Double-layer Devices Research Articles

Analysis of Carrier Behaviors in Double-layer Organic Devices by Displacement Current Measurement and Electric-field-induced Optical Second-harmonic Generation Measurement

Analysis of Carrier Behaviors in Double-layer Organic Devices by Displacement Current Measurement and Electric-field-induced Optical Second-harmonic Generation Measurement

Anomalous low-temperature Coulomb drag in graphene-GaAs heterostructures.

Vertical heterostructures combining different layered materials offer novel opportunities for applications and fundamental studies. Here we report a new class of heterostructures comprising a single-layer (or bilayer) graphene in close proximity to a quantum well created in GaAs and supporting a high-mobility two-dimensional electron gas. In our devices, graphene is naturally hole-doped, thereby allowing for the investigation of electron-hole interactions. We focus on the Coulomb drag transport measurements, which are sensitive to many-body effects, and find that the Coulomb drag resistivity significantly increases for temperatures <5-10 K. The low-temperature data follow a logarithmic law, therefore displaying a notable departure from the ordinary quadratic temperature dependence expected in a weakly correlated Fermi-liquid. This anomalous behaviour is consistent with the onset of strong interlayer correlations. Our heterostructures represent a new platform for the creation of coherent circuits and topologically protected quantum bits.

Silver nanoparticles with 4,4′-dicyanamidobiphenyl ligand: Synthesis, photoluminescent and electroluminescent properties and DFT calculations

In this research, a new complex of 4,4′-dicyanamidobiphenyl (bpH2) ligand, Ag(bpH) has been synthesized and characterized by FT-IR, UV–Vis spectroscopies, and elemental analysis. The morphology and size of complex were determined by the scanning electron microscopy (SEM) image. The diameter of the complex was about 61nm and the nanoparticles were spherical. TGA result indicated that this complex was thermally stable. Also, photoluminescent and electroluminescent properties of Ag(bpH) were reported. The blue–green light emission was observed by fabricating double-layer devices using Ag(bpH) as electron-transport and NPB as hole-transport material. In addition, this complex was optimized by B3LYP/LANL2DZ method. The calculated FT-IR and UV–Vis data are in better agreement with the experimental results. In addition to DFT calculations, natural bond orbital (NBO) was also performed at the same level of theory.

Localized resistive switching in a ZnS–Ag/ZnS double-layer memory

Bipolar resistive-switching memories were demonstrated based on a Ag/ZnS/ITO stack structure, where the electrolyte layer is made into a double-layer structure composed of a Ag-nanoclusters-embedded ZnS layer and a pure ZnS layer. The introduction of Ag-nanoclusters results in thicker Ag conducting filaments (CFs) formed in ZnS–Ag than in ZnS. This difference in CFs’ size between two sub-layers makes the resistive switching localized in the ZnS sub-layer. By reducing its thickness, the switching region is reduced and the degree of localization is enhanced. Therefore, compared with the single-layer ZnS memory, the double-layer device shows improved switching uniformity, reduced forming/switching voltages and better endurance.

Multi-layer Ceramic-Pillar Composite Structures for Piezoelectric Energy Harvesters

In this paper, we will discuss piezoelectric energy harvesters of multi-layered ceramic-pillar composite structures to be used as the energy source of small size electronic devices. 0.2(PbMg1/3Nb2/3O3)-0.8(PbZr0.475Ti0.525O3) ceramics and polydi-methylsiloxane (PDMS) were employed as piezoelectric ceramic-pillar composite, which prepared in forms of rectangular pillar structures. The piezoelectric ceramic-pillar composites were sintered separately and attached on the metallic bottom electrode with poled states. Symmetric upper electrodes were attached on the other side. Subsequently, the ceramic-pillar structures of composite material were manufactured with single-layer, double-layer and triple-layer devices. Both of multi-layer and single layer devices will be compared with several analysis methods. Piezoelectric properties will be discussed including the piezoelectric constant. Also, the output power of the multi-layer and single layer devices will be studied.

