Microsyst Technol DOI 10.1007/s00542-014-2100-4
Technical Paper
Component design and testing for a miniaturised autonomous sensor based on a nanowire materials platform Giorgos Fagas · Michael Nolan · Yordan M. Georgiev · Ran Yu · Olan Lotty · Nikolay Petkov · Justin D. Holmes · Guobin Jia · Björn Eisenhawer · Annett Gawlik · Fritz Falk · Naser Khosropour · Elizabeth Buitrago · Montserrat Fernández‑Bolaños Badia · Francois Krummenacher · Adrian M. Ionescu · Maher Kayal · Adrian M. Nightingale · John C. de Mello · Erik Puik · Franc van der Bent · Rik Lafeber · Rajesh Ramaneti · Hien Duy Tong · Cees van Rijn Received: 16 August 2013 / Accepted: 24 January 2014 © Springer-Verlag Berlin Heidelberg 2014
Abstract We present the design considerations of an autonomous wireless sensor and discuss the fabrication and testing of the various components including the energy harvester, the active sensing devices and the power management and sensor interface circuits. A common materials platform, namely, nanowires, enables us to fabricate stateof-the-art components at reduced volume and show chemical sensing within the available energy budget. We demonstrate a photovoltaic mini-module made of silicon nanowire solar cells, each of 0.5 mm2 area, which delivers a power of 260 μW and an open circuit voltage of 2 V at one sun illumination. Using nanowire platforms two sensing applications are presented. Combining functionalised suspended Si nanowires with a novel microfluidic fluid delivery system, fully integrated microfluidic–sensor devices are examined as sensors for streptavidin and pH, whereas, using a microchip modified with Pd nanowires provides a power efficient and fast early hydrogen gas detection method. Finally, an ultra-low power, efficient solar energy harvesting and G. Fagas (*) · M. Nolan · Y. M. Georgiev · R. Yu · O. Lotty · N. Petkov · J. D. Holmes Tyndall National Institute, University College Cork, Cork, Ireland e-mail:
[email protected] G. Jia · B. Eisenhawer · A. Gawlik · F. Falk Institute of Photonic Technology, Jena, Germany N. Khosropour · E. Buitrago · M. F.-B. Badia · F. Krummenacher · A. M. Ionescu · M. Kayal Ecole Polytechnique Federal Lausanne, Lausanne, Switzerland A. M. Nightingale · J. C. de Mello Imperial College London, London, UK E. Puik · F. van der Bent · R. Lafeber · R. Ramaneti · H. D. Tong · C. van Rijn Nanosens BV, Berkelkade 11, 7201 JE Zutphen, The Netherlands
sensing microsystem augmented with a 6 mAh rechargeable battery allows for less than 20 μW power consumption and 425 h sensor operation even without energy harvesting.
