rus
Issue #3-4/2024
E.V.Panfilova, V.A.Diubanov, A.R.Ibragimov, D.Yu.Shramko
LABORATORY COMPLEX FOR OBTAINING COLLOIDAL PHOTONIC-CRYSTAL STRUCTURES. PART 1
LABORATORY COMPLEX FOR OBTAINING COLLOIDAL PHOTONIC-CRYSTAL STRUCTURES. PART 1
DOI: https://doi.org/10.22184/1993-8578.2024.17.3-4.190.198
Colloidal photonic crystal structures are a promising material for nanoengineering. The goal of the work was to create a set of scalable equipment for the synthesis of monodisperse colloidal particles and the production of superlattices from them. The authors presented a description of the kit, the results of a study of the structures and formulated recommendations for the design of equipment and the implementation of technological processes.
Colloidal photonic crystal structures are a promising material for nanoengineering. The goal of the work was to create a set of scalable equipment for the synthesis of monodisperse colloidal particles and the production of superlattices from them. The authors presented a description of the kit, the results of a study of the structures and formulated recommendations for the design of equipment and the implementation of technological processes.
INTRODUCTION
The structures obtained on the basis of colloidal photonic-crystalline (PC) films belong to the basic materials for technologies realised by the bottom-up nanoengineering principle. The prospects for their use are associated with photonics, optoelectronics, laser technology, and a number of other fields [1]. In [2–4] sensor devices were presented, the operation principle is based on the photonic forbidden zone (PFZ) position displacement under mechanical, chemical and electrostatic effects on the structure. In [5], a sensor of solution acidity based on the change of functional charge groups when the hydrogen index changes is described. In [6–7], variants of using inverse colloidal structures with integrated biomimetic materials in storage devices and robotics are considered. In [8–10] samples of light-activated actuators were presented, the operation principle of is based on the phenomena of cis-trans isomerisation, photocyclisation or intramolecular mass transfer. In [11–14] the colloidal structures application in forming the colour displays, including those based on liquid crystals, plasmonic structures and photonic crystals, such displays consume less power during operation and require less operating voltage compared to conventional LC displays. The structures obtained on the basis of colloidal photonic crystals allow increasing quantum yield, improving biological activity, reducing toxicity and improving catalytic properties of the substance [15–16]. In addition to creating functional structures, colloidal layers of a given configuration are used as a mask in microsphere lithography processes [17].
The diversity of applications of colloidal PC films highlights the problem of preparing the various combinations of particle packing and the number of layers to be formed. For example, when forming SERS-active substrates [18] or a masking layer for microsphere lithography, it is strictly necessary to obtain an ordered monolayer of particles. Whereas in a photonic crystal strain sensor [19], a structure with a thickness of a few microns is required, which, with characteristic particle sizes of hundreds of nanometres, means that at least 10 layers must be formed.
The process of obtaining ordered periodic colloidal layers is based on the phenomenon of self-organisation (self-assembly) of colloidal particles. External perturbing influences on the self-organising system transfer self-assembly into a controlled direction, affecting coagulation of colloidal microspheres, their movement in solution and the assembly rate into a crystal. The corresponding processes can be used to obtain structures with specified properties, including photonic bandgap (PBG) characteristics. PBG indicators are determined by the colloidal superlattice structure, which, in turn, is related to the colloidal system properties, monodispersity of the particles contained in it and the character of self-assembly. The latter depends on the method of obtaining colloidal particles, as well as modes of their deposition and parameters of the equipment used in this process. The coagulation of colloidal particles occurring in this process is described in detail by the well-known theory of Deryagin, Landau, Fervay, Overbeck (DLFO). However, the current problem of preparing the defect-free single-crystal structures on an area of hundreds of micrometres sufficient for device fabrication hinders the transfer of developments in this area of nanoengineering into practical applications. For its solution it is necessary to have special equipment realising controlled and manageable processes of synthesis of monodisperse colloidal particles and obtaining superlattices from them. Therefore, the aim of this work was to develop and create a set of scalable equipment and special tooling for laboratories of scientific and educational institutions, as well as to determine the rational modes of their operation.
METHODS AND MATERIALS
The fabrication process of colloidal PC structures consists of the stages of substrate cleaning; synthesis of colloidal particles; their self-assembly and formation of colloidal monolayers, films or 3D structures; hardening heat treatment; fine fitting of monolayer lattice parameters by plasma etching and microscopic and spectral control (Fig.1).
