Atomically-thin Semiconductors Research Articles

Manipulating nonlinear exciton polaritons in an atomically-thin semiconductor with artificial potential landscapes

Exciton polaritons in atomically thin transition-metal dichalcogenide microcavities provide a versatile platform for advancing optoelectronic devices and studying the interacting Bosonic physics at ambient conditions. Rationally engineering the favorable properties of polaritons is critically required for the rapidly growing research. Here, we demonstrate the manipulation of nonlinear polaritons with the lithographically defined potential landscapes in monolayer WS2 microcavities. The discretization of photoluminescence dispersions and spatially confined patterns indicate the deterministic on-site localization of polaritons by the artificial mesa cavities. Varying the trapping sizes, the polariton-reservoir interaction strength is enhanced by about six times through managing the polariton–exciton spatial overlap. Meanwhile, the coherence of trapped polaritons is significantly improved due to the spectral narrowing and tailored in a picosecond range. Therefore, our work not only offers a convenient approach to manipulating the nonlinearity and coherence of polaritons but also opens up possibilities for exploring many-body phenomena and developing novel polaritonic devices based on 2D materials.

Impacts of dielectric screening on the luminescence of monolayer WSe2

Single layers of transition metal dichalcogenides (TMDCs), such as WSe2 have gathered increasing attention due to their intense electron–hole interactions, being considered promising candidates for developing novel optical applications. Within the few-layer regime, these systems become highly sensitive to the surrounding environment, enabling the possibility of using a proper substrate to tune desired aspects of these atomically-thin semiconductors. In this scenario, the dielectric environment provided by the substrates exerts significant influence on electronic and optical properties of these layered materials, affecting the electronic band-gap and the exciton binding energy. However, the corresponding effect on the luminescence of TMDCs is still under discussion. To elucidate these impacts, we used a broad set of materials as substrates for single-layers of WSe2, enabling the observation of these effects over a wide range of electrical permittivities. Our results demonstrate that an increasing permittivity induces a systematic red-shift of the optical band-gap of WSe2, intrinsically related to a considerable reduction of the luminescence intensity. Moreover, we annealed the samples to ensure a tight coupling between WSe2 and its substrates, reducing the effect of undesired adsorbates trapped in the interface. Ultimately, our findings reveal how critical the annealing temperature can be, indicating that above a certain threshold, the heating treatment can induce adverse impacts on the luminescence. Furthermore, our conclusions highlight the influence the dielectric properties of the substrate have on the luminescence of WSe2, showing that a low electrical permittivity favours preserving the native properties of the adjacent monolayer.

Impurity-induced pairing in two-dimensional Fermi gases

We study induced pairing between two identical fermions mediated by an attractively interacting quantum impurity in two-dimensional systems. Based on a stochastic variational method (SVM), we investigate the influence of confinement and finite interaction range effects on the mass ratio beyond which the ground state of the quantum three-body problem undergoes a transition from a composite bosonic trimer to an unbound dimer-fermion state. We find that confinement as well as a finite interaction range can greatly enhance trimer stability, bringing it within reach of experimental implementations such as found in ultracold atom systems. In the context of solid-state physics, our solution of the confined three-body problem shows that exciton-mediated interactions can become so dominant that they can even overcome detrimental Coulomb repulsion between electrons in atomically-thin semiconductors. Our paper thus paves the way towards a universal understanding of boson-induced pairing across various fermionic systems at finite density, and opens perspectives towards realizing unexplored forms of electron pairing beyond the conventional paradigm of Cooper pair formation.

Optical classification of excitonic phases in molecular functionalized atomically-thin semiconductors

The excitonic insulator is an elusive electronic phase exhibiting a correlated excitonic ground state. Materials with such a phase are expected to have intriguing properties such as excitonic high-temperature superconductivity. However, compelling evidence on the experimental realization is still missing. Here, we theoretically propose hybrids of two-dimensional semiconductors functionalized by organic molecules as prototypes of excitonic insulators, with the exemplary candidate ${\mathrm{WS}}_{2}$-F6TCNNQ. This material system exhibits an excitonic insulating phase at room temperature with a ground state formed by a condensate of interlayer excitons. To address an experimentally relevant situation, we calculate the corresponding phase diagram for the important parameters: temperature, gap energy, and dielectric environment. Further, to guide future experimental detection, we show how to optically characterize the different excitonic phases via far-infrared to terahertz spectroscopy valid also for monolayer materials.

Asymmetric magnetic proximity interactions in MoSe2/CrBr3 van der Waals heterostructures.

