Microscopic evidence of strong interactions between chemical vapor deposited 2D MoS2 film and SiO2 growth template

Wafer-Scale Growth and Transfer of High-Quality MoS2 Array by Interface Design for High-Stability Flexible Photosensitive Device

Transition metal disulfide compounds (TMDCs) emerges as the promising candidate for new-generation flexible (opto-)electronic device fabrication. However, the harsh growth condition of TMDCs results in the necessity of using hard dielectric substrates, and thus the additional transfer process is essential but still challenging. Here, an efficient strategy for preparation and easy separation-transfer of high-uniform and quality-enhanced MoS2 via the precursor pre-annealing on the designed graphene inserting layer is demonstrated. Based on the novel strategy, it achieves the intact separation and transfer of a 2-inch MoS2 array onto the flexible resin. It reveals that the graphene inserting layer not only enhances MoS2 quality but also decreases interfacial adhesion for easy separation-transfer, which achieves a high yield of ≈99.83%. The theoretical calculations show that the chemical bonding formation at the growth interface has been eliminated by graphene. The separable graphene serves as a photocarrier transportation channel, making a largely enhanced responsivity up to 6.86 mA W-1, and the photodetector array also qualifies for imaging featured with high contrast. The flexible device exhibits high bending stability, which preserves almost 100% of initial performance after 5000 cycles. The proposed novel TMDCs growth and separation-transfer strategy lightens their significance for advances in curved and wearable (opto-)electronic applications.

Recent Advances in Non-Ti MXenes: Synthesis, Properties, and Novel Applications

One of the most fascinating 2D nanomaterials (NMs) ever found is various members of MXene family. Among them, the titanium-based MXenes, with more than 70% of publication-related investigations, are comparatively well studied, producing fundamental foundation for the 2D MXene family members with flexible properties, familiar with a variety of advanced novel technological applications. Nonetheless, there are still more candidates among transitional metals (TMs) that can function as MXene NMs in ways that go well beyond those that are now recognized. Systematized details of the preparations, characteristics, limitations, significant discoveries, and uses of the novel M-based MXenes (M-MXenes), where M stands for non-Ti TMs (M = Sc, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, and Lu), are given. The exceptional qualities of the 2D non-Ti MXene outperform standard Ti-MXene in several applications. There is many advancement in top-down as well as bottom-up production of MXenes family members, which allows for exact control of the M-characteristics MXene NMs to contain cutting-edge applications. This study offers a systematic evaluation of existing research, covering everything in producing complex M-MXenes from primary limitations to the characterization and selection of their applications in accordance with their novel features. The development of double metal combinations, extension of additional metal candidates beyond group-(III-VI)B family, and subsequent development of the 2D TM carbide/TMs nitride/TM carbonitrides to 2D metal boride family are also included in this overview. The possibilities and further recommendations for the way of non-Ti MXene NMs are in the synthesis of NMs will discuss in detail in this critical evaluation.

Development of Self-Doped Monolayered 2D MoS2 for Enhanced Photoresponsivity

Transition metal dichalcogenides (TMDs) exist in two distinct phases: the thermodynamically stable trigonal prismatic (2H) and the metastable octahedral (1T) phase. Phase engineering has emerged as a potent technique for enhancing the performance of TMDs in optoelectronics applications. Nevertheless, understanding the mechanism of phase transition in TMDs and achieving large-area synthesis of phase-controlled TMDs continue to pose significant challenges. This study presents the synthesis of large-area monolayered 2H-MoS2 and mixed-phase 1T/2H-MoS2 by controlling the growth temperature in the chemical vapor deposition (CVD) method without use of a catalyst. The field-effect transistors (FETs) devices fabricated with 1T/2H-MoS2 mixed-phase show an on/off ratio of 107. Photo response devices fabricated with 1T/2H-MoS2 mixed-phase show ≈55 times enhancement in responsivity (from 0.32 to 17.4 A W-1) and 102 times increase in the detectivity (from 4.1 × 1010 to 2.48 × 1012 cm Hz W-1) compare to 2H-MoS2. Introducing the metallic 1T phase within the 2H phase contributes additional carriers to the material, which prevents the electron-hole recombination and thereby increases the carrier density in the 1T/2H-MoS2 mixed-phase in comparison to 2H-MoS2. This work provides insights into the self-doping effects of 1T phase in 2H MoS2, enabling the tuning of 2D TMDs properties for optoelectronic applications.

