rus
DOI: 10.22184/1993-7296.FRos.2025.19.1.28.38
The article continues the review of single photon sources while considering various methods for the single photon sources (SPS) development. Earlier, the first part of the review (Photonics Russia. 2024; 18(5): 376–396) discussed the requirements for single-photon sources and their characterization criteria, described the single-ion and single-atom-based single-photon sources. The SPSs based on the quantum dots and color centers in the crystals were considered in the second part of the review (Photonics Russia. 2024; 18(8): 610–620). The third part considers the single-photon sources based on the carbon nanotubes and their defects (defect engineering in the nanotubes), on nanocrystals and layered nanocrystals.
The article continues the review of single photon sources while considering various methods for the single photon sources (SPS) development. Earlier, the first part of the review (Photonics Russia. 2024; 18(5): 376–396) discussed the requirements for single-photon sources and their characterization criteria, described the single-ion and single-atom-based single-photon sources. The SPSs based on the quantum dots and color centers in the crystals were considered in the second part of the review (Photonics Russia. 2024; 18(8): 610–620). The third part considers the single-photon sources based on the carbon nanotubes and their defects (defect engineering in the nanotubes), on nanocrystals and layered nanocrystals.
Теги: carbon nanotubes chirality indices low dimensional nanostructures nanocrystals nanotube defect engineering quantum dots индексы хиральности инженерия дефектов в нанотрубках квантовые точки нанокристаллы низкоразмерные наноструктуры углеродные нанотрубки
Single Photon Sources. A Review. Part 3
V. G. Krishtop
Institute of Microelectronics Technology and High Purity Materials RAS, Chernogolovka, Moscow region, Russia.
JSC “InfoTeСS”, Moscow.
Moscow Institute of Physics and Technology, Dolgoprudny, Moscow Region, Russia.
The article continues the review of single photon sources while considering various methods for the single photon sources (SPS) development. Earlier, the first part of the review (Photonics Russia. 2024; 18(5): 376–396) discussed the requirements for single-photon sources and their characterization criteria, described the single-ion and single-atom-based single-photon sources. The SPSs based on the quantum dots and color centers in the crystals were considered in the second part of the review (Photonics Russia. 2024; 18(8): 610–620). The third part considers the single-photon sources based on the carbon nanotubes and their defects (defect engineering in the nanotubes), on nanocrystals and layered nanocrystals.
Keywords: quantum dots, carbon nanotubes, nanotube defect engineering, chirality indices, nanocrystals, low dimensional nanostructures
Article received: August 01, 2024
Article accepted: August 16, 2024
Carbon nanotubes (CNT)
Carbon nanotubes can be used as the single photon sources [140]. The carbon nanotubes are one or more layers of graphene rolled into a cylinder. Despite their apparent simplicity, the nanotubes represent an entire class of various nanoobjects with a wide variety of properties. Due to their unique structure, they simultaneously demonstrate the properties of one-dimensional, two-dimensional, and three-dimensional materials. For example, carbon nanotubes have demonstrated superconductivity and the memristor effect. Despite the fact that nanotubes have been theoretically predicted and observed experimentally quite a long time ago, the diversity of nanotubes with various geometries and modifications continues to expand. Their properties are still being studied, and new horizons are opening up along this path.
The carbon nanotubes can be single-walled or multi-walled, open or closed, their diameter and length can vary widely (Fig. 13). The diameter of nanotubes can be from one to several tens of nanometers, and their length reaches several microns.
The single-walled carbon nanotubes (SWCNT) have such a parameter as chirality (Fig. 14) that can be simply represented as an angle relative to the crystallographic axes, at which a section of graphene is “cut” from the plane. This nanotube is rolled into a cylinder from such section. In general, the nanotubes, like single-layer graphene, show metallic conductivity. However, the single-walled nanotubes with the certain chirality indices demonstrate the semiconducting properties.