Planar fluxgate layouts using a single layer of coils

This work describes the development of planar fluxgate magnetometers consisting of a single coils’ layer. The constructed sensors can achieve analogous noise levels to a similar double layer device, while reducing the manufacture complexity. The tread-off is an early limit in noise reduction by increasing the excitation amplitude, caused by the non-directly excited core part. But placing extra excitation coils parallel to the pick-up ones can avoid this minimum threshold. Fabrication and characterization are presented and discussed.

Solution-processed cathode-interlayer-free deep blue organic light-emitting diodes

Efficient non-doped deep blue organic light-emitting diodes (OLEDs) were fabricated by solution-processing method by using a series of small molecules consisting of various contents of triphenylamine and phosphonate-featured fluorene units as the emitting layer. Without any electron-injection layer, one of the optimal devices with a simple double-layer device configuration exhibits a maximal current efficiency of 2.59cdA−1 at 6.8V (1.72mAcm−2) with a CIE coordinates of (0.163, 0.097). These double-layer devices are demonstrated with excellent color-stability under a wide range of operating current density. The current work indicates that electron-rich triphenylamine moiety incorporated with phosphonate-featured fluorene units could be utilized as building blocks to construct a multi-functional platform combining good electron-injection property, carrier-transport property, and efficient electroluminescence. It also provides an approach to achieve a structure-simplified color-stable efficient blue OLED.

New generation of wearable goniometers for motion capture systems.

BackgroundMonitoring joint angles through wearable systems enables human posture and gesture to be reconstructed as a support for physical rehabilitation both in clinics and at the patient’s home. A new generation of wearable goniometers based on knitted piezoresistive fabric (KPF) technology is presented.MethodsKPF single-and double-layer devices were designed and characterized under stretching and bending to work as strain sensors and goniometers. The theoretical working principle and the derived electromechanical model, previously proved for carbon elastomer sensors, were generalized to KPF. The devices were used to correlate angles and piezoresistive fabric behaviour, to highlight the differences in terms of performance between the single layer and the double layer sensors. A fast calibration procedure is also proposed.ResultsThe proposed device was tested both in static and dynamic conditions in comparison with standard electrogoniometers and inertial measurement units respectively. KPF goniometer capabilities in angle detection were experimentally proved and a discussion of the device measurement errors of is provided. The paper concludes with an analysis of sensor accuracy and hysteresis reduction in particular configurations.ConclusionsDouble layer KPF goniometers showed a promising performance in terms of angle measurements both in quasi-static and dynamic working mode for velocities typical of human movement. A further approach consisting of a combination of multiple sensors to increase accuracy via sensor fusion technique has been presented.

Effects of ion concentration on thermally-chargeable double-layer supercapacitors

The concept of thermally-chargeable supercapacitor was discussed and validated experimentally. As two double-layer supercapacitor-type devices were placed at different temperatures and connected, due to the thermal dependence of surface charge structures, the electrode potentials became different, and thermal energy could be harvested and stored as electric energy. The important effect of ion concentration was investigated. The results were quite different from the prediction of conventional surface theory, which should be attributed to the unique behaviors of the ions confined in the nanoporous electrodes.

Lithographically Patterned Thin Activated Carbon Films as a New Technology Platform for On-Chip Devices

Continuous, smooth, visibly defect-free, lithographically patterned activated carbon films (ACFs) are prepared on the surface of silicon wafers. Depending on the synthesis conditions, porous ACFs can either remain attached to the initial substrate or be separated and transferred to another dense or porous substrate of interest. Tuning the activation conditions allows one to change the surface area and porosity of the produced carbon films. Here we utilize the developed thin ACF technology to produce prototypes of functional electrical double-layer capacitor devices. The synthesized thin carbon film electrodes demonstrated very high capacitance in excess of 510 F g(-1) (>390 F cm(-3)) at a slow cyclic voltammetry scan rate of 1 mV s(-1) and in excess of 325 F g(-1) (>250 F cm(-3)) in charge-discharge tests at an ultrahigh current density of 45,000 mA g(-1). Good stability was demonstrated after 10,000 galvanostatic charge-discharge cycles. The high values of the specific and volumetric capacitances of the selected ACF electrodes as well as the capacity retention at high current densities demonstrated great potential of the proposed technology for the fabrication of various on-chip devices, such as micro-electrochemical capacitors.

