Effect of Ultraviolet-Ozone Treatment on MoS2 Monolayers: Comparison of Chemical-Vapor-Deposited Polycrystalline Thin Films and Mechanically Exfoliated Single Crystal Flakes
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We report the different oxidation behavior between polycrystalline chemical-vapor-deposited and mechanically exfoliated single crystal MoS2 monolayers by ultraviolet-ozone treatment. As ultraviolet-ozone treatment time increased from 0 to 5 min, photoluminescence emission and Raman modes of both MoS2 disappeared, suggesting structural degradation by oxidation. Analysis with optical absorbance and X-ray photoelectron spectroscopy suggested the formation of MoO3 in both MoS2 after ultraviolet-ozone treatment. In addition, ultraviolet-ozone treatment possibly led to the formation of oxygen vacancies, molybdenum oxysulfide, or molybdenum sulfates in chemical-vapor-deposited MoS2. The measurement of electrical resistance after ultraviolet-ozone treatment suggested the transformation of chemical-vapor-deposited MoS2 into doped MoO3 and of mechanically exfoliated MoS2 into negligibly doped MoO3. These results demonstrate that the crystallinity of monolayer MoS2 can strongly influence the effect of ultraviolet-ozone treatment, providing important implications on the device integration of MoS2 and other two-dimensional semiconductors.
KeywordsMoS2 Monolayer Single crystal Polycrystalline UV-O3 treatment
Chemical vapor deposition
Resistance measured after ultraviolet-ozone treatment
Resistance measured before ultraviolet-ozone treatment
Transition metal dichalcogenides
X-ray photoelectron spectroscopy
There is a great interest in transition metal dichalcogenides (TMDs), such as MoS2, since they offer an attractive possibility for various device applications including transistors, optoelectronic devices, heterojunction structures, sensors, and electrocatalysis [1, 2]. The existence of direct bandgaps in monolayer TMDs makes these two-dimensional semiconductors especially promising for optoelectronic devices [3, 4]. However, critical challenges to fabricate TMD-based optoelectronic devices such as phototransistors include the deposition of high-k dielectrics on TMDs and the doping of TMDs. Because of the absence of dangling bonds on the surface of TMDs, it is challenging to deposit high-k dielectrics on TMDs . Moreover, the doping of TMDs is also challenging as the substitutional doping used for bulk semiconductors such as silicon modifies the two-dimensional structure and properties of monolayer TMDs .
To overcome these difficulties, surface functionalization of TMDs by O2 plasma [7, 8] or ultraviolet-ozone (UV-O3) [9, 10, 11] has been suggested. While these methods can functionalize the surface of MoS2 by surface oxidation, they can simultaneously influence the structure and properties of monolayer MoS2 [12, 13, 14, 15, 16]. For example, oxidation by O2 plasma or UV-O3 treatment altered the Raman vibration modes and photoluminescence (PL) emission of monolayer MoS2 [12, 16]. However, as most studies were based on micrometer-scale monolayer MoS2 flakes obtained by mechanical exfoliation from bulk single crystals, little has been known on their interaction with large-area monolayer MoS2 thin films, which are typically polycrystalline. Grain boundaries in polycrystalline monolayer MoS2 may allow higher reactivity with UV-O3 than that of single crystal, resulting in different oxidation behavior. Therefore, in this study, we explore the effect of UV-O3 treatment on MoS2 monolayers by directly comparing the oxidation behavior of polycrystalline chemical vapor deposition (CVD) thin films and mechanically exfoliated single crystal flakes. We systematically investigate the PL and Raman spectra of both MoS2 monolayers for different duration of UV-O3 exposure. We also investigate the oxidation behavior of both MoS2 monolayers during UV-O3 treatment with X-ray photoelectron spectroscopy (XPS). We further measure electrical resistance of pristine and UV-O3-treated MoS2 monolayers to understand the effect of UV-O3 treatment on MoS2 monolayers.
MoS2 monolayers were exposed to UV-O3 (SEN LIGHTS PL16–110, 185 nm and 254 nm) for 0–5 min at the irradiance of 58 mW cm−2. Optical absorbance was measured by UV-visible spectroscopy (PerkinElmer Lambda 35). Raman/PL spectroscopy (Horiba Jobin-Yvon LabRam Aramis) were measured on pristine and UV-O3-treated MoS2 monolayers with a 532-nm laser and a beam power of 0.5 mW. XPS (Thermo Scientific K-Alpha) was carried out using a monochromatic Al Kα x-ray source (hν = 1486.7 eV) with a take-off angle of 45°, a pass energy of 40 eV, and a spot size of 400 μm in diameter. For all samples, C 1s and O 1s were observed presumably because they are exposed to atmosphere before loaded to ultrahigh vacuum chamber for XPS analysis. Adventitious carbon (C 1s at 284.8 eV) was used as a charge correction reference for XPS spectra. The energy resolution is 0.7 eV measured using the full width at half-maximum intensity of the Ag 3d5/2 peak. MoS2 samples were exposed to atmosphere while they were brought to XPS equipment. Although in situ XPS analysis could provide more accurate information, it was unavailable in this work. For peak deconvolution and background subtraction, Thermo Scientific Avantage Data System software was used. Gaussian functions were used to fit XPS spectra.
