Browsing by Author "Weckman, Timo"

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Atomic Layer Deposition of Zinc Oxide: Diethyl Zinc Reactions and Surface Saturation from First Principles
(2016) Weckman, Timo; Laasonen, Kari
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Zinc oxide thin films grown via atomic layer deposition have been under intense research for the past few years. Here we present a comprehensive density functional theory study on the atomic layer deposition of zinc oxide. The adsorption of diethyl zinc and subsequent surface reactions are studied on an ideal (100) ZnO surface as well as on a stepped surface in order to compare an ideal and a nonideal surface structures. Our results show that diethyl zinc adsorbs and reacts rapidly on the surface to form monoethyl zinc. Our calculations also show that the initial ligand-exchange reactions are preferred on the planar surface over the step surface. Further reaction from monoethyl zinc to adsorbed zinc atoms has a high reaction barrier. We present two surface structures for the saturated zinc oxide surface at the end of the diethyl zinc pulse corresponding to a low and a high temperature approximations that are in good agreement with the experiments.
Atomic Layer Deposition of Zinc Oxide: Study on the Water Pulse Reactions from First Principles
(2018) Weckman, Timo; Laasonen, Kari
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Atomic layer deposition (ALD) of zinc oxide thin films has been under intense research in the past few years. The most common precursors used in this process are diethyl zinc (DEZ) and water. The surface chemistry related to the growth of a zinc oxide thin film via atomic layer deposition is not entirely clear and the ideal model of the process has been contradicted by experimental data e.g. the incomplete elimination of the ethyl-ligands from the surface and the non-negative mass change during the water pulse. In this work we investigate the surface reactions of water during the atomic layer deposition of zinc oxide. The adsorption and ligand-exchange reactions of water are studied on ethyl-saturated surface structures to grasp the relevant surface chemistry contributing to the deposition process. The complex ethyl-saturated surface structures are adopted from a previous publication on the DEZ/\ce{H2O}-process and different configurations are sampled using \emph{ab initio} molecular dynamics in order to find a suitable minimum structure. Water molecules are found to adsorb exothermically onto the ethyl-covered surface at all the ethyl-concentrations considered. We do not observe an adsorption barrier for water at 0 K, however, the adsorption energy for any additional water molecules decreases rapidly at high ethyl-concentrations. Ligand-exchange reactions are studied at various surface ethyl-coverages. The water pulse ligand-exchange reactions have overall larger activation energies than surface reactions for diethyl zinc pulse. For some of the configurations considered the reaction barriers may be inaccessible at the process conditions, suggesting that some ligands may be inert towards ligand-exchange with water. The activation energies for the surface reactions show only a weak dependence on the surface ethyl-concentration. The sensitivity of the adsorption of water at high ethyl-coverages suggests that at high ligand-coverages the kinetics may be somewhat hindered due to steric effects. Calculations on the ethyl-covered surfaces are compared to a simple model containing a single monoethyl zinc group. The calculated activation energy for this model is in line with calculations done on the complex model, but the adsorption of water is poorly described. The weak adsorption bond onto a single monoethyl zinc is probably due to a cooperative effect between the surface zinc atoms. A cooperative effect between water molecules is also observed, however, the effect on the activation energies is not as significant as has been reported for other ALD processes.
Enhanced performance of a silicon microfabricated direct methanol fuel cell with PtRu catalysts supported on few-walled carbon nanotubes
(2014-02-01) Borghei, Maryam; Scotti, Gianmario; Kanninen, Petri; Weckman, Timo; Anoshkin, Ilya V.; Nasibulin, Albert G.; Franssila, Sami; Kauppinen, Esko I.; Kallio, Tanja; Ruiz, Virginia
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Silicon micro fuel cells (Si-MFCs) are promising power supplies for microelectronic applications, however their development is still at early stages compared to the conventional proton exchange membrane fuel cells (PEMFCs). There are not many published reports on the durability of Si-MFCs and those available only projected the life-time of standard Vulcan based catalysts. However, the limited durability resulting from carbon corrosion is one of the crucial issues in fuel cells. In this study, Si-MFC with an integrated silicon nanograss diffusion layer is used for the direct methanol fuel cell investigations. The long-term (3-day) performance of PtRu catalysts supported on different carbon supports, namely Vulcan, Graphitized carbon nanofibers (GNFs) and Few-walled carbon nanotubes (FWCNTs), was studied. PtRu-FWCNTs and PtRu-GNFs exhibited respectively 471% (20.0 mW cm-2) and 274% (13.1 mW cm-2) power density enhancements compared to PtRu-Vulcan (3.5 mW cm-2). After 3-day durability measurements, power density stayed at 72, 68 and 91% of the initial value, respectively for PtRu-FWCNTs, PtRu-GNFs and PtRu-Vulcan. To evaluate the influence of carbon supports as well as the distribution and the size of the nanoparticles on the overall performance of Si-MFCs, further characterizations with Raman, BET, XRD, SEM and TEM were performed.
