Real-time plasmon spectroscopy study of the solid-state oxidation and Kirkendall void formation in copper nanoparticles

Green Cu particles for functional and biodegradable food packaging solutions

This study demonstrates a simple approach to synthesize green Cu particles stabilized by poly(n-vinyl)pyrrolidone (PVP): the latter acts as stabilizer and dispersant, and its presence in solution eliminates the need for an inert atmosphere. Synthetic parameters were tuned to obtain particles with diameters >200 nm, to be human-safe and prevent nano-cytotoxicity. PVP and reductant concentrations, with reaction times, were varied to investigate their effect on colloidal stability, kinetics, and particles size. Particles were fully characterized, morphologically and spectroscopically. Cu@PVP colloids were washed, to remove unbound PVP and reactions byproducts, and then embedded in chitosan (CS) polymer matrix, to prepare self-standing films for food packaging applications. Films were obtained by a simple solvent evaporation protocol. Pellicles were characterized by common analytical techniques; viscoelastic properties, water uptake, and Cu2+ ionic release were investigated, as well. The films antimicrobial efficacy was also tested against three different model fungi responsible for agrifood spoilage.

Mapping the local stoichiometry in Cu nanoparticles during controlled oxidation by STEM-EELS spectral imaging

Copper nanoparticles (NPs) can be coupled with cuprous oxide, combining photoelectrocatalytic properties with a broad-range optical absorption. In the present study, we aimed to correlate changes in morphology, electronic structure and plasmonic properties of Cu NPs at different stages of oxidation. We demonstrated the ability to monitor the oxidation of NPs at the nanometric level using STEM-EELS spectral maps, which were analyzed with machine learning algorithms. The oxidation process was explored by exposing Cu NPs to air plasma, revealing systematic changes in their morphology and composition. Initial plasma exposure created a Cu2O shell, while prolonged exposure resulted in hollow structures with a CuO shell. This study identified procedures to obtain a material with Cu2O surface stoichiometry and absorption extended into the near-infrared range. Moreover, this study introduced a novel application of machine learning clustering techniques to analyze the morphological and chemical evolution of a nanostructured sample.

Switching of electrochemical selectivity due to plasmonic field-induced dissociation

Electrochemical reactivity is known to be dictated by the structure and composition of the electrocatalyst-electrolyte interface. Here, we show that optically generated electric fields at this interface can influence electrochemical reactivity insofar as to completely switch reaction selectivity. We study an electrocatalyst composed of gold-copper alloy nanoparticles known to be active toward the reduction of CO2 to CO. However, under the action of highly localized electric fields generated by plasmonic excitation of the gold-copper alloy nanoparticles, water splitting becomes favored at the expense of CO2 reduction. Real-time time-dependent density functional tight binding calculations indicate that optically generated electric fields promote transient-hole-transfer-driven dissociation of the O─H bond of water preferentially over transient-electron-driven dissociation of the C─O bond of CO2. These results highlight the potential of optically generated electric fields for modulating pathways, switching reactivity on/off, and even directing outcomes.

Spark Discharge Aerosol-Generated Copper-Based Nanoparticles: Structural & Optical Properties; Application on the Antiviral (SARS-CoV-2) and Antibacterial Improvement of Face Masks

Nanoparticle formation by Spark Discharge Aerosol Generation offers low-cost fabrication of nanoparticles, without the use of chemicals or vacuum. It produces aerosol particles of a few nanometers in size with high purity. In this work, copper-based -CuO (tenorite) and Cu- nanoparticles are produced, characterized and used to modify face mask air filters, achieving the introduction of antibacterial and antiviral properties. A range of characterization techniques have been employed, down to the atomic level. The majority of the particles are CuO (of a few nanometers in size that agglomerate to form aggregates), the remainder being a small number of larger Cu particles. The particles were deposited on various substrates, mainly fiber filters in order to study them and use them as biocidal agents. On face masks, their antibacterial activity against Escherichia coli (E.coli) results in a 100 % decrease in bacteria cell viability. Their antiviral activity on face masks results in a 90 % reduction of the Severe Acute Respiratory Syndrome Corona Virus 2 (SARS-CoV-2) viability, 15 minutes post the application of the virus stock solution. This highlights the effectiveness of this approach, its simplicity, its low cost and its excellent environmental credentials.

