Cerium-doped Tb3Al5O12(TAG:Ce3+) colloidal nanocrystals were synthesized by pulsed laser ablation (PLA) in de-ionized water and lauryl dimethylaminoacetic acid betain (LDA) aqueous solution for luminescent bio-labeling application. The influence of LDA molecules on the crystallinity, crystal morphology, crystallite size, and luminescent properties of the prepared TAG:Ce3+colloidal nanocrystals was investigated in detail. When the LDA solution was used, smaller average crystallite size, narrower size distribution, and enhanced luminescence were observed. These characteristics were explained by the effective role of occupying the oxygen defects on the surface of TAG:Ce3+colloidal nanocrystal because the amphoteric LDA molecules were attached by positively charged TAG:Ce3+colloidal nanocrystals. The blue-shifted phenomena found in luminescent spectra of the TAG:Ce3+colloidal nanocrystals could not be explained by previous crystal field theory. We discuss the 5d energy level of Ce3+with decreased crystal size with a phenomenological model that explains the relationship between bond distance with 5d energy level of Ce3+based on the concept of crystal field theory modified by covalency contribution.
KeywordsTAG:Ce3+nanocrystal Pulsed laser ablation LDA Luminescence Blue-shift
Integrated sensors for biological application and a variety of molecular-sensing schemes characterized by electrochemistry, refractive index, UV absorption, and luminescence have been devised in an effort to understand the spatio-temporal interplay of biomolecules in biology [1–4]. In particular, luminescent labeling for cellular imaging and assay detection is a standard technique used to understand the interaction of biomolecules in biology [5–7]. However, organic dyes, luminescent proteins or lanthanide chelates as bio-labeling agents may cause problems such as broad spectrum profiles, low photo-bleaching thresholds, and poor photochemical stability.
Ce3+-doped terbium aluminum garnet (Tb3Al5O12:Ce3+, TAG:Ce3+), which has been widely used as a white solid-state light emitting device, may be an alternative label to overcome these problems because of its chemical and thermal stability . We believe that TAG:Ce3+ nanocrystal can be substituted for chalcogenide phosphor as a bio-label for several reasons. Not only does TAG:Ce3+ have the potential to obtain luminescent quantum efficiency up to 40% by the allowed 4f → 5d transition of Ce3+, but it is also considered as a nontoxic material . In addition, in the in vivo experiment, the cells are not degraded by blue light, the excitation source for emission from TAG:Ce3+.
Generally, the conventional solid-state reaction method has been used to fabricate TAG:Ce3+ crystal. However, this method requires a relatively long processing time and temperature over 1500 °C to obtain single-phase product. Moreover, there have been few reports on the preparation of nano-sized TAG:Ce3+ crystals . Under these circumstances, a novel preparation method for TAG:Ce3+ nanocrystal would be greatly valuable. One novel technique for synthesizing TAG:Ce3+ nanocrystals is pulsed laser ablation (PLA) in liquid media. This technique provides remarkable advantages: simplicity in the synthesis process, the availability of various liquid media, and the absence of chemical reagents in solution [11–13]. Moreover, PLA is a convenient technology for synthesize nanostructured materials, and some researchers have made achievements on this research field [14–16].
In general, luminescence efficiency of phosphor materials goes down with diminution of crystallite size into nanometer range, because an increase in surface-to-volume ratio seriously induces nonradiative processes related to surface defects. In our previous report, Ce3+-doped yttrium aluminum garnet (YAG:Ce3+) nanoparticles were successfully fabricated by laser ablation in de-ionized water . However, luminescence efficiency was considerably low compared to bulk target, which were due to increased surface defects. Therefore, it is necessary to cap such defects by an appropriate surfactant to enhance luminescence efficiency. In this work, we report a new synthetic approach to directly produce highly dispersed and strongly luminescent TAG:Ce3+ colloidal nanocrystals for application to luminescent bio-labeling by PLA in aqueous solution. Moreover, the change of 5d electron energy level of Ce3+ in nanoscaled TAG crystal, which has attracted much discussion in recent years [18, 19], was investigated in detail.
The colloidal suspension was dropped on a copper mesh coated with amorphous carbon film to observe the microstructure and shape of nanocrystals by a transmission electron microscope (JEOL JEM-2010). The size of the nanocrystals was statistically analyzed using 160 nanoparticles in an TEM image. The precipitates of colloidal suspension were repeatedly centrifuged at 30,000 rpm by an ultracentrifuge (Hitachi CS100GXL) for the X-ray diffraction (XRD, Rigaku RAD-C, CuK α radiation) and X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI-5600ci) measurement. The binding energy was determined with reference to the C 1s line at 184.6 eV of adventitious carbon. Standard atomic concentration calculation provided content of doped Ce atoms in the colloidal nanocrystals. Luminescence spectra of the TAG:Ce3+nanocrystal-dispersed suspensions were measured using a fluorescence spectrometer (PerkinElmer LS-45) at room temperature. The excitation wavelength used for measuring emission spectra was 460 nm, and the excitation spectra were measured at emission maxima.
