Monolayer Of Atoms Research Articles

Realization of a Hole-Doped Mott Insulator on a Triangular Silicon Lattice.

The physics of doped Mott insulators is at the heart of some of the most exotic physical phenomena in materials research including insulator-metal transitions, colossal magnetoresistance, and high-temperature superconductivity in layered perovskite compounds. Advances in this field would greatly benefit from the availability of new material systems with a similar richness of physical phenomena but with fewer chemical and structural complications in comparison to oxides. Using scanning tunneling microscopy and spectroscopy, we show that such a system can be realized on a silicon platform. The adsorption of one-third monolayer of Sn atoms on a Si(111) surface produces a triangular surface lattice with half filled dangling bond orbitals. Modulation hole doping of these dangling bonds unveils clear hallmarks of Mott physics, such as spectral weight transfer and the formation of quasiparticle states at the Fermi level, well-defined Fermi contour segments, and a sharp singularity in the density of states. These observations are remarkably similar to those made in complex oxide materials, including high-temperature superconductors, but highly extraordinary within the realm of conventional sp-bonded semiconductor materials. It suggests that exotic quantum matter phases can be realized and engineered on silicon-based materials platforms.

Saturation of tungsten surfaces with hydrogen: A density functional theory study complemented by low energy ion scattering and direct recoil spectroscopy data

Herein, we investigate the saturation limits of hydrogen on the (110) and (100) surfaces of tungsten via Density Functional Theory (DFT) and complement our findings with experimental measurements. We present a detailed study of the various stable configurations that hydrogen can adopt upon the surfaces at coverage ratios starting below 1.0, up to the point of their experimental coverage ratios, and beyond. We provide the many low-energy configurations that exist at all coverages along with the energy landscape they form. Our findings allow us to estimate that the saturation limit on each surface exists with one monolayer of hydrogen atoms adsorbed. In the case of (110) this corresponds to a coverage ratio of one hydrogen atom per tungsten atom, while in the case of (100) a full monolayer is present at a coverage ratio of 2.0 hydrogen atoms per tungsten atoms. Preliminary Low Energy Ion Scattering (LEIS) and Direct Recoil Spectroscopy (DRS) measurements complement this work on the W(110) surface. These results and some previously published measurements obtained on the W(100) surface confirm the findings obtained by DFT. In particular, the saturation limits on each surface, the preferred adsorption sites on both surfaces up to saturation, and the reconstruction of the bare and unsaturated (100) surface.

Assessment of graphene as an alternative microporous layer material for proton exchange membrane fuel cells

Microporous layer (MPL) is a key component in the management of water (and to a lesser degree, heat) for enhanced performance, reliability and durability of proton exchange membrane (PEM) fuel cells at high current densities for a wide range of commercial applications. In this study, as an alternative to conventional nanomaterials, such as Ketjenblack, graphene, a monolayer of carbon atoms arranged in a two-dimensional lattice, is considered to be an ideal MPL material due to its unique characteristics, including excellent electrical and thermal conductivity. A graphene-based MPL has been prepared and investigated for its effective water management and performance enhancement for PEM fuel cells through morphological, structural, physical and electrochemical characterization and performance testing for single large cells. Comparison studies are also conducted with the MPL made of conventional material, Ketjenblack. It is shown that the graphene-based MPL demonstrates comparable performance to the one made of Ketjenblack under high-humidity operation, while exhibiting excellent performance superiorities (up to a peak power density improvement of approximately 55%) under low- and intermediate-humidity operation. Overall, the graphene-based MPL has significant potential to meet performance demand under a wide range of operating conditions.

