Air-Stable Surface Charge Transfer Doping of MoS₂ by Benzyl Viologen

Air-stable doping of transition metal dichalcogenides is of fundamental importance to enable a wide range of optoelectronic and electronic devices while exploring their basic material properties. Here we demonstrate the use of benzyl viologen (BV), which has one of the highest reduction potentials of all electron-donor organic compounds, as a surface charge transfer donor for MoS₂ flakes. The n-doped samples exhibit excellent stability in both ambient air and vacuum. Notably, we obtained a high electron sheet density of ∼1.2 × 10¹³ cm^–2, which corresponds to the degenerate doping limit for MoS₂. The BV dopant molecules can be reversibly removed by immersion in toluene, providing the ability to control the carrier sheet density as well as selective removal of surface dopants on demand. By BV doping of MoS₂ at the metal junctions, the contact resistances are shown to be reduced by a factor of >3. As a proof of concept, top-gated field-effect transistors were fabricated with BV-doped n⁺ source/drain contacts self-aligned with respect to the top gate. The device architecture, resembling that of the conventional Si transistors, exhibited excellent switching characteristics with a subthreshold swing of ∼77 mV/decade

Field-Effect Transistors Built from All Two-Dimensional Material Components

We demonstrate field-effect transistors using heterogeneously stacked two-dimensional materials for all of the components, including the semiconductor, insulator, and metal layers. Specifically, MoS₂ is used as the active channel material, hexagonal-BN as the top-gate dielectric, and graphene as the source/drain and the top-gate contacts. This transistor exhibits n-type behavior with an ON/OFF current ratio of >10⁶, and an electron mobility of ∼33 cm²/V·s. Uniquely, the mobility does not degrade at high gate voltages, presenting an important advantage over conventional Si transistors where enhanced surface roughness scattering severely reduces carrier mobilities at high gate-fields. A WSe₂–MoS₂ diode with graphene contacts is also demonstrated. The diode exhibits excellent rectification behavior and a low reverse bias current, suggesting high quality interfaces between the stacked layers. In this work, all interfaces are based on van der Waals bonding, presenting a unique device architecture where crystalline, layered materials with atomically uniform thicknesses are stacked on demand, without the lattice parameter constraints. The results demonstrate the promise of using an all-layered material system for future electronic applications

Strain-Induced Indirect to Direct Bandgap Transition in Multilayer WSe₂

Transition metal dichalcogenides, such as MoS₂ and WSe₂, have recently gained tremendous interest for electronic and optoelectronic applications. MoS₂ and WSe₂ monolayers are direct bandgap and show bright photoluminescence (PL), whereas multilayers exhibit much weaker PL due to their indirect optical bandgap. This presents an obstacle for a number of device applications involving light harvesting or detection where thicker films with direct optical bandgap are desired. Here, we experimentally demonstrate a drastic enhancement in PL intensity for multilayer WSe₂ (2–4 layers) under uniaxial tensile strain of up to 2%. Specifically, the PL intensity of bilayer WSe₂ is amplified by ∼35× , making it comparable to that of an unstrained WSe₂ monolayer. This drastic PL enhancement is attributed to an indirect to direct bandgap transition for strained bilayer WSe₂, as confirmed by density functional theory (DFT) calculations. Notably, in contrast to MoS₂ multilayers, the energy difference between the direct and indirect bandgaps of WSe₂ multilayers is small, thus allowing for bandgap crossover at experimentally feasible strain values. Our results present an important advance toward controlling the band structure and optoelectronic properties of few-layer WSe₂ via strain engineering, with important implications for practical device applications

Dynamic Changes in Heparan Sulfate Nanostructure in Human Pluripotent Stem Cell Differentiation

Heparan sulfate (HS) is a heterogeneous, cell-surface polysaccharide critical for transducing signals essential for mammalian development. Imaging of signaling proteins has revealed how their localization influences their information transfer. In contrast, the contribution of the spatial distribution and nanostructure of information-rich, signaling polysaccharides like HS is not known. Using expansion microscopy (ExM), we found striking changes in HS nanostructure occur as human pluripotent stem (hPS) cells differentiate, and these changes correlate with growth factor signaling. Our imaging studies show that undifferentiated hPS cells are densely coated with HS displayed as hair-like protrusions. This ultrastructure can recruit fibroblast growth factor for signaling. When the hPS cells differentiate into the ectoderm lineage, HS is localized into dispersed puncta. This striking change in HS distribution coincides with a decrease in fibroblast growth factor binding to neural cells. While developmental variations in HS sequence were thought to be the primary driver of alterations in HS-mediated growth factor signaling, our high-resolution images indicate a role for the HS nanostructure. Our study highlights the utility of high-resolution glycan imaging using ExM. In the case of HS, we found that changes in how the polysaccharide is displayed link to profound differences in growth factor binding

