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Ultrafast Anisotropic Exciton Dynamics In Nanopatterned MoS Sheets Andrea Camellini, Carlo Mennucci, Eugenio Luigi Cinquanta, Christian Martella, Andrea Mazzanti, Alessio Lamperti, Alessandro Molle, Francesco Buatier de Mongeot, Giuseppe Della Valle, and Margherita Zavelani-Rossi ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00621 • Publication Date (Web): 10 Jun 2018 Downloaded from http://pubs.acs.org on June 10, 2018

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Ultrafast Anisotropic Exciton Dynamics In Nanopatterned MoS2 Sheets

Andrea Camellini,† Carlo Mennucci, ‡ Eugenio Cinquanta, §& Christian Martella,# Andrea Mazzanti,§ Alessio Lamperti,# Alessandro Molle,# Francesco Buatier de Mongeot, ‡ Giuseppe Della Valle,§&* and Margherita Zavelani-Rossi†&* †

Dipartimento di Energia, Politecnico di Milano, via G. Ponzio 34/3, I-20133 Milano, Italy

‡

Dipartimento di Fisica, Università di Genova, Via Dodecaneso 33, I-16146 Genova, Italy &

#

§

IFN-CNR, Piazza L. da Vinci 32, I-20133 Milano, Italy

IMM-CNR, Unit of Agrate Brianza, via C. Olivetti 2, I-20864 Agrate Brianza, Italy

Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci 32, I-20133 Milano, Italy

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

We study the optical properties of an anisotropic ripple-shaped two-dimensional molybdenum disulphide (MoS2) nanosheet deposited by chemical vapor deposition onto a nanopatterned silica (SiO2) substrate. We unveil a giant anisotropic optical response in the linear and nonlinear regime by a combination of optical extinction measurements, ultrafast broadband transient absorption experiments and finite element method numerical simulations. In steady state optical measurements, such anisotropy appears as a polarization-dependent extinction in correspondence to the characteristic excitonic peaks (A, B, C and D) of MoS2. Along with spectral changes, ultra-fast measurements strikingly exhibit the onset of an anisotropic relaxation dynamic in the region of the C exciton. Numerical simulations indicate that the observed polarization dependent optical response is dictated by the nanopattering of MoS2, with peculiar features belonging to the out–of–plane component of the dielectric tensor of MoS2 that is made effective through the rippled configuration. Our findings give a rationale to the anisotropic exciton response and show that morphology manipulation represents a valuable option for the tuning of optical and electronic properties in transition-metal dichalcogenides.

KEYWORDS two-dimensional materials, transition metal dichalcogenides, anisotropy, nanopatterning, transient absorption spectroscopy, polarization.


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Transition metal dichalcogenides (TMDs) and their van der Waals heterostructures exhibit a variety of peculiar electronic and optical properties at the two-dimensional (2D) level that are outstandingly appealing for a tremendously increasing number of applications in nanoelectronics and nanophotonics, herein including logic circuits and optoelectronic components.1–10 In particular, a thickness-dependent strong light-matter interaction, in the spectral region ranging from the ultraviolet to the terahertz, both in the linear and non-linear regime, enables TMDs to be an ideal platform for innovative devices such as custom-tailored photodetectors,5,11 light emitting diodes,6 and optical modulators.12 A further interesting advance in this respect could be obtained by introducing a controlled shape anisotropy in the spatial arrangement,13,14 as an effective path for exciton tuning and band structure engineering15–17 with widespread technology potential in light-harvesting and photocatalysis,13 optical waveplates,18 single photon emitters,19 thermoelectrics,20 and so forth. In this context, the application of uniaxial strain deformation on an elastomeric substrate and lithographic nanoscale processes on exfoliated flakes were shown to induce significant changes in the band-structure of molybdenum disulphide

