Heteroepitaxial growth of core-shell and core-multishell nanocrystals composed of palladium and gold

The heteroepitaxial growth of the core-shell nanocrystals composed of Pd and Au is developed. Pd nanocubes or Au nano-octahedrons are utilized as the cores. The growths of the Au and Pd shells are realized under similar conditions, where the same reducing agent and stabilizing surfactant are employed. The preparation is highly controllable, and the epitaxial growth is repeated up to three times to yield Pd@Au@Pd@Au and Au@Pd@Au@Pd core-trishell nanocrystals. The thickness of each shell is readily varied by changing the amount of the metal salts used for growth. All of the nanocrystal products have narrow size distributions and are single crystalline. The plasmon resonance properties of these nanocrystals are mainly determined by the exterior shell. The plasmon of the Pd-shell-terminated nanocrystals is suppressed, while that of the Au-shell-terminated ones is recovered and is stronger when the Au shell becomes thicker. This growth method can potentially be extended to other metals for the synthetic design of more complex core-multishell metal nanostructures with desirable optical, catalytic, and magnetic properties.     

Preparation methodologies and nano/microstructural evaluation of metal/semiconductor thin films

Metal/semiconductor thin films are a class of unique materials that are widespread technological applications, particularly in the field of microelectronic devices. Assessment strategies of fractal and tures are of fundamental importance in the development of nano/microdevices. This review presents the preparation methodologies and nano/microstructural evaluation of metal/semiconductor thin films including Au/Ge bilayer films and Pd-Ge alloy thin films, which show in the form of fractals and nanocrystals. Firstly, the extended version of Au/Ge thin films for the fractal crystallization of amorphous Ge and the formation of nanocrystals developed with improved micro- and nanostructured features are described in Section 2. Secondly, the nano/microstructural characteristics of Pd/Ge alloy thin films during annealing have been investigated in detail and described in Section 3. Finally, we will draw the conclusions from the present work as shown in Section 4. It is expected that the preparation methodologies developed and the knowledge of nano/microstructural evolution gained in metal/semiconductor thin films, including Au/Ge bilayer films and Pd-Ge alloy thin films, will provide an important fundamental basis underpinning further interdisciplinary research in these fields such as physics, chemistry, materials science, and nanoscience and nanotechnology, leading to promising exciting opportunities for future technological applications involving these thin films.     

Electronic properties of single Au nanocrystals and synthesis of 1-dimensional nanocrystal arrays

The current-voltage characteristics of citrate- and octanethiol-capped Au nanocrystals assembled on planar Au substrates under ambient conditions in air and in solution are reported. We have found that single electron tunneling (SET) behavior is observed for citrate-capped Au crystals in air. In high dielectric media, however, SET is only seen for nanocrystals protected by the hydrophobic alkane ligands and not for the relatively bare citrate-capped nanocrystals. In addition, we report a new strategy for assembling these nanocrystals into spatially well-organized 1-dimensional (1D) arrays. The method involves trapping the crystals inside 20 nm diameter pores of porous filtration membranes. The 1D linked nanocrystals confined inside the membrane pores were characterized by ultraviolet-visible spectroscopy. “Shrinkwrapping” the nanocrystals with the organic conductive polymer polypyrrole or covalently linking the particles with alkyldithiols and subsequent dissolution of the membrane provided a suspension suitable for characterization by transmission electron microscopy. These studies are important in determining the role of spatial arrangement and chemical environment on the electronic properties of this important class of crystalline material     

Shape control of PbSe nanocrystals using noble metal seed particles

We demonstrate that the shape of PbSe nanocrystals can be controlled systematically by seeding their growth with noble metal nanoparticles (Au, Ag, or Pd) and varying the seed and precursor concentrations. Cylinders (quantum rods), cubes, crosses, stars, and branched structures were produced in high yield at 150 degrees C in reaction times of a few minutes. Although their absorption spectrum does not exhibit sharp features, the quantum rods exhibit significant photogeneration efficiency, enabling infrared sensitization of a polymeric photoconductive nanocomposite.     

