Glowing White: Solvent-free luminescent organic liquids for organic electronics

Current approaches to organic electronics mainly involve plastic film supports with conducting paths and components made of organic molecules inexpensively printed or glued on. Organic electronics are interesting as potential “disposable electronics” for applications like electronic price tags. Even more intriguing are devices that cannot be produced with standard electronics, such as flexible films with integrated circuits for use as novel flat-panel displays or “electronic paper”. A third area of interest involves applications such as photovoltaics that are dependent on economical mass production in order to be profitable.

The development of large components like displays requires organic coatings that emit white light and are inexpensive to produce. Previous gel- or solvent-based liquid “dyes” are easy to apply, but are often not colorfast or are barely luminescent after drying. For solids, on the other hand, processing is often too complex.

A team led by Takashi Nakanishi at the National Institute for Materials Science in Tsukaba (Japan) has taken a different approach: they use uncharged organic substances that are luminescent liquids at room temperature and require no solvent. The electronically active parts of the molecules consist of linear chains of carbon atoms linked by ?-conjugated double bonds. This means that electrons can move freely over a large portion of the molecule. The core is shielded by low-viscosity organic side chains that ensure that the core areas do not interact with each other and that the substance remains liquid.

The researchers were able to prepare a liquid that fluoresces blue under UV light. They then dissolved green- and orange-emitting dyes in this solvent-free liquid. This results in a durable, stable white-emitting paste whose glow can be adjusted from a “cool” bluish white to a “warm” yellowish white by changing the ratio of the dyes. It is possible to use this ink directly in a roller-ball pen for writing, or to apply it with a brush on a wide variety of surfaces. Application to a commercially available UV-LED allowed the researchers to produce white light-emitting diodes.

More information: Takashi Nakanishi, Solvent-Free Luminescent Organic Liquids, Angewandte Chemie International Edition, http://dx.doi.org/10.1002/anie.201108853

Provided by Wiley (news : web)

Novel nanocrystals with advanced optical properties developed for use as luminescent biomarkers

 Upconversion emission materials are ideal for bioimaging due to its effectiveness as contrast agents for the detection of cancer cells, more so when the background emission of non-cancerous tissues can be minimised. These materials could be used as biomarkers for luminescent labeling of cancerous cells. Opaque tissues can be turned into glassy, transparent substances by using these biomarkers which rely on near-infrared excitation.

The Singapore research team led by Associate Professor Xiaogang Liu and its co-researchers from Saudi Arabia and China succeeded in developing an efficient upconversion process in nanoparticles, ensuring a broad tunability of light emission that could be used in imaging applications. They found a chemical structure that can exhibit efficient upconversion properties through a special arrangement of energy levels. Their synthesis of lanthanide-doped core-shell nanocrystals which resulted in advanced optical properties that can control light, proved to be a novel approach.

For sensing applications, separating optical signals from the background can be tricky when the signal and noise occur at the same wavelength. This problem can be solved with upconversion -- a nonlinear optical process -- where two low-energy photons of an incident beam can be converted into a single photon of higher energy, which can then be easily distinguished from the background.

The ability to convert light using these nanomaterials for heating also offers promising applications in photodynamic therapy and drug delivery.

The work of Assoc Prof Liu and team was reported in Nature Materials on 23 October 2011. His team comprises research fellow Dr Feng Wang and graduate students Renren Deng and Juan Wang from the National University of Singapore's (NUS) Department of Chemistry. They worked alongside researchers from King Abdullah University of Science and Technology and Fujian Institute of Research on the Structure of Matter. Assoc Prof Liu and Dr Feng Wang are also scientists at the Institute of Materials Research and Engineering (IMRE), a research institute of Singapore's Agency for Science, Technology and Research (A*STAR).

The published research work was funded by Singapore's A*STAR and Ministry of Education.

A novel approach to cancer detection

The team of researchers focuses on controlling the optical properties of nanomaterials by doping rare-earth metals in confined layer-by-layer structures. The nanoparticle shell can be doped with different rare earth metals, resulting in a broad tunability of the upconverted emission.

By producing nanoparticles with tunable emission which should also have a low toxicity, the researchers have made a great leap in the development of upconverting materials.

