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On Friday, Xu Qiu took Mo Wenlin to test the exciton binding energy of the IDIC sample again. The test method was the same as yesterday's ITIC sample, a low-temperature fluorescence luminescence experiment.

The second test was also successful once, but the time consumed did not decrease too much. I started working from 9:00 am to 6:00 pm, and finally got the result.

The results show that the exciton binding energy of IDIC is 112 meV, which is numerically comparable to that of ITIC at 117 meV.

Based on the results of these two times and the distribution of HOMO/LUMO energy levels on molecules obtained from the previous DFT simulation, Xu Qiu probably had a guess:

The low exciton binding energy of the ADA non-fullerene acceptor material may be due to the molecular structure of A-D-A.

For the excitons generated in the molecule, the positive charge will be concentrated on the D unit, and the negative charge will be concentrated on the A unit, which will lead to the preliminary splitting of the positive and negative charges inside the molecule, reducing it to completely split into free The energy required for the charge, the apparent phenomenon is that the exciton binding energy is low.

It is more difficult to verify this conjecture.

It is almost impossible to prove it directly, and it can only be corroborated by testing the exciton binding energy data of different ADA non-fullerene acceptor materials.

Among them, once a counterexample appears, the conjecture will be overturned, or the expression needs to be modified and patched.

This is also normal.

After all, the essence of science is to constantly establish theories, overthrow theories, and improve theories.

Just like the establishment of Newton's three laws at the beginning, it is established in the macroscopic and low-speed world.

But in the microcosmic and high-speed world, it is not established. Therefore, quantum mechanics and relativity are supplemented separately. These theories also jointly build the cornerstone of the modern physics department building.

Of course, these are all current theories, and perhaps they will continue to be overthrown and improved in the future.

After all, human beings are mortals with naked eyes, without X-ray eyes, and cannot move at near the speed of light, so it is difficult to say exactly what microscopic and high-speed things look like.

Even if it is tested by an instrument, is the data displayed by the instrument necessarily true?

Furthermore, is what we directly observe necessarily true?

You can't think deeply about this kind of thing, because when you think about it in the end, you will inevitably fall into an infinite loop of doubts.

It is true that many things cannot be proved or falsified.

This may also be the reason why most foreign scientists convert to religion, and some people say that the end of science is theology.

The more you explore and the closer you get to the boundaries of cognition, the more you may feel that you need an absolute creator to support the existence of all this.

On Sunday, Xu Qiu started another test of the exciton diffusion distance.

There are many methods for this test. A few days ago, he searched the literature and found four methods in total, and finally chose the method of using highly oriented pyrolytic graphite (HOPG) substrate to test the fluorescent signal.

The reason is also very simple. Wei Xingsi had done this experiment before when he was in the beautiful country. Xu Qiu directly copied Teacher Wei's skills. The proficiency is quite satisfactory, the fourth level is 0%. The possible reason is that Teacher Wei practiced it at that time. The number of times is not many, or because the experiment is not carried out in the front line for a long time, the proficiency does not increase but decreases.

But in any case, even if there is only a proficiency of the second and third levels, compared to other methods, it is much better. After all, Xu Qiu has a system, and the system image is much more vivid than the text description on the reference.

After spending a few points, Xu Qiu looked at the experimental operation of the fifth-order 100% HOPG method to test the exciton diffusion distance.

The operation method does not seem to be difficult, you only need to spin-coat the sample on HOPG, then test the fluorescence signals of samples with different thicknesses, and finally perform the fitting.

The experimental principle is also relatively simple. Once the excitons generated by the photoelectric material are diffused onto the surface of HOPG, they will be quenched with a probability of more than 99%, and the energy contained in it will be released in the form of heat. If the excitons are excited If the muons have recombined before diffusing to the HOPG surface, a fluorescent signal will be emitted.

In other words, if the exciton diffusion distance of the material is relatively long, such as more than 100 nanometers, then for a sample film of about 10 nanometers, the fluorescence signal will be almost 0, because the average excitons can diffuse 100 nanometers, and the film only 10 nanometers, in this process, the probability of excitons moving to the position of the boundary HOPG is very high, and the excitons are all eaten by HOPG, naturally unable to generate fluorescent signals.

Conversely, if the exciton diffusion distance is relatively short, such as 10 nanometers, then a thin film of about 10 nanometers can naturally generate a fluorescent signal. At this time, the fluorescence quenching efficiency is about 50%, that is, the luminous intensity is half of the normal intensity. .

In the afternoon, the HOPG that Xu Qiu bought in advance had been delivered by courier to the concierge on the second floor of Material One.

HOPG refers to pyrolytic graphite, which is a new type of graphite whose performance is close to that of single crystal graphite after high temperature treatment.

