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.     

Conduction mechanism in a molecular hydrogen contact

We present first principles calculations for the conductance of a hydrogen molecule bridging a pair of Pt electrodes. The transmission function has a wide plateau with T approximately 1 which extends across the Fermi level and indicates the existence of a single, robust conductance channel with nearly perfect transmission. Through a detailed Wannier function analysis we show that the H(2) bonding state is not involved in the transport and that the plateau forms due to strong hybridization between the H(2) antibonding state and states on the adjacent Pt atoms. The Wannier functions furthermore allow us to derive a resonant-level model for the system with all parameters determined from the fully self-consistent Kohn-Sham Hamiltonian.     

Electron transport through single Mn12 molecular magnets

We report transport measurements through a single-molecule magnet, the Mn12 derivative [Mn12O12(O2C-C6H4-SAc)16(H2O)4], in a single-molecule transistor geometry. Thiol groups connect the molecule to gold electrodes that are fabricated by electromigration. Striking observations are regions of complete current suppression and excitations of negative differential conductance on the energy scale of the anisotropy barrier of the molecule. Transport calculations, taking into account the high-spin ground state and magnetic excitations of the molecule, reveal a blocking mechanism of the current involving nondegenerate spin multiplets.     

Ab-initio study of the coadsorption of Li and H on Pt(001), Pt(110) and Pt(111) surfaces

The coadsorption of Li and H atoms on Pt(001), Pt(110) and Pt(111) surfaces is studied using density functional theory with generalised gradient approximation. In all calculations Li, H and the two topmost layers of the metal were allowed to relax. At coverage of 0.25 mono-layer in a p(2×2) unit cell, lithium adsorption at the hollow site for the three surfaces is favoured over top and bridge sites. The most favoured adsorption sites for H atom on the Pt(001) and Pt(110) surfaces are the top and bridge sites, while on Pt(111) surface the fcc site appears to be slightly favoured over the hcp site. The coadsorption of Li and atomic hydrogen shows that the interaction between the two adsorbates is stabilising when they are far from each other. The analysis of Li, H and Pt local density of states shows that Li strongly interacts with the Pt surfaces.     

Structural dependence of intermediate species for the hydrogen evolution reaction on single crystal electrodes of Pt

Adsorbed hydrogen and water were measured during the hydrogen evolution reaction (HER) on the low and high index planes of Pt in 0.5 M H₂SO₄ using infrared reflection absorption spectroscopy. Hydrogen is adsorbed at the atop site (atop H) on Pt(110) during the HER, whereas adsorbed hydrogen at the asymmetric bridge site (bridge H) is found on Pt(100). The band intensity of the adsorbed hydrogen depends on temperature, indicating that the bands are due to the intermediate species for the HER. The band of the atop H appears on stepped surfaces with (110) step, whereas the asymmetric bridge H is observed on Pt(211) = 3(111)–(100) and Pt(311) = 2(111)–(100) that have (100) step. The absence of the atop H on Pt(100), Pt(211), and Pt(311) can be attributed to the relative stability of the bridge site.     

Mechanical fabrication of metal nano-contacts showing conductance quantization under electrochemical potential control

We have mechanically fabricated Ni and Cu nano-constrictions in solution to study their quantized conductance behavior under electrochemical potential control. Conductance quantization was observed at both metals in solution at room temperature for the first time. The conductance of Cu nano-constriction was quantized in units of G₀(=2e²/h). A sharp 1G₀ peak was observed in the conductance histogram. For Ni, a rather broad peak at 1–1.5G₀ was observed in the histogram. The conductance quantization behavior was discussed by comparing previously documented results of nano-constrictions fabricated in air or ultra-high vacuum conditions, with those fabricated in solution.     

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.     

