Nanostructures. Self-assembled domain patterns

Thermal stability and secondary aggregation of self-limiting, geometrically frustrated assemblies: Chain assembly of incommensurate polybricks

In geometrically frustrated assemblies, equilibrium self-limitation manifests in the form of a minimum in the free energy per subunit at a finite, multisubunit size which results from the competition between the elastic costs of frustration within an assembly and the surface energy at its boundaries. Physical realizations-from ill-fitting particle assemblies to self-twisting protein superstructures-are capable of multiple mechanisms of escaping the cumulative costs of frustration, resulting in unlimited equilibrium assembly, including elastic modes of "shape flattening" and the formation of weak, defective bonds that screen intra-assembly stresses. Here we study a model of one-dimensional chain assembly of incommensurate "polybricks" and determine its equilibrium assembly as a function of temperature, concentration, degree of shape frustration, elasticity, and interparticle binding, notably focusing on how weakly cohesive, defective bonds give rise to strongly temperature-dependent assembly. Complex assembly behavior derives from the competition between multiple distinct local minima in the free-energy landscape, including self-limiting chains, weakly bound aggregates of self-limiting chains, and strongly bound, elastically defrustrated assemblies. We show that this scenario, in general, gives rise to anomalous multiple aggregation behavior, in which disperse subunits (stable at low concentration and high temperature) first exhibit a primary aggregation transition to self-limiting chains (at intermediate concentration and temperature) which are ultimately unstable to condensation into unlimited assembly of finite-chains through weak binding beyond a secondary aggregation transition (at low temperature and high concentration). We show that window of stable self-limitation is determined both by the elastic costs of frustration in the assembly as well as energetic and entropic features of intersubunit binding.

Controlling the Shape of Small Clusters with and without Macroscopic Fields

Despite major advances in the understanding of the formation and dynamics of nanoclusters in the past decades, theoretical bases for the control of their shape are still lacking. We investigate strategies for driving fluctuating few-particle clusters to an arbitrary target shape in minimum time with or without an external field. This question is recast into a first passage problem, solved numerically, and discussed within a high temperature expansion. Without field, large-enough low-energy target shapes exhibit an optimal temperature at which they are reached in minimum time. We then compute the optimal way to set an external field to minimize the time to reach the target, leading to a gain of time that grows when increasing cluster size or decreasing temperature. This gain can shift the optimal temperature or even create one. Our results could apply to clusters of atoms at equilibrium, and colloidal or nanoparticle clusters under thermo- or electrophoresis.

Self-Assembly of Human Serum Albumin: A Simplex Phenomenon

Spontaneous self-assemblies of biomolecules can generate geometrical patterns. Our findings provide an insight into the mechanism of self-assembled ring pattern generation by human serum albumin (HSA). The self-assembly is a process guided by kinetic and thermodynamic parameters. The generated protein ring patterns display a behavior which is geometrically related to a n-simplex model and is explained through thermodynamics and chemical kinetics.

Strain-driven mound formation of substrate under epitaxial nanoparticles

We observe the growth of crystalline SiC nanoparticles on Si(001) at 900 °C using in situ electron microscopy. Following nucleation and growth of the SiC, there is a massive migration of Si, forming a crystalline Si mound underneath each nanoparticle that lifts it 4-5 nm above the initial growth surface. The volume of the Si mounds is roughly five to seven times the volume of the SiC nanoparticles. We propose that relaxation of strain drives the mound formation. This new mechanism for relieving interfacial strain, which involves a dramatic restructuring of the substrate, is in striking contrast to the familiar scenario in which only the deposited material restructures to relieve strain.

Nonclassical "explosive" nucleation in Pb/Si(111) at low temperatures

Classically, the onset of nucleation is defined in terms of a critical cluster of the condensed phase, which forms from the gradual aggregation of randomly diffusing adatoms. Experiments in Pb/Si(111) at low temperature have discovered a dramatically different type of nucleation, with perfect crystalline islands emerging "explosively" out of the compressed wetting layer after a critical coverage Θ_{c}=1.22  ML is reached. The unexpectedly high island growth rates, the directional correlations in the growth of neighboring islands and the persistence in time of where mass is added in individual islands, suggest that nucleation is a result of the highly coherent motion of the wetting layer, over mesoscopic distances.

