rus
Issue #2/2022
A. I. Arzhanov, A. O. Savostianov, K. A. Magaryan, K. R. Karimullin, A. V. Naumov
Photonics of Semiconductor Quantum Dots: Applied Aspects
Photonics of Semiconductor Quantum Dots: Applied Aspects
DOI: 10.22184/1993-7296.FRos.2022.16.2.96.112
Semiconductor nanocrystals (quantum dots, QDs) have unique photophysical properties, which opens up wide possibilities for their applications in the methods and tools of modern photonics. This article discusses possible applications of QDs. Both existing devices and prospects for the development of new methods and photonics devices are discussed. Innovative approaches to the use of quantum dots in various areas of modern photonic technologies are considered: optoelectronics, biophysics, quantum optics, sensorics, photovoltaics.
Semiconductor nanocrystals (quantum dots, QDs) have unique photophysical properties, which opens up wide possibilities for their applications in the methods and tools of modern photonics. This article discusses possible applications of QDs. Both existing devices and prospects for the development of new methods and photonics devices are discussed. Innovative approaches to the use of quantum dots in various areas of modern photonic technologies are considered: optoelectronics, biophysics, quantum optics, sensorics, photovoltaics.
Теги: biolabels fluorescence nanoscopy integrated photonics optical switches photovoltaics quantum dot detectors quantum dot lasers quantum dot leds quantum dots quantum size effect targeted drug delivery адресная доставка лекарств биомаркеры детекторы на квантовых точках интегральная фотоника квантово-размерный эффект квантовые точки лазеры на квантовых точках оптические переключатели светодиоды на квантовых точках флуоресцентная наноскопия фотовольтаика
Photonics of Semiconductor Quantum Dots: Applied Aspects
A. I. Arzhanov, A. O. Savostianov, K. A. Magaryan, K. R. Karimullin, A. V. Naumov
Moscow State Pedagogical University, Moscow, Russia
Institute of Spectroscopy RAS, Troitsk, Moscow, Russia
Lebedev Physical Institute, RAS, Troitsk Branch, Troitsk, Moscow, Russia
Semiconductor nanocrystals (quantum dots, QDs) have unique photophysical properties, which opens up wide possibilities for their applications in the methods and tools of modern photonics. This article discusses possible applications of QDs. Both existing devices and prospects for the development of new methods and photonics devices are discussed. Innovative approaches to the use of quantum dots in various areas of modern photonic technologies are considered: optoelectronics, biophysics, quantum optics, sensorics, photovoltaics.
Keywords: quantum size effect, quantum dots, quantum dot LEDs, integrated photonics, quantum dot lasers, optical switches, quantum dot detectors, photovoltaics, fluorescence nanoscopy, biolabels, targeted drug delivery
Received on: 10.01.2022
Accepted on: 24.01.2022
INTRODUCTION
Semiconductor nanocrystals (quantum dots, QDs) have unique photophysical properties that depend on their morphology and chemical composition, as well as thermodynamic parameters and environmental properties (electromagnetic field, pressure, acidity, humidity, chemical composition, etc.) (1). The fundamental regularities found in the studies (these issues were covered in detail in the first part of this review in (1)), which determine these relationships, make it possible to carry out the engineering and controlled synthesis of QDs and nanocomposites based on them with desired properties. Thus, QDs are increasingly being used in various branches of science and technology, and, first of all, in various applications of photonics. Conventionally, there are several main areas of application of QDs (see the diagram in Fig. 1):
Light sources (including light-emitting diodes, laser sources, light converters, non-classical light sources).
Displays and multi-component screens.
Luminescent labels – nanotags for various applications, including nanodiagnostics of condensed matter, medical diagnostics and theranostics, targeted drug delivery.
New nanomaterials for photonics, including quasi-two-dimensional nanostructures, nature-like materials (e. g., neuromorphic structures).
Light converters, including the basic elements of nonlinear optics and spectroscopy, light switches (optical transistors), detectors for various spectral range.
Basic elements of quantum technologies, including non-classical light sources, quantum memory and quantum sensors.
Basic elements of solar energy (photovoltaic cells).
In this article, these applications of QDs will be considered in detail, regarding to the fundamental characteristics of new materials described in our recent paper (1).
Light sources and LED
QDs can be used as bright light emitters as an alternative to organic phosphors (2). Due to their high quantum yield, LEDs based on QDs (QLEDs) are more efficient than organic compounds (3). Organic phosphors are characterized by a wide emission spectrum, what limits the ability of manufacturers to fine-tune the color temperature of a light source by mixing emission of several phosphors (e. g., blue and yellow). The spectral properties of QD-based phosphors can be controlled due to the quantum size effect by selecting the luminescence spectrum using a given size distribution of QDs. Such an emitter can be excited with a conventional blue LED due to the presence of a wide absorption spectrum.
QLEDs are, becoming the basis of a technology that has come to replace the LC displays. QDs used as radiation sources in QLED technology make it possible to create displays with high brightness and enhanced color rendering. The development of a new generation of displays (flexible, transparent) based on QDs for wearable wireless electronics is already underway (4).
The possibilities of creating wearable devices with displays based on QDs arrays for displaying various biofunctional parameters of living organisms (e. g., remote medical diagnostics) have also been demonstrated. QDs arrays grown on an elastomeric surface can adhere to the human skin and exhibit stretchable properties and low power consumption. This device is capable of visually displaying body movement and skin temperature signals from peripheral sensors (5).
The main disadvantage that limits the wide introduction of this technology is the fact that the QD array re-radiates light from an external excitation source (an array of LEDs), and is not an independent source of radiation. Currently, there is an active search for ways to create individual RGB pixels based on electroluminescent semiconductor nanocrystals, but today the technology is limited by using nanoparticles as radiation converters (6).
Light converters and detectors
Let’s discuss the possibilities of radiation conversion using QDs as detectors. At present, elements based on silicon, or other semiconductor heterostructures, are most widely used in various photodetectors and matrix sensors operating in the visible spectral range. However, such detectors have a limited operating spectral range from 400 to 1 000 nm. Extending the range to the near (1.3–1.5 μm) and middle (20–200 μm) IR regions can become possible due to covering photoactive Si surface with QDs (7). Interest in promoting highly sensitive detector technology in the IR range is associated with the rapid development of optoelectronic technologies and telecommunications, appearance of the new challenges of quantum optics and non-invasive medical diagnostics.
