Quantum Dots: Status, Opportunities and Challenges
Professor Peng Xiaogang of the Department of Chemistry "Based on the new quantum dots, we have seen the first subversive quantum dot application by working with the Associate Professor Jin Yizheng and the Najing Technology Company of the Zhejiang University Materials Department. That is the excellent performance. 'Quantum Point LED' (QLED)."
Deep natural resource crisis
I believe that quantum dots are an important frontier of modern science.
Why do you say that? In 2002, there was an article in the Proceedings of the National Academy of Sciences, which made an analysis that the Earth is a closed system, and the only thing that comes in is solar energy. Therefore, the entire planet is completely dependent on solar energy regeneration. However, by the time of the 1990s, humans used about 70% of the earth's regenerative capacity, and by the 1980s it had reached 100%. By the end of the 1990s, it had reached 120%, which meant that it had exceeded the load capacity of the Earth.
This is why there are so many environmental problems today, such as smog. Look carefully, if everyone lives in the American way, by 2002, we need two planets. In 2009, I once spoke about this report in the UK. A scholar told me that two earths are not enough. According to the standard of living in the United States, humans should need five earths. Think about it, this statement is similar. Because the United States accounts for 5% of the world's population, it uses 40% of the world's resources. Therefore, the Chinese themselves need two earths, and the entire human race needs five earths. From this point of view, it is reasonable for Americans, Europeans and other developed countries to call the energy crisis. But we Chinese and even humanity are not only facing the energy crisis, but the broader resource crisis. If the Chinese are blindly following the "energy crisis" with the Americans and other countries with high resource share, and ignoring the deeper resource crisis, then we are selling ourselves to help others count the money.
The real problem facing humanity is the resource crisis, not just the energy crisis. We live in the way of Americans, unrealistic, and there is darkness in front. One of the answers to this major challenge for chemists around the world is high-tech chemistry. It is impossible to build a planet. There is a different way for chemists to find, design and create high-tech materials for human beings. This is the core idea of ​​high-tech materials chemistry, and it has also led to the recent development of high-tech materials chemistry.
Thomson Reuters' list of highly cited scientists, China's top chemical and materials disciplines accounted for a large proportion. This is not surprising, which shows that our scientists have a conscience. They really think about problems, thinking about creating new materials for human beings, turning useless things into useful things, using useful things to the extreme, and running out of no burden on nature. This is a major trend in the entire chemistry, even the entire scientific community: high-tech materials chemistry.
There is also an interesting phenomenon. In 2011, according to the citation of the published paper, Thomson Reuters made a ranking of 100 high-impact chemists in the first 10 years of the 21st century. About 60% of the scholars are related to nanomaterials. why? Because nano is not just a scale, but nanomaterials represent the current main direction of high-tech materials chemistry.
A class of star materials - quantum dots
Quantum dots are one of a large class of new materials—solution nanocrystals. The reason for "new" is that, under normal circumstances, chemists are mainly doing things dissolved in solution. The same is true for biologists, which cannot survive without a solution organism. Because there are some functions that only crystals can have, chemists are not familiar with these functions and have no effort to open up their potential applications. However, if the crystals are made small to the nanometer scale, they will enter the solution like a protein. Thus, humans have a new class of materials that have the dual nature of seed crystals and solutions. From a chemical point of view, even a new class of molecules. From the future of materials, it represents many new possibilities.
Among all solution nanocrystals, there is a class of star nanomaterials, and materials that will soon have breakthrough industrial applications. That is the quantum dot.
Unlike other nanocrystalline materials, quantum dots are based on semiconductor crystals. The size is between 1 and 100 nanometers, and each particle is a single crystal. The name of a quantum dot is derived from the quantum confinement effect of a semiconductor nanocrystal or a quantum size effect. When semiconductor crystals are small to nanometer scale (1 nanometer is about one ten thousandth of the width of the hairline), different sizes can emit light of different colors. For example, cadmium selenide, a semiconductor nanocrystal, emits blue light at 2 nm, red light when it reaches 8 nm, and green yellow orange in the middle. This chemical composition, the illuminating color can cover the entire visible region from blue light to red light, and the color purity is high and continuously adjustable.
