High-throughput 3-dimensional time-resolved spectroscopy: simultaneous characterisation of luminescence properties in spectral and temporal domains

Abstract

Lanthanide luminescence is presented in full spectral and temporal detail by challenging the limits of low-light sensing and high-speed data acquisition. A robust system is demonstrated, capable of constructing high-resolution time-resolved spectra with high throughput processing. This work holds real value in advancing characterisation capability to decode interesting insights within lanthanide materials.By meeting the stringent requirements in lighting, telecommunication, electroluminescent devices, analytical sensors and bioimaging set-ups, the luminescence properties of lanthanide-doped materials have attracted significant research efforts both at the fundamental and applied levels.1 The unique luminescent properties of lanthanide materials offer sharp emission spectrum, large Stokes shifts, exceptionally long lifetime, and upconversion generation that stem from their ladder-like energy levels and sophisticated energy transfer processes. Traditional spectroscopic approaches of either luminescence spectra or lifetime decay rates at individual wavelengths are insufficient to understand the rich chemical physics. This deficiency as result of lacking of robust instrumentation development remains a major barrier for the field of material science and applications. 16 In the NaYF 4 :Yb/Er system, a network of closely spaced Yb ions sensitises with infrared radiation at a wavelength of 980 nm and couples via non-radiative resonance to neighbouring Er ions. In contrast with organic dye fluorophores, the Er ion has multiple excited states with remarkably long (subms) lifetimes, so that upconversion nanocrystals are able to stepwise absorb two or more near-infrared photons, and display blueshifted emission in the visible spectrum. Fundamental to fully understand the mechanisms behind upconversion process in Ln-doped nanocrystals, one must overcome the difficulties that result from multiple energy levels of the lanthanide material itself and the complicated photon relaxation process associated with multiple sensitised photons travelling within a network of thousands of interacting nano-scaled sensitisers and emitters, which together are influenced by the large surface-to-volume ratio common in nanoparticle science. 17 Practically, there are two major properties of the upconversion luminescence that provide opportunities for advanced bioimaging. 12 Firstly, in the spectral domain, narrow-bandwidth anti-Stokes-shifted emission allows efficient colour separation from the autofluorescence. The second advantage lies in a millionfold difference between the much longer lifetimes of upconversion nanocrystals and shorter endogenous fluorophores lifetimes that make up the autofluorescence background (y1 ms cf. y3 ns). Therefore an optical time-gated approach allows almost complete suppression of the autofluorescence and excitation background. From an application perspective, it becomes critical to characterise simultaneously the spectral and lifetime characteristics of Lndoped nanocrystals as a unifying picture. There have been ...

Measuring Activation and Luminescence Time Scales of Upconverting NaYF4:Yb,Er Nanocrystals

“…As shown in Figure , all series of RENs in this work exhibited three characteristic transitions, a green emission at 530 nm corresponding to 2 H 11/2 , 4 S 3/2 → 4 I 15/2 , and a red emission at 650 nm, corresponding to 4 F 9/2 → 4 I 15/2 . The two emission peaks were characteristic of the hexagonal β-phase NaY 0.78 F 4 :Yb 0.2 ,Er 0.02 RENs, as widely known from prior investigation. ,, As seen in the spectrum in Figure , the red-to-green peak intensity ratio decreased as size of RENs increased. This observation is consistent with a previous study, and such phenomena are due to nonradiative decay processes across the relevant energy gaps, which are reported to be more efficient for smaller particles. ,, …”

“…By assuming the tail of lifetime decay curve is attributed to the luminescence process, conventional studies use single-exponential fitting to obtain lifetime of RENs. , The lifetime response would be expressed as …”

“…Upconversion luminescence using rare-earth-doped nanocrystals (REN) is a rapidly growing field with potential applications in bioimaging, − volumetric displaying, , anticounterfeiting technology, , optical storage, solid-state lasers, − and solar cells. , These nanomaterials can be excited by near-infrared (NIR) radiation, which can penetrate several centimeters into biological tissues, , and they produce comparatively strong emissions in the visible range. − Understanding the effects of fabrication processes and structure on luminescence properties often requires time-resolved measurements. Extracting time scales of the processes involved includes fitting using luminescence lifetime models. , Because of the complexity of performing sophisticated fits, , most often single-exponential decay lifetime models on the tails or rise times of the measured response are used to fit the measurements. ,, In these approaches, the initial rise was attributed to the upconversion process, and the tail to the luminescence process . As shown in this work, this assumption is not always justified and can lead to incorrect conclusions on the effects of fabrication processes and structure on the luminescence and upconversion properties.…”

Emission stability and reversibility of upconversion nanocrystals

“…The pH values of UCNPs dispersed in aqueous solution were adjusted using Milli-Q water and the PBS solutions with pH 3 and 12 respectively. The luminescence lifetimes were measured using a purpose-built high-throughput 3-dimensional time-resolved spectrometer, 47 with pulsed 980 nm laser excitation (100 mW) at a repetition rate of 50 Hz. Fig.…”

New method for measuring time-resolved spectra of lanthanide emission using square-wave excitation

“…Equipped with multi-channel photon-sensing and high-speed data acquisition devices (such as intensified CCD camera), three-dimensional time-resolved luminescence spectra could be obtained for certain special use. 14 This method is universally valid and can be applied to characterize any CW lasers, including He-Ne laser, Ar + laser, Kr + laser, Ti:sapphire laser, etc.…”