Co-reporter:Gregory S. Hutchings, Jin-Hao Jhang, Chao Zhou, David Hynek, Udo D. Schwarz, and Eric I. Altman
ACS Applied Materials & Interfaces March 29, 2017 Volume 9(Issue 12) pp:11266-11266
Publication Date(Web):March 10, 2017
DOI:10.1021/acsami.7b01369
Epitaxial strain can be a powerful parameter for directing the growth of thin films. Unfortunately, conventional materials only offer discrete choices for setting the lattice strain. In this work, it is demonstrated that epitaxial growth of transition metal alloy solid solutions can provide thermally stable, high-quality growth substrates with continuously tunable lattice constants. Molecular beam epitaxy was used to grow NixPd1–x(111) alloy films with lattice constants between 3.61 and 3.89 Å on the hexagonal (0001) basal planes of α-Al2O3 and Cr2O3 (grown as epitaxial films on α-Al2O3 (0001)). The Cr2O3 acted as an adhesion layer, which not only improved the high-temperature stability of the films but also produced single-domain films with NixPd1–x [112̅] parallel to Cr2O3 [112̅0], in contrast to growth on α-Al2O3 that yielded twinned films. Surface characterization by electron diffraction and scanning tunneling microscopy (STM) as well as bulk X-ray diffraction analysis indicated that the films are suitable as inexpensive and stable substrates for thin-film growth and for surface science studies. To demonstrate this suitability, bilayer SiO2, a two-dimensional van der Waals material, was grown on a NixPd1–x(111) film tuned to closely match the film’s lattice constant, with STM and electron diffraction results revealing a highly ordered, single-phase crystalline state.Keywords: alloy; bilayer silica; molecular beam epitaxy; nickel; palladium; thin-film growth; two-dimensional materials;
Co-reporter:Eric I. Altman
The Journal of Physical Chemistry C August 3, 2017 Volume 121(Issue 30) pp:16328-16328
Publication Date(Web):July 7, 2017
DOI:10.1021/acs.jpcc.7b04394
The ability of group III phosphates to adopt a two-dimensional van der Waals (2D VDW) structure observed for SiO2 was evaluated using density functional theory. The energies to form 2D hexagonal bilayers of corner-sharing tetrahedra did not follow a monotonic trend: the energies for AlPO4 and GaPO4 were similar to silica, while for BPO4 it was more than a factor of 2 larger and for InPO4 nearly another factor of 2 larger. The larger In atom favors octahedral coordination, accounting for the high energy of the 2D InPO4 structure. Meanwhile, boron’s small size leads to a different favored bulk structure than AlPO4 or GaPO4 which competes much more successfully with the 2D phase. The implication is a sweet spot in the cation size for forming 2D tetrahedral oxides that spans Si to Ga. The 2D BPO4 and GaPO4 structures displayed alternating rotations of the B(Ga)O4 and PO4 tetrahedra which allowed the B(Ga)–O–P bond angles to match those seen in the favored bulk compounds; no such rotations were required for 2D AlPO4 and SiO2 to match bond angles in bulk compounds. The interactions of AlPO4 and GaPO4 with Rh(111) as a prototypical growth substrate were also investigated with the results revealing adhesion dominated by VDW interactions. Alternate structures were considered with results mimicking those seen for SiO2: introducing larger rings of corner-sharing tetrahedra decreases the density, allowing the structure to be tuned by applying tensile strain. In comparison to SiO2, however, only even-membered rings are possible for the phosphates, restricting the range of structures and defects that can form. Finally, it was found that Mg2+ could readily replace Al3+ in AlPO4 in the process, creating ion exchange sites. The results highlight the great promise for adding AlPO4 and GaPO4 to the family of 2D VDW materials.
Co-reporter:Jin-Hao Jhang;Chao Zhou;Omur E. Dagdeviren;Gregory S. Hutchings;Udo D. Schwarz;Eric I. Altman
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 21) pp:14001-14011
Publication Date(Web):2017/05/31
DOI:10.1039/C7CP02382K
Two-dimensional (2D) silica (SiO2) and aluminosilicate (AlSi3O8) bilayers grown on Pd(111) were fabricated and systematically studied using ultrahigh vacuum surface analysis in combination with theoretical methods, including Auger electron spectroscopy, X-ray photoelectron spectroscopy, low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory. Based on LEED results, both SiO2 and AlSi3O8 bilayers start ordering above 850 K in 2 × 10−6 Torr oxygen. Both bilayers show hexagonal LEED patterns with a periodicity approximately twice that of the Pd(111) surface. Importantly, the SiO2 bilayer forms an incommensurate crystalline structure whereas the AlSi3O8 bilayer crystallizes in a commensurate structure. The incommensurate crystalline SiO2 structure on Pd(111) resulted in a moiré pattern observed with LEED and STM. Theoretical results show that straining the pure SiO2 bilayer to match Pd(111) would cost 0.492 eV per unit cell; this strain energy is reduced to just 0.126 eV per unit cell by replacing 25% of the Si with Al which softens the material and expands the unstrained lattice. Furthermore, the missing electron created by substituting Al3+ for Si4+ is supplied by Pd creating a chemical bond to the AlSi3O8 bilayer, whereas van der Waals interactions predominate for the SiO2 bilayer. The results reveal how the interplay between strain, doping, and charge transfer determine the structure of metal-supported 2D silicate bilayers and how these variables may potentially be exploited to manipulate 2D materials structures.