Is luminescent material important to humans? Very important, this is because we are species that evolved under the sun. The importance of luminescent materials to humans determines that quantum dots will become star materials. Searching in the SCI paper database with "quantum dots" as the key word, the number of articles published in one year has reached more than 10,000. This amount is very large. Of course, only basic research is not a feature of the discipline of high-tech materials chemistry. To solve the problem of human resources, high-tech materials must become industrial applications. Based on this, when I returned to China in 2009, I started a company in Hangzhou with several people who are interested in exploring high-tech entrepreneurship in China, called Najing Technology Co., Ltd. Najing's goal is to design, manufacture and industrialize optoelectronic devices and biological and medical testing products with quantum dot materials as the core.
Quantum dots can be used in the biomedical field. We can use quantum dots to fully display the skeleton of the cell. Compared with other types of detection methods, quantum dot luminescent materials are certainly advantageous for detection. We can easily use multiple colors of quantum dots to simultaneously detect multiple pathogens or pesticide residues. Moreover, because the quantum dot absorption capacity is very large, the sensitivity can be greatly improved. The early pregnancy test paper that everyone is familiar with is the use of gold nanocrystals for color development. In this sense, nanotechnology has long since entered life, but people often don’t know. Why use luminescent quantum dots instead of gold nanocrystals? This is because the gold nanocrystal coloring method cannot be quantified, and quantitative detection can be performed by using quantum dots for detection. It is estimated that around February next year, Najing's biomedical rapid detection products will be seen in the market.
Lighting is also a big industry, consuming nearly 20% of the energy. The light efficiency of current artificial light sources is very low. For example, incandescent lamps with high illumination quality have a luminous efficacy of only 2%. If you can increase the efficiency to 20%, it means you can save 20% of energy consumption. In 2010, Najing Technology demonstrated a quantum dot-based LED lamp at the American Light Show. The US Department of Energy's solid-state lighting roadmap wrote a paragraph: quantum dots will play an important role in human lighting. The energy efficiency and light quality of that product are outstanding, but because of the traditional GaN LED, its price is high. From the current situation, quantum dots are used for lighting and require some research and development work to become a competitive product.
The industry where quantum dots can bring significant changes is shown. The current first generation will be a combination of GaN LED and quantum dot backlight products. Najing and two high-tech companies in the United States have entered the commercial stage. This new type of backlight makes the display color high in purity and saturated in color, which is difficult to match with other display technologies. As far as I know, a large domestic TV manufacturer will launch this new color TV at the end of this year or early next year.
From the origin to the boom
The origin of the field of quantum dots was around the end of the 70s. At the time, chemists in Western countries were affected by the oil crisis and wanted to find a new generation of photocatalytic and photoelectric conversion systems that could use solar energy. Drawing on the principles of semiconductor solar cells, chemists began experimenting with the preparation of small semiconductor crystals in solution and studying their optoelectronic properties. Representative figures include BARD and BRU in the United States, Ekimov in the former Soviet Union, and HENGLEIN in Germany.
In the lab, they found a very strange phenomenon. For example, the large single crystal of lead sulfide is always familiar black, but the color of the nanocrystals that chemists make in solution is different, some are yellow, some are red, some are black, some are even without color. . What strange things happened?
Finally, the American scientist BRUS E-FROS of the former Soviet Union gave a beautiful explanation, which is the "quantum confinement effect" theory. The publication time of their articles was somewhat poor, but because of the isolation of the former Soviet Union, they did not know each other's work.
