title: Multi-color colloidal quantum dot based light emitting diodes micropatterned on silicon hole transporting layers.
abstract: We present a colloidal quantum dot based light emitting diode (QD-LED) which utilizes the p-type silicon substrate as the hole transporting layer. A microcontact printing technique was introduced to pattern self-assembled CdSe/ZnS QD films, which allowed creation of an LED with well-defined geometry suitable for monolithic integration on silicon substrates. Our QD-LED consists of multi-layers of inorganic materials: a combination of Au (thickness: 5 nm) and Ag (12 nm) as the cathode, a ZnO:SnO(2) mixture (ratio 3:1, 40 nm) as the electron transporting layer, CdSe/ZnS QDs as the light emission layer, 1 nm SiO(2) as an energy barrier layer, and p-type silicon as the hole transporting layer. These printed QD-LEDs are capable of multi-color emission peaked at wavelengths of 576 nm, 598 nm, and 622 nm, corresponding to sizes of the embedded QDs with the diameters of 8.4 nm, 9.0 nm, and 9.8 nm respectively. The optimal thickness of the quantum dot layers needed for light emission is characterized using atomic force microscopy: for 8.4 nm QDs, the value is 33 nm (+/- 5 nm) or approximately 4 ML (monolayers). Larger turn on voltages were measured (2, 4 and 5 V) for the smaller average particle diameters (9.8 nm, 9.0 nm and 8.4 nm, respectively). The mixture ratio of Zn and Sn was optimized (40% Zn and 25% Sn) to maintain proper hole-electron recombination at the QD layer and avoid the yellowish-white emission from ZnO/SnO(2).
authors.affiliation: Department of Biomedical Engineering, The University of Texas at Austin, Austin, TX 78758, USA.
authors.forename: Ashwini
authors.initials: A
authors.lastname: Gopal
authors.forename: Kazunori
authors.initials: K
authors.lastname: Hoshino
authors.forename: Sunmin
authors.initials: S
authors.lastname: Kim
authors.forename: Xiaojing
authors.initials: X
authors.lastname: Zhang
mesh_terms: D003102:Colloids; D003116:Color; D004867:Equipment Design; D049449:Luminescence; D018625:Microscopy, Atomic Force; D046529:Microscopy, Electron, Transmission; D053758:Nanoparticles / Q000648:ultrastructure*; D010316:Particle Size; D045663:Quantum Dots; D012825:Silicon / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D003102:Colloids; D012825:Silicon