A. Paul Alivisatos

Colloidal nanocrystal synthesis and the organic-inorganic interface.

Colloidal nanocrystals are solution-grown, nanometre-sized, inorganic particles that are stabilized by a layer of surfactants attached to their surface. The inorganic cores possess useful properties that are controlled by their composition, size and shape, and the surfactant coating ensures that these structures are easy to fabricate and process further into more complex structures. This combination of features makes colloidal nanocrystals attractive and promising building blocks for advanced materials and devices. Chemists are achieving ever more exquisite control over the composition, size, shape, crystal structure and surface properties of nanocrystals, thus setting the stage for fully exploiting the potential of these remarkable materials.

Nanomechanical oscillations in a single-C60 transistor

The motion of electrons through quantum dots is strongly modified by single-electron charging and the quantization of energy levels. Much effort has been directed towards extending studies of electron transport to chemical nanostructures, including molecules, nanocrystals and nanotubes. Here we report the fabrication of single-molecule transistors based on individual C60 molecules connected to gold electrodes. We perform transport measurements that provide evidence for a coupling between the centre-of-mass motion of the C60 molecules and single-electron hopping—a conduction mechanism that has not been observed previously in quantum dot studies. The coupling is manifest as quantized nano-mechanical oscillations of the C60 molecule against the gold surface, with a frequency of about 1.2 THz. This value is in good agreement with a simple theoretical estimate based on van der Waals and electrostatic interactions between C 60 molecules and gold electrodes.

A single-electron transistor made from a cadmium selenide nanocrystal

The techniques of colloidal chemistry permit the routine creation of semiconductor nanocrystals, whose dimensions are much smaller than those that can be realized using lithographic techniques. The sizes of such nanocrystals can be varied systematically to study quantum size effects or to make novel electronic or optical materials with tailored properties. Preliminary studies of both the electrical and optical properties of individual nanocrystals have been performed recently. These studies show clearly that a single excess charge on a nanocrystal can markedly influence its properties. Here we present measurements of electrical transport in a single-electron transistor made from a colloidal nanocrystal of cadmium selenide. This device structure enables the number of charge carriers on the nanocrystal to be tuned directly, and so permits the measurement of the energy required for adding successive charge carriers. Such measurements are invaluable in understanding the energy-level spectra of small electronic systems, as has been shown by similar studies of lithographically patterned quantum dots and small metallic grains.

Colloidal nanocrystal heterostructures with linear and branched topology

The development of colloidal quantum dots has led to practical applications of quantum confinement, such as in solution-processed solar cells, lasers and as biological labels. Further scientific and technological advances should be achievable if these colloidal quantum systems could be electronically coupled in a general way. For example, this was the case when it became possible to couple solid-state embedded quantum dots into quantum dot molecules. Similarly, the preparation of nanowires with linear alternating compositions—another form of coupled quantum dots—has led to the rapid development of single-nanowire light-emitting diodes and single-electron transistors. Current strategies to connect colloidal quantum dots use organic coupling agents, which suffer from limited control over coupling parameters and over the geometry and complexity of assemblies. Here we demonstrate a general approach for fabricating inorganically coupled colloidal quantum dots and rods, connected epitaxially at branched and linear junctions within single nanocrystals. We achieve control over branching and composition throughout the growth of nanocrystal heterostructures to independently tune the properties of each component and the nature of their interactions. Distinct dots and rods are coupled through potential barriers of tuneable height and width, and arranged in three-dimensional space at well-defined angles and distances. Such control allows investigation of potential applications ranging from quantum information processing to artificial photosynthesis.

Localized surface plasmon resonances arising from free carriers in doped quantum dots

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.

Observation of Single Colloidal Platinum Nanocrystal Growth Trajectories

Mergers and Acquisitions The crystallization of small molecules or polymers is often described in terms of a nucleation stage, where initial clusters form, followed by a distinct growth stage. Growth can come from the addition of unbound molecules, or through “Ostwald ripening” where larger crystals grow at the expense of smaller ones due to thermodynamic effects. Zheng et al. (p. 1309) studied the growth of platinum nanocrystals inside a transmission electron microscope using a special liquid cell, allowing observation of crystal growth in situ. Both monomer addition to growing particles and the coalescence of two particles were observed. The specific growth mechanism appeared to be governed by the size of each of the particles. The combination of growth processes makes it possible for an initially broad distribution of particles to narrow into an almost uniform one. Transmission electron microscopy provides details of the growth mechanisms of platinum nanocrystals in solution. Understanding of colloidal nanocrystal growth mechanisms is essential for the syntheses of nanocrystals with desired physical properties. The classical model for the growth of monodisperse nanocrystals assumes a discrete nucleation stage followed by growth via monomer attachment, but has overlooked particle-particle interactions. Recent studies have suggested that interactions between particles play an important role. Using in situ transmission electron microscopy, we show that platinum nanocrystals can grow either by monomer attachment from solution or by particle coalescence. Through the combination of these two processes, an initially broad size distribution can spontaneously narrow into a nearly monodisperse distribution. We suggest that colloidal nanocrystals take different pathways of growth based on their size- and morphology-dependent internal energies.

Fabrication of metallic electrodes with nanometer separation by electromigration

A simple yet highly reproducible method to fabricate metallic electrodes with nanometer separation is presented. The fabrication is achieved by passing a large electrical current through a gold nanowire defined by electron-beam lithography and shadow evaporation. The current flow causes the electromigration of gold atoms and the eventual breakage of the nanowire. The breaking process yields two stable metallic electrodes separated by ∼1 nm with high efficiency. These electrodes are ideally suited for electron-transport studies of chemically synthesized nanostructures, and their utility is demonstrated here by fabricating single-electron transistors based on colloidal cadmium selenide nanocrystals.

Nanoantenna-enhanced gas sensing in a single tailored nanofocus

We demonstrate antenna-enhanced hydrogen sensing at the single-particle level. We place a single palladium nanoparticle near the tip region of a gold nanoantenna and detect the changing optical properties of the system upon hydrogen exposure.

Biological applications of colloidal nanocrystals

Due to their interesting properties, research on colloidal nanocrystals has moved in the last few years from fundamental research to first applications in materials science and life sciences. In this review some recent biological applications of colloidal nanocrystals are discussed, without going into biological or chemical details. First, the properties of colloidal nanocrystals and how they can be synthesized are described. Second, the conjugation of nanocrystals with biological molecules is discussed. And third, three different biological applications are introduced: (i) the arrangement of nanocrystal–oligonucleotide conjugates using molecular scaffolds such as single-stranded DNA, (ii) the use of nanocrystal–protein conjugates as fluorescent probes for cellular imaging, and (iii) a motility assay based on the uptake of nanocrystals by living cells.

High-resolution EM of colloidal nanocrystal growth using graphene liquid cells.

Liquid Nanocrystals In high-resolution transmission electron microscopy, grid materials are used to support solid samples while providing a means for preventing a build-up of static charge. Liquids are difficult to study at the same atomic resolution and require encapsulation to prevent excess sample movement, sample damage, or evaporation. Materials that have been used for liquid cells, like silicon nitride or silicon oxide, need thick layers and have poor electron transmittance at the thicknesses required because they contain high atomic number elements. Yuk et al. (p. 61; see the Perspective by Colliex) show that liquids can be encapsulated in graphene sheets, and through this technique, they studied the formation of platinum nanocrystals with atomic resolution. The crystals could be tracked as they selectively coalesced, modified their shape, and formed surface facets. Encapsulating a liquid film between two graphene layers allows the film and growing crystals from the graphene sheets to be studied at an atomic scale. 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.