Single-phased emission-tunable Mg and Ce co-doped ZnO quantum dots for white LEDs

(Mg,Mn)-dual doping synergism towards luminescence and electrical properties of ZnO/p-Si heterojunction diodes

In this study, we investigated the impact of divalent dual dopants on ZnO by examining the structural and spectroscopic properties of (Mg,Mn)-doped ZnO thin films deposited using spray deposition technique. Also, we analysed the current-voltage (I-V) characteristics of (Mg,Mn)-doped ZnO/p-Si heterojunctions for potential light-emitting applications. X-ray diffraction of (Mg,Mn)-doped ZnO on glass substrates reveals a compression along the c-axis and a reduction in crystallite size compared to the Mn-doped ZnO film. Moreover, the band gap of Mn-doped ZnO samples increases from 3.29 eV to 3.35 eV with the addition of the Mg dopant. The optical disorder, as estimated through the Urbach tail, increases from 0.33 eV to 0.5 eV with an incremental increase in the concentration of Mg. XPS studies confirmed the substitution of Mn2+ and Mg2+ into Zn2+ in MnMg:ZnO samples. A dominant color of yellow with wavelength 585 nm was recorded, suitable for yellow emitting devices. In the set of fabricated heterojunctions of MnMg:ZnO/Si, 2 at% Mg doped MnZnO film showed a low knee voltage of ∼1.8 V. It was observed that all the MnMg:ZnO/p-Si heterojunctions showed good rectifying behaviour. Various diode parameters were found using transport models such as TE and Norde, wherein a barrier height of ∼0.6-0.7 eV and an ideality factor in the range of ∼1.5-3 was observed. Retention of good crystallinity, slight band gap tuning, apt barrier height, low sheet resistance, and better emission properties were identified for the prepared MgMn:ZnO thin films that find application in optoelectronic devices.

Self-Reduction-Related Defects, Long Afterglow, and Mechanoluminescence in Centrosymmetric Li2ZnGeO4:Mn2

Mechanoluminescent materials have shown great application potential in the fields of stress detection, anti-counterfeiting, and optical storage; however, its development is hindered by the unclear mechanism. Different from the mainstream exploration of new mechanoluminescent materials in non-centrosymmetric structures, a centrosymmetric mechanoluminescent material Li₂ZnGeO₄:Mn²⁺ is synthesized by a standard high-temperature solid-state reaction in an ambient atmosphere. Combined with the Rietveld refinement, photoluminescence, electron spin resonance, and X-ray photoelectron spectroscopy, it is proved that the increase in oxygen vacancies is accompanied by the self-reduction process from Mn⁴⁺ to Mn²⁺, and the mechanism of mechanoluminescence is clarified through the afterglow and thermoluminescence spectra. The carriers trapped by the shallow traps participate in the mechanoluminescence process through the tunneling effect, while the carriers trapped by the deep traps take part in the mechanoluminescence process via conduction band or tunneling. A signature anti-counterfeiting application is designed using the new mechanoluminescent material Li₂ZnGeO₄:0.004Mn²⁺. Utilizing the afterglow characteristics of Li₂ZnGeO₄:xMn²⁺ phosphors, we designed an intelligent long-persistent luminescence quick response code (QR-code) and visualized information encoding/decoding model, which provides a fast, simple, and effective method for information encryption, transformation, and dynamic anti-counterfeiting. This study not only analyzes the self-reduction and mechanoluminescence processes in detail but also breaks the limitation of crystal symmetry and provides a new strategy for the exploration of novel mechanoluminescent materials.

Exploration of yellow-emitting phosphors for white LEDs from natural resources

White light-emitting diodes (LEDs) are widely used in various lighting fields as a part of energy-efficient technology. However, some shortcomings of luminescent materials for white LEDs, such as complexity of synthesis, high cost, and harmful impact on the environment, limit their practical applications to a large extent. In this respect, the present work aims to study the ability of using Berberine (BBR) chloride extracted from Rhizoma coptidis and Phellodendron Chinese herbs as yellow phosphor for white LEDs. For this, white LEDs were successfully fabricated by applying 0.006 g of BBR chloride onto the blue LED chips (450 nm). The produced LEDs exhibited good luminescence properties at a voltage of 2.4 V along with eco-friendly characteristics and low cost. The Commission International de l'Eclairage chromaticity, the correlated color temperature, and the color rendering index were determined to be (${x} = {0.32}$, ${y} = {0.33}$), 5934 K, and 74, respectively. Therefore, BBR chloride is a suitable environmentally friendly and easily accessible yellow phosphor for white LEDs.

Journey of ZnO quantum dots from undoped to rare-earth and transition metal-doped and their applications

Currently, developments in the field of quantum dots (QDs) have attracted researchers worldwide. A large variety of QDs have been discovered in the few years, which have excellent optoelectronic, antibacterial, magnetic, and other properties. However, ZnO is the single known material that can exist in the quantum state and can hold all the above properties. There is a lot of work going on in this field and we will be shorthanded if we do not accommodate this treasure at one place. This manuscript will prove to be a milestone in this noble cause. Having a tremendous potential, there is a developing enthusiasm toward the application of ZnO QDs in diverse areas. Sol-gel method being the simplest is the widely-favored synthetic method. Synthesis via this method is largely affected by a number of factors such as the reaction temperature, duration of the reaction, type of solvent, pH of the solution, and the precipitating agent. Doping enhances the optical, magnetic, anti-bacterial, anti-microbial, and other properties of ZnO QDs. However, doping elements reside mostly on the surface of the QDs. The presence of doping elements inside the core is still a major challenge for doping techniques. In this review article, we have focused on pure, rare-earth, and transition metal-doped ZnO QD properties, and the various synthetic processes and applications. Quantum confinement effect is present in nearly every aspect of the QDs. The effect of quantum confinement has also been summarized in this manuscript. Furthermore, the doping of rare earth elements and transition metal, synthetic methods for different organic molecule-capped ZnO QDs, mechanisms for reactive oxygen species (ROS) generation, drug delivery system for cancer treatment, and many more application are discussed in this paper.

White light emission from a single plant source extract with tunable photoluminescence

Blue, yellow and red emissions from the extract of a single plant source (pomegranate), under NUV light excitation have been reported. The blue emission (450 nm) was attributed to baicalin and protein, whereas the yellow (550 nm) and red (665 nm) emissions were due to two kinds of anthocyanin components (A1 and A2, respectively). Both the green-to-white and yellow-to-white photoluminescences were tuned by variation of excitation wavelengths (350-400 nm). This change in photoluminescence was due to the occurrence of Forster resonance energy transfer from baicalin to A1. White light emission with good CIE color coordinates (0.34, 0.33) was obtained from the pomegranate pulp extract solution (12% w/v) at excitation of 350 nm. The results demonstrated that white light emission could be achieved from a single plant source, which would provide a new method for the design and fabrication of WLE with simple, green, and low-cost materials.