Tuomo Suntola

Surface chemistry of materials deposition at atomic layer level

Abstract Structures in modern semiconductor devices are getting smaller and smaller, coming close to atomic dimensions. The demand for materials processing at atomic layer level can be approached from extreme process control or from delicate utilization of surface chemistry. The opportunity in the surface chemistry approach is to create conditions for monoatomic layer buildup through saturated surface reactions. Material layer processing through sequentially performed saturated surface reactions is generally referred to as atomic layer epitaxy (ALE). ALE has been successfully applied in commercial manufacturing of thin film electroluminescent displays. Also, extensive scientific work has been done for applying atomic layer controlled growth of epitaxial layers and superlattice structures of III–V and II–VI semiconductors. Surface controlled build-up of molecular structures has recently been applied to porous supports for heterogeneous catalysts. For further progress in atomic layer level controlled materials processing well understood surface chemistry is of major importance.

Chemisorption of chromium acetylacetonate on porous high surface area silica

Abstract Atomic layer epitaxy (ALE) reactions (i.e. saturating gas-solid reactions) of chromium acetylacetonate (Cr(acac)3) at 200–280°C with silica preheated at 200–820°C were studied by determining chromium and carbon concentrations, recording FTIR spectra, and reacting Cr(acac)3 with the silylated silica surface. Cr(acac)3 was found to be selectively chemisorbed to silica through reaction with the isolated OH groups, leading to release of one acac ligand. The relatively large size of the supported chromium complex that formed had a highly controlling effect on the amount of chromium atoms bound. In addition to this steric hindrance, the saturation density of chromium could be further regulated by the preheat temperature of the silica, which determines the number of OH groups, and by the reaction temperature. The reaction with the silylated silica surface provided a means for achieving an even lower saturation density of chromium and confirmed that the strongly H-bonded OH groups present on silica preheated at 200°C were only partly reactive. The ligands of the surface complex could be removed by water vapor and air treatment.

Preparation of Ni/Al2O3 catalysts from vapor phase by atomic layer epitaxy

Ni/Al2O3 catalysts were prepared by saturating gas-solid reactions as an atomic layer epitaxy (ALE) process. Vaporized Ni(acac)2 was chemisorbed on a porous alumina support, and the produced surface complex was then air treated to remove the ligand residues. The nickel content could be precisely controlled by repeating this reactor cycle. On alumina preheated at 800°C, the nickel content varied from 3 to 21 wt%, when the number of reaction cycles was increased from one to ten. The performance of the Ni-catalysts was evaluated in the gas-phase hydrogenation of toluene. The preheat temperature of alumina influenced the activity of the catalyst, and a maximum in the activity was observed for catalysts prepared from alumina preheated at 875°C. Catalysts prepared by four reaction cycles, containing about 10 wt% nickel, gave the highest utilization of nickel.

Atomic layer growth of TiO2 on silica

Abstract Study was made of the initial stages of TiO2 on porous high-surface-area silica support by atomic layer epitaxy (ALE). The effects of the number and types of surface bonding sites and the reaction temperature on the growth rate and structure of TiO2 were determined. The reactants were TiCl4 and H2O. At 175°C binding of TiCl4 takes place to one or to two hydroxyl groups, but at 450°C only to two isolated hydroxyl groups. The number and type of binding sites available and the reaction temperature selected determine how the growth will proceed. XRD peaks at the 2θ values of anatase and rutile were detected after one reaction cycle of TiCl4 and H2O at 450°C.

Advanced Materials Processing by Adsorption Control

Precise control and knowledge of surface structures are essential inorder to meet the requirements of todays and future materials. One possiblegrowth technique capable of meeting the requirements is atomic layer epitaxy(ALE). ALE is based on sequentially applied saturated gas-solid reactions,which provide the means for adsorption controlled material deposition atatomic layer level. In this paper the potentiality of the use of porousmaterials in a detailed study of adsorption controlled growth is discussed.At the same time the study promotes the application of adsorption controlledmaterials processing for advanced catalysts manufacturing.

Surface coverage of ALE precursors on oxides

The work with single molecular layers on porous materials adds to the atomic level understanding of the film growth in ALE. In this paper the primary factors affecting the surface coverage of three different ALE precursors (TiCl4, Cr(acac)3 and hexamethyldisilazane (HMDS) on porous, high surface area SiO2 were studied by element determinations and FTIR spectroscopy. Also a preliminary low-energy ion scattering (LEIS) study on Ti/silica samples was carried out. First, the surface coverage of metal species achievable at a certain reaction temperature on an oxide is determined by the number of specific reactive sites for the precursor. TiCl4 utilizes both isolated and H-bonded OH groups while Cr(acac)3 and HMDS react preferentially with isolated OH groups. Second, the bonding mode affected the surface coverage of titanium species. The bifunctional reactivity of TiCl4, on the one hand, consumed two OH groups per one TiCl4 molecule lowering the surface coverage on silica preheated at elevated temperatures, and on the other hand, enabled the utilization of the H-bonded OH groups present on silica preheated at lower temperatures thereby increasing the surface coverage. Third, the steric hindrance between the precursor and the already chemisorbed metal complexes was shown to have an effect on the surface coverage of Cr(acac)3 because of the size of two remaining acac ligands of bound chromium complex. Hence, at saturating reaction conditions of compound precursors, where undecomposed, reproducible and selectively chemisorbed surface species are obtained, the full monolayer coverage on oxides is seldom achieved.

Processing of catalysts by atomic layer epitaxy : modification of supports

Abstract Different supports were modified with titania, zirconia and chromia by the atomic layer epitaxy technique (ALE). In ALE, a metal precursor is bound to the support in saturating gas-solid reactions. Surface oxides are grown by alternating reactions of the metal precursor and an oxidizing agent. Growth mechanisms differ depending on the precursor-support pair and the processing conditions. In this work, the influences of the support, precursor and reaction temperature were investigated by comparing the growth of titania from Ti(OCH(CH 3 ) 2 ) 4 on silica and alumina, titania from TiCl 4 and Ti(OCH(CH 3 ) 2 ) 4 on silica, and zirconia from ZrCl 4 on silica and alumina. The modification of porous oxides supported on metal substrates (monoliths) was demonstrated for the growth of chromia from Cr(acac) 3 .

Growth mechanisms of mixed oxides on alumina

In this paper the growth mechanisms of mixed oxides on alumina are examined. Two combinations of both LaCeCuO/alumina and MgMnCoO/alumina were processed by the atomic layer epitaxy (ALE) technique by carrying out from five to nine reaction cycles. Each cycle of growth consisted of the reaction sequences of the corresponding metal beta-diketonate and air. The layer-by-layer growth was followed by element determinations and FTIR spectroscopy. X-ray diffraction (XRD), scanning electron microscopy with elemental mapping (SEM-EDS) and surface area measurements were carried out on the mixed oxides after the growth was completed. It was shown that mixed oxides can be processed on alumina in a reproducible fashion by ALE using the metal beta-diketonates as precursors. Owing to the high alumina surface area, a submonolayer of atoms bound on a surface could be precisely determined and the growth rate followed stepwise after each reaction sequence. In addition, during the growth, the stability and also the tendency of the metal beta-diketonates to etch the oxide layers could be evaluated. Although etching played an important role in the growth, it was also found to be reproducible. On the whole, it is suggested that the growth on high surface area materials can be used as a tool to deepen our understanding of the growth mechanisms involved in ALE.