Fourier-transform microwave spectroscopy of the deuterated acetylene dimers: The interconversion tunneling motions of (DCCD)2, (DCCH)2, DCCD---DCCH, DCCH---DCCD, HCCH---DCCD, and HCCH---DCCH

Microwave spectra of the deuterated acetylene dimers, produced in a molecular beam at 1 K from samples of HCCH, DCCH, and DCCD, were observed using a Fourier transform microwave spectrometer. We observed all variations of deuterated acetylene dimers in which a deuterium atom participates in the hydrogen bond; i.e., (DCCD)₂, (DCCH)₂, DCCD---DCCH, DCCH---DCCD, HCCH---DCCD, and HCCH---DCCH. Deuterated acetylene dimers with the hydrogen atom located in the hydrogen bond could not be detected. Precise molecular constants were determined for each species. Among the dimers identified, (DCCD)₂, (DCCH)₂, DCCD---DCCH, and DCCH---DCCD showed evidence of an interconversion tunneling motion like the tunneling observed for (HCCH)₂. The tunneling potential of (DCCD)₂ was analyzed using a one-coordinate model and the potential depth was determined to be V₄ = 35.577 cm⁻¹, which is 2.371 cm⁻¹ deeper than that of (HCCH)₂ studied by Fraser et al. (J. Chem. Phys. 89, 6028–6045 (1988)). A one-coordinate model was also applied to the other deutered acetylene dimers by adopting a further assumption of a composite potential.     

Infrared spectra of acetylene dimers and acetylene–nitrogen: (DCCD)2, H-bonded DCCD–HCCH, and DCCD–NN in the 4.1 μm region

Infrared spectra of the weakly-bound T-shaped acetylene dimers DCCD–DCCD and DCCD–HCCH are studied in the region of the DCCD ν₃ fundamental (∼2440 cm⁻¹) using a pulsed supersonic slit-jet expansion and a tunable diode laser probe. The K_a = 0 ← 1 and 1 ← 0 subbands, corresponding to the vibration of the DCCD monomer at the “top” of the T, are analyzed. Compared to the analogous spectrum of HCCH–HCCH, the present results are much less perturbed. The tunneling splitting for (DCCD)₂ in the excited state is determined to be 141 MHz, a big reduction from the previously determined ground state value of 424 MHz. The dimer A rotational constants show a large apparent increase upon vibrational excitation, and we discuss whether this increase is real. The linear DCCD–NN complex is also observed as an impurity in the spectrum, and it too is found to be unperturbed, in contrast with HCCH–NN.     

Prediction and characterisation of a chalcogen···π interaction with acetylene as a potential electron donor in XHS···HCCH and XHSe···HCCH (X = F,Cl, Br,CN, OH,OCH3, NH2, CH3) σ-hole complexes

It is well-known that many covalently bonded atoms of group VI have specific positive regions of electrostatic potential (σ-holes) through which they can interact with Lewis bases. This interaction is called ‘chalcogen bond’ by analogy with halogen bond and hydrogen bond. In this study, ab initio calculations are performed to predict and characterise chalcogen···π interactions in XHS···HCCH and XHSe···HCCH complexes, where X = F, Cl, Br, CN, OH, OCH₃, NH₂, CH₃. For the complexes studied here, XHS(Se) and HCCH are treated as a Lewis acid and a Lewis base, respectively. The CCSD(T)/aug-cc-pVTZ interaction energies of this type of σ-hole bonding range from ?1.18 to ?4.83 kcal/mol. The calculated interaction energies tend to increase in magnitude with increasing positive electrostatic potential on the extension of X–S(Se) bond. The stability of chalcogen···π complexes is attributed mainly to electrostatic and correlation effects. The nature of chalcogen···π interactions is unveiled by means of the atoms in molecules, natural bond orbital, and electron localisation function analyses.     

Combined experimental and theoretical DFT study of molecular nanowires negative differential resistance and interaction with gold clusters

Electric-field-assisted assembly has been used to place rod-shaped metal nanowires containing 4-[[2-nitro-4-(phenylethynyl) phenyl] ethynyl] benzenthiol molecules onto lithographically defined metal pads. These junctions exhibited negative differential resistance. The quantum chemical approach was used to compare the properties of Au-bonded 4-[[2-nitro-4-(phenylethynyl) phenyl] ethynyl] benzenthiol molecule and a molecule that does not exhibit the negative differential resistance, Au-bonded 4-[[4-(phenylethynyl) phenyl] ethynyl] benzenthiol. The influence of the static electric field and charge variation were modelled for both systems.     

Optimization of three-dimensional TROSY-type HCCH NMR correlation of aromatic (1)H-(13)C groups in proteins.

