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Natural sciences can be difficult to grasp because physical and chemical phenomena can take place across time and length scales that are beyond the reach of human perception. This problem is particularly true for students attempting to learn about microfluidics, a discipline that involves intricate engineering methods and fluid phenomena that are unintuitive and unique to the microscopic scale. New learning paradigms that combine established principles from the learning sciences and mixed reality (MR) technologies may facilitate the understanding of microfluidics and help connect the experimental methods to the underlying physical and chemical processes. Yet only a few studies have implemented learning sciences principles into the design of MR experiences for university laboratory courses. We thus created ALETHA, an interactive and immersive MR learning platform to help students learn about microfluidics and microfabrication techniques. We designed ALETHA to include scaffolding, gamification, control-of-variables, and multimodal representation strategies that are known to enhance intuition building and learning. We hypothesized that MR will enhance student understanding of microfluidics and microfabrication and help them build intuitions about the processes involved at that scale. To test whether ALETHA improved affective learning outcomes, we employed quizzes and surveys and compared the performance of students that participated in the course with MR to that of a cohort using traditional paper protocols. Overall, we measured a greater building of intuition and engagement for MR students. Our new learning platform provides a useful and practical example of how MR can be implemented to learn challenging interdisciplinary topics such as microfluidics., Journal of Chemical Education, 99 (3), ISSN:0021-9584, ISSN:1938-1328

Nanoparticles are key to a range of applications, due to the properties that emerge as a result of their small size. However, their size also presents challenges to their processing and use, especially in relation to their immobilization on solid supports without losing their favorable functionalities. Here, we present a multifunctional polymer-bridge-based approach to attach a range of presynthesized nanoparticles onto microparticle supports. We demonstrate the attachment of mixtures of different types of metal-oxide nanoparticles, as well as metal-oxide nanoparticles modified with standard wet chemistry approaches. We then show that our method can also create composite films of metal and metal-oxide nanoparticles by exploiting different chemistries simultaneously. We finally apply our approach to the synthesis of designer microswimmers with decoupled mechanisms of steering (magnetic) and propulsion (light) via asymmetric nanoparticle binding, aka Toposelective Nanoparticle Attachment. We envision that this ability to freely mix available Nanoparticle ability freely nanoparticles to produce composite films will help bridge the fields of catalysis, nanochemistry, and active matter toward new materials and applications., Chemistry of Materials, 35 (9), ISSN:0897-4756

Nanoparticles are key to a range of applications, due to the properties that emerge as a result of their small size. However, their size also presents challenges to their processing and use, especially in relation to their immobilization on solid supports without losing their favourable functionalities. Here, we present a multifunctional polymer-bridge-based approach to attach a range of pre-synthesised nanoparticles onto microparticle supports. We demonstrate the attachment of mixtures of different types of metal-oxide nanoparticles, as well as metal-oxide nanoparticles modified with standard wet chemistry approaches. We then show that our method can also create composite films of metal and metal-oxide nanoparticles by exploiting different chemistries simultaneously. We finally apply our approach to the synthesis of designer microswimmers with decoupled mechanisms of steering (magnetic) and propulsion (light) via asymmetric nanoparticle binding, aka Toposelective Nanoparticle Attachment. We envision that the ability to freely mix available nanoparticles to produce composite films will help bridge the fields of catalysis, nanochemistry, and active matter towards new materials and applications.

The active Brownian particle (ABP) model is widely used to describe the dynamics of active matter systems, such as Janus microswimmers. In particular, the analytical expression for an ABP's mean-squared displacement (MSD) is useful as it provides a means to describe the essential physics of a self-propelled, spherical Brownian particle. However, the truncated or "short-time" form of the MSD equation is typically fitted, which can lead to significant problems in parameter estimation. Furthermore, heteroscedasticity and the often statistically dependent observations of an ABP's MSD lead to a situation where standard ordinary least-squares regression leads to biased estimates and unreliable confidence intervals. Instead, we propose here to revert to always fitting the full expression of an ABP's MSD at short timescales, using bootstrapping to construct confidence intervals of the fitted parameters. Additionally, after comparison between different fitting strategies, we propose to extract the physical parameters of an ABP using its mean logarithmic squared displacement. These steps improve the estimation of an ABP's physical properties and provide more reliable confidence intervals, which are critical in the context of a growing interest in the interactions of microswimmers with confining boundaries and the influence on their motion.

