seedless-growth-bismuth-nanowire-array-via-vacuum-thermal

Seedless Growth of Bismuth Nanowire Array via Vacuum Thermal Evaporation

Here a seedless and template-free technique is demonstrated to scalably grow bismuth nanowires, through thermal evaporation in high vacuum at RT. ...

Brookhaven National Laboratory

The diameter-dependent Young's modulus, E, and quality factor, Q, of GaN nanowires were measured using electromechanical resonance analysis in a transmission electron microscope. E is close to the theoretical bulk value ( approximately 300 GPa) for a large diameter nanowire (d=84 nm) but is significantly smaller for smaller diameters. At room temperature, Q is as high as 2,800 for d=84 nm, significantly greater than what is obtained from micromachined Si resonators of comparable surface-to-volume ratio. This implies significant advantages of smooth-surfaced GaN nanowire resonators for nanoelectromechanical system (NEMS) applications. Two closely spaced resonances are observed and attributed to the low-symmetry triangular cross section of the nanowires.

Diameter-dependent electromechanical properties of GaN nanowires.

In this work, VO2 nanorods have been initially generated as reactive nanoscale precursors to their subsequent conversion to large quantities of highly crystalline V2O3 with no detectable impurities. Structural changes in VO2, associated with the metallic-to-insulating transition from the monoclinic form to the rutile form, have been investigated and confirmed using X-ray diffraction and synchrotron data, showing that the structural transition is reversible and occurs at around 63 degrees C. When this VO2 one-dimensional sample was subsequently heated to 800 degrees C in a reducing atmosphere, it was successfully transformed into V2O3 with effective retention of its nanorod morphology. We have also collected magnetic and transport data on these systems that are comparable to bulk behavior and consistent with trends observed in previous experiments.

Synthesis and characterization of V2O3 nanorods.

A core phthalocyanine platform allows engineering of the solubility properties the band gap, shifting the maximum absorption toward the red. A simple method for increasing the efficiency of heterojunction solar cells uses a self-organized blend of phthalocyanine chromophores fabricated by solution processing.

Phthalocyanine blends improve bulk heterojunction solar cells.

Realizziamo un&#39;architettura verticale canale polimero semiconduttore campo effetto transistor da confinare il materiale organico all&#39;interno di reticoli di trincee cosparse. Il risparmio di spazio geometrico di un orientamento perpendicolare canale si traduce in dispositivi sourcing areal densit&agrave; di corrente superiori a 40 mA/cm(2), utilizzando una tensione di alimentazione volt uno e mantenendo il dispositivo vicino-ideale le caratteristiche di funzionamento. Transistori con canale verticale hanno una mobilit&agrave; elettronica simile a quella del planar dispositivi usando il semiconduttore stesso polimero, coerenza con un riorientamento molecolare all&#39;interno di imprigionare trincee che comprendiamo attraverso misure di dispersione dei raggi x.

Funzionamento One-volt ad alta corrente verticale canale polimero semiconduttore campo &ndash; transistor ad effetto.

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We realize a vertical channel polymer semiconductor field effect transistor architecture by confining the organic material within gratings of interdigitated trenches. The geometric space savings of a perpendicular channel orientation results in devices sourcing areal current densities in excess of 40 mA/cm(2), using a one-volt supply voltage, and maintaining near-ideal device operating characteristics. Vertical channel transistors have a similar electronic mobility to that of planar devices using the same polymer semiconductor, consistent with a molecular reorientation within confining trenches we understand through X-ray scattering measurements.

One-volt operation of high-current vertical channel polymer semiconductor field-effect transistors.

