Scanning Tunneling Spectroscopy Measurements Research Articles

Formation of atomically ordered and chemically selective Si-O-Ti monolayer on Si0.5Ge0.5(110) for a MIS structure via H2O2(g) functionalization.

Si0.5Ge0.5(110) surfaces were passivated and functionalized using atomic H, hydrogen peroxide (H2O2), and either tetrakis(dimethylamino)titanium (TDMAT) or titanium tetrachloride (TiCl4) and studied in situ with multiple spectroscopic techniques. To passivate the dangling bonds, atomic H and H2O2(g) were utilized and scanning tunneling spectroscopy (STS) demonstrated unpinning of the surface Fermi level. The H2O2(g) could also be used to functionalize the surface for metal atomic layer deposition. After subsequent TDMAT or TiCl4 dosing followed by a post-deposition annealing, scanning tunneling microscopy demonstrated that a thermally stable and well-ordered monolayer of TiOx was deposited on Si0.5Ge0.5(110), and X-ray photoelectron spectroscopy verified that the interfaces only contained Si-O-Ti bonds and a complete absence of GeOx. STS measurements confirmed a TiOx monolayer without mid-gap and conduction band edge states, which should be an ideal ultrathin insulating layer in a metal-insulator-semiconductor structure. Regardless of the Ti precursors, the final Ti density and electronic structure were identical since the Ti bonding is limited by the high coordination of Ti to O.

Pulsed laser deposition of two-dimensional ZnO nanocrystals on Au(111): growth, surface structure and electronic properties

Two-dimensional (2D) ZnO structures have been deposited on the Au(111) surface by means of the pulsed laser deposition technique. In situ scanning tunneling microscopy and scanning tunneling spectroscopy measurements have been performed to characterize morphological, structural and electronic properties of 2D ZnO at the nanoscale. Starting from a sub-monolayer coverage, we investigated the growth of ZnO, identifying different atomic layers (up to the fifth). At low coverage, we observed single- and bi-layer nanocrystals, characterized by a surface moiré pattern that is associated to a graphene-like ZnO structure. By increasing the coverage, we revealed a morphological change starting from the fourth layer, which was attributed to a transition toward a bulk-like structure. Investigation of the electronic properties revealed the semiconducting character of 2D ZnO. We observed a dependence of the density of states (DOS) and, in particular, of the conduction band (CB) on the ZnO thickness, with a decreasing of the CB onset energy for increasing thickness. The CB DOS of 2D ZnO shows a step-like behaviour which may be interpreted as due to a 2D quantum confinement effect in ZnO atomic layers.

Charge density wave modulation and gap measurements inCeTe3

We present a study of charge density wave (CDW) ordering in CeTe$_3$ at room temperature using a scanning tunneling microscope and Raman spectroscopy. Two characteristic CDW ordering wavevectors obtained from the Fourier analysis are assessed to be $|{\bf \textbf{c}}^\ast-{\bf q}|=4.19\,{\rm nm}^{-1}$ and $|{\bf q}|=10.26\,{\rm nm}^{-1}$ where $|{\bf c}^\ast| = 2\pi/c$ is the reciprocal lattice vector. The scanning tunneling spectroscopy measurements, along with inelastic light (Raman) scattering measurements, show a CDW gap $\Delta_{\rm max}$ of approximately 0.37 eV. In addition to the CDW modulation, we observe an organization of the Te sheet atoms in an array of alternating V- and N- groups along the CDW modulation, as predicted in the literature.

The role of electron confinement in Pd films for the oscillatory magnetic anisotropy in an adjacent Co layer

We demonstrate the interplay between quantum well states in Pd and the magnetic anisotropy in Pd/Co/Cu (0 0 1) by combined scanning tunneling spectroscopy (STS) and magneto optical Kerr effect (MOKE) measurements. Low temperature scanning tunneling spectroscopy reveals occupied and unoccupied quantum well states (QWS) in atomically flat Pd films on Co/Cu (0 0 1). These states give rise to sharp peaks in the differential conductance spectra. A quantitative analysis of the spectra reveals the electronic dispersion of the Pd (0 0 1) d-band (-type) along the -X direction. In situ MOKE experiments on Pd/Co/Cu (1, 1, 13) uncover a periodic variation of the in-plane uniaxial magnetic anisotropy as a function of Pd thickness with a period of 6 atomic layers Pd. STS shows that QWS in Pd cross the Fermi level with the same periodicity of 6 atomic layers. Backed by previous theoretical work we ascribe the variation of the magnetic anisotropy in Co to QWS in the Pd overlayer. Our results suggest a novel venue towards tailoring uniaxial magnetic anisotropy of ferromagnetic films by exploiting QWS in an adjacent material with large spin–orbit coupling.

