Introduction

Approximately 20% of global electricity consumption is used only for illumination,¹ and in response to the ever-increasing energy demands coupled with serious concern for global warming, there has been an immense interest in the generation of light sources that can save electrical energy consumption, reduce operating expenses, and perform better. High-efficiency light-emitting diodes (LEDs) are^2-4 being explored intensely in this connection. Clearly, the availability of high-quality phosphors is the single most-important requirement for better performance of such LEDs. II-VI semiconducting nanocrystals (NCs) have recently emerged as better phosphors compared to traditional phosphors.^5-7 These nanophosphors have broader and stronger absorption and higher resistance to photooxidation compared to the common emissive materials, such as organic dyes and inorganic phosphors; also for the NCs with sizes less than 10 nm, loss of energy due to scattering is strongly reduced.⁸ Another important aspect of these NCs is solution processability;⁹ the surface of NCs can be functionalized using various organic molecules, making them soluble in both polar and nonpolar solvents. In recent years, transition-metal-doped nanocrystals have come up as a new class^10-13 of light-emitting materials that retain all of the advantages of undoped NCs and also overcome some of the intrinsic disadvantages such as self-absorption and sensitivity to thermal, chemical, and photochemical disturbances compared to their undoped counterpart.^14,15

White-light phosphor is not new, and different schemes have already been reported^16-22 to generate white light. One of the most obvious ways to generate white phosphor is to blend red, green, and blue phosphor in an appropriate ratio.^17,21 Unfortunately, this intuitively most appealing approach is fraught with several disadvantages. Other than the complexity involved in maintaining the appropriate proportions of the individual component in the blend, a major demerit of such system is the overall self-absorption. Although the individual components can reabsorb its own emission, because of the small stokes shift between the absorption and band-edge emission energies, reducing the quantum yield, the more-important concern is the absorption of shorter wavelengths by the particles with smaller band gaps. For example, blue emission from blue-emitting NCs is significantly absorbed by red- and green-emitting NCs, present in the blend. Consequently, a change in solution concentration keeping the ratio of the three components constant can alter the extent of self-absorption of the blue and green light by the other components of the blend with lower bandgaps, resulting in uncontrolled changes in the chromaticity, or the "whiteness" of the emitted light. There is the additional disadvantageous requirement of maintaining the size of each type of nanoparticle in such a blend within a very narrow range in this approach, due to the rapid change in the emission wavelength with the particle size. In a different approach,²⁰ Bowers II et al. generated white light combining the broad surface-state emissions and band-edge emission from magic-sized CdSe NCs with ~2% emission efficiency. The low efficiency of emission is not perceived as an intrinsic disadvantage at these early, exploratory stages in this field with the hope that a particular approach can be subsequently fine-tuned to enhance the efficiency. However, this approach is not suitable for generating different shades of white lights by tuning the chromaticity because of the difficulty in controlling the surface states; this method also requires a high degree of control on the size, which may eventually prove to be an impediment to large-scale technological applications. In a very recent article,²² two routes to white-light generation has been demonstrated. In one case, the core-shell-shell type of multilayer structure has been used. In this case, the material and the size of core and those of the shell are so chosen that emissions of different wavelengths from these two combine to give white light. In essence, this approach is similar to the one based on blending with all of the advantages and disadvantages of that method. The other approach involves generation of white light combining surface-state and band-edge emission, similar to the report²⁰ of Bowers II et al. Here we introduce the idea of generation of white light by combining surface-state emissions of nanocrystalline host and inner-core transitions from dopant centers, with an example of Mn²⁺-doped CdS NCs; the addition of Mn²⁺ as a dopant helps in extending the emission to longer wavelengths. This approach offers several advantages. Because the surface-state emission as well as the inner-core transition at the dopant sites are relatively less sensitive to a size variation compared to the band-gap emissions, the chromaticity of the light generated is not critically dependent on the particle size or its distribution, thereby making it possible to use a sample with larger size distributions. Although neither the surface state nor the dopant emission can be tuned, the chromaticity of the white light can be significantly tuned by altering the relative proportion of each of these two emissions. Additionally, these NCs can be excited over a wide range of excitation wavelengths without disturbing the chromaticity of emission. Finally, this approach does not suffer from self-absorption because of substantial stokes shifts of the component emissions compared to absorption, therefore producing white light both as a dilute solution and in the solid form and proving itself to be an ideal material for a white-light-emitting intrinsic layer in a WLED. We, however, note here that the measured quantum efficiency of about 2% is very small for any practical application at present; hopefully, the quantum efficiency can also be increased within this approach in the near future.

