Identification of individual and few layers of WS2 using Raman Spectroscopy
Identifieur interne : 000315 ( Pmc/Checkpoint ); précédent : 000314; suivant : 000316Identification of individual and few layers of WS2 using Raman Spectroscopy
Auteurs : Ayse Berkdemir [États-Unis] ; Humberto R. Gutiérrez [États-Unis] ; Andrés R. Botello-Méndez [Belgique] ; Néstor Perea-L Pez [États-Unis] ; Ana Laura Elías [États-Unis] ; Chen-Ing Chia [États-Unis] ; Bei Wang [États-Unis] ; Vincent H. Crespi [États-Unis] ; Florentino L Pez-Urías [États-Unis, Mexique] ; Jean-Christophe Charlier [Belgique] ; Humberto Terrones [États-Unis] ; Mauricio Terrones [États-Unis, Japon]Source :
- Scientific Reports [ 2045-2322 ] ; 2013.
Abstract
The Raman scattering of single- and few-layered WS2 is studied as a function of the number of S-W-S layers and the excitation wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A1g(Γ) phonon mode monotonically decreases with the number of layers. For single-layer WS2, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode (LA(M)). This resonance results from a coupling between the electronic band structure and lattice vibrations. First-principles calculations were used to determine the electronic and phonon band structures of single-layer and bulk WS2. The reduced intensity of the 2LA mode was then computed, as a function of the laser wavelength, from the fourth-order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS2 films.
Url:
DOI: 10.1038/srep01755
PubMed: NONE
PubMed Central: 3639451
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Crespi</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation></author><author><name sortKey="L Pez Urias, Florentino" sort="L Pez Urias, Florentino" uniqKey="L Pez Urias F" first="Florentino" last="L Pez-Urías">Florentino L Pez-Urías</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation><affiliation wicri:level="1"><nlm:aff id="a6">Current address: On leave from Advanced Materials Department, IPICYT, Camino a Presa San José 2055, Col. Lomas 4a Sección, San Luis Potosí, México.</nlm:aff><country xml:lang="fr" wicri:curation="lc">Mexique</country><wicri:regionArea>Current address: On leave from Advanced Materials Department, IPICYT, Camino a Presa San José 2055, Col. Lomas 4a Sección, San Luis Potosí</wicri:regionArea><wicri:noRegion>San Luis Potosí</wicri:noRegion></affiliation></author><author><name sortKey="Charlier, Jean Christophe" sort="Charlier, Jean Christophe" uniqKey="Charlier J" first="Jean-Christophe" last="Charlier">Jean-Christophe Charlier</name><affiliation wicri:level="3"><nlm:aff id="a2"><institution>Institute of Condensed Matter and Nanosciences (IMCN), Université catholique de Louvain (UCL)</institution>, Chemin des étoiles 8, 1348 Louvain-la-Neuve, Belgium</nlm:aff><country xml:lang="fr">Belgique</country><wicri:regionArea>, Chemin des étoiles 8, 1348 Louvain-la-Neuve</wicri:regionArea><placeName><region type="land" nuts="2">Vienne (Autriche)</region><settlement type="city">Vienne (Autriche)</settlement></placeName></affiliation></author><author><name sortKey="Terrones, Humberto" sort="Terrones, Humberto" uniqKey="Terrones H" first="Humberto" last="Terrones">Humberto Terrones</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation></author><author><name sortKey="Terrones, Mauricio" sort="Terrones, Mauricio" uniqKey="Terrones M" first="Mauricio" last="Terrones">Mauricio Terrones</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation><affiliation wicri:level="1"><nlm:aff id="a3"><institution>Department of Materials Science and Engineering and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA & Research Center for Exotic Nanocarbons (JST), Shinshu University</institution>, Wakasato 4-17-1, Nagano 380-853, Japan</nlm:aff><country xml:lang="fr">Japon</country><wicri:regionArea>, Wakasato 4-17-1, Nagano 380-853</wicri:regionArea><wicri:noRegion>Nagano 