So far, chemists are playing a leading role in this field, and synthesizing quantum dots with performance requirements is still the most critical thing in the field. Prior to 1990, synthetic methods were based on traditional chemical methods for preparing colloidal small particles, such as coprecipitation, microemulsions, micelles, and the like. These methods are capable of controlling the size to a desired extent to some extent, but the optical performance is very poor and substantially does not emit light.
Quantum dot research took place a very important thing between 1990 and 1993. A new synthetic method called “metal organic-coordinating solvent-high temperature†was introduced. This method is highly toxic and very Unstable dimethyl cadmium is used as a cadmium source to synthesize high quality cadmium selenide at high temperature (about 300 degrees Celsius) and organic coordination solvent. This method was first invented by Brus and Stigwald at Bell Labs, and the first article was published in 1990. Because the description is very simple, it has no effect. Later, after Dr. Brus, Bawendi went to MIT to teach him to lead his students to systematically and seriously, and finally became a repeatable synthetic reaction. This molding reaction was published in the American Chemical Society's Journal in 1993, which made high-quality cadmium selenide quantum dots possible. In the late 1990s, when I was at the University of California at Berkeley, I improved this method, some of which was published in Nature. However, I think that my own work (including the article published in Nature) is far less important than the previous two work. At present, everyone attaches great importance to Nature and Sci-ence, but the excessive interpretation of the performance of the magazine is "the layman watching the lively, the insider watching the doorway."
The two ground-breaking work of 1990 and 1993 is a milestone for the entire field. However, this also leaves a challenge for the field. The raw materials they use are derived from "metal organic vapor deposition", in which dimethyl cadmium is explosive, even at room temperature, it is very toxic and costly. These factors have led to the development of this field not being fast in the next 10 years, and only one material can be made.
Later, when I arrived at the University of Arkansas, we found a “green†organic solvent route. The value of this method is that the simple synthesis of quantum dots has entered the labs around the world. As long as there is an ordinary chemical synthesis laboratory, it can be done in China. Further, we systematically explored the quantum dot growth mechanism, and the range of relatively high quality quantum dots was gradually extended to other types of semiconductors. For these reasons, this “green†route was quickly promoted around the world (both industrial and academic), which in one way caused a high citation rate for our published work.
Looking back now, the basic idea of ​​the MIT and Bell Labs methods is not right. In fact, we later proved that the three factors of the “metal organic-coordinating solvent-high temperature†route are not true elements. On the contrary, our recent experimental results at Zhejiang University show that quantum dot synthesis will do better under relatively mild conditions. So, is the “metal organic-coordinating solvent-high temperature†route a detour?
I believe that scientific research falls into two categories, namely, "forward-looking exploration" and "systematic research." The above-mentioned work of Bell Labs in 1990 is a typical forward-looking exploration. The work of our laboratory in this century is closer to systematic research, and the work of MIT is in the role of linking up and down. The unknown world facing scientific research does not have the standard answer like an exam. Therefore, we can neither deny forward-looking exploration nor despise systematic research. At present, Chinese scientific research has a tendency to place too much emphasis on the former and pay too much attention to scientific hotspots.
Entrepreneurial wave
Since it is a functional material, it is not good to just look good. Young American students are quite different from young Chinese scholars. If they think a technology is useful, go to the company after the doctor's graduation (even before graduation). This is a graduate of a prestigious school. They go to start a business and provide employment opportunities for others. China's higher education is worthy of reflection in this respect. How to educate students not to become a social employment burden, but to become an entrepreneur?
The first influential quantum dot company was founded in 1999, and its main business is quantum dots for biomedical markers. But unfortunately, the company's end result was to become a martyr. After a few years of struggle, it was acquired by a big company, and its purchase price was lower than the cost of starting a business. In American society, there is no shame in the failure of entrepreneurship. Employers may also think that their experience is valuable. If China wants to achieve industrial upgrading, then I think it should allow startups to become martyrs. Of course, they are called martyrs because their sacrifices provide experience for the success of latecomers.
