Photoconductivity in Metal–Organic Framework (MOF) Thin Films

Photoconductivity is a characteristic property of semi‐conductors. Herein, we present a photo‐conducting crystalline metal–organic framework (MOF) thin film with an on–off photocurrent ratio of two orders of magnitude. These oriented, surface‐mounted MOF thin films (SURMOFs), contain porphyrin in the framework backbone and C₆₀ guests, loaded in the pores using a layer‐by‐layer process. By comparison with results obtained for reference MOF structures and based on DFT calculations, we conclude that donor–acceptor interactions between the porphyrin of the host MOF and the C₆₀ guests give rise to a rapid charge separation. Subsequently, holes and electrons are transported through separate channels formed by porphyrin and by C₆₀, respectively. The ability to tune the properties and energy levels of the porphyrin and fullerene, along with the controlled organization of donor–acceptor pairs in this regular framework offers potential to increase the photoconduction on–off ratio.     

Photoconductivity and photoluminescence in chemically deposited films of (Cd–Zn)S:CdCl2, Ho

Results of SEM, XRD, optical absorption/reflectance, photoconductivity (PC), and photoluminescence (PL) are presented for (Cd–Zn)S:CdCl₂, Ho films prepared by chemical deposition technique by direct dipping either at room temperature (RT) or at 60 °C in a water bath. SEM studies show the presence of non-uniform distribution of particles. XRD studies show lines of CdS, ZnS along with lines of CdCl₂ and Ho. PL is found to be pronounced in films of RT preparation and for those prepared at 60 °C, PC is higher. Optical absorption/reflectance studies show the presence of Ho corresponding to the transition ⁵I₈→⁵F₁/⁵G₆. Emissions corresponding to the transitions ⁵S₂/⁵ F₄→⁵ I₈, ⁵G₅→⁵I₇, and ³H₅→⁵I₈ are observed in PL emission spectra.     

杂质与缺陷对GaMnAs光学特性影响的研究

提高GaMnAs材料中Mn的含量可以提高其居里温度, 但随之而来也会引入很多缺陷。为了研究高含量Mn引入的缺陷对稀磁半导体材料的影响, 本文对低温分子束外延技术（LT-MBE）生长的GaMnAs外延层进行了光电导以及红外等光谱的分析。通过对样品的光谱分析, 发现样品中存在大量的As反位缺陷（AsGa）、Mn的间隙位缺陷（MnI）、以及在生长和退火过程中产生的Mn以及MnAs团簇等缺陷, 这些缺陷都会影响外延层的光谱特性, 同时也会影响器件的电学性能。     

Discotic derivatives of 6-oxoverdazyl radical

Substitution of each phenyl in 1,3,5-triphenyl-6-oxoverdazyl with three alkoxy groups induces an ordered columnar hexagonal phase (Col_h(o)) below 130°C in 1b[n], while in the alkylsulfanyl analogues 1a[n] additional periodicity along the columns was found rendering the phase a true three-dimensional columnar hexagonal phase (Col_h(3D)) below 60°C. Both series exhibit broad absorption bands in the visible region with maxima at 540 and 610 nm in series 1a[n] and at 486 and 614 nm in series 1b[n]. Unusual reversible thermochromism is observed in series 1b[n], in which the dark green isotropic phase turns red in the discotic phase. Analysis of 1a[8] revealed redox potentials E^0/+1_1/2 = +0.99 V and E^{0/ ?1}_1/2 = –0.45 V vs. saturated calomel electrode (SCE), while the potentials in the alkoxy analogue 1b[8] are shifted cathodically by 0.16 V. Photovoltaic studies of 1a[8] demonstrated hole mobility of μ_h = 1.52 × 10^?3 cm² V^?1 s^?1 in the mesophase with an activation energy E_a = 0.06 ± 0.01 eV. Magnetisation studies of 1a[8] revealed nearly ideal paramagnetic behaviour in either the solid or fluid phase above 200 K and weak antiferromagnetic interactions at low temperatures. In contrast, a noticeable drop of about 4% in μ_eff was observed during the I→Col phase transition in 1b[8], which coincide with the thermochromic effect.     