Magnetic proximity interactions between atomically thin semiconductors and two-dimensional magnets provide a means to manipulate spin and valley degrees of freedom in non-magnetic monolayers, without using applied magnetic fields1-3. In such van der Waals heterostructures, magnetic proximity interactions originate in the nanometre-scale coupling between spin-dependent electronic wavefunctions in the two materials, and typically their overall effect is regarded as an effective magnetic field acting on the semiconductor monolayer4-8. Here we demonstrate that magnetic proximity interactions in van der Waals heterostructures can in fact be markedly asymmetric. Valley-resolved reflection spectroscopy of MoSe2/CrBr3 van der Waals structures reveals strikingly different energy shifts in the K and K' valleys of the MoSe2 due to ferromagnetism in the CrBr3 layer. Density functional calculations indicate that valley-asymmetric magnetic proximity interactions depend sensitively on the spin-dependent hybridization of overlapping bands and as such are likely a general feature of hybrid van der Waals structures. These studies suggest routes to control specific spin and valley states in monolayer semiconductors9,10.

Doping transition metal in PdSeO3 atomic layers by aqueous cation exchange: A new doping protocol for a new 2D photocatalyst

Elemental doping confined in atomically-thin 2D semiconductors offers a compelling strategy for constructing high performance photocatalysts. Although impressive progress has been achieved based on co-thermolysis method, the choices of dopants as well as semiconductor hosts are still quite limited to yield the elaborate photocatalyst with atomic-layer-confined doping defects, owing to the difficulty in balancing the reaction kinetics of different precursors. This study shows that the cation exchange reaction, which is dictated by the Pearson's hard and soft acids and bases (HSAB) theory and allowed to proceed at mild temperatures, can be developed into a conceptually new protocol for engineering elemental doping confined in semiconductor atomic layers. To this aim, the two atomic layers of a new type of 2D photocatalyst PdSeO3 (PdSeO3 2ALs, 1.1 nm) are created by liquid exfoliation and exploited as a proof-of-concept prototype. It is demonstrated that the Mn(II) dopants with controlled concentrations can be incorporated into PdSeO3 2ALs via topological Mn2+-for-Pd2+ cation exchange performed in water/isopropanol solution at 30 °C. The resulting Mn-doped PdSeO3 2ALs present enhanced capacity for driving photocatalytic oxidation reactions in comparison with their undoped counterparts. The findings here suggest that the new route mediated by post synthetic cation exchange promises to give access to manifold 2D confined-doping photocatalysts, with little perturbations on the thickness, morphology, and crystal structure of the atomically-thin semiconductor hosts.

Bright single photon emitters with enhanced quantum efficiency in a two-dimensional semiconductor coupled with dielectric nano-antennas

Single photon emitters in atomically-thin semiconductors can be deterministically positioned using strain induced by underlying nano-structures. Here, we couple monolayer WSe2 to high-refractive-index gallium phosphide dielectric nano-antennas providing both optical enhancement and monolayer deformation. For single photon emitters formed on such nano-antennas, we find very low (femto-Joule) saturation pulse energies and up to 104 times brighter photoluminescence than in WSe2 placed on low-refractive-index SiO2 pillars. We show that the key to these observations is the increase on average by a factor of 5 of the quantum efficiency of the emitters coupled to the nano-antennas. This further allows us to gain new insights into their photoluminescence dynamics, revealing the roles of the dark exciton reservoir and Auger processes. We also find that the coherence time of such emitters is limited by intrinsic dephasing processes. Our work establishes dielectric nano-antennas as a platform for high-efficiency quantum light generation in monolayer semiconductors.

Motional narrowing, ballistic transport, and trapping of room-temperature exciton polaritons in an atomically-thin semiconductor

Monolayer transition metal dichalcogenide crystals (TMDCs) hold great promise for semiconductor optoelectronics because their bound electron-hole pairs (excitons) are stable at room temperature and interact strongly with light. When TMDCs are embedded in an optical microcavity, excitons can hybridise with cavity photons to form exciton polaritons, which inherit useful properties from their constituents. The ability to manipulate and trap polaritons on a microchip is critical for applications. Here, we create a non-trivial potential landscape for polaritons in monolayer WS2, and demonstrate their trapping and ballistic propagation across tens of micrometers. We show that the effects of dielectric disorder, which restrict the diffusion of WS2 excitons and broaden their spectral resonance, are dramatically reduced for polaritons, leading to motional narrowing and preserved partial coherence. Linewidth narrowing and coherence are further enhanced in the trap. Our results demonstrate the possibility of long-range dissipationless transport and efficient trapping of TMDC polaritons in ambient conditions.