(MoS2),

affecting

Raman

modes,

photoluminescence

and

exciton

confinement.14,17,19,21–23 Very recently, a novel bottom-up approach to anisotropy design in TMDs has been implemented24 on a large area (cm2-scale) MoS2 nanosheets. So far, the effects of the induced anisotropy have been tested and considered only in relation to Raman phonon modes or steady state photoluminescence and absorption,14,17,21–23 whereas no dynamic transient behavior on a broad spectral region has been reported despite the potential to elucidate a polarization-dependent exciton dynamics. Here we report on the linear and transient non-linear optical response of few-layer anisotropic MoS2 nanosheets deposited by chemical vapor deposition (CVD) on a SiO2 template


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which has been textured with an uniaxial ripple pattern by defocused ion beam sputtering (IBS) through a self-organised stencil mask.25 By means of linear extinction measurements and ultrafast transient absorption (TA) spectroscopy, we probed a broad spectral region extending from 1.6 to 3.4 eV, covering the A, B, C and D absorption peaks of MoS2. We investigated the effects of the uniaxial corrugation of the MoS2 layers as a function of the polarization of the incident light beam with respect to the main axis of the rippled substrate (defined by the projection of ion beam during sputtering process). Steady-state optical transmission measurements show a strong polarization-dependent extinction all over the probed spectral region with intensity variations among the different resonance peaks. Full-vectorial finite element method (FEM) numerical simulations reveal the major role of the shape anisotropy induced by nanopattering and material anisotropy contributions ascribed to the out–of–plane component of the dielectric tensor of MoS2. We also investigate the optical anisotropy in the transient nonlinear regime by polarization resolved TA measurements with sub-picosecond time resolution. We disclose a rich scenario in terms of spectral and temporal anisotropic features, with a highly dispersive character and a strikingly anisotropic dynamic behavior of the C exciton, thus opening a promising path for exciton tuning by morphology design. Few layers of MoS2 were grown by CVD with high degree of control of the thickness down to the atomic level,26 both on nanopatterned and flat silica substrates (the latter one used as a reference). The nanopatterns are created by grazing angle defocused IBS25 (see Methods for further details) which leads to the self-organized formation of a uniaxial corrugations (Figure 1(a)) in the form of one-dimensional nanoripples (see topographic line profile in Figure 1(b)). A four-layer thick MoS2 film was conformally grown on such template by CVD24 (see Fig. S1 in the Supporting Information (SI) document), resulting in a homogeneous coverage of the surface


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on the cm2 scale with randomly oriented MoS2 polycrystalline grains with average size of about 50 nm. From the atomic force microscopy (AFM) analysis we extracted the lateral correlation length (ξ) and the root mean square roughness (σ) of the nano-pattern (see Methods) that were used to model the corrugation along the x-axis by a sinusoid with periodicity P = 2ξ and peak-tovalley amplitude A = 2√2σ.

Figure 1. (a) Atomic force microscopy (6µm×6µm) of the rippled MoS2 nanosheets and corresponding topographic line profile in (b); the black arrow indicates the projection of the ion beam and the direction of the ripple main axis. (c) Sketch of the system: and represent the electric fields of the incident linearly polarized light with linear polarization parallel or orthogonal to the ripple main axis ( axis). (d) Sketch of the MoS2 metasheet with twisting optical axis employed to model the effective contribution of MoS2 out-of-plane permittivity (orange arrows).

The nearly 1D corrugation of the nanosheets defines two non-equivalent directions for the linear polarization of the wave impinging on the sample. We refer to a transverse electric (TE) wave when the electric field is parallel to the ripple main axis (y-axis), and to a transverse magnetic (TM) wave when the electric field is orthogonal to this axis, as sketched in Figure 1(c). The linear optical transmission of the MoS2 samples is characterized by extinction measurements employing a linearly-polarized light beam coupled to a spectrometer through an optical fiber. Figure 2(a) shows the experimental results both for flat and rippled substrates for


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different angles of incidence and polarizations. The spectra are normalized to the transmission of the bare silica substrate in order to reject spurious contributions from the SiO2 template.

