Low-temperature Scanning Tunneling Spectroscopy Research Articles

An ultracompact scanning tunneling microscope within a Φ 10 piezo tube in a 20T superconducting magnet.

Low-temperature scanning tunneling microscopy and spectroscopy (STM/S) help to better understand the fundamental physics of condensed matter. We present an ultracompact STM within a Φ 10 piezo tube in a 20T superconducting magnet. The carefully cut piezo tube contains the STM's coarse-positioning assembly. Loading an STM tip-sample mechanical loop into the piezo tube with special cut openings enables an ultracompact pencil-size dimension down to Φ 10mm, in which fine-machined nonmagnetic parts are assembled to enable slide-stick motion and xyz-scanning procedures. The small size leads to a higher resonant frequency, a typical feature of a rigid STM instrument, increasing its vibration immunity. Scanning by moving the sample while keeping the tip stationary improves the stability of the tip-sample junction compared to moving the tip. Taking advantage of its high-field compatibility and rigid design, our STM captures the atomically resolved topography of highly oriented pyrolytic graphite (HOPG) at 1.5K and in magnetic fields up to 17T. The topography of graphene lattice and graphite is simultaneously recorded on an atomic terrace of HOPG, unveiling a modified local charge density at a surface defect. The superconducting energy gaps of layered type-II superconductors NbSe2 and PdBi2 are well resolved through dI/dV tunneling spectra at sub-2K. Our unique STM is highly suitable for potential STM/S applications in world-class high-field facilities where the strong magnetic field can exceed 30T.

Designing 2D stripe winding network through crown-ether intermediate Ullmann coupling on Cu(111) surface.

Chemical synthesis typically yields the most thermodynamically stable ordered arrangement, a principle also governing surface synthesis on an atomically level two-dimensional (2D) surface, fostering the creation of structured 2D formations. The linear connection arising from energetically stable chemical bonding precludes the generation of a 2D random network comprised of one-dimensional (1D) convoluted stripes through on-surface synthesis. Nonetheless, we underscored that on-surface synthesis possesses the capability not solely to fashion a 2D ordered linear network but also to fabricate a winding 2D network employing a precursor with a soft ring and intermediate state bonding within the Ullmann reaction. Here, on-surface synthesis was exhibited on Cu(111) employing a 2D self-assembled monolayer array of 4,4',5,5'-tetrabromodibenzo[18]crown-6 ether (BrCR) precursors. These precursors were purposefully structured, with a crown ether ring at the core and Br atoms positioned at the head and tail ends, facilitating preferential connections along the elongated axis to foster a 1D stripe configuration. We illustrate how adjustments in the quantities of the intermediate state, serving as a primary linkage, can yield a labyrinthine, convoluted winding 2D network of stripes. The progression of growth, underlying mechanisms, and electronic structures were scrutinized using an ultrahigh vacuum low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) setup combined with density functional theory (DFT) calculations. This experimental evidence opens a novel functionality in leveraging on-surface synthesis for the formation of a 2D random network. This discovery holds promise as a pioneering constituent in the construction of a ring host supramolecule, augmenting its capability to ensnare guest atoms, molecules, or ions.

Electronic Janus lattice and kagome-like bands in coloring-triangular MoTe2 monolayers

Polymorphic structures of transition metal dichalcogenides (TMDs) host exotic electronic states, like charge density wave and superconductivity. However, the number of these structures is limited by crystal symmetries, which poses a challenge to achieving tailored lattices and properties both theoretically and experimentally. Here, we report a coloring-triangle (CT) latticed MoTe2 monolayer, termed CT-MoTe2, constructed by controllably introducing uniform and ordered mirror-twin-boundaries into a pristine monolayer via molecular beam epitaxy. Low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) together with theoretical calculations reveal that the monolayer has an electronic Janus lattice, i.e., an energy-dependent atomic-lattice and a Te pseudo-sublattice, and shares the identical geometry with the Mo5Te8 layer. Dirac-like and flat electronic bands inherently existing in the CT lattice are identified by two broad and two prominent peaks in STS spectra, respectively, and verified with density-functional-theory calculations. Two types of intrinsic domain boundaries were observed, one of which maintains the electronic-Janus-lattice feature, implying potential applications as an energy-tunable electron-tunneling barrier in future functional devices.

