Enhanced Light Emission from Monolayer Semiconductors by Forming Heterostructures with ZnO Thin Films

Abstract

Monolayer transition-metal dichalcogenides (1L-TMDs) are atomically thin direct band gap semiconductors, from which the emission of light is determined by optical transitions of exciton complexes such as neutral excitons and trions. While the quantum yields of 1L-TMDs are quite low, the ability to control the populations of exciton complexes in 1L-TMDs through various doping processes is an interesting advantage, and provides ample possibilities for engineering the optical properties of these semiconductor monolayers. Here we demonstrate a simple method of controlling the populations of excitons and trions to enhance the light emission of 1L-TMDs by having them form heterostructures with ZnO thin films (TFs). 1Ls of MoS or MoSe showed up to 17-fold increases in photoluminescence (PL) when they were placed on ∼50 nm thick ZnO TFs. This enhancement of the PL was due to charge exchanges occurring through the 1L-TMD/ZnO interface. The PL enhancements and changes in the PL spectra of the 1L-TMDs were greater when the 1L-TMD/ZnO heterostructures were subjected to 355 nm wavelength laser excitation than when they were excited with a 514 nm wavelength laser, which we attributed to the onset of energy transfer by photoexcited excitons and/or the additional p-doping by photoexcited holes in ZnO. The p-doping phenomenon and the enhanced light emission of 1L-TMD/ZnO heterostructures were unambiguously visualized in spatially resolved PL and Raman spectral maps. Our approach using the 1L-TMD/ZnO TF heterostructure suggests that a rich variety of options for engineering the optical properties of 1L-TMDs may be made available by carrying out simple and intuitive manipulations of exciton complexes, and these endeavors may yield practical applications for 1L-TMDs in nanophotonic devices.

Measurement of Quantum Yields of Monolayer TMDs Using Dye-Dispersed PMMA Thin Films

“…The estimated QY of 1L-WS 2 is 0.73 ± 0.11%, which is in a similar range to the previously reported QY of 1L-WS 2 [29]. Figure 3a illustrates the confocal PL spectra of the 1L-MoSe 2 exfoliated on a quartz substrate with varying laser intensities, wherein the peak at~785 nm is consistent with previous PL results for 1L-MoSe 2 (for the Raman spectra, see the Supplementary Materials, Figure S2b) [34,47]. As laser-intensity-dependent PL spectra are illustrated in Figure 3a, the PL spectra are shown to be quite symmetrical even at higher laser intensities in comparison with 1L-WS 2 ; however, the influence of the recombination of trions [34] can be clearly observed in the deconvoluted PL spectra illustrated in Figure 3b,c.…”

“…All 1L-TMDs were exfoliated on quartz substrates using bulk single crystals (2D Semiconductors Inc. Scottsdale, AZ, USA). 1L-MoS 2 samples were grown using the chemical vapor deposition (CVD) method on Si/SiO 2 substrates and transferred onto quartz substrates using the wet transfer method, wherein hydrofluoric acid (HF) was used as the SiO 2 etchant, according to previously reported methods [13,15,34,35]. The CVD-grown 1L-MoS 2 samples were chemically treated using TFSI (Sigma-Aldrich, St. Louis, MO, USA) molecules after being transferred onto the quartz substrates and further optical characterizations were conducted [13,14].…”

“…The excitation light source was a 150-W Xenon lamp (Hamamatsu Photonics, Hamamatsu City, Japan) with a selectable wavelength or 514 nm laser line, respectively. For the confocal PL and transmission spectroscopy measurements, a lab-made laser confocal microscope combined with a spectrometer was used [15,[34][35][36]. The laser light was focused using a 0.9-NA objective lens, and the scattered light was collected using the same objective lens and guided to a 50-cm-long monochromator equipped with a cooled CCD.…”

Absorption and emission modulation in a MoS2–GaN (0001) heterostructure by interface phonon–exciton coupling

“…The conduction band of MoS2 captures the free-carriers in GaN. These excess carriers in MoS2 can also be captured by the defect states in GaN [9]. Optical excitation in this region enhances the charge transfer between the semiconductors without momentum change [10], [14].…”

Progress of Large‐Scale Synthesis and Electronic Device Application of Two‐Dimensional Transition Metal Dichalcogenides

“…The results indicate that charge transfer plays an important role, and the PL intensity can be drastically altered by different substrates. Such substrate dependence was also studied in other films such as ZnO and VO 2 . Electron injection or extraction can effectively tune the exciton charge state that is correlated to modification of PL spectroscopy …”