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.

Drew recently obtained the Ph.D. Degree from Boston University, where he worked in the group of Prof. Rick Averitt.

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

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.

In a recent paper published in Physical Review Letters, we investigate one of the crucial mechanisms which limit the efficiency of polymer solar cells. We use a unique optical technique, known as ultrafast time-resolved THz spectroscopy, which directly monitors the electric conductivity of the electrical charges generated by light in the solar cell with unprecedented time resolution. Our measurements reveal that within the polymer solar cell, only one third of the electric charges generated by the incoming light are available for current generation in the solar cell device, while the remaining two thirds of the charges are trapped. A modification of this ratio will directly influence the overall efficiency of the solar cell. This gives solar cell developers an invaluable experimental tool in the quest to increase the efficiency of next generation solar cells by new polymer chemistry and device design.

The investigation was spearheaded by Dr. David Cooke, who is now heading the Ultrafast THz Laboratory at McGill University, Montreal, Canada, in close collaboration with world-leading polymer solar cell expert, Prof. Frederik Krebs, Department of Energy Conversion and Storage at the Technical university of Denmark.

Read the official DTU press release (in Danish).

Citation:
D. G. Cooke, F. C. Krebs, and P. U. Jepsen, “Direct Observation of Sub-100 fs Mobile Charge Generation in a Polymer-Fullerene Film,” Phys. Rev. Lett. 108, 056603 (2012)

The code is available as freeware, under very favorable conditions – cite us if you use it.

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