Highly stable localized surface plasmon resonance of Cu nanoparticles obtained via oxygen plasma irradiation

Copper nanoparticles (CuNPs) possess strong localized surface plasmon resonance (LSPR) in visible light. However, CuNPs are not chemically stable in air, which has seriously hindered the applications based on the LSPR of CuNPs. We developed an artificial method to passivate CuNPs as Al naturally does in air, preventing the oxidation of CuNPs through swift oxidation of the surface atoms via oxygen plasma irradiation. A hemispheric core-shell structure of CuNPs uniformly covered by a dense CuO shell (CuNPs@d-CuO) was constructed. The 4 nm d-CuO shell can prevent CuNPs from further oxidation. As a result, the LSPR of the CuNPs is stable in air over 180 days.

In situ formation of CuxO/ZnO photocatalysts for efficient simultaneous oxidation of As (III) and adsorption of As (V): Effect of Cu loading

The introduction of Cu ions onto ZnO leads to alterations in the electrical, optical, and magnetic characteristics of ZnO. These transformations, in turn, result in heightened photocatalytic activity and enhanced stability when employed in the degradation of both organic and inorganic pollutants. Here, a novel photocatalytic-adsorbent system is developed using zinc oxide (ZnO) nanostructures modified with Cu (II) ions in an aqueous solution containing 40 mg/L of As (III). The system utilizes UV-A light (365 nm) as the irradiation source, and the weight percentage of Cu (II) in the composite varies from 0 to 20%. The experimental results reveal significant adsorption of As (III), ranging from 20 to 50%, depending on the solution's Cu (II) content. Remarkably, the ZnO10%Cu composite exhibits the highest photocatalytic activity, achieving 40% adsorption and complete oxidation of As (III) within 25 min of irradiation. Characterization of the composite after the photocatalytic treatment reveals the effective adsorption of As (V) within its structure. Furthermore, no traces of Cu (II) ions are detected in the solution after the reaction, indicating their successful adsorption onto the ZnO surface as Cu (I) and Cu (II) ions. This research marks a significant advancement in harnessing innovative materials for efficient arsenic removal, offering promising insights into the development of novel photocatalytic-adsorbent systems.

Spectral analysis of oxidation on localized surface plasmon resonance of copper nanoparticles thin film

Copper nanoparticles (CuNPs) possess localized surface plasmon resonance (LSPR) effect. Cu thin films composed of individual CuNPs exhibit stronger LSPR than the individual CuNPs due to the LSPR coupling among CuNPs. However, CuNPs are easy to be oxidized, which results in the rapid LSPR damping of the CuNPs thin films. Simulation of the variations of the coupled LSPR of two adjacent CuNPs with the thickness of oxide shells formed during oxidation is of great importance for understanding the mechanisms of the strong LSPR of CuNPs thin films and its rapid attenuation. In this paper, Discrete-dipole approximation method is used to simulate the extinction spectra of two adjacent spherical CuNPs as a function of the shell thickness (t), the ambient refractive index (n), the diameter (D) of the CuNPs, and the inter-nanoparticle spacing (L). The calculation is validated by experimental results. According to our model, for a definite CuNPs thin films, the oxide shell thickness of CuNPs can be calculated only if the extinction spectra and the morphology are provided. Further, it is found when the oxide shell thickness is small (t/R< 0.3), increasing n and decreasing L/D have an obvious synergistic effect on enhancing the coupled LSPR, but this synergistic effect weakens with the deepening of oxidation, and disappeared when t/R > 0.5. This study provides a calculation method for coupled core-shell nanoparticles and throws light on the role of oxidation on the rapid damped LSPR of CuNPs thin films.

