Large Bond Energy Research Articles

Synthesis of a novel polysiloxane as an inhibitor in water-based drilling fluids and an assessment of the inhibition mechanism

In oil and gas well engineering, shale wellbore stability poses significant challenges. Developing shale inhibitors with high inhibition and resistance to elevated temperatures is crucial to address wellbore stability in deep wells. In this study, an amphoteric polysiloxane known as SC-POTD was synthesized as a heat-resistant shale inhibitor for water-based drilling fluid. The inhibition properties were evaluated using linear swelling, rolling recovery, and rheological tests, and the results were compared with the properties of potassium chloride and commercial polyamines. Various analysis methods were conducted to understand the interaction mechanism between the clay particles and polymer molecules, including X-ray diffraction test, Zeta potential determination, particle size analysis, contact angle measurement, and scanning electron microscope observation. SC-POTD with an amphoteric structure can effectively adsorb on the clay surface through electrostatic force, compressing the electric double layer of clay and forming a hydrophobic layer. Furthermore, SC-POTD can embed in the crystal interlayer, exchange cations and discharge interlayer water, effectively inhibiting hydration. With the flexibility of the Si-O backbone and the small steric hindrance of methyl side groups, SC-POTD may have a better affinity with bentonite containing siloxane tetrahedrons, and cooperate with the quaternary ammonium group to enhance clay interlayer interaction. In addition, the temperature resistance of SC-POTD can be improved by the siloxane backbone with larger bond energy. The results indicated that modified polysiloxane may find a promising application as heat-resistant drilling fluid additives.

Impact of 100 MeV high-energy proton irradiation on β-Ga2O3 solar-blind photodetector: Oxygen vacancies formation and resistance switching effect

β-Ga2O3 based solar-blind photodetectors have strong radiation hardness and great potential applications in Earth's space environment due to the large bandgap and high bond energy. In this work, we investigated the photoelectric properties influence of β-Ga2O3 photodetector irradiated by 100 MeV high-energy protons which are the primary components in the inner belt of the Van Allen radiation belts where solar-blind photodetectors mainly worked. After proton irradiation, due to the formation of more oxygen vacancies and their migration driven by bias at the metal/semiconductor interface, transportation of carriers transforms with electron tunneling conduction for low-resistance state and thermionic emission for high resistance state. As a result, the current–voltage curves of β-Ga2O3 solar-blind photodetectors exhibit apparent hysteresis loops. The photoresponsivity of β-Ga2O3 photodetectors slightly increases from 1.2 × 103 to 1.4 × 103 A/W after irradiation, and the photoresponse speed becomes faster at a negative voltage while slower at positive voltage. The results reveal the effects of high-energy proton irradiation on β-Ga2O3 solar-blind photodetectors and provide a basis for the study of their use in a radiation harsh environment.

Effect of Co3+ Doping on Crystal Structure and Electrochemical Properties of Spinel LiMn2O4 Cathode Material

To inhibit the influence of structural distortion caused by the Jahn–Teller effect on LiMn2O4 (LMO), the spinel LMO doped with cobalt ions is prepared by a simple solid-state calcining method. The effects of different cobalt doping amounts on the structure and electrochemical properties of LMO are investigated. The XRD refinement results show that Co3+ successfully enters the lattice of LMO, and its spinel structure is not changed. Due to the smaller radius of Co3+ and the larger bond energy, the unit cell of the LMO material shrinks, and the spinel structure is more stable. The initial capacity of the material decreases with the increase of Co doping content, but the cycle performance is significantly improved. Impressively, when Co doped at x = 0.04, the initial discharge specific capacity of the LMO-0.04 sample is 115.5 mAh·g–1 at 0.1C, and the capacity retention rate is still 81.04% after 500 cycles. The cycle times are much higher than that of the pure LMO sample. Additionally, the diffusion coefficient of Li+ calculated by the Randles–Sevcik equation shows that the doping of Co3+ significantly improves the mobility of Li+ compared with pure LMO. Hence, Co3+ doping can effectively inhibit the structural distortion caused by the Jahn–Teller effect and improve the electrochemical performance of LMO.

