Polymer Solar Cells and Graphene Transistors Are On a Roll
What's Hot in September/October 2011
by Simon Mitton
Paper #1, on high-efficiency solar cells, having reached the mandatory two-year retirement age for Hot Papers, is making its final appearance in the Physics Top Ten. During the latest bimonthly period, March-April 2011, it notched up 116 citations. The organic solar cell described in #1 is a huge advance on the inorganic silicon cell. That’s because the conducting polymers at the heart of the device are low-cost and easy to fabricate using roll-to-roll technology. The paper is highly cited because engineers and applied physicists are now focused on developing high-speed processing methods that are scalable, and solar cells with better functionality.
The properties of graphene continue to get a lot of attention because the material has unique electrical properties and great potential for electronics. Paper #6 is about the operation of graphene transistors at 100 GHz. Yu-Ming Lin of the IBM Corporation had already demonstrated operation at 26 GHz of a monolayer of carbon atoms arranged as a honeycomb lattice. In that case the graphene was exfoliated from a graphite crystal.
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The novelty in #6 is the use of epitaxial graphene formed on the Si face of a SiC wafer by thermal annealing at 1460 °C. A top gate stack of polymer was deposited on the graphene to fabricate field-effect transistors (FETs). Signals propagated at up to 100 GHz, a new record for graphene FETs as well as for Si transistors. Interest in these graphene FETs centers on the very high cut-off frequency and the ability of the FETs to amplify signals. Like the organic solar cells, transparent graphene films can be made in commercial quantities by the roll-to-roll production.
Paper #7 is also with us for a final appearance, and has made a strong finish with 46 citations in this period. It describes a graphene FET with a widely tunable bandgap (up to 250 meV). That is of high importance in achieving commercial transistors that can be tuned. The citations are mainly from research groups that are making large commitments to understanding as much as possible about the electronic properties of bilayer graphene.
This period welcomes back, after an absence of nearly a year, paper #8 on parton distributions for the Large Hadron Collider (LHC). For Science Watch, group leader Alan Martin of the department of physics at the University of Durham, U.K., explained the renewed interest in this paper: "The LHC is performing excellently and the luminosity is increasing rapidly. Right now the focus is on comparing LHC results for W and Z bosons, top quarks, etc., with the Standard Model of particle physics. Our parton distributions are considered to be the most reliable, so they are widely used in benchmark comparisons. That’s why they get widely cited in experimental talks and papers."
A rather different Standard Model, the lambda cold dark matter cosmology (flat universe with adiabatic Gaussian fluctuations) is a remarkably good fit to data on the cosmic microwave background (CMB), and this is the main interest in the new paper at #2 on seven-year data from the Wilkinson Microwave Anisotropy Probe (WMAP). Further refinements to the cosmological parameters are now at the stage of tweaking the third significant figure, but nevertheless they have delivered a 50% tighter limit on the Standard Model.
The significance of #2 mainly lies in the polarization data which are dramatically improved on earlier WMAP results. The polarization vectors around hot and cold spots in the CMB follow the visual pattern expected in the Standard Model. There are now improved limits on any parity-violating effects in the universe. An excellent result has been obtained on the primordial helium abundance at a redshift z ~ 1000; helium was present long before the first stars formed, and it was synthesized in the first few minutes after the big bang. WMAP has exquisitely confirmed a key prediction of the hot big bang.
WMAP has also detected a key signature of inflation: large-scale fluctuations are slightly more intense than small-scale ones. That’s a prediction of some models of inflation. Evidence is mounting that dark energy is in fact a cosmological constant; WMAP data allow this inference without it being necessary to invoke the supernova Type Ia data. We are inside a flat universe dominated by a cosmological constant.
Paper #2 is one of six that report on the analysis of WMAP seven-year data. It’s no surprise that this paper on cosmological implications is well ahead of the pack.
Dr. Simon Mitton is a Fellow of St. Edmund’s College, University of Cambridge, U.K.
What's Hot in Physics | |||
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Rank | Paper |
Cites This Period Mar-Apr 11 |
Rank Last Period Jan-Feb 11 |
1 | S.H. Park, et al., "Bulk heterojunction solar cells with internal quantum efficiency approaching 100%," Nature Photonics, 3(5): 297-302, May 2009. [U. Calif., Santa Barbara; Gwangju Inst. Sci. & Tech., S. Korea; U. Laval, Quebec City, Canada] *447UY | 116 | 1 |
2 | E. Komatsu, et al., "Seven-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Cosmological interpretation," Astrophys. J. Suppl. Ser., 192(2): No. 18, February 2011. [14 U.S., U.K., and Canadian institutions] *706BL | 89 | † |
3 | H.Y. Chen, et al., "Polymer solar cells with enhanced open-circuit voltage and efficiency," Nature Photonics, 3(11): 649-53, November 2009. [Solarmer Energy, Inc., El Monte, CA; U. Calif., Los Angeles; U. Chicago, IL] *526PG | 87 | 3 |
4 | K.N. Abazajian, et al., "The Seventh Data Release of the Sloan Digital Sky Survey," Astrophys. J. Suppl. Ser., 182(2): 543-58, June 2009. [110 institutions worldwide] *448UE | 74 | 2 |
5 | S. Reineke, et al., "White organic light-emitting diodes with fluorescent tube efficiency," Nature, 459(7244): 234-8, 14 May 2009. [Inst. Angew. Photophys., Dresden, Germany] *445FR | 51 | 5 |
6 | Y.M. Lin, et al., "100-GHz transistors from wafer-scale epitaxial graphene ," Science, 327(5966): 662, 5 February 2010. [IBM T.J. Watson Res. Ctr., Yorktown Height, NY] *551ZD | 47 | † |
7 | Y.B. Zhang, et al., "Direct observation of a widely tunable bandgap in bilayer graphene," Nature, 459(7248): 820-3, 11 June 2009. [U. Calif., Berkeley; Lawrence Berkeley Natl. Lab., CA] *459EV | 46 | 10 |
8 | A.D. Martin, et al., "Parton distributions for the LHC ," Eur. Phys. J., 63(2): 189-285, September 2009. [U. Durham, U.K.; U. Cambridge, U.K.; U. Coll. London, U.K.] *495BC | 44 | † |
9 | Y.L. Chen, et al., "Experimental realization of a three-dimensional topological insulator, Bi2Te3", Science, 325(5937): 178-81, 10 July 2009. [Stanford U., CA; Lawrence Berkeley Natl. Lab., CA; Chinese Acad. Sci., Beijing] *468FK | 41 | † |
10 | W.B. Atwood, et al., "The Large Area Telescope on the Fermi Gamma-Ray Space Telescope mission," Astrophys. J., 697(2): 1071-1102, 1 June 2009. [57 institutions worldwide] *446YT | 38 | 6 |
SOURCE: Thomson Reuters Hot Papers Database. Only papers indexed by Clarivate since May 2009 are tracked. A dagger indicates that the paper was not ranked in the Top Ten during the last period. In the event that two or more papers collected the same number of citations in the most recent bimonthly period, total citations to date determine the rankings. |