Metal Monolayers Research Articles

Investigation of Interface Interactions Between Monolayer MoS2 and Metals: Implications on Strain and Surface Roughness.

Achieving a low contact resistance has been an important issue in the design of two-dimensional (2D) semiconductor-metal interfaces. The metal contact resistance is dominated by interfacial interactions. Here, we systematically investigate 2D semiconductor-metal interfaces formed by transferring monolayer MoS2 onto prefabricated metal surfaces, such as Au and Pd, using X-ray photoelectron spectroscopy (XPS), atomic force microscopy, and Raman spectroscopy. In contrast to the MoS2/HOPG interface, the interfaces of MoS2/Au and MoS2/Pd feature the formation of weak covalent bonds. The XPS spectra reveal distinct peak positions for S-Au and S-Pd, indicating a higher doping concentration at the S-Au interface. This difference is a key factor in understanding the electronic interactions at the metal-MoS2 interfaces. Additionally, we observe that the metal surface roughness is a critical determinant of the adhesion behavior of transferred monolayer MoS2, resulting in different strains and doping concentrations. The strain on transferred MoS2 increases with an increase in substrate roughness. However, the strain is released when the roughness of metal surface surpasses a certain threshold. The dependence of the contact material and the influence of the substrate roughness on the contact interface provide critical information for improving 2D semiconductor-metal contacts and device performance.

Seeding the vertical growth of laterally coherent coordination polymers on the rutile-TiO2(110) surface.

Coordination polymers may be synthesized by linear bridging ligands to metal ions with conventional chemistry methods (e.g. in solution). Such complexes can be hardly brought onto a substrate with the chemical, spatial and geometrical homogeneity required for device integration. Instead, we follow an in situ synthesis approach, where the anchoring points are provided by a monolayer of metal(II)-tetraphenylporphyrin (M-TPP, M = Cu, Zn, Co) grown in vacuum on the rutile-TiO2(110) surface. We probed the metal affinity to axial coordination by further deposition of symmetric dipyridyl-naphthalenediimide (DPNDI). By NEXAFS linear polarization dichroism, we show that DPNDI stands up on Zn- and Co-TPP thanks to axial coordination, whereas it lies down on the substrate for Cu-TPP. Calculations for a model pyridine ligand predict strong binding to Zn and Co cations, whose interaction with the O anions underneath is disrupted by surface trans effect. The weaker interactions between pyridine and Cu-TPP are then overcome by the strong attraction between TiO2 and DPNDI. The binding sites exposed by the homeotropic alignment of the ditopic DPNDI ligand on Zn- and Co-TPP are the foundations to grow coordination polymers preserving the lateral coherence of the basal layer.

A metallic two-dimensional b-BS2 monolayer as a superior Na/K-ion battery anode.

Two-dimensional (2D) materials with light weight and ultra-high electrical conductivity are expected to exhibit high capacity as anodes of batteries. We have explored the curved square lattice BS2 (b-BS2) monolayer possessing a space group similar to the transition metal chloride (space group: P4̄M2). The electrochemical performance of the b-BS2 monolayer as the anode for various metal-ion batteries (Li, Na, K, Mg, Ca, and Al) has been investigated. It exhibits low diffusion energy barrier, high theoretical capacity, and low open circuit voltage for Na/K-ion batteries (SIBs/PIBs). The ultrahigh energy densities of 2146.08 and 715.36 mA h g-1 can be achieved with the stoichiometries BS2Na6 and BS2K2, respectively. Furthermore, the b-BS2 monolayer may be synthesized on the surfaces of metal substrate materials (Ag(111), Al(111), and Au(111)). These results indicate that the b-BS2 monolayer is a good candidate for the anode material of SIBs/PIBs.

Enriching 2D transition metal borides via MB XMenes (M = Fe, Co, Ir): Strong correlation and magnetism.

