Low-temperature Scanning Tunneling Microscopy Research Articles

Study of the Cu(111) Surface by Scanning Tunneling Microscopy: The Morphology Evolution, Reconstructions, Superstructures and Line Defects.

The Cu(111) surface is an important substrate for catalysis and the growth of 2D materials, but a comprehensive understanding of the preparation and formation of well-ordered and atomically clean Cu(111) surfaces is still lacking. In this work, the morphology and structure changes of the Cu(111) surface after treatment by ion bombardment and annealing with a temperature range of 300-720 °C are investigated systematically by using in situ low-temperature scanning tunneling microscopy. With the increase of annealing temperature, the surface morphology changes from corrugation to straight edge, the number of screw dislocations changes from none to numerous, and the surface atomic structure changes from disordered to ordered structures (with many reconstructions). In addition, the changing trend of step width and step height in different stages is different (first increased and then decreased). A perfect Cu(111) surface with a step height of one atom layer (0.21 nm) and a width of more than 150 nm was obtained. In addition, two interesting superstructures and a new surface phase with a large number of line defects were found. This work serves as a strong foundation for understanding the properties of Cu(111) surface, and it also provides important guidance for the effective pretreatment of Cu(111) substrates, which are widely used.

Controlling the Spin States of FeTBrPP on Au(111).

Spin-flip excitations of iron porphyrin molecules on Au(111) are investigated with a low-temperature scanning tunneling microscope. The molecules adopt two distinct adsorption configurations on the surface that exhibit different magnetic anisotropy energies. Density functional theory calculations show that the different structures and excitation energies reflect unlike occupations of the Fe 3d levels. We demonstrate that the magnetic anisotropy energy can be controlled by changing the adsorption site, the orientation, or the tip-molecule distance.

Selective Activation of Aromatic C-H Bonds Catalyzed by Single Gold Atoms at Room Temperature.

Selective activation and controlled functionalization of C-H bonds in organic molecules is one of the most desirable processes in synthetic chemistry. Despite progress in heterogeneous catalysis using metal surfaces, this goal remains challenging due to the stability of C-H bonds and their ubiquity in precursor molecules, hampering regioselectivity. Here, we examine the interaction between 9,10-dicyanoanthracene (DCA) molecules and Au adatoms on a Ag(111) surface at room temperature (RT). Characterization via low-temperature scanning tunneling microscopy, spectroscopy, and noncontact atomic force microscopy, supported by theoretical calculations, revealed the formation of organometallic DCA-Au-DCA dimers, where C atoms at the ends of the anthracene moieties are bonded covalently to single Au atoms. The formation of this organometallic compound is initiated by a regioselective cleaving of C-H bonds at RT. Hybrid quantum mechanics/molecular mechanics calculations show that this regioselective C-H bond cleaving is enabled by an intermediate metal-organic complex which significantly reduces the dissociation barrier of a specific C-H bond. Harnessing the catalytic activity of single metal atoms, this regioselective on-surface C-H activation reaction at RT offers promising routes for future synthesis of functional organic and organometallic materials.

The composition and structure of the ubiquitous hydrocarbon contamination on van der Waals materials

The behavior of single layer van der Waals (vdW) materials is profoundly influenced by the immediate atomic environment at their surface, a prime example being the myriad of emergent properties in artificial heterostructures. Equally significant are adsorbates deposited onto their surface from ambient. While vdW interfaces are well understood, our knowledge regarding atmospheric contamination is severely limited. Here we show that the common ambient contamination on the surface of: graphene, graphite, hBN and MoS2 is composed of a self-organized molecular layer, which forms during a few days of ambient exposure. Using low-temperature STM measurements we image the atomic structure of this adlayer and in combination with infrared spectroscopy identify the contaminant molecules as normal alkanes with lengths of 20-26 carbon atoms. Through its ability to self-organize, the alkane layer displaces the manifold other airborne contaminant species, capping the surface of vdW materials and possibly dominating their interaction with the environment.

