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Master degree on Nanostructured Materials for Nanotechnology Applications USE OF HIGHLY CONJUGATED ORGANIC COMPOUNDS FOR THE FABRICATION OF METAL / MONOLAYER / METAL DEVICES Jorge Trasobares Sánchez SEPTEMBER 2012 2 PILAR CEA MINGUEZA profesor titular del Departamento de Química Física de la Universidad de Zaragoza y SANTIAGO MARTÍN SOLÁNS, contratado Ramón y Cajal del Departamento de Física de la Materia Condensada de dicha Universidad CERTIFICAN: Que el trabajo presentado en esta memoria por Jorge Trasobares Sánchez como Proyecto Final del Máster Universitario en Materiales Nanoestructurados para Aplicaciones Nanotecnológicas y que lleva como título “Use of highly conjugated organic compounds for the fabrication of metal / monolayer / metal devices”, ha sido realizado en el Departamento de Química Física de la Facultad de Ciencias de la Universidad de Zaragoza, bajo su dirección, autorizando la presentación de la misma para su calificación por el Tribunal correspondiente. Y para que así conste, se expide el presente certificado, en Zaragoza, a 3 de septiembre de 2012. Fdo: Pilar Cea Mingueza Fdo: Santiago Martín Soláns 3 Content Resume ............................................................................................................................. 4 1 Introduction ................................................................................................................... 5 1.1 Revision of the State of the Art .......................................................................................... 5 1.2 Objectives ............................................................................................................................ 11 2 Experimental procedures ............................................................................................. 12 2.1 Materials .............................................................................................................................. 12 2.2 Fabrication and Characterization Techniques ................................................................ 14 2.2.1. Fabrication techniques................................................................................... 14 2.2.2. Characterization techniques .......................................................................... 15 3 Rupture of a Metal organic compound to form the Top contact electrode.................. 19 4 Deposition of a Metal layer by Stencil Lithography ................................................... 26 5 Graphene Oxide ..................................................................................................................... 31 6 Conclusions ................................................................................................................. 35 Appendix 1 ..................................................................................................................... 36 Acknowledgements ........................................................................................................ 37 References ...................................................................................................................... 38 4 Resume The progressive miniaturization of the components used in electronic devices has allowed the development of technology as it has been described by Moore's law. However, silicon technologies is reaching the limits of miniaturization; thus, exploring new fields is necessary to progress in the manufacture of smaller, more efficient and with new properties devices. Molecular electronics is presented as an important technology for developing a hybrid technology (Si-electronics) in the near future and projecting a complete replacement of silicon technology in a far future. On the other hand, with the emergence of sophisticated assembly techniques as a consequence of the progress in Nanoscience, it is expected that molecular electronics maintain the current miniaturization process. Therefore, this project is focused on the assembly of molecules by the Langmuir-Blodgett technique with special attention on one of the challenges set by the ITRS (International Technology Roadmap for Semiconductors): the fabrication of a top electrode contact onto an organic monolayer without damaging it. To achieve this objective, this work presents three different approaches. The first one, is the rupture of an organometallic compound containing a gold atom in the backbone; the breakdown of these molecules by annealing produce the reduction of the gold atom generating gold nanoparticles which remain onto the monolayer acting as the top metal electrode. The second approach is to use the new technology "Stencil Lithography". This lithography consists on using a mask for evaporating small amounts of metal onto a monolayer, minimizing the possibility of generating a short circuit. Finally, we have studied the formation of graphene oxide monolayers by Langmuir- Blodgett technique; these monolayers would be deposited on an organic monolayer and after reduction acts as the top semiconductor electrode. 