Anomalous surface diffusion in nanoscale direct deposition processes

Confined sandpile in two dimensions: Percolation and singular diffusion

We investigate the properties of a two-state sandpile model subjected to a confining potential in two dimensions. From the microdynamical description, we derive a diffusion equation, and find a stationary solution for the case of a parabolic confining potential. By studying the systems at different confining conditions, we observe two scale-invariant regimes. At a given confining potential strength, the cluster size distribution takes the form of a power law. This regime corresponds to the situation in which the density at the center of the system approaches the critical percolation threshold. The analysis of the fractal dimension of the largest cluster frontier provides evidence that this regime is reminiscent of gradient percolation. By increasing further the confining potential, most of the particles coalesce in a giant cluster, and we observe a regime where the jump size distribution takes the form of a power law. The onset of this second regime is signaled by a maximum in the fluctuation of energy.

Instability in reaction-superdiffusion systems

We study the effect of superdiffusion on the instability in reaction-diffusion systems. It is shown that reaction-superdiffusion systems close to a Turing instability are equivalent to a time-dependent Ginzburg-Landau model and the corresponding free energy is introduced. This generalized free energy which depends on the superdiffusion exponent governs the stability, dynamics, and the fluctuations of reaction-superdiffusion systems near the Turing bifurcation. In addition, we show that for a general n-component reaction-superdiffusion system, a fractional complex Ginzburg-Landau equation emerges as the amplitude equation near a Hopf instability. Numerical simulations of this equation are carried out to illustrate the effect of superdiffusion on spatiotemporal patterns. Finally, the effect of superdiffusion on the instability in the Brusselator model, as a special case of reaction-diffusion systems, is studied. In general, superdiffusion introduces a new parameter that changes the behavior of the system near the instability.

Normal and Anomalous Diffusion: An Analytical Study Based on Quantum Collision Dynamics and Boltzmann Transport Theory

Diffusion, an emergent nonequilibrium transport phenomenon, is a nontrivial manifestation of the correlation between the microscopic dynamics of individual molecules and their statistical behavior observed in experiments. We present a thorough investigation of this viewpoint using the mathematical tools of quantum scattering, within the framework of Boltzmann transport theory. In particular, we ask: (a) How and when does a normal diffusive transport become anomalous? (b) What physical attribute of the system is conceptually useful to faithfully rationalize large variations in the coefficient of normal diffusion, observed particularly within the dynamical environment of biological cells? To characterize the diffusive transport, we introduce, analogous to continuous phase transitions, the curvature of the mean square displacement as an order parameter and use the notion of quantum scattering length, which measures the effective interactions between the diffusing molecules and the surrounding, to define a tuning variable, η. We show that the curvature signature conveniently differentiates the normal diffusion regime from the superdiffusion and subdiffusion regimes and the critical point, η = ηc, unambiguously determines the coefficient of normal diffusion. To solve the Boltzmann equation analytically, we use a quantum mechanical expression for the scattering amplitude in the Boltzmann collision term and obtain a general expression for the effective linear collision operator, useful for a variety of transport studies. We also demonstrate that the scattering length is a useful dynamical characteristic to rationalize experimental observations on diffusive transport in complex systems. We assess the numerical accuracy of the present work with representative experimental results on diffusion processes in biological systems. Furthermore, we advance the idea of temperature-dependent effective voltage (of the order of 1 μV or less in a biological environment, for example) as a dynamical cause of the perpetual molecular movement, which eventually manifests as an ordered motion, called the diffusion.

Plasmonic gold nanodiscs fabricated into a photonic-crystal nanocavity

We fabricate and characterise an optical structure consisting of a photonic crystal L3 nanocavity containing two gold nanodisks placed close to a field antinode. We use finite difference time domain (FDTD) modelling to show that the optical properties of the nanocavity are sensitive to the physical separation between the gold nanodisks, and that at reduced separation, the q-factor of a cavity mode polarised parallel to the dimer long-axis is reduced, indicating coupling between the cavity mode and a localised plasmon. Preliminary experimental measurements indeed indicate a damping of the cavity mode in the presence of the dimer; a result consistent with the FDTD modelling. Such a scheme may be used to integrate plasmonic systems into all-optical photonic circuits.

