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

The original title: "can studying physics make us young forever?" "

For the vast number of boys and girls

A lot of people must be hoping

Stay young, handsome and beautiful for a long time.

Meanwhile,

In the long history of mankind

And created a lot of works of art.

For human beings,

It seems impossible to stay young forever.

But for works of art,

Maybe we can make it beautiful forever.

The beautiful colors in Part I art many works of art cannot be preserved for a long time. For example, paints and adhesives in oil paintings inevitably degrade due to fluctuations in light, humidity and temperature. Artists mixing incompatible paints can also make paint unstable over time. The specific mechanism of color degradation has been discussed in detail in the previous article "discoloration method reward".

Pictures of faded murals in the Mogao Grottoes Source: Baidu pictures but ancient artists also created works of art that will not fade.

Ancient Roman glassmakers made a magical Lycurgus cup in the 4th century AD: when the light shines from the front of the glass, the glass appears green and red when the light passes through the back of the glass.

Ancient Rome magical discoloration wine glass (a) light shines from the front (b) light shines from the rear Photo Source: reference [1] into modern times, scientists through analysis found that the glass of the wine glass into the gold, silver and other metal particles, the diameter of about 50nm, causing it to absorb blue, green and other shorter wavelengths of light. So it shows a strange two-color effect [1].

At the same time, in the Middle Ages, the use of gold and silver particles mixed sol method for decoration has long been a trend. For example, the stained glass of the Chapel in Paris is also mixed with nano-metal particles.

These brilliant colors also come from the scattering and absorption of sunlight by these nanoparticles [2].

In addition to Baidu Encyclopedia, about 500 years ago, artists decorated with ornate gold leaves in what is now the Alhambra Palace in Spain.

After centuries of wear and tear, purple spots have been found on inert gold that are difficult to understand by common damage mechanisms.

Purple spots on gold ornaments Source: reference [3] Modern scientists have found metal gold particles with a diameter of 70nm in the gold-plated area by using scanning electron microscopy, x-ray diffraction and Raman spectroscopy.

These nanoparticles exhibit local plasmon oscillations on the surface of gold and platinum, which interact with the incident visible light and appear purple [3].

Figure: source of gold nanoparticles on the surface of gold ornaments: reference [3] these gold nanoparticles are formed by redox, dissolution and deposition under the interaction of worn gold and platinum, silver impurities, tin layer in jewelry, moist and chlorine-containing air in Granada, Spain, and so on.

We can find that the peculiar color characteristics of these works of art are all due to the existence of nano-scale metal particles.

The role of nanoparticles is to form local surface plasmon resonance on the surface of the object, and the resonance results in three effects: photon absorption, photon scattering and photoelectric field enhancement, among which the first two effects determine the color of metal nanoparticles.

So what is local surface plasmon resonance?

Part II romantic collective oscillation light to see such a long name may be a lot of friends have fainted, this is not a scientist to look high-end and hard name (dog head).

Each of these words has a specific physical connotation, let's look at it step by step.

1. There are free electrons in the plasmon metal, which means that a part of the outer electrons in each atom can move freely in space without the bondage of the nucleus. Free electrons are uniformly distributed in the metal space, which is the free electron gas model.

The interaction between free electrons and electrons uniformly distributed in metals is Coulomb interaction.

We can imagine that if one of the electrons moves, the distance between the electron and the other electrons will change, thus changing the magnitude and direction of the Coulomb force, so that the other electrons will also move.

Therefore, due to the interaction between electrons, the motion of electrons in solids is often a collective effect.

We also know that light is an alternating electromagnetic field. When an AC electric field acts on a free electron, the motion of the electron can be understood by the damped forced vibration in classical physics.

By solving the equation of forced vibration, we can get the inherent vibration frequency of a free electron, which is called the plasma frequency. The collective motion of free electrons in metals is called plasma oscillation or plasmon.

When the frequency of the incident light is equal to the plasma frequency, the plasma resonance occurs, and the corresponding metal dielectric constant is 0.

The source of the wavelength corresponding to the plasma frequency of some metals: it can be found from the table in reference [4] that the light wavelength corresponding to the plasma frequency of the metal is about in the ultraviolet band. Because only when the frequency of the incident light is equal to or greater than the plasma frequency, the metal can absorb photons, that is, light cannot transmit to the metal.

So the metals we see in our daily life can well reflect most of the visible light, which is why many metal surfaces are bright white.

Bright white metal plate source: webpage 2. Surface plasmon

The surface plasmon refers to the phenomenon that the electromagnetic wave is strongly coupled with the free electrons on the metal surface and forms a collective oscillation on the metal surface.