Co-reporter:Xiaodong Zhu;Jin-Hao Jhang;Chao Zhou;Omur E. Dagdeviren;Zheng Chen;Udo D. Schwarz;Eric I. Altman
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 48) pp:32492-32504
Publication Date(Web):2017/12/13
DOI:10.1039/C7CP06059A
The ability to affect the surface properties of non-polar Cr2O3 films through polar ZnO(0001) and (000) supports was investigated by characterizing the polarity of ZnO films grown on top of the Cr2O3 surfaces. The growth and geometric and electronic structures of the ZnO films were characterized with X-ray photoelectron spectroscopy, ultra-violet photoelectron spectroscopy, reflection high-energy electron diffraction, low-energy electron diffraction, and X-ray diffraction. The ZnO growth mode was Stranski–Krastanov, which can be attributed to the ZnO layers initially adopting a non-polar structure with a lower surface tension before transitioning to the polar bulk structure with a higher surface energy. A similar result has been reported for ZnO growth on α-Al2O3(0001), which is isostructural with Cr2O3. The polarity of the added ZnO layer was determined by examining the surface morphology following wet chemical etching with atomic force microscopy and by characterizing the surface reactivity via temperature-programmed desorption of alcohols, which strongly depends on the ZnO polarization direction. Consistent with prior work on ZnO growth on bulk Cr2O3(0001), both measurements indicate that thick Cr2O3 layers support ZnO(000) growth regardless of the underlying ZnO substrate polarization; however, the polarization direction of ZnO films grown on Cr2O3 films less than three repeat units thick follows the direction of the underlying substrate polarization. These findings show that it is possible to manipulate the surface properties of non-polar materials with a polar substrate, but that the effect does not penetrate past just a couple of repeat units.
Co-reporter:Andrei Malashevich, Sohrab Ismail-Beigi, and Eric I. Altman
The Journal of Physical Chemistry C 2016 Volume 120(Issue 47) pp:26770-26781
Publication Date(Web):November 2, 2016
DOI:10.1021/acs.jpcc.6b07008
Density functional theory was used to assess the viability of approaches to controlling the structure of a recently discovered two-dimensional form of SiO2. In accord with prior work, a hexagonal bilayer of mirror image planes of corner-sharing SiO4 tetrahedra in six-membered rings yielded only a slightly higher energy than α-quartz. Structures including four- through eight-membered rings were evaluated and in certain cases found to be as little as 17 meV/Si higher in energy than the hexagonal bilayer. When either biaxial or uniaxial tensile strain was applied, combinations of eight-, six-, and four-membered rings became favored due to the lower density of structures with larger rings. These findings, together with experiments that reveal expansion of silica bilayers to match the lattice of metal substrates, suggest that epitaxial strain may be used to control the bilayer structure. Replacement of Si with Ge and Al as prototypical tetravalent and trivalent dopants was also investigated. Substituting Ge for Si was energetically unfavorable and offered no obvious advantage for structural control over pure SiO2 bilayers. In contrast, Al substitution was energetically favorable and only minimally distorted the bilayer. It was found that while the hexagonal bilayer remained favored, the extra-framework electron donors K and H that accompany each Al preferred to occupy larger rings when possible, thus forcing Al to reside in large rings as well. This suggests that the bilayer structure may be controlled through substitution of Si for trivalent dopants and selection of extra-framework electron donors that favor larger rings.
Co-reporter:Eric I. Altman, Mehmet Z. Baykara, and Udo D. Schwarz
Accounts of Chemical Research 2015 Volume 48(Issue 9) pp:2640
Publication Date(Web):August 24, 2015
DOI:10.1021/acs.accounts.5b00166
Although atomic force microscopy (AFM) was rapidly adopted as a routine surface imaging apparatus after its introduction in 1986, it has not been widely used in catalysis research. The reason is that common AFM operating modes do not provide the atomic resolution required to follow catalytic processes; rather the more complex noncontact (NC) mode is needed. Thus, scanning tunneling microscopy has been the principal tool for atomic scale catalysis research. In this Account, recent developments in NC-AFM will be presented that offer significant advantages for gaining a complete atomic level view of catalysis.The main advantage of NC-AFM is that the image contrast is due to the very short-range chemical forces that are of interest in catalysis. This motivated our development of 3D-AFM, a method that yields quantitative atomic resolution images of the potential energy surfaces that govern how molecules approach, stick, diffuse, and rebound from surfaces. A variation of 3D-AFM allows the determination of forces required to push atoms and molecules on surfaces, from which diffusion barriers and variations in adsorption strength may be obtained. Pushing molecules towards each other provides access to intermolecular interaction between reaction partners. Following reaction, NC-AFM with CO-terminated tips yields textbook images of intramolecular structure that can be used to identify reaction intermediates and products.Because NC-AFM and STM contrast mechanisms are distinct, combining the two methods can produce unique insight. It is demonstrated for surface-oxidized Cu(100) that simultaneous 3D-AFM/STM yields resolution of both the Cu and O atoms. Moreover, atomic defects in the Cu sublattice lead to variations in the reactivity of the neighboring O atoms. It is shown that NC-AFM also allows a straightforward imaging of work function variations which has been used to identify defect charge states on catalytic surfaces and to map charge transfer within an individual molecule.These advances highlight the potential for NC-AFM-based methods to become the cornerstone upon which a quantitative atomic scale view of each step of a catalytic process may be gained. Realizing this potential will rely on two breakthroughs: (1) development of robust methods for tip functionalization and (2) simplification of NC-AFM instrumentation and control schemes. Quartz force sensors may offer paths forward in both cases. They allow any material with an atomic asperity to be used as a tip, opening the door to a wide range of surface functionalization chemistry. In addition, they do not suffer from the instabilities that motivated the initial adoption of complex control strategies that are still used today.