Fundamental aspects in surface self-assembly: theoretical implications of molecular polarity and shape

We investigate fundamental aspects of structure formation in molecular self-assembly, by examining the emergence of order upon adsorption of a series of model molecules. It is known that strongly polar diatomic molecules form three-dimensional crystals in the absence of a substrate. This tendency can be disrupted upon assembly on a solid surface, and various other types of order may arise. Depending on the relative strength of the interactions, disordered phases, two-dimensional crystals commensurate to the surface, and unmodified crystals were observed upon adsorption of simple dipoles in the present work. Introduction of steric features, in the form of a longer backbone or substituents external to the polar pair, led to even richer phase diagrams. The formation of two-dimensional phases with nematic (parallel) or antiparallel alignment was accomplished by altering the polarity of the end groups on needle-like molecules, whereas embedded charged groups made two-dimensional structure unstable for even very long molecules. These molecules preferred to align in long, often desorbed, molecular wires. The wealth of phases observed here parallel the results of experimental systematic or incidental studies of the relationships between molecular interactions and self-assembled patterns, and provide some insight into the molecular handles that self-assembly researchers can wield to guide the process towards a desired structural outcome.

Ink transport modelling in Dip-Pen Nanolithography and Polymer Pen Lithography

Dip-pen nanolithography (DPN) and Polymer pen lithography (PPL) are powerful lithography techniques being able to pattern a wide range of inks. Transport and surface spreading depend on the ink physicochemical properties, defining its diffusive and fluid character. Structure assembly on surface arises from a balance between the entanglement of the ink itself and the interaction with the substrate. According to the transport characteristics, different models have been proposed. In this article we review the common types of inks employed for patterning, the particular physicochemical characteristics that make them flow following different dynamics as well as the corresponding transport mechanisms and models that describe them.

A diffusive ink transport model for lipid dip-pen nanolithography

Despite diverse applications, phospholipid membrane stacks generated by dip-pen nanolithography (DPN) still lack a thorough and systematic characterization that elucidates the whole ink transport process from writing to surface spreading, with the aim of better controlling the resulting feature size and resolution. We report a quantitative analysis and modeling of the dependence of lipid DPN features (area, height and volume) on dwell time and relative humidity. The ink flow rate increases with humidity in agreement with meniscus size growth, determining the overall feature size. The observed time dependence indicates the existence of a balance between surface spreading and the ink flow rate that promotes differences in concentration at the meniscus/substrate interface. Feature shape is controlled by the substrate surface energy. The results are analyzed within a modified model for the ink transport of diffusive inks. At any humidity the dependence of the area spread on the dwell time shows two diffusion regimes: at short dwell times growth is controlled by meniscus diffusion while at long dwell times surface diffusion governs the process. The critical point for the switch of regime depends on the humidity.

Dip-pen microarraying of molecular beacon probes on microgel thin-film substrates

Micron-sized spots of molecular beacon probes are patterned on PEG microgel thin films using dip-pen nanolithography.

Locally altering the electronic properties of graphene by nanoscopically doping it with Rhodamine 6G

We show that Rhodamine 6G (R6G), patterned by dip-pen nanolithography on graphene, can be used to locally n-dope it in a controlled fashion. In addition, we study the transport and assembly properties of R6G on graphene and show that in general the π-π stacking between the aromatic components of R6G and the underlying graphene drives the assembly of these molecules onto the underlying substrate. However, two distinct transport and assembly behaviors, dependent upon the presence or absence of R6G dimers, have been identified. In particular, at high concentrations of R6G on the tip, dimers are transferred to the substrate and form contiguous and stable lines, while at low concentrations, the R6G is transferred as monomers and forms patchy, unstable, and relatively ill-defined features. Finally, Kelvin probe force microscopy experiments show that the local electrostatic potential of the graphene changes as function of modification with R6G; this behavior is consistent with local molecular doping, highlighting a path for controlling the electronic properties of graphene with nanoscale resolution.

