Free-standing Films Research Articles

Comparison of Au Nanoparticle/Poly(9-vinylcarbazole) Thin-Film Electrogeneration at 3 Distinct Liquid/Liquid Interfaces: Water/1,2-Dichloroethane, /α,α,α-Trifluorotoluene, Or/Ionic Liquid.

Metal nanoparticle (NP) incorporated conductive polymer films are attractive for their mechanical stability for biomedical applications and as heterogeneous electrocatalysis materials. Novel approaches to generate these materials with tunable properties are still being sought. Herein, the interface between two immiscible electrolyte solutions (ITIES) has been employed as a molecularly sharp and reproducible platform for simultaneous Au NP and poly(9-vinylcarbazole) generation. Three interfaces have been compared, including between water|1,2-dichloroethane (w|DCE), water|α,α,α-trifluorotoluene (w|TFT), and water|ionic liquid (w|IL). In this case the IL was P8888TB (tetraoctylphosphonium tetrakis(pentafluorophenyl)borate). 9-Vinylcarbazole (VC) can polymerize via two routes, either propagating through the vinyl substituent or the aryl rings. The former gives rise to a white semiconducting polymer with a wide bandgap, while the latter produces a green, conducting polymer. External potential control through voltammetric cycling was found to generate the film more rapidly favoring heterogeneous electron transfer with formation of the green poly(VC) variant at the ITIES. This was a free-standing film that could be easily removed from the interface. In the absence of external control, white polymer crystals formed within the oil phase spontaneously likely via AuCl4- w → o transfer followed by a homogeneous electron transfer reaction mechanism. Scanning electrochemical microscopy probe approach curve experiments were used to quantify the electroactivity of the film and are complemented by direct conductivity measurements.

Anomalously High Apparent Young's Modulus of Ultrathin Freestanding PZT Films Revealed by Machine Learning Empowered Nanoindentation.

The mechanical properties at small length scales are not only significant for understanding the intriguing size-dependent behaviors but also critical for device applications. Nanoindentation via atomic force microscopy is widely used for small-scale mechanical testing, yet determining the Young's modulus of quasi-2D films from freestanding force-displacement curve of nanoindentation remains challenging, complicated by both bending and stretching that are highly nonlinear. To overcome these difficulties, a machine learning model is developed based on the back propagation (BP) neural network and finite element training to accurately determine the Young's modulus, pretension, and thickness of freestanding films from nanoindentation force-displacement curves simultaneously, improving the computational efficiency by two orders of magnitude over conventional brute force curve fitting. Using this technique, anomalously high apparent Young's modulus is discovered of 2.8nm thick PbZr0.2Ti0.8O3 (PZT) film as large as 229.4±12.1GPa, much higher than the bulk value. The enhancement can be attributed to the strain gradient-induced flexoelectric effect, and the corresponding flexoelectric coefficient is estimated to be ≈200nCm-1. The method is developed to enable an artificial intelligence (AI) system to study the mechanical properties of a wide range of low-dimensional materials.

Efficient Growth of 1D Van der Waals Heterostructures on Zeolite-Supported SWCNTs.

The controlled synthesis of 1D van der Waals (1D vdW) heterostructures, specifically single-walled carbon nanotubes encapsulated within boron nitride nanotubes (SWCNT@BNNT), presents a challenge due to an incomplete understanding of the factors influencing BNNT growth. This study investigates the growth yield of SWCNT@BNNT heterostructures produced using zeolite-supported SWCNT templates on SiO2-coated Transmission Electron Microscopy (TEM) grids, which enable in situ synthesis and thorough evaluation of each step without compromising the nanotube structure. The high-resolution TEM analysis reveals a significant improvement in BNNT coverage on individual nanotubes, increasing from 9% to 42%, through optimization of the ammonia borane precursor amount. A thorough comparison of BNNT growth efficiency is performed using zeolite-supported SWCNT templates against the current benchmark of free-standing SWCNT films, highlighting comparable synthesis yields despite different morphologies and SWCNT diameter distributions. By integrating all Raman and TEM data from the studied SWCNT@BNNT samples, a correlation between BNNT coverage and the intensity of the buckling R-mode of BNNTs is established, providing a reliable criterion for evaluating BNNT growth efficiency in 1D vdW heterostructures. This work advances the understanding and characterization of 1D vdW heterostructures, offering insights into synthesizing hetero-nanotubes using other types of SWCNT templates and paving the way for their diverse applications.

