Ultrathin Polymer Films Research Articles

Metal-Insulator Transition of Ultrathin Sputtered Metals on Phenolic Resin Thin Films: Growth Morphology and Relations to Surface Free Energy and Reactivity.

Nanostructured metal assemblies on thin and ultrathin polymeric films enable state of the art technologies and have further potential in diverse fields. Rational design of the structure–function relationship is of critical importance but aggravated by the scarcity of systematic studies. Here, we studied the influence of the interplay between metal and polymer surface free energy and reactivity on the evolution of electric conductivity and the resulting morphologies. In situ resistance measurements during sputter deposition of Ag, Au, Cu and Ni films on ultrathin reticulated polymer films collectively reveal metal–insulator transitions characteristic for Volmer–Weber growth. The different onsets of percolation correlate with interfacial energy and energy of adhesion weakly but as expected from ordinary wetting theory. A more pronounced trend of lower percolation thickness for more reactive metals falls in line with reported correlations. Ex situ grazing incidence small angle X-ray scattering experiments were performed at various thicknesses to gain an insight into cluster and film morphology evolution. A novel approach to interpret the scattering data is used where simulated pair distance distributions of arbitrary shapes and arrangements can be fitted to experiments. Detailed approximations of cluster structures could be inferred and are discussed in view of the established parameters describing film growth behavior.

Formation of Flat‐on Lamellar Crystals in Absence of Nanoconfinement

AbstractFlat‐on lamellar crystals are crucial for gas‐barrier polymer films. The formation of this lamellar crystalline structure with chains perpendicular to the substrate is currently understood as the result of confined crystallization, e.g., in block copolymers and ultrathin polymer films. In this paper, it is demonstrated that these flat‐on lamellar crystals of various thermoplastic polymers may form on stainless steel and silicon wafer surfaces without the presence of confined crystallization. Atomic force microscopy, high‐resolution scanning electron microscopy, and polarized light hot‐stage microscopy are used to characterize the formation of the lamellar crystals. Further results show that the surface physicochemical properties of the substrates strongly influence the formation of these lamellar crystals. A hypothesis, based on the heterogeneous crystallization theory, is proposed to explain the formation of such flat‐on lamellar crystals. These results are crucial for a fundamental understanding of the formation of lamellar crystals and may provide a new approach to fabricate such structures.

B/N-Enriched Semi-Conductive Polymer Film for Micro-Supercapacitors with AC Line-Filtering Performance.

Microsupercapacitors (MSCs) have drawn great attention for use as miniaturized electrochemical energy storage devices in portable, wearable, as well as implantable electronics. Many materials have been developed as electrodes for MSCs. However, the thin-film fabrication for most of these materials involves multistep operations, including filtration, spray coating, and sputtering. Most importantly, these methods present challenges for the preparation of thin films at the atomic or molecular scale. Therefore, the understanding of performance of ultrathin-film-based MSCs remains challenge. Herein, a B/N-enriched polymer film is successfully prepared using the photoassisted interfacial approach. The as-synthesized polymer film exhibits typical semiconductive characteristics and can be easily scaled up to a large area of up to tens of square centimeters. This ultrathin polymer film can be directly transferred to silicon wafers to fabricate MSC through laser scribing. The prepared MSC exhibits specific volumetric capacitance as high as 20.9 F cm-3, corresponding to volumetric energy density of 2.9 mWh cm-3 (at 0.1 V s-1). Moreover, the volumetric power density can reach 1461 W cm-3, surpassing most existing semiconductive polymer film-based MSC devices. In addition, the prepared MSC exhibits typical AC line-filtering ability (-67° at 120 Hz). This study offers a facile interfacial approach to preparing semiconductive polymer films with aromatic moieties for microsized energy storage devices.