Nanoscale crystallization of phase change Ge2Sb2Te5 film with AFM lithography

We have made nanoindents on Ge(2)Sb(2)Te(5)(GST) films using electric field-assisted atomic force microscope (AFM) lithography. GST shows increase of material density and electric conductivity as it changes from amorphous to crystalline phases. By applying electric field between AFM probe-tip and GST surface, nanoscale crystallization could be induced on tip contact area. As the crystallized GST exhibits increase of material density, that is to say depression of volume, nanoindented surface with crystallization is created on host amorphous GST (a-GST) film. For the AFM lithography, a highly conductive tip, which showed voltage-switching characteristics in current-voltage spectroscopy of GST film, was found to be very suitable for recording and sensing crystallized nanoindents on the GST film. By varying sample bias voltages, we performed nanoscale crystallization, and measured the nanostructured film in AFM conductance-image (C-image) mode and topography-image (T-image) mode, simultaneously. Two types of crystallized wires were fabricated on (a-GST) film. Type-I was sensed in only C-image, whereas Type-II was sensed in both C-image and T-image. These nanowires are discussed in terms of crystallization of GST and sensitivity of current (or topography) sensing. By repeated lithography, larger size of nanoindented wires were also produced, which indicates line-dimension controllability of AFM lithography.

"Force-feedback" leveling of massively parallel arrays in polymer pen lithography

Polymer pen lithography is a recently developed molecular printing technique which can produce features with diameters ranging from 80 nm to >10 microm in a single writing step using massively parallel (>10(7) pens) arrays of pyramidal, elastomeric pens. Leveling these pen arrays with respect to the surface to produce uniform features over large areas remains a considerable challenge. Here, we describe a new method for leveling the pen arrays that utilizes the force between the pen arrays and the surface to achieve leveling with a tilt of less than 0.004 degrees, thereby producing features that vary by only 50 nm over 1 cm.

Enabling individualized therapy through nanotechnology

Individualized medicine is the healthcare strategy that rebukes the idiomatic dogma of 'losing sight of the forest for the trees'. We are entering a new era of healthcare where it is no longer acceptable to develop and market a drug that is effective for only 80% of the patient population. The emergence of "-omic" technologies (e.g. genomics, transcriptomics, proteomics, metabolomics) and advances in systems biology are magnifying the deficiencies of standardized therapy, which often provide little treatment latitude for accommodating patient physiologic idiosyncrasies. A personalized approach to medicine is not a novel concept. Ever since the scientific community began unraveling the mysteries of the genome, the promise of discarding generic treatment regimens in favor of patient-specific therapies became more feasible and realistic. One of the major scientific impediments of this movement towards personalized medicine has been the need for technological enablement. Nanotechnology is projected to play a critical role in patient-specific therapy; however, this transition will depend heavily upon the evolutionary development of a systems biology approach to clinical medicine based upon "-omic" technology analysis and integration. This manuscript provides a forward looking assessment of the promise of nanomedicine as it pertains to individualized medicine and establishes a technology "snapshot" of the current state of nano-based products over a vast array of clinical indications and range of patient specificity. Other issues such as market driven hurdles and regulatory compliance reform are anticipated to "self-correct" in accordance to scientific advancement and healthcare demand. These peripheral, non-scientific concerns are not addressed at length in this manuscript; however they do exist, and their impact to the paradigm shifting healthcare transformation towards individualized medicine will be critical for its success.

The effects of organic vapor on alkanethiol deposition via dip-pen nanolithography

Dip-pen nanolithography (DPN) is a scanning probe-based technique that allows for direct delivery of molecules to a range of substrates with sub-50 nm resolution. This study describes the effect of organic solvent vapor on the deposition rate and feature size of nanostructures deposited via DPN. The transport rate of model molecular inks, 1-octadecanethiol, and 16-mercaptohexadecanoic acid were examined under atmospheres of ethanol, methanol, hexane, and dichloromethane. In all cases, presence of an organic vapor increased deposition rate and feature size, in some cases by an order of magnitude. This underscores how the environment can be used to regulate molecular transport rates in a DPN experiment.

Molecular simulation of the water meniscus in dip-pen nanolithography

We report a molecular dynamics simulation of the nanometer water meniscus formed in dip-pen nanolithography (DPN). When an atomic force microscope tip is in contact with a surface, the meniscus is significantly asymmetric around the tip axis. The meniscus as a whole can move away from the tip axis due to surface diffusion. The structure of the meniscus fluctuates and its periphery has a finite thickness as large as 25% of its width. We simulated the transport of nonpolar hydrophobic molecules through a water meniscus. Molecules move on the surface of, not dissolving into the interior of, the meniscus. As a result, an annular pattern forms in DPN. Even if the meniscus is cylindrically symmetric, the molecular flow from the tip and the subsequent pattern growth on the surface are anisotropic at the nanosecond timescale.