Route to Enhancing Remote Epitaxy of Perovskite Complex Oxide Thin Films.

Remote epitaxy is taking center stage in creating freestanding complex oxide thin films with high crystallinity that could serve as an ideal building block for stacking artificial heterostructures with distinctive functionalities. However, there exist technical challenges, particularly in the remote epitaxy of perovskite oxides associated with their harsh growth environments, making the graphene interlayer difficult to survive. Transferred graphene, typically used for creating a remote epitaxy template, poses limitations in ensuring the yield of perovskite films, especially when pulsed laser deposition (PLD) growth is carried out, since graphene degradation can be easily observed. Here, we employ spectroscopic ellipsometry to determine the critical factors that damage the integrity of graphene during PLD by tracking the change in optical properties of graphene in situ. To mitigate the issues observed in the PLD process, we propose an alternative growth strategy based on molecular beam epitaxy to produce single-crystalline perovskite membranes.

Supercritical mechano-exfoliation process

The intricate balance among cost, output, and quality has substantially hindered the practical application of graphene within the downstream industry chain. Here we present a scalable and green supercritical CO2-assisted mechano-exfoliation (SCME) process that omits the use of organic solvents and oxidants throughout the production lifecycle, including exfoliation, separation, and purification. The SCME process achieves graphene powder space-time yields exceeding 40 kg/(m³·day) at laboratory (0.06–0.2 kg) and pilot scales ( > 4 kg), with resultant free-standing films showing conductivities up to 5.26 × 10⁵ S/m. Further kinetic investigations propose general guidelines for grinding-assisted exfoliation: (1) the macroscopic optimizing ability of mechanotechnics for mass transfer frequency and stress distribution and (2) the microscopic multiplication ability of exfoliation medium for shear-delamination. The comprehensive techno-economic analysis also underscores the economic viability of the SCME process for large-scale production.

Epitaxial Thin Film Growth on Recycled SrTiO3 Substrates Toward Sustainable Processing of Complex Oxides.

Complex oxide thin films cover a range of physical properties and multifunctionalities that are critical for logic, memory, and optical devices. Typically, the high-quality epitaxial growth of these complex oxide thin films requires single crystalline oxide substrates such as SrTiO3 (STO), MgO, LaAlO3, a-Al2O3, and many others. Recent successes in transferring these complex oxides as free-standing films not only offer great opportunities in integrating complex oxides on other devices, but also present enormous opportunities in recycling the deposited substrates after transfer for cost-effective and sustainable processing of complex oxide thin films. In this work, the surface modification effects introduced on the recycled STO are investigated, and their impacts on the microstructure and properties of subsequently grown epitaxial oxide thin films are assessed and compared with those grown on the pristine substrates. Detailed analyses using high-resolution scanning transmission electron microscopy and geometric phase analysis demonstrate distinct strain states on the surfaces of the recycled STO versus the pristine substrates, suggesting a pre-strain state in the recycled STO substrates due to the previous deposition layer. These findings offer opportunities in growing highly mismatched oxide films on the recycled STO substrates with enhanced physical properties. Specifically, yttrium iron garnet (Y3Fe5O12) films grown on recycled STO present different ferromagnetic responses compared to that on the pristine substrates, underscoring the effects of surface modification. The study demonstrates the feasibility of reuse and redeposition using recycled substrates. Via careful handling and preparation, high-quality epitaxial thin films can be grown on recycled substrates with comparable or even better structural and physical properties toward sustainable process of complex oxide devices.

Multiferroic Microstructure Created from Invariant Line Constraint

AbstractFerroic materials enable a multitude of emerging applications, and optimum functional properties are achieved when ferromagnetic and ferroelectric properties are coupled to a first‐order ferroelastic transition. In bulk materials, this first‐order transition involves an invariant habit plane, connecting coexisting phases: austenite and martensite. Theory predicts that this plane should converge to a line in thin films, but experimental evidence is missing. Here, the martensitic and magnetic microstructure of a freestanding epitaxial magnetic shape memory film is analyzed. It is shown that the martensite microstructure is determined by an invariant line constraint using lattice parameters of both phases as the only input. This line constraint explains most of the observable features, which differ fundamentally from bulk and constrained films. Furthermore, this finite‐size effect creates a remarkable checkerboard magnetic domain pattern through multiferroic coupling. The findings highlight the decisive role of finite‐size effects in multiferroics.

