Mo-O bond doping and related-defect assisted enhancement of photoluminescence in monolayer MoS2

In this work, we report a strong photoluminescence (PL) enhancement of monolayer MoS2 under different treatments. We find that by simple ambient annealing treatment in the range of 200 °C to 400 °C, the PL emission can be greatly enhanced by a factor up to two orders of magnitude. This enhancement can be attributed to two factors: first, the formation of Mo-O bonds during ambient exposure introduces an effective p-doping in the MoS2 layer; second, localized electrons formed around Mo-O bonds related defective sites where the electrons can be effectively localized with higher binding energy resulting in efficient radiative excitons recombination. Time resolved PL decay measurement showed that longer lifetime of the treated sample consistent with the higher quantum efficiency in PL. These results give more insights to understand the luminescence properties of the MoS2.

In this work, we report a strong photoluminescence (PL) enhancement of monolayer MoS 2 under different treatments.We find that by simple ambient annealing treatment in the range of 200 • C to 400 • C, the PL emission can be greatly enhanced by a factor up to two orders of magnitude.This enhancement can be attributed to two factors: first, the formation of Mo-O bonds during ambient exposure introduces an effective p-doping in the MoS 2 layer; second, localized electrons formed around Mo-O bonds related defective sites where the electrons can be effectively localized with higher binding energy resulting in efficient radiative excitons recombination.Time resolved PL decay measurement showed that longer lifetime of the treated sample consistent with the higher quantum efficiency in PL.These results give more insights to understand the luminescence properties of the MoS 2 .C 2014 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.[http://dx.doi.org/10.1063/1.4897522]Transition metal chalcogenide are widely studied because of their attractive properties in the electronic and optoelectronic applications. 1MoS 2 provides a promising basis for developing high performance photo detector, 2,3 light emitting device 4 and high mobility field effect transistor. 5 Bulk MoS 2 crystal is a layered compound bounded by the weak van der Waals forces.From the bulk to the monolayer MoS 2 (1L-MoS 2 ), the bandgap becomes wider and evolves into a direct bandgap profile that leads to stronger photoluminescence (PL). 6,7The as-exfoliated single layer MoS 2 is usually n-type. 8,9Charged excitons (i.e.trions) are easy to form due to an enhanced Coulomb interaction in the single layer structure, which results in a faster non-radiative recombination and low PL efficiency. 10y electric field gating 11 or chemical doping, 12 free excitons can be emancipated from the trions and thus an enhanced and tunable PL can be observed.It has been found that exposure of 1L-MoS 2 to O 2 and H 2 O gases can lead to drastic PL enhancement. 13This is because the physically adsorbed molecules can deplete the extra electrons and promote radiative recombination of the excitons and efficient light emission.Meanwhile, it is found that structure defects in 1L-MoS 2 sheet play also an important role for the PL process, 14 though the underlying mechanism remains poorly understood.
In this letter, we present a systematical investigation of the 1L-MoS 2 PL performance under different ambient annealing treatments.We found that the PL intensity of the treated samples can vary over two orders of magnitude with the introduction of Mo-O bonds and related defects.We note that this enhancement is even higher than that achieved by chemical and electrical doping effects in previous reports. 11,12To explain these phenomena, we propose a new mechanism centering at the roles of Mo-O bonds' formation that help to deplete the extra electrons in 1L-MoS 2 to facilitate conversion of trions into excitons, and at the same time the localized electrons with higher binding energy avoiding auger non-radiation recombination for the excitons.The combination of these effects results in a much enhanced PL.This model has been further supported by our time-resolved PL characterizations of the treated samples, where we found that the introduction of Mo-O bonds during the annealing treatment is correlated with a longer PL lifetime.The 1L-MoS 2 film were prepared on the n-type Si substrate covered with 300 nm SiO 2 film from bulk MoS 2 crystal by mechanical exfoliation.The monolayer samples were found by optical contrast under a microscope and confirmed by the atomic force microscopy (AFM) and micro-Raman spectroscopy (LabRAMHR).Room temperature PL and Raman spectra were measured in ambient by using the same micro-Raman setup excited by an Ar ion laser with the 514.5 nm line.The laser spot size is around 1 µm.The power has been limited to 50µW in order to avoid local heating during the measurement.PL was detected by a CCD detector cooled around 200K.Time-resolved PL measurement was excited by OPO laser pulses with pulse width of less than 4 pecoseconds (ps) at wavelength of 555 nm.An avalanche photodiode (APD) detector was used to detect the time-resolved PL signals, with a time resolution of the setup of ∼200 ps.X-ray photoelectron spectroscopy (XPS) was also used to analysis the chemical composition of MoS 2 surface.
