Departamento de Química-Física. Facultad de Ciencias Químicas - PDF

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Departamento de Química-Física Facultad de Ciencias Químicas SELF-ASSEMBLED SYSTEMS OF NANOMATERIALS ON LANGMUIR-BLODGETT FILMS Siisttemas Autto-ensambllados de Nanomatter riialles en Pellíícullas Langmuiir-Bllodgetttt

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Departamento de Química-Física Facultad de Ciencias Químicas SELF-ASSEMBLED SYSTEMS OF NANOMATERIALS ON LANGMUIR-BLODGETT FILMS Siisttemas Autto-ensambllados de Nanomatter riialles en Pellíícullas Langmuiir-Bllodgetttt Beeaattrri izz Maa aarrttí ín Gaarrccí íaa Saal laamaan nccaa 2013 FACULTAD DE CIENCIAS QUÍMICAS Departamento de Química-Física SELF-ASSEMBLED SYSTEMS OF NANOMATERIALS ON LANGMUIR-BLODGETT FILMS SISTEMAS AUTOENSAMBLADOS DE NANOMATERIALES EN PELÍCULAS LANGMUIR-BLODGETT BEATRIZ MARTÍN GARCÍA Salamanca, 2013 FACULTAD DE CIENCIAS QUÍMICAS Departamento de Química-Física SELF-ASSEMBLED SYSTEMS OF NANOMATERIALS ON LANGMUIR-BLODGETT FILMS SISTEMAS AUTOENSAMBLADOS DE NANOMATERIALES EN PELÍCULAS LANGMUIR-BLODGETT Memoria que para optar al grado de Doctor por la Universidad de Salamanca presenta la licenciada Beatriz Martín García Salamanca, 15 de Abril de 2013 Fdo. Beatriz Martín García Dª. Mª Mercedes Velázquez Salicio, Catedrática de Universidad en el Departamento de Química-Física de la Universidad de Salamanca, INFORMA: Que el trabajo presentado como Tesis Doctoral por la licenciada en Ingeniería Química Beatriz Martín García para optar al grado de Doctor por la Universidad de Salamanca, titulado Self-assembled Systems of Nanomaterials on Langmuir-Blodgett Films / Sistemas Auto-ensamblados de Nanomateriales en Películas Langmuir-Blodgett ha sido realizado en el laboratorio del G.I.R de Coloides e Interfases en el Dpto. Química-Física de la Facultad de Ciencias Químicas de la Universidad de Salamanca (España) y en las instalaciones del Grupo de Fotoquímica Molecular del Instituto Superior Técnico de Lisboa (Portugal). Como directora del trabajo, autoriza la presentación del mismo, al considerar que se han alcanzado los objetivos marcados. Y para que conste, firmo el presente en Salamanca a 15 de Abril de Fdo. Mª Mercedes Velázquez Salicio El trabajo que engloba la presente Memoria se ha realizado durante el periodo de disfrute de una Beca de Colaboración (curso 2006/2007) en el Departamento de Química-Física del Ministerio de Educación y Ciencia y una ayuda a la contratación de Personal Investigador de Reciente Titulación Universitaria (2008/2012) de la Junta de Castilla y León concedida al amparo de la Orden EDU/330/2008 de 3 de marzo. El trabajo desarrollado dentro del G.I.R. de Coloides e Interfases del Departamento de Química-Física ha sido financiado por los proyectos: MAT , MAT y MAT del Ministerio de Educación y Ciencia; y SA038A05 y SA038A08 de la Junta de Castilla y León. ʺI am among those who think that science has great beauty. A scientist in his laboratory is not only a mere technician: he is also a child confronting natural phenomena which impress him like a fairy taleʺ by Marie Sklodowska Curie Acknowledgments En estas líneas quiero expresar mi agradecimiento a todas aquellas personas que de una manera u otra han intervenido en el desarrollo del presente trabajo y por tanto, lo han hecho posible. Espero no olvidarme de nadie, pero por si acaso, pido disculpas por anticipado. Agradecerles su compañía en esta difícil tarea y su ayuda a crecer como persona tanto humana y como científicamente. En primer lugar quisiera agradecer a mi Directora, Prof. Dra. Dª. Mercedes Velázquez Salicio, por brindarme la oportunidad de introducirme en el mundo de la investigación dentro del Grupo de Coloides e Interfases de la Universidad de Salamanca y transmitirme cada día su ilusión por este trabajo. Así como su inestimable ayuda en el trabajo de cada día y en la elaboración de esta memoria. Dentro del Departamento de Química-Física, quisiera agradecer también a la Prof. Dra. Dª. Mª