Two-dimensional transition metal dichalcogenides (TMDCs) are promising materials for next-generation optoelectronic devices, yet their implementation is hindered by limited sample stability and challenges in forming reliable electrical contacts. Here, by utilizing time-domain THz emission spectroscopy we directly probe charge carrier dynamics in monolayer WS2 on gold (Au) and fused silica (SiO2) as a function of interface morphology. For laser excitation above the band gap of WS2, we independently extract effective transport times for both electrons and holes and find that discontinuous WS2 contacts on rough Au generate larger net photocurrents than uniform, strongly coupled interfaces - a counterintuitive observation attributed to imbalanced electron and hole transfer from WS2 to Au. Crucially, we demonstrate that ultrafast charge extraction and separation suppress recombination-driven energy release and thereby prevent photo-induced degradation under ambient conditions, eliminating the need for encapsulation. These findings redefine interfacial design as a central control parameter for both performance and stability in 2D optoelectronic devices.

We investigate interfacial charge carrier dynamics and photo-induced degradation in monolayer WS2 on gold (Au) and fused silica (SiO2) substrates using time-domain THz emission spectroscopy, with a focus on the role of interface morphology. For excitation with photon energies above the band gap of WS2, we observe a stronger net transient photocurrent - dominated by hole transfer from WS2 to Au - in samples with a rugged, discontinuous van der Waals (vdW) interface. This counterintuitive result arises from asymmetric charge flow across the tunneling barrier, whereas smoother, uniform interfaces facilitate balanced electron-hole transfer, which cancels the net current. Inefficient charge separation in WS2 on insulating SiO2 is further linked to rapid photo-induced degradation under ambient conditions, attributed to exciton recombination at defect sites, which in turn can initiate surface chemical reactions. This study highlights the critical importance of metal-semiconductor interface morphology in governing both the dynamics of photo-generated charge carriers and sample stability of 2D material-based devices.

Excitons in monolayer transition metal dichalcogenides (1L-TMDs) exhibit a Rydberg-like series of bound states, including optically dark levels. Using a three-pulse scheme, we directly track the dynamics of the 2p dark exciton in 1L-WS_{2} through the Autler-Townes splitting of the bright 1s state. The splitting, induced by a midinfrared field resonant with the 1s-2p transition, reveals distinct 1s and 2p responses to photoexcitation. This approach provides new insight into many-body effects and offers a versatile experimental framework to probe driven excitonic dynamics in quantum materials.

Van der Waals stacking presents new opportunities for nonlinear optics with its remarkable tunability and scalability. However, the fundamental role of interlayer interactions in modifying the overall nonlinear optical susceptibilities remains elusive. In this paper, we report anisotropic enhancement of second-harmonic generation (SHG) from a WS2/ReSe2 heterobilayer, where the individual composite layers possess distinctive crystal phases. We investigate polarization-resolved response and twist-angle dependence in SHG and reveal that band alignment alone is insufficient to explain the observed anisotropy in the modified SHG response. Spectral shifts in excitonic features highlight band renormalization, supporting the role of hybridization between the two layers. Furthermore, SHG enhancement is highly anisotropic and can even be suppressed in some orientations, suggesting possible intensity-borrowing mechanisms within the heterostructure. Our work demonstrates the ability to tune both the intensity and polarization dependence of nonlinear optical responses with van der Waals stacking of distinctive crystal phases.

Environmentally friendly tin halide perovskite (THP) solar cells have recently attracted more and more research efforts as a promising alternative to toxic lead-based perovskites. Incorporating more stable and uniformly oriented two-dimensional (2D) phase components into the fast-crystallizing and oxidation-prone three-dimensional (3D) phases as a growth template is a common strategy to enhance THP film quality. However, the simultaneous crystallization of the 2D phase with the 3D phase greatly undermines ordered crystallization for high-performance THP devices. Herein, a fullerene derivative ligand is introduced to establish distinct ligand-cation interaction modes with the 2D and 3D components, thereby programming the A-site cation‑mediated sequential assembly and crystallization. The 2D phase crystallizes first and serves as a template to guide the ordered epitaxial growth of the 3D phase, yielding a highly crystalline and oriented THP film with a homogeneous 2D/3D heterojunction as an efficient interface. The resulting devices achieve a significantly enhanced champion power conversion efficiency of 16.6%. Moreover, unencapsulated devices operating at maximum power point under continuous 1-sun illumination and 50°C exhibit a nine-fold longer T90 lifetime (882 h vs. 99 h) compared to control devices.

