Effet de l'oxydation en surface et en volume sur les spectres de pertes caractéristiques d'énergie d'électrons dans les couches minces de titane

Transmission and reflection electron energy loss spectra, together with electronic diffraction patterns and Auger spectra have been registered on evaporated titanium thin films. The optical, crystallographic and chemical properties for the bulk and the surface of the films are compared. The 25 eV energy loss observed in the spectra of the low-energy electrons seems to give evidence for the formation of a superficial oxide layer. This assumption is confirmed by the transmission energy loss spectrum of pure rutile which is also presented.     

Coexistence of rocksalt and wurtzite structure in nanosized CoO films

Cobalt oxide (CoO) films epitaxially grown on Ir(100) in (111) orientation were investigated by means of quantitative low-energy electron diffraction and scanning tunneling microscopy. We find with high crystallographic precision that in the bulk of the films the rocksalt structure prevails while near the surface there is a switch towards the wurtzite structure. As a consequence, nanosized CoO cannot be considered as a single structural phase. The film surfaces prove to be metallic, apparently connected with polarity compensation.     

Oberflächenstrukturen von Silber-Aufdampfschichten

Using low-energy electron diffraction (LEED) technique surface structures of thin epitaxial silver films have been investigated. The surface of films, evaporated onto freshly cleaved mica sheets is found to be the undisturbed (111)-plane of silver, all crystallites of the film are well ordered within 1?. Two different superstructures caused by oxygen adsorption have been observed: One at room temperature, the other at 400? C with a unit mesh of 3 and √3 times the interatomic distance of silver, respectively.     

Atomic structures by direct transform of diffraction patterns

We propose all the diffraction patterns can be directly transformed to provide three-dimensional atomic structures for the system studied. Depending on the scattering process, either the holography or Patterson transform scheme is used. For diffraction patterns which are generated from a localized emitter source or dominated by an inelastic-scattering feature like core-level photoelectron or low-energy Kikuchi electron, holography transform is needed. On the other hand, for diffraction patterns which were dominated by elastic-scattering, like grazing-incidence X-ray diffraction, electron correlated thermal diffuse scattering or low-energy electron diffraction curves, Patterson transform is needed. To prove our point, high-fidelity and artifact-free three-dimensional atomic structures obtained by transform of low-energy Kikuchi electron patterns and low-energy electron diffraction curves are presented. The future of these direct methods by transforming diffraction patterns will be discussed.     

Effect of surface ZnO coatings on oxidation and thermal stability of zinc films

The effect of thin ZnO coatings grown on Zn films on further oxidation and thermal stability of Zn films deposited on Mo(110) substrate was in situ investigated under ultrahigh vacuum by photoelectron spectrometries and low-energy electron diffraction. The results indicated that ZnO layers formed by oxidizing Zn films had at least a thickness of 3–5 monolayers. Further oxidation of Zn films was confined by as-formed ZnO coatings due to a surface passivation. It was of advantage to explain the difficulty in growing low oxygen-deficient ZnO films. The surface ZnO coatings strongly enhanced the thermal stability of Zn films, which was useful for understanding the underlying application of Zn/ZnO materials, such as Zn/ZnO nanocables with Zn core and ZnO shell.     

Magnetic and magneto-optical properties of Co/Nb-codoped TiO2 films deposited by gas flow sputtering

Titanium dioxide thin films codoped with Co and Nb (Co/Nb-codoped TiO₂ films) were fabricated by a low-energy deposition process, gas flow sputtering. No metallic Co or Nb phase was detected by X-ray diffraction and X-ray photoelectron spectroscopy, suggesting that the Co and Nb ions have the oxidation states 2+ and 5+, respectively. The films show a relatively large Faraday rotation and magnetic circular dichroism on the order of 10³ deg/cm.     

Electron beam damage of chemisorbed surface species: A tunneling spectroscopy study

Electron beam damage is a problem for low-energy electron diffraction, Auger electron spectroscopy, and electron microscopy. We have used tunneling spectroscopy to study the damage caused by 30 keV incident electrons on chemisorbed monolayer films of hexanoic acid, 2,4-hexadienoic acid, and benzoic acid. Our results on monolayer films are compared to existing work on bulk samples. Damage cross sections are similar to bulk values; molecules with more delocalized electrons are more resistant to damage. In contrast to bulk results, however, we find little if any conjugation or cross linking after irradiation.     

Low-energy electron diffraction study of the surface structures of adsorbed amino acid monolayers and ordered films deposited on copper crystal surfaces

Monolayer structures and ordered multilayer films of several amino acids on single-crystal substrates were studied using low-energy electron diffraction. At monolayer coverage, ordered layers of glycine, alanine, D- and L-tryptophan were observed on both Cu(100) and Cu(111). With increasing film thickness, ordered multilayer structures of the tryptophans were also seen on both substrates. The monolayer structures are considered in terms of molecular size and packing. The optically isomeric D- and L-tryptophans yielded surface structures which were related by mirror inversion. Ordered multilayer films, apparently unlike the bulk crystals, were obtained only for tryptophan. Electron beam damage was serious for both glycine and alanine multilayer films while tryptophan films remained unaffected in agreement with previously observed stabilization by conjugated electron systems.     

