Capping ligand infrared absorption and dopant photoluminescence spectroscopy provides a comprehensive picture to probe dopant spatial location in semiconductor nanoparticles

Gouranga H. Debnath, Arijita Chakraborty and Prasun Mukherjee*
Centre for Research in Nanoscience and Nanotechnology, University of Calcutta, JD-2, Sector-III, Salt Lake, Kolkata-700106, West Bengal, India. E-mail: pmukherjee12@gmail.com

Received 5th August 2016 , Accepted 31st August 2016

First published on 31st August 2016


Abstract

The spatial location of a foreign species (dopant) in a semiconductor nanoparticle matrix is probed by monitoring the infrared absorption and photoluminescence spectroscopy of the capping ligand and dopant moieties respectively at room temperature. The results have been rationalized within the domain of observables in each experiment and argued to provide a comprehensive picture in tracking the core and surface localized terbium cations that are incorporated in zinc sulfide nanoparticles. The progression of luminescence quantum yield of the core and surface localized terbium cations with doping extent induced variation has been found to be different. The generality of the experimental observations has been demonstrated with the corresponding europium incorporated nanoparticles.


Introduction

Incorporating trivalent lanthanide cations (Ln3+) as a foreign species (dopant) into a semiconductor nanoparticle matrix is an emerging method to develop novel lanthanide based luminophores.1–12 The nanoparticles, while acting as an optical antenna and protective matrix, help to overcome the challenges associated with parity forbidden 4f–4f weak direct optical excitation of lanthanide cations13 and the quenching of Ln3+ luminescence by the vibrational overtones of –OH, –NH and –CH bonds present in immediate ligand and solvent molecules.14 This generates long lived (microsecond to millisecond lifetime) lanthanide cation centered narrow emission bands which have minimum intra and inter lanthanide cation spectral overlap and are distributed over the entire visible and near-infrared (NIR) spectral region. Lanthanide cation incorporated semiconductor nanoparticle luminophores offer (i) incorporation of multiple Ln3+ in a given matrix, thus generating brighter luminescence, (ii) formation of multiplex assays, and (iii) availability of low frequency vibrations from inorganic host; the aspects otherwise not easily realizable in a lanthanide based luminophore with organic ligands. Lanthanide luminophores find applications in the fields of biological imaging, bio-analytical and sensing applications, optoelectronics, lasers and telecommunications.13,15–19

In the context of doping Ln3+ in a semiconductor nanoparticle matrix, charge and size mismatch of the host and guest cation pair needs consideration. For example, Bol and co-workers20 discussed about the difficulties associated to dope Ln3+ in semiconductor nanoparticle matrices with charge and size mismatched pairs and argued that in such cases the dopants might only populate the surface sites of the nanoparticles. However, it is noteworthy that various reports in the literature demonstrate the feasibility of doping charge and size mismatched pairs of cationic host-guest (dopant) ingredients, indicating that with proper synthetic methodologies it is indeed possible to dope cations that differ in charge and size with respect to the original host cation.6,21–24 Specifically, Chen and co-workers21,22 demonstrated doping of Ln3+ in TiO2 nanoparticles (ionic radii for six co-ordinated Ln3+ 100–117 pm, Ti4+ 75 pm).25 In a case study, these authors further discussed that such a doping of Eu3+ in anatase TiO2 nanoparticles result in a change of symmetry of Ti4+ from D2d to either D2 or C2v for internal sites and C1 for disordered external surface related sites.21 Banin and co-workers23 discussed successful doping of Cu2+, Ag+ and Au3+ separately in InAs nanoparticles, (ionic radii for six co-ordinated In3+ 94 pm, Cu2+ 87 pm, Ag+ 129 pm, Au3+ 99 pm),25 with further demonstration of doping Cu2+ in the interstitial sites of InAs nanoparticles.24 The question on charge mismatch between the cationic ingredients might be visualized by a charge compensation that is mediated by the presence of appropriate defects in the nanoparticle or by the nanoparticle's surface capping ligands. The size mismatch between the two cations could even result in lattice distortion as has been discussed by Banin and co-workers, with the ability to dope InAs nanoparticles with Cu2+, Ag+ and Au3+ separately23 and Chen and co-workers in the context of doping Eu3+ in TiO2 nanoparticles.21

Based on electron microscopy, photoluminescence (steady-state, time-gated and time-resolved) and infrared absorption spectroscopy experiments, we have been arguing to successfully incorporate Ln3+ in the II–VI semiconductor nanoparticles.2,26–29 This rationalization is based on the following experimental observations; (i) identification of Tb3+ in the energy dispersive X-ray spectrum (EDS), which has also been observed in the corresponding spectrum from scanning transmission electron microscopy (STEM) mode covering only a few particles,27 (ii) increase in the Tb3+ and Eu3+ emission in presence of the Ln3+ incorporated nanoparticles compared to that in the corresponding free Ln(III) precursor in bulk solvent,26 (iii) appearance of a broad excitation profile upon monitoring the Ln3+ emission bands which closely resembles to that of the zinc sulfide centered emission, without significant contribution from direct intra-configurational 4f–4f excitation bands (an optical antenna effect),2,26,27 (iv) observation of a shorter and longer Ln3+ luminescence lifetime in the Zn(Ln)S [Ln = Tb, Eu] nanoparticles, suggesting a lesser (presumably surface localized) and better (presumably core localized) protected Ln3+ co-ordination environment in the nanoparticles,2,26,27 with the longer lifetime component values even longer than the corresponding values reported for well protected Ln3+ molecular complexes, being 1.3 and 0.78 ms respectively comprising with Tb3+ and Eu3+ based complexes,30 (v) near constancy of the infrared absorption characteristics of the symmetric and asymmetric stretching frequencies originating from the longer alkyl chain of the capping ligand in absence (undoped) and presence of dopants (lanthanide incorporated) in the nanoparticles argue in favour of insignificant role of Ln3+ being intercalated in the long alkyl chain of the capping ligands28 and (vi) difference in IR absorption signatures of Zn(Ln)S [Ln = Sm, Dy] nanoparticles with the corresponding Ln(III) stearate salts indicate negligible contribution from Ln(III) stearate salts in modulating the signature in the Zn(Ln)S nanoparticles (for representative spectral comparison see ESI).28 Various other researchers have also presented experimental evidence supporting the incorporation of Ln3+ in II–VI semiconductor nanostructures.1,4–6,9,10,31,32

A well-orchestrated doped semiconductor nanoparticle would require knowledge of dopant spatial location (core versus surface) in the host matrix. Deciphering the dopant location helps understanding the physical and chemical properties of the material under investigation and has important implications towards the development of novel doped nanomaterials. Significant difference in dopant properties might be expected from a core localized and surface related dopant moiety. Few researchers addressed this important aspect of understanding the dopant location in semiconductor nanoparticles.24,26,33–36 A majority of the previous studies on this front focus on monitoring the luminescence of the species under interest26,33–36 and in some cases involves low temperature measurements for site selective spectroscopy.21,35,36 From a structural viewpoint, in a case study with Cu2+ doping in InAs nanoparticles, Banin and co-workers24 provided insight from X-ray spectroscopy and correlated the experimental observations with density functional theory calculations. The approach discussed in this study differs in that it considers monitoring different spectral properties from two different species; namely the infrared (IR) absorption of the capping ligand moieties and the dopant photoluminescence. The surface localized dopant cations are potentially eligible to alter the IR absorption characteristics of the capping ligand molecules located on or near the surface of the nanoparticles. Moreover, the luminescence quantum yield and the luminescence lifetime of surface localized dopant cations would be expected to be lesser than the corresponding core localized counterparts. In short, this account provides an unprecedented opportunity to compare easily accessible different spectral properties from different moieties in a given system at room temperature, in order to address the same question of identifying the dopant location in the semiconductor nanoparticle matrix.