Evidence of Nodal Superconductivity in Monolayer 1H-TaS2 with Hidden Order Fluctuations.

Unconventional superconductors represent one of the fundamental directions in modern quantum materials research. In particular, nodal superconductors are known to appear naturally in strongly correlated systems, including cuprate superconductors and heavy-fermion systems. Van der Waals materials hosting superconducting states are well known, yet nodal monolayer van der Waals superconductors have remained elusive. Here, using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) experiments, it is shown that pristine monolayer 1H-TaS2 realizes a nodal superconducting state. Non-magnetic disorder drives the nodal superconducting state to a conventional gapped s-wave state. Furthermore, many-body excitations emerge close to the gap edge, signalling a potential unconventional pairing mechanism. The results demonstrate the emergence of nodal superconductivity in a van der Waals monolayer, providing a building block for van der Waals heterostructures exploiting unconventional superconductingstates.

Ring-Closing Reactions of Porphyrins on Noble Metal Surfaces

On-surface chemistry protocols provide elegant routes to individual molecular complexes, oligomers, and hybrid structures at confining interfaces, yielding distinct chemical, physical, and structural properties [1]. In this context, porphyrins and other tetrapyrroles represent versatile precursors that can undergo metalation, ring-opening, and ring-closing reactions. Furthermore, they are often used as functional tectons for self-assembled molecular films, metal-organic coordination networks, and covalent architectures at interfaces [2].We will report on temperature-induced ring-closing reactions of porphyrins on single crystalline Ag and Au surfaces in an ultrahigh vacuum environment. Examples covered include dehydrogenative homocoupling as well as intramolecular ring-closing. Specifically, distinct oligomers including square-type porphyrin tetramers featuring a planar, central antiaromatic cyclooctatetraene (COT) moiety were achieved on a Ag(100) support by employing unsubstituted Zn- and free-base porphines (see Figure) as precursors [3]. Low-temperature scanning tunnelling microscopy (STM), spectroscopy (STS), and bond-resolved atomic force microscopy (AFM) supported by complementary theoretical modelling were used to comprehensively characterize chemical structure and electronic properties of the square-type tetramers with sub-molecular resolution. Furthermore, the synthesis of π-extended oxygen-embedded porphyrins via O-annulation on Au(111) will be discussed. This surface chemical approach yields high selectivity and yield for planar porphyrin products with distinct symmetries and oxygen content that is not achieved by solution chemistry. The bond-resolved AFM characterization is complemented by X-ray photoelectron spectroscopy measurements. These studies thus give access, and insights, to novel porphyrinoids with distinct properties on noble metal supports.

Characterization of dissipative regions of a N-doped superconducting radio-frequency cavity

We report radio-frequency measurements of quality factors and temperature mapping of a nitrogen doped Nb superconducting RF cavity. Cavity cutouts of hot and cold spots were studied with low temperature scanning tunneling microscopy and spectroscopy, X-ray photoelectron spectroscopy and secondary electron microscopy. Temperature mapping revealed a substantial reduction of the residual resistance upon cooling the cavity with a greater temperature gradient and hysteretic losses at the quench location, pointing to trapped vortices as the dominant source of residual surface resistance. Analysis of the tunneling spectra in the framework of a proximity effect theory shows that hot spots have a reduced pair potential and a wider distribution of the contact resistance between the Nb and the top Nb oxide. Alone, these degraded superconducting properties account for a much weaker excess dissipation as compared with the vortex contribution. Based on the correlation between the quasiparticle density of states and temperature mapping, we suggest that degraded superconducting properties may facilitate vortex nucleation or settling of trapped flux during cooling the cavity through the critical temperature.

