Quantification of defects engineered in single layer MoS2

Inkjet-Printed, High-Performance MoS2 Transistors and Unipolar Logic Electronics

Two-dimensional (2D) semiconductor field-effect film transistors combine large carrier mobility with mechanical flexibility and therefore can be ideally suitable for wearable electronics or at the sensor interfaces of smart sensor systems. However, such applications require large-area solution processing as opposed to single-flake devices, where the critical challenge to overcome is the high interflake resistance values. In this report, using a narrow-channel, near-vertical transport device architecture, we have fabricated inkjet-printed sub-20 nm channel electrolyte-gated transistors with predominantly intraflake carrier transport. Therefore, the electronics transport in these transistors is not dominated by the high interflake resistance, and the intraflake material properties including doping density, defect concentration, contact resistance, and threshold voltage modulation can be examined and optimized independently to achieve a current density as high as 280 μA·μm-1. In addition, through the passivation of the sulfur vacancies with a tailored surface treatment, we demonstrate an impressive On-Off current ratio exceeding 1 × 107, complemented by a low subthreshold swing of 100 mV·decade-1. Next, exploiting these high-performance transistors, unipolar depletion-load-type inverters have been fabricated that show a maximum gain of 31. Furthermore, we have realized NAND, NOR, and OR gates, demonstrating their seamless operation at a frequency of 1 kHz. Therefore, this work represents an important step forward to realize electronic circuits based on printed 2D thin film transistors.

Characterization of 2D Transition Metal Dichalcogenides Through Anisotropic Exciton Behaviors

This study reports the first attempt to characterize the quality, defects, and strain of as-grown monolayer transition metal dichalcogenide (TMDC)-based 2D materials through exciton anisotropy. A standard ellipsometric parameter (Ψ) to observe anisotropic exciton behavior in monolayer 2D materials is used. According to the strong exciton effect from phonon-electron coupling processes, the change in the exciton in the Van Hove singularity is sensitive to lattice distortions such as defects and strain. In comparison with Raman spectroscopy, the variations in exciton anisotropy in Ψ are more sensitive for detecting slight changes in the quality and strain of monolayer TMDC films. Moreover, the optical power requirement for TMDC characterization through exciton anisotropy in Ψ is ≈10-5 mW cm-2, which is significantly less than that of Raman spectroscopy (≈106 mW cm-2). The standard deviation of the signals varies with strain (defects) in Raman spectra and exciton anisotropies in Ψ are 0.700 (0.795) and 0.033 (0.073), indicating that exciton anisotropy is more sensitive to slight changes in the quality of monolayer TMDC films.

Metrology for 2D materials: a perspective review from the international roadmap for devices and systems

The International Roadmap for Devices and Systems (IRDS) predicts the integration of 2D materials into high-volume manufacturing as channel materials within the next decade, primarily in ultra-scaled and low-power devices. While their widespread adoption in advanced chip manufacturing is evolving, the need for diverse characterization methods is clear. This is necessary to assess structural, electrical, compositional, and mechanical properties to control and optimize 2D materials in mass-produced devices. Although the lab-to-fab transition remains nascent and a universal metrology solution is yet to emerge, rapid community progress underscores the potential for significant advancements. This paper reviews current measurement capabilities, identifies gaps in essential metrology for CMOS-compatible 2D materials, and explores fundamental measurement science limitations when applying these techniques in high-volume semiconductor manufacturing.

Defects and Defect Engineering of Two-Dimensional Transition Metal Dichalcogenide (2D TMDC) Materials

As layered materials, transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) materials. Interestingly, the characteristics of these materials are transformed from bulk to monolayer. The atomically thin TMDC materials can be a good alternative to group III-V and graphene because of their emerging tunable electrical, optical, and magnetic properties. Although 2D monolayers from natural TMDC materials exhibit the purest form, they have intrinsic defects that limit their application. However, the synthesis of TMDC materials using the existing fabrication tools and techniques is also not immune to defects. Additionally, it is difficult to synthesize wafer-scale TMDC materials for a multitude of factors influencing grain growth mechanisms. While defect engineering techniques may reduce the percentage of defects, the available methods have constraints for healing defects at the desired level. Thus, this holistic review of 2D TMDC materials encapsulates the fundamental structure of TMDC materials, including different types of defects, named zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D). Moreover, the existing defect engineering methods that relate to both formation of and reduction in defects have been discussed. Finally, an attempt has been made to correlate the impact of defects and the properties of these TMDC materials.

