rus
Issue #3-4/2025
E.V.Panfilova, V.A.Diubanov, A.R.Ibragimov, O.M.Ibragimova, D.Yu.Shramko, Cao Van Hoa, I.I.Yurasova, A.N.Dvinyaninov
FORMATION OF SiO2 PHOTONIC CRYSTAL FILMS WITH A SPECIFIED NUMBER OF LAYERS BY CONTROLLED SELFASSEMBLY METHODS
FORMATION OF SiO2 PHOTONIC CRYSTAL FILMS WITH A SPECIFIED NUMBER OF LAYERS BY CONTROLLED SELFASSEMBLY METHODS
DOI: https://doi.org/10.22184/1993-8578.2025.18.3-4.212.220
Photonic crystal superlattices based on silicon dioxide microspheres have unique structural, mechanical, chemical and optical properties. Due to the cost-effective self-assembly technology of their formation and a wide range of possible applications in electronics, photonics and laser technology, they are one of the most promising materials in nanoengineering. To move to the practical use of developments in this area, it is necessary to learn how to obtain structures with specified parameters. Therefore, the goal of this work was to develop scientific and technical solutions that allow the controlled self-organization of microspheres into a film structure with a specified number of layers.
Photonic crystal superlattices based on silicon dioxide microspheres have unique structural, mechanical, chemical and optical properties. Due to the cost-effective self-assembly technology of their formation and a wide range of possible applications in electronics, photonics and laser technology, they are one of the most promising materials in nanoengineering. To move to the practical use of developments in this area, it is necessary to learn how to obtain structures with specified parameters. Therefore, the goal of this work was to develop scientific and technical solutions that allow the controlled self-organization of microspheres into a film structure with a specified number of layers.
Теги: nanoengineering photonic crystal films self-organisation silicon dioxide spherical particles диоксид кремния наноинженерия самоорганизация сферические частицы фотонно-кристаллические пленки
INTRODUCTION
The prospects for modern electronics development are associated with nanostructured materials. The ability of colloidal spherical microparticles of silicon dioxide SiO2 (silica) ranging in size from 100 to 700 nm to self-organise into ordered photonic crystal (PC) superlattices has made them one of the most promising materials of technologies implemented according to the bottom-up nanoengineering principle [1, 2]. Silicon dioxide superlattices are characterised by mechanical, chemical and thermal stability, which is not typical for structures based on organic materials, such as polystyrene or polymethylmethacrylate. Well-proven technologies of plasma etching, electroplating and vacuum deposition of materials are used to obtain structures based on them [3-5]. It is estimated that colloidal photonic crystals are 10...100 times cheaper than "classical" crystals [6] and require 10...50 times less time to produce.
In the recent years publications, special attention is paid to transfer developments in this field into practical applications, the processes of obtaining prototype devices: LEDs [7], GCR-active substrates [8], emission structures [9], sensor elements [10] are described, and the results of the study of their functional characteristics are presented. The response of PC structures can be regulated by changing the superlattice parameters. For example, the number of structure layers significantly affects the characteristics of a photonic crystal and is determined by its intended use. To create a planar array of dots using colloidal lithography methods, a monolayer of micro- or nanospheres is required; to form a photodiode cell, a structure with a thickness of 2...4 layers is required; to fabricate a waveguide, a structure with a thickness of more than 5 layers is required. Increasing the number of layers usually leads to an increase in reflectivity, including at the wavelength corresponding to the photonic bandgap (PBG). By varying the number of layers, it is possible to fine-tune the photonic bandgap width, the peak position of which depends on the size of the microparticles and the distance between the layers. In practice, when manufacturing samples of sensor elements and filters, the optimal number of layers for each specific task is used to ensure a balance between selectivity and intensity.
Thus, in order for self-organising silica-based structures to become active and widely used in production, it is necessary to implement the technology of controlled superlattice formation. Therefore, the aim of this work is to develop scientific and technical solutions that allow controlled self-organisation of microspheres into a film structure with a controllable number of layers.
METHODS AND MATERIALS
The studies described in this work were realised on the basis of a laboratory complex for obtaining colloidal photonic-crystalline structures [3, 4]. Currently, it is used both in the educational process and in research work to obtain structures based on silicon dioxide SiO2 particles and polystyrene monodisperse latex PS with diameters from 100 to 500 nm.
To realise the controlled synthesis of monodisperse silica microspheres, the Stober method was used, which is characterised by the theoretical possibility of obtaining particles in a wide size range from 50 nm to 2000 nm with a size deviation of up to 4...6%. The method is based on obtaining silica microspheres by their stepwise growth from organosols, in particular, by hydrolysis of tetraethoxysilane in organic solvents.
