金属衬底上石墨烯的红外近场光学

对石墨烯/铜体系开展了系统性的近场光学实验研究,成功观测到了区别于铜衬底的、来自石墨烯的近场光学响应信号,发现在表面台阶几何参数相同的铜衬底上的不同石墨烯样品表现出了截然不同的近场光学响应.     

石墨烯等离激元的光学性质及其应用前景

石墨烯等离激元由于其独特的电学可调性、本征低衰减以及局域光场高度增强等特性, 引起了广泛的关注并迅速成长为一门新的学科分支--石墨烯表面等离激元光子学. 本文介绍了石墨烯等离激元的一些基本性质, 包括色散关系、局域的等离激元和传导的等离激元以及石墨烯等离激元对其周边介电环境的敏感性等. 在此基础上, 进一步介绍了石墨烯等离激元在太赫兹到中红外频段的应用, 比如有源光调制器的一些功能器件和增强的红外光谱探测等.     

Electronic properties and interfacial coupling in Pb islands on single-crystalline graphene

Introducing metal thin films on two-dimensional (2D) material may present a system to possess exotic properties due to reduced dimensionality and interfacial effects. We deposit Pb islands on single-crystalline graphene on a Ge(110) substrate and studied the nano- and atomic-scale structures and low-energy electronic excitations with scanning tunneling microscopy/spectroscopy (STM/STS). Robust quantum well states (QWSs) are observed in Pb(111) islands and their oscillation with film thickness reveals the isolation of free electrons in Pb from the graphene substrate. The spectroscopic characteristics of QWSs are consistent with the band structure of a free-standing Pb(111) film. The weak interface coupling is further evidenced by the absence of superconductivity in graphene in close proximity to the superconducting Pb islands. Accordingly, the Pb(111) islands on graphene/Ge(110) are free-standing in nature, showing very weak electronic coupling to the substrate.     

化学气相沉积石墨烯薄膜的洁净转移

石墨烯因其优异的性能在很多领域具有广阔的应用前景.目前石墨烯薄膜主要是以铜作为催化基底,通过化学气相沉积法制备.这种方法制备的石墨烯薄膜需要被转移到目标基底上进行后续应用,而转移过程则会对石墨烯造成污染,进而影响石墨烯的性质及器件的性能.如何减少或避免污染,实现石墨烯的洁净转移,是石墨烯薄膜转移技术研究的重要课题,也是本综述的主题.本综述首先简单介绍了石墨烯的转移方法;进而重点讨论由于转移而引入的各种污染物及其对石墨烯性质的影响,以及如何抑制污染物的引入或如何将其有效地去除;最后总结了石墨烯洁净转移所存在的挑战,展望了未来的研究方向和机遇.本综述不仅有助于石墨烯薄膜转移技术的研究,对整个二维材料器件的洁净制备也将有重要参考价值.     

等离激元增强的石墨烯光吸收

石墨烯中等离激元具有特殊的光电性质,其和入射光的强烈耦合可以引起光吸收的增强.本文基于时域有限差分法和多体自洽场理论研究了等离激元对处于光学谐振腔中的石墨烯光吸收的影响.由于石墨烯中等离激元与入射光动量和能量不匹配而不能直接相互作用,因此石墨烯上施加了金属光栅结构.研究发现光栅结构能够对入射光进行动量补偿并且能够引起其下石墨烯中的电场强度产生很大程度增强,从而导致在该石墨烯结构中太赫兹等离激元和入射光发生强烈耦合而产生太赫兹等离极化激元,同时引起石墨烯光吸收的增强.希望本文能够加深对石墨烯光电特性的理解以及可以为基于石墨烯的太赫兹光电装置提供一定的理论依据.     

