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We report the observation of fully magnetically polarized ultrathin La 0.8 Sr 0.2 MnO 3 films by using LaMnO 3 and La 0.45 Sr 0.55 MnO 3 buffer layers grown epitaxially on SrTiO 3 (001) substrates by molecular beam epitaxy. Specifically, we show that La 0.8 Sr 0.2 MnO 3 films grown on 12-unit-cell LaMnO 3 have bulk-like magnetic moments starting from a single unit cell thickness, while for the 15-unit-cell La 0.45 Sr 0.55 MnO 3 buffer layer, the La 0.8 Sr 0.2 MnO 3 transitions from an antiferromagnetic state to a fully spin-polarized ferromagnetic state at 4 unit cells. The magnetic results are confirmed by X-ray magnetic circular dichroism, while linear dichroic measurements carried out for the La 0.8 Sr 0.2 MnO 3 /La 0.45 Sr 0.55 MnO 3 series show the presence of an orbital reorganization at the transition from the antiferromagnetic to ferromagnetic state corresponding to a change from a preferred in-plane orbital hole occupancy, characteristic of the A-type antiferromagnetic state of La 0.45 Sr 0.55 MnO 3 , to preferentially out of plane. We interpret our findings in terms of the different electronic charge transfers between the adjacent layers, confined to the unit cell in the case of insulating LaMnO 3 and extended to a few unit cells in the case of conducting La 0.45 Sr 0.55 MnO 3 . Our work demonstrates an approach to growing ultrathin mixed-valence manganite films that are fully magnetically polarized from the single unit cell, paving the way to fully exploring the unique electronic properties of this class of strongly correlated oxide materials.

A potential approach to enhance the suppressed proton conductivity of nanoscale ultrathin Nafion films is to adjust the ionomer structure via regulating the catalyst-ionomer interaction. To understand the interaction between substrate surface charges and Nafion molecules, self-assembled ultrathin films (∼20 nm) were prepared on the SiO 2 model substrates, which were modified with silane coupling agents to carry either negative (COO - ) or positive (NH 3 ) charges. Specifically, the surface energy, phase separation, and proton conductivity were investigated by contact angle measurements, atomic force microscopy, and microelectrodes to illuminate the relationship between the substrate surface charge, thin-film nanostructure, and proton conduction. Compared to electrically neutral substrates, ultrathin films formed faster on the negatively charged substrate with an 83% increase in proton conductivity but formed more slowly on the positively charged substrate, with proton conductivity decreased by 35% at 50 °C. The surface charges interact with sulfonic acid groups of Nafion molecules to alter molecular orientation, resulting in different surface energies and phase separation, which are responsible for proton conductivity variation.+ ) charges. Specifically, the surface energy, phase separation, and proton conductivity were investigated by contact angle measurements, atomic force microscopy, and microelectrodes to illuminate the relationship between the substrate surface charge, thin-film nanostructure, and proton conduction. Compared to electrically neutral substrates, ultrathin films formed faster on the negatively charged substrate with an 83% increase in proton conductivity but formed more slowly on the positively charged substrate, with proton conductivity decreased by 35% at 50 °C. The surface charges interact with sulfonic acid groups of Nafion molecules to alter molecular orientation, resulting in different surface energies and phase separation, which are responsible for proton conductivity variation.

Fabrication of ultrathin films of dielectric (with particular reference to materials with high dielectric constants) materials has significance in many advanced technological applications including hard protective coatings, sensors, and next-generation logic devices. Current state-of-the-art in microelectronics for fabricating these thin films is a combination of atomic layer deposition and photolithography. As feature size decreases and aspect ratios increase, conformality of the films becomes paramount. Here, we show a polymer brush template-assisted deposition of highly conformal, ultrathin (sub 5 nm) high-κ dielectric metal oxide films (hafnium oxide and zirconium oxide) on topographically patterned silicon nitride substrates. This technique, using hydroxyl terminated poly-4-vinyl pyridine (P4VP-OH) as the polymer brush, allows for conformal deposition with uniform thickness along the trenches and sidewalls of the substrate. Metal salts are infiltrated into the grafted monolayer polymer brush films via solution deposition. Tailoring specific polymer interfacial chemistries for ion infiltration combined with subsequent oxygen plasma treatment enabled the fabrication of high-quality sub 5 nm metal oxide films.

The formation of ultrathin films of Rh-based porous metal-organic polyhedra (Rh-MOPs) by the Langmuir-Blodgett method has been explored. Homogeneous and dense monolayer films were formed at the air-water interface either using two different coordinatively alkyl-functionalized Rh-MOPs (HRhMOP(diz) 12 and HRhMOP(oiz) 12 ) or by in situ incorporation of aliphatic chains to the axial sites of dirhodium paddlewheels of another Rh-MOP (OHRhMOP) at the air-liquid interface. All these Rh-MOP monolayers were successively deposited onto different substrates in order to obtain multilayer films with controllable thicknesses. Aliphatic chains were partially removed from HRhMOP(diz) 12 films post-synthetically by a simple acid treatment, resulting in a relevant modification of the film hydrophobicity. Moreover, the CO 2 /N 2 separation performance of Rh-MOP-supported membranes was also evaluated, proving that they can be used as selective layers for efficient CO 2 separation.

