Up-Conversion Emissions from HfO2: Er3+, Yb3+ Nanoparticles Synthesized by the Hydrothermal Method

Up-conversion emission from HfO2 nanoparticles, as a host lattice, doped with Er3+ and Yb3+ ions and codoped with alkaline cations Li+ and Na+ obtained. The HfO2 nanoparticles, about 80 nm in diameter, were synthesized by the hydrothermal method at 200 °C for 1.3 h, and an additional heat treatment at 1000 °C was necessary to ensure the dopants incorporation into the host lattice. These nanoparticles were studied by means of XRD, Raman Spectroscopy, SEM, EDS, PL, CL, and up-conversion luminescence. First, the doping was performed with Er3+ ions in different percentages. The photoluminescence and cathodoluminescence studies showed an inefficient emission, and only at 7 at % Er3+ ions, the sample presented emissions at 522, 545, and 656 nm corresponding to the transitions of the Er3+ ions. So, codoping was carried out, and HfO2: Er3+/Yb3+ generated an efficient conversion process. The atom percentage of Yb3+ ions was fixed (7 at % Yb3+), and the Er3+ content was varied, showing the highest emission intensity at 3 at % Er3+ ions. Subsequently, the up-conversion emission intensity was optimized by varying the percentage of Yb3+ ions and keeping the Er3+ ion content fixed (3 at %). Adding cations such as Na+ and Li+ in different percentages, a notable improvement of the up-conversion emission intensities in the HfO2: Er3+/Yb3+ nanoparticles was obtained. The up-conversion emission bands observed were located at ∼523 and 544 nm, corresponding to the electronic transitions 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2, respectively. While the bands at ∼652 and 673 nm correspond to the transition 4F9/2 → 4I15/2, respectively. The excitation of these materials with infrared radiation (980 nm) produced noticeable emission bands in the red spectral range, whereas excitation with accelerated electrons (CL) generated prominent bands in the green region.


■ INTRODUCTION
Study of luminescent nanomaterials has been growing exponentially, thanks to the multiple applications they have from smart displays, 1 scintillators, 2 CMOS devices, 3 lightemitting diodes, 4−6 fluorescent lamps, 7 sensors, 8 optoelectronic devices, 9 and solar cells 10,11 to medical and/or biological applications such as theragnostic or bioimaging. 12,13Luminescent nanomaterials present energy emission phenomena called Stokes or anti-Stokes shifts.The first one presents an emission wavelength longer than the exciting radiation, which is commonly on the order of UV (ultraviolet), and this process is also known as down conversion (DC).In the second one, the opposite process occurs, the emission wavelength is shorter than the excited radiation one, which, in this case, is in the IR (infrared) range, and this effect is known as up-conversion (UC). 4To obtain each of these phenomena, a host lattice and inserted activator ions are needed.−21 Metal oxides are known for their good physical and chemical stability, low phonon frequency, and large energy gap, to mention some of their most important characteristics. 21,22−30 The characteristics that it possesses are its wide ban gap (5.68 eV), 2 a high refractive index, 8 transparency in the visible range, 31,32 great dielectric constant, 7 high melting temperature (2774 °C), good chemical stability, 33 high crystallographic density, and so forth, 3,34 making it suitable for optical applications. 35Among its various applications in engineering, 8,31 it is found as an ideal candidate for the creation of luminescent materials where it plays the role of host lattice.−39 There are different works that report the use of HfO 2 as a host lattice doped with Ba, 8 Er 3+ , 25 Ce 3+ , 31 Tb 3+ , 34 Eu 3+ , 40 Au, 41 Al, 42 Cu, 43 Mn 2+ , 44 Gd, 45 Ti, 46 La, and 47 Sm 3+48 and codoped such as Er 3+ /Yb 3+ , 7 Ce 3+ /Dy 3+ , 49 Lu 3+ /Eu 3+ , 50 Er 3+ / Yb 3+ /Li + , 51 and La/Er 3+ /Yb 3+ /Li + , 3 confirming that it is a good host lattice for Ln 3+ ions.