Nous r&eacute;alisons une canal vertical polym&egrave;re semi-conducteur domaine effet transistor architecture en limitant la mati&egrave;re organique au sein de r&eacute;seaux de tranch&eacute;es interdigit&eacute;es. Les &eacute;conomies de l&#39;espace g&eacute;om&eacute;trique d&#39;une orientation perpendiculaire canal entra&icirc;ne dispositifs sourcing des densit&eacute;s de courant surfaciques sup&eacute;rieure &agrave; 40 mA/cm(2), utilisant une tension d&#39;alimentation v un et entretien pr&egrave;s-id&eacute;al appareil caract&eacute;ristiques de fonctionnement. Transistors canal vertical ont une mobilit&eacute; &eacute;lectronique semblable &agrave; celle des dispositifs planaires en utilisant le m&ecirc;me semi-conducteur de polym&egrave;re, conforme &agrave; une r&eacute;orientation mol&eacute;culaire au sein de confiner les tranch&eacute;es que nous comprendre &agrave; travers des mesures de diffusion des rayons x.

V une op&eacute;ration de haute intensit&eacute; canal vertical polym&egrave;re semi-conducteur field - effect transistors.

Wir realisieren eine vertikale Kanal Polymer Halbleiter Feld Effekt Transistor Architektur durch das organische Material im Gitter der ineinandergreifenden Rinnen zu beschr&auml;nken. Die geometrischen Raum Einsparungen von einer senkrechten Kanal-Orientierung f&uuml;hrt Ger&auml;te Beschaffung areal Stromdichten von mehr als 40 mA/cm(2), mit Hilfe einer ein-Volt Versorgungsspannung und Wartung nahezu ideale Ger&auml;t Laufeigenschaften. Vertikalen Kanal Transistoren haben eine &auml;hnliche elektronische Mobilit&auml;t der planar-Ger&auml;te mit die gleichen Polymer-Halbleiter, mit einer molekularen Neuausrichtung innerhalb der Gr&auml;ben, die wir durch R&ouml;ntgen Streuung Messungen verstehen zu beschr&auml;nken.

Eins-Volt-Betrieb von Hochstrom-vertikalen Kanal Polymer Halbleiter &ndash; Feldeffekt-Transistoren.

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Realizamos una arquitectura campo de transistor de efecto pol&iacute;mero semiconductor canal vertical por el confinamiento de la materia org&aacute;nica dentro de rejillas de trincheras interdigitados. Los ahorros de espacio geom&eacute;trico de una orientaci&oacute;n perpendicular canal resulta en dispositivos abastecimiento corriente densidades superiores a 40 mA/cm(2), con una tensi&oacute;n de alimentaci&oacute;n uno voltios y mantener casi ideales dispositivo caracter&iacute;sticas de funcionamiento. Transistores de canal vertical tienen una movilidad electr&oacute;nica similar a la de dispositivos planares, por el mismo pol&iacute;mero semiconductor, consistente con una reorientaci&oacute;n molecular en el confinamiento de trincheras que entendemos a trav&eacute;s de medidas de dispersi&oacute;n de rayos x.

Voltios una operaci&oacute;n de canal vertical de alta corriente pol&iacute;mero semiconductor transistores efecto de campo.

PEDOT:PSS thin films are successfully patterned using water-vapor-assisted nanoimprinting, a process compatible with organic electronic devices. The imprinted patterns are characterized via grazing-incident small-angle X-ray scattering and scanning electron microscopy. Superior pattern transfer quality of water-vapor assisted nanoimprinting over conventional thermal nanoimprinting is demonstrated.

Water-vapor-assisted nanoimprinting of PEDOT:PSS thin films.

We report chemical modification of self-assembled block copolymer thin films by ultraviolet light that enhances the block-selective affinity of organometallic precursors otherwise lacking preference for either copolymer block. Sequential precursor loading and reaction facilitate formation of zinc oxide, titanium dioxide, and aluminum oxide nanostructures within the polystyrene domains of both lamellar- and cylindrical-phase modified polystyrene-block-poly(methyl methacrylate) thin film templates. Near-edge X-ray absorption fine structure measurements and Fourier transform infrared spectroscopy show that photo-oxidation by ultraviolet light creates Lewis basic groups within polystyrene, resulting in an increased Lewis base-acid interaction with the organometallic precursors. The approach provides a method for generating both aluminum oxide patterns and their corresponding inverses using the same block copolymer template.

