Non-Invasive Near-Field Photocurrent Nanoscopy Enables Graphene Device Quality Control

Achim Woessner ., ICFO - Institut de Ciencies Fotoniques, Barcelona, Spain Pablo Alonso-González ., CIC nanoGUNE, San Sebastian, Spain Mark B. Lundeberg ., ICFO - Institut de Ciencies Fotoniques, Barcelona, Spain Gabriele Navickaite ., ICFO - Institut de Ciencies Fotoniques, Barcelona, Spain Yuanda Gao Department of Mechanical Engineering, Columbia University, New York, NY, USA Qiong Ma Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA Davide Janner ., ICFO - Institut de Ciencies Fotoniques, Barcelona, Spain Kenji Watanabe ., National Institute for Materials Science, Tsukuba, Japan Takashi Taniguchi ., National Institute for Materials Science, Tsukuba, Japan Valerio Pruneri ., ICFO - Institut de Ciencies Fotoniques, Barcelona, Spain Pablo Jarillo-Herrero Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA James Hone Department of Mechanical Engineering, Columbia University, New York, NY, USA Rainer Hillenbrand ., CIC nanoGUNE and UPV/EHU, San Sebastian, Spain ., IKERBASQUE, Basque Foundation for Science, Bilbao, Spain Frank Koppens ., ICFO - Institut de Ciencies Fotoniques, Barcelona, Spain

Graphene is a promising material for optoelectronic applications as its lack of a bandgap leads to a broad band absorption that spans the visible, near-infrared, mid-infrared and THz regime.[1,2] For applications it is of great importance to know the exact optoelectronic properties of the devices used. With common far-field methods the large size of the laser spot after focusing prevents a spatial resolution below the diffraction limit. This leads to smearing of the spatial photocurrent maps, which can mask important details. Here we introduce a photocurrent measurement technique which is not limited by the diffraction limit. Using a scattering-type scanning near-field optical microscope (s-SNOM) [3,4] with a mid-infared laser source we excite a strong near-field at the apex of a metallized atomic force microscope probe tip, which acts as a local heat source, generating a temperature gradient in the graphene. This temperature gradient together with a change in Seebeck coefficient leads to a photothermoelectric photocurrent that can be measured spatially. [5] Here we show how near-field photocurrent measurements with extremely high spatial resolution can be used for characterizing optoelectronic devices made of graphene and graphene heterostructures.[6] We show photocurrent measurements at grain boundaries intrinsic to graphene grown by chemical vapor deposition [7] and extract their polarity. Furthermore we use this unique tool to measure photocurrent from charge puddles [8] of exfoliated graphene on silicon dioxide and show a photocurrent resolution of sub-30 nm. This proofs the extremely high spatial resolution which can be obtained, which ultimately is only limited by the radius of the tip apex. Finally we use the near-field photocurrent technique to confirm the spatial uniformity of the charge neutrality point of graphene encapsulated in hexagon boron nitride.[9] In summary, in this talk we introduce the novel near-field photocurrent mapping technique and show its potential applications in device characterization and quality control.

References
[1] F.H.L. Koppens et al., Nature Nanotechnology, 9 (2014) 780-793.
[2] M. Badioli et al., Nano Letters, 14 (2014) 6374-6381.
[3] J. Chen et al., Nature, 487 (2012) 77–81.
[4] Z. Fei et al., Nature, 487 (2012) 82-85.
[5] N. Gabor et al., Science, 334 (2011) 648-652.
[6] A.K. Geim et al., Nature, 499 (2013) 419-425.
[7] A.W. Cummings et al., Advanced Materials, 30 (2014) 5079-5094.
[8] J. Martin et al., Nature Physics, 4 (2008) 144-148.
[9] L. Wang et al., Science, 342 (2013) 614-617.

achim.woessner@icfo.es