石墨烯纳米卷的各向异性光响应

Anisotropic Optical Response of Graphene Nanoscrolls

单层石墨烯独特的能带结构赋予了其宽谱响应、高迁移率、超快响应等优异特性,这使得其在光电技术领域备受关注。然而,石墨烯具有高对称性,缺乏各向异性的光响应,这一点限制了其在偏振探测、量子通信、光学传感等领域的应用。本课题组利用石墨烯卷曲制备石墨烯纳米卷,破缺了石墨烯原有的高对称性,实现了各向异性的光响应。通过优化石墨烯纳米卷的制备和表征工艺,在拉曼与非线性过程中观察到了显著的各向异性光响应,说明石墨烯纳米卷形成过程破缺了石墨烯原有的晶格对称性。利用上述对称性破缺效应,制备了基于石墨烯纳米卷的各向异性光电器件,其光电流偏振比为0.33。此外,该器件还呈现出空间异质光电响应特性,结合拉曼光谱成像、近场显微成像技术推断该效应来源于缺陷、应力和掺杂分布等。本研究对于理解石墨烯纳米卷的各向异性光响应及其在光电器件中的潜在应用具有重要意义。

Objective 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 latticestructure 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.Methods 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.Results and Discussions 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 nearfield microscopy imaging,suggests that the effect is attributed primarily to variations in the defect and doping distributions.Conclusions 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.