Identifying the Correct HostâGuest Combination To Sensitize Trivalent

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Identifying the Correct Host - Guest Combination to Sensitize Trivalent Lanthanide (Guest) Luminescence: Titanium Dioxide Nanoparticles as a Model Host System Arijita Chakraborty, Gouranga H. Debnath, Nayan Ranjan Saha, Dipankar Chattopadhyay, David H. Waldeck, and Prasun Mukherjee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08421 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 29, 2016

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The Journal of Physical Chemistry

Identifying the Correct Host − Guest Combination to Sensitize Trivalent Lanthanide (Guest) Luminescence: Titanium Dioxide Nanoparticles as a Model Host System Arijita Chakraborty,1 Gouranga H. Debnath,1 Nayan Ranjan Saha,2 Dipankar Chattopadhyay,2 David H. Waldeck3,* and Prasun Mukherjee1,* 1

Centre for Research in Nanoscience and Nanotechnology, University of Calcutta, JD-2, Sector-

III, Salt Lake, Kolkata-700106, West Bengal, India 2

Department of Polymer Science and Technology, University of Calcutta, 92 A. P. C. Road,

West Bengal-700009, India 3

Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA

E-mail: [email protected], [email protected] * To whom correspondence should be addressed.

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Abstract This work develops a rationale for effective sensitization of trivalent lanthanide cation (Ln3+) luminescence in a semiconductor nanoparticle by examining the luminescence characteristics of Ln3+ dopants in a titanium dioxide nanoparticles [Ti(Ln)O2] [Ln = praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb)], as a representative model system.

For excitation of the TiO2 host at 350 nm the intra-configurational 4f-4f sharp

luminescence bands are observed for the Nd, Sm, Eu, Ho, Er, Tm and Yb incorporated (doped) nanoparticles, and no such luminescence is observed for the Pr, Gd, Tb and Dy containing nanoparticles. While host sensitized luminescence of lanthanide ions dominates the emission in the Nd and Sm incorporated nanoparticles, the host sensitization effect is less pronounced for the Eu and Yb containing systems, and for the Ho, Er, and Tm doped nanocrystals only a subset of the dopant ions’ luminescence bands is sensitized. The experimental observations of the host sensitized Ln3+ luminescence properties in the [Ti(Ln)O2] nanoparticles can be rationalized by considering that the dopant ions act as charge traps in the host lattice and associated environment induced luminescence quenching effects. Using these results, an energy offset between the trap site and the nanoparticle’s band edge that will generate an optimal host sensitized dopant emission is proposed.

The approach presented necessarily improves over a combinatorial

approach to select the host and dopant moieties, with the benefit of providing scientific insight regarding the nature of photophysical processes in a given host (semiconductor nanoparticle) – guest (Ln3+) composite system. Keywords: Trivalent Lanthanides, Semiconductor Nanoparticles, Host-Guest Combination, Charge Trapping. 2 ACS Paragon Plus Environment

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Introduction Trivalent lanthanide cations (Ln3+) are attractive luminophores because of their unique spectral characteristics, namely long luminescence lifetime (microsecond to millisecond), resistance to photobleaching, sharp luminescence bands that are spaced across the entire visible and near infrared (NIR) spectral region depending on the choice of the Ln3+, and minimal intra and inter Ln3+ emission spectral band overlap.

1-6

These unique properties allow the realization

of multiplex assays, time-gated measurements which can eliminate nanosecond timescale background, and longer experiment times, thus providing an avenue to obtain a better signal to noise ratio.

Moreover, NIR emitting lanthanides are especially advantageous in biological

imaging applications because of their deeper penetration in tissues. The use of Ln3+ luminophores in applications requires that one overcome the low oscillator strength of the f-f transitions and the quenching of the lanthanide luminescence by the vibrational overtones of −OH, −NH, and −CH bonds of adjacent ligand and solvent molecules. 7 To overcome these limitations, researchers are searching for an Ln3+ coordination environment that allows for an appropriate energy feeding mechanism from a moiety with a higher molar extinction coefficient (optical antenna effect 8, 9) and provides protection of the excited Ln3+ ion from environmental quenching effects.

