Fim observation of an iron deposited tungsten tip

An iron-deposited tungsten tip was observed with a field ion microscope. The observed features were classified into five types corresponding to the various substrate temperatures. Epitaxial growth was observed at substrate temperatures ranging from 100°C to 500°C, though the parallelism was not perfect. An epitaxially grown film was observed only on one side of a tip cap at a low temperature. The film spread to the entire tip cap at a higher temperature. Diffusivity of the deposited iron was estimated from the observed migration of the iron atoms. Images which implied the formation of an alloy of kon and tungsten were obtained at 550–660°C. When the substrate temperature exceeded 700°C, iron atoms were not observed on the tip cap. These sequential change corresponding to the substrate temperatures will be discussed in relation to the surface diffusion of the deposited iron and substrate tungsten atoms.     

Impact of Mo and Ce on growth of single-walled carbon nanotubes by chemical vapour deposition using MgO-supported Fe catalysts

A series of nine catalysts containing Ce/Fe and Mo/Fe at various loadings on MgO supports have been studied as catalysts for chemical vapour deposition (CVD) of single-walled carbon nanotubes (SWCNTs) using a methane carbon source. Our results show that the Ce/Fe system is very suitable as a catalyst that favours SWCNT growth, and we question the special importance that has been attributed to Mo as an additive to Fe-based catalysts for SWCNT growth, as it appears that Ce is equally effective. Our results indicate that dehydroaromatization (DHA) is not a defining step for the growth mechanism, as has been suggested for Mo/Fe systems previously, and show that Ce and Mo do not seriously perturb the well-known Fe/MgO system for growth of high quality SWCNT. Using Raman spectroscopy, we have shown that the Ce/Fe/MgO catalyst system favours growth of SWCNTs with a different distribution of chiralities compared to the analogous Mo/Fe/MgO system.     

Effect of adding W to Fe catalyst on the synthesis of SWCNTs by arc discharge

Combining iron (Fe) and tungsten (W) as a bimetallic catalyst, we synthesized high-yield single-wall carbon nanotubes (SWCNTs) of narrow diameter distribution by a hydrogen–argon arc discharge method. Raman spectra indicate that the diameters of SWCNTs prepared using the Fe–W catalysts are about 0.5 nm smaller than those using Fe catalyst alone. The transmission electron microscopy and X-ray diffraction studies show that the SWCNTs prepared by the bimetallic catalyst coexist with few graphite flakes and other amorphous carbon. At the W content of 2–4 at%, tungsten cannot be found in the SWCNT samples. Thus by using a simple two-step purification process, high-purity SWCNT samples can be obtained. We have demonstrated the growth mechanism for the high melting metal (such as W, Mo)–Fe catalyst synthesis of SWCNTs by the arc discharge method.     

Mechanism of single-walled carbon nanotube growth on natural magnesite

Fe K-edge X-ray absorption fine structure (XAFS) measurements were performed in order to elucidate the formation mechanism of single-walled carbon nanotubes (SWCNTs) grown on natural magnesite by pyrolyzing methane gas. It was clearly shown by XAFS analyses that iron metal fine particles, which were reduced from iron oxides by methane gas, worked as a catalyst for SWCNT growth. Structural characteristics of the initial iron state in the natural magnesite were also discussed.     

Single-walled carbon nanotube formation on iron oxide catalysts in diffusion flames

Single-walled carbon nanotubes (SWCNTs) are shown to grow rapidly on iron oxide catalysts on the fuel side of an inverse ethylene diffusion flame. The pathway of carbon in the flame is controlled by the flame structure, leading to formation of SWCNTs free of polycyclic aromatic hydrocarbons (PAH) or soot. By using a combination of oxygen-enrichment and fuel dilution, fuel oxidation is favored over pyrolysis, PAH growth, and subsequent soot formation. The inverse configuration of the flame prevents burnout of the SWCNTs while providing a long carbon-rich region for nanotube formation. Furthermore, flame structure is used to control oxidation of the catalyst particles. Iron sub-oxide catalysts are highly active toward SWCNT formation while Fe and Fe₂O₃ catalysts are less active. This can be understood by considering the effects of particle oxidation on the dissociative adsorption of gas-phase hydrocarbons. The optimum catalyst particle composition and flame conditions were determined in near real-time using a scanning mobility particle sizer (SMPS) to measure the catalyst and SWCNT size distributions. In addition, SMPS results were combined with flame velocity measurement to measure SWCNT growth rates. SWCNTs were found to grow at rates of over 100 μm/s.     

