Excitons in epitaxially grown WS2 on Graphene: a nanometer-resolved EELS and DFT study

In this study, we investigate excitonic properties of epitaxially grown WS2, which is of particular interest for various applications due to its potential for upscaling to wafer sized structures. Understanding the effect of the dielectric environment due to changing layer numbers and multi-material heterostructures on the optical properties is crucial for tailoring device properties. Monochromated electron energy loss spectroscopy in a scanning transmission electron microscope is employed to characterize the excitonic spectrum of WS2 on graphene grown by metal organic chemical vapor deposition. This technique provides the required spatial resolution at the nanometer scale in combination with high quality spectra. To complement the experimental results, theoretical investigations using density functional theory and applying the Bethe-Salpeter equations are conducted. We find that by transitioning from mono- to bi- to multilayers of WS2 the spectra show redshifts for both, the K-valley excitons at about 2.0 and 2.4 eV as well as excitonic features of higher energies. The latter features originate from so called band nesting of transitions between the Gamma- and K-point. In summary, this study provides valuable insights into the excitonic properties of WS2 in different layer configurations and environments, which are realistically needed for future device fabrication and property tuning. Finally, we can show that nanometer scale electron spectroscopy supported by careful theoretical modelling can successfully link atomic structure and optical properties, such as exciton shifts, in non-idealized complex material systems like multilayer 2D heterostructures.

[1]  A. Marty,et al.  Mapping domain junctions using 4D-STEM: toward controlled properties of epitaxially grown transition metal dichalcogenide monolayers , 2023, 2D Materials.

[2]  J. D. Thomsen,et al.  Direct Visualization of Subnanometer Variations in the Excitonic Spectra of 2D/3D Semiconductor/Metal Heterostructures. , 2023, Nano letters.

[3]  A. Vescan,et al.  Nucleation and coalescence of tungsten disulfide layers grown by metalorganic chemical vapor deposition , 2023, Journal of Crystal Growth.

[4]  P. Ajayan,et al.  Mapping Modified Electronic Levels in the Moiré Patterns in MoS2/WSe2 Using Low-Loss EELS. , 2021, Nano letters.

[5]  T. Pichler,et al.  Probing Exciton Dispersions of Freestanding Monolayer WSe_{2} by Momentum-Resolved Electron Energy-Loss Spectroscopy. , 2019, Physical review letters.

[6]  A. Bostwick,et al.  Rigid Band Shifts in Two-Dimensional Semiconductors through External Dielectric Screening. , 2019, Physical review letters.

[7]  Bjarke S. Jessen,et al.  Wafer-Scale Synthesis of Graphene on Sapphire: Toward Fab-Compatible Graphene. , 2019, Small.

[8]  J. Maultzsch,et al.  Phonon dispersion in MoS2 , 2018, Physical Review B.

[9]  Michael Walter,et al.  The atomic simulation environment-a Python library for working with atoms. , 2017, Journal of physics. Condensed matter : an Institute of Physics journal.

[10]  Timothy C. Berkelbach,et al.  Coulomb engineering of the bandgap and excitons in two-dimensional materials , 2017, Nature Communications.

[11]  J. Coleman,et al.  Probing the local nature of excitons and plasmons in few-layer MoS2 , 2017, npj 2D Materials and Applications.

[12]  K. Novoselov,et al.  2D materials and van der Waals heterostructures , 2016, Science.

[13]  Daniel G. A. Smith,et al.  Revised Damping Parameters for the D3 Dispersion Correction to Density Functional Theory. , 2016, The journal of physical chemistry letters.

[14]  M. Knupfer,et al.  Nongeneric dispersion of excitons in the bulk of WSe 2 , 2016, 1604.01895.

[15]  Xiaohao Zhou,et al.  Coupling and Interlayer Exciton in Twist‐Stacked WS2 Bilayers , 2015 .

[16]  K. Thygesen,et al.  Simple Screened Hydrogen Model of Excitons in Two-Dimensional Materials. , 2015, Physical review letters.

[17]  S. Blugel,et al.  Wannier function approach to realistic Coulomb interactions in layered materials and heterostructures , 2015, 1504.05230.

[18]  J. Hone,et al.  Measurement of the optical dielectric function of monolayer transition-metal dichalcogenides: MoS 2 , Mo S e 2 , WS 2 , and WS e 2 , 2014, 1610.04671.

[19]  Timothy C. Berkelbach,et al.  Exciton binding energy and nonhydrogenic Rydberg series in monolayer WS(2). , 2014, Physical review letters.

[20]  Gurpreet Singh,et al.  MoS2/graphene composite paper for sodium-ion battery electrodes. , 2014, ACS nano.

