Monatomic Layer Research Articles

Flexural rigidity of layers and its manifestation in the vibrational characteristics of strongly anisotropic layered crystals. Characteristic frequencies and stability conditions in quasi-two-dimensional systems

The reasons for the appearance of flexural rigidity of layers in strongly anisotropic layered crystals are investigated. Structures consisting of loosely bound monatomic layers (specifically, graphite) as well as formed loosely bound structural elements each consisting of several monolayers which are coupled much more strongly with one another (for example, in dichalcogenide transition metals) are examined. The effect of the flexural rigidity on the phonon spectra of these compounds—quasi-flexural bending of the dispersion curves of phonon modes polarized in a direction normal to the layers—and the particularities appearing in the phonon densities of states as a result of quasi-flexural vibrational branches crossing low-frequency optical modes are analyzed.

Effect of Sputtering Deposition Process on Magnetic Properties of Magnetic Multilayers

The energy distribution of energetic particles produced in magnetron sputtering plasma was measured by varying the sputtering gas species and pressure, and Co/Pt, Gd/Fe, and Co/Cu multilayers were fabricated using the same sputtering cathodes as the diagnosed one in order to discuss the relationship between the plasma parameters and the magnetic properties of the multilayers. When a heavy mass target such as W or Pt was used in Ar gas sputtering, the energy of the recoiled Ar atoms from the target was found to reach up to 150 eV. The energy and flux of the recoiled atoms were reduced by using Kr or Xe gas in place of Ar gas, whereas they are rather independent of the sputtering pressure in the range of 0.3–0.5 Pa. Low-pressure sputtering was effective for increasing the energy of sputtered atoms, which was at most 10 eV. Layered structures of the magnetic multilayers were significantly influenced by the sputtering conditions. The atomic intermixing at the interface, i.e., degradation of the interface sharpness, was controlled mainly by using a heavy sputtering gas, while the surface or interface flatness mainly depended on the sputtering pressure. The perpendicular magnetic anisotropy of Co/Pt and Gd/Fe multilayers was enhanced by suppressing atomic intermixing rather than by improving the interface flatness, and the use of a heavy inert gas such as Kr or Xe was found to be effective to enhance the anisotropy. When Kr or Xe gas was used, the monatomic layered structure of the Co (1 ML)/Pt (1 ML) (ML: monatomic layer) multilayer was found to survive, and positive perpendicular anisotropy was observed. In contrast, the magnetoresistance ratio of Co (1.5 nm)/Cu (t nm) multilayers (t∼0.9 nm) increased to around 50% when they were sputtered in an inert gas at low pressure, while the use of Xe gas degraded their magnetoresistance ratio. In the Co/Cu multilayers, the interface flatness rather than the sharpness was critical for enhancing the magnetoresistance ratio.

Atomically precise impurity identification and modification on the manganese doped GaAs(110) surface with scanning tunneling microscopy

Cross-sectional scanning tunneling microscopy (STM) measurements on molecular beam epitaxy grown Mn doped GaAs(110) at 5 and 77 K are presented. The enhanced mechanical stability of the STM at low temperature allows a detailed study of the electronic contrast of Mn atoms in the GaAs(110) surface. According to reproducible and distinguishable contrast patterns of single Mn atoms, we present statistical evidence for a layer by layer identification of Mn atoms embedded in the first few monatomic layers of the crystal. A comparison with a bulklike theoretical approach reveals a semiquantitative agreement with the measurements. Remaining differences between theory and experiment indicate the influence of the surface as an important factor to understand the contrast of impurities close to the surface. Furthermore, we report the injection of transition-metal atoms into the surface. Finally, reproducible complexes consisting of a surface Mn and an adsorbate atom are found and manipulated.

