C. B. Murray

Self-Organization of CdSe Nanocrystallites into Three-Dimensional Quantum Dot Superlattices

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.

Investigation of the surface morphology of capped CdSe nanocrystallites by 31P nuclear magnetic resonance

Room temperature 31P magic angle sample spinning (MAS) nuclear magnetic resonance (NMR) is used to study the phosphine chalconide species coordinating the surface of CdSe nanocrystallites. Two surface species are identified: trioctylphosphine oxide (TOPO) and trioctylphosphine selenide (TOPSe). The TOPO coordinates Cd surface sites while trioctylphosphine (TOP) coordinates Se to produce surface TOPSe species. Chemical removal of TOPSe linkages produces CdSe nanocrystallites passivated exclusively with TOPO. A double resonance experiment (31P and 77Se) characterizes the TOPSe species. Quantitative studies indicate coordination to nearly all surface Cd atoms and allows the development of a model for the morphology of an average crystallite surface.

Electrospray organometallic chemical vapor deposition—A novel technique for preparation of II–VI quantum dot composites

A novel technique combining electrospray and organometallic chemical vapor deposition (OMCVD) has been developed for the synthesis of new II–VI quantum dot composites. CdSe nanocrystals (quantum dots) of selected size are dispersed in a pyridine/acetonitrile mixture. The nanocrystals are transferred by electrospray into the growth zone of an OMCVD reactor and codeposited on a ZnSe matrix grown from hydrogen selenide and diethyl zinc. Composites consisting of CdSe nanocrystals and an amorphous or polycrystalline ZnSe matrix have been deposited on glass substrates at temperatures of 150–250u2009°C. Room‐temperature absorption and photoluminescence spectra show optical transitions characteristic of the initial nanocrystal dispersions. The emission wavelength may be tuned in a broad spectral region by incorporating nanocrystals of varying sizes. The composites have been characterized by Auger electron spectroscopy, high resolution transmission electron microscopy, and scanning transmission electron microscopy.

Stark spectroscopy of CdSe nanocrystallites: The significance of transition linewidths

We use Stark spectroscopy to examine the nature of the excited states of CdSe nanocrystallites. The Stark spectra we obtain are in the small coupling limit in which the changes induced by the electric field to the absorption spectrum are small compared to the transition linewidths. Within this limit, we theoretically examine the dependence of the line shape of Stark difference spectra on the linewidth of the transitions involved. For systems such as CdSe nanocrystallites, which have overlapping transitions coupled by the electric field, we find that the usual association of derivatives of absorption features with dipole moments and polarizabilities is problematic. We show that the Stark absorption spectrum of the CdSe nanocrystallites can be explained by polarizable and delocalized nonpolar excited states.

Size-dependent optical spectroscopy of II-VI semiconductor nanocrystallites (quantum dots)

We use low temperature (10K) optical hole-burning and fluorescence line narrowing spectroscopy to investigate the electronic properties of CdSe nanocrystallites (quantum dots) as a function of crystallite diameter (20–80Å). We discuss how the homogeneous linewidth of the HOMO-LUMO transition, the energy shift between the absorbing and emitting state, and the LO phonon frequency vary with nanocrystallite size.

Surface electronic properties of CdSe nanocrystallites

The surface electronic properties of CdSe nanocrystallites have been probed using low temperature and Zeeman spectroscopies. Fluorescence line narrowed spectra show dramatic changes between 1.75 and 10 K and also as a function of applied magnetic field. These effects are attributed to the localization of the photogenerated charge carriers on the surface. A simple model has been constructed to calculate the charge distribution within the nanocrystallite.

Preparation of II–VI quantum dot composites by electrospray organometallic chemical vapor deposition

Abstract New thin film composites consisting of a ZnSe matrix and CdSe nanocrystals (NCs) have been prepared by a novel technique combining electrospray and organometallic chemical vapor deposition (OMCVD). CdSe NCs, synthesized in solution by controlled growth of CdSe nuclei, were derivatized with pyridine or overcoated with a thin ZnSe layer. The derivatized NCs were dispersed in a pyridine/acetonitrile mixture and transferred into the growth zone of an OMCVD reactor using an electrospray. The transferred NCs were co-deposited with ZnSe grown by OMCVD from hydrogen selenide and diethyl zinc at temperatures ranging from 150 to 250 °C. The absorption and emission spectra of the composites show characteristic transitions of the NCs. The emission wavelength can be tuned by selecting the size of the NCs. A pre-formed ZnSe passivation layer, also synthesized in solution, improves thermal stability of the NCs during the co-deposition, and enhances the photoluminescence emission efficiency of the composites. The elemental composition and microstructure of the materials are probed by Auger electron spectroscopy, X-ray fluorescence spectroscopy, and high-resolution transmission electron microscopy.

Characterization of CdSe nanocrystallite dispersions by small angle x‐ray scattering

We use x‐ray scattering at small angles (SAXS) to study interparticle interactions in dilute dispersions of CdSe nanocrystallite particles. In particular, we probe the dependence of these interactions on a few key parameters, including the nature of the cap molecules attached to the surface of the particles and the solvent. Alkyl capped nanocrystallites with relatively long hydrocarbon chains are stable in a range of solvents. Nanocrystallites capped with smaller molecules show attractive interparticle interactions. Existence of association (e.g., dimers and trimers) in dispersions characterized by attractive interactions is observed in the scattering data for pyridine capped particles.

Synthesis and structural characterization of II–VI semiconductor nanocrystallites (quantum dots)

A methodology for the production of II–VI semiconductor nanocrystallites employing organometallic precursors has been developed. The rapid pyrolysis of reagents in a coordinating solvent provides temporally discrete nucleation. Subsequent controlled growth allows the production of macroscopic quantities of nanocrystallites with consistent structure, surface derivatization and a high degree of monodispersity. The samples produced are structurally characterized with a combination of X-ray and Electron Beam based techniques.

Size-dependent spectroscopy and photodynamics of some II-VI semiconductor nanocrystallites (quantum dots)

The relaxation dynamics of photoexcited CdSe nanocrystallites (quantum dots) are dominated by the surface. Surface electronic properties of CdSe nanocrystallites have been probed using low temperature fluorescence line narrowing and time resolved luminescence. We find that the surface structure creates a random potential for the hole with a size dependent barrier for site to site hopping.