5 1 Introduction This work, which constitutes the final master project of the Master in Nanostructured Materials for Nanotechnology Applications of the Zaragoza University, has been developed at the Physical Chemistry Department of Zaragoza University and also at the National Center of Microelectronics of Barcelona (CNM-CSIC). The objective of this project is to open new research lines to provide some insight into one of the most challenge bottlenecks in molecular electronics: the construction of the topcontact electrode without inducing any damage to the organic monolayer beneath. To do so, both top-down and bottom-up approaches have been used. In the next section a review of the State of the Art on molecular electronics is presented together with the objectives of this work. In chapter 2, the experimental procedures, together with the main techniques and materials used along this project are shown, in chapter 3, the experimental results for the rupture of a metal organic compound to form the top-contact electrode are presented, and in chapter 4, the results related to the deposition of a metal layer by stencil lithography are reported. In chapter 5, graphene is presented as a possible alternative to form the top-contact electrode. Finally in chapter 6 summarizes the conclusions of this work. 1.1 Revision of the State of the Art Manipulating matter on an atomic scale, arbitrarily located between 100 and 0.1 nm[1], is a key technology for the present and the future which is known as Nanotechnology. Accordingly, billions of dollars have been invested in research areas related to Nanotechnology with an exponential grown in the number of publications [2- 3]. Nevertheless, Nanotechnology is not exactly new; as one goes deeper into the subject more old uses of the properties of matter at nanoscale one can find. For example, nanoparticles (NPs) of gold and silver coloring of clear-cut yellow, red or blue stained glass windows of medieval churches, where color depends on the nature and diameter of the NPs. Romans also hid their white hair applying a paste of chalk and lead oxide 6 (procedure not recommended due the lead toxicity, but interesting because the reaction of the sulfur in hair reacts with the oxide, growing black nanocrystals of lead sulfide). It was Richard Feynman the first scientist to notice that there is nothing in physics that prevents the use of atoms and molecules as if they were bricks with which to construct molecular buildings. His famous lecture at the American Physical Society Meeting at Calttech on the 29th December 1959 is well known. Scientific research groups are attracted by the possibility of direct manipulation of individual atoms or molecules and companies are interested in the profits that this knowledge can offer. Although Richard Feynman suggested the possibility of making nanoscale machines, the term Nanotecnology (first used by Norio Taniguchi, 1974) was popularized by K. Eric Drexter in his popular 1986 book, “Engines of Creation: The Coming Era of Nanotechnology”. This author suggested that these tiny machines could make copies of themselves via computer control instead of being control by human operators [2]. There are many different and multidisciplinary fields of applications of Nanotechnology and the ability to build with atomic precision has already let developing materials with new and improved magnetic, tensile, thermal and electrical properties. A lot of sectors are benefited of this knowledge, such as medicine (e.g. drug delivery systems or medical diagnostic tools, such as cancer tagging mechanisms and lab on a chip providing real time diagnostics); energy and environment where nanostructured photovoltaic cells are being developed as well as filters incorporating NPs which have excellent properties for liquid filtration. Textile industry, production of artificial skin and muscles among others fields also take advantage of the Nanotechnology [4]. Another important field of applications is Nanoelectronics, which studies the phenomena of transport and distribution of charge and spin on the nanometer scale [5]. This project is intimately related to the field of Nanoelectronics, and more specifically, on Molecular Electronics which hunts for incorporating molecular components as functional elements in electronic devices [6]. Moore´s Law was coined around 1970 by the Caltech professor Gordon Earl Moore. In his publication “Cramming more components