380-853</wicri:noRegion></affiliation></author></titleStmt><publicationStmt><idno type="wicri:source">PMC</idno><idno type="pmc">3639451</idno><idno type="url">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3639451</idno><idno type="RBID">PMC:3639451</idno><idno type="doi">10.1038/srep01755</idno><idno type="pmid">NONE</idno><date when="2013">2013</date><idno type="wicri:Area/Pmc/Corpus">000031</idno><idno type="wicri:Area/Pmc/Curation">000031</idno><idno type="wicri:Area/Pmc/Checkpoint">000315</idno></publicationStmt><sourceDesc><biblStruct><analytic><title xml:lang="en" level="a" type="main">Identification of individual and few layers of WS<sub>2</sub> using Raman Spectroscopy</title><author><name sortKey="Berkdemir, Ayse" sort="Berkdemir, Ayse" uniqKey="Berkdemir A" first="Ayse" last="Berkdemir">Ayse Berkdemir</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation><affiliation><nlm:aff id="a4">These authors contributed equally to this work.</nlm:aff></affiliation></author><author><name sortKey="Gutierrez, Humberto R" sort="Gutierrez, Humberto R" uniqKey="Gutierrez H" first="Humberto R." last="Gutiérrez">Humberto R. Gutiérrez</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation><affiliation><nlm:aff id="a4">These authors contributed equally to this work.</nlm:aff></affiliation><affiliation wicri:level="2"><nlm:aff id="a5">Current address: Department of Physics & Astronomy, University of Louisville, Louisville, KY 40292 USA.</nlm:aff><country xml:lang="fr">États-Unis</country><placeName><region type="state">Kentucky</region></placeName><wicri:cityArea>Current address: Department of Physics & Astronomy, University of Louisville, Louisville</wicri:cityArea></affiliation></author><author><name sortKey="Botello Mendez, Andres R" sort="Botello Mendez, Andres R" uniqKey="Botello Mendez A" first="Andrés R." last="Botello-Méndez">Andrés R. Botello-Méndez</name><affiliation wicri:level="3"><nlm:aff id="a2"><institution>Institute of Condensed Matter and Nanosciences (IMCN), Université catholique de Louvain (UCL)</institution>, Chemin des étoiles 8, 1348 Louvain-la-Neuve, Belgium</nlm:aff><country xml:lang="fr">Belgique</country><wicri:regionArea>, Chemin des étoiles 8, 1348 Louvain-la-Neuve</wicri:regionArea><placeName><region type="land" nuts="2">Vienne (Autriche)</region><settlement type="city">Vienne (Autriche)</settlement></placeName></affiliation></author><author><name sortKey="Perea L Pez, Nestor" sort="Perea L Pez, Nestor" uniqKey="Perea L Pez N" first="Néstor" last="Perea-L Pez">Néstor Perea-L Pez</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation></author><author><name sortKey="Elias, Ana Laura" sort="Elias, Ana Laura" uniqKey="Elias A" first="Ana Laura" last="Elías">Ana Laura Elías</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation></author><author><name sortKey="Chia, Chen Ing" sort="Chia, Chen Ing" uniqKey="Chia C" first="Chen-Ing" last="Chia">Chen-Ing Chia</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation></author><author><name sortKey="Wang, Bei" sort="Wang, Bei" uniqKey="Wang B" first="Bei" last="Wang">Bei Wang</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation></author><author><name sortKey="Crespi, Vincent H" sort="Crespi, Vincent H" uniqKey="Crespi V" first="Vincent H." last="Crespi">Vincent H. Crespi</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation></author><author><name sortKey="L Pez Urias, Florentino" sort="L Pez Urias, Florentino" uniqKey="L Pez Urias F" first="Florentino" last="L Pez-Urías">Florentino L Pez-Urías</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation><affiliation wicri:level="1"><nlm:aff id="a6">Current address: On leave from Advanced Materials Department, IPICYT, Camino a Presa San José 2055, Col. Lomas 4a Sección, San Luis Potosí, México.