In situ doping effect in monolayer MoS2 via laser irradiation

Two-dimensional (2D) semiconducting materials with a single atomic layer display exceptional structural symmetry and band structures, making them the most promising candidates for investigating the spin-valley coupling effect and fabricating novel optoelectronic devices. Their atomic thinness also makes it easy to adjust their excitonic optical response through plasma treatment or thermal annealing. In this study, we present a simple technique for modifying the optical properties of monolayer MoS2 by briefly exposing it to laser irradiation in ambient conditions. Initially, this exposure resulted in a nearly twofold increase in photoluminescence (PL) intensity, with the neutral exciton intensity increasing while the trion exciton intensity decreased. We propose that oxygen-related functional groups, such as O2 and H2O from the surrounding air, adsorb onto MoS2 and extract extra electrons, which enhances exciton emission while reducing trion emission. In a subsequent stage, both exciton intensities decreased as all extra electrons were depleted. Additionally, any structural distortions or potential damage were found to decrease the PL intensity, and these changes were linked to alterations in the Raman spectra.

Monolayer hexagonal molybdenum disulfide (2H-MoS 2 ), a member of the TMDs family, has been extensively investigated in recent years [29,30].Bound quasiparticles (excitons and trions) are generally susceptible to factors like intrinsic dopants [31][32][33][34], molecules on the surface of monolayer MoS 2 [34][35][36][37][38], and carrier trapping at the interface between monolayer MoS 2 and the substrate [39].The high sensitivity to the surrounding environment has led to the discovery of novel properties [31,[40][41][42].Previous studies have demonstrated the enhancement or quenching of the PL intensity through defect engineering, adsorption of foreign molecules, thermal annealing, and structural disorder [32,43].Detailed investigations have also explored changes in the Raman spectra of monolayer MoS 2 under strain, such as the shift of the A 1g mode, which can be used to probe doping type and intensity, and the sensitivity of the E 1 2g mode to surface reconstruction and strain [44].Optical spectroscopy, a widely used technique, involves irradiating the material with a laser of suitable energy, followed by monitoring PL or Raman emission.The excitation laser can profoundly impact the optical/electrical properties of the materials [45][46][47].Therefore, it is crucial to understand the effects of the excitation laser on MoS 2 as devices are typically exposed to light in practical applications.Recently, a way to controllably dope CVD-grown monolayer MoS 2 on SiO 2 substrates upon laser irradiation was reported [45].It focused on the S-vacancies passivation mechanism.An aging process was performed to yield a high density of chalcogen vacancies and an eventual loss of PL intensity in monolayer TMDs.In addition, the enhancement of excitonic emission was introduced to interpret the increment of PL emission using laser treatment [46].We conducted a systematic investigation of PL and Raman spectra changes using a 532 nm excitation laser on as-grown monolayer MoS 2 flakes as a function of irradiation time under ambient conditions.Our analysis revealed that the blueshift of the A 1g mode, along with the decrease in full-width at half-maximum (FWHM) and the increase in integrated intensity, is associated with p-doping effects in monolayer MoS 2 .The gradual increase in PL intensity is attributed to the transformation of charges between intrinsic electrons and the laser-induced physisorption of functional groups.With prolonged laser irradiation, the PL intensity reaches a maximum and decreases due to possible oxidation accompanying structural degradation in MoS 2 .

Experimental
CVD growth of monolayer MoS 2 on SiO 2 /Si We grew monolayer MoS 2 flakes on a Si substrate with a 300 nm top layer of SiO 2 using chemical vapor deposition (CVD) in a 1-inch quartz tube.An aluminum oxide crucible containing 1 mg of MoO 3 powder was placed in the center of the furnace.Another aluminum crucible containing 20 mg of S powder was placed upstream of the tube furnace at a temperature of 200 °C.The distance between the S powder and the MoO 3 precursor was approximately 18 cm.The SiO 2 /Si substrate was face-down on the crucible containing the MoO 3 powder.
Before growth, the whole CVD system was purged with Ar for 10 min.The furnace was elevated from room temperature to 680 °C in 20 min and kept for 12 min in a typical growth process.Sulfur vapor, introduced by a flow of 40 sccm of Ar gas, reacted with the molybdenum oxide at the elevated temperature, leading to the growth of monolayer MoS 2 through a sulfurization reaction at atmospheric pressure.Traces of NaCl, mixed with MoO 3 precursor, were used as promoters to enhance the synthesis of large-size, high-quality monolayer MoS 2 .After the growth process, we introduced 120 sccm of Ar gas into the furnace to rapidly cool the system to room temperature.

Characterization of monolayer MoS 2
We used atomic force microscopy (AFM, NT-MDT NEXT) characterization to explore the thickness of MoS 2 flakes.We characterized monolayer MoS 2 using confocal Raman microscopy (NT-MDT NTEGRA Spectra) to examine its optical properties.We employed a continuous wave 532 nm laser with a power output of 3.5 mW and a spot size of 800 nm for extended illumination.Both Raman and PL spectra were acquired with the laser in the on-state, and data collection times were set at 10 s and 5 s for Raman and PL spectra, respectively.To explore the stability of the changes, the shift of peak position and FWHM were averaged from 5 different flakes.To assess the surface morphology of the as-grown monolayer MoS 2 , we employed an optical microscope (OLYMPUS BX51).

