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An improved method is described for the production of graphene oxide nanoribbons (GONRs) via longitudinal unzipping of multiwalled carbon nanotubes. The method produces GONRs with fewer defects and/or holes on the basal plane, maintains narrow ribbons <100 nm wide, and maximizes the high aspect ratio. Changes in the reaction conditions such as acid content, time, and temperature were investigated. The new, optimized method which introduces a second, weaker acid into the system, improves the selectivity of the oxidative unzipping presumably by in situ protection of the vicinal diols formed on the basal plane of graphene during the oxidation, and thereby prevents their overoxidation and subsequent hole generation. The optimized GONRs exhibit increased electrical conductivity over those chemically reduced nanoribbons produced by previously reported procedures.
ACS Nano 2010 Apr 27
PMID:Lower-defect graphene oxide nanoribbons from multiwalled carbon nanotubes. 2042 Apr 68

The new metalloporphyrin Pt(tptnp), where tptnp = tetraphenyltetranaphtho[2,3]porphyrin, has been prepared and subjected to photophysical and electrooptical device studies. In degassed toluene solution at room temperature Pt(tptnp) features efficient phosphorescence emission with lambda(max) 883 nm with a quantum efficiency of 0.22. The complex has been used as the active phosphor in polymer and organic light-emitting diodes. Polymer light-emitting diodes based on a spin-coated emissive layer consisting of a blend of Pt(tptnp) doped in poly(9-vinylcarbazole) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole exhibit near-IR emission with lambda(max) 896 nm, with a maximum external quantum efficiency (EQE) of 0.4% and a maximum radiant emittance of 100 muW/cm(2). Organic light-emitting diodes prepared via vapor deposition of all layers and that feature an optimized multilayer hole injection and electron blocking layer heterostructure with an emissive layer consisting of 4,4'-bis(carbazol-9-yl)biphenyl (CBP) doped with Pt(tptnp) exhibit a maximum EQE of 3.8% and a maximum radiant emittance of 1.8 mW/cm(2). The polymer and organic light-emitting diodes characterized in this study exhibit record high efficiency for devices that emit in the near-IR at lambda >800 nm.
ACS Appl Mater Interfaces 2009 Feb
PMID:Efficient near-infrared polymer and organic light-emitting diodes based on electrophosphorescence from (tetraphenyltetranaphtho[2,3]porphyrin)platinum(II). 2035 14

We present first-principles transport calculations of graphene nanoribbons with chemically reconstructed edge profiles. Depending on the geometry of the defect and the degree of hydrogenation, spectacularly different transport mechanisms are obtained. In the case of monohydrogenated pentagon (heptagon) defects, an effective acceptor (donor) character results in strong electron-hole conductance asymmetry. In contrast, weak backscattering is obtained for defects that preserve the benzenoid structure of graphene. Based on a tight-binding model derived from ab initio calculations, evidence for large conductance scaling fluctuations are found in disordered ribbons with lengths up to the micrometer scale.
ACS Nano 2010 Apr 27
PMID:Quantum transport in graphene nanoribbons: effects of edge reconstruction and chemical reactivity. 2035 32

Two novel soluble compounds T and A that contain a central dihexyloxy-p-phenylenevinylene unit, intermediate moieties of thiophene or anthracene, respectively, and terminal cyano-vinylene nitrophenyls were synthesized and characterized. They showed moderate thermal stability and relatively low glass transition temperatures. These compounds displayed similar optical properties. Their absorption was broad and extended up to about 750 nm with the longer-wavelength maximum around 640 nm and an optical band gap of approximately 1.70 eV. From the current-voltage characteristics of the devices using both compounds T and A, it was concluded that both compounds behave as p-type organic semiconductors with hole mobility on the order of 10(-5) cm(2)/(V s). The power conversion efficiency (PCE) of the devices based on these compounds was 0.019% and 0.013% for compounds A and T, respectively. When compounds A and T were blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the PCE dramatically increased up to 1.66% and 1.36% for devices with A:PCBM and T:PCBM, respectively. The efficiencies of the devices were further enhanced upon thermal annealing up to 2.49% and 2.33% for devices based on A:PCBM and T:PCBM, respectively.
ACS Appl Mater Interfaces 2009 Aug
PMID:Novel p-phenylenevinylene compounds containing thiophene or anthracene moieties and cyano-vinylene bonds for photovoltaic applications. 2035 87

