Quantifying size and dimensions on a nanometer scale is often a key task for investigating the nano-world or developing nanoscale devices. In their latest paper in JPhysD, Peter Schellenberg and his colleagues demonstrate that the quenching of an excited dye molecule by energy transfer to a graphene surface can be used as an accurate molecular ruler, allowing us to characterize distances in the ‘hard-to-measure’ zone of several nanometers to a few tens of nanometers.
This range of distances can often be difficult to measure using available techniques, especially in soft matter or biological samples where the procedures associated with X-ray diffraction or electron microscopy can be fairly intrusive or even destructive. Optical microscopy can be employed under ambient conditions, but using conventional techniques the resolution is limited by the Abbe diffraction limit. Even the numerous recently developed super resolution techniques are typically limited to several tens of nanometer resolution. On the other hand, the well-known Förster Resonance Energy Transfer (FRET) is most accurate over distances of a few nanometers. FRET provides efficient coupling of energy acceptors and donors mediated by the non-propagating, evanescent modes, i.e., near-field dipolar Coulomb interaction.
Our work is essentially an adaptation of the FRET technique. We use optically excited molecules of the common dye perylene as the energy donor while a single layer graphene sheet at a well-defined distance acts as the energy acceptor, see Figure. By carefully selecting the dye and the spacer dielectric, we are able to observe coupling between the dye and graphene at distances exceeding 60 nm. The value extends to a distance that is 20% of the fluorescence wavelength in the used spacer dielectric and is roughly an order of magnitude larger than typical distances associated with molecular FRET between donor and acceptor dye probes.
The use of graphene as an energy acceptor has numerous advantages. First, the flat geometry of graphene stretching macroscopically in two dimensions assures that all the localized dye molecules couple in an identical manner to the electronic states of the acceptor. Second, the broad absorption band of graphene stretching over the whole visible range makes the process wavelength independent. Third, the transparency and thinness of graphene allows for probing from different directions. Last, but not least, the Förster coupling of graphene is especially strong and, unlike metals, universal, only depending on fundamental constants. Together these properties distinguish graphene as an excellent fundamental nano-ruler for a wide-range of applications.
The ability to accurately characterize distances in this range may open new applications. For example, graphene may be exploited in microscopy for the investigation of biological specimens as a long range molecular ruler or as a homogeneous transparent quenching layer. It could also form the core of various types of detectors based on energy-transfer or plasmonics. This investigation may also find use in solar energy harvesting applications by facilitating the transfer of electronic excitation of the primary absorbers over an extended range of distances.
About the authors
Hugo Gonçalves received the B.S. degree in Physics in 2011 and the M.S. degree in Biophysics in 2013 from University of Minho, Portugal. He is currently a PhD student of the joint doctoral programme in Physics of Universities of Minho, Aveiro and Porto. His research interests include graphene, molecular physics, nanomaterials, biophysics and nonlinear optics.
César Bernardo received his B.S. degree in Chemistry in 2011 and his M.S. in Medicinal Chemistry in 2013 both from the University of Minho, Portugal. He is currently a PhD student of the joint doctoral programme in Physics of Universities of Minho, Aveiro and Porto. His research interests include ultrafast spectroscopy, nonlinear optics and nanomaterials.
Cacilda Moura, received the graduation degree from the University of Minho, Braga, Portugal in 1984 and the Ph.D. degree in Physics from the University of Minho, Portugal and University Paul Sabatier, Toulouse, France, in 1998. She is Assistant Professor at the University of Minho, working in the field of materials characterization by Raman spectroscopy.
Maria Rute de Amorim e Sá Ferreira André got her Ph.D. in physics in 2002 and the Agregação in Physics in 2012, both from University of Aveiro. She is an Associated Professor of Physics at the University of Aveiro, and is vice-president of the Scientific Council for Exact Sciences and Engineering of the National Science Foundation. Her scientific interests include optoelectronic properties of hybrid materials and semiconducting nanoparticles for lighting, integrated optics and energy conversion.
Paulo Sérgio de Brito André received the B.S. in physics engineering (1996), the Ph.D. degree in physics (2002) and the Agregação title in 2011, all from the University of Aveiro, Portugal. In 2013, he joined the Instituto Superior Técnico, University of Lisbon as an Associate Professor. Currently, he is the vice director of the Department of electronics. His research interests include the study and simulation of photonic and optoelectronic components, optical sensors, integrated optics, photonics applications, multi-wavelength optical communications systems and passive optical networks.
Tobias Stauber obtained his Diploma in Physics from the University of Göttingen, Germany in 1998 and his PhD in Theoretical Physics from the University of Heidelberg, Germany in 2002. He was Assistant Research Professor at the University of Minho and the University Autónoma of Madrid. He is currently a tenured scientist at the Material Science Institute of Madrid of the Spanish National Research Council. His research focus is on the optical response of graphene and related two-dimensional materials.
Michael Belsley received his Ph.D. from the University of Colorado in 1986. Since 1991 he has been an Associate Professor of Physics at the University of Minho. His research interests include ultrafast spectroscopy, nonlinear optics, quantum optics, coherent effects in highly scattering media and optical instrumentation.
Peter Schellenberg obtained his Diploma in Chemistry from the University of Mainz in 1990 and the PhD degree in Physics from the University of Bayreuth in 1994. In 2008, he joined the University of Minho as an Assistant Research Professor, working in femtosecond laser spectroscopy and nonlinear optics of graphene, quantum nanodots and organic molecules.
This work is licensed under a Creative Commons Attribution 3.0 Unported License. Image taken from Hugo Gonçalves et al 2016 J. Phys. D: Appl. Phys. 49 315102, © IOP Publishing, All Rights Reserved.
Categories: Journal of Physics D: Applied Physics