Chemically enhancing block copolymers for block-selective synthesis of self-assembled metal oxide nanostructures.

Systematic changes in the exocyclic substiution of core phthalocyanine platform tune the absorption properties to yield commercially viable dyes that function as the primary light absorbers in organic bulk heterojunction solar cells. Blends of these complementary phthalocyanines absorb a broader portion of the solar spectrum compared to a single dye, thereby increasing solar cell performance. We correlate grazing incidence small angle x-ray scattering structural data with solar cell performance to elucidate the role of nanomorphology of active layers composed of blends of phthalocyanines and a fullerene derivative. A highly reproducible device architecture is used to assure accuracy and is relevant to films for solar windows in urban settings. We demonstrate that the number and structure of the exocyclic motifs dictate phase formation, hierarchical organization, and nanostructure, thus can be employed to tailor active layer morphology to enhance exciton dissociation and charge collection efficiencies in the photovoltaic devices. These studies reveal that disordered films make better solar cells, short alkanes increase the optical density of the active layer, and branched alkanes inhibit unproductive homogeneous molecular alignment.

Controlling Morphology and Molecular Packing of Alkane Substituted Phthalocyanine Blend Bulk Heterojunction Solar Cells.

We report the synthesis of a tellurophene-containing low-bandgap polymer, PDPPTe2T, by microwave-assisted palladium-catalyzed ipso-arylative polymerization of 2,5-bis[(α-hydroxy-α,α-diphenyl)methyl]tellurophene with a diketopyrrolopyrrole (DPP) monomer. Compared with the corresponding thiophene analog, PDPPTe2T absorbs light of longer wavelengths and has a smaller bandgap. Bulk heterojunction solar cells prepared from PDPPTe2T and PC71 BM show PCE values of up to 4.4%. External quantum efficiency measurements show that PDPPTe2T produces photocurrent at wavelengths up to 1 µm. DFT calculations suggest that the atomic substitution from sulfur to tellurium increases electronic coupling to decrease the length of the carbon-carbon bonds between the tellurophene and thiophene rings, which results in the red-shift in absorption upon substitution of tellurium for sulfur.

Polymerization of tellurophene derivatives by microwave-assisted palladium-catalyzed ipso-arylative polymerization.

We report high-yield room-temperature growth of vertical single-crystalline bismuth nanowire array by vacuum thermal evaporation of bismuth over a choice of arbitrary substrate coated with a thin interlayer of nanoporous vanadium. The nanowire growth is the result of spontaneous and continuous expulsion of nanometer-sized bismuth domains from the vanadium pores, driven by their excessive surface energy that suppresses the melting point of bismuth close to room temperature. The simplicity of the technique opens a new avenue for the growth of nanowire arrays of a variety of materials.

Surface-energy induced formation of single crystalline bismuth nanowires over vanadium thin film at room temperature.

We report an efficiency of 6.1% for a solution-processed non-fullerene solar cell using a helical perylene diimide (PDI) dimer as the electron acceptor. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor-acceptor interfaces, indicating that charge carriers are created from photogenerated excitons in both the electron donor and acceptor phases. Light-intensity-dependent current-voltage measurements suggested different recombination rates under short-circuit and open-circuit conditions.

Efficient organic solar cells with helical perylene diimide electron acceptors.

The vertical integration of graphene with inorganic semiconductors, oxide semiconductors, and newly emerging layered materials has recently been demonstrated as a promising route toward novel electronic and optoelectronic devices. Here, we report organic thin film transistors based on vertical heterojunctions of graphene and organic semiconductors. In these thin heterostructure devices, current modulation is accomplished by tuning of the injection barriers at the semiconductor/graphene interface with the application of a gate voltage. N-channel devices fabricated with a thin layer of C60 show a room temperature on/off ratio >10(4) and current density of up to 44 mAcm(-2). Because of the ultrashort channel intrinsic to the vertical structure, the device is fully operational at a driving voltage of 200 mV. A complementary p-channel device is also investigated, and a logic inverter based on two complementary transistors is demonstrated. The vertical integration of graphene with organic semiconductors via simple, scalable, and low-temperature fabrication processes opens up new opportunities to realize flexible, transparent organic electronic, and optoelectronic devices.