The authors acknowledge support from an ETH Research Grant ETH-15 17-1 (R. S.), from an ETH Postdoctoral Fellowship FEL-21 15-2 and SNSF PRIMA Grant 179834 (E. S.), from a Postdoctoral fellowships programme “Beatriu de Pinós”, funded by the Secretary of Universities and Research (Government of Catalonia) and by the Horizon 2020 programme of research and innovation of the European Union under the Marie Sklodwoska-Curie grant agreement no. 801370 (Grant 2018 BP 00029) (M. A. F. R.) and from a Gordon and Betty Moore Foundation Investigator Award on Aquatic Microbial Symbiosis (grant GBMF9197) (R. S.). The authors thank Dr. Heiko Wolf at IBM Research Zurich for insightful discussions., Colloidal patterning enables the placement of a wide range of materials into prescribed spatial arrangements, as required in a variety of applications, including micro- and nano-electronics, sensing, and plasmonics. Directed colloidal assembly methods, which exploit external forces to place particles with high yield and great accuracy, are particularly powerful. However, currently available techniques require specialized equipment, which limits their applicability. Here, we present a microfluidic platform to produce versatile colloidal patterns within a microchannel, based on sequential capillarity-assisted particle assembly (sCAPA). This new microfluidic technology exploits the capillary forces resulting from the controlled motion of an evaporating droplet inside a microfluidic channel to deposit individual particles in an array of traps microfabricated onto a substrate. Sequential depositions allow the generation of a desired spatial layout of colloidal particles of single or multiple types, dictated solely by the geometry of the traps and the filling sequence. We show that the platform can be used to create a variety of patterns and that the microfluidic channel easily allows surface functionalization of trapped particles. By enabling colloidal patterning to be carried out in a controlled environment, exploiting equipment routinely used in microfluidics, we demonstrate an easy-to-build platform that can be implemented in microfluidics labs., ETH Research Grant ETH-15 17-1, ETH Postdoctoral Fellowship FEL-21 15-2, SNSF PRIMA Grant 179834, Postdoctoral fellowships programme "Beatriu de Pinos" - Government of Catalonia, Horizon 2020 programme of research and innovation of the European Union under the Marie Sklodwoska-Curie grant 801370 2018 BP 00029, Gordon and Betty Moore Foundation Investigator Award on Aquatic Microbial Symbiosis GBMF9197

Microswimmers are small particles capable of converting available energy sources into propulsion owing to their compositional asymmetry and are promising for applications ranging from targeted delivery to enhanced mixing at the microscale. However, current fabrication techniques demonstrate limited scalability and/or rely on the excessive use of expensive precursor materials. Here, a scalable Pickering-wax emulsion technique is combined with galvanic electrochemistry, to grow platinum films from copper nanoparticles asymmetrically seeded onto SiO\textsubscript{2} microparticle supports. In this manner, large quantities of Pt-SiO\textsubscript{2} Janus microswimmers are obtained. Utilising copper as a templating material not only reduces synthesis time, material costs, and toxic waste, but also facilitates the further extension of this methodology to a range of functional materials. This electrochemical approach builds upon previous attempts to overcome the current limitations in microswimmer synthesis and offers exciting opportunities for their future development.

Monolayers of soft colloidal particles confined at fluid interfaces are at the core of a broad range of technological processes, from the stabilization of responsive foams and emulsions to advanced lithographic techniques. However, establishing a fundamental relation between their internal architecture, which is controlled during synthesis, and their structural and mechanical properties upon interfacial confinement remains an elusive task. To address this open issue, which defines the monolayer's properties, we synthesize core-shell microgels, whose soft core can be chemically degraded in a controlled fashion. This strategy allows us to obtain a series of particles ranging from analogues of standard batch-synthesized microgels to completely hollow ones after total core removal. Combined experimental and numerical results show that our hollow particles have a thin and deformable shell, leading to a temperature-responsive collapse of the internal cavity and a complete flattening after adsorption at a fluid interface. Mechanical characterization shows that a critical degree of core removal is required to obtain soft disk-like particles at an oil-water interface, which present a distinct response to compression. At low packing fractions, the mechanical response of the monolayer is dominated by the outer polymer chains forming a corona surrounding the particles within the interfacial plane, regardless of the presence of a core. By contrast, at high compression, the absence of a core enables the particles to deform in the direction orthogonal to the interface and to be continuously compressed without altering the monolayer structure. These findings show how fine, single-particle architectural control during synthesis can be engineered to determine the interfacial behavior of microgels, enabling one to link particle conformation with the resulting material properties., ACS Nano, 15 (8), ISSN:1936-0851, ISSN:1936-086X

