Free-standing Membranes Research Articles

Assembly of ultra-thin membranes with inherent reinforcement-structure as component of the 2D-material

The invention consists of a special synthesis concept for the assembly of reinforced, ultra-thin membranes made from so called 2-dimensional materials: A primary, 1–10 atomic layer thick film of the 2-dim material is synthesized and lithographically pre-structured towards a film containing a periodic hole structure. A secondary ultra-thin (1–5 atomic layer) film of the same material is then synthesized and merged with the holey film with contact at the nm length scale to result in a novel 2-dim membrane material with periodically alternating film thickness. The basic structure of such an assembly is shown in the TOC Figure.The resulting new material combines the high mechanical stability of the thick membrane sections with the properties of the thinner membrane segments. Already a doubled membrane thickness leads to a significant stability increase in these thickness regimes, while the thin regions are highly required in applications where the electronic properties rapidly change with decreasing thickness or where the thin membrane sections turn into an electron transparent material.The novel 2-dim material can be used to span large holes with ultra-thin, freestanding membranes, but may also be used to achieve a highly reliable transfer protocol for the assembly of devices in planar technology.

Chemical synthesis of complex oxide thin films and freestanding membranes.

Oxides offer unique physical and chemical properties that inspire rapid advances in materials chemistry to design and nanoengineer materials compositions and implement them in devices for a myriad of applications. Chemical deposition methods are gaining attention as a versatile approach to develop complex oxide thin films and nanostructures by properly selecting compatible chemical precursors and designing an accurate cost-effective thermal treatment. Here, upon describing the basics of chemical solution deposition (CSD) and atomic layer deposition (ALD), some examples of the growth of chemically-deposited functional complex oxide films that can have applications in energy and electronics are discussed. To go one step further, the suitability of these techniques is presented to prepare freestanding complex oxides which can notably broaden their applications. Finally, perspectives on the use of chemical methods to prepare future materials are given.

Freestanding Block Copolymer Membranes with Tunable Pore Sizes Promoted by Subnanometer Nanowires.

Block copolymers (BCPs) have enduring appeal for its intriguing assembly behaviors. Nevertheless, the unsatisfactory mechanical properties of BCPs make it a problem to fabricate freestanding membranes and hindered practical applications. Herein, a freestanding membrane with tunable pore size is prepared simply by co-assembly of BCPs and subnanometer nanowires (SNWs), combining the abundant function of BCPs and prominent mechanical properties of SNWs. Benefited from synergy of the components and the hierarchical structure, the tensile strength of composite membrane is promoted by two orders of magnitude compared to that of BCPs. With the columnar pores aligning vertically to surfaces and the pore size regulated by processing conditions, the membranes exhibit precise size-selected effect in ultrafiltration of Au nanoparticles (Au NPs) and can distinct NPs with diameter difference as tiny as 5nm, demonstrating the promising prospect in separation technology and even widespread fields.

Fabrication of a self-standing supramolecular membrane by a "soft spray" technique.

We report a one-step method to fabricate a free-standing supramolecular membrane composed of melamine and barbituric acid coordinated with silver nitrate (Mba-Ag) at the gas/liquid interface by a soft spray technique. MBa-Ag exhibits a folded two-dimensional layered morphology and thickness of 4.5 μm. The shortwave IR transmittance of MBa-Ag is as high as 95%, which is much higher than the transmittance of UV and visible light, and has the potential for electromagnetic wave transmission.

Effects of crowding on the diffusivity of membrane adhered particles.

The lateral diffusion of cell membrane inclusions, such as integral membrane proteins and bound receptors, drives critical biological processes, including the formation of complexes, cell-cell signaling, and membrane trafficking. These diffusive processes are complicated by how concentrated, or "crowded", the inclusions are, which can occupy between 30-50% of the area fraction of the membrane. In this work, we elucidate the effects of increasing concentration of model membrane inclusions in a free-standing artificial cell membrane on inclusion diffusivity and the apparent viscosity of the membrane. By multiple particle tracking of fluorescent microparticles covalently tethered to the bilayer, we show the transition from expected Brownian dynamics, which accurately measure the membrane viscosity, to subdiffusive behavior with decreased diffusion coefficient as the particle area fraction increases from 1% to around 30%, approaching physiological levels of crowding. At high crowding, the onset of non-Gaussian behavior is observed. Using hydrodynamic models relating the 2D diffusion coefficient to the viscosity of a membrane, we determine the apparent viscosity of the bilayer from the particle diffusivity and show an increase in the apparent membrane viscosity with increasing particle area fraction. However, the scaling of this increase is in contrast with the behavior of monolayer inclusion diffusion and bulk suspension rheology. These results demonstrate that physiological levels of model membrane crowding nontrivially alter the dynamics and apparent viscosity of the system, which has implications for understanding membrane protein interactions and particle-membrane transport processes.