</nlm:aff><country xml:lang="fr" wicri:curation="lc">Mexique</country><wicri:regionArea>Current address: On leave from Advanced Materials Department, IPICYT, Camino a Presa San José 2055, Col. Lomas 4a Sección, San Luis Potosí</wicri:regionArea><wicri:noRegion>San Luis Potosí</wicri:noRegion></affiliation></author><author><name sortKey="Charlier, Jean Christophe" sort="Charlier, Jean Christophe" uniqKey="Charlier J" first="Jean-Christophe" last="Charlier">Jean-Christophe Charlier</name><affiliation wicri:level="3"><nlm:aff id="a2"><institution>Institute of Condensed Matter and Nanosciences (IMCN), Université catholique de Louvain (UCL)</institution>, Chemin des étoiles 8, 1348 Louvain-la-Neuve, Belgium</nlm:aff><country xml:lang="fr">Belgique</country><wicri:regionArea>, Chemin des étoiles 8, 1348 Louvain-la-Neuve</wicri:regionArea><placeName><region type="land" nuts="2">Vienne (Autriche)</region><settlement type="city">Vienne (Autriche)</settlement></placeName></affiliation></author><author><name sortKey="Terrones, Humberto" sort="Terrones, Humberto" uniqKey="Terrones H" first="Humberto" last="Terrones">Humberto Terrones</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation></author><author><name sortKey="Terrones, Mauricio" sort="Terrones, Mauricio" uniqKey="Terrones M" first="Mauricio" last="Terrones">Mauricio Terrones</name><affiliation wicri:level="2"><nlm:aff id="a1"><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</nlm:aff><country xml:lang="fr">États-Unis</country><wicri:regionArea>, PA 16802</wicri:regionArea><placeName><region type="state">Pennsylvanie</region></placeName></affiliation><affiliation wicri:level="1"><nlm:aff id="a3"><institution>Department of Materials Science and Engineering and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA & Research Center for Exotic Nanocarbons (JST), Shinshu University</institution>, Wakasato 4-17-1, Nagano 380-853, Japan</nlm:aff><country xml:lang="fr">Japon</country><wicri:regionArea>, Wakasato 4-17-1, Nagano 380-853</wicri:regionArea><wicri:noRegion>Nagano 380-853</wicri:noRegion></affiliation></author></analytic><series><title level="j">Scientific Reports</title><idno type="eISSN">2045-2322</idno><imprint><date when="2013">2013</date></imprint></series></biblStruct></sourceDesc></fileDesc><profileDesc><textClass></textClass></profileDesc></teiHeader><front><div type="abstract" xml:lang="en"><p>The Raman scattering of single- and few-layered WS<sub>2</sub> is studied as a function of the number of S-W-S layers and the excitation wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A<sub>1g</sub>(Γ) phonon mode monotonically decreases with the number of layers. For single-layer WS<sub>2</sub>, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode (LA(M)). This resonance results from a coupling between the electronic band structure and lattice vibrations. First-principles calculations were used to determine the electronic and phonon band structures of single-layer and bulk WS<sub>2.</sub> The reduced intensity of the 2LA mode was then computed, as a function of the laser wavelength, from the fourth-order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS<sub>2</sub> films.</p></div></front><back><div1 type="bibliography"><listBibl><biblStruct><analytic><author><name sortKey="Geim, A K" uniqKey="Geim A">A. K. 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<front><journal-meta><journal-id journal-id-type="nlm-ta">Sci Rep</journal-id><journal-id journal-id-type="iso-abbrev">Sci Rep</journal-id><journal-title-group><journal-title>Scientific Reports</journal-title></journal-title-group><issn pub-type="epub">2045-2322</issn><publisher><publisher-name>Nature Publishing Group</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="pmc">3639451</article-id><article-id pub-id-type="pii">srep01755</article-id><article-id pub-id-type="doi">10.1038/srep01755</article-id><article-categories><subj-group subj-group-type="heading"><subject>Article</subject></subj-group></article-categories><title-group><article-title>Identification of individual and few layers of WS<sub>2</sub> using Raman Spectroscopy</article-title></title-group><contrib-group><contrib contrib-type="author"><name><surname>Berkdemir</surname><given-names>Ayse</given-names></name><xref ref-type="aff" rid="a1">1</xref><xref ref-type="aff" rid="a4">4</xref></contrib><contrib contrib-type="author"><name><surname>Gutiérrez</surname><given-names>Humberto R.