Size- and shape-dependent energetics of nanocrystal interfaces: experiment and simulation

We study the interface energetics of Ag nanocrystals on a H-passivated Si(111) surface by a transmission electron microscopy experiment and molecular dynamics simulations. The annealed nanocrystals are oriented with Ag(111)||Si(111). Azimuthally, epitaxy is preferred for nanocrystals with an interface larger than a coincident-site-lattice (CSL) cell. The equilibrium orientation, or interface energy minimum, depends on the interface size and shape. For interfaces approaching a CSL cell in size ( approximately 2 nm nanocrystals), fluctuations of a single atom at an interface can lead to large variations in nanocrystal orientations.     

Selected growth of cubic and hexagonal GaN epitaxial films on polar MgO(111)

Selected molecular beam epitaxy of zinc blende (111) or wurtzite (0001) GaN films on polar MgO(111) is achieved depending on whether N or Ga is deposited first. The cubic stacking is enabled by nitrogen-induced polar surface stabilization, which yields a metallic MgO(111)-(1 x 1)-ON surface. High-resolution transmission electron microscopy and density functional theory studies indicate that the atomically abrupt semiconducting GaN(111)/MgO(111) interface has a Mg-O-N-Ga stacking, where the N atom is bonded to O at a top site. This specific atomic arrangement at the interface allows the cubic stacking to more effectively screen the substrate and film electric dipole moment than the hexagonal stacking, thus stabilizing the zinc blende phase even though the wurtzite phase is the ground state in the bulk.     

Oxidation of low-index FeAl surfaces

The oxidation of the (100), (110) and (111) surfaces of the intermetallic compound FeAl has been investigated using LEED and XPS. On all three surfaces, oxidation at room temperature leads to the formation of an amorphous oxide film on top of an Al-depleted interlayer. The film growth can be divided into two regions of differing kinetics, i.e. the initial formation of a closed oxide film and a subsequent thickening. In the first region, the oxygen-uptake rate varies significantly with surface orientation, while in the thickening regime the uptake is the same for all surfaces. The maximum thickness as well as the composition of the oxide films were found to depend on the initial oxidation rate. At higher oxidation temperatures, ordered oxide films of around 5–8 Å in thickness are formed, very similar to those observed on NiAl. Photoemission spectra from these ordered phases showed evidence for Al atoms in two different chemical environments, i.e. the well-known oxide species in the interior of the film and an additional species present at the oxide/alloy interface.     

Layer-by-layer epitaxial growth of polar FeO(111) thin films on MgO(111)

We report on the structural properties of epitaxial FeO layers grown by molecular beam epitaxy on MgO(111). The successful stabilization of polar FeO films as thick as 16 monolayers (ML), obtained by deposition and subsequent oxidation of single Fe layers, is presented. FeO/MgO(111) thin films were chemically and structurally characterized using low-energy electron diffraction, Auger electron spectroscopy and conversion electron Mössbauer spectroscopy (CEMS). Detailed in situ CEMS measurements as a function of the film thickness demonstrated a size-effect-induced evolution of the hyperfine parameters, with the thickest film exhibiting the bulk-wüstite hyperfine pattern. Ex situ CEMS investigation confirmed existence of magnetic ordering of the wüstite thin film phase at liquid nitrogen temperature.     

Epitaxial growth and magnetic exchange anisotropy in Fe3O4<Emphasis Type="Bold">/NiO bilayers grown on MgO(001) and Al</Emphasis>2O3(0001)

Epitaxial Fe3O4/NiO bilayers were epitaxially grown on MgO(001) and Al2O3(0001) substrates to investigate the influence of the fully spin compensated (001) and the non-compensated (111) NiO interface planes between the ferromagnetic (F) and antiferromagnetic (AF) layers on the AF/F exchange coupling. Bilayers of different magnetite thicknesses and constant NiO thickness were investigated. The structural characterizations indicate a perfect epitaxy of the two layers for the both growth directions in the two Fe3O4/NiO/MgO(001) and NiO/Fe3O4/Al2O3(0001) systems. An epitaxial ferrimagnetic (Ni,Fe)Fe2O4 phase is observed at the AF/F interface when the NiO oxide is grown on the top of the Fe3O4 layer while a perfectly flat AF/F interface is observed in the Fe3O4/NiO/MgO(001) system exhibiting only a very slight interdiffusion. Magnetic measurements indicate a relative strong bias at 300 K for the bilayers grown on Al2O3(0001), which decreases with the inverse of the ferrimagnetic layer thickness as theoretically expected. On the contrary, a zero exchange biasing is observed at 300 K for the bilayers grown on MgO(001).     

