Interlayer Registry Research Articles

Production of Lipid Constructs by Design via Three-Dimensional Nanoprinting.

Atomic force microscopy (AFM) in conjunction with microfluidic delivery was utilized to produce three-dimensional (3D) lipid structures following a custom design. While AFM is well-known for its spatial precision in imaging and 2D nanolithography, the development of AFM-based nanotechnology into 3D nanoprinting requires overcoming the technical challenges of controlling material delivery and interlayer registry. This work demonstrates the concept of 3D nanoprinting of amphiphilic molecules such as 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Various formulations of POPC solutions were tested to achieve point, line, and layer-by-layer material delivery. The produced structures include nanometer-thick disks, long linear spherical caps, stacking grids, and organizational chiral architectures. The POPC molecules formed stacking bilayers in these constructions, as revealed by high-resolution structural characterizations. The 3D printing reached nanometer spatial precision over a range of 0.5 mm. The outcomes reveal the promising potential of our designed technology and methodology in the production of 3D structures from nanometer to continuum, opening opportunities in biomaterial sciences and engineering, such as in the production of 3D nanodevices, chiral nanosensors, and scaffolds for tissue engineering and regeneration.

Effective Method for a Graphene Oxide with Impressive Selectivity in Carboxyl Groups.

The development of new applications of graphene oxide in the biomedical field requires the covalent bonding of bioactive molecules to a sheet skeleton. Obtaining a large carboxyl group population over the surface is one of the main targets, as carboxyl group concentration in conventional graphene oxide is low among a majority of non-useful sp3-C-based functionalities. In the present work, we propose a selective method that yields an impressive increase in carboxyl group population using single-layer, thermally reduced graphene oxide as a precursor in a conventional Hummers–Offemann reaction. When starting with a reduced graphene oxide with no interlayer registry, sulfuric acid cannot form a graphite intercalated compound. Then, potassium permanganate attacks in in-plane (vacancies or holes) structural defects, which are numerous over a thermally reduced graphene oxide, as well as in edges, yielding majorly carboxyl groups without sheet cutting and unzipping, as no carbon dot formation was observed. A single-layer precursor with no ordered stacking prevents the formation of an intercalated compound, and it is this mechanism of the potassium permanganate that results in carboxyl group formation and the hydrophilic character of the compound.

Pattern Development and Control of Strained Solitons in Graphene Bilayers

Engineering strain and interlayer registry in 2D crystals have been demonstrated as effective controls of their properties. Separation of domains with different interlayer registries in graphene bilayer has been reported, but the pattern control of strained solitons has not yet been achieved. We show here that, by pulling a graphene bilayer apart, soliton structures with a regularly modulated interlayer registry arise from the competition between elastic deformation in monolayers and local slip at the van der Waals interfaces. The commensurate-incommensurate transition with strain localization is identified as the interlayer overlap exceeds a critical size, where the continuum description of load transfer through the tension-shear chain breaks down. Birth, development and annihilation processes of the strained solitons can be controlled by the loading conditions. The effects of lattice symmetry and mechanical constraints are also discussed, completing the picture for microstructural evolution processes in the homo- or heterostructures of 2D crystals.

Investigating the Formation of MoSe2 and TiSe2 Films from Artificially Layered Precursors.