Figure 2. Transmission spectra for MoS2 nanosheets normalized to the transmission of the silica substrate. (a) Experimental data versus (b) FEM simulations for different angles of incidence (0° or 45°), different polarizations (TE or TM), and different substrates (flat or rippled). In the numerical simulations, solid curves refer to the model of Fig.1(c) with isotropic MoS2 permittivity, whereas the dashed curve refers to the metasheet of Fig.1(d), having two nondegenerate in-plane and out-of-plane components of MoS2 permittivity. Black arrows point out the blue shift around the C and D peaks under TM polarization.

We first recorded the transmission spectrum of a flat sample at normal incidence (implying TE/TM degeneracy). The spectrum (black curve in Figure 2(a)) is characterized by four clear dips around the so-called A (1.85eV), B (2.02eV), C (2.73eV) excitonic resonances and D resonance (3.1eV), in agreement with previous studies based on few-layers and monolayer CVD-


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grown MoS2.27,28 The band edge excitonic resonances, i.e. A and B, arise from direct-gap transitions from the twofold split valence band to the conduction band around K point of the Brillouin zone.29 The valence band splitting is due to interlayer interaction as reported for MoS2 films with even number of layers and for bulk substrates.30 The C resonance is conversely determined by a van Hove singularity in the joint density of states which arises from the almost parallel conduction and valence bands between K and points,31,32 and it is referred to as having a localized nature in real space.30 The origin of the D resonance is not thoroughly investigated: to our knowledge, it is reported by Klots et al.33 as a minimum in the quasiparticle band structure along K – direction near the C exciton region and has been recently identified by broadband transient absorption spectroscopy in a CVD MoS2 mono and multilayer.28 In addition to these general features, the optical transmission spectra of MoS2 deposited on a rippled substrate show a clear evidence of polarization dependence on the whole visible spectral range. Figure 2(a) displays the different spectra collected for TE and TM polarizations at normal incidence (red and green lines respectively). We observe a sizeable reduction of transmission for TE polarization (red curve in Fig. 2(a)) and a strong increase for TM polarization (green curve in Fig. 2(a)), with respect to the flat MoS2 sheet, thus giving rise to a giant optical anisotropy at normal incidence out of a few nm thin MoS2 layer in the rippled geometry configuration. Most interestingly, these variations are strongly dispersed in wavelength. As an example, the difference between the TM and TE spectra, relative to the TE, is less than 5% at the A, B excitons and amounts to about 12% and 5% at around the C and D resonances (Fig. S2(a) in SI), leading to a substantial distortion of the TM spectrum with respect to the TE one (Fig. S2(b)). Note also that, despite a smaller dichroism as compared to the C peak, the D peak exhibits a sizeable blue shift, of about 100 meV, in the TM spectrum. We emphasize that such a variation of the transmission spectra


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with polarization cannot be ascribed to a mere geometrical effect due to the fact that a substantial fraction of the nanosheet is tilted. Actually, according to the topography analysis reported above, the average tilt (in modulus) would be about 45° for the rippled MoS2 sheet of Figure 1. However, the optical anisotropy of a flat MoS2 sheet under 45° angle of incidence looks different: the TE (blue curve in Figure 2(a)) and TM (magenta curve in Figure 2(a)) spectra have different amplitudes, but the same shape and no spectral distortion is observed (see also Fig. S2(b)). In particular, no spectral distortion around the C exciton and no shift of the D exciton can be retrieved when tilting a flat MoS2. Note that similar amplitude variations were predicted in calculations of optical absorption from MoS2 monolayers for TM-polarized light at various angles of incidence.34 On the other hand, the variations of the spectral shape observed in Figure 2(a) is a strikingly new effect which is univocally due to the anisotropic nanopatterning. To get a physical insight into the polarization dependent anisotropy induced by nanopatterning in the amplitude and spectral features of the optical spectra, we performed numerical investigations via FEM analysis. We assumed continuous translational invariance along y-direction and the Bloch-Floquet periodicity along x-direction (Figure 1(c)). The silica substrate is assumed to be lossless and nondispersive isotropic with permittivity εsub = 2.1, whereas for the MoS2 nanosheet we employed a dispersive complex permittivity. For the latter, we assumed the ab-initio calculated spectrum reported for a monolayer MoS235 in the 2.2-3.6 eV range, complemented by the experimental data reported for a 3-layer MoS236 in the 1.6-2.2 eV spectral region (Figure S3 (a) and (c) “in-plane component”). Unlike previous studies,34 our model allowed to properly include the A-B excitons contribution. The rippled substrate was approximated by a cosine profile with 250 nm period and 100 nm peak-to-valley amplitude, in accord with the morphological characterization detailed above. Using this low order model, the