Specific features of the formation of ytterbium films on the Si(111) surface at room temperature

The processes accompanying the formation of ytterbium films on the Si(111) surface at room temperature are investigated by the contact potential difference method, Auger electron spectroscopy, low-energy electron diffraction, and thermal desorption spectroscopy. It is shown that the grown metal films are uniform in thickness and that Si atoms virtually do not dissolve in the films. The atoms of the silicon substrate can diffuse in limited amounts into the Yb metal film only when the surface is bombarded by high-energy primary electron beams employed in Auger electron spectroscopy. The results obtained permit the conclusion that the previously observed oscillations of the work function in Yb-Si(111) thin-film structures cannot originate from dissolution of silicon atoms in the ytterbium film.     

Growth of β-MnO2 films on TiO2(110) by oxygen plasma assisted molecular beam epitaxy

We have used oxygen plasma assisted MBE to grow epitaxial films of pyrolusite (β-MnO₂) on TiO₂(110) for thicknesses of one to six bilayers (BL). We define a bilayer to be a layer of Mn and lattice O and an adjacent layer of bridging O within the rutile structure. The resulting surfaces have been characterized in situ by reflection high-energy electron diffraction, low-energy electron diffraction, X-ray photoelectron spectroscopy and diffraction, and atomic force microscopy. Well-ordered, pseudomorphic overlayers form for substrate temperatures between 400 and 500°C. Mn–Ti intermixing occurs over the time scale of film growth (1 BL/min) for substrate temperatures in excess of 500°C. Films grown at 400–500°C exhibit island growth, whereas intermixed films grown at temperatures of 500–600°C are more laminar. 1 BL films grown at 450°C are more laminar than multilayer films grown at the same temperature, and form a well-ordered surface cation layer of Mn on the rutile structure with at most 10% indiffusion to the second cation layer.     

Morphology and composition of Au films on Si(100)

Spot profile analysis low-energy electron diffraction, low-energy ion scattering and Auger electron spectroscopy were employed to study the morphology and composition of Au films on Si(100). After annealing, two distinct surface reconstructions were observed: a two-domain c(8×2) phase and a four-domain incommensurate (5×3.2)R5.7° phase. During the transition from the c(8×2) to the (5×3.2)R5.7° phase, the subsurface composition changes drastically from Au-rich to Si-rich, whereas the outermost layer composition remains almost constant (about 65 at.% Au). Detailed information concerning the domain structure for the two phases is subtracted from the profiles of the LEED spots.     

Interaction of thin silicon layers with the (0001) surface of rare-earth metals

The electronic and crystalline structures of the systems formed upon deposition of silicon layers onto the Gd(0001) and Dy(0001) surfaces of single-crystal films annealed subsequently at T=450–500°C have been studied by low-energy electron diffraction (LEED) and also by the Auger electron and angle-resolved photoelectron spectroscopy of the valence band and the Si(2p) core level. It is shown that the systems thus produced can be described as starting single-crystal films of Gd and Dy, with 3D islands of the silicides of these metals on the surface of the corresponding metalfillms.     

Effect of Pretreatment of TaN Substrates on Atomic Layer Deposition Growth of Ru Thin Films

The polycrystalline ruthenium films are grown on TaN substrates by atomic layer deposition （ALD） using bis（cyclopentadienyl） ruthenium [RuCp2] and oxygen as ruthenium precursor and reactant respectively at a deposition temperature of 330℃. The low-energy Ar ion bombardment and Ru pre-deposition are performed to the underlying TaN substrates before ALD process in order to improve the Ru nucleation. X-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscopy and atomic force microscopy are carried out to characterize the properties of ALD Ru films. The results show that the nucleation density of Ru films with Ar^＋ bombardment to the underlying TaN substrates is much higher than that of the ones without any pretreatment. The possible reasons are discussed.     

锂离子注入对V2O5薄膜光吸收的影响

采用真空蒸发经热处理制备了Ｖ２Ｏ５薄膜,使用二电极恒流法从１Ｍ／ＬＬｉＣｌＯ４的ＰＣ电解质溶液向Ｖ２Ｏ５薄膜注入锂离子,形成ＬｉｘＶ２Ｏ５（０≤ｘ≤０．５４）,测量了Ｖ２Ｏ５薄膜近垂直反射和透射光谱,计算了光吸收系数。Ｘ射线衍射分析表明薄膜为微晶结构。吸收系数与光子能量关系曲线中存在两个不同的变化区域,光子能量较高部分,（αｈν）１２与ｈν有线性关系;较低部分,吸收光谱存在一个尾巴。这两个区域的分界能量取决于电子和锂离子的注入量。研究结果表明Ｖ２Ｏ５薄膜阳极电致变色起源于吸收边缘的移动,而阴极电致变色则来源于小极化子的吸收     

