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1. Strained two-dimensional tungsten diselenide for mechanically tunable exciton transport

   https://doi.org/10.1038/s41467-024-55135-8  

   Kim, Jin Myung; Jeong, Kwang-Yong; Kwon, Soyeong; So, Jae-Pil; Wang, Michael Cai; Snapp, Peter; Park, Hong-Gyu; Nam, SungWoo (December 2024, Nature Communications)

   Abstract Tightly bound electron-hole pairs (excitons) hosted in atomically-thin semiconductors have emerged as prospective elements in optoelectronic devices for ultrafast and secured information transfer. The controlled exciton transport in such excitonic devices requires manipulating potential energy gradient of charge-neutral excitons, while electrical gating or nanoscale straining have shown limited efficiency of exciton transport at room temperature. Here, we report strain gradient induced exciton transport in monolayer tungsten diselenide (WSe₂) across microns at room temperature via steady-state pump-probe measurement. Wrinkle architecture enabled optically-resolvable local strain (2.4%) and energy gradient (49 meV/μm) to WSe₂. We observed strain gradient induced flux of high-energy excitons and emission of funneled, low-energy excitons at the 2.5 μm-away pump point with nearly 45% of relative emission intensity compared to that of excited excitons. Our results strongly support the strain-driven manipulation of exciton funneling in two-dimensional semiconductors at room temperature, opening up future opportunities of 2D straintronic exciton devices. 

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2. Probing excitons in transition metal dichalcogenides by Drude-like exciton intraband absorption

   https://doi.org/10.1039/C8NR03135E  

   Zhao, Siqi; He, Dawei; He, Jiaqi; Zhang, Xinwu; Yi, Lixin; Wang, Yongsheng; Zhao, Hui (January 2018, Nanoscale)

   Understanding excitonic dynamics in two-dimensional semiconducting transition metal dichalcogenides is important for developing their optoelectronic applications. Recently, transient absorption techniques based on resonant excitonic absorption have been used to study various aspects of excitonic dynamics in these materials. The transient absorption in such measurements originates from phase-space state filling, bandgap renormalization, or screening effects. Here we report a new method to probe excitonic dynamics based on exciton intraband absorption. In this Drude-like process, probe photons are absorbed by excitons in their intraband excitation to higher energy states, causing a transient absorption signal. Although the magnitude of the transient absorption is lower than that of the resonant techniques, the new method is less restrictive on the selection of probe wavelength, has a larger linear range, and can provide complementary information on photocarrier dynamics. Using the WS 2 monolayer and bulk samples as examples, we show that the new method can probe exciton–exciton annihilation at high densities and reveal exciton formation processes. We also found that the exciton intraband absorption cross section of the WS 2 monolayer is on the order of 10 −18 cm 2 . 

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3. Extension Doping with Low‐Resistance Contacts for P‐Type Monolayer WSe ₂ Field‐Effect Transistors

   https://doi.org/10.1002/aelm.202400843  

   Chen, Sihan; Zhang, Yue; King, William P; Bashir, Rashid; van_der_Zande, Arend M (December 2024, Advanced Electronic Materials)

   Abstract Source/Drain extension doping is crucial for minimizing the series resistance of the ungated channel and reducing the contact resistance of field‐effect transistors (FETs) in complementary metal–oxide–semiconductor (CMOS) technology. 2D semiconductors, such as MoS₂and WSe₂, are promising channel materials for beyond‐silicon CMOS. A key challenge is to achieve extension doping for 2D monolayer FETs without damaging the atomically thin material. This work demonstrates extension doping with low‐resistance contacts for monolayer WSe₂p‐FETs. Self‐limiting oxidation transforms a bilayer WSe₂into a hetero‐bilayer of a high‐work‐function WO_xSe_yon a monolayer WSe₂. Then, damage‐free nanolithography defines an undoped nano‐channel, preserving the high on‐current of WO_xSe_y‐doped FETs while significantly improving their on/off ratio. The insertion of an amorphous WO_xSe_yinterlayer under the contacts achieves record‐low contact resistances for monolayer WSe₂over a hole density range of 10¹²to 10¹³cm⁻²(1.2 ± 0.3 kΩ µm at 10¹³cm⁻²). The WO_xSe_y‐doped extension exhibits a sheet resistance as low as 10 ± 1 kΩ □⁻¹. Monolayer WSe₂p‐FETs with sub‐50 nm channel lengths reach a maximum drain current of 154 µA µm⁻¹with an on/off ratio of 10⁷–10⁸. These results define strategies for nanometer‐scale selective‐area doping in 2D FETs and other 2D architectures. 

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4. Long-range transport of 2D excitons with acoustic waves

   https://doi.org/10.1038/s41467-022-29042-9  

   Peng, Ruoming; Ripin, Adina; Ye, Yusen; Zhu, Jiayi; Wu, Changming; Lee, Seokhyeong; Li, Huan; Taniguchi, Takashi; Watanabe, Kenji; Cao, Ting; et al (December 2022, Nature Communications)

   Abstract Excitons are elementary optical excitation in semiconductors. The ability to manipulate and transport these quasiparticles would enable excitonic circuits and devices for quantum photonic technologies. Recently, interlayer excitons in 2D semiconductors have emerged as a promising candidate for engineering excitonic devices due to their long lifetime, large exciton binding energy, and gate tunability. However, the charge-neutral nature of the excitons leads to weak response to the in-plane electric field and thus inhibits transport beyond the diffusion length. Here, we demonstrate the directional transport of interlayer excitons in bilayer WSe 2 driven by the propagating potential traps induced by surface acoustic waves (SAW). We show that at 100 K, the SAW-driven excitonic transport is activated above a threshold acoustic power and reaches 20 μm, a distance at least ten times longer than the diffusion length and only limited by the device size. Temperature-dependent measurement reveals the transition from the diffusion-limited regime at low temperature to the acoustic field-driven regime at elevated temperature. Our work shows that acoustic waves are an effective, contact-free means to control exciton dynamics and transport, promising for realizing 2D materials-based excitonic devices such as exciton transistors, switches, and transducers up to room temperature. 

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5. Probing phonon focusing, thermomechanical behavior, and moiré patterns in van der Waals architectures using surface acoustic waves

   https://doi.org/10.1038/s41524-024-01315-5  

   Baradwaj, Nitish; Aditya, Anikeya; Mishra, Ankit; Burns, Kory; Lang, Eric; Hachtel, Jordan A; Hattar, Khalid; Aitkaliyeva, Assel; Nakano, Aiichiro; Vashishta, Priya; et al (December 2024, npj Computational Materials)

   Abstract Surface acoustic waves (SAWs) propagate along solid-air, solid-liquid, and solid-solid interfaces. Their characteristics depend on the elastic properties of the solid. Combining transmission electron microscopy (TEM) experiments with molecular dynamics (MD) simulations, we probe atomic environments around intrinsic defects that generate SAWs in vertically stacked two-dimensional (2D) bilayers of MoS₂. Our joint experimental-simulation study provides insights into SAW-induced structural and dynamical changes and thermomechanical responses of MoS₂bilayers. Using MD simulations, we compute mechanical properties from the SAW velocity and thermal conductivity from thermal diffusion of SAWs. The results for Young’s modulus and thermal conductivity of an MoS₂monolayer are in good agreement with experiments. The presence of defects, such as nanopores which generate SAWs, reduces the thermal conductivity of 2D-MoS₂by an order of magnitude. We also observe dramatic changes in moiré patterns, phonon focusing, and cuspidal structures on 2D-MoS₂layers. 

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