The reaction of ultrathin layers of Mo and Ti with Se was investigated, and significantly different reaction pathways were found. However, in both systems postdeposition annealing results in smooth dichalcogenide films with specific thicknesses determined by the precursor. X-ray diffraction (XRD) patterns of as-deposited Mo|Se films around a 1:2 ratio of Mo to Se contain weak, broad reflections from small and isolated MoSe2 crystallites that nucleated during deposition and a sharper intensity maximum resulting from the composition modulation created from the alternating deposition of Mo and Se layers. In contrast, as-deposited Ti|Se films around a 1:2 ratio of Ti to Se contain narrow and intense 00l reflections from TiSe2 crystallites and do not contain a Bragg reflection from the sequence of deposited Ti|Se layers. The as-deposited TiSe2 crystallites have a larger c-axis lattice parameter than was previously reported for TiSe2, however, which suggests a poor vertical interlayer registry and/or high defect densities including interstitial atoms. In-plane XRD patterns show the nucleation of both TiSe2 and Ti2Se during deposition, with the Ti2Se at the substrate. For both systems, annealing the precursors decreases the peak width and increases the intensity of reflections from crystalline TiSe2 and MoSe2. Optimized films consist of a single phase after the annealing and show clear Laue oscillations in the specular XRD patterns, which can only occur if a majority of the diffracting crystallites in the film consist of the same number of unit cells. The highest quality films was obtained when an excess of ∼10% Se was deposited in the precursor, which presumably acts as a flux to facilitate diffusion of metal atoms to crystallite growth fronts and compensates for Se loss to the open system during annealing.

Semiempirical van der Waals method for two-dimensional materials with incorporated dielectric functions

A density functional theory based semiempirical van der Waals (vdW) method with dielectric functions being incorporated is developed for two-dimensional materials. The coefficients of interatomic pairwise potentials are derived from atomic polarizabilities obtained via a Clausius-Mossotti relation dedicated for layered crystals. The method not only can efficiently describe the dispersion energy for a range of planar graphene-like materials at nearly the same accuracy as the adiabatic connection fluctuation-dissipation theorem, but also rationalizes experimentally measured relative interfacial strengths of heterostructures and interlayer registry of hexagonal boron nitride that have plagued other vdW methods.

Interlayer locking and atomic-scale friction in commensurate small-diameter boron nitride nanotubes

Density functional theory applied to small-diameter boron nitride nanotubes (BNNTs) finds out-of-plane structural buckling in contrast to large-diameter tubes that exhibit faceting. Buckling significantly affects interlayer interactions in commensurate double-walled BNNTs. Energy corrugation amplitudes in relative motions of BNNT walls change up to fourfold, depending on interlayer registry relaxation, in marked contrast to carbon nanotubes. Large differences between relaxed and unrelaxed energy corrugations of BNNTs could yield energy dissipation via the strain-induced anelastic relaxation of interlayer locking (or ``lattice kinks''), which can be exploited for mechanical damping applications.

Multiwalled nanotube faceting unravelled.

Nanotubes show great promise for miniaturizing advanced technologies. Their exceptional physical properties are intimately related to their morphological and crystal structure. Circumferential faceting of multiwalled nanotubes reinforces their mechanical strength and alters their tribological and electronic properties. Here, the nature of this important phenomenon is fully rationalized in terms of interlayer registry patterns. Regardless of the nanotube identity (that is, diameter, chirality, chemical composition), faceting requires the matching of the chiral angles of adjacent layers. Above a critical diameter that corresponds well with experimental results, achiral multiwalled nanotubes display evenly spaced extended axial facets whose number equals the interlayer difference in circumferential unit cells. Elongated helical facets, commonly observed in experiment, appear in nanotubes that exhibit small interlayer chiral angle mismatch. When the wall chiralities are uncorrelated, faceting is suppressed and outer layer corrugation, which is induced by the Moiré superlattice, is obtained in agreement with experiments. Finally, we offer an explanation for the higher incidence of faceting in multiwalled boron nitride nanotubes with respect to their carbon-based counterparts.

Three-Dimensional Nanoprinting via Scanning Probe Lithography-Delivered Layer-by-Layer Deposition

Three-dimensional (3D) printing has been a very active area of research and development due to its capability to produce 3D objects by design. Miniaturization and improvement of spatial resolution are major challenges in current 3D printing technology development. This work reports advances in miniaturizing 3D printing to the nanometer scale using scanning probe microscopy in conjunction with local material delivery. Using polyelectrolyte polymers and complexes, we have demonstrated the concept of layer-by-layer nanoprinting by design. Nanometer precision is achieved in all three dimensions, as well as in interlayer registry. The approach enables production of designed functional 3D materials with nanometer resolution and, as such, creates a platform for conducting scientific research in designed 3D nanoenvironments as well. In doing so, it enables production of nanomaterials and scaffolds for photonics devices, biomedicine, and tissue engineering.