1 Introduction The continuous development of portable devices in terms of complexity and functionality, in ever decreasing volumes is inciting a new technological revolution. Miniaturised devices that communicate wirelessly and are self-powered, that is, autonomous, thereby removing the burden of battery replacement and reducing installation and maintenance costs, will drive this revolution. Many autonomous wireless sensor networks (WSN) solutions have been deployed in various applications, including health and lifestyle, automotive, smart buildings, predictive maintenance (e.g., of machines and infrastructure), and active RFID tags (Vullers et al. 2010; Ó Mathúna et al. 2008; Barton et al. 2008). The WSN platforms face the main technological challenges of miniaturization, autonomy and manufacturing cost (Penders et al. 2008). To achieve long lifetime and small form factors, the emerging autonomous sensors have to maintain ultra-low power (ULP) duty cycles and incorporate an energy harvesting source, an energy storage device and electronic circuits for power management, sensing and communication into mm-scale systems. Also, enabling chemical sensing of a variety of (bio-)molecules will enlarge the range of commercial applications in environmental, security and health monitoring. To this end, the authors have been working together within the European Union funded SiNAPS project (SiNAPS (2013)) to develop state of the art miniaturised components for an autonomous mote. The SiNAPS mote is based on a common “materials
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platform” that is, using high aspect ratio nanowires in multiple components. Nanowires allow: (1) reducing the volume of individual active devices while following lowcost processing and device integration strategies compatible with complementary metal–oxide–semiconductor (CMOS) processes, and (2) the demonstration of (bio-) chemical sensing while keeping the power consumption within the available energy budget. A preliminary account of this work was presented at the SPIE Microtechnologies 2013 (Kohsro Pour et al. 2013a). In this paper, we discuss in more detail our results on optimising and miniaturizing the individual components of the SiNAPS mote, that is (1) the nanowire solar energy harvester, (2) the nanowire sensing and (3) the CMOS electronics for the power management unit and the sensor interface. We present first results for proof-of-principle of two sensing applications enabled by nanowire platforms: biosensing using functionalised silicon nanowires and microfluidic delivery and H2 gas detection in the low ppm and broad temperature ranges using Pd nanowires. Within the SiNAPS mote a solar minimodule was developed based on silicon nanowire solar cells. Solar energy is the most abundant and practical form of ambient energy. In outdoor applications, the source power can reach 100 mW/cm2, while for indoor applications, illumination levels are on the order of 100 μW/cm2. Thanks to high efficiencies, solar cells are good energy sources for autonomous wireless sensor nodes and nanowires can be a low-cost photovoltaic (PV) material, also enabling miniaturisation. Today’s major PV technology is based on crystalline silicon (generation I) with a market share of 90 %. Generation II uses thin film technology to produce solar cells at lower cost in €/W, but to date these suffer from lower module efficiency, around 7 % for a-Si and up to 12 % for CdTe, CIGS or Si-tandem. Much effort is now focused on new, potentially lower cost solutions based on nanotechnology (Conibeer 2007; Peng and Lee 2011) including nanocrystals, nanorods, and nanowires. Efficiencies competitive with generation II PV technologies have been obtained from materials based on the radial core/shell nanoarchitecture of Si nanowires (SiNWs) (Peng and Lee 2011; Jia et al. 2013; Steglich et al. 2012; Song et al. 2012; Green et al. 2012). Si nanowires can be grown by well known techniques, namely metal assisted wet chemical etching on a silicon wafer (Peng et al. 2001), vapour–liquid–solid (VLS) and chemical vapour deposition (CVD). Apart from more cost effective processing methods, using nanostructures allows scaling down the device thickness to the micrometre scale and below, and separating more efficiently the charge carriers. The required solar cell area depends on the power needed by the sensor device but the volume and the weight of the device can be rather low if it can be made rather thin.
