Modifying Quantum Well States of Pb Thin Films via Interface Engineering

We demonstrate the importance of interface modification on improving electron confinement by preparing Pb quantum islands on Si（111） substrates with two different surface reconstructions, i.e., Si（111）-7 ×7 and Si（111）- Root3×Root3-Pb （hereafter, 7 ×7 and R3）. Characterization with scanning tunneling microscopy/spectroscopy shows that growing Pb films directly on a 7 × 7 surface will generate many interface defects, which makes the lifetime of quantum well states （QWSs） strongly dependent on surface locations. On the other hand, QWSs in Pb films on an R3 surface are well defined with small variations in linewidth on different surface locations and are much sharper than those on the 7 × 7 surface. We show that the enhancement in quantum confinement is primarily due to the reduced electron-defect scattering at the interface.     

Robust surface state of intrinsic topological insulator Bi2Te2Se thin films: a first-principles study

Bi(2)Te(2)Se, a ternary tetradymite compound, has recently been identified to be a three-dimensional topological insulator. In this paper, we theoretically study the electronic structures of bulk and thin films of Bi(2)Te(2)Se employing spin-orbit coupling (SOC) self-consistently with density-functional theory. It is found that SOC plays an important role in determining the electronic properties of Bi(2)Te(2)Se. A finite bandgap opens up in the surface states of Bi(2)Te(2)Se thin films due to the hybridization of the top and bottom surface states of films. The intrinsic Bi(2)Te(2)Se thin films of three or more quintuple layers exhibit a robust topological nature of electronic structure with the Fermi energy intersecting the Dirac cone of the surface states only once between time-reversal-invariant momenta. These characteristics of Bi(2)Te(2)Se are similar to the topological behavior of Bi(2)Te(3), promising a variety of potential applications in nanoelectronics and spintronics.     

Robustness of topologically protected surface states in layering of Bi2Te3 thin films

Bulk Bi2Te3 is known to be a topological insulator. We investigate surface states of Bi2Te3(111) thin films of one to six quintuple layers using density-functional theory including spin-orbit coupling. We construct a method to identify topologically protected surface states of thin film topological insulators. Applying this method to Bi2Te3 thin films, we find that the topological nature of the surface states remains robust with the film thickness and that the films of three or more quintuple layers have topologically nontrivial surface states, which agrees with experiments.     

Oscillatory electron phonon coupling in Pb/Si（111）deduced by temperature-dependent quantum well states

Photoemission study of atomically flat Pb films with a thickness from 15 to 24 monolayers （ML） have been performed within a temperature range 75-270K. Well-defined quantum well states （QWSs） are observed, which exhibit interesting temperature-dependent behaviours. The peak position of the QWSs shifts towards higher binding energy with increasing substrate temperature, whereas the peak width broadens linearly due to enhanced electron-phonon coupling strength （λ）. An oscillatory A with a period of 2ML is deduced. Preliminary analysis shows that the oscillation can be explained in terms of the interface induced phase variations, and is thus a manifestation of the quantum size effects.     

Band Structures of Ultrathin Bi(110) Films on Black Phosphorus Substrates Using Angle-Resolved Photoemission Spectroscopy

The band structures of two-monolayer Bi(110) films on black phosphorus substrates are studied using angleresolved photoemission spectroscopy. Within the band gap of bulk black phosphorus, the electronic states near the Fermi level are dominated by the Bi(110) film. The band dispersions revealed by our data suggest that the orientation of the Bi(110) film is aligned with the black phosphorus substrate. The electronic structures of the Bi(110) film strongly deviate from the band calculations of the free-standing Bi(110) film, suggesting that the substrate can significantly affect the electronic states in the Bi(110) film. Our data show that there are no non-trivial electronic states in Bi(110) films grown on black phosphorus substrates.     

Stable nontrivial Z2 topology in ultrathin Bi (111) films: a first-principles study

Recently, there have been intense efforts in searching for new topological insulator materials. Based on first-principles calculations, we find that all the ultrathin Bi (111) films are characterized by a nontrivial Z(2) number independent of the film thickness, without the odd-even oscillation of topological triviality as commonly perceived. The stable nontrivial Z(2) topology is retained by the concurrent band gap inversions at multiple time-reversal-invariant k points with the increasing film thickness and associated with the intermediate interbilayer coupling of the Bi film. Our calculations further indicate that the presence of metallic surface states in thick Bi (111) films can be effectively removed by surface adsorption.     