Experimental Section

Syntheses of Mn²⁺-doped CdS NCs and the postsynthesis size-selective precipitation were carried out following the methods¹⁵ developed by Nag et al. and is discussed in detail in the Supporting Information. Perkin-Elmer's Lambda 35 uv/visible spectrometer and Perkin-Elmer's LS 55 Luminescent spectrometer were used for UV-visible absorption and photoluminescence (PL) experiments, respectively. Quantum efficiency was measured using a reference dye; a detailed description of the procedure is provided in the Supporting Information. The molar percentage of manganese with respect to that of cadmium was obtained using Perkin-Elmer's AAnalyst 200, atomic absorption spectrometer, equipped with Perkin-Elmer Lumina lamps for manganese and cadmium at wavelengths 279.5 and 228.8 nm, respectively.

Results and Discussion

Scheme 1 briefly shows the strategy to synthesize CdS NCs with different extents of Mn²⁺ doping. It has been known¹⁵ for some time that in contrast to the high solubility of Mn²⁺ in the bulk CdS lattice, the extent of doping in nanocrystalline CdS even in presence of excess Mn²⁺ ions is very poor, leaving most of manganese in the supernatant solution obtained after the precipitation and centrifugation of the product NCs. First, two samples of Mn²⁺-doped CdS NCs were synthesized with 2 and 3% nominal dopant concentrations in the solution. Size-selective precipitation was carried out for both of these samples of Mn²⁺-doped CdS NCs separately, which allows us to separate the fractions of larger- and smaller-sized NCs within the achieved narrow size distribution for a given reaction, producing a total of four samples. Efficacy of the postsynthesis size-selective precipitation generating two samples from a single synthesis was confirmed by a blue shift in the UV-visible absorption spectrum of the smaller particles compared to that of the larger particles as shown in Figure 1 of the Supporting Information. Atomic absorption spectroscopy (AAS) shows the presence of 0.10, 0.19, 0.28, and 0.36% manganese in the final product NCs; with the average sizes of these four being estimated from the UV-visible absorption edges as 1.8, 1.8, 2.1, and 2.1 nm, respectively, in diameter. As shown in Scheme 1, the percentage of Mn²⁺ is higher in the larger particles compared to smaller particles obtained after size-selective precipitation; this observation agrees with the established¹⁵ fact that for a given reaction condition Mn²⁺ ions are preferentially incorporated in the larger-sized particles within the given size distribution of the synthesized NCs. Most interestingly, 0.10 and 0.19% Mn²⁺-doped CdS NCs emit white light of different shades upon UV irradiation; the photographs of the white-light emission (quantum efficiency ~2%) from these NCs dispersed in dimethyl sulfoxide (DMSO) with the excitation wavelength at 365 nm are shown in Scheme 1. Further increase in the Mn²⁺ concentration to 0.28 and 0.36% vitiates the whiteness of the emitted light perceptively, as shown in the same scheme.

Scheme 1: Reaction Strategy Following Which Mn2+-Doped CdS NCs Have Been Synthesized to Emit Lights of Different Colorsa a Photographs of the emitted light showing 0.10 and 0.19% Mn2+-doped CdS NCs producing white light of different shades upon excitation at 365 nm, whereas 0.28 and 0.36% doping produces yellow emission upon excitation at 383 nm.

UV-visible absorption and PL spectra of 0.10 and 0.19% Mn²⁺-doped CdS NCs are shown in Figure 1a. The absorption spectra are similar to each other with a point of inflection at ~366 ± 1 nm corresponding to a band gap of 3.39 ± 0.01 eV. Substantial increase in the band gap of the NCs compared to the bulk CdS band gap of 2.42 eV (513 nm) is a consequence of quantum-confinement effect, and the particle size was estimated to be 1.8 nm using the reported²³ correlation of band gap with size in the nanometric regime. Polydispersity in size of NCs correspond to a distribution of band gap contributing to the spread of the absorption spetra;^24,25 we estimate²⁴ a size dispersity of 9.7% in these samples from the observed absorption spectra. Detailed characterization of this series of NCs using transmission electron microscopy, powder X-ray diffraction, and electron paramagnetic resonance (EPR) studies were done, and the results are similar to the previous report.¹⁵

Figure 1 (a) UV-visible absorption and PL spectra of 0.10 and 0.19% Mn2+-doped CdS NCs. (b) PL spectra of 0.10 and 0.19% Mn2+-doped CdS NCs. with a time delay of 50 s. All of the PL spectra were obtained by exciting at 365 nm, and the spectra were normalized at their emission peak. (c) CIE diagram showing the chromaticity coordinates of the produced white light of different shades corresponding to the PL spectra of Figure 1a.