The development of such detectors is still underway, in particular, it is possible to create light converters not only using epitaxial QDs in nanoheterostructures (8), but also using colloidal QDs (9). It was shown, that the deposition of colloidal QDs on the silicon surface makes it possible to reduce the transparency window of the semiconductor in the IR range due to the creation of impurity states in the bandgap of silicon. Taking into account the fact that colloidal QDs are quite simple to manufacture and have wide possibilities for controlling spectral properties, it can be assumed that research in this direction can give a powerful stimulus to the improvement of silicon optoelectronic devices.
Significant progress in the development of light detectors has been achieved using colloidal QDs PbS, PbSe, Bi2S3, In2S3 (10), that are used in flexible electronics and CMOS technologies.
Being easily combined with various metamaterials and heterostructures, QDs make it possible to proceed to the design of a new generation integrated photonics devices. Thus, in (11), the possibility of creating a highly sensitive camera for the mid-IR range based on an array of plasmonic structures integrated with a system of QDs included in quantum wells was demonstrated.
QDs-based structures can also be used for more complex conversion of light fluxes. Thus, in (12), it was proposed to use semiconductor CdSe / CdS / ZnS QDs exciton levels for writing and reading two-quantum transient holograms. In turn, the nanometer dimensions of QDs allow to use it in photonic and optoelectronic converters of integrated optics. For example, in (13), the effect of plasmon switching was considered for surface plasmon polaritons in a graphene waveguide integrated with a pin nanocavity with QDs. The detected effect potentially provides the ability to control the light fluxes of the IR range, localized in a device with a size of 20 nm.
Sensor technologies
The high sensitivity of QD optical parameters to the characteristics of the external environment opens up the possibility of developing various QD based sensor devices. For example, the selective change in the physical and chemical properties of composites with QDs upon contact with various substances is the basis of gas sensors (e. g. NO2) (14).
The temperature dependence of the spectral characteristics of QDs can become the basis for temperature sensors (15, 16) and nanosensors (17, 18).
The creation of composites with QDs provides a significant increase in the efficiency of photocatalytic systems (19).
The conjugation of QDs with complex nanocomposites enables highly sensitive detection of other physicochemical parameters of media; e. g., it was proposed in (20) to use a polymer–QD–graphene oxide composite as a highly sensitive pH sensor.
Another example of QD usage as sensors of the material characteristics of a medium is associated with local field effects, which manifest themselves in the dependence of the lifetime of the excited state of a QD on the value of the refractive index of the medium (permittivity, magnetic susceptibility) (21, 22). In this regard, luminescent QDs can be used in the original micro-refractometry technique to determine the refractive index of a medium and map its fluctuations, including on the sub-micrometer scale (23, 24).
Solar energy (photovoltaics)
In recent years, solar energy technologies have been actively developing all over the world. Recently, silicon has been common material for solar panels, as the most efficient radiation converters used in solar energy have been developed and commercialized. According to the National Renewable Energy Laboratory (NREL) (see Fig. 3), silicon photovoltaic cells have a fairly high energy conversion efficiency (up to 27.6%) (25). Engineering the same multi-junction photovoltaic cells provides an increase in conversion efficiency up to 47%.
At the same time, a significant problem of silicon optoelectronics should be noted – the high cost and complexity of production.
To date, there has been an intensive growth in the total capacity of electricity generation using solar power plants (see Fig. 4). At the same time, the growing capacities of the resource-intensive and energy-consuming semiconductor production (silicon photovoltaic cells) lead to climate change on the planet, in connection with which it the problem of finding new materials for the manufacture of solar cells becomes acute.
Increasingly, semiconductor nanocrystals – QDs and composites (including hybrid ones) based on them act as the basis for light-converting elements (photovoltaic cells). Due to the wide absorption spectrum, QDs can effectively absorb from the UV to the far IR spectral range, which makes them promising for use in solar energy. QDs already occupy a certain niche in this area, and every year the efficiency of energy conversion is growing. According to (26), an efficiency value of 16.6% has been achieved for colloidal perovskite QDs. The properties of colloidal solutions of nanocrystals make it possible to use them for the manufacture of exotic photovoltaic materials (e. g., flexible solar cells) (27). To date, there are many methods for manufacturing solar panels from colloidal solutions, and the roll-to-roll (R2R) technology which is among them should be highlighted (28). The essence of the method is the deposition of colloidal solutions by blade coating, spraying or injection application on a flexible base, followed by baking to obtain a homogeneous layer of a photoactive substance. Thanks to this technology, it is possible to create ultrathin solar cells, and also to reduce the cost of production of photovoltaic cells based on colloidal QDs significantly.
Quantum dot lasers
In the past six decades since the invention of the laser, semiconductor materials have proven to be excellent laser media. After significant progress in the synthesis of colloidal QDs in the mid‑1990s, the question of the prospects for their application for the manufacture of highly efficient, but at the same time extremely compact laser media, became especially acute. The main advantages of nanocrystals over traditional bulk materials are, associated with size quantization effects, which make it possible to achieve a sufficiently low lasing threshold with high thermal stability and the ability to adjust the radiation wavelength using QDs of various sizes.
Research into the study of laser generation in systems based on QDs was started quite a long time ago, and in the late 1990s prototypes of such lasers came to light (29, 30). It turned out, however, that achieving generation in semiconductor nanocrystals is a very difficult challenge. To create a population inversion in a QD, it is necessary to excite at least a pair of excitons. It wouldn’t seem that difficult to achieve by exciting the QDs with high-power femtosecond laser pulses, however, as it was found, excitons in QDs with a size of the order of several nm can exchange energy quite quickly (the so-called Auger recombination), which prevents the creation of a population inversion necessary to achieve laser radiation.
