Synthesis, structure and luminescence of Er3+-doped ... - UPV

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Fig. 3FTIR and Raman spectra of the YGG nano-garnets.consists of two main bands with superimposed peaks whoseenergies range from 520 to 1100 cm 1 , in addition to differentbroad peaks at higher energies. The peaks with maxima ataround 590, 620, 640, 685 and 840 cm 1 correspond to theformation of the nanocrystalline YGG and are assigned tothe characteristic metal–oxygen stretching vibrational modes ofthe GaO 4 tetrahedra in the garnet structure. 23,30The Raman spectrum of the YGG nano-garnets has beenmeasured in the 100–2500 cm 1 range and the observed modesare summarized in Table 2. The primitive cell of a garnet latticecontains four formula units (80 atoms) 31 in which, from a factorgroup analysis, there are 25 Raman-active modes that can beclassified as 3 A 1g ,8E g and 14 T 2g modes. 32 It is worth notingthat out of 25 predicted Raman-active modes, 17 of them areexperimentally observed for nanocrystalline YGG and arecompared with vibrational modes of YGG single crystals 29 (seeTable 2). A quite good agreement is observed between thevibrational modes of the nano-YGG and the YGG single-crystaland, in general, only slight shifts in frequency have been foundbetween the Raman modes of nanocrystals and those of the bulkTable 2 Comparison of the energies of the experimental Raman vibrationalmodes (in cm 1 ) of the YGG nano-garnets and single crystalVibrational phononmodeYGG nano-garnet(present work)YGG singlecrystal (ref. 29)T 2g 119 119 (theory)E g 133 134T 2g 151 155 (theory)T 2g 175 176T 2g 187 180T 2g 243 243T 2g 272 274E g 286 287E g 300 324A 1g 357 356T 2g 393 392E g 411 412T 2g 420 432A 1g 528 529T 2g 594 596T 2g 612 611E g — 635T 2g 752 754material, and these are within the accuracy limits of themeasurements.The Raman spectrum of garnets has been interpreted on thebasis of the vibrational modes of the tetrahedral GaO 4 andoctahedral GaO 6 units, considering that the vibrations of thedifferent polyhedra are strongly coupled to each other. 33,34 Whilethe assignment of the different Raman modes to the differentvibrations in terms of eigenmodes is very clear in the Y 3 Al 5 O 12(YAG) crystal, 35 the same situation is not so evident in theY 3 Ga 5 O 12 (YGG) crystal. 29 The Raman spectrum of YGGcan be divided into two main regions: the low-frequency region(100–340 cm 1 ) and the high-frequency region (340–800 cm 1 ).Starting with the latter and according to Saine et al. (1982), 34 thebands comprised in the 340–450 cm 1 and the 580–700 cm 1ranges can be attributed to the antisymmetric stretching modesof the GaO 6 and GaO 4 polyhedra, respectively. However, theband at 360 cm 1 can be assigned to the symmetric stretchingmode of the GaO 4 tetrahedron coupled with a rotational modeinvolving the whole tetrahedron. Finally, the band observed ataround 760 cm 1 is mainly due to the symmetric stretching modeof the GaO 4 tetrahedron, even if a weaker contribution due to theantisymmetric stretching mode of the same polyhedron could bepresent. 34The bands in the low energy region are mainly due to theO–Ga–O bending modes of the Ga-related polyhedra and tolattice modes related to the movements of the Ga-related polyhedraagainst the Y 3+ ions, which can also be considered asstretching or bending modes of the YO 8 dodecahedra. In thissense, it is important to note that the stretching and bendingforces of Y 3+ ions in YO 8 dodecahedra are considerably smallerthan those of GaO 4 tetrahedra and similar to those of GaO 6octahedra in garnets. 29 In particular, the two lowest-energy E gphonons at 133 and 286 cm 1 have frequencies that exhibita strong dependence on the lattice parameter and correspondinglyon the RE 3+ mass, being the smallest in frequency attributedto the translation of YO 8 units. On the other hand, the T 2gmodes at 187 and 272 cm 1 have been shown to be almostvolume-independent and this behavior could be related tocanceling contributions due to the intermixing of octahedral anddodecahedral molecular modes. 29 Therefore, all these lowfrequencyE g and T 2g modes could yield important informationregarding the occupation of the dodecahedral sites by the Er 3+ions. In fact, since this is a low-energy phonon site, the multiphononrelaxation probabilities in the YGG nano-garnets maybe considered one of the lowest found in oxide matrices, andcould yield high quantum efficiencies for the emitting levels ofthe Er 3+ ions.Despite the measured FTIR and Raman spectra confirmingthe good crystalline quality of these nanocrystals of garnetstructure, there are a few Raman and IR peaks above 1100 cm 1whose explanation is not easy, although it is accepted that theirpresence is related to the anions attached to the nano-garnetcrystal surface during the synthesis process. In principle, thepresence of these anionic groups on the nanoparticles surface ispotentially undesirable since they may act as quenching traps,diminishing the luminescence and lifetimes of the RE 3+ emittinglevels. The peak at around 1500 cm 1 2could be assigned to CO 3carbonate groups, while the bands in the range from 2000 to2350 cm 1 could arise from the carbon dioxide in air. Finally,This journal is ª The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22, 13788–13799 | 13791