Interaction of iron atoms with the silicon surface coated with a native oxide layer

The solid-phase epitaxy of iron silicide on the Si(111) surface coated with a native oxide layer is studied by high-resolution photoelectron spectroscopy using synchrotron radiation and by atomic force microscopy. The iron deposition dose changes up to 1 nm, and the annealing temperature changes up to 500°C. At room temperature, the native oxide layer is shown to be impermeable to Fe atoms and an iron film grows on the sample surface. An increase in the annealing temperature to ～100°C results in a change in the film morphology, increasing its heterogeneity. As the annealing temperature increases to ～250°C, Fe and Si atoms diffuse through the oxide layer and undergo a solid-phase reaction. As a result, stable iron monosilicide ?-FeSi forms.     

超长定向含铁多壁碳纳米管阵列的制备

采用无模板化学气相沉积法,以二茂铁为催化剂,二甲苯为碳源,利用单温炉加热装置制备了定向碳纳米管阵列。运用扫描电子显微镜、透射电子显微镜、拉曼光谱和X射线衍射仪等对定向碳纳米管阵列的形貌、成分和物相进行细致的分析和表征。结果表明:制得的碳纳米管阵列具有良好的定向性和多壁管状结构,并且石墨化程度高;碳纳米管中除碳元素外,管中包含有少量以纳米颗粒和纳米线形式存在的铁及其化合物,主要成分是铁和碳化铁。结合碳纳米管的制备和透射电子显微镜分析表征结果,认为超长碳纳米管阵列的生长模式为底部生长方式,即经历催化剂分解、催化、成核、长大、中毒、凝聚成粒和连接成线的循环过程,正是由于碳源和催化剂的连续供应促成了碳纳米管阵列的快速定向生长。     

An investigation of the surface chemistry of methyl pyruvate on Cu(1 1 1)

The surface chemistry of an α-ketoester, methyl pyruvate, has been studied on a model Cu(1 1 1) single crystal surface. Monolayers of methyl pyruvate at 180 K consist predominately (ca. 66%) of a chemisorbed methyl pyruvate moiety, with its keto-carbonyl bonded to the surface in a η² configuration, this moiety desorbs intact at 365 K. The rest of the monolayer contains weakly adsorbed methyl pyruvate, which desorbs at 234 K, which interacts with the surface through the lone pair electrons of the oxygen atoms of the CO groups, adopting a η¹ configuration. Previous studies of simple ketones on model noble metal surfaces have only observed weakly bonded η¹ configurations. The observation of a strongly chemisorbed moiety in the present study is attributed to the activation of the keto-carbonyl by the electron withdrawing ester group. This behaviour is consistent with the homogeneous inorganic chemistry of ketones. Given both the formation of a η² bonded methyl pyruvate moiety on Cu(1 1 1) and the known activity of Cu as a selective hydrogenation catalyst, it is suggested that it maybe worthwhile considering the possibility of testing the effectiveness of chirally modified supported Cu as an enantioselective catalyst.     

Spin-polarized ion scattering spectroscopy of CCl4 adsorption on Fe(0 0 1) surfaces

The interaction of CCl₄ molecules with Fe(0 0 1) surfaces was investigated by spin-polarized ion scattering spectroscopy (SP-ISS). It was observed that CCl₄ molecules adsorb dissociatively on the surface at ambient temperature (290 K), and consequently, iron and chlorine were major surface constituents. It was found that the chlorine adatoms are located atop of iron atoms of the second surface layer (hollow sites of the surface). It is indicated that the spin state of iron atoms at the surface is not affected by exposure to a CCl₄ atmosphere, while almost no spin is induced in the chlorine adatoms. Similar behavior is observed in the spin states of iron and chlorine on an oxygen preadsorbed-Fe(0 0 1) surface. The difference in the spin states of iron and chlorine clarifies the local property of the incidence ion neutralization and element selectivity of SP-ISS in this CCl₄/Fe system.     