This difficulty was first overcome in the work of the V. Klimov scientific group (31), where the possibility of obtaining population inversion in close-packed layers of CdSe nanocrystals with the size of 1.2 nm was demonstrated for the first time. The characteristic times of the nonradiative transition between the excited level and required to populate one, measured for these QDs, amounted to impressive hundreds of femtoseconds for both electrons and holes, which is an order of magnitude shorter than the Auger recombination times (~6 ps for CdSe QDs of the indicated radius).
Subsequently, a method was proposed for implementing laser generation in QDs without excitation of multiexcitons (single-exciton optical gain in semiconductor nanocrystals). The material and diameter of the QD (CdS / ZnSe) were chosen so that after excitation by a laser pulse, exciton spatial separation is carried out in such a way: the electron turned out to be localized in the core of the QD, whereas the hole moved to its shell. The resulting local electric field is strong enough to significantly (~100 meV) change the energy of the exciton produced during the following excitation process. As a result, in QDs excited by successive pulses, a shift of absorption bands is observed, which, in turn, makes it possible to create a population inversion in them. Understanding the nature of the processes has opened the way to the creation of low-threshold QD lasers, the implementation of optical amplification by electrical injection, and the development of optically pumped lasers, a standard light-emitting diode with electrical excitation, and on-chip lasers (32–34).
Efficient lasing can be achieved in structures with distributed Bragg gratings on quantum wells, dots, and with quantum cascades (35). Integration of InAs QDs into a planar GaAs semiconductor nanostructure waveguide coupled to an electrically pumped semiconductor laser based on AlAs / AlGaAs microcavity with GaAs quantum wells in the active region makes it possible to realize circularly polarized radiation (36).
Optical switches
An actual task of using QD is the development of new photonic logic elements and the engineering of computational architectures. Optically active nanoobjects come to replace the basic logical elements of electronics – transistors. Due to their scalability and quantum behavior, they are of great interest in the creation of photonic signal processing devices (37). Numerous nanoscale logic devices have been developed to date, operating on systems of trapped ions (38), single atoms (39), nonlinear materials (40), single molecules (41), plasmonic nanoparticles (42), vacancy color centers in diamonds (43), and others. Semiconductor colloidal QDs, which are sources of single photons, can also be used as an active material for such systems.
Fully functioning logic was created on single semiconductor QDs which consisted of high-precision quantum gates (44), photonic switches (45), and repeaters (46). By analogy with transistors, optical logic elements can be used to design and build all-optical integrated circuits, which will greatly affect the progress of quantum photonic technologies soon.
Fluorescence nanoscopy, biolabels and targeted drug delivery
QDs due to their unique physical and chemical characteristics are used as efficient fluorescent labels, for drug delivery, monitoring of the process of drug metabolism in the body. Other biomedical applications of QDs include developing sensitive sensors for disease detection and performing fluorescent assays for drug design (47–49).
In recent years, methods of fluorescent nanoscopy (microscopy of superhigh spatial resolution) have been actively developed, where QDs can act as point-like emitters (labels) (50, 51). Nanoscopy methods are also developing in the direction of three-dimensional visualization of the nanostructure of the object under study, tracking of nanoparticles (52).
Since the late 1990s number of different applications of QDs in biology and medicine is growing exponentially. Semiconductor nanoparticles are actively used as fluorescent labels for bioanalytical purposes, such as detection of DNA, proteins, biomolecules, and cells. For analysis of binding or energy transfer from biological structures (e. g., proteins), Förster resonant energy transfer (FRET) mechanisms are used (53). To implement the FRET mechanism, a biological object under study (e. g., a protein molecule) is used as a donor, and QDs can be used as an acceptor. For these purposes, a QD surface is functionalized with specially grown ligands capable of attaching to a specific complex molecule, and the distance from the center of a nanocrystal to a molecule should be from 1 to 10 nm. When a donor-acceptor pair is formed due to the dipole-dipole interaction, energy is transferred without intermediate emission of a photon. This means that the transfer process is non-radiative, and the energy will be transferred to the acceptor (e. g., a QD), which will subsequently lead to creation of a QD luminescence photon. The efficiency of energy transfer is higher not only due to the spatial proximity of two objects (the FRET efficiency depends on the distance as R6), but also due to overlap of the donor excitation spectrum and the acceptor absorption spectrum. QDs are very promising acceptors due to the wide absorption spectrum in the blue region of the spectrum, appropriate for many biological objects.
Another opportunity to use QDs for biological purposes is based on bactericidal properties of silver-based nanocrystals. The ability of silver ions to block action of various types of viruses, as well as to have a therapeutic effect in the treatment of a numerous viral diseases, is well known. In this regard, the development of methods for QDs synthesis using silver, as well as hybrid associates based on them (54), will make it possible to create drugs with antimicrobial activity in the future.
New materials of photonics
One of the most interesting new materials are carbon-based QDs, including so-called graphene nanocrystals (55). Graphene QDs should not be confused with fullerenes – spheres of carbon atoms. Typically, such nanoobjects are synthesized from fragments of a graphene ribbon or nanotube (56) which makes them look more like flakes. Due to the natural chemical inertness and amphiphilicity graphene QDs are potentially applicable in various biological applications. Like colloidal semiconductor nanocrystals Graphene QDs exhibit band gap dependent fluorescence, which arises due to the quantum confinement effect. Together, these features make graphene QDs excellent candidates for the role of biocompatible markers.
Another interesting quantum-well object is the completely inorganic perovskite QD (PeQD) which has opened up great prospects for the development of light-emitting devices without toxic substances (cadmium). Such nanocrystals have similar properties to semiconductor QDs and have an adjustable bandgap throughout the entire visible spectrum. Also, they demonstrate ultra-high effective quantum yields (93%) (57). Due to their photophysical properties, perovskite QDs have found applying in various optical applications: optical memory cells (58), photodetectors (59), solar cells (60), lasers (61), LEDs (62).
Quantum dots in quantum technologies
Single-Photon Sources
High-performance sources of single photons are key elements for the implementation, for instance, of quantum communication lines (63). Along with single atoms, organic molecules, color centers in diamonds, etc., semiconductor QDs can be used as such controlled sources (64). The main method for manufacturing sources of single-photon emission based on them is molecular epitaxy, which makes it possible to place QDs on substrates with high accuracy, as well as inside optical waveguides and microcavities (65, 66, 67). The other innovative methods of controlled synthesis of such structures, for example, direct laser writing (68), also exist.