Fabrication and characterization of bioresorbable, electroactive and highly regular nanomodulated cell interfaces

Biomaterial-based implantable scaffolds capable of promoting physical and functional reconnection of injured spinal cord and nerves represent the latest frontier in neural tissue engineering. Here, we report the fabrication and characterization of self-standing, biocompatible and bioresorbable substrates endowed with both controlled nanotopography and electroactivity, intended for the design of transient implantable scaffolds for neural tissue engineering. In particular, we obtain conductive and nano-modulated poly(D,L-lactic acid) (PLA) and poly(lactic-co-glycolic acid) free-standing films by simply iterating a replica moulding process and coating the polymer with a thin layer of poly(3,4-ethylendioxythiophene) polystyrene sulfonate. The capability of the substrates to retain both surface patterning and electrical properties when exposed to a liquid environment has been evaluated by atomic force microscopy, electrochemical impedance spectroscopy and thermal characterizations. In particular, we show that PLA-based films maintain their surface nano-modulation for up to three weeks of exposure to a liquid environment, a time sufficient for promoting axonal anisotropic sprouting and growth during neuronal cell differentiation. In conclusion, the developed substrates represent a novel and easily-tunable platform to design bioresorbable implantable devices featuring both topographic and electrical cues.

Mechanically Stable and Damage Resistant Freestanding Ultrathin Silver Nanowire Films with Closely Packed Crossed-Lamellar Structure

Mechanically Stable and Damage Resistant Freestanding Ultrathin Silver Nanowire Films with Closely Packed Crossed-Lamellar Structure

Free-standing sodium niobate nanowires film: Morphological and chemical surface characterization

This work presents the development and characterization of a novel free-standing sodium niobate nanowires (SNN) film. The material is obtained by hydrothermal synthesis, and a methodology to detach the SNN film from the substrate was developed. The obtained free-standing film exhibits flexibility and two distinct surfaces, the bulk-side and the free-side. Another aspect of the SNN film is its adhesive property, which is observed only on the free-side. The FEG-SEM characterization showed that the bulk-side has a spongy morphology, whereas the free-side presents a nanostructured surface in the form of nanowires with a thickness of 28.96 ± 5.4 nm and the cross-sectional image of the film revealed a thickness of 16.25 µm ± 1.01 µm. Wettability was also determined, and it shows that the bulk-side is less hydrophilic than the free-side. Besides, it was possible to determine that the two surfaces exhibited distinct chemical compositions by XPS and ToF-ERDA analyses. In addition, it is worth highlighting that the interplay between the free-standing nature, the adhesive property, and the nanostructured morphology of the SNN film can provide great versatility of applications.

Mechanistic Insights into the Potentiodynamic Electrosynthesis of PEDOT Thin Films at a Polarizable Liquid|Liquid Interface.

Conducting polymer (CP) thin films find widespread use, for example in bioelectronic, energy harvesting and storage, and drug delivery technology. Electrosynthesis at a polarizable liquid|liquid interface using an aqueous oxidant and organic soluble monomer provides a route to free-standing and scalable CP thin films, such as poly(3,4-ethylenedioxythiophene) (PEDOT), in a single step at ambient conditions. Here, using the potentiodynamic technique of cyclic voltammetry, interfacial electrosynthesis involving ion exchange, electron transfer, and proton adsorption charge transfer processes is shown to be mechanistically distinct from CP electropolymerization at a solid electrode|electrolyte interface. During interfacial electrosynthesis, the applied interfacial Galvani potential difference controls the interfacial concentration of the oxidant, but not the CP redox state. Nevertheless, typical CP electropolymerization electrochemical behaviors, such as steady charge accumulation with each successive cycle and the appearance of a nucleation loop, were observed. By combining (spectro)electrochemical measurements and theoretical models, this work identifies the underlying mechanistic origin of each feature on the cyclic voltammograms (CVs) due to charge accumulated from Faradaic and capacitive processes as the PEDOT thin film grows. To prevent overoxidation during interfacial electrosynthesis with a powerful cerium aqueous oxidant, scan rates in excess 25 mV·s-1 were optimal. The experimental methodology and theoretical models outlined in this article provide a broadly generic framework to understand evolving CVs during interfacial electrosynthesis using any suitable oxidant/monomer combination.