C(sp3)–F Bond Transformation of Perfluoroalkyl Compounds Mediated by Visible-Light Photocatalysis: Spin-Center Shifts and Radical/Polar Crossover Processes via Anionic Intermediates

AbstractDue to its large bond energy, precisely controllable C–F bond activation is a significant challenge in organic synthesis. A single C(sp3)–F bond transformation of perfluoroalkyl groups is particularly desirable to supply functionalized perfluoroalkyl compounds offering properties that are potentially useful in pharmaceutical and materials chemistry. Recently, the single defluorinative transformation of perfluoroalkyl compounds has been developed via visible-light photocatalysis. Herein, we summarize this field via two main topics. Topic 1 covers the transformations of C(sp3)–F bonds in either perfluoroalkylarenes or perfluoroalkane carbonyl compounds via a defluorinative spin-center shift in the radical anion intermediates. Topic 2 addresses the defluorinative transformations of α-trifluoromethyl alkenes to give gem-difluoroalkenes via a radical/polar crossover process.1 Introduction2 C(sp3)–F Transformations via Defluorinative Spin-Center Shifts3 C(sp3)–F Transformations via a Radical/Polar Crossover Process4 Conclusions

Chemically coupled electrocatalyst for nitrogen reduction to ammonia

The efficient electrochemical production of ammonia (NH3) is a major subject of research for both, academic and industrial pursuits. The NN triple bond has large bond energy and eventually, the sluggish kinetics make the electrochemical nitrogen (N2) reduction to the NH3 process elusive. Electrochemical N2 fixation requires effective and simultaneous adsorption and activation of N2 molecules on the surface of the electrocatalyst. To drive the N2 reduction to NH3, a chemically coupled hematite-rGO electrocatalyst has been synthesized by a redox hydrothermal technique. The hematite phase formation was confirmed by the presence of diffraction peaks (012), (104), (110), (113), (024), (116), (214), (300) corresponding to the rhombohedral structure of α-Fe2O3. The chemical coupling of the hematite-rGO was revealed by the increasing ID/IG ratio (from 1.01 to 1.30) in Raman Spectrum and presence of peak at 568 cm−1 corresponding to FeOC bond in the FTIR spectrum, upon hematite coupling with the rGO. Hematite-rGO exhibits selective electrocatalytic N2 reduction to NH3 with the yield rate of 7.77 µg h−1 mg−1 at −0.1 V and Faradic efficiency (FE) of 2.28% at −0.1 V under ambient conditions.

Engineering approach toward catalyst design for solar photocatalytic CO 2 reduction: A critical review

Solar-driven converting CO2 to value-added chemicals can not only address the ever-growing energy crisis, but also simultaneously mitigate CO2 emission. Although much progress has been made in the last decade, it still remains great challenge to achieve the efficient reduction of CO2 with desirable productivity and high product selectivity because CO2 is a thermodynamically stable and inert molecule with large bond energy. Design and synthesis of efficient catalyst play a pivotal role in promoting the activity and selectivity of photocatalytic CO2 reduction. Apart from the active sites engineering, manipulation of the charge separation and light harvesting efficiency or prolonging the lifetime of photogenerated carriers also greatly matters. Consequently, this review summarizes recent advances in photocatalyst design for CO2 conversion from two major aspects, namely active sites engineering and carriers lifespan prolonging. Firstly, we sort out the fundamental principles and merits of artificial photosynthesis in order to provide readers with a current snapshot of this rapidly developed field. Subsequently, various catalyst engineering strategies, including doping, alloying, Z-scheme structure construction, are discussed in detail. Finally, some challenging issues as well as insights into the future development of artificial photosynthesis are presented.

Study on the mechanism of auto-oxidation of Jatropha biodiesel and the oxidative cleavage of C[sbnd]C bond

Jatropha biodiesel was obtained according to the continuous preparation process which included vapor esterification - transesterification - methanol steam distillation. Accelerated oxidation of small Jatropha biodiesel was obtained by the Rancimat method. GC–MS and liquid phase micro-extraction were used to study and analyze the components in the oxidation process of Jatropha curcas biodiesel. The electronic effects of the related reactants and products were calculated by density functional theory, followed by the deduction of the related chemical reaction paths. Experimental investigation shows that methyl linoleate is the main factor affecting the oxidation stability of the Jatropha biodiesel. The main volatile products at the initial stages of the oxidation of methyl linoleate are hexanal, methyl octanoate, styrene, and 2-heptenal. The cis/trans-3-octyl-oxiranyl octanoic acid methyl ester (18.03% yield) is produced by the reaction of peroxy acid and methyl oleate during the oxidation of methyl oleate. The hydrogen extraction reaction is difficult to occur, and the oxidation reaction energy barrier is relatively high due to the relatively large bond energy of the CH bond in the methyl stearate molecule. In this manuscript, the auto-oxidation mechanism of the biodiesel fatty acid methyl esters at the initial stage of oxidation, the path of oxidative cleavage of the CC bond of Jatropha biodiesel and the formation process of ethylene oxide structure are obtained through DFT calculation and analysis of the oxidation products.