Van der Waals Heteroepitaxy of Germanene Islands on Graphite.

We fabricated flat, two-dimensional germanium sheets showing a honeycomb lattice that matches that of germanene by depositing submonolayers of Ge on graphite at room temperature and subsequent annealing to 350 °C. Scanning tunneling microscopy shows that the germanene islands have a small buckling with no atomic reconstruction and does not give any hints for alloy formation and hybridization with the substrate. Our density functional theory calculations of the structural properties agree well with our experimental findings and indicate that the germanene sheet interacts only weakly with the substrate underneath. Our band structure calculations confirm that the Dirac cone of free-standing germanene is preserved for layers supported on graphite. The germanene islands show a small but characteristic charge transfer with the graphite substrate which is predicted by our ab initio simulations in excellent agreement with scanning tunneling spectroscopy measurements.

Epitaxial Growth of Single Layer Blue Phosphorus: A New Phase of Two-Dimensional Phosphorus.

Blue phosphorus, a previously unknown phase of phosphorus, has been recently predicted by theoretical calculations and shares its layered structure and high stability with black phosphorus, a rapidly rising two-dimensional material. Here, we report a molecular beam epitaxial growth of single layer blue phosphorus on Au(111) by using black phosphorus as precursor, through the combination of in situ low temperature scanning tunneling microscopy and density functional theory calculation. The structure of the as-grown single layer blue phosphorus on Au(111) is explained with a (4 × 4) blue phosphorus unit cell coinciding with a (5 × 5) Au(111) unit cell, and this is verified by the theoretical calculations. The electronic bandgap of single layer blue phosphorus on Au(111) is determined to be 1.10 eV by scanning tunneling spectroscopy measurement. The realization of epitaxial growth of large-scale and high quality atomic-layered blue phosphorus can enable the rapid development of novel electronic and optoelectronic devices based on this emerging two-dimensional material.

Large area self-ordered parallel C60 molecular nanowire arrays on Si(110) surfaces

We report that the large-scale self-organization of well-ordered parallel C60 molecular nanowire arrays with tunable structures on Si(110) surfaces can be achieved through a site-selective anchoring method. Our scanning tunneling microscopy investigations show that the preferential trapping of C60 molecules atop Si pentagons on upper terraces of the C60-induced Si(110) reconstructed surface leads to the formation of long-range ordered C60 molecular nanowires with a C60 triplet as a repeat unit along each Si upper terrace (i.e., a Si nanowire). Scanning tunneling spectroscopy measurements show a large energy gap of ∼3.0 eV between the highest occupied molecular orbital and the lowest unoccupied molecular orbital of the C60 molecules within a C60 triplet, indicating a relatively weak C60–Si(110) interaction that is the main driving force for the site-selective anchoring atop Si pentagons via the geometric matching. The formation of ordered parallel C60-tetramer nanowire arrays upon further C60 adsorption confirms this self-ordering mechanism. Such mesoscopically ordered hybrid C60/Si nanojunction arrays with the unmodified C60 on periodic Si nanowires were not yet observed before. The ability to grow the large-area parallel fullerene nanowire arrays with tunable structures on Si(110) represents an important step toward the engineering of advanced Si-based molecular nanoelectronics and quantum computation.