Tungsten diselenide (WSe2) is a promising p-type semiconductor limited by high contact resistance (RC) and the lack of a reliable doping strategy. Here, we demonstrate that exposing WSe2 to chloroform provides simple and stable p-type doping. In monolayer WSe2 transistors with Pd contacts, chloroform increases the maximum hole current by over 100× (>200 µA/µm), reduces RC to 2.5 kΩ·μm, and retains an on/off ratio of 1010 at room temperature. These improvements persist for over 8 months, survive a 150 °C thermal anneal, and remain effective down to 10 K, enabling a cryogenic RC of 1 kΩ·μm. Density functional theory indicates that chloroform strongly physisorbs to WSe2, inducing hole doping with minimal impact on the electronic states between the valence band and conduction band edges. Auger electron spectroscopy and atomic force microscopy suggest that chloroform intercalates at the WSe2 interface with the gate oxide, contributing to doping stability and mitigating interfacial dielectric disorder, though further studies are needed to conclusively confirm this mechanism. This robust, scalable approach enables high-yield WSe2 transistors with good p-type performance. 2D p-type transistors are essential for the realization of complementary circuits for post-silicon electronics. Here, the authors report a chloroform doping strategy to fabricate p-type monolayer WSe2 transistors with high performance and long-term stability.

Strong-field ionization in two-color ( ω + 2 ω ) elliptically polarized laser fields highlights subcycle electron dynamics in the angular distribution of photoelectrons. In these distributions, a characteristic alternating angular pattern is observed, in which successive above-threshold ionization peaks and sidebands exhibit opposite angular shifts and are controllable through the relative phase between the two driving fields. This pattern originates from subcycle interference between adjacent ionization bursts, where a weak-field-induced phase modulation changes sign from one subcycle to the next, thereby directly controlling the emission angle. We further identify abrupt sideband shifts arising from the sensitivity to the relative phase between the two fields. Time-dependent Schrödinger equation simulations show that the long-range Coulomb interaction selectively suppresses electron emission counterrotating with respect to the driving field, with a pronounced dependence on the field helicity. These results establish angular shifts as a direct observable of subcycle phase modulation and demonstrate their utility as a sensitive interferometric probe of attosecond electron wave-packet dynamics.

The work required to drive a system from one state to another comprises both the equilibrium free energy difference and the dissipation associated with irreversibility. As physical processes-such as computing-approach fast limits, calculating this excess dissipation becomes increasingly critical. Yet, precisely quantifying dissipation, more specifically, entropy production, in strongly driven, time-dependent, realistic nanoscale systems remains a considerable challenge. Consequently, previous studies have largely been limited to either idealized Markovian systems under time-dependent driving or non-Markovian steady-state systems under constant driving. Here we measure the full dynamics of trajectory-level entropy production in a non-stationary, non-Markovian material arising from time-dependent driving. We use machine learning to extract the entropy produced by a quantum dot stochastically blinking under a stepwise control protocol. The entropy produced corresponds to the loss of memory in the material as the carrier distribution evolves. In addition, our approach quantifies both information insertion and dissipation under a quenched protocol. This work demonstrates a simple and effective approach for visualizing dissipation dynamics following a fast quench and serves as a stepping stone towards optimizing energy costs in the control of real materials and devices.

