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  • Published on: 15th October 2019
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The disulphides and diselenides of molybdenum and tungsten form a class of semiconducting layered compounds with triagonal prismatic coordination of the metal atoms. The optical gap range from about 1eV for MoTe2 to about 2eV for WS2.This semiconducting gap results from the combined effect of ligand 'field splitting and the d-d hybridization of Mo 4d states. This ligand-field splitting of d-levels in a triagonal prismatic coordination can be described better as an effect of metal 'd-nonmetal 'p covalancy rather than crystal field effect.

Fig 'differential reflection of the 750nm probe as function of the probe delay in the bulk WS2 measured with a 405nm pump fluence of o.3''j /cm^2 the inset provides a closer views over the early probe delay.

For comparison we study a thick flake on the same substrate ,its thickness is not accurately determined however its not transprent for visible light hence atleast a 10 nm thickness and can be treated as a bulk sample the experimental condition are the same.with pumpfluence of 0.3''j/cm^2 ,resul as shown in above.a similar fast rise and fast decay follpwed by slower decay.an exponential fit red line gives a decay constant of 160+-10ps which is a factor of 8 longer than the monolayer the enhanced exciton recombination in monolayer observed here is consistent with recent PL measurement that shows much higher PL yield in monolayer WSe2 than the few layered bulk sanmple[21,22].

Transient absorption measurement of monolatyers of MoS2 ,spectra of the transient absorption is found to significantly different from the the absorption spectrfa of the sample[50,51,53].hence its interesting to measure transient absorption in WSe2 with a fixed 405 nm pump fluence of 0.7''j/cm^2,measuring the differential reflection of the monolayer flakes as a function of the probe delay,with a various probe wavelength. The red circles in the above fig show the  magnitude of the signal at a fixed probe delay of 5ps togather with the PL spectrum of the monolayer sample (gray line)measured under excitation of 632.8 nm He-Ne laser beam,on the longer wasve length side of the PL peak the differential reflection peaks agrees very well with PL line shape . how ever on the short wave length side there appears to be negative component that is super imposed to the resonance in a small wavelength range around 730nm the signal becomes negative.

Generally speaking the transient absorption on exciton resonance in semiconductors can be induced by several mechanism. Such as pase space state filling [54] screening of coulomb interaction[54]and band gap re normalisation [55],the phase space state filling effect reduces the exciton transition strength while the other to cause the exciton transition to shift and broaden. Our results indicate that on the long wave length side the effects predominately phase space state filling ,while on the sort wave length side other mechanism are also involved. Also measured the transient absorption spectrum of the bulk sample under the same condition result are shown in the fig 3 blue triangle. Compared to PL monolayer the peak is broader and the slightly shifted to longer wave length the negative component  in the short wave length side is not seen.

 

FIG-OPTICAL IMAGE OF SINGLE AND FEW LAYER MoSe2

2.3.1   Transport properties study

To study the transport properties of excitons in WSe2, spatially and temporally resolve the differential reflection signal,

in this measurement the 405 nm pump and 705 nm probe pilses are both focussed to a spot of about 1.3''m with pump fluence of 0.7''j/cm^2 we adjust the distance between the centeres of the pump and probe spots on the sample ,and at each distance we measure the differential reflection signal as a function of the probe delay.figure is plotted in bleow fig 4a to analyse the evolution of profile we fit the profile corresponding to each probe delay by Gaussian function to determine its 1/ e width ,the exciton injected by the tightly focussed pump pulse diffuse in the monolayer WSe2 ,durin these process excitons also recombine randomly and independent of each other,such process can be explained by classical diffusion aquation[56],since the initially injected profile is determined by the pump laser spot,which has a Gaussian intensity profile .the exciton densityprofile reminds Gaussian and broaden with time .quantitatively

''^2(t)=''^2(0)+4D(t)

where D and ''0 are the exciton diffusion coefficient and the initial width respectively[56] by using a linear fit we obtain an exciton diffusion coefficient 15  plus or minus  5 cm^2/s.

Fig 4 a differential reflection signal as a function of both the probe delay and the probe position measured from the monolayer WSe2 sample

Fig differential reflection of signals as a function of both probe delay and probe position measured from the bulk WSe2 sample.

The measurement is performed with a low density exciton system,the peak density at the centre of the profile is 2.4*10^10/cm^2 corresponding to an average distance of 63nm between two exciton under this condition the exciton transport is does not influenced by the exciton exciton interaction.it is determined by the thermal motion of excitons and the exciton scattering with its environment, including phonon ,lattice imperfection and substrate. the exciton diffusion coefficient indicates the strength of these interactions. From the measured diffusion coefficient, we deduce a mean free time, of about 0.24 ps and a mean free path of 17nm togather with a lifetime of t=18ps. We obtain diffusion length using from time resolved measurement, these fundamental parameters on exciton dynamics are important for understanding exciton physics.

Recently study shows that PL from from monolayer WSe2 at a low temperature is linearlypolariszed.indicating a valley coherence time that is much larger than the exciton life time. measurement of exciton lifetime can help estimate the valley coherence time.

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