The PL spectra in Figure 1a appear broad and significantly red-shifted compared to the absorption edge with hardly any band-edge emission. This is attributed to an extensive presence of noncoordinated surface-related midgap states; the photoexcited electron and hole are transferred to these midgap states, before recombining radiatively to give rise to the broad emission spectrum red-shifted from the absorption spectrum. Decrease in particle size increases the surface-to-volume ratio, resulting in more available surface-related states; for 2 nm CdS NCs, surface-state emissions are the dominant mode of radiative relaxation with no measurable contribution from band-edge emission as shown in Figure 2 of the Supporting Information. Broad surface-state emissions of the undoped NCs spread over the entire visible range generating light close to white in color. In general, it is difficult to tune the energies arising from such defect states, rendering it difficult to obtain any specific desired shades of white-light emission by rational synthesis. The broad emissions for 0.10 and 0.19% Mn²⁺-doped CdS NCs represented by red and blue lines, respectively, in Figure 1a are due to the overlap of surface-state emissions with the orange/red emission coming from ⁴T₁-⁶A₁ transitions of Mn²⁺ d electrons, which is absent in the undoped nanocrystallites. When the same nanocrystal dispersion was excited at 480 nm (2.58 eV), that is with a substantially lower excitation energy than the average absorption edge of ~366 nm (3.39 eV), it does not give rise to any Mn²⁺ d related emission, as shown in Figure 3 in the Supporting Information, confirming the fact that energy transfer from the host CdS nanocrystal excites Mn²⁺ ions that eventually de-excite emitting at ~620 nm involving Mn²⁺ d states. We have explicitly confirmed significant contributions from Mn²⁺ d emission in both samples with delayed PL spectra. We have collected PL emissions of 0.10 and 0.19% Mn²⁺-doped CdS NCs with a 50-s delay time after the initial excitation; these delayed PL spectra, shown in Figure 1b, exhibit a drastically different line shape compared to the time-integrated total PL spectra in Figure 1a. The lifetime associated with the surface-state emissions is much lower than 50 s and is therefore entirely absent in the spectra with a 50-s delay. In contrast, the lifetime associated with the spin-forbidden ⁴T₁-⁶A₁ transition of Mn²⁺ d electrons is on the order of milliseconds.^14,15 Thus, the PL spectra collected with a fixed delay of 50 s with respect to the excitation are contributed by the Mn²⁺ d emission, resulting in the emission peak at 620 nm. These delayed spectra representing Mn²⁺ d emission appear almost identical in spectral shape for the two samples.

The emission properties of these Mn²⁺-doped CdS NCs have been tuned targeting the white-light emission by controlling the dopant concentrations. The total PL spectra corresponding to 0.10 and 0.19% Mn²⁺-doped CdS NCs have chromaticity coordinates of (0.30, 0.40) and (0.35, 0.40), respectively, which are within the white region of the 1931 CIE (Commission Internationale de L'Eclairaged) diagram²⁶ as shown in Figure 1c. The existence of two different CIE coordinates, both falling in the white region, confirm the generation of white lights with two different shades, which is also visible in the photographs of the two white-light emissions corresponding to 0.10 and 0.19% Mn²⁺-doped CdS NCs in Scheme 1. However, light emitted by 0.28 and 0.36% Mn²⁺-doped CdS NCs have chromaticity coordinates of (0.36, 0.43) and (0.37, 0.43), respectively, corresponding to the yellow region of visible light, as evidenced from the respective photographs in Scheme 1. Increase or decrease in Mn²⁺ concentration increases or decreases the contribution from orange/red emission, thereby allowing us to modify the emission spectrum controllably and producing white lights of desired shades.