Dolores Merchán Moreno el apoyo y los consejos recibidos durante este tiempo. También hacer un especial agradecimiento a los Prof. Drs. José Luis Usero, Mª Ángeles del Arco, Carmen Izquierdo, Julio Casado y Emilio Calle, por su colaboración en determinados momentos. Al Dpto. de Química Orgánica, en especial a los Prof. Drs. D. Francisco Bermejo y Dª. Josefa Anaya por su asesoramiento en síntesis; y al Prof. Dr. Joaquín R. Morán por compartir su sabiduría a la hora resolver cuestiones puntuales sobre productos y disolventes. Al Dpto. de Química Inorgánica, concretamente a Prof. Drs. D. Vicente Rives y Dª. Raquel Trujillano por su disponibilidad y facilidades para utilizar varios equipos. Al Dpto. de Bioquímica y Biología Molecular, en especial a la Prof. Dra. Dª. Nieves Pérez por su amabilidad y accesibilidad para el uso de las centrífugas. Al Servicio de Microscopía, con mención a sus técnicos Juan y en los últimos tiempos, a Marta por su fantástica predisposición. Al Dpto. de Química Analítica, por las pruebas en las centrífugas, algunos compuestos químicos y el agua cuando fallaba el MilliQ, y sobre todo como ellos dicen por salvarme la vida . Al CLPU y todo su equipo por facilitar la realización de las medidas de AFM, pero con especial hincapié en José y Juan, por su paciencia y dedicación. Y continuando con la Física, al Prof. Dr. D. Enrique Diez y su doctorando, Cayetano S. Cobaleda, del Laboratorio de Bajas Temperaturas (Universidad de Salamanca) por adentrarme en el mundo del grafeno. In questa linea, vorrei anche ringraziare il Prof. Dr. Vittorio Bellani ed il Dr. Francesco Rosella (Università di Pavia, Italia) per la loro inestimabile collaborazione ed il loro appoggio. Aquí, también me gustaría dedicar unas palabras de agradecimiento a la gente del ISOM-UPM, en especial a Mª Mar, David, Maika, Alicia y José Antonio. Al Grupo de Dispositivos Semiconductores (Universidad de Salamanca), en especial al Prof. Drs. D. Tomás González y al D. Ignacio Íñiguez de la Torre, por hacer posibles las medidas de conductividad y su dedicación. Al Grupo de Sistemas Complejos de la Universidad Complutense de Madrid (UCM), en especial a los Prof. Drs. Francisco Ortega y Ramón G. Rubio por darme la oportunidad de realizar una estancia en el grupo y permitir el acceso al uso del elipsómetro, y al Dr. José E.F. Rubio por las medidas. Destacar la fantástica acogida y las charlas con Eduardo, Armando, Hernán, Marta y Mónica. También al Prof. Dr. D. Valentín García Baonza de la UCM, por algunas pruebas de Espectroscopía Raman. Asimismo, agradecer a la Prof. Dra. Dª. Margarita González Prolongo y a su equipo del Departamento de Materiales y Producción Aeroespacial de la Escuela Técnica Superior de Ingenieros Aeronáuticos de la Universidad Politécnica de Madrid (UPM) por permitirnos utilizar el equipo de Calorimetría Diferencial de Barrido (DSC). Al Prof. Dr. José Luis G. Fierro del Instituto de Catálisis y Petroquímica (CSIC), por su colaboración y discusiones de las medidas de XPS. Al Prof. Dr. Albert Cicera y a Sergi Claramunt del Departmento de Electrónica de la Universitat de Barcelona, por facilitar la realización de las medidas de FE-SEM. À Professora Silvia M.B. Costa, do Instituto Superior Tecnico da Lisboa, pela oportunidade de fazer uma estadia no seu grupo de investigação, a sua implicação no desenvolvimento do trabalho, preocupação e atenções. Também por deixa-me participar nas reuniões do grupo, como um membro mais, e poder apresentar e discutir o nosso trabalho. Ao Dr. Pedro M.R. Paulo pela sua dedicação, ajuda, ensinamentos e longas discussões dos resultados durante e depois da minha estadia. Com licença e hulmidade, gostaria de poder lhes dedicar o capitulo da tese da fotofísica dos quantum dots, onde agradeço a ajuda na elabora redacção ao Dr. Pedro M.R. Paulo. Muito obrigada também à tudos os membros do Grupo de Fotoquímica Molecular pelo trato recibido durante a minha estadia. Para el final querría hacer un punto de inflexión, para agradecer los momentos divertidos y de