In recent years, significant attention has been devoted to the mechanical behaviour of thin crystalline membranes in layered van der Waals materials, motivated by their extreme mechanical compliance and by the strong coupling between deformation, electronic structure, and vibrational properties. Within this broad context, the formation of bubbles and blisters has emerged as a recurrent phenomenon whenever atomically thin or few-layer membranes are brought into contact with substrates or subjected to intercalation processes. Although long regarded as unavoidable morphological defects associated with imperfect interfaces or processing-induced contamination, blisters are now increasingly recognised as physically meaningful systems governed by general principles of membrane mechanics. In two-dimensional materials, such as graphene and other van der Waals crystals, blisters have been extensively investigated as pressurised membranes enabling controlled strain engineering and the exploration of coupled mechanical, electronic, and vibrational phenomena. In contrast, blisters formed in bulk layered materials, and in particular in electrochemically intercalated highly oriented pyrolytic graphite (HOPG), have received comparatively limited attention as individual physical entities, with most studies focusing on surface morphology or electrochemical signatures rather than on their mechanical behaviour and internal strain state. This thesis addresses this gap by investigating blisters formed in electrochemically intercalated HOPG as confined and mechanically active systems embedded within a multilayer graphite crystal. A multiscale experimental approach combining atomic force microscopy (AFM), correlative AFM–Raman spectroscopy, and tip-enhanced Raman spectroscopy (TERS) is employed to probe blister mechanics across different length scales, from their global elastic response to nanoscale strain heterogeneity. AFM-based nanoindentation and mechanical manipulation experiments demonstrate that blistered regions behave as compliant pressurised membranes with an effective mechanical response that is markedly different from that of the surrounding basal plane. Correlative AFM–Raman measurements reveal that blister formation is associated with a well-defined strain state, which can be quantitatively accessed through strain-induced shifts of the G and 2D Raman bands. By adopting a reference framework based on few-layer graphene, the spectroscopic response of individual blisters is interpreted in terms of finite-thickness membranes subjected to internal pressure. The application of TERS provides direct access to the local vibrational response within single blisters, revealing pronounced spatial variations of strain that cannot be resolved by far-field spectroscopy. These measurements confirm that blister swelling does not act as a source of lattice defects and demonstrate that strain is heterogeneously distributed across the blistered membrane, with distinct behaviour at the centre and near the perimeter. Taken together, the results presented in this thesis establish a coherent multiscale description of blisters in intercalated HOPG as confined mechanical systems. Beyond refining existing models of blister formation, this work redefines the role of blisters in bulk layered materials and highlights their relevance as model systems for studying strain, confinement, and membrane mechanics beyond strictly two-dimensional crystals.
Negli ultimi anni, notevole attenzione è stata dedicata al comportamento meccanico delle membrane cristalline sottili nei materiali stratificati di tipo van der Waals, motivata dalla loro estrema cedevolezza meccanica e dal forte accoppiamento tra deformazione, struttura elettronica e proprietà vibrazionali. In questo ampio contesto, la formazione di bolle e blister è emersa come un fenomeno ricorrente ogniqualvolta membrane atomiche o a pochi strati vengono messe a contatto con substrati o sottoposte a processi di intercalazione. Sebbene a lungo considerati come difetti morfologici inevitabili associati a interfacce imperfette o a contaminazioni indotte dai processi di fabbricazione, i blister sono oggi sempre più riconosciuti come sistemi fisicamente significativi, governati da principi generali della meccanica delle membrane. Nei materiali bidimensionali, come il grafene e altri cristalli di van der Waals, i blister sono stati ampiamente studiati come membrane pressurizzate che consentono un’ingegneria controllata della deformazione e l’esplorazione di fenomeni accoppiati di natura meccanica, elettronica e vibrazionale. Al contrario, i blister formati in materiali stratificati bulk, e in particolare nella grafite altamente orientata pirolitica (HOPG) intercalata elettrochimicamente, hanno ricevuto un’attenzione relativamente limitata come entità fisiche individuali, con la maggior parte degli studi focalizzati sulla morfologia superficiale o sulle firme elettrochimiche, piuttosto che sul loro comportamento meccanico e sullo stato di deformazione interno. Questa tesi affronta tale lacuna studiando i blister formati in HOPG intercalata elettrochimicamente come sistemi confinati e meccanicamente attivi, incorporati all’interno di un cristallo di grafite multilayer. Viene adottato un approccio sperimentale multiscala che combina microscopia a forza atomica (AFM), spettroscopia Raman correlata AFM–Raman e spettroscopia Raman esaltata da punta (TERS), al fine di investigare la meccanica dei blister su diverse scale di lunghezza, dalla