For the single-photon sources, the single-walled semiconductor carbon nanotubes are applied. In such nanotubes, the excitons are formed when exposed to the external radiation or electric current. An exciton is an uncharged hydrogen-like quasiparticle consisting of an electron and a hole bound together. The excitons in the semiconductor carbon nanotubes are recombined with the photon emission, thus, the semiconductor nanotubes under the influence of optical or electrical excitation emit photons in the visible and infrared ranges [144], i. e. they show the photoluminescence and electroluminescence properties.
To develop a single photon source, it is necessary to introduce a special defect into the semiconductor single-walled nanotube that will promote the exciton recombination near this defect. In this case, if the exciton size and the nanotube diameter are comparable, then near the defect (in the area of stimulated recombination) there is recombination of exactly one exciton and emission of exactly one photon. The exciton lifetime in the defect-free areas of a nanotube is significantly higher, and the spontaneous recombination probability is significantly lower. Another way is to arrange an exciton trap in a nanotube. Having fallen into such a trap, the exciton becomes localized, after which it only has to be successfully recombined at the end of its lifetime.
In addition, the excitons in nanotubes (similar to the incident reactants), can bind in pairs and generate an exciton molecule that also emits one photon upon recombination. Recombination with a single photon emission during the annihilation of two excitons moving along a nanotube towards each other [145] and an exciton annihilation (recombination) at a zero-dimentional state in a nanotube [146] (Fig. 15) have also been demonstrated.
A wide variety of items can be applied as the optically active defects that force the excitons to recombine. The invention of such items and production of nanotubes containing these items have even received a special name: defect engineering of carbon nanotubes [147].
Defects in the carbon nanotube structure are divided into several categories: topological defects; defects related to the carbon atom rehybridization; and defects of unsaturated (dangling) bonds.
The topological defects of the crystal structure can be extremely diverse, for example, formation of a pentagon and a heptagon in a hexagonal structure (Stone-Wales defect); elbow defects that lead to the nanotube bending (for example, formation of a heptagon-pentagon pair on various nanotube sides opposite to each other); a ring shearing defect similar to the dislocation in three-dimensional crystals, rolled into a ring (bamboo-like bridge); or narrowing of the nanotube (Fig. 16).
The crystalline defect related to the hybridization of the electron orbitals of carbon is rather popular for extracting single photons from the single-walled nanotubes [151, 152]. The fact is that the electron carbon orbitals can be hybridized in different ways (Fig. 17). The carbon atoms available in the regular hexagonal lattice of graphene have sp2‑hybridization. However, with the help of special chemical reactions it is possible to attach to a single atom in the crystalline lattice, and then the hybridization of this single atom in the lattice will be changed to sp3. In this case, the sp3‑hybridized carbon atom will develop a non-uniformity in the ideal lattice of sp2 atoms. Therefore, a stoichiometrically regular nanotube will have a non-uniformity in the electron density distribution.
Another class of defects includes the defects of unsaturated (dangling) bonds (Fig. 18). Such defects are formed in the large quantities during thermal treatment of the nanotubes at high temperatures. For example, Fig. 18 shows some pyrolytic defects during the nanotube annealing process in air at a temperature of 600–700 degrees. Along with nitrogen and oxygen, the nanotubes can be doped with fluorine.
An important quality of single-photon sources based on the carbon nanotubes is the possible single-photon emission at a room temperature [153–157]. It is also interesting that the emission frequency depends on the nanotube diameter that makes it possible to set the source frequency in a wide range of values.
The possibilities listed above are far from exhaustive. The so-called nanotube functionalization methods allow the development of chemically modified nanotubes in a variety of ways. The nanotubes can be doped with the impurity atoms. Additional atoms and radicals can be attached to the outer surface of nanotubes. The inner part of the nanotubes can contain the chains of single atoms or organic molecules. The peapods, namely the nanotubes stuffed with the fullerenes like a pea pod, are currently being actively studied. The nanotubes of various diameters and chiralities can be cross-linked to each other at different angles (for example, semiconductor and metallic nanotubes). They can be branched out in all sorts of ways (the Y-type, H-type and X-type nanotubes) or rolled into a torus. The nanotubes can be decorated with fullerenes like the nut leaf galls. The DNA strands (polynucleotides) synthesized with a regular order of nucleotides and wrapped around the nanotube can be used as the contacts for the nanotube.