Fe–Ni Surface Alloy Formation on Ni(111) Investigated by Scanning Tunneling Microscopy

The structure of Fe films grown on Ni(111) with an Fe coverage of 3.2 monolayers and its conversions during heating in ultrahigh vacuum have been investigated by scanning tunneling microscopy (STM). The Fe films consist of elongated domains stacked with four to six monatomic layers of Fe, forming ridgelike structures. A phase transition from fcc(111) to bcc(110) with the Nishiyama–Wassermann orientation relationship occurs from at least the third monatomic layer. A triangular network structure and large accumulated bcc-Fe islands are formed on the surface after heating the Fe films at 550 K. The network is considered to be misfit dislocation loops caused by the lattice misfit between the fcc-Fe films and the Ni substrate. A surface structure consisting of two distinct randomly distributed spots and a well-ordered (5 ×5) superstructure are observed after further heating at 620 and 700 K, respectively, and they are caused by the formation of the Fe–Ni surface alloy on the Ni(111) surface.

Fullerene on Nitrogen-Adsorbed Cu(001) Nanopatterned Surfaces: From Preferential Nucleation to Layer-by-Layer Growth

Nitrogen (N)-adsorbed Cu(001)−c(2 × 2) nanopatterned surfaces are used as templates to guide the growth of low-dimensional C60 molecular nanostructures. At room temperature and during the initial stages of growth, C60 molecules preferentially adsorb on the bare Cu regions on a partially N-covered grid surface. Subsequently, a two-dimensional molecular nanomesh is formed at low (∼0.28 monatomic layer) C60 coverages. Further deposition leads to C60 growth on the c(2 × 2)−N surface until the first molecular layer is completed. For a N-saturated surface with trench structures, the <010> steps of these structures serve as initial anchoring sites for C60 growth. From there, the growth proceeds two-dimensionally until a single C60 layer is achieved due to island coalescence. In contrast, no nucleation site was observed when the <110> steps were predominant on the surface. At least up to 6 monatomic layers, the growth proceeds layer-by-layer (i.e., the overlayer morphologies are directed by the underlying substrate pattern). Four rotational domains are observed for the quasi-hexagonally close-packed C60 overlayer with a nearest-neighbor C60−C60 distance of 1.02 nm. It was found that the interaction between C60 and the c(2 × 2)−N surface is fairly weak, likely dominated by van der Waals forces, whereas the C60−Cu interface is chemisorbed. Site-specific electronic effects between these two regions can be resolved by STM even for thick films.

Electronic excitation in anAr7+ion traversing a graphene sheet: Molecular dynamics simulations

We studied the interaction between an ${\mathrm{Ar}}^{7+}$ ion and a graphene sheet by combining real-time propagation of electron wave functions with molecular dynamics simulations. Our calculation supports the fluorescence from ${\mathrm{Ar}}^{7+}$ ions penetrating the carbon foil [S. Bashkin et al., Phys. Rev. A 25, 417 (1982)], and it suggests that this fluorescence also occurs even when the thickness of the target is only the monatomic layer and when the incident kinetic energy corresponds to $500\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. Monitoring the fluorescence is a useful way to detect graphene sheets even with monatomic thickness.

Fivefoldi−Al−Pd−Mnsurface as template for growing monatomic quasiperiodic layers: First-principles simulations for adatoms from groups one to three

By using ab initio density-functional calculations, we have explored the conditions for the formation of quasiperiodic overlayers on top of a fivefold surface of an icosahedral Al-Pd-Mn quasicrystal. We calculate binding energies of single adatoms at special sites of the surface, with the aim to construct the potential-energy landscape of the quasiperiodic surface. We show that the energetically favorable adsorption sites are located at the vertices of a DHBS tiling [consisting of decagons (D), hexagons (H), boats (B), and stars (S)] describing the quasiperiodic ordering of the surface. An idealized structural model for the adlayer is constructed by decorating the interior of these tiles such that atoms are placed again in energetically favorable locations and that the local arrangement does not break the fivefold symmetry of the underlying substrate. The stability of this idealized structure is probed for elements from groups one to three of the Periodic Table by performing a conjugate-gradient relaxation under the action of the ab initio calculated forces. We demonstrate that in addition to a strong binding of the adatoms to the substrate, the size of the adatoms and the achievement of a dense surface coverage are the most important factors influencing the stability of quasiperiodic ordering in the adsorbed monolayer. From the analysis of the geometrical distribution of the adsorption sites, we conclude that a dense quasiperiodic overlayer with a density of $0.066\phantom{\rule{0.3em}{0ex}}\text{atoms}∕{\mathrm{\AA{}}}^{2}$ (corresponding to a coverage $\ensuremath{\Theta}\ensuremath{\approx}0.50$ monolayers) can be formed by adatoms with an atomic size of $3.7\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}\ifmmode\pm\else\textpm\fi{}0.4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Atoms with this size are Na, Ca, Y, and La. We suggest that these elements and presumably also most rare-earth elements are good candidates for the formation of highly regular quasiperiodic adlayers with a structure described by a decagonal DHBS tiling consisting of decagons, hexagons, boats, and pentagonal stars.