Vacancy-enabled N2 activation for ammonia synthesis on an Ni-loaded catalyst

Ammonia (NH3) is pivotal to the fertilizer industry and one of the most commonly produced chemicals1. The direct use of atmospheric nitrogen (N2) had been challenging, owing to its large bond energy (945 kilojoules per mole)2,3, until the development of the Haber-Bosch process. Subsequently, many strategies have been explored to reduce the activation barrier of the N≡N bond and make the process more efficient. These include using alkali and alkaline earth metal oxides as promoters to boost the performance of traditional iron- and ruthenium-based catalysts4-6 via electron transfer from the promoters to the antibonding bonds of N2 through transition metals7,8. An electride support further lowers the activation barrier because its low work function and high electron density enhance electron transfer to transition metals9,10. This strategy has facilitated ammonia synthesis from N2 dissociation11 and enabled catalytic operation under mild conditions; however, it requires the use of ruthenium, which is expensive. Alternatively, it has been shown that nitrides containing surface nitrogen vacancies can activate N2 (refs. 12-15). Here we report that nickel-loaded lanthanum nitride (LaN) enables stable and highly efficient ammonia synthesis, owing to a dual-site mechanism that avoids commonly encountered scaling relations. Kinetic and isotope-labelling experiments, as well as density functional theory calculations, confirm that nitrogen vacancies are generated on LaN with low formation energy, and efficiently bind and activate N2. In addition, the nickel metal loaded onto the nitride dissociates H2. The use of distinct sites for activating the two reactants, and the synergy between them, results in the nickel-loaded LaN catalyst exhibiting an activity that far exceeds that of more conventional cobalt- and nickel-based catalysts, and that is comparable to that of ruthenium-based catalysts. Our results illustrate the potential of using vacancy sites in reaction cycles, and introduce a design concept for catalysts for ammonia synthesis, using naturallyabundant elements.

Photocatalytic CO2 conversion: What can we learn from conventional COx hydrogenation?

Solar-driven reduction of CO2 into fuels/feedstocks is a promising strategy for addressing energy and CO2 emission issues. Despite great research efforts, it still remains a grand challenge to achieve efficient and highly selective reduction of CO2 owing to the large bond energy of CO2 and the diversity of reduction products. In addition to the control of light harvesting and charge transfer like photocatalytic water splitting, the design of catalytically active sites is highly important to promote CO2 reduction activity and selectivity (e.g., C-C coupling). In fact, we can learn a lot from conventional CO2 hydrogenation and syngas conversion in terms of active site design. In this article, we demonstrate how to design catalytically active sites for efficient and highly selective photocatalytic reduction of CO2 by sorting out the rules from the existing research on conventional COx hydrogenation, with a focus on enhancing C[double bond, length as m-dash]O activation and C-C coupling to form value-added products. This article aims to highlight the challenges in the field of photocatalytic CO2 conversion and the connection of photocatalysis with conventional catalytic systems, providing the readers the opportunities to join the research.

(Rg = He ∼ Rn, n = 1–4): In quest of the potential trapping ability of the aromatic ring

AbstractA new series of divalent boron‐rare gas cations (Rg = He ∼ Rn, n = 1–4) have been predicted theoretically at the B3LYP, MP2, and CCSD(T) levels to present the structures, stability, charge distributions, bond natures, and aromaticity. The RgB bond energies are quite large for heavy rare gases and increase with the size of the Rg atom. Because of steric hindrance new Rg atoms introduced to the B4 ring will weaken the RgB bond. Thus in the RgB bond has the largest binding energy 90–100 kcal/mol. p‐ has a slightly shorter RgB bond length and a larger bond energy than o‐ . NBO and AIM analyses indicate that for the heavy Rg atoms Ar ∼ Rn the BRg bonds have character of typical covalent bonds. The energy decomposition analysis shows that the σ‐donation from rare gases to the boron ring is the major contribution to the RgB bonding. Adaptive natural density partitioning and nuclear‐independent chemical shift analyses suggest that both and have obvious aromaticity.

