Investigation of upconversion and photoacoustic properties of NIR activated Er³⁺/Yb³⁺ doped [RE]VO₄ (RE = Y, Gd) phosphors for photothermal conversion applications

Abstract

Er³⁺/Yb³⁺ doped YVO₄ and GdVO₄ phosphors were prepared through the combustion technique and were characterized through various techniques. XRD has revealed a tetragonal structure for YVO₄ and GdVO₄ phosphors. The 980 nm excited upconversion emission has resulted in intense green upconversion emission from both samples. However, the GdVO₄ sample has shown higher emission intensity. Photoacoustic heat generation in samples has been studied using the same 980 nm excitation, and in this case, the YVO₄ phosphor has shown higher heat generation. The YVO₄ phosphor again has shown lower heat dissipation and, hence, higher photo-thermal conversion efficiency upon irradiation of the sample.

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1. Introduction

The extensive study of luminous materials has been driven by the quest for novel substances capable of functioning effectively in applications such as lighting, sensing, visualisation, nonlinear optics, bioimaging and therapy.¹ A class of luminous materials known as upconversion (UC) phosphors take advantage of the unique wavelength-converting properties of rare-earth/lanthanide (Ln³⁺) ions.² Rare earth-based phosphors are efficient UC materials that absorb multiple photons in the low-frequency infrared region (IR) and have emission spectra in the higher energy visible region.³ In this anti-Stokes luminescence, multiple photons with lower energy are transformed into a single photon having higher energy. This process involves the transfer of accumulated energy in the system, typically absorbed by sensitiser ions, to activator ions.⁴

Phosphors based on rare-earth/lanthanide (Ln³⁺) elements hold appeal due to their distinctive spectral characteristics, including narrow emission and absorption lines and comparatively prolonged emission durations. When incorporated into an inorganic host lattice, the long-lived ladder-like 4f–4f intermediate energy states of these ions enable them to absorb multiphotons to yield anti-Stokes luminescence.⁵ The efficiency of phosphors is influenced by factors such as the host composition, crystallinity, concentration of structural defects, component compositions, distribution of dopant ions, etc.⁶

Dopants of rare earths can be introduced into various organic as well as inorganic host materials. Among them, the rare-earth orthovanadates [RE]VO₄ (such as YVO₄, GdVO₄, etc.) as inorganic hosts are notable for their innate self-activation properties. Vanadate-based materials have a significantly better thermal stability, even above 1300 K, than their fluoride counterparts, which usually begin to oxidise and break down after 700 K.⁷ Their physical and chemical stabilities and favourable band gap and phonon frequency conditions make them excellent host material choices.⁸ They are outstandingly flexible hosts for lanthanide (Ln³⁺) optical centres with down and upconversion emissions. When lanthanide ions (Ln³⁺) are incorporated into their internal structure, these materials demonstrate the luminescence phenomenon upon suitable photo-excitation, displaying distinct, sharp, and narrow emission bands corresponding to the Ln³⁺ 4f–4f transitions.⁵

For years, significant progress has been achieved in developing luminescent materials utilising GdVO₄ and YVO₄ as host lattices. Other lanthanides can easily replace Gd³⁺ and Y³⁺ because of their similar valence and ionic radii. GdVO₄ and YVO₄ crystallise into a tetragonal zircon-type structure under ambient conditions.⁹ Comprehensive research has been done on materials doped with erbium (Er³⁺) and ytterbium (Yb³⁺) ions. This is because Er³⁺ ions have beneficial metastable energy levels with longer lifetimes and emissions in the green and red wavelength ranges. These materials are widely used in various fields, such as security inks and temperature sensing, and in biological applications. Yb³⁺ ions have a broad absorption cross-section around 980 nm, facilitating the effective transfer of excitation energy to Er³⁺ ions.³