Covalent organic framework assisted interlocked graphene oxide based thin-film composite membrane for effective water remediation

2D materials like graphene oxide (GO) based free-standing membranes, although have shown excellent salt rejection at short time scales, suffer from structural stability and swelling on prolonged use.

A bio-inspired versatile free-standing membrane for oral cavity microenvironmental monitoring and remineralization to prevent dental caries.

The fast monitoring of oral bacterial infection, bacterial clearance and repairing of enamel damage caused by dental caries relies on an effective way of monitoring, killing and repairing in situ, but presents a major challenge in oral healthcare. Herein, we developed a bio-inspired versatile free-standing membrane by filling TiO2 nanotube arrays with β-sheet-rich silk fibroin and cleaving them from Ti foil, as inspired by nacre or enamel-like structures. The robust transparent membrane exhibited good mechanical properties, and could indicate acid-base microenvironment variation and the infection of S. mutans in a 5 min test by loading cyanidin cations in the membrane. Meanwhile, it can be used for photocatalysis and nanoreservoirs ascribed to TiO2 nanotubes, to kill and remove 99% of S. mutans bacteria under interval UV irradiation with low-power density, and load functional peptide to induce the remineralization on the etched-enamel for long-term treatment, tested in vitro and in vivo. The mechanical property of repaired enamel is improved in comparison. This bio-inspired constructed membrane would be applied in the prevention and treatment of oral cavity related diseases, such as enamel demineralization and dental caries, etc.

Ferroelectric Domain Control of Nonlinear Light Polarization in MoS2 via PbZr0.2 Ti0.8 O3 Thin Films and Free-Standing Membranes.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as MoS2 exhibit exceptionally strong nonlinear optical responses, while nanoscale control of the amplitude, polar orientation, and phase of the nonlinear light in TMDCs remains challenging. In this work, by interfacing monolayer MoS2 with epitaxial PbZr0.2 Ti0.8 O3 (PZT) thin films and free-standing PZT membranes, the amplitude and polarization of the second harmonic generation (SHG) signal are modulated via ferroelectric domain patterning, which demonstrates that PZT membranes can lead to in-operando programming of nonlinear light polarization. The interfacial coupling of the MoS2 polar axis with either the out-of-plane polar domains of PZT or the in-plane polarization of domain walls tailors the SHG light polarization into different patterns with distinct symmetries, which are modeled via nonlinear electromagnetic theory. This study provides a new material platform that enables reconfigurable design of light polarization at the nanoscale, paving the path for developing novel optical information processing, smart light modulators, and integrated photonic circuits.

Controlled Iodine Phase Transfer of Covalent Organic Framework Membranes for Instant but Sustained Disinfection.

Freestanding membranes of CuCl2-implanted TpPa covalent organic frameworks (COFs) were mechanochemically produced. The resulting membrane had a high I2 adsorption capacity (566.78 g·mol-1) in cyclohexane, which corresponds to 2.2I2 per unit cell with 1.3I2 immobilized on 3Cl- ions (60%) and 0.9 on 3N atoms (40%). Upon being placed in aqueous media, the membrane released 61.1% of its loaded I2 mainly by its Cl- ions within 10 min and the remaining 38.9% mainly from its N atoms within about 5 h. Thanks to that, the COF membranes loaded with 1.5 mg of I2 could be repetitively utilized to kill about 108 CFU/mL E. coli in 0.5-3 min at least five times, after which the membranes could retain their bactericidal activity for 4 h against 108 CFU/mL E. coli. This highlights the promising application of I2-loaded TpPa-CuCl2 COF membranes for instant and sustained disinfection.

