Particle-substrate Interactions Research Articles

Adsorption of Spherical Polyelectrolyte Brushes: from Interactions to Surface Patterning

Abstract Adsorption of colloidal particles constitutes an attractive route to tailor the properties of surfaces. However, for efficient material design full control over the particle-substrate interactions is required. We investigate the interaction of spherical polyelectrolyte brushes (SPB) with charged substrates based on adsorption studies and atomic force spectroscopy. The brush layer grafted from the colloidal particles allows a precise adjustment of their adsorption behavior by varying the concentration of added salt. We find a pronounced selectivity between oppositely and like-charged surfaces for ionic strengths up to 10 mM. Near the transition from the osmotic to the salted brush regime at approximately 100 mM attractive secondary interactions become dominant. In this regime SPB adsorb even to like-charged surfaces. To determine the adhesion energy of SPB on charged surfaces directly, we synthesize micrometer-sized SPB. These particles are used in colloidal probe AFM studies. Measurements on oppositely charged surfaces show high forces of adhesion for low ionic strengths that can be attributed to an entropy gain by counterion release. Transferring our observations to charge patterned substrates, we are able to direct the deposition of SPB into two-dimensional arrays. Considering that numerous chemical modifications have been reported for SPB, our studies could open exiting avenues for the production of functional materials with a hierarchical internal organization.

Back Cover: Self-Assembly of Vertically Aligned Gold Nanorod Arrays on Patterned Substrates (Angew. Chem. Int. Ed. 35/2012)

The self-assembly of gold nanorods into standing arrays was achieved with precise placement over large areas. In their Communication on page 8732 ff., U. Bach et al. report on a nanofabrication process based on convective and capillary forces. The functionalization of the nanorods can be used to fine-tune the interparticle and particle–substrate interaction energies, leading to high hexagonal close-packed ordering.

Post-fabrication Voltage Controlled Resonance Tuning of Nanoscale Plasmonic Antennas

Voltage controlled wavelength tuning of the localized surface plasmon resonance of gold nanoparticles on an aluminum film is demonstrated in single particle microscopy and spectroscopy measurements. Anodization of the Al film after nanoparticle deposition forms an aluminum oxide spacer layer between the gold particles and the Al film, modifying the particle-substrate interaction. Darkfield microscopy reveals ring-shaped scattering images from individual Au nanoparticles, indicative of plasmon resonances with a dipole moment normal to the substrate. Single particle scattering spectra show narrow plasmon resonances that can be tuned from ~580 to ~550 nm as the anodization voltage increases to 12 V. All observed experimental trends could be reproduced in numerical simulations. The presented approach could be used as a general postfabrication resonance optimization step of plasmonic nanoantennas and devices.

Effect of Carbon Supports on Electrocatalytic Reactivity of Au–Pd Core–Shell Nanoparticles

The role of particle-substrate interactions on the reactivity of bimetallic nanostructures is investigated in the case of Au–Pd core–shell nanoparticles supported on Vulcan XC-72R (Vulcan). Core–shell nanostructures (CS) featuring 19 nm Au cores and Pd shells with thicknesses between ca. 1 and 10 nm were synthesized by controlled colloidal methods and subsequently incorporated in the carbon support. X-ray diffraction, energy dispersive X-ray analysis, and high resolution transmission electron microscopy confirmed the CS nature of the nanostructures, which remain unaffected upon incorporation onto the carbon matrix. Their electrochemical properties toward CO and HCOOH electro-oxidation were studied, using cyclic voltammetry and chronoamperometry. The results show that the CO stripping potential becomes independent of the average Pd lattice strain in the case of Vulcan supported CS. This behavior is significantly different to the trend observed in CS assemblies at In-doped SnO2 electrodes. Formic acid oxida...

Self-alignment of Gold Nanoparticles through the Control of Particle-substrate and Particle-particle Interactions

Gold (Au) nanostructures have been fabricated on various substrates, such as Si, ITO and quartz glass, through the position-selective deposition of Au nanoparticles. This self-aligned deposition of the nanoparticles is promoted through the electrostatic interaction between the nanoparticles and the deposition sites, which are charged negatively and positively, respectively. Prior to the nanoparticle deposition, the positively charged sites where covered with aminosilane self-assembled monolayer had been constructed on the substrate with an artificial pattern design by a lithographic method. A photolithographic process, which was based on direct photochemical modification of organic substances induced by irradiating with a vacuum ultra-violet (VUV) light, was employed so that sub-μm features down to 500nm have been successfully fabricated and provided as templates for aligning Au nanoparticles. The second method for patterning was electrochemical line drawing using a scanning probe microscope. One-dimensional arrays of Au nanoparticles with a line thickness of a single nanoparticle diameter have been successfully fabricated. In addition, by replacing a surface charged layer consisting of citric acid molecules surrounding the Au nanoparticles with alkanethiol molecules, the Au nanoparticles were neutralized so that the fabricated Au nanoparticle array in which most of the nanoparticles were separated each other was re-organized to a more closely packed structure.

