石墨上纳米尺度金岛的形成

用扫描隧道显微镜研究了真空蒸镀在高定向热解石墨上金岛的形成和形状．随着蒸镀量的增加及时间的推移，金原子在表面通过扩散而逐渐合并成越来越大的原子团，以至岛，甚至岛群．虽然几个纳米大小的原子团仍十分可动，但在蒸镀量大于20单层时的岛或岛群已十分稳定．研究发现，同一蒸镀量下，各个金岛具有非常接近的宽度和高度，用薄膜成核的圆柱状模型计算岛的宽度与高度之比表明，金岛非常接近热力学平衡状态．这些形状各异的通过生长而自组装形成的纳米尺度金岛可用来进行介观物理的研究 关键词：     

Kinetics of desorption from adsorbed layers formed by two-dimensional islands and by a two-dimensional gas

The kinetics of desorption from the adsorbed layers formed by two-dimensional (2D) islands and by a 2D gas of single atoms is considered for the case when the flux of adsorbate atoms from the gaseous phase reaches the surface. The effect of this flux on the desorption kinetics is analyzed. It is shown that this effect can be disregarded when the islands are in equilibrium with the surrounding 2D gas. The possibilities of determining the energy required for the removal of atoms from 2D islands to a vacuum are considered for the case of the absence of equilibrium in adsorbed layers. The conditions under which such a determination is possible are formulated, and the corresponding recommendations for experimental studies are worked out.     

Formation of ordered arrays of Ag nanowires and nanodots on Si(5 5 7) surface

The ordered arrays of Ag nanowires and nanodots have been grown in ultra-high vacuum on the Si(5 5 7) surface containing regular steps of three bilayer height. Formation of Ag nanostructures have been studied by scanning tunneling microscopy, low energy electron diffraction and Auger electron spectroscopy at room temperature. It was shown that a sample exposure in the vacuum before Ag growth affects the shape of the forming Ag islands. This effect is caused by oxygen adsorption on the silicon surface from the residual atmosphere in the vacuum chamber. When Ag is deposited on the clean silicon surface the islands, overlapping several (1 1 1) neighboring terraces, form. The arrays of silver nanowires elongated along steps and silver nanodots, arranged in lines parallel to the steps, can be formed on the Si(5 5 7) surface depending on the amount of adsorbed oxygen.     

Structures and chemistry of the annealed SrTiO3(001) surface

Surface structures and chemistry of the annealed SrTiO₃(001) have been studied by scanning tunneling microscopy and Auger electron spectroscopy. Our STM results show that islands arc formed on an originally smooth surface after annealing in ultra high vacuum at a temperature of 1300°C. Tunneling spectra show that the islands exhibit a different electronic structure than that of SrTiO₃, with an apparent bandgap of 6 eV rather than 3.2 eV. Auger electron spectroscopy indicates that annealing yields strontium enrichment at the surface. The nature of the islands and change of surface chemistry are discussed in terms of correlation of the STM and Auger results.     

Effect of oxygen on the stability of Ag islands on Si(111)-7 × 7

We have used scanning tunneling microscopy to probe the effect of oxygen exposure on an ensemble of Ag islands separated by a Ag wetting layer on Si(111)-7 × 7. Starting from a distribution dominated by islands that are 1 layer high (measured with respect to the wetting layer), coarsening in ultrahigh vacuum at room temperature leads to growth of 2-layer islands at the expense of 1-layer islands, which is expected. If the sample is exposed to oxygen, 3-layer islands are favored, which is unexpected. There is no evidence for oxygen adsorption on top of Ag islands, but there is clear evidence for adsorption in the wetting layer. Several possible explanations are considered.     

Reduction of Cu2O islands grown on a Cu(100) surface through vacuum annealing

The reduction of Cu2O islands grown on Cu(100) surfaces through vacuum annealing was visualized by an in situ ultrahigh vacuum transmission electron microscope. The shrinkage of the island followed a linear decay behavior. The complete reduction of the oxide islands leads to the formation of nanoindentations on the Cu surfaces. A simple phenomenological kinetic model based on the dissociation along the island perimeter suitably describes the reduction behavior of the surface oxide islands.     

Epitaxial growth of C<Subscript>60</Subscript> thin films on the Bi(0001)/Si(111) surface

The morphology and atomic structure of C₆₀ fullerene films on the Bi(0001)/Si(111)?7 × 7 surface with different coverages have been studied by scanning tunneling microscopy and spectroscopy and low-energy electron microscopy in high vacuum. It is shown that the most favorable sites for nucleation of C₆₀ islands are double steps and domain boundaries on the surface of epitaxial Bi film.     