Electronic states of moiré modulated Cu films

We examined by low-energy electron diffraction and scanning tunneling microscopy the surface of thin Cu films on Pt(111). The Cu/Pt lattice mismatch induces a moiré modulation for films from 3 to about 10?ML thickness. We used angle-resolved photoemission spectroscopy to examine the effects of this structural modulation on the electronic states of the system. A series of hexagonal- and trigonal-like constant energy contours is found in the proximity of the Cu(111) zone boundaries. These electronic patterns are generated by Cu sp-quantum well state replicas, originating from multiple points of the reciprocal lattice associated with the moiré superstructure. Layer-dependent strain relaxation and hybridization with the substrate bands concur to determine the dispersion and energy position of the Cu Shockley surface state.     

YCo2: intrinsic magnetic surface of a paramagnetic bulk material

Here we report on results of a spin-resolved photoelectron spectroscopic (SRPES) study of YCo2 thin films (150 A-thick) grown on a W(110) substrate. The films were prepared by co-deposition of stoichiometric amounts of Y and Co onto a clean W surface followed by thermal annealing leading to (2x2) overstructure with respect to W(110) in the low-energy electron diffraction pattern indicated formation of a structurally ordered YCo2(111) surface. While no clear spin asymmetry was observed for bulk-sensitive SRPES data taken at hnu=1253.6 eV, the more surface-sensitive SRPES data obtained at hnu=21.2 eV photon energy revealed a clear spin-asymmetry probing the validity of the recent theoretical prediction.     

Initial stages of formation of a Yb-Si(111) interface

The initial stages of formation of an Yb-Si(111) interface are investigated by several methods: thermal desorption spectroscopy, atomic beam modulation, and low-energy electron diffraction. The structure of the adsorbed films and ytterbium silicide films is analyzed over a wide range of surface coverage ratios, along with the desorption kinetics of Yb atoms. The desorption activation energies of Yb atoms are measured for 3×2, 5×1, and 2×1 submonolayer structures. The temperature interval in which ytterbium silicide decomposes and the activation energy of this process are determined. It is shown that the Yb-Si(111) phase interface evolves by a mechanism similar to the Stransky-Krastanov mechanism. Fiz. Tverd. Tela (St. Petersburg) 39, 256–263 (February 1997)     

High quality PdTe_2 thin films grown by molecular beam epitaxy

PdTe_2,a member of layered transition metal dichalcogenides(TMDs),has aroused significant research interest due to the coexistence of superconductivity and type-II Dirac fermions.It provides a promising platform to explore the interplay between superconducting quasiparticles and Dirac fermions.Moreover,PdTe_2 has also been used as a substrate for monolayer antimonene growth.Here in this paper,we report the epitaxial growth of high quality PdTe_2 films on bilayer graphene/SiC(0001)by molecular beam epitaxy(MBE).Atomically thin films are characterized by scanning tunneling microscopy(STM),X-ray photoemission spectroscopy(XPS),low-energy electron diffraction(LEED),and Raman spectroscopy.The band structure of 6-layer PdTe_2 film is measured by angle-resolved photoemission spectroscopy(ARPES).Moreover,our air exposure experiments show excellent chemical stability of epitaxial PdTe_2 film.High-quality PdTe_2 films provide opportunities to build antimonene/PdTe_2 heterostructure in ultrahigh vacuum for future applications in electronic and optoelectronic nanodevices.     

Structural manipulation of ultrathin organic films on metal surfaces: the case of decane thiol/Au(111)

3 (CH₂)₉]SH) have been adsorbed on Au(111) single crystals both via vacuum deposition and from an ethanolic solution. The epitaxial structure of the ultrathin organic films has been identified at room temperature via low-energy electron diffraction to be c(4×2)R30° for the solution grown film and rectangular c(23×) for the vacuum deposited film. These structures correspond to molecules adsorbed on the surface with their carbon chains flat on the surface (vacuum deposited) and nearly perpendicular (solution grown). It is demonstrated that this orientation can be changed reversibly in vacuum via either annealing the films or exposing them to additional gas. Received: 7 February 1997/Accepted: 27 May 1997     

The growth of Ag films on a TiO2(110)-(1×1) surface

We report here the growth of Ag film and its thermal stability on the TiO₂(1 1 0)-(1×1) surface using combination techniques of low-energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At a surface temperature as low as 125 K, a 2D growth of Ag films seems to occur for submonolayer coverages up to ∼0.8 ML. Annealing of low temperature grown Ag films to 500 K for coverage of 1–2.4 ML would result in the formation of metastable Ag layers with rest of Ag forming 3D needle-like islands on top of this Ag film.