Bozhi Tian

Coaxial silicon nanowires as solar cells and nanoelectronic power sources

Solar cells are attractive candidates for clean and renewable power; with miniaturization, they might also serve as integrated power sources for nanoelectronic systems. The use of nanostructures or nanostructured materials represents a general approach to reduce both cost and size and to improve efficiency in photovoltaics. Nanoparticles, nanorods and nanowires have been used to improve charge collection efficiency in polymer-blend and dye-sensitized solar cells, to demonstrate carrier multiplication, and to enable low-temperature processing of photovoltaic devices. Moreover, recent theoretical studies have indicated that coaxial nanowire structures could improve carrier collection and overall efficiency with respect to single-crystal bulk semiconductors of the same materials. However, solar cells based on hybrid nanoarchitectures suffer from relatively low efficiencies and poor stabilities. In addition, previous studies have not yet addressed their use as photovoltaic power elements in nanoelectronics. Here we report the realization of p-type/intrinsic/n-type (p-i-n) coaxial silicon nanowire solar cells. Under one solar equivalent (1-sun) illumination, the p-i-n silicon nanowire elements yield a maximum power output of up to 200 pW per nanowire device and an apparent energy conversion efficiency of up to 3.4 per cent, with stable and improved efficiencies achievable at high-flux illuminations. Furthermore, we show that individual and interconnected silicon nanowire photovoltaic elements can serve as robust power sources to drive functional nanoelectronic sensors and logic gates. These coaxial silicon nanowire photovoltaic elements provide a new nanoscale test bed for studies of photoinduced energy/charge transport and artificial photosynthesis, and might find general usage as elements for powering ultralow-power electronics and diverse nanosystems.

Three-dimensional, Flexible Nanoscale Field Effect Transistors as Localized Bioprobes

Nanoprobes of Cell Potential Direct electrical measurements of cell potentials usually face design compromises. Microelectrodes probe within the cytosol of cells but have a minimum size (hundreds of nanometers in width) for obtaining useful signals. Nanoscale field effect transistors (FETs) can have an active probe size of only tens of nanometers but generally allow only the outer cell potential to be measured. Tian et al. (p. 830) fabricated nanowires in which kinks could be introduced to create a sharp probe tip pointing away from the fabrication substrate. Coating the tip with a phospholipid bilayer allowed the probe to be inserted through the membranes of beating cardiac cells, where it could be used to follow temporal changes in cell potential. Kinked nanowire transistors can measure intracellular electric potentials. Nanoelectronic devices offer substantial potential for interrogating biological systems, although nearly all work has focused on planar device designs. We have overcome this limitation through synthetic integration of a nanoscale field-effect transistor (nanoFET) device at the tip of an acute-angle kinked silicon nanowire, where nanoscale connections are made by the arms of the kinked nanostructure, and remote multilayer interconnects allow three-dimensional (3D) probe presentation. The acute-angle probe geometry was designed and synthesized by controlling cis versus trans crystal conformations between adjacent kinks, and the nanoFET was localized through modulation doping. 3D nanoFET probes exhibited conductance and sensitivity in aqueous solution, independent of large mechanical deflections, and demonstrated high pH sensitivity. Additionally, 3D nanoprobes modified with phospholipid bilayers can enter single cells to allow robust recording of intracellular potentials.

Single and Tandem Axial p-i-n Nanowire Photovoltaic Devices

Nanowires represent a promising class of materials for exploring new concepts in solar energy conversion. Here we report the first experimental realization of axial modulation-doped p-i-n and tandem p-i-n(+) -p(+)-i-n silicon nanowire (SiNW) photovoltaic elements. Scanning electron microscopy images of selectively etched nanowires demonstrate excellent synthetic control over doping and lengths of distinct regions in the diode structures. Current-voltage (I-V) characteristics reveal clear and reproducible diode characteristics for the p-i-n and p-n SiNW devices. Under simulated one-sun solar conditions (AM 1.5G), optimized p-i-n SiNW devices exhibited an open circuit voltage (Voc) of 0.29 V, a maximum short-circuit current density of 3.5 mA/cm(2), and a maximum efficiency of 0.5%. The response of the short-circuit current versus Voc under varying illumination intensities shows that the diode quality factor is improved from n=1.78 to n=1.28 by insertion of the i-type SiNW segment. The temperature dependence of Voc scales as -2.97 mV/K and extrapolates to the crystalline Si band gap at 0 K, which is in excellent agreement with bulk properties. Finally, a novel single SiNW tandem solar cell consisting of synthetic integration of two photovoltaic elements with an overall p-i-n(+) -p(+)-i-n structure was prepared and shown to exhibit a Voc that is on average 57% larger than that of the single p-i-n device. Fundamental studies of such well-defined nanowire photovoltaics will enable their intrinsic performance limits to be defined.

