Palladium diselenide (PdSe2), a peculiar noble metal dichalcogenide, has emerged as a new two-dimensional material with high predicted carrier mobility and a widely tunable band gap for device applications. The inherent in-plane anisotropy endowed by the pentagonal structure further renders PdSe2 promising for novel electronic, photonic, and thermoelectric applications. However, the direct synthesis of few-layer PdSe2 is still challenging and rarely reported. Here, we demonstrate that few-layer, single-crystal PdSe2 flakes can be synthesized at a relatively low growth temperature (300 °C) on sapphire substrates using low-pressure chemical vapor deposition (CVD). The well-defined rectangular domain shape and precisely determined layer number of the CVD-grown PdSe2 enable us to investigate their layer-dependent and in-plane anisotropic properties. The experimentally determined layer-dependent band gap shrinkage combined with first-principle calculations suggest that the interlayer interaction is weaker in few-layer PdSe2 in comparison with that in bulk crystals. Field-effect transistors based on the CVD-grown PdSe2 also show performances comparable to those based on exfoliated samples. The low-temperature synthesis method reported here provides a feasible approach to fabricate high-quality few-layer PdSe2 for device applications.

A moirésuperlattice formed in twisted van der Waals bilayers has emerged as a new tuning knob for creating new electronic states in two-dimensional materials. Excitonic properties can also be altered drastically due to the presence of moireṕ otential. However, quantifying the moirépotential for excitons is nontrivial. By creating a large ensemble of MoSe 2 /MoS 2 heterobilayers with a systematic variation of twist angles, we map out the minibands of interlayer and intralayer excitons as a function of twist angles, from which we determine the moireṕ otential for excitons. Surprisingly, the moirépotential depth for intralayer excitons is up to ∼130 meV, comparable to that for interlayer excitons. This result is markedly different from theoretical calculations based on density functional theory, which show an order of magnitude smaller moirépotential for intralayer excitons. The remarkably deep intralayer moirépotential is understood within the framework of structural reconstruction within the moiréunit cell.

We investigated hybrid zero-dimensional core–shell CdSe/ZnS quantum dot (QD)/two-dimensional monolayer WSe2 semiconductors with an Ag nanodisk (ND) for manipulating plasmonic-enhanced photoluminescence (PL) and color conversion efficiency. The absorption spectrum of the local surface plasmon resonance (LSPR) effectively overlaps with that of QDs or monolayer WSe2 to considerably enhance PL. The broad absorption spectrum of the LSPR simultaneously overlapped with the emission spectrum of QDs and the absorption spectrum of excitons in WSe2 to enhance the color conversion efficiency. The highest efficiency of color conversion from QDs to WSe2 with Ag ND was 53%. In the future, hybrid QD/transition metal dichalcogenide light emitters could be further integrated with GaN-based white light-emitting diodes to manipulate the color temperature and expand the color gamut to develop a miniature white light-emitting diode.

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Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have shown promise for a variety of optoelectronic applications due to a wide range of optical, electrical, and mechanical properties. Large-area chemical vapor deposition (CVD)-grown TMDC flakes could be useful in such devices. However, the defects present in large-area TMDC flakes can significantly influence carrier dynamics and transport properties. Here, the ultrafast carrier dynamics of monolayer tungsten disulfide (WS 2 ) covering a large area of the substrate was explored using transient absorption spectroscopy. By monitoring the transient optical response, exciton trapping by oxygen-induced defects has been identified in monolayer WS 2 . We observe excitation-densitydependent exciton decay dynamics for both band-edge and above band-edge excitations due to exciton−exciton annihilation. Our results demonstrate the impact of defect states on carrier recombination in CVD-grown TMDCs, which could pave the way for utilizing such materials in optoelectronic device applications.

Laser direct writing is an attractive method for patterning 2D materials without contamination. Literature shows that the ultrafast ablation threshold of graphene across substrates varies by an order of magnitude. Some attribute it to the thermal coupling to the substrates, but it remains by and large an open question. For the first time the effect of substrates on the femtosecond ablation of 2D materials is studied using MoS2 as an example. We show unambiguously that femtosecond ablation of MoS2 is an adiabatic process with negligible heat transfer to the substrates. The observed threshold variation is due to the etalon effect which was not identified before for the laser ablation of 2D materials. Subsequently, an intrinsic ablation threshold is proposed as a true threshold parameter for 2D materials. Additionally, we demonstrate for the first time femtosecond laser patterning of monolayer MoS2 with sub-micron resolution and mm/s speed. Moreover, engineered substrates are shown to enhance the ablation efficiency, enabling patterning with low-power ultrafast oscillators. Finally, a zero-thickness approximation is introduced to predict the field enhancement with simple analytical expressions. Our work clarifies the role of substrates on ablation and firmly establishes ultrafast laser ablation as a viable route to pattern 2D materials.

We report an ultrafast increase of the quasi-particle bandgap and effective mass in photoexcited monolayer MoS2 on HOPG, utilizing extreme-ultraviolet time- and angle-resolved photoemission spectroscopy (XUV-trARPES). Combined with theoretical models, we attribute these compelling band renormalizations to the excitonic effects from bound electron-hole pairs.