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Epitaxial Materials 2015

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A special issue of Materials (ISSN 1996-1944).

Deadline for manuscript submissions: closed (31 July 2015) | Viewed by 74101

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Prof. Dr. Jan Ingo Flege

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Applied Physics and Semiconductor Spectroscopy, Brandenburg University of Technology Cottbus–Senftenberg, Konrad-Zuse-Strasse 1, D-03046 Cottbus, Germany
Interests: surface physics and chemistry; model heterogeneous catalysis; transition metal and rare-earth oxides; low-dimensional materials
Special Issues, Collections and Topics in MDPI journals

Dr. Sanjaya Senanayake

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Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA

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Dear Colleagues,

Ever increasing technological demands in the 21st century have been met with aggressive research proliferation for the discovery of new materials and in the adaptation of universally used materials, better tailored towards more effective and targeted phenomenological functionality. An important class of novel materials is the epitaxially architectured metal and oxide systems that have been incorporated in cross-field and interdisciplinary investigations, which have yielded a rich source of fundamental physics, chemistry, and materials specific knowledge. These epitaxial systems exploit, in a systematic manner, careful consideration of nanoscale, mesoscale, and interfacial structure and chemical moieties in the stabilization of two or more materials (oxides or metals) in which properties differ substantially from their isolated nanostructures, or bulk counterparts. These properties are readily probable with either well-defined and constrained model-like or heterogeneous and multicomponent powder materials using high-resolution experimental methods (diffraction, microscopy, and spectroscopy employing X rays, electrons, infrared and optical photons). This Special Issue aims to galvanize the multiple communities in the fields of heterogeneous catalysis, semiconductor physics, as well as photo and electrochemical conversion, that are actively engaged in world leading efforts associated with fundamental and applied investigations of such materials, the characterization of their fundamental properties, and tailoring of their functionalities. We will devote subsections specific to mono-functional and multi-functional materials that can deliver unique physical and chemical properties either individually or collectively in the form of nanoparticles and films, nanocrystalline materials with oxide or metal supports, multilayer and laterally strained materials, as well as intermetallic or mixed oxide systems.

Dr. Jan Ingo Flege
Dr. Sanjaya Senanayake
Guest Editor

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Keywords

Heteroepitaxial systems and interfaces
structure-function relationship
rational design of nanoscale materials
interfacial strain engineering
interface- and defect-mediated physics and chemistry
redox coupled phenomena
spillover and interdiffusion processes
catalytically active sites

Published Papers (11 papers)

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9750 KiB

Open AccessFeature PaperArticle

Correlation of High Magnetoelectric Coupling with Oxygen Vacancy Superstructure in Epitaxial Multiferroic BaTiO₃-BiFeO₃ Composite Thin Films

by Michael Lorenz, Gerald Wagner, Vera Lazenka, Peter Schwinkendorf, Michael Bonholzer, Margriet J. Van Bael, André Vantomme, Kristiaan Temst, Oliver Oeckler and Marius Grundmann

Materials 2016, 9(1), 44; https://0-doi-org.brum.beds.ac.uk/10.3390/ma9010044 - 13 Jan 2016

Cited by 16 | Viewed by 8327

Abstract

Epitaxial multiferroic BaTiO₃-BiFeO₃ composite thin films exhibit a correlation between the magnetoelectric (ME) voltage coefficient α_ME and the oxygen partial pressure during growth. The ME coefficient α_ME reaches high values up to 43 V/(cm·Oe) at 300 K and at 0.25 mbar oxygen growth pressure. The temperature dependence of α_ME of the composite films is opposite that of recently-reported BaTiO₃-BiFeO₃ superlattices, indicating that strain-mediated ME coupling alone cannot explain its origin. Probably, charge-mediated ME coupling may play a role in the composite films. Furthermore, the chemically-homogeneous composite films show an oxygen vacancy superstructure, which arises from vacancy ordering on the {111} planes of the pseudocubic BaTiO₃-type structure. This work contributes to the understanding of magnetoelectric coupling as a complex and sensitive interplay of chemical, structural and geometrical issues of the BaTiO₃-BiFeO₃ composite system and, thus, paves the way to practical exploitation of magnetoelectric composites. Full article

