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Nanocrystals twinkle no longer
Nano Today (2009) 4, 283—287
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanotoday
NEWS AND OPINIONS
Nanocrystals twinkle no longer Cordelia Sealy
The tunable optical properties of semiconductor nanocrystals have made them attractive for lasers, light-emitting diodes, solar cells and biological imaging but their habit of ‘blinking’ — rather like twinkling stars — has curtailed actual applications. But now Todd D. Krauss from the University of Rochester, together with colleagues from Cornell, Eastman Kodak Company and the Naval Research Laboratory, have reported non-blinking semiconductor nanocrystals [X. Wang, et al., Nature (2009), doi:10.1038/nature08072]. The core/shell CdZnSe/ZnSe nanocrystals exhibit continuous, non-blinking photoluminescence over the course of hours. The new nanocrystals are also unusual because they are charged — casting doubt on the idea that blinking is the result of extra charges enhancing nonradiative decay rates. ‘‘We have discovered an exciting new class of colloidal semiconductor nanocrystals that show a complete absence of single molecule photoluminescence blinking,’’ says Krauss. ‘‘For about 15 years people have been observing the blinking effect in nanocrystals and trying to control or mitigate it — we are the first to do that completely.’’ The structure of the nanocrystals appears to be key to their unusual behaviour. While the nanocrystals comprise a CdZnSe core capped with a ZnSe shell, the transition between the two layers is not abrupt, as is usually the case, but is a continuous gradient from the core to the outer shell (Fig. 1). This soft quantum confinement, believe the researchers, helps suppress Auger recombination, which is thought to be responsible for the ‘off’ periods during photoluminescence. ‘‘That gradient squelches the processes that prevent photons from radiating, and the result is a stream of emitted photons as steady as the stream of absorbed photons,’’ explains Krauss. Not only could non-blinking nanocrystals be a boon to biological imaging, they could also have significantly lower laser
Figure 1 A rendition of the new non-blinking nanocrystal. (Credit: Todd D. Krauss, University of Rochester.)
thresholds opening up the possibility of electrically pumped lasers. ‘‘This is a very important result that addresses one of the main issues regarding colloidal quantum dots,’’ says Benoit Dubertret of ESPCI in Paris.
284 Boldizsar Janko of the University of Notre Dame agrees that the work is a ‘‘clear and massive experimental advance’’. But there are some hurdles to overcome first. The CdZnSe/ZnSe nanocrystals have broad photoluminescence spectra containing multiple peaks, which could make their use for biological imaging tricky because of the potential for
News and opinions overlap between different spectra. If this issue can be overcome, however, the new nanocrystals could be produced reliably in large volumes, Krauss told Nano Today. E-mail address: [email protected] 1748-0132/$ — see front matter doi: 10.1016/j.nantod.2009.06.009
Nanorods take data storage to the next dimension Cordelia Sealy A new optical storage device, which harnesses Au nanorods to record data in five dimensions, could push information density beyond 1012 bits per cm3 (1 Tbit/cm3 ) [P. Zijlstra, et al., Nature (2009) 459, 410]. Current storage devices use three spatial dimensions to record data, but with Au nanorods, which form surface plasmons when hit by light, researchers from Swinburne University of Technology in Australia have introduced two extra dimensions — the wavelength of light and its polarization.
‘‘We have shown how nanostructured material can be incorporated into a disk in order to increase data capacity, without increasing the physical size of the disk,’’ says researcher Min Gu. The device consists of 1 m thick recording layers of Au nanorods embedded in a polymer matrix of spin-coated polyvinyl alcohol on a glass substrate. Multiple recording layers are separated by 10 m thick transparent layers (Fig. 1). When the recording layers are blasted with a high-power laser pulse, the nanorods heat up and change
Figure 1 Normalized two-photon luminescence (TPL) raster scans of 18 patterns encoded in the same area using two laser light polarizations and three different laser wavelengths. Patterns were written in three layers spaced by 10 m. The recording laser pulse properties are indicated (wavelength on left, polarization on bottom). The recordings were retrieved by detecting the TPL excited with the same wavelength and polarization as employed for the recording. The size of all images is 100 m × 100 m, and the patterns are 75 × 75 pixels. Reprinted by permission from Macmillan Publishers Ltd.: Nature, advance online publication, 21 May 2009 (doi:10.1038/nature08053).
