Quantum Confinement of Surface Electrons by Molecular Nanohoop Corrals

Abstract

Quantum confinement of two-dimensional surface electronic states has been explored as a way for controllably modifying the electronic structures of a variety of coinage metal surfaces. In this Letter, we use scanning tunneling microscopy and spectroscopy (STM/STS) to study the electron confinement within individual ring-shaped cycloparaphenylene (CPP) molecules forming self-assembled films on Ag(111) and Au(111) surfaces. STM imaging and STS mapping show the presence of electronic states localized in the interiors of CPP rings, inconsistent with the expected localization of molecular electronic orbitals. Electronic energies of these states show considerable variations correlated with the molecular shape. These observations are explained by the presence of localized states formed due to confinement of surface electrons by the CPP skeletal framework, which thus acts as a molecular electronic "corral". Our experiments suggest an approach to robust large-area modification of the surface electronic structure via quantum confinement within molecules forming self-assembled layers.

Nanoscale Crystalline Sheets and Vesicles Assembled from Nonplanar Cyclic π -Conjugated Molecules

“…To gain insight into the subtle arrangement of CPP molecules in the nanosheets on molecular resolution, [8]CPP, [8]CPP-pyrene, and [8]CPP-TPE were deposited on the surface of highly oriented pyrolytic graphite (HOPG) from THF solution (2×10 −5 M) and observed with high-resolution STM by using a high vacuum cryogenic STM system. Unexpectedly, in sharp contrast to a bright cyclic structure of flat-lying [8]CPPs in STM images observed at Au substrate surface in previous work [41], the bright short rods and dark elliptical cycles were clearly visualized in the molecular resolved STM images of [8]CPP, [8]CPP-pyrene, and [8]CPP-TPE three samples (Figures 4(a)–4(c)), suggesting again that the cyclic scaffold of individual [8]CPP molecules does not lie flat on the HOPG substrate. Instead, the CPP molecules stand up on the HOPG surface with certain tilt angle.…”

Open-boundary reflection of quantum well states at Pb(111)

“…Since the first experimental evidence for QWS was reported, using the reflection of low-energy electrons from Au films deposited on Ir(111), 7 there has followed a wealth of experimental and theoretical work on QWS for a wide variety of film-substrate combinations, 8 and the importance of QWS has been unraveled for, amongst others, monitoring film quality, 9 chemical reactivity, 10,11 crystal growth, 12 magnetic interactions, [13][14][15] and electron correlation effects such as thinfilm superconductivity [16][17][18] and the Kondo effect. 19,20 Lateral electron confinement to two dimensions has likewise been observed, with real-space images of surface electron standing waves in the vicinity of noble-metal step edges [21][22][23] constituting seminal work that has seen artificially fabricated atom assemblies used as quantum corrals for electronic surface states, [24][25][26] and nanometer-scaled clusters, 27,28 vacancies 29,30 and molecular networks 31,32 shown to effectively confine electron motion. Laterally confined surface states have also been observed on narrow terraces 33 and vicinal surfaces, [34][35][36] and recently electron confinement to graphene nanostructures has attracted considerable interest.…”

Theoretical Insights for Materials Properties of Cyclic Organic Nanorings

“…Actually, the self‐assembled (solid‐state) samples obtained for these systems has led to unique materials with impressive, distinctive, and tunable size‐dependent applications [ 43,44 ] such as: i) size‐selective encapsulation of optically active (e.g., fluorescent) guest molecules [ 45–50 ] and its fluorophore bioapplications [ 51 ] or its role as capturing agents [ 52 ] ; ii) controlled porosity andadsorption of small molecules in the cavities formed upon tubular‐like self‐assembly of the nanorings and its sensing applications [ 53–55 ] ; iii) creation of mechanically interlocked nanomachines and nanosystems, [ 56–60 ] macrocyclic homopolimerized compounds, [ 61 ] lemmiscular systems, [ 62,63 ] and Möbius belt‐shaped nanorings [ 64,65 ] ; iv) performance as organic molecular semiconductors with competitive properties (i.e., hole charge‐transport mobilities) compared with other state‐of‐the‐art organic systems; [ 66 ] v) quantum confinement of surface‐induced electronic states, [ 67 ] etc. Note that these ambitious goals go well beyond the envisioned pioneering use of these nanorings as molecular templates for the controlled growth of carbon nanotubes (CNTs) of defined size, chirality, and edges.…”