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

Design of Surfactant–Substrate Interactions for Roll-to-Roll Assembly of Carbon Nanotubes for Thin-Film Transistors

Controlled assembly of single-walled carbon nanotube (SWCNT) networks with high density and deposition rate is critical for many practical applications, including large-area electronics. In this regard, surfactant chemistry plays a critical role as it facilitates the substrate–nanotube interactions. Despite its importance, detailed understanding of the subject up until now has been lacking, especially toward tuning the controllability of SWCNT assembly for thin-film transistors. Here, we explore SWCNT assembly with steroid- and alkyl-based surfactants. While steroid-based surfactants yield highly dense nanotube thin films, alkyl surfactants are found to prohibit nanotube assembly. The latter is attributed to the formation of packed alkyl layers of residual surfactants on the substrate surface, which subsequently repel surfactant encapsulated SWCNTs. In addition, temperature is found to enhance the nanotube deposition rate and density. Using this knowledge, we demonstrate highly dense and rapid assembly with an effective SWCNT surface coverage of ∼99% as characterized by capacitance–voltage measurements. The scalability of the process is demonstrated through a roll-to-roll assembly of SWCNTs on plastic substrates for large-area thin-film transistors. The work presents an important process scheme for nanomanufacturing of SWCNT-based electronics

Dual-Gated MoS₂/WSe₂ van der Waals Tunnel Diodes and Transistors

Two-dimensional layered semiconductors present a promising material platform for band-to-band-tunneling devices given their homogeneous band edge steepness due to their atomically flat thickness. Here, we experimentally demonstrate interlayer band-to-band tunneling in vertical MoS₂/WSe₂ van der Waals (vdW) heterostructures using a dual-gate device architecture. The electric potential and carrier concentration of MoS₂ and WSe₂ layers are independently controlled by the two symmetric gates. The same device can be gate modulated to behave as either an Esaki diode with negative differential resistance, a backward diode with large reverse bias tunneling current, or a forward rectifying diode with low reverse bias current. Notably, a high gate coupling efficiency of ∼80% is obtained for tuning the interlayer band alignments, arising from weak electrostatic screening by the atomically thin layers. This work presents an advance in the fundamental understanding of the interlayer coupling and electron tunneling in semiconductor vdW heterostructures with important implications toward the design of atomically thin tunnel transistors

Measuring the Edge Recombination Velocity of Monolayer Semiconductors

Understanding edge effects and quantifying their impact on the carrier properties of two-dimensional (2D) semiconductors is an essential step toward utilizing this material for high performance electronic and optoelectronic devices. WS₂ monolayers patterned into disks of varying diameters are used to experimentally explore the influence of edges on the material’s optical properties. Carrier lifetime measurements show a decrease in the effective lifetime, τ_effective, as a function of decreasing diameter, suggesting that the edges are active sites for carrier recombination. Accordingly, we introduce a metric called edge recombination velocity (ERV) to characterize the impact of 2D material edges on nonradiative carrier recombination. The unpassivated WS₂ monolayer disks yield an ERV ∼ 4 × 10⁴ cm/s. This work quantifies the nonradiative recombination edge effects in monolayer semiconductors, while simultaneously establishing a practical characterization approach that can be used to experimentally explore edge passivation methods for 2D materials

General Thermal Texturization Process of MoS₂ for Efficient Electrocatalytic Hydrogen Evolution Reaction

Molybdenum disulfide (MoS₂) has been widely examined as a catalyst containing no precious metals for the hydrogen evolution reaction (HER); however, these examinations have utilized synthesized MoS₂ because the pristine MoS₂ mineral is known to be a poor catalyst. The fundamental challenge with pristine MoS₂ is the inert HER activity of the predominant (0001) basal surface plane. In order to achieve high HER performance with pristine MoS₂, it is essential to activate the basal plane. Here, we report a general thermal process in which the basal plane is texturized to increase the density of HER-active edge sites. This texturization is achieved through a simple thermal annealing procedure in a hydrogen environment, removing sulfur from the MoS₂ surface to form edge sites. As a result, the process generates high HER catalytic performance in pristine MoS₂ across various morphologies such as the bulk mineral, films composed of micron-scale flakes, and even films of a commercially available spray of nanoflake MoS₂. The lowest overpotential (η) observed for these samples was η = 170 mV to obtain 10 mA/cm² of HER current density

High Luminescence Efficiency in MoS₂ Grown by Chemical Vapor Deposition

One of the major challenges facing the rapidly growing field of two-dimensional (2D) transition metal dichalcogenides (TMDCs) is the development of growth techniques to enable large-area synthesis of high-quality materials. Chemical vapor deposition (CVD) is one of the leading techniques for the synthesis of TMDCs; however, the quality of the material produced is limited by defects formed during the growth process. A very useful nondestructive technique that can be utilized to probe defects in semiconductors is the room-temperature photoluminescence (PL) quantum yield (QY). It was recently demonstrated that a PL QY near 100% can be obtained in MoS₂ and WS₂ monolayers prepared by micromechanical exfoliation by treating samples with an organic superacid: bis­(trifluoromethane)­sulfonimide (TFSI). Here we have performed a thorough exploration of this chemical treatment on CVD-grown MoS₂ samples. We find that the as-grown monolayers must be transferred to a secondary substrate, which releases strain, to obtain high QY by TFSI treatment. Furthermore, we find that the sulfur precursor temperature during synthesis of the MoS₂ plays a critical role in the effectiveness of the treatment. By satisfying the aforementioned conditions we show that the PL QY of CVD-grown monolayers can be improved from ∼0.1% in the as-grown case to ∼30% after treatment, with enhancement factors ranging from 100 to 1500× depending on the initial monolayer quality. We also found that after TFSI treatment the PL emission from MoS₂ films was visible by eye despite the low absorption (5–10%). The discovery of an effective passivation strategy will speed the development of scalable high-performance optoelectronic and electronic devices based on MoS₂

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