Semiconductor Nanocrystal Quantum Dots

Abstract

AbstractSemiconductor quantum dots (QDs) bridge the gap between cluster molecules and bulk materials. The boundaries between molecular, QD, and bulk regimes are not well defined and are strongly material dependent. However, a range from ∼100 to ∼10 000 atoms per particle can be considered as a rough estimate of sizes for which the nanocrystal regime occurs. The lower limit of this range is determined by the stability of the bulk crystalline structure with respect to isomerization into molecular structures. The upper limit corresponds to sizes for which energy level spacings approach thermal energy, kT. In this nanosize regime, the semiconductor energy‐ or band gap is dependent upon the particle size. This phenomenon is known as thequantum‐size effect, and it allows band‐gap‐dependent material properties to be tuned with particle size. Specifically, semiconductor absorption and emission wavelengths can be controllably blueshifted from their bulk semiconductor values over hundreds of nanometers.Semiconductor quantum dots (QDs) have been prepared by a variety of ‘physical’ and ‘chemical’ methods. Here, we review the synthetic chemistry of QDs prepared by colloidal chemical routes. These QDs, known as nanocrystal quantum dots (NQDs), comprise an inorganic core overcoated with a layer of organic ligand molecules. The organic capping provides electronic and chemical passivation of surface dangling bonds, prevents uncontrolled growth and agglomeration of the nanoparticles, and allows NQDs to be chemically manipulated like large molecules with solubility and reactivity determined by the identity of the surface ligand. In contrast, with substrate‐bound epitaxial QDs prepared by physical deposition methods (e.g. molecular beam epitaxy and metal‐organic chemical vapor deposition), NQDs are ‘freestanding.’ In this discussion, we concentrate on the most successful synthesis methods, where success is determined by high crystallinity, adequate surface passivation, solubility in nonpolar or polar solvents, and good size monodispersity. Size monodispersity is critical as it permits the study and, ultimately, the use of materials‐size‐effects to define novel electronic, optical, and structural materials properties. Further, we examine many of the most significant contributions in the areas of NQD chemical surface modification (organic and inorganic), particle shape control, and phase control.