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Given an energy harvester, there is the key question of using the available energy efficiently. Presently, complex wireless sensor platforms, e.g. iMote (Imote2 Datasheet 2009), are realized on printed circuit boards (PCB) and cannot be used for mm-scale sensors (Barton et al. 2008). Bulky batteries are needed to provide the required peak and average power during sensing and data transmission. Additionally, many standard wireless transmission protocols, e.g. Zigbee transceivers (Gislason 2008), require cm-scale antennas. To replace the bulky batteries with miniaturized storage options, e.g. thin film Li-ion batteries, stringent ULP requirements need to be met by proper circuit design of all power-hungry components such as the wireless transceiver and the sensor interface unit (SIU). Also, the energy harvester circuit must be designed for high efficiency energy transfer from energy source to storage. Different architectures are possible, including inductor-based DC–DC converters (Qiu et al. 2011) and switched capacitor DC–DC converters (Chen et al. 2011). Due to the small target size (on the order of mm3) the harvested energy is a few hundred microwatts and this limited power budget affects the system level solution. Theoretically, inductorbased DC–DC converters have the highest efficiency but their power consumption is normally the total power budget of the mote. In the SiNAPS mote design, direct charging is implemented using a PV mini-module that provides the matched voltage to the battery leading to the highest overall efficiency. For miniaturised biosensors, SiNWs boast excellent electrical and mechanical properties which, combined with their high surface area to volume ratio, make them attractive candidates for applications such as field effect transistors (FETs) (Fasoli and Milne 2012; Cui and Lieber 2001) and sensing devices (Cui et al. 2001; Patolsky et al. 2006). Chemically grown NWs have been used as FET sensors (Cui et al. 2001). These NWs can be fabricated in large quantities but their device integration is non-trivial. The simpler approach of producing the NWs in situ using “top-down” fabrication is employed here. This also allows increased control over NW length, width, thickness, number, and crystallographic orientation, all of which are important parameters for sensing applications (Buitrago et al. 2013a; Nair and Alam 2007; Park et al. 2007, 2010). As a demonstration of the microfluidic channel implementation and the functionalisation methods, the SiNW platform is characterised for pH and streptavidin sensing. Streptavidin is the prototype target protein for biosensing. For the gas sensing application platform, palladium nanowires are used as the sensing element as Pd is strongly selective to H2. Hydrogen sensing is important due to the increased need for reliable, inexpensive and low power hydrogen sensors for applications such as the
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hydrogen leak detection in the future hydrogen economy. Hughes and Schubert (1992) demonstrated the first practical device using a 50 nm thick Pd–Ni alloy thin film. Nanowire-based devices are seen as the next generation of sensors with shorter response times and higher sensitivity due to their large surface-to-volume ratio. There have been a number of approaches to PdNW growth. Top down micro-fabrication processes, combined with patterning using UV-DUV photolithography and/or combined with nanolithography (e-beam) or focused-ion-beam (FIB) can be realized down to a few nanometers. The principle of Pd H2 sensing is based on detecting directly the change in resistivity or the electrostatic coupling via FETs (Lundstrom et al. 1975; Hughes and Schubert 1992; Hübert et al. 2011). The resistivity change upon exposure to hydrogen is attributed to the formation of Pd hydride (Von Lewis 1967; Flanagan and Oates 1991). In the case of FET-based sensors the Pd layer interacts with the gas phase and the result of this interaction, a dipole layer formation at the Pd-oxide interface, is measured in terms of the changes in drain current/threshold voltage of sourcedrain channel. The Pd H2 sensor in SiNAPS uses the change in resistivity for H2 detection. 2 System and component fabrication and experimental methods 2.1 Nanowire PV cell fabrication For the PV solar cell, nanowires are prepared by metal assisted wet chemical etching on an n-type silicon wafer (Peng et al. 2001). This method produces random arrangements of nanowires with diameters in the 20 to 200 nm range. The length of the nanowires is determined by the etching time. For etching we use a solution of HF/AgNO3. When AgNO3 contacts the silicon, silver nanoparticles form which act as catalysts for the silicon etching. Etching occurs only at positions where these particles are located so that at the particle-free regions nanowires remain. After etching the silver particles are carefully removed. Around the nanowires hydrogenated amorphous silicon is deposited by conventional PECVD. First a thin 1 nm intrinsic a-Si:H layer is deposited, followed by 10 nm of p-type a-Si:H to form the p–n junction as in Sanyo’s HIT (heterojunction with thin intrinsic layer) concept (Mishima et al. 2011). For passivation, the active structure is further coated by a 1 nm thin Al2O3 layer, deposited by atomic layer deposition (ALD). For contacting the structure, the space between the nanowires is filled with a transparent conductive oxide. We use aluminum doped ZnO deposited by ALD (Steglich et al. 2012). The complete solar cell structure is shown in the TEM cross section of Fig. 1.