金属衬底上高质量大面积石墨烯的插层及其机制

石墨烯作为一种新型二维材料,因其优异的性质,在科学和应用领域具有非常重要的意义.而其超高的载流子迁移率、室温量子霍尔效应等,使其在信息器件领域备受关注.如何获得高质量并且与当代硅基工艺兼容的石墨烯功能器件,是未来将石墨烯应用于电子学领域的关键.近年来,研究人员发展了一种在外延石墨烯和金属衬底之间实现硅插层的技术,将金属表面外延石墨烯高质量、大面积的特点与当代硅基工艺结合起来,实现了无需转移且无损地将高质量石墨烯置于半导体之上.通过系统的实验研究并结合理论计算,揭示了插层过程包含四个主要阶段:诱导产生缺陷、异质原子插层、石墨烯自我修复和异质原子扩散成膜,并证实了这一插层机制的普适性.拉曼和角分辨光电子能谱实验结果表明,插层后的石墨烯恢复了本征特性,接近自由状态.此外,还实现了多种单质元素的插层.不同种类的原子形成不同的插层结构,从而构成了多种石墨烯/插层异质结.这为调控石墨烯的性质提供了实验基础,也展现了该插层技术的普适性.     

Theoretical study of broadband near-field optical spectrum of twisted bilayer graphene

We theoretically study the broadband near-field optical spectrum of twisted bilayer graphene (TBG) at various twist angles near the magic angle using two different models. The spectrum at low Fermi energy is characterized by a series of peaks that are almost at the same energies as the peaks in the far-field optical conductivity of TBG. When the Fermi energy is near a van Hove singularity, an additional strong peak appears at finite energy in the near-field spectrum, which has no counterpart in the optical conductivity. Based on a detailed calculation of the plasmon dispersion, we show that these spectroscopic features are associated with interband and intraband plasmons, which can provide critical information about the local band structure and plasmonic excitations in TBG. The near-field peaks evolve systematically with the twist angle, so they can serve as fingerprints for identifying the spatial dependent twist angle in TBG samples. Our findings pave the way for future experimental studies of the novel optical properties of TBG in the nanoscale.     

氧化石墨烯的多色发光及其在荧光成像中的应用

氧化石墨烯作为石墨烯的一种带隙打开的衍生物,极大地丰富了其光学性质,并拓展了它在传感和成像方面的应用,特别是氧化石墨烯限域的π共轭结构对构建发光碳材料提供了十分便利的条件。目前,有大量的研究工作报道了氧化石墨烯及其衍生物能够产生多种颜色的荧光信号,然而,系统地总结这些研究去揭示氧化石墨烯发光机理的相关工作还比较少。本文总结了关于发光氧化石墨烯纳米材料的合成及其在光学成像方面应用的大量研究工作,为进一步开发新型的发光氧化石墨烯材料提供一些建设性意见。     

过渡金属掺杂的扶手椅型氮化硼纳米带的磁电子学特性及力-磁耦合效应

基于密度泛函理论的第一性原理计算方法,研究了多种过渡金属(TM)掺杂扶手椅型氮化硼纳米带(ABNNR-TM)的结构特点、磁电子特性及力-磁耦合效应.计算的结合能及分子动力学模拟表明ABNNRTM的几何结构是较稳定的,同时发现对于不同的TM掺杂,ABNNRs能表现出丰富的磁电子学特性,可以是双极化磁性半导体、一般磁性半导体、无磁半导体或无磁金属.双极化磁性半导体是一种重要的稀磁半导体材料,它在巨磁阻器件和自旋整流器件上有重要的应用.此外,力-磁偶合效应研究表明:ABNNR-TM的磁电子学特性对应力作用十分敏感,能实现无磁金属、无磁半导体、磁金属、磁半导体、双极化磁性半导体、半金属等之间的相变.特别是呈现的宽带隙半金属对于发展自旋电子器件有重要意义.这些结果表明:可以通过力学方法来调控ABNNR-TM的磁电子学特性.     

六方氮化硼薄膜对石墨烯-碳化硅结构近场热辐射的增强

六方氮化硼(hBN)具有跟石墨烯类似的层状结构和晶格参数,研究发现hBN薄膜具有良好的热传导、电绝缘、光学和力学等性能。本文从理论上研究了hBN薄膜对石墨烯-碳化硅(G/S)结构的近场热辐射的影响。研究发现在红外频段.hBN薄膜在低频率区和高频率区会增强G/S结构的近场热辐射,经计算在G/S结构中加入厚度为10 nm的hBN薄膜时获得的辐射热流是同物理条件下G/S结构的1.5倍;而在中频率区hBN薄膜的厚度阻碍了石墨烯表面等离激元和碳化硅表面声子极化激元的耦合,使得近场热辐射热流随hBN薄膜厚度增加而逐渐减弱。本研究的结果可为下一步实验与应用中对hBN薄膜厚度的选择提供理论基础。     

