In a recent paper published in ACS Nano Letters, we report on the use of terahertz time-domain spectroscopy for contact-free measurement of the electrical conductivity of single-layer graphene.

In spite of the atomically thin structure, impressive progress towards large-scale fabrication of films of graphene with high contuctivity, high transparency to visible light, and high mechanical robustness. Graphene is therefore a strong candidate for next-generation touch-screen display technology and top electrode material in solar cells.

With the development of large-area graphene films (approaching square meters), there is a high demand for rapid screening technology for quality inspection of such films. Rapid screening requires contactless measurement technology, and this is where THz technology enters the picture.

Here we have demonstrated that a tightly focused beam of THz radiation can be used for rapid determination of the local electrical conductivity of single-layer graphene. The measurements have been carefully compared to another state-of-the-art conductivity characterization technology, namely micro-four point probes. 

The left panel of the Figure shows a THz-TDS based conductivity map of a single-layer graphene flake. The scale bar is 2 mm. The right panel shows the corresponding micro-four point probe map of the same flake.  

The report focuses on the close correlation between the THz-frequency measurements (AC conductivity) and the M4PP measurements (DC conductivity), and among other things, we demonstrate that THz-frequency measurements are sensitive to the nanoscopic conductivity while the M4PP technique is sensitive to the conductivity on the length scale of the pitch of the electrodes of the M4PP sampling head. This fundamental difference makes the two techniques complementary, and the combination of the two techniques is thus a unique characterization tool.

Together with the Nanointegration group at DTU Nanotech, we carried out the investigation in collaboration with researchers at McGill University, Montreal, Canada, and Chalmers University of Technology, Gothenburg, Sweden.

Citation: J. D. Buron, D. H. Petersen, P. Bøggild, D. G. Cooke, M. Hilke, J. Sun, E. Whiteway, P. F. Nielsen, O. Hansen, A. Yurgens, and P. U. Jepsen, “Graphene Conductance Uniformity Mapping,” Nano Lett. DOI: 10.1021/nl301551a (2012)

For additional information, please contact Prof. Peter Uhd Jepsen.

Tianwu Wang started on his Ph.D. Project in our group on June 15th, after being guest student for a couple of months. We are very happy to welcome him as a more long-term member of the group. Tianwu will be working on ultrafast photoinduced dynamics in water, studied with THz spectroscopy. He is funded by the Danish Council for Independent Research|Natural Sciences via the THz-BREW project.

The European Optical Society held the 3rd Topical Meeting on Terahertz Science and Technology in Prague on June 17-20, 2012. The THz group was present at the meeting with two student presentations by Jonas Buron and Maksim Zalkovskij, speaking about their work on graphene and metamaterials, respectively. Former member of the group, Dr. David Cooke (now at McGill University, Montreal, Canada), delivered an invited talk on his work on polymer solar cells which was performed in the group.

Group leader Prof. Peter Uhd Jepsen, was chairman of the conference together with Prof. Petr Kuzel, Chech Academy of Sciences.

We have recently invested in a rapid THz scanner system (Picometrix T-Ray 4000) for fast, nondestructive testing of materials. With this system we can now perform 2D and 3D reconstruction of the outer and inner structure of a wide range of materials.

The image below shows the result of THz tomography of a toy horse, revealing the inner structure of the horse. Most prominent are air voids in the central part of the body.

For more information on the application of THz imaging in your own setting (such as industry, art and cultural heritage conservation), please contact us for further details.