Finally, the nanotubes may not only be the carbon ones. The nanotubes made of boron nitride, boron carbide, silicon carbide, and carbon nitride; of transition metal oxides and dichalcogenides; of zinc oxide that is a high-quality piezoelectric, have been obtained. Moreover, there are the multilayer carbon nanotubes coated with zinc oxide; gallium nitride and selenide; molybdenum disulfide and tungsten. There is also a Prince technology for rolling up the strained epitaxial heterostructures into a tube, for example, AlGaAs/GaAs, GaAs/InGaAs, Si/SiGe (named after the Russian scientist V. Ya. Prince).
All these methods make it possible to obtain new items and materials with the advanced and as yet experimentally unstudied properties. In principle, the similar construction principles of single-photon sources can be applicable to them.
Nanocrystals, low dimensional nanostructures
Various nanostructures and nanoobjects are very promising for use as a single-photon source, since when moving to the nanoscale in a low-dimensional structure (two-dimensional or one-dimensional, or even zero-dimensional), as well as in the quantum dot, the dimensional quantization levels occur that can be controlled and operated in a quantum mode. At present, an enormous number of various nanostructures have been invented for arrangement of a single-photon emission mode. Moreover, there is not even a hope to review them in any complete way in an article.
Let us give a couple of examples. Fig. 19 shows an optical resonator cavity in the form of an erbium-doped waveguide with a regular structure of nanometer holes. The erbium atoms in a needle-like silicon crystal emit light with a wavelength of 1 536 nm that corresponds to the short edge of the standard C-band in the fiber-optic communication.
Fig. 20 shows a gallium nitride nanowire with an indium gallium nitride quantum dot. This nanowire implements a single-photon emission mode with the linear polarization and electrical pumping [160].
Layered nanocrystals
The core-shell semiconducting nanocrystals (CSSNCs) represent a new class of materials that demonstrates the properties being intermediate between those of small individual molecules and those of bulk crystalline semiconductors. These nanocrystals consist of a semiconductor core (a quantum dot) and a shell made of a separate semiconductor material. The core and shell are typically composed of II–VI, IV–VI, and III–V type semiconductors with the configurations such as CdS/ZnS, CdSe/ZnS, CdSe/CdS, and InAs/CdSe (typical designation: core/shell) [161,162]. The organically passivated quantum dots have low fluorescence quantum yields due to the surface-bound traps [163]. The CSSNCs address this issue since the shell increases the quantum yield by passivating the surface states [163]. Moreover, the shell provides protection against the environmental changes, photooxidative degradation and provides another route to modularity [163, 164]. Precise control of the dimensions, shape, and composition of both core and shell allows the emission wavelength to be set in a wider range of wavelengths than is possible with any single semiconductor.
The relative simplicity of obtaining CSSNC nanocrystals by purely chemical methods in the colloidal solutions, without any application of expensive microelectronic process units, is rather attractive. Another interesting possibility is the simplicity of nanocrystal application to the prepared microchip surface in the form of a suspension, and the ability of colloidal nanocrystals to be self-organized into a monolayer.
More subtle possibilities for controlling the radiation/emission stability and frequency are provided by the three-layer nanocrystals where a spacer (buffer) layer of a specially selected material is introduced between the core and the shell (fig. 21). For example, the quantum dots represent a core made of cadmium selenide, an interlayer of mercury sulfide in the shell of cadmium sulfide: CdSe/HgS/CdS. The single-photon emission at a room temperature has already been demonstrated in such systems (Fig. 21) [165].