Improved methods for three-dimensional TROSY-Type HCCH correlation involving protons of negligible CSA are presented. The TROSY approach differs from the conventional approach of heteronuclear decoupling in evolution and detection periods by not mixing fast and slowly relaxing coherences and usually suppressing the former. Pervushin et al. (J. Am. Chem. Soc. 120, 6394-6400 (1998)) have proposed a 3D TROSY-type HCCH experiment where the TROSY approach is applied only in one of the (13)C dimensions. A new pulse sequence applying the TROSY approach in both indirect dimensions is advantageous when the TROSY effect of the carbons is large or when a relatively high resolution is required. For lower resolutions or moderate TROSY effects we show that it is possible to combine the best of both worlds, namely to suppress heteronuclear couplings without mixing fast and slowly relaxing coherences while at the same time superimpose the two components and thus have both contribute to the detected signal. That is possible using the novel technique of Spin-State-Selective Time-Proportional Phase Incrementation (S(3) TPPI). The new 3D S(3) TPPI TROSY HCCH method is demonstrated on a (13)C,(15)N-labeled protein sample, RAP 18-112 (N-terminal domain of alpha(2)-macroglobulin receptor associated protein), at 750 MHz and average sensitivity enhancements of 10% are obtained for the cross peaks in comparison to methods based on conventional decoupling on one of the carbons or on TROSY on both carbons.     

Synthesis of graphene oxide nanosheets and its application to construct a modified carbon paste electrode as a hydroxylamine electrochemical sensor

Ionics - A ferrocene-derivative compound, 2, 7-bis (ferrocenyl ethynyl) fluoren-9-one (2,7-BFE), was synthesized and used to construct a modified graphene paste electrode. The electrooxidation of...     

The heats of formation of the haloacetylenes XCCY [X,Y = H,F, Cl]: basis set limit ab initio results and thermochemical analysis

The heats of formation of haloacetylenes are evaluated using the recent W1 and W2 ab initio computational thermochemistry methods. These calculations involve CCSD and CCSD(T) coupled cluster methods, basis sets of up to spdfgh quality, extrapolations to the one-particle basis set limit, and contributions of inner-shell correlation, scalar relativistic effects. and (where relevant) first-order spin-orbit coupling. The heats of formation determined using W2 theory are: δH₁ ²⁹⁸(HCCH) = 54.48 kcal mol^?1, δH_f ²⁹⁸(HCCH) = 25.15 kcal mol, δH_f ²⁹⁸(FCCF) = 1.38 kcal mol^?1, δH_f ²⁹⁸(HCCC1) = 54.83 kcal mol^?1, δH_f ²⁹⁸(CICCC1) = 56.21 kcal mol^?1, and δH_f ²⁹⁸(FCCC1) = 28.47 kcal mo1^?1. Enthalpies of hydrogenation and destabilization energies relative to acetylene were obtained at the WI level of theory. So doing we find the following destabilization order for acetylenes: FCCF > ClCCF > HCCF > ClCCCl > HCCCI > HCCH. By a combination of WI theory and isodesmic reactions. we show that the generally accepted heat of formation of 1,2-dichloroethane should be revised to ?31.8 ± 0.6 kcal mol^?1, in excellent agreement with a very recent critically evaluated review. The performance of compound thermochemistry schemes, such as G2, G3, G3X and CBS-QB3 theories, has been analysed.     

Optimization of Three-Dimensional TROSY-Type HCCH NMR Correlation of Aromatic H–C Groups in Proteins

Improved methods for three-dimensional TROSY-Type HCCH correlation involving protons of negligible CSA are presented. The TROSY approach differs from the conventional approach of heteronuclear decoupling in evolution and detection periods by not mixing fast and slowly relaxing coherences and usually suppressing the former. Pervushin et al. (J. Am. Chem. Soc. 120, 6394–6400 (1998)) have proposed a 3D TROSY-type HCCH experiment where the TROSY approach is applied only in one of the ¹³C dimensions. A new pulse sequence applying the TROSY approach in both indirect dimensions is advantageous when the TROSY effect of the carbons is large or when a relatively high resolution is required. For lower resolutions or moderate TROSY effects we show that it is possible to combine the best of both worlds, namely to suppress heteronuclear couplings without mixing fast and slowly relaxing coherences while at the same time superimpose the two components and thus have both contribute to the detected signal. That is possible using the novel technique of Spin-State-Selective Time-Proportional Phase Incrementation (S³ TPPI). The new 3D S³ TPPI TROSY HCCH method is demonstrated on a ¹³C,¹⁵N-labeled protein sample, RAP 18–112 (N-terminal domain of α₂-macroglobulin receptor associated protein), at 750 MHz and average sensitivity enhancements of 10% are obtained for the cross peaks in comparison to methods based on conventional decoupling on one of the carbons or on TROSY on both carbons.     