A wide choice of semiconductor materials makes it possible to obtain light sources with different emission wavelengths: QDs based on GaN / AlGaN can obtain generation in the near UV range (69, 70) (~280–330 nm), CdSe / ZnSe (71) and CdSe / ZnSSe (72) in the visible range (~550 nm), InGaAs / GaAs (73) (900 nm – 1.55 µm) and InAs / InP (74) (~1.55 µm) cover the IR range, including the optical fiber transparency windows at 1 310 and 1 550 nm. At the same time, the radiative properties of QDs can be controlled, for example, by interacting with plasmonic nanostructures (75).
The main technical parameters that determine the efficiency of single-photon sources are the values of the second-order autocorrelation function
, (1)
where ( and are the creation and annihilation operators of the optical field, respectively) and the temperature range in which single-photon generation is possible. The quality of the latter is determined by the value of g(2)(0). For single-photon radiation, in the ideal case, g(2)(0) = 0 (the so-called “antibunching of photons”), for real sources, g(2)(0) takes values from 0 to 0.5, which is due to non-ideal fabricated QDs and the impossibility of eliminating the detection of a noise signal during the experiment (65). Today, at temperatures of several K, the typical values of g(2)(0) for QDs are 0.02–0.25, while the maximum achievable operating temperatures vary significantly for QDs of various chemical compositions and can be within tens of K (76) and reach values of 300–350 K (77). The quality of single-photon generation expectedly worsens with increasing temperature and corresponds to g(2)(0) values within 0.2–0.5 for the upper limits of the operating temperature range.
With the controlled synthesis of small ensembles of interacting QDs with strong coupling, it is possible to carry out the engineering of multiphoton (entangled) states. For instance, in the case of shell QDs with a double radiating core (78) or paired QDs (79), one can expect the generation of biphoton states, which are also in demand in problems of quantum informatics.
Quantum memory
Another promising application of QDs in quantum technologies is their use in the implementation of quantum memory devices. In the general case, the principle of operation of quantum memory is based on the processes of controlled absorption and subsequent re-emission of a photon (80). Devices that implement the operation of quantum memory require the most accurate transfer of the initial quantum state; for a number of applications, the storage time of the quantum state and the possibility of simultaneous storage of several states play a role (81, 82).
An example of the practical implementation of quantum memory based on InAs QDs grown in a GaAs matrix using the epitaxy and placed inside an optical cavity, which is a one-dimensional photonic crystal (GaAs / AlAs), is given in (83). In such a scheme, a thin QD layer is located inside the p-n junction formed by GaAs / AlAs layers additionally doped with carbon and silicon atoms, respectively. The process of writing to quantum memory is triggered by a weak circularly polarized light pulse that excites an exciton with a certain spin state in the QD. The selected configuration of potential barriers leads to the spatial localization of an electron, while a hole rather quickly (compared to the characteristic exciton recombination times) tunnels outside a QD. The transition to the stage of reading information (recorded with the help of the electron spin) is triggered by applying an electric pulse that returns a couple of holes to the QD. One of the holes recombines with the electron, leading to the emission of a photon with a polarization determined by the spin of the electron (and hence the polarization of the previously absorbed photon). In the above experiment, it was possible to achieve a spin state storage time of ~1 μs at a temperature of ~10 K.
Based on QDs, it is possible to realize qubits, for example, spin qubits (84).
Quantum sensors
Finally, one more important area of applied use of quantum dots in quantum technologies is quantum sensors. The sensitivity of the optical-spectral characteristics of QDs to the local environment and external parameters is used in quantum sensors to characterize nanodefects (85), for measuring electric fields (86), and temperature (16),(87).
Conclusion
Semiconductor nanocrystals (quantum dots) and new composite materials based on them, while continuing to be the subject of intensive fundamental research, are already being actively implemented in various photonic applications. The dependence of the functional properties of QD materials on their morphology and chemical composition provides the possibility of engineering and deterministic synthesis with predetermined (calculated) physicochemical characteristics. In turn, these properties can depend in a known way on the physicochemical parameters of the local environment, thermodynamic and field characteristics. All this explains the growing interest in the applied use of QDs in various photonics methods and tools. QDs has already found its high-performance applications in the light source industry and related technologies (light-emitting diodes, displays, laser sources), solar energy (high-efficiency and low-cost photovoltaic cells), security systems and anti-counterfeiting techniques. QDs are one of the most effective materials for the implementation of the element base of quantum technologies (sources of non-classical light for quantum computers and quantum telecommunications, the element base of quantum memory, quantum sensors), new optical and optoelectronic devices (nonlinear optical converters, switches, converters, detectors), including in rapidly developing integrated (on-chip) optics technologies. The use of QDs as nanolabels seems promising in solving a wide range of biophotonics problems (fluorescent nanoscopy, flow cytometry, theranostics, nature-like technologies). The solution of many applied problems is associated with the search for new materials and synthesis methods for the production of QDs and nanocomposites based on them (thin films, isolated QDs and their ordered ensembles, hybrid structures and metamaterials), including two-dimensional nanostructures (graphene and graphene-like materials), QDs based on carbon and germanium, diamond QDs with impurity color centers, nanoparticles from materials with a perovskite structure, QDs with complex geometry and morphology (multilayer heterostructural particles, tetrapods, nanorods and nanoplates).
Acknowledgements
The review was prepared as part of the State Assignment of the Moscow State Pedagogical University (MSPU) “Physics of nanostructured materials: fundamental research and applications in materials science, nanotechnologies and photonics” with the support of the Ministry of Education of the Russian Federation (AAAA-A20-120061890084-9). The authors are members of the Leading Scientific School of the Russian Federation (grant of the President of the Russian Federation НШ‑776.2022.1.2).
AUTHORS
Arzhanov A. I., Moscow Pedagogical State University, Moscow; Institute of Spectroscopy of the Russian Academy of Sciences, Troitsk, Moscow; P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Moscow, Troitsk, Russia.