The Role of Grain Boundary Sites for the Oxidation of Copper Catalysts during the CO Oxidation Reaction

The oxidation of transition metal surfaces is a process that takes place readily at ambient conditions and that, depending on the specific catalytic reaction at hand, can either boost or hamper activity and selectivity. Cu catalysts are no exception in this respect since they exhibit different oxidation states for which contradicting activities have been reported, as, for example, in the catalytic oxidation of CO. Here, we investigate the impact of low-coordination sites on nanofabricated Cu nanoparticles with engineered grain boundaries on the oxidation of the Cu surface under CO oxidation reaction conditions. Combining multiplexed in situ single particle plasmonic nanoimaging, ex situ transmission electron microscopy imaging, and density functional theory calculations reveals a distinct dependence of particle oxidation rate on grain boundary density. Additionally, we found that the oxide predominantly nucleates at grain boundary-surface intersections, which leads to nonuniform oxide growth that suppresses Kirkendall-void formation. The oxide nucleation rate on Cu metal catalysts was revealed to be an interplay of surface coordination and CO oxidation behavior, with low coordination favoring Cu oxidation and high coordination favoring CO oxidation. These findings explain the observed single particle-specific onset of Cu oxidation as being the consequence of the individual particle grain structure and provide an explanation for widely distributed activity states of particles in catalyst bed ensembles.

Electron Beam Induced Enhancement and Suppression of Oxidation in Cu Nanoparticles in Environmental Scanning Transmission Electron Microscopy

We have investigated the effects of high-energy electron irradiation on the oxidation of copper nanoparticles in environmental scanning transmission electron microscopy (ESTEM). The hemispherically shaped particles were oxidized in 3 mbar of O2 in a temperature range 100-200 °C. The evolution of the particles was recorded with sub-nanometer spatial resolution in situ in ESTEM. The oxidation encompasses the formation of outer and inner oxide shells on the nanoparticles, arising from the concurrent diffusion of copper and oxygen out of and into the nanoparticles, respectively. Our results reveal that the electron beam actively influences the reaction and overall accelerates the oxidation of the nanoparticles when compared to particles oxidized without exposure to the electron beam. However, the extent of this electron beam-assisted acceleration of oxidation diminishes at higher temperatures. Moreover, we observe that while oxidation through the outward diffusion of Cu+ cations is enhanced, the electron beam appears to hinder oxidation through the inward diffusion of O2- anions. Our results suggest that the impact of the high-energy electrons in ESTEM oxidation of Cu nanoparticles is mostly related to kinetic energy transfer, charging, and ionization of the gas environment, and the beam can both enhance and suppress reaction rates.

Optical Characteristics of MgAl2O4 Single Crystals Irradiated by 220 MeV Xe Ions

In In this study, the optical properties of magnesium-aluminate spinel were examined after being irradiated with 220 MeV Xe ions. The research aimed to simulate the impact of nuclear fuel fission fragments on the material. The following measurements were taken during the experiments: transmission spectra in the IR region (190-7000) nm, optical absorption spectra in the range (1.2-6.5) eV, and Raman spectra were measured along the depth of ion penetration from the surface to 30 µm. A peak with a broad shape at approximately 5.3 eV can be observed in the optical absorption spectrum of irradiated spinel crystals. This band is linked to the electronic color centers of F+ and F. Meanwhile, the band with a maximum at ~(3-4) eV is attributed to hole color centers. Apart from the typical Raman modes of an unirradiated crystal, additional modes, A1g* (720 cm-1), and Eg* (385 cm-1), manifested mainly as an asymmetric shoulder of the main Eg mode, are also observed. In addition, the Raman spectroscopy method showed that the greatest disordering of crystallinity occurs in the near-surface layer up to 4 μm thick. At the same time, Raman scattering spectroscopy is sensitive to structural changes almost up to the simulated value of the modified layer, which is an excellent express method for certifying the structural properties of crystals modified by swift heavy ions.