Co-reporter:Eric I. Altman;Udo D. Schwarz
Advanced Materials Interfaces 2014 Volume 1( Issue 7) pp:
Publication Date(Web):
DOI:10.1002/admi.201400108
Co-reporter:M.W. Herdiech, H. Mönig, E.I. Altman
Surface Science 2014 Volume 626() pp:53-60
Publication Date(Web):August 2014
DOI:10.1016/j.susc.2014.04.004
•The Lewis acid BF3 weakly interacts with LiNbO3 independent of poling direction.•Despite similar BF3 adsorption, the F 1s peak position depends on polarization.•Opposite surface dipoles on oppositely poled surfaces cause the F 1s peak shifts.•The surface charge of ferroelectric surfaces can be far from equilibrium.Adsorption of the strong Lewis acid BF3 was investigated to probe the sensitivity of the Lewis basicity of surface oxygens on LiNbO3 (0001) to the ferroelectric polarization direction. Adsorption and desorption were characterized by using X-ray photoelectron spectroscopy (XPS) to monitor the intensity and binding energy of the F 1s core level as a function of BF3 exposure and temperature. The results indicate that both BF3 uptake and desorption are very similar on the positively and negatively poled surfaces. In particular, BF3 only weakly adsorbs with the majority of the adsorbed BF3 desorbing below 200 K. Despite the similarities in the uptake and desorption behavior, the binding energy of the F 1s peak relative to the substrate Nb 3d5/2 peak was sensitive to the polarization direction, with the F 1s peak occurring at a binding energy up to 0.3 eV lower on positively poled than negatively poled LiNbO3 for equivalent BF3 exposures. Rather than reflecting a difference in bonding to the surface, however, this shift could be associated with oppositely oriented dipoles at the positively and negatively poled surfaces creating opposite band offsets between the adsorbate and the substrate. A similar effect was observed with lead zirconate titanate thin films where the Pb 4f XPS peak position changes as a function of temperature as a result of the pyroelectric effect which changes the magnitude of the surface and interface dipoles.
Co-reporter:Eric I. Altman, Jan Götzen, Niveditha Samudrala, and Udo D. Schwarz
The Journal of Physical Chemistry C 2013 Volume 117(Issue 49) pp:26144-26155
Publication Date(Web):November 18, 2013
DOI:10.1021/jp4101152
Silica films grown on Pd(100) were characterized by Auger electron spectroscopy, low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). While no evidence of long-range order could be detected for films grown below 600 K, STM images of these films nevertheless revealed flat surfaces through which the step-terrace structure of the substrate could be seen. Annealing the films in 10–6 Torr of O2 above 975 K resulted in crystalline bilayers that produced hexagonal LEED patterns with a periodicity twice that of the substrate and with one of the overlayer close-packed directions paralleling Pd[011]. The extent of the crystalline domains was limited to typically five repeat units along two of the three close-packed directions of the film but was tens of repeat units long along the third. The lattice matching to the substrate expands the spacing in the bilayer on Pd(100) compared to bulk crystalline SiO2 and bilayers observed on other substrates; as a consequence, it is suggested that the regular domain boundaries that form help relieve stress. The dominant features in high-resolution STM images were dark pores surrounded by six other pores; consistent with prior studies, these features are assigned to six-membered rings of corner-sharing SiO4 tetrahedra. Elongation of the pores at the domain boundaries is attributed to insertion of edge-sharing tetrahedra into the rings. Ab initio calculations on freestanding bilayers were performed to understand the effect of the substantial strain on the growth and structure of the film. The results indicate that relaxation orthogonal to the commensurate direction can greatly reduce the strain energy; as a consequence, the square substrate promotes epitaxial growth of crystalline SiO2 by providing an incommensurate direction along which the film can relax.
Co-reporter:K. Garrity;A. M. Kolpak;S. Ismail-Beigi;E. I. Altman
Advanced Materials 2010 Volume 22( Issue 26-27) pp:2969-2973
Publication Date(Web):
DOI:10.1002/adma.200903723
Abstract
It has been recognized since the 1950s that the polar and switchable nature of ferroelectric surfaces can potentially lead to polarization direction-dependent surface chemistry. Recent theoretical studies and advances in growing high quality epitaxial ferroelectric thin films have motivated a flurry of experimental studies aimed at creating surfaces with switchable adsorption and catalytic properties, as well as films whose polarization direction switches depending on the gas phase environment. This research news article briefly reviews the key findings of these studies. These include observations that the adsorption strengths, and in certain cases the activation energies for reactions, of polar molecules on the surfaces of ferroelectric materials are sensitive to the polarization direction. For bare ferroelectric surfaces, the magnitudes of these differences are not large, but are still comparable to the energy barrier required to switch the polarization of ∼10 nm thick films. Highlights of a recent study where chemical switching of a thin film ferroelectric was demonstrated are presented. Attempts to use the ferroelectric polarization to influence the behavior of supported catalytic metals will also be described. It will be shown that the tendency of the metals to cluster into particles makes it difficult to alter the chemical properties of the metal surface, since it is separated from the ferroelectric by several layers of metal atoms. An alternate approach to increasing the reactivity of ferroelectric surfaces is suggested that involves modifying the surface with atoms that bind strongly to the surface and thus remain atomically dispersed.