Kinetic features of the oxide formation on {111} polar planes upon anode treatment of <Emphasis Type="Italic">n</Emphasis>-GaAs

The mechanism and kinetics of anode destruction of {111} polar planes of n-GaAs and morphological features of forming oxide films in the potentiostatic mode of polarization in weakly acid solutions of electrolytes have been studied. It has been found that anode polarization of the gallium plane (111) Ga provides the formation of a porous structure of both the single-crystal matrix and oxide film, which has a planar topology. In this case, the pore density is always commensurable with the surface dope concentration. In contrast to the gallium plane, the anode polarization of the arsenic plane \(\overline {\left( {111} \right)} \) As provides the tangential mechanism of destruction of the semiconductor matrix and the island-type morphology of the oxide. Equal crystallographic orientation of islands is determined by the directive action of the family of oxidized planes \(\left\{ {1\overline {11} } \right\}\) GaAs. However, regardless of the crystallographic orientation of the polar plane, the forming oxide is represented by polycrystalline As₂O₃ and amorphous Ga₂O₃.     

Tuning the growth orientation of epitaxial films by interface chemistry

The support of epitaxial films frequently determines their crystallographic orientation, which is of crucial importance for their properties. We report a novel way to alter the film orientation without changing the substrate. We show for the growth of CoO on the Ir(100) surface that, while the oxide grows in (111) orientation on the bare substrate, the orientation switches to (100) by introducing a single (or a few) monolayer(s) of Co between the oxide and substrate. This tunability of the orientation of epitaxial films by the appropriate choice of interface chemistry most likely is a general feature.     

Frozen low-spin interface in ultrathin Fe films on Cu(111)

In ultrathin film systems, it is a major challenge to understand how a thickness-driven phase transition proceeds along the cross-sectional direction of the films. We use ultrathin Fe films on Cu(111) as a prototype system to demonstrate how to obtain such information using an in situ scanning tunneling microscope and the surface magneto-optical Kerr effect. The magnetization depth profile of a thickness-driven low-spin to high-spin magnetic phase transition is deduced from the experimental data, which leads us to conclude that a low-spin Fe layer at the Fe/Cu interface stays live upon the phase transition. The magnetically live low-spin phase is believed to be induced by a frozen fcc Fe layer that survives a thickness-driven fcc-->bcc structural transition.     

Iron oxide thin film growth on Al2O3/NiAl(1 1 0)

The electronic structure and the growth morphology of iron oxide thin films were studied by means of Synchrotron Radiation Photoelectron Spectroscopy (SRPES) and Low Energy Electron Diffraction (LEED). A thin well-ordered alumina film on a NiAl(1 1 0) single crystal surface as a template for iron oxide growth was employed. Two different methods of iron oxide film preparation were applied. In the first attempt, iron deposited at room temperature was subsequently annealed in oxygen. Even though a whole layer of iron was oxidized, an expected long-range order was not achieved. The second attempt was to perform reactive deposition. For this reason iron was evaporated in oxygen ambient at elevated substrate temperature. This method turned out to be more efficient. Diffused but clear LEED patterns of six-fold symmetry indicating hexagonal surface atoms arrangement were observed. From the PES measurements, binding energies for Fe2p for grown iron oxide film were established as well as energy distribution curves for the valence band. Growth curves based on Fe3p core-level peak intensities for iron and iron oxide were plotted identifying type of film growth for both deposition methods. Based upon these results we have found evidence for interdiffusion in the interface between alumina and iron oxide at the early stages of growth. Further deposition led to formation of Fe₃O₄(1 1 1) (magnetite) overlayer. Moreover, the quality of the film could also be improved by long-time annealing at temperatures not exceeding 575 K. Higher annealing temperature caused disappearance of LEED pattern indicating loss of long-range ordering.     

Using polarity for engineering oxide nanostructures: structural phase diagram in free and supported MgO(111) ultrathin films

Using an ab initio total energy approach, we study the stability of free and Ag(111)-supported MgO(111) ultrathin films. We unravel a novel microscopic mechanism of stabilization of polar oxide orientations, based on a strong modification of the MgO structural phase diagram with respect to the bulk material. We predict that, at low thickness, films which are either unsupported or deposited on Ag(111) display a graphitelike Bk structure rather than the expected rocksalt one. Our results provide a consistent interpretation of recent experimental findings, exemplify the efficiency of this novel stabilization mechanism, and suggest new methods to engineer oxide nanostructures.     