(This article belongs to the Special Issue Epitaxial Materials 2015)

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1926 KiB

Open AccessArticle

Single-Crystal Y₂O₃ Epitaxially on GaAs(001) and (111) Using Atomic Layer Deposition

by Y. H. Lin, C. K. Cheng, K. H. Chen, C. H. Fu, T. W. Chang, C. H. Hsu, J. Kwo and M. Hong

Materials 2015, 8(10), 7084-7093; https://0-doi-org.brum.beds.ac.uk/10.3390/ma8105364 - 19 Oct 2015

Cited by 18 | Viewed by 6926

Abstract

Single-crystal atomic-layer-deposited (ALD) Y\(_{\mathrm{2}}\)O\(_{\mathrm{3}}\) films 2 nm thick were epitaxially grown on molecular beam epitaxy (MBE) GaAs(001)-4 \(\times\) 6 and GaAs(111)A-2 \(\times\) 2 reconstructed surfaces. The in-plane epitaxy between the ALD-oxide films and GaAs was observed using \textit{in-situ} reflection high-energy electron diffraction in our uniquely designed MBE/ALD multi-chamber system. More detailed studies on the crystallography of the hetero-structures were carried out using high-resolution synchrotron radiation X-ray diffraction. When deposited on GaAs(001), the Y\(_{\mathrm{2}}\)O\(_{\mathrm{3}}\) films are of a cubic phase and have (110) as the film normal, with the orientation relationship being determined: Y\(_{\mathrm{2}}\)O\(_{\mathrm{3}}\)(\(110\))[\(001\)][\(\overline{1}10\)]//GaAs(\(001\))[\(110\)][\(1\overline{1}0\)]. On GaAs(\(111\))A, the Y\(_{\mathrm{2}}\)O\(_{\mathrm{3}}\) films are also of a cubic phase with (\(111\)) as the film normal, having the orientation relationship of Y\(_{\mathrm{2}}\)O\(_{\mathrm{3}}\)(\(111\))[\(2\overline{1}\overline{1}\)] [\(01\overline{1}\)]//GaAs (\(111\)) [\(\overline{2}11\)][\(0\overline{1}1\)]. The relevant orientation for the present/future integrated circuit platform is (\(001\)). The ALD-Y\(_{\mathrm{2}}\)O\(_{\mathrm{3}}\)/GaAs(\(001\))-4 \(\times\) 6 has shown excellent electrical properties. These include small frequency dispersion in the capacitance-voltage CV curves at accumulation of ~7% and ~14% for the respective p- and n-type samples with the measured frequencies of 1 MHz to 100 Hz. The interfacial trap density (Dit) is low of ~10\(^{12}\) cm\(^{−2}\)eV\(^{−1}\) as extracted from measured quasi-static CVs. The frequency dispersion at accumulation and the D\(_{it}\) are the lowest ever achieved among all the ALD-oxides on GaAs(\(001\)). Full article

(This article belongs to the Special Issue Epitaxial Materials 2015)

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1094 KiB

Open AccessArticle

Engineered Heusler Ferrimagnets with a Large Perpendicular Magnetic Anisotropy

by Reza Ranjbar, Kazuya Suzuki, Atsushi Sugihara, Terunobu Miyazaki, Yasuo Ando and Shigemi Mizukami

Materials 2015, 8(9), 6531-6542; https://0-doi-org.brum.beds.ac.uk/10.3390/ma8095320 - 22 Sep 2015

Cited by 6 | Viewed by 4654

Abstract

Synthetic perpendicular magnetic anisotropy (PMA) ferrimagnets consisting of 30-nm-thick D0₂₂-MnGa and Co₂MnSi (CMS) cubic Heusler alloys with different thicknesses of 1, 3, 5, 10 and 20 nm, buffered and capped with a Cr film, are successfully grown epitaxially on MgO substrate. Two series samples with and without post annealing at 400 °C are fabricated. The (002) peak of the cubic L2₁ structure of CMS films on the MnGa layer is observed, even for the 3-nm-thick CMS film for both un-annealed and annealed samples. The smaller remnant magnetization and larger switching field values of CMS (1–20 nm)/MnGa (30 nm) bilayers compared with 30-nm-thick MnGa indicates antiferromagnetic (AFM) interfacial exchange coupling (J_ex) between MnGa and CMS films for both un-annealed and annealed samples. The critical thickness of the CMS film for observing PMA with AFM coupling in the CMS/MnGa bilayer is less than 10 nm, which is relatively large compared to previous studies. Full article