Atomic level tailoring of the electrocatalytic activity of Au-Pt core-shell nanoparticles with controllable Pt layers toward hydrogen evolution reaction

The hydrogen evolution reaction (HER) is a critical step in water splitting and demands efficient and low-cost catalysts for real applications. Pt catalyzes HER efficiently, however, its scarcity and high cost inspire researchers to explore metallic catalysts with core-shell nanostructures. Precise tailoring of Pt catalysts at atomic monolayer level would shed insights on the relationship between electronic structure and electrocatalytic properties. Herein, we report the preparation of Au-Pt core-shell nanoparticles deposited on graphene sheets with tunable Pt monolayer coverage by controlling Cu underpotential deposition (UPD)-Galvanic replacement cycles. The electrochemical results suggest that Au-Pt with monolayer Pt shows superior electrocatalytic activity toward HER. Increasing the thickness of Pt shell results in the decrease of the electrocatalytic activity, which might be attributed to the decreased electronic interactions of Au core with outmost surface Pt atoms as confirmed by X-ray photoelectron spectroscopic characterization. The present research would offer a design criterion for metallic nanostructures in realizing catalysts with high performance.

Hetero-diffusion of Au epitaxy on stepped Ag(110) surface: Study of the jump rate and diffusion coefficient

We have used molecular dynamics simulations combined with an interatomic potential derived from the embedded atom method, to investigate the hetero-diffusion of Au adatom near a stepped Ag(110) surface with the height of one monoatomic layer. The activation energies for different diffusion processes, which occur on the terrace and near the step edge, are calculated both by molecular statics and molecular dynamics simulations. Static energies are found by the drag method, whereas the dynamic barriers are computed at high temperature from the Arrhenius plots. Our numerical results reveal that the jump process requires very high activation energy compared to the exchange process either on the terrace or near the step edge. In this work, other processes, such as upward and downward diffusion at step edges, have also been discussed.

Structural, electronic and photocatalytic properties of atomic defective BiI3 monolayers

The structural, electronic and photocatalytic properties of five vacancy-containing 2D BiI3 monolayers are investigated by the first-principle calculations. The electronic structures show that the five structures are stable and have comparable binding energies to that of the pristine BiI3 monolayer, and the defects can tune the band gaps. Optical spectra indicate that the five structures retain high absorption capacity for visible light. The spin-orbit coupling (SOC) effect is found to play an important role in the band edge of defective structures, and the VBi and VBi-I3 defective BiI3 monolayers can make absolute band edges straddle water redox potentials more easily.

Formation Mechanism of Secondary Electron Contrast of Graphene Layers on a Metal Substrate.

Scanning electron microscopy (SEM) is widely used to observe graphene on metal substrates. However, the origin of the SEM image contrast of graphene is not well understood. In this work, we performed in situ SEM imaging of layer-number-controlled graphene on a Ni substrate using a high-pass energy filter for secondary electrons. We found that the graphene layer contrast was maximized at 15–20 eV, corresponding to the π–σ* interband transition in graphene. Our results indicate that the SEM image of graphene is produced by attenuation of the electrons emitted from the metal substrate by the monoatomic layers of graphene.

The “critical limits for crystallinity” in nanoparticles of the elements: A combined thermodynamic and crystallographic critique

The theme of the present work is the procedure for evaluating the minimum size for the stability of a crystalline particle with respect to the same group of atoms but in the amorphous state. A key goal of the study is the critical analysis of an extensively quoted paper by F.G. Shi [J. Mater. Res. 9 (1994) 1307–1313], who presented a criterion for evaluating a “crystallinity distance” (h) through its relation with the “critical diameter” (dC) of a particle, i.e., the diameter below which no particles with the crystalline structure are expected to exist at finite temperatures. Key assumptions of Shi's model are a direct proportionality relation between h and dC, and a prescription for estimating h from crystallographic information. In the present work the accuracy of the Shi model is assessed with particular reference to nanoparticles of the elements. To this end, an alternative way to obtain h, that better realizes Shi's idea of this quantity as “the height of a monolayer of atoms on the bulk crystal surface”, is explored. Moreover, a thermodynamic calculation of dC, which involves a description of the bulk- and the surface contributions to the crystalline/amorphous relative phase stability for nanoparticles, is performed. It is shown that the Shi equation does not account for the key features of the h vs. dC relation established in the current work. Consequently, it is concluded that the parameter h obtained only from information about the structure of the crystalline phase, does not provide an accurate route to estimate the quantity dC. In fact, a key result of the current study is that dC crucially depends on the relation between bulk- and surface contributions to the crystalline/amorphous relative thermodynamic stability.

