Free-standing Multilayers Research Articles

Oxidic 2D Materials.

The possibility of producing stable thin films, only a few atomic layers thick, from a variety of materials beyond graphene has led to two-dimensional (2D) materials being studied intensively in recent years. By reducing the layer thickness and approaching the crystallographic monolayer limit, a variety of unexpected and technologically relevant property phenomena were observed, which also depend on the subsequent arrangement and possible combination of individual layers to form heterostructures. These properties can be specifically used for the development of multifunctional devices, meeting the requirements of the advancing miniaturization of modern manufacturing technologies and the associated need to stabilize physical states even below critical layer thicknesses of conventional materials in the fields of electronics, magnetism and energy conversion. Differences in the structure of potential two-dimensional materials result in decisive influences on possible growth methods and possibilities for subsequent transfer of the thin films. In this review, we focus on recent advances in the rapidly growing field of two-dimensional materials, highlighting those with oxidic crystal structure like perovskites, garnets and spinels. In addition to a selection of well-established growth techniques and approaches for thin film transfer, we evaluate in detail their application potential as free-standing monolayers, bilayers and multilayers in a wide range of advanced technological applications. Finally, we provide suggestions for future developments of this promising research field in consideration of current challenges regarding scalability and structural stability of ultra-thin films.

Indentation size effects in graphene oxide under suspended nanoindentation

In this study, measurements of the elastic properties and the pre-stress of free-standing multilayers graphene oxide (GO) sheets were performed by suspended nanoindentation tests, using two indenter radius with values of approximately 50 nm and 636 nm, in an atomic force microscope and a triboindenter system respectively. The GO sheets were obtained by a modified Hummers’ method and deposited by drop-casting method over a specialized transmission electronic microscopy (TEM) grid. The GO sheets had about 3 layers, an average area of 3 μm2, and a roughness average, ra, of approximately 3 nm. The results showed that the GO sheets had differences in the magnitude of the mechanical properties, according to the number of layers. However, once the measurements were normalized based on their thickness, the bulk elastic modulus from both types of nanoindentation systems had a different value of approximately 130 GPa. Thus, it can be argued that also the mechanical behavior was influenced by the roughness as well as by magnitude of contact depth according to the tip radius size used. Besides, finite element (FE) simulations of both nanoindentations performed on a representative GO sheet, were carried out to verify the elastic modulus obtained experimentally, and then to approximate the pre-stresses involved in the regarding suspended nanoindentations. It was demonstrated that GO sheet roughness, as the tip radius, were also fundamental factors that played a significant role in the different mechanical properties obtained by the considered experimental and computational procedures.

Self-Assembly of Metal-Organic Framework (MOF) Nanoparticle Monolayers and Free-Standing Multilayers.

We report the first self-assembled porous monolayer and free-standing multilayer films composed of metal-organic framework (MOF) nanoparticles. Self-assembled MOF monolayers (SAMMs) were assembled at a liquid-air interface to produce films that are 87 wt % (89 vol %) MOF. Monolayer self-assembly was aided by growing a layer of poly(methyl methacrylate) (PMMA) on the particle surface using a histamine anchor. SAMMs could be stacked to obtain MOF multilayers, including alternating MOF/polymer heterostructures. SAMMs were coated on silicon microparticles, and a MOF film constructed of only five stacked layers could be manipulated as a free-standing, opalescent film. These monolayers are a significant advancement for obtaining highly functional porous membranes and coatings.

Semicoherent heterophase interfaces with core-spreading dislocation structures in magneto-electro-elastic multilayers under external surface loads

