Effect of bias voltage and electrodes size on the Pd conductance histograms

Conductance histograms of palladium nanocontacts in ultra high vacuum (UHV) were experimentally studied at room temperature using scanning tunneling microscope (STM). Our results show that the resolution of the pure Pd peaks, at 1.8 and around 3G₀, in the histograms, depends on the bias voltage and the electrodes size. The size of the electrodes should be as small as possible to achieve the higher extraction of hydrogen from them and therefore preventing the diffusion of H from the bulk to the nanocontacts during the conductance measurements, particularly at low bias voltage. This could explain why peaks have not been observed previously in the Pd histograms, using STM techniques in UHV.     

Quantized conductance behavior of Pt metal nanoconstrictions under electrochemical potential control

We studied the quantized conductance behavior of mechanically fabricated Pt nanoconstrictions under electrochemical potential control in H₂SO₄, Na₂SO₄, and NaOH solutions. There was no clear feature in the conductance histogram, when the electrochemical potential of the nanoconstrictions was kept at the double layer or the under potential deposited hydrogen potential. At the hydrogen evolution potential, the conductance histograms showed clear features around 0.5 and 1 G₀ in the H₂SO₄ solution. In Na₂SO₄, and NaOH solutions, a 1 G₀ feature with a shoulder appeared in the histogram. The quantized conductance behavior of Pt nanoconstrictions could be controlled by the electrochemical potential and solution pH.     

Conductance through atomic point contacts between fcc(100) electrodes of gold

Electrical conductance through various nanocontacts between gold electrodes is studied by using the density functional theory, scalar-relativistic pseudopotentials, generalized gradient approximation for the exchange-correlation energy and the recursion-transfer-matrix method along with channel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips and 1 to 5 gold atoms between the tips. Upon elongation of the contact by adding gold atoms between the tips, the conductance at Fermi energy E_F evolves from G ≈ 3G₀ to G ≈ 1G₀ (G₀ = 2e/h²). Formation of a true one-atom point contact, with G ≈ 1G₀ and only one open channel, requires at least one atom with coordination number 2 in the wire. Tips that share a common vertex atom or tips with touching vertex atoms have three partially open conductance channels at E_F, and the symmetries of the channels are governed by the wave functions of the tips. The long 5-atom contact develops conductance oscillations and conductance gaps in the studied energy range -3 ≤ E-E_F ≤ 5 eV, which reflects oscillations in the local density of electron states in the 5-atom linear “gold molecule" between the electrodes, and a weak coupling of this “molecule" to the tips.     

Metal atomic contact under electrochemical potential control

Electric conductance of the metal atomic contacts of Au and Pd was investigated using a scanning tunneling microscope (STM) in solution under electrochemical potential control. At the hydrogen evolution potential, a fractional conductance peak appeared around 0.5 G(0) (G(0) = 2e(2)/h) in the conductance histograms of Au contacts. For Pd contacts, peaks appeared around 1.0 G(0) in the conductance histograms at the hydrogen evolution potential. The conductance behavior and atomic configuration of the metal atomic contacts at hydrogen evolution potential were discussed based on previously reported experimental results and theoretical calculation results. We have proposed the formation of hydrogen adsorbed metal Au and Pd contacts in solution at the hydrogen evolution potential.     

Magnetic field effects on total and partial conductance histograms in Cu and Ni nanowires

Conductance histograms have become a powerful tool for studying transport properties of metallic nanowires. However, the individual conductance curves display a very rich structure that might be concealed by the statistical procedure of finding preferred conductance values by building conductance occurrence histograms using consecutive nanocontact breakage experiments. This is particularly true when it comes to discerning 1/2G₀=e²/hquantization in magnetic nanowires. The effect of disorder, added to possible magnetic sources of scattering, and different magnetic states of different nanowires, might hide its appearance as histogram peaks. This work analyzes and compares Ni and Cu nanowire experimental histograms at room temperature (RT). Those obtained with no curve selection criteria are basically unaffected by the presence of a magnetic field. A selection of particular sets of conductance curves shows that conductance quantization could occur in steps of e²/h and 2e²/h in Ni as well as in Cu in the presence or absence of a magnetic field. Sorting out curves in sets that present conductance plateaus at half integer and integer values, and compiling statistics on the number of such curves that appear, depending on the applied magnetic field, results in differences between the behaviour of Cu and Ni. While for Cu, the magnetic field keeps the ratio of curves that present plateaus at 1/2G₀with respect those presenting G₀ plateaus unchanged; for Ni, the number of curves which exhibit plateaus at just G₀ almost disappears with the applied field. This experimental fact might indicate that the magnetic field removes spin degeneracy in these magnetic nanowires. PACS 72.25.Ba; 73.40.Jn; 73.63.Rt; 75.75.+a     

From favorable atomic configurations to supershell structures: a new interpretation of conductance histograms.

