Zinc blende GaAsSb nanowires grown by molecular beam epitaxy

We report the growth of GaAsSb nanowires (NWs) on GaAs(111)B substrates by Au-assisted molecular beam epitaxy. The structural characteristics of the GaAsSb NWs have been investigated in detail. Their Sb mole fraction was found to be about?25%. Their crystal structure was found to be pure zinc blende (ZB), in contrast to the wurtzite structure observed in GaAs NWs grown under similar conditions. The ZB GaAsSb NWs exhibit rotational twins around their [111]B growth axis, with twin-free segments as long as 500?nm. The total volumes of GaAsSb segments with twinned and un-twinned orientations, respectively, were found to be equal by x-ray diffraction analysis of NW ensembles.     

The effect of ferromagnetic Mn-delta-doped layer on the emission properties of GaAsSb/GaAs and InGaAs/GaAsSb/GaAs heterostructures

We have studied the emission characteristics and circularly polarized electroluminescence of light-emitting diodes based on heterostructures with a single (GaAs/GaAsSb/GaAs) or two-layer (GaAs/InGaAs/GaAsSb/GaAs) quantum well (QW) and a Mn-delta-doped layer in the GaAs barrier. The ferromagnetic effect of the delta-layer of Mn on the spin polarization of carriers in QWs based on type-II heterostructures has been observed and studied for the first time. The observed phenomena are described using a model of the exchange interaction of Mn ions in the barrier and holes in the QW.     

Au(111)电极表面溴离子吸附诱导的表面应力的理论研究

以Br~-为例,应用格子气模型,建立了阴离子吸附层对Au(111)电极表面应力贡献的统计热力学理论,计算了吸附层Br~-间的相互作用能及表面应力的贡献.计算结果表明,总的表面应力是压缩性的;在高覆盖度区域,表面应力与覆盖度近似呈直线关系;在表面吸附层应力的多种物理起源中,通过底物的分子间作用力有着决定性的贡献,揭示了分子的吸附能间接地起着重要作用.     

Effects of polycrystalline cu substrate on graphene growth by chemical vapor deposition

Chemical vapor deposition of graphene on Cu often employs polycrystalline Cu substrates with diverse facets, grain boundaries (GBs), annealing twins, and rough sites. Using scanning electron microscopy (SEM), electron-backscatter diffraction (EBSD), and Raman spectroscopy on graphene and Cu, we find that Cu substrate crystallography affects graphene growth more than facet roughness. We determine that (111) containing facets produce pristine monolayer graphene with higher growth rate than (100) containing facets, especially Cu(100). The number of graphene defects and nucleation sites appears Cu facet invariant at growth temperatures above 900 °C. Engineering Cu to have (111) surfaces will cause monolayer, uniform graphene growth.     

Determining polytype composition of silicon carbide films by UV ellipsometry

A universal ellipsometric model is proposed that describes the optical properties of silicon carbide (SiC) films grown on Si substrates by the method of atomic substitution due to a chemical reaction between the substrate and gaseous carbon monoxide. According to the proposed three-layer model, Si concentration decreases in a stepwise manner from the substrate to SiC film surface. The ellipsometric curves of SiC/Si(111), SiC/Si(100), and SiC/Si(110) samples grown under otherwise identical conditions have been measured in a 1.35–9.25 eV range using a VUV-VASE (J.A. Woollam Co.) ellipsometer with a rotating analyzer. Processing of the obtained spectra in the framework of the proposed model allowed the polytype composition of SiC films to be determined for the first time. It is established that SiC grown on Si(111) is predominantly cubic, while SiC on Si(110) is predominantly hexagonal (with cubic polytype admixture) and SiC on Si(100) has a mixed polytype composition.     

Photoreflectance spectroscopy for the study of GaAsSb/InP heterojunction bipolar transistors

This work is devoted to a photoreflectance (PR) study of single GaAsSb layers grown pseudomorphically on InP. Such antimonide alloys included in the base of heterojunction bipolar transistors (HBT) are expected to lead to high cut-off frequency transistors. However, many important parameters such as the nature of the InP/GaAsSb interface, the conduction band offset and the Fermi level pinning at GaAsSb surface are not well known in this system. In this study, they are determined from optical measurements performed between 8 and 300 K. Unusual evolution of the photovoltage as a function of temperature is reported in GaAsSb. Finally, a PR study of a complete HBT structure shows the potential of this technique to non-destructively characterize complex epitaxial layers, and further on to study fully processed devices under operation.     