Effect of MgO Underlying Layer on the Growth of GaOx Tunnel Barrier in Epitaxial Fe/GaOx/(MgO)/Fe Magnetic Tunnel Junction Structure.

We investigated the effect of a thin MgO underlying layer (~3 monoatomic layers) on the growth of GaOx tunnel barrier in Fe/GaOx/(MgO)/Fe(001) magnetic tunnel junctions. To obtain a single-crystalline barrier, an in situ annealing was conducted with the temperature being raised up to 500 °C under an O2 atmosphere. This annealing was performed after the deposition of the GaOx on the Fe(001) bottom electrode with or without the MgO(001) underlying layer. Reflection high-energy electron diffraction patterns after the annealing indicated the formation of a single-crystalline layer regardless of with or without the MgO layer. Ex situ structural studies such as transmission electron microscopy revealed that the GaOx grown on the MgO underlying layer has a cubic MgAl2O4-type spinel structure with a (001) orientation. When without MgO layer, however, a Ga-Fe-O ternary compound having the same crystal structure and orientation as the crystalline GaOx was observed. The results indicate that the MgO underlying layer effectively prevents the Fe bottom electrode from oxidation during the annealing process. Tunneling magneto-resistance effect was observed only for the sample with the MgO underlying layer, suggesting that Ga-Fe-O layer is not an effective tunnel-barrier.

In Situ Monitoring of Cation-Exchange Reaction Shell Growth on Nanocrystals

We demonstrate how in situ monitoring of the photoluminescence during shell growth around colloidal nanocrystals (NCs) can be used to develop a detailed and quantitative model for this process. We apply it here to study cation-exchange based growth of ZnS on a Cu–In–Se NC to form Cu–In–Se/ZnSe1–xSx alloyed NCs. We determine that this process begins with the Zn precursor binding to the outer layer of the NC followed by diffusion of Zn cations into successive atomic monolayers of the NC. At temperatures below 100 °C, Zn cations can only diffuse into the outermost atomic monolayer of the Cu–In–Se NCs. At growth temperatures above 100 °C, the second monolayer also becomes thermally accessible and can be filled with Zn cations. Our results provide an understanding of cation-exchange shell growth at the atomic level via optical analysis. The approach and mathematical model described here can be applied to other core/shell nanostructures and allows selection of optimal synthesis conditions to achieve desired cor...

Ab initio Calculation of the Depth‐Dependent Optical Reflectance From Layer‐by‐Layer Atomic Disorder

We present a simple model to study the effects of displacing a single atomic monolayer on the linear optical properties of a material. As an example, we calculate the change in reflectance of a Si(111)(1 × 1):H slab after disordering successively deeper atomic layers. We find that the reflectance varies significantly at photon energies above 2.0 eV, and that the disordered slab produces a larger reflectance than the relaxed slab. The results also show a quantitative difference in the contribution from the odd and even atomic layers to the calculated reflectance. This simplified model is a first approach; also, it can be extended to consider more realistic systems that can be probed by techniques such as coherent acoustic phonon (CAP) spectroscopy, and can also be applied to two‐dimensional materials.

Renaissance of an Old Topic: From Borazines to BN-doped Nanographenes.

