Photocarrier dynamics in transition metal dichalcogenide alloy Mo 0 . 5 W 0 . 5 S 2

We report a transient absorption study of photocarrier dynamics in transition metal dichalcogenide alloy, Mo0.5W0.5S2. Photocarriers were injected by a 400-nm pump pulse and detected by a 660-nm probe pulse. We observed a fast energy relaxation process of about 0.7 ps. The photocarrier lifetime is in the range of 50 100 ps, which weakly depends on the injected photocarrier density and is a few times shorter than MoS2 and WS2, reflecting the relatively lower crystalline quality of the alloy. Saturable absorption was also observed in Mo0.5W0.5S2, with a saturation energy fluence of 32 μJ cm−2. These results provide important parameters on photocarrier properties of transition metal dichalcogenide alloys. © 2015 Optical Society of America OCIS codes: (320.7150) Ultrafast spectroscopy; (160.4670) Optical materials; (160.6000) Semiconductor materials. References and links 1. Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. 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The fast progress in developing and optimizing fabrication techniques allows further studies on the properties of TMD alloys.For example, lattice structure and distribution of Mo and W atoms in Mo 1−x W x S 2 monolayers were investigated by scanning transmission electron microscopy [35].Effects of alloy engineering on controlling the detects were studied in Mo 1−x W x Se 2 [34].The electronic structure of monolayer Mo 1−x W x S 2 was revealed by scanning tunneling microscopy and spectroscopy [36].In contrast, optical studies of TMD alloys are rare and are limited to steady state measurements, such as photoluminescence [37], Raman spectroscopy [37], and photocurrent generation [27].
Here we report the first time-resolved study of carrier dynamics in TMD alloys by using femtosecond transient absorption technique.As an example, we investigated Mo 1−x W x S 2 bulk crystals with x = 0.5.We deduced the lifetime of photocarriers and energy relaxation time of hot carriers.The photocarrier dynamics in Mo 1−x W x S 2 is also compared with MoS 2 and WS 2 .These measurements provide important parameters to understand performance of these TMD alloys in various applications in optoelectronic devices.

Sample preparation
We studied bulk alloy of Mo 0.5 W 0.5 S 2 provided by 2D Semiconductors.To ensure that the measurements were performed on a single-crystal domain, a mechanical exfoliation procedure was used to obtain films of Mo 0.5 W 0.5 S 2 , which were transferred to a silicon substrate with an oxide layer.The films studied have thicknesses on the order of 100 nm, and can be treated as bulk materials.All the measurements were performed with the sample at room temperature.

Transient absorption
The dynamics of the photocarriers in Mo 0.5 W 0.5 S 2 was studied by transient absorption in the reflection geometry.As shown schemitically in Fig. 1, the setup is based on an 80 MHz Ti:sapphire laser that generates 100 fs pulses with a central wavelength of 800 nm.A portion of this beam is focused to a beta barium borate (BBO) crystal to generate its second harmonic at 400 nm.This beam is focused to the sample by a microscope objective lens.This pump pulse injects photocarriers to the sample by interband absorption.The other portion of the 800 nm beam is coupled to a highly chirped photonic crystal fiber (PCF) to generate a supercontinuum radiation in the visible and near infrared range.A dielectric bandpass filter with a central passing wavelength of 660 nm and a bandwidth of 10 nm is used to select that wavelength component from the supercontinuum.This probe beam is combined with the pump beam and focused to the sample through the same objective lens.The focused spot size is about 2.4 μm for both pump and probe beams, which is determined by an imaging system with a precision of about 0.2 μm.The reflected probe is sent to a silicon photodiode, which output is measured by a lock-in amplifier.A long-pass filter prevents the reflected pump beam from reaching the photodiode.A mechanical chopper modulates the pump intensity at about 2 KHz, in order to improve the signal-to-noise ratio.The pump and the probe pulses are orthogonally linearly polarized.
With this configuration, we measure the differential reflection of the probe, ΔR/R 0 = (R − R 0 )/R 0 , where R and R 0 are the reflection of the probe with and without the presence of the pump beam, respectively.The differential reflection is measured as a function of the probe delay, which is defined as the arrival time of the probe pulse at the sample with respect to the pump pulse.This quantity is controlled by a using a linear stage, which moves a reflector along a line, in the pump arm to vary the pump beam path length.