Different growth behaviors of ambient pressure chemical vapor deposition graphene on Ni(111) and Ni films: A scanning tunneling microscopy study

Graphene growth on the same metal substrate with different crystal morphologies, such as single crystalline and polycrystalline, may involve different mechanisms. We deal with this issue by preparing graphene on single crystal Ni(111) and on ∼300 nm thick Ni films on SiO₂ using an ambient pressure chemical vapor deposition (APCVD) method, and analyze the different growth behaviors for different growth parameters by atomically-resolved scanning tunneling microscopy (STM) and complementary macroscopic analysis methods. Interestingly, we obtained monolayer graphene on Ni(111), and multilayer graphene on Ni films under the same growth conditions. Based on the experimental results, it is proposed that the graphene growth on Ni(111) is strongly templated by the Ni(111) lattice due to the strong Ni-C interactions, leading to monolayer graphene growth. Multilayer graphene flakes formed on polycrystalline Ni films are usually stacked with deviations from the Bernal stacking type and show small rotations among the carbon layers. Considering the different substrate features, the inevitable grain boundaries on polycrystalline Ni films are considered to serve as the growth fronts for bilayer and even multilayer graphene.      

Wetting and adhesion at Mg/MgO interfaces

The wetting of (100), (110), and (111) MgO single crystals by molten Mg was studied in a flowing Ar atmosphere at 1073 K using an improved sessile drop method. The results show that the contact angle and work of adhesion are mildly dependent on MgO substrate orientation. However, the work of separation calculated by density functional theory shows a sequence of (100) < (110) < (111). This discrepancy was explained by a model considering the possible effects of evaporation and deposition of Mg at the substrate surface on the interfacial adhesion. For Mg on the clean MgO surfaces, the adhesion consists primarily of Mg–O ionic bonds with some Mg–Mg metallic bonds. In addition, a small amount of covalent bonds are present in the metal slab adjacent to the Mg/MgO (110) and Mg/MgO (111) interfaces. The different work of separation at three interfaces depends on the strength and amounts of Mg–O and Mg–Mg bonds as well as on the surface free energy of respective planes. For the MgO surfaces pre-deposited by a monolayer of Mg atoms, the interfaces are dominated by the Mg–Mg bonds, thus mitigating the effect of substrate orientation on the wettability and adhesion.     

All-(111) surface silicon nanowires: selective functionalization for biosensing applications

We demonstrate the utilization of selective functionalization of carbon-silicon (C-Si) alkyl and alkenyl monolayers covalently linked to all-(111) surface silicon nanowire (Si-NW) biosensors. Terminal amine groups on the functional monolayer surfaces were used for conjugation of biotin n-hydroxysuccinimide ester. The selective functionalization is demonstrated by contact angle, X-ray photoelectron spectroscopy (XPS), and high-resolution scanning electron microscopy (HRSEM) of 5 nm diameter thiolated Au nanoparticles linked with streptavidin and conjugated to the biotinylated all-(111) surface Si-NWs. Electrical measurements of monolayer passivated Si-NWs show improved device behavior and performance. Furthermore, an analytical model is presented to demonstrate the improvement in detection sensitivity of the alkyl and alkenyl passivated all-(111) Si-NW biosensors compared to conventional nanowire biosensor geometries and silicon dioxide passivation layers as well as interface design and electrical biasing guidelines for depletion-mode sensors.     

Electron excitation in the interaction of slow ions and electrons with metals and monolayer graphite on Ni(111) surfaces

We report on the experimental energy distributions of electrons emitted in the interaction of slow ions and electrons with aluminium and with monolayer graphite deposited on a Ni(111) surface. Measurements on the Al surface clarify the role of plasmon decay in secondary electron emission. In contrast with theoretical calculations, our experiments indicate that the electron collision cascade inside the solid produced by electrons excited by plasmon decay does not contribute significantly to electron emission.The energy distribution of electrons emitted in the interaction of He⁺ ions with monolayer graphite reveal a high-energy feature that can be attributed to the autoionization decay of excitonic states populated by electron promotion in close He-C encounters.     

Ab initio explanation of tunneling line shapes for the kondo impurity state

We report an ab initio study of the Kondo states formed from a Co adatom on Cu(111) and Cu(100). The model consists of a CoCun cluster ( n = 5-19) embedded in (111) and (100) Cu slabs. An embedding potential derived from density functional theory treats the interaction between the periodic crystal surroundings and the CoCun cluster, while strong electron correlations within CoCun are explicitly accounted for via configuration interaction (CI) methods. Analysis of the embedded CI wave function provides insight into the nature of the Kondo state, specifically into the influence of the crystal host on the Co d-electronic structure. We predict that different d-orbitals are preferentially singly occupied in Co on Cu(111) versus Cu(100) as a result of the different crystalline environment. We propose that these variations in the local d-electronic structure on Co, not accounted for in previous theories, are responsible for the drastically different Kondo resonance line shapes observed in scanning tunneling microscopy experiments on these two surfaces.     