The presence of inherently strong spin-orbit coupling in bismuth, its unique layer-dependent band topology and high carrier mobility make it an interesting system for both fundamental studies and applications. Theoretically, it has been suggested that strong quantum size effects should be present in the Bi(110) films, with the possibility of Dirac Fermion states in the odd-bilayer (BL) films, originating from dangling p z orbitals and quantum-spin hall (QSH) states in the even-bilayer films. However, the experimental verification of these claims has been lacking. Here, we study the electronic structure of Bi(110) films grown on a high- T c superconductor, Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) using angle-resolved photoemission spectroscopy (ARPES). We observe an oscillatory behavior of electronic structure with the film thickness and identify the Dirac-states in the odd-bilayer films, consistent with the theoretical predictions. In the even-bilayer films, we find another Dirac state that was predicted to play a crucial role in the QSH effect. In the low thickness limit, we observe several extremely one-dimensional states, probably originating from the edge-states of Bi(110) islands. Our results provide a much needed experimental insight into the electronic and structural properties of Bi(110) films.

Surface termination is known to play an important role in determining the physical properties of materials. It is crucial to know how surface termination affects the metal-insulator transition (MIT) of V 2 O 3 films for both fundamental understanding and its applications. By changing growth parameters, we achieved a variety of surface terminations in V 2 O 3 films that are characterized by low-energy electron diffraction (LEED) and photoemission spectroscopy techniques. Depending upon the terminations, our results show that MIT can be partially or fully suppressed near the surface region due to the different fillings of the electrons at the surface and subsurface layers and the change of screening length compared to the bulk. Across MIT, a strong redistribution of spectral weight and its transfer from a high-to-low-binding energy regime is observed in a wide energy scale. Our results show that the total spectral weight in the low-energy regime is not conserved across MIT, indicating a breakdown of the "sum rules of spectral weight", signature of a strongly correlated system. Such a change in spectral weight is possibly linked to the change in hybridization, lattice volume ( i.e ., effective carrier density), and the spin degree of freedom in the system that occurs across MIT. We find that MIT in this system is strongly correlation-driven, where the electron-electron interactions play a pivotal role. Moreover, our results provide better insight into the understanding of the electronic structure of strongly correlated systems and highlight the importance of accounting for surface effects during interpretation of the physical property data mainly using surface-sensitive probes, such as surface resistivity.

While the bulk structure of vapor-deposited glasses has been extensively studied, structure at buried interfaces has received little attention, despite being important for organic electronic applications. To learn about glass structure at buried interfaces, we study the structure of vapor-deposited glasses of the organic semiconductor DSA-Ph (1,4-di-[4-( N , N -diphenyl)amino]styrylbenzene) as a function of film thickness; the structure is probed with grazing incidence X-ray scattering. We deposit on silicon and gold substrates and span a film thickness range of 10-600 nm. Our experiments demonstrate that interfacial molecular packing in vapor-deposited glasses of DSA-Ph is more disordered compared to the bulk. At a deposition temperature near room temperature, we estimate ∼8 nm near the substrate can have modified molecular packing. Molecular dynamics simulations of a coarse-grained representation of DSA-Ph reveal a similar length scale. In both the simulations and the experiments, deposition temperature controls glass structure beyond this interfacial layer of a few nanometers.

Fabricating plasmon-enhanced organic nanomaterials with technologically relevant supporting architectures on planar solids remains a challenging task in the chemistry of thin films and interfaces. In this work, we report a bottom-up assembly of ultrathin layered composites of conductive polymers with photophysical properties enhanced by gold nanoparticles. The polydiacetylene component was formed by photopolymerization of a catanionic mixture of pentacosadiynoic surfactants on a surface of citrate-stabilized gold hydrosol monitored by a fiber optic spectrometer. Microscopic examination of the 3 nm thick solid-immobilized film showed that gold nanoparticles (AuNPs) do not aggregate within the monolayer upon polymerization. This polydiacetylene/AuNPs monolayer was coupled with 60 nm thick polyaniline-based layer deposited atop. The resulting polymer composite with an integrated 4-stripe electric cell showed nonadditive electric behavior due to the formation of electron-hole pairs with increased charge carrier mobility at the interface between the polymer layers. Under visible light irradiation of the composite film, a plasmonic effect of the gold nanoparticles was observed at the onset of photoconductivity, although neither polydiacetylene nor the polyaniline component alone are photoconductive polymers. The results indicate that our bottom-up strategy can be expanded to design other plasmon-enhanced ultrathin polymer composites with potential applications in optoelectronics and photovoltaics.

