Electronic properties of free and supported metal clusters studied by photoelectron spectroscopy

Abstract

Abstract. The electronic properties of free and supported metal clusters are studied by photoelectron spectroscopy. Experimental as well as theoretical results clearly demonstrate a dramatic dependence of the level structure on the cluster size. By this an interesting way might be opened to modify the electronic, optical and chemical properties of surfaces. 36.40.+d; 79.60.-i; 82.65.-i Optical and electronic properties of materials may change upon reduction of their sizes. This phenomenon is wellknown in the study of, e.g., thin films where the electrical conductivity is influenced by the confinement of the charge carriers to two dimensions. A three-dimensional confinement to a few A in diameter will induce discrete energy levels. Figure 1 schematically sketches the change in the density of states upon a reduction of the dimensionality. For the very small sizes, experimental and theoretical evidence suggests a picture of the valence electrons (e.g., in an alkali cluster) as a system of Fermi particles, being quantized in a mean field made up by the positive atomic cores. The order and spacing of the levels depends on the geometry of the confining volume as well as the details of the effective potential seen by the electrons. Such "quantum size effects" are well-known from, e.g., nuclear physics. They also play an important role in the transition regime between atom and solid-state physics [1,2]. PACS:Semiconductor quantum devices (e.g., ballistic transistors) are already used to tune electronic properties. The potential scope of these new devices gives rise to hope that simple binary switches might be replaced by complex multiswitches. As building blocks in integrated circuits they could lead to a dramatic increase of the frequency of logic steps as well as to a further decrease of the system size [3]. Metal quantum devices, on the other hand, would need even further reduced distances (~ 10 A compared to about 150 A for the case of semiconductors).Gas-phase metal clusters are well suited to model such confined systems: Early evidence has come, in particular,

Quantized Electrodes: Atomic Palladium and Gold in Polyaniline

“…[5][6][7][8] Atomic metal clusters can be prepared using electrochemical techniques [9][10][11] or by utilizing mass-selective beam technology. [12][13][14] With the latter technique, a beam containing mixtures of atomic clusters is separated by mass and selected clusters are deposited on an inert, insulating substrate and characterized spectroscopically. [15] This is an example of "top-down" method, and relies on the resolution power of mass spectroscopy.…”

“…In that state the difference between “bulk” and “surface” atoms disappears and such atomic clusters may exhibit unusual catalytic properties [5–8] . Atomic metal clusters can be prepared using electrochemical techniques [9–11] or by utilizing mass‐selective beam technology [12–14] . With the latter technique, a beam containing mixtures of atomic clusters is separated by mass and selected clusters are deposited on an inert, insulating substrate and characterized spectroscopically [15] .…”

Two photon photoemission of deposited silver clusters

“…4. With the exception of Ag 3 , it reveals an odd/even alternation that is well known from gas phase photoelectron spectroscopy of small silver clusters [26,27]. The odd/even effect was found for the ionization potential (IP) of neutral gas phase silver clusters [28,29] as well as for the electron affinity or the vertical detachment energy (VDE) of negatively charged gas phase silver clusters [27,30].…”

k parallel -resolved electronic structure of quasi-free 2-dimensional HfS 2 clusters

“…The electronic structure of clusters was of great interest in the last decade [1]. Research dealt, e.g., with band gaps tunable with cluster size [2][3][4], electron wave vector quantization [5], collective electron excitations [6], the evolution of valence band edges and core levels [7][8][9][10] and showed a strong dependence of the electronic density of states on cluster size [11][12][13]. In contrast to these investigations, focusing on electronic properties regardless of electron momentum, only little work has been done on the momentum (k )-resolved electronic structure of clusters [14].…”

“…[2]). However, strong bonding between cluster and substrate complicates or even prevents the investigation of intrinsic properties of the cluster itself [13].…”