D. L.
Inglefield
Jr.
a,
T. R.
Merritt†
b,
B. A.
Magill†
b,
T. E.
Long
a and
G. A.
Khodaparast
*b
aDepartment of Chemistry, Virginia Tech, Blacksburg, VA 24061, USA
bDepartment of Physics, Virginia Tech, Blacksburg, VA 24061, USA. E-mail: khoda@vt.edu
First published on 8th May 2015
Lanthanide-doped upconverting nanoparticles (UCNPs) have the ability to convert low energy photons into high energy photons, making this material appealing for a variety of scientific pursuits, from solar energy conversion to bioimaging. A combination of polymers and nanocomposites increases the utility of these upconverting nanoparticles allowing nanoparticles to be added to any device compatible with polymer coatings. Here, trifluoroacetate salt decomposition enables Er/Yb doped NaYF4 upconverting nanoparticle synthesis. The subsequent deposition of a silica nanoshell yields polar silica-coated upconverting nanoparticles, enabling composite formation with polar urea-containing methacrylic polymers. Hydrogen bonding between urea groups in the polymer and the silica-coated nanoparticles allowed for dispersion of the upconverting nanoparticles to form upconverting composite films. These films exhibit desirable upconversion comparable to the nanoparticles dispersed in methanol. Urea-containing polymers are promising candidates for matrices in nanocomposites formed with polar silica nanoparticles due to favorable polymer–nanoparticle interactions. This architecture is superior to urea-methacrylate homopolymers, since the central low glass transition temperature block will provide critical ductility to the film, thus rendering the film to be durable for optical applications.
While various anti-Stokes mechanisms exist, such as the widely employed processes of simultaneous two photon absorption (TPA) and second harmonic generation (SHG), upconversion based on energy transfer conversion (ETU) remains an appealing alternative for the generation of anti-Stokes emission.1,13,14 The former processes generate upconverted emission through virtual intermediate excited states, a mechanism reliant on high-density excitation, a requirement typically fulfilled by ultrafast lasers.13,14 In contrast, ETU involves unspecified non-radiative energy transfer between real, long-lived intermediate excited states of neighboring lanthanide ions; as a result, the requirement of high-density excitation is largely waived for ETU and, as a consequence, high conversion efficiency is allowed without the need for intense coherent excitation sources.13,14
In particular, lanthanide doped glasses are attractive ETU systems due to their ability to possess emission wavelengths that spam the entire visible region; this feature is achieved through the judicious choice of the activator ion.14,15 The most efficient species of these upconversion materials are fluoride glasses co-doped with the sensitizer–activator pair of Yb3+ and Er3+, namely hexagonal NaYF4:Yb,Er,16 which exhibits upconversion quantum yields of up to 3% in bulk glasses.17 This maximal quantum yield is attributable to the high absorption cross-section of Yb3+ and the spectral overlap of Yb3+ solitary optical transition with the 4I15/2 → 4I11/2 transition of erbium, as well as the low optical phonon energies of hexagonal NaYF4, which results in the considerable suppression of multiphonon relaxation processes.1,16,18
Although few studies investigate the use of UCNPs in polymer composites, the initial results appear to be promising.17,19 These upconverting composites represent materials with possible applications in light curable polymers and solar energy technology. For example, upconversion offers the potential for increasing the efficiency of photovoltaic cells through converting light below the optical band gap to light within the photovoltaic cells' optical band gap. Here we present our initial results toward the goal of utilizing urea-containing polymers in upconverting nanocomposites.
Urea-containing polymers appear to be particularly promising for applications using silica and metal coated UCNPs, due to the high levels of hydrogen bonding between urea groups with silica and the well-known urea–metal interaction.20–23 The ABA triblock copolymer architecture is a well-recognized sequence for the formation of mechanically ductile films and coatings. This architecture is superior to urea-methacrylate homopolymers, since the central low glass transition temperature (Tg) block will provide critical ductility to the film, thus rendering the film to be durable for optical applications. Most importantly, the nanoscale phase separated block copolymer morphology provides a template for nanoparticle organization, locating the nanoparticles in specific sequences containing the urea sites for coordination and creating new compositional options for macromolecular composites whose architecture is controlled by polymer chemistry.24–26
In addition to using these nanocomposites as an upconverting coating for solar cells, these composites could also prove useful for fabricating hybrid nanocomposite structures where upconverting particles are suspended near a metal surface27,28 or other metal nanoparticles,7,29 in self-aligned chains or layers where the particle density and orientation are controlled by an external chemical matrix similar to what has been done with semiconductor nanoparticles26,30,31 or in planar structures constructed using nano-imprint lithography.28 These hybrid structures take advantage of the surface plasmon resonance in the nearby metal to enhance either the emission or absorption by the UCNPs in the hybrid material.
