Facile synthesis of bimetallic nanoplates consisting of Pd cores and Pt shells through seeded epitaxial growth

Pd-Pt core-shell nanoplates with hexagonal and triangular shapes were synthesized through the heterogeneous, epitaxial growth of Pt on Pd nanoplates. The Pd nanoplates were synthesized by reducing Na2PdCl4 precursor with PVP as a reducing agent, which then served as seeds for the nucleation of Pt atoms formed by reducing H2PtCl6 with citric acid. Characterization of the as-prepared Pd-Pt nanoplates by scanning transmission electron microscopy and high-resolution transmission electron microscopy reveals that a thin, uniform Pt shell was formed around the Pd nanoplate, demonstrating the layer-by-layer epitaxial growth of Pt on Pd surface in this approach. The close lattice match between Pd and Pt (lattice mismatch of only 0.77%) and the slow reduction rate associated with the mild reducing power of citric acid play key roles in achieving the epitaxial growth of Pt shells on Pd nanoplates.     

Sodium lanthanide fluoride core-shell nanocrystals: A general perspective on epitaxial shell growth

Understanding the structural characteristics and growth mechanism（s） are essen-tial for generating core-shell nano-heterostructures with distinctive properties. Especially in lanthanide-based nanocrystals, rational design of the core-shell composition can be utilized to enhance/tune the optical properties of the final nanostructure, or can be used to integrate multiple functional applications （e.g., luminescent/magnetic）. In this article, we review the progress in our current understanding of the epitaxial shell growth in sodium lanthanide fluoride （NaLnF4） nanocrystals. In order to understand epitaxial shell growth the core nanocrystals have to be uniform, and to date the synthesis of high quality near uniform size/shape dispersion controlled synthesis of lanthanide-based nanocrystals has been achieved mainly with this class of nanocrystals. The progress in core-shell synthesis and the epitaxial shell growth mechanism in this class of nanocrystals （NaLnF4） are reviewed, and a general perspective is provided on the core-shell morphology based on different characterization techniques. While there has been tremendous progress in studying the impact of core-shell structures in various functional applications, this review also highlights, in our view, the still limited understanding of ways to control the core-shell morphology and it emphasizes some important, unanswered questions that remain to be addressed to maximize their performance.     

Suppression of inhomogeneous segregation in graphene growth on epitaxial metal films

Large-scale uniform graphene growth was achieved by suppressing inhomogeneous carbon segregation using a single domain Ru film epitaxially grown on a sapphire substrate. An investigation of how the metal thickness affected growth and a comparative study on metals with different crystal structures have revealed that locally enhanced carbon segregation at stacking domain boundaries of metal is the origin of inhomogeneous graphene growth. Single domain Ru film has no stacking domain boundary, and the graphene growth on it is mainly caused not by segregation but by a surface catalytic reaction. Suppression of local segregation is essential for uniform graphene growth on epitaxial metal films.     

Octopods versus concave nanocrystals: control of morphology by manipulating the kinetics of seeded growth via co-reduction

Au/Pd octopods and concave core@shell Au@Pd nanocrystals have been prepared by coupling for the first time a seed-mediated synthetic method with co-reduction. The integration of these two methods is central to the formation of these binary Au/Pd nanocrystals wherein the kinetics of seeded growth are manipulated via the co-reduction technique to control the final morphology of the nanocrystals. Significantly, the synthesis of these structures under similar reaction conditions illustrates that they are structurally related kinetic products. Detailed characterization by STEM-EDX analysis highlights the unique structural features of these nanocrystals and indicates that Pd localizes on the higher-energy features of the nanocrystals. Optical and electrocatalytic characterization also demonstrates their promise as a new class of multifunctional nanostructures.     

A general approach to binary and ternary hybrid nanocrystals

We describe and demonstrate a general strategy for engineering binary and ternary hybrid nanoparticles based on spontaneous epitaxial nucleation and growth of a second and third component onto seed nanoparticles in high-temperature organic solutions. Multifunctional hybrid nanoparticles that combine magnetic, plasmonic, and semiconducting properties and that are tunable in size and morphology can be realized, as demonstrated for combinations of Au, Fe3O4 and PbS or PbSe. The properties of each component within the hybrids can be modulated strongly by the conjugating component(s) aided by the coherent interfaces between them.     

