Ratchets for Electronic Transport
People: Ofer Kedem, Mohamad Kodaimati
Nano-structured electronic devices have scattering mechanisms that randomize electron transport and inhibit directional current. Biological systems face similar challenges where enzymes and proteins, such as the molecular motors found in muscles interact with a strong thermal bath while operating with low energy input on a per-motor and per-unit time basis. However, molecular motors are able to achieve directed motion without any significant backsteps by rectifying the random motion that is induced by the thermal fluctuations of their surroundings. This principle by which the molecular motors works has been coined a “ratchet”. Ratchets are non-equilibrium systems that bias the motion of randomly moving particles by breaking time-reversal and spatial symmetries in the direction of transport through application of a time-dependent potential with repeating, asymmetric features.
We aim to use ratchets to improve charge transport in devices. Our research focuses on elucidating the behavior of electrons in ratchets, by studying the impact of a variety of parameters on the current, as well as examining their time-dependent trajectories. Since the ratchet current is sensitive to variation of any parameter, studying large parameter spaces offers the most insight. We investigate these electron ratchets using theoretical simulations as well as experimental fabrication of electron ratchets.
Figure 1. An ON-OFF ratchet; in the ON state of the potential, the particle distribution (or wavefunction) is localized in the wells. In the OFF state it freely diffuses in both directions. Flashing between the two states at an appropriate rate results in net transport.
Quantum model of electron ratchet
We simulate the behavior of damped non-interacting electrons, modeled by a quantum Lindblad master equation, within a one-dimensional flashing ratchet. We examine systems where the potential energy surface is represented by a biharmonic Fourier series (Fig. 2) over the complete space of all biharmonic potential shapes and a large range of oscillation frequencies to identify different modes of ratchet operation. We are able to identify ratchet shapes that produce positive current, zero current, and negative current (Fig. 3)
Figure 2. A biharmonic potential, very commonly used in ratchet research.
Figure 3. Representative potential shapes that lead to positive current (panel a), zero current (panel b), and negative current (panel c) for “slow” ratchets.
Ratcheting of Photo-generated Carriers
We also develop experimental ratchet systems. Specifically, we ratchet thermally- and photo-generated carriers in a bulk heterojunction (BHJ) film. An oscillating potential, applied to electrodes with an asymmetric thickness profile, produces a flashing asymmetric periodic potential in the OPV, ratcheting carriers between source and drain electrodes and producing directional transport without application of a bias. We find that the oscillating ratchet potential indeed results in ratchet current, which can reverse as a function of flashing frequency or amplitude.
Figure 4. Schematic illustrating the operating principle of a one-dimensional on/off flashing ratchet.
Figure 5. Model of the experimental system.
Designing & Characterizing Non-equilibrium Systems
Nature is highly adept at utilizing energy input and subsequent dissipation to yield to produce unique structures from dynamically controlled systems. Most synthetic approaches, in contrast, rely on systems that lack dynamic control and depend only on initial or static conditions. The component mixtures formed simply approach their equilibrium or kinetically-controlled statistical structures, while other structures remain largely inaccessible. These non-equilibrium or lower entropy structures may only be accessible through dynamic processes. To access these structures, the design and characterization of non-equilibrium systems with assemblies dictated by dynamically controllable stimuli must be achieved.
In one proposed system, light pulse sequences excite photoacids or photobases to transiently modulate the pH of a quantum dot mixture. These pH jumps trigger the reversible assembly or disassembly of pH-responsive quantum dots and modulate the structures formed. The assemblies formed under these dynamic pH conditions, which correspond to specific pulse sequences, may consist of non-equilibrium or lower entropy structures.
Lau, B.; Kedem, O.; Ratner, M.A.; Weiss, E.A. Identification of Two Mechanisms for Current Production in a Biharmonic Quantum Flashing Ratchet, Phys. Rev. E, 93, 062128 (2016)
Lau, B.; Kedem, O.; Schwabacher, J.; Kwasnieski, D.; Weiss, E.A. An Introduction to Ratchets in Chemistry and Biology, Materials Horizons, Advance Article, DOI: 10.1039/C7MH00062F (2017)
Kedem, O.; Lau, B.; Ratner, M.A.; Weiss, E.A. A Light-Responsive Organic Electron Flashing Ratchet, Proc. Natl. Acad. Sci., in press
Kedem, O.; Lau, B.; Weiss, E.A. Mechanisms of Symmetry Breaking in a Multidimensional Flashing Particle Ratchet, ACS Nano, ASAP
This work is supported by the Center for Bio-Inspired Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0000989.