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Graphene plasmons, collective oscillation modes of electrons in graphene, have recently attracted intense attention in both the fundamental researches and the applications because of their strong field confinement, low loss and excellent tunability. The dispersion of graphene plasmons can be significantly modified in the system of graphene on metal substrate, in which the screening of the long-range part of the electron-electron interactions by nearby metal can lead to many novel quantum effects, such as acoustic plasmons, quantum nonlocal effects and renormalization of band structure. Scattering-type scanning near-field optical microscopy (s-SNOM) which consists of a laser coupled to the tip of an atomic force microscopy (AFM), is an effective technique to directly probe plasmons in two-dimensional materials including graphene, and the graphene plasmons can be observed visually by real-space imaging. But so far the detailed s-SNOM studies of graphene/metal system have not been reported. One potential challenge is that the near-field response of highly conductive metal substrate may partially or entirely obscure that of graphene, making it difficult to further explore graphene by using s-SNOM. Here in this paper, we report the direct observation of near-field optical response of graphene in a graphene/metal system excited by a mid-infrared quantum cascade laser. From a close examination of the data of graphene/Cu compared with that of h-BN/Cu, we are able to identify experimental features due to the near-field response of graphene. Surprisingly, two completely different behaviors are observed in the s-SNOM data for different graphene samples on Cu substrates with similar surface step geometries. These results suggest that the near-field response of graphene/metal system is not completely dominated by the metal substrate, and that two completely different near-field response behaviors of graphene may be attributed to their intrinsic properties affected by metal substrates themselves rather than surface step geometries of metal substrate. In addition, following this approach it is possible to distinguish the near-field optical responses of graphene from that of graphene/metal system. Our work reveals the clear signatures of the near-field optical response of graphene on metal substrate, which provides the foundation for probing plasmons in these systems by using the s-SNOM and understanding many novel quantum phenomena therein.
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Keywords:
- graphene, metal substrate /
- near-field optics
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图 1 s-SNOM示意图 一束红外光(IR)聚焦于针尖和样品之间, 针尖以频率
$ \varOmega $ 和振幅A在竖直方向振动, 针尖与样品的距离为$H(t) ={H_0} + A(1 + {\rm{cos}}(\varOmega t))$ , 其中${H_0}$ 表示针尖和样品的最小距离, 测量信号是针尖-样品系统的散射光${E_{{\rm{sc}}}}$ Fig. 1. Schematic of s-SNOM. An infrared light is focused between the sample and the probe tip, which oscillate vertically with frequency
$ \varOmega$ and amplitude A. The tip-sample distance is$H(t) ={H_0} + A(1 + {\rm{cos}}(\varOmega t))$ , where${H_0}$ is the minimum tip-sample distance. Incident light${E_{{\rm{in}}}}$ interacts with tip-sample system, and is elastically scattered. The scattered field${E_{{\rm{sc}}}}$ encodes the properties of the sample surface.
图 2 生长在铜衬底上的石墨烯拉曼光谱2D峰呈现了单个的洛伦兹峰型, 表明是单层石墨烯; D峰没有出现, 表明是无缺陷的高质量石墨烯
Fig. 2. Raman spectra of graphene on Cu substrate. The Lorentzian shape of 2D peak manifested that it is a monolayer graphene. Absence of D peak indicated its defect-free and high-quality nature.
图 3 h-BN/Cu台阶成像图 (a) 针尖扫过台阶时的不同位置, ①和④表示针尖处于远离台阶的位置, ②和③分别表示信号峰值(两个热点形成处)和信号谷值对应的针尖位置, 图中的针尖与台阶以等比例画出, 针尖中的红色圆圈表示直径D = 25 nm的针尖尖端; (b) (a)中位置②处针尖与台阶边缘间隔的局部放大图, 红色箭头指出热点形成的位置; (c) 左侧: h-BN/Cu台阶的形貌(上)和s-SNOM成像(下); 右侧: 左侧图中蓝色实线对应的形貌(上)和s-SNOM信号(下), 其中标签①—④与(a)中的标签相对应, 黑色和红色虚线分别指出s-SNOM信号的不同特征(峰值、谷值和平台)
Fig. 3. s-SNOM images of a h-BN/Cu step: (a) Different tip positions when the tip is scanned across a step. ① and ④ are the tip positions far away from the step, ② and ③ are the positions where a peak (corresponding to the formation of two hot-spots) and a dip appear in the s-SNOM signal, respectively. The tip and the step are shown in proportion. Red circle drew on the tip represents its apex with diameter of D = 25 nm; (b) Zoom-in on the gap between tip and sample at tip position ② in (a), which shows the two hot-spots by red arrows; (c) left panel: topography (upper) and s-SNOM image (lower) of h-BN/Cu at a surface step. Right panel: topography (upper) and s-SNOM line-profiles (lower) corresponding to the blue solid lines in the left panel. Labels ①–④ are corresponding to those in (a). Black and red dash lines indicate different features (peak, dip and plateau) in the s-SNOM line-profile.