[21]  K. Thygesen,et al.  How dielectric screening in two-dimensional crystals affects the convergence of excited-state calculations: Monolayer MoS2 , 2013, 1311.1384.

[22]  A. Neto,et al.  Band nesting and the optical response of two-dimensional semiconducting transition metal dichalcogenides , 2013, 1305.6672.

[23]  L. Chu,et al.  Evolution of electronic structure in atomically thin sheets of WS2 and WSe2. , 2012, ACS nano.

[24]  A. Krasheninnikov,et al.  Effects of confinement and environment on the electronic structure and exciton binding energy of MoS2 from first principles , 2012 .

[25]  Keliang He,et al.  Control of valley polarization in monolayer MoS2 by optical helicity. , 2012, Nature nanotechnology.

[26]  Wang Yao,et al.  Valley polarization in MoS2 monolayers by optical pumping. , 2012, Nature nanotechnology.

[27]  Ji Feng,et al.  Valley-selective circular dichroism of monolayer molybdenum disulphide , 2012, Nature Communications.

[28]  Hisato Yamaguchi,et al.  Photoluminescence from chemically exfoliated MoS2. , 2011, Nano letters.

[29]  Yingchun Cheng,et al.  Giant spin-orbit-induced spin splitting in two-dimensional transition-metal dichalcogenide semiconductors , 2011 .

[30]  Stefan Grimme,et al.  Effect of the damping function in dispersion corrected density functional theory , 2011, J. Comput. Chem..

[31]  K. Jacobsen,et al.  Linear density response function in the projector augmented wave method: Applications to solids, surfaces, and interfaces , 2011, 1104.1273.

[32]  N. A. Romero,et al.  Electronic structure calculations with GPAW: a real-space implementation of the projector augmented-wave method , 2010, Journal of physics. Condensed matter : an Institute of Physics journal.

[33]  S. Grimme,et al.  A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. , 2010, The Journal of chemical physics.

[34]  J. Shan,et al.  Atomically thin MoS₂: a new direct-gap semiconductor. , 2010, Physical review letters.

[35]  A. Splendiani,et al.  Emerging photoluminescence in monolayer MoS2. , 2010, Nano letters.

[36]  C. Beenakker,et al.  Valley filter and valley valve in graphene , 2006, cond-mat/0608533.

[37]  E. Gross,et al.  Exact coulomb cutoff technique for supercell calculations , 2006, cond-mat/0601031.

[38]  K. Jacobsen,et al.  Real-space grid implementation of the projector augmented wave method , 2004, cond-mat/0411218.

[39]  V. Barone,et al.  Toward reliable density functional methods without adjustable parameters: The PBE0 model , 1999 .

[40]  Steven G. Louie,et al.  Electron-Hole Excitations in Semiconductors and Insulators , 1998 .

[41]  Stefan Albrecht Lucia Reining Rodolfo Del Sole Giovanni Onida.  Ab Initio Calculation of Excitonic Effects in the Optical Spectra of Semiconductors , 1998, cond-mat/9803194.

[42]  Eric L. Shirley,et al.  Optical Absorption of Insulators and the Electron-Hole Interaction: An Ab Initio Calculation , 1998 .

[43]  K. Burke,et al.  Rationale for mixing exact exchange with density functional approximations , 1996 .

[44]  Burke,et al.  Generalized Gradient Approximation Made Simple. , 1996, Physical review letters.

[45]  Blöchl,et al.  Projector augmented-wave method. , 1994, Physical review. B, Condensed matter.

[46]  Haas,et al.  Electronic structure of MoSe2, MoS2, and WSe2. II. The nature of the optical band gaps. , 1987, Physical review. B, Condensed matter.

[47]  Louie,et al.  Electron correlation in semiconductors and insulators: Band gaps and quasiparticle energies. , 1986, Physical review. B, Condensed matter.

[48]  H. Tributsch.  Layer‐Type Transition Metal Dichalcogenides — a New Class of Electrodes for Electrochemical Solar Cells , 1977 .

[49]  H. G. Smith,et al.  Lattice dynamics of hexagonal Mo S 2 studied by neutron scattering , 1975 .

[50]  Jean. Steinier,et al.  Smoothing and differentiation of data by simplified least square procedure. , 1964, Analytical chemistry.

[51]  Junliang Liu,et al.  Two-dimensional heterostructures and their device applications: progress, challenges and opportunities—review , 2021, Journal of Physics D: Applied Physics.

[52]  W. M. Sears,et al.  Photovoltaic effect and optical absorption in MoS2 , 1982 .