CdS nanoparticles capped with tri-n-octylphosphine oxide (TOPO) have been synthesised by a single-source route using cadmium ethylxanthate as a precursor. The nanoparticles obtained show quantum size effects in the optical absorption spectra and the photoluminescence shows an emission maximum that is characteristically red shifted in relation to the band edge. The X-ray diffraction (XRD) pattern shows the material to be hexagonal with a calculated particle size (from the Scherrer equation) of 3.9 nm. The size of the particles is confirmed by the transmission electron microscope (TEM) image, which shows well-defined, spherical particles with an average size of 4.2 nm ¡ 10%.

A spectroscopic study is reported of relaxation in the exciton fine structure of CdSe nanorods measured using ultrafast cross-polarized heterodyned third-order transient grating (CPH-3TG) spectroscopy. The CPH-3TG spectroscopy probes the dynamics of population transfer between states accompanying exciton spin flip (radiationless transitions in the exciton fine structure associated with changes in the sign of the total angular momentum). Analysis of the data enabled elucidation of pathways for exciton fine structure relaxation (EFSR) involving all of the exciton fine structure states. The mechanism and origin of the EFSR dynamics in CdSe nanorods are discussed in the exciton state picture with an analogy to molecular radiationless transition processes such as internal conversion and intersystem crossing. Key conclusions are that the fast transitions are between states with total angular momentum of (1 and -2 and that the rate of this transition follows an inverse diameter to the fourth power size dependence, which originates from the Dresselhaus spin-orbit coupling effect.

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 dynamics of exciton spin relaxation in CdSe nanorods of various sizes and shapes are measured by an ultrafast transient polarization grating technique. The measurement of the third-order transient grating (3-TG) signal utilizing linear cross-polarized pump pulses enables us to monitor the history of spin relaxation among the bright exciton states with a total angular momentum of F = +/-1. From the measured exciton spin relaxation dynamics, it is found that the effective mechanism of exciton spin relaxation is sensitive to the size of the nanorod. Most of the measured cross-polarized 3-TG signals show single-exponential spin relaxation dynamics, while biexponential spin relaxation dynamics are observed in the nanorod of the largest diameter. This analysis suggests that a direct exciton spin flip process between the bright exciton states with F = +/-1 is the dominant spin relaxation mechanism in small nanocrystals, and an indirect spin flip via the dark states with F = +/-2 contributes as the size of the nanocrystal increases. This idea is examined by simulations of 3-TG signals with a kinetic model for exciton spin relaxation considering the states in the exciton fine structure. Also, it is revealed that the rate of exciton spin relaxation has a strong correlation with the diameter, d, of the nanorod, scaled by the power law of 1/d4, rather than other shape parameters such as length, volume, or aspect ratio.

Size-dependent optical properties of semiconductor nanocrystals are of great interest because of the myriad of phenomena stemming from them. The preparation of more complex colloidal shapes will facilitate the systematic study of shape-dependent phenomena. It is shown that a strategy to obtain systematically more complex nanocrystal structures is to exert a sequence of shape-directing steps during the colloidal growth. Using experiments based on multiple reagent injections we show how changes in the type of surfactant introduced during growth of CdSe nanocrystals promotes shape evolution. On this basis, we propose a means to achieve a further generation of shape design in nanometer-sized colloids by using a series of growth steps, each one building from the previous conditions of shape as well as surface-specific reactivity. To understand the shape formation and stability in nanocrystalline colloids, and particularly the importance of surface ligands, we introduce an analogy with the thermodynamics of droplets.

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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We report the preparation of CdS nanorods using a thiosemicarbazide complex of cadmium [Cd(NH2CSNHNH2)2Cl2]. The precursor was decomposed in tri-n-octylphosphine oxide (TOPO) at 280 degrees C to give TOPO capped CdS nanoparticles; nano-dimensional rods of the material are clearly visible in transmission electron microscopy (TEM); the particles have been further characterised by X-ray diffraction (XRD) and selected area electron diffraction (SAED) and optical measurements.