Synthesis and properties of oxetane monomers and oligomers with electro‐active pendent groups

BACKGROUND: Monomers and polymers carrying pendent electro‐active fragments are widely studied due to their application in various optoelectronic devices. Monomers containing triphenylamino, triphenyldiamino and carbazol‐9‐yl fragments with vinyl, epoxy or acryl functional groups are mostly used. The synthesized materials are used for preparation of hole transport layers as well as host materials for electrophosphorescent light‐emitting diodes. Much fewer presentations are reported on the preparation of monomers containing other electro‐active or functional groups. RESULTS: Here we describe oxetane monomers and their oligomers containing various electro‐active pendent groups. The weight‐average molecular weights of the oligomers are in the range 1420–3250 g mol^?1 with a molecular weight distribution of 1.7–4.1. The electron photoemission spectra of amorphous layers of the compounds established ionization potentials of 5.55–5.85 eV. Room temperature hole drift mobility in the layers of some oligomers exceeds 10^?7 cm² V^?1 s^?1 at high electric fields. CONCLUSION: The synthesized oligomers exhibit promising thermal and film‐forming properties. Amorphous layers of some of the materials demonstrate suitable ionization potentials and sufficient hole transport properties for their application in optoelectronic devices. Copyright © 2008 Society of Chemical Industry     

Synthesis and properties of new derivatives of poly[9‐(2,3‐epoxypropyl)carbazole]

BACKGROUND: Carbazole derivatives are well known to exhibit interesting electro‐ and photo‐active properties due to their hole‐transporting ability, strong absorption in the ultraviolet spectral region and blue‐light emission. One of the most widely studied materials among carbazole‐containing oligomers is poly[9‐(2,3‐epoxypropyl)carbazole] (PEPK). The main field of application of this oligomer is electrophotographic microfilming. It is also used for the manufacture of multicolour slides and in the photothermoplastic recording of information. Unfortunately, due to its high ionization potential, which reaches 5.86 eV, the possibilities of application of this compound in optoelectronic devices are rather limited. RESULTS: PEPK‐based charge transporting oligomers, incorporating hydrazone moieties, are reported. The oligomers were prepared by chemical modification of PEPK. The materials obtained were examined using various techniques including differential scanning calorimetry and ultraviolet, infrared and NMR spectroscopy. Electron photoemission spectra of layers of the synthesized oligomers showed ionization potentials (I_p) in the range 5.4–5.5 eV. CONCLUSION: The synthesized oligomers possess a larger π‐conjugated system and show ionization potentials of ca 5.4 eV. Therefore, they are more suitable for use in optoelectronic devices with quicker photoresponse than unmodified PEPK. Copyright © 2008 Society of Chemical Industry     

Structural and electronic properties of bilayer and trilayer graphdiyne

Stimulated by the recent experimental synthesis of a new layered carbon allotrope-graphdiyne film, we provide the first systematic ab initio investigation of the structural and electronic properties of bilayer and trilayer graphdiyne and explore the possibility of tuning the energy gap via a homogeneous perpendicular electric field. Our results show that the most stable bilayer and trilayer graphdiyne both have their hexagonal carbon rings stacked in a Bernal way (AB and ABA style configuration, respectively). Bilayer graphdiyne with the most and the second most stable stacking arrangements have direct bandgaps of 0.35 eV and 0.14 eV, respectively; trilayer graphdiyne with stable stacking styles have bandgaps of 0.18-0.33 eV. The bandgaps of the semiconducting bilayer and trilayer graphdiyne generally decrease with increasing external vertical electric field, irrespective of the stacking style. Therefore, the possibility of tuning the electronic structure and optical absorption of bilayer and trilayer graphdiyne with an external electric field is suggested.     

Improved field emission from multiwall carbon nanotubes with nano-size defects produced by ultra-low energy ion bombardment

Multiwalled carbon nanotubes were irradiated with ultra-low energy (few eV) nitrogen and hydrogen ions using a microwave discharge. These ultra-low energy plasma-ions remain confined to the nanotube walls, transferring their maximum energy to the carbon atoms, and produce extraordinary structural changes to the carbon nanotube pillars as well as within the carbon nanotubes. Conical shaped emitters and nanotube structures with nano-defects are produced that exhibit remarkable field emission with ultra-low turn-on electric field (∼0.16 V/μm) and a >300-fold increase in the maximum emission current density compared to non-irradiated nanotubes. Doping of nitrogen is also identified due to such irradiation processes.     