In Situ Electron Microscopy of Transformations of Copper Nanoparticles under Plasmonic Excitation

Metal nanoparticles are attracting interest for their light-absorption properties, but such materials are known to dynamically evolve under the action of chemical and physical perturbations, resulting in changes in their structure and composition. Using a transmission electron microscope equipped for optical excitation of the specimen, the structural evolution of Cu-based nanoparticles under simultaneous electron beam irradiation and plasmonic excitation was investigated with high spatiotemporal resolution. These nanoparticles initially have a Cu core-Cu₂O oxide shell structure, but over the course of imaging, they undergo hollowing via the nanoscale Kirkendall effect. We captured the nucleation of a void within the core, which then rapidly grows along specific crystallographic directions until the core is hollowed out. Hollowing is triggered by electron-beam irradiation; plasmonic excitation enhances the kinetics of the transformation likely by the effect of photothermal heating.

Competing oxidation mechanisms in Cu nanoparticles and their plasmonic signatures

Chemical reactions involving nanoparticles often follow complex processes. In this respect, real-time probing of single nanoparticles under reactive conditions is crucial for uncovering the mechanisms driving the reaction pathway. Here, we have captured in situ the oxidation of single Cu nanoparticles to unravel a sequential competitive activation of different mechanisms at temperatures 50-200 °C. Using environmental scanning transmission electron microscopy, we monitor the evolution of oxide formation with sub-nanometre spatial resolution, and show how the prevalence of oxide island nucleation, Cabrera-Mott, Valensi-Carter and Kirkendall mechanisms under different conditions determines the morphology of the particles. Moreover, using in situ electron energy-loss spectroscopy, we probe the localised surface plasmons of individual particles during oxidation, and with the aid of finite-difference time-domain electrodynamic simulations investigate the signature of each mechanism in their plasmonic response. Our results shed light on the rich and intricate processes involved in the oxidation of nanoparticles, and provide in-depth insight into how these processes govern their morphology and optical response, beneficial for applications in catalysis, sensing, nanomedicine and plasmonics.

Dynamics of the nanocrystal structure and composition in growth solutions monitored by in situ lab-scale X-ray diffraction

In situ characterization of nanoparticle (NP) growth has become the state-of-the-art approach for studying their growth mechanisms; there is broad consensus on the reliability and precision of in situ characterization techniques compared to more traditional ex situ ones. Nonetheless, most of the currently available methods require the use of sophisticated setups such as synchrotron-based X-ray sources or an environmental liquid transmission electron microscopy (TEM) cell, which are expensive and not readily accessible. Herein, we suggest a new approach to study NP growth mechanisms: using a commercially available heating chamber for time-resolved X-ray diffraction (TR-XRD) measurements of NP growth in solution. We demonstrate how this lab-scale in situ XRD can be used to study NP growth mechanisms when complemented by standard ex situ techniques such as TEM and UV-vis spectroscopy. TR-XRD reveals the crystallographic phase and real-time evolution of NP size, shape, and composition. A detailed analysis allows determining the growth mechanism and measuring the alloying kinetics of multinary nanocrystals, demonstrated herein for a colloidal Cd_xZn_1-xS system. This approach proves itself as a promising strategy for NP growth research and could be expanded to related fields that study dynamic changes as the formation and evolution of crystalline materials in solutions.