Microphase formation at a 2D solid-gas phase transition

Density modulated micro-separated phases (microphases) occur at 2D liquid interfaces in the form of alternating regions of high and low density domains. Brewster angle microscopy (BAM) images demonstrate the existence of microphases in cluster, stripe, and mosaic morphologies at the buried interface between hexane and water with fluoro-alkanol surfactant dissolved in the bulk hexane. At high temperature, the surfactant assembles at the interface in a 2D gaseous state. As the system is cooled additional surfactants condense onto the interface, which undergoes a 2D gas-solid phase transition. Microphase structure is observed within a few degrees of this transition in the form of clusters and labyrinthine stripes. Microphases have been observed previously in a number of other systems; nevertheless, we demonstrate that adsorption transitions at the liquid-liquid interface provide a convenient way to observe a full sequence of temperature-dependent 2D phases, from gas to cluster to stripe to mosaic to inverted stripe phases, as well as coexistence between some of these microphases. Cracking and fracture of the clusters reveal that they are a solid microphase. Theories of microphases often predict a single length scale for cluster and stripe phases as a result of the competition between an attractive and a repulsive interaction. Our observation that two characteristic length scales are required to describe clusters whose diameter is much larger than the stripe period, combined with the solid nature of the clusters, suggests that a long-range elastic interaction is relevant. These results complement earlier X-ray measurements on the same system.

Epigenetic regulation of the Plasmodium falciparum genome

Recent research has highlighted some unique aspects of chromatin biology in the malaria parasite Plasmodium falciparum. During its erythrocytic lifecycle P. falciparum maintains its genome primarily as unstructured euchromatin. Indeed there is no clear role for chromatin-mediated silencing of the majority of the developmentally expressed genes in P. falciparum. However discontinuous stretches of heterochromatin are critical for variegated expression of contingency genes that mediate key pathogenic processes in malaria. These range from invasion of erythrocytes and antigenic variation to solute transport and growth adaptation in response to environmental changes. Despite lack of structure within euchromatin the nucleus maintains functional compartments that regulate expression of many genes at the nuclear periphery, particularly genes with clonally variant expression. The typical components of the chromatin regulatory machinery are present in P. falciparum; however, some of these appear to have evolved novel species-specific functions, e.g. the dynamic regulation of histone variants at virulence gene promoters. The parasite also appears to have repeatedly acquired chromatin regulatory proteins through lateral transfer from endosymbionts and from the host. P. falciparum chromatin regulators have been successfully targeted with multiple drugs in laboratory studies; hopefully their functional divergence from human counterparts will allow the development of parasite-specific inhibitors.

Structural transformations in Pb/Si(111) phases induced by C₆₀ adsorption

Structural transformations at the Pb/Si(111) surface occurring upon C₆₀ adsorption onto Pb/Si(111)1 × 1 phase at room temperature and Pb/Si(111)[Formula: see text] at low temperatures between 30 and 210 K, have been studied using scanning tunneling microscopy and low-energy electron diffraction observations. Typically, C₆₀ fullerenes agglomerate into random molecular islands nucleated at the surface defects. C₆₀ island formation is accompanied by expelling Pb atoms to the surrounding surface area where more dense Pb/Si(111) phases form. Productivity of C₆₀-induced expelling of Pb atoms is controlled by surface defects and is suppressed dramatically when regular ('crystalline') C₆₀ islands self-assemble at the defect-free Pb/Si(111) surface. When Pb atoms are ejected by the random C₆₀ islands, extended structural transformations involving reordering of numerous Pb atoms are fully completed at the surface within the shortest possible time (a few dozen seconds) to reapproach and image the surface after C₆₀ deposition. Estimations show that the observed transformations cannot be controlled by random walk diffusion of Pb adatoms, which implies a highly correlated motion of the Pb atom displacements within the layer.