Low-voltage organic electronics based on a gate-tunable injection barrier in vertical graphene-organic semiconductor heterostructures.

A major challenge in nanofabrication is to pattern unconventional substrates that cannot be processed for a variety of reasons, such as incompatibility with spin coating, electron beam lithography, optical lithography, or wet chemical steps. Here, we present a versatile nanofabrication method based on re-usable silicon membrane hard masks, patterned using standard lithography and mature silicon processing technology. These masks, transferred precisely onto targeted regions, can be in the millimetre scale. They allow for fabrication on a wide range of substrates, including rough, soft, and non-conductive materials, enabling feature linewidths down to 10 nm. Plasma etching, lift-off, and ion implantation are realized without the need for scanning electron/ion beam processing, UV exposure, or wet etching on target substrates.

Nanofabrication on unconventional substrates using transferred hard masks.

We present a methodology for probing light-matter interactions in prototype photovoltaic devices consisting of an organic semiconductor active layer with a semitransparent metal electrical contact exhibiting surface plasmon-based enhanced optical transmission. We achieve high-spectral irradiance in a spot size of less than 100 μm using a high-brightness laser-driven light source and appropriate coupling optics. Spatially resolved Fourier transform photocurrent spectroscopy in the visible and near-infrared spectral regions allows us to measure external quantum efficiency with high sensitivity in small-area devices (<1 mm(2)). This allows for rapid fabrication of variable-pitch sub-wavelength hole arrays in metal films for use as transparent electrical contacts, and evaluation of the evanescent and propagating mode coupling to resonances in the active layer.

Characterization of plasmonic hole arrays as transparent electrical contacts for organic photovoltaics using high-brightness Fourier transform methods.

Despite numerous organic semiconducting materials synthesized for organic photovoltaics in the past decade, fullerenes are widely used as electron acceptors in highly efficient bulk-heterojunction solar cells. None of the non-fullerene bulk heterojunction solar cells have achieved efficiencies as high as fullerene-based solar cells. Design principles for fullerene-free acceptors remain unclear in the field. Here we report examples of helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor-acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometres in diameter for efficient exciton separation and charge transport. This study describes a new motif for designing highly efficient acceptors for organic solar cells.

Molecular helices as electron acceptors in high-performance bulk heterojunction solar cells.

Excitonic energy transfer (ET) offers exciting opportunities for advances in optoelectronic devices such as solar cells. While recent experimental attempts have demonstrated its potential in both organic and inorganic photovoltaics (PVs), what remains to be addressed is quantitative understanding of how different ET modes contribute to PV performance and how ET contribution is differentiated from the classical optical coupling (OC) effects. In this study, we implement an ET scheme using a PV device platform, comprising CdSe/ZnS nanocrystal energy donor and 500 nm-thick ultrathin Si acceptor layers, and present the quantitative mechanistic description of how different ET modes, distinguished from the OC effects, increase the light absorption and PV efficiency. We find that nanocrystal sensitization enhances the short circuit current of ultrathin Si solar cells by up to 35%, of which the efficient ET, primarily driven by a long-range radiative mode, contributes to 38% of the total current enhancement. These results not only confirm the positive impact of ET but also provide a guideline for rationally combining the ET and OC effects for improved light harvesting in PV and other optoelectronic devices.

Enhanced photovoltaic performance of ultrathin Si solar cells via semiconductor nanocrystal sensitization: energy transfer vs. optical coupling effects.

Chang-Yong Nam

Center for Functional Nanomaterials

Brookhaven National Laboratory

Seedless Growth of Bismuth Nanowire Array via Vacuum Thermal Evaporation

Chang-Yong Nam

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Center for Functional Nanomaterials

Brookhaven National Laboratory

Seedless Growth of Bismuth Nanowire Array via Vacuum Thermal Evaporation

Center for Functional Nanomaterials