January 19, 2007 – QD Vision, a startup founded by MIT scientists in 2004 says it has received a US patent for “Stabilized Semiconductor Nanocrystals,” materials that the company says can enhance the performance of quantum dots, for use in quantum dot light-emitting devices (LED) for flat displays.
“The issuance of this patent represents a key milestone toward our goal of enabling display manufacturers to produce the next generation of displays,” said Mark Comerford, QD Vision’s CEO, in a statement.
The patented technology describes a semiconductor nanocrystal stabilized with a polydentate ligand on its surface. Monodentate ligands can exchange and diminish emissions from the nanocrystal, and such nanocrystals embedded in a nonpassivating environment tend to lose their high luminescence and initial chemical inertness, including phase separation.
By contrast, polydentate ligands (a polyphosphine, a polyphosphine oxide, a polyphosphinic acid, or a polyphosphonic acid, or a salt thereof) bind more strongly to the surface of the nanocrystal, stabilizing it and preserving high luminescence. In addition, the outer portion of the polydentate ligand can be chosen to be compatible with the nanocrystal’s surrounding environment, e.g., organic solvent, aqueous media, or polymer matrix.
The semiconductor forming the core of the nanocrystal can include a variety of Group II-VI, Group II-V, Group III-VI, Group III-V, Group IV-VI, Group I-III-VI, Group II-IV-VI, and Group II-IV-V compounds.
Comparison of nanocrystal stability in the presence of oligomeric phosphine ligands vs. monomeric ligands is shown in the image below, taken from the US Patent and Trademark Office’s filing for this patent. Stabilities of photoluminescence are used to measure the different binding affinities and passivating powers of the ligands on nanocrystal surface. The top panel shows that nanocrystals dispersed in THF, passivated by oligomeric phosphine with hexadecyl alkyl chain (solid line) are more stable than those passivated by trioctylphosphine (dotted line). In the bottom panel, in aqueous 0.1 M potassium hydroxide, nanocrystals passivated by oligomeric phosphine with carboxylic acid (solid line) are greatly stabilized compared to nanocrystals passivated by mercaptoundecanoic acid (dotted line).
QD Vision was founded in August 2004 by a trio of MIT scientists to commercialize their work on quantum-dot display materials, processes, and devices.
Last summer the company revealed it had fabricated a 32×64 pixel red quantum-dot device achieving external quantum efficiencies (EQEs) of 2.2% and luminous efficiencies of 2.7 lm/W (3.7 Cd/A) at a brightness of 100 nits and with CIExy color coordinates of (0.65, 0.32), exceeding NTSC standards. The quantum dots were printed within a sandwich of organic semiconductor thin films, which deliver energy to the quantum dots enabling light emission. In November the company said it boosted EQE to 3.1%, and luminous efficiencies to 3.4 lm/W (4.2 Cd/A) at a brightness of 210 nits, and with CIExy color coordinates of (0.66, 0.33). In October the company announced it had made a green quantum-dot LED, with a blue quantum-dot LED is still in development.