Spectral characterization methods were performed using a pulsed Ti:Sapphire laser, as the excitation source, (Coherent Chameleon Ultra II) operating at 980 nm with a pulse duration of 140 fs and a repetition rate of 80 MHz. The average excitation power was controlled using an acousto-optical modulator and set to 30 mW for all measurements. A 40 mm aspheric lens was used to both excite and collect emission from hexane or water UCNP dispersions contained in 1 cm-thick cells. With the aid of an 800 nm edge long-pass dichroic beam splitter, the photoluminescence (PL) was separated from the co-aligned excitation–emission beam. After this spectral separation, the PL was delivered, through a 100 m wide slit, to a 0.55 m focal length spectrometer (HORIBA Jobin Yvon iHR550) equipped with a 600 grooves per mm, 500 nm blaze grating, and measured using a liquid-nitrogen cooled charged-coupled device (CCD) (HORIBA Jobin Yvon Symphony II).
Lifetimes for our UCNPs were found using the Chameleon ultra fast laser to excite UCNP cores at 980 nm through the 20× objective lens of the Zeiss LSM 510. The light was chopped at 5 Hz using a mechanical chopper to provide a signal that varied over a time interval longer than the expected PL lifetime and within the range of time that our detector could resolve. The light was gathered through the same objective, the laser beam was removed via a pair of dichroic filters and the CCD in the Zeiss LSM 510 was used to measure the photoluminescence as a function of time.
For PL measurements, UCNP cores were measured in cyclohexane and UCNP@SiO2 were measured in methanol. Solvents with similar solubility were chosen to disperse the nanoparticles for optical measurements. Methanol was used in place of water because of water's absorbance in the IR range. The weight percentage of the UCNP cores in cyclohexane and UCNP@SiO2 in methanol was kept constant at 16 mg mL−1.
Fed molar ratio of ArUrMA to HUrMA | Obtained ratio by 1H NMR | M n (g mol−1) | M w (g mol−1) | PDI | T g (°C) |
---|---|---|---|---|---|
100![]() ![]() |
100![]() ![]() |
150![]() |
171![]() |
1.14 | 128 |
80![]() ![]() |
78![]() ![]() |
162![]() |
185![]() |
1.14 | 114 |
60![]() ![]() |
58![]() ![]() |
145![]() |
166![]() |
1.14 | 106 |
40![]() ![]() |
39![]() ![]() |
123![]() |
150![]() |
1.21 | 91 |
20![]() ![]() |
20![]() ![]() |
140![]() |
174![]() |
1.24 | 86 |
0![]() ![]() |
0![]() ![]() |
255![]() |
315![]() |
1.23 | 77 |
Based on the physical properties and solubility of poly(HUrMA), and the high Tg and sustainability of poly(ArUrMA) for TEM analysis, a copolymer of these monomers was targeted for use in RAFT triblock synthesis. To ensure the random polymerization behavior and miscibility of these monomers, a series of copolymers was prepared using free radical polymerization (Scheme 2). Table 2 summarizes the properties of these copolymers. The copolymers are of sufficiently high molecular weight, and the molar ratio of each monomer charged in the polymerization is very close to the molar ratio in the resultant polymer as determined by 1H NMR spectroscopy, upon comparing the integrations of HUrMA urea protons with ArUrMA urea protons. The copolymers closely follow the Fox equation:
![]() | (1) |
Polymers | M n (g mol−1) | wt% outer blocks | T g (°C) UrMa | T d5% (°C) |
---|---|---|---|---|
Free radical poly(2-EHMA) | 132![]() |
— | — | 270 |
MacroCTA | 49![]() |
0 | — | 182 |
UrMA-12 | 55![]() |
12 | — | 178 |
UrMA-41 | 82![]() |
41 | 84 | 184 |
UrMA-45 | 82![]() |
45 | 67 | 184 |
UrMA-51 | 100![]() |
51 | 67 | 190 |
UrMA-56 | 112![]() |
56 | 79 | 196 |
UrMA-73 | 179![]() |
— | 73 | 201 |
Free radical 80![]() ![]() |
140![]() |
0 | 86 | 224 |
![]() | ||
Fig. 1 Fit of urea-containing copolymers to the Fox equation; dots indicate measured values while the line indicates the expected value from theory. |
Polymerization of 2-EHMA in the presence of dCDP in dioxane using V-501 as an initiator (Scheme 2) enabled the controlled growth of difunctional poly(2-EHMA) macro CTA. This RAFT polymerization provided near linear molecular weight growth over time and acceptably low PDIs at the longer reaction times according to THF SEC and offline determined dn/dc giving absolute Mw values. Dialysis ensured complete removal of residual monomers, and purification of this polymer by precipitation from dioxane into cold methanol proves to be difficult because of the melting temperature of dioxanes. The molecular weight of 49000 g mol−1 obtained from a 10 h polymerization served as the macroCTA for subsequent triblock synthesis due to its high molecular weight and low PDI.