Gold nanoparticles have been extensively studied for their applications in catalysis. For Au nanoparticles to be catalytically active, controlling the particle size is crucial. Here we present a low temperature (105 °C) thermal atomic layer deposition approach for depositing gold nanoparticles on TiO2 with controlled size and loading using trimethylphosphino-trimethylgold(III) and two co-reactants (ozone and water) in a fluidized bed reactor. We show that the exposure time of the precursors is a variable that can be used to decouple the Au particle size from the loading. Longer exposures of ozone narrow the particle size distribution, while longer exposures of water broaden it. By studying the photocatalytic activity of Au/TiO2 nanocomposites, we show how the ability to control particle size and loading independently can be used not only to enhance performance but also to investigate structure–property relationships. This study provides insights into the mechanism underlying the formation and evolution of Au nanoparticles prepared for the first time via vapor phase atomic layer deposition. Employing a vapor deposition technique for the synthesis of Au/TiO2 nanocomposites eliminates the shortcomings of conventional liquid-based processes opening up the possibility of highly controlled synthesis of materials at large scale., Nanoscale, 12 (16), ISSN:2040-3364, ISSN:2040-3372

In industrial catalysis, the sintering of supported nanoparticles (NPs) is often associated with the loss of catalyst activity and thus with periodic plant downtime and economic burdens. Yet, sintering mechanisms are at play also during the synthesis of the catalyst itself. They can, in fact, determine the size distribution of the NPs, and thus the activity and the stability of the catalyst. Here, we examine the role of nanoparticle sintering in a technique borrowed from the semiconductor industry that promises to reconcile atomic-scale precision with scalability: atomic layer deposition. By modeling the cyclic influx of single atoms in concomitance with NP sintering via either dynamic coalescence or Ostwald ripening, we establish the “signature” of different growth regimes: the size distribution. In contrast, we show that integral quantities such as the mean diameter, the number of NPs per unit area, and the material loading are poor indicators of the underlying growth mechanism. In particular, a constant number of NPs cannot be interpreted as a sign of no sintering. Finally, we argue that NP sintering, if properly understood, can open up new avenues for the control over the size distribution of NPs, and thus over their catalytic activity and stability.

Understanding the spontaneous organization of atoms on well-defined surfaces promises to enable control over the shape and size of supported nanostructures. Atomic layer deposition (ALD) boasts atomic-scale control in the synthesis of thin films and nanoparticles. Yet, the possibility to control the shape of ALD-grown nanostructures remains mostly unexplored. Here, we report on the bottom-up formation of both linear and V-shaped anatase TiO2 nanorods (NRs) on graphene nanoplatelets during TiCl4/H2O ALD carried out at 300 °C. NRs as large as 200 nm form after only five ALD cycles, indicating that diffusional processes rather than layer-by-layer growth are behind the NR formation. In particular, high-resolution transmission electron microscopy reveals that the TiO2 NRs and graphene nanoplatelets are in rotational alignment as a result of lattice matching. Crucially, we also show that individual nanocrystals can undergo in-plane oriented attachment.

The realization of non-close-packed nanoscale patterns with multiple feature sizes and length scales via colloidal self-assembly is a highly challenging task. We demonstrate here the creation of a variety of tunable particle arrays by harnessing the sequential self-assembly and deposition of two differently sized microgel particles at the fluid-fluid interface. The two-step process is essential to achieve a library of 2D binary colloidal alloys, which are kinetically inaccessible by direct co-assembly. These versatile binary patterns can be exploited for a range of end-uses. Here we show that they can for instance be transferred to silicon substrates, where they act as masks for the metal-assisted chemical etching of binary arrays of vertically aligned silicon nanowires (VA-SiNWs) with fine geometrical control. In particular, continuous binary gradients in both NW spacing and height can be achieved. Notably, these binary VA-SiNW platforms exhibit interesting anti-reflective properties in the visible range, in agreement with simulations. The proposed strategy can also be used for the precise placement of metallic nanoparticles in non-close-packed arrays. Sequential depositions of soft particles enable therefore the exploration of complex binary patterns, e.g. for the future development of substrates for biointerfaces, catalysis and controlled wetting.