</given-names></name><xref ref-type="aff" rid="a1">1</xref><xref ref-type="aff" rid="a4">4</xref><xref ref-type="aff" rid="a5">5</xref></contrib><contrib contrib-type="author"><name><surname>Botello-Méndez</surname><given-names>Andrés R.</given-names></name><xref ref-type="aff" rid="a2">2</xref></contrib><contrib contrib-type="author"><name><surname>Perea-López</surname><given-names>Néstor</given-names></name><xref ref-type="aff" rid="a1">1</xref></contrib><contrib contrib-type="author"><name><surname>Elías</surname><given-names>Ana Laura</given-names></name><xref ref-type="aff" rid="a1">1</xref></contrib><contrib contrib-type="author"><name><surname>Chia</surname><given-names>Chen-Ing</given-names></name><xref ref-type="aff" rid="a1">1</xref></contrib><contrib contrib-type="author"><name><surname>Wang</surname><given-names>Bei</given-names></name><xref ref-type="aff" rid="a1">1</xref></contrib><contrib contrib-type="author"><name><surname>Crespi</surname><given-names>Vincent H.</given-names></name><xref ref-type="aff" rid="a1">1</xref></contrib><contrib contrib-type="author"><name><surname>López-Urías</surname><given-names>Florentino</given-names></name><xref ref-type="aff" rid="a1">1</xref><xref ref-type="aff" rid="a6">6</xref></contrib><contrib contrib-type="author"><name><surname>Charlier</surname><given-names>Jean-Christophe</given-names></name><xref ref-type="aff" rid="a2">2</xref></contrib><contrib contrib-type="author"><name><surname>Terrones</surname><given-names>Humberto</given-names></name><xref ref-type="aff" rid="a1">1</xref></contrib><contrib contrib-type="author"><name><surname>Terrones</surname><given-names>Mauricio</given-names></name><xref ref-type="corresp" rid="c1">a</xref><xref ref-type="aff" rid="a1">1</xref><xref ref-type="aff" rid="a3">3</xref></contrib><aff id="a1"><label>1</label><institution>Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park</institution>, PA 16802, USA</aff><aff id="a2"><label>2</label><institution>Institute of Condensed Matter and Nanosciences (IMCN), Université catholique de Louvain (UCL)</institution>, Chemin des étoiles 8, 1348 Louvain-la-Neuve, Belgium</aff><aff id="a3"><label>3</label><institution>Department of Materials Science and Engineering and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA & Research Center for Exotic Nanocarbons (JST), Shinshu University</institution>, Wakasato 4-17-1, Nagano 380-853, Japan</aff><aff id="a4"><label>4</label>These authors contributed equally to this work.</aff><aff id="a5"><label>5</label>Current address: Department of Physics & Astronomy, University of Louisville, Louisville, KY 40292 USA.</aff><aff id="a6"><label>6</label>Current address: On leave from Advanced Materials Department, IPICYT, Camino a Presa San José 2055, Col. Lomas 4a Sección, San Luis Potosí, México.</aff></contrib-group><author-notes><corresp id="c1"><label>a</label><email>mut11@psu.edu</email></corresp></author-notes><pub-date pub-type="epub"><day>30</day><month>04</month><year>2013</year></pub-date><pub-date pub-type="collection"><year>2013</year></pub-date><volume>3</volume><elocation-id>1755</elocation-id><history><date date-type="received"><day>12</day><month>12</month><year>2012</year></date><date date-type="accepted"><day>15</day><month>04</month><year>2013</year></date></history><permissions><copyright-statement>Copyright © 2013, Macmillan Publishers Limited. All rights reserved</copyright-statement><copyright-year>2013</copyright-year><copyright-holder>Macmillan Publishers Limited. All rights reserved</copyright-holder><license license-type="open-access" xlink:href="http://creativecommons.org/licenses/by-nc-nd/3.0/"><pmc-comment>author-paid</pmc-comment>
