Global Optimization of Molybdenum Subnanoclusters on Graphene: a Consistent Approach Towards Catalytic Applications
Yao Wei, Alejandro Santana-Bonilla, Lev Kantorovich
备注：
39 pages, 8 figures, due to the limitation "The abstract field cannot be longer than 1,920 characters", the abstract appearing here is slightly shorter than that in the PDF file
The development of novel sub-nanometer clusters (SNCs) catalysts with superior catalytic performance depends on the precise control of clusters' atomistic sizes, shapes, and accurate deposition onto surfaces. The intrinsic complexity of the adsorption process complicates the ability to achieve an atomistic understanding of the most relevant structure-reactivity relationships hampering the rational design of novel catalytic materials. In most cases, existing computational approaches rely on just a few structures to draw conclusions on clusters' reactivity thereby neglecting the complexity of the existing energy landscapes thus leading to insufficient sampling and, most likely, unreliable predictions. Moreover, modelling of the actual experimental procedure that is responsible for the deposition of SNCs on surfaces is often not done even though in some cases this procedure may enhance the significance of certain (e.g., metastable) adsorption geometries. This study proposes a novel systematic approach that utilizes global search techniques, specifically, the particle swarm optimization (PSO) method, in conjunction with \textit{ab-initio} calculations, to simulate {\it all stages} in the beam experiments, from predicting the most relevant SNCs structures in the beam and on a surface, to their reactivity. To illustrate the main steps of our approach, we consider the deposition of Molybdenum SNC of 6 Mo atoms on a free-standing graphene surface, as well as their catalytic properties with respect to the CO molecule dissociation reaction. Even though our calculations are not exhaustive and serve only to produce an illustration of the method, they are still able to provide insight into the complicated energy landscape of Mo SNCs on graphene demonstrating the catalytic activity of Mo SNCs and the importance of performing statistical sampling of available configurations...
Direct growth of flat micrometer-sized bilayer graphene islands in between molybdenum disulfide sheets is achieved by chemical vapor deposition of ethylene at about 800 °C. The temperature assisted decomposition of ethylene takes place mainly at molybdenum disulfide step edges. The carbon atoms intercalate at this high temperature, and during the deposition process, through defects of the molybdenum disulfide surface such as steps and wrinkles. Post growth atomic force microscopy images reveal that circular flat graphene islands have grown at a high yield. They consist of two graphene layers s…
Molybdenum carbide (Mo2C) nanoparticles and thin films are particularly suitable catalysts for catalytic fast pyrolysis (CFP) as they are effective for deoxygenation and can catalyze certain reactions that typically occur on noble metals. Oxygen deposited during deoxygenation reactions may alter the carbide structure leading to the formation of oxycarbides, which can determine changes in catalytic activity or selectivity. Despite emerging spectroscopic evidence of bulk oxycarbides, so far there have been no reports of their precise atomic structure or their relative stability with respect to o…
Edge structures are highly relevant to the electronic, magnetic and catalytic properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) and their one dimensional (1D) counterpart, i.e., nanoribbons, which should be precisely tailored for the desirable applications. In this work, we report the formation of novel Mo6S6 nanowire (NW) terminated edges in a monolayer molybdenum disulfide (MoS2) via an e-beam irradiation process combined with high temperature heating in a scanning transmission electron microscope (STEM). Atomic structures of NW terminated edges and the dynamic forma…
We show the merits of plasma enhanced atomic layer deposition (PEALD) of catalytic substrate for chemical vapour deposition (CVD) graphene growth. The high quality multilayer graphene (MLG) on molybdenum carbide ($MoC_{x}$) thin film exhibits excellent uniformity and layer homogeneity over a large area. Moreover, we demonstrate how to achieve control of graphene layers thickness and properties, by varying the specific catalytic film chemical and physical properties. The control of growth is not digital, but is broad ranged from few layer graphene to a graphitic film of $\sim{75}$ graphene laye…
In most envisioned applications, full utilization of a graphene-carbon nanotube (CNT) construct requires maintaining the integrity of the graphene layer during the CNT growth step. In this work, we exhibit an approach towards controlled CNT growth atop graphene substrates where the reaction equilibrium between source hydrocarbon decomposition and carbon saturation into and precipitation from the catalyst nanoparticles shifts towards CNT growth rather than graphene consumption. By utilizing C2H4 feedstock, we demonstrate that the low temperature growth permissible with this gas suppresses undes…
