Nanometre-size inorganic dots, tubes and wires exhibit a wide range of electrical and optical properties that depend sensitively on both size and shape, and are of both fundamental and technological interest. In contrast to the syntheses of zero-dimensional systems, existing preparations of one-dimensional systems often yield networks of tubes or rods which are difficult to separate. And, in the case of optically active II-VI and III-V semiconductors, the resulting rod diameters are too large to exhibit quantum confinement effects. Thus, except for some metal nanocrystals, there are no methods of preparation that yield soluble and monodisperse particles that are quantum-confined in two of their dimensions. For semiconductors, a benchmark preparation is the growth of nearly spherical II-VI and III-V nanocrystals by injection of precursor molecules into a hot surfactant. Here we demonstrate that control of the growth kinetics of the II-VI semiconductor cadmium selenide can be used to vary the shapes of the resulting particles from a nearly spherical morphology to a rod-like one, with aspect ratios as large as ten to one. This method should be useful, not only for testing theories of quantum confinement, but also for obtaining particles with spectroscopic properties that could prove advantageous in biological labelling experiments and as chromophores in light-emitting diodes.

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We demonstrate that hollow nanocrystals can be synthesized through a mechanism analogous to the Kirkendall Effect, in which pores form due to the difference in diffusion rates between two components in a diffusion couple. Cobalt nanocrystals are chosen as a primary example to show that their reaction in solution with oxygen, sulfur or selenium leads to the formation of hollow nanocrystals of the resulting oxide and chalcogenides. This process provides a general route to the synthesis of hollow nanostructures of large numbers of compounds. A simple extension of this process yields platinum-cobalt oxide yolk-shell nanostructures which may serve as nanoscale reactors in catalytic applications.

Localized surface plasmon resonances (LSPRs) typically arise in nanostructures of noble metals resulting in enhanced and geometrically tunable absorption and scattering resonances. LSPRs, however, are not limited to nanostructures of metals and can also be achieved in semiconductor nanocrystals with appreciable free carrier concentrations. Here, we describe well-defined LSPRs arising from p-type carriers in vacancy-doped semiconductor quantum dots (QDs). Achievement of LSPRs by free carrier doping of a semiconductor nanocrystal would allow active on-chip control of LSPR responses. Plasmonic sensing and manipulation of solid-state processes in single nanocrystals constitutes another interesting possibility. We also demonstrate that doped semiconductor QDs allow realization of LSPRs and quantum-confined excitons within the same nanostructure, opening up the possibility of strong coupling of photonic and electronic modes, with implications for light harvesting, nonlinear optics, and quantum information processing.

biomolecules with up to zeptomole sensitivity in bulk experiments 7,8,13 . Moreover, the binding of proteins to single functionalized particles can be followed by dark-field microscopy, by exploiting the dependence of the plasmon resonance wavelength on the refractive index of the particle's surroundings 14,15 . The plasmon resonance wavelength of a metal nanoparticle is also affected by other nanoparticles that are in its immediate environment. When two nanoparticles are brought into proximity, their plasmons couple, which shifts the resonance wavelength depending on the particle separation. Since this effect is well known theoretically 16, 17 and experimentally observed for fixed distances 18, 19 , we sought to explore its use as molecular ruler.We applied this 'plasmon ruler' to study the dynamics of DNA hybridization on a single We employed 40 nm diameter gold and silver nanoparticles. The particle diameter was chosen to ensure a sufficiently intense light scattering signal while minimizing any effects of the particles on proximal biomolecules. However, we subsequently found that a reduction in particle size to 20 nm for silver and 30 nm for gold is possible with the current technique.Particles were illuminated with unpolarized white light and light scattered by individual particles was collected by a darkfield microscope in transmission mode (Fig. 1a) 25 . Upon introduction of streptavidin-functionalized particles into the BSA-biotin coated glass chamber, we immediately observed numerous scattering sources adhering to the chamber surfaces. Nanoparticles were vividly colored: individual silver nanoparticles were blue (Fig. 1b), gold nanoparticles were green ( Fig. 1c), aggregates were red-shifted compared to individual particles (typically by about 50 nm for gold, 150nm for silver), and dust and scratches were white.Our first application of plasmon coupling was to monitor the directed assembly of functionalized particle pairs. We used the surface immobilized particles (Figs. 1b and c) as anchors for single stranded DNA (ssDNA) functionalized particles. The 33-nucleotide ssDNA molecules had a biotin at their 3' end, allowing them to bind to the streptavidin coated anchor particles (Fig. 1a). Shortly after introducing the ssDNA-functionalized particles into the chamber, some scattering centers suddenly changed color due to dimer formation. Silver particles turned from blue to green (Fig. 1b), gold particles turned from green to orange (Fig. 1c). The spectral shift upon dimer formation is considerably larger for the silver particles (102 nm) than for gold particles (23 nm, Fig. 1d). The fraction of surface immobilized particles that captured a ssDNAfunctionalized particle ranged from 10% to 86 % depending on the time the samples had been stored, with fresher particles performing better. Aggregates of more than two particles were easily identified by their intensity and distinct color with multiple peaks in the spectrum (see for example the purple dot in Fig 1b). To avoid these aggregates of more than two partic...

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We introduce a new type of liquid cell for in situ transmission electron microscopy (TEM) based on entrapment of a liquid film between layers of graphene. The graphene liquid cell facilitates atomic-level resolution imaging while sustaining the most realistic liquid conditions achievable under electron-beam radiation. We employ this cell to explore the mechanism of colloidal platinum nanocrystal growth. Direct atomic-resolution imaging allows us to visualize critical steps in the process, including site-selective coalescence, structural reshaping after coalescence, and surface faceting.

We have demonstrated that seeded growth of nanocrystals offers a convenient way to design nanoheterostructures with complex shapes and morphologies by changing the crystalline structure of the seed. By using CdSe nanocrystals with wurtzite and zinc blende structure as seeds for growth of CdS nanorods, we synthesized CdSe/CdS heterostructures nanorods and nano-tetrapods, respectively. Both of these structures showed excellent luminescent properties, combining high photoluminescence efficiency (~80% and ~50% for nanorods and nano-tetrapods, correspondingly), giant extinction coefficients (~2·10 7 M -1 cm -1 and ~1.5·10 8 M -1 cm -1 at 350 nm for nanorods and nano-tetrapods, correspondingly) and efficient energy transfer from the CdS arms into the emitting CdSe core. † Current address: