Adsorbed Alkali Atoms Research Articles

Quantum-Confined Lifshitz Transition on Weyl Semimetal Td-MoTe2.

Adsorption of alkali atoms onto material surfaces is widely utilized for controlling electronic properties and is particularly effective for two-dimensional materials. While tuning the chemical potential and band gap and creating quantum-confined states are well established for alkali adsorption on semiconductors, the effects on semimetallic systems remain largely elusive. Here, utilizing angle-resolved photoemission spectroscopy measurements and density functional theory calculations, we disclose the creation of two-dimensional electron gas and the quantum-confined Lifshitz transition at the surface of a Weyl semimetal Td-MoTe2 by potassium adsorption. Electrons from potassium adatoms are shown to be transferred mainly to the lowest unoccupied band within the gapped part of the Brillouin zone, which, in turn, induces strong surface band bending and quantum confinement in the topmost layer. The quantum-confined topmost layer evolves from a semimetal to a strong metal with a Lifshitz transition departing substantially from the bulk band. The present finding and its underlying mechanism can be exploited for the creation of electronic heterojunctions in van der Waals semimetals.

Hexagonal boron phosphide as a potential anode nominee for alkali-based batteries: A multi-flavor DFT study

The adsorption of alkali atoms (Li, Na, and K) on graphene and hexagonal boron phosphide (h-BP) is studied by means of various density functionals including semi-local GGA, dispersion corrected DFT-D, meta-GGA, and many of the available non-local vdW-DF functionals. The accurate interaction energies are crucial for energy storage and battery applications as it is directly related to key properties in electrochemistry, such as storage capacities in general, and open circuit voltages (OCVs), in particular. A wider range of adsorption strength is predicted depending on the choice of non-local vdW-DF functionals. Furthermore, the performance of vdW-DF functionals was found to be independent of the choice of pseudopotentials used and especially if the choice is revPBE. Additionally, an excellent agreement is found between the Gaussian, VASP, and QE codes. Moreover, the h-BP is found to be an exceptional anode material for alkali batteries which can compete with any other available anode material. The small change in the adsorption energies as a function of increasing concentration of alkali atoms is a unique characteristic of h-BP. The exceptional 1283 mAh/g storage capacity not only for Li but in contrary with the previous study, also for Na in addition to the 642 mAh/g for K at vdW-DF/DZP level makes it a prominent candidate to be used as anode material. The average open circuit voltage for Li, Na, and K was also found to be in superb range. However, the values are found to be sensitive to the choice of functional, and in some cases, the storage capacity can be predicted as high as twice of the actual values. Therefore, the accurate description of the interaction is crucial and this study can be used to further refine the non-local DFT functionals. Moreover, by virtue of these properties, h-BP can be the best choice to be used as an anode material not only in LIBS but also in SIBs and KIBs.

Potassium and Water Coadsorption on TiO2(110): OH-Induced Anchoring of Potassium and the Generation of Single-Site Catalysts

Potassium deposition on TiO2(110) results in reduction of the substrate and formation of loosely bound potassium species that can move easily on the oxide surface to promote catalytic activity. The results of density functional calculations predict a large adsorption energy (∼3.2 eV) with a small barrier (∼0.25 eV) for diffusion on the oxide surface. In scanning tunneling microscopy images, the adsorbed alkali atoms lose their mobility when in contact with surface OH groups. Furthermore, K adatoms facilitate the dissociation of water on the titania surface. The K-(OH) species generated are good sites for the binding of gold clusters on the TiO2(110) surface, producing Au/K/TiO2(110) systems with high activity for the water-gas shift.

