Near‐Infrared Electroluminescence from HgTe Nanocrystals

Abstract

Composites based on conjugated polymers and strongly luminescent semiconductor nanocrystals (NCs) are considered to be promising materials for optoelectronic applications such as large area light-emitting devices (LEDs). A number of LEDs for the visible spectral range utilizing CdSe [1][2][3][4] or CdTe [5,6] NCs have been reported. If extended to the near-infrared (NIR) region, they could potentially provide active optical components compatible with existing telecommunication technology utilizing the low-loss fiber windows at 1.3 and 1.55 microns. [7] Currently available molecular and polymer dyes generally emit at wavelengths considerably shorter than 1 mm, thus posing an intrinsic limit to the development of all-organic NIR LEDs and other optoelectronic devices. The first examples of the NCbased NIR LEDs were demonstrated very recently utilizing core-shell InAs/ZnSe, [8] PbS [9] and PbSe [10] NCs. Among conjugated polymers, poly(para-phenylene vinylene) derivates were the most widely used so far for the fabrication of both visible and NIR emitting NC-based composites, owing to their holeconducting properties and the possibility of processing both from organics [1,2,8,9] and from aqueous solutions.[3] Herein, we demonstrate, for the first time, NIR LEDs based on a composite of HgTe NCs and a highly ordered ladder-type conjugated polymer. HgTe NCs can be easily produced by room-temperature aqueous synthesis [11][12][13] providing a material with strong fluorescence, ranging from 40 to 50 % quantum efficiency at room temperature, whose emission wavelength is tunable between 900 nm and 2000 nm by changing the size of the NCs. The combination with a wide band-gap hole-transporting conjugated polymer is readily demonstrated and allows the fabrication of composite films by spin casting techniques, which are well established for processing of organic polymers into large area thin films.

Near-infrared electroluminescent devices based on colloidal HgTe quantum dot arrays

“…[8][9][10][11][12] As a result, an initial demonstration of phenomena, such as size-tunable NIR luminescence, electroluminescence ͑EL͒ and optical gain, has been possible. [13][14][15][16][17] Concerning HgTe, in particular, this material has an inverted band structure and an essentially negative band gap of around Ϫ0.15 eV at 295 K. 18 It is expected that the effect of quantum confinement in HgTe quantum dots should increase the effective band gap of the material and give rise to NIR luminescence. To address this issue, colloidal HgTe quantum dots were recently synthesized in an aqueous solution at room temperature by wet chemical means, and were shown to be very stable toward oxidation due to effective surface passivation.…”

“…This measured external quantum efficiency is an order of magnitude higher than that reported for NIR-emitting organic lightemitting diode devices fabricated using HgTe dots blended into a conjugated polymer matrix. 16 This efficiency also represents a minimum value as no optical corrections were made for absorption, wave guiding, or other effects that might reduce light output, e.g., ITO reabsorption ͑a ϳ 100 nm thick ITO film has only 70% transparency at 1500 nm͒. 15 In summary, EL devices based on colloidal HgTe quantum dot arrays have been fabricated.…”

Infrared Emitting HgTe Quantum Dots and Their Waveguide and Optoelectronic Devices

“…Working with Kotov we found HgTe QDs could readily be incorporated into Layer by Layer (LbL) deposited films along with polyelectrolytes [19] and still retain their IR emission and these films even had sufficient conductivity to allow rudimentary IR LEDs to be fabricated and weak electroluminescence (EL) obtained with HgTe [20] (see Figure 3). This was built on earlier similar work on CdTe LbL films [21], which were likewise shown to be electroluminescent [22].…”

NIR-emitting nanocrystals for photonic applications

“…In ref. [39] a schematic of a NIR-LED device is outlined, based on a blend of a conjugated polymer, which is a methyl-substituted ladder-type poly(para-phenylene) (LPPP) and HgTe NCs processed from toluene solution. NIR electroluminescence was readily observable from diodes fabricated from the polymer-NC blend.…”