Since colloidal solutions have a tendency to age, and unstrengthened structures degrade rapidly, there is an obvious need to create a complex of equipment characterised by a unity of place, implementing the above operations (Fig.2). The equipment used in it should provide possibility of monitoring and control of key process parameters. We have created a laboratory complex that meets these requirements. Currently, it is used both in the educational process and in research work to produce structures based on silicon dioxide (silica) particles and polystyrene monodisperse latex PS with diameters from 100 to 500 nm. Steps such as media purification and preparation, heat treatment and plasma etching are realised therein using commercial equipment. For the stages of colloidal solution synthesis and colloidal layer deposition, original plants, stands and tooling were designed and manufactured. The methods used in this process are described below.
Silicon dioxide SiO2, polystyrene PS and polymethyl methacrylate PMMA microspheres are used in practice to obtain colloidal FC structures. Organic microspheres are characterised by high monodispersity, silicon dioxide is thermally and chemically stable, and is easily integrated into optoelectronic circuits. Therefore, the presented complex is designed to operate with both organic materials and silicon dioxide. We use commercial standard polystyrene samples. Colloidal solution of silicon dioxide is synthesised on the basis of the laboratory complex.
The synthesis of silicon dioxide particles (SDP) is carried out by the modernised Stober method in terms of the nucleation stage (Fig.3). It is this stage that determines the monodispersity of the obtained particles, for its increase the pretreatment of initial tetraethoxysilane (TEOS) is used. The size of formed SDPs is determined by the hydrolysis ratio and condensation rates, which, in turn, can be set by varying concentration of ammonia in the mixture.
To increase accuracy of the obtained results, the synthesis process is realised under conditions of strict control of a large number of factors determining the size and monodispersity of particles, such as: purity of the reagents used and their concentration, mixing protocol, reaction temperature, mixing frequency, and process duration.
To obtain colloidal films under conditions of externally controlled self-assembly, the laboratory complex uses electrophoresis, centrifugation, Langmuir – Blodgett and vertical deposition methods, realised either by pumping the solution or by pulling the substrate out of it (so-called vertical pulling). Schemes of the mentioned methods are shown in Fig.3. The laboratory complex implements all of the above methods of controlled action.
According to the DLFO theory, interaction of colloidal particles possessing a double electric layer occurs due to electrostatics and intermolecular van der Waals forces. The former leads to desire of particles to separate from each other, however, they can form complexes during collisions. Accordingly, the colloidal crystal formation process can be divided into three phases. First phase: the particles are in solution in free movement, at a distance from each other. Second phase: the particles are brought closer together by evaporation of the solution and/or external forces, in particular gravity, they are in the zone of action of the forces of mutual attraction and repulsion between the particles. Third phase: the particles have formed a lattice, the solution has practically evaporated, but capillary bridges remain between the particles. The task of external influence on the colloidal system is to form conditions for self-assembly of particles and prevent their coagulation both in the solution thickness and near the substrate. The methods realising this approach differ from each other in the way of realisation of external influence on the particle interaction pattern at the first and second stages of colloidal crystal formation.
The vertical pulling method allows to obtain structurally ordered films with a given number of layers and packing density. The essence of the method consists in slow pulling of a vertically arranged substrate from a colloidal solution (Fig.3a). In this process, a meniscus area is formed at the three media air – colloidal solution – substrate interface due to surface tension forces. Evaporation of the solution results in a flow of particles towards the meniscus, forming the highest concentration of particles in this region. Formation begins with a single row of particles deposited on the substrate surface, stopped by frictional and capillary forces, taking into account mutual attraction and repulsion with other particles approaching the substrate [20]. Thus, as the substrate is stretched, hexagonal layers of particles are formed stepwise. To find the number of film layers (k) obtained by self-assembly methods, one can use the formula linking the average evaporation rates of solution and particles to the process parameters [20–21]:
, (1)
where β is a constant depending on the ratio of solute to meniscus velocities, l is the evaporation length, j is the evaporation intensity, φp is the particle volume fraction, v is the particle pulling velocity, and r is the particle radius.
Based on theoretical calculation and mathematical modelling, the dependence of the number of layers on the pulling speed was plotted for 5% colloidal silica solution with a particle size of 200 nm (Fig.4).
