Free-standing Nanomembranes Research Articles

Ion and Molecular Sieving With Ultrathin Polydopamine Nanomembranes.

In contrast to biological cell membranes, it is still a major challenge for synthetic membranes to efficiently separate ions and small molecules due to their similar sizes in the sub-nanometer range. Inspired by biological ion channels with their unique channel wall chemistry that facilitates ion sieving by ion-channel interactions, the first free-standing, ultrathin (10-17nm) nanomembranes composed entirely of polydopamine (PDA) are reported here as ion and molecular sieves. These nanomembranes are obtained via an easily scalable electropolymerization strategy and provide nanochannels with various amine and phenolic hydroxyl groups that offer a favorable chemical environment for ion-channel electrostatic and hydrogen bond interactions. They exhibit remarkable selectivity for monovalent ions over multivalent ions and larger species with K+/Mg2+ of ≈4.2, K+/[Fe(CN)6]3- of ≈10.3, and K+/Rhodamine B of ≈273.0 in a pressure-driven process, as well as cyclic reversible pH-responsive gating properties. Infrared spectra reveal hydrogen bond formation between hydrated multivalent ions and PDA, which prevents the transport of multivalent ions and facilitates high selectivity. Chemically rich, free-standing, and pH-responsive PDA nanomembranes with specific interaction sites are proposed as customizable high-performance sieves for a wide range of challenging separation requirements.

Spatially reconfigurable antiferromagnetic states in topologically rich free-standing nanomembranes

Antiferromagnets hosting real-space topological textures are promising platforms to model fundamental ultrafast phenomena and explore spintronics. However, they have only been epitaxially fabricated on specific symmetry-matched substrates, thereby preserving their intrinsic magneto-crystalline order. This curtails their integration with dissimilar supports, restricting the scope of fundamental and applied investigations. Here we circumvent this limitation by designing detachable crystalline antiferromagnetic nanomembranes of α-Fe2O3. First, we show—via transmission-based antiferromagnetic vector mapping—that flat nanomembranes host a spin-reorientation transition and rich topological phenomenology. Second, we exploit their extreme flexibility to demonstrate the reconfiguration of antiferromagnetic states across three-dimensional membrane folds resulting from flexure-induced strains. Finally, we combine these developments using a controlled manipulator to realize the strain-driven non-thermal generation of topological textures at room temperature. The integration of such free-standing antiferromagnetic layers with flat/curved nanostructures could enable spin texture designs via magnetoelastic/geometric effects in the quasi-static and dynamical regimes, opening new explorations into curvilinear antiferromagnetism and unconventional computing.

Self-Rolled-Up Ultrathin Single-Crystalline Silicon Nanomembranes for On-Chip Tubular Polarization Photodetectors.

Freestanding single-crystalline nanomembranes and their assembly have broad application potential in photodetectors for integrated chips. However, the release and self-assembly process of single-crystalline semiconductor nanomembranes still remains a great challenge in on-chip processing and functional integration, and photodetectors based on nanomembrane always suffer from limited absorption of nanoscale thickness. Here, a non-destructive releasing and rolling process is employed to prepare tubular photodetectors based on freestanding single-crystalline Si nanomembranes. Spontaneous release and self-assembly are achieved by residual strain introduced by lattice mismatch at the epitaxial interface of Si and Ge, and the intrinsic stress and strain distributions in self-rolled-up Si nanomembranes are analyzed experimentally and computationally. The advantages of light trapping and wide-angle optical coupling are realized by tubular geometry. This Si microtube device achieves reliable Ohmic contact and exhibits a photoresponsivity of over 330mA W-1 , a response time of 370 µs, and a light incident detection angle range of over 120°. Furthermore, the microtubular structure shows a distinct polarization angle-dependent light absorption, with a dichroic ratio of 1.24 achieved at 940nm. The proposed Si-based microtubes provide new possibilities for the construction of multifunctional chips for integrated circuit ecosystems in the More than Moore era.

