A photoinduced method for converting large quantities of silver nanospheres into triangular nanoprisms is reported. The photo-process has been characterized by time-dependent ultraviolet-visible spectroscopy and transmission electron microscopy, allowing for the observation of several key intermediates in and characteristics of the conversion process. This light-driven process results in a colloid with distinctive optical properties that directly relate to the nanoprism shape of the particles. Theoretical calculations coupled with experimental observations allow for the assignment of the nanoprism plasmon bands and for the first identification of two distinct quadrupole plasmon resonances for a nanoparticle. Unlike the spherical particles they are derived from that Rayleigh light-scatter in the blue, these nanoprisms exhibit scattering in the red, which could be useful in developing multicolor diagnostic labels on the basis not only of nanoparticle composition and size but also of shape.

Semiconductor quantum dots, due to their small size, mark the transition between molecular and solid-state regimes, and are often described as`arti®cial atoms' (refs 1±3). This analogy originates from the early work on quantum con®nement effects in semiconductor nanocrystals, where the electronic wavefunctions are predicted 4 to exhibit atomic-like symmetries, for example`s' and`p'. Spectroscopic studies of quantum dots have demonstrated discrete energy level structures and narrow transition linewidths 5±9 , but the symmetry of the discrete states could be inferred only indirectly. Here we use cryogenic scanning tunnelling spectroscopy to identify directly atomic-like electronic states with s and p character in a series of indium arsenide nanocrystals. These states are manifest in tunnelling current±voltage measurements as two-and six-fold single-electron-charging multiplets respectively, and they follow an atom-like Aufbau principle of sequential energy level occupation 10 .InAs nanocrystals for this study were prepared using a solutionphase pyrolytic reaction of organometallic precursors. These nanocrystals are nearly spherical in shape, with size controlled between 10 and 40 A Ê in radius 11 . The nanocrystal surface is passivated by organic ligands which also provide chemical accessibility for quantum dot (QD) manipulation: for example, to prepare``nanocrystal molecules'' 12,13 , nanocrystal-based light-emitting diodes 14 , and to fabricate single-electron tunnelling devices 15 . For the tunnelling spectroscopy studies we link the nanocrystals to a gold ®lm via hexane dithiol molecules 16 . Figure 1a (left inset) shows a scanning tunnelling microscope (STM) topographic image of an isolated InAs QD, 32 A Ê in radius. Also shown in Fig. 1a is a tunnelling current±voltage (I±V) curve that was acquired after positioning the STM tip above the QD and disabling the scanning and feedback controls, realizing a doublebarrier tunnel junction (DBTJ) con®guration 17±19 (Fig. 1a, right inset). A region of suppressed tunnelling current is observed around zero bias, followed by a series of steps at both negative and positive bias. In Fig. 1b we show the tunnelling conductance spectrum (that is, dI/dV versus V), which is proportional to the tunnelling density of states (DOS) 20 . A series of discrete single-electron tunnelling peaks is clearly observed, where the separations are determined by both single-electron charging energy (addition spectrum) and the discrete level spacings (excitation spectrum) of the QD. The I±V characteristics were acquired with the tip retracted from the QD to a distance where the bias voltage is dropped largely across the tip±QD junction. Under these conditions, conduction (valence) band states appear at positive (negative) sample bias, and the excitation peak separations are equal to the real QD level spacings 19 .On the positive-bias side of Fig. 1, two closely spaced peaks are observed immediately after current onset, followed by a larger spacing and a group of six nearly equidistant peaks. We ...

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With high quality and in high yield: The synthesis presented here affords CdSe and CdTe semiconductor nanocrystals without the need of a precursor injection. It allows the detailed control of the size and shape of the nanocrystals, as can be seen from the tetrahedral CdSe nanocrystals that have been prepared (TEM image). The method is suitable for industrial‐scale preparations.

Colloidal superparticles are nanoparticle assemblies in the form of colloidal particles. The assembly of nanoscopic objects into mesoscopic or macroscopic complex architectures allows bottom-up fabrication of functional materials. We report that the self-assembly of cadmium selenide-cadmium sulfide (CdSe-CdS) core-shell semiconductor nanorods, mediated by shape and structural anisotropy, produces mesoscopic colloidal superparticles having multiple well-defined supercrystalline domains. Moreover, functionality-based anisotropic interactions between these CdSe-CdS nanorods can be kinetically introduced during the self-assembly and, in turn, yield single-domain, needle-like superparticles with parallel alignment of constituent nanorods. Unidirectional patterning of these mesoscopic needle-like superparticles gives rise to the lateral alignment of CdSe-CdS nanorods into macroscopic, uniform, freestanding polymer films that exhibit strong photoluminescence with a striking anisotropy, enabling their use as downconversion phosphors to create polarized light-emitting diodes.