Real-time microscopy of graphene growth on epitaxial metal films: role of template thickness and strain

Epitaxial transition metal films have recently been introduced as substrates for the scalable synthesis of transferable graphene. Here, real-time microscopy is used to study graphene growth on epitaxial Ru films on sapphire. At high temperatures, high-quality graphene grows in macroscopic (>100 μm) domains to full surface coverage. Graphene nucleation and growth characteristics on thin (100 nm) Ru films are consistent with a pure surface chemical vapor deposition process, without detectable contributions from C segregation. Experiments on thicker (1 μm) films show a systematic suppression of the C uptake into the metal to levels substantially below those expected from bulk C solubility data, consistent with a strain-induced reduction of the C solubility due to gas bubbles acting as stressors in the epitaxial Ru films. The results identify two powerful approaches--i) limiting the template thickness and ii) tuning the interstitial C solubility via strain--for controlling graphene growth on metals with high C solubility, such as Ru, Pt, Rh, Co, and Ni.     

Kinetics of epitaxial growth and roughening

We review some recent results on epitaxial growth and surface roughening. Particular emphasis is placed on the concept of the critical island size in submonolayer growth and on the existence of scaling in both the submonolayer and multilayer growth regimes. The use of scaling ideas as well as Monte Carlo simulations and continuum equations is shown to be effective in understanding experimental results for submonolayer growth and surface roughening.     

Au-free epitaxial growth of InAs nanowires

III-V nanowires have been fabricated by metal-organic vapor-phase epitaxy without using Au or other metal particles as a catalyst. Instead, prior to growth, a thin SiOx layer is deposited on the substrates. Wires form on various III-V substrates as well as on Si. They are nontapered in thickness and exhibit a hexagonal cross-section. From high-resolution X-ray diffraction, the epitaxial relation between wires and substrates is demonstrated and their crystal structure is determined.     

Role of steps in epitaxial growth

The role of steps in the epitaxial growth of quantum structures is discussed. We present experimental results and theoretical predictions of growth on stepped surfaces. Scanning tunneling microscopy (STM) images of molecular beam epitaxy grown GaAs(001) surfaces misoriented by 1° and 2° towards the (111) A direction show non-uniform terraces with a peak in the terrace width distribution at 40 Å. Simple models of atoms landing on a step and attaching at the ascending step edge, however, predict an equalization of terrace widths. A thermodynamic model which allows the steps to move freely with the constraint that it costs energy to form a kink predicts step bunching for high kink energies. Steps on vicinal surfaces have been utilized for growing quantum wire structures using a technique where fractional monolayers of different materials are deposited on a stepped surface, leading to the creation of a lateral superlattice (LSL). The terrace width uniformity is observed by STM to improve dramatically with the growth of an AlAs---GaAs LSL. Cross-sectional transmission electron microscopy of LSLs shows good segregation of the composite layers.     

One-step Ge/Si epitaxial growth

Fabricating a low-cost virtual germanium (Ge) template by epitaxial growth of Ge films on silicon wafer with a Ge(x)Si(1-x) (0 < x < 1) graded buffer layer was demonstrated through a facile chemical vapor deposition method in one step by decomposing a hazardousless GeO(2) powder under hydrogen atmosphere without ultra-high vacuum condition and then depositing in a low-temperature region. X-ray diffraction analysis shows that the Ge film with an epitaxial relationship is along the in-plane direction of Si. The successful growth of epitaxial Ge films on Si substrate demonstrates the feasibility of integrating various functional devices on the Ge/Si substrates.     

Advances in metal halide perovskite nanocrystals: Synthetic strategies,growth mechanisms,and optoelectronic applications

Metal halide perovskite nanocrystals, as a new class of light-harvesting and light-emitting materials, have recently attracted a lot of attention for an impressive variety of optoelectronic applications. Some advantages of perovskite nanocrystals include there being low-cost, easy-to-perform synthetic routes, convenient solution processability, precise bandgap tunability over the entire visible spectral range, and exceptionally high photoluminescence quantum yields. In this review, we summarize recent advances of perovskite nanocrystals with an emphasis on the synthetic methods, growth mechanisms, optical properties, and related applications. The focus is placed on emerging new results in terms of the increasing diversity in the synthetic methodologies, ability to control nanoparticle shapes, stability enhancement strategies (including direct syntheses in water), and on the particle formation mechanisms. The basic design principles and up-to-date performance of optoelectronic devices based on perovskite nanocrystals are considered, with a main focus on light-emitting diodes, but also touching upon solar cells, photodetectors, and lasers. We finish the review with a critical outlook into the open issues and future perspective of this versatile, still rapidly developing field.     