another band at around 3550 cm 1 (not shown) is associated withthe OH anionic groups arising from the water adsorbed duringthe exposure of dried powder to air. The presence of high energyvibrations may indicate that the heat treatment was not enoughto remove either the carbonate or the hydroxyl ions from thenanocrystal surface, since these bands have been shown todisappear after synthesis at a higher temperature. 23C.2 Absorption and luminescenceFor optical applications special interest is devoted to the analysisof the local structure around the RE 3+ ions in the matrix, since itrules the fine structure splitting of the free-ion multiplets and theforced intra-configurational 4f–4f electric-dipole transitionprobabilities in the visible range. The optical properties of theRE 3+ ions and, therefore, their interest for optical applicationsdepend on the final environment of these ions. 36As already mentioned, the Er 3+ ions will predominantlyenter the distorted dodecahedral sites by replacing the Y 3+ ions.Thus the eight oxygen ligands surrounding the optically activeEr 3+ ion create a local environment with orthorhombic D 2point symmetry. As a consequence, the D 2 crystal-field interactionfelt by the optically active ion will completely remove thedegeneracy of the 2S+1 L J multiplets of the free-Er 3+ ion, giving riseto (2J + 1)/2 Stark, or crystal-field, levels labeled according to theirreducible representations obtained from group theory. 36The room temperature (RT) diffuse reflectance spectrum ofthe YGG5Er nano-garnet in the UV-visible-NIR range is givenin Fig. 4. The peaks observed correspond to intra-configurational4f 11 –4f 11 electronic transitions starting from the 4 I 15/2ground state to the different excited levels of the Er 3+ ion. All thetransitions are assumed to be electric dipole in nature, except forthat to the 4 I 13/2 first excited state that shows a magnetic dipolecontribution. 37 The labels of the different transitions of the Er 3+ion in the YGG nano-garnet have been assigned according to thewell-known Dieke’s diagram for this ion in the LaCl 3 crystal 38and the energies of the Stark levels of Er 3+ in a YGG singlecrystal summarized by Morrison and Leavitt. 39 A partial energylevel diagram of the Er 3+ ion in the YGG nano-garnet is given inFig. 5. The sharp peak profiles found for all the electronicFig. 4 Diffuse reflectance spectrum of the YGG5Er nano-garnet powderin the UV-visible-NIR range at RT. All transitions start from the 4 I 15/2ground state to the indicated levels.Fig. 5 Partial energy level diagram of the Er 3+ ion showing differentradiative transitions (downward solid lines) related to the luminescenceafter a 4 I 15/2 / 4 F 7/2 ground state absorption (GSA) under a direct laserexcitation at 488 nm. The multiphonon (zig-zag lines) and energy transfer(dashed lines) non-radiative relaxation processes are also shown.transitions confirms that the Er 3+ ions are incorporated in thenanocrystalline structure of the garnet. From these data, and theemission bands presented later, it can be stated that theabsorption transitions at RT start mainly from the four Starklevels of the 4 I 15/2 ground state with lowest energies, which areseparated around 350 cm 1 from the Stark levels with higherenergies. The intense band at around 240 nm, strongly overlappedwith the absorbance of the YGG matrix, can be assignedto the Er 3+ –O 2 charge transfer state, although different4f 11 / 4f 10 5d inter-configurational transition bands of theEr 3+ ion have also been observed in this range. 40When the Er 3+ ions are resonantly excited to the 4 F 7/2 level witha cw 488 nm blue laser radiation, the luminescent 2 H 11/2 and 4 S 3/2thermalized levels are immediately populated through nonradiativemultiphonon relaxation processes and green, red, andNIR luminescence are measured (see Fig. 6). However, theseemissions not only start from the 2 H 11/2 and 4 S 3/2 thermalizedlevels but also from the 4 F 9/2 , 4 I 11/2 , and 4 I 13/2 multiplets populatedthrough different non-radiative de-excitations. The onlynon-emitting level is the 4 I 9/2 one, from which the expectedemissions at around 800 and 1700 nm are not observed, evenunder a direct excitation of the Er 3+ ions to this level. Theemission bands are assigned in Fig. 6 and their sharp peakprofiles further confirm that the Er 3+ ions are incorporated inthe YGG nano-crystals. Similar observations are found forEr 3+ -doped Lu 3 Ga 5 O 12 (LuGG) 3 and Gd 3 Ga 5 O 12 (GGG)nanocrystals 5 and single crystals. 41Of special interest are the emission bands from the 4 I 13/2multiplet at around 1.5 mm shown in Fig. 6. The peak positionsand spectral profiles of these bands are similar to those in theabsorption spectrum, although their relative intensities strongly13792 | J. Mater. Chem., 2012, 22, 13788–13799 This journal is ª The Royal Society of Chemistry 2012
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