Dispersive growth and laser-induced rippling of large-area singlelayer MoS2 nanosheets by CVD on c-plane sapphire substrate

A zinc-containing porphyrin aluminum MOF in sorption of diethyl sulfide vapor: mechanistic experimental and computational study

We report a mechanistic study of the interactions in the sorption of volatile organic sulfur compound (VOSC) diethyl sulfide (DES) by zinc porphyrin aluminum MOF (actAl-MOF-TCPPZn) compound 3. First, interactions were studied under dynamic conditions with the vapor of DES in flowing air, using in situ time-dependent ATR-FTIR spectroscopy in a controlled atmosphere with a new facile spectroscopic mini-chamber. The first binding site includes μ(O-H) and COO- groups as detected by characteristic peak shifts. Control experiments with a model compound, which lacks porosity and these groups, show no peak shifts. An additional insight was obtained by DFT computations using small clusters. The kinetics of sorption of DES by compound 3 is of the Langmuir adsorption model and pseudo-first order with rate constant robs = 0.442 ± 0.056 min-1. Sorption of DES under static conditions in saturated vapor results in stoichiometric adsorption complex [Al-MOF-TCPPZn]1(DES)4 characterized by spectroscopic, structural and gravimetric methods; the adsorbed amount is very high (381 mg g-1 sorbent). The repetitive sorption and desorption of DES are conducted, with facile regeneration. Finally, the mechanistic details were determined by Raman and photoluminescence (PL) spectroscopy using a confocal Raman microscope. Photoexcitation of compound 3 at 405 nm into the Soret band of the metalloporphyrin linker shows the characteristic PL peaks of Q-bands: the purely electronic Q(0-0) and first vibronic Q(0-1) bands. Upon interaction with DES, preferential quenching of PL from the Q(0-0) band occurs with a significant increase of the signal of the vibronic Q(0-1) band, reflecting bonding to the metalloporphyrin ring. Compound 3 is of interest to mechanistic studies of VOSCs, their removal from air, and optical chemo-sensing.

Laser-Synthesized 2D-MoS2 Nanostructured Photoconductors

The direct laser synthesis of periodically nanostructured 2D transition metal dichalcogenide (2D-TMD) films, from single source precursors, is presented here. Laser synthesis of MoS₂ and WS₂ tracks is achieved by localized thermal dissociation of Mo and W thiosalts, caused by the strong absorption of continuous wave (c.w.) visible laser radiation by the precursor film. Moreover, within a range of irradiation conditions we have observed occurrence of 1D and 2D spontaneous periodic modulation in the thickness of the laser-synthesized TMD films, which in some cases is so extreme that it results in the formation of isolated nanoribbons with a width of ~200 nm and a length of several micrometers. The formation of these nanostructures is attributed to the effect that is known as laser-induced periodic surface structures (LIPSS), which is caused by self-organized modulation of the incident laser intensity distribution due to optical feedback from surface roughness. We have fabricated two terminal photoconductive detectors based on nanostructured and continuous films and we show that the nanostructured TMD films exhibit enhanced photo-response, with photocurrent yield increased by three orders of magnitude as compared to their continuous counterparts.

Optical Modification of 2D Materials: Methods and Applications

2D materials are under extensive research due to their remarkable properties suitable for various optoelectronic, photonic, and biological applications, yet their conventional fabrication methods are typically harsh and cost-ineffective. Optical modification is demonstrated as an effective and scalable method for accurate and local in situ engineering and patterning of 2D materials in ambient conditions. This review focuses on the state of the art of optical modification of 2D materials and their applications. Perspectives for future developments in this field are also discussed, including novel laser tools, new optical modification strategies, and their emerging applications in quantum technologies and biotechnologies.

Fast MoS$$_2$$ thickness identification by transmission imaging

Determining the thickness of a few-layer 2D material is a tough task that often involves complex and time consuming measurements. Here we discuss a rapid method for determining the number of layers of molybdenum disulfide, MoS$$_2$$ 2 , flakes based on microscopic transmission imaging. By analyzing the contrast of the red, blue and green channels of the flake image against the background, we show that it is possible to unequivocally determine the number of layers. The presented method is based on the light absorption properties of MoS$$_2$$ 2 and its validity is confirmed by micro-Raman measurements. The main advantage of this method against traditional methods is to quickly determine the thickness of the material in the early stages of the experimental process with low cost apparatus.