Adsorption of alkali adatoms on graphene supported by the Au/Ni(111) surface

We study alkali-metal adsorption on supported graphene by means of density-functional-theory calculations that include dispersion corrections. Graphene supported by the Au/Ni(111) surface is an important system for fundamental studies because this surface allows one to support graphene, preserving the electronic properties of freestanding graphene. We investigate the binding energetics as well as the structural and electronic properties of Li, Na, K, and Rb atoms adsorbed or intercalated at the graphene/Au/Ni(111) interfaces and compare the results to those obtained on freestanding graphene. Both the adsorption and intercalation of the alkali atoms induce electron doping to the graphene $\ensuremath{\pi}$ bands that display a quasirigid-energy shift. Electron doping by alkali atoms preserves the Dirac cone of graphene, which shifts downwards due to the negative polarity of the doping. The metallic or ionic character of the alkali dopant is controlled by its position relative to graphene. All of the investigated alkali atoms display an energetic preference to intercalate between graphene and the Au/Ni support. These results shed light on the relationship between graphene electron doping, the alkali-atom adsorption site, and the charge transfers at the graphene/support interface.

Free energies of absorption of alkali ions onto beidellite and montmorillonite surfaces from constrained molecular dynamics simulations

Ab initio Molecular dynamics have been performed to study the free energy of adsorption of alkali atoms onto smectite clay surfaces and to identify the most favourable region in the interlayer for the cations. This is achieved by potential of mean force calculations using a constraint method to determine the lowest free energy configurations of lithium, potassium and sodium beidellite and sodium montmorillonite clays with a monolayer and bilayer of water present in the interlayer region. The constraint method has allowed us to examine the changes in the lowest free energy configuration for each ion with increasing hydration. From this, we can interpret the likelihood of clay swelling from the monolayer to bilayer coverage and compare with experimental observations. We find, that with a bilayer of water present, both lithium beidellite and sodium montmorillonite have their free energy minimum in the centre of the interlayer. For monolayer coverage, the free energy minimum for lithium, sodium and potassium beidellite is approximately the mid-point of the interlayer. Na-beidellite has a lowest free energy region at 6.1Å from the centre of the clay layer for both mono- and bi-layer coverage, while for the potassium counter-ion, commonly used as swelling inhibitor, the free energy profile for K-beidellite shows peaks close to both surfaces at approximately 6.2 and 8.6Å. We find that for systems where the free energy minimum remains in the middle of the interlayer when the hydration levels increase from monolayer to bilayer, it is known experimentally that these systems will swell in contact with water. The move to the middle of the interlayer with increasing hydration is associated with the full hydration sphere of the ion being composed purely of water oxygen atoms, and no clay surface oxygen atoms.

Self-consistent approach for spectral properties of single alkali adatoms on Cu(111)

We investigate the nature of the adatom-surface interaction in the adsorption of alkali atoms on Cu(111). Our calculation, exploiting the embedding approach in a density functional theory framework, describes an isolated atom on a metallic surface, which is modeled via a one-dimensional modulated potential. The absence of empirical term in the Hamiltonian guaranties a completely ab initio determination of the electronic properties of the system. The role of the projected energy gap in determining lifetime and binding energy of the adatom resonances is evidenced. On the basis of the electronic properties of the adsorbed alkali atoms the covalent/ionic nature of the bonding with the surface is analyzed.

Theoretical investigation on the alkali-metal doped BN fullerene as a material for hydrogen storage

First-principles calculations have been used to investigate hydrogen adsorption on alkali atom doped B 36N 36 clusters. The alkali atom adsorption takes place near the six tetragonal bridge sites available on the cage, thereby avoiding the notorious clustering problem. Adsorption of alkali atoms involves a charge transfer process, creating positively charged alkali atoms and this polarizes the H 2 molecules thereby, increasing their binding energy. Li atom has been found to adsorb up to three hydrogen molecules with an average binding energy of 0.189 eV. The fully doped Li 6B 36N 36 cluster has been found to hold up to 18 hydrogen molecules with the average binding energy of 0.146 eV. This corresponds to a gravimetric density of hydrogen storage of 3.7 wt.%. Chemisorption on the Li 6B 36N 36 has been found to be an exothermic reaction, in which 60 hydrogen atoms chemisorbed with an average chemisorption energy of −2.13 eV. Thus, the maximum hydrogen storage capacity of Li doped BN fullerene is 8.9 wt.% in which 60 hydrogen atoms were chemisorbed and 12 hydrogen molecules were adsorbed in molecular form.

