Atomic Tunneling Research Articles

Structure-resistive property relationships in thin ferroelectric BaTiO_{3} films

A combined study of local structural, electric and ferroelectric properties of SrTiO_{3}/La_{0.7}Sr_{0.3}MnO_{3}/BaTiO_{3} heterostructures was performed by Piezoresponse Force Microscopy, tunneling Atomic Force Microscopy and Scanning Tunneling Microscopy in the temperature range 30–295 K. The direct correlation of film structure (epitaxial, nanocrystalline or polycrystalline) with local electric and ferroelectric properties was observed. For polycrystalline ferroelectric films the predominant polarization state is defined by the peculiarity of screening the built-in field by positively charged point defects. Based on Scanning Tunneling Spectroscopy results, it was found that a sequent voltage application provokes the modification of local resistive properties related to the redistribution of point defects in thin ferroelectric films. A qualitative analysis of acquired Piezoresponse Force Microscopy, tunneling Atomic Force Microscopy and Scanning Tunneling Microscopy images together with Scanning Tunneling Spectroscopy measurements enabled us to conclude that in the presence of structural defects the competing processes of electron injection, trap filling and the drift of positively charged point defects drives the change of resistive properties of thin films under applied electric field. In this paper, we propose a new approach based on Scanning Tunneling Microscopy/Spectroscopy under ultrahigh vacuum conditions to clarify the influence of point defects on local resistive properties of nanometer-thick ferroelectric films.

Combined atomic force microscope and scanning tunneling microscope with high optical access achieving atomic resolution in ambient conditions.

Performing atomic force microscopy (AFM) and scanning tunneling microscopy (STM) with atomic resolution under ambient conditions is challenging due to enhanced noise and thermal drift. We show the design of a compact combined atomic force and scanning tunneling microscope that uses qPlus sensors and discuss the stability and thermal drift. By using a material with a low thermal expansion coefficient, we can perform constant height measurements and achieve atomic resolution in both AFM and STM on various samples. Moreover, the design allows a wide angle optical access to the sensor and the sample that is of interest for combining with optical microscopes or focusing optics with a high numerical aperture.

Coexistence of High Electron Conduction and Low Heat Conduction in Tungsten Oxide Epitaxial Films with 1D Atomic Defect Tunnels

Materials showing unusual electron and heat conduction such as the coexistence of high electron conduction and low heat conduction are essentially important to realize efficient thermal management ...

Energetically stretching proteins on patterned two dimensional nanosheets

Driven by increasing interests in sequencing proteins, several single molecule methods (such as the measurement of electron tunneling currents through each amino acid) have been recently proposed and demonstrated in experiment. While promising, they are still in early stages and challenges such as the lack of protein stretching and large thermal fluctuations of amino acids remain to be solved. Here, we show that a patterned two dimensional (2D) in-plane heterostructure comprising both graphene and hexagonal boron nitride (hBN) domains can be utilized to stretch the intrinsically disordered Aβ42 protein because of the chemical potential difference between the graphene and hBN surfaces. Additionally, thanks to the physisorption of the flexible Aβ42 protein on the 2D surface, thermal fluctuations of amino acids are substantially reduced. Therefore, this stretched protein conformation with amino acids adsorbed on the 2D surface may facilitate protein sequencing using the high resolution atomic force microscopy (AFM) or scanning tunneling microscope (STM), with improved signal-to-noise ratios. We further show that the 2D-heterostructure-based protein electrophoresis can help detect the phosphorylation in a single protein molecule. We expect that the patterned 2D heterstructure could serve as a new versatile platform for various single-protein analyses.

