Schottky barrier based silicon nanowire ph sensor with live sensitivity control

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1 Nano Research Nano Res 1 DOI /s Schottky barrier based silicon nanowire ph sensor with live sensitivity control Felix M. Zörgiebel, Sebastian Pregl, Lotta Römhildt, Jörg Opitz, W. Weber, T. Mikolajick, Larysa Baraban ( ), and Gianaurelio Cuniberti Nano Res., Just Accepted Manuscript DOI: /s on November 22, 2013 Tsinghua University Press 2013 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.

2 TABLE OF CONTENTS (TOC) Schottky barrier based silicon nanowire ph sensor with live sensitivity control Felix M. Zörgiebel, Sebastian Pregl, Lotta Römhildt, Jörg Opitz, W. Weber, T. Mikolajick, Larysa Baraban,* and Gianaurelio Cuniberti Institute for Materials Science and Max Bergmann Center of Biomaterials, TU Dresden, Dresden, Germany We demonstrate a ph sensor based on ultrasensitive nanosized Schottky junctions formed within bottom up grown dopant free arrays of assembled silicon nanowires and present a new measurement concept allowing to perform experiments in the optimum sensitivity regime. Provide the authors website if possible. 1

3 Nano Research DOI (automatically inserted by the publisher) Research Article Please choose one Schottky barrier based silicon nanowire ph sensor with live sensitivity control Felix M. Zörgiebel, 1,5 Sebastian Pregl, 1,5 Lotta Römhildt, 1 Jörg Opitz, 3 W. Weber, 2,5 T. Mikolajick, 4,5 Larysa Baraban, 1 (*) and Gianaurelio Cuniberti 1,5 1 Institute for Materials Science and Max Bergmann Center of Biomaterials, TU Dresden, Dresden (Germany) 2 NaMLab GmbH, Dresden (Germany) 3 Fraunhofer Institute IZFP Dresden, Dresden (Germany) 4 Institute for Semiconductors and Microsystems Technology, TU Dresden, 01187, Dresden (Germany) 5 Dresden, Germany, Center for Advancing Electronics Dresden, TU Dresden, Dresden, Germany Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) Tsinghua University Press and Springer Verlag Berlin Heidelberg 2011 ABSTRACT We demonstrate a ph sensor based on ultrasensitive nanosized Schottky junctions formed within bottom up grown dopant free arrays of assembled silicon nanowires. A new measurement concept relying on continuous gate sweep is presented, which allows to determine straightforwardly the point of maximum sensitivity of the device and to perform sensing experiments in the optimum regime. Integration of devices into a portable fluidic system and electrode isolation strategy permits stable environment and enables the long time robust FET sensing measurements in a liquid environment. Investigations of the physical and chemical sensitivity of our devices for changing ph value and their comparison to theoretical limits are discussed as well. We believe that such a combination of the nanofabrication and engineering advances make this Schottky barrier powered silicon nanowires lab on a chip platform suitable for efficient biodetection and even for more complex biochemical analysis. KEYWORDS Silicon nanowires, field effect transistor, sub threshold regime, nanosensors, ph sensor, bottom up fabrication, maximum sensitivity of sensor Address correspondence to L. Baraban, larysa.baraban@nano.tu-dresden.de 2

4 1. Introduction Biosensors relying on electrical signal readout have attracted great attention in the last decades since they can provide rich quantitative information for medical and biotechnological assays without pretreatment and specific labeling of analyte solutions. Sensing of chemical and biological species using field effect transistors (FET) goes back to the 1970s [1], showing that such electronic configuration can represent a key technology for chemical and biodetection area because of its high sensitivity and CMOS compatibility. One of the prominent examples, a so called ion sensitive field effect transistor has been used for measuring ion concentrations, i.e. protons in solution. In this configuration, change of the transistor current is detected, upon the changes of ph