In a recent work we discussed the modulation of capping ligand [strearate and trioctylphosphine oxide (TOPO)] IR absorption by Ln3+ cations in the lanthanide cation incorporated ZnS [Zn(Ln)S] [Ln = Sm, Eu, Tb, Dy] nanoparticles.28 A comparison of the IR absorption characteristics in the Zn(Ln)S nanoparticles to that in the undoped ZnS and zinc stearate (ZnSt) reveals that lanthanide cation incorporation (a) decreases the intensity of the carboxylate asymmetric (as) and symmetric (s) stretching bands (νas[COO] = 1538 cm−1 and νs[COO] = 1398 cm−1) and (b) induces the emergence of a new carboxylate symmetric stretching band (νs[COO] = 1372 cm−1) in the Zn(Ln)S nanoparticles.

The present study discusses the doping extent induced change in the trend of IR absorption of capping ligand moiety (stearate) in the Tb3+ incorporated ZnS [Zn(Tb)S] nanoparticles and the corresponding variation in their luminescence properties, in the context of assessing the spatial location of dopant cations in the semiconductor nanoparticle matrix and the corresponding luminescence properties. A doping percent of 1%, 5%, 10%, 15%, 20% and 30% have been considered. The choice of the nanoparticle system for the present work has been considered based on our previous study,2 where a relative energy level schematic identifies ZnS nanoparticles as the most promising host candidate to sensitize the Tb3+ luminescence among the II–VI sulfide and selenide materials. The experiments follow an analysis to monitor and characterize the photophysical properties of core and surface localized lanthanide cations. In order to examine the generality of the trend in experimental observations, corresponding Zn(Eu)S nanoparticles with 5%, 15% and 30% doping extent have been considered.

The primary objective of this study which is to decipher the possible location of the Ln3+ dopant and understanding the luminescence properties of the dopants those are spatially placed in different locations in the zinc sulfide nanoparticle matrix with the combined use of capping ligand FTIR and dopant photoluminescence spectroscopy, maximizing the Ln3+ luminescence in the semiconductor nanoparticle matrix is desirable. A previous study by our group on the various sized Zn(Tb)S nanoparticles27 identifies a Zn(Tb)S particle size of 2.0 ± 0.3 nm in diameter as an optimum dimension in which maximum Tb3+ luminescence could be realized, with an increase in particle size resulting decreased Tb3+ luminescence from the Zn(Tb)S nanoparticles. These results have been rationalized with the relative energy level positions of the Tb3+ ground and excited states with respect to the valence and conduction band of the host material, with a smaller sized particle providing the favorable energetic alignment, with concomitant minimization of competitive pathways like autoionization of the excited electrons. This optimization of Zn(Tb)S nanoparticle size to achieve maximum Tb3+ luminescence from the nanoparticles lead us to select the Zn(Tb)S nanoparticles with smaller dimension for the current work. A smaller particle size as has been studied in the present work implies the importance of surface atoms, due to the larger surface to volume ratio. The possibility of co-localization of dopant moieties at the core (internal) and surface (external) sites in the nanoparticles connects with the objective of this study to evaluate the spatial location of the Ln3+ (dopant) in the interior and exterior sites of the ZnS (host) nanoparticles, both sites being populated. Such an indication of simultaneous population of Tb3+ in the Zn(Tb)S nanoparticles at the core and surface sites of the nanoparticles has been inferred from a Stern–Volmer (SV) quenching experiment, in which using methanol as the quencher for the Tb3+ luminescence resulted multiple slopes in the SV plot.28 The uniqueness of the analyses presented in the current study lies in the fact that while capping ligand infrared absorption selectively probes the surface localized Ln3+,28 dopant photoluminescence may arise from both the core and surface sites.

Materials and methods

Chemicals

Tetracosane (99%), trioctylphosphine oxide (TOPO) (90%), trioctylphosphine (TOP) (90%), zinc stearate (tech.), octadecene (90%) (tech.), potassium bromide (KBr) (IR grade) was purchased from Sigma-Aldrich. Terbium(III) nitrate hydrate (99.9%) and europium(III) nitrate hydrate (99.9%) were purchased from Alfa Aesar. Sulfur (99.999%) was purchased from Fisher Scientific. 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 synthesis of terbium incorporated (doped) ZnS nanoparticles [Zn(Tb)S] was performed using a hot-injection method based on a protocol reported by Peng and co-workers37 for undoped particles and with later modifications by Waldeck, Petoud and co-workers2,26 for synthesizing the Zn(Ln)S nanoparticles [Ln = Eu, Tb]. For a typical synthesis of Zn(Tb)S nanoparticles with 15% Tb3+ as dopant, 2.0 g tetracosane, 1.7 g TOPO, 0.68 mmol zinc stearate and 3 mL octadecene were added to a three neck round bottomed flask. The mixture was stirred and refluxed at 300 °C under an argon atmosphere. The reaction was allowed to continue for ∼1.5–2 hours which was followed by the addition of lanthanide stock solution (0.12 mmol terbium(III) nitrate dispersed in 3 mL TOP, by sonication) through injection into the reaction mixture. A period of approximately 45 minutes to 1 hour was allowed after the injection of lanthanide stock solution followed by the injection of a sulfur stock solution (0.40 mmol sulfur dissolved in 2.5 mL octadecene, by sonication) into the reaction mixture. The reaction was brought to a completion by removing the heat source after a growth time of 20 minutes was allowed. Five additional syntheses were carried out by changing the extent of Tb3+ doping to 1%, 5%, 10%, 20% and 30% respectively by varying the concentrations of zinc stearate and terbium nitrate accordingly, while keeping the concentrations/volumes of the remaining precursors/solvents identical to the aforementioned reaction. The purification process involves the dispersion of the synthesized nanoparticles in chloroform and addition of methanol to facilitate the precipitation of the nanoparticles. The volume ratio of methanol to chloroform was maintained at ∼5[thin space (1/6-em)]:[thin space (1/6-em)]1. The purification steps were repeated two times for all the synthesized Zn(Tb)S nanoparticles. Repeated washing of the as synthesized nanoparticles gets rid of the free Ln3+ and the observed spectroscopic signatures have indeed been generated from the Ln3+ that have interacted with the nanoparticles, as evidenced by the FTIR spectroscopy where characteristic IR absorption signature from free or otherwise un-reacted Tb(III) nitrate was not observed in the nanoparticles investigated (vide infra and in the ESI). In all cases, the nominal doping extent has been considered to present the experimental data. While these nominal values have only been used to present the spectra, all data analysis from the IR and photoluminescence measurements were performed directly by using the core and surface related spectral parameters derived from the experimental data within the framework of the experimental observables (vide infra, Table 2). Moreover, it is important to note that the signature from free or otherwise un-reacted Tb3+ was not observed in the IR spectra of Zn(Tb)S nanoparticles suggesting the absence of free Tb3+ in the Zn(Tb)S nanoparticles investigated and the observed photoluminescence spectra should have negligible contribution from free Tb3+ (if any) due to the very inefficient direct excitation of Tb3+ that is associated with extremely low molar extinction co-efficient (<10 M−1 cm−1) of Tb3+ in bulk solvents. The europium incorporated ZnS [Zn(Eu)S] and undoped ZnS nanoparticles have been synthesized in a similar manner, with the inclusion of Eu(III) nitrate and omission of lanthanide cation precursor for the respective syntheses.