Simulated minimum cross-section histograms of breaking Al nanocontacts are produced using molecular dynamics. The results allow a new interpretation of the controverted conductance histogram peaks based on preferential geometrical arrangements of nanocontact necks. As temperature increases, lower conductance peaks decrease in favor of broader and higher conductance structures. This reveals the existence of shell and supershell structures favored by the increased mobility of Al atoms.     

Theoretical study of conductance in stretched monatomic nanowires

Recent experiments showed that the last, single channel conductance step in monatomic gold contacts exhibits significant fluctuations as a function of stretching. From simulations of a stretched gold nanowire linked to deformable tips, we determine the distribution of the bond lengths between atoms forming the nanocontact and analyze its influence on the electronic conductance within a simplified single channel approach. We show that the inhomogeneous distribution of bond lengths can explain the occurrence and the 5% magnitude of conductance fluctuations below the quantum conductance unit g₀=2e²/h.     

ATR-SEIRAS--an approach to probe the reactivity of Pd-modified quasi-single crystal gold film electrodes

Quasi-single crystalline gold films of 20 nm thickness and preferential (1 1 1) orientation on Si hemispheres were modified by controlled potentiostatic deposition of Pd (sub-ML, ML, multi-L) from sulphate and/or chloride-containing electrolyte. The electrochemical properties of these model electrodes were characterised for hydrogen and (hydrogen-) sulphate adsorption as well as for surface oxide formation by cyclic voltammetry. Conditions were developed to fabricate defined and stable Pd monolayers. In situ ATR-SEIRAS (Attenuated Total Reflection Surface Enhanced Infrared Reflection Absorption Spectroscopy) experiments were carried out to describe the electrochemical double layer of Pd modified gold film electrodes in contact with aqueous 0.1 M H₂SO₄ with focus on interfacial water and anion adsorption. Based on an analysis of the non-resonant IR background signal the potential of zero charge is estimated to 0.10-0.20 V (vs. RHE). CO was found to be weakly physisorbed in atop sites on Au(1 1 1-20 nm)/0.1 M H₂SO₄ only in CO saturated electrolyte. CO, deposited on a quasi-single crystal gold film modified with 1 ML Pd, is chemisorbed with preferential occupation of bridge sites and atop positions at step edges. Saturated CO adlayers, as obtained by deposition at 0.10 V, contain isolated water species and are covered by a second layer of hydrogen bonded water. Potentiodynamic SEIRAS experiments of CO electro-oxidation on Pd-modified gold film electrodes demonstrate clearly the existence of a “pre-oxidation” region. They also provide spectroscopic evidence that isolated water and weakly hydrogen bonded water are consumed during the reaction and that atop CO on defect sites is a preferential reactant. The simultaneous in situ monitoring of the potential- and time-dependent evolution of characteristic vibrational modes in the OH- and CO-stretching regions are in agreement with the Gilman (“reactant pair”) mechanism of CO oxidation.     

Ionic shell and subshell structures in aluminum and gold nanocontacts

Conductance histograms of aluminum and gold nanocontact rupture are studied experimentally and simulated using embedded atom potentials to assess the interplay between electronic and structural properties at room temperature. Our results reveal a crossover from quantized conductance structures to crystalline faceting or geometric shell/subshell structures at 300 K. The absence of electronic shell structure in gold and aluminum is in stark contrast with the behavior of alkaline metal nanowires which emulate their cluster counterparts. Semiclassical arguments suggest why rapid dominance of ionic structures takes place, and possible nanowire architectures are proposed in consistency with both the experimental and simulated nanocontact data.     

Magnetism-induced ballistic conductance changes in palladium nanocontacts

We present first-principles calculations of the effects of magnetism on the ballistic conductance of a model Pd nanocontact, made of a short Pd monatomic stretched chain placed between two Pd leads, simulated by semi-infinite (100) slabs. The stretching makes the suspended Pd chain generally ferromagnetic. The spin-resolved ballistic conductance, calculated according to the Landauer-Büttiker formula is found to be 0.85G₀ for the spin-up and 1.15G₀ for the spin-down electrons (G₀ = 2e²/h is the conductance quantum). The total conductance ~2G₀ is lower, but still relatively close to that of the nonmagnetic Pd nanocontact with the same geometry, calculated to be 2.3G₀. To illustrate how magnetism and conductance depend on structural details, we change the three atom chain docking from the top to a hollow surface site, where at the same stress the Pd contact is nonmagnetic and the conductance decreases to 1.8G₀. Overall we find these calculated ballistic conductance values of very similar magnitude to the first histogram peak in the experimental data obtained for Pd at low temperature in mechanically controllable break junctions. We conclude that the 15% conductance changes caused by the onset or the demise of local magnetism, similar in magnitude to geometry-related conductance changes, are probably too small to be used as a diagnostic for the presence or absence of nanocontact magnetism.     