Recently, two-dimensional (2D) FeSe-like anti-MXenes (or XMenes), composed of late d-block transition metal M and p-block nonmetal X elements, have been both experimentally and theoretically investigated. Here, we select three 2D borides FeB, CoB and IrB for a deeper investigation by including strong correlation effects, as a fertile ground for understanding and applications. Using a combination of Hubbard corrected first-principles calculations and Monte Carlo simulations, FeB and CoB are found to be ferro- and anti-ferro magnetic, contrasting with the non-magnetic nature of IrB. The metallic FeB XMene monolayer, superior to most of the MXenes or MBenes, exhibits robust ferromagnetism, driven by intertwined direct-exchange and super-exchange interactions between adjacent Fe atoms. The predicted Curie temperature (TC) of the FeB monolayer via the Heisenberg model reaches an impressive 425 K, with the easy-axis oriented out-of-plane and high magnetic anisotropic energy (MAE). The asymmetry in the spin-resolved transmission spectrum induces a thermal spin current, providing an opportunity for spin filtration. This novel 2D FeB material is expected to hold great promise as an information storage medium and find applications in emerging spintronic devices.

Sign-flipping intrinsic anomalous Hall conductivity with Berry curvature tunability in a half-metallic ferromagnet NbSe2-VSe2 lateral heterostructure.

Single-layer half-metal magnets offer exciting scope in spin electronic quantum applications owing to improved spin transport, reduced interfacial resistance and streamlined device fabrication. Herein, we report the emergence of sign-flipping intrinsic anomalous Hall conductivity (AHC) as a result of changes in Berry curvature under an external electric field and half metallicity in a lateral heterostructure composed of centrosymmetric metallic monolayers 1T-NbSe2 and 1T-VSe2. The metallic monolayers 1T-NbSe2 and 1T-VSe2 laterally interfaced along the zigzag orientation break inversion symmetry at the interface and result in distinctive Berry curvature features. Furthermore, the half-metallic character was prominent with gapped states in the spin-up channel, while the spin-down state remained conductive; we observed the unique manifestation of sign-flipping intrinsic AHC at the Fermi level in addition to the electron- and hole-doped regions. This sign-flipping aspect of AHC at the Fermi level is of fundamental importance from the prospect of real-time device applications as it eliminates the necessity of supplementary actions, such as doping and strain engineering, which are traditionally employed to achieve AHC sign reversal. Additionally, a phase transition from half metal to metal occurs at a field of 0.5 V Å-1 and beyond. Half metallicity with sign switching AHC via external electric field makes the lateral NbSe2-VSe2 heterostructure a potential candidate for real-time energy-efficient low-power spintronic devices.

Quantum structure of the surface layer of metals

In this paper, we propose a model for determining the thickness of the surface layer of metals and the quantum structure of this layer. An atomically smooth metal is represented as a diagram: nanolayer → mesolayer → bulk phase, which differ from each other in the nature of size effects. There is no size effect in the bulk phase. The thickness of the surface layer of metals R(I) has a size from 1 nm to 10 nm, except for cesium, i.e. they represent a nanostructure. It is shown that the energy levels En of the nanolayer are determined by one fundamental parameter - the lattice constant of the metal à. As soon as the parameter à stops changing, the spectrum of quantum states passes into a continuous spectrum. The nanolayer R(I) is a step function in En, which can be easily reduced to the Lebesgue integral, which plays an important role in quantum theory. Quantum threads (quantum planes) in the nanolayer R(I) can be interpreted as solitons, crowdions, discrete breathers turning into nanocracks. Quantum threads in the R(I) nanolayer and the nanolayer itself can be represented as a nanoparticle with a diameter of R(I) and pre-melting occurs in it in a stepwise manner. It is finally shown that all metal monolayers differ significantly from each other.

A novelty in local electrochemical analysis with a pressing sensor cell (a review)