The detection of specific biomolecular interactions with micro-Hall magnetic sensors

The detection of reagent-free specific biomolecular interactions through sensing of nanoscopic magnetic labels provides one of the most promising routes to biosensing with solid-state devices. In particular, Hall sensors based on semiconductor heterostructures have shown exceptional magnetic moment sensitivity over a large dynamic field range suitable for magnetic biosensing using superparamagnetic labels. Here we demonstrate the capability of such micro-Hall sensors to detect specific molecular binding using biotin-streptavidin as a model system. We apply dip-pen nanolithography to selectively biotinylate the active areas of InAs micro-Hall devices with nanoscale precision. Specific binding of complementarily functionalized streptavidin-coated superparamagnetic beads to the Hall crosses occurs via molecular recognition, and magnetic detection of the assembled beads is achieved at room temperature using phase sensitive micro-Hall magnetometry. The experiment constitutes the first unambiguous demonstration of magnetic detection of specific biomolecular interactions with semiconductor micro-Hall sensors, and the selective molecular functionalization and resulting localized bead assembly demonstrate the possibility of multiplexed sensing of multiple target molecules using a single device with an array of micro-Hall sensors.

Nanowire and nanotube transistors for lab-on-a-chip applications

Implementation of one-dimensional nanostructure-based devices in the lab-on-a-chip framework can allow us to impart various functionalities such as highly-sensitive sensors to a single chip. However, it is still extremely difficult to position nanowires or nanotubes on a defined area of solid substrates to build integrated functional devices. Herein, we review promising strategies for the massive integration of nanowires/nanotubes on lab-on-a-chip and their practical applications to sensors. The theoretical understanding and sensor characteristics of nanowire/nanotube-based devices are also discussed.

Dip-pen-nanolithographic patterning of metallic, semiconductor, and metal oxide nanostructures on surfaces

Dip-pen nanolithography (DPN) is a powerful method to pattern nanostructures on surfaces by the controlled delivery of an "ink" coating the tip of an atomic force microscope upon scanning and contacting with surfaces. The growing interest in the use of nanoparticles as structural and functional elements for the fabrication of nanodevices suggests that the DPN-stimulated patterning of nanoparticles on surfaces might be a useful technique to assemble hierarchical architectures of nanoparticles that could pave methodologies for functional nanocircuits or nanodevices. This Review presents different methodologies for the nanolithographic patterning of metallic, semiconductor, and metal oxide nanostructures on surfaces. The mechanisms involved in the formation of the nanostructures are discussed and the effects that control the dimensions of the resulting patterns are reviewed. The possible applications of the nanostructures are also addressed.

Scanning probe lithography of polymers: tailoring morphology and functionality at the nanometer scale

This article reviews the patterning of the polymer via scanning probe lithography (SPL). Several different lithographies are characterized by the source of the patterned material, whether a mechanical, electrical, or thermal field is used, and whether the lithography modifies morphology, functionality, or both. The merits of the different strategies are discussed with respect to the fabrication goals.

Selective assembly and guiding of actomyosin using carbon nanotube network monolayer patterns

We report a new method for the selective assembly and guiding of actomyosin using carbon nanotube patterns. In this method, monolayer patterns of the single-walled carbon nanotube (swCNT) network were prepared via the self-limiting mechanism during the directed assembly process, and they were used to block the adsorption of both myosin and actin filaments on specific substrate regions. The swCNT network patterns were also used as an efficient barrier for the guiding experiments of actomyosin. This is the first result showing that inorganic nanostructures such as carbon nanotubes can be used to control the adsorption and activity of actomyosin. This strategy is advantageous over previous methods because it does not require complicated biomolecular linking processes and nonbiological nanostructures are usually more stable than biomolecular linkers.

Dip-pen nanolithography of high-melting-temperature molecules

Direct nanopatterning of a number of high-melting-temperature molecules has been systematically investigated by dip-pen nanolithography (DPN). By tuning DPN experimental conditions, all of the high-melting-temperature molecules transported smoothly from the atomic force microscope (AFM) tip to the surface at room temperature without tip preheating. Water meniscus formation between the tip and substrate is found to play a critical role in patterning high-melting-temperature molecules. These results show that heating an AFM probe to a temperature above the ink's melting temperature is not a prerequisite for ink delivery, which extends the current "ink-substrate" combinations available to DPN users.