Poly(ethylene oxide)-Based Copolymer-IL Composite Membranes for CO2 Separation.

Poly(ethylene oxide) (PEO)-based copolymers are at the forefront of advanced membrane materials for selective CO2 separation. In this work, free-standing composite membranes were prepared by blending imidazolium-based ionic liquids (ILs) having different structural characteristics with a PEO-based copolymer previously developed by our group, targeting CO2 permeability improvement and effective CO2/gas separation. The effect of IL loading (30 and 40 wt%), alkyl chain length of the imidazolium cation (ethyl- and hexyl- chain) and the nature of the anion (TFSI-, C(CN)3-) on physicochemical and gas transport properties were studied. Among all composite membranes, PEO-based copolymer with 40 wt% IL3-[HMIM][TFSI] containing the longer alkyl chain of the cation and TFSI- as the anion exhibited the highest CO2 permeability of 46.1 Barrer and ideal CO2/H2 and CO2/CH4 selectivities of 5.6 and 39.0, respectively, at 30 °C. In addition, almost all composite membranes surpassed the upper bound limit for CO2/H2 separation. The above membrane showed the highest water vapor permeability value of 50,000 Barrer under both wet and dry conditions and a corresponding H2O/CO2 ideal selectivity value of 1080; values that are comparable with those reported for other highly water-selective PEO-based polymers. These results suggest the potential application of this membrane in hydrogen purification and dehydration of CO2 gas streams.

Investigating the Degradation of EUV Transmittance of an EUV Pellicle Membrane.

The extreme ultraviolet (EUV) pellicle is a freestanding membrane that protects EUV masks from particle contamination during EUV exposure. Although a high EUV transmittance of the pellicle is required to minimize the loss of throughput, the degradation of EUV transmittance during the extended exposure of the pellicle has been recently reported. This may adversely affect the throughput of the lithography process. However, the cause of this phenomenon has not yet been clarified. Therefore, we investigated the cause of the degradation in the EUV transmittance by observing the compositional change when the Ru/SiNx pellicle composite was heated in an emulated EUV scanner environment. The Ru thin film that was deposited at high pressure had more void networks but was not oxidized, whereas the SiNx thin film was oxidized after heating. This was because the void network in the Ru thin film served as a preferential diffusion path for oxygen and caused oxidation of the SiNx thin film. It was confirmed that the degradation of the EUV transmittance was due to the oxidation of SiNx. The results verified the effect of diffusivity in the thin film due to the void network on oxidation and EUV transmittance.

Preparation and Characterization of Free-Standing Composite Cellulose-Based Carbon Molecular Sieve Membranes

Carbon membranes with excellent strength and mechanical stability were prepared using a cellulose precursor regenerated from an ionic liquid solution with a low loading of boehmite ceramic nanoparticles, ca. 0.1 wt. %. The precursor films show an asymmetrical structure while, after carbonization, the corresponding carbon molecular sieve membranes (c-CMSM) show a dense structure with a uniform thickness of ca. 22 µm. The c-CMSM exhibited excellent separation performance, well above the Robeson Upper Bound. The permeability of the c-CMSM to hydrogen increased from 300 to 530 barrer and the H2/CH4 permselectivity increased to 473, after adding boehmite nanoparticles. Boehmite nanoparticles act as a microporosity-forming agent without compromising the width of the ultramicropores. The prepared c-CMSM is extremely promising for several applications, namely for hydrogen recovery from methane – H2/CH4 (Robeson index:θ=13.5); biogas upgrading – CO2/CH4 (θ=6.5); CO2 removal from combustion flue gas – CO2/N2 (θ=1.7); and hydrogen recovery from ammonia processes – H2/N2 (θ=4.8).

Landing Proteins on Graphene Trampoline Preserves Their Gas-Phase Folding on the Surface.