Local electrochemical analysis (LEA) with a pressure cell-sensor (CSC) is based on electrodecrystallization of the material studied (metal, alloy, powder composition, semiconductor structures, etc.) in an area limited by the size of the hole in the graphite CSC filled with the appropriate electrolyte, and recording the dissolution current strength. The selected surface of the analyzed material or product is the working electrode, and the graphite body of the cell or a special electrode inserted into the inert body of the cell is the counter electrode. Choice of the electrolyte and dissolution mode determines the possibility of using LEA with CSC for studying thickness of the layer, surface composition or the distribution of the composition over the thickness of the material layer in the controlled area, with a diameter of 1–2 mm. Design of the CSC and methods of its application were developed in the 60s of the last century by a team of authors from Kuibyshev Polytechnical Institute (now SamSTU) for coulometric control of monolayer metal coatings on watch cases, which resulted in the corresponding requirements to the size of the cell body and its contact hole. Subsequent studies showed possibility of using LEA with CSC for controlling thickness of multilayer metal coatings with individual metals on various products, composition of binary alloy coatings by selective dissolution of their components in a potentiodynamic mode, as well as powders and powder compositions pressed into a pellet electrode, control of the distribution profile of the alloying impurity in silicon epitaxial structures, thickness of oxide films, phase composition of metal alloy samples and a number of other application parameters. The present work provides an overview of advances in the development of theory, methodology, tools and practical applications of local electrochemical analysis over the last decade.

Metallic B2Si monolayer as a flexible anode material for Li, Na and K-ion batteries: A first principles study

Silicon-based 2D materials have garnered significant attention as outstanding anodes of alkali metal ion batteries (MIBs); nevertheless, the poor electrical conductivity and mechanical property have severely hindered their practical application. In this work, we conducted a detailed first principles calculation to examine a newly-developed 2D silicon-boron binary compound (named B2Si) as the anode of MIBs. The B2Si anode harbors the intrinsic metallicity and exhibits low Li/Na/K diffusion barriers of 0.26, 0.14 and 0.09 eV, which makes a great contribution to the high rate performance. The storage capacities for Li/Na/K are predicted to be 1908.57, 583.48 and 431.97 mA h/g, respectively, superior to the commercial graphite anode. Moreover, the B2Si anode has good mechanical property and structural flexibility, which could avoid destroying its structure on the metal ion intercalation/deintercalation process. These fascinating features render the B2Si monolayer an exceptional anode of MIBs.

(Keynote) Iridium Deposition By Galvanic Displacement of Cu on a One-Pot Configuration

Water electrolysis with proton exchange membranes (PEMWE) is expected to play a vital role in the future hydrogen infrastructure. The hydrogen evolution is effectively catalyzed by platinum, and iridium oxide is a highly active and reasonably stable catalyst for the oxygen-evolution reation (OER) [1]. However, iridium is costly and scarce, and efficient utilization of it is necessary for large-scale deployment of PEMWE technology. This can be achieved in various ways, such as dilution with other and cheaper compounds such as Ta2O5 [2] and alloying with more abundant metals followed by post-synthesis-treatment such as de-alloying or surface enrichment [3]. A possible solution is the application of ultra-thin layers of iridium oxide on a less costly substrate. A suitable method for achieving such layers is that of surface-limited galvanic displacement [4], i.e. the underpotential deposition of a metal M followed by its oxidation by another and more noble metal N.In this work we have investigated Ir plating onto a polycrystalline gold electrode by successive galvanic displacement of Cu monolayers in a one-pot method, i.e by performing the Cu underpotential-deposition step in a solution also containing iridium precursors. The process is indicated in the figure. By using low concentrations of the precursors H2IrCl6 and IrCl3 in H2SO4, Cu submonolayers could be formed on a Au(poly) rotating disc electrode by underpotential deposition without any significant direct electrochemical formation of Ir or IrO2. The Cu submonolayer formed was then available for reaction with the Ir precursor before a new Cu submonolayer was formed. By evaluating the amount of Cu deposited in each submonolayer and the resulting Ir deposit, our results indicate that H2IrCl6 is reduced onto the Au electrode as metallic Ir via the formation of Ir(III). Due to transport of Ir(III) away from the reaction surface, this resulted in a low yield. No Ir deposited was achieved from IrCl3 solutions, possibly due to adsorption of irreducible species onto the Au surface.The results will be discussed in terms of possible reaction paths and the relative stability of iridium complexes in chloride-containing solutions. Acknowledgements This work was performed within the project "Metal(-oxide) catalyst-monolayer as cost-effective electrocatalysts for PEM water electrolysis" funded by the Research Council of Norway, contract no. 254976/E20. References Carmo, D. L. Fritz, J. Mergel, and D. Stolten, “A comprehensive review on PEM water electrolysis,” Int. J. Hydrogen Energy, 38, pp. 4901–4934, 2013.Comninellis and G. P. Vercesi, “Characterization of DSA®-type oxygen evolving electrodes: Choice of a coating,” J Appl Electrochem, 21, pp. 335–345, 1991.N. Nong, L. Gan, E. Willinger, D. Teschner, and P. Strasser, “IrOx core-shell nanocatalysts for cost- and energy-efficient electrochemical water splitting,” Chem. Sci., 5, pp. 2955–2963, 2014.Brankovic, J. X. Wang, and R. R. Adžic, “Metal monolayer deposition by replacement of metal adlayers on electrode surfaces,” Surface Science, vol. 474, L173–L179, 2001. Figure 1