Synthesis of spatially uniform metal alloys nanocrystals via a diffusion controlled growth strategy: The case of Au-Pd alloy trisoctahedral nanocrystals with tunable composition

Rational synthesis of bimetallic alloy nanocrystals (NCs) is still a great challenge. Especially, spatially uniform alloy NCs are very difficult to achieve because of the different reduction rates of the individual alloy components. Herein we propose a facile wet chemical synthetic strategy to prepare uniform bimetallic alloy NCs with tunable composition by controlling the growth of alloy NCs under diffusion controlled conditions. Using this strategy, we successfully synthesized trisoctahedral (TOH) Au-Pd alloy NCs enclosed by {hhl} high-index facets with uniform spatial distributions and different compositions. Significantly, using our strategy, the composition of the as-prepared Au-Pd alloy NCs is identical to the ratio of the two metal precursors in the reaction solution over a wide range. Investigation of the composition-dependent electrochemical behavior of the as-prepared TOH Au-Pd alloy NCs showed that the TOH Au-Pd alloy NCs containing 14.1 atom% Pd exhibited the best activity.      

Controlled growth of tetrapod-branched inorganic nanocrystals

Nanoscale materials are currently being exploited as active components in a wide range of technological applications in various fields, such as composite materials, chemical sensing, biomedicine, optoelectronics and nanoelectronics. Colloidal nanocrystals are promising candidates in these fields, due to their ease of fabrication and processibility. Even more applications and new functional materials might emerge if nanocrystals could be synthesized in shapes of higher complexity than the ones produced by current methods (spheres, rods, discs). Here, we demonstrate that polytypism, or the existence of two or more crystal structures in different domains of the same crystal, coupled with the manipulation of surface energy at the nanoscale, can be exploited to produce branched inorganic nanostructures controllably. For the case of CdTe, we designed a high yield, reproducible synthesis of soluble, tetrapod-shaped nanocrystals through which we can independently control the width and length of the four arms.     

Diameter-dependent growth direction of epitaxial silicon nanowires

We found that silicon nanowires grown epitaxially on Si (100) via the vapor-liquid-solid growth mechanism change their growth direction from 111 to 110 at a crossover diameter of approximately 20 nm. A model is proposed for the explanation of this phenomenon. We suggest that the interplay of the liquid-solid interfacial energy with the silicon surface energy expressed in terms of an edge tension is responsible for the change of the growth direction. The value of the edge tension is estimated by the product of the interfacial thickness with the surface energy of silicon. For large diameters, the direction with the lowest interfacial energy is dominant, whereas for small diameters the surface energy of the silicon nanowire determines the preferential growth direction.     

Molecular beam epitaxial growth of topological insulators

With the molecular beam epitaxy technique, layer-by-layer growth of atomically flat topological insulator Bi(2) Te(3) and Bi(2) Se(3) thin films has been realized on Si(111) and graphene substrates, respectively. The growth criteria by which intrinsic topological insulators can readily be obtained is established. By using in situ angle-resolved photoemission spectroscopy and scanning tunneling microscopy measurements, the band structure and surface morphology of Bi(2) Te(3) and Bi(2) Se(3) thin films of different thickness can be studied. Molecular beam epitaxy technique was shown to not only provide an excellent method to prepare high quality topological insulators but show possibilities of engineering their electronic and spin structures as well, which is of significant importance for potential applications of topological insulators based on well-developed Si technology.     

Preparation and spectroscopy study of monodispersed metal oxide nanocrystals

SnO₂ nanocrystals with an average particle size of 3.5 nm are obtained by heating the mixture of hydrous SnO₂ nanoparticles and SrCO₃ particles. SrCO₃ can be removed from the product by washing with diluted HNO₃ acid. A series of SnO₂ nanocrystal samples with different particle size are prepared by heating at different temperature. All the samples obtained are characterized by XPS and Raman spectra. XPS result shows that the binding energy of Sn3d_5/2 decreases as the particle size increases. There is a surface-related Raman peak observed, which increases in intensity when the SnO₂ particles size decreases. The size-related change of the surface structure is responsible for the properties of nanocrystals.     

Transfer-free electrical insulation of epitaxial graphene from its metal substrate

High-quality, large-area epitaxial graphene can be grown on metal surfaces, but its transport properties cannot be exploited because the electrical conduction is dominated by the substrate. Here we insulate epitaxial graphene on Ru(0001) by a stepwise intercalation of silicon and oxygen, and the eventual formation of a SiO(2) layer between the graphene and the metal. We follow the reaction steps by X-ray photoemission spectroscopy and demonstrate the electrical insulation using a nanoscale multipoint probe technique.