Scanning Tunneling Microscopy Spectroscopy Research Articles

Structure and quantum well states in silicene nanoribbons on Ag(110)

We have performed scanning tunneling microscopy/spectroscopy (STM/STS) study and first-principles calculations to investigate the atomic structure and electronic properties of silicon nanoribbons (SiNRs) grown on Ag(110). Despite the extensive research on SiNRs in the last decades, its atomic structure is still not fully understood so far. In this report we determine that the structure of SiNRs/Ag(110) is armchair silicene nanoribbon with reconstructed edges. Meanwhile, pronounced quantum well states (QWS) in SiNRs were observed and their energy spectrum was systematically measured. The QWS are due to the confinement of quasiparticles perpendicular to the nanoribbon and can be well explained by the theory of one-dimensional (1D) “particle-in-a-box” model in quantum mechanics.

Microscopic basis for the band engineering of Mo1-xWxS2-based heterojunction.

Transition-metal dichalcogenide layered materials, consisting of a transition-metal atomic layer sandwiched by two chalcogen atomic layers, have been attracting considerable attention because of their desirable physical properties for semiconductor devices, and a wide variety of pn junctions, which are essential building blocks for electronic and optoelectronic devices, have been realized using these atomically thin structures. Engineering the electronic/optical properties of semiconductors by using such heterojunctions has been a central concept in semiconductor science and technology. Here, we report the first scanning tunneling microscopy/spectroscopy (STM/STS) study on the electronic structures of a monolayer WS2/Mo1−xWxS2 heterojunction that provides a tunable band alignment. The atomically modulated spatial variation in such electronic structures, i.e., a microscopic basis for the band structure of a WS2/Mo1−xWxS2 heterojunction, was directly observed. The macroscopic band structure of Mo1−xWxS2 alloy was well reproduced by the STS spectra averaged over the surface. An electric field of as high as 80 × 106 Vm−1 was observed at the interface for the alloy with x = 0.3, verifying the efficient separation of photoexcited carriers at the interface.

Emergence of an incipient ordering mode in FeSe

The structurally simplest Fe-based superconductor FeSe with a critical temperature $T_{c}\approx$ 8.5 K displays a breaking of the four-fold rotational symmetry at a temperature $T_{s}\approx 87$ K. We investigated the electronic properties of FeSe using scanning tunneling microscopy/spectroscopy (STM/S), magnetization, and electrical transport measurements. The results indicated two new energy scales (i) $T^{*} \approx$ 75 K denoted by an onset of electron-hole asymmetry in STS, enhanced spin fluctuations, and increased positive magnetoresistance; (ii) $T^{**} \approx$ 22 - 30 K, marked by opening up of a partial gap of about 8 meV in STS and a recovery of Kohler's rule. Our results reveal onset of an incipient ordering mode at $T^{*}$ and its nucleation below $T^{**}$. The ordering mode observed here, both in spin as well as charge channels, suggests a coupling between the spins with charge, orbital or pocket degrees of freedom.

Grain Boundary Structures and Electronic Properties of Hexagonal Boron Nitride on Cu(111).

Grain boundaries (GBs) of hexagonal boron nitride (h-BN) grown on Cu(111) were investigated by scanning tunneling microscopy/spectroscopy (STM/STS). The first experimental evidence of the GBs composed of square-octagon pairs (4|8 GBs) was given, together with those containing pentagon-heptagon pairs (5|7 GBs). Two types of GBs were found to exhibit significantly different electronic properties, where the band gap of the 5|7 GB was dramatically decreased as compared with that of the 4|8 GB, consistent with our obtained result from density functional theory (DFT) calculations. Moreover, the present work may provide a possibility of tuning the inert electronic property of h-BN via grain boundary engineering.

Investigation of local tunneling current noise spectra on the silicon crystal surfaces by means of STM/STS

We report on a careful analysis of the local tunneling conductivity by means of ultra-high vacuum scanning tunneling microscopy/spectroscopy (STM/STS) technique in the vicinity of low-dimensional structures on the Si(111)–(7 × 7) and Si(110)–(16 × 2) surfaces. The power-law exponent α of low-frequency tunneling current noise spectra is investigated for different values of the tunneling contact parameters: relaxation rates, the localized state coupling, and the tunneling barrier width and height.

