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Title: Surface Electronic Properties and Site-Specific Laser Desorption Processes of Highly Structured Nanoporous MgO Thin Films

Abstract

The surface electronic properties of metal oxides critically depends on low-coordinated sites, such as kinks, corners and steps [1]. In order to characterize experimentally those surface states as well as their role for laser desorption processes, we prepare defect enriched surfaces by growing thin MgO films using reactive ballistic deposition [2] on crystalline dielectric substrates. With samples held at room temperature, the resulting MgO films are highly textured and consist of porous columns with column lengths ranging from tens of nanometers up to six micrometers. Measurements by x-ray photoelectron spectroscopy (XPS) are carried out in-situ for MgO films, vacuum-cleaved MgO crystals, and water vapor exposed samples. In the case of thin films, we observe O 1s spectra with a significant shoulder feature at 2.3 eV higher binding energy (HBE) than the corresponding peak at 530.0 eV representing regular lattice oxygen. We evaluate this feature in terms of non-stoichiometric oxygen and formation of an oxygen-rich layer at the topmost surface of the MgO columns. In contrast, no HBE-features are detectable from clean single crystal MgO surfaces, while the hydroxyl O 1s band peaks at 531.6 eV. Under excitation with 266-nm-laser-pulses, known to be resonant with low-coordinated surface anions [3], we observemore » preferential depletion of defective oxygen-states (HBE signal) and temporary restoration of ideal surface stoichiometry. Furthermore, auxiliary signals are observed on several micrometer thick films, acting like satellites to major photoelectron-peaks (O 1s, Mg 2s, and Mg 2p) but shifted by approximately 4 eV towards lower kinetic energy. These features are depleted by UV-light exposure, pointing to the occurrence of surface-charge imbalance, accompanied by photon stimulated charge-transfer reactions. These results are in line with desorption experiments of neutrals, stimulated by laser excitation at 266 nm. According to the low-coordination nature of nanoporous MgO films, we find that the laser fluence required for oxygen desorption is much lower in comparison to the single-crystal MgO (100) surface. The detected neutral oxygen desorption occurs with a single photon power dependence from nanoporous MgO thin films. This contrasts to the two-photon power dependence observed from MgO single crystals. In fact, the single photon energy of the laser lies within the charge-transfer transition in surface ion pairs with three-coordinated anions ({approx} 4.7 eV), while a two-photon absorption regime easily lies within the four-coordinated anion charge transfer transition near 5.4 eV [3]. In summary, our XPS studies and laser desorption experiments indicate new surface-site-specific excitation regimes, which might eventually allow for site-specific manipulation of surface morphology.« less

Authors:
; ; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
859973
Report Number(s):
PNNL-SA-44464
Journal ID: ISSN 0039-6028; SUSCAS; 1365; 4792; KC0301020; TRN: US200608%%302
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
Journal Name:
Surface Science
Additional Journal Information:
Journal Volume: 593; Journal Issue: 1-3; Journal ID: ISSN 0039-6028
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ANIONS; BINDING ENERGY; DESORPTION; DIELECTRIC MATERIALS; ION PAIRS; KINETIC ENERGY; LASERS; MONOCRYSTALS; MORPHOLOGY; OXIDES; THIN FILMS; WATER VAPOR; X-RAY PHOTOELECTRON SPECTROSCOPY; Environmental Molecular Sciences Laboratory

Citation Formats

Henyk, Matthias, Beck, Kenneth M, Engelhard, Mark H, Joly, Alan G, Hess, Wayne P, and Dickinson, J T. Surface Electronic Properties and Site-Specific Laser Desorption Processes of Highly Structured Nanoporous MgO Thin Films. United States: N. p., 2005. Web. doi:10.1016/j.susc.2005.06.090.
Henyk, Matthias, Beck, Kenneth M, Engelhard, Mark H, Joly, Alan G, Hess, Wayne P, & Dickinson, J T. Surface Electronic Properties and Site-Specific Laser Desorption Processes of Highly Structured Nanoporous MgO Thin Films. United States. doi:10.1016/j.susc.2005.06.090.
Henyk, Matthias, Beck, Kenneth M, Engelhard, Mark H, Joly, Alan G, Hess, Wayne P, and Dickinson, J T. Sun . "Surface Electronic Properties and Site-Specific Laser Desorption Processes of Highly Structured Nanoporous MgO Thin Films". United States. doi:10.1016/j.susc.2005.06.090.
@article{osti_859973,
title = {Surface Electronic Properties and Site-Specific Laser Desorption Processes of Highly Structured Nanoporous MgO Thin Films},
author = {Henyk, Matthias and Beck, Kenneth M and Engelhard, Mark H and Joly, Alan G and Hess, Wayne P and Dickinson, J T},
abstractNote = {The surface electronic properties of metal oxides critically depends on low-coordinated sites, such as kinks, corners and steps [1]. In order to characterize experimentally those surface states as well as their role for laser desorption processes, we prepare defect enriched surfaces by growing thin MgO films using reactive ballistic deposition [2] on crystalline dielectric substrates. With samples held at room temperature, the resulting MgO films are highly textured and consist of porous columns with column lengths ranging from tens of nanometers up to six micrometers. Measurements by x-ray photoelectron spectroscopy (XPS) are carried out in-situ for MgO films, vacuum-cleaved MgO crystals, and water vapor exposed samples. In the case of thin films, we observe O 1s spectra with a significant shoulder feature at 2.3 eV higher binding energy (HBE) than the corresponding peak at 530.0 eV representing regular lattice oxygen. We evaluate this feature in terms of non-stoichiometric oxygen and formation of an oxygen-rich layer at the topmost surface of the MgO columns. In contrast, no HBE-features are detectable from clean single crystal MgO surfaces, while the hydroxyl O 1s band peaks at 531.6 eV. Under excitation with 266-nm-laser-pulses, known to be resonant with low-coordinated surface anions [3], we observe preferential depletion of defective oxygen-states (HBE signal) and temporary restoration of ideal surface stoichiometry. Furthermore, auxiliary signals are observed on several micrometer thick films, acting like satellites to major photoelectron-peaks (O 1s, Mg 2s, and Mg 2p) but shifted by approximately 4 eV towards lower kinetic energy. These features are depleted by UV-light exposure, pointing to the occurrence of surface-charge imbalance, accompanied by photon stimulated charge-transfer reactions. These results are in line with desorption experiments of neutrals, stimulated by laser excitation at 266 nm. According to the low-coordination nature of nanoporous MgO films, we find that the laser fluence required for oxygen desorption is much lower in comparison to the single-crystal MgO (100) surface. The detected neutral oxygen desorption occurs with a single photon power dependence from nanoporous MgO thin films. This contrasts to the two-photon power dependence observed from MgO single crystals. In fact, the single photon energy of the laser lies within the charge-transfer transition in surface ion pairs with three-coordinated anions ({approx} 4.7 eV), while a two-photon absorption regime easily lies within the four-coordinated anion charge transfer transition near 5.4 eV [3]. In summary, our XPS studies and laser desorption experiments indicate new surface-site-specific excitation regimes, which might eventually allow for site-specific manipulation of surface morphology.},
doi = {10.1016/j.susc.2005.06.090},
journal = {Surface Science},
issn = {0039-6028},
number = 1-3,
volume = 593,
place = {United States},
year = {2005},
month = {11}
}