Fang Qian

Nanowire electronic and optoelectronic devices

Electronic and optoelectronic devices impact many areas of society, from simple household appliances and multimedia systems to communications, computing, and medical instruments. Given the demand for ever more compact and powerful systems, there is growing interest in the development of nanoscale devices that could enable new functions and/or greatly enhanced performance. Semiconductor nanowires are emerging as a powerful class of materials that, through controlled growth and organization, are opening up substantial opportunities for novel nanoscale photonic and electronic devices. We review the broad array of nanowire building blocks available to researchers and discuss a range of electronic and optoelectronic nanodevices, as well as integrated device arrays, that could enable diverse and exciting applications in the future.

Nitrogen-Doped ZnO Nanowire Arrays for Photoelectrochemical Water Splitting

We report the rational synthesis of nitrogen-doped zinc oxide (ZnO:N) nanowire arrays, and their implementation as photoanodes in photoelectrochemical (PEC) cells for hydrogen generation from water splitting. Dense and vertically aligned ZnO nanowires were first prepared from a hydrothermal method, followed by annealing in ammonia to incorporate N as a dopant. Nanowires with a controlled N concentration (atomic ratio of N to Zn) up to approximately 4% were prepared by varying the annealing time. X-ray photoelectron spectroscopy studies confirm N substitution at O sites in ZnO nanowires up to approximately 4%. Incident-photon-to-current-efficiency measurements carried out on PEC cell with ZnO:N nanowire arrays as photoanodes demonstrate a significant increase of photoresponse in the visible region compared to undoped ZnO nanowires prepared at similar conditions. Mott-Schottky measurements on a representative 3.7% ZnO:N sample give a flat-band potential of -0.58 V, a carrier density of approximately 4.6 x 10(18) cm(-3), and a space-charge layer of approximately 22 nm. Upon illumination at a power density of 100 mW/cm(2) (AM 1.5), water splitting is observed in both ZnO and ZnO:N nanowires. In comparison to ZnO nanowires without N-doping, ZnO:N nanowires show an order of magnitude increase in photocurrent density with photo-to-hydrogen conversion efficiency of 0.15% at an applied potential of +0.5 V (versus Ag/AgCl). These results suggest substantial potential of metal oxide nanowire arrays with controlled doping in PEC water splitting applications.

Multi-quantum-well nanowire heterostructures for wavelength-controlled lasers

Rational design and synthesis of nanowires with increasingly complex structures can yield enhanced and/or novel electronic and photonic functions. For example, Ge/Si core/shell nanowires have exhibited substantially higher performance as field-effect transistors and low-temperature quantum devices compared with homogeneous materials, and nano-roughened Si nanowires were recently shown to have an unusually high thermoelectric figure of merit. Here, we report the first multi-quantum-well (MQW) core/shell nanowire heterostructures based on well-defined III-nitride materials that enable lasing over a broad range of wavelengths at room temperature. Transmission electron microscopy studies show that the triangular GaN nanowire cores enable epitaxial and dislocation-free growth of highly uniform (InGaN/GaN)n quantum wells with n=3, 13 and 26 and InGaN well thicknesses of 1-3 nm. Optical excitation of individual MQW nanowire structures yielded lasing with InGaN quantum-well composition-dependent emission from 365 to 494 nm, and threshold dependent on quantum well number, n. Our work demonstrates a new level of complexity in nanowire structures, which potentially can yield free-standing injection nanolasers.

Double-sided CdS and CdSe quantum dot co-sensitized ZnO nanowire arrays for photoelectrochemical hydrogen generation.

