What's in my pixel for you: creating nanopixels using plasmon metasurfaces

    Take a look at the screen. What do you see? Website page with text and pictures, right? But, if you dig deeper? All these elements, different in semantic load and in the way of presentation, consist of digital visual “atoms” called pixels. The more pixels, the better, with the exception of some indie games. Pixels, like any "atom" in the Universe, have their own specific properties and limitations. At least it was before. Today we will get acquainted with a study that describes a method for creating a new type of pixel, hundreds of times smaller and better than current ones. How exactly did the scientists succeed, what amazing characteristics do the new pixels possess and could such pixels help us make out what is happening in the darkness of the third series of the eighth season of Game of Thrones? We will look for answers in the report of the research group. Go.

    Study basis

    We hear the word pixel quite often from a variety of sources. A new smartphone with a camera of 20 megapixels (megapixels), a new pixel indie game, pixel art, the not very successful film "Pixels" in 2015 with Tyrion Lannister, i.e. with Peter Dinklage (sorry, PTSD after the Game of Thrones marathon), etc.

    In scientific terms, a pixel is the smallest logical element of a two-dimensional image (voxels play this role in three-dimensional). If you compare any picture on your screen with the sea, then a pixel is a drop of seawater, exaggerated saying.

    Pixels are round or rectangular (square) in shape. Unlike spy movies and TV shows about super-detectives, if you enlarge a digital image, sooner or later it will turn into a bunch of squares of different colors, and not a super clear image.

    Game of Thrones poster with King of the Night.

    The word pixel itself has a slightly astronomical origin. In 1965, Frederick Billingsley from the Jet Propulsion Laboratory first used this word to describe graphic elements of video images from the space probes of the Moon and Mars. At the same time, Mr. Billingsley was not a pioneer in the field of word formation, because before him this word was used by Keith MacFarland in 1963. The English version of “ pixel ” can be divided into two components - “pix "( picture - image) and" el "( element - element).

    History is history, but we have not gathered here for its sake, but for the sake of fresh discoveries.

    This research is based on the metasurfaces previously touched by us in previous articles.
    Metamaterial * is a composite (of several components), the properties of which depend not so much on the properties of its constituent elements as on its general structure (topology, architecture, etc.).

    In turn, metasurfaces are a two-dimensional type of metamaterials that are characterized by low losses when working with light and ease of manufacture.
    Recently, scientists are paying more and more attention to plasmon * (not to be confused with plasma) metasurfaces.
    Plasmon * is a quasiparticle corresponding to the quantization of plasma oscillations, which are collective oscillations of a free electron gas.
    However, there have always been difficulties in working with plasmon metasurfaces, despite all the technical advantages.

    In this study, scientists describe a method for creating a new type of scalable, electrically controlled metasurfaces. In the process of creating new items, the bottom-up approach was used (the formation of nanoparticles from smaller elements, that is, from smaller to larger). And now in more detail.

    Sample preparation

    Scientists remind us that plasmon resonances, in combination with nanostructures of noble metals, have become an excellent tool for improving certain optical phenomena and processes.

    The use of plasmons in nanolithography for creating displays is also very promising, since plasmon components have a wide color spectrum and a very small size, even smaller than ordinary pixels. But to this day, it was possible to realize exclusively static colors using a very complex process of tuning and arranging scattering elements to overcome the dependence on light polarization, viewing angle, and illumination. In other words, previously it was possible, but damn difficult.

    If we want to get active plasmon colors, scientists say, it is necessary to control the optical properties of the environment from the outside. For example, if plasmonic metasurfaces are used in conjunction with electrochromic materials (conductive polymers and materials with a phase transition), then one can obtain “on / off” when the charge state of the electrochromic material changes. And this already doubles the refresh rate and optical contrast in comparison with systems where only electrochromic materials are available.

    Considering that the size of plasmons controls the color generation of RGB * pixels, scientists have used electro / chemical means to make plasmon nanoparticles function as small optical switches / pixels.
    RGB * (red, green, blue) or GLC (red, green, blue) is an additive color model.
    For example, Au (gold) nanostructures coated with an Ag (silver) shell exhibit wide color dynamics due to electrochemical control of the Ag shell thickness or redox reactions. However, such nanostructures are very short-lived (no more than 1 month), and their switching speed is very low (more than 0.5 s).