As mentioned earlier, metals generally have good reflectivity for electromagnetic waves with lower wavelengths. Although electromagnetic waves cannot pass through metal, they can still penetrate the metal surface and attenuate to a certain depth, which is called the skin depth of electromagnetic waves.

Source of skin depth map: Baidu encyclopedia so the plasmon on the metal surface (the collective oscillation of surface free electrons) can propagate along the surface direction and attenuate in the vertical surface direction (the amplitude of the collective oscillation).

Propagation pattern source of surface plasmon: reference [1] 3. The skin depth of local surface plasma oscillating electromagnetic wave on metal surface is generally in the scale of tens of nanometers.

Imagine that if bulk metals are turned into nanoscale particles so that their size is smaller than the skin depth of electromagnetic waves, then the propagation of surface plasmons is limited.

In other words, the coupling process between electromagnetic waves and free electrons can only occur in this narrow area of nanometer scale. This is the concept of locality.

When the incident light irradiates on the metal nanoparticles which are much smaller than its wavelength, the free electrons in the metal nanoparticles will shift relative to the positive ion center under the action of the incident photoelectric field.

As a result, positive and negative charges are accumulated on both sides of the surface of metal nanoparticles, and then a local recovery electric field is formed inside.

The free electron oscillates collectively in this local electric field [5]. This process is the local surface plasma oscillation.

Source of local surface plasmon resonance: reference [1] when the frequency of incident light is equal to the frequency of collective oscillation, resonance occurs, thus realizing the phenomenon of local surface plasmon resonance.

Source of resonance wavelength range of surface plasmon of commonly used metals: reference [5] generally speaking, for the same metal, the size and shape of nanoparticles, the distance between nanoparticles, the medium environment around the nanoparticles, the angle of incident light and other factors will affect the resonance wavelength of local surface plasmon.

Source of absorbance spectra of gold nanorods with different aspect ratios: reference [1] localized surface plasmon resonance beyond Part III beauty not only allows us to see a variety of colorful colors, but also helps us to achieve a more powerful detection method: surface enhanced Raman scattering.

When light shines on an object, it not only reflects, transmits and refracts, but also scatters when it interacts with particles in the matter.

In the process of light scattering from atoms and molecules, the photoelectric field shifts the positive and negative charges inside, forming a dipole or light field that directly interacts with polar molecules.

Source of the electric dipole diagram: the Baidu encyclopedia dipole itself will have an inherent vibration frequency, and the light field will be coupled with this vibration, thus changing the wavelength.

The specific process of coupling is as follows:

The electron absorbs photons from the initial energy level to a virtual energy level, and then the electron falls back from the virtual energy level to a lower energy level.

If the final state energy level of the electron is the same as the initial energy level, the photon wavelength emitted by the transition process remains the same, which is Rayleigh scattering.

If the final state energy level of the electron is different from the initial energy level, the photon wavelength emitted by the transition process changes, and the wavelength change is the vibration frequency of the dipole, which is called Raman scattering.

The schematic diagram of light scattering with atoms and molecules Raman scattering can be used to analyze the chemical bonds and lattice vibration in substances, which plays an important role in the fields of biology, chemistry and materials.

However, the intensity of Raman scattering is generally very low, which makes it difficult to observe the Raman spectra of materials.

We mentioned earlier that the local surface plasmon resonance process not only produces photon absorption and photon scattering determines the color of metal nanoparticles, but also enhances the scattered photoelectric field, which is the basis of surface enhanced Raman scattering.

Source of surface-enhanced Raman scattering: reference [1] the intensity of surface-enhanced Raman scattering can be 100, 000 to 1 million times stronger than ordinary Raman scattering, so it can be used more widely.

For example, monitoring the electron transfer process of catalytic reaction in chemistry; real-time manipulation and detection of the behavior of biological macromolecules; detection of Raman signals of small biological tissues; characterization of nanomaterials and so on.

Conclusion through the manufacture of metal nanoparticles

Can make works of art have a variety of colorful colors.

The beauty of these works of art

The freeze has been realized from the moment of birth.

Of course it's a cool thing.

But,

The mark left by time

It is also a kind of beauty!

References:

Huang Yu, finite element simulation and analysis of surface plasma near-field imaging, doctoral thesis of Tsinghua University, 2016.

Mark I. Stockman, Nanoplasmonics: The physics behind the applications, Physics Today 64, 2, 39 (2011). [3] https://physicstoday.scitation.org/do/10.1063/PT.6.1.20220915a/full/

Fang Rongchuan, solid State Spectroscopy, University of Science and Technology of China Press, 2001 first Edition

Bian Yajie, Local Surface plasmon Regulation of Metal Nanostructures, Ph. D. thesis, East China normal University, 2022.

Note: memes and some pictures come from the Internet.

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop). Author: Xiao Ming, who wants to be handsome. Editor: Garrett.

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