Growth and Modification of Cluster-Assembled Thin Films

Thin film applications have become increasingly important in our search for multifunctional and economically viable technological solutions of the future. Thin film coatings can be used for a multitude of purposes, ranging from a basic enhancement of aesthetic attributes to the addition of a complex surface functionality. Anything from electronic or optical properties, to an increased catalytic or biological activity, can be added or enhanced by the deposition of a thin film, with a thickness of only a few atomic layers at the best, on an already existing surface. Thin films offer both a means of saving in materials and the possibility for improving properties without a critical enlargement of devices. Nanocluster deposition is a promising new method for the growth of structured thin films. Nanoclusters are small aggregates of atoms or molecules, ranging in sizes from only a few nanometers up to several hundreds of nanometers in diameter. Due to their large surface to volume ratio, and the confinement of atoms and electrons in all three dimensions, nanoclusters exhibit a wide variety of exotic properties that differ notably from those of both single atoms and bulk materials. Nanoclusters are a completely new type of building block for thin film deposition. As preformed entities, clusters provide a new means of tailoring the properties of thin films before their growth, simply by changing the size or composition of the clusters that are to be deposited. Contrary to contemporary methods of thin film growth, which mainly rely on the deposition of single atoms, cluster deposition also allows for a more precise assembly of thin films, as the configuration of single atoms with respect to each other is already predetermined in clusters. Nanocluster deposition offers a possibility for the coating of virtually any material with a nanostructured thin film, and therein the enhancement of already existing physical or chemical properties, or the addition of some exciting new feature. A clearer understanding of cluster-surface interactions, and the growth of thin films by cluster deposition, must, however, be achieved, if clusters are to be successfully used in thin film technologies. Using a combination of experimental techniques and molecular dynamics simulations, both the deposition of nanoclusters, and the growth and modification of cluster-assembled thin films, are studied in this thesis. Emphasis is laid on an understanding of the interaction between metal clusters and surfaces, and therein the behaviour of these clusters during deposition and thin film growth. The behaviour of single metal clusters, as they impact on clean metal surfaces, is analysed in detail, from which it is shown that there exists a cluster size and deposition energy dependent limit, below which epitaxial alignment occurs. If larger clusters are deposited at low energies, or cluster-surface interactions are weaker, non-epitaxial deposition will take place, resulting in the formation of nanocrystalline structures. The effect of cluster size and deposition energy on the morphology of cluster-assembled thin films is also determined, from which it is shown that nanocrystalline cluster-assembled films will be porous. Modification of these thin films, with the purpose of enhancing their mechanical properties and durability, without destroying their nanostructure, is presented. Irradiation with heavy ions is introduced as a feasible method for increasing the density, and therein the mechanical stability, of cluster-assembled thin films, without critically destroying their nanocrystalline properties. The results of this thesis demonstrate that nanocluster deposition is a suitable technique for the growth of nanostructured thin films. The interactions between nanoclusters and their supporting surfaces must, however, be carefully considered, if a controlled growth of cluster-assembled thin films, with precisely tailored properties, is to be achieved.Tunna filmer, som bäst några atomlager smala skikt av material, uppfyller en central roll inom nanovetenskapen, en ny vetenskapsgren som fokuserar på egenskaperna och tillämpningen av material på en atomär eller molekylär skala - en längdskala på omkring 1-100 nanometer. Med hjälp av tunna filmer kan de fysikaliska och kemiska egenskaperna av en yta fullständigt förändras, utan att nämnvärt öka på storleken av det föremål som täckts med filmen. Nanoklusterdeponering är en lovande metod för tillväxten av tunna filmer. Nanokluster, nanometerstora sammanhopningar av atomer, har exotiska egenskaper vilka kan skilja sig markant från egenskaperna av både de enskilda atomerna och större stycken av materialet i fråga. På grund av den stora ytan, i förhållandet till volymen av ett kluster, samt den begränsade volymen i vilken atomerna och deras elektroner kan röra sig, kan ett kluster bete sig som ett helt nytt grundämne, med egenskaper vilka starkt beror på det specifika antalet atomer i just det klustret. Hoppet om att överföra dessa spännande egenskaper på tunna filmer, genom tillväxt av så kallade nanostrukturerade tunna filmer, är en drivkraft för utvecklingen av nanoklusterdeponering. I denna avhandling studerades tillväxten av tunna filmer genom deponering av metallnanokluster med hjälp av både experiment och datorsimuleringar. Både den grundläggande växelverkan mellan enskilda kluster och ytor samt mekanismerna som styr klustrens växelverkningar vid deponering av ett flertal kluster undersöktes i detalj. Hela processen av filmdeponering, från de första klustren till ett tjockare lager av kluster i en film, kartlades noggrannt. Strukturen av filmer tillverkade med nanoklusterdeponering är starkt beroende av både storleken på klustren samt den energi med vilken de kolliderar med ytan. Hur bland annat dessa faktorer påverkar de slutliga filmerna bestämdes i avhandlingen. Samtidigt fastställdes även ett gränsvärde för dessa parametrar, vid vilket tillväxten av nanostrukturerade filmer överhuvudtaget kan ske. Ytterligare modifiering av de deponerade filmerna, med målet att förbättra deras mekaniska egenskaper, utan att negativt påverka deras nanostruktur, presenteras även. Bombardemang med jonstrålar introduceras som en möjlig metod för detta. Slutligen undersöktes även själv-arrangemang av kluster på olika ytor; hur naturliga variationer i växelverkningen mellan kluster och ytor kan leda till de mest varierande slutresultat. Resultaten av denna avhandling visar hur nanokluster effektivt kan användas för tillväxten av nanostrukturerade tunna filmer. Avhandlingen visar att växelverkningen mellan kluster och ytor noggrannt måste beaktas, ifall en kontrollerbar använding av nanoklusterdeponering skall vara möjlig