In situ STEM study on the morphological evolution of copper-based nanoparticles during high-temperature redox reactions

Despite the broad relevance of copper nanoparticles in industrial applications, the fundamental understanding of oxidation and reduction of copper at the nanoscale is still a matter of debate and remains within the realm of bulk or thin film-based systems. Moreover, the reported studies on nanoparticles vary widely in terms of experimental parameters and are predominantly carried out using either ex situ observation or environmental transmission electron microscopy in a gaseous atmosphere at low pressure. Hence, dedicated studies in regards to the morphological transformations and structural transitions of copper-based nanoparticles at a wider range of temperatures and under industrially relevant pressure would provide valuable insights to improve the application-specific material design. In this paper, copper nanoparticles are studied using in situ Scanning Transmission Electron Microscopy to discern the transformation of the nanoparticles induced by oxidative and reductive environments at high temperatures. The nanoparticles were subjected to a temperature of 150 °C to 900 °C at 0.5 atm partial pressure of the reactive gas, which resulted in different modes of copper mobility both within the individual nanoparticles and on the surface of the support. Oxidation at an incremental temperature revealed the dependency of the nanoparticles' morphological evolution on their initial size as well as reaction temperature. After the formation of an initial thin layer of oxide, the nanoparticles evolved to form hollow oxide shells. The kinetics of formation of hollow particles were simulated using a reaction-diffusion model to determine the activation energy of diffusion and temperature-dependent diffusion coefficient of copper in copper oxide. Upon further temperature increase, the hollow shell collapsed to form compact and facetted nanoparticles. Reduction of copper oxide was carried out at different temperatures starting from various oxide phase morphologies. A reduction mechanism is proposed based on the dynamic of the reduction-induced fragmentation of the oxide phase. In a broader perspective, this study offers insights into the mobility of the copper phase during its oxidation-reduction process in terms of microstructural evolution as a function of nanoparticle size, reaction gas, and temperature.

Copper catalysis at operando conditions-bridging the gap between single nanoparticle probing and catalyst-bed-averaging

In catalysis, nanoparticles enable chemical transformations and their structural and chemical fingerprints control activity. To develop understanding of such fingerprints, methods studying catalysts at realistic conditions have proven instrumental. Normally, these methods either probe the catalyst bed with low spatial resolution, thereby averaging out single particle characteristics, or probe an extremely small fraction only, thereby effectively ignoring most of the catalyst. Here, we bridge the gap between these two extremes by introducing highly multiplexed single particle plasmonic nanoimaging of model catalyst beds comprising 1000 nanoparticles, which are integrated in a nanoreactor platform that enables online mass spectroscopy activity measurements. Using the example of CO oxidation over Cu, we reveal how highly local spatial variations in catalyst state dynamics are responsible for contradicting information about catalyst active phase found in the literature, and identify that both surface and bulk oxidation state of a Cu nanoparticle catalyst dynamically mediate its activity.

The Effect of Nanosizing on the Oxidation of Partially Oxidized Copper Nanoparticles

Copper nanoparticles are of great interest in various applications, such as catalysis, cooling fluids, conductive inks or for their antibacterial activity. In this paper, the thermal behavior of copper nanoparticles was studied using thermogravimetry, differential thermal analysis and differential scanning calorimetry. Original Cu samples as well as the products of oxidation were analysed by X-ray diffraction, scanning/transmission electron microscopy and energy dispersive spectroscopy. A step-by-step oxidation mechanism during the oxidation of Cu nano-powders was observed. The Cu-nano oxidation starts slightly above 150 °C when bulk copper does not yet react. The dominant oxidation product in the first step is Cu₂O while CuO was identified as the final state of oxidation. Our results confirm an easier oxidation process of Cu-nano than Cu-micro particles, which must be attributed to kinetic not thermodynamic aspects of oxidation reactions.

Metal/semiconductor interfaces in nanoscale objects: synthesis, emerging properties and applications of hybrid nanostructures

Hybrid nanostructures, composed of multi-component crystals of various shapes, sizes and compositions are much sought-after functional materials. Pairing the ability to tune each material separately and controllably combine two (or more) domains with defined spatial orientation results in new properties. In this review, we discuss the various synthetic mechanisms for the formation of hybrid nanostructures of various complexities containing at least one metal/semiconductor interface, with a focus on colloidal chemistry. Different synthetic approaches, alongside the underlying kinetic and thermodynamic principles are discussed, and future advancement prospects are evaluated. Furthermore, the proved unique properties are reviewed with emphasis on the connection between the synthetic method and the resulting physical, chemical and optical properties with applications in fields such as photocatalysis.