In situ Real-time Monitoring of Electrochemical Ag Deposition on a Reconstructed Au(111) Surface Studied by Scanning Tunneling Microscopy

Electrochemical deposition of Ag on a 23 × √3 reconstructed surface of Au(111) electrode at various potentials was followed by scanning tunneling microscopy (STM) in situ in real time. At −0.2 V (vs Ag/AgCl), line shaped Ag deposits with the height of 0.46 ± 0.03 nm, which is equivalent to 2 atomic height, were observed. The center of each Ag line was located in the hcp domain of the reconstructed structure. They then grew two-dimensionally so that the other regions, i.e., bridge and fcc domains, of the reconstructed Au surface were gradually covered with the Ag bilayer. As the deposition proceeded, another Ag layer started to nucleate and grow on the Ag bilayer. This layer was one atomic height and grew not linearly but two-dimensionally from the beginning. At 0.3 V, the monatomic layer of Ag was formed preferentially in the hcp domain and the Ag growth stopped at ca. 1 ML. The potential dependent stabilities of the deposited bi- and mono-atomic Ag layers were confirmed by the potential step measurements. The structural conversion from the bi- to mono-atomic layer of Ag was observed when the potential was stepped from −0.2 to +0.3 V. At 0 V, an intermediate potential, both the bi- and mono-atomic Ag layers were observed at the initial stage of Ag deposition. These results revealed that the biatomic Ag layer was more favored at more negative potentials in the range of −0.2 to +0.3 V. The growth mode of the potentiostatic electrochemical deposition of Ag on the reconstructed Au (111) electrode surface observed in this study is quite different from those previously reported for the electrochemical deposition on the reconstruction-lifted Au(111) electrode surface and deposition under ultrahigh vacuum (UHV) condition on the reconstructed Au(111) surface, showing the importance of structure of substrate surface and electrode potential on the growth mode.

Electrocatalytic Oxidation of Formic Acid and Methanol on Pt Deposits on Au(111)

This work presents characteristics of Pt deposits on Au(111) obtained by the use of spontaneous deposition and investigated by electrochemical scanning tunneling microscopy (EC-STM). On such prepared and STM characterized Au(111)/Pt surfaces, we studied electrocatalytic oxidation of formic acid and methanol. We show that the first monatomic layer of Pt displays a (square root 3 x square root 3)R30 degrees surface structure, while the second layer is (1 x 1). After prolonged deposition, multilayer Pt deposits are formed selectively on Au(111) surface steps and are 1-20 nm wide and one to five layers thick. On the optimized Au(111)/Pt surface, formic acid oxidation rates are enhanced by a factor of 20 compared to those of pure Pt(111). The (square root 3 x square root 3)R30 degrees-Pt yields very low methanol oxidation rates, but the rates increase significantly with further Pt growth.

Fermiology and transport in metallic monatomic layers on semiconductor surfaces

We review Fermi surfaces and band dispersion of metallic monatomic layers (surfacesuperstructures) on semiconductor substrates studied by photoemission spectroscopy, inconnection with the surface electronic transport properties. By using various surfacesuperstructures such as two- and quasi-one-dimensional metallic ones, the conductivity, itsanisotropy, and the Hall effect in the surface states are understood quantitativelyby the Fermi surfaces and band dispersion using the Boltzmann picture. Theexperimental techniques and analysis procedures are described, which are applicable tovarious surface systems. This demonstrates that surface atomic layers providean intriguing platform for the study of atomic-scale low dimensional electrondynamics.