Permutationally invariant polynomial potential energy surfaces for tropolone and H and D atom tunneling dynamics

We report permutationally invariant polynomial (PIP) fits to energies and gradients for 15-atom tropolone. These include standard, augmented, and fragmented PIP bases. Approximately, 6600 energies and their associated gradients are obtained from direct-dynamics calculations using DFT/B3LYP/6-31+G(d) supplemented by grid calculations spanning an energy range up to roughly 35 000 cm-1. Three fragmentation schemes are investigated with respect to efficiency and fit precision. In addition, several fits are done with reduced weight for gradient data relative to energies. These do result in more precision for the H-transfer barrier height. The properties of the fits such as stationary points, harmonic frequencies, and the barrier to H-atom transfer are reported and compared to direct calculations. A previous 1D model is used to obtain the tunneling splitting for the ground vibrational state and qualitative predictions for excited vibrational states. This model is applied to numerous fits with different barrier heights and then used to extrapolate the H and D atom tunneling splittings to values at the CCSD(T)-F12 barrier. The extrapolated values are 2.3 and 0.14 cm-1, respectively for H and D. These are about a factor of two larger than experiment, but within the expected level of agreement with experiment for the 1D method used and the level of the electronic structure theory.

Atomic Force and Scanning Tunneling Microscopy of Ordered Ionic Liquid Wetting Layers from 110 K up to Room Temperature

Ionic liquids (ILs) are used as ultrathin films in many applications. We studied the nanoscale arrangement within the first layer of 1,3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C1C1Im] [Tf2N]) on Au(111) between 400 and 110 K in ultrahigh vacuum by scanning tunneling and noncontact atomic force microscopy with molecular resolution. Compared to earlier studies on similar ILs, a different behavior is observed, which we attribute to the small size and symmetrical shape of the cation: (a) In both AFM and STM only the anions are imaged; (b) only long-range-ordered but no amorphous phases are observed; (c) the hexagonal room-temperature phase melts 30–50 K above the IL’s bulk melting point; (d) at 110 K, striped and hexagonal superstructures with two and three ion pairs per unit cell, respectively, are found. AFM turned out to be more stable at higher temperature, while STM revealed more details at low temperature.

Covalent Functionalization of Carbon Surfaces: Diaryliodonium versus Aryldiazonium Chemistry

Covalently modified carbon surfaces remain central to a number of applications. Surface modification is often achieved using aryldiazonium chemistry, which has become the gold standard for the functionalization of carbon-based surfaces. However, the higher reactivity and thus the lower stability of diazonium precursors at carbon interfaces have prompted a search for alternative chemistries. Diaryliodonium salts have emerged as stable alternatives to the conventional aryldiazonium precursors. In this contribution, we provide a detailed comparison between the two types of chemistries by carrying out electrochemical covalent grafting of graphite substrates using nitrophenyl groups generated from the corresponding aryldiazonium and diaryliodonium salts under identical experimental conditions. The electrochemical process was studied using cyclic voltammetry, whereas the covalently grafted substrates were characterized using a battery of surface analytical techniques spanning multiple length scales. Analysis of the modified substrates using Raman spectroscopy revealed that the efficiency of covalent grafting is higher for iodonium chemistry than that for aryldiazonium chemistry. This observation was further corroborated by detailed morphological characterization of the covalent films using atomic force microscopy and molecular-resolution scanning tunneling microscopy, which revealed higher surface coverage for iodonium modified substrates. The chemical composition of the films was probed using X-ray photoelectron spectroscopy. The detailed and systematic comparison presented here clearly demonstrates that diaryliodonium chemistry presents a robust and reliable alternative to the widely used aryldiazonium chemistry.

Comment on “Computational evidence for sulfur atom tunneling in the ring flipping reaction of S4N4”

In a recent article (Chem. Phys. Lett. 749 (2020), 137440), Kumar and Mallick predict by computations that a sulfur atom tunneling mechanism leads to a ring flipping reaction of S4N4 below 70 K. Herein, on the basis of their tunneling computations, it is shown that the sulfur atom tunneling is not viable, and therefore impossible to be probed experimentally. Based on this, we discuss the prerequisites for a chemical reaction to be driven by heavy atom tunneling.

Strumming a Single Chemical Bond.