Salt-regulated attraction and repulsion of spherical polyelectrolyte brushes towards polyelectrolyte multilayers

Adsorption of colloidal particles presents an interesting alternative to the modification of surfaces using covalent coupling or physisorption of molecules. However, to tailor the properties of these materials full control over the effective particle-substrate interactions is required. We present a systematic investigation of the adsorption of spherical polyelectrolyte brushes (SPB) onto polyelectrolyte multilayers (PEM). A brush layer grafted from colloidal particles allows the incorporation of various functional moieties as well as the precise adjustment of their adsorption behaviour. In the presence of oppositely charged surfaces the amount of adsorbed SPB monotonically increases with the ionic strength, whereas equally charged substrates efficiently prevent colloidal attachment below a threshold salt concentration. We found that the transition from the osmotic to the salted brush regime at approximately 100 mM coincided with a complete loss of substrate selectivity. In this regime of high ionic strength, attractive secondary interactions become dominant over electrosteric repulsion. Due to the soft polyelectrolyte corona a surface coverage exceeding the theoretical jamming limit could be realized. Both the adsorption kinetics and the resulting thin film morphologies are discussed. Our study opens avenues for the production of two-dimensional arrays and three-dimensional multilayered structures of SPB particles.

Electrochemical performance of Pd and Au–Pd core–shell nanoparticles on surface tailored carbon black as catalyst support

The present work studies the influence of the surface chemistry of carbon supports on the electrochemical behaviour of Pd and Au–Pd core–shell (CS) nanoparticles. Vulcan XC-72R was chemically modified by different acid treatments, inducing changes in the volume of the mesopores and surface density of oxygenated species. The CS nanostructures featuring 19 nm Au cores and 10 nm Pd shells were synthesized by colloidal methods and subsequently incorporated to the carbons supports. Pd nanoparticles were prepared by impregnating a Pd precursor into the modified carbons followed by reduction with sodium borohydride. The use of different preparation methods allowed the independent study of the effect of the support on the morphology/distribution of the nanoparticles and on the reactivity of the nanoparticles, through their interaction with organic molecules. The electrocatalysts were characterised by XRD, EDX, Raman spectroscopy and contact angle measurements. CO and formic acid (HCOOH) electro-oxidation were studied using cyclic voltammetry and chronoamperometry. The effect of the carbon support on the electrocatalytic activity was highly dependent on the method of preparation. Pd nanoparticles obtained by impregnation showed higher HCOOH oxidation currents when supported on the highly oxidised Vulcan support. This is due to the generation of smaller particle sizes (2.3 nm) as a result of the high density of oxygenated functional groups. On the other hand, the CS nanostructures are significantly less active in highly oxidised Vulcan as a results of specific chemical interactions which may be related to the formation of oxides. The implication of these findings towards rationalising particle–substrate interactions are briefly discussed.

Robust tracking control of an array of nanoparticles moving on a substrate

Control of nanosystems with frictional dynamics using feedback control methods is important to a wide range of applications of nanotribology. This paper studies the tracking control problem of an array of nanoparticles moving on a substrate with friction between the substrate and the particles. The focus of this study is on control design and stability analysis. The major challenges in this problem include nonlinearities and uncertainties in the frictional dynamics and limited availability of measurable states in nanosystems. The particle–substrate interaction is considered to be unknown, and the unknown effect of unmodeled particle dynamics on the dynamics of the center of mass of the array is also considered. A nonlinear identifier is first developed to identify these unmodeled dynamics. A feedback controller is then developed based on the identifier to control the center of mass of the particles to track a desired trajectory. Boundedness of the closed-loop states and semiglobal asymptotic stability of the tracking error are proven using Lyapunov theory for the case of linear inter-particle interactions. An example with more general Morse-type inter-particle interactions is included to provide some level of confidence that the results are general but not assuredness that they are. Numerical simulation results are provided to demonstrate the performance of the developed identification and control law.