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanotoday
NEWS AND OPINIONS
Nanocrystals twinkle no longer Cordelia Sealy
The tunable optical properties of semiconductor nanocrystals have made them attractive for lasers, light-emitting diodes, solar cells and biological imaging but their habit of ‘blinking’ — rather like twinkling stars — has curtailed actual applications. But now Todd D. Krauss from the University of Rochester, together with colleagues from Cornell, Eastman Kodak Company and the Naval Research Laboratory, have reported non-blinking semiconductor nanocrystals [X. Wang, et al., Nature (2009), doi:10.1038/nature08072]. The core/shell CdZnSe/ZnSe nanocrystals exhibit continuous, non-blinking photoluminescence over the course of hours. The new nanocrystals are also unusual because they are charged — casting doubt on the idea that blinking is the result of extra charges enhancing nonradiative decay rates. ‘‘We have discovered an exciting new class of colloidal semiconductor nanocrystals that show a complete absence of single molecule photoluminescence blinking,’’ says Krauss. ‘‘For about 15 years people have been observing the blinking effect in nanocrystals and trying to control or mitigate it — we are the first to do that completely.’’ The structure of the nanocrystals appears to be key to their unusual behaviour. While the nanocrystals comprise a CdZnSe core capped with a ZnSe shell, the transition between the two layers is not abrupt, as is usually the case, but is a continuous gradient from the core to the outer shell (Fig. 1). This soft quantum confinement, believe the researchers, helps suppress Auger recombination, which is thought to be responsible for the ‘off’ periods during photoluminescence. ‘‘That gradient squelches the processes that prevent photons from radiating, and the result is a stream of emitted photons as steady as the stream of absorbed photons,’’ explains Krauss. Not only could non-blinking nanocrystals be a boon to biological imaging, they could also have significantly lower laser
Figure 1 A rendition of the new non-blinking nanocrystal. (Credit: Todd D. Krauss, University of Rochester.)
thresholds opening up the possibility of electrically pumped lasers. ‘‘This is a very important result that addresses one of the main issues regarding colloidal quantum dots,’’ says Benoit Dubertret of ESPCI in Paris.
284 Boldizsar Janko of the University of Notre Dame agrees that the work is a ‘‘clear and massive experimental advance’’. But there are some hurdles to overcome first. The CdZnSe/ZnSe nanocrystals have broad photoluminescence spectra containing multiple peaks, which could make their use for biological imaging tricky because of the potential for
News and opinions overlap between different spectra. If this issue can be overcome, however, the new nanocrystals could be produced reliably in large volumes, Krauss told Nano Today. E-mail address: [email protected] 1748-0132/$ — see front matter doi: 10.1016/j.nantod.2009.06.009
Nanorods take data storage to the next dimension Cordelia Sealy A new optical storage device, which harnesses Au nanorods to record data in five dimensions, could push information density beyond 1012 bits per cm3 (1 Tbit/cm3 ) [P. Zijlstra, et al., Nature (2009) 459, 410]. Current storage devices use three spatial dimensions to record data, but with Au nanorods, which form surface plasmons when hit by light, researchers from Swinburne University of Technology in Australia have introduced two extra dimensions — the wavelength of light and its polarization.
‘‘We have shown how nanostructured material can be incorporated into a disk in order to increase data capacity, without increasing the physical size of the disk,’’ says researcher Min Gu. The device consists of 1 m thick recording layers of Au nanorods embedded in a polymer matrix of spin-coated polyvinyl alcohol on a glass substrate. Multiple recording layers are separated by 10 m thick transparent layers (Fig. 1). When the recording layers are blasted with a high-power laser pulse, the nanorods heat up and change
Figure 1 Normalized two-photon luminescence (TPL) raster scans of 18 patterns encoded in the same area using two laser light polarizations and three different laser wavelengths. Patterns were written in three layers spaced by 10 m. The recording laser pulse properties are indicated (wavelength on left, polarization on bottom). The recordings were retrieved by detecting the TPL excited with the same wavelength and polarization as employed for the recording. The size of all images is 100 m × 100 m, and the patterns are 75 × 75 pixels. Reprinted by permission from Macmillan Publishers Ltd.: Nature, advance online publication, 21 May 2009 (doi:10.1038/nature08053).