Mo Adatoms Research Articles

Detailed study on MOCVD of wafer-scale MoS2 monolayers: From nucleation to coalescence

Metal–organic chemical vapour deposition (MOCVD) has become one of the most promising techniques for the large-scale fabrication of 2D transition metal dichalcogenide (TMDC) materials. Despite efforts devoted to the development of MOCVD for TMDC monolayers, the whole picture of the growth process has not been fully unveiled yet. In this work, we employ a commercial AIXTRON CCS MOCVD tool for the deposition of MoS2 on sapphire using standard precursors and H2 as carrier gas. Adsorption and diffusion of Mo adatoms on the substrate are found to be decisive for nucleation. By lowering temperature from 650 to 450 °C, a uniform distribution of nuclei on sapphire terraces is achieved. Full coalescence of MoS2 monolayers with limited bilayer formation (~ 15%) is then realized at 700 °C. This study highlights the importance of understanding the details of film formation mechanisms and developing multi-stage MOCVD processes for 2D TMDC films.Graphical abstract

The effect of grain boundary in hexagonal boron nitride on catalytic activity of nitrogen reduction reaction

Designing high-efficiency electrocatalysts is of great significance to produce ammonia via electroreduction reaction of nitrogen (NRR). To this end, grain boundary catalysis (GBC) has emerged as a promising strategy experimentally. Herein, based on first principles calculations, we modeled the grain boundary (GB) structures of hexagonal boron nitride (h-BN), then 12 transition metal atoms (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Y, Zr, Mo, Ru, and Rh) were anchored on the GB structures to construct single-atom catalysts, the most ideal overpotential of NRR is as low as 0.13 V for Mo adatoms on GB. Additionally, more sophisticated situations in regard to h-BN GB were considered, meanwhile electronic structure analysis was also conducted. We found that, the configuration and density of GB can hardly influence the catalytic activity at GB, while the coordinated B bonded with Mo strongly modified the spin states and the bonding strengths within certain intermediates, leading to this excellent NRR catalytic activity. This work not only provides a new strategy for the development of highly efficient and stable NRR electrocatalysts, but also enhances the understanding on the origin of GBC.

Influence of thickness on physical and optoelectronic properties of DC sputtered bilayer molybdenum thin films for CIGS solar cell

For rear ohmic contact of CIGS solar cell, bilayer Mo thin film on 2 sq- inch glass substrate was deposited using DC sputtering technique. Thickness of bilayer Mo thin film was 300 nm, 500 nm, 1000 nm, and substrate temperature were maintained constant at 215 °C and room temperature. From XRD, FESEM, optical and electrical analysis it was found that Mo bilayer with low thickness (300 nm) have low optical reflectivity 35% and high sheet resistance 2.03 Ω/□ compared to Mo bilayer having high thickness (500 nm and 1000 nm) i.e. about 72% and 0.52 Ω/□. XRD analysis reveals that 300 nm have low crystallite size 6.62 nm while Mo 1000 nm have 12.24 nm. Micro-structure of Mo 500 has closed dense pack dendric structure while 300 nm has columnar growth uneven in size with small circular grains. Slight wide columns appeared for 1000 nm due to interdiffusion of Mo adatoms deposited at pressure 1 mTorr onto 10 mTorr forming homogenous structure.

Mo Concentration Controls the Morphological Transitions from Dendritic to Semicompact, and to Compact Growth of Monolayer Crystalline MoS2 on Various Substrates.

The domain morphology in the growth of transition-metal dichalcogenides (TMDCs) is mostly triangular but rarely dendritic. Here, we report a robust chemical vapor deposition method to fabricate atomic-thin 2H-phase MoS2 dendrites on several single-crystalline substrates with different lattice structures, such as rutile-TiO2(001), SrTiO3(001), and sapphire(0001). It is found that by tuning the concentration of Mo adatoms, the morphology of MoS2 domains on these substrates evolves from tridentate dendrites at a low Mo concentration to semicompact fractal domains at an intermediate Mo concentration, and to a compact triangular shape at a high Mo concentration. First-principles calculations reveal that the edge diffusion barrier of Mo is comparable to the attachment barrier, inhibiting fast Mo atom diffusion along the edge. Kinetics Monte Carlo simulations with varying Mo concentrations well reproduce the experimental results. Our combined experimental and theoretical analyses evidently show that the growth of MoS2 dendritic domains at a low Mo concentration is a nonequilibrium process, which is dominated by the kinetics of Mo adatoms. Our study presents an effective route to control the morphology of TMDCs by simply tuning the transition-metal adatom concentration.