Polarization-Dependent Scanning Photoionization Microscopy: Ultrafast Plasmon-Mediated Electron Ejection Dynamics in Single Au Nanorods

This work investigates plasmon-enhanced multiphoton scanning photoelectron emission microscopy (SPIM) of single gold nanorods under vacuum conditions. Striking differences in their photoemission properties are observed for nanorods deposited either on 2 nm thick Pt films or 10 nm thick indium tin oxide (ITO) films. On a Pt support, the Au nanorods display fourth-order photoionization when excited at 800 nm, a wavelength corresponding to their plasmon resonance in aqueous solution. A cos(8)(θ) dependence of the photoelectron flux on laser polarization implies photoemission mediated by the dipolar plasmon; however, no plasmon resonance signature is exhibited over the 750-880 nm range. Electromagnetic simulations confirm that the resonance is severely broadened compared to aqueous solution, indicative of strong interactions between the Au nanorod and propagating surface plasmon modes in the Pt substrate. On ITO substrates, by way of contrast, sharp plasmon resonances in the photoemission from individual Au nanorods are observed, with widths limited only by fundamental internal electron collision processes. Furthermore, the ensemble-averaged plasmon resonance for Au nanorods on ITO is almost unshifted compared to its frequency in solution. Both findings suggest that plasmonic particle-substrate interactions are suppressed in the Au/ITO system. However, Au nanorods on ITO exhibit a surprising third-order photoemission (observed neither in Au nor ITO by itself), indicating that electrostatic interactions introduce a substantial shift in the work function for this fundamental nanoparticle-substrate system.

Interaction of bi-dispersed particles with contact line in an evaporating colloidal drop

The deposition behavior of an inkjet-printed aqueous colloidal mixture of micro- and nanoparticles onto a glass substrate with systematically varied wettability has been investigated using fluorescence microscopy. Real-time bottom-view images show that particles inside an evaporating drop rearrange themselves near the drop contact line according to their sizes, where smaller particles tend to deposit closer to the contact line compared to the larger ones. By increasing substrate wettability, particles in the bi-dispersed mixture can be further separated compared to those on substrates of poor wettability. This is primarily because, during different stages of evaporation, the interplay of surface tension, drag due to evaporative flow, and particle–substrate interactions rearrange particles inside a colloidal drop near the contact line region. Forces acting on particles determine the extent to which particles enhance contact line pinning, which ultimately determines the final deposition morphology of particles from a bi-dispersed colloidal mixture.

Effects of particle contamination and substrate interaction on the Raman response of unintentionally doped graphene

We investigated the inhomogeneities in the charge density of unintentionally doped graphene on SiO2 prepared by mechanical exfoliation. From the analysis of the G, D, and 2D phonon modes of the Raman spectra after displacing contaminants on graphene surface, and measuring the separation monolayer-substrate distance among zones with different doping levels, we deduce that the interaction with the substrate is the main cause of doping in graphene rather than particle contamination. In particular, we show how graphene doping levels vary within the same flake depending on the distance between graphene and the substrate.

Lifting and Sorting of Charged Au Nanoparticles by Electrostatic Forces in Atomic Force Microscopy

Manipulation of nanoparticles and biomolecules is an upcoming fi eld with potential applications in electronic and optical devices [ 1,2 ] and a variety of biosensors. [ 3,4 ] Ever since the fi rst particle manipulation experiment done by Eigler and Schweizer using scanning tunnel microscope, [ 5 ] there has been tremendous effort around the world to slide, lift or deposit nanoparticles with a high degree of specifi city and positioning them on desired substrates. [ 6–9 ] The atomic force microscope (AFM) is by far the most promising tool to manipulate single nanoparticles or nanoparticle conjugates. Depending on the path of the moving nanoparticle, manipulation can be categorized as horizontal or vertical. In the case of horizontal nanomanipulation, fewer factors affect the manipulation process (e.g., gravitational force, particle-substrate detachment force, and particle-probe attachment force could be ignored). The sliding nature of horizontal movement makes the particle-substrate interactions and particle-probe interactions less demanding. In recent works, nanostructures of various materials, sizes [ 10–13 ] and shapes [ 14 ] have been horizontally manipulated at ambient atmospheric conditions or even in fl uid environments. [ 15 ] Both contact and non-contact modes [ 16 ] of AFM have been successfully used for such particle manipulations. Vertical manipulation on the other hand is strongly dependent on both the probe-particle interaction and the particle-substrate interaction forces. The vertical deposition and patterning of small organic molecules on inorganic substrates has been widely reported using direct-writing techniques like dip pen nanolithography (DPN). [ 17,18 ] Removal or patterning of larger molecules like proteins and DNA require additional factors like mechanical forces as in AFM based nanografting [ 19,20 ] or electric forces as in electrochemical DPN, [ 21 ] nanopipetting [ 22,23 ] and dielectrophoretic deposition of DNA. [ 24 ] Thus far vertical lifting or deposition