Structure and Electronic Properties of Alkali−C60 Nanoclusters

I investigated the structural and electronic properties of both Na(n)C(60) and Li(n)C(60) (n < or = 12) clusters in the framework of density-functional theory. The behavior of the alkali atoms over the C(60) surface is analyzed. The hypotheses for either an homogeneous coating of the C(60) surface by the alkali atoms or the growth of an alkali droplet not wetting the fullerene surface are discussed. Lithium atoms are found to coat homogeneously the fullerene on the C(60) surface via pentagonal sites, contrary to sodium atoms, which prefer to form 4-atom islands on the surface. The charge transfer, the energetics, and the dipole moments are discussed in relation with available results. The adsorption of alkali atoms on the C(60) surface considerably enhances the capacity of C(60) to accept electrons. The arrangement of lithium atoms seems more favorable for the hydrogen storage than that of sodium atoms.

Low-coverage alkali metal adsorption on the Ge(0 0 1)-p(1 [formula omitted]2) surface

Alkali metals adsorbed on Ge(0 0 1)-p(1 × 2) exhibit a variety of atomic arrangements depending on the coverage. We present theoretical investigation of a quasi-isolated alkali atom adsorption on the Ge(0 0 1)-p(1 × 2) surface, with and without a missing-dimer defect. Stable adsorption sites are found and the corresponding adsorption energies are compared. For completeness, STM images are simulated for the energetically most favourable atomic configurations.

Tuning the work function of ultrathin oxide films on metals by adsorption of alkali atoms

We report a theoretical investigation of the adsorption of alkali metal atoms deposited on ultrathin oxide films. The properties of Li, Na, and K atoms adsorbed on SiO(2)/Mo(112) and of K on MgO / Ag(100) and TiO(2)/Pt(111) have been analyzed with particular attention to the induced changes in the work function of the system, Phi. On the nonreducible SiO(2) and MgO oxide films there is a net transfer of the outer ns electron of the alkali atom to the metal substrate conduction band; the resulting surface dipole substantially lowers Phi. The change in Phi depends (a) on the adsorption site (above the oxide film or at the interface) and (b) on the alkali metal coverage. Deposition of K on reducible TiO(2) oxide films results in adsorbed K(+) ions and in the formation of Ti(3+) ions. No charge transfer to the metal substrate is observed but also in this case the surface dipole resulting from the K-TiO(2) charge transfer has the effect to considerably reduce the work function of the system.

Gap opening in the surface electronic structure of graphite induced by adsorption of alkali atoms: Photoemission experiments and density functional calculations

The adsorbate-substrate interaction for Na and K on highly oriented pyrolytic graphite (HOPG) has been investigated by photoemission and by density functional calculations. With increasing coverage a peculiar spectral structure develops below the Fermi level reminiscent of the one observed for $\mathrm{Cu}∕\mathrm{Xe}∕\mathrm{HOPG}$ and formerly interpreted in terms of a Fano resonance. In contrast, we show that this unusual spectral feature reflects a surprising gap opening in the surface electronic structure of graphite owing to charge transfer upon alkali metal adsorption. This finding allows us to put forward a unified picture for the electronic structure changes upon adsorption of alkali metals (Na, K) on HOPG as well as of alkali and alkalilike metals on noble gas spacer layers supported on HOPG.