Resolving single Cu nanoparticle oxidation and Kirkendall void formation with in situ plasmonic nanospectroscopy and electrodynamic simulations

Copper nanostructures are ubiquitous in microelectronics and heterogeneous catalysis and their oxidation is a topic of high current interest and broad relevance. It relates to important questions, such as catalyst active phase, activity and selectivity, as well as fatal failure of microelectronic devices. Despite the obvious importance of understanding the mechanism of Cu nanostructure oxidation, numerous open questions remain, including under what conditions homogeneous oxide layer growth occurs and when the nanoscale Kirkendall void forms. Experimentally, this is not trivial to investigate because when a large number of nanoparticles are simultaneously probed, ensemble averaging makes rigorous conclusions difficult. On the other hand, when (in situ) electron-microscopy approaches with single nanoparticle resolution are applied, concerns about beam effects that may both reduce the oxide or prevent oxidation via the deposition and cross-linking of carbonaceous species cannot be neglected. In response we present how single particle plasmonic nanospectroscopy can be used for the in situ real time characterization of multiple individual Cu nanoparticles during oxidation. Our analysis of their optical response combined with post mortem electron microscopy imaging and detailed Finite-Difference Time-Domain electrodynamics simulations enables in situ identification of the oxidation mechanism both in the initial oxide shell growth phase and during Kirkendall void formation, as well as the transition between them. In a wider perspective, this work presents the foundation for the application of single particle plasmonic nanospectroscopy in investigations of the impact of parameters like particle size, shape and grain structure with respect to defects and grain boundaries on the oxidation of metal nanoparticles.

Thermal effects on the surface plasmon resonance of Cu nanoparticles in phosphate glass: impact on Cu+ luminescence

An evaluation of the effects of temperature on the optical properties of phosphate glass containing Cu nanoparticles (NPs) and Cu⁺ ions was carried out by means of optical absorption and photoluminescence (PL) spectroscopy measurements performed jointly in situ in the 298 to 573 K range. The surface plasmon resonance (SPR) of Cu NPs displayed a strong dampening effect with temperature, consistent with the thermal expansion of Cu NPs and an increase in the electron-phonon scattering rate. The PL of Cu⁺ ions in the glass with Cu NPs showed the thermal quenching effect connected with an increase in non-radiative relaxation processes. Moreover, a comparison with the precursor glass without NPs revealed that a lower activation energy for the thermal quenching of Cu⁺ PL results in the presence of Cu NPs for Cu⁺ sites emitting in resonance with the SPR. It is suggested that the increase in electron-phonon interaction in Cu NPs with temperature impacts the PL quenching of Cu⁺ ions the most. The current results suggest that a Cu⁺ → Cu NP resonant energy transfer supports a deactivation of the Cu⁺ emitting states with increasing temperature.

Heterodimers for in Situ Plasmonic Spectroscopy: Cu Nanoparticle Oxidation Kinetics, Kirkendall Effect, and Compensation in the Arrhenius Parameters

