Advances in Ma terials Physics and Che mist ry, 2012, 2, 60-62
doi:10.4236/ampc.2012.24B017 Published Online December 2012 (htt p://
Copyright © 2012 SciRes. AMPC
Morphology and Electronic Properties of Hybrid
Organic-Inorganic System: Ag Nanoparticles
Embedded into CuPc Matrix
I. M. Aristova1, O.Yu.Vilkov2, A. Pietzsch3, M . Tchaplyguine3, O.V. Molodtsova4, V.Yu. Aristov1,5
1Institute of Solid State Physics, Russian Academy of Sc ien ces, Cherno gol ov ka, R u ss ia
2Helmholtz Zentrum Berlin Mat&Energy, Berl in, Germa ny
3Lund University, MAX-lab, Lund, Sweden
4Hasylab, Desy, Hamburg, Germany
5European XFEL GmbH, Hamburg, Germany
Received 2012
Materials with a high on-off resistance ratio could become the basis for resistive random-access memor y (RRAM). It i s assu med th at
one of RRAM types can be based on hybrid organic-inorganic systems, while particular attention is focused on hybrid systems con-
sisting of metal nanoparticles (NP) embedded in organic matrix (OM). In this investigation we created and studied the hybrid organ-
ic-inorgan ic syste ms made of met al (Ag) nanop articles embedded in organic semicon ductor materia l CuPc. The LEED p atterns and
NEXAFS data demonstrate that the CuPc films deposited on Au(001) substrate are highly ordered and molecular planes lie paralle l to
the gold sur face. Th e metal at oms wer e depo s ited on the outer surfa ce of the or ganic mol ecular film an d self-assembled into nanopar-
ticles due to surface and bulk diffusion. The properties of nano-composite materials seem to be significantly dependent on the micro-
struct ure, i. e. the size, concen tration , bulk- and size-distribution of nanoparticles; therefore we have studied by high resolution trans-
mission electron microscopy the evolution of morphology of nano-composite films as a function of nominal metal deposition. The
filled and empty electronic states of the hybrid organic-inorganic syste ms, energ y level al ign ment at in terfaces formed between metal
nanop articles and th e organic semicond uctor CuP c as well as the chemical in teractio n at the NP/OM in terface were stud ied by UPS,
XPS and NEXAFS methods.
Keywords: Hybrid Materials; Nanoparticles; Organic Matrix; Morphology; Electronic Properties; Nonvolatile Memory
Nowadays there are tremendous worldwide efforts to develop
new memory devices for long-term storage of the information
[1,2]. The new kind of memory based on switchable resistive
materials is commonly ranked as resistive random-access
memory (RRAM). Hybrid systems, mainly consisting of inor-
ganic nanoparticles (NP) blended into an organic matrix have
been proposed as one type of RRAM [3,4]. Possible memory
architecture is obtained if an organic thin film containing metal
nanoparticls is sandwiched between cross-point arrays of elec-
trodes that consist of narrow metal stripes, running in perpen-
dicu lar directions above and below the film (see Figure 1).
The resistivity at “cross-points” can be switched either a
high- or low-conductivity state by applying a voltage, suitable
to write or erase.
Figure 1. Cross-point memory archit ectu r e
Above a thr eshol d volt age the device sudd enl y switches fro m
a high -i mpedan ce state t o a low-impedan ce state an d remains i n
that state even when the power is off. The resistivity in the
high- and low conductivity state can differ by 6-8 orders of
magnitude [5]. The high-impedance state can be recovered by
applying a voltage in the reverse direction. The corresponding
state can then be probed by measuring the current across the
crosspoints at some lower voltages. By defining the two states
it is possible to create digital memory devices. These two s tates
can be viewed as the r ealizatio n of no n-volatile electrical mem-
ory, thus rendering the structur es suitable fo r data storage.
Albeit, there is a rapid development in this area but the pre-
cise memor y mechani sm is st ill unclear an d many quest io ns left
unanswered [6-8]. Properties of the materials formed with par-
ticipation of nanometer-sized (tens or units nanometers) struc-
tural elements, are not identical with the properties of bulk
matter, so it is possible to consider such nanostructures as a
special state of substance. The properties of such composite
films are strongly linked to the particles’ nanostructure, i.e. the
size, concentration, bulk- and size-distribution of respective
nanoparticles. The most fertile model for mechanism that is
responsible for the large difference in resistivity between the
high and low conductivity states is based on charging of the NP,
which leads to strong modifications of the properties of the
organi c matrix mat er ial (see Review [5] and references therein).
Copyright © 2012 SciRes. AMPC
It is impossible to understand the underlying mechanism with-
out comprehensive information about the specific electron
structure and interface interactions between the NP’s and the
thin organic film matrix. The aim of the present work was to
investigate the diverse morphologically defined hybrid systems
composed of silver NP’s (Ag NP’s) distributed in a CuPc (or-
ganic semiconductor copper phthalocyanine) matrix, which is
formed by molecules with a wide energy gap and delocalized
The organic semiconductor copper phthalocyanine matrix, CuPc
thin film, for hybrid organic-inorganic system was grown on an
atomically clean Au (001) surface by deposition of organic
molecules evaporated from an effusion cell under ultrahigh-
vacuum (UHV) conditions.