Correlation between OH density and Fe electronic states of H/Fe3O4(001) film surfaces studied by scanning tunneling microscopy/spectroscopy

We report two types of adsorption structures in H/Fe3O4(001) film surfaces and the correlation between OH density and Fe electronic states, which have been studied by scanning tunneling microscopy/spectroscopy (STM/STS). Two types of bright protrusions (BPs), whose lengths along the atomic rows are different, are observed in the STM images. The shorter and longer BPs consist of Fe atoms with one and with two OH groups neighbor, respectively. In addition, STS measurements show the higher local density of states (LDOS) just below the Fermi level of Fe atoms with increasing neighboring OH groups. The variation can be attributed to the difference in the gain of electrons from H atoms, which is due to the difference in the number of neighboring OH groups. These results reveal that surface OH density is a factor for determining the LDOS just below the Fermi level of surface Fe atoms.

Cr impurity-induced electronic states in ZnTe(110) surface

The impurity states of Cr atoms, which substituted Zn sites in the topmost layer of a p-type ZnTe(110) surface, were investigated by scanning tunneling microscopy/spectroscopy (STM/STS) and we firstly observed Cr-induced impurity states in the energy gap region of the host ZnTe including the unoccupied states by STS. Furthermore, we compared the observed energy levels and spatial distributions of the local density of states with those in the previous theoretical study [Katayama-Yoshida et al., Phys. Status Solidi A 204, 15 (2007)] and successfully identified the impurity states as the respective spin-polarized impurity states predicted by the theoretical study.

Observation of mode-like features in tunneling spectra of iron-based superconductors**Project supported by the National Basic Research Program of China (Grant Nos. 2011CBA00110 and 2012CB821403), the National Natural Science Foundation of China (Grant Nos. 11174349 and 11322432), and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB07020300).

We report scanning tunneling microscopy/spectroscopy (STM/STS) studies on iron-based superconductors of Ba1−xKxFe2As2 and nearly optimally doped Fe(Te,Se). Mode-like features were observed universally outside the superconducting gaps in the tunneling spectra, which are similar to our previous observations in other samples and can be ascribed to the interaction between electrons and spin excitations. Furthermore, an almost linear relationship between the superconducting gaps and the superconducting transition temperatures was noted and should also be taken into account in understanding the mechanism of iron-based superconductors.

Transition metal dichalcogenide and hexagonal boron nitride heterostructures grown by molecular beam epitaxy

Heterostructures coupling transition metal dichalcogenides (TMDs) and insulating hexagonal boron nitride (h-BN) were grown by molecular beam epitaxy (MBE) demonstrating the unique opportunities for fabricating all 2D heterostructures with the desired band alignments for novel nanoelectronic devices. Structural and chemical characterization of the TMDs and h-BN was conducted via reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning tunneling microscopy/spectroscopy (STM), and X-ray photoelectron spectroscopy (XPS).

The spatial distributions of large gap-like structure on Fe(Se,Te) single crystals observed by STM/STS

The nanoscale spatial distributions of large gap-like structure on superconducting FeSe1−xTex were investigated by scanning tunneling microscopy/spectroscopy (STM/STS). The STM topography shows regular atomic lattice arrangements with the lattice spacing ∼0.38nm, together with the randomly distributed large spots due to the excess Fe atoms. From the STS measurements, the small gap structures of Δ∼7meV were partly observed. On the other hand, the high-bias dI/dV curves exhibit the broad peak structures at the negative biases of VPG=−200 to −400mV in the measured whole surface area. The average of these large gaps is |VPGave|∼305mV with the standard deviation of σ∼48mV. The spatial distributions of the VPG exhibit the domain structures consisting of the relatively smaller gaps (<250meV), which correspond to the excess Fe positions. The small gap Δ∼7meV is also observed at those positions, suggesting that the excess Fe affects the electronic structures of FeSe1−xTex.