Formation of interfacial iron silicides on the oxidized silicon surface during solid-phase epitaxy

The early stages of iron silicide formation in the Fe/SiO _x /Si(100) ternary system during solid-phase epitaxy are studied by high-resolution (～100 meV) photoelectron spectroscopy using synchrotron radiation. The spectra of core and valence electrons taken after a number of isochronous heat treatments of the samples at 750°C are analyzed. It is found that the solid-phase reaction between Fe and Si atoms proceeds in the vicinity of the SiO _x /Si interface, which metal atoms reach when deposited on the sample surface at room temperature. Iron silicide starts forming at 60°C. Solid-phase synthesis is shown to proceed in two stages: the formation of the metastable FeSi interfacial phase with a CsCl-like structure and the formation of the stable β-FeSi₂ phase. During annealing, structural modification of the silicon oxide occurs, which shows up in the growth of the Si⁺⁴ peaks and attenuation of the Si⁺² peaks.     

Controlling the size and the activity of Fe particles for synthesis of carbon nanotubes

The properties of carbon nanotubes (CNTs) are controlled by their structure and morphology. Therefore, their selective synthesis, using catalytic chemical vapor deposition, requires precise control of a number of parameters including the size and activity of the catalyst nanoparticles. Previously, an environmental scanning transmission electron microscope (ESTEM) has been used to demonstrate that electron beam-induced decomposition (EBID) of Fe containing precursor molecules can be used to selectively deposit Fe catalyst nanoparticles that are active for CNT growth. We have extended these in situ ESTEM observations to further our understanding of the EBID parameters, such as deposition time, and substrate temperature, that control the size and placement of Fe catalyst particles for two precursors, namely diiron nonacarbonyl (Fe(2)(CO)(9)) and ferrocene (Fe(C(5)H(5))(2)). We found that the diameter of deposited particles increased with increasing deposition time. Electron energy-loss spectra, collected during deposition, show the incorporation of C in the Fe particles. The C content decreased as the substrate temperature was increased and was negligible at 100°C for Fe(2)(CO)(9). However, C and Fe were co-deposited at all temperatures (up to 450°C) when Fe(C(5)H(5))(2) was used as an iron source. After deposition, the substrate was heated to the CNT growth temperature in flowing hydrogen to remove the co-deposited C, which was an important step to activate the deposited Fe catalyst for the growth using acetylene. Our measurements revealed that the Fe nanoparticles fabricated from Fe(2)(CO)(9) had higher activity for CNT growth compared to the ones fabricated using Fe(C(5)H(5))(2). We also found that the co-deposited carbon could not be removed by heating in hydrogen in the case of Fe(C(5)H(5))(2). The particles deposited from Fe(C(5)H(5))(2) at 300°C to 450°C formed a core-shell structure with Fe surrounded by graphitic carbon. We speculate that the reduced activity for Fe(C(5)H(5))(2) is due to the C content in the deposit.     

An in-situ study of structure and magnetic properties of Fe films on the sulphur passivated Ge(100) surface at 150°C

Ultra-thin Fe films have been grown on the sulphur passivated Ge(100) surface at 150°C. The growth, structure and the magnetic properties of the thin films were studied with LEED, AES, angle resolved AES and in-situ magneto-optical Kerr effect measurements. For the first five monolayers of Fe, no long-range order was observed . However, angle resolved AES data suggest that local order occurs with a small fraction of the Fe atoms adsorbed on bcc sites. For thicker Fe films, the growth becomes ordered. A comparison of the present study and with a previous study of the growth of Fe on sputter cleaned Ge(100)(2×1) surface, shows that sulphur passivation effectively prevents Fe–Ge intermixing. During the Fe deposition process, most of the sulphur atoms migrate to the growing surface, thus acting as surfactants. The presence of sulphur at the surface also affects the growth and magnetic properties of the thin films.     

