Resistance Controllability in Alkynylgold(III) Complex‐Based Resistive Memory for Flash‐Type Storage Applications

Owing to the demands of state‐of‐the‐art information technologies that are suitable for vast data storage, the necessity for organic memory device (OMD) materials is highlighted. However, OMDs based on metal complexes are limited to several types of transition‐metal complex systems containing nitrogen‐donor ligands. Herein, attempts are made to introduce novel alkynylgold(III) materials into memory devices with superior performance. In this respect, an alkynyl‐containing coumarin gold(III) complex, [(C₁₉N₅H₁₁)Au−C≡C−C₉H₅O], has been synthesized and integrated into a sandwiched Al/[(C₁₉N₅H₁₁)Au−C≡C−C₉H₅O]/indium tin oxide device. By precisely controlling the compliance current (I _cc), the devices show different switching characteristics from flash‐type binary resistance switching (I _cc≤10⁻³ A) to WORM‐type (WORM=write once read many times) ternary resistance switching (I _cc=10⁻² A). This work explores electrical gold(III) complex based memories for potential use in organic electronics.     

Impact of Top Electrodes (Cu,Ag, and Al) on Resistive Switching behaviour of Cu-rich Cu2ZnSnS4 (CZTS) Ideal Kesterite

Cu₂ZnSnS₄ (CZTS) active material-based resistive random-access memory (RRAM) devices are investigated to understand the impact of three different Cu, Ag, and Al top electrodes. The dual resistance switching (RS) behaviour of spin coated CZTS on ITO/Glass is investigated up to 10² cycles. The stability of all the devices (Cu/CZTS/ITO, Ag/CZTS/ITO, and Al/CZTS/ITO) is investigated up to 10³ sec in low- (LRS) and high- (HRS) resistance states at 0.2 V read voltage. The endurance up to 10² cycles with 30 msec switching width shows stable write and erase current. Weibull cumulative distribution plots suggest that Ag top electrode is relatively more stable for set and reset state with 33.61 and 25.02 shape factors, respectively. The charge carrier transportation is explained by double logarithmic plots, Schottky emission plots, and band diagrams, substantiating that at lower applied electric field intrinsic copper ions dominate in Cu/CZTS/ITO, whereas, at higher electric filed, top electrodes (Cu and Ag) dominate over intrinsic copper ions. Intrinsic Cu⁺ in CZTS plays a decisive role in resistive switching with Al electrode. Further, the impedance spectroscopy measurements suggest that Cu⁺ and Ag⁺ diffusion is the main source for the resistive switching with Cu and Ag electrodes.     

A Solution‐Processable Donor–Acceptor Compound Containing Boron(III) Centers for Small‐Molecule‐Based High‐Performance Ternary Electronic Memory Devices

A novel small‐molecule boron(III)‐containing donor–acceptor compound has been synthesized and employed in the fabrication of solution‐processable electronic resistive memory devices. High ternary memory performances with low turn‐on (V_Th1=2.0 V) and distinct threshold voltages (V_Th2=3.3 V), small reading bias (1.0 V), and long retention time (>10⁴ seconds) with a large ON/OFF ratio of each state (current ratio of “OFF”, “ON1”, and “ON2”=1:10³:10⁶) have been demonstrated, suggestive of its potential application in high‐density data storage. The present design strategy provides new insight in the future design of memory devices with multi‐level transition states.     

Highly Emissive Fused Heterocyclic Alkynylgold(III) Complexes for Multiple Color Emission Spanning from Green to Red for Solution‐Processable Organic Light‐Emitting Devices

A new class of fused heterocyclic tridentate ligand‐containing alkynylgold(III) complexes with tunable emission color has been successfully designed and synthesized. Structural modification of the σ‐donating fused heterocyclic alkynyl ligands, including substituted fluorene, carbazole, and triphenylamine, enables a large spectral shift of about 110 nm (ca. 3310 cm^?1) that covers the green to red region to be realized with the same tridentate ligand‐containing alkynylgold(III) complexes in solid‐state thin films. Interestingly, the energy of the excimeric emission can be controlled by the rational design of the fused heterocyclic alkynyl ligands. Superior solution‐processable organic light‐emitting devices (OLEDs) with high external quantum efficiencies (EQEs) of 12.2, 13.5, 9.3, and 5.2 % were obtained with green, yellow, orange, and red emission. These high EQE values are comparable to those of the vacuum‐deposited OLEDs based on structurally related alkynylgold(III) complexes.     

