V. A. L. Roy

Luminescent and structural properties of ZnO nanorods prepared under different conditions

The morphology and optical properties of ZnO nanostructures prepared by thermal evaporation of Zn under different conditions was investigated. ZnO nanostructures prepared in air, dry and humid argon flow, and dry and humid nitrogen flow were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and photoluminescence. Tetrapod nanorods were obtained for fabrication in air, while for fabrication in argon or nitrogen flow nanowires and tetrapod nanorods were obtained. Growth of nanowires from the end of the tetrapod nanorod was observed. Influence of the preparation conditions on the structure and the room-temperature photoluminescence is discussed.

Magnetic properties of Mn doped ZnO tetrapod structures

ZnO tetrapod nanostructures were prepared by evaporating Zn metal under humid argon flow. After the fabrication, Mn diffusion doping was performed at two different temperatures (600 and 800u200a°C). The samples were characterized by scanning electron microscopy, transmission electron microscopy, x-ray fluorescence, x-ray diffraction (XRD), superconducting quantum interference device magnetometer, and photoluminescence. Diffusion doping resulted in the increase of the size of tetrapods, but no new peaks were found in XRD spectrum. Mn doped ZnO tetrapod structures were found to be ferromagnetic with Curie temperature ∼50 K, and showed large coercive field (∼3500 Oe for 800u200a°C sample, ∼5500 Oe for 600u200a°C sample).

Supramolecular Polymers and Chromonic Mesophases Self‐Organized from Phosphorescent Cationic Organoplatinum(II) Complexes in Water

Soft matter with a hard core: Through PtII⋅⋅⋅PtII and hydrophobic interactions, planar organoplatinum(II) cations with chloride or sulfate as counterions self‐organize themselves in water into red‐emissive, chromonic, and viscoelastic mesophases from which aligned films and discrete uniaxial microfibers with cofacial molecular orientations can be readily prepared.

Semiconducting and electroluminescent nanowires self-assembled from organoplatinum(II) complexes

Organometallic nanowires with luminescent and current‐modulating properties were self‐assembled from cyclometalated/terpyridyl platinum(II) complexes with auxiliary arylisocyanide/arylacetylide ligands and incorporated into a compact organic light‐emitting field‐effect transistor (see picture) by solution‐processable protocols. The nanowires exhibit both electron and hole mobilities of 0.1u2005cm2u2009V−1u2009s−1.

Nanocomposite field effect transistors based on zinc oxide/polymer blends

Significant progress is being made in the realization of thin-film transistors (TFTs) for application in various electronic devices and circuits [1-5]. Currently, one of the important challenges in this area is to design low-cost and stable organic semiconductors that possess high field-effect mobilities for constructing low-power high-speed transistor devices. However, there are only limited stable and cheap organic semiconductors that are applicable for OTFT applications. Here, we report the work in our laboratory that focus on stable, inexpensive and high field-effect mobility nano-composite materials for the potential application in OTFT technologies. Solution processed polymer based nano-composite field effect transistors with wide band gap semi-conducting ZnO nano-tetrapods and nano-crystals dispersed in the polymer matrix were utilized to study the field effect behaviour. The electrical characteristics of polymer based wide band gap nano-crystal or nano-tetrapod composite devices exhibit an increase in the hole mobility up to two orders of magnitude higher than the pristine polymer. The fabricated devices that contained a layer of MEH-PPV only exhibited p-channel behaviour with a hole mobility up to 10-4 cm2/Vs, similar to previously reported.3 Figures la and lb show the TEM (transmission electron microscope) images of ZnO nanocrystals or tetrapods dispersed in MEH-PPV solutions, respectively. The size of the nanocrystals is around 5 nm (Figure la) and the legs of the tetrapods are around 100 nm in width (Figure lb). Figure 2 shows the electrical behaviour of the devices fabricated from MEH-PPV and nanocomposite with ZnO nanocrystals or tetrapods. In Figure 2, the I-V characteristics and the transfer curves of the devices based on MEH-PPV (Figures 2a and 2b), 9 mg of ZnO nanocrystals in 10 mg of MEH-PPV or 47% of ZnO in weight (Figures 2c and 2d) and 9 mg of ZnO tetrapods in 10 mg of MEH-PPV or 47% of ZnO in weight (Figures 2e and 2f) are depicted. A saturation of the hole mobility is observed in the nanocomposite devices when the concentration of ZnO tetrapods or nanocrystal exceeds 40% in weight as shown in Figure 3. From the I-V characteristics, incorporation of ZnO nanocrystals or tetrapods in the polymer enhances the drain current and the mobility. The calculated hole mobility was up to 0.08 cm2/Vs for the ZnO nanocrystals / MEH-PPV devices and up to 0.15 cm2/Vs for the ZnO tetrapods / MEH-PPV devices, at the saturation regime. Where as in the linear regime, the hole mobility was up to 0.071 cm2/Vs for the ZnO nanocrystals / MEH-PPV devices and up to 0.096 cm2/Vs for the ZnO tetrapods / MEH-PPV devices. A decrease in the threshold voltage up to -15 V was found for both nanocomposite devices (ZnO nanocrystals or ZnO tetrapods / MEH-PPV). The sub-threshold swing was found to be 2 V per decade for the ZnO / MEH-PPV nanocomposite devices and up to 10 V per decade for the MEH-PPV devices. The on/off ratio was calculated as 105 for the nanocomposite devices where it was only 103 for MEH-PPV devices. Furthermore, a reduction in density of traps, given by NT = VT Con/q, has been observed, as shown in the inset of Figure 3, while the weight percentage of ZnO increases in the polymer. However, the trap density seems to saturate when the concentration of ZnO tetrapods or nanocrystal in the polymer exceeds 40% in weight. Incorporation of ZnO nanomaterials (nanocrystals or tetrapods) into the MEH-PPV polymer -a p-type semiconductor -did not change the nature of charge transport, as the nanocomposite devices were found to behave as p-channel transistors. However the hole mobility was enhanced in the nanocomposite devices, in addition, the band diagram of MEH-PPV and ZnO are well known. The highest occupied molecular orbital (HOMO, 5.3 eV) and lowest unoccupied molecular orbital (LUMO, 3.0 eV) levels of MEH-PPV and the valence (7.6 eV) and conduction (4.4 eV) bands of ZnO shows clearly that a huge energy barrier exists for holes to be transferred from ZnO to MEH-PPV for transport. Consequently, holes are confined in MEH-PPV and we suggest that the effect of ZnO is to reduce the density of traps in the polymer which probably is a reason for the enhanced mobility and the reduced threshold voltage.

