Soft-Templated Room Temperature Fabrication of Nanoscale

Article pubs.acs.org/JPCC

Soft-Templated Room Temperature Fabrication of Nanoscale Lanthanum Phosphate: Synthesis, Photoluminescence, and EnergyTransfer Behavior Sayantani Chall, Soumya Sundar Mati, Soumyadipta Rakshit, and Subhash Chandra Bhattacharya* Department of Chemistry, Jadavpur University, Kolkata 700032, India S Supporting Information *

ABSTRACT: We herein report a simple and effective soft template mediated synthesis protocol for the room temperature preparation of highly crystalline cerium (Ce3+) and terbium (Tb3+) doped lanthanum phosphate (LaPO4) nanorods (NRs) and nanoflowers. Anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/n-alkane soft template was chosen since it forms stable reverse micelles (RMs). Transmission electron microscopy (TEM) analysis corroborated successful formation of LaPO4:Ce3+,Tb3+ NRs having different aspect ratios (ranging from 2.8:1 to 7.6:1) under varying reaction conditions. The crystalline nature of the nanomaterials (NMs) was ascertained by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and small-area electron diffraction (SAED) studies. Energy-dispersive X-ray (EDX) and Fourier transform infrared (FTIR) spectroscopy further verified the elemental existence of the desirably synthesized nanostructures. Experimental results from thermogravimetric-differential thermal analysis (TG-DTA) revealed thermal stability of the NRs up to 800 °C. The prepared NRs exhibit strong yellowish-green photoluminescence (PL) at 544 nm (5D4 → 7F5) when excited at 270 nm. Furthermore, timeresolved decay analysis along with steady-state PL measurements indicated an efficient energy transfer (ET) phenomenon between Ce3+ and Tb3+ ions doped in LaPO4 (LAP) host matrix.

1. INTRODUCTION On account of comprehensive utilization as superconductors,1−4 magnets,1,3 catalysts,2,5 biological sensors,6,7 cell labels,8−10 drug carriers,11,12 etc., demand for designing and fabrication of multifunctional, smart f-block rare earth (RE) nanostructures is still a competing field of research. Usually, the optical functionality is afforded by quantum dots (QDs),13,14 fluorescent dyes,13,15 and/or rare-earth-based materials;7,16 however, the latter are being preferred owing to several intriguing aptitudes such as sharp emission bands, high photochemical stability, long luminescence lifetimes (≫10−6 s), appreciably large Stokes shifts (≫10−9 m), and lower toxicity.8 Lanthanide (Ln)-doped RE phosphate stands out as a fascinating and complementary but relevant optical (and/or imaging) fluorescent probe owing to its unique luminescent behavior in the presence of suitable dopants (for example, Ce3+, Tb3+, Eu3+, Gd3+, etc), thus acting as an efficient alternative to the traditional semiconductor QDs containing cadmium. In addition, Ln-doped large band gap nanocrystals also provide a rigid crystal environment for the dopant ions, resulting in a higher photoluminescence (PL) quantum yield (QY).3,17 LaPO4 (LAP) also gains paramount importance in many fields such as laser materials, catalysts, heat-resistant materials, proton conductors, versatile biological labels, and photon upconversion materials.18−20 More importantly, in the recent past, doped LAP nanorods have been successfully used as cellular markers, demonstrating localization in the cytoplasm of © 2013 American Chemical Society

cells without any reports of induced toxicity after internalization.3,21,22 To date, numerous methods such as hydrothermal, microwave heating, and solid-state reactions have been employed for the synthesis of LaPO4:RE3+; however, these syntheses require severe conditions of high temperatures and/or high pressure.1,8 These disadvantages also limit the potential applications of these nanomaterials (NMs).23 Therefore, low-temperature synthesis, especially room-temperature growth of RE nanocrystals, has drawn tremendous attention in research interest since it provides numerous attractive advantages over the commonly used methods.23 Unfortunately, with significant paucity of literature reports, room-temperature synthesis is still extremely challenging in the contemporary research field.18,22,24 In view of these perspectives, this report provides a simple yet efficient synthesis methodology by carefully choosing surfactant-driven, soft-template strategy for preparing nanoscale RE phosphate with controlled size and morphology at room temperature. Reverse micelles (RMs), having excellent advantage for particle fabrication (controlling growth, kinetics, interaction dynamics, etc.), are extensively used for devising a typical membrane-mimetic system. Fundamental concepts of reverse micelles explain that polar or ionic components which are Received: September 4, 2013 Revised: October 23, 2013 Published: October 29, 2013 25146

dx.doi.org/10.1021/jp408850m | J. Phys. Chem. C 2013, 117, 25146−25159

The Journal of Physical Chemistry C

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added to the isotropic liquid mixtures of water, oil and surfactant will become distributed into the central cores of these RMs, affording fine dispersion of inorganic nanosized grains in oil.25,26 Versatile structure variation (both in size and shape) can easily be achieved by varying water-to-surfactant ratio (ω), surfactant film flexibility, reactant concentrations, bulk solvents, added electrolytes, etc. In previous years, pioneering works on nanoparticle (NP) synthesis using RM soft templates had been successfully carried out by Pileni,27 Eastoe,25 Lopez-Quintela,28 Capek,29 Holmberg,30 and Uskokovic.31 Xing et al.32 in their earlier work reported microemulsion-based synthesis of cerium phosphate at room temperature. In another work, Li et al.33 used ionic liquid based microemulsions for preparing rare earth phosphates. In our present work, we intend toward focusing on the systematic synthesis of Ce3+,Tb3+-doped LaPO4 (we preferred Ce3+ and Tb3+ ion pair since Ce3+ effectively sensitizes bright green Tb3+ luminescence6,7) NMs employing AOT/n-alkane RM. It is well established that the solvent properties also play an important role in the dynamics of NP formation because of different interactions between the solvent and the surfactant tails.25 In corroboration of this fact, we performed a detailed study on the effect of bulk solvents in controlling particle morphology or aspect ratios thereby making our present work justifiably significant. Furthermore, the synthesized nanomaterial was characterized using X-ray diffraction (XRD), thermogravimetric-differential thermal analysis (TG-DTA), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FTIR), and dynamic light scattering (DLS). Effects of various synthetic parameters on the photoluminescence (PL) properties of the LAP:Ce3+,Tb3+ were also exemplified. In a nutshell, we have made a modest endeavor to investigate the role of Ce3+ ions in enhancing the luminescence intensity of the Tb3+ ions within reverse micellar nanocavities along with the corresponding energy-transfer (ET) mechanism possibly occurring between them.

can briefly be described as follows (Scheme 1): Reverse micelles of AOT (0.1 mol dm−3)/n-alkane were prepared; one part contained aqueous solutions of La3+, Ce3+ and Tb3+ and the other consisted of NaH2PO4.

