An STM/FM-AFM investigation of selected organic and inorganic 2D systems on metallic surfaces

Abstract

Organic/inorganic interfaces play a key role in organic electronic devices such as organic light emitting diodes (OLED), organic field effect transistors (OFET) and organic solar cells (OSC). In these interfaces crucial processes such as charge injection or extraction take place. Improving the performance of these devices has the potential to result in more efficient sources of lighting, printable electronics, and highly scalable solar energy harvesting. With this aim, a solid understanding at atomic level of the structural and electronic properties of organic/inorganic interfaces is needed.

The research presented in this thesis is based on scanning probe microscopies which are powerful techniques to probe and manipulate the electronic and structure at atomic scale. The structural and electronic properties of selected organic and inorganic 2D systems on metallic surfaces have been investigated by a combined scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM) in ultra-high vacuum conditions and at room temperature. The combination of these local probe techniques permits elucidating the interface structure at atomic level and disentangle electronic from topographic information. Particular attention has been dedicated to the measure and interpretation of the local work function.

Cuprous oxide (Cu2O) is an intrinsic p-type semiconductor. Copper oxide ultrathin films have been suggested to be candidates as low resistance electrodes, catalysts, sensing materials and semiconductor materials for solar cell transformation. However the properties of ultrathin film may differ from those of the bulk material. With the aim of increasing the actual knowledge, an atomic thin film of copper oxide has been grown on Cu(111) by air-enriched argon sputtering plus annealing. Several structures have been found and the local work function has been evaluated by contact potential difference and distance spectroscopies. Its structural and electronic properties are presented in Chapters 5 and 6.

Several organic molecules have been investigated on different surfaces: perylene-3,4,9,10-tetracarboxylic anhydride (PTCDA), diindenoperylene (DIP) and chloroaluminum phthalocyanine (ClAlPc). All of them are small pi-conjugated molecules that have been grown by molecular beam deposition and are served as model systems for a basic understanding of organic/inorganic interfaces. The choice of the selected molecules has been motivated by the distinct chemical and physical properties: PTCDA is a perylene derivate molecule with two anhydride end groups that has an electrical quadrupole moment and forms ordered films by hydrogen bonding, ClAlPc is a phthalocyanine that has a dipole moment perpendicular to the pi-plane of the molecule and DIP is another perylene derivate that is composed only by carbon and hydrogen atoms and forms films only by van der Waals interaction. The structural and electronic properties of a monolayer of PTCDA on Si(111)7x7 and AgSi(111) have been studied and are presented in Chapter 7. A monolayer of DIP on the Cu(111) surface and a monolayer and bilayer of ClAlPc on Au(111) are shown in Chapter 8 and Chapter 9, respectively. The final structure of these molecules on the surfaces is a competition between intermolecular and molecule-substrate forces. In the organic/metal interface many complex processes can occur such as charge transfer, charge rearrangement and push back effect, affecting the work function in a non trivial way what has been evaluated in all the mentioned systems.

En los dispositivos basados en semiconductores, las interfases entre material metálico y material semiconductor juegan un papel importante en el funcionamiento final de dichos dispositivos. Algunos ejemplos de dispositivos son las celdas solares, los diodos emisores de luz y los transistores de efecto campo. En las interfases metal/semiconductor se producen muchos de los procesos fundamentales para el correcto funcionamiento de éstos, como la inyección de carga o la separación de excitones. La optimización de dichos procesos requiere un sólido conocimiento a nivel atómico de las interfases desde un punto de vista estructural y electrónico. Por consiguiente, en esta tesis se han estudiado una serie de sistemas bidimensionales orgánicos e inorgánicos crecidos sobre diferentes superficies metálicas mediante microscopía de sonda próxima, una de las técnicas más potentes en el campo de la nanotecnología. Concretamente se ha utilizado un microscopio combinado de efecto túnel (STM) y de fuerzas atómicas (AFM), en condiciones de ultra alto vacío y a temperatura ambiente.

Capas delgadas de óxido de cobre (Cu2O) han sido ampliamente utilizadas por sus óptimas propiedades en catálisis y como material semiconductor en celdas solares. Con el fin de estudiar las propiedades estructurales y electrónicas, se han crecido capas ultra delgadas (un átomo de grosor) de Cu2O sobre una superficie de cobre (111). Diferentes técnicas han sido utilizadas para su caracterización estructural y electrónica.

Por otro lado, otro de los materiales semiconductores utilizados en el desarrollo de futuras celdas solares son capas finas formadas por moléculas orgánicas semiconductoras. A pesar de que se podrían utilizar muchas moléculas para la fabricación de dispositivos orgánicos, las moléculas pequeñas conjugadas son especialmente interesantes debido al bajo peso molecular, su estabilidad ante la polimerización y ante la descomposición térmica. Dichas moléculas pueden ser sublimadas en condiciones de ultra alto vacío mediante crecimiento epitaxial por haces de moléculas orgánicas. En el transcurso de esta tesis, varias moléculas orgánicas han sido crecidas sobre diferentes superficies metálicas: perileno tetracarboxílico dianhídrido (PTCDA), diindenoperileno (DIP) y ftalocianina de cloro y aluminio (ClAlPc). Su caracterización estructural y la medida e interpretación de la función de trabajo local se han presentado en esta tesis.