Atomic-scale Contacts Research Articles

Halide adsorption influences snapback distance in Scanning Tunnelling Microscope break junctions

‘Snapback distance’ refers to the rapid increase in the size of the gap formed immediately after breaking an atomic-scale metallic contact. It is a commonly observed phenomenon in Scanning Tunnelling Microscope break junction (STM-BJ) and mechanically controlled break junction (MCBJ) experiments. Here, we show that the snapback distance measured for a gold break junction in pure water was significantly reduced in an electrolyte containing halide anions. In the case of Br−, experiments under electrochemical control provided clear evidence that this reduction was caused by halide adsorption on the surface of the gold.

Rapid synthesis of phosphor-glass composites in seconds based on particle self-stabilization

Phosphor-glass composites (PGC) are excellent candidates for highly efficient and stable photonic converters; however, their synthesis generally requires harsh procedures and long time, resulting in additional performance loss and energy consumption. Here we develop a rapid synthetic route to PGC within about 10 seconds, which enables uniform dispersion of Y3Al5O12:Ce3+ (YAG:Ce) phosphor particles through a particle self-stabilization model in molten tellurite glass. Thanks for good wettability between YAG:Ce micro-particles and tellurite glass melt, it creates an energy barrier of 6.94 × 105 zJ to prevent atomic-scale contact and sintering of particles in the melt. This in turn allows the generation of YAG:Ce-based PGC as attractive emitters with high quantum efficiency (98.4%) and absorption coefficient (86.8%) that can produce bright white light with luminous flux of 1227 lm and luminous efficiency of 276 lm W−1 under blue laser driving. This work shows a generalizable synthetic strategy for the development of functional glass composites.

Decorated NiFeOOH on high entropy perovskite oxide by interface engineering for efficient oxygen evolution and overall water splitting

High entropy perovskite oxide (HEPO) has proved to be one of the most promising candidates in the exploration of non-precious oxygen evolution reaction (OER) catalysts. Herein, we report a chemical bath deposition method for decorating NiFeOOH on lanthanide-based high entropy perovskite oxide (La-HEO) to form a rational core–shell heterostructure through direct atomic-scale contact to further enhance the OER activity of HEPO. Compared with La-HEO and NiFeOOH, the resulting La-HEO@NiFeOOH showed excellent OER performance, requiring an overpotential of 262 mV (10 mA cm−2) and a Tafel slope of 38 mV dec−1, which owes much to the accelerated charge transfer and the optimized adsorption energy of reaction intermediates caused by synergistic effects. Meanwhile, the water splitting electrolyzer demanded a current density of 10 mA cm−2 at 1.574 V for La-HEO@NiFeOOH(1:3)||Pt/C, which is lower than that of La-HEO||Pt/C. This study illustrates the effectiveness of interface engineering to boost the OER performance of HEPO, and reveals the potential of La-HEO@NiFeOOH as a promising OER electrocatalyst.

Electrically Controlled Bimetallic Junctions for Atomic-Scale Electronics.

Forming atomic-scale contacts with attractive geometries and material compositions is a long-term goal of nanotechnology. Here, we show that a rich family of bimetallic atomic-contacts can be fabricated in break-junction setups. The structure and material composition of these contacts can be controlled by atomically precise electromigration, where the metal types of the electron-injecting and sink electrodes determine the type of atoms added to, or subtracted from, the contact structure. The formed bimetallic structures include, for example, platinum and aluminum electrodes bridged by an atomic chain composed of platinum and aluminum atoms as well as iron-nickel single-atom contacts that act as a spin-valve break junction without the need for sophisticated spin-valve geometries. The versatile nature of atomic contacts in bimetallic junctions and the ability to control their structure by electromigration can be used to expand the structural variety of atomic and molecular junctions and their span of properties.

Maximizing the Interface of Dual Active Sites to Enhance Higher Oxygenate Synthesis from Syngas with High Activity

Selective synthesis of higher oxygenates from syngas provides a promising route for the conversion of nonpetroleum carbon resources into valuable chemicals. However, it remains a grand challenge to design highly efficient and stable dual-sites structures to promote the production of higher oxygenates. Herein, we reported an effective method to maximize the interface of dual active sites via designing the structure of alloy carbide derived from the FeCo layered double hydroxide precursor. Cobalt atoms were well-distributed and doped into Fe2C to form (FexCoy)2C alloy carbide. The atomic-scale contact Fe–Co interfacial sites could achieve a >35% oxygenate selectivity at a CO conversion of >80% during 200 h of running, and a high space–time yield of 183.9 mg/gcat./h for oxygenates with 95.6% being the C2+OH fraction was obtained. The kinetic study confirmed that the apparent activation energy of (FexCoy)2C alloy carbide was lower than that of separated Fe2C-Co2C dual sites. This work provides a strategy for the design of an effective catalyst for selective synthesis of higher oxygenates from syngas by tuning the interface of dual active sites at an atomic level.