Fig. 1 TEM cross section of nanowire solar cell. The nanowires are single crystalline and n-type with the p–n junction in core shell configuration by covering the nanowires with a shell of p-type amorphous silicon. Parts a and b show different magnifications, indicated by the scale bars
2.2 Silicon nanowire biosensing platform 2.2.1 Vertically stacked Si nanowire sensor fabrication process Two different, CMOS-compatible top-down process schemes have been developed for Si-NW sensor fabrication in SiNAPS. The first one involves the fabrication of a junctionless nanowire transistor (JNT, a highly doped, ultrathin device with no source/channel/drain junctions) and has been reported elsewhere (Khosro Pour et al. 2013a; Buitrago et al. 2013a; Georgiev et al. 2013). The second fabrication process involves the development of a 3D sensor using SiNW FETs with junctions, that is, a high density (7 × 20 NWs) SiNW array featuring fully depleted, ultra-thin, suspended channels (Buitrago et al. 2012). Figure 2 shows a schematic of a junctionless SiNW sensor compared with a junction-based FET setup. Figure 2c also shows a schematic of the vertically stacked SiNW sensor with 3 NWs. Higher utilization of the Si substrate is possible with the 3D stacking. Furthermore, high currents and higher opportunities for a sensing event are possible as the number of channels increases in two directions. With the NWs suspended, the whole surface is available for sensing. In any case, both biosensor fabrication schemes allow for the efficient heterogeneous integration with other components as necessary for a low power sensing device. The vertically stacked silicon nanowire structures were fabricated by using the natural scalloping effect resulting from consecutive BOSCH cycles (deep reactive ion etch cycles, DRIE: isotropic SF6 plasma etch followed by a passivation C4F8 step and O2 cleaning step) as first proposed by (Ng et al. 2007, 2009). SiNWs are formed by the thermal oxidation of the resulting scalloped columns. The NWs
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Fig. 2 Schematic of SiNW sensor structure based on a junctionless SiNWs, b junction based SiNW FETs, and c vertically stacked SiNW FETs with junctions (3 NWs)
Fig. 3 a SEM top side tilted view of structure after BOSCH, b cross section view of scalloped columns, c SiNWs after thermal oxidation, d top side tilted view of structure with implant mask openings, e top side view of structure after metallization, f suspended SiNWs after BHF bath, g closeup view of suspended SiNWs, and h top view of finished structure
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Microsyst Technol Fig. 4 Schematic showing the layout of the microfluidic channel geometry used to supply the sensors with fluid for the vertically stacked NW structure. The width of each microfluidic channel is 150 μm
Fig. 5 a Picture showing the PDMS microfluidic stamp attached to a sensor device. b Microscope image showing the microfluidic channel (positioned top to bottom) above a sensor window
form at the intersection of two consecutive cycles. This approach is inexpensive and utilizes conventional semiconductor and micro electromechanical system (MEMS) fabrication processes (Buitrago et al. 2012). SOI wafers with a BOX and device layer thickness of 1 μm each, p-type boron, and resistivity between 1–10 Ωcm were utilised. Images of the structure after various process steps are shown in Fig. 3. Chemical modification of the sensors is used to render them sensitive towards specific targeted analytes. For the streptavidin sensing experiments described here, the sensors were chemically modified with a surface coating of biotin which irreversibly binds to the nanowires. This was achieved using a two stage process. Firstly samples were functionalised with 3-(aminopropyl)triethoxysilane (APTES) by immersion in a 50 °C solution of 5 v/v % APTES in anhydrous toluene for 3 h. They were then rinsed with anhydrous toluene, deionised water and dried under nitrogen. In the second stage, the aminosilanised devices (Si-APTES) were immersed in 2 ml phosphate buffered silane (PBS, pH 7.5) and 100 μl of E, Z link-NHS-LC-Biotin in DMF (1 mg/ml) was added. The samples were left to react for 3 h at room temperature. Surfaces were rinsed with PBS and deionised water and dried under nitrogen. 2.2.2 Microfluidic delivery and sensing experiments Microfluidic channels attached to the device allow delivery of a fluid analyte directly to the SiNW sensors, with only a small quantity (μls) of analyte required for analysis. The propulsion of the fluid through the channel can either be achieved by passive delivery via capillary action or using external pumping equipment. While the former option is far preferable for the