2. EXPERIMENTAL SECTION Materials. AR grade salts lanthanum(III) nitrate hexahydrate (La(NO3)3·6H2O), terbium(III) nitrate hydrate (Tb(NO3)3·xH2O), and cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O) with a purity of 99.999% were purchased from Alfa Aesar and Aldrich, respectively. AR grade phosphate sources sodium dihydrogen phosphate (NaH2PO4·H2O), disodium hydrogen phosphate (Na2HPO4·2H2O), trisodium phosphate (Na3PO4·12H2O), and orthophosphoric acid (H3PO4) were procured from E. Merck. For oil medium, spectroscopic grade solvents, n-hexane (n-hex), n-heptane (nhep), cyclohexane (CyHx), n-octane (n-oct), isooctane (isooct), and n-decane (n-dec) were acquired from E. Merck. Before using in synthesis, each solvent was dried following standard procedure.34 Surfactant bis(2-ethylhexyl) sulfosuccinate (AOT) (96%) was purchased from Aldrich. Ethanol (E. Merck) and acetone (Aldrich) were used for extracting and washing powdered samples from the RM medium. Millipore-Q purified water was used throughout the experiment. All the purchased chemical reagents were used as received without further purification. Synthesis of Nanomaterials. Synthesis of LAP:Ce3+,Tb3+ nanostructures in reverse micelles was simply achieved by the reaction of reactant ions solubilized in the aqueous cores of anionic RMs at room temperature. The preparation procedure

Na2HPO4, Na3PO4, and H3PO4 were also used in place of NaH2PO4 during the experiment. Concentrations of stock solution of each of the reactants were kept at 0.5 mol dm−3. After that, the two RMs were mixed together and subjected to rapid mechanical stirring at 1000 rpm for 4 h. Owing to the dynamic character of the nanoreactor, the reactants within the water pools of the two RMs were distributed evenly over the entire droplet population so that reaction could occur inside the droplets. In consequence of such incessant coalescence and decoalescence process, Ce3+- and Tb3+-doped nanosized LAP were formed with controlled morphology. However, the whole reaction system in the RM medium was maintained overnight. The product was then extracted, centrifuged, and washed repeatedly by using acetone and ethanol. The obtained solid was then dried, stored in a vacuum desiccator, and finally used for various characterizations. Synthesis of the desired nanomaterials was critically investigated under varying reaction parameters. To observe the effect of surfactant concentration, [AOT] were varied from 0.1 to 0.5 mol dm−3. Yet again, a number of linear chain nalkanes (−(CH2)n−; n = 4, 5, 6, 8), cyclohexane, and isooctane were used as oil medium to observe the solvent effect on the particle morphology and its various characteristics. Since ω is another important size and/or shape controlling parameter, its effect was also studied within a wide range varying from 5 to 30.

Scheme 1. Soft Chemical Synthesis Scheme of LAP:Ce3+,Tb3+ Nanorods Employing AOT/n-Alkane Reverse Micelles

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Figure 1. XRD pattern of LAP:Ce3+,Tb3+ (a) synthesized in n-heptane; after thermal treatment at temperatures (b) 300 °C; (c) 600 °C; and (d) 800 °C; XRD pattern of LAP:Ce3+,Tb3+ in different solvents, (e) n-hexane, (f) cyclohexane, (g) n-octane, and (h) n-decane; in all cases, ω = 25; [AOT] = 0.1 mol dm−3; [La3+] = 0.5 mol dm−3; [Ce3+] = 0.5 mol dm−3; [Tb3+] = 0.5 mol dm−3; [H2PO4−] = 0.5 mol dm−3.

Spectrum RXI equipment with a resolution of 2 cm−1. The spectrum was recorded from 400 to 4000 cm−1. Thermogravimetric-differential thermal analysis was carried out in a PerkinElmer Pyris Diamond TGA/DTA system in air with a heating rate 10 °C min−1. Time-resolved fluorescence measurements were performed from time-resolved intensity decay by the method of time-correlated single-photon-counting using a diode laser at 300 nm (IBH Nanoled-03) as light source with TBX-04 detector (all IBH, UK). Detailed procedure of the time-resolved decay analysis is provided in the Supporting Information.

During the synthesis, the host ion concentrations, i.e., [La3+], were varied within the range from 0.05 to 0.5 mol dm−3. Dopant ions (Ce3+, Tb3+) concentrations with respect to the host matrix were varied from 1.0 mol % to a maximum of 33.0 mol %. For the energy-transfer study between Ce3+ and Tb3+ ions, the host ion (La3+) and donor (Ce3+) concentrations were kept at 7.268 × 10−4 and 2.907 × 10−4 mol dm−3, respectively. Acceptor ion (Tb3+) concentrations were varied from 0 to 7.27 × 10−5 mol dm−3. Characterization. Absorption spectra were recorded on a Shimadzu UV-1700 spectrophotometer using quartz cells of 1 cm path length. The photoluminescence measurements of LAP:Ce3+,Tb3+ were carried out on a Fluorolog F-IIA spectrofluorimeter (Spex Inc., NJ) with a slit width of 2.5 mm. The average particle size of LAP:Ce3+,Tb3+ NMs was initially determined by a dynamic light scattering instrument (Model DLS-nano ZS90, Zetasizer Nanoseries, Malvern Instruments). Samples were filtered several times through a 0.22 μm Millipore membrane filter prior to measurements to avoid the possible presence of any dust particle. Electron microscopy images were obtained with a FEI make Tecnai Stwin and JEOL-JEM-2100 transmission electron microscope at an accelerating voltage of 200 kV. The analyte solution was sonicated for 45 min and a drop of the solution was placed on a 300 mesh copper grid coated with a thin amorphous carbon film. The grid was dried carefully, keeping it in a vacuum desiccator for 24 h and was finally examined under the electron microscope to study the structural features of the NMs. SAED patterns of the NMs were collected from both the TEM instruments. EDX analysis was done in the field emission scanning electron microscopy (Hitachi S-4800, Japan). The crystalline structure of the prepared samples was assessed from X-ray diffraction patterns (XRD; D-8 Advanced, Bruker) collected at intervals of 0.02° (2θ). The FTIR spectrum of the powdered sample (using KBr pellets of the sample at a KBr−sample ratio of 100:1) was recorded with Perkin-Elmer