values of the liquid placed on the device [2 4]. This concept was a technological novelty and represented a more sensitive alternative to the existing method, i.e. ph indicators employing halochromic compounds [5]. Biological species ranging from DNA [6 10] up to proteins (isolated and as viral surface protein) [11, 12], cells [13], and cultured neurons [14, 15] have since been measured using FET devices, ranging from metal oxide semiconductor field effect transistors (MOSFETs) [16] to nanoribbons, [17, 18] doped nanowires [19] and carbon nanotubes [20]. During the past decade one dimensional nanostructures, in particular semiconductor nanowires, have attracted attention as highly efficient sensor elements due to their high surface tovolume ratio and electronic properties [21 23], which enables the detection of biochemical species down to single molecules [2, 11, 12]. Some of the main issues, which impede the straightforward commercialization of nanowire based sensor devices are related to (i) device to device variations in current and sensitivity of bottom up wires and hence calibration problems, (ii) low current output, and (iii) electronic signal drifts and quick device degradation. Here we introduce the first bottom up fabricated Schottky barrier FET consisting of parallel arrays of silicon nanowires, suitable for robust sensing applications in liquid environment. Furthermore, we introduce a new measurement approach making a maximum of information available during the experiment. The method relies on a continuous gate sweep and allows us to follow the region of highest sensitivity during the measurement. As a first application we demonstrate the performance of Schottky barrier (SB) based silicon nanowire FET device for sensing the ph values of a solution. 2. Results and discussions 1) Fabrication of Schottky barrier SiNW sensor Fabrication procedure of the FET devices is summarized in Figure 1. Sensor devices consist of parallel arrays of pre assembled bottom up fabricated Schottky barrier (SB) silicon nanowires (SiNWs), covered by a 6 nm thin layer of thermal oxide. Devices are produced at the p doped silicon wafer with 100 nm and 400 nm back gate dielectric thicknesses (see below). In contrast to top down fabricated SB FETs [24], we fabricate Schottky junctions using bottom up approach, by thermal annealing of silicon nanowires assembled between nickel electrodes [25, 26]. A nanoscopic metal semiconductor interface appears within the nanowire due to axial diffusion of nickel and local formation of nickel silicide. This interface is not buried below a metal electrode, but is exposed to a liquid phase during ph measurements. Figure 1A shows an electron microscopy image of a small part of a parallel array of SiNW FETs with two exemplary Schottky junctions marked by yellow circles. The manufacturing of such nanosized SB is highly reproducible, since it depends only on the nanowire diameter, which is well controlled by a synthesis procedure, as well as on annealing time and temperature [26]. Therefore, the silicidation length and, thus the length of the channel of FET is similar for all nanowires in a parallel array of SiNWs. According to the statistical analysis, presented in our previous work [25], a device can consist of up to 10 3 contacted nanowires in parallel. More details on device fabrication are provided in the section Supporting information (see Figure S3). Because of avoidance of dopants during nanowires synthesis, the Debye screening length of the channel is substantially exceeding the nanowire diameter [29, 30]. Therefore gate fields can efficiently penetrate in the silicon channel and Schottky contacts formed at 3

