Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor

Journal name:
Nature Materials
Volume:
13,
Pages:
1091–1095
Year published:
DOI:
doi:10.1038/nmat4061
Received
Accepted
Published online

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are emerging as a new platform for exploring 2D semiconductor physics1, 2, 3, 4, 5, 6, 7, 8, 9. Reduced screening in two dimensions results in markedly enhanced electron–electron interactions, which have been predicted to generate giant bandgap renormalization and excitonic effects10, 11, 12, 13. Here we present a rigorous experimental observation of extraordinarily large exciton binding energy in a 2D semiconducting TMD. We determine the single-particle electronic bandgap of single-layer ​MoSe2 by means of scanning tunnelling spectroscopy (STS), as well as the two-particle exciton transition energy using photoluminescence (PL) spectroscopy. These yield an exciton binding energy of 0.55 eV for monolayer ​MoSe2 on graphene—orders of magnitude larger than what is seen in conventional 3D semiconductors and significantly higher than what we see for ​MoSe2 monolayers in more highly screening environments. This finding is corroborated by our ab initio GW and Bethe–Salpeter equation calculations14, 15 which include electron correlation effects. The renormalized bandgap and large exciton binding observed here will have a profound impact on electronic and optoelectronic device technologies based on single-layer semiconducting TMDs.

At a glance

Figures

  1. Morphology of monolayer MoSe2 on bilayer graphene.
    Figure 1: Morphology of monolayer ​MoSe2 on bilayer graphene.

    a, Top and side view sketches of monolayer ​MoSe2, including the substrate. b, High-resolution STM image of ​MoSe2 (Vs = −1.53 V, It = 3 nA, T = 5 K). c, STM image (typical resolution) of monolayer ​MoSe2 showing a 9.7 Å × 9.7 Å moiré pattern with an angle of 3° between the moiré pattern and the ​MoSe2 lattice (Vs = −0.9 V, It = 20 pA, T = 5 K). d, High-resolution STM image of BLG (Vs = −0.5 V, It = 30 pA, T = 5 K). Unreconstructed unit cells are indicated in green for ​MoSe2 and dark red for BLG. Approximate moiré pattern unit cell for ​MoSe2 is outlined in orange.

  2. Electronic structure of monolayer MoSe2 on bilayer graphene.
    Figure 2: Electronic structure of monolayer ​MoSe2 on bilayer graphene.

    a, Energy diagram schematically indicating the electronic bandgap (Eg), the optical bandgap (Eopt), and the exciton binding energy (Eb). b, STM dI/dV spectrum acquired on monolayer ​MoSe2/BLG showing the electronic bandgap and nearby electronic features: V1−4 in the valence band (VB) and C1 in the conduction band (CB) (f = 873 Hz, It = 5 nA, Vrms = 3 mV, T = 5 K). c, Close-up view of ​MoSe2 STS (boxed region in b and d) showing the valence band maximum (VBM) and V1 feature (f = 873 Hz, It = 4 nA, Vrms = 2 mV, T = 5 K). d, Logarithm of a typical dI/dV spectrum used in the statistical analysis to obtain Eg (f = 873 Hz, It = 5 nA, Vrms = 3 mV, T = 5 K).

  3. Optical characterization of monolayer MoSe2 on bilayer graphene.
    Figure 3: Optical characterization of monolayer ​MoSe2 on bilayer graphene.

    Representative photoluminescence spectra acquired at room temperature (RT; black) and at 77 K (green) for 0.8 ML ​MoSe2 on a BLG/​SiC substrate. The photoluminescence at room temperature is centred at 1.55 eV. The peak shifts to 1.63 eV at 77 K. This photoluminescence peak corresponds to the lowest-energy exciton transition (Eopt) in single-layer ​MoSe2 (inset).

  4. Comparison between ab initio excited-state calculations and single-layer MoSe2 experiment.
    Figure 4: Comparison between ab initio excited-state calculations and single-layer ​MoSe2 experiment.

    ac, Relevant energy levels sketched for GW–BSE calculation without substrate (a), GW–BSE calculation with BLG substrate (b) and experimental data (c). d, Calculated optical absorbance of single-layer ​MoSe2 with and without electron–hole interactions, including BLG screening. A constant Gaussian broadening of σ = 50 meV (30 meV) was used in the curve with (without) electron–hole interactions. The shaded grey area corresponds to energies above the single-particle electronic gap. The experimental differential reflectivity spectrum measured at 77 K is shown in green. e, Spatial map of the exciton wavefunction corresponding to the excitonic peak labelled A in ad (wavefunction is shown with the hole (black circle) fixed in space). Mo atoms are small black squares, Se atoms not shown.

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Author information

  1. These authors contributed equally to this work.

    • Miguel M. Ugeda,
    • Aaron J. Bradley &
    • Su-Fei Shi

Affiliations

  1. Department of Physics, University of California at Berkeley, Berkeley, California 94720, USA

    • Miguel M. Ugeda,
    • Aaron J. Bradley,
    • Su-Fei Shi,
    • Felipe H. da Jornada,
    • Diana Y. Qiu,
    • Wei Ruan,
    • Feng Wang,
    • Steven G. Louie &
    • Michael F. Crommie
  2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Felipe H. da Jornada,
    • Diana Y. Qiu,
    • Feng Wang,
    • Steven G. Louie &
    • Michael F. Crommie
  3. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Yi Zhang,
    • Sung-Kwan Mo &
    • Zahid Hussain
  4. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

    • Yi Zhang &
    • Zhi-Xun Shen
  5. State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China

    • Wei Ruan
  6. Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA

    • Zhi-Xun Shen
  7. Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Feng Wang &
    • Michael F. Crommie

Contributions

M.M.U., A.J.B., S-F.S., F.W. and M.F.C. conceived the work and designed the research strategy. M.M.U. and A.J.B. measured and analysed the STM/STS data in collaboration with W.R. for the ​MoSe2/HOPG experiments. S-F.S. carried out the photoluminescence and Raman experiments. F.H.d.J. and D.Y.Q. performed the theoretical calculations. Y.Z. and S-K.M. performed the MBE growth and characterization (LEED, RHEED, core-level spectroscopy) of the samples. Z.H. and Z-X.S. supervised the MBE and sample characterization. F.W. supervised the optical measurements. S.G.L. supervised the theoretical calculations. M.F.C. supervised the STM/STS experiments. M.M.U. wrote the paper with help from A.J.B. and M.F.C. M.M.U. and M.F.C. coordinated the collaboration. All authors contributed to the scientific discussion and manuscript revisions.

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The authors declare no competing financial interests.

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