(This article belongs to the Special Issue Epitaxial Materials 2015)

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707 KiB

Open AccessArticle

Plasma Enhanced Complete Oxidation of Ultrathin Epitaxial Praseodymia Films on Si(111)

by Olga Kuschel, Florian Dieck, Henrik Wilkens, Sebastian Gevers, Jari Rodewald, Christian Otte, Marvin Hartwig Zoellner, Gang Niu, Thomas Schroeder and Joachim Wollschläger

Materials 2015, 8(9), 6379-6390; https://0-doi-org.brum.beds.ac.uk/10.3390/ma8095312 - 18 Sep 2015

Cited by 4 | Viewed by 6863

Abstract

Praseodymia films have been exposed to oxygen plasma at room temperature after deposition on Si(111) via molecular beam epitaxy. Different parameters as film thickness, exposure time and flux during plasma treatment have been varied to study their influence on the oxygen plasma oxidation process. The surface near regions have been investigated by means of X-ray photoelectron spectroscopy showing that the plasma treatment transforms the stoichiometry of the films from Pr2O3 to PrO2. Closer inspection of the bulk properties of the films by means of synchrotron radiation based X-ray reflectometry and diffraction confirms this transformation if the films are thicker than some critical thickness of 6 nm. The layer distance of these films is extremely small verifying the completeness of the plasma oxidation process. Thinner films, however, cannot be transformed completely. For all films, less oxidized very thin interlayers are detected by these experimental techniques. Full article

(This article belongs to the Special Issue Epitaxial Materials 2015)

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4955 KiB

Open AccessArticle

Methanol Adsorption and Reaction on Samaria Thin Films on Pt(111)

by Jin-Hao Jhang, Andreas Schaefer, Volkmar Zielasek, Jason F. Weaver and Marcus Bäumer

Materials 2015, 8(9), 6228-6256; https://0-doi-org.brum.beds.ac.uk/10.3390/ma8095302 - 17 Sep 2015

Cited by 5 | Viewed by 5974

Abstract

We investigated the adsorption and reaction of methanol on continuous and discontinuous films of samarium oxide (SmO_x) grown on Pt(111) in ultrahigh vacuum. The methanol decomposition was studied by temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), while structural changes of the oxide surface were monitored by low-energy electron diffraction (LEED). Methanol dehydrogenates to adsorbed methoxy species on both the continuous and discontinuous SmO_x films, eventually leading to the desorption of CO and H₂ which desorbs at temperatures in the range 400–600 K. Small quantities of CO₂ are also detected mainly on as-prepared Sm₂O₃ thin films, but the production of CO₂ is limited during repeated TPD runs. The discontinuous film exhibits the highest reactivity compared to the continuous film and the Pt(111) substrate. The reactivity of methanol on reduced and reoxidized films was also investigated, revealing how SmO_x structures influence the chemical behavior. Over repeated TPD experiments, a SmO_x structural/chemical equilibrium condition is found which can be approached either from oxidized or reduced films. We also observed hydrogen absence in TPD which indicates that hydrogen is stored either in SmO_x films or as OH groups on the SmO_x surfaces. Full article

(This article belongs to the Special Issue Epitaxial Materials 2015)

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387 KiB

Open AccessArticle

X-Ray Spectroscopy of Ultra-Thin Oxide/Oxide Heteroepitaxial Films: A Case Study of Single-Nanometer VO2/TiO2

by Nicholas F. Quackenbush, Hanjong Paik, Joseph C. Woicik, Dario A. Arena, Darrell G. Schlom and Louis F. J. Piper

Materials 2015, 8(8), 5452-5466; https://0-doi-org.brum.beds.ac.uk/10.3390/ma8085255 - 21 Aug 2015