To measure the electrical resistance of MoS2 monolayers, Au contacts (100 × 100 μm2, 70 nm thick) were deposited on top of MoS2 by electron-beam evaporation. Spin-coated photoresist on top of Au layer was then patterned by conventional photolithography to form opening areas for subsequent etching. After Au in opening areas was removed by wet etching in aqua regia, remaining photoresist was removed in acetone. Then, the devices were annealed at 200 °C for 2 h in a tube furnace (100 sccm Ar and 10 sccm H2) to remove photoresist residue and to decrease contact resistance. Electrical resistance was calculated with current-voltage (I–V) measurement (Keithley 4200-SCS) in atmospheric environments.
Results and Discussion
Beside AFM measurement, PL and Raman spectra are measured to confirm the formation of MoS2 monolayers. Because of its direct bandgap, MoS2 monolayers allow PL emission at ~ 1.88 eV [3, 4]. In addition, the frequency difference between the two characteristic Raman A1g and E12g modes of MoS2 monolayers is less than 20 cm−1 . In Fig. 3, the PL emission of pristine MoS2 at ~ 1.88 eV indicates that both MoS2 are monolayers. In Fig. 4, pristine MoS2 exhibits the frequency difference between 19.6 and 19.9 cm−1 implying monolayer MoS2. XRD and TEM analysis indicated the single crystal nature of bulk MoS2 crystals and polycrystalline nature of our monolayer MoS2 thin films (Additional file 1: Figure S1). The grain size of monolayer MoS2 thin films is ~ 10 nm .
In S 2p region, the existence of S2−-state can be observed from the binding energy of S 2p1/2 and S 2p3/2 orbitals in pristine MoS2. The binding energy of S2−-state in single crystal MoS2 shows further positive shift than that in CVD MoS2 thin films suggesting higher n-type doping . Although S-O bond is observed at ~ 165 eV in UV-O3-treated single crystal MoS2, it is below the detection limit in CVD thin films. Instead, a new doublet peak of sulfur oxidation state appears at higher binding energy (~ 169 eV) in CVD thin films after UV-O3 treatment for 3 min. This new doublet corresponds to the S 2p peaks of oxidized sulfur S6+, suggesting possibly the formation of various molybdenum sulfates Mo (SO4)x . While the intensity of S2− doublet keeps decreasing with longer UV-O3 exposure, the intensity of S6+ doublet further increases after 5-min UV-O3 treatment, suggesting further conversion of S2− into higher oxidation state (S6+) by oxidation. Similarly with Mo4+ peaks, the intensity of S2− peaks does not change with UV-O3 treatment time in large MoS2 single crystals. The existence of S6+-state after O2 plasma or UV-O3 treatment is inconsistent in literature. Its existence was reported in polycrystalline multilayer MoS2 thin films after O2 plasma treatment . However, it was not observed in other polycrystalline multilayer MoS2 thin films [26, 29] or single crystals [9, 16, 30] after O2 plasma or UV-O3 treatment. While this inconsistency may be related to dose- and time-dependence of MoS2 oxidation , more systematic investigation is needed to clarify this in the future.
The different XPS behavior may be related to the difference of composition and crystallinity between single crystals and CVD thin films. The composition of Mo:S is 1:1.97 in bulk single crystals and 1:1.5 in CVD thin films, suggesting higher concentration of S vacancies in CVD thin films. The higher concentration of S vacancies, combined with the existence of grain boundaries in CVD thin films, may allow higher reactivity to oxygen than that in single crystals.
In summary, we investigated the effect of UV-O3 treatment on polycrystalline CVD thin films and single crystal flakes of monolayer MoS2. Monolayer MoS2 becomes transparent after UV-O3 treatment suggesting the formation of wide bandgap semiconductor MoO3. As UV-O3 treatment time increases, the intensity of PL and Raman spectra significantly decreased, suggesting the formation of oxides or defects. In both MoS2, XPS analysis indicated the formation of Mo-O bonds and MoO3. However, in CVD MoS2 thin films, the conversion of Mo4+-and S2−-states into Mo5+- and S6+-states was also observed after UV-O3 treatment, suggesting the possible existence of oxygen vacancies, MoOxSy, or Mo (SO4)x. As the electrical resistance of single crystal MoS2 monolayers significantly increased with longer UV-O3 treatment time, the oxidation of single crystal MoS2 into MoO3 seems to provide negligible doping. In contrast, the electrical resistance of CVD MoS2 monolayers decreased with longer UV-O3 treatment time, suggesting that the oxidation of CVD MoS2 into MoO3 provides doping. These results demonstrate the significant impact of crystallinity on the effect of UV-O3 treatment on MoS2 monolayers, providing possibly interesting implications on fabricating heterojunction structures based on two-dimensional nanomaterials.
CJ and HIY contributed equally to this work. WC initiated the research. CJ worked on the growth and characterization of CVD thin films. HIY worked on the fabrication and characterization of mechanically-exfoliated single-layer flakes. WC wrote the manuscript. All authors read and approved the manuscript.
This work was supported by the National Research Foundation of Korea (Grant NRF-2013K1A4A3055679, NRF-2016R1A2B4014369, and NRF-2019R1F1A1057293).
The authors declare that they have no competing interests.
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