Publication: Electrostatic characterization of disordered 2D materials through force microscopy
Authors
Navarro Rodríguez, Mario
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Escuela Internacional de Doctorado
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Palacios Lidón, Elisa
Publisher
Universidad de Murcia
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DOI
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info:eu-repo/semantics/doctoralThesis
Description
Abstract
El estudio de materiales bidimensionales constituye uno de los campos más activos en la física y la ciencia de materiales contemporáneas, impulsado por su enorme potencial en aplicaciones optoelectrónicas, energéticas y biotecnológicas. Entre ellos, el óxido de grafeno (GO) y el óxido de grafeno reducido (RGO) destacan por su versatilidad y propiedades altamente sintonizables. Sin embargo, la comprensión detallada de sus mecanismos de transporte y de las interacciones electrónicas que emergen en presencia de desorden estructural sigue siendo un desafío. En este contexto, la caracterización a escala nanométrica resulta esencial para establecer la conexión entre morfología, propiedades eléctricas y comportamiento funcional. Para abordar este problema, en esta Tesis Doctoral se emplea la microscopía de fuerzas atómicas (AFM) como herramienta central combinando mediciones electrostáticas y ópticas para arrojar luz a esta problemática.
El primer bloque de resultados aborda el proceso de reducción del GO y su evolución hacia RGO. Se desarrolla un método reproducible y controlado para obtener materiales con distintos grados de reducción, lo que permite estudiar de forma sistemática cómo la eliminación progresiva de grupos oxigenados transforma las propiedades del material. A partir de medidas electrostáticas se identifican magnitudes experimentales que distinguen inequívocamente entre GO y RGO, demostrando que pueden emplearse como indicadores equivalentes a magnitudes usadas tradicionalmente. Asimismo, se identifican estados intermedios de reducción, en los que coexisten regiones aislantes y conductoras dentro de una misma lámina, abriendo la posibilidad de emplearlas como plantillas para funcionalización selectiva.
En el segundo bloque se desarrolla un modelo teórico para describir la evolución temporal de cargas en superficie, incorporando simultáneamente los efectos de conducción en volumen y superficie. Este marco permite explicar observaciones como el decaimiento no uniforme de la carga o la aparición de dinámicas dependientes de la escala espacial. El modelo es validado experimentalmente, demostrando que puede emplearse para caracterizar cuantitativamente la conductividad de superficie y volumen.
El tercer bloque examina cómo la dinámica de carga influye directamente en la medida de altura aparente de materiales bidimensionales y en las medidas electrostáticas. Se demuestra que las diferencias de espesor observadas entre GO y RGO no se deben únicamente a efectos capilares, como se asumía tradicionalmente, sino también a procesos disipativos de origen electrónico, interpretados como disipación Joule en la nanoescala. Además, se demuestra que las dinámicas de carga inducidas por el sensor influyen significativamente en las medidas electrostáticas, pudiendo llevar a interpretaciones erróneas si no se consideran. Sin embargo, caracterizando adecuadamente estos efectos de conductividad superficial finita, es posible estimar la conductividad superficial de materiales 2D. Experimentos a temperatura variable muestran, además, que la conductividad superficial disminuye monótonamente con la temperatura, en concordancia con el comportamiento esperado para materiales electrónicos desordenados.En el último bloque se explora la funcionalización no covalente de GO y RGO con protoporfirina IX (PPIX). Los resultados muestran que la organización molecular y la respuesta óptica dependen fuertemente del grado de reducción del material: mientras que en GO la fluorescencia de PPIX se mantiene, aunque alterada respecto a la observada en solución, en RGO sin embargo, esta desaparece debido a una transferencia de electrones hacia la lámina. Simulaciones de dinámica molecular confirman que esta diferencia proviene de la distinta naturaleza de las interacciones entre molécula y superficie, que determinan la geometría de adsorción y la eficiencia en la transferencia de carga.
En conjunto, esta tesis establece un marco experimental y conceptual para el estudio de materiales bidimensionales desordenados. Los resultados demuestran que las propiedades electrónicas de GO y RGO están estrechamente ligadas a su estructura local y a la dinámica de carga superficial, aportando herramientas y modelos que permiten cuantificar estos efectos con precisión.
