Interfaces - Molecular Organic Materials - page 149

surface reconstructions. In the case of graphite no reconstruction or relaxation is
observed because of its low γ s value. This is the reason why graphite is very easily

mechanically cleaved with e.g., sticky tape, exposing atomically flat surfaces with
micrometre-range terraces. In addition the generated surfaces are hydrophobic. The
stability of surfaces of many MOMs upon exposure to air is clearly evidenced by
the quality of the measured STM and AFM images, which exhibit molecular or
even submolecular resolution as thoroughly discussed in Magonov & Whangbo,
1996. This stability often allows ex situ analysis of such surfaces, avoiding the
rather involved in situ UHV measurements, although investigations performed as
a function of temperature impose the operation in UHV.
The STM was developed by G. Binnig and H. Rohrer (Binnig et al. , 1982) and
its working principle is based on the quantum mechanical tunnelling effect. When
two conducting samples are brought into close proximity (few nm), electrons from
one sample may flow (tunnel) into the other. The probability of an electron to
get through the tunnelling barrier decreases exponentially with the distance, hence
the tunnelling current I t at a constant applied bias V t is an extremely sensitive
measure of the distance between the two conducting samples. The STMmakes use
of this sensitivity. A sharp metal tip is brought mechanically into close proximity
of a flat conducting surface and V t is applied between the sample and the tip. In
a STM electrons flow from the tip to the sample for V t 0, thus exploring the

sample's unoccupied electronic states, and from the sample to the tip for V t

>

0,
thus exploring the sample's occupied electronic states. This is closely related to
inverse and direct photoemission, respectively. Representing the tip by an atom
with a single s -orbital and assuming small bias voltages, J. Tersoff and D. R.
Hamann (Tersoff & Hamann, 1985) showed that the spatial variation of I t can be
described by that of the partial local electron DOS associated with the energy levels
lying in the vicinity of E F . For metals the highest occupied and lowest unoccupied
states (around E F ) are similar in nature, so that the characteristics of their STM
images are independent of the bias polarity. Typically

<

1 V and I t lies in
the pA-nA range. Note that the applied electric field is rather large,

| V t |≤

1Vnm − 1

=
10 9 Vm − 1 , which can strongly perturb the surface molecular arrangement. In fact
this effect is used for the manipulation of single molecules.
Few examples of surface relaxation have been reported for MOMs. One of them
corresponds to the ac -surface of

∼

-(BEDT-TTF) 2 PF 6 (Ishida et al. , 2001), an or-
ganic 1D conductor along the c -axis exhibiting a metal-insulator transition at

β

297
K associated with the doubling of the c -parameter below the transition temperature
(see discussion in Section 1.7 concerning the band gap opening at E F ). The max-
imum value of

∼

− 1 cm − 1 and below the transition σ along the c -axis is ∼ 10-20

temperature E a 2 eV (Kobayashi et al. , 1983). In the crystal, BEDT-TTF and

PF 6 layers are alternately stacked along the b -axis. In the case of PF 6 -terminated

0 .