62 results on '"Roland Gillen"'
Search Results
2. Robustness of momentum-indirect interlayer excitons in MoS2/WSe2 heterostructure against charge carrier doping
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Ekaterina Khestanova, Tatyana Ivanova, Roland Gillen, Alessandro D’Elia, Oliver Nicholas Gallego Lacey, Lena Wysocki, Alexander Grüneis, Vasily Kravtsov, Wlodek Strupinski, Janina Maultzsch, Viktor Kandyba, Mattia Cattelan, Alexei Barinov, José Avila, Pavel Dudin, and Boris V. Senkovskiy
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Condensed Matter - Materials Science ,Condensed Matter - Mesoscale and Nanoscale Physics ,Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,Electrical and Electronic Engineering ,Atomic and Molecular Physics, and Optics ,Biotechnology ,Electronic, Optical and Magnetic Materials - Abstract
Monolayer transition-metal dichalcogenide (TMD) semiconductors exhibit strong excitonic effects and hold promise for optical and optoelectronic applications. Yet, electron doping of TMDs leads to the conversion of neutral excitons into negative trions, which recombine predominantly non-radiatively at room temperature. As a result, the photoluminescence (PL) intensity is quenched. Here we study the optical and electronic properties of a MoS2/WSe2 heterostructure as a function of chemical doping by Cs atoms performed under ultra-high vacuum conditions. By PL measurements we identify two interlayer excitons and assign them to the momentum-indirect Q-Gamma and K-Gamma transitions. The energies of these excitons are in a very good agreement with ab initio calculations. We find that the Q-Gamma interlayer exciton is robust to the electron doping and is present at room temperature even at a high charge carrier concentration. Submicrometer angle-resolved photoemission spectroscopy (micro-ARPES) reveals charge transfer from deposited Cs adatoms to both the upper MoS2 and the lower WSe2 monolayer without changing the band alignment. This leads to a small (10 meV) energy shift of interlayer excitons. Robustness of the momentum-indirect interlayer exciton to charge doping opens up an opportunity of using TMD heterostructures in light-emitting devices that can work at room temperature at high densities of charge carriers.
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- 2023
3. Oxidation and phase transition in covalently functionalized MoS2
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Narine Moses Badlyan, Nina Pettinger, Niklas Enderlein, Roland Gillen, Xin Chen, Wanzheng Zhang, Kathrin C. Knirsch, Andreas Hirsch, and Janina Maultzsch
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- 2022
4. Covalent Bisfunctionalization of Two‐Dimensional Molybdenum Disulfide
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Janina Maultzsch, Stefan Wolff, Cian Bartlam, Kathrin C. Knirsch, Narine Moses Badlyan, Andreas Hirsch, Vicent Lloret, Tanja Stimpel-Lindner, Xin Chen, Roland Gillen, and Georg S. Duesberg
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Iodide ,02 engineering and technology ,010402 general chemistry ,01 natural sciences ,Catalysis ,Adduct ,chemistry.chemical_compound ,Nanosheets ,Reactivity (chemistry) ,Molybdenum disulfide ,Research Articles ,Alkyl ,chemistry.chemical_classification ,010405 organic chemistry ,Aryl ,General Medicine ,General Chemistry ,021001 nanoscience & nanotechnology ,Combinatorial chemistry ,0104 chemical sciences ,reactivity ,covalent functionalization ,chemistry ,Covalent bond ,Electrophile ,MoS2 ,0210 nano-technology ,hybrid structures ,Research Article ,ddc:547 - Abstract
Covalent functionalization of two‐dimensional molybdenum disulfide (2D MoS2) holds great promise in developing robust organic‐MoS2 hybrid structures. Herein, for the first time, we demonstrate an approach to building up a bisfunctionalized MoS2 hybrid structure through successively reacting activated MoS2 with alkyl iodide and aryl diazonium salts. This approach can be utilized to modify both colloidal and substrate supported MoS2 nanosheets. We have discovered that compared to the adducts formed through the reactions of MoS2 with diazonium salts, those formed through the reactions of MoS2 with alkyl iodides display higher reactivity towards further reactions with electrophiles. We are convinced that our systematic study on the formation and reactivity of covalently functionalized MoS2 hybrids will provide some practical guidance on multi‐angle tailoring of the properties of 2D MoS2 for various potential applications., For the first time, we demonstrate an approach to building up a bisfunctionalized MoS2 hybrid structure through successively treating activated MoS2 with alkyl iodide and aryl diazonium salt. This approach can be used to modify both colloidal and substrate‐supported MoS2 nanosheets.
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- 2021
5. First- and second-order Raman spectroscopy of monoclinic β−Ga2O3
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Benjamin M. Janzen, Roland Gillen, Zbigniew Galazka, Janina Maultzsch, and Markus R. Wagner
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Physics and Astronomy (miscellaneous) ,General Materials Science - Published
- 2022
6. Vibrational Properties and Charge Transfer in the Misfit-Layer Compound LaS–CrS2
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Leela S. Panchakarla, Janina Maultzsch, Reshef Tenne, Felix Kampmann, and Roland Gillen
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Materials science ,Charge (physics) ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Molecular physics ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Transfer (group theory) ,General Energy ,Density functional theory ,Physical and Theoretical Chemistry ,0210 nano-technology ,Layer (electronics) ,Electronic properties - Abstract
We present density functional theory calculations of the vibrational and electronic properties of the misfit-layer compound (MLC) LaS–CrS2 and its isolated sublayers to identify the vibrational mod...
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- 2021
7. Comprehensive Raman study of orthorhombic κ/ε-Ga2O3 and the impact of rotational domains
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Linus P. Grote, Oliver Bierwagen, Markus R. Wagner, Vivien F. S. Peltason, Roland Gillen, Roberto Fornari, Benjamin M. Janzen, Piero Mazzolini, and Janina Maultzsch
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Materials science ,Condensed matter physics ,Band gap ,Phonon ,General Chemistry ,Crystal structure ,Ferroelectricity ,Condensed Matter::Materials Science ,symbols.namesake ,Phase (matter) ,Materials Chemistry ,symbols ,Orthorhombic crystal system ,Spectroscopy ,Raman spectroscopy - Abstract
Gallium oxide (Ga2O3) is an ultra-wide bandgap material, which has recently attracted widespread attention for holding promising applications in power electronics and solar blind UV photodetectors, outclassing GaN or SiC in terms of a larger bandgap and higher breakdown voltages. The orthorhombic κ phase (also referred to as e) has sparked particular interest for offering higher symmetry than β, while featuring ferroelectric behavior paired with a large predicted spontaneous polarization, paving the way to fabricating high-quality two-dimensional electron gases for application in heterostructure field effect transistors. The presently available κ phase samples are characterized by a domain structure, in which orthorhombic domains are rotated 120° against each other within the c-plane forming a pseudo-hexagonal structure, which has previously often been ascribed to e-Ga2O3 and incorrectly been viewed as this polymorph's true crystal structure. A detailed investigation into the phonon modes of orthorhombic κ-Ga2O3 provides insights into fundamental material properties such as crystal structure and orientation as well as the vibrational symmetries of Raman active modes. We investigate the Raman active phonon modes of an MBE-grown orthorhombic κ-Ga2O3 thin film featuring the domain structure deposited on (0001)-Al2O3 by experiment and theory: Polarized micro-Raman spectroscopy measurements in conjunction with density functional perturbation theory (DFPT) calculations enable the identification of both the frequencies and vibrational symmetries of the Raman active phonons. Presenting comprehensive Raman spectra of the orthorhombic κ phase, the experimental frequencies of more than 90 Raman modes are determined and correlated with the 117 modes predicted by the calculations. Angular-resolved Raman measurements are utilized to provide an experimental verification of phonon mode symmetries. We present an analytical tool to deal with the domain structure and its effect on the obtained Raman spectra.