Two-dimensional ferroelectrics and homogeneous switching. On the 75th anniversary of the Landau–Ginzburg theory of ferroelectricity

Within the framework of the Landau–Ginzburg (LG) theory, the kinetics of polarization switching in ferroelectric crystals and the transition from domain switching to homogeneous switching in nanosized single-crystal films are considered. It is shown that homogeneous (nondomain) switching can be described in LG theory terms only for two-dimensional ferroelectrics. A review of the experimental results for two-dimensional films of a ferroelectric polymer and barium titanate is given. For ultrathin polymer films, these results are also confirmed by first principles calculations.

Quantification of molecular weight discrimination in grafting to reactions from ultrathin polymer films by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

In the present study, a reliable and robust method was developed to quantify the molecular weight discrimination that can occur in grafting to reactions via indirect MALDI-TOF quantification of the molecular weights of grafted chains by comparing the characteristics of the polymeric material before the grafting reaction with those of the unreacted material recovered after grafting. Two polystyrene samples with different molecular weights and narrow molecular weight distributions were employed to prepare model blends that were grafted to silicon wafers and an analytical method was developed and validated to assess and quantify the modification of the molecular weight distribution that takes place during the grafting to process. Particular attention was paid to the standardization of the sample treatment and to find the best data collection and calibration methodologies in order to have statistically significant data even in the presence of a very scarce amount of the sample. Furthermore, to evaluate the accuracy of the analytical procedure, the lack of suitable standard and certified materials required a further experiment to be carried out by comparing the new optimized MALDI-TOF method and direct measurements using TGA-GC-MS on a model blend containing deuterated and hydrogenated polystyrene samples with appropriate molecular weights and distributions. The optimized method was applied on samples obtained by a thermally induced grafting to reaction from ultrathin polymer films and, for the first time, to our knowledge, an enrichment effect occurring in the ultrathin grafted layer obtained from a melt was evidenced.

Визуализация проводящих каналов в полимерных слоях методом атомно-силовой микроскопии с проводящим зондом

The results of an experimental study of the local electrophysical properties of ultrathin polymer films by atomic force microscopy with a conducting probe are presented. It is established that visualization of current flow sites (conducting channels) is possible in areas from which the surface layer has been mechanically removed. The conducting channels in the current image have the form of individual points with a height corresponding to the locally flowing current. It is found that the location of the observed channels correlates well with the model of conductivity along the grain boundaries of the supramolecular structure of the polymer.

Non-linear dynamics and self-similarity in the rupture of ultra-thin viscoelastic liquid coatings

The influence of viscoelasticity on the dewetting of ultrathin polymer films is unraveled by means of theory and numerical simulations in the inertialess limit. Three viscoelastic models are employed to analyse the dynamics of the film, namely the Oldroyd-B, Giesekus, and FENE-P models. We revisit the linear stability analysis first derived by [Tomar et al., Eur. Phys. J. E., 2006, 20, 185-200] for a Jeffrey's film to conclude that all three models formally share the same dispersion relation. For times close to the rupture singularity, the self-similar regime recently discovered [Moreno-Boza et al., Phys. Rev. Fluids, 2020, 5, 014002], where the dimensionless minimum film thickness scales with the dimensionless time until rupture as hmin = 0.665τ1/3, is asymptotically established independently of the rheological model. The spatial structure of the flow is characterised by a Newtonian core and a thin viscoelastic boundary layer at the free surface, where polymeric stresses become singular as τ → 0. The Deborah number and the solvent-to-total viscosity ratio affect the rupture time but not the length scale of the resulting dewetting pattern and asymptotic flow structure close to rupture, which is thus shown to be universal. Our three-dimensional simulations lead us to conclude that bulk viscoelasticity alone does not explain the experimental observations of dewetting of polymeric films near the glass transition.

Interfacial Dynamics in Supported Ultrathin Polymer Films-From the Solid to the Free Interface.