At present, there are quite a few companies in the world that claim to focus on quantum dots. Some of them, I don't think they will become martyrs. Judging from the current situation, there is strong intellectual property support from universities. For example, the technical foundation of QDVISION in the United States comes from MIT, and NANOSYS in the United States is based on Berkeley's technology. The previous technology of China's Najing Technology was obtained from the University of Arkansas and completed through the acquisition of NN-Labs.
Subversive progress
As mentioned earlier, synthetic chemistry is the decisive factor in this field. After returning to Zhejiang University, I slowly realized that the real core problem of quantum dot synthesis chemistry is excited state control. This is because, as a luminescent material, its performance can only be achieved in an excited state. For traditional synthetic chemistry, chemists only care about the ground state. Based on this new understanding, we have adopted some new synthetic control methods. As a result, we have some quantum dots that have never been seen before.
Based on these new quantum dots, we have seen the first subversive quantum dot application by working with the Associate Professor Jin Yizheng and the Najing Technology Company of the Zhejiang University Materials Department. That is the quantum dot LED (QLED) with excellent performance. After applying for a patent, we submitted the first article to Natue magazine. Already published online.
Light-emitting diode (LED) LEDs are changing our lives, and energy-saving effects in lighting and display have been recognized. This is the foundation of this year's Nobel Prize in Physics (GaN-Blu-ray LED). Gallium nitride blue LEDs have been mass-produced, and related intellectual property rights have been firmly controlled by Japanese, American and European companies. However, the technology of GaN blue LED is based on epitaxial growth of a multilayer semiconductor single crystal on a sapphire single crystal substrate, requiring high vacuum equipment, ultra high purity raw materials, and high energy consumption in the preparation process. Therefore, its basic cost is large.
In addition to GaN blue LEDs, Chinese American scholar Professor Deng Qingyun reported in 1979 an LED (OLED) with organic molecules as the illuminating center. The biggest difference between OLED and GaN LED is that it can be processed by common chemical methods (such as solution method), so the cost can be very low. At present, Samsung has made a commercial mobile phone display with OLED, and its excellent color is unmatched by the GaN LED display. But unfortunately, the thermal stability and photostability of organic light-emitting molecules of OLEDs are almost inaccessible. This has led to a low yield of OLED displays, and it is not a problem to make a mobile phone screen, but large-screen applications such as televisions and computer displays are basically impossible.
If quantum dot synthesis meets the requirements for LED optoelectronic performance, then quantum dot LEDs are expected to combine the advantages of both GaN LEDs and OLEDs. Our recent work confirms this vision. Nature's reviewers gave several indicators to allow us to make a horizontal comparison with OLED and other solution processing LEDs. The results show that although our QLEDs were prepared by solution method under relatively simple conditions, our devices almost completely outperformed. To be conservative, our QLED commercialization opportunities will at least not be lost to OLEDs. It should be pointed out that in addition to the quantum dots to be done, it is crucial to try to understand what quantum dots should pay attention to in the processing of devices. This is called processing science.
LED is also the core component of the lighting industry. But compared with sunlight, the current white LED lamp is defective. It is artificial white light and has many high-energy photons. The impact of high-energy photons on human health has been shown to be unfavorable. In addition, the current white LED fever is more obvious, this is not good news. The white light of QLED can be completely consistent with the ideal illumination source, closer to natural light, and the heat is greatly reduced. The progress of our recent work shows that one day quantum dot LEDs will contribute to the lighting industry.
The field of quantum dots has now evolved to a level that requires deeper, more systematic, and more integrated (or more intersecting) levels. At present, the research of quantum dots is developing from the guerrillas' zero-breaking to the group-formed regular military operations. Can Chinese entrepreneurs, government leaders, and scholars have enough wisdom to support the formation of a true regular army in China? For example, our QLED technology is currently in the international leading position and has established its own intellectual property rights. However, competition from MIT (QDVision), SAMSUMG, etc. is not to be underestimated.
(October 27, 2014) I have reviewed it according to the recording.
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