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- 2021
8. NaGdSi2O6 – A novel antiferromagnetically coupled silicate with Vierer chain structure
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Franz Kamutzki, Maged F. Bekheet, Sören Selve, Felix Kampmann, Konrad Siemensmeyer, Delf Kober, Roland Gillen, Markus Wagner, Janina Maultzsch, Aleksander Gurlo, and Dorian A.H. Hanaor
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Inorganic Chemistry ,Materials Chemistry ,Ceramics and Composites ,Physical and Theoretical Chemistry ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials - Published
- 2023
9. Nagdsi2o6 – a Novel Antiferromagnetic Silicate with Vierer Chain Structure
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Franz Kamutzki, Maged F. Bekheet, Sören Selve, Felix Kampmann, Konrad Siemensmeyer, Delf Kober, Roland Gillen, Markus Wagner, Janina Maultzsch, Aleksander Gurlo, and Dorian A. H. Hanaor
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History ,Polymers and Plastics ,Business and International Management ,Industrial and Manufacturing Engineering - Published
- 2022
10. Dark exciton-exciton annihilation in monolayer WSe2
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Daniel Erkensten, Samuel Brem, Koloman Wagner, Roland Gillen, Raül Perea-Causín, Jonas D. Ziegler, Takashi Taniguchi, Kenji Watanabe, Janina Maultzsch, Alexey Chernikov, and Ermin Malic
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Other Physics Topics ,Condensed Matter - Mesoscale and Nanoscale Physics ,Condensed Matter::Other ,Atom and Molecular Physics and Optics ,FOS: Physical sciences ,02 engineering and technology ,Condensed Matter Physics ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,01 natural sciences ,Condensed Matter::Materials Science ,Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ,0103 physical sciences ,010306 general physics ,0210 nano-technology - Abstract
The exceptionally strong Coulomb interaction in semiconducting transition-metal dichalcogenides (TMDs) gives rise to a rich exciton landscape consisting of bright and dark exciton states. At elevated densities, excitons can interact through exciton-exciton annihilation (EEA), an Auger-like recombination process limiting the efficiency of optoelectronic applications. Although EEA is a well-known and particularly important process in atomically thin semiconductors determining exciton lifetimes and affecting transport at elevated densities, its microscopic origin has remained elusive. In this joint theory-experiment study combining microscopic and material-specific theory with time- and temperature-resolved photoluminescence measurements, we demonstrate the key role of dark intervalley states that are found to dominate the EEA rate in monolayer WSe$_2$. We reveal an intriguing, characteristic temperature dependence of Auger scattering in this class of materials with an excellent agreement between theory and experiment. Our study provides microscopic insights into the efficiency of technologically relevant Auger scattering channels within the remarkable exciton landscape of atomically thin semiconductors., 17 pages, 6 figures
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- 2021
11. Interlayer excitonic spectra of vertically stacked MoSe$_2$/WSe$_2$ heterobilayers
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Roland Gillen
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Materials science ,Exciton ,Binding energy ,Stacking ,FOS: Physical sciences ,02 engineering and technology ,01 natural sciences ,Molecular physics ,Spectral line ,Condensed Matter::Materials Science ,0103 physical sciences ,Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ,ddc:530 ,Perturbation theory ,010302 applied physics ,Condensed Matter - Materials Science ,Condensed Matter - Mesoscale and Nanoscale Physics ,Condensed Matter::Other ,Materials Science (cond-mat.mtrl-sci) ,Heterojunction ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Electronic, Optical and Magnetic Materials ,Direct and indirect band gaps ,0210 nano-technology ,Excitation - Abstract
The optical spectra of vertically stacked MoSe$_2$/WSe$_2$ heterostructures contain additional 'interlayer' excitonic peaks that are absent in the individual monolayer materials and exhibit a significant spatial charge separation in out-of-plane direction. Extending on a previous study, we used a many-body perturbation theory approach to simulate and analyse the excitonic spectra of MoSe$_2$/WSe$_2$ heterobilayers with three stacking orders, considering both momentum-direct and momentum-indirect excitons. We find that the small oscillator strengths and corresponding optical responses of the interlayer excitons are significantly stacking-dependent and give rise to high radiative lifetimes in the range of 5-200\,ns (at T=4\,K) for the 'bright' interlayer excitons. Solving the finite-momentum Bethe-Salpeter Equation, we predict that the lowest-energy excitation should be an indirect exciton over the fundamental indirect band gap (K$\rightarrow$Q), with a binding energy of 220\,meV. However, in agreement with recent magneto-optics experiments and previous theoretical studies, our simulations of the effective excitonic Land\'e g-factors suggest that the low-energy momentum-indirect excitons are not experimentally observed for MoSe$_2$/WSe$_2$ heterostructures. We further reveal the existence of 'interlayer' C excitons with significant exciton binding energies and optical oscillator strengths, which are analogous to the prominent band nesting excitons in mono- and few-layer transition-metal dichalcogenides., Comment: 20 pages, 14 figures, 3 tables
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- 2021
12. Tailoring Coulomb correlations in twisted WSe2 bilayers
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Ermin Malic, Rupert Huber, Fabian Mooshammer, Roland Gillen, Philipp Merkl, Janina Maultsch, Leonard Weigl, Anna Girnghuber, John M. Lupton, Chaw-Keong Yong, Samuel Brem, and Kai-Qiang Lin
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Physics ,Red shift ,Trace (linear algebra) ,Exciton ,Binding energy ,Coulomb ,Twist angle ,Electron ,Molecular physics ,Photon counting - Abstract
Phase-locked few-cycle mid-infrared pulses trace how the twist angle alone renormalizes the binding energy of excitons in twisted WSe2 homobilayers by a factor of two and tunes their lifetime by a factor of twenty.
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- 2021
13. Isotopic study of Raman active phonon modes in β-Ga$_{2}$O$_{3}$
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Piero Mazzolini, Janina Maultzsch, Markus R. Wagner, Benjamin M. Janzen, Roland Gillen, Andreas Falkenstein, Hans Tornatzky, Oliver Bierwagen, and Manfred Martin
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Materials science ,Band gap ,Phonon ,02 engineering and technology ,Crystal structure ,01 natural sciences ,Molecular physics ,symbols.namesake ,0103 physical sciences ,Materials Chemistry ,Raman active phonon modes ,Isotopologue ,ddc:530 ,Spectroscopy ,phonon modes ,010302 applied physics ,Dopant ,General Chemistry ,530 Physik ,021001 nanoscience & nanotechnology ,Crystallographic defect ,power electronics ,symbols ,0210 nano-technology ,Raman spectroscopy - Abstract
Journal of materials chemistry / C 9(7), 2311-2320 (2021). doi:10.1039/D0TC04101G, Published by RSC, London [u.a.]
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- 2021
- Full Text
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14. Hybridized intervalley moiré excitons and flat bands in twisted WSe
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Samuel, Brem, Kai-Qiang, Lin, Roland, Gillen, Jonas M, Bauer, Janina, Maultzsch, John M, Lupton, and Ermin, Malic
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The large surface-to-volume ratio in atomically thin 2D materials allows to efficiently tune their properties through modifications of their environment. Artificial stacking of two monolayers into a bilayer leads to an overlap of layer-localized wave functions giving rise to a twist angle-dependent hybridization of excitonic states. In this joint theory-experiment study, we demonstrate the impact of interlayer hybridization on bright and momentum-dark excitons in twisted WSe2 bilayers. In particular, we show that the strong hybridization of electrons at the Λ point leads to a drastic redshift of the momentum-dark K-Λ exciton, accompanied by the emergence of flat moiré exciton bands at small twist angles. We directly compare theoretically predicted and experimentally measured optical spectra allowing us to identify photoluminescence signals stemming from phonon-assisted recombination of layer-hybridized dark excitons. Moreover, we predict the emergence of additional spectral features resulting from the moiré potential of the twisted bilayer lattice.