Molecular dynamics in ultrathin layers is investigated using nanostructured electrodes to perform broadband dielectric spectroscopy measurements, and by atomistic molecular dynamics simulations. Using poly(vinyl acetate) as the model system and taking advantage of access to the distribution of relaxation times in an extended temperature range above the glass transition temperature, Tg, we demonstrate that while the mean rates of the segmental relaxation remain bulklike down to 12 nm film thickness, modified molecular mobilities arise in the interfacial zones. Combining results from simulations and experiments, we show unambiguously that both the slow relaxations arising from adsorbed polymer segments and the faster modes attributed to segments in the vicinity of the free interface have non-Arrhenius temperature activation. These interfacial regions span thicknesses of ∼1.5 nm each just above the calorimetric Tg independent of molecular weight and film thickness. These deviations at interfaces are relevant for applications of polymers in adhesion, coatings, and polymer nanocomposites.

Ultrathin polymeric films for interfacial passivation in wide band-gap perovskite solar cells

Wide band-gap perovskite solar cells have the potential for a relatively high output voltage and resilience in a degradation-inducing environment. Investigating the reasons why high voltages with adequate output power have not been realized yet is an underexplored part in perovskite research although it is of paramount interest for multijunction solar cells. One reason is interfacial carrier recombination that leads to reduced carrier lifetimes and voltage loss. To further improve the Voc of methylammonium lead tri-bromide (MAPbBr3), that has a band-gap of 2.3 eV, interface passivation technique is an important strategy. Here we demonstrate two ultrathin passivation layers consisting of PCBM and PMMA, that can effectively passivate defects at the TiO2/perovskite and perovskite/spiro-OMeTAD interfaces, respectively. In addition, perovskite crystallization was investigated with the established anti-solvent method and the novel flash infrared annealing (FIRA) with and without passivation layers. These modifications significantly suppress interfacial recombination providing a pathway for improved VOC’s from 1.27 to 1.41 V using anti solvent and from 1.12 to 1.36 V using FIRA. Furthermore, we obtained more stable devices through passivation after 140 h where the device retained 70% of the initial performance value.

Electrolyte Transfer at a Liquid-Solid Polypyrrole Interface Quantified By in Situ X-Ray Reflectometry

In situ characterization of the electrochemical (EC) intercalation mechanism of ions in conductive polymer films is one of the most important aspects in the development of novel organic supercapacitors. Compared to atomic force microscopy and quartz microbalance measurements, synchrotron-based in situ X-ray reflectometry (XRR) allows deeper and more detailed insights into electrically controlled transfer processes of electrolytes across liquid-solid interfaces. The low roughness of the layer system to be investigated, which is required for XRR, is a particular challenge: Electrochemically deposited polypyrrole (PPy) films naturally possess a rough surface, the typical cauliflower structure. In the present study, PPy was deposited on a silicon crystal with a fast-varying voltage to obtain a PPy film with a very smooth surface, which led in parallel to a significantly denser film, which also increases the XRR contrast. This 30 nm thin PPy film was measured in situ in a 0.1M HClO4 environment with XRR at different voltage levels between -0.2 and +0.5V vs Ag/AgCl and the film thickness and electron density were determined. A linear and reversible change in thickness and electron density was measured, which is typical for the potential-dependent intercalation process of anions. The number of electrons transferred across the electrolyte-PPy interface was determined from the change in thickness and electron density, from which the transport of matter was derived. The correlation with the electrically measured transferred charge resulted in a ratio that was significantly below the expected value for exclusive anion exchange. Considering all present and available matter, it can be deduced that proton transport partially compensates for the charge and that 2-3 water molecules per incorporated perchlorate anion are removed from the film. Fast XRR measurements on the same PPy film during applied cyclovoltammetry showed two intervals of different behavior of the PPy film: In the higher voltage range anion intercalation dominates, the PPy film shows a very high pseudo-capacity due to its high density, while in the lower voltage range proton exchange starts and reduces the pseudo-capacity. Electrochemical impedance spectroscopy confirmed these results: Above the transition of these two voltage ranges, the conductivity of the PPy film increases dramatically due to the intercalation of perchlorate anions. With the presented synchrotron-based, in situ EC-XRR setup the film thickness, the electron density, and the change of both, as well as the pseudo-capacitance and conductivity as a function of the applied voltage can be measured simultaneously on ultra-thin electrochemical polymer films. Especially for the development of new electrochemical systems this is a highly potent measurement technique for in situ characterization.

