Momentum-resolved hot electron dynamics at the $2H\text{−}{\mathrm{MoS}}_2$ surface

Time- and angle-resolved photoelectron spectroscopy (trARPES) is employed to study hot electron dynamics in the conduction band of photoexcited $2H\text{−}{\mathrm{MoS}}_2$. Momentum-dependent rise times of up to 150 fs after near-ultraviolet photoexcitation and decay times of the order of several-hundred fs allow us to locate areas of light absorption in the conduction-band energy landscape as well as to track the relaxation of hot electrons into the lowest-energy states. The conduction-band minima are finally depopulated within $\approx 1$ ps, although a residual population remains up to the maximum investigated pump-probe delay of 15 ps. The presence of the fast depopulation channel differs from the results of experiments of bulk ${\mathrm{MoS}}_2$ performed with all-optical methods. It conforms, however, with recent findings for monolayer ${\mathrm{MoS}}_2$. We attribute this similarity to defect and surface states being of considerable relevance for the near-surface electron dynamics of bulk ${\mathrm{MoS}}_2$, as probed in a trARPES experiment.

Figure 1

(a) Side view of the 2$H_b$ crystal structure of bulk ${\mathrm{MoS}}_2$ [15]. $2H\text{−}{\mathrm{MoS}}_2$ consists of covalently bonded S-Mo-S layers that are held together via van der Waals–like interactions. (b) Surface-projected Brillouin zone of $2H\text{−}{\mathrm{MoS}}_2$. The momentum cuts investigated in the present study are indicated by red lines.

Figure 2

TrARPES data of $2H\text{−}{\mathrm{MoS}}_2$ around $\overline{M}$. (a) ARPES intensity map of the valence (bottom) and conduction (top) band recorded with HHG light $(h\nu =22.1$ eV) at 300 K in equilibrium, i.e., 1 ps before pump excitation. Separate color scales are used for the valence-band and conduction-band signal. (b)–(e) ARPES snapshots of the conduction-band region recorded at different time delays. For comparison, results of band structure calculations for the conduction band from Ref. [42] along the M-K (black line) and L-H (red line) direction are added. (f) Photoemission-intensity transient generated by integration of the photoemission signal over an energy and momentum region centered at $\overline{M}$, as indicated by the black dashed box in (a). The solid line is a fit to the experimental data by the model function described in the text. Dashed vertical lines mark the time delays of the snapshots shown in (b)–(e).

Figure 3

TrARPES data of $2H\text{−}{\mathrm{MoS}}_2$ around 0.53 $\overline{\mathrm{\Gamma }}\overline{K}$. (a) ARPES intensity map of the valence (bottom) and conduction (top) band recorded along the $\overline{\mathrm{\Gamma }}\overline{K}$ direction with HHG light $(h\nu =22.1$ eV) at 300 K in equilibrium, i.e., 1 ps before pump excitation. Separate color scales are used for the valence-band and conduction-band signal. (b)–(e) ARPES snapshots of the conduction band recorded at different time delays. For comparison, results of band structure calculations for the conduction band from Ref. [42] along the $\mathrm{\Gamma }$-K (black line) and A-H (red line) direction are added. (f) Photoemission-intensity transient generated by integration of the photoemission signal over an energy and momentum region centered at 0.53 $\overline{\mathrm{\Gamma }}\overline{K}$, as indicated by the black dashed box in (a). The solid line is a fit to the experimental data by the model function described in the text. Dashed vertical lines mark the time delays of the snapshots shown in (b)–(e). The inset compares energy distribution curves at different time delays obtained by integration of ARPES intensity maps along $\overline{\mathrm{\Gamma }}\overline{K}(0.35\le k_x\le 0.85$ Å${}^{-1})$.

Figure 4

(a) Calculated band structure of $2H\text{−}{\mathrm{MoS}}_2$ for the $\mathrm{\Gamma }$MK (black) and ALH (red) planes, adopted from Ref. [42]. Black arrows indicate photoexcitation by absorption of 395 nm light. Blue arrows illustrate intraband relaxation of photoexcited electrons into the local conduction-band minima. The black dashed boxes indicate integration areas used to generate the photoemission-intensity transients shown in Figs. 2 and 3. (b) Characteristic photoemission-intensity rise times ${\tau }_{\mathrm{rise}}$ and fall times ${\tau }_{\mathrm{fall}}$ as a function of momentum, as determined from fits to momentum-dependent photoemission transients. The horizontal dashed line indicates the effective time resolution of the experiment $\left(33\mathrm{fs}\right)$ as given by the FWHM of the pump-probe cross-correlation signal.

Figure 5

Schematic illustration of relaxation pathways of hot electrons in the conduction band of $2H\text{−}{\mathrm{MoS}}_2$ after photoexcitation with 395 nm laser pulses, as identified within the present study.