Bias-dependence of the conductance of Au nanocontacts at 4 K

The bias-dependence of the quantized conductance in Au nanocontacts has been measured at liquid He temperature. A well-defined 1G₀ peak (G₀=2e²/h is the conductance quantum unit) appears in the conductance histogram. With increasing the bias, the 1G₀ peak decreases in height, while its position remains unshifted. This behavior of the 1G₀ peak is just the same as that observed at room temperature [H. Yasuda, A. Sakai, Phys. Rev. B 56 (1997) 1069]. The critical bias V_bc at which the 1G₀ peak disappears is ∼2.2 V, which is not much different from V_bc at 300 and 77 K. This weak temperature dependence of V_bc is compatible with high-current contact instability due to electromigration of contact atoms.     

Electron-vibration interaction in single-molecule junctions: from contact to tunneling regimes

Point contact spectroscopy on a H(2)O molecule bridging Pt electrodes reveals a clear crossover between enhancement and reduction of the conductance due to electron-vibration interaction. As single-channel models predict such a crossover at a transmission probability of tau=0.5, we used shot noise measurements to analyze the transmission and observed at least two channels across the junction where the dominant channel has a tau=0.51 +/- 0.01 transmission probability at the crossover conductance, which is consistent with the predictions for single-channel models.     

Macroscopic quantum coherence in a single molecular magnet and Kondo effect of electron transport

We report a Kondo-effect study of electron transport through a quantum dot with embedded biaxial single-molecule magnet based on slave boson mean-field theory and non-equilibrium Green-function technique. It is found the macroscopic quantum coherence of molecule-magnet results in the Kondo peak split of differential conductance due to interaction between electron and molecular magnet. It is also demonstrated that both the peak height and position can be controlled by the sweeping magnetic field and polarization of ferromagnetic electrodes. The characteristic peak split may be used to identify the macroscopic quantum coherence and develop molecule devices.     

Berry-phase oscillations of the Kondo effect in single-molecule magnets

We show that it is possible to topologically induce or quench the Kondo resonance in the conductance of a single-molecule magnet (S>1/2) strongly coupled to metallic leads. This can be achieved by applying a magnetic field perpendicular to the molecule easy axis and works for both full- and half-integer spin cases. The effect is caused by the Berry-phase interference between two quantum tunneling paths of the molecule's spin. We have calculated the renormalized Berry-phase oscillations of the Kondo peaks as a function of the transverse magnetic field as well as the conductance of the molecule by means of the poor man's scaling method. We propose to use a new variety of the single-molecule magnet Ni4 for the experimental observation of this phenomenon.     

Electronic transport in single-molecule magnets on metallic surfaces

An electron transport is studied in the system that consists of a scanning tunneling microscopy, single-molecule magnet metal. Because of quantum tunneling of magnetization in a single-molecule magnet, linear response conductance exhibits stepwise behavior with increasing longitudinal field, and each step is maximized at a certain value of field sweeping speed. The conductance at each step oscillates as a function of the additional transverse magnetic field along the hard axis. A rigorous theory is presented that combines the exchange model with the Landau-Zener model.     

Pt纳米微粒自组装体系的电化学和原位FTIR反射光谱研究

用化学还原法制备了铂金属纳米微粒 ,透射电子显微镜 (TEM)表征纳米Pt微粒的平均直径为 2 5nm。通过二硫醇将Pt纳米微粒组装到多晶金电极表面。以Fe(CN) 4- 3-6 的氧化还原作为探针反应的电化学研究表明 ,Au表面组装二硫醇后抑制了电极 /溶液界面的电子传递过程 ,而在二硫醇上再组装铂纳米微粒后 ,电子传递又可进行。运用电化学FTIR反射光谱研究了Pt纳米微粒组装电极在酸性介质中CO的吸附 ,检测到CO的线型、桥式吸附态 ,分别在 2 0 30和 184 5cm- 1 附近给出红外吸收谱峰 ,并且有增强红外效应。此外 ,还观察到Pt纳米微粒上的CO孪生吸附态。红外吸收峰位于 2 10 0cm- 1 附近。     

Evidence of a radiation-induced defect level in n-type InSb

Capacitance bridge measurements of loss on irradiated n-type InSb specimens at low temperatures are reported. A peak in a.c. conductance at 51 K is seen after irradiation and it is attributed to a radiation induced level.     

