The distinct band structure of monolayer graphene endows it with exceptional properties such as broadband response, high carrier mobility, and ultrafast response, thus rendering graphene highly promising in the field of optoelectronics. However, graphene exhibits isotropic optical responses owing to its high symmetry, which limits its application in devices for polarization detection, quantum communication, and optical sensing. This study proposes methods for the spontaneous curling of graphene to reduce its dimensionality and for forming graphene nanoscrolls, thereby breaking the high symmetry of graphene and achieving anisotropic optical responses. By optimizing the preparation and characterization processes of graphene nanoscrolls, we observed significant anisotropic optical responses in both the Raman and nonlinear processes, which indicates a change in the original lattice-structure symmetry of graphene during curling, thus resulting in lattice-symmetry breaking in the graphene nanoscrolls. By applying symmetry-breaking effects, we prepared an optoelectronic device based on graphene nanoscrolls with anisotropic responses and confirmed a photocurrent anisotropic ratio of 0.33. We further observed spatially dependent photocurrent responses in the device by comparing Raman spectra and near-field microscopy imaging results. We inferred that this spatial heterogeneity originates from defects, strain, and doping. This study is important for understanding the anisotropic properties of graphene nanoscrolls and their applications in optoelectronic devices.

First, an appropriate monolayer of graphene was prepared on a silicon-dioxide layer via mechanical exfoliation. Subsequently, an isopropanol (IPA) aqueous solution (volume ratio of IPA to water is 1∶3) was deposited onto the graphene monolayer to induce spontaneous curling, thereby resulting in graphene nanoscrolls. We employed various spectroscopic and microscopic techniques to comprehensively characterize the anisotropic structure and optical response of the nanoscrolls. Additionally, we fabricated a graphene nanoscroll device via dry transfer and then investigated its optoelectronic response.

The prepared graphene nanoscrolls feature a large central radius and a multilayered structure. As the lattice structure of graphene changes during curling, we observed the anisotropic lattice vibration mode of the graphene nanoscrolls using Raman spectroscopy. Specifically, the intensity of the G peak is maximized when the polarization of the incident light is aligned with the curling axis of the nanoscroll. Further spectral analysis shows the splitting of degenerate vibrational levels in the graphene nanoscrolls, which signifies variations in the phonon vibration symmetry in the anisotropic curling structure of the nanoscrolls. Subsequently, we performed a nonlinear spectroscopy mapping study, which shows that the graphene nanoscrolls exhibit stronger nonlinear signals than graphene. The presence of SHG (second harmonic generation) signals in the graphene nanoscroll regions further confirms the breaking of the intrinsic lattice symmetry of graphene during the formation of the graphene nanoscrolls. Subsequently, we fabricated optoelectronic devices based on the graphene nanoscrolls. By applying the symmetry-breaking effect, we measured the anisotropic optoelectronic response of the nanoscroll devices and obtained a photocurrent ratio of 0.33. Additionally, we observed a spatially heterogeneous photocurrent response within the device. This observation, in addition to confirmation based on advanced near-field microscopy imaging, suggests that the effect is attributed primarily to variations in the defect and doping distributions.

In this study, graphene nanoscrolls with anisotropic properties were successfully fabricated via the spontaneous curling of graphene. Atomic force microscopy (AFM) characterization of the graphene nanoscrolls shows their large central radius and multilayered structure. The Raman signal splitting and anisotropic G^{peak response of the graphene nanoscrolls indicate changes in the lattice symmetry. Further confirmation of symmetry breaking along the curling direction is indicated by the SHG of the graphene nanoscrolls. By applying the symmetry-breaking effect inherent in graphene nanoscrolls, we fabricated a graphene-nanoscroll device with significant anisotropic optoelectronic responses. This achievement underscores the potential of this device for diverse applications. By breaking the symmetry constraints of the original samples and artificially fabricating anisotropic nanoscroll materials, high carrier mobility and strong anisotropic magnetoresistance can be achieved. This approach offers numerous application advantages and can potentially further enhance device performance. By examining the anisotropy in both the lattice structure and optical responses of graphene nanoscrolls, this study provides critical insights that can facilitate the future advancement of innovative optoelectronic polarization detectors and the development of optically anisotropic nanomaterials derived from graphene nanoscrolls.}