In situ synthesis of stable silicon carbide-reinforced silicon nanosheets from organoclay for high-performance lithium-ion battery anodes

The extreme volume change of silicon anode causes fast capacity decay and short cycle life of lithium-ion batteries (LIBs). Thus, the development of stable Si-based anodes to avoid fractures of electrode materials is critical to their commercial applications. Herein, we designed a mechanically stable silicon carbide-reinforced silicon (Si/SiC) material via a facile molten salt-assisted magnesiothermic reduction of the carbonized organoclay. The as-prepared Si/SiC sample inherited the intrinsic layered structure of clay minerals. Benefiting from the atomic-scale direct contact of silicon oxide and carbon in organoclay precursor, β-SiC nanoparticles grew in situ in the Si nanosheets to form a heterostructure with strong bonding, thanks to their similar cubic crystal structure. The Si/SiC anodes exhibited enhanced cycling performance as anodes in LIBs, as compared with pure nanosilicon, which was attributed to the incorporation of SiC nanoparticles, the robust interface structure, and the hierarchical pore structure. Specifically, the Si/SiC nanocomposites containing 11 wt% SiC exhibited a high gravimetric capacity of 1507 mAh g−1, with 72% capacity being retained after 100 cycles at 1 A g−1. Coupled with the LiCoO2 cathode, the full cell showed a high capacity of 148 mAh g−1 after 100 cycles at 0.5C, demonstrating its potential for application in LIBs.

Definition of Atomic-Scale Contact: What Dominates the Atomic-Scale Friction Behaviors?

The definition of atomic-scale contact is a very ambiguous issue owing to the discrete atomic arrangement, which hinders the development of contact theory and nano-tribological techniques. In this work, we studied the atomic-scale contact area and their correlations with friction force based on three distinct contact definitions (interatomic distance, force, and interfacial chemical bonds) by performing large-scale atomistic simulations on a typical ball-on-disk contact model. In the simulations, the measured contact areas defined by interatomic distance, force, and interfacial chemical bonds (referred as to Adist, Aforce, and Abond, respectively) are not equivalent at all, while we interestingly clarify that only Adist is consistent with the one calculated by continuum Hertz contact mechanics, and moreover, only Abond is proportional to the friction force indicating that Abond is the dominant one for determining materials' frictional behaviors. The above fundamental insights into the atomic-scale contact problems are useful to deeply understand the origins of tribological phenomena and contribute to the further prediction of atomic-scale friction.

Fluorine-Induced Surface Metallization for Ammonia Synthesis under Photoexcitation up to 1550 nm.

The first observation of surface metallization of TiO2-x induced by fluoride ions is presented. The emerging metallic states are contributed by the 3d orbital of surface Ti and the 2p orbital of surface bridging F, which are intrinsically originated from the strong electron repulsion between F- and adjacent Ti3+ . The metalized TiO2-x with reduced work function and downward band bending possesses high electron-donating power to supported Ru species via atomic-scale ohmic contacts, exhibiting unprecedented photocatalytic performances for ammonia synthesis across the entire solar spectrum region (200-1550 nm) at room temperature. Mechanism and kinetic analysis revealed that the loaded Ru could behave as efficient electron sinks to accumulate photogenerated electrons and that the metallic surface markedly enhanced the dissociation of H2 and N2 by the hot electrons generated by the visible or even infrared light irradiation.

Dynamical Coulomb Blockade as a Local Probe for Quantum Transport.

Quantum fluctuations are imprinted with valuable information about transport processes. Experimental access to this information is possible, but challenging. We introduce the dynamical Coulomb blockade (DCB) as a local probe for fluctuations in a scanning tunneling microscope (STM) and show that it provides information about the conduction channels. In agreement with theoretical predictions, we find that the DCB disappears in a single-channel junction with increasing transmission following the Fano factor, analogous to what happens with shot noise. Furthermore we demonstrate local differences in the DCB expected from changes in the conduction channel configuration. Our experimental results are complemented by abinitio transport calculations that elucidate the microscopic nature of the conduction channels in our atomic-scale contacts. We conclude that probing the DCB by STM provides a technique complementary to shot noise measurements for locally resolving quantum transport characteristics.

