Molecularly defined photodetectors in FeCl3-intercalated graphene
Adolfo DE SANCTIS (University of Exeter, UK)
Graphene, a single layer of carbon atoms with honeycomb structure, has emerged as a new paradigm in condensed matter physics due to the breadth of unique properties which also make it the ideal platform for novel transparent and flexible opto-electronic devices. These unique properties can be further tailored by means of chemical bonding of a molecule or a chemical element to the pristine graphene [1, 2]. The most recent example of the potential of chemical functionalization is the intercalation with FeCl3 of few-layer graphene (FLG). In this case a strong charge transfer occurs between the graphene and the intercalant layers, resulting in heavy p-doping of graphene.
This gives rise to a new system which is the best known flexible and transparent conductive material, with a sheet resistance of 8.8Ω/ and an optical transmittance as high as 84% [2]. It has been also shown that FeCl3-FLG can withstand to the harsh conditions of relative humidity of up to 100% for weeks, as well as temperatures of up to 150 ◦C in air or as high as 620 ◦C in vacuum [3]. The previously unknown durability to extreme conditions position FeCl3-FLG as a viable and attractive replacement to indium tin oxide (ITO), the main transparent conductive material currently used in electronics. In this contribution we report novel opto-electronic properties of FeCl3-FLG and opto-electronic devices which can be realized using FeCl3-FLG. A key requirement for flexible transparent conductors in modern electronics is for their work function to be similar to that of ITO. We report the first study of the work function of large-area (9mm2 ) FeCl3-FLG grown by chemical vapor deposition on Nickel, which results in values as large as 5.1eV. Furthermore we report the analysis of the Raman spectrum of this material from which a charge density of ∼ 5 · 1013 cm−2 can be extrapolated as a result of the intercalation process.
This large charge density is promising for the use of this material as a platform for plasmonics applications in the near-infrared region. Finally, we show for the first time, the ability to control the arrangement of FeCl3 molecules in the lattice of FeCl3-FLG by direct laser writing micrometer-scale patterns. This gives us control of doping in FeCl3-FLG resulting in controlled p-p’ junctions with spatial resolution down to 0.3 µm. We find enhanced photoresponse at these planar junctions in the wavelength range from UV-A to visible with at least ten times larger linear dynamical range than previously reported in graphenebased photodetectors. The photoresponse of FeCl3-FLG junctions is entirely dominated by the photoelectric effect without any contribution from the photothermoelectric effect.
These findings are in stark contrast to the dominating photothermoelectric effect observed in low-doped graphene junctions. Our experiments pave the way to the molecular design of atomically thin fexible opto-electronic devices ideally suited for sensing applications where high environmental stability and low footprint is required.
[1] F. Withers, T. H. Bointon, M. Dubois, S. Russo and M. F. Craciun; Nano Letters, 11, 3912 (2011). [2] I. Khrapach, F. Withers, T. H. Bointon, D. K. Polyushkin, W. L. Barnes, S. Russo and M. F. Craciun; Adv. Mat., 24, 2844 (2012). [3] D. J. Wehenkel, T. H. Bointon, T. Booth, P. Boggild, M. F. Craciun, and S. Russo; Sci. Rep., 5, 7609 (2015).