Adjusting the Proportion of Electron‐Withdrawing Groups in a Graft Functional Polymer for Multilevel Memory Performance

A polymer containing aldehyde active groups (PVB) was synthesized by atom transfer radical polymerization (ATRP), acting as a polymer precursor to graft a functional moiety via nucleophilic addition reaction. DHI (2‐(1,5‐dimethyl‐hexyl)‐6‐hydrazino‐benzo[de]isoquinoline‐1,3‐dione) and NPH (nitrophenyl hydrazine) groups, which contain naphthalimides that act as narrow traps and nitro groups that act as deep traps, were anchored onto the PVB at different ratios. A series of graft polymers were obtained and named PVB‐DHI, PVB‐DHI₄‐NPH, PVB‐DHI‐NPH₄, and PVB‐NPH. The chemical composition of the polymers was analyzed by ¹H‐NMR spectroscopy and X‐ray photoelectron spectroscopy (XPS). Memory devices were prepared from the polymers, and I–V characteristics were measured to determine the performance. By adjusting the ratio of different electron acceptors (DHI and NPH) to 4:1, ternary memory behavior was achieved. The relationship between memory behavior of PVB‐DHI_xNPH_y and acceptor groups as well as their conduction mechanism were studied in detail.     

Performance Improvement of Organic Light Emitting Diodes Using Poly(N-vinylcarbazole) (PVK) as a Blocking Layer

High efficiency organic light‐emitting‐devices (OLED) have been fabricated by incorporation of a polymeric layer as a controller of the unbalanced charge. In device configuration of ITO/PEDOT:PSS/PVK/Alq₃/LiF:Al, poly(N‐vinylcarbazole) (PVK) was selected as a block‐ing layer (BL) because it has a hole transporting property and a higher band gap, especially a lower LUMO level than the emitting layer (Alq₃) and a higher HOMO level than the hole injection layer (PEDOT: PSS). As a result, the optimal structure with this bl layer showed a peak efficiency of 6.89 cd/A and 2.30 lm/W compared to the device without the PVK layer of 1.08 cd/A, 0.27 lm/W. This result shows that the PVK layer could effec‐tively block the electrons from metal cathode and confine them in the emitting layer and accomplish the charge balance, which leads to enhanced hole‐electron balance for achieving high recombination efficiency.     

Boron(III)‐Containing Donor–Acceptor Compound with Goldlike Reflective Behavior for Organic Resistive Memory Devices

A small‐molecule‐based boron(III)‐containing donor–acceptor compound has been designed and synthesized. Interesting goldlike reflective behavior was observed in the neat thin‐film sample from simple spin‐coating preparation, which can serve as a potential organic thin‐film optical reflector. The small thickness in nanometer range and the relatively smooth surface morphology, together with simple preparation and easy solution processability, are attractive features for opening up new avenues for the fabrication of reflective coatings. Moreover, this donor–acceptor compound has been employed in the fabrication of organic resistive memory device, which exhibited good performance with low turn‐on voltage, small operating bias, large ON/OFF ratio, and long retention time.     

Thienopyrazine‐Based Low‐Bandgap Poly(heteroaryleneethynylene)s for Photovoltaic Devices

Summary: Low‐bandgap π‐conjugated polymers that consist of alkyl thiophene/alkoxy phenylene and 2,3‐diphenylthieno[3,4‐b]pyrazine units have been prepared in high yields by a Sonogashira polycondensation. The copolymers are characterized by NMR, IR, UV, GPC, and elemental analysis. Thin films of the polymers P1 , P2 , and P3 exhibit an optical bandgap of ≈1.57–1.60 eV. Under simulated AM 1.5 conditions P2/PCBM devices on polyester foil provide a short circuit current of I_SC = 10.72 mA · cm⁻², an open circuit voltage of V_oc = 0.67 V, and a power conversion efficiency of 2.37%.

Schematic of the photovoltaic device made from the polymers synthesized here.     