Along with the radiative transitions in materials the non-radiative transitions also occur at the same time and these non-radiative transitions can be detected through the photoacoustic (PA) spectroscopy technique, which was discovered by Alexander Graham Bell.¹⁰ This technique initiated a pathway for previously unexplored non-radiative routes of energy transitions in light and matter interaction.¹¹ This technique is widely employed in various applications, including optical and thermal property analysis, quality assessment, trace gas detection, and bioimaging.^12,13 In recent times, there has been a remarkable and rapid growth in research on photoacoustic imaging (PAI), driven by its appealing characteristics as a non-invasive, non-ionising, and multiparametric imaging technique. PAI involves the application of temporally confined optical illumination to a target, generating acoustic waves that convey information about the target's photothermal conversion efficiency and spatial geometry.¹⁴ The primary advantage of photoacoustic spectroscopy (PAS) lies in its immunity to interferences due to scattered, reflected, or transmitted light. Also, a strong laser source can be used to increase the sensitivity of the device because the strength of the acoustic signal is proportional to the absorbed light intensity.¹⁵

There are few studies on the upconversion emission and photoacoustic properties of rare-earth co-doped orthovanadate phosphors. Among the reported upconversion phosphors, the first red-emitting Eu³⁺ doped YVO₄ phosphors facilitated the advancement of colour television display technology.¹⁶ Sheng et al. showed the use of rare earth-doped phosphors as contrast agents in dual-modality luminescence–photoacoustic (PA) imaging to improve the signal-to-noise ratio (SNR) and resolution. Using a single NIR excitation wavelength within biological windows, they simultaneously exploited both radiative and non-radiative de-excitations to record contrast signals in PA and luminescence.¹⁷

Thao et al. developed an efficient process for synthesising large quantities of Yb³⁺, Er³⁺, and Eu³⁺ triplet-doped YVO₄. The prepared material showed strong red emission when excited by ultraviolet light (254 nm) and intense green emission when excited by infrared light (980 nm). Its bioimaging feasibility was also investigated by modifying its surface with L-cysteine (Cys) and L-glutathione (GSH).¹⁸ Mahalingam et al. investigated the effect of increasing the Yb³⁺ concentration in Er³⁺/Yb³⁺-doped GdVO₄ nanocrystals to intensify the green-to-red emission ratio.¹⁹ Stopikowska et al. employed a combination of hydrothermal and calcination methods to synthesise single and co-doped YVO₄:Yb³⁺,Er³⁺. When subjected to continuous laser excitations at 785 nm and 975 nm, the synthesised phosphors exhibited pure green up-conversion luminescence.⁷

Using the solid-state reaction technique, our research group previously successfully synthesised the monoclinic phase LaVO₄ and the tetragonal phase GdVO₄ co-doped with Tm³⁺ and Yb³⁺.²⁰ The non-thermally coupled levels, ³F₃ (700 nm) and ¹G₄ (475 nm), were used for optical thermometry under 980 nm laser excitation, and the LaVO₄:Tm³⁺/Yb³⁺ phosphor showed a higher sensing sensitivity than GdVO₄:Tm³⁺/Yb³⁺. Roy et al. reported a comparative investigation of the upconversion (UC) emissions. They tuned the emission colour and intensity of YVO₄, YTaO₄, and YNbO₄ doped with Ho³⁺/Yb³⁺ by altering the vanadate(V) concentration.²¹

In this work, 2%Er³⁺ and 10%Yb³⁺ co-doped rare-earth orthovanadate ([RE]VO₄, RE = Y, Gd) phosphors were synthesized via a low-temperature solution-combustion process. The structural, morphological, surface and absorption characteristics of the prepared samples were investigated using powder X-ray diffraction (XRD), FTIR, SEM, and UV-vis-NIR absorption techniques. The radiative and non-radiative emission characteristics were analyzed using upconversion (UC) and photoacoustic (PA) spectroscopic techniques. The thermal conversion efficiency of the samples was also determined.

2. Experimental

2.1 Chemicals used

For synthesis, yttrium nitrate (Y(NO₃)·xH₂O; 99.9%; CDH, India), gadolinium(III) nitrate (Gd(NO₃)·6H₂O; 99.9%; CDH, India), ytterbium(III) oxide (Yb₂O₃; 99.99%; Alfa Aesar, USA), erbium(III) oxide (Er₂O₃; 99.99%; Alfa Aesar, USA), ammonium-m-vanadate (NH₄VO₃; 99%; CDH, India), urea (NH₂CONH₂; 99.5% SRL, India) and nitric acid (75.0%, Finar, India) were used. Deionised (DI) water (Millipore Milli-Q water purifier) was used throughout the synthesis. All materials and chemicals were used as obtained and were not subjected to any purification processes.

