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A Bright Future for Quantum Dots

By Mary Page Bailey |

The novel performance characteristics and tunability of quantum dots make them a promising nanomaterial in numerous emerging applications

Quantum dots (QDs) represent a broad group of semiconducting nanoparticles that feature a unique combination of optical and electronic properties. For example, QDs provide many of the same benefits as organic dyes in existing applications, but they are more robust in terms of light-conversion capabilities and can also withstand harsh chemical solvents, higher temperatures and corrosion, while generally providing a broader light-absorption band. Perhaps QDs are most prominently known for their ability to emit extremely pure colors for long durations (Figure 1) — they have been widely used to improve the display characteristics of high-end televisions. But QDs’ applicability stretches far beyond consumer electronics into wide-ranging potential end uses from solar power to agriculture to water purification.

FIGURE 1. Quantum dots are known for their ability to instill brilliant colors into electronics displays, but their chemistry and tunability make them ripe for innovation in many other areas

First continuous processing method

Quantum Materials Corp. LLC (QMC; San Marcos, Tex.; www.qmcdots.com) has developed what is said to be the world’s first continuous production process for QD manufacturing. QMC’s technology uses all fluidized materials and continuous-flow reactors, whereas the vast majority of QD production depends on batch production. “We use microflow reactors, which can achieve high rates of heat and mass transfer, resulting in a very uniform product. Therefore, very little post-processing is required,” explains Krishna Kowlgi, senior research engineer at QMC. In addition to product homogeneity, another benefit of continuous QD processing over batch is time — the entire continuous-flow process can be automated and requires much less manual intervention. “From start to finish, it takes a minimum of 5 s to a maximum of 19 min, based on the QD material type, to convert raw materials to functional QDs. We think this is the fastest QD production currently on the market. The numbers in literature for batch QD processes range from multiple hours to days or even weeks,” adds Kowlgi. He emphasizes that QMC’s reactors are flexible enough to produce different QDs for different end uses. “The light-emission capabilities of the QD depends on its geometry and chemical composition. A 4-nm spherical QD would produce different emissions than one with a 3-nm diameter,” he adds. Due to the fluidized flow chemistry of QMC’s process, QDs are produced in solution, so they are either filtered or undergo some other type of post-processing separation step.

Currently, QMC has production capabilities to manufacture around 3 kg/h of QDs at its site in San Marcos (Figure 2). In late 2018, the company signed a license agreement to construct a large-scale QD plant in Assam, India. QDs produced at the Assam site are expected to mainly be deployed in solar power and display and lighting applications. Once operational, the Assam site will be among the world’s largest manufacturing plants for active nanoparticles, and the only industrial-scale site to employ continuous QD production.

FIGURE 2. QMC’s continuous process for QD manufacturing enables a considerably faster conversion from feedstock to functional materials
Quantum Materials

Moving forward, QMC is continuing to push the boundaries of its continuous-flow process. “Because the flow reactors have a very small footprint, they are well suited for handling extreme process conditions,” explains Kowlgi. By examining different combinations of processing conditions, more robust QDs can be tailored for a broader range of end uses. Kowlgi lists anti-counterfeit inks (Figure 3) and tags, digital camera sensors and photovoltaics as important emerging application areas for QDs in the coming years.

FIGURE 3. An emerging application for QDs is in anti-counterfeit inks, where their unique color signatures can strengthen security measures
Quantum Materials

QDs for sustainability

QDs also hold promise in improving environmental sustainability. UbiQD, Inc. (Los Alamos, N.M.; www.ubiqd.com) has developed specialized QDs for agricultural and solar energy applications. According to Hunter McDaniel, founder and CEO of UbiQD, the company’s QDs bring together a unique semiconducting composition and a novel luminescence mechanism into a low-toxicity formula — based on zinc or copper rather than commonly used, more toxic materials, such as cadmium (Cd) or lead (Pb) — to enable QDs’ use in applications that were not previously accessible at a commercial scale. “Our main focus is large-area applications that involve the manipulation of sunlight. Greenhouse agriculture and commercial building windows are our priorities. In both cases, we are partially absorbing sunlight, changing the color, and then re-routing that light energy to either boost crop yield or generate electricity.” explains McDaniel. The UbiGro greenhouse-focused product line is commercially available, while the solar-window materials are still in the development phase. UbiGro is an active QD-based film used on greenhouses (Figure 4) that red-shifts the sun’s spectrum. McDaniel says that UbiGro has shown effectiveness in improving yield and quality over a variety of climate conditions with many crops, including tomatoes, cucumbers, leafy greens, hemp and more.

FIGURE 4. QD films in greenhouses can adjust sunlight to improve crop yield
UbiQD

UbiQD utilizes a liquid-phase batch reaction to make about 1 kg of QDs per 10 L of reactor each week. McDaniel says that the key to manufacturing the company’s unique QDs is in the chemical recipes and post-processing steps, and that the process requires lower costs and results in higher-stability materials than other QD manufacturing processes.

Although QDs are still mainly thought of as display components, UbiQD believes that the agricultural market will quickly become another major application area, with solar energy not far behind. “Quantum dots are already making the world a better place, but there is huge untapped potential,” emphasizes McDaniel.

