Bar-coated high-performance organic thin-film transistors based on ultrathin PDFDT polymer with molecular weight independence

We report high-performance organic thin-film transistors (OTFTs) with an ultrathin active layer of difluorobenzothiadiazole-dithienosilole copolymer (PDFDT) form by using the wire bar-coating process. The top-gate/bottom contact (TG/BC) OTFTs based on bar-coated PDFDT polymer as channel material and poly(methyl methacrylate) (PMMA) as gate dielectric show a hole mobility of up to 2.2 cm² V⁻¹s⁻¹ with a current ON/OFF ratio (I_on/I_off) of 10⁴∼10⁵, with the mobility being two times larger than that of the spin-coated PDFDT based OTFTs. The higher mobility of the bar-coated PDFDT polymer films can be attributed to the well-organized fibril structures of the polymer chains. Importantly, two different molecular weight polymers (M_n = 23 and 34 kDa) were employed to conduct these experiments and both batches showed about the same performance, which mitigates the typical batch-to-batch variation in OTFT performance. Furthermore, we explored the operational stability of the bar-coated OTFTs in ambient air and nitrogen environments. The bias-stress and cycling tests between the ON/OFF states of the bar-coated devices showed high stability in both nitrogen and air. Conclusively, here we demonstrate that (i) a simple bar-coating process is a better method to control and obtain good polymer morphology in comparison to spin-coating, and (ii) the PDFDT polymer has great potential to provide good reproducibility and stability in large-area OTFT devices.     

Concentration effect of Nd ion on the spectroscopic properties of Er/Nd co-doped LiYF4 single crystal

High quality Er³⁺/Nd³⁺:LiYF₄ single crystals were grown by a Bridgman method. Their spectroscopic properties were studied to understand the Nd³⁺ concentration effect upon excitation of an 800 nm laser diode. The intensest 2.7 μm emission was observed in the LiYF₄ crystal codoped with 0.99 mol% Er³⁺ and 0.62 mol% Nd³⁺. Meanwhile, the emission intensity for the green up-conversion and 1.5 μm downconversion of Er³⁺ decreased with increasing of the Nd³⁺ concentration. The modified Inokuti–Hirayama model was used to analyze the decay curves of the 1.06 (Nd³⁺) and 1.5 (Er³⁺) μm emissions. The results indicated that the energy transfer process (Er³⁺:⁴I_13/2 + Nd³⁺:⁴I_9/2 → Er³⁺:⁴I_15/2 + Nd³⁺:⁴I_15/2) is mainly due to the electric dipole–dipole interaction. The energy transfer efficiencies between Nd³⁺ and Er³⁺ ions were calculated. All results suggested that the Er³⁺/Nd³⁺:LiYF₄ single crystals may have potential applications in mid-infrared lasers.     

Optical Spectra and Gain Properties of Ho3＋/Pr3＋ Co-doped LiYF4 Crystal

The LiYF4 single crystals singly doped Ho3＋ and co-doped Ho3＋, Pr3＋ ions were grown by a modified Bridgman method. The Judd-Ofelt strength parameters （Ω2, Ω4, Ω6） of No3＋ were calculated according to the absorption spectra and the Judd-Ofelt theory, by which the radiative transition probabilities （A）, fluorescence branching ratios （β） and radiative lifetime （τ rad） were obtained. The radiative lifetimes of 5/6 and 5/7 levels in Ho3＋ （1 mol%）：LiYF4 are 10.89 and 20.19 ms, respectively, while 9.77 and 18.50 ms in Ho3＋/pr3＋ doped crystals. Hence, the τ rad of 5/7 level decreases significantly by introduction of Pr3＋ into Ho3＋：LiYF4 crystal which is beneficial to the emission of 2.9 μm. The maximum emission cross section of Ho3＋：LiYF4 crystal located at 2.05 μm calculated by McCumber theory is 0.51 ×10-20 cm2 which is compared with other crystals. The maximum emission cross section at 2948 nm in Ho3＋/pr3＋ co-doped LiYF4 crystal obtained by Fuchtbauer- Ladenburg theory is 0.68 × 10-20 cm2, and is larger than the value of 0.53 × 10-20 cm2 in Ho3＋ singly doped LiYF4 crystal. Based on the absorption and emission cross section spectra, the gain cross section spectra were calculated. In the Ho3- ions singly doped LiYF4 crystal, the gain cross sections for 2.05 μm infrared emission becomes positive once the population inversion level reaches 30%. It means that the pump threshold for obtaining 2.05 μm laser is probably lower which is an advantage for Ho3＋-doped LiYF4 2.05 μm infrared lasers. The calculated gain cross section for 2.9 μm mid-infrared emission does not become positive until the population inversion level reaches 40% in Ho3＋/pr3＋：LiYF4 crystal, but 50% in Ho3＋ singly doped LiYF4 crystal, indicating that a low pumping threshold is achieved for the H03＋：5/6 → 5/7 laser operation with the introduction of Pr3＋ ions. It was also demonstrated that Pr3＋ ion can deplete rapidly the lower laser Ho3＋：5/7 level and has influence on t     

White light emission from Dy3+-doped LiLuF4 single crystal grown by Bridgman method

Lithium lutetium fluoride （LiLuF4） single crystals doped with different Dy3＋ ion concentrations were grown by Bridgman method. The Judd-Ofelt （J-O） strength parameters （Ω2, Ω4, Ω6） of Dy3＋ in LiLuF4 crystal are calculated according to the measured absorption spectra and the J-O theory, by which the asymmetry of the Dy3＋：LiLuF4 single crystal and the possibility of attaining stimulated emission from 4F9/2 level are analyzed. The capability of the Dy3＋：LiLuF4 crystal in generating white light by simultaneous blue and yellow emissions under excitation with ultra- violet light is produced. The effects of excitation wavelength and doping concentration on chromaticity coordinates and photoluminescence intensity are also investigated. Favorable CIE coordinates, x=0.319 3 and y=0.349 3, can be obtained for Dy3＋ ion in 2.701% molar doping concentration under excitation of 350 nm.     