The Optical Visibility of Graphene: Interference Colors of Ultrathin Graphite on SiO2

Monatomic layers of graphite are emerging as building blocks for novel optoelectronic devices. Experimental studies on a single graphite layer (graphene) are today possible since very thin graphite can be identified on a dielectric substrate using a normal optical microscope. We investigate the mechanism behind the strong visibility of graphite, and we discuss the importance of substrates and of the microscope objective used for the imaging.

Varying the Schottky barrier of thin film Mg∕H:p-Si(111) contacts: Properties and applications

Different types of Mg∕p-Si diodes were prepared by thermal evaporation of thin Mg layers on H-passivated Si(111) surfaces at low temperatures followed by various annealing procedures. After deposition at 190K, the current-voltage characteristics of the diodes are typical for interfaces with lateral inhomogeneity and high ideality factors. The homogeneous barrier height is measured as 0.8eV. This value is much larger than expected from the metal-induced gap states and electronegativity concept. The deviation is explained by interface dipoles due to a monatomic hydrogen layer between Mg and Si. Hydrogen is removed by annealing the diodes to room temperature. This procedure leads to the formation of an intermediate Mg silicide layer and reduces the homogeneous barrier height to 0.69eV. The annealed diodes show excellent current-voltage properties. Applications of the diodes for chemical sensing such as chemicurrents and for internal photoemission are discussed.

Perpendicular magnetization of L1-ordered FePt films in the thinnest limit

L 1 0 -ordered FePt ultrathin films with thickness down to subnanometer range were fabricated by alternate monatomic layer (ML) deposition of Fe and Pt on Pt (001) substrate and were capped by Pt. Perpendicular Fe magnetic moment under magnetic field, together with that under remanence, was detected by soft x-ray magnetic circular dichroism. Pt∕(Fe1ML∕Pt1ML)3∕Pt (001), whose FePt layer is ∼1nm thick, showed perpendicular remanent magnetization at room temperature. At low temperatures, perpendicular remanent magnetization was found even at the thinnest end, i.e., 1 ML Fe sandwiched by Pt (001).

One-dimensional and spherical polarization analysis of artificially confined spin-density waves in Cr/Sn multilayers

In Cr/Sn(0 0 1) multilayers in which monatomic Sn nonmagnetic layers are periodically embedded, spin-density waves (SDWs) are formed below the Néel temperature like that in bulk Cr. They are, however, quite different to the SDW in bulk Cr. For instance the wavelength of the SDW is defined not simply by the nesting vector of Cr, but also by the artificial periodicity introduced by the Sn layers. We performed the spherical neutron-polarization analysis of these anomalous SDW structures by using CRYOPAD (CRYOgenic Polarization Analysis Device) as well as the one-dimensional polarization analysis in order to get more detailed magnetic structural information which could not be obtained with unpolarized neutron experiments.

Structure and magnetic properties for L1 0-ordered FeNi films prepared by alternate monatomic layer deposition

Alternate monatomic layer (ML) deposition technique has been used to fabricate a metastable L1 0-ordered phase of FeNi alloys. The films were prepared by alternating Fe (0 0 1) and Ni (0 0 1) MLs on MgO (0 0 1) substrates at various temperatures T S in the range between 80 and 400 °C. It has been found that the degree of long-range order S evaluated by X-ray diffraction increases with T S, and it has a maximum at T S=240 °C. With further increase of T S, S shows a drastic decrease, and almost disappears at T S=280 °C. 280 °C is close to the order–disorder transformation temperature in the thermal equilibrium. The variation of K u shows the same tendency as that of S, suggesting the formation of L1 0-ordered FeNi alloy in the range of T S=200–260 °C. Maximum values of S=0.6±0.2 and K u=6.3×10 6 (erg/cm 3) are obtained at T S=240 °C.

