Lithographically Patterned Nanowire Electrodeposition Research Articles

A Nose for Hydrogen Gas: Fast, Sensitive H2 Sensors Using Electrodeposited Nanomaterials.

Hydrogen gas (H2) is odorless and flammable at concentrations above 4% (v/v) in air. Sensors capable of detecting it rapidly at lower concentrations are needed to "sniff" for leaked H2 wherever it is used. Electrical H2 sensors are attractive because of their simplicity and low cost: Such sensors consist of a metal (usually palladium, Pd) resistor. Exposure to H2 causes a resistance increase, as Pd metal is converted into more resistive palladium hydride (PdHx). Sensors based upon Pd alloy films, developed in the early 1990s, were both too slow and too insensitive to meet the requirements of H2 safety sensing. In this Account, we describe the development of H2 sensors that are based upon electrodeposited nanomaterials. This story begins with the rise to prominence of nanowire-based sensors in 2001 and our demonstration that year of the first nanowire-based H2 sensor. The Pd nanowires used in these experiments were prepared by electrodepositing Pd at linear step-edge defects on a graphite electrode surface. In 2005, lithographically patterned nanowire electrodeposition (LPNE) provided the capability to pattern single Pd nanowires on dielectrics using electrodeposition. LPNE also provided control over the nanowire thickness (±1 nm) and width (±10-15%). Using single Pd nanowires, it was demonstrated in 2010 that smaller nanowires responded more rapidly to H2 exposure. Heating the nanowire using Joule self-heating (2010) also dramatically accelerated sensor response and recovery, leading to the conclusion that thermally activated H2 chemisorption and desorption of H2 were rate-limiting steps in sensor response to and recovery from H2 exposure. Platinum (Pt) nanowires, studied in 2012, showed an inverted resistance response to H2 exposure, that is, the resistance of Pt nanowires decreased instead of increased upon H2 exposure. H2 dissociatively chemisorbs at a Pt surface to form Pt-H, but in contrast to Pd, it stays on the Pt surface. Pt nanowires showed a faster response to H2 exposure than Pd nanowires operating at the same elevated temperature, but they had a surprising disadvantage: The resistance change observed for Pt nanowires was exactly the same for all H2 concentrations. Electron surface scattering was implicated in the mechanism for these sensors. Work on Pt nanowires lead in 2015 to the preparation of Pd nanowires that were electrochemically modified with thin Pt layers (Pd@Pt nanowires). Relative to Pd nanowires, Pt@Pd nanowires showed accelerated response and recovery to H2 while retaining the same high sensitivity to H2 concentration seen for sensors based upon pure Pd nanowires. A new chapter in H2 sensing (2017) involves the replacement of metal nanowires with carbon nanotube ropes decorated with electrodeposited Pd nanoparticles (NPs). Even higher sensitivity and faster sensor response and recovery are enabled by this sensor architecture. Sensor properties are strongly dependent on the size and size monodispersity of the Pd NPs, with smaller NPs yielding higher sensitivity and more rapid response/recovery. We hope the lessons learned from this science over 15 years will catalyze the development of sensors based upon electrodeposited nanomaterials for gases other than H2.

100k Cycles and Beyond: Extraordinary Stability for Au@MnO2 core@Shell nanowires Imparted By Gel Electrolyte

Nanowire electrodes for batteries and super-capacitors can produce superior power relative to films of the same materials. But the Achilles Heel of nanowires is their susceptibility to degradation and failure. This is particularly true for materials that employ Faradaic reactions requiring ion insertion/deinsertion for energy storage. Here we describe the preparation of arrays of ultra-long (2.5 mm) Gold@δ-MnO2, core@shell nanowires. Gold@δ-MnO2 nanowires are prepared on glass surfaces using a variant of the Lithographically Patterned Nanowire Electrodeposition process. All nanowire capacitors prpared from these ultra-long nanowires produce state-of-the-art metrics for energy and power in MnO2-based systems, but they demonstrate irreversible capacity loss exceeding 20% in ≈4000 - 8000 cycles. This cycle stability nevertheless represents the state-of-the-art for MnO2 based electrodes. Now we have discovered that the addition of poly(methylmethacrylate)(PMMA) to the propylene carbonate (PC) electrolyte extends cycle stability to 100,000 cycles and beyond. Other energy storage metrics of these systems are not degraded by the PMMA. In this talk, I describe the results of our experiments, and present data on the charactrerization of these Gold@δ-MnO2 nanowire systems in PC electrolyte and PC electrolyte containing PMMA, before and after cycling.

