Tailoring excitonic complexes in layered materials

Abstract

Layered transition metal dichalcogenides (TMDCs) host a variety of strongly bound exciton complexes that control the optical properties in these materials. Apart from spin and valley, layer index provides an additional degree of freedom in a few-layer-thick lm. While in the 1H monolayer TMD inversion symmetry is broken, and the reflection symmetry is maintained but, in the bilayer, it is reversed. Trions are excitonic species with a positive or negative charge, and thus, unlike neutral excitons, the flow of trions can generate a net detectable charge current. Trions under favourable doping conditions can be created in a coherent manner using resonant excitation. The neutral biexciton (bound state of two excitons) can assemble further to create a charged state with another electron or hole. Generally, in W-based TMDs these ve-particle quinton states dominate the population density and this can also be engineered to produce photocurrent at cryogenic temperature. In the firrst work, we show that in a few-layer TMDC lm, the wave functions of the conduction and valence-band-edge states contributing to the K(K0) valley are spatially con ned in the alternate layers - giving rise to direct (quasi-)intralayer bright exciton and lower-energy interlayer dark excitons. Depending on the spin and valley con figuration, the bright-exciton state is further found to be a coherent superposition of two layer- induced states, one (E type) distributed in the even layers and the other (O type) in the odd layers. The intralayer nature of the bright exciton manifests as a relatively weak dependence of the exciton binding energy on the thickness of the few-layer lm, and the binding energy is maintained up to 50 meV in the bulk limit - which is an order of magnitude higher than conventional semiconductors. Fast Stokes energy transfer from the intralayer bright state to the interlayer dark states provides a clear signature in the layer-dependent broadening of the photoluminescence peak and plays a key role in the suppression of the photoluminescence intensity observed in TMDCs with thickness beyond a monolayer. In the second work, we show that bilayer WS2 exhibits a quantum con ned Stark effect (QCSE) that is linear with the applied out-of-plane electric field, in contrast to a quadratic one for a monolayer because of the contrasting symmetries between monolayer and bilayer. The interplay between the unique layer degree of freedom in the bilayer and the field-driven partial interconversion between intralayer and interlayer excitons generates a giant tunability of the exciton oscillator strength. This makes bilayer WS2 a promising candidate for an atomically thin, tuneable electro-absorption modulator at the exciton resonance, particularly when stacked on top of a graphene layer that provides an ultrafast nonradiative relaxation channel. By tweaking the biasing confi guration, we further show that the excitonic response can be largely tuned through electrostatic doping, by efficiently transferring the oscillator strength from neutral to charged exciton. In the third and fourth work, we demonstrate interlayer charge transport from top few-layer graphene to bottom monolayer graphene, mediated by a coherently formed trion state using a few-layer graphene/monolayer WS2/monolayer graphene vertical het- erojunction. This is achieved by using a resonant excitation and varying the sample temperature. The resulting change in the WS2 bandgap allows us to scan the excitation around the exciton-trion spectral overlap with high spectral resolution. By correlating the vertical photocurrent and in situ photoluminescence features at the heterojunction as a function of the spectral position of the excitation, we show that (1) trions are anoma- lously stable at the junction even up to 463 K due to enhanced doping, and (2) the photocurrent results from the ultrafast formation of a trion through exciton-trion coher- ent coupling, followed by its fast interlayer transport. Further, the resonant photocurrent thus generated can be effectively controlled by a back gate voltage applied through the incomplete screening of the bottom monolayer graphene, and the photocurrent strongly correlates with the gate dependent trion intensity, while the non-resonant photocurrent exhibits only a weak gate dependence. We estimate a sub-100 fs switching time of the device. In the final work, we have used the pulsed laser excitation to create the quinton states in monolayer WS2 while resonantly exciting the exciton and trion states at low temperature. Strong light absorption by the charged biexciton under spectral resonance, coupled with its charged nature, makes it intriguing for photodetection - an area that is hitherto unexplored. Using the high built-in vertical electric eld in an asymmetrically designed few-layer graphene encapsulated 1L-WS2 heterostructure, here we report, for the rst time, a large, highly nonlinear photocurrent arising from the strong absorption by two charged biexciton species under zero external bias (self-powered mode). Time- resolved measurement reveals that the photoresponse is ultra-fast, on the order of sub-5 ps. By using single- and two-color photoluminescence excitation spectroscopy, we show that the two biexcitonic peaks originate from bright-dark and bright-bright exciton-trion combinations. The possibility of electrical manipulation and detection of a charged exciton (trion) before its radiative recombination makes it promising for excitonic devices. The demon- stration of coherent formation, high stabilization, vertical transportation, and electrical detection of trions marks a step toward room-temperature trionics. Following the same the ve-particle charged quinton can also be efficiently generated and electrically de- tected. They can be used in electrical detection of constituting bright and dark states and quantum manipulation of the coupled spin-valley physics. Such innate nonlinearity in the photocurrent due to its biexcitonic origin, coupled with the ultra-fast response due to swift inter-layer charge transfer exempli fies the promise of manipulating many- body effects in monolayers. Also, the findings are prospective toward highly tunable, atomically thin, compact, and light on chip, re-confi gurable components and promising for several applications such as higher harmonics generation of the modulating signal, receiver design in microwave photonics and visible light communication, square-law cir- cuits, and also in nonlinear next generation optoelectronics.