First Principles Multiscale Modelling of the Atomic Layer Deposition of Al2O3 and ZnO
(2018) Weckman, Timo
School of Chemical Engineering | Doctoral dissertation (article-based)
The rapid development of nanotechnology, especially in the field of microelectronics, and ever shrinking dimensions of device components set high requirements for the manufacturing of the necessary nanostructures. Many microscopic components, e.g. transistors, are constructed layer-by-layer from thin film. An important tool 21st century technique for the fabrication of such thin films is the atomic layer deposition. Atomic layer deposition, originally developed in Finland, is based on sequential self-limiting gas-pulses, resulting in a uniform, pin-hole free thin film, with thickness control at the atomic level. Computational modeling is an important part of modern chemistry. Research can be conducted theoretically - without empirical parameters - with the application of quantum mechanics. With quantum mechanical calculations it is possible to model the electronic structure of molecules and to study the bonding and interactions of molecules as well as different molecular mechanisms. In this work, the deposition of aluminium and zinc oxides were studied using computational chemistry. Both oxides have wide range of applications e.g. in transistors and solar cells. Aluminium oxide is usually deposited using a trimethylaluminium-water-process. The surface chemistry was studied on a realistic hydroxylated surface model and trimethylaluminium was observed to react rapidly with surface hydroxyl groups to produce monomethylaluminium. Monomethylaluminium was estimated to be relatively inert and to convert to aluminium only at high temperatures. Subsequent water pulse mechanisms were also studied at low methyl-coverage. Direct dimethylaluminium--water reactions were accessible at process conditions, but the elimination of monomethylaluminium by water requires a complex cooperative mechanism. Zinc oxide is usually deposited using a diethylzinc-water-process. Diethylzinc was found to convert rapidly into monoethylzinc but the elimination of monoethylzinc was found to be a slow process. Based on the calculations, two ethyl-saturated surface structures were constructed, corresponding to low and high temperature estimations. These saturated surfaces were used in a subsequent study on the water pulse reactions, resulting in a reaction network for a complete ALD cycle. The growth of the zinc oxide thin film was then modeled in macroscopic scale using a kinetic Monte Carlo model. The kinetic modelling enables a direct comparison with experimental measurements. The kinetic model, built upon the theoretical calculations, accurately predicted the temperature-dependency of the film growth. Also, the predicted growth per cycle is in good agreement with experimental data.
First principles study of the atomic layer deposition of alumina by TMA-H2O-process
(2015) Weckman, Timo; Laasonen, Kari
School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Atomic layer deposition (ALD) is a coating technology used to produce highly uniform thin films. Aluminiumoxide, Al2O3, is mainly deposited using trimethylaluminium (TMA) and water as precursors and is the most studied ALD-process to date. However, only few theoretical studies have been reported in the literature. The surface reaction mechanisms and energetics previously reported focus on a gibbsite-like surface model but a more realistic description of the surface can be achieved when the hydroxylation of the surface is taken into account using dissociatively adsorbed water molecules. The adsorbed water changes the structure of the surface and reaction energetics change considerably when compared to previously studied surface model. Here we have studied the TMA/H2O process using density functional theory on a hydroxylated alumina surface and reproduced the previous results for comparison. Mechanisms and energetics during both the TMA and the subsequent water pulse are presented. TMA is found to adsorb exothermically onto the surface. The reaction barriers for the ligand-exchange reactions between the TMA and the surface hydroxyl groups were found to be much lower compared to previously presented results. TMA dissociation on the surface is predicted to saturate at monomethylaluminium. Barriers for proton diffusion between surface sites are observed to be low. TMA adsorption was also found to be cooperative with the formation of methyl bridges between the adsorbants. The water pulse was studied using single water molecules reacting with the DMA and MMA surface species. Barriers for these reactions were found to reasonable in the process conditions. However, stabilizing interactions amongst water molecules were found to lower the reaction barriers and the dynamical nature of water is predicted to be of importance. It is expected that these calculations can only set an upper limit for the barriers during the water pulse.
