Showing papers in "Advanced electronic materials in 2015"

Effective Approaches to Improve the Electrical Conductivity of PEDOT:PSS: A Review

Abstract: The rapid development of novel organic technologies has led to significant applications of the organic electronic devices such as light-emitting diodes, solar cells, and field-effect transistors. There is a great need for conducting polymers with high conductivity and transparency to act as the charge transport layer or electrical interconnect in organic devices. Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS), well-known as the most remarkable conducting polymer, has this role owing to its good film-forming properties, high transparency, tunable conductivity, and excellent thermal stability. In this Review, various of interesting physical and chemical approaches that can effectively improve the electrical conductivity of PEDOT:PSS are summarized, focusing especially on the mechanism of the conductivity enhancement as well as applications of PEDOT:PSS films. Prospects for future research efforts are also provided. It is expected that PEDOT:PSS films with high conductivity and transparency could be the focus of future organic electronic materials breakthroughs.

Flexible, Stretchable and Wearable Multifunctional Sensor Array as Artificial Electronic Skin for Static and Dynamic Strain Mapping

Abstract: Artificial electronic skin consists of mechanically flexible and stretchable sensor networks that can accommodate irregular surfaces and spatially map/quantify various stimuli, such as strains, pressures, and temperatures to imitate the human somatosensory system. Here, a flexible/wearable multifunctional sensor array is designed and fabricated in a cost-effective manner through simple fabrication procedures for highly-sensitive contact/pressure/strain detections. Composed of PET-based Ag serpentine-shaped electrodes, the sensor array is implemented for static and dynamic mapping of spatial contact/pressure/strain distributions in large-scale, with a detection limit of 6 Pa (corresponding to 0.5 mg). By attaching the flexible/wearable devices on human body, different motions are recognized/distinguished for gesture control applications. Combining the easy-fabricated and low-cost features, these sensor arrays may become promising candidate for highly-sensitive force detections, gesture controls, imaging of spatial pressure distributions, and find potential applications in advanced robotics, human-machine interfaces, next-generation prosthetics and healthcare monitoring devices.

Giant Electric Energy Density in Epitaxial Lead‐Free Thin Films with Coexistence of Ferroelectrics and Antiferroelectrics

Abstract: Ferroelectrics/antiferroelectrics with high dielectric breakdown strength have the potential to store a great amount of electrical energy, attractive for many modern applications in electronic devices and systems. Here, it is demonstrated that a giant electric energy density (154 J cm−3, three times the highest value of lead-based systems and five times the value of the best dielectric/ferroelectric polymer), together with the excellent fatigue-free property, good thermal stability, and high efficiency, is realized in pulsed laser deposited (Bi1/2Na1/2)0.9118La0.02Ba0.0582(Ti0.97Zr0.03)O3 (BNLBTZ) epitaxial lead-free relaxor thin films with the coexistence of ferroelectric (FE) and antiferroelectric (AFE) phases. This is endowed by high epitaxial quality, great relaxor dispersion, and the coexistence of the FE/AFE phases near the morphotropic phase boundary. The giant energy storage effect of the BNLBTZ lead-free relaxor thin films may make a great impact on the modern energy storage technology.

Prospective of Semiconductor Memory Devices: from Memory System to Materials

Abstract: The ever-increasing demand for higher-capacity digital memory shows no sign of declining. The conventional strategy for meeting such demand, i.e. shrinking of the memory cell size, will no longer be useful at some point in the future, owing to economic reasons and performance degradation. Nevertheless, performance of computing systems will keep improving for the next generation information technology. This indicates the necessity to consider a fundamentally disparate approach to enhance memory technology. Here, the current status of computer memory chips is reviewed and the pros and cons of the present technology are discussed from computing system, fabrication technology, and materials points of view. Based on this knowledge, the limitations of the present technologies are described, and the possible solutions suggested up to now are reassessed. Finally, a shift in the fundamental computational paradigm from von Neumann computing to other alternatives such as neuromorphic computing and material implication, is commented upon.