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- 2020
15. Hybridized intervalley moir\'e excitons and flat bands in twisted WSe$_2$ bilayers
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Jonas M. Bauer, Kai-Qiang Lin, Janina Maultzsch, Ermin Malic, John M. Lupton, Roland Gillen, and Samuel Brem
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Physics ,Photoluminescence ,Condensed Matter - Mesoscale and Nanoscale Physics ,Exciton ,Bilayer ,ddc:530 ,Stacking ,02 engineering and technology ,Electron ,021001 nanoscience & nanotechnology ,530 Physik ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,01 natural sciences ,Molecular physics ,Condensed Matter::Materials Science ,0103 physical sciences ,Monolayer ,General Materials Science ,Twist ,010306 general physics ,0210 nano-technology ,Wave function - Abstract
The large surface-to-volume ratio in atomically thin 2D materials allows to efficiently tune their properties through modifications of their environment. Artificial stacking of two monolayers into a bilayer leads to an overlap of layer-localized wave functions giving rise to a twist angle-dependent hybridization of excitonic states. In this joint theory-experiment study, we demonstrate the impact of interlayer hybridization on bright and momentum-dark excitons in twisted WSe$_2$ bilayers. In particular, we show that the strong hybridization of electrons at the $\Lambda$ point leads to a drastic redshift of the momentum-dark K-$\Lambda$ exciton, accompanied by the emergence of flat moir\'e exciton bands at small twist angles. We directly compare theoretically predicted and experimentally measured optical spectra allowing us to identify photoluminescence signals stemming from phonon-assisted recombination of layer-hybridized dark excitons. Moreover, we predict the emergence of additional spectral features resulting from the moir\'e potential of the twisted bilayer lattice.
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- 2020
16. Tunable infrared light emission from MoS2/WSe2 heterostructures
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Tzach Yaffe, Elyse Barré, Takashi Taniguchi, Katayun Barmak, Bumho Kim, Sze Cheung Lau, Eric Yue Ma, Janina Maultzsch, Roland Gillen, Tony F. Heinz, Lior Gal, Meir Orenstein, Kenji Watanabe, Ralph H. Page, and Ouri Karni
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Materials science ,Infrared ,business.industry ,Astrophysics::High Energy Astrophysical Phenomena ,Exciton ,Heterojunction ,02 engineering and technology ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,01 natural sciences ,010309 optics ,Condensed Matter::Materials Science ,Electric field ,0103 physical sciences ,Monolayer ,Optoelectronics ,Light emission ,Stimulated emission ,0210 nano-technology ,business ,Visible spectrum - Abstract
We report light emission around 1200 nm from a vertical heterostructure consisting of M0S2 and WSe2 monolayers. The emission, arising from the fundamental interlayer exciton, can be tuned by nearly 100 nm by electrical gating.
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- 2020
17. Unveiling the oxidation behavior of liquid-phase exfoliated antimony nanosheets
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Mhamed Assebban, Félix Zamora, Gonzalo Abellán, Stefan Wolff, Iñigo Torres, Carlos Gibaja, Kian Ping Loh, Michael Fickert, Erik Weinreich, Sherman Tan Jun Rong, Maria Varela, Janina Maultzsch, Roland Gillen, Enrique G. Michel, UAM. Departamento de Física de la Materia Condensada, and UAM. Departamento de Química Inorgánica
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Antimony ,Materials science ,Band gap ,Annealing (metallurgy) ,Sonication ,Liquid phase ,chemistry.chemical_element ,02 engineering and technology ,010402 general chemistry ,01 natural sciences ,X-ray photoelectron spectroscopy ,Chemical Analysis ,Nanosheets ,Monolayer ,General Materials Science ,Ultraviolet Photoelectron Spectroscopy ,Materials ,Mechanical Engineering ,Física ,General Chemistry ,Química ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,0104 chemical sciences ,Chemical engineering ,chemistry ,Mechanics of Materials ,ddc:540 ,Chemical stability ,0210 nano-technology ,Surface Chemistry - Abstract
Antimonene, a monolayer of β-antimony, is increasingly attracting considerable attention, more than that of other monoelemental two-dimensional materials, due to its intriguing physical and chemical properties. Under ambient conditions, antimonene exhibits a high thermodynamic stability and good structural integrity. Some theoretical calculations predicted that antimonene would have a high oxidation tendency. However, it remains poorly investigated from the experimental point of view. In this work, we study the oxidation behavior of antimonene nanosheets (ANS) prepared by ultrasonication-assisted liquid-phase exfoliation. Using a set of forefront analytical techniques, a clear effect of sonication time on the surface chemistry of prepared ANS is found. A dynamic oxidation behavior has been observed, which upon annealing at moderate temperature (210 °C) resulted in a semiconducting behavior with a bandgap of approximately 1 eV measured by ultraviolet photoelectron spectroscopy. This study yields valuable information for future applications of antimonene and paves the way towards novel modification approaches in order to tailor its properties and complement its limitations.
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- 2020
18. Twist-tailoring Coulomb correlations in van der Waals homobilayers
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Rupert Huber, Janina Maultzsch, Marlene Liebich, Kai-Qiang Lin, Fabian Mooshammer, Leonard Weigl, John M. Lupton, Ermin Malic, Anna Girnghuber, Chaw-Keong Yong, Samuel Brem, Roland Gillen, and Philipp Merkl
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Phase transition ,Nonlinear optics ,Solid-state physics ,Science ,Exciton ,Atom and Molecular Physics and Optics ,General Physics and Astronomy ,02 engineering and technology ,Two-dimensional materials ,01 natural sciences ,Resonance (particle physics) ,Physical Chemistry ,General Biochemistry, Genetics and Molecular Biology ,Article ,symbols.namesake ,Condensed Matter::Materials Science ,Ultrafast photonics ,0103 physical sciences ,Coulomb ,lcsh:Science ,010306 general physics ,Electronic band structure ,Physics ,Multidisciplinary ,Condensed matter physics ,ddc:530 ,General Chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,530 Physik ,3. Good health ,symbols ,Quasiparticle ,lcsh:Q ,van der Waals force ,0210 nano-technology - Abstract
The recent discovery of artificial phase transitions induced by stacking monolayer materials at magic twist angles represents a paradigm shift for solid state physics. Twist-induced changes of the single-particle band structure have been studied extensively, yet a precise understanding of the underlying Coulomb correlations has remained challenging. Here we reveal in experiment and theory, how the twist angle alone affects the Coulomb-induced internal structure and mutual interactions of excitons. In homobilayers of WSe2, we trace the internal 1s–2p resonance of excitons with phase-locked mid-infrared pulses as a function of the twist angle. Remarkably, the exciton binding energy is renormalized by up to a factor of two, their lifetime exhibits an enhancement by more than an order of magnitude, and the exciton-exciton interaction is widely tunable. Our work opens the possibility of tailoring quasiparticles in search of unexplored phases of matter in a broad range of van der Waals heterostructures., The crystallographic orientation of monolayers in van der Waals multi-layers controls their electronic and optical properties. Here the authors show how the twist angle affects Coulomb correlations governing the internal structure and the mutual interaction of excitons in homobilayers of WSe2.
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- 2020
19. Infrared Interlayer Exciton Emission in MoS2/WSe2 Heterostructures
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Eric Yue Ma, Janina Maultzsch, Elyse Barré, Ralph H. Page, Kenji Watanabe, Bumho Kim, Tony F. Heinz, Ouri Karni, Sze Cheung Lau, Takashi Taniguchi, Katayun Barmak, and Roland Gillen
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Silicon photonics ,Materials science ,Condensed matter physics ,Infrared ,Exciton ,General Physics and Astronomy ,Heterojunction ,Electronic structure ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,01 natural sciences ,Transition metal dichalcogenide monolayers ,Condensed Matter::Materials Science ,Dipole ,0103 physical sciences ,Light emission ,010306 general physics - Abstract
We report light emission around 1 eV (1240 nm) from heterostructures of MoS_{2} and WSe_{2} transition metal dichalcogenide monolayers. We identify its origin in an interlayer exciton (ILX) by its wide spectral tunability under an out-of-plane electric field. From the static dipole moment of the state, its temperature and twist-angle dependence, and comparison with electronic structure calculations, we assign this ILX to the fundamental interlayer transition between the K valleys in this system. Our findings gain access to the interlayer physics of the intrinsically incommensurate MoS_{2}/WSe_{2} heterostructure, including moire and valley pseudospin effects, and its integration with silicon photonics and optical fiber communication systems operating at wavelengths longer than 1150 nm.
