Atomic layer deposition (ALD) of platinum (Pt) has gained significant interest in the recent years due to its capability of depositing various Pt nanostructures for applications in different fields, such as Pt nanoparticles (NPs) for catalytic reactions and energy devices and Pt thin films for microelectronic technology. Among various developed processes, Pt ALD using MeCpPtMe₃as the precursor has been most popularly employed owing to the high reactivity, volatility, and thermal stability of the precursor, which enable controlled deposition of Pt nanostructures in a broad range of temperatures. Typical MeCpPtMe₃-based Pt ALD processes use O₂and H₂as the coreactants. In this study, we explore atomic hydrogen as an alternative and reveal its exceptional reactivity that outperforms H₂and O₂. Specifically, atomic hydrogen enables the deposition of highly dispersed Pt NPs with narrow particle size distributions (i.e., standard deviation <0.3 nm) on various oxide surfaces, including TiO₂, SiO₂, CeO₂and V₂O₅, which is unattainable with H₂under identical experimental conditions. In addition, it facilitates the deposition of Pt NPs with improved size uniformity and accelerates the closure of Pt films compared to ALD processes using O₂as the coreactant. The results demonstrate a significant potential of atomic hydrogen as a highly effective coreactant for ALD of Pt NPs and thin films.

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.

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.

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.

Atomic layer deposition (ALD) is a gas-phase thin film technology that boasts atomic-level control over the amount of material being deposited. A great deal of research effort has been devoted to the exploitation of ALD precision for the synthesis of nanostructures other than thin films such as supported nanoparticles (NPs). ALD is not only precise but also scalable to high-surface-area supports such as powders, which are relevant to a wide range of applications in fields spanning catalysis, energy storage and conversion, and medicine. Yet, translating the precision of ALD of thin films to the synthesis of NPs is not straightforward. In fact, ALD is mostly understood in terms of self-limiting surface reactions leading to a layer-by-layer conformal growth. However, the formation and growth of NPs is bound to be dictated by atomistic processes other than ALD surface reactions, such as the diffusion and aggregation of atoms and NPs. Understanding the role of such non-equilibrium processes is the key to achieving atomic-level control over the morphology of ALD-grown NPs and, in particular, their particle size distribution (PSD) and shape. This thesis is aimed at expanding our atomic-scale understanding of the mechanisms behind the formation of NPs during ALD. In particular, this thesis is based on experiments and models that were devised with an eye to scalability.

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.

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.

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 (P_O2 × 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) little to no Pt is deposited after the first cycle and most of the Pt is atomically dispersed. Increasing the oxygen exposure above 120 mbar s results in a rapid increase in the Pt loading, which saturates at 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 P_O2 (≥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 P_O2.

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.

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.

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.

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.

We report the fabrication of ultrathin aluminum-based overcoats on supported platinum catalysts using atomic layer deposition and hydrid atomic/molecular layer deposition. Tuning the process parameters and applying heat treatment for porosity induction can result to controllably accesible model catalysts. We examined the effect of the number of deposition cycles (1-25), co-reactant choice (water, ethylene glycol, 1,5-pentanediol) and exposure time on the characteristics of the alumina and alucone stabilizing films. The use of fluidized bed reactors for atomic layer deposition applied for active phase deposition and overcoat stabilization paves the way for large-scale gas-phase production of tailored nanostructured catalysts.