onto integrated circuits” [7] Electronics Magazine 19th April 1965 we can read that “The complexity for minimum component costs has increased at a rate of roughly a factor of two per year...” it means 7 that the number of transistor can be doubled every year. In 1975, Moore altered his projection to a doubling every two years. Consequently with this miniaturization, the International Technology Roadmaps for Semiconductor (ITRS) [8], which gives guidance on research and development in the semiconductor field, notices three main research domains in its Nanoelectronics Roadmap, called: “More Moore”, “More than Moore” and “Beyond CMOS1”, Figure 1.1. These two latter domains evade the aggressive Moore’s law which is not anymore the unique or the main driver. Figure 1.1. Scheme suggesting in a qualitative way the evolution of the device size relevant to this division of nanolectronics [9]. The first domain, called “More Moore”, trends for increased performances with technologies related to the nanometer CMOS. This trail has been leading the silicon technology, but its time is close to be over due to technological reasons. In addition, the huge investments needed to build new production lines in this silicon technology are expected to be so high that this technology might not be profitable in the near future. At least in the next 15 or 20 years, the use of dielectrics like Hafnium or the optimization of the surface into a wafer is not enough to front some significant limitations to this technology [10]. The second trend is designated as “Moore than Moore”. It is characterized by functional diversification of semiconductor-based devices. These non-digital functionalities do contribute to the miniaturization of electronic systems, although they do not necessarily scale at the same rate as the one that describes the development of digital functionality. This domain based on the engineering of complex systems 1 CMOS: Complementary Metal-Oxide-Silicon 8 combines various technologies (not exclusively electronics) in order to meet certain needs and challenging specifications of advanced applications. Finally, the “Beyond CMOS” domain, addresses with disruptive technology and device principles with the ability to scale the functional performances of information processing beyond the ones of ultimately scaled CMOS in order to truly exploit atomicscale technology (from semiconductor to molecular technology). Novel switches, architectures for universal memory and new interconnect approaches are some of the identified targets. In this context, Molecular Electronics pursues the use of molecules to build up electronic devices which in the future might substitute the conventional CMOS devices. Molecular Electronics involves the study and application of molecular building blocks for the fabrication of electronic components using monomolecular films incorporating functional organic materials as well as single-molecule electronic components. The assembly of molecular building blocks for the fabrication of electronic components includes both passive (e.g. resistive wires) and active components such as transistors and molecular-scale switches. The seminal work from Aviram and Ratner in 1974 is considered the origin of molecular electronics. This contribution generated many expectations about preferential conduction in one direction in molecular materials. Additionally, conductive polymers and some basic ideas about the electronic behavior of organic compounds were emerging in the literature of the time. A remarkable event was the discovery of conductive polymers, awarded with the Nobel Prize in Chemistry in 2000 shared by Alan J. Heeger [11], Alan G. Mac Diarmid [12] and Hideki Shirakawa[13]. Some organic materials have been demonstrated to offer electrical properties similar to those of metals and semiconductors, but with improved mechanical properties, processing advantages and low production costs. Organic molecules assembled into electronic devices can offer advantages such as their small size, high speed of process, low costs of fabrication, less consume, etc. Limitations include the low stability of organic materials at high temperatures [14-15] and the necessity of fabricating exceptional robust contacts between metal and molecules. 