Self-organized nanostructures generated on metal surfaces under electron irradiation at low temperatures

抄録

Introduction

Generation of nanosized structures is technologically important, and the controlled formation of structures in solids on a nanometer scale is critical to modern technology. Such structures may have properties different from those of the bulk materials. Nanohole-based nanomaterials with a size less than the wavelength of an excitation laser beam, for example, are promised for applications such as chemical and biological sensing, membrane biorecognition, unique optical responses under laser excitation, etc.

High-energy particle irradiation is one of method to generate interesting nanomaterials. Aggregation of surface vacancies produced homogeneously by ion sputtering may produce pits that can become rather deep. Electron irradiation, which can induce back sputtering on the surface of thin foil specimens, is also one of the techniques used for nanometer scale etching, lithography and hole formation, and intense convergent electron beams have been utilized, so far. Parallel electron beams of 500–1000 nm diameter have been shown to produce pits on the exit surface of Au(111) foils by sputtering over the electron energy range of 0.4–1.1 MeV.

Self-organization is a method to produce characteristic structures and several self-organization phenomena of defect clusters under high-energy particle irradiations such as voids, bubbles, and stacking fault tetrahedra have been reported so far. Spontaneous well-ordered periodicity can be developed on a broad surface by ion beam sputtering and a numerical model has been proposed as the formation process.

Here, we present our studies on the evolution of nanosized structures resulting from the sputtering of atoms from the exit surface of thin metal specimens during homogeneous electron irradiation, focusing on a novel self-organization phenomenon, which can occur on the electron irradiated surface [1–6] and give data on the temperature dependence of the formation of nanoholes for gold.

Results and Discussion

We found a new type of self-organized nanostructure formation on the exit surface of thin gold foils irradiated with high doses of 360–1250 keV electrons at temperatures of about 100 K. Fig. 1 shows a self-organized nanostructure generated on Au(001) foil surface at 95 K. The structure consists of aligned nanogrooves, which develop parallel to the surface, and nanoholes and hillocks, which grow parallel to the electron beam. The nanogrooves show strong irradiation-direction dependencies on their growth. They grow along [100] and [010] directions for [001] irradiation, along [100] for the [011] irradiation, whereas no clear grooves are formed for [111] irradiation. The widths of nanogrooves and holes are between about 1 and 2 nm, which are the smallest ones generated on metal surfaces so far. The final structures of the thin foils under electron irradiation are nanoparticles or nanowires. This method has been utilized to produce long gold nanowires for investigations of the interesting physics such as the electron transport properties and the multi-shell structure. Temperature dependence of the nanostructure for gold indicates that the effect of surface diffusion becomes significant above 240 K.

Furthermore, the self-organized structures for silver, copper, nickel, and iron are investigated. The formation of nanoholes and nanogrooves basically originates in the sputtering at the electron exit surface. The difference in the anisotropic growth of the nanogrooves and nanoholes among the kinds of metals should be attributed to the irradiation-induced anisotropic flow of point defects and some related factors, which are attributed to the nature of metals.

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