2.2 Synthesis of the samples

Due to its rapid reaction times and minimal temperature demands, the combustion method is feasible for generating luminous materials, especially those infused with rare earth (RE) dopants.²² In the present study, Er³⁺/Yb³⁺doped rare earth vanadate ([RE]VO₄) phosphors were synthesised via the solution combustion method. Urea served as the fuel agent in the process. The composition comprised 88 mol% [RE]VO₄, 2 mol% Er₂O₃, and 10 mol% Yb₂O₃.

Fig. 1 summarises the different stages involved in the synthesis process. The oxide precursors were converted into equivalent nitrates by processing them with concentrated nitric acid at 80 °C until a clear solution was obtained. The various nitrate precursors were then dissolved in deionised water and mixed at 90 °C while constantly stirring. Once they were blended properly, NH₄VO₃ in DI water was added, giving a pale-yellow colour to the mixture. After a few minutes, urea was added, and the mixture was heated further until the mixture turned into gel form. After this, the gel was put into an alumina crucible and kept inside a furnace at 500 °C to initiate the combustion process. Within a few minutes, dense fumes started to appear, followed by combustion, resulting in the formation of yellow foamy products. Using a pestle and mortar, the obtained products were ground well into fine powder and annealed for 2 hours at a temperature of 800 °C.

Fig. 1 A summary of the different stages involved in the synthesis process.

2.3 Instrumentation

Powder X-ray diffraction was performed for the phase identification of the samples using a high-resolution X-ray diffractometer (Rigaku Smartlab). Field scanning electron microscopy (Zeiss, Supra-55) was used to investigate the morphological characteristics. The absorbance spectra were measured using a UV-vis-NIR absorption spectrometer (Agilent Cary-5000). The PerkinElmer spectrum 2.0 spectroscope was used to record the FT-IR spectra in the form of KBr particles to investigate the vibrational bands and luminescent quenching centres that existed in the sample. A spectrometer based on a charge-coupled device (Avantes, USA, ULS2048X64 spectrometer) and a 980 nm laser source were used to record the UC emission spectra. The photoacoustic spectroscopic data were recorded using a home-assembled photoacoustic spectrometer setup.²³ A diagram illustrating the configuration of the photoacoustic spectroscopic system is provided in Fig. 2. The excitation sources were a xenon lamp and a 980 nm laser. Light modulation was achieved using an optical chopper (model SR540) operating within a frequency range of 4 Hz to 3.7 kHz. The acoustic signals produced inside the custom-made photoacoustic cell were detected and amplified using a very sensitive microphone-amplifier setup (Bruel & Kjaer, 50 mV Pa⁻¹). These signals were then processed by a high-end lock-in amplifier (SRS560), which was connected to the chopper controller before being recorded by the computer.

Fig. 2 Schematic diagram of a home-assembled photoacoustic spectrometer setup.

3. Results and discussion

3.1 X-ray diffraction (XRD) analysis

The powder XRD data of the samples were recorded in the 2θ range of 10–80°. The structure and phase of the prepared samples annealed at 800 °C were identified from the obtained diffraction spectra. The XRD patterns, as well as the standard card of doped [RE]VO₄ samples, are depicted in Fig. 3(a and a′).

Fig. 3 (a and a′) XRD pattern and (b and b′) crystal structure of the unit cell and (c and c′) Rietveld refinement of the XRD patterns of 2%Er³⁺/10%Yb³⁺ co-doped YVO₄ and GdVO₄.

The XRD results demonstrated that both samples are crystallised in their typical zircon tetragonal structure. The diffraction peaks matched well with the typical tetragonal phase of YVO₄ (ICSD 98-001-5604) and GdVO₄ (ICSD 98-008-1703) with the space group I41/amd.^24–26 The most prominent peaks are detected around 2θ values of approximately 25°, 33°, and 49°, corresponding to the (hkl) planes (200), (112), and (312), respectively. Although they are less intense, both samples contained some feeble impurity peaks in their XRD spectra. In YVO₄, a few peaks were found in the 2θ range of 15–32°. Among them, peaks around 20°, 23°, 26°, and 32° correspond to the (011), (101), (111), and (200) planes of orthorhombic phased yttrium vanadate(III) (ICSD 98-015-5477), while the peak around 15° may be due to the presence of cubic phased diyttrium divanadate (ICSD 98-016-0600). In the case of GdVO₄, only one impurity peak was observed around 20°, which indicates the presence of the orthorhombic phased gadolinium vanadate(III) (ICSD 98-004-0391).

