Self-Assembled Organic Films on Semiconductor
Surfaces
C. Emanuelsson
Institutionen för ingenjörsvetenskaper och fysik.
Tel.:054-7001807, E-mail: christian.emanelsson@kau.se
Abstract: Organic electronics have the potential to become an important resource to create
electronic devices in a wider variety of ways than today. One way of creating these
devices is to build layers of semiconductors and organic molecules. Knowing the
properties of the molecular layer at the interface give us important information about the
device. Scanning tunneling microscopy is one way of studying these interfaces. The thin
layer of PTCDI on silver covered silicon is well structured and grows layer by layer which
make it useful for device applications.
INTRODUCTION
In our modern world we are surrounded by different
electrical devices. Many of these are at some level
dependent on semiconductors like silicon. Examples
of devices are solar cells, light emitting diodes and
computer chips. In the last decades researchers have
studied organic semiconductors and how to use them
instead of inorganic semiconductors in various
electronic applications (Forrest, 1997).
There are several reasons why organic electronics
could be beneficial to use next to conventional
electronics. The organic electronics could be made
lighter and flexible meaning that they could be
applied to a wider array of applications than their
inorganic counterpart. Device engineers would also
have a huge number of organic semiconductors to
choose from so they can pick those that are best
suited for their application. Organic electronics also
open up for new and interesting ways to produce
devices. For instance you could have a printer that
could print a number of organic molecules and use
that to print electric circuits. And finally, organic
electronics could in principle be cheaper than
conventional electronics.
A common technique to create electronic devices
using organic semiconductors is to sandwich
different layers of molecules, semiconductors or
metals.
In my research I focus on the interface between two
of these layers. I study how organic films are grown
on different substrates. The substrates are
semiconductors, like silicon and germanium, covered
by a thin layer of metal like silver or tin. Several
important properties of these devices depend on the
quality of the molecular layer and its structure. Some
questions about the layer need to be answered: Are
the molecules well-ordered or random? Are they
lying down or standing up? Are there several
different structures or is the film homogeneous? Do
the film grow layer by layer or in as islands?
Examples of different film properties are presented in
Figure 1.
Fig. 1. Examples of different organic film properties
EXPERIMENTAL METHOD
To study the quality and structure of thin organic
films it is necessary to actually see the film and how
the molecules arrange themselves in it. This requires
an experimental technique that allows us to study
objects at the nanoscale. To do this we use what is
called Scanning Tunneling Microscopy, STM. The
basic principle is that if you have two conducting
objects with an voltage applied between them and
bring them close enough electrons can “jump” from
one to the other, this process is called tunneling. The
resulting current is very sensitive to the distance
between the objects and is therefore used to precisely
measure the distance between them.
In our experiments one of the objects is a sharp tip
and the other is the sample. The microscope is then
set to keep a constant current between and the tip and
the sample and it does that by keeping the distance
between the sample and the tip. By moving the tip
over the sample the movement of the tip resembles
the height of the sample and we can use this to create
an image of surface at the nanoscale, see Figure 2.
Fig. 2. The basic principle of STM
SOME RESULTS
The combination of PTCDI and this particular
surface has been studied before with less than one
complete layer of PTCDI. The hydrogen and oxygen
at the end of the molecules interacts to create onedimensional rows and two-dimensional islands. In
the islands the molecules also bind to each other in
rows but the molecules in adjacent rows are tilted
differently (Swarbrick et. al., 2005), see Figure 4 a).
In my work I wanted to see if the structure and
quality in the film changes as the film thickness
grows. I started from less than one complete layer
and went up to five complete layers. An example
STM image of the molecular layer is presented in
Figure 4 b). I could not observe any other structures
at low coverage or for higher coverage, so the film
keeps it structure for higher coverage. Also each
layer grew layer by layer, no island formation was
observed. Furthermore the higher layers were aligned
with the layer underneath.
In one of my experiments I studied the organic
semiconductor perylene tetracarboxylic diimide
(PTCDI). It has the interesting property that it can be
functionalized by changing the hydrogens connected
to the nitrogen at the ends to some other group, see
Figure 3. By functionalizing the molecule the
electrical, optical and charge-transport properties of
the molecule can be tuned and it has therefore been
studied for different types of applications (Delgado
et. al., 2010).
Fig. 4. a) Model of the structure found in the 2D
islands. b) STM image of the molecular film
REFERENCES
Delgado, M.C.R., E. Kim, D.A. da Silva Filho and J.
Bredas (2010), Tuning the Charge-Transport
Parameters of Perylene Diimide Single Crystals
via End and/or Core Functionalization: A Density
Functional Theory Investigation, Vol. 132, pp.
3375-3387.
Forrest, R. (1997), Ultrathin Organic Films Grown
by Organic Molecular Beam Deposition and
Related Techniques, Chemical Reviews, Vol. 97,
1793-1896.
Fig. 3. Regular PTCDI (left) and PTCDI
functionalized with methyl (right)
In the experiment PTCDI molecules were evaporated
onto a silicon substrate with one atomic layer of
silver on it. A pure silicon surface reacts strongly
with the molecules and they cannot move around
freely which prohibits the creation of well-ordered
films. The silver makes the surface less interactive so
the molecules can move around, interact with each
other and create well-ordered films.
Swarbrick J.C., J. Ma, J.A. Theobald, N.S. Oxtoby,
J.N. O'Shea, N.R. Champness and P.H. Beton
(2005), Square, Hexagonal and Row Phases of
PCDA and PTCDI on Ag-Si(111) √3 x√3 R30°,
Journal of Physical Chemistry B, Vol. 109,
12167-12174.