Chain extension of the difunctional poly(2-EHMA) macroCTA using an 80:
20 molar ratio of HUrMA
:
ArUrMA yielded urea-containing ABA triblock copolymers as shown in Scheme 2. Because of the complex solubility of the obtained triblocks with a non-polar central block and polar outer blocks, a 65
:
35 dioxane
:
DMF (v
:
v%) solvent mixture was used to ensure solvation of the macroCTA and final triblocks at the polymerization temperature. Precipitation into 4
:
1 methanol
:
water (v
:
v%) purified the triblock copolymers. SEC could not reliably determine molecular weights because of the complex solubility. Therefore, the triblock copolymer molecular weight was determined from the known molecular weight of the macro CTA by 1H NMR spectroscopy comparing the integration of ester adjacent methylene protons (present in all repeating units) to the urea adjacent aromatic protons (present in the ArUrMA repeating units). This calculation assumed 20 mol% incorporation of ArUrMA into the outer blocks, which is a reasonable assumption from the incorporation in poly(HUrMA-co-ArUrMA) conventional free-radical copolymers (Table 2). The molecular weights are summarized in Table 1 and the polymerization behavior displays a nearly linear molecular weight growth over time, indicating good control of the reaction. However, PDIs could not be determined without SEC data. Each triblock composition retained THF solubility which was used to cast films.
DSC and TGA investigated the thermal properties of the ABA triblock copolymers and free radical controls of each block, which are summarized in Table 2. The Td5% of 270 °C of the free radical poly(2-EHMA) is significantly higher than the Td5% of the poly(2-EHMA) macroCTA at 184 °C because of the trithiocarbonate functionality that begins degrading around this temperature, greatly reducing the thermal stability of the macroCTA. Increasing the incorporation of UrMA outer blocks increases the Td5%, due to dilution of the trithiocarbonate endgroups. The Td5% of the poly(UrMA) free radical control polymer at 224 °C is significantly lower than the free radical poly(2-EHMA), likely due to the urea group reducing the thermal stability. DSC analysis reveals the presence of poly(UrMA) phases in triblocks with significant outer block incorporation, however with slightly depressed Tgs compared to the poly(UrMA) free radical control which is likely the consequence of some phase mixing during the DSC experiment. However, the Tg of poly(2-EHMA) was not observable under these DSC conditions, even in the case of poly(2-EHMA) homopolymers. The literature value of Tg of poly(2-EHMA) is about −13 °C.35 The lack of an observable transition here in DSC is likely due to the broad nature of this transition, which was further elucidated in DMA.
DMA and AFM further investigated the phase separated morphology and mechanical properties of the triblock copolymers. Although the triblocks were fairly brittle which limited their mechanical testing, the triblocks with lower incorporations of outer blocks were evaluated by DMA. DMA of annealed films revealed a more defined rubbery plateau and an increase in the rubbery plateau storage modulus with an increase in outer block incorporation, as shown in Fig. 2a. The UrMA-12 triblock copolymer did not contain enough outer blocks to form a rubbery plateau region, and its behavior resembled that of the poly(2-EHMA) free radical control, which contains a broad Tg transition around −20 °C and did not flow until 40 °C. The triblocks with a defined rubbery plateau also showed this broad poly(2-EHMA) transition around −20 °C, and flowed around 100 °C after going through the Tg of poly(UrMA). The tan delta in Fig. 2b clearly shows the nature of these transitions, with a very broad poly(2-EHMA) transition and flow after going through the Tg of poly(UrMA).
