Gold Nanojunctions Research Articles

Understanding the rheology of nanocontacts

Mechanical stiffness, as opposed to softness, is a fundamental property of solids. Its persistence or rheological evolution in vibrating solid-solid nanocontacts is important in physics, materials science and technology. A puzzling apparent liquefaction under oscillatory strain, totally unexpected at room temperature, was suggested by recent experiments on solid gold nano-junctions. Here we show theoretically that realistically simulated nanocontacts actually remain crystalline even under large oscillatory strains. Tensile and compressive slips, respectively of “necking” and “bellying” types, do take place, but recover reversibly even during fast oscillatory cycles. We also show that, counterintuitively, the residual stress remains tensile after both slips, driving the averaged stiffness from positive to negative, thus superficially mimicking a liquid’s. Unlike a liquid, however, rheological softening occurs by stick-slip, predicting largely frequency independent stiffness with violent noise in stress and conductance, properties compatible with experiments. The baffling large amplitude rheology of gold nanocontacts and its consequences should apply, with different parameters, to many other metals.

Intrinsic luminescence blinking from plasmonic nanojunctions

Plasmonic nanojunctions, consisting of adjacent metal structures with nanometre gaps, can support localised plasmon resonances that boost light matter interactions and concentrate electromagnetic fields at the nanoscale. In this regime, the optical response of the system is governed by poorly understood dynamical phenomena at the frontier between the bulk, molecular and atomic scales. Here, we report ubiquitous spectral fluctuations in the intrinsic light emission from photo-excited gold nanojunctions, which we attribute to the light-induced formation of domain boundaries and quantum-confined emitters inside the noble metal. Our data suggest that photoexcited carriers and gold adatom - molecule interactions play key roles in triggering luminescence blinking. Surprisingly, this internal restructuring of the metal has no measurable impact on the Raman signal and scattering spectrum of the plasmonic cavity. Our findings demonstrate that metal luminescence offers a valuable proxy to investigate atomic fluctuations in plasmonic cavities, complementary to other optical and electrical techniques.

Driven Dynamics of Long-Time Bond-Breaking Events.

We present a predict-correct trajectory propagation (PCTP) method for simulating nonequilibrium driven dynamics of the long-time bond-breaking event in an atomic force microscope (AFM). Whereas the parallel replica method can extend the time scale of molecular dynamics (MD) simulations of infrequent-event systems with high parallel efficiency, the second aspect of the time scale difficulty in MD simulations for slow-driven systems, namely, the unphysically high attempt frequency of an infrequent event, cannot be resolved by this method. Here, we take a gold nanojunction under mechanical pulling as a simulation system and demonstrate that the PCTP simulation is capable of capturing key transition dynamics of bond breaking predicted by accurate MD simulations in the activationless regime, such as at cryogenic temperature with high pulling rates. The PCTP algorithm includes three steps: (i) the trajectory prediction of the AFM tip, (ii) the atomic structural relaxation of the nanojunction, and (iii) the trajectory correction of the tip motion predicted by step (i). Application of the PCTP simulation to the bond breaking at AFM experimental pulling rates reveals different rupture mechanisms, depending on temperature and single-atom nanojunction structures, which are consistent with recent AFM bond-breaking observations.

Stark control of electrons along nanojunctions

Ultrafast control of currents on the nanoscale is essential for future innovations in nanoelectronics. Recently it was experimentally demonstrated that strong non-resonant few-cycle 4 fs laser pulses can be used to induce phase-controllable currents along gold–silica–gold nanojunctions in the absence of a bias voltage. However, since the effect depends on a highly non-equilibrium state of matter, its microscopic origin is unclear and the subject of recent controversy. Here we present atomistically detailed (time-dependent non-equilibrium Green’s function) electronic transport simulations that recover the main experimental observations and offer a simple intuitive picture of the effect. The photoinduced currents are seen to arise due to a difference in effective silica-metal coupling for negative and positive field amplitudes induced by lasers with low temporal symmetry. These insights can be employed to interpret related experiments, and advance our ability to control electrons in matter using lasers.