Results and discussion
Figure 1(a) illustrates the laser irradiation process for excitation, where a diode-pumped solid-state laser was focused on the as-grown monolayer MoS 2 along the normal direction of the sample.Figure 1(b) shows an optical image of the as-grown monolayer MoS 2 on a SiO 2 /Si substrate.The consistent contrast between the substrate and nanosheets highlights good uniformity of the as-grown flake, with edge lengths in the tens of microns.The thickness of 0.68 nm confirms that the flake is a monolayer (see figure S1 in Supporting Information).Figure 1(c) presents the Raman spectra of monolayer MoS 2 for various laser irradiation times under ambient conditions.The spectral resolution for Raman measurement is estimated to be 0.9 cm −1 , and the 520 cm −1 phonon mode from the Si substrate is used for calibration.The Raman peaks of the E 1 2g and A 1g modes are initially located at 382.91 cm −1 and 402.14 cm −1 , respectively, with a difference of 19.23 cm −1 , indicating monolayer thickness [6,9].As the laser irradiation time increases, the A 1g mode gradually shifts to a higher frequency compared to pristine, while the E 1 2g mode shifts to a lower frequency.To illustrate the time evolution of Raman spectra with laser excitation on the monolayer MoS 2 flake, we employed overlapping peak resolution.Figure 2 provides quantitative data on the peak position and integrated  intensity for all observed Raman modes.Figure 2(a) shows the redshift of the E 1 2g mode from 382.91 cm −1 to 381.84 cm −1 , along with a broadening full-width at half-maximum (FWHM).Figure 2(b) demonstrates the gradual blueshift of the A 1g peak from 402.14 cm −1 in pristine monolayer MoS 2 to 402.73 cm −1 at 25 min of laser irradiation, after which it remains nearly constant with extended irradiation time.The inset shows that the FWHM of the A 1g mode decreased from 5.73 cm −1 to 5.12 cm −1 .Figures 2(c) and (d) illustrate an increase in the integrated intensity of the E 1 2g and A 1g modes, respectively.Typically, Raman spectroscopy is a powerful tool for detecting the doping effect in MoS 2 and provides information about structural-related distortions.The out-of-plane A 1g Raman mode is susceptible to electron extraction or injection, and the in-plane E 1 2g Raman mode is more sensitive to lattice deformation and strain [47,48].The blueshift of the A 1g mode is attributed to p-doping in monolayer MoS 2 during the first 25 min of laser irradiation [49][50][51].The blueshift of the 2LA mode (see figure S2 in Supporting Information), a secondorder longitudinal acoustic mode at the M-point of the Brillouin zone, provides additional evidence of p-doping in monolayer MoS 2 [52].The redshift of the E 1 2g mode is associated with possible structural distortion induced by laser irradiation or tensile strain [47].Further evidence from experimental and theoretical research is needed.
The study provides evidence that the adsorption of O 2 or H 2 O causes electron depletion in MoS 2 [51], leading to a decrease in the concentration of intrinsic electrons.Further, it is observed that the doping strength is subject to the concentration of electrons.With increased irradiation time, the concentration of intrinsic electrons gradually decreases until it reaches its limit.In the experimental conditions of this study, the charge transfer between intrinsic electrons and functional groups primarily occurred within 25 min, hardening the peak position of the A 1g mode.In the subsequent 25-30 min, the doping effect gradually became insensitive to laser illumination time as the concentration of intrinsic electrons decreased.After 30 min, the position of the A 1g mode remained almost constant due to the complete depletion of residual electrons in monolayer MoS 2 .Additionally, the A 1g phonon is strongly influenced by the coupling between electrons and phonons in monolayer MoS 2 and is extremely sensitive to electron concentration.The decrease in FWHM for the A 1g mode is also associated with the depletion of residual electrons.The increased integrated intensity of the E 1 2g and A 1g modes is related to reduced screening due to the depletion of electrons.
We utilized micro-PL measurements to further evaluate the changes in the optical properties of monolayer MoS 2 over time with laser irradiation under ambient conditions.Figure 3 displays the time-dependent evolution of PL spectra, which showcase the well-known A and B peaks [38,53], indicating the exceptional quality of monolayer MoS 2 at various irradiation times.Interestingly, under ambient conditions, the PL intensity of the A peak initially experiences a twofold enhancement with increasing laser irradiation time.However, it is subsequently suppressed until it reaches a maximum value, while the PL intensity of the B peak remains relatively unchanged.The positions of all the peaks remain largely unaffected during laser irradiation.In fact, the laser power impacts the changes.Using low laser power (∼1.5 mW), the absence of changes in PL spectra hints the crystal structure and optical properties of monolayer MoS 2 are maintained (see figure S3 in Supporting Information).
The A peak found in the PL spectra of TMDs materials is highly responsive to reduced charge screening.When tightly bound excitons capture additional charges, they can form trions, which are charged excitons.The shape of the A peak in the PL spectra is asymmetric, coupled with the emergence of the B exciton, strongly suggesting the presence of the trion exciton.To elucidate the spectral distinctions, we deconvoluted the A peak in the PL spectra into two Lorentzian peaks corresponding to the neutral exciton (A 0 ) and the trion exciton (A -), respectively.The use of a relatively high-power laser during spectral measurements for this study resulted in variations in the components of PL emission throughout laser irradiation.
It is clear that in the pristine monolayer MoS 2 , the intensity of A -is significantly greater than that of A 0 .This difference is likely due to intrinsic n-doping effects caused by defects such as S vacancies in the CVD growth samples [23,24].As laser irradiation time increases, the A peak in the PL spectra gradually shifts toward dominance by the A 0 exciton, with a diminishing contribution from the A -exciton.After several to more than ten minutes, both contributions from A 0 exciton and A -exciton are less to the PL intensity of the A peak, although A 0 exciton still prevails.The increasing PL intensity ratio between A 0 and A during this period indicates p-type doping in monolayer MoS 2 under laser irradiation, likely due to the physisorption of O 2 or H 2 O groups.The top graph in figure 3 demonstrates that both intensities of A 0 exciton and A -exciton are reduced, resulting in a substantial decrease in the total A peak.The decline in the PL intensity of A 0 exciton and A -exciton is attributed to potential structural distortion or damage in MoS 2 .The appearance of Raman peak, corresponding to Raman mode from MoO 3 [54], provides evidence of the deterioration of molybdenum disulfide after 15 min of laser irradiation (see figure S4 in Supporting Information).
Figure 4 presents the time-dependent changes in the integrated PL intensity of the A 0 exciton and A -exciton, and the FWHM of each exciton.From figures 4(a) and (b), the following observations can be made: (1) During the initial 0-6 min, the PL intensity of the A 0 exciton increases significantly, while that of the A -exciton experiences a slight decrease.(2) In the intermediate period of 6-12 min, the intensity of the neutral exciton starts to decrease, while the intensity of the trion exciton shows a slight decrement.(3) After 12 min, both intensities decrease.The FWHM of both excitons sharply decreases within the first 6 min and gradually increases until it nearly saturates as the irradiation time increases (figures 4(c) and (d)).All the observations strongly indicate chemical modification via exposing monolayer MoS 2 to laser.
Figures 3 and 4 depict that functional groups draw residual electrons from MoS 2 during the initial 0-6 min of laser irradiation.The charge transfer leads to an increased PL intensity of A0 and a decreased PL intensity of A -, resulting in an overall increase in total emission.Our findings are aligned with previous research that associates the heightened PL intensity with an increase in A 0 exciton intensity alongside a decrease in trion intensity [38,51,55].The PL intensity of A 0 reaches its maximum at 6 min, constrained by the concentration of intrinsic electrons, and subsequently starts to decrease.After 12 min, structural distortion or possible damage to monolayer MoS 2 suppresses the intensities of both A 0 and A − excitons, causing a decline in PL emission.Furthermore, we attribute the decrease in FWHM to the reduction in exciton-carrier scattering as electron depletion progresses [56]