Co-reporter:Eric I. Altman;Udo D. Schwarz
Advanced Materials 2010 Volume 22( Issue 26-27) pp:2854-2869
Publication Date(Web):
DOI:10.1002/adma.200903927
Abstract
Advances in scanning probe microscopies (SPM) have allowed the mechanisms and rates of adsorption, diffusion and reactions on surfaces to be characterized by directly observing the motions of the individual atoms and molecules involved. The importance of oxides as thermal and photocatalysts, chemical sensors, and substrates for epitaxial growth has motivated dynamical SPM studies of oxide surfaces and their formation. Work on the TiO2 (110) surface is reviewed as an example of how dynamic SPM studies have revealed unexpected interactions between adsorbates and defects that influence macroscopic reaction rates. Studies following diffusion, adsorption and phase transitions on bulk and surface oxides are also discussed. A perspective is provided on advanced SPM techniques that hold great promise for yielding new insights into the mechanisms and rates of elemental processes that take place either during oxidation or on oxide surfaces, with particular emphasis on methods that extend the time and chemical resolution of dynamical SPM measurements.
Co-reporter:Y. Yun, N. Pilet, U.D. Schwarz, E.I. Altman
Surface Science 2009 Volume 603(Issue 20) pp:3145-3154
Publication Date(Web):15 October 2009
DOI:10.1016/j.susc.2009.08.030
To investigate the possibility of manipulating the surface chemical properties of finely dispersed metal films through ferroelectric polarization, the interaction of palladium with oppositely poled LiNbO3(0 0 0 1) substrates was characterized. Low energy ion scattering indicated that the Pd tended to form three-dimensional clusters on both positively and negatively poled substrates even at the lowest coverages. X-ray photoelectron spectroscopy (XPS) showed an upward shift in the binding energy of the Pd 3d core levels of 0.9 eV at the lowest Pd coverages, which slowly decayed toward the bulk value with increasing Pd coverage. These shifts were independent of the poling direction of the substrate and similar to those attributed to cluster size effects on inert supports. Thus, the spectroscopic data suggested that Pd does not interact strongly with LiNbO3 surfaces. The surface chemical properties of the Pd clusters were investigated using CO temperature programmed desorption. On both positively and negatively poled substrates, CO desorption from freshly deposited Pd showed a splitting of the broad 460 K desorption peak characteristic of bulk Pd into distinct peaks at 270 and 490 K as the Pd coverage was decreased below 1.0 ML; behavior that also resembles that seen on inert supports. It was found that a small fraction of the adsorbed CO may dissociate (<2%) for Pd on both positively and negatively poled substrates. The thermal response of the smaller Pd clusters on the LiNbO3 surfaces, however, was different from that of inert substrates. In a manner similar to Nb2O5, when CO desorption experiments were carried out a second time, the adsorption capacity decreased and the higher temperature desorption peak shifted from 490 K to below 450 K. This behavior was independent of the substrate poling direction. Thus, while there was evidence that LiNbO3 does not behave as a completely inert support, no significant differences between positively and negatively poled surfaces were observed. This lack of sensitivity of the surface properties of the Pd to the poling direction of the substrate is attributed to the three-dimensional Pd clusters being too thick for their surfaces to be influenced by the polarization of the underlying substrate.
Co-reporter:Eric I. Altman
Surface Science 2009 Volume 603(Issue 17) pp:2669-2670
Publication Date(Web):1 September 2009
DOI:10.1016/j.susc.2009.07.001
Co-reporter:Y. Yun, M. Li, D. Liao, L. Kampschulte, E.I. Altman
Surface Science 2007 Volume 601(Issue 19) pp:4636-4647
Publication Date(Web):1 October 2007
DOI:10.1016/j.susc.2007.08.001
The effect of ferroelectric poling direction on the structure and electronic properties of the LiNbO3 (0 0 0 1) surface was characterized. Low energy and reflection high energy electron diffraction indicated that both the positively and negatively poled surfaces were (1 × 1) with no evidence of longer range periodic reconstructions. Low energy ion scattering spectra from both surfaces were dominated by scattering from oxygen atoms. X-ray and ultraviolet photoelectron spectra also showed little difference between the positively and negatively poled surfaces, with the exception of a high binding energy shoulder on the O 1s core level of the negative surface. Exposure of the surfaces to atomic hydrogen caused reduction of the surface Nb rather than an increase in intensity on the high binding energy side of the O 1s peak, indicating that the shoulder on the O 1s peak on the negative surface was not due to surface hydroxyl groups. Temperature programmed desorption measurements indicated that the nearly stoichiometric LiNbO3 samples were susceptible to loss of Li2O starting at temperatures as low as 500 K, independent of the poling direction. An adatom/vacancy model is proposed in which oxygen ad-anions accumulate on one side of the crystal while oxygen anion vacancies are created on the opposite surface. This model can explain the apparent oxygen termination of both surfaces and the observed (1 × 1) periodicity of the surfaces, and also effectively screens the thickness dependent electric field associated with the polar orientation of the crystal.