3. RESULTS AND DISCUSSION 3.1. Structural Investigations of LaPO4:Ce3+,Tb3+. XRD. The XRD patterns of the prepared LaPO4 doped with Ce3+ and Tb3+ ions and calcined at 300, 600, and 800 °C are illustrated in Figure 1a−d. The powder XRD data of the as-synthesized product shows characteristic diffraction peaks (24.9°, 28.8°, 31.0°, 37.3°, 38.2°, 40.9°, 46.5°, 48.1°, 51.4°, 52.6°, 58.7°) in 2θ range between 20° and 75°. Accordingly, XRD analyses (Figure 1e−h) exhibit similar structural pattern for all the set of samples synthesized by using different solvents at room temperature. Even after the thermal treatment at the said temperatures (Figure 1b−d), no phase transformation of the as-prepared powdered material was observed. We have carefully measured the fwhm (full width at half-maxima) values for different XRD plots at different temperatures and it reveals insignificant variation of fwhm values with respect to the XRD spectra of the LaPO4:Ce3+,Tb3+ taken at room temperature. This finding was further substantiated by FTIR analysis in the latter part. Analyses of XRD patterns (Figure 1a) were almost perfectly indexed to the monoclinic unit cell (a = 8.25 Å, b = 7.09 Å, c = 6.42 Å; space group P21/a (14); JCPDS card no. 73-0188). Assignment of the diffraction peaks to the monoclinic type structure indicated that the obtained samples were of single phase and doping of Ce3+ (Figure S1 in the Supporting 25148



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Scheme 2. Schematic Representation of (a) LAP and (b) LAP:Ce3+,Tb3+ Crystal Structure

Figure 2. FTIR spectra of LAP:Ce3+,Tb3+ (a) synthesized in n-heptane; after treatment at temperatures (b) 300 °C; (c) 600 °C; and (d) 800 °C; FTIR spectra of LAP:Ce3+,Tb3+ in different solvents, (e) n-hexane, (f) cyclohexane, (g) n-octane, and (h) n-decane; in all cases, ω = 25; [AOT] = 0.1 mol dm−3; [La3+] = 0.5 mol dm−3; [Ce3+] = 0.5 mol dm−3; [Tb3+] = 0.5 mol dm−3; [H2PO4−] = 0.5 mol dm−3.

Information (SI)) and Tb3+ ions did not cause any significant change in the host structure (Figure 1a). A slight shift of the peak values from the standard JCPDS data may be due to the difference in ionic radii of Ce3+ (1.14 Å), Tb3+ (1.09 Å), and La3+ (1.15 Å).35 On the basis of similar effective ionic radius and valence of the cations, we can therefore suggest that Ce3+/ Tb3+ ions essentially prefer to occupy La3+ sites (Scheme 2a,b).35 The La ion is 9-fold coordinated to PO4 tetrahedra. The PO4 tetrahedral may have to change its orientations to accommodate the coordination requirements of the cations.35 As depicted in Figure 1, 12-1 and 11-2/21-2 planes can be indexed to the mostly intense peak, thus suggesting a preferential growth of the nanostructures toward a specific direction. XRD patterns were also analyzed for the samples prepared by means of different surfactant concentrations and phosphate sources (i.e., Na2HPO4, Na3PO4, and H3PO4) (Figure S2a−f in the SI). However, in comparison to Na2HPO4 and Na3PO4, use of H3PO4 can only hold good for the crystalline nature of the sample. EDX. Analysis of the powdered samples by energy-dispersive X-ray (Figure S3 in the SI) confirmed the presence of La, P, Ce, Tb, and O. The atom percentage ratio of La/P/O was found to

be 13.8:18.6:62.9 as obtained from the EDX data which is very next to the standard atom ratio (1:1:4), further indicating that the NMs are indeed LaPO4. Because of very small particle size, elemental mapping (Figure S4 in the SI) did not provide any explicit evidence for the location of specified atoms in a particular selected area; it rather afforded the whole distribution of the atoms within the nanostructures. Elemental composition of the calcined product powder became unaffected even at 800 °C as confirmed by EDX analysis. Here also, the atom ratio of the host material resulted very close to 1:1:4. Overall, the EDX analysis revealed successful formation of pure phase Ce3+- and Tb3+-doped desired LaPO4 as the final product. FTIR. Synthesis of pure phase LAP:Ce3+,Tb3+ was further investigated by FTIR spectroscopy. The bond characterization was carried out by the IR spectrum presented in Figure 2a−i. Vibration bands at ∼542 cm−1 (O−P−O), ∼572 cm−1 (O−P− O), and ∼617 cm−1 (O=P−O) can be well assigned to the asymmetric bending vibration mode of PO43− group in the ν4 region.36 The peak at ∼1055 cm−1 is due to P−O asymmetric stretching vibration (ν1) of the PO43− group. The band in the region of 3250−3750 cm−1 may be correlated to the stretching vibrations of crystal water and that at 1640 cm−1 is ascribable to 25149



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Table 1. Assignment of the FTIR Absorption Bands of the As-Prepared LAP:Ce3+,Tb3+, Synthesized in Different Solventsa

s = sharp; m = medium; w = weak, b = broad, vw = very weak; [AOT] = 0.1 mol dm−3; [La3+] = 0.5 mol dm−3; [Ce3+] = 0.5 mol dm−3; [Tb3+] = 0.5 mol dm−3; [H2PO4−] = 0.5 mol dm−3; ω = 25. a

Figure 3. (a) TG-DTA of LAP:Ce3+,Tb3+ illustrate thermal stability of the nanorods up to 800 °C; (b) DLS study reveals particle size distribution of LAP:Ce3+,Tb3+ in AOT/n-heptane reverse micelle medium at ω = 25.

