Topmost Surface Layer Research Articles

Zinc blende ZnS (001) surface structure investigated by XPS, LEED, and DFT

The formation and stability of the zinc blende ZnS (001) single crystal surface have been examined by x-ray photoemission spectroscopy (XPS), low-energy electron diffraction (LEED), and density-functional theory (DFT) calculations. LEED patterns obtained from the ZnS (001) surface prepared in ultrahigh vacuum conditions at 1420 K reveal the formation of a (1 × 2) reconstructed structure. Surface chemical characterization performed by XPS indicates that the surface region consists of a Zn-deficient (sulfur-rich) phase. Our DFT theoretical calculations confirm that the ZnS (001) (1x2) surface structure is energetically favorable and consists of a Zn-terminated topmost surface layer stabilized by Zn-vacancies. The calculations also suggest that the remaining Zn-cations on the surface are displaced inward, driven by surface energy minimization related to zinc-blend ZnS (001) polar nature. Notably, the observed surface structural modifications have potential implications for the ZnS physical and chemical properties.

Quantum-Confined Tunable Ferromagnetism on the Surface of a Van der Waals Antiferromagnet NaCrTe2.

The surface of three-dimensional materials provides an ideal and versatile platform to explore quantum-confined physics. Here, we systematically investigate the electronic structure of Na-intercalated CrTe2, a van der Waals antiferromagnet, using angle-resolved photoemission spectroscopy and ab initio calculations. The measured band structure deviates from the calculation of bulk NaCrTe2 but agrees with that of ferromagnetic monolayer CrTe2. Consistently, we observe unexpected exchange splitting of the band dispersions, persisting well above the Néel temperature of bulk NaCrTe2. We argue that NaCrTe2 features a quantum-confined 2D ferromagnetic state in the topmost surface layer due to strong ferromagnetic correlation in the CrTe2 layer. Moreover, the exchange splitting and the critical temperature can be controlled by surface doping of alkali-metal atoms, suggesting the feasibility of tuning the surface ferromagnetism. Our work not only presents a simple platform for exploring tunable 2D ferromagnetism but also provides important insights into the quantum-confined low-dimensional magnetic states.

Spin and lattice dynamics of the two-dimensional van der Waals ferromagnet CrI3

Chromium tri-iodide (CrI3) is a prototypical ferromagnetic van der Waals insulator with its genuinely two-dimensional (2D) long-range magnetic order below 45 K demonstrated recently. The underlying magnetic anisotropy has not been completely understood while both the Dzyaloshinskii-Moriya (DM) interaction and the Kitaev−Γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-\\Gamma$$\\end{document} type interaction have been considered as the relevant magnetic Hamiltonian. In addition, the relation between the crystal structure and the magnetic order needs to be further elucidated concerning their possible coupling in few-layer samples and in the topmost surface layers of bulk samples. Here, we investigate these issues via temperature- and magnetic field-dependent terahertz spectroscopy on bulk CrI3 single crystals, focusing on the dynamics of ferromagnetic resonances (FMRs) and optical phonons in the terahertz (THz) region from 4 to 120 cm−1 (from 0.5 to 15 meV). We narrow down the possible ranges of the interaction parameters such as the off-diagonal symmetric terms and the single-ion anisotropy. The accurate values of these parameters significantly constrain the magnitude of possible Kitaev−Γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-\\Gamma$$\\end{document} exchange interaction and the topological magnon gap. Moreover, the structure-magnetism relationship was critically analyzed based on the temperature- and field-dependences of two Eu\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\\rm{E}}}_{{\\rm{u}}}$$\\end{document} in-plane optical phonon modes, which shows that the commonly believed structural phase diagram of CrI3, derived from surface-preferential data, has to be seriously modified.