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. The TEM samples were prepared by placing a drop of colloidal solution (obtained by dispersing a small amount of nanoparticles in chloroform) on carbon-coated copper grids which were dried to remove the extra solution. The energy dispersive X-ray spectrum (EDS) was collected using the TEM instrument, with the samples that have been sonicated for at least thirty minutes, by which we anticipate that any loosely bound Tb3+ would be removed and correspondingly the observed signal in the EDS is an indication of the presence of Tb3+ in the Zn(Tb)S nanoparticles studied.

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. Typically ∼1 mg nanoparticles sample was mixed with ∼100 mg KBr and the pellet was prepared by the application of pressure. All data analysis was performed using the software provided by Jasco.

Photoluminescence measurements

The steady-state luminescence spectra were collected in the Horiba Fluorolog 3-22 luminescence spectrometer. For collecting the emission spectra, the nanoparticles were excited at 295 nm. The excitation at this wavelength results in predominant excitation of the nanoparticles without significant contribution from higher lying 4f–5d Tb3+ excitation band. The excitation spectra were collected by monitoring the nanoparticle's surface trap state related (vide infra) emission at 400 nm and the Tb3+ or Eu3+ luminescence at 545 or 616 nm respectively. A delay time and detection window of 0.05 and 5 ms respectively were used to collect the time-resolved emission decay profiles. The complete decay of the nanosecond lived components in the sample can be achieved by the delay time while only the long lived Tb3+ luminescence can be captured by the gate time. The luminescence spectra were 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 (measured using the Perkin Elmer Lambda 1050 UV-Vis-NIR absorption spectrophotometer). The spectra were corrected for the absorbance at the excitation wavelength. All measurements were performed at room temperature. The luminescence spectral data analyses were performed using the Origin 8.5 software.

Quantum yield

The quantum yields were calculated based on a relative method (using the known quantum yield of a conventional fluorophore as the reference)38 using eqn (1).
 
image file: c6ra19823f-t1.tif(1)
where the subscripts x and r stand for sample and reference respectively, A is the absorbance at the excitation wavelength (λ), Iex is the intensity of the excitation light at the same wavelength, η is the refractive index (η = 1.446 for chloroform and η = 1.327 for methanol) and Iem([small nu, Greek, macron]) is the luminescence intensity in wavenumber scale. The Zn(Ln)S [Ln = Tb, Eu] samples were excited at 295 nm. All the samples were dispersed in chloroform. For the calculation of Ln3+ [Ln = Tb, Eu] luminescence quantum yield values, the corresponding steady-state emission spectra were used, with the broad emission contribution from the ZnS nanoparticle band subtracted. For example, to extract the Tb3+ emission at 545 nm, the experimentally acquired steady-state emission spectrum was first interpolated with two data points in the range from 520 to 570 nm, which was subsequently interpolated with a data interval of 1 nm (same as the experimental condition used for acquiring the spectra). This interpolated line was subsequently subtracted from the experimentally acquired spectrum (having contributions from both the broad nanoparticle and sharp Tb3+ based emissions respectively) to obtain the sole contribution of Tb3+ emission. The Tb3+ and Eu3+ luminescence quantum yield values were calculated considering the 5D47Fn [n = 3–6] and 5D07Fn [n = 4–0] emission bands. Coumarin-153 (C153) dissolved in methanol was used as the reference for the quantum yield calculations, for which a value of 0.42 has been considered.39

Results and discussion

Electron microscopy

A representative transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) image for the Zn(Tb)S nanoparticles with 10% doping is shown in Fig. 1a and b respectively, indicating the formation of crystalline phases. The corresponding energy dispersive X-ray spectrum (EDS) shown in Fig. 1c confirms the presence of terbium in the nanoparticles and corroborates with the observations from luminescence spectroscopy (vide infra). The size distribution histograms summarized in Fig. 1d–i indicate an average particle diameter of 2.2 ± 0.5, 2.0 ± 0.4, 1.9 ± 0.4, 2.0 ± 0.3, 2.1 ± 0.3 and 2.2 ± 0.3 nm for the 1%, 5%, 10%, 15%, 20% and 30% doping extent respectively. This reveals that the size of the Zn(Tb)S nanoparticles remain unaffected as a function of varying Tb(III) content, as summarized from the corresponding TEM images, see Fig. S1 in the ESI. The same size and hence same bandgap of Zn(Tb)S nanoparticles ensures that the relative energetic alignment of the valence and conduction bands of the host lattice and the Tb3+ ground and excited states is not a variable in determining the Tb(III) luminescence in the systems studied. This provides an opportunity to evaluate the photophysical properties of Zn(Tb)S nanoparticles with same band gap with differing Tb3+ extent. The scenario presented in the current study differs from our previous work in which Zn(Tb)S nanoparticles with different sizes having similar Tb3+ extent has been discussed. The importance of nanoparticle size in controlling the Tb(III) luminescence in Zn(Tb)S nanoparticles has been discussed in this previous study,27 which identifies a Zn(Tb)S nanoparticle of diameter 2.0 ± 0.3 nm as an optimum particle dimension to realize maximum Tb3+ luminescence from such particles.
image file: c6ra19823f-f1.tif
Fig. 1 The TEM and HRTEM images of the Zn(Tb)S particles with 10% doping are shown in panels (a) and (b) respectively. The corresponding energy dispersive X-ray spectrum (EDS) is shown in panel (c), confirming the presence of terbium in the nanoparticles. The presence of copper in the EDS spectrum is most likely associated with its presence in the TEM grid. The size distribution histograms of the Zn(Tb)S nanoparticles with 1%, 5%, 10%, 15%, 20% and 30% doping extent respectively are shown in panels (d–i). The data for the Zn(Tb)S nanoparticles with 15% doping extent has been reported in our earlier work.27

It is important to appreciate that while the identification of the presence of lanthanide cations in the purified Zn(Ln)S nanoparticles by either EDS spectroscopy or inductively coupled plasma atomic emission spectroscopy (ICPAES) (an alternate but similar approach to determine the elemental composition of a species) confirms the presence of a particular element in the nanoparticles, these techniques do not differentiate between the core and surface localized dopant moieties. A detailed insight on structural perspective; dopants being in the substitutional, interstitial or surface related sites of the lanthanide cation incorporated II–VI semiconductor nanoparticles are not yet fully characterized. These structural aspects will be reported comprehensively in a future study.

Fourier transform infrared (FTIR) spectroscopy

The dopant extent induced capping ligand IR absorption spectra of the Zn(Tb)S nanoparticles in the carboxylate stretching region are shown in Fig. 2, the corresponding spectra of zinc stearate and undoped ZnS have also been included. For representative complete spectra of the capping ligands (stearate and TOPO), see Fig. S2 in the ESI. Both the asymmetric and symmetric stretching bands have been reduced in intensity in the Zn(Tb)S nanoparticles as compared to that in the pure zinc stearate molecule and undoped ZnS nanoparticles. In presence of Tb3+ in the Zn(Tb)S nanoparticles, an additional absorption band evolves that has been interpreted to a carboxylate symmetric stretching IR absorption. Interestingly, this band at 1372 cm−1 has been observed to increase in intensity with respect to the intrinsic carboxylate symmetric stretching at 1398 cm−1 with an increase in the extent of doping. The parameter of consideration is the ratio of integrated absorption of these two bands (defined as R, eqn (2)) as a function of doping extent, summarized in Table 1, and is a gauge of the relative concentration of the species responsible for these two IR absorption bands. Based on a hypothesis that the tuning of capping ligand IR absorption in the Zn(Tb)S nanoparticles predominantly originates from the Tb3+ cations that are located on or near the surface of the nanoparticles (vide infra), an increase in dopant extent would be expected to modulate the capping ligand IR absorption to a larger extent.
 