All Transparent Metal Oxide Ultraviolet Photodetector

Abstract: A high-performing UV photodetector that uses large energy bandgap materials of p-type NiO and n-type ZnO without an opaque metal electrode is reported. A quality heterojunction is formed by large-area applicable sputtering deposition method that has an extremely low saturation current density of 0.1 μA cm−2. This abrupt p-NiO/n-ZnO heterojunction device is visible-light transparent and shows the fastest photoresponse time of 24 ms among NiO-based UV photodetectors, along with the highest responsivity (3.85 A W−1) and excellent detectivity (9.6 × 1013 Jones) properties. Structural, physical, optical, and electrical properties of nanocrystalline NiO are systematically investigated. Mott–Schottky analyses are applied to develop the interface of NiO and ZnO by establishing energy diagrams. Defects existing inside the nanocrystalline NiO film enhance the UV detection performance by defect-assisted carrier transportation. The results provide a solid scheme of manipulation of NiO defects for functional photoelectric device applications.

Three-Dimensional SnO2 Nanowire Networks for Multifunctional Applications: From High-Temperature Stretchable Ceramics to Ultraresponsive Sensors

Abstract: Stretchable ceramic networks built from quasi‐one‐dimensional (Q1D) structures are important candidates for various applications of nanostructures in real‐world technologies. A flame transport synthesis approach is developed that enables versatile synthesis of interconnected SnO2 nanowire networks. These SnO2 structures exhibit interesting defects that are very relevant for the oxide material‐engineering community. Devices based on Q1D SnO2 structure networks show enormous potential for gas/UV sensing.

Large Strain in Relaxor/Ferroelectric Composite Lead-Free Piezoceramics

Abstract: A lead-free relaxor (RE)/ferroelectric (FE) 0–3 composite was developed with a large strain that resulted from the electric-field-induced ergodic relaxor-to-ferroelectric phase transition at a relatively low operational field of 4 kV mm−1. The composite comprised of 70 vol% 0.91Bi1/2Na1/2TiO3–0.06BaTiO3–0.03AgNbO3 RE matrix and 30 vol% 0.93Bi1/2Na1/2TiO3–0.07BaTiO3 FE seed shows a normalized strain, , of 824 pm V−1 at room temperature. In order to explore the underlying mechanism of this composite effect, two multilayer ceramics with alternating RE and FE layers are also prepared, one with the layers parallel (polarization-coupled multilayer) and the other with the layers perpendicular (strain-coupled multilayer) to the electroded surfaces. It is found that in addition to polarization coupling, the strain coupling effect also plays a critical role in the reduction of the RE–FE phase transition field. The switching dynamics is highlighted with time-dependent piezoforce microscopy in the vicinity of the FE/RE interface.

Thin PZT‐Based Ferroelectric Capacitors on Flexible Silicon for Nonvolatile Memory Applications

Abstract: A flexible version of traditional thin lead zirconium titanate ((Pb1.1Zr0.48Ti0.52O3)-(PZT)) based ferroelectric random access memory (FeRAM) on silicon shows record performance in flexible arena. The thin PZT layer requires lower operational voltages to achieve coercive electric fields, reduces the sol-gel coating cycles required (i.e., more cost-effective), and, fabrication wise, is more suitable for further scaling of lateral dimensions to the nano-scale due to the larger feature size-to-depth aspect ratio (critical for ultra-high density non-volatile memory applications). Utilizing the inverse proportionality between substrate's thickness and its flexibility, traditional PZT based FeRAM on silicon is transformed through a transfer-less manufacturable process into a flexible form that matches organic electronics' flexibility while preserving the superior performance of silicon CMOS electronics. Each memory cell in a FeRAM array consists of two main elements; a select/access transistor, and a storage ferroelectric capacitor. Flexible transistors on silicon have already been reported. In this work, we focus on the storage ferroelectric capacitors, and report, for the first time, its performance after transformation into a flexible version, and assess its key memory parameters while bent at 0.5 cm minimum bending radius.

