Highly luminescent copper-doped ultrathin CdSe nanoplatelets for white-light generation

Highlights

• Thin populations of CdSe NPLs grown at temperatures below 200 °C emit white-light.

• Emission properties of CdSe NPLs can be tailored by varying the growth temperature.

• Cu-doped ultrathin CdSe NPLs generate white-light with PL QY up to 95%.

• The re-absorption problem in WLEDs can be avoided by using Cu-doped ultrathin NPLs.

Abstract

In this work, the potential of ultrathin pure and copper-doped CdSe nanoplatelets (NPLs) for white-light generation is studied. It is shown that thin populations of CdSe NPLs synthesized at relatively low growth temperatures generate white-light due to combining the band-edge and broad trap-assisted emissions. It is found that the luminescence and photometric properties of the CdSe NPLs can be tailored in a wide range simply by changing their growth temperature. As-grown CdSe NPLs generate white light with a color rendering index (CRI) as high as 75 and color coordinates of (0.31, 0.33) at a correlated color temperature (CCT) of 6438K. Further, to avoid the well-known re-absorption problem and increase photoluminescence quantum yield (PL QY), the thinnest population of CdSe NPLs is doped with copper. It is demonstrated that by copper-doping, the band-edge emission of ultrathin CdSe NPLs can be fully suppressed. Copper-induced emission covers a wavelength range of 400–700nm with PL QY reaching values up to 95%. By integrating the undoped and copper-doped ultrathin CdSe NPLs whith a UV chip, light-emitting devices (LED) emitting within the white-light region of the CIE 1931 chromaticity diagram are fabricated.

Introduction

Colloidal semiconductor nanoparticles have attracted a great deal of attention thanks to their quantum confinement effects and size-dependent photoemisson properties[1,2]. Relatively high PL QY, narrow and tunable emission of colloidal semiconductor nanoparticles make them promising candidates for solid-state lighting and thin-film display applications [[3], [4], [5]]. White-light emitting devices (WLED) by semiconductor nanoparticles are mainly obtained by replacing the conventional down-converting phosphors (usually YAG:Ce) in color-conversion LEDs by a mixture of different color emitting colloidal semiconductor nanoparticles. In these devices, UV- or blue-emitting LEDs are usually used to optically–excite the emissive layer. Thanks to the precisely controlled and pure emission of colloidal semiconductor nanoparticles their applying as down-converting materials results in significant improvements of photometric properties of the devices[[6], [7], [8]]. Although by applying different color emitting nanoparticles, WLEDs with excellent photometric properties were obtained, their fabrication is quite difficult because of complexity involved in maintaining the appropriate proportions of the nanoparticles in the blend [9,10]. On the other hand, the WLEDs made by a mixture of nanoparticles suffer from the self-absorption problem between the different color emitting nanoparticles, which reduces the overall quantum efficiency. For instance, the blue light emitted by smaller nanoparticles would be partially absorbed by the green and red-emitting nanoparticles[11,12]. Besides the self-absorption between different nanoparticles, the emitted light can also be re-absorbed by nanoparticles themselves due to the relatively small Stokes shift between the absorption and emission energies[13,14]. Alternatively, WLEDs can be designed using a single type nanoparticle that emits white light owing to the coexistence of band-edge and broad trap-assisted emissions[13]. The main advantage of WLEDs obtained by a single type nanoparticle is the significantly reduced self-absorption effects. There are also other ways of dissipation of excitation energy such as non-radiative recombination of charges, Förster energy transfer between closely located nanoparticles [15], scattering losses and an energy loss associated with the conversion of high energy photons to low energy photons. However, the self-absorption is one of the main parameters, which determine the quantum efficiency of the WLEDs obtained even by using a single type nanoparticle. Therefore, it is clear that an ideal material for WLEDs should be highly emissive nanoparticles with well-separated emission and absorption spectra that leads to reduction of the re-absorption of the emitted light by the nanoparticles themselves. Up to now, WLEDs by employing a single type nanoparticle were found mostly by using CdSe [[16], [17], [18]], and CdS [13,19] spherical nanoparticles. By adjusting the growth conditions, the ratio between the band-edge and trap related emissions can be changed resulting in different visual properties including color coordinates, CRI and CCT [19,20]. However, complete suppression of the band-edge emission cannot be achieved in single-phase nanoparticles. Sarma et al.[21] proposed a method to eliminate the self-absorption problem using manganese doped CdS nanoparticles. They showed that the incorporating of manganese ions as a dopant into the host nanoparticles results in a broad and large Stokes shift emission separated from the absorption spectrum, thereby overcoming the intrinsic problem of the undoped nanoparticles. However, PL QY of this doped nanoparticles was found to be about 2%, which is low for practical applications. Wang et al. fabricated a WLED by combining a yellow-emitting (YAG:Ce) phosphor and red-emitting Cu-doped CdS/ZnS spherical nanoparticles on a blue LED chip [22]. They demonstrated that the self-absorption problem of multiphase phosphors is successfully eliminated thanks to the large Stokes shift red emission from the Cu-doped CdS/ZnS nanoparticles. Thus, the metal-ion doped nanoparticles are of potential interest for designing WLEDs thanks to their tunable and large Stokes shift emissions.

Two-dimensional cadmium chalcogenide nanoparticles known as nanoplatelets (NPLs) are a new class of colloidal semiconductor nanoparticles with superior optical properties[23,24]. Thanks to their precisely controlled few-atomic monolayer (ML) thicknesses they exhibit giant oscillator strength [25,26], ultra-narrow emission together with relatively high PL QY [27,28] and large absorption cross section [29,30], making them particularly useful for color-converting applications and lasers [31]. Until now, several papers were published regarding the use of CdSe NPLs and different heterostructures based on them as active materials for ultra-pure color emitting electrically driven LEDs[[32], [33], [34]]. To our knowledge, the possibility of white-light generation using CdSe NPLs is not well investigated yet.

In this paper, we comprehensively studied the optical properties of three thin populations of CdSe NPLs to analyze their performances for white-light generation. We demonstrated that thin populations of CdSe NPLs obtained at relatively low growth temperatures generate white light due to combining the broad trap-assisted emission and band-edge emission. It is shown that the luminescence properties of the samples can be tailored by changing the growth temperature, and white-light with color coordinates of (0.31, 0.33), a CRI of 75 at a CCT of 6438K can be obtained using single-phase CdSe NPLs. However, the PL QY of the best samples was about 10% that is low for any practical applications. On the other hand, a small Stokes shift (~5nm) between the absorption and band-edge emission peaks, which is characteristic for NPLs leads to efficient re-absorption of the emitted light. Thus, we further developed the synthesis of Cu-doped ultrathin CdSe NPLs to completely suppress their band-edge emission and increase PL QY. It is shown that by the addition of Cu ions into the ultrathin CdSe NPLs, the band-edge emission can be completely eliminated. It is demonstrated that the dopant induced emission covers a wavelength range of 400–700nm, providing white-light with PL QY as high as 95%. Finally, we fabricated WLEDs, using the undoped and Cu-doped ultrathin CdSe NPLs. The obtained devices showed color coordinates of (0.37, 0.45) for undoped, and (0.31, 0.44) for 0.05% Cu-doped ultrathin CdSe NPLs, which are within the white-light region of the Commission Internationale de l’Eclairage (CIE).