image file: c6ra19823f-t2.tif(2)

image file: c6ra19823f-f2.tif
Fig. 2 FTIR spectra of the Zn(Tb)S nanoparticles in the cabroxylate stretching region is shown in the left panel. An enlarged part of the two IR absorption bands of consideration is shown in the right panel, in the absorbance scale. The corresponding spectra of ZnS nanoparticles and zinc stearate (ZnSt) are also shown. The spectra for the ZnS and Zn(Tb)S nanoparticles with 15% doping extent has been reported first in our earlier work.28
Table 1 Relevant spectral properties of the different systems studieda
System Capping ligand infrared absorption Dopant photoluminescence
R ΦTb a1c
a The values reported are the average and standard deviation from multiple measurements. For the photoluminescence lifetime decay fitting, the adjusted R2 values were found to be ≥0.994.b ΦT refers to the Tb3+ luminescence quantum yield.c a1 refers to the relative amplitude of the shorter luminescence lifetime component, with the shorter (τ1) and longer (τ2) lifetime components fixed at 1.0 and 3.7 ms respectively. The sum of relative amplitudes (a1 and a2) has been scaled to unity.
Zn(Tb)S 1% 0.37 ± 0.05 (0.008 ± 0.002) × 10−3 0.40 ± 0.03
Zn(Tb)S 5% 0.89 ± 0.18 (0.24 ± 0.07) × 10−3 0.34 ± 0.02
Zn(Tb)S 10% 0.65 ± 0.13 (0.50 ± 0.02) × 10−3 0.32 ± 0.03
Zn(Tb)S 15% 1.56 ± 0.12 (2.0 ± 0.3) × 10−3 0.36 ± 0.02
Zn(Tb)S 20% 3.10 ± 0.14 (2.2 ± 0.4) × 10−3 0.41 ± 0.01
Zn(Tb)S 30% 3.20 ± 0.14 (2.3 ± 0.5) × 10−3 0.42 ± 0.01
ZnS 0.15 ± 0.01


It is noteworthy that while McQuillan and co-workers40 reported the carboxylate symmetric stretching band at 1407 cm−1 in the oleate salt, the corresponding band appears at 1416 cm−1 in the oleate capped CdS quantum dots with an emergence of an additional IR absorption band at 1378 cm−1 (which was originally not assigned to specific bond vibration in their study). This band at 1378 cm−1 in their study is consistent with the emergence of an additional IR absorption band at 1372 cm−1 in the stearate capped zinc sulfide nanoparticles studied in the current work. Also, our independent effort to make oleate capped CdSe nanoparticles (Fig. S3 and S4) resulted R values of 1.06 ± 0.02 and 2.45 ± 0.30 for the CdSe and Cd(Tb)Se nanoparticles respectively, suggesting the dopant Tb3+ induced increase of the 1376 cm−1 IR absorption band with respect to the intrinsic carboxylate symmetric stretching band located at 1410 cm−1. Moreover, the attenuation of carboxylate asymmetric and symmetric stretching bands at 1538 and 1398 cm−1 respectively with the formation of Zn(Tb)S nanoparticles, compared to the corresponding signals in the unbound stearate moiety observed in the current work is also consistent with the report by McQuillan and co-workers.40 These results collectively indicate that the attenuation of carboxylate asymmetric and symmetric stretching bands (as judged by the relative intensity ratio of these bands to the long chain methylene IR absorption band) in the IR absorption spectrum with an emergence of an additional band at lower energy with the formation of nanoparticles could be considered as general observation for carboxylate ligand capped II–VI semiconductor nanoparticles.

Additionally, we note that an analysis of nanoparticle formation induced changes in the IR absorption characteristics of capping ligand and its possible tuning in presence of dopant moieties is theoretically possible for nanoparticles capped with other ligands having different functional groups. However, such an analysis first requires an understanding of the spectral modulation of capping ligand IR absorption characteristics in nanoparticles, compared to the unbound capping ligand molecule. We anticipate that the nanoparticle formation induced changes and its potential tuning of the capping ligand IR absorption characteristics could be dependent on the identity of the ligand. For example, our study on 1-thioglycerol capped zinc sulfide nanoparticles have shown an attenuation of S–H stretching band intensity at 2556 cm−1 in both the undoped and Ln3+ [Ln = Sm, Eu, Tb, Dy] incorporated nanoparticles (with respect to that in the 1-thioglycerol molecule) and an Ln3+ induced change in IR absorption characteristics have been observed in the region of 615–635 cm−1, which was assigned to a C–S stretching band.29 Such a detailed analysis of capping ligand IR absorption characteristics with various functional groups in the ligand moiety is beyond the scope of the current work.

In the perspective of populating the sites by Tb3+ in the Zn(Tb)S nanoparticles with increase in dopant extent, at least two possibilities are relevant, (i) sequential gradient population of surface from core sites; that is, at lower dopant extent the Tb3+ locate at the core sites of the Zn(Tb)S nanoparticles and with an increase in dopant concentration the surface sites gradually gets populated and (ii) random population in both core and surface sites; that is, the core and surface sites of the nanoparticles gets populated simultaneously in a range of dopant concentration. The localization of the capping ligands being on the surface of the nanoparticles, it is reasonable to assume that the capping ligand IR absorption is not sensitive to the Tb3+ cations that are located at the core of the Zn(Tb)S nanoparticles and the observed trend is a reflection of the steady-state population of Tb3+ cations that are located on or near the surface of the Zn(Tb)S nanoparticles. Accordingly the capping ligand IR absorption selectively probes the concentration of surface located dopants and hence is unable to differentiate the two cases mentioned above. Dopant luminescence, on the other hand, is sensitive to both the core and surface located Tb3+ cations and hence would be able to differentiate between the two possibilities. The Tb3+ cations that are located on or near the surface of the Zn(Tb)S nanoparticles being more exposed to the immediate ligand and solvent molecules would be more prone to the non-radiative decay mechanisms and hence are less bright compared to the corresponding core located counterpart.

Luminescence spectroscopy

The luminescence spectral data of the Zn(Tb)S nanoparticles as a function of doping extent are shown in Fig. 3. Typically the emission spectrum of the Zn(Tb)S nanoparticles consist of a broad blue emission around 400 nm originating from surface trap states of the ZnS nanoparticles and the Tb3+ intra-configurational 4f–4f emission bands at 490, 545, 585 and 620 nm respectively. The broad blue emission feature observed with the ZnS based materials studied are in corroboration with the data previously reported by us2,26,27 and various other researchers.41–43 By considering the reactant lanthanide cation concentration, an estimation of the Tb3+ concentration in the Zn(Tb)S nanoparticles in solution (with which the spectra were acquired) poses an upper limit of 50 micromolar. It is important to note that the observation of luminescence spectral signatures from free Tb3+ in bulk solvents under the same experimental conditions would require a concentration in the range of millimolar order. The difficulty to observe intra-configurational 4f–4f luminescence bands from free Tb3+ is associated with the extremely low molar extinction co-efficient of Tb3+ (<10 M−1 cm−1). The incorporation of Tb3+ in the Zn(Tb)S nanoparticles relaxes the concentration requirement by virtue of higher molar extinction co-efficient of the semiconductor nanoparticle matrix and associated energy feeding from the nanoparticles to the Tb3+ centers (optical antenna effect). The emission intensity of Tb3+ increases with an increase in the dopant extent upto 15%, further increase in Tb3+ content did not produce significant increase in dopant luminescence. The extent of dopant emission increase, as summarized in Table 1 in the form of luminescence quantum yield (ΦT), suggests that the Tb3+ gets populated simultaneously in the core and surface related sites in the Zn(Tb)S nanoparticles. These results indicate that significant amount of Tb3+ (at least approximately upto 15% doping extent) could be accommodated in the Zn(Tb)S nanoparticles without noticeable luminescence quenching. Towards this line, Wang and co-workers have also observed Eu3+ luminescence increase upto 16 mol% in the TiO2 particles.44
image file: c6ra19823f-f3.tif
Fig. 3 The steady state luminescence spectra of the Zn(Tb)S nanoparticles are shown in the top (left, right) and bottom (left) panels. The spectra in the bottom (left) panel have been normalized with respect to the relative quantum yield value, that is the areas under the spectra correspond to the quantum yield values reported in Table 1. The corresponding Tb3+ luminescence decay profiles are shown in the bottom (right) panel.