    Such disadvantages are associated mainly with silver. When it is precipitated too often or it often goes through the oxidation / reduction process, ion diffusion is slower and leads to rapid morphological changes in the nanoscale. It turns out that the method is good and working, but not very durable.

    Another way to achieve the desired is to use a multilayer plasmonic composite with dielectric gasket (NPoM) inside.
    NPoM - nanoparticle-on-mirror (nanoparticle-on-mirror).
    Another good thing is that such composites can be created without the use of problem lithography, but the accuracy will be down to the atomic level.

    Image No. 1

    The main advantage of this structure is that the nanoparticles strongly limit the light inside their individual cells to the underlying mirror and, thus, create extremely localized optical resonators (image above). Thus, the nanoparticles become independent of each other and insensitive to the angle and polarization of the incident light.

    Scientists note that a similar technology has not previously been used to create displays. And their main task is to realize the ability to produce NPoM on a large scale, while maintaining the independence of individual nanopixels.

    In their work, scientists describe the creation ofeNPoM - electrochromic nanoparticles-on-mirrors formed from gold nanoparticles encapsulated in a conductive polymer shell of polyaniline.

    The biggest achievements are the performance and energy efficiency of eNPoM. Switching the charge state of the shell allows you to quickly shift the color of the resonance scattering eNPoM in the wavelength range> 100 nm. An active nanopixel in such a system requires only ~ 0.2 fJ (femtojoule, 1 fJ = ​​10 −15 J) of energy for each wavelength shift of 1 nm.

    ENPoM Theory

    Color dynamics based on local surface plasmon resonance ( LSPR ) works by changing the refractive index of the medium surrounding the plasmon nanomaterial, shifting the position of the LSPR peak. The corresponding color adjustment can be deduced from the LSPR sensitivity:

    where λ is the resonator wavelength, x is the shape factor of the metal nanoparticle (if 2, then this is a sphere), ℇ m is the dielectric constant of the metal nanoparticle, and n is the refractive index of the medium surrounding the nanoparticle. In the best case scenario, ∆n should be large, providing n ~ 1 to maintain LSPR resonance in the middle of the visible region, and allowing ∆λ * to tune to the entire visible spectrum.

    The use of plasmonic nanoparticles is a logical solution in this situation, however, there are a number of problems. Inorganic materials with large ∆n have a shape factor> 2. Because of this, their LSPR resonances are in the near infrared (NIR) and are not suitable for plasmon color applications. Sensitive polymers with n <1.7 can be used. But with such materials it is difficult to adjust and adjust the color.

    It turns out that it is impossible to apply classical methods, or rather it is possible, but the result will be weak. That is why scientists used eNPoM ( 1a), consisting of Au nanoparticles encapsulated in a polyaniline shell (hereinafter referred to as PANI). Such an NPoM topology manifests itself as a dimeric pair of plasmon particles that do not interact with each other, which causes amplification of the optical field coupling in the gap, known as a “hot spot” ( 1b ). This region leads to the formation of an additional coupled resonance and a transverse mode of about 550 nm, supported by Au nanoparticles alone.

    Changing the surrounding optical medium allows you to adjust this resonance, and the transverse mode at this moment practically does not change. Change in the redox state of the ultralow volume of the PANI shell surrounding each nanoparticle (~ 3x10 -4 μm -3 ).

    After modeling using the finite time difference method ( 1s ), the researchers suggested that the full redox effect of PANI in eNPoM can lead to visible shifts of scattering wavelengths> 100 nm, i.e. 300% more than those supported exclusively by nanoparticles ( without the participation of a polyaniline shell). In the reduced state of PANI 0, the associated eNPoM resonance appears at c 0 = 675 nm, and when oxidized to PANI 2+ , a shift to blue occurs at c 2+ = 575 nm.

    Optimum eNPoM scattering predicts a 100nm color range with 43% adjustable contrast ( 1s) Such observations indicate a real opportunity to obtain custom / switchable colors with low optical loss and high spatial resolution, which was confirmed by experiments on devices with one nanopixel ( 1d ).

