September 2014
Name: Ana Brown
Department: Applied Physics
School: California Institute of Technology
Project: Exploring the Plasmoelectric Effect - a Novel Mechanism for Solar Energy Conversion
Research Advisor: Harry Atwater
A plasmon is a coherent oscillation of free carriers due to optical excitation, which is useful for concentrating optical fields. In recent years, plasmonic elements have been incorporated in photovoltaic devices and fuel cells as passive elements to increase absorption and concentrate light and electric fields. In contrast to such passive applications of plasmonic nanostructures, we have recently discovered a new optoelectronic phenomenon that further utilizes the large plasmonic field enhancement (arXiv:1202.0301). Off-resonant optical excitation of plasmons results in spontaneous electrostatic charging in a conducting nanostructure. I propose further investigation of this effect, termed the 'plasmoelectric effect', which could allow the use of plasmonic structures as active optoelectronic elements in light-to-electric energy devices such as solar or fuel cells. For example, plasmoelectric elements could be used to produce a light-induced electric field (P-N junction) in a solar cell or could be the center-piece of a semiconductor-less solar cell, by appropriately controlling incident radiation.
The plasmoelectric effect generates an optically induced electrochemical potential in plasmonic nanostructures that is a consequence of the dependence of the plasmon resonance frequency on charge density. When incident radiation is at frequencies higher than the frequency of maximum absorption by a neutral plasmonic resonator, electron density will increase, manifesting a negative plasmoelectric potential and enhanced absorption compared with the neutral structure. Similarly, radiation at lower frequencies induces a lower carrier density, a positive plasmoelectric potential, and enhanced absorption. The magnitude of these trends scales with incident power. Therefore, a plasmonic resonator can behave as a heat engine that converts absorbed off-resonant optical power into a DC electrostatic potential.
Recently, (paper in review) we have experimentally verified this behavior through electrical and optical experiments. We used Kelvin probe force microscopy to monitor surface electrostatic potential of both Au colloid particles on ITO and Au nano-hole arrays under monochromatic optical excitation. We measured negative induced potentials on the plasmonic structures during excitation to the high frequency side of the neutral plasmon resonance, and positive potentials during excitation on the low frequency side of the neutral resonance. The signal magnitude showed the expected power dependence. We also measured increased optical absorption, indicating plasmoelectric potentials, in Au colloid particles when optically pumped with radiation to either side of the neutral plasmon resonance. These results demonstrate that plasmonic absorption in nanostructures can directly induce electrochemical potentials.
I propose further optical and electrical investigations that will provide us with better quantitative understanding of the magnitude of charging and times cales of the plasmoelectric effect. This information is essential for developing and improving technologies that will incorporate this fundamentally new mechanism of optical power conversion in active plasmoelectric elements. These elements could function as light-induced P-N junctions to augment conventional solar and fuel cell modules, as well as enable light-to-electric power conversion in systems made entirely of conductors by replacing the usual function of doped or gated semiconductor components with nanostructured low-cost materials like Aluminum or Graphene irradiated off-resonance. The latter approach is facilitated by the remarkable spectral tailorability of plasmonic nanostructures.
Fundamentally, the plasmoelectric mechanism provides active control of the electrochemical potential of a conductor using incident radiation. By further exploring this new effect, we hope to increase efficiency in conventional sustainable energy technology and open brand new avenues for light-to-electrical power conversion.
Name: John Helveston
Department: Engineering and Public Policy
School: Carnegie Mellon University
Project: Quantifying the Environmental Implications of Consumer Preferences and Policy Incentives for Plug-in Vehicles in China
Research Advisor:Jeremy Michalek
China has the largest passenger car market in the world. Cars now consume approximately half of all crude oil used in China and also contribute to over half of all harmful pollutants in China. In an effort to reduce some of the negative impacts from passenger cars, the Chinese government has promoted the use of plug-in hybrid (PHEV) and battery electric (BEV) vehicles through federal subsidies and other incentives. This study aims to quantify the effectiveness of such policies by estimating the potential environmental impacts from plug-in vehicle adoption in China.
This research will use discrete choice models to estimate market demand for plug-in vehicles based on consumer preferences and policy incentives and then use life-cycle assessment (LCA) to estimate the expected reductions in emissions and oil consumption from the adoption of these vehicles. LCA accounts for all energy consumed and emissions released from vehicle production until end of life. The resulting models will enable policy makers to quantify the expected environmental outcomes from policy decisions such as vehicle subsidies in terms of dollars per avoided (or increased) damages. Expected results include a general approach for incorporating consumer preferences and policy incentives into LCA and an assessment of the potential reductions in emissions and oil consumption from vehicle subsidies.
Name: Christopher W. Roske
Department: Chemistry Department
School: California Institute of Technology
Project: Earth-Abundant Hydrobromic Acid Splitting Device
Research Advisor:Harry Gray and Nathan Lewis
The fundamental challenge facing widespread adoption of renewable energy sources such as photovoltaic devices or wind power is intermittency. Significant investments would be necessary in electricity transmission infrastructure and large changes to the fleet of baseload power generation stations would need to be made to accommodate variable outputs by fuel-less methods. Ensuring a higher penetration of the electricity generation sector by these renewables requires a way to store the collected energy and release it exactly in tune with demand.
One proposed method to do this is by building a fully integrated light collector and chemical storage system. If light collection, storage, and extraction are all efficient processes, then these systems would ideally be distributed across the country—bringing additional stability to the grid, enabling maximum collection from the sun, and encouraging increased renewable energy usage. We are working on experiments that leads to a device that will store solar energy by splitting HBr into Br2 and H2. The device will use radial junction silicon microwire arrays as photoelectrodes in a side-by-side configuration; protect the photoelectrodes using inorganic films; and interface electrocatalysts to the protected microwires. We will raise the bar in the field higher by making an earth-abundant, stable, efficient, and scalable integrated solar energy system.