Monodispersity of the colloidal solution significantly affects defectivity of the formed superlattice. If the particle size deviates to a smaller side from the average value, vacancies are formed in the structure, and cracks are formed to a larger side. Therefore, the solution synthesised by Stober method was subjected to high-speed zonal centrifugation to reduce particle size dispersion. By varying the density gradient of the buffer solution, the centrifuge rotation speed and the process duration, the method allows for rapid separation of the solution into fractions with improved monodispersity relative to the initial monodispersity.
The best quality of self-organising photonic crystal structures is obtained when they are obtained by natural sedimentation. However, this method, firstly, is very time-consuming – obtaining a layer of several microns takes several days, and, secondly, does not allow to complete the process promptly. Therefore, to obtain film superlattices with a given number of microsphere layers, we used methods of controlled self-organisation: vertical pulling of the substrate from the solution, centrifugation and electrophoresis.
The vertical pulling process consists of slowly pulling the substrate out of the solution at a rate of up to 0.5 mm/min and moving the particles into the meniscus region at the phase boundary due to the surface tension forces of the liquid. In this case, if the area of the substrate area, pulled out of the liquid phase per unit time, exceeds the area of projection of the layer of particles brought to the meniscus area on the substrate, the film is not formed. When these areas are approximately equal, a monolayer is formed, and when the second of the above areas is larger, a number of layers is formed equal to the number of particles brought in divided by the number of particles fitting on the substrate in the meniscus region.
The electrophoretic deposition method consists in layer-by-layer deposition of charged particles under the action of an external controlled electric field formed by applying an electric potential of 3...20 V to the conductive parts of the substrate. Shielding of the external electric potential in the particle solution and rapid fading of the exposure energy with increasing distance from the nonmetallic particle to the substrate surface leads to this energy of the external electric field ceases to change significantly with increasing potential, which limits the number of film layers formed from the nonmetallic dispersed phase. The limit on the number of layers that can be deposited for given parameters of solution, particles and external applied potential depends on the energy distribution over the distance from the particle to the substrate, and conditions for deposition of particles of the i-th layer and the magnitude of the potential barriers of the particles. When forming a superlattice on a non-conductive coating to obtain structures of greater thickness, it is advisable to use a pulse mode of voltage supply, the switching frequency of which is determined in accordance with the formation time of the shielding layer.
The essence of the process of obtaining photonic crystal structures using the classical centrifugation scheme consists in deposition of colloidal particles on a substrate placed at the bottom of a test tube fixed on a multi-position rotor under the action of centrifugal forces while rotating the rotor. Varying the rotation frequency from 2000 to 2500 rpm and varying the process duration upwards from 3 min, it is possible to obtain films with thickness from monolayer and higher.
The formed superlattice samples were studied by scanning electron microscopy (SEM).
RESULTS AND DISCUSSION
SEM study of the obtained solution samples has shown that at increase of ammonia NH3 content in the system from 0.1 M to 1.5 M (Fig.1a) and corresponding increase of initial value of hydrogen index pH from 11.3 to 12.0 the particle size increases from 161 to 271 nm, at the same time the particle size scattering decreases from 12 to 2%, and the character of dependences undergoes a change at the value of NH3 concentration 0.5 M/L, which confirms the proposed model of the process. According to it, the process of particle formation consists of the stages of nucleation and subsequent particle growth, and at NH3 concentration less than 0.5 M/L, nucleation occurs during the whole process. The obtained logarithmic dependences of particle size on synthesis time can be used to control the particle growth process and obtain particles of a given size. The growth rate was found to increase with increasing ammonia content from 1 mL to 10.5 mL, at which it was found to be 2.72 ± 0.49 nm/min (Fig.1b).
Analysis of the results of high-speed zone centrifugation of the synthesised SiO2 solutions at 600 rpm through a sucrose gradient solution demonstrated the ability to separate the particles into fractions with a diameter variation step of 2 nm and a standard deviation of the mean of 0.5 nm.
The choice of the method of subsequent deposition of PC films was based on the requirements to the structure. The centrifugation method was used to obtain the most mechanically strong densely packed films. Using relatively low rotor speeds (2000 rpm) it is possible to obtain a monolayer. Increasing the process duration above 2 min and frequency up to 2500 rpm expectedly entails an increase in the number of deposited layers (Fig.2). To remove defects in the form of individual particles on the surface of the formed structure, it is recommended to treat the sample by spin-coating centrifugation at the end of the process.
Electrophoresis is characterised by the highest rate of film deposition and possibility of obtaining both monolayers and, using a pulsed mode of potential supply with a frequency of 1 Hz to 1 kHz, films with thicknesses of several tens of layers of microspheres (Fig.3a). In this case, continuous structures are formed on areas of thousands of square micrometres (Fig.3b). To fine-tune the number of layers, the hydrogen pH and temperature of the solution, as well as the magnitude of the potential, should be varied (Fig.4).