5 the Si/NiSi2 interfaces, leading to a FET behavior with high on/off current ratios [29]. Electrical sensitivity of the nanowire FETs to changes of the electric field in the liquid is localized at the Schottky junctions, which have been already shown by probing SBs in dry states with top gates, scanning gate atomic force microscopy (AFM) measurements and several theoretical investigations [26 29]. The high reproducibility of the production process enables us to contact large numbers of wires in parallel without substantially sacrificing electrical performance of the complete device. This revolutionizes bottom up fabrication of SB based silicon nanowire biosensors for measurements in liquid surrounding. Note that previously reported SB nanowire FETs were mainly suited for dry state measurements because the sensitive Schottky junctions were situated at the metal contact pads, which were either not electrically isolated against electrochemical reactions and thus non usable for measurements in liquid [30, 32], or isolated and thus inaccessible for molecules at the sensitive sites, yielding low surface charge sensitivity [24]. The electrical isolation of metal leads of SB based nanowires sensors is provided by a 100 nm thick layer of photoresist (AR N 4340 S5, Allresists) with microfabricated windows to expose the nanowires and SBs to the liquid environment. The photoresist passivation alignment is shown in the confocal microscope (Keyence VK X200) in Figure 1B. The alignment accuracy together with the well known length of the NiSi2 phases of the wires permits a complete exposure of Schottky barriers to the liquid to be measured. A fluidic channel manufactured in polydimethylsiloxane (PDMS, Dow Corning Sylgard 184 ) was finally attached to the chip by mechanical pressure using a custom made mechanical device, as shown in Figure 1C. The potential of the liquid is controlled by a commercial Ag/AgCl reference electrode (Microelectrodes Inc., USA) that is built into the fluidic capillary tubing in the close vicinity to the sensor chip. The source and drain electrodes are contacted in a tip probe station. 2) Sensor characterization The electrical wiring scheme of the sensor is shown in Figure 1D. The origin and physical meaning of the elements in the scheme are introduced below. The physical mechanism for nanowire based sensor signal is caused by surface charge induced modulation of the gating field in the nanowire, i.e. as for typical ion sensitive field effect transistors [29, 33]. Once exposed to solutions with various ph values, the gating field in the FET is generated by a backgate potential Vbg, the liquid potential Vliquid, and the surface potential Vsurface, which is affected by the ph changes as: Vsurface =Vliquid α 59.5mV (ph pi), (1) where α is the relative surface sensitivity according to the site binding model with α 1 defining the Nernst limit of the surface potential kbt/e ln(10)=59.5mv/ph; and pi is the isoelectric point of the surface. The electric potential in the active region of the FET can be described by coupling capacitance weighted addition of the backgate potential and the surface potential with capacitances Cbg and Csurface (Figure 1D) [34, 35] Φ = (Vbg Cbg + Vsurface Csurface) / (Cbg + Csurface). (2) In the sub threshold regime, the logarithm of the current at fixed source drain voltage, further on abbreviated with deci = log10isd, is linearly dependent on the electrical potential Φ due to the thermal motion of electrons, with the curve steepness limited by the same numerical constant as the Nernst limit [30] Φ/ deci = β 59.5mV. In this equation β 1, the gate coupling factor, determines the effectiveness of applied electric potentials, with β=1 for the case of ideal device. The minus sign results from the positive charge of holes, which contribute to the FET conduction close to 0V gate voltage, although Schottky barrier based FET devices used in our experiment are ambipolar [29]. In order to study the gate coupling efficiency the electrical characteristics of parallel arrays of Schottky barrier SiNW FETs were measured in dry conditions and in phosphate buffer. The source drain current Isd versus gate voltage curves in both conditions are summarized in Figure 2. In this graph the horizontal (voltage) axis was scaled to display the two measurements according to the fitted slopes in the sub threshold regime. Blue curve displays the I V 4