<license-p>This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit <ext-link ext-link-type="uri" xlink:href="http://creativecommons.org/licenses/by-nc-nd/3.0/">http://creativecommons.org/licenses/by-nc-nd/3.0/</ext-link></license-p></license></permissions><abstract><p>The Raman scattering of single- and few-layered WS<sub>2</sub> is studied as a function of the number of S-W-S layers and the excitation wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A<sub>1g</sub>(Γ) phonon mode monotonically decreases with the number of layers. For single-layer WS<sub>2</sub>, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode (LA(M)). This resonance results from a coupling between the electronic band structure and lattice vibrations. First-principles calculations were used to determine the electronic and phonon band structures of single-layer and bulk WS<sub>2.</sub> The reduced intensity of the 2LA mode was then computed, as a function of the laser wavelength, from the fourth-order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS<sub>2</sub> films.</p></abstract></article-meta></front><floats-group><fig id="f1"><label>Figure 1</label><caption><p>(a) SEM image of a monolayer WS<sub>2</sub> triangular island.(b) TEM image of a WS<sub>2</sub> island transferred to a lacey carbon grid with inset showing the electron diffraction pattern of the entire region. (c) High-resolution phase-contrast TEM image showing the hexagonal arrangement of the WS<sub>2</sub> structure with inset showing the Fourier transform of the micrograph. (d) AFM image of a WS<sub>2</sub> island and the corresponding height profile along the dashed line. (e) and (f) Room-temperature Raman spectra from a monolayer WS<sub>2</sub> region, using the 514.5 and 488 nm laser excitation, respectively, including Lorentzian peak fits for 514.5 nm data. The inset in (f) shows the atomic displacements for the in-plane phonon mode E<sup>1</sup><sub>2g</sub>(Γ) and the out-of-plane phonon mode A<sub>1g</sub>(Γ) for two adjacent layers, the dashed line represents the weak inter-layer van der waals interaction.</p></caption><graphic xlink:href="srep01755-f1"></graphic></fig><fig id="f2"><label>Figure 2</label><caption><p>(a) AFM image of a WS<sub>2</sub> triangular island, which varies from four to one layers thick.Also Raman mappings of the same WS<sub>2</sub> island (using 514.5 nm laser excitation): (b) intensity of the 2LA second-order mode, (c) frequency and (d) intensity of the A<sub>1g</sub> mode. (e) Line profiles along the red horizontal line in each image corresponding to height, 2LA intensity, A<sub>1g</sub> intensity and A<sub>1g</sub> peak position (ω). The Raman mappings as well as the intensity profiles refer to the maximum intensity of the peaks (not the integrated intensities).</p></caption><graphic xlink:href="srep01755-f2"></graphic></fig><fig id="f3"><label>Figure 3</label><caption><title>Raman spectra collected from regions with different WS<sub>2</sub> thicknesses (1L, 2L 3L and bulk) using three different excitation wavelengths: (a) 488 nm, (b) 514.5 nm and (c) 647 nm.</title><p>For each individual wavelength all the spectra, including that from the bulk, were acquired using the same experimental conditions of laser power and collection time.</p></caption><graphic xlink:href="srep01755-f3"></graphic></fig><fig id="f4"><label>Figure 4</label><caption><title>Intensity ratios and peak frequencies of WS<sub>2</sub> Raman modes.