The science and technology dynamic silver is one of the metals known and used in ancient times and is also an important precious metal. Silver's physicochemical properties are relatively stable, with good thermal and electrical conductivity, soft and ductile properties, and it has also been one of our circulation currencies.
In the era of endless nanomaterials, silver at nanometer size also has a very attractive feature - surface plasmon resonance can be generated in visible and near-infrared light at a certain size, which is the most visible and near-infrared light transmission. Good plasmon material. However, it is very difficult to obtain a very thin and stable silver film of high quality, which also restricts the application and development of the silver film as a nano-optical device material.
The research team from the University of Michigan developed a thin film of silver atoms that can exist in the air, and the surface of the material is very smooth. They ingeniously added aluminum atoms during the silver film deposition process, and succeeded in obtaining an ultra-thin and flat silver film that was exposed to the air without discoloration. After deposition of a layer of antireflective coating, the transparency of the silver film reached 92.4%. Such good transparency and conductivity are used to make new touch screens, flat screens, and flexible displays.
The research results were published in the form of papers in Advanced Materials. The research team also stated that the material also has excellent performance as an optical waveguide: since light is transmitted through surface plasmon resonance on the surface of the silver thin film, light has a particularly small propagation loss in which the distance traveled is in other metal waveguides. Spread 10 times.
Relying on this principle, the research team also used such silver thin films for the production of super lenses with resolutions that could not be achieved with general optical microscopes, and even comparable to electron microscopes. Such a lens can be used for photolithography of electronic chips and can even replace today's deep ultraviolet lithography. This is undoubtedly a big push for the development of computer hardware technology.
Touch screen material battle
We all know that the display screen needs a lot of transparent electrodes to control the darkness and brightness of the pixels, and the touch screen needs these electrodes. We can see from the composition of the touch screen. In simple terms, the touch screen consists of a non-conductive glass or plastic layer plus a transparent conductive layer.
Taking the capacitive screen as an example, in the normal state, the screen's conductive condition or charge distribution is constant, and once a conductor such as our finger touches the screen, the situation will change, and the system will detect the corresponding signal.
Touch screen function diagram
Nowadays, the most used material for the conductive layer on touch screens is “Indium Tin Oxide (ITO)â€, which is a kind of metal oxide. When it is coated on the surface of glass or plastic, a transparent film can be obtained.
In response, L.Jay Guo, a professor of electrical engineering and computer engineering at the University of Michigan, said: “The market for transparent electrodes has been monopolized by this type of material. However, due to the continuous growth of the touch screen market, this material has also changed. It's getting more and more expensive. It used to be cheap, but now the price has soared."
For touch screen applications, silver thin films do have the ability to replace ITO materials. However, high-quality, high-performance silver films are not easy to do. Prof. Guo explained: “Usually, it is very difficult to make thin silver films less than 15 nanometers thick or less. This is only 100 silver atoms thick, and the film is continuous. Sex is not good.In this case, the silver atom always likes a group of people to gather to form an island instead of a complete atomic layer film."
However, the research team eventually found a flaw - adding 6% of aluminum atoms during deposition, and successfully growing a 7-nanometer silver film (which is half the theoretical limit). Even more amazing is that they exposed it to the air, and the aluminum-silver-doped film did not produce the same color change as the pure silver film. And after a few months, the aluminum-doped silver film still maintains its original electrical conductivity and transparency, and does not fall off from the glass. Even with the use of transparent tape, aluminum-silver-doped silver film does not fall off.
Surface Plasmon Resonance
In addition to its use in display applications, Silver also has superior waveguide capability, in particular the ability to transmit visible and infrared light on its surface. This form of propagation is a surface plasmon, which is the oscillation of the electromagnetic field on the surface of the silver thin film caused by incident light.
In simple terms, when a light wave (electromagnetic wave) is irradiated on the surface of a silver thin film, the charge on the surface of the silver atomic layer can couple with the light wave. If the silver film size matches the incident light frequency, the charge on the surface of the silver atomic layer will also oscillate. This forms a special electromagnetic mode, Surface Plasmon Resonance, where the light wave acts as an electromagnetic wave. It will be confined to the metal surface and enhance.
This oscillation is related to the size of the silver film, and there are also only special wavelengths of light that can be excited, so the surface plasmon resonance can show the frequency of light from the side. Then we can use this principle to use light to transmit information. This is actually the same as the effect of fiber. It encodes and emits an optical signal at one end, then receives the signal at the other end and decodes the transmitted information.
However, under Moore’s Law, today’s electronic circuits are getting smaller and smaller, but the size of optical fibers cannot be reduced to the size of wires in electronic chips. Plasma optical waveguides also assume the task of small-size world “optical fibersâ€. It's faster than electronic transmission of information. Moreover, the plasma transmission has almost no loss of signal, which also makes the distance of the silver thin film to transmit data become very long. A flat silver film can be transmitted at a distance of 1 cm, which is enough for the computer chip.
The properties of the silver thin film plasma can also be used to make metamaterials. This is a material with unconventional optical properties. The refractive index can be negative. It is a material that produces "Perfect Lens". The object is imaged below the diffraction limit. As shown in the figure, a metamaterial superlens composed of a metamaterial can converge the light emitted from the object and image it. This is not achievable with conventional optical lenses, which is why ultra-high resolution lenses have super-high resolution.
Metamaterials and Supermaterial Superlenses
The combination of a silver thin film with some dielectrics, such as glass, can also be made into another super lens. As shown in the figure below, it is the working principle of the super lens. The total reflection of the incident light collected in the lens at the interface between the air and the glass will produce an evanescent wave, which belongs to the category of near-field optics. The silver film now acts as an "optical sensor", detecting the interaction between such evanescent waves and the sample, and obtaining the sample size information.
Super lens principle
In general, a super lens made of a silver thin film is an optical lens with a resolution far exceeding that of a normal microscope. It can see objects smaller than the wavelength of light, and can also be used for precise laser cutting on a chip.
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