A simple way to prepare monolayer films of the magnetite nanocrystal by the Langmuir-Blodgett-like technique

A simple preparation of water-soluble magnetite nanoparticles protected by sodium oleate has been demonstrated. The magnetite particles are nanocrystal, which are confirmed with XRD. This kind of magnetite nanocrystal can be changed into amphiphilic particles and steadily exist at the interface of water/toluene. The amphiphilic nanocrystal can form monolayer film on hydrophilic substrate by the Langmuir-Blodgett-like technique. The results of SEM indicate that the magnetite nanocrystal film is a uniform compact monolayer film that is composed of nanosized magnetite particles. We also proposed a possible mechanism for the formation of the nanocrystal monolayer film.     

Conductivity and band alignment of LaCrO_3/SrTiO_3(111) heterostructure

In this work,we investigate the electrical transport property and electronic structure of oxide heterostructure LaCrO_3/SrTiO_3(111).The interface grown under relatively low oxygen partial pressure is found to be metallic with a conducting critical thickness of 11 unit cells of LaCrO_3.This criticality is also observed by x-ray photoelectron spectroscopy,in which the Ti~(3+) signal intensity at the spectrum edge of the Ti-2p_(3/2) core level increases rapidly when the critical thickness is reached.The variations of the valence band offset and full width at half maximum of the core-level spectrum with LaCrO_3 thickness suggest that the built-in fields exist both in LaCrO_3 and in SrTiO_3.Two possible origins are proposed:the charge transfer from LaCrO_3 and the formation of a quantum well in SrTiO_3.Our results shed light on the understanding of the doping mechanism at the polar/non-polar oxide interface.Moreover,due to the interesting lattice and spin structure of LCO in the(111) direction,our work provides a basis for further exploring the novel topological quantum phenomena in this system.     

Study of the wurtzite zinc-blende mixed-structured GaAs nanocrystals grown on Si (111) substrates

The structure and growth mechanism of GaAs nanocrystals grown on Si (111) substrates by using the molecular beam epitaxy method have been studied using transmission electron microscopy. The isolated nanocrystals had hexangular shapes, with aspect ratio ～1 and high symmetry. The crystal structure of the GaAs nanocrystals contains a mixture of a stable state of zinc-blende and a metastable state of wurtzite. A number of thin wurtzite layers parallel to the Si (111) plane are introduced into the zinc-blende GaAs nanocrystals as stacking faults. Formation of partial dislocations near the GaAs/Si interface and the small difference in the Gibbs free energy between the zinc-blende and wurtzite structures could cause formation of wurtzite as stacking faults in the zinc-blende structure     

不同晶化工艺对非晶PZT纳米薄膜形核取向生长机理的影响

射频磁控溅射法室温下在Pt/Ti/SiO2/Si上制备非晶Pb(Zr048Ti052)O3薄膜，非晶PZT薄膜分别经常规炉退火(CFA)处理和快速热退火(RTA)处理晶化为（100），（111）不同择优取向的多晶薄膜. 采用x射线衍射测定了薄膜相组分、择优取向度；用原子力显微镜和压电响应力显微镜观察了薄膜表面形貌，以及对应区域由自发极化形成的铁电畴像，观察了不同取向薄膜的电畴分布特征. 结果表明，RTA晶化过程钙钛矿结构PZT结晶主要以PZT/Pt界面处的PtPb化合物为成核点异质形核并类似外延的结晶生长，沿界面结晶速率远大于垂直膜面结晶速率，而CFA晶化样品成核发生在膜内杂质缺陷处，以同质成核为主. 不同的成核机理导致了不同晶面择优取向生长. 关键词： PZT薄膜 结晶 形核 力显微技术     

Chemically abrupt interface between Ce oxide and Fe films

A chemically abrupt Fe/Ce oxide interface can be formed by initial oxidation of an Fe film followed by deposition of Ce metal. Once a Ce oxide layer is formed on top of Fe, it acts a passivation barrier for oxygen diffusion. Further deposition of Ce metal followed by its oxidation preserve the abrupt interface between Ce oxide and Fe films. The Fe and Ce oxidation states have been monitored at each stage using X-ray photoelectron spectroscopy.     