    Creating eNPoM

    Image No. 2

    The process of creating eNPoM consists of two stages of the bottom-up method: enveloping Au nanoparticles with a PANI coating in solution; low tide Au flat mirror.

    Colloidal Au nanoparticles were encapsulated in an integral thin PANI shell by chemical oxidative polymerization using a surfactant (insert in the upper right corner at 2b ).

    Further, the obtained samples were embedded in electrochemical chambers (cells) created in the laboratory, which were optimized for simultaneous tracking of optical and electrical dynamics.

    The Au mirror forms a working electrode, and the redox state of the PANI shells is controlled by changing the voltage from -0.2 to 0.6 V with a scanning speed of 50 mV / s. Cyclic voltammetry curves averaged over 90 cycles ( 2a ) show two sets of oxidized (upper) and reduced peaks (lower) from three different redox forms of PANI: PANI 0 - completely reduced; PANI 1+ is semi-oxidized and PANI 2+ is fully oxidized. Therefore, the complete oxidation and reduction of eNPoM occurs only in the potential range ∆V <1 V. At the same time, the “dark-field” scattering spectrum of one eNPoM is measured ( 2b and 1d)

    The application of a negative potential causes a decrease in the PANI shell (PANI 0 ), which leads to a scattering peak at c 0 = 642 nm. And the reverse of the potential leads to a resonance shift to c 2+ = 578 nm, while ∆λ * = 64 nm is consistent with the modeling carried out earlier ( 1s ).

    Further observation of the dark-field scattering spectrum during cyclic voltammetry showed highly stable and reversible optical switching ( 2c ) with fully reproducible dynamics ( 2d ).

    An even more important observation is the identity of all eNPoMs in terms of optical dynamics: if the conditions for all nanopixels are the same, then their optical dynamics will be the same, which is extremely important for large-scale homogeneous metasurfaces.

    Different clearances on eNPoM

    Image No. 3

    After the preparatory work, the scientists decided to check how structural parameters of eNPoM affect color switching, in particular, how eNPoM gaps, determined by the thickness of the shell on the surface of the Au nanoparticle, influence this process. For this, several eNPoM test samples were created with different gaps, while the shell thickness was increased from 10 to 20 nm.

    As a result, 4 types of eNPoM nanopixels were obtained: 11, 13, 18, and 20 nm ( 3a ). Scientists have evaluated their electrical ( 3b ) and optical dynamics ( 3c - 3f ).

    Simulations and actual experiments with different nanopixels showed similar results - reversible blue shifts ( 3d) and a decrease in intensity by ~ 50% ( 3e ) upon oxidation.

    In theory, according to scientists, with a decrease in gaps, the length of the resonant wave and the range of its spectral tuning should increase. In reality, everything turned out differently - the thinning of the PANI shell led to a smaller color range during the redox cycle. Researchers explain this with additional structural factors that were not taken into account in modeling (in theory):

    • imperfection of the spherical shape and size of Au nanoparticles;
    • differences in the optical properties of PANI of different thicknesses;
    • heterogeneity of the PANI shell covering the nanoparticle;
    • ~ 30% change in shell thickness during the redox process;
    • heterogeneity of the redox process of the PANI shell molecules in the gap.

    As a result, NPoM with a thicker shell (more than 15 nm) showed excellent color characteristics with high accuracy, consistent with mathematical modeling.

    Redox monitoring

    A color change upon a change in the redox state of a conductive polymer opens up the possibility of tracking the associated electron dynamics in a tiny channel under individual single nanoparticles in the NPoM geometry ( 4a ).

    Image No. 4

    This allows you to understand how many electrons are transported through the gap in eNPoM and at what speed.

    The electron transfer rate between PANI and the Au mirror is quite high due to the fact that this process proceeds precisely in nano-gaps with insignificant mass transfer. This ensures that the redox system is electrochemically reversible. Peak Current i POn the cyclic voltammetry curve in the oxidized (or reduced) state, eNPoM is linearly proportional to the potential scanning speed n with a limited peak shift.

