Our group attends the APS March Meeting this year in Los Angeles: Sridhar gives a talk on his upcoming work on controlling non-equilibrium energy dissipation in two-dimensional materials, Pierre gives a talk on our recently published work and is the principal focus session organizer for session 13.1.2 on Electron, Exciton, and Heat Transport at the Nanoscale, covering 6 sessions and 5 invited speakers from Physics, Chemistry, Materials Science departments, as well as two of the Nanoscience Research Centers: the Center for Functional Nanomaterials at Brookhaven National Laboratory, and the Molecular Foundry at Lawrence Berkeley Laboratory, outlining the importance of the NSRCs in the world-wide multidisciplinary research on charge and energy transport. The great Alex Weber-Bargioni and Pramod Reddy were coorganizers, and provided much needed expertise and depth on exciton and heat transport.

The number of abstract received for this focus session increased from 63 in the past year to 81. Speakers were nominated by the community (23 nominations received, in steep decline from the 58 nominations received two years ago for the same session).

Sridhar and Pierre will be session chairs (for sessions F.15 and E.15, respectively), and so will be some of our collaborators: Dr. Colin Van Dyck will chair A.15 “Charge Transport at the Nanoscale”, Dr. Zhenfei Liu B.15, Prof. Sahar Sharifzadeh C.15, and Dr. Yi Xia G.15. Thanks to all for helping!

Our former CNM colleague Prof. Jonathan Foley IV of William Paterson University was also the main organizer of focus session 13.1.1 on Nanostructures and Metamaterials.

Yonatan “Yoni” Dubi from Ben Gurion University started the day with an interesting talk on molecular diodes and single molecule photoswitches, both of these works in collaboration with the experimental group of Bingqian Xu at the University of Georgia. The later topic (and related article was particularly interesting to me:

In short, since the work of Meir and Wingreen in 1992, and the subsequent work of Galperin and Nitzan in 2005, the current induced by an optical transition occuring on a molecule is generally understood as the result an opening of an additional current channel not originally in the bias window but “activated” by the non-equilibrium distribution function generated by the optical transition.

Mathematically, that means the non-equilibrium Green’s function G lesser becomes F_eq * A(w) + F {neq} A(w), the later part being responsible for the current in a term \int dw (\Gamma_L - \Gamma_R) F{neq}A(w), hence proportional to the asymmetry of the contacts at these frequencies. This can be an attractive view, but it neglect the fact that the non-equilibrium density matrix may significantly differ from the equilibrium one and will impact the spectral function A(w) through the renormalization of the single particle level energies (as the charged excitations on the molecule will see a very different potential if the molecule has been photoexcited). Dubi explains this effect as being of “excitonic” nature, as photoexcited electrons will “pull” the holes towards them, closing the gap of the molecule. As the conductance of a molecular junction approximately goes as 1/(E_Fermi - E_Frontier)^2 (where E_Frontier is the energy of the molecular orbital closest to Fermi), the photoconductance of the molecule is much higher than its nominal conductance. With a lot of sagacity, Dubi and coworkers used a symmetrically coupled molecule (which implies the (\Gamma_L - Gamma_R) term of the Galperin and Nitzan model is equal to 0, and demonstrated a large increase of the conductance.

While I find the overall explanation extremely convincing, I am not convinced about the “exciton” terminology here (it is not impossible for me to imagine a molecule in which the energy gap would increase upon irradiation, making the molecule less conductive in the presence of an excitation. In any case, it was a brilliant talk (and Yoni Dubi is definitely the speaker you want for starting a conference at 8am on a Monday!).

Also seen today:

  1. E-Dean Fung from Latha Venkataraman’s group demonstrated hysteresis associated with charging in a single molecule, proposing a polaron model to explain how the charging state would persist the very large timescale (typically ms) associated with the measurement of an IV characteristics. The data were convincing though I must say it is hard for me to imagine how a localized charge on a molecule contacted to metallic electrodes could be stable for so long.
  2. Matt Sfeir from the CFN discussed his collaborative studies on singlet fission in organic dimers –and gave a very convincing explanation on how organic molecules are much closer to the theoretical limit of multiexciton generation than other systems (such as quantum dots).

Rant(s) of the day:

  1. Regrettably, no woman was featured as an invited speaker in focus session 13.1.2… (One was originally invited by the organizers, but could not attend). An overwhelming number of the nominations were given to men (>90%). Possibly even more worrisome, the only nominated women were at the mid- and senior-career level, leaving the highly-visible and sometimes career-deciding invited talks to men. This is sadly echoing a very general trends and our experience organizing the same focus session two years ago (though, at the time, 14 out of 58 nominees were women, only 2 were nominated at the junior level) . For the record, the topics covered by the focus session feature many leading women, e.g. Latha Venkataraman, Elke Scheer, Gemma Solomon, Naomi Halas, Naomi Ginsberg, Emily Weiss, and Laura Herz. Evaluating people at the junior level is of course more subjective, though it is difficult to believe the nomination process will be fairer to junior women faculty. The fact this is not reflected in the APS invited talks is a problem that needs to be specifically addressed by the APS. The biases in this matter have been identified before, and we will make some suggestions to the APS in order to bridge this gap.
  2. Another trend that I deplore (though far less outrageous) is the large number of faculty “subbing” in their postdocs at the last minute. In my third consecutive year as a session chair and/or organizer, this is at least the third time it happens to me. While there are tons of well-intended reasons to do that (starting with the willingness of promoting a talented early career researcher soon-to-be on the job market), this is quite unfair to the junior faculties competing for these spots.