Engineering Vacancies for the Creation of Antisite Defects in Chemical Vapor Deposition Grown Monolayer MoS2 and WS2 via Proton Irradiation

It is critical to understand the laws of quantum mechanics in transformative technologies for computation and quantum information science applications to enable the ongoing second quantum revolution calls. Recently, spin qubits based on point defects have gained great attention, since these qubits can be initiated, selectively controlled, and read out with high precision at ambient temperature. The major challenge in these systems is controllably generating multiqubit systems while properly coupling the defects. To address this issue, we began by tackling the engineering challenges these systems present and understanding the fundamentals of defects. In this regard, we controllably generate defects in MoS2 and WS2 monolayers and tune their physicochemical properties via proton irradiation. We quantitatively discovered that the proton energy could modulate the defects' density and nature; higher defect densities were seen with lower proton irradiation energies. Three distinct defect types were observed: vacancies, antisites, and adatoms. In particular, the creation and manipulation of antisite defects provides an alternative way to create and pattern spin qubits based on point defects. Our results demonstrate that altering the particle irradiation energy can regulate the formation of defects, which can be utilized to modify the properties of 2D materials and create reliable electronic devices.

Towards Low-Temperature CVD Synthesis and Characterization of Mono- or Few-Layer Molybdenum Disulfide

Molybdenum disulfide (MoS2) transistors are a promising alternative for the semiconductor industry due to their large on/off current ratio (>1010), immunity to short-channel effects, and unique switching characteristics. MoS2 has drawn considerable interest due to its intriguing electrical, optical, sensing, and catalytic properties. Monolayer MoS2 is a semiconducting material with a direct band gap of ~1.9 eV, which can be tuned. Commercially, the aim of synthesizing a novel material is to grow high-quality samples over a large area and at a low cost. Although chemical vapor deposition (CVD) growth techniques are associated with a low-cost pathway and large-area material growth, a drawback concerns meeting the high crystalline quality required for nanoelectronic and optoelectronic applications. This research presents a lower-temperature CVD for the repeatable synthesis of large-size mono- or few-layer MoS2 using the direct vapor phase sulfurization of MoO3. The samples grown on Si/SiO2 substrates demonstrate a uniform single-crystalline quality in Raman spectroscopy, photoluminescence (PL), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy. These characterization techniques were targeted to confirm the uniform thickness, stoichiometry, and lattice spacing of the MoS2 layers. The MoS2 crystals were deposited over the entire surface of the sample substrate. With a detailed discussion of the CVD setup and an explanation of the process parameters that influence nucleation and growth, this work opens a new platform for the repeatable synthesis of highly crystalline mono- or few-layer MoS2 suitable for optoelectronic application.

Exfoliation procedure-dependent optical properties of solution deposited MoS2 films

The development of high-precision large-area optical coatings and devices comprising low-dimensional materials hinges on scalable solution-based manufacturability with control over exfoliation procedure-dependent effects. As such, it is critical to understand the influence of technique-induced transition metal dichalcogenide (TMDC) optical properties that impact the design, performance, and integration of advanced optical coatings and devices. Here, we examine the optical properties of semiconducting MoS2 films from the exfoliation formulations of four prominent approaches: solvent-mediated exfoliation, chemical exfoliation with phase reconversion, redox exfoliation, and native redox exfoliation. The resulting MoS2 films exhibit distinct refractive indices (n), extinction coefficients (k), dielectric functions (ε1 and ε2), and absorption coefficients (α). For example, a large index contrast of Δn ≈ 2.3 is observed. These exfoliation procedures and related chemistries produce different exfoliated flake dimensions, chemical impurities, carrier doping, and lattice strain that influence the resulting optical properties. First-principles calculations further confirm the impact of lattice defects and doping characteristics on MoS2 optical properties. Overall, incomplete phase reconfiguration (from 1T to mixed crystalline 2H and amorphous phases), lattice vacancies, intraflake strain, and Mo oxidation largely contribute to the observed differences in the reported MoS2 optical properties. These findings highlight the need for controlled technique-induced effects as well as the opportunity for continued development of, and improvement to, liquid phase exfoliation methodologies. Such chemical and processing-induced effects present compelling routes to engineer exfoliated TMDC optical properties toward the development of next-generation high-performance mirrors, narrow bandpass filters, and wavelength-tailored absorbers.