Two-dimensional molybdenum disulphide on graphene grown by chemical vapour deposition is a promising van der Waals system for applications in optoelectronics and catalysis. To extend the fundamental understanding of growth and intrinsic properties of molybdenum disulphide on graphene, molybdenum disulphide on highly oriented pyrolytic graphite is a suitable model system. Here we show, experimentally and by density-functional-theory calculations, that molybdenum disulphide flakes grow in two orientations. One of the orientations is energetically preferred, the other one is rotated by 30 degree.…
The development of wafer-scale continuous single-crystal graphene layers is key in view of its prospective applications. To this end, here we pave the way for a graphene growth model in the framework of the Langmuir adsorption theory and two dimensional crystallization. In specific, we model the nucleation and growth of graphene on copper using methane as carbon precursor. The model leads to identification of the range of growth parameters (temperature and gas pressures) that uniquely entails the final surface coverage of graphene. This becomes an invaluable tool to address the fundamental pro…
Heterostructures made by stacking different materials on top of each other are expected to exhibit unusual properties and new phenomena. Interface of the heterostructures plays a vital role in determining their properties. Here, we report the observation of a two-dimensional (2D) spinodal interface in graphene-molybdenum carbide (α-Mo2C) heterostructures, which arises from spinodal decomposition occurring at the heterointerface, by using scanning tunneling microscopy. Our experiment demonstrates that the 2D spinodal interface modulates graphene into whispering gallery resonant networks filled …
We report electron-beam activated motion of a catalytic nanoparticle along a graphene step edge and associated etching of the edge. This approach enables beam-controlled etching of matter through activated electrocatalytic processes. The applications of electron-beam control as a paradigm for molecular-scale robotics are discussed.
Currently, the fundamental factors that control the oxygen reduction reaction (ORR) activity of graphene itself, in particular the dependence of the ORR activity on the number of exposed edge sites remain elusive, mainly due to limited synthesis routes of achieving small size graphene. In this work, the synthesis of low oxygen content (< 2.5 +/-0.2 at %), few layer graphene nanosheets with lateral dimensions smaller than a few hundred nm was achieved using a combination of ionic liquid assisted grinding of high purity graphite coupled with sequential centrifugation. We show for the first time,…
A novel route for the synthesis of large area graphene by an indigenously built electron cyclotron resonance (ECR) plasma enhanced chemical vapor deposition (PECVD) setup has been reported in this work. A unique 2.45 GHz permanent magnet type ECR PECVD system has been designed and developed for the growth of various nano-structures and films. The apparatus has a unique advantage to synthesize large area graphene at much lower additional substrate heating than heat supplied by an external oven in conventional thermal CVD method. A major amount of heat is supplied here by ECR plasma. Polycrystal…
Multilayer graphene (MLG) films were grown by chemical vapour deposition (CVD) on molybdenum carbide ($MoC_{x}$) substrates. We fabricated the catalytic $MoC_{x}$ films by plasma enhanced atomic layer deposition (PEALD). The mechanism of graphene growth is studied and analysed for amorphous and crystalline $MoC_{x}$ films. In addition, the unique advantages of catalytic substrate PEALD are demonstrated in two approaches to graphene device fabrication. First, we present a complete bottom up, resist-free patterned graphene growth (GG) on pre-patterned $MoC_{x}$ PEALD performed at 50$^{\circ}C$. …
We derive, from an empirical interaction potential, an analytic formula for the elastic bending modulus of single-layer MoS2 (SLMoS2). By using this approach, we do not need to define or estimate a thickness value for SLMoS2, which is important due to the substantial controversy in defining this value for two-dimensional or ultrathin nanostructures such as graphene and nanotubes. The obtained elastic bending modulus of 9.61 eV in SLMoS2 is significantly higher than the bending modulus of 1.4 eV in graphene, and is found to be within the range of values that are obtained using thin shell theory…
Graphene is, in principle, a promising material for consideration as component (support, active site) of electrocatalytic materials, particularly with respect to reduction of oxygen, an electrode reaction of importance to low-temperature fuel cell technology. Different concepts of utilization, including nanostructuring, doping, admixing, preconditioning, modification or functionalization of various graphene-based systems for catalytic electroreduction of oxygen are elucidated, as well as important strategies to enhance the systems' overall activity and stability are discussed.