AuPt合金的两种典型生长机理研究

分别采用晶种生长法和快速共还原法制备得到了AuPt合金。采用晶种生长法制备AuPt合金时,Au晶种的尺寸对Au-Pt双金属结构的形成有关键性影响,当用小尺寸的Au纳米颗粒(3nm)作为晶种时才会形成AuPt合金结构,当Au晶种尺寸较小时,由于颗粒内纳米尺度的原子快速扩散导致合金结构的形成。高温下油胺对Au和Pt前体的快速共还原会使Au和Pt在极短的时间内同时得到还原,避免了两相的分离,得到了AuPt合金结构。     

Facet‐Dependent Optical Properties Revealed through Investigation of Polyhedral Au–Cu2O and Bimetallic Core–Shell Nanocrystals

The ability to prepare Au–Cu₂O core–shell nanocrystals with precise control over particle size and shape has led to the discovery of facet‐dependent optical properties in cuprous oxide crystals. The use of Au cores not only allows the successful formation of Au–Cu₂O core–shell nanocrystals with tunable sizes, but also enables the observation of facet‐dependent optical properties in these crystals through the Au localized surface plasmon resonance (LSPR) absorption band. By tuning the Cu₂O shell morphology from rhombic dodecahedral to octahedral and cubic structures, and thus the exposed facets, the Au LSPR band position can be widely tuned. Such facet‐dependent optical effects are not observed in bimetallic Au–Ag and Au–Pd core–shell nanocrystals with the same precisely tuned particle sizes and shapes. It is believed that similar facet‐dependent optical properties could be observed in other ionic solids and other metal–metal oxide systems. The unusually large degree of plasmonic band tuning covering from the visible to the near‐infrared region in this type of nanostructure should be quite useful for a range of plasmonic applications.     

Size and shape effects on the strength of platinum nanoparticles

Journal of Materials Science - Several previous studies demonstrated that defect-free faceted nanocrystals of face-centered cubic metals (such as Au, Ni, and Pd) exhibit extraordinarily high...     

Interfacial reactivity of Au, Pd, and Pt on BiI3 (001): implications for electrode selection

Understanding the contact-semiconductor interface is important in determining the performance of a semiconductor device. This study investigated the contact chemistry of BiI(3) single crystal with Au, Pd, and Pt electrodes using X-ray photoelectron spectroscopy (XPS), a technique widely used to probe the interfacial chemistry of many materials. Chemical reactions were identified on the BiI(3) surface for the case of Pd and Pt contacts, while Au showed no reactivity with BiI(3). The difference in reactivities correlated with different surface morphologies of the contact on the BiI(3) surface, which was evidenced by atomic force microscopy (AFM) characterization. The dark resistivity of the BiI(3) crystal with above contact materials was measured by I-V characterization. The highest resistivity was obtained when Au was employed as the contact. These results suggest that Au is better than Pd and Pt as the contact material for BiI(3) single crystal.     

Ag+-assisted heterogeneous growth of concave Pd@Au nanocubes for surface enhanced Raman scattering (SERS)

AgNO3 is often used in the preparation of Au nanostructures since Ag-based substances (AgBS) can selectively be adsorbed on Au(100) and significantly modulate the growth of Au nanocrystals.High-index-faceted Au nanostructures have demonstrated excellent performance in catalysis and surface enhanced Raman scattering (SERS),thus attracting the interest of many researchers in the past several decades.Herein,high-index-faceted Pd@Au concave nanocubes (CNCs) were prepared using AgBS as growth-directing agents in the heterogeneous growth of Au on Pd nanocubes (NCs).During the growth of Pd@Au CNCs,Au atoms are initially deposited on the Pd{100} facets leading to the formation of thin Au shells,and then AgBS are quickly adsorbed on the formed Au(100),favoring the growth along 〈111〉 and the formation of Pd@Au CNCs.Transmission electron microscopy (TEM),high resolution transmission electron microscopy (HRTEM),energy dispersive spectroscopy (EDS),high angle annular dark field (HAADF),and scanning transmission electron microscopy EDS (STEMEDS) were used to systematically investigate the growth of Pd@Au CNCs.We also demonstrated that the high-index-faceted Pd@Au CNCs exhibited excellent SERS performances.     

