Scanning Tunneling Microscope Tip Research Articles

Tunable Lattice Reconstruction, Triangular Network of Chiral One-Dimensional States, and Bandwidth of Flat Bands in Magic Angle Twisted Bilayer Graphene.

The interplay between interlayer van der Waals interaction and intralayer lattice distortion can lead to structural reconstruction in slightly twisted bilayer graphene (TBG) with the twist angle being smaller than a characteristic angle θ_{c}. Experimentally, the θ_{c} is demonstrated to be very close to the magic angle (θ≈1.08°). Here we address the transition between reconstructed and unreconstructed structures of the TBG across the magic angle by using scanning tunneling microscopy (STM). Our experiment demonstrates that both structures are stable in the TBG around the magic angle. By using a STM tip, we show that the two structures can be changed to each other and a triangular network of chiral one-dimensional states hosted by domain boundaries can be switched on and off. Consequently, the bandwidth of the flat band, which plays a vital role in the emergent strongly correlated states in the magic angle TBG, is tuned. This provides an extra control knob to manipulate the exotic electronic states of the TBG near the magic angle.

Lattice Dynamics Driven by Tunneling Current in 1T′ Structure of Bilayer VSe2

Interesting lattice dynamics were recently reported in 1T′ structure of bilayer VSe2 attributed to the vibration of Se atoms, but it was limited to topographic observation using scanning tunneling microscopy (STM). We conduct further systematic STM study to understand the origin of lattice dynamics in bilayer VSe2. Time spectroscopy of tunneling current is measured with the feedback loop disabled, while holding the position of the STM tip over a single Se atom. The time spectroscopy indeed shows random telegraph signals that tunneling current fluctuates between high and low currents. Such fluctuations of current reflect the vibrational motion of the Se atom. Statistical analysis described by the Markov process provides average residence time (τ) for high and low current states, and transition rate (R = 1/τ). Interestingly, the transition rate as a function of tunneling current (I) follows a power law of R = IN (N = constant), and the N value is close to 1. This linearity of transition rate indicates that the lattice dynamics are mostly induced by tunneling current. Our result confirms the role of tunneling current that drives the observed lattice dynamics in the bilayer VSe2.

Mott Metal-Insulator Transition from Steady-State Density Functional Theory.

We present a computationally efficient method to obtain the spectral function of bulk systems in the framework of steady-state density functional theory (i-DFT) using an idealized scanning tunneling microscope (STM) setup. We calculate the current through the STM tip and then extract the spectral function from the finite-bias differential conductance. The fictitious noninteracting system of i-DFT features an exchange-correlation (XC) contribution to the bias which guarantees the same current as in the true interacting system. Exact properties of the XC bias are established using Fermi-liquid theory and subsequently implemented to construct approximations for the Hubbard model. We show for two different lattice structures that the Mott metal-insulator transition is captured by i-DFT.

Force Spectroscopy of Iron Tetraphenylporphyrin Molecules with Cl Tips

At low sample voltages, Cl ions may be bidirectionally transferred between a Fe tetraphenylporphyrin (FeTPP) molecule on Au(111) and the tip of a low-temperature scanning tunneling microscope that ...

Experimental observation of Dirac cones in artificial graphene lattices

Artificial lattices provide a tunable platform to realize exotic quantum devices. A well-known example is artificial graphene (AG), in which electrons are confined in honeycomb lattices and behave as massless Dirac fermions. Recently, AG systems have been constructed by manipulating molecules using scanning tunnelling microscope tips, but the nanoscale size typical for these constructed systems are impossible for practical device applications and insufficient for direct investigation of the electronic structures using angle-resolved photoemission spectroscopy (ARPES). Here, we demonstrate the synthesis of macroscopic AG by self-assembly of C$_{60}$ molecules on metal surfaces. Our theoretical calculations and ARPES measurements directly confirm the existence of Dirac cones at the $K$ ($K^\prime$) points of the Brillouin zone (BZ), in analogy to natural graphene. These results will stimulate ongoing efforts to explore the exotic properties in artificial lattices and provide an important step forward in the realization of novel molecular quantum devices.

