Avalanche ionization is shown to be the dominant carrier generation mechanism in the ultrafast ablation of transition metal dichalcogenides. Carrier densities reaching 22% of the total valence band ...population are needed for ablation to occur.
Transition metal dichalcogenides are known to possess large optical nonlinearities and driving these materials at high intensities is desirable for many applications. Understanding their optical ...responses under repetitive intense excitation is essential to improve the performance limit of these strong-field devices and to achieve efficient laser patterning. Here, we report the incubation study of monolayer MoS\({}_{2}\) and WS\({}_{2}\) induced by 160 fs, 800 nm pulses in air to examine how their ablation threshold scales with the number of admitted laser pulses. Both materials were shown to outperform graphene and most bulk materials; specifically, MoS\({}_{2}\) is as resistant to radiation degradation as the best of the bulk thin films with a record fast saturation. Our modeling provides convincing evidence that the small reduction in threshold and fast saturation of MoS\({}_{2}\) originates in its excellent bonding integrity against radiation-induced softening. Sub-ablation damages, in the forms of vacancies, lattice disorder, and nano-voids, were revealed by transmission electron microscopy, photoluminescence, Raman, and second harmonic generation studies, which were attributed to the observed incubation. For the first time, a sub-ablation damage threshold is identified for monolayer MoS\({}_{2}\) to be 78% of single-shot ablation threshold, below which MoS\({}_{2}\) remains intact for many laser pulses. Our results firmly establish MoS\({}_{2}\) as a robust material for strong-field devices and for high-throughput laser patterning.
Laser direct writing is an attractive method for patterning 2D materials without contamination. Literature shows that the femtosecond 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 femtosecond ablation of 2D materials is studied using MoS\(_{2}\) as an example. We show unambiguously that femtosecond ablation of MoS\(_{2}\) 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 MoS\(_{2}\) with sub-micron resolution and mm/s speed. Moreover, engineered substrates are shown to enhance the ablation efficiency, enabling patterning with low-power femtosecond 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 femtosecond laser ablation as a viable route to pattern 2D materials.
MoS2 Devices
In article number 2106411, Wen‐Wei Wu and co‐workers reveal the direct observation of an MoS2 device under biasing via powerful in situ transmission electron microscopy (TEM). During in ...situ TEM biasing, the MoS2 is etched vertically and horizontally; the former is dominated by knock‐on damage, while the latter involves atomic migration induced by Joule heating. Also, the long cracks that form by thermal stress, which are discovered in both in situ and ex situ biasing at 10 V, are discussed in this research. It is believed that insights of material damage can push the limit of material properties and broaden the range of MoS2‐based device applications.
A moiré superlattice 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 moiré potential. However, quantifying the moiré potential for excitons is nontrivial. By creating a large ensemble of MoSe
/MoS
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 moiré potential for excitons. Surprisingly, the moiré potential 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 moiré potential for intralayer excitons. The remarkably deep intralayer moiré potential is understood within the framework of structural reconstruction within the moiré unit cell.
The adsorption and desorption of electrolyte ions strongly modulates the carrier density or carrier type on the surface of monolayer-MoS
catalyst during the hydrogen evolution reaction (HER). The ...buildup of electrolyte ions onto the surface of monolayer MoS
during the HER may also result in the formation of excitons and trions, similar to those observed in gate-controlled field-effect transistor devices. Using the distinct carrier relaxation dynamics of excitons and trions of monolayer MoS
as sensitive descriptors, an in situ microcell-based scanning time-resolved liquid cell microscope is set up to simultaneously measure the bias-dependent exciton/trion dynamics and spatially map the catalytic activity of monolayer MoS
during the HER. This operando probing technique used to monitor the interplay between exciton/trion dynamics and electrocatalytic activity for two-dimensional transition metal dichalcogenides provides an excellent platform to investigate the local carrier behaviors at the atomic layer/liquid electrolyte interfaces during electrocatalytic reaction.
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