Results and discussion
The electronic bandstructure of Mo 1−x W x S 2 alloys has not been precisely determined yet.However, since the bandgaps of MoS 2 and WS 2 bulk crystals are 1.2 and 1.4 eV, respectively [38], the bandgap of Mo 0.5 W 0.5 S 2 is expected to be in the range of 1.2 -1.4 eV, and be close to 1.3 eV.In our experiment, the 400-nm pump pulse with a photon energy of 3.1 eV injects photocarriers with an excess energy of about 1.8 eV, which is distributed between the electrons and the holes.These hot carriers injected are expected to relax their energy rapidly to the lattice, moving to the low-energy states in the conduction and valance bands.This energy relaxation process will be followed by a slower recombination process of the photocarriers, determined by their lifetime.The dynamics of these injected photocarriers are monitored by the 660-nm probe pulse with a photon energy of about 1.9 eV.Hence, it directly probes states in the conduction and valance bands that are about 0.3 eV above the band edges.The observed differential reflection signal is shown in Fig. 2 for short (left) and long (right) time ranges.Different symbols represent measurements with different values of pump fluence, as labeled in the figure.So far, the absorption coefficient of TMD alloys has not been precisely measured, preventing us from accurately determining the injected photocarrier density from the pump fluence.If we assume that the absorption coefficient of Mo 0.5 W 0.5 S 2 at 400 nm is close to WS 2 , which is about 7 × 10 7 m −1 [39], we can roughly estimate that a pump fluence of 1 μJ cm −2 corresponds to a photocarrier density of 1.5 × 10 18 cm −3 at the sample surface.
As shown in the left panel of Fig. 2, the differential reflection signal reaches to a peak rapidly in a few picoseconds.This process can be attributed to the relaxation of photocarriers from the initially injected high energy states to the states probed.We model the rising part of the signal as N 0 (1 − e −t/τ X ), where N 0 is the peak signal and τ X describes the associated energy relaxation time [40].By fitting the signals with probe delays from 0 to 2 ps, we find this simple model is consistent with the data, as indicated by the solid red curves.From these fits, we obtained the relaxation times and the peak values, as summarized in Figs.3(a) and 3(b).No dependence of the relaxation time on the pump fluence is observed.This is expected, since the energy relaxation of carriers is achieved by phonon emission, which does not strongly depend on the injected carrier density.We obtain an average values of τ X = 0.68 ± 0.10 ps.We also find that the peak signal increases with the pump fluence linearly when the fluence is low, and saturates at higher fluences, as shown in Fig. 3(b).This dependence can be satisfactorily described by the saturation model [41], ΔR/R 0 ∝ F/(F + F s ), where F is the pump fluence and F s the saturation fluence.From a fit, indicated as the red line, we obtain F s = 32 ± 3 μJ cm −2 , which corresponds to a saturation density of about 5×10 19 cm −3 .This quantity describes the effectiveness of photocarriers in saturating the absorption coefficient, and is important for applications as saturable absorbers.Similar saturation effects have been observed in TMDs, too [42,43].
The right panel of Fig. 2 shows the differential reflection signal on a long time scale.The decay of the signal is caused by loss of photocarrier population due to their recombination.Since the exciton binding energies in TMD bulk crystals are much higher than the room temperature thermal energy [44], the photocarriers are expected to be mostly excitons during this process.We fit the data with a single exponential function, N 0 e −t/τ R + N R , as shown as the solid red curves.Clearly, the decay can be satisfactorily described by the single exponential function.The deduced photocarrier lifetime, τ R , is plotted as a function of the pump fluence in Fig. 3(c).We found that the lifetime decreases with the fluence: it changes from about 100 ps at relatively low fluence to about 50 ps at the highest fluence used in this study.Such an enhanced recombination at high injected carrier densities can be attributed to multi-carrier processes, such as excitonexciton annihilation, that have been observed in transient metal dichalcogenides [45,46].
As shown in the right panel of Fig. 2, there is a significant residual of the signal after this single exponential decay process.The magnitude of this residual is proportional to the pump fluence, as shown in Fig. 3(d).This indicates that there is a slower process induced by the pump pulse.Since no signal is observed at negative probe delays, this residual decays on a time scale shorter than 13 ns, which is the separation of two sequential pump pulses in the measurement.It is unlikely that this residual is caused by trapped carriers at defect sites, since it shows no saturation at rather high carrier densities.We attribute this residual to thermal effect left by the carriers, which causes the excited region of the sample to reach a higher lattice temperature.
Finally, we compare the photocarrier dynamics in Mo 0.5 W 0.5 S 2 with those in MoS 2 and WS 2 bulk crystals.Samples of MoS 2 and WS 2 films were prepared from bulk crystals with a similar procedure as Mo 0.5 W 0.5 S 2 .For each sample, we injected photocarriers with a pump pulse of 400 nm wavelength and 4-μJ cm −2 fluence, and used the same 660-nm pulse as the probe.The results are shown in Fig. 4 for short (left) and long (right) time scales.Under the same conditions, the signals from WS 2 and Mo 0.5 W 0.5 S 2 have similar magnitudes, while MoS 2 produced a signal of a few times larger.Both MoS 2 and WS 2 have a small peak right after zero delay, which is absent in Mo 0.5 W 0.5 S 2 .
We are mostly interested in comparing the recombination of photocarriers in the three materials.As shown in the right panel of Fig. 4, the decay of the signal in MoS 2 and WS 2 is noticeably slower than Mo 0.5 W 0.5 S 2 .By fitting all the three curves with single exponential functions (red curves), we obtained photocarrier lifetimes of 360 ± 10 and 240 ± 20 ps in MoS 2 and WS 2 , respectively, both are much longer than the 100 ps obtained in Mo 0.5 W 0.5 S 2 alloy.The shorter lifetime in Mo 0.5 W 0.5 S 2 can be attributed to its alloy nature, which results in lower crystalline quality of the lattice.This result also suggests that the recombination lifetime of photocarriers in all these materials are dominated by nonradiative recombination, and reflects the quality of the samples.The radiative recombination time of these indirect semiconductors are expected to be much longer.

