High-Energy Radiation Hardness of Isotopically Pure Monolayer MoS₂ Probed by Raman Spectroscopy

Two-dimensional (2D) semiconductors have gained significant interest due to their atomically thin structure, sensitivity to environmental stimuli, and potential utility for radiation-hard space applications. Molybdenum disulfide, MoS₂, is a promising n-type semiconductor currently being investigated for next-generation computing and logic applications [1]. However, little is known about the effects of high energy radiation on these materials.

Radiation hardness is a key property for electronics in extreme environments, as ionizing radiation can damage or degrade such systems. 2D materials have been predicted to be innately radiation hard because their atomic thinness minimizes the interaction volume through which charged particles can pass [2]. Previous work has shown that radiation-induced substrate effects, such as interface defects and oxide fixed charges, can cause degradation in 2D semiconductor devices [2]. However, the effect of radiation on Raman and photoluminescence (PL) properties of these materials has not yet been studied.

Here, we study the effect of high-energy radiation on monolayer MoS₂. We grow samples of monolayer MoS₂ on SiO₂/Si substrates with solid-source chemical vapor deposition at a growth temperature of 750°C. To investigate the effect of isotopic purity on radiation hardness, we grew MoS₂ with isotopically pure ⁹⁸Mo source in addition to the “natural” Mo source with average isotopic mass of ~96 amu. We then expose the samples to high-energy gamma radiation from a Co-60 source, such that the samples receive a total ionizing dose (TID) of 5, 25, 50, 100, and 220 krad. The samples were exposed to radiation in nitrogen ambient to prevent potential in situ oxidation or defect-passivation effects [3]. After radiation, we measure the Raman and PL emission spectra with a 532 nm laser in a Horiba LabRAM.

We find that the first-order E’ and A₁’ Raman peaks in natural MoS₂ redshift by 0.06 ± 0.002 cm^-1/10 krad and 0.04 ± 0.001 cm^-1/10 krad, respectively. In addition, the A exciton peak in the PL spectrum shifts by 0.6 ± 0.08 meV/10 krad. Previous work suggests that red shifting of both the E’ and A₁’ modes and of the PL peak is indicative of n-type doping in 2D materials [4]. In addition, some studies have attributed the E’ left shoulder peak to increased disorder in the material, potentially correlating to defect density [5]. We observe that the intensity ratio between the E’ left shoulder and the E’ peak increases by 0.004 ± 0.002 per 10 krad, suggesting that high-energy radiation is damaging to MoS₂. We hypothesize that radiation induces very deep traps in the MoS₂ band gap, consistent with previous radiation studies on ceramics [6].

In MoS₂ grown with ⁹⁸Mo, the E’ and A’ peaks also redshift by 0.03 ± 0.004 cm^-1/10 krad and 0.05 ± 0.004 cm^-1/10 krad, while the A exciton peak blueshifts by 1.5 ± 0.1 meV/10 krad. However, there was no statistically significant change in the E’ shoulder intensity ratio between the four doses. This result may suggest that the MoS₂ grown with ⁹⁸Mo may sustain less radiation damage compared to the natural MoS₂.

Our results challenge the assumption that 2D materials are innately radiation hard. We show that Raman and PL spectroscopy can characterize the damage caused by high-energy radiation for monolayer MoS₂. Future work will investigate the physical mechanisms behind this radiation-induced damage and extend the technique to other 2D semiconductors. This work was supported in part by the ARCS Foundation Fellowship (J.A.Y.), by the NSF MPS-Ascend Fellowship (T.P.), and by the Stanford SystemX Alliance (other authors).

[1] S. Das et al., Nat. Electron. 4, 786 (2021).
[2] A. J. Arnold et al., ACS Appl. Mater. Interfaces 11, 8391 (2019).
[3] A. V. Krasheninnikov, Nanoscale Horiz. 5, 1447 (2020).
[4] A. Michail et al., Appl. Phys. Lett., 108, 173102 (2016).
[5] K. Schauble et al., ACS Nano, 14, 14798 (2020).
[6] N. J. Kreidl, Mat. Sci. Res. 5, 521 (1971).