Lithographically Patterned PEDOT Nanowires for the Detection of Iron(III) with Nanomolar Sensitivity.

Arrays of nanowires of an electronically conductive polymeric affinity medium tailored to the detection of Fe(III) are prepared, and their properties for detecting Fe(III) are evaluated. This polymeric affinity medium consists of poly(3,4-ethylenedioxythiophene) (PEDOT) into which an iron chelator, deferoxamine (DFA), has been doped during the polymerization process. PEDOT-DFA nanowires are potentiostatically deposited from a solution containing both EDOT and DFA using lithographically patterned nanowire electrodeposition (LPNE). The through-nanowire electrical resistance of PEDOT-DFA nanowires is measured as a function of the Fe(III) concentration. In parallel with measurements on PEDOT-DFA nanowire arrays, the electrochemical impedance of PEDOT-DFA films is characterized as a function of the Fe(III) concentration and the frequency of the impedance measurement in order to better understand the mechanism of transduction. PEDOT-DFA nanowires detect Fe(III) from 10(-4) to 10(-8) M with a limit of detection of 300 pM (calculated) and 10 nM (measured).

A 30 μm Coaxial Nanowire Photoconductor Enabling Orthogonal Carrier Collection.

We describe the preparation and properties of a coaxial, three-layer, gold-CdSe-gold nanowire 30 μm in length that functions as a monolithic photodetector. The gold (Au) electrode core of this sandwich structure is prepared using the lithographically patterned nanowire electrodeposition (LPNE) method on a glass surface. A CdSe shell of defined thickness, dCdSe, from 200 to 280 nm is then electrodeposited on this Au nanowire. Finally, a conformal gold layer is electrodeposited on top of the CdSe shell. The two concentric gold electrodes within this architecture measure the photoconductivity of the ultrathin CdSe absorbing layer in the direction orthogonal to the nanowire axis. This architecture enables accelerated response/recovery of the nanowire to light while simultaneously maximizing the photoconductive gain without relinquishing any of the photoresponsive area of a "bare" nanowire. Characterization by scanning electron microscopy (SEM) of focused ion beam (FIB) cross sections together with electron dispersive X-ray spectroscopy (EDS) reveal the distinct core-multishell nanostructure, layer thicknesses, and layer compositions. The position-dependent photoresponse along the axis of the nanowire, probed using a laser spot, shows that the Au nanoshell significantly enhances the photocurrent. The performance of Au-CdSe-Au core-multishell nanowire photodetectors depend sensitively on the thickness of CdSe nanoshell over the range of from 200 nm < dCdSe < 280 nm. The highest performance was obtained for the dCdSe = 250 nm this device, which showed a photoconductive gain of 2172, a responsivity of 209 A·W(-1), a response time of 17 μs, and a recovery time of 96 μs.