Transition metal (Mn, Fe, Co, Ni) and nitrogen co-doping for improving the photocatalytic activity of monolayer MoS2

Exploring efficient photocatalyst is important for environmental protection. In this work, we systematically investigate the doping configuration, formation energy, band structures, density of state, recombination rate, catalytic active site, work function, redox potential, and optical absorption properties of transition metal and nitrogen co-doped monolayer MoS2. In transition doped MoS2, transition metal atoms replace Mo. Transition metal and nitrogen atoms substitute Mo and S atoms in transition metal and nitrogen co-doped MoS2, respectively. Compared with undoped MoS2, transition metal and nitrogen co-doped MoS2 have low photogenerated carrier recombination rate, abundant catalytic active sites, small work function, suitable redox potentials, and strong light absorption, especially Mn-N and Ni-N co-doped MoS2.

Computational verification of conductive Be2Zn monolayer as a superior anode for alkali and alkaline ion batteries

Availability of high-performance anode materials remains a great challenge for the clean energy storage systems. Here, we predict the planar hexacoordinate Be2Zn monolayer by first-principles calculations as a highly conductive metallic conductor with all-round structural stabilities. More impressively, the material delivers ultrahigh theoretical specific capacities of 1285, 1928, 1285, and 3856 mA h g−1 for Li, Na, K, and Ca atoms, respectively, with low migration barriers of 18, 7, 4, and 119 meV as well as with favorite average open-circuit voltages of 0.346, 0.366, 0.322, and 0.151 V. Its structural reversibility at the maximum loading concentrations is jointly confirmed by ab-initio molecular dynamics simulations at the upper temperature of 400 K for commercial battery operation under normal ambient environments and subsequent energy minimization calculations. Moreover, the monolayer can tolerate heavy tensile strain up to around 20% far surpassing the structural deformations at different charging stages, which convincingly guarantees its superlative cycle stability during the charging and discharging processes. Taken together, these appealing findings show that the metallic Be2Zn monolayer has great potential to be used as a universal Li, Na, K, and Ca ion battery anode.

Correlating Experimentally Determined Hydrogen Binding Energy with Hydrogen Evolution Activity over Metal Monolayers on Molybdenum Nitride

Correlating Experimentally Determined Hydrogen Binding Energy with Hydrogen Evolution Activity over Metal Monolayers on Molybdenum Nitride

A first-principles study of hydrogen storage on pristine and Li-decorated aluminium monolayer

Hydrogen storage is a major challenge for the development of hydrogen energy technology, and this has inspired the search for novel materials with high storage capacity and reversibility. In this work, we study the use of pristine and Li-decorated metallic honeycomb aluminium monolayer for hydrogen storage, using density functional theory based methods. Our findings reveal that the hydrogen adsorption energies are significantly improved upon Li decoration, from an average of −0.045 eV for pristine monolayer to an average of −0.18 eV. Further, the Li-decorated monolayer can adsorb up to 3H2 molecules per Li atom, and the binding remains almost constant for different adsorbing sites for the Li atom. Additionally, we studied the impact of pressure and temperature on the hydrogen storage capability of the decorated monolayer and found that all of the H2 molecules adsorbed on the monolayer are stable at mild pressure at room temperature. The DOE's target of 5.5 wt % for hydrogen gravimetric densities in 2025 may be exceeded as a result of the metal decoration on both sides of the monolayer. Our results highlights Li-decorated low-cost metallic Aluminum sheets as potential candidates for hydrogen storage.