The study of two-dimensional materials represents one of the most active fields in contemporary physics and materials science, driven by their enormous potential for optoelectronic applications among many others. In particular, graphene oxide (GO) and reduced graphene oxide (RGO) stand out for their versatility and highly tunable properties. However, a detailed understanding of their transport mechanisms and the electronic interactions that emerge in the presence of structural disorder remains a major challenge. In this context, nanoscale characterization is essential to establish the connection between morphology, electrical properties, and functional behavior. To address this, this Doctoral Thesis employs atomic force microscopy (AFM) as the central tool, combining electrostatic and optical measurements to shed light on these phenomena. The first block of results focuses on the reduction process of GO and its evolution into RGO. A reproducible and controlled method is developed to obtain materials with different degrees of reduction, enabling a systematic study of how the progressive removal of oxygen groups transforms the material's properties. From electrostatic measurements, experimental magnitudes that unequivocally distinguish between GO and RGO are identified, demonstrating that they can serve as reliable indicators equivalent to traditionally used metrics like the carbon-to-oxygen ratio. Intermediate reduction states are also identified, in which insulating and conducting regions coexist within a single sheet, opening the possibility of using them as templates for selective functionalization.In the second block, a theoretical model is developed to describe the temporal evolution of surface charge distributions, simultaneously incorporating the effects of bulk and surface conduction. This framework explains phenomena such as nonuniform charge decay and the emergence of scale-dependent dynamics. The model is experimentally validated, demonstrating its applicability for quantitative characterization of both surface and bulk conductivity. The third block examines how charge dynamics directly influence the apparent height of two-dimensional materials and their electronic properties. It is shown that the observed thickness differences between GO and RGO are not solely due to capillary effects, as traditionally assumed, but also to dissipative processes of electronic origin, interpreted as nanoscale Joule dissipation. Furthermore, it is demonstrated that sensor-induced charge dynamics significantly affect electrostatic measurements, potentially leading to misinterpretations if not properly accounted for. However, by adequately characterizing these finite surface conductivity effects, it becomes possible to estimate the surface conductivity of 2D materials. Temperature-dependent experiments further show that surface conductivity decreases monotonically with temperature, consistent with the expected behavior of disordered electronic materials. The final block explores the noncovalent functionalization of GO and RGO with protoporphyrin IX (PPIX). The results show that molecular organization and optical response strongly depend on the reduction degree of the material. In GO, the PPIX fluorescence is preserved, although altered compared to that in solution, while in RGO it is almost completely quenched due to electron transfer to the sheet. Molecular dynamics simulations confirm that this difference arises from the distinct nature of molecule-surface interactions, which determine adsorption geometry and charge transfer efficiency.Overall, this thesis establishes an experimental and conceptual framework for studying disordered two-dimensional materials. The results demonstrate that the electronic properties of GO and RGO are closely linked to their local structure and surface charge dynamics, providing tools and models that enable these effects to be quantified with precision.
The study of two-dimensional materials represents one of the most active fields in contemporary physics and materials science, driven by their enormous potential for optoelectronic applications among many others. In particular, graphene oxide (GO) and reduced graphene oxide (RGO) stand out for their versatility and highly tunable properties. However, a detailed understanding of their transport mechanisms and the electronic interactions that emerge in the presence of structural disorder remains a major challenge. In this context, nanoscale characterization is essential to establish the connection between morphology, electrical properties, and functional behavior. To address this, this Doctoral Thesis employs atomic force microscopy (AFM) as the central tool, combining electrostatic and optical measurements to shed light on these phenomena. The first block of results focuses on the reduction process of GO and its evolution into RGO. A reproducible and controlled method is developed to obtain materials with different degrees of reduction, enabling a systematic study of how the progressive removal of oxygen groups transforms the material's properties. From electrostatic measurements, experimental magnitudes that unequivocally distinguish between GO and RGO are identified, demonstrating that they can serve as reliable indicators equivalent to traditionally used metrics like the carbon-to-oxygen ratio. Intermediate reduction states are also identified, in which insulating and conducting regions coexist within a single sheet, opening the possibility of using them as templates for selective functionalization.In the second block, a theoretical model is developed to describe the temporal evolution of surface charge distributions, simultaneously incorporating the effects of bulk and surface conduction. This framework explains phenomena such as nonuniform charge decay and the emergence of scale-dependent dynamics. The model is experimentally validated, demonstrating its applicability for quantitative characterization of both surface and bulk conductivity. The third block examines how charge dynamics directly influence the apparent height of two-dimensional materials and their electronic properties. It is shown that the observed thickness differences between GO and RGO are not solely due to capillary effects, as traditionally assumed, but also to dissipative processes of electronic origin, interpreted as nanoscale Joule dissipation. Furthermore, it is demonstrated that sensor-induced charge dynamics significantly affect electrostatic measurements, potentially leading to misinterpretations if not properly accounted for. However, by adequately characterizing these finite surface conductivity effects, it becomes possible to estimate the surface conductivity of 2D materials. Temperature-dependent experiments further show that surface conductivity decreases monotonically with temperature, consistent with the expected behavior of disordered electronic materials. The final block explores the noncovalent functionalization of GO and RGO with protoporphyrin IX (PPIX). The results show that molecular organization and optical response strongly depend on the reduction degree of the material. In GO, the PPIX fluorescence is preserved, although altered compared to that in solution, while in RGO it is almost completely quenched due to electron transfer to the sheet. Molecular dynamics simulations confirm that this difference arises from the distinct nature of molecule-surface interactions, which determine adsorption geometry and charge transfer efficiency.Overall, this thesis establishes an experimental and conceptual framework for studying disordered two-dimensional materials. The results demonstrate that the electronic properties of GO and RGO are closely linked to their local structure and surface charge dynamics, providing tools and models that enable these effects to be quantified with precision.
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