The ability to study oxidation, reduction, and other chemical transformations of nanoparticles in real time and under realistic conditions is a nontrivial task due to their small dimensions and the often challenging environment in terms of temperature and pressure. For scrutinizing oxidation of metal nanoparticles, visible light optical spectroscopy based on the plasmonic properties of the metal has been established as a suitable method. However, directly relying on the plasmonic resonance of metal nanoparticles as a built-in probe to track oxidation has a number of drawbacks, including the loss of optical contrast in the late oxidation stages. To address these intrinsic limitations, we present a plasmonic heterodimer-based nanospectroscopy approach, which enables continuous self-referencing by using polarized light to eliminate parasitic signals and provides large optical contrast all the way to complete oxidation. Using Au-Cu heterodimers and combining experiments with finite-difference time-domain simulations, we quantitatively analyze the oxidation kinetics of ca. 30 nm sized Cu nanoparticles up to complete oxidation. Taking the Kirkendall effect into account, we extract the corresponding apparent Arrhenius parameters at various extents of oxidation and find that they exhibit a significant compensation effect, implying that changes in the oxidation mechanism occur as oxidation progresses and the structure of the formed oxide evolves. In a wider perspective, our work promotes the use of model-system-type in situ optical plasmonic spectroscopy experiments in combination with electrodynamics simulations to quantitatively analyze and mechanistically interpret oxidation of metal nanoparticles and the corresponding kinetics in demanding chemical environments, such as in heterogeneous catalysis.

Size Distribution Control of Copper Nanoparticles and Oxides: Effect of Wet-Chemical Redox Cycling

In this work, we studied the effect of liquid-phase redox cycling on the size of Cu nanoparticles and oxides. The mixed solution of sodium hydroxide and ammonium persulfate was applied as the oxidation system at room temperature, and ascorbic acid was used as reduction agent at 80 °C in the cycling process. It was found that pristine copper particles with average size of around 800 nm and wide distribution from 300 to 1300 nm could be turned into the resulting particles with the average size of around 162.3 nm with the distribution from 75 to 250 nm after 5 redox cycles. It was also observed that uniform copper oxide nanowires formed after 5 oxidation cycles could be easily reduced into fine copper nanoparticles. The critical tuning factors including the precursor size, morphology, defects, reaction time, and the way of adding oxidant were investigated. It was suggested that the synergetic driving effect of chemical reduction and nanostructure thermodynamic instability in solution accounted for the size reformation of the copper nanoparticles. This proposed method of size-shrinking could be developed as a general strategy for large-scale tuning the properties of copper nanoparticles for wide applications and extended to other metal particles as well.

Active control of plasmonic colors: emerging display technologies

In recent years there has been a growing interest in the use of plasmonic nanostructures for color generation, a technology that dates back to ancient times. Plasmonic structural colors have several attractive features but once the structures are prepared the colors are normally fixed. Lately, several concepts have emerged for actively tuning the colors, which opens up for many new potential applications, the most obvious being novel color displays. In this review we summarize recent progress in active control of plasmonic colors and evaluate them with respect to performance criteria for color displays. It is suggested that actively controlled plasmonic colors are generally less interesting for emissive displays but could be useful for new types of electrochromic devices relying on ambient light (electronic paper). Furthermore, there are several other potential applications such as images to be revealed on demand and colorimetric sensors.

Kirkendall effect in the two-dimensional lattice-gas model

Customarily, the Kirkendall effect is associated with the vacancy-mediated balance of diffusion fluxes of atoms at the interface between two metals. Nowadays, this effect attracts appreciable attention due to its crucial role in the formation of various hollow nanoparticles via oxidation of metal nanocrystallites. The understanding of the physics behind this effect in general and especially in the case of nanoparticles is still incomplete due to abundant complicating factors. Herein, the Kirkendall effect is illustrated in detail at the generic level by performing two-dimensional (2D) lattice Monte Carlo simulations of diffusion of A and B monomers with attractive nearest-neighbor interaction for times up to 10^{7} Monte Carlo steps. Initially, A monomers are considered to form a close-packed array, while B monomers are in the 2D-gas state. The A-B interaction is assumed to be stronger compared to the other interactions, so that thermodynamically the c(2×2) A-B phase is preferable compared to the close-packed A phase (as in the case of metal oxidation). Depending on the relative rate of the diffusion jumps of A and B monomers, the patterns observed at the late stage of the formation of the mixed phase are shown to range from a single array without voids to those with appreciable disintegration of the initial array. In this way, the model predicts a single array with numerous small voids, a few moderate voids, or a single large void inside.