Quantum dots (QDs) are semiconductor nanocrystals with peculiar optoelectronic properties. Their wide application in light-emitting diodes, solar cells, and the medical and defense fields makes them a potential candidate in the area of photonics and biophotonics. In this feature issue of Optical Materials Express, together with Optics Express we focus on different aspects of semiconducting nanocrystals research, especially on the advances in the synthesis, physical properties, and application of QDs.
© 2012 OSA
Semiconductor nanocrystals, especially quantum dots (QDs), have attracted much attention in the past decades, and their unique properties due to the quantum confinement are intensely studied . Different QDs have been synthesized by using groups II-VI, III-V, IV-VI, IV elements and their alloys. The size of QDs is on nanometer scale, and the diameter is less than twice the Bohr radius of electronic particles or excitons in the bulk material . In this case the excitons are confined by potential barriers in all three dimensions. Two-dimensional quantum confinement produces quantum wires or rods, and one-dimensional confinement produces quantum films . The probability of multiple exciton generation in QDs per single excitation makes them a potential candidate in photodriven applications . Different experimental techniques based on femtosecond to nanosecond spectroscopy, such as transient IR absorption, tetrahertz spectroscopy, time-resolved photoluminescence, etc., are used by many research groups as evidence of multiple exciton generation. No free electrons and holes exist in an isolated QD, and they can be generated only by dissociation of the excitons followed by the separation of charge carriers in a device. An important area of semiconductor nanoscience is the formation of QD arrays and studying the charge transport and other properties . Advances in the nanomaterial synthesis and nanoscale characterizations help us to investigate their structure-property relationship in a better way . Introduction of mild reaction conditions, one-pot synthesis, size and shape control, and appropriate functionalizations have improved the synthesis of QDs today.
This feature issue presents reviews and research articles in the area of QDs, which helps to widen the understanding of their behavior and applications. In Optical Materials Express, Lee et al. present a review on the application of QDs in photonics . In this review the authors provide a detailed description about the synthesis, functionalization, and processing of QDs mainly for photonic applications. Among the various photonic devices, their study concentrated mainly on photodetectors, photovoltaics, photorefractive devices, and organic light-emitting diodes (OLEDs). This review gives an idea about how the quality of QD-based photonic devices can be improved by enhancing the charge separation and carrier dynamics in these devices. The influence of nanotubes, small molecules, and polymer semiconductors in the enhancement of charge separation and charge carrier dynamics has been detailed. Lim et al. provide a detailed review about the progress in the preparation of QDs, the structural design of electroluminescent devices using them, and the printing technique for full-color display of QDs . The nature of QDs, device structures, and processing techniques are closely related to each other, and we need to optimize all these factors to get a high-quality photonic device. This review gives an insight about the methods to accelerate the realization of full-color QD display even though there are obstacles arising from the use of heavy metals, large hole injection barrier, and imperfect printing, which limit the practical applications of QD-based devices.
Because of the toxicity, heavy-metal-free semiconductor materials like silicon nanoparticles (Si-NPs) are necessary for biomedical applications. Intartaglia et al. synthesized luminescent silicon nanoparticles by ultrashort pulsed laser ablation in liquid for bioimaging . In this article the authors report on a simple and effective method for the generation of luminescent silicon QDs in colloidal form and investigation of photoemissive properties. Jeon et al. describe the fabrication and optoelectric properties of blue OLEDs with a hole transport layer based on QDs embedded in a poly(N-vinyl cabazole) (PVK) layer . They have synthesiszed CdSe and CdSe/Zns QDs for their studies on optical and electrical properties. The origin of temperature broadband light emission in the UV to red from silver ion-implanted Si-NPs is studied by Singh et al. . In their studies, it was observed that the spectral characteristics in the UV and blue region remain unchanged by annealing at different temperatures except for the light emission intensity enhancement.
The preparation of solution-processable and -photopatternable QDs from core-only CdSe as well as core-shell type QDs of CdS/ZnS, CdSe/ZnS, and CdSe/ZnSe is reported by Jang et al. . These QDs were able to spin cast on organic and inorganic substrates. Three-dimensional microstructure fabrication using these materials by two-photon nanostereo-lithography is also reported in this article. Moreels et al. present spectra of the dielectric function of PbS QDs in a glass matrix in the 200–1800 nm range . The dielectric function of QDs with diameters of 3.5–5.0 nm is determined by Maxwell-Garnett effective medium theory combined with Kramers-Kronig analysis. By comparing the results from QDs in a glass matrix with that in colloidal form it is found that the optical properties are comparable. The authors provide an important input for the modeling of photonic devices from PbS QD-doped glasses, by calculating both the intrinsic QD refractive index and extinction coefficient and data on the effective optical constants. Laurand et al. report on the steady-state and optical modulation characteristics of luminescent colloidal QD nanocomposite suitable for integration with gallium nitride optoelectronics . These modulation characteristics are in correlation with the lifetime of charge carriers. Their work gives some guidelines to the construction of hybrid LEDs for various applications.
The feature issue articles featured in Optics Express also represent the intensive research being conducted the QD field. Kuo et al. fabricate and analyze the properties of ZnO thin films with nanocrystalline Si QDs. These new thin films retain the essential optical properties of ZnO at short and long wavelengths while demonstrating the appearance of a sub-bandgap formation in ZnO thin film associated with nanocrystalline Si QDs . It is well known that the energy level transitions in a QD can be designed for detection of a wide range of wavelengths. In the topic of detectors, Sandall et al. demonstrate that InAs QDs grown on a Si substrate exhibit low bulk dark current with strong quantum stark shift effect at 1.3 μm communication wavelength . Foell et al. utilize a photonic crystal microcavity to couple the excitonic emission from PbSe colloidal QDs into a photonic circuit fabricated in a silicon-on-insulator wafer using a CMOS-compatible process .
In the area of sensors, Ling et al. demonstrate how a dual-band infrared QD photodetector, covering the midwave and longwave IR wavelengths, can be used for temperature measurements. The ratio of the photocurrents measured at different applied bias was shown to vary as the target temperature was lowered from 1000°C to 27°C . QDs can enhance the performance of photovoltaics as reported by Kim et al. . In this paper the authors utilize ZnSe QDs to enhance the power conversion efficiency of an organic photovoltaic cell. Relative to a control sample without QDs, they were able to double the power conversion efficiency as well as enhance the short circuit current density and open circuit voltage.
In conclusion, we present the synthesis and potential applications of QDs in this feature issue. We thank all the authors and reviewers who strengthen the importance of this special issue with their valuable contributions.
References and links
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