Detecting Electric Dipoles Interaction at the Interface of Ferroelectric and Electrolyte Using Graphene Field Effect Transistors.

Graphene was inserted into the interface between electric dipole layers from DEME-TFSI ionic liquid (top-gate) and ferroelectric Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT, back-gate) to probe the interface dipole-dipole interaction in response to DC and pulsed gate voltages. A highly complicated behavior of the interface dipole-dipole interaction has been revealed as a combination of electrostatic and electrochemical effects. The interfacial polar molecules in the DEME-TFSI electrical double layer are pinned with assistance from the PLZT back-gate in response to a DC top-gate pump, leading to strong nonlinear electrochemical behavior. In contrast, depinning of these molecules can be facilitated by a faster pulsed top-gate pump, which results in a characteristic linear electrostatic behavior. This result not only sheds light on the dynamic dipole-dipole interactions on the interface between functional materials but also prototypes a unique pump and probe approach using graphene field effect transistors to detect the interface dipole-dipole interaction.

Graphene/GaSe-Nanosheet Hybrid: Towards High Gain and Fast Photoresponse.

While high photoconductive gain has been recently achieved in graphene-based hybrid phototransistors using semiconductor two-dimensional transition/post-transition metal dichalcogenides or quantum dots sensitizers, obtaining fast photoresponse simutaneously remains a challenge that must be addressed for practical applications. In this paper we report a graphene/GaSe nanosheets hybrid photodetector, in which GaSe nanosheets provide a favorable geometric link to graphene conductive layer through van Der Waals force. After a vacuum annealing process, a high gain in exceeding 10(7) has been obtained simitaneously with a dynamic response time of around 10 ms for both light on and off. We attribute the high performance to the elimination of possible deep charge traps, most probably at the graphene/GaSe nanosheets interface. This result demonstrates high photoconductive gain and fast photoresponse can be achieved simultaneously and a clean interface is the key to the high performance of these hybrid devices.

Detangling extrinsic and intrinsic hysteresis for detecting dynamic switch of electric dipoles using graphene field-effect transistors on ferroelectric gates.

A transition in source-drain current vs. back gate voltage (ID-VBG) characteristics from extrinsic polar molecule dominant hysteresis to anti-hysteresis induced by an oxygen deficient surface layer that is intrinsic to the ferroelectric thin films has been observed on graphene field-effect transistors on Pb0.92La0.08Zr0.52Ti0.48O3 gates (GFET/PLZT-Gate) during a vacuum annealing process developed to systematically remove the polar molecules adsorbed on the GFET channel surface. This allows the extrinsic and intrinsic hysteresis on GFET/PLZT-gate devices to detangle and the detection of the dynamic switch of electric dipoles using GFETs, taking advantage of their high gating efficiency on ferroelectric gate. A model of the charge trapping and pinning mechanism is proposed to successfully explain the transition. In response to pulsed VBG trains of positive, negative, as well as alternating polarities, respectively, the source-drain current ID variation is instantaneous with the response amplitude following the ID-VBG loops measured by DC VBG with consideration of the remnant polarization after a given VBG pulse when the gate electric field exceeds the coercive field of the PLZT. A detection sensitivity of around 212 dipole per µm(2) has been demonstrated at room temperature, suggesting the GFET/ferroelectric-gate devices provide a promising high-sensitivity scheme for uncooled detection of electrical dipole dynamic switch.

Dimension effect on the performance of carbon nanotube nanobolometers.

Carbon nanotube (CNT) film nanobolometers take advantages of high infrared absorption of CNTs, proving a promising alternative for low-cost, uncooled infrared detection. The performance of the CNT nanobolometers is determined by the optoelectronic process on CNTs at a microscopic scale, which links intimately to the diameter of the CNT-a critical parameter that intrinsically affects the band gap and hence infrared absorption, as well as extrinsically affects the surface oxygen adsorption effect and thermal-link of the CNT detector element to the environment. Both the intrinsic and extrinsic factors play important roles in the photoresponse, noise spectrum and the figure-of-merit detectivity D* of the CNT nanobolometers and their interplay determines the device's ultimate performance. In this work, we present a systematic study of the effect of CNT diameter in the range of 1-50 nm on the physical properties relevant to CNT nanobolometers. The optimal CNT diameter was found to be in the range of 2-12 nm with the D* up to 3.3 × 10(7) cm(Hz)(1/2) W(-1), which represents an order of magnitude improvement over the best D* reported previously on CNT film nanobolometers.

Integrating atomic layer deposition and ultra-high vacuum physical vapor deposition for in situ fabrication of tunnel junctions.

Atomic Layer Deposition (ALD) is a promising technique for growing ultrathin, pristine dielectrics on metal substrates, which is essential to many electronic devices. Tunnel junctions are an excellent example which require a leak-free, ultrathin dielectric tunnel barrier of typical thickness around 1 nm between two metal electrodes. A challenge in the development of ultrathin dielectric tunnel barriers using ALD is controlling the nucleation of dielectrics on metals with minimal formation of native oxides at the metal surface for high-quality interfaces between the tunnel barrier and metal electrodes. This poses a critical need for integrating ALD with ultra-high vacuum (UHV) physical vapor deposition. In order to address these challenges, a viscous-flow ALD chamber was designed and interfaced to an UHV magnetron sputtering chamber via a load lock. A sample transportation system was implemented for in situ sample transfer between the ALD, load lock, and sputtering chambers. Using this integrated ALD-UHV sputtering system, superconductor-insulator-superconductor (SIS) Nb-Al/Al2O2/Nb Josephson tunnel junctions were fabricated with tunnel barriers of thickness varied from sub-nm to ~1 nm. The suitability of using an Al wetting layer for initiation of the ALD Al2O3 tunnel barrier was investigated with ellipsometry, atomic force microscopy, and electrical transport measurements. With optimized processing conditions, leak-free SIS tunnel junctions were obtained, demonstrating the viability of this integrated ALD-UHV sputtering system for the fabrication of tunnel junctions and devices comprised of metal-dielectric-metal multilayers.

