Željka Antić*a,
K. Prashanthi*b,
Sanja Kuzmana,
Jovana Perišaa,
Zoran Ristića,
V. R. Palkarc and
Miroslav D. Dramićanina
aUniversity of Belgrade, Vinča Institute of Nuclear Sciences, P.O. Box 522, Belgrade, Serbia. E-mail: zeljkaa@gmail.com
bUniversity of Alberta, Department of Chemical & Materials Engineering, Edmonton, Canada. E-mail: kovur@ualberta.ca
cIndian Institute of Technology Bombay (IIT-B), Mumbai, India
First published on 30th April 2020
A strategy for optical nanothermometry using the negative thermal quenching behavior of intrinsic BiFeO3 semiconductor nanoparticles has been reported here. X-ray diffraction measurement shows polycrystalline BiFeO3 nanoparticles with a rhombohedral distorted perovskite structure. Transmission electron microscopy shows agglomerated crystalline nanoparticles around 20 nm in size. Photoluminescence measurements show that intensity of the defect level emission increases significantly with temperature, while the intensity of near band emission and other defect levels emissions show an opposite trend. The most important figures of merit for luminescence nanothermometry: the absolute (Sa) and the relative sensor sensitivity (Sr) and the temperature resolution (ΔTm) were effectively resolved and calculated. The relative sensitivity and temperature resolution values are found to be 2.5% K−1 and 0.2 K, respectively which are among the highest reported values observed so far for semiconductors.
Scientific and technological progress creates an ongoing need for the development of new measuring concepts and instruments. There is now an immediate need for non-contact thermometry of moving or contact-sensitive objects, bodies that are difficult to access or in hazardous locations. Temperature measurements resulting from changes in materials' optical properties are considered as a promising route to meet the needs. Raman scattering, optical interferometry, thermoreflectance, near-field scanning optical microscopy and photoluminescence (PL) spectroscopy are the optical methods of interest.1
Temperature strongly affects photoluminescence features of optical materials such as peak energy and intensity, band shape, excited states lifetimes, and rise-times, which can be exploited to measure temperature. The temperature read-out method most commonly used in the present practice of luminescence thermometry is one based on the determination of the ratio of intensities of different emission bands of luminescent material. Ratiometric measurements are self-referencing and prone to fluctuations in excitation sources and detection electronics.1
To date, luminescence-based nanothermometry has been performed mainly using organic dyes, lanthanides (Ln) and transition metal (TM)-doped nanoparticles, and semiconductor quantum dots (QDs). In recent years researchers have started to explore the potential of luminescent materials' negative thermal quenching effect for non-contact temperature sensing. Lei et al. showed in a series of NaGdF4@Ca/Yb/Er:NaGdF4 core–shell nanocrystals that confining doping ions in two-dimensional space and introducing defect energy level via low-valence doping benefit the negative thermal quenching effect in the upconversion nanocrystals.3 Zou et al. reported a strategy for enhancing photon upconversion at high temperatures by taking advantage of negative thermal expansion host materials. They reported a 29-fold enhancement of green upconversion luminescence in the Er3+-doped orthorhombic Yb2W3O12 crystals when temperature is increased from 303 to 573 K.4 Also, authors have focused on uncommon thermal quenching effects in thermochromic materials and Covalent Organic Frameworks (COF). Mazza et al. have shown that the spontaneous opening and closing of oxazine heterocycles might become a viable mechanism to design fluorescence sensors capable of responding ratiometrically to temperature.5 Kaczmarek et al. have proposed a new class of materials, Ln3+-grafted Covalent Organic Frameworks (LnCOF), employed for temperature sensing applications. In Eu/Tb systems they observed a quite unusual and rarely reported behavior in which there is no thermal quenching of the Tb3+ emission, as a result of the absence of ion-to-ligand/host energy back transfer.6
In semiconductors, the intensity of photoluminescence normally decreases with higher temperature (thermal quenching) mainly due to thermally-induced delocalization of charge carriers followed by non-fluorescent trapping.7,8 Here, an approach for monitoring temperature variations using defect emission from intrinsic semiconductor BiFeO3 (BFO) nanoparticles (NPs) is reported. Generally, materials made up of nanoparticles have a larger surface area when compared to the same volume of material made up of larger particles. It means that the surface-to-volume ratio increases as the radius of the sphere decreases. In BFO nanostructures, surface states play a vital role in optical and electrical characteristics due to the high surface-to-volume ratio when compared with bulk material.9–12 They serve as traps for carriers and hence affect the recombination and transport of charge carriers.13–16 Unlike conventional optical thermometry with semiconductors where luminescent signal decreases at high temperatures, we have observed that BFO NPs exhibit an emission-related negative thermal quenching (NTQ), where the photoluminescence intensity increases with temperature. Here, we propose strategy for temperature sensing based on the NTQ mechanism of intrinsic BFO NPs with an excellent performance of thermal sensitivity and resolution.
Structural analysis of BFO NPs was done by X-ray diffraction measurements on a Rigaku X-ray diffractometer using Cu Kα radiation (λ = 1.546 Å).
The PL thermometry data were collected using a Fluorolog-3 Model FL3-221 spectrofluorometer system (Horiba-Jobin-Yvon) over the temperature range from room temperature (RT = 293 K) to 453 K under continuous excitation using 450 W xenon lamp (λex = 320 nm). Time-resolved PL measurements were acquired utilizing a Xenon–Mercury pulsed lamp as excitation source. For PL measurements, the BFO NPs were prepared in a form of pallet. The BFO NPs pallet was placed on a custom-made temperature controlled furnace, and emission spectra were collected via an optical fiber bundle. The temperature of the samples was controlled within the accuracy of ±0.5 K by a temperature control system utilizing a proportional-integral-derivative feedback loop equipped with the T-type thermocouple for temperature monitoring.