Resistance changes in thin film integrated circuits caused by interdiffusion

The resistance changes in the thin film Au/Pd and Al/Au systems are investigated at a temperature of 350°C (Au/Pd) and in the range 100–200°C (Al/Au). Except for the original Au/Pd and Al/Au data, the experimental Au/Pd values published in the paper by Hall et al. are evaluated. The correct mathematical solutions for lateral diffusion from grain boundaries into grains and for interdiffusion in two-film samples are used in interpreting the experimental data. The evaluated values of the diffusion coefficient D satisfy the Arrhenius relationships and yield reliable information on the activation enthalpies H of the interdiffusion processes. However, the values of the frequency factors D₀ are less reliable, owing to their direct dependence on the grain size (Au/Pd) or on the film thicknesses (Al/Au).     

Self-assembly directed synthesis of poly(ortho-toluidine)-metal(gold and palladium) composite nanospheres

Poly(ortho-toluidine) (POT)-gold (Au) and palladium (Pd) composite nanospheres were successfully synthesized by the reaction of o-toluidine with the corresponding metal (Au or Pd) colloidal solution through self-assembly process in the presence of dodecylbenzenesulfonic acid (DBSA), which acts as both a dopant and surfactant, and ammonium peroxydisulfate as an oxidizing agent. The composites (POT-DBSA/Au or Pd) were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, UV-Visible (UV-Vis) spectroscopy, and electrical conductivity measurements. TEM images of the nanocomposites reveal that metal (Au or Pd) nanoparticles were well dispersed on POT spheres. TGA and XRD results show that the composites exhibit high thermal stability and are more crystalline compared with pristine POT. It was found that the electrical conductivity of the POT-DBSA/Au or Pd composites is 2 orders of magnitude higher than that of pristine polymer. Also, the POT-DBSA/Pd composite exhibits magnetic property. The formation mechanism of the POT-DBSA/Au or Pd composite nanosphere is discussed.     

金/钯哑铃状纳米晶的制备及其催化对硝基苯酚还原研究

采用晶种-溶液生长法制备了单分散性良好、长径比均一的Au纳米棒, 利用H₂PdCl₄作为前驱体, CTAC作为软模版, 抗坏血酸作为还原剂对Au纳米棒进行改性合成了金/钯哑铃状结构纳米晶(Au/Pd NDs)。采用透射电子显微镜(TEM)、X射线能谱仪(EDS)和紫外-可见分光光度计(UV-Vis-NIR)对样品的结构和形貌进行表征, 探讨了铃铛状结构形成的机理, 并研究了其对硼氢化钠还原对硝基苯酚反应的催化性能。结果表明: 大量的多晶钯颗粒定向选择生长在金纳米棒(AuNRs)两端, 形成哑铃状结构; 通过调控还原剂与前驱体的比例, 铃铛尺寸连续可调。当钯的分散性好且总的催化活性位点多时, 金/钯哑铃状结构纳米晶催化对硝基苯酚还原的效率高。钯颗粒尺寸为20.7 nm的Au/Pd NDs(0.04 mg/mL)催化对硝基苯酚还原的反应速率常数可达0.44 min^-1, 证明其是一种非常有效的催化剂。     

Mesoscopic assembly of magnetic Pd-Ni nanocrystals into ordered arrays by using alkane thiols

Two-dimensional arrays of core-shell Pd561 Nin (n = 38, 280, 561, 800, 1500, 3000, 4500) nanocrystals have been prepared by using alkanethiols. The arrays of Pd561 Nin nanocrystals are particularly well ordered; those with high Ni coverage show a tendency to form multilayers or crystallites. Two-dimensional arrays also have been obtained of Pd561 Ni3000 nanocrystals further covered by a Pd layer with a Pd561 Ni3000 Pd1500 composition.     