Adsorption stage in the formation of Eu-Si(111) thin-film structures

The adsorption stage in the formation of the Eu-Si(111) interface has been studied within a broad temperature range by thermal and isothermal desorption spectroscopy, low-energy-electron diffraction, Auger electron spectroscopy, and the contact potential difference method. It is shown that the ordering of an adsorbed europium film is accompanied by silicon surface reconstruction throughout the coverage range studied, 0<θ≤1.8. This self-organized process is also shown to be thermally activated. Ordered adsorbed europium layers have been found to be made up of 2D islands, whose structure depends on the amount of the metal deposited on the surface. The energy required to remove atoms from an island to vacuum has been determined. This energy decreases with decreasing 2D lattice constant of the islands. This pattern of its variation is accounted for, in the final count, by the decrease of the number of the Si surface atoms not bound directly to Eu atoms.     

Graphene traps for cesium atoms

A significant increase in the surface concentration of cesium atoms intercalated under graphene islands on rhodium has been revealed when annealing the adsorbed layer in a ultrahigh vacuum. Heating leads to a decrease in the area of graphene islands due to the solution of carbon atoms in the metal bulk. At the same time, the edge carbon atoms in the islands, which are coupled with the surface by chemisorptive forces, prevent the leakage of the alkali metal from under the islands. This leads to the significant compression of cesium and to an increase in its surface concentration under the islands by a factor of almost 10. The desorption of cesium is observed only after the complete thermal destruction of graphene islands.     

蓝宝石基片的处理方法对ZnO薄膜生长行为的影响

采用反应射频磁控溅射方法,在经过不同方法处理的蓝宝石基片上,在同一条件下沉积了ZnO薄膜.利用原子力显微镜、X射线衍射、反射式高能电子衍射等分析技术,对基片和薄膜的结构、表面形貌进行了系统表征.研究结果显示,不同退火条件下的蓝宝石基片表面结构之间没有本质的差异,均为α-Al₂O₃ (001)晶面,但基片表面形貌的变化较大.在不同方法处理的蓝宝石基片上生长的ZnO薄膜均具有高c轴取向的织构特征,但薄膜的表面形貌差异较大.基片经真空退火处 关键词： ZnO薄膜 反应磁控溅射 基片处理 形貌分析     

Insulator-metal transition in a conservative system: An evidence for mobility coalescence in island silver films

Aging, which manifests itself as an irreversible increase in electrical resistance in island metal films is of considerable interest from both academic as well as applications point of view. Aging is attributed to various causes, oxidation of islands and mobility of islands followed by coalescence (mobility coalescence) being the main contenders. The effect of parameters like substrate temperature, substrate cleaning, residual gases in the vacuum chamber, ultrasonic vibration of the substrate, suggest that the mobility coalescence is responsible for the aging in island metal films. Electron microscopy studies show evidence for mobility of islands at high substrate temperatures. The comparison of aging data of island silver films deposited on glass substrates in ultra high vacuum and high vacuum suggests that the oxidation of islands, as being responsible for aging in these films, can be ruled out. Further, under certain conditions of deposition, island silver films exhibit a dramatic and drastic fall in electrical resistance, marking the insulator-metal transition. This interesting transition observed in a conservative system — after the stoppage of deposition of the film — is a clear evidence for mobility coalescence of islands even at room temperature. The sudden fall in resistance is preceded by fluctuations in resistance with time and the fluctuations are attributed to the making and breaking of the percolation path in the film.     

Formation of relaxed SiGe on the buffer consists of modified SiGe stacked layers by Si pre-intermixing

High-quality relaxed SiGe films on Si (0 0 1) have been demonstrated with a buffer layer containing modified SiGe (m-SiGe) islands in ultra-high vacuum chemical vapor deposition (UHV/CVD) system. The m-SiGe islands are smoothened by capping an appropriate amount of Si and the subsequent annealing for 10 min. This process leads to the formation of a smooth buffer layer with non-uniform Ge content. With the m-SiGe-dot multilayer as a buffer layer, the 500-nm-thick uniform Si_0.8Ge_0.2 layers were then grown. These m-SiGe islands can serve as effective nucleation centers for misfit dislocations to relax the SiGe overlayer. Surface roughness, strain relaxation, and crystalline quality of the relaxed SiGe overlayer were found to be a function of period's number of the m-SiGe-dot multilayer. By optimizing period number in the buffer, the relaxed Si_0.8Ge_0.2 film on the 10-period m-SiGe-dot multilayer was demonstrated to have a threading dislocation density of 2.0 × 10⁵ cm⁻² and a strain relaxation of 89%.     

Single layer gold islands at the interface between a self-assembled monolayer and the Au(111) substrate: A high-resolution STM study

Physical vapor deposition of gold onto a self-assembled monolayer (SAM) of octanethiol on Au(111) has been studied at the molecular level in ultra-high vacuum (UHV) using atomic-resolution scanning tunneling microscopy (STM). A specially prepared SAM with not only the usual etch pits but also co-existing phases and domain boundaries is used for the purpose of studying details of the nucleation process. Etch pits are found to be filled by deposited Au atoms. At the same time, preferential nucleation and growth of gold islands at intersections of different domains, as well as inside the domains of the less dense striped phase, is observed. We find no gold islands within the densely-packed (√3 × √3)R30° phase. High-resolution STM imaging shows that the SAM over the newly formed gold islands adopts the same structure as that in the immediate surroundings.     