Scanning Tunneling Microscopy and Density Functional Theory Study on Zinc(II)-Phthalocyanine Tetrasulfonic Acid on Bilayer Epitaxial Graphene on Silicon Carbide(0001)

Zinc(II)-phthalocyanine tetrasulfonic acid (Zn-PcS) molecules physisorbed on bilayer epitaxial graphene on silicon carbide (SiC(0001)) were studied by using scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT). Two different methods were used to deposit Zn-PcS molecules and regardless of the method being used, the surface coverage stayed very low indicating the weakness of surface-molecule interaction. STS measurements revealed that derivative of tunneling current with respect to voltage (dI/dV) measured on Zn-PcS molecules did not exhibit the characteristic dip observed on dI/dV curves of pristine bilayer epitaxial graphene. DFT calculations show that the energy of the lowest unoccupied molecular orbital (LUMO) of the Zn-PcS molecule is below the Dirac point of graphene which enhances local density of states (LDOS). We attribute the disappearance of the dip in the dI/dV curves measured on the Zn-PcS/bilayer system to the LUMO of Zn-PcS. Charge density calculations along Zn-PcS/graphene interface reveal that there is a small charge transfer from graphene to the molecule. Calculated adsorption energy (3.13 eV) of the molecule is notably low and is consistent with the observed low surface coverage at room temperature.

Activating Nonreducible Oxides via Doping.

Nonreducible oxides are characterized by large band gaps and are therefore unable to exchange electrons or to form bonds with surface species, explaining their chemical inertness. The insertion of aliovalent dopants alters this situation, as new electronic states become available in the gap that may be involved in charge-transfer processes. Consequently, the adsorption and reactivity pattern of doped oxides changes with respect to their nondoped counterparts. This Account describes scanning tunneling microscopy (STM) and photoelectron spectroscopy (XPS) experiments that demonstrate the impact of dopants on the physical and chemical properties of well-defined crystalline oxide films. For this purpose, MgO and CaO as archetypical rocksalt oxides have been loaded either with high-valence (Mo, Cr) or low-valence dopants (Li). While the former generate filled states in the oxide band gap and serve as electron donors, the latter produce valence-band holes and give rise to an acceptor response. The dopant-related electronic states and their polarization effect on the surrounding host material are explored with XPS and STM spectroscopy on nonlocal and local scales. Moreover, charge-compensating defects were found to develop in the oxide lattice, such as Ca and O vacancies in Mo- and Li-doped CaO films, respectively. These native defects are able to trap the excess charges of the impurities and therefore diminish the desired doping effect. If noncompensated dopants reside in the host lattice, electron exchange with surface species is observed. Mo ions in CaO, for example, were found to donate electrons to surface Au atoms. The anionic Au strongly binds to the CaO surface and nucleates in the form of monolayer islands, in contrast to the 3D growth prevailing on pristine oxides. Charge transfer is also revealed for surface O2 that traps one Mo electron by forming a superoxo-species. The activated oxygen is characterized by a reinforced binding to the surface, an elongated O-O bond length, and a reduced barrier for dissociation, and represents an important intermediate for oxidation reactions. The charge-transfer processes described here are quenched if Li is inserted into the oxide lattice, neutralizing the effect of the extra electrons. The specific behavior of doped oxides has been explored on a mechanistic level, i.e. on thin-film model systems at ultrahigh vacuum and low temperature. We believe, however, that our results are transferrable to realistic conditions and doping might thus develop into a powerful method to improve the performance of nonreducible oxides in surface-catalyzed reactions.

Dynamic probe of ZnTe(110) surface by scanning tunneling microscopy

The reconstructed surface structure of the II–VI semiconductor ZnTe (110), which is a promising material in the research field of semiconductor spintronics, was studied by scanning tunneling microscopy/spectroscopy (STM/STS). First, the surface states formed by reconstruction by the charge transfer of dangling bond electrons from cationic Zn to anionic Te atoms, which are similar to those of IV and III–V semiconductors, were confirmed in real space. Secondly, oscillation in tunneling current between binary states, which is considered to reflect a conformational change in the topmost Zn–Te structure between the reconstructed and bulk-like ideal structures, was directly observed by STM. Third, using the technique of charge injection, a surface atomic structure was successfully fabricated, suggesting the possibility of atomic-scale manipulation of this widely applicable surface of ZnTe.

