Shulou(Shulou.com)11/24 Report--

After enduring a long cold wave and low temperature,

Beijing finally ushered in the first snow a few days ago

The freeze of the editor has finally paid off.

There is no need to "play with snow" from moments.

There is no point in not playing with the snow. (shock sound)

Snow scene of the Institute of Physics | Photography: Huang Shuiji except for the snow-capped scenery

Take out a snowflake alone to enjoy it

It is also a beautiful pattern.

A single snowflake is also a beautiful pattern | Source: pixabay, etc.

Why is it mentioned that snowflakes are patterns seen on the front?

Why is there so little view of snowflakes from the side?

Is snowflake a second dimension?!

01. Snowflake: strive to grow into a cool look when people look at the snowflake pattern, they will be surprised to find that the variety of snowflake is far beyond human imagination. As early as the 1930s, snowflakes were divided into 21 categories, and the number increased over time, and by 2013, there were 121 categories of snowflakes.

Of course, 121 classification is too complex, for the general snow appreciation and research, the classification of 35 snowflakes listed below is representative enough.

35 types of snowflakes | Image Source: snowcrystals.com has very little information about the thickness of snowflakes, but we can simply calculate this figure. The diameter of a snowflake is usually between 0.05~4.6mm, and the mass of a single snowflake is 0.2~0.5mg. Consider a snowflake with a diameter of 2mm and a mass of 0.4mg. The density is 0.92 g / cm3 of ice, and its thickness is about 0.01mm, less than the thickness of a hair. The diameter is 200 times the thickness, so it is no wonder that the side is usually not considered when studying snowflakes.

Of course, this is not absolute. For example, the 25 types of snowflakes listed above, such as simple prisms, twin columns and skeletal forms, are three-dimensional shapes, and the thickness of snowflakes in these snowflakes can not be ignored.

Of all the shapes, the most common one is a hexagonal star. The shape of this hexagonal star is not random, it has something to do with the structure of water molecules. A water molecule consists of one oxygen atom and two hydrogen atoms, and the two bonds are at an angle. When water molecules are combined into crystals, the oxygen atoms of water molecules will form hydrogen bonds with the hydrogen atoms of other water molecules. this hydrogen bond structure determines the crystal composed of water molecules, which is macroscopically hexagonal star.

When the air full of water vapor encounters a low temperature environment, the water vapor will gather around the dust in the air, which is the nucleation point. The water vapor first liquefied into small droplets, which is the origin of Rain Water; if the temperature is low enough, the small droplets will solidify into small ice crystals, which are first in the shape of a hexagonal prism, and the ice crystals grow faster on the edge. finally, hexagonal star-shaped snowflakes fall.

Real snow crystal growth process | Picture Source: reference 1 in this growth process, due to the influence of hydrogen bond pairing, water molecules tend to combine horizontally, so snow crystals grow faster horizontally and slowly longitudinally, forming very thin hexagonal star crystals.

Graphene: although born in two dimensions, snowflakes are very thin, but they are still more than ten microns thick, about tens of thousands of atoms. Considering that individual atoms can now be directly manipulated through STM, snowflakes can obviously be further divided longitudinally, not really "paper snow".

By the way, the thickness of standard A4 paper is 0.104mm. By this standard, most snowflakes can be said to be "paper snow". On the other hand, the paper is still too thick, so please don't say that your two-dimensional wife is a "paper man" in the future. Manual dog head)

Considering the difficulty of manufacturing large accelerators, we can think that the diameter of a single atom is the smallest order of magnitude of the material size. So if a material is only one atom thick and its area is very large, far beyond the order of magnitude of thickness, isn't this material a two-dimensional material?

Do two-dimensional materials exist? Of course, for example-graphene (Graphene).

In 2004, the Geim team at the University of Manchester in the UK discovered that a material with only one layer of carbon atoms could be obtained by repeatedly pasting and folding graphite with adhesive tape, which they named graphene. The discovery of this new material has opened up a new direction of material science research-two-dimensional materials. Up to now, the research and development and industrialization of two-dimensional materials are still going on.

The most common preparation method of graphene is the mechanical stripping method mentioned above, which uses adhesive tape to peel off the graphite crystal directly. This method is simple and fast, and the area of graphene is large, so it is a necessary skill for the study of graphene thin films.