Effective Electronic Mechanisms for Optimizing the Thermoelectric Properties of GeTe-Rich Alloys

Abstract: Most of the recent methods for thermoelectric (TE) enhancement are focused on reduction of the lattice thermal conductivity values, with an only limited success on efficiency enhancement due to an improved electronic doping action. This is attributed partly to the correlation between the involved electronic properties, contradicting each other in terms of the TE efficiency, and partly to the difficulty of effectively doping such alloys in the optimal 1019 cm−3 carrier concentration range. Even a positive application of an optimal electronic doping level usually results in a narrow maximal figure of merit (ZT) values range, which is followed by an onset of an intrinsic conduction, degrading these values at higher temperatures. Here, a novel BiTe- and Cu-co-doping of GeTe is reported. Incorporation of Cu electron donors into GeTe–BiTe vacancies results in an effective compensation of the inherent high holes' concentration of GeTe toward the optimal range with pronounced carriers scattering mechanism of ionized impurities, in addition to the known acoustic phonons mechanism, while enhancing of Seebeck coefficient. Adverse high temperature intrinsic conduction effects are suppressed by the rhombohedral-to-cubic phase transition of GeTe, extending the maximal ZTs range of ≈1.55 ± 0.1 over a large temperature range.

Unprecedentedly Wide Curie-Temperature Windows as Phase-Transition Design Platform for Tunable Magneto-Multifunctional Materials

Abstract: A series of unprecedentedly wide Curie-temperature windows (CTWs) between 40 and 450 K are realized by employing the isostructural alloying principle for the strongly coupled magnetostructural phase transitions in a single host system. The CTWs provide a design platform for magneto-multifunctional multiferroic alloys that can be manipulated in a quite large temperature space in various scales and patterns, as well as by multiple physical fields.

Hydrogen Bistability as the Origin of Photo-Bias-Thermal Instabilities in Amorphous Oxide Semiconductors

Abstract: Zinc-based metal oxide semiconductors have attracted attention as an alternative to current silicon-based semiconductors for applications in transparent and fl exible electronics. Despite this, metal oxide transistors require signifi cant improvements in performance and electrical reliability before they can be applied widely in optoelectronics. Amorphous indium‐zinc‐tin oxide (a-IZTO) has been considered an alternative channel layer to a prototypical indium‐gallium‐zinc oxide (IGZO) with the aim of achieving a high mobility (>40 cm 2 Vs −1 ) transistors. The effects of the gate bias and light stress on the resulting a-IZTO fi eldeffect transistors are examined in detail. Hydrogen impurities in the a-IZTO semiconductor are found to play a direct role in determining the photo-bias stability of the resulting transistors. The Al 2 O 3 -inserted IZTO thin-fi lm transistors (TFTs) are hydrogen-poor, and consequently show better resistance to the external-bias-thermal stress and photo-bias-thermal stress than the hydrogenrich control IZTO TFTs. First-principles calculations show that even in the amorphous phase, hydrogen impurities including interstitial H and substitutional H can be bistable centers with an electronic deep-to-shallow transition through large lattice relaxation. The negative threshold voltage shift of the a-IZTO transistors under a negative-bias-thermal stress and negative-bias-illumination stress condition is attributed to the transition from the acceptor-like deep interstitial H i − (or substitutional H-DX − ) to the shallow H i + (or H O + ) with a high (low) activation energy barrier. Conclusively, the delicate controllability of hydrogen is a key factor to achieve the high performance and stability of the metal oxide transistors.

Stretchable and Conformable Oxide Thin-Film Electronics

Abstract: Stretchable large-area high-performance amorphous oxide thin-film electronics fabricated using locally reinforced composite elastomers or wavy structures are functional while elongated by >200% and after 4000 stretching and relaxation cycles. 2D stretchable sensors, amplifiers, and circuits for wireless power transmission are conformably wrapped around arbitrary 3D structures and enable sensor systems for electronic implants and skins.