Size Effects in Single- and Few-Layer MoS2 Nanoflakes: Impact on Raman Phonons and Photoluminescence

The high optical absorption and emission of bidimensional MoS₂ are fundamental properties for optoelectronic and biodetection applications and the opportunity to retain these properties in high quality nano-sized flakes would bring further possibilities. Here, a large set of single-layer and few-layer (2–3 layers) MoS₂ flakes with size in the range from 10 nm to 20 μm are obtained on sapphire by vapor deposition techniques and evaluated combining the information from the Raman phonons with photoluminescence (PL) and absorption bands. The flakes have triangular shape and are found to be progressively relaxed from the tensile strain imposed by the sapphire substrate as their size is reduced. An increasing hole doping as size decreases is deduced from the blue shift of the A_1g phonon, related to charge transfer from adsorbed oxygen. No clear correlation is observed between defects density and size, therefore, doping would be favored by the preferential adsorption of oxygen at the edges of the flakes, being progressively more important as the edge/surface ratio is incremented. This hole doping also produces a shift of the PL band to higher energies, up to 60 meV. The PL intensity is not found to be correlated to the size but to the presence of defects. The trends with size for single-layer and for 2–3 layer samples are found to be similar and the synthesis method does not influence PL efficiency which remains high down to 40 nm being thus promising for nanoscale photonics.

Multidimensional Imaging Reveals Mechanisms Controlling Multimodal Label-Free Biosensing in Vertical 2DM-Heterostructures

Two-dimensional materials and their van der Waals heterostructures enable a large range of applications, including label-free biosensing. Lattice mismatch and work function difference in the heterostructure material result in strain and charge transfer, often varying at a nanometer scale, that influence device performance. In this work, a multidimensional optical imaging technique is developed in order to map subdiffractional distributions for doping and strain and understand the role of those for modulation of the electronic properties of the material. As an example, vertical heterostructures comprised of monolayer graphene and single-layer flakes of transition metal dichalcogenide MoS₂ were fabricated and used for biosensing. Herein, the optical label-free detection of doxorubicin, a common cancer drug, is reported via three independent optical detection channels (photoluminescence shift, Raman shift, and graphene enhanced Raman scattering). Non-uniform broadening of components of multimodal signal correlates with the statistical distribution of local optical properties of the heterostructure. Multidimensional nanoscale imaging allows one to reveal the physical origin for such a local response and propose the best strategy for the mitigation of materials variability and future device fabrication, enabling multiplexed biosensing.

Optical Inspection of 2D Materials: From Mechanical Exfoliation to Wafer-Scale Growth and Beyond

Optical inspection is a rapid and non-destructive method for characterizing the properties of two-dimensional (2D) materials. With the aid of optical inspection, in situ and scalable monitoring of the properties of 2D materials can be implemented industrially to advance the development and progress of 2D material-based devices toward mass production. This review discusses the optical inspection techniques that are available to characterize various 2D materials, including graphene, transition metal dichalcogenides (TMDCs), hexagonal boron nitride (h-BN), group-III monochalcogenides, black phosphorus (BP), and group-IV monochalcogenides. First, the authors provide an introduction to these 2D materials and the processes commonly used for their fabrication. Then they review several of the important structural properties of 2D materials, and discuss how to characterize them using appropriate optical inspection tools. The authors also describe the challenges and opportunities faced when applying optical inspection to recently developed 2D materials, from mechanically exfoliated to wafer-scale-grown 2D materials. Most importantly, the authors summarize the techniques available for largely and precisely enhancing the optical signals from 2D materials. This comprehensive review of the current status and perspective of future trends for optical inspection of the structural properties of 2D materials will facilitate the development of next-generation 2D material-based devices.

Bandgap recovery of monolayer MoS2 using defect engineering and chemical doping

Two-dimensional transition metal dichalcogenide materials have created avenues for exciting physics with unique electronic and photonic applications. Among these materials, molybdenum disulfide is the most known due to extensive research in understanding its electronic and optical properties. In this paper, we report on the successful growth and modification of monolayer MoS₂ (1L MoS₂) by controlling carrier concentration and manipulating bandgap in order to improve the efficiency of light emission. Atomic size MoS₂ vacancies were created using a Helium Ion Microscope, then the defect sites were doped with 2,3,5,6-tetrafluro7,7,8,8-tetracyanoquinodimethane (F4TCNQ). The carrier concentration in intrinsic (as-grown) and engineered 1L MoS₂ was calculated using Mass Action model. The results are in a good agreement with Raman and photoluminescence spectroscopy as well as Kelvin probe force microscopy characterizations.

Two-dimensional transition metal dichalcogenide materials have created avenues for exciting physics with unique electronic and photonic applications.

Effect of interfacial defects on the electronic properties of MoS2 based lateral T–H heterophase junctions

Our systematic study shows significant improvement in transport properties of MoS2-based lateral T–H heterophase junctions when interfacial defects are present.