Microswimmers are small particles capable of converting available energy sources into propulsion owing to their compositional asymmetry, and are promising for applications ranging from targeted delivery to enhanced mixing at the microscale. However, current fabrication techniques demonstrate limited scalability and/or rely on the excessive use of expensive precursor materials. Here, a scalable Pickering-wax emulsion technique is combined with galvanic electrochemistry, to grow platinum films from copper nanoparticles asymmetrically seeded onto SiO₂ microparticle supports. In this manner, large quantities of Pt-SiO₂ Janus microswimmers are obtained. Utilizing copper as a templating material not only has the potential to reduce synthesis time, material costs, and toxic waste, but also facilitates the further extension of this methodology to a range of functional materials. This electrochemical approach builds upon previous attempts to overcome the current limitations in microswimmer synthesis and offers exciting opportunities for their future development., Particle & Particle Systems Characterization, 39 (2), ISSN:0934-0866, ISSN:1521-4117

Microswimmers hold promise for applications ranging from targeted delivery to enhanced mixing at the microscale. However, current fabrication techniques suffer from limited throughput and material selection. Here, a versatile route enabling the synthesis of microswimmers from off-the-shelf micro- and nano-particles is demonstrated. The protocol hinges on the toposelective attachment of photocatalytic nanoparticles onto microparticles, exploiting a multi-functional polymer and a Pickering-wax emulsification step, to yield large quantities of photo-responsive active Janus particles. The polymer presents both silane and nitrocatechol groups, binding silica microspheres to a range of metal oxide nanoparticles. The Pickering-wax emulsions protect part of the microspheres' surface, enabling asymmetric functionalization, as required for self-propulsion. The resulting photocatalytic microswimmers display a characteristic orientation-dependent 3D active motion upon ultra-violet illumination, different from that conventionally described in the literature. By connecting the extensive library of heterogeneous nanoparticle photocatalysts with the nascent field of active matter, this versatile material platform lays the groundwork toward designer microswimmers, which can swim by catalyzing a broad range of chemical reactions with potential for future applications., Advanced Functional Materials, 32 (7), ISSN:1616-3028, ISSN:1616-301X

Area-selective deposition (ASD) enables the growth of materials on target regions of patterned substrates for applications in fields ranging from microelectronics to catalysis. Selectivity is often achieved through surface modifications aimed at suppressing or promoting the adsorption of precursor molecules. Here, we show instead that varying the surface composition can enable ASD by affecting surface diffusion rather than adsorption. Ru deposition from (carbonyl)-(alkylcyclohexadienyl)Ru and H2 produces smooth films on metal nitrides, and nanoparticles on SiO2. The latter form by surface diffusion and aggregation of Ru adspecies. Kinetic modeling shows that changing the surface termination of SiO2 from −OH to −CH3, and thus its surface energy, leads to larger and fewer nanoparticles because of a 1000-fold increase in surface diffusion rates. Kinetic Monte Carlo simulations show that even surface diffusion alone can enable ASD because adspecies tend to migrate from high- to low-diffusivity regions. This is corroborated by deposition experiments on three-dimensional (3D) TiN–SiO2 nanopatterns, which are consistent with Ru migrating from SiO2 to TiN. Such insights not only have implications for the interpretation of experimental results but may also inform new ASD protocols, based on chemical vapor and atomic layer deposition, that take advantage of surface diffusion. © 2020 American Chemical Society., Chemistry of Materials, 32 (22), ISSN:0897-4756

