Light-emitting transistor structures based on semiconducting polymers and inorganic nanoparticles

One of the new directions in organic electronics is the development of light-emitting organic field-effect transistors, which combine the light-emitting properties of organic light-emitting diodes and the switching properties of organic field-effect transistors. Optical and electronic properties of novel nanocomposite materials based on semiconducting polymers and inorganic nanoparticles and designed for applications in organic electronics devices were investigated. Light-emitting organic field-effect transistors with composite active layers based on the soluble semiconducting polymers PFO and MEH-PPV and ZnO nanoparticles and having asymmetric electrodes (Al and Au) that inject electrons into ZnO and holes into PFO and MEH-PPV were prepared and investigated. The data are interpreted in the context of the possibility of organic field-effect transistors based on PFO: ZnO and MEH-PPV: ZnO composite films to work in both the unipolar regime and the ambipolar regime. It is shown that the mobility of charge carriers in light-emitting organic field-effect transistors based on PFO: ZnO at 300 K reaches ～0.02 for electrons and ～0.03 cm²/(V s) for holes, increasing with an increase in the concentration of nanoparticles up to ～2 cm²/(V s), a value that is comparable to the maximum mobility values for conducting polymers. Light-emitting organic field-effect transistors based on PFO: ZnO and MEH-PPV: ZnO emit light in the green and orange ranges of the optical spectrum, respectively, their electroluminescence intensities rising with an increase in either the negative bias or the positive bias at the source-drain and the gate as well as with an increase in the concentration of ZnO nanoparticles. The results indicate that light-emitting organic field-effect transistors based on soluble conjugated polymers and semiconducting ZnO nanoparticles are examples of multifunctional devices whose production technology is compatible with the modern ink-jet printing technology of organic electronics.     

Synthesis of zinc oxide nanostructures on graphene/glass substrate by electrochemical deposition: effects of current density and temperature

The electrochemical growth of zinc oxide (ZnO) nanostructures on graphene on glass using zinc nitrate hexahydrate was studied. The effects of current densities and temperatures on the morphological, structural, and optical properties of the ZnO structures were studied. Vertically aligned nanorods were obtained at a low temperature of 75°C, and the diameters increased with current density. Growth temperature seems to have a strong effect in generating well-defined hexagonal-shape nanorods with a smooth top edge surface. A film-like structure was observed for high current densities above -1.0 mA/cm² and temperatures above 80°C due to the coalescence between the neighboring nanorods with large diameter. The nanorods grown at a temperature of 75°C with a low current density of -0.1 mA/cm² exhibited the highest density of 1.45 × 10⁹ cm^-2. X-ray diffraction measurements revealed that the grown ZnO crystallites were highly oriented along the c-axis. The intensity ratio of the ultraviolet (UV) region emission to the visible region emission, I_UV/I_VIS, showed a decrement with the current densities for all grown samples. The samples grown at the current density below -0.5 mA/cm² showed high I_UV/I_VIS values closer to or higher than 1.0, suggesting their fewer structural defects. For all the ZnO/graphene structures, the high transmittance up to 65% was obtained at the light wavelength of 550 nm. Structural and optical properties of the grown ZnO structures seem to be effectively controlled by the current density rather than the growth temperature. ZnO nanorod/graphene hybrid structure on glass is expected to be a promising structure for solar cell which is a conceivable candidate to address the global need for an inexpensive alternative energy source.     

Angle-dependent photodegradation over ZnO nanowire arrays on flexible paper substrates

In this study, we grew zinc oxide (ZnO) nanowire arrays on paper substrates using a two-step growth strategy. In the first step, we formed single-crystalline ZnO nanoparticles of uniform size distribution (ca. 4 nm) as seeds for the hydrothermal growth of the ZnO nanowire arrays. After spin-coating of these seeds onto paper, we grew ZnO nanowire arrays conformally on these substrates. The crystal structure of a ZnO nanowire revealed that the nanowires were single-crystalline and had grown along the c axis. Further visualization through annular bright field scanning transmission electron microscopy revealed that the hydrothermally grown ZnO nanowires possessed Zn polarity. From photocatalytic activity measurements of the ZnO nanowire (NW) arrays on paper substrate, we extracted rate constants of 0.415, 0.244, 0.195, and 0.08 s^-1 for the degradation of methylene blue at incident angles of 0°, 30°, 60°, and 75°, respectively; that is, the photocatalytic activity of these ZnO nanowire arrays was related to the cosine of the incident angle of the UV light. Accordingly, these materials have promising applications in the design of sterilization systems and light-harvesting devices.     