Controlling Near-Surface Ni Composition in Octahedral PtNi(Mo) Nanoparticles by Mo Doping for a Highly Active Oxygen Reduction Reaction Catalyst.

We report and study the translation of exceptionally high catalytic oxygen electroreduction activities of molybdenum-doped octahedrally shaped PtNi(Mo) nanoparticles from conventional thin-film rotating disk electrode screenings (3.43 ± 0.35 A mgPt-1 at 0.9 VRHE) to membrane electrode assembly (MEA)-based single fuel cell tests with sustained Pt mass activities of 0.45 A mgPt-1 at 0.9 Vcell, one of the highest ever reported performances for advanced shaped Pt alloys in real devices. Scanning transmission electron microscopy with energy dispersive X-ray analysis (STEM-EDX) reveals that Mo preferentially occupies the Pt-rich edges and vertices of the element-anisotropic octahedral PtNi particles. Furthermore, by combining in situ wide-angle X-ray spectroscopy, X-ray fluorescence, and STEM-EDX elemental mapping with electrochemical measurements, we finally succeeded to realize high Ni retention in activated PtNiMo nanoparticles even after prolonged potential-cycling stability tests. Stability losses at the anodic potential limits were mainly attributed to the loss of the octahedral particle shape. Extending the anodic potential limits of the tests to the Pt oxidation region induced detectable Ni losses and structural changes. Our study shows on an atomic level how Mo adatoms on the surface impact the Ni surface composition, which, in turn, gives rise to the exceptionally high experimental catalytic ORR reactivity and calls for strategies on how to preserve this particular surface composition to arrive at performance stabilities comparable with state-of-the-art spherical dealloyed Pt core-shell catalysts.

Electron-Beam-Induced Synthesis of Hexagonal 1 H-MoSe2 from Square β-FeSe Decorated with Mo Adatoms.

Two-dimensional (2D) materials have generated interest in the scientific community because of the advanced electronic applications they might offer. Powerful electron beam microscopes have been used not only to evaluate the structures of these materials but also to manipulate them by forming vacancies, nanofragments, and nanowires or joining nanoislands together. In this work, we show that the electron beam in a scanning transmission electron microscope (STEM) can be used in yet another way: to mediate the synthesis of 2D 1 H-MoSe2 from Mo-decorated 2D β-FeSe and simultaneously image the process on the atomic scale. This is quite remarkable given the different crystal structures of the reactant (square lattice β-FeSe) and the product (hexagonal lattice 1 H-MoSe2). The feasibility of the transformation was first explored by theoretical calculations that predicted that the reaction is exothermic. Furthermore, a theoretical reaction path to forming a stable 1 H-MoSe2 nucleation kernel within pure β-FeSe was found, demonstrating that the pertinent energy barriers are smaller than the energy supplied by the STEM electron beam.

Point defects in the 1T′ and 2H phases of single-layer MoS2 : A comparative first-principles study

The metastable $1{T}^{\ensuremath{'}}$ phase of layered transition metal dichalcogenides has recently attracted considerable interest due to electronic properties, possible topological phases, and catalytic activity. We report a comprehensive theoretical investigation of intrinsic point defects in the $1{T}^{\ensuremath{'}}$ crystalline phase of single-layer molybdenum disulfide ($1{T}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$) and provide comparison to the well-studied semiconducting $2H$ phase. Based on density functional theory calculations, we explore a large number of configurations of vacancy, adatom, and antisite defects and analyze their atomic structure, thermodynamic stability, and electronic and magnetic properties. The emerging picture suggests that, under thermodynamic equilibrium, $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ is more prone to hosting lattice imperfections than the $2H$ phase. More specifically, our findings reveal that the S atoms that are closer to the Mo atomic plane are the most reactive sites. Similarly to the $2H$ phase, S vacancies and adatoms in $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ are very likely to occur while Mo adatoms and antisites induce local magnetic moments. Contrary to the $2H$ phase, Mo vacancies in $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ are expected to be an abundant defect due to the structural relaxation that plays a major role in lowering the defect formation energy. Overall, our study predicts that the realization of high-quality flakes of $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ should be carried out under very careful laboratory conditions but at the same time the facile defects introduction can be exploited to tailor physical and chemical properties of this polymorph.

Direct Imaging of Kinetic Pathways of Atomic Diffusion in Monolayer Molybdenum Disulfide.