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- 2019
20. Infrared Interlayer Exciton Emission in MoS_{2}/WSe_{2} Heterostructures
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Ouri, Karni, Elyse, Barré, Sze Cheung, Lau, Roland, Gillen, Eric Yue, Ma, Bumho, Kim, Kenji, Watanabe, Takashi, Taniguchi, Janina, Maultzsch, Katayun, Barmak, Ralph H, Page, and Tony F, Heinz
- Abstract
We report light emission around 1 eV (1240 nm) from heterostructures of MoS_{2} and WSe_{2} transition metal dichalcogenide monolayers. We identify its origin in an interlayer exciton (ILX) by its wide spectral tunability under an out-of-plane electric field. From the static dipole moment of the state, its temperature and twist-angle dependence, and comparison with electronic structure calculations, we assign this ILX to the fundamental interlayer transition between the K valleys in this system. Our findings gain access to the interlayer physics of the intrinsically incommensurate MoS_{2}/WSe_{2} heterostructure, including moiré and valley pseudospin effects, and its integration with silicon photonics and optical fiber communication systems operating at wavelengths longer than 1150 nm.
- Published
- 2019
21. Phonon Dispersion in MoS2 by Inelastic X-ray Scattering
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Hans Tornatzky, Roland Gillen, Janina Maultzsch, and Hiroshi Uchiyama
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Materials science ,Condensed matter physics ,Scattering ,Phonon ,Dispersion (optics) ,X-ray - Published
- 2019
22. Two-Dimensional Antimony Oxide
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Mhamed Assebban, Stefan Wolff, Gonzalo Abellán, Roland Gillen, and Janina Maultzsch
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Materials science ,Oxide ,General Physics and Astronomy ,chemistry.chemical_element ,FOS: Physical sciences ,01 natural sciences ,chemistry.chemical_compound ,symbols.namesake ,Antimony ,0103 physical sciences ,Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ,010306 general physics ,Materials ,Condensed Matter - Materials Science ,Condensed Matter - Mesoscale and Nanoscale Physics ,business.industry ,Física ,Materials Science (cond-mat.mtrl-sci) ,3. Good health ,Semiconductor ,chemistry ,Chemical physics ,Topological insulator ,symbols ,Direct and indirect band gaps ,Density functional theory ,Antimony oxide ,business ,Raman spectroscopy - Abstract
Two-dimensional (2D) antimony, so-called antimonene, can form antimonene oxide when exposed to air. We present different types of single- and few-layer antimony oxide structures, based on density functional theory (DFT) calculations. Depending on stoichiometry and bonding type, these novel 2D layers have different structural stability and electronic properties, ranging from topological insulators to semiconductors with direct and indirect band gaps between 2.0 and 4.9 eV. We discuss their vibrational properties and Raman spectra for experimental identification of the predicted structures., Comment: 4 figures
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- 2019
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23. Vibrational signatures of diamondoid dimers with large intramolecular London dispersion interactions
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Tobias Hückstaedt, Andrey A. Fokin, Nataliya A. Fokina, Peter R. Schreiner, Tommy Otto, Christoph Tyborski, Janina Maultzsch, and Roland Gillen
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Condensed Matter - Materials Science ,Materials science ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Diamondoid ,01 natural sciences ,London dispersion force ,0104 chemical sciences ,Bond length ,symbols.namesake ,Chemical physics ,Intramolecular force ,Molecular vibration ,symbols ,Single bond ,General Materials Science ,Density functional theory ,0210 nano-technology ,Raman spectroscopy - Abstract
We analyze the vibrational properties of diamondoid compounds via Raman spectroscopy. The compounds are interconnected with carbon-carbon single bonds that exhibit exceptionally large bond lengths up to 1.71 A. Attractive dispersion interactions caused by well-aligned intramolecular H--H contact surfaces determine the overall structures of the diamondoid derivatives. The strong van-der-Waals interactions alter the vibrational properties of the compounds in comparison to pristine diamondoids. Supported by dispersion-corrected density functional theory (DFT) computations, we analyze and explain their experimental Raman spectra with respect to unfunctionalized diamondoids. We find a new set of dispersion-induced vibrational modes comprising characteristic CH/CH$_{2}$ vibrations with exceptionally high energies. Further, we find structure-induced dimer modes that are indicative of the size of the dimers., Comment: 8 pages, 6 figures
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- 2019
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24. Phonon dispersion of MoS$_2$
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Hans Tornatzky, Janina Maultzsch, Hiroshi Uchiyama, and Roland Gillen
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Condensed Matter - Materials Science ,Materials science ,Condensed matter physics ,business.industry ,Scattering ,Phonon ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,01 natural sciences ,Displacement (vector) ,Brillouin zone ,Crystal ,Condensed Matter::Materials Science ,Semiconductor ,Condensed Matter::Superconductivity ,0103 physical sciences ,Thermal ,Dispersion (optics) ,010306 general physics ,0210 nano-technology ,business - Abstract
Transition metal dichalcogenides like MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$ have attracted enormous interest during recent years. They are van-der-Waals crystals with highly anisotropic properties, which allows exfoliation of individual layers. Their remarkable physical properties make them promising for applications in optoelectronic, spintronic, and valleytronic devices. Phonons are fundamental to many of the underlying physical processes, like carrier and spin relaxation or exciton dynamics. However, experimental data of the complete phonon dispersion relations in these materials is missing. Here we present the phonon dispersion of bulk MoS$_2$ in the high-symmetry directions of the Brillouin zone, determined by inelastic X-ray scattering. Our results underline the two-dimensional nature of MoS$_2$. Supported by first-principles calculations, we determine the phonon displacement patterns, symmetry properties, and scattering intensities. The results will be the basis for future experimental and theoretical work regarding electron-phonon interactions, intervalley scattering, as well as phonons in related 2D materials.
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- 2018
25. Native point defects of semiconducting layered Bi2O2Se
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Xintong Xu, Yi Zhang, Roland Gillen, John Robertson, Huanglong Li, Luping Shi, Gillen, Roland [0000-0002-7913-0953], and Apollo - University of Cambridge Repository
- Subjects
Electron mobility ,Materials science ,lcsh:Medicine ,FOS: Physical sciences ,Nanotechnology ,Context (language use) ,02 engineering and technology ,01 natural sciences ,4016 Materials Engineering ,Article ,symbols.namesake ,chemistry.chemical_compound ,0103 physical sciences ,Nano ,Electronics ,lcsh:Science ,40 Engineering ,010302 applied physics ,Condensed Matter - Materials Science ,Multidisciplinary ,lcsh:R ,Materials Science (cond-mat.mtrl-sci) ,Fermi energy ,021001 nanoscience & nanotechnology ,5104 Condensed Matter Physics ,Crystallographic defect ,Phosphorene ,chemistry ,symbols ,lcsh:Q ,van der Waals force ,0210 nano-technology ,51 Physical Sciences - Abstract
Bi2O2Se is an emerging semiconducting, air-stable layered material (Nat. Nanotechnol. 2017, 12, 530; Nano Lett. 2017, 17, 3021), potentially exceeding MoS2 and phosphorene in electron mobility and rivalling typical Van der Waals stacked layered materials in the next-generation high-speed and low-power electronics. Holding the promise of functional versatility, it is arousing rapidly growing interest from various disciplines, including optoelectronics, thermoelectronics and piezoelectronics. In this work, we comprehensively study the electrical properties of the native point defects in Bi2O2Se, as an essential step toward understanding the fundamentals of this material. The defect landscapes dependent on both Fermi energy and the chemical potentials of atomic constituents are investigated. Along with the bulk defect analysis, a complementary inspection of the surface properties, within the simple context of charge neutrality level model, elucidates the observed n-type characteristics of Bi2O2Se based FETs. This work provides important guide to engineer the defects of Bi2O2Se for desired properties, which is key to the successful application of this emerging layered material27.