No residual contamination was detected in core-level photoe-
mission (CL-PES) spectra. Morphology and ordering of the
CuPc overlayer were evaluated by means of LEED and near-
edge x-ray absorption fine structure spectroscopy (NEXAFS)
[9,10], which demonstrate that the CuPc films are highly or-
dered and molecular planes lie parallel to the gold substrate.
Figure 2 reveal a very strong angular dependence of the N 1s
π* intensities (396404 eV). At grazing incidence, when pola-
rization vector E is almost perpendicular to the subbstrate, the
intensities of these π* signals show a maximum, while reaching
a minimum, when E is almost parallel to the substrate. The
angular dependence of the N 1s − σ* intensities (404420 eV)
reveals the opposite trend. This behavior together with regis-
tered LEED pattern s demons trate s, that the CuP c molecules ar e
well ordered with the molecular planes lying parallel to the Au
(001) substrate. This observation proves that the CuPc–Au(001)
substrate coupling is stronger than the CuPcCuPc interaction.
Resistive evaporation of a high-purity silver wire wound
around a thoroughly degassed tungsten filament was used for
metal deposition on the outer surface of the organic film. Due
to surface and bulk diffusion of deposited Ag atoms the em-
bedded silver nanoparticles were self-assembled forming a
three-dimensional Ag NP’s distribution in the bulk of the or-
ganic semiconductor.
The size, concentration, size-distribution and shape of the
resulting nanoparticles were studied using transmission electron
microscopy (TEM) JEOL JEM-2100 operated at 200 kV. Thus
the microstructure and evolution of the morphology of the na-
no-composite films as a function of nominal silver coverage
were studied ( s ee Figures 3 and 4).
The hybrid organic (CuPc) inorganic (Ag) systems for
TEM investigations were prepared under UHV conditions on
cleaved NaCl single crystals as substrate using the same UHV
chamber, evaporators and the same parameters (temperature,
rate of CuPc deposition, organic film thickness, rate of silver
deposition, nominal silver coverage, etc.) as for the systems
grown on Au(001) surface. In order to bind the nano-composite
thin films these samples were then coated by an ultra thin over-
layer of amorphous carbon. Film separation from a substrate
always occurs easily by dissolution salt in water. Preliminary
the film was cut by square pieces with the side of about 2-3 mm.
Then samples are transferred on 250-mesh (250 lines/inch)
copper grids for TEM investigations.
Figures 3-4 presents bright-field TEM images of nanocom-
posite thin films microstructure. It composed of silver particles
(the black dots in the picture) embedded in an organic semi-
conductor matrix CuPc for nominal Ag deposition of 0.4 and
5.7 nm. The electron diffraction patterns for selected areas of
the samples with corresponding silver deposition prove that the
diffraction originates from silver NP’s. Figures 3-4 demon-
strate that the size of the Ag NP’s strongly depends on silver
coverage. For nominal Ag deposition of 0.4 nm (Figure 3)
silver nanoparticles form some particle distribution, while metal
nanoparticles are spherical. With further deposition one ob-
serves strong growth of individual grains. Large particles have
an irregular shape. The analysis of size distribution of silver
nanoparticles allows to assume the following: due to increase of
nominal coverage of sil ver some n earby nanop articles (b ecause
of increase their sizes) come to contact and coalesce, forming
larger particles.
395 400 405 410 415
α = 20
α = 50
α = 90
Photon energy (eV)
Intensity (arb. units)
Figure 2. N 1s NEXAFS spectra taken from CuPc film (7 nm thick),
deposited o n the Au(001) surface, ta ken at 3 diffe rent angles al pha
between the light polarization vector E a nd the normal to the sa m-
ple surface n. The inset indicates the experimental geometry. The
lower energy features (396404 eV) represent the π* resonances,
whereas the features above 404 eV are related to the σ* resonances.
Figure 3. Microstructure of nanocomposite thin film composed of
silver particles embedded in CuPc matrix (TEM) for 2 different (a
and b) magnifications. Nominal Ag deposition is 0.4 nm.
Copyright © 2012 SciRes. AMPC
Figure 4. Microstructure of nanocomposite thin film composed of
silver particles embedded i n CuPc matrix (TEM) fo r 2 different (a
and b) magnifications. Nominal Ag deposition is 5.7 nm.
Coalescen ce of two or more small p articles in one l arge par-
ticle lead s to reduct ion of parti cles surface are a and th erefore to
reduction of particles surface energy. Thus, we observe the
coalescence process of silver nanoparticles embedded in the
CuPc film surface. It leads to increase of average diameter of
the particles and thereby has an influence on size distribution of
silver nanoparticles.
The electronic structure (occupied and unoccupied states) of
the hybrid organic-inorganic systems, the energy level align-
ment at interfaces formed between Ag NP’s and the organic
semiconductor CuPc, as well as the ch emical in teraction at this
interface were stud ied by CL-PES, valence b and P ES (VB-PES)
and NEXAFS methods. The work function (WF) changes in-
duced by silver deposition on CuPc thin films were determined
from VB-PES data using the cut-off procedure. All PES and
NEXAFS measurements were performed at BESSY (Berlin)
and MAX-Lab (Lund).
This work was supported by the RFBR under grant no.
10-02-00269 and grant no. 11-02-01253.
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