Using the extended Stroh formalism combining with the Fourier-transform and dual variable and position techniques, semicoherent interfaces are introduced to simulate the appropriate structural interface conditions in three-dimensional magneto-electro-elastic (MEE) multilayered composites under external loads. The field solutions caused by both internal and external actions are expressed in terms of biperiodic Fourier series expansions in multilayered systems that are made of dissimilar, linear and anisotropic plates with planar lattice mismatches. For non-perfectly bonded internal interfaces, the semicoherent heterophase interfaces are atomistically described by infinitely-long, parallel, periodically-spaced, and core-spreading intrinsic dislocation structures in free-standing multilayers. On the other hand, subsequent solutions for general loads on the external surfaces with coherent interfacial conditions are consistently derived and superposed to the former dislocation-induced fields.Three application examples in bi-, tri-, and six-layered MEE systems are investigated. The first bilayered case illustrates the significant effects of the semicoherent interface with dislocation core-spreading widths on the elastic, electric, and magnetic fields in the CoFe2O4/BaTiO3 composite materials. The second example is a trilayered system made of A/B/A with two discrete sets of intrinsic dislocation dipoles on their semicoherent interfaces, analyzed by proposing a novel energy based criterion. Critical dislocation spacings and thicknesses of capped interlayers are quantified in nanoscale MEE heterostructures with different stacking sequences, providing possible guides for stable growth of layered nanostructures. Finally, the complex distribution of field solutions in a six-layered MEE system with a prescribed externally mechanical load shows the crucial role played by the topological defects in localizing the coupled electric and magnetic components at the semicoherent interfaces, which indicates the important interplay between the internal and external actions. These resulting features open up novel opportunities for designing atomic-scale heterostructures with unprecedented MEE properties by engineering the interfacial dislocation patterns when the multilayered structures are free or under external action.

Intrinsic core level photoemission of suspended monolayer graphene

X-ray photoelectron spectroscopy of graphene is important both for its characterization and as a model for other carbon materials. Despite great recent interest, the intrinsic photoemission of its single layer has not been unambiguously measured, nor is the layer-dependence in free-standing multilayers accurately determined. We combine scanning transmission electron microscopy and Raman spectroscopy with synchrotron-based scanning photoelectron microscopy to characterize the same areas of suspended graphene samples down to the atomic level. This allows us to assign spectral signals to regions of precisely known layer number and purity. The core level binding energy of the monolayer is measured at 284.70 eV, thus 0.28 eV higher than that of graphite, with intermediate values found for few layers. This trend is reproduced by density functional theory with or without explicit van der Waals interactions, indicating that intralayer charge rearrangement dominates, but in our model of static screening the magnitudes of the shifts are underestimated by half.

THz time-domain spectroscopic investigations of thin films

THz time domain spectroscopy is a powerful technique enabling the investigation of different materials in the far-infrared frequency range. Even if nowadays this technique is well established, its application to very thin films remains particularly difficult. We investigated the utilization of THz spectroscopy on samples of micrometric thickness with the aim to disentangle multiple reflections and to measure with high accuracy the absolute values of the material parameters. We implemented an experimental and data analysis procedure that can be applied to free-standing single-layers or multi-layers samples.Specifically, we report on the experimental investigation by THz time domain spectroscopy of two samples: a test sample made of two layers of known thickness and materials; and a second sample, that is of a great interest for cultural heritage studies, made of a thin film of ink layered on a thicker support. Moreover, we describe in details the data analysis and fitting procedures needed to extract the material parameters from the experimental results.

Asymmetry of the free-standing polyelectrolyte multilayers

Free-standing polyelectrolyte multilayers (PEMs) triggered an intense research effort to develop functional capsules and membranes, nevertheless, the comprehensive understanding of the surface distinctions between the two sides of the free-standing PEMs has been rarely studied. In this paper, we demonstrate the asymmetric surface morphologies, compositions, surface charge and wetting properties of the free-standing PEMs made of alternating deposition of poly(diallyldimethylammonium chloride) (PDDA) and poly(styrenesulfonate) (PSS) on uncharged trimethoxy (octyl) silane modified silicon wafer (TMOS-Si) substrates. The growth behaviors (e.g., linear or exponential growth) of the (PDDA/PSS)n PEMs deposited on TMOS-Si substrates are similar to those deposited on negatively charged substrates except a slower evolution in morphology and thickness, evidenced by atomic force microscopy (AFM) and ellipsometry. The surface charge of the two sides can be tuned as+/+, −/-, and+/− by selecting the first and the terminate layer of the polyelectrolyte during the deposition process, indicated by X-ray photoelectron spectroscopy (XPS) and the surface zeta potential measured by spinning disk method. Further, water contact angle measurements exhibit a strong difference between the back side (the bottom side that was initially contacted with the substrate) (74±4°) and front side (33±2° and 48±3° for PDDA and PSS as the outmost layer, respectively) of the PEMs. The larger water contact angle of the back side is probably resulted from the orientation of the alkyl chains of the polyelectrolytes as the hydrophobic–hydrophobic interactions played significant roles in deposition of a polyelectrolyte onto an uncharged substrate. This paper may gain new insights on understanding surface properties of the PEMs.