Coaxial Group III−Nitride Nanowire Photovoltaics

Coaxial core/shell nanowires represent an important class of nanoscale building blocks with substantial potential for exploring new concepts and materials for solar energy conversion. Here, we report the first experimental realization of coaxial group III-nitride nanowire photovoltaic (PV) devices, n-GaN/i-In(x)Ga(1-x)N/p-GaN, where variation of indium mole fraction is used to control the active layer band gap and hence light absorption. Current-voltage data reveal clear diode characteristics with ideality factors from 3.9 to 5.6. Electroluminescence measurements demonstrate tunable emission from 556 to 371 nm and thus confirm band gap variations in the In(x)Ga(1-x)N active layer from 2.25 to 3.34 eV as In composition is varied. Simulated one-sun AM 1.5G illumination yielded open-circuit voltages (V(oc)) from 1.0 to 2.0 V and short-circuit current densities (J(sc)) from 0.39 to 0.059 mA/cm(2) as In composition is decreased from 0.27 to 0 and a maximum efficiency of approximately 0.19%. The n-GaN/i-In(x)Ga(1-x)N/p-GaN nanowire devices are highly robust and exhibit enhanced efficiencies for concentrated solar light illuminations as well as single nanowire J(sc) values as high as 390 mA/cm(2) under intense short-wavelength illumination. The ability to rationally tune the structure and composition of these core/shell III-nitride nanowires will make them a powerful platform for exploring nanoenabled PVs in the future.

High‐Yield Synthesis of Periodic Mesoporous Silica Rods and Their Replication to Mesoporous Carbon Rods

Ordered mesoporous carbon rods have been obtained in high yield using silica rods as templates. The inorganic salts that are used for the synthesis of the silica templates control the morphology and adjust the wall structure of the mesoporous silica materials and thus of the final carbon rods. The Figure shows a TEM image of a carbon sample synthesized using SBA-15-100 silica as a template.

Single crystalline kinked semiconductor nanowire superstructures

The ability to control and modulate the composition1–4, doping1,3–5, crystal structure6–8 and morphology9,10 of semiconductor nanowires during the synthesis process has allowed researchers to explore various applications of nanowires11–15. However, despite advances in nanowire synthesis, progress towards the ab initio design and growth of hierarchical nanostructures has been limited. Here we demonstrate a ‘nanotectonic’ approach that provides iterative control over the nucleation and growth of nanowires and use it to grow kinked or zigzag nanowires in which the straight sections are separated by triangular joints. Moreover, the lengths of the straight sections can be controlled and the growth direction remains coherent along the nanowire. We also grow dopant-modulated structures in which specific device functions, including p-n diodes and field-effect transistors, can be precisely localized at the kinked junctions in the nanowires.

Intracellular recordings of action potentials by an extracellular nanoscale field-effect transistor

The ability to make electrical measurements inside cells has led to many important advances in electrophysiology1-6. The patch clamp technique, in which a glass micropipette filled with electrolyte is inserted into a cell, offers both high signal-to-noise ratio and temporal resolution1,2. Ideally the micropipette should be as small as possible to increase the spatial resolution and reduce the invasiveness of the measurement, but the overall performance of the technique depends on the impedance of the interface between the micropipette and the cell interior1,2, which limits how small the micropipette can be. Techniques that involve inserting metal or carbon microelectrodes into cells are subject to similar constraints4,7-9. Field-effect transistors (FETs) can also record electric potentials inside cells10, and since their performance does not depend on impedance11,12, they can be made much smaller than micropipettes and microelectrodes. Moreover, FET arrays are better suited for multiplexed measurements. Previously we have demonstrated FET-based intracellular recording with kinked nanowire structures10, but the kink configuration and device design places limits on the probe size and the potential for multiplexing. Here we report a new approach where a SiO2 nanotube is synthetically integrated on top of a nanoscale FET. After penetrating the cell membrane, the SiO2 nanotube brings the cell cytosol into contact with the FET and enables the recording of intracellular transmembrane potential. Simulations show that the bandwidth of this branched intracellular nanotube FET (BIT-FET) is high enough for it to record fast action potentials even when the nanotube diameter is decreased to 3 nm, a length scale which is well below that accessible with other methods1,2,4. Studies of cardiomyocyte cells demonstrate that when brought close, the nanotubes of phospholipid-modified BIT-FETs spontaneously penetrate the cell membrane to yield stable, full-amplitude intracellular action potential recording, showing that a stable tight seal forms between the nanotube and cell membrane. We also show that multiple BIT-FETs can record multiplexed intracellular signals from both single cells and networks of cells.