The doping sites of Er³⁺/Yb³⁺ ions within the host materials can be determined using the percentage radius variance (Δr) resulting from the doping process, which is given by,²⁷

where R_h(CN) and R_d(CN) represent the ionic radii of the host and doping ions, associated with their corresponding coordination numbers. It has been suggested that the effective substitution of host lattice sites by dopant ions is possible when the Δr value is below 30%.²⁸ Applying this formula, the percentage radius variance (Δr) value of V⁵⁺ (0.54 Å, CN = 6) ions is obtained as 64% with Er³⁺ (0.89 Å, CN = 6) and 60.74% with Yb³⁺ (0.868 Å, CN = 6). Meanwhile, the Δr (%) values of Y³⁺ (0.9 Å, CN = 6) and Gd³⁺ (0.938 Å, CN = 6) host ions paired with Er³⁺ and Yb³⁺ were calculated as 1.11%, 3.55%, 5.11%, and 7.46% respectively. Therefore, it is reasonable to infer that both dopants, Er³⁺ and Yb³⁺, occupy positions within the Y³⁺ and Gd³⁺ sites in their respective host matrices.²⁹Fig. 3(b and b′) illustrates the schematic representation of the unit cell of the crystal structure as predicted by VESTA software.³⁰ In this structure, each vanadium ion (V⁵⁺) within the [VO₄]³⁻ groups is coordinated tetrahedrally to oxygen ions (O²⁻). Also, the rare-earth ions (Y³⁺ and Gd³⁺) are positioned within a dodecahedron composed of eight oxygen ions.³¹

Further, Rietveld refinement of the XRD patterns of 2%Er³⁺ and 10%Yb³⁺ doped YVO₄ and GdVO₄ samples was performed to elucidate the phase formation. Fig. 3(c and c′) presents the observed Bragg's diffraction peaks, fitted curve and the corresponding differences of YVO₄ and GdVO₄ samples. The reliability of the fitting was confirmed by the goodness of the fit parameters R_p, R_wp and GOF. A comprehensive overview of the structural properties of the synthesized samples is presented in Table 1.

Table 1 Refinement parameters of 2%Er³⁺/10%Yb³⁺ doped YVO₄ and GdVO₄

Parameters	YVO4	GdVO4
Space group	I41/amd	I41/amd
Cell parameters	a = b = 7.10953(79) Å	a = b = 7.18879(12) Å
	c = 6.28431(69) Å	c = 6.33402(11) Å
	α = β = γ = 90°	α = β = γ = 90°
	V = 317.644 Å^3	V = 327.334(13) Å^3
Reliability factors	R p = 6.21	R p = 4.57
	R wp = 8.16	R wp = 5.84
	GOF = 1.73	GOF = 1.28

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3.2 FE-SEM analysis

The surface morphology and particle size of the prepared samples were examined through FE-SEM analysis. Fig. 4(a and b) presents the FE-SEM images of the rare-earth orthovanadates co-doped with Er³⁺ and Yb³⁺. The images revealed that the samples were synthesised in irregular shapes. The particle size distribution was calculated from the FE-SEM images using ImageJ software and is given in Fig. 4(a′ and b′). The average diameter of the samples was found to fall within the 0.8–1.4 μm range.

Fig. 4 (a) and (b) FE-SEM images and (a′) and (b′) particle size distribution of Er³⁺/Yb³⁺ co-doped YVO₄ and GdVO₄.