Flipped polyelectrolyte multilayer films: accessing the buried interface.

Little is known concerning the interface between a polyelectrolyte multilayer, PEMU, and its substrate. Recent models suggest that excess polymer charge, compensated by counterions, remains buried within the PEMU, especially for thicker films having a nonlinear component to their growth. We report a novel approach for making free-standing multilayers of poly(diallyldimethylammonium) (PDADMA) and poly(styrenesulfonate) (PSS): after assembly on aluminum substrates, films were released by brief immersion in aqueous alkali. The multilayers were then flipped, allowing access to the initially buried substrate/PEMU interface. Experiments were performed to show that this method of release, one of many established for PEMUs, perturbed the surface and bulk of the film minimally. Film/solution and film/substrate interfaces were compared using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM was used to record topography and perform nanoindentation, while XPS provided surface elemental composition. All three methods revealed data consistent with an excess of PDADMA at the buried interface. This excess PDADMA was then complexed with additional PSS to yield "nanosandwiches" of nonstoichiometric PEMU between layers of stoichiometric PEMU.

Ferroelectric-ferromagnetic multilayers: A magnetoelectric heterostructure with high output charge signal

Multiferroic composites and heterostructures comprising ferroelectric and ferromagnetic materials exhibit room-temperature magnetoelectric (ME) effects greatly exceeding those of single-phase magnetoelectrics known to date. Since these effects are mediated by the interfacial coupling between ferroic constituents, the ME responses may be enhanced by increasing the density of interfaces and improving their quality. A promising material system providing these features is a ferroelectric-ferromagnetic multilayer with epitaxial interfaces. In this paper, we describe theoretically the strain-mediated direct ME effect exhibited by free-standing multilayers composed of single-crystalline ferroelectric nanolayers interleaved by conducting ferromagnetic slabs. Using a nonlinear thermodynamic approach allowing for specific mechanical boundary conditions of the problem, we first calculate the polarization states and dielectric properties of ferroelectric nanolayers in dependence on the lattice mismatch between ferroic constituents and their volume fractions. In these calculations, the ferromagnetic component is described by a model which combines linear elastic behavior with magnetic-field-dependent lattice parameters. Then the quasistatic ME polarization and voltage coefficients are evaluated using the theoretical strain sensitivity of ferroelectric polarization and measured effective piezomagnetic coefficients of ferromagnets. For Pb(Zr0.5Ti0.5)O3−FeGaB and BaTiO3−FeGaB multilayers, the ME coefficients are calculated numerically as a function of the FeGaB volume fraction and used to evaluate the output charge and voltage signals. It is shown that the multilayer geometry of a ferroelectric-ferromagnetic nanocomposite opens the way for a drastic enhancement of the output charge signal. This feature makes biferroic multilayers advantageous for the development of ultrasensitive magnetic-field sensors for technical and biomedical applications.

Thermodynamically destabilized hydride formation in “bulk” Mg–AlTi multilayers for hydrogen storage

Thermodynamic destabilization of MgH2 formation through interfacial interactions in free-standing Mg-AlTi multilayers of overall "bulk" (0.5 μm) dimensions with a hydrogen capacity of up to 5.5 wt% is demonstrated. The interfacial energies of Mg-AlTi and Mg-Ti (examined as a baseline) are calculated to be 0.81 and 0.44 J m(-2). The enhanced interfacial energy of AlTi opens the possibility of creating ultrathin alloy interlayers that provide further thermodynamic improvements in metal hydrides.

Low-energy electron transmission imaging of clusters on free-standing graphene

We investigated the utility of free-standing graphene as a transparent sample carrier for imaging nanometer-sized objects by means of low-energy electron holography. The sample preparation for obtaining contamination-free graphene as well as the experimental setup and findings are discussed. For incoming electrons with 66 eV kinetic energy, graphene exhibits 27% opacity per layer. Hence, electron holograms of nanometer-sized objects adsorbed on free-standing graphene can be recorded and numerically reconstructed to reveal the object's shapes and distribution. Furthermore, a Moiré effect has been observed with free-standing graphene multi-layers.

