Size-Dependent Surface States of Strained Cobalt Nanoislands on Cu(111)

Abstract

Low-temperature scanning tunneling spectroscopy over Co nanoislands on Cu(111) showed that the surface states of the islands vary with their size. Occupied states exhibit a sizeable downward energy shift as the island size decreases. The position of the occupied states also significantly changes across the islands. Atomic-scale simulations and ab inito calculations demonstrate that the driving force for the observed shift is related to size-dependent mesoscopic relaxations in the nanoislands.PACS numbers: 73.20.At, It was recognized quite early that metallic particles exhibit unique properties that differ significantly from their bulk counterparts [1]. Nanoislands grown on metal surfaces, in particular, have been a matter of intense research for decades in view of prospective applications in a vast variety of domains ranging from magnetoelectronics, catalysis, optoelectronics, to data storage technology. The electronic, magnetic and chemical properties of a nanoisland are governed by the size, shape and structure of the island. These, in turn, are profoundly influenced by the lattice mismatch with the metal substrate and, for heteroepitaxial systems, also by the bonding interactions in the island/substrate interface (ligand effects). Metal islands tend to adopt the lattice parameter of the underlying surface [2], and as a consequence the bond lengths between the metal atoms in the supported nanoisland are different than those in the parent metals, resulting in changes due to strain. A theoretical study on homoepitaxial double layer Cu islands on Cu(111) [3], has shown that the strain produces an inhomogeneous distribution of bond lengths over the nanoisland, the average bond length varying with island size. The nanoislands also locally distort the surface and induce a displacement pattern in the substrate which affects the diffusion of atoms and, ultimately, the growth of the nanoislands.Despite these studies, our knowledge of how strain affects the properties of a metallic nanoisland, especially the electronic states, remains very limited. The fundamental problem of the change in energy upon lattice distortion in solids has first been addressed by J. Friedel within a simple model [4], and latter on extended to various problems of lattice contractions at metal surfaces and clusters [5]. On the same lines, it has been shown for thin films that strain effects, along with ligand effects, can cause a shift of the surface d band [6,7,8], resulting in chemical properties that are significantly different from those of the pure overlayer metal. Recently, a modification of electronic states due to a local strain field induced by a nanopattern formation has been observed for Cu(100) covered with N atoms [9].In this Letter, we specifically focus on the interplay between strain-induced structural relaxations and the surface states of Co nanoislands on Cu(111). These nanoislands constitute a reference system that has been extensively investigated by Scanning Tunneling Microscopy/Spectroscopy (STM/STS) [10,11,12,13,14]. By ...

Quantum interference effects in molecular spin hybrids

“…The electronic properties show no significant difference for faulted and unfaulted Co islands. Only the dlike surface state shifts slightly in energy (< 50 mV) for differently oriented Co island as well as for different island sizes 60 . These small variations are specific for each Co island and, thus, necessitate that the spin-polarized measurements, namely for magnetically parallel and antiparallel configuration, must always be carried out on the same island and molecule, because otherwise the data are not comparable.…”

Direct proof of mesoscopic misfit in nanoscale islands by x-ray absorption spectroscopy

“…It has been found that mesoscopic relaxations in a substrate can induce surprisingly fast diffusion of small clusters 43 , has a strong influence on the surface electronic structure of a nitrogen-covered metal substrate 56 and on the rates of chemical reactions on noble-metal surfaces 57 . The surface states over nanoscale islands are modified as the lateral size of an island changes, and this effect is also related to the size-dependent mesoscopic misfit 29,58 . Despite some indirect experimental evidence from stress 59,60 and reflection high-energy electron diffraction 61,62 experiments, the experimental verification of the concept of 'mesoscopic relaxations' came only recently using surface x-ray diffraction (SXRD) 47,48 by analyzing the static disorder of the Co atoms relative to the fourfold hollow sites of the Cu(001) substrate.…”

Engineering Negative Differential Conductance with the Cu(111) Surface State

“…In our setup, the counterelectrode can be conveniently chosen by posi- tioning a C 60 tip above nanoscale objects supported by Cu(111) or by changing the metal surface. Several counterelectrodes were inspected, either having a featureless electronic structure over the energy range of interest such as a Au atom on Cu(111) or the Cu(100) surface, either presenting a marked electronic structure such as a cobalt islands on Cu(111) [22], or a C 60 molecule on Cu(111).…”