Cobweb-like, Ultrathin Porous Polymer Films for Ultrasensitive NO2 Detection.

Gas sensors based on polymer field-effect transistors (FETs) have drawn much attention owing to the inherent merits of specific selectivity, low cost, and room temperature operation. Ultrathin (<10 nm) and porous polymer semiconductor films offer a golden opportunity for achieving high-performance gas sensors. However, wafer-scale fabrication of such high-quality polymer films is of great challenge and has rarely been realized before. Herein, the first demonstration of 4 in. wafer-scale, cobweb-like, and ultrathin porous polymer films is reported via a one-step phase-inversion process. This approach is extremely simple and universal for constructing various ultrathin porous polymer semiconductor films. Thanks to the abundant pores, ultrathin size, and high charge-transfer efficiency of the prepared polymer films, our gas sensors exhibit many superior advantages, including ultrahigh response (2.46 × 106%), low limit of detection (LOD) (<1 ppm), and excellent selectivity. Thus, the proposed fabrication strategy is exceptionally promising for mass manufacturing of low-cost high-performance polymer FET-based gas sensors.

Capillary deformation of ultrathin glassy polymer films by air nanobubbles

Confined glasses and their anomalous interfacial rheology raise important questions in fundamental research and numerous practical applications. In this Letter, we study the influence of interfacial air nanobubbles on the free surface of ultrathin high-molecular-weight glassy polystyrene films immersed in water, in ambient conditions. In particular, we reveal the counterintuitive fact that a soft nanobubble is able to deform the surface of a rigid glass, forming a nanocrater with a depth that increases with time. By combining in-situ atomic-force-microscopy measurements and a modified lubrication model for the liquid-like layer at the free surface of the glass, we demonstrate that the capillary pressure in the nanobubble together with the liquid-like layer at the free surface of the glass determine the spatiotemporal growth of the nanocraters. Finally, from the excellent agreement between the experimental profiles and the numerical solutions of the governing glassy thin-film equation, we are able to precisely extract the surface mobility of the glass. In addition to revealing and quantifying how surface nanobubbles deform immersed glasses, until the latter eventually dewet from their substrates, our work provides a novel, precise, and simple measurement of the surface nanorheology of glasses.

How Does Monomer Structure Affect the Interfacial Dynamics of Supported Ultrathin Polymer Films?

We utilize recently introduced chemically specific but coarse-grained models of poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) to explore the influence of monomer architecture on t...

Stability of Fluid Ultrathin Polymer Films in Contact with Solvent-Loaded Gels for Cultural Heritage.

The removal of ultrathin amorphous polymer films in contact with an aqueous gelled solution containing small amounts of good solvent is addressed by means of specular and off-specular neutron reflectometry. The distribution of heavy water and benzyl alcohol is revealed inside Laropal A81, often employed as a protective varnish layer for Culture Heritage in the restoration of easel paintings. The swelling kinetics, interface roughness, and film morphologies were recorded as a function of temperature and increasing benzyl alcohol concentration in the dispersion of Pemulen TR-2, a hydrophobically modified acrylic acid copolymer. The addition of small amounts of good solvent results in the appearance of water-filled cavities inside the varnish, which grow with time. It is shown that while increasing the solvent concentration greatly enhances the hole growth kinetics, an increase in temperature above the glass transition temperature does not have such a big effect on the kinetics.