Graphene quantum dot states near defects

Smoothly confined graphene quantum dots (GQDs) localize Dirac electrons with conserved spin and valley degrees of freedom. Recent experimental realization of such structures using a combination of magnetic fields and a scanning tunneling microscope tip showcased their potential in locally probing and adjusting the valley degree of freedom. The present work models the influence of lattice defects on the level structure of GQDs. We study both the adiabatic level spacing ``landscape''---orbital splitting and valley splitting---as well as transition dynamics between GQD states. The system is modeled using a tight-binding approach with on-site and hopping parameters in the vicinity of the defect region extracted from density functional theory via Wannier orbitals while time propagation is done using Magnus operators. Different defect types, such as double vacancy, Stone-Wales, flower, and Si substitution, are considered. We predict tunable valley splittings of the order of 2--20 meV. The level structure can thus be tailored at will by engineering appropriate defect geometries.

Theoretical investigation of the scanning tunneling microscopy of Majorana bound states in topological superconductor vortices

Scanning tunneling microscopy (STM) is an indispensable tool in detecting Majorana bound states (MBSs) in vortices of topological superconductors. By reducing the computational complexity via non-uniform grids, we systematically study the tunnel coupling as well as the temperature dependence of the differential conductance of MBSs in two dimensional devices. Numerical results show that the conductance peak approaches the quantized value 2e 2/h in strong coupling limit at low temperatures which are characteristic features of MBSs. More interestingly, a conductance local minimum in the spatially scanning is observed when the STM tip is placed at the vortex center. The dip structure can be enhanced with increased temperature or enlarged vortex size. We ascribe this observation to the sensitivity of the Andreev reflection processes of carriers at the vortex center where the thermal energy could be comparable to the vanishing pair potential. We also investigate the STM of two-vortex systems where the hybridization of the vortices can lead to oscillatory behavior of the state energy. With small inter-vortex distances, the original MBSs in vortices can merge into topologically trivial states and the conductance peak can be significantly suppressed.

Longitudinal and transverse electron paramagnetic resonance in a scanning tunneling microscope.

Electron paramagnetic resonance (EPR) spectroscopy is widely used to characterize paramagnetic complexes. Recently, EPR combined with scanning tunneling microscopy (STM) achieved single-spin sensitivity with sub-angstrom spatial resolution. The excitation mechanism of EPR in STM, however, is broadly debated, raising concerns about widespread application of this technique. We present an extensive experimental study and modeling of EPR-STM of Fe and hydrogenated Ti atoms on a MgO surface. Our results support a piezoelectric coupling mechanism, in which the EPR species oscillate adiabatically in the inhomogeneous magnetic field of the STM tip. An analysis based on Bloch equations combined with atomic-multiplet calculations identifies different EPR driving forces. Specifically, transverse magnetic field gradients drive the spin-1/2 hydrogenated Ti, whereas longitudinal magnetic field gradients drive the spin-2 Fe. Also, our results highlight the potential of piezoelectric coupling to induce electric dipole moments, thereby broadening the scope of EPR-STM to nonpolar species and nonlinear excitation schemes.

Fabrication and manipulation of nanosized graphene homojunction with atomically-controlled boundaries

Controlling the atomic configurations of structural defects in graphene nanostructures is crucial for achieving desired functionalities. Here, we report the controlled fabrication of high-quality single-crystal and bicrystal graphene nanoislands (GNI) through a unique top-down etching and post-annealing procedure on a graphite surface. Low-temperature scanning tunneling microscopy (STM) combined with density functional theory calculations reveal that most of grain boundaries (GBs) formed on the bicrystal GNIs are 5-7-5-7 GBs. Two nanodomains separated by a 5-7-5-7 GB are AB stacking and twisted stacking with respect to the underlying graphite substrate and exhibit distinct electronic properties, forming a graphene homojunction. In addition, we construct homojunctions with alternative AB/twisted stacking nanodomains separated by parallel 5-7-5-7 GBs. Remarkably, the stacking orders of homojunctions are manipulated from AB/twist into twist/twist type through a STM tip. The controllable fabrication and manipulation of graphene homojunctions with 5-7-5-7 GBs and distinct stacking orders open an avenue for the construction of GBs-based devices in valleytronics and twistronics.