Direct deposition and assembly of gold colloidal particles using a nanofountain probe

We report the direct delivery and assembly of negatively charged gold colloidal particles atop positively charged amino-terminated silicon oxide surfaces using a nanofountain atomic force microscopy probe. The experimental results and fluid simulations indicate that the flow of nanoparticles is confined to the core tip region of the probe. This leads to the assembly of high-resolution submicron patterns (200 nm) on the substrate with feature sizes dependent on the tip-substrate contact time. A diffusion mechanism for the patterning is proposed and discussed.

Bio-nanopatterning of Surfaces

Bio-nanopatterning of surfaces is a very active interdisciplinary field of research at the interface between biotechnology and nanotechnology. Precise patterning of biomolecules on surfaces with nanometre resolution has great potential in many medical and biological applications ranging from molecular diagnostics to advanced platforms for fundamental studies of molecular and cell biology. Bio-nanopatterning technology has advanced at a rapid pace in the last few years with a variety of patterning methodologies being developed for immobilising biomolecules such as DNA, peptides, proteins and viruses at the nanoscale on a broad range of substrates. In this review, the status of research and development are described, with particular focus on the recent advances on the use of nanolithographic techniques as tools for biomolecule immobilisation at the nanoscale. Present strengths and weaknesses, as well future challenges on the different nanolithographic bio-nanopatterning approaches are discussed.

Applications of dip-pen nanolithography

The ability to tailor the chemical composition and structure of a surface at the sub-100-nm length scale is important for studying topics ranging from molecular electronics to materials assembly, and for investigating biological recognition at the single biomolecule level. Dip-pen nanolithography (DPN) is a scanning probe microscopy-based nanofabrication technique that uniquely combines direct-write soft-matter compatibility with the high resolution and registry of atomic force microscopy (AFM), which makes it a powerful tool for depositing soft and hard materials, in the form of stable and functional architectures, on a variety of surfaces. The technology is accessible to any researcher who can operate an AFM instrument and is now used by more than 200 laboratories throughout the world. This article introduces DPN and reviews the rapid growth of the field of DPN-enabled research and applications over the past several years.

Directed-assembly of single-walled carbon nanotubes using self-assembled monolayer patterns comprising conjugated molecular wires

Self-assembled monolayer (SAM) patterns on electrodes are often utilized to guide the assembly of single-walled carbon nanotubes (SWCNTs) onto the electrodes to form desired device structures. In this case, the SWCNTs are in contact with the electrodes through the SAM which comprises molecular wires. Presumably, it is desirable to use conjugated molecular wires for a low contact resistance because they have been reported as a better electric conductor than non-conjugated ones. However, until now, the directed-assembly of SWCNTs has been driven mostly via molecular wires with alkane backbones which are known to be relatively poor conductors. Herein, we report large-scale directed-assembly of SWCNTs utilizing SAM patterns comprising conjugated molecular wires. We achieved highly selective adsorption and precision alignment of SWCNTs utilizing polar SAM patterns comprising conjugated molecular wires, while SAM patterns with non-polar terminal groups efficiently prevented adsorption of SWCNTs. Furthermore, we developed a process for assembling a SWCNT across two electrodes coated with conjugated molecular wires, and the electrical conduction through the SWCNT was measured via a conducting atomic force microscope. This result could be an important guideline for large-scale directed-assembly of SWCNT-based devices in the future.

Isometric graphing and multidimensional scaling for reaction-diffusion modeling on regular and fractal surfaces with spatiotemporal pattern recognition

Heterogeneous surface reactions exhibiting complex spatiotemporal dynamics and patterns can be studied as processes involving reaction-diffusion mechanisms. In many realistic situations, the surface has fractal characteristics. This situation is studied by isometric graphing and multidimensional scaling (IGMDS) of fractal surfaces for extracting geodesic distances (i.e., shortest scaled distances that obtain edges of neighboring surface nodes and their interconnections) and the results obtained used to model effects of surface diffusion with nonlinear reactions. Further analysis of evolved spatiotemporal patterns may be carried out by IGMDS because high-dimensional snapshot data can be efficiently projected to a transformed subspace with reduced dimensions. Validation of the IGMDS methodology is carried out by comparing results with reduction capabilities of conventional principal component analysis for simple situations of reaction and diffusion on surfaces. The usefulness of the IGMDS methodology is shown for analysis of complex patterns formed on both regular and fractal surfaces, and using generic nonlinear reaction-diffusion systems following FitzHugh Nagumo and cubic reaction kinetics. The studies of these systems with nonlinear kinetics and noise show that effects of surface disorder due to fractality can become very relevant. The relevance is shown by studying properties of dynamical invariants in IGMDS component space, viz., the Lyapunov exponents and the KS entropy for interesting situations of spiral formation and turbulent patterns.

