Atmospheric methane levels increased over geological formations in Northern Siberia following a heat wave in 2020, suggesting that permafrost thaw releases methane from reservoirs.
Image credit: Wikimedia Commons.
Contributed by Mark A. Ratner, July 5, 2017 (sent for review April 11, 2017; reviewed by Ignacio Franco and Eran Rabani)
Significance
Transport of objects in man-made systems typically relies on energy gradients that span the distance over which the particles must travel. Biological systems do not have these large-scale gradients available and instead transport particles with repeated local interactions between asymmetric structures, powered by nondirectional sources of energy like heat or chemical reactions. This process is called “ratcheting.” Here, we exploit the ratcheting mechanism to transport charge-carrying particles, electrons, through an amorphous organic film in which local structural inhomogeneities disrupt the gradients needed for typical electrical conduction to occur efficiently. This work explores basic mechanisms by which energy that is normally wasted in an electronic or photoelectrical device could be harnessed to do electrical work.
Abstract
Ratchets are nonequilibrium devices that produce directional motion of particles from nondirectional forces without using a bias, and are responsible for many types of biological transport, which occur with high yield despite strongly damped and noisy environments. Ratchets operate by breaking time-reversal and spatial symmetries in the direction of transport through application of a time-dependent potential with repeating, asymmetric features. This work demonstrates the ratcheting of electrons within a highly scattering organic bulk-heterojunction layer, and within a device architecture that enables the application of arbitrarily shaped oscillating electric potentials. Light is used to modulate the carrier density, which modifies the current with a nonmonotonic response predicted by theory. This system is driven with a single unbiased sine wave source, enabling the future use of natural oscillation sources such as electromagnetic radiation.
Footnotes
- ↵1To whom correspondence may be addressed. Email: e-weiss{at}northwestern.edu or Ratner{at}northwestern.edu.
Author contributions: O.K., B.L., M.A.R., and E.A.W. designed research; O.K. performed research; O.K., B.L., M.A.R., and E.A.W. analyzed data; and O.K., B.L., M.A.R., and E.A.W. wrote the paper.
Reviewers: I.F., University of Rochester; and E.R., University of California, Berkeley.
The authors declare no conflict of interest.
Data deposition: The measurement data supporting this work are available from the Northwestern University Libraries Archive (https://doi.org/10.21985/N2KD32).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1705973114/-/DCSupplemental.
Light-responsive flashing electron ratchet
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- Physical Sciences
- Chemistry
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