Confinement Of Surface State Electrons Research Articles

Engineering quantum states and electronic landscapes through surface molecular nanoarchitectures

Surfaces are at the frontier of every known solid. They provide versatile supports for functional nanostructures and mediate essential physicochemical processes. Being intimately related with 2D materials, interfaces and atomically thin films often feature distinct electronic states with respect to the bulk, which are key for many relevant properties, such as catalytic activity, interfacial charge-transfer, or crystal growth mechanisms. Of particular interest is reducing the surface electrons' dimensionality and spread with atomic precision, to induce novel quantum properties via lateral scattering and confinement. Both atomic manipulation and supramolecular principles provide access to custom-designed molecular superlattices, which tailor the surface electronic landscape and influence fundamental chemical and physical properties at the nanoscale. Herein, we review the confinement of surface state electrons focusing on their interaction with molecule-based scaffolds created by molecular manipulation and self-assembly protocols under ultrahigh vacuum conditions. Starting from the quasi-free 2D electron gas present at the (111)-terminated surface planes of noble metals, we illustrate the enhanced molecule-based structural complexity and versatility compared to simple atoms. We survey low-dimensional confining structures in the form of artificial lattices, molecular nanogratings or quantum dot arrays, which are constructed upon appropriate choice of their building constituents. Whenever the realized (metal-)organic networks exhibit long-range order, modified surface band structures with characteristic features emerge, revealing intriguing physical properties, such as discretization, quantum coupling or energy and effective mass renormalization. Such collective electronic states can be additionally modified by positioning guest species at the voids of open nanoarchitectures [...].

Dichotomous Array of Chiral Quantum Corrals by a Self-Assembled Nanoporous Kagomé Network

The confinement of surface-state electrons by a complex supramolecular network is studied with low-temperature scanning tunneling microscopy and rationalized by electronic structure calculations using a boundary element method. We focus on the self-assembly of dicarbonitrile-sexiphenyl molecules on Ag(111) creating an open kagomé topology tessellating the surface into pores with different size and symmetry. This superlattice imposes a distinct surface electronic structure modulation, as observed by tunneling spectroscopy and thus acts as a dichotomous array of quantum corrals. The inhomogenous lateral electronic density distribution in the chiral cavities is reproduced by an effective pseudopotential model. Our results demonstrate the engineering of ensembles of elaborate quantum resonance states by molecular self-assembly at surfaces.

Confinement of surface state electrons in self-organized Co islands on Au(111)

We report on detailed low temperature scanning tunneling spectroscopy measurements performed on nanoscale Co islands on Au(111) films. At low coverages, Co islands self-organize in arrays of mono- and bilayer nanoscale structures that often have an hexagonal shape. The process of self-organization is induced by the Au(111) ‘herringbone’ reconstruction. By means of mapping of the local density of states with lock-in detection, electron standing wave patterns are resolved on top of the atomically flat Co islands. The surface state electrons are observed to be strongly confined laterally inside the Co nanosized islands, with their wavefunctions reflecting the symmetry of the islands. To complement the experimental work, particle-in-a-box calculations were performed. The calculations are based on a newly developed variational method that can be applied to ‘2D boxes’ of arbitrary polygonal shape. The experimental patterns are found to fit nicely to the calculated wavefunctions for a box having a symmetry corresponding to the experimental island symmetry. The small size of the Co islands under study (down to 7.7 nm2) is observed to induce a strong discretization of the energy levels, with very large energy separations between the eigenstates up to several 100 meV. The observed standing wave patterns are identified either as individual eigenstates or as a ‘mixture’ of two or more energetically close-lying eigenstates of the cobalt island. Additionally, the Co surface state appears not to be limited to mono- and bilayer islands, but this state remains observable for multilayered islands up to five monolayers of Co.

Observing the lateral confinement of surface state electrons in room temperature stable metallic nanostructures

The lateral confinement of the surface state electrons of Cu(111) has been studied by Scanning Tunnelling Microscopy and Spectroscopy at low temperature. The confining nanostructures are Cu(111) islands embedded in a semiconducting Cu3N(111) film which completely isolate them from each other. The standing wave pattern observed reflect the shape of the edge of the islands, i.e. the positions of the confining potential as long as the islands are larger than twice the Fermi wavelength of the surface electrons. The interference pattern in smaller islands is more complex, reflecting the collective behavior of the electrons. When the width of the islands is, at least in one dimension, smaller than the Fermi wavelength, there is a clear shift in the energy of the bottom of the surface band towards the Fermi level. The depopulation of the surface state produced by lateral confinement might have important consequences with respect to the reactivity of these nanostructures.

Confinement of surface state electrons on Cu and Au field emitters

Field-emission images of clean Cu and Au emitters show a peculiar halo-like ring centered at the {1 1 1} pole. Typical diameter and width of the ring are ∼10 nm and ∼2 nm, respectively. Since we found no geometrical features around the {1 1 1} pole that gives rise to such a ring pattern, we interpreted the ring pattern as representing enhanced emission from an annular terrace that resonantly confines surface electrons. A simple analysis shows that the observed ring pattern appears at a terrace whose width nearly matches the confinement condition.

Confinement of Surface State Electrons in Fabry-Pérot Resonators

Ag(111) surface state electrons have been confined in symmetric and asymmetric Fabry-Perot resonators formed by two atomically parallel step edges. The local density of states in the resonators has been measured by means of low-temperature scanning tunneling spectroscopy and can perfectly be explained with a simple Fabry-Perot-like model. The energy dependent reflection amplitudes and scattering phase shifts of the different kinds of Ag(111) step edges have been determined with high accuracy. The model character of the resonators opens up quantitative electron scattering experiments at test structures brought into the resonator.