Quantum dots mechanism

Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

Bawendl M G, Stelgerwald M L and Brus L E 1990 The quantum mechanics of larger semiconductor clusters ( quantum dots ) Ann. Rev. Phys. Chem. 41 477... 

Ruach-Nir I, Wagner HD, Rubinstein I, Hodes G (2003) Structural effects in the electrodeposition of CdSe quantum dots on mechanically strained gold. Adv Eunct Mater 13 159-164... 

Tang, J. and Marcus, R. A. (2005) Diffusion-controlled electron transfer processes and power-law statistics of fluorescence intermittency of nanoparticles. Phys. Rev. Lett, 95, 107401-1-107401-4 Tang, J. and Marcus, R. A. (2005) Mechanisms of fluorescence blinking in semiconductor nanocrystal quantum dots./. Chem. Phys., 123,054704-1-054704-12. 

A detailed mechanism of the photoactivated enhancement of photoluminescence in quantum dots was proposed by Jones et al. [31]. They found an increase in the... 

Under electron irradiation (or by other mechanisms) it is possible to generate carbon vacancies leading to the formation of extended defect domains (with the presence of pentagonal and heptagonal, and even four-membered carbon rings) showing semiconductor character. This is the mechanism of formation of semiconductor properties in quantum-dot carbon nanoparticles or graphene nanoribbon. The mechanism... 

The fuzzy frontier between the molecular and the nanometric level can be elucidated from an electronic point of view. Molecules and small clusters can be described as systems in which the metal atoms form well-defined bonding and antibonding orbitals. Large clusters or small nanoparticles (quantum dots) with dimensions of a few nanometers are intermediate between the size of molecules and bulk material, presenting discrete energy levels with a small band gap owing to quantum-mechanical rules. Finally, larger particles tend to lose this trend and display a typical band structure similar to that of the bulk material. 

A schematic view of the nanomechanical GMR device to be considered is presented in Fig. 1. Two fully spin-polarized magnets with fully spin-polarized electrons serve as source and drain electrodes in a tunneling device. In this paper we will consider the situation when the electrodes have exactly opposite polarization. A mechanically movable quantum dot (described by a time-dependent displacement x(t)), where a single energy level is available for electrons, performs forced harmonic oscillations with period T = 2-k/uj between the leads. The external magnetic field is perpendicular to the orientation of the magnetization in both leads. 

Semiconducting nanocrystals, usually CdSe-ZnS crystals a few nanometers in diameter, called quantum dots, can be useful for in vivo imaging of biochemical dynamics but still suffer from three limitations.51 The most severe problem is that quantum dots blink at probability distributions that lead to loss of continuity in keeping track of individual molecules, limiting the ability to measure the dynamics and mechanisms of biophysical chemistry in vivo and ex vivo. Some significant fraction of fabricated quantum dots appear to be totally dark, reducing... 

In conclusion, semiconductor quantum dots play the role of mechanical supports, photonic antennae, and fluorophores. Such versatility results from unique properties of semiconductor nanoparticles. 

Our interest in quantum dot-sensitized solar cells (QDSSC) is motivated by recent experiments in the Parkinson group (UW), where a two-electron transfer from excitonic states of a QD to a semiconductor was observed [32]. The main goal of this section is to understand a fundamental mechanism of electron transfer in solar cells. An electron transfer scheme in a QDSSC is illustrated in Figure 5.22. As discussed in introduction, quantum correlations play a crucial role in electron transfer. Thus, we briefly describe the theory [99] in which different correlation mechanisms such as e-ph and e-e interactions in a QD and e-ph interactions in a SM are considered. A time-dependent electric field of an arbitrary shape interacting with QD electrons is described in a dipole approximation. The interaction between a SM and a QD is presented in terms of the tunneling Hamiltonian, that is, in... 

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