Impurity Effects on the Energy Gap in Fe-doped Bi2212

We performed scanning tunnelling microscopy/spectroscopy (STM/STS) on Fe-doped Bi2212. The Fe substitution for Cu causes a strong spatial inhomogeneity in STS spectra. The energy gap (Δ1 ∼80mV) has a sub-gap (Δ2 ∼70mV) in some distinct locations on the sample surface. We find that the gap edge peaks are largely depressed and only the sub-gap survives across the region where the spatial modulation of the local density of states is stronger. This indicates, that Δ1 anti-correlates with Δ2.

Investigation of scanning tunneling spectra on iron-based superconductor FeSe0.5Te0.5

FeSe0.5Te0.5 single crystals with superconducting critical temperature of 13.5 K are investigated by scanning tunneling microscopy/spectroscopy (STM/STS) measureflents in detail. STM image on the top surface shows an atomically resolved square lattice consisted by white and dark spots with a constant of about 3.73 0.03 which is consistent with the lattice constant 3.78 . The Se and Te atoms with a height difference of about 0.35 are successfully identified since the sizes of the two kinds of atoms are different. The tunneling spectra show very large zero-bias conductance value and asymmetric coherent peaks in the superconducting state. According to the positions of coherence peaks, we determine the superconducting gap 2 = 5.5 meV, and the reduced gap 2/kBTc = 4.9 is larger than the value predicted by the weak-coupling BCS theory. The zero-bias conductance at 1.7 K only have a decrease of about 40% compared with the normal state conductance, which may originate from some scattering and broadening mechanism in the material. This broadening effect will also make the superconducting gap determined by the distance between the coherence peaks larger than the exact gap value. The asymmetric structure of the tunneling spectra near the superconducting gap is induced by the hump on the background. This hump appears at temperature more than twice the superconducting critical temperature. This kind of hump has also been observed in other iron pnictides and needs further investigation. A possible bosonic mode outside the coherence peak with a mode energy of about 5.5 meV is observed in some tunneling spectra, and the ratio between the mode energy and superconducting transition temperature /kBTc 4.7 is roughly consistent with the universal ratio 4.3 in iron-based superconductors. The high-energy background of the spectra beyond the superconducting gaps shows a V-shape feature. The slopes of the differential conductance spectra at high energy are very different in the areas of Te-atom cluster and Se-atom cluster, and the difference extends to the energy of more than 300 meV. The differential conductance mapping has very little information about the quasi-particle interference of the superconducting state, which may result from the other strong scattering mechanism in the sample.

Temperature dependence of the superconducting proximity effect quantified by scanning tunneling spectroscopy

Here, we present the first systematic study on the temperature dependence of the extension of the superconducting proximity effect in a 1–2 atomic layer thin metallic film, surrounding a superconducting Pb island. Scanning tunneling microscopy/spectroscopy (STM/STS) measurements reveal the spatial variation of the local density of state on the film from 0.38 up to 1.8 K. In this temperature range the superconductivity of the island is almost unaffected and shows a constant gap of a 1.20 ± 0.03 meV. Using a superconducting Nb-tip a constant value of the proximity length of 17 ± 3 nm at 0.38 and 1.8 K is found. In contrast, experiments with a normal conductive W-tip indicate an apparent decrease of the proximity length with increasing temperature. This result is ascribed to the thermal broadening of the occupation of states of the tip, and it does not reflect an intrinsic temperature dependence of the proximity length. Our tunneling spectroscopy experiments shed fresh light on the fundamental issue of the temperature dependence of the proximity effect for atomic monolayers, where the intrinsic temperature dependence of the proximity effect is comparably weak.

Three axis vector magnet set-up for cryogenic scanning probe microscopy.

