Scanning Transmission Electron Microscopy Analysis Research Articles

Kinetics of antimony biogeochemical processes under pre-definite anaerobic and aerobic conditions in a paddy soil

While the transformation of antimony (Sb) in paddy soil has been previously investigated, the biogeochemical processes of highly chemical active Sb in the soil remain poorly understood. In addition, there is a lack of quantitative understanding of Sb transformation in soil. Therefore, in this study, the kinetics of exogenous Sb in paddy soils were investigated under anaerobic and aerobic incubation conditions. The dissolved Sb(V) and the Sb(V) extracted by diffusive gradient technique decreased under anaerobic conditions and then increased under aerobic conditions. The redox reaction of Sb occurred, and Sb bioavailability significantly decreased after 55 days of incubation. The kinetics of Fe and the scanning transmission electron microscopy analysis revealed that the Fe oxides were reduced and became dispersed under anaerobic conditions, whereas they were oxidized and re-aggregated during the aerobic stage. In addition, the redox processes of sulfur and nitrogen were detected under both anaerobic and aerobic conditions. Based on these observations, a simplified kinetic model was established to distinguish the relative contributions of the transformation processes. The bioavailability of Sb was controlled by immobilization as a result of S reduction and by mobilization as a result of Fe reductive dissolution and S oxidation, rather than the pH. These processes coupled with the redox reaction of Sb jointly resulted in the complex behavior of Sb transformation under anaerobic and aerobic conditions. The model-based method and findings of this study provide a comprehensive understanding of the Sb transformation in a complex soil biogeochemical system under changing redox conditions.

On the Contact Optimization of ALD-Based MoS2 FETs: Correlation of Processing Conditions and Interface Chemistry with Device Electrical Performance

Despite the extensive ongoing research on MoS2 field effect transistors (FETs), the key role of device processing conditions in the chemistry involved at the metal-to-MoS2 interface and their influence on the electrical performance are often overlooked. In addition, the majority of reports on MoS2 contacts are based on exfoliated MoS2, whereas synthetic films are even more susceptible to the changes made in device processing conditions. In this paper, working FETs with atomic layer deposition (ALD)-based MoS2 films and Ti/Au contacts are demonstrated, using current–voltage (I–V) characterization. In pursuit of optimizing the contacts, high-vacuum thermal annealing as well as O2/Ar plasma cleaning treatments are introduced, and their influence on the electrical performance is studied. The electrical findings are linked to the interface chemistry through X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) analyses. XPS evaluation reveals that the concentration of organic residues on the MoS2 surface, as a result of resist usage during the device processing, is significant. Removal of these contaminations with O2/Ar plasma changes the MoS2 chemical state and enhances the MoS2 electrical properties. Based on the STEM analysis, the observed progress in the device electrical characteristics could also be associated with the formation of a continuous TiSx layer at the Ti-to-MoS2 interface. Scaling down the Ti interlayer thickness and replacing it with Cr is found to be beneficial as well, leading to further device performance advancements. Our findings are of value for attaining optimal contacts to synthetic MoS2 films.

The Investigation of Microwave-Assisted Greener Synthesis of Ag/Ti/Zn Trimetallic Nanoparticles and Carbon Quantum Dots Nanocomposites in the Application of Solar Cell

In this communication, we report a greener microwave method for synthesizing Ag/Ti/Zn trimetallic nanoparticles and carbon quantum dots nanocomposites (Ag/Ti/Zn TNPs and CQDs NCs). The morphology, topography, and size of the Ag/Ti/Zn TNPs and CQDs nanocomposites were determined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. The optical properties of the as-synthesized Ag/Ti/Zn TNPs and CQDs NCs were examined using UV–Vis spectroscopy and bandgap analysis. Finally, the Ag/Ti/Zn TNPs and CQDs NCs were fabricated by microwave-assisted greener synthesis, which was used as a photosensitizer, the copper plate as a counter electrode, and polysulfide as an electrolyte was assembled into the solar cell. The natural pigments extracted from the fruit of Solanum lycopersicium extract were used in Ag/Ti/Zn TNPs and CQDs NCs based solar cells. The as-fabricated solar cell's conversion efficiency was 5.47% with a Voc of 0.74 V, Jsc of 12.1 mA/cm2, and an FF of 0.60. Thus, this study highlights that the use of Ag/Ti/Zn TNPs and CQDs NCs has the great potential to be used as a photosensitizer.