apoyo de los que considero mis compañeros y mis amigos. Primeramente, a Teresa, con la que empecé mi andadura en el doctorado y con la que he tenido la oportunidad de trabajar y compartir buenos y malos momentos, estancias y congresos, un gran apoyo y amiga, simplemente gracias. A mis asesores ... por el apoyo y los sabios consejos. A mis amigos, por las velas, las oraciones y la confianza en mí. Por lo importante que ha sido el entorno de trabajo, a mis compañeros de laboratorio, David, Rubén y Sofía. A los compañeros de Departamento, tanto a los que están como a los que afortunadamente han encontrado un post-doc o trabajo, por hacer más ameno el día a día, el paintball, las discusiones científicas y conseguir artículos, Pablo, Jesús, Susana, Marina, Jorge, Mario, Rafa, Fabián, Manso, Teresa, Jessica, Nico y Ruth. Sin poderme olvidar tampoco de los orgánicos de Villarriba en especial de Ángel, gracias por todo, aquí incluiré también a Sole y a Juan, por las cenas, el argón y los disolventes; ni de las analíticas, Sara, un poco culpable de que me dedique a esto de la investigación, y Ana. A Cayetano, por su ayuda durante mi estancia en la sala blanca y sus mal-logrados intentos en Bilbao junto a Mario de convertirme al QHE. Gostaria também de agradecer à gente da Lisboa: as minhas Raqueis, o Pedro, com licença, a Vanda, o Vipin, a Sofía, a Marta, o André, o Sergio, o Quaresma,..., Jose Antonio e Elisa, a tudos por fazer que estivera como na minha casa durante a estadia, imensamente obrigada sempre. Contents Contents i Contents Aims and Scope of the Thesis 1 I. State of the Art 7 I.1. Self-assembly of Polymers at the Air-Water Interface 11 I.2. Self-assembly of Nanoparticles at the Air-Water Interface 12 I.3. Self-assembly of Carbon Allotropes at the Air-Water Interface 14 II. A General Overview 15 II.1. Langmuir Monolayers 15 II.2. Mixed Langmuir Monolayers 21 II.3. Langmuir-Blodgett Films 22 II.4. Polymer Langmuir Monolayers 26 II.5. Langmuir Monolayers of Nanoparticles 32 II.6. Langmuir Monolayers of Graphene Derivatives 37 III. Experimental Section 41 III.1. Materials and Reagents 41 III.2. Langmuir Monolayers: Preparation Procedure 45 III.3. Experimental Techniques 47 III.3.1. Langmuir Trough 47 III.3.2. Surface Potential: Kelvin Probe 54 III.3.3. Brewster Angle Microscopy 58 III.3.4. Langmuir-Blodgett Trough 62 III.3.5. Atomic Force Microscopy 63 III.3.6. Electronic Microscopy 67 III.3.7. Ellipsometry 70 III.3.8. Micro-Raman Spectroscopy 75 ii Contents III.3.9. UV-vis Spectrofotometry 78 III Fourier Transform Infrared Spectroscopy 79 III X-Ray Photoelectron Spectroscopy 80 III Differential Scanning Calorimetry 83 III Four-point Probe Conductivity Measurements 84 III Dual Focused Ion Beam/Scanning Electron Microscopy 87 III Fluorescence Lifetime Imaging Microscopy 90 III Electron Beam Lithography 97 IV. Polymer Monolayers 103 IV.1. PS-MA-BEE Monolayers 105 IV.2. PS-b-MA Monolayers 123 V. Preparation and Properties of QDs Films 133 V.1. Experimental Section 136 V.2.Preparation of QDs Films 139 V.2.1. Langmuir and Langmuir-Blodgett Films of QDs and PS- 139 MA-BEE V.2.2. LB Films of QDs transferred onto a LB Film of Polymer 154 V.2.3. Surface Ligand Exchange: PSMABEE-capped QDs 161 V Surface Ligand Exchange Process 161 V Langmuir and Langmuir-Blodgett Films of PSMABEE- 165 capped QDs V.3. Dynamic Properties of QD/PS-MA-BEE Mixed Systems 172 V.3.1. Effect of Shearing on Film Morphology and Monolayer 174 Reorganization V.4. Photoluminescence of QDs in Langmuir-Blodgett Films 191 V.4.1. Experimental Details 193 Contents iii V.4.2. Selection of the Experimental Conditions by Steady-State 195 Measurements V.4.3. Photoluminescence Dynamics of QDs 197 V Photoluminescence Dynamics of QDs in Solution 198 V Effect of the Excitation Energy on QDs LB Films 201 V Effect of the Exposure Time on QDs LB films 204 V.4.4. Photoluminescence Dynamics of QD/PS-MA-BEE LB 209 Films V Interpretation of the QDs Photoluminescence 215 Dynamics V Effect of Capping Exchange on the QDs 229 Photoluminescence