Broadening mechanisms for the adsorbate-induced resonance in the Na/Cu(1 1 1) and Cs/Cu(1 1 1) systems

Adsorption of alkali atoms on the (1 1 1) and (1 0 0) noble metal surfaces has been shown recently to induce long-lived resonances located inside the surface projected band gap. However, the width of these resonances, as it appears in two-photon photo-emission experiments, is much larger than the inverse of their lifetime. We report on a theoretical study of some broadening mechanisms of these resonance lines in the Na/Cu(1 1 1) and Cs/Cu(1 1 1) systems at low coverage, including the homogeneous natural line broadening and the inhomogeneous statistical broadenings due to the distribution of adsorption heights associated to the quantal vibration of the alkali adsorbate and to the lateral disorder of alkali adsorption on the surface. The inhomogeneous mechanisms are shown to induce a very large broadening of the resonance line, in quantitative agreement with experimental results. The most important broadening effect appears to be the effect of the distribution of alkali adsorption heights.

Interaction of alkali atoms with water multilayers adsorbed on TiO 2(1 1 0): a study with MIES and UPS

The chemistry of alkali atoms (Li, Na, K, Cs) embedded in a multilayer aqueous environment was studied with metastable impact electron spectroscopy (MIES) and ultraviolet photoemission spectroscopy (UPS) (HeI and II) under ultra high vacuum (UHV) conditions. The water multilayers were grown at 130 K on a rutile, 1×1 reconstructed, TiO 2(1 1 0) single crystal. The behavior of the multilayer system was investigated as a function of the temperature (130–500 K). Due to the relatively large escape depth of the emitted electrons UPS provides spectroscopic information about several layers in contrast to MIES which is only sensitive to the outermost layer. This allows us to discriminate between species adsorbed at the water multilayer and species, which are embedded in the solvent or at the substrate–solvent interface. Furthermore, MIES is, in contrast to UPS, very sensitive to the outermost s-states of adsorbed alkali atoms, which are considered to be responsible for the high reactivity of these metals. The present study gives insight into the complicated chemistry of alkali atoms added to an aqueous multilayer system. The chosen combination of MIES and UPS allows us to distinguish clearly between various phases depending on the amount of offered alkali atoms. For low alkali concentrations the alkali atoms penetrate the water surface whereby they dissociate some water. With increasing exposure more and more water molecules become dissociated, whereby the outermost water layer remains intact. Finally, the chemistry between water and alkali atoms takes place at the outermost surface too, which is manifested by the formation of OH-groups at the surface. With further increasing alkali concentration the atoms start to adsorb as neutral atoms; whereby the spectrum of the alkali species is then strongly influenced by the underlying solvent system. For very high exposure the observed spectra are not influenced by the underlying aqueous system anymore; the formation of an alkali metal film is observed. Under annealing, the entire multilayer system desorbs around 200 K; a titania surface covered by hydroxyl groups and alkali ions is remaining. This adlayer desorbs under further annealing above room temperature (around 400 K, but dependent on the particular alkali atom) resulting in a titania surface.

High-coverage adsorption of alkali metals at the CaO and MgO (100) surfaces

The interaction of alkali metals (Li and Na) with the defect-free (100) surface of CaO and MgO has been studied in the framework of density functional theory with periodic boundary conditions for the coverages 1.0 and 0.5 ML (Li) and 0.5 ML (Na). The lateral interaction among adsorbed alkali atoms contributes significantly to the stabilization of the metallic layer. The binding energy of Li for the 1.0 ML coverage is comparable for CaO and MgO and is larger than the cohesive energy of bulk Li. Hence, this monolayer is stable with respect to formation of a three-dimensional metallic island, and a small lattice mismatch between MgO(100) and the bcc Li indicates the possibility of creating a low-energy interface between these two materials under proper growth conditions. The chemical effects in the metal–substrate interaction are not significant.

Theoretical Study of c(2×2) Structure in Li/Al(001) System

Stability of Li/Al(001)-c(2×2) structure in which Li atoms are adsorbed substitutionally on Al(001) surface is studied by performing first-principles total-energy calculations. Our results for optimized structural parameters of the system are in reasonably good agreement with experiment. It is found that the c(2×2) structure is stabilized mainly due to a binding-energy gain of Li with Al atoms. Density of states of the c(2×2) structure of Li/Al(001) is compared with that of Na/Al(001) system to clarify bonding nature of alkali-atom adsorption.