Structure and hardness of in situ synthesized nano-oxide strengthened CoCrFeNi high entropy alloy thin films

In this study, we report on face-centered cubic structured CoCrFeNi high-entropy alloy thin films with finely dispersed nano-oxide particles which are formed by internal oxidation. Analytical scanning transmission electron microscopy imaging found that the particles are Cr2O3. The oxide particles contribute to the hardening of the film increasing its hardness by 14% compared to that of the film without precipitates, through the Orowan-type strengthening mechanism. Our novel approach paves the way to design medium- and high-entropy alloys with high strength by making use of oxide phases.

Defect localization in high-power vertical cavity surface emitting laser arrays by means of reverse biased emission microscopy

Applying a reverse bias near the breakdown voltage results in photon emission at the pn-junction in vertical cavity surface emitting lasers (VCSELs). This radiation can be collected with an emission microscope. Here, this technique is employed to investigate a high-power two dimensional (2D) VCSEL array with a large number of emitters at a non-degraded state and after high electrical stress. It has been found that non-degraded arrays show varying photon intensities across all emitters at breakdown condition while degraded arrays exhibit more intense electroluminescence at areas with faulty emitters containing defects in the active area as confirmed by plan view scanning transmission electron microscopy analysis.

The Effect of Tin Addition to Platinum Catalysts with Different Morphologies towards Methanol Electrooxidation in Alkaline Media

Direct methanol fuel cells (DMFCs) using anion exchange membranes have been attracting great interest. Thus, it is very important to develop electrocatalysts for methanol electrooxidation reaction (MEOR) in alkaline media. In this study, the effects of platinum nanoparticle morphology and presence of tin oxide in the electrocatalysts were investigated. Platinum and platinum-tin nanoparticles with cubic-like shape and (100) preferential orientation supported on carbon, Pt/C(100) and PtSnO2/C(100), were synthesized by the alcohol-reduction process using KBr as the shape-directing agent. Polycrystalline nanoparticles supported on carbon, Pt/C, and PtSnO2/C were also synthesized. The catalytic activity of the nanomaterials toward MEOR in alkaline media was investigated by cyclic voltammetry (CV) and chronoamperometry (CA). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis showed cubic Pt nanoparticles and SnO2 dispersed on the carbon support. X-ray diffraction (XRD) showed peaks of SnO2 cassiterite and peaks of the face-centered cubic (FCC) structure of Pt. CV experiments showed that the maximum current density from MEOR on Pt/C(100) (current density of about 108 mA cm–2 mg–1 of Pt) is about 2.4, 2.1, and 1.1 times higher than on Pt/C, PtSnO2/C, and PtSnO2/C(100), respectively. The same trend was observed in CA experiments. The higher catalytic activity of Pt(100) might be related to the OHads species coverage on it. SnO2 increases the catalytic activity on polycrystalline platinum, probably due to the OHads species provided by it. On the other hand, for Pt(100), the presence of SnO2 results in a decrease of the catalytic activity towards MEOR.

Thermal Behavior and Determination of the Heated Structure of 11Å Anomalous Tobermorite by in situ X‐ray Diffraction

AbstractThis article presents the thermal transitions of a tobermorite‐bearing sample when heated from 30°C up to 1200° C, both in vacuum and in static air, including tobermorite transforming to wollastonite, aragonite to calcite and calcite to lime. Characteristics obtained by in situ high temperature X‐ray diffraction, field emission scanning electron microscopy and scanning transmission electron microscopy analyses jointly indicate that the investigated tobermorite is anomalous. The variations along the a, b, c axes and the volume changes of tobermorite with increasing temperature are described, and its thermal shrinkage coefficients therefore determined. The comparison between the refined structures at 30°C and 800°C demonstrates that the shrinkage degree (Δa/a0) along the a axis is higher than those (Δb/b0, Δc/c0) along the b and c axes. The wollastonite is formed in two ways: Tobermorite converting to wollastonite and lime reacting with quartz to form wollastonite.

Tailoring bolometric properties of a TiOx/Ti/TiOx tri-layer film for integrated optical gas sensors.