Incorporating Ln3+ in a matrix of semiconductor

nanoparticles or other supramolecular assemblies provides one strategy to overcome these challenges.

10-22

Energy transfer from excited semiconductor nanoparticles (host) to lanthanide

dopants (guest) can be used to overcome the low molar extinction coefficient of lanthanides by an optical antenna effect while simultaneously acting as a rigid matrix that protects the lanthanide from coupling to high frequency vibrations. Moreover, an inorganic semiconductor host provides the possibility to incorporate multiple and oftentimes distinct Ln3+ ions in a given 3 ACS Paragon Plus Environment


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host matrix, thus making the overall nanoparticle luminophore brighter and opening up the avenue for multiplex assays. Among various semiconductor nanoparticles, titanium dioxide (TiO2) is known for its range of applications.

23-30

Researchers have also investigated the usefulness of TiO2 based

materials as the host moiety for sensitizing Ln3+ luminescence, particularly for Nd3+, 31, 33-37

Eu3+, 31, 32, 37-42 Tb3+, 43 Er3+,

31, 32, 44, 45

and Yb3+

31, 32

31-33

Sm3+,

dopants. The mismatch in size and

charge of the Ln3+ and Ti4+ generates concomitant lattice distortion and charge compensation upon doping. In a case study, Chen and co-workers have investigated the doping of Eu3+ in anatase TiO2 nanoparticles and identified three distinct Eu3+ related sites, corresponding to two core sites in which the local Ti4+ coordination site symmetry changes from a D2d point group to either D2 or C2v symmetry and one surface related site in which the symmetry changes to a C1 symmetry.

42

These previous reports on Ti(Ln)O2 materials motivated the current study which

aims to develop a model that can guide the choice of host-guest combinations for optimal host sensitized Ln3+ luminescence. Currently, no such understanding exists to explain whether a given host semiconductor (such as TiO2) will provide good sensitization for a Ln3+ cation. By considering Ti(Ln)O2 nanoparticles as a model system this study aims to provide a rationale for choosing an appropriate host-guest combination. First we present our experimental observations of Ln3+ 4f-4f intra-configurational sharp luminescence bands in the Ti(Ln)O2 [Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb] nanoparticles, and then we describe the correlation between the experimental observations and the position of the Ln3+ dopant energy levels with respect to the valence and conduction bands of the TiO2 host. The corresponding cases with La, Ce, and Lu are not considered because of their intrinsic electronic structure and associated absence of 4f-4f intra-configurational spectral bands, and the case with Pm was not 4 ACS Paragon Plus Environment

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considered because of its radioactive nature.

These observations are shown to support a

proposed mechanism for the luminescence sensitization and to predict the relevant energy level offsets that are necessary to realize optimum host sensitized lanthanide luminescence. Materials and Methods Chemicals. Tetra(n-butyl) titanate and lanthanide acetate hydrates (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) (99.9%) were purchased from Alfa Aesar. Methanol, ethanol, and acetone were purchased from Merck. Potassium bromide (KBr) IR grade and coumarin 153 (C153) were purchased from Sigma Aldrich.

All chemicals were used as purchased without additional

purification. Water used for all the experiments was obtained from a Millipore system with a resistivity of 18.2 MΩ cm at 25°C. Nanoparticle Synthesis. Ln (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) incorporated TiO2 (titanium dioxide) nanoparticles were synthesized by a sol-gel solvothermal method based on the report of Chen and co-workers. 33, 42 In a typical synthesis, 58 micromoles of a particular lanthanide acetate hydrate were dissolved in 20 ml of absolute ethanol and 0.2 ml of distilled water with stirring at room temperature. To this mixture, 1 ml of tetra(n-butyl) titanate dissolved in 20 ml absolute ethanol was added. The solution was stirred vigorously with a magnetic stirrer for three hours at room temperature. The mixture turned cloudy during continuous stirring. This cloudy mixture was transferred to a 50 ml Teflon lined autoclave and was subjected to the solvothermal treatment for five hours at 120°C. The resultant mixture was cooled to room temperature. The as synthesized materials were washed three times with absolute ethanol. The white precipitates were collected by centrifugation and dried overnight at 60°C. The obtained sample was annealed at 500°C for two hours.

