Yttrium vanadate has a scheelite-like structure, which is represented with the general formula ABO 4, where A = Y, Yb, Lu, and Nd and B = V with a tetragonal structure. (27) Unit-Cell-Win software was used to calculate the lattice parameters for the annealed nanophosphor materials. Here, ions such as Ho 3+ ( r = 1.015 Å), Yb 3+ ( r = 0.985 Å), and K + ( r = 1.51 Å) get occupied at the Y 3+ ( r = 1.019 Å) sites of the YVO 4 lattice since they almost have the same ionic radii with a coordination number (CN) of eight, where eight O 2– ions surround the Y 3+ ion. YVO 4:Ho 3+/Yb 3+/K + shows a strong peak in the (200) plane. Upon K + ion codoping, there is an improvement in the crystallinity. This suggests that the codoped ions may occupy the cationic sites of the host lattice structure.
Upon Ho 3+, Yb 3+, and K + ion codoping, it is observed that the patterns have no significant changes in the number of peaks and no extra peaks are observed. The patterns are indexed with the standard data of pure YVO 4 (JCPDS No. Figure 2(a) shows the room temperature XRD patterns of the annealed samples of YVO 4:Ho 3+/Yb 3+ nanoparticles codoped with different at.% of K + ions. Ho 3+ (1 at.%), Yb 3+ (10 at.%), and K + (0, 2, 5, 7, and 10 at.%)-codoped YVO 4 nanophosphor samples were prepared at a low temperature (150 ☌ for 2 h) and further annealed at 900 ☌ for 4 h. (22−24) However, if the above limitations are overcome, it has many advantages over UV light excitation because of deep penetration of the NIR light and thus materials having a few cm thickness can be detected. The number of literature studies using NIR light excitation is very less because of many limitations such as a very small spot size of the laser (1–2 mm diameter) and need of a proper collimator and proper filters. However, UV light is hazardous for the surrounding environment including the persons who are working. (21) have reported dual-mode color-tunable Y 2O 3:Er 3+/Yb 3+ for anticounterfeit applications using a UV source.
Many materials such as rare-earth-doped luminescent nanomaterials, metal–organic frameworks, quantum dots (semiconductor and carbon based), and plasmonic nanomaterials were used as a security ink in anticounterfeit applications for many years. Generally, vanadate and molybdate hosts are also used in the upconversion process due to low phonon vibrations possessed by them. Using UC nanoparticles, detection of uranyl down to 20 ppm has been achieved.
The particles are invisible in normal light but visible upon 980 nm excitation and are useful in display devices, advanced anticounterfeiting purposes, and therapy of cancer via hyperthermia and bioimaging (since it shows red emission at ∼650 nm). Both UC and hybrid nanoparticles show interesting security ink properties upon excitation by a 980 nm laser. A hyperthermia temperature is achieved from this hybrid. In addition, the polyethylene glycol-coated UC nanoparticles are highly water-dispersible and their hybrid with Fe 3O 4 nanoparticles shows magnetic-luminescence properties. Upon 300 nm excitation, the downconversion emission spectrum shows a broad peak in the 400–500 nm range (related to the charge transfer band of V–O) along with Ho 3+ peaks. Upon 980 nm laser excitation, the UC emission spectrum shows a sharp bright peak at ∼650 nm of Ho 3+ ion and the luminescence intensity increases twofold upon K + codoping. In this work, we report a polyol route for easy synthesis of upconversion (UC) phosphor nanoparticles, YVO 4:Ho 3+-Yb 3+-K +, which enables large-scale production and enhancement of luminescence.