- Published
- 2018
26. Interlayer excitons in MoSe2/WSe2 heterostructures from first principles
- Author
-
Roland Gillen and Janina Maultzsch
- Subjects
Physics ,Condensed matter physics ,Band gap ,Exciton ,Binding energy ,Order (ring theory) ,Heterojunction ,02 engineering and technology ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,01 natural sciences ,Spectral line ,Brillouin zone ,Condensed Matter::Materials Science ,symbols.namesake ,0103 physical sciences ,Rydberg formula ,symbols ,010306 general physics ,0210 nano-technology - Abstract
Based on ab initio theoretical calculations of the optical spectra of vertical heterostructures of ${\mathrm{MoSe}}_{2}$ (or ${\mathrm{MoS}}_{2}$) and ${\mathrm{WSe}}_{2}$ sheets, we reveal two spin-orbit-split Rydberg series of excitonic states below the $A$ excitons of ${\mathrm{MoSe}}_{2}$ and ${\mathrm{WSe}}_{2}$ with a significant binding energy on the order of 250 meV for the first excitons in the series. At the same time, we predict from accurate many-body ${\mathrm{G}}_{0}{\mathrm{W}}_{0}$ calculations that crystallographically aligned ${\mathrm{MoSe}}_{2}\text{/}{\mathrm{WSe}}_{2}$ heterostructures exhibit an indirect fundamental band gap. Due to the type-II nature of the ${\mathrm{MoSe}}_{2}\text{/}{\mathrm{WSe}}_{2}$ heterostructure, the indirect transition and the exciton Rydberg series corresponding to a direct transition exhibit a distinct interlayer nature with spatial charge separation of the coupled electrons and holes. Our calculations confirm the recent experimental observation of a doublet nature of the long-lived states in photoluminescence spectra of $\mathrm{Mo}{X}_{2}\text{/}\mathrm{W}{Y}_{2}$ heterostructures, and we attribute these two contributions to momentum-direct interlayer excitons at the $K$ point of the hexagonal Brillouin zone and to momentum-indirect excitons at the indirect fundamental band gap. Our calculations further suggest a noticeable effect of stacking order on the electronic band gaps and on the peak energies of the interlayer excitons and their oscillation strengths.
- Published
- 2018
27. Electronic and Vibrational Properties of Diamondoid Oligomers
- Author
-
Roland Gillen, Christoph Tyborski, Christian Thomsen, Natalie A. Fokina, Janina Maultzsch, Tetyana V. Koso, Heike Hausmann, Peter R. Schreiner, V. N. Rodionov, and Andrey A. Fokin
- Subjects
Double bond ,Adamantane ,Resonance Raman spectroscopy ,02 engineering and technology ,010402 general chemistry ,Photochemistry ,Diamondoid ,01 natural sciences ,symbols.namesake ,chemistry.chemical_compound ,Physics::Atomic and Molecular Clusters ,Physics::Chemical Physics ,Physical and Theoretical Chemistry ,chemistry.chemical_classification ,021001 nanoscience & nanotechnology ,3. Good health ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Crystallography ,General Energy ,chemistry ,Molecular vibration ,symbols ,Density functional theory ,0210 nano-technology ,Raman spectroscopy ,Diamantane - Abstract
We analyzed the vibrational and electronic properties of diamondoid oligomers via resonance Raman spectroscopy. The compounds consist of lower diamondoids such as adamantane or diamantane that are interconnected with double bonds. Therefore, all oligomers have ethylene-like centers strongly influencing the character of the optical transitions. The double bond localizes the HOMO (highest occupied moluecular orbital) in between the diamondoids accompanied by a significant decrease of optical transition energies. Comparing Raman spectra of the compounds to pristine diamondoids, we find several characteristic modes originating from the ethylene moieties. Supported by DFT (density functional theory) computations, we attribute these modes to highly localized vibrations that can partially be derived from the vibrational modes of parent ethylene. We further observe two new Raman modes in the compounds: a dimer breathing mode and a rotational mode of the entire ethylene moieties.
- Published
- 2017
28. Effects of annealing on optical and structural properties of zinc oxide nanocrystals
- Author
-
Matthew R. Phillips, Janina Maultzsch, André Kliem, Sumin Choi, Roland Gillen, Cuong Ton-That, Sevak Khachadorian, and Axel Hoffmann
- Subjects
Materials science ,Photoluminescence ,Annealing (metallurgy) ,Inorganic chemistry ,chemistry.chemical_element ,Zinc ,Condensed Matter Physics ,Crystallographic defect ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,symbols.namesake ,Chemical engineering ,Nanocrystal ,chemistry ,Physics::Atomic and Molecular Clusters ,symbols ,Density functional theory ,Raman spectroscopy ,Raman scattering - Abstract
The optical and vibrational properties of zinc oxide nanocrystals (ZnO NCs), grown by chemical precipitation method, were investigated following thermal annealing treatments in oxygen, argon, and zinc vapors at temperatures up to 900∘C. Raman scattering and photoluminescence techniques were utilized to show the effect of temperature annealing in various environments on optical and structural properties of ZnO NCs, referring them to the reduction and increase of certain intrinsic defect concentrations. The experimental results are complemented by density functional theory calculations for understanding the correlation between Raman modes induced with different annealing procedures and certain intrinsic point defects.
- Published
- 2015
29. Electronic properties of MoS2/h-BN heterostructures: Impact of dopants and impurities
- Author
-
Roland Gillen, John Robertson, and Janina Maultzsch
- Subjects
Materials science ,Condensed matter physics ,Dopant ,Spintronics ,Band gap ,Doping ,Heterojunction ,Nanotechnology ,Electronic structure ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Condensed Matter::Materials Science ,Condensed Matter::Superconductivity ,Monolayer ,Condensed Matter::Strongly Correlated Electrons ,Density functional theory - Abstract
We report density functional theory calculations of the electronic properties of doped heterostructures of monolayer molybdenum disulphide and hexagonal boron nitride (h-BN). Extending on a recent study, we investigate the effect of doping the h-BN layer on the electronic structure of the MoS2 subsystem by a number of impurites. We consider the intrinsic n-type dopants S and Mo, the p-type dopant Mg, and interstitial K and Mn atoms. Magnesium and molybdenum substitutional impurities induce tails of occupied defect states into the band gap of the compound that reach up to the conduction band minimum of MoS2. This suggests easy charge transfer between the layers in these cases. Additionally, the d-electrons in the studied transition metal impurities induce localized magnetic momenta into the MoS2 subsystem, which might be exploited in spintronics applications. Our results contribute to the understanding of the interaction of MoS2 in vertical heterostructures with nearby impurities and offer insights into possible origins of intrinsic doping of experimental samples of MoS2.
- Published
- 2014
30. Study of CeO2 and Its Native Defects by Density Functional Theory with Repulsive Potential
- Author
-
Bolong Huang, Roland Gillen, and John Robertson
- Subjects
Valence (chemistry) ,Chemistry ,education ,chemistry.chemical_element ,Crystallographic defect ,Oxygen ,Molecular physics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Ion ,Pseudopotential ,General Energy ,Kröger–Vink notation ,Frenkel defect ,Density functional theory ,Physical and Theoretical Chemistry ,Atomic physics - Abstract
We investigated the native point defects in CeO2 by the density functional theory with repulsive potential (DFT+U) method and by use of a nonlinear core-corrected norm-conserving Ce pseudopotential. We find the neutral oxygen vacancy (VO0) in CeO2 to have a very low formation energy of only 0.39 eV in the O-poor limit. It is a deep donor with negative U behavior, stable only in its neutral and doubly positive states. The anion Frenkel defect is found to be the lowest energy disorder defect, with a formation energy of only 2.08 eV/defect site. These low formation energies arise from the improved transferability of our Ce pseudopotential for its +3 and +4 valence states. The negative U behavior of VO leads to excellent photocatalytic behavior, while the low formation energy of the anion Frenkel defect leads to a superior oxygen storage-and-release capability.