The organic compounds of such multilayer nanocrystals and special organic radicals containing the rare earth ions are also rather interesting. For example, a hybrid material consisting of three-layer nanocrystals CdSe/CdS/ZnS, surrounded by a shell made of a specially selected comprehensive compound of rare earth neodymium ion with organic molecules has been developed. The organic strapping is used as a kind of accumulator for pumping energy and a core excitation catalyst [166].
In the remaining part of the review, the single-photon sources on the collective states in the ensemble systems, on the single molecules, on the metal ions in a polymer matrix, as well as the sources on nonlinear crystals will be considered.
ABOUT AUTHOR
V. G. Krishtop: e-mail: vladimir.krishtop@infotecs.ru.
ORCID: 0000-0001-6063-2657
V. G. Krishtop
Institute of Microelectronics Technology and High Purity Materials RAS, Chernogolovka, Moscow region, Russia.
JSC “InfoTeСS”, Moscow.
Moscow Institute of Physics and Technology, Dolgoprudny, Moscow Region, Russia.
The article continues the review of single photon sources while considering various methods for the single photon sources (SPS) development. Earlier, the first part of the review (Photonics Russia. 2024; 18(5): 376–396) discussed the requirements for single-photon sources and their characterization criteria, described the single-ion and single-atom-based single-photon sources. The SPSs based on the quantum dots and color centers in the crystals were considered in the second part of the review (Photonics Russia. 2024; 18(8): 610–620). The third part considers the single-photon sources based on the carbon nanotubes and their defects (defect engineering in the nanotubes), on nanocrystals and layered nanocrystals.
Keywords: quantum dots, carbon nanotubes, nanotube defect engineering, chirality indices, nanocrystals, low dimensional nanostructures
Article received: August 01, 2024
Article accepted: August 16, 2024
Carbon nanotubes (CNT)
Carbon nanotubes can be used as the single photon sources [140]. The carbon nanotubes are one or more layers of graphene rolled into a cylinder. Despite their apparent simplicity, the nanotubes represent an entire class of various nanoobjects with a wide variety of properties. Due to their unique structure, they simultaneously demonstrate the properties of one-dimensional, two-dimensional, and three-dimensional materials. For example, carbon nanotubes have demonstrated superconductivity and the memristor effect. Despite the fact that nanotubes have been theoretically predicted and observed experimentally quite a long time ago, the diversity of nanotubes with various geometries and modifications continues to expand. Their properties are still being studied, and new horizons are opening up along this path.
The carbon nanotubes can be single-walled or multi-walled, open or closed, their diameter and length can vary widely (Fig. 13). The diameter of nanotubes can be from one to several tens of nanometers, and their length reaches several microns.
The single-walled carbon nanotubes (SWCNT) have such a parameter as chirality (Fig. 14) that can be simply represented as an angle relative to the crystallographic axes, at which a section of graphene is “cut” from the plane. This nanotube is rolled into a cylinder from such section. In general, the nanotubes, like single-layer graphene, show metallic conductivity. However, the single-walled nanotubes with the certain chirality indices demonstrate the semiconducting properties.
For the single-photon sources, the single-walled semiconductor carbon nanotubes are applied. In such nanotubes, the excitons are formed when exposed to the external radiation or electric current. An exciton is an uncharged hydrogen-like quasiparticle consisting of an electron and a hole bound together. The excitons in the semiconductor carbon nanotubes are recombined with the photon emission, thus, the semiconductor nanotubes under the influence of optical or electrical excitation emit photons in the visible and infrared ranges [144], i. e. they show the photoluminescence and electroluminescence properties.
To develop a single photon source, it is necessary to introduce a special defect into the semiconductor single-walled nanotube that will promote the exciton recombination near this defect. In this case, if the exciton size and the nanotube diameter are comparable, then near the defect (in the area of stimulated recombination) there is recombination of exactly one exciton and emission of exactly one photon. The exciton lifetime in the defect-free areas of a nanotube is significantly higher, and the spontaneous recombination probability is significantly lower. Another way is to arrange an exciton trap in a nanotube. Having fallen into such a trap, the exciton becomes localized, after which it only has to be successfully recombined at the end of its lifetime.