In-situ examination of the selective etching of an alkanethiol monolayer covered Au{111} surface

An examination of the selective etching mechanism of a 1-alkanethiol self-assembled monolayer (SAM) covered Au{111} surface using in-situ atomic force microscopy (AFM) and molecular resolution scanning tunnelling microscopy (STM) is presented. The monolayer self-assembles on a smooth Au{111} surface and typically contains nanoscale non-uniformities such as pinholes, domain boundaries and monatomic depressions. During etching in a ferri/ferrocyanide water-based etchant, selective and preferential etching occurs at SAM covered Au(111) terrace and step edges where a lower SAM packing density is observed, resulting in triangular islands being relieved. The triangular islands are commensurate with the Au(111) lattice with their long edges parallel to its [11¯0] direction. Thus, SAM etching is selective and preferential attack is localized to defects and step edges at sites of high molecular density contrast.     

Optical gain of 1.3 /spl mu/m GaAsSbN/GaAs quantum well lasers

Optical gain of 1.3 mum GaAsSbN/GaAs quantum well (QW) structure is investigated using the multiband effective mass theory. The results are compared with those of 1.3 mum InGaNAs/GaAs and GaAsSb/GaAs QW structures. The optical gain of the GaAsSbN/GaAs QW structure is found to be similar to that of the InGaAsN/GaAs QW structure. In contrast, GaAsSbN/GaAs and InGaNAs/GaAs QW structures show significantly larger optical gain than the GaAsSb/GaAs QW structure. This is mainly attributed to the fact that the former has a larger optical matrix element than the latter. In addition, GaAsSbN/GaAs and InGaNAs/GaAs QW structures have much smaller threshold current density than the GaAsSb/GaAs QW structure. This is because the Auger recombination current density gives dominant contribution to the threshold current density and the former has smaller threshold carrier density than the latter. On the contrary, the threshold current density of the GaAsSbN/GaAs QW structure is shown to be similar to that of the InGaAsN/GaAs QW structure     

纳米纤维素分散的高稳定性单片层黏土分散液的制备及其在透明柔性薄膜的应用

以天然的纳米纤维素（NFC）为分散剂,利用其亲水亲油的特性,将其用于剥离和分散片层黏土,成功攻克单片层黏土易发生絮聚的缺陷,高获得率制备出NFC分散的具有优异稳定性的单片层黏土分散液,并采用AFM和TEM对NFC和单片层黏土的形貌进行表征。最后,将单片层黏土分散体与NFC结合成功制备出一种高透明柔性薄膜。当单片层黏土含量为50wt%时,NFC/单片层黏土复合薄膜在600~800 nm波长下的透光率高达90%,且呈现较低的雾度。此外,该薄膜还具有优异的紫外阻隔能力,在紫外区能完全阻隔短波紫外线（UVC,波长为100~290 nm）,阻隔大部分中波紫外线（UVB,波长为290~320 nm）。NFC分散的具有优异稳定性的单片层黏土分散液的成功制备,将有助于拓展纳米黏土的应用领域及提高单片层黏土基复合材料的性能。     

X-ray-diffraction characterization of Pt(111) surface nanopatterning induced by C60 adsorption

Understanding the adsorption mechanisms of large molecules on metal surfaces is a demanding task. Theoretical predictions are difficult because of the large number of atoms that have to be considered in the calculations, and experiments aiming to solve the molecule-substrate interaction geometry are almost impossible with standard laboratory techniques. Here, we show that the adsorption of complex organic molecules can induce perfectly ordered nanostructuring of metal surfaces. We use surface X-ray diffraction to investigate in detail the bonding geometry of C(60) with the Pt(111) surface, and to elucidate the interaction mechanism leading to the restructuring of the Pt(111) surface. The chemical interaction between one monolayer of C(60) molecules and the clean Pt(111) surface results in the formation of an ordered sqrt[13] x sqrt[13]R13.9 degrees reconstruction based on the creation of a surface vacancy lattice. The C(60) molecules are located on top of the vacancies, and 12 covalent bonds are formed between the carbon atoms and the 6 platinum surface atoms around the vacancies. In-plane displacements induced on the platinum substrate are of the order of a few picometres in the top layer, and are undetectable in the deeper layers.     