Electric-field control of conductance in metal quantum point contacts by electric-double-layer gating

An electric-field control of quantized conductance in metal (gold) quantum point contacts (QPCs) is demonstrated by adopting a liquid-gated electric-double-layer (EDL) transistor geometry. Atomic-scale gold QPCs were fabricated by applying the feedback-controlled electrical break junction method to the gold nanojunction. The electric conductance in gold QPCs shows quantized conductance plateaus and step-wise increase/decrease by the conductance quantum, G0 = 2e2/h, as EDL-gate voltage is swept, demonstrating a modulation of the conductance of gold QPCs by EDL gating. The electric-field control of conductance in metal QPCs may open a way for their application to local charge sensing at room temperature.

A DFT study on surface-enhanced Raman spectroscopy of aromatic dithiol derivatives adsorbed on gold nanojunctions

A computational study on aromatic dithiol derivatives (HS-Ar-X-Ar-SH, X=O, S, Se, NH, CH2, NN, CHCH, CC) interacting with gold cluster(s) was presented to investigate the chemical enhancement mechanism related to surface-enhanced Raman spectroscopy (SERS) for molecular junctions. Density functional theory (DFT) were performed on derivatives molecules as well as their single-end-linked (SEL) or double-end-linked (DEL) complexes for geometric, spectra, electronic and excitation properties, leading to discussions on dominant factor during SERS process. The resulted enhancement factors of SEL and DEL complexes exhibited specific dependency on linking atom or functional group between two phenyls, which was in accordance with the variation of polarizabilities and molecule-cluster transition energy.

Origin of the periodic structure in the conductance curve of gold nanojunctions in hydrogen environment

The evolution of the atomic structure and the vibrational and electronic transport properties of gold atomic junctions incorporating molecular and atomic hydrogen upon elongation have been investigated with the nonequilibrium Green's function formalism combined with density functional theory. Our calculations show that for the case of gold junctions doped with a single ${\mathrm{H}}_{2}$ molecule the low-bias conductance drops rapidly with the electrodes' separation, while it remains almost constant if a single H atom replaces the molecule. In contrast, when one considers two H atoms adsorbed on a gold monatomic chain forming an Au-H-Au-H-Au double-bridge structure, the low-bias conductance increases first and then shows a plateau upon stretching the junction, in perfect agreement with experiments on gold nanocontacts in hydrogen environment. Furthermore, also the distribution of the calculated vibrational energies of the two H atoms is consistent with the experimental result in the low-conductance region, demonstrating clear evidence that hydrogen molecules can dissociate on stretched gold monatomic chains. These findings are helpful to improve our understanding of the structure-property relation of gold nanocontacts and also provide a new prospect for gold nanowires being used as chemical sensors and catalysts.

Breaking Force and Conductance of Gold Nanojunctions: Effect of Humidity.

Forces acting on elongated gold nanojunctions and their electric conductance were simultaneously measured by current-sensing force spectroscopy in an atmosphere with controlled humidity. The breaking force of "thick" nanojunctions with conductance >20G0 is not affected by the environmental humidity. The presence of ambient water stabilizes "thin" nanojunctions with conductance <15G0, whose breaking force of 10-15 nN was higher than that in a dry atmosphere due to the capillary forces. The observed effect of humidity would not be possible to distinguish by techniques measuring only forces or only conductance in nanojunctions.

Conformation-dependent conductance through a molecular break junction

Ab initio molecular dynamics simulations have been performed of a gold-1,4-benzenedithiol (BDT)-gold nanojunction under mechanical stress. For three different pulling rates between 10 and 40 m s(-1), it is found that the nanowire always ruptures between the second and third Au atom from the thiol sulfur. Larger rupture forces and longer extensions are required at higher pulling rates and vice versa. The electrical conductance was calculated along a pulling trajectory using the DFT-NEGF method to study the effect of thermal and stress-induced structural changes on the electrical transport properties. While the mechanically induced stretching of the junction is seen to lower the time-averaged conductance, thermal conformational changes are capable of altering the conductance by one order of magnitude. No single geometric quantity could be identified as the main contributor to the conductance fluctuations. Small modulations, however, can be explained in terms of C=C double bond vibrations in the BDT molecule. The dependence of the conductance on different geometric variables has further been investigated systematically by performing constrained geometry optimizations along a number of angle and dihedral coordinates. The largest changes in the conductance are observed when the Au-S-C angle and the Au-S-C-C dihedral are simultaneously constrained.