Investigation of Charge Injection and Relaxation in Multilayer Dielectric Stacks for Capacitive RF MEMS Switch Application

This paper proposes a new approach to the problem of irreversible stiction of capacitive radio frequency (RF) microelectromechanical (MEMS) switch attributed to the dielectric charging. We investigate how charge accumulates in multi- and single-layer dielectric structures for a capacitive RF MEMS switch using metal-insulator-semiconductor (MIS) capacitor structure. Two multidielectric-layers are structured, which are SiO2+Si3N4 and SiO2+Si3N4+SiO2 stack films. Meanwhile, Si3N4 single-layer dielectric structure is also fabricated for comparison. In the experiments, the space charges are first injected into the dielectric layers by stressing MIS devices with a dc bias; then the injected charge kinetics are monitored by capacitance-voltage measurement before and after charge injection. We found that the polarity of charge accumulated in the dielectric is strongly influenced by the dielectric structure. When the metal electrode is positively biased, a negative charge accumulates in the single and triple-layer devices, while a positive charge accumulates in the double-layer devices. Furthermore, the experiment results also show that the lowest charge accumulation can be achieved using double-layer dielectric structure even though the fastest relaxation process takes place in triple-layer dielectric structure.

Highly efficient green phosphorescent organic light emitting diodes with simple structure

A series of simple structures is investigated for realization of the highly efficient green phosphorescent organic light emitting diodes with relatively low voltage operation. All the devices were fabricated with mixed host system by using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) which were known to be hole and electron type host materials due to their great hole and electron mobilities [μh(TAPC): 1×10−2cm2/Vs and μe(TpPyPB): 7.9×10−3cm2/Vs] [1]. The optimized device with thin TAPC (5–10nm) as an anode buffer layer showed relatively high current and power efficiency with low roll-off characteristic up to 10,000cd/m2. The performances of the devices; with buffer layer were compared to those of simple devices with single layer and three layers. Very interestingly, the double layer device with TAPC buffer layer showed better current and power efficiency behavior compared to that of three layer device with both hole and electron buffer layers (TAPC, TpPyPB, respectively).

Record Low-Power Organic RRAM With Sub-20-nA Reset Current

In this letter, organic resistive random access memory (RRAM) devices based on double-layer polychloro-para-xylylene (parylene-C) are fabricated, which show stable bipolar resistive switching behavior, excellent data retention, and high scalability. Moreover, extremely low reset current of sub-20 nA and set current of 0.15 μA are obtained with adequate switching margin for the first time in the field of organic RRAM, almost 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> times lower than that of the single-layer parylene-C cells, exhibiting great potentials for future low-power applications. Possible mechanism for the ultralow operating current of double-layer device is discussed.

PVK空穴传输层对Ndq3近红外发光二极管性能的影响

A three-layered near-infrared OLED with structure of ITO/PEDOT∶PSS/PVK/neodymium tris-(8-hydroxyquinoline)(Ndq3)/Al was fabricated,in which poly(N-vinylcarbazole)(PVK) was hole transport layer,and poly(3,4-ethylene dioxythiophene)∶poly(styrenesulfonate)(PEDOT∶PSS) was hole injection layer.The EL spectrum shows three emission peaks at 905,1 064,1 340 nm in the near-infrared range which are correspond to Nd3+ transitions of 4F3/2→4I9/2,4F3/2→4I11/2,4F3/2→4I13/2,respectively.The influence mechanism of functional layers was discussed with help of I-V curves,in reference with single layer device(ITO/Ndq3/Al) and double layer device(ITO/PVK/Ndq3/Al).The increase in device current corresponding to the reduction of the series resistance was observed and was related to the high conductivity of PEDOT∶PSS layer.The joint action on the hole injection barrier by PVK and PEDOT∶PSS was contributed to the improvement of the charge carriers balance of device and EL emission intensity.Also,the decrease of the ITO surface roughness was realized as one of the key issue for the performance improvement.