The growth of single and multilayer graphene nano-flakes on MgO and ZrO2 at low temperatures is shown through transmission electron microscopy. The graphene nano-flakes are ubiquitously anchored at step edges on MgO (100) surfaces. Density functional theory investigations on MgO (100) indicate C2H2 decomposition and carbon adsorption at step-edges. Hence, both the experimental and theoretical data highlight the importance of step sites for graphene growth on MgO.
Single atom catalysts (SACs) present the ultimate level of catalyst utilization, which puts them in the focus of current research. For this reason, their understanding is crucial for the development of new efficient catalytic systems. Using Density Functional Theory calculations, model SACs consisted of nine metals (Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt and Au) on four different supports (pristine graphene, N- and B-doped graphene and graphene with single vacancy) were analyzed. Among them, only graphene with a single vacancy enables the formation of SACs, which are stable in terms of aggregation and…
Large-area ($\sim$cm$^2$) films of vertical heterostructures formed by alternating graphene and transition-metal dichalcogenide(TMD) alloys are obtained by wet chemical routes followed by a thermal treatment at low temperature (300 $^\circ$C). In particular, we synthesized stacked graphene and W$_x$Mo$_{1-x}$S$_2$ alloy phases that were used as hydrogen evolution catalysts. We observed a Tafel slope of 38.7 mV dec$^{-1}$ and 96 mV onset potential (at current density of 10 mA cm$^{-2}$) when the heterostructure alloy is annealed at 300 $^o$C. These results indicate that heterostructure formed b…
Understanding the nature and hierarchy of on surface reactions is a major chal- lenge for designing coordination and covalent nanostructures by means of multistep synthetic routes. In particular, intermediates and final products are hard to predict since reaction paths and their activation windows depend on the choice of both the molecular precursor design and the substrate. Here we report a systematic study of the effect of the catalytic metal surface to reveal how a single precursor can give rise to very distinct polymers that range from coordination and covalent non planar polymer chains of…
We study by high-resolution transmission electron microscopy the structural response of bilayer graphene to electron irradiation with energies below the knock-on damage threshold of graphene. We observe that one type of divacancy, which we refer to as the butterfly defect, is formed for radiation energies and doses for which no vacancies are formed in clean monolayer graphene. By using first principles calculations based on density-functional theory, we analyze two possible causes related with the presence of a second layer that could explain the observed phenomenon: an increase of the defect …
Pristine graphene is thought lack of catalytic activity up to date, although using graphene-plus-heteroatom materials as catalysts has become a subject of intensive research because it can be metal saving, eco-friendly and ultimately sustainable. Here we report observations of catalytic reactions of high-quality, clean, pristine graphene when immersed into organics containing C=O bonds, like acetone, acetic acid and acetaldehyde. The C=O bonds were found to break and form polymers including polyethers. The reaction rate is highly temperature dependent. The reaction products mainly physically a…
Direct growth of large-area vertically stacked two-dimensional (2D) van der Waal (vdW) materials is a prerequisite for their high-end applications in integrated electronics, optoelectronics and photovoltaics. Currently, centimetre- to even metre-scale monolayers of single-crystal graphene (MLG) and hexagonal boron nitride (h-BN) have been achieved by epitaxial growth on various single-crystalline substrates. However, in principle, this success in monolayer epitaxy seems extremely difficult to be replicated to bi- or few-layer growth, as the full coverage of the first layer was believed to term…
Chemical vapour deposition (CVD) is a promising method for producing large-scale graphene (Gr). Nevertheless, microscopic inhomogeneity of Gr grown on traditional metal substrates such as copper or nickel results in a spatial variation of Gr properties due to long wrinkles formed when the metal substrate shrinks during the cooling part of the production cycle. Recently, molybdenum (Mo) has emerged as an alternative substrate for CVD growth of Gr, mainly due to a better matching of the thermal expansion coefficient of the substrate and Gr. We investigate the quality of multilayer Gr grown on Mo…
Single layered molybdenum disulfide with a direct bandgap is a promising two-dimensional material that goes beyond graphene for next generation nanoelectronics. Here, we report the controlled vapor phase synthesis of molybdenum disulfide atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers. Furthermore, a nucleation-controlled strategy is established to systematically promote the formation of large-area single- and few-layered films. The atomic structure and morphology of the grains and their boundaries in th…