Ultrathin carbon nanotube films have gathered attention for flexible electronics applications. Unfortunately, their network structure changes significantly even under small applied strains. We perform mesoscopic distinct element method simulations and develop an atomic-scale picture of the network stress relaxation. On this basis, we put forward the concept of mesoscale design by the addition of excluded-volume interactions. We integrate silicon nanoparticles into our model and show that the nanoparticle-filled networks present superior stability and mechanical response relative to those of pure films. The approach opens new possibilities for tuning the network microstructure in a manner that is compatible with flexible electronics applications.

Atomic layer deposition (ALD) of ultrathin aluminum oxide (AlO x ) films was systematically studied on supported chemical vapor deposition (CVD) graphene. We show that by extending the precursor residence time, using either a multiple-pulse sequence or a soaking period, ultrathin continuous AlO x films can be achieved directly on graphene using standard H 2 O and trimethylaluminum (TMA) precursors even at a high deposition temperature of 200 °C, without the use of surfactants or other additional graphene surface modifications. To obtain conformal nucleation, a precursor residence time of >2s is needed, which is not prohibitively long but sufficient to account for the slow adsorption kinetics of the graphene surface. In contrast, a shorter residence time results in heterogeneous nucleation that is preferential to defect/selective sites on the graphene. These findings demonstrate that careful control of the ALD parameter space is imperative in governing the nucleation behavior of AlO x on CVD graphene. We consider our results to have model system character for rational two-dimensional (2D)/non-2D material process integration, relevant also to the interfacing and device integration of the many other emerging 2D materials.

Because of their immense scalability and manufacturability potential, the HfO2-based ferroelectric films attract significant attention as strong candidates for application in ferroelectric memories and related electronic devices. Here, we report the ferroelectric behavior of ultrathin Hf0.5Zr0.5O2 films, with the thickness of just 2.5 nm, which makes them suitable for use in ferroelectric tunnel junctions, thereby further expanding the area of their practical application. Transmission electron microscopy and electron diffraction analysis of the films grown on highly doped Si substrates confirms formation of the fully crystalline non-centrosymmetric orthorhombic phase responsible for ferroelectricity in Hf0.5Zr0.5O2. Piezoresponse force microscopy and pulsed switching testing performed on the deposited top TiN electrodes provide further evidence of the ferroelectric behavior of the Hf0.5Zr0.5O2 films. The electronic band lineup at the top TiN/Hf0.5Zr0.5O2 interface and band bending at the adjacent n(+)-Si bottom layer attributed to the polarization charges in Hf0.5Zr0.5O2 have been determined using in situ X-ray photoelectron spectroscopy analysis. The obtained results represent a significant step toward the experimental implementation of Si-based ferroelectric tunnel junctions.

Sorption of compressed gases into thin polymeric films is essential for applications including gas sensors and membrane based gas separation. For glassy polymers, the sorption behavior is dependent on the nonequilibrium status of the polymer. The uptake of molecules by a polymer is generally accompanied by dilation, or swelling, of the polymer material. In turn, this dilation can result in penetrant induced plasticization and physical aging that affect the nonequilibrium status of the polymer. Here, we investigate the dilation and sorption behavior of ultrathin membrane layers of a hybrid inorganic-organic network material that consists of alternating polyhedral oligomeric silsesquioxane and imide groups, upon exposure to compressed carbon dioxide and methane. The imide precursor contains fluoroalkene groups that provide affinity toward carbon dioxide, while the octa-functionalized silsesquioxane provides a high degree of cross-linking. This combination allows for extremely high sorption capacities, while structural rearrangements of the network are hindered. We study the simultaneous uptake of gases and dilation of the thin films at high pressures using spectroscopic ellipsometry measurements. Ellipsometry provides the changes in both the refractive index and the film thickness, and allows for accurate quantification of sorption and swelling. In contrast, gravimetric and volumetric measurements only provide a single parameter; this does not allow an accurate correction for, for instance, the changes in buoyancy because of the extensive geometrical changes of highly swelling films. The sorption behavior of the ultrathin hybrid layers depends on the fluoroalkene group content. At low pressure, the apparent molar volume of the gases is low compared to the liquid molar volume of carbon dioxide and methane, respectively. At high gas concentrations in the polymer film, the apparent molar volume of carbon dioxide and methane exceeds that of the liquid molar volume, and approaches that of the gas phase. The high sorption capacity and reversible dilation characteristics of the presented materials provide new directions for applications including gas sensors and gas separation membranes.

We present a new strategy to form thickness-adjusted and ultrasmooth films of very large and unwrinkled graphene oxide (GO) flakes through the transfer of both hemispherical and vertical water films stabilized by surfactants. With its versatility in terms of substrate type (including flexible organic substrates) and in terms of flake density (from isolated flakes to continuous and multilayer films), this wafer-scale assembly technique is adapted to a broad range of experiments involving GO and rGO (reduced graphene oxide). We illustrate its use through the evaluation of transparent rGO electrodes.