“Sliding kinetics” of single-walled carbon nanotubes on self-assembled monolayer patterns: Beyond random adsorption

We present the experimental results and theoretical model describing new adsorption kinetics of single-walled carbon nanotubes (swCNTs) onto self-assembled monolayer (SAM) including their sliding motion. The adsorption behavior of swCNTs on large-size SAM patterns is similar to the Langmuir isotherm, while that on nanoscale patterns shows a significant deviation which can be explained by the sliding motion of adsorbed nanotubes. The "sliding chamber" experiment confirms that swCNTs can align along the SAM patterns by sliding motion right above the SAM surfaces. This result provides new scientific insights regarding the adsorption kinetics of one-dimensional nanostructures, and, from a practical point of view, it can be an important guideline to design SAM patterns to assemble carbon nanotubes and nanowires into desired device structures.

Selective Adsorption and Alignment Behaviors of Double- and Multiwalled Carbon Nanotubes on Bare Au and SiO2 Surfaces

We present the study of selective adsorption and alignment behaviors of double- and multiwalled carbon nanotubes (dwCNTs and mwCNTs) on self-assembled monolayer (SAM) patterns, bare Au, and SiO2 surfaces. dwCNTs and mwCNTs exhibited stronger affinity to polar SAMs, bare Au, and SiO2 surfaces than to nonpolar SAM surfaces. Furthermore, we found the adsorption probability of smaller carbon nanotubes (CNTs) was higher than that of larger CNTs. As proof of concept, we successfully assembled and aligned dwCNTs and mwCNTs on Au and SiO2 substrates without relying on external forces and demonstrated wafer-scale fabrication of back-gate transistors based on dwCNTs with a high yield.

Modeling collective behavior of molecules in nanoscale direct deposition processes

We present a theoretical model describing the collective behavior of molecules in nanoscale direct deposition processes such as dip-pen nanolithography. We show that strong intermolecular interactions combined with nonuniform substrate-molecule interactions can produce various shapes of molecular patterns including fractal-like structures. Computer simulations reveal circular and starlike patterns at low and intermediate densities of preferentially attractive surface sites, respectively. At large density of such surface sites, the molecules form a two-dimensional invasion percolation cluster. Previous experimental results showing anisotropic patterns of various chemical and biological molecules correspond to the starlike regime [P. Manandhar et al., Phys. Rev. Lett. 90, 115505 (2003); J.-H. Lim and C. A. Mirkin, Adv. Mater. (Weinheim, Ger.) 14, 1474 (2002); D. L. Wilson et al., Proc. Natl. Acad. Sci. U.S.A. 98, 13660 (2001); M. Su et al., Appl. Phys. Lett. 84, 4200 (2004); R. McKendry et al., Nano Lett. 2, 713 (2002); H. Zhou et al., Appl. Surf. Sci. 236, 18 (2004); G. Agarwal et al., J. Am. Chem. Soc. 125, 580 (2003)].

Growth Dynamics of Self-Assembled Monolayers in Dip-Pen Nanolithography

Using molecular dynamics simulations, we studied the growth mechanism of self-assembled monolayers in dip-pen nanolithography. A molecule dropping from the tip kicks out a molecule sitting on the substrate, and the displaced molecule in turn kicks out a molecule next to it. This kicking propagates and finally stops when it hits the periphery of the monolayer. This monolayer growth is faster than predicted from the previous diffusion theory. Increasing the molecule-substrate binding strength enhances the molecular deposition rate and makes the monolayer well-ordered.

Phase of molecular ink in nanoscale direct deposition processes

We report the first observation of a phase transition in a nanoscale direct deposition process. This transition involves the melting of molecular ink layers in dip-pen nanolithography, and it is observed by measuring the temperature dependence of the growth rate of the deposited pattern. The results are interpreted using a diffusion equation approach in conjunction with a "double-molecular-layer" model of the adsorbed molecules on the atomic force microscope tip. The theory provides a qualitative explanation for the dependence of the pattern growth rate on solvent and adsorbed water as well as on temperature.