Origin of surface and subband states at the InAs(111)A surface

The atomic structure of surfaces and interfaces plays a vital role in the electronic quality and properties of quantum devices. The interplay between the surface and confined bulk subband states in terms of their susceptibility has been investigated in relation to crystal defects on an InAs(111)A-$(2\ifmmode\times\else\texttimes\fi{}2)$ reconstructed surface, using low-temperature scanning tunneling microscopy and spectroscopy. We measure the two-dimensional quantized subband states arising from the confined potential imposed by downward bending of the conduction band edge. Furthermore, we show evidence of the existence of surface Bloch states within the confined bulk band gap projected on the surface spectrum which have originated from the surface reconstruction. As expected, larger confined bulk band gaps at the surface and conduction band offset are measured to be 0.58 and 0.31 eV, respectively. We further show the scattering of these quantum states at different surface defects and demonstrate that surface states are more susceptible to the defect potential when compared with the corresponding subband states. This apparent contrast follows from the length scale at which these defect potentials actively interact on or near the surface. Our observed experimental results are supported by empirical tight-binding simulations for the subband states and first-principles density functional theory simulations for the surface states present on the surface.

Scanning tunneling microscopy and spectroscopy characterization of Nb films for quantum applications

Niobium thin films are key components of superconducting microwave resonators. Interest in these devices has increased dramatically because of their application in quantum systems. Despite tremendous effort to improve their performance, loss mechanisms are still not well understood. Nb/substrate and Nb/air interfaces are likely culprits in contributing to decoherence and ultimately limiting the performance of superconducting devices. Here, we investigate the Nb/substrate interface by studying the effect of hydrogen-passivated H:Si(111) substrates on the local superconducting properties of ∼40 nm thick Nb films compared to Nb films grown on typical Si(001) substrates. Specifically, low-temperature scanning tunneling microscopy and spectroscopy are employed to compare nanoscale material properties. The atomically flat monohydride H:Si(111) substrates are found to yield a smoother and less defective interface with the Nb film. Correspondingly, the Nb films grown on H:Si(111) substrates present more uniform superconducting properties and exhibit less quasiparticle broadening.

Visualizing Atomic Quantum Defects in Ultrathin 1T-PtTe2.

Defects are of significant importance to determine and improve the distinct properties of 2D materials, such as electronic, optical, and catalytic performance. In this report, we observe four types of point defects in atomically thin flakes of 1T-PtTe2 by using low-temperature scanning tunnelling microscopy and spectroscopy (STM/S). Through the combination of STM imaging and simulations, such defects are identified as a single tellurium vacancy from each side of the top PtTe2 layer and a single platinum vacancy from the topmost and next layer. The density functional theory (DFT) calculations reveal that the platinum vacancies from both the monolayer and bilayer exhibit a local magnetic moment. In bilayer PtTe2, the interlayer coulomb screening effect reduces the local magnetic momentum of the single platinum vacancy. Our research provides meaningful guidance for further experiments about the effects of intrinsic defects on potential functions of thin 1T-PtTe2, such as catalysis and spintronic applications.

From a wide band gap to the superconducting proximity effect: Fe on Pb(111)

Epitaxially grown Fe nanostructures on Pb(111) were studied by low-temperature scanning tunneling microscopy and spectroscopy. The deposited Fe assemblies are classified into two groups according to their electronic behavior close to the Fermi energy. One group exhibits a wide energy gap of 0.7 eV that is independent of the temperature ranging from 5 K to room temperature. These Fe islands indicate the absence of the superconductivity proximity effect in their interior. The other group shows a metallic behavior at the Fermi level. The substrate superconducting phase locally enters into these islands, which is evidenced by a sharp resonance at the Fermi energy presumably signaling Andreev reflection at the magnet–superconductor interface.