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numerical simulations (Figure 2(b)) turned out to be in good qualitative agreement with the experimental data (Figure 2(a)). In particular, the general features of the spectra are well reproduced for the flat MoS2, especially with regard to the position of the transmission dips and their amplitude variations with the angle of incidence (compare black, blue and magenta curves in Figure 2(a) and 2(b)). Quantitative deviations are observed in terms of the widths of the resonances, since numerical simulations retrieve a smaller broadening of MoS2 absorption resonances compared to the experimental ones. This can be easily understood when considering that the model basically disregards inhomogeneous broadening mechanisms due to dislocations and grain boundary effects. Most interestingly, the model is capable of grasping the giant optical anisotropy of the nanopatterned sheets at normal incidence, showing a reduced transmittance for TE polarization (red curve in Figure 2(b)) and a much higher transmittance under TM polarization (green curve in Figure 2(b)) as compared to the flat nanosheet (black curve in Figure 2(b)). However, the major distortion observed around the C peak as well as the blue shift of the D peak under TM polarization are not reproduced. A further insight into the origin of such a peculiar behavior is provided by FEM simulations with a different model, that takes into account the anisotropy of MoS2 permittivity tensor, which actually exhibits two non-degenerate components, one for the electric field tangential to the layer (so-called in-plane component) and one for the electric field orthogonal to the layer (so-called out-of-plane component). Usually, the latter one is neglected since it is not accessible in flat MoS2 sheets at normal incidence, but this is not the case for the rippled configuration. Therefore, we considered both the in-plane and the out-of plane components of the permittivity tensor. In particular, for the in-plane permittivity we used the same data used in the isotropic model, from Ref. 36 in the 1.6-2.2 eV range and from Ref. 35 in the 2.2-3.6 eV range, (Fig. S3 (a) and (c)), and for the out-of-plane we considered the


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spectra given in Ref 35 (Fig. S3 (b) and (d)), that describe the spectral region (2.2-3.6 eV) in which we observed the anisotropic effects. As such, this approach enables a clear-cut comparison between isotropic and anisotropic models. To disclose a possible role of the out-of-plane permittivity in the giant optical anisotropy observed in our experiments we assumed, for the sake of simplicity, a flat configuration (i.e. we neglect shape anisotropy) with periodic rotation of MoS2 optical axis according to the sinusoidal behaviour extracted from AFM analysis, as sketched in Figure 1(d). With this assumption, the interaction of the impingent fields with the MoS2 sheet is affected by the two components of the permittivity. The TM spectrum generated by such a kind of metasheet (dashed green curve in Figure 2(b)) exhibits the enhanced anisotropy observed in the experiment (cf. green curve in Figure 2(a)) around the C peak and the blue shift of the D peak, even though just about 40 meV. Moreover, the simulations indicate that the spectral distortion of the C exciton in the TM spectrum is partially ascribable to a blue shift of the C peak of about 120 meV, not observed in the experiments possibly because of the larger broadening of the C resonance in the recorded spectra. These results suggest that the rippled morphology switches the anisotropic nature of the MoS2 permittivity on, by enabling the coupling of the TM incident electric field with the out-of-plane component of the tensor. Actually, whilst the TE radiation can couple just with the in plane (y-component) of the dielectric tensor, the TM radiation has non-vanishing projections on both the x and z components, i.e. on both in plane and out-of plane-permittivity (Figure 1(c)). Remarkably, the out-of-plane component (see Fig. S3 in the SI document) is almost negligible up to 2.8 eV whereas it rapidly increases for higher energies, that is the spectral region where we observe the most intriguing optical anisotropy. Our phenomenological model of an effective MoS2 metasheet is capable of simply rationalizing the measured optical anisotropy without resorting to other effects.21,22 In any