First of all, we measured and show the PL spectra of the as-exfoliated 1L-MoS 2 sample in Fig. 1, where two luminescent bands can be observed, the stronger one is centered at 1.84 eV (peak A1) and the other at around 2 eV (peak B1) due to the spin-orbital splitting of valence band. 6As we can see, the initial luminescence intensity is rather weak.Then, the sample was annealed in an Ar and H 2 gas mixture at 400 • C for 30 min, which causes the PL intensity of the post-annealing sample to increase by a factor of 10 times.For the as-exfoliate 1L-MoS 2 sample is n-type caused by the defects or substrate dielectric doping, 8 during the PL process, the charged excitons (named as trions, with binding energy ∼30 meV) makes very low efficient light emission mainly due to the auger non-radiation recombination.This annealing treatment can remove the contamination caused by the scotch tape during the sample preparation and, importantly, makes the sample surface active for the air adsorption (dominated by the physical adsorption of O 2 and H 2 O).The air molecular adsorption have the p-doping effect to the sample: they can localize or stabilize the electrons and emancipate trions to excitons resulting in efficient light emission and stronger PL.Meanwhile, we notice a blue shifting of the peak A1 from 1.84 eV to 1.87 eV. which could indicate a transition of the trions into excitons in the 1L-MoS 2 . 11n parallel, we also carried out annealing treatment of the 1L-MoS 2 samples in ambient in the range of 200 to 400 • C for different time (t a ).Strikingly, as we can see in Fig. 1(a), the PL intensity (I PL ) gets greatly enhanced with the increase of t a , up to a factor of ∼200 compared to the as-exfoliated samples after 20 min annealing at 300 • C, but decreasing at even longer annealing.We found that annealing treatments shift the peak A1 with 30 meV higher to around 1.87 eV, remaining basically insensitive to following annealing (normalized PL curves are plotted in the inset of Fig. 1(a)), while the emissions at peak B1 gradually disappear.It is important to emphasize that this effect is readily reproducible for several separate trials.While annealing at 200 or 400 • C, the treatment is not as efficient as that of 300 • C in term of time and the PL intensity enhancement.Fig. 1(b) gives the data about the PL intensity evolution with t a under annealing temperature of 200,300 and 400 • C in air.The PL intensity increase only about 6 times for t a = 50 min, only about 10 times for t a = 120 min under 200 • C treatment.During the annealing at 400 • C in air, the sample is easily damaged and give a moderate PL enhancement.Further analysis will be given later on in the context.
Raman spectroscopy is a powerful tool to analyze and to determine, not only the layer number of the MoS2, but also the doping level. 15,16P/N doping can be revealed from the blue/red shift of the out-of plane vibration mode of A 1g mode due to changes of the electron-phonon interaction.While the in-plane vibration mode of E 2g peak is inert to this change.Fig. 2 gives the Raman spectra of the same sample series at different annealing treatment stages.The A 1g mode around 404 cm −1 is blue-shifted under annealing treatment, while the position of E 2g mode is not influenced.These observations indicate an effectively p-doping established during the annealing treatment. 3The sample annealed in Ar/H 2 at 400 • C has the p-doping effect due to the ambient molecular physical adsorption under exposure in air.For the samples annealed in air, from t a = 5 min the doping effect is already prominent, longer time treatment does not change much the position of the peak.While the Raman intensity is almost unchanged up to t a = 20 min.For longer time the annealing, the Raman intensity starts to decrease implying the excessive damage of the film.To verify this point, we adopted AFM does not show clear signs of etching, we believe that the etching already starts and the holes are too small to be observed under AFM.As reported that MoS 2 will be decomposed during the annealing treatment in air at 300 • C, and sulfide vacancies will be created. 17,18It is also well known that vacancies in the basal plane and edges of the MoS 2 have high catalytic activity for oxygen and water chemisorption at room temperature. 19Oxygen and water molecules in air can be adsorbed by the defective sites forming Mo-O bonds under this condition, which can lead to p-doping effect in the MoS 2 film. 17The Mo-O bonds are also confirmed by the XPS analysis shown in the Fig. 3(d 4, the overall lifetime is very short for all three samples (less than 1 ns), mainly due to the quantum confinement and the rich non-radiative channels. 20,21ach curve include multi-exponential decay process.According to the Ref. 20, there are three typical recombination processes: the fastest one around several ps due to the fast surface trapping centers, the exciton-phonon interaction around 100 ps and the slow direct inter-band recombination in several hundred ps process.We cannot extract the lifetime precisely because of the resolution limitation of this setup, but it is clear that the PL lifetime of the sample annealed in air is longer than the other two samples.To our common sense, defects are usually make the PL lifetime shorter because they are scattering centers for the excitons.But here, we got the longer PL lifetime of the defective sample which show greatly enhanced PL.On the one hand, excitons are effectively localized at the Mo-O bond sites resulting in less non-radiative Auger-recombination, on the other hand, the non-radiative trapping centers are probably passivated 22 by oxygen or water during the annealing treatment.
Based on the experimental evidence, we propose that the great enhancement of PL is due to a combination of two effects.First, it is well known that the Mo-O bonds can effectively deplete the extra electrons and cause p-type doping in MoS 2 . 17This effect will convert excitons from trions, resulting in PL enhancement.Compared to previously reported results on doped sample by physic-sorption, the enhancement is much greater, indicating that Mo-O bonds are more effective doping source.Second, it is expected that the electrons can be effectively localized at the Mo-O bonds related defects with higher binding energy in this low dimensional material 23,24 which can suppress the thermal activation of excitons to auger non-radiative recombination and result in improved quantum efficiency.
In conclusion, we have investigated the PL properties of the 1L-MoS 2 film under annealing treatment in ambient.We find that the PL intensity can be greatly improved up to ∼200 times after annealing in air for a short time.The enhancement is mainly attributed to the p-type doping by Mo-O bonds as well as the defects-assisted localization of the electrons to suppress non-radiative processes and possible passivation of the non-radiative centers during the treatment.These results give further insights to the PL process of dichalcogenides.

