We use time-, wavelength-, temperature-, polarization-resolved luminescence to elucidate the nature of the absorbing and ‘‘band edge’’ luminescing states in 32 Å diameter wurtzite CdSe quantum crystallites. Time-resolved emission following picosecond size-selective resonant excitation of the lowest excited state shows two components—a temperature insensitive 100 ps component and a microsecond, temperature sensitive component. The emission spectrum, showing optic phonon vibrational structure, develops a ∼70 wave number red shift as the fast component decays. Photoselection shows the slow component to be reverse polarized at 10 K, indicating this component to be the result of a hole radiationless transition. The 100 ps emitting state is repopulated thermally as temperature increases from 10 to 50 K. All available data are interpreted by postulating strong resonant mixing between a standing wave molecular orbital delocalized inside the crystallite and intrinsic surface Se lone pair states. The apparent exciton transition is assigned to a ∼130 wave number wide band of eigenstates with the hole localized principally on the surface. The band contains strongly emitting ‘‘doorway’’ states and weakly emitting ‘‘background’’ states. The hole becomes mobile among these states as T increases to 50 K. It is suggested that such resonant mixing may be general in II–VI and III–V crystallites.

Composite semiconductor crystallites involving CdSe grown on an ZnS seed,and vice versa, have been synthesized and "capped" with organic ligands in inverse micelle solutions. These composite particles, as well as capped seed crystallites of CdSe and ZnS, are isolated, purified, and characterized for relative atomic composition, structure, and electronic properties. The Debye X-ray scattering equation, when solved for these layered particles, shows that powder X-ray scattering is insensitive to a small foreign inclusion. A simple theoretical model for the LUMO and HOMO of layered crystallites shows that a small (<15-A diameter) interior foreign seed causes only small shifts of the lowest excited state, to either higher or lower energies. The capped CdSe seed and the capped CdSe portion of the layered particle grown on a ZnSe seed undergo low-temperature (169 "C) annealing to give near-single-crystal X-ray scattering. However, CdSe annealing is blocked by a surface ZnS layer which is ca. 4 A thick. While growth to make composite particles does occur, neither particle shows evidence for epitaxial growth.Semiconductor quantum crystallites (20-60-A diameter) have incomplete band structure development, even though the bulk unit cell is present. Their optical and electronic properties are size dependent, and within the past 5 years synthetic procedures have been developed to prepare and stabilize uniform crystallites.I~* Inverse micelle solutions provide a medium for synthesis of stable, size-selected semiconductor colloid^.^ CdSe crystallites in inverse micelles do not fuse with one another because of the surfactant, yet they can grow larger if either ionic or organometallic sources of Cd and Se atoms are added.4 This surface reactivity was exploited in an organic "capping" reaction:(1) Steigerwald, M. L.; Brus, L. E.

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We use transient optical hole burning and photoluminescence to investigate the static and dynamic electronic properties of 32-A CdSe quantum dots. We observe a number of discrete electronic transitions, resolve LO-phonon progressions, and obtain homogeneous linewidths and electron-LO-phonon couplings. We find that the band-edge luminescence is not from the exciton state but from a surface trapped state. Rapid (-160 fsec) trapping into these surface states results in long-lived (-10-100 nsec) bleach and induced-absorption features in pump-probe experiments.PACS numbers: 73.20.Dx, 36.40.+d, 73.20.At Semiconductor crystallites which are small compared to the bulk exciton Bohr radius exhibit three-dimensional electron and hole confinement. ' They show discrete, large-molecule-like electronic states that shift to higher energy with smaller particle size. They are commonly referred to as nanometer-scale clusters or quantum dots. Extensive research on semiconductor quantum wells has shown that restriction of excitons to two dimensions results in spectral confinement of the oscillator strength which can be exploited in the design of nonlinear optical devices. In principle, the further reduction of dimensionality in quantum dots should lead to enhanced nonlinear optical properties.The presence of the crystallite surface as a boundary and source of surface states makes nanometer-scale clusters fundamentally diferent in principle from epitaxial low-dimensional structures such as quantum wells and quantum wires. This aspect and the question of the response of a small finite lattice to a resident electron-hole pair remain poorly understood.The eAective-mass model, including the eA'ects of Coulomb correlations and the internal structure of the valence band, has been applied extensively to nanometer-scale clusters. ' The lowest excited state, labeled 1s,-1sp and commonly referred to as the exciton state, in principle has both carriers in totally symmetric particle-in-a-sphere eigenstates with nodes at the crystallite surface. The true homogeneous linewidth and electronic character of this state in well characterized nearly monodisperse CdSe crystallites is the central issue of this Letter. Fast transient optical studies of CdS, CdS"-Se] -,and CdSe crystallites, principally in silicate glasses and polymer films, have previously revealed a fast bleaching of exciton absorption, induced absorption attributed to biexciton formation, and photophysical hole burning.'' These samples typically emit from deep trap states, and the observed spectra are complicated by averaging over various and most uncharacterized distributions, such as those of size, structure, defects, stoichiometry, and surface chemistry.We study chemically synthesized CdSe crystallites dispersed in a Lewis base organic solvent (90% Bu3P, 10% Bu3PO). This type of solvent complexes (e.g. , bonds to) metal ions such as Cd and in our system most likely passivates surface Cd atoms. TEM and x-ray analysis sho~s that this solution contains CdSe crystallites having the bu...

The resonance Raman spectrum of 45 (+-3) A diameter CdSe clusters was measured. The incident photons were resonant with the HOMO-LUMO transition in the clusters. At low temperature, one mode at 205 cm-I is observed, as well as two overtones, with the integrated area~ un~er thes~ pea~s in.the ratio of9:3:1. This mode is assigned as the longest wavelength longltudmal optical vibratIOn of the cluster. The strength of the coupling between the lowest electronic excited state and the LO vibration is found to be 20 times weaker in these clusters than in the bulk solid. The CdSe cluster resonance Raman spectrum is shown to be consistent with the recently measured homogeneous cluster absorption spectrum.