Lette nanowire devices and potential reductions through, for example, improvements in junction quality and reduction of the surface defects Localized detection using our kinked p-n probes was first xplored in single nanoparticle sensing experiments. Specifi V.=+5V V.=5V substrate,was coupled to poly( dimethylsiloxane)(PDMs) microfluidic channel to control the solution flow over the devices. Conductance versus time traces recorded simulta- neously from two independent devices following the troduction of a 1.2 nm solut b solution of 100 nm diameter chargee fluorescent polystyrene nanobeads( Figure 3a) exhibit several b Figure 2. tmSGM and water-gate experiments of kinked p-n nanowire devices. (a) Superposition of tmSGM n AFM topographic images of a representative kinked p-n nanowire device under Vip of +5 V (left, scanning direction from top down)and -5V (right, scanning direction from bottom p), respectively. Scale bar, 0.5 Am. The blue/red arrows indicate the p-type and n-type depletion/ accumulation regions (left panel), respectively; the same positions show accumulation/depletion in the right panel Insets: line profiles of the tmSGM signal along the white dashed lines about these p-type and n-type regions. (b) Conductance versus water-gate reference potential data recorded from a representative kinked p-n nanowire device Ix phosphate buffer saline(PBS). Inset: schematic of conductance water"gate experiment. Figure 3. Fluorescent polystyrene nanobead sensing experiment. (a)(Left)Conductance vs time data recorded simultaneously from size of the sensitive region without optimization is comparable two independent kinked p-n nanowire devices with nanobeads in to the best value reported in our previous work, we note that deionized(Di)water introduced into the microfluidic channel. Black theoretical spatial resolution of a gated p-n device, charac- arrows mark the on/off points of the terized by the thickness of the depletion region, is 10-30 nm for lultaneously from the same two devices with highly doped silicon and thus could be improved in the future nly Di water in the microfluidic channel. (b)Schematic of fluorescent polystyrene nanobead sensing process using kinked p-n nanowire evices and the corresponding schematic of time-dependent change in fabricated through multiple EBL, metallization, and passivation evice conductance. Black arrows mark the on/off points of the signal. steps similar to previous reports, ,10, 12,26 using Cr/Pd/Cr for (c)(Top)Simultaneous confocal microscopy and conductance vs time data recorded from a kinked p-n nanowire device in a nanobead electrodes from the aqueous medium. The sensitivity of kinked solution flow. Red arrow marks the charge sensing signal. Blue arrow nanowire p-n devices in solution was assessed by water-gate xperiments"(inset, Figure 2b), where the p-n junction was P-n junction for images I and 2. The green arrows in the images highlight the positions of the nanowire junction and fluorescent forward biased at 1.0 V and a Ag/AgCl electrode was used to nanobead in both images. In Image 1, the two green arrows overlap control the chemical potential (vp) of the solution. Representa (Bottom) Simultaneous confocal microscopy and conductance vs time tive conductance versus Ve data (Figure 2b)demonstrate a D) demonstrate a data recorded from the same device without nanobeads in solution. p-type response and sensitivity of 620 nS/V. The p-type Blue arrows indicate photocurrent peaks due to the laser scanning ov esponse is consistent with the tmSGM results. The water-gate the p-n junction. The green arrow in the image highlights the position results also exhibit an increase in noise with increasing device f the nanowire junction. All the electrical data were filtered through a conductance, which could be due to increased recombination .00 Hz low-pass digital filter. Scale bars, 5 um at higher carrier concentrations within the depletion region. From a practical perspective, such water-gate data can be used to key features. First, when nanobeads choose an optimal operating regime (ie, where the sensitivity/ the device area, uncorrelated"pulse noise ratio is maximized), although future work should also observed from both devices(red trace address fundamental origin of noise in these p-n junction duration time of the"on"state of the 1713 dxdoloran0.1021/n300256 rI Nono Lett.2012.12.171-1716size of the sensitive region without optimization is comparable to the best value reported in our previous work, we note that theoretical spatial resolution of a gated p−n device, charac￾terized by the thickness of the