(Invted) Catalytically Accelerated Hydrogen Gas Detection at Electrodeposited Pd@Pt Nanowires

Platinum (Pt)-modified palladium (Pd) nanowires (or Pd@Pt nanowires) are prepared with control of the Pt coverage. These Pd@Pt nanowires are used as resistive gas sensors for the detection of hydrogen gas in air, and the influence of the Pt surface layer is assessed. Pd nanowires with dimensions of 60 nm(h) × 200 nm(w) × 20 μm(l) are first prepared using lithographically-patterned nanowire electrodeposition (LPNE). A thin Pt surface layer is elec- trodeposited conformally onto a Pd nanowire at coverages, θPt , of 0.10 monolayer (ML), 1.0 ML, and 10 ML. X-ray photoelectron spectroscopy coupled with scanning electron microscopy and electrochemical measurements are consistent with a layer-by-layer deposition mode for Pt on the Pd nanowire surface. The resistance of a single Pd@Pt nanowire is measured during the exposure of these nanowires to pulses of hydrogen gas in air at concentrations ranging from 0.04 to 5.0 vol%. Both Pd nanowires and Pd@Pt nanowires show a prompt increase in resistance upon exposure to H2 in air, caused by the conversion of Pd to more resistive PdHx. The Pt surface layer present at Pd@Pt nanowires modifies the response to H2 in two ways: First, the amplitude of the resistance change is reduced as θPt is increased, and second, the rate at which the nanowire resistance attains a new steady-state value upon a change in the H2 concentration - characterized by the response time and the recovery time - is accelerated. An optimum Pt coverage of θPt = 1.0 ML is seen for which nanowire response is accelerated by a factor of 2-3 while the recovery from H2 exposure is faster by an order of magnitude as compared to a pure Pd nanowire operating at the same elevated temperature.

Gold Nanowire Thermophones

We report the investigation of thermophones consisting of arrays of ultralong (mm scale) polycrystalline gold nanowires. Arrays of ∼4000 linear gold nanowires are fabricated at 5 μm pitch on glass surfaces using lithographically patterned nanowire electrodeposition (LPNE). The properties of nanowire arrays for generating sound are evaluated as a function of frequency (from 5–120 kHz), angle from the plane of the nanowires, input power (from 0.30–2.5 W), and the width of the nanowires in the array (from 270 to 500 nm). Classical theory for thermophones based on metal films accurately predicts the measured properties of these gold nanowire arrays. Angular “nodes” for the off-axis sound pressure level (SPL) versus frequency data, predicted by the directivity factor, are faithfully reproduced by these nanowire arrays. The maximum efficiency of these arrays (∼10–10 at 25 kHz), the power dependence, and the frequency dependence is independent of the lateral dimensions of these wires over the range from 270 to 5...

Single Tellurium Nanoribbon-Based NOx Gas Sensor

We investigated sensing properties on NOx (g) of single tellurium (Te) nanoribbon-based nanodevices. The synthesized Te nanoribbons were aligned at well-defined locations using a lithographically patterned nanowire electrodeposition (LPNE) technique. The shape and length were feasibly modified by the designed lithographical pattern, and the width was tailored by the applied electrochemical parameters. Temperature-dependent electrical resistance was analyzed with as-synthesized and annealed Te nanoribbons in the temperature ranging from 15 K to 300 K, where the calculated thermal activation energies of as-synthesized and annealed Te nanoribbons were 35.7 meV and 19.2 meV, respectively. Room-temperature sensor performance of as-synthesized and annealed single Te nanoribbons on detecting NOx (g) was investigated as a function of the tailored concentration of NOx (g). Compared to a sensitivity of 16±2.9% on detecting NOx (g) of 10 ppm in the as-synthesized single Te nanoribbon sensor aligned between Au/Cr electrodes, the sensitivity of 21±3.2% on detecting NOx (g) of 10 ppm at room temperature was demonstrated by single Te nanoribbon-based sensor annealed at 200oC for 1 hour in 5 % H2/N2 (g). The effects of annealing on sensing properties have demonstrated the improved sensitivity in the annealed Te nanoribbons..