ORCID: 0000-0001-9305-067X
Savostianov A. O., Moscow Pedagogical State University, Moscow; Institute of Spectroscopy of the Russian Academy of Sciences, Troitsk, Moscow; P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Moscow, Troitsk, Russia.
ORCID: 0000-0001-8815-8440
Magaryan K. A., Moscow Pedagogical State University, Moscow, Russia.
ORCID: 0000-0003-4754-4657
Karimullin K. R., Moscow Pedagogical State University, Moscow; Institute of Spectroscopy of the Russian Academy of Sciences, Troitsk, Moscow; P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Moscow, Troitsk, Russia.
ORCID: 0000-0001-6799-2479
Naumov A. V., Moscow Pedagogical State University, Moscow; Institute of Spectroscopy of the Russian Academy of Sciences, Troitsk, Moscow; P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Moscow, Troitsk, Russia. www.single-molecule.ru
ORCID: 0000-0001-7938-9802
A. I. Arzhanov, A. O. Savostianov, K. A. Magaryan, K. R. Karimullin, A. V. Naumov
Moscow State Pedagogical University, Moscow, Russia
Institute of Spectroscopy RAS, Troitsk, Moscow, Russia
Lebedev Physical Institute, RAS, Troitsk Branch, Troitsk, Moscow, Russia
Semiconductor nanocrystals (quantum dots, QDs) have unique photophysical properties, which opens up wide possibilities for their applications in the methods and tools of modern photonics. This article discusses possible applications of QDs. Both existing devices and prospects for the development of new methods and photonics devices are discussed. Innovative approaches to the use of quantum dots in various areas of modern photonic technologies are considered: optoelectronics, biophysics, quantum optics, sensorics, photovoltaics.
Keywords: quantum size effect, quantum dots, quantum dot LEDs, integrated photonics, quantum dot lasers, optical switches, quantum dot detectors, photovoltaics, fluorescence nanoscopy, biolabels, targeted drug delivery
Received on: 10.01.2022
Accepted on: 24.01.2022
INTRODUCTION
Semiconductor nanocrystals (quantum dots, QDs) have unique photophysical properties that depend on their morphology and chemical composition, as well as thermodynamic parameters and environmental properties (electromagnetic field, pressure, acidity, humidity, chemical composition, etc.) (1). The fundamental regularities found in the studies (these issues were covered in detail in the first part of this review in (1)), which determine these relationships, make it possible to carry out the engineering and controlled synthesis of QDs and nanocomposites based on them with desired properties. Thus, QDs are increasingly being used in various branches of science and technology, and, first of all, in various applications of photonics. Conventionally, there are several main areas of application of QDs (see the diagram in Fig. 1):
Light sources (including light-emitting diodes, laser sources, light converters, non-classical light sources).
Displays and multi-component screens.
Luminescent labels – nanotags for various applications, including nanodiagnostics of condensed matter, medical diagnostics and theranostics, targeted drug delivery.
New nanomaterials for photonics, including quasi-two-dimensional nanostructures, nature-like materials (e. g., neuromorphic structures).
Light converters, including the basic elements of nonlinear optics and spectroscopy, light switches (optical transistors), detectors for various spectral range.
Basic elements of quantum technologies, including non-classical light sources, quantum memory and quantum sensors.
Basic elements of solar energy (photovoltaic cells).
In this article, these applications of QDs will be considered in detail, regarding to the fundamental characteristics of new materials described in our recent paper (1).
Light sources and LED
QDs can be used as bright light emitters as an alternative to organic phosphors (2). Due to their high quantum yield, LEDs based on QDs (QLEDs) are more efficient than organic compounds (3). Organic phosphors are characterized by a wide emission spectrum, what limits the ability of manufacturers to fine-tune the color temperature of a light source by mixing emission of several phosphors (e. g., blue and yellow). The spectral properties of QD-based phosphors can be controlled due to the quantum size effect by selecting the luminescence spectrum using a given size distribution of QDs. Such an emitter can be excited with a conventional blue LED due to the presence of a wide absorption spectrum.
QLEDs are, becoming the basis of a technology that has come to replace the LC displays. QDs used as radiation sources in QLED technology make it possible to create displays with high brightness and enhanced color rendering. The development of a new generation of displays (flexible, transparent) based on QDs for wearable wireless electronics is already underway (4).
The possibilities of creating wearable devices with displays based on QDs arrays for displaying various biofunctional parameters of living organisms (e. g., remote medical diagnostics) have also been demonstrated. QDs arrays grown on an elastomeric surface can adhere to the human skin and exhibit stretchable properties and low power consumption. This device is capable of visually displaying body movement and skin temperature signals from peripheral sensors (5).
The main disadvantage that limits the wide introduction of this technology is the fact that the QD array re-radiates light from an external excitation source (an array of LEDs), and is not an independent source of radiation. Currently, there is an active search for ways to create individual RGB pixels based on electroluminescent semiconductor nanocrystals, but today the technology is limited by using nanoparticles as radiation converters (6).
Light converters and detectors
Let’s discuss the possibilities of radiation conversion using QDs as detectors. At present, elements based on silicon, or other semiconductor heterostructures, are most widely used in various photodetectors and matrix sensors operating in the visible spectral range. However, such detectors have a limited operating spectral range from 400 to 1 000 nm. Extending the range to the near (1.3–1.5 μm) and middle (20–200 μm) IR regions can become possible due to covering photoactive Si surface with QDs (7). Interest in promoting highly sensitive detector technology in the IR range is associated with the rapid development of optoelectronic technologies and telecommunications, appearance of the new challenges of quantum optics and non-invasive medical diagnostics.
The development of such detectors is still underway, in particular, it is possible to create light converters not only using epitaxial QDs in nanoheterostructures (8), but also using colloidal QDs (9). It was shown, that the deposition of colloidal QDs on the silicon surface makes it possible to reduce the transparency window of the semiconductor in the IR range due to the creation of impurity states in the bandgap of silicon. Taking into account the fact that colloidal QDs are quite simple to manufacture and have wide possibilities for controlling spectral properties, it can be assumed that research in this direction can give a powerful stimulus to the improvement of silicon optoelectronic devices.
Significant progress in the development of light detectors has been achieved using colloidal QDs PbS, PbSe, Bi2S3, In2S3 (10), that are used in flexible electronics and CMOS technologies.