Electronic noise generated by a temperature gradient across a hybrid normal metal–superconductor nanojunction

Understanding electronic noise in hybrid junctions is essential for designing efficient nanoscale electronics at the quantum limit. Recently, a new (previously overlooked) noise contribution, generated by the temperature difference across a conducting heterostructure and denoted as delta-T noise, was predicted and revealed experimentally in atomic-scale contacts (Shein Lambroso et al. in Nature 562:240–244, 2018). While the new type of electronic noise and the well-known voltage-generated shot noise have the same partition origin, they are activated by different stimuli. In the paper by Shein Lambroso et al. (2018), transmission probabilities for electrons flowing through the considered setups were assumed to be constant, a very good approximation for normal metal contacts within the used temperature range. In this work, we propose and study theoretically the delta-T noise in hybrid normal metal–superconductor nanojunctions where it has to be more pronounced at low temperatures due to the strongly nonlinear energy dependence of interface scattering characteristics in the gap region. Such experiments can be useful to probe quantum effects and to reveal small temperature variations along electronic nanoscale circuits.

Nanocontacts and Gaussian Filters

The inherent surface roughness at atomic scale contacts has profound effects on their local stresses, contact areas, and so on, which yield deviations from the predictions of continuum models. While these deviations can be interpreted as a breakdown of the models, attempts have been made to reconcile the two via different paths, e.g., proposing new methods for estimating the contact area, or filtering the contact stress fields. The applicability of the latter to randomly rough nanocontacts, however, has not yet been explored hitherto. Here, we show that this new method should be employed with care. Investigating two smooth and rough contacts, we find that the filtering method can make the stresses resemble each other by varying the filter’s resolution. Our results demonstrate that the filtering method improves comparisons between atomistic contacts and Hertz theory, while accounting for roughness in the rough contact results in more accurate solutions.

Modeling Atomic-Scale Electrical Contact Quality Across Two-Dimensional Interfaces.

Contacting interfaces with physical isolation and weak interactions usually act as barriers for electrical conduction. The electrical contact conductance across interfaces has long been correlated with the true contact area or the "contact quantity". Much of the physical understanding of the interfacial electrical contact quality was primarily based on Landauer's theory or Richardson formulation. However, a quantitative model directly connecting contact conductance to interfacial atomistic structures still remains absent. Here, we measure the atomic-scale local electrical contact conductance instead of local electronic surface states in graphene/Ru(0001) superstructure, via atomically resolved conductive atomic force microscopy. By defining the "quality" of individual atom-atom contact as the carrier tunneling probability along the interatomic electron transport pathways, we establish a relationship between the atomic-scale contact quality and local interfacial atomistic structure. This real-space model unravels the atomic-level spatial modulation of contact conductance, and the twist angle-dependent interlayer conductance between misoriented graphene layers.

Atomic rheology of gold nanojunctions.

Despite extensive investigations of dissipation and deformation processes in micro- and nano-sized metallic samples1-7, the mechanisms at play during the deformation of systems with ultimate (molecular) size remain unknown. Although metallic nanojunctions, which are obtained by stretching metallic wires down to the atomic level, are typically used to explore atomic-scale contacts5,8-11, it has not been possible until now to determine the full equilibrium and non-equilibrium rheological flow properties of matter at such scales. Here, by using an atomic-force microscope equipped with a quartz tuning fork, we combine electrical and rheological measurements on ångström-size gold junctions to study the non-linear rheology of this model atomic system. By subjecting the junction to increasing subnanometric deformations we observe a transition from a purely elastic regime to a plastic one, and eventually to a viscous-like fluidized regime, similar to the rheology of soft yielding materials12-14, although orders of magnitude different in length scale. The fluidized state furthermore exhibits capillary attraction, as expected for liquid capillary bridges. This shear fluidization cannot be captured by classical models of friction between atomic planes15,16 and points to an unexpected dissipative behaviour of defect-free metallic junctions at ultimate scales. Atomic rheology is therefore a powerful tool that can be used to probe the structural reorganization of atomic contacts.

Barrier Inhomogeneities in Atomic Contacts on WS2.

The down-scaling of electrical components requires a proper understanding of the physical mechanisms governing charge transport. Here, we have investigated atomic-scale contacts and their transport characteristics on WS2 using conductive atomic force microscopy (c-AFM). We demonstrate that c-AFM can provide true atomic resolution, revealing atom vacancies, adatoms, and periodic modulations arising from electronic effects. Moreover, we find a lateral variation of the surface conductivity that arises from the lattice periodicity of WS2. Three distinct sites are identified, i.e., atop, bridge, and hollow. The current transport across these atomic metal-semiconductor interfaces is understood by considering thermionic emission and Fowler-Nordheim tunnelling. Current modulations arising from point defects and the contact geometry promise a novel route for the direct control of atomic point contacts in diodes and devices.