6 characteristics of the SB silicon nanowires device measured in dry state, revealing the slope of about 950mV/decI. Red curve demonstrates the Isd dependence in liquid state with the slope 127mV/ deci, respectively. The back gate and liquid electrode were set to the same potential Vg = Vbg = Vliquid. The gate coupling increased by a factor of 7.5 in liquid conditions and corresponding the device quality parameter becomes β = The gate capacitance ratio for SB silicon nanowires devices, fabricated at wafers with back gate dielectrics of 100 nm and 400 nm thickness and with taking into account the thickness of an oxide shell of nanowires 6 nm, is expected to be Cbg/Csurface=0.05 and , respectively. 3) Continuous gate sweeping Conventionally, the sensing measurements FET configurations are realized with the fixed gate voltage Vg. In order to carry out quantitative measurements in optimal regime, we proposed a new approach to detect signal changes in the FET sensor by continuously sweeping the gate voltage with a triangular signal and recording the sourcedrain current during each sweep (100 data points per sweep). This method allows the extraction of the threshold voltage at a fixed source drain current from the recorded data. The voltage range is chosen such that the complete switching characteristic of the FET device is recorded in each sweep. The extraction of the threshold voltage at a fixed source drain current from the recorded data is possible. The benefits of this method are: (i) all information available in Isd (Vg) can be obtained; (ii) since a large range of currents is recorded, the threshold current with maximum sensitivity can be chosen for threshold voltage analysis; (iii) random drifts within the device hysteresis are reduced since maxima and minima of the hysteresis are passed in each sweep (drifts from other sources are not eliminated by this). We provide the comparative ph sensing measurements and sensitivity analysis using new gate sweeping approach and conventional constantgate potential method. 4) ph sensing with SB SiNW device Physical aspects: maximizing sensitivity As introduced in the previous section and Eq. (1) and (2), influence of ph values of the liquid on the surface potential Vsurface determines the physical basis for the sensitivity of the nanowires based devices. Sensitivity of current change versus ph changes is represented as S = deci/ ph = (α/β) (1 + Cbg/Csurface) 1 (3) Thus, the maximum current sensitivity Smax=1 can be achieved only for a fully activated surface (α=1), an ideal FET device (β=1), and a dominating surface capacitance, Csurface Cbg. The interesting consequence of the Eq. (3) is that use of the ideal FET device with the large nanowires surface capacitance leads to a linear scaling of the current with the ion concentration in solution. The estimated current sensitivity of FET devices fabricated for our experiments is limited to S= of the linear limit due to the high back gate capacitance. We applied the gate sweeping method to the detection of ph changes with our silicon nanowire sensor devices (see Figure 3). The source drain current versus gate voltage Vg and time during the course of a ph sensing experiment on a sensor chip with 100 nm back gate dielectric is demonstrated in Figure 3A. In order to better visualize the modulation of the current upon ph and gate voltage changes, we employed the color mapping of the recorded signal. Source drain current Isd was extracted from these data at Vg = 0V and is plotted as a function of ph values (Figure 3B, blue crosses). Linear fitting of the obtained curve (dashed line) for low ph values and low currents, i.e. in the subthreshold regime, allows to derive a maximum sensitivity S 1/3 of the SB based device. This is on the order of magnitude of the theoretical limit S = 1 (or deci ph), displayed in Figure 3B by the dotdashed line and greatly exceeds sensitivities previously reported for top down fabricated Schottky barrier silicon nanowire ph sensors [24]. The non linearity of the Isd obtained for the range of higher ph and current values is caused by the typical nonlinearity of the FET switching behavior. In order to investigate in detail the sensitivity of the device in solutions with ph= values, we 5