Luminescent nanomaterials can generate UC or DC emissions, depending on the excitation source and nature of the Ln 3+ ions used.The UC is given by the absorption of two or more lower energy photons, producing a higher energy photon. 3,52The main advantage of UC is its excitation source that operates in the infrared region, which implies a lower cost and a long lifetime of the devices. 7Er 3+ and Yb 3+ ion pair is known for their good UC emission using a 980 nm excitation source.Er 3+ ion is known for its long lifetime in the 4 I 11/2 state 33,51 and emissions by green and red lights.In general, the efficiency of UC in Er 3+ is low, and it is highly dependent on the amount of dopant ions as well as the aid of a sensitizer such as Yb 3+ .The combination of the Yb 3+ and Er 3+ ions as dopants in phosphors to obtain UC was reported in different matrices: NaYF4:Er 3+ /Yb 3+ , 53,54 BaYO:Er 3+ /Yb 3+ , 55 K 0.3 Bi 0.7 F 2.4 :Yb 3+ / Er 3 + , 5 6 NaGdF 4 :Yb 3 + /Er 3 + , 5 7 Sr 2 GdF 7 :Yb 3 + /Er 3 + , 5 8 KNbO 3 :Yb 3+ /E 3+ , 59 KGdF 4 :Yb 3+ /Er 3+ , 60 BaY 2 O 4 :Yb 3+ /Er 3+ , 19 YVO 4 :Yb 3+ /Er 3+ , 61 Gd 2 O 3 :Yb 3+ /Er 3+ , 20 Y 2 O 3 : Yb 3+ /Er 3+ , 21 LaNbO 4 :Yb 3+ /Er 3+ , 62 and ZrO 2 :Er 3+ /Yb 3+ . 4In the works, the good coupling between Er 3+ and Yb 3+ ions stood out.For example, in 2018, Masashi Hanioka et al. concluded that the UC emission with these ions is dependent on the host lattice structure and the energy transfer between them, as well as the separation distance that they have in relation to the composition of the host lattice. 52In other studies, it was reported that the incorporation of Li + can considerably increase the UC emission, since adding this ion modifies the crystalline structure, distorting the crystalline field around the Er 3+ ion, which means that prohibited transitions can be carried out, and with this, the UC emission can be improved. 51,63,64Just as Li + , Na + , and K + can perform this type of effect when they are added to UC materials.However, Li + stands out as the most appropriate, according to its low ionic radius.When Li + ions are incorporated in the host lattice, the interatomic distance between Er 3+ ions increases, and it reduces relaxation processes. 3here are scarce reports of HfO 2 codoped with Yb 3+ /Er 3+ ; in 2010, L. A. Gomez et al. studied the structural differences and UC luminescence of ZrO 2 and HfO 2 doped with Er 3+ and Yb 3+ synthesized by the sol−gel technique, under the 980 nm excitation. 7In 2015, Carmona Tellez et al. reported the behavior of HfO 2 powders doped with Er 3+ /Yb 3+ /Li + obtained by the simple evaporation method, and this material was embedded inside a polyester film to fabricate the UC films. 51n 2020, Mariscal Becerra et al. studied the structural behavior, UC, and DC luminescence of HfO 2 doped with La 3+ /Er 3+ /Yb 3+ and Li + synthesized by the solvent evaporation technique, where the main objective was to study host lattice modification with the addition of La 3+ ions. 3ne of the limitations of this type of material is the UC efficiency.Therefore, in this work, the main objective was to obtain the optimal emission thorough the variation of concentration of dopants and codopants (HfO 2 :Er 3+ , HfO 2 : Er 3+ /Yb 3+ , HfO 2 :Er 3+ /Yb 3+ /Na + , and HfO 2 :Er 3+ /Yb 3+ /Li + ) and to reach an improvement in efficiency.The red emission due to the UC process was possible to observe by the naked eye at natural light when phosphor nanoparticles were excited by the 980 nm wavelength.Also, green emissions were obtained when the material was excited by accelerated electrons.

