Observation of two distinct negative trions in tungsten disulfide monolayers

Abstract

Ultrafast pump-probe spectroscopy of two-dimensional tungsten disulfide monolayers (2D-WS 2 ) grown on sapphire substrates revealed two transient absorption spectral peaks that are attributed to distinct negative trions at ~2.02 eV (T 1 ) and ~1.98 eV (T 2 ). The dynamics measurements indicate that trion formation by the probe is enabled by photodoped 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 X A and X B at ~2.03 and ~2.40 eV, respectively, were separately monitored and compared to the photoinduced absorption features. Selective excitation of the lowest exciton level X A using λ pump <2.4 eV forms only trion T 1 implying that the electron remaining from dissociation of exciton X A is involved in the creation of this trion with a binding energy ~ 10 meV with respect to X A . The absorption peak corresponding to trion T 2 appears when λ pump >2.4 eV, which is just sufficient to excite exciton X B . The dynamics of trion T 2 formation are found to correlate with the disappearance of the bleach of X B exciton, indicating the involvement of holes participating in the bleach dynamics of exciton X B . Static electrical-doping photoabsorption measurements confirm the presence of an induced absorption peak similar to that of T 2 . 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.

Nanosculpting of Atomically Thin 2D Materials for Site‐Specific Photoluminescence Modulation

“…It is reported that defect sites could dissociate excitons by trapping the holes and leave the electron in the conduction band to bind with photo generated electron‐hole pairs to form negative trion. Further, the pump probe measurements carried out with different laser energy fluence indicates that the observed trion amplitudes are more correlated to the number of trapping sites in comparison to the production of excitons . The similar trionic emission in 2D system is not shown when the monolayer MoS 2 is under biaxial strain with suspended geometry due to absence of substrate effect.…”

“…Typically, such negative trions are known to emerge when excess electrons are introduced in photo‐excited excitons . Ultrafast pump probe spectroscopic study clearly revealed the origin of negative trions and its relation with defect sites . Dynamic measurements suggested that trions are formed by photodoping of WS 2 monolayers with residual electrons that are remaining after trapping of holes or free electron‐hole pair at defect sites.…”

Orbital, spin and valley contributions to Zeeman splitting of excitonic resonances in MoSe 2 , WSe 2 and WS 2 Monolayers

“…This peculiar evolution originates, as discussed previously in literature [43][44][45][46][47][48], from contribution of two different CX states to the feature observed in absorption-type spectra. The existence of two CX states in 'darkish' materials is a consequence of spin alignment in conduction subbands, which allows an excess electron from either K+ or K− valley to accompany the photo-created electron-hole pair and form a bound CX state.…”

Long-Lived Valley Polarization of Intravalley Trions in Monolayer WSe2

“…The exceptionally large binding energies of these quasiparticles (i.e. 200-500 meV for excitons and an additional binding energy of 20-40 meV for trions) further promise room temperature valleytronic applications [2,3,[7][8][9][10][11][12][13].High efficiency valley initialization and a long lifetime of valley polarization are preferred in valleytronic applications [14][15][16][17]. Initial experiments based on steadystate photoluminescence have shown the possibility of creating a near-unity valley polarization in MoS2 and WSe2 via exciton resonances [4,18].…”