Theoretical investigation of the O2 adsorption effect in the electron transport of single Fe-porphyrin molecule

The effect of O₂ adsorption on the electron transport behavior of Fe-porphyrin molecule is investigated by the first-principles computational approach. The current-voltage characteristics of Fe-porphyrin and O₂ adsorbed Fe-porphyrin between gold electrodes are calculated. We find that the conductance of the Fe-porphyrin decreases dramatically upon the adsorption of O₂, which suggests that this system has potential application as a molecular sensor or a switch. This switching-behavior is analyzed from the evolutions of the transmission spectra and the molecular projected self-consistent Hamiltonian states of the molecular systems.     

真空热处理碳纳米管的储氢性能研究

研究了真空热处理对多壁碳纳米管(MWNTs)电化学储氢性能的影响.采用化学气相沉积法(CVD)制备碳纳米管，碳纳米管与LaNi₅储氢合金按质量比1∶10混合，制作成CNTs-LaNi₅电极.电解池采用三电极体系，6mol/L KOH为电解液，Ni(OH)₂为正极，Hg/HgO为参比电极.实验结果表明，在相同的充放电条件下，850℃时CNTs-LaNi₅电极的储氢性能最好，克容量最大为503.6mAh/g，相应的平台电压高达1.18V.从500—850℃随着温度升高，放电量有较大幅度的增加，但到950℃时放电量反而下降.由此可见，碳纳米管的热处理温度对碳纳米管的电化学储氢性能有着较大的影响. 关键词： 碳纳米管(CNTs) 储氢性能 5合金')" href="#">LaNi₅合金 化学气相沉积法(CVD法)     

Conductance of atom-sized contacts of transition metals at room temperature

Exploiting the mechanically controllable break junction technique, we have measured the conductance of atom-sized contacts of Fe, Co, and Ni at room temperature under ultrahigh vacuum conditions. The conductance histogram of Fe exhibits a broad peak around 2.5 G₀ (G₀ ≡ 2e²/h), whereas those of Co and Ni show no conductance peaks. However, the histograms of Co and Ni display different structures: While the Co histogram is simply flat, the Ni histogram reveals an appreciable background. Our experimental results are compared with previous results obtained at cryogenic and room temperatures, and the observed peak missing in our room-temperature histograms of Co and Ni is discussed.     

Breaking processes in nickel nanocontacts: a statistical description

In this work we perform a statistical study of favorable atomic configurations of nickel nanocontacts during their stretching at 4 K and 300 K. Nanowire breaking events are simulated using molecular dynamics (MD) where atomic interactions are represented with state-of-the-art embedded atom (EAM) interatomic potentials. The full determination of atomic positions during the contact evolution allows determination of the evolution of the minimum-cross section S_m during stretching. By accumulating many breaking traces, we built minimum cross-section histograms H(S_m). These simulated histograms reveal the presence of preferential geometrical arrangements during the nanocontact breaking, showing that no remarkable differences should appear between the low (4 K) and room temperature (300 K) situations. These results show that differences observed between low and room temperature experimental Ni conductance histograms, are not caused by the different structural evolution and, that therefore, other phenomena are involved. PACS  81.07.Lk; 68.65.-k; 73.63.Rt; 31.15.Qg     

Electrochemical fabrication and characterization of nanocontacts and nm-sized gaps

Copper nanocontacts and molecular-sized nanogaps were prepared and characterized at electrified solid/liquid interfaces employing lithographic and electrochemical techniques. A dedicated four-electrode potentiostat was developed for controlling the electrochemical fabrication process and for monitoring the electrical characteristics of the nanostructures created. The formation and breaking of Cu nanocontacts exhibits conductance quantization characteristics. The statistical analysis of conductance histograms revealed a preferential stability of nanocontacts with integer values of G₀, with a clear preference for 1 G₀, 2 G₀ and 3 G₀. The growth of molecular-sized gaps shows quantized tunneling current, which is attributed to the discrete nature of Cu atoms, water molecules, and specifically adsorbed ions. PACS 73.23Ad; 73.63.Rt; 82.45.Yz; 85.35.-p     

