Current research into semiconductor clusters is focused on the properties of quantum dots-fragments of semiconductor consisting of hundreds to many thousands of atoms-with the bulk bonding geometry and with surface states eliminated by enclosure in a material that has a larger band gap. Quantum dots exhibit strongly size-dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery.

Semiconductor Clusters, Nanocrystals, and Quantum Dots

Semiconductor nanocrystals exhibit a wide range of size-dependent properties. Variations in fundamental characteristics ranging from phase transitions to electrical conductivity can be induced by controlling the size of the crystals. The present status and new opportunities for research in this area of materials physical chemistry are reviewed.

Perspectives on the Physical Chemistry of Semiconductor Nanocrystals

The synthesis of epitaxially grown, wurtzite CdSe/CdS core/shell nanocrystals is reported Shells of up to three monolayers in thickness were grown on cores ranging in diameter from 23 to 39 A Shell growth was controllable to within a tenth of a monolayer and was consistently accompanied by a red shift of the absorption spectrum, an increase of the room temperature photoluminescence quantum yield (up to at least 50%), and an increase in the photostability Shell growth was shown to be uniform and epitaxial by the use of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and optical spectroscopy The experimental results indicate that in the excited state the hole is confined to the core and the electron is delocalized throughout the entire structure The photostability can be explained by the confinement of the hole, while the delocalization of the electron results in a degree of electronic accessibility that makes these nanocrystals

Epitaxial Growth of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability and Electronic Accessibility

Nanoparticles of CdTe were found to spontaneously reorganize into crystalline nanowires upon controlled removal of the protective shell of organic stabilizer. The intermediate step in the nanowire formation was found to be pearl-necklace aggregates. Strong dipole-dipole interaction is believed to be the driving force of nanoparticle self-organization. The linear aggregates subsequently recrystallized into nanowires whose diameter was determined by the diameter of the nanoparticles. The produced nanowires have high aspect ratio, uniformity, and optical activity. These findings demonstrate the collective behavior of nanoparticles as well as a convenient, simple technique for production of one-dimensional semiconductor colloids suitable for subsequent processing into quantum-confined superstructures, materials, and devices.

https://science.sciencemag.org/content/sci/297/5579/237.full.pdf

Spontaneous organization of single CdTe nanoparticles into luminescent nanowires.

High temperature, solution phase reduction of cobalt chloride in the presence of stabilizing agents was employed to produce magnetic colloids (ferrofluids) of cobalt nanocrystals. We systematically synthesized and isolated nearly monodisperse nanocrystal samples ranging in size from 2 to 11 nm while maintaining better than a 7% std. dev. in diameter. As synthesized cobalt particles are each a single crystal with a complex cubic structure related to the beta phase of elemental manganese (e-Co). Annealing the nanocrystals at 300 °C converts them quantitatively to the more common hexagonal-close-packed crystal form. Deposition of these uniform cobalt particles on solid substrates by evaporation of the carrier solvent results in the spontaneous assembly of two-dimensional and three-dimensional magnetic superlattices (colloidal crystals). A combination of x-ray powder diffraction, transmission electron microscopy, and superconducting quantum interference device magnetometry were used to characterize both the d...

Synthesis of monodisperse cobalt nanocrystals and their assembly into magnetic superlattices (invited)

The kinetics of solid-solid phase transitions are explored using pressure-induced structural transformations in Si nanocrystals. In agreement with the predictions of homogeneous deformation theories, large elevations in phase transition pressure are observed in nanocrystals as compared to bulk Si, and high pressure x-ray diffraction peak widths indicate an overall change in nanocrystal shape upon transformation. In addition, unlike the BC8 phase recovered in bulk Si, amorphous Si nanoclusters are obtained upon release of pressure, providing an example of kinetic size control over solid phases. {copyright} {ital 1996 The American Physical Society.}

Pressure-induced structural transformations in Si nanocrystals: Surface and shape effects.

X-ray diffraction was used to monitor the structure of 45 A diameter CdSe nanocrystals as they transformed repeatedly between fourfold and sixfold coordinated crystal structures. Simulations of the diffraction patterns reveal that a shape change occurs as the crystals transform. They also show that stacking faults are generated in the transition from the high- to the low-pressure phase. The shape change and stacking fault generation place significant constraints on the possible microscopic mechanism of the phase transition.

Shape change as an indicator of mechanism in the high-pressure structural transformations of CdSe nanocrystals

The size dependent electronic absorption spectra of CdSe nanocrystals have been measured in a diamond anvil cell. Under pressure, these nanocrystals are reversibly converted from a direct gap wurtzite structure to a rock salt structure which has an indirect gap in the bulk. It is thus possible to compare the influence of quantum confinement on direct and indirect transitions in nanocrystals of the same size. The ratio of oscillator strength between direct and indirect structures does not change with size, indicating that zero-phonon transitions are not occurring in the indirect nanocrystals.

Comparison of quantum confinement effects on the electronic absorption spectra of direct and indirect gap semiconductor nanocrystals.

Erratum: Shape Change as an Indicator of Mechanism in the High-Pressure Structural Transformations of CdSe Nanocrystals [Phys. Rev. Lett. 84, 923 (2000)]

Pressure-induced phase transitions in CdSe, InP and Si nanocrystals of 2 to 50 nm in diameter were studied with high pressure X-ray diffraction and optical absorption measurements. All samples were found to transform via a single nucleation event. In addition, all samples transformed at an elevated pressure with a large hysteresis compared to that of the bulk. These phenomena are explained by an overall change in shape of the crystallite after it goes through the phase transition. Evidence for this shape change is shown in the elevation of the phase transition pressure in all samples and in the high pressure shape of the Si nanocrystals. These experiments present an opportunity to investigate first order solid-solid phase transitions on a nanometer scale.

Amy B. Herhold

Papers

Pressure-induced structural transformations in Si nanocrystals: Surface and shape effects.

Shape change as an indicator of mechanism in the high-pressure structural transformations of CdSe nanocrystals

Comparison of quantum confinement effects on the electronic absorption spectra of direct and indirect gap semiconductor nanocrystals.

Erratum: Shape Change as an Indicator of Mechanism in the High-Pressure Structural Transformations of CdSe Nanocrystals [Phys. Rev. Lett. 84, 923 (2000)]

A Comparison of Pressure-Induced Structural Transformations in CdSe, InP, and Si Nanocrystals