Molecule-surface collisions are known to initiate dynamics that lead to products inaccessible by thermal chemistry. These collision dynamics, however, have mostly been examined on bulk surfaces, leaving vast opportunities unexplored for molecular collisions on nanostructures, especially on those that exhibit mechanical properties radically different from those of their bulk counterparts. Probing energy-dependent dynamics on nanostructures, particularly for large molecules, has been challenging due to their fast time scales and high structural complexity. Here, by examining the dynamics of a protein impinging on a freestanding, single-atom-thick membrane, we discover molecule-on-trampoline dynamics that disperse the collision impact away from the incident protein within a few picoseconds. As a result, our experiments and ab initio calculations show that cytochrome c retains its gas-phase folded structure when it collides onto freestanding single-layer graphene at low energies (∼20 meV/atom). The molecule-on-trampoline dynamics, expected to be operative on many freestanding atomic membranes, enable reliable means to transfer gas-phase macromolecular structures onto freestanding surfaces for their single-molecule imaging, complementing many bioanalytical techniques.

Large-Scale Silver Sulfide Nanomesh Membranes with Ultrahigh Flexibility.

The growth of flexible semiconductor thin films and membranes is highly desirable for the fabrication of next-generation wearable devices. In this work, we have developed a one-step, surface tension-driven method for facile and scalable growth of silver sulfide (Ag2S) membranes with a nanomesh structure. The nanomesh membrane can in principle reach infinite size but only limited by the reactor size, while the thickness is self-limited to ca. 50 nm. In particular, the membrane can be continuously regenerated at the water surface after being transferred for mechanical and electronic tests. The free-standing membrane demonstrates exceptional flexibility and strength, resulting from the nanomesh structure and the intrinsic plasticity of the Ag2S ligaments, as revealed by robust manipulation, nanoindentation tests and a pseudo-in situ tensile test under scanning electron microscope. Bendable electronic resistance-switching devices are fabricated based on the nanomesh membrane.

Two routes for N-rich solid polymer electrolyte for all-solid-state lithium-ion batteries

Very few works have been realized on Li-ion all-solid-state batteries using solid polymer electrolytes (SPE) based on nitrogen-rich polymers (NRP)s, especially poly(ethyleneimine) (PEI) and poly(oxazoline) (POx). Synergizing the advantages of the two polymers, two complementary routes with blends of polymers (POx:BPEI) and the synthesis of copolymers POx-co-PEI by partial hydrolysis of POx were highlighted in this work. Free-standing membranes with low glass transition temperature exhibited valuable compromise between mechanical properties, thermal stability and effective Li+ (LiTFSI) transport. Comparable ionic conductivity to the PEO (structural counterpart of PEI) (2,7.10−5 S cm−1 at 60 °C) was reached and explained by complexation and interactions between lithium salt and several electro-donor sites of NRPs during a FTIR spectroscopy investigation.

Impact of Non-Accelerated Aging on the Properties of Parylene C

The polymer Parylene combines a variety of excellent properties and, hence, is an object of intensive research for packaging applications, such as the direct encapsulation of medical implants. Moreover, in the past years, an increasing interest for establishing new applications for Parylene is observed. These include the usage of Parylene as a flexible substrate, a dielectric, or a material for MEMS, e.g., a bonding adhesive. The increasing importance of Parylene raises questions regarding the long-term reliability and aging of Parylene as well as the impact of the aging on the Parylene properties. Within this paper, we present the first investigations on non-accelerated Parylene C aging for a period of about five years. Doing so, free-standing Parylene membranes were fabricated to investigate the barrier properties, the chemical stability, as well as the optical properties of Parylene in dependence on different post-treatments to the polymer. These properties were found to be excellent and with only a minor age-related impact. Additionally, the mechanical properties, i.e., the Young's modulus and the hardness, were investigated via nano-indentation over the same period of time. For both mechanical properties only, minor changes were observed. The results prove that Parylene C is a highly reliable polymer for applications that needs a high long-term stability.