Alkali Promotion ofN2Dissociation over Ru(0001)

Using self-consistent density functional calculations, we show that adsorbed Na and Cs lower the barrier for dissociation of N2 on Ru(0001). Since N2 dissociation is a crucial step in the ammonia synthesis reaction, we explain in this way the experimental observation that alkali metals promote the ammonia synthesis reaction over Ru catalysts. We also show that the origin of this effect is predominantly a direct electrostatic attraction between the adsorbed alkali atoms and the dissociating molecule. [S0031-9007(98)06016-5]

One-Dimension Chains of Structural Defects on the Surface of Sapphire and their Role in Adsorption of Alkali Atoms

Alkali adsorption on the (0001) surface of the monocrystalline α- Al 2 O 3 was studied by means of laser-simulated desorption. It was found that alkali atoms are adsorbed mostly on the centers with the surface concentration of about 1013 cm -2. Although the temperature dependence of surface coverage is consistent with the conventional Langmuir model for adsorption on the isolated centers, the estimated adsorption energy of about 1 eV is too large to account for the quantum yield of the photodesorption process, which is as high as 5 ×10-5. To resolve this contradiction we assumed that adsorption centers make up chains along the steps on the surface. Lateral interactions between neighboring adatoms were accounted for by means of methods developed for the Ising model. Theoretical results are in agreement with the experiment.

Desorption kinetics of alkali atoms from transition metals

A model for the adsorption of alkali atoms on transition metals is proposed and thermal desorption spectra are analysed for the systems K/Ni(111), K/Pt(111) and K/Fe(111). The basic assumption is that the interaction energy between alkali atoms, being of dipole - dipole type, attenuates as coverage increases. The attenuation function, obtained by fitting experimental thermal desorption spectra, is used to calculate the variation with coverage of the dipole moment and the work function through the Topping model. These calculations show an excellent agreement with independent experimental measurements of the dipole moment and work function over the whole coverage range.

Bonding of an Isolated K atom to a Surface: Experiment and Theory

We present a new and general technique with which the local properties of the unoccupied states of an adsorbed alkali atom in the low coverage limit can be studied. The method, based on a combination of experimental core level spectroscopy data and calculations, is demonstrated for the K/graphite system. We show that core excitation and decay spectra of adsorbed Ar combined with the calculations form a most powerful tool in order to determine the unoccupied density of states for an isolated K adatom. We find that the 4s level of an isolated K atom on graphite is placed 2.7eV above E{sub F} , with a hybridization width of the order of only 0.1eV. {copyright} {ital 1997} {ital The American Physical Society}

Erratum: Inner-shell promotions in low-energy Li+-Al collisions at clean and alkali-covered Al(100) surfaces

Spectra of scattered ions and ion-induced electron emissions are used to investigate the electronic processes that occur during 0.4--5.0-keV Li[sup +] bombardment of clean and alkali-atom-covered Al(100). The results show that Li 1[ital s] electrons are promoted during hard Li-Al collisions, but not during Li-K or Li-Na collisions. Consequences of the inner-shell electron promotions are evident in the inelastic loss, neutralization behavior, and electron emissions of the scattered Li particles. Spectra of scattered Li[sup +] ions exhibit discrete-loss features, which are resistant to the usual increase in resonant neutralization that accompanies the deposition of alkali atoms on the surface. The loss features are due to Li 1[ital s] excitation via electron promotion, while the production of ions away from the surface via autoionization is responsible for their lack of response to alkali-atom adsorption. Spectra of ion-induced electron emissions confirm that Li[sup *](1[ital s]2[ital s][sup 2]) is produced and that it undergoes autoionization.