Electrical Recording from Hearts with Flexible Nanowire Device Arrays

We show that nanowire field-effect transistor (NWFET) arrays fabricated on both planar and flexible polymeric substrates can be reproducibly interfaced with spontaneously beating embryonic chicken hearts in both planar and bent conformations. Simultaneous recordings from glass microelectrode and NWFET devices show that NWFET conductance variations are synchronized with the beating heart. The conductance change associated with beating can be tuned substantially by device sensitivity, although the voltage-calibrated signals, 4-6 mV, are relatively constant and typically larger than signals recorded by microelectrode arrays. Multiplexed recording from NWFET arrays yielded signal propagation times across the myocardium with high spatial resolution. The transparent and flexible NWFET chips also enable simultaneous electrical recording and optical registration of devices to heart surfaces in three-dimensional conformations not possible with planar microdevices. The capability of simultaneous optical imaging and electrical recording also could be used to register devices to a specific region of the myocardium at the cellular level, and more generally, NWFET arrays fabricated on increasingly flexible plastic and/or biopolymer substrates have the potential to become unique tools for electrical recording from other tissue/organ samples or as powerful implants.

Controlled Synthesis of Millimeter-Long Silicon Nanowires with Uniform Electronic Properties

We report the nanocluster-catalyzed growth of ultralong and highly uniform single-crystalline silicon nanowires (SiNWs) with millimeter-scale lengths and aspect ratios up to approximately 100,000. The average SiNW growth rate using disilane (Si 2H 6) at 400 degrees C was 31 microm/min, while the growth rate determined for silane (SiH 4) reactant under similar growth conditions was 130 times lower. Transmission electron microscopy studies of millimeter-long SiNWs with diameters of 20-80 nm show that the nanowires grow preferentially along the 110 direction independent of diameter. In addition, ultralong SiNWs were used as building blocks to fabricate one-dimensional arrays of field-effect transistors (FETs) consisting of approximately 100 independent devices per nanowire. Significantly, electrical transport measurements demonstrated that the millimeter-long SiNWs had uniform electrical properties along the entire length of wires, and each device can behave as a reliable FET with an on-state current, threshold voltage, and transconductance values (average +/-1 standard deviation) of 1.8 +/- 0.3 microA, 6.0 +/- 1.1 V, 210 +/- 60 nS, respectively. Electronically uniform millimeter-long SiNWs were also functionalized with monoclonal antibody receptors and used to demonstrate multiplexed detection of cancer marker proteins with a single nanowire. The synthesis of structurally and electronically uniform ultralong SiNWs may open up new opportunities for integrated nanoelectronics and could serve as unique building blocks linking integrated structures from the nanometer through millimeter length scales.

Nanowire transistor arrays for mapping neural circuits in acute brain slices

Revealing the functional connectivity in natural neuronal networks is central to understanding circuits in the brain. Here, we show that silicon nanowire field-effect transistor (Si NWFET) arrays fabricated on transparent substrates can be reliably interfaced to acute brain slices. NWFET arrays were readily designed to record across a wide range of length scales, while the transparent device chips enabled imaging of individual cell bodies and identification of areas of healthy neurons at both upper and lower tissue surfaces. Simultaneous NWFET and patch clamp studies enabled unambiguous identification of action potential signals, with additional features detected at earlier times by the nanodevices. NWFET recording at different positions in the absence and presence of synaptic and ion-channel blockers enabled assignment of these features to presynaptic firing and postsynaptic depolarization from regions either close to somata or abundant in dendritic projections. In all cases, the NWFET signal amplitudes were from 0.3–3 mV. In contrast to conventional multielectrode array measurements, the small active surface of the NWFET devices, ∼0.06 μm2, provides highly localized multiplexed measurements of neuronal activities with demonstrated sub-millisecond temporal resolution and, significantly, better than 30 μm spatial resolution. In addition, multiplexed mapping with 2D NWFET arrays revealed spatially heterogeneous functional connectivity in the olfactory cortex with a resolution surpassing substantially previous electrical recording techniques. Our demonstration of simultaneous high temporal and spatial resolution recording, as well as mapping of functional connectivity, suggest that NWFETs can become a powerful platform for studying neural circuits in the brain.