Conclusion
In summary, we have time resolved photocarrier dynamics in TMD alloys.Differential reflection signals were observed in a Mo 0.5 W 0.5 S 2 bulk crystal, following interband absorption of an ultrafast pump pulse.An energy relaxation process shorter than 1 ps was observed.The photocarrier lifetime was found to be in the range of 50 -100 ps, depending on the injected photocarrier density.This is a few times shorter than MoS 2 and WS 2 , reflecting the relatively lower crystalline quality in the alloy.Saturable absorption of TMD alloys was also observed, with a saturation fluence of 32 μJ cm −2 .This effect can be utilized in nonlinear photonic and optoelectronic devices.These results provide important parameters on photocarrier properties in TMD alloys.

# 253223 Received 3 Fig. 2 .
Fig. 2. Differential reflection signal over short (left panel) and long (right panel) time scales, with different values of the pump fluence as labeled.The solid red lines are fits to the data.

Fig. 3 .
Fig. 3. (a) The rising times of the differential reflection signal deduced from fits shown in Fig. 2 (left panel).The red line indicates an average value of 0.68 ps.(b) The peak value of differential reflection.The red line shows a fit to the data, corresponding to a saturation fluence of 32 μJ cm −2 .(c) Decay times of the differential reflection signal.(d) The values of the residual differential reflection signal at long probe delays.