One-step rolling fabrication of VO2 tubular bolometers with polarization-sensitive and omnidirectional detection.

Uncooled infrared detection based on vanadium dioxide (VO2) radiometer is highly demanded in temperature monitoring and security protection. The key to its breakthrough is to fabricate bolometer arrays with great absorbance and excellent thermal insulation using a straightforward procedure. Here, we show a tubular bolometer by one-step rolling VO2 nanomembranes with enhanced infrared detection. The tubular geometry enhances the thermal insulation, light absorption, and temperature sensitivity of freestanding VO2 nanomembranes. This tubular VO2 bolometer exhibits a detectivity of ~2 × 108 cm Hz1/2 W-1 in the ultrabroad infrared spectrum, a response time of ~2.0 ms, and a calculated noise-equivalent temperature difference of 64.5 mK. Furthermore, our device presents a workable structural paradigm for polarization-sensitive and omnidirectional light coupling bolometers. The demonstrated overall characteristics suggest that tubular bolometers have the potential to narrow performance and cost gap between photon detectors and thermal detectors with low cost and broad applications.

Direct observation of large-area strain propagation on free-standing nanomembranes

Investigations on epitaxial nanostructures with size of tens of nanometers have been a challenging issue for techniques that present high strain sensitivity but restricted spatial resolution. This is the case of recently developed x-ray nanoprobe techniques. Despite its inherent nondestructive character, submicron x-ray spots have only been successfully applied to the study of individual nanostructures which are either strain free or present extremely mild spatial lattice parameter gradients. Such limitation, with an uttermost barrier given by the diffraction limit, leads to voxel or pixel sizes between 5 and 10 nm obtained in coherent diffraction imaging or ptychographic reconstructions of real-space objects. Whenever the strain field of a nanostructure is successfully reconstructed from reciprocal space measurements, it cannot vary considerably in short distances since this would induce diffraction peak broadening and cause abrupt phase variations, leading to convergence issues on reconstruction algorithms. Here we show how epitaxial systems with large lattice mismatch and appreciable interfacial strain can be identified and directly analyzed throughout their strain field propagation in nanometer-thin crystalline membrane platforms, using the InGaAs/GaAs Stranski-Krastanov system as a model. The strain-induced footprint becomes observable along a few microns if the membrane thickness is comparable to the nanostructure size. It is possible to retrieve both interfacial strain and nanostructure size by probing individual objects.

Electrostatic Repulsive Features of Free-Standing Titanium Dioxide Nanotube-Based Membranes in Biofiltration Applications.

This study presents the electrostatic repulsive features of electrochemically fabricated titanium dioxide nanotube (NT)-based membranes with different surface nanomorphologies in cross-flow biofiltration applications while maintaining a creatinine clearance above 90%. Although membranes exhibit antifouling behavior, their blood protein rejection can still be improved. Due to the electrostatically negative charge of the hexafluorotitanate moiety, the fabricated biocompatible, superhydrophilic, free-standing, and amorphous ceramic nanomembranes showed that about 20% of negatively charged 66 kDa blood albumin was rejected by the membrane with ∼100 nm pores. As the nanomorphology of the membrane was shifted from NTs to nanowires by varying fabrication parameters, pure water flux and bovine serum albumin (BSA) rejection performance were reduced, and the membrane did not lose its antifouling behavior. Herein, nanomembranes with different surface nanomorphologies were fabricated by a multi-step anodic oxidation process and characterized by scanning electron microscopy, atomic force microscopy, water contact angle analysis, X-ray diffraction, and energy-dispersive X-ray spectroscopy. The membrane performance of samples was measured in 3D printed polyethylene terephthalate glycol flow cells replicating implantable artificial kidney models to determine their blood toxin removal and protein loss features. In collected urine mimicking samples, creatinine clearances and BSA rejections were measured by the spectrophotometric Jaffe method and high-performance liquid chromatography.

