Are Quantum Dots flat? – An interview with Nexdot

After numerous false starts, Quantum Dot (QD) technologies finally took off less than a decade ago and have since seen a meteoric rise in consumer display applications. First encountered exclusively in high-end flagship TVs, quantum dots conversion films have now cascaded into numerous mid-range LCD TVs and monitors, enabling better performance in terms of color gamut, color volume, and power consumption. More importantly, new applications are emerging on a regular basis: QD recently entered the OLED world with Samsung Display’s QD-OLED panels, already available in flagship TVs and monitors from Sony, Samsung, and Dell: see Yole Développement’s first impressions at the CES 2022. But the rise of Quantum Dots might not stop here.

As illustrated by last year’s acquisition of microLED pioneer Glo by QD leader Nanosys, the microLED revolution could leverage QD color conversion to enable or accelerate adoption in many applications. Yole Développement (Yole) analyzed in depth the status of the MicroLED industry, combining market, technologies, and IP data, with two dedicated reports: MicroLED Displays – Market, Industry and Technology Trends and MicroLED Displays – Intellectual Property Landscape and Analysis.

Ultimately, some even see Electroluminescence as the end game technology for displays. But QDs are not stopping here and have also started to propagate in general lighting, imaging, and photovoltaic applications.

In this context, Nexdot, a little-known French start-up, has been quietly developing a portfolio of unique technologies aimed at solving some of the most vexing problems associated with further advancing QD in displays and other applications. Following a variety of recent breakthroughs, Benoit Dubertret, founder and CEO of Nexdot, has taken the time to answer some questions from Yole’s display analysts, Zine Bouhamri and Eric Virey.

Eric Virey (EV): Could you first introduce yourself and Nexdot to our readers?

Benoit Dubertret (BD): Sure, I am Benoit Dubertret, founder and CEO of Nexdot. I was trained as a material scientist. I spent six years in the US (MIT, Princeton, the Rockefeller University) and 15 years in CNRS at ESPCI in Paris, France. I am ranked among the 50 top-cited French scientists in material science.

I founded Nexdot in 2010 at ESPCI, a world-class research facility that has produced 5 Nobel prizes. Nexdot was founded after the discovery of quantum plates, colloidal quantum wells with exceptional optoelectronic properties.

For those interested in colloidal semiconductor history, colloidal quantum dots were first synthesized as spheres (with 3D exciton confinement) in 1993 by Chris Murray and David Norris in the group of Mungi Bawendi at MIT. This work led to the creation of QDVision. In 2000, X. Peng, in the group of P. Alivisatos, synthesized the first quantum rods (with 2D confinement). This work is at the origin of Nanosys. In 2007, S. Ithurria, one of my Ph.D. students, synthesized quantum plates (1D confinement). Nexdot was born. After incubation at ESPCI for a few years, Nexdot moved to Romainville, close to Paris, to a campus dedicated to chemical and pharmaceutical companies. Nexdot is now a rich team of young, enthusiastic, and dedicated scientists and technicians.

EV: What are the key technologies Nexdot is working on, and how do you differentiate from the competition?

BD: Nexdot has been developing a portfolio of technologies along three main axes: quantum plates, encapsulation, and pixelization. We have been developing these technological bricks for more than 12 years and have now reached a maturity level compatible with the most stringent requirement of the microLED industry. Nexdot has invented and patented quantum plates, the equivalent of colloidal quantum wells. We are the only company in the world developing quantum plates and have invented an industrial encapsulation process similar to packaging that embeds the nanocrystals in a matrix that protects them from degradation (water, oxygen, and free radicals).

Zine Bouhamri (ZB): Starting with the platelet structure, what unique advantages does it offer compared to dots?

BD: Quantum plates (QPs) are a new generation of colloidal semiconductor nanoparticles with a confinement in just one dimension. Their thickness is controlled with atomic precision: chemistry reaches a structural limit. This structural control has many advantages: quantum plates emit colors with very good purity (20-30 nm FWHM), have directional light emission, and the absorption coefficient is higher than for spherical QDs. In collaboration with the CEA LETI, we have demonstrated that quantum plates can absorb light three times more efficiently than spherical QDs. Combined with a resin-free pixelization technique, this allows us to make pixels that convert 90% of the blue light into red light with a thickness of just 250 nm (see figure 1). This is three times thinner than what theoretically could be achieved with spherical QDs.

In addition, QPs have excellent light flux stability. In combination with encapsulation, we have demonstrated that our QPs have L80 close to 25,000 hrs under 0.2 W/cm2 excitation at 455 nm for red-emitting QPs and an L80 close to 5,000 hrs for green-emitting QPs under the same conditions. The results for the red-emitting QPs are in agreement with external measurements done by CEA-LETI (figure 2). To the best of our knowledge, these results demonstrate that quantum plates are very suitable color converters for the microLED industry.

Quantum plates convert blue light into red light much more efficiently than Quantum Dots. With red QPs, 90% of the blue light can be converted into red light with a 250 nm thick QPs layer – Courtesy of Nexdot, 2022

ZB: Do your material contain any heavy metals? What is the status regarding RoHS compliance for the different configurations and applications?

BD: The best performances are achieved when some heavy metals are used, but we are very close to RoHS compliance and will be RoHS compliant if the latest exemption requests are accepted.

EV: Is the stability intrinsic to the platelet structure or enabled by the encapsulation?

BD: We have developed the encapsulation of QPs to protect them from chemical degradation and enhance the durability of their emission under continuous and intense excitation. Our encapsulation process is compatible with any other kind of nanoparticles and has the advantage that virtually any matrix can be used. When applied to semiconductor nanoparticles, Nexdot has developed a process that preserves almost entirely the fluorescent properties of the semiconductor nanocrystals and yet protects them efficiently against degradation due to water, oxygen, or even free radicals. Thanks to the encapsulation, the QDs can be manipulated much more easily, incorporated into a different polymer matrix or resin without needing an additional protecting layer that can be expensive. It is really the combination of QPs and encapsulation that gives the best stability under flux.

