Control over Memory Performance of Layer-by-Layer Assembled Metal Phthalocyanine Multilayers via Molecular-Level Manipulation

We herein report on the nonvolatile memory properties of iron phthalocyanine multilayers prepared using an electrostatic layer-by-layer assembly method. Cationic poly­(allylamine hydrochloride) (PAH) and anionic iron­(III) phthalocyanine-4, 4,′ 4″, 4′″-tetrasulfonic acid (Fe-TsPc) were alternately deposited onto quartz glass, indium tin oxide (ITO), or platinum-coated silicon substrates via electrostatic interactions. The electrochemical response of the PAH/Fe-TsPc, which was obtained from cyclic voltammograms (CV) in solution, indicated that redox reactions occurred at the phthalocyanine unit and at the metallic center. It was found that these redox reactions of the PAH/Fe-TsPc multilayer films in solution could be extended to resistive switching nonvolatile memory based on a charge trap/release mechanism in air. The PAH/Fe-TsPc multilayers sandwiched between the bottom (platinum) and top (Ag or tungsten) electrodes exhibited the characteristics of a resistive switching memory at a relatively low operating voltage of less than 2 V, with a switching speed of about 100 ns and an ON/OFF current ratio of ∼10³. Additionally, it is confirmed using kelvin probe force microscopy (KPFM) that the reversible resistance changes in the PAH/Fe-TsPc multilayers are mainly caused by the externally applied voltage as a result of the trapping and release of charges at redox sites within the Fe-TsPc. Furthermore, in the case where insulating layers of about 2 nm in thickness are inserted between adjacent Fe-TsPc layers, it is demonstrated that these devices can exhibit further improvements in memory performance (ON/OFF current ratio of ∼10⁶) and a lower power consumption in comparison with PAH/Fe-TsPc multilayers

Multifunctional Colloids with Reversible Phase Transfer between Organic and Aqueous Media via Layer-by-Layer Assembly

We report the successful multifunctional colloids that enable reversible phase transfer between organic and aqueous phases via layer-by-layer (LbL) assembly. These colloids exhibited a high level of dispersion stability in a variety of solvents ranging from nonpolar to aqueous media, based on the type of outermost layer adsorbed onto the colloids. Hydrophobic nanoparticles (NPs) synthesized using carboxylic acid or ammonium moiety-based ligands (i.e., oleic acid or tetraoctylammonium) in a nonpolar solvent (toluene, hexane, or chloroform) were directly deposited onto dendrimer-coated SiO₂ colloids via ligand exchange between the hydrophobic ligands and the amine-functionalized dendrimers in the same organic solvent. Additionally, these hydrophobic NPs were adsorbed onto the colloids forming the densely packed layer structure that could not be easily achieved by conventional electrostatic LbL assembly. The subsequent adsorption of amine-functionalized dendrimers onto hydrophobic NP-coated colloids led to well-dispersed colloids in aqueous media as well as alcohol solvent and possibly induced the deposition of electrostatic LbL-assembled films, such as (cationic Au_NP/anionic polyelectrolyte (PE))_<i>n</i> or (cationic PE/anionic enzyme)_<i>n</i> multilayers. Furthermore, the additional deposition of ligand exchange-induced multilayers (i.e., (dendrimer/hydrophobic NP)_<i>n</i>) onto electrostatic multilayer-coated colloids produced colloids with highly dispersible properties in organic media. Given that previous approaches to the reversible phase transfer of colloids have depended on the physicochemical properties of selective ligands under limited and specific conditions, our approach may provide a basis for the design and exploitation of high-performance colloids with tailored functionality in a variety of solvents

Solvent-Free Nanocomposite Colloidal Fluids with Highly Integrated and Tailored Functionalities: Rheological, Ionic Conduction, and Magneto-Optical Properties

We introduce a unique and facile strategy for the preparation of solvent-free nanocomposite colloidal fluids that allows accurate control over the integration of functionalities as well as the composition and dimensions of the nanocomposite structure. For the preparation of colloidal fluids with highly integrated functionalities, oleic acid (OA)-stabilized magnetic nanoparticles (i.e., OA-Fe₃O₄ NPs) and CdSe@ZnS quantum dots (QDs) were first synthesized in nonpolar solvent. In this case, OA-QDs dispersed in toluene were successively phase transferred to thiol-functionalized imidazolium-type ionic liquid (IL-SH) media with rheological and ionic conduction properties. After the functional NPs were synthesized, amine-functionalized dendrimers and OA-Fe₃O₄ NPs were alternately deposited onto silica colloids (i.e., SiO₂/(dendrimer/OA-Fe₃O₄)<i>_n</i>) using a ligand-exchange-induced LbL-assembly in organic media. Electrostatic LbL-assembled (anionic polyelectrolyte (PE)/cationic IL-SH-QD)<i>_n</i> multilayers were then sequentially adsorbed onto the outermost dendrimer layer of the magnetic colloids. The resulting functional colloidal fluids were devoid of colloidal aggregation and exhibited strong superparamagnetic, fluorescent, rheological, and ionic conduction properties at room temperature. Furthermore, mixtures of photoluminescent colloidal fluids with and without OA-Fe₃O₄ NPs behaved effectively as magneto-optically separable colloidal fluids. Because a variety of inorganic NPs ranging from metal to transition-metal oxides can be easily incorporated into colloidal substrates via LbL-assembly, our approach provides a basis for exploiting and designing functional colloidal fluids with liquidlike behavior at room temperature