Interfacial Shear Strength of Multilayer Graphene Oxide Films.

Graphene oxide (GO) is considered as one of the most promising layered materials with tunable physical properties and applicability in many important engineering applications. In this work, the interfacial behavior of multilayer GO films was directly investigated via GO-to-GO friction force microscopy, and the interfacial shear strength (ISS) was measured to be 5.3 ± 3.2 MPa. Based on high resolution atomic force microscopy images and the available chemical data, targeted molecular dynamics simulations were performed to evaluate the influence of functional structure, topological defects, and interlayer registry on the shear response of the GO films. Theoretical values for shear strength ranging from 17 to 132 MPa were predicted for the different structures studied, providing upper bounds for the ISS. Computational results also revealed the atomic origins of the stochastic nature of friction measurements. Specifically, the wide scatter in experimental measurements was attributed to variations in functional structure and topological defects within the sliding volume. The findings of this study provide important insight for understanding the significant differences in strength between monolayer and bulk graphene oxide materials and can be useful for engineering topological structures with tunable mechanical properties.

Chemical Mass Production of Graphene Nanoplatelets in ∼100% Yield.

Successful application of graphene is hampered by the lack of cost-effective methods for its production. Here, we demonstrate a method of mass production of graphene nanoplatelets (GNPs) by exfoliation of flake graphite in the tricomponent system made by a combination of ammonium persulfate ((NH4)2S2O8), concentrated sulfuric acid, and fuming sulfuric acid. The resulting GNPs are tens of microns in diameter and 10-35 nm in thickness. When in the liquid phase of the tricomponent media, graphite completely loses its interlayer registry. This provides a ∼100% yield of GNPs from graphite in 3-4 h at room temperature or in 10 min at 120 °C.

Mechanism of Graphene Oxide Formation

Despite intensive research, the mechanism of graphene oxide (GO) formation remains unclear. The role of interfacial interactions between solid graphite and the liquid reaction medium, and transport of the oxidizing agent into the graphite, has not been well-addressed. In this work, we show that formation of GO from graphite constitutes three distinct independent steps. The reaction can be stopped at each step, and the corresponding intermediate products can be isolated, characterized, and stored under appropriate conditions. The first step is conversion of graphite into a stage-1 graphite intercalation compound (GIC). The second step is conversion of the stage-1 GIC into oxidized graphite, which we define as pristine graphite oxide (PGO). This step involves diffusion of the oxidizing agent into the preoccupied graphite galleries. This rate-determining step makes the entire process diffusive-controlled. The third step is conversion of PGO into conventional GO after exposure to water, which involves hydrolysis of covalent sulfates and loss of all interlayer registry.

Logarithmic Domain Growth in Ternary Mixture Lipid Multilayer Systems

Cholesterol, one essential constituent of cell membrane, is shown to perform an important function as to maintaining the membrane integrity and fluidity. However, the role of cholesterol in mixed membrane phase behavior is still not completely understood. In our study of model membrane multilayer systems consisting of 1:1 DPPC/DOPC with 0∼30% Cholesterol, we have studied the kinetics of domain growth as a function of time. Both the DOPC rich domains and DPPC rich domains grow vertically via interlayer registry, and also laterally as domains coarsen. X-ray diffraction measurements reveal two different time scales of domain growth: fast growth at small time scale of minutes, and slow growth at large time scale which exhibits a logarithmic growth law for the single domain volume. AFM measurements were also performed to demonstrate the initial fast growth of the domains in real-time hydrated condition. The logarithmic growth occurs at different time scales for different cholesterol concentrations. This logarithmic growth law may indicate that the barriers to domain growth increase linearly with the domain size [1,2]. Our detailed analysis of domain lipid structure from electron density profiles with different cholesterol concentrations, and comparison of the growth law time constants should shed new light on the effects of cholesterol on membrane domains.[1] J.D. Shore, M. Holzer, and J.P. Sethna, Phys. Rev. B 46, 11376 (1992).[2] Z.W. Lai, G.F. Mazenko, and O.T. Valls, Phys. Rev. B 37, 9481 (1988).