risposta elastica globale fino all’eterogeneità della deformazione su scala nanometrica. Esperimenti di nanoindentazione e manipolazione meccanica basati su AFM dimostrano che le regioni contenenti blister si comportano come membrane pressurizzate cedevoli, con una risposta meccanica efficace marcatamente diversa rispetto al piano basale circostante. Le misure correlate AFM–Raman mostrano che la formazione dei blister è associata a uno stato di deformazione ben definito, accessibile quantitativamente attraverso gli spostamenti indotti dalla deformazione delle bande Raman G e 2D. Adottando un quadro di riferimento basato sul grafene a pochi strati, la risposta spettroscopica dei singoli blister viene interpretata in termini di membrane a spessore finito soggette a pressione interna. L’applicazione della TERS consente di accedere direttamente alla risposta vibrazionale locale all’interno dei singoli blister, rivelando marcate variazioni spaziali della deformazione che non possono essere risolte mediante spettroscopia in campo lontano. Tali misure confermano che il rigonfiamento dei blister non costituisce una sorgente di difetti del reticolo e dimostrano che la deformazione è distribuita in modo eterogeneo all’interno della membrana, con comportamenti distinti tra il centro e la regione prossima al perimetro. Nel complesso, i risultati presentati in questa tesi stabiliscono una descrizione multiscala coerente dei blister in HOPG intercalata come sistemi meccanici confinati. Oltre a raffinare i modelli esistenti di formazione dei blister, questo lavoro ridefinisce il ruolo dei blister nei materiali stratificati bulk e ne evidenzia la rilevanza come sistemi modello per lo studio della deformazione, del confinamento e della meccanica delle membrane oltre il limite dei cristalli strettamente bidimensionali.
Blisters in intercalated HOPG: mechanical properties of individual structures studied by combined high-resolution microscopy and spectroscopy
MENEGAZZO, MARCO
2025/2026
Abstract
In recent years, significant attention has been devoted to the mechanical behaviour of thin crystalline membranes in layered van der Waals materials, motivated by their extreme mechanical compliance and by the strong coupling between deformation, electronic structure, and vibrational properties. Within this broad context, the formation of bubbles and blisters has emerged as a recurrent phenomenon whenever atomically thin or few-layer membranes are brought into contact with substrates or subjected to intercalation processes. Although long regarded as unavoidable morphological defects associated with imperfect interfaces or processing-induced contamination, blisters are now increasingly recognised as physically meaningful systems governed by general principles of membrane mechanics. In two-dimensional materials, such as graphene and other van der Waals crystals, blisters have been extensively investigated as pressurised membranes enabling controlled strain engineering and the exploration of coupled mechanical, electronic, and vibrational phenomena. In contrast, blisters formed in bulk layered materials, and in particular in electrochemically intercalated highly oriented pyrolytic graphite (HOPG), have received comparatively limited attention as individual physical entities, with most studies focusing on surface morphology or electrochemical signatures rather than on their mechanical behaviour and internal strain state. This thesis addresses this gap by investigating blisters formed in electrochemically intercalated HOPG as confined and mechanically active systems embedded within a multilayer graphite crystal. A multiscale experimental approach combining atomic force microscopy (AFM), correlative AFM–Raman spectroscopy, and tip-enhanced Raman spectroscopy (TERS) is employed to probe blister mechanics across different length scales, from their global elastic response to nanoscale strain heterogeneity. AFM-based nanoindentation and mechanical manipulation experiments demonstrate that blistered regions behave as compliant pressurised membranes with an effective mechanical response that is markedly different from that of the surrounding basal plane. Correlative AFM–Raman measurements reveal that blister formation is associated with a well-defined strain state, which can be quantitatively accessed through strain-induced shifts of the G and 2D Raman bands. By adopting a reference framework based on few-layer graphene, the spectroscopic response of individual blisters is interpreted in terms of finite-thickness membranes subjected to internal pressure. The application of TERS provides direct access to the local vibrational response within single blisters, revealing pronounced spatial variations of strain that cannot be resolved by far-field spectroscopy. These measurements confirm that blister swelling does not act as a source of lattice defects and demonstrate that strain is heterogeneously distributed across the blistered membrane, with distinct behaviour at the centre and near the perimeter. Taken together, the results presented in this thesis establish a coherent multiscale description of blisters in intercalated HOPG as confined mechanical systems. Beyond refining existing models of blister formation, this work redefines the role of blisters in bulk layered materials and highlights their relevance as model systems for studying strain, confinement, and membrane mechanics beyond strictly two-dimensional crystals.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/256438