In this study, large-area hexagonal-packed Si nanorod (SiNR) arrays in conjunction with Au nanoparticles (AuNPs) were fabricated for surface-enhanced Raman spectroscopy (SERS). We have achieved ultrasensitive molecular detection with high reproducibility and spatial uniformity. A finite-difference time-domain simulation suggests that a wide range of three-dimensional electric fields are generated along the surfaces of the SiNR array. With the tuning of the gap and diameter of the SiNRs, the produced long decay length (>130 nm) of the enhanced electric field makes the SERS substrate a zero-gap system for ultrasensitive detection of large biomolecules. In the detection of R6G molecules, our SERS system achieved an enhancement factor of >10 with a relative standard deviation as small as 3.9-7.2% over 30 points across the substrate. More significantly, the SERS substrate yielded ultrasensitive Raman signals on long amyloid-β fibrils at the single-fibril level, which provides promising potentials for ultrasensitive detection of amyloid aggregates that are related to Alzheimer's disease. Our study demonstrates that the SiNRs functionalized with AuNPs may serve as excellent SERS substrates in chemical and biomedical detection.

We report on a combination of lithography and self-assembly techniques which results in long-range two-dimensionally ordered Ge islands. Island lattices with perpendicular but also with obliquely oriented unit vectors were realized. Quantitative analysis of the island topographies demonstrates that the size dispersion of these islands is smaller than that found on flat substrates. Furthermore, island formation on the patterned substrates is observed for a smaller amount of Ge deposition. However, with further Ge deposition an increasing amount is incorporated into the sidewalls.

Two-dimensional (2D) periodic arrays of Ge islands were realized on prepatterned Si (001) substrates by solid-source molecular-beam epitaxy. Atomic-force microscopy images demonstrate that the Ge islands are formed in the 2D laterally ordered pits of patterned substrates. The 2D periodicity of the substrate pattern is replicated throughout a stack of Ge island layers by strain-driven vertical ordering. Photoluminescence spectra of the ordered Ge islands show well-resolved peaks of the no-phonon signal and the transverse-optical phonon replica. These peaks are observed at nearly the same energy as those of random Ge islands deposited under the same conditions on unpatterned Si substrates.

Unimodal SiGe islands with dominant f111g facets were grown coherently on pit-patterned Si (001) substrates by molecular beam epitaxy. With increasing Ge deposition, the f111g pyramids evolve from dome-shaped islands, reaching significantly larger volumes than are coherently possible on flat substrates. Finite element calculations and molecular dynamics simulations show that SiGe islands in pits can have less misfit strain with respect to islands of the same shape on flat substrates. The injection of dislocations is thus delayed, allowing for the observed development of coherent islands with a very high aspect ratio. DOI: 10.1103/PhysRevLett.98.176102 PACS numbers: 81.07.ÿb, 68.37.Lp, 68.37.Ps, 71.15.Pd Since the first evidence of coherent SiGe island formation [1,2], self-assembled SiGe islands on Si (001) have been extensively studied due to their compatibility with standard Si technology [3]. It is now well established that the evolution of SiGe islands with deposition results from a competition between the relaxation of misfit strain and the increase of the surface energy. After the initial formation of prepyramids [4] or mounds [5], islands bounded by f105g facets appear. They form square-based pyramids, or rectangular-based hut clusters [1], with a height-to-base aspect ratio (a=r) of 0.1. Further volume increase results in multifaceted dome islands with dominant f15; 3; 23g and f113g facets [6 -8], and an a=r of about 0.2. The increased average steepness of large islands is attributed to more efficient strain relaxation [9,10]. One would expect an island bounded solely by f111g facets to be the natural evolution at large enough volumes, due to a large steepness combined with a close-packed surface. However, at large volumes, strain relaxation by dislocation injection becomes competitive. Hence, under most experimental conditions, domes are followed by dislocated superdomes [11,12], which show an inverted trend in the a=r vs volume. The formation of coherent islands with a higher a=r than domes has only been reported for SiGe alloys of low Ge content [13][14][15]. Still, the barn-shaped islands in Refs. [13,15] have a lower a=r than the ones reported here .In this Letter, we show that a unimodal distribution of coherent SiGe pyramids with dominant f111g facets and a=r 0:37 can be realized on patterned Si (001) substrates by solid source MBE. Such f111g pyramids are the result of a shape transition from domes, accompanied by a substantial increase of the a=r. Molecular dynamics (MD) simulations and elasticity-theory calculations are used to explain the role played by the substrate pattern in altering the strain distribution and favoring coherence at high a=r.Patterned Si(001) substrates with two-dimensionally (2D) periodic pits along two orthogonal h110i directions were obtained by holographic lithography and reactive ion etching [16,17]. To create a well-defined starting surface, a 130 nm thick Si buffer layer was grown with 0:05 nm=s while ramping the temperature from 450 C to 600 C. On sample A only ...