Liquid-phase catalyst pre-seeding for controlled growth of layered MoS2 films over a large area via chemical vapor deposition

We introduce an innovative method that facilitates precise control of high-quality molybdenum disulfide (MoS2) growth, extending up to three layers, on a large scale. This scalable growth is realized by employing solution-based catalysts and precursors in conjunction with chemical vapor deposition (CVD). The catalyst not only diminishes the precursor's activation energy and melting temperature but also augments the overall reaction rate. By regulating the concentration ratio, we directly manipulate the precursor concentrations, thereby promoting clean growth. This unique control mechanism, as delineated in this study, is unprecedented. Our findings confirm that the catalyst introduction does not compromise the quality of the resulting samples. Field effect transistors (FETs) fabricated from the synthesized MoS2 display superior electrical properties; they exhibit a high carrier mobility of 32.1 cm2 V-1 s-1 and an on/off current ratio of 108, signifying their promising electrical performance. Accordingly, our findings suggest that the solution-based CVD strategy presented herein can be potentially utilized for the integration of FETs into a multitude of practical applications.

Large area MoS2 thin film growth by direct sulfurization

In this study, we present the growth of monolayer MoS₂ (molybdenum disulfide) film. Mo (molybdenum) film was formed on a sapphire substrate through e-beam evaporation, and triangular MoS₂ film was grown by direct sulfurization. First, the growth of MoS₂ was observed under an optical microscope. The number of MoS₂ layers was analyzed by Raman spectrum, atomic force microscope (AFM), and photoluminescence spectroscopy (PL) measurement. Different sapphire substrate regions have different growth conditions of MoS₂. The growth of MoS₂ is optimized by controlling the amount and location of precursors, adjusting the appropriate growing temperature and time, and establishing proper ventilation. Experimental results show the successful growth of a large-area single-layer MoS₂ on a sapphire substrate through direct sulfurization under a suitable environment. The thickness of the MoS₂ film determined by AFM measurement is about 0.73 nm. The peak difference between the Raman measurement shift of 386 and 405 cm^-1 is 19.1 cm^-1, and the peak of PL measurement is about 677 nm, which is converted into energy of 1.83 eV, which is the size of the direct energy gap of the MoS₂ thin film. The results verify the distribution of the number of grown layers. Based on the observation of the optical microscope (OM) images, MoS₂ continuously grows from a single layer of discretely distributed triangular single-crystal grains into a single-layer large-area MoS₂ film. This work provides a reference for growing MoS₂ in a large area. We expect to apply this structure to various heterojunctions, sensors, solar cells, and thin-film transistors.

Understanding the 2D-material and substrate interaction during epitaxial growth towards successful remote epitaxy: a review

Remote epitaxy, which was discovered and reported in 2017, has seen a surge of interest in recent years. Although the technology seemed to be difficult to reproduce by other labs at first, remote epitaxy has come a long way and many groups are able to consistently reproduce the results with a wide range of material systems including III-V, III-N, wide band-gap semiconductors, complex-oxides, and even elementary semiconductors such as Ge. As with any nascent technology, there are critical parameters which must be carefully studied and understood to allow wide-spread adoption of the new technology. For remote epitaxy, the critical parameters are the (1) quality of two-dimensional (2D) materials, (2) transfer or growth of 2D materials on the substrate, (3) epitaxial growth method and condition. In this review, we will give an in-depth overview of the different types of 2D materials used for remote epitaxy reported thus far, and the importance of the growth and transfer method used for the 2D materials. Then, we will introduce the various growth methods for remote epitaxy and highlight the important points in growth condition for each growth method that enables successful epitaxial growth on 2D-coated single-crystalline substrates. We hope this review will give a focused overview of the 2D-material and substrate interaction at the sample preparation stage for remote epitaxy and during growth, which have not been covered in any other review to date.

A critical review of fabrication challenges and reliability issues in top/bottom gated MoS2field-effect transistors