7 fabricated a device with 400 nm back gate dielectric, which lead to a more linear current response. The current sensitivity versus ph change was determined for all applied gate voltages by linear fitting S= deci/ ph to the measured data. The evolution of the sensitivity versus gate voltage Vg is plotted in Figure 3C, exhibiting a maximum at Vg = 0.25V (red circle in Figure 3C), in the sub threshold regime, similar to values reported by Gao et al [30]. Current versus ph values graphs for three exemplary gate voltages (0.2V, 0.5V, and 0.7V) are shown in the insets with the respective gate voltage indicated. Naturally, the sensitivity of the device can be only judged in relation to their standard deviations σs and signal to noise ratio S/σS, which were analyzed from the fitting procedure based on the standard deviation of the currents measured for each ph value. The signal to noise ratio has a plateau like shape for low values of gate voltages Vg (from 0.2V to 0.2V), and sharply declines for higher voltages (see gray plot in Figure 3C). The maximal sensitivity S of the reported device and signal to noise ratio S/σS are thus overlapping only for a small gate voltage range. The reason for this behavior is related to the absolute values of the Isd current. The highest sensitivity is measured at the highest slope of the FET switching characteristics; however this point coincides with low Isd levels. On the other hand, lower sensitivities S in conjunction with higher current levels lead to the same quality of sensing. This statement allows to conclude that the previously assumed importance of the sub threshold regime for optimized sensing [30], is rather relative. In order to prove the efficiency of the developed gate sweeping approach for FET sensing, we further compared our technique with the conventional constant gate potential sensing method. This is realized by subsequent gate sweep and constant liquid gate potential (Vg = 0V) experiments, applied to the same device for the same ph solutions. The responses of the device are summarized in Figure 3D, where source drain currents Isd are plotted versus ph values. A sensitivity of S = 0.08 was determined for the fixed gating voltage measurement, while the sensitivity extracted from the gate sweep at Vg=0V was higher (S = 0.122). It must be noted that the current levels for both measurements were differing. The difference of the sensitivity values can therefore be explained by a signal drift between the two measurements, but not a general change of experimental conditions, which were held constant. This result underlines that fixing a constant gate voltage might result in measurements out of the range of optimal gate voltage regime. Chemical aspects: surface potential measurement More suitable for ph sensing experiments is the measurement of the surface potential on the ion sensitive FET. In such configuration the threshold gate voltage Vt, that fixes the source drain current Isd to a constant threshold value It, is measured continuously. In our setup, where the back gate and the liquid electrode are set to the same potential the threshold voltage change with ph Vt, can be represented as Vt / ph = α 59.5mV (1 + Cbg/Csurface) 1, according to Eqs. (1) and (2) [36]. Changes of the surface potential in ion sensitive FET can be therefore determined as ΔVsurface = ΔVt (1 + Cbg/Csurface) (4) The absolute value of the surface potential is obtained by determining isoelectric point of the nanowires surface pi, which defines Vsurface(pI) = 0V and thus Vsurface = ΔVsurface ΔVsurface(pI). In order to determine the pi value, we measured zeta potential of silicon nanowires in solution at different ph values and conclude that isoelectric point of silicon nanowires is