Emergence of surface states in nanoscale Cu2N islands

The bulk electronic band structure of solids first emerges from atomic orbitals in nanoscale clusters. The scanning tunneling microscope (STM) is a useful tool for studying the properties of nanoclusters on surfaces ranging in size from 1 to >10 4 atoms. Tunneling spectroscopy can probe the valence orbitals of single adatoms 1 and the evolution of quantumconfinementinatomicchains 2 oradatomislands 3,4 on metal surfaces. Recently, there has been considerable interest in isolating such structures from the metal surface by using an intervening, few-monolayer-thick insulating film (e.g., Cu2N, NaCl, Al2O3 ,M gO). 5‐7 Ultrathin insulating films are themselvesaformofnanocluster;priorstudieshaveshownthat a bulk-like band gap is already established in few-layer NaCl 8 and MgO films. 9 While these studies probed the influence of quantumconfinementinonedimensiondowntothenanometer scale, the lateral sizes of such films were too large to see additional confinement effects. Here, we study the emergence of electronic states in one-atomic-layer-thick Cu2N films, ranging in lateral area from quasi-continuous monolayers to sub-nm 2 islands. STM measurements reveal the quantum-confined blueshift of an unoccupied state with decreasing island area down to ∼1.5 nm 2 , corresponding to ∼50 atoms. While similar quantum-confined energy shifts have been observed in a variety of STM studies, 4 our studies extend down to smaller islands (0.5 nm 2 or 12 atoms), where the state itself is no longer observed. This trend is qualitatively reproduced in our density functional theory (DFT) calculations of the local density of states (LDOS). In contrast, a higher-energy state exhibits no systematic shift with area and persists down to the smallest islands studied. Our DFT calculations suggest that this contrasting behavior reflects the different effective masses of the two Cu2N bands, and their proximity in energy to a Cu surface state.