For the synthesis of the undoped TiO2



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nanoparticles, the same synthetic protocol was adopted, without the addition of lanthanide precursor. X-Ray Diffraction.

X-Ray powder diffraction measurements were carried out at room

temperature using a PANalytical X’pert PRO (X-PERT-PRO Panalytical) diffractometer, operated at a generator voltage of 40 keV and a current of 30 mA. The diffraction patterns were collected using Cu Kα radiation (λ = 0.154 nm) as the X-ray source with a scan speed of 1°/ min within the 2θ scan range of 15° to 100°. Fourier Transform Infrared Spectroscopy. Fourier transform infrared spectroscopy was carried out on the obtained samples with a Jasco FTIR 6300 spectrometer. The samples were prepared by using the KBr pellet method. The resolution of the measured spectra was 4 cm−1. The spectra which are presented were acquired from an average of 64 scans. Typically, 1 mg of the sample was mixed uniformly with 100 mg of dried KBr in a mortar and pestle. The KBr pellet was prepared by the application of pressure. All spectra were recorded at room temperature. The data analysis was performed with the analysis software provided with the instrument. Electron Microscopy Measurements. Transmission electron microscopy (TEM) was performed on the samples by using a TEM instrument from JEOL (Model JEM-2100) operated with an acceleration potential of 200 kV.

The sample for TEM measurements was prepared by

dispersing a small amount of nanoparticles in water and placing a drop of solution on a carbon coated copper grid. The extra solution was removed by drying the grid for half an hour under an infrared lamp. The energy dispersive X-ray spectra (EDS) were acquired using the Zeiss Model EVO 18 scanning electron microscopy instrument. For performing the experiments, the samples were coated with platinum before mounting in the instrument.


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Luminescence Spectroscopy. The luminescence spectra of the samples were acquired using a Horiba Fluorolog 3-22 luminescence spectrometer. For collecting the emission spectra, the nanoparticles were excited at 350 nm and the excitation spectra were acquired by collecting the emission of Ln3+ band at appropriate wavelengths. To obtain spectra in the visible spectral region, the excitation and emission slits were kept at a 2 nm spectral resolution; in cases with low luminescence intensity the corresponding slits were opened to a 4 nm spectral resolution. For the NIR spectral regime, slit widths corresponding to 8 nm and 4 nm spectral resolutions for the excitation and emission path, respectively, were typically used. For the samples with low luminescence signal in the NIR spectral region, the corresponding slits were broadened to spectral resolutions of 14 and 40 nm respectively. The R-928 photomultiplier tube (PMT) and a liquid nitrogen cooled indium gallium arsenide (InGaAs) detector [Model: DSS-IGA(1-9)010L] were used to collect the emission in the visible and NIR spectral region respectively. Samples were dispersed in water for the measurements. All measurements were performed at room temperature. Relative quantum yields for the Ln3+ band centered luminescence in the visible spectral range were calculated by comparison to the quantum yield of coumarin 153 (C153) dissolved in methanol (Φ Φ =0.42

46

). Relative quantum yields Φ were calculated with all of the observed

Ln3+ emission bands by way of Eqn 1; namely ∞

Φ Φ

=

(ν)ν (λ ) (λ ) η , ∞ (λ ) (λ ) η , (ν )ν

(1)

in which the subscripts x and r stand for sample and reference respectively, A is the absorbance at the excitation wavelength (λ), I is the intensity of the excitation light at the same wavelength, η is the refractive index (η = 1.333 for water and η = 1.327 for methanol) and 7 ACS Paragon Plus Environment