Atomic force microscopy and scanning tunneling microscopy can image the internal structure of molecules adsorbed on surfaces. One reliable method is to terminate the tip with a nonreactive adsorbate, often a single CO molecule, and to collect data at a close distance where Pauli repulsion plays a strong role. Lateral force microscopy, in which the tip oscillates laterally, probes similar interactions but has the unique ability to pull the CO over a chemical bond, load it as a torsional spring, and release it as it snaps over with each oscillation cycle. This produces measurable energy dissipation. The dissipation has a characteristic decay length in the vertical direction of 4 pm, which is 13 times smaller than the decay length in typical STM or AFM experiments.

Resolving Quinoid Structure in Poly(para-phenylene) Chains.

The quinoid structure, a resonance structure of benzenoid, gives rise to peculiar chemical reactivity and physical properties. A complete characterization of its geometric and electronic properties on the atomic scale is of vital importance to understand and engineer the chemical and physical properties of quinoid molecules. Here, we report a real-space structural and electronic characterization of quinoid poly(para-phenylene) (PPP) chains by using noncontact atomic force microscopy and scanning tunneling microscopy. Our results reveal that quinoid PPP chains adopt a coplanar adsorption configuration on Cu(111) and host in-gap states near Fermi level. In addition, intra- and interchain hopping of quinoid structure are observed, indicative of a quasiparticle behavior originating from charge-lattice interactions. The experimental results are nicely reproduced by tight-binding calculations. Our study provides a comprehensive understanding of the structural and electronic properties of quinoid PPP chains in real space and may be further extended to address the dynamics of nonlinear excitations in quinoid molecules.

(Invited) Chemical Functionalization and Transformation of 2D Layered Chalcogenide Materials

Layered chalcogenide materials are a diverse class of crystalline materials that consist of covalently bound building blocks held together by van der Waals forces. Among these materials are the transition metal dichalcogenides (TMDCs) which can be exfoliated into two-dimensional (2D) nanosheets, and the pnictogen chalcogenides (PCs) which can be exfoliated into one-dimensional (1D) nanoribbons and 2D nanosheets. Controlling the electronic and optical properties of these materials, as well as forming interfaces with other materials structures, is important for a wide range of potential applications, and can be achieved using a set of strategies for chemical modification that has been pursued in our laboratory. This talk will highlight our recent work on covalent chemical functionalization, structural and geometric conversion, and self-assembly of organic van der Waals heterostructures.In the first strategy, semiconducting MoS2 is covalently functionalized using aryl diazonium salts that form out-of-plane bonds on the basal plane. Detailed imaging studies of the reaction under a range of parameters combined with density functional theory (DFT) calculations show that the functionalization initiates at point defects but then propagates in a chain-like morphology across the MoS2 surface. The reaction is also shown to be well-described by Freundlich and Temkin isotherm models, and to have pseudo-second-order kinetics. The diazonium salts can also be modified to enable tethering of proteins to the MoS2 surface. The same reaction scheme is also extended across the TMDC and PC layered chalcogenides including MoSe2, WS2, WSe2, Bi2S3, and Sb2S3. In the second strategy, MoO3 and WO3 nanoscrolls are formed by transformation from 2D MoS2 and WS2 nanosheets, respectively, via atmospheric plasma treatment. The combination of different radicals in the plasma simultaneously oxidize the layered disulfides and initiate delamination from the substrate, so that the strain of the altered crystal structure is relieved by rolling into nanoscroll geometries. In the final strategy, we form a self-assembled monolayer of fullerene molecules on WSe2, and use atomic resolution scanning tunneling microscopy to show that the molecules exhibit a 2x2 rotational superstructure due to the interplay of molecule-molecule, molecule-substrate, van der Waals, and Coulomb interactions. The system forms a charge redistribution network with long-range ordering.

Engineering of Magnetic Coupling in Nanographene.