Green H2O2 activation of electrospun polyimide-based carbon nanofibers towards high-performance free-standing electrodes for supercapacitors

Free-standing carbon nanofibrous membranes are prepared by carbonization and H 2 O 2 activation of electrospun polyimide nanofibrous membranes. The green and facile H 2 O 2 activation method not only regulates the pore structure of carbon nanofibers but also introduces rich oxygen species, rendering a hierarchically porous and heteroatom-doped carbon electrode for supercapacitors. The optimal electrode (PI800-8) shows a high specific capacitance of 339.9 F g −1 (0.5 A g −1 , 6 M KOH electrolyte) and excellent cycle durability with a capacitance retention of 98.4 % after 50,000 cycles (5 A g −1 ). Particularly, a non-aqueous symmetric supercapacitor assembled with 1 M Et 4 NBF 4 electrolyte shows a high operating voltage of 2.8 V, and delivers the maximum energy/power density of 37.3 Wh kg −1 and 28.0 kW kg −1 , respectively. The H 2 O 2 activation method provides a green, efficient, and versatile strategy to improve the pore properties and chemical compositions of porous carbon materials towards energy applications. • The N/O-codoped carbon nanofibrous membranes were prepared by carbonization/activation of commercial electrospun polyimide nanofibrous membranes. • The green and cost-effective H 2 O 2 activation was proposed to ameliorate the pore and chemical properties of carbon nanofibrous membranes. • Excellent comprehensive performance was presented for both aqueous/non-aqueous symmetric supercapacitor.

Achieving high initial coulombic efficiencies and cycle stability of free-standing anodes by chemical prelithiation of carbon matrix

The influence of carbon matrix in freestanding electrodes for flexible lithium-ion batteries (LIBs) cannot be neglected because of the strong intercalation ability of Li+ which could result in low initial columbic efficiencies (ICEs). Based on electrostatic potential, inorganic lithium salts and graphene oxide (GO) with opposite zeta potential in ethanol is utilized as chemical prelithiation reagent, which enables a successful prelithiation of GO-based SnTiS3 anodes. By molecular dynamic calculation and experimental evaluation, the prelithiated GO matrix with enlarged layer distance enables an improved ion conductivity and suppresses Li consumption. Consequently, prelithiated free-standing membranes as anodes for LIBs can demonstrate significantly improved ICEs and capacity utilization compared with their counterparts without prelithiation. This strategy can shed new light on prelithiated free-standing electrodes with high performance for flexible wearable energy storage devices.

High-Performance All-Solid-State Proton Rectifier Using a Heterogeneous Membrane Composed of Coordination Polymer and Layered Double Hydroxide.

Rational control of unidirectional proton transport is highly challenging, primarily owing to the difficulty in introducing an asymmetric factor into proton conducting media. In this study, free-standing membranes of a proton-conducting two-dimensional porous coordination polymer, Cu2 (CuTCPP) (H2 TCPP: 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin) and a hydroxide ion-conducting layered double hydroxide, Mg-Al-LDH(NO3 ), were combined to generate a pH gradient in the conducting media. The current-voltage measurements revealed that the heterogeneous membrane exhibits a significant unidirectional proton transport with a proton rectification ratio exceeding 200 under 90 % relative humidity in the initial voltage scan. This value is the highest among the reported all-solid-state proton rectifiers. The high designability of both components with well-defined structures, which is in contrast to the organic polymers used so far, provides a new avenue for developing and understanding the proton-rectifying behavior in the solid state.

Fabrication and Characterization of Free-Standing and Flexible Polyaniline Membranes: Role of Graphene Nanoscrolls

Wearable technologies can contribute to the early and accurate detection of chronic diseases which can be achieved by the integration of biosensors into wearable technologies. However, the challenges associated with the performance of current electrode materials—i.e., flexibility, conductivity, and mechanical stability, made from conducting polymers are preventing their widespread usage. Herein, we report a freestanding and flexible electrode synthesized from polyaniline (PANI) and graphene nanoscrolls (GNS). The PANI-GNS nanohybrid membranes were synthesized via chemical oxidative polymerization and characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), nanoindentation (NI), and four-point probe techniques. FTIR results showed an increase in conjugation length of the PANI after the addition of GNS into the mixture which can be indicative of an enhancement of electrical properties. Nanoindentation studies showed an elastic modulus and hardness of 2.6 GPa and 0.17 GPa, respectively, for PANI-GNS-5 nanocomposite, compared to 1.9 GPa and 0.08 GPa, for pure PANI. This was later confirmed by the four-point probe technique as the addition of GNS increased the conductivity of electrodes up to 9 S/cm at a 5% weight ratio. Moreover, SEM results of the PANI-GNS showed an open porous morphology of the polymer matrix in comparison with pure PANI samples which would readily translate into higher amounts of enzyme immobilization on the surface.