Diffusion and growth of nickel,iron and magnesium adatoms on the aluminum truncated octahedron: A molecular dynamics simulation

The structure of nickel (Ni), iron (Fe), and magnesium (Mg) adatoms on the aluminum (Al) truncated octahedron is studied using molecular dynamics and the analytic embedded atom method. First, the energy barriers of several typical diffusion processes of Ni, Fe, and Mg adatoms on the Al truncated octahedral cluster were calculated using the nudged elastic band method. The calculated energy barriers were found to be related to the surface energy and atomic radius of the adatom and substrate atom. The result shows that the incorporation of Ni and Fe atoms into Al core easily occurs, and the Mg atom should segregate at the surface of the Al cluster. Thus, the growth of Ni, Fe and Mg on the Al truncated octahedron with 1289 atoms was simulated at several temperatures. In the Ni–Al and Fe–Al cases, the core-shell structure was not obtained. For the Mg–Al system, a good Mg shell on the Al core was found at lower temperatures, and an almost perfect truncated octahedron with more Al shells emerged with an increase in temperature.     

Size-selected Ni catalyst islands for single-walled carbon nanotube arrays

Many properties of single-walled carbon nanotube (SWCNT) arrays are determined by the size and surface coverage of the metal catalyst islands from which they are nucleated. Methods using thermal fragmentation of continuous metal films frequently fail to produce size-uniform islands. Hybrid numerical simulations are used to propose a new approach to controlled self-assembly of Ni islands of the required size and surface coverage using tailored gas-phase generated nanocluster fluxes and adjusted surface temperatures. It is shown that a maximum surface coverage of 0.359 by 0.96–1.02 nm Ni catalyst islands can be achieved at a low surface temperature of 500 K. Optimized growth of Ni catalyst islands can lead to fabrication of size-uniform SWCNT arrays, suitable for numerous nanoelectronic applications. This approach is deterministic and is applicable to a range of nanoassemblies where high surface coverage and island size uniformity are required.

     

硫在Fe(111)面吸附的密度泛函研究

采用基于密度泛函理论的第一性原理方法,系统研究了不同覆盖度下硫在Fe（111）表面的吸附构型和吸附特性,计算并分析了硫在Fe（111）表面的吸附能、电荷密度、分波态密度、电荷布局、电子局域化函数等数据. 研究结果表明：S在Fe（111）面的H位吸附最稳定,并且吸附能随着覆盖度的增加而增加. 另外,电子态密度、电子局域化函数和布局分析表明Fe、S之间呈较弱的共价键,这种作用力主要是Fe的3d轨道和S的2p轨道杂化所贡献,而随着覆盖度的增加,Fe、S之间的作用力逐渐减弱,这可能是由于S原子之间的排斥力减弱了Fe、S之间的作用. S在Fe（111）、Fe（110）和Fe（100）这三个晶面上吸附情况的对比分析发现,S与Fe（111）表面的相互作用最强,Fe（100）面次之,而Fe（110）面最弱.     

ATLEED研究6H-SiC(0001)-(3×3)R30°重构表面

低能电子衍射(LEED)对6H-SiC(0001)-(3×3)R30°表面的研究结果表明,该表面有1/3单层的Si原子吸附在T4空位上与第一个SiC复合层中的三个Si原子键接,它们之间的垂直距离为0.171nm.通过对该表面10个非等价垂直入射衍射束的自动张量低能电子衍射(ATLEED)计算,得到“最佳结构”由于表面SiC复合层堆积顺序不同而产生的三种表面终止状态(surface termination)的混合比例为S1∶S2∶S3=15∶15∶70,理论计算与实验I-V曲线比较得到可靠性因子RVHT=0.165,RP=0.142,表明表面生长符合能量最小化的台阶生长机制.     