9 In 2009 the International Technology Roadmap for Semiconductor, ITRS [8], has pointed out the main bottlenecks that should be overcome before Molecular Electronics can be incorporated in the market. The most important challenges in molecular electronics are: • Look for new metal/molecules interfaces in order to get efficient contacts. • Understand and control the behavior of metal/molecules/metal interfaces under an electric field. • Developing easy, robust, and efficient methods to deposit the top-contact electrode onto the organic monolayer without damage of the monolayer and without penetration of the metal through the organic material producing a short circuit. • Define protocols or design assembly strategies for precise placement of molecular materials within device structures. The research group Platon at the University of Zaragoza is working on these challenges. In particular this project is focused on the development of new alternatives for the fabrication of the top-contact electrode. Therefore, in the following lines, a revision of the main methods used in the literature for the deposition of this top-contact electrode is presented together with their limitations which highlight the need of finding new protocols for the deposition of the second electrode. Until now there is no technique to achieve a robust contact between the molecule and the top-contact electrode [16]. It is possible to evaporate directly the metal over the monolayer however generally short-circuits are formed in this process due to the penetration of the metal through the monolayer and contact with the underlying electrode. In order to decrease the energy of the metal atoms when reach the monolayer surface there are some alternative methods such as: refrigeration of the substrate (~100 K), introduction of an inert gas in the evaporator chamber or the use of deflectors that block the direct pass from the evaporator to the sample [17-18]. Another approach consists on the deposition of a drop of mercury on the monolayer; the equipment is simple and due to the high surface tension of mercury no penetration of the metal through the layer occurs. However, mercury tends to form oxides which change the electrical properties of the devices and additionally mercury 16 (1) where  is the Brewster angle and n1 and n2 are the refraction index of air and water, respectively. When the monolayer is spread onto the water surface and the angle of incidence is kept constant, a small part of the incident light beam is reflected due to the floating material (change in the refraction index) and by means of appropriated software, pictures of the monolayer can be obtained. The photographs show different levels of grey depending on the monolayer thickness and orientation. UV-Vis reflection spectroscopy Reflection spectroscopy allows determining the presence and orientation of chromophore units on the surface. The measurements were performed using the spectrophotometer RefSpec Nanofilm (Göttingen, Germany). The light source is a lamp DTM FiberLight 6/50 comprises two deuterium and tungsten lamps installed in a ceramic sensor. The working range of the spectrophotometer is 240 to 1000 nm. The reflection spectroscopy technique consists on radiation impinging perpendicularly on the surface (a light trap is placed inside the aqueous subphase to avoid contamination by reflection from said subphase). The reflection does not change significantly unless the molecules absorb radiation, but when the molecules absorb the reflection increases according with equation 3: (2) where RS and RD,S represent the intensity of the beam reflected by the aqueous surface before and after the film formation, respectively; ΔR is the variation of the radiation and AD is the absorbance characteristic of the monolayer. This equation is only valid for small values of AD. The parameters to determine experimentally are RD,S and RS, meanwhile ΔR is obtained directly. Equation 3 shows a linear dependence of the increase of the reflection respect to the chromophore density at the interface. A reflection spectrum has the same shape as an absorption spectrum. 17 Quartz Crystal Microbalance (QCM) Quartz Crystal Microbalance measurements were carried out using a Stanford Research Systems instrument. The sensor is a thin disk of -quartz, with a thickness of about 331 µm, which is cut in the direction AT (35° 15’). The circular disc presents gold electrodes on both sides and its nominal frequency of oscillation is ca. 5 MHz. Due to the piezo-effect, an AC voltage across the electrodes induces a shear deformation and vice versa. By using the Sauerbrey equation [32] the variation in mass can be determined (and consequently the surface density of the film deposited onto the QCM substrate): (3) where f0 is the fundamental resonant frequency of 5 MHz, Δm is the mass change (g), A is the electrode area, ρq is the density of the quartz (2.65 g·cm-3), and μq is the shear modulus (2.95 · 1011 dyn·cm-2). Atomic Force Microscopy (AFM) Atomic Force Microscopy is one of a large number of scanning probe microscope methods (SPMs) which permits the determination of images of a surface using a physical probe which scans the sample. The scanning is obtained by mechanically moving the probe, line