Enhanced Atomic Precision Fabrication by Adsorption of Phosphine into Engineered Dangling Bonds on H-Si Using STM and DFT.

The doping of Si using the scanning probe hydrogen depassivation lithography technique has been shown to enable placing and positioning small numbers of P atoms with nanometer accuracy. Several groups have now used this capability to build devices that exhibit desired quantum behavior determined by their atomistic details. What remains elusive, however, is the ability to control the precise number of atoms placed at a chosen site with 100% yield, thereby limiting the complexity and degree of perfection achievable. As an important step toward precise control of dopant number, we explore the adsorption of the P precursor molecule, phosphine, into atomically perfect dangling bond patches of intentionally varied size consisting of three adjacent Si dimers along a dimer row, two adjacent dimers, and one single dimer. Using low temperature scanning tunneling microscopy, we identify the adsorption products by generating and comparing to a catalog of simulated images, explore atomic manipulation after adsorption in select cases, and follow up with incorporation of P into the substrate. For one-dimer patches, we demonstrate that manipulation of the adsorbed species leads to single P incorporation in 12 out of 12 attempts. Based on the observations made in this study, we propose this one-dimer patch method as a robust approach that can be used to fabricate devices where it is ensured that each site of interest has exactly one P atom.

Adsorption structures of catechol on the ZnO(10-10) surface

The details of catechol interacting with the ZnO surfaces are critical for understanding how the organic chromophores anchor on and hence functionalize the ZnO materials. In this work, we apply low-temperature scanning tunneling microscopy (LT-STM) in combination with density functional theory (DFT) calculations to investigate the adsorption state and interface charge transfer of catechol on a ZnO(10-10) surface. For the molecules deposited at liquid nitrogen temperature, the fully-protonated (FP) status prevails in the isolated molecules despite the synergistic formation of half-protonated (HP) and fully-deprotonated (FD) ones. However, annealing to elevated temperature drives all the molecules to assemble into either linear or a c(2 × 2) superstructure composed of only FD catechols. Such spontaneous deprotonation of catechol is different from the predicted water-assisted process and plays critical roles in switching the catechol from electron acceptors to electron donors relative to the ZnO surface. These results may bring new insights into the design of optoelectronic and biomedical applications of the ZnO-organic compositional system.

Two-dimensional tellurium superstructures on Au(111) surfaces.

Two-dimensional (2D) allotropes of tellurium (Te), recently coined as tellurene, are currently an emerging topic of materials research due to the theoretically predicted exotic properties of Te in its ultrathin form and at the single atomic layer limit. However, a prerequisite for the production of such new and single elemental 2D materials is the development of simple and robust fabrication methods. In the present work, we report three different 2D superstructures of Te on Au(111) surfaces by following an alternative experimental deposition approach. We have investigated the superstructures using low-temperature scanning tunneling microscopy and spectroscopy, Auger electron spectroscopy (AES), and field emission AES. Three superstructures (13 × 13, 8 × 4, and √11 × √11) of 2D Te are observed in our experiments, and the formation of these superstructures is accompanied by the lifting of the characteristic 23 × √3 surface reconstruction of the Au(111) surface. Scanning tunneling spectroscopy reveals a strong dependence of the local electronic properties on the structural arrangement of the Te atoms on the Au(111) support, and we observe superstructure-dependent electronic resonances around the Fermi level and below the Au(111) conduction band. In addition to the appearance of the new electronic resonances, the emergence of band gaps with a p-type charge character has been evidenced for two out of three Te superstructures (13 × 13 and √11 × √11) on the Au(111) support.

