Determining Up Conversion in Lanthanides (Rare Earth Metals) Using Fluorescence Spectroscopy

The up-conversion process is mainly used in critical applications relating to photodynamic therapy, optical sensors, biological labels, lasers, solar cells, and light emitting diodes. In this process, photons are added through sequential absorption and transfer of energy between excited ions, with successive emission of higher energy photons.

Conversely, the up conversion process is not linear with excitation spectra, with the up-converted emission saturating at high excitation powers.

Methods and Materials

A FLS980 fluorescence spectrometer was employed to determine the excitation spectra. This instrument was fitted with a 450 W Xe lamp along with double excitation and emission monochromators. A 976.4 nm diode laser and a PM-1 modulator to accommodate constant and pulsed excitation were employed to acquire emission and time-resolved spectra.

Photomultiplier tube (PMT) detectors in the near-infrared (NIR) (Hamamatsu, R5509-72) and visible (Hamamatsu, R928P) were used to detect emission. Erbium-ytterbium doped sodium yttrium fluoride (NaYF4:YbEr) is an efficient and extensively researched phosphors for NIR to visible up-conversion emission.

To measure photoluminescence, a thin layer of Sigma Aldrich 756555-25G, NaY.77Yb .20Er .03F4 – an up conversion phosphor - was fastened between quartz glass slides and positioned in a front-face holder. The absolute photoluminescence quantum yield (PLQY) was measured with a well-known technique using an integrated sphere.

Results

Powerful up conversion emission was displayed by NaYF4:Yb3+Er3+ around 550 to 650nm as depicted in Figure 1.The emission spectrum determined in the FLS980 spectrometer under NIR excitation and changeable irradiance shows distinct narrow emissions due to the electronic configuration of Er3+.

To determine this spectrum, a spectrometer is required that is capable of resolving the narrow emission lines from the partly forbidden changed accruing in lanthanides.

Emission spectra of NaYF:YbEr from the S3/2 and F9/2 to I15/2 upon 976.4nm excitation

Figure 1. Emission spectra of NaYF:YbEr from the S3/2 and F9/2 to I15/2 upon 976.4nm excitation. Δ&lambdaem=0.1nm, step=0.2nm.

Figure 2 shows the emission from the 4I13/24I15/2 transition under the same excitation conditions, and detected between 1400 nm and 1650 nm with a liquid nitrogen cooled NIR-PMT. The integrated intensity slope were 1.68 and 1.91 for the S3/2 to I15/2 and for the 4F9/2 to 4I15/2transition, respectively. This is in agreement with two-photon up-conversion.

Emission spectra of NaYF:YbEr from the I13/2 – I15/2 upon 976.4nm excitation.

Figure 2. Emission spectra of NaYF:YbEr from the I13/2 – I15/2 upon 976.4nm excitation. Δλem=0.5nm, step=1nm.

Table 1. shows the absolute up-conversion quantum yield of NaYF4:Yb3+Er3+ transitions resulting in red and green up-conversion emission. A PLQY of 3.09% was calculated at an irradiance of 20 W/cm2

Table 1. Absolute up-conversion quantum yield of NaYF4:Yb3+Er3+ measured with an integrating sphere in the FLS980 fluorescence spectrometer.

Irradiance 4S3/24I15/2 (green) 4F9/24I15/2 (red)
10W/cm2 1.54% 0.48%
13W/cm2 2.66% 0.95%
20W/cm2 3.09% 1.17%

The narrow excitation spectra in contrast to the previous processes help in distinguishing the up conversion from cooperative processes. The order of up conversion is also revealed by the narrowing of the excitation spectrum. Figure 3 shows the progressively narrow excitation spectrum for higher orders in the normalized spectra.

Excitation spectra from 900nm to 1020nm at the peak wavelengths of the main emission bands.

Figure 3. Excitation spectra from 900nm to 1020nm at the peak wavelengths of the main emission bands. The experimental conditions were Δλexc=15nm, Δλem=5 nm and a step of 1nm.

Sequential absorption within an ion and energy transfer between ions can also be distinguished by recording the temporal evolution of the radiative decays.

Decay curves of the S3/2 and F9/2 to I15/2 (green and red, respectively) upon 976.4nm and 30µs pulsed excitation

Figure 4. Decay curves of the S3/2 and F9/2 to I15/2 (green and red, respectively) upon 976.4nm and 30µs pulsed excitation.

Time-resolved measurements shown in Figure 4 reveal the energy transfer between Er3+ and Yb3+ ions. Faster rise time observed before the decays is a distinctive feature of the energy transfer process.

This information has been sourced, reviewed and adapted from materials provided by Edinburgh Instruments.

For more information on this source, please visit Edinburgh Instruments.

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