Atomic layer deposition of Al-doped ZnO/Al2O3 double layers on vertically aligned carbon nanofiber arrays.

High-aspect-ratio, vertically aligned carbon nanofibers (VACNFs) were conformally coated with aluminum oxide (Al2O3) and aluminum-doped zinc oxide (AZO) using atomic layer deposition (ALD) in order to produce a three-dimensional array of metal-insulator-metal core-shell nanostructures. Prefunctionalization before ALD, as required for initiating covalent bonding on a carbon nanotube surface, was eliminated on VACNFs due to the graphitic edges along the surface of each CNF. The graphitic edges provided ideal nucleation sites under sequential exposures of H2O and trimethylaluminum to form an Al2O3 coating up to 20 nm in thickness. High-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy images confirmed the conformal core-shell AZO/Al2O3/CNF structures while energy-dispersive X-ray spectroscopy verified the elemental composition of the different layers. HRTEM selected area electron diffraction revealed that the as-made Al2O3 by ALD at 200 °C was amorphous, and then, after annealing in air at 450 °C for 30 min, was converted to polycrystalline form. Nevertheless, comparable dielectric constants of 9.3 were obtained in both cases by cyclic voltammetry at a scan rate of 1000 V/s. The conformal core-shell AZO/Al2O3/VACNF array structure demonstrated in this work provides a promising three-dimensional architecture toward applications of solid-state capacitors with large surface area having a thin, leak-free dielectric.

High photoresponse in hybrid graphene-carbon nanotube infrared detectors.

Efficient exciton dissociation is crucial to obtaining high photonic response in photodetectors. This work explores implementation of a novel exciton dissociation mechanism through heterojunctions self-assembled at the graphene/MWCNT (multiwall carbon nanotube) interfaces in graphene/MWCNT nanohybrids. Significantly enhanced near-infrared photoresponsivity by nearly an order of magnitude has been achieved on the graphene/MWCNT nanohybrids as compared to the best achieved so far on carbon nanotube (CNT) only infrared (IR) detectors. This leads to a high detectivity up to 1.5 × 10(7) cm·Hz(1/2)·W(-1) in the graphene/MWCNT nanohybrid, which represents a 500% improvement over the best D* achieved on MWCNT film IR detectors and may be further improved with optimization on the interfacial heterojunctions. This approach of the self-assembly of graphene/CNT nanohybrids provides a pathway toward high-performance and low-cost carbon nanostructure IR detectors.

Extraordinary photocurrent harvesting at type-II heterojunction interfaces: toward high detectivity carbon nanotube infrared detectors.

Despite the potentials and the efforts put in the development of uncooled carbon nanotube infrared detectors during the past two decades, their figure-of-merit detectivity remains orders of magnitude lower than that of conventional semiconductor counterparts due to the lack of efficient exciton dissociation schemes. In this paper, we report an extraordinary photocurrent harvesting configuration at a semiconducting single-walled carbon nanotube (s-SWCNT)/polymer type-II heterojunction interface, which provides highly efficient exciton dissociation through the intrinsic energy offset by designing the s-SWCNT/polymer interface band alignment. This results in significantly enhanced near-infrared detectivity of 2.3 × 10(8) cm·Hz(1/2)/W, comparable to that of the many conventional uncooled infrared detectors. With further optimization, the s-SWCNT/polymer nanohybrid uncooled infrared detectors could be highly competitive for practical applications.

Development of nanopatterned fluorine-doped tin oxide electrodes for dye-sensitized solar cells with improved light trapping.

Transparent conductors (TCs) are an important component of optoelectronic devices and nanoscale engineering of TCs is important for optimization of the device performance through improved light trapping. In this work, patterned periodic arrays of nanopillars and nanolines of pitch size of ~700 nm were created on fluorine-doped tin oxide (FTO) using nanoimprint lithography and reactive ion etching using environmentally friendly gases. The patterned FTO exhibits enhanced light trapping as compared to the unpatterned FTO, which agrees well with simulations based on Finite-Difference Time-Domain method for up to a distance of 4 µm. Dye sensitized solar cells (DSSCs) fabricated on the patterned FTO exhibited improved performance (fill factor and power conversion efficiency), which can be attributed to enhanced light absorption in the range 400-650 nm. Further, electrochemical impedance measurements revealed lower recombination resistance for the patterned FTO/TiO(2) electrode compared to the unpatterned FTO electrode/TiO(2) electrode as a result of better light capturing properties of patterned FTO. The direct fabrication of nanopatterns on TCs developed in the present study is expected to be a viable scheme for achieving improved performance in many other optoelectronic devices.

A comparative study of 1/f noise and temperature coefficient of resistance in multiwall and single-wall carbon nanotube bolometers.

The 1/f noise and temperature coefficient of resistance (TCR) are investigated in multiwall carbon nanotube (MWCNT) film bolometers since both affect the bolometer detectivity directly. A comparison is made between the MWCNT film bolometers and their single-wall carbon nanotube (SWCNT) counterparts. The intrinsic noise level in the former has been found at least two orders of magnitude lower than that in the latter, which outweighs the moderately lower TCR absolute values in the former and results in higher bolometer detectivity in MWCNT bolometers. Interestingly, reduced noise and enhanced TCR can be obtained by improving the inter-tube coupling using thermal annealing in both SWCNT and MWCNT films, suggesting much higher detectivity may be achieved via engineering the inter-tube coupling.