Ambipolar organic thin-film transistors with a bilayer structure of poly(3-hexylthiophene) and [6,6]phenyl C(61) butyric acid methyl ester were fabricated using a simple solution-based, contact-film-transfer method. The transistors exhibited balanced electron and hole mobilities of 2.1 x 10(-2) and 1.1 x 10(-2) cm(2) V(-1) s(-1), respectively. Complementary inverters based on two identical ambipolar transistors showed good performance with a gain of 14.
ACS Appl Mater Interfaces 2009 Sep
PMID:Bilayer ambipolar organic thin-film transistors and inverters prepared by the contact-film-transfer method. 2035 7

5,7,12,14-Tetrachloro-6,13-diaza-6,13-dihydropentacene (TCDAHP) and 5,7,12,14-tetrachloro-6,13-diazapentacene (TCDAP) were synthesized and assessed as the active channel materials for thin-film transistor applications. Analyses of the crystal structures of these molecules revealed that both exhibited slipped pi-pi stacking of the long and fused aromatic moiety. Although the packing features of the two compounds are basically identical, their highest occupied molecular orbitals, which are relevant to hole transport, are very different. Better mobility was predicted for TCDAHP over TCDAP based on the dimeric structure in the X-ray coordinates. The morphologies of thin films of TCDAHP and TCDAP prepared by thermal evaporation depend critically on the substrate on which the molecules were deposited: from the amorphous state on a SiO(2)/Si surface to the crystalline state on a pentacene buffer layer surface. The performance of thin-film transistors prepared on various substrate surfaces was studied. While no field-effect mobility was observed for these films deposited on SiO(2)/Si, a high mobility of 1.4 cm(2)/(V s) for the TCDAHP film was achieved when deposited on a pentacene buffer layer prepared on a rubbed monolayer of n-nonyltrichlorosilane on a SiO(2)/Si surface. A similar device prepared from TCDAP gave a mobility of 0.13 cm(2)/(V s).
ACS Appl Mater Interfaces 2009 Sep
PMID:Diazapentacene derivatives as thin-film transistor materials: morphology control in realizing high-field-effect mobility. 2035 35

The charge balance in blue-phosphorescent devices was studied using single-carrier devices, and the results show that the transport is highly hole dominant. The effect of the charge balance on the device performance was further demonstrated using different electron-transport materials with different electron mobilities. By optimization of the charge balance, a maximum current efficiency of 60 Cd A(-1) at a luminance of 500 cd m(-2) was achieved.
ACS Appl Mater Interfaces 2009 Jun
PMID:Effect of the charge balance on high-efficiency blue-phosphorescent organic light-emitting diodes. 2035 9

We report herein a detailed study of the thermal and hole-transport properties of poly[2,7-(9,9-dihexylfluorene)-alt-bithiophene] (F6T2) and its photovoltaic performance in a bulk-heterojunction (BHJ) solar cell. This crystalline polymer has a high weight-average molecular weight (M(w) = 52 400) with a polydispersity index of 1.99. With a band gap of 2.36 eV, F6T2 exhibits strong absorption in the 300-500 nm region. BHJ solar cells blending F6T2 with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) (1:3 weight ratio) as the active layer present a high open-circuit voltage (V(oc) approximately 0.9 V) and a promising power conversion efficiency of 2.4% under simulated solar light AM1.5G (100 mW/cm(2)). Furthermore, F6T2 shows sufficient hole mobility [ca. 8.4 x 10(-5) cm(2)/(V s) at 310 K and 2.5 x 10(5) V/cm applied electric field] by a time-of-flight transient photocurrent technique, allowing efficient charge extraction and a good fill factor for solar cell application. Nanoscale phase separation was observed in F6T2/PCBM films with a surface roughness lower than 60 nm.
ACS Appl Mater Interfaces 2009 Jul
PMID:Hole transport in Poly[2,7-(9,9-dihexylfluorene)-alt-bithiophene] and high-efficiency polymer solar cells from its blends with PCBM. 2035 50