Improvement of fatigue life of Ti-6Al-4V alloy treated by double-sided symmetric oblique laser shock peening

To improve the anti-foreign object damage (FOD) ability of Ti-6Al-4V alloy blisk blades, a blade simulation specimen was designed and subjected to double-sided symmetrical oblique laser shock peening (DSOLSP) treatment. Surface roughness, residual stress distribution, and microstructural evolution of simulated specimens of Ti-6Al-4V alloy without and with DSOLSP were investigated, and the strengthening effect was evaluated by flexural fatigue test. The results showed that after DSOLSP treatment, surface roughness of simulated specimens was reduced from Ra0.97 µm to Ra0.6 µm. DSOLSP treatment generated a compressive residual stress (CRS) field along the entire thickness direction of the simulated specimen. Nanostructures with a depth of ∼500 nm was observed at topmost surface layer, and the average size of the nanograins was about 35.2 nm. Compared with the base material, the flexural average fatigue life of the simulated specimen was increased by 8.45 times. Moreover, mathematical statistical analysis confirms that DSOLSP has a significant effect on the fatigue life of simulated samples. The development of CRS fields throughout the entire thickness direction, the grain refinement on the surface, and the reduction of surface roughness are primarily responsible for the enhancement of the flexural fatigue life of the DSOLSP simulated specimen.

Control of Biological Surface States on Chlorine-Doped Amorphous Silica Particles and Their Effective Absorptive Ability for Antibody Protein.

Amorphous silica particles (ASPs) have low biotoxicity and are used in foodstuffs; however, the adsorption states of proteins on their surfaces have not yet been clarified. If the adsorption states can be clarified and controlled, then a wide range of biological and medical applications can be expected. The conventional amorphous silica particles have the problem of protein adsorption due to the strong interaction with their dense silanol groups and denaturation. In this study, the surfaces of amorphous silica particles with a lower silanol group density were modified with a small amount of chlorine during the synthesis process to form a specific surface layer by adsorbing water molecules and ions in the biological fluid, thereby controlling the protein adsorption state. Specifically, the hydration state on the surface of the amorphous silica particles containing trace amounts of chlorine was evaluated, and the surface layer (especially the hydration state) for the adsorption of antibody proteins while maintaining their steric structures was evaluated and discussed. The results showed that the inclusion of trace amounts of chlorine increased the silanol groups and Si-Cl bonds in the topmost surface layer of the particles, thereby inducing the adsorption of ions and water molecules in the biological fluid. Then, it was found that a novel surface layer was formed by the effective adsorption of Na and phosphate ions, which would change the proportion of the components in the hydration layer. In particular, the proportion of the free water component increased by 21% with the doping of chlorine. Antibody proteins were effectively adsorbed on the particles doped with trace amounts of chlorine, and their steric adsorption states were evaluated. It was found that the proteins were clearly adsorbed and maintained the steric state of their secondary structure. In the immunoreactivity tests using streptavidin and biotin, biotin bound to the chlorine-doped particles showed efficient reactivity. In conclusion, this study is the first to discover the surface layer of the amorphous silica particles to maintain the steric structures of adsorbed proteins, which is expected to be used as a carrier particle for antibody test kits and immunochromatography.

Effect of surface contamination on electrochemical corrosion behavior of ultrasonic shot peened TA2 alloy

Ultrasonic shot peening (USSP) is a technique to tune the performance of metallic materials by generating gradient nanocrystalline structure (GNS) as well as residual compressive stress. However, surface contamination can also be introduced during USSP. The nature of this surface contamination layer and how it influences the corrosion behavior of alloy require further investigation. In this work, we found that a Fe-rich oxide layer with a thickness of around 50 nm, mainly in an amorphous state, was introduced on the topmost surface layer of TA2 alloy during USSP treatment. Electrochemical corrosion results indicate that this Fe-rich oxide significantly decreases the corrosion resistance of TA2 alloy. By polishing off this surface contamination layer, the corrosion resistance of the peened sample becomes superior compared to the untreated counterpart, indicating that the introduction of the Fe-rich oxide layer conceals the beneficial effect of GNS caused by USSP. Additionally, the corrosion resistance of TA2 alloy after USSP treatment exhibits a downward trend with the increasing repeated use of steel shots. The corrosion mechanism and potential use of this surface contamination layer are also briefly discussed.

Tailoring Hot-Carrier Distributions of Plasmonic Nanostructures through Surface Alloying.

