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July 2012

Nanometers and Nanoseconds, Together at Last

Elegant yet powerful method measures fast transient events with nanometer-scale resolution

Rajiv Giridharagopal

Measuring fast transient effects with atomic force microscopy. A light pulse is applied to a solar device. The change in AFM tip position is mathematically converted to frequency. This process is repeated at each point to generate a photocharging image, thereby showing which parts of the solar cell convert light to electrical current most efficiently.

A team from the Center for Interface Science: Solar Electric Materials, or CISSEM, recently published a technique in Nano Letters for measuring events as fast as 100 nanoseconds on areas as small as 80 nanometers. This technique is based on atomic force microscopy, or AFM, a popular method for taking images of surfaces with resolution of billionths of a meter. However, AFM is poor at measuring how quickly events happen. For example, AFM can provide a detailed image of a plastic solar cell, but it provides little information on how efficiently sunlight is converted into electricity. The CISSEM team's new method combines the high-resolution images of AFM with 1000-fold improvement in time resolution.

Traditional approaches for measuring fast events in AFM often involve expensive, ultrafast lasers and complex optical alignment. The CISSEM team use affordable equipment and mathematical techniques to achieve the same end. The nanosecond time-resolution means it is now possible to investigate nanoscale dynamics that affect a wide range of materials and interfaces previously inaccessible—including the local origins of photocurrent in a solar cell—at a fraction of the cost.

Measuring Charging Times Helps Improve Solar Devices: A solar cell converts incident light into electricity. If you shine a pulse of light on an inefficient material, current is generated slowly; on an efficient material, current is generated quickly. Because of the complicated local structure in many modern solar devices, particularly plastic solar cells, the efficiency varies considerably throughout the material.

Combining measurements of charging times with nanoscale images lets scientists pinpoint the regions where solar materials generate current the fastest, with sensitivity far beyond other tools. The reported technique can be used on full devices under realistic conditions, thereby directly correlating device performance with charging times measured over nanometer-sized regions.

A Simple Solution for Fast Measurements: In AFM, a sharp metal tip vibrates a few nanometers above a surface at a specific rate called its "resonance frequency." Imagine a tuning fork that rings when you strike it; the note you hear depends on its resonance frequency. The resonance frequency of the AFM tip depends on how much electrical charge is in the surface underneath. Additionally, the time it takes the AFM tip’s resonance frequency to change depends on how quickly charge is built up underneath it.

To measure this charging time, the reported technique records the AFM tip position as a function of time as a light pulse is applied to the sample. The light pulse is controlled to happen only at a specific point in time. The position signal is numerically converted to a frequency versus time signal. By measuring how quickly the resonance frequency changes, it is possible to determine the charging time to 100 billionths of a second. The novelty of the CISSEM team’s technique is in its application to solar energy, and in acquiring the frequency versus time using only a combination of mathematics and time-synced averaging while at the same time achieving time-resolution less than a single up-and-down motion of the AFM tip.

The Next Application: The CISSEM team is looking at how different molecules used deliberately to modify interfaces affect solar cells. New types of molecules can be used to create more efficient solar cells by modifying the electrodes used. The reported technique can measure the rate of both charge generation and decay in systems using new molecules. Additionally, many next-generation solar cell materials break down when exposed to air. The AFM technique can show where on the surface this occurs and at what rate. The ultimate goal is a better understanding of how nanometer-scale phenomena affect device efficiency.

More Information

Giridharagopal R, GE Rayermann, G Shao, DT Moore, OG Reid, AF Tillack, DJ Masiello, and DS Ginger. 2012. "Sub-Microsecond Time Resolution Atomic Force Microscopy for Probing Nanoscale Dynamics." Nano Letters 12(2)893-898. DOI: 10.1021/nl203956q

Acknowledgments

The Center for Interface Science: Solar Electric Materials, an Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, supported the new instrumentation and hardware development to probe interfacial phenomena with high resolution. Additional previous funding was from Air Force Office of Sponsored Research, supporting the purchase of the atomic force microscope hardware, and the National Science Foundation, supporting the numerical modeling. OG Reid was supported by a National Science Foundation fellowship during portions of this work. GE Rayermann received additional support from a fellowship from the University of Washington.

About the author(s):

  • Rajiv Giridharagopal is a postdoctoral research associate in David S. Ginger’s group at the University of Washington and a member of the Center for Interface Science: Solar Electric Materials. His current research interests include using scanning probe tools to examine organic electronic and photovoltaic interfaces.

Catching a Hold of Time

Reactions occurring in billionths of seconds can no longer hide from affordable, conventional microscope

This new approach measure effects as fast as 100 nanoseconds occurring on areas as small as 80 billionths of a meter.

Whether turning agricultural waste into fuel or sunlight into electricity, electrons shift and molecules break in mere billionths of seconds, or nanoseconds. To control reactions, scientists want high-quality images of what is happening every hundred nanoseconds. Atomic force microscopes provide clear images but cannot see events happening that quickly. Scientists devised a straightforward approach that can measure effects as fast as 100 nanoseconds on features as small as 80 billionths of a meter. This technique does not rely on custom probes or expensive hardware. It uses complex calculations and data already collected by the microscope. Scientists are using this method to study materials and interfaces for improved solar cells. The Center for Interface Science: Solar Electric Materials, led by the University of Arizona, did the work.

More Information

Giridharagopal R, GE Rayermann, G Shao, DT Moore, OG Reid, AF Tillack, DJ Masiello, and DS Ginger. 2012. "Sub-Microsecond Time Resolution Atomic Force Microscopy for Probing Nanoscale Dynamics." Nano Letters 12(2)893-898. DOI: 10.1021/nl203956q

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.