Anh Tuan Hoang

Nanoscale transistors demand aggressive scaling of all channel dimensions—length, width and thickness. Two-dimensional semiconductors (2DS) provide the ultimate thickness limit, yet good device performance has largely remained restricted to micrometre-wide channels. Here we report monolayer 2DS nanoribbon transistors with both n- and p-type operation, fabricated by a top-down multipatterning process that includes ‘anchored’ contacts to limit nanoribbon delamination. This approach achieves channel lengths and widths down to 25–30 nm, with minimal edge degradation confirmed through nanoscale characterization, including tip-enhanced photoluminescence. Integrated with thin high- κ gate dielectrics, the devices deliver on-state currents up to 560, 420 and 130 µA µm ⁻¹ at a drain-to-source voltage of 1 V for n-type MoS ₂ , n-type WS ₂ and p-type WSe ₂ , respectively. These results exceed prior single-gated 2DS nanoribbon reports, with WS ₂ improving by more than two orders of magnitude, even for normally off (enhancement-mode) operation. Overall, these findings position top-down patterned 2DS nanoribbons as promising building blocks for future nanosheet transistor architectures.

Two-dimensional semiconductors are promising candidates for next-generation electronics. However, characterizing these materials at technologically relevant dimensions remains underexplored. Here, we use tip-enhanced Raman spectroscopy (TERS) to map lithographically-patterned monolayer MoS2 nanoribbons down to 50 nm widths. The surface sensitivity of TERS enables direct nanoscale assessment of the MoS2 surface after a vacuum annealing procedure that removes resist residues. Subsequently, we find a consistently strong TERS response across the nanoribbon, indicating good quality MoS2 and a clean interface to the Au substrate. The good spatial resolution of TERS (down to ∼10 nm) uncovers small, 50–100 nm regions of inhomogeneities, likely arising from the growth process with a higher intensity and redshifted 2LA(M) peak. We also find a 0.5 cm−1 redshift of the A1′ mode at nanoribbon edges, consistent with a fixed negative charge. Our study highlights how advanced nanoscale metrology can be leveraged for future devices and fabrication process optimization.

Electronic and optoelectronic applications of two-dimensional (2D) semiconductors demand precise control over material quality, including thickness, composition, doping, and defect density. Conventional benchmarking methods (e.g. charge transport, confocal mapping, electron or scanning probe microscopy) are slow, perturb sample quality, or involve trade-offs between speed, resolution, and scan area. To accelerate assessment of 2D semiconductors, we demonstrate a broadband, wide-field hyperspectral optical microscope for 2D materials that rapidly captures a spatial-spectral data cube within seconds. The data cube includes x – y spatial coordinate (a 300 × 300 µ m ² field, with ∼1 µ m resolution) and a selectable wavelength range between 200 nm to 1100 nm at each pixel. Using synthesized films and heterostructures of transition metal dichalcogenides (MoS ₂ , WS ₂ , V _x W _{1− x} S ₂ , and WSe ₂ ), we show that this cost-effective technique detects spectral fingerprints of material identity, doping, grain boundaries, and alloy composition, and enables advanced analysis, including unsupervised machine learning for spatial segmentation.

Rapid and quantitative characterization of atomic defects in two-dimensional (2D) semiconductors and transistors is crucial for growth optimization and understanding of device behavior. However, such defect metrology remains challenging due to limitations of existing characterization methods, which are generally destructive and slow or lack the necessary sensitivity. Here, we use nondestructive lateral force microscopy (LFM) to directly map surface defects in monolayer WSe₂ and WS₂ on different growth substrates (SiO₂ and sapphire), as well as in WSe₂ transistors. Through LFM measurements on various WSe₂ layers, we show that this technique can detect defect densities well below the range of typical Raman measurements on this material. We also demonstrate mapping of spatial variation of defect density within as-grown WSe₂ and that the LFM technique can detect defects on suspended and polymer-supported monolayers, expanding the application space. Applied to WSe₂ transistors, LFM uncovers defect densities over double that of similar as-grown films, suggesting that defects can be introduced by common fabrication processes. This work demonstrates the applications of LFM as a nondestructive defect characterization method for monitoring 2D material growth and device fabrication.

Tungsten diselenide (WSe ₂ ) is a promising p -type semiconductor limited by high contact resistance ( R _C ) and the lack of a reliable doping strategy. Here, we demonstrate that exposing WSe ₂ to chloroform provides simple and stable p -type doping. In monolayer WSe ₂ transistors with Pd contacts, chloroform increases the maximum hole current by over 100× (>200 µA/µm), reduces R _C to ~ 2.5 kΩ·μm, and retains an on/off ratio of 10 ¹⁰ 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 R _C of ~ 1 kΩ·μm. Density functional theory indicates that chloroform strongly physisorbs to WSe ₂ , 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 WSe ₂ 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 WSe ₂ transistors with good p -type performance.