Effect of Interfacial Adsorption on the Stability of Thin Polymer Films in a Solvent-induced Process

The stability of ultrathin polymer films plays a crucial role in their technological applications. Here, we systematically investigated the influence of interfacial adsorption in physical aging and the stability of thin polymer films in the solvent-induced process. We further identify the stability mechanism from the theory of thin film stability. Our results show that the aging temperature and film thickness can strongly influence the stability of thin PS films in acetone vapor. Physical aging can greatly improve the stability of thin polymer films when the aging temperature Taging1 > Tg. A thinner PS film more quickly reaches a stable state via physical aging. At short aging time, the formation of the adsorbed layer can reduce the polar interaction; however, it slightly influences the stability of thin polymer films in the solvent-induced process. At later aging stage, the conformational rearrangement of the polymer chains induced by the interfacial effect at the aging temperature Taging1 plays an important role in stabilizing the thin polymer films. However, at Taging2 < Tg, the process of physical aging slightly influences the stability of the thin polymer films. The formation of the adsorbed layer at Taging2 can reduce the short-range polar interaction of the thin film system and cannot suppress the instability of thin polymer films in the solvent-induced process. These results provide further insight into the stable mechanism of thin polymer films in the solvent-induced process.

Superior Dynamic Penetration Resistance of Nanoscale Multilayer Polymer/Metal Films

Abstract Recent advances in experimental techniques have enabled impact tests of ultrathin films. For example, microprojectile impact tests of ultrathin polymer films have revealed that their specific penetration energy is about ten times more than that of the conventional armor materials. On the other hand, metallic nanostructures have demonstrated extraordinary mechanical properties. These observations suggest that multilayer arrangements of nanoscale polymer and metal films could possess superior ballistic impact resistance. In order to test this hypothesis, we simulated the impact tests of multilayer aluminum-polyurea nanostructures using molecular dynamics (MD). Our simulations demonstrate that the ballistic limit velocity (V50) and the specific penetration energy of the multilayers and aluminum nanofilms are significantly higher than the experimentally measured values for any material. In order to further investigate the mechanisms associated with the observed superior ballistic performance of multilayers, we computed their V50 using an existing membrane model and another analytical model reflecting a two-stage penetration process. Our results demonstrate a potential bottom-up design pathway for developing flexible barrier materials with superior dynamic penetration resistance.

Polarization Switching in 2D Nanoscale Ferroelectrics: Computer Simulation and Experimental Data Analysis.

The polarization switching kinetics of nanosized ferroelectric crystals and the transition between homogeneous and domain switching in nanoscale ferroelectric films are considered. Homogeneous switching according to the Ginzburg-Landau-Devonshire (LGD) theory is possible only in two-dimensional (2D) ferroelectrics. The main condition for the applicability of the LGD theory in such systems is its homogeneity along the polarization switching direction. A review is given of the experimental results for two-dimensional (2D) films of a ferroelectric polymer, nanosized barium titanate nanofilms, and hafnium oxide-based films. For ultrathin 2D ferroelectric polymer films, the results are confirmed by first-principle calculations. Fitting of the transition region from homogeneous to domain switching by sigmoidal Boltzmann functions was carried out. Boltzmann function fitting data enabled us to correctly estimate the region sizes of the homogeneous switching in which the LGD theory is valid. These sizes contain several lattice constants or monolayers of a nanosized ferroelectrics.

