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Jun 27, 2019 - Electron beam or photonic excitation of localized surface plasmon resonances (LSPRs) in gold nanoparticles and their subsequent ...
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Cite This: J. Phys. Chem. C 2019, 123, 17943−17950

Charge-Induced Lattice Compression in Monolayer MoS2 Astha Singh, Geeta Sharma, Bhanu Pratap Singh,* and Parinda Vasa*

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Department of Physics, Indian Institute of Technology Bombay, 400076 Mumbai, India ABSTRACT: Electron beam or photonic excitation of localized surface plasmon resonances (LSPRs) in gold nanoparticles and their subsequent nonradiative relaxation create a nonuniform charge distribution near the metal−semiconductor interface via hot-electron injection from gold nanoparticles to the monolayer MoS2 conduction band states. These charge domains induce lattice compression in the MoS2 lattice because of the inverse piezoelectric response arising due to the noncentrosymmetric nature of the crystal lattice. Our high-resolution transmission electron microscopy studies reveal that as much as 9−10% biaxial compressive strain is generated in free-standing monolayer MoS2 in the vicinity of gold nanoparticles, which relaxes rapidly within 50 nm from the edge of the gold nanoparticle. The metal interface-induced hot-electron injection via photonic excitation of LSPRs and the resultant lattice distortion also manifest in shifts as well as broadening of A1g(Γ) and E12g(Γ) Raman modes of the monolayer MoS2 in presence of gold nanoparticles. These findings may aid in tuning the lattice structure of MoS2 via hot-electron injection.

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INTRODUCTION Atomically thin (bi/monolayer) 2-dimensional (2D) transition metal dichalcogenide semiconductors are fast emerging as potential materials for new generation of optoelectronic and photonic devices.1 These materials possess intriguing properties, which stem from their crystal structure, large spin−orbit coupling, and strong Coulomb interaction. For instance, monolayer MoS2 is a direct band gap semiconductor with a band gap (∼2.0 eV) in the visible region of the spectrum.2 It exhibits promising optical properties associated with tightly confined electronic states having large oscillator strength for excitonic transitions.2−6 Such optical properties along with high carrier mobility makes it promising for developing optoelectronic,7,8 photovoltaic,9,10 and photocatalysis11,12 applications and devices for solar energy harvesting as well as for fabricating high on/off contrast field effect transistors8 in stretchable electronics. Furthermore, novel phenomena, which are a manifestation of strong spin−valley coupling, broken inversion symmetry,13,14 and an additional degree of freedom addressable through nonzero Berry’s curvature and orbital magnetic moments13−17 are defining a paradigm shift towards valleytronic devices.18−20 In many of these applications, valley pseudo-spin14 can be used as an information carrier rather than a charge. Demonstrations of optical manipulation of spin− valley polarization and valley quantum coherence at an ultrafast time scale in 2D semiconductors including monolayer MoS2 have paved the way for optically driven spintronics and quantum information processing hardware.21−23 An interesting feature of these 2D materials is that their optical, electronic, and magnetic properties can be easily and reversibly tuned within a significant extent through strain engineering owing to their high mechanical strength and carrier doping possibility.24−32 In particular, monolayer MoS2 can withstand up to 11% tensile strain as determined by nanoindentation measurements performed on a stretched membrane.33 However, Lloyd et al. have shown that the strain © 2019 American Chemical Society