Strain in epitaxial graphene visualized by intercalation

Intercalation of Eu under graphene on Ir(111) results in patterns oriented along the graphene moiré and quantized in size by its unit mesh. The patterns are formed by stripes, compact islands, and channels. Over a wide range of intercalated amounts the step concentration of the pattern has a rather constant saturation value. These findings are explained by the chemically modulated binding of graphene to the substrate and the preexisting strain in graphene due to its cooldown from the growth temperature. Local variations in the intercalation step density appear to reflect local variations in the preexisting strain.

Superdiffusive motion of the Pb wetting layer on the Si(111) surface

Mass transport in the Pb wetting layer on the Si(111) surface is investigated by observing nonequilibrium coverage profile evolution with low energy electron microscopy and microlow energy electron diffraction. Equilibration of an initial coverage step profile occurs by the exchange of mass between oppositely directed steep coverage gradients that each move with unperturbed shape. The bifurcation of the initial profile, the shape of the profile between the two moving edges, and the time dependence of equilibration are all at odds with expectations for classical diffusion behavior. These observations signal a very unusual coverage dependence of diffusion or may even reveal an exceptional collective superdiffusive mechanism.

Spin reorientation transitions and structures of electrodeposited Ni/Cu(100) ultrathin films with and without Pb additives

Magnetic properties and surface structures of Ni/Cu(100) ultrathin films are studied by means of magneto-optical Kerr effect and in situ scanning tunneling microscopy in combination with cyclic voltammetry. At the initial stage of Ni deposition on a Cu(100) electrode, nickel atoms attach onto the steps and the surface shows single atomic steps corresponding to a layer-by-layer growth. For thicker Ni/Cu(100) films, nanometer-size clusters are randomly distributed on the surface showing a three-dimensional island growth. For thinner Ni layers in the coherent region, the magnetic anisotropy energy of the Cl-electrolyte/Ni interface is small. The reduction of squareness of the hysteresis loops is related to the inhomogeneous growth of the Ni layers. For thicker Ni layers in the incoherent region, the negative value of interface anisotropy for the Cl-electrolyte/Ni interface has a strong impact on perpendicular magnetic anisotropy and plays an important role on the reduction of the Ni thickness for spin reorientation transition in the electrolyte condition. By adding Pb additives, the deposition of a Pb wetting layer causes a defaceting phenomenon and the hydrogen evolution reaction is reduced. As the Ni thickness increases, the growth of Ni changes from layer-by-layer to quasi-two-dimensional islands with a flat top layer. With a Pb additive, the spin reorientation transitions of the Ni/Cu(100) system are not significantly influenced. However, due to the change of the growth mode by Pb atoms as a surfactant, the squareness of the hysteresis loops is enhanced for all the Ni thicknesses.

Phase transition driven discontinuity in thermodynamic size selection

We show how an order-disorder phase transition in a two-dimensional system can discontinuously alter the shape and size of stress-stabilized self-assembled nanostructures. Low energy electron microscopy was used to study the dealloying of the Cu(111)-sqrt[3]×sqrt[3]-R30°-Bi surface alloy. The gradual expulsion of embedded bismuth from the alloy with increasing temperature induces a hard-hexagon-type order-disorder transition in the surface alloy. Our low energy electron microscopy results demonstrate how the loss of long-range order induces enormous changes in the domain patterns that the alloy forms with a Bi overlayer phase. We propose that the occurrence of phase transitions in one of the two surface phases that constitute a self-assembled domain pattern, provides a general, largely unexplored, mechanism that can be used to influence the morphological details of two-dimensional nanostructures.

Spontaneous propagation of self-assembly in a continuous medium

We report a mechanism in which self-assembly propagates spontaneously in a continuous medium, enabling the delivery of local order information to distance. In a large stable system a locally self-assembled structure as a precursor destabilizes its surrounding areas through a dipole interaction. The newly formed structures inherit the same order information from the precursor and further activate the self-assembly of their neighbors. This process causes spatial extension of self-assembly and replication of the order, producing extremely long-range ordered superlattice without defects.