Graphene is one of the leading materials in today's science, but the lack of a band gap limits its application to replace semiconductors in optoelectronic devices. To overcome this limitation, the replacement of C=C bonds by isostructural and isoelectronic bonds is emerging as an effective strategy to open a band gap in monoatomic graphene layers. First prepared by Stock and Pohland in 1926, borazine is the isoelectronic and isostructural inorganic analogue of benzene, where the C=C bonds are replaced by B-N couples. The strong polarity of the BN bonds widens the molecular HOMO-LUMO gap, imparting strong UV-emission/absorption and electrical insulating properties. These properties make borazine a valuable molecular scaffold to be inserted as doping units in graphitic-based carbon materials to tailor a relevant band gap. It is with this objective that we became interested in the development of new synthetic organic methodologies to gain access to functionalized borazine derivatives. In particular, we have described the synthesis of borazine derivatives that, featuring aryl substituents at the B-centers bearing ortho-functionalities, are exceptionally stable against hydrolysis. Building on these structural motifs, we prepared hybrid BN-doped polyphenylene nanostructures featuring controlled doping patterns, both as dosage and orientation. Finally, exploiting the Friedel-Craft electrophilic aromatic substitution, we could develop the first rational synthesis of the first soluble hexa-peri-hexabenzoborazinocoronene and measured its optoelectronic properties, showing a widening of its gap compared to its full-carbon congener.

Another approach to the problem of room temperature superconductivity

The superconductivity at room temperature $$T_{\mathrm{m}}$$ can be accomplished by pursuing two distinct strategies. One approach is to create a new superconductor, whose critical temperature $$T_{\mathrm{c}} \sim T_{\mathrm{m}}$$ .Here we discuss viability of another approach involving integration of a conventional superconductor (S) with the Peltier cooler (P) made of an atomic monolayer (AM). Owing to remarkable properties of AM, the hybrid SP thermoelectric device promises a tremendously high efficiency, sufficient to diminish the local temperature of S below its $$T_{\mathrm{c}}$$ amid the ambient temperature $$\sim T_{\mathrm{m}}>>T_{\mathrm{c}}$$ . In particular, the chirality and Klein tunneling of electric charge carriers in graphene allow creating topologically protected quantized state, immune to electron scattering on lattice imperfections. Efficient doping with the gate electrodes allows changing the local carrier concentration in wide limits, leading to a high thermopower. The nanostructuring and alloying diminish the phonon part of the thermal flux by orders of magnitude. By combining the above methods, one accomplishes the nanoscale cooling of S locally down to $$T_{\mathrm{c}}<<T_{\mathrm{m}}$$ .

Probing the Interlayer Exciton Physics in a MoS2/MoSe2/MoS2 van der Waals Heterostructure.

Stacking atomic monolayers of semiconducting transition metal dichalcogenides (TMDs) has emerged as an effective way to engineer their properties. In principle, the staggered band alignment of TMD heterostructures should result in the formation of interlayer excitons with long lifetimes and robust valley polarization. However, these features have been observed simultaneously only in MoSe2/WSe2 heterostructures. Here we report on the observation of long-lived interlayer exciton emission in a MoS2/MoSe2/MoS2 trilayer van der Waals heterostructure. The interlayer nature of the observed transition is confirmed by photoluminescence spectroscopy, as well as by analyzing the temporal, excitation power, and temperature dependence of the interlayer emission peak. The observed complex photoluminescence dynamics suggests the presence of quasi-degenerate momentum-direct and momentum-indirect bandgaps. We show that circularly polarized optical pumping results in long-lived valley polarization of interlayer exciton. Intriguingly, the interlayer exciton photoluminescence has helicity opposite to the excitation. Our results show that through a careful choice of the TMDs forming the van der Waals heterostructure it is possible to control the circular polarization of the interlayer exciton emission.

Te Monolayer-Driven Spontaneous van der Waals Epitaxy of Two-dimensional Pnictogen Chalcogenide Film on Sapphire.

Demands on high-quality layer structured two-dimensional (2D) thin films such as pnictogen chalcogenides and transition metal dichalcogenides are growing due to the findings of exotic physical properties and potentials for device applications. However, the difficulties in controlling epitaxial growth and the unclear understanding of van der Waals epitaxy (vdWE) for a 2D chalcogenide film on a three-dimensional (3D) substrate have been major obstacles for the further advances of 2D materials. Here, we exploit the spontaneous vdWE of a high-quality 2D chalcogenide (Bi0.5Sb1.5Te3) film by the chalcogen-driven surface reconstruction of a conventional 3D sapphire substrate. It is verified that the in situ formation of a pseudomorphic Te atomic monolayer on the surface of sapphire, which results in a dangling bond-free surface, allows the spontaneous vdWE of 2D chalcogenide film. Since this route uses the natural surface reconstruction of sapphire with chalcogen under vacuum condition, it can be scalable and easily utilized for the developments of various 2D chalcogenide vdWE films through conventional thin-film fabrication technologies.

