Article pubs.acs.org/JPCC
Controlled Terbium(III) Luminescence in Zinc Sulfide Nanoparticles: An Assessment of Competitive Photophysical Processes Gouranga H. Debnath,† Arijita Chakraborty,† Ankita Ghatak,† Madhuri Mandal,‡ and Prasun Mukherjee*,† †
Centre for Research in Nanoscience and Nanotechnology, University of Calcutta, JD-2, Sector-III, Salt Lake, Kolkata 700098, West Bengal, India ‡ S. N. Bose National Centre for Basic Sciences, JD Block, Sector-III, Salt Lake, Kolkata 700098, West Bengal, India S Supporting Information *
ABSTRACT: The lanthanide photoluminescence in the trivalent terbium (Tb3+) incorporated zinc sulfide nanoparticles [Zn(Tb)S] has been reported with the nanoparticle size varying from 2.0 ± 0.3 to 14 ± 3 nm in diameter as a function of reaction temperature. In all the nanoparticles, the Tb3+ luminescence has been sensitized by the nanoparticle acting as an optical antenna. The relative contribution of different excitation bands in sensitizing Tb3+ luminescence in the Zn(Tb)S nanoparticles has been found to be dependent on the size of the nanoparticles. The observed Tb3+ luminescence efficiency in the Zn(Tb)S nanoparticles has been rationalized by competing factors: (i) the sensitization efficiency that is guided by the relative energy level position of the Tb3+ ground and excited states with respect to the valence and conduction bands of the ZnS and (ii) the extent of incorporation of Tb3+ in the nanoparticles. Additionally, it has been argued that the spectral overlap between the nanoparticle (donor) emission and Tb3+ (acceptor) absorption is not a prerequisite in determining the Tb3+ emission in the Zn(Tb)S nanoparticles studied.
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realize the Ln3+ luminescence. Specifically, the nanoparticles with high molar extinction coefficient absorb light and transfer the energy nonradiatively to the lanthanide center, and concomitantly an incorporation of the Ln3+ into the nanoparticle matrix ensures the decrease of the contribution from the various nonradiative decay channels. In order to utilize the Ln3+ luminescence in a given material, it is important to understand and optimize the underlying photophysical processes. It has been argued that the Ln3+ acts as charge traps in the semiconductor materials, and the nonradiative recombination at these charge traps populates the excited state of the lanthanide center.16 For such a process to be operational, the relative energy level positions of the Ln3+ ground and excited states with respect to the valence and conduction bands of the host material is of importance. Such energy level diagrams for various II−VI semiconductor materials have been constructed in a previous study.2 Based on this, the experimental trend of lanthanide band centered luminescence efficiency has been rationalized in the zinc and cadmium containing sulfide and selenide nanoparticles. This analysis identifies zinc sulfide as the most promising host candidate to sensitize the Tb 3+ luminescence. For example, this kind of schematic relative energy level diagram reveals Tb3+ is a potential hole trap in zinc sulfide (ZnS), and the excited electrons suffer a minimum effect
INTRODUCTION Doped semiconductor nanoparticles are useful due to the possible modulation of chemical, optical, electrical, and magnetic properties of the materials. In the perspective of optically active doped semiconductor nanoparticles, trivalent lanthanide (Ln3+) incorporated semiconductor nanoparticles find wide attraction.1−6 The luminescence that originates from the intraconfigurational 4f−4f transitions in Ln3+ is unique compared to that in the conventional luminophores and finds usage in various luminescence-based applications.7−14 The luminescence of lanthanide cations exhibit sharp emission bands spanning the entire visible and near-infrared spectral region allowing multiplex assays; longer (microseconds to milliseconds) lifetime which makes time-gated measurements feasible; and resistance to photobleaching mechanisms, hence allowing longer experiment time thereby increasing the signal-to-noise ratio. In order to detect the Ln3+-based luminescence, two significant challenges need to be overcome. First, the molar extinction coefficient of these cations is at least 1000 times smaller than the corresponding values for the conventional organic fluorophores, allowing the direct excitation of the lanthanide cations to the higher lying excited state difficult. In addition to this, the Ln3+ luminescence is quenched by the vibrational overtones of various chemical bonds present in the solvent and ligand molecules of the immediate environment.15 Incorporating Ln3+ in a semiconductor nanoparticle matrix offers a way to overcome these challenges.1 In such a system, the nanoparticle matrix acts as an optical antenna and protective matrix simultaneously in order to © XXXX American Chemical Society
Received: July 24, 2015 Revised: September 30, 2015
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DOI: 10.1021/acs.jpcc.5b07182 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
the LaMer model, the rate of nucleation depends on three variables: namely, the supersaturation, temperature, and surface free energy. The growth of the particles depends on three variables: namely, the number of nuclei subjected to grow, total amount of diffusible particles, and the diffusion coefficient. The total amount of diffusible particles (C0) can be realized as the difference between the initial amount (CSS) and the solubility of the corresponding supercooled liquid (CS). This difference is temperature dependent due to its dependence on the relevant solubility. The diffusion coefficient (D) is inversely proportional to the viscosity of the medium and hence directly proportional to temperature. The parameter of interest in determining the growth of the particles is DC0, in which the effects of temperature are opposing in nature. In short, temperature has an effect in modulating the formation of the nanocrystals, the effect of which may require evaluation by specific case under investigation. This work aims to study the reaction temperature mediated changes in the size of the zinc sulfide (ZnS) nanoparticles and the corresponding control of the terbium (Tb3+) luminescence in the lanthanide incorporated Zn(Tb)S nanoparticles, in order to provide a rationalization of the photophysical processes for the sensitization of Tb3+ luminescence in the Zn(Tb)S nanoparticles.