Water purification is another area where QDs can improve sustainability. A group of researchers from the University of South Carolina (Columbia; www.sc.edu) and the SmartState Center of Catalysis for Renewable Fuels (www.smartstatesc.org) have successfully integrated QDs into a reverse-osmosis (RO) membrane designed for high-flux desalination processes. The team used nitrogen-doped graphene-oxide quantum dots (N-GOQDs) as an additive in the fabrication of a polyamide membrane. “The very small size of GOQDs and their rich functional groups make them an excellent additive in polymeric membranes, or even as basic building blocks for GOQD membranes if packed and crossed appropriately,” says Miao Yu, one of the lead researchers on the project, and currently a professor of chemical and biological engineering at Rensselaer Polytechnic Institute (Troy, N.Y.; www.rpi.edu; yumiaorpi.wixsite.com/mysite). According to Yu, this work is the first to investigate GOQDs in membrane applications. “There are many nanoparticles that have been added to RO membrane matrices, but much of this work is still at the lab stage, and the water flux increase is not as significant as that seen with the addition of N-GOQDs,” he adds. The research showed that adding just 0.02 wt.% of GOQDs tripled the polyamide membrane’s water permeability, while still rejecting salts at a comparable rate to an untreated polyamide membrane.

Yu says that N-GOQDs’ functional groups can be easily crosslinked into an RO membrane matrix without introducing large cavities, and that their hydrophilic nature helps to further facilitate water transport, increasing flux. Thus far, a membrane with a permeation area of 10 cm 2 has been demonstrated, but Yu says that the fabrication process, based on interfacial polymerization, is compatible with industrial membrane-preparation processes, making its scaleup more promising. The team has applied for a patent and plans to work with an industry partner on larger-scale production.

Advanced imaging and beyond

In China, NajingTech (Hangzhou; www.najingtech.com) is working to extend the performance of QDs for both photoluminescent (photon-to-photon) and electroluminescent (electron-to-photon) uses. Founded in 2002 by QD pioneer and former professor of chemistry at the University of Arkansas, Xiaogang Peng, NajingTech’s U.S. subsidiary, NN-Labs LLC (Fayetteville, Ark.; ww.nn-labs.com) was among the first commercial providers of QD nanocrystals. Currently, NajingTech is focused on the deployment of QD light-converting devices (QLCD) and QD light-converting films (QLCF), as well as commercializing electroluminescent QD light-emitting diode (QLED) technologies (Figure 5), along with emerging work in the fields of photovoltaics, sensors and detectors, lasers, bio-imaging and more, says Wei Huang, senior researcher at NajingTech. On the biotechnology front, NajingTech is currently working to develop QD-based immunochromatography technologies and QD-linked immunosorbent assays. The company is also investigating ways to minimize or eliminate the Cd content in QD materials for large-scale applications and electroluminescent devices, since Cd introduces toxicity and limits materials’ ability to be used in certain commercial end markets. For example, NN-Labs has developed copper indium sulfide/zinc sulfide (CuInS/ZnS) core-shell QD products for use in biomedical, solar energy and LED lighting applications. The company also offers a line of indium phosphide/zinc sulfide-based (InP/ZnS) QDs as an alternative to Cd-based formulations.

FIGURE 5. Refining electroluminescent properties is the next step in implementing QDs into more complex LED applications

“We use a solution-based ‘green synthesis’ method to make QDs by using the non-coordinating solvent octadecene along with stable and safe precursor materials,” explains Huang. This synthesis method enables precise tuning of nanocrystal size and size distribution, and the use of a non-coordinating solvent is said to make the process simpler and more environmentally friendly than other techniques. Since being developed by Peng and patented in 2006, many QD companies have also adopted this process. In the future, says Huang, the company plans to deploy a printing-based technology to fabricate QLED-based technologies.

MilliporeSigma (Burlington, Mass.; www.sigmaaldrich.com) and EMD Performance Materials, two of the North American operating companies of Merck KGaA (Darmstadt, Germany; www.merckgroup.com), supply QD materials for researchers working on photovoltaics, LEDs, lasers, bio-imaging and more. To facilitate such a wide range of end uses, ensuring consistency in surface chemistry and other functional properties is critically important. “Composition, size and geometry are all important factors that are customized to fit all of these diverse applications,” says Bryce Nelson, head of materials science at MilliporeSigma. “For example, PbS-based QDs exhibit fluorescence in the infrared regime, while InP/ZnS and Cd-based QDs possess fluorescence emission in the ultraviolet-visible range. Adjustments in size allow the fine-tuning of fluorescence properties,” he continues. And depending on the end market, further customization may be required, such as optimizing surface chemistry for compatibility with a surrounding polymer or matrix in display applications. The QD synthesis method is also critical in tailoring materials for new end-use applications — common techniques include the hot-injection method and the heat-up method, among many others.

“Solar cells and lasers come to mind as areas with a lot of research activity right now. For in-vivo applications, QDs with infrared emission capabilities enable enhanced single-cell imaging, as well as deep-tissue mono- and multi-photon imaging,” says Nelson. However, he emphasizes that the use of QDs in-vivo is still hampered by challenges related to toxicity and lack of biodegradability, further highlighting the importance of reducing heavy-metal content. According to Nelson, the Performance Materials business of Merck is working toward Cd-free QDs for display applications, which can provide the increased color range of QDs without the toxicity of heavy metals. EMD Performance Materials is also currently developing new QD technologies for next-generation organic LED (OLED) displays, which will require complex chemical formulations to vastly improve color quality. While these new technologies, says Nelson, are still a few years from commercialization and significant formulation challenges remain, the combination of OLED and QD technologies has the potential to bring a new standard of performance in terms of brightness, efficiency and viewing angle.

From displays to biomedical imaging to solar energy and agriculture, the flurry of activity around tailoring QDs for ever-expanding application fields is certainly going to continue in the coming years, with an increasing number of novel end-uses expected to appear, as formulation and synthesis techniques are further honed. ■

Mary Page Bailey

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