Single-atom Contacts Research Articles

Direct observation of conductance fluctuations of a single-atom tunneling contact

Using a cryogenic scanning tunneling microscope, the dynamics of lateral translations of a single silver atom adsorbed on Ag(111) were investigated with a time resolution of 10 ns. Elevated tunneling currents passed through the tip close to the adsorbed atom induce sudden adsorption site changes, which are reflected by two-level fluctuations of the tunneling conductance. The voltage dependence of the fluctuation rate reveals different local heating of the adsorbed atom in different positions relative to the tip.

Controlled single atom and single molecule contacts

The scanning tunnelling microscope (STM) may be used to prepare contacts to single atoms or molecules on surfaces and to investigate the transport of electrons through these junctions. The imaging capabilities of STM along with spectroscopic modes of measurement enable a rather detailed characterisation of the geometric and electronic properties of the contacts. This perspective presents recent investigations into the quantum transport through magnetic atoms and C(60) molecules.

Scanning Tunneling Microscopic Investigations into the Conductance of Single-Atom Junctions

Experiments using a scanning tunneling microscope to controllably fabricate single-atom contacts are reviewed. An important feature of this approach is that the electronic and geometric properties of the junctions can be characterized prior to and after contact formation. Investigations on magnetic contacts have led to the observation of unusual conductance values. Spectroscopy of the electronic states of single atoms in contact with the tip is demonstrated through the observation of the Kondo effect and of surface state localization at adsorbed atoms. Time-resolved two-level fluctuations of a single-atom tunneling contact reveal heating of the junction in an otherwise cryogenic environment.

Single-atom contacts with a scanning tunnelling microscope

The tip of a cryogenic scanning tunnelling microscope is used to controllably contact single atoms adsorbed on metal surfaces. The transition between tunnelling and contact is gradual for silver, while contact to adsorbed gold atoms is abrupt. The single-atom junctions are stable and enable spectroscopic measurements of, e.g., the Abrikosov–Suhl resonance of single Kondo impurities.

Spin fluctuation effects on the conductance through a single Pd atom contact

A controversy about the conductance through single atoms still exists.There are many experiments where values lower than the quantum unityG0 = 2e2/h have been found associated to Kondo regimes with high Kondo temperatures.Specifically in the Pd single atom contact, conductance values close toG0/2 at room temperature have been reported. In this work we propose a theoreticalanalysis of a break junction of Pd where the charge fluctuation in the single atomcontact is limited to the most probable one: . The projected density of states and the characteristics of the electron transport arecalculated by using a realistic description of the interacting system. A Kondo regime isfound where the conductance values and their dependence on temperature are in goodagreement with the experimental trends observed in the conduction of single moleculetransistors based on transition metal coordination complexes.

Electron-Plasmon and Electron-Electron Interactions at a Single Atom Contact

The transition from tunneling to contact is investigated by detecting light emitted from Au(111) in a scanning tunneling microscope. Optical spectra reflect single and multielectron processes and their distinct evolutions as a single-atom contact is formed. The experimental data are analyzed in terms of plasmon excitation and hot-hole processes.

Point-contact spectroscopy on aluminium atomic-size contacts: longitudinal and transverse vibronic excitations

We report a point-contact spectroscopy investigation of aluminium atomic-size contacts produced with the help of the mechanically controllable break-junction technique at low temperature. The derivatives of the differential conductance traces (d2I/dV2 versus V) display point-symmetric structures at certain voltage values, which move upon stretching the contact. These excitations may give rise to either enhancements or reductions of the conductance, indicating that they couple to highly or weakly transmissive electronic channels, respectively. By analyzing the sign of the conductance change and the strain dependence of the amplitudes and the energies we suggest that transverse as well as longitudinal vibronic modes are detected in the transport through single-atom contacts. Our findings are in agreement with symmetry arguments for the coupling of electronic and vibronic excitations.