Mesoscopic 2D molecular self-assembly on an insulator

Two-dimensional (2D) nanostructures and nanomaterials offer potential for a wide range of technological applications in electronics, optoelectronics, data storage, sensing and catalysis. On-surface molecular self-assembly—where organic molecules act as building blocks and where surfaces play the role of supporting templates—allows for the bottom-up synthesis of such 2D systems with tuneable atomically precise morphologies and tailored electronic properties. These self-assembly protocols are well established on metal surfaces, but remain limited on electronically gapped substrates (insulators, semiconductors). The latter are useful for preventing electronic coupling (that is, hybridization between molecular assembly and underlying surface) and for avoiding quenching of optical processes, necessary for prospective electronic and optoelectronic applications. In particular, molecular self-assembly on surfaces other than weakly interacting metals can be challenging due to substrate reactivity, defects and inhomogeneities, resulting in intricate energy landscapes that limit the growth kinetically and hampers the synthesis of large-area defect-free 2D systems. Here, we demonstrate the self-assembly of a 2D, atomically thin organic molecular film on a model wide bandgap 2D insulator, single-layer hexagonal boron nitride (hBN) on Cu(111). The molecular film consists of flat, aromatic 9,10-di-cyano-anthracene (DCA) molecules. Our low-temperature scanning tunnelling microscopy and spectroscopy measurements revealed mesoscopic (>100 × 100 nm2), topographically homogeneous crystalline molecular domains resulting from flat molecular adsorption and noncovalent in-plane cyano-ring bonding, with electronically decoupled molecular orbitals (MOs) lying within the hBN electronic gap. These MOs exhibit an energy level spatial modulation (∼300 meV) that follows the moiré work function variation of hBN on Cu(111). These findings pave the way for large-area, atomically precise, highly crystalline 2D organic (and metal-organic) nanomaterials on electronically functional wide bandgap insulators.

Synthesizing Cr-Based Two-Dimensional Conjugated Metal-Organic Framework Through On-Surface Substitution Reaction.

A single-layer Cr3 (HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) conjugated metal-organic framework (c-MOF) issynthesizedunder ultrahigh vacuum conditions by substituting Cr for Ni in Ni3 (HITP)2 template. As revealed by low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy, while codeposition of Cr atoms and 2,3,6,7,10,11-hexaaminotriphenylene precursors produces irregular branches, crystalline Cr3 (HITP)2 frameworks are obtained by depositing Cr atoms to the Ni3 (HITP)2 templates. Thedensity functional theory calculations reveal that the binding energy between Cr and HITP ligands is much higher than that for Ni, which hampers the growth of crystalline Cr3 (HITP)2 frameworks through direct coordination assembly but makes the substitution reaction energetically favorable. Thiswork demonstrates a new strategy to prepare high-quality early-transition-metal-based c-MOFs under ultrahigh vacuum conditions.

A 2D Bismuth-Induced Honeycomb Surface Structure on GaAs(111).

