Achieving mastery over the synthesis of metal nanocrystals has emerged as one of the foremost scientific endeavors in recent years. This intense interest stems from the fact that the composition, size, and shape of nanocrystals not only define their overall physicochemical properties but also determine their effectiveness in technologically important applications. Our aim is to present a comprehensive review of recent research activities on bimetallic nanocrystals. We begin with a brief introduction to the architectural diversity of bimetallic nanocrystals, followed by discussion of the various synthetic techniques necessary for controlling the elemental ratio and spatial arrangement. We have selected key examples from the literature that exemplify critical concepts and place a special emphasis on mechanistic understanding. We then discuss the composition-dependent properties of bimetallic nanocrystals in terms of catalysis, optics, and magnetism and conclude the Review by highlighting applications that h...

Bimetallic Nanocrystals: Syntheses, Properties, and Applications

Microscopic view of epitaxial metal growth: nucleation and aggregation

Physical and chemical properties of bimetallic surfaces

With octahedral Au nanocrystals as seeds, highly monodisperse Au@Pd and Au@Ag core-shell nanocubes were synthesized by a two-step seed-mediated method in aqueous solution. Accordingly, we have preliminarily proposed a general rule that the atomic radius, bond dissociation energy, and electronegativity of the core and shell metals play key roles in determining the conformal epitaxial layered growth mode.

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Epitaxial growth of heterogeneous metal nanocrystals: from gold nano-octahedra to palladium and silver nanocubes.

Ordered anion adlayers on metal electrode surfaces.

We discuss the physical phenomena fundamental to the understanding of the structure and growth of crystalline superlattices: (i) the growth mode as determined by surface energies, supersaturation, and lattice misfit; and (ii) the dependence of epitaxial orientation on lattice matching, atomic bonding, and film thickness, in the topical case of epitaxy at (111) fcc/(110) bcc interfaces. For uniformity monolayer-by-monolayer [Frank--van der Merwe (FM)] growth is desirable. This may be adversely affected by the formation of misfit dislocations. Continued FM growth may be achieved with alternate A and B layers at moderate supersaturation, provided that the surface energies ${\ensuremath{\gamma}}_{A}$ and ${\ensuremath{\gamma}}_{B}$ are compatible. The suggestion that it is possible otherwise at sufficiently high supersaturation is a misconception. The main epitaxial orientations in the present case---the Nishiyama-Wassermann (NW) and the Kurdjumov-Sachs (KS) orientations---have been previously predicted on the (energetically justified) basis of geometrical relationships alone. The predictive power of this model is demonstrated for hexagonal interfaces. Ideally, to predict the evolution of the structure and orientation of a growing (thickening) film atomic forces must be allowed for. We model these forces by means of crystallinity and harmonicity of film, and by a truncated--Fourier-series adsorbate-substrate interaction. Various forms of homogeneous and oscillatory film strains, affecting orientation and structure, are illustrated graphically. We conclude that a good guideline for superlattice formation is the following: (a) growth at moderate supersaturations of metal pairs with comparable \ensuremath{\gamma}'s in the unique NW orientation (0.8\ensuremath{\lesssim}b/a\ensuremath{\lesssim}1.0, a and b are nearest-neighbor distances); or, possibly, (b) nucleation and growth along unidirectional steps.

Structure and growth of crystalline superlattices: From monolayer to superlattice.

A parabolic interaction potential has been used to develop a model for calculating the misfit dislocation (MD) energy in the case of a superlattice of alternating layers of materials with equal elastic constants and thicknesses. The model, which is believed to be a good one for small misfits and to have some merit for covalent bonded materials, is exactly solvable for the critical thickness above which it is energetically favorable to lose coherency by the introduction of MDs into the interfaces. It was found, for a given misfit f, that the critical thickness for epitaxial superlattices free from their substrate is somewhat more than four times that for a single epilayer on a thick substrate. Furthermore, the critical thickness varies almost inversely with misfit to the power 1.22 when Poisson's ratio is 1/3. It was also shown that the critical misfit f(c) obtained by equating maximal misfit strain and MD energies is a significant overestimate of f(c). The results for a superlattice are compared with those of a thin layer on a thick substrate.

An exactly solvable model for calculating critical misfit and thickness in epitaxial superlattices: Layers of equal elastic constants and thicknesses

Low energy dislocation structures in epitaxy

Epitaxy is the phenomenon in which the orientation of a crystal 0 which grows on a crystalline substrate S, usually different from 0, is related to that of S, even though their normal lattices do not match at their common interface. Apart from its fundamental interest, epitaxy is important in view of its application in the fabrication of devices where structural perfection is essential for performance and stability. Misfit is detrimental to crystal perfection.

The role of lattice misfit in epitaxy

The coherency-incoherency transition in epitaxial crystals is said to take place at a critical misfit f(c) or, for a system in which a monolayer is subcritical, at a critical thickness h(c). In this paper, the physical principles and models used to predict critical parameters are analyzed and put into perspective. The dependence of the relevant principles on the equilibrium-nonequilibrium conditions under which the quantities are measured in practice is stressed. The advantages and disadvantages of the models used (essentially the Frenkel-Kontorowa and Volterra models) are highlighted.

Jan H. van der Merwe

Papers

Structure and growth of crystalline superlattices: From monolayer to superlattice.

An exactly solvable model for calculating critical misfit and thickness in epitaxial superlattices: Layers of equal elastic constants and thicknesses

Low energy dislocation structures in epitaxy

The role of lattice misfit in epitaxy

The prediction and confirmation of critical epitaxial parameters