Conductance of atom-sized contacts of dilute Al alloys

We have measured the conductance of atom-sized contacts of some dilute Al alloys in ultrahigh vacuum at room temperature. Measurements on Al-1.1at%Si, Al-2.8at%Mg, and Al-1.1at%Mg-0.6at%Si show no significant alloying effects: their conductance histograms are substantially the same as that of pure Al and exhibit a first peak corresponding to the single-atom contact of Al. On the other hand, a multicomponent Al alloy, Al-4.2at%Zn-3.6at%Mg-0.7at%Cu-0.01at%Ag (“meso10”), yields a conductance histogram which is totally different from others and shows no single-atom conductance peak of Al. We consider that in this high-strength alloy, the influence of alloying on the conductance would be mechanical rather than electronic. [DOI: 10.1380/ejssnt.2009.741]

Conductance quantization in nanowires formed at the metal-semiconductor interface

We demonstrate that step-like conductance traces resulting from quantum efiects such as the conductance quantization or formation of single-atom contacts are possible in the nanowires formed at the metal-semiconductor junction. In the macroscopic metal-semiconductor interfaces the Schottky barrier is usually risen. For the nanowires formed at the contact between the germanium surface and cobalt or gold tip, the conductance traces are characterized by a high reproducibility and long duration of the conductance plateaus. The current-voltage characteristics of these contacts are strongly nonlinear similarly to a junction with the Schottky barrier.

Scaling of1∕fnoise in tunable break junctions

We have studied the $1/f$ voltage noise of gold nanocontacts in electromigrated and mechanically controlled break junctions having resistance values $R$ that can be tuned from $10\text{ }\ensuremath{\Omega}$ (many channels) to $10\text{ }\text{k}\ensuremath{\Omega}$ (single-atom contact). The noise is caused by resistance fluctuations as evidenced by the ${S}_{V}\ensuremath{\propto}{V}^{2}$ dependence of the power-spectral density ${S}_{V}$ on the applied dc voltage $V$. As a function of $R$ the normalized noise ${S}_{V}/{V}^{2}$ shows a pronounced crossover from $\ensuremath{\propto}{R}^{3}$ for low-Ohmic junctions to $\ensuremath{\propto}{R}^{1.5}$ for high-Ohmic ones. The measured powers of 3 and 1.5 are in agreement with $1/f$ noise generated in the bulk and reflect the transition from diffusive to ballistic transport.

Cluster-based density-functional approach to quantum transport through molecular and atomic contacts

We present a cluster-based approach to model charge transport through molecular and atomic contacts. The electronic structure of the contacts is determined in the framework of density functional theory, and the parameters needed to describe transport are extracted from finite clusters. A similar procedure, restricted to nearest-neighbor interactions in the electrodes, has been presented by Damle et al (2002 Chem. Phys. 281 171). Here, we show how to systematically improve the description of the electrodes by extracting bulk parameters from sufficiently large metal clusters. In this way, we avoid problems arising from the use of nonorthogonal basis functions. For demonstration, we apply our method to electron transport through Au contacts with various atomic-chain configurations and to a single-atom contact of Al.

Kondo Effect in Single Atom Contacts: The Importance of the Atomic Geometry

Co single atom junctions on copper surfaces are studied by scanning tunneling microscopy and ab initio calculations. The Kondo temperature of single cobalt atoms on the Cu(111) surface has been measured at various tip-sample distances ranging from tunneling to the point contact regime. The experiments show a constant Kondo temperature for a whole range of tip-substrate distances consistently with the predicted energy position of the spin-polarized d levels of Co. This is in striking difference to experiments on Co/Cu(100) junctions, where a substantial increase of the Kondo temperature has been found. Our calculations reveal that the different behavior of the Co adatoms on the two Cu surfaces originates from the interplay between the structural relaxations and the electronic properties in the near-contact regime.

Stabilizing Single Atom Contacts by Molecular Bridge Formation

Gold-molecule-gold junctions can be formed by carefully breaking a gold wire in a solution containing dithiolated molecules. Surprisingly, there is little understanding on the mechanical details of the bridge formation process and specifically on the role that the dithiol molecules play themselves. We propose that alkanedithiol molecules have already formed bridges between the gold electrodes before the atomic gold-gold junction is broken. This leads to stabilization of the single atomic gold junction, as observed experimentally. Our data can be understood within a simple spring model.

