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Anomalous Lattice Vibrations of Single- and Few-Layer MoS<sub>2</sub>
Lee, Changgu,Yan, Hugen,Brus, Louis E.,Heinz, Tony F.,Hone, James,Ryu, Sunmin American Chemical Society 2010 ACS NANO Vol.4 No.5
<P>Molybdenum disulfide (MoS<SUB>2</SUB>) of single- and few-layer thickness was exfoliated on SiO<SUB>2</SUB>/Si substrate and characterized by Raman spectroscopy. The number of S−Mo−S layers of the samples was independently determined by contact-mode atomic force microscopy. Two Raman modes, E<SUP>1</SUP><SUB>2g</SUB> and A<SUB>1g</SUB>, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2010/ancac3.2010.4.issue-5/nn1003937/production/images/medium/nn-2010-003937_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn1003937'>ACS Electronic Supporting Info</A></P>
Tuning the graphene work function by electric field effect.
Yu, Young-Jun,Zhao, Yue,Ryu, Sunmin,Brus, Louis E,Kim, Kwang S,Kim, Philip American Chemical Society 2009 Nano letters Vol.9 No.10
<P>We report variation of the work function for single and bilayer graphene devices measured by scanning Kelvin probe microscopy (SKPM). By use of the electric field effect, the work function of graphene can be adjusted as the gate voltage tunes the Fermi level across the charge neutrality point. Upon biasing the device, the surface potential map obtained by SKPM provides a reliable way to measure the contact resistance of individual electrodes contacting graphene.</P>
Synthesis and Electrical Characterization of Magnetic Bilayer Graphene Intercalate
Kim, Namdong,Kim, Kwang S.,Jung, Naeyoung,Brus, Louis,Kim, Philip American Chemical Society 2011 Nano letters Vol.11 No.2
<P>We report synthesis and transport properties of the minimal graphite intercalation compound, a ferric chloride (FeCl<SUB>3</SUB>)<SUB><I>n</I></SUB> island monolayer inside bilayer graphene. Chemical doping by the intercalant is simultaneously probed by micro-Raman spectroscopy and Hall measurements. Quantum oscillations of conductivity originate from microscopic domains of intercalated and unintercalated regions. A slight upturn in resistance related to magnetic transition is observed. Two-dimensional intercalation in bilayer graphene opens new possibilities to engineer two-dimensional properties of intercalates.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2011/nalefd.2011.11.issue-2/nl104228f/production/images/medium/nl-2010-04228f_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl104228f'>ACS Electronic Supporting Info</A></P>
Raman Spectroscopy of Lithographically Patterned Graphene Nanoribbons
Ryu, Sunmin,Maultzsch, Janina,Han, Melinda Y.,Kim, Philip,Brus, Louis E. American Chemical Society 2011 ACS NANO Vol.5 No.5
<P>Nanometer-scale graphene objects are attracting much research interest because of newly emerging properties originating from quantum confinement effects. We present Raman spectroscopy studies of graphene nanoribbons (GNRs), which are known to have nonzero electronic bandgap. GNRs of width ranging from 15 to 100 nm have been prepared by e-beam lithographic patterning of mechanically exfoliated graphene followed by oxygen plasma etching. Raman spectra of narrow GNRs can be characterized by an upshifted G band and a prominent disorder-related D band originating from scattering at the ribbon edges. The D-to-G band intensity ratio generally increases with decreasing ribbon width. However, its decrease in width of <25 nm, partly attributed to amorphization at the edges, provides a valuable experimental estimate on D mode relaxation length of <5 nm. The upshift in the G band of the narrowest GNRs can be attributed to confinement effect or chemical doping by functional groups on the GNR edges. Notably, GNRs are much more susceptible to photothermal effects resulting in reversible hole doping caused by atmospheric oxygen than bulk graphene sheets. Finally we show that the 2D band is still a reliable marker in determining the number of layers of GNRs despite its significant broadening for very narrow GNRs.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2011/ancac3.2011.5.issue-5/nn200799y/production/images/medium/nn-2011-00799y_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn200799y'>ACS Electronic Supporting Info</A></P>