Structural and electronic properties of Pt induced nanowires on Ge(110)

The structural and electronic properties of Pt induced nanowires on Ge(110) surfaces have been studied by scanning tunneling microscopy and low energy electron microscopy. The deposition of a sub-monolayer amount of Pt and subsequent annealing at 1100 (±30) K results into nanowires which are aligned along the densely packed [1–10] direction of the Ge(110) surface. With increasing Pt coverage the nanowires form densely packed arrays with separations of 1.1±0.1nm, 2.0±0.1nm and 3.4±0.1nm. Ge pentagons reside in the troughs for nanowire separations of 3.4nm, however for smaller nanowire separations no pentagons are found. Spatially resolved scanning tunneling spectroscopy measurements reveal a filled electronic state at −0.35eV. This electronic state is present in the troughs as well as on the nanowires. The −0.35eV state has the strongest intensity on the pentagons. For Pt depositions exceeding two monolayers, pentagon free nanowire patches are found, that coexist with Pt/Ge clusters. Upon annealing at 1040K these Pt/Ge clusters become liquid-like, indicating that we are dealing with eutectic Pt0.22Ge0.78 clusters. Low energy electron microscopy videos reveal the formation and spinodal decomposition of these eutectic Pt/Ge clusters.

Structural and Electronic Properties of Germanene on MoS_{2}.

To date germanene has only been synthesized on metallic substrates. A metallic substrate is usually detrimental for the two-dimensional Dirac nature of germanene because the important electronic states near the Fermi level of germanene can hybridize with the electronic states of the metallic substrate. Here we report the successful synthesis of germanene on molybdenum disulfide (MoS_{2}), a band gap material. Preexisting defects in the MoS_{2} surface act as preferential nucleation sites for the germanene islands. The lattice constant of the germanene layer (3.8±0.2 Å) is about 20% larger than the lattice constant of the MoS_{2} substrate (3.16Å). Scanning tunneling spectroscopy measurements and density functional theory calculations reveal that there are, besides the linearly dispersing bands at the K points, two parabolic bands that cross the Fermi level at the Γ point.

Epitaxial iron oxide nanocrystals with memory function grown on Si substrates

High-density Fe3O4−δ nanocrystals (NCs) were epitaxially grown on Si substrates by molecular beam epitaxy with epitaxial Ge NCs being used as nucleation sites. Scanning tunneling spectroscopy measurements showed that the surface bandgap of the as-grown Fe3O4−δ NCs was ∼0.2 eV, consistent with that reported for Fe3O4−δ films. Conductive atomic force microscopy measurements of the NCs revealed hysteresis in the voltage–current curves, indicating bipolar resistive switching behavior. The measurement results established the superiority of the NCs to thin conventional polycrystalline Fe3O4−δ films/Si in terms of resistive switching characteristics. This demonstrated the possibility of developing resistance random access memory devices composed of ubiquitous Fe3O4−δ NC materials.

Single-gap superconductivity inβ−Bi2Pd

beta-Bi2Pd compound has been proposed as another example of a multi-gap superconductor [Y. Imai et al., J. Phys. Soc. Jap. 81, 113708 (2012)]. Here, we report on measurements of several important physical quantities capable to show a presence of multiple energy gaps on our superconducting single crystals of beta-Bi2Pd with the critical temperature Tc close to 5 K. The calorimetric study via a sensitive ac technique shows a sharp anomaly at the superconducting transition, however only a single energy gap is detected. Also other characteristics inferred from calorimetric measurements as the field dependence of the Sommerfeld coefficient and the temperature and angular dependence of the upper critical magnetic field point unequivocally to standard single s-wave gap superconductivity. The Hall-probe magnetometry provides the same result from the analysis of the temperature dependence of the lower critical field. A single-gapped BCS density of states is detected by the scanning tunneling spectroscopy measurements. Then, the bulk as well as the surface sensitive probes evidence a standard conventional superconductivity in this system where the topologically protected surface states have been recently detected by ARPES [M. Sakano et al., Nature Comm. 6, 8595 (2015)] .

Coexistence of charge and ferromagnetic order in fcc Fe.