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I (ν ) is the luminescence intensity as a function of wavenumber ν . For the calculation of relative quantum yields in the NIR spectral region, the PMT and InGaAs detectors were calibrated with respect to the common band of Tm3+ emission which is centered around 810 nm. Results and Discussion X-Ray Diffraction. X-ray diffraction (XRD) patterns for some of the TiO2 based nanoparticles are shown in Figure 1. The XRD data indicate an anatase phase for the synthesized TiO2 nanoparticles, with characteristic signals at 2θ = 25.4, 38.0, 48.1, 54.1, 55.2, 62.9, 68.9, 70.7 and 75.3 that originate from the (101), (004), (200), (105), (211), (204), (116), (220) and (215) planes, respectively.

Moreover, the synthetic protocol yields particles without noticeable

contribution from the rutile phase, as judged by the absence of the signature (110) plane at 2θ = 27.4. Note that the band profiles for the Ln3+-doped TiO2 nanoparticles are broader than that for the undoped TiO2 nanoparticles. The full width at half maximum (FWHM) values were found to be 0.32, 0.39, 0.39 and 0.65 for the TiO2 and Ti(Ln)O2 [Ln = Sm, Tb, Yb] nanoparticles respectively. The peaks in the XRD pattern are generally broadened by either a size reduction of the crystallites and/or strain associated with the lattice distortion. 47, 48 The signal broadening in the Ti(Ln)O2 nanoparticles investigated with respect to the undoped TiO2 nanoparticles are most likely associated with the introduction of lattice strain arising from the incorporation of charge and size mismatched Ln3+ in the TiO2 nanoparticles.


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Figure 1. Representative XRD profiles are shown for the TiO2 and [Ti(Ln)O2] [Ln = Sm, Tb, Yb] nanoparticles, showing the formation of the anatase phase for both the TiO2 and Ti(Ln)O2 nanoparticles. The Joint Committee on Powder Diffraction Standards (JCPDS) card number has been included in the bottom panel. Fourier Transform Infrared (FTIR) Spectroscopy.

FTIR spectra for TiO2 nanoparticles,

Ti(Ln)O2 [Ln = Sm, Tb] nanoparticles, and terbium acetate are shown in Figure 2. All the nanoparticles show strong characteristic IR absorption bands in the region of 400-1000 cm−1 that is a signature of Ti−O−Ti stretching modes. Prominent IR absorption peaks appear at 496 cm−1 and 682 cm−1 for the TiO2 nanoparticles, and they shift to 470-490 cm−1 and 660 cm−1 in the Ti(Ln)O2 [Ln = Sm, Tb] nanoparticles. The red shift of the IR absorption band maxima is consistent with an increase of reduced mass in the presence of Ln3+ cations in the TiO2 lattice. Moreover, a visual comparison of the nanoparticle spectra with the corresponding spectrum of Tb(III) acetate (considered as a representative lanthanide precursor) demonstrates the absence of free Ln3+ precursor in the Ti(Ln)O2 nanoparticles investigated; as judged by the absence of 9 ACS Paragon Plus Environment


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characteristic acetate asymmetric and symmetric stretching frequencies at 1550 and 1460 cm−1 respectively.

Collectively, these results indicate an interaction of Ln3+ with the TiO2

nanoparticle host lattice.

Figure 2. Representative FTIR spectra are shown for the TiO2 and Ti(Ln)O2 [Ln = Sm, Tb] nanoparticles. The corresponding spectrum for the lanthanide precursor is shown, considering Tb(III) acetate as a representative case. Electron Microscopy.