In addition, the excitons in nanotubes (similar to the incident reactants), can bind in pairs and generate an exciton molecule that also emits one photon upon recombination. Recombination with a single photon emission during the annihilation of two excitons moving along a nanotube towards each other [145] and an exciton annihilation (recombination) at a zero-dimentional state in a nanotube [146] (Fig. 15) have also been demonstrated.
A wide variety of items can be applied as the optically active defects that force the excitons to recombine. The invention of such items and production of nanotubes containing these items have even received a special name: defect engineering of carbon nanotubes [147].
Defects in the carbon nanotube structure are divided into several categories: topological defects; defects related to the carbon atom rehybridization; and defects of unsaturated (dangling) bonds.
The topological defects of the crystal structure can be extremely diverse, for example, formation of a pentagon and a heptagon in a hexagonal structure (Stone-Wales defect); elbow defects that lead to the nanotube bending (for example, formation of a heptagon-pentagon pair on various nanotube sides opposite to each other); a ring shearing defect similar to the dislocation in three-dimensional crystals, rolled into a ring (bamboo-like bridge); or narrowing of the nanotube (Fig. 16).
The crystalline defect related to the hybridization of the electron orbitals of carbon is rather popular for extracting single photons from the single-walled nanotubes [151, 152]. The fact is that the electron carbon orbitals can be hybridized in different ways (Fig. 17). The carbon atoms available in the regular hexagonal lattice of graphene have sp2‑hybridization. However, with the help of special chemical reactions it is possible to attach to a single atom in the crystalline lattice, and then the hybridization of this single atom in the lattice will be changed to sp3. In this case, the sp3‑hybridized carbon atom will develop a non-uniformity in the ideal lattice of sp2 atoms. Therefore, a stoichiometrically regular nanotube will have a non-uniformity in the electron density distribution.
Another class of defects includes the defects of unsaturated (dangling) bonds (Fig. 18). Such defects are formed in the large quantities during thermal treatment of the nanotubes at high temperatures. For example, Fig. 18 shows some pyrolytic defects during the nanotube annealing process in air at a temperature of 600–700 degrees. Along with nitrogen and oxygen, the nanotubes can be doped with fluorine.
An important quality of single-photon sources based on the carbon nanotubes is the possible single-photon emission at a room temperature [153–157]. It is also interesting that the emission frequency depends on the nanotube diameter that makes it possible to set the source frequency in a wide range of values.
The possibilities listed above are far from exhaustive. The so-called nanotube functionalization methods allow the development of chemically modified nanotubes in a variety of ways. The nanotubes can be doped with the impurity atoms. Additional atoms and radicals can be attached to the outer surface of nanotubes. The inner part of the nanotubes can contain the chains of single atoms or organic molecules. The peapods, namely the nanotubes stuffed with the fullerenes like a pea pod, are currently being actively studied. The nanotubes of various diameters and chiralities can be cross-linked to each other at different angles (for example, semiconductor and metallic nanotubes). They can be branched out in all sorts of ways (the Y-type, H-type and X-type nanotubes) or rolled into a torus. The nanotubes can be decorated with fullerenes like the nut leaf galls. The DNA strands (polynucleotides) synthesized with a regular order of nucleotides and wrapped around the nanotube can be used as the contacts for the nanotube.
Finally, the nanotubes may not only be the carbon ones. The nanotubes made of boron nitride, boron carbide, silicon carbide, and carbon nitride; of transition metal oxides and dichalcogenides; of zinc oxide that is a high-quality piezoelectric, have been obtained. Moreover, there are the multilayer carbon nanotubes coated with zinc oxide; gallium nitride and selenide; molybdenum disulfide and tungsten. There is also a Prince technology for rolling up the strained epitaxial heterostructures into a tube, for example, AlGaAs/GaAs, GaAs/InGaAs, Si/SiGe (named after the Russian scientist V. Ya. Prince).