Spontaneous transformations of the magnetic structure of a film nanocontact

The magnetization distributions in a symmetric magnetic film nanocontact for oppositely magnetized ferromagnetic electrodes are analyzed based on numerically solving the Landau-Lifshitz and magnetostatic equations as a function of magnetic and geometrical factors. It is found that a symmetric magnetic configuration is unstable when the head-to-head domain wall dividing the regions with opposite orientations of magnetization is located at the center of the nanocontact. The instability arises when the uniaxial magnetic anisotropy constant reaches a certain critical value K _c below which it spontaneously leaves the center of the nanocontact. The transition from the symmetric state (wall at the center) to an asymmetric one can be continuous (second order) or discrete (first order), depending on the geometrical and physical parameters of the nanocontact (length to width ratio, anisotropy constant, and saturation magnetization). The phase diagram is constructed in terms of the variable’s nanocontact length vs. anisotropy constant. This diagram divides the symmetric and asymmetric magnetic configurations of the system. The occurrence of a tricritical point in the phase diagram is its characteristic feature.     

Hydrogen insertion into Pd–Pt–Rh alloy limited volume electrodes (LVEs)☆☆Keynote Lecture.

Limited volume electrodes have been used to examine the processes of hydrogen electrosorption by Pd–Pt–Rh alloys under conditions of cyclic voltammetric (CV) experiments. Hydrogen adsorption and hydrogen absorption signals well separated from surface oxides generation and oxides reduction currents are seen on CV curves recorded in the full potential range. The possibility is demonstrated of simultaneous investigations of bulk processes of hydrogen insertion/removal and surface processes of carbon oxides adsorption. Due to different adsorption characteristics towards CO₂ exhibited by the alloy components hydrogen adsorption and hydrogen absorption signals can be distinguished. Adsorbed CO₂ causes partial blocking of hydrogen adsorbed on Pt and Rh surface atoms. The presence of adsorbed CO₂ on the electrode surface does not influence significantly hydrogen insertion into the alloy. CO adsorption results in a strong inhibition of hydrogen adsorption. Hydrogen insertion into the bulk is not totally blocked but proceeds much slower than in the absence of CO adsorbates.     

Autocatalysis by the intermediate surface hydroxide formed during hydrogen peroxide reduction on silver electrodes

Recent electrochemical studies of the cathodic reduction of hydrogen peroxide (H₂O₂) on silver electrodes in acidic electrolyte (HClO₄) revealed a novel autocatalytic reaction path. Adsorbed hydroxyl groups, OH_ad, were proposed to act as the catalyst. To gather further clarity about this mechanism, in particular about the presence and nature of the postulated adsorbate OH_ad, in the present work surface science experiments on Ag(1 1 1) electrodes are evaluated. It is concluded that the species OH_ad is identical with the surface AgOH formed in alkaline solution as a relatively stable intermediate in the anodic oxidation of OH^? to surface-Ag₂O.     

Antioxidant behaviour of 2′‐hydroxy‐chalcones: a study of their electrochemical properties

The electrochemical behaviour of 13 chalcone analogues was systematically studied by means of cyclic voltammetry and chronoamperometry at a glassy carbon (GC), gold and platinum working electrodes using two different supporting electrolyte/solvent combinations. It was found that chalcone analogues can be easily oxidized at both GC and gold working electrodes, but not at a platinum electrode. Principal component analysis was further employed to reveal similarities/dissimilarities between oxidation potentials, chronoamperometric signals and ability of the compounds to scavenge the reactive oxygen species H₂O₂. The study reveals the inverse proportional relationship between the scavenging ability of H₂O₂, expressed as IC₅₀, and chronoamperometric signal at 800 mV using gold as working electrode. Copyright © 2012 John Wiley & Sons, Ltd.     

Electric conductance of metal nanowires at mechanically controllable break junctions under electrochemical potential control

We have developed the mechanically controllable break junction setup with an electrochemical cell (EC-MCBJ) to measure the electric conductance of metal nanowires under electrochemical potential control. The electric conductance of Au nanowires was investigated in 0.1 M Na₂SO₄ solution using EC-MCBJ. The conductance of the Au nanowires was quantized in units of G₀ (=2e²/h), showing clear features in the conductance histogram. The atomic contact with a specific conductance value was kept for >5 s, indicating the relatively high stability of the present EC-MCBJ system.