Investigation and Manipulation of C60 on a Si Surface Using a Scanning Tunneling Microscope

We have demonstrated that the tip of a scanning tunnelling microscope (STM) may be used to position individual C₆₀ molecules on a Si(111) surface. This work is reviewed together with more recent results on STM modification of C₆₀multilayers. The chemical passivation of Si(111) by a C₆₀ monolayer is also discussed.     

Electroless preparation of macroporous Cu thin film from large-scale-orderly colloid monolayer

A facile method to fabricate the highly ordered colloid monolayer of a large scale is demonstrated. The colloidal template involves the ordered array of ∼ 100 nm silica colloids functioned with 3-mercaptopropyltrimethoxysilane (MPTMS) by spin coating on the silicon wafer etched square pattern with an ∼ 100 nm depth. Based on the new nanostructure, highly ordered macroporous Cu thin film can be prepared from Pd nanoparticles-based electroless deposition. By the deposition time, the macroporous thin film of various thicknesses can be prepared. Additionally, the Cu film with (111) strong texture is deposited on the gold substrate of single crystalline (111) orientation.     

Si(111)7×7表面Na沉积过程的光学二次谐波法研究

用原位光学二次谐波方法监测了室温时Na在Si(111)7×7表面沉积过程中的信号变化。室温下Na沉积量小于饱和单层时信号变化比较复杂,随Na沉积量的变化依次出现三个特征峰。覆盖度约0.45 ML时出现较弱的峰A,其产生可能与碱金属诱导的占据态F和没有被占据的态A间的跃迁所引起的共振增强有关;随后在约0.9 ML的覆盖度下出现峰B并在沉积量大于饱和单层后信号快速增大,形成陡峭的峰C。相测量表明峰B与峰C信号之间存在明显的相位差,说明两者的起源有不同之处。峰C的出现与等离激元的激发密切相关。在峰B所处的覆盖度下,二维及二维-三维过渡结构的存在可能产生两种以上不同起源的二次谐波信号,其相干叠加促成了峰B的出现。     

4He Films near Monolayer Completion

We study the role played by the substrate–helium interaction in determining whether a ⁴He film is fluid or solid near monolayer completion. In order to glean information concerning the possible fluid-solid transition we calculate the equation of state as well as the behavior of phonons and rotons for a high density monolayer fluid film. By analyzing the behavior of the long-wavelength excitations and the roton gap, we argue that we can infer instability in the fluid phase. We study a model Lennard-Jones two parameter potential for the substrate–helium interaction. We map out in the two-parameter space the separatrix between those weak substrates that can only support a fluid monolayer from those strong substrates that support a solid. Our approach utilizes a combination of information from both variational calculations and also correlated basis function theory to examine in detail the excitation structure in the monolayer liquid as a function of film coverage and substrate potential.     

Magnetic Ordering and Exchange Interaction of Laves Compounds Sm0.9Pr0.1（Fe1—xCox）2

Structure,magnetic properties and magnetostriction of Sm0.9Pr0.1(Fe1-xCox)2 compounds have been investi-gated by means of X-ray diffraction,a.c. initial susceptibility, extracting sample magnetometer,Mossbauer spec-troscopy and standard strain gauge techniques.The lattice parameter a of the MgCu2-type Laves compounds Sm0.9Pr0.1(Fe1-xCox)2 decreases nonlinearly with increasing Co concentration,deviating from the Vegardˊs law.Curie temperature Tc increases initially from 668 K for x=0 to 694 K for x=0.2 and then decreases to 200 K for x=1.0.The saturation magnetization Ms at temperatures 1.5K, 77K and 300K have the same variation tendency as the composition dependence of Curie temperature,in consistence with rigid-band model.The easy magnetization direction(EMD) od Sm0.9Pr0.1(Fe1-xCox)2 lies along [111] direction in the range x≤0.6,and changes to [110] for x=0.8 ,while Sm0.9Pr0.1(Fe1-xCox)2 stays in the paramagnetic state at room temperature.The composition dependence of the average hyperfine field,Hhf,demonstrates a similar variation tendency as that of the saturation magnetiza-tion Ms and Curie temperature Tc .The spontaneous magnetostricton λ111 increases with increasing Co content.The saturation magnetostriction λs decreases monotonically with increasing x,which is caused by the increase of magnetostriction constant λ100 with opposite sign to that of λ111.A two-sublattice model has been proposed to understand the intermediate region between the [111]and [110] spin configurations ,which can also be used to explain the temperature dependence of magnetization.