by line, and recording the probe-surface interaction as a function of the position. There are two modes of operation: contact mode and tapping mode. In this work, the images of the monomolecular films were obtained using an AFM Multimode 8 (Veeco) belonging to the Laboratory of Advanced Microscopies (LMA). The characterization of the films was carried out using the tapping mode with a silicon tip from Bruker with a resonance frequency of 300 kHz and a constant force of 40 mN and a sweep rate of 1 Hz. To perform the electrical measurements a Veeco conducting AFM (C-AFM) belonging to the CNM was used in contact mode, applying a voltage from -4 to 4 V. 18 Optical microscopy Optical microscopy is a type of microscopy which uses light and a system of lenses to magnify images of small samples. In this work, a Nikon Eclipse ME600 is used to check the state of stencil shadows and also to localize the top-electrodes deposited through them. Scanning Electron Microscopy (SEM) Scanning Electron Microscopy (SEM) is a type of electron microscope which images a sample by scanning it with an electron beam. Electrons interact with atoms in the sample resulting in the emission of secondary or back-scattered electrons, which provide information about the topography and composition of the sample. The equipment used in this work is a JEOL JSM 6400 instrument which belongs to the Laboratory of Advanced Microscopies (LMA). The resolution is 3.5 nm using a 35 keV electron beam and a focal distance of 8 mm. X-Ray Photoelectron Spectroscopy (XPS) The measurements were carried out using a Kratos AXIS Ultra DLD spectrometer with a monochromatic X-ray source of aluminum [Al Ka (1486.6 eV)]. The spectrometer is located in the laboratories of the Institute of Nanoscience of Aragon (INA). The XPS spectra are obtained when a sample is irradiated with X-rays while the kinetic energy is measured and the number of electrons is stripped from the analyzed surface. Finally, the analysis photoelectrons energies and intensities provide information of the oxidation state of certain constituents of the material incorporated in the film. 19 3 Rupture of a Metal organic compound to form the Top contact electrode In this section, the results obtained for one of the strategies used to form the top contact electrode, rupture of a metal organic compound, are showed. The molecule chosen for carrying out this study was presented in chapter 2 as well as the technique used to ensemble this compound: the Langmuir-Blodgett method. Additionally to the condition exposed in chapter 2, in this section a MeOPhAu- Tolan-NH2 10-5 M in chloroform was used and a volume of the spreading solution that yields an initial surface coverage of 1.25 nm2·molecule-1. In addition, the solution was sonicated for 10 minutes before the spreading process to minimize aggregation phenomena. Figure 3.1 shows a reproducible surface pressure-area per molecule (  -A) isotherm of MeOPhAu-Tolan-NH2 at the air-water interface. The π-A isotherm is characterized by a zero surface pressure in the 1.4-0.45 nm2·molecule-1 range, featuring a lift-off at 0.45 nm2·molecule-1 followed by an increase of the surface pressure upon compression. Changes in the slope reveal a progressive orientation and/or reorientation of the molecules at the interface during compression. Figure 3.1. Surface pressure vs. area per molecule (π-A) isotherm of MeOPhAu-Tolan-NH2 at 20 ºC. 20 Brewster angle microscopy (BAM) images were recorded upon compression process of the monolayer to get more information about its characteristics, Figure 3.2. It is important to note here that in spite of the tendency of this compound to aggregate as a consequence of - lateral interactions and aurophilic lateral interactions between MeOPhAu-Tolan-NH2 molecules no three-dimensional aggregates were observed in BAM images under the experimental conditions used to fabricate these films. Nevertheless, a rapid increment in the brightness of the images upon compression after the lift-off is observed indicating that the film thickness of the monolayer increases. In addition, at higher surface pressures (e.g., 14 mN·m-1) BAM images show that the monolayer covers practically all the water surface. Figure 3.2. BAM images of MeOPhAu-Tolan-NH2 at the air-water interface at the indicated surface pressures. Molecular organization of Langmuir films upon compression was investigated in situ by UV-vis reflection spectroscopy through the reflection of light under normal incidence. A normalized spectrum is used to eliminate the dependence with the surface density (ΔRnorm= ΔR·Area per molecule in which the ΔR is the reflection). Normalized spectra are showed in Figure 3.3. As can be observed, the intensity of the normalized spectra decreases upon compression of the monolayer until a surface pressure of 16 mN·m-1 revealing a more vertical orientation of the molecules respect to the air-water interface during the compression process. At higher surface pressures, the general trend is not fulfilled and the normalized reflection spectra intensity increases slightly or keeps constant indicating that the collapse of the monolayer could have been achieved. 