Semiconductor nanocrystals (NCs) are colloidal single crystals that have been widely examined in recent years to elucidate the origins and applications of size-tunable properties. Size-tunable optical properties are particularly desirable for applications in light-emitting devices, lasers, and biological labeling. [1][2][3][4][5] Strong confinement effects, characteristic of many NCs (''quantum dots''), give them an electronic structure more like molecules than semiconductors and this is of special interest in areas such as quantum information.[6] However, while all colloidal quantum dots are NCs, not all NCs are colloidal quantum dots. This distinction between bulk and confined nanocrystals is not always obvious depending on the characteristics of the bulk band structure. In this paper we show that the optical spectra of bulk semiconductor NCs can exhibit surprising features that may be confused with quantum confinement effects. We compare the optical properties of two bulk nanocrystal systems, Ag 2 S and EuS.Bulk semiconductors have revolutionized a breadth of technologies as a consequence of the way electronic levels form delocalized bands. Although the development of new quantum dot systems have dominated semiconductor nanocrystal research, we suggest that the field of bulk NC synthesis and characterization is complementary to the well-established field of quantum-confined NCs, and offers great potential for the discovery of materials that exploit the desirable electronic and magnetic attributes of bulk semiconductors on the nanoscale. A key advantage foreseen for these NCs is that they are easily processed and they can potentially be programmed for intelligent self-assembly. [7] The realization of the scope and potential of nanoscience has stimulated the discovery of many routes for semiconductor quantum dot synthesis. Many of these reports seek to demonstrate quantum-confinement effects; that is, size-tunable absorption features as demonstrated in compelling pioneering work. [8,9] Compared to the extensive amount of research directed towards creating novel quantum dot systems, little work has been directed towards thinking about the potential of colloidal synthesis for the miniaturization of semiconductors while retaining the essential attributes of the bulk material. There are some notable exceptions to this including nanocrystalline TiO 2 which has been extensively studied for use in photovoltaics.[10] Bulk semiconductors have been, and continue to be, essential components in almost all aspects of technology. They differ from quantum-confined semiconductors in that carriers are located in bands rather than discrete energy levels, thus producing the well-known electronic properties. Nanocrystals that have these same properties could potentially find wide application in micro-and nanoelectronics because they may facilitate processing of devices on small length scales without introducing complicating quantum effects. These materials would exploit the advantages of nanocrystals, such as enhanced processabilit...

The acoustic phonon modes of colloidal CdSe nanoparticles in solution (293 K) are passively measured by a third order ultrafast heterodyne cross-polarized transient grating measurement. Using the observed size-dependence of the acoustic phonon frequency, the elastic properties of the nanoparticles are determined. The size-dependence of the elastic modulus is then used to ascertain information about the relative surface energies of the nanocrystals and suggests the extent and depth of surface reconstruction.

Evidence for an interaction between the quantum dot exciton fine structure states F = +/-1 is obtained by measuring the dynamics of transitions among those states, exciton spin relaxation or flipping. An ultrafast transient grating experiment based on a crossed-linear polarization grating is reported. By using the quantum dot selection rules for absorption of circularly polarized light, it is demonstrated that it is possible to detect transitions between nominally degenerate fine structure states, even in a rotationally isotropic system. The results for colloidal CdSe quantum dots reveal a strong size dependence for the exciton spin relaxation rate from one bright exciton state (F = +/-1) to the other in CdSe colloidal quantum dots at 293 K, on a time scale ranging from femtoseconds to picoseconds, depending on the quantum dot size. The results are consistent with an interaction between those states attributed to a long-range contribution to the electron-hole exchange interaction.

The rate of exciton spin relaxation (flips) between the bright exciton states (F = +/-1) of CdSe nanocrystals is reported as a function of shape, for dots and nanorods. The spin relaxation is measured using an ultrafast transient grating method with a crossed linearly polarization sequence. It is found that the spin relaxation rate depends on the radius, not length, of the nanocrystals. That observation is explained by deriving an expression for the electronic coupling matrix element that mixes the bright exciton states.

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