Enhanced reducibility of well-defined Cr oxide nanostructures on Au(111) through Zn doping

Mixed oxides play important roles in many heterogeneous catalytic processes, and understanding of interaction between different oxide components is crucial for synthesizing the catalysts with high performances. Here, we introduce trace of metallic Zn species into well-defined chromium oxide (Cr oxide) nanostructures grown on Au(111) including CrO2-bilayer nanoislands and Cr2O7dinuclear clusters and study the impact of the dopant on reducibility of the Cr oxides. Combining low-temperature scanning tunneling microscopy (LT-STM) and X-ray photoelectron spectroscopy (XPS), we demonstrate that incorporation of 2.5 at.% Zn can lower the initial reduction temperature of Cr oxides by 70–160 K upon heating in ultrahigh vacuum. The reduction degree of Zn-doped Cr oxide is much larger than the pure oxide at the same heating temperature. Density functional theory (DFT) calculations reveal that the enhanced reducibility of the pristine oxide via introduction of Zn dopant mainly comes from the promoted activity of oxygen atoms due to Zn substitution of surface Cr.

Surface Characterization of the Solution‐Processed Organic–Inorganic Hybrid Perovskite Thin Films

The surface properties of organic-inorganic hybrid perovskites can strongly affect the efficiency and stability of corresponding devices. Even though different surface passivation methods are developed, the microscopic structures of solution-processed perovskite film surfaces are not systematically studied. This study uses low-temperature scanning tunneling microscopy to study the organic-inorganic hybrid perovskite thin films, MA0.4 FA0.6 PbI3 and MAPbI3 , synthesized by the spin-coating method.Flat surface structures, atomic steps, and crystal grain boundaries are resolved at an atomic resolution. The surface imperfections are also characterized, as well as the dominant defects.Simulations on different types of iodine vacancy configurations are performed by density functional theory calculations.In addition, it is observed that the surface iodine lattice structure is unstable during scanning. Tip scanning can also cause the vertical migration of surface iodine ions. The measurements provide the direct visualizations of the surface imperfections of the solution-processed perovskite films. They are essential for understanding the surface-related optoelectronic effects and rationally designing more efficient surface passivation methods.

Percolative superconductivity in electron-doped Sr1−xEuxCuO2+y films

Electron-doped infinite-layer Sr$_{1-x}$Eu$_{x}$CuO$_{2+y}$ films over a wide doping range have been prepared epitaxially on SrTiO$_3$(001) using reactive molecular beam epitaxy. In-plane transport measurements of the single crystalline samples reveal a dome-shaped nodeless superconducting phase centered at $x$ $\sim$ 0.15, a Fermi-liquid behavior and pronounced upturn in low temperature resistivity. We show that the resistivity upturn follows square-root temperature dependence, suggesting the emergence of superconductivity via a three-dimensional percolation process. The percolative superconductivity is corroborated spectroscopically by imaging the electronic phase separation between superconducting and metallic phases with low-temperature scanning tunneling microscopy. Furthermore, we visualize interstitial and apical oxygen anions that rapidly increase in number as $x>$ 0.12, and elucidate their impacts on the superconductivity and normal-state resistivity.

On-Surface Synthesis and Spectroscopy of Aluminum Phthalocyanine on Superconducting Lead.

Large ordered islands of aluminum phthalocyanine (AlPc) molecules, which are unstable in air, are synthesized from ClAlPc on Pb(100) via dechlorination. Low-temperature scanning tunneling microscopy reveals that isolated AlPc molecules lose their spin moment on superconducting Pb(100). Molecular magnetism, which is detected via Yu-Shiba-Rusinov (YSR) resonances, may be restored by surrounding a molecule with an array of neighbor molecules in artificial arrays or in a self-assembled monolayer. Unlike phthalocyanine (H2Pc) or lead phthalocyanine (PbPc) monolayers, where the YSR energy was found to depend strongly on the detailed configuration of the neighboring molecules, we find a similar magnetic moment on every second molecule for AlPc. In addition, YSR resonances lead to unusually high conductance peaks that are due to vibrational excitations. Twelve vibrational modes are resolved and discussed with respect to similar results from PbPc. The enhancement of the inelastic transitions is tentatively attributed to the large amplitude of the YSR resonances and the long lifetime of electrons in the molecular bound state. By assembling neighboring molecules into configurations that differ from those of the monolayer, the YSR energy may be fine-tuned, and a simple spin-state switching device is constructed.