Ferromagnetism in bare gold nanoagglomerates produced by nanocluster deposition

Abstract Recent research has shown unconventional magnetic properties in nanosized gold systems. These effects have mostly been detected in functionalized gold nanoparticles as well as in gold nanocrystalline films. We demonstrate ferro- and superparamagnetic behaviour in assemblies of bare gold nanoclusters. This is demonstrated by the characteristic ferromagnetic hysteresis with the temperature dependent saturation magnetization, remanence and coercivity in aggregates of small clusters. The detected magnetization is caused by the interaction between the separate clusters exhibiting an core-shell structure, and dependent on the total amount of gold confined in the samples. The behaviour is analogous to that of transition metal clusters.Peer reviewe

Metallization of self-assembled organic monolayer surfaces by Pd nanocluster deposition

A condensation-cell-type cluster aggregation source was used to deposit Pd clusters on the self-assembled monolayers of four different types of organic molecules. Mica slides covered by Au (111) were coated with self assembled monolayer films of n-dodecanethiol, 4-mercaptopyridine, dimethyldithiocarbamate, and diethyldithiocarbamate, and the behaviour of Pd clusters on these surfaces, as well as on the bare Au (111) surface, was characterized using scanning tunnelling microscopy and X-ray photoelectron spectroscopy. The aim of this study was to present an alternative means for the metallization of organic layers, through the use of preformed clusters that, unlike single adatoms, are less likely to penetrate the organic layer and cause unwanted interfacial layers or short circuits, and deduce its suitability for the various types of organic self-assembled monolayers. Our experimental results indicate that all of the studied self-assembled monolayers are impenetrable for Pd clusters deposited at thermal energies. Contrary to most methods of metallization, Pd cluster deposition therefore provides an efficient means of growing metal overlayers on organic self-assembled monolayers.Peer reviewe

Rhenium Metal and Rhenium Nitride Thin Films Grown by Atomic Layer Deposition

Abstract Rhenium is both a refractory metal and a noble metal that has attractive properties for various applications. Still, synthesis and applications of rhenium thin films have been limited. We introduce herein the growth of both rhenium metal and rhenium nitride thin films by the technologically important atomic layer deposition (ALD) method over a wide deposition temperature range using fast, simple, and robust surface reactions between rhenium pentachloride and ammonia. Films are grown and characterized for compositions, surface morphologies and roughnesses, crystallinities, and resistivities. Conductive rhenium subnitride films of tunable composition are obtained at deposition temperatures between 275 and 375 °C, whereas pure rhenium metal films grow at 400 °C and above. Even a just 3 nm thick rhenium film is continuous and has a low resistivity of about 90 µΩ cm showing potential for applications for which also other noble metals and refractory metals have been considered.Peer reviewe

Atomic Structure of a Spinel-Like Transition Al2O3(100) Surface

We study a crystalline epitaxial alumina thin film with the characteristics of a spinel-type transition Al2O3(100)surface by using atom-resolved noncontact atomic force microscopy and density functional theory. It is shown that the films are terminated by an Al-O layer rich in Al vacancies, exhibiting a strong preference for surface hydroxyl group formation in two configurations. The transition alumina films are crystalline and perfectly stable in ambient atmospheres, a quality which is expected to open the door to new fundamental studies of the surfaces of transition aluminas.Peer reviewe

Crystalline tungsten sulfide thin films by atomic layer deposition and mild annealing