Water vapor responsiveness of chitosan: An experimental and simulation analysis.

Stimuli-responsive polymers have gained significant research interest in recent years owing to their potential applications in diverse areas. Here, we present a study on the actuation characteristics of chitosan-based free-standing films that exhibit full reversibility and repeatability in response to water vapor exposure. The effect of pH of the water and the degree of cross-linking of the chitosan films on the actuation performance is studied. In the case of free-standing polymer film-based actuators, the primary driving force behind actuation is understood to be the differential strain induced by the gradient in volume changes across the thickness of the film. To understand it further, we conducted full atomistic molecular dynamics simulation studies to explore water absorption and adsorption into the chitosan matrix. Our simulations revealed an accumulation of water molecules in the surface layer that rapidly desorb when shielded from water vapor. Furthermore, estimates of the energy gain resulting from the adsorption of water on the surface suggest that it is adequate to drive the shape change of the actuator when subjected to asymmetric exposure to water vapor. This finding supports the fact that the adsorbed layer of water on the surface of the chitosan film plays a role in actuation.

All-day freshwater harvesting enabled by designing benzoxazine molecule with intrinsic photothermal conversion and radiation cooling ability

The importance of clean water resources for human health and economic development cannot be overlooked. Providing a way for fully passive all-day freshwater harvesting presents a significant challenge. Herein, we propose a strategy for all-day freshwater harvesting passively through a freestanding Janus film which combining solar-driven interfacial desalination during day time and dew water generation driven by passive radiative cooling at night. Specifically, the core of the strategy was a new bio-based benzoxazine which containing high-infrared emittance in the atmospheric window and the resultant cured black freestanding film owns a steady high light absorbance in the whole range of the solar irradiation wavelength. The home-made device made by the Janus film achieved a drinkable freshwater collection efficiency of 90.9 % when desalinating seawater under 1 sun illumination and achieved a dew water collection rate of 68.36 g m−2 day−1 at night. Moreover, the film has an excellent antibacterial effect, which ensures the quality of water collection and enhances the service life. Combining the advantages of bio-based raw materials, a simple fabrication process and scalability of the film, the all-day freshwater collection system makes a promising solution to alleviate freshwater shortage in many arid regions, islands and even boats sailing on the sea.

Burst plasma preparation of metallic nanoparticles on carbon fabrics for antibacterial and electrocatalytic applications

Metal nanoparticles have extraordinary properties, but their integration into mesostructures has been challenging. Producing uniformly dispersed nanoparticles attached to substrates in industrial quantities is difficult. Herein, a “plasmashock” method was developed to synthesize metal nanoparticles anchored on different types of carbonaceous substrates using liquid salt solution precursors. These self-supporting, nanoparticle-loaded carbon fabrics are mechanically robust and have been tested as antibacterial substrates and electrocatalysts for reducing carbon dioxide and nitrite. A piece of silver–carbon nanotube paper with a silver loading of ~0.13 mg cm−2 treated after a few-second plasmashock presents good antibacterial and electrocatalytic properties in wastewater, even after 20 bactericidal immersion cycles, due to the strong bonding of the nanoparticles to the substrate. The results prove the effectiveness of this plasmashock method in creating free-standing functional composite films or membranes.

Preparation of freestanding BaTiO3 nanocrystalline films and investigation of the effect of sintering on their dielectric properties

A paste was developed using commercially available BaTiO3 nanoparticles and subsequently used to fabricate 5 cm × 5 cm freestanding films via a simple screen-printing method. The dielectric properties of the resultant freestanding films were investigated using sintering temperature as a parameter. The results showed that the desired bulk behavior could not be attained in films sintered at 900 °C. When the sintering temperature was increased to 1000 °C and 1100 °C, the films exhibited bulk and single-crystal-like behaviors; sintering at 1200 °C resulted in a film with dielectric properties superior to those of bulk and single-crystalline BaTiO3. In addition, the dielectric loss was less than 0.04 in the temperature range from room temperature to 160 °C, indicating that the film is a highly practical freestanding film. The newly developed technology for forming freestanding films with thicknesses ranging from tens to hundreds of micrometers can be used to develop not only materials for multilayered ceramic capacitors but also materials for piezoelectric transducers, actuators, positive-temperature-coefficient thermistors, and ultrasonic devices.