Dynamic bonding influenced by the proximity of adatoms to one atom high step edges

Low-temperature scanning tunneling microscopy is used here to study dynamic bonding of gold atoms on surfaces under low coordination conditions. In the experiments, using an atomically-sharp gold tip, a gold adatom is deposited onto a gold surface with atomic precision either on the first hollow site near a step edge, or far away from it. Classical molecular dynamics simulations at 4.2 K and density functional theory calculations serve to elucidate the difference in the bonding behavior between these two different placements, while also providing information on the crystalline classification of the STM tips based on their experimental performance.

Observation of robust zero-energy state and enhanced superconducting gap in a trilayer heterostructure of MnTe/Bi2Te3/Fe(Te, Se).

The interface between magnetic material and superconductors has long been predicted to host unconventional superconductivity, such as spin-triplet pairing and topological nontrivial pairing state, particularly when spin-orbital coupling (SOC) is incorporated. To identify these unconventional pairing states, fabricating homogenous heterostructures that contain such various properties are preferred but often challenging. Here, we synthesized a trilayer-type van der Waals heterostructure of MnTe/Bi2Te3/Fe(Te, Se), which combined s-wave superconductivity, thickness-dependent magnetism, and strong SOC. Via low-temperature scanning tunneling microscopy, we observed robust zero-energy states with notably nontrivial properties and an enhanced superconducting gap size on single unit cell (UC) MnTe surface. In contrast, no zero-energy state was observed on 2-UC MnTe. First-principle calculations further suggest that the 1-UC MnTe has large interfacial Dzyaloshinskii-Moriya interaction and a frustrated AFM state, which could promote noncolinear spin textures. It thus provides a promising platform for exploring topological nontrivial superconductivity.

Planar bridging an atomically precise surface trench with a single molecular wire on an Au(1 1 1) surface

In a bridge configuration, a single graphene nanoribbon (GNR) is positioned with a picometer precision over a trench in between two monoatomic steps on an Au(111) surface. This GNR molecular wire adopts a deformed conformation towards the down terrace in between the two contact step edges. Using differential conductance dI/dV mapping from a low-temperature scanning tunneling microscope, it is demonstrated how the electronic delocalization along GNR is cut at each contact by its down curvature. It points out the need to bring conductive nanocontacts backside of the support for preserving the front side GNR planar conformation.

Spin-related electronic pathway through single molecule on Au(111)

Spin properties of organic molecules have attracted great interest for their potential applications in spintronic devices and quantum computing. Fe-tetraphenyl porphyrin (FeTPP) is of particular interest for its robust magnetic properties on metallic substrates. FeTPP is prepared in vacuum via on-surface synthesis. Molecular structure and spin-related transport properties are characterized by low-temperature scanning tunneling microscope and spectroscopy at 0.5 K. Density functional theory calculations are performed to understand molecular adsorption and spin distribution on Au(111). The molecular structure of FeTPP is distorted upon adsorption on the substrate. Spin excitations of FeTPP are observed on the Fe atom and high pyrrole groups in differential conductance spectra. The calculated spin density distribution indicates that the electron spin of FeTPP is mainly distributed on the Fe atom. The atomic transmission calculation indicates that electrons transport to substrate is mediated through Fe atom, when the tip is above the high pyrrole group.

Direct observation of multiband charge density waves in NbTe2

The electronic structure of periodic lattice distortion (PLD) in ${\mathrm{NbTe}}_{2}$ was examined using low-temperature scanning tunneling spectroscopy and microscopy. The striped PLDs with $3\ifmmode\times\else\texttimes\fi{}1$ and $1\ifmmode\times\else\texttimes\fi{}\frac{9}{2}$ superstructures were characterized in real and reciprocal space. The simultaneous formation of momentum-specific suppressions in the spectral weight and phase shifts of the wavefront related to the superstructures were observed at multiple energies. These unusual energy dependencies are well agreed with the charge density waves (CDWs) formed in the multiband electronic structure of ${\mathrm{NbTe}}_{2}$. Fermi-surface nesting and reconstruction of the reciprocal lattice were suggested in developing the multiband CDWs in ${\mathrm{NbTe}}_{2}$.