Direct and quantitative understanding of the non-Ohmic contact resistance in organic and oxide thin-film transistors

We explore the device physics of thin film transistors (TFTs) with non-Ohmic contacts and develop a simple and fast method for evaluating the contact properties TFTs through output characteristics. Using one single output scan, the quantitative relationship between contact resistances and drain voltage were evaluated, revealing the property of interfacial injection at non-Ohmic contacts. This is demonstrated and validated in both TFT simulations and experiments employing inorganic and organic TFTs. The approach can be applied to general TFTs with arbitrary materials and configurations conveniently and enables faster and improved understanding of TFT operation and device physics.     

Co2+,Zn2+双掺杂铌酸锂晶体的坩埚下降法生长及其光谱特性

在LiNbO3中掺入0．1％（摩尔分数．下同）的CoO与3％,6％的ZnO,采用坩埚下降法技术生长了双掺杂的Co^2＋：Zn^2＋：LiNbO3晶体。用X射线衍射与差热分析表征了获得的晶体。测定了晶体不同部位在350～2500nm波段的吸收光谱。研究表明：Co离子位于LiNbO3晶体的崎变的氧八面体中,呈现＋2价,沿着生长方向Co^2＋在晶体中的浓度逐渐减少。ZnO的掺杂有效地抑制了Co^2＋离子在晶体中的掺入,使得Co^2＋在晶体中浓度的分布变得均匀。同时发现：ZnO的掺入使得上部晶体的熔点升高。从化学组分的角度解释了产生熔点变化的原因。观测到了750nm波长的荧光发射带。[编者按]     

Enhancement of photoluminescence in ZnS/ZnO quantum dots interfacial heterostructures

ZnS/ZnO quantum dots (QDs) were synthesized by controlled oxidation of ZnS nanoparticles. HRTEM image showed small nanocrystals of size 4 nm and the magnified image of single quantum dot shows interfacial heterostructure formation. The optical absorption spectrum shows a blue shift of 0.19 and 0.23 eV for ZnO and ZnS QDs, respectively. This is due to the confinement of charge carries within the nanostructures. Enormous enhancement in UV emission (10 times) is reported which is attributed to interfacial heterostructure formation. Raman spectrum shows phonons of wurtzite ZnS and ZnO. Phonon confinement effect is seen in the Raman spectrum wherein LO phonon peaks of ZnS and ZnO are shifted towards lower wavenumber side and are broadened.     

Photoluminescence properties of Tm~(3+)/Tb~(3+)/Sm~(3+)tri-doped Na_5Y_9F_(32)single crystal with high thermal stability for white light-emitting diodes

A novel Tm~(3+)/Tb~(3+)/Sm~(3+)tri-doped Na_5 Y_9 F_(32) single crystal was synthesized by a modified Bridgman method for the propose of white light emitting diodes.The fluorescence spectra of various Sm~(3+)ion concentrations and fixed 0.4 mol% Tm~(3+) and 0.5 mol% Tb~(3+) were measured and studied systematically excited by near-ultraviolet light of 355 nm.The Sm3+ion concentration takes apparent effect on the relative intensity of peaks in the visible region and the color coordinate combining from these emission bands.A near pure white light emission with color coordinates(0.3295,0.3057) and color temperature(5657 K) can be obtained when the concentrations of Tm~(3+),Tb~(3+) and Sm~(3+) ions are 0.4 mol%,0.5 mol%and 0.8 mol%,respectively.Furthermore,the practical down-conversion internal quantum yield was measured by integrating spheres at about 14.39%.The tri-doped Na_5 Y_9 F_(32) single crystal shows a high thermal stability inferring from the temperature dependent emission in which the integrated emission intensities are reduced only by～3% with the increase of temperature from 280 to 450 K.The present results demonstrate that the Tm~(3+)/Tb~(3+)/Sm~(3+)tri-doped Na_5 Y_9 F_(32) single crystal may provide a promising candidate for white light-emitting diodes,luminescent materials and fluorescent display devices.     

Thick film ZnO based varistors prepared by screen printing

Thick film varistors based on the system ZnO–Bi₂O₃–Sb₂O₃ have been prepared by screen printing technology on dense alumina substrates. Different processing strategies have been designed in order to control the excessive volatilization of Bi₂O₃ in varistor films during the sintering, due to the high area–volume ratio, and as a means to improving their electrical response. Starting powders were selected and pre-treated in different ways to obtain different phases and control the Bi-rich liquid phase formation. Significant differences have been observed in the electrical properties which are related to the selection of the starting powders.