Spatial zigzag evolution of cracks in moving sapphire initiated by bursts of picosecond laser pulses for ultrafast wafer dicing

Spatial zigzag evolution of cracks in moving sapphire wafer was observed after irradiation with sequences of picosecond laser pulses (bursts). The Gaussian beam was tightly focused inside the sapphire. The spatial position of laser initiated cracks moved in vertical and horizontal directions when a wafer was translated at a controllable speed perpendicular to the beam propagation direction. The cracking plane consisting of the periodically repeating inclined modifications and cracks was observed. The period of modifications and the inclination angle had a linear dependence on the wafer translation speed. The model of spatial zigzag crack evolution was created and the physical origin of modification growth at a measured speed of 1.3 ± 0.1 m s-1 is discussed. The zigzag cracking was applied for ultrafast stealth dicing and cleavage of the sapphire: dicing speed 300 mm s-1, wafer thickness 430 μm, laser power 5.5 W, repetition rate 100 kHz, sub-pulse duration 9 ps, the temporal distance between sub-pulses in burst 26.7 ns, and the number of sub-pulses 13.

Coherent phonon dynamics in a c-plane sapphire crystal before and after intense femtosecond laser irradiation

Femtosecond pump-probe experiments with a ∼6.4 fs time-resolution were performed to investigate the coherent phonon dynamics in a c-plane sapphire crystal before and after intense 800 nm femtosecond laser irradiation. The intense femtosecond laser induced defect/distortion and even re-crystallization of crystalline structures, which result in the appearance of new peaks and relative intensity change in coherent phonon and Raman spectra. The combination of these two spectra was found to be beneficial to evidence the variation of crystalline structure and further to differentiate the origins of new Raman peaks after irradiation. Further analysis of time-dependent differential absorbance with damped cosine function fitting and Fourier transfer calculation yields the vibrational parameters, including periods, damping times and initial phases, before and after irradiation. With these parameters, the defect-effects on damping time and the mechanism of coherent phonon generation were addressed.

Effect of Compressive Prestrain on the Anti-Pressure and Anti-Wear Performance of Monolayer MoS2: A Molecular Dynamics Study

The effects of in-plane prestrain on the anti-pressure and anti-wear performance of monolayer MoS₂ have been investigated by molecular dynamics simulation. The results show that monolayer MoS₂ observably improves the load bearing capacity of Pt substrate. The friction reduction effect depends on the deformation degree of monolayer MoS₂. The anti-pressure performance of monolayer MoS₂ and Pt substrate is enhanced by around 55.02% when compressive prestrain increases by 4.03% and the anti-wear performance is notably improved as well. The improved capacities for resisting the in-plane tensile and out-of-plane compressive deformation are responsible for the outstanding lubrication mechanism of monolayer MoS₂. This study provides guidelines for optimizing the anti-pressure and anti-wear performance of MoS₂ and other two-dimension materials which are subjected to the in-plane prestrain.

Nonlayered Two-Dimensional Defective Semiconductor γ-Ga2S3 toward Broadband Photodetection

Two-dimensional (2D) materials exhibit high sensitivity to structural defects due to the nature of interface-type materials, and the corresponding structural defects can effectively modulate their inherent properties in turn, giving them a wide application range in high-performance and functional devices. 2D γ-Ga₂S₃ is a defective semiconductor with outstanding optoelectronic properties. However, its controllable preparation has not been implemented yet, which hinders exploring its potential applications. In this work, we introduce nonlayered γ-Ga₂S₃ into the 2D materials family, which was successfully synthesized via the space-confined chemical vapor deposition method. Its intriguing defective structure are revealed by high-resolution transmission electron microscopy and temperature-dependent cathodoluminescence spectra, which endow the γ-Ga₂S₃-based device with a broad photoresponse from the ultraviolet to near-infrared region and excellent photoelectric conversion capability. Simultaneously, the device also exhibits excellent ultraviolet detection ability ( R_λ = 61.3 A W^-1, I_on /I_off = 851, EQE = 2.17× 10⁴ %, D* = 1.52× 10¹⁰ Jones @350 nm), and relatively fast response (15 ms). This work provides a feasible way to fabricate ultrathin nonlayered materials and explore the potential applications of a 2D defective semiconductor in high-performance broadband photodetection, which also suggests a promising future of defect creation in optimizing photoelectric properties.