Dynamics: PSMABEE-capped QDs LB Films V.4.5. Imaging Characterization of Mixed QD/PS-MA-BEE 233 Films V QD TOPO /PS-MA-BEE Langmuir-Blodgett Films 234 V QD P /PS-MA-BEE Langmuir-Blodgett Films 240 VI. Chemically derived Graphene 243 VI.1. Oxidation and Reduction Procedures of Graphitic Material 247 VI.1.1. Graphite Oxide Production 247 VI.1.2. Reduction of Graphite Oxide 252 VI Chemical Reduction of GO with Hydrazine 257 VI Chemical Reduction of GO with Vitamin C 258 VI Chemical Reduction of GO assisted by the Surfactant 258 DDPS VI.2. RGO samples: Characterization and Langmuir-Blodgett 259 Deposition VI.2.1. Characterization of RGO samples 259 iv Contents VI.2.2. Langmuir-Blodgett Films of Graphite Oxide 265 VI.2.3. Langmuir-Blodgett Films of RGO samples 267 VI.2.4. Langmuir-Blodgett deposited Sheets of RGO 273 functionalized with the Surfactant DDPS VI.2.5. Electric Conductivity Measurements 276 VI.2.6. Electron Beam Lithography: RGO sheets Au-contacted 279 VII. Conclusions 283 VIII. References 289 Articles / Manuscripts 335 Appendix Nomenclature i-vii Resumen (Summary) I-XLV Aims and Scope of the Thesis Aims and Scope of the Thesis 1 Aims and Scope of the Thesis In recent years, the development and study of the nanomaterials have focused the attention of the scientists to use them as building blocks looking for novel properties inside technological and biological applications. The small size of the materials leads to unique properties that allow the construction of small devices ranging from nanometers to a few micrometers. Within this field one of the most important issue is the control of the size and shape of these structures in order to use them. In this sense, two of the challenges are the knowledge and understanding of the structures formation to tune the architecture of the nanomaterial system looking for the modulation of the properties. In some applications, as the construction of optoelectronic devices such as sensors, LEDs or photovoltaic cells, nanomaterials are deposited onto solids. In these cases it becomes necessary to develop the proper methodology to achieve a good coverage, avoid nanomaterials 3D agglomeration and allow the variation of the density of nanomaterials, spacing and even arrangement. An effective and convenient method to pattern a surface on an nm-µm scale without the use of lithographic processes, is the self-assembly approach. It is a low-cost, large-area scalable and solution-processing technique that does not require sophisticated equipments. In the self-assembly the behaviour of the nanomaterial at the interface, in which it is placed, plays a decisive role. Thus, to obtain a good quality or optimize the assembly formation, it is important to understand the mechanism or forces involved in the self-assembly process. Therefore, the study of the behaviour of nanomaterials at the interfaces, by means of the equilibrium and dynamic properties, is the starting point to achieve the assembly modulation. In this sense, the overall objective of this thesis is to study the self-assembly process of three different nanomaterials at the air-water interface and onto solids. The systems proposed were polymers, CdSe quantum dots (QDs) and chemically derived graphene. The common aspect between them is the use of the Langmuir 2 Aims and Scope of the Thesis and Langmuir-Blodgett (LB) techniques to evaluate the effect of the equilibrium and dynamic properties on their self-assembly process. These techniques render the self-assembly process of different nanomaterials at the air-water interface under well controlled and reproducible conditions. The LB technique was chosen because it has proved to be a versatile and interesting method to obtain thin films that allows a control of the surface concentration, which can be readily modified by compressing or expanding the film using barriers. Moreover, some dewetting processes have been observed in the preparation of the LB films that could be used to pattern at the nanoscale. [1-3] From this picture, the thesis