Solution phase syntheses and size-selective separation methods to prepare semiconductor and metal nanocrystals, tunable in size from ∼1 to 20 nm and monodisperse to ≤5%, are presented. Preparation of monodisperse samples enables systematic characterization of the structural, electronic, and optical properties of materials as they evolve from molecular to bulk in the nanometer size range. Sample uniformity makes it possible to manipulate nanocrystals into close-packed, glassy, and ordered nanocrystal assemblies (superlattices, colloidal crystals, supercrystals). Rigorous structural characterization is critical to understanding the electronic and optical properties of both nanocrystals and their assemblies. At inter-particle separations 5-100Å, dipole-dipole interactions lead to energy transfer between neighboring nanocrystals, and electronic tunneling between proximal nanocrystals gives rise to dark and photoconductivity. At separations <5Å, exchange interactions cause otherwise insulating assemblies to become semiconducting, metallic, or superconducting depending on nanocrystal composition. Tailoring the size and composition of the nanocrystals and the length and electronic structure of the matrix may tune the properties of nanocrystal solid-state materials. MURRAY KAGAN BAWENDI the electronic and optical properties of metals (1-4) and semiconductors (5-9) strongly depend on crystallite size in the nanometer size regime. Efforts to explore structures on the nanometer length scale unite the frontiers of materials chemistry, physics, and engineering. It is in the design and characterization of advanced materials that the importance of new interdisciplinary studies may be realized. Uncovering and mapping size-dependent materials properties requires synthetic routes to prepare homologous size series of monodisperse nanometer size crystals, known as nanocrystals (NCs). NC samples must be monodisperse in terms of size, shape, internal structure, and surface chemistry. A diverse set of structural probes is combined to characterize and develop consistent structural models of NC samples. Optical, electrical, and magnetic studies of well-defined NC samples reveal the unique size-dependent properties of materials in this intermediate, nanometer size regime between molecular species and bulk solid. When atoms or molecules organize into condensed systems, new collective phenomena develop. Cooperative interactions produce the physical properties we recognize as characteristic of bulk materials. Like atoms or molecules, but in the next level of hierarchy, NCs may also be used as the building blocks of condensed matter. Routes enabling controlled manipulation of NCs into the glassy and ordered states of matter lead to the preparation of close-packed NC solids. Assembling NCs into solids opens up the possibilities of fabricating new solidstate materials and devices with novel physical properties, as interactions between proximal NCs give rise to new collective phenomena. Building upon rigorous understanding of the physical proper...

The self-organization of CdSe nanocrystallites into three-dimensional semiconductor quantum dot superlattices (colloidal crystals) is demonstrated. The size and spacing of the dots within the superlattice are controlled with near atomic precision. This control is a result of synthetic advances that provide CdSe nanocrystallites that are monodisperse within the limit of atomic roughness. The methodology is not limited to semiconductor quantum dots but provides general procedures for the preparation and characterization of ordered structures of nanocrystallites from a variety of materials.T h e engineering. of materials and devices --on the nanometer scale is of considerable current interest in electronics ( I ) , optics (2), catalysis (3), ceramics (4), and magnetic storage (5). Nanometer-sized crystallites (nanocrystallites) can display optical, electronic, and structural properties that often are not present in either isolated molecules or macroscopic solids. Well-defined ordered solids prepared from tailored nanocrystalline building blocks provide opportunities for optimizing properties of materials and offer possibilities for observing interesting and potentially useful new collective physical ~henomena. Uniform particles, whether atomic, molecular, or colloidal, organize to form ordered solids when attractive and repulsive . interparticle forces are properly balanced. Ordered colloids or "colloidal crystals" have attracted scientific attention for more than 50 vears and remain an active area of researkh (6). The inherent tendency for monodisperse lyophobic colloids to self-organize provides a general route to new nanostructured materials.Nanocrystallites of semiconductors have discrete electronic transitions that are tunable with size. Thev are often referred to as quantum dots (QDs) or artificial atoms. Their highly polarizable excited states are potentially useful in optoelectronic applica-have been predicted for three-dimensional (3D) ordered arrays (superlattices) of QDs (7). Two-dimensional arrays of substantially larger dots fabricated with the use of lithogr a~h v show that unusual electronic behav-. , ior results from dot interactions (8).In this reDort we demonstrate that suDerlattices of nanometer-sized QDs can be generated either in solution as the "crystallization" of a monodisperse colloid or at a solid or liquid interface as a thin, ordered superlattice of dots. In both cases, the size and spacing of the dots are controlled with a precision that is limited by atomic roughness. Assembly at an interface has the added benefit that both the superlattice and the individual dots have their crystallographic axes oriented with respect to the interface, presenting an opportunity to control and study optical anisotropy in the individual crystallites and in the ordered arrays.Nanocrystallites of CdSe are ideal building blocks for the formation of Q D superlattices. Svnthet ic methods ~roduce macroscopic quantities of single-domain wurtzite crystallites with low defect densities, a uniform shape (slightly p...

Organic-inorganic hybrid materials promise both the superior carrier mobility of inorganic semiconductors and the processability of organic materials. A thin-film field-effect transistor having an organic-inorganic hybrid material as the semiconducting channel was demonstrated. Hybrids based on the perovskite structure crystallize from solution to form oriented molecular-scale composites of alternating organic and inorganic sheets. Spin-coated thin films of the semiconducting perovskite (C(6)H(5)C(2)H(4)NH(3))(2)SnI(4) form the conducting channel, with field-effect mobilities of 0.6 square centimeters per volt-second and current modulation greater than 10(4). Molecular engineering of the organic and inorganic components of the hybrids is expected to further improve device performance for low-cost thin-film transistors.

Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today's strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.

The continued growth of mobile and interactive computing requires devices manufactured with low-cost processes, compatible with large-area and flexible form factors, and with additional functionality. We review recent advances in the design of electronic and optoelectronic devices that use colloidal semiconductor quantum dots (QDs). The properties of materials assembled of QDs may be tailored not only by the atomic composition but also by the size, shape, and surface functionalization of the individual QDs and by the communication among these QDs. The chemical and physical properties of QD surfaces and the interfaces in QD devices are of particular importance, and these enable the solution-based fabrication of low-cost, large-area, flexible, and functional devices. We discuss challenges that must be addressed in the move to solution-processed functional optoelectronic nanomaterials.