final applications (e.g., point-of-care health diagnostics) the latter method is convenient for laboratory testing in a probestation, allowing for easy filling and cleaning of the channels during sensing measurements. As such, hydrodynamic flow was deemed to be a more appropriate method of fluid delivery during experiments. The inlet/outlets of the microfluidic stamp were connected to lengths of polytetrafluoroethylene (PTFE) tubing so that fluid could be administered and collected from the outside of the probe station. Figure 4 shows the suspended Si-NW chip layout and the microfluidic channel design applied to it. The layout consists of two parallel lines of devices can be seen. Access to the sensor structures is possible through small windows on the SU8 isolation layer at the centre. Contact access is possible on the outer sides of the chip. Fluid delivery to the sensor structures is enabled by bonding a polydimethylsiloxane (PDMS) stamp composed of two separate 150 μm wide channels to the top of the chip, as shown in Fig. 5. Access holes with diameters ~400 μm were drilled on the top and sides of the stamp to link the channel to external tubing. Figure 6 shows the cross section schematic of the vertically stacked structure with a microfluidic channel sitting on top of the device. The microfluidic stamps were fabricated using standard procedures (Duffy et al. 1998). The stamps were attached to the devices using the “stamp and stick” technique (Satyanarayana et al. 2005) in which a thin layer of uncured-wet PDMS is added to the underside of the stamp before it is positioned on the device. Positioning of the devices uses a custom-made micrometer-controlled positioning rig. Curing at 60 °C for 2 h produces a strong but non-permanent bond to realize the integrated fluid delivery/ sensing system (Fig. 5).
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into the probe station. Standard fittings from Upchurch Scientific were used for all connections. Electrical measurements of devices as well as the sensing experiments were done using cascade manual probe station and Agilent semiconductor analyser B1500. 2.3 Palladium nanowire hydrogen‑sensing platform
Fig. 6 Cross section schematic of vertically stacked SiNW structure with microfluidic channel on top of device
With the microfluidic stamp attached, liquid analyte can be delivered directly to the sensors. For the streptavidin sensing experiments, small quantities of streptavidin were introduced within a continuous stream of buffer solution. PBS solution was delivered to the sensor by an external pump at a rate of 200 μl/min from a syringe (BD Plastipak, 10 ml) propelled by a pump (Harvard, Pump 11+). From the syringe, the flow passed through polyethylene (PE) tubing (ID 0.4 mm, OD 1.0 mm). Inside the probe station (Fig. 7a), the tubing was downsized (OD 0.4 mm ID 0.1 mm PTFE tubing) using interconnect junctions fabricated in-house from PDMS. Commercially available interconnects can also be used (e.g., from Upchurch Scientific). The smaller tubing was required to interface to the PDMS stamp by inserting into the pre-drilled holes, but could not be used for the entire length of the fluid supply lines due to excessive back-pressure. The analyte solution can be injected separately into the main channel solution by the use of a T-junction located about 10 cm away from the syringe exit (Fig. 7b). From the T-junction, the flow continued along the PE tubing (~1 m) Fig. 7 a Image showing an integrated device within the probe station before testing. The device is connected to tubing ready for fluid delivery. b Image showing the syringes and syringe pump used to deliver fluid for the streptavidin sensing experimentation