The voyage of semiconductor industry to decrease the size of transistors to achieve superior device performance seems to near its physical dimensional limitations. The quest is on to explore emerging material systems that offer dimensional scaling to match the silicon- based technologies. The discovery of atomic flat two-dimensional materials has opened up a completely new avenue to fabricate transistors at sub-10 nanometer level which has the potential to compete with modern silicon-based semiconductor devices. Molybdenum disulfide (MoS₂) is a two-dimensional layered material with novel semiconducting properties at atomic level seems like a promising candidate that can possibly meet the expectation of Moore's law. This review discusses the various 'fabrication challenges' in making MoS₂based electronic devices from start to finish. The review outlines the intricate challenges of substrate selection and various synthesis methods of mono layer and few-layer MoS₂. The review focuses on the various techniques and methods to minimize interface defect density at substrate/MoS₂interface for optimum MoS₂-based device performance. The tunable band-gap of MoS₂with varying thickness presents a unique opportunity for contact engineering to mitigate the contact resistance issue using different elemental metals. In this work, we present a comprehensive overview of different types of contact materials with myriad geometries that show a profound impact on device performance. The choice of different insulating/dielectric gate oxides on MoS₂in co-planar and vertical geometry is critically reviewed and the physical feasibility of the same is discussed. The experimental constraints of different encapsulation techniques on MoS₂and its effect on structural and electronic properties are extensively discussed.

Steep-Slope Transistor with an Imprinted Antiferroelectric Film

The effect of negative capacitance (NC), which can internally boost the voltage applied to a transistor, has been considered to overcome the fundamental Boltzmann limit of a transistor. To stabilize the NC effect, the dielectric (DE) must be integrated into a heterostructure with a ferroelectric (FE) film. However, in a multidomain hafnia, the charge boosting effect is reduced owing to a lowering of the depolarization field originating from the stray field at each domain, and simultaneously, the operating voltage increases owing to the voltage division at the DE. Here, we demonstrate core approaches to the gate stack of energy-efficient device technology using a transient NC. Electrical measurements of the transistor with imprinted antiferroelectric and high C_DE/C_FE structures exhibit low subthreshold slopes below 20 mV/dec, a low voltage operation of 0.5 V, a fast operation of 20 ns, hysteresis-free I_d-V_g, and high endurance characteristics of 10¹² cycles. We expect that this will lead to the rapid implementation of the NC effect in high-speed switching device applications with significantly improved energy efficiency.

Selector-less Ferroelectric Tunnel Junctions by Stress Engineering and an Imprinting Effect for High-Density Cross-Point Synapse Arrays

In the quest for highly scalable and three-dimensional (3D) stackable memory components, ferroelectric tunnel junction (FTJ) crossbar architectures are promising technologies for nonvolatile logic and neuromorphic computing. Most FTJs, however, require additional nonlinear devices to suppress sneak-path current, limiting large-scale arrays in practical applications. Moreover, the giant tunneling electroresistance (TER) remains challenging due to their inherent weak polarization. Here, we present that the employment of a diffusion barrier layer as well as a bottom metal electrode having a significantly low thermal expansion coefficient has been identified as an important way to enhance the strain, stabilize the ferroelectricity, and manage the leakage current in ultrathin hafnia film, achieving a high TER of 100, negligible resistance changes even up to 10⁸ cycles, and a high switching speed of a few tens of nanoseconds. Also, we demonstrate that the usage of an imprinting effect in a ferroelectric capacitor induced by an ionized oxygen vacancy near the electrode results in highly asymmetric current-voltage characteristics with a rectifying ratio of 1000. Notably, the proposed FTJ exhibits a high density array size (>4k) with a securing read margin of 10%. These findings provide a guideline for the design of high-performance and selector-free FTJ devices for large-scale crossbar arrays in neuromorphic applications.

Generating large out-of-plane piezoelectric properties of atomically thin MoS2via defect engineering

We calculated the piezoelectric properties of asymmetrically defected MoS₂ using density functional theory. By creating uneven numbers of defects on either side of two-dimensional MoS₂, the out-of-plane centrosymmetry of the charge distribution is clearly broken, and the out-of-plane piezoelectric response is induced. The largest out-of-plane piezoelectric response is associated with the highest defect ratio for MoS₂ to be semiconducting. We calculated the critical defect density of the metal-insulator transition of the asymmetrically defected MoS₂ to be 9.90 × 10¹⁴ cm^-2 and chemical formula MoS_1.22. The d₃₃ of the multilayer of optimally defected MoS₂ is found to be greater than those of AlN and ZnO, and in the same order of magnitude as lead zirconate titanate. All two-dimensional transition metal dichalcogenides can in principle be fabricated as piezoelectric with this approach. The required defect engineering is readily available with various types of ion irradiation or plasma treatment. By controlling the dose of the ion, the defect ratio and hence the piezoelectricity can be tuned. Such asymmetrically defected transition metal dichalcogenides can easily be integrated into two-dimensional transition metal dichalcogenide based devices, which is hard for conventional piezoelectric thin films to rival.