Alloyed metal nanoparticles are a promising platform for plasmonically enabled hot-carrier generation, which can be used to drive photochemical reactions. Although the non-plasmonic component in these systems has been investigated for its potential to enhance catalytic activity, its capacity to affect the photochemical process favorably has been underexplored by comparison. Here, we study the impact of surface alloy species and concentration on hot-carrier generation in Ag nanoparticles. By first-principles simulations, we photoexcite the localized surface plasmon, allow it to dephase, and calculate spatially and energetically resolved hot-carrier distributions. We show that the presence of non-noble species in the topmost surface layer drastically enhances hot-hole generation at the surface at the expense of hot-hole generation in the bulk, due to the additional d-type states that are introduced to the surface. The energy of the generated holes can be tuned by choice of the alloyant, with systematic trends across the d-band block. Already low surface alloy concentrations have a large impact, with a saturation of the enhancement effect typically close to 75% of a monolayer. Hot-electron generation at the surface is hindered slightly by alloying, but here a judicious choice of the alloy composition allows one to strike a balance between hot electrons and holes. Our work underscores the promise of utilizing multicomponent nanoparticles to achieve enhanced control over plasmonic catalysis and provides guidelines for how hot-carrier distributions can be tailored by designing the electronic structure of the surface through alloying.

Ultrasonic Vibration assisted Silver Integration by Powder Mixed EDM for Antibacterial Surfaces

Powder mixed electrical discharge machining (PMEDM) with silver nano-powder has demonstrated its potential capability in antibacterial surface modification. Analyses in the cross-sectional layer showed that silver was mainly integrated near the topmost surface layer, and its content is reduced within the layer towards the substrate. However, to achieve a long-term antibacterial efficacy, silver needs to be deeply integrated in the modified layer.Previous studies indicated that ultrasonic vibration (UV) assisted PMEDM can homogenize the silver content in the deposited layer. In this study, the effect of UV on the silver distribution within the modified layer is investigated. Ti6Al4V workpieces were stimulated by UV with a frequency of 22.3 kHz and an amplitude of 2.5 µm. Hydrocarbon-based dielectric fluid, mixed with silver nano-powder with concentrations up to 15 g/L, were internally flushed through a hollow tool with a pressure of 4 bar. PMEDM without UV was investigated for comparison. The layer thickness, micro-cracks and the cross-sectional distribution of deposited silver were analysed by confocal microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Results indicate that applying UV leads to the highest silver content in the middle region of the layer, a significant decrease of micro-cracks and a slight reduction of the layer thickness.

Enhance corrosion resistance of 304 stainless steel using nanosecond pulsed laser surface processing

In this study, a novel strategy for nanosecond (ns) laser surface processing was designed to improve the corrosion resistance of commercial 304 stainless steel (304 SS). The influence of different ns-laser parameters on pitting corrosion resistance was systematically investigated. An optimal pitting potential of ∼887 mV, from the origin of 503 mV, was obtained at a laser power of 20 W and a beam size of 116 µm, which was superior to those treated by traditional laser methods. Based on the microstructure observations, it was found that the gradient structured 304 SS was produced by the remelting effect of ns-laser surface processing. Along the direction of laser irradiation, the modified microstructure was composed of ultrafine grains in the topmost surface layer, dislocation accumulations in the sublayer, and coarse grains with nanotwins in the matrix. Additionally, the mechanisms of improved corrosion resistance were also revealed. The experimental results demonstrated that the formation of gradient microstructure containing ultrafine grains was responsible for the exceptional corrosion resistance. It is expected that the advanced ns-laser surface processing will be applied on various metallic materials to meet the practical demands for high corrosion resistance across various industries.

Hydrogen Evolution Reaction on AuNi Surface Alloy Formed on Single Crystal Au Electrodes

AbstractThe hydrogen evolution reaction (HER) on Au with high chemical stability is activated by alloying it with Ni. We evaluated the HER activity of the AuNi surface alloy prepared at different alloying temperatures on single‐crystal Au electrodes in 0.05 M H2SO4. The potential cycle, including the region of Ni oxidative dissolution, further enhanced the HER activity by dealloying Ni, and the activity depended on the surface structure of the Au substrate in the following order: AuNi/Au(100)<AuNi/Au(111)<AuNi/Au(110). The surface structure and atomic composition were investigated using X‐ray photoelectron spectroscopy and surface X‐ray diffraction. Surface structural analysis revealed that the dissolution of Ni from the AuNi surface‐alloy layer resulted in a decrease in the atomic occupancy of the topmost surface layer. Ni dealloying creates low‐coordination Au sites on the AuNi surface alloy, which activates the HER.