Free-standing spectral purity filters for extreme ultraviolet lithography

Free-standing multilayer films consisting of Si, Zr, Mo and silicides of both metals have been fabricated and studied as spectral purity filters (SPF) for extreme ultraviolet (EUV) (13.5 nm) lithography tools. Comparative tests of multilayer SPF structures of various compositions have been performed at high power loads. It was found that a Mo/ZrSi2 structure with MoSi2 capping layers is featured with capability to withstand prolonged heating in vacuum (10−7 mbar) at 900-950°C. A technique of fabrication of large aperture free-standing multilayers was developed, and a pilot sample of the above film structure of 160 mm in diameter, 50 nm thick, with transparency at 13.5 nm above 70% was fabricated as a conceptual prototype of SPFs with large dimensions.

Free-standing ferroelectric multilayers: Crossover from thin-film to bulk behavior

Ferroelectric films usually have phase states and physical properties very different from those of bulk ferroelectrics. Here we propose free-standing ferroelectric-elastic multilayers as a bridge between these two material systems. Using a nonlinear thermodynamic theory, we determine phase states of such multilayers as a function of temperature, misfit strain, and volume fraction φp of passive elastic material. The numerical calculations performed for two classical ferroelectrics—PbTiO3 and BaTiO3—demonstrate that polarization states of multilayers in the limiting cases φp→1 and φp→0 coincide with those of thin films and bulk crystals. At intermediate volume fractions, however, the misfit strain-temperature phase diagrams of multilayers differ greatly from those of epitaxial films. Remarkably, a ferroelectric phase not existing in thin films and bulk crystals can be stabilized in BaTiO3 multilayers. Owing to additional tunable parameter and reduced clamping, ferroelectric multilayers may be superior for a wide range of device applications.

Biocompatible, Detachable, and Free-Standing Polyelectrolyte Multilayer Films

Self-assembled polyelectrolyte multilayers have gained tremendous popularity over the past decade and have been incorporated in diverse applications. However, the fabrication of detachable and free-standing polyelectrolyte multilayers (PEMs) has proven to be difficult. We report the design of detachable, free-standing, and biocompatible PEMs comprised of hyaluronic acid (anionic PE) and chitosan (cationic PE). These PEMs can be detached from an underlying inert substrate without any postprocessing steps. Our approach enables the fabrication of detachable PEMs from a wide range of polyelectrolytes. Cross-linked PEMs exhibited greater than 95% weight retention when maintained in phosphate buffered saline at 37 °C over a seven day period. The PEM thickness was approximately 3 μm for dried films and increased 2-fold under hydration. A unique feature of the detachable, free-standing PEMs is their optical transparency in the 400-900 nm range under hydrated conditions. The Young's modulus of the cross-linked films ranged from 300-400 MPa, rendering these detachable free-standing multilayers ideal for biomaterial applications. BALB/c 3T3 fibroblasts adhered on the PEMs and colonized the entire surface over a six day period. The cellular responses, as well as the physical properties, demonstrate that the detachable PEM films exhibit tremendous potential for applications in biomaterials and tissue engineering.

Free-Standing Nanocomposite Multilayers with Various Length Scales, Adjustable Internal Structures, and Functionalities

We introduce an innovative and robust method for the preparation of nanocomposite multilayers, which allows accurate control over the placement of functional groups as well as the composition and dimensions of individual layers/internal structure. By employing the photocross-linkable polystyrene (PS-N(3), M(n) = 28.0 kg/mol) with 10 wt % azide groups (-N(3)) for host polymer and/or the PS-N(3)-SH (M(n) = 6.5 kg/mol) with azide and thiol (-SH) groups for capping ligands of inorganic nanoparticles, nanocomposite multilayers were prepared by an efficient photocross-linking layer-by-layer process, without perturbing underlying layers and nanostructures. The thickness of individual layers could be controlled from a few to hundreds of nanometers producing highly ordered internal structure, and the resulting nanocomposite multilayers, consisting of polymer and inorganic nanoparticles (CdSe@ZnS, Au, and Pt), exhibit a variety of interesting physical properties. These include prolonged photoluminescent durability, facile color tuning, and the ability to prepare functional free-standing films that can have the one-dimensional photonic band gap and furthermore be patterned by photolithography. This robust and tailored method opens a new route for the design of functional film devices based on nanocomposite multilayers.