Tungsten disulfide (WS2) is a semiconducting 2D material, which is gaining increasing attention in the wake of graphene and MoS2 owing to its exciting properties and promising performance in a multitude of applications. Herein, the authors deposited WSx thin films by atomic layer deposition using W-2(NMe2)(6) and H2S as precursors. The films deposited at 150 degrees C were amorphous and sulfur deficient. The amorphous films crystallized as WS2 by mild postdeposition annealing in H2S/N-2 atmosphere at 400 degrees C. Detailed structural characterization using Raman spectroscopy, x-ray diffraction, and transmission electron microscopy revealed that the annealed films consisted of small (Peer reviewe

Effect of Particle Morphology on the Ripening of Supported Pt Nanoparticles

To improve the understanding of sintering in diesel and lean-burn engine exhaust aftertreatment catalysts, we examined oxygen-induced sintering in a model catalyst consisting of Pt nanoparticles supported on a planar, amorphous Al2O3 substrate. After ageing at increasing temperatures, transmission electron microscopy analysis reveals that highly monodisperse ensembles of nanoparticles transformed into ensembles with bimodal and subsequently Lifshitz-Slyozov-Wagner particle size distributions. Moreover, scanning transmission electron microscopy and atomic force microscopy analysis suggest that the Pt nanoparticles have size-dependent morphologies after sintering in oxidizing environment. The evolution of the particle sizes is described by a simple kinetic model for ripening and the size-dependent particle morphology is proposed as an explanation for the observed bimodal particle size distribution shapes

Atomic Layer Deposition of Crystalline MoS2 Thin Films : New Molybdenum Precursor for Low-Temperature Film Growth

Molybdenum disulfide (MoS2) is a semiconducting 2D material, which has evoked wide interest due to its unique properties. However, the lack of controlled and scalable methods for the production of MoS2 films at low temperatures remains a major hindrance on its way to applications. In this work, atomic layer deposition (ALD) is used to deposit crystalline MoS2 thin films at a relatively low temperature of 300 degrees C. A new molybdenum precursor, Mo(thd)(3) (thd = 2,2,6,6-tetramethylheptane-3,5-dionato), is synthesized, characterized, and used for film deposition with H2S as the sulfur precursor. Self-limiting growth with a low growth rate of approximate to 0.025 angstrom cycle(-1), straightforward thickness control, and large-area uniformity are demonstrated. Film crystallinity is found to be relatively good considering the low deposition temperature, but the films have significant surface roughness. Additionally, chemical composition as well as optical and wetting properties are evaluated. MoS2 films are deposited on a variety of substrates, which reveal notable differences in growth rate, surface morphology, and crystallinity. The growth of crystalline MoS2 films at comparably low temperatures by ALD contributes toward the use of MoS2 for applications with a limited thermal budget.Peer reviewe

Low-Temperature Wafer-Scale Deposition of Continuous 2D SnS2 Films

Semiconducting 2D materials, such as SnS2, hold immense potential for many applications ranging from electronics to catalysis. However, deposition of few-layer SnS2 films has remained a great challenge. Herein, continuous wafer-scale 2D SnS2 films with accurately controlled thickness (2 to 10 monolayers) are realized by combining a new atomic layer deposition process with low-temperature (250 degrees C) postdeposition annealing. Uniform coating of large-area and 3D substrates is demonstrated owing to the unique self-limiting growth mechanism of atomic layer deposition. Detailed characterization confirms the 1T-type crystal structure and composition, smoothness, and continuity of the SnS2 films. A two-stage deposition process is also introduced to improve the texture of the films. Successful deposition of continuous, high-quality SnS2 films at low temperatures constitutes a crucial step toward various applications of 2D semiconductors.Peer reviewe

Effect of atomic layer deposited zinc promoter on the activity of copper-on-zirconia catalysts in the hydrogenation of carbon dioxide to methanol

The development of active catalysts for carbon dioxide (CO2) hydrogenation to methanol is intimately related to the creation of effective metal-oxide interfaces. In this work, we investigated how the order of addition of copper and zinc on zirconia influences the catalytic properties, the catalytic activity and selectivity toward methanol. Regarding the carbon dioxide conversion and methanol production, the catalysts on which the promoter (zinc) was atomically deposited after copper impregnation (i.e., ZnO/Cu/ZrO2 and ZnO/Cu/ZnO/ZrO2) were superior catalysts compared to the reverse copper-after-zinc catalyst (Cu/ZnO/ZrO2). Temperature-programmed experiments and in situ diffuse reflectance infrared Fourier transform-spectroscopy (DRIFTS) experiments allowed us to elucidate the benefits of the zinc-after-copper pair to store CO2 as carbonate species and further convert them into formate species, key intermediates in the formation of methanol. This research provides insights into the potential of atomic layer deposition in the development of tailored heterogeneous catalysts for efficient CO2 valorization to methanol.Peer reviewe