石墨烯纳米泡的充电态操控

Manipulating charge states of single⁃layer graphene nanobubbles

可控调节二维材料纳米结构电学行为的能力,可为创造新颖功能型应用提供有效手段。本文利用扫描隧道显微技术实现了对SiO_(2)衬底上单层石墨烯纳米泡充电行为的连续偏压操控。通过形貌表征,观测到了转移到绝缘体SiO_(2)衬底上的化学汽相沉积单层石墨烯中纳米泡结构的存在。进一步,证实了石墨烯纳米泡的非平面结构引起局域电子态密度的连续增强,并有效屏蔽衬底掺杂效应,形成类量子点结构。扫描隧道显微谱学成像表明,在正负偏压下,石墨烯纳米泡边界处均会出现微分电导增强的环状结构,且其直径变化趋势在狄拉克点两侧具有明显不同的行为:低于狄拉克点时,随着偏压降低,环形结构的尺寸逐渐增大;而在高于狄拉克点一侧,随着偏压增大,其尺寸变化很小。这一结果与实验测定的石墨烯纳米泡电子态密度在正负能量区间的分布特征保持一致。基于针尖的局域栅压效应,考虑石墨烯纳米泡电子态随着偏压变化持续地越过费米面引起的电导增强现象,可以很好地解释实验结果。与半导体中杂质态的充电行为不同的是,所观测到的石墨烯纳米泡的电离可以发生在正负偏压,且没有偏压阈值,这为未来研究其类量子点的新奇量子限域效应提供了新的调控手段。

Impurities,including defects,dopants and structural deformations,provide the primary source of disorder and scattering that limit the mobility and conductance of two⁃dimensional(2D)materials,which thus play a key role in determining the electrical transport properties and their potential applications.Thus,it is of great importance to probe and manipulate the electronic properties of single impurities.Previously,scanning tunneling microscopy(STM)has been applied to manipulate the charge states of single atomic dopants and defects in various 2D materials.Compared to atomic impurities,nanoscale deformations can cause stronger influences on the behaviors of 2D materials,which also make the manipulation harder.Recently,there is a growing interest in studying the nanoscale deformation of the folded and other buckled structures in 2D materials,such as wrinkles and bubbles in graphene and transition⁃metal dichalcogenide(TMD)materials.While the ability to control the charge states of these nearly isolated nanoscale deformed structures can provide another tunnability for exploring the novel quantum confinement effects,it is still unknown whether they can be manipulated with the STM tip.Here,we report the manipulation of the charge states of graphene nanobubbles(GNBs)with an STM tip.The monolayer graphene was grown by chemical vapor deposition method on the Cu foils and transferred onto the 300 nm SiO_(2)/Si substrates.GNBs can form after 300℃annealing in ultrahigh vacuum conditions,due to the large roughness of the monolayer graphene on the SiO_(2)/Si substrate.The out⁃of⁃plane deformation enhances the local density of states(LDOS)of the GNB continuously in the energy ranges both below and above the Dirac point(E_(D)).What’s more,the energy of E_(D) was found to be lower than that of the regions off the GNB,which can be attributed to the out⁃of⁃plane buckled structure that reduces the doping effect from the SiO_(2) substrate.These results confirm the formation of a quantum⁃dot⁃like structure in the GNB.A serial of ring⁃like structures with enhanced LDOSs were observed in the differential conductance(dI/dV)mappings continuously in energies from the negative to positive biases across the Fermi level(E_(F)).The ring sizes showed apparently different trends between the bias voltages below and above the E_(D) of the GNB region.These behaviors can be well understood by adopting the tip⁃induced local gating effect,based on the difference between the experimentally observed LDOSs of the GNB at energies below and above the E_(D).Considering the exhibited novel phenomena in graphene quantum dots,the tunability of the charge state with an STM tip demonstrated in this work paves an avenue to exploring the quantum confinement effects in the quantum⁃dot⁃like GNBs at controllable charging states.