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Elias C, Valvin P, Pelini T, Summerfield A, Mellor CJ, Cheng TS, Eaves L, Foxon CT, Beton PH, Novikov SV, Gil B, Cassabois G. Direct band-gap crossover in epitaxial monolayer boron nitride. Nat Commun 2019; 10:2639. [PMID: 31201328 PMCID: PMC6572751 DOI: 10.1038/s41467-019-10610-5] [Citation(s) in RCA: 48] [Impact Index Per Article: 9.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/25/2019] [Accepted: 05/22/2019] [Indexed: 12/01/2022] Open
Abstract
Hexagonal boron nitride is a large band-gap insulating material which complements the electronic and optical properties of graphene and the transition metal dichalcogenides. However, the intrinsic optical properties of monolayer boron nitride remain largely unexplored. In particular, the theoretically expected crossover to a direct-gap in the limit of the single monolayer is presently not confirmed experimentally. Here, in contrast to the technique of exfoliating few-layer 2D hexagonal boron nitride, we exploit the scalable approach of high-temperature molecular beam epitaxy to grow high-quality monolayer boron nitride on graphite substrates. We combine deep-ultraviolet photoluminescence and reflectance spectroscopy with atomic force microscopy to reveal the presence of a direct gap of energy 6.1 eV in the single atomic layers, thus confirming a crossover to direct gap in the monolayer limit.
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Affiliation(s)
- C Elias
- Laboratoire Charles Coulomb, UMR5221 CNRS-Université de Montpellier, 34095, Montpellier, France
| | - P Valvin
- Laboratoire Charles Coulomb, UMR5221 CNRS-Université de Montpellier, 34095, Montpellier, France
| | - T Pelini
- Laboratoire Charles Coulomb, UMR5221 CNRS-Université de Montpellier, 34095, Montpellier, France
| | - A Summerfield
- School of Physics and Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - C J Mellor
- School of Physics and Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - T S Cheng
- School of Physics and Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - L Eaves
- School of Physics and Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - C T Foxon
- School of Physics and Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - P H Beton
- School of Physics and Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - S V Novikov
- School of Physics and Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - B Gil
- Laboratoire Charles Coulomb, UMR5221 CNRS-Université de Montpellier, 34095, Montpellier, France
| | - G Cassabois
- Laboratoire Charles Coulomb, UMR5221 CNRS-Université de Montpellier, 34095, Montpellier, France.
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Albar JD, Summerfield A, Cheng TS, Davies A, Smith EF, Khlobystov AN, Mellor CJ, Taniguchi T, Watanabe K, Foxon CT, Eaves L, Beton PH, Novikov SV. An atomic carbon source for high temperature molecular beam epitaxy of graphene. Sci Rep 2017; 7:6598. [PMID: 28747805 PMCID: PMC5529545 DOI: 10.1038/s41598-017-07021-1] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2017] [Accepted: 06/20/2017] [Indexed: 12/04/2022] Open
Abstract
We report the use of a novel atomic carbon source for the molecular beam epitaxy (MBE) of graphene layers on hBN flakes and on sapphire wafers at substrate growth temperatures of ~1400 °C. The source produces a flux of predominantly atomic carbon, which diffuses through the walls of a Joule-heated tantalum tube filled with graphite powder. We demonstrate deposition of carbon on sapphire with carbon deposition rates up to 12 nm/h. Atomic force microscopy measurements reveal the formation of hexagonal moiré patterns when graphene monolayers are grown on hBN flakes. The Raman spectra of the graphene layers grown on hBN and sapphire with the sublimation carbon source and the atomic carbon source are similar, whilst the nature of the carbon aggregates is different - graphitic with the sublimation carbon source and amorphous with the atomic carbon source. At MBE growth temperatures we observe etching of the sapphire wafer surface by the flux from the atomic carbon source, which we have not observed in the MBE growth of graphene with the sublimation carbon source.
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Affiliation(s)
- J D Albar
- School of Physics & Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - A Summerfield
- School of Physics & Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - T S Cheng
- School of Physics & Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - A Davies
- School of Physics & Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK.,Nanoscale and microscale research centre (NMRC) and School of Chemistry, University of Nottingham, Nottingham, NG7 2RD, UK
| | - E F Smith
- Nanoscale and microscale research centre (NMRC) and School of Chemistry, University of Nottingham, Nottingham, NG7 2RD, UK
| | - A N Khlobystov
- Nanoscale and microscale research centre (NMRC) and School of Chemistry, University of Nottingham, Nottingham, NG7 2RD, UK
| | - C J Mellor
- School of Physics & Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - T Taniguchi
- The National Institute for Materials Science, Advanced Materials Laboratory, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan
| | - K Watanabe
- The National Institute for Materials Science, Advanced Materials Laboratory, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan
| | - C T Foxon
- School of Physics & Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - L Eaves
- School of Physics & Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - P H Beton
- School of Physics & Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK
| | - S V Novikov
- School of Physics & Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK.