Graphene/ MoS2 heterostructures as templates for growing two-dimensional metals: Predictions from ab initio calculations

Preparation of single-atom-thick layers of ordinary metals has been a challenging task since their closely packed atoms lack layered structure with highly anisotropic bonding. Using computational modeling based on density functional theory we showed that graphene/${\mathrm{MoS}}_{2}$ heterostructures can be used as suitable templates to grow stable two-dimensional (2D) clusters, as well as extended monoatomic layers of metals with nonlayered structure in the bulk. Considering gold and lithium as two metals with markedly different properties, we found that Li intercalants strengthen coupling between graphene (G) and ${\mathrm{MoS}}_{2}$, mainly due to electrostatic attraction of 2D materials with positively charged Li atoms. However, intercalation with large Au atoms gives rise to a significant increase in the distance between G and ${\mathrm{MoS}}_{2}$ and thus, weakens their interaction. In addition to strong preference for 2D growth, we demonstrated that Au intercalants weakly interact with both G and ${\mathrm{MoS}}_{2}$, and hence $\mathrm{G}/{\mathrm{MoS}}_{2}$ vertical heterostructures could be a promising framework to prepare gold 2D structures with electronic properties closely resembling those of the hypothetical free-standing hexagonal gold monolayer.

(Invited) Carbon Dioxide Electroreduction at Ultrathin Palladium Layers Deposited on Copper Electrode

Electrochemical conversion of carbon dioxide into eligible fuels has become the subject of particular fundamental interest. Recently, we have reported electrochemical studies on CO2 electroreduction at multilayered catalyst composed of the monatomic layer of copper covering palladium overlayers (0.8-10 monolayers) deposited on the well-defined Au(111) surface [1]. In this study Carbon dioxide electroreduction has been studied at ultrathin palladium layers deposited on copper electrode. The Pd deposits have been prepared by using chemical methods. The spontaneous galvanic displacement and resulting uncontrolled deposition of the palladium on copper has been entirely eliminated by the use of palladium(II). This method allowed us for controlled and precise deposition of palladium layers by electrochemical methods. The deposits coverage was changed within the range 0.5 to 5 atomic monolayers. The effectiveness of the method and the durability of the obtained ultrathin palladium layers have been confirmed by X-ray photoelectron spectroscopy (XPS). XPS has been also used for detailed analysis of the electronic properties of the Pd/Cu systems. Significant chemical interactions of both elements resulted in a partial shift of the electron density from palladium to copper and in the changes in the symmetry of 3d palladium signals characteristic for two-dimensional palladium structures (Pd monolayers). A detailed analysis of the valence band structure of Pd/Cu was correlated to the catalytic activity of the studied systems. The unique properties of the valence band structure cannot be attributed to the simple superposition of pure metal bands (Pd and Cu). It has been shown that the increase in the electron density near the Fermi level results in a significant decrease in the overpotential of the carbon dioxide electroreduction. Analysis of the gaseous products obtained in the CO2 electroreduction was performed by gas chromatography. Adsorption of carbon dioxide on Pd/Cu layers was demonstrated by XPS measurements in the presence of 1 mbar CO2 in XPS measurements under near ambient pressure XPS (NAP-XPS). [1] A. Januszewska, R. Jurczakowski, P. Kulesza, Langmuir 30(2014)14314-14321

