Particles are widely used in the chemical industry as raw material and end product. In many applications of particles, it is advantageous to give them a coating that can either protect the particle from outside influences or give it an additional function. Technologies to provide micron-sized particles with relatively thick coatings (in the order of ?m) are readily available. However, for many applications it would be attractive to apply coatings to fine particles (of the order of 50 ?m or smaller) and to make these coatings thinner (<< 1 ?m). This opens possibilities for new properties. In addition, a lower amount of the compound would be needed to coat the material of interest. This thesis focuses on a technique for depositing ultrathin and conformal films, with control down to nanometre scale, with high a utilisation of the precursors: atomic layer deposition (ALD) applied to fine particles. ALD is a gas-phase coating technique that relies on two self terminating reactions, that form a coating cycle, after which a fraction of a monolayer of compound is deposited on the substrate. This cycle can be repeated a desired number of times to deposit thicker layers. Amongst the wide variety of compounds that have been deposited with ALD, alumina (Al2O3) has been the favourite one to study, due to the ideal layer-by-layer growth mode that is observed when used trimethyl aluminium (TMA) and water as precursors, and its broad applicability in different fields. Thin Al2O3 ALD films have been used, for example, in the fabrication of semiconductor devices, in the production of catalysts and the passivation of the cathode of the Li-ion batteries. Besides these applications, other materials such as polymers or biological products could benefit from the deposition of ultrathin alumina films. However, alumina has been typically deposited at relatively high temperatures, i.e., ~170 °C and < 1 mbar, inhibiting its application to heat-sensitive materials. This is our main motivation to investigate the deposition of alumina films at room temperature and atmospheric pressure. Working at ambient conditions would, in first place, permit the coating of heat sensitive materials, and, in second place, facilitate the coating process and improve the scaling-up prospects, since less complex equipment would be required. Atomic layer deposition provides accurate control over the film thickness based on the selfterminating nature of the ALD reactions. That means that deposition only takes place as long as there are surface species available for reaction. The combination of the operating temperature and pressure, and the purging of the reactor with nitrogen after each reaction, ensure the removal of unreacted precursor molecules from the surface of the substrate. However, the physisorption of these unreacted molecules is the main challenge that we face when depositing alumina at 25 °C and 1 bar, since the normal boiling temperature of TMA and water is 125 and 100 °C, respectively. In Al2O3 ALD processes at 170 °C and < 1 mbar, where no physisorption occurs, the thickness of the deposited films depends solely on the number of coating cycles, since once the active sites on the surface are depleted, no more reaction takes place, regardless of the amount of precursors fed to the reactor in excess. In contrast, at ambient conditions, the thickness of the alumina film will depend on the number of cycles and on the dosing time of the precursors due to the accumulation of the unreacted precursor molecules that are fed in excess. In this work, we aimed at controlling the physisorption of the precursors at ambient conditions by adjusting the dosing time of the precursors to the reactor. By feeding the amount of precursor molecules needed to saturate the active sites present on the surface of the substrate (TiO2 nanoparticles in our case), we obtained similar growths as in pure ALD, i.e., 0.1-0.2 nm per cycle, even if the reactions are not self-limiting. Complementarily, having a faster growth of the alumina film could be done by dosing both precursors in excess due to the physisorption of unreacted molecules. These findings were applied in the remainder of the thesis to deposit alumina, at ambient conditions, on three different substrates: polymeric powder coating paint, lead-selenide quatum dot (PbSe QD) films, and silicon carbide (SiC) particles. Two of these materials, the powder coating and the QD films, are heat sensitive, thus working at room temperature was essential. Al2O3 films were used to tune the surface appearance of a powder coating (i.e., dry paint without any solvent). The polymeric core particles were coated with alumina films of different thicknesses. The alumina layers partially confined the core material when it softened above the glass transition temperature. As the softened core did not flow during the curing of the paint, it created roughness of the surface, and therefore, a matte surface appearance. The mechanical properties of the matte powder coating were similar to the ones of the originally glossy one. Thin alumina films were used to tune the final appearance of the powder coating paint without the use of foreign particles that would produce the same effect. Additionally, films of PbSe QD crystals, which are nanosized crystals with very interesting properties for semiconductor applications, were passivated using alumina. QDs films have very promising applications, e.g., as high-efficiency photovoltaic material. However, they are air and heat sensitive and irreversibly oxidize after a short exposure to air, losing their good properties. Previous work showed the passivation of the QDs with alumina at temperatures between 25 and 90 °C, and pressures smaller than 1 mbar. In this work, alumina was deposited at 25 °C and 1 bar to efficiently passivate the QDs. At ambient conditions, a fast deposition of alumina took place as a consequence of different groups that can react with TMA, such as hydroxyl and amine groups. Coating with alumina at ambient conditions can be easily coupled with the fabrication of the QD films, which also is done at room temperature and atmospheric pressure. The final application we considered is the production of radioactive tracer particles. For fluidized bed studies, we would like to label SiC particles with a minimum change to their other physical properties. SiC particles have a rather inert surface towards radioactive ions. We used the Al2O3 coating to enhance the labelling efficiency of this material and produce a tracer. Typically, a completely different material is used for the tracer particles, such as ?-Al2O3. However, this would produce a mismatch in properties such as size, shape and density between the tracer and the material of study. In this work, alumina films were deposited on SiC particles to mimic the affinity of the ?-Al2O3 particles towards the radioactive ions, and produce a SiC-based tracer. Since the labelling efficiency depends on the alumina film thickness, TMA and water were fed in large excess, achieving a growth per cycle of alumina of about 10 nm. The resulting core-shell particles, with an alumina coating of about 400 nm, had only a slightly lower relative activity as the ?-Al2O3 particle. Nevertheless, this SiC based tracer was used in a particle tracking experiment, having enough activity to be detected by the sensors and reconstruct its trajectory during fluidization. One might argue whether the studied deposition process at atmospheric pressure and room temperature is still true ALD, since the reactions are not self-terminating and we deposit several layers of alumina in each cycle. Nevertheless, it gives the possibility of depositing alumina on heat sensitive materials, with good tunability and control over the films thickness, at operating conditions that would simplify the scaled-up of this technology. The findings in this thesis could be expanded to other applications, such as the coating of biological compounds, that would require low-temperature processing, or the deposition of other compounds at ambient conditions, i.e., zinc oxide (ZnO). This is further discussed in the outlook of this thesis.