Dynamic Changes in Heparan Sulfate Nanostructure in Human Pluripotent Stem Cell Differentiation

Heparan sulfate (HS) is a heterogeneous, cell-surface polysaccharide critical for transducing signals essential for mammalian development. Imaging of signaling proteins has revealed how their localization influences their information transfer. In contrast, the contribution of the spatial distribution and nanostructure of information-rich, signaling polysaccharides like HS is not known. Using expansion microscopy (ExM), we found striking changes in HS nanostructure occur as human pluripotent stem (hPS) cells differentiate, and these changes correlate with growth factor signaling. Our imaging studies show that undifferentiated hPS cells are densely coated with HS displayed as hair-like protrusions. This ultrastructure can recruit fibroblast growth factor for signaling. When the hPS cells differentiate into the ectoderm lineage, HS is localized into dispersed puncta. This striking change in HS distribution coincides with a decrease in fibroblast growth factor binding to neural cells. While developmental variations in HS sequence were thought to be the primary driver of alterations in HS-mediated growth factor signaling, our high-resolution images indicate a role for the HS nanostructure. Our study highlights the utility of high-resolution glycan imaging using ExM. In the case of HS, we found that changes in how the polysaccharide is displayed link to profound differences in growth factor binding

Dynamic Changes in Heparan Sulfate Nanostructure in Human Pluripotent Stem Cell Differentiation

Heparan sulfate (HS) is a heterogeneous, cell-surface polysaccharide critical for transducing signals essential for mammalian development. Imaging of signaling proteins has revealed how their localization influences their information transfer. In contrast, the contribution of the spatial distribution and nanostructure of information-rich, signaling polysaccharides like HS is not known. Using expansion microscopy (ExM), we found striking changes in HS nanostructure occur as human pluripotent stem (hPS) cells differentiate, and these changes correlate with growth factor signaling. Our imaging studies show that undifferentiated hPS cells are densely coated with HS displayed as hair-like protrusions. This ultrastructure can recruit fibroblast growth factor for signaling. When the hPS cells differentiate into the ectoderm lineage, HS is localized into dispersed puncta. This striking change in HS distribution coincides with a decrease in fibroblast growth factor binding to neural cells. While developmental variations in HS sequence were thought to be the primary driver of alterations in HS-mediated growth factor signaling, our high-resolution images indicate a role for the HS nanostructure. Our study highlights the utility of high-resolution glycan imaging using ExM. In the case of HS, we found that changes in how the polysaccharide is displayed link to profound differences in growth factor binding

Air Stable p‑Doping of WSe₂ by Covalent Functionalization

Covalent functionalization of transition metal dichalcogenides (TMDCs) is investigated for air-stable chemical doping. Specifically, p-doping of WSe₂ <i>via</i> NO_<i>x</i> chemisorption at 150 °C is explored, with the hole concentration tuned by reaction time. Synchrotron based soft X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) depict the formation of various WSe_{2–<i>x</i>–<i>y</i>}O_<i>x</i>N_<i>y</i> species both on the surface and interface between layers upon chemisorption reaction. <i>Ab initio</i> simulations corroborate our spectroscopy results in identifying the energetically favorable complexes, and predicting WSe₂:NO at the Se vacancy sites as the predominant dopant species. A maximum hole concentration of ∼10¹⁹ cm^–3 is obtained from XPS and electrical measurements, which is found to be independent of WSe₂ thickness. This degenerate doping level facilitates 5 orders of magnitude reduction in contact resistance between Pd, a common p-type contact metal, and WSe₂. More generally, the work presents a platform for manipulating the electrical properties and band structure of TMDCs using covalent functionalization

Engineering Light Outcoupling in 2D Materials

When light is incident on 2D transition metal dichalcogenides (TMDCs), it engages in multiple reflections within underlying substrates, producing interferences that lead to enhancement or attenuation of the incoming and outgoing strength of light. Here, we report a simple method to engineer the light outcoupling in semiconducting TMDCs by modulating their dielectric surroundings. We show that by modulating the thicknesses of underlying substrates and capping layers, the interference caused by substrate can significantly enhance the light absorption and emission of WSe₂, resulting in a ∼11 times increase in Raman signal and a ∼30 times increase in the photoluminescence (PL) intensity of WSe₂. On the basis of the interference model, we also propose a strategy to control the photonic and optoelectronic properties of thin-layer WSe₂. This work demonstrates the utilization of outcoupling engineering in 2D materials and offers a new route toward the realization of novel optoelectronic devices, such as 2D LEDs and solar cells