Transmission electron microscopy (TEM) data are shown for the

Ti(Sm)O2 nanoparticles in Figure 3. An analysis of the image gives an average nanoparticle size of 3.5 ± 0.4 nm in diameter. The high resolution transmission electron microscopy (HRTEM) image reveals crystalline phases that have been attributed to the (101) plane of the anatase TiO2 crystal. The energy dispersive X-ray spectrum (EDS) of Ti(Sm)O2 nanoparticles shows the presence of samarium in the nanoparticles. The corresponding EDS for all other nanoparticles 10 ACS Paragon Plus Environment

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investigated in this study are provided in Figure S1. It is important to note that the EDS signals originate from the lanthanides in the nanoparticles and is not contaminated by free Ln3+, as the FTIR spectra do not display IR absorption bands from Ln(III) acetate (vide supra).

This

observation is consistent with the repeated washing of the nanoparticles during the synthesis, which eventually removes any free or otherwise un-reacted Ln(III) precursors.

Figure 3. TEM image of Ti(Sm)O2 nanoparticles with the corresponding size distribution are shown in the top left and right panels respectively. The HRTEM image in the bottom left panel identifies the crystalline phases in the nanoparticles. A representative EDS of Ti(Sm)O2 nanoparticles is shown in the bottom right panel.

Luminescence Spectroscopy.

The steady-state luminescence emission spectra have been

collected for solutions of each of the Ti(Ln)O2 nanoparticles by excitation at 350 nm, and the corresponding excitation spectra (obtained by monitoring the Ln3+ sharp intra-configurational 4f4f emission bands, vide infra) were acquired to ascertain the excitation mechanism that is responsible for generating the Ln3+ luminescence. The major excitation pathways that can give



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rise to Ln3+ luminescence from optical excitation of the Ln3+ incorporated semiconductor nanoparticles are (i) the direct excitation of intra-configurational Ln3+ 4f-4f bands, (ii) excitation of the Ln3+ 4f-5d bands, (iii) excitation of a ligand-to-metal charge transfer (LMCT) band, and (iv) excitation of the host lattice, the optical antenna effect. Because of the parity forbidden nature for the electric dipole part of the 4f-4f transitions, process (i) is inefficient for populating the luminescent state of Ln3+, however they can contribute at some small level. Although the 4f5d transitions (process ii) are parity allowed for the electric part of the incident radiation, they often lie out of the UV-visible-NIR spectral window. For example, the 4f-5d transition falls in the range of 4.0-4.5 eV for Ce3+ and Tb3+, while for all the other Ln3+ ions the corresponding value is more than 5 eV.

49

Excitation of LMCT bands (process iii) and energy transfer from

ligand excited states (process iv) are viable pathways that numerous workers have investigated, 2, 8, 10, 12, 13, 19-21, 50-57

however the charge transfer bands in oxide materials lie in the deep UV; based

on Jørgensen’s empirical relationship

58

and the Pauling electronegativity scale the charge

transfer energy from the valence band of oxide based materials to Eu3+ lies at an energy of 5.37 eV and for the other Ln3+ the corresponding value is even higher.59

Consequently, the

engineering of correct host-guest combinations and the operation of the optical antenna effect (process iv) is the most viable strategy for sensitizing Ln3+ luminescence in oxides. Most germane to the current study, the use of an oxide host material provides an opportunity to evaluate the role of the optical antenna effect solely, as the charge transfer bands which might operate in other types of materials lie out of the visible spectral window. The steady-state luminescence excitation and emission spectra for the Ti(Ln)O2 nanoparticles are shown in Figures 4 and 5. To present these data the Ln3+ ions have been divided into two sub-categories, based on the number of major luminescent energy levels. The 12 ACS Paragon Plus Environment