reached at ph=4.8 (see Figure S2 in Supporting Information). With the new gate sweep approach we can, in parallel to current measurements, extract the shift of the threshold voltage from each measured curve, and therefore determine the surface potential in timedomain. We developed and employed an analysis method that enables to extract automatically all necessary parameters of the measurements (e.g. a threshold current, sensitivity and signal to noise ratio), utilizing the full gate sweep data with highest sensitivity to gate voltage regions (see Supporting information, Figure S1, Figure S3, S4). To derive the surface potential changes, silicon nanowire sensor devices were exposed to buffer solutions between ph = 1 and ph = 12 using gate 6

8 sweep regime of measurements. Source drain current Isd was recorded at a frequency of 0.81 s 1, as a function of gate voltage. Figure 4 displays the surface potentials, which are plotted for two devices with 100 nm thick (main plot) and 400 nm thick (inset) back gate dielectric. Dashed lines are linear fits to the data, the dash dotted lines mark the Nernst limit of 59.5 mv/ph. Two principal regimes can be discerned: below ph=6, the slope was fitted to 37.17mV/pH, while above ph=6 the corresponding value is only mv/ph for 100 nm and mv/ph for 400 nm back gate dielectric. Accordingly, the surface activation parameter α for the two regimes can be estimated as α = and α = 0.350, respectively, with small deviations for the different back gate dielectric thicknesses. Note that previously reported value of relative surface sensitivity for silicon α 0.5 is comparable to our estimates [30, 37, 38]. Sensitivity values are also consistent with the measurements of current sensitivity and device quality shown above in Figure 3. Results on zeta potential of SiNWs in solution for ph values below 6 are in good agreement with our measurements of surface potential changes Vsurface (Figure 4), as well (see also Figure S2 in Supporting information). Furthermore, low slope of the surface potential was reported for low ph values and a higher slope for larger ph values, respectively. 2 However one has to respect that silicon oxide shows a hysteretic behaviour for ph sweeping, i.e. a remanence of the surface potential, which leads to a higher slope in a range of low ph values. 3. Conclusions In conclusion, we demonstrated the bottom up manufactured parallel arrays of Schottky barrier silicon nanowire field effect transistors, used for ph sensing with superior sensitivity [24] and accuracy. Excellent device performance is caused by the sensitive nanosized atomically sharp Si/NiSi2 metalsemiconductor junctions (Schottky barriers), formed within silicon nanowires by a thermal annealing, and exposed to the liquid environment during sensing. We introduced and employed the new measurement concept of continuous gate sweeps, which incorporates optimum current sensitivity to ph and, in parallel, accurate potentiometric measurements for quantitative information gain. Remarkably, the combined analysis of the sensitivity S and signal to noise ratio S/σS enabled to conclude that the subthreshold regime, commonly considered as an optimal one [30] is not compulsory for the best sensing measurements. We showed that the lower sensitivities in conjunction with higher Isd current levels yield comparable or higher signal to noise ratio. The developed bottom up manufactured architecture relies on assembled parallel arrays of silicon nanowires, helping to increase the current output and to decrease the device to device variation, and thus is a good candidate to be integrated into existing bio nanoelectronic detecting chips. In particular, the fabrication of FETs using the nanowire printing technique enables the easy transfer of such sensor technology onto flexible and stretchable substrates [39, 40]. Finally we believe that the proposed highly sensitive platform, representing a smart conjunction of bottom up nanofabrication techniques and measurement concept can represent a