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High-Strength Chemical-Vapor–Deposited Graphene and Grain Boundaries
Lee, Gwan-Hyoung,Cooper, Ryan C.,An, Sung Joo,Lee, Sunwoo,van der Zande, Arend,Petrone, Nicholas,Hammerberg, Alexandra G.,Lee, Changgu,Crawford, Bryan,Oliver, Warren,Kysar, Jeffrey W.,Hone, James American Association for the Advancement of Scienc 2013 Science Vol.340 No.6136
<P><B>Graphene Staying Strong</B></P><P>Although exfoliated graphene can be extremely strong, it is produced on too small a scale for materials application. Graphene can be produced on a more practical scale by chemical vapor deposition, but the presence of grain boundaries between crystallites apparently weakens the material. <B>Lee <I>et al.</I></B> (p. 1073) show that postprocessing steps during the removal of the graphene sheets can oxidize the grain boundaries and weaken them. If these steps are avoided, the material is comparable in strength to exfoliated graphene.</P>
Lee, Gwan-Hyoung,Yu, Young-Jun,Cui, Xu,Petrone, Nicholas,Lee, Chul-Ho,Choi, Min Sup,Lee, Dae-Yeong,Lee, Changgu,Yoo, Won Jong,Watanabe, Kenji,Taniguchi, Takashi,Nuckolls, Colin,Kim, Philip,Hone, James American Chemical Society 2013 ACS NANO Vol.7 No.9
<P>Atomically thin forms of layered materials, such as conducting graphene, insulating hexagonal boron nitride (hBN), and semiconducting molybdenum disulfide (MoS<SUB>2</SUB>), have generated great interests recently due to the possibility of combining diverse atomic layers by mechanical “stacking” to create novel materials and devices. In this work, we demonstrate field-effect transistors (FETs) with MoS<SUB>2</SUB> channels, hBN dielectric, and graphene gate electrodes. These devices show field-effect mobilities of up to 45 cm<SUP>2</SUP>/Vs and operating gate voltage below 10 V, with greatly reduced hysteresis. Taking advantage of the mechanical strength and flexibility of these materials, we demonstrate integration onto a polymer substrate to create flexible and transparent FETs that show unchanged performance up to 1.5% strain. These heterostructure devices consisting of ultrathin two-dimensional (2D) materials open up a new route toward high-performance flexible and transparent electronics.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2013/ancac3.2013.7.issue-9/nn402954e/production/images/medium/nn-2013-02954e_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn402954e'>ACS Electronic Supporting Info</A></P>
Lee, Youngbin,Lee, Jinhwan,Bark, Hunyoung,Oh, Il-Kwon,Ryu, Gyeong Hee,Lee, Zonghoon,Kim, Hyungjun,Cho, Jeong Ho,Ahn, Jong-Hyun,Lee, Changgu The Royal Society of Chemistry 2014 Nanoscale Vol.6 No.5
We describe a method for synthesizing large-area and uniform molybdenum disulfide films, with control over the layer number, on insulating substrates using a gas phase sulfuric precursor (H2S) and a molybdenum metal source. The metal layer thickness was varied to effectively control the number of layers (2 to 12) present in the synthesized film. The films were grown on wafer-scale Si/SiO2 or quartz substrates and displayed excellent uniformity and a high crystallinity over the entire area. Thin film transistors were prepared using these materials, and the performances of the devices were tested. The devices displayed an on/off current ratio of 10(5), a mobility of 0.12 cm(2) V (1) s (1) (mean mobility value of 0.07 cm(2) V-1 s(-1)), and reliable operation.
Structural and Optical Properties of Single- and Few-Layer Magnetic Semiconductor CrPS<sub>4</sub>
Lee, Jinhwan,Ko, Taeg Yeoung,Kim, Jung Hwa,Bark, Hunyoung,Kang, Byunggil,Jung, Soon-Gil,Park, Tuson,Lee, Zonghoon,Ryu, Sunmin,Lee, Changgu American Chemical Society 2017 ACS NANO Vol.11 No.11
<P>Atomically thin binary two-dimensional (2D) semiconductors exhibit diverse physical properties depending on their composition, structure, and thickness. By adding another element in these materials, which will lead to formation of ternary 2D materials, the property and structure would greatly change and significantly expanded applications could be explored. In this work, we report structural and optical properties of atomically thin chromium thiophosphate (CrPS<SUB>4</SUB>), a ternary antiferromagnetic semiconductor. Its structural details were revealed by X-ray and electron diffraction. Transmission electron microscopy showed that preferentially cleaved edges are parallel to diagonal Cr atom rows, which readily identified their crystallographic orientations. Strong in-plane optical anisotropy induced birefringence that also enabled efficient determination of crystallographic orientation using polarized microscopy. The lattice vibrations were probed by Raman spectroscopy and exhibited significant dependence on thickness of crystals exfoliated down to a single layer. Optical absorption determined by reflectance contrast was dominated by d–d-type transitions localized at Cr<SUP>3+</SUP> ions, which was also responsible for the major photoluminescence peak at 1.31 eV. The spectral features in the absorption and emission spectra exhibited noticeable thickness dependence and hinted at a high photochemical activity for single-layer CrPS<SUB>4</SUB>. The current structural and optical investigation will provide a firm basis for future study and application of this kind of atomically thin magnetic semiconductors.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2017/ancac3.2017.11.issue-11/acsnano.7b04679/production/images/medium/nn-2017-04679m_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn7b04679'>ACS Electronic Supporting Info</A></P>
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>
Lee, Youngbin,Kim, Hyunmin,Lee, Jinhwan,Yu, Seong Hun,Hwang, Euyheon,Lee, Changgu,Ahn, Jong-Hyun,Cho, Jeong Ho American Chemical Society 2016 Chemistry of materials Vol.28 No.1
<P>We studied the surface-enhanced Raman scattering of an organic fluoropore (Rhodamine 6G, R6G) monolayer adsorbed onto graphene and two-dimensional (2D) molybedenium disulfides (MoS2) phototransistors and compared the results with the Raman scattering of R6G on 2D tungsten diselenides system (WSe2). The Raman enhancement factor of the R6G film adsorbed onto WSe2 was comparable to the corresponding value on graphene at 1365 cm(-1) and was approximately twice this value at 615 cm(-1). The amplitude of the charge transfer was estimated in situ by measuring the photocurrent produced in a hybrid system consisting of physisorbed R6G layer and the 2D materials. We found that the enhanced Raman scattering of R6G adsorbed onto the 2D materials was closely correlated with the charge transfer between the adsorbed molecules and the 2D materials. We also revealed that the intensity of Raman scattering generally decreased as the layer number of the 2D materials increased. For the R6G on the MoS2 nanosheet, a single layer system provided a maximum Raman enhancement factor, and this value decreased pseudolinearly with the number of layers. By contrast, the Raman enhancement factor of the R6G on WSe2 was greatest for both the mono- and bilayers, and it decreased dramatically as the number of layers increased. We provide qualitative theoretical explanations for these trends based on the electric field enhancement for the multile Fresnel phases and energy band diagrams of both systems.</P>