Nanographenes with sublattice imbalance host a net spin according to Lieb's theorem for bipartite lattices. Here, we report the on-surface synthesis of atomically precise nanographenes and their atomic-scale characterization on a gold substrate by using low-temperature noncontact atomic force microscopy and scanning tunneling spectroscopy. Our results clearly confirm individual nanographenes host a single spin of S=1/2 via the Kondo effect. In covalently linked nanographene dimers, two spins are antiferromagnetically coupled with each other as revealed by inelastic spin-flip excitation spectroscopy. The magnetic exchange interaction in dimers can be well engineered by tuning the local spin density distribution near the connection region, consistent with mean-field Hubbard model calculations. Our work clearly reveals the emergence of magnetism in nanographenes and provides an efficient way to further explore the carbon-based magnetism.

Computational evidence for sulfur atom tunneling in the ring flipping reaction of S4N4

It has been shown by means of ab initio calculations and kinetic studies that the sulfur atom tunneling can play a crucial role in the ring flipping reaction of S4N4, which is a main precursor of the superconducting material polythiazyl. The investigations indicate that, below 70 K, the reaction predominantly occurs via tunneling, and at 150 K temperature, the tunneling increases the reaction rate by approximately two times.

New imaging modes for analyzing suspended ultra-thin membranes by double-tip scanning probe microscopy

Scanning probe microscopy (SPM) techniques are amongst the most important and versatile experimental methods in surface- and nanoscience. Although their measurement principles on rigid surfaces are well understood and steady progress on the instrumentation has been made, SPM imaging on suspended, flexible membranes remains difficult to interpret. Due to the interaction between the SPM tip and the flexible membrane, morphological changes caused by the tip can lead to deformations of the membrane during scanning and hence significantly influence measurement results. On the other hand, gaining control over such modifications can allow to explore unknown physical properties and functionalities of such membranes. Here, we demonstrate new types of measurements that become possible with two SPM instruments (atomic force microscopy, AFM, and scanning tunneling microscopy, STM) that are situated on opposite sides of a suspended two-dimensional (2D) material membrane and thus allow to bring both SPM tips arbitrarily close to each other. One of the probes is held stationary on one point of the membrane, within the scan area of the other probe, while the other probe is scanned. This way new imaging modes can be obtained by recording a signal on the stationary probe as a function of the position of the other tip. The first example, which we term electrical cross-talk imaging (ECT), shows the possibility of performing electrical measurements across the membrane, potentially in combination with control over the forces applied to the membrane. Using ECT, we measure the deformation of the 2D membrane around the indentation from the AFM tip. In the second example, which we term mechanical cross-talk imaging (MCT), we disentangle the mechanical influence of a scanning probe tip (e.g. AFM) on a freestanding membrane by means of independently recording the response of the opposing tip. In this way we are able to separate the tip-induced membrane deformation topography from the (material-dependent) force between the tip and the membrane. Overall, the results indicate that probing simultaneously both surfaces of ultra-thin membranes, such as suspended 2D materials, could provide novel insights into the electronic properties of the materials.

Creation of New Materials by Compressing Nanoparticles and Exciting Atoms Chemically Bonded to the Surface

The possibility of obtaining new materials as a result of tunneling chemical reactions that occur on the surfaces of compressible nanoparticles is shown. The chemical bonds of surface atoms can be excited. Silicon nanoparticles covered with hydrogen atoms are used as a model. Quantum tunneling of hydrogen atoms of different nanoparticles towards each other takes place under the branches of two intersecting parabolas. A semiclassical approach is chosen for simulation. The branches of the parabolas are directed upwards. In compressed nanoparticles, the pressure at the points of their contact can increase by a factor of a few hundred or even a few thousand. Free bonds that appear after hydrogen detachment transform into chemical bonds between nanoparticles.

