for in-vitro
Luis Botaya was born in Zaragoza, Spain, in 2984. He obtained his grade in Electronic engineering in Zaragoza, in 2007 and his Mater’s in Biomedical engineering in 2011. Actually he is research fellow in advance technologies applied to biology and biomedical devices. In 2014 he began his activity as an associated teacher at University of Barcelona. His research interests focus in biosensor devices and their development.