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Surface-Effect-Induced Optical Bandgap Shrinkage in GaN Nanotubes
Park, Young S.,Lee, Geunsik,Holmes, Mark J.,Chan, Christopher C. S.,Reid, Benjamin P. L.,Alexander-Webber, Jack A.,Nicholas, Robin J.,Taylor, Robert A.,Kim, Kwang S.,Han, Sang W.,Yang, Woochul,Jo, Y. American Chemical Society 2015 NANO LETTERS Vol.15 No.7
<P>We investigate nontrivial surface effects on the optical properties of self-assembled crystalline GaN nanotubes grown on Si substrates. The excitonic emission is observed to redshift by ∼100 meV with respect to that of bulk GaN. We find that the conduction band edge is mainly dominated by surface atoms, and that a larger number of surface atoms for the tube is likely to increase the bandwidth, thus reducing the optical bandgap. The experimental findings can have important impacts in the understanding of the role of surfaces in nanostructured semiconductors with an enhanced surface/volume ratio.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2015/nalefd.2015.15.issue-7/acs.nanolett.5b00924/production/images/medium/nl-2015-00924p_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl5b00924'>ACS Electronic Supporting Info</A></P>
Two-Dimensional Excitonic Photoluminescence in Graphene on a Cu Surface
Park, Youngsin,Kim, Yooseok,Myung, Chang Woo,Taylor, Robert Anthony,Chan, Christopher C. S.,Reid, Benjamin P. L.,Puchtler, Tim J.,Nicholas, Robin J.,Singh, Laishram Tomba,Lee, Geunsik,Hwang, Chan-Cuk American Chemical Society 2017 ACS NANO Vol.11 No.3
<P>Despite having outstanding electrical properties, graphene is unsuitable for optical devices because of its zero band gap. Here, we report two-dimensional excitonic photoluminescence (PL) from graphene grown on a Cu(111) surface, which shows an unexpected and remarkably sharp strong emission near 3.16 eV (full width at half-maximum <= 3 meV) and multiple emissions around 3.18 eV. As temperature increases, these emissions blue shift, displaying the characteristic negative thermal coefficient of graphene. The observed PL originates from the significantly suppressed dispersion of excited electrons, in graphene caused by hybridization of graphene pi and Cu d orbitals of the first and second Cu layers at a shifted saddle point 0.525(M+K) of the Brillouin zone. This finding provides a pathway to engineering optoelectronic graphene devices, while maintaining the outstanding electrical properties of graphene.</P>
Optical polarization in mono and bilayer MoS2
박영신,Nannan Li,Christopher C.S. Chan,Benjamin P.L. Reid,Robert A. Taylor,임현식 한국물리학회 2017 Current Applied Physics Vol.17 No.9
Optical anisotropy in monolayer- and bilayer-MoS2 was investigated by polarization resolved photoluminescence measurements. The photoluminescence of monolayer-MoS2 is found to be partially polarized at 4.2 K and maintains this polarization characteristic up to room temperature, while the photoluminescence of bilayer-MoS2 shows no obvious polarization. This polarization anisotropy is due to strain effects at the interface between the MoS2 layer and the SiO2 substrate, causing symmetry breaking of the MoS2 charge distribution. Calculations using density functional theory of the electron density distribution of the monolayer- and bilayer-MoS2 in the in-plane direction are also presented, giving support to our qualitative analysis.