from autoionization, hence making the terbium incorporated ZnS nanoparticles a better luminophore compared to that in the corresponding cadmium selenide (CdSe) nanoparticles, where autoionization could play a significant role in minimizing the Tb3+ luminescence. While the previous work identifies a suitable host−guest combination,2 it does not identify any possible sizedependent tuning of Tb3+ dopant emission in the Zn(Tb)S nanoparticles. This is the main objective of this study that may help developing a novel lanthanide-based semiconductor nanoparticle luminophore with controlled dopant emission. There are several factors (growth time, reaction temperature, reactant concentration, pH, etc.) that may affect the size and shape of a nanoparticle in a chemical synthesis. Growth time dependent nanoparticle size variation has a remarkable effect for the cadmium containing semiconductor nanoparticles;17 however, this approach is not drastic in the zinc sulfide nanoparticles2 where similar luminescence spectra have been observed with 1 and 20 min growth time. This difficulty is most likely associated with a smaller exciton Bohr radius of ZnS as compared to for example CdSe and significant contribution of surface states in the luminescence emission spectrum of the ZnS nanoparticles. In order to explore alternative avenues to control the size of the ZnS nanoparticles in a systematic way, this study discusses the role of reaction temperature in modulating the size of the synthesized terbium incorporated (doped) zinc sulfide [Zn(Tb)S] nanoparticles. The corresponding nanoparticle size dependent impact on the sensitization of the Tb3+ luminescence has been discussed. Various researchers have devoted attention in understanding the reaction temperature dependent tuning of nanoparticle size and shape.18−29 Alivisatos and co-workers reported an effect of reaction temperature on the transition metal cupferron complex mediated syntheses of γ-Fe2O3, Mn3O4, and Cu2O nanoparticles.18 For example, for the synthesis of γ-Fe2O3 nanoparticles from the iron cupferron complex precursor, while an injection and refluxing temperature of 250 and 200 °C resulted in the particle diameter of 6.7 ± 1.4 nm, the combination of 300 and 225 °C produced particles with marginal decrease of diameter to 5.2 ± 1.5 nm. For a synthesis of CdS nanoparticles from metal salt of alkyl xanthates as precursor and hexadecylamine as solvent, Efrima and co-workers reported the formation of particles at 70 and 90 °C with diameter 5.2 ± 0.6 and 3.5 ± 0.4 nm, respectively.19 Park and co-workers have synthesized Mn3O4 nanoparticles with 6 nm diameters at 150 °C, while an alteration of reaction temperature to 250 °C resulted particles with 15 nm diameter.20 In this study, an increase in particle diameter with an increase in reaction temperature has also been observed for the MnO nanoparticles. Sun and co-workers argued that for the magnetite nanoparticles lower reaction temperature mediated syntheses in aqueous medium are limited to access the sub-20 nm size regime and demonstrated the formation of particles with 4 nm size with 265 °C reaction temperature.21 Various other studies discussed the reaction temperature dependent changes during the formation of magnetite,22 hydroxyapatite,23 silver,24−28 and gold29 nanomaterials. From the discussion above, it is evident that the reaction temperature has a pronounced effect on the reaction rate and hence impacts the particle formation characteristics. The formation mechanism of monodispersed hydrosols has been extensively discussed by LaMer and co-workers30 and Hyeon and co-workers.31 Monodispersed nanocrystals are oftentimes prepared by the “hot injection” method,17 in which the production of the materials is guided by nucleation and growth, which are considered as separated phenomenon. According to