promising future alternative for the state of the art technology in the area of biodetection and diagnostics. Acknowledgements This work was supported by the European Union (European Social Fund) and the Free State of Saxony (Sächsische Aufbaubank) in the young researcher group InnovaSens (SAB Nr ). Further we acknowledge support from the German Excellence Initiative via the Cluster of Excellence EXC1056 Center for Advancing Electronics Dresdenʺ (cfaed). We thank Kai Meine (Keyence Deutschland GmbH) for providing the laser scanning microscope, Anja Caspari and Dr. Cornelia Bellmann (Leibniz Institute, IPF) for the support in zeta potential measurements. Finally, we thank Dr. Robin Ohmann for his comments and fruitful discussions. Electronic Supplementary Material: Supplementary material on device fabrication, i.e. printing and lithography, electrical measurements, zeta potential (automatically inserted by the publisher). 7

9 References 1. Bergveld, P. Development of an Ion sensitive solid state device for neurophysiological measurement. IEEE T Bio Med Eng 1970, BM 17, 70 &. 2. Bergveld, P. The impact of MOSFET based sensors. Sensors and Actuators 1985, 8 (2), Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Nanowire nanosensors for highly sensitive and selective detection of biological and chemical species. Science 2001, 293, Spijkman, M. J.; Brondijk, J. J.; Geuns, T. C. T.; Smits, E. C. P.; Cramer, T.; Zerbetto, F.; Stoliar, P.; Biscarini, F.; Blom, P. W. M.; Leeuw, D. M. D. Dual Gate Organic Field Effect Transistors as Potentiometric Sensors in Aqueous Solution. Adv. Funct. Mater. 2010, 20, Zumdahl, S. Chemical Principles (6th ed.). New York: Houghton Mifflin Company 2009, Hahm, J.; Lieber, C. M. Direct ultrasensitive electrical detection of DNA and DNA sequence variations using nanowire nanosensors. Nano Lett. 2004, 4, Gao, Z.; Agarwal, A.; Trigg, A. D.; Singh, N.; Fang, C.; Tung, C. H.; Fan, Y.; Buddharaju, K. D.; Kong, J. Silicon nanowire arrays for label free detection of DNA. Anal Chem 2007, 79, Cattani Scholz, A.; Pedone, D.; Dubey, M.; Neppl, S.; Nickel, B.; Feulner, P.; Schwartz, J.; Abstreiter, G.; Tornow, M. Organophosphonate based PNA functionalization of silicon nanowires for label free DNA detection. ACS Nano 2008, 2, Gao, A.; Lu, N.; Dai, P.; Li, T.; Pei, H.; Gao, X.; Gong, Y.; Wang, Y.; Fan, C. Silicon Nanowire Based CMOS Compatible Field Effect Transistor Nanosensors for Ultrasensitive Electrical Detection of Nucleic Acids. Nano Lett. 2011, 11, Kurkina, T.; Vlandas, A.; Ahmad, A.; Kern, K.; Balasubramanian, K. Label Free Detection of Few Copies of DNA with Carbon Nanotube Impedance Biosensors. Angew. Chem. Int. Edit. 2011, 50, Patolsky, F.; Zheng, G.; Hayden, O.; Lakadamyali, M.; Zhuang, X.; Lieber, C. M. Electrical detection of single viruses. Proc. Natl. Acad. Sci. USA 2004, 101, Zheng, G.; Patolsky, F.; Cui, Y.; Wang, W. U.; Lieber, C. M. Multiplexed electrical detection of cancer markers with nanowire sensor arrays. Nat. Biotechnol. 2005, 23, Susloparova, A.; Koppenhöfer, D.; Vu, X. T.; Weil, M.; Ingebrandt, S. Impedance spectroscopy with field effect transistor arrays for the analysis of anti cancer drug action on individual cells. Biosensors and Bioelectronic 2012, Patolsky, F.; Timko, B.; Yu, G.; Fang, Y.; Greytak, A.; Zheng, G.; Lieber, C. Detection, stimulation, and inhibition of neuronal signals with high density nanowire transistor arrays. Science 2006, 313, Lambacher, A.; Vitzthum, V.; Zeitler, R.; Eickenscheidt, M.; Eversmann, B.; Thewes, R.; Fromherz, P. Identifying firing mammalian neurons in networks with highresolution multitransistor array (MTA). Appl. Phys. A 2011, 102, Esashi, M.; Matsuo, T. Integrated micro multi ion sensor using field effect of semiconductor. IEEE T Bio Med Eng. 1978, BME 25, Elfström, N.; Karlström, A.; Linnros, J. Silicon nanoribbons for electrical detection of biomolecules. Nano Lett. 2008, 8, Vu, X. T.; Ghoshmoulick, R.; Eschermann, J. F.; Stockmann, R.; Offenhaeusser, A.; Ingebrandt, S. Fabrication and application of silicon nanowire transistor arrays for biomolecular detection. Sensor Actuat B Chem. 2010, 144, Patolsky, F.; Zheng, G.; Lieber, C. M. Fabrication of silicon nanowire devices for ultrasensitive, label free, realtime detection of biological and chemical species. Nat. Protocols. 