Seed/catalyst-free growth of zinc oxide nanostructures on multilayer graphene by thermal evaporation

We report the seed/catalyst-free growth of ZnO on multilayer graphene by thermal evaporation of Zn in the presence of O₂ gas. The effects of substrate temperatures were studied. The changes of morphologies were very significant where the grown ZnO structures show three different structures, i.e., nanoclusters, nanorods, and thin films at 600°C, 800°C, and 1,000°C, respectively. High-density vertically aligned ZnO nanorods comparable to other methods were obtained. A growth mechanism was proposed based on the obtained results. The ZnO/graphene hybrid structure provides several potential applications in electronics and optoelectronics.     

Grain size dependence of dielectric relaxation in cerium oxide as high-k layer

Cerium oxide (CeO₂) thin films used liquid injection atomic layer deposition (ALD) for deposition and ALD procedures were run at substrate temperatures of 150°C, 200°C, 250°C, 300°C, and 350°C, respectively. CeO₂ were grown on n-Si(100) wafers. Variations in the grain sizes of the samples are governed by the deposition temperature and have been estimated using Scherrer analysis of the X-ray diffraction patterns. The changing grain size correlates with the changes seen in the Raman spectrum. Strong frequency dispersion is found in the capacitance-voltage measurement. Normalized dielectric constant measurement is quantitatively utilized to characterize the dielectric constant variation. The relationship extracted between grain size and dielectric relaxation for CeO₂ suggests that tuning properties for improved frequency dispersion can be achieved by controlling the grain size, hence the strain at the nanoscale dimensions.     

Structural,electrical, and optical properties of Ti-doped ZnO films fabricated by atomic layer deposition

High-quality Ti-doped ZnO films were grown on Si, thermally grown SiO₂, and quartz substrates by atomic layer deposition (ALD) at 200°C with various Ti doping concentrations. Titanium isopropoxide, diethyl zinc, and deionized water were sources for Ti, Zn, and O, respectively. The Ti doping was then achieved by growing ZnO and TiO₂ alternately. A hampered growth mode of ZnO on TiO₂ layer was confirmed by comparing the thicknesses measured by spectroscopic ellipsometry with the expected. It was also found that the locations of the (100) diffraction peaks shift towards lower diffraction angles as Ti concentration increased. For all samples, optical transmittance over 80% was obtained in the visible region. The sample with ALD cycle ratio of ZnO/TiO₂ being 20 had the lowest resistivity of 8.874 × 10⁻⁴ Ω cm. In addition, carrier concentration of the prepared films underwent an evident increase and then decreased with the increase of Ti doping concentration.     

High carrier mobility low-voltage ZnO thin film transistors fabricated at a low temperature via solution processing

Wide-bandgap ZnO TFTs have many potential applications in large-area, flexible electronics and transparent devices because of their low cost, high performance and excellent optical transmittance. High-performance ZnO TFTs fabricated via simple solution processing have been widely studied. However, the key issues of solution-processable ZnO TFTs are the relatively high processing temperature (> 300?°C) and the high operating voltage for achieving the desired electrical properties. Here, we successfully fabricated low-voltage ZnO TFTs at an annealing temperature of ≤?250?°C. The resulting ZnO transistors with high-k terpolymer P(VDF-TrFE-CFE) showed a mobility of up to 5.3?cm² V^?1 s^?1 and an on/off ratio of >?10⁵ at 3?V. Furthermore, the influence of the dielectric constant on the carrier mobility was investigated. A lower k-value dielectric resulted in a high carrier mobility under the same carrier density. Therefore, with a low-k CYTOP dielectric applied to modify the interface between the ZnO semiconductor and the P(VDF-TrFE-CFE) layer, ZnO transistors annealed at 250?°C showed an electron mobility of 13.6?cm² V^?1 s^?1 and an on/off ratio of >?10⁵ at 3?V. To the best of our knowledge, this mobility is the highest value reported to date among the low-voltage solution-processable undoped ZnO TFTs annealed at temperatures of ≤?300?°C.     

