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Scientific Report 2007-2009
Condensed matter physics and biophysics
C34. Design of electronic properties at hybrid organic-inorganic
systems
The appealing optical, magnetic and transport properties
of organometallic materials, the virtually unlimited
choice of organic molecules and the processing flexibility
of molecular films has led to exceptionally rapid progress
in the development of organic devices over the past
decade. Engineering of these devices requires an atomic
level understanding of the parameters that control the
structure and the function of these flexible molecular architectures.
Today, several open questions enliven the
scientific debate, primarily referred to organic/inorganic
interface control (e.g. charge carrier injection, interaction
at the interfaces, metal-semiconductor transition,
spin coupling, etc.). Recent research has focussed on aromatic
oligomers, whose pi-conjugation guarantees charge
delocalization and electron mobility, an important issue
for possible band-transport, and whose typical energy
gaps lie in the visible energy range, with potential application
in novel opto-electronic devices. A crucial issue
for these organic-inorganic systems is the achievement
of long-range order in exotic configurations (twodimensional
arrays, one-dimensional wires) such as to
allow formation of exemplary hybrid structures with peculiar
electronic properties.
at the Fermi level (Fig. 1), as confirmed by highresolution
angular-resolved photoemission (HR-ARUPS)
and ab-initio calculations [1, 2]. The control of the electron/hole
injection barrier has been determined using an
organic buffer single-layer [3], whose mechanisms have
also been explained by a theoretical model valid for a
wide class of organic hetherojunctions[3].
Among the aromatic oligomers, metalphthalocyanines
(MPcs, M-C 32 H 16 N 8 ) are promising
active elements for many optical, electronic and
magnetic applications, and the central metal atom
in MPcs can play a crucial role to establish the electronic/magnetic
properties of the interface. A careful
control of the nature and character of the induced electronic
states at the interface with the metal substrate
is a crucial issue. We have succeded in building-up
highly-ordered MPc arrays on Au(110), and we determined
the energy band diagram by HR-ARUPS (Fig.
2). In particular, by using alkali-metal intercalation we
could tailor the energy gap, adjusting the hole-injection
barrier and observing electron correlation effects due to
the electron-injection into the localised states [4].
Figure 2: CuPc single-layer on Au(110): (left) interface band
dispersion; (right) spectral density of electronic states as a
function of K doping [4].
Figure 1: Pentacene nano-rails grown on Cu(119): (left)
STM image; (right) electronic spectral density of states [1].
Within this appealing research field, in the LOTUS
aboratory we have studied one-dimensional (1D) and
two-dimensional (2D) highly-ordered structures of π-
conjugated molecules assembled on single crystal metal
surfaces, presenting nanometer-scale patterning. Wellordered
nano-rails of pentacene (C 22 H 14 ) have been
grown on the Cu(119) vicinal surface, whose electronic
structure shows an enhanced density of electronic states
The control of the spin coupling of MPc molecules
with a central magnetic atom with the underlying metal
substrate can give rise to enhanced magnetic moments,
with new 1D and 2D architectures for the fabrication of
molecular spintronic devices. Objective of the on-going
work is the study of MPcs formed by a magnetic
central atom that can be used as chemical ”cage”
for anchoring the magnetic ion to a metal surface,
so that the spin-state of the central atom could couple
with the underlying magnetic or non magnetic metal.
References
1. A. Ferretti, et al., Phys. Rev. Lett. 99, 046802 (2007).
2. M. Chiodi, et al., Phys. Rev. B 77, 115321 (2008).
3. M.G. Betti, et al., Phys. Rev. Lett. 100, 027601 (2008).
4. A. Calabrese, et al., Phys. Rev. B 79, 115446 (2009).
Authors
M.G. Betti, C. Mariani, P. Gargiani
http://server2.phys.uniroma1.it/gr/lotus/index.htm
Sapienza Università di Roma 87 Dipartimento di Fisica