Vibrational characterization of carbon monoxide oxidation on the Pt(111) surface

The surface reaction between coadsorbed carbon monoxide and atomic oxygen has been characterized using high resolution electron energy loss spectroscopy, coupled with temperature programmed reaction spectroscopy on a Pt(111) surface characterized using Auger electron spectroscopy and low energy electron diffraction. Preferential oxidation of bridge bonded CO is not observed despite the fact that bridge bonded CO is adsorbed less vigorously than linearly bound CO. Saturation of the Pt(111) surface with one quarter of a monolayer of atomic oxygen completely suppresses the adsorption of bridge bonded CO. However, substantial coverages of bridge bonded CO can be coadsorbed if the Pt(111) surface is only partially saturated with atomic oxygen. The vibrational data for reaction of coadsorbed CO and atomic oxygen is consistent with a reaction mechanism involving reaction of mobile CO along oxygen island perimeters.     

GaAs(100)表面(4×1)结构的UPS研究

用偏振的紫外光源测量了GaAs(100)表面(4×1)结构的UPS谱,从清洁表面和吸附氧以后UPS的差谱中辨别出了在价带顶以下2eV以内的表面态峰,根据用偏振光所得到的谱和跃迁选择定则的讨论,认为表面态包含了三个峰,价带顶以下0.5eV处有一个对应于表面Ga原子桥键态的峰,在0.7eV处有对应于表面As原子桥键态的峰,而在1.3eV处的峰则同表面原子的悬键态相联系。 关键词：     

Laser induced desorption and interconversion of linear and bridge bonded CO on Pt(100) : progress towards time-resolved IR spectroscopy of surface processes

Laser induced thermal desorption has been tested as a method of modulating the coverage of CO in planned time-resolved FTIR experiments. CO can be completely desorbed in ~ 10 ns from Pt(100) without damaging the surface or indeed causing the (1 × 1) to hex transition, even though peak temperatures of ~ 1600 K are reached. In thermal transients in which no desorption occurs, conversion of linearly bonded to bridge bonded CO is observed, suggesting that the species are not in equilibrium at room temperature.     

Resonant vibrations, peak broadening, and noise in single molecule contacts: the nature of the first conductance peak

We carry out experiments on single-molecule junctions at low temperatures, using the mechanically controlled break junction technique. Analyzing the results obtained with various molecules, the nature of the first peak in the differential conductance spectra is elucidated. We observe an electronic transition with a vibronic fine structure, if the first peak occurs at small voltages. This regime can accurately be described by the resonant tunneling model. At higher voltages, additional smearing is observed and no fine structure can be resolved. A detailed analysis of the noise signal indicates that the onset of current is associated with strong fluctuations as a precursor of current flow. The data indicate that a complex fluctuation-driven transport mechanism takes over in this regime.     

缺陷对Pt在Graphene上吸附影响的研究

本文利用第一原理方法计算了空位缺陷和硼（B）掺杂时对Pt在graphene上吸附的影响.结果表明：Pt在graphene上吸附的稳定位置是Pt吸附在桥位;悬挂键的存在极大的增强了Pt在graphene空位处的吸附;B替位掺杂有利于Pt原子在杂质附近的吸附.     

缺陷对Pt在Graphene上吸附影响的研究

本文利用第一原理方法计算了空位缺陷和硼（B）掺杂时对Pt在graphene上吸附的影响.结果表明：Pt在graphene上吸附的稳定位置是Pt吸附在桥位；悬挂键的存在极大的增强了Pt在graphene空位处的吸附；B替位掺杂有利于Pt原子在杂质附近的吸附.