First principles study of the atomic layer deposition of alumina by TMA/H2O-process
(2015) Weckman, Timo; Laasonen, Kari
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Kinetic Monte Carlo Study of the Atomic Layer Deposition of Zinc Oxide
(2018-11-29) Weckman, Timo; Shirazi, Mahdi; Elliott, Simon D.; Laasonen, Kari
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Atomic layer deposition (ALD) has emerged as an important technique for thin film deposition in the last two decades. Zinc oxide thin films, usually grown via DEZ/H2O-process, have seen much interest both in application and in theoretical research. The surface processes related to the growth of the thin film are not entirely understood and the conceptual picture of the ALD process has been contradicted by recent experiments where ligands from the zinc pulse persist on the surface even after extended water pulse exposures. In this work, we investigate the overall growth of the zinc oxide thin films grown via DEZ/H2O-process by modelling the surface chemistry using first principles kinetic Monte Carlo for the first time. The kinetic Monte Carlo allows us to implement density functional theory calculations conducted on zinc oxide (100) surface into a kinetic model and extract data directly comparable to experimental measurements. The temperature dependent growth profile obtained from our model is in good qualitative agreement with the experimental data. The onset of thin film growth is offset from the experimentally data due to the underestimation of the reaction barriers within density functional theory. The growth per cycle of the deposited film is overestimated by 18% in the kinetic model. Mass-gain during an ALD cycle is in qualitative agreement with experimental quartz-crystal microbalance data. The main mass-gain within an ALD cycle is obtained during the DEZ pulse and mass-change during the water pulse negligible. The cause of low film growth at low temperatures is due to the high reaction barriers for ethyl-elimination during the water pulse. This kinetic barrier results in low film growth as no new DEZ can adsorb to the ethyl-saturated surface. At elevated temperatures ethyl-elimination becomes accessible, resulting in the ideal layer-by-layer growth of the film. However, a large fraction of ethyl-ligands persist on the surface after each ALD cycle even at high temperatures. This results in ethyl-ligands being encapsulated into the film lattice. This is likely due to an incomplete set of reaction pathways and it is likely that some yet unidentified process is responsible for the elimination of the ethyl-ligands from the surface as the deposition process progresses.
Laskennallinen tutkimus TMA/H2O ALD-prosessista
(2014-02-11) Weckman, Timo
Kemian tekniikan korkeakoulu | Master's thesis
Alkujaan suomalaisperäinen ALD-prosessi (atomic layer deposition, atomikerroskasvatus) on erityisesti pienelektroniikassa tärkeä sovellus, jolla on mahdollista syntetisoida jopa vain muutaman atomin paksuisia ohutkalvoja. Prosessilla valmistettavien atomikerrostumien paksuutta ja rakennetta on mahdollista kontrolloida hyvin tarkasti. Ohutkalvoille on lukuisia käyttökohteita joista kaupallisesti tärkeimmät ovat puolijohdeteollisuudessa, esimerkiksi transistoreiden portti-oksidien valmistuksessa. Laskennallinen mallintaminen on noussut luonnontieteissä tärkeäksi komponentiksi teoreettisen ja kokeellisen tutkimuksen rinnalle. Kemiassa systeemejä voidaan mallintaa \emph{ab initio}, alusta alkaen ilman varsinaista empiiristä tietoa systeemistä, käyttämällä kvanttimekaniikkaa. Laskennallisella tutkimuksella voidaan paitsi verifioida tai falsifoida olemassa olevia malleja, myös tutkia esimerkiksi uusia reaktiomekanismeja ja tuottaa kvantitatiivista tietoa. Trimetyylialumiini--vesi-prosessi on ehkä tutkituin ALD-prosessi. Prosessilla kasvatetaan alumiinioksidiohutkalvoja. Teoreettinen tarkastelu systeemistä on kuitenkin ollut melko vähäistä. Osa tutkimuksista on pohjautunut alkuaskeleisiin piidioksidin pinnalla, osa vastaavasti jatkoreaktioihin muodostuneella alumiinioksidipinnalla. Aikaisemmat tutkimukset kasvusta aluminalla ovat pohjautuneet klusterimalleihin tai prosessiolosuhteiden kannalta epärealistiseen kuvaukseen pinnasta. Tässä työssä on toistettu aiemmin kirjallisuudessa esitetyt mallit prosessista ja esitetään reaktiomekanismit realistisemmalla alumiinioksidi-pintamallilla.