■ EXPERIMENTAL DETAILS
The HfO 2 :Er 3+ , HfO 2 :Yb 3+ /Er 3+ , HfO 2 :Yb 3+ /Er 3+ /Na + , and HfO 2 :Yb 3+ /Er 3+ /Li + UCNPs were synthesized by the hydrothermal method, using solutions 0.04 M of HfCl 4 (Sigma-Aldrich 98% purity) and NaOH (Golden Bell 97% purity) as precipitating agent; an optimized pH = 14 was used.The dopant ion sources were YbCl 3 •6H 2 O (Alfa Aesar 99.9% purity), Er(OOCCH 3 ) 3 •4H 2 O (Alfa Aesar 99.9% purity), and LiCl (Alfa Aesar 98% purity).All precursor solutions prepared using deionized water as a solvent were placed into a Teflon container inside a 30 mL capacity stainless steel autoclave and heat-treated at 200 °C for 1.3 h.The autoclave was then allowed to cool to room temperature, and, subsequently, the resulting precipitates were rinsed several times with deionized water and dried at 85 °C for approximately 3 h.Finally, the obtained nanoparticles were subjected to thermal treatment (TT) at 1000 °C for 2 h to ensure the dopant ions incorporation into the host lattice.Structural properties of the samples were characterized by X-ray diffraction (XRD) with a Bruker D8 diffractometer.Photoluminescence was evaluated by a JOBIN YVON HORIBA spectrofluorometer, and in the case of UC measurements, the samples were irradiated at 980 nm with a BWF1 Fiber Coupled Laser System.Cathodoluminescence was tested with the use of a cathode ray tube, applying a voltage of 5 kV with a current of 0.3 mA.Raman spectra were recorded by a Thermo Scientific with a 532 nm excitation laser.SEM images were obtained by a scanning electron microscope JEOL JSM-7600, and the chemical composition was determined by means of energy dispersive spectroscopy (EDS).
■ RESULTS AND DISCUSSION Crystallographic Properties. Figure 1 shows the XRD diffractograms recorded for HfO 2 nanoparticles, undoped and codoped with Er 3+ , Yb 3+ , and Na + , as obtained by hydrothermal synthesis + TT at 110°.In all the diffractograms of the analyzed samples, the (111) plane at 28°typical of the monoclinic phase was found, 43,48,49 and the change from the monoclinic to the cubic phase is observed upon addition of the lanthanide ions (Ln 3+ ) of Er 3+ and Yb 3+ and subsequently upon addition of the Na + cation.
It was reported that the addition of lanthanide ions stabilizes the cubic phase at room temperature, and this was attributed to the presence of oxygen vacancies. 24The sites of Hf 4+ ions are replaced by Ln 3+ ions in both the monoclinic and cubic lattices. 36,65The cubic phase has, as its characteristic, the (111) reflex located at 30°. 3,24,36,65 Therefore, phase mixing is present after the addition of the dopants into the host lattice, and this behavior is in good agreement with JCPDS No. 00-043-1017 66−70 for the monoclinic phase and JCPDS No. 01-070-2831 71 for the cubic phase.In Table 1, the fwhm values and the grain size calculated using the Scherrer formula are listed.For the grain size analysis, the (111) plane was used.There is a decrease in grain size with the addition of Ln 3+ ions as well as a shift of the peaks, indicating micro strains in the crystalline lattice.The presence of Na + ions induces a deformation in the crystal lattice, causing an increase in grain size, and this behavior was observed with the presence of other alkaline ions such as Li + .The increase in the intensity of the (111) peak indicates a stabilization of the cubic phase.The diffractograms corroborate what was seen by Raman, the decrease of the Ag Hf−O mode characteristic of the monoclinic phase suggests a phase mixture. 35igure 2 shows the Raman spectra of HfO 2 nanoparticles.1][42][43][44]50,65 The peaks located at lower wavenumbers are mainly due to vibrations of hafnium ions (Hf 4+ ), and these peaks are located below ∼300 cm −1 . The peakafter the most intense peak (∼495 cm −1 Ag) are due to exclusive vibrations of oxygen (O 2− ) ions, and this is due to the accommodation of the atoms in a monoclinic cell (Figure 2a).While the modes activated by the dopant ions can be appreciated at Figure 2b, phonons due to Yb 3+ are located at 100−200 cm −1 , whereas Er 3+ vibrational modes have dispersions since 500−1100 cm −1 .72 Therefore, there is a peak superposition between HfO 2 and Er 3+ vibrational modes, which could be attributed to a possible substitution of the Hf 4+ ions by Ln 3+ ions, which