Letter pubs.acs.org/NanoLett
High-Power Genuine Ultraviolet Light-Emitting Diodes Based On Colloidal Nanocrystal Quantum Dots Jeonghun Kwak,†,‡ Jaehoon Lim,§,‡ Myeongjin Park,∥,‡ Seonghoon Lee,*,⊥ Kookheon Char,*,§ and Changhee Lee*,∥ †
Department of Electronic Engineering, Dong-A University, Busan 604-714, Korea School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Korea ∥ Department of Electrical and Computer Engineering, Inter-university Semiconductor Research Center (ISRC), Seoul National University, Seoul 151-744, Korea ⊥ Department of Chemistry, Seoul National University, Seoul 151-747, Korea §
S Supporting Information *
ABSTRACT: Thin-film ultraviolet (UV) light-emitting diodes (LEDs) with emission wavelengths below 400 nm are emerging as promising light sources for various purposes, from our daily lives to industrial applications. However, current thin-film UVemitting devices radiate not only UV light but also visible light. Here, we introduce genuine UV-emitting colloidal nanocrystal quantum dot (NQD) LEDs (QLEDs) using precisely controlled NQDs consisting of a 2.5-nm-sized CdZnS ternary core and a ZnS shell. The effective core size is further reduced during the shell growth via the atomic diffusion of interior Cd atoms to the exterior ZnS shell, compensating for the photoluminescence red shift. This design enables us to develop CdZnS@ZnS UV QLEDs with pure UV emission and minimal parasitic peaks. The irradiance is as high as 2.0−13.9 mW cm−2 at the peak wavelengths of 377−390 nm, several orders of magnitude higher than that of other thin-film UV LEDs. KEYWORDS: ultraviolet, ultraviolet light-emitting diodes, quantum dots, quantum dot light-emitting diodes
U
deposited via epitaxy onto Si or sapphire substrates with a complicated metal−insulator−semiconductor or quantum well structure.1−3 They also require bulky heat sinks for heat dissipation. For years, thin-film-based UV LEDs have been demonstrated using novel materials such as semiconducting organic compounds and metal oxide nanoparticles. Organic lightemitting diodes (OLEDs) with a wide bandgap molecule in the emitting layer were introduced several years ago.4−7 These UV OLEDs exhibit a primary electroluminescence (EL) peak at approximately 400 nm, but their emission profiles extend significantly into the visible region and contain multiple vibronic peaks because of the strong electron−phonon coupling of organic materials. A few papers have reported the preparation of UV LEDs based on metal oxide nanoparticles (e.g., ZnO and SnO2) as active materials via economic solution processing.8−11 Although such metal oxides have the benefit of generating UV radiation based on their intrinsically wide bandgaps, oxygen vacancies in the metal oxide lattice produce
ltraviolet (UV) light, the type of electromagnetic radiation that possesses high photon energies from 3 to 124 eV, is essential to a broad range of applications in industry, hygienic and medical treatments, and even our daily lives.1−3 In particular, ultraviolet A (UVA), which lies in the spectral range between 315 and 400 nm, is widely used for various purposes such as the curing of inks and adhesives, 3D printing, tanning, hygienic water cleaning, light therapy, entertainment (black light), photocatalytic purification, counterfeit detection, and bug zappers. For these applications, in most cases, mercurybased lamps are used as the UV light source. However, the emission spectrum produced by mercury vapor discharge includes not only UVA but also high-energy UV (