Associative Learning with Temporal Contiguity in a Memristive Circuit for Large‐Scale Neuromorphic Networks

Abstract: Memristors, acting as artificial synapses, have promised their prospects in neuromorphic systems that imitate the brain's computing paradigm. However, most studies focused on the understanding of the memristive mechanism and how to optimize the synaptic performance, and the implementations of higher-order cognitive functions are quite limited. Here the experimental demonstration of a representative network level learning function, i.e., associative learning and extinction, in a compact memristive neuromorphic circuit with only one memristor is reported. The association of the conditioned and unconditioned stimulus is established within a temporal window through the spike-timing-dependent plasticity rule, whereas the extinction of the formed memory is due to the synaptic depression. The temporal contiguity consists with biological behaviors and reflects nature's cause and effect rule. An efficient methodology of integrating memristors into large-scale neuromorphic systems for massively parallel computing, such as pattern recognition, is provided herein.

Lead Replacement in CH3NH3PbI3 Perovskites

Abstract: Superior photovoltaic performance in organic–inorganic hybrid perovskite is based on the unique properties of each moiety contined within it. Identifying the role of metal atoms in the perovskite is of great importance to explore the low-toxicity lead-free perovskite solar cells. By using the first-principle calculations, four types of AMX3 (A = CH3NH3, M = Pb, Sn, Ge, Sr, X = I) perovskite materials are investigated and an attempt is made to understand the structural and electronic influences of the metal atoms on the properties of perovskites. Then, the solutions to the replacement of Pb are discussed. It is found that for the small radius metal atoms as compared with Pb, the strong geometry distortion will result in a less p–p electron transition and larger carrier effective mass. The outer ns2 electrons of the metal ions play critical roles on the modulation of the optical and electronic properties for perovskite materials. These findings suggest that the solutions to the Pb replacement might be metal or metallic clusters that have effective ionic radius and outer ns2 electrons configuration on the metal ions with low ionization energy similar to Pb2+. Based on this, lead-free perovskite solar cells are expected to be realized in the near future.

Electroluminescence from Organometallic Lead Halide Perovskite‐Conjugated Polymer Diodes

Abstract: We gratefully acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC). A.K. acknowledges NRF-Singapore for a scholarship.

High‐Performance Solution‐Processed Amorphous‐Oxide‐Semiconductor TFTs with Organic Polymeric Gate Dielectrics

Abstract: We gratefully acknowledge financial support from the European Commission through the POINTS project (FP7-NMP-2010-Small-4).

Novel Optical and Electrical Transport Properties in Atomically Thin WSe2/MoS2 p–n Heterostructures

Abstract: Vertically stacked Van der Waals heterojunctions of atomically thin transition metal dichalcogenides (TMDs) offer new physical properties and new strategies for designing novel device functionalities that are vastly different from homostructured TMDs. The Raman intensity is strongest and frequency difference is largest in monolayer WSe2 compared with that in few-layers, which is opposite to MoS2 and WS2. In the WSe2/MoS2 bilayer heterostructures, inefficient charge transfer quenches light emission of monolayer WSe2 but strengthens those of MoS2 monolayer. Interestingly, rectification and ambipolar effects emerge due to tunneling-assisted interlayer recombination and dual conducting channels of p-WSe2 and n-MoS2 in the heterojunctions system. Gate-induced holes tunneling also leads to a novel “anti-bipolar” behavior with a sharp current peak. Under light illumination, charge transfer competes with the holes tunneling between the WSe2 and MoS2 layers, which can greatly influence the electrical transport leading to the disappeared rectifying and “anti-bipolar” properties.

