Nano Focus: Room-temperature terahertz detectors fabricated using graphene field-effect transistors
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False-color scanning electron micrographs in (a) and (b) show that the detector consists of a logperiodic circular tooth antenna located between the source and gate of a graphene field-effect transistor. The line running to the bonding pad is the drain. A Cr/Au top gate is located in the middle of the graphene channel, over the insulating layer. In (c), off-axis parabolic mirrors focus the terahertz radiation. Reproduced with permission from Nature Mater. 11 (2012), DOI: 10.1038/ NMAT3417; p. 865. © 2012 Macmillan Publishers Ltd.
line as a drain running to the bonding pad (see figure). After depositing an insulating layer of HfO2, the researchers then used e-beam lithography to define an identical antenna lobe for the top gate. Calculation showed that the antenna has resonant frequencies of 0.4, 0.7, 1, and 1.4 THz. The researchers then measured the conductivity and photoresponse to terahertz radiation at room temperature in singlelayer and bilayer graphene devices while varying the gate voltage, and showed that even though a considerable fraction of the radiation field is not funneled into the GFETs, the nonlinear response to the oscillating radiation field at the gate electrode is exploited with both thermoelectric and photoconductive contributions. The noise equivalent powers (NEPs)—a figure of merit for photodetectors that corresponds to the lowest detectable power in a 1-Hz output bandwidth—are about 200 nWHz–1/2 and 30 nWHz–1/2 for single-layer and bilayer devices, respectively. Although these are one to two orders of magnitude larger than those for commercial detectors, the researchers said that these are upper limits; correcting for the coupling efficiency of the radiation into the nanosized transistor element would result in much smaller NEPs. The researchers demonstrated that, even without optimization, their devices can perform large-area, fast imaging of realistic samples. Furthermore, the researchers said that their GFETs “have the potential for investigations of fundamental physics, such as the hydrodynamic behavior of chiral electron plasmas and their nonlinear instability, chirality-assisted electronic cloaking, and Zener-tunneling-induced negative differential conductivity.” Steven Trohalaki
ers working with Thomas Zeuch from the University of Göttingen, Petr Slavíček from the Technical University in Prague, and Udo Buck from the Göttingen-based Max Planck Institute for Dynamics and Self-Organization has now shown experimentally that crystalline order starts to form with just 275 water molecules, and that only 475 can generate a real crystalline structure.
The water molecules in ice crystals arrange themselves in a hexagonal lattice in which each water molecule forms hydrogen bonds to four adjacent molecules, and which occupies more space than liquid water, which is an unusual behavior. In the experiments performed, clusters below the minimum size for a crystal are generated with temperatures of around −180°C to −160°C. As they are
Nano Focus Room-temperature terahertz detectors fabricated usin
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