Encoding multistate charge order and chirality in endotaxial heterostructures

High-density phase change memory (PCM) storage is proposed for materials with multiple intermediate resistance states, which have been observed in 1T-TaS2 due to charge density wave (CDW) phase transitions. However, the metastability responsible for this behavior makes the presence of multistate switching unpredictable in TaS2 devices. Here, we demonstrate the fabrication of nanothick verti-lateral H-TaS2/1T-TaS2 heterostructures in which the number of endotaxial metallic H-TaS2 monolayers dictates the number of resistance transitions in 1T-TaS2 lamellae near room temperature. Further, we also observe optically active heterochirality in the CDW superlattice structure, which is modulated in concert with the resistivity steps, and we show how strain engineering can be used to nucleate these polytype conversions. This work positions the principle of endotaxial heterostructures as a promising conceptual framework for reliable, non-volatile, and multi-level switching of structure, chirality, and resistance.

Metallic BSi3 monolayer as a high rate-capacity anode material for flexible potassium ion batteries: A first-principles study

Potassium ion batteries (PIBs) have attracted great attention as a competitive substitute for lithium ion batteries (LIBs) because of the good safety and low cost; nevertheless, their practical application is severely impeded by the lack of a high-performance anode material. Motivated by the high capacity of silicon and excellent stability of boron, we investigate a 2D B-Si binary compound (h-BSi3 and o-BSi3) as the anode material of PIBs. Our first-principles calculation results demonstrate that BSi3 monolayers have the intrinsic metallicity with high electrical conductivity, which is of great importance for the rapid electronic transport during electrochemical process. Benefitting from the strong affinity of potassium, the h-BSi3 (o-BSi3) anode achieves a high specific capacity of 1268.45 (1190.15) mA h/g and a low averaged open circuit voltage of 0.38 (0.48) V. The potassium diffusion barrier on the h-BSi3 (o-BSi3) anode is as low as 0.02 (0.04) eV. Moreover, a small lattice expansion is observed for the BSi3 anodes without metal dendrite formation at the final potassiated stage, which is beneficial to prolong the cycle life of PIBs. All the unique features suggest that BSi3 monolayers are promising anode candidates for flexible PIBs.

Catalytic reduction of CO2 in the interface formed by monolayer graphene and metal atom (Pt, Ni, Pd, Co) doped Cu-nanoclusters: A theoretical design and investigation

In this study, pristine Cu13 nanocluster and a series of metal atoms (Pt, Ni, Pd and Co) doped Cu12X were designed and placed on the top of a monolayer graphene to form novel catalysts for CO2 reduction. Based on the designed catalysts, we studied the reaction mechanisms of five catalysts for CO2 reduction leading to three different products: CH4, CH3OH and HCOOH. By comparing the activation energies of different reaction channels of different catalysts, we obtained the three different product rate-determining steps of CO2 reduction of CH4, CH3OH and HCOOH. The selectivity order of CO2 reduction products catalyzed by graphene-supported Cu13/G and doped Cu12X/G catalysts is CH4>CH3OH>HCOOH. The order of catalyst activity is Cu12Co/G>Cu12Ni/G>Cu12Pd/G>Cu12Pt/G>Cu13/G. The doping of the four metals Pt, Ni, Pd and Co are all beneficial for improving the catalytic activity of CO2 reduction, and the catalytic activity of metal Co doping is the optimal. These results agree well with the observed phenomena. Our study reveals a correlation between the CO2 reduction catalytic activity of graphene-supported Cu13/Cu12X nanoclusters and their physical properties.

Main group tin single atom catalyst supported by C5N monolayer for oxygen evolution reactions

Single atom catalysts (SACs) supported by carbonaceous materials present extraordinary activity for oxygen evolution reaction (OER), which usually contain transition metal atoms serving as active sites. However, when it turns to the SACs containing main group metal atoms, the OER performance is believed to be significantly decreased. We here proposed a strategy of designing SACs with p-block element of Sn as active sites, which exhibit excellent OER performance without nitrogen coordination of the metal centers. Density functional theory calculations demonstrate that atomically dispersed Sn atoms supported by the metallic monolayers of C5N can efficiently catalyze OER, of which the Sn center coordinated with four C atoms give rise to low overpotential of 0.54 V comparable or even lower than those given by the transition-metal-based SACs. Moreover, correlation between the overpotentials and some intrinsic properties of SACs, which demonstrates that the OER performance is not strongly correlated with the charge state of Sn atoms and Sn-N/C bond length. This study paves a new way of theoretically designing SACs containing main group elements toward OER or even broader range of electrochemical reactions.