Cited by 24 | Viewed by 9156

Abstract

Epitaxial ultra-thin oxide films can support large percent level strains well beyond their bulk counterparts, thereby enabling strain-engineering in oxides that can tailor various phenomena. At these reduced dimensions (typically < 10 nm), contributions from the substrate can dwarf the signal from the epilayer, making it difficult to distinguish the properties of the epilayer from the bulk. This is especially true for oxide on oxide systems. Here, we have employed a combination of hard X-ray photoelectron spectroscopy (HAXPES) and angular soft X-ray absorption spectroscopy (XAS) to study epitaxial VO2/TiO2 (100) films ranging from 7.5 to 1 nm. We observe a low-temperature (300 K) insulating phase with evidence of vanadium-vanadium (V-V) dimers and a high-temperature (400 K) metallic phase absent of V-V dimers irrespective of film thickness. Our results confirm that the metal insulator transition can exist at atomic dimensions and that biaxial strain can still be used to control the temperature of its transition when the interfaces are atomically sharp. More generally, our case study highlights the benefits of using non-destructive XAS and HAXPES to extract out information regarding the interfacial quality of the epilayers and spectroscopic signatures associated with exotic phenomena at these dimensions. Full article

(This article belongs to the Special Issue Epitaxial Materials 2015)

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1899 KiB

Open AccessArticle

Growth of Ceria Nano-Islands on a Stepped Au(788) Surface

by Teng Ma, Svetlozar Surnev and Falko P. Netzer

Materials 2015, 8(8), 5205-5215; https://0-doi-org.brum.beds.ac.uk/10.3390/ma8085205 - 11 Aug 2015

Cited by 8 | Viewed by 6440

Abstract

The growth morphology and structure of ceria nano-islands on a stepped Au(788) surface has been investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Within the concept of physical vapor deposition, different kinetic routes have been employed to design ceria-Au inverse model catalysts with different ceria nanoparticle shapes and arrangements. A two-dimensional superlattice of ceria nano-islands with a relatively narrow size distribution (5 ± 2 nm²) has been generated on the Au(788) surface by the postoxidation method. This reflects the periodic anisotropy of the template surface and has been ascribed to the pinning of ceria clusters and thus nucleation on the fcc domains of the herringbone reconstruction on the Au terraces. In contrast, the reactive evaporation method yields ceria islands elongated in [01-1] direction, i.e., parallel to the step edges, with high aspect ratios (~6). Diffusion along the Au step edges of ceria clusters and their limited step crossing in conjunction with a growth front perpendicular to the step edges is tentatively proposed to control the ceria growth under reactive evaporation conditions. Both deposition recipes generate two-dimensional islands of CeO₂(111)-type O–Ce–O single and double trilayer structures for submonolayer coverages. Full article

(This article belongs to the Special Issue Epitaxial Materials 2015)

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1946 KiB

Open AccessArticle

Soft X-ray Exposure Promotes Na Intercalation in Graphene Grown on Si-Face SiC

by Somsakul Watcharinyanon, Chao Xia, Yuran Niu, Alexei A. Zakharov, Leif I. Johansson, Rositza Yakimova and Chariya Virojanadara

Materials 2015, 8(8), 4768-4777; https://0-doi-org.brum.beds.ac.uk/10.3390/ma8084768 - 28 Jul 2015

Cited by 2 | Viewed by 4533

Abstract

An investigation of how electron/photon beam exposures affect the intercalation rate of Na deposited on graphene prepared on Si-face SiC is presented. Focused radiation from a storage ring is used for soft X-ray exposures while the electron beam in a low energy electron microscope is utilized for electron exposures. The microscopy and core level spectroscopy data presented clearly show that the effect of soft X-ray exposure is significantly greater than of electron exposure, i.e., it produces a greater increase in the intercalation rate of Na. Heat transfer from the photoelectrons generated during soft X-ray exposure and by the electrons penetrating the sample during electron beam exposure is suggested to increase the local surface temperature and thus the intercalation rate. The estimated electron flux density is 50 times greater for soft X-ray exposure compared to electron exposure, which explains the larger increase in the intercalation rate from soft X-ray exposure. Effects occurring with time only at room temperature are found to be fairly slow, but detectable. The graphene quality, i.e., domain/grain size and homogeneity, was also observed to be an important factor since exposure-induced effects occurred more rapidly on a graphene sample prepared in situ compared to on a furnace grown sample. Full article