Microstructure and Mechanical Properties of Gradient Nanostructured Q345 Steel Prepared by Ultrasonic Severe Surface Rolling

In this work, ultrasonic severe surface rolling (USSR), a new surface nanocrystallization technique, is used to prepare gradient nanostructure (GNS) on the commercial Q345 structural steel. The microstructure of the GNS surface layer is characterized by employing EBSD and TEM, and the result indicates that a nanoscale substructure is formed at the topmost surface layer. The substructures are composed of subgrains and dislocation cells and have an average size of 309.4 nm. The GNS surface layer after USSR processing for one pass has a thickness of approximately 300 μm. The uniaxial tensile measurement indicates that the yield strength of the USSR sample improves by 25.1% compared to the as-received sample with slightly decreased ductility. The nanoscale substructure, refined grains, high density of dislocations, and hetero-deformation-induced strengthening are identified as responsible for the enhanced strength. This study provides a feasible approach to improving the mechanical properties of structural steel for wide applications.

The effect of the structural parameters on the friction and wear properties of some FCC metals

Friction and wear are very complex thermo-mechano-chemical processes of contact interaction. For a better understanding of the mechanisms of plastic deformation affecting friction and wear, four face-centered cubic (FCC) metals (Ag, Cu, Ni, and Al) were studied using a pin-on-disk rig. The topmost surface layer presents nanocrystalline (NC) and ultrafine grain (UFG) structures with the transition to a sublayer with a lamellar structure parallel to the direction of friction. The thickness of the topmost layers is mainly determined by stacking fault energy (SFE), hydrostatic pressure, and shear stress. The thermo-mechanical control of plastic flow during the steady friction state was considered. The effective shear stress as the activation barrier, the strain rate sensitivity, and the contact temperature were calculated. The values of the strain rate sensitivity, m, indicate that NC and UFG surface layers for Ag, Cu, and Ni, exhibit low capacities for strain hardening. The friction of FCC metals in lubricated conditions is characterized by a switch from dislocation-mediated plasticity to grain boundary (GB) plasticity in the topmost surface layers. The similarity between friction with low and high coefficients of friction during dry and lubricated conditions was considered. The wear resistance of studied materials can be compared to strength improvements and easy shearing of the topmost surface layer under dry and lubricated conditions. Dislocation glide and grain growth play a key role in the hardening/softening of surface layers both in dry and lubricated conditions.

Preparing Thick Gradient Surface Layer in Cu-Zn Alloy via Ultrasonic Severe Surface Rolling for Strength-Ductility Balance.

A gradient structure (GS) design is a prominent strategy for strength-ductility balance in metallic materials, including Cu alloys. However, producing a thick GS surface layer without surface damage is still a challenging task limited by the available processing technology. In this work, a gradient structure (GS) surface layer with a thickness at the millimeter scale is produced in the Cu-38 wt.% Zn alloy using ultrasonic severe surface rolling technology at room temperature. The GS surface layer is as thick as 1.1 mm and involves the gradient distribution of grain size and dislocation density. The grain size is refined to 153.5 nm in the topmost surface layer and gradually increases with increasing depth. Tensile tests indicate that the single-sided USSR processed alloy exhibits balanced strength (467.5 MPa in yield strength) and ductility (10.7% in uniform elongation). Tailoring the volume fraction of the GS surface layer can tune the combination of strength and ductility in a certain range. The high strength of GS surface layer mainly stems from the high density of grain boundaries, dislocations and dislocation structures, deformation twins, and GS-induced synergistic strengthening effect. Our study elucidates the effect of the thick GS surface layer on strength and ductility, and provides a novel pathway for optimizing the strength-ductility combination of Cu alloys.

Hydrogen adsorption and diffusion on doped Zr(0001) surfaces: A first-principles study

The hydrogen adsorption and diffusion behaviors on the clean and a series of element doped Zr(0001) surfaces are studied through first-principles calculations. Among the studied doping elements, Cu, Co, Y, and Mg prefer to substitute Zr on the topmost surface layer, Al, Pd, Ir, and Si are favored from topmost layer to several surface layers down, while Mo is not favored. Independent of the substitution energies, Mo, Co, and Ir induce a symmetry-breaking local distortion surface structure. Based on the obtained geometries, it is found that most dopants promote the hydrogen adsorption on their next nearest neighbor sites but hinder it on the nearest neighbor sites. Most of the dopants also promote both the hydrogen diffusion on the surface plane and the hydrogen penetration into the subsurface layers. The results indicate that element doping may facilitate the hydride nucleation in Zr alloys.