ZB: How complex is this encapsulation process? How does it work with the ligands and other surface treatments required to ensure proper behavior of the dots in the polymer matrix, inks, or resist they are dispersed into for the different applications?

BD: The encapsulation process is very easy in principle. It is compatible with virtually any type of ligands or surface chemistry for the QPs. The surface chemistry of our Quantum Pearls can be easily modified so that our pearls can be dispersed in various types of polymer matrix, ink, resist, or liquid. This technology greatly extends the possible applications of nanocrystals. Without encapsulation, fluorescent nanocrystals have to be protected from degradation due to water and oxygen using barrier layers or films, as is the case in QLED TVs. With encapsulation, the nanocrystals can be used without any additional protection. And they become much more robust fluorescent emitters able to withstand harsh chemical processes. We are indeed taking advantage of the encapsulation to penetrate markets where the use of QDs was hampered by their sensitivity to oxygen and water.

Red and green quantum plates fluorescence emission when excited with a 455 nm light @ 0.2 W/cm2. Red (green) QPs fluorescence loses 20% of its intensity after 25,000 hrs (5,000 hrs) – Courtesy of Nexdot, 2022

EV: How thick can the quantum platelets film be, or more importantly, for display applications, what kind of aspect ratio can you get?

BD: Thanks to the QPs’ efficient color conversion ability, a film of 250 nm of red QPs can convert 90 % of the blue light into red light (see figure 2). For green-emitting QPs, a film of ~1 um is needed. For a 5um pixel pitch, this results in an aspect ratio of 1/20 for red pixels and 1/5 for green pixels.

ZB: What is Nexdot’s business model? Are your encapsulation and patterning technologies for internal use, or are you open to making them available to other material manufacturers? Are you working with major partners?

BD: Nexdot’s business model is to develop solutions with material manufacturers using our main technological bricks that can be implemented in their production lines with as little modification as possible. Our solutions are developed internally to make them available to other material manufacturers. Yes, we are already working with major partners.

EV: Would you be operating as a “foundry”, providing encapsulation and patterning services, or would you be granting licenses to partners so they can set up their own operations?

BD: In our experience, co-development is necessary to adapt and fine-tune our solutions so that they deliver the performances expected by our partners. It is only when you master synthesis, surface chemistry, and transformation (encapsulation, patterning, etc…) for nanoparticles that you can provide a solution that can be successfully integrated into a product.

ZB: The world has been a strange place over the last couple of years. How has Nexdot operated during the pandemic? Any major impact on your activities and partnerships?

BD: As we mentioned, Nexdot’s team is young and very motivated. Because they always felt secure at Nexdot, we never stopped our activities, though relationships with our partners have proven more complicated mainly because of travel restrictions.

EV: Is there anything you’d like to add for our readers?

BD: I would like to tell them that colloidal semiconductor applications are coming of age. Korean, American, and Chinese players are well known; Europe has remained quite unnoticed so far in the field of light emission. We discussed briefly three technological bricks developed at Nexdot that could change the way semiconductor nanocrystals can be used as color converters for the microLED industry. But as Werner Heisenberg said, « the whole is greater than the sum of the parts ». Come and visit us at Display Week beginning of May at booth 229: we have more to show !  

About the interviewee

Benoit Dubertret is founder and CEO of Nexdot. Prior to Nexdot, Benoit worked at ESPCI, CNRS (Paris, France), as a Director of Research. Between 1998 and 2022, he occupied a post-Doctorate position at the NEC Research Institute in Princeton and at the Rockefeller University (New York, USA).

Benoit graduated with a Ph.D. in Physics, from the Université Louis Pasteur (Strasbourg, France). He has published more than 190 articles in refereed journals.

About the interviewers

Eric Virey, Ph.D., serves as a Principal Display Market and Technologies Analyst within the Photonics, Sensing & Display division at Yole Développement (Yole).
Eric is a daily contributor to the development of the Display activities at Yole, with an extensive collection of market and technology reports on display technologies, Quantum Dots, MicroLEDs, and TFT backplanes, as well as multiple custom consulting projects: business strategy, identification of investments or acquisition targets, due diligence (buy/sell side), market and technology analysis, cost modeling, technology scouting, etc.

Eric has spoken at more than 50 industry conferences worldwide over the last ten years. He has been interviewed and quoted by leading media worldwide, including: The Wall Street Journal, CNN, Fox News, CNBC, Bloomberg, Financial Review, Forbes, Technology Review, etc. He is also a regular contributor to various display industry media and organizations.
Previously Eric has held various R&D, engineering, manufacturing, and business development positions with Fortune 500 Company Saint-Gobain in France and the United States.
Eric Virey holds a Ph.D. in Optoelectronics from the National Polytechnic Institute of Grenoble. He is currently based in Portland, OR.

Zine Bouhamri, Ph.D., is Team Lead Analyst, Imaging & Display Activities at Yole Développement (Yole).

Zine manages the expansion of the technical expertise and market know-how of the company.
In addition, he actively assists and supports the development of dedicated imaging collection of market & technology reports and monitors as well as custom consulting projects.
Prior to Yole, Zine oversaw numerous R&D programs at Aledia. For more than three years, he developed strong technical expertise as well as a detailed understanding of the display industry. He is the author or co-author of several papers and patents.
Zine Bouhamri holds an Electronics Engineering Degree from the National Polytechnic Institute of Grenoble (FR), one from the Politecnico di Torino (IT), and a Ph.D. in RF & Optoelectronics from Grenoble University (FR).

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