electron micrographs (TEMS) of medium length
(3.3x 23 nm) CdSe core nanorods (a) and the
same cores with different thickness shells
of Cd/ZnS (b-d). The shell thickness is 2
monolayers (b), 4.5 monolayers (c), and 6.5
emitting diodes (LEDs), laser diodes, liquid
crystal displays (LCDs)
- Biological Imaging
electrodes in optically pumped devices
magnetic materials for aligning shaped nanocrystals
and tracing that takes advantage of linearly
polarized light emission
polarized emission switching
the luminescence efficiency of nanorods from
below 1% to up to 20-25%
the applications of nanocrystals by combining
materials with a variety of electronic, optical,
and physical properties
can be grown on complex shapes
linearly polarized light
the optical gain threshold of nanorods
in air and under UV light
Alivisatos, Erik Scher, and Liberato Manna have
grown graded shells on CdSe core nanorods. Traditional
techniques have only succeeded in growing shells
on spherical nanocrystal cores. The Berkeley Lab
researchers have also grown a nanorod of one material
out of the end of a nanorod of a different material
to create simple linear heterostructures. Both
of these methods enable the synthesis of single
nanostructures that can combine the desired electronic,
optical, and other shape and size dependent properties
of semiconductors, metals, or insulators in a
variety of nested shapes or adjacent rod-like
In the Berkeley Lab synthesis, multiple materials
are simultaneously injected into a colloidal solution
containing core nanorods. In the case of the shell
configurations, they self-order according to the
degree to which their lattice matches the core
material or the shell layer onto which it is depositing.
Using this principle of interfacial segregation,
a primary CdS shell and a secondary ZnS shell
were grown on CdSe core nanorods with aspect ratios
from 2:1 to 10:1. The method allows for variation
in shell thickness between one and six monolayers.
After photochemical annealing, the resulting shell
increases the luminescence efficiency of nanorods
from below 1% to up to 20-25%, while preserving
their solubility in a wide range of solvents.
Epitaxial growth of an inorganic shell on a nanocrystal
when the two materials have a close lattice match
removes the surface trap-states of the nanocrystal,
raising the probability of radiative recombination.
Because CdSe nanorods emit linearly polarized
light, the highly luminescent Berkeley Lab materials
could eliminate the need for one of the light
polarizers typically used in liquid crystal displays
(LCDs), making them thinner and more efficient.
These materials can also be used for labeling
and tracing applications that take advantage of
their linearly polarized light emission. For example,
they could be used to follow the conformational
dynamics of a large molecule having a graded core/shell
nanorod attached to it.
Nanorods are potentially superior to quantum dots
for lasing because they have a lower optical gain
threshold. Adding the graded shells lowers the
gain threshold further, making them even better
emitters. Unlike spherical quantum dots, the Berkeley
Lab graded shell nanorods and simple heterostructures
are ideal for contacting and connecting electrodes
in electrically pumped devices. They can also
be used for LEDs, linearly polarized emission
switching, as well as for biological imaging.
Published Patent Application. Available for licensing or collaborative
research. A license to this technology does
not include a license to the Berkeley Lab process
for making core nanrods.
L., Scher, E.C., Li, Liang-Shi, Alivisatos, A.P., "Epitaxial Growth and Photochemical Annealing
of Graded CdS/ZnS Shells on Colloidal CdSe Nanorods,"
Journal of the American Chemical Society
2002, 124, 7136-45.
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