A Lithographically Patterned Capacitor with Horizontal Nanowires of Length 2.5 mm

A symmetrical hybrid capacitor consisting of interdigitated, horizontal nanowires is described. Each of the 750 nanowires within the capacitor is 2.5 mm in length, consisting of a gold nanowire core (40 × ≈200 nm) encapsulated within a hemicylindrical shell of δ-phase MnO2 (thickness = 60-220 nm). These Au@δ-MnO2 nanowires are patterned onto a planar glass surface using lithographically patterned nanowire electrodeposition (LPNE). A power density of 165 kW/kg and energy density of 24 Wh/kg were obtained for a typical nanowire array in which the MnO2 shell thickness was 68 ± 8 nm. Capacitors incorporating these ultralong nanowires lost ≈10% of their capacity rapidly, during the first 20 discharge cycles, and then retained 90% of their maximum capacity for the ensuing 6000 cycles. The ability of capacitors consisting of ultralong Au@δ-MnO2 nanowires to simultaneously deliver high power and high capacity with acceptable cycle life is demonstrated.

Laser Annealing of Nanocrystalline Gold Nanowires

The efficacy of laser annealing for the thermal annealing of nanocrystalline gold nanowires is evaluated. Continuous laser illumination at 532 nm, focused to a 0.5 μm diameter spot, was rastered perpendicular to the axis of nanocrystalline gold nanowire at ∼2 kHz. This rastered beam was then scanned down the nanowire at velocities from 7 to 112 nm/s. The influence on the electrical resistance of the gold nanowire of laser power, polarization, translation speed, and nanowire width were evaluated. Nanocrystalline gold nanowires were prepared on glass surfaces using the lithographically patterned nanowire electrodeposition (LPNE) method. These nanowires had a rectangular cross section with a height of 20 (± 3) nm and widths ranging from 76 to 274 nm. The 4-contact electrical resistance of the nanowire is measured in situ during laser annealing and a real-time decrease in electrical resistance of between 30 and 65% is observed, depending upon the laser power and scan rate along the nanowire. These resistance decreases are associated with an increase in the mean grain diameter within these nanowires, measured using transmission electron microscopy, of up to 300%. The observed decrease in the electrical resistance induced by laser annealing conforms to classical predictions based upon the reduction in grain boundary scattering induced by grain growth.

Polypyrrole nanoribbon based chemiresistive immunosensors for viral plant pathogen detection

Label-free chemiresistive sensors based on a polypyrrole (PPy) nanoribbon (width: 500 nm, thickness: 25–100 nm) were batch-fabricated by a lithographically patterned nanowire electrodeposition (LPNE) technique. A plant pathogen specific antibody was covalently conjugated on the surface of the structure via N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (EDC)/N-hydrosuccinimide (NHS) crosslinking. The sensing performance was investigated by the detection of cucumber mosaic virus (CMV). The sensitivity of the nano-immunosensors was enhanced by reducing the electrical conductivity from 1 to 0.005 S cm−1 or by decreasing the thickness of the nanoribbon from 100 nm to 25 nm. The reduction in the ionic strength of the pH buffer solutions (i.e., 10 mM PBS to 10 mM PB) also enhanced the sensitivity. However, the reliability and reproducibility of the sensors were significantly reduced by the buffer change. The optimum sensor showed excellent sensitivity with a low and upper detection limit of 10 ng ml−1 and 100 μg ml−1, respectively, which is much lower than the low detection limit of traditional enzyme-linked immunosorbent assays (ELISAs) (i.e., 3 μg ml−1).

Field-Effect Transistors from Lithographically Patterned Cadmium Selenide Nanowire Arrays