Their novel approach involves the designing of core-shell nanoparticles that separates the upconversion process from that of light emission. Photons are absorbed in the core of the nanoparticles and turned into excited electrons, after which they cascade from the core of the nanoparticles into the excited state of rare earth dopants in the shell. While there, these electrons relax and emit light.

Although such sequential energy transfer has been investigated for certain semiconductor nanoparticles and nanowires for solar energy applications, it has not been done so before for rare earth-doped nanoparticles.

Assoc Prof Liu pointed out that effort to find upconverting ions that emit in a wide-ranging spectral region has been unsuccessful until now. This is because an efficient photon upconversion has generally been restricted to a small number of lanthanide ions with emitted light signal detectable by the naked eye.

Explaining his successful approach, Assoc Prof Liu said: "We perform photon upconversion on an array of rare-earth metals. Photon upconversion turns low energy near-infrared light into higher energy made visible with the rational design and chemical synthesis of a core-shell nanostructure."

Assoc Prof Liu and team prepared nanoparticles which could demonstrate an upconversion emission ranging from violet, blue, green to red yellow, with significantly longer infrared excitation wavelengths of up to 980 nm. An important aspect of using light with 980 nm wavelength is such that the transparency of living tissues is high in infrared. This enhances the opportunity for the use of these nanoparticles for cancer detection. Furthermore, the multiple emission colours demonstrated in this research can potentially be used for a more reliable biological diagnostics application, for instance, multiple cell markers.

Opportunities for wider use

The ability to convert low energy near-infrared light into higher energy visible emission, along with low levels of toxicity to cells, and ease of processing, will turn nanometer-sized lanthanide-doped crystals into ideal materials for numerous applications.

According to the group from NUS, the results indicate that a large "library" of luminescent upconversion nanocrystals with distinguishable spectroscopic fingerprints can now be established. When coupled with biological molecules, these nanomaterials would provide a platform for a rapid and reliable route to multiplex detection of cancer or other diseases. The ability of these nanomaterials to induce light-control release of drugs for site-specific delivery also bodes well for future medicine -- fewer or reduced side effects can be expected as lanthanide-doped crystals have been tested to be non toxic.

"This work made me confident that we will see exciting new applications for these particles soon," says Thomas Nann, a research professor from the University of South Australia whose research is in this same field. Prof Nann adds that "Up-converting nanoparticles are materials with a tremendous potential for application. However, due to the need for a rigorous selection of usable up-converting ions, Science appeared not to have made any headway for some time prior to this discovery."

Assoc Prof Liu and co-researchers noted the uniqueness of their design, which is the use of core-shell nanostructures and gadolinium ions for energy migration that enhances the ability to produce a wide range of lanthanide-doped nanocrystals to yield upconverted luminescence.

"Benefiting from the sub-lattice of gadolinium ions as a network for energy migration, these judiciously-designed nanoparticles light up those less commonly used lanthanide ions like terbium, europium, and samarium under near-infrared excitation," explains Professor Chun-Hua Yan, a chemistry professor and well known scientist in the same field in Peking University, China. Adding, Prof Yan says "I do believe that this model, with its uniqueness and versatility, will vastly enrich the currently available upconversion materials, and will have impact on relevant fields such as luminescent biolabelling, multiplexed data storage and display."

The Singapore group has recently filed a related patent for their ground-breaking discovery. Currently, they are working with clinicians to develop clinical diagnostic models for use in a practical manner.

Story Source:

The above story is reprinted from materials provided by National University of Singapore, via AlphaGalileo.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Feng Wang, Renren Deng, Juan Wang, Qingxiao Wang, Yu Han, Haomiao Zhu, Xueyuan Chen, Xiaogang Liu. Tuning upconversion through energy migration in core–shell nanoparticles. Nature Materials, 2011; DOI: 10.1038/NMAT3149

Note: If no author is given, the source is cited instead.

Progress in quantum computing: Researchers control rate of photon emission from luminescent imperfections in diamond

Engineers and physicists at Harvard have managed to capture light in tiny diamond pillars embedded in silver, releasing a stream of single photons at a controllable rate.