(qualified friends can also try it at home, just buy a graphite single crystal from the Internet, then tear off a small piece of tape, fold the tape repeatedly and press it on the silicon wafer, put it under the light microscope and you can see the thinner graphene. )

Monolayer graphene obtained by mechanical stripping method | Picture Source: reference 2 the monolayer graphene was obtained from graphite crystal by mechanical stripping method, which is a "top-down" preparation method. In addition to the mechanical stripping method, the liquid phase stripping method also belongs to the top-down preparation method.

The principle of the "top-down" preparation method is that the interaction between different graphene layers is van der Waals interaction, while the carbon atoms in the same graphene layer are combined by covalent bonds. High school chemistry tells us that the strength of covalent bonds is much stronger than the van der Waals interaction. Therefore, single-layer graphene can be obtained from graphite crystals by destroying the interaction between layers.

Since there is a "top-down" approach, there is naturally a "bottom-up" approach. The so-called "bottom-up" is to start from a single carbon atom, constantly combine other carbon atoms, expand the area horizontally on the substrate, and eventually grow into a monolayer graphene film.

(because the material in this process is getting bigger and bigger as if it were growing up, the material prepared in the laboratory is often called "long material". )

Graphene growth diagram, its growth can be divided into several stages: nucleation-island growth-island connection-thickness increase. So why does graphene attract so many people to study? Of course, it is because of its excellent nature. For example, it is one of the materials with the highest known strength and good toughness; its thermal conductivity and electrical conductivity are very good, and it has a broad application prospect in the thermal and electrical fields; from the point of view of energy band, graphene has a special energy band structure of Dirac cone, which can be used as a platform to study quantum Hall effect.

03. Two-way foil: dimension is not love, you can buy it if you want to buy it. "in the process of two-dimensional, every point on a three-dimensional object is projected onto a two-dimensional plane according to precise geometric rules, so that the two-dimensional body becomes the two most complete and accurate drawings of the original three-dimensional spaceship and three-dimensional human body, and all its internal structures are arranged on the plane without any hiding, but its mapping rules are completely different from engineering drawings. It is visually difficult to reproduce the original three-dimensional shape by imagination. "

Two-way foil, the most poetic name brings the greatest shock to people, directly manipulating the dimensions of the universe to achieve a real "dimensionality reduction strike". The opening paragraph of this section is one of the detailed descriptions of the two-dimensional process of the solar system under the action of two-dimensional foil in the book "three-body".

So where is the development of two-dimensional materials now, and how far is it from the "two-way foil"?

As mentioned before, most of today's two-dimensional materials are prepared by "top-down" or "bottom-up" methods. The direct dimensionality reduction such as bi-directional foil can not be realized by existing human theory and technology.

Since the discovery of graphene, the development of two-dimensional materials is mainly focused on the preparation, characterization and application fields. So far, many kinds of two-dimensional materials such as hexagonal boron nitride, transition metal chalcogenide, main metal chalcogenide, silene, germanene and so on have been discovered. important breakthroughs have been made in the applications of high frequency transistors, field effect transistors, high efficiency luminescence and photodetectors.

But this is still a long way from the mature application of two-dimensional materials, not to mention two-dimensional foil. As Geim, the father of graphene, said in a recent interview: "there are hundreds of so-called 'graphene products' on the market, but I don't think they are revolutionary and subversive products in a practical sense. Graphene has played a role in it, but it is not so amazing that we cannot do without graphene."

When human beings can make, encapsulate and use two-way foil skillfully, human beings may be able to reach the technological level of singer civilization, taking space as the sword and time as the song.

"I saw my love.

I flew to her side.

I held out a present for her

That's a small piece of solidified time.

There are beautiful stripes in time

Feel it as soft as the mud of a shallow sea

She smeared time all over her body

Then pull me up and fly to the edge of existence

This is a spiritual flight.

The stars in our eyes are like ghosts

We are also like ghosts in the eyes of the stars. "

-- "three-body"

Reference:

Https://mp.weixin.qq.com/s/VhcoI6Tfzd1uRnSvVxX8Ig?scene=25#wechat_redirect

Xu Hong, Meng Lei, Li Yang, Yang Tianzhong, Bao Lihong, Liu Guodong, Zhao Lin, Liu Sheng, Xing Jie, Gao Hongjun, Zhou Xingjiang, Huang Yuan. Application of new mechanical cleavage method in the study of two-dimensional materials. Journal of Physics, 2018, 67 (21): 218201. Doi: 10.7498 / aps.67.20181636

Https://mp.weixin.qq.com/s/YuKDQSb8QPCg7u1XlNxqng

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: happy Superman

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