Monolayer molybdenum disulfide (MoS 2 ) has attracted tremendous attention due to its promising applications in high-performance field-effect transistors, phototransistors, spintronic devices and nonlinear optics. The enhanced photoluminescence effect in monolayer MoS 2 was discovered and, as a strong tool, was employed for strain and defect analysis in MoS 2 . Recently, large-size monolayer MoS 2 has been produced by chemical vapour deposition, but has not yet been fully explored. Here we systematically characterize chemical vapour deposition-grown MoS 2 by photoluminescence spectroscopy and mapping and demonstrate non-uniform strain in single-crystalline monolayer MoS 2 and strain-induced bandgap engineering. We also evaluate the effective strain transferred from polymer substrates to MoS 2 by three-dimensional finite element analysis. Furthermore, our work demonstrates that photoluminescence mapping can be used as a non-contact approach for quick identification of grain boundaries in MoS 2 .

131wileyonlinelibrary.com COMMUNICATION www.MaterialsViews.com www.advopticalmat. de Being entangled in controlling the electronic properties of graphene for next-generation electronics, [ 1,2 ] monolayer transition metal dichalcogenides such as MS 2 (M = Mo, W) are attracting great interest as 2D semiconductors with a native direct-energy gap in the visible frequency range. [ 3,4 ] Monolayers of other layered materials such as h -BN, GaS, GaSe, TaSe 2 , and so on, have also attracted much attention because of their unique properties when scaled down to monolayers. [5][6][7][8] There are comprehensive and intensive studies on monolayer MoS 2 , including its optical and electronic properties, [9][10][11][12][13][14][15][16][17] valleytronics, [18][19][20][21] strain effects, [22][23][24] thermal effects, [ 25 ] and so on. However, investigations of WS 2 have just started. Similar to 2H-MoS 2 , monolayer 2H-WS 2 can be constructed by sandwiching two atomic layers of S and one atomic layer of W through covalent W-S bonds, where W locates at the body center of a trigonal-prismatic case formed by six S atoms. Confi nement of charge carriers inside the horizontal atomic plane gradually enlarges energy gaps when thinning WS 2 layers. [ 26 ] Instead of an indirect energy gap for multiple layers, a direct energy gap of ∼ 2 eV at the corners (K and K' points) of the Brillouin Zone could be formed in monolayer WS 2 as clearly demonstrated by both theoretical and experimental studies. [ 9,[27][28][29] The immediate consequence, also a benefi t of the existence of such direct bandgap, is the signifi cant enhancement of visible light emission. In WS 2 monolayers, breaking inversion symmetry leads to the strong spin-orbit coupling and the splitting of valence bands at K/K' points with a sub-gap of around 0.4 eV. [ 30 ] Furthermore, the split spins at the time-reversed K and K' valleys have the opposite signs. Thus, such spin-valley coupling offers an extra degree of freedom to charge carriers in WS 2 monolayers. Though it has not been reported in monolayer WS 2 , theory predicts and experiments have observed in monolayer MoS 2 a non-equilibrium charge carrier imbalance at two valleys, revealed by the remarkable difference of absorption of left-( σ -) and right-handed ( σ +) circular polarized lights at the two valleys. [ 9,[18][19][20][21]31 ] All these interesting and important properties, plus the newly revealed potential in the fl exible heterostructures of graphene-WS 2 stacks [ 32,33 ] guarantee a promising future of WS 2 as the candidate of nextgeneration nanoelectronics, spintronics, valleytronics, and optoelectronics. [ 34 ] However, compared to graphene, it is very diffi cult to prepare MS 2 monolayers, and atomically thin MS 2 fl akes made by mechanical exfoliation are much smaller, in fact too small to be well characterized and processed for devices. Most recently, chemical vapor deposition (CVD) has been used to successfully grow large-area single crystals of monolayer MoS 2 . [ 11,[35][36][37][38] However, the c...

The false-color (3D type) image of the intensity of the Raman spectra of monolayer MoS2 versus both peak positions and polar angles is plotted. It shows that the strongest E2g (1+) and E2g (1-) peaks appear at different angles, reflected as the alternation of the maxima of the intensity within the frequency range of the E2g (1) mode, which is the consequence of the crystallographic orientation relevant to the strain direction as predicted by theoretical analysis.

Plasmon-free surface enhanced Raman scattering (SERS) based on the chemical mechanism (CM) is drawing great attention due to its capability for controllable molecular detection. However, in comparison to the conventional noble-metal-based SERS technique driven by plasmonic electromagnetic mechanism (EM), the low sensitivity in the CM-based SERS is the dominant barrier toward its practical applications. Herein, we demonstrate the 1T' transition metal telluride atomic layers (WTe and MoTe) as ultrasensitive platforms for CM-based SERS. The SERS sensitivities of analyte dyes on 1T'-W(Mo)Te reach EM-comparable ones and become even greater when it is integrated with a Bragg reflector. In addition, the dye fluorescence signals are efficiently quenched, making the SERS spectra more distinguishable. As a proof of concept, the SERS signals of analyte Rhodamine 6G (R6G) are detectable even with an ultralow concentration of 40 (400) fM on pristine 1T'-W(Mo)Te, and the corresponding Raman enhancement factor (EF) reaches 1.8 × 10 (1.6 × 10). The limit concentration of detection and the EF of R6G can be further enhanced into 4 (40) fM and 4.4 × 10 (6.2 × 10), respectively, when 1T'-W(Mo)Te is integrated on the Bragg reflector. The strong interaction between the analyte and 1T'-W(Mo)Te and the abundant density of states near the Fermi level of the semimetal 1T'-W(Mo)Te in combination gives rise to the promising SERS effects by promoting the charge transfer resonance in the analyte-telluride complex.

No abstract