The History and Future of Quantum Dots

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(Source: Materials Research Center, NajingTech, Hangzhou, China) This mini-report elaborates the fundamentals of quantum dots (QDs) materials and the development of QDs from the stage of “Seed” to “Maturity” as well as commercialization around the world.

QDs, QLED, NajingTech, QD Vision, Nanosys, Nanoco, Commercialization

What exactly is a quantum dot?

Quantum Dots (QDs), also known as semiconductor nanocrystals, are quasi-zero dimensional nanoparticles with the size between 2 and 10 nm. The dimensions of QDs are smaller than the Fermi wavelength of electrons inside of QDs materials, and the movement of electrons in all directions is limited, thus leading to a remarkable quantum confinement effect (QCE) or quantum size effect. The continuous energy band structure is changed to be discrete due to the QCE, which can generate fluorescence under the excitation at specific wavelength. As a new type of fluorescent material, QDs are known as “the best luminescent material ever discovered by humans”, Dr. Xiaogang Peng said, a professor from Department of Chemistry, Zhejiang University (Hangzhou, China).

QDs are based on semiconductor crystals, and each particle is a single crystal. When the semiconductor crystal is as small as nanoscale, i.e., <100 nm, so-called nanocrystals, different sizes of nanocrystals can emit the light with different colors. For example, cadmium selenide (CdSe) can emit the blue light at 2 nm, red light at 8 nm, and green, yellow, orange between 2-8 nm.

From the stage of “Seed” to “Maturity”

In the crisis of petroleum at the end of 1970’s, chemists from western countries were looking for a new generation of solar energy conversion like photocatalysis, photovoltaics. Benefiting from the mechanism of semiconductor solar cells, scientists, who are Dr. Allen J. Bard and Dr. Louis E. Brus from US, Dr. Alexey Ekimov from former Soviet Union and Dr. Friedrich Arnim Henglein from Germany, began to synthesize the small semiconductor crystals in solution and investigate their optoelectronic properties. There was something very interesting. The bulk PbS always shows a black color, but PbS nanocrystals in solution exhibit the colors from red to blue (blueshift) with decreasing size of nanocrystals. What’s happened? The “quantum confinement effect” was thought to be the main reason of blueshift, first explained by American chemist Louis E. Brus and American-Russian physicist Alexei L. Efros.

Prior to 1990’s, the synthesis of colloidal nanoparticles was based on traditional chemical methods, such as co-precipitation, micro-emulsions. This is capable to control the size of nanocrystals, but their optical performance is very poor, “without any fluorescence”. During the period of 1990-1993, a new synthesis method “metal organic -coordinating solvent -high temperature” was first used by researchers in Bell Lab of US. They achieved the synthesis of high quality CdSe nanocrystals by using dimethylcadmium (CdMe2) in organic coordinating solvent at a high temperature of 300 degree Celsius. However, CdMe2 is explosive even at room temperature, very toxic and expensive, thus causing a slow progress in the next 10 years.

Until the beginning of 21st century, Prof. Xiaogang Peng who worked at Department of Chemistry, University of Arkansas first conceived a “Green Synthesis” method, i.e., CdMe2 was replaced by CdO with non-coordinating solvent ODE, which allowed “QDs synthesis” to enter labs and industries around the world.

In 2009, Prof. Peng left University of Arkansas (UARK, US) and joined Zhejiang University. He continued to study QDs materials and realized that the critical issue for chemical synthesis is about the control of excited state of QDs which effectively influences their fluorescent emission, contrasting to traditional chemical synthesis only concerning the steady state of QDs. Later, Prof. Peng, collaborated with NajingTech (Najing Technology, Hangzhou, China), first proposed a new application related to the electroluminescent QDs-based devices (quantum dots light-emitting diode, QLED), in addition to photoluminescent (PL).

Commercialization around the world

In 2014, the Nobel Prize in physics offered “for the invention of efficient blue light-emitting diodes (LED), which has enabled bright and energy-saving white light sources”. GaN blue-LED has been already mass-produced by using an epitaxial growth of a multilayer semiconductor single crystal on a sapphire substrate at high vacuum and temperature. However this is really expensive! If the QDs-based devices (i.e., QLED) can achieve high performance comparable with GaN-LED, it’s expected to combine the advantages of both GaN-LED and organic light-emitting diode (OLED). Recent work from Peng’s group confirmed this idea by using a “solution processed synthesis method”, and obtained a high performance and low-cost QLED (i.e., Nature, 2014, 515 (7525), 96).

To commercialize QDs technology from lab to market in China, Dr. Peng created a startup “NajingTech” in Hangzhou in 2009. Prior to that, he founded NN-Labs LLC in Arkansas US in early 2002, and now NN-Labs is a wholly-owned subsidiary of NajingTech. So far NajingTech possesses the core patent families related to basic QDs materials and more than hundred peripheral and core patents related to QDs-based devices, including tens of patents outside of China partially authorized by UARK. All of these make the company to be comparable with other QDs companies: US-based QD Vision (founded in 2004) and Nanosys (in 2001), and UK-based Nanoco (in 2001), known as Top 4 in the world.

In the recent decade, photoluminescent QDs technology was largely commercialized in lighting and display. For example, Quantum-dot Light Converting Device (QLCD) was used for backlight unit in LCD (Liquid Crystal Display) screen. QD vision and NajingTech fabricated tube-shaped QLCD (QD tube) to their customers: Sony, Hisense, TCL, TPV, HKC and Konka. Quantum Dot Enhancement Film (QDEF) was first out from NajingTech, 3M and Nanosys, and has been already mounted into most of high-end TVs. Moreover, electricity-driven QDs technology (i.e., QLED) is more or less catching up in recent years. South Korea’s Samsung and Chinese TCL announced their plans of “QLED TV” at CES 2017 and 2018, respectively. Up to date, Samsung will first unveil a real “QLED TV” in the world in 2019.

Note: Najing Technology Co., LTD.: http://www.najingtech.com/
NN-labs: https://www.nn-labs.com/

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Wei Huang
Najing Tech
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Wei Huang