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Research

Current work - ratchets

Motivation. My current work focuses on ratchets – far-from-equilibrium devices that transport particles using local asymmetries, rather than overall biases. Ratchets are rectifiers – they extract directional motion from non-directed sources of energy, like chemical energy and Brownian motion. Biological motors in the body use ratchet mechanisms, and produce motion very efficiently, even in the highly-damped biological conditions, where the noise is actually orders of magnitude stronger than the chemical energy available. We want to understand how the ratcheting applies to electrons, especially under highly-damped conditions, like in low-mobility organic semiconductors. Very little experimental work has been done on electron ratchets, and so we mainly seek to improve our understanding of the mechanism, with an eye toward possible future applications in solar cells or other electronic devices.

This work is part of an Energy Frontiers Research Center (EFRC) of the Department of Energy, called the Center for Bio-inspired Energy Science.

Ratchet principles. Three conditions are needed to achieve transport: (i) breaking the spatial inversion symmetry, for instance by applying an electric field with periodic asymmetric repeat units (e.g., a sawtooth potential); (ii) breaking temporal symmetry, typically assured by energy dissipation; (iii) energy input to keep the system far-from-equilibrium (otherwise rectification would be prohibited by the 2nd law of thermodynamics). In a flashing ratchet, we combine conditions (i) and (iii) by oscillating (flashing) a periodic sawtooth-shaped potential, so that both spatial asymmetry and energy are applied together. All of our work is in damped systems, so condition (ii) is satisfied by the dissipative energy loss. As an aside, some researchers have constructed non-dissipative ratchets (sometimes called Hamiltonian ratchets) using optical traps and cold atom clusters, so they must use asymmetric temporal drives to achieve transport.

We have recently published an introduction to ratchets, accessible to a wide scientific audience, and I highly recommend perusing that work for a detailed explanation of ratcheting, and some thoughts my colleagues and I have on the field (Lau, Kedem et al., Mater. Horiz. 2017).

A new design. My main work is designing, fabricating and characterizing an experimental flashing electron ratchet. The device is similar to field effect transistors, in a co-planar, bottom-gate geometry. An array of “finger electrodes” (FEs) with an asymmetric thickness profile applied an electric field to an organic semiconductor transport layer above, inducing a current between source and drain electrodes made of the same metal. As a ratchet, the device produces a short-circuit current – current flows between the source and drain electrodes with no applied source-drain bias, or work-function difference between them. The current is produced due to the local asymmetries in the applied field, but without an overall asymmetry (bias). The current is very sensitive to the oscillation frequency, as ratcheting is a dynamic steady-state. The ratchet design we developed enables the application of a wide range of potential shapes, and allows us to examine general behaviors. Our use of a photoactive transport layer enables the modulation of charge carrier density using light, and revealed that sometimes, increasing the carrier density can actually reduce the ratchet current. For the first time in a ratchet device, the ratchet is powered by an unbiased temporal drive, a sine wave, rather than an on/off or sin^2 drive. This will enable the future use of energy source such as electromagnetic radiation to power a ratchet. We explore the symmetry breaking mechanism responsible in another study. We have detailed the design, fabrication, and characterization of the new ratchet device in a 2017 PNAS paper (Kedem et al., PNAS 2017).

Temporal drive. We initially powered our ratchet using a simple sine wave; work in the literature used basic on/off drives. But, surely the temporal drive can be optimized? There has been very little work in the literature, so we set out to explore the topic. We replaced the sine wave with a square wave, and modified the duty ratio (the fraction of time spent in the positive state). We find a rather complicated relationship, but we find the behavior can be approximated very well with a previously published analytical model. With it, we explain the symmetry-breaking mechanism that enables the use of unbiased drives (sine, 0.5 duty ratio square), and explain how to optimize the current (Kedem et al., Nano Lett. 2017).

Ratcheting beyond 1D. Almost all ratchet theory papers use 1D models. However, experimental ratchets typically have a 3D transport layer. Usually the electric field is applied by electrodes under the transport layer, meaning the field is asymmetric in the thickness dimension. In this work we classically simulated charged nanoparticles in water, ratcheted by oscillating ratchet potentials. We found that the non-uniformity in the z-direction (thickness) means the thickness of the transport layer is extremely important to effective transport. In agreement with the experimental studies on electron ratchet, we found that the decay in the z-direction allows us to use sine wave temporal drive, something which is not possible in 1D models (Kedem et al., ACS Nano 2017).

Potential shape. Typically, both theory and experiments in the ratchet field use a simple sawtooth shape, either a piecewise linear (sharp sawtooth), or a biharmonic (sum of two sine waves) with a specific set of pre-factors. But how sensitive is the performance to the shape itself? What if instead of one asymmetric shape, we choose another, slightly different one? My colleague, Bryan Lau, led a study to answer this question, using quantum-mechanical simulations of an electron ratchet. He varied the shape of the potential, as well as the damping strength in the system. We identified two different ratcheting mechanisms, at low and high damping regimes, and found that the ratchet current is extremely sensitive to the shape of the potential – a small change of the shape can result in a nullification or even a reversal of the current (Lau et al., Phys. Rev. E 2016).

A new photovoltaic design. Regular solar-cells (inorganic or organic) rely on exciting electrons over a bandgap. Any energy beyond the band-gap is lost, and photons with energy below the bandgap cannot be used. My colleague, Bryan Lau, performed a study exploring a new type of photovoltaic device, which can use below-bandgap photons (far-IR, THz range) to produce a current. The work used semi-classical simulations of a slab of SiGe with an asymmetric doping profile with Ge to create strain in the Si lattice. This strain causes splitting and shifting of conduction band energy levels. If we use a periodic doping profile with asymmetric repeat units (e.g., sawtooth), we get an asymmetric strain profile, and thus potential. The simulated device can harness below-bandgap, unpolarized, incoherent light. Though the calculated efficiency was very low, this proof-of-principle is very promising, and future simulation and experimental work will improve the performance (Lau et al., Adv. Energy Mater. 2017).