Strain solitons and topological defects in bilayer graphene.

Bilayer graphene has been a subject of intense study in recent years. The interlayer registry between the layers can have dramatic effects on the electronic properties: for example, in the presence of a perpendicular electric field, a band gap appears in the electronic spectrum of so-called Bernal-stacked graphene [Oostinga JB, et al. (2007) Nature Materials 7:151-157]. This band gap is intimately tied to a structural spontaneous symmetry breaking in bilayer graphene, where one of the graphene layers shifts by an atomic spacing with respect to the other. This shift can happen in multiple directions, resulting in multiple stacking domains with soliton-like structural boundaries between them. Theorists have recently proposed that novel electronic states exist at these boundaries [Vaezi A, et al. (2013) arXiv:1301.1690; Zhang F, et al. (2013) arXiv:1301.4205], but very little is known about their structural properties. Here we use electron microscopy to measure with nanoscale and atomic resolution the widths, motion, and topological structure of soliton boundaries and related topological defects in bilayer graphene. We find that each soliton consists of an atomic-scale registry shift between the two graphene layers occurring over 6-11 nm. We infer the minimal energy barrier to interlayer translation and observe soliton motion during in situ heating above 1,000 °C. The abundance of these structures across a variety of samples, as well as their unusual properties, suggests that they will have substantial effects on the electronic and mechanical properties of bilayer graphene.

Interlayer Registry to Determine the Sliding Potential of Layered Metal Dichalcogenides: The Case of 2H-MoS2

We provide a simple and intuitive explanation for the interlayer sliding energy landscape of metal dichalcogenides. Based on the recently introduced registry index (RI) concept, we define a purely geometrical parameter which quantifies the degree of interlayer commensurability in the layered phase of molybdenum disulphide (2HMoS2). A direct relation between the sliding energy landscape and the corresponding interlayer registry surface of 2H-MoS2 is discovered thus marking the registry index as a computationally efficient means for studying the tribology of complex nanoscale material interfaces in the wearless friction regime.

Fabrication of three-dimensional imprint lithography templates by colloidal dispersions

Self-aligned imprint lithography (SAIL) enables the patterning and alignment of submicron-sized features on metre-scaled flexible substrates in the roll-to roll (R2R) environment. SAIL solves the problem of precision interlayer registry on a moving web by encoding all the geometry information required for the entire patterning steps into a monolithic three-dimensional mask that is imprinted on the thin film stack deposited on a flexible substrate. Soft molds made of plastics or elastomers cast on a silicon master have been used as stamps to pattern the 3D masks because of their low cost and ease of fabrication. However, the durability of these stamps is one factor that limits their efficiency in a R2R process. Fluorothermoplastics are low cost imprint stamp materials with great mechanical strength and chemical compatibility but with low gas permeability that trap air bubbles in the photopolymer during the imprint process. This paper describes the strategy for increasing gas permeability of fluorothermoplastics by introducing voids or pores in the stamp material by fabricating the stamps with aqueous colloidal dispersions of tetrafluoroethylene–hexafluoropropylene copolymer (FEP) nanoparticles. The basic idea is that the hard fluorinated particles, whose modulus is too high to deform during drying, remain as hard spheres and lead to a porous packing when drying is complete. The selection of suitable additives to eliminate cracks created by capillary stresses during water evaporation is also described in this paper.

Roll‐to‐roll manufacturing of electronics on flexible substrates using self‐aligned imprint lithography (SAIL)

Abstract— The manufacture of large‐area arrays of thin‐film transistors on polymer substrates using roll‐to‐roll (R2R) processes exclusively is being developed. Self‐aligned imprint lithography (SAIL) enables the patterning and alignment of submicron‐sized features on meter‐scaled flexible substrates in the R2R environment. SAIL solves the problem of precision interlayer registry on a moving web by encoding all the geometry information required for the entire patterning steps into a monolithic three‐dimensional imprint with discrete thickness modulation. The pre‐aligned multiple‐step mask structure maintains its alignment regardless of subsequent substrate distortion. Challenges are encountered in relation to the novel nature of using flexible substrates and building toolsets for the R2R processing. In this paper, methods of the SAIL process, the resulting active‐matrix backplanes, the trajectory of SAIL process development, and the remaining issues for production are presented.

