Properties of Quantum Dots

As a result of their unique optical and electronic properties, particularly their ability to fluoresce at discrete wavelengths directly proportional to their sizes and material compositions, QDs have found use in many fields, including electronics, biology, medicine, and even cosmetics. The first attempts to modify their surface characteristics to make them water-soluble and biocompatible eventually led to their use as fluorescent labels for biomolecules in many applications (Rogach et al., 1996 Bruchez et al., 1998 Chan and Nie, 1998). 

The emission properties of QDs can be adjusted based upon core diameter and nanoparticle composition. Nanoparticle diameters typically are carefully controlled during manufacture to be between 2 and 10 nm. In addition, the band gap energy or energy of fluorescence emission is inversely proportional to the diameter of the QD particle. Thus, the smaller the particle, the 

One major advantage of DHLA modification is that the diameter of the QD remains as small as possible, while still creating a water-soluble particle. QDs of 10nm diameter have been created using this process and were successfully used to image intracellular proteins (Jaiswal et al., 2003). 

Water-soluble QDs now are available from a number of manufacturers (Invitrogen, Evident Technologies, and Crystalplex). Each supplier uses their own proprietary methods of surface pacification to create biocompatible particles. Even coated QD clusters are available that contain hundreds of particles bound together in a polymer matrix (Crystalplex). These form intensely bright labels for biomolecules, because the nanocrystals do not quench when clustered together at high density. 

QDs have been used successfully in many biological applications, which exploit their best properties of brightness, photostability, and multiplex capability. There are many publications 

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As briefly mentioned in the previous section, PCS provides quantitative information on the lifetime of the non-radiative state for molecules in solution in the time range from sub-microseconds to seconds. This method can, potentially, be applied to the characterization of the photophysical properties of quantum dots freely diffusing in solution with higher temporal resolution than the previous SPD. 

In the 1980s, CdSe quantum dots vere prepared by top-dovm techniques such as lithography ho vever, size variations, crystal defects, poor reproducibility, and poor optical properties of quantum dots made them inadequate for advanced applications. Introduction of bottom-up colloidal synthesis of CdSe quantum dots by Murray et al. [3] and its further advancements brought radical changes in the properties of quantum dots and their applications in devices and biology. The colloidal syntheses of CdSe quantum dots are broadly classified into organic-phase synthesis and aqueous-phase synthesis. 

Before discussing the experimental evidence, it is worthwhile to consider lasing-related properties of quantum dots from a fundamental point of view. The theoretical description of the optical gain in bulk and quantum well semiconductors is discussed in Datareview C5.3. 

In conclusion, photoelectric phenomena in multilayer quantum dot structures are determined by complicated multistage mechanism provided by the geometry of the system, properties of quantum dots and matrix as well as the interface between them and residual elastic strains in heterojunctions. 

Electronic and Optical Properties of Quantum Dots, 563 Type I and Type II Core-Shell Quantum Dots, 565 The Absorption Cross-Sections of Quantum Dots, 565 Absorption and Emission Maxima of Quantum Dots, 567 Luminescence Lifetimes of Q-Dots, 567 Electrochemistry of Nanoparticles, 569 Conclusion, 571 Further Reading, 571 References, 572 Problems, 575 Answers, 576... 

Quantum dots are semiconductors composed of atoms from groups II-VI or III-V elements of the periodic table, for example, CdSe, CdTe, and InP (39). Their brightness is attributed to the quantization of energy levels due to confinement of an eleetron in a three-dimensional box. The optical properties of quantum dots can be manipulated by synthesizing a (usually stabilizing) shell. Such Q-dots are known as core-shell quantum dots, for example, CdSe-ZnS, InP-ZnS, and InP-CdSe. In this section, we will discuss the different properties of quantum dots based on their size and composition. 

The optical properties of quantum dots have been extensively investigated. They have discrete atomic-like energy levels that alter predictably as the size of the particle changes, so that the light emitted can be tuned. This finds application in many areas, including displays and lasers. 

S Some (Electrical) Transport Properties of Quantum Dots 33... 

Fig. 6.1. Absorption and emission spectra of (a) rhodamine red and a genetically encoded protein (DsRed2), (b) six different ZnS-CdSe quantum dot dispersions, (c) A photograph demonstrating the size-tunable fluorescence properties of quantum dot dispersions (reproduced with permission from [887])...

Quantum dots (QD) play an important role in nano-scaled electronic devices. In order to fully understand the transport properties of quantum dots, phenomena such as tunneling of electrons must be characterized. 

In the early 1980s, Efros described the size-dependent electronic properties of quantum dots, first delineating that the bandgap, , will increase from the bulk... 

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