Being easily combined with various metamaterials and heterostructures, QDs make it possible to proceed to the design of a new generation integrated photonics devices. Thus, in (11), the possibility of creating a highly sensitive camera for the mid-IR range based on an array of plasmonic structures integrated with a system of QDs included in quantum wells was demonstrated.
QDs-based structures can also be used for more complex conversion of light fluxes. Thus, in (12), it was proposed to use semiconductor CdSe / CdS / ZnS QDs exciton levels for writing and reading two-quantum transient holograms. In turn, the nanometer dimensions of QDs allow to use it in photonic and optoelectronic converters of integrated optics. For example, in (13), the effect of plasmon switching was considered for surface plasmon polaritons in a graphene waveguide integrated with a pin nanocavity with QDs. The detected effect potentially provides the ability to control the light fluxes of the IR range, localized in a device with a size of 20 nm.
Sensor technologies
The high sensitivity of QD optical parameters to the characteristics of the external environment opens up the possibility of developing various QD based sensor devices. For example, the selective change in the physical and chemical properties of composites with QDs upon contact with various substances is the basis of gas sensors (e. g. NO2) (14).
The temperature dependence of the spectral characteristics of QDs can become the basis for temperature sensors (15, 16) and nanosensors (17, 18).
The creation of composites with QDs provides a significant increase in the efficiency of photocatalytic systems (19).
The conjugation of QDs with complex nanocomposites enables highly sensitive detection of other physicochemical parameters of media; e. g., it was proposed in (20) to use a polymer–QD–graphene oxide composite as a highly sensitive pH sensor.
Another example of QD usage as sensors of the material characteristics of a medium is associated with local field effects, which manifest themselves in the dependence of the lifetime of the excited state of a QD on the value of the refractive index of the medium (permittivity, magnetic susceptibility) (21, 22). In this regard, luminescent QDs can be used in the original micro-refractometry technique to determine the refractive index of a medium and map its fluctuations, including on the sub-micrometer scale (23, 24).
Solar energy (photovoltaics)
In recent years, solar energy technologies have been actively developing all over the world. Recently, silicon has been common material for solar panels, as the most efficient radiation converters used in solar energy have been developed and commercialized. According to the National Renewable Energy Laboratory (NREL) (see Fig. 3), silicon photovoltaic cells have a fairly high energy conversion efficiency (up to 27.6%) (25). Engineering the same multi-junction photovoltaic cells provides an increase in conversion efficiency up to 47%.
At the same time, a significant problem of silicon optoelectronics should be noted – the high cost and complexity of production.
To date, there has been an intensive growth in the total capacity of electricity generation using solar power plants (see Fig. 4). At the same time, the growing capacities of the resource-intensive and energy-consuming semiconductor production (silicon photovoltaic cells) lead to climate change on the planet, in connection with which it the problem of finding new materials for the manufacture of solar cells becomes acute.
Increasingly, semiconductor nanocrystals – QDs and composites (including hybrid ones) based on them act as the basis for light-converting elements (photovoltaic cells). Due to the wide absorption spectrum, QDs can effectively absorb from the UV to the far IR spectral range, which makes them promising for use in solar energy. QDs already occupy a certain niche in this area, and every year the efficiency of energy conversion is growing. According to (26), an efficiency value of 16.6% has been achieved for colloidal perovskite QDs. The properties of colloidal solutions of nanocrystals make it possible to use them for the manufacture of exotic photovoltaic materials (e. g., flexible solar cells) (27). To date, there are many methods for manufacturing solar panels from colloidal solutions, and the roll-to-roll (R2R) technology which is among them should be highlighted (28). The essence of the method is the deposition of colloidal solutions by blade coating, spraying or injection application on a flexible base, followed by baking to obtain a homogeneous layer of a photoactive substance. Thanks to this technology, it is possible to create ultrathin solar cells, and also to reduce the cost of production of photovoltaic cells based on colloidal QDs significantly.
Quantum dot lasers
In the past six decades since the invention of the laser, semiconductor materials have proven to be excellent laser media. After significant progress in the synthesis of colloidal QDs in the mid‑1990s, the question of the prospects for their application for the manufacture of highly efficient, but at the same time extremely compact laser media, became especially acute. The main advantages of nanocrystals over traditional bulk materials are, associated with size quantization effects, which make it possible to achieve a sufficiently low lasing threshold with high thermal stability and the ability to adjust the radiation wavelength using QDs of various sizes.
Research into the study of laser generation in systems based on QDs was started quite a long time ago, and in the late 1990s prototypes of such lasers came to light (29, 30). It turned out, however, that achieving generation in semiconductor nanocrystals is a very difficult challenge. To create a population inversion in a QD, it is necessary to excite at least a pair of excitons. It wouldn’t seem that difficult to achieve by exciting the QDs with high-power femtosecond laser pulses, however, as it was found, excitons in QDs with a size of the order of several nm can exchange energy quite quickly (the so-called Auger recombination), which prevents the creation of a population inversion necessary to achieve laser radiation.
This difficulty was first overcome in the work of the V. Klimov scientific group (31), where the possibility of obtaining population inversion in close-packed layers of CdSe nanocrystals with the size of 1.2 nm was demonstrated for the first time. The characteristic times of the nonradiative transition between the excited level and required to populate one, measured for these QDs, amounted to impressive hundreds of femtoseconds for both electrons and holes, which is an order of magnitude shorter than the Auger recombination times (~6 ps for CdSe QDs of the indicated radius).
Subsequently, a method was proposed for implementing laser generation in QDs without excitation of multiexcitons (single-exciton optical gain in semiconductor nanocrystals). The material and diameter of the QD (CdS / ZnSe) were chosen so that after excitation by a laser pulse, exciton spatial separation is carried out in such a way: the electron turned out to be localized in the core of the QD, whereas the hole moved to its shell. The resulting local electric field is strong enough to significantly (~100 meV) change the energy of the exciton produced during the following excitation process. As a result, in QDs excited by successive pulses, a shift of absorption bands is observed, which, in turn, makes it possible to create a population inversion in them. Understanding the nature of the processes has opened the way to the creation of low-threshold QD lasers, the implementation of optical amplification by electrical injection, and the development of optically pumped lasers, a standard light-emitting diode with electrical excitation, and on-chip lasers (32–34).