Limits of open circuit voltage in organic photovoltaic devices

Open circuit voltage (Voc) of organic photovoltaic devices has been interpreted with either the metal-insulator-metal (MIM) model or the energy offset between highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor (HOMOD-LUMOA). To elucidate the relation between Voc and the two models, we have used electrodes of a wide range of work functions to connect the CuPc/C60 organic photovoltaic devices. We found that when the work function difference (Δϕelectrodes) between ITO and Al electrode is in the range −3 and 0 eV, Voc increases linearly with Δϕelectrodes as prescribed by the MIM model. Outside this range, Voc saturates with values close to that given by the HOMOD-LUMOA less the exciton binding energy.

A high-performance organic field-effect transistor based on platinum(II) porphyrin: peripheral substituents on porphyrin ligand significantly affect film structure and charge mobility.

Organic field-effect transistors incorporating planar pi-conjugated metal-free macrocycles and their metal derivatives are fabricated by vacuum deposition. The crystal structures of [H2(OX)] (H(2)OX=etioporphyrin-I), [Cu(OX)], [Pt(OX)], and [Pt(TBP)] (H2TBP=tetra-(n-butyl)porphyrin) as determined by single crystal X-ray diffraction (XRD), reveal the absence of occluded solvent molecules. The field-effect transistors (FETs) made from thin films of all these metal-free macrocycles and their metal derivatives show a p-type semiconductor behavior with a charge mobility (mu) ranging from 10(-6) to 10(-1) cm(2) V(-1) s(-1). Annealing the as-deposited Pt(OX) film leads to the formation of a polycrystalline film that exhibits excellent overall charge transport properties with a charge mobility of up to 3.2 x 10(-1) cm(2) V(-1) s(-1), which is the best value reported for a metalloporphyrin. Compared with their metal derivatives, the field-effect transistors made from thin films of metal-free macrocycles (except tetra-(n-propyl)porphycene) have significantly lower mu values (3.0 x 10(-6)-3.7 x 10(-5) cm(2) V(-1) s(-1)).

Exchange bias and the origin of magnetism in Mn-doped ZnO tetrapods

Wurtzite-type ZnO tetrapod nanostructures were prepared by evaporating Zn metal under humid argon flow. After the fabrication, Mn was doped into ZnO nanostructures by diffusion at 600°C. The average concentration of Mn was determined to be 8.4mol% by x-ray fluorescence. X-ray diffraction patterns obtained from the doped and undoped samples are almost the same. High-resolution transmission electron microscopy observations reveal the existence of surface layers. Magnetic measurements show that the sample has a very large coercivity HC=5500Oe at 5.5K and a Curie temperature TC=43K, which may suggest that ferrimagnetic (Zn,Mn)Mn2O4 exists at the surface. Exchange bias is clearly observed below 22K. Exchange bias is attributed to the exchange interaction between ferrimagnetic (Zn,Mn)Mn2O4 and spin-glass-like (or antiferromagnetic) phase in manganese oxides.

Improving efficiency of organic photovoltaic cells with pentacene-doped CuPc layer

We have fabricated efficient heterojunction organic photovoltaic (OPV) cells based on pentacene-doped copper(II) phthalocyanine (CuPc) layer as donor and fullerene (C60) layer as acceptor. The power conversion efficiency of 4% pentacene-doped CuPc∕C60 OPV cell (3.06%) is increased by 77% compared with that of the standard CuPc∕C60 OPV cell (1.73%). The efficiency improvement can be attributed to the higher carrier mobility instead of the stronger photon absorption of the pentacene-doped CuPc layer.

Influence of the donor/acceptor interface on the open-circuit voltage in organic solar cells

The donor/acceptor interface in a standard CuPc/C60 organic solar cell was modified by insertion of a thin layer of molybdenum trioxide (MoO3). An ultrathin layer of MoO3 between the donor and acceptor increased the open-circuit voltage (VOC) from 0.45 to 0.85 V. The enhancement in VOC is explained by the increase in the energy level offset between the lowest unoccupied molecular orbital of the acceptor and the highest occupied molecular orbital of the donor (EDHOMO-EALUMO). The explanation is supported by the energy level analysis of the donor/acceptor interface by ultraviolet photoemission spectroscopy and x-ray photoemission spectroscopy.