The luminescence excitation spectrum while monitoring the Tb3+ emission at 545 nm provides an avenue to evaluate the excitation pathways in order to sensitize the Tb3+ luminescence in a given system. It has been argued previously that the luminescence sensitization by semiconductor nanoparticles (optical antenna effect) is an important path for Tb3+ luminescence sensitization in the Zn(Tb)S nanoparticles, while the contribution from direct excitation of intra-configurational 4f–4f excitation bands are negligible, see Fig. S5 in the ESI.2,27 The corresponding excitation spectra in all the Zn(Tb)S nanoparticles as a function of Tb3+ doping extent corroborate with the general observation from the previous studies. It is remarkable to note that the normalized excitation spectra reveal a nearly identical profile for doping extent upto 15%, which has been attributed to the size independence of the Zn(Tb)S nanoparticles with varying dopant extent. This rules out any variation in luminescence of different Zn(Tb)S nanoparticles that may originate from the difference in size of the nanoparticles27 and suggests that the observed luminescence is a major reflection of spatial location of Tb3+ in the Zn(Tb)S nanoparticles. The corresponding excitation spectra for the Zn(Tb)S nanoparticles with doping extent 20% and 30% respectively reveal significant difference in the spectral profile compared to the corresponding cases upto 15% doping extent. The increase in dopant extent increases the relative contribution of direct excitation bands, as judged by the increase in relative excitation intensity in the wavelength range of 350–380 nm. The increase in the proportion of direct excitation bands in the Zn(Tb)S nanoparticles with 20% and 30% doping extent corroborates with more surface related Tb3+ and is consistent with near constancy of the Tb3+ emission quantum yield values for the Zn(Tb)S nanoparticles with 15%, 20% and 30% doping extent respectively. Moreover, a comparison of spectral profiles (Fig. S6) reveals the full width at half maximum (FWHM) values in the range of 348 ± 13 cm−1 for the Tb3+ emission at 545 nm for the Zn(Tb)S nanoparticles with varying doping extent. This at least grossly suggests that the observed Tb3+ emission most likely monitors similar chemical species throughout the doping extent studied and is not having significant contribution from aggregated Tb3+ moiety.

The Tb3+ luminescence lifetime decay profiles were found to be bi-exponential. The decay profiles, I(t), were fitted by a sum of two decaying exponentials with time constant values fixed at 1.0 ± 0.05 and 3.7 ± 0.1 ms respectively according to eqn (3), where the shorter and longer lifetime components have been attributed to originate from the surface (lesser protected) and core (more protected) localized dopant cations in the nanoparticles and is in corollary with our previous reports.2,26,27 The lifetime components were fixed in the fitting process in order to better compare the relative amplitudes. Table S1 summarizes a representative detailed comparison of fitting parameters from fitting with all parameters varying and only relative amplitudes varying in the fitting process. The decrease in average luminescence lifetime with increase in dopant extent likely reflects an increase in non-radiative processes that is induced by the concentration quenching effect resulted by high local dopant concentration.

 
image file: c6ra19823f-t3.tif(3)

It is remarkable to note that in a site-selective spectroscopic analysis at 10 K, Chen and co-workers36 have observed two lifetime components of 0.69 and 1.16 ms respectively in Eu3+ doped ZnO nanocrystals, which has been correlated to surface and inner lattice sites respectively. The observation of two lifetime components even at 10 K further emphasizes the importance of understanding the properties associated with the core and surface related sites. The trend observed from Tb3+ luminescence lifetime analysis also corroborate with a simultaneous population of core and surface sites in the Zn(Tb)S nanoparticles, in which similar shorter lifetime amplitude has been expected. The amplitude of the shorter lifetime component (a1), as summarized in Table 1, was found to be similar for the doping extent upto 15%, without a noticeable systematic trend with respect to the doping extent. A case where the surface sites preferentially and sequentially gets populated with an increase in dopant extent would generate an increase in the amplitude of the shorter lifetime component, due to the higher non-radiative decay processes originating from the surface related sites of the nanoparticles. The additional non-radiative decay pathways originate from the interaction of surface localized Tb3+ with the vibrational overtones of the high frequency vibrations at the nearby ligand and solvent molecules. This has indeed been observed for the nanoparticles with 20% and 30% doping extent respectively, where a distinct increase in the non-radiative rate is evident, as judged by the increase in the amplitude of shorter lifetime component and is consistent with the appearance of some direct 4f–4f intra-configurational excitation bands observed in the steady-state excitation spectra while monitoring the Tb3+ luminescence. We anticipate that the quenching effect by the surface defects of the Zn(Tb)S nanoparticles would be similar, as the surface to volume ratio is same that is guided by the near constancy of the Zn(Tb)S nanoparticle's size with varying Tb3+ extent (an average particle diameter of 2.2 ± 0.5, 2.0 ± 0.4, 1.9 ± 0.4, 2.0 ± 0.3, 2.1 ± 0.3 and 2.2 ± 0.3 nm for the 1%, 5%, 10%, 15%, 20% and 30% doping extent respectively).

A unified picture towards tracking dopant spatial location and corresponding luminescence properties

The results obtained from the infrared absorption of the capping ligand and the dopant photoluminescence (Table 1) were used to develop a unified picture that tracks the dopant spatial location in the Tb3+ incorporated ZnS [Zn(Tb)S] nanoparticles and corresponding luminescence properties. As discussed above, this analysis hypothesizes that the capping ligand spectral signature probes steady-state population of the surface localized lanthanide cations solely and the dopant photoluminescence signal originates from both the core and surface localized dopant cations. Correspondingly, the relative concentration of surface localized Tb3+ in the Zn(Tb)S nanoparticles have been estimated from the IR observables. The relative concentration of the core localized Tb3+ in the Zn(Tb)S nanoparticles have been estimated using the proportion of core and surface localized dopant atoms calculated based on empirical relations (vide infra).

For spherical ZnS particles (Fig. 1) with wurtzite phase,28 the diameter (d) is given by

 
image file: c6ra19823f-t4.tif(4)
where the unit cell parameters are given by a = b = 0.382 nm and c = 0.626 nm. The total number of atoms in the nanoparticles has been represented by NT.

Volume of the nanoparticle (VNP) is given by

 
VNP = NTVZnS (5)

Simple mathematical calculation reduces eqn (5) to

 
image file: c6ra19823f-t5.tif(6)
where r represents the corresponding radius of the species.

The surface area (SNP) of the nanoparticles is represented by

 
SNP = 4πrNP2 (7)

The number of surface localized atoms (NS) is given by

 
image file: c6ra19823f-t6.tif(8)

The NT and NS values for a nanoparticle with 2 nm diameter are tabulated in Table 2. Importance of significant contribution from surface localized atoms is obvious and relates to small size of the nanoparticles. The total number of cationic and anionic ingredients in the nanoparticles has been estimated assuming a simple 1[thin space (1/6-em)]:[thin space (1/6-em)]1 stoichiometry. The total number of Tb3+ [N(Tb3+)T] has been calculated according to the doping percentage. The similar luminescence lifetime values observed in a range of doping extent of the Zn(Tb)S nanoparticles (Table 1) indicates that the core and surface sites of the nanoparticles are randomly populated by the dopants. Based on this both the core (C) and surface (S) localized Tb3+ cations [N(Tb3+)C/S] have been calculated according to the NS and NT proportion for the given particle diameter.

