FisMat2017 - Submission - View

Abstract's title: Mechanical exfoliation and layer number identification of MoS2 revisited
Submitting author: Luca Ottaviano
Affiliation: Dipartimento di Scienze Fisiche e Chimiche Università degli studi dell'Aquila
Affiliation Address: Via Vetoio 10 67100 L'Aquila
Country: Italy
Oral presentation/Poster (Author's request): Oral presentation
Other authors and affiliations: S. Palleschi, F. Perrozzi, G. D’Olimpio, F. Priante, M. Donarelli, P. Benassi, M. Nardone Dipartimento di Scienze Fisiche e Chimiche (DSFC) Università dell’Aquila, Via Vetoio 10 67100 L’Aquila, Italy CNR-SPIN UOS L’Aquila, Via Vetoio 10 67100 L’Aquila, Italy M. Gonchigsuren School of Applied Sciences, MUST, Bagatoiruu, Ulaanbaatar 14191, Mongolia M. Gombosuren Department of Physics, National University of Mongolia, Ulaanbaatar 14201, Mongolia A. Lucia, G. Moccia, O. A. Cacioppo LFoundry, a SMIC company, Via A. Pacinotti 7, 67051 Avezzano (AQ), Italy

 In this paper we accurately revisit the mechanical exfoliation and layer

number determination of MoS2 . By modelling the exfoliation itself as a random vertical

(lateral) exfoliation (fragmentation) phenomenon, a rationale is given to optimise

the number of iterations in the scotch-tape peeling technique and we rigorously

demonstrate that since the second cleavage the occurrence of monolayers is the most

likely. For the unambiguous layer number identification, experiments have been carried

out with a unified complementary approach based on optical microscopy, Atomic Force

Microscopy, resonant and non resonant Raman spectroscopy, and Photo-Luminescence

spectroscopy. The experimental analysis has been carried out on a statistically

significant set of few-layer MoS flakes (from one to five layers). The work stresses the

strong need of such complementary multi-technique approach to really unambiguously

determine the layer number of flakes (that neither optical microscopy, nor AFM alone

can give). Optical microscopy contrast analysis experiments (carried out on flakes

deposited onto 270 nm SiO2 /Si(100) substrates) demonstrate that for few-layer MoS2

 (from the mono to the epta-layer) the optical contrast is weakly depending on the

radiation wavelength and is a non-monotonic function of the layer number. Thus, flakes

from ten to twelve layers exhibit the same contrast of monolayers, as demonstrated by

parallel AFM analysis. AFM clearly shows that the stacking between the silicon oxide

substrate and the first MoS layer is strongly unpredictable and likely depending on

the degree of surface contamination of the substrate. A parallel systematic resonant

 Raman study is performed ranging from one to five layers clearly exhibiting systematic

layer dependent spectral features in the one-five layers range. For the first time a layer

estimation number based on a quantitative analysis of resonant Raman and PL spectra

is proposed.