Graphite is one of the most chemically inert materials. Its elementary constituent, monolayer graphene, is generally expected to inherit most of the parent material's properties including chemical inertness. Here we show that, unlike graphite, defect-free monolayer graphene exhibits a strong activity with respect to splitting molecular hydrogen, which is comparable to that of metallic and other known catalysts for this reaction. We attribute the unexpected catalytic activity to surface corrugations (nanoscale ripples), a conclusion supported by theory. Nanoripples are likely to play a role in …
Exploring an upper limit of magnetic anisotropy in two-dimensional materials, such as graphene and transition metal dichalcogenides, is at the heart of spintronics research. Herein, an atomic-scale perpendicular magnetic anisotropy up to an order of 100 meV per atom, which is far beyond the ordinarily obtained value in graphene and pristine transition metal dichalcogenides, is demonstrated in individual ruthenium and osmium adatoms at a monosulfur vacancy in molybdenum disulfide. We further propose a phenomenological model where a spin state transition that involves hybridization between molyb…
While freestanding clean graphene is essential for various applications, existing technologies for removing the polymer layer after transfer of graphene to the desired substrate still leave significant contaminations behind. We discovered a method for preparing ultra-clean freestanding graphene utilizing the catalytic properties of platinum metals. Complete catalytic removal of polymer residues requires annealing in air at a temperature between 175 and 350°C. Low-energy electron holography investigations prove that this method results in ultra-clean freestanding graphene.
Hybrid systems composed of the reduced graphene oxide-supported platinum and multiwall carbon nanotubes-supported iridium (both noble metals utilized at low loadings on the level of 15 and < 5 microg cm-2, respectively) have been considered as catalytic materials for the reduction of oxygen in acid media (0.5 mol dm-3 H2SO4). The electrocatalytic activity toward reduction of oxygen and formation of hydrogen peroxide intermediate have been tested using rotating ring-disk electrode voltammetric experiments. The efficiency of the proposed catalytic systems has also been addressed by performing ga…
Previous studies indicate that the properties of graphene oxide (GO) can be significantly improved by enhancing its graphitic domain size through thermal diffusion and clustering of functional groups. Remarkably, this transition takes place below the decomposition temperature of the functional groups and thus allows fine-tuning of graphitic domains without compromising with the functionality of GO. By studying the transformation of GO under mild thermal treatment, we directly observe this size enhancement of graphitic domains from originally 40 nm2 to 200 nm2 through an extensive transmission …
We present a transfer-free preparation method for graphene on hexagonal boron nitride (h-BN) crystals by chemical vapor deposition of graphene via a catalytic proximity effect, i.e. activated by a Cu catalyst close-by . We demonstrate the full coverage by monolayer graphene of half-millimeter-sized hexagonal boron nitride crystals exfoliated on a copper foil prior to growth. We demonstrate that the proximity of the copper catalyst ensures high yield with the growth rate estimated between of 2μm/min to 5μm/min . Optical and electron microscopies together with confocal micro-Raman mapping confir…
The use of a novel three-dimensional graphene structure allows circumventing the limitations of the two-dimensional nature of graphene and its application in hydrogen absorption. Here we investigate hydrogen-bonding on monolayer graphene conformally grown via the epitaxial growth method on the (0001) face of a porousified 4H-SiC wafer. Hydrogen absorption is studied via Thermal Desorption Spectroscopy (TDS), exposing the samples to either atomic (D) or molecular (D2) deuterium. The graphene growth temperature, hydrogen exposure temperature, and the morphology of the structure are investigated …
Here we describe a computational study undertaken in an effort to elucidate the reaction mechanisms behind the experimentally observed oxidations and hydrations catalyzed by graphene oxide (GO). Using the oxidation of benzyl alcohol to benzaldehyde as a model reaction, density functional theory (DFT) calculations revealed that this reactivity stemmed from the transfer of hydrogen atoms from the organic molecule to the GO surface. In particular, neighbouring epoxide groups decorating GO's basal plane were ring-opened, resulting in the formation of diols, followed by dehydration. Consistent with…