Phase coexistence phenomena have been intensively studied in strongly correlated materials where several ordered states simultaneously occur or compete. Material properties critically depend on external parameters and boundary conditions, where tiny changes result in qualitatively different ground states. However, up to date, phase coexistence phenomena have exclusively been reported for complex compounds composed of multiple elements. Here we show that charge- and magnetically ordered states coexist in double-layer Fe/Rh(001). Scanning tunnelling microscopy and spectroscopy measurements reveal periodic charge-order stripes below a temperature of 130 K. Close to liquid helium temperature, they are superimposed by ferromagnetic domains as observed by spin-polarized scanning tunnelling microscopy. Temperature-dependent measurements reveal a pronounced cross-talk between charge and spin order at the ferromagnetic ordering temperature about 70 K, which is successfully modelled within an effective Ginzburg–Landau ansatz including sixth-order terms. Our results show that subtle balance between structural modifications can lead to competing ordering phenomena.

Symmetry-selected spin-split hybrid states inC60/ferromagneticinterfaces

The understanding of orbital hybridization and spin-polarization at the organic-ferromagnetic interface is essential in the search for efficient hybrid spintronic devices. Here, using first-principles calculations, we report a systematic study of spin-split hybrid states of C$_{60}$ deposited on various ferromagnetic surfaces: bcc-Cr(001), bcc-Fe(001), bcc-Co(001), fcc-Co(001) and hcp-Co(0001). We show that the adsorption geometry of the molecule with respect to the surface crystallographic orientation of the magnetic substrate as well as the strength of the interaction play an intricate role in the spin-polarization of the hybrid orbitals. We find that a large spin-polarization in vacuum above the buckyball can only be achieved if the molecule is adsorbed upon a bcc-(001) surface by its pentagonal ring. Therefore bcc-Cr(001), bcc-Fe(001) and bcc-Co(001) are the optimal candidates. Spin-polarized scanning tunneling spectroscopy measurements on single C$_{60}$ adsorbed on Cr(001) and Co/Pt(111) also confirm that both the symmetry of the substrate and of the molecular conformation have a strong influence on the induced spin polarization. Our finding may give valuable insights for further engineering of spin filtering devices through single molecular orbitals.

Strong dependence of flattening and decoupling of graphene on metals on the local distribution of intercalated oxygen atoms

We show that the flattening of a highly corrugated graphene layer grown on Rh(111) is linked to its decoupling from the metal substrate taking place during oxygen intercalation in the lowest moiré sites. We have been able to track this process at the atomic scale by combining scanning tunneling microscopy experiments and first-principles calculations. Initially, intercalated and non intercalated areas can coexist through a careful experimental control of the oxygen dosage and temperature. This allows a direct comparison of the corrugation profiles of both areas. Although the corrugation increases for very low coverages, higher oxygen concentrations provoke the occupation of the lowest moiré sites leading to a flattening of the graphene sheet. This mechanism is confirmed by calculations of the equilibrium geometries for different oxygen concentrations. Electronic structure calculations and scanning tunneling spectroscopy measurements reveal that the flattening of the graphene layer is linked with the local uncoupling from the metal induced by the increasing oxygen concentration. This process, completed only when the lowest moiré sites are filled with oxygen, finally converts a strongly coupled system into a free-standing like p-doped graphene layer.

Origins of enhanced thermoelectric power factor in topologically insulating Bi0.64Sb1.36Te3 thin films

In this research, we report the enhanced thermoelectric power factor in topologically insulating thin films of Bi0.64Sb1.36Te3 with a thickness of 6–200 nm. Measurements of scanning tunneling spectroscopy and electronic transport show that the Fermi level lies close to the valence band edge, and that the topological surface state (TSS) is electron dominated. We find that the Seebeck coefficient of the 6 nm and 15 nm thick films is dominated by the valence band, while the TSS chiefly contributes to the electrical conductivity. In contrast, the electronic transport of the reference 200 nm thick film behaves similar to bulk thermoelectric materials with low carrier concentration, implying the effect of the TSS on the electronic transport is merely prominent in the thin region. The conductivity of the 6 nm and 15 nm thick film is obviously higher than that in the 200 nm thick film owing to the highly mobile TSS conduction channel. As a consequence of the enhanced electrical conductivity and the suppressed bipolar effect in transport properties for the 6 nm thick film, an impressive power factor of about 2.0 mW m−1 K−2 is achieved at room temperature for this film. Further investigations of the electronic transport properties of TSS and interactions between TSS and the bulk band might result in a further improved thermoelectric power factor in topologically insulating Bi0.64Sb1.36Te3 thin films.