in MoS2 causes transition from a direct to an indirect band gap and reduces the photoluminescence emission efficiency.25 While strain engineering,25,28 can be useful for developing piezotronic devices, strain in the contact regions of metal electrodes can adversely affect their local transport properties.8,30 On the other hand, creation of 2D semiconductor− metal hybrid nanostructures can lead to enhancement in photoelectronic properties and control of light−matter interaction because of the field localization in metal nanostructures.31,32 In 2D semiconductor−metal hybrid nanostructures, resonant interaction with light, can excite localized surface plasmon resonances (LSPRs), which are responsible for light localization and charge transfer phenomena. These plasmonic excitations can also be induced by a stream of energetic electrons. Unlike photons, which can excite only the bright modes, electron beam can excite both bright and dark plasmon modes.34,35 These LSPRs can relax either by radiative relaxation by photon re-emission or nonradiatively by hot carrier generation via Landau damping.31,32 Bright SPRs can also be suppressed by excitation of dark plasmon modes.36 Thus, plasmonic nanostructures can dissipate energy to adjacent materials by transfer of hot electrons generated via nonradiative plasmon relaxation. These hot electrons are known to influence the carrier distribution in 2D materials.37 Hence, metal-induced hot-electron injection in presence of light or electron beam offers the unique possibility of spatially selective and localized dynamical doping as well as active control and manipulation of 2D-semiconductor properties.31,32,38−41 Such hot-electron injection has fascinating applications in developing photodetectors,32,42 and photocatalysts.43 However, carrier doping is inevitably accompanied by modifications in the Coulomb interaction. There are Received: March 11, 2019 Revised: June 25, 2019 Published: June 27, 2019 17943

DOI: 10.1021/acs.jpcc.9b02308 J. Phys. Chem. C 2019, 123, 17943−17950

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The Journal of Physical Chemistry C

Figure 1. (a) HRTEM image of a monolayer MoS2 in the absence of gold nanoparticles. The image shows the Mo lattice exhibiting a hexagonal pattern (lattice constant = 3.12 Å) with atomic resolution, whereas the inset shows a relatively lower resolution image. White arrows represent the basis vectors of a hexagonal lattice and dashed lines identify the mutually perpendicular atomic orientations for which the interplanar separations are evaluated. The square marks the area shown with a higher resolution. (b) TEM images of spherical, elliptical, and rod-shaped gold nanoparticles used in this study. (c) Typical HRTEM image of a monolayer MoS2 film in the vicinity of a gold nanorod. The square marks the area over which the lattice constant is evaluated using FFT analysis. The distance from the gold nanoparticle is measured along the surface normal taken at the nearest edge of the particle. It is the distance between the nearest point on the surface to the center of the rectangle used to perform the FFT analysis. (d) High-resolution image of the area marked by the square in (c). The distance from the nanoparticle is ∼4 nm. There is a significant reduction in the interplanar separation for (010) and (110) planes due to which the lattice constant is decreased to 2.83 Å. (e−g) Diffraction pattern and spatial frequency patterns obtained by FFT analysis of the region shown in (d). The lattice constants presented in this work have been evaluated using similar FFT analysis of the selected area of HRTEM images. The reduction in interplanar separations for two mutually perpendicular sets of crystallographic planes is strongly suggestive of a biaxial strain.

in piezoelectric MoS2 (unlike graphene) is likely to yield untenable results. This is because there is a possibility that the strong tip−sample interactions can induce further lattice distortions and affect the intimately related charge distributions. In this paper, we report on high-resolution transmission electron microscopy (HRTEM) and far-field Raman spectroscopy studies on the lattice distortion in a monolayer MoS2 film induced by charge transfer between MoS2 and physisorbed gold nanoparticles. Our intrinsically different studies directly probe the extent and distribution of the strain with very high spatial resolution as well its origin in a 2D material.