    It follows that i P = vF 2 fA / RT with the participation of two electrons, where F is the Faraday constant (C / mol), R is the ideal gas constant (J / (mol ∙ K)), T is the temperature of the system (K ), A is the area of ​​the working electrode (m 2 ), f is the surface area of ​​the particles on the electrode (mol / m 2 ).

    Given a linear relationship with n, f is constant and gives the number of PANI molecules undergoing electron transfer, which is given by the number eNPoM on the electrode. This will allow you to calibrate the number of input / output electrons from each NPoM ( 4b) Thus, one can see the electron dynamics in the gaps of individual NPoMs associated with three different PANI redox states. Approximately 30,000 electrons in each nanoparticle are transported. Measurements of optical dynamics showed two distinct transitions that ideally correspond to electron dynamics ( 4c ).

    The main conclusion from the above observations is the energy efficiency of nanopixels - ~ 80 and ~ 200 AJ (attojoule, 1 AJ = 10 −18 J) per 1 nm shift are required for color switching from c 0 to c 1+ and from 1 + to c 2+ wavelengths.

    Next, scientists analyzed the optical switching of single eNPoMs with faster rectangular electrical modulation ( 4d from above) to determine the time response. In the case of applying a voltage jump from 0.6 to -0.2 V, causing fast shifts in the coupled mode from c 0 to c 2+ , a sharp redox transition of the polymer was observed ( 4d from the bottom).

    The switching time was 32 ms (oxidation) and 143 ms (decrease) with a change in intensity of 47%. Reversible color switching at the level of single nanoparticles is observed in response to rectangular potentials of increasing the frequency up to 50 Hz ( 4e , 4f ).

    Due to the stability of the PANI charge states, bistability (two equilibrium states) of eNPoM was observed. In addition, the resonance modes at c 2+ and c 0 persist for> 10 minutes. And this is one of the factors in reducing energy consumption for a device based on this technology.

    Scaling eNPoM metasurfaces

    Energy efficiency is, of course, good, but scalability is also needed. It is even better to combine these two indicators, avoiding lithography in production, as the researchers say. To achieve this, a new method of assembling nanoparticles through meniscus * guidance was applied .
    The meniscus * is a concave-convex or convex-concave lens bounded by two spherical surfaces.
    The volume fraction of particles in the solution used for coating determines the particle density (fraction of filling) on ​​the mirror substrate (image No. 5). Surfaces consisting of randomly distributed eNPoMs with a fill fraction of 20% are obtained using 0.3% of the volume fraction of the original colloid.

    Image No. 5

    A ~ 100 nm interval provides the minimum near-field optical coupling between nanoparticles ( 5a ). Colors are controlled solely by the gaps under each eNPoM. The resulting increased eNPoM metasurface also showed excellent color switching with Δλ * = 79 nm and 57% contrast switching over the entire surface ( 5b - 5e) In other words, the metasurface from eNPoM exhibits the same properties and behavior as a single eNPoM.

    The color range and dynamics in the metasurface can be improved by mixing different nanoparticles or by using ultraviolet plasmon nanoparticles.

    Image No. 6 The

    graphs above show how good the characteristics of the developed system based on eNPoM nanopixels are. The setting of the visible wavelength, ultra-small pixel size and switching speed correspond to modern requirements (green area at 5a ).

    Scientists note that the developed metasurface has been working for 3 months (at the time of writing of the report) at power densities below 300 mW / cm 2 and at a pixel density of 109 per square inch.

    For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of the research group and additional materials to it.


    The study we examined today falls into the category of improving existing technologies. However, at the same time, scientists used very radical, as they call them, nanotechnology methods to achieve the desired result. Light behaves very unusual at the nano level, and understanding of its properties and characteristics allows you to create new devices and improve existing ones.

    The developed nanopixels can find their application in various fields - from displays the size of a house to camouflage materials. Scientists themselves are confident in this. They will continue to work on their invention, expanding its capabilities and improving its characteristics.

    Friday off-top:

    The confrontation between the avengers and Thanos ("War of Infinity") in the old-school form.

    Off-top 2.0 (Valar Morgulis)

    What would an old-school fighting game look like with the heroes from Game of Thrones.

    Thank you for your attention, stay curious and have a great weekend everyone, guys! :)

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