一种三苯胺多枝化合物的合成及光电性能研究

本文设计合成了一种含供电基团的三苯胺多枝化合物,并通过IR、MS、NMR进行了结构表征,用紫外-吸收光谱、荧光发射光谱、循环伏安法(CV)等手段研究了它的光电性能,分析了它的电化学行为,计算出其电离势(P1)为5.64eV,带隙(Eg)为2.79eV,结果表明,该化合物是优良的空穴高效注入和传输的蓝色有机电致发光材料.     

Microstructure and ferroelectric properties of Ta-doped Bi3.25La0.75Ti3O12/ZnO thin film capacitors

Ta-doped Bi_3.25La_0.75Ti₃O₁₂(BLTT)/ZnO films were fabricated on Pt(111)/Ti/SiO₂/Si substrates by a magnetron sputtering method. Firstly, ZnO crystal thin films were grown on the substrates by a reactive sputtering method. Then, BLTT thin films were deposited on the ZnO layers at room temperature and post-annealed at 600 °C. The micromorphology, ferroelectric and dielectric properties of BLTT/ZnO films were analyzed. The XRD analysis shows that ZnO buffer layer significantly reduces the crystallization temperature of BLTT thin film. The TEM results show that lamellar BLTT grains are grown on ZnO layer at a certain angle with few elements diffusion at the interface of ZnO phase and Bi₄Ti₃O₁₂ phase. The ferroelectric properties indicate that BLTT/ZnO films exhibit different remanent polarization and coercive fields under electric field with different directions. The novel mechanism of tailoring ferroelectric properties may open new possibilities for designing special ferroelectric devices.     

Sulfur doped nanocrystalline diamond films as field enhancement based thermionic emitters and their role in energy conversion

Sulfur doped nanocrystalline diamond films, like other nanostructured carbon films, exhibit electron emission characterized by a spatial non-uniformity of the field enhancement factor. While field emission effects are observed at room temperature, an increase in emitter temperature is accompanied by an amplified emission current with a simultaneous drop in the threshold field. At low extraction fields a fit of the emission current to the Richardson equation indicates a material work function of 2.5 eV. The Schottky formula describes thermionic emission at a moderate field and is utilized to determine the work function at an electric field of 0.8 V/μm with a value of 1.7 eV and a concurrently reduced Richardson constant. This significant difference in the work function of 2.5 and 1.7 eV for 0.5 and 0.8V/μm, respectively can be attributed to field enhancement effects.     

New derivatives of indazole as electronically active materials

Synthesis and thermal, optical, electrochemical and photoelectrical properties of new indazole-based electroactive materials are reported. 1-Phenyl-5(6)-[N,N-(bisphenyl)]aminoindazoles and their methoxy-substituted analogues exhibit high thermal stabilities with the onset temperatures of thermal degradation ranging from 352 to 424 °C. The synthesized indazole derivatives form glasses with glass transition temperatures ranging from 35 to 39 °C. The synthesized compounds are electrochemically stable: their cyclic voltammograms show one reversible oxidation couple and no reduction waves. The ionization potentials of the solid samples of the synthesized materials are in the range of 5.3-5.9 eV. Methoxy-substituted derivatives show lower ionization potentials. Time-of-flight hole drift mobilities of 50% solid solution of 1-(4-methoxyphenyl)-5-{N,N-[bis(4-methoxyphenyl)]}aminoindazole in bisphenol Z polycarbonate reach 10⁻⁵ cm²/V s at high electric fields.     

Influence of yttrium doping on the structural,morphological and optical properties of nanostructured ZnO thin films grown by spray pyrolysis

This study reports on the deposition of highly transparent, n-type ZnO thin films on glass substrate at 450?°C using spray pyrolysis processing, with the simultaneous insertion of yttrium (Y) at different percentages (0, 2, 5, 7?at%) as a dopant. The effect of Y doping on the structure, morphology and optical properties of Y doped ZnO (ZnO:Y) was investigated for optoelectronic applications. The obtained thin films were characterized by means of X-ray diffraction, field-emission scanning electron microscopy (FESEM), UV–visible absorbance measurements, photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. The as-prepared films exhibit well-defined hexagonal wurtzite structure grown along [002]. Field emission scanning electron microscope micrographs of the pure ZnO and ZnO:Y showed that the films acquired a dominance of hexagonal-like grains, the morphology was influenced by Y incorporation. All the films showed high transparency in the visible domain with an average transmittance of 83%. The band gap energy, E_g, increased from 3.12?eV to 3.18?eV by increasing the Y doping concentration up to 5?at% and then decreased to 3.15?eV for 7?at% Y content. The PL and CL measurements reveal a strong ultraviolet (UV) emission, suggesting that the as-prepared ZnO:Y thin films can potentially be used in optoelectronic devices.     

