Observation of two distinct negative trions in tungsten disulfide monolayers

Ultrafast pump-probe spectroscopy of two-dimensional tungsten disulfide monolayers (2DWS2) grown on sapphire substrates revealed two transient absorption spectral peaks that are attributed to distinct negative trions at ∼2.02eV (T1) and ∼1.98eV (T2). The dynamics measurements indicate that trion formation by the probe is enabled by photodoped 2D WS2 crystals with electrons remaining after trapping of holes from excitons or free electron-hole pairs at defect sites in the crystal or on the substrate. Dynamics of the characteristic absorption bands of excitons XA and XB at ∼2.03 and ∼2.40eV, respectively, were separately monitored and compared to the photoinduced absorption features. Selective excitation of the lowest exciton level XA using λpump<2.4eV forms only trion T1, implying that the electron remaining from dissociation of exciton XA is involved in the creation of this trion with a binding energy ∼10meV with respect to XA. The absorption peak corresponding to trion T2 appears when λpump<2.4eV, which is just sufficient to excite exciton XB. The dynamics of trion T2 formation are found to correlate with the disappearance of the bleach of the XB exciton, indicating the involvement of holes participating in the bleach dynamics of exciton XB. Static electrical-doping photoabsorption measurements confirm the presence of an induced absorption peak similar to that of T2. Since the proposed trion formation process here involves exciton dissociation through hole trapping by defects in the 2D crystal or substrate, this discovery highlights the strong role of defects in defining optical and electrical properties of 2D metal chalcogenides, which is relevant to a broad spectrum of basic science and technological applications.