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MATERIALS AND METHODS Chemicals. Trioctylphosphine (TOP) (90%), trioctylphosphine oxide (TOPO) (90%), zinc stearate (tech.), tetracosane (99%), and octadecene (90%) (tech.) were purchased from Sigma-Aldrich. Sulfur (99.999%) was purchased from Fisher Scientific. Terbium(III) nitrate hydrate (99.9%) was purchased from Alfa Aesar. Chloroform and methanol were purchased from Merck. Argon was purchased from Hindustan Gases and Welding. All chemicals were used without performing any additional purification. Nanoparticle Synthesis. The terbium incorporated (doped) ZnS nanoparticles [Zn(Tb)S] were synthesized based on a protocol reported by Peng and co-workers32 for undoped particles and later modifications by Waldeck, Petoud, and coworkers2,3 for synthesizing the Zn(Ln)S nanoparticles [Ln = Eu, Tb]. Based on the electron microscopy associated with energy dispersive X-ray (EDX) spectroscopy, steady-state, time-gated, and time-resolved luminescence spectroscopy experiments, and infrared absorption spectroscopy,2,3,33 the particles synthesized based on this synthetic protocol have been demonstrated to establish the interaction between the lanthanide cation center and the host nanoparticle matrix, where the ZnS nanoparticles act as an optical antenna and a protector matrix to sensitize the 4f−4f intraconfigurational Tb3+ luminescence in the Zn(Tb)S nanoparticles. The synthetic protocol used in the present study has been based on the general recipe reported previously by Waldeck, Petoud, and co-workers.2,3 Specifically, in a typical synthesis of terbium incorporated zinc sulfide nanoparticles [Zn(Tb)S], 2.0 g of tetracosane, 1.7 g of TOPO, 0.68 mmol of zinc stearate, and 3 mL of octadecene were added. The mixture was refluxed at 300 °C while stirring under an argon atmosphere. The reaction was allowed to continue for ∼1.5−2 h followed by the addition of lanthanide stock solution (0.12 mmol of terbium(III) nitrate dispersed in 3 mL of TOP, by sonication) which was injected into the reaction mixture. A sulfur stock solution (0.40 mmol of sulfur dissolved in 2.5 mL of octadecene, by sonication) was injected into the reaction mixture approximately 45 min−1 h after the injection of lanthanide stock solution. A growth time of 20 min B
DOI: 10.1021/acs.jpcc.5b07182 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
lived Tb3+ luminescence only. The luminescence spectra were recorded with 2 nm each excitation and emission slits and corrected for instrument response. A long pass filter was used to remove the harmonic peak of the excitation source. The solutions for acquiring the luminescence spectra were prepared so that the absorbance at 295 nm was ∼0.1. The luminescence spectra were corrected for the absorbance at the excitation wavelength. All optical measurements were performed at room temperature.
was allowed after which the heat source was removed to bring the reaction to a completion. Two additional syntheses were carried out in which the reaction temperature was changed from 300 °C to 280 and 260 °C, respectively, while keeping the concentrations/volumes of precursors/solvents identical to the aforementioned reaction at 300 °C. For the purification of the nanoparticles, the synthesized nanoparticles were first dispersed in chloroform, and methanol was added to this dispersion as an antisolvent, which results in the precipitation of the nanoparticles. The volume ratio of methanol to chloroform was maintained at ∼5:1. For all the synthesized Zn(Tb)S nanoparticles, the purification steps were repeated two times. The resulting purified nanoparticles did not show characteristic signatures of free reactant molecules, as judged by the infrared absorption spectroscopy (vide inf ra). Fourier Transform Infrared (FTIR) Spectroscopy. Fourier transform infrared spectra of the samples were acquired with a Jasco FTIR 6300 spectrometer. An average of 64 scans was obtained for each spectrum, with a resolution of 4 cm−1. The samples were prepared using a KBr pellet method, and the measurements were performed at room temperature. Electron Microscopy Measurements. The morphology of the Zn(Tb)S nanoparticles were characterized by transmission electron microscope (TEM) [Model: JEOL, JEM-2100] operated with an acceleration potential of 200 kV. The images were processed with Digital Micrograph 2.3 software. To prepare the TEM samples, a drop of colloidal solution (obtained by dispersing a very small amount of nanoparticles in chloroform) on carbon-coated copper grids was placed, and the extra solution had been removed by drying the grid. The energy dispersive Xray (EDX) spectra of the Zn(Tb)S nanoparticles were collected using a field emission scanning electron microscope (FESEM) [Model: JEOL, JSM-7600F], and the spectra were acquired from at least four different areas of the sample, each of which consists an area of 1.0 × 0.8 μm. This procedure covers a wide range of spatial location in the sample. The average terbium content was considered for an estimation of elemental composition. EDX spectra from various spatial positions of a sample containing nanoparticles have been known to provide