We report the design and characterization of a novel double-sided CdS and CdSe quantum dot cosensitized ZnO nanowire arrayed photoanode for photoelectrochemical (PEC) hydrogen generation. The double-sided design represents a simple analogue of tandem cell structure, in which the dense ZnO nanowire arrays were grown on an indium-tin oxide substrate followed by respective sensitization of CdS and CdSe quantum dots on each side. As-fabricated photoanode exhibited strong absorption in nearly the entire visible spectrum up to 650 nm, with a high incident-photon-to-current-conversion efficiency (IPCE) of approximately 45% at 0 V vs Ag/AgCl. On the basis on a single white light illumination of 100 mW/cm(2), the photoanode yielded a significant photocurrent density of approximately 12 mA/cm(2) at 0.4 V vs Ag/AgCl. The photocurrent and IPCE were enhanced compared to single quantum dot sensitized structures as a result of the band alignment of CdS and CdSe in electrolyte. Moreover, in comparison to single-sided cosensitized layered structures, this double-sided architecture that enables direct interaction between quantum dot and nanowire showed improved charge collection efficiency. Our result represents the first double-sided nanowire photoanode that integrates uniquely two semiconductor quantum dots of distinct band gaps for PEC hydrogen generation and can be possibly applied to other applications such as nanostructured tandem photovoltaic cells.

GaN nanowire lasers with low lasing thresholds

We report optically pumped room-temperature lasing in GaN nanowires grown by metalorganic chemical vapor deposition (MOCVD). Electron microscopy images reveal that the nanowires grow along a nonpolar ⟨11-20⟩ direction, have single-crystal structures and triangular cross sections. The nanowires function as free-standing Fabry–Perot cavities with cavity mode spacings that depend inversely on length. Optical excitation studies demonstrate thresholds for stimulated emission of 22kW∕cm2 that are substantially lower than other previously reported GaN nanowires. Key contributions to low threshold lasing in these MOCVD GaN nanowire cavities and the development of electrically pumped GaN nanowire lasers are discussed.

Supercapacitors Based on Three-Dimensional Hierarchical Graphene Aerogels with Periodic Macropores

Graphene is an atomically thin, two-dimensional (2D) carbon material that offers a unique combination of low density, exceptional mechanical properties, thermal stability, large surface area, and excellent electrical conductivity. Recent progress has resulted in macro-assemblies of graphene, such as bulk graphene aerogels for a variety of applications. However, these three-dimensional (3D) graphenes exhibit physicochemical property attenuation compared to their 2D building blocks because of one-fold composition and tortuous, stochastic porous networks. These limitations can be offset by developing a graphene composite material with an engineered porous architecture. Here, we report the fabrication of 3D periodic graphene composite aerogel microlattices for supercapacitor applications, via a 3D printing technique known as direct-ink writing. The key factor in developing these novel aerogels is creating an extrudable graphene oxide-based composite ink and modifying the 3D printing method to accommodate aerogel processing. The 3D-printed graphene composite aerogel (3D-GCA) electrodes are lightweight, highly conductive, and exhibit excellent electrochemical properties. In particular, the supercapacitors using these 3D-GCA electrodes with thicknesses on the order of millimeters display exceptional capacitive retention (ca. 90% from 0.5 to 10 A·g(-1)) and power densities (>4 kW·kg(-1)) that equal or exceed those of reported devices made with electrodes 10-100 times thinner. This work provides an example of how 3D-printed materials, such as graphene aerogels, can significantly expand the design space for fabricating high-performance and fully integrable energy storage devices optimized for a broad range of applications.

Solar-driven microbial photoelectrochemical cells with a nanowire photocathode.

We report a self-biased, solar-driven microbial photoelectrochemical cell (solar MPC) that can produce sustainable energy through coupling the microbial catalysis of biodegradable organic matter with solar energy conversion. The solar MPC consists of a p-type cuprous oxide nanowire-arrayed photocathode and an electricigen (Shewanella oneidensis MR-1)-colonizing anode, which can harvest solar energy and bioenergy, respectively. The photocathode and bioanode are interfaced by matching the redox potentials of bacterial cells and the electronic bands of semiconductor nanowires. We successfully demonstrated substantial current generation of 200 μA from the MPC device based on the synergistic effect of the bioanode (projected area of 20 cm2) and photocathode (projected area of 4 cm2) at zero bias under white light illumination of 20 mW/cm2. We identified the transition of rate-limiting step from the photocathode to the bioanode with increasing light intensities. The solar MPC showed self-sustained operation for more than 50 h in batch-fed mode under continuous light illumination. The ability to tune the synergistic effect between microbial cells and semiconductor nanowire systems could open up new opportunities for microbial/nanoelectronic hybrid devices with unique applications in energy conversion, environmental protection, and biomedical research.