has been organized in three different part according to the nanomaterials studied. The first part is focused on polymer thin films. Research on thin polymer films has revealed that various physical properties, such as unexpected instabilities, chain conformations, dewetting processes or glass-transition temperature variations, exhibit characteristics strongly deviating from their bulk behaviour, with major implications for most technological applications based on such nanoscopic films. [4] Despite the extensive research work a clear understanding of thin polymer film properties has not yet been reached. Therefore, in order to prepare good quality films for the construction of devices it is necessary previously to understand the equilibrium and dynamic properties of the monolayers precursors of the LB films. In this setting, we decided to study styrene-maleic anhydride copolymers films because these polymers have shown potential application in optical waveguides, electron beam resists and photodiodes. [5, 6] The polymers selected were the block copolymer poly (styreneco-maleic anhydride) partial 2-buthoxy ethyl ester cumene terminated, PS-MA- BEE, and poly (styrene-co-maleic anhydride) cumene terminated, PS-b-MA. Consequently, they could be used as a pattern for the fabrication of layered molecular electronic devices. Moreover, the interfacial rheology is interesting due Aims and Scope of the Thesis 3 to the polymer films are exposed to external disturbances. Thus, the stability properties of the films are important in applications such as coating or adhesion processes. In this sense, the aim was to study the effect of the addition of electrolytes in the aqueous subphase and temperature on the equilibrium and dynamic properties of Langmuir monolayers of the two polymers selected. Moreover, Langmuir-Blodgett films prepared from polymer monolayers onto different substrates have been characterized by different techniques to analyze the influence of different factors such as subphase composition, temperature and polymer nature on the film formation. The second system studied was hydrophobic CdSe QDs films. These nanoparticles present attractive optical applications in the fabrication of solar cells or LEDs due to their band-gap tunability. QDs show size-dependent optoelectronic properties that allow modulating the match with the solar spectrum in photovoltaic devices or improving the emission efficiency producing white (or coloured) light in LEDs. The most important optical advantages are a broad and continuous absorbance spectrum (from the UV to the far-ir), a narrow emission spectrum whose maximum position depends on the QD size, ligandaffected physico-chemical properties and high light stability. However, optoelectronic device applications based on QDs may either involve a very large number of dots in an ensemble with controllable architecture to avoid the deterioration of film quantum efficiency. Therefore, the thickness and uniformity of the assembled QD films are crucial factors in the emission properties of the films. [7-11] Some theoretical arguments suggest that the interactions between particles and a self assembled material can produce ordered structures. [12] Thus, diblock copolymers are known to self-assemble spontaneously into structures in the order of tens of nanometers in length, and these structures can be transferred onto substrates by LB or dip-coating methods. [13] Some research revealed that the organization of nanoparticles is governed by molecular interactions between the diblock copolymers and nanoparticles that constitute the mixed monolayers at the 4 Aims and Scope of the Thesis air-water interface. [14, 15] Despite some successful results, more research must be carried out to develop nanometric structures that may provide new properties associated with the reduction of the materials size. [16] Thus, the objective in this part was to use the copolymers ability of self-assembly at the air-water in
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