Wafer-scale detachable monocrystalline germanium nanomembranes for the growth of III-V materials and substrate reuse.

Germanium (Ge) is increasingly used as a substrate for high-performance optoelectronics, photovoltaics, and electronic devices. These devices are usually grown on thick and rigid Ge substrates manufactured by classical wafering techniques. Nanomembranes (NMs) provide an alternative to this approach while offering wafer-scale lateral dimensions, weight reduction, waste limitation, and cost effectiveness. Herein, we introduce the Porous germanium Efficient Epitaxial LayEr Release (PEELER) process, which consists of the fabrication of wafer-scale detachable Ge NMs on porous Ge (PGe) and substrate reuse. We demonstrate the growth of Ge NMs with monocrystalline quality as revealed by high-resolution transmission electron microscopy (HRTEM) characterization. Together with the surface roughness below 1 nm, it makes the Ge NMs suitable for growth of III-V materials. Additionally, the embedded nanoengineered weak layer enables the detachment of the Ge NMs. Finally, we demonstrate the wet-etch-reconditioning process of the Ge substrate, allowing its reuse, to produce multiple free-standing NMs from a single parent wafer. The PEELER process significantly reduces the consumption of Ge in the fabrication process, paving the way for a new generation of low-cost flexible optoelectronic devices.

Freestanding epitaxial SrTiO 3 nanomembranes via remote epitaxy using hybrid molecular beam epitaxy

The epitaxial growth of functional oxides using a substrate with a graphene layer is a highly desirable method for improving structural quality and obtaining freestanding epitaxial nanomembranes for scientific study, applications, and economical reuse of substrates. However, the aggressive oxidizing conditions typically used in growing epitaxial oxides can damage graphene. Here, we demonstrate the successful use of hybrid molecular beam epitaxy for SrTiO3 growth that does not require an independent oxygen source, thus avoiding graphene damage. This approach produces epitaxial films with self-regulating cation stoichiometry. Furthermore, the film (46-nm-thick SrTiO3) can be exfoliated and transferred to foreign substrates. These results open the door to future studies of previously unattainable freestanding oxide nanomembranes grown in an adsorption-controlled manner by hybrid molecular beam epitaxy. This approach has potentially important implications for the commercial application of perovskite oxides in flexible electronics and as a dielectric in van der Waals thin-film electronics.

Freestanding Metal Nanomembranes and Nanowires by Template Transfer with a Soluble Adhesive.

The fabrication of nanostructures usually involves chemical processes that have in certain steps. Especially, it is necessary to use the chemical etching method to release the as-patterned structures from the substrate in most of the transfer techniques. Here, a novel scheme of template transfer as developed for the fabrication of freestanding Au nanomembranes and nanowires by using a soluble PVP adhesive. The nanomembranes feature the periodic nanohole arrays with high uniformity. Without the substrates, these plasmonic nanohole arrays show symmetric and antisymmetric resonance modes with bright and dark spectral features, respectively, in transmission. Through the spectral analysis for reflection, we have disclosed that the usual dark mode in transmission is not really dark, but it reveals a distinct feature in reflection. Two coupling modes present distinct spectral features in transmission and reflection due to their different loss channels. To show their versatility, the freestanding nanomembranes were also employed as secondary templates to form Si nanowire arrays by the metal-assisted chemical etching method.