indications that these modifications can strain the lattice of 2D-layered materials. Can charge transfer-induced strain in a 2D-transition metal dichalcogenide lattice be used to engineer its material properties or can it affect the performance of the devices involving hot-election injection in an undesirable manner? These questions also become pertinent for the influence of carrier doping from electrical contacts in determining the robustness of spin−valley coupling in spintronic and valleytronic devices, even in the case of the lattice-matched structures. However, to the best of our knowledge, the issue related to lattice strain-induced by carrier doping in monolayer MoS2, if any, has not been addressed so far. Optical spectroscopy and electron energy loss spectroscopy (EELS), both have emerged as powerful tools for the characterization of spatial distribution of plasmons and have been used extensively. EELS has been used to study augmented plasmonic damping due to hot-electron injection in gold nanoparticles covalently bonded to nanosheets of WS2, another representative of the transition metal dichalcogenides family.44 DeJarnette and Roper used EELS to study changes in plasmon resonances and their decay by hot-electron transfer from gold nanoparticles to adjacent graphene using a 125 kV scanning transmission electron microscope equipped with a 50 meV resolution GATAN monochromator and measured a hotelectron transfer efficiency of 20%.45 In addition, electron microscopy has also been used for the study of the strain in 2D materials,46 as it offers a high spatial resolution without exerting any force of its own on their lattice. Though optical spectroscopy techniques are very efficient, it is challenging to get sub-10 nm resolution using these techniques including near-field microscopy and tip-enhanced Raman spectroscopy.47,48 The doping concentration as a function of distance from the gold nanoparticles has been probed in graphene by scanning probe techniques.37 However, use of these techniques

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EXPERIMENTAL SECTION

In our investigations, chemical vapor deposition (CVD)-grown monolayer MoS2 samples on fused silica substrates were procured from 6Carbon Technology, China. To prepare the samples for HRTEM, as-purchased MoS2 samples (on fused silica substrate) were soaked in few drops of water for about 30 s. The water film exfoliates MoS2 flakes as they are only loosely bound to the substrate. The water drop containing MoS2 flakes was collected with the help of a syringe and drop-casted over a lacey-carbon transmission electron microscopy (TEM) grid. The sample was kept for drying under an infrared (IR) lamp for about 30 min. A drop of aqueous colloid containing gold nanoparticles was then drop-casted over the TEM grid containing free-standing MoS2 flakes and was again dried under the IR lamp. Hybrid nanostructures for these studies were prepared with three different shapes of gold nanoparticles. The aqueous gold colloids (Nanopartz Inc, USA) used here contained either gold nanospheres or nanorods or nanoellipsoids (with cetyl trimethylammonium bromide or CTAB as the surfactant) in distilled water, with EMoS2 − EAu, where ESPR = 2.33 eV corresponding to laser excitation. Once again, our far-field Raman measurements confirm that physisorbed gold nanoparticles on the surface induce compressive strain in the monolayer MoS2 film on a rigid substrate, which cannot be caused by purely mechanical loading effects. Earlier measurements of Raman modes of monolayer MoS2 have shown that because of the short tunneling lengths of electrons, a close contact is necessary for enhancing charge transfer effects.56 Because in both the hybrid structures studied here: free-standing monolayer dotted with nanoparticles and a monolayer on the rigid substrate coated with nanoparticles, MoS2 is in direct contact with the nanoparticles, they are optimized for maximizing charge transfer effects. Following similar analysis involving Raman shifts, induced strain and doping concentration,25,54,55 we estimate the electron doping concentration ∼3 × 1014 cm−2 corresponding to the maximum strain seen in HRTEM experiments (∼9%) in the vicinity of gold nanoparticles. Of course, the values of strain and charge doping obtained from Raman measurements are at least an order of magnitude