The equipment requirements for the validation of theoretical studies are smooth and uniform pulling of the substrate during the entire process, as well as the absence of vibrations and minimisation of the influence of external factors that can affect the process of self-assembly of particles.
The combination of methods of vertical deposition (Fig.3b) and electrophoresis (Fig.3c) is used to solve the problem of uniform colloidal structures formation of millimetre area on the substrate. In this case, in order to ensure structures reproducibility over the substrate area, it is necessary to make conditions under colloidal solution stability will not be disturbed in the volume, while particles coagulation of the dispersed phase should take place directly in the deposition zone. In accordance with the extended DLFO theory, in order to exclude coagulation, i.e. direct interaction between particles, it is necessary to carry out the ordering of microspheres in solution by applying an external influence that keeps microspheres in the sedimentation zone. The energy of such influence should be higher than the chaotic motion energy of microspheres, but less than the energy of the potential barrier characterising the adhesion of particles. Directionality of the impact will not prevent the particles from travelling along the deposition zone. Thus, self-assembly of particles into the structure occurs before their coagulation. Since the interaction energy of particles depends on temperature, decreasing temperature of the system can provide an increase in the potential barrier with energy decrease of thermal motion, i.e. increase the range of control of the restraining effect. Conversely, increasing of temperature makes it possible to realise self-assembly in highly stable colloidal systems with the application of smaller external influences. Adjustment of formed structure thickness in this case is carried out by controlled limitation of the electric potential with its decrease at a distance from the substrate due to screening by charges in the solution. As a result, when the final layer is reached, the energy of external electrostatic influence We is reduced to a value smaller than interaction energy of the particle Wвзаим with its neighbours, the number of which is equal to n:
Wэ < nWвзаим. (2)
Strengthening of the obtained colloidal crystals is carried out by heating to temperature corresponding to the beginning of melting of the particle material. In this case colloidal medium and organic residues are removed from interspherical voids and "bridges" are formed between particles. Thus, the colloidal crystal structure becomes stronger and its optical characteristics are improved. In the process of heat treatment of polystyrene particles like "bridges" are formed due to emergence of new chemical bonds in the course of thermal degradation and destruction of macromolecular bonds.
Fine tuning of spherical particle diameters to a given size as well as correction of interspherical distances to obtain non close-packed colloidal crystals (NCPCC) is carried out by plasma etching in a mixture of argon Ar and oxygen O2. The etching process and, consequently, the resulting diameter of microspheres are most influenced by the gas flow rate, etching time and power of the source.
The obtained samples are controlled by electron and atomic force microscopy on a ZEISS Crossbeam 550 scanning electron microscope (Carl Zeiss Microscopy, Germany) and a Solver Next scanning probe microscope (NT-MDT, Russia), respectively, and spectrometry on an EPSILON optical spectrophotometer (IZOVAK, Belarus).
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 structures obtained on the basis of colloidal photonic-crystalline (PC) films belong to the basic materials for technologies realised by the bottom-up nanoengineering principle. The prospects for their use are associated with photonics, optoelectronics, laser technology, and a number of other fields [1]. In [2–4] sensor devices were presented, the operation principle is based on the photonic forbidden zone (PFZ) position displacement under mechanical, chemical and electrostatic effects on the structure. In [5], a sensor of solution acidity based on the change of functional charge groups when the hydrogen index changes is described. In [6–7], variants of using inverse colloidal structures with integrated biomimetic materials in storage devices and robotics are considered. In [8–10] samples of light-activated actuators were presented, the operation principle of is based on the phenomena of cis-trans isomerisation, photocyclisation or intramolecular mass transfer. In [11–14] the colloidal structures application in forming the colour displays, including those based on liquid crystals, plasmonic structures and photonic crystals, such displays consume less power during operation and require less operating voltage compared to conventional LC displays. The structures obtained on the basis of colloidal photonic crystals allow increasing quantum yield, improving biological activity, reducing toxicity and improving catalytic properties of the substance [15–16]. In addition to creating functional structures, colloidal layers of a given configuration are used as a mask in microsphere lithography processes [17].
The diversity of applications of colloidal PC films highlights the problem of preparing the various combinations of particle packing and the number of layers to be formed. For example, when forming SERS-active substrates [18] or a masking layer for microsphere lithography, it is strictly necessary to obtain an ordered monolayer of particles. Whereas in a photonic crystal strain sensor [19], a structure with a thickness of a few microns is required, which, with characteristic particle sizes of hundreds of nanometres, means that at least 10 layers must be formed.
