Self-assembly of nanometer-scale magnetic dots with narrow size distributions on an insulating substrate

Improvement of the uniformity and dipole ferromagnetism in Co nanodots assemblies on Pb/Si(111) via step tuned dimensionality variation

We fabricated quasi-one-dimensional Co nanochain assemblies and two-dimensional Co nanodot assemblies on Pb/Si(111) substrates by step decoration. The morphology and magnetic properties of these two kinds of Co nanodot assemblies were investigated by in situ scanning tunneling microscopy and magneto-optical Kerr effect measurements. It was found that the steps cannot only improve the uniformity of the Co nanodots, but also increase the critical temperature T(c). Monte Carlo simulation indicates that the ferromagnetism mainly originates from the dipolar interactions and the critical temperature T(c) can be enhanced by introducing an in-plane uniaxial magnetic anisotropy via the step tuned dimensionality variation of the nanodot assemblies.

Manipulated nucleation of Fe nanostructures on Au(111) with combined growth methods

The manipulated nucleation of Fe on hexagonal close-packed (hcp) and face centered cubic (fcc) areas of an Au(111) herringbone surface was demonstrated with the combined growth methods of Xe buffer layer, seed effect and low temperature deposition. Besides the herringbone kinks, bi-layer stacking at fcc gaps was observed. This preference resulted in various nucleation patterns, such as zigzag stripes and doublet islands, by properly choosing the growth condition. The idea of nucleation control through combined growth methods can be applied in other nanopatterned templates for the fabrication of self-assembled nanostructures.

Pd atomic chain formation as a result of submonolayer deposition of 3d metals on Pd(110)

Submonolayer deposition of 3d transition metals such as Cr, Mn, Fe, Co, and Ni on Pd(110) at room temperature causes the formation of monoatomic chains of Pd as identified with scanning tunneling microscopy and spectroscopy. In agreement with recent theoretical predictions [Phys. Rev. B 79, 155410 (2009)], the substitution of Pd substrate atoms with the deposited atoms of 3d metals is found to be responsible for the formation of Pd atomic chains. This finding clarifies the long-debated issue about the chemical composition of the atomic chains grown on Pd(110) and points out the intriguing processes in the formation of self-assembled and self-organized nanostructures.

Self-limiting cyclic growth of gallium droplets on Si(111)

In this paper the growth of Ga droplets on a vicinal Si(111) surface is discussed. We report that the droplet size can be controlled down to the sub-100 nm range by carefully chosen deposition conditions. In addition, the dependence of the droplet size on deposition time is not monotonic, but shows self-limiting behaviour: with increasing amount of Ga on the surface the droplets stop increasing in lateral dimension and, instead, additional droplets of the same size are formed on the surface. Further, in the case of deposition at 300 °C substrate temperature it has been found that the growth proceeds in cycles. Thus, not only the size but also the droplet concentration can be controlled. In this way, non-ordered arrays of metallic droplets with a very narrow size distribution can be grown.

Distinctive fluctuations in a confined geometry

Spurred by recent theoretical predictions [Phys. Rev. E 69, 035102(R) (2004)10.1103/PhysRevE.69.035102; Surf. Sci. Lett. 598, L355 (2005)10.1016/j.susc.2005.09.023], we find experimentally using STM line scans that the fluctuations of the step bounding a facet exhibit scaling properties distinct from those of isolated steps or steps on vicinal surfaces. The correlation functions go as t0.15 +/- 0.03 decidedly different from the t0.26 +/- 0.02 behavior for fluctuations of isolated steps. From the exponents, we categorize the universality, confirming the prediction that the nonlinear term of the Kardar-Parisi-Zhang equation, long known to play a central role in nonequilibrium phenomena, can also arise from the curvature or potential-asymmetry contribution to the step free energy.

Nanopantography: a new method for massively parallel nanopatterning over large areas

We report a radically different approach to the versatile fabrication of nanometer-scale preselected patterns over large areas. Standard lithography, thin film deposition, and etching are used to fabricate arrays of ion-focusing microlenses (e.g., small round holes through a metal/insulator structure) on a substrate such as a silicon wafer. The substrate is then placed in a vacuum chamber, a broad-area collimated beam of ions is directed at the substrate, and electric potentials are applied to the lens arrays such that the ions focus at the bottoms of the holes (e.g., on the wafer surface). When the wafer is tilted off normal (with respect to the ion beam axis), the focal points in each hole are laterally displaced, allowing the focused beamlets to be rastered across the hole bottoms. In this "nanopantography" process, the desired pattern is replicated simultaneously in many closely spaced holes over an area limited only by the size of the broad-area ion beam. With the proper choice of ions and downstream gaseous ambient, the method can be used to deposit or etch materials. Data show that simultaneous impingement of an Ar(+) beam and a Cl(2) effusive beam on an array of 950-nm-diam lenses can be used to etch 10-nm-diam features into a Si substrate, a reduction of 95x. Simulations indicate that the focused "beamlet" diameters scale directly with lens diameter, thus a minimum feature size of approximately 1 nm should be possible with 90-nm-diam lenses that are at the limit of current photolithography. We expect nanopantography to become a viable method for overcoming one of the main obstacles in practical nanoscale fabrication: rapid, large-scale fabrication of virtually any shape and material nanostructure. Unlike all other focused ion or electron beam writing techniques, this self-aligned method is virtually unaffected by vibrations, thermal expansion, and other alignment problems that usually plague standard nanofabrication methods. This is because the ion focusing optics are built on the wafer.

Tunable metastability of surface nanostructure arrays

A Fokker-Planck equation is used to model the coarsening of surface nanostructure arrays. Metastable states are identified which are associated with a narrow size distribution and a coverage dependent mean island size. This is a general feature linked to nanostructures which, as a function of island size, are associated with a minimum in formation energy per atom and a positive chemical potential gradient. This has important implications for the self-organization of quantum dots.

Reabsorption effect and spatial ordering in island ripening

A model for detachment-limited island ripening is analyzed in terms of a reabsorption (or pedophagous) effect where islands releasing monomers can reabsorb their own offspring. When the effect is artificially turned off, ripened island arrays are spatially disordered and the island growth rates and size distribution are easily understood in terms of a classic mean field theory. With the pedophagous effect included, the arrays develop a spatial ordering consistent with experimental studies. The ordering is quantified through a correlation between island size and the number of nearby neighbors, yielding modified mean field island growth rates and a much-improved expression for the island size distribution. It is suggested that the spatial ordering and our explanation are widely applicable to ripening systems.