only observe the E12g(Γ) mode at ∼385 cm−1 and A1g(Γ) at ∼404 cm−1 modes as the interlayer shear mode (∼32 cm−1) is out of the scan range of our spectrometer and the E1g(Γ) mode is forbidden in the backscattering geometry. The peak positions of Raman modes were determined by fitting. Voigt lineshape was chosen for fitting to account for the inhomogeneous broadening arising from the ensemble averaging. The fits yielded accuracy better than ±0.1 cm−1 for E12g(Γ) and A1g(Γ) modes. The fitted values of Raman mode frequencies are obtained to be 385.19 cm−1 for E12g(Γ) and 404.3 cm−1 for A1g(Γ) modes, respectively. These are consistent with the reported values for unstrained and undoped monolayer MoS2.53 On the area of drop-casted elliptical gold nanoparticles, E12g(Γ) is seen to be blue-shifted by 0.7 cm−1 to 385.89 cm−1 while the A1g(Γ) mode is redshifted by 1.6 cm−1 to 402.7 cm−1. There is also considerable broadening of both the modes in presence of gold nanoparticles. The measurements performed at different locations on the coated region yielded spectra similar to those shown in Figure 4. It has been shown that electron−phonon coupling matrix elements nearly vanish for the in-plane, E12g(Γ) and hence it is relatively immune to carrier doping effects, while out-of-plane, A1g(Γ) mode exhibits strong electron-phonon coupling, hence is affected by the doping.54−56 As a result, A1g(Γ) mode frequency is strongly sensitive to carrier doping. Increased (decreased) electron occupancy of conduction band states by n-type doping (p-type doping) softens (stiffens) this mode.53,54 On the other hand, the frequency of the E12g(Γ) mode is much more sensitive to strain than the relatively immune A1g(Γ) mode.25 Previous studies on the strain response of the E12g mode have shown it to stiffen (soften) under compressive (tensile) strain.56 In view of the above, the observed red-shift of the A1g mode and blue-shift of the E12g mode imply n-type doping by hot-electron injection and compressive strain in the monolayer MoS2 lattice in vicinity of physisorbed gold nanoparticles. The increased broadening in presence of gold nanoparticles corroborates with inhomogeneous broadening due to averaging over the varying 17947

DOI: 10.1021/acs.jpcc.9b02308 J. Phys. Chem. C 2019, 123, 17943−17950

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each having unique dependence on the shape of the particle, distance, and the origin responsible for creating the nonuniform charge distribution locally. Based on our experimental results and analysis, we infer that physisorption of gold nanoparticles results in the localized biaxial compressive strain as a result of ion-charge polarization created in monolayer MoS2 by hot-electron injection from the nonradiative relaxation of LSPRs excited in gold nanoparticles. Such a lattice distortion can strongly affect the electronic and optical properties of monolayer MoS2 like nature and the energy of band gap, excitonic transitions, as well as the photoluminescence yield. A wide variety of metal−semiconductor hybrid nanostructures involving LSPRs have been proposed as devices to take advantage of the stronger light−matter interactions and faster carrier dynamics.60,61 Hence, the role of charge transfer effects in structures involving metal electrodes needs to be addressed thoroughly while designing and developing monolayer MoS2 devices. Though it is challenging to prevent the lattice distortion, it may be used as an effective means to manipulate the material properties of 2D semiconductors.