The structure and bonding of furan on Pd(111)

The structure and bonding of molecular furan, C₄H₄O, on Pd(111) has been investigated using density functional theory (DFT) calculations and the results compared with those of a recent experimental investigation using scanned-energy mode photoelectron diffraction (PhD). The DFT results confirm the orientation of the molecular plane to be essentially parallel to the surface and show a clear energetic preference for one of the two possible structures identified in the PhD study, namely that with the molecule centred over the hollow sites of the surface. Two slightly different geometries at the hollow sites are found to be essentially energetically equivalent; in both cases, one Pd surface atom bonds to two C atoms, while two other Pd atoms each bond to one C atom. These structures differ in that in one case the pair of C atoms bonding to a single Pd atom are both β-C (C atoms not bonded to O in the furan molecule), whereas in the second case this pair of C atoms comprises one β-C and one α-C (adjacent to the O atom in furan). In both structures the C–Pd bonding is accompanied by displacements of the H and O atoms away from the surface and out of the molecular plane and local C–Pd coordination consistent with a rehybridisation of the C bonding to sp³ character.     

Mössbauer analysis of Fe−N austenites

Analysis of transmission Mössbauer spectra for high nitrogen Fe?N austenites is done in terms of five iron environments dealing with first and second nearest neighbour shells. The similar analysis for Fe?C austenites which displays only three iron environments led to the conclusion that carbon atoms are ordered whereas nitrogen atoms are more randomly distributed.     

Iron-zirconium oxide catalysts for the hydrogenation of carbon monoxide: In situ studies by iron-57 Mössbauer spectroscopy

Some unsupported iron-zirconium oxide catalysts have been prepared by the calcination in air of precipitates containing 15 mole % iron. The catalyst formed at 500°C was shown by powder X-ray diffraction to consist of a non-equilibriated solid solution of iron(III) in a tetragonal or cubic zirconium dioxide structure whereas the catalyst formed at 1000°C was found to contain a zirconium-doped α-iron(III) oxide, or a magnetically ordered iron-zirconium oxide, in combination with an iron-containing monoclinic polymorph of zirconium dioxide. The⁵⁷Fe Mössbauer spectra recorded in situ following the pretreatment of the solids in nitrogen, carbon monoxide and hydrogen showed that little change is induced in the catalysts under such conditions. The⁵⁷Fe Mössbauer spectra also showed that the pretreated catalysts were unchanged by exposure to a 1:1 mixture of carbon monoxide and hydrogen at 270°C and 1 atmosphere pressure but were partially converted to iron carbide when used for the hydrogenation of carbon monoxide at 330°C and at 20 atmospheres pressure. The hydrocarbon product distribution showed Schulz-Flory α-values of 0.73 to 0.76 which were higher than the α-values obtained from pure iron catalysts which had been prepared and pretreated in a similar fashion. The⁵⁷Fe Mössbauer spectra and the results of the catalytic evaluation may be associated with an interaction between zirconium(IV) and the electron-rich atoms of the reactant carbon monoxide.     

Geometrical and chemical environment effects on the magnetism of stepped surfaces of V and V over Fe

By means of a real-space tight-binding calculation we obtain the spin-polarization of vicinal (1, 0, 2n?1) vanadium surface, and of one V layer deposited on vicinal (1, 0, 2n?1) iron surfaces (n=1,2,3,4). These geometries can be viewed as a staircase on the (001) surface with steps of monoatomic height andn-atoms width. Forn≤2, the pure V stepped surfaces do not show magnetism which is consistent with the absence of spinpolarization at the (101) surface. By contrast, magnetism is always obtained when a V monolayer is deposited on Fe stepped substrate, which is due to the hybridization of d-orbitals of V and Fe. Furthermore, in all the cases where magnetism is obtained, the surface V atoms at the edge of the step result to be antiferromagnetically coupled with all kink atoms. The effect of this local defect on the magnetic structure seems to remain when approaching the (001) surface (n→∞).