Sensing mechanism for a fluorescent off–on chemosensor for cyanide anion

In this article, the sensing mechanism of cyanide anion chemosensor 2-((2-phenyl-2H-1,2,3-triazol-4-yl)methylene)malononitrile (M1) has been investigated through the density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The theoretical results demonstrate that the reaction barrier of 13.02kcal/mol means a favorable response speed of the chemosensor M1 for cyanide anion. Cyanide anion attacks C=C double bond and hinders the ICT process from the malononitrile moiety to the fluorophore phenyl ring. The high viscosity of DMSO restrains the twisting of the group, inhibits the formation of the ICT state in the first excited state. Due to weak ICT character, the nucleophilic addition product shows the dramatic “off–on” fluorescence enhancement. Meanwhile, intramolecular charge transfer (ICT) mechanism accounts for how different solvents influence the fluorescence spectra. That is, more obvious ICT character of product in EtOH causes fluorescence quenching. The “reaction-based” recognition mode and large bond energy between M1 and cyanide anion minimize the interference by other anions, such as F−, AcO−. Thus, the chemosensor M1 has a high selectivity for cyanide.

Molecularly Tuning the Radicaloid N-H···O═C Hydrogen Bond.

Substituent effects on the open shell N-H···O═C hydrogen-bond has never been reported. This study examines how 12 functional groups composed of electron donating groups (EDG), halogen atoms and electron withdrawing groups (EWG) affect the N-H···O═C hydrogen-bond properties in a six-membered cyclic model system of O═C(Y)-CH═C(X)N-H. It is found that group effects on this open shell H-bonding system are significant and have predictive trends when X = H and Y is varied. When Y is an EDG, the N-H···O═C hydrogen-bond is strengthened; and when Y is an EWG, the bond is weakened; whereas the variation in electronic properties of X group do not exhibit a significant impact upon the hydrogen bond strength. The structural impact of the stronger N-H···O═C hydrogen-bond are (1) shorter H and O distance, r(H···O) and (2) a longer N-H bond length, r(NH). The stronger N-H···O═C hydrogen-bond also acts to pull the H and O in toward one another which has an effect on the bond angles. Our findings show that there is a linear relationship between hydrogen-bond angle and N-H···O═C hydrogen-bond energy in this unusual H-bonding system. In addition, there is a linear correlation of the r(H···O) and the hydrogen bond energy. A short r(H···O) distance corresponds to a large hydrogen bond energy when Y is varied. The observed trends and findings have been validated using three different methods (UB3LYP, M06-2X, and UMP2) with two different basis sets.

Evidence for Emergent Chemical Bonding in Au+−Rg Complexes (Rg = Ne, Ar, Kr, and Xe)

Evidence is presented that there is a clear covalent component in the bonding of Au+ to Kr and Au+ to Xe, with some evidence that there may be such bonding between Au+ and Ar; for Au+ and Ne, there is no such evidence, and the bonding seems to be entirely physical. A model potential analysis shows that when all attractive inductive and dispersive terms out to R-8 are properly included in the Au+-Ne case, with an Ae(-bR) Born-Mayer repulsive term, essentially all the bonding in Au+-Ne can be rationalized by physical attraction alone. This is consistent with a natural bond order (NBO) analysis of the Au+-Ne ab initio wavefunctions, which shows the charge on Au+ to be very close to 1.0. In contrast, similar model potential and NBO analyses show quite clearly that physical interactions alone cannot account for the large bond energy values for the Au+-Kr and Au+-Xe complexes and are consistent with covalent contributions to the Au+-Kr and Au+-Xe interactions. Au+-Ar is seen to lie on the borderline between these two limits. In performing the model potential analyses, high-level ab initio calculations are employed [CCSD(T) energies, extrapolated to the complete basis set limit], to obtain reliable values of Re, De and omegae as input. A comparison of the gold-Xe bond distances in several solid-state Au(I, II and III) oxidation-state complex ions, containing "ligand" Xe atoms, prepared by Seppelt and co-workers, with that of the "free" Au+-Xe gas-phase ion is made, and a discussion of the trends is presented.