Interplay between subsurface ordering, surface segregation, and adsorption on Pt-Ti(111) near-surface alloys

Using the first-principles cluster expansion (CE) method, we studied the subsurface ordering of Pt/Pt-Ti(111) surface alloys and the effect of this ordering on segregation and adsorption behavior. The clusters included in the CE are optimized by a genetic algorithm to better describe the interactions between Pt and Ti atoms in the subsurface layer. Similar to bulk Pt-Ti alloys, Pt-Ti(111) subsurface alloys show a strong ordering tendency. A series of stable ordered Pt-Ti subsurface structures are identified from the two-dimensional (2D) CE. As an indication of the connection between the 2D and the bulk ordering, the CE predicts a ground-state Pt(8)Ti structure in the (111) subsurface layer, which is the same ordering as the close-packed plane of the bulk Pt(8)Ti compound. We carried out Monte Carlo simulations (MC) using the CE Hamiltonian to study the finite temperature stability of the Pt-Ti subsurface structures. The MC results show that subsurface structures in the Pt-rich range have higher order-disorder transition temperatures than their Ti-rich subsurface counterparts. We calculate the binding energy of different adsorbates (O, S, H, and NO) on Pt-terminated and Ti-segregated surfaces of ordered PtTi and Pt(8)Ti subsurface alloys. The binding of these adsorbates is generally stronger on Ti-segregated surfaces than Pt-terminated surfaces. The adsorption-induced Ti surface segregation is determined by two factors: (i) the unfavorable energy penalty for the Ti atom to segregate to the clean surface and (ii) the favorable energy decrease from stronger adsorbate binding on the Ti-segregated surface. The two factors introduce similar magnitude in energy change for the S and NO adsorption on Ti-segregated surfaces of PtTi subsurface alloys. We predict an adsorption-induced Ti surface segregation that is dependent on the atomic configurations of the Ti-segregated surfaces resulting from the competition of the two factors.

Nanoscale surface pattern evolution in heteroepitaxial bimetallic films

Nanoscale self-assembly dynamics of submonolayer bimetallic films was studied through simulation of a coarse-grained mesoscopic model. Simulations predict a phase transition sequence (hexagonal→stripe→inverse hexagonal) consistent with experimental observations of Pb/Cu(111) heteroepitaxial growth. Post-transition ordering dynamics of hexagonal and inverse hexagonal patterns was simulated and quantified in order to predict pattern quality and evolution mechanisms. Correlation length scaling laws and nanoscale evolution mechanisms were predicted through simulation of experimentally relevant length (≈1 μm(2)) and time scales, with findings supporting evidence of universal pattern behavior with other hexagonal systems. Results provide detailed dynamics and structure of this novel self-assembly process applicable to the design and optimization of functional bimetallic materials, such as bimetallic catalysts.

Dichroic coherent diffractive imaging

Understanding electronic structure at the nanoscale is crucial to untangling fundamental physics puzzles such as phase separation and emergent behavior in complex magnetic oxides. Probes with the ability to see beyond surfaces on nanometer length and subpicosecond time scales can greatly enhance our understanding of these systems and will undoubtedly impact development of future information technologies. Polarized X-rays are an appealing choice of probe due to their penetrating power, elemental and magnetic specificity, and high spatial resolution. The resolution of traditional X-ray microscopes is limited by the nanometer precision required to fabricate X-ray optics. Here we present a novel approach to lensless imaging of an extended magnetic nanostructure, in which a scanned series of dichroic coherent diffraction patterns is recorded and numerically inverted to map its magnetic domain configuration. Unlike holographic methods, it does not require a reference wave or precision optics. In addition, it enables the imaging of samples with arbitrarily large spatial dimensions, at a spatial resolution limited solely by the coherent X-ray flux, wavelength, and stability of the sample with respect to the beam. It can readily be extended to nonmagnetic systems that exhibit circular or linear dichroism. We demonstrate this approach by imaging ferrimagnetic labyrinthine domains in a Gd/Fe multilayer with perpendicular anisotropy and follow the evolution of the domain structure through part of its magnetization hysteresis loop. This approach is scalable to imaging with diffraction-limited resolution, a prospect rapidly becoming a reality in view of the new generation of phenomenally brilliant X-ray sources.