Photoconductivity and photoluminescence in chemically deposited (Cd‐Zn)S:CdCl2,Tb films

Results of SEM, EDX and XRD studies, Optical Absorption spectra, Photoconductivity (PC) rise and decay, PC excitation spectra and Photoluminescence (PL) emission spectra are presented for (Cd‐Zn)S:CdCl₂, Tb films prepared by chemical deposition method on glass substrates either at room temperature (RT) or at 60 °C in a water bath (WB). SEM studies show ball type structure which is related to layered growth. EDX measurements show excess of Cd in such preparations along with the presence of Tb. XRD studies show prominent diffraction lines of CdS and ZnS along with lines of CdCl₂ and impurity. The values of strain (ε), grain size (D), and dislocation density (δ) are evaluated from XRD studies and the nature of crystallinity of the films are discussed. Optical Absorption spectra also show the presence of Tb in the lattice corresponding to the transition ⁷F₆(4f⁸) → ⁵D₀(5d¹4f⁷) of Tb³⁺ ions. From results of optical absorption spectra, the band gaps are determined, whose values are quite similar to those obtained from PC excitation spectra. Sufficiently high photo current (I_pc) to dark current (I_dc) ratios with a maximum value of the order of 10⁶ are observed. This high photosensitisation is related to increase in mobility and lifetime of carriers due to photo excitation. PL emission spectra consist of peaks due to transitions in Tb levels. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surface Conductivity of DR1-Functionalized Organic-Inorganic Sol-Gel Materials

In recent years, in looking for an important photorefractive application, several studies on electro-optic and photoconductive sol-gel responses have been done. A very important effort has been addressed to establish the appropriate induced-orientation procedure, in order to get the highest electro-optic coefficient. In this way a very high coefficient of 48 pm/V at 831 nm in sol-gel has been already found. Similarly, the importance of the non-linear chromophore concentration into the material electro-optic behavior has been studied. However, the influence of the orientation procedure and the chromophore concentration over the photoconductive response has not been performed. In this work we study the vacuum-surface-charge-transport under and without illumination after poling times of 10, 30 and 120 min on DR1-functionalized sol-gel thin films of 1.3 m in thickness with a suitable concentration of DR1. We include the measures before poling for other chromophore concentrations. We found the largest density of photocurrent at 633 nm for a poling time of 30 min. We also measured the order parameter in order to follow the Corona induced orientation evolution as function of time for each case. The saturation found into this parameter and into the photoconduction show the existence of an optimal poling time.     

用三硝基芴酮高浓度掺杂树脂做电荷传输层的正充电有机光导体

本研究采用三硝基芴酮(TNF)高浓度掺杂的高分子树脂作为电荷传输层(CTL)、ε-CuPc的高分子分散体系作为电荷产生层(CGL)制作了电子传输型正充电有机光导体。TNF质量分数为10～50%的高分子树脂薄膜在80℃下干燥8h,再在常温下保存近半年仍然处于完全透明状态,表明该掺杂体系比较好地解决了通常TNF与高分子之间相容性差,难以实现高浓度掺杂的问题。实验结果表明这种光导体具有良好的表面电荷保持能力,在所考察的掺杂浓度范围内饱和电压一般都在+950V以上;通过调节CGL的高分子材料暗衰减速度可以控制在3.3V/s;光导体在整个可见光范围内表现出良好的感光度,特别在550、650和750nm处感光度出现峰值,在750nm处半衰曝光量为1.93μJ/cm²,在800nm处为2.8μJ/cm²。     

转化乳剂的离子电导和光电导研究

本文研究了AgBrCl转化乳剂的离子电导和光电导。随着乳剂中溴含量的增高,其离子电导也随之升高,而光电子寿命则降低。AgBrCl乳剂的离子电导主要取决于其组成;AgBrCl转化乳剂的光电子寿命主要受隙间银子浓度的影响,隙间银离子浓度越大,光电子寿命越短。     

Photoelectrical properties of crystalline titanium dioxide thin films after thermo‐annealing

This paper reports the photoelectrical properties of sol gel derived titanium dioxide (TiO₂) thin films annealed at different temperatures (425‐900°C). The structure of the as‐grown film was found to be amorphous and it transforms to crystalline upon annealing. The trap levels are studied by thermally stimulated current (TSC) measurements. A single trap level with activation energy of 1.5 eV was identified. The steady state and transient photocurrent was measured and the results are discussed on the basis of structural transformation. The photocurrent was found to be maximum for the films annealed at 425°C and further it decreases with annealing at higher temperatures. The photoconduction parameters such as carrier lifetime, lifetime decay constant and photosensitivity were calculated and the results are discussed as a function of annealing temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoconduction and transport mechanisms in polycrystalline zincphthalocyanine thin films

Zincphthalocyanine (ZnPc) thin films were prepared by the vacuum evaporation method under a pressure of 10^‐6 mbar. The X‐ray diffraction analysis of vacuum evaporated ZnPc films reveals that the structure of the films is polycrystalline in nature. The photoconduction properties have been studied in the wavelength range 400 –800nm using suitable masks. The Photoconductivity of the films as a function of light intensity and applied voltage were studied and results were discussed in detail. The photoconduction was found to increase with higher light illumination and maximum at the band edge of the ZnPc thin film. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)