Competition between agostic WCH2+ and HWCH+: A joint experimental and theoretical study

We present both theoretical and experimental photodissociation results on the products of the methane dehydrogenation by W+ in the gas phase. We show that the reaction may lead to two isomers: whereas only the methylidenetungsten WCH2+ had been proposed, we show that the hydridomethylidynetungsten HWCH+ can also be formed. Both density functional and highly correlated ab initio quantum chemical calculations have been performed using a relativistic core potential for W+. Spin–orbit couplings have been evaluated semiempirically. We found the HWCH+ and the WCH2+ isomers to be nearly degenerate, the latter structure exhibiting a strong agostic distortion. Photodissociation of the mass selected [W,C,2H]+ product of the reaction has been carried out in a Fourier transform ion cyclotron resonance mass spectrometer. The H+WCH+ channel has been observed as the major photofragmentation channel and a photodissociation threshold of 2.5±0.1 eV has been derived. This low-energy value is in good agreement with the thermodynamic threshold determined theoretically. These results suggest a very large bond energy associated with the triple WC bond in WCH+ (about 158 kcal/mol).

Characteristic Changes of Bond Energies for Gas-Phase Cluster Ions of Halide Ions with Methane and Chloromethanes

Gas-phase equilibria for clustering reactions of both halide ions (X-) with methane and chloride ions with chloromethanes (CH4-mClm) were measured with a pulsed electron-beam high-pressure mass spectrometer. The bond energies were found to show irregular decreases for F-(CH4)n, with n = 6 and 8, for Cl-(CH3Cl)n, with n = 2, 4, and 6, and for Cl-(CH2Cl2)n, with n = 2 and 4. These even numbers indicate that the core ions are preferably solvated by the ligands with these n values. The theoretical calculation revealed that the cluster ion Cl-(CCl4) has the structure of [Cl...ClCCl3]- rather than Cl-...Cl3CCl. The unexpectedly large bond energy for Cl-(CCl4 ) (13.4 kcal/mol) is due to the charge dispersal in the complex [Cl...ClCCl3]-.

NUCLEATION AND GROWTH OF Cu/Ni(100): A VARIABLE TEMPERATURE STM STUDY

A detailed study of the nucleation and growth of Cu on Ni(100) as a function of substrate temperature and deposition rate by variable temperature STM is reported. By the quantitative analysis of the saturation island density as a function of temperature and flux for submonolayer coverages, we have deduced the migration barrier (0.35 eV), the dimer bond energy (0.46 eV) as well the sizes of the the critical nuclei. Because the dimer bond energy is large with respect to the migration barrier, a well-defined transition from a critical nucleus of 1 to 3 has been observed, as expected from the adsorption site geometry on square lattices. The large dimer bond energy of Cu on Ni(100) is one of the physical reasons for the recently uncovered strain relief mechanism via internal {111} faceting. The substantially increased island density on mono- and bilayer copper films on Ni(100) with respect to multilayer films might also be attributed to the enhanced lateral bonding.

Ab initio molecular-orbital calculation for C70 and seven isomers of C80.

We investigate stable structures and electronic properties for isolated ${\mathrm{C}}_{70}$ and nonisolated ${\mathrm{C}}_{80}$, by performing an ab initio molecular-orbital calculation based on a nonlocal density-functional formalism and a Harris-functional approximation. We first show the stable structure for ${\mathrm{C}}_{70}$ and give the explanation for the experimental inconsistency in structure by revealing the existence of a metastable structure. Concerning the seven isomers of ${\mathrm{C}}_{80}$ satisfying an isolated pentagon rule, it is found that the isomer with nearly ${\mathrm{D}}_{2}$ symmetry (${\mathrm{D}}_{2\mathrm{\ensuremath{-}}}$${\mathrm{C}}_{80}$) has a large bond energy and highest occupied molecular-orbital--lowest unoccupied molecular-orbital (HOMO-LUMO) energy gap which are comparable to those for isolated higher fullerenes. Therefore we propose that ${\mathrm{D}}_{2\mathrm{\ensuremath{-}}}$${\mathrm{C}}_{80}$ should be detectable in an experiment. Since ${\mathrm{C}}_{80}$ does not belong to the sequence of fullerenes with magic numbers, we discuss the problem of whether it is really unstable or not from the viewpoint of the electronic properties. Further, we consider the structures of ${\mathrm{C}}_{80}$ ions and show the great stability of the ${\mathrm{C}}_{80}^{6\mathrm{\ensuremath{-}}}$ with nearly ${\mathit{I}}_{\mathit{h}}$ symmetry which has a larger HOMO-LUMO gap than both ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$.