Self‐assembly affords simpler synthetic routes to heterostructures compared with manual layer‐by‐layer stacking, yet controlling interlayer twist angles in a bulk solid remains an outstanding challenge. We report two new single‐crystal heterostructures: (Sn 2 Cl 2 )(CYS) 2 SnCl 4 (CYS = + NH 3 (CH 2 ) 2 S – ; Sn_CYS ) and (Sn 2 Cl 2 )(SeCYS) 2 SnCl 4 (SeCYS = + NH 3 (CH 2 ) 2 Se – ; Sn_SeCYS ) synthesized in solution, with alternating perovskite and intergrowth layers. Notably, compared to the recently reported lead analog, (Pb 2 Cl 2 )(CYS) 2 PbCl 4 ( Pb_CYS ), the tin heterostructures feature a twist between the perovskite and intergrowth layers. We trace this twist to local distortions at the Sn centers, which change the interfacial lattice‐matching requirements compared to those of the Pb analog. Electronic band structure calculations show that the striking differences in the relative energies of perovskite‐ and intergrowth‐derived bands in Sn_CYS and Pb_CYS arise from structural and not compositional differences. The structural anisotropy of Sn_CYS is also reflected in a large in‐plane photoluminescence linear anisotropy ratio. Interfacial strain further affords differential incorporation of Pb into the perovskite and intergrowth layers of the Sn heterostructures, resulting in redshifted optical absorption onsets. Thus, we posit that local structural distortions may be exploited to manipulate the twist angle and interfacial strain in bulk heterostructures, providing a new handle for tuning the band alignments of bulk quantum‐well electronic structures.

Moiré superlattices, engineered through precise stacking of van der Waals (vdW) layers, hold immense promise for exploring strongly correlated and topological phenomena. However, these applications have been held back by the common preparation method: tear-and-stack of Scotch tape exfoliated monolayers, which suffer from low efficiency and reproducibility, twist angle inhomogeneity, interfacial contamination, and micrometer sizes. Here, we report an effective strategy to construct highly consistent mixed-dimensional and twisted bilayer vdW moiré structures with high production throughput, near-unity yield, pristine interfaces, precisely controlled twist angles, and macroscopic scale (up to centimeters) with enhanced thermal stability. We demonstrate the versatility across various vdW materials, including transition metal dichalcogenides, graphene, and hBN. The expansive size and high quality of moiré structures enable reciprocal-space high-resolution mapping of the superlattices and back-folded moiré mini band structures with low energy electron diffraction (LEED) and angle-resolved photoemission spectroscopy (ARPES). In particular, we identify the backfolded bands at the K point of twisted transition metal dichalcogenide moiré structures. This technique will have broad applications in both fundamental studies and the mass production of twistronic devices.

We report on the experimental characterization of the second-order susceptibility in MoSe2/WS2 heterobilayers, including their hidden complex phases. To this end, we developed a heterodyne-detection scheme for second-harmonic generation and applied it to macroscale heterobilayer samples prepared using the gold-tape exfoliation method. The heterodyne scheme enabled us to distinguish the relative orientation of the crystal domains, and furthermore, it allowed us to characterize the complex phases of the susceptibility relative to a reference quartz sample. By comparing the results from the monolayer regions and the heterobilayer region over several hundred microns of the sample area, we determined that the contribution of interlayer effects to second-harmonic generation is within the experimental uncertainty arising from the sample inhomogeneity. The results here provide fundamental quantitative information necessary for the precise design of nanophotonic systems based on stacking engineering.

Achieving ultra-low thermal conductivity under ambient conditions is a fundamental challenge constrained by classical heat transport limits and material design trade-offs. Here, we introduce a new class of nano-bubble wrap architectures that achieve exceptionally low thermal conductivity by integrating nanoscale gas confinement with atomically thin, weakly coupled van der Waals solids. Using scalable patterning of 2D monolayers into periodic nano-bubbles and nano-wrinkles, we construct materials with structural analogies to macroscopic bubble wrap but engineered at length scales much shorter than the mean free path of air and the mean free path of phonons in the atomically thin monolayers. Time-domain thermoreflectance measurements reveal out-of-plane thermal conductivities nearly an order of magnitude lower than that of air and commercial aerogels, reaching critical values below 0.001 W · M^-1K^-1 under room temperature and atmospheric pressure. This extreme thermal resistance arises from the combined suppression of gas-phase conduction, phonon transport, and interfacial coupling. Our findings establish nano-bubble wraps as a versatile platform for tuning heat flow in ultrathin materials and open new pathways for designing thermal metamaterials and energy-efficient technologies.

Van der Waals stacking presents new opportunities for nonlinear optics with its remarkable tunability and scalability. However, the fundamental role of interlayer interactions in modifying the overall nonlinear optical susceptibilities remains elusive. In this letter, we report an anisotropic enhancement of second-harmonic generation (SHG) from a WS_2/ReSe_2 heterobilayer, where the individual composite layers possess distinctive crystal phases. We investigate polarization-resolved response and twist-angle dependence in SHG and reveal that band alignment alone is insufficient to explain the observed anisotropy in the modified SHG response. Spectral shifts in excitonic features highlight band renormalization, supporting the role of hybridization between the two layers. Furthermore, SHG enhancement is highly anisotropic and can even be suppressed in some orientations, suggesting possible intensity-borrowing mechanisms within the heterostructure. Our work demonstrates the ability to tune both the intensity and polarization dependence of nonlinear optical responses with van der Waals stacking of distinctive crystal phases.