- Published
- 2014
31. From isolated diamondoids to a van-der-Waals crystal: A theoretical and experimental analysis of a trishomocubane and a diamantane dimer in the gas and solid phase
- Author
-
Tobias Bischoff, Andrey A. Fokin, Thomas Möller, V. N. Rodionov, Robert Richter, Peter R. Schreiner, Christian Thomsen, Roland Gillen, Andrea Merli, Tetyana V. Koso, Reinhard Meinke, Christoph Tyborski, Janina Maultzsch, Andre Knecht, and Torbjörn Rander
- Subjects
Resonance Raman spectroscopy ,Analytical chemistry ,General Physics and Astronomy ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Molecular physics ,0104 chemical sciences ,Bond length ,chemistry.chemical_compound ,symbols.namesake ,chemistry ,Excited state ,Physics::Atomic and Molecular Clusters ,symbols ,Density functional theory ,Molecular orbital ,Physical and Theoretical Chemistry ,0210 nano-technology ,Raman spectroscopy ,HOMO/LUMO ,Diamantane - Abstract
The electronic properties of sp 2/sp 3 diamondoids in the crystalline state and in the gas phase are presented. Apparent differences in electronic properties experimentally observed by resonance Raman spectroscopy in the crystalline/gas phase and absorption measurements in the gas phase were investigated by density functional theory computations. Due to a reorganization of the molecular orbitals in the crystalline phase, the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy gaps are lowered significantly by 0.5 eV-1 eV. The π → π∗ transition is responsible for large absorption in both gas and crystalline phases. It further causes a large increase in the Raman intensity of the C=C stretch vibration when excited resonantly. By resonance Raman spectroscopy we were able to determine the C=C bond length of the trishomocubane dimer to exhibit 1.33 Å in the ground and 1.41 Å in the excited state.
- Published
- 2017
32. Fundamental Insights into the Degradation and Stabilization of Thin Layer Black Phosphorus
- Author
-
Janina Maultzsch, Frank Hauke, Stefan Wild, Nils Scheuschner, Udo Mundloch, Andreas Hirsch, Roland Gillen, Vicent Lloret, Maria Varela, and Gonzalo Abellán
- Subjects
Passivation ,Chemistry ,Analytical chemistry ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Biochemistry ,Instability ,Catalysis ,Article ,0104 chemical sciences ,chemistry.chemical_compound ,symbols.namesake ,Colloid and Surface Chemistry ,Ionic liquid ,Microscopy ,ddc:540 ,symbols ,Degradation (geology) ,0210 nano-technology ,Anisotropy ,Raman spectroscopy ,Visible spectrum - Abstract
Herein, we have developed a systematic study on the oxidation and passivation of mechanically exfoliated black phosphorus (BP). We analyzed the strong anisotropic behavior of BP by scanning Raman microscopy providing an accurate method for monitoring the oxidation of BP via statistical Raman spectroscopy. Furthermore, different factors influencing the environmental instability of the BP, i.e., thickness, lateral dimensions or visible light illumination, have been investigated in detail. Finally, we discovered that the degradation of few-layer BP flakes of
- Published
- 2017
33. Raman spectroscopy of intercalated and misfit layer nanotubes
- Author
-
Christian Thomsen, Thomas Heine, Vladimir Bacic, Matthias Staiger, Roland Gillen, Janina Maultzsch, Gal Radovsky, Konstantin Gartsman, and Reshef Tenne
- Subjects
symbols.namesake ,Materials science ,Analytical chemistry ,symbols ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,0210 nano-technology ,Raman spectroscopy ,01 natural sciences ,Layer (electronics) ,0104 chemical sciences - Published
- 2016
34. Light-Matter Interactions in Two-Dimensional Transition Metal Dichalcogenides: Dominant Excitonic Transitions in mono- and few-layer MoX$_2$ and Band Nesting
- Author
-
Roland Gillen and Janina Maultzsch
- Subjects
Physics ,Condensed Matter - Materials Science ,Absorption spectroscopy ,Condensed matter physics ,Exciton ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,Fermi energy ,Optical polarization ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,Brillouin zone ,Condensed Matter::Materials Science ,Ab initio quantum chemistry methods ,0103 physical sciences ,Monolayer ,Density of states ,Electrical and Electronic Engineering ,010306 general physics ,0210 nano-technology - Abstract
We report ab initio calculations of the dielectric function of six mono- and bilayer molybdenum dichalcogenides based in a Bethe Salpether equation+G$_0$W$_0$ ansatz, focussing on the excitonic transitions dominating the absorption spectrum up to an excitation energy of 3\,eV. Our calculations suggest that switching chalcogen atoms and the strength of interlayer interactions should affect the detailed composition of the high 'C' peaks in experimental optical spectra of molybdenum dichalcogenides and cause a significant spin-orbit-splitting of the contributing excitonic transitions in monolayer MoSe$_2$ and MoTe$_2$. This can be explained through changes in the electronic dispersion around the Fermi energy along the chalcogen series S$\rightarrow$Se$\rightarrow$Te that move the van-Hove singularities in the density of states of the two-dimensional materials along the \textit{$\Gamma$}-\textit{K} line in the Brillouin zone. Further, we confirm the distinct interlayer character of the '\textsl{C}' peak transition in few-layer MoS$_2$ that was predicted before from experimental data and show that a similar behaviour can be expected for MoSe$_2$ and MoTe$_2$ as well., Comment: Submitted manuscript + supplementary material
- Published
- 2016
35. Revealing the origin of high-energy Raman local mode in nitrogen doped ZnO nanowires
- Author
-
Roland Gillen, Axel Hoffmann, Cuong Ton-That, Liangchen Zhu, Sevak Khachadorian, Janina Maultzsch, and Matthew R. Phillips
- Subjects
Materials science ,Scanning electron microscope ,Phonon ,Doping ,Nanowire ,Nanotechnology ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Acceptor ,Molecular physics ,symbols.namesake ,0103 physical sciences ,symbols ,General Materials Science ,Density functional theory ,010306 general physics ,0210 nano-technology ,Raman spectroscopy ,Raman scattering ,Applied Physics - Abstract
Scanning electron microscopy (SEM) image of ZnONWs. Raman scattering experiments complemented by density functional theory (DFT) calculations of phonon frequencies have been performed to understand the origin of observed high-energy local Raman modes at 2269 cm–1 and 2282 cm–1 on N-plasma treated ZnO nanowires (NWs). We show that these modes increase in intensity with prolonged N-plasma treatment. Our results reveal that the origin of the high-energy Raman local mode is a loosely bound N2 molecule in the vicinity of a zinc vacancy, which according to our latest work acts as a shallow acceptor and leads to the donor-acceptor-pair transition at 3.232 eV [Phys. Rev. B 92, 024103 (2015)]. Moreover the results provide a more thorough description of nitrogen related complexes in ZnO NWs. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)
- Published
- 2016
36. The screening effects of the screened exchange hybrid functional in surface systems: A case study on the CO/Pt(111) problem
- Author
-
Huanglong Li, John Robertson, Roland Gillen, and Apollo - University of Cambridge Repository
- Subjects
Work (thermodynamics) ,010304 chemical physics ,Band gap ,Chemistry ,General Physics and Astronomy ,Crystal structure ,01 natural sciences ,lcsh:QC1-999 ,Hybrid functional ,Metal ,Adsorption ,Transition metal ,Computational chemistry ,Chemical physics ,visual_art ,0103 physical sciences ,visual_art.visual_art_medium ,Molecule ,010306 general physics ,51 Physical Sciences ,lcsh:Physics ,40 Engineering - Abstract
The screened exchange (sX) hybrid functional has been widely used in computational material science. Although it has widely been studied in bulk systems, less is known about its functional behavior in surface systems which are crucial to many technologies such as materials synthesis and nano-electronic devices. Assessing the screening dependent functional behaviors in the surface systems is therefore important for its application in such systems. In this work, we investigate the screening effects of the sX in CO adsorption on Pt(111) surface. The differences between the sX and Heyd-Scuseria-Ernzerhof (HSE06) hybrid functionals, and the effects of screening parameters are studied. The screening has two effects: first, the HOMO-LUMO gap is screening dependent. This affects the site preference most significantly. In this work, atop adsorption of CO/Pt(111) is predicted by the hybrid functionals with screened exchange potential. The sX(1.44) gives the largest HOMO-LUMO gap for the isolated CO molecule. The adsorption energy difference between the atop and fcc site is also the largest by the sX(1.44) which is explained by the reduced metal d states to the CO 2π* state back-donation, with stronger effect for the fcc adsorption than for the atop adsorption; second, the adsorption energy is screening dependent. This can be seen by comparing the sX(2.38) and HSE06 which have different screening strengths. They show similar surface band structures for the CO adsorption but different adsorption energies, which is explained by the stronger CO 5σ state to the metal d states donation or the effectively screened Pauli repulsion. This work underlines the screening strength as a main difference between sX and HSE06, as well as an important hybrid functional parameter for surface calculation.