![]() | ||
Fig. 2 DMA of triblock copolymers: (a) storage modulus. (b) Tan delta. Copolymers are labeled according to the wt% of outer block incorporation. |
AFM revealed the phase separated surface morphology of the triblock copolymers. AFM imaged both the unannealed film, as shown in Fig. 3, and the annealed film (100 °C in vacuo for 16 h) as shown in Fig. 4. In the unannealed films, the surface morphology follows a logical trend with increasing wt% of outer block incorporation. The hard domains formed by the poly(UrMA) outer blocks grow in size, starting as small spherical domains and growing into lamella-like structures lacking long-range order, and eventually forming large domains with inclusions of soft segments formed by the poly(2-EHMA) inner block. Annealing significantly changes the surface morphology, serving to coalesce the hard domains, yielding larger soft domains on the surface of the film. Although different in appearance compared to the unannealed films, the annealed films also follow a visible trend with increasing outer block incorporation. Unfortunately, limited contrast in small-angle X-ray scattering (SAXS) prevented SAXS analysis of the bulk morphology.
![]() | ||
Fig. 3 AFM of unannealed triblock copolymers (copolymers are labeled according to the wt% of outer block incorporation). |
![]() | ||
Fig. 4 AFM of annealed triblock copolymers (copolymers are labeled according to the wt% of outer block incorporation). |
The morphology and the particle size of the bare and silica capped NaYF4:Yb,Er nanocrystals were characterized by TEM studies. Fig. 5 shows a bright-field TEM micrograph of a typical sample. The dimensions, where r1 indicates the radius of the UCNP, r2 indicates the outer radius of the silica shell, and the shell thickness, from examination of a small (<50) sample set of UCNPs, given by r1 − r2, were determined to be between 10–15 nm.
![]() | ||
Fig. 5 TEM of upconverting nanoparticles: (a) Er/Yb doped NaYF4 core. (b) Er/Yb doped NaYF4 core with the silica shell. |
The emission from NaYF4:Yb,Er is the result of a two photon absorption process mediated by the Yb3+ sensitizers in an energy transfer upconversion scheme.1,13 Upon the absorption of a 980 nm (1.27 eV) photon, the Yb3+ ion is elevated from the ground state to the excited state of 2F5/2. The Yb3+ ion relaxes back down to the ground state, transferring the energy of the excited state to the adjacent Er3+ ion, which, in turn, promotes the Er3+ from the ground state 4I15/2 to the 4I11/2 level. The absorption of a second photon populates the 4F7/2 level, resulting from either a second energy transfer or by the direct excitation of the Er3+ ion. The Er3+ ion can then nonradiatively relax to the 2H11/2 and 4S3/2 levels from which it radiatively relaxes, resulting in the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 green emission bands, centered at 520 and 540 nm, respectively. Alternatively, the Er3+ can further nonradiatively relax from these levels to the 4F9/2 level, ultimately producing the 4F9/2 → 4I15/2 red emission band centered at 660 nm. In addition, there is an alternative optical route that can lead to this red emission. While in the 4I11/2 state, the Er3+ ion can decay to the 4I13/2 state and are then subsequently promoted to the 4F9/2 state through an energy transfer from the excited Yb3+ ions. Once again, radiative relaxation from this level produces the 4F9/2 → 4I15/2 red emission.