Physical attachment of fluorescent protein particles to atomic force microscopy probes in aqueous media: Implications for surface pH, fluorescence, and mechanical properties studies

Transfer of a fluorescently labeled protein particle from a surface to a microsized scanning probe has been induced by repetitive scanning in aqueous medium. The so-attached particle can in turn act as a probing tool to study particle-substrate and particle-particle interactions. Attachment of the fluorescent particle occurs at the apical region of an atomic force microscope (AFM) cantilever tip and it endures repetitive loading-unloading cycles against the sample surface. Fluorescence microscopy has been used to address the exact location of the attached particle in the cantilever and to identify the moment when the particle contacts the sample. Moreover, we have observed that fluorescence intensity at the contact point is lower when the probing particle contacts another fluorescent particle than when it contacts the nonfluorescent substrate. The change in fluorescence is attributed to local changes of pH and interparticle-quenching of fluorophores in the contact region. These findings are promising since they constitute a chemical-free way to attach bioparticles to AFM probes under physiological conditions. The atomic force microscopy combined with fluorescence microscopy provides a straight forward method to study particle/particle and particle/substrate interactions, as well as to investigate mechanical properties of biocolloids.

Influence of Particle−Substrate Interaction on Localized Plasmon Resonances

We present a theory for determining the localized surface plasmon resonance shifts of arbitrarily shaped metal nanoparticles on a substrate. Using a pseudoparticle concept, an expression for the particle-substrate interaction is derived, providing both physical insight and formulas to estimate the shifted plasmon resonance. The theory is verified against measured scattering spectra of nanorods on substrates. Simple formulas are provided to calculate the resonance of nanorods, spheres, and ellipsoids on dielectric substrate.

Silver Nanoparticles with Controlled Dispersity and Their Assembly into Superstructures

In this paper we report on the influence of particle size distribution, particle substrate interaction, and drying behavior on the self-assembly process using ligand stabilized silver particles. Two-dimensional particle arrays were characterized using transmission electron microscopy and extensive image analysis. The formation of such structures was observed in situ using an environmental scanning electron microscope in WET-STEM mode. The results confirm that a small particle size distribution is crucial for the formation of regular particle patterns with long range order, but also the particle substrate interaction and the particle density have an influence on the degree of ordering. Additionally, we find that separated binary particle assemblies keep the orientation of their two-dimensional hexagonal lattices over alternating domains of small and big particles. This is probably enabled due to the formation of dislocations and a small change of the course of the lattice lines within the respective boundary.

The Effect of HVOF Particle-Substrate Interactions on Local Variations in the Coating Microstructure and the Corrosion Resistance

Splashing and redeposition of droplets occur during thermal spray processing, which affects the coating porosity and morphology. Therefore, this phenomenon is important from a practical point of view such as corrosion. Particle interaction with substrate is a function of the particle velocity, viscosity, temperature, as well as the substrate temperature, chemistry, roughness, and geometry. In the present study, the splashing phenomenon was studied on CrC-NiCr and stainless steel materials deposited using the high velocity oxygen fuel process. The effect of particle splashing on the coating microstructure was investigated with respect to the corrosion properties. Particle behavior during impact was explained based on in-flight particle velocity and temperature measurements. It was found that the conditions that favor particle splashing promote occurrence of localized porosity. The localized porosity was a strong function of the substrate curvature and originated from the substrate asperities.

