Characterization of Layered GaSe Crystals Intercalated with RbNO3 Ferroelectric Salt and their Application for Electric Capacitors
Abstract
XRD, SEM, EDX investigations as well as wide temperature (T = 5–300 K) photoluminescence measurements of GaSe single crystals intercalated from the melt of RbNO3 ferroelectric salt at various temperatures and exposure times are performed in this work. Intercalation by this method results in GaSe matrix fragmentation by separate polycrystals 1 mm in size which consist of bulk GaSe segments (with sizes up to 50.0 mkm) and veins of RbNO3 ferroelectric salt with thickness reaching 2–3 mkm. Within the GaSe segments are inclusions of nano-sized phases consisting of RbNO3 salt whose diameter does not exceed 120 nm. It has been shown that the creation of GaSe hybrid structure has an insignificant influence on the optical properties of GaSe matrix, since in the photoluminescence spectra of GaSe at T = 300 K one can observe emission of free excitons which is typical for GaSe single crystals. The electrical investigations performed indicate that the intercalated GaSe or GaSe are capable of accumulating electric energy, and prototypes of supercapacitors based on them possess: specific long-time energy 105 kJ/kg and resource of cycles > 106
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Introduction
Anisotropy of chemical bonds in layered crystals, weak van der Waals bonds between crystalline layers, and strong covalent bonds between adjacent atoms inside the layers define anisotropy of electric and dielectric properties of these crystals, which results in instability of synthesis when forming the crystalline lattice of these compounds. The following intercalation of the interlayer space in these crystals with foreign atoms or molecules, beside obtaining some expected changes in physical-andchemical properties of hybrid materials based on them, finds its practical applications in power engineering, in particular, as solid-state hydrogen storages [1, 2], accumulators of electric energy [3], and sensors of physical fields [4]; moreover, heterostructures with high photosensitivity based on these materials can be applied in solar cells [5–7].
In this paper we present our recent investigation of some physical and chemical properties of GaSe single crystals intercalated in melts of RbNO3 ferroelectric salt and done some estimates of their application as working materials for solid state supercapacitors in the up-to-date semiconductor element base.
Conclusion
From the performed investigations of processes taking place in GaSe crystals when intercalating them in the melt of ferroelectric salt RbNO3 and studying the physical and chemical properties of the prepared GaSe intercalates, the following features could be ascertained:
1. Non-monotonous growth of the mass and thickness of the crystal takes place over time. This process consists of three stages: i) the mass and thickness of the sample grow; ii) the mass and thickness of the sample are not practically changed with time; iii) both parameters increase to the limit when matrix destruction is observed.
In accord with the classical theory of the strength of materials, intercalation of the GaSe matrix in the first stage obeys Hooke’s law: nanoparticles of RbNO3 salt are formed in this matrix, resulting in growth of the mass and thickness of the sample. In the second stage, plastic deformation of the GaSe matrix takes place with a possible transition to another modification, when GaSe bulk polycrystals are created. In the third stage, we observe a transition to final fragmentation of the GaSe matrix when veins of RbNO3 salt are created between these fragments. Further intercalation is completed by destruction of the GaSe matrix.
2. In the process of intercalation, the GaSe matrix becomes polycrystalline, and these polycrystals (with the sizes up to 1 mm) consist of segments (with sizes of up to 50 mkm), between which ferroelectric salt veins of a complex 3D form are created, whose thickness is close to 2 – 3 mkm. Inside the matrix segments, one can observe non-uniformly distributed inclusions of ferroelectric salt with a mean size of 120 nm. The concentration of these inclusions in the matrix segments reaches 2% to 10% and becomes 100% in the veins between segments.
3. The unit cell of the GaSe single crystal in ε-modification makes the transition into δ-modification, which consist of four crystalline layers with the following parameters: a = 3.7550 Å and c = 31.8903 Å. This lowering of the parameter с takes place due to a reduction in the width of the van der Waals gap caused by pressure related with embedding the veins and nano-sized RbNO3 inclusions in the GaSe matrix.
4. Both parameters of the unit cell of the ferroelectric salt RbNO3are increased to a = 10.4996 Å and с = 7.3810 Å.
5. Despite the creation of a large numbers of defects (veins and nanoparticles of RbNO3) during intercalation by using the “from the melt” method, the GaSe polycrystalline matrix retains the optical properties inherent in the GaSe single crystal. At the same time, the number of point and spatial defects becomes larger, which creates deep acceptor levels responsible for the processes of radiative recombination of electrons from direct/indirect conduction bands with participation of one to several optical phonons.
6. The electrical investigations performed indicate that the intercalated GaSe or GaSe can find application as working materials for solid state supercapacitors in the up-to-date semiconductor element base. They are capable of accumulating electric energy, and prototypes of supercapacitors based on them possess: specific longtime energy 105 kJ/kg and resource of cycles > 106 .