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Engineering Optical and Electronic Properties of WS<sub>2</sub> by Varying the Number of Layers
Kim, Hyun-Cheol,Kim, Hakseong,Lee, Jae-Ung,Lee, Han-Byeol,Choi, Doo-Hua,Lee, Jun-Ho,Lee, Wi Hyoung,Jhang, Sung Ho,Park, Bae Ho,Cheong, Hyeonsik,Lee, Sang-Wook,Chung, Hyun-Jong American Chemical Society 2015 ACS NANO Vol.9 No.7
<P>The optical constants, bandgaps, and band alignments of mono-, bi-, and trilayer WS<SUB>2</SUB> were experimentally measured, and an extraordinarily high dependency on the number of layers was revealed. The refractive indices and extinction coefficients were extracted from the optical-contrast oscillation for various thicknesses of SiO<SUB>2</SUB> on a Si substrate. The bandgaps of the few-layer WS<SUB>2</SUB> were both optically and electrically measured, indicating high exciton-binding energies. The Schottky-barrier heights (SBHs) with Au/Cr contact were also extracted, depending on the number of layers (1–28). From an engineering viewpoint, the bandgap can be modulated from 3.49 to 2.71 eV with additional layers. The SBH can also be reduced from 0.37 eV for a monolayer to 0.17 eV for 28 layers. The technique of engineering materials’ properties by modulating the number of layers opens pathways uniquely adaptable to transition-metal dichalcogenides.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2015/ancac3.2015.9.issue-7/acsnano.5b01727/production/images/medium/nn-2015-017277_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn5b01727'>ACS Electronic Supporting Info</A></P>
Lee, Seonwoo,Kim, Kangwon,Dhakal, Krishna P.,Kim, Hyunmin,Yun, Won Seok,Lee, JaeDong,Cheong, Hyeonsik,Ahn, Jong-Hyun American Chemical Society 2017 Nano letters Vol.17 No.12
<P>We report on the thickness-dependent Raman spectroscopy of ultrathin silicon (Si) nanomembranes (NMs), whose thicknesses range from 2 to 18 nm, using several excitation energies. We observe that the Raman intensity depends on the thickness and the excitation energy due to the combined effects of interference and resonance from the band-structure modulation. Furthermore, confined acoustic phonon modes in the ultrathin Si NMs were observed in ultralow-frequency Raman spectra, and strong thickness dependence was observed near the quantum limit, which was explained by calculations based on a photoelastic model. Our results provide a reliable method with which to accurately determine the thickness of Si NMs with thicknesses of less than a few nanometers.</P>
Raman Signatures of Polytypism in Molybdenum Disulfide
Lee, Jae-Ung,Kim, Kangwon,Han, Songhee,Ryu, Gyeong Hee,Lee, Zonghoon,Cheong, Hyeonsik American Chemical Society 2016 ACS NANO Vol.10 No.2
<P>Since the stacking order sensitively affects various physical properties of layered materials, accurate determination of the stacking order is important for studying the basic properties of these materials as well as for device applications. Because 2H-molybdenum disulfide (MoS2) is most common in nature, most studies so far have focused on 2H-MoS2. However, we found that the 2H, 3R, and mixed stacking sequences exist in few-layer MoS2 exfoliated from natural molybdenite crystals. The crystal structures are confirmed by HR-TEM measurements. The Raman signatures of different polytypes are investigated by using three different excitation energies that are nonresonant and resonant with A and C excitons, respectively. The low-frequency breathing and shear modes show distinct differences for each polytype, whereas the high-frequency intralayer modes show little difference. For resonant excitations at 1.96 and 2.81 eV, distinct features are observed that enable determination of the stacking order.</P>
Ising-Type Magnetic Ordering in Atomically Thin FePS<sub>3</sub>
Lee, Jae-Ung,Lee, Sungmin,Ryoo, Ji Hoon,Kang, Soonmin,Kim, Tae Yun,Kim, Pilkwang,Park, Cheol-Hwan,Park, Je-Geun,Cheong, Hyeonsik American Chemical Society 2016 NANO LETTERS Vol.16 No.12
<P>Magnetism in two-dimensional materials is not only of fundamental scientific interest but also a promising candidate for numerous applications. However, studies so far, especially the experimental ones, have been mostly limited to the magnetism arising from defects, vacancies, edges, or chemical dopants which are all extrinsic effects. Here, we report on the observation of intrinsic antiferromagnetic ordering in the two-dimensional limit. By monitoring the Raman peaks that arise from zone folding due to antiferromagnetic ordering at the transition temperature, we demonstrate that FePS3 exhibits an Ising-type antiferromagnetic ordering down to the monolayer limit, in good agreement with the Onsager solution for two-dimensional order disorder transition. The transition temperature remains almost independent of the thickness from bulk to the monolayer limit with T-N similar to 118 K, indicating that the weak interlayer interaction has little effect on the antiferromagnetic ordering.</P>