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first sub-category comprises Ln3+ ions (Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+ and Yb3+) with one major luminescent energy level. In this subcategory, only Nd3+, Sm3+, Eu3+ and Yb3+ showed characteristic 4f-4f intra-configurational sharp luminescence bands when doped in the TiO2 nanoparticles; no emission bands were observed from the Gd3+, Tb3+ and Dy3+ doped TiO2 nanoparticles. The second subcategory comprises Ln3+ with two or more major luminescent energy levels (Pr3+, Ho3+, Er3+ and Tm3+). In this subcategory the Ln3+ band centered sharp luminescence was observed from the Ho3+, Er3+ and Tm3+ containing nanoparticles; no such emission was observed from the corresponding Pr3+ containing nanoparticles. Note that the concentrations of the lanthanide ions in these solutions are much lower than that commonly used for observing lanthanide ion luminescence in solution. Assuming the complete incorporation of the precursor materials into the TiO2, an estimation of the [Ln3+] used in the luminescence experiments is < 20 micromolar, whereas millimolar concentrations are typically required (molar extinction co-efficient of Ln3+ is ≤ 10 M−1cm−1) to observe the luminescence from Ln3+ in free solution. By analyzing the luminescence excitation spectra for each of the major Ln3+ emission bands, it is possible to identify the excitation pathways (processes i to iv, vide supra) that lead to sensitization of the different Ln3+ emission bands in the corresponding Ti(Ln)O2 nanoparticles. For the Ti(Sm)O2 nanoparticles, the luminescence excitation spectra (Fig 4) display identical broad excitation profiles with a peak near 350 nm for each of the Sm3+ emission lines at 585, 612 and 664 nm, suggesting a common excitation pathway for sensitization of all the Sm3+ luminescence bands. Fitzmaurice and co-workers

60

have discussed the increase in band gap of

the TiO2 nanoparticles with particles less than 5 nm diameter. Consequently, the appearance of the broad excitation profile centered around 350 nm for these nanoparticles with 3.5 nm average 13 ACS Paragon Plus Environment


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diameter and the lack of intra-configurational 4f-4f sharp excitation bands is taken as strong evidence for operation of the optical antenna effect in sensitizing the Sm3+ luminescence by the electronic levels of the TiO2 nanoparticles (see Figure S2 in the supporting information for a representative electronic absorption spectrum of the TiO2 nanoparticles). The excitation spectra for the Ti(Nd)O2 nanoparticles luminescence bands at 915 and 1094 nm (Fig 4) are very similar to that found for Ti(Sm)O2, a broad excitation profile centered around 350 nm. Correspondingly, the Ti(Yb)O2 show a broad excitation profile in the near UV (around 350 nm) for the 975 nm band and for the structured emission between 990 nm and 1050 nm, however the signal to noise in these materials is less and suggests that the sensitization is less effective (vide infra). Note that the nature of the structured Yb3+ emission is consistent with previous reports.

4, 31

These

results suggest that Ln3+ luminescence bands of the Ti(Ln)O2 [Ln = Nd, Sm, Yb] nanoparticles are sensitized by the electronic states of the TiO2 nanoparticle matrix; an optical antenna effect. In contrast to the Ti(Ln)O2 [Ln = Nd, Sm] nanoparticles, the Ti(Eu)O2 nanoparticles do not display the optical antenna effect as the major contribution. The excitation profiles in the Ti(Eu)O2 nanoparticles (Fig 4) upon monitoring the Eu3+ emission at either 590 or 616 nm reveal an excitation pathway that is dominated by the direct 4f-4f intra-configurational sharp excitation bands with a somewhat weaker contribution from the band centered around 350 nm. Thus, the optical antenna effect for the Eu3+ emission is no more effective a sensitization pathway for Eu3+ luminescence than is the direct optical pumping of the 4f-4f transitions.