Seedless growth of zinc oxide flower-shaped structures on multilayer graphene by electrochemical deposition

A seedless growth of zinc oxide (ZnO) structures on multilayer (ML) graphene by electrochemical deposition without any pre-deposited ZnO seed layer or metal catalyst was studied. A high density of a mixture of vertically aligned/non-aligned ZnO rods and flower-shaped structures was obtained. ML graphene seems to generate the formation of flower-shaped structures due to the stacking boundaries. The nucleation of ZnO seems to be promoted at the stacking edges of ML graphene with the increase of applied current density, resulting in the formation of flower-shaped structures. The diameters of the rods/flower-shaped structures also increase with the applied current density. ZnO rods/flower-shaped structures with high aspect ratio over 5.0 and good crystallinity were obtained at the applied current densities of −0.5 and −1.0 mA/cm². The growth mechanism was proposed. The growth involves the formation of ZnO nucleation below 80°C and the enhancement of the growth of vertically non-aligned rods and flower-shaped structures at 80°C. Such ZnO/graphene hybrid structure provides several potential applications in sensing devices.     

Electrical and optical properties of Al-doped ZnO and ZnAl2O4 films prepared by atomic layer deposition

ZnO/Al₂O₃ multilayers were prepared by alternating atomic layer deposition (ALD) at 150°C using diethylzinc, trimethylaluminum, and water. The growth process, crystallinity, and electrical and optical properties of the multilayers were studied with a variety of the cycle ratios of ZnO and Al₂O₃ sublayers. Transparent conductive Al-doped ZnO films were prepared with the minimum resistivity of 2.4 × 10⁻³ Ω·cm at a low Al doping concentration of 2.26%. Photoluminescence spectroscopy in conjunction with X-ray diffraction analysis revealed that the thickness of ZnO sublayers plays an important role on the priority for selective crystallization of ZnAl₂O₄ and ZnO phases during high-temperature annealing ZnO/Al₂O₃ multilayers. It was found that pure ZnAl₂O₄ film was synthesized by annealing the specific composite film containing alternative monocycle of ZnO and Al₂O₃ sublayers, which could only be deposited precisely by utilizing ALD technology.     

Transparent metal oxide nanowire transistors

With the features of high mobility, a high electric on/off ratio and excellent transparency, metal oxide nanowires are excellent candidates for transparent thin-film transistors, which is one of the key technologies to realize transparent electronics. This article provides a comprehensive review of the state-of-the-art research activities that focus on transparent metal oxide nanowire transistors. It begins with the brief introduction to the synthetic methods for high quality metal oxide nanowires, and the typical nanowire transfer and printing techniques with emphasis on the simple contact printing methodology. High performance transparent transistors built on both single nanowires and nanowire thin films are then highlighted. The final section deals with the applications of transparent metal oxide nanowire transistors in the field of transparent displays and concludes with an outlook on the current perspectives and future directions of transparent metal oxide nanowire transistors.     

Open structure ZnO/CdSe core/shell nanoneedle arrays for solar cells

Open structure ZnO/CdSe core/shell nanoneedle arrays were prepared on a conducting glass (SnO₂:F) substrate by solution deposition and electrochemical techniques. A uniform CdSe shell layer with a grain size of approximately several tens of nanometers was formed on the surface of ZnO nanoneedle cores after annealing at 400°C for 1.5 h. Fabricated solar cells based on these nanostructures exhibited a high short-circuit current density of about 10.5 mA/cm² and an overall power conversion efficiency of 1.07% with solar illumination of 100 mW/cm². Incident photo-to-current conversion efficiencies higher than 75% were also obtained.     

Effect of UV/ozone treatment on polystyrene dielectric and its application on organic field-effect transistors

The influence of UV/ozone treatment on the property of polystyrene (PS) dielectric surface was investigated, and pentacene organic field-effect transistors (OFETs) based on the treated dielectric was fabricated. The dielectric and pentacene active layers were characterized by atomic force microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The results showed that, at short UVO exposure time (<10 s), the chemical composition of PS dielectric surface remained the same. While at long UVO exposure time (>60 s), new chemical groups, including alcohol/ether, carbonyl, and carboxyl/ester groups, were formed. By adjusting the UVO exposure time to 5 s, the hole mobility of the OFETs increased to 0.52 cm²/Vs, and the threshold voltage was positively shifted to -12 V. While the time of UVO treatment exceeded 30 s, the mobility started to shrink, and the off-current was enlarged. These results indicate that, as a simple surface treatment method, UVO treatment could quantitatively modulate the property of PS dielectric surface by controlling the exposure time, and thus, pioneered a new way to modulate the characteristics of organic electronic devices.     

Performance enhancement of multiple-gate ZnO metal-oxide-semiconductor field-effect transistors fabricated using self-aligned and laser interference photolithography techniques

The simple self-aligned photolithography technique and laser interference photolithography technique were proposed and utilized to fabricate multiple-gate ZnO metal-oxide-semiconductor field-effect transistors (MOSFETs). Since the multiple-gate structure could improve the electrical field distribution along the ZnO channel, the performance of the ZnO MOSFETs could be enhanced. The performance of the multiple-gate ZnO MOSFETs was better than that of the conventional single-gate ZnO MOSFETs. The higher the drain-source saturation current (12.41 mA/mm), the higher the transconductance (5.35 mS/mm) and the lower the anomalous off-current (5.7 μA/mm) for the multiple-gate ZnO MOSFETs were obtained.     