The XRD pattern, Fig. 1(d), corresponds to polycrystalline BFO of the R3C rhombohedral distorted perovskite structure (ICDD 01-070-5668), and no noticeable diffractions can be observed for impurity phases. The average crystallite size of ∼30 nm was estimated from the full widths at half maximum (FWHMs) of the resolved reflections in the X-ray powder diffraction pattern using the Debay-Scherrer approximation. A Scherrer constant of K = 0.89 was taken as an average for every reflection although it was shown that this constant is anisotropic and its value may vary for different (hkl).19
One should note that the X-ray diffraction is a structural technique that provides the analysis at a micron scale, while the TEM reveals the structure and morphology at the atomic level (Fig. 1(c)). Therefore, the calculated average crystallite size of ∼30 nm from the XRD is in agreement with the 10 to 50 nm particle size obtained from TEM measurements.
By preparing the material with no traces of impurity phases, one can be ascertain in the reproducibility of photoluminescent measurements and, therefore, in the usability of the material as a luminescent thermal probe.
Fig. 2 (a) Experimental setup used for optical nanothermometry and (b) enlarged PL spectra of BFO nanoparticles over 293–453 K temperature range (temperature increment 10 K). |
Fig. 3 (A) PL emission spectra of BFO NPs at RT (293 K) and at 453 K (λex = 320 nm) and (B) CIE diagram of the BFO NPs in the RT – 453 K temperature range. |
Oxygen vacancy related defect emissions and NTQ behavior has been reported in some oxide systems.20–27 Moreover, in our previously published work on multifferoic BFO nanowires (NWs) reasonable interpretation of the photoluminescence NTQ process in BFO NWs and detailed representation of the energy levels and E1, E2, E3, and E4 bands is given.15 Briefly, according to the multi-level model developed by Shibata28 temperature dependent PL intensity can be expressed by activation energy for the process that increases the PL intensity with increasing temperature – NTQ and activation energy for the non-radiative channels . The activation energies of NTQ process in BFO nanowires were found to be a few times greater for E2 than for E1, E3, and E4. Therefore, as temperature increases, the carriers with lower activation energy will start to escape into the barrier where they can recombine non-radiatively leading to decrease in the PL intensity while carriers with higher activation energy lead to NTQ behavior. In addition to recombination, one should note that energy states are closely spaced in energy so, according to Boltzmann distribution, redistribution of electron population between states exists and is higher at higher temperature (being proportional to kT). The E2 emission comes from the trap state of highest energy which is thermally coupled to lower E3 and E4 energy state. The increase in temperature is followed by a redistribution of electrons between the traps so that high-energy trap (E2) increases in population on account of lower-energy traps (E3 and E4).
The most important figures of merits for luminescence nanothermometry are: the absolute sensor sensitivity, Sa, defined as the slope of the signal change with temperature; the relative sensor sensitivity, Sr, defined as the normalized absolute sensor sensitivity with respect to the measured value and the temperature resolution (ΔTm) indicating the minimum temperature that can be effectively resolved (where σ represents the uncertainty of the FIR measurement, here 0.5):29
(1) |
(2) |
(3) |
The absolute and relative sensitivities of BFO NPs, as well as temperature resolution calculated from eqn (1)–(3) are plotted in Fig. 6 and summarized in Table 1. The maximal value of the relative sensitivity of 2.5% K−1 is found at room temperature for E2/E3 ratio and the value decreases with temperature to 0.4% K−1 at 398 K. Relative sensitivities achieved with BFO NPs are among the highest recorded for semiconductor nanothermometers, but with larger measurement range (for a comparison see Table 2).
Fig. 6 (A) Relative (solid line) and absolute (dashed line) sensitivity of FIR thermometry and (B) temperature resolution. |
FIR ratio | Temperature range; Tm (K) | Thermal sensitivity | Temperature resolution | |
---|---|---|---|---|
Sa (K−1) | Sr (% K−1) | @ 293 K (K) | ||
E2/E1 | 293–453 (293) | 7.4 × 10−3 | 1.1 | 0.45 |
E2/E3 | 293–453 (293) | 1.3 × 10−2 | 2.5 | 0.2 |
E2/E4 | 293–453 (293) | 2 × 10−2 | 1.7 | 0.3 |
Semiconductor material/form | T range (ΔT, K) with the T at which Sr is maximum (Tm, K) | Relative sensitivity (Sr, % K−1) | Resolution (K) | Reference |
---|---|---|---|---|
CdSe/NCs | 82–280 | 0.44 | 8 | |
ZnS-caped CdSe/anodized-aluminum paint QDs | 100–500 (245) | 1.3 | 30 | |
CdSe–ZnS/QDs | 287–316 | 1.6 | 0.3 | 31 |
CdSe–ZnS/QDs | 278–313 | 1.3 | 32 | |
ZnS–AgInS/NPs | 293–353 | 1 | 33 | |
CdSe/ZnS and CdTe/ZnS/QDs | 287–320 (320) | 0.9 | 34 | |
CdTe@NaCl/QDs | 80–360 (199) | 0.61 | 35 | |
BiFeO3/NPs | 293–453 (293) | 2.5 | 0.2 | Present work |
Estimated temperature resolutions (see Fig. 6(B)) were better than 1 K for all ratios and temperatures up to 350 K with 0.2 K maximal resolution at room temperature for E2/E3 ratio. Note that temperature resolution estimates were based on the standard deviation of measurements obtained from the equipment described in the experimental section. In practice, the resolution will depend on the used instruments and the measurement conditions.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra01896a |
This journal is © The Royal Society of Chemistry 2020 |