Figure 4. Transmission electron microscopy illustrating particle distribution of (a) LaPO4; (b) LaPO4:Ce3+; (c) LaPO4:Ce3+,Tb3+ and (d) LaPO4:Ce3+,Tb3+, ω = 10. (e) Magnified view of (d), inset depicts SAED pattern of the nanoflowers; (f) LaPO4:Ce3+,Tb3+, [AOT] = 0.02 mol dm−3; Images in panels a, b, c, and f were taken at ω = 25, and in all cases, solvent is n-heptane; RE ions and H2PO4− concentrations were 0.5 mol dm−3.

flexural vibrations of crystal water. At a higher temperature, two new peaks arise at 952 and 1050 cm−1 (Figure 2b−d) which are ascribable to the ν3 peaks, further confirming that the structures are typically monoclinic. IR analysis of the samples, prepared by using different phosphate sources (i.e., Na2HPO4, Na3PO4, and

H3PO4), gives insignificant variation, therefore suggesting the formation of LAP:Ce3+,Tb3+ in each case. Assignment of the IR peaks is summarized in Table 1. TGA/DTA. Figure 3a depicts the TGA/DTA curves of LAP:Ce3+,Tb3+ (prepared in AOT/n-heptane RMs at ω = 25) 25150



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Figure 5. (a) Particle size distribution (inset represents a single nanorod), (b) high-resolution TEM (arrows indicate growth of the NR in two different directions), and (c) SAED pattern of LAP:Ce3+,Tb3+ synthesized using cyclohexane solvent. (d) Distribution, (e) high-resolution TEM, and (f) SAED pattern of LAP:Ce3+,Tb3+ of nanorods synthesized in isooctane solvent; ω was maintained at 25; RE ions and H2PO4− concentrations were 0.5 mol dm−3.

Table 2. Summary of TEM Analysis of Cerium and Terbium Ion Doped Lanthanum Phosphates, Synthesized in Different Reaction Conditionsa

a [La3+] = 0.5 mol dm−3; [Ce3+] = 0.5 mol dm−3; [Tb3+] = 0.5 mol dm−3; [H2PO4−] = 0.5 mol dm−3. (a) [AOT] = 0.1 mol dm−3; solvent = nheptane; ω = 25. (b) [AOT] = 0.1 mol dm−3; ω = 25. (c) Solvent = n-heptane; ω = 25. (d) [AOT] = 0.1 mol dm−3; ω = 10; solvent = n-heptane

describing thermal stability of the nanorods up to 800 °C. At 125 and 216 °C, there are two obvious weight losses. The former is ascribed to desorption of the physical adsorbed water due to the incomplete drying of the final product. The latter is attributed to the dehydration of the monoclinic LaPO4. Meanwhile, obvious endothermic effects can be clearly observed in the DTA curve at the above-mentioned temperature regions. DLS and TEM. In order to understand the trend in particle size domains of the LAP:Ce3+,Tb3+ under diverse experimental conditions, dynamic light scattering (DLS) technique was used primarily. Figure 3b represents a typical DLS plot of the asprepared samples dispersed in AOT/n-heptane reverse micelle medium. Results showed that average size of nanomaterials was about 60 nm when synthesized in the above specified template. However, it is quite difficult to predict the exact size variation by DLS measurement since it averages out the dimensions in the form of an equivalent sphere irrespective of the actual shape of the particles.

The morphologies of the as-prepared samples were eventually confirmed by analyzing TEM images, as represented in Figures 4 and 5. Structural inclinations of lanthanide phosphates prepared at varying reaction parameters were found to remain alike; however, dimensions (length and width) of the nanostructures are different. Undoped LaPO4 has initially rodlike morphology which overall adopt a net structure (Figure 4a). Ce3+-doped LAP has grown up as tiny nanorods (NRs) with length and width 38.3 (±5) and 4.3 (±0.5) nm, respectively (aspect ratio 8.9:1; Figure 4b). Finally, TEM images revealed that Ce3+- and Tb3+-doped LAPs have distinct rodlike morphology at ω = 25 (Figures 4c,f and 5a,d, and Figure S5a−f in the SI). Interestingly, at lower ω (=10), initially formed NRs have taken a flowerlike structure after their complete growth. Overall dimensions of these nanoflowers were approximately 341.4 nm and the rodlike building blocks of such nanoflowers have length 77.6 (±12) nm and width 9.4 (±1.5) nm (aspect ratio 8.2:1). Structural details and dimensions of all of the particles are presented in the Table 2. 25151



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Figure 6. (a) HRTEM and (b) SAED image LAP:Ce3+,Tb3+ synthesized in AOT (0.1 mol dm−3)/n-heptane RM template; (c) HRTEM and (d) SAED image LAP:Ce3+,Tb3+ synthesized using 0.02 mol dm−3 AOT/n-heptane; ω = 25.

Figure 7. Photoluminescence spectra of (a) LAP:Ce3+ (inset shows excitation spectra of LAP:Ce3+, λmaxex = 355 nm); (b) LAP:Tb3+; and (c) LAP:Ce3+,Tb3+ (inset shows yellowish green emission of the prepared nanomaterials under UV light excitation) demonstrate various electronic transitions between Ce3+ and Tb3+ ions. (d) Chromaticity diagram represents different color positions of the samples (λex = 270 nm); all spectra were taken in AOT/n-heptane at ω = 25.

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ω = 25. The host matrix itself (undoped LaPO4) is very weakly luminescent (Figure S6a in the SI) having emission maxima at 3+ 335 nm (λmax ions in AOT/n-heptane ex = 275 nm). Bare La RMs have λabs at 295 nm and the corresponding emission is observed at 414 nm. Assuming the excited state of La3+ arising from a P→d electron transition, we may say that there occurs a 1 S0 → 1D2 transition in the La3+ ions which is of lowest energy transition and is a Laporte-forbidden transition, 1A1g → 1Eg (A and E symbolize the singlet and doublet state of the excitedstate energy levels of the La3+ ion)5 which accounts for the lowintensity emission spectra of La3+ ion. However, the change in absorption or emission property of La3+ may be attributed to the different ionic environment. Ce3+ ions have only one electron in their 4f shell, which can be excited to the 5d orbitals upon UV irradiation. Affected by the crystal field, the excited 5d state of Ce3+ splits into two spin−orbit components, designated as 2D5/2 and 2D3/2. Owing to very low energy gap between these two spin−orbit components, they are frequently assigned as 2D energy band (Scheme 3).35,39,40 Absorption spectrum of bare cerium ions