3.3 FTIR analysis

Any foreign volatile impurity present in samples along with the maximum phonon frequency of the host can be identified using the Fourier-transform infrared spectroscopy (FTIR) analysis technique.³²Fig. 5 illustrates the FTIR spectra recorded within the 450–4000 cm⁻¹ range. The peak at 824 cm⁻¹ is due to the antisymmetric stretching mode of V–O bands present in VO₄³⁻ units.³³ On the other hand, the peaks around 1635 and 3428 cm⁻¹ correspond to the hydroxyl group's stretching (ϑOH) and bending (δOH) vibrations, likely caused by the inherent moisture-absorbing properties of our materials.³⁴ It can be concluded that samples have the same maximum lattice frequency of 824 cm⁻¹.

Fig. 5 Fourier-transform infrared spectroscopy (FTIR) of 2%Er³⁺/10%Yb³⁺ co-doped YVO₄ and GdVO₄ phosphors.

3.4 UV-visible-NIR absorption spectroscopy

The absorption properties of synthesized phosphors were recorded and analysed in the UV-visible-NIR region. Fig. 6 illustrates the optical absorption spectra of the co-doped rare-earth orthovanadates recorded within the 250–2000 nm wavelength range.

Fig. 6 UV-visible-NIR optical absorption spectra of Er³⁺/Yb³⁺ co-doped GdVO₄ and YVO₄.

Both phosphors exhibited wide absorption bands spanning 250–500 nm, with two prominent peaks centred around 264 and 312 nanometers. The charge transfer state (CTS) transitions between O²⁻ and V⁵⁺ ions are the source of these peaks.³⁵ The charge transfer arises from the transition of 2p electrons of excited oxygen ligands (O²⁻) to the empty 3d orbitals of the central vanadium atom (V⁵⁺) in the VO₄³⁻ groups. As per the molecular orbital theory, this involves the transitions from the ¹A₂(¹T₁) ground state to two close-lying ¹A₁(¹E) and ¹E(¹T₂) excited electronic states of the VO₄³⁻ ions in crystalline hosts. The crystal field reduces the original T_d symmetry of VO₄³⁻ (free ion) to D_2d, causing a splitting of the degenerate levels of VO₄³⁻.^36–38 The broad band in the range of 400 nm in the spectra may be due to the overlapping of different bands of VO₄³⁻.³⁹ The absorption band observed at 980 nm is a result of the ²F_5/2 to ²F_7/2 transition of the Yb³⁺ ion.⁴⁰ No other 4f–4f transition of doped ions is seen due to weak absorption by doped ions.

3.5 Upconversion emission study

The anti-Stoke frequency upconversion emission characteristics of the Er³⁺/Yb³⁺ co-doped phosphor subjected to the laser excitation at 980 nm are illustrated in Fig. 7(a). The spectra consist of two prominent green emission peaks approximately at 525 nm and 553 nm, originating from the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of the Er³⁺ ion. Meanwhile, the red emission peak at 660 nm is due to the Er³⁺ ion's ⁴F_9/2 → ⁴I_15/2 transitions.^40,41 Among YVO₄ and GdVO₄, the latter exhibited the highest emission intensities for all three peaks.

Fig. 7 (a) Upconversion emission spectra of Er³⁺/Yb³⁺ co-doped [RE]VO₄, (RE = Y, Gd) phosphors. (b) Energy level diagram and emission mechanisms of 980 nm excited Er³⁺ and Yb³⁺ ions.

The observed UC emissions of the prepared phosphors result from several processes. Various excitation and emission processes, such as ground state absorption (GSA), excited state absorption (ESA), energy transfer (ET), and cross-relaxation (CR), are involved. Fig. 7(b) illustrates the energy level diagram that explains the diverse processes responsible for the observed emissions. The larger absorption cross-section of Yb³⁺ ions allowed them to absorb a significant portion of the incoming near-infrared photons, leading to their transition from ²F_7/2 to ²F_5/2 levels. Simultaneously, a certain amount of NIR photons was also absorbed by the Er³⁺ ions. This induces a ground-state absorption (GSA) process in Er³⁺ ions through which they undergo a transition from the ⁴I_15/2 to ⁴I_11/2 level. The perfect energy resonance among the ²F_5/2 and the ⁴I_11/2 levels of the Yb³⁺ and Er³⁺ ions facilitated an effective energy transfer from Yb³⁺ to Er³⁺ ions. The energy gained by the Er³⁺ ions in this process resulted in their transitions to the higher energy ⁴F_7/2 state. Most of the Er³⁺ ions populated at the ⁴F_7/2 level via excited state absorption (ESA) de-excited non-radiatively to the ²H_11/2 level. Among them, a few transited down again into the ⁴S_3/2 level non-radiatively. The radiative transitions of Er³⁺ ions into the ⁴I_15/2 level from ²H_11/2 and ⁴S_3/2 levels resulted in two green UC emission peaks, one around 525 and the other at 553 nm. A few Er³⁺ ions populated at ⁴I_11/2 by ground-state absorption de-excited non-radiatively into the ⁴I_13/2 level. Subsequently, from there, they undergo a transition into a higher ⁴F_9/2 state via ESA. The ⁴F_9/2 was also populated by the non-radiative transitions from the ⁴F_7/2 and ⁴S_3/2. These Er³⁺ ions occupied in the ⁴F_9/2 level underwent radiative transitions into the ⁴I_15/2 ground state, resulting in the 660 nm red emission.^42–45