Growth of a self-assembled monolayer decoupled from the substrate: nucleation on-command using buffer layers.

Structural polymorphism is ubiquitous in physisorbed self-assembled monolayers formed at the solution–solid interface. One of the ways to influence network formation at this interface is to physically decouple the self-assembled monolayer from the underlying substrate thereby removing the influence of the substrate lattice, if any. Here we show a systematic exploration of self-assembly of a typical building block, namely 4-tetradecyloxybenzoic acid at the 1-phenyloctane–graphite interface in the presence and in the absence of a buffer layer formed by a long chain alkane, namely n-pentacontane. Using scanning tunneling microscopy (STM), three different structural polymorphs were identified for 4-tetradecyloxybenzoic acid at the 1-phenyloctane–graphite interface. Surprisingly, the same three structures were formed on top of the buffer layer, albeit at different concentrations. Systematic variation of experimental parameters did not lead to any new network in the presence of the buffer layer. We discovered that the self-assembly on top of the buffer layer allows better control over the nanoscale manipulation of the self-assembled networks. Using the influence of the STM tip, we could initiate the nucleation of small isolated domains of the benzoic acid on-command in a reproducible fashion. Such controlled nucleation experiments hold promise for studying fundamental processes inherent to the assembly process on surfaces.

Real-Space Studies of Plasmon-Induced Dissociation Reactions with an STM

Abstract Molecular bond dissociation and formation reactions induced by localized surface plasmons of metal nanostructures are promising reactions in terms of the effective utilization of sunlight. The plasmon has a potential not only to enhance photochemical reactions but also to enable efficient novel reaction pathways. However, the reaction mechanism is still veiled because it is difficult to directly observe the reactions caused at the localized field of the plasmon near the metal surfaces. For the visualization of the reactions induced by the plasmon at a single-molecule level, we have applied a scanning tunneling microscope (STM). Bond dissociation reactions were induced by the plasmon excited at a nanogap between the STM tip and a metal substrate under light irradiation. The STM analyses combined with density functional theory calculations provided mechanistic insights into the plasmon-induced dissociation reactions.

On the mystery of the absence of a spin-orbit gap in scanning tunneling microscopy spectra of germanene

Germanene, the germanium analogue of graphene, shares many properties with its carbon counterpart. Both materials are two-dimensional materials that host Dirac fermions. There are, however, also a few important differences between these two materials: (1) graphene has a planar honeycomb lattice, whereas germanene’s honeycomb lattice is buckled and (2) the spin-orbit gap in germanene is predicted to be about three orders of magnitude larger than the spin-orbit gap in graphene (24 meV for germanene versus 20 μeV for graphene). Surprisingly, scanning tunneling spectra recorded on germanene layers synthesized on different substrates do not show any sign of the presence of a spin-orbit gap. To date the exact origin of the absence of this spin-orbit gap in the scanning tunneling spectra of germanene has remained a mystery. In this work we show that the absence of the spin-orbit gap can be explained by germanene’s exceptionally low work function of only 3.8 eV. The difference in work function between germanene and the scanning tunneling microscopy tip (the work functions of most commonly used STM tips are in the range of 4.5 to 5.5 eV) gives rise to an electric field in the tunnel junction. This electric field results in a strong suppression of the size of the spin-orbit gap.

Quantum-well and modified image-potential states in thin Pb(111) films: an estimate for the local work function

Quantum-confined electronic states such as quantum-well states (QWS) inside thin Pb(111) films and modified image-potential states (IPS) above the Pb(111) films grown on Si(111) 7 × 7 substrate were studied by means of low-temperature scanning tunnelling microscopy (STM) and spectroscopy (STS) in the regime of constant current I. By plotting the position of the nth emission resonances Un versus n2/3 and extrapolating the linear fit for the dependence Un(n2/3) in the high-n limit towards n = 0, we estimate the local work function for the Pb(111) film: W ≃ 3.8 ± 0.1 eV. We experimentally demonstrate that modifications of the shape of the STM tip can change the number of the emission peaks associated with the resonant tunnelling via quantized IPS levels for the same Pb terrace; however it does not affect the estimate of the local work function for the flat Pb terraces. We observe that the maxima in the spectra of the differential tunnelling conductance dI/dU related to both the QWS and the modified IPS resonances are less pronounced if the STM tip becomes more blunt.

