Quantum Size Effects in the Growth and Properties of Ultrathin Metal Films, Alloys, and Related Low-Dimensional Structures

Abstract

This chapter addresses the quantum mechanical nature of the formation, stability, and properties of ultrathin metal films, metallic alloys, and related lowdimensional structures, with Pb as a primary elemental example. The emphasis is on the contribution to the overall energetics from the electronic degrees of freedom of the low-dimensional systems. As a metal film reduces its thickness, the competition between quantum confinement, charge spilling, and Friedel oscillations, all of electronic origin, can dictate whether an atomically smooth film is marginally, critically, or magically stable or unstable against roughening during the growth of such metal films. The "electronic growth" mode as emphasized here serves as an intriguing addition to the three well-established classic modes of crystal growth. In exploring electronic growth, Pb(111) films represent a particularly compelling example, not only because their stability exhibits unusually strong quantum oscillations but also because their physical and chemical properties can be tuned with great precision by controlling the film thickness or the chemical composition. Recent advances and the perpectives in this active area of film growth will be reviewed, with results from both theoretical and experimental studies. Historical ReviewLow-dimensional materials have been an important platform for the discovery of novel quantum phenomena, such as the quantized conductance and quantum Hall effects in semiconductor heterostructures [1] and giant magneto-resistance in metallic magnetic superlattices [2]. In these examples, fabrication of the low-dimensional medium was accomplished via classical thin film growth; yet the emerging properties are clearly quantum mechanical in nature. More recently, it has been shown that the size quantization of itinerant electrons in an ultrathin metal film also plays a decisive role in the early formation stages and kinetic stability of the films [3][4][5][6][7][8][9].

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

“…At specific film thicknesses standing electron waves are established between the interfaces, giving rise to quantum well states (QWS). 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.…”

Quantum confinement of electrons at metal surfaces

“…[5] Electron confinement gives rise to new electron states, which can alter the physical properties of the solid in a manner that is beneficial to applications such as spintronics, [6] quantum computing, [7] optoelectronics, [8,9] photovoltaics [10] and catalysis. [11] In particular, QWS play an important role in monitoring film quality, [12] chemical reactivity, [13,14] crystal growth, [15] magnetic interactions, [16][17][18][19][20][21][22] oscillatory quantum size effects [23] and electron correlation effects such as thinfilm superconductivity [24][25][26][27][28] and the Kondo effect. [29,30] The experimental real-space observation of electron standing waves at surfaces belongs to the most fascinating capabilities of scanning probe methods.…”