Conclusions
In this study, we systematically investigated the changes in optical properties of monolayer MoS 2 under laser irradiation in ambient conditions.Notably, we observed distinct modification behaviors that depend on the duration of laser irradiation.During the initial several to tens of minutes, we observed a significant increase in the photoluminescence (PL) intensity of the neutral exciton alongside a gradual intensity decrease in the trion exciton.This led to an overall enhancement in the total PL emission.This effect was attributed to the p-doping of monolayer MoS 2 caused by the adsorption of oxygen-related functional groups.Once the extra electrons were depleted, both exciton intensities diminished.Raman measurements revealed that structural distortion or potential damage mechanisms might play a critical role in suppressing PL emission.Our findings provide valuable insights into the effects of light exposure on 2D TMDs materials-based optoelectronic devices.

Figure 1 .
Figure 1.The characteristic of the as-grown monolayer MoS 2 while the laser remains in the on-state.(a) Depicts a schematic illustration of laser irradiation on MoS 2 under ambient conditions, with the laser spot size focused to less than 800 nm and a laser power of 3.5 mW.(b) Shows an optical image of the as-grown monolayer MoS 2 on a SiO 2 /Si substrate.(c) Presents Raman spectra obtained at different laser on-state durations: 0 min, 15 min, 30 min, and 50 min, respectively.

Figure 2 .
Figure 2. Time evolution of the Raman spectra of monolayer MoS 2 , acquired using a 532 nm laser with a power of 3.5 mW.Panels (a) and (c) depict the peak position and integrated intensity for the E 1 2g mode.Panels (b) and (d) display the A 1g mode peak position and integrated intensity.Insets in Panels (a) and (b) are the corresponding FWHM.

Figure 3 .
Figure 3. Analysis of the PL spectral shapes of pristine monolayer MoS 2 and monolayer MoS 2 after laser irradiation (for 6 and 50 min).All the PL spectra were deconvoluted into three Lorentzian peaks, representing the B exciton, neutral exciton (A 0 ), and trion exciton (A − ).