Co-reporter:J. Wang, Y. Yun, E.I. Altman
Surface Science 2007 Volume 601(Issue 16) pp:3497-3505
Publication Date(Web):15 August 2007
DOI:10.1016/j.susc.2007.06.063
The oxidation of Pd(1 0 0) by an oxygen plasma was characterized using X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (ISS), temperature programmed desorption (TPD), and low energy electron diffraction (LEED). The oxygen uptake followed a typical parabolic profile with oxygen coverages reaching 32 ML after 1 h in the plasma; a factor of 40 higher than could be achieved by dosing molecular oxidants in ultra high vacuum. Even after adsorbing 32 ML of oxygen, XPS revealed both metallic Pd and PdO in the surface region. The (5×5)R27o LEED pattern previously attributed to a surface oxide monolayer, slowly attenuated with oxygen coverage indicating that the PdO formed poorly ordered three dimensional clusters that slowly covered the ordered surface oxide. While XPS revealed the formation of bulk PdO, only small changes in the ISS spectra were observed once the surface oxide layer was completed. The leading edges of the O2 TPD curves showed only small shifts with increasing oxygen coverage that could be explained in terms of the lower thermodynamic stability of small oxide clusters. The desorption curves, however, could not be adequately described as simple zero order decomposition of PdO. There has been an ongoing debate in the literature about the relative catalytic activities of PdO and oxygen phases on Pd, the results indicate that any differences in the reactivity between bulk PdO and surface oxides are not associated with differences in the density of exposed Pd atoms or the decomposition kinetics of these two phases.
Co-reporter:W. Gao, E.I. Altman
Surface Science 2006 Volume 600(Issue 12) pp:2572-2580
Publication Date(Web):15 June 2006
DOI:10.1016/j.susc.2006.04.022
The interaction of vanadium oxide with epitaxial anatase films exposing (1 0 1) terraces was characterized. The TiO2 films were grown on vicinal LaAlO3 (1 1 0) substrates by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE); reflection high energy and low energy electron diffraction (RHEED and LEED) indicated that the films exposed (1 0 1) terraces of the anatase TiO2 polymorph. When a vanadium oxide monolayer was deposited onto the anatase surface by OPA-MBE at 725 K, only (1 × 1) RHEED and LEED patterns were observed. The V X-ray photoelectron spectroscopy (XPS) peak intensities indicated that the monolayer wetted the anatase surface and so the diffraction patterns were attributed to an epitaxial vanadia layer. Analysis of the vanadium oxide monolayer by X-ray and ultraviolet photoelectron spectroscopies revealed that the V was predominantly 5+. When the vanadia coverage was increased at 725 K, Auger electron spectra showed only very slow attenuation of the anatase Ti peaks while spots began to develop in RHEED patterns recorded along the LaAlO3[1¯10] direction; both indicative of 3-D cluster formation. In the orthogonal direction, the RHEED patterns showed unusual diagonal streaks. Meanwhile, the (1 × 1) LEED pattern persisted even after 30 nm of vanadia was deposited. This was attributed to gaps between the 3-D clusters exposing the epitaxial monolayer. Core level XPS spectra of the 3-D clusters revealed a broad V 2p3/2 peak that was centered at the position expected for V4+ but could be deconvoluted into three peaks corresponding to V3+, V4+, and V5+. It is shown that crystallographic shear that accommodates such variations in the oxygen content of V oxides can lead to the diagonal streaks in RHEED patterns recorded along the LaAlO3 [0 0 1] direction even as the pattern in the orthogonal direction shows sharp transmission spots. The results show that vanadia growth on anatase (1 0 1) proceeds through the Stranski–Krastanov mode with a strong vanadia–titania interaction stabilizing a dispersed vanadia monolayer. The results are compared with previous data for vanadia growth on anatase (0 0 1) where smooth, epitaxial VO2 films grow ad infinitum.
Co-reporter:J. Wang, M. Li, E.I. Altman
Surface Science 2005 Volume 596(1–3) pp:126-143
Publication Date(Web):10 December 2005
DOI:10.1016/j.susc.2005.09.009
Gold growth on Ge(0 0 1) was studied as a function of coverage and growth temperature using scanning tunneling microscopy (STM). Initial Au deposition at 475 K resulted only in two-dimensional Ge islands and numerous vacancies on the surface. The Ge islands were attributed to Ge atoms ejected onto the surface by Au moving sub-surface. At higher temperatures the vacancies ordered into (1 + 2 + 1) dimer vacancy complexes, and step structures not seen on clean Ge surfaces were observed. When more Au was deposited between 475 and 800 K, alternating bright and dim chains appeared on the surface that were attributed to Au–Au and Au–Ge dimer rows, respectively. An increase in the density of double-height steps was observed as the chains formed suggesting that the Ge atoms incorporated into the mixed dimer rows were extracted from step edges. Increasing the Au coverage further caused the chains to organize into well-ordered (4 × 2) domains. After depositing 1.5 ML of Au at 675 K the surface was completely covered by the chains. Similar chains have been observed during Pt growth on Ge but not during Ag growth, supporting the idea that the 5d metals favor the formation of one-dimensional chains. After the surface was covered with chains, additional Au aggregated into three-dimensional clusters. The smaller of these clusters exhibited a rectangular shape consistent with the formation of Au(1 1 0) clusters. The [1 1 0]-orientation was confirmed through atomic resolution imaging of the tops of larger clusters. Larger clusters, however, adopted a different shape—an asymmetric octagon—and had their top surfaces tilted with respect to the Ge(0 0 1) surface. The tilt was attributed to stress relief due to the lattice mismatch, which broke the equivalence between the surface energies of inclined facets on opposite sides of the cluster, accounting for the asymmetric shape. Annealing the Au-covered surfaces to 1000 K caused all of the Au to move into the bulk leaving a defective Ge surface.