21 Figure 3.3. Normalized reflection spectra of MeOPhAu-Tolan-NH2 upon compression at the indicated surface pressures. According to the results obtained, we can say that an adequate surface pressure to transfer the Langmuir layer onto a substrate could be 14 mN·m-1. In addition, monomolecular films were transferred onto mica at several surface pressures to corroborate the optimum surface pressure of transference and to check the morphology of the LB films. Figure 3.4 AFM images and sections of a one-layer LB film of MeOPhAu-Tolan-NH2 transferred at the indicated surface pressures. Root mean square (RMS) roughness values are also indicated for each film. 22 The optimum surface pressure to transfer the Langmuir layer onto a substrate was demonstrated to be 14 mN·m-1 since lower or higher surface pressures yield to films with a higher root mean square (RMS) roughness and the evidence of holes or three dimensional aggregates in the film, see Figure 3.4. In addition, atomic force microscopy images also revealed the formation of highly homogeneous films with low defect densities at 14 mN·m-1. Therefore, Langmuir monolayer were transferred onto solid substrates, initially immersed in the sub-phase, by vertical dipping method at the surface pressure of 14 mN·m-1. One or several layers can be transferred by this method, see table 3.1. At this surface pressure, the transfer ratio (the relationship between the surface coverage of the monolayer at the air-water interface and the surface coverage in the substrate) was 1. The experimental surface coverage as well as the transfer ratio was also verified by the QCM; the frequency change (Δf) for a QCM substrate before and after the transference (Δf = - 48 Hz) provides the mass change (Δm) making use of the Sauerbrey equation, equation 2.2. With this value a surface coverage of Γ = 4.68·1014 molecules·cm-2 was obtained. The theoretical surface coverage is given from the molecular area of the MeOPhAu-Tolan-NH2 monolayer at the air-water interface at the surface pressure of 14 mN·m-1 which is Γ = 4.76·1014 molecules·cm-2. This result confirms that the transfer ratio is close to 1. Table 3.1. Transference ratios calculated by QCM for several layers. Nº layer Transference ratio 1 1,018 2 0,997 3 0,997 4 0,958 5 0,977 These values are very close to one for all transferences revealing that multilayers of this compound can be transferred, as has already said above. As mentioned in the objectives of this work, the purpose of these studies is to fabricate the top contact electrode. In this sense, the experimental results detailed in the next paragraphs will demonstrate that annealing the films leads to the rupture of C-Au- 23 C bonds as has been already observed previously for solutions of MOC compounds by Chico et al.[33]. This process implies the reduction of gold(III) to gold(0) with the subsequent deposition of these gold nanoparticles (NPs) on top of the organic film while the -methoxy-isocyanidebencene group is eliminated by thoroughly rinsing the film with chloroform, scheme 3.1. Scheme 3.1. Rupture of C-Au-C bonds after annealing and formation of gold NPs on the film surface. The 4-methoxy-isocyanidebencene group is eliminated by thoroughly rinsing the film with chloroform. The tolan group remains attached to the gold substrate by chemisorption of the amine group onto gold substrates. A systematic study in which the films were annealed at different temperatures and during different periods of time was performed in Platon´s Group [34]. This study shown that the optimum conditions to fabricate the top contact electrode by annealing a organometallic compound by the rupture of the C-Au-C bond were to anneal the monolayer at 150 ºC during 2 hours. Therefore, these conditions were the used in this work to try to fabricate the top contact electrode with our compound. QCM measurements were performed before and after the annealing process to verify the possible rupture of the C-Au-C bond. The variation of the frequency was Δf =13 Hz which would correspond to the loss of the 4-methoxy-isocyanidebencene group as is required to achieve our objective. Nevertheless, to be sure that this group is eliminated during the annealing process and that the reduction of gold(III) to gold(0) occurs experiments using XPS were performed.For XPS experiments, Langmuir films were transferred at a pressure of 14 mN·m-1 onto quartz substrates and keeping in a desiccators for 24 hours. In these experiments, our attention was focused on the nitrogen and gold atoms. Figure 3.5 shows the XPS spectra of MeOPhAu-Tolan-NH2. 