Fabrication and electrical properties of MoS2 nanodisc-based back-gated field effect transistors

Two-dimensional (2D) molybdenum disulfide (MoS₂) is an attractive alternative semiconductor material for next-generation low-power nanoelectronic applications, due to its special structure and large bandgap. Here, we report the fabrication of large-area MoS₂ nanodiscs and their incorporation into back-gated field effect transistors (FETs) whose electrical properties we characterize. The MoS₂ nanodiscs, fabricated via chemical vapor deposition (CVD), are homogeneous and continuous, and their thickness of around 5 nm is equal to a few layers of MoS₂. In addition, we find that the MoS₂ nanodisc-based back-gated field effect transistors with nickel electrodes achieve very high performance. The transistors exhibit an on/off current ratio of up to 1.9 × 10⁵, and a maximum transconductance of up to 27 μS (5.4 μS/μm). Moreover, their mobility is as high as 368 cm²/Vs. Furthermore, the transistors have good output characteristics and can be easily modulated by the back gate. The electrical properties of the MoS₂ nanodisc transistors are better than or comparable to those values extracted from single and multilayer MoS₂ FETs.     

Se-doping dependence of the transport properties in CBE-grown InAs nanowire field effect transistors

We investigated the transport properties of lateral gate field effect transistors (FET) that have been realized by employing, as active elements, (111) B-oriented InAs nanowires grown by chemical beam epitaxy with different Se-doping concentrations. On the basis of electrical measurements, it was found that the carrier mobility increases from 10³ to 10⁴ cm²/(V × sec) by varying the ditertiarybutyl selenide (DtBSe) precursor line pressure from 0 to 0.4 Torr, leading to an increase of the carrier density in the transistor channel of more than two orders of magnitude. By keeping the DtBSe line pressure at 0.1 Torr, the carrier density in the nanowire channel measures ≈ 5 × 10¹⁷ cm^-3 ensuring the best peak transconductances (> 100 mS/m) together with very low resistivity values (70 Ω × μm) and capacitances in the attofarad range. These results are particularly relevant for further optimization of the nanowire-FET terahertz detectors recently demonstrated.PACS: 73.63.-b, 81.07.Gf, 85.35.-p     

Novel Insights into Inkjet Printed Silver Nanowires Flexible Transparent Conductive Films

Silver nanowire (AgNWs) inks for inkjet printing were prepared and the effects of the solvent system, wetting agent, AgNWs suspension on the viscosity, surface tension, contact angle between ink droplet and poly(ethylene) terephthalate (PET) surface, and pH value of AgNWs ink were discussed. Further, AgNWs flexible transparent conductive films were fabricated by using inkjet printing process on the PET substrate, and the effects of the number printing layer, heat treatment temperature, drop frequency, and number of nozzle on the microstructures and photoelectric properties of AgNWs films were investigated in detail. The experimental results demonstrated that the 14-layer AgNWs printed film heated at 60 °C and 70 °C had an average sheet resistance of 13 Ω∙sq⁻¹ and 23 Ω∙sq⁻¹ and average transparency of 81.9% and 83.1%, respectively, and displayed good photoelectric performance when the inkjet printing parameters were set to the voltage of 20 V, number of nozzles of 16, drop frequency of 7000 Hz, droplet spacing of 15 μm, PET substrate temperatures of 40 °C and nozzles of 35 °C during printing, and heat treatment at 60 °C for 20 min. The accumulation and overflow of AgNWs at the edges of the linear pattern were observed, which resulted in a decrease in printing accuracy. We successfully printed the heart-shaped pattern and then demonstrated that it could work well. This showed that the well-defined pattern with good photoelectric properties can be obtained by using an inkjet printing process with silver nanowires ink as inkjet material.     