Co-reporter:W. Gao, R. Klie, E.I. Altman
Thin Solid Films 2005 Volume 485(1–2) pp:115-125
Publication Date(Web):1 August 2005
DOI:10.1016/j.tsf.2005.03.041
The heteroepitaxial growth and surface structures of TiO2 films on vicinal and flat LaAlO3 (110) were characterized. The films were grown by oxygen plasma assisted molecular beam epitaxy at low growth rates and temperatures between 825 and 875 K. Bulk characterization by low energy electron loss spectroscopy indicated that only the anatase structure formed. Both X-ray diffraction and scanning transmission electron microscopy indicated that the films grew with anatase (102) planes parallel to the interface. The surfaces of the films were characterized by reflection high energy electron diffraction (RHEED) during growth, and photoelectron spectroscopy, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM) following growth. The photoelectron spectra were consistent with the growth of stoichiometric TiO2. Both RHEED and LEED, however, showed patterns expected of anatase (101) surfaces, not anatase (102). Further, STM images revealed the oblique unit cell expected of anatase (101) along with many parallel steps. The (101) surface is the lowest energy anatase surface and it is suggested that the film surface facets towards (101) to reduce the surface energy. Anatase (101) is also a much better geometric match to LaAlO3 (110) than anatase (102); however, unlike the [102] orientation, the [101] orientation places Ti cations directly above substrate cations increasing the interfacial energy. Thus the film structure and surface morphology is determined by the interplay between geometric and electronic matching at the interface and the energy of the free surface.
Co-reporter:W. Gao, C.M. Wang, H.Q. Wang, V.E. Henrich, E.I. Altman
Surface Science 2004 Volume 559(2–3) pp:201-213
Publication Date(Web):20 June 2004
DOI:10.1016/j.susc.2004.04.028
Oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) of vanadium oxide on (1 × 4)-reconstructed anatase (0 0 1) thin films was studied using reflection high energy electron diffraction (RHEED), low energy electron diffraction (LEED), X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). XPS and UPS results showed that the vanadium was predominantly in the 5+ oxidation state after deposition of a monolayer at 525 K. After 1 ML of vanadia was deposited, the anatase (1 × 4)/(4 × 1) LEED and RHEED patterns were replaced by (1 × 1) patterns indicating that the vanadia lifts the reconstruction and suggesting that the monolayer is pseudomorphic. At 525 K, the V5+ oxidation state predominated in thicker films, however, no discernible LEED or RHEED patterns were seen after a few monolayers were deposited indicating that V2O5 epitaxy cannot be continued beyond 1 ML. When the growth temperature was increased to 750 K, RHEED patterns indicated no change in the surface structure after more than 20 ML of vanadia were deposited. Under these conditions, XPS peak positions were consistent with VO2. After growth at 775 K a c(2 × 2) LEED pattern attributed to half a monolayer of adsorbed oxygen on the VO2 surface was observed. The surface characterization data all pointed towards pseudomorphic growth of VO2 with a half monolayer of capping oxygen allowing the monolayer to achieve the V2O5 stoichiometry while maintaining the anatase structure. Bulk XRD data, however, were consistent with VO2(B), V6O13, and rutile VO2; none of which expose surfaces with the periodicity observed with RHEED and LEED. The reasons for the differences between the surface and bulk characterization are discussed.
Co-reporter:E.I. Altman
Surface Science 2003 Volume 547(1–2) pp:108-126
Publication Date(Web):10 December 2003
DOI:10.1016/j.susc.2003.10.010
The effect of oxygen coverage and surface structure on the total oxidation of propene over Pd(1 0 0) was studied using temperature-programmed reaction (TPR), isothermal kinetic measurements, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). TPR revealed CO2 production peaks at 430 and 550 K. When the surface was covered by chemisorbed oxygen in (2 × 2) structures the 550 K peak dominated, while only the 430 K peak was seen at low propene doses and high oxygen coverages (∼0.8 ML) where a ()R27° reconstruction covered the surface. Regardless of the oxygen coverage, water desorbed at 430 K indicating that the 550 K CO2 peak was due to oxidation of C deposited by propene dissociation. The initial propene sticking coefficient at 330 K was a factor of five greater for (2 × 2)-O surfaces than for the ()R27°-O surface, thus the lower activation energy pathway favored at high oxygen coverages did not necessarily translate into higher reaction rates. Above 450 K, the isothermal kinetic measurements showed that the reaction rate increases with decreasing oxygen coverage until the reaction becomes starved for oxygen. LEED measurements showed that the rate increases as the ()R27° structure is replaced by the (2 × 2) structures. At lower temperatures, however, the oxidation rate of C deposited on the surface by propene dissociation at low oxygen coverages is slow and so higher rates were seen at high oxygen coverages. At room temperature, STM images showed that propene initially slowly randomly adsorbs atop the ()R27° surface. As the propene coverage increased, however, adsorbates tended to cluster together forming disordered regions; as the surface disordered the adsorption rate increased. At 550 K, STM movies recorded during propene exposure to oxygen-covered surfaces showed the slow removal of ()R27° domains followed by rapid dissolution of islands formed during oxygen adsorption. The results indicate that oxidizing the Pd surface affects hydrocarbon oxidation in two opposing ways: it decreases the hydrocarbon adsorption probability but also favors an easier oxidation pathway once the molecules adsorb.