causes changes in the HfO 2 atomic structure, which consequently generates a change in the vibration of the bonds; the peaks are sharper, and such a behavior has already been reported.73,74 The change of the vibrational modes with respect to the doping level is presented in Table 2, and it has been proven that the variation of the doping level does not affect the intensity of the activated Raman modes in an increasing or decreasing way.However, the fwhm of the AgHf-O (∼500 cm −1 ) presents a notorious decrease (77%), which indicates that there is an increase in the crystallinity of the material, and this is characteristic of the doped matrices.A Raman shift of 5 cm −1 of this mode is probably due to the stress generated by the Er 3+ and Yb 3+ ions inside the host lattice.On the other hand, it has been proved that the variation of the doping level does not affect the intensity of the activated Raman modes in an increasing or decreasing way. Hwever, there is a slight Raman shift of the AgHf-O mode in the undoped HfO 2 matrix to HfO 2 codoped samples at 3, 5 y 7% Er 3+ to 9 y 15% Er 3+ .The shift is due to the stress generated by the Er 3+ and Yb 3+ ions inside the host lattice, taking into account that the equipment error is 0.1 cm −1 .There are reports in which the phase change is mentioned with the addition of ions from monoclinic phase to cubic.3,23,31,34,35,50 Morphological Properties.SEM micrographs of doped and undoped HfO 2 nanoparticles are displayed in Figure 3. Figure 3a shows the HfO 2 nanoparticles without heat treatment (at 1000 °C), with a nanoparticle size of ∼60 nm, and their surface is formed of agglomerated spheroidal particles throughout the entire sample.The HfO 2 nanoparticles which were subjected to a heat treatment of 1000 °C for 2 h are shown in Figure 3b, and in this case, a diameter of ∼80 nm is observed.Here, a small increase in the size of the nanoparticles is found that is probably due to the thermal treatment.In this case, some type of nucleation is observed, changing the surface morphology from a spherical shape to a shape more like an oval or "egg" by the union of the spheroidal nanoparticles.75−77 The surface of the HfO 2 : Er 3+ (7 at %) nanoparticles heat-    506.0 cm −1 14.0 treated at 1000 °C for 2 h is presented in Figure 3c.The surface looks porous, formed by particles of approximately 90 nm, like the one shown in Figure 3b but larger.Finally, the HfO 2 nanoparticles doped with 7 at % of Yb 3+ and 3 at % of Er 3+ are shown in Figure 3d, and this sample displays agglomerated and porous morphology with particles size of ∼90 to 110 nm.Evidently, the thermal treatment and doping modify the morphology of the samples by promoting the binding of the spheroidal nanoparticles of the initial material.
−80 Figure 4 shows the SEM micrographs of the codoped samples.The micromorphology of the HfO 2 : 3 at % Er 3+ + 20 at % Yb 3+ nanoparticles is shown in Figure 4a.This surface shows agglomerated particles which are shaped like peanuts   combined with spheroidal particles with an average size of ∼100 nm and greater.The pattern of HfO 2 : 3 at % Er 3+ + 7 at % Yb 3+ + 20 at % Na + is displayed in Figure 4b.The addition of sodium produces a greater nucleation of nanoparticles and a decrease in their homogeneity, presenting particles of 200 nm and more.Evidently, the excessive size of the particles is associated with the presence of sodium ions.The chemical composition (EDS) results for doped and undoped HfO 2 nanoparticles are given in Table 3.The undoped HfO 2 sample shows no traces of impurities, such as chlorine or sodium, despite being precursors of the hydrothermal synthesis, and it is due to the high temperatures of the thermal treatment 75,76 and to the washing processes during the synthesis.The sample doped nominally with 7 at % Er 3+ was characterized by the presence of Er, indicating that it was incorporated into the host lattice.According to the results obtained by this technique, it is observed that the stoichiometry of the material is affected by the inclusion of the Er 3+ ion in comparison with the pure HfO 2 sample, with a greater variation in the percentage of Hf present in samples doped with 7 at % of Er 3+ .This behavior is similar in the sample codoped with 7 at % Er 3+ and 3 at % Yb 3+ with respect to the Yb 3+ ion.