Scanning Tunnelling Microscopy and Spectroscopy of the Layered Nitride Superconductor α-NaxTiNCl

Low-temperature scanning tunnelling microscopy and spectroscopy (STM/STS) measurements have been carried out on α-NaxTiNCl (x = 0.16) superconductors with Tc of 18K. The STM image clearly reveals the streak patterns along b-axis with the spacing of period of ≈ 0.39nm (= a0) and the circular spots that form a rectangular lattice inside streak patterns. The conductance dI/dV dependences on the voltage V show the typical superconducting gap value of Δ ≈ 9.5 meV, leading to a large gap to Tc ratio 2Δ/kBTc ≈ 12. The dI/dV maps show both a0 streak patterns and the long-periodic modulation with the period of ≈ 1.6nm, which is about 5 times b0 length. Such a long period of the observed charge density wave correlates with the doping degree of Na ≈ 0.2 and is probably associated with the inverse wave vector of the Fermi surface nested section.

Photoluminescence through in-gap states in phenylacetylene functionalized silicon nanocrystals.

Optoelectronic properties of Si nanocrystals (SiNCs) were studied by combining scanning tunneling spectroscopy (STS) and optical measurements. The photoluminescence (PL) of phenylacetylene functionalized SiNCs red shifts relative to hexyl- and phenyl-capped counterparts, whereas the absorption spectra and the band gaps extracted from STS are similar for all surface groups. However, an in-gap state near the conduction band edge was detected by STS only for the phenylacetylene terminated SiNCs, which can account for the PL shift via relaxation across this state.

Formation of all-oxide solar cells in atmospheric condition based on Cu2O thin-films grown through SILAR technique

We report formation of Cu2O thin-films through successive ionic layer adsorption and reaction (SILAR) process. We have fully characterized the thin-films grown in atmospheric condition with a non-vacuum process. Post-deposition annealing in oxygen environment has led to formation of CuO thin-films. We have formed pn-junctions with a layer of the p-type copper oxides and another layer of n-type SnO2 nanoparticles. Such pn-junctions, along with other oxide semiconductors as carrier blocking layers acted as all-oxide thin-film solar cells which were formed in an atmospheric condition. We report the effect of NiO as a hole-transporting and electron-blocking layer and ZnO as a buffer layer between p- and n-layers to forbid chemical changes occurring at the interface. From scanning tunneling spectroscopy (STS) measurements we could locate band-edges of the materials with respect to their Fermi energy. The NiO/Cu2O/ZnO/SnO2 heterojunction devices having a staircase-like energy level led to facile transport of electrons and holes to the opposite electrodes and thereby acting as all-oxide thin-film solar cells yielding an energy conversion efficiency in excess to 1%.

Evolution of the superconducting properties inFeSe1−xSx

We present scanning tunneling microscopy and spectroscopy measurements on ${\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{S}}_{x}$ single crystals with $x=0$, $0.04$, and $0.09$. The S substitution into the Se site is equivalent to a positive chemical pressure, since S and Se have the same valence and S has a smaller ionic radius than Se. The subsequent changes in the electronic structure of FeSe induce a decrease of the structural transition temperature and a small increase in the superconducting critical temperature. The evolution of the gaps with increasing S concentration suggests an increase of the hole Fermi surface. Moreover, the vortex core anisotropy, that likely reflects the Fermi surface anisotropy, is strongly suppressed by the S substitution.

Superconductivity dichotomy in K-coated single and double unit cell FeSe films onSrTiO3

We report the superconductivity evolution of one unit cell (1-UC) and 2-UC FeSe films on SrTiO3(001) substrates with potassium (K) adsorption. By in situ scanning tunneling spectroscopy measurement, we find that the superconductivity in 1-UC FeSe films is continuously suppressed with increasing K coverage, whereas non-superconducting 2-UC FeSe films become superconducting with a gap of ~17 meV or ~11 meV depending on whether the underlying 1-UC films are superconducting or not. This work explicitly reveals that the interface electron-phonon coupling is strongly related to the charge transfer at FeSe/STO interface and plays vital role in enhancing Cooper pairing in both 1-UC and 2-UC FeSe films.