Sara D. Costa

Heating Isotopically Labeled Bernal Stacked Graphene: A Raman Spectroscopy Study

One of the greatest issues of nanoelectronics today is how to control the heating of the components. Graphene is a promising material in this area, and it is essential to study its thermal properties. Here, the effect of heating a bilayer structure was investigated using in situ Raman spectroscopy. In order to observe the effects on each individual layer, an isotopically labeled bilayer graphene was synthesized where the two layers were composed of different carbon isotopes. Therefore, the frequency of the phonons in the Raman spectra was shifted in relation to each other. This technique was used to investigate the influence of different stacking order. It was found that in bilayer graphene grown by chemical vapor deposition (CVD), the two layers behave very similarly for both Bernal stacking and randomly oriented structures, while for transferred samples, the layers act more independently. This highlights a significant dependence on the sample preparation procedure.

Fluorination of Isotopically Labeled Turbostratic and Bernal Stacked Bilayer Graphene

Fluorination of graphene opens up a bandgap, which creates opportunities for optoelectronics, and also paves the way for the creation of extremely thin insulating layers, which can be important for applications in devices. However, in spite of many interesting features offered by, for example, unequally doped layers in multilayered systems, most of the work has concerned the fluorination of graphene monolayers. Here, the fluorination process of graphene bilayers is investigated through high-resolution Raman mapping followed by analysis of more than 10,000 spectra of bilayer graphene. Isotopically labeled bilayers are used, allowing each individual layer in bilayer graphene to be addressed unambiguously. The fluorinated graphene is prepared through exposure to XeF2. Monolayer graphene is found to be significantly more sensitive to fluorination than bilayer graphene. Through comparison of the D/G area ratio and the position of the G band for turbostratic and Bernal stacked (AB) bilayers, it is found that the fluorination process is more effective for turbostratic than for AB-stacked bilayer graphene. The fluorination changes the electronic structure similarly for the top and bottom layers in turbostratic bilayers. However, the top layer is more sensitive than the bottom layer in AB-stacked bilayers.

Monitoring the doping of graphene on SiO2/Si substrates during the thermal annealing process

Various doping levels of graphene on SiO2/Si substrates are reported in the literature. We show by in situ Raman spectroscopy that the heating of chemical vapor deposited graphene on SiO2/Si during the transfer process is the main factor causing this unintended doping of graphene samples. Large areas of graphene were analyzed using Raman spectroscopy, before and after the thermal treatment, to demonstrate that the effects of heating are spread throughout the graphene layer. The perturbations caused by the exposure of supported graphene during the first heating cycle (in vacuum) are irreversible, even though the samples were later in contact with the atmosphere. These results clarify deviations found in the Raman data obtained for transferred chemical vapor deposited graphene by different authors.

Do defects enhance fluorination of graphene

Controlling the functionalization of graphene is essential for many applications. Here, we probed the reactivity of monolayer and bilayer graphene samples with intentionally prepared specific numbers of defects. Fluorination was used as a model reaction. We demonstrate that the reactivity of the single-layer graphene is not significantly affected by oxygen plasma-generated defects. On the other hand in the case of graphene bilayers, a decrease in the reactivity was observed for a small number of new defects, while an increase in reactivity was found for a larger number of defects. The long-term stability test of the fluorinated samples showed that minor changes in the sample occur during the first week after fluorination.

Addressing Raman features of individual layers in isotopically labeled Bernal stacked bilayer graphene

In this report important Raman modes for the evaluation of strain in graphene (the 2D and 2D) are analyzed. The isotope labeling is used to disentangle contribution of individual graphene layers of graphene bilayer to the studied Raman modes. It is shown that for Bernal-stacked bilayers, the 2D and the 2D Raman modes have three distinct components that can be assigned to processes originating solely from the top graphene layer, bottom graphene layer, and from a combination of processes originating both from the top and bottom layers. The reported results thus enable addressing the properties of individual graphene layers in graphene bilayer by Raman spectroscopy.