Self-assembled growing ultrathin Ag@NiO core-shell nanowires for stable freestanding transparent conductive colorless polyimide nanomembranes

Among the alternatives to indium tin oxide, silver nanowire (Ag NW) networks have received widespread attention for their superior optoelectronic properties. However, the rapid degradation of Ag under ambient conditions essentially limits its practical application. In this paper, we innovatively propose the synthesis of ultrathin Ag@NiO core-shell nanowires by a simple and low-cost solution method to enhance the thermal, electrical and chemical stability of NWs. The Ni2+ and Ag atoms undergo a redox reaction in the presence of water, and a reasonable mechanism of NiO formation is proposed. The Ag@NiO NW networks show low sheet resistance of 12.1 Ω/sq. and high transmittance of 84.8%. The encapsulation of the NiO shell can significantly improve their resistance to heat (85 °C) and humidity (85%) for 96 h, and NaCl solution for 200 min, while their resistance remains similar to the original values. The stable free-standing transparent conductive nanomembrane (FTCNM), consisting of an Ag@NiO NW nanowire network embedded in an ultrathin colorless polyimide, is fabricated by all-solution-process, which is perfectly conformal with human skin. We also present a skin-attachable capacitive nanomembrane sensor based on the FTCNM, the sensor shows remarkable pressure-sensing capability.

Nano-LED driven phase change evolution of layered chalcogenides for Raman spectroscopy investigations

We present a device driving testing platform based on vertically integrated nano light emitting diodes (nano-LEDs). The nano-LEDs with a peak wavelength emission centered at ∼ 445 nm were arranged in arrays and conditioned using a laser-micro-annealing process to individually tune their intensity. They were coupled with freestanding monocrystalline Ge1Sb2Te4 nano-membranes with three different thicknesses (∼40, ∼60 and ∼ 90 nm) with the aim of initializing ultrafast switching processes and of observing phase changed states simultaneously by Raman spectroscopy. Raman spectroscopy studies reveal that the optical pulses emitted from the nano-LEDs induce substantial, local changes in the nano-membranes’ states of the Ge1Sb2Te4 layered material. Beside the crystalline state in non-exposed areas (as-grown material), amorphous and different intermediate states were identified in exposed areas as island-like structures with diameters ranging from ∼ 300 nm up to ∼ 1.5 µm. The latter confirms the nano-LEDs’ emission role in both near- and far-field regimes, depending on the distance between nano-LED and nano-membrane, for driving i.e. inducing the phase change process. The results presented demonstrate the suitability and potential of the vertically integrated nano-LEDs as the key components for a testing platform/for electro-optical convertors driving phase change processes in optically active media. They could also play an important role in the development of future, e.g., non-volatile data storage as well as in optical and neuromorphic computing architectures based on transmistor devices.

Ballistic dynamics of flexural thermal movements in a nanomembrane revealed with subatomic resolution.

Flexural oscillations of freestanding films, nanomembranes, and nanowires are attracting growing attention for their importance to the fundamental physical and optical properties and device applications of two-dimensional and nanostructured (meta)materials. Here, we report on the observation of short–time scale ballistic motion in the flexural mode of a nanomembrane cantilever, driven by thermal fluctuation of flexural phonons, including measurements of ballistic velocities and displacements performed with subatomic resolution, using a free electron edge-scattering technique. Within intervals <10 μs, the membrane moves ballistically at a constant velocity, typically ~300 μm/s, while Brownian-like dynamics emerge for longer observation periods. Access to the ballistic regime provides verification of the equipartition theorem and Maxwell-Boltzmann statistics for flexural modes and can be used in fast thermometry and mass sensing during atomic absorption/desorption processes on the membrane.

Highly stable silver nanowire networks with tin oxide shells for freestanding transparent conductive nanomembranes through all‐solution processes

Freestanding nanomembranes (NMs) with nanoscale thickness, are mechanically flexible, which are novel building blocks with potential applications in future wearable conformal electronics. Here, we demonstrate a highly stable, ultrathin, freestanding, transparent, and conductive hybrid NM, consisting of a silver nanowire network with tin oxide shell embedded in a colorless polyimide (cPI). The entire fabrication processes are all solution based and non-vacuum, which are simple, scalable and low-cost, enabling facile large-area integration with wearable electronic devices. Our hybrid NMs possess high optical transmittance of 86.7% with low sheet resistance of 9.6 Ω/sq. and can be conformal attached to human skin. In addition, our hybrid NMs exhibit excellent stability to resist moisture, long-term storage, solvent damage, strong oxidant, ultrasonication and acid/base should stem. Particularly, the hybrid NMs are robust and able to withstand high temperature of 300 ℃, without deterioration of performance. As a conformal/skin-attachable device application, a skin-attachable capacitive NM sensor is presented, which provides excellent pressure sensing capabilities.