Growing large nanostructured superlattices from a continuum medium by sequential activation of self-assembly

We propose a mechanism to grow a large superlattice of phase domains from a continuum homogenous binary film by sequential activation of self-assembly. Self-assembly was initiated in a small mobile region (where atoms could diffuse) to form a seed pattern, and then the mobile region was shifted gradually. This process led to a long-range ordered superlattice regardless whether the seed was perfect or not, since the pattern quickly improved to a perfect superlattice along with the sequential activation. At a bistable state the scanning velocity controlled the type of superlattice. Further exploration led to an intriguing finding that we call the self-activation of self-assembly, a domino effect where the self-assembly in a small region causes a long-range interaction that destabilizes its homogeneous neighbor and triggers the propagation of self-assembly to the entire system.

Trends in low energy electron microscopy

Low energy electron microscopy (LEEM) and spin polarized LEEM (SPLEEM) are two powerful in situ techniques for the study of surfaces, thin films and other surface-supported nanostructures. Their real-time imaging and complementary diffraction capabilities allow the study of structure, morphology, magnetism and dynamic processes with high spatial and temporal resolution. Progress in methods, instrumentation and understanding of novel contrast mechanisms that derive from the wave nature and spin degree of freedom of the electron continue to advance applications of LEEM and SPLEEM in these areas and beyond. We review here the basic imaging principles and recent developments that demonstrate the current capabilities of these techniques and suggest potential future directions.

Experimental phase diagram of perpendicularly magnetized ultrathin ferromagnetic films

We image the domain patterns in perpendicularly magnetized ultrathin Fe films on Cu(100) as a function of the temperature T and the applied magnetic field H. Between the low-field stripe phase and the high-field uniform phase we find a bubble phase, consisting of reversed circular domains in a homogeneous background. The curvature of the transition lines in the H-T parameter space is in contrast to the general expectations. The pattern transformations show yet undetected scaling properties.

Geometrical tile design for complex neighborhoods

Recent research has showed that tile systems are one of the most suitable theoretical frameworks for the spatial study and modeling of self-assembly processes, such as the formation of DNA and protein oligomeric structures. A Wang tile is a unit square, with glues on its edges, attaching to other tiles and forming larger and larger structures. Although quite intuitive, the idea of glues placed on the edges of a tile is not always natural for simulating the interactions occurring in some real systems. For example, when considering protein self-assembly, the shape of a protein is the main determinant of its functions and its interactions with other proteins. Our goal is to use geometric tiles, i.e., square tiles with geometrical protrusions on their edges, for simulating tiled paths (zippers) with complex neighborhoods, by ribbons of geometric tiles with simple, local neighborhoods. This paper is a step toward solving the general case of an arbitrary neighborhood, by proposing geometric tile designs that solve the case of a “tall” von Neumann neighborhood, the case of the f-shaped neighborhood, and the case of a 3 × 5 “filled” rectangular neighborhood. The techniques can be combined and generalized to solve the problem in the case of any neighborhood, centered at the tile of reference, and included in a 3 × (2k + 1) rectangle.

Creating perfectly ordered quantum dot arrays via self-assembly

Several applications involving quantum dots require perfect long-range ordered arrays. Unfortunately, self-assembly (the choice method to fabricate quantum dots) leads to patterns that, although short range ordered, exhibit defects equivalent to grain boundaries and dislocations on a large scale. We note that rotational invariance of film growth is one reason for formation of defects, and hence study an anisotropic model of quantum dot formation. However, nonlinear stability analysis shows that even in the extreme limit of anisotropy, square arrays whose orientations are in a finite range are linearly stable; consequently structures created in the film continue to have defects. Building on insights developed by the authors earlier on a simpler monolayer self-assembly model, we propose controlling the deposition through a mask to generate ordered quantum dots arrays. General principles to estimate geometrical characteristics of the mask are given. Numerical integration of the model shows that perfectly ordered square arrays of quantum dots can indeed be created using masked deposition.