</title><p>(a) Frequencies of the <italic>A<sub>1g</sub></italic>, <inline-formula id="m6"><inline-graphic id="d33e906" xlink:href="srep01755-m6.jpg"></inline-graphic></inline-formula>, , and 2<italic>LA</italic> Raman modes as a function of thickness (number of WS<sub>2</sub> monolayers) for the three excitation laser lines. Each point represents an average over ten different positions and the error bars correspond to the standard deviations (b) Thickness-dependent intensity ratios of <inline-formula id="m7"><inline-graphic id="d33e915" xlink:href="srep01755-m7.jpg"></inline-graphic></inline-formula> (for λ<sub>exc</sub> = 514.5 nm) and <inline-formula id="m8"><inline-graphic id="d33e922" xlink:href="srep01755-m8.jpg"></inline-graphic></inline-formula> (for λ<sub>exc</sub> = 488 and 647 nm).</p></caption><graphic xlink:href="srep01755-f4"></graphic></fig><fig id="f5"><label>Figure 5</label><caption><title>Phonon dispersion plus density of states [left]; and electronic band structures [right] for WS<sub>2</sub> monolayer [top] and bulk [bottom], respectively.</title><p>The electronic band structure in (b) was calculated by LDA with 1.94 eV band gap; the dashed line corresponds to the bottom of the conduction band with a rigid shift to match the experimental 2.1 eV band gap. Although the phonon dispersions for monolayer and bulk are very similar, their band structures are clearly different. While the direct gap at the K point remains almost the same; the electronic bands between Γ-M and Γ-K undergo major changes associated with the indirect-to-direct bandgap transition from bulk to monolayer. The horizontal dashed lines in (b) and (d) are guides for the eye to visualize better these differences.</p></caption><graphic xlink:href="srep01755-f5"></graphic></fig><fig id="f6"><label>Figure 6</label><caption><title>Schematic of the double-resonant Raman process that generates the 2LA(M) experimental peak.</title><p>Calculated data from the valence (blue) and conduction (red) bands are used for the visualization. An incident photon with energy <bold><italic>hν<sub>i</sub></italic></bold> produces a vertical electronic transition (black vertical arrow) from valence to conduction band. The electron in the conduction band then experiences two electron-phonon scattering events, the first scattering event involving a phonon with momentum +q and the second involving a phonon with momentum –q (red dashed double arrow). Finally another vertical transition from conduction to valence band emits a Raman-shifted photon with energy <bold><italic>hν<sub>f</sub></italic></bold>.</p></caption><graphic xlink:href="srep01755-f6"></graphic></fig><fig id="f7"><label>Figure 7</label><caption><title>Reduced Raman intensity of the 2LA Raman line at the vicinity of K and M as a function of laser energy using a double-resonant model for both bulk and monolayer WS<sub>2 </sub>.</title><p>Although the reduced intensities were also investigated around 488 and 647 nm (not shown), the monolayer system shows a resonant peak only for the vicinity of 514.7 nm.</p></caption><graphic xlink:href="srep01755-f7"></graphic></fig><table-wrap position="float" id="t1"><label>Table 1</label><caption><title>Summary of the frequency for the main Raman modes as well as the intensity ratio for the most intense peaks, as a function of the number of layers and the excitation wavelength</title></caption><table frame="hsides" rules="groups" border="1"><colgroup><col align="left"></col><col align="center"></col><col align="center"></col><col align="center"></col><col align="center"></col><col align="center"></col></colgroup><thead valign="bottom"><tr><th align="justify" valign="top" charoff="50">λ<sub>Exc</sub></th><th align="center" valign="top" charoff="50">Phonon modes</th><th align="center" valign="top" charoff="50">1-layer</th><th align="center" valign="top" charoff="50">2-layers</th><th align="center" valign="top" charoff="50">3-layers</th><th align="center" valign="top" charoff="50">Bulk</th></tr></thead><tbody valign="top"><tr><td align="justify" valign="top" charoff="50">488 nm</td><td align="char" valign="top" char="(" charoff="50">A<sub>1g</sub>(Γ) (cm<sup>−1</sup> )</td><td