Electronic structure and reactivity of ultra-thin Fe films on a Rh(100) surface

The electronic structure and reactivity of ultra-thin Fe films on a Rh(100) surface have been investigated by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and thermal desorption spectroscopy (TDS). The dispersion of d-bands of a clean Rh surface is consistent with the projection of the bulk band structure. One monolayer of Fe film shows a systematic shift of d-bands toward large binding energy by 0.4 eV and a large reduction in the density of states just below the Fermi level, in particular, around the M point. In accordance with the decrease in the density of states at the Fermi level, the bonding energy of hydrogen is greatly reduced to 245 kJ/mol on a 1 ML (ML = monolayer) Fe film, although the sticking coefficient is still in the range of 0.1–0.3. The successive increase in activation energy for the desorption of hydrogen with the increase of Fe film thickness from 1 to 3 ML is associated with a recovery of the density of states at the Fermi level.     

Spectra of quantum states in thin metal films and their modification: Al/W(110) system

The modification of spectra of quantum well states of sp-type in thin Al films on the W(110) surface was experimentally investigated by angular-resolved photoelectron spectroscopy both during deposition and in dependence of the detection angle. Quantum well states are observed for the partially filled band of valence states in the range of binding energies from 4.4 eV to the Fermi level. An Al film with a thickness of 11 monolayers exhibits a jump of the dispersion relations of quantum well states in the local W(110) band gap in the ΓS direction and splitting of these relations due to the effect of substrate electronic structure on the formed spectrum of quantum states and their possible spin polarization.     

Fermi surface and anisotropic spin-orbit coupling of Sb(111) studied by angle-resolved photoemission spectroscopy

High-resolution angle-resolved photoemission spectroscopy has been performed on Sb(111) to elucidate the origin of anomalous electronic properties in group-V semimetal surfaces. The surface was found to be metallic despite the semimetallic character of bulk. We clearly observed two surface-derived Fermi surfaces which are likely spin split, demonstrating that the spin-orbit interaction plays a dominant role in characterizing the surface electronic states of group-V semimetals. The universality or dissimilarity of the electronic structure in Bi and Sb is discussed in relation to the granular superconductivity, electron-phonon coupling, and surface charge or spin density wave.     

Electronic properties and interfacial coupling in Pb islands on single-crystalline graphene

Introducing metal thin films on two-dimensional (2D) material may present a system to possess exotic properties due to reduced dimensionality and interfacial effects. We deposit Pb islands on single-crystalline graphene on a Ge(110) substrate and studied the nano- and atomic-scale structures and low-energy electronic excitations with scanning tunneling microscopy/spectroscopy (STM/STS). Robust quantum well states (QWSs) are observed in Pb(111) islands and their oscillation with film thickness reveals the isolation of free electrons in Pb from the graphene substrate. The spectroscopic characteristics of QWSs are consistent with the band structure of a free-standing Pb(111) film. The weak interface coupling is further evidenced by the absence of superconductivity in graphene in close proximity to the superconducting Pb islands. Accordingly, the Pb(111) islands on graphene/Ge(110) are free-standing in nature, showing very weak electronic coupling to the substrate.     

Origin of the metallic to insulating transition of an epitaxial Bi(111) film grown on Si(111)

Transport characteristics of single crystal bismuth films on Si(111)-7×7 are found to be metallic or insulating at temperatures below or above T_C, respectively. The transition temperature T_C decreases as the film thickness increases. By combining thickness dependence of the films resistivity, we find the insulating behaviour results from the states inside film, while the metallic behaviour originates from the interface states. We show that quantum size effect in a Bi film, such as the semimetal-to-semiconductor transition, is only observable at a temperature higher than T_C.     

Two-photon resonance in optical second harmonic generation from quantum well states in ultra-thin Ag films grown on Si(1 1 1) surfaces

We studied optical second harmonic generation (SHG) oscillations during the growth of Ag films on Si(1 1 1) 7 × 7 clean and H-terminated surfaces. In the growth on the 7 × 7 surfaces at room temperature, the second and third peaks of the oscillation shift towards the thinner side with an increase in pump photon energy. Our analysis revealed that these peaks are caused by two-photon resonant transitions from the n = 1 and 2 occupied quantum well states (QWSs) in the Ag film to the Ag/Si interface at 1.9 eV above the Fermi level (E_f). In Ag growth on the hydrogen-terminated surfaces, the SHG oscillation was similar to that on the 7 × 7 surfaces at room temperature. However, the QWS-related peak was suppressed in the growth at 300 °C. This is attributed to an inhibited intrusion of the interface state into the Ag layers.     