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case, the nanopatterning turned out to be the key-ingredient in the control of the linear optical anisotropy of MoS2 nanosheets and correspondingly substantial modifications of the transient and non-linear optical properties are expected as well. In order to characterize the transient optical properties and to test the possible anisotropic behavior of rippled MoS2 in the nonlinear regime, we performed polarization-sensitive TA measurements, through the pump-probe technique. We excited the sample with ξ, the height variations are instead spread randomly.51 In case of the Figure 1(a) we measure σ =35nm and a statistical lateral correlation length ξ =125 nm, which can be interpreted as the half period of the closely packed ripple structure. Linear Optical Transmission. The optical characterization of the textured substrates has been performed with the aim of assessing the interplay between surface morphology and MoS2 optical properties. Extinction spectra have been measured by illuminating the samples at normal


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incidence from the back flat side by means of a compensated deuterium-halogen lamp (DH2000-BAL, Mikropak). The light directly transmitted through the sample is collected by a lens and conveyed by a fibre to a PC controlled high resolution solid state spectrometer (HR4000, Ocean Optics), with 1 nm resolution. Measurements acquired on rippled MoS2 nanosheets have been normalized to the extinction spectrum acquired on the same spot before the MoS2 growth to minimize the scattering contribution due to the presence of nanostructures in the diffractive size regime. MoS2 Synthesis. MoS2 was synthesized following the procedure described in Ref. 26 starting from MoO3 films (thickness 4 nm) grown by means of e-beam evaporation onto rippled and flat SiO2 substrates. The Mo oxide films were placed in the centre of a quartz tube in a tubular furnace. Sulfur powder (1-2 gr, by Sigma Aldrich) was placed in a quartz boat and introduced upstream the quartz tube. This procedure allowed the sulfur powder to start evaporating (around 170 °C) when the substrates were at temperature above 400 °C. An Ar flow of 0.2-0.3 l/h was used as carrier gas for S vapor towards Mo films. The furnace was heated up to 850°C with a 5 °C/min rate and kept at this temperature for 10 minutes. After this process, the furnace was naturally cooled down. Relative Differential Transmission Measurements. For ultrafast pump−probe measurements, the laser system employed was based on a Ti:Sapphire laser source with chirped pulse amplification (Coherent LIBRA HE), which provides pulses with a maximum output energy of about 2 mJ, at 2 kHz repetition rate, a central wavelength of 800 nm and a pulse duration of ~ 95 fs. Pump pulses at 3.1 eV were obtained by second harmonic generation in a β-barium borate 1mm thick crystal and were focused on the sample in a spot of (390 x 235) µm2. The probe beam was a white light supercontinuum generated by focusing a small fraction of the fundamental


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beam

onto

a

3

mm

∆⁄ = − !

CaF2

plate.