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While e-beam and FIB allow nm-scale Pd nanowire fabrication, they cannot be scaled up for industrial scale processes; alternatives including nanoimprint lithography or nanostencils are compromised regarding lateral dimensions and reproducibility. Our solution to improve dimensional control is to precisely define a cavity that permits controlled removal of part of the metal layer with an angled wafer level ion beam etch that resembles a nanostencil structure patterned directly on the wafer service, which minimizes the lateral spread of the deposited metal (Tong et al. 2009). This process is indicated schematically in Fig. 8. The generic process of deposition and etching at an angle (DEA) is applied here to fabricate Pd nanowires on thermal SiO2/Si based silicon substrates and the patterned devices can be addressed individually or in a array using two point contacts (Tong et al. 2009). A typical layout of the sensing device is shown in Fig. 9 (Van der Bent and Van Rijn 2010). For testing this component, a typical measurement consists of using a source measure unit (Keithley 2400) with a voltage bias applied between 50–800 mV and controlled remotely via GBIP interface. The recorded signal (real time current/resistance change) can be used to do current–voltage sweeps under different operating conditions. A temperature controller enables temperature dependent measurement. Noise spectra from the devices are measured and analysed using the Agilent 35665A dynamic signal analyzer. The measurement resolution is determined by the noise level from the nanowires and that of the readout electronics. Measurements of the inherent noise levels from nanowires and the noise spectrum of single nanowires for different bias currents (1–180 µA) have been made to estimate the low frequency resistor noise. The
Microsyst Technol Fig. 8 The microfabrication process of Pd nanowire devices adapted from (Tong et al. 2009)
Fig. 9 Layout of the PdNW sensing device with a magnified SEM image of a single nanowire
nanowire noise spectrum shows 1/f behavior, from which the measurement resolution was determined to be 0.03 % of the nanowire resistance (R ~ 7 kΩ). The integration of the nanowires into a one chip solution will decrease the resolution to approximately 1–2 %, which is still sufficient for our purposes. In the present work, a Pd nanowire chip was fabricated using Deposition and Etching under an angle (Tong et al. 2010). Two identical nanowires with a width of 200 nm, a height of 20 nm, and a length of 50 μm were selected, giving a wire resistance of approximately 8,300 Ω for each nanowire (293 K, 25 % 0.955 132 120 23 13 74 176 180 790 0.58 5.55
Complete system
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simulation results for power consumption of the power management and the sensor interface circuits in different system operation modes, denoted SL3 to SL0. Vbat is detected when the battery is discharged by a high current during wireless data transmission. The remaining battery charge is determined from the battery discharge curve (Varta V6HR Datasheet 2013). In SL3 mode, with the highest Vbat, the system operates at 1 MHz and the wireless transceiver sends the measurement results for H2 concentration and temperature to a base station every 15 s. In this operating mode, the average power consumption of the energy harvester is less than 300 nW. Upon detection of a lower battery voltage, the system switches to a lower clock frequency; for example, in SL0 mode the circuit operates at 125 kHz frequency and the average power consumption of the energy harvester drops to less than 110 nW thanks to reducing the power consumption of the clock generation and DCU blocks; the simulated power consumption of the clock generator and the DCU is reduced linearly with operating frequency, reaching 23 and 13 nW, at 125 kHz (Khosro Pour et al. 2013b). The average power consumption of the incremental and SAR ADC is reduced almost linearly as seen in Table 2. Finally, in this mode measurement results are sent every 120 s. As another example, 330 nW is used for biasing the bipolar junction transistors in the integrated temperature sensor and generating VCM common mode voltage for ADC in SL3 mode, which is reduced to 176 nW in SL0 mode. As a resistive divider has been used to generate VCM, the power consumption of the VCM generator is almost constant; however as a frequency-proportional current source has been used to bias the BJT transistors in the temperature sensor, the power consumption is reduced in SL0 mode.