Table 2 Parameters related to evaluation of dopant spatial location and the corresponding photoluminescence properties
System Number of atoms (N) NT = 367, NS = 209 Capping ligand infrared absorption [Tb3+](x)C Dopant photoluminescence
N[Ln3+]T N[Ln3+]C N[Ln3+]S [Tb3+](x)S ΦS (%) ΦS ΦC
Zn(Tb)S 1% 2 1 1 0.22 ± 0.05 0.22 ± 0.05 15 ± 2 (0.0012 ± 0.0003) × 10−3 (0.0068 ± 0.0017) × 10−3
Zn(Tb)S 5% 9 4 5 0.74 ± 0.18 0.59 ± 0.18 12 ± 1 (0.03 ± 0.01) × 10−3 (0.21 ± 0.06) × 10−3
Zn(Tb)S 10% 18 8 10 0.50 ± 0.13 0.40 ± 0.13 11 ± 1 (0.06 ± 0.01) × 10−3 (0.44 ± 0.02) × 10−3
Zn(Tb)S 15% 28 12 16 1.41 ± 0.12 1.06 ± 0.12 13 ± 1 (0.26 ± 0.04) × 10−3 (1.74 ± 0.26) × 10−3
Zn(Tb)S 20% 37 16 21 2.95 ± 0.14 2.25 ± 0.14 16 ± 1 (0.35 ± 0.07) × 10−3 (1.85 ± 0.34) × 10−3
Zn(Tb)S 30% 55 24 31 3.05 ± 0.14 2.36 ± 0.14 16 ± 1 (0.37 ± 0.08) × 10−3 (1.93 ± 0.42) × 10−3


The presence of capping ligands on the surface of the nanoparticles infers that the infrared absorbance of the capping ligand probes the concentration of the surface localized Ln3+ in the Zn(Ln)S nanoparticles. As shown in Fig. 2 and Table 1, the infrared absorption signal at 1372 cm−1 emerges with an increase in the Tb3+ doping extent. Accordingly, this analysis hypothesizes that the trend in R value (eqn (2)) from the Zn(Tb)S and ZnS nanoparticles may be used as a gauge to monitor the concentration of surface localized Tb3+ ions, [Tb3+](x)S, in the Zn(Tb)S nanoparticles, mathematically this has been expressed by

 
[Tb3+](x)S ∝ (R[Zn(Tb)S] − R[ZnS]) ∝ N(Tb3+)S (9)

The construction of eqn (9) considers the molar extinction coefficient of Zn(Tb)S nanoparticles as being independent of doping extent and that the pathlength of the samples remain same.

From luminescence spectroscopy the steady-state emission intensity at 545 nm (5D47F5 of Tb3+) [ISS(545 nm)] is given by,

 
image file: c6ra19823f-t7.tif(10)

Correspondingly, the contribution from the surface localized Tb3+ [ΦS (%)] in the overall quantum yield (ΦT) is expressed by

 
image file: c6ra19823f-t8.tif(11)

It is to note that the ΦT [as computed considering the 5D47Fn (n = 3–6) Tb3+ emission bands] would be proportional to ISS(545 nm). Accordingly, from the ΦT values the corresponding ΦS and ΦC have been calculated. It has been inferred that [Tb3+](x)S is responsible for the ΦS. The corresponding core localized Tb3+ correlate with the ΦC.

In order to evaluate the trend in Tb3+ luminescence quantum yield values with the relative concentrations of core (more protected) and surface (lesser protected) localized Tb3+, the ΦS/C are plotted as a function of [Tb3+](x)S/C. The corresponding plot from such an analysis has been shown in Fig. 4. In a case where the effect of Tb3+ localization in either the core or surface of the Zn(Tb)S nanoparticles merely monotonously increases; the Tb3+ emission, a plot as shown in Fig. 4, would reveal a linear dependence. However, significant deviation from linear behavior has been observed for both the core and surface localized Tb3+. Moreover, the extent of increase in the Tb3+ luminescence quantum yield for the surface localized dopants was found to be more flattened compared to that for the corresponding core localized counterpart. This implies that an increase in concentration of dopant (Tb3+) moieties does not result the core and surface localized Tb3+ equally brighter in the Zn(Tb)S nanoparticles. The less dramatic concentration dependence on the surface localized dopants is most likely associated with the environmental quenching effect from the capping ligand and solvent molecules in the immediate local environment.


image file: c6ra19823f-f4.tif
Fig. 4 The increase in the Tb3+ luminescence quantum yield with respect to the relative concentration of the core and surface localized Tb3+ moieties. A second order polynomial fitting results in the following empirical relations, ΦS[Tb3+] = −0.07 + 0.31 × [Tb3+]S − 0.05 × [Tb3+]S2 and ΦC[Tb3+] = −0.67 + 3.20 × [Tb3+]C − 0.91 × [Tb3+]C2. The adjusted R2 values were found to be 0.992 and 0.987 for the black and red curves respectively. The value for 5% doping extent has been ignored in the fitting process.

The second order polynomial fitting while represents a satisfactory mathematical representation of the data with qualitative indication of deviation from linear increase of the quantum yield values (which most likely results from Tb3+ that interacts in a confined spatial location thereby introducing non-radiative decay mechanisms); we do not attribute any specific physical model associated with the second order polynomial fitting. While the real picture may well have contributions from higher order polynomials, we intend not to complicate the physical interpretation without prior knowledge of the exact dependence. In short, the second order polynomial fitting broadly captures the experimental trend that while the initial increase in Tb3+ incorporation content increases the Tb3+ luminescence quantum yield, it deviates from linearity beyond a certain Tb3+ incorporation extent. Moreover, the parameters of the fitting indicates that with the progression of the relative amount of core and surface localized Tb3+ moiety, the quantum yield values for the core localized components have been found to be at least 10 times more than the corresponding changes of the surface localized counterparts. The lesser sensitivity of the surface localized components most likely originates from the quenching of Tb3+ by adjacent ligand and solvent molecules that induces additional non-radiative decay pathways. The parameters from the fitting provides an idea of the relative changes of the core and surface localized Tb3+ in the Zn(Tb)S nanoparticles studied, however these numbers are not intended to have a quantitative meaning in an absolute scale due to the proportionality of Tb3+ concentration extracted from IR spectra with the corresponding numbers of Tb3+ (eqn (9)) (vide infra).

The analysis to probe the core (more protected) and surface (lesser protected) localized Tb3+ in the Zn(Tb)S nanoparticles studied with the corresponding luminescence properties presented in the current work relies on the following assumptions, the validity of which is justified by the following arguments; (a) the infrared absorption of the capping ligand probes the steady-state population of the surface localized Tb3+ which is guided by the confinement of the capping ligand moiety on the surface of the nanoparticles. Various other researchers have investigated the surface chemistry of nanoparticles by monitoring the IR spectral characteristics of the nanoparticles;40,45–58 (b) the two luminescence lifetime components can be correlated with the core (more protected) and surface (lesser protected) Tb3+ moieties, which is guided by the fact the surface sites being less ordered compared to the core sites induces additional non-radiative decay pathways due to the vibrational overtones of the adjacent ligand and solvent molecules. It is emphasized that the co-ordination environment of Tb3+ in the core and surface sites would be different which results the two luminescence lifetime components. Various works relate multiple Ln3+ luminescence lifetime components to multiple co-ordination environments.5,59–61 To this end, we further stress that while a fitting with two lifetime components might present a simplistic representation of the luminescence lifetime decay profiles, most likely there exists a distribution of lifetime components resulting from a gradient population of Tb3+ from the core towards the surface of the nanoparticles;26,28 (c) the proportionality of the contribution from the surface localized components extracted from the IR spectra to the corresponding number of Tb3+ extracted from a simple analysis from the diameter of the nanoparticles (eqn (9)). This proportionality imposes an important implication of the data presented in Fig. 4 being a relative trend of the luminescence characteristics of the core and surface localized components without necessarily emphasizing on the absolute values of the fitting parameters (see Fig. S7 for such a representative analysis).