Photoluminescence Properties.Down Conversion.The PL spectra of DC (down conversion) for HfO 2 :Er 3+ (0, 3, 5, 7, 9, and 15 at %) nanoparticles under the excitation at 372 nm are presented in Figure 5.In the HfO 2 :Er 3+ (0 at %) sample, a broad band is noted from 400 to about 600 nm, having a maximum at 475 nm.The band is attributed to V O4 and mainly V O3 oxygen vacancies, and this is the intrinsic PL emission of the HfO 2 nanoparticles.As the Er 3+ content increases, this broad band decreases in intensity, and the typical green bands of Er 3+ ions appear because with the increase of Er 3+ ions, the number of defects in the matrix decreases, thus decreasing the cold light characteristic emission of HfO 2 . 81The PL emission bands at 530, 553, and 564 nm correspond to the electronic transitions 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2 of Er 3+ ions.The red emissions from the Er 3+ ions are practically negligible.The presence of these bands corroborates that the Er 3+ ions are embedded in the crystal lattice of the host matrix.It is evident that the maximum PL intensity is reached at 3 at % of Er 3+ ions, and that in this case, these emissions coexist with the already diminished intrinsic PL emissions from the host lattice.It is also observed that, in the samples doped with 7, 9, and 15 at % of Er ions, the bands in the green region have greater intensity than the broadband from undoped HfO 2 .The rise of doping increases the green PL emission intensity, decreasing, in theory, the number of characteristic oxygen vacancies of HfO 2 .It is known that the trivalent Er 3+ ions replace the tetravalent Hf 4+ ions in the crystal lattice, causing the existing vacancies to decrease by the coupling of these ions into the host lattice. 23,81p-Conversion. Figure 6a shows the UC (up-conversion) emission spectra for codoped samples: HfO 2 : Yb 3+ (7 at %) + different atomic concentrations of Er 3+ ions (0, 1, 3, 5, 7, 9, and 15 at %).The excitation wavelength is 980 nm.The codoping was performed by fixing the Yb 3+ ion at 7 at %.This was based on other studies performed in HfO 2 34,40,82,83 and studies performed with different doping concentrations of Yb 3+ ions in other host matrices. 7,10,33,51,55,84,85In this contribution, it was observed that the increase of Er 3+ ions raised the UC luminescent intensity, having its maximum threshold at 3 at % of Er 3+ ions.A decrease of the emission intensity after exceeding this percentage due to the concentration quenching effect was observed.The bands observed at ∼523, 544, 652, and 673 nm correspond to the electronic transitions 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 , and 4 F 9/2 → 4 I 15/2 , respectively, generated by the different energy transfer mechanisms of the upconversion effect (ESA = excited absorption state, CR = cross relaxation, and ET = energy transfer). 86,87Evidently, the UC emissions generated come from the electronic transitions in the Er 3+ ions. 51,52,55,85It was found that varying the amount of Er 3+ doping in the codoping mostly changes the red emission intensity of Er 3+ ions, which is of utmost importance for bioimaging applications as it allows to modify the emission intensity because there is a decrease of the UC luminescence in this type of applications. 85On the other hand, the red emission allows a greater penetration into the skin, thanks to the infrared excitation which is within the biological window (650−950 nm), which benefits the interaction with the organic tissues. 4The inset exhibits an amplification of the green band zone of the UC emission spectrum generated from the Er 3+ ions.Figure 6b shows the UC emission spectra for codoped samples HfO 2 : Er 3+ (3 at %) with different atom concentrations of Yb 3+ ions (0, 3, 5, 7, 9, 15, 17, 20, 23, and 25 at %).The excitation wavelength was 980 nm.Here, the typical emissions from the Er 3+ ions are observed with greater intensity in the red range, and the green bands are almost imperceptible.The highest UC emission intensity is reached for 20 at % of Yb 3+ ions; for higher values, a concentration quenching phenomenon is observed.
