纳米级别下的颜色

If nanoscience were television, we'd be in the 1950s. Although scientists can make and manipulate nanoscale objects with increasingly awesome¹ control, they are limited to black-and-white imagery for examining those objects. Information about nanoscale chemistry and interactions with light -- the atomic-microscopy equivalent to color -- is tantalizingly²（逗人地） out of reach to all but the most persistent³ researchers. But that may all change with the introduction of a new microscopy tool from researchers at the Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) that delivers exquisite⁴ chemical details with a resolution once thought impossible. The team developed their tool to investigate solar-to-electric energy conversion⁵ at its most fundamental level, but their invention promises to reveal new worlds of data to researchers in all walks of nanoscience.

 

"We've found a way to combine the advantages of scan/probe microscopy with the advantages of optical spectroscopy," says Alex Weber-Bargioni, a scientist at the Molecular⁶ Foundry, a DOE nanoscience center at Berkeley Lab. "Now we have a means to actually look at chemical and optical processes on the nanoscale where they are happening."

 

Weber-Bargioni is corresponding author of a paper reporting this research, published in Science. The paper is titled, "Mapping local charge recombination heterogeneity⁷（异质性） by multidimensional nanospectroscopic imaging." Co-authoring the paper are Wei Bao, Mauro Meli, Frank Ogletree, Shaul Aloni, Jeffrey Bokor, Stephano Cabrini, Miquel Salmeron, Eli Yablonovitch, and James Schuck of Berkeley Lab; Marco Staffaroni of the University of California, Berkeley; Hyuck Choo of Caltech; and their colleagues in Italy, Niccolo Caselli, Francesco Riboli, Diederik Wiersma, and Francesca Intoni.

 

"If you want to characterize materials, particularly nanomaterials, the way it's traditionally been done is with electron microscopies and scan/probe microscopies because those give you really high, sub-atomic spatial⁸ resolution," says co-author James Schuck, a nano-optics researcher at the Molecular Foundry. "Unfortunately, what they don't give you is chemical, molecular-level information."

 

For chemical information, researchers typically turn to optical or vibrational⁹ spectroscopy. The way a material interacts with light is dictated¹⁰ to large part by its chemical composition, but for nanoscience the problem with doing optical spectroscopy（光谱学） at relevant scales is the diffraction limit, which says you can't focus light down to a spot smaller than approximately half its wavelength¹¹, due to the wave-nature of light.

 

To get around the diffraction limit, scientists employ "near-field" light. Unlike the light we can see, near-field light decays exponentially away from an object, making it hard to measure, but it contains very high resolution -- much higher than normal, far-field light.

 

Says Schuck, "The real challenge to near-field optics, and one of the big achievements in this paper, is to create a device that acts as a transducer of far-field light to near-field light. We can squeeze it down and get very enhanced local fields that can interact with matter. We can then collect any photons that are scattered¹² or emitted due to this interaction, collect in the near field with all this spatial frequency information and turn it back into propagating, far-field light."

 

The trick for that conversion is to use surface plasmons: collective oscillations of electrons that can interact with photons. Plasmons on two surfaces separated by a small gap can collect and amplify¹³ the optical field in the gap, making a stronger signal for scientists to measure.

 

Researchers have exploited these effects to make near-field probes with a variety of geometries, but the experiments typically require painstaking¹⁴ optical alignment¹⁵, suffer from background noise, only work for narrow frequency ranges of light and are limited to very thin samples.

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