1. |
- Boucly, Anthony, et al.
(author)
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Soft X-ray Heterogeneous Radiolysis of Pyridine in the Presence of Hydrated Strontium-Hydroxyhectorite and its Monitoring by Near-Ambient Pressure Photoelectron Spectroscopy
- 2018
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In: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 8
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Journal article (peer-reviewed)abstract
- The heterogeneous radiolysis of organic molecules in clays is a matter of considerable interest in astrochemistry and environmental sciences. However, little is known about the effects of highly ionizing soft X-rays. By combining monochromatized synchrotron source irradiation with in situ Near Ambient Pressure X-ray Photoelectron Spectroscopy (in the mbar range), and using the synoptic view encompassing both the gas and condensed phases, we found the water and pyridine pressure conditions under which pyridine is decomposed in the presence of synthetic Sr2+-hydroxyhectorite. The formation of a pyridine/water/Sr2+ complex, detected from the Sr 3d and N 1s core-level binding energies, likely presents a favorable situation for the radiolytic breaking of the O-H bond of water molecules adsorbed in the clay and the subsequent decomposition of the molecule. However, decomposition stops when the pyridine pressure exceeds a critical value. This observation can be related to a change in the nature of the active radical species with the pyridine loading. This highlights the fact that the destruction of the molecule is not entirely determined by the properties of the host material, but also by the inserted organic species. The physical and chemical causes of the present observations are discussed.
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2. |
- Chaudhary, Shilpi, et al.
(author)
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Real-Time Study of CVD Growth of Silicon Oxide on Rutile TiO2(110) Using Tetraethyl Orthosilicate
- 2015
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:33, s. 19149-19161
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Journal article (peer-reviewed)abstract
- The interaction of the ruffle TiO2(110) surface with tetraethyl orthosilicate (TEOS) in the pressure range from UHV to 1 mbar as well as the TEOS-based chemical vapor deposition of SiO2 on the TiO2(110) surface were monitored in real time using near-ambient pressure X-ray photoelectron spectroscopy. The experimental data and density functional theory calculations confirm the dissociative adsorption of TEOS on the surface already at room temperature. At elevated pressure, the ethoxy species formed in the adsorption process undergoes further surface reactions toward a carboxyl species not observed in the absence of a TEOS gas phase reservoir. Annealing of the adsorption layer leads to the formation of SiO2, and an intermediate oxygen species assigned to a mixed titanium/silicon oxide is identified. Atomic force microscopy confirms the morphological changes after silicon oxide formation.
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3. |
- D'acunto, Giulio, et al.
(author)
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Bimolecular Reaction Mechanism in the Amido Complex-Based Atomic Layer Deposition of HfO2
- 2023
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In: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 35:2, s. 529-538
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Journal article (peer-reviewed)abstract
- The surface chemistry of the initial growth during the first or first few precursor cycles in atomic layer deposition is decisive for how the growth proceeds later on and thus for the quality of the thin films grown. Yet, although general schemes of the surface chemistry of atomic layer deposition have been developed for many processes and precursors, in many cases, knowledge of this surface chemistry remains far from complete. For the particular case of HfO2 atomic layer deposition on a SiO2 surface from an alkylamido-hafnium precursor and water, we address this lack by carrying out an operando atomic layer deposition experiment during the first cycle of atomic layer deposition. Ambient-pressure X-ray photoelectron spectroscopy and density functional theory together show that the decomposition of the metal precursor on the stoichiometric SiO2 surface in the first half-cycle of atomic layer deposition proceeds via a bimolecular reaction mechanism. The reaction leads to the formation of Hf-bonded methyl methylene imine and free dimethylamine. In addition, ligand exchange takes place involving the surface hydroxyls adsorbed at defect sites of the SiO2 surface.
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4. |
- Head, Ashley, et al.