Epitaxial stabilization of ferromagnetism in the nanophase of FeGe

Epitaxial nanocrystals of FeGe have been stabilized on Ge(111). The nanocrystals assume a quasi-one-dimensional shape as they grow exclusively along the <110> direction of the Ge(111) substrate, culminating in a compressed monoclinic modification of FeGe. Whereas monoclinic FeGe is antiferromagnetic in the bulk, the nanowires are surprisingly strong ferromagnets below approximately 200 K with an average magnetic moment of 0.8 microB per Fe atom. Density functional calculations indicate an unusual stabilization mechanism for the observed ferromagnetism: lattice compression destabilizes the antiferromagnetic Peierls-like ground state observed in the bulk while increased p-d hybridization suppresses the magnetic moments and stabilizes ferromagnetism.     

Ab initio computer simulation of adsorption of a Fe monolayer on Si(111)

Computer simulation of adsorption of an Fe monolayer on Si(111) is carried out in the generalized gradient approximation in the density functional theory. It is shown that mixing of Fe and Si atoms followed by the replacement of Si atoms and the formation of a silicide-like film containing two types of domains (with the A8 and B8 structure) takes place in the course of deposition. Analysis of the atomic structure of this compound shows that the formation of this interface makes it possible to obtain an FeSi film (with the CsCl-type structure) as well as FeSi₂ film (with a CaF₂-type structure).     

Microstructure evolution of zircaloy-4 during Ne ion irradiation and annealing: An in situ TEM investigation

The microstructural evolution of zircaloy-4 was studied, including the amorphization and recrystallization of Zr（Fe, Cr）2 precipitates, and the density of dislocations under in situ Ne ion irradiation and post annealing. The results show that irradiation at a relatively high temperature and dose induces the formation of nanocrystals in pre-amorphized Zr（Fe, Cr）2 precipitates. The recrystallized nanocrystals also have the structure of hcp-Zr（Fe, Cr）2. The formation of the nanocrystals is thought to be the consequence of competition between atomistic disordering and the recrystallization of precipitates under ion irradiation. The free energy of the nanocrystal is lower than that of the amorphous state, which is another reason for the recrystallization of the precipitates. With increased annealing temperature, the density of the nanocrystals is increased. The dislocation density sharply decreases with the increase in the annealing temperature, and its size increases.     

Growth of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> films on the Si(111) surface covered by a thin SiO<Subscript>2</Subscript> layer

Magnetite polycrystalline films are grown by variously oxidizing a Fe film on the Si(111) surface covered by a thin (1.5 nm) SiO₂ layer. It is found that defects in the SiO₂ layer influence silicidation under heating of the Fe film. The high-temperature oxidation of the Fe film results in the formation of both Fe₃O₄ and iron monosilicide. However, the high-temperature deposition of Fe in an oxygen atmosphere leads to the growth of a compositionally uniform Fe₃O₄ film on the SiO₂ surface. It is found that such a synthesis method causes [311] texture to arise in the magnetite film, with the texture axis normal to the surface. The influence of the synthesis method on the magnetic properties of grown Fe₃O₄ films is studied. A high coercive force of Fe₃O₃ films grown by Fe film oxidation is related to their specific morphology and compositional nonuniformity.     

Controlled growth of complex polar oxide films with atomically precise molecular beam epitaxy

At heterointerfaces between complex oxides with polar discontinuity, the instability-induced electric field may drive electron redistribution, causing a dramatic change in the interfacial charge density. This results in the emergence of a rich diversity of exotic physical phenomena in these quasi-two-dimensional systems, which can be further tuned by an external field. To develop novel multifunctional electronic devices, it is essential to control the growth of polar oxide films and heterointerfaces with atomic precision. In this article, we review recent progress in control techniques for oxide film growth by molecular beam epitaxy (MBE). We emphasize the importance of tuning the microscopic surface structures of polar films for developing precise growth control techniques. Taking the polar SrTiO₃ (110) and (111) surfaces as examples, we show that, by keeping the surface reconstructed throughout MBE growth, high-quality layer-by-layer homoepitaxy can be realized. Because the stability of different reconstructions is determined by the surface cation concentration, the growth rate from the Sr/Ti evaporation source can be monitored in real time. A precise, automated control method is established by which insulating homoepitaxial SrTiO₃ (110) and (111) films can be obtained on doped metallic substrates. The films show atomically well-defined surfaces and high dielectric performance, which allows the surface carrier concentration to be tuned in the range of ~10¹³/cm². By applying the knowledge of microstructures from fundamental surface physics to film growth techniques, new opportunities are provided for material science and related research.