Self-assembled monolayers of microparticles encoding Archimedean and non-regular tessellations promise unprecedented structure-property relationships for a wide spectrum of applications in fields ranging from optoelectronics to surface technology. Yet, despite numerous computational studies predicting the emergence of exotic structures from simple interparticle interactions, the experimental realization of non-hexagonal patterns remains challenging. Not only kinetic limitations often hinder structural relaxation, but also programming the inteparticle interactions during assembly, and hence the target structure, remains an elusive task. Here, we demonstrate how a single type of soft polymeric microparticle (microgels) can be assembled into a wide array of complex structures as a result of simple pairwise interactions. We first let microgels self-assemble at a water-oil interface into a hexagonally packed monolayer, which we then compress to varying degrees and deposit onto a solid substrate. By repeating this process twice, we find that the resultant structure is not the mere stacking of two hexagonal patterns. The first monolayer retains its hexagonal structure and acts as a template into which the particles of the second monolayer rearrange to occupy interstitial positions. The frustration between the two lattices generates new symmetries. By simply varying the packing fraction of the two monolayers, we obtain not only low-coordination structures such as rectangular and honeycomb lattices, but also rhomboidal, hexagonal, and herringbone superlattices which display non-regular tessellations. Molecular dynamics simulations show that these structures are thermodynamically stable and develop from short-ranged repulsive interactions, making them easy to predict, and thus opening new avenues to the rational design of complex patterns.

We demonstrate the deposition of crystalline palladium nanoparticles on graphene nanoplatelets (Pd/graphene) via atmospheric pressure atomic layer deposition (ALD) carried out in a fluidized bed reactor. The nucleation and growth of Pd nanoparticles on the inert graphene surface was enabled by applying an ozone pretreatment step, without significantly affecting the graphene crystalline structure. Uniform nucleation on both basal planes and edges of the graphene was obtained. The Pd loading and dispersion as well as the average particle size could be controlled by varying the number of ALD cycles. By analyzing the evolution of the particle size distribution and spatial density, we obtained insights into the nucleation and growth of Pd ALD on graphene. Furthermore, by shortening the pretreatment time, selective growth of Pd nanoparticles on the edges of the graphene was achieved. The Pd/graphene fabricated with our method showed a significantly lower level of impurities compared to the Pd/graphene synthesiz...

Atomic layer deposition (ALD) is a gas-phase coating technique that can be used to coat nanoparticles in a fluidized bed reactor. ALD is based on the alternating supply of two precursors, which makes it an inherent dynamic process. We discuss a multi-scale, multiphase mass transfer-diffusion-reaction model capable of predicting the evolution of surface coverage of particles at different local operating conditions. The dynamic ALD-reactor model can be extended with operational scenarios. The reactor design combined with the scenarios has many degrees of freedom, yielding ample opportunities to optimize the process with efficient utilization of precursors.

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Understanding the growth mechanisms during the early stages of atomic layer deposition (ALD) is of interest for several applications including thin film deposition, catalysis, and area-selective deposition. The surface dependence and growth mechanism of (ethylbenzyl)(1-ethyl-1,4-cyclohexadienyl)ruthenium and O2 ALD at 325 °C on HfO2, Al2O3, OH, and SiOSi terminated SiO2, and organosilicate glass (OSG) are investigated. The experimental results show that precursor adsorption is strongly affected by the surface termination of the dielectric, and proceeds most rapidly on OH terminated dielectrics, followed by SiOSi and finally SiCH3 terminated dielectrics. The initial stages of growth are characterized by the formation and growth of Ru nanoparticles, which is mediated by the diffusion of Ru species. Mean-field and kinetic Monte Carlo modeling show that ALD on OSG is best described when accounting for (1) cyclic generation of new nanoparticles at the surface, (2) surface diffusion of both atomic species and nanoparticles, and (3) size-dependent nanoparticle reactivity. In particular, the models indicate that precursor adsorption initially occurs only on the dielectric substrate, and occurs on the Ru nanoparticles only when these reach a critical size of about 0.85 nm. This phenomenon is attributed to the catalytic decomposition of oxygen requiring a minimum Ru nanoparticle size. ispartof: ADVANCED MATERIALS INTERFACES vol:5 issue:24 status: published

We present a multiscale dynamic model as a means of understanding and optimizing the precursor utilization during atomic layer deposition (ALD) on nanoparticles and micron-sized nano-porous particles in fluidized bed reactors. We used as case study the deposition of alumina using trimethylaluminum and water on both, titania nanoparticles and micron-sized nano-porous γ -alumina particles under low (∼1 mbar) and atmospheric pressure. In doing so, we assess the effect of the precursor transport, from the inlet of the reactor to the particles active surface, on the precursor utilization efficiency. Our results show that, at proper operating conditions, fast ALD reaction kinetics enables the saturation of the particles surface area with hardly any loss of precursors. Finally, simple scaling rules for the optimization of the precursor utilization are proposed.

