Huarui Sun

Evolution of dislocation arrays in epitaxial BaTiO3 thin films grown on (100) SrTiO3

Dislocation arrays and dislocation half-loops in BaTiO3 thin films were characterized using transmission electron microscopy (TEM). BaTiO3 films with thicknesses ranging from 2 to 20 nm were grown on (100) SrTiO3 by reactive molecular beam epitaxy (MBE). The critical thickness for dislocations to occur in this system was found to lie between 2 and 4 nm. The misfit dislocations are mainly 〈100〉 type. The average spacing between the dislocations in the array becomes smaller when the film is thicker, which indicates gradual relaxation of mismatch strain with increasing film thickness.

Hexagonal close-packed Ni nanostructures grown on the (001) surface of MgO

We report the in situ microscopy observation of an unnatural phase of Ni, a highly strained hexagonal close-packed (hcp) form which we believe is stabilized by heteroepitaxial growth on the (001) face of MgO. We find that the nanosized hcp nickel islands transform into the normal face-centered cubic structure when the size of the islands exceeds a critical value (about 2.5 nm thick with a lateral size of ∼5nm). The structural transition proceeds via a martensitic change in the stacking sequence of the close-packed planes. The formation of hcp Ni nanostructures with an unusually large crystallographic c∕a ratio (∼6% larger than ideal hcp) is very interesting for spintronic and recording applications where large uniaxial anisotropies are desirable.

Thermal boundary resistance of copper phthalocyanine-metal interface

Systems containing interfaces between dissimilar materials can exhibit lower thermal conductivity than their pure constituents, with important implications for thermal management and thermoelectric energy conversion. However, the heat transfer processes at such interfaces, in particular those between organic and inorganic materials, remain for the most part uncharacterized. We use vacuum thermal evaporation to grow archetypal multilayer thin films of copper phthalocyanine (CuPc) and Ag or Al, and measure their thermal conductivity as a function of interface density. We observe large thermal boundary resistance values (7.8×10−8 m2 K/W for CuPc/Ag and 2.0×10−8 m2 K/W for CuPc/Al), attributable to acoustic mismatch, heat carrier mismatch, and weak bonding.

Interface structure and strain relaxation in BaTiO3 thin films grown on GdScO3 and DyScO3 substrates with buried coherent SrRuO3 layer

To obtain the electrical properties of strained ferroelectric thin films, bottom electrodes with lattice constants and thermal coefficients matched to both films and substrates are needed. The interface structure, strain configuration, and strain relaxation in such bilayer systems are different from those in single layer systems. Here, we report transmission electron microscopy studies of epitaxial BaTiO3 films grown on GdScO3 and DyScO3 substrates with buried SrRuO3 layers. We found that the different strain relaxation behaviors observed in the bilayer are mainly dependent on lattice mismatch of each layer to the substrate and the thicknesses of each layer.

Partial encapsulation of Pd particles by reduced ceria-zirconia

Direct observation of metal-oxide interfaces with atomic resolution can be achieved by cross-sectional high-resolution transmission electron microscopy (HRTEM). Using this approach to study the response of a model, single-crystal thin film automotive exhaust-gas catalyst, Pd particles supported on the (111) ceria-zirconia (CZO) surface, to a redox cycle, we have found two distinct processes for the partial encapsulation of the Pd particles by the reduced CZO surface that depend on their relative crystallographic orientations. In the case of the preferred orientation found for Pd particles on CZO, Pd(111)[110]∕∕CZO(111)[110], a flat and sharp metal/oxide interface was maintained upon reduction, while ceria-zirconia from the adjacent surface tended to accumulate on and around the Pd particle. In rare cases, Pd particles with other orientations tended to sink into the oxide support upon reduction. Possible mechanisms for these encapsulation processes are proposed.

Formation and coarsening of Ga droplets on focused-ion-beam irradiated GaAs surfaces

We have investigated the formation and coarsening of Ga droplets on focused-ion-beam (FIB) irradiated GaAs surfaces. To separately examine formation and coarsening, Ga droplets were fabricated by Ga+ FIB irradiation of GaAs substrates with and without pre-patterned holes. We determined the droplet growth rate and size distribution as a function of FIB energy following irradiation. The data suggest a droplet formation mechanism that involves Ga precipitation from a Ga-rich layer, followed by droplet coarsening via a combination of diffusion and Ostwald ripening or coalescence via droplet migration (dynamic coalescence).

Formation and evolution of epitaxial Co5Ge7 on Ge(001) surface by reactive deposition inside an ultrahigh-vacuum transmission electron microscope

Cobalt was deposited on single-crystal Ge(001) surface at ∼350°C by electron-beam evaporation in an ultrahigh-vacuum transmission electron microscope. The deposited Co reacts with Ge to form nanosized islands with the cobalt germanide Co5Ge7 phase. The Co5Ge7 islands show square and rectangular shapes. Two epitaxial orientation relationships between Co5Ge7 and Ge were observed: Co5Ge7 ⟨110⟩(001)‖Ge⟨100⟩(001) and Co5Ge7⟨001⟩(110)‖Ge⟨100⟩(001).

Formation and evolution of epitaxial Co5Ge7 film on Ge (001) surface by solid-state reaction in an in situ ultrahigh-vacuum transmission electron microscope

A thin metallic cobalt (Co) layer was deposited on a single-crystal Ge (001) surface at room temperature by the electron-beam evaporation of a pure Co metal source in an ultrahigh-vacuum transmission electron microscope. The formation and epitaxial growth of a cobalt germanide Co5Ge7 phase on the Ge (001) surface was studied in situ by gradually heating the sample from room temperature to ∼350°C. The occurrence of an epitaxial hexagonal-close-packed Co and the reaction between Co and Ge were observed at ∼225°C. After annealing at ∼300°C for 26.5 h, a continuous epitaxial Co5Ge7 film formed on the Ge (001) substrate. With further annealing at a higher temperature, the continuous Co5Ge7 layer broke up and formed three-dimensional islands in order to relieve the strain energy in the epitaxial Co5Ge7 layer. Two epitaxial relationships between Co5Ge7 and Ge, i.e., Co5Ge7⟨110⟩(001)∕∕Ge⟨100⟩(001) and Co5Ge7⟨001⟩(110)∕∕Ge⟨100⟩(001) were found by electron diffraction.

Perturbation analysis of acoustic wave scattering at rough solid-solid interfaces

We apply a boundary perturbation approach to model the scattering of acoustic waves at rough solid-solid interfaces. This method incorporates mode conversion and allows straightforward separation of specular and diffuse components, the relative fractions of which play an important role in determining thermal boundary resistance and the performance of acoustic phonon devices. The effect of acoustic impedance mismatch on the distribution of scattered waves is investigated, as well as the coupling to interface modes.

Surface plasmon resonances of Ga nanoparticle arrays

We have examined the influence of particle and chain diameter on surface plasmon resonance (SPR) energy of 2D and 1D Ga nanoparticle (NP) arrays fabricated using focused-ion-beam irradiation of GaN surfaces. Maxima in the extinction spectra suggest the presence of SPR at visible and near-infrared wavelengths. The SPR energies increase with decreasing NP or chain diameter, due to particle diameter-dependent dipole interactions within the metallic NPs. The SPR quality factors are comparable to those reported from Ag and Au NPs, suggesting Ga NPs as a promising alternative plasmonic material.