We conducted low-temperature photoluminescence (PL) spectroscopy experiments on individual WS2 and MoSe2 nanoribbons prepared by gold-assisted exfoliation from the slanted surface of bulk crystals with a vicinal and stepwise pattern. The nanoribbons are predominantly monolayer and have widths varying from hundreds of nanometers down to tens of nanometers. Most MoSe2 NRs display an emission profile similar to 2D excitons of MoSe2 monolayers. In contrast, WS2 nanoribbons are characterized with sharp emission peaks that can be attributed to the emission from localized excitons or trions. Moreover a broad low energy emission peak can be also observed from some of the WS2 nanoribbons, which originates from bilayer regions. In this manuscript, we analyze spectral diffusion behavior along with pump power and temperature dependence of the localized exciton emission peaks, shedding light on potential of TMDC nanoribbons in sensing and opto-electronic applications.

Beam steering metasurfaces are ultra-compact optical coatings that offer on-demand redirection of optical power to specific diffraction orders. To achieve this, spatial gradients are commonly introduced in the phase of light scattered by plasmon or Mie resonant nanoparticles within the metasurface grating's unit cell. However, these phase gradients are oftentimes difficult to tune post-fabrication. Recently, excitons in monolayer 2D semiconductors have emerged as a new metasurface building block, due to their strong and electrically-tunable resonant light-matter interaction. These 2D excitonic metasurfaces offer the tantalizing prospect of beam switching within a single monolayer. Here, it is demonstrated how the 2D analog of binary blazed gratings enables such beam switching by mere nanopatterning of a large monolayer WS2, even though nanoscale ribbons of WS2 do not support geometrical resonances. By introducing a gradient in the nanoribbon width within the metasurface unit cell, an amplitude gradient combined with a small phase gradient in the scattered fields results in asymmetric diffraction efficiencies. Using a scattered-field analysis, it is shown that these gradients can be further engineered via interference effects with the substrate reflection. Finally, the electrical tunability of the exciton resonance is leveraged to achieve selective and dynamic beam switching with an atomically-thin metasurface.

In situ tensile testing using transmission electron microscopy (TEM) is a powerful technique to probe structure-property relationships of materials at the atomic scale. In this work, a facile tensile testing platform for in situ characterization of materials inside a transmission electron microscope is demonstrated. The platform consists of: 1) a commercially available, flexible, electron-transparent substrate (e.g., TEM grid) integrated with a conventional tensile testing holder, and 2) a finite element simulation providing quantification of specimen-applied strain. The flexible substrate (carbon support film of the TEM grid) mitigates strain concentrations usually found in free-standing films and enables in situ straining experiments to be performed on materials that cannot undergo localized thinning or focused ion beam lift-out. The finite element simulation enables direct correlation of holder displacement with sample strain, providing upper and lower bounds of expected strain across the substrate. The tensile testing platform is validated for three disparate material systems: sputtered gold-palladium, few-layer transferred tungsten disulfide, and electrodeposited lithium, by measuring lattice strain from experimentally recorded electron diffraction data. The results show good agreement between experiment and simulation, providing confidence in the ability to transfer strain from holder to sample and relate TEM crystal structural observations with material mechanical properties.

Transition metal dichalcogenides (TMDs) are considered as perfect candidates for the realization of ultrafast optoelectronic devices thanks to their unique physical properties, particularly interesting in monolayer crystals. The interaction between TMDs and ultrashort, intense laser pulses is dominated by coupled carrier-exciton dynamics and unfolds on a few-femtosecond time scale. Attoscience offers the unique opportunity to investigate such fast dynamics in real time [1], probing the material response in the extreme-ultraviolet (XUV) and soft X-ray energy range. However, attosecond-resolved electron dynamics in few-layer TMDs still remains a challenging task, also because their optical response in this photon energy region is yet to be characterized.