聚乙烯醇基聚合物材料在多元驱动方式下的形状记忆行为

形状记忆聚合物（SMPs）是近些年发展起来的一种环境响应型智能材料。在外界刺激驱动下分子内或分子间会发生物化变化,分子结构和形态的改变使形变后的材料在宏观上回复到起始形态。常见的SMPs有聚乙烯、聚氨酯、聚己内酯等,而聚乙烯醇（PVA）的形状记忆效应是在热致型形状记忆凝胶被发现以来才引起人们关注的。由于PVA侧链富含大量羟基,化学活性高、易与官能团进行功能化改性,因此可设计出满足不同驱动方式的分子结构。目前研究者已采用冻融循环、化学或辐射交联、接枝改性及共混复合等多种方法制备了多种刺激源（如温度,溶剂、光、电、微波及超声波等）驱动下的形状记忆聚乙烯醇（SM-PVA）、PVA衍生物及复合材料。本文综述了近年来不同刺激源驱动下SM-PVA的研究进展,阐述了不同材料的结构性能、回复机理及存在的问题,并展望了PVA在该领域的发展和应用前景。     

The Mechanisms of Rectification in Au|Molecule|Au Devices Based on Langmuir–Blodgett Monolayers of Iron(III) and Copper(II) Surfactants

Langmuir–Blodgett films of metallosurfactants were used in Au|molecule|Au devices to investigate the mechanisms of current rectification.     

Synthesis,Electrochemistry, and Photophysical Studies of Ruthenium(II) Polypyridine Complexes with D–π–A–π–D Type Ligands and Their Application Studies as Organic Memories

A new class of ruthenium(II) polypyridine complexes with a series of D–π–A–π–D type (D=donor, A=acceptor) ligands was synthesized and characterized by ¹H NMR spectroscopy, mass spectrometry, and elemental analysis. The photophysical and electrochemical properties of the complexes were also investigated. The newly synthesized ruthenium(II) polypyridine complexes were found to exhibit two intense absorption bands at both high‐energy (λ=333–369 nm) and low‐energy (λ=520–535 nm) regions. They are assigned as intraligand (IL) π→π* transitions of the bipyridine (bpy) and π‐conjugated bpy ligands, and IL charge‐transfer (CT) transitions from the donor to the acceptor moiety with mixing of dπ(Ru^II)→π*(bpy) and dπ(Ru^II)→π*(L) MLCT characters, respectively. In addition, all complexes were demonstrated to exhibit intense red emissions at approximately λ=727–744 nm in degassed dichloromethane at 298 K or in n‐butyronitrile glass at 77 K. Nanosecond transient absorption (TA) spectroscopy has also been carried out, establishing the presence of the charge‐separated state. In order to understand the electrochemical properties of the complexes, cyclic voltammetry has also been performed. Two quasi‐reversible oxidation couples and three quasi‐reversible reduction couples were observed. One of the ruthenium(II) complexes has been utilized in the fabrication of memory devices, in which an ON/OFF current ratio of over 10⁴ was obtained.     

Rational Design of Poly(2,7‐Carbazole) Derivatives for Photovoltaic Applications

A reliable model that can be used to estimate the electronic properties (i.e., the HOMO, LUMO, and band gap energies) of conjugated polymers would be a great tool for applications in organic electronics such as light‐emitting diodes, field‐effect transistors, and photovoltaic cells. Recently, poly(2,7‐carbazole) derivatives have shown promising results when used as an active donor layer in bulk heterojunction photovoltaic cells with power conversion efficiency exceeding 7%. By using a simple correlation between density functional theory (DFT) theoretical calculations performed on six model compounds (using the repeating unit) and experimental data from the six corresponding polymers, an accurate estimation of the HOMO energy level, the LUMO energy levels, and the band gap of several poly(2,7‐carbazole) derivatives was obtained. According to the theoretical data obtained for more than one hundred repeating units, fourteen new copolymers that can be used as p‐type materials in bulk heterojunction solar cells were selected and synthesized. Experimental data obtained from these materials were then used to refine the correlation between DFT and experimental data of poly(2,7‐carbazole) derivatives.

     

Conjugated Donor‐Acceptor‐Acceptor (D‐A‐A) Molecule for Organic Nonvolatile Resistor Memory

A new donor‐acceptor‐acceptor (D‐A‐A) type of conjugated molecule, N‐(4‐(N′,N′‐diphenyl)phenylamine)‐4‐(4′‐(2,2‐dicyanovinyl)phenyl) naphthalene‐1,8‐dicarboxylic monoimide ( TPA‐NI‐DCN ), consisting of triphenylamine (TPA) donors and naphthalimide (NI)/dicyanovinylene (DCN) acceptors was synthesized and characterized. In conjunction with previously reported D ‐A based materials, the additional DCN moiety attached as end group in the D‐A‐A configuration can result in a stable charge transfer (CT) and charge‐separated state to maintain the ON state current. The vacuum‐deposited TPA‐NI‐DCN device fabricated as an active memory layer was demonstrated to exhibit write‐once‐read‐many (WORM) switching characteristics of organic nonvolatile memory due to the strong polarity of the TPA‐NI‐DCN moiety.     