Efficient lasing can be achieved in structures with distributed Bragg gratings on quantum wells, dots, and with quantum cascades (35). Integration of InAs QDs into a planar GaAs semiconductor nanostructure waveguide coupled to an electrically pumped semiconductor laser based on AlAs / AlGaAs microcavity with GaAs quantum wells in the active region makes it possible to realize circularly polarized radiation (36).
Optical switches
An actual task of using QD is the development of new photonic logic elements and the engineering of computational architectures. Optically active nanoobjects come to replace the basic logical elements of electronics – transistors. Due to their scalability and quantum behavior, they are of great interest in the creation of photonic signal processing devices (37). Numerous nanoscale logic devices have been developed to date, operating on systems of trapped ions (38), single atoms (39), nonlinear materials (40), single molecules (41), plasmonic nanoparticles (42), vacancy color centers in diamonds (43), and others. Semiconductor colloidal QDs, which are sources of single photons, can also be used as an active material for such systems.
Fully functioning logic was created on single semiconductor QDs which consisted of high-precision quantum gates (44), photonic switches (45), and repeaters (46). By analogy with transistors, optical logic elements can be used to design and build all-optical integrated circuits, which will greatly affect the progress of quantum photonic technologies soon.
Fluorescence nanoscopy, biolabels and targeted drug delivery
QDs due to their unique physical and chemical characteristics are used as efficient fluorescent labels, for drug delivery, monitoring of the process of drug metabolism in the body. Other biomedical applications of QDs include developing sensitive sensors for disease detection and performing fluorescent assays for drug design (47–49).
In recent years, methods of fluorescent nanoscopy (microscopy of superhigh spatial resolution) have been actively developed, where QDs can act as point-like emitters (labels) (50, 51). Nanoscopy methods are also developing in the direction of three-dimensional visualization of the nanostructure of the object under study, tracking of nanoparticles (52).
Since the late 1990s number of different applications of QDs in biology and medicine is growing exponentially. Semiconductor nanoparticles are actively used as fluorescent labels for bioanalytical purposes, such as detection of DNA, proteins, biomolecules, and cells. For analysis of binding or energy transfer from biological structures (e. g., proteins), Förster resonant energy transfer (FRET) mechanisms are used (53). To implement the FRET mechanism, a biological object under study (e. g., a protein molecule) is used as a donor, and QDs can be used as an acceptor. For these purposes, a QD surface is functionalized with specially grown ligands capable of attaching to a specific complex molecule, and the distance from the center of a nanocrystal to a molecule should be from 1 to 10 nm. When a donor-acceptor pair is formed due to the dipole-dipole interaction, energy is transferred without intermediate emission of a photon. This means that the transfer process is non-radiative, and the energy will be transferred to the acceptor (e. g., a QD), which will subsequently lead to creation of a QD luminescence photon. The efficiency of energy transfer is higher not only due to the spatial proximity of two objects (the FRET efficiency depends on the distance as R6), but also due to overlap of the donor excitation spectrum and the acceptor absorption spectrum. QDs are very promising acceptors due to the wide absorption spectrum in the blue region of the spectrum, appropriate for many biological objects.
Another opportunity to use QDs for biological purposes is based on bactericidal properties of silver-based nanocrystals. The ability of silver ions to block action of various types of viruses, as well as to have a therapeutic effect in the treatment of a numerous viral diseases, is well known. In this regard, the development of methods for QDs synthesis using silver, as well as hybrid associates based on them (54), will make it possible to create drugs with antimicrobial activity in the future.
New materials of photonics
One of the most interesting new materials are carbon-based QDs, including so-called graphene nanocrystals (55). Graphene QDs should not be confused with fullerenes – spheres of carbon atoms. Typically, such nanoobjects are synthesized from fragments of a graphene ribbon or nanotube (56) which makes them look more like flakes. Due to the natural chemical inertness and amphiphilicity graphene QDs are potentially applicable in various biological applications. Like colloidal semiconductor nanocrystals Graphene QDs exhibit band gap dependent fluorescence, which arises due to the quantum confinement effect. Together, these features make graphene QDs excellent candidates for the role of biocompatible markers.
Another interesting quantum-well object is the completely inorganic perovskite QD (PeQD) which has opened up great prospects for the development of light-emitting devices without toxic substances (cadmium). Such nanocrystals have similar properties to semiconductor QDs and have an adjustable bandgap throughout the entire visible spectrum. Also, they demonstrate ultra-high effective quantum yields (93%) (57). Due to their photophysical properties, perovskite QDs have found applying in various optical applications: optical memory cells (58), photodetectors (59), solar cells (60), lasers (61), LEDs (62).
Quantum dots in quantum technologies
Single-Photon Sources
High-performance sources of single photons are key elements for the implementation, for instance, of quantum communication lines (63). Along with single atoms, organic molecules, color centers in diamonds, etc., semiconductor QDs can be used as such controlled sources (64). The main method for manufacturing sources of single-photon emission based on them is molecular epitaxy, which makes it possible to place QDs on substrates with high accuracy, as well as inside optical waveguides and microcavities (65, 66, 67). The other innovative methods of controlled synthesis of such structures, for example, direct laser writing (68), also exist.
A wide choice of semiconductor materials makes it possible to obtain light sources with different emission wavelengths: QDs based on GaN / AlGaN can obtain generation in the near UV range (69, 70) (~280–330 nm), CdSe / ZnSe (71) and CdSe / ZnSSe (72) in the visible range (~550 nm), InGaAs / GaAs (73) (900 nm – 1.55 µm) and InAs / InP (74) (~1.55 µm) cover the IR range, including the optical fiber transparency windows at 1 310 and 1 550 nm. At the same time, the radiative properties of QDs can be controlled, for example, by interacting with plasmonic nanostructures (75).