The factors controlling the ultimate size and shape of particles grown within the reverse micelles remain an area of significant interest. Noteworthy works by various groups were reported in the literature exemplifying how the ultimate particle size and particle growth rate in the AOT/n-alkane reverse micelles are functions of ω and bulk solvent type.37 In reference to the previous works, particle sizes are largely controlled by solvent stabilization where the surfactant acts as a stabilizing ligand.37 TEM investigations in the present work clearly justify that variation of the properties of the bulk solvent can induce a change in the growth rate and the ultimate particle size obtained (Table 2), which can be explained through the solvation interactions between the surfactant tails and the bulk phase. The solvent and its interaction with the surfactant tails, however, is a contributing factor for the stabilization of particles in solution and the maximum particle size obtainable.37,38 The particle size shows a general increase as the solvent carbon chain length decreases. For solvent n-hexane, particle length is 64.5 nm (aspect ratio 6.9:1) and it reduced to 54.2 nm (aspect ratio 5.0:1) for solvent n-decane. However, comparatively larger particle sizes were obtained for both isooctane (70.6 nm; aspect ratio 6.5:1) and cyclohexane (80.2 nm; aspect ratio 6.2:1) solvents compared to all other n-alkane solvents. Structurally, cyclohexane is larger than that of isooctane, indicating that the potentially more rigid micellar interface created by the cyclohexane molecule might support formation of a larger particle37 (Table 2). Again, at lower ω, with insufficient water content, the micelle interface is said to be “rigid”, consequently lowering intermicellar exchange and thus growth rate.37,38 As ω is raised, increasing water makes the film more labile and hence enhances the rate of growth.37 Therefore, it certainly justifies the formation of large size nanoflowers at lower ω values (ω = 10). From a similar viewpoint, formations of large size particles (length 95.7 nm, width 12.6 nm; aspect ratio 7.6:1) at lower surfactant concentration (0.02 mol dm−3) compared to those (length 27.1 nm, width 9.7 nm; aspect ratio 2.8:1) formed at higher AOT concentration (0.5 mol dm −3 ) can be authenticated. Moreover, this observation is of particular interest and may be the subject of future experimentation since systematic study on the effect of different reverse micelles parameters on the growth rate and formation mechanism is still rare in the literature. In addition, HRTEM and SAED were also analyzed to get a comprehensive view about the phase pattern of the sample. Panels a,b and c,d of Figure 6 represent the HRTEM and SAED patterns of LAP:Ce3+,Tb3+ prepared in 0.1 and 0.02 mol dm−3 AOT/n-heptane RMs, respectively. The SAED patterns of both the samples show clear reflections, suggesting the high-quality crystalline nature of the LAP:Ce3+,Tb3+ nanorods. A concomitant analysis of the HRTEM images in Figure 6 demonstrated that the interplanar spacing (dhkl) of synthesized LAP:Ce3+,Tb3+ was 2.87 Å (21-2/11-2) and 3.57 Å (12-1) when synthesized in 0.02 and 0.1 mol dm −3 AOT concentrations, respectively (JCPDS data card no. 73-0188). Analyses of the SAED and HRTEM patterns of other sets of samples are also in good agreement with those obtained from XRD analysis. 3.2. UV−Visible Spectroscopy and Photoluminescence Activity of LAP:Ce3+,Tb3+. Luminescence spectra of individually doped LAP:RE3+ (RE = Ce3+, Tb3+) were studied (Figure 7a,b) to comprehend various electronic transitions of individual ions. Figure 7c highlights the PL spectrum of LAP:Ce3+,Tb3+ prepared in AOT/n-heptane reverse micelle at

Scheme 3. Illustration of Energy Transfer in Conjunction with Various Electronic Transitions between Ce3+−Tb3+ Ions in LaPO4 Host Structures

showed peaks at 250, 260, and 300 nm. However, Ce3+ doped within LaPO4 matrix exhibits a strong and single excitation maxima at 270 nm (corresponding to the transitions from the ground 2F5/2 state to excited 5d states of the Ce3+ ions; Figure 7a, inset) with consequent PL maxima at 355 nm. Usually, Ce3+ emission shows two bands due to the duplet character of the 4f1 ground state (spin−orbit components are 2F5/2 and 2F7/2 states); one is 2D (5d) → 2F5/2 (4f) (342−341 nm) and the other is 2D (5d) → 2F7/2 (4f) (460−458 nm) where both the transitions are parity allowed.35,39 However, the emission transition 2D → 2F5/2 appears to be more intense and broader in all Ce3+:phosphors.39 Even with the increase of Ce3+ concentration, splitting of two emission bands grows indistinguishably. The reason for such variation is probably due to a variation in the spectral overlap between the Ce3+ emission and the Ce3+ absorption.35,39 The ground state of the Tb3+ ions having 4f8 electron configuration is on the 7F6 level, and its 4f75d1 excitation levels have high-spin 9DJ and the low-spin 7DJ states (Scheme 3). The PL and photoluminescence excitation spectra (PLE) of 25153