Injected coherence in the tunneling of ultracold atoms through a two-photon mazer in the squeezed vacuum and coherent field distributions

We examine the traversal aspect of ultracold two-level atoms through a high-quality two-photon mazer with squeezed vacuum and coherent field distributions. Here the net cavity potential is constituted by the coherent addition of multiple barriers and well potentials, which is quite different from the potential offered by a cavity in the Fock or vacuum state. For squeezed vacuum and coherent field distributions, one gets different phase times ( t p h ) for different modes of the distribution, which are then averaged by taking the weighted mean on the t p h over the n-dependent transmission probability. It is found that the nature, intensity of the field, and injected coherence of the incident atoms have a decisive role in controlling peak values and sub- and superclassical traversals of the tunneling atoms. For two of the system’s dressed states ( | Φ b 0 ⟩ , | Φ b 1 ⟩ ), the cavity offers reflectionless transmission, as the atoms experience zero potential due to the dark state formation. Moreover, for a somewhat intense cavity field, mazer action (scattering-type behavior) may be obtained for an extended range of energies due to increased cavity potential in the course of atom–cavity interaction.

Centimeter-Scale and Highly Crystalline Two-Dimensional Alcohol: Evidence for Graphenol (C6OH).

We report a chemical route to synthesize centimeter-scale stoichiometric "graphenol (C6OH1)", a 2D crystalline alcohol, via vapor phase hydroxylation of epitaxial graphene on Cu(111). Atomic resolution scanning tunneling microscopy revealed this highly-ordered configuration of graphenol and low energy electron diffraction studies on a large-area single crystal graphene film demonstrated the feasibility of the same superstructure being achieved at the centimeter length scale. Periodic density functional theory (DFT) calculations about the formation of C6(OH)1 and its electronic structure are also reported.

Room-Temperature Transport Properties of Graphene with Defects Derived from Oxo-Graphene.

In recent years, graphene oxide has been considered as a soluble precursor of graphene for electronic applications. However, the performance lags behind that of graphene due to lattice defects. Here, the relation between the density of defects in the range of 0.2 % and 1.5 % and the transport properties is quantitatively studied. Therefore, the related flakes of monolayers of graphene were prepared from oxo‐functionalized graphene (oxo‐G). The morphologic structure of oxo‐G was imaged by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). Field‐effect mobility values were determined to range between 0.3 cm2 V−1 s−1 and 33.2 cm2 V−1 s−1, which were inversely proportional to the density of defects. These results provide the first quantitative description of the density of defects and transport properties, which plays an important role for potential applications.

Atomic structures and scanning tunnelling microscopy of nitrogen-doped carbon nanotubes

We report on atomic structures, energetics, and scanning tunnelling microscopy images of nitrogen defects in carbon nanotubes (CNTs) based on first-principles density-functional study. As defect configurations in CNTs, not only the substitutional nitrogen defect but also the nitrogen-vacancy complex defects (pyridine-type defects) are considered. The substitutional nitrogen defect is found to be energetically the most stable defect. On the other hand, for the pyridine-type defects, the tetramerized and the trimerized configurations are found to be the possible configurations in energy. The scanning tunnelling microscopy (STM) images for defect configurations in N-doped CNTs are studied, and the various N defects in CNTs are expected to be identified by STM experiments.

Особенности двумерных бифуркаций при диссипативном туннелировании электронов в массивах Au наночастиц

Abstract. In framework of the 2D - dissipative tunneling theory in approximation of a rarefied gas of the «instanton - antiinstanton pairs» at a finite temperature under the conditions of an external electric field, the features of tunneling transport for planar structures with quantum dots (QDs) from colloidal gold, that have metamaterial properties, have been studied. It was experimentally shown that, depending on the positioning of the cantilever needle of a combined atomic force and scanning tunneling microscope (AFM / STM), either above a single quantum dot or between two neighboring quantum dots, either a single or double effect of 2D tunneling bifurcations have been observed, respectively. It is such a double bifurcation regime, as our theoretical model has shown, that is associated with the manifestation of the metamaterial properties by the structure under study. A convincing qualitative agreement between the experimental I – V characteristics and the field dependence of the 2D - dissipative tunneling probability in the two studied modes, taking into account the observed quantum beats in the vicinity of the 2D bifurcation points, has been obtained.