Model of grain-boundary diffusion with allowance for near-boundary layers of equilibrium composition

Fisher model of intercrystalline diffusion is refined based on an analysis of previously obtained results of emission Mossbauer studies of grain boundaries. It is shown that under conditions of dominating intercrystalline diffusion, the existence of monatomic near-boundary layers of equilibrium composition should be taken into account. It has been found that the allowance for these layers does not change the expressions for the distribution of the concentration of the diffusing impurity in a polycrystal in the case of a type-B kinetic regime, whereas in the type-C regime the corresponding expressions change somewhat. The technique that was used previously for the determination of parameters of grain-boundary diffusion and segregation, which is based on the results of emission Mossbauer investigations of grain boundaries, has been modified; the application of the new approach has been demonstrated on the example of processing data of Mossbauer investigations of grain boundaries in polycrystalline Nb at 119mSn nuclei.

Contact-induced magnetism in nanostructures based on chromium with nonmagnetic metal monolayers

Magnetic ordering is studied in Cr/X (1 ML) layer structures with monatomic layers (ML) of nonmagnetic metals (X = Sn, V, Ag, Au) incorporated into a chromium matrix. The mechanism associated with a redistribution of the charge and spin densities near the Cr/X interface and through which a spin-density wave (SDW) is induced and modified in these systems is analyzed. A semiphenomenological model is considered in detail in terms of which SDW structure near a single planar nonmagnetic defect can be described qualitatively using the Ginzburg-Landau functional. The spatial SDW configuration in a [Cr(t/X(1 ML)] superlattice is calculated, and the dependences of the SDW parameters on the temperature T, type of metal X, and superlattice period t are established. Based on the results of the study, experimental Mossbauer spectroscopy data are interpreted.

Angle-resolved soft X-ray magnetic circular dichroism in a monatomic Fe layer facing an MgO(0 0 1) tunnel barrier

The electronic and magnetic states of a monatomic Fe(0 0 1) layer directly facing an MgO(0 0 1) tunnel barrier were studied by angle-resolved X-ray magnetic circular dichroism (XMCD) at the Fe L 2,3 edges in the longitudinal (L) and transverse (T) arrangements. A strong XMCD reveals no oxidation of the 1-ML Fe, showing its crucial role in giant tunnel magnetoresistance effects in Fe/MgO/Fe magnetic tunnel junctions. Sum-rule analyses of the angle-resolved XMCD give values of a spin moment, in-plane and out-of-plane orbital and magnetic dipole moments. Argument is given on their physical implication.

X-ray absorption and x-ray magnetic circular dichroism studies on a monatomic bcc-Co(001) layer facing an amorphous Al–O tunnel barrier

We studied the electronic states and magnetic moments of a bcc-Co(001) monatomic layer facing an amorphous Al–O tunneling barrier using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the L2,3 core edges of Co. Both the XAS and XMCD measurements revealed that 1 monolayer (ML) Co is not oxidized. The magnetic moment of 1 ML bcc Co(001) facing the amorphous Al–O was observed to be almost the same as that of bulk Co. In contrast, the total magnetic moment of 1 ML bcc Co(001) facing a metallic Al layer was reduced by the effect of the proximity to Al atoms.

Atomic-scale yield and dislocation nucleation in KBr

Atomic-scale plastic deformation on a KBr(100) surface has been produced and characterized by use of atomic force microscopy (AFM) in ultrahigh vacuum. The structure of displaced material was imaged using noncontact mode AFM after first implementing the sharp silicon tip as an indenter. After indentation the KBr(100) surface is found to exhibit monatomic terraces which are formed via dislocation nucleation and glide. Discontinuities in the force-distance curves recorded during indentation are correlated to the creation of dislocation loops in the crystal. Incipient dislocation nucleation has been characterized as the abrupt monatomic layer displacement of the tip into the sample and the corresponding creation of monatomic terraces. The indenter radius has been found to significantly influence the lateral extent of the dislocation structure and the distribution of force discontinuities during indentation. The shear stress at the yield point was experimentally determined to be $2.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ which is consistent with recent theoretical predictions for the ideal shear stress of KBr.