Au-Pd-Pt Multilayer Catalyst for Miniature Fuel Cells

We have developed miniature fuel cells with monolithically fabricated Si electrodes. In the fuel cells, two Si electrodes with porous Pt catalyst layer, which Pt usage was 3.6mg/cm2, were hot-pressed onto either side of a PEM (polymer electrolyte membrane), and the prototype fuel cells showed promising performances of over 500mW/cm2 output at 313K with H2-O2. However, recent improvements of Li-ion battery are lowering expectation to the miniature fuel cells. The advantage of the fuel cell, i.e. clean and efficient energy source, should be strengthened. Bio-mass is an ultimate energy source. Hydrogen can be produced by gasification of the bio-mass in reducing environment, but high CO concentration in the bio-mass derived hydrogen is inevitable. Catalyst poisoning by CO is problematic. We found that our Pd-Pt catalyst obtained by using UPD-SLRR had promising feature. Though the catalyst layer used only 6μg/cm2 of Pt, high tolerance to CO was observed. But Pd and Pt are rare metals, and both amounts should be minimized. In this study, a novel Au-Pd-Pt multilayer catalyst is proposed. Though Au is also a precious metal, reserve of Au is much larger than Pd and Pt, and porous Au can be formed on a Si substrate by similar process to porous Pt. There is no clear evidence, but it is assumed that high CO tolerance of the Pd-Pt catalyst is realized with sub-monolayer Pt on Pd. Hydrogen absorption or stress near the interface between Pd and Pt are sensitive to the number of atomic layer of Pd, and fine control of Pd and Pt deposition on Au is crucial. In our previous study, sub-monolayer Pt was deposited with Cu-UPD, but hydrogen also absorbs on some metal surfaces and form atomic monolayer at slightly under the equilibrium potential. The H-UPD-SLRR can be performed just in an acidic plating bath containing Pd2+ or Pt4+. There is no need to replace the plating bath, and repetition of the atomic layer deposition process will be easy. In this study, basic fabrication strategy of the Au-Pd-Pt multilayer catalyst using H-UPD-SLRR process was examined using Au wire. Preliminary results of cyclic voltammetry suggested that Pd and Pt were successfully deposited on Au wire, and only one Pt deposition cycle on the Au wire with Pd, deposited by several UPD-SLRR cycles, showed large hysteresis loop in the CV, which related to catalytic performance.

Effect of Mo concentration on shape and size of monolayer MoS2 crystals by chemical vapor deposition

Monolayer molybdenum disulfide (MoS2), a new two-dimensional direct-bandgap semiconductor, has attracted research interests into applications in atomically thin electronics and optoelectronics. Growing monolayer MoS2 film by chemical vapor deposition is the most commonly used approach. Little is known, however, about the cause of the shape and size evolution. Here, we explore how the precursor’s concentration affects the MoS2 crystals’ shape and size. When S concentration is stable, the shape of the MoS2 domain evolves from triangle to hexagon and then truncated triangle, finally back to a triangle as the concentration of Mo elevates. Regulating the concentration of Mo leads to the controllable growth of MoS2 crystal. By controlling the concentration of Mo, we eventually synthesized over 100 µm monolayer MoS2 crystals. Our results are a significant step forward in realizing the ultimate promise of large atomic MoS2 monolayer crystals for flexible, electronic, optoelectronic devices.

Enhanced periodic modulation of electronic states in a hexagonal iron-nitride monolayer on Cu(001) via interfacial interaction

Metal nitride has robust bonding between metal and nitrogen atoms. We have synthesized a monoatomic layer of a hexagonal-type iron nitride with strong Fe-N bond on the Cu(001) substrate with a fourfold lattice symmetry. From atomic-scale observation using scanning tunneling microscopy, we have found that the Fe-N film exhibits a strip structure consisting of regular and deformed hexagonal lattices. The lattices are deformed along a hypothetical Moir\'e pattern made by a perfect hexagonal lattice on the square Cu(001) lattice. With increasing the lattice deformation, the strain energy increases, whereas the periodic deformation of the Fe-N lattice can minimize the total lattice energy including the strain energy of the Fe-N in-plane bonding and interfacial bonding energy between the Fe-N layer and the Cu substrate. In $\mathrm{d}I/\mathrm{d}V$ spectra, periodic spatial change of the unoccupied density of states due to the lattice strain was observed. This is attributed to an energy shift of the antibonding states due to compressive strain.