Versatility of electrochemically grown dendrites in the undergraduate laboratory

We describe an experiment to fabricate atomic-scale contacts using electrochemically grown silver wires. The formation of a single-wire junction is directly observed and captured by an optical microscope, while electrical conductance of the wire, simultaneously recorded, is shown to be quantized. Further, a diffusion-limited aggregate (DLA) simulation is performed to compare the observed fractal formed by the silver dendrites. Our experiment directly exposes undergraduate students to exciting contemporary physics ranging from atomic-scale switches to fractal formation, all on a single experimental platform.

Designing nanoindentation simulation studies by appropriate indenter choices: Case study on single crystal tungsten

Atomic simulations are widely used to study the mechanics of small contacts for many contact loading processes such as nanometric cutting, nanoindentation, polishing, grinding and nanoimpact. A common assumption in most such studies is the idealisation of the impacting material (indenter or tool) as a perfectly rigid body. In this study, we explore this idealisation and show that active chemical interactions between two contacting asperities lead to significant deviations of atomic scale contact mechanics from predictions by classical continuum mechanics. We performed a testbed study by simulating velocity-controlled, fixed displacement nanoindentation on single crystal tungsten using five types of indenter (i) a rigid diamond indenter (DI) with full interactions, (ii) a rigid indenter comprising of the atoms of the same material as that of the substrate i.e. tungsten atoms (TI), (iii) a rigid diamond indenter with pairwise attraction turned off, (iv) a deformable diamond indenter and (v) an imaginary, ideally smooth, spherical, rigid and purely repulsive indenter (RI). Corroborating the published experimental data, the simulation results provide a useful guideline for selecting the right kind of indenter for atomic scale simulations.

Macro-mechanics controls quantum mechanics: mechanically controllable quantum conductance switching of an electrochemically fabricated atomic-scale point contact

Here, we present a silver atomic-scale device fabricated and operated by a combined technique of electrochemical control (EC) and mechanically controllable break junction (MCBJ). With this EC-MCBJ technique, we can perform mechanically controllable bistable quantum conductance switching of a silver quantum point contact (QPC) in an electrochemical environment at room temperature. Furthermore, the silver QPC of the device can be controlled both mechanically and electrochemically, and the operating mode can be changed from ‘electrochemical’ to ‘mechanical’, which expands the operating mode for controlling QPCs. These experimental results offer the perspective that a silver QPC may be used as a contact for a nanoelectromechanical relay.

Fano profiles in palladium nanoconstrictions

We have performed the differential conductance (dI/dV) measurements of palladium (Pd) nanoconstrictions made by a mechanically controllable break junction technique to examine the origin of Fano resonance observed in atomic sized contacts of ferromagnetic metals such as Ni. As a quasimagnetic metal of Pd undergoing ferromagnetic transition by downsizing, the dI/dV spectra exhibit zero-bias anomalies with a dip, peak or asymmetric shape and are well-fitted by the Fano formula. Moreover, the amplitude of the anomalies varies with the temperature in logarithmic scale, suggesting that the origin of the anomaly is caused by the Kondo effect. These results indicate that the appearance of Kondo effect would be ubiquitous in ferromagnetic atomic scale contacts.

Quantized edge modes in atomic-scale point contacts in graphene

The zigzag edges of single- or few-layer graphene are perfect one-dimensional conductors owing to a set of gapless states that are topologically protected against backscattering. Direct experimental evidence of these states has been limited so far to their local thermodynamic and magnetic properties, determined by the competing effects of edge topology and electron-electron interaction. However, experimental signatures of edge-bound electrical conduction have remained elusive, primarily due to the lack of graphitic nanostructures with low structural and/or chemical edge disorder. Here, we report the experimental detection of edge-mode electrical transport in suspended atomic-scale constrictions of single and multilayer graphene created during nanomechanical exfoliation of highly oriented pyrolytic graphite. The edge-mode transport leads to the observed quantization of conductance close to multiples of G0 = 2e2/h. At the same time, conductance plateaux at G0/2 and a split zero-bias anomaly in non-equilibrium transport suggest conduction via spin-polarized states in the presence of an electron-electron interaction.

Kondo-Fano resonance in atomic-scale contacts for ferromagnetic metals

The electrical conductance of cobalt (Co) nano-scale contacts prepared by a Mechanically Controllable Break Junction (MCBJ) technique has been studied to understand the origin of Fano resonance in ferromagnetic atomic-sized contacts. In an atomic-scale contacts, the zero-bias anomaly is well-fitted by the Fano formula. The characteristic temperature, which is introduced as the fitting parameter of that formula, exhibits the log-normal distribution. These indicate that the anomaly is likely caused by Kondo effect. Moreover, the zero-bias anomaly is observed in large scale contacts where the bulk ferromagnetic properties should be retained. This suggests the coexistence of Kondo effect and ferromagnetism.