quantitative information on the elemental composition.34 The EDX spectrum under high resolution was obtained from the TEM instrument. Moreover, it is to note that all the nanoparticles were washed during the purification procedure and the purified nanoparticles did not show signature of free reactant molecules (vide inf ra), indicating that the observed EDX spectra originate from the elements in the nanoparticles and not a mere reflection of free ions. Ultraviolet−Visible Absorption and Luminescence Measurements. The absorption spectra were collected in the PerkinElmer Lambda 1050 absorption spectrometer. The steady-state luminescence spectra were collected in the Horiba Fluorolog 3-22 luminescence spectrometer. For the emission spectra, the nanoparticles were excited at 295 nm. Exciting the samples at this wavelength ensures predominant nanoparticle excitation without significant contribution from higher lying 4f− 5d Tb3+ excitation band (vide inf ra). The excitation spectra were collected by monitoring either the broad blue nanoparticle centered emission at 400 nm or the Tb3+ band centered luminescence at 545 nm. The time-resolved emission decay profiles were collected with the same instrument, with a delay time and detection window of 0.05 and 5 ms, respectively. The delay time ensures the complete decay of the nanosecond lived components in the sample, and the gate time captures the long-
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GENERAL CHARACTERIZATION Fourier Transform Infrared (FTIR) Spectroscopy. The Fourier transform infrared (FTIR) spectra of the different systems studied are shown in Figure 1. The IR spectra identify
Figure 1. Representative FTIR spectra of the Zn(Tb)S nanoparticles of diameters d = 2.0 ± 0.3 nm (green), d = 3.0 ± 0.5 nm (blue), and d = 14 ± 3 nm (magenta) are shown, with the corresponding spectra of the pure zinc stearate (ZnSt) (black), TOPO (red), and Tb(III) nitrate (cyan), respectively. The spectra of the Zn(Tb)S nanoparticles indicate both stearate and TOPO to be acting as the capping ligands.
stearate and TOPO as the capping ligand moieties on the surface of the nanoparticles studied. A comparison of the corresponding spectra with the pure zinc stearate (ZnSt) and TOPO molecule reveals a significant attenuation of carboxylate asymmetric, carboxylate symmetric, and PO IR stretching bands at 1538, 1398, and 1146 cm−1 with an emergence of an additional band at 1372 cm−1 in the presence of Tb3+ in the Zn(Tb)S nanoparticles. The PO stretching band is also broadened in the Zn(Tb)S nanoparticles, compared to that in the pure TOPO molecule, suggesting multiple coordination environment of the capping ligands in the nanoparticles. These results have been found to be consistent with our previous study,33 which reports a tuning of surface capping ligand IR absorption characteristics by the lanthanide cations that are located on or near the surface of the nanoparticles. All the Zn(Tb)S nanoparticles studied show characteristic asymmetric and symmetric stretching IR absorption bands from the longer methylene chain of the capping ligand moieties in the spectral range of 2800−3000 cm−1. The observations from the IR spectra support the hydrophobic nature of the Zn(Tb)S nanoparticles studied. Moreover, the IR absorption signature observed in the Zn(Tb)S nanoparticles studied differ drastically compared to that in the Tb(III) nitrate (typical nitrate IR absorption bands appear at 1042 and 1335 cm−1), suggesting the absence of free terbium precursor compound in the Zn(Tb)S nanoparticles studied. Electron Microscopy. The electron micrographs along with the corresponding size distributions are shown in Figure 2 for the C
DOI: 10.1021/acs.jpcc.5b07182 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 2. Transmission electron microscopy (TEM) images of the Zn(Tb)S nanoparticles synthesized with reaction temperature of 300 °C (left panel), 280 °C (middle panel), and 260 °C (right panel). For clarity, a few particles have been labeled by white circles in the top left panel. The corresponding size distribution is shown for each panel. The size of the nanoparticles increased from 2.0 ± 0.3 to 14 ± 3 nm in diameter with a decrease in reaction temperature. For the particles with 14 ± 3 nm in diameter, the size distribution histogram has been developed using multiple images, one of which is presented in Figure S1. The corresponding representative EDX spectra are shown demonstrating the presence of Tb3+ in the Zn(Tb)S nanoparticles.