2006, 1, Balasubramanian, K.; Lee, E. J. H.; Weitz, R. T.; Burghard, M.; Kern, K. Carbon nanotube transistors chemical functionalization and device characterization. Phys. Stat. Solidi A 2008, 205, Iijima, S. Helical microtubules of graphitic carbon. Nature 1991, 354, Wu, Y.; Cui, Y.; Huynh, L.; Barrelet, C. J.; Bell, D. C.; Lieber, C. M. Controlled Growth and Structures of Molecular Scale Silicon Nanowires. Nano Lett. 2004, 4, Nerowski, A.; Poetschke, M.; Bobeth, M.; Opitz, J.; Cuniberti, G. Dielectrophoretic Growth of Platinum Nanowires: Concentration and Temperature Dependence of the Growth Velocity. Langmuir 2012, 28, Shin, K. S.; Pan, A.; Chui, C. O. Channel length dependent sensitivity of Schottky contacted silicon nanowire field effect transistor sensors. Appl. Phys. Lett. 2012, 100, Pregl, S.; Weber, W. M.; Nozaki, D.; Kunstmann, J.; Baraban, L.; Opitz, J.; Mikolajick, T.; Cuniberti, G. Parallel arrays of Schottky barrier nanowire field effect transistors: Nanoscopic effects for macroscopic current output. Nano Research 2013, 6, Weber, W. M.; Geelhaar, L.; Graham, A.; Unger, E.; Duesberg, G. S.; Liebau, M.; Pamler, W.; Cheze, C.; Riechert, H.; Lugli, P. Silicon nanowire transistors with intruded nickel silicide contacts. Nano Lett. 2006, 6, Heinzig, A.; Slesazeck, S.; Kreupl, F.; Mikolajick, T.; Weber, W. M. Reconfigurable Silicon Nanowire Transistors. Nano Lett. 2012, 12, Martin, D.; Heinzig, A.; Grube, M.; Geelhaar, L.; Mikolajick, T.; Riechert, H.; Weber, W. M. Direct Probing of Schottky Barriers in Si Nanowire Schottky Barrier Field Effect Transistors. Phys Rev Lett. 2011, 107,

10 29. Nozaki, D.; Kunstmann, J.; Zörgiebel, F. M.; Weber, W. M.; Mikolajick, T.; Cuniberti, G. Multiscale modeling of nanowire based Schottky barrier field effect transistors for sensor applications. Nanotechnol. 2011, 22, Gao, X. P. A.; Zheng, G.; Lieber, C. M. Subthreshold regime has the optimal sensitivity for nanowire FET biosensors. Nano Lett. 2010, 10, Hu, Y.; Zhou, J.; Yeh, P. H.; Li, Z.; Wei, T. Y.; Wang, Z. L. Supersensitive, Fast Response Nanowire Sensors by Using Schottky Contacts. Adv. Mater. 2010, 22, Skucha, K.; Fan, Z.; Jeon, K.; Javey, A.; Boser, B. Palladium/silicon nanowire Schottky barrier based hydrogen sensors. Sensor Actuat: B. Chem. 2010, 145, Bergveld, P. Thirty years of ISFETOLOGY What happened in the past 30 years and what may happen in the next 30 years. Sensor Actuat: B Chem. 2003, 88, Knopfmacher, O.; Tarasov, A.; Fu, W.; Wipf, M.; Niesen, B.; Calame, M.; Schönenberger, C. Nernst Limit in Dual Gated Si Nanowire FET Sensors. Nano Lett. 2010, 10, Spijkman, M. J.; Smits, E. C. P.; Cillessen, J. F. M.; Biscarini, F.; Blom, P. W. M.; de Leeuw, D. M. Beyond the Nernst limit with dual gate ZnO ion sensitive field effect transistors. Appl. Phys. Lett. 2011, 98, Measurement of the threshold voltage with the back gate increases this sensitivity by the factor Csurface/Cbg, as shown by Knopfmacher et al. in 2010 and Spijkman et al. in 2011, however this does not influence the signal to noise ratio of the measurement. 37. Bergveld, P. ISFET, Theory and Practice Tarasov, A.; Wipf, M.; Bedner, K.; Kurz, J.; Fu, W.; Guzenko, V. A.; Knopfmacher, O.; Stoop, R. L.; Calame, M.; Schönenberger, C. True Reference Nanosensor Realized with Silicon Nanowires. Langmuir 2012, 28, Fan, Z.; Ho, J.; Jacobson, Z.; Yerushalmi, R.; Alley, R.; Razavi, H.; Javey, A. Wafer scale assembly of highly ordered semiconductor nanowire arrays by contact printing. Nano Lett. 2008, 8, Ishikawa, F.; kang Chang, H.; Ryu, K.; chiang Chen, P.; Badmaev, A.; Arco, L. G. D.; Shen, G.; Zhou, C. Transparent electronics based on transfer printed aligned carbon nanotubes on rigid and flexible substrates. ACS Nano 2009, 3,