In this paper, we systematically investigated tailoring bolometric properties of a proposed heat-sensitive TiOx/Ti/TiOx tri-layer film for a waveguide-based bolometer, which can play a significant role as an on-chip detector operating in the mid-infrared wavelength range for the integrated optical gas sensors on Ge-on-insulator (Ge-OI) platform. As a proof-of-concept, bolometric test devices with a TiOx single-layer and TiOx/Ti/TiOx tri-layer films were fabricated by varying the layer thickness and thermal treatment condition. Comprehensive characterization was examined by the scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses in the prepared films to fully understand the microstructure and interfacial properties and the effects of thermal treatment. Quantitative measurements of the temperature- and time-dependent resistance variations were conducted to deduce the minimum detectable change in temperature (ΔTmin) of the prepared films. Furthermore, based on these experimentally obtained results, limit-of-detection (LoD) for the carbon dioxide gas sensing was estimated to demonstrate the feasibility of the proposed waveguide-based bolometer with the TiOx/Ti/TiOx tri-layer film as an on-chip detector on the Ge-OI platform. It was found that the LoD can reach ∼3.25 ppm and/or even lower with the ΔTmin of 11.64 mK in the device with the TiOx/Ti/TiOx (47/6/47 nm) tri-layer film vacuum-annealed at 400 °C for 15 min, which shows great enhancement of ∼7.7 times lower value compared to the best case of TiOx single-layer films. Our theoretical and experimental demonstration for tailoring bolometric properties of a TiOx/Ti/TiOx tri-layer film provides fairly useful insight on how to improve LoD in the integrated optical gas sensor with the bolometer as an on-chip detector.

Electronic and Structural Transitions of LaAlO3 /SrTiO3 Heterostructure Driven by Polar Field-Assisted Oxygen Vacancy Formation at the Surface.

The origin of 2D electron gas (2DEG) at LaAlO3/SrTiO3 (LAO/STO) interfaces has remained highly controversial since its discovery. Various models are proposed, which include electronic reconstruction via surface‐to‐interface charge transfer and defect‐mediated doping involving cation intermixing or oxygen vacancy (V O) formation. It is shown that the polar field‐assisted V O formation at the LAO/STO surface plays critical roles in the 2DEG formation and concurrent structural transition. Comprehensive scanning transmission electron microscopy analyses, in conjunction with density functional theory calculations, demonstrate that V O forming at the LAO/STO surface above the critical thickness (t c) cancels the polar field by doping the interface with 2DEG. The antiferrodistortive (AFD) octahedral rotations in LAO, which are suppressed below the t c, evolve with the formation of V O above the t c. The present study reveals that local symmetry breaking and shallow donor behavior of V O induce the AFD rotations and relieve the electrical field by electron doping the oxide heterointerface.

Chemical durability and surface alteration of lanthanide zirconates (A2Zr2O7: A = La-Yb)

Chemical durability of lanthanide zirconates (A2Zr2O7) (A = La-Yb) under near-field environments is important for evaluating their application as potential nuclear waste forms. In this work, A2Zr2O7 (A = La-Yb) are synthesized by spark plasma sintering with controlled microstructure and their chemical durability are evaluated in a nitric acid solution (pH = 1). Scanning transmission electron microscopy analysis reveals an amorphous passivation film either enriched with Zr or lanthanide. The complex chemistry of the passivation films can be correlated with a transition in corrosion mechanisms from a preferential release of lanthanide in La2Zr2O7 to a preferential release of Zr in Er2Zr2O7 and Yb2Zr2O7. These results suggest a dominant mechanism of incongruent dissolution and surface reorganization for the formation of passivation films. Strong correlations are identified between the leaching rates and cation ionic size, ionic potential, electronegativity differences between A-site cation and Zr, and bonding valence sum of oxygen, suggesting important impacts of structural and bonding characteristics in controlling chemical durability of lanthanide zirconates.

Lattice-Match Stabilization and Magnetic Properties of Metastable Epitaxial Permalloy-Disilicide Nanostructures on a Vicinal Si(111) Substrate.

We report the epitaxial formation of metastable γ-(FexNi1−x)Si2 nanostructure arrays resulting from the reaction of Ni80Fe20 permalloy with vicinal Si(111) surface atoms. We then explore the effect of structure and composition on the nanostructure’s magnetic properties. The low-temperature annealing (T < 600 °C) of a pre-deposited permalloy film led to solid-phase epitaxial nucleation of compact disk-shaped island nanostructures decorating <110> ledges of the stepped surface, with either (2 2) or () R30° reconstructed flat top faces. High resolution scanning transmission electron microscopy analysis demonstrated fully coherent epitaxy of the islands with respect to the substrate, consistent with a well-matched CaF2-prototype structure associated with γ-FeSi2, along perfect atomically sharp interfaces. Energy dispersive spectroscopy detected ternary composition of the islands, with Fe and Ni atoms confined to the islands, and no trace of segregation. Our magnetometry measurements revealed the superparamagnetic behavior of the silicide islands, with a blocking temperature around 30 K, reflecting the size, shape, and dilute arrangement of the islands in the assembly.