Nanostructures with long-range order in monolayer self-assembly

A key ingredient for continued expansion of nanotechnologies is the ability to create perfectly ordered arrays on a small scale with both site and size control. Self-assembly-i.e., the spontaneous formation of nanostructures-is a highly promising alternative to traditional fabrication methods. However, efforts to obtain perfect long-range order via self-assembly have been frustrated in practice as ensuing patterns contain defects. We use an idea based on the fundamental physics of pattern formation to introduce a strategy to consistently obtain perfect patterns.

Simulation of self-assembled nanopatterns in strained 2D alloys on the face centered cubic (111) surface

We investigate the formation of nanostructures in 2D strained alloys on face centered cubic (111) surfaces by means of equilibrium Monte Carlo simulations. In the framework of an off-lattice model, we consider one monolayer of two bulk-immiscible adsorbates A and B with negative and positive misfit relative to the substrate, respectively. Simulations show that the adsorbates partly self-organize into island or stripe-like patterns. We show how these structures depend on the relative misfits, interaction, and concentration of components. The morphology is quite different for phase separation and intermixing regimes.

Low-temperature ultrafast mobility in systems with long-range repulsive interactions: Pb/Si(111)

A realization of the numerous phases predicted in systems with long-range repulsive interactions was recently found in Pb/Si(111). Surprisingly, these numerous phases can be grown at low temperatures approximately 40 K over macroscopic distances. This unusual observation can be explained from theoretical calculations of the collective diffusion coefficient D(c) in systems with long-range repulsive interactions. Instead of a gradual dependence of D(c) on coverage, it was found that D(c) has sharp maxima at low temperatures for every stable phase (i.e., for every rational value of the coverage theta=p/q) in agreement with the experiment.

Role of surface elastic relaxations in an O-induced nanopattern on Pt(110)-(1 x 2)

Scanning tunneling microscopy shows that a nanopattern forms as the Pt(110)-(1 x 2) surface is exposed to oxygen at room temperature or above. The nanopattern consists of [11[over]0] oriented O-induced stripes assembling into a (11 x 2) superstructure at high O coverage. The stripes form because the O adsorption energy increases by expanding the Pt lattice along the ridges of the surface as compared to the bulk. From interplay with density functional theory calculations, we show that the O-induced nanoscale periodicity is caused by short-ranged elastic relaxations confined to the surface.

How Pb-overlayer islands move fast enough to self-assemble on Pb-Cu surface alloys

Low-energy electron microscopy reveals that two-dimensional, approximately 50 000 atom, Pb-overlayer and vacancy islands both have diffusion coefficients of 25.6+/-0.8 nm2/sec at 400 degrees C on Pb-Cu surface alloys. This high mobility, key to self-assembly in this system, results from the fast transport of Pb atoms on the surface alloy and of Cu through the Pb overlayer. A high Pb vacancy concentration, predicted by ab initio calculations, facilitates the latter.

Self-assembled MnN superstructure

Self-assembled MnN nanoislands have been prepared on Cu(001) substrate. The nanoislands show a square shape and a well-defined size. They are regularly arrayed with a periodicity of (3.5+/-0.1) nanometer and form a two-dimensional square superstructure. The MnN island superstructure is stabilized by a short-range mechanism. A structural model has been proposed to explain the self-assembly and the high quality of the superstructure.

Classification of hexagonal adlayer arrangements by means of collective geometrical properties

Unequal-sphere packing model is applied for the simulation of large number of hexagonal adlayer structures with surface coverage between theta=13 and theta=1 on the hexagonal substrate, with atomic radius of the adsorbate and substrate atoms as the only input. Each structure is characterized with respect to collective adlayer properties: the average adlayer height and the adlayer roughness. The distribution of hexagonal arrangements is presented in a special plot, which can be used for identification and characterization of hexagonal adlayers of different surface coverages and atomic registries. The most likely structures are related to the extreme values of our model parameters. The usefulness of this methodology is successfully demonstrated by comparison with some real adsorbate-substrate systems, i.e., halogens and rare gases adsorbed on (111) surface. Besides the agreement with experimental results, our model offers new insight into the formation of atomic adlayers and detailed analysis of the atomic registry. We believe that our approach will be of use for identification of probable structures among the large number of combinatorial possibilities in theoretical studies and for better interpretation of experimental results (i.e., scanning-tunneling microscopy images of atomic adlayers).