Atomic structure of Ge quantum dots on the Si(001) surface

In situ morphological investigation of the “105” faceted Ge islands on the Si(001) surface (hut clusters) have been carried out using an ultra high vacuum instrument integrating a high resolution scanning tunnelling microscope and a molecular beam epitaxy vessel. Both species of hut clusters-pyramids and wedges-were found to have the same structure of the “105” facets which was visualized. Structures of vertexes of the pyramidal clusters and ridges of the wedge-shaped clusters were revealed as well and found to be different. This allowed us to propose a crystallographic model of the “105” facets as well as models of the atomic structure of both species of the hut clusters. An inference is made that transitions between the cluster shapes are impossible.     

Time evolution of adatom and vacancy clusters on Ag(110)

The time evolution of adatom and vacancy islands on an Ag(110) surface is studied by using a variable-temperature ultrahigh vacuum scanning tunneling microscope. The islands are created by Ar⁺ ion sputtering at low temperatures and at very low fluxes (0.004 ML s⁻¹). The area of both kinds of island decreases linearly as a function of time, at least for small islands. The experimental results are compared with kinetic Monte Carlo simulations.     

Formation of quasi-one-dimensional Cu2O structures by in situ oxidation of Cu(100)

Epitaxial Cu2O islands on a Cu(100) surface formed through oxidizing Cu(100) films at 600 degrees C in an ultrahigh vacuum transmission electron microscope were observed to undergo a shape transition from initially square shaped islands to elongated islands at a critical size of approximately 110 nm. Our experimental data on the elongation of Cu2O islands agree with the energetic calculations based on the balance between surface and interface energies and the elastic stress relaxation in the three-dimensional islands. We developed a kinetic model based on oxygen surface diffusion that fits well with the observed volume evolution of the Cu2O islands.     

Synthesis and characterization of palladium nanoislands on the silicon surface

Samples representing a system of Pd nanoparticles on a Si surface is obtained. The metal is deposited onto the substrate by thermal evaporation in vacuum. The system of nanoparticles is formed using a thin screen located at an angle to the substrate and by vacuum annealing of ultrathin continuous palladium films. The chemical composition of the deposited substance is studied by Auger electron spectroscopy. The size and topology of the islands on the surface is established by scanning electron microscopy.     

Formation of Ge islands from a Ge layer on Si substrate during post-growth annealing

We have deposited a 12 nm thick Ge layer on Si(1 0 0) held at 200 °C by thermal evaporation under high vacuum conditions. Upon subsequent thermal annealing in vacuum, self-assembled growth of nanostructural Ge islands on the Ge layer occurred. Atomic force microscopy (AFM) and grazing incidence small-angle X-ray scattering (GISAXS) were used to characterize such layers. GISAXS measurements evidenced the formation of cylinder shaped structures upon annealing at 700 °C, which was confirmed by AFM measurements with a very sharp tip. A Ge mass transport from the layer to the islands was inferred by X-ray reflectivity and an activation energy of 0.40 ± 0.10 eV for such a process was calculated.     

Photoemission studies of the surfactant-aided growth of Ge on Te-terminated Si(100)

The interactions of Ge adatoms with a Si(100) surface terminated by an ordered layer of Te have been studied in detail using XPS, SXPS, STM and LEED. It has been demonstrated that the Te layer has a surfactant action on the growth mode of the Ge in that the two dimensional growth regime is extended to at least 200 Å and the Te is seen to segregate to the growing Ge surface. The surface reconstruction of the Ge layer changes from (1 × 1) in the initial stages to (2 × 2) as growth proceeds and the surface population of Te is reduced. SXPS line shape analysis has indicated that the initial stages of Ge incorporation are characterised by the formation of small islands above those surface Si sites not fully coordinated with Te. Continued growth of such islands is, however, restricted due to their high surface free energy with respect to the surrounding Te-terminated areas. Ge atoms therefore site-exchange with Te atoms in bridge sites, thus becoming incorporated onto the Si lattice and displacing the Te to bridge sites on the growing surface. In this manner islanding is prevented and two-dimensional growth continues beyond the critical thickness. No evidence is seen for any significant incorporation of the Te within the growing Ge layer.     

Individual bit island reversal and switching field distribution in perpendicular magnetic bit patterned media

The switching of single bit magnetic islands in bit patterned media (BPM) for two cases with 10 times difference in coercivity, as well as the switching field distribution (SFD) of the islands, has been studied using magnetic force microscopy (MFM) measurements. The intrinsic SFD is measured to be ∼9-11% of the remanence coercivity (H_cr), which contributes only ∼20-50% of the total SFD broadening (∼23-41% of H_cr). High resolution MFM observations clearly showed the influence of surrounding islands on the switching behaviour and switching fields of individual bit islands, resulting in significant contributions in SFD broadening due to non-intrinsic dipolar interactions. It was further observed that single magnetic islands could be switched within a very narrow switching field range as small as 4 Oe, which indicates very sharp and uniform switching for each individual island of BPM.