Dextran based amphiphilic self-assembled biopolymeric macromolecule synthesized via RAFT polymerization as indomethacin carrier

This work demonstrates a facile pathway to develop a biopolymer based amphiphilic macromolecule through reversible addition–fragmentation chain transfer (RAFT) polymerization, using dextran (a biopolymer) as starting material. Also, a new hydrophobic monomer [2-methyl-acrylic acid 1-benzyl-1H-[1,2,3] triazol-4-ylmethyl ester (MABTE)] has been synthesized using methacrylic acid via “click” approach. The resultant copolymer displays controlled radical polymerization characteristics: narrow polydispersity (Ð) and controlled molecular weight as obtained through advanced polymer chromatography (APC) analysis. In aqueous solution, the copolymer can proficiently be self-assembled to provide micellar structure, which has been evidenced from field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analyses. The in-vitro cytotoxicity study illustrates the nontoxic nature of the copolymer up to 100 μg/mL polymer concentration. The copolymer has been found to be worthy as an efficient carrier for the sustained release of hydrophobic drug: Indomethacin (IND).

Unveiling local atomic bonding and packing of amorphous nanophases via independent component analysis facilitated pair distribution function

Amorphous nanophases play a significant role for the properties of a variety of nanoscale heterogeneous materials. Experimental characterization of the atomic arrangement of the amorphous structure, including nanoscale structural variations, is one of the main challenges limiting the rational design of the materials. Here, an approach to characterize local bonding and atomic packing in complex nanomaterials is introduced. Building on scanning transmission electron microscopy (STEM) and pair distribution function analysis (PDF) to record local diffraction information with nanometer spatial resolution, we show that independent component analysis for “blind source separation” of mixed information due to projection effects in STEM-PDF, enables full separation of these signals. The unprecedented information allows determining the structure of individual nanoscale phases and identifying the compounds inside. We analyzed a FeZr/ZrO2 multilayer as proof of principle, and discovered differently coordinated FeOx in the interfacial region. The approach was applied to Fe25Sc75 nanoglass and revealed Fe–Fe bonding concealed in the Sc-rich matrix. Finally, analysis of a shear band in a deformed Cu/CuZr nanolaminate confirmed Cu enrichment and reduced medium-range order in the shear band.

Facile fabrication of CuO nanosheets for photocatalytic applications

The development of efficient photocatalysts for the removal of pollutants is the need of the hour. In this work, we report surfactant-free easy synthesis of CuO nanosheets and account detailed investigations on the morphological, structural and optical properties of CuO nanosheets for the photocatalytic removal of pollutants. The morphology and microstructure of the prepared nanostructures were explored by transmission electron microscopy (TEM), high resolution TEM and scanning TEM analyses which showed CuO nanosheets in the prepared sample. X-ray diffraction and Raman analyses confirmed monoclinic crystal structure of CuO nanosheets with crystallite size of 12 nm. UV–visible diffuse reflectance spectroscopy (UV–Vis-DRS) studies revealed band gap of 1.98 eV of CuO nanosheets while Brunauer–Emmett–Teller surface area analysis showed high surface area of 29.1 m2/g. Fabricated CuO nanosheets acted as efficient photocatalysts for the elimination of methyl orange and methylene blue from wastewater. Growth process of CuO nanosheets is proposed and photocatalytic mechanism for the removal of dyes by CuO nanosheets is thoroughly examined.

Characterizing the spectral, microstructural, and chemical effects of solar wind irradiation on the Murchison carbonaceous chondrite through coordinated analyses

