Tuning of the electronic and vibrational properties of epitaxial MoS₂ through He-ion beam modification

Abstract

Atomically thin transition metal dichalcogenides (TMDs), like MoS2 with high carrier mobilities and tunable electron dispersions, are unique active material candidates for next generation opto-electronic devices. Previous studies on ion irradiation show great potential applications when applied to two-dimensional (2D) materials, yet are limited to micron size exfoliated flakes or smaller. To demonstrate the scalability of this method for industrial applications, we report the application of relatively low power (50 keV) 4He+ ion irradiation towards tuning the optoelectronic properties of an epitaxially grown continuous film of MoS2 at the wafer scale, and demonstrate that precise manipulation of atomistic defects can be achieved in TMD films using ion implanters. The effect of 4He+ ion fluence on the PL and Raman signatures of the irradiated film provides new insights into the type and concentration of defects formed in the MoS2 lattice, which are quantified through ion beam analysis. PL and Raman spectroscopy indicate that point defects are generated without causing disruption to the underlying lattice structure of the 2D films and hence, this technique can prove to be an effective way to achieve defect-mediated control over the opto-electronic properties of MoS2 and other 2D materials.

Ion Treatments on Two-Dimensional Molybdenum Disulfide

“…Figure 6.7 shows the accumulated normalized PL signal intensities for different treatment times at fixed energy of 5 KeV as a function of emission wavelength.In this graph can be observed the emission peak varies slightly from one sample to another, but its overall shape is preserved. This observation contrasts what was observed by Parida et al[53], where they observed the apparition of a new bound exciton while the pristine emission was quenched, possibly due to higher ion dose and energy applied. The graph in figure6.7 by itself might lead one to think the Ion Beam treatment is causing considerable peak emission wavelength shifts.…”

“…Figure 3.3 PL spectra of MoS 2 irradiated by 4 He + ion beams with different fluences given in displacements per atom (dpa) show emergence of the bound exciton, X b , peak around 1.7 eV upon irradiation attributed to generation of point lattice defects. Spectra are shown normalized to the intensity of sapphire peaks and were recorded at 4 K. Adapted from [53]. The tubular furnace used to grow the samples and its controller.…”

“…What is less highlighted in general, but is of equal importance is the experiment's geometry, the ion source-to-sample distance, and the working pressure which influences the ions density. The creation of point defects in monolayer MoS 2 can increase photoluminescence through different mechanisms: they can introduce localized electronic states within the bandgap (either close to the valence band or the conduction band), thus facilitating the radiative transition; they can promote charge transfer between the defect sites and the crystal lattice, thus altering the electronic band and eventually facilitating the radiative emission; the defect sites can also create localized excitons (as seen in [53]) that enhance the PL emission; the modification of the band structure can also increase the light absorption, increasing the quantity of electron-hole pairs and thus the PL emission. Proposal Among the Two-dimensional Transition Metal Dichalcogenides representatives this work concerns monolayer MoS 2 .…”

Towards Low-Temperature CVD Synthesis and Characterization of Mono- or Few-Layer Molybdenum Disulfide

“…A photoluminescence map was recorded from the same molybdenum sample area ( Figure 6 a). It shows significant quenching for multilayer ( Figure 6 b, blue spectrum) and bulk ( Figure 6 b, cyan spectrum) MoS 2 , not only due to the gradual transition from the monolayer direct bandgap to the bulk indirect bandgap [ 46 , 47 , 48 ], but also due to defects and terminations [ 49 , 50 , 51 ]. Monolayer MoS 2 shows a strong photoluminescence peak at 1.85–1.90 eV ( Figure 6 b red spectrum).…”