Zn(Eu)S nanoparticles

The results obtained with the Zn(Tb)S nanoparticles with different doping extent reveals a trend where the luminescence intensity of the lanthanide band gradually increases upto a doping extent of 15%, further increase of the dopant moiety increases the Tb3+ luminescence only marginally. In order to check the generality of the observation, experiments have been undertaken with the Zn(Eu)S nanoparticles. Selected doping extent of 5%, 15% and 30% have been chosen. The results obtained from the Zn(Eu)S nanoparticles studied are summarized in Table 3. Representative infrared and photoluminescence spectra are shown in Fig. S2 and S8 respectively. The data with the Zn(Eu)S nanoparticles reveal the following trends, (i) the R values as computed from the IR spectra increases with respect to the doping extent, (ii) the increase in Eu3+ luminescence as a function of Eu3+ doping extent has been found to be non-linear and inclined towards the abscissa and (iii) the luminescence lifetime values were found to be nearly similar without a major dependence on doping extent. In summary, the trend in results obtained with the Zn(Eu)S nanoparticles are in general agreement, at least qualitatively, with that obtained for the corresponding Zn(Tb)S nanoparticles, suggesting the generality of the observations.
Table 3 Relevant spectral properties of the different systems studieda
System Capping ligand infrared absorption Dopant Photoluminescence
R ΦT a1b
a The values reported are the average and standard deviation from multiple measurements. For the photoluminescence lifetime decay fitting, the adjusted R2 values were found to be ≥0.994.b The lifetime values were kept fixed at 0.80 ± 0.05 and 3.7 ± 0.1 ms respectively during the fitting process.
Zn(Eu)S 5% 1.24 ± 0.23 (0.23 ± 0.04) × 10−4 0.66 ± 0.04
Zn(Eu)S 15% 1.56 ± 0.11 (2.6 ± 0.5) × 10−4 0.70 ± 0.02
Zn(Eu)S 30% 2.00 ± 0.20 (5.1 ± 0.2) × 10−4 0.75 ± 0.03


Conclusions

To summarize, monitoring both the dopant induced IR absorption characteristics of the surface capping ligands and the dopant PL would provide complementary insight towards assessing the dopant spatial location and the corresponding luminescence properties in the doped semiconductor nanoparticles. While the capping ligand IR absorption characteristics selectively probe the surface related dopants, the luminescence measurements provide insight on both the core and surface related dopants in the semiconductor nanoparticles. A different behavior between photophysical properties of the core (more protected) and surface (lesser protected) localized dopant moieties has been observed. The results obtained with the terbium incorporated zinc sulfide nanoparticles are in general agreement with the corresponding europium incorporated nanoparticles. The uniqueness of the analyses presented lies in the fact that it extracts information by probing two different entities in the doped semiconductor nanoparticles of interest by using relatively easily accessible spectroscopic equipment at room temperature. A comparison of two independent experimental data also helps eliminating false positive interpretation. For example, the indication of presence of surface related Tb3+ dopants in the Zn(Tb)S nanoparticles may only strengthen the interpretation of the origin of the two lifetime components being the core and surface localized dopants and probably not an intrinsic bi-exponential decay kinetics originating from a single Tb3+ population in the Zn(Tb)S nanoparticles, an important avenue that could only be directly addressable from a single particle luminescence decay kinetics measurement.

Acknowledgements

The authors gratefully acknowledge financial assistance from the Science and Engineering Research Board (SERB), Department of Science and Technology (DST) (SB/S1/PC-040/2013). The authors thank Ms Urmila Goswami for help in the electron microscopy measurements.