A high temperature for some catalytic reactions like synthesis of large area and high quality graphene is required. The mentioned graphene can be obtained by a chemical vapour deposition (CVD) process on copper foil at 800 1000. Here, we describe a room temperature synthesis method from different carbon sources for the first time, including acetylene, carbon dioxide and carbon monoxide, on copper foil by using a heuristic method which was inspired from the role of some electronic promoters in catalyst science. Promoters are substances that increase the catalytic activity, but they are not cata…
Graphene nanoribbons (GNRs) with widths of a few nanometres are promising candidates for future nano-electronic applications due to their structurally tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability. However, the direct growth of micrometre-long GNRs on insulating substrates, which is essential for the fabrication of nano-electronic devices, remains an immense challenge. Here, we report the epitaxial growth of GNRs on an insulating hexagonal boron nitride (h-BN) substrate through nanoparticle-catalysed chemical vapor deposition (CVD). Ultra-narrow GNRs with lengths of…
Active sites and catalytic mechanism of nitrogen-doped graphene in oxygen reduction reaction (ORR) have been extensively studied but are still inconclusive, partly due to the lack of an experimental method that can detect the active sites. It is proposed in this report that the active sites on nitrogen-doped graphene can be determined via the examination of its chemical composition change before and after ORR. Synchrotron-based X-ray photoelectron spectroscopy analyses of three nitrogen-doped multilayer graphene samples reveal that oxygen reduction intermediate OH(ads) which should chemically …
We synthesized three-dimensional nanoporous graphene films by a chemical vapor deposition method with nanoporous copper as a catalytic substrate. The resulting nanoporous graphene has the same average pore size as the underlying copper substrate. Our surface-enhanced Raman scattering (SERS) investigation indicates that the nanoporosity of graphene significantly improves the SERS efficiency of graphene as a substrate as compared to planar graphene substrates.
Metal heteroatoms dispersed in nitrogen-doped graphene display promising catalytic activity for fuel cell reactions such as the hydrogen evolution reaction (HER). Here we explore the effects of dopant concentration on the synergistic catalytic behaviour of a paired metal atom active site comprised of Co and Pt atoms. The metals are coordinated to six atoms in a vacancy of N-doped graphene. We find that HER activity is enhanced with increasing N concentration, where the free energy of hydrogen atom adsorption ranges from 0.23 to -0.42 eV as the doping changes from a single N atom doped in the p…
Utilizing the strengths of nitrogen doped graphene quantum dot (N-GQD) as a substrate, here in, we have shown that one can stabilize the catalytically more active planar Au 20 (P-Au 20 ) compared to the thermodynamically more stable tetrahedral structure (T-Au 20 ) on an N-GQD. Clearly, this simple route avoids the usage of traditional transition metal oxide substrates which have been suggested and used for stabilizing the planar structure for a long time. Considering the experimental success in the synthesis of N-GQDs and in the stabilization of Au nanoparticles on N-doped graphene, we expect…
Innovative applications based on two-dimensional solids require cost-effective fabrication processes resulting in large areas of high quality materials. Chemical vapour deposition is among the most promising methods to fulfill these requirements. However, for 2D materials prepared in this way it is generally assumed that they are of inferior quality in comparison to the exfoliated 2D materials commonly used in basic research. In this work we challenge this assumption and aim to quantify the differences in quality for the prototypical transition metal dichalcogenide MoS$_2$. To this end single …
We investigate the microscopic processes leading to graphene growth by the chemical vapor deposition of propane in the argon atmosphere at the SiC surface. Experimentally, it is known that the presence of argon fastens the dehydrogenation processes at the surface, in high temperature of about 2000K. We perform ab-initio calculations, at zero temperature, to check whether chemical reactions can explain this phenomenon. Density functional theory and supporting quantum chemistry methods qualitatively describe formation of the graphene wafers. We find that the 4H-SiC(0001) surface exibits large ca…
Using first-principles calculations based on density functional theory and non-equilibrium Green's functions, we characterized the effect of surface termination on the electronic transport properties of nanoscale Cu slabs. With ideal, clean (111) surfaces and oxidized ones as baselines we explore the effect of capping the slabs with graphene, hexagonal boron nitrate, molybdenum disulfide and stanene. Surface oxide suppresses balistic conductance by a factor of 10 compared to the ideal surface. Capping the ideal copper surface with graphene slightly increase conductance but MoS$_2$ and stanene …