All these methods make it possible to obtain new items and materials with the advanced and as yet experimentally unstudied properties. In principle, the similar construction principles of single-photon sources can be applicable to them.
Nanocrystals, low dimensional nanostructures
Various nanostructures and nanoobjects are very promising for use as a single-photon source, since when moving to the nanoscale in a low-dimensional structure (two-dimensional or one-dimensional, or even zero-dimensional), as well as in the quantum dot, the dimensional quantization levels occur that can be controlled and operated in a quantum mode. At present, an enormous number of various nanostructures have been invented for arrangement of a single-photon emission mode. Moreover, there is not even a hope to review them in any complete way in an article.
Let us give a couple of examples. Fig. 19 shows an optical resonator cavity in the form of an erbium-doped waveguide with a regular structure of nanometer holes. The erbium atoms in a needle-like silicon crystal emit light with a wavelength of 1 536 nm that corresponds to the short edge of the standard C-band in the fiber-optic communication.
Fig. 20 shows a gallium nitride nanowire with an indium gallium nitride quantum dot. This nanowire implements a single-photon emission mode with the linear polarization and electrical pumping [160].
Layered nanocrystals
The core-shell semiconducting nanocrystals (CSSNCs) represent a new class of materials that demonstrates the properties being intermediate between those of small individual molecules and those of bulk crystalline semiconductors. These nanocrystals consist of a semiconductor core (a quantum dot) and a shell made of a separate semiconductor material. The core and shell are typically composed of II–VI, IV–VI, and III–V type semiconductors with the configurations such as CdS/ZnS, CdSe/ZnS, CdSe/CdS, and InAs/CdSe (typical designation: core/shell) [161,162]. The organically passivated quantum dots have low fluorescence quantum yields due to the surface-bound traps [163]. The CSSNCs address this issue since the shell increases the quantum yield by passivating the surface states [163]. Moreover, the shell provides protection against the environmental changes, photooxidative degradation and provides another route to modularity [163, 164]. Precise control of the dimensions, shape, and composition of both core and shell allows the emission wavelength to be set in a wider range of wavelengths than is possible with any single semiconductor.
The relative simplicity of obtaining CSSNC nanocrystals by purely chemical methods in the colloidal solutions, without any application of expensive microelectronic process units, is rather attractive. Another interesting possibility is the simplicity of nanocrystal application to the prepared microchip surface in the form of a suspension, and the ability of colloidal nanocrystals to be self-organized into a monolayer.
More subtle possibilities for controlling the radiation/emission stability and frequency are provided by the three-layer nanocrystals where a spacer (buffer) layer of a specially selected material is introduced between the core and the shell (fig. 21). For example, the quantum dots represent a core made of cadmium selenide, an interlayer of mercury sulfide in the shell of cadmium sulfide: CdSe/HgS/CdS. The single-photon emission at a room temperature has already been demonstrated in such systems (Fig. 21) [165].
The organic compounds of such multilayer nanocrystals and special organic radicals containing the rare earth ions are also rather interesting. For example, a hybrid material consisting of three-layer nanocrystals CdSe/CdS/ZnS, surrounded by a shell made of a specially selected comprehensive compound of rare earth neodymium ion with organic molecules has been developed. The organic strapping is used as a kind of accumulator for pumping energy and a core excitation catalyst [166].
In the remaining part of the review, the single-photon sources on the collective states in the ensemble systems, on the single molecules, on the metal ions in a polymer matrix, as well as the sources on nonlinear crystals will be considered.
ABOUT AUTHOR
V. G. Krishtop: e-mail: vladimir.krishtop@infotecs.ru.
ORCID: 0000-0001-6063-2657
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