Huie Zhu

Ferroelectricity of poly(vinylidene fluoride) homopolymer Langmuir–Blodgett nanofilms

As-deposited poly(vinylidene fluoride) (PVDF) Langmuir–Blodgett (LB) nanofilms with a complete β phase show a remarkably high remanent polarization (Pr), about 6.6 μC cm−2 at 81 nm. Extrinsic switching characteristics of the nanofilms are demonstrated. Results also show that highly oriented PVDF homopolymer LB nanofilms down to 12 nm with no post-treatment retain robust room-temperature ferroelectric polarization switching that is greater than 105 operation cycles, with longer standing fatigue endurance than those of PVDF copolymer nanofilms.

Room temperature magnetoresistance effects in ferroelectric poly(vinylidene fluoride) spin valves

Ferroelectric poly(vinylidene fluoride) (PVDF) nanofilms have been fabricated by the Langmuir–Blodgett technique, possessing mainly a ferroelectric active phase and a controllable film thickness of 2.3 nm per layer. Atomic force microscopy and Fourier transform infrared spectroscopy were utilized to characterize the film properties. Importantly, the PVDF films could act as barrier layers to prepare spin transport devices using Fe3O4 and Co as bottom and top ferromagnetic electrodes, respectively. Spin-dependent electron transport behaviors were systemically studied in these devices by varying the PVDF film thickness from 3 layers (7 nm) to 13 layers (30 nm). With increasing PVDF layer numbers, the magnetoresistance (MR) response decreases likely due to the change in spin transport from tunneling to hopping transport. We further investigated the MR dependence on operation temperatures (150 K, 200 K, 250 K and 300 K). It is noteworthy that the MR effect was observed even at 300 K with an MR ratio exceeding 2.5%, which is achieved for the first time in such organic devices. The device performance could be further improved at lower operation temperatures. The MR ratios, device resistances and electron transport mechanisms in the present devices were also discussed to analyze the spin transport behaviors. The results indicate that the ferroelectric PVDF nanofilms are promising candidates for spin devices operated at room temperature, thereby shedding light on the design of organic ferroelectric spintronics with a higher performance.

A versatile platform of catechol-functionalized polysiloxanes for hybrid nanoassembly and in situ surface enhanced Raman scattering applications

Inspired by the marine mussel strategy of adhesion in aqueous environments, catechol-functionalized polysiloxane (CFPS) was synthesized. Facile dip-coating showed good film forming ability on various organic and inorganic substrates with a surface roughness of 4.6 nm (50 μm × 50 μm). Silver nanoparticles (AgNPs) are anchored onto CFPS-modified substrates using a dip coating process. The scanning electron microscopy images revealed that AgNPs were distributed homogeneously on numerous substrates. Moreover, the surface number density and average interspace of the AgNPs were tuned easily by controlling the concentration of AgNP dispersions. A substrate with high-density AgNPs exhibited excellent surface enhanced Raman scattering (SERS) performance with an enhancement factor as high as 7.89 × 107 and an ultra-low detection limitation of 10−10 M, which can be ascribed to the “hotspots” formed between the adjacent AgNPs controlled by CFPS-induced self-assembly. The AgNP structures prepared on different substrates modified using CFPS show a similar high intensity, suggesting a versatile platform of the CFPS film for AgNP assemblies. In addition, AgNPs were anchored to be extremely stable on substrates because of strong hydrogen bonding interactions provided by the high surface-density catechol units of CFPS, which made the substrate a promising SERS sensor for practical applications. In situ SERS detection was also demonstrated on apple peel with no damage to AgNP structures.

Asymmetric Ferroelectric Switching Based on an Al/PVDF Langmuir-Blodgett Nanofilm/PEDOT:PSS/Al Device

We prepared highly oriented ferroelectric poly(vinylidene fluoride) (PVDF) Langmuir–Blodgett (LB) nanofilms by an assistance of amphiphilic poly(N-dodecylacrylamide) (pDDA) nanosheets. In the nanofilms, semi-crystalline PVDF contains abundant polar β crystals, a parallel packing of PVDF all-trans molecules. Combing semi-conductive PEDOT:PSS and ferroelectric PVDF LB nanofilms, we fabricated a sandwiched capacitor (Al/PVDF LB nanofilm/PEDOT:PSS/Al). The capacitor shows asymmetric hysteresis curves and high remanent polarization values.

Superhydrophobic surfaces with fluorinated cellulose nanofiber assemblies for oil–water separation

A Cellulose nanofiber (CNF) is an amazing nanomaterial produced from ubiquitous sources with outstanding mechanical, chemical, and barrier properties. This report describes facile functionalization of CNF with trichloro(1H,1H,2H,2H-heptadecafluorodecyl)silane (THFS). The paste form CNF was solvent exchanged from initial water to AK-225 (a mixture of 3,3-dichloro-1,1,1,2,2-pentafluoropropane and 1,3-dichloro-1,1,2,2,3-pentafluoropuropane) before surface modification with THFS. The modified CNFs were dispersed uniformly in AK-225. The CNF film, which was prepared using simple drop casting, showed a superhydrophobic surface with a water contact angle of 160° and oleophilicity with a hexane contact angle of less than 35°. The modified CNF assembly is thermally stable, optically transparent and resistant to corrosive environment (acidic, basic and seawater solutions). The separation of oil and water mixtures was demonstrated using steel mesh coated with modified CNF. Separation efficiency greater than 99% was achieved by simple gravitational force for hydrocarbons and organic solvents. The as-prepared mesh can be used repeatedly more than 50 times with the same efficiency as the initial state.