The process of obtaining ordered periodic colloidal layers is based on the phenomenon of self-organisation (self-assembly) of colloidal particles. External perturbing influences on the self-organising system transfer self-assembly into a controlled direction, affecting coagulation of colloidal microspheres, their movement in solution and the assembly rate into a crystal. The corresponding processes can be used to obtain structures with specified properties, including photonic bandgap (PBG) characteristics. PBG indicators are determined by the colloidal superlattice structure, which, in turn, is related to the colloidal system properties, monodispersity of the particles contained in it and the character of self-assembly. The latter depends on the method of obtaining colloidal particles, as well as modes of their deposition and parameters of the equipment used in this process. The coagulation of colloidal particles occurring in this process is described in detail by the well-known theory of Deryagin, Landau, Fervay, Overbeck (DLFO). However, the current problem of preparing the defect-free single-crystal structures on an area of hundreds of micrometres sufficient for device fabrication hinders the transfer of developments in this area of nanoengineering into practical applications. For its solution it is necessary to have special equipment realising controlled and manageable processes of synthesis of monodisperse colloidal particles and obtaining superlattices from them. Therefore, the aim of this work was to develop and create a set of scalable equipment and special tooling for laboratories of scientific and educational institutions, as well as to determine the rational modes of their operation.
METHODS AND MATERIALS
The fabrication process of colloidal PC structures consists of the stages of substrate cleaning; synthesis of colloidal particles; their self-assembly and formation of colloidal monolayers, films or 3D structures; hardening heat treatment; fine fitting of monolayer lattice parameters by plasma etching and microscopic and spectral control (Fig.1).
Since colloidal solutions have a tendency to age, and unstrengthened structures degrade rapidly, there is an obvious need to create a complex of equipment characterised by a unity of place, implementing the above operations (Fig.2). The equipment used in it should provide possibility of monitoring and control of key process parameters. We have created a laboratory complex that meets these requirements. Currently, it is used both in the educational process and in research work to produce structures based on silicon dioxide (silica) particles and polystyrene monodisperse latex PS with diameters from 100 to 500 nm. Steps such as media purification and preparation, heat treatment and plasma etching are realised therein using commercial equipment. For the stages of colloidal solution synthesis and colloidal layer deposition, original plants, stands and tooling were designed and manufactured. The methods used in this process are described below.
Silicon dioxide SiO2, polystyrene PS and polymethyl methacrylate PMMA microspheres are used in practice to obtain colloidal FC structures. Organic microspheres are characterised by high monodispersity, silicon dioxide is thermally and chemically stable, and is easily integrated into optoelectronic circuits. Therefore, the presented complex is designed to operate with both organic materials and silicon dioxide. We use commercial standard polystyrene samples. Colloidal solution of silicon dioxide is synthesised on the basis of the laboratory complex.
The synthesis of silicon dioxide particles (SDP) is carried out by the modernised Stober method in terms of the nucleation stage (Fig.3). It is this stage that determines the monodispersity of the obtained particles, for its increase the pretreatment of initial tetraethoxysilane (TEOS) is used. The size of formed SDPs is determined by the hydrolysis ratio and condensation rates, which, in turn, can be set by varying concentration of ammonia in the mixture.
To increase accuracy of the obtained results, the synthesis process is realised under conditions of strict control of a large number of factors determining the size and monodispersity of particles, such as: purity of the reagents used and their concentration, mixing protocol, reaction temperature, mixing frequency, and process duration.
To obtain colloidal films under conditions of externally controlled self-assembly, the laboratory complex uses electrophoresis, centrifugation, Langmuir – Blodgett and vertical deposition methods, realised either by pumping the solution or by pulling the substrate out of it (so-called vertical pulling). Schemes of the mentioned methods are shown in Fig.3. The laboratory complex implements all of the above methods of controlled action.
According to the DLFO theory, interaction of colloidal particles possessing a double electric layer occurs due to electrostatics and intermolecular van der Waals forces. The former leads to desire of particles to separate from each other, however, they can form complexes during collisions. Accordingly, the colloidal crystal formation process can be divided into three phases. First phase: the particles are in solution in free movement, at a distance from each other. Second phase: the particles are brought closer together by evaporation of the solution and/or external forces, in particular gravity, they are in the zone of action of the forces of mutual attraction and repulsion between the particles. Third phase: the particles have formed a lattice, the solution has practically evaporated, but capillary bridges remain between the particles. The task of external influence on the colloidal system is to form conditions for self-assembly of particles and prevent their coagulation both in the solution thickness and near the substrate. The methods realising this approach differ from each other in the way of realisation of external influence on the particle interaction pattern at the first and second stages of colloidal crystal formation.