Surface effects on the magnetic properties of Co2FeAl(0 0 1): An ab initio study

Electronic structures of the Co₂FeAl(0 0 1) surface are studied theoretically via first-principles calculations based on density functional theory. It is found that the minority spin band gap at the Fermi level in bulk Co₂FeAl disappears at the surface due to space localization of the states. However, beneath the surface, the density of states of individual atoms shows a trend of minority spin gap opening at the Fermi level, which indicates that the electronic structures become close to that of bulk Co₂FeAl. The termination of FeAl is more favorable for spin polarization of Co₂FeAl films than that of Co. Accordingly, we present a composite tri-layer model to illustrate the fading of the half-metallic property in Co₂FeAl films against the ideal character in bulk materials.     

Electronic quasiparticles and evolution of Fermi level spin states in thin magnetic layers

Here we report on high-resolution photoemission of iron layers grown on a W(1 1 0) substrate. The evolution of the substrate states upon sub-monolayer adsorption of Fe atoms leads to a shift in surface state binding energy. For thicker (1 1 0) films, sharp metallic surface states are obtained. Their dispersion displays the signature of quasiparticle renormalization due to dressing with excitations. The energy scale is characteristic for the spin wave spectrum in iron, thereby giving evidence of electron-magnon coupling. Furthermore, it is found that quantum well states occur as a function of layer thickness. These modify the spin density of states at the Fermi level in the ferromagnetic film.     

Low-dimensional nanostructures and fullerene films on semiconductor surface

The morphology and atomic structures of C₆₀ fullerene films on a Bi(0001)/Si(111)-7 × 7 surface and adsorption of fluorofullerene C₆₀F _x molecules on a Si(111)-7 × 7 surface have been studied by scanning tunneling microscopy/spectroscopy and low-energy electron microscopy under ultra high-vacuum conditions. It has been shown that initial nucleation of C₆₀ islands on the surface of an epitaxial Bi film occurs on double steps and domain boundaries, while tunnel spectra do not exhibit any significant charge transfer to the lowest unoccupied molecular orbital states. Fluorofullerene molecules allow local (at the nanoscale level) modification of Si surface through local etching.     

Electronic structure of the conduction band upon the formation of ultrathin fullerene films on the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band in the energy range 5–25 eV above the Fermi level E_F and the interfacial potential barrier upon deposition of aziridinylphenylpyrrolofullerene (APP-C₆₀) and fullerene (C₆₀) films on the surface of the real germanium oxide ((GeO₂)Ge) have been presented. The content of the oxide on the (GeO₂)Ge surface has been determined using X-ray photoelectron spectroscopy. The electronic properties have been measured using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode. The regularities of the change in the fine structure of total current spectra (FSTCS) with an increase in the thickness of the APP-C₆₀ and C₆₀ coatings to 7 nm have been investigated. A comparison of the structures of the FSTCS maxima for the C₆₀ and APP-C₆₀ films has made it possible to reveal the energy range (6–10 eV above the Fermi level E_F) in which the energy states are determined by both the π* and σ* states and the FSTCS spectra have different structures of the maxima for the APP-C₆₀ and unsubstituted C₆₀ films. The formation of the interfacial potential barrier upon deposition of APP-C₆₀ and C₆₀ on the (GeO₂)Ge surface is accompanied by an increase in the work function of the surface E_vac–E_F by the value of 0.2–0.3 eV, which corresponds to the transfer of the electron density from the substrate to the organic films under investigation. The largest changes occur with an increase in the coating thickness to 3 nm, and with further deposition of APP-C₆₀ and C₆₀, the work function of the surface changes only slightly.     