超声雾化辅助微波法合成氧化锌纳米颗粒及其光催化性能

氧化锌(ZnO)具有宽带隙、高光催化效率和稳定的化学性质,已成为处理水体中有机污染物的常用材料.采用超声雾化辅助微波法成功合成了ZnO纳米颗粒,XRD图谱和TEM照片表明超声雾化抑制了ZnO结晶,其结构为无序状,得到的ZnO纳米颗粒为非晶态.非晶态ZnO纳米颗粒紫外最强吸收峰为300nm,紫外吸收边发生红移,禁带宽度降...     

Phenothiazinyl- and 4-diethylaminophenyl-substituted diethylenes as fluorescent and hole-transporting molecular materials

A series of phenothiazinyl- and 4-diethylaminophenyl-substituted diethylenes were synthesized and characterised; their thermal, optical and photoelectrical properties were investigated. TGA showed that the compounds display high thermal stability, achieving 5% mass loss at temperatures up to 385 °C; DSC revealed that many of the compounds exhibited glass transition temperatures ranging from 20 to 124 °C. Dilute solutions of the diethylenes exhibited fluorescence emission in the green-blue region with an efficiency reaching 99%. Electron photoemission spectrometry and time of flight revealed ionization potentials of 5.34–5.52 eV; the room temperature hole drift mobility of one of the compounds molecularly doped in bisphenol Z polycarbonate exceeded 10^?5 cm²/Vs under a high electric field.     

Semiconducting properties of cup-stacked carbon nanotubes

The current-voltage characteristics of individual cup-stacked carbon nanotubes (CSCNTs) were investigated in situ inside the transmission electron microscope. Different from other quasi-1D carbon structures such as multi-walled carbon nanotubes, carbon nanofibers or graphitic fibers that normally behave as a metallic conductor of electrons, individual CSCNTs were found to exhibit unexpectedly semiconducting behaviors due to the special stacking microstructure of graphene layers. The band gap of the CSCNTs was obtained with the value of about 0.44 eV, in contrast to the zero-gap semiconducting quasi-2D graphene. These findings provide new information about the effect of the stacking graphene layers on their electronic properties, and will widen the usefulness of such stacking structure for the application in nanoelectronics.     

Role of acetylenic bonds in the mechanical,electronic and optical properties of yne-diamonds

Yne-diamonds are novel carbon allotropes designed by inserting acetylenic bonds into the framework of diamond. Varying the ratio of acetylenic bonds yields a new family of carbon allotropes consisting of sp- and sp³-hybridized atoms. The study of these novel carbon frameworks is becoming a topic of increasing interest. Here, we report our systematic studies on the stability, mechanical, electronic and optical properties of yne-diamonds. Our calculations indicate that yne-diamonds are mechanically stable, although they are energetically less favorable than diamond due to sp-hybridized carbon atoms in the frameworks. In contrast to early estimation, yne-diamonds are unlikely superhard materials, but exhibit good ductility. The band gaps of yne-diamonds vary from 0.165 eV to 4.850 eV, as the ratio of acetylenic bonds increases. The energetically most favorable yne-diamond has a direct band gap of 2.916 eV. The optical properties of these carbon allotropes are also discussed.     

Highly efficient field emission from aligned ZnO/TiN core-shell nanorods

Novel polycrystalline TiN coated ZnO core-shell heterostructure nanorods have been prepared on carbon paper substrate via a low-temperature hydrothermal and sputtering process. The core-shell emitters exhibit a highly efficient field-emission performance with a low threshold of electric field ∼0.72 V/μm, and a high emission current density ~16.41 mA/cm², a level that is the highest of ZnO emitters reported to date. The improved field-emission characteristics may be attributed to the unique materials combination of ZnO core and TiN shell, resulting in the conductivity enhancement, emission sites increase, and the work function reduction. Our results demonstrate that ZnO/TiN core-shell emitter will be a distinguished candidate for electronic source devices.     