Metallinanopartikkelin koon ja pintarakenteen merkitys katalyysissä
(2011) Weckman, Timo
Kemian tekniikan korkeakoulu | Bachelor's thesis
Modelling the growth reaction pathways of zincone ALD/MLD hybrid thin films: a DFT study
(2024-06-28) Mäkinen, Mario; Weckman, Timo; Laasonen, Kari
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
ALD/MLD hybrid thin films can be fabricated by combining atomic layer deposition (ALD) and molecular layer deposition (MLD). Even though this deposition method has been extensively used experimentally, the computational work required to acquire the reaction paths during the thin film deposition process is still in dire demand. We investigated hybrid thin films consisting of diethyl zinc and either 4-aminophenol or hydroquinone using both gas-phase and surface reactions to gain extensive knowledge of the complex phenomena occurring during the process of hybrid thin film deposition. We used density functional theory (DFT) to obtain the activation energies of these kinetic-dependent deposition processes. Different processes of ethyl ligand removal as ethane were discovered, and we found that the hydroxyl group of 4-aminophenol was more reactive than the amino group in the migration of hydrogen to an ethyl ligand within a complicated branching reaction chain.
Sinkkioksidipohjaisten hybridiohutkalvojen mallinnus tiheysfunktionaaliteorian avulla
(2019-07-31) Mäkinen, Mario
Kemian tekniikan korkeakoulu | Master's thesis
Atomi- ja molekyylikerroskasvatus ovat menetelmiä, joilla valmistetaan ohutkalvoja. Nämä menetelmät yhdistämällä voidaan valmistaa hybridiohutkalvoja. Hybridimateriaalien mahdollisia tulevaisuuden sovelluksia ovat valosähköiset laitteet, sensorit, aurinkopaneelit, suojaavat pinnoitteet sekä taipuisa elektroniikka. Hybridiohutkalvojen valmistus atomi- ja molekyylikerroskasvatuksen avulla on vielä vaillinaisesti tunnettu prosessi. Näiden ohutkalvojen kasvatus perustuu adsorptioreaktioihin,joita voidaan mallintaa laskennallisilla menetelmillä. Tässä diplomityössä adsorptioreaktioiden mallinnuksessa käytettiin tiheysfunktionaaliteoriaa. Mallinnetut hybridiohutkalvot koostuivat dietyylisinkistä sekä orgaanisesta fenolista: joko 4-aminofenolista tai hydrokinonista. Näiden hybridiohutkalvojen kasvureaktioita mallinnettiin kaasufaasissa sekä pintamallilla, jossa pintarakenne koostui etyyliligandeilla saturoituneesta sinkkioksidista. Tutkittu hybridiohutkalvon kasvureaktio eteni odotetusti: prekursori fysisorptoitui eksotermisesti, mitä seurasi dissosiatiivinen kemisorptioreaktio tai ligandinvaihtoreaktio.Hybridiohutkalvon kasvua heikensi pinnan korkea etyyliligandikonsentraatio: korkea etyyliligandikonsentraatio nosti aktivoitumisenergioita, heikensi muodostuvia kemiallisia sidoksia sekä esti dissosiatiivisen adsorption pinnalla. Kaasufaasimalli ei kuvaa hybridiohutkalvojen kasvureaktioita riittävän hyvin. Kasvureaktioiden aktivoitumisenergia oli pintamallissa kaasufaasireaktioita matalampi, kun ligandikonsentraatio oli matala. Todennäköisin syy matalammalle aktivoitumisenergialle oli sinkkioksidipinnan reagoivaa sinkkiatomia ympäröivät happiatomit, jotka vetävät elektronegatiivisuutta puoleensa.