图 4 衬底台阶几何相近的两个石墨烯/铜样品的s-SNOM成像. 两种情况中, 左侧: 形貌(上)和s-SNOM成像(下); 右侧: 左侧图中蓝色实线对应的形貌(上)和s-SNOM信号(下) (a) 情况一, 台阶高度和宽度分别约为 5.3 nm和50 nm, 峰值和谷值分别对应台阶下方和下方边缘; (b)情况二, 台阶高度和宽度分别约为 5.5 nm和55 nm, 仅出现一个对应于台阶下方边缘的峰值
Fig. 4. s-SNOM images of two graphene/Cu samples with similar surface step geometries. In both cases, left panel: topography (upper) and s-SNOM image (lower); right panel: line-profiles of topography (upper) and s-SNOM line-profiles (lower) corresponding to the blue solid lines in left panel. (a) Case 1 (peak-dip): step height of about 5.3 nm and width of about 50 nm. Signal peak and dip appear corresponding to the lower and upper edge of the step, respectively; (b) Case 2 (peak): step height of about 5.5 nm and width of about 55 nm. Only a signal peak appears corresponding to the lower edge of the step.
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[1] Basov D N, Fogler M M, de Abajo F J G 2016 Science 354 6309
Google Scholar
[2] Chen X, Hu D, Mescall R, You G, Basov D N, Dai Q, Liu M 2019 Adv. Mater. 31 1804774
Google Scholar
[3] Atkin J M, Berweger S, Jones A C, Raschke M B 2012 Adv. Phys. 61 745
Google Scholar
[4] Low T, Avouris P 2014 ACS Nano 8 1086
Google Scholar
[5] 段嘉华, 陈佳宁 2019 68 110701
Google Scholar
Duan J H, Chen J N 2019 Acta Phys. Sin. 68 110701
Google Scholar
[6] Lundeberg M B, Gao Y, Asgari R, Tan C, Duppen B V, Autore M, Alonso González P, Woessner A, Watanabe K, Taniguchi T, Hillenbrand R, Hone J, Polini M, Koppens F H L 2017 Science 357 187
Google Scholar
[7] Principi A, Asgari R, Polini M 2011 Solid State Commun. 151 1627
Google Scholar
[8] Principi A, Loon E v, Polini M, Katsnelson M I 2018 Phys. Rev. B. 98 035427
Google Scholar
[9] Alkorre H, Shkerdin G, Stiens J, Vounckx R 2015 J. Opt. 17 045003
Google Scholar
[10] Chen J, Badioli M, Alonso González P, Thongrattanasiri S, Huth F, Osmond J, Spasenović M, Centeno A, Pesquera A, Godignon P, Elorza A Z, Camara N, de Abajo F J G, Hillenbrand R, Koppens F H K 2012 Nature 487 77
Google Scholar
[11] Fei Z, Rodin A S, Andreev G O, Bao W, McLeod A S, Wagner M, Zhang L M, Zhao Z, Thiemens M, Dominguez G, Fogler M M, Neto A H C, Lau C N, Keilmann F, Basov D N 2012 Nature 487 82
Google Scholar
[12] Ni G X, Wang H, Wu J S, Fei Z, Goldflam M D, Keilmann F, Özyilmaz B, Neto A H C, Xie X M, Fogler M M, Basov D N 2015 Nat. Mater. 14 1217
Google Scholar
[13] Dai S, Ma Q, Liu M K, Andersen T, Fei Z, Goldflam M D, Wagner M, Watanabe K, Taniguchi T, Thiemens M, Keilmann F, Janssen G C A M, Zhu S E, Jarillo Herrero P, Fogler M M, Basov D N 2015 Nat. Nanotechnol. 10 682
Google Scholar
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Google Scholar
[15] Yang X, Zhai F, Hu H, Hu D, Liu R, Zhang S, Sun M, Sun Z, Chen J, Dai Q 2016 Adv. Mater. 28 2931
Google Scholar
[16] Jiang L, Shi Z, Zeng B, Wang S, Kang J H, Joshi T, Jin C, Ju L, Kim J, Lyu T, Shen Y R, Crommie M, Gao H J, Wang F 2016 Nat. Mater. 15 840
Google Scholar
[17] Sunku S S, Ni G X, Jiang B Y, Yoo H, Sternbach A, McLeod A S, Stauber T, Xiong L, Taniguchi T, Watanabe K, Kim P, Fogler M M, Basov D N 2018 Science 362 1153
Google Scholar
[18] Jiang B Y, Ni G X, Addison Z, Shi J K, Liu X, Zhao S Y F, Kim P, Mele E J, Basov D N, Fogler M M 2017 Nano Lett. 17 7080
Google Scholar
[19] Jiang B Y, Zhang L M, Neto A H C, Basov D N, Fogler M M 2016 J. Appl. Phys. 119 054305
Google Scholar
[20] Xu X, Zhang Z, Qiu L, Zhuang J, Zhang L, Wang H, Liao C, Song H, Qiao R, Gao P, Hu Z, Liao L, Liao Z, Yu D, Wang E, Ding F, Peng H, Liu K 2016 Nat. Nanotechnol. 11 930
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Google Scholar
[24] Babicheva V E, Gamage S, Stockman M I, Abate Y 2017 Opt. Express 25 23935
Google Scholar
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Google Scholar
[26] Costa S D, Weis J E, Frank O, Kalbac M 2015 Carbon. 93 793
Google Scholar
[27] Frank O, Vejpravova J, Holy V, Kavan L, Kalbac M 2014 Carbon. 68 440
Google Scholar
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Google Scholar
[29] Lyu B, Li H, Jiang L, Shan W, Hu C, Deng A, Ying Z, Wang L, Zhang Y, Bechtel H A, Martin M C, Taniguchi T, Watanabe K, Luo W, Wang F, Shi Z 2019 Nano Lett. 19 1982
Google Scholar
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