Field emission characteristics of ZnO nanowires grown by catalyst-assisted MOCVD on free-standing inorganic nanomembranes

ZnO field emitters on freely suspended inorganic nanomembranes (NMs) were synthesized by catalyst-assisted metal organic chemical vapor deposition (MOCVD) using the precursors zinc acetylacetonate hydrate and oxygen. The morphology and the possibly involved growth mechanisms of the randomly distributed ZnO nanostructures were investigated by scanning electron microscopy and by energy-dispersive x-ray spectroscopy. The findings indicate a growth process that involves the vapor–liquid–solid as well as the vapor–solid–solid mechanism. The field emission (FE) properties of such ZnO nanowire (NW) samples showed to be highly reproducible and independent of the investigated Si-based substrate types. Herein, a new type of flexible substrate for the MOCVD process has been introduced that allows for the direct growth of ZnO NWs on free-standing membranes for potential FE-based sensor applications. A turn-on field of 4.1 V µm−1 was detected for a macroscopic emission current density of 10 µA cm−2 and the stability test revealed fluctuations of only 9% around the mean emission current over a duration of 3 h proving a reliable and stable operation of such devices. Moreover, approaches were identified to further enhance the FE characteristics of the ZnO NWs by variation of the synthesis parameters and by enlargement of the NM area.

Combining 2D organic and 1D inorganic nanoblocks to develop free-standing hybrid nanomembranes for conformable biosensors

We report a simple approach to fabricate free-standing perforated 2D nanomembranes hosting well-ordered 1D metallic nanostructures to obtain hybrid materials with nanostructured surfaces for flexible electronics. Nanomembranes are formed by alternatively depositing perforated poly(lactic acid) (PLA) and poly(3,4-ethylenedioxythiophene) layers. Copper metallic nanowires (NWs) were incorporated into the nanoperforations of the top PLA layer by electrodeposition and further coated with silver via a transmetallation reaction. The combination of 2D polymeric nanomembranes and aligned 1D metallic NWs allows merging the flexibility and conformability of the ultrathin soft polymeric nanomembranes with the good electrical properties of metals for biointegrated electronic devices. Thus, we were able to tailor the nanomembrane surface chemistry as it was corroborated by SEM, EDX, XPS, CV, EIS and contact angle. The obtained hybrid nanomembranes were flexible and conformable showing sensing capacity towards H2O2 with good linear concentration range (0.35–10 mM), sensitivity (120 µA cm−2 mM−1) and limit of detection (7 μm). Moreover, the membranes showed good stability, reproducibility and selectivity towards H2O2.

Optical properties of freestanding GaN nanomembranes using monochromated valence-EELS

In the present study, the dielectric function of freestanding GaN nanomembranes was described using valence electron energy loss spectroscopy coupled to a monochromated electron probe in a Sub-Electron-volt Sub-Angstrom transmission electron Microscope. Such a technique has proven to be very efficient in handling nanometer-scale freestanding nanomembranes and provided local and direct measurements of optical characteristics. Indeed, core-state along with outer-shell electrons transitions were first measured in the configuration where the GaN c→-axis was normal to the transferred momentum q→. With the help of Kramers-Kroning analyses and density functional theory, critical points energies were extracted and assigned. Finally, optical data such as the real refractive index n, extinction coefficient k, absorption coefficient α and reflectivity R were numerically deduced from the real and imaginary parts of the dielectric function.