24 Figures 3.5a and b show XPS in N1s range before and after annealing. It can be appreciated that while Figure 3.5a shows two peaks at 400.04 and 399.12 eV corresponding to CN group and amine (NH2) group, respectively; in Figure 3.5b only a peak at 399.89 eV is observed revealing the loss of the 4-methoxy-isocyanidebencene group. On the other hand, in Figure 3.5c, Au4f region, two peaks at 84.09 and 87.77 eV are observed which correspond to gold(III). However, in Figure 3.5d, after annealing the film, we observe how the contribution of these peaks decreases and how two new peaks at at 87.16 and 83.54 eV appear. These new peaks are associated to the presence of gold(0) in the sample, so this result reveals the reduction of gold(III) to gold(0), although the reduction is not completed, and confirm the generation of gold nanoparticles onto the monolayer. Figure 3.5 XPS spectra of MeOPhAu-Tolan-NH2. (A) XPS in N1s range before annealing, (B) after annealing, (C) XPS in Au4f range before annealing and (D) after annealing. Further evidence of the formation of gold NPs was provided by AFM and SEM experiments. Figure 3.6 illustrates AFM images before and after the annealing process. As can be appreciated, there is a great presence of brightness spots and an increase of the roughness (RMS) once the annealing process had taken place which, together with the XPS results, reveal that gold NPs have been formed onto the molecular surface. 25 Figure 3.6 AFM images of MeOPhAu-Tolan-NH2. Before (A) and after (B) annealing. Finally, the images obtained with a SEM before and after the annealing process, Figure 3.7, also showed the presence of gold NPs in the monolayer, brightness spots are observed after annealing the sample in contrast with the absence of these for the image without annealing, Figure 3.7a. Figure 3.7 SEM images of MeOPhAu-Tolan-NH2. Before (A) and after (B) annealing. 32 functional groups containing oxygen attached to various sites; carboxylic acids at the edges, and hydroxyl or epoxide groups at the basal plane, Figure 5.1. These dispersions can be processed and assembled by spin and spray coating [47], transfer printing [48], dip-coating [49], electrophoretic deposition [49] or Langmuir-Blodget technique [50]; method used in this work Figure 5.1 Schematic representation of the structure of a GO layer. Hydroxyl groups and epoxy disrupt electron conjugation graphitic network through sp3 bond formation. Dispersions of GO 8.8 mg/L in 5:1:6; methanol:water:chloroform or ethanol:water:chloroform (which were sonicated during 5 minutes before spreading) were used to assemble this material by LB technique, Figure 5.2 shows a representative surface pressure-area (  -A) isotherm of both GO dispersions at the air-water interface. Figure 5.2. Surface pressure mN·m-1 vs. area (π-A) isotherm of both GO dispersions at the airwater interface. As can be observed, four stages well differentiated and defined by changes in the slope are obtained. First stage, from 600 to 250 cm2 correspond to a gaseous phase 33 where there are not interactions between the GO sheets. During the second one, from 250 to 190 cm2, the surface pressure starts to increase upon compression revealing first interactions between GO sheets. From 190 to 120 cm2 other slope change is observed, third stage, in which BAM image (Figure 5.3) shows a packed stage although with areas without covering and how some sheets start to fold at the touching points along their edges (instead of overlapping) [50]. Finally, from 120 to 46 cm2 BAM image shows how the monolayer cover practically all the interface although GO sheets continue overlapping and wrinkling. Figure 5.3 BAM images of GO dispersions at the air-water interface (A) and (B) correspond to the first stage, (C) to the second stage, (D) to the third one and (E) to the fourth stage. Once observed and studied the formation of a Langmuir film of GO at the airwater interface, the next step was to transfer the monolayer onto a substrate (mica) and to check the morphology of the film by AFM. Taking into account the isotherm, three different surface pressures were selected to make the transference: 5, 20 and 35 mN·m-1. Figure 5.4. AFM images of a LB film of GO transferred at (A)5 m·Nm-1 (B) 20 m·Nm-1 and (C) 35 m·Nm-1. 