Results and Discussion
Synthesis of TAG:Ce3+Colloidal Nanocrystals
According to previous studies of laser ablation at liquid–solid interfaces [24–26], the initial stage of interaction between the pulsed laser beam and the target surface generates hot-plasma plume over the laser spot on the ceramic target, resulting in intense evaporation from the melt surface, similar to that of PLA in the gas phase or vacuum background. However, because the plasma is confined in the liquid during the laser ablation, it expands outside adiabatically at a supersonic velocity creating shock waves in front, which induces elevated pressure and further increase of plasma temperature. This extremely high transient pressure in the plasma plume can lead to forceful impingement of the ablated species on the confining liquid. In our case, the active species of TAG:Ce3+ nanoclusters could strongly react with water molecules at the interfacial region between the plasma and liquid. The initially generated plasma plume is called hot-plasma zone, and the expanded plasma–liquid interfacial region is called reactive quenching zone. This initial cluster served as nuclei and rapidly quenched into the liquid solution together with other radicals or ions such as OH· and H+. It was most likely that the directly quenched cluster of TAG:Ce3+ had an amorphous phase due to the transient reaction and rapid quenching. In addition, the quenched clusters in the solution generally had a positively charged surface, because oxygen vacancies were generated easily as a result of the dissociation of oxygen induced by instantaneous high temperature over melting temperature during laser ablation. The pH measurement demonstrated that the liquid solutions were changed from a neutral state (6.8) to a weak acidic state (5.0) after laser ablation.
Luminescence and Blue-Shift Phenomena of TAG:Ce3+Colloidal Nanocrystals
Moreover, it was clearly observed that the excitation and emission spectra of TAG:Ce3+ colloidal nanocrystals were shifted to the blue wavelength region compared to those of the bulk target. There have been many reports [27, 28] on the blue-shift phenomena in the luminescent spectra of nano-scaled semiconductor materials. However, few reports have focused on nano-sized rare-earth doped phosphor materials. The mechanism of luminescence in the semiconductor is the recombination of electrons in the conduction band and holes in the valence band. A larger band-gap is necessary for quantum confinement to cause the blue-shift in luminescence. However, the luminescence in TAG:Ce3+ is due to the transition between energy levels of Ce3+ atoms as the luminescence center [8–10]. Therefore, a different interpretation is required to explain the blue-shifts in the luminescence spectra of the TAG:Ce3+ nanocrystals.
where 〈 〉 denotes the expectation value with respect to the subscript eigenstate, and α is the polarizability of the ligand located at a distance R from Ce3+ in a lattice. This model demonstrates the general red-shifting phenomena of rare-earth doped nanocrystalline compounds. These red-shifting phenomena in 4f ↔ 5d transitions have been observed in several Ce3+-doped compounds [32, 33] and interpreted by this crystal field model. However, with this model, it is difficult to interpret the blue-shifting luminescence spectra observed in our TAG:Ce3+ colloidal nanocrystal.
where it is assumed that both π antibonding energy for the t2g level and σ antibonding energy for the e g level vary exponentially with the nearest-neighbor distance R with the same decay distance b. In effect, C1C2C3A B, and b are phenomenological parameters adjusted to the measured dependence of 10 Dq and ΔEc on R in a series of iso-structural hosts.
Figure 8(b) depicts the dependence of 5d energy levels and their centroid on bond distance for a cubic structure qualitatively using Eq. (1–3). The range of bond distances in Fig. 8(b) can be divided into three regimes, which are distinguishable in terms of qualitative dependence on the lattice constant. First, in the covalency negligible regime ①, both thet2glevel and the 5d centroid decrease as the lattice constant decreases. In the covalency intermediate regime ②, the 5d centroid begins to rise with decreasing lattice constant; thet2glevel is near the minimum of the curve as shown in Fig. 8(b), and is therefore insensitive to the lattice constant. It is clear that in the covalency dominant regime ③, thet2glevel rises with decreasing lattice constant due to overwhelming antibonding and exclusive effects.
The decrease of bond distance from regimes ① to ② increases the crystal splitting energy and lowers the centroid energy, thus leading the emission band to shift to a longer wavelength (red-shift). However, this phenomena occurring in the range from ① to ② do not seem to be related with the luminescent results of TAG:Ce3+nanocrystals prepared by PLA, because the change of bond distance in regimes ① and ② cannot explain the blue-shifted luminescence observed in prepared TAG:Ce3+colloidal nanocrystals. Therefore, it can be concluded that the bond distance of the Ce3+in TAG decreased from regime ② into ③ as the crystal size decreased to nanoscale, and induced the rising of centroid andt2genergy levels. Consequently, the changes of energy levels in the centroid andt2gshifted excitation and emission spectra to a shorter wavelength as confirmed in Fig. 7. In the synthesis section, we revealed that the TAG:Ce3+colloidal nanocrystal prepared in LDA solution had smaller average crystallite size and a narrower size distribution than those prepared in de-ionized water. Therefore, blue-shift into shorter wavelength in luminescence of the TAG:Ce3+colloidal nanocrystals prepared in LDA solution compared to those prepared in de-ionized water, and bulk target can be explained well by this modified crystal field model.
TAG:Ce3+colloidal nanocrystals were fabricated by an one-step simple route using PLA in de-ionized water and LDA solution for luminescent bio-labeling application. In LDA solution, the crystallite size and the standard deviation of crystallite size decreased, whereas emission intensity increased remarkably. This result indicated that anionic oxygen in the LDA molecules effectively occupied the oxygen vacancy sites on the surface of TAG:Ce3+colloidal nanocrystal by the charge matching with the positively charged colloidal nanocrystals. The origin of blue-shift in luminescence spectra was investigated in detail by a phenomenological model based on the covalency contribution to the crystal field splitting energy of Ce3+with decreasing of bond length. It is worthy that this is a new account of room-temperature synthesis of TAG:Ce3+colloidal nanocrystal using PLA in liquid media. Furthermore, the experimental evidences of blue-shifts in luminescence spectra can be helpful in understanding the nano-size effect on the 5d energy level of Ce3+-doped compounds.