Continuous polymer films deposited on top of porous substrates using plasma-enhanced atomic layer deposition and molecular layer deposition

Molecular layer deposition (MLD) is able to produce ultrathin polymer films with control over thickness, cross-linking, and chemical composition. With these capabilities, MLD should be useful in the fabrication of novel polymer membranes on porous supports. However, confining a continuous MLD film to the surface of porous substrates is difficult because of MLD film growth in the pores. The deposition in the pores lowers the conductance of the porous support. This paper presents a method to deposit continuous polymer films on top of porous substrates. In this method, Al2O3 plasma-enhanced atomic layer deposition (PE-ALD) using trimethylaluminum and oxygen plasma as the reactants was first used to cap the pores of the substrate. Subsequently, a polyamide MLD film was deposited on the Al2O3 PE-ALD capping layer using m-phenylenediamine and trimesoyl chloride as the reactants. The Al2O3 pore caps were then removed from the porous substrate by etching from the backside using a timed exposure to a dilute sodium hydroxide solution. This method was demonstrated using anodic aluminum oxide (AAO) and polyethersulfone (PES) porous substrates. Al2O3 PE-ALD film growth was limited to the top of the porous substrate, resulting in rapid surface recombination or high sticking coefficients for the reactive plasma species within the pores. Gas permeance measurements confirmed the pore capping of the AAO substrates. The reopening of the pores by dissolving the Al2O3 pore caps with a sodium hydroxide solution was monitored using gas permeance versus etch time. The removal of the Al2O3 pore caps from the PES substrates could also dissolve the Al2O3 layer underneath the MLD film. The loss of this Al2O3 layer led to the detachment of the MLD film from the PES substrate. However, the MLD film could be anchored to the PES support at fractures located in the Al2O3 film prior to the MLD. The Al2O3 film fracture allowed the MLD film to anchor firmly to the PES substrate by MLD in the pores of the PES porous substrate. The distance between the anchor points was a function of fracture density. This distance could be controlled by applying a tensile stress to the Al2O3 PE-ALD film to fracture the film through sample bending. This method produced firmly anchored polymer MLD films on top of the PES porous substrates.

Molecular dynamics study of the mechanical behaviour of ultrathin polymer–metal multilayers under extreme dynamic conditions

Recent microprojectile impact tests have revealed that ultra-thin polymer films possess superior specific penetration energy, about ten times more than that of conventional armour materials. On the other hand, it is well established that metallic nanostructures demonstrate extraordinary mechanical properties. Therefore, it may be possible to achieve structures with superior dynamic performance by arranging ultrathin polymer and metal films in multilayer architectures. In order to test this hypothesis, we conducted molecular dynamics simulations of ballistic impact tests of multilayer aluminium-polyurea nanostructures. The computed specific penetration energy of a polymer–metal bilayer is 42% higher than the highest experimentally measured value for any material at a similar impact speed. Moreover, an aluminium layer confined between two polyurea layers has approximately 40% less impact-induced dislocations compared to an aluminium layer which is in contact with only one polyurea layer. This observation clearly demonstrates that the polymer exerts remarkable resistance on the dislocation nucleation in the metallic layer. Our results propose a potential bottom-up design pathway to develop structures with superior dynamic performance.

Imprint and transfer fabrication of freestanding plasmonic membranes

This paper reports an imprint and transfer approach for the rapid and inexpensive fabrication of the ultra-thin freestanding plasmonic membrane (FPM) that supports surface plasmon resonances. The imprint and transfer fabrication method involves the soft imprint lithography on an ultrathin polymer film, transfer of the perforated polymer film to a supporting frame, subsequent deposition of gold, and final removal of the polymer film. Without using any sophisticated lithography and etching processes, the imprint and transfer method can produce freestanding gold membranes with 2D arrays of submicrometer-sized holes that support plasmonic modes in the mid-wavelength infrared (mid-IR) range. Two FPM devices with an array constant of 4.0 and 2.5 μm have been simulated, fabricated, and measured for their transmittance characteristics. The fabricated FPMs exhibit surface plasmon polariton Bloch mode and extraordinary optical transmission (EOT) with the enhanced local field around the membrane. The effects of membrane thickness and angle dispersion on the FPM were investigated to show the tuning of EOT modes in IR. Furthermore, we demonstrated the refractometric sensing and enhanced IR absorption of the FPM device for its potential in chemical and biomolecule sensing applications.