The advance represents a milestone on the road to quantum networks in which information can be encoded in spins of electrons and carried through a network via light, one photon at a time.

The finding was published in Nature Photonics, appearing online Oct. 9.

"We can make the emission of photons faster, which will allow us to do more processing per second -- for example, more computations -- in the future quantum network," explains principal investigator Marko Lončar, Associate Professor of Electrical Engineering at the Harvard School of Engineering and Applied Sciences (SEAS).

The device Lončar's research team has built consists of parallel rows of tiny, nanofabricated diamond posts, embedded in a layer of silver, that can each act as a single photon source.

By removing the silver wrapping from their nanostructures, the team was also able to achieve a slower release of photons, which is of interest for probing the dynamics of the quantum system.

The breakthrough takes advantage of imperfections in the diamond's crystal lattice, where carbon atoms are replaced by other elements. To the naked eye, these imperfections can appear as discolorations in the diamond, turning it yellow in the case of nitrogen. Occasionally, there is also a vacancy (missing carbon atom) next to the nitrogen atom.

Each nitrogen-vacancy imperfection can serve as a nearly perfect quantum emitter, capable of emitting red photons one by one, even at room temperature. The technology is a promising candidate for realization of scalable, on-chip quantum networks.

"The color centers in diamond are very interesting as qubits for quantum information processing, where they can be used as memory to store information," says Lončar. "More importantly, they can be interrogated -- they can be written into and read out -- with light."

Lončar's team fabricates diamond posts that contain negatively charged nitrogen vacancy centers, which can absorb light and hold its energy for a given amount of time, finally releasing it in the form of photons.

"The rate at which photons are emitted can be controlled by carefully nano-engineering the center's surrounding," says co-author Irfan Bulu, a research associate in the Lončar group. Attaining fine control of that release, however, has been difficult.

"One of the main challenges has been the efficiency with which you can write information into the spin of these color centers, as well as the efficiency with which you can collect photons emitted from the color centers," explains co-author Jennifer Choy, a graduate student in Lončar's lab at SEAS. "The other challenge has been the rate -- how quickly you can perform these processes."

Previous work from Lončar's group solved the collection efficiency problem by using diamond nanowires to channel and direct the flow of photons. The new research manipulates the radius of diamond pillars and adds the silver coating. The diamond-silver construction acts as an optical nanoresonator, creating a strong electromagnetic field around the emitter and offering a new level of control over the rate of emission.

Moreover, the device functions at room temperature -- an essential requirement for practical computing applications -- and the nanostructured chips are fully scalable.

"We've designed everything in parallel in a massive system, which allows us to make thousands or millions of devices with more or less the same properties, and we use conventional microfabrication and nanofabrication techniques, unlike what has been done in this field before," says Birgit Hausmann, a graduate student in Lončar's lab at SEAS and one of the co-authors.

In addition to Lončar, Choy, Hausmann, and Bulu, co-authors included Tom Babinec, a graduate student at SEAS; Mughees Khan, a staff scientist at the Wyss Institute for Biologically Inspired Engineering at Harvard; Patrick Maletinsky, a fellow of the Department of Physics at Harvard; and Amir Jacoby, Professor of Physics in the Harvard Faculty of Arts and Sciences.

The work was supported by grants and fellowships from the U.S. Department of Defense, the Defense Advanced Research Projects Agency (DARPA) QuEST program, the National Science Foundation (NSF), the King Abdullah University of Science and Technology (KAUST), the Sloan Foundation, and the NSF-supported Nanoscale Science and Engineering Center (NSEC) at Harvard. Fabrication took place at the NSF-supported Center for Nanoscale Systems (CNS) at Harvard.

The above story is reprinted (with editorial adaptations) from materials provided by Harvard University.

Journal Reference:

Jennifer T. Choy, Birgit J. M. Hausmann, Thomas M. Babinec, Irfan Bulu, Mughees Khan, Patrick Maletinsky, Amir Yacoby, Marko Lončar. Enhanced single-photon emission from a diamond–silver aperture. Nature Photonics, 2011; DOI: 10.1038/nphoton.2011.249