Two-dimensional (2D) topological insulators have fascinating physical properties which are promising for applications within spintronics. In order to realize spintronic devices working at room temperature, materials with a large nontrivial gap are needed. Bismuthene, a 2D layer of Bi atoms in a honeycomb structure, has recently attracted strong attention because of its record-large nontrivial gap, which is due to the strong spin-orbit coupling of Bi and the unusually strong interaction of the Bi atoms with the surface atoms of the substrate underneath. It would be a significant step forward to be able to form 2D materials with properties such as bismuthene on semiconductors such as GaAs, which has a band gap size relevant for electronics and a direct band gap for optical applications. Here, we present the successful formation of a 2D Bi honeycomb structure on GaAs, which fulfills these conditions. Bi atoms have been incorporated into a clean GaAs(111) surface, with As termination, based on Bi deposition under optimized growth conditions. Low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/S) demonstrates a well-ordered large-scale honeycomb structure, consisting of Bi atoms in a √3 × √3 30° reconstruction on GaAs(111). X-ray photoelectron spectroscopy shows that the Bi atoms of the honeycomb structure only bond to the underlying As atoms. This is supported by calculations based on density functional theory that confirm the honeycomb structure with a large Bi-As binding energy and predict Bi-induced electronic bands within the GaAs band gap that open up a gap of nontrivial topological nature. STS results support the existence of Bi-induced states within the GaAs band gap. The GaAs:Bi honeycomb layer found here has a similar structure as previously published bismuthene on SiC or on Ag, though with a significantly larger lattice constant and only weak Bi-Bi bonding. It can therefore be considered as an extreme case of bismuthene, which is fundamentally interesting. Furthermore, it has the same exciting electronic properties, opening a large nontrivial gap, which is the requirement for room-temperature spintronic applications, and it is directly integrated in GaAs, a direct band gap semiconductor with a large range of (opto)electronic devices.

Dimensional crossover of charge order in IrTe2 with strong interlayer coupling

Tuning dimensionality in van der Waals materials with finite interlayer coupling has introduced various electronic phase transitions by conventional mechanical exfoliation. Particularly when the electronic order is tied to the modulation of the interlayer coupling, such dimensional tunability has a strong impact on its stability and properties, which has rarely been investigated experimentally. Here, we demonstrate a dimensional crossover of charge order in IrTe$_2$ from genuine two- to quasi-three-dimension using low-temperature scanning tunneling microscopy and spectroscopy. Employing atomically thin IrTe$_2$ flakes ranging from monolayer to multilayer, we observe a gradual phase transition of charge order and exponential decay of Coulomb gap with increasing thickness. Moreover, we find a suppression of the density of states emerging at an abrupt lateral interface between two- and three-dimension. These findings are attributed to the interplay between the strongly coupled layers and substrate-driven perturbation, which can provide a new insight into the dimensional crossover of strongly coupled layered materials with hidden electronic phases.

Embedded pseudo graphene nanoribbons oriented via Ge(110) surface reconstruction

Graphene nanoribbon, which is the one-dimensional form of graphene, is an attractive nano structure for next generation electronic devices. Herein, we studied the electronic properties of pseudo-GNRs in large-scale graphene sheets grown on Ge(110) using low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Based on our STM results, the pseudo-GNRs are aligned in the <112> direction of the Ge surface; the alignment of the pseudo-GNRs is controlled by the surface reconstruction of Ge(110) substrate, which can be an important merit in terms of the mass fabrication. Bandgap energies ranging from ∼0.12 to ∼0.3 eV were measured via STS on pseudo-GNRs in graphene/Ge(110), while the surrounding graphene area outside the pseudo-GNR growth region shows the typical electronic structure of graphene, verifying the spontaneous formation of metallic-semiconducting-metallic junction nanostructure. This study unveils the geometric and electronic properties of pseudo-GNRs in graphene/Ge(110), providing essential information for the realization of next-generation nanoelectronic devices. • Pseudo-GNRs aligned in the <112> direction on Ge surface. • Alignment controlled by surface reconstruction of Ge substrate. • Bandgap energies from ∼0.12 to ∼0.3 eV measured via STS on pseudo-GNRS. • Graphene outside growth regions showed typical electronic structure of graphene.

When Molecular Dimerization Induces Magnetic Bi‐Stability at the Metal–Organic Interface (Adv. Phys. Res. 1/2023)

Molecular Dimers with Induced Magnetic Bi-Stability In article number 2200005, Jean-Pierre Bucher and co-workers present an on-surface assembly method to create metal–organic dimer complexes. Their method, which uses low-temperature scanning tunneling microscopy and spectroscopy, represents an experimental route towards dimer-based materials with complex magnetic structures. These materials could find future applications in spintronics, logics, and computing.

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.

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.

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 ${\mathrm{NbTe}}_{2}$.