Transport Rates Vary with Deposition Time in Dip-Pen Nanolithography

By patterning with dip-pen nanolithography using tip dwell times ranging from 15 s to 2 h over a period of 19 h, we show that the transport rate for smaller patterns is different than for larger ones. This transport behavior is found for both 1-octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHDA) inks on gold substrates. Additionally, MHDA shows an overall decrease in transport rate as a function of total writing time during such experiments. These results indicate that measurements with short dwell times are insufficient to determine transport rates for larger features.

Structural Behavior and Self-Assembly of Lennard-Jones Clusters on Rigid Surfaces

The phase behavior and surface pattern formation for intermediate size Lennard-Jones clusters on rigid surfaces are examined. We use a parallel tempering Monte Carlo algorithm, in the canonical ensemble. Tempering is done over the temperature domain in most of the calculations. A two-dimensional temperature and Hamiltonian tempering algorithm is also implemented, to examine its usefulness in investigating this type of problem. In general, we observe gas phase systems as they undergo a condensation transition on the surface, followed by a freezing transition. The final solid state pattern formed by the cluster on the surface is the result of a number of competing effects. First, there is a competition between attraction within the cluster and that between cluster and surface atoms. Second, a monolayer of Lennard-Jones atoms tends to pack in a hexadic geometry. This geometry is frustrated on a surface with a different symmetry. The molecular organization of the substrate has a serious impact on the cluster packing. The surface morphology and the size mismatch between cluster and surface atoms, along with the relative interaction strengths, determine which of the effects prevail. When the surface atoms are small enough, the interactions within the cluster determine the symmetry of the pattern. In such a case, the substrate behaves similarly to a continuous surface, and the low-temperature pattern is a hexadic monolayer. When the sizes of the surface and cluster atoms are comparable, the low-temperature adsorbed geometry mimics the substrate symmetry. On a face-centered cubic surface, face-centered cubic monolayers or droplets are obtained.

Molecular transfer and transport in noncovalent microcontact printing

Microcontact printing is commonly used to create patterned films of molecules covalently bonded to substrates (e.g., thiols on gold). Here we describe microcontact printing of several types of noncovalently bonding molecules on mica. Due to the weaker interaction of the molecules with the substrate, environmental factors such as temperature and relative humidity play an important role. The vapor pressure of the inks also had a large impact on the fidelity of the stamped patterns. Fingering instabilities were observed for monolayers of octadecanol, docosanol, stearylamine, and stearic acid stamped at moderate relative humidity. The fidelity of the stamped pattern generally increased with the headgroup-surface interaction strength. These stamped monolayer films shed light on molecular transfer and two-dimensional spreading mechanisms.

Dip-pen nanolithography of reactive alkoxysilanes on glass

The use of organofunctional silane chemistry is a flexible and general method for immobilizing biomolecules on silicon oxide surfaces, including fabricating DNA, small-molecule, and protein microarrays. The biggest hurdle in employing dip-pen nanolithography (DPN) for extending this general approach to the nanoscopic domain is the tendency of trialkoxy- and trichlorosilanes to rapidly polymerize due to hydrolysis reactions. The control of the local water concentration between the substrate surface and the scanning AFM tip is critical, both to the physical and chemical processes involved in DPN writing and to the ability to form well-defined thin layers of reactive silanes without extensive polymerization induced disorder. We found that we could control the degree of polymerization through careful choice of the alkoxysilane used as the "ink" for DPN and through control of the relative humidity during inking and writing with the coated AFM tip. As a proof-of-principle, we demonstrate that areas patterned with an alkoxysilane on glass with DPN are functional for subsequent immobilization of fluorescently labeled streptavidin via covalent attachment of biotin. This preliminary result sets the stage for the ability to capture proteins in their fully hydrated state from buffered solution, by molecular recognition onto previously written reactive nanoscopic regions on oxidized silicon and glass.

Nanotube electronics: large-scale assembly of carbon nanotubes

Nanoscale electronic devices made from carbon nanotubes, such as transistors and sensors, are much smaller and more versatile than those that rely on conventional microelectronic chips, but their development for mass production has been thwarted by difficulties in aligning and integrating the millions of nanotubes required. Inspired by biomolecular self-assembly processes, we have created chemically functionalized patterns on a surface, to which pre-grown nanotubes in solution can align themselves in huge numbers. This method allows wafer-scale fabrication of millions of carbon-nanotube circuits with single-nanotube precision, and may enable nanotube-based devices, such as computer chips and high-density sensor arrays, to be produced industrially.