1
Key Laboratory of Photonic and Electronic Materials and School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, P. R.China 2 National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing 210093, P. R.China (Received 24 July 2014; accepted 2 September 2014; published online 6 October 2014)

123004- 3 Wei 4 Wei
FIG. 1. (a)The PL spectra of the samples: as-exfoliated, annealed in the protecting gas Ar(95)/H 2 (5) at 400 • C, and the samples annealed in air at 300 • C for different time (ta) with ta=5,10,15,20 min as well as the spectra of the sample after ta=20 min treatment (with greatly enhanced PL) exposed in air for 10 and 25 min.Inset are the normalized PL spectra of the samples according to the Fig.1(a); spectra in the same color are the same treatment condition.(b) are the PL intensity of the samples under treatment in air at 200, 300 and 400 • C, horizontal axis is the treatment time.
).Both of the as-exfoliated and the annealed samples have the Mo 4+ 3d 3/2 , Mo 4+ 3d 5/2 , and S 2s peaks at around 233.2, 230.1 and 227.2 eV, respectively.But the signal of the Mo 6+ peak is significantly enhanced for the annealed one.Meanwhile, the intensity of the O 1S peak around 532 eV (both physical and chemical adsorptions) increase dramatically after the treatment (inset of the Fig.3(d)).The greatly increase in the Mo 6+ and oxygen concentration indicate the formation of Mo-O bonds.There might be the MoO 3 forming during the annealing in the air at 300 • C. Back to the discussion of the I PL vs. t a in Fig. 1(b), for sample annealed at 200 • C, the temperature is too low to effectively create Mo-O bonds, while the sample are easily decomposed in air at such a high temperature as 400 • C.So 300 • C is an appropriate for this kind of treatment condition.Furthermore, we use time-resolved PL measurement to study the excitons dynamics.Three 1L-MoS 2 samples are studied: as-exfoliated, annealed in Ar/H 2 gas at 400 • C and annealed in ambient at 300 • C for 20 min treatment.As shown in Fig.

FIG. 3 .
FIG. 3. The AFM pictures of the samples: (a) as-exfoliated, (b) annealed in air at 300 • C for 20 min with the greatly enhanced PL, and (c) the sample annealed in air at 300 • C for 60 min.(d) the XPS spectra of samples of the pristine and annealed in air for 30 min.

FIG. 4 .
FIG.4.Time-resolved PL decay spectra of the monolayer MoS 2 of the samples: annealed in air at 300 • C (red one), annealed in Ar/H 2 at 400 • C (green one), the sample of as-exfoliation (blue one) and the laser (yellow one).The black solid lines are fit lines with a tri-exponential decay function.