Fusing bithiophene units with a benzo moiety, benzo[2,1-b:3,4-b']dithiophene (BDT), was projected by theoretical calculations to lower the highest occupied molecular orbital (HOMO) energy level of the resulting polymers compared with that of the bithiophene unit, which would enhance the open circuit voltage of bulk heterojunction photovoltaic cells fabricated from BDT-based polymers blended with PCBM. The homopolymer of BDT (HMPBDT) and alternating copolymer of BDT with 2,1,3-benzothiadiazole (PBDT-BT) were therefore synthesized and fully characterized. Both the homopolymer (HMPBDT) and the copolymer (PBDT-BT) were experimentally confirmed to have low HOMO energy levels (-5.70 eV for HMPBDT and -5.34 eV for PBDT-BT). Introducing the acceptor moiety (2,1,3-benzothiadiazole) successfully lowered the optical band gap of the copolymer from 2.31 eV (HMPBDT) to 1.78 eV (PBDT-BT). Bulk heterojunction photovoltaic devices were fabricated from blends of these structurally related polymers with PBCM to investigate the photovoltaic performances. The optimized device of HMPBDT:PCBM (1:3, 180 nm) exhibited an improved open circuit voltage (V(oc)) of 0.76 V, a short circuit current (J(sc)) of 0.34 mA/cm(2), and a fill factor (FF) of 0.40, offering an overall efficiency of 0.10%. The observed large phase separation of the thin film by AFM and the large band gap were accountable for the small current. The optimized device of PBDT-BT:PCBM (1:3, 55 nm) demonstrated a better efficiency of 0.6%, with V(oc) = 0.72 V, J(sc) = 2.06 mA/cm(2), and FF = 0.42. The much improved current was attributed to the lower bandgap and better film morphology. However, the low hole mobility limited the thickness of the PBDT-BT:PCBM film, making inaccessible the thicker film which would utilize more light and enhance the current. Further improvements are expected if the mobility and film morphology can be improved by the new materials design, together with low band gap and low HOMO energy level.
ACS Appl Mater Interfaces 2009 Jul
PMID:Conjugated polymers based on benzo[2,1-b:3,4-b']dithiophene with low-lying highest occupied molecular orbital energy levels for organic photovoltaics. 2035 69

Nanocrystalline mesoporous gamma-Al2O3 film of high thickness has been developed and characterized. The films were prepared on ordinary glass substrates by a single dip-coating method using boehmite (AlOOH) sols derived from aluminum tri-sec-butoxide in presence of cetyltrimethylammonium bromide (CTAB) as structure-directing agent. The dried films were heat-treated at 500 degrees C in air to remove the organics and strengthen the network. The GIXRD of the heat-treated (500 degrees C) film shows a broad peak in the low-angle region supporting the formation of worm-hole-like disordered mesostructures. The high-angle GIXRD, FTIR, and TEM of the films confirm the formation of gamma-Al2O3. N2 adsorption-desorption analyses showed that the heat-treated (500 degrees C) film has a BET surface area of 171 m(2) g(-1) with a pore volume of 0.188 cm(3) g(-1) and mean pore diameter 4.3 nm. Pt nanoparticles (NPs) (approximately 2.7 mol % with respect to the equivalent AlO(1.5)) were generated inside the mesopores of the heat-treated films simply by soaking H2PtCl6 solutions into it, and followed by thermal decomposition at 500 degrees C. The surface area and pore volume of the Pt-incorporated film have been reduced to 101 m(2) g(-1) and 0.119 cm(3) g(-1) respectively, confirming the inclusion of Pt NPs inside the pores. FESEM and TEM studies revealed uniform distribution of Pt NPs (2-8.5 nm; average diameter 4.9 nm) in the films. Catalytic properties of the Pt-incorporated films were investigated in two model (one inorganic and other organic) systems: reduction of hexacyanoferrate(III) ions by thiosulfate to ferrocyanide, and p-nitrophenol to p-aminophenol. In both the cases, the catalyst showed excellent activities, and the reduction reactions followed smoothly, showing isosbestic points in the UV-visible spectra. The catalyst films can be separated easily after the reactions and reused several times.
ACS Appl Mater Interfaces 2009 Apr
PMID:Synthesis of thick mesoporous gamma-alumina films, loading of Pt nanoparticles, and use of the composite film as a reusable catalyst. 2035 9


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