Plasmonic-Induced Luminescence of MoSe₂ Monolayers in a Scanning Tunneling Microscope

Abstract

We report on a scanning tunneling microscopy-induced luminescence in MoSe2 monolayers supported by uniform and nanopatterned gold substrates. Luminescence intensity mappings, recorded with a nanometric spatial resolution, and spectroscopy measurements were performed and analyzed in terms of photon emission processes taking place within the tip–surface gap region, which supports strongly localized optical and electronic states. We found that, excited by tunneling electrons, the light emission is due to the radiative recombinations of free excitons confined within the MoSe2 monolayer. Light emission is observed at positive and negative bias voltages with very different emission rates. The results are interpreted in terms of charge carrier injection in the MoSe2 layer. Additionally, electrodynamic simulations also stress that resonance between the emitted radiation and the surface plasmons formed in the tip–surface gap region plays a critical role in the emission process. When the MoSe2 layer lays on a nanopatterned gold substrate, its luminescence intensity, induced by the tunneling electrons, is enhanced by nearly an order of magnitude. Such an effect is observed here for the first time in scanning tunneling microscopy experiments. The luminescence enhancement is attributed to the surface plasmon properties of the nanopatterned gold substrate, which spectrally match the excitonic transition of the MoSe2 layer. We show that this surface-enhanced scanning tunneling microscope-induced light emission (SESTM-LE) effect is very useful for investigating the photon emission from localized emitters (e.g., quantum wells, quantum dots, molecules) with a subnanometer spatial resolution.

Electroluminescence of monolayer WS2 in a scanning tunneling microscope: Effect of bias polarity on spectral and angular distribution of emitted light

“…Moreover, TMDs offer unique opportunities for fundamental research on exciton physics in 2D materials. Recently, scanning tunneling microscopy (STM)-induced luminescence (STML) has emerged as a promising nanoscopic probe of the optoelectronic properties of monolayer TMDs [6][7][8][9][10][11]. There is experimental evidence that, under certain conditions, STML reveals the intrinsic excitonic species of the excited TMD monolayer [7].…”

Local strain and tunneling current modulate excitonic luminescence in MoS2 monolayers

“…Accounting for the geometric collection efficiency of the lens system and the photon counter's detection efficiency, the quantum efficiency (QE) of STM-LE of monolayer MoS 2 is estimated to be 3.7 × 10 −6 photons per electron, similar to the value reported for MoSe 2 /Au (∼4 × 10 −6 photons per electron). 24 …”

“…Accounting for the geometric collection efficiency of the lens system and the photon counter's detection efficiency, the quantum efficiency (QE) of STM-LE of monolayer MoS 2 is estimated to be 3.7 Â 10 À6 photons per electron, similar to the value reported for MoSe 2 /Au ($4 Â 10 À6 photons per electron). 24 Tunneling current-induced exciton emission Exciton emission of TMDCs has been reported to depend on current. Examples include multiple-exciton-exciton interactions in a MoS 2 diode 30 and neutral exciton and trion emission controlled by tunneling current.…”

“…The STM tip works as a low-energy electron source that excites the sample locally. Recently, STM-induced light emission (STM-LE) has been applied to detect excitonic luminescence in monolayer MoSe 2 , 23,24 tunneling-currentcontrolled charged and neutral exciton emission in monolayer WSe 2 . 25 Submolecular resolution in STM-induced electroluminescence has been reported for excitonic and vibronic features of molecules.…”

Local strain and tunneling current modulated excitonic luminescence in MoS2 monolayers

“…The STM tip works as a low-energy electron source that excites the sample locally. Recently, STM-induced light emission (STM-LE) has been applied to detect excitonic luminescence in monolayer MoSe2 [23,24], tunneling-current-controlled charged and neutral exciton emission in 3 monolayer WSe2 [25], radiative plasmonic modes in free-standing monolayer MoS2 [26], and photon emission from atomic defects in monolayer WS2 [27].…”