A fundamental understanding of the interplay between ligand-removal kinetics and metal aggregation during the formation of platinum nanoparticles (NPs) in atomic layer deposition of Pt on TiO

We tailored the size distribution of Pt nanoparticles (NPs) on graphene nanoplatelets at a given metal loading by using low-temperature atomic layer deposition carried out in a fluidized bed reactor operated at atmospheric pressure. The Pt NPs deposited at low temperature (100 °C) after 10 cycles were more active and stable towards the propene oxidation reaction than their high-temperature counterparts. Crucially, the gap in the catalytic performance was retained even after prolonged periods of time (>24 hours) at reaction temperatures as high as 450 °C. After exposure to such harsh conditions the Pt NPs deposited at 100 °C still retained a size distribution that is narrower than the one of the as-synthesized NPs obtained at 250 °C. The difference in performance correlated with the difference in the number of facet sites as estimated after the catalytic test. Our approach provides not only a viable route for the scalable synthesis of stable supported Pt NPs with tailored size distributions but also a tool for studying the structure-function relationship.

We present an atomistic understanding of the evolution of the size distribution with temperature and number of cycles in atomic layer deposition (ALD) of Pt nanoparticles (NPs). Atomistic modeling of our experiments teaches us that the NPs grow mostly via NP diffusion and coalescence rather than through single-atom processes such as precursor chemisorption, atom attachment, and Ostwald ripening. In particular, our analysis shows that the NP aggregation takes place during the oxygen half-reaction and that the NP mobility exhibits a size- and temperature-dependent scaling. Finally, we show that contrary to what has been widely reported, in general, one cannot simply control the NP size by the number of cycles alone. Instead, while the amount of Pt deposited can be precisely controlled over a wide range of temperatures, ALD-like precision over the NP size requires low deposition temperatures (e.g., T < 100 °C) when growth is dominated by atom attachment.

Atomic layer deposition (ALD) is a gas-phase deposition technique that, by relying on self-terminating surface chemistry, enables the control of the amount of deposited material down to the atomic level. While mostly used in semiconductor technology for the deposition of ceramic oxides and nitrides on wafers, ALD lends itself to the deposition of a wealth of materials on virtually every substrate. In particular, ALD and its organic counterpart molecular layer deposition (MLD), have opened up attractive avenues for the synthesis of novel nanostructured materials. However, as most ALD processes were developed and optimized for semiconductor technology, these might not be optimal for applications in fields such as catalysis, energy storage, and health. For this reason, novel applications for ALD often require new surface chemistries, process conditions, and reactor types. As a result, recent developments in ALD technology have marked a considerable departure from the standard set by well-established ALD processes. The aim of this review is twofold: firstly, to capture the recent departure of ALD from its original development; and secondly, to pinpoint the unexplored paths through which ALD can advance further in terms of synthesis of novel materials. To that end, we provide a review of the recent developments of ALD and MLD of materials that are gaining increasing attention on various substrates, with particular emphasis on high-surface-area substrates. Furthermore, we present a critical review of the effects of the process conditions, namely, temperature, pressure, and time on ALD growth. Finally, we also give a brief overview of the recent advances in ALD reactors and energy-enhanced ALD processes.

A fundamental understanding of the interplay between ligand-removal kinetics and metal aggregation during the formation of platinum nanoparticles (NPs) in atomic layer deposition of Pt on TiO2 nanopowder using trimethyl(methylcyclo-pentadienyl)platinum(IV) as the precursor and O2 as the coreactant is presented. The growth follows a pathway from single atoms to NPs as a function of the oxygen exposure (PO2 × time). The growth kinetics is modeled by accounting for the autocatalytic combustion of the precursor ligands via a variant of the Finke–Watzky two-step model. Even at relatively high oxygen exposures (> 120 mbar s. The deposition of more Pt leads to the formation of NPs that can be as large as 6 nm. Crucially, high PO2 (≥5 mbar) hinders metal aggregation, thus leading to narrow particle size distributions. The results show that ALD of Pt NPs is reproducible across small and large surface areas if the precursor ligands are removed at high PO2.