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Figure 4. Steady-state luminescence excitation (left) and emission spectra (right) are shown for Ti(Ln)O2 [Ln = Nd, Sm, Eu, Yb] nanoparticles dispersed in water. In the Ti(Sm)O2 emission spectrum, the region near 700 nm is broken to eliminate the contribution from the harmonic band of the excitation source. These lanthanide cations have one major luminescent energy level and one or more final states. For Ln3+ ions comprised of two or more major luminescent energy levels (see Figure 5); an emission wavelength dependent excitation profile has been observed in selected cases. In this subcategory, both Ho3+ and Er3+ exhibit multiple emission bands. For Ti(Er)O2 nanoparticles, monitoring the Er3+ band centered emission at 565 nm reveals a significant contribution from sharp 4f-4f intra-configurational direct excitation bands centered at around 380 and 525 nm and a significant contribution from a broad profile centered at around 350 nm. When monitoring the 665 nm Er3+ emission band the relative contribution of the direct excitation bands is significantly decreased as compared to the broad 350 nm band, and for the excitation spectrum obtained while monitoring the emission at 1550 nm only the contribution from the broad 350 nm band is seen; i.e., a pure optical antenna effect without the signature from sharp direct excitation bands. A similar behavior in the excitation spectra is evident for the Ho3+ containing TiO2 nanoparticles (Fig 5), where an increase in the monitoring wavelength from 545 to 670 nm reveals a weaker contribution from direct excitation bands as compared to the broad 350 nm band. The bottom panel of Figure 5, shows the excitation and emission spectra for dispersions of Ti(Tm)O2 nanoparticles, and it reveals that the Tm3+ band centered luminescence around 808 nm is sensitized by way of a broad excitation band centered around 350 nm, namely the optical antenna effect.


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Figure 5. Steady-state luminescence excitation and emission spectra are shown for aqueous dispersions of Ti(Ln)O2 [Ln = Ho, Er, Tm] nanoparticles. In the Ti(Tm)O2 excitation spectrum, the region near 404 nm is broken to eliminate the contribution from the harmonic band. The lanthanide cations have been categorized based on the presence of two or more major luminescent energy levels. A Photophysical Rationalization.

The sensitization mechanism is proposed to involve

photoexcited charge carrier trapping at the dopant sites; i.e., the Ln3+ ions act as trap sites for 17 ACS Paragon Plus Environment


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charge carriers [electron (e−) and/or hole (h+)] in the semiconductor nanoparticle matrix.12, 61, 62 Other researchers have discussed similar sensitization processes in doped bulk semiconductor materials,63-66 and a redox reaction based energy transfer mechanism has been proposed for molecular d-f complexes.67,

68

Based on luminescence experiments and the calculation of the

spectral overlap integral in the Zn(Tb)S, Zn(Eu)S, Zn(Tb)Se, Cd(Tb)S, Cd(Tb)Se nanoparticles 12

and Zn(Tb)S nanoparticles with different sizes 61 from our previous works, we showed that the

spectral overlap between the emission of the nanoparticle (donor) and absorption of a Ln3+ (acceptor) is not a good predictor for the extent of Ln3+ luminescence sensitization in Ln3+ doped semiconductor nanoparticles. Figure 6 shows a Jablonski diagram illustrating the key photophysical processes in the luminescence sensitization scheme. In this energy scheme the ground state (GS) and excited luminescent state (LS) of the lanthanide are assumed to lie within the band gap of the semiconductor host with a valence band edge VB and conduction band edge CB. The vertical arrow I identifies the process for optical excitation of an electron from the valence band to the conduction band of the host nanoparticle leaving a hole in the valence band, and the vertical arrow labeled V indicates the process for optical excitation of the lanthanide ion (4f-4f transition). The diagram also shows the two important charge trapping processes: hole trapping at the Ln3+ ground state is indicated by the dashed arrow IIforward (detrapping of the hole is indicated by the IIback process) and electron trapping at the excited state energy level is indicated by the process IIIforward (as well as possible electron detrapping by the IIIback process). In order to generate an excited state lanthanide ion the electron and hole must be captured at the dopant ion within a short amount of time, i.e., a timescale that can compete with other nonradiative recombination processes, which are represented by the squiggly lines and involve recombination 18 ACS Paragon Plus Environment