Highly efficient blue phosphorescent and electroluminescent Ir(iii) compounds

Three new Ir(III) compounds with deep-blue phosphorescence have been synthesized. These molecules have the general formula of Ir(C∧N)2(L∧X), where C∧N = 2′,6′-difluoro-2,3′-bipyridine (dfpypy) and L∧X = ancillary ligand such as 2-picolinate, pic (1), acetylacetonate, acac (2), or dipivaloylmethanoate, dpm (3). The ancillary ligands have been found to significantly destabilize both HOMO and LUMO levels of the Ir(III) complexes, compared to Ir(dfpypy)3, without significantly changing the phosphorescence energy. Compounds 1–3 emit bright blue phosphorescence with λmax = 440–460 nm and quantum efficiencies of 0.60–0.95 in solution and the solid state. Double-layer electroluminescent devices using compounds 1–3 as the dopant, CDBP (4,4′-bis(9-carbazolyl)-2,2′-dimethylbiphenyl) as the host/hole transporting layer, and TPBi (1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene) as the electron transport layer have been fabricated. These EL devices show pure blue colour with high efficiency. The EL device of compound 3 at the doping level of 20 wt% shows the best performance with EQE of 10–15% at the brightness of 10–1000 cd m−2 and the maximum current efficiency of 22 cd A−1.

Efficient monochromatic red-light-emitting PLEDs based on a series of nonconjugated Eu-polymers containing a neutral terpyridyl ligand

Three novel Eu-polymers containing hole transporting units and light-emitting units in the main chain have been designed and synthesized via radical copolymerization of N-vinylcarbazole and polymerizable Eu(TTA)3vinyl-tpy, where TTA is 2-thenoyltrifluoroacetonate. The chemical structure and composition of the Eu-polymers were characterized by FTIR, UV-vis, 1H NMR, 13C NMR spectroscopy, GPC, ESI-MS, and elemental analysis. The geometry of the polymerizable monomer Eu(TTA)3vinyl-tpy was predicted using the Sparkle/PM6 model and suggested to be in a chemical environment with very low symmetry around the Eu3+ ions (C1), in agreement with the fluorescence spectrum. All Eu-polymers exhibited good solubility, as well as good thermal stability and high glass transition temperatures. The photoluminescence (PL) properties of the copolymers in solution and in the solid state were investigated in detail. Intramolecular energy transfer from the carbazole groups to the europium complexes occurred even in diluted solutions. The efficiency of this process also depended on the composition of the Eu-polymers. In the solid state, emission from the carbazole groups was suppressed and the absorbed excitation energy was transferred effectively to the europium complexes in the Eu-polymers. Most importantly, the EL performances of eight pure red-emission PLEDs based on P1, P2, and P3 as the emitting layer have been studied in detail. Bright electroluminescence with a maximum luminance of 68.2 cd m−2 from the double-layer devices of P1 was demonstrated. Although the EL performance was only the third-best among those of the Eu-chelated polymers reported so far, to the best of our knowledge, this is the first example of electroluminescent devices of Eu-polymers based on tpy as a neutral ligand. These results illustrate the potential application of polymerizable tpy ligands in high performance EL Eu-chelated polymers.

Conjugated polymers containing trifluoren-2-ylamine, trifluoren-2-ylbenzene and trifluoren-2-yltriazine for electroluminescence