Graphene quantum dots directly generated from graphite via magnetron sputtering and the application in thin-film transistors

This work presents a novel method to prepare graphene quantum dots (GQDs) directly from graphite. A composite film of GQDs and ZnO was first prepared using the composite target of graphite and ZnO via magnetron sputtering, followed with hydrochloric acid treatment and dialysis. Morphology and optical properties of the GQDs were investigated using a number of techniques. The as-prepared GQDs are 4–12 nm in size and 1–2 nm in thickness. They also exhibited typical excitation-dependent properties as expected in carbon-based quantum dots. To demonstrate the potential applications of GQDs in electronic devices, pure ZnO and GQD–ZnO thin-film transistors (TFTs) using ZrO_x dielectric were fabricated and examined. The ZnO TFT incorporating the GQDs exhibited enhanced performance: an on/off current ratio of 1.7 × 10⁷, a field-effect mobility of 17.7 cm²/Vs, a subthreshold swing voltage of 90 mV/decade. This paper provides an efficient, reproducible and eco-friendly approach for the preparation of monodisperse GQDs directly from graphite. Our results suggest that GQDs fabricated using magnetron sputtering method may envision promising applications in electronic devices.     

Molecular aggregation–performance relationship in the design of novel cyclohexylethynyl end-capped quaterthiophenes for solution-processed organic transistors

The synthesis and characterization of cyclohexylethenyl end-capped quaterthiophenes is reported. Additionally, an investigation of the performance of organic field-effect transistors based on these quaterthiophenes in view of the relationship between the solid-state (or aggregate) order and the electronic performance is described. UV–vis absorption measurements revealed that the quaterthiophene with an asymmetrically substituted cyclohexylethynyl end-group induced the formation of H-type aggregates, whereas the quaterthiophene with a symmetrically substituted cyclohexylethynyl end-groups favored the formation of J-type aggregates. Two-dimensional grazing-incidence wide-angle X-ray scattering studies were performed to support the molecular structure-dependent packing of films of the new quaterthiophenes. Solution-processed quaterthiophenes were tested as the active layers of p-type organic field-effect transistors with a bottom gate/top contact geometry. The field-effect mobility of devices that incorporated asymmetric quaterthiophene molecules was quite high, exceeding 0.02 cm²/V s, due to H-aggregation and good in-plane ordering. In contrast, the field-effect mobility of devices that incorporated symmetrical quaterthiophenes, was low, above 5 × 10^?4 cm²/(V s), due to the formation of J-aggregates and poor in-plane ordering. A comparison of the symmetrical and asymmetrical quaterthiophene derivatives revealed that the molecular aggregation-dependent packing, determined by the cyclohexylethynyl end groups, was responsible for influencing the organic field-effect transistor performance.     

ZnO nanosheet arrays constructed on weaved titanium wire for CdS-sensitized solar cells

Ordered ZnO nanosheet arrays were grown on weaved titanium wires by a low-temperature hydrothermal method. CdS nanoparticles were deposited onto the ZnO nanosheet arrays using the successive ionic layer adsorption and reaction method to make a photoanode. Nanoparticle-sensitized solar cells were assembled using these CdS/ZnO nanostructured photoanodes, and their photovoltaic performance was studied systematically. The best light-to-electricity conversion efficiency was obtained to be 2.17% under 100 mW/cm² illumination, and a remarkable short-circuit photocurrent density of approximately 20.1 mA/cm² was recorded, which could attribute to the relatively direct pathways for transportation of electrons provided by ZnO nanosheet arrays as well as the direct contact between ZnO and weaved titanium wires. These results indicate that CdS/ZnO nanostructures on weaved titanium wires would open a novel possibility for applications of low-cost solar cells.     

Direct formation of gold nanoparticles on substrates using a novel ZnO sacrificial templated-growth hydrothermal approach and their properties in organic memory device

This study describes a novel fabrication technique to grow gold nanoparticles (AuNPs) directly on seeded ZnO sacrificial template/polymethylsilsesquioxanes (PMSSQ)/Si using low-temperature hydrothermal reaction at 80°C for 4 h. The effect of non-annealing and various annealing temperatures, 200°C, 300°C, and 400°C, of the ZnO-seeded template on AuNP size and distribution was systematically studied. Another PMMSQ layer was spin-coated on AuNPs to study the memory properties of organic insulator-embedded AuNPs. Well-distributed and controllable AuNP sizes were successfully grown directly on the substrate, as observed using a field emission scanning electron microscope followed by an elemental analysis study. A phase analysis study confirmed that the ZnO sacrificial template was eliminated during the hydrothermal reaction. The AuNP formation mechanism using this hydrothermal reaction approach was proposed. In this study, the AuNPs were charge-trapped sites and showed excellent memory effects when embedded in PMSSQ. Optimum memory properties of PMMSQ-embedded AuNPs were obtained for AuNPs synthesized on a seeded ZnO template annealed at 300°C, with 54 electrons trapped per AuNP and excellent current–voltage response between an erased and programmed device.