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LaPO4:Tb3+ are illustrated in Figure 7b and Figure S6b in the SI. Under the optimum excitation wavelength of 270 nm, the PL spectrum exhibits four line emissions peaking at 490, 544, 588, and 627 nm, which are assigned to the 5D4 → 7FJ (J = 6, 5, 4, and 3; presented in Scheme 3), characteristic transitions of Tb3+ ions, respectively. Among these four PL peaks, the strongest one appears at 544 nm. Monitored at 544 nm, the PLE spectrum consists of a group of absorption bands between 200 and 390 nm, among which the excitation bands observed in the range of 200−275 nm can be attributed to the spin-allowed 4f8 → 4f75d1 transition of Tb3+ and those of feebly intense lines in the range of 300−400 nm peaking at 283, 302, 317, 340, 350, 369, and 378 nm are due to the forbidden 4f → 4f transitions of Tb3+ ions from 7F6 to 5IJ, 5HJ, 5D0,1, 5G2,3,4, 5D2, 5L10, and 5D3 levels, respectively.35,39,40 In this context, the Judd−Ofelt (JO) theory is a most useful one for predicting the probability of the forced electric dipole transitions of RE ions in various environments. Three phenomenological intensity parameters in the JO theory, Ω2, Ω4, and Ω6, can be determined experimentally from the measurements of absorption spectra and refractive index of host material.41 These parameters are important for studying several important optical properties. The JO parameters, however, exhibit the influence of the host on the transition probabilities since they contain the crystal-field parameters, interconfigurational radial integrals, and the interaction between the central ion and the intermediate environment.41 A least-squares fitting of measured oscillator strength (f) to calculated values provided the following values for the three JO parameters Ω2 = 1.296 × 10−20 cm2, Ω4 = 2.975 × 10−20 cm2, and Ω6 = 0.184 × 10−20 cm2. The value of f for 7F6 → 5F3 (270 nm) was observed as 16.2 × 10−7. It is noteworthy to mention that with increased Tb3+ concentration, the emissions from the 5D3 to the 7FJ levels were quenched gradually by the cross-relaxation process between neighboring Tb3+ ions.35 For the Tb3+ ion, the energy gap between the 5D3 and 5D4 levels is close to that between the 7 F6 and 7F0 levels. As a result, when the Tb3+ concentration is high enough, the higher-energy-level emission can be easily quenched in favor of the lower-energy-level emission. This cross-relaxation process produces the rapid population of the 5 D4 level at the expense of the 5D3 level, thereby resulting in a strong emission from the 5D4 to the 7FJ levels. LAP:Ce3+,Tb3+. Figure 7c demonstrates PL spectrum of LAP:Ce3+,Tb3+ where emission lines arrived from transitions between f-electron and d-electron states of cerium and between different f-electron states of terbium (Scheme 3). Upon UV excitation, colloidal solutions of LAP:Ce3+,Tb3+ NRs exhibit distinctly yellowish green luminescence (inset, Figure 7c) due to transitions between the excited 5D4 state and the 7FJ (J = 6, 5, 4, and 3) ground states of terbium. It was observed that particle size effects on the luminescence are weak, since transitions of the well-shielded f electrons are mainly affected by the local symmetry of the crystal site. The Commission Internationale de L’Eclairage (CIE) chromaticity coordinates for LAP:Ce3+,Tb3+ excited at 270 nm are presented in Figure 7d (CIE coordinates x = 0.34, y = 0.57; Table 3). Calculated QY of the material in AOT/n-heptane was 38.0% (Table 3). In the present work, total doping concentration was carefully optimized to 33.0 mol % with respect to the host (LAP) concentration (i.e., 2:1 mol ratio). Incorporation of 33.0% Ce3+ ion to the LAP matrix enhances PL intensity of LAP by 20 times. Subsequently, a 4:1 Ce3+ to Tb3+ mole ratio (26.7% Ce3+

Table 3. Quantum Yields and Chromaticity Coordinates (x,y) of LAP:Ce3+,Tb3+ Samples under UV Excitation (λex = 270 nm) samples LaPO4:Ce3+,Tb3+; LaPO4:Ce3+,Tb3+; LaPO4:Ce3+,Tb3+; LaPO4:Ce3+,Tb3+; LaPO4:Ce3+,Tb3+; LaPO4:Ce3+,Tb3+; LaPO4:Ce3+,Tb3+; LaPO4:Ce3+,Tb3+;

n-hep n-hex CyHX n-oct isooct n-dec 0.02 M AOT 0.5 M AOT

quantum yield (%)

CIE coordinates (x,y)

38.6 41.3 36.5 40.8 38.7 83.7 35.9 50.5

0.34,0.57 0.32,0.55 0.32,0.59 0.35,0.56 0.34,0.57 0.43,0.49 0.38,0.34 0.34,0.33

and 6.7% Tb3+) was standardized since at this ratio terbium exhibits maximum photoluminescence. Discrete results were obtained for bulk solvent variation. Figure 8a represents PL spectra of LAP:Ce3+,Tb3+ prepared in six different solvents. In AOT/cyclohexane RMs, Tb3+ showed maximum intensity (545 nm) while Ce3+ had maximum intensity (350 nm) in AOT/ndecane RMs (Figure 8a). Interestingly, PL of NRs synthesized in n-decane shifted to the more yellow region as revealed in the CIE color diagram in Figure 8b (x = 0.43, y = 0.49). NRs in AOT/n-decane also yielded high QY value, 83.0%. However, decay analysis of the samples in six different solvents showed insignificant variation in excited-state stability for both the doped ions in LAP host matrix. For Ce3+ emission at 350 nm, average lifetime (τav) was found to vary within time range from 12.5 to 14.9 ns. It was experimentally observed that, with increasing ω, PL intensity of the NRs gradually increases (Figure 8c). As the host concentration varies from 0.05 to 0.25 mol dm−3, PL intensity of both Ce3+ and Tb3+ increases (Figure S6c in the SI). Variation in surfactant (AOT) concentration (0.02−0.4 mol dm−3) produced reverse results for the Ce3+ (at 350 nm) and Tb3+ emission (at 544 nm) (Figure 8d). Monitoring Ce3+ emission at 350 nm, the calculated quantum yield of LAP:Ce3+,Tb3+ is summarized in Table 3. Another important outcome from this experimental analysis is that only orthophosphoric acid can act as a suitable replacement for NaH2PO4 or Na3PO4 because synthesis employing H3PO4 can only embrace sufficient PL property in the finally desired nanostructures. Energy-Transfer (ET) Behavior. PL intensity of weak emission lines of Tb3+ (Figure S6b in the SI) enhanced significantly in the presence of cerium ions. For understanding the energy-transfer process between Ce3+ and Tb3+, Ce3+ concentration was kept fixed at 2.907 × 10−4 mol dm−3 and PL spectra were recorded with increasing [Tb3+] at ω = 25 (Figure 9a). The 5d−4f transition of Ce3+ is electric dipole allowed and is sufficiently stronger than the f−f transitions of Tb3+. Therefore, the Ce3+ ions can strongly absorb UV light, become excited, and then efficiently transfer energy to the Tb3+ ions. As a result, the excitation into the Ce3+ band at 270 nm yields both the emissions of the Ce3+ and Tb3+ ions (Scheme 3). The energy-transfer efficiency (ηT) from a sensitizer to an activator can be calculated by eq 135,39 I ηT = 1 − S IS0 (1) in which IS and IS0 are the luminescence intensities of the sensitizer (Ce3+) with and without the activator (Tb3+), 25154



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Figure 8. Photoluminescence spectra of LAP:Ce3+,Tb3+ (a) in six different solvents; (b) chromaticity diagram in different solvents; (c) at varying ω; (d,e) indicate decreasing and increasing PL intensity of Tb3+ and Ce3+, respectively, with increasing AOT concentration (0.02−0.5 mol dm−3).