Significant Fowler–Nordheim tunneling across ZnO – Nanorod based nanojunctions for nanoelectronic device applications

We demonstrate significant Fowler–Nordheim (FN) tunneling across Al/Al₂O₃/ZnO metal–insulator–semiconductor (MIS) and Ag/ZnO metal–semiconductor (MS) nanojunctions. The transport properties of ZnO nanostructures in the form of urchins and randomly distributed nanorods were investigated in terms of various conduction mechanism. The minimum voltage necessary for triggering Fowler–Nordheim (FN) tunneling, under forward biasing, was ～1.2 V and ～3.4 V; respectively, below which only direct tunneling and thermionic emission events were evident. Mediated through Al₂O₃ layer, the FN tunneling was more prominent across MIS junction than MS one. The weak FN tunneling across MS junction was owing to interfacial charge transfer process through the atomic scale gapping between adjacent nanostructures. The extent of such type of tunneling is found to be nanostructure morphology dependent and largely rely on the free electrons donated by the native donor defects in the crystal structure of ZnO. The significant FN tunneling across the MIS and MS junctions has a direct relevance in designing nanoscale field emission devices/components working at low voltage with high throughputs.     

Properties of photon tunneling through single-negative materials

The photon tunneling phenomena in the composite barriers of single-negative materials were analyzed. It was found that the tunneling through such a barrier shifts TE- and TM-polarization light waves laterally (parallel to the material interface) in two opposite directions, causing them to be divided into two waves after tunneling. This property could not be obtained with double-positive and (or) double-negative materials.     

Ultrafast dynamics of photoionized acetylene

Acetylene cations [HCCH](+) produced in the A(2)Σ(g)(+) state by extreme ultraviolet (XUV) photoionization are investigated theoretically, based on a mixed quantum-classical approach. We show that the decay of the A(2)Σ(g)(+) state occurs via both ultrafast isomerization and nonradiative electronic relaxation. We find a time scale for hydrogen migration and electronic decay of about 60 fs, in good agreement with recent XUV-pump/XUV-probe time-resolved experiments on the same system [Phys. Rev. Lett. 105, 263002 (2010)]. Moreover, we predict an efficient vibrational energy redistribution mechanism that quickly transfers excess energy from the isomerization coordinates to slower modes in a few hundred femtoseconds, leading to a partial regeneration of acetylenelike conformations.     

(CdZnTe,ZnSeTe)/ZnTe复合量子阱中激子隧穿过程

设计了(CdZnTe,ZnSeTe)/ZnTe复合量子阱结构,并用吸收光谱、室温光致发光谱和飞秒脉冲抽运-探测方法研究了该复合结构中的激子隧穿过程.分别测量了该结构中CdZnTe/ZnTe量子阱层和ZnSeTe/ZnTe量子阱层中激子衰减时间.观察到从CdZnTe/ZnTe量子阱层向ZnSeTe/ZnTe量子阱层的快速激子隧穿,隧穿时间为5.5ps. 关键词： (CdZnTe ZnSeTe)/ZnTe复合量子阱 激子 隧穿 抽运-探测     

Current-distance-voltage characteristics of electron tunneling through an electrochemical STM junction

We have studied the electron tunneling process through an electrochemical scanning tunneling microscopic (STM) junction formed by a gold tip and a gold electrode immersed in an inert NaClO₄ solution. Current-distance-voltage characteristics of the tunneling process are examined by simultaneous measurement of tunneling current, voltage, and distance. The results indicate that the tunneling voltage across the junction changes with tunneling distance; however, tunneling conductance is an inverse exponential function of distance over the entire investigated range of tunneling current, voltage, and distance. The results provide clear evidence for the validity of a one-dimensional tunneling model for the aqueous tunneling process. Implications of the observation are mentioned with regard to the distance-dependent STM imaging and the origin of a low tunneling barrier height.     

Single-molecule reaction and characterization by vibrational excitation

Controlled chemical reaction of single trans-2-butene molecules on the Pd(110) surface was realized by dosing tunneling electrons from the tip of a scanning tunneling microscope at 4.7 K. The reaction product was identified as a 1,3-butadiene molecule by inelastic electron tunneling spectroscopy. Threshold voltage for the reaction is approximately 365 mV, which coincides with the vibrational excitation of the C-H stretching mode. The reaction was ascertained to be caused by C-H bond dissociation by multiple vibrational excitations of the C-H stretching mode via inelastic electron tunneling process.     