Deterministic positioning of three-dimensional structures on a substrate by film growth

A process to fabricate three-dimensional crystalline structures at controlled locations on a substrate during film growth and annealing is demonstrated. Low-energy electron microscopy reveals that silver is transported to regions on a tungsten surface with closely spaced atomic steps. By controlling the substrate topography using a focused ion beam to machine small holes, this general mechanism produces an array of cylinders as a silver film dewets the substrate.

Nonequilibrium domain pattern formation in mesoscopic magnetic thin film elements assisted by thermally excited spin fluctuations

The dynamic behavior in the evolving pattern of thermally assisted, nonequilibrium domains in magnetic thin-film elements undergoing ultrafast 180 degrees magnetization reversal was studied. Magnetization reversal enters a fully dynamic regime when the external field conditions are changed much faster than the sample is able to respond. The dynamic pathway develops a complexity not seen in quasistatic reversal but still retains a high level of order with well-developed dynamic domain patterns formed in response to subnanosecond transitions of the external applied magnetic field.

Self-assembled patterns and strain-induced instabilities for modulated systems

The self-assembled domain patterns of modulated systems are characteristic of a wide variety of chemical and physical systems, and are the result of competing interactions. From a technological point of view, there is considerable interest in these domain patterns, as they form suitable templates for the fabrication of nanostructures. We have analyzed the domains and instabilities that form in modulated systems, and show that a large variety of patterns--based on long-lived metastable or glassy states--may be formed as a compromise between the required equilibrium modulation period and the strain present in the system. The strain results from topologically constrained trajectories in phase space, that effectively preclude the equilibrium configuration.

Patterning multilayers of molecules via self-organization

The electric dipole interaction among adsorbate molecules may cause them to form regular nanopatterns. In a multilayer system, the self-organization of each layer is also influenced by the underlying layers. This Letter develops a phase field model to simulate the molecular patterning process. The study reveals self-alignment, scaling down of size, and the effect of guided self-assembly with embedded electrodes.

New and exotic self-organized patterns for modulated nanoscale systems

The self-assembled domain patterns of modulated systems are the result of competing short-range attractive and long-range repulsive interactions found in diverse physical and chemical systems. From an application point of view, there is considerable interest in these domain patterns, as they form templates suitable for the fabrication of nanostructures. In this work we have generated a variety of new and exotic patterns, which represent either metastable or glassy states. These patterns arise as a compromise between the required equilibrium modulation period and the strain resulting from topologically constrained trajectories in phase space that effectively preclude the equilibrium configuration.

Macroscopically Uniform Nanoperiod Alloy Multilayers Formed by Coupling of Electrodeposition with Current Oscillations

The electrodeposition from an acidic solution containing Cu(2+), Sn(2+), and a cationic surfactant gave a negative differential resistance (NDR) and a current oscillation in a narrow potential region of about 20 mV lying slightly more negative than the onset potential for Sn-Cu alloy deposition. Scanning Auger microscopic inspection has indicated that alloy films deposited during the oscillation have a clear alternate multilayer structure composed of two alloy layers of different compositions. The multilayer had the period of thickness of 40-90 nm and was uniform over a macroscopically wide area of about 1 mm x 1 mm. Detailed investigations have revealed that the NDR arises from adsorption of a cationic surfactant (acting as an inhibitor for diffusion of metal ions) on the alloy surface, and the oscillation comes from coupling of the NDR with the ohmic drop in the electrolyte.

Monolayer pattern evolution via substrate strain-mediated spinodal decomposition

Investigations of octylsilane (C8H17SiH3) monolayer pattern formation on Au(111) are reported. Scanning tunneling microscopy data display the evolution of a approximately 6 nm scale pattern of interwoven features concomitant with ejection of surface Au atoms and relaxation of the Au(111) 23xsqrt[3] surface reconstruction. Numerical simulations suggest the surface dynamics are governed by a substrate strain-mediated spinodal decomposition mechanism, novel to organic monolayer formation. Collectively, the experimental and theoretical data indicate strain-inducing Si-Au bond interactions drive the pattern formation and the alkyl chains play a negligible role.

