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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://www.SciRP.org/journal/ampc) 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 ABSTRACT 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). I. M. ARISTOVA ET AL. Copyright © 2012 SciRes. AMPC 61 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 π-orbitals. 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 (396–404 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 (404—420 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 CuPc–CuPc 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 E α hν α = 20 α = 50 α = 90 Photon energy (eV) Intensity (arb. units) NEXAFS N 1s CuPc/Au(001) 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 (396–404 eV) represent the π* resonances, whereas the features above 404 eV are related to the σ* resonances. (a) (b) 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. I. M. ARISTOVA ET AL. Copyright © 2012 SciRes. AMPC 62 (a) (b) 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). Acknowledgements This work was supported by the RFBR under grant no. 10-02-00269 and grant no. 11-02-01253. REFERENCES [1] Z. Liu, A.A. Yasseri, J.S. Lindsey, D.F. Bocian, Molecular Memories That Survive Silicon Device Processing and Real-World Operation , Science, vol. 302, 2003, pp. 1543-1545. [2] J.C. Scott, Is There an Immortal Memory?, Science, vol. 304, 2004, pp. 62-63; J.C. Scott and L.D. 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