Intercalation of metals and silicon at the interface of epitaxial graphene and its substrates

Intercalations of metals and silicon between epitaxial graphene and its substrates are reviewed. For metal intercala- tion, seven different metals have been successfully intercalated at the interface of graphene/Ru（O001） and form different intercalated structures. Meanwhile, graphene maintains its original high quality after the intercalation and shows features of weakened interaction with the substrate. For silicon intercalation, two systems, graphene on Ru（O001） and on Ir（l I 1）, have been investigated. In both cases, graphene preserves its high quality and regains its original superlative properties after the silicon intercalation. More importantly, we demonstrate that thicker silicon layers can be intercalated at the interface, which allows the atomic control of the distance between graphene and the metal substrates. These results show the great potential of the intercalation method as a non-damaging approach to decouple epitaxial graphene from its substrates and even form a dielectric layer for future electronic applications.     

不同衬底条件下石墨烯结构形核过程的晶体相场法研究

利用可描述气-固转变的三模晶体相场模型,在原子尺度上研究了不同衬底条件下石墨烯结构的形核过程.结果表明:无论衬底存在与否,气态原子均是先聚集为无定形过渡态团簇,随着气态原子的不断堆积和固相团簇中原子位置的不断调整,过渡态团簇逐渐转变为有序的石墨烯晶核,在此过程中,五元环结构具有重要的过渡作用;石墨烯在结构匹配较好的衬底(如面心立方(face-centered cubic,FCC)结构(111)和(110))上生长时,可形成几乎没有结构缺陷单晶石墨烯岛;在无衬底或结构匹配性较差的衬底(如FCC结构(100)面)上生长时,形成的石墨烯岛结构缺陷和晶界较多,不利于高质量石墨烯的制备.     

石墨烯与金属的欧姆接触理论研究

石墨烯材料应用于多种电子器件时不可避免地要与金属电极接触,它们之间的接触电阻直接影响了器件的性能.为了揭示影响金属电极与石墨烯间接触电阻的因素,提出有效地抑制这些影响的措施,本文建立了一种求解接触电阻的物理模型,将载流子的输运分为金属与正下方石墨烯之间、正下方石墨烯与邻近石墨烯之间的两个过程,分别研究各个过程的输运概率;结合金属电极与石墨烯接触对载流子分布的影响分析接触电阻,据此分别探讨了金属电极材料、栅极电压、掺杂浓度、金属与石墨烯原子距离等对接触电阻的影响.为验证理论分析结果的正确性,制作了金与石墨烯接触的实验样品,实验测得的接触电阻与理论分析结果符合.理论分析结果表明,可通过选择与石墨烯功函数接近的金属材料,降低二氧化硅层厚度,增加载流子平均自由程,改进金属材料的表面形态使其更光滑,减小金属与石墨烯耦合长度等方法降低石墨烯与金属电极的接触电阻.     

悬空石墨烯对其表面金纳米膜形貌的影响

石墨烯与金属间的相互作用是石墨烯器件研究中的关键问题之一,其涉及石墨烯器件的电学接触、锂离子电池石墨烯电极、石墨烯金属光学等方面.本文重点研究了不同层数的悬空石墨烯表面金纳米膜退火前后的形貌演化过程,观测到两个重要的现象:1)排除基底影响后的悬空石墨烯层数可以通过金纳米膜的形貌特征进行确认,但其随层数的变化趋势与有基底支撑的石墨烯正好相反;2)退火处理后的悬空石墨烯上的金纳米膜形貌演化过程具有类似水滴在荷叶上的行为.对悬空石墨烯表面金属纳米膜在退火前后的形貌变化规律及其现象背后的物理机理进行了详细的讨论和理论解释.     