Based on combination of experimental measurements and first-principles calculations we report a novel carbon-based catalytic material and describe significant acceleration of the hydrogenation of magnesium at room temperature in presence of nickel nanoparticles wrapped in multilayer graphene. Increase of the rate of magnesium hydrogenation in contrast to the mix of graphite and nickel nanoparticles evidences intrinsic catalytic properties of explored nanocomposites. Results of simulations demonstrate that the doping from metal substrate and the presence of Stone-Wales defects turn multilayer g…
Understanding the influence that copper substrate surface symmetry and oxygen impurities have on the growth of graphene by chemical vapor deposition is important for developing techniques for producing high quality graphene. Therefore, we have studied the growth of graphene by catalytic decomposition of ethylene in an ultra-high vacuum chamber on both a clean Cu(100) surface and a Cu(100) surface pre-dosed with a layer of chemisorbed oxygen. The crystal structure of the graphene films was characterized with \emph{in-situ} low energy electron diffraction. By heating the clean Cu(100) substrate …
A fully self-consistent density-functional theory (DFT) with improved functionals is used to provide a comprehensive account of structural, electronic, and optical properties of C$_{3}$N$_{4}$ polymorphs. Using our recently developed van Leeuwen-Baerends (vLB) corrected local-density approximation (LDA), we implemented LDA+vLB within full-potential N$^{th}$-order muffin-tin orbital (FP-NMTO) method and show that it improves structural properties and band gaps compared to semi-local functionals (LDA/GGA). We demonstrate that the LDA+vLB predicts band-structure and work-function for well-studied…
The integration of two-dimensional (2D) materials with functional non-2D materials such as metal oxides is of key importance for many applications, but underlying mechanisms for such non-2D/2D interfacing remain largely elusive at the atomic scale. To address this, we here investigate the nucleation stage in atomic layer deposition (ALD) of the important metal oxide HfO2 on chemical vapor deposited graphene using atomically resolved and element specific scanning transmission electron microscopy (STEM). To avoid any deleterious influence of polymer residues from pre-ALD graphene transfers we em…
The outstanding electrical and mechanical properties of graphene make it very attractive for several applications, Nanoelectronics above all. However a reproducible and non destructive way to produce high quality, large-scale area, single layer graphene sheets is still lacking. Chemical Vapour Deposition of graphene on Cu catalytic thin films represents a promising method to reach this goal, because of the low temperatures (T < 900 Celsius degrees) involved during the process and of the theoretically expected monolayer self-limiting growth. On the contrary such self-limiting growth is not comm…
Graphene-covered copper surfaces have been exposed to borazine, (BH)3(NH)3, with the resulting surfaces characterized by low-energy electron microscopy. Although the intent of the experiment was to form hexagonal boron nitride (h-BN) on top of the graphene, such layers were not obtained. Rather, in isolated surface areas, h-BN is found to form micrometer-size islands that substitute for the graphene. Additionally, over nearly the entire surface, the properties of the layer that was originally graphene is observed to change in a manner that is consistent with the formation of a mixed h-BN/graph…
Organohalide perovskite with a variety of surface structures and morphologies have shown promising potential owing to the choice of the type of heterostructure dependent stability. We systematically investigate and discuss the impact of 2-dimensional molybdenum-disulphide (MoS2), molybdenum-diselenide (MoSe2), tungsten-disulphide (WS2), tungsten-diselenide (WSe2), boron- nitiride (BN) and graphene monolayers on band-gap and energy stability of organic-inorganic halide perovskites. We found that MAPbI3ML deposited on BN-ML shows room temperature stability (-25 meV~300K) with an optimal bandgap …
Vanadium disulfide (VS_{2}) attracts elevated interests for its charge-density wave (CDW) phase transition, ferromagnetism, and catalytic reactivity, but the electronic structure of monolayer has not been well understood yet. Here we report synthesis of epitaxial 1T VS_{2} monolayer on bilayer graphene grown by molecular-beam epitaxy (MBE). Angle-resolved photoemission spectroscopy (ARPES) measurements reveal that Fermi surface with six elliptical pockets centered at the M points shows gap opening at low temperature. Temperature-dependence of the gap size suggests existence of CDW phase transi…