Properties of Pd nanocrystals prepared by PVD method

Pd nanocrystals obtained by the physical vacuum deposition (PVD) method were studied by X-ray diffraction, transmission electron microsocpy (TEM), atomic force microsocpy (AFM) and optical absorption methods. These Pd fcc nanocrystals are embedded in carbonaceous matrix. The object of study was the structure of Pd nanocrystals and interaction between Pd and carbon matrix interface. In the research a metal-organic crystalline structure was found.The structural model of Pd fcc type nanocrystals with different sizes (adopting the full-shell model) was also proposed.     

Ni/Au、Pd/Au与p-AlGaN材料的接触特性

在AlGaNpin型日盲紫外探测器结构中的p-AlGaN层上生长了Ni/Au和Pd/Au,并在600～850℃温度下进行快速热退火,测量其退火前后传输线模型中各金属接触间的电学性质。实验发现,Ni/Au与Pd/Au在p-AlGaN上表现出了不同的接触性能。为了更好的说明金属与p-AlGaN材料接触之间在退火后电流的变化,还测量了p-AlGaN材料裸片两点之间I-V曲线在退火前后的变化。实验表明,比起Ni/Au来,Pd/Au在p-AlGaN材料上制备欧姆接触具有一定的优势,并在文中进行了分析。     

In Situ Observations of Shell Growth and Oxidative Etching Behaviors of Pd Nanoparticles in Solutions by Liquid Cell Transmission Electron Microscopy

It is demonstrated that the redox reaction behaviors of Pd nanoparticles in HAuCl₄ solutions can be substantially modified by the introduction of hexadecyl trimethyl ammonium bromide (CTAB) agents through systematic liquid cell transmission electron microscopy (LCTEM) investigations. The gradual dissolution of Pd nanoparticles is observed when HAuCl₄ solution is pumped into liquid flow cells, the etching characteristics of which are depended on both HAuCl₄ concentrations and incident electron doses. In comparison, with the presence of CTAB agents, the dominated phenomenon appears to be the precipitation of Au species and incorporation onto the surface of Pd seeds. It is also observed that the rapid growth of Au on Pd seeds occurs by loading Pd and HAuCl₄ solutions into static liquid cells. The resultant Au shells exhibit rather sparse structural configurations and are formed possibly by homogeneous nucleation/coalescence of Au species as well as monomer attachments. The observed Au‐shell growth instead of Pd dissolution is attributed to the presence of the residual regents, which may be also responsible for the initially already existing small Au adsorptions at the corner/edge sites of Pd seeds. The study provides a useful reference for the convenient fabrication of complex nanostructures and functional nanomaterials in a controllable way.     

Understanding the Photothermal Conversion Efficiency of Gold Nanocrystals

Plasmon‐based photothermal therapy is one of the most intriguing applications of noble metal nanostructures. The photothermal conversion efficiency is an essential parameter in practically realizing this application. The effects of the plasmon resonance wavelength, particle volume, shell coating, and assembly on the photothermal conversion efficiencies of Au nanocrystals are systematically studied by directly measuring the temperature of Au nanocrystal solutions with a thermocouple and analyzed on the basis of energy balance. The temperature of Au nanocrystal solutions reaches the maximum at ～75°C when the plasmon resonance wavelength of Au nanocrystals is equal to the illumination laser wavelength. For Au nanocrystals with similar shapes, the larger the nanocrystal, the smaller the photothermal conversion efficiency becomes. The photothermal conversion can also be controlled by shell coating and assembly through the change in the plasmon resonance energy of Au nanocrystals. Moreover, coating Au nanocrystals with semiconductor materials that have band gap energies smaller than the illumination laser energy can improve the photothermal conversion efficiency owing to the presence of an additional light absorption channel.     