Recent Developments in Controlled Vapor-Phase Growth of 2D Group 6 Transition Metal Dichalcogenides

An overview of recent developments in controlled vapor-phase growth of 2D transition metal dichalcogenide (2D TMD) films is presented. Investigations of thin-film formation mechanisms and strategies for realizing 2D TMD films with less-defective large domains are of central importance because single-crystal-like 2D TMDs exhibit the most beneficial electronic and optoelectronic properties. The focus is on the role of the various growth parameters, including strategies for efficiently delivering the precursors, the selection and preparation of the substrate surface as a growth assistant, and the introduction of growth promoters (e.g., organic molecules and alkali metal halides) to facilitate the layered growth of (Mo, W)(S, Se, Te)₂ atomic crystals on inert substrates. Critical factors governing the thermodynamic and kinetic factors related to chemical reaction pathways and the growth mechanism are reviewed. With modification of classical nucleation theory, strategies for designing and growing various vertical/lateral TMD-based heterostructures are discussed. Then, several pioneering techniques for facile observation of structural defects in TMDs, which substantially degrade the properties of macroscale TMDs, are introduced. Technical challenges to be overcome and future research directions in the vapor-phase growth of 2D TMDs for heterojunction devices are discussed in light of recent advances in the field.

New Insights into Planar Defects in Layered α-MoO3 Crystals

The observation of regular ( h0 l) planar defects in α-MoO₃ crystals can be traced back to over 60 years ago. Two mechanisms have been proposed to interpret the formation of the planar defects. One is related to the diffusion of oxygen vacancies because of thermal-driven release of oxygen atoms in vacuum and the consequent crystallographic shear of α-MoO₃. The other is associated with redox reactions of moisture and/or hydrocarbons that give rise to H _xMoO₃ precipitates. Here, we report that regularly spaced (302) planar defects can be introduced into α-MoO₃ belt crystals by heating in liquid sulfur at 300 °C. These defects are undetectable by both atomic force microscopy and scanning electron microscopy at the crystal surface. Raman scattering enhancement and weakening have been observed for different phonon modes of α-MoO₃ at the (302) planar defects as probed from the (010) surface. Their comparisons with the Raman scattering enhancements at the edges and the argon-plasma-induced Raman spectral evolutions of the as-grown α-MoO₃ belt crystals provide new insights into the planar defects with regard to their formation and characteristics.

Modification of Vapor Phase Concentrations in MoS2 Growth Using a NiO Foam Barrier

Single-layer molybdenum disulfide (MoS₂) has attracted significant attention due to its electronic and physical properties, with much effort invested toward obtaining large-area high-quality monolayer MoS₂ films. In this work, we demonstrate a reactive-barrier-based approach to achieve growth of highly homogeneous single-layer MoS₂ on sapphire by the use of a nickel oxide foam barrier during chemical vapor deposition. Due to the reactivity of the NiO barrier with MoO₃, the concentration of precursors reaching the substrate and thus nucleation density is effectively reduced, allowing grain sizes of up to 170 μm and continuous monolayers on the centimeter length scale being obtained. The quality of the monolayer is further revealed by angle-resolved photoemission spectroscopy measurement by observation of a very well resolved electronic band structure and spin-orbit splitting of the bands at room temperature with only two major domain orientations, indicating the successful growth of a highly crystalline and well-oriented MoS₂ monolayer.

High-throughput Identification and Characterization of Two-dimensional Materials using Density functional theory

We introduce a simple criterion to identify two-dimensional (2D) materials based on the comparison between experimental lattice constants and lattice constants mainly obtained from Materials-Project (MP) density functional theory (DFT) calculation repository. Specifically, if the relative difference between the two lattice constants for a specific material is greater than or equal to 5%, we predict them to be good candidates for 2D materials. We have predicted at least 1356 such 2D materials. For all the systems satisfying our criterion, we manually create single layer systems and calculate their energetics, structural, electronic, and elastic properties for both the bulk and the single layer cases. Currently the database consists of 1012 bulk and 430 single layer materials, of which 371 systems are common to bulk and single layer. The rest of calculations are underway. To validate our criterion, we calculated the exfoliation energy of the suggested layered materials, and we found that in 88.9% of the cases the currently accepted criterion for exfoliation was satisfied. Also, using molybdenum telluride as a test case, we performed X-ray diffraction and Raman scattering experiments to benchmark our calculations and understand their applicability and limitations. The data is publicly available at the website http://www.ctcms.nist.gov/~knc6/JVASP.html.