We performed H+ and He+ irradiation experiments on slabs of the Murchison CM2 meteorite to simulate solar wind irradiation of carbonaceous asteroids. Two separate 6 mm × 6 mm regions were irradiated with 1 keV H+ and 4 keV He+, respectively, to fluences of 8.1 × 1017 ions/cm2 for H+ and 1 × 1018 ions/cm2 for He+. Unirradiated and irradiated surfaces were analyzed using X-ray photoelectron spectroscopy (XPS), visible to near-infrared spectroscopy (VNIR; 0.35–2.5 μm), and microprobe two-step laser-desorption mass spectrometry (μL2MS). We also performed analytical field-emission scanning transmission electron microscopy (FE-STEM) on focused ion beam (FIB) cross-sections extracted from olivine grains and matrix material within the H+- and He+-irradiated regions. In situ XPS analyses suggest that ion irradiation results in the removal of most surface carbon and the partial reduction of surface iron to lower oxidation states. In response to He+-irradiation, we observe brightening (longward of ~0.75 μm) and reddening of reflectance spectra, which is a departure from typical lunar-style space weathering. Additionally, H+- and He+-irradiation have opposing effects on organic carbon content: H+-irradiation increases the abundance of some low-molecular-weight free organic species by breaking down macromolecular material while He+-irradiation causes a decrease in overall organic content by cleaving bonds and sputtering constituent atoms. This suggests that solar wind H+-irradiation and solar wind He+-irradiation change the organic functional group chemistry of asteroidal regolith in different ways. In contrast to some previous experimental space weathering studies, we observe an increase in H2O and OH− abundances in our sample in response to both types of ion irradiation. FE-STEM and energy dispersive X-ray spectroscopy (EDX) analyses show complete amorphization of matrix phyllosilicates in ion-affected rims, partial amorphization of olivine, and changes in Si and Mg concentrations at and/or near the surface. We discuss the implications of these results for understanding the complex nature of space weathering on primitive, carbon-rich asteroids and for analyzing future returned samples from carbonaceous asteroids Bennu and Ryugu.

Eco-friendly synthesis of reduced graphene oxide as sustainable photocatalyst for removal of hazardous organic dyes

Synthetic dyes are widely used as coloring agents in the textile, food, paper, leather, and printing industries. Sustainable removal of these dye molecules is a challenging task due to their toxic nature to the environment as well as for living organisms. In the present study, a simple hydrothermal method is carried out to synthesize reduced graphene oxide (rGO) using the leaves extract of Murraya koenigii. Further, different characterization techniques such as X-ray diffraction, Raman spectroscopy, UV–vis spectroscopy, and Fourier transform infrared spectroscopy (FT-IR) are used to confirm the physicochemical properties of synthesized rGO. Raman analysis confirms the reduction of graphene oxide by the increase in ID/IG ratio significantly. Field emission scanning electron microscopy (FE-SEM) and Transmission electron microscopy (TEM) analysis show well-exploited rGO morphology. Further, newly synthesized rGO is used as a photocatalyst for the removal of methylene blue (MB) and methyl orange (MO) dyes. UV–vis spectrophotometer is used for monitoring the degradation efficiency. Catalyst MKrGO shows 80% of MO and 77% of MB degradation within 120 min of sunlight exposure. The sustainability of this catalyst is checked by recyclability in five subsequent degradation cycles and noticed a stable and significant degradation activity.

Green Synthesis, Structural Characterization and Photocatalytic Activities of Chitosan-ZnO Nano‐composite

Chitosan was isolated from chitin, a direct derivative of snail shell, and further used to form a heterostructure with ZnO nanoparticles (ZnO NPs). This study was carried out to utilize green nanochemistry in the purification of waste water. The obtained ZnO-chitosan nanocomposite was made by precipitation method and characterized by Fourier Transform Infrared (FTIR), powder X-ray diffraction (pXRD), Energy dispersive X-ray (EDX), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) analyses. The FTIR spectra among other peaks revealed bands around 1735–1740 cm−1 in all the spectra due to C=O stretching band. The XRD patterns showed the crystalline nature of ZnO and the ZnO-chitosan nanocomposites with low intensities in the peaks of the nanocomposites, an indication of reduced crystallinity. The SEM micrographs showed thin strands of the chitin and chitosan whereas the ZnO NPs appeared as clustered spheroids with the ZnO-chitosan nanocomposites revealing the anchoring of the ZnO spheroids on the smooth strands of the chitosan. The EDX spectra showed various elemental compositions with 54.82% Zn in the ZnO NPs and 17.27% Zn in the ZnO-chitosan nanocomposites. TEM studies showed spherical images of the ZnO NPs (3.69 nm) and the quasi-spherical nature of the ZnO-chitosan nanocomposites (8.91 nm). The photodegradation of methylene blue dye by ZnONPs recorded gradual decomposition of the dye while in the composite, a tremendous change was observed within the first 15 min of the reaction.