As the name implies, this type of graphite is highly oriented and can be manipulated experimentally to obtain a very smooth, high-conductivity surface that can be examined in a scanning tunneling microscope or used as a substrate for other materials being studied.

According to the inlay angle (mosaic diffusion angle) of HOPG, it can be divided into three grades: A, B, and C.

Grade A has the smallest inlay angle, usually around 0.5 degrees, the best quality, and the closest to the properties of single crystals, but of course the price is also the most expensive.

Grade C has the largest inlay angle, usually above 1.5 degrees, and the worst quality, but the cheapest price.

The setting angle of grade B is between A and C, usually around 0.8 degrees.

Xu Qiu shopped online and finally found a domestic HOPG manufacturer.

The smallest size they sell is 10*10*1 mm in length, width, and height, which means an area of ​​one square centimeter and a height of one millimeter.

With such a specification, A-level ones cost 2,000 yuan, and C-level ones only cost 700 yuan.

If the specifications are larger, such as 20*20*1 mm in length, width and height, the A-level one will cost 8,000 yuan, and of course the C-level one will also cost 4,000 yuan. If the length, width, and height are 20*20*2 mm, the price will be about tens of thousands.

Of course, regardless of its small size, if you buy a small piece of HOPG, you can use it dozens or hundreds of times. Otherwise, if this thing is disposable, no one can afford it.

After all, Xu Qiu needs to prepare more than a dozen samples to test the exciton diffusion distance of a system.

Feeling the price of the scientific research circle, Xu Qiu finally chose the smallest C-grade HOPG, and first tried the water with a cheap one of 700 yuan.

Theoretically, what he is doing now is an ordinary fluorescence test, which mainly uses the quenching effect of graphite on excitons, and the requirements for the substrate should not be as high as those of the scanning tunneling microscope.

Xu Qiu took the courier from the concierge, looked at the courier box, and said casually, This courier box is so random, it's just an ordinary small paper box.

Afterwards, he put on disposable PE gloves and began to unpack the express delivery. There was a centrifuge tube inside, and HOPG was wrapped in paper and placed inside.

After unwrapping the wrapping paper, the HOPG I got is similar to the description I saw on the Internet before, with a size of 10*10*1 mm, one side is dark gray, and the other side is bright gray.

Among them, the dark gray side is the reverse side, and the light gray side is the front side.

HOPG looks a lot like a chip, or an SD memory card in an early mobile phone.

However, Xu Qiu knew that this thing was high-purity carbon.

Graphite is just carbon.

With HOPG in hand, the next step is sample preparation.

However, the “thick” HOPG surface needs to be stripped before spin-coating the samples, resulting in a fresh substrate with a smooth surface.

There are some non-mainstream methods, such as direct peeling with tweezers, which are more difficult and can easily damage the surface of HOPG.

The more mainstream method is to use adhesive tape to stick it directly on the HOPG, and then tear it apart.

Although this method is relatively safe, Xu Qiu did not rush to try it in reality. Instead, he first went to the simulation laboratory and tried it with 3M tape.

At the beginning, there will be uneven surfaces, or barbs.

After repeated attempts many times, Xu Qiu finally mastered the technique.

That is to press the tape as tightly and evenly as possible, so that the probability of getting a smooth surface is relatively high, because if some parts are not glued, they will remain on the surface of the original HOPG body and form barbs.

After peeling, there will be a layer of broken graphite flakes on the tape, and a fresh, smooth surface on the HOPG matrix.

This method of sticking with tape sounds relatively earthy, not so high-end, not very scientific, but it is precisely the method of preparing graphene that was first discovered at the time.

In 2004, professors Andre Geim and Konstantin Novoselov, physicists at the University of Manchester in Great Britain, stuck graphite to graphite, and then tore it off, and found that a thin layer of graphite flakes.

This is nothing new, but they had a whim, took the second piece of tape to stick the graphite flakes left on the first piece of tape, at this time the two pieces of tape were stuck together, and then the two pieces of tape were torn apart, It turned out that the remaining graphite flakes on the tape were thinned.

At this time, many people will feel bored, isn't it just tearing the tape, and after the two professors found that the graphite sheet became thinner, they took the third piece of tape to stick the graphite on the second piece of tape, and then used the fourth tape to stick it Three pieces of tape...

Just sticking and tearing the tape over and over again, they finally got tiny graphite flakes that couldn't be thinner, only one layer of atoms thick.

This is a single layer of graphite, also known as graphene.

After the birth of single-layer graphene, many scientists are actively exploring the ability to produce larger-scale and higher-quality graphene.

In addition to sticking with tape, there are many other graphene preparation methods, such as micromechanical exfoliation, chemical exfoliation, and chemical vapor deposition.

Moreover, the amazing material properties of graphene have also attracted extensive research by a large number of scientists in the past ten years.

In the end, those two professors also won the 2010 Nobel Prize in Physics.