The electronic and geometric structures of the early transition metal cations MCH +, MCH 2+ and MCH 3+

The electronic and geometric structures of several transition metal cations MCH 3 +, MCH 2 + and MCH +, where M is Sc, Ti, V and Cr, have been studied by ab initio methods (MCSCF and MCSCF+1+2). The calculated lengths of the metal-carbon single, double and triple bonds are somewhat insensitive to the metal, having average values of 2.19±0.06 Å, 1.96±0.04 Å and 1.76±0.01 Å respectively. This is to be contrasted with the calculated bond strengths which vary from 46-18 kcal mol −in the MCH 3 + series; from 68-39 kcal mol −1 in the MCH 2 + series; and from 96-71 kcal mol −1 in the MCH + series. In all cases the bond energy decreases as one goes from Sc + to Cr +. The trend in the calculated bond strengths/calculated bond lengths for this series of molecules is interesting in that the Sc compounds always have the larger bond length as well as the larger bond energy, a situation not normally found in molecules composed entirely of main group elements.

Low temperature reactions at Si/metal interfaces; What is going on at the interfaces?

Si-metal contacts play one of the most important roles in Si-device or integrated circuit technology represented by IC, LSI and VLSI. The role of the contacts is to interconnect individual devices (transistors, diodes and so on) in a Si chip and connect them as a whole to the outer circuit. These contacts are numerous in the chip and can easily total more than one million. Therefore, the establishment of criteria for providing stable and reproducible electrical (ohmic or Schottky barrier) contacts has been a key problem in Si-LSI technology. Si is a typical covalent semiconductor with a large bond energy (≈eV/ bond) and consequently its melting point is high (≈1440 °C). However,crucial to the Si-metal contact problem is that when Si is in contact with metal it readily reacts with it at a temperature as low as ≈100 °C. This interfacial reaction induces large scale transport of materials across the Si/metal interface to give rise to several interesting phenomena. Examples of this are thick (≈1000 Å) SiO 2 growth for a short time (≈10 min) due to Si-Au reaction and uniform silicide layer formation at Si/Pd, Pt, Ni interfaces. Interesting point is that since the Si-Si covalent bonding is very strong without the presence of such effect of metal to weaken the Si bond adjacent to the metal, the above contact or interfacial reactions can rarely occur. As possible mechanism of this bond-weakening, several models have been proposed. One of them is by the present author postulating electronic screening of Coulomb interaction responsible for the covalent bonding due to mobile electrons in the metal films. In the present article, this “screening model” is critically discussed and compared with other models on the basis of experiments done by several laboratories on microscopic or atomic-scale observation of initial stages of the Si-Au and Si-Pd reactions by both electron and ion scattering spectroscopies. In addition new usage of the channeling effect of MeV He + ions is demonstrated to be a powerful tool for interface and surface studies.

1857. Silicon-gold potential barriers made by localized sputtering method: Z Majewski, Bull Acad Polon Sci Sér Sci Tech, 15 (4), 1967, 343–346 ( in English)

Si-metal contacts play one of the most important roles in Si-device or integrated circuit technology represented by IC, LSI and VLSI. The role of the contacts is to interconnect individual devices (transistors, diodes and so on) in a Si chip and connect them as a whole to the outer circuit. These contacts are numerous in the chip and can easily total more than one million. Therefore, the establishment of criteria for providing stable and reproducible electrical (ohmic or Schottky barrier) contacts has been a key problem in Si-LSI technology. Si is a typical covalent semiconductor with a large bond energy (≈eV/ bond) and consequently its melting point is high (≈1440 °C). However,crucial to the Si-metal contact problem is that when Si is in contact with metal it readily reacts with it at a temperature as low as ≈100 °C. This interfacial reaction induces large scale transport of materials across the Si/metal interface to give rise to several interesting phenomena. Examples of this are thick (≈1000 Å) SiO2 growth for a short time (≈10 min) due to Si-Au reaction and uniform silicide layer formation at Si/Pd, Pt, Ni interfaces. Interesting point is that since the Si-Si covalent bonding is very strong without the presence of such effect of metal to weaken the Si bond adjacent to the metal, the above contact or interfacial reactions can rarely occur. As possible mechanism of this bond-weakening, several models have been proposed. One of them is by the present author postulating electronic screening of Coulomb interaction responsible for the covalent bonding due to mobile electrons in the metal films. In the present article, this “screening model” is critically discussed and compared with other models on the basis of experiments done by several laboratories on microscopic or atomic-scale observation of initial stages of the Si-Au and Si-Pd reactions by both electron and ion scattering spectroscopies. In addition new usage of the channeling effect of MeV He+ ions is demonstrated to be a powerful tool for interface and surface studies.