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of an electron and hole in the conduction and valence band of the TiO2 respectively, an electron in the conduction band of the TiO2 and a hole at the Ln3+ trap site, and an electron in the Ln3+ trap site and a hole in the valence band. When an electron and a hole are trapped together at the Ln3+ trap site, an Ln3+ excited state can be created and it can decay radiatively (downward vertical arrow VI) or nonradiatively (represented by squiggly lines). In this scheme, the host sensitized Ln3+ luminescence is most effective when the Ln3+ trap sites dominate the exciton recombination. 63-66

Figure 6. The schematic energy level diagram shows the key processes for realization of host sensitized Ln3+ luminescence in semiconductor nanoparticles. The dashed arrows indicate processes for charge carrier trapping and detrapping at Ln3+ sites; the vertical arrows indicate radiative processes; and the squiggly arrows show nonradiative recombination processes. As Figure 6 suggests, the relative positions of Ln3+ ground and excited states with respect to the valence and conduction band of the host lattice are important for determining whether the charge trapping will be an efficient sensitization mechanism. To this end, a method proposed by Dorenbos and co-workers,

59, 69-71

and later adopted by us,

12, 61, 62

can be used to estimate the

relative energy level placements for the lanthanide ions in the semiconductor matrix. This method relies on two assumptions: (i) because the f electrons are well shielded, the trend in 19 ACS Paragon Plus Environment


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binding energies through the lanthanide series is universal and largely independent of the host and (ii) the charge transfer energy between the anion valence band and the Eu3+ ground state is equal to the energy difference between the valence band of the host lattice and the Eu2+ ground state. Based on Jørgensen’s empirical relationship 58 and the Pauling electronegativity scale, the charge transfer energy from the valence band of an oxide based material to the Eu3+ ground state has been estimated as 5.37 eV.

59

Furthermore, knowledge of the difference between the Eu2+

and Eu3+ ground states (considered to be 5.7 eV for nanoparticle bandgap values < 6 eV

59, 69

)

allows one to construct the entire relative energy level scheme. The higher lying Ln3+ energy levels have been placed based on the corresponding values for the Ln3+ aqua complexes reported by Rajnak and co-workers.

72-75

The relevant energy parameters for the Ti(Ln)O2 nanoparticles

studied in the present work are tabulated in Table 1. The relative energy scheme for the Ln3+ ground states with respect to the valence band and conduction band of the anatase TiO2 nanoparticles are shown in Figure 7. This diagram shows how the energy offset between the Pr3+, Nd3+, Pm3+, Sm3+, Dy3+, Ho3+, Er3+, Tm3+ and Yb3+ ions’ ground states change relative to the TiO2 through the series. To this end, it is worthwhile to compare similar efforts by other researchers. Note that the placement of the Sm3+ and Gd3+ energy levels are in agreement with those reported by Qiu and co-workers 76 and Paik and co-workers 77 in TiO2 nanoparticles.


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Figure 7. The relative energy levels of the Ln3+ ground state with respect to the valence band and conduction band of the host TiO2 nanoparticles is shown. The relative energy level positions of the Ln3+ ground state and the luminescent states with respect to the valence and conduction band of the host TiO2 nanoparticles have been constructed and are shown in Figure 8 for Ln = Nd, Sm, Eu, Gd, Tb, Dy and Yb and in Figure 9 for Ln = Pr, Ho, Er and Tm. Using these energy level diagrams, a complete and consistent explanation for the Ln3+ luminescence in TiO2 can be provided. In these energy level schemes, the important energy offsets to consider are (i) the energy difference between the top of the TiO2 valence band and the Ln3+ ground state ∆E1, which affects the hole trapping efficiency; and (ii) the energy difference between the bottom of the TiO2 conduction band and the Ln3+ luminescent state ∆E2, which affects the electron trapping efficiency. The optimum values for these energy offsets are those that allow efficient carrier co-localization, so that radiative recombination via the lanthanide dopant levels is possible. Clearly, a negative value for ∆E1 and a positive value for ∆E2 will be optimal.



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Note that another important consideration