Chirp-free

differential

transmission

spectra

at different pump-probe delays were acquired by a fast optical

multichannel analyser operating at the full repetition rate. and

are the transmissions of

the probe through the perturbed and unperturbed sample. All the measurements were carried out at room temperature. Relative Differential Transmission Simulations. The relative differential transmission ∆⁄

as a function of the probe wavelength # and time delay t was simulated according to a perturbative approach (see for example Ref. 52). More precisely, Δ #, %; '() = *#; '()Δ+ , #, % + .#; '()Δ+ ,, #, % 2

where pol is either the TE or TM polarization of the probe, Δ+′#, % and Δ+′′#, % are, respectively, the real and imaginary parts of MoS2 permittivity variation induced by pump absorption. For simplicity, we neglected permittivity variations for the out-of-plane component of MoS2 permittivity, i.e. Δ+′#, % + 1Δ+′′#, % = Δ+23 #, % = Δ+#, %. In above equation,

* and . are polarization-dependent spectral coefficients given by FEM numerical computation

of, respectively, the derivatives d/d+′ and d/d+′′, evaluated at the probe wavelength. The variation Δ+#, % of the in-plane component of MoS2 permittivity is the sum of two contributions, Δ+ #, % and Δ+ 7 #, %, arising, respectively, from two distinct processes: (i) a

broadening of the permittivity spectra caused by the excitons created by the pump (with density NE), and (ii) a band-gap renormalization and subsequent shift of the permittivity spectra caused by the separated charges (with density NC) created by exciton decay. In formulas: Δ+ #, % = 8#Θ % 3 ACS Paragon Plus Environment

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Δ+ 7 #, % = +23 ;# − Γ7 %= − +23 λ 4 where +23 λ is the unperturbed in-plane permittivity of MoS2, 8# = @ +23 λ/dλ , and

Θ = 1.17 ⋅ 10D nm5 and Γ = 20 nm4 two parameters whose values are fitted on the experimental relative differential transmission.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: … Transmission electron microscopy image of nanopatterned MoS2 sheet, relative transmission and normalized extinction spectra, dielectric permittivity tensor of MoS2 , ∆T/T spectra at different probe delays, comparison between experimental and simulated ∆T/T spectra, ∆T/T dynamics of A, B, D resonances and time constant of the C ∆T/T dynamic.

ACKNOWLEDGMENTS All authors acknowledge the project MIUR-PRIN 2015 Grant No. 2015WTW7J3 (HotPlasMoS2) for financial support. F. Buatier de Mongeot acknowledges financial support from the Compagnia di San Paolo in the framework of Project ID ROL 9361 and from the MAECI in the framework of the Italy–Egypt bilateral protocol. We acknowledge P. Targa and D. Codegoni (STMicroelectronics, Agrate) for TEM measurements. A. Molle acknowledges partial funding from Regione Lombardia – Fondazione CARIPLO, Project “Crystel” (ref. no. 2016-


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0978). E. Cinquanta acknowledges financial support from Fondazione CARIPLO, Project “GREENS” (ref. no. 2013-0656).

ABBREVIATIONS 2D, two-dimensional; TMDs, transition metal dichalcogenides; MoS2, molybdenum disulfide; SiO2, sulfide dioxide; CVD, chemical vapor deposition; IBS, ion beam sputtering; TA, transient absorption; TE, transverse electric; TM, transverse magnetic; FEM, finite element; AFM, atomic force microscopy; PB, photobleaching; PA, photo-induced absorption.

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For Table of Contents Use Only

Ultrafast Anisotropic Exciton Dynamics In Nanopatterned MoS2 Sheets Andrea Camellini, Carlo Mennucci, Eugenio Cinquanta,

Christian Martella, Andrea

Mazzanti, Alessio Lamperti, Alessandro Molle, Francesco Buatier de Mongeot, Giuseppe Della Valle, and Margherita Zavelani-Rossi

The uniaxial rippled MoS2 nanosheet, created by self-organized nanopattering of silica substrate, gives rise to giant optical dichroism at normal incidence and ultrafast anisotropic exciton dynamics. A keyrole of the out-of-plane component of MoS2 permittivity tensor is revealed.


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Ultrafast Anisotropic Exciton Dynamics In ... - ACS Publications

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