边界对石墨烯量子点非线性光学性质的影响

石墨烯作为一种新型非线性光学材料,在光子学领域具有重要的应用前景,引起研究人员的极大兴趣.本文运用量子化学计算方法研究了边界引入碳碳双键(C=C)和掺杂环硼氮烷(B₃N₃)环对石墨烯量子点非线性光学性质和紫外-可见吸收光谱的影响.研究发现,扶手椅边界上引入C=C双键后,六角形石墨烯量子点分子结构对称性降低,电荷分布对称性发生破缺,导致分子二阶非线性光学活性增强.石墨烯量子点在从扶手椅型边界向锯齿型边界过渡的过程中,随着边界C=C双键数目的增加,六角形石墨烯量子点和B3N3掺杂六角形石墨烯量子点的极化率和第二超极化率分别呈线性增加.此外,边界对石墨烯量子点的吸收光谱也有重要影响.无论是石墨烯量子点还是B₃N₃掺杂石墨烯量子点,扶手椅型边界上引入C=C双键导致最高占据分子轨道能级升高,最低未占分子轨道能级的降低,前线分子轨道能级差减小,因而最大吸收波长发生了红移.中心掺杂B₃N₃环后会增大石墨烯量子点的分子前线轨道能级差,导致B3N3掺杂后的石墨烯量子点紫外-可见吸收光谱发生蓝移.本文研究为边界修饰调控石墨烯量子点非线性光学响应提供了一定的理论指导.     

Lattice plasmon mode excitation via near-field coupling

The optical response of metal nanoparticles can be modified through near-field or far-field interaction,yet the lattice plasmon modes(LPMs)considered can only be excited from the latter.Here instead,we present a theoretical evaluation for LPM excitation via the near-field coupling process.The sample is an arrayed structure with specific units composed of upper metal disks,a lower metal hole and a sandwiched dielectric post.The excitation process and underlying mechanism of the LPM and the influence of the structure parameters on the optical properties have been investigated in detail by using a finite-difference time-domain(FDTD)numerical method.Our investigation presented here should advance the understanding of near-field interaction of plasmon modes for LPM excitation,and LPMs could find some potential applications,such as in near-field optical microscopes,biosensors,optical filters and plasmonic lasers.     

Graphene and metal hybridized terahertz metasurfaces toward tunable plasmon-induced transparency effects

Tunable metasurfaces have attracted much attention recently, owing to their capability of actively controlling electromagnetic waves. Here, we propose tunable graphene-metal hybridized terahertz (THz) metasurfaces exhibiting plasmon induced transparency (PIT) effects, driven alternately by metal and graphene resonators. The proposed meta-molecule comprises a central metal (graphene) strip placed orthogonally to two side graphene (metal) strips. Then, near-field coupling between the strips is probed by translating the central strip along the length of the side strips and the evolution of the PIT effect is investigated for orthogonal polarizations of probing THz. The coupled oscillator-based analytical model explains the coupling mechanism in these metasystems. We observe the strongest coupling and the most prominent PIT effect for edge-to-edge coupling (threshold position) between the central and side strips. Further, active PIT response is achieved by altering graphene's Fermi energy. These metasurfaces can contribute in designing compact and active metasurface-based hybrid devices for THz domain.     

Nano-infrared imaging of localized plasmons in graphene nano-resonators

We conduct in-situ near-field imaging of propagating and localized plasmons(cavity and dipole modes) in graphene nano-resonator. Compared with propagating graphene plasmons, the localized modes show twofold near-field amplitude and high volume confining ability(~ 10~6). The cavity resonance and dipole mode of graphene plasmons can be effectively controlled through optical method. Furthermore, our numerical simulation shows quantitative agreement with experimental measurements. The results provide insights into the nature of localized graphene plasmons and demonstrate a new way to study the localization of polaritons in Van der Waals materials.     

石墨烯-硅基混合光子集成电路

近年来硅基光子学已经慢慢走向成熟,它被认为是未来取代电子集成电路,实现下一代更高性能的光子集成电路的关键技术.这得益于硅基光子器件与现代的互补金属氧化物半导体工艺相兼容,能够实现廉价的大规模集成.然而,由于受硅材料本身的光电特性所限,在硅基平台上实现高性能的有源器件仍然存在着巨大挑战.石墨烯-硅基混合光子集成电路的发展为解决这一问题提供了可行的方案.这得益于石墨烯作为一种兼具高载流子迁移率、高电光系数和宽带吸收等优点的二维光电材料,能够方便地与现有硅基器件相集成,并充分发挥自身的光电性能优势.本文结合我们课题组在该领域研究的一些最新成果,介绍了国际上在石墨烯-硅基混合光子集成电路上的一些重要研究进展,涵盖了光源、光波导、光调制器和光探测器四个重要组成部分.