Improving Memory Performances by Adjusting the Symmetry and Polarity of O‐Fluoroazobenzene‐Based Molecules

Three O‐fluoroazobenzene‐based molecules were chosen as memory‐active molecules: FAZO‐1 with a D–A2–D symmetric structure, FAZO‐2 with an A1–A2–A1 symmetric structure, and FAZO‐3 with a D–A2–A1 asymmetric structure. Both FAZO‐1 and FAZO‐2 had a lower molecular polarity, whereas FAZO‐3 had a higher polarity. The fabricated indium–tin oxide (ITO)/ FAZO‐1 /Al (Au) and ITO/ FAZO‐2 /Al (Au) memory devices both exhibited volatile static random access memory (SRAM) behavior, whereas the ITO/ FAZO‐3 /Al (Au) device showed nonvolatile ternary write‐once‐read‐many‐times (WORM) behavior. It should be noted that the reproducibility of these devices was considerably high, which is significant for practical application in memory devices. In addition, the different memory performances of the three active materials were determined to be attributable to the stability of electric‐field‐induced charge‐transfer complexes. Therefore, the switching memory behavior could be tuned by adjusting the molecular polarity.     

Impact of Charge Transfer Complex on the Dielectric Relaxation Processes in Poly(methyl methacrylate) Polymer

The impact of the charge transfer complex on the dielectric relaxation processes in free poly(methyl methacrylate) (PMMA) polymer sheets was investigated. The frequency dependence of dielectric properties was obtained over the frequency range 0.1 Hz–1 MHz at temperatures ranging between 303 K and 373 K for perylene dye and acceptors (picric acid (PA) and chloranilic acid (CLA)) in an in situ PMMA polymer. The TG/dTG technique was used to investigate the thermal degradation of the synthesized polymeric sheets. Additionally, the kinetic parameters have been assessed using the Coats–Redfern relation. The dielectric relaxation spectroscopy of the synthesized polymeric sheets was analyzed in terms of complex dielectric constant, dielectric loss, electrical modulus, electrical conductivity, and Cole–Cole impedance spectroscopy. α- and β-relaxation processes were detected and discussed. The σ(ω) dispersion curves of the synthesized polymeric sheets show two distinct regions with increasing frequency. The impedance data of the synthesized polymeric sheets can be represented by the equivalent circuit (parallel RC).     

EDOT‐diketopyrrolopyrrole copolymers for high bulk hole mobility and near infrared absorption

To obtain novel low‐bandgap materials with tailored hole‐transport properties and extended absorption, electron rich 3,4‐ethylenedioxythiophene is introduced as a comonomer in diketopyrrolo[3,4‐c]pyrrole copolymers with different aryl flanking units. The polymers are characterized by absorption and photoluminescence spectroscopy, dynamic scanning calorimetry, cyclic voltammetry, and X‐ray diffraction. The charge transport properties of these new materials are studied carefully using an organic field effect transistor geometry where the charge carriers are transported over a narrow channel at the semiconductor/dielectric interface. These results are compared to bulk charge carrier mobilities using space‐charge limited current (SCLC) measurements, in which the charge carrier is transported through the complete film thickness of several hundred nanometers. Finally, charge carrier mobilities are correlated with the electronic structure of the compounds. We find that in particular the thiophene‐flanked copolymer PDPP[T]₂‐EDOT is a very promising candidate for organic photovoltaics, showing an absorption response in the near infrared region with an optical bandgap of 1.15 eV and a very high bulk hole mobility of 2.9 × 10^?4 cm² V^?1 s^?1 as measured by SCLC. This value is two orders of magnitudes higher than SCLC mobilities reported for other polydiketopyrrolopyrroles and is in the range of the well‐known hole transporting polymer poly(3‐hexylthiophene). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 639–648     

Judicious Choice of N-Heterocycles for the Realization of Sky-Blue- to Green-Emitting Carbazolylgold(III) C^C^N Complexes and Their Applications for Organic Light-Emitting Devices

