Coating processes – Particles – flakes – or granules coated or encapsulated – Inorganic base
Reexamination Certificate
1999-08-05
2001-03-27
Le, H. Thi (Department: 1773)
Coating processes
Particles, flakes, or granules coated or encapsulated
Inorganic base
C427S226000, C427S592000, C428S403000
Reexamination Certificate
active
06207229
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to luminescent nanocrystalline materials which emit visible light over a very narrow range of wavelengths. The invention further relates to materials which emit visible light over a narrow range tunable over the entire visible spectrum.
BACKGROUND OF THE INVENTION
Semiconductor nanocrystallites (quantum dots) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of quantum dots shift to the blue (higher energies) as the size of the dots gets smaller.
Bawendi and co-workers have described a method of preparing monodisperse semiconductor nanocrystallites by pyrolysis of organometallic reagents injected into a hot coordinating solvent (
J. Am. Chem. Soc.,
115:8706 (1993)). This permits temporally discrete nucleation and results in the controlled growth of macroscopic quantities of nanocrystallites. Size selective precipitation of the crystallites from the growth solution provides crystallites with narrow size distributions. The narrow size distribution of the quantum dots allows the possibility of light emission in very narrow spectral widths.
Although semiconductor nanocrystallites prepared as described by Bawendi and co-workers exhibit near monodispersity, and hence, high color selectivity, the luminescence properties of the crystallites are poor. Such crystallites exhibit low photoluminescent yield, that is, the light emitted upon irradiation is of low intensity. This is due to energy levels at the surface of the crystallite which lie within the energetically forbidden gap of the bulk interior. These surface energy states act as traps for electrons and holes which degrade the luminescence properties of the material.
In an effort to improve photoluminescent yield of the quantum dots, the nanocrystallite surface has been passivated by reaction of the surface atoms of the quantum dots with organic passivating ligands, so as to eliminate forbidden energy levels. Such passivation produces an atomically abrupt increase in the chemical potential at the interface of the semiconductor and passivating layer (See, A. P. Alivisatos,
J. Phys. Chem.
100:13226 (1996)). Bawendi et al. (
J. Am. Chem. Soc.,
115:8706 (1993)) describe CdSe nanocrystallites capped with organic moieties such as tri-n-octyl phosphine (TOP) and tri-n-octyl phosphine oxide (TOPO) with quantum yields of around 5-10%.
Passivation of quantum dots using inorganic materials also has been reported. Particles passivated with an inorganic coating are more robust than organically passivated dots and have greater tolerance to processing conditions necessary for their incorporation into devices. Previously reported inorganically passivated quantum dot structures include CdS-capped CdSe and CdSe-capped CdS (Tian et al.,
J. Phys. Chem.
100:8927 (1996)); ZnS grown on CdS (Youn et al.,
J. Phys. Chem.
92:6320 (1988)); ZnS on CdSe and the inverse structure (Kortan et al.,
J. Am. Chem. Soc.
112:1327 (1990)); and SiO
2
on Si (Wilson et al.,
Science
262:1242 (1993)). These reported quantum dots exhibit very low quantum efficiency and hence are not commercially useful in light emitting applications.
M. A. Hines and P. Guyot-Sionnest report the preparation of ZnS-capped CdSe nanocrystallites which exhibited a significant improvement in luminescence yields of up to 50% quantum yield at room temperature (
J. Phys. Chem.
100:468 (1996)). However, the quality of the emitted light remained unacceptable because of the large size distribution (12-15% rms) of the core of the resulting capped nanocrystallites. The large size distribution resulted in light emission over a wide spectral range. In addition, the reported preparation method does not allow control of the particle size obtained from the process and hence does not allow control of color.
Danek et al. report the electronic and chemical passivation of CdSe nanocrystals with a ZnSe overlayer (
Chem. Materials
8:173 (1996)). Although it might be expected that such ZnSe-capped CdSe nanocrystallites would exhibit as good as or better quantum yield than the ZnS analogue due to the better unit cell matching of ZnSe, in fact, the resulting material showed only disappointing improvements in quantum efficiency (≦0.4% quantum yield).
Thus there remains a need for semiconductor nanocrystallites capable of light emission with high quantum efficiencies throughout the visible spectrum, which possess a narrow particle size (and hence with narrow photoluminescence spectral range).
It is the object of the invention to provide semiconductor nanocrystallites which overcome the limitations of the prior art and which exhibit high quantum yields with photoluminescence emissions of high spectral purity.
SUMMARY OF THE INVENTION
In one aspect of the invention, a coated nanocrystal capable of light emission includes a substantially monodisperse core selected from the group consisting of CdX, where X=S, Se, Te; and an overcoating of Zn Y, where Y=S, Se, and mixtures thereof uniformly deposited thereon, said coated core characterized in that when irradiated the particles emit light in a narrow spectral range of no greater than about 40 nm at full width half max (FWHM). In some embodiments, the narrow spectral range is selected from the spectrum in the range of about 470 nm to about 620 nm and the particle size of the core is selected from the range of about 20 Å to about 125 Å.
In other embodiments of the invention, the coated nanocrystal is characterized in that the nanocrystal exhibits less than a 10% and preferably less than 5%, rms deviation in diameter of the core. The nanocrystal preferably exhibits photoluminescence having quantum yields of greater than 30%, and most preferably in the range of about 30 to 50%.
In another embodiment of the invention, the overcoating comprises one to two monolayers of ZnY. The nanocrystal may further comprise an organic layer on the nanocrystal outer surface. The organic layer may be comprised of moieties selected to provide compatibility with a suspension medium, such as a short-chain polymer terminating in a moiety having affinity for a suspending medium, and moieties which demonstrate an affinity to the quantum dot surface. The affinity for the nanocrystal surface promotes coordination of the organic compound to the quantum dot outer surface and the moiety with affinity for the suspension medium stabilizes the quantum dot suspension.
In another aspect of the invention, a method of preparing a coated nanocrystal capable of light emission includes introducing a substantially monodisperse first semiconductor nanocrystal and a precursor capable of thermal conversion into a second semiconductor material into a coordinating solvent. The coordinating solvent is maintained at a temperature sufficient to convert the precursor into the second semiconductor material yet insufficient to substantially alter the monodispersity of the first semiconducting nanocrystal and the second semiconductor material has a band gap greater than the first semiconducting nanocrystal. An overcoating of the second semiconductor material is formed on the first semiconducting nanocrystal.
In one embodiment of the invention, the monodispersity of the nanocrystal is monitored during conversion of the precursor and overcoating of the first semiconductor nanocrystal. In another embodiment, an organic overcoating is present on the outer nanocrystal surface, obtained by exposing the nanocrystal to an organic compound having affinity for the nanocrystal surface, whereby the organic compound displaces the coordinating solvent.
In addition to having higher quantum efficiencies, ZnS overcoated particles are more robust than organically passivated nanocrystallites and are potentially more useful for optoelectronic devi
Bawendi Moungi
Dabbousi Bashir O.
Jensen Klavs F.
Mikulec Frederic Victor
Rodriguez-Viejo Javier
Fish & Richardson P.C.
Le H. Thi
Massachusetts Institute of Technology
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