The absence of long-range order in the SEM images in Fig.4 may be explained by the fact that thickness control was performed along the edge of the structure. It is possible to obtain continuous films from unfractionated solution by electrophoresis, using in such processes low values of potential (2..3 V), since its value is directly proportional to the number of defects.
The vertical drawing method was used mainly to obtain monolayers and structures with a thickness not exceeding 5 layers. Their number is determined primarily by the substrate movement speed, which should be varied from 0.01 to 0.5 mm/min [4]. The highest value corresponds to a monolayer (Fig.5).
The vertical deposition method produces defect-free single crystal films that are free of cracks, vacancies and dislocations.
CONCLUSIONS
The SiO2 film superlattices with an adjustable number of microsphere layers represent an important class of nanostructured materials with a wide potential for practical applications. Further research in this area will be aimed at optimising the modes of their formation in order to scale up the process.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The prospects for modern electronics development are associated with nanostructured materials. The ability of colloidal spherical microparticles of silicon dioxide SiO2 (silica) ranging in size from 100 to 700 nm to self-organise into ordered photonic crystal (PC) superlattices has made them one of the most promising materials of technologies implemented according to the bottom-up nanoengineering principle [1, 2]. Silicon dioxide superlattices are characterised by mechanical, chemical and thermal stability, which is not typical for structures based on organic materials, such as polystyrene or polymethylmethacrylate. Well-proven technologies of plasma etching, electroplating and vacuum deposition of materials are used to obtain structures based on them [3-5]. It is estimated that colloidal photonic crystals are 10...100 times cheaper than "classical" crystals [6] and require 10...50 times less time to produce.
In the recent years publications, special attention is paid to transfer developments in this field into practical applications, the processes of obtaining prototype devices: LEDs [7], GCR-active substrates [8], emission structures [9], sensor elements [10] are described, and the results of the study of their functional characteristics are presented. The response of PC structures can be regulated by changing the superlattice parameters. For example, the number of structure layers significantly affects the characteristics of a photonic crystal and is determined by its intended use. To create a planar array of dots using colloidal lithography methods, a monolayer of micro- or nanospheres is required; to form a photodiode cell, a structure with a thickness of 2...4 layers is required; to fabricate a waveguide, a structure with a thickness of more than 5 layers is required. Increasing the number of layers usually leads to an increase in reflectivity, including at the wavelength corresponding to the photonic bandgap (PBG). By varying the number of layers, it is possible to fine-tune the photonic bandgap width, the peak position of which depends on the size of the microparticles and the distance between the layers. In practice, when manufacturing samples of sensor elements and filters, the optimal number of layers for each specific task is used to ensure a balance between selectivity and intensity.
Thus, in order for self-organising silica-based structures to become active and widely used in production, it is necessary to implement the technology of controlled superlattice formation. Therefore, the aim of this work is to develop scientific and technical solutions that allow controlled self-organisation of microspheres into a film structure with a controllable number of layers.
METHODS AND MATERIALS
The studies described in this work were realised on the basis of a laboratory complex for obtaining colloidal photonic-crystalline structures [3, 4]. Currently, it is used both in the educational process and in research work to obtain structures based on silicon dioxide SiO2 particles and polystyrene monodisperse latex PS with diameters from 100 to 500 nm.
To realise the controlled synthesis of monodisperse silica microspheres, the Stober method was used, which is characterised by the theoretical possibility of obtaining particles in a wide size range from 50 nm to 2000 nm with a size deviation of up to 4...6%. The method is based on obtaining silica microspheres by their stepwise growth from organosols, in particular, by hydrolysis of tetraethoxysilane in organic solvents.
Monodispersity of the colloidal solution significantly affects defectivity of the formed superlattice. If the particle size deviates to a smaller side from the average value, vacancies are formed in the structure, and cracks are formed to a larger side. Therefore, the solution synthesised by Stober method was subjected to high-speed zonal centrifugation to reduce particle size dispersion. By varying the density gradient of the buffer solution, the centrifuge rotation speed and the process duration, the method allows for rapid separation of the solution into fractions with improved monodispersity relative to the initial monodispersity.
The best quality of self-organising photonic crystal structures is obtained when they are obtained by natural sedimentation. However, this method, firstly, is very time-consuming – obtaining a layer of several microns takes several days, and, secondly, does not allow to complete the process promptly. Therefore, to obtain film superlattices with a given number of microsphere layers, we used methods of controlled self-organisation: vertical pulling of the substrate from the solution, centrifugation and electrophoresis.