Piezotronic and piezo-phototronic effects of atomically-thin ZnO nanosheets

Abstract Piezotronic and piezo-phototronic effects have attracted much attention as promising approaches for active electronic/optoelectronic devices. However, the piezotronic and piezo-phototronic devices in previous reports are mainly based on nanowires or two-dimensional transition-metal dichalcogenides that have size of several micrometers along the polarization direction. As the fast development of nanoelectronics and nano-optoelectronics, exploring the piezotronic effect and piezo-phototronic effect at nanometer scale for ultrathin nanodevices and nanosystems is valuable. Here, we investigated the piezotronic and piezo-phototronic effects of atomically thin ZnO nanosheet, and revealed the dominant mechanism. Experiments were performed on the atomically thin ZnO field effect transistor, which showed enhanced electronic transport characteristic under pressure. Theoretical analysis revealed that the change of electronic transport behavior was caused by pressure induced modulation on the effective thickness of the transport channel and the Schottky barrier between ZnO and contact electrodes. Meanwhile, the atomically-thin ZnO film exhibited enhanced response to ultraviolet light under pressure with a high photoresponsivity of 300 AW−1 (Vds = 2 V). This value was improved 230% than the response of the same device under strain-free condition, and more than 103 times higher than the performance of commercial ultraviolet photodetectors, indicating the effectiveness of piezo-phototronic effect in nanoscale. This study shows great promises of the ultrathin devices based on piezotronic and piezo-phototronic effects, which paves the way for atomically-thin semiconductors with out-of-plane piezoelectricity for applications in novel electronics/optoelectronics.

Coulomb-bound four- and five-particle intervalley states in an atomically-thin semiconductor

As hosts for tightly-bound electron-hole pairs carrying quantized angular momentum, atomically-thin semiconductors of transition metal dichalcogenides (TMDCs) provide an appealing platform for optically addressing the valley degree of freedom. In particular, the valleytronic properties of neutral and charged excitons in these systems have been widely investigated. Meanwhile, correlated quantum states involving more particles are still elusive and controversial despite recent efforts. Here, we present experimental evidence for four-particle biexcitons and five-particle exciton-trions in high-quality monolayer tungsten diselenide. Through charge doping, thermal activation, and magnetic-field tuning measurements, we determine that the biexciton and the exciton-trion are bound with respect to the bright exciton and the trion, respectively. Further, both the biexciton and the exciton-trion are intervalley complexes involving dark excitons, giving rise to emissions with large, negative valley polarization in contrast to that of the two-particle excitons. Our studies provide opportunities for building valleytronic quantum devices harnessing high-order TMDC excitations.

SiC/MoS2 layered heterostructures: Promising photocatalysts revealed by a first-principles study

Van der Waals coupling is emerging as a powerful method to engineer atomically-thin semiconductor materials, which leads to unusual properties and plays an increasingly key role in catalysis and solar energy conversion. The electronic properties of these two-dimensional (2D) layers are critically important for their applications. Herein the electronic, interfacial and optical properties of 2D SiC/MoS2 van der Waals heterostructures have been investigated by using density functional theory with the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. Both the SiC/MoS2 heterostructure (SiC monolayer coupled with MoS2 monolayer) and SiC/BL-MoS2 heterostructure (SiC monolayer combined with MoS2 bilayer) demonstrate narrower band gaps than the two involved constituents, with a energy gap value of 1.46 eV and 1.36 eV, respectively, which mainly due to the formation of type-II heterojunctions for them and the sensitive electronic properties of SiC to lattice constant variations. Moreover, for the two vdW heterostructures, the electron transition mainly occurs between different constituents, which greatly facilitates effective separation for photogenerated charge carrier. The contradiction of the electrostatic potential and work functions difference in the nanocomposite is another key factor to improve the photocatalytic activity and stability. This work noth only reveals that the SiC/MoS2 and SiC/BL-MoS2 vdW heterostructure are both promising photocatalyst, but also provide an insight into the underlying mechanism in SiC/MoS2 semiconductor photocatalytic nanomaterials.

Nanoantenna-Enhanced Light–Matter Interaction in Atomically Thin WS2

We present a hybrid system consisting of an atomically-thin semiconductor and a plasmonic nanoantenna. The intense optical near-fields provided by the antenna increase the photoluminescence intensity of WS 2 by more than one order of magnitude.

Optical Manipulation and Electrical Control of Excitons and Valleys in Atomically-Thin Semiconductors

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