Regioselective Synthesis of Eight-Armed Cyclosiloxane Amphiphile for Functional 2D and 3D Assembly Motifs

A crystalline tetramethylcyclotetrasiloxane (TMCS)-derived amphiphile was regioselectively synthesized with eight peripheral hydrophilic amide groups and hydrophobic dodecyl chains by Pt(0)-catalyzed hydrosilylation and amidation reactions. The as-synthesized materials showed ordered lamellar structure formation in the powder form. It also exhibits superior two-dimensional (2D) monolayer formation properties at the air-water interface with unexpectedly high collapse surface pressure and elastic modulus. The monolayers act as two-dimensional building blocks with finely controllable thickness on a several nanometer scale irrespective of the substrate type and properties. The amphiphile forms nanofibers spontaneously by good-poor solvent strategies, which contributes to porous three-dimensional (3D) structures possessing superhydrophobic surface wettability.

Highly oriented poly(vinylidene fluoride-co-trifluoroethylene) ultrathin films with improved ferroelectricity

We fabricated poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE) 75/25 mol% Langmuir–Blodgett (LB) nanofilms by the assistance of amphiphilic poly(N-dodecylacrylamide) (pDDA) nanosheets. The nanosheet is formed based on a well-organized hydrogen bonding network among pDDA amide groups at the air–water interface. By the introduction of the pDDA nanosheet, the film stability of the P(VDF-TrFE) Langmuir film was greatly improved with a twofold increase in the collapse surface pressure to 57 mN m−1 of pure P(VDF-TrFE). Then, the P(VDF-TrFE) Langmuir film was endowed a good transfer ability with a unity transfer ratio irrespective of its non-amphiphilicity. Absorbance of the amide group of pDDA in UV-vis spectra shows a good linear relation with the film thickness. The result indicates that the multilayered film takes a uniform layered structure. The β-crystal content in as-prepared LB nanofilms with no post-treatment is up to 80%, one of the highest values ever reported. The monolayer thickness was determined as 3.5 nm by AFM measurements. The good film properties make the as-prepared P(VDF-TrFE)/pDDA LB ultrathin films (18 nm) available for ferroelectricity measurement using macroscopic methods such as the typical Sawyer–Tower circuit, which is usually challenging for other ultrathin films. The measurements demonstrate improved ferroelectricity, with a high remanent polarization value of 5.0 μC cm−2 at 10 Hz.

Angle-resolved X-ray photoelectron spectroscopy study of poly(vinylidene fluoride)/poly(N-dodecylacrylamide) Langmuir–Blodgett nanofilms

Our earlier research prepared ferroelectric poly(vinylidene fluoride) (PVDF) homopolymer monolayers at the air–water interface using amphiphilic poly(N-dodecylacrylamide) (pDDA) nanosheets with Langmuir–Blodgett (LB) technique. However, the miscibility of solvent for PVDF with the water sub-phase in the Langmuir trough makes the film composition unclear in spite of the feeding ratio of (). In this study, angle-resolved X-ray photoelectron spectroscopy (AR-XPS) was used to investigate the surface chemical composition and the depth profile of the PVDF/pDDA LB nanofilms. The X-ray photoelectron spectroscopy (XPS) spectra confirmed by the detection of fluorine atoms that PVDF molecules were deposited successfully onto the substrate. The constant chemical composition with increasing takeoff angle from 15 to 75° reflects a well-regular layer structure of the PVDF LB nanofilm. The mixing ratio of is , which contributes 89.8 wt % PVDF and 10.2 wt % in the PVDF/pDDA LB nanofilms.

Resistive non-volatile memories fabricated with poly(vinylidene fluoride)/poly(thiophene) blend nanosheets

Ferroelectric poly(vinylidene fluoride)/semiconductive polythiophene (P3CPenT) blend monolayers were developed at varying blend ratios using the Langmuir–Blodgett technique. The multilayered blend nanosheets show much improved surface roughness that is more applicable for electronics applications than spin-cast films. Because of the precisely controllable bottom-up construction, semiconductive P3CPenT were well dispersed into the ferroelectric PVDF matrix. Moreover, the ferroelectric matrix contains almost 100% β crystals: a polar crystal phase responsible for the ferroelectricity of PVDF. Both the good dispersion of semiconductive P3CPenT and the outstanding ferroelectricity of the PVDF matrix in the blend nanosheets guaranteed the success of ferroelectric organic non-volatile memories based on ferroelectricity-manipulated resistive switching with a fresh high ON/OFF ratio and long endurance to 30 days.

Organic spin valves with poly(vinylidene fluoride) barriers

Poly(vinylidene fluoride) (PVDF) was used as an organic spacer to fabricate spin-valve devices for the first time. Their magnetoresistance (MR) effects with different layer numbers of PVDF were investigated at both room and low temperatures. The device resistances and MR ratios increase with decreasing measurement temperature. It is noted that the MR ratios at room temperature are over 2% and 0.5% for devices with 3 layers and 13 layers of PVDF, respectively.