(author)
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Near Ambient Pressure X-ray Photoelectron Spectroscopy Study of the Atomic Layer Deposition of TiO2 on RuO2(110)
- 2016
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:1, s. 243-251
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Journal article (peer-reviewed)abstract
- The atomic layer deposition (ALD) of TiO2 on a RuO2(110) surface from tetrakis(dimethylamido) titanium and water at 110 degrees C was investigated using near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) at precursor pressures up to 0.1 mbar. In addition to the expected cyclic surface species, evidence for side reactions was found. Dimethylamine adsorbs on the surface during the TDMAT half-cycle, and a second species, likely methyl methylenimine, also forms. The removal of the amide ligand and the formation of an alkyammonium species during the water half-cycle were found to be pressure dependent. The O 1s, Ru 3d, and Ti 2p spectra show the formation of the Ru-O-Ti interface, and the binding energies are consistent with formation of TiO2 after one full ALD cycle. Dosing TDMAT on the RuO2(110) surface at room temperature promotes a multilayer formation that begins to desorb at 40 degrees C. The imine species is not seen until 60 degrees C. These insights into the ALD mechanism and precursor pressure dependence on reactivity highlight the utility of NAP-XPS in studying ALD processes and interface formation.
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5. |
- Jones, Rosemary, et al.
(author)
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Operando study of HfO2 atomic layer deposition on partially hydroxylated Si(111)
- 2024
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In: Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films. - 0734-2101. ; 42:2
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Journal article (peer-reviewed)abstract
- The introduction of atomic layer deposition (ALD), to the microelectronics industry has introduced a large number of new possible materials able to be deposited in layers with atomic thickness control. One such material is the high-κ oxide HfO2; thermally stable and ultrathin HfO2 films deposited by ALD are a significant contender to replace SiO2 as the gate oxide in capacitor applications. We present a mechanistic study of the first deposition cycle of HfO2 on the Si(111) surface using tetrakis(dimethylamido) hafnium (TDMAHf) and water as precursors using operando ambient pressure x-ray photoelectron spectroscopy. Here, we show that the hydroxylation of the clean Si(111) surface by residual water vapor, resulting in a 0.3 monolayer coverage of hydroxyls, leads to instantaneous full surface coverage of TDMAHf. The change in the atomic ratio of Hf to C/N found during the first deposition half-cycle, however, does not match the assumed immediate ligand loss through reaction with surface hydroxyls. One would expect an immediate loss of ligands, indicated by a Hf:N ratio of approximately 1:3 as TDMAHf deposits onto the surface; however, a Hf:N ratio of 1:3.6 is observed. The partial hydroxylation on the Si(111) surface leads to binding through the TDMAHf ligand N atoms resulting in both N and CH3 being found remaining on the surface post water half-cycle. Although there is evidence of ligand exchange reactions occurring at Si-OH sites, it also seems that N binding can occur on bare Si, highlighting the complexity of the substrate/precursor reaction even when hydroxyls are present. Moreover, the initial low coverage of Si-OH/Si-H appears to severely limit the amount of Hf deposited, which we hypothesize is due to the specific geometry of the initial arrangement of Si-OH/Si-H on the rest- and adatoms.
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6. |
- Shayesteh, Payam, et al.
(author)
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Experimental and theoretical gas phase electronic structure study of tetrakis(dimethylamino) complexes of Ti(IV) and Hf(IV)
- 2019
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In: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048. ; 234, s. 80-85
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Journal article (peer-reviewed)abstract
- The gas phase electronic structure of two transition metal alkylamino complexes, M(N(CH3)2)4, where M = Ti, Hf, was studied using photoelectron spectroscopy and density functional theory (DFT). These studies are a first step for predicting atomic layer and chemical vapor deposition reactions on surfaces, which are common applications of these molecules. The valence photoemission spectra of these two complexes were collected with 50 and 150 eV photon energies. Comparison of calculated ionization energies and our experiments yielded good agreement. Analysis of calculated molecular orbitals provides insight into the π-donation interaction between the lone pair of electrons on the amino ligands and the empty metal orbitals. In addition to the valence structure, the core level photoemission spectra were analyzed. The π-interaction was found to influence core level ionization energies. Vibrational structure in the C 1s spectra are reported with insight from DFT calculations. The comprehensive experimental and theoretical characterization of the electronic structure of these complexes provide a robust foundation to progress to detailed spectroscopic studies of the interactions between these molecules and surfaces.
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