Fabrication and Characterization of Pure and Pb-doped ZnO Thin Films Prepared by Sol–Gel and Dip-coeting Method

In this paper, undoped and Pb-doped ZnO thin films have been prepared by sol gel method and deposited on glass substrate using dip-coating technique. The structural, morphological, and optical properties of the films were investigated as a function of Pb doping. The results of the structural tests showed that these films are of a polycrystalline hexagonal structure with a preferred orientation in the (002) direction. The grain size values of Pb-doped films were lower than that of pure ZnO, but the strain and the dislocation density values inecrease with increase Pb doping ratio. The atomic force microscopy (AFM) images showed that the particle size and Root Mean Square (RMS) of ZnO decrease with increasing Pb doping. The optical band gap values were found to increase from (3.19 to 3.30 eV) and the Urbach energy decrease from (322 to 313 meV). PL spectra exhibit an increased amount of defects with increasing Pb, which leads to a red shift in the UV region.

     

Fabrication of Mg-doped ZnO nanofibers with high purities and tailored band gaps

The tailored doping levels towards the band gap tunability are one of the challenges to push forward the potential application of one-dimensional (1D) ZnO nanostructures in the opto/electric nanodevices. In present work, we reported the exploration of Mg-doped ZnO nanofibers via electrospinning of polyvinylpyrrolidone (PVP), Zn(CH₃COO)₂ (ZnAc) and Mg(CH₃COO)₂ (MgAc), followed by calcination in air. The resultant products were systematically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscope (HRTEM), and X-ray photoelectron spectroscopy (XPS). The optical measurements (UV–vis) of the Mg-doped ZnO nanofibers suggested that the optical band gaps of the ZnO nanofibers could be tuned from 3.33 to 3.40 eV as a function of the Mg doing levels. This tunability of the band gap of ZnO nanofibers with an intentional impurity could eventually be useful for optoelectronic applications.     

Copolymers containing electronically isolated indolyl fragments as materials for optoelectronics

Monomer and its copolymers containing electronically isolated indolyl fragments were synthesized by multi-step synthetic route. The materials were examined by various techniques including thermogravimetry, differential scanning calorimetry, UV and fluorescence spectrometry as well as electron photoemission technique. The copolymers exhibit initial mass loss temperatures in the range of 259–321 °C and form amorphous films with glass transition temperatures of 102–122 °C. Thin layers of the materials demonstrate ionization potentials of about 5.7 eV. The copolymers were tested as host materials in electro-phosphorescent devices.     

Charge-transporting 3,4-ethylenedioxythiophene-based hydrazone monomers and oligomers

3,4-Ethylenedioxythiophene-based hydrazones, containing reactive functional groups were synthesized and their optical, thermal, photophysical and electrochemical properties were studied. The monomers were subjected to cationic polymerization using boron trifluoride etherate as an initiator. The ionization potentials of the films of the hydrazones, measured by the electron photoemission technique, range from 5.53 to 5.62 eV. Room temperature time-of-flight hole mobilities in the solid solutions of 3,4-ethylenedioxythiophene hydrazone monomers in bisphenol-Z polycarbonate exceed 10^?5 cm² V^?1 s^?1 at high applied electric fields.     

Doped 1D Nanostructures of Transition-metal Oxides: First-principles Evaluation of Photocatalytic Suitability

The splitting of water molecules under the influence of solar light on semiconducting electrodes is a clean and renewable source for the production of hydrogen fuel. Its efficiency depends on the relative position of the band-gap edges or the induced defect levels with a proper band alignment relative to the redox H⁺/H₂ and O₂/H₂O potentials. For example, TiO₂ and ZnO bulk, as well as thick slabs (whose band gaps are ∼3.2–3.4 eV), can be active only for photocatalytic applications under UV irradiation (possessing ∼1 % solar energy conversion efficiency). Nevertheless, by adjusting the band gap through formation of nanostructures and further doping, the efficiency can be increased up to ∼15 % (for 2.0–2.2 eV band gap). We analyse results of DFT (density functional theory) calculations on TiO₂ nanotubes and ZnO nanowires, both pristine and doped (e.g., by Ag_Zn, C_O, Fe_Ti, N_O and S_O substitutes). To reproduce the energies of one-electron states better, we have incorporated the Hartree-Fock (HF) exchange into the hybrid DFT+HF Hamiltonian. Both the atomic and electronic structure of nanomaterials, simulated by us, are analysed to evaluate their photocatalytic suitability, including positions of the redox potential levels inside the modified band gap, the width of which corresponds to visible-light energies. Analysis of the densities of states (DOS) for considered nanostructures clearly shows that photocatalytic properties can be significantly altered by dopants. The chosen hybrid methods of first-principles calculations significantly simplify selection of suitable nanomaterials possessing the required photocatalytic properties under solar light irradiation.