Electrically Bistable Properties of Layer-by-Layer Assembled Multilayers Based on Protein Nanoparticles

Electrochemical properties of redox proteins, which can cause the reversible changes in the resistance according to their redox reactions in solution, are of the fundamental and practical importance in bioelectrochemical applications. These redox properties often depend on the chemical activity of transition metal ions as cofactors within the active sites of proteins. Here, we demonstrate for the first time that the reversible resistance changes in dried protein films based on ferritin nanoparticles can be caused by the externally applied voltage as a result of charge trap/release of Fe^III/Fe^II redox couples. We also show that one ferritin nanoparticle of about 12 nm size can be operated as a nanoscale-memory device, and furthermore the layer-by-layer assembled protein multilayer devices can be extended to bioinspired electronics with adjustable memory performance <i>via</i> molecular level manipulation

Hydrophobic Nanoparticle-Based Nanocomposite Films Using <i>In Situ</i> Ligand Exchange Layer-by-Layer Assembly and Their Nonvolatile Memory Applications

A robust method for preparing nanocomposite multilayers was developed to facilitate the assembly of well-defined hydrophobic nanoparticles (<i>i.e.</i>, metal and transition metal oxide NPs) with a wide range of functionalities. The resulting multilayers were stable in both organic and aqueous media and were characterized by a high NP packing density. For example, inorganic NPs (including Ag, Au, Pd, Fe₃O₄, MnO₂, BaTiO₃) dispersed in organic media were shown to undergo layer-by-layer assembly with amine-functionalized polymers to form nanocomposite multilayers while incurring minimal physical and chemical degradation of the inorganic NPs. In addition, the nanocomposite multilayer films formed onto flat and colloidal substrates could directly induce the adsorption of the electrostatically charged layers without the need for additional surface treatments. This approach is applicable to the preparation of electronic film devices, such as nonvolatile memory devices requiring a high memory performance (ON/OFF current ratio >10³ and good memory stability)

Acetylene-containing highly birefringent rod-type reactive liquid crystals based on 2-methylhydroquinone

<p>New highly birefringent reactive liquid crystal materials based on the 2-methylhydroquinone core were designed and synthesised. Rod-type liquid crystal compounds bearing photo-crosslinkable reactive group of acryloyl, methacryloyl, cinnamoyl, furylacryloyl group were synthesised by introducing acetylene groups via Sonogashira coupling to obtain high birefringence, and lateral groups such as fluoro and methyl to adjust the temperature of the liquid crystal phase. The synthesised compounds were characterised using nuclear magnetic resonance spectroscopy, mass spectrometry and elemental analysis. In addition, their thermal behaviour was investigated using differential scanning calorimetry and polarised optical microscopy. After aligning the synthesised compounds, liquid crystal films were prepared by photo-irradiation. Photo-elastic modulator results showed that the obtained liquid crystal films had high birefringence (Δ<i>n</i>) values of 0.32–0.40.</p

Water-Based Thixotropic Polymer Gel Electrolyte for Dye-Sensitized Solar Cells

For the practical application of dye-sensitized solar cells (DSSCs), it is important to replace the conventional organic solvents based electrolyte with environmentally friendly and stable ones, due to the toxicity and leakage problems. Here we report a noble water-based thixotropic polymer gel electrolyte containing xanthan gum, which satisfies both the environmentally friendliness and stability against leakage and water intrusion. For application in DSSCs, it was possible to infiltrate the prepared electrolyte into the mesoporous TiO₂ electrode at the fluidic state, resulting in sufficient penetration. As a result, this electrolyte exhibited similar conversion efficiency (4.78% at 100 mW cm^–2) and an enhanced long-term stability compared to a water-based liquid electrolyte. The effects of water on the photovoltaic properties were examined elaborately from the cyclic voltammetry curves and impedance spectra. Despite the positive shift in the conduction band potential of the TiO₂ electrode, the open-circuit voltage was enhanced by addition of water in the electrolyte due to the greater positive shift in the I^–/I₃^– redox potential. However, due to the dye desorption and decreased diffusion coefficient caused by the water content, the short-circuit photocurrent density was reduced. These results will provide great insight into the development of efficient and stable water-based electrolytes

Colloidal Spherical Quantum Wells with Near-Unity Photoluminescence Quantum Yield and Suppressed Blinking

Thick inorganic shells endow colloidal nanocrystals (NCs) with enhanced photochemical stability and suppression of photoluminescence intermittency (also known as blinking). However, the progress of using thick-shell heterostructure NCs in applications has been limited due to the low photoluminescence quantum yield (PL QY ≤ 60%) at room temperature. Here, we demonstrate thick-shell NCs with CdS/CdSe/CdS seed/spherical quantum well/shell (SQW) geometry that exhibit near-unity PL QY at room temperature and suppression of blinking. In SQW NCs, the lattice mismatch is diminished between the emissive CdSe layer and the surrounding CdS layers as a result of coherent strain, which suppresses the formation of misfit defects and consequently permits ∼100% PL QY for SQW NCs with a thick CdS shell (≥5 nm). High PL QY of thick-shell SQW NCs is preserved even in concentrated dispersion and in film under thermal stress, which makes them promising candidates for applications in solid-state lightings and luminescent solar concentrators