Figure 9. (a) Energy-transfer profile of Ce3+ to Tb3+ ions in LaPO4 host matrix. PL intensity of Ce3+ at 350 nm is decreasing whereas that at 544 nm for Tb3+ is increasing, λex = 270 nm, ω = 25. (b) Variation of energy-transfer efficiency as a function of Tb3+ ion concentrations.

respectively. The ηT values of LAP:Ce3+,Tb3+ were obtained as a function of [Tb3+] and are presented in Figure 9b, in which ηT was found to increase gradually with an increase in Tb3+ dopant content. At maximum [Tb3+] (7.27 × 10−5 mol dm−3), ET efficiency was 0.53. There are many instances showing that concentration quenching is due to the ET from one activator to another until the energy sink in the lattice is reached. Blasse suggested that the average separation RCe−Tb of energy transfer can be estimated from eq 242

⎡ 3V ⎤1/3 R Ce−Tb ≈ 2⎢ ⎣ 4πxN ⎥⎦

in which V is the volume of the unit cell, N is the number of the host cations in the unit cell, and x is the total concentration of Ce3+ and Tb3+ ions. The critical concentration (xc) is that at which the luminescence intensity of sensitizer (Ce3+) is half of that in the sample in the absence of activator (Tb3+), i.e., at xc, ηT = 0.5. For the LaPO4 host, N is 1 and V is estimated to be 303.186 Å3 with the assumption that the lattice parameters are almost constant with Ce3+ and Tb3+ doping levels. Using eq 2, the critical distance of energy transfer R is estimated to be about 11.9 Å. The ET process from a sensitizer to an activator may take place through (i) radiative transfer, (ii) exchange interaction, and iii) electric multipolar interaction.35,39 The absence of the

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8/3 10/3 Figure 10. Dependence of IS0/IS of Ce3+ on (a) C6/3 Ce+Tb (dipole−dipole), (b) CCe+Tb (dipole−quadrupole), and (c) CCe+Tb (quadrupole− quadrupole).

dips in the emission peak of the Ce3+ ions that correspond to the f−f absorption lines of the Tb3+ ions showed that radiative energy transfer between the sensitizer and the acceptor can be neglected. Conversely, large direct or indirect overlap between donor and acceptor orbitals leads to easy electronic exchange thereby aiding exchange interaction. The transition of the Ce3+ ions from the 5d excited state to the 2F5/2 and 2F7/2 ground states are pure electric dipolar transitions, but the 4f8 → 4f75d Tb3+ transitions have simultaneously dipolar and quadrupolar character.35,39,40 Therefore, both dipole−dipole and dipole− quadrupole mechanisms may be involved in the energy transfer from the Ce3+ to Tb3+ ions. According to Dexter’s energytransfer formula of multipolar interaction and Reisfeld’s approximation, eq 3 can be represented as43,44 ηS0 ηS

∝

n /3 CCe + Tb

in which fd is the oscillator strength of the involved absorption transition of the acceptor, τS is the radiative decay time of the sensitizer (S symbolizes sensitizer), R is the sensitizer−acceptor average distance (Å), E is the energy involved in the transfer (eV), and ∫ Fs(E)FA(E) dE / E4 represents the spectral overlap between the normalized shapes of the Ce3+ emission FS(E) and the Tb3+ excitation FA(E), and in our case it is calculated to be about 0.0085 eV−4. The critical distance (Rc) of the energy transfer from the sensitizer to the acceptor is defined as the distance for which the probability of transfer equals the probability of radiative emission of donor, i.e., PCe−TbτS0 = 1. Hence, Rc can be obtained from eq 535 R c 6 = 3.024 × 1012fd

fd R6τS

∫

E4

(5)

⎛ λS ⎞2 fq P dq ⎜ ⎟ ≈ ⎝ R ⎠ fd P dd

(6)

in which λS (Å) is the wavelength position of the sensitizer’s emission, and fd and fq are the oscillator strengths of the electric dipole and quadrupole transitions, respectively. Using eqs 4 and 6, we obtain eq 735 R c 8 = 3.024 × 1012λS2fq

∫

FS(E)FA(E) dE E4

(7) 3+

However, the oscillator strength of the Tb quadrupole transition (fq) has still not been obtained.35 It was suggested by Verstegen et al.45 that the ratio fq/fd is about 10−3−10−2. Using

FS(E)FA(E) dE E4

FS(E)FA(E) dE

Now, using values of the oscillator strength (fd) corresponding to Tb3+ electric dipole transition (in the order of 10−6) and the calculated spectral overlap in eq 5, the critical distance Rc for a dipole−dipole interaction mechanism is calculated to be 4.1 Å. This value largely deviates from that anticipated from the concentration quenching data (11.9 Å), in consequence further indicating that the electric dipole−dipole interaction is not primarily responsible for the ET from the Ce3+ to the Tb3+ ions. Hence, one has to consider electric dipole−quadrupole interaction as the most feasible mechanism for an energy transfer. The relationship of the transfer probability between Pdq of the dipole−quadrupole interaction and Pdd of the dipole− dipole interaction is given by eq 635

(3)

in which ηS0 is the intrinsic luminescence quantum efficiency of the Ce3+ ions and ηS is the luminescence quantum efficiency of the Ce3+ ions with the activator (Tb3+) present. The values of ηS0/ηS can be approximately calculated by the ratio of related luminescence intensities (IS0/IS), CCe+Tb is the total dopant concentration of Ce3+ and Tb3+, and n = 6, 8, and 10 are dipole−dipole, dipole−quadrupole, and quadrupole−quadrupole interactions, respectively. The IS0/IS versus Cn/3 plots are further demonstrated in Figure 10a−c, and the relationships were observed when n = 6, 8, and 10. Linear relation was obtained only when n = 8. This undoubtedly signifies the occurrence of energy transfer from the Ce3+ to the Tb3+ ions via dipole−quadrupole mechanism. Therefore, the electric dipole− quadrupole interaction predominates in the ET mechanism from the Ce3+ to the Tb3+ ions in LAP:Ce3+,Tb3+ which is similar to the literature values.35,39,40 To further investigate the characteristics of multipolar interactions such as dipole−dipole, dipole−quadrupole, and higher-order interactions, the energy-transfer probability for each multipolar interaction was considered. The dipole−dipole energy-transfer probability (Pdd Ce−Tb) from a sensitizer to an acceptor is given by eq 442,44 dd 12 PCe − Tb = 3.024 × 10

