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Fabrication of Highly Luminescent Graded Core/Shell Nanocrystals and Simple Heterostructures
IB-1830
   
     
  Transmission 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 monolayers (d).  

APPLICATIONS:

  • Light emitting diodes (LEDs), laser diodes, liquid crystal displays (LCDs)
  • Biological Imaging
  • Connecting electrodes in optically pumped devices
  • Growing magnetic materials for aligning shaped nanocrystals on semiconductors
  • Labeling and tracing that takes advantage of linearly polarized light emission
  • Linearly polarized emission switching

ADVANTAGES:

  • Increases the luminescence efficiency of nanorods from below 1% to up to 20-25%
  • Expands the applications of nanocrystals by combining materials with a variety of electronic, optical, and physical properties
  • Shells can be grown on complex shapes
  • Emits linearly polarized light
  • Lowers the optical gain threshold of nanorods
  • Solution processable
  • Stable in air and under UV light

ABSTRACT:

Paul 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 configurations.

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.

STATUS:

  • 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.

PUBLICATION:

Manna, 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.

REFERENCE NUMBER: IB- 1830

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