3.5.1 Pump power-dependent UC emission. The influence of pump power on the emission characteristics of samples was explored using a 980 nm laser. The power of the excitation source was varied in the range of 49 to 273 mW, and the corresponding variations in the intensities of emissions were recorded. Fig. 8(a and b) depicts the pump power-dependent upconversion emission spectra of YVO₄ and GdVO₄ doped with Er³⁺ and Yb³⁺. The relation between emission intensity and pump power is already established using the equation,⁴⁶

I = Pⁿ (2)

Fig. 8 Pump power-dependent UC emission spectra of Er³⁺/Yb³⁺ co-doped (a) YVO₄ and (b) GdVO₄ and (c) ln(P)–ln(I) plot associated with the 525 nm emission.

At a moderate pump power, the upconversion luminescence intensity (I) exhibits a proportional relationship to the laser power (P) raised to the power of n, where ‘n’ represents the number of photons participating in a specific upconversion emission process. The slope of the linear fit of the ln(P)–ln(I) graph would be the value of n. Fig. 8(c) depicts the relationship between the pump power and the emission intensity of upconversion associated with the 525 nm emission of the prepared phosphors on a logarithmic scale. The obtained values were around 1.5, deviating from the anticipated involvement of two photons, as indicated by the energy level diagram. This divergence is attributed to the participation of different energy transfer processes, such as cross-relaxation (CR) and other non-radiative emissions. At higher concentrations of the activator ions, the average distance between them decreases. This facilitates the energy transfer process between them. Also, the presence of chemical functional groups such as O–H and C–O with high vibrational frequency modes present in the sample increases the nonradiative transitions. These eventually reduce the slope values in UC emissions. A similar behaviour in slope values has been found in certain theoretical investigations, which explain that due to intermediate-level saturation, the computed slopes deviate from the expected theoretical values.^47,48

3.6 Constant wavelength mode photoacoustic spectroscopic study

Prepared samples were studied for chopping frequency dependent photoacoustic signals. This study will reveal the intensity of heat generation and its diffusion in the samples. Samples were excited using a 980 nm laser. The acoustic signals were generated inside the cell due to the periodic heat flow from periodically heated samples to the surrounding gas. The photoacoustic (PA) signal was recorded using a highly sensitive microphone at chopping frequencies of 5 Hz to 60 Hz. The variation in the photoacoustic signal intensity with the variation in chopping frequency in Er³⁺/Yb³⁺ doped YVO₄ and GdVO₄ at 18.23 W cm⁻² excitation power density is depicted in Fig. 9(a). As the chopping frequency increases, the PA signal amplitude decreases due to a decrease in the energy density of the light beam.^23,49 YVO₄ exhibited the highest PA signal amplitude among the two samples, specifically at lower chopping frequencies, indicating that the YVO₄ lattice generated more heat compared to the GdVO₄ lattice. This can be correlated with the upconversion emission; as the GdVO₄ phosphor shows higher emission, it should have lower non-radiative emission in principle.

Fig. 9 (a) Variation of the photoacoustic signal amplitude with chopping frequency at an excitation power density of 18.23 W cm⁻² and (b) ln(chopping frequency)–ln(PA signal) graph for transition frequency calculation.