∫

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This double-exponential decay of LaPO4:Ce3+,Tb3+ can suggest two types of luminescence centers in nanopowders, i.e., “surface”- and “bulk”-related ions. If Ce3+ (or Tb3+) ions are located in the “bulk” region of the nanoparticle, then their positions in the crystal lattice and their local surroundings were similar to the positions and the local surroundings of the impurity ions in the bulk material. However, dopants in the surface region were strongly perturbed by the surface states. Importantly, their local symmetry is lower than that in the bulk region. Consequently, the decay kinetics becomes shorter due to surface activators; i.e., the observed double-exponential decay kinetics arises from dopants being in two different regions of the nanoparticle.47

these values and the calculated spectral overlap in eq 7, the critical distance Rc for a dipole−quadrupole interaction mechanism was estimated to be 11.4 Å. This value, however, approximately matches that obtained by using the concentration-quenching method (11.9 Å). In addition to the steady-state PL technique, time-resolved luminescence of Ce3+ and Tb3+ doped nanocrystalline LaPO4 were also studied to further elucidate the energy-transfer process between dopants ions.46,47 Because of the allowed 5d− 4f transition in Ce3+, it decays relatively fast; i.e., lifetime lies in the nanosecond time range. Experimentally, it was observed in our study that bare Ce3+ emission (λem = 350 nm) decays single-exponentially with a lifetime value 27 ns. In this context, it is necessary to mention that energy transfer from Ce3+ to Tb3+ should influence the decay kinetics of Ce3+ emission.47 We observed that energy transfer from Ce3+ to Tb3+ ions leads to the intensity suppression of Ce3+ emission in LAP matrix along with biexponential decay kinetics of Ce3+. In contrast to the allowed d−f transition of cerium, f−f transitions in Tb3+ are spin and parity forbidden ensuing very long Tb3+ luminescence decay (in our study, microsecond time range). It is well-known that, during the ET process, the lifetime of the sensitizer decreases regularly with increasing concentration of an activator.48 In the Ce3+ and Tb3+ doped LAP host matrix, excited-state lifetime (τav) of Tb3+ (activator) was found to increase gradually (from 247.4 to 272.2 μs; Figure S7 in the SI) in conjunction with steady decrease of Ce3+ (sensitizer) lifetime (τav) from 18.3 to 10.2 ns (Figure 11, Table 4).

4. CONCLUSION In summary, this work underlines an efficient synthesis of highly crystalline, monoclinic, rod-shaped and flowerlike rare earth phosphate employing reverse micelle soft template at room temperature. Synthesized NRs displayed very good monodispersity and excellent solubility in AOT/n-alkane RMs to form stable and clear colloidal solutions. Analysis of the material by XRD, FTIR, EDX, and electron microscopy revealed pure phase synthesis of the material. This work illustrates the formation of nanorods with different aspect ratios or growth of nanoflower with the change in fluidity of the micellar interface. It was seen that the material displayed yellowish green photoluminescence (CIE coordinate x = 0.34, y = 0.57) when excited at 270 nm. In addition, an efficient energy transfer from Ce3+ to Tb3+ occurred in Ce3+ and Tb3+ ion doped LAP host which is supposed to be a resonant type via dipole−quadrupole mechanism. The critical distance of Ce3+ to Tb3+ ions in LAP was calculated to be 11.4 Å. Nanosized highly luminescent LaPO4:Ce3+,Tb 3+ is nowadays one of the promising materials for biomedical applications such as fluorescence resonance energy-transfer assays, optical imaging, etc. Enhanced hydrophilic nature is a primary requirement for biological applications of NMs which can easily be achieved by microemulsion methodology. Besides, this technique renders very precise control over structure and size of the LAP:Ce3+,Tb3+ nanorods, which is imperative to conclude the successful application of these NRs in many fields. On the basis of the experimental findings, we honestly believe that this work will be quite informative and unique toward designing of such highly fluorescent NRs utilizing reverse micelle microenvironment for future applications.

Figure 11. Time-resolved decay study reveals gradual decrease of Ce3+ lifetime with increasing Tb3+ concentration, therefore confirming energy transfer between Ce3+ and Tb3+.

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Table 4. Time-Resolved Photoluminescence Data of the LAP:Ce3+,Tb3+ Samples Indicating an Efficient Energy Transfer from Ce3+ to Tb3+ Ions 3+

350 nm (λem of Ce )

S Supporting Information *

XRD plot of Ce 3+-doped LaPO4, and LaPO 4 :Ce3+,Tb 3+ synthesized by means of different surfactant concentrations and various PO4 sources, EDX spectrum and elemental mapping of LaPO4:Ce3+,Tb3+, TEM, HRTEM, and SAED pattern of the nanorods prepared in different solvents, PL spectrum of undoped LaPO4, PLE spectrum of Tb3+ doped of LaPO4, PL spectra at different La3+ host ion concentrations, and photoluminescence decay of LaPO4:Ce3+,Tb3+. This material is available free of charge via the Internet at http:// pubs.acs.org.

3+

545 nm (λem of Tb )

τ1 (ns)

τ2 (ns)

τav (ns)

τ1 (μs)

τ2 (μs)

τav (μs)

6.4 7.5 6.8 6.0 5.6

28.7 26.5 24.4 22.5 20.7

18.3 16.9 14.2 12.0 10.2

0 388.1 410.6 432.5 452.3

0 113.1 108.9 105.5 101.9

0 247.4 255.9 264.7 272.2

ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], scbhattacharyya@chemistry. jdvu.ac.in. Phone: +91(033) 24146223. Fax: +91(033) 24146584. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.C. acknowledges DST (SR/S1/PC-63/2009) for providing a SRF. S.S.M. and S.R. are thankful to UGC for providing SRF and JRF, respectively. We are also grateful to Dr. K. K. Chattopadhyay and Mr. N. S. Das, Jadavpur University, India, and Mr. P. Ray, SINP, Kolkata, for their kind help in electron microscopy measurements. We thank Mr. D. Sarkar for EDX analysis and Mr. S. Bardhan for mathematical analysis.

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Soft-Templated Room Temperature Fabrication of Nanoscale

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