Hardness and corrosion behavior of an Al-2Mn alloy with both microstructural and chemical gradients

An Al–2Mn binary alloy with gradient microstructure and chemistry near its surface was fabricated by combining surface mechanical treatment and post-ageing treatment. TEM results indicate that the minimum grain size of the topmost surface layer is below 100 nm. As revealed by SIMS results, Mn is depleted in the surface layer with ~2 μm in thickness, which is due to the “short-circuit” diffusion along grain boundaries and dislocation pipes. Microhardness and corrosion testing results revealed that both hardness and corrosion resistance increase substantially with this gradient design. XPS and Mott–Schottky results demonstrate that the oxide film of the gradient Al–Mn alloy is thinner and denser than that of the coarse-grained sample. Our design method of obtaining gradient distribution both in microstructure and chemistry near metal surface lights a pathway for overcoming the trade-off between properties such as strength and corrosion in 3000 series Al alloys.

3D characterization of a nanostructured Al-Cu-Mg alloy

Three-dimensional (3D) characterization of variations in crystallography and chemistry of nanostructured metals will provide vital information to understand their mechanical and thermal behaviours. This study applied a surface sliding friction treatment (SSFT) at liquid nitrogen temperature to produce nanostructured surface layers in a peak-aged Al-Cu-Mg alloy. The nanostructured surface was characterized by means of 3D orientation mapping in the transmission electron microscope (3D-OMiTEM) and atom probe tomography (APT). 3D-OMiTEM results revealed a lamellar structure with an average lamellar boundary spacing of 26 nm at the topmost surface layer (depth < 20 μm), which is much finer than normally achievable in commercial purity Al deformed to high strain levels. Based on the 3D-OMiTEM data, a five-parameter grain boundary character analysis was carried out. It was found that low angle grain boundaries dominate the nanoscale structure and that the grain boundary plane distribution of high angle lamellar grain boundaries shows a preference around {101}. APT analysis showed segregation of Cu and Mg atoms at lamellar boundaries, which is believed to play a role in stabilizing the boundaries and enhancing the structural refinement during SSFT.

Laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy with significantly enhanced wear resistance

• Gradient nanostructured layer on TiZrHfTaNb refractory high-entropy alloy was fabricated by laser surface remelting technique. • The average grain size in gradient nanostructured layer is refined from ∼ 200 µm in matrix to only ∼ 8 nm in the top surface. • The microhardness is increased from ∼ 240 HV in matrix to ∼ 650 HV in the top surface layer. • The decomposition-mediated gradient refinement mechanisms were revealed mainly by HRTEM. • The gradient nanostructure shows significantly enhanced wear resistance, reducing the wear rate with an order of magnitude. Heterogeneous gradient nanostructured metals have been shown to achieve the strength-ductility synergy, thus potentially possessing the enhanced tribological performance in comparison with their homogeneous nanograined counterparts. In this work, a facile laser surface remelting-based surface treatment technique is developed to fabricate a gradient nanostructured layer on a TiZrHfTaNb refractory high-entropy alloy. The characterization of the microstructural evolution along the depth direction from the matrix to the topmost surface layer shows that the average grain size in the ∼100 µm-thick gradient nanostructured layer is dramatically refined from the original ∼200 µm to only ∼8 nm in the top surface layer. The microhardness is therefore gradually increased from ∼240 HV in matrix to ∼650 HV in the topmost surface layer, approximately 2.7 times. Noticeably, the original coarse-grained single-phase body-centered-cubic TiZrHfTaNb refractory high-entropy alloy is gradually decomposed into TiNb-rich body-centered-cubic phase, TaNb-rich body-centered-cubic phase, ZrHf-rich hexagonal-close-packed phase and TiZrHf-rich face-centered-cubic phase with gradient distribution in grain size along the depth direction during the gradient refinement process. As a result, the novel laser surface treatment-introduced gradient nanostructured TiZrHfTaNb refractory high-entropy alloy demonstrates the significantly improved wear resistance, with the wear rate reducing markedly by an order of magnitude, as compared with the as-cast one. The decomposed multi-phases and gradient nanostructures should account for the enhanced wear resistance. Our findings provide new insights into the refinement mechanisms of the laser-treated refractory high-entropy alloys and broaden their potential applications via heterogeneous gradient nanostructure engineering.

