Research Projects

Why f-f transition yields vibration-free narrow near-infrared (NIR) emission but d-d transition is vibrationally coupled and broad? Lanthanide (Yb3+, Nd3+, Pr3+, Er3+) doped in different solid matrices show ultra-narrow NIR emissions in the range of 900-1600 nm. The shielded f-electrons do not interact with surrounding ligands. So, the excitation/de-excitation of f-electrons via f-f transitions do not influence the metal-ligand (M-L) bond length. In other words, the f-f transition is purely electronic in nature, without involving any change in vibrational states. This vibration-free, atomic-like f-f transition yields ultra-narrow emissions, and also, does not follow the Kasha's rule. In contrast, typical d-d electronic transitions like t2g³eg⁰ to t2g²eg¹ change the geometric distribution of d-electrons around the metal ion. Therefore, the metal-ligand (M-L) bond length changes with d-d transitions, since the d-electrons interact with the ligands. So, a typical d-d electronic transition is accompanied with vibrational changes, leading to a broad emission, that follows the Kasha's rule. Can d-d transitions behave like f-f transitions? The answer is yes, provided the d-d transition is an intra-configurational (like t2g³eg⁰ to t2g³eg⁰) spin-flip transition. The t2g³eg⁰ to t2g³eg⁰ transition does not change the electronic configuration around the metal ion, and therefore, M-L bond length in the ground and excited state remains unchanged. Only the electronic spin is changed during the transition. So, such intra-configurational spin-flip transitions are purely electronic and free from vibrations. Therefore, such unique d-d transitions behave like f-f transitions. Cr3+ and Mn4+ with 3d³ electrons show f-f-like ultra-narrow visible emission (around red), leading to commercial Ruby laser and white LEDs. What is the gap and prospect of ultra-narrow d-d transitions NIR emission? To extend this intra-configurational (like t2g³eg⁰ to t2g³eg⁰) spin-flip transitions from (Cr3+ and Mn4+) visible to NIR region, one needs to use dopants with 4d or 5d electrons that have high crystal field and smaller Racah parameter. For example, Mo3+ (4d³), W3+(5d³) or Re4+(5d³) can yield ultra-narrow NIR emission, but only in air-free (vacuum) conditions at low temperatures (50 K or below). This is because these 4d and 5d dopants easily oxidize in air (Mo3+ (4d³) to Mo6+ (4d⁰)), completely losing the NIR emission in ambient conditions. Our present proposal is to overcome this issue by designing new materials, yielding stable, intense, and ultra-narrow d-d transition emission in the wavelength range of 800-1600 nm. The stability in ambient conditions will open up the scopes of applications like ultra-narrow NIR LED, lasers and optical fiber. Why is the ultra-narrow d-d NIR emission significant? The proposed materials will show f-f-like d-d narrow NIR emissions, but at wavelengths of the d-d emissions will be different from f-f emissions. Therefore, one can get ultra-narrow d-d NIR emissions beyond the scopes of lanthanides like Nd3+ (Nd:YAG lasers) or Er3+ (Er based optical fiber). So, the proposed materials can lead to new ultra-narrow NIR LEDs, NIR lasers or optical fibers operating at different wavelengths. Importantly, this project will provide insights about the pros and cons of f-f and d-d transitions based vibration-free ultra-narrow NIR emissions. Also, in general, the transition metals are more abundant than the rare-earth lanthanides. What are the main focuses of the project? Material design (Mo3+, W3+, Re4+, Ru5+, Ni2+, etc doped halides and oxide hosts) will remain the main emphasis of the project, followed by the photophysical understanding of the excitation and emission processes of the ultra-narrow NIR emission. The newly discovered materials and NIR emissions will be explored for applications like NIR LEDs, NIR lasers, and optical fibers.