Patterns in melting snow and vapor deposited layers

We have observed a natural periodic pattern occurring in partially melted snow lying on the ground under certain atmospheric conditions. We explain this phenomenon quantitatively by considering heat flow through this layer coexisting in two phases (snow and water). Our model equations exhibit a range of patterns depending on the average density of snow. Strikingly similar patterns have been observed by Plass and co-workers in monolayer depositions of Pb on heated PbCu substrates. We argue that the physics of the two phenomena, differing in length scales by 7 orders of magnitude, is similar.

Programmable motion and patterning of molecules on solid surfaces

Adsorbed on a solid surface, a molecule can migrate and carry an electric dipole moment. A nonuniform electric field can direct the motion of the molecule. A collection of the same molecules may aggregate into a monolayer island on the solid surface. Place such molecules on a dielectric substrate surface, beneath which an array of electrodes is buried. By varying the voltages of the electrodes individually, it is possible to program molecular patterning, direct an island to move in a desired trajectory, or merge several islands into a larger one. The dexterity may lead to new technologies, such as reconfigurable molecular patterning and programmable molecular cars. This paper develops a phase field model to simulate the molecular motion and patterning under the combined actions of dipole moments, intermolecular forces, entropy, and electrodes.

Self-assembled periodical polycrystalline-ZnO/a-C nanolayers on Zn nanowire

Zn nanowires with an epitaxial thin surface layer of zinc oxide were dispersed onto amorphous carbon films and stored at room temperature. After 1500 h, a self-organized equal-spaced zinc oxide (approximately 2 nm)/carbon (approximately 2.5 nm) multilayer structure was found to form outside the Zn nanowire, taking the place of the original ZnO surface layer. We carried a systematic study to clarify the self-formation mechanism of the periodical multilayers outside the Zn nanowire and found out that such a configuration originated from a chemical reaction between Zn and CO2 and were formed via a gas phase diffusion-interfacial chemical reaction-phase separation process.

Dynamic in situ characterization of organic monolayer formation via a Novel substrate-mediated mechanism

Ultrahigh vacuum scanning tunneling microscopy data investigating octylsilane (C8H17SiH3) monolayer pattern formation on Au(111) are presented. The irregular monolayer pattern exhibits a 60 A length scale. Formation of the octylsilane monolayer relaxes the Au(111) 23 x square root3 surface reconstruction and ejects surface Au atoms. Au adatom diffusion epitaxially extends the Au(111) crystal lattice via step edge growth and island formation. The chemisorbed monolayer covers the entire Au surface at saturation exposure. Theoretical and experimental data suggest the presence of two octylsilane molecular adsorption phases: an atop site yielding a pentacoordinate Si atom and a surface vacancy site yielding a tetracoordinate Si atom. Theoretical simulations investigating two-phase monolayer self-assembly dynamics on a solid surface suggest pattern formation results from strain-induced spinodal decomposition of the two adsorption phases. Collectively, the theoretical and experimental data indicate octylsilane monolayer pattern formation is a result of interfacial Au-Si interactions and the alkyl chains play a negligible role in the monolayer pattern formation mechanism.

Precursor film controlled wetting of Pb on Cu

Wetting in a system where the kinetics of drop spreading are controlled by the rate of formation of a precursor film is modeled for the first time at the atomistic scale. Molecular dynamics simulations of Pb(l) wetting Cu(111) and Cu(100) show that a precursor film of atomic thickness evolves and spreads diffusively. This precursor film spreads significantly faster on Cu(111) than on Cu(100). For Cu(100), the kinetics of drop spreading are dramatically decreased by slow advancement of the precursor film. Slow precursor film kinetics on Cu(100) are partly due to the formation of a surface alloy at the solid-liquid interface which does not occur on Cu(111).