Effect of the electrostatic field due to the surface monolayer metal atoms on the dissociation of homonuclear diatomic gases in gas-metal surface reactions

The present paper examines the role of an electrostatic field generated by the outermost monolayer of metal ions prior to gas adsorption in aiding the dissociation of homonuclear diatomic gas molecules. The interaction of five homonuclear diatomic gases, Cl2, F2, H2, N2 and O2, with several pure metals are examined using Coulomb's law to calculate the attractive and repulsive forces between the electrons or ions in the gas molecule and the free surface metallic electrons or ions assuming a Bohr model. These calculations demonstrate that the total energy of the electrostatic fields from the metals can exceed the molecular binding energies of Cl2, F2, and O2 at some distance from the metallic surface thereby suggesting that these gases interact primarily with the metallic surface in their atomic states after molecular bond dissociation prior to reaction. In contrast, H2 and N2 gases do not dissociate prior to reaching the metal surface due to the fact that the effective charge of the gas ions is less than the total electron charge at the outermost electronic shell. The present results correlate linearly with the electrochemical series standard reduction potential as well as with the Pauling electronegativity for several metals.

Realization of Fermi level unpinning and high-quality p-type contacts for 2D β-TeO2 by a built-in intercalation

Contriving an industry-compatible method for fabricating purely high-quality p-type two-dimensional (2D) field effect transistors (FETs) has been proved to be pressing but challenging due to the strong interfacial coupling and thus tight Fermi level pinning (FLP). Herein, by employing density functional theory calculations, a straightforward but effective strategy is proposed to achieve high-quality p-type contact in few-layer TeO2–Pt junction by using a build-in intercalation to suppress the FLP and shield the damage from electrodes to the excellent electrical properties of TeO2 such as its ultra-high hole mobility. Specifically, the increase of TeO2 layer number will make the charge distribution of VBM and CBM be more lumped in the inner core layer, as a result, the outlying Te–O sublayer can act as an inserting layer like BN to protect the semiconducting electronic states and avoid gap-state pinning. Thus, as the TeO2 layer increases from 1 L to 3 L, (Ⅰ) strong FLP in metal-TeO2 interfaces is gradually unpinned; (Ⅱ) the polarity of Pt–TeO2 contact reverses from n-type to p-type; (Ⅲ) the excellent electrical properties of TeO2 such as hole effective mass is preserved as original despite strong charge transfer occurring at the interface. Besides, the tunnelling barrier at the interface is close to zero. This is quite different from introducing an extern BN intercalation, in which case the tunnelling through BN will prominently reduce the overall hole current. In addition, the contacts between 2D metal electrodes and monolayer TeO2 are found to obey the Schottky-Mott rule, and thereby p-type Ohmic contacts can be obtained at TeO2-VS2, -NbS2 interfaces. Whereas, a relatively large tunnelling barrier exists at the 2D metal-TeO2 interfaces. Our findings are quite essential and beneficial for designing high-performance p-type 2D FETs.

Magnetism and magnetic properties of 3d transition metal monolayer on Pd(1 0 0)

First-principles calculations based on density functional theory (DFT) are utilized to examine the effect of 3d transition metal (TM) monolayer on the Pd (001) substrate. Four transition metal (Cr, Mn, Fe, and Co) monolayers are investigated in the middle of a 1 × 1 × 7face-centered cubic supercell. Two different structures are considered, one with the TM monolayer sandwiched between the Pd atoms in the supercell creating a substrate-cap structure (Pd/TM/Pd), while the other structure is left uncapped (Pd/TM). We found that the magnetocrystalline anisotropy energy of the (Pd/TM/Pd) structure is more significant than the uncapped structure (Pd/TM). Furthermore, the ground state of the Cr and Mn monolayers is antiferromagnetic, while the ferromagnetic ordering is the ground state for the Fe and Co monolayers. In addition, magnetic moments, anisotropy field, and maximum energy products are studied.