New physics occurs in semiconductors when one or more dimensions of the crystal are reduced to a size comparable to bulk electron delocalization lengths (tens to hundreds of angstroms). The properties of "quantum dots" or semiconductor nanocrystals are now being studied, as techniques to fabricate the crystallites are developed. Temperature-dependent electron diffraction studies on nanocrystals of CdS show a large depression in the melting temperature with decreasing size, as a larger fraction of the total number of atoms is on the surface. Thermal stability may play a role in determining the uses of semiconductor nanocrystals.

https://science.sciencemag.org/content/sci/256/5062/1425.full.pdf

Melting in semiconductor nanocrystals.

A nanocrystal capable of light emission includes a nanoparticle having photoluminescence having quantum yields of greater than 30%.

Highly luminescent color-selective nanocrystalline materials

A semiconductor nanocrystal compound and probe are described. The compound is capable of linking to one or more affinity molecules. The compound comprises (1) one or more semiconductor nanocrystals capable of, in response to exposure to a first energy, providing a second energy, and (2) one or more linking agents, having a first portion linked to the one or more semiconductor nanocrystals and a second portion capable of linking to one or more affinity molecules. One or more semiconductor nanocrystal compounds are linked to one or more affinity molecules to form a semiconductor nanocrystal probe capable of bonding with one or more detectable substances in a material being analyzed, and capable of, in response to exposure to a first energy, providing a second energy. Also described are processes for respectively: making the semiconductor nanocrystal compound; making the semiconductor nanocrystal probe; and treating materials with the probe.

Semiconductor nanocrystal probes for biological applications and process for making and using such probes

UPS of Buckminsterfullerene and other large clusters of carbon

In this paper we address some of the unique, novel and basic features of clusters, which involve the physical and chemical consequences of their large surface/volume ratio and the size dependence of the properties of large finite systems Energetic, quantum, electronic, spectroscopic, and electrodynamic size effects in clusters were quantified by cluster size equations (CSEs), which describe the gradual “transition” from the large finite cluster to the infinite bulk system, with increasing the cluster size Some progress was also accomplished in the description of the “transition” from cluster microsurfaces to macrosurfaces, which can be described in terms of surface CSEs The analysis implies a nonuniversality principle for cluster size effects, with different physical properties being described in terms of distinct CSEs The CSEs provide a quantitative answer to a central question in the area of cluster chemical physics: What is the minimal cluster size for which its properties become size invariant and do not differ in any significant way from those of the macroscopic sample of the same material? A unified (but not universal) description is advanced for the merging between the properties of microscopic large finite systems and those of a macroscopic bulk material

Cluster size effects

Larger argon atomic clusters undergo a size-specific transition from rigid to fluid forms as the cluster temperature is varied The observed nature of this phase change, detected by measurement of the spectral line shape of a benzene dopant molecule, favors the phase-coexistence model of the cluster melting transition

Rigid-Fluid Transition in Specific-Size Argon Clusters

Magic numbers in CN+ and CN− abundance distributions

Exciton fusion in molecular clusters.

Spectroscopic manifestations of structural shell filling in (benzene) N clusters, N=1-20

Photoionization measurements on Al · ArN clusters (N < 150) in a cold cluster beam have been used to determine ionization threshold energies El, th(N) of the metal atom in a variable-size dielectric medium. The El, th(N) values descend from the atomic value [5.99 eV (1 eV ≈ 1.602 × 10–19 J)] to below 5.5 eV near N≈ 55. The El, th(N) functional form is compared to dielectric continuum predictions and to N-specific Monte Carlo calculations incorporating polarization and model-potential interactions. Satisfactory agreement is obtained when a strong neutral Al–Ar interaction is assumed. Measurements of the slow fragmentation probability indicate that near-threshold ionization of smaller clusters (N < 25) produces negligible fragmentation, resulting in N-specific threshold measurements. Comparison of neutral cluster distributions, measured by threshold photoionization to the corresponding cold Al+·ArN distribution (produced in the source) supports a predominant charge-invariance of the icosahedral-based structures, also found in Monte Carlo simulations. By contrast, photo-absorption spectra in bound states, as exemplified by R2PI spectra of the Al 3p → 4s transition, exhibit only a small fraction (ca. 0.12 eV) of the bulk shift (ca. 0.6 eV). This result may be partly explicable in terms of the distinction between sudden and adiabatic polarization energies in the two experiments.

M. Y. Hahn

Papers

Rigid-Fluid Transition in Specific-Size Argon Clusters

Magic numbers in CN+ and CN− abundance distributions

Exciton fusion in molecular clusters.

Spectroscopic manifestations of structural shell filling in (benzene) N clusters, N=1-20

Photoionization and excitation energies of an Al atom in ArN clusters