Figure 3. High-resolution transmission microscopy (HRTEM) image of the Zn(Tb)S nanoparticles with 3.0 ± 0.5 nm diameter is shown in the top left panel. An image of the nanoparticles under STEM mode is shown in the top right panel. The marked area in the STEM image was used to generate the EDX spectrum. The corresponding EDX spectrum is shown in the bottom panel. The EDX spectrum has been corrected for the baseline due to the low signal intensity.
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RESULTS AND DISCUSSION Steady-State Luminescence Spectra. The normalized luminescence excitation and emission spectra of Zn(Tb)S nanoparticles of various size are shown in Figure 4a−c. Exciting the nanoparticles at 295 nm results a broad blue emission centered at ∼400 nm in all the nanoparticles studied and originates from surface trap states.2,3,35−37 With increase in nanoparticle size, there is a blue-shift in the broad band. This blue-shift has been attributed to the lesser contribution of surface
Zn(Tb)S nanoparticles synthesized at 300, 280, and 260 °C. The size of the nanoparticles was found to be 2.0 ± 0.3 nm (average size ± standard deviation) in diameter with the reaction temperature of 300 °C and is in agreement with the previous reports on the Zn(Tb)S nanoparticles synthesized using a similar synthetic protocol.2,3,33 A decrease in reaction temperature to 280 °C increases the size of the synthesized nanoparticles to 3.0 ± 0.5 nm in diameter. The Zn(Tb)S nanoparticles synthesized with 260 °C resulted significant increase in the diameter, with a value of 14 ± 3 nm. For all the Zn(Tb)S nanoparticles synthesized at different reaction temperatures, the particles were found to be spherical in shape. The representative EDX spectra shown in Figure 2 indicate the presence of Tb3+ in the nanoparticles and is consistent with the observations from steady-state and time-resolved luminescence spectroscopy, where an interaction between the Tb3+ dopant in the Zn(Tb)S nanoparticles is evident (vide inf ra). In short, a decrease in reaction temperature from 300 to 260 °C resulted in Zn(Tb)S nanoparticles from 2.0 ± 0.3 to 14 ± 3 nm in diameter, without noticeable alteration from spherical shape. As a representative case, the high resolution transmission electron microscopy (HRTEM) image of the Zn(Tb)S nanoparticles with 3.0 ± 0.5 nm diameter has been shown in Figure 3, demonstrating the crystalline nature of the particles. The corresponding EDX spectrum under scanning transmission electron microscopy (STEM) mode shows the presence of terbium cations in the nanoparticles. This analysis identifies the presence of terbium even from a smaller area that covers only a few particles.
Figure 4. Luminescence excitation and emission spectra of the Zn(Tb)S nanoparticles studied are shown in panels a−c. In panels a−c, all spectra are represented with respect to the maximum intensity for the Zn(Tb)S nanoparticles with 2.0 ± 0.3 nm diameter, which is set to unity. The corresponding normalized excitation spectra are shown in panel d. The nanoparticles were dispersed in chloroform. D
DOI: 10.1021/acs.jpcc.5b07182 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C states in a larger nanoparticle and concomitant involvement of the core states in the emission spectrum. In addition to the broad emission, all the Zn(Tb)S nanoparticles exhibits characteristic sharp Tb3+ 4f−4f intraconfigurational emission bands at 490, 545, 585, and 620 nm, which originates from the 5D4 → 7Fn (n = 6−3) transitions, respectively. In order to evaluate the excitation pathway for the Tb3+ luminescence, the excitation spectra were recorded monitoring the Tb3+ band centered emission at 545 nm. The excitation spectra were found to be generally broad in nature without significant contribution from the sharp direct 4f− 4f excitation bands, having significant overlap with the corresponding excitation spectra monitoring the nanoparticle emission at 400 nm. This indicates that for all the Zn(Tb)S nanoparticles studied, the Tb3+ luminescence is sensitized by the nanoparticles acting as an optical antenna. The Tb3+ band centered luminescence spectra in the Zn(Tb)S nanoparticles reveal features that argue in favor of the emission being originated from the nanoparticles and not from free Tb3+ in solution. This argument is based on the following: (a) The Tb3+ concentration in the dispersion of Zn(Tb)S nanoparticles is