smaller than those obtained from HRTEM measurements. This is because Raman measurements yield an averaged value of the spatially varying strain over the entire laser excited area (∼1 μm) of the monolayer MoS2 film coated with gold nanoparticles. However, the concentration of gold nanoparticles is not uniform throughout and there are regions which are not coated with the nanoparticles. On the other hand, HRTEM measurements give the local strain at a point in the vicinity of a gold nanoparticle. Moreover, hot-electron doping concentrations can be much larger to begin with in the case of HRTEM experiments, because the electron beaminduced generation of plasmons leads to excitation of both dark and bright plasmon modes as against excitation of only bright modes by photons (in Raman measurements). These bright modes relax predominantly by re-emission of photons, while hot-electron are generated by nonradiative relaxation of plasmons. Dark modes are known to generate more hotelectrons due to the suppression of radiative plasmon relaxation.36 Monolayer MoS2 being a noncentrosymmetric structure, it has been theoretically predicted51 and experimentally demonstrated57−59 to exhibit strong piezoelectricity arising from strain induced ion-charge polarization associated with the lattice distortion. It is then natural to expect that polarization domains created by local hot-electron injection in monolayer MoS2 will generate compressive strain via the inverse piezoelectric effect. Thus, the observed compressive strain in TEM and Raman experiments is attributed to hot-electron injection via the nonradiative relaxation of LSPRs excited by the electron beam in the former, and the laser beam in the latter case. In either experiment, there is an electron transfer from the populated LSPR level to the MoS2 lattice. In Raman experiments, as the excitation photon energy is larger than the band gap, there can be other conduction band electronic levels of MoS2 involved in charge redistribution. Absence of any direct electron-beam or laser-induced doping effects and lattice distortion in the MoS2 film without LSPR levels is ruled out by measurements on monolayer MoS2 in the absence of and far away from gold nanoparticles. These measurements confirm the undistorted lattice. The origin of nonuniform charge or polarization domains created at the interface certainly warrants further experimental investigations that are outside the scope of the present work. Our results suggest that the strength of the charge domains is related to the size and shape of gold nanoparticles. Because of the comparatively higher curvature, the nanosphere gives rise to a larger lattice compression compared to the nanorod (Figures 2 and 3). There is also a possibility of charge transfer accompanying the alignment of Fermi levels at the metal−semiconductor interface, which will result in electron transfer from MoS2 to gold nanoparticles.56 However, this would have resulted in p-type doping, which is not observed in Raman measurements. This is expected as the hot-electron injection via LSPR levels is a dominant effect compared to Fermi level alignment in plasmonic metal− semiconductor hybrid nanostructures. In any case, irrespective of the mechanism of charge transfer (hot-electron injection or Fermi level alignment), doping would result in the creation of localized charge or polarization domains in the vicinity of the gold nanoparticles that can deform the lattice via the inverse piezoelectric response in noncentrosymmetric monolayer MoS2. Because the induced strain as a function of the distance from the interface (Figure 3) can be satisfactorily fitted to a quadratic polynomial, there can be more than one mechanism,

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SUMMARY We have investigated the effect of physisorbed gold nanoparticles on the free-standing monolayer MoS2 lattice using highly spatially resolved HRTEM imaging. The observations are supported by far-field Raman spectroscopy measurements performed on an ensemble. Electron microscopy reveals the emergence of as much as 9−10% biaxial compressive strain in the vicinity of gold nanoparticles, which disperses away within ∼50 nm distance from the nearest metal−semiconductor interface. Raman spectroscopy measurements confirm compressive strain and establish that charge transfer occurring at the interface to be the cause for lattice distortion. The compressive strain is attributed to inverse piezoelectric effect resulting from polarization domains generated because of the hot-electron injection via nonradiative LSPR relaxation of gold nanoparticles. Such lattice distortion effects at the metalsemiconductor interface are anticipated to have implications on the performance of valleytronic and spintronic devices as well as for the hot-electron injection detectors and photocatalytic energy harvesting applications. The strain effects can also be used as an effective means to manipulate the material properties of 2D semiconductors.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.P.S.). *E-mail: [email protected] (P.V.). ORCID

Parinda Vasa: 0000-0002-3182-0736 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Sophisticated Analytical Instrument Facility (SAIF) and Centre for Research in Nanotechnology and Science (CRNTS) at IIT Bombay for providing synthesis and characterization facilities. We also acknowledge financial support from the Science and Engineering Board (SERB), Department of Science and Technology to P.V. under the project grant no. CRG/2018/000157. We are grateful to Dr. 17948

DOI: 10.1021/acs.jpcc.9b02308 J. Phys. Chem. C 2019, 123, 17943−17950

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The Journal of Physical Chemistry C

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Pranab Mohapatra, Irfana Neyaz Ansari, Dr. Mohan Chandra Mathpal, and other group members for their help.

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DOI: 10.1021/acs.jpcc.9b02308 J. Phys. Chem. C 2019, 123, 17943−17950

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DOI: 10.1021/acs.jpcc.9b02308 J. Phys. Chem. C 2019, 123, 17943−17950