A series of light-emitting conjugated polymers (LEPs) based on building blocks of electron-donating trifluoren-2-ylamine (TFA), electrically neutral 1,3,5-trifluoren-2-ylbenzene (TFB), and electron-withdrawing 2,4,6-trifluoren-2-yltriazine (TFT) were successfully synthesized via palladium-catalyzed Suzuki cross-coupling polycondensation. Their structure–property relationships were thoroughly studied. For P1 containing electron-withdrawing backbone and electron-donating pendants, their photophysical properties in solution are strongly dependent on the solvent polarity due to the intense intramolecular charge-transfer (ICT) interaction. In addition, their emission colors and energy levels could be effectively tuned by changing the structure of the main chain as well as the substituent at the pendent fluorene of TFT, TFB, and TFA. To evaluate their electroluminescence properties, double-layer devices with a configuration of ITO/PEDOT:PSS (40 nm)/emitting layer (EML) (70–80 nm)/TPBI (30 nm)/CsF (1.5 nm)/Al were fabricated, where the developed polymers were used as an EML and TPBI (2,2′,2″-(1,3,5-benzenetriyl)-tris-(1-phenyl-1H-benzimidazole)) was used as an electron-transport and hole-block layer. Deep-blue light emission was achieved for the device based on P1d, a polymer based on TFT and the second generation (G2) of carbazole dendrimer pendant, that shows a maximum current efficiency (CE) of 1.26 cd A−1, corresponding to external quantum efficiency (EQE) of 1.27%, with Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.14). In comparison, due to the intense ICT interaction between the main chain and the side chain, light-green emission was achieved for the device based on P1c, a polymer based on TFT and bis(9,9-dioctyl-9H-fluoren-2-yl)amine pendant, giving the highest maximum CE of 4.10 cd A−1, corresponding to EQE of 2.45%, with CIE coordinates of (0.24,0.51), although either of the main chain and the side chain emits blue light.

Plasmons in layered structures including graphene

We investigate the optical properties of layered structures with graphene at the interface for arbitrary linear polarization at finite temperature including full retardation by working in the Weyl gauge. As a special case, we obtain the full response and the related dielectric function of a layered structure with two interfaces. We apply our results to discuss the longitudinal plasmon spectrum of several single- and double-layer devices such as systems with finite and zero electronic densities. We further show that a nonhomogeneous dielectric background can shift the relative weight of the in-phase and out-of-phase modes, and discuss how the plasmonic mode of the upper layer can be tuned into an acoustic mode with a specific sound velocity.

Infrared conductivity of hole accumulation and depletion layers in (Ga,Mn)As- and (Ga,Be)As-based electric field-effect devices

We have fabricated electric double-layer field-effect devices to electrostatically dope our active materials, either $x$=0.015 Ga$_{1-x}$Mn$_x$As or $x$=3.2$\times10^{-4}$ Ga$_{1-x}$Be$_x$As. The devices are tailored for interrogation of electric field induced changes to the frequency dependent conductivity in the accumulation or depletions layers of the active material via infrared (IR) spectroscopy. The spectra of the (Ga,Be)As-based device reveal electric field induced changes to the IR conductivity consistent with an enhancement or reduction of the Drude response in the accumulation and depletion polarities, respectively. The spectroscopic features of this device are all indicative of metallic conduction within the GaAs host valence band (VB). For the (Ga,Mn)As-based device, the spectra show enhancement of the far-IR itinerant carrier response and broad mid-IR resonance upon hole accumulation, with a decrease of these features in the depletion polarity. These later spectral features demonstrate that conduction in ferromagnetic (FM) Ga$_{1-x}$Mn$_x$As is distinct from genuine metallic behavior due to extended states in the host VB. Furthermore, these data support the notion that a Mn-induced impurity band plays a vital role in the electron dynamics of FM Ga$_{1-x}$Mn$_x$As. We add, a sum-rule analysis of the spectra of our devices suggests that the Mn or Be doping does not lead to a substantial renormalization of the GaAs host VB.

Accurate analysis of conduction and resistive-switching mechanisms in double-layered resistive-switching memory devices

Resistive-switching and current conduction mechanisms have been studied in TiN/Ti/TiOx/HfOx/TiN resistive-switching random access memories (RRAMs). From I-V characteristics and temperature measurement, thermionic emission is found to be the most appropriate mechanism representing the dominant current conduction in all the bias regions and resistance states. Low-frequency noise power spectrum is measured to analyze accurately the conduction mechanism, which corroborates the thermionic-emission. Also, using the migration of oxygen ions depending on the polarity of the applied field, we propose the resistive-switching model of a double-layered RRAM to explain the unique resistive-switching characteristics.