Enhanced photon emission by field emission resonances and local surface plasmon in tunneling junction

We investigated the photon emission spectra on Ag (111) surface excited by tunneling electrons using a low temperature scanning tunneling microscope in ultrahigh vacuum. Characteristic plasmon modes were illustrated as a function of the bias voltage. The one electron excitation process was revealed by the linear relationship between the luminescence intensity and the tunneling current. Luminescence enhancement is observed in the tunneling regime for the relatively high bias voltages, as well as at the field emission resonance with bias voltage increased up to 9 V. Presence of a silver (Ag) nanoparticle in the tunneling junction results in an abnormally strong photon emission at the high field emission resonances, which is explained by the further enhancement due to coupling between the localized surface plasmon and the vacuum. The results are of potential value for applications where ultimate enhancement of photon emission is desired.     

Electron transport in granular amorphous silicon dioxide films with ferromagnetic nanoparticles placed in a magnetic field

Electron transport in amorphous silicon dioxide films with embedded nanoparticles (Co, Nb, Ta) was studied. The mean number of localized states in the interparticle tunneling channel was derived from the temperature dependence of conductivity for various grain concentrations under the assumption of the electron transport being governed by resonance tunneling in a chain of localized states between grains. To confirm the assumption of the inelastic character of tunneling, the dependences of the magnetoresistance on grain concentration, temperature, and magnetic field were studied. Accepting the single-orbital model, where the intergrain tunneling magnetoresistance is determined by s-s tunneling, it was found that the existence of weakly split localized states in the tunneling channel results in a lack of magnetoresistance saturation in strong magnetic fields. The combined effect of a decrease in the s-s tunneling coefficient and of growth in the probability of inelastic electron spin scattering with increasing length of the chain of localized states between particles in which the electron is tunneling accounts for the characteristic temperature-concentration dependences of the magnetoresistance. The experimental observation of these features provides an argument for the electron transport in a-SiO₂(Co,Nb,Ta) structures being governed by inelastic resonance tunneling through intergrain localized states.     

Photo-induced tunneling spectroscopy of ReS2: Dramatic increase of the quantum efficiency by chemical treatment

In this work we present a systematic study of the local photovoltaic properties of ReS₂, using a scanning tunneling microscope (STM). The tunneling junction of the STM was optically illuminated during the tunneling process. The phase sensitive detected photo-induced tunneling current (PITC) was studied as a function of wavelength and surface topography. In order to improve the performance of ReS₂ solar cells, the samples were treated with NaI/I₂ and EDTA solutions. Relative to the untreated sample, the EDTA-treated samples show an increase in the photo-induced tunneling current by a factor of 8–10 in the whole spectral range, the NaI/I-treated samples by 2–3. Two dimensional mapping of the PITC was performed on an atomic scale and compared to the surface topography.     

Three-dimensional scanning force/tunneling spectroscopy at room temperature

We simultaneously measured the force and tunneling current in three-dimensional (3D) space on the Si(111)-(7 × 7) surface using scanning force/tunneling microscopy at room temperature. The observables, the frequency shift and the time-averaged tunneling current were converted to the physical quantities of interest, i.e. the interaction force and the instantaneous tunneling current. Using the same tip, the local density of states (LDOS) was mapped on the same surface area at constant height by measuring the time-averaged tunneling current as a function of the bias voltage at every lateral position. LDOS images at negative sample voltages indicate that the tip apex is covered with Si atoms, which is consistent with the Si-Si covalent bonding mechanism for AFM imaging. A measurement technique for 3D force/current mapping and LDOS imaging on the equivalent surface area using the same tip was thus demonstrated.     

Effects of structural disorders on sequential tunneling in multiple quantum wells

Based on an isotropic random distribution model, the effects of structural disorders embedded in the barriers on the sequential electron tunneling in multiple quantum wells were studied at low temperatures. By using a sequential tunneling model [Stievenard et al., Appl. Phys. Lett. 61 (1992) 1582], the transmission coefficient through a single barrier was calculated using a finite-difference method and averaged over random configurations of disorders. To compute the tunneling current, a self-consistent calculation for the electronic states was performed, including the Hartree and exchange interactions and non-parabolic energy dispersion. Both disorder-assisted and disorder-impeded electron tunneling phenomena were found as a function of the activation energy. The effects of electric field, barrier width, and temperature were also studied. The predicted resonant disorder-assisted electron tunneling should be large enough to be observable at low temperatures in an experiment.     

弱耦合GaAs/AlGaAs/InGaAs双势阱隧穿结构的磁隧穿特性研究

研究了低温(15 K)条件下弱耦合GaAs/AlGaAs/InGaAs双势阱结构的纵向磁隧穿特性. 研究表明,器件在零偏压下处于共振状态. 通过分析不同偏压下的磁电导振荡曲线,可以得到双量子阱中的基态束缚能级随偏压的变化规律,从而可以确定隧穿电流峰对应的隧穿机制. 所得结果可为弱耦合双量子点器件的制备提供基础. 关键词： 双量子阱 隧穿结构 磁电导振荡