Multi-photon absorption enhancement by dual-wavelength double-pulse laser irradiation for efficient dicing of sapphire wafers

The evidence of multi-photon absorption enhancement by the dual-wavelength double-pulse laser irradiation in transparent sapphire was demonstrated experimentally and explained theoretically for the first time. Two collinearly combined laser beams with the wavelengths of 1064 nm and 355 nm, inter-pulse delay of 0.1 ns, and pulse duration of 10 ps were used to induce intra-volume modifications in sapphire. The theoretical prediction of using a particular orientation angle of 15 degrees of the half-wave plate for the most efficient absorption of laser irradiation is in good agreement with the experimental data. The new innovative effect of multi-photon absorption enhancement by dual-wavelength double-pulse irradiation allowed utilisation of the laser energy up to four times more efficiently for initiation of internal modifications in sapphire. The new absorption enhancement effect has been used for efficient intra-volume dicing and singulation of transparent sapphire wafers. The dicing speed of 150 mm/s was achieved for the 430 μm thick sapphire wafer by using the laser power of 6.8 W at the repetition rate of 100 kHz. This method opens new opportunities for the manufacturers of the GaN-based light-emitting diodes by fast and precise separation of sapphire substrates.

Topography preserved microwave plasma etching for top-down layer engineering in MoS2 and other van der Waals materials

A generic and universal layer engineering strategy for van der Waals (vW) materials, scalable and compatible with the current semiconductor technology, is of paramount importance in realizing all-two-dimensional logic circuits and to move beyond the silicon scaling limit. In this letter, we demonstrate a scalable and highly controllable microwave plasma based layer engineering strategy for MoS₂ and other vW materials. Using this technique we etch MoS₂ flakes layer-by-layer starting from an arbitrary thickness and area down to the mono- or the few-layer limit. From Raman spectroscopy, atomic force microscopy, photoluminescence spectroscopy, scanning electron microscopy and transmission electron microscopy, we confirm that the structural and morphological properties of the material have not been compromised. The process preserves the pre-etch layer topography and yields a smooth and pristine-like surface. We explore the electrical properties utilising a field effect transistor geometry and find that the mobility values of our samples are comparable to those of the pristine ones. The layer removal does not involve any reactive gasses or chemical reactions and relies on breaking the weak inter-layer vW interaction making it a generic technique for a wide spectrum of layered materials and heterostructures. We demonstrate the wide applicability of the technique by extending it to other systems such as graphene, h-BN and WSe₂. In addition, using microwave plasma in combination with standard lithography, we illustrate a lateral patterning scheme making this process a potential candidate for large scale device fabrication in addition to layer engineering.

Large-Scale Synthesis and Systematic Photoluminescence Properties of Monolayer MoS2 on Fused Silica

Monolayer MoS2, with fascinating mechanical, electrical, and optical properties, has generated enormous scientific curiosity and industrial interest. Controllable and scalable synthesis of monolayer MoS2 on various desired substrates has significant meaning in both basic scientific research and device application. Recent years have witnessed many advances in the direct synthesis of single-crystalline MoS2 flakes or their polycrystalline aggregates on numerous diverse substrates, such as SiO2-Si, mica, sapphire, h-BN, and SrTiO3, etc. In this work, we used the dual-temperature-zone atmospheric-pressure chemical vapor deposition method to directly synthesize large-scale monolayer MoS2 on fused silica, the most ordinary transparent insulating material in daily life. We systematically investigated the photoluminescence (PL) properties of monolayer MoS2 on fused silica and SiO2-Si substrates, which have different thermal conductivity coefficients and thermal expansion coefficients. We found that there exists a stronger strain on monolayer MoS2 grown on fused silica, and the strain becomes more obvious as temperature decreases. Moreover, the monolayer MoS2 grown on fused silica exhibits the unique trait of a fractal shape with tortuous edges and has stronger adsorbability. The monolayer MoS2 grown on fused silica may find application in sensing, energy storage, and transparent optoelectronics, etc.