Static properties of STM tip in response to magnetic field

A nano size pyramidal superconducting tip is characterized theoretically. Vortex states are achieved in the framework of three-dimensional (3D) numerical discretization of Ginzburg-Landau (GL) formalism. Systematic investigation is performed according to the geometric parameters of the tip and different GL parameter κ. Meissner state and critical field of the tip are obtained in the presence of external magnetic field which is perpendicular to the square base of the tip. It is found that upper critical field Hc2 comes to lower with increasing κ but superconductivity persists up to higher filed in case of a much sharp tip. The results are important to comprehend the properties of superconducting tips, which are presently employed in scanning tunneling microscopy (STM), in the presence of an external magnetic field.

Phase-Resolved Detection of Ultrabroadband THz Pulses inside a Scanning Tunneling Microscope Junction.

Coupling phase-stable single-cycle terahertz (THz) pulses to scanning tunneling microscope (STM) junctions enables spatiotemporal imaging with femtosecond temporal and Ångstrom spatial resolution. The time resolution achieved in such THz-gated STM is ultimately limited by the subcycle temporal variation of the tip-enhanced THz field acting as an ultrafast voltage pulse, and hence by the ability to feed high-frequency, broadband THz pulses into the junction. Here, we report on the coupling of ultrabroadband (1–30 THz) single-cycle THz pulses from a spintronic THz emitter (STE) into a metallic STM junction. We demonstrate broadband phase-resolved detection of the THz voltage transient directly in the STM junction via THz-field-induced modulation of ultrafast photocurrents. Comparison to the unperturbed far-field THz waveform reveals the antenna response of the STM tip. Despite tip-induced low-pass filtering, frequencies up to 15 THz can be detected in the tip-enhanced near-field, resulting in THz transients with a half-cycle period of 115 fs. We further demonstrate simple polarity control of the THz bias via the STE magnetization and show that up to 2 V THz bias at 1 MHz repetition rate can be achieved in the current setup. Finally, we find a nearly constant THz voltage and waveform over a wide range of tip–sample distances, which by comparison to numerical simulations confirms the quasi-static nature of the THz pulses. Our results demonstrate the suitability of spintronic THz emitters for ultrafast THz-STM with unprecedented bandwidth of the THz bias and provide insight into the femtosecond response of defined nanoscale junctions.

Tunable Plasmonic Quantum Light Source with Silver Nanoclusters on a Silver Surface

Scanning tunneling luminescence (STL) can be used to investigate the optical properties of nanostructures with high spatial resolution beyond the diffraction limit of light. To get appropriate STL spectra, one needs to modify the tip repeatedly. However, such irreversible tip modification often leads to very unstable tips. Here, by using a scanning tunneling microscope (STM) tip, we demonstrate that STL spectra are tunable via silver nanocluster arrays fabricated directly on a silver surface. The silver nanoclusters are created by transferring silver atoms from the silver tip to the silver surface under STM. We found that the STL spectra were enhanced (suppressed) at higher (lower) energy on the clusters and that they showed opposite behaviors between the nanoclusters. Without relying on the irreversible tip shape modification, our findings indicate that we can tune the STL spectra simply by moving the STM tip over the nanoclusters. Our method can be used to provide a tunable light source for systematic nano-optical experiments and for nanoscale optical devices.

Absence of Nonlocal Manipulation of Oxygen Atoms Inserted below the Si(111)-7×7 Surface.