34 Figure 5.4 shows the recorded AFM images. At 5 mN·m-1, Figure 5.4a, we can observe the presence of single GO sheets as well as large sheets, around 12 µm2, which are not monolayers. As surface pressure increases, 20 mN·m-1, GO sheets start to close existing interactions between them, Figure 5.4b, and the sheets are folded since these are soft and flexibl5 mN·m-1e and start to wrinkly and overlap. Finally, at a surface pressure of 35 mN·m-1, Figure 5.4c, a great number of winkling sheets, folding points and some sheets overlapping were observed. Due to not formation of a monomolecular film as it has been observed in Figure 5.4, a new spreading process using a GO dispersion in solvents with more purity was carried: ethanol:water:chloroform (5:1:6) or methanol:water:chloroform (5:1:6) and the Lamguir film was transferred at 15 mN·m-1 onto a mica to be studied by AFM, Figure 5.5 and Figure 5.6, respectively. The image in Figure 5.5 shows large GO sheets composed by several layers (a monolayer has a tickness of 1 nm), see section analysis in Figure 5.5, and also the presence of impurities due to probably a pieces of small GO sheets or rest of the solvents which were yet observed in Figure 5.4. The same result was obtained when GO was solved in a more pure methanol:water:chloroform (5:1:6) mixture, see Figure 5.6. Figure 5.5. AFM image of a LB film of GO transferred at 15 mN·m-1 using more pure solvents: (A) ethanol:water:chloroform (5:1:6); (B) methanol:water:chloroform (5:1:6). 35 6 Conclusions We can conclude that the three different methods studied in this work for the fabrication of the top contact electrode in metal/molecule/metal devices reveal that the three routes are feasible and promising to achieve the objective. Although the three ones are in a different level of development and need a further work. Nevertheless, the main conclusions obtained from this work are: The Rupture of a MOC has permitted to fabricate a metal/molecule/metal device being an efficient method for the fabrication of the top contact electrode and minimizing the appearance of short circuits which are a rather common problem in other traditional methods used for the metallization of monolayers. In addition, this technique avoids expensive and damage deposition techniques for the fabrication of the top contact electrode such as chemical vapour deposition (CVD) or atomic layer deposition (ALD). Although electrical measurements need to be performed to test the real effectiveness of the method. The use of Stencil technique lithography as possible method to fabricate the top contact electrode has concluded that: i) I-V curves not depend on the size of the top electrode in contrast to the expected. ii) The gold substrate roughness does not allow good electrical measurements. Nevertheless, this roughness does not explain the different I-V curves obtained depending on the placement of the tip and why the same I-V curve is performed for all the top-electrode sizes. iii) Damage of the layer during the evaporation process. Although the evaporation process is fast the temperature reached in the chamber can damage the layer. iv) Generation of little short-circuits. Even when the probabilities of generating a short-circuit decreases by SL, the energy of the gold atoms evaporated which pass through shadows and reach the molecular layer could be high enough to create bridges between the top and bottom electrode (red line). 36 Finally, the use of GO as possible top contact electrode has concluded that GO can be transferred onto solid substrates using LB technique; although an optimization of the conditions is required to get homogenous GO sheets which can be reduced to form graphene without damaging the film and, thus, can be used as a top-contact electrode. Appendix 1 37 Acknowledgements Jorge Trasobares Sánchez, NANOMAT master student in the academic year 2011-12, is grateful for financial assistance from INTERREG IV B SUDOE; Transpyrenees Action on Advanced Infrastructures for Nanosciences and Nanotechnologies TRAIN2; and Ministerio de Ciencia e Innovación (grant number: 5108900). Special gratitude to Dr. Pilar Cea and Dr. Santiago Martin Solans, supervisors of this work. It has been a pleasure to work with you at this time. Also special thanks to PhD students Luz Mariana and Javier Cortes and to Platon´s Group for your support and companionship. Thanks to Dr. Wolfgang Maser, Dr. Ana Benito and Dr. Pere Castell from Instituto de Carboquímica of Zaragoza by providing "GO" samples and by the helpful meetings. Thanks to Dr. Francesc Pérez Murano and his group Lorea Oria, Marc Sansa and Nerea Alayo for your welcome to the CNM and useful meetings. It was a pleasure to meet you and share your knowledge. 38 References [1] André-Yves Portnoff, "Claves para el Nanomundo", Cotec, 27,2008. [2] M. L. Grieneisen, M. Zhang, Nature Nanotechnology , 7, 273, 2012. [3] http://en.wikipedia.org/wiki/History_of_nanotechnology [4] G. A. Waychunas, Reviews in Mineralogy and Geochemistry, 44, 2001. 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