| 1. |
- Timm, Rainer, et al.
(author)
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Self-cleaning and surface chemical reactions during hafnium dioxide atomic layer deposition on indium arsenide
- 2018
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 9:1
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Journal article (peer-reviewed)abstract
- Atomic layer deposition (ALD) enables the ultrathin high-quality oxide layers that are central to all modern metal-oxide-semiconductor circuits. Crucial to achieving superior device performance are the chemical reactions during the first deposition cycle, which could ultimately result in atomic-scale perfection of the semiconductor-oxide interface. Here, we directly observe the chemical reactions at the surface during the first cycle of hafnium dioxide deposition on indium arsenide under realistic synthesis conditions using photoelectron spectroscopy. We find that the widely used ligand exchange model of the ALD process for the removal of native oxide on the semiconductor and the simultaneous formation of the first hafnium dioxide layer must be significantly revised. Our study provides substantial evidence that the efficiency of the self-cleaning process and the quality of the resulting semiconductor-oxide interface can be controlled by the molecular adsorption process of the ALD precursors, rather than the subsequent oxide formation.
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| 2. |
- Chaudhary, Shilpi, et al.
(author)
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Adsorption of 3-(triethoxysilyl)propionitrile on a rutile TiO2(110) surface : An X-ray photoelectron spectroscopy study
- 2020
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In: DAE Solid State Physics Symposium 2019. - : AIP Publishing. - 1551-7616 .- 0094-243X. - 9780735420250 ; 2265
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Conference paper (peer-reviewed)abstract
- The adsorption of 3-(triethoxysilyl)propionitrile (TESP) on a reduced rutile TiO2(110) surface has been investigated using synchrotron-based X-ray photoelectron spectroscopy in ultrahigh vacuum (UHV). We have studied TESP adsorption on two surface preparation of rutile TiO2 (110) to explore the role of surface defects. In the first preparation, the adsorption of TESP was studied on reduced TiO2 (110) surface at room temperature. In the second experiment TESP was adsorbed on the oxygen-treated TiO2 surface to quench the oxygen vacancies generated by UHVannealing of the TiO2 crystal. The molecular footprints of the TESP molecules confirms the adsorption on both types of TiO2(110) surfaces. In the case of the reduced surface, temperature-dependent XPS measurements show the thermal stability of TESP molecules up to 600 °C. The comparison of the nitrogen and carbon lineshapes for both preparations suggests different adsorption geometries on the reduced and oxygen-dosed surfaces. To the best of our knowledge, the UHV preparations and measurements of TESP adsorption on rutile TiO2 (110) in the present study are reported for the first time.
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| 3. |
- Chaudhary, Shilpi, et al.
(author)
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X-ray photoelectron spectroscopy study of adsorption of (3-mercaptopropyl)trimethoxysilane and n-propyltriethoxysilane on a rutile TiO2(110) surface
- 2015
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In: Advanced Materials Letters. - 0976-3961. ; 6:4, s. 279-283
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Journal article (peer-reviewed)abstract
- We have studied the adsorption of two silane compounds, (3-mercaptopropyl) trimethoxysilane (MPTMS) and n-propyltriethoxysilane (PTES), on a rutile TiO2(110) surface using angle dependent X-ray photoelectron spectroscopy. The observation of the S 2p line, in the case of MPTMS, and the C 1s line for both MPTMS and PTES confirms the adsorption of the molecules. For a dose of 122 Langmuirs of MPTMS we find room temperature coverage of 0.55 monolayers, while for a 60 Langmuir dose of PTES the coverage is found to be 0.89 monolayers. Thus, MPTMS has a considerably lower sticking coefficient on the rutile TiO2(110) surface than PTES. Both PTES and MPTES are found to bind dissociatively to the surface. An analysis of angle dependent data further suggests that for MPTMS the thiol group and thus alkyl chain points away from the surface, while for a 0.5 monolayer coverage of PTES the alkyl chain is oriented towards the surface. A higher coverage, ~1 monolayer, the behavior seems to be reversed for at least a fraction of all molecules. Temperature programed XPS measurements suggest that the oxy groups of both molecules desorb from the surface at 550 K, which is in accordance with literature. The present study thus provides information on how these silane coupling agents bind to titanium oxide and what their molecular orientation is on the surface.