align="char" valign="top" char="." charoff="50">417.5</td><td align="char" valign="top" char="." charoff="50">418.9</td><td align="char" valign="top" char="." charoff="50">419.5</td><td align="char" valign="top" char="." charoff="50">420.2</td></tr><tr><td align="char" valign="top" char="(" charoff="50"> </td><td align="char" valign="top" char="(" charoff="50">E<sup>1</sup><sub>2g</sub>(Γ) (cm<sup>−1</sup> )</td><td align="char" valign="top" char="." charoff="50">355.9</td><td align="char" valign="top" char="." charoff="50">355.3</td><td align="char" valign="top" char="." charoff="50">355</td><td align="char" valign="top" char="." charoff="50">355.8</td></tr><tr><td align="char" valign="top" char="(" charoff="50"> </td><td align="char" valign="top" char="(" charoff="50">2LA(M) (cm<sup>−1</sup> )</td><td align="char" valign="top" char="." charoff="50">351.7</td><td align="char" valign="top" char="." charoff="50">350.3</td><td align="char" valign="top" char="." charoff="50">350</td><td align="char" valign="top" char="." charoff="50">351</td></tr><tr><td align="char" valign="top" char="(" charoff="50"> </td><td align="char" valign="top" char="(" charoff="50"><inline-formula id="m9"><inline-graphic id="d33e1048" xlink:href="srep01755-m9.jpg"></inline-graphic></inline-formula> (Intensity ratio)</td><td align="char" valign="top" char="." charoff="50">0.78</td><td align="char" valign="top" char="." charoff="50">0.62</td><td align="char" valign="top" char="." charoff="50">0.59</td><td align="char" valign="top" char="." charoff="50">0.53</td></tr><tr><td align="justify" valign="top" charoff="50">514.5 nm</td><td align="char" valign="top" char="(" charoff="50">A<sub>1g</sub>(Γ) (cm<sup>−1</sup> )</td><td align="char" valign="top" char="." charoff="50">417.2</td><td align="char" valign="top" char="." charoff="50">418.4</td><td align="char" valign="top" char="." charoff="50">419</td><td align="char" valign="top" char="." charoff="50">420.1</td></tr><tr><td align="char" valign="top" char="(" charoff="50"> </td><td align="char" valign="top" char="(" charoff="50">E<sup>1</sup><sub>2g</sub>(Γ) (cm<sup>−1</sup> )</td><td align="char" valign="top" char="." charoff="50">355.2</td><td align="char" valign="top" char="." charoff="50">355</td><td align="char" valign="top" char="." charoff="50">354.4</td><td align="char" valign="top" char="." charoff="50">355.3</td></tr><tr><td align="char" valign="top" char="(" charoff="50"> </td><td align="char" valign="top" char="(" charoff="50">2LA(M) (cm<sup>−1</sup> )</td><td align="char" valign="top" char="." charoff="50">351.1</td><td align="char" valign="top" char="." charoff="50">350.7</td><td align="char" valign="top" char="." charoff="50">349.7</td><td align="char" valign="top" char="." charoff="50">350</td></tr><tr><td align="char" valign="top" char="(" charoff="50"> </td><td align="char" valign="top" char="(" charoff="50"><inline-formula id="m10"><inline-graphic id="d33e1116" xlink:href="srep01755-m10.jpg"></inline-graphic></inline-formula> (Intensity ratio)</td><td align="char" valign="top" char="." charoff="50">2.2</td><td align="char" valign="top" char="." charoff="50">1.01</td><td align="char" valign="top" char="." charoff="50">0.72</td><td align="char" valign="top" char="." charoff="50">0.47</td></tr><tr><td align="justify" valign="top" charoff="50">647 nm</td><td align="char" valign="top" char="(" charoff="50">A<sub>1g</sub>(Γ) (cm<sup>−1</sup> )</td><td align="char" valign="top" char="." charoff="50">417.2</td><td align="char" valign="top" char="." charoff="50">418.7</td><td align="char" valign="top" char="." charoff="50">419.2</td><td align="char" valign="top" char="." charoff="50">420.5</td></tr><tr><td align="char" valign="top" char="(" charoff="50"> </td><td align="char" valign="top" char="(" charoff="50">E<sup>1</sup><sub>2g</sub>(Γ) (cm<sup>−1</sup> )</td><td align="char" valign="top" char="." charoff="50">355.3</td><td