Surfactant‐Concentration‐Dependent Shape Evolution of Au–Pd Alloy Nanocrystals from Rhombic Dodecahedron to Trisoctahedron and Hexoctahedron

The surface structure‐controlled synthesis of noble metal nanocrystals (NCs) bounded by high‐index facets has become a hot research topic due to their potential to significantly improve catalytic performance. This study reports the preparation of monodisperse Au–Pd alloy NCs with systematic shape evolution from rhombic dodecahedral (RD) to trisoctahedral (TOH), and hexoctahedral (HOH) structures by varying the concentration of surfactant in the surfactant‐mediated synthesis. The as‐prepared three kinds of alloy NCs possess almost the same size and composition as each other. It is suggested that the surfactant containing long‐chain octadecyltrimethyl ammonium (OTA⁺) ions plays a key role in the formation of high index facets, and the crystal growth kinetics may also have an effect on the formation of different nanocrystal morphologies. In addition, the catalytic activities of these NCs are evaluated by structure‐sensitive reactions, including ethanol electro‐oxidation and the catalytic reduction of 4‐nitrophenol (4‐NPh). These three types of Au–Pd alloy NCs exhibit different catalytic selectivities towards these two reactions. The catalytic activities toward electro‐oxidation of ethanol are in the order of HOH > RD > TOH, which follows the order of their corresponding surface energies. However, the activities toward catalytic reduction of 4‐NPh are in the order of RD > TOH > HOH, which should be related to the local structure of the surfaces.     

Formation of Si nanocrystals utilizing a Au nanoscale island etching mask

Si nanocrystals were formed by using a Au nanoscale island etching mask. A high-resolution transmission electron microscopy image showed that the Si nanocrystals were created on a SiO_x layer, and the luminescence peak related to Si nanocrystals was observed in the cathodoluminescence spectrum. Capacitance-voltage measurements demonstrate a metal-insulator-semiconductor behavior with a flatband voltage shift for the Al/SiO₂/nanocrystalline Si/SiO₂/p-Si structures, indicative of the existence of the Si nanocrystals embedded into the SiO_x layer. These results indicate that Si nanocrystals embedded into the SiO_x layer can be formed by using a Au island etching mask.     

Microbial synthesis of core/shell gold/palladium nanoparticles for applications in green chemistry

We report a novel biochemical method based on the sacrificial hydrogen strategy to synthesize bimetallic gold (Au)–palladium (Pd) nanoparticles (NPs) with a core/shell configuration. The ability of Escherichia coli cells supplied with H₂ as electron donor to rapidly precipitate Pd(II) ions from solution is used to promote the reduction of soluble Au(III). Pre-coating cells with Pd(0) (bioPd) dramatically accelerated Au(III) reduction, with the Au(III) reduction rate being dependent upon the initial Pd loading by mass on the cells. Following Au(III) addition, the bioPd–Au(III) mixture rapidly turned purple, indicating the formation of colloidal gold. Mapping of bio-NPs by energy dispersive X-ray microanalysis suggested Au-dense core regions and peripheral Pd but only Au was detected by X-ray diffraction (XRD) analysis. However, surface analysis of cleaned NPs by cyclic voltammetry revealed large Pd surface sites, suggesting, since XRD shows no crystalline Pd component, that layers of Pd atoms surround Au NPs. Characterization of the bimetallic particles using X-ray absorption spectroscopy confirmed the existence of Au-rich core and Pd-rich shell type bimetallic biogenic NPs. These showed comparable catalytic activity to chemical counterparts with respect to the oxidation of benzyl alcohol, in air, and at a low temperature (90°C).