Fig. 6 shows the emission spectra of bare UCNP cores in cyclohexane solution and UCNP@SiO2 dispersed in triblock co-polymers, respectively. Both spectra feature two strong emission bands at 545 and 660 nm (2H11/2 → 4I15/2 and 4F9/2 → 4I15/2), respectively, a weaker emission band at 520 nm (2H11/2 → 4I15/2), and a small artifact at 700 nm from the laser. While the inclusion of the silica shell did not alter the peak positions or spectral shapes of these bands, it did result in an increase in Rr/g (Ratio of red to green) from 0.92 for the bare cores to 1.28 for the Si coated cores, the intensity ratio between the 540 nm-centered green emission and the 680 nm-centered red emission, opposite to our expectation that the Si shell would decrease Rr/g. The main expectation from the deposition of a silica shell onto the UCNP was the depletion of proximate surface quenching centers and their associated high vibrational quanta.36 This would affect the phonon-assisted relaxations in the Er3+activator and, hence, alter the relative populations of the excited states. Due to the greater reliance of the red upconversion pathways on nonradiative relaxations, particularly in the 4I11/2 → 4I13/2 transition, the value of Rr/g was expected to decrease in this case.33,37 The increase in Rr/g we report may be due to the corpulent silica shell layer that is well beyond the thicknesses used in previous work, 5–20 nm.12,17,36 In contrast to their thinner counterparts, thick silica shells readily absorb the pumped photons as well as the emitted upconverted photons, detrimentally affecting the luminescence yield; in addition, an amorphous silica shell suffers from considerable lattice mismatch with the UCNP core, producing interface defects that give rise to additional phonon modes36 which, in turn, may selectively promote the red emission and lead to an increase in Rr/g reducing the effect of proximate surface quenching from the silica shell.
![]() | ||
Fig. 6 Photoluminescene spectra of bare UCNP cores in solution versus Si coated cores in tri-block polymers. Both spectra were acquired using 980 nm excitation with an average power of 300 mW. |
Direct comparison of the upconversion from cores in solution to cores in polymers is difficult due to the difference in transmittivity between the solution and air, the density of the cores in the solution versus in the polymer, as well as the polymer being easily focused on in comparison to the cores in solution. Thus, to evaluate the effect of the polymer on upconversion we compared the overall lineshape of the emission before and after incorporation of the cores into the polymer. This comparison is shown in Fig. 7, and we observed that the incorporation of the UCNPs into the polymer did not change the overall shape or the location of the emission peaks of the UCNP's PL. The incorporation of the cores into the polymer did cause a reduction in the intensity of two of the green emission peaks, in comparison to the red emission peaks, resulting in a Rr/g = 1.094 for the cores in triblock polymers and a decrease from 1.28 observed for Si coated cores in solvent. This suggests that the addition of the polymer resulted in an increase in the surface quenching of the green emission.
In Fig. 7, we compare the PL spectra of UCNPs in a tri-block copolymer versus UCNPs embedded in a homo-polymer when excited with the same intensity of 980 nm light. Compared to the triblock copolymer, the Rr/g ratio decreased for the particles in the homo-polymer to an Rr/g = 0.753. The overall shape of both emissions is the same and the emission peaks are located at the same wavelengths. In the homo-polymer the highest energy green peak is also the largest, similar to the cores in the triblock polymer. The greater decrease in Rr/g, as well as the overall smaller emission intensity, observed for the cores in the homo-polymer suggests that our tri-block polymer affects the PL spectra less than the homo-polymer.
In upconverting nanoparticles the emission lifetime can be related directly to the quantum efficiency. As such, we compared the fluorescence lifetimes of our polymer–UCNP composites to lifetimes of polymer encapsulated NaYF4 UCNPs in previous studies.17,38 This allows us to evaluate the quantum efficiency of our UCNPs after incorporation into the copolymer and determine whether our triblock polymers are suitable for constructing polymer–UCNP composites.
To measure the UCNP lifetimes we modulated the laser before the microscope using an optical chopper at 2 krpm, and then the fluorescence lifetimes were found by an exponential fit of the decay of the PL signal that occurs after the laser beam was blocked using the chopper. For the green (red) decay time we used a dichroic filter to remove the red (green) portion of the PL. The decay time of the full PL as well as the decay times of the green and the red portion of the PL are shown in Table 3. The life times of our polymer embedded UCNPs are longer than similar UCNPs with surfaces modified by phosphate ligands (76 μs)17 but they are shorter by a factor of 8–10 of similar bare UCNPs in solution.38
Samples | Lifetimes (μs) |
---|---|
Bare cores in the homo-polymer | 98.5 ± 8.6 |
Silica coated cores in the tri-block copolymer | 99.6 ± 5.8 |
Silica coated cores in the tri-block copolymer (red emission) | 112.5 ± 9.3 |
Silica coated cores in the tri-block copolymer (green emission) | 98.5 ± 9.4 |
Footnote |
† These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2015 |