The vertical pulling method allows to obtain structurally ordered films with a given number of layers and packing density. The essence of the method consists in slow pulling of a vertically arranged substrate from a colloidal solution (Fig.3a). In this process, a meniscus area is formed at the three media air – colloidal solution – substrate interface due to surface tension forces. Evaporation of the solution results in a flow of particles towards the meniscus, forming the highest concentration of particles in this region. Formation begins with a single row of particles deposited on the substrate surface, stopped by frictional and capillary forces, taking into account mutual attraction and repulsion with other particles approaching the substrate [20]. Thus, as the substrate is stretched, hexagonal layers of particles are formed stepwise. To find the number of film layers (k) obtained by self-assembly methods, one can use the formula linking the average evaporation rates of solution and particles to the process parameters [20–21]:
, (1)
where β is a constant depending on the ratio of solute to meniscus velocities, l is the evaporation length, j is the evaporation intensity, φp is the particle volume fraction, v is the particle pulling velocity, and r is the particle radius.
Based on theoretical calculation and mathematical modelling, the dependence of the number of layers on the pulling speed was plotted for 5% colloidal silica solution with a particle size of 200 nm (Fig.4).
The equipment requirements for the validation of theoretical studies are smooth and uniform pulling of the substrate during the entire process, as well as the absence of vibrations and minimisation of the influence of external factors that can affect the process of self-assembly of particles.
The combination of methods of vertical deposition (Fig.3b) and electrophoresis (Fig.3c) is used to solve the problem of uniform colloidal structures formation of millimetre area on the substrate. In this case, in order to ensure structures reproducibility over the substrate area, it is necessary to make conditions under colloidal solution stability will not be disturbed in the volume, while particles coagulation of the dispersed phase should take place directly in the deposition zone. In accordance with the extended DLFO theory, in order to exclude coagulation, i.e. direct interaction between particles, it is necessary to carry out the ordering of microspheres in solution by applying an external influence that keeps microspheres in the sedimentation zone. The energy of such influence should be higher than the chaotic motion energy of microspheres, but less than the energy of the potential barrier characterising the adhesion of particles. Directionality of the impact will not prevent the particles from travelling along the deposition zone. Thus, self-assembly of particles into the structure occurs before their coagulation. Since the interaction energy of particles depends on temperature, decreasing temperature of the system can provide an increase in the potential barrier with energy decrease of thermal motion, i.e. increase the range of control of the restraining effect. Conversely, increasing of temperature makes it possible to realise self-assembly in highly stable colloidal systems with the application of smaller external influences. Adjustment of formed structure thickness in this case is carried out by controlled limitation of the electric potential with its decrease at a distance from the substrate due to screening by charges in the solution. As a result, when the final layer is reached, the energy of external electrostatic influence We is reduced to a value smaller than interaction energy of the particle Wвзаим with its neighbours, the number of which is equal to n:
Wэ < nWвзаим. (2)
Strengthening of the obtained colloidal crystals is carried out by heating to temperature corresponding to the beginning of melting of the particle material. In this case colloidal medium and organic residues are removed from interspherical voids and "bridges" are formed between particles. Thus, the colloidal crystal structure becomes stronger and its optical characteristics are improved. In the process of heat treatment of polystyrene particles like "bridges" are formed due to emergence of new chemical bonds in the course of thermal degradation and destruction of macromolecular bonds.
Fine tuning of spherical particle diameters to a given size as well as correction of interspherical distances to obtain non close-packed colloidal crystals (NCPCC) is carried out by plasma etching in a mixture of argon Ar and oxygen O2. The etching process and, consequently, the resulting diameter of microspheres are most influenced by the gas flow rate, etching time and power of the source.
The obtained samples are controlled by electron and atomic force microscopy on a ZEISS Crossbeam 550 scanning electron microscope (Carl Zeiss Microscopy, Germany) and a Solver Next scanning probe microscope (NT-MDT, Russia), respectively, and spectrometry on an EPSILON optical spectrophotometer (IZOVAK, Belarus).
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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