We describe a three axis vector magnet system for cryogenic scanning probe microscopy measurements. We discuss the magnet support system and the power supply, consisting of a compact three way 100 A current source. We obtain tilted magnetic fields in all directions with maximum value of 5T along z-axis and of 1.2T for XY-plane magnetic fields. We describe a scanning tunneling microscopy-spectroscopy (STM-STS) set-up, operating in a dilution refrigerator, which includes a new high voltage ultralow noise piezodrive electronics and discuss the noise level due to vibrations. STM images and STS maps show atomic resolution and the tilted vortex lattice at 150 mK in the superconductor β-Bi2Pd. We observe a strongly elongated hexagonal lattice, which corresponds to the projection of the tilted hexagonal vortex lattice on the surface. We also discuss Magnetic Force Microscopy images in a variable temperature insert.

Quantitative connection between the nanoscale electronic inhomogeneity and the pseudogap of Bi2Sr2CaCu2[formula omitted] superconductors

We have found a quantitative connection between the evolution of the inhomogeneous nanoscale electronic gaps (INSEG) state detected in Bi2Sr2CaCu2O8+δ by scanning tunneling microscopy/spectroscopy (STM/S) and the two universal, the upper and the lower, pseudogaps in high-temperature cuprate superconductors (HTCS). When the doping and temperature dependent INSEG map were analyzed by using our proposed hole-scale, we find that the two pseudogaps are connected to two specific coverages of the CuO2 plane by INSEG: the 50% and 100% coverages of the CuO2 planes by INSEG correspond to the upper and lower pseudogaps, respectively. This quantitative connection to the two pseudogaps indicates that the origin of the measured pseudogap energies and temperatures are intimately related to the geometrical coverage of the CuO2 planes by the INSEG state. We find that INSEG and superconductivity coexist in the underdoped to the overdoped regimes. We suggest that pseudogap states are microscopically inhomogeneous and 100% coverage of the CuO2 planes by the INSEG is a necessary condition for the high-Tc superconductivity.

Topological properties determined by atomic buckling in self-assembled ultrathin Bi(110).

Topological insulators (TIs) are a new type of electronic materials in which the nontrivial insulating bulk band topology governs conducting boundary states with embedded spin-momentum locking. Such edge states are more robust in a two-dimensional (2D) TI against scattering by nonmagnetic impurities than in its three-dimensional (3D) variant, because in 2D the two helical edge states are protected from the only possible backscattering. This makes the 2D TI family a better candidate for coherent spin transport and related applications. While several 3D TIs are already synthesized experimentally, physical realization of 2D TI is so far limited to hybrid quantum wells with a tiny bandgap that does not survive temperatures above 10 K. Here, combining first-principles calculations and scanning tunneling microscopy/spectroscopy (STM/STS) experimental studies, we report nontrivial 2D TI phases in 2-monolayer (2-ML) and 4-ML Bi(110) films with large and tunable bandgaps determined by atomic buckling of Bi(110) films. The gapless edge states are experimentally detected within the insulating bulk gap at 77 K. The band topology of ultrathin Bi(110) films is sensitive to atomic buckling. Such buckling is sensitive to charge doping and could be controlled by choosing different substrates on which Bi(110) films are grown.

Restoring pristine Bi2Se3 surfaces with an effective Se decapping process

High quality thin films of topological insulators (TI) such as Bi2Se3 have been successfully synthesized by molecular beam epitaxy (MBE). Although the surface of MBE films can be protected by capping with inert materials such as amorphous Se, restoring an atomically clean pristine surface after decapping has never been demonstrated, which prevents in-depth investigations of the intrinsic properties of TI thin films with ex situ tools. Using high resolution scanning tunneling microscopy/spectroscopy (STM/STS), we demonstrate a simple and highly reproducible Se decapping method that allows recovery of the pristine surface of extremely high quality Bi2Se3 thin films grown and capped with Se in a separate MBE system then exposed to the atmosphere during transfer into the STM system. The crucial step of our decapping process is the removal of the surface contaminants on top of amorphous Se before thermal desorption of Se at a mild temperature (∼210 °C). This effective Se decapping process opens up the possibility of ex situ characterizations of pristine surfaces of interesting selenide materials and beyond using cutting-edge techniques.