In order to illustrate that this route to white-light generation does not require a high degree of monodispersity, thereby significantly simplifying the synthesis processes, we have deliberately synthesized NCs with a size distribution as large as 17% by lowering the reaction temperature to 50 C and also by a faster rate of addition of sulfer precursor (10 mL in ~2 min), leading to a doping of 0.17% manganese. This sample emits white light with a chromaticity coordinate (0.33, 0.38), which is in between the two white-light chromaticities shown in Figure 1c, presumably because of its manganese content is intermediate of those in the previous two samples. UV-visible absorption and PL spectra of the sample are shown in Figure 4 of the Supporting Information. These results establish the feasibility of working with samples containing a wide size distribution, unlike the earlier literature-reported methods of generating white light, as discussed in the introduction section. Additionally, we have examined the stability of the white emission, when the sample is exposed continuously to UV radiation. The PL spectral shapes as well as the intensity obtained from the sample after UV exposure of various time durations are shown in Figure 5 of the Supporting Information. Clearly, the emission is very much stable with respect to both spectral shape and quantum efficiency under a continuous exposure of UV radiation over 15 h.

Figure 2a compares the PL spectrum of solid powders of 0.10% Mn²⁺-doped CdS NCs with that of their DMSO dispersion. Evidently, the PL spectrum of nanocrystalline powder is essentially identical to that of the DMSO dispersion. This clearly suggests that self-absorption is not perceptible in the present system. This is a consequence of the large stokes shift of the surface-state emissions and Mn²⁺ d emission compared to the absorption energy, as evident from Figure 1a. 0.19% Mn²⁺-doped CdS NCs also exhibit the same behavior as shown by the PL spectra in Figure 6 of the Supporting Information. Another unique feature of semiconducting nanocrystals is the wide range of excitation wavelengths over which sizable PL intensity can be observed, in sharp contrast to traditional organic phosphors; specifically, the present samples exhibit significant PL over the excitation range of 260-410 nm, as shown in Figure 7 of the Supporting Information. Figure 2b shows the PL spectra of 0.10% Mn²⁺-doped CdS NCs with a few specific excitation wavelengths. We have also included the PL emission, labeled "average" in Figure 2b, by averaging over individual PL spectra with a large number of excitation wavelengths, such that this spectrum represents the expected spectrum when the system is excited with usual UV sources without any wavelength selectivity. All of the PL spectra are normalized at their emission maximum for easy comparison of the spectral shape, illustrating a remarkable constancy of the spectral shape and, consequently, the chromaticity of the exact shade of the emitted light independent of the excitation wavelength. Evidently, a large variety of cheap UV-LEDs, even with a wide energy/wavelength nonselective excitation can be used to generate white light of the desired shade from these samples, maintaining the color purity of the emitted light. 0.19% Mn²⁺-doped CdS NCs also shows the similar behavior as shown in Figure 8 of the Supporting Information.

Figure 2 (a) Comparison of PL spectra of 0.10% Mn2+-doped CdS nanocrystalline powder with that of their DMSO dispersion keeping the excitation at 365 nm in both the cases. (b) PL spectra of 0.10% Mn2+-doped CdS NCs dispersed in DMSO with excitation wavelengths 260, 300, and 365 and an average spectrum obtained by averaging the absolute PL intensity with 260, 300, 365, 385, and 400 nm. All of the PL spectra were normalized at their emission maximum.

Conclusions

White-light emission has been produced from Mn²⁺-doped CdS NCs with an average size of 1.8 nm. The broad PL spectra of these NCs with contributions from both surface-state emissions and Mn²⁺ d emission can be tuned by controlling the dopant concentration, and white lights of different shades were produced. Mn²⁺-doped CdS NCs (0.10 and 0.19%) produce white lights with chromaticity coordinates (0.30, 0.40) and (0.35, 0.40), which is within the white region of the 1931 CIE diagram. These NCs exhibit a huge stokes shift between the absorption and emission spectra minimizing the well-known self-absorption problem, resulting in no change in the emission spectrum of a given sample either in solution or in the solid state. These NCs can be excited over a wide range of excitation wavelengths without compromising the chromaticity.

Acknowledgment

We acknowledge the Department of Science and Technology and the Board of Research in Nuclear Sciences, Government of India, for funding the project. D.D.S. acknowledges the National J. C. Bose Fellowship. A.N. acknowledges CSIR, Government of India for a fellowship.