Molecular dynamics simulations of the stable structures of single atomic contacts in gold nanojunctions

Molecular dynamics simulations are performed to investigate the breaking forces and stable atomic contacts in different gold nanojunctions. These forces and structures are obtained through repeated jump-to-contact and jump-out-of-contact trainings with the aid of a simple driving-spring model, which can reasonably represent the long-range elasticity of the gold electrode. For the three stable trained atomic contacts obtained in this work, we find that the breaking force and binding energy of a gold dimer in the 3-1-1-3 contact are 0.87 nN and 0.38 eV, respectively, which are much smaller than those of the 3-1-3 and 3-1-2 contacts. The binding energies of these stable nanojunctions vary from 0.38--0.74 eV, consistent with other experimental measurements.

Deformation-mediated phase transformation in gold nano-junction

To further advance our understanding of the mechanical behavior of nano-sized crystals, it is important to visualize the deformation process in the real material system. In this work, body-centered-tetragonal (BCT) lattice was observed during in-situ tension of gold nano-junction in a transmission electron microscope, which is well explained by the proposed deformation mechanism. In contrast to the well-known Bain model, the phase transformation can be realized by a progressively slipping process with a vector of 1/12<112>. The lattice constant a can be accurately predicated from a hard-sphere model and c is about 5% larger than that in MD simulations using a more realistic interatomic potential.

Spectroscopic analysis of electromigration at gold nanojunctions

We have investigated the electromigration process at gold nanojunctions by introducing a novel spectroscopic approach. When the junction voltage exceeded certain critical values, conductance showed successive drops by one quantum conductance, corresponding to one-by-one removal of gold atoms. The observed critical voltages agree with the activation energies for surface diffusion of gold atoms. This fact clearly indicates that the elementary process of electromigration is the self-diffusion of metal atoms driven by microscopic kinetic energy transfer from a single conduction electron to a single metal atom. This new finding can be applied to reproducible formation of atomic-spacing gaps for single molecular junctions and also to the failure-tolerant interconnects for VLSIs.

Nonthermal origin of electromigration at gold nanojunctions in the ballistic regime

We have investigated the electromigration process at gold nanojunctions as small as several tens of atoms. Junction conductance showed successive drops by one conductance quantum, corresponding to one-by-one removal of gold atoms, only when the junction voltage exceeded certain critical values. The peak position in the histogram of the observed critical voltages agreed with the activation energies for surface diffusion of gold atoms. This fact indicates that the elementary process of electromigration in such small junctions is the self-diffusion of metal atoms driven by microscopic kinetic energy transfer from a single conduction electron to a single metal atom.

Substituent Effect on the Mechanical Properties of Au−N Nanojunctions

We report on the calculated binding energies and rupture forces of a series of ortho-substituted pyrazines bonded to gold nanoclusters. This system is proposed as a model for the 4,4-bipyridine−gold nanojunction to assess the effect of the substituent on the mechanical stability and the rupture modes of such nanojunction. The present results show that although there is a definite trend to a reduced rupture force for a more electron withdrawing substituent on the pyrazine molecule, this dependence is small, allowing for the construction of nanojunctions with different transport properties but similar mechanical stabilities. A clear-cut correlation is found between the rupture force of the nanojunction and the pKa value of the structurally related pyridines, providing with a simple empirical way of predicting the stability of the system. The possibility of attaching the molecule to the cluster through a monatomic gold chain is also explored, with the finding that the Au−Au rupture at the chain is a possible...

