Red Light-Emitting Diode Based on Blue InGaN Chip with CdTexS(1 − x) Quantum Dots

Thioglycolic acid-capped CdTexS(1 − x) quantum dots (QDs) were synthesized through a one-step approach in an aqueous medium. The CdTexS(1 − x) QDs played the role of a color conversion center. The structural and luminescent properties of the obtained CdTexS(1 − x) QDs were investigated. The fabricated red light-emitting hybrid device with the CdTexS(1 − x) QDs as the phosphor and a blue InGaN chip as the excitation source showed a good luminance. The Commission Internationale de L’Eclairage coordinates of the light-emitting diode (LED) at (0.66, 0.29) demonstrated a red LED. Results showed that CdTexS(1 − x) QDs can be excited by blue or near-UV regions. This feature presents CdTexS(1 − x) QDs with an advantage over wavelength converters for LEDs.


Background
One of the main challenges in communication and illumination industries is the development of full color displays and solid-state light-emitting devices. White light-emitting diodes (LEDs), which are considered as the next-generation solid-state illuminants, have recently gained considerable attention because of their high efficiency, long service life, and environmental protection [1][2][3]. At present, white LEDs (WLEDs) are fabricated by combining blue-emitting InGaN chips with yellow-emitting Ce 3+ -doped Y 3 Al 5 O 12 phosphors. However, Ce 3+ -doped Y 3 Al 5 O 12 phosphorbased WLEDs have certain disadvantages, such as low luminous efficiency and a poor color rendering index owing to their red spectral deficiency [4][5][6][7]. A variety of red phosphors have been studied to increase red emissions [8][9][10][11][12][13]. Among the various new red phosphors, II-VI or III-V semiconductor nanoparticles have been widely investigated for wavelength converters [14,15]. Compared with binary quantum dots (QDs) (CdSe, ZnSe, CdTe, etc.), ternary alloy QDs have received a great deal of attention because they can be used in device fields because of their photoluminescence (PL) properties that can be tuned by controlling particle size and the composition of the alloy QDs [16,17]. Cadmium sulfur (CdS) is one of the most important group II-VI nanoparticle (NC) semiconductors and displays a wide direct bandgap (2.42 eV). Compared with CdSe, CdTe QDs have greater exciton Bohr radius (7.3 nm) and stronger quantum size effect [18]. The band gap and lattice parameters of CdTe x S (1 − x) ternary alloy QDs can be varied by adjusting the concentration of S and Te in the CdTe x S (1 − x) compound. In addition, semiconductor nanoparticles can be excited by any optical source with an energy larger than their exciton energy [19].
In the present study, a facile method was developed to synthesize water-soluble red-emitting CdTe x S (1 − x) alloyed QDs by using thioglycolic acid (TGA) as a stabilizer. Compared with the traditional two step aqueous synthesis, the approach proposed in the current study is simpler and more environment-friendly. The effects of reaction time and Te:S mole ratio on the maximum emission wavelength, full width at half maximum (FWHM), and PL quantum yield (QY) were also investigated. Furthermore, a red LED was fabricated by combining a 460-nm emitting InGaN chip with CdTe x S (1 − x) NCs. The performance of the fabricated red LED was then evaluated.

Methods
The red-emitting CdTe x S (1 − x) QDs were synthesized through a one-step approach in an aqueous medium using TeO 2 , Na 2 S, NaBH 4 , and CdCl 2 ·2.5H 2 O as precursors. Exactly 0.3 mL TGA and 100 mL CdCl 2 ·2.5H 2 O solution were added to 250 mL of three-necked flask solution and then mixed under stirring. The solution was then adjusted to pH 10.5 with the dropwise addition of 1 mol/L NaOH solution. TeO 2 , Na 2 S, and NaBH 4 were then injected into the original solution under stirring. The resulting mixture solution was heated to 100°C and refluxed for different periods to control the size of the CdTe x S (1 − x) QDs. These CdTe x S (1 − x) NCs were precipitated with the excess absolute ethyl alcohol added to the CdTe x S (1 − x) QD aqueous solution, centrifuged, and then dried at room temperature.
PL and UV-Vis absorption spectra were measured using a FluoroMax-4 fluorescence spectrometer and Cary 5000 spectrometer, respectively. The PL QY was determined using Rhodamine 6G as reference. Highresolution transmission electron microscopy (HRTEM) images were obtained with Tecnai G2 F20. X-ray diffraction (XRD) analysis was performed using Rigaku/ Dmax-2500 (Cu Kα = 1.5406 Å). LED parameters were measured in an integrating sphere, which was connected to a CCD detector (HAAS-1200) under 20 mA forward bias.

Results and Discussion
The effect of reflux time on the optical properties of the CdTe x S (1 − x) QDs was investigated. Figure 1 shows the PL and corresponding QY of the CdTe x S (1 − x) QDs for different reflux times (varying from 0.5 h to 7 h). The Te:S molar ratio was 0.3:1.7, and the temperature of the system was maintained at 100°C. With an increase in reaction time from 0.5 to 7 h, the maximum emission peak exhibited an evident red shift from 544 nm to a long wavelength of 644 nm because of the quantum confinement effect. The size of CdTe x S (1 − x) QDs grown at different reaction times was measured by 3D LS Spectrometer. The size of particles is 3.87, 3.98, 4.18, 4.32, and 4.43 nm. This is further evidence that the particle size of CdTe x S (1 − x) QDs increases as the prolonged reaction times. The FWHM of the PL spectra was between 64 and 81 nm. As shown in Fig. 1b,