The main technical parameters that determine the efficiency of single-photon sources are the values of the second-order autocorrelation function
, (1)
where ( and are the creation and annihilation operators of the optical field, respectively) and the temperature range in which single-photon generation is possible. The quality of the latter is determined by the value of g(2)(0). For single-photon radiation, in the ideal case, g(2)(0) = 0 (the so-called “antibunching of photons”), for real sources, g(2)(0) takes values from 0 to 0.5, which is due to non-ideal fabricated QDs and the impossibility of eliminating the detection of a noise signal during the experiment (65). Today, at temperatures of several K, the typical values of g(2)(0) for QDs are 0.02–0.25, while the maximum achievable operating temperatures vary significantly for QDs of various chemical compositions and can be within tens of K (76) and reach values of 300–350 K (77). The quality of single-photon generation expectedly worsens with increasing temperature and corresponds to g(2)(0) values within 0.2–0.5 for the upper limits of the operating temperature range.
With the controlled synthesis of small ensembles of interacting QDs with strong coupling, it is possible to carry out the engineering of multiphoton (entangled) states. For instance, in the case of shell QDs with a double radiating core (78) or paired QDs (79), one can expect the generation of biphoton states, which are also in demand in problems of quantum informatics.
Quantum memory
Another promising application of QDs in quantum technologies is their use in the implementation of quantum memory devices. In the general case, the principle of operation of quantum memory is based on the processes of controlled absorption and subsequent re-emission of a photon (80). Devices that implement the operation of quantum memory require the most accurate transfer of the initial quantum state; for a number of applications, the storage time of the quantum state and the possibility of simultaneous storage of several states play a role (81, 82).
An example of the practical implementation of quantum memory based on InAs QDs grown in a GaAs matrix using the epitaxy and placed inside an optical cavity, which is a one-dimensional photonic crystal (GaAs / AlAs), is given in (83). In such a scheme, a thin QD layer is located inside the p-n junction formed by GaAs / AlAs layers additionally doped with carbon and silicon atoms, respectively. The process of writing to quantum memory is triggered by a weak circularly polarized light pulse that excites an exciton with a certain spin state in the QD. The selected configuration of potential barriers leads to the spatial localization of an electron, while a hole rather quickly (compared to the characteristic exciton recombination times) tunnels outside a QD. The transition to the stage of reading information (recorded with the help of the electron spin) is triggered by applying an electric pulse that returns a couple of holes to the QD. One of the holes recombines with the electron, leading to the emission of a photon with a polarization determined by the spin of the electron (and hence the polarization of the previously absorbed photon). In the above experiment, it was possible to achieve a spin state storage time of ~1 μs at a temperature of ~10 K.
Based on QDs, it is possible to realize qubits, for example, spin qubits (84).
Quantum sensors
Finally, one more important area of applied use of quantum dots in quantum technologies is quantum sensors. The sensitivity of the optical-spectral characteristics of QDs to the local environment and external parameters is used in quantum sensors to characterize nanodefects (85), for measuring electric fields (86), and temperature (16),(87).
Conclusion
Semiconductor nanocrystals (quantum dots) and new composite materials based on them, while continuing to be the subject of intensive fundamental research, are already being actively implemented in various photonic applications. The dependence of the functional properties of QD materials on their morphology and chemical composition provides the possibility of engineering and deterministic synthesis with predetermined (calculated) physicochemical characteristics. In turn, these properties can depend in a known way on the physicochemical parameters of the local environment, thermodynamic and field characteristics. All this explains the growing interest in the applied use of QDs in various photonics methods and tools. QDs has already found its high-performance applications in the light source industry and related technologies (light-emitting diodes, displays, laser sources), solar energy (high-efficiency and low-cost photovoltaic cells), security systems and anti-counterfeiting techniques. QDs are one of the most effective materials for the implementation of the element base of quantum technologies (sources of non-classical light for quantum computers and quantum telecommunications, the element base of quantum memory, quantum sensors), new optical and optoelectronic devices (nonlinear optical converters, switches, converters, detectors), including in rapidly developing integrated (on-chip) optics technologies. The use of QDs as nanolabels seems promising in solving a wide range of biophotonics problems (fluorescent nanoscopy, flow cytometry, theranostics, nature-like technologies). The solution of many applied problems is associated with the search for new materials and synthesis methods for the production of QDs and nanocomposites based on them (thin films, isolated QDs and their ordered ensembles, hybrid structures and metamaterials), including two-dimensional nanostructures (graphene and graphene-like materials), QDs based on carbon and germanium, diamond QDs with impurity color centers, nanoparticles from materials with a perovskite structure, QDs with complex geometry and morphology (multilayer heterostructural particles, tetrapods, nanorods and nanoplates).
Acknowledgements
The review was prepared as part of the State Assignment of the Moscow State Pedagogical University (MSPU) “Physics of nanostructured materials: fundamental research and applications in materials science, nanotechnologies and photonics” with the support of the Ministry of Education of the Russian Federation (AAAA-A20-120061890084-9). The authors are members of the Leading Scientific School of the Russian Federation (grant of the President of the Russian Federation НШ‑776.2022.1.2).
AUTHORS
Arzhanov A. I., Moscow Pedagogical State University, Moscow; Institute of Spectroscopy of the Russian Academy of Sciences, Troitsk, Moscow; P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Moscow, Troitsk, Russia.
ORCID: 0000-0001-9305-067X
Savostianov A. O., Moscow Pedagogical State University, Moscow; Institute of Spectroscopy of the Russian Academy of Sciences, Troitsk, Moscow; P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Moscow, Troitsk, Russia.
ORCID: 0000-0001-8815-8440
Magaryan K. A., Moscow Pedagogical State University, Moscow, Russia.
ORCID: 0000-0003-4754-4657
Karimullin K. R., Moscow Pedagogical State University, Moscow; Institute of Spectroscopy of the Russian Academy of Sciences, Troitsk, Moscow; P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Moscow, Troitsk, Russia.
ORCID: 0000-0001-6799-2479
Naumov A. V., Moscow Pedagogical State University, Moscow; Institute of Spectroscopy of the Russian Academy of Sciences, Troitsk, Moscow; P. N. Lebedev Physical Institute of the Russian Academy of Sciences, Troitsk Branch, Moscow, Troitsk, Russia. www.single-molecule.ru
ORCID: 0000-0001-7938-9802
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