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Wang JY, Suddards ME, Mellor CJ, Owers-Bradley JR. Lung function measurement with multiple-breath-helium washout system. Med Eng Phys 2012; 35:457-69. [PMID: 22835436 DOI: 10.1016/j.medengphy.2012.06.010] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2011] [Revised: 06/11/2012] [Accepted: 06/16/2012] [Indexed: 10/28/2022]
Abstract
Multiple-breath-washout (MBW) measurements are regarded as a sensitive technique which can reflect the ventilation inhomogeneity of respiratory airways. Typically nitrogen is used as the tracer gas and is washed out by pure oxygen in multiple-breath-nitrogen washout (MBNW) tests. In this study, instead of using nitrogen, (4)He is used as the tracer gas with smaller gas density which may be able to reach deeper into our lungs in a given time and the helium washout results may be more sensitive to the ventilation inhomogeneity in small airways. A multiple-breath-helium-washout (MBHW) system developed for the lung function study is also presented. Quartz tuning forks with a resonance frequency of 32,768Hz have been used for detecting the change of the respiratory gas density. The resonance frequency of the quartz tuning fork decreases linearly with increasing density of the surrounding gas. Knowing the CO2 concentration from the infrared carbon dioxide detector, the helium concentration can be determined. Results from 14 volunteers (3 mild asthmatics, 4 tobacco smokers, 1 with asthma history, 1 with COPD history, 5 normal) have shown that mild asthmatics have higher ventilation inhomogeneity in either conducting or acinar airways (or both). A feature has been found in washout curve of single breaths from 4 tobacco smokers with different length of smoking history which may indicate the early stage of respiratory ventilation inhomogeneity in acinar airways.
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Affiliation(s)
- J-Y Wang
- School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK.
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Baumgartner A, Suddards ME, Mellor CJ. Low-temperature and high magnetic field dynamic scanning capacitance microscope. Rev Sci Instrum 2009; 80:013704. [PMID: 19191438 DOI: 10.1063/1.3069289] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
We demonstrate a dynamic scanning capacitance microscope (DSCM) that operates at large bandwidths, cryogenic temperatures, and high magnetic fields. The setup is based on a noncontact atomic force microscope (AFM) with a quartz tuning fork sensor for the nonoptical excitation and readout in topography, force, and dissipation measurements. The metallic AFM tip forms part of a rf resonator with a transmission characteristics modulated by the sample properties and the tip-sample capacitance. The tip motion gives rise to a modulation of the capacitance at the frequency of the AFM sensor and its harmonics, which can be recorded simultaneously with the AFM data. We use an intuitive model to describe and analyze the resonator transmission and show that for most experimental conditions it is proportional to the complex tip-sample conductance, which depends on both the tip-sample capacitance and the sample resistivity. We demonstrate the performance of the DSCM on metal disks buried under a polymer layer and we discuss images recorded on a two-dimensional electron gas in the quantum Hall effect regime, i.e. at cryogenic temperatures and in high magnetic fields, where we directly image the formation of compressible stripes at the physical edge of the sample.
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Affiliation(s)
- A Baumgartner
- School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom.
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Leturcq R, L'Hôte D, Tourbot R, Mellor CJ, Henini M. Resistance noise scaling in a dilute two-dimensional hole system in GaAs. Phys Rev Lett 2003; 90:076402. [PMID: 12633254 DOI: 10.1103/physrevlett.90.076402] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/24/2002] [Indexed: 05/24/2023]
Abstract
We have measured the resistance noise of a two-dimensional (2D) hole system in a high mobility GaAs quantum well, around the 2D metal-insulator transition (MIT) at zero magnetic field. The normalized noise power S(R)/R(2) increases strongly when the hole density p(s) is decreased, increases slightly with temperature (T) at the largest densities, and decreases strongly with T at low p(s). The noise scales with the resistance, S(R)/R(2) approximately R2.4, as for a second order phase transition such as a percolation transition. The p(s) dependence of the conductivity is consistent with a critical behavior for such a transition, near a density p(*) which is lower than the observed MIT critical density p(c).
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Affiliation(s)
- R Leturcq
- Service de Physique de l'Etat Condensé, CEA/DSM, CE Saclay, F-91191 Gif-sur-Yvette, France
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Mellor CJ, Eyles RH, Digby JE, Kent AJ, Benedict KA, Challis LJ, Henini M, Foxon CT, Harris JJ. Phonon absorption at the magnetoroton minimum in the fractional quantum Hall effect. Phys Rev Lett 1995; 74:2339-2342. [PMID: 10057903 DOI: 10.1103/physrevlett.74.2339] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
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Cooper J, Ouali FF, Challis LJ, Mellor CJ. Quantum oscillations in the cyclotron phonon emission from a heated two-dimensional electron gas. Phys Rev B Condens Matter 1995; 51:7085-7089. [PMID: 9977267 DOI: 10.1103/physrevb.51.7085] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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Kent AJ, McKitterick DJ, Challis LJ, Hawker P, Mellor CJ, Henini M. Imaging nonequilibrium phonon-induced backscattering in the quantum Hall regime. Phys Rev Lett 1992; 69:1684-1686. [PMID: 10046287 DOI: 10.1103/physrevlett.69.1684] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
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Akimov AV, Challis LJ, Cooper J, Mellor CJ, Moskalenko ES. Phonon emission from the first and second subbands of a two-dimensional electron gas in silicon detected by exciton luminescence. Phys Rev B Condens Matter 1992; 45:11387-11390. [PMID: 10001077 DOI: 10.1103/physrevb.45.11387] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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