Electronic structure of the conduction band of the interface region of ultrathin films of substituted perylenedicarboximides and the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band and the interfacial potential barrier during the formation of interfaces of dioctyl-substituted perylenedicarboximide (PTCDI-C₈) and diphenyl-substituted perylenedicarboximide (PTCDI-Ph) ultrathin films with the oxidized germanium surface have been presented. The experimental results have been obtained using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode at energies in the range from 5 to 20 eV above the Fermi level E_F. The positions of the maxima of the fine structure of total current spectra (FSTCS) of the PTCDI-C₈ and PTCDI-Ph films differ significantly in the energy range from 9 to 20 eV above the Fermi level E_F, which can be associated with the difference between the substituents of the chosen molecules, dioctyl- and diphenyl-, respectively. At the same time, the positions of the lowenergy maxima in the FSTCS spectra at an energy 6–7 eV above the Fermi level E_F for the PTCDI-C₈ and PTCDI-Ph films almost coincide with each other. It has been suggested that these maxima are attributed to the electronic states of the perylene core of the molecules under investigation. The process of the formation of interfacial potential barriers of the PTCDI-C₈ and PTCDI-Ph films with the oxidized germanium surface has been analyzed. It has been found that the work functions of the surface, E_vac–E_F, differ little from 4.6 ± 0.1 eV over the entire range of organic coating thicknesses from 0 to 6 nm.     

Initial stages of Bi/Ge(111) interface formation: A detailed STM study

We present a detailed scanning tunneling microscopy investigation of ultra-thin Bi films on Ge(111)-c(2 × 8) in the range up to 1.5 ML. During growth at 300 K, the second/third atomic layer of Bi already starts to nucleate before the completion of the first/second layer correspondingly. Laterally isolated first layer Bi atoms, clusters and islands posses no electronic states in the range ~ 0.5 eV above the Fermi level of the substrate. In contrast, metallic electronic properties are found for continuous films when Bi coverage nears 1 ML. Annealing the as-deposited Bi films at 450 K causes lateral redistribution of Bi due to surface diffusion: coarsening of two-dimensional Bi islands with no long range order in the adsorbate layer is observed up to 1 ML; long range ordered (√3 × √3)-Bi/Ge(111) interface plus three-dimensional Bi clusters are obtained for coverages in excess of 1 ML.     

Quantum confinement and surface-state effects in bismuth nanowires

Bulk bismuth is an efficient thermoelectric material. Assuming intrinsic conditions, the theory of quantum confinement of bismuth nanowires by Hicks and Dresselhaus predicts a semimetal-to-semiconductor transformation for critical diameters of around 50 nm. For nanowires of diameters below the critical diameter, electronic states can be considered to be one dimensional and therefore the thermopower can be very large. However, angle-resolved photoemission spectroscopy (ARPES) studies of Bi planar surfaces present direct evidence of heavy mass surface states that can inhibit the semimetal-to-semiconductor transformation. We present a study of the Fermi surface of Bi nanowires of diameters ranging between 200 and 30 nm employing the Shubnikov–de Haas method. Our results can be understood in terms of the model of surface states. For 30 nm nanowires we find that the Fermi surface is spherical, that the carriers have high effective mass, and that the number of carriers corresponds to that inferred from ARPES measurements.     

STM and XPS investigations of bismuth islands on HOPG

The growth of bismuth thin films on highly oriented pyrolitic graphite (HOPG) was studied using ultra high vacuum (UHV) scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The locations of the main XPS peaks, and also the plasmon energy, are in good agreement with results recorded on bulk bismuth. The shape of the Bi 4f doublet, as well as the well developed Fermi edge, indicates the metallic character of the deposited film. One of the observed shoulders is identified with a quantum-well subband characteristic of thin bismuth films. There is no evidence for the existence of Bi–C bonds, consistent with weak bonding between the Bi islands and the HOPG. The height of the islands and their crystallographic orientation were investigated as a function of surface coverage. The Bi islands grow with the (110) plane parallel to the substrate. The observed heights (3, 5, 7, 9 ML) indicate that the preferred crystal structure involves paired layers on an intermediate mono-atomic Bi layer. There is evidence both for and against the Black Phosphorus like allotrope, and the nature of both the layer pairing and the intermediate layer are yet to be resolved. The islands exhibit stripes oriented along the $<1\overline{1}0>$ axis of the Bi crystal, which is a fast growth direction due to the existence of strongly bonded zig-zag chains of atoms. The surface unit cell and the parameters of the rhombohedral bulk unit cell are estimated based on atomic resolution images. In the case of 2 ML stripes on top of a 5 ML base, the expansion of the outer atomic rows was estimated at 27%. Asymmetries in the growth of the islands are observed. Based on low coverage depositions at reduced substrate temperatures, it is proposed that there is a second fast growth direction corresponding to preferential attachment of atoms to one of the faces of the asymmetric, rhombic cross-section of the (110) crystal.