Characterization of vein-like structures formed in nitrided layers during plasma nitriding of 8Cr4Mo4V steel

In this study, nitrided 8Cr4Mo4V was characterized, using optical microscopy, scanning electron microscopy, transmission electron microscopy (TEM), atom probe tomography (APT), electron probe microanalysis, and Thermo-Calc software, to study the vein-like structures formed during plasma nitriding. The structures existed in a certain zone in the nitrided layers, namely the posterior end of the N-content plateau and the anterior end of the N-content rapid-decline zones. The structures disappeared in the topmost surface layer with increasing nitriding time. For the first time, APT was used to observe these structures. Together, the APT and TEM images revealed that the main components of the structures were Fe3C and alloy nitrides. The structures and matrix were related through the (220) Fe3C//(110)α-Fe, [110] Fe3C//[001]α-Fe orientations. The appearance and disappearance of the structures were closely related to the C and N contents. In the alloy carbides, N replaced C to form alloy carbonitrides and released C and Fe to form Fe3C which appeared as the vein-like structures. In the topmost surface of the nitrided layers, the C content decreased because of decarburization. The Fe3C and alloy carbides were transformed into ferrite and alloy nitrides, and the structures disappeared.

Formation of nanostructured surface layer, the white layer, through solid particles impingement during slurry erosion in a martensitic medium-carbon steel

The extremely altered topmost surface layer, known as the white layer, formed in a medium-carbon low-alloy steel as result of impacts by angular 10–12 mm granite particles during the slurry erosion process is comprehensively investigated. For this purpose, the characteristics, morphology, and formation mechanism of this white layer are described based on the microstructural observations using optical, scanning and transmission electron microscopies as well as nanoindentation hardness measurements and modelling of surface deformation. The white layer exhibits a nanocrystalline structure consisting of ultrafine grains with an average size of 200 nm. It has a nanohardness level of around 10.1 GPa, considerably higher than that of untempered martensitic bulk material (5.7 GPa) achieved by an induction hardening treatment. The results showed that during the high-speed slurry erosion process, solid particle impacts brought forth conditions of high strain, high strain rate, and multi-directional strain paths. This promoted formation of a cell-type structure at first and later, after increasing the number of impacts, development of subgrains following by subgrain rotation and eventually formation of a nanocrystalline structure with ultra-high hardness. The model confirmed that high strain conditions - much higher than required for the onset of plastic deformation - can be achieved on the surface resulting in severe microstructural and property changes during the slurry erosion test.

Metallic Transport in Monolayer and Multilayer Molybdenum Disulfides by Molecular Surface Charge Transfer Doping.

Carrier modulation in transition-metal dichalcogenides (TMDCs) is of importance for applying electronic devices to tune their transport properties and controlling phases, including metallic to superconductivity. Although the surface charge transfer doping method has shown a strong modulation ability of the electronic structures in TMDCs and a degenerately doped state has been proposed, the details of the electronic states have not been elucidated, and this transport behavior should show a considerable thickness dependence in TMDCs. In this study, we characterize the metallic transport behavior in the monolayer and multilayer MoS2 under surface charge transfer doping with a strong electron dopant, benzyl viologen (BV) molecules. The metallic behavior transforms to an insulative state under a negative gate voltage. Consequently, metal-insulator transition (MIT) was observed in both monolayer and multilayer MoS2 correlating with the critical conductivity of order e2/h. In the multilayer case, the BV molecules strongly modulated the topmost surface layer in the bulk MoS2; the transfer characteristics suggested a crossover from a heterogeneously doped state with a doped topmost layer to doping in the deep layers caused by the variation in the gate voltage. The findings of this work will be useful for understanding the device characteristics of thin-layered materials and for applying them to the controlling phases via carrier modulation.