Precise Synthesis of Concentric Ring, Helicoid, and Ladder Metallo-Polymers with Chevron-Shaped Monomers.

Molecular geometry represents one of the most important structural features and governs physical properties and functions of materials. Nature creates a wide array of substances with distinct geometries but similar chemical composition with superior efficiency and precision. However, it remains a formidable challenge to construct abiological macromolecules with various geometries based on identical repeating units, owing to the lack of corresponding synthetic approaches for precisely manipulating the connectivity between monomers and feasible techniques for characterizing macromolecules at the single-molecule level. Herein, we design and synthesize a series of tetratopic monomers with chevron stripe shape which serve as the key precursors to produce four distinct types of metallo-macromolecules with well-defined geometries, viz., the concentric hexagon, helicoid polymer, ladder polymer, and cross-linked polymer, via platinum-acetylide couplings. Concentric hexagon, helicoid, and ladder metallo-polymers are directly visualized by transmission electron microscopy, atomic force microscopy, and ultra-high-vacuum low-temperature scanning tunneling microscopy at the single-molecule level. Finally, single-walled carbon nanotubes (SWCNTs) are selected as the guest to investigate the structure-property relationship based on such macromolecules, among which the helicoid metallo-polymer shows high efficiency in wrapping SWCNTs with geometry-dependent selectivity.

Negative Differential Resistance in Spin-Crossover Molecular Devices.

We demonstrate, based on low-temperature scanning tunneling microscopy (STM) and spectroscopy, a pronounced negative differential resistance (NDR) in spin-crossover (SCO) molecular devices, where a FeII SCO molecule is deposited on surfaces. The STM measurements reveal that the NDR is robust with respect to substrate materials, temperature, and the number of SCO layers. This indicates that the NDR is intrinsically related to the electronic structure of the SCO molecule. Experimental results are supported by density functional theory (DFT) with nonequilibrium Green's function (NEGF) calculations and a generic theoretical model. While the DFT+NEGF calculations reproduce NDR for a special atomically sharp STM tip, the effect is attributed to the energy-dependent tip density of states rather than the molecule itself. We, therefore, propose a Coulomb blockade model involving three molecular orbitals with very different spatial localization as suggested by the molecular electronic structure.

Enhanced photon emission by field emission resonances and local surface plasmon in tunneling junction

We investigated the photon emission spectra on Ag (111) surface excited by tunneling electrons using a low temperature scanning tunneling microscope in ultrahigh vacuum. Characteristic plasmon modes were illustrated as a function of the bias voltage. The one electron excitation process was revealed by the linear relationship between the luminescence intensity and the tunneling current. Luminescence enhancement is observed in the tunneling regime for the relatively high bias voltages, as well as at the field emission resonance with bias voltage increased up to 9 V. Presence of a silver (Ag) nanoparticle in the tunneling junction results in an abnormally strong photon emission at the high field emission resonances, which is explained by the further enhancement due to coupling between the localized surface plasmon and the vacuum. The results are of potential value for applications where ultimate enhancement of photon emission is desired.

Ubiquitous stripe phase and enhanced electron pairing in the interfacial high- Tc superconductor FeSe/BaTiO3

Monolayer FeSe grown on SrTiO$_3$ provides a new playground for high-Tc superconductivity. A recent study shows the importance of stripy instability for superconductivity enhancement in FeSe/SrTiO3. However, it is still under debate whether such a stripe phase is ubiquitously bound to the interfacial superconductivity. Here, we report on molecular beam epitaxy growth and low-temperature scanning tunneling microscopy study of FeSe films on BaTiO$_3$(001). We find that the stripe phase is more prominent in FeSe/BaTiO$_3$. Long-range stripes exist in bilayer and triple-layer FeSe films at 4 K and even persist up to 77 K in the case of bilayer FeSe, with enlarged superconducting gaps upon alkali-metal deposition. The results point out an intimate correlation between the interfacial superconductivity and the stripe phase.