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| 4. |
- 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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| 5. |
- Eren, Baran, et al.
(author)
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Contamination Issues in Ambient Pressure Experiments
- 2021
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In: Ambient Pressure Spectroscopy in Complex Chemical Environments. - Washington, DC : American Chemical Society. - 0097-6156 .- 1947-5918. - 9780841298125 - 9780841298118 ; 1396, s. 267-295
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Book chapter (peer-reviewed)abstract
- Contamination is the most common and arguably the most significant problem scientists are facing in experimental surface science research that is practiced in the presence of gases. It is fair to say that contamination problems are often worse with ambient pressures compared to conventional experiments in vacuum. It is one of the main reasons for poor reproducibility in this field and in relevant basic and applied research fields like heterogeneous catalysis and electrochemistry. Whilst some type of contaminants are more innocent and only hinder quantitative analysis, some are harmful as they change the outcome of the experiments. In this chapter, the potential sources of contamination are summarized and some solutions are suggested. Examples of commonly observed contaminants such as hydrocarbons, oxygenated hydrocarbons, and adsorbed species of traces gases are presented. The scope of this chapter is restricted to ambient pressure x-ray photoelectron spectroscopy and infrared reflection absorption spectroscopy studies on single crystal surfaces, but similar problems exist on other sample surfaces or with other techniques such as x-ray absorption spectroscopy and sum frequency generation spectroscopy.
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| 6. |
- Head, Ashley R., et al.
(author)
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In situ characterization of the deposition of anatase TiO2 on rutile TiO2(110)
- 2018
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In: Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 36:2
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Journal article (peer-reviewed)abstract
- Growing additional TiO2 thin films on TiO2 supstrates in ultrahigh vacuum (UHV)-compatible chambers have many applications for sample preparation, such as smoothing surface morphologies, templating, and covering impurities. However, there has been little study into how to control the morphology of TiO2 films deposited onto TiO2 supstrates, especially using atomic layer deposition (ALD) precursors. Here, the authors show the growth of a TiO2 film on a rutile TiO2(110) surface using titanium tetraisopropoxide (TTIP) and water as the precursors at pressures well below those used in common ALD reactors. X-ray absorption spectroscopy suggests that the relatively low sample temperature (175 °C) results in an anatase film despite the rutile template of the supstrate. Using ambient pressure x-ray photoelectron spectroscopy, the adsorption of TTIP was found to be self-limiting, even at room temperature. No molecular water was found to adsorb on the surface. The deposited thickness suggests that an alternate chemical vapor deposition growth mechanism may be dominating the growth process. This study highlights the possibility that metal oxide film deposition from molecular precursors is an option for sample preparations in common UHV-compatible chambers.
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| 7. |
- Head, Ashley R., et al.
(author)
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UHV and Ambient Pressure XPS : Potentials for Mg, MgO, and Mg(OH)2 Surface Analysis
- 2016
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In: JOM. - : Springer Science and Business Media LLC. - 1047-4838 .- 1543-1851. ; 68:12, s. 3070-3077
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Journal article (peer-reviewed)abstract
- The surface sensitivity of x-ray photoelectron spectroscopy (XPS) has positioned the technique as a routine analysis tool for chemical and electronic structure information. Samples ranging from ideal model systems to industrial materials can be analyzed. Instrumentational developments in the past two decades have popularized ambient pressure XPS, with pressures in the tens of mbar now commonplace. Here, we briefly review the technique, including a discussion of developments that allow data collection at higher pressures. We illustrate the information XPS can provide by using examples from the literature, including MgO studies. We hope to illustrate the possibilities of ambient pressure XPS to Mg, MgO, and Mg(OH)2 systems, both in fundamental and applied studies.