References

  1. D. A. Chengelis, A. M. Yingling, P. D. Badger, C. M. Shade and S. Petoud, J. Am. Chem. Soc., 2005, 127, 16752–16753 CrossRef CAS PubMed.
  2. P. Mukherjee, C. M. Shade, A. M. Yingling, D. N. Lamont, D. H. Waldeck and S. Petoud, J. Phys. Chem. A, 2011, 115, 4031–4041 CrossRef CAS PubMed.
  3. R. Martín-Rodríguez, R. Geitenbeek and A. Meijerink, J. Am. Chem. Soc., 2013, 135, 13668–13671 CrossRef PubMed.
  4. J. Planelles-Aragó, B. Julián-López, E. Cordoncillo, P. Escribano, F. Pellé, B. Viana and C. Sanchez, J. Mater. Chem., 2008, 18, 5193–5199 RSC.
  5. J. Planelles-Aragó, E. Cordoncillo, R. A. S. Ferreira, L. D. Carlos and P. Escribano, J. Mater. Chem., 2011, 21, 1162–1170 RSC.
  6. X. Chen, W. Luo, Y. Liu and G. Liu, J. Rare Earths, 2007, 25, 515–525 CrossRef.
  7. J. R. Dethlefsen, A. A. Mikhailovsky, P. T. Burks, A. Døssing and P. C. Ford, J. Phys. Chem. C, 2012, 116, 23713–23720 CAS.
  8. W. Luo, Y. Liu and X. Chen, Sci. China Mater., 2015, 1–32 Search PubMed.
  9. L. Chen, J. Zhang, S. Lu, X. Ren and X. Wang, Chem. Phys. Lett., 2005, 409, 144–148 CrossRef CAS.
  10. N. Jing-hua, H. Rui-nian, L. Wen-lian, L. Ming-tao and Y. Tian-zhi, J. Phys. D: Appl. Phys., 2006, 39, 2357–2360 CrossRef.
  11. L. Sun, C. Yan, C. Liu, C. Liao, D. Li and J. Yu, J. Alloys Compd., 1998, 275–277, 234–237 CrossRef CAS.
  12. S. Sivakumar, F. C. J. M. van Veggel and M. Raudsepp, ChemPhysChem, 2007, 8, 1677–1683 CrossRef CAS PubMed.
  13. J.-C. G. Bünzli, Chem. Rev., 2010, 110, 2729–2755 CrossRef PubMed.
  14. A. Beeby, I. M. Clarkson, R. S. Dickins, S. Faulkner, D. Parker, L. Royle, A. S. de Sousa, J. A. G. Williams and M. Woods, J. Chem. Soc., Perkin Trans. 2, 1999, 493–503 RSC.
  15. S. V. Eliseeva and J.-C. G. Bünzli, Chem. Soc. Rev., 2010, 39, 189–227 RSC.
  16. K. Binnemans, Chem. Rev., 2009, 109, 4283–4374 CrossRef CAS PubMed.
  17. C. P. Montgomery, B. S. Murray, E. J. New, R. Pal and D. Parker, Acc. Chem. Res., 2009, 42, 925–937 CrossRef CAS PubMed.
  18. C. Bouzigues, T. Gacoin and A. Alexandrou, ACS Nano, 2011, 5, 8488–8505 CrossRef CAS PubMed.
  19. S. V. Eliseeva and J.-C. G. Bünzli, New J. Chem., 2011, 35, 1165–1176 RSC.
  20. A. A. Bol, R. v. Beek and A. Meijerink, Chem. Mater., 2002, 14, 1121–1126 CrossRef CAS.
  21. W. Luo, R. Li, G. Liu, M. R. Antonio and X. Chen, J. Phys. Chem. C, 2008, 112, 10370–10377 CAS.
  22. W. Luo, R. Li and X. Chen, J. Phys. Chem. C, 2009, 113, 8772–8777 CAS.
  23. D. Mocatta, G. Cohen, J. Schattner, O. Millo, E. Rabani and U. Banin, Science, 2011, 332, 77–81 CrossRef CAS PubMed.
  24. Y. Amit, H. Eshet, A. Faust, A. Patllola, E. Rabani, U. Banin and A. I. Frenkel, J. Phys. Chem. C, 2013, 117, 13688–13696 CAS.
  25. M. W. Barsoum, Series in Materials Science and Engineering Fundamentals of Ceramics, Taylor and Francis Group, Boca Raton, 2002 Search PubMed.
  26. P. Mukherjee, R. F. Sloan, C. M. Shade, D. H. Waldeck and S. Petoud, J. Phys. Chem. C, 2013, 117, 14451–14460 CAS.
  27. G. H. Debnath, A. Chakraborty, A. Ghatak, M. Mandal and P. Mukherjee, J. Phys. Chem. C, 2015, 119, 24132–24141 CAS.
  28. A. Ghatak, G. H. Debnath, M. Mandal and P. Mukherjee, RSC Adv., 2015, 5, 32920–32932 RSC.
  29. A. Chakraborty, G. H. Debnath, M. Ahir, S. Bhattacharya, P. Upadhyay, A. Adhikary and P. Mukherjee, RSC Adv., 2016, 6, 43304–43315 RSC.
  30. S. Petoud, G. Muller, E. G. Moore, J. Xu, J. Sokolnicki, J. P. Riehl, U. N. Le, S. M. Cohen and K. N. Raymond, J. Am. Chem. Soc., 2007, 129, 77–83 CrossRef CAS PubMed.
  31. G. Ehrhart, B. Capoen, O. Robbe, F. Beclin, P. Boy, S. Turrell and M. Bouazaoui, Opt. Mater., 2008, 30, 1595–1602 CrossRef CAS.
  32. L. Dong, Y. Liu, Y. Zhuo and Y. Chu, Eur. J. Inorg. Chem., 2010, 2504–2513 CrossRef CAS.
  33. V. Sudarsan, F. C. J. M. van Veggel, R. A. Herring and M. Raudsepp, J. Mater. Chem., 2005, 15, 1332–1342 RSC.
  34. A. Nag, R. Cherian, P. Mahadevan, A. V. Gopal, A. Hazarika, A. Mohan, A. S. Vengurlekar and D. D. Sarma, J. Phys. Chem. C, 2010, 114, 18323–18329 CAS.
  35. O. Lehmann, K. Kömpe and M. Haase, J. Am. Chem. Soc., 2004, 126, 14935–14942 CrossRef CAS PubMed.
  36. Y. Liu, W. Luo, R. Li and X. Chen, Opt. Lett., 2007, 32, 566–568 CrossRef CAS PubMed.
  37. L. S. Li, N. Pradhan, Y. Wang and X. Peng, Nano Lett., 2004, 4, 2261–2264 CrossRef CAS.
  38. J. R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd edn, 2006 Search PubMed.
  39. J. E. Lewis and M. Maroncelli, Chem. Phys. Lett., 1998, 282, 197–203 CrossRef CAS.
  40. A. G. Young, N. Al-Salim, D. P. Green and A. J. McQuillan, Langmuir, 2008, 24, 3841–3849 CrossRef CAS PubMed.
  41. W. G. Becker and A. J. Bard, J. Phys. Chem., 1983, 87, 4888–4893 CrossRef CAS.
  42. D. E. Dunstan, A. Hagfeldt, M. Almgren, H. O. G. Siegbahn and E. Mukhtar, J. Phys. Chem., 1990, 94, 6197–6804 CrossRef.
  43. K. Sooklal, B. S. Cullum, S. M. Angel and C. J. Murphy, J. Phys. Chem., 1996, 100, 4551–4555 CrossRef CAS.
  44. L. Li, C.-K. Tsung, Z. Yang, G. D. Stucky, L. Sun, J. Wang and C. Yan, Adv. Mater., 2008, 20, 903–908 CrossRef CAS.
  45. M. E. Abrishami, A. Kompany, S. M. Hosseini and N. G. Bardar, J. Sol-Gel Sci. Technol., 2012, 62, 153–159 CrossRef.
  46. W. Wang, X. Chen and S. Efrima, J. Phys. Chem. B, 1999, 103, 7238–7246 CrossRef CAS.
  47. A. Gupta, C. Schulz and H. Wiggers, J. Optoelectron. Adv. Mater., 2010, 12, 518–522 CAS.
  48. B. S. Kim, L. Avila, L. E. Brus and I. P. Herman, Appl. Phys. Lett., 2000, 76, 3715–3717 CrossRef CAS.
  49. J. Chen, Q. Meng, P. S. May, M. T. Berry and C. Lin, J. Phys. Chem. C, 2013, 117, 5953–5962 CAS.
  50. T. Grzyb, M. Runowski, A. Szczeszak and S. Lis, J. Phys. Chem. C, 2012, 116, 17188–17196 CAS.
  51. A. P. Duarte, L. Mauline, M. Gressier, J. Dexpert-Ghys, C. Roques, J. M. A. Caiut, E. Deffune, D. C. G. Maia, I. Z. Carlos, A. A. P. Ferreira, S. J. L. Ribeiro and M.-J. Menu, Langmuir, 2013, 29, 5878–5888 CrossRef CAS PubMed.
  52. T. Posati, F. Costantino, L. Latterini, M. Nocchetti, M. Paolantoni and L. Tarpani, Inorg. Chem., 2012, 51, 13229–13236 CrossRef CAS PubMed.
  53. W. W. Yu, Y. A. Wang and X. Peng, Chem. Mater., 2003, 15, 4300–4308 CrossRef CAS.
  54. S.-L. Iconaru, M. Motelica-Heino and D. Predoi, J. Spectrosc., 2013, 284285 Search PubMed.
  55. Q. Wang, T. Fang, P. Liu, B. Deng, X. Min and X. Li, Inorg. Chem., 2012, 51, 9208–9213 CrossRef CAS PubMed.
  56. X. Wu, D. Wang and S. Yang, J. Colloid Interface Sci., 2000, 222, 37–40 CrossRef CAS PubMed.
  57. W. Xu, B. A. Bony, C. R. Kim, J. S. Baeck, Y. Chang, J. E. Bae, K. S. Chae, T. J. Kim and G. H. Lee, Sci. Rep., 2013, 3, 3210 Search PubMed.
  58. Á. I. López-Lorente and B. Mizaikoff, TrAC, Trends Anal. Chem., 2016 DOI:10.1016/j.trac.2016.01.012.
  59. J. P. Cross, M. Lauz, P. D. Badger and S. Petoud, J. Am. Chem. Soc., 2004, 126, 16278–16279 CrossRef CAS PubMed.
  60. J. Zhang, C. M. Shade, D. A. Chengelis and S. Petoud, J. Am. Chem. Soc., 2007, 129, 14834–14835 CrossRef CAS PubMed.
  61. K. A. White, D. A. Chengelis, M. Zeller, S. J. Geib, J. Szakos, S. Petoud and N. L. Rosi, Chem. Commun., 2009, 4506–4508 RSC.

Footnote

Electronic supplementary information (ESI) available: TEM images, representative FTIR and luminescence spectra, luminescence lifetime decay fitting parameters have been presented in the electronic supplementary information. See DOI: 10.1039/c6ra19823f

This journal is © The Royal Society of Chemistry 2016
Click here to see how this site uses Cookies. View our privacy policy here.