Field-effect transistors (NWFETs) have been prepared from arrays of polycrystalline cadmium selenide (pc-CdSe) nanowires using a back gate configuration. pc-CdSe nanowires were fabricated using the lithographically patterned nanowire electrodeposition (LPNE) process on SiO(2)/Si substrates. After electrodeposition, pc-CdSe nanowires were thermally annealed at 300 °C × 4 h either with or without exposure to CdCl(2) in methanol-a grain growth promoter. The influence of CdCl(2) treatment was to increase the mean grain diameter from 10 to 80 nm as determined by grazing incidence X-ray diffraction and to convert the crystal structure from cubic to wurtzite. Measured transfer characteristics showed an increase of the field effect mobility (μ(eff)) by an order of magnitude from 1.94 × 10(-4) cm(2)/(V s) to 23.4 × 10(-4) cm(2)/(V s) for pc-CdSe nanowires subjected to the CdCl(2) treatment. The CdCl(2) treatment also reduced the threshold voltage (from 20 to 5 V) and the subthreshold slope (by ~35%). Transfer characteristics for pc-CdSe NWFETs were also influenced by the channel length, L. For CdCl(2)-treated nanowires, μ(eff) was reduced by a factor of eight as L increased from 5 to 25 μm. These channel length effects are attributed to the presence of defects including breaks and constrictions within individual pc-CdSe nanowires.

Lithographically Patterned Gold/Manganese Dioxide Core/Shell Nanowires for High Capacity, High Rate, and High Cyclability Hybrid Electrical Energy Storage

We describe the fabrication of arrays of nanowires on glass in which a gold core nanowire is encapsulated within a hemicylindrical shell of manganese dioxide. Arrays of linear gold (Au) nanowires are first prepared on glass using the lithographically patterned nanowire electrodeposition (LPNE) method. These Au nanowires have a rectangular cross-section with a width and height of ≈200 and 40 nm, respectively, and lengths in the 1 mm to 1 cm range. Au nanowires are then used to deposit MnO2 by potentiostatic electrooxidation from Mn2+ solution, forming a conformal, hemicylindrical shell with a controllable diameter ranging from 50 to 300 nm surrounding each Au nanowire. This MnO2 shell is δ-phase and mesoporous, as revealed by X-ray diffraction and Raman spectroscopy. Transmission electron microscopy (TEM) analysis reveals that the MnO2 shell is mesoporous (mp-MnO2), consisting of a network of ≈2 nm fibrils. The specific capacitance, Csp, of arrays of gold:mp-MnO2 nanowires is measured using cyclic voltammetry. For a mp-MnO2 shell thickness of 68 ± 3 nm, core:shell nanowires produce a Csp of 1020 ± 100 F/g at 5 mV/s and 450 ± 70 F/g at 100 mV/s. The cycle stability of this Csp, however, is extremely limited in aqueous electrolyte, decaying by >90% in 100 scans, but after oven drying and immersion in dry 1.0 M LiClO4, acetonitrile, dramatically improved cycle stability is achieved characterized by the absence of Csp fade for 1000 cycles at 100 mV/s. Core:shell nanowires exhibit true hybrid energy storage, as revealed by deconvolution of Csp into insertion and noninsertion components.

Wafer-Scale Fabrication of Nanofluidic Arrays and Networks Using Nanoimprint Lithography and Lithographically Patterned Nanowire Electrodeposition Gold Nanowire Masters

Wafer scale (cm(2)) arrays and networks of nanochannels were created in polydimethylsiloxane (PDMS) from a surface pattern of electrodeposited gold nanowires in a master-replica process and characterized with scanning electron microscopy (SEM), atomic force microscopy (AFM), and fluorescence imaging measurements. Patterns of gold nanowires with cross-sectional dimensions as small as 50 nm in height and 100 nm in width were prepared on silica substrates using the process of lithographically patterned nanowire electrodeposition (LPNE). These nanowire patterns were then employed as masters for the fabrication of inverse replica nanochannels in a special formulation of PDMS. SEM and AFM measurements verified a linear correlation between the widths and heights of the nanowires and nanochannels over a range of 50 to 500 nm. The PDMS replica was then oxygen plasma-bonded to a glass substrate in order to create a linear array of nanofluidic channels (up to 1 mm in length) filled with solutions of either fluorescent dye or 20 nm diameter fluorescent polymer nanoparticles. Nanochannel continuity and a 99% fill success rate was determined from the fluorescence imaging measurements, and the electrophoretic injection of both dye and nanoparticles in the nanochannel arrays was also demonstrated. Employing a double LPNE fabrication method, this master-replica process was also used to create a large two-dimensional network of crossed nanofluidic channels.