Advanced monitoring of neural responses has deepened our understanding of brain functions. However, balancing sampling fidelity and spatial resolution of electrocorticography mapping remains a major challenge in neuroscience and neuroelectronics. Here we describe a flexible, two-dimensional molybdenum disulfide (MoS₂)-based active array for electrocorticography sensing that uses a dual-transistor multiplexed source follower array architecture. We grow wafer-scale trilayer MoS₂ directly on polyimide substrates for a scalable fabrication of active arrays and simultaneously scaling matrices with ultrahigh mapping resolution (up to 51 pixels mm⁻²). In vitro recordings of epileptiform activities and sinusoidal potential distributions demonstrate the high spatiotemporal resolution for full-bandwidth electrocorticography recordings using the 50 × 50 MoS₂-based active arrays, which result from the fast on-site multiplexing speeds (τ ≈ 20 ns) and the megahertz response bandwidth of the transducer. In living mice, the active arrays enable high-fidelity electrocorticography monitoring of multiscale neural processes, including auditory-evoked potentials, tonotopic maps and localized multiunit activities at high frequency. Overall, these results suggest that the MoS₂-based active arrays are promising minimally invasive high-fidelity tools for spatiotemporal neuronal monitoring.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) possess distinct optical and electronic properties, making them promising candidates for optoelectronic applications. Recently, major advances in the wafer-scale growth of TMDs using the metal-organic chemical vapor deposition (MOCVD) have enabled their integration with standard electronics. However, such materials continue to suffer from defects and unwanted doping, which lower semiconductor performance, as exemplified by poor photoluminescence (PL) yield. Chemical treatment protocols have been shown to improve the PL yield in exfoliated and CVD-grown materials. Here, using PL, Raman microscopy, X-ray photoemission spectroscopy (XPS) and density functional theory (DFT) calculations, we develop chemical treatment protocols for wafer-scale MOCVD-grown monolayer MoS₂. The postgrowth treatment uses sulfide and TFSI-based ionic salts delivered via a solution process. We demonstrate a substantial PL enhancement ranging from 23 to 50 times, depending on the underlying MOCVD growth method of the MoS₂. We present design rules for tuning chemical treatment protocols, depending on the defect densities and doping levels, allowing for successful passivation and large PL enhancements across different growth conditions. Our results demonstrate the versatility of these chemical treatment protocols and their potential to improve PL in device-relevant wafer-scale MOCVD-grown monolayer TMDs.

Integration of ultrathin, high-quality gate insulators is critical to the success of two-dimensional (2D) semiconductor transistors in next-generation nanoelectronics. Here, we investigate the impact of atomic layer deposition (ALD) precursor choice on the nucleation and growth of insulators on monolayer MoS₂. Surveying a series of aluminum (AlO_x) precursors, we observe that increasing the length of the ligands reduces the nucleation delay of alumina on monolayer MoS₂, a phenomenon that we attribute to improved van der Waals dispersion interactions with the 2D material. Using the precursor triisobutylaluminum (TIBA), we achieve uniform coverage of ∼3 nm AlO_x on MoS₂ after just 30 cycles. We also build top-gated transistors with alumina seed layers grown by different precursors, demonstrating how the nucleation behavior of the seed layer influences the device behavior. With a bilayer stack of TIBA-AlO_x and HfO₂ as the top-gate insulator, we achieve n-type MoS₂ transistors with negligible hysteresis, small and positive threshold voltage, ∼80 mV/dec subthreshold swing at room temperature, and a top-gate equivalent oxide thickness of 0.95 nm. Through this work, we develop a simple, industry-compatible, all-ALD process for depositing a top-gate insulator directly on monolayer MoS₂, and we elucidate critical insights into how the ALD chemistry can be tuned to improve insulator deposition.

Defect engineering in transition metal dichalcogenide (TMD) monolayers enables applications in single-photon emission, sensing, and photocatalysis. These functionalities critically depend on defect type, density, spatial distribution, relative energy, and the dynamics of exciton trapping at the defect sites. The latter are mediated by coupling to optical phonons through mechanisms not yet fully understood. Traditionally, exciton or carrier trapping at defects in inorganic crystals has been described by incoherent multiphonon emission within the Born-Oppenheimer approximation─an approach that underpins the widely used Shockley-Read-Hall framework for nonradiative recombination. Here, we use impulsive vibrational spectroscopy to investigate exciton trapping in defect-modified monolayers of WS₂ grown through metal-organic chemical vapor deposition. We find that the phonon coherences of the Raman-active A' and E' modes persist throughout the ultrafast (∼100 fs) exciton trapping process, indicating a continuous evolution of the excitonic wave function. This observation is consistent with a conical intersection-mediated trapping process, in which a potential energy surface crossing between the free and trapped excitonic states acts as a funnel to drive this nonadiabatic transition. Such a molecular-like, vibronically coherent mechanism lies beyond the Born-Oppenheimer approximation, in stark contrast to classical, incoherent trapping models in solids. Moreover, the faster dephasing of the E' mode in the trapped exciton state compared to the free exciton suggests it acts as a vibrational coordinate that promotes the trapping process. These findings provide mechanistic insights into exciton-phonon interactions at defects in TMD monolayers and inform strategies for engineering quantum and energy functionalities.

Low-power transistors based on two-dimensional (2D) semiconductors require ultrathin gate insulators, whose atomic layer deposition (ALD) has been difficult without adequate surface preparation. Here, we achieve sub-1 nm equivalent oxide thickness (EOT) on monolayer MoS₂ using HfO₂ and a simple, commonly available Si seed. We first investigate six seed layer candidates (Si, Ge, Hf, La, Gd, Al₂O₃) and find that only Si and Ge cause no measurable damage to the MoS₂. With these, we build monolayer MoS₂ transistors using ALD of HfO₂ top-gate dielectric and find that the Si seed provides the better, low-hysteresis interface. The thickness of this interfacial layer also controls the threshold voltage, enabling normally-off, well-behaved transistors. The thinnest gate stack reached low EOT ≈ 0.9 nm with low leakage (2) and ∼80 mV/dec subthreshold swing at room temperature. This represents a simple top-gate dielectric deposition approach, achievable within many common nanofabrication facilities.