The length within the sample over which the entire thermal energy is dispersed is called the thermal diffusion length (μ_s). The sample is considered as thermally thin or thick based on whether the thermal diffusion length is greater than or less than the sample thickness (l). With the chopping frequency, the depth at which thermal energy penetrates the sample also varies.⁵⁰ The thermal transition and characteristic frequency (f_c) of the samples were determined from the ln(chopping frequency) vs. ln(PAS amplitude) graph illustrated in Fig. 9(b). A sudden change in the slope of the graph was observed at a certain point. The antilog of the ln(chopping frequency) value at which this change occurs gives the value of f_c.⁵¹ The transition frequencies were found to be 25 Hz and 28 Hz for YVO₄ and GdVO₄, respectively. These transition frequencies indicate that at these frequencies the samples are changed from thermally thin to thermally thick state. This characteristic frequency is related to the thermal diffusivity (α) of the sample by the relationship α = f_cl².⁵² Lower transition frequency will result in lower heat dissipation to the environment and hence higher temperature could be achieved upon irradiation. This property is beneficial for photo-thermal applications of such phosphors in biology.

3.7 Photo-thermal conversion efficiency measurement

Due to their NIR absorption and efficient conversion of light into heat, rare-earth-based photothermal converting agents are beneficial for photo-thermal therapy applications.^53,54 The photothermal conversion efficiency, η, is a measure of the amount of heat produced after light radiation is absorbed by the sample, which is given by,where T_∞ and T_max represent the ambient temperature and the maximum temperatures achieved by the sample after exposure to laser energy, C_p is the specific heat capacity of the phosphor dispersion. τ_p and P denote the time constant and laser power. A_λ refers to the absorbance of the upconversion nanoparticle at the excitation wavelength λ. Q₀ is the heat dissipated by the sample holder after interacting with radiation. The value of Q₀ is obtained by measuring the temperature variation profile of the holder solvent system without the phosphor.where τ₀, C_p,0, and T_max,0 represent the system's time constant, specific heat capacity, and maximum temperature achieved during laser irradiation, respectively. Therefore, the equation¹⁰ is given by

Here, the value of η is computed by analysing the temperature changes over time in the suspension during the laser ON state (temperature rise profile of the system). For highly diluted phosphor fluids (where there is a negligible amount of sample in the suspension), C_p is approximately equal to C_p,0, yielding;^46,55

Fig. 10 illustrates the temperature variations of the prepared phosphor samples with time under 980 nm laser irradiation. From the temperature rise profile, the photothermal conversion efficiencies are calculated to be 31% and 22.83% for YVO₄ and GdVO₄, respectively.

Fig. 10 Temperature variation with time under 980 nm laser irradiation of Er³⁺/Yb³⁺ doped (a) YVO₄ and (b) GdVO₄.

4. Conclusions

Er³⁺/Yb³⁺ doped YVO₄ and GdVO₄ phosphors were prepared through the combustion technique and were characterized through various techniques. Both the prepared samples have shown good upconversion emission, and among the two, the GdVO₄ phosphor has shown higher emission intensity despite having a similar phonon frequency. Both samples were also studied for photoacoustic heat generation, and it was found that the YVO₄ phosphor showed a higher heat generation compared to the other. The YVO₄ phosphor again has shown lower heat dissipation and, hence, higher photo-thermal conversion efficiency upon irradiation.

Author contributions

Cinumon K. V.: conceptualisation, methodology, investigation, formal analysis, data curation and writing – original draft; Minarul I. Sarkar: investigation, formal analysis, and data curation; and Kaushal Kumar: conceptualisation, supervision, data curation, validation, resources, and writing – reviewing and editing.

Data availability

Data will be made available on request.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Cinumon K. V. is thankful to the Indian Institute of Technology (Indian School of Mines) Dhanbad for providing financial support in terms of a research fellowship. The authors acknowledge the Central Research Facility and the FTIR lab at the Department of Chemistry and Chemical Biology, IIT (ISM) Dhanbad. The authors are also thankful to Ms Chandni Kumari, Department of Physics IIT(ISM), for assistance with the Rietveld refinement analysis. Prof. Kaushal Kumar acknowledges the Department of Science and Technology (DST) Science and Engineering Research Board (SERB), New Delhi, India (as project DST (CRG/2023/006161) for financial support.

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