Foldable Conductive Cellulose Fiber Networks Modified by Graphene Nanoplatelet-Bio-Based Composites

Abstract: Truly foldable flexible electronic components require a foldable substrate modified with a conducting material that can retain its electrical conductivity and mechanical integrity even after hard mechanical manipulations and multiple folding events. Here, such a material exploiting the combination of all-biodegradable components (substrate and the polymer matrix) and graphene nanoplatelets is designed and fabricated. A commercially available thermoplastic starch-based polymer (Mater-Bi) and graphene nanoplatelets are simultaneously dispersed in an organic solvent to formulate conductive inks. The inks are spray painted on pure cellulose sheets and hot-pressed into their fiber network after drying. The resultant nanostructured flexible composites display excellent isotropic electrical conductivity, reaching very low sheet resistance value ≈10 Ω sq−1, depending on the relative concentration between the biopolymer and the graphene nanoplatelets. Transmission electron microscopy results indicated that during hot-pressing, graphene nanoplatelets are physically embedded into the cellulose fibers, resulting in high electrical conductivity of the flexible composite. The paper-like flexible conductors can withstand many severe folding events, maintaining their mechanical and electrical properties and showing only a slight decrease of their electrical conductivity with respect to the unfolded counterparts. Unlike conductive paper technologies, the proposed paper-like flexible conductors demonstrate both sides isotropic conductivity due to pressure-induced impregnation.

Enhanced Thermoelectric Performance of Ultrathin Bi2Se3 Nanosheets through Thickness Control

Abstract: Large-scale Bi2Se3 nanosheets with controllable thickness have been synthesized by a microwave-assisted solvothermal method. Through detailed structural characterizations, high-quality Bi2Se3 nanosheets with average thickness of 1, 4, 7, and 13 nm have been fabricated. Their thermoelectric performance has been detailed investigated by experiments and fundamental nonparabolic Kane models. A significantly reduced thermal conductivity (only 0.41 W m(-1) K-1), and enhanced powder factor (4.71 x 10(-4) W m(-1) K-2 with a Seebeck coeffi cient of -155.32 mu V K-1 and an electrical conductivity of 1.96 x 10(4) S m(-1)) are observed in the pellet composed of single-layered Bi2Se3 nanosheets. Such an enhanced thermoelectric performance is ascribed to the broadened bandgap and optimized Fermi level in ultrathin Bi2Se3 nanosheets.

Ligand-induced control of photoconductive gain and doping in a hybrid graphene-quantum dot transistor

Abstract: Recent advances in graphene-based electronics range from the discovery of fundamental physical phenomena to the development of new high-performance photosensitive devices. [ 1–5 ] These applications exploit not only the unique electronic properties of graphene but also additional functionalities that can be achieved by capping the graphene layer with another material or nanostructure, e.g., atomically thin fi lms, [ 6,7 ] carbon nanotubes [ 8 ] or inorganic nanoparticles. [ 8–10 ] Colloidal semiconductor quantum dots (QDs) are of particular interest as their optical properties can be fi ne-tuned by varying their size and/or composition. [ 11 ] In addition, colloidal synthesis enables QDs to be functionalized by doping [ 12 ] and/or surface encapsulation. [ 13 ] Recently, a photoresponsivity of 10 7 A W −1 was achieved by depositing QDs on graphene and explained in terms of trapping of photoexcited carriers on the QDs and charge transfer between them and the graphene layer. [ 9,10 ] This mechanism should be strongly dependent on the interface between the QDs and graphene. Furthermore, the ligands that encapsulate the QDs may provide a means of modifying the transfer of electronic charges and enhancing the electronic properties of the graphene layer. Here, we investigate the properties of single layer graphene (SLG) functionalized with an overlayer of near-infrared PbS colloidal quantum dots capped with thioglycerol/dithioglycerol or polyethylene glycol (PEG500 and PEG2000). We demonstrate that the polarity of the conductivity and the carrier concentration can be modifi ed, and photoresponsivity of SLG can be signifi cantly enhanced by the choice of ligands. By reducing the length of capping ligands, hence the thickness of the dielectric barrier between the QDs and the SLG, and by preserving the integrity of the ligand layer, we achieve the effi - cient transfer of photogenerated carriers from the QDs to the graphene before recombining, resulting in enhanced photoresponsivities of up to ≈10 9 A W −1 .