A new class of sky-blue- to green-emitting carbazolylgold(III) C^C^N complexes containing pyrazole or benzimidazole moieties has been successfully designed and synthesized. Through the judicious choice of the N-heterocycles in the cyclometalating ligand and the tailor-made carbazole moieties, maximum photoluminescence quantum yields of 0.52 and 0.39 have been realized in the green- and sky-blue-emitting complexes, respectively. Solution-processed and vacuum-deposited organic light-emitting devices (OLEDs) based on the benzimidazole-containing complexes have been prepared. The sky-blue-emitting device shows an emission peaking at 484 nm with a narrow full-width at half-maximum of 57 nm (2244 cm⁻¹), demonstrating the potential of this class of complexes in the application of OLEDs with high color purity. In addition, high maximum external quantum efficiencies of 12.3 % and a long operational half-lifetime of over 5300 h at 100 cd m⁻² have been achieved in the vacuum-deposited green-emitting devices.     

Graphene Enhances the Shape Memory of Poly (acrylamide‐co‐acrylic acid) Grafted on Graphene

Acrylamide and acrylic acid are grafted on graphene by free‐radical polymerization to produce a series of graphene–poly(acrylamide‐co‐acrylic acid) hybrid materials with different contents of graphene. The materials demonstrate shape memory effect and self‐healing ability when the content of graphene is in the range of 10%–30% even though poly(acrylamide‐co‐acrylic acid) itself had poor shape memory ability. The permanent shape of the materials can be recovered well after 20 cycles of cut and self‐healing. The result is attributed to the hard–soft design that can combine nonreversible “cross‐link” by grafting copolymer on graphene and reversible “cross‐link” utilizing the “zipper effect” of poly(acrylamide‐co‐acrylic acid) to form or dissociate the hydrogen‐bond network stimulated by external heating.

     

DpgC-Catalyzed Peroxidation of 3,5-Dihydroxyphenylacetyl-CoA (DPA-CoA): Insights into the Spin-Forbidden Transition and Charge Transfer Mechanisms**

Despite being a very strong oxidizing agent, most organic molecules are not oxidized in the presence of O₂ at room temperature because O₂ is a diradical whereas most organic molecules are closed-shell. Oxidation then requires a change in the spin state of the system, which is forbidden according to non-relativistic quantum theory. To overcome this limitation, oxygenases usually rely on metal or redox cofactors to catalyze the incorporation of, at least, one oxygen atom into an organic substrate. However, some oxygenases do not require any cofactor, and the detailed mechanism followed by these enzymes remains elusive. To fill this gap, here the mechanism for the enzymatic cofactor-independent oxidation of 3,5-dihydroxyphenylacetyl-CoA (DPA-CoA) is studied by combining multireference calculations on a model system with QM/MM calculations. Our results reveal that intersystem crossing takes place without requiring the previous protonation of molecular oxygen. The characterization of the electronic states reveals that electron transfer is concomitant with the triplet–singlet transition. The enzyme plays a passive role in promoting the intersystem crossing, although spontaneous reorganization of the water wire connecting the active site with the bulk presets the substrate for subsequent chemical transformations. The results show that the stabilization of the singlet radical-pair between dioxygen and enolate is enough to promote spin-forbidden reaction without the need for neither metal cofactors nor basic residues in the active site.     

Judicious Choice of N‐Heterocycles for the Realization of Sky‐Blue‐ to Green‐Emitting Carbazolylgold(III) C^C^N Complexes and Their Applications for Organic Light‐Emitting Devices

A new class of sky‐blue‐ to green‐emitting carbazolylgold(III) C^C^N complexes containing pyrazole or benzimidazole moieties has been successfully designed and synthesized. Through the judicious choice of the N‐heterocycles in the cyclometalating ligand and the tailor‐made carbazole moieties, maximum photoluminescence quantum yields of 0.52 and 0.39 have been realized in the green‐ and sky‐blue‐emitting complexes, respectively. Solution‐processed and vacuum‐deposited organic light‐emitting devices (OLEDs) based on the benzimidazole‐containing complexes have been prepared. The sky‐blue‐emitting device shows an emission peaking at 484 nm with a narrow full‐width at half‐maximum of 57 nm (2244 cm^?1), demonstrating the potential of this class of complexes in the application of OLEDs with high color purity. In addition, high maximum external quantum efficiencies of 12.3 % and a long operational half‐lifetime of over 5300 h at 100 cd m^?2 have been achieved in the vacuum‐deposited green‐emitting devices.