Figure 7a,b shows the UC emission bands and possible energy transfer mechanism for HfO 2 :Er 3+ / Yb 3+ nanoparticles.This phenomenon begins with the excitation of electrons from the ground state (GSA) 2 F 7/2 of the Yb 3+ ion to the excited state 2 F 5/2 , followed by the transfer of this energy to an Er 3+ ion.The bands located at ∼523 and 544 nm, which correspond to the electronic transitions 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2 , respectively, are generated mainly by the energy transfer occurring from Yb 3+ to Er 3+ .The excitation of the 4 I 15/2 ground state of Er 3+ with the laser energy (980 nm) yields the 4 I 11/2 → 4 I 15/2 transition, and the energy transfer (ET) from Yb 3+ ( 2 F 5/2 → 2 F 7/2 ) to Er 3+ at 4 I 11/2 provides the photon summation, which makes the 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2 transitions possible.While the bands at ∼652 and 673 nm corresponding to the 4 F 9/2 → 4 I 15/2 transition occur due to the cross relaxation (CR) of radiation existing at higher sublevels ( 4 F 3/2 ,    so this transition is generated in two emission bands ((a) ∼ 652 and (b) ∼ 673 nm).It is said that for this transition, rigid sublevels are generated, which are caused by the thermal coupling of the ions. 55he difference of emission generated in UC in the doped and codoped samples is displayed in Figure 8a.The decrease in the spectrum generated by the oxygen vacancies existing in the HfO 2 host lattice is observed by adding the Er 3+ (7 at %) ions, and one can see the up-conversion effect.The Er 3+ ion is reported as one of the lanthanide capable of generating both up-conversion and down conversion emissions because it has appropriate localized states. 3In the spectrum of the HfO 2 :Er 3+ sample, a wavelength of 372 nm was used generating down conversion emissions, and the energy absorption in the sample originates a step in the electrons in the levels 4 I 11/12 and 4 F 7/2 from where the electron decays in a nonradiative way mainly in the levels 4 S 3/2 and 2 H 11/2 .When the IR radiation (980 nm) absorber (Yb 3+ ) is introduced, emissions in the red zone of the Er emission spectrum appear prominently.This comparative study was mainly done to emphasize the large differences in band intensities in the red region when Yb 3+ ions are introduced.The colors of these emissions are identified with the help of the CIE (Commission International de L'Eclairage) chromatic diagram showing the coordinates (x, y) for the UC emissions of HfO 2 , HfO 2 : Er (7 at %), and HfO 2 : Er (3 at %) + Yb (7 at %) in Figure 8b.Points A and B are in the white region of the chromatic diagram, although it must be clarified that their emission intensities are low.A very strong intensity is reached in the case of point C located in the red-orange region.Figure 8c shows a photograph taken by ourselves of the up-conversion emission for the HfO 2 : Er (3 at %) + Yb (7 at %) sample excited with a portable IR laser (980 nm, 0.5 W).In this case, the strong intensity of the red-orange UC emission is detected.
The up-conversion emission spectra of the samples containing alkaline ions are shown in Figure 9.It is known in the literature that the incorporation of alkaline cations (Li, Na, K, and so forth) as codopants improves the intensity of UC emission.In Figure 9a, UC emission spectra for HfO 2 : Er 3+ (3 at %) + Yb 3+ (7 at %) nanoparticles codoped with different  lithium concentrations (0, 1.5, 3, and 6 at %) are exhibited.The excitation wavelength was 980 nm.These UC emission spectra present the distinctive emission bands of the Er 3+ ions and show different UC emission intensities as the content of Li + ions is increased, and the highest emission intensity is reached with 3 at % Li + ions.The UC emission spectra for HfO 2 : Er 3+ (3 at %) + Yb 3+ (7 at %) nanoparticles codoped with different sodium concentrations (0, 1.5, 5, 7, 10, 15, 20, and 30 at %) are displayed in Figure 9b.As in the previous case, the UC emission spectra show the characteristic bands of the Er 3+ ions with the predominating bands in the red range.Here, it is observed that the maximum emission intensity is reached at 20 at % Na + ions.Presumably, the Li + ions incorporated in the host lattice cause an increase in the emission intensity attributed to the increase of the interatomic distance between the Er 3+ ions as the Li + ions have a smaller radius with respect to the Hf 4+ ions.However, further research is necessary to clarify this point. 3,51igure 10a shows the UC emission spectra for codoped HfO 2 : Er 3+ + Yb 3+ with the addition of stabilizer ions, such as Li + and Na + (in all cases, the excitation wavelength was 980 nm).In this figure, curves A and B are for samples without stabilizing ions (Li + and Na + ).Here, it can be seen that in the case of curve B (with a greater amount of Yb 3+ ions), its UC emission intensity is much higher than that of curve A. The stabilizing ions (Li + and Na + ) were incorporated into a sample like the one represented by curve A to better view the effect of their incorporation.It is possible to observe that the incorporation of these ions notably improves the intensity of the red emission coming from the Er 3+ ions (curves C and D).It is evident that the effect is more notable with the incorporation of Na + ions (curve D).The intensities of curves B and D are very similar.The CIE chromatic diagram for the UC emissions from doped and codoped HfO 2 nanoparticles is presented in Figure 10b.It should be noted, again, that the emission intensity for the sample represented by point A is very weak compared to that of points B, C, and D, which are much more intense.