The vertical pulling process consists of slowly pulling the substrate out of the solution at a rate of up to 0.5 mm/min and moving the particles into the meniscus region at the phase boundary due to the surface tension forces of the liquid. In this case, if the area of the substrate area, pulled out of the liquid phase per unit time, exceeds the area of projection of the layer of particles brought to the meniscus area on the substrate, the film is not formed. When these areas are approximately equal, a monolayer is formed, and when the second of the above areas is larger, a number of layers is formed equal to the number of particles brought in divided by the number of particles fitting on the substrate in the meniscus region.
The electrophoretic deposition method consists in layer-by-layer deposition of charged particles under the action of an external controlled electric field formed by applying an electric potential of 3...20 V to the conductive parts of the substrate. Shielding of the external electric potential in the particle solution and rapid fading of the exposure energy with increasing distance from the nonmetallic particle to the substrate surface leads to this energy of the external electric field ceases to change significantly with increasing potential, which limits the number of film layers formed from the nonmetallic dispersed phase. The limit on the number of layers that can be deposited for given parameters of solution, particles and external applied potential depends on the energy distribution over the distance from the particle to the substrate, and conditions for deposition of particles of the i-th layer and the magnitude of the potential barriers of the particles. When forming a superlattice on a non-conductive coating to obtain structures of greater thickness, it is advisable to use a pulse mode of voltage supply, the switching frequency of which is determined in accordance with the formation time of the shielding layer.
The essence of the process of obtaining photonic crystal structures using the classical centrifugation scheme consists in deposition of colloidal particles on a substrate placed at the bottom of a test tube fixed on a multi-position rotor under the action of centrifugal forces while rotating the rotor. Varying the rotation frequency from 2000 to 2500 rpm and varying the process duration upwards from 3 min, it is possible to obtain films with thickness from monolayer and higher.
The formed superlattice samples were studied by scanning electron microscopy (SEM).
RESULTS AND DISCUSSION
SEM study of the obtained solution samples has shown that at increase of ammonia NH3 content in the system from 0.1 M to 1.5 M (Fig.1a) and corresponding increase of initial value of hydrogen index pH from 11.3 to 12.0 the particle size increases from 161 to 271 nm, at the same time the particle size scattering decreases from 12 to 2%, and the character of dependences undergoes a change at the value of NH3 concentration 0.5 M/L, which confirms the proposed model of the process. According to it, the process of particle formation consists of the stages of nucleation and subsequent particle growth, and at NH3 concentration less than 0.5 M/L, nucleation occurs during the whole process. The obtained logarithmic dependences of particle size on synthesis time can be used to control the particle growth process and obtain particles of a given size. The growth rate was found to increase with increasing ammonia content from 1 mL to 10.5 mL, at which it was found to be 2.72 ± 0.49 nm/min (Fig.1b).
Analysis of the results of high-speed zone centrifugation of the synthesised SiO2 solutions at 600 rpm through a sucrose gradient solution demonstrated the ability to separate the particles into fractions with a diameter variation step of 2 nm and a standard deviation of the mean of 0.5 nm.
The choice of the method of subsequent deposition of PC films was based on the requirements to the structure. The centrifugation method was used to obtain the most mechanically strong densely packed films. Using relatively low rotor speeds (2000 rpm) it is possible to obtain a monolayer. Increasing the process duration above 2 min and frequency up to 2500 rpm expectedly entails an increase in the number of deposited layers (Fig.2). To remove defects in the form of individual particles on the surface of the formed structure, it is recommended to treat the sample by spin-coating centrifugation at the end of the process.
Electrophoresis is characterised by the highest rate of film deposition and possibility of obtaining both monolayers and, using a pulsed mode of potential supply with a frequency of 1 Hz to 1 kHz, films with thicknesses of several tens of layers of microspheres (Fig.3a). In this case, continuous structures are formed on areas of thousands of square micrometres (Fig.3b). To fine-tune the number of layers, the hydrogen pH and temperature of the solution, as well as the magnitude of the potential, should be varied (Fig.4).
The absence of long-range order in the SEM images in Fig.4 may be explained by the fact that thickness control was performed along the edge of the structure. It is possible to obtain continuous films from unfractionated solution by electrophoresis, using in such processes low values of potential (2..3 V), since its value is directly proportional to the number of defects.
The vertical drawing method was used mainly to obtain monolayers and structures with a thickness not exceeding 5 layers. Their number is determined primarily by the substrate movement speed, which should be varied from 0.01 to 0.5 mm/min [4]. The highest value corresponds to a monolayer (Fig.5).
The vertical deposition method produces defect-free single crystal films that are free of cracks, vacancies and dislocations.
CONCLUSIONS
The SiO2 film superlattices with an adjustable number of microsphere layers represent an important class of nanostructured materials with a wide potential for practical applications. Further research in this area will be aimed at optimising the modes of their formation in order to scale up the process.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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