Co-reporter:G. Zheng, E.I. Altman
Surface Science 2002 Volume 504() pp:253-270
Publication Date(Web):20 April 2002
DOI:10.1016/S0039-6028(02)01104-4
The oxidation of Pd(1 0 0) was characterized using temperature programmed desorption (TPD), low-energy electron diffraction (LEED), and in situ variable-temperature scanning tunneling microscopy (STM). The results indicate that Pd(1 0 0) oxidation proceeds through four stages involving up to five surface phases. In the first stage, oxygen chemisorbs atop the Pd surface resulting in p(2×2) and c(2×2) overlayers that desorb at 800 and 700 K, respectively. As the overlayers saturated, island and peninsula growth was observed in STM movies. The islands are one Pd atom high and so the growth is attributed to Pd atoms ejected onto the terraces which then either nucleated islands or attached to preexisting steps. During this second stage, no change in the surface periodicity was observed. After growth stops, the surface reconstructs in the third stage. Above 475 K, STM images revealed the formation of a () R27° reconstruction. The images of the () R27° structure are twofold symmetric, consistent with the formation an epitaxial PdO(0 0 1)-like layer. In addition, a (5×5) reconstruction was observed with LEED prior to the () R27° reconstruction. As the oxygen coverage entered the regime where these reconstructions were observed, a new desorption peak appeared at 650 K. This temperature is higher than that expected for PdO dissociation but lower than that observed for chemisorption suggesting that the Pd–O bond strength in the reconstructed surfaces is intermediate between the bulk oxide and chemisorbed oxygen. In the fourth stage, the LEED patterns began to fade, the STM images showed three-dimensional clusters, and a low-temperature shoulder consistent with PdO decomposition developed in TPD traces. Thus the surface roughens as the bulk oxide forms. The mechanism outlined above is similar to that recently observed for Pd(1 1 1) oxidation.
Co-reporter:R.E. Tanner, Y. Liang, E.I. Altman
Surface Science 2002 Volume 506(Issue 3) pp:251-271
Publication Date(Web):May 2002
DOI:10.1016/S0039-6028(02)01388-2
The structure of the anatase TiO2(0 0 1) surface and its reactivity towards carboxylic acids were studied using in situ scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS) and temperature-programmed desorption (TPD). Following annealing in ultra-high vacuum, a (1×4) reconstruction developed that was visible in STM and LEED. At densest packing, a (2×4) overlayer of dissociated carboxylate groups was seen with STM. From the images, it was deduced that single carboxylates bind to positions centered atop (1×4) rows. TPD experiments revealed a range of reactivity depending on the preparation conditions. The fully oxidized anatase (0 0 1)-(1×4) surface bonded carboxylate groups strongly with no TPD products seen below 750 K. Surfaces that were not fully oxidized, or had been sputtered, converted formic acid to CO at 540 K and formaldehyde at 450 K. Sequential TPD experiments changed the surface activity, suppressing CO production, but not significantly affecting the sites responsible for formaldehyde production. The defective surface converted adsorbed acetate to CO at 535 K and ketene at 560 K. Sequential acetic acid TPD runs caused the reactivity towards both CO and ketene to diminish. These results are attributed to sputtering creating under-coordinated Ti4+ and Ti3+ sites that were revealed in XPS spectra and STM images. No bimolecular coupling reactions of acetic acid took place, indicating that the formaldehyde was produced via a reduction pathway. Both the location of the adsorbates and the lack of reaction products associated with fourfold coordinated Ti4+ raise doubts about current models of the (1×4) reconstruction.
Co-reporter:E.I. Altman, T. Droubay, S.A. Chambers
Thin Solid Films 2002 Volume 414(Issue 2) pp:205-215
Publication Date(Web):22 July 2002
DOI:10.1016/S0040-6090(02)00487-X
The growth of MoO3 films on SrLaAlO4(0 0 1), a substrate lattice-matched to β-MoO3, by oxygen plasma assisted molecular beam epitaxy was characterized using reflection high-energy electron diffraction (RHEED), X-ray photoelectron spectroscopy, X-ray diffraction (XRD), and atomic force and scanning tunneling microscopies (AFM and STM). It was found that the flux of reactive oxygen species to the surface was not high enough to maintain the proper stoichiometry, even at the lowest measurable deposition rates. Therefore, the films were grown by depositing Mo in small increments and then allowing the Mo to oxidize. At 675 K, the films grew epitaxially but in a three-dimensional manner. XRD of films grown under these conditions revealed a tetragonal structure that has not been previously observed in bulk MoO3 samples. Decreasing the growth temperature to 535 K led to polycrystalline α-MoO3 preferentially aligned with the [0 1 0] and [1 0 0] directions of the grains oriented normal to the substrate. By manipulating the initial growth conditions, relatively flat, epitaxial MoO3 films could be grown on SrLaAlO4(0 0 1). In this case, the equivalent of one layer of β-MoO3(0 0 1) was deposited at 675 K before adding several layers of MoO3 at 550 K. Unlike growth solely at 675 K, the MoO3 formed in this manner did not dewet the surface when reheated to 675 K and RHEED indicated that continued MoO3 growth at this temperature proceeded epitaxially. XRD and AFM indicated that films grown in this manner contained α-MoO3 crystallites in addition to an epitaxial phase that accounted for most of the surface area of the film. STM images of the film that revealed step heights expected for β-MoO3(0 0 1), along with RHEED results that revealed in-plane lattice constants consistent with β-MoO3, indicated that epitaxial β-MoO3 films can be grown and that geometric matching to the substrate can stabilize the β phase at temperatures where it is not stable in the bulk.
