Since the discovery of graphene, two-dimensional materials have gained significant interest due to their unique properties associated with their low dimensionality. Moreover, these materials possess mechanical flexibility that allows for the manipulation of the system's dimensionality through curvature. Notably, in graphene, curvature-induced strain has been shown to generate a controllable pseudomagnetic field, presenting opportunities in the emerging field of straintronics. This thesis is based on a comprehensive literature review on this topic and aims to experimentally investigate the effects of curvature on graphene's properties. Curvature is induced by depositing graphene or 2D heterostructures onto non-flat substrates. The conformation of 2D materials to the substrate is analyzed, revealing its dependence on factors like substrate pattern shape and dimensionality, as well as material thickness. Thereafter, for the samples where good conformation is observed, Raman spectroscopy is utilized to study the effects of curvature on strain and doping. Although most samples yield inconclusive results, one sample exhibits both unexpectedly lower p-doping concentration and tensile strain, near the area of curvature. Hypotheses to explain this anomaly are discussed. Moreover, straintronic devices, consisting of hBN-encapsulated graphene on a Gaussian substrate, are fabricated and characterized. One device shows a peculiar resistance hysteresis, localized at the charge neutrality point, exclusively when the current flows along the Gaussian fold, but not across it. Additionally, resistance peaks are observed at low gate voltages. For both phenomena, common causes are excluded, and thus curvature is speculated to be the underlying factor. The conclusions drawn from the key areas of observation elucidate the conformation of 2D materials on underlying substrates and provide insights on the effects of curvature on strain, doping, and electronic properties of graphene. In addition, further studies are proposed to address unanswered questions raised by this work, and to use the outcomes to develop innovative applications.
Dalla scoperta del grafene, i materiali bidimensionali hanno riscosso un notevole interesse per le loro proprietà uniche. Questi materiali possiedono una flessibilità meccanica che consente la manipolazione della dimensionalità del sistema attraverso la curvatura. In particolare, per il grafene, è stato dimostrato che la deformazione indotta dalla curvatura genera un campo pseudomagnetico, offrendo opportunità nel campo emergente della straintronica. Questa tesi si basa su un'esaustiva revisione della letteratura e mira a indagare sperimentalmente gli effetti della curvatura sulle proprietà del grafene. La curvatura è indotta depositando grafene o eterostrutture 2D su substrati non piani. Viene analizzata la conformazione dei materiali 2D al substrato, rivelando la sua dipendenza da fattori come la forma e la dimensionalità del motivo del substrato, e lo spessore del materiale. Per i campioni in cui si osserva una buona conformazione, viene utilizzata la spettroscopia Raman per studiare l'effetto della curvatura su deformazione e drogaggio. Sebbene la maggior parte dei campioni fornisca risultati inconcludenti, un campione presenta una concentrazione di drogaggio p e una deformazione tensile inaspettatamente inferiori vicino all'area di curvatura. Vengono fabbricati e caratterizzati dispositivi straintronici, costituiti da grafene hBN-incapsulato su un substrato gaussiano. Un dispositivo mostra una peculiare isteresi di resistenza, localizzata al punto di Dirac, esclusivamente quando la corrente scorre lungo la gaussiana, ma non quando scorre attraverso di essa. Inoltre, si osservano picchi di resistenza a bassi voltaggi di gate. Per entrambi i fenomeni sono escluse le cause più comuni, quindi si ipotizza che la curvatura sia il fattore sottostante. Le conclusioni tratte dalle aree chiave di osservazione chiariscono la conformazione dei materiali 2D su substrati nanostrutturati e forniscono informazioni sugli effetti della curvatura su deformazione, drogaggio e proprietà elettroniche del grafene. Inoltre, vengono proposti ulteriori studi per affrontare le domande sollevate da questo lavoro e utilizzare i risultati per sviluppare applicazioni innovative.
Curvature engineering of 2D materials using substrate nanopatterning
Meucci, Giulia
2022/2023
Abstract
Since the discovery of graphene, two-dimensional materials have gained significant interest due to their unique properties associated with their low dimensionality. Moreover, these materials possess mechanical flexibility that allows for the manipulation of the system's dimensionality through curvature. Notably, in graphene, curvature-induced strain has been shown to generate a controllable pseudomagnetic field, presenting opportunities in the emerging field of straintronics. This thesis is based on a comprehensive literature review on this topic and aims to experimentally investigate the effects of curvature on graphene's properties. Curvature is induced by depositing graphene or 2D heterostructures onto non-flat substrates. The conformation of 2D materials to the substrate is analyzed, revealing its dependence on factors like substrate pattern shape and dimensionality, as well as material thickness. Thereafter, for the samples where good conformation is observed, Raman spectroscopy is utilized to study the effects of curvature on strain and doping. Although most samples yield inconclusive results, one sample exhibits both unexpectedly lower p-doping concentration and tensile strain, near the area of curvature. Hypotheses to explain this anomaly are discussed. Moreover, straintronic devices, consisting of hBN-encapsulated graphene on a Gaussian substrate, are fabricated and characterized. One device shows a peculiar resistance hysteresis, localized at the charge neutrality point, exclusively when the current flows along the Gaussian fold, but not across it. Additionally, resistance peaks are observed at low gate voltages. For both phenomena, common causes are excluded, and thus curvature is speculated to be the underlying factor. The conclusions drawn from the key areas of observation elucidate the conformation of 2D materials on underlying substrates and provide insights on the effects of curvature on strain, doping, and electronic properties of graphene. In addition, further studies are proposed to address unanswered questions raised by this work, and to use the outcomes to develop innovative applications.File | Dimensione | Formato | |
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2023_07_Meucci_01.pdf
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Descrizione: Tesi
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2023_07_Meucci_02.pdf
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Descrizione: Executive summary
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https://hdl.handle.net/10589/207536