The injection of electrons from the scanning tunneling microscope tip can be used to perform nanoscale chemistry and study hot electron transport through surfaces. While nonlocal manipulation has been demonstrated primarily for aromatic adsorbates, here we confirm that oxygen atoms bonded to the Si(111) surface can also be nonlocally manipulated, and we fit the measured manipulation data to a single channel decay model. Unlike aromatic adsorption systems, oxygen atoms also insert below the surface of silicon. Although the inserted oxygen can be manipulated when the tip is directly over the relevant silicon adatom, it is not possible to induce nonlocal manipulation of inserted oxygen atoms at the same bias. We attribute this to the electrons injected at +4 eV initially relaxing to couple to the highest available surface state at +3.4 eV before laterally transporting through the surface. With a manipulation threshold of 3.8 eV for oxygen inserted into silicon, once carriers have undergone lateral transport, they do not possess enough energy to manipulate and remove oxygen atoms inserted beneath the surface of silicon. This result confirms that nonlocal nanoscale chemistry using the scanning tunneling microscope tip is dependent not only on the energy required for atomic manipulation, but also on the energy of the available surface states to carry the electrons to the manipulation site.

Comprehensive Electrostatic Modeling of Exposed Quantum Dots in Graphene/Hexagonal Boron Nitride Heterostructures.

Recent experimental advancements have enabled the creation of tunable localized electrostatic potentials in graphene/hexagonal boron nitride (hBN) heterostructures without concealing the graphene surface. These potentials corral graphene electrons yielding systems akin to electrostatically defined quantum dots (QDs). The spectroscopic characterization of these exposed QDs with the scanning tunneling microscope (STM) revealed intriguing resonances that are consistent with a tunneling probability of 100% across the QD walls. This effect, known as Klein tunneling, is emblematic of relativistic particles, underscoring the uniqueness of these graphene QDs. Despite the advancements with electrostatically defined graphene QDs, a complete understanding of their spectroscopic features still remains elusive. In this study, we address this lapse in knowledge by comprehensively considering the electrostatic environment of exposed graphene QDs. We then implement these considerations into tight binding calculations to enable simulations of the graphene QD local density of states. We find that the inclusion of the STM tip’s electrostatics in conjunction with that of the underlying hBN charges reproduces all of the experimentally resolved spectroscopic features. Our work provides an effective approach for modeling the electrostatics of exposed graphene QDs. The methods discussed here can be applied to other electrostatically defined QD systems that are also exposed.

Scanned Single-Electron Probe inside a Silicon Electronic Device.

Solid-state devices can be fabricated at the atomic scale, with applications ranging from classical logic to current standards and quantum technologies. Although it is very desirable to probe these devices and the quantum states they host at the atomic scale, typical methods rely on long-ranged capacitive interactions, making this difficult. Here, we probe a silicon electronic device at the atomic scale using a localized electronic quantum dot induced directly within the device at a desired location, using the biased tip of a low-temperature scanning tunneling microscope. We demonstrate control over short-ranged tunnel coupling interactions of the quantum dot with the device's source reservoir using sub-nanometer position control of the tip and the quantum dot energy level using a voltage applied to the device's gate reservoir. Despite the ∼1 nm proximity of the quantum dot to the metallic tip, we find that the gate provides sufficient capacitance to enable a high degree of electric control. Combined with atomic-scale imaging, we use the quantum dot to probe applied electric fields and charge in individual defects in the device. This capability is expected to aid in the understanding of atomic-scale devices and the quantum states realized in them.

Charge State Control of F16CoPc on h‐BN/Cu(111)

AbstractThe use of molecular materials in solar cells and nano‐electronics demands a fundamental understanding and control of their electronic properties. Particularly relevant is the molecular response to the environment, that is, the interaction with the support and adjacent molecules, as well as the influence of electrostatic gating. Here, the control of molecular level alignment and charge states of fluorinated cobalt phthalocyanines (F16CoPc) on atomically thin hexagonal boron nitride (h‐BN) sheets on Cu(111) is reported using scanning tunneling microscopy (STM) and spectroscopy (STS), as well as atomic force microscopy (AFM) and complementary density functional theory (DFT) calculations. Three parameters that govern the electronic level alignment of F16CoPc orbitals are investigated: i) template‐induced gating by the work function variation of the h‐BN/Cu(111) substrate, ii) gating by the STM tip, and iii) screening by neighboring molecules. The interplay of these parameters influences the charge distribution in the studied molecular arrangements and thus provides the possibility to tune their physicochemical behavior, for instance, the response toward electronic or optical excitation, charge transport, or binding of axial adducts.