The controlled large-area synthesis of two dimensional metals

The rise of nanotechnology has been propelled by low dimensional metals. Albeit the long perceived importance, synthesis of freestanding metallic nanomembranes, or the so-called 2D metals, however has been restricted to simple metals with a very limited in-plane size (<10 μm). In this work, we developed a low-cost method to synthesize 2D metals through polymer surface buckling enabled exfoliation. The 2D metals so obtained could be as chemically complex as high entropy alloys while possessing in-plane dimensions at the scale of bulk metals (>1 cm). With our approach, we successfully synthesized a variety of 2D metals, such as 2D high entropy alloy and 2D metallic glass, with controllable geometries and morphologies. Moreover, our approach can be readily extended to non-metals and composites, thereby opening a large window to the fabrication of a wide range of 2D materials of technologic importance which have never been reported before.

Free-Standing Large-Area Nanoperforated Gold Membranes Fabricated by Hopping Electrodeposition

A room-temperature two-step cost-effective electrochemical technology is proposed for the preparation of free-standing Au nanomembranes. A thin Au film with the thickness less than 100 nm was deposited by pulsed electroplating on a GaAs substrate in the first step, while electrochemical etching was applied in the second technological step to introduce porosity into the GaAs substrate underneath the Au film. It has been shown that detachment of the film from the substrate occurs at optimized parameters of anodic etching. Scanning electron microscopy imaging of the deposited Au film revealed its nanoparticulate structure generated via the mechanism of hopping electrodeposition, i.e. the film proved to consist of a monolayer of Au nanoparticles with the mean diameter around 20–30 nm. It was found that nanoholes with the diameter controlled by the duration of negative voltage pulses can be introduced into the Au film during electroplating. The purity of the detached Au nanomembranes was demonstrated by the energy dispersive X-ray analysis. The flexibility, nanoparticulate structure along with possibilities to transfer the prepared nanomembranes to various substrates make them promising for new optical, plasmonic and electronic applications.

Molecular transport in ionic liquid/nanomembrane hybrids.

Ionic liquids and nanoscale membranes are both considered as promising functional components to design next-generation gas separation technologies. Herein, we combine free-standing carbon nanomembranes (CNMs) with [bmim][Tf2N] ionic liquid having affinity to carbon dioxide, and explore molecular permeation through such a composite membrane. Gas transport measurements reveal an increase in the transmembrane flux of carbon dioxide as compared to that of bare CNMs, whereas passage of helium is found to be suppressed in accordance with the solubility constants. Upon exposure to water vapor, the behavior of the hybrid membrane appears to differ strikingly as hydrophilic properties of CNMs are camouflaged by the hydrophobic nature of the ionic liquid. Kinetic simulations are conducted to account for the change in permeation mechanism, and the results agree with the experimental data obtained. Our study confirms that molecular transport in two-dimensional membranes can be tailored by imparting chemical functionalities, but at the same time highlights practical challenges in surface modification.

When finite-size effects dictate the growth dynamics on strained freestanding nanomembranes.

We investigate the influence of strain-sharing and finite-size effects on the morphological instability of hetero-epitaxial nanomembranes made of a thin film on a thin freestanding substrate. We show that long-range elastic interactions enforce a strong dependence of the surface dynamics on geometry. The instability time-scale τ is found to diverge as (e/H)−α with α = 4 (respectively 8) in thin (resp. thick) membranes, where e (resp. H) is the substrate (resp. nanomembrane) thickness, revealing a huge inhibition of the dynamics as strain sharing decreases the level of strain on the surface. Conversely, τ vanishes as H2 in thin nano-membranes, revealing a counter-intuitive strong acceleration of the instability in thin nanomembranes. Similarly, the instability length-scale displays a power-law dependence as (e/H)−β, with β = α/4 in both the thin and thick membrane limits. These results pave the way not only for experimental investigation, but also, for the dynamical control of the inescapable morphological evolution in epitaxial systems.