To estimate the total power consumption of the sensor, the average power consumption of the sensor biasing circuit and the wireless transceiver are required. Before measuring the H2 concentration, the Pd nanowires are biased with a 7 μA bias current for 10 s; since the ADC conversion takes less than 1 ms, the total power consumption of the sensor interface circuit is determined by the sensor bias circuit. The TZ1053 transceiver consumes 5 μA during standby and 3.3 mA in 20 ms to send data with the minimum payload size of 55 bytes (Toumaz 2012). In SL3 mode, the average current consumption of the Pd nanowires and the wireless transceiver are 4.67 and 9.4 μA, respectively. By sending the samples every 120 s in SL0 mode, these values will be reduced to 0.58 and 5.6 μA, respectively. The average current consumption of the whole sensor is 14.1 μA in SL3 mode and in SL0 mode this is reduced to 6.2 μA. To evaluate the autonomous operation of the sensor, the power delivered to the battery by the PV module can be simulated with an equivalent circuit model. The 4 mm2 area PV module can provide a maximum power of 319.5 μW at its Vmpp under AM1.5 illumination level. Figure 24 shows that under simulated 10 % of AM 1.5 illumination, the PV module delivers an average power of 24.45 μW to the battery and efficiency is 90.7 %. When the battery is almost fully discharged, the system operates in SL0 mode and 21.05 μW is delivered to the battery with 78.1 % efficiency (Fig. 24), while the average current consumption of the complete system is only 6.2 μA. As a result the battery gets charged and Vbat increases gradually. By increasing the battery voltage, efficiency is improved and more power is delivered to the battery. Therefore, even at 10 % light intensity, the harvested energy is enough for autonomous operation of the complete system. Finally, as the whole sensor consumes 14.1 μA for sensing and data transmission, the
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Fig. 24 Power delivered to the battery under simulated 10 % of AM1.5 illumination
target 6 mAh battery can provide enough power for approximately 425 h continuous operation, even without energy harvesting.
4 Concluding remarks In summary we prepared a photovoltaic mini-module consisting of silicon nanowire solar cells, each of 0.5 mm2 area. The efficiency of the module reaches 10 % and it delivers a power of 260 μW and an open circuit voltage of 2 V at one sun illumination. At 1 % of one sun illumination, the PV module delivers 1.4 V, which makes this Si nanowire based solar cell module useful as an energy harvester for charging the battery in the SiNAPS mote. We have tested fully integrated microfluidic–sensor devices, combining functionalized suspended SiNWs with a novel microfluidic fluid delivery system to examine their potential as sensors for streptavidin and pH. In this integrated device, analyte solutions are delivered to the NW sensor via microfluidics and the electrical response noted. For the streptavidin sensing, NWs are functionalized with biotin and after addition of a 0.4 μM solution of streptavidin, the current drop by over two orders of magnitude. For pH sensing, APTES-modified NW devices were used. On adding different buffered solutions (pH 4, 7 and 10) the current reproducibly shifts, with the direction of the shift correlating to the pH of the buffer. An ultra-low power, efficient solar energy harvesting and sensing microsystem has been proposed. An area- and power-efficient solar energy harvester stores energy harvested from the PV module in an MiMH microbattery. This circuit also scales the power consumption and performance of the complete system for autonomous operation of the sensor at low battery charge. A fully integrated sensor
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interface circuit measures H2 concentration by measuring the conductance changes of a miniaturised Pd nanowire sensor and temperature measurement allows sensor calibration. A new incremental ADC converts measurement data to 13-bit digital values, that are transmitted to a base station. Even 10 % light intensity is sufficient for the system to operate autonomously. The target 6 mAh battery provides power for 425 h sensor operation even without energy harvesting. A power efficient and fast early hydrogen detection method has been presented. This is achieved by employing a microchip with two Pd nanowires and demonstrating sensing under varying temperature in a test chamber, with controlled H2 concentration. The dual Pd nanowire, in which one acts as H2 sensor and the other as a temperature sensor, is found to be the best choice as input for an adequate temperature compensation algorithm. Selective and fast detection over a wide temperature range is attainable if the relevant temperature time constraints of the different components are met. A detection limit of 100 ppm H2 has been demonstrated under temperature fluctuations. Acknowledgments We acknowledge support from the European Commission Framework 7 ICT-FET-Proactive funded project SiNAPS (contract number 257856) for financial support of this work.
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