Dynamics in Tethered Particle Motion: Interpreting the Observations

Tethered Particle Motion (TPM) enables the researcher to examine the properties of semi-flexible polymers at the single molecule level. In TPM, a small reporter particle is tethered to a substrate using the polymer of interest. The particle motion reflects the mechanical properties of the tethering polymer.We will present a framework in which the influence of the different experimental aspects on the measurement outcome are quantified. The key elements are tether length, particle size, exposure time, fluid properties and frame rate.Here, we use 80 nanometer diameter gold particles, tethered to a glass slide by double stranded DNA, that are visualized by dark field microscopy. The recorded images of these highly scattering particles have a high contrast and signal-to-noise ratio; therefore the particle can be tracked with high spatial resolution (5-20 nm).High temporal resolution is necessary to distinguish between different diffusion regimes. At very short time scales the particle is freely diffusing and on longer time scales its motion is influenced and eventually restricted by the retracting harmonic force of the tether. We will show that the diffusion coefficient of the free motion on short time scales is composed of the diffusion properties of both the particle and tether and that the harmonic potential stems from the entropic elasticity of the tether molecule. Motion blur caused by the finite exposure time has to be considered for computing the diffusion constant. Lastly, the choice of tether length and particle size play an important role as well. They determine how often the particle will be in proximity of the substrate where particle-substrate interactions such as van-der-Waals and electrostatic forces play a bigger role.

Order formation of colloidal nanoparticles adsorbed on a substrate with friction

We examine the adsorption process and order formation of colloidal nanoparticles on a planar surface with friction. We perform Brownian dynamics simulations with a three-dimensional cell model in which the particle–particle and particle–substrate interactions are modeled on the DLVO theory, and examine the effects of the friction acting between the adsorbed particles and the substrate on the adsorbed structure formed on the substrate. The results obtained are as follows: when the friction is so strong that the adsorbed particles are stuck to the substrate, ordered structures never form, which seems to be quite natural. However, when the magnitude of the frictional force is moderate, an ordered structure can form even with low coverage because the frictional force aids order formation. This is because the friction counterbalances the particles’ Brownian motion, which would otherwise disturb the ordered structure. Furthermore, through a detailed examination of the distribution of the Brownian motion, it is demonstrated that an increase in the friction has a similar effect as a decrease in temperature.

Is Surface Roughness a “Scapegoat” or a Primary Factor When Defining Particle−Substrate Interactions?

Extended DLVO interaction potentials were determined for spherical particles approaching nanopatterned substrates using the numerical surface element integration (SEI) technique. In most cases, nanopatterned ("rough") surfaces produced smaller interaction potentials than chemically identical planar ("smooth") surfaces. For unfavorable scenarios, electrostatic double layer and acid-base potentials were reduced to a greater extent than van der Waals potentials, which made rough surfaces "more attractive" than smooth ones. Two influential surface morphological descriptors emerged: (1) the ratio of particle size to asperity size, a/r, and (2) the ratio of asperity separation to asperity size, p/r. As a/r increased, particle-substrate interaction energy decreased, while the opposite was true for p/r. The simple morphological descriptors gave rise to an analytical model based on the Derjaguin integration (DI) method that compared reasonably well with numerical SEI results, where the size and density of nanopatterned surface features dictated the magnitude of interaction potentials. In fact, changes in the size of nanopatterned surface features impacted the magnitudes of interaction potentials to the same extent as similar changes in the magnitudes of acid-base free energy and zeta potential, which begs the question, "is surface morphology a 'scapegoat' or a primary consideration when defining particle-substrate interactions?"

The persistence length of double stranded DNA determined using dark field tethered particle motion

The wormlike chain model describes the micromechanics of semiflexible polymers by introducing the persistence length. We propose a method of measuring the persistence length of DNA in a controllable near-native environment. Using a dark field microscope, the projected positions of a gold nanoparticle undergoing constrained Brownian motion are captured. The nanoparticle is tethered to a substrate using a single double stranded DNA (dsDNA) molecule and immersed in buffer. No force is exerted on the DNA. We carried out Monte Carlo simulations of the experiment, which give insight into the micromechanics of the DNA and can be used to interpret the motion of the nanoparticle. Our simulations and experiments demonstrate that, unlike other similar experiments, the use of nanometer instead of micrometer sized particles causes particle-substrate and particle-DNA interactions to be of negligible effect on the position distribution of the particle. We also show that the persistence length of the tethering DNA can be estimated with a statistical error of 2 nm, by comparing the statistics of the projected position distribution of the nanoparticle to the Monte Carlo simulations. The persistence lengths of 45 single molecules of four different lengths of dsDNA were measured under the same environmental conditions at high salt concentration. The persistence lengths we found had a mean value of 35 nm (standard error of 2.8 nm), which compares well to previously found values using similar salt concentrations. Our method can be used to directly study the effect of the environmental conditions (e.g., buffer and temperature) on the persistence length.