Tuning Electrical and Optoelectronic Properties of Single Cadmium Telluride Nanoribbon

Cadmium telluride nanoribbons were synthesized electrochemically and formed into nanodevices using lithographically patterned nanowire electrodeposition (LPNE) that integrated synthesis and device fabrication together. By controlling the composition of the CdTe nanoribbons via adjusting the electrodeposition potential, electrical properties (i.e., electrical resistivity and field-effect transistor (FET) mobility) and optoelectronic property (photocurrent) were tuned and compared. Electrical resistivity of CdTe nanoribbons was strongly dependent on the Te content as higher Te content provided higher carrier concentration. Electrical conduction was limited by grain boundary traps due to polycrystalline structure. FET mobility was correlated to electrical resistivity and photocurrent, and annealed samples showed improved FET mobility, electrical resistivity, and photocurrent response.

Virus–Polymer Hybrid Nanowires Tailored to Detect Prostate-Specific Membrane Antigen

We demonstrate the de novo fabrication of a biosensor, based upon virus-containing poly(3,4-ethylene-dioxythiophene) (PEDOT) nanowires, that detects prostate-specific membrane antigen (PSMA). This development process occurs in three phases: (1) isolation of a M13 virus with a displayed polypeptide receptor, from a library of ≈10(11) phage-displayed peptides, which binds PSMA with high affinity and selectivity, (2) microfabrication of PEDOT nanowires that entrain these virus particles using the lithographically patterned nanowire electrodeposition (LPNE) method, and (3) electrical detection of the PSMA in high ionic strength (150 mM salt) media, including synthetic urine, using an array of virus-PEDOT nanowires with the electrical resistance of these nanowires for transduction. The electrical resistance of an array of these nanowires increases linearly with the PSMA concentration from 20 to 120 nM in high ionic strength phosphate-buffered fluoride (PBF) buffer, yielding a limit of detection (LOD) for PSMA of 56 nM.

Trace Detection of Dissolved Hydrogen Gas in Oil Using a Palladium Nanowire Array

The electrical resistance, R, of an array of 30 palladium nanowires is used to detect the concentration of dissolved hydrogen gas (H(2)) in transformer oil over the temperature range from 21 to 70 °C. The palladium nanowire array (PdNWA), consisting of Pd nanowires ∼100 nm (width), ∼20 nm (height), and 100 μm (length), was prepared using the lithographically patterned nanowire electrodeposition (LPNE) method. The R of the PdNWA increased by up to 8% upon exposure to dissolved H(2) at concentrations above 1.0 ppm and up to 2940 ppm at 21 °C. The measured limit-of-detection for dissolved H(2) was 1.0 ppm at 21 °C and 1.6 ppm at 70 °C. The increase in resistance induced by exposure to H(2) was linear with [H(2)](oil)(1/2) across this concentration range. A PdNWA sensor operating in flowing transformer oil has functioned continuously for 150 days.

Mesoporous Manganese Oxide Nanowires for High-Capacity, High-Rate, Hybrid Electrical Energy Storage

Arrays of mesoporous manganese dioxide, mp-MnO(2), nanowires were electrodeposited on glass and silicon surfaces using the lithographically patterned nanowire electrodeposition (LPNE) method. The electrodeposition procedure involved the application, in a Mn(ClO(4))(2)-containing aqueous electrolyte, of a sequence of 0.60 V (vs MSE) voltage pulses delineated by 25 s rest intervals. This "multipulse" deposition program produced mp-MnO(2) nanowires with a total porosity of 43-56%. Transmission electron microscopy revealed the presence within these nanowires of a network of 3-5 nm diameter fibrils that were X-ray and electron amorphous, consistent with the measured porosity values. mp-MnO(2) nanowires were rectangular in cross-section with adjustable height, ranging from 21 to 63 nm, and adjustable width ranging from 200 to 600 nm. Arrays of 20 nm × 400 nm mp-MnO(2) nanowires were characterized by a specific capacitance, C(sp), of 923 ± 24 F/g at 5 mV/s and 484 ± 15 F/g at 100 mV/s. These C(sp) values reflected true hybrid electrical energy storage with significant contributions from double-layer capacitance and noninsertion pseudocapacitance (38% for 20 nm × 400 nm nanowires at 5 mV/s) coupled with a Faradaic insertion capacity (62%). These two contributions to the total C(sp) were deconvoluted as a function of the potential scan rate.