Atomic Spalling of a van der Waals Nanomembrane.

ConspectusThe vertical integration of van der Waals nanomembranes (vdW NMs), composed of two-dimensional (2D) layered materials and three-dimensional (3D) freestanding films with vdW surfaces, opens new avenues for exploring novel physical phenomena and offers a promising pathway for prototyping ultrathin, superior-performance electronic and optoelectronic applications with unique functionalities. Achieving the desired functionality through vdW integration necessitates the production of high-quality individual vdW NMs, which is a fundamental prerequisite. A profound understanding of the synthetic strategies for vdW NMs, along with their fundamental working principles, is crucial in guiding the experimental design toward 3D integrated heterostructures. The foremost synthetic challenges in fabricating high-quality vdW NMs are achieving exact control over thickness and ensuring surface planarity on the atomic scale. Despite the development of numerous chemical and mechanical approaches to tackle these issues, an all-encompassing solution has yet to be realized. To address these challenges, we have developed advanced spalling techniques, specifically known as atomic spalling or 2D material-based layer transfer, which emerge as a promising technology for achieving both atomically precise thickness-engineered and atomically smooth vdW NMs. These techniques involve engineering the interfacial fracture toughness and strain energy in the vdW system, allowing for precise control over the initiation and the propagation of cracks within the vdW material based on controlled spalling theory.In this Account, we summarize our recent advancements in the atomic precision spalling technique for the preparation of vdW NMs and their applications. We begin by introducing the fundamentals of advanced spalling techniques, which are based on spalling mode fracture in bilayer systems. Following this, we succinctly describe the preparation methods for source materials for vdW NMs, with a primary focus on chemical synthesis approaches. We then delve into the working principles underlying our recent contributions to advanced spalling techniques, providing insights into how this method attains unprecedented atomic-precision control compared to other fabrication methods with a particular emphasis on tuning the interface between the stressor and the vdW system. Subsequently, we highlight cutting-edge applications based on vdW heterostructures, which combine our spalled vdW NMs. Finally, we discuss the current challenges and future directions for advanced spalling techniques, underscoring their potential to be established as a robust methodology for the preparation of high-quality vdW NMs. Our advanced spalling strategy not only ensures the reliable production of vdW NMs with exceptional control over thickness and atomic-level flatness but also provides a robust theoretical framework essential for producing high-quality vdW NMs.

TiO2 Films with Macroscopic Chiral Nematic-Like Structure Stabilized by Copper Promoting Light-Harvesting Capability for Hydrogen Generation.

Cellulose nanocrystals (CNCs) have inspired the synthesis of various advanced nanomaterials, opening opportunities for different applications. However, a simple and robust approach for transferring the long-range chiral nematic nanostructures into TiO2 photocatalyst is still fancy. Herein, a successful fabrication of freestanding TiO2 films maintaining their macroscopic chiral nematic structures after removing the CNCs biotemplate is reported. It is demonstrated that including copper acetate in the sol avoids the epitaxial growth of the lamellar-like structure of TiO2 and stabilizes the chiral nematic structure instead. The experimental results and optical simulation demonstrate an enhancement at the blue and red edges of the Fabry-Pérot reflectance peak located in the visible range. This enhancement arises from the light scattering effect induced by the formation of the chiral nematic structure. The nanostructured films showed 5.3 times higher performance in the photocatalytic hydrogen generation, compared to lamellar TiO2, and benefited from the presence of copper species for charge carriers' separation. This work is therefore anticipated to provide a simple approach for the design of chiral nematic photocatalysts and also offers insights into the electron transfer mechanisms on TiO2/CuxO with variable oxidation states for photocatalytic hydrogen generation.