Cathodoluminescence Properties. Figure 11 shows the cathodoluminescence emission spectra of the Er 3+ -doped HfO 2 nanoparticles.The spectra of HfO 2 : Er 3+ (0, 3, 5, and 7 at %) are presented in Figure 11a and those of HfO 2 : (Er 3+ + Yb 3+ ) codoped nanoparticles in Figure 11b.The emission was excited by accelerated electrons (V = 5 kV, I = 0.3 mA).For HfO 2 : Er 3+ (0, 3, 5, and 7 at %), the spectrum of the undoped sample exhibits a very broad band that covers almost the entire visible region and shows a maximum around 470 nm.This band is generally associated with structural defects in the host lattice such as oxygen vacancies.As the Er 3+ ions are incorporated, sharp bands appear in the band, which are typical of this ion. 87t should be noted that the bands in the green region are more intense than those in the red region.The emission intensity reaches its maximum for 7 at % Er 3+ ions.In Figure 11b, the HfO 2 : Yb 3+ (7 at %) + Er 3+ (0, 3, 5, and 7 at %) samples were studied.The sample without Er 3+ ions presents a very broad band without characteristic sharp peaks.As the Er 3+ ions were incorporated, their typical bands appeared, with those in the green range being more intense.The maximum intensity is reached at 5 at % Er 3+ ions; for higher contents of Er 3+ ions, a decrease in the CL emission intensity is seen, probably due to the concentration quenching effect.Here, an intense green emission is observed, and the central spot that looks white is due to the high intensity of the emitted light.
Significant Results.HfO 2 nanoparticles codoped by Yb 3+ (7 at %) + Er 3+ synthesized by hydrothermal and thermal treatment at 110 °C have the particle size (less than 100 nm) optimum to be used for interaction with cells; others morphology characteristics as monodispersion are considered.HfO 2 possesses oxygen vacancies that play an important role in the cooping process.UCNP has a great red-orange emission, at room environment.Red emission was improved by the study of different compositions and active cations like Na + and K + ; additionally, UCNP nanoparticles possess an exponential redorange emission increase compared with green emission.Therefore, all these properties suggest that our UCNP could be applied as biomarkers or in phototherapy.

■ CONCLUSIONS
Using the simple and economical hydrothermal technique, doped and undoped HfO 2 nanoparticles were synthesized.The crystalline structure of these nanoparticles is mostly monoclinic in the undoped samples and evolves to a mixture of the monoclinic and cubic phases when dopant ions are introduced.Regarding the structural properties, the decrease in the Ag Hf−O mode observed in Raman suggests phase mixing of monoclinic and cubic phases, which was corroborated by XRD results.With the addition of trivalent ions, stabilization of the cubic phase was obtained at low temperatures, and a higher stabilization of the cubic phase was observed with the addition of Na + ions.The EDS measurements of the chemical composition of the nanoparticles confirm correct stoichiometry.The surface morphology of HfO 2 nanoparticles was characterized by the presence of spheroidal particles and pores before thermal treatment, and it changed to larger peanutshaped particles after the thermal treatment.These nanoparticles have notable luminescence characteristics.On the one hand, they show luminescence emissions when only doped with Er 3+ ions and excited with ultraviolet radiation (272 nm), and in this case, emissions in the green range of the spectrum prevail.On the other hand, when they were doped with Er 3+ and Yb 3+ ions, strong red emission was obtained under the excitation with infrared radiation (980 nm).The UC emission intensity of these samples was notably increased with the help of stabilizers such as Li + and Na + , and this effect was stronger for Na + ions.Cathodoluminescence measurements show strong emissions preferably in the green region of the spectrum.Variations in the atomic concentrations of the dopant ions allowed us to determine the optimal values of each of them and to know the thresholds at which the concentration quenching effect occurs.
Finally, HfO 2 nanoparticles doped and codoped with Er, Yb, Li, and Na ions show very versatile and interesting luminescence properties since they allow obtaining strong emissions excited either with ultraviolet, infrared radiation, or accelerated electrons.These characteristics allow a variety of applications of these materials; particularly, they are excellent candidates to be utilized in medical applications, specifically, for cancer biomarker applications.

Figure 5 .
Figure 5. DC photoluminescence of erbium-doped HfO 2 nanoparticles as a function of the Er 3+ content.The excitation wavelength is 372 nm.

Figure 6 .
Figure 6.UC luminescence spectra from codoped HfO 2 nanoparticles under the excitation at 980 nm: (a) Yb 3+ content is fixed at 7 at %, and Er 3+ content is varied.(b) Er 3+ content is fixed at 3 at %, and Yb 3+ content is varied.

Table 2 .
Raman Shift Variation and FWHM in Codoped Samples