Characterization and Application of Surface Plasmon-Enhanced Optical Diffraction from Electrodeposited Gold Nanowire Arrays

Arrays of gold nanowires formed by the process of lithographically patterned nanowire electrodeposition (LPNE) were characterized by a combination of SEM, polarized UV-visible absorption spectroscopy and optical diffraction measurements. A transverse localized surface plasmon resonance (LSPR) was observed for gold nanowire arrays with an absorption maximum (λ(max)) that varied with nanowire width. Transmission optical diffraction measurements were measured with the even and odd diffraction orders creating an alternating, out of phase sinusoidal intensity pattern characteristic of the LPNE nanowire arrays. The intensities of the even diffraction order maxima were the strongest for nanowires with a width of 115 ± 10 nm; nanowires of this width exhibit a λ(max) of 635 ± 10 nm, verifying that the transverse LSPR has enhanced the optical diffraction signal. Real time total internal reflection diffraction intensity measurements were used to monitor in situ the electrodeposition of silver monolayers onto the gold nanowire arrays.

Enhanced Thermoelectric Metrics in Ultra-long Electrodeposited PEDOT Nanowires

The Seebeck coefficient, S, and the electrical conductivity, σ, of electrodeposited poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires and thin films are reported. PEDOT nanowires were prepared by electropolymerizing 3,4-ethylenedioxythiophene (EDOT) in aqueous LiClO(4) within a template prepared using the lithographically patterned nanowire electrodeposition (LPNE) process. These nanowires were 40-90 nm in thickness, 150-580 nm in width, and 200 μm in length. σ and S were measured from 190 K to 310 K by fabricating heaters and thermocouples on top of arrays of 750 PEDOT nanowires. Such PEDOT nanowire arrays consistently produced S values that were higher than those for PEDOT films: up to -122 μV/K (310 K) for nanowires and up to -57 μV/K (310 K) for films. The sample-to-sample variation in S for 14 samples of PEDOT nanowires and films, across a wide range of critical dimensions, is fully explained by variations in the carrier concentrations in accordance with the Mott equation. In spite of their higher |S| values, PEDOT nanowires also had higher σ than films, on average, because electron mobilities were greater in nanowires by a factor of 3.

High-Throughput Measurement of the Seebeck Coefficient and the Electrical Conductivity of Lithographically Patterned Polycrystalline PbTe Nanowires

A high-throughput method for measuring the Seebeck coefficient, S, and the electrical conductivity, σ, of lithographically patterned nanowire arrays is described. Our method involves the microfabrication of two heaters and two Ag/Ni thermocouples, literally on top of an array of polycrystalline PbTe nanowires synthesized on a Si3N4 wafer using the lithographically patterned nanowire electrodeposition (LPNE) method. This strategy eliminates the transfer and manipulation of nanowires as a prerequisite for carrying out measurements on these wires of thermoelectric metrics. With these devices, we have measured the influence of the thermal annealing temperature on the thermoelectric properties of nine arrays of 60 nm ×200 nm × 200 μm PbTe nanowires, and we find that at an optimum annealing temperature of 453 K, the S at 300 K is increased from −41 μV/K for unannealed wires to −479 μV/K, 80% larger in magnitude than the S (−260 μV/K) of bulk PbTe.