- Published
- 2016
37. Advances in understanding of transparent conducting oxides
- Author
-
Stewart J. Clark, Roland Gillen, and John Robertson
- Subjects
Materials science ,Dopant ,Condensed matter physics ,Band gap ,Doping ,Metals and Alloys ,Fermi energy ,Nanotechnology ,02 engineering and technology ,Surfaces and Interfaces ,021001 nanoscience & nanotechnology ,01 natural sciences ,Semimetal ,Band offset ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,0103 physical sciences ,Materials Chemistry ,Density of states ,Vacuum level ,010306 general physics ,0210 nano-technology - Abstract
The band structures of some transparent conducting oxides are calculated using the screened exchange hybrid functional. The optical properties and band gaps of the CuAlO 2 defossalite family follow the expected chemical trends. The limits to the doping of n- and p-type oxides are examined in terms of the dopant compensation by native defects. The accessible range of the Fermi energy under doping is that for which compensating native defects have a positive formation energy. These energy limits are aligned on a band offset diagram. N-dopable oxides have a conduction band minimum that lies deep below the vacuum level, while p-dopable oxides have a valence band top that lies high towards the vacuum level. The nature of electron conduction in amorphous InGaZnO x type oxides is discussed, in terms of the nature of localisation and the density of states at the mobility edge. © 2011 Elsevier B.V. All rights reserved.
- Published
- 2012
38. The screened-exchange approximation as alternative method for DFT calculations on graphene structures
- Author
-
Roland Gillen and John Robertson
- Subjects
Materials science ,Condensed matter physics ,Band gap ,Graphene ,Fermi energy ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,law.invention ,Zigzag ,law ,Density functional theory ,Electronic band structure ,Ground state ,Graphene nanoribbons - Abstract
We present results from calculations of the electronical band structures of graphene and zigzag graphene nanoribbons employing the sX-LDA approximation to the electron–electron interaction. The sX-LDA functional includes non-local electron exchange and is thus expected to yield more accurate results than standard LDA or GGA functionals. We show that sX-LDA can make up for the shortcomings of LDA in terms of the electronical band structure of graphene materials and can correctly describe the value of the Fermi velocity in graphene and improvements on the spin-polarized ground state in zigzag nanoribbons and the resulting spin-induced band gaps.
- Published
- 2010
39. Raman-active modes in graphene nanoribbons
- Author
-
Marcel Mohr, Roland Gillen, and Janina Maultzsch
- Subjects
Materials science ,Condensed matter physics ,Phonon ,Graphene ,Condensed Matter Physics ,Symmetry (physics) ,Spectral line ,Electronic, Optical and Magnetic Materials ,law.invention ,symbols.namesake ,Zigzag ,law ,symbols ,Raman spectroscopy ,Graphene nanoribbons - Abstract
We present the symmetry representations of the Raman-active modes in graphene nanoribbons (GNRs) with pure armchair (AGNR) or zigzag edges (ZGNR) and explain the representations in terms of fundamental modes and overtones in the Γ-point phonon spectrum of GNRs. This provides possible means to assign Raman peaks in measured polarization-dependent spectra. We show simulated Raman spectra in zz-polarization for a 7-AGNR and a 4-ZGNR.
- Published
- 2010
40. Lattice vibrations in graphene nanoribbons from density functional theory
- Author
-
Marcel Mohr, Janina Maultzsch, Christian Thomsen, and Roland Gillen
- Subjects
Materials science ,Condensed matter physics ,Graphene ,Phonon ,Context (language use) ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,law.invention ,Brillouin zone ,Optical properties of carbon nanotubes ,Condensed Matter::Materials Science ,Zigzag ,law ,Condensed Matter::Superconductivity ,Condensed Matter::Strongly Correlated Electrons ,Density functional theory ,Physics::Chemical Physics ,Graphene nanoribbons - Abstract
This work shows results of our investigation of the lattice vibrations in graphene nanoribbons with zigzag and armchair type edges, which we performed by the use of density functional theory. The boundary conditions induced by the nanoribbon edges allow for an interpretation of the phonons as fundamental modes and their respective overtones. The calculated phonons show a characteristic dependence on the nanoribbon width. In this context, we considered a possible influence of the antiferromagnetic ground state of zigzag nanoribbons with respect to the phonons. Furthermore, we demonstrate that a mapping of the calculated r-point phonon frequencies of nanoribbons onto the phonon dispersion of graphene corresponds to an "unfolding" of the nanoribbons' Brillouin zone onto that of graphene and yields good agreement. We further show symmetry properties of armchair and zigzag nanoribbons and full phonon dispersions for small-width nanoribbons.
- Published
- 2009
41. Beyond double-resonant Raman scattering: Ultraviolet Raman spectroscopy on graphene, graphite, and carbon nanotubes
- Author
-
Roland Gillen, Christoph Tyborski, Janina Maultzsch, and Felix Herziger
- Subjects
Materials science ,FOS: Physical sciences ,chemistry.chemical_element ,02 engineering and technology ,Carbon nanotube ,medicine.disease_cause ,01 natural sciences ,law.invention ,Condensed Matter::Materials Science ,symbols.namesake ,law ,0103 physical sciences ,medicine ,Graphite ,010306 general physics ,Condensed Matter - Materials Science ,business.industry ,Graphene ,Materials Science (cond-mat.mtrl-sci) ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,chemistry ,symbols ,Optoelectronics ,0210 nano-technology ,business ,Raman spectroscopy ,Carbon ,Raman scattering ,Ultraviolet ,Excitation - Abstract
We present an analysis of deep-UV Raman measurements of graphite, graphene and carbon nanotubes. For excitation energies above the strong optical absorption peak at the $M$ point in the Brillouin zone ($\approx 4.7\,\text{eV}$), we partially suppress double-resonant scattering processes and observe the two-phonon density of states of carbon nanomaterials. The measured peaks are assigned to contributions from LO, TO, and LA phonon branches, supported by calculations of the phonon dispersion. Moreover, we gain access to the infrared-active $E_{1u}$ mode in graphite. By lowering the excitation energy and thus allowing double-resonant scattering processes, we demonstrate the rise of the \textit{2D} mode in graphite with ultra-short phonon wave vectors.
- Published
- 2015
42. Interlayer resonant Raman modes in few-layerMoS2
- Author
-
Nils Scheuschner, Matthias Staiger, Roland Gillen, and Janina Maultzsch
- Subjects
Physics ,symbols.namesake ,Condensed matter physics ,symbols ,Thin film ,Condensed Matter Physics ,Raman spectroscopy ,Layer (electronics) ,Group theory ,Electronic, Optical and Magnetic Materials ,Metrology - Abstract
Two new first-order Raman-active modes are experimentally observed in few-layer MoS${}_{2}$ samples. The authors of this paper show that these modes should appear in virtually all few-layer systems. Since these new modes are symmetry-forbidden in single-layer structures, the results are going to be useful for metrology of layered thin films.
- Published
- 2015
43. Splitting of monolayer out-of-planeA1′Raman mode in few-layerWS2
- Author
-
Marika Schleberger, Oliver Ochedowski, Christian Thomsen, Nils Scheuschner, Felix Kampmann, Matthias Staiger, Janina Maultzsch, and Roland Gillen
- Subjects
Physics ,Condensed matter physics ,Phonon ,Resonance ,Nanotechnology ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,symbols.namesake ,Homogeneous space ,Monolayer ,symbols ,Density functional theory ,Raman spectroscopy ,Excitation ,Energy (signal processing) - Abstract
We present Raman measurements of mono- and few-layer ${\mathrm{WS}}_{2}$. We study the monolayer ${A}_{1}^{\ensuremath{'}}$ mode around $420{\mathrm{cm}}^{\ensuremath{-}1}$ and its evolution with the number of layers. We show that with increasing layer number there are an increasing number of possible vibrational patterns for the out-of-plane Raman mode: in $N$-layer ${\mathrm{WS}}_{2}$ there are $N$ $\mathrm{\ensuremath{\Gamma}}$-point phonons evolving from the ${A}_{1}^{\ensuremath{'}}$ monolayer mode. For an excitation energy close to resonance with the $A$ excitonic transition energy, we were able to observe all of these $N$ components, irrespective of their Raman activity. Density functional theory calculations support the experimental findings and make it possible to attribute the modes to their respective symmetries. The findings described here are of general importance for all other phonon modes in ${\mathrm{WS}}_{2}$ and other layered transition-metal dichalcogenide systems in the few-layer regime.