Nanofabricated adjustable multicontact devices on membranes

Adjustable atomic size contacts realized by break junctions have become a standard tool during the last decade. Although nanofabricated break junctions may in principle be incorporated onto complex electronic circuits, a fundamental drawback of the standard break junction technique is its limitation to a single adjustable junction per device. We have fabricated single break junctions as well as devices containing two break junctions on a silicon membrane. The junctions are adjusted by positioning a fine tip via piezocontrol on the rear side of the membrane. We describe the fabrication process of the membranes and the devices and present results obtained on circuits made of gold and platinum. We show that the junctions can be addressed independently by a suitable choice of the tip position. Single-atom contacts, vacuum tunneling contacts as well as larger contacts can be stabilized.

Measurement Environment Dependency of Single Molecule Conductance

Abstract In order to investigate the measurement environment dependency of single molecule conductance, the lifetime of Au single-atom contacts and the conductance of a metal complex were investigated in solution and vacuum using a nanofabricated, mechanically controllable break junction (nano-MCBJ). The lifetime of Au single-atom contacts in a solution is shorter than that in vacuum. Single molecule conductance of a metal complex was found to be independent of the measurement environment.

Thermodynamic stability of single molecule junctions

Thermodynamic stability of Au-benzenedithiolate-Au single molecule junctions is explored at a room temperature in a vacuum by evaluating stretching rate dependence of the contact lifetime down to 0.0006nm∕s using nanofabricated mechanically controllable break junctions. The measured single molecule conductance is ∼0.01G0 irrespective of the displacement speeds. The molecular junctions demonstrate long-term durability compared to Au single atom contacts with natural lifetime of 14.6h, the characteristics of which is of immense significance from the viewpoint of device applications.

Formation and Self-Breaking Mechanism of Stable Atom-Sized Junctions

The self-breaking mechanism of gold junctions is studied by investigating stability of the atom-sized contacts. The single atom contact lifetime increases from about 0.02 to 200 s upon decreasing the junction stretching speed, while at the same time, the breaking force diminishes logarithmically. We find that the junction self-breaking processes involve sufficient atomic rearrangements, which thereby allow complete self-compensation of externally introduced strain at 0.8 pm/s. The present results have important implications on fabrication of stable single molecule junctions.

Development of Mechanically Controllable Break-junction Combined with Transmission Electron Microscopy for Single-Atom Contact

Extract Extended abstract of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007

Conduction eigenchannels of atomic-sized contacts:Ab initioKKR Green’s function formalism

We develop a formalism for the evaluation of conduction eigenchannels of atomic-sized contacts from first-principles. The multiple scattering Korringa-Kohn-Rostoker (KKR) Green's function method is combined with the Kubo linear response theory. Solutions of the eigenvalue problem for the transmission matrix are proven to be identical to eigenchannels introduced by Landauer and B{\"u}ttiker. Applications of the method are presented by studying ballistic electron transport through Cu, Pd, Ni and Co single-atom contacts. We show in detail how the eigenchannels are classified in terms of irreducible representations of the symmetry group of the system as well as by orbital contributions when the channels wave functions are projected on the contact atom.

Distribution of 1G0 Plateau Length of Au Contacts at Room Temperature

We have measured the 1G0 (G0 is the conductance quantum unit) plateau length τ of Au breaking contacts at room temperature and examined the plateau-length distribution P(τ) for biases 0.1–2.0 V. We observed that P(τ) exhibits a long tail and obeys the power-law distribution P(τ)∝τ-γ with γ∼1, indicating a strong diversity in the atomic configuration of the Au single-atom contacts. The exponent γ little varies with the bias voltage and ranges between 0.8 and 1.1. We changed the contact breaking conditions, by slowing down the breaking speed and lowering the contact current, and again observed the same power-law distribution for P(τ) with γ∼1, whereas the average plateau length <τ> increases at slower breaks. We also found that the Au 1G0 contacts in air show longer <τ> than those in ultrahigh vacuum, suggesting contact stabilization by ambient air molecules.