Polyelectrolyte multilayer hollow capsules studied by small-angle neutron scattering (SANS)

Silica-templated polyelectrolyte (PE) hollow capsules with various template diameters and PE layer numbers were fabricated by using the layer-by-layer technique and subsequent core dissolution in hydrofluoric acid. The properties of the resulting freestanding polyelectrolyte multilayers (PEM) were characterized in an aqueous environment by means of SANS and compared with scanning force microscopy (SFM) data. The thickness of the capsule wall was found to be about 25% thicker and with correspondingly higher water content than the template-supported PE layers. The wall thickness increase as a result of core dissolution was anisotropic. The average single layer thickness of hollow capsules was independent of the surface curvature and decreased slightly with increasing PE layer number. SANS was used to determine whether the capsules were empty or contained the rest of the core. An annealing at 70 °C for 4 h induced capsule shrinking by about 20% at an ionic strength of 1 mol L−1. Furthermore, the capsule wall upon annealing increased in thickness by about 38%. These changes corresponded to a wall densification of about 13%.

Sputter deposited nanocrystalline Ni-25Al alloy thin films and Ni/Ni 3Al multilayers

Thin films of nominal composition Ni-25at%Al have been sputter deposited from a target of the intermetallic compound Ni 3Al at different substrate deposition temperatures. The film deposited on an unheated substrate exhibited a strongly textured columnar growth morphology and consisted of a mixture of metastable phases. Nanoindentation studies carried out on this film exhibited a strong strain hardening tendency. In contrast, the film deposited at 200 °C exhibited a recrystallized non-textured microstructure consisting of grains of a partially ordered Ni 3Al phase. At higher deposition temperatures (∼400 °C), larger grains of the bulk equilibrium, long-range ordered, Ll 2 Ni 3Al phase were observed in the film. Unlike the film deposited on an unheated substrate, the films deposited at elevated temperatures did not exhibit any dependence of the hardness on the indentation depth and, consequently no strain hardening. The average hardness of the film deposited at 200 °C was higher than the one deposited at 400 °C. In addition to monolithic Ni-25Al thin films, multilayered Ni/Ni3Al thin films were also deposited. Multilayers deposited non-epitaxially on unheated substrates exhibited a strong {111} fiber texture while those deposited epitaxially on (001) NaCl exhibited a {001} texture. Free-standing multilayers of both types of preferred orientations as well as of different layer thicknesses were deformed in tension untill fracture. Interestingly, the {111} oriented multilayers failed primarily by a brittle fracture while the {001} multilayers exhibited features of ductile fracture.

On the size dependence of the critical point of nanoscale interstitial solid solutions

It is shown that a size dependence of the critical temperature Tc of the miscibility gap in nanocrystalline and nanoscale particle interstitial solid solutions results from stress owing to elastic interaction of the bulk with layers of material at the grain boundaries or surfaces which have a small solute susceptibility (i.e. a weak dependence of the concentration on the chemical potential) at the phase transition. When the volume fraction occupied by the interfacial layers is not too large, then the changes in Tc and in the critical concentration xc can be predicted on the basis of a series expansion of the solute chemical potential in the bulk about the critical point. The model can be extended to free-standing thin films and coherent multilayers. The dependence of the pressure on the hydrogen concentration in the crystal lattice of nanocrystalline palladium-hydrogen is measured on the basis of X-ray diffraction data. The result agrees with the predictions of the theory.

The dynamics of interface relaxation and destratification in AgNi multilayers

Resistance and length changes were measured during thermal treatments on freestanding AgNi multilayers. The experiments allowed a clear separation of the successive stages of the microstructural evolution of these stratified structures: interfacial relaxation, destratification and grain growth. The kinetics of these processes were extracted by monitoring resistivity and length during thermal cycles and isotherms. A substantial densification was revealed during the interface relaxation, that results in a slowing down of the local reorganization process. An early diffusional creep was also shown; its occurrence is restricted to the destratification region.