Direct observation of atomic migration both on and below surfaces is a long-standing but important challenge in materials science as diffusion is one of the most elementary processes essential to many vital material behaviors. Probing the kinetic pathways, including metastable or even transition states involved down to atomic scale, holds the key to the underlying physical mechanisms. Here, we applied aberration-corrected transmission electron microscopy (TEM) to demonstrate direct atomic-scale imaging and quasi-real-time tracking of diffusion of Mo adatoms and vacancies in monolayer MoS2, an important two-dimensional transition metal dichalcogenide (TMD) system. Preferred kinetic pathways and the migration potential-energy landscape are determined experimentally and confirmed theoretically. The resulting three-dimensional knowledge of the atomic configuration evolution reveals the different microscopic mechanisms responsible for the contrasting intrinsic diffusion rates for Mo adatoms and vacancies. The new insight will benefit our understanding of material processes such as phase transformation and heterogeneous catalysis.

Comprehensive structural and optical characterization of MBE grown MoSe2 on graphite, CaF2 and graphene

We report the structural and optical properties of a molecular beam epitaxy (MBE) grown 2-dimensional (2D) material molybdenum diselenide (MoSe2) on graphite, CaF2 and epitaxial graphene. Extensive characterizations reveal that 2H–MoSe2 grows by van-der-Waals epitaxy on all three substrates with a preferred crystallographic orientation and a Mo:Se ratio of 1:2. Photoluminescence at room temperature (∼1.56 eV) is observed in monolayer MoSe2 on both CaF2 and epitaxial graphene. The band edge absorption is very sharp, <60 meV over three decades. Overcoming the observed small grains by promoting mobility of Mo adatoms would make MBE a powerful technique to achieve high quality 2D materials and heterostructures.

Metal desorption from Fe(1 1 0) and its alloyed surfaces

We have performed first-principles calculations to study the surface metal–metal bonding and energetics that control desorption on metal and alloy surfaces. The adsorbed and desorbed states of Fe, Cr, and Mo adatoms on the Fe(110) surface were examined. Their formation and extraction were investigated both as neutral particles and as metal complexes. In addition, the roles which alloying elements play on adatom-surface binding is also a critical question that needs to be addressed. The alloying effect was found to relate to the nature of charge transfer between alloying elements and host metals.

Molybdenum Clusters on a TiO2(110) Substrate Studied by Density Functional Theory

A theoretical study on molybdenum clusters adsorbed on a rutile TiO 2 (110) substrate is reported. Using density functional theory, equilibrium geometries, atomic charges, and total energies have been calculated for clusters containing up to five Mo atoms. Isolated Mo adatoms are strongly oxidized and repel each other. The Mo oxidation state is considerably lowered as soon as the first short Mo-Mo bond is formed. The relative stability of different cluster geometries can be understood from the competition between Mo-Mo and Mo-O bonding. Some low-energy structures for two and three Mo atoms involve large displacements of a substrate oxygen atom. The most stable five-atom cluster, a square-based pyramid, is identified as the nucleus of epitaxial growth of body-centered cubic molybdenum films on TiO 2 (110).

Photon-induced chemical vapor deposition of molybdenum on Pt(111)

Abstract Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) have been employed to study the adsorption and photon-induced decomposition of Mo(CO) 6 . Mo(CO) 6 adsorbs molecularly on a Pt(1 1 1) surface with weak interaction at 100 K and desorbs intact at 210 K without undergoing thermal decomposition. Adsorbed Mo(CO) 6 undergoes decarbonylation to form surface Mo(CO) x ( x ⩽ 5) under irradiation of ultraviolet light. The Mo(CO) x species can release further CO ligands to form Mo adatoms with CO desorption at 285 K. In addition, a fraction of the released CO ligands transfers onto the Pt surface and subsequently desorbs at 350–550 K. The resulting Mo layer deposited on the Pt surface is nearly free of contamination by C and O. The deposited Mo adatoms can diffuse into the bulk Pt at temperatures above 1070 K.

Structural details of copper seed cones revealed by high-resolution electron microscopy

High-resolution transmission electron microscopy revealed that Mo seed layers deposited on obliquely evolving Cu cones were crystallized epitaxially, even at room temperature. At the interface, Mo {110} planes smoothly junctioned with Cu {111} planes, presenting a monocrystal-like structure extending over the Mo and Cu domains. Impacting ions presumably induced the diffusion of Mo adatoms, thereby transporting the adatoms toward the more energetically favorable lattice sites. Sputtering thus involves a diffusional process akin to that involved in conventional vapor-phase crystal growth.