Atomically Dispersed Pt on TiO2 Nanosheets for Catalytic Gaseous Acetaldehyde Abatement

Increasing volatile organic compounds (VOCs) in the atmosphere have triggered intensive research on photocatalytic oxidation technology to safely photodegrade these volatile organic pollutants. Downsizing metal catalysts up to an atomic level on oxide supports can provide more active sites on the surface for different reactions, i.e., CO oxidation, CO2 reduction, removal of organic pollutants as well as decrease the use of precious metals. Here, we designed atomically dispersed metal catalysts (ADMCs), i.e., Pt on TiO2 nanosheets, which produced a robust and stable photocatalytic degradation of the gas-phase acetaldehyde. The loading of Pt was carried out using different deposition times, such as 2, 4, and 6 h, designated as Pt/T xh at 70 °C. This was followed by vacuum drying (60 °C) and subsequent calcination at 125 °C in an inert atmosphere. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis confirmed the presence of atomically dispersed Pt on TiO2 nanosheets. The sample Pt/T 4h displayed a high photodegradation efficiency (100%) than pure TiO2 (38%) for 500 ppm of acetaldehyde with a flow rate of 10 sccm. Also, 700 ppm of CO2 was produced by Pt/T 4h in contrast to 340 ppm by pure TiO2. The experimental data for the sample Pt/T 4h was further characterized using Langmuir–Hinshelwood kinetic models to determine the reaction rates, adsorption equilibrium constants, and water adsorption constant. This study provides a unique opportunity to develop metal-decorated TiO2-based photocatalysts for the robust and efficient removal and mineralization of acetaldehyde from the indoor atmosphere.

Synthesis of LiNbO3 nanocrystals by microwave-assisted hydrothermal method: formation mechanism and application to hydrogen evolution reaction

Lithium niobate (LiNbO3) is an important ferroelectric material with a wide range of applications. In this work, the use of microwave radiation to improve the conditions of hydrothermal synthesis of pure nanocrystalline LiNbO3 was proposed. The structural characterization showed that nanoparticles of a pure LiNbO3 phase were synthesized. The scanning electron microscopy and transmission electron microscopy analysis revealed a high-quality nanoparticle with an average diameter of ca. 42 and 65 nm for 2 and 3 h of synthesis time, respectively. These results represent a time-saving using a microwave-assisted hydrothermal method for LiNbO3 preparation, with excellent quality at the nanometer scale. The nanoparticles were employed as photocatalysts for hydrogen evolution reaction through water splitting, reaching the rate of 10.5 µmol h−1 g−1. It had shown to be a potential candidate for water photolysis due to the small particle size and high crystallinity. Moreover, it also allows the direct and fast synthesis of LiNbO3 on carbon fiber, multi-walled carbon nanotubes and nanocellulose, which were used as supporting materials for the fabrication of novel nanocomposites.

Quantification and Healing of Defects in Atomically Thin Molybdenum Disulfide: Beyond the Controlled Creation of Atomic Defects.

Atomically thin 2D materials provide an opportunity to investigate the atomic-scale details of defects introduced by particle irradiation. Once the atomic configuration of defects and their spatial distribution are revealed, the details of the mesoscopic phenomena can be unveiled. In this work, we created atomically small defects by controlled irradiation of gallium ions with doses ranging from 4.94 × 1012 to 4.00 × 1014 ions/cm2 on monolayer molybdenum disulfide (MoS2) crystals. The optical signatures of defects, such as the evolution of defect-activated LA-bands and a broadening of the first-order (E' and A'1) modes, can be studied by Raman spectroscopy. High-resolution scanning transmission electron microscopy (HR-STEM) analysis revealed that most defects are vacancies of few-molybdenum atoms with surrounding sulfur atoms (VxMo+yS) at a low ion dose. When increasing the ion dose, the atomic vacancies merge and form nanometer-sized holes. Utilizing HR-STEM and image analysis, we propose the estimation of the finite crystal length (Lfc) via the careful quantification of 0D defects in 2D systems through the formula Lfc = 4.41/, where ηion corresponds to the ion dose. Combining HR-STEM and Raman spectroscopy, the formula to calculate Lfc from Raman features, I(LA)/I(A'1) = 5.09/Lfc2, is obtained. We have also demonstrated an effective route to healing the ion irradiation-induced atomic vacancies by annealing defective MoS2 in a hydrogen disulfide (H2S) atmosphere. The H2S annealing improved the crystal quality of MoS2 with Lfc greater than the calculated size of the A exciton wave function, which leads to a partial recovery of the photoluminescence signal after its quenching by ion irradiation.