Co-reporter:C.Y. Nakakura, E.I. Altman
Surface Science 1999 Volume 424(2–3) pp:244-261
Publication Date(Web):1 April 1999
DOI:10.1016/S0039-6028(99)00007-2
The effect of surface steps on CuBr nucleation, growth, and sublimation was studied by comparing in situ variable temperature scanning tunneling microscopy (STM) and temperature-programmed desorption (TPD) results for Cu(100) and Cu(11 1 0). Bromine chemisorption caused the Cu steps to facet parallel to 〈100〉 directions. The faceted steps supplied Cu atoms for the reaction with Br2 to form CuBr. STM movies showed that the halide did not accumulate where the reaction took place but rather diffused across the surface, nucleating and growing halide clusters independent of the reaction step. At room temperature, the halide formed flat (111)-oriented γ-CuBr islands at step facet corners. The islands grew by preferential addition of CuBr to {100} island edges, resulting in a triangular shape during growth that relaxed to an asymmetric hexagonal shape when growth was stopped. The halide did not block further reaction of the underlying Cu; STM movies showed Cu steps being consumed beneath halide layers. High-resolution images of CuBr islands revealed a (1×1) periodicity, whereas images of multilayer films displayed a (2×2) periodicity attributed to an ordered vacancy structure that maintains neutrality of the polar (111) surface. With time, or on annealing, the two-dimensional islands roughened into three-dimensional clusters. Although the high step density of Cu(11 1 0) did not substantially alter the nucleation, growth, relaxation, or structure of the CuBr, when Br2 was dosed at 325 K, a low temperature CuBr TPD peak was observed for Cu(11 1 0) but not Cu(100). This low temperature peak disappeared when the CuBr films were roughened by increasing the reaction temperature to 380 K, suggesting that the low temperature peak is associated with the two-dimensional film structure. Thus, the low temperature peak was attributed to the steps on the Cu(11 1 0) surface introducing defects into the CuBr films because of the large mismatch between the Cu and CuBr step heights.
Co-reporter:Y. Yun, J. Wang, E.I. Altman
Journal of Catalysis (25 January 2008) Volume 253(Issue 2) pp:295-302
Publication Date(Web):25 January 2008
DOI:10.1016/j.jcat.2007.10.027
The reactivity of bulk PdO clusters produced by plasma oxidation of Pd(100) towards propene oxidation was characterized using temperature programmed desorption (TPD) and isothermal oxygen titration. The TPD results were dominated by simultaneous CO2 and water desorption in a peak at 490 K. The only other product observed was a small amount of CO near saturation propene coverages that also desorbed at 490 K. The propene coverage saturated at exposures between 0.5–1 l, indicating a sticking coefficient close to one. In the titration experiments, CO2 production peaked almost immediately upon exposure to propene, indicating that the propene oxidation rate fell as the surface was reduced. Above 450 K, virtually all of the propene was completely oxidized to CO2 and water, while at lower temperatures small amounts of CO were observed and unreacted propene fragments accumulated on the surface. In comparison, previous results for a well-ordered surface oxide on Pd(100) were similar in that CO2 and water also desorbed simultaneously indicating a similar mechanism, but different in that the sticking coefficient on the surface oxide was a factor of 20 lower, and the desorption peaked 60 K lower. These differences cause the bulk oxide to be far more active at higher temperatures than the surface oxide, but the surface oxide displays some activity down to lower temperatures where propene simply accumulates on the bulk oxide surface.
Co-reporter:Jin-Hao Jhang, Chao Zhou, Omur E. Dagdeviren, Gregory S. Hutchings, Udo D. Schwarz and Eric I. Altman
Physical Chemistry Chemical Physics 2017 - vol. 19(Issue 21) pp:NaN14011-14011
Publication Date(Web):2017/05/11
DOI:10.1039/C7CP02382K
Two-dimensional (2D) silica (SiO2) and aluminosilicate (AlSi3O8) bilayers grown on Pd(111) were fabricated and systematically studied using ultrahigh vacuum surface analysis in combination with theoretical methods, including Auger electron spectroscopy, X-ray photoelectron spectroscopy, low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory. Based on LEED results, both SiO2 and AlSi3O8 bilayers start ordering above 850 K in 2 × 10−6 Torr oxygen. Both bilayers show hexagonal LEED patterns with a periodicity approximately twice that of the Pd(111) surface. Importantly, the SiO2 bilayer forms an incommensurate crystalline structure whereas the AlSi3O8 bilayer crystallizes in a commensurate structure. The incommensurate crystalline SiO2 structure on Pd(111) resulted in a moiré pattern observed with LEED and STM. Theoretical results show that straining the pure SiO2 bilayer to match Pd(111) would cost 0.492 eV per unit cell; this strain energy is reduced to just 0.126 eV per unit cell by replacing 25% of the Si with Al which softens the material and expands the unstrained lattice. Furthermore, the missing electron created by substituting Al3+ for Si4+ is supplied by Pd creating a chemical bond to the AlSi3O8 bilayer, whereas van der Waals interactions predominate for the SiO2 bilayer. The results reveal how the interplay between strain, doping, and charge transfer determine the structure of metal-supported 2D silicate bilayers and how these variables may potentially be exploited to manipulate 2D materials structures.
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