Time-Resolved Picosecond Luminescence Spectroscopy of ZnO Tetrapods: Opposite Ultrafast Energy Shifts of UV Stimulated and Visible Defect Emission

Time resolved picosecond measurements of the UV and visible emissions from two ZnO tetrapod sample variants, synthesized with the flame transport synthesis method under two different conditions, were investigated upon UV light excitation. Spontaneous emissions of both regions were found, in line with previous measurements, to be long-lived with two time constants for the UV emission decay. Upon excitation with 340 nm, 100 fs pulses at fluences up to 3.3 × 10–4 J cm–2 (≈1.5 × 1015 photons cm–2 per pulse, corresponding to an estimated carrier density of ≈1 × 1019 cm–3), both ultraviolet (≈ 380 nm) and visible (≈ 490–510 nm) emissions were observed and studied. The UV emission exhibits stimulated characteristics, including spectral narrowing and a superlinear increase with pump fluence, with temporal full widths at half-maximum (FWHM) approaching the instrument response (≈0.8 ps). Wavelength-resolved TRPL reveals opposite early time spectral shifts: the UV emission red-shifts with increasing delay (consistent with exciton–electron scattering and electron–hole plasma formation with band gap renormalization), while the visible emission blue-shifts (assigned to progressive filling of defect-related trap states). Overall, these changes were seen to be nearly identical for both ZnO tetrapods (with minor differences), indicating that both synthesis variants delivered materials with similar optical response, ensuring high reliability of the flame method. These results demonstrate the opposite ultrafast dynamics of excitonic and defect-assisted processes in ZnO tetrapods and highlight their potential for applications requiring tunable emission in both the ultraviolet and visible ranges. Interpretations of the principles behind these events are presented in detail.