depletion region, is 10−30 nm for highly doped silicon31 and thus could be improved in the future. The devices used for sensing experiments in solution were fabricated through multiple EBL, metallization, and passivation steps similar to previous reports,2,10,12,26 using Cr/Pd/Cr for contacts and SU8 as the passivation layer to isolate the metal electrodes from the aqueous medium. The sensitivity of kinked nanowire p−n devices in solution was assessed by water-gate experiments2,10 (inset, Figure 2b), where the p−n junction was forward biased at 1.0 V and a Ag/AgCl electrode was used to control the chemical potential (Vg) of the solution. Representa￾tive conductance versus Vg data (Figure 2b) demonstrate a p-type response and sensitivity of 620 nS/V. The p-type response is consistent with the tmSGM results. The water-gate results also exhibit an increase in noise with increasing device conductance, which could be due to increased recombination at higher carrier concentrations within the depletion region.22 From a practical perspective, such water-gate data can be used to choose an optimal operating regime (i.e., where the sensitivity/ noise ratio is maximized), although future work should also address fundamental origin of noise in these p−n junction nanowire devices and potential reductions through, for example, improvements in junction quality and reduction of the surface defects. Localized detection using our kinked p−n probes was first explored in single nanoparticle sensing experiments. Specifi￾cally, an array of kinked p−n nanowires probes on a SiO2 substrate2,10,12 was coupled to poly(dimethylsiloxane) (PDMS) microfluidic channel to control the solution flow over the devices.32 Conductance versus time traces recorded simulta￾neously from two independent devices following the introduction of a 1.2 nM solution of 100 nm diameter charged fluorescent polystyrene nanobeads (Figure 3a) exhibit several key features. First, when nanobeads solutions flow through the device area, uncorrelated “pulse” (on/off) signals were observed from both devices (red traces, Figure 3a). The time duration time of the “on” state of the pulses ranged from 50 to Figure 2. tmSGM and water-gate experiments of kinked p−n nanowire devices. (a) Superposition of tmSGM images on AFM topographic images of a representative kinked p−n nanowire device under Vtip of +5 V (left, scanning direction from top down) and −5 V (right, scanning direction from bottom up), respectively. Scale bar, 0.5 μm. The blue/red arrows indicate the p-type and n-type depletion/ accumulation regions (left panel), respectively; the same positions show accumulation/depletion in the right panel. Insets: line profiles of the tmSGM signal along the white dashed lines about these p-type and n-type regions. (b) Conductance versus water-gate reference potential data recorded from a representative kinked p−n nanowire device in 1× phosphate buffer saline (PBS). Inset: schematic of conductance vs water-gate experiment. Figure 3. Fluorescent polystyrene nanobead sensing experiment. (a) (Left) Conductance vs time data recorded simultaneously from two independent kinked p−n nanowire devices with nanobeads in deionized (DI) water introduced into the microfluidic channel. Black arrows mark the on/off points of the signals. (Right) Conductance vs time data recorded simultaneously from the same two devices with only DI water in the microfluidic channel. (b) Schematic of fluorescent polystyrene nanobead sensing process using kinked p−n nanowire devices and the corresponding schematic of time-dependent change in device conductance. Black arrows mark the on/off points of the signal. (c) (Top) Simultaneous confocal microscopy and conductance vs time data recorded from a kinked p−n nanowire device in a nanobead solution flow. Red arrow marks the charge sensing signal. Blue arrows mark the photocurrent peaks caused by the laser scanning over the p−n junction for images 1 and 2. The green arrows in the images highlight the positions of the nanowire junction and fluorescent nanobead in both images. In Image 1, the two green arrows overlap. (Bottom) Simultaneous confocal microscopy and conductance vs time data recorded from the same device without nanobeads in solution. Blue arrows indicate photocurrent peaks due to the laser scanning over the p−n junction. The green arrow in the image highlights the position of the nanowire junction. All the electrical data were filtered through a 100 Hz low-pass digital filter. Scale bars, 5 μm. Nano Letters Letter 1713 dx.doi.org/10.1021/nl300256r | Nano Lett. 2012, 12, 1711−1716