A room temperature low-threshold ultraviolet plasmonic nanolaser

Constrained by large ohmic and radiation losses, plasmonic nanolasers operated at visible regime are usually achieved either with a high threshold (10(2)-10(4) MW cm(-2)) or at cryogenic temperatures (4-120 K). Particularly, the bending-back effect of surface plasmon (SP) dispersion at high energy makes the SP lasing below 450 nm more challenging. Here we demonstrate the first strong room temperature ultraviolet (~370 nm) SP polariton laser with an extremely low threshold (~3.5 MW cm(-2)). We find that a closed-contact planar semiconductor-insulator-metal interface greatly lessens the scattering loss, and more importantly, efficiently promotes the exciton-SP energy transfer thus furnishes adequate optical gain to compensate the loss. An excitation polarization-dependent lasing action is observed and interpreted with a microscopic energy-transfer process from excitons to SPs. Our work advances the fundamental understanding of hybrid plasmonic waveguide laser and provides a solution of realizing room temperature UV nanolasers for biological applications and information technologies.

Rational growth of branched nanowire heterostructures with synthetically encoded properties and function

Branched nanostructures represent unique, 3D building blocks for the “bottom-up” paradigm of nanoscale science and technology. Here, we report a rational, multistep approach toward the general synthesis of 3D branched nanowire (NW) heterostructures. Single-crystalline semiconductor, including groups IV, III–V, and II–VI, and metal branches have been selectively grown on core or core/shell NW backbones, with the composition, morphology, and doping of core (core/shell) NWs and branch NWs well controlled during synthesis. Measurements made on the different composition branched NW structures demonstrate encoding of functional p-type/n-type diodes and light-emitting diodes (LEDs) as well as field effect transistors with device function localized at the branch/backbone NW junctions. In addition, multibranch/backbone NW structures were synthesized and used to demonstrate capability to create addressable nanoscale LED arrays, logic circuits, and biological sensors. Our work demonstrates a previously undescribed level of structural and functional complexity in NW materials, and more generally, highlights the potential of bottom-up synthesis to yield increasingly complex functional systems in the future.

A mechanistic study into the catalytic effect of Ni(OH)2 on hematite for photoelectrochemical water oxidation

We report a mechanistic study of the catalytic effect of Ni(OH)2 on hematite nanowires for photoelectrochemical water oxidation. Ni compounds have been shown to be good catalysts for electrochemical and photoelectrochemical water oxidation. While we also observed improved photocurrents for Ni-catalyst decorated hematite photoanodes, we found that the photocurrents decay rapidly, indicating the photocurrents were not stable. Importantly, we revealed that the enhanced photocurrent was due to water oxidation as well as the photo-induced charging effect. In addition to oxidizing water, the photoexcited holes generated in hematite efficiently oxidize Ni(2+) to Ni(3+) (0.35 V vs. Ag/AgCl). The instability of photocurrent was due to the depletion of Ni(2+). We proposed that the catalytic mechanism of the Ni(II) catalyst for water oxidation is a two-step process that involves the fast initial oxidation of Ni(2+) to Ni(3+), and followed by the slow oxidation of Ni(3+) to Ni(4+), which is believed to be the active catalytic species for water oxidation. The catalytic effect of the Ni(II) catalyst was limited by the slow formation of Ni(4+). Finally, we elucidated the real catalytic performance of Ni(OH)2 on hematite for photoelectrochemical water oxidation by suppressing the photo-induced charging effect. This work could provide important insights for future studies on Ni based catalyst modified photoelectrodes for water oxidation.