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| 8. |
- Hu, Tianhao, et al.
(author)
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Interaction of Anisole on Alumina-Supported Ni and Mo Oxide Hydrodeoxygenation Catalysts
- 2023
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In: Journal of Physical Chemistry C. - 1932-7447. ; 127:39, s. 19440-19450
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Journal article (peer-reviewed)abstract
- The conversion of biomass to transportation fuels and value-added chemicals is a promising method to reduce the reliance on fossil fuels. Mo-based catalysts have been shown to be highly active in the hydrodeoxygenation of biomass-derived phenolic compounds. The catalyst active phase, surface species, and the effect of adding additional metals are not comprehensively understood. Here we compare the temperature-dependent adsorption behavior of the model compound anisole on an alumina-supported mixed nickel molybdenum oxide catalyst with two reference catalysts, molybdenum oxide and nickel oxide. Raman spectroscopy showed that the catalysts contain significant amounts of molybdates and molybdoaluminates, in addition to NiMoO4 in the nickel molybdenum catalyst and MoO3 in the molybdenum-only catalyst. Using transmission infrared spectroscopy under a controlled environment, we find that anisole chemisorbed largely through the oxygen in the methoxy group to form surface-bound phenoxy and methoxy species on all of the catalysts. Ambient pressure X-ray photoelectron spectroscopy measurements of the catalysts in anisole vapor showed reduced Mo atoms are the binding sites. The surface interaction and removal temperature of these species varied with the metal composition. The MoOx component dominated the adsorption behavior in both MoOx and NiMoOx catalysts. The formation of new aromatics, including methylated rings, depended on the Ni composition. Upon the addition of hydrogen to induce the hydrodeoxygenation of anisole, undesirable polynuclear aromatic species were quickly formed on the Mo-based catalysts. These results suggest that the molybdenum oxide controls the adsorption and reactivity of the surface species with a cooperative effect by Ni.
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| 9. |
- Kersell, Heath, et al.
(author)
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Some Future Perspectives in Ambient Pressure X-ray Spectroscopies : Atmospheric Pressure, Spatially Resolved and Multi-modal Experiments
- 2021
-
In: Ambient Pressure Spectroscopy in Complex Chemical Environments. - Washington, DC : American Chemical Society. - 0097-6156 .- 1947-5918. - 9780841298125 - 9780841298118 ; 1396, s. 333-358
-
Book chapter (peer-reviewed)abstract
- This chapter introduces a selection of novel and emerging concepts in ambient pressure X-ray spectroscopies. Technical considerations and first photoemission experiments performed in and above 1 bar pressure are described. Then, X-ray spectroscopy experiments that provide high lateral (microscopy) and depth (X-ray standing-waves) resolution in ambient environments are discussed. The final part of the chapter introduces recent developments in multimodal instrumentation, such as the combination of ambient pressure X-ray Photoelectron Spectroscopy with Infrared spectroscopy and X-ray scattering.
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| 10. |
- Shavorskiy, Andrey, et al.
(author)
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Time Resolved Ambient Pressure X-ray Photoelectron Spectroscopy
- 2021
-
In: Ambient Pressure Spectroscopy in Complex Chemical Environments. - Washington, DC : American Chemical Society. - 0097-6156 .- 1947-5918. ; 1396, s. 219-248
-
Book chapter (peer-reviewed)abstract
- This chapter describes basic principles and recent advances in time-resolved X-ray Photoelectron Spectroscopy with particular emphasis on studies conducted in complex reaction environments. Specifically, it discusses different ways for obtaining time information and achieving different time resolutions in tr-(AP)XPS experiment, covering timescales from fs to s. This includes experimental setups at the free-electron lasers as well as electron storage rings and high-harmonic generation light sources. A variety of perturbations such as optical, mechanical, gas composition, temperature, and pressure is discussed. A number of examples are described that demonstrate the power of tr-APXPS in studies combining various timescales and perturbation methods.
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| 11. |
- 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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