(This article belongs to the Special Issue Epitaxial Materials 2015)

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1011 KiB

Open AccessArticle

Investigation of the InAs/GaAs Quantum Dots’ Size: Dependence on the Strain Reducing Layer’s Position

by Manel Souaf, Mourad Baira, Olfa Nasr, Mohamed Helmi Hadj Alouane, Hassen Maaref, Larbi Sfaxi and Bouraoui Ilahi

Materials 2015, 8(8), 4699-4709; https://0-doi-org.brum.beds.ac.uk/10.3390/ma8084699 - 24 Jul 2015

Cited by 31 | Viewed by 6741

Abstract

This work reports on theoretical and experimental investigation of the impact of InAs quantum dots (QDs) position with respect to InGaAs strain reducing layer (SRL). The investigated samples are grown by molecular beam epitaxy and characterized by photoluminescence spectroscopy (PL). The QDs optical transition energies have been calculated by solving the three dimensional Schrödinger equation using the finite element methods and taking into account the strain induced by the lattice mismatch. We have considered a lens shaped InAs QDs in a pure GaAs matrix and either with InGaAs strain reducing cap layer or underlying layer. The correlation between numerical calculation and PL measurements allowed us to track the mean buried QDs size evolution with respect to the surrounding matrix composition. The simulations reveal that the buried QDs’ realistic size is less than that experimentally driven from atomic force microscopy observation. Furthermore, the average size is found to be slightly increased for InGaAs capped QDs and dramatically decreased for QDs with InGaAs under layer. Full article

(This article belongs to the Special Issue Epitaxial Materials 2015)

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Review

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2671 KiB

Open AccessReview

Heteroepitaxy of Cerium Oxide Thin Films on Cu(111)

by Josef Mysliveček, Vladimir Matolín and Iva Matolínová

Materials 2015, 8(9), 6346-6359; https://0-doi-org.brum.beds.ac.uk/10.3390/ma8095307 - 18 Sep 2015

Cited by 10 | Viewed by 6990

Abstract

An important part of fundamental research in catalysis is based on theoretical and modeling foundations which are closely connected with studies of single-crystalline catalyst surfaces. These so-called model catalysts are often prepared in the form of epitaxial thin films, and characterized using advanced material characterization techniques. This concept provides the fundamental understanding and the knowledge base needed to tailor the design of new heterogeneous catalysts with improved catalytic properties. The present contribution is devoted to development of a model catalyst system of CeO₂ (ceria) on the Cu(111) substrate. We propose ways to experimentally characterize and control important parameters of the model catalyst—the coverage of the ceria layer, the influence of the Cu substrate, and the density of surface defects on ceria, particularly the density of step edges and the density and the ordering of the oxygen vacancies. The large spectrum of controlled parameters makes ceria on Cu(111) an interesting alternative to a more common model system ceria on Ru(0001) that has served numerous catalysis studies, mainly as a support for metal clusters. Full article

(This article belongs to the Special Issue Epitaxial Materials 2015)

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3421 KiB

Open AccessReview

Structure, Morphology and Reducibility of Epitaxial Cerium Oxide Ultrathin Films and Nanostructures

by Paola Luches and Sergio Valeri

Materials 2015, 8(9), 5818-5833; https://0-doi-org.brum.beds.ac.uk/10.3390/ma8095278 - 31 Aug 2015

Cited by 25 | Viewed by 6694

Abstract

Cerium oxide is a very interesting material that finds applications in many different fields, such as catalysis, energy conversion, and biomedicine. An interesting approach to unravel the complexity of real systems and obtain an improved understanding of cerium oxide-based materials is represented by the study of model systems in the form of epitaxial ultrathin films or nanostructures supported on single crystalline substrates. These materials often show interesting novel properties, induced by spatial confinement and by the interaction with the supporting substrate, and their understanding requires the use of advanced experimental techniques combined with computational modeling. Recent experimental and theoretical studies performed within this field are examined and discussed here, with emphasis on the new perspectives introduced in view of the optimization of cerium oxide-based materials for application in different fields. Full article

(This article belongs to the Special Issue Epitaxial Materials 2015)

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