align="char" valign="top" char="." charoff="50">354.2</td><td align="char" valign="top" char="." charoff="50">354</td><td align="char" valign="top" char="." charoff="50">354.7</td></tr><tr><td align="char" valign="top" char="(" charoff="50"> </td><td align="char" valign="top" char="(" charoff="50">2LA(M) (cm<sup>−1</sup> )</td><td align="char" valign="top" char="." charoff="50">350.4</td><td align="char" valign="top" char="." charoff="50">349.5</td><td align="char" valign="top" char="." charoff="50">349.5</td><td align="char" valign="top" char="." charoff="50">351.1</td></tr><tr><td align="char" valign="top" char="(" charoff="50"> </td><td align="char" valign="top" char="(" charoff="50"><inline-formula id="m11"><inline-graphic id="d33e1185" xlink:href="srep01755-m11.jpg"></inline-graphic></inline-formula> (Intensity ratio)</td><td align="char" valign="top" char="." charoff="50">0.5</td><td align="char" valign="top" char="." charoff="50">0.8</td><td align="char" valign="top" char="." charoff="50">0.8</td><td align="char" valign="top" char="." charoff="50">0.4</td></tr></tbody></table></table-wrap></floats-group></pmc><affiliations><list><country><li>Belgique</li><li>Japon</li><li>Mexique</li><li>États-Unis</li></country><region><li>Kentucky</li><li>Pennsylvanie</li><li>Vienne (Autriche)</li></region><settlement><li>Vienne (Autriche)</li></settlement></list><tree><country name="États-Unis"><region name="Pennsylvanie"><name sortKey="Berkdemir, Ayse" sort="Berkdemir, Ayse" uniqKey="Berkdemir A" first="Ayse" last="Berkdemir">Ayse Berkdemir</name></region><name sortKey="Chia, Chen Ing" sort="Chia, Chen Ing" uniqKey="Chia C" first="Chen-Ing" last="Chia">Chen-Ing Chia</name><name sortKey="Crespi, Vincent H" sort="Crespi, Vincent H" uniqKey="Crespi V" first="Vincent H." last="Crespi">Vincent H. Crespi</name><name sortKey="Elias, Ana Laura" sort="Elias, Ana Laura" uniqKey="Elias A" first="Ana Laura" last="Elías">Ana Laura Elías</name><name sortKey="Gutierrez, Humberto R" sort="Gutierrez, Humberto R" uniqKey="Gutierrez H" first="Humberto R." last="Gutiérrez">Humberto R. Gutiérrez</name><name sortKey="Gutierrez, Humberto R" sort="Gutierrez, Humberto R" uniqKey="Gutierrez H" first="Humberto R." last="Gutiérrez">Humberto R. Gutiérrez</name><name sortKey="L Pez Urias, Florentino" sort="L Pez Urias, Florentino" uniqKey="L Pez Urias F" first="Florentino" last="L Pez-Urías">Florentino L Pez-Urías</name><name sortKey="Perea L Pez, Nestor" sort="Perea L Pez, Nestor" uniqKey="Perea L Pez N" first="Néstor" last="Perea-L Pez">Néstor Perea-L Pez</name><name sortKey="Terrones, Humberto" sort="Terrones, Humberto" uniqKey="Terrones H" first="Humberto" last="Terrones">Humberto Terrones</name><name sortKey="Terrones, Mauricio" sort="Terrones, Mauricio" uniqKey="Terrones M" first="Mauricio" last="Terrones">Mauricio Terrones</name><name sortKey="Wang, Bei" sort="Wang, Bei" uniqKey="Wang B" first="Bei" last="Wang">Bei Wang</name></country><country name="Belgique"><region name="Vienne (Autriche)"><name sortKey="Botello Mendez, Andres R" sort="Botello Mendez, Andres R" uniqKey="Botello Mendez A" first="Andrés R." last="Botello-Méndez">Andrés R. Botello-Méndez</name></region><name sortKey="Charlier, Jean Christophe" sort="Charlier, Jean Christophe" uniqKey="Charlier J" first="Jean-Christophe" last="Charlier">Jean-Christophe Charlier</name></country><country name="Mexique"><noRegion><name sortKey="L Pez Urias, Florentino" sort="L Pez Urias, Florentino" uniqKey="L Pez Urias F" first="Florentino" last="L Pez-Urías">Florentino L Pez-Urías</name></noRegion></country><country name="Japon"><noRegion><name sortKey="Terrones, Mauricio" sort="Terrones, Mauricio" uniqKey="Terrones M" first="Mauricio" last="Terrones">Mauricio Terrones</name></noRegion></country></tree></affiliations></record>Pour manipuler ce document sous Unix (Dilib)
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