Dynamic Characterization of the Postbreaking Behavior of a Nanowire

The dynamic evolution of a gold nanojunction has been investigated by incorporation of molecular dynamics (MD) simulation and mechanically controllable break junction (MCBJ) in a wide range of strain rates, covering nonequilibrium, quasi-equilibrium, and equilibrium tensile states. Theoretical simulations corresponding to the nonequilibrium and quasi-equilibrium states demonstrated that the metallic nanojunction spontaneously grew in the time scale of ∼1 ns. In the final stage, the gap increment revealed a unique stepwise feature, corresponding to the atomic-resolved tip reconstruction. When strain rate varied from 489.6 to 0.245 m/s, the gap was reduced from 4.0 to 1.3 nm. In the regime of equilibrium stretching, i.e., several nanometers per second, the experimental MCBJ results gave a mean gap size of 0.4 ± 0.1 nm, which showed less strain rate dependence. This value corresponded well to the absolute value of the stepwise increment of the gap due to the local tip recrystallization as estimated by the MD simulation. The agreement between theoretical modeling and experimental measurement demonstrates that the combination is a good strategy in nanoscience.

Conductance of gold nanojunctions thinned by electromigration

We investigate nanocontact formation by thermally assisted electromigration of gold nanowires. An automatic cycling process allows us to follow a line of constant power dissipated at the nanocontact up to resistances corresponding to 10–20 conductance quanta. The contacts are thinned in a controlled way by voltage-induced heating. In the ballistic regime, oscillations of the conductance histograms show oscillations typical for atomic discreteness.

An ab initio study of the field-induced position change of aC60 molecule adsorbed on a gold tip

Recent current/voltage (I/V) curve measurements suggest thatC60 molecules deposited in gold nanojunctions change their adsorption configuration when afinite voltage in a two-terminal setting is applied. This is of interest for molecularelectronics because a robust molecular transistor could be based on such junctions if themechanism of the process is understood. We present density functional theorybased plane wave calculations, where we studied the energetics of the molecule’sadsorption under the influence of an external field. Particular emphasis was placed oninvestigating a possible lightning rod effect which might explain the switchingbetween configurations found in experiments. We also analyse our results for theadsorption energetics in terms of an electrostatic expression for the total energy,where the dependence of the polarizability of the junction on the position of theC60 molecule was identified as a crucial property for the field-induced change of adsorptionsite.

Pulling gold nanowires with a hydrogen clamp: Strong interactions of hydrogen molecules with gold nanojunctions

In this paper an experimental study of the interaction of hydrogen molecules with gold nanowires is presented. Our results show that chains of Au atoms can also be pulled in hydrogen environment; however, in this case the conductance of the chain is strongly reduced compared to the perfect transmission of pure Au chains. The comparison of our experiments with recent theoretical prediction for the hydrogen welding of Au nanowires implies that a hydrogen molecule can even be incorporated in the gold nanocontact, and this hydrogen clamp is strong enough to pull a chain of gold atoms.

Suppression of Spin-Orbit Scattering in Strongly Disordered Gold Nanojunctions

We discovered that spin-orbit scattering in strongly disordered gold nanojunctions is strongly suppressed relative to that in weakly disordered gold thin films. This property is unusual because in weakly disordered films spin-orbit scattering increases with disorder. Granularity and freezing of spin-orbit scattering inside the grains explains the suppression of spin-orbit scattering. We propose a generalized Elliot-Yafet relation that applies to a strongly disordered granular regime.

Quantum interference structures in the conductance plateaus of gold nanojunctions

The conductance of breaking metallic nanojunctions shows plateaus alternated with sudden jumps, corresponding to the stretching of stable atomic configurations and atomic rearrangements. We investigate the structure of the conductance plateaus both by measuring the voltage dependence of the plateaus' slope on individual junctions and by a detailed statistical analysis on a large amount of contacts. Though the atomic discreteness of the junction plays a fundamental role in the evolution of the conductance, we find that the fine structure of the conductance plateaus is determined by quantum interference phenomenon to a great extent.