Surface-deformability dependent contact time of bouncing droplets on sessile soap bubbles

HypothesisThe curvature of the free-standing liquid film is expected to modify its surface deformability, thereby affecting droplet bouncing dynamics and possibly tuning the liquid repellency performance in practical applications. ExperimentsIn this study, the bouncing dynamics of water droplets on sessile soap bubbles with different curvatures has been experimentally investigated using high-speed camera. FindsTo resist the impacting droplets, the soap bubbles is observed to show two types of deformation: the geometrical deformation caused by the total impacting force and the pressure distribution induced deformation from the droplet dynamics. The variation trend of the contact time of the droplet with the impact velocity is found to be highly dependent on the surface deformability of the soap bubble. This trend becomes non-monotonic when the soap bubble is large and more deformable. The decreasing contact time with increasing impact velocity can be well captured by a phenomenological model of coupled springs. The increasing contact time for the large soap bubbles at high impact velocities is due to the further compression of the soap bubble by the recoiling droplet, leading to the decoupling of the above two types of bubble deformation.

Significantly enhanced electrochemical performance of Al-Ti3C2Tx MXene synthesized from Al-Ti3AlC2 MAX phase

MXenes, a type of two-dimensional nanomaterials consisting of transition metal carbides/nitrides, have emerged as promising electrode materials for supercapacitors. Especially, Ti3C2Tx MXene prepared from Ti3AlC2 MAX phase shows great application potential in flexible supercapacitors due to its facile synthesis, excellent conductivity and film-forming property. In contrast to conventional Ti3AlC2, excessive aluminum is incorporated in the synthesis process to produce Al-Ti3AlC2. In this study, Ti3AlC2 and Al-Ti3AlC2 MAX phases are used to synthesize Ti3C2Tx and Al-Ti3C2Tx MXenes, respectively, by the same method of HF:HCl etching and LiCl intercalation. The freestanding Ti3C2Tx and Al-Ti3C2Tx film electrode is separately prepared through simple vacuum filtration. The presence of excess aluminum during the Al-Ti3AlC2 synthesis facilitates to form more homogeneous grains structure with relatively larger particle sizes and an improved stoichiometric MAX phase with a Ti:C ratio closer to 3:2. The resulting Al-Ti3C2Tx nanosheets show improved oxidation resistance with relatively uniform and larger dimensions, which facilitate the higher conductivity with slightly increased interlayer spacing for the Al-Ti3C2Tx film. Therefore, the Al-Ti3C2Tx film electrode shows more efficient ions transport and charge transfer, and thus achieves significantly enhanced capacitance (399 F g−1 at 1 A g−1) and rate performance (63.6 % retention from 1 to 10 A g−1) compared to the Ti3C2Tx film electrode (342 F g−1 at 1 A g−1, 55.6 % retention from 1 to 10 A g−1). Moreover, the Al-Ti3C2Tx-based flexible sandwiched supercapacitor also exhibits superior energy storage performance. The impressive results indicate that Al-Ti3C2Tx MXene synthesized from Al-Ti3AlC2 MAX phase possesses the great application potential in flexible supercapacitors.

Self-Repeatable snapping liquid–crystal-elastomer actuator

Liquid crystal elastomers (LCEs) show potential for use in sensors and robotics owing to their reversible stimuli response. However, current actuators have limitations such as low power and slow recovery. Incorporating snap-through transitions induced by mechanical instability into motion can enhance the speed and power of soft actuators. However, the snap-through transitions’ single-shot motion response without external assistance at complicates their application to continuous and self-repeatable actuation. This study introduces a promising and simple approach for creating a rapid and self-repeatedly regulated snapping motion in an LCE actuator by confining the LCE film in a frame. We explore the confinement effect of a freestanding pre-curved LCE film that smoothly changes its bending direction and shows anticlastic buckling. Interestingly, a self-repeating snapping motion is generated by attaching a rigid film onto the LCE film, thereby inducing synclastic deformation. Remarkably, the up-and-down bending snapping motion occurs at a very high speed (within 30 ms) and is self-regulated depending on the temperature of the bottom surface. By utilizing this thermally responsive snapping LCE actuator, we successfully demonstrate an organic-based novel fire alarm and a self-propelled soft-walk mechanism.