Structure of Fe( [formula omitted]) studied by quantitative LEED analysis and pseudopotential DFT calculations

In view of the discrepancy in the literature with regard to the structure of Fe(3 1 0), we have investigated this surface by a refined quantitative LEED analysis and a pseudopotential DFT study. Similar results to a previous LEED analysis have been obtained by our new LEED analysis. Our pseudopotential calculations have reproduced the relaxation sequence of the interlayer spacings determined by our LEED analysis, i.e., − + − + with the largest discrepancy in the structural parameters less than 0.04 Å. A large lateral relaxation of the interlayer registry between the first and second layers has been confirmed by our LEED analysis and pseudopotential calculations.

Tin(iv) sulfide–alkylamine composite mesophase: a new class of thermotropic liquid crystals

The synthesis, structural characterization, thermal, electronic, optical birefringence and electrical properties of a new class of semiconducting tin sulfide based thermotropic liquid crystals (LCs), Meso-SnS-n, are described for the first time. The as-synthesized crystalline form of Meso-SnS-1 has a structure that is based upon well registered yet poorly ordered porous tin(iv) sulfide layers between which are sandwiched well organized hexadecylamine bilayers. On warming this material to around 45 °C the hexadecylamine bilayer first becomes disordered while the porous tin(iv) sulfide sheets remain registered. This is followed, around 85 °C, by a transition where both the alkylamine bilayer and the porous tin(iv) sulfide lamellae become liquid crystalline at which point the intralayer but not the interlayer registry is lost. The liquid-crystal organic–inorganic composite phase has either a nematic or smectic C structure. Electrically, the room-temperature ordered phase has a conductivity of 5.3×10–8 Ω–1 cm–1 which increases by more than a 1000 times on transforming to the LC phase where it behaves as a semiconducting metallogen. The conductivity of Meso-SnS-1 cycles reversibly with temperature and displays discontinuities that are coincident with the crystal–semiliquid crystal and semiliquid crystal–liquid crystal thermal transitions that are defined by differential scanning calorimetry and variable-temperature powder X-ray diffraction. Meso-SnS-1 readily forms electrically conducting thin films which are able to reversibly adsorb molecules like H2O and CO2 . These properties bode well for the use of this new class of inorganic semiconducting LCs for electro-optical displays and chemical sensing applications.

Multilayer relaxation of the Al(331) surface

The surface structure of Al(331) has been determined by means of a systematic optimization of the agreement between experimental LEED intensity-energy spectra, measured at normal incidence and 115 K, and spectra calculated using multiple-scattering theory, as a function of the structural and non-structural variables of the calculations. Relaxation from the bulk structure has been investigated for the first six atomic layers. Substantial relaxations of the first four interlayer spacings d 1 to d 4 and the first interlayer registry r 1 have been found. The relaxations, expressed as deviations from the respective distances in the bulk, are determined to be: Δd 1 = −11.7±2.3%, Δd 2 = −4.1±3.1%, Δd 3 = +10.3±2.7%, Δd 4 = −4.8±4.1% and Δr 1 = −2.8±3.7%. These results are discussed in the context of recent model calculations of surface structure.

Multilayer Relaxation of Interlayer Registry and Spacing at High-Index Metal Surfaces

Oscillatory multilayer relaxation of both interlayer spacing and registry at certain high-index metal surfaces is predicted via minimization of a simple model for the total energy of a semi-infinite crystal. Results for the (210) and (211) surfaces of bcc and fcc simple metals indicate that the relaxation parallel to the surface plane moves the surface layers toward more symmetrical configurations with respect to adjacent layers.