- Published
- 2015
44. Electronic structure of lanthanide oxide high K gate oxides
- Author
-
John Robertson and Roland Gillen
- Subjects
Lanthanide ,Materials science ,Band gap ,Rare earth ,Inorganic chemistry ,Oxide ,Electronic structure ,Condensed Matter Physics ,Atomic and Molecular Physics, and Optics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Threshold voltage ,Metal ,chemistry.chemical_compound ,chemistry ,Chemical physics ,visual_art ,visual_art.visual_art_medium ,Electrical and Electronic Engineering ,High-κ dielectric - Abstract
Graphical abstractDisplay Omitted Electronic structure of LnOx oxides.Explaining when 4f states enter band gap.Simplest first principles calculation of Ln oxides to date.How to choose Ln oxides for high K application. Lanthanide (rare earth) oxides are used in gate stacks as high K gate oxides and for threshold voltage control. It is important that such oxides do not have states in the main band gap due to localized 4f states. We present hybrid density functional calculations of the oxides which give an electronic structure in agreement with previous empirical models and that show which metal oxides have gap states.
- Published
- 2013
45. Defect densities inside the conductive filament of RRAMs
- Author
-
John Robertson and Roland Gillen
- Subjects
Resistive touchscreen ,Materials science ,Condensed matter physics ,Oxide ,Nanotechnology ,Condensed Matter Physics ,Atomic and Molecular Physics, and Optics ,Oxygen vacancy ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Resistive random-access memory ,Formalism (philosophy of mathematics) ,chemistry.chemical_compound ,chemistry ,Vacancy defect ,Conductive filament ,Electrical and Electronic Engineering - Abstract
Graphical abstractHighly localized oxygen vacancy state as calculated by screened exchange.Display Omitted Estimate of defect densities in the conductive filament of an HfO2 RRAM.Discussion of defect localisation and its effects on percolation threshold.Defect states in HfO2. Oxide-based resistive random access memories (RRAM) are based on a conductive filament of oxygen vacancies across the film. It is pointed out that the density of vacancies must be high in order to form a percolating path due to the highly localised nature of the vacancy defect orbital, which is much more localised than that given by local density formalism.
- Published
- 2013
46. Indirect doping effects from impurities inMoS2/h-BNheterostructures
- Author
-
Janina Maultzsch, Roland Gillen, and John Robertson
- Subjects
Materials science ,FOS: Physical sciences ,02 engineering and technology ,Substrate (electronics) ,01 natural sciences ,Condensed Matter::Materials Science ,chemistry.chemical_compound ,Impurity ,Condensed Matter::Superconductivity ,Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ,0103 physical sciences ,010306 general physics ,Condensed Matter - Mesoscale and Nanoscale Physics ,Dopant ,Condensed matter physics ,Doping ,Heterojunction ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,chemistry ,Boron nitride ,Condensed Matter::Strongly Correlated Electrons ,Density functional theory ,0210 nano-technology ,Ground state - Abstract
We performed density functional theory calculations on heterostructures of single layers of hexagonal BN and MoS$_2$ to assess the effect of doping in the BN sheet and of interstitial Na atoms on the electronic properties of the adjacent MoS$_2$ layer. Our calculations predict that $n$-doping of the boron nitride subsystem by oxygen, carbon and sulfur impurities causes noticeable charge transfer into the conduction band of the MoS$_2$ sheet, while $p$-doping by beryllium and carbon leaves the molybdenum disulphide layer largely unaffected. Intercalated sodium atoms lead to a significant increase of the interlayer distance in the heterostructure and to a metallic ground state of the MoS$_2$ subsystem. The presence of such $n$-dopants leads to a distinct change of valence band and conduction band offsets, suggesting that doped BN remains a suitable substrate and gate material for applications of $n$-type MoS$_2$., Comment: 8 pages, 7 figures
- Published
- 2014
47. Photoluminescence of freestanding single- and few-layer MoS 2
- Author
-
Roland Gillen, Oliver Ochedowski, Marika Schleberger, Nils Scheuschner, Janina Maultzsch, and Anne-Marie Kaulitz
- Subjects
Condensed Matter - Materials Science ,Photoluminescence ,Materials science ,Condensed matter physics ,business.industry ,Exciton ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,Substrate (electronics) ,Physik (inkl. Astronomie) ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,symbols.namesake ,symbols ,Optoelectronics ,Direct and indirect band gaps ,Photoluminescence excitation ,van der Waals force ,Trion ,business ,Energy (signal processing) - Abstract
We present a photoluminescence study of freestanding and Si/${\text{SiO}}_{2}$ supported single- and few-layer ${\text{MoS}}_{2}$. The single-layer exciton peak (A) is only observed in freestanding ${\text{MoS}}_{2}$. The photoluminescence of supported single-layer ${\text{MoS}}_{2}$ instead originates from the A${}^{\ensuremath{-}}$ (trion) peak as the ${\text{MoS}}_{2}$ is $n$-type doped from the substrate. In bilayer ${\text{MoS}}_{2}$, the van der Waals interaction with the substrate decreases the indirect band gap energy by up to $\ensuremath{\approx}80$ meV. Furthermore, the photoluminescence spectra of suspended ${\text{MoS}}_{2}$ can be influenced by interference effects.
- Published
- 2014
48. Rücktitelbild: Few-Layer Antimonene by Liquid-Phase Exfoliation (Angew. Chem. 46/2016)
- Author
-
Maria Varela, Roland Gillen, Carlos Gibaja, Janina Maultzsch, Félix Zamora, Pablo Ares, Julio Gómez-Herrero, David Rodríguez-San-Miguel, Frank Hauke, Andreas Hirsch, and Gonzalo Abellán
- Subjects
Materials science ,Chemical engineering ,Liquid phase ,02 engineering and technology ,General Medicine ,010402 general chemistry ,021001 nanoscience & nanotechnology ,0210 nano-technology ,01 natural sciences ,Layer (electronics) ,Exfoliation joint ,0104 chemical sciences - Published
- 2016
49. Back Cover: Few-Layer Antimonene by Liquid-Phase Exfoliation (Angew. Chem. Int. Ed. 46/2016)
- Author
-
Frank Hauke, Félix Zamora, Pablo Ares, Carlos Gibaja, David Rodríguez-San-Miguel, Julio Gómez-Herrero, Andreas Hirsch, Roland Gillen, Maria Varela, Gonzalo Abellán, and Janina Maultzsch
- Subjects
010302 applied physics ,Materials science ,Atomic force microscopy ,Liquid phase ,02 engineering and technology ,General Chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Exfoliation joint ,Catalysis ,symbols.namesake ,Chemical engineering ,0103 physical sciences ,symbols ,Cover (algebra) ,0210 nano-technology ,Raman spectroscopy ,Layer (electronics) - Published
- 2016
50. Nature of the electronic band gap in lanthanide oxides
- Author
-
Roland Gillen, Stewart J. Clark, and John Robertson
- Subjects
Physics ,Lanthanide ,Condensed Matter - Materials Science ,Period (periodic table) ,Band gap ,Electronic band ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,Nanotechnology ,Electron ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Functional methods ,Atomic physics - Abstract
Accurate electronic structures of the technologically important lanthanide/rare earth sesquioxides (Ln2O3, with Ln=La,...,Lu) and CeO2 have been calculated using hybrid density functionals HSE03, HSE06 and screened-exchange (sX-LDA). We find that these density functional methods describe the strongly correlated Ln f-electrons as well as the recent G0W0@LDA+U results, generally yielding the correct band gaps and trends across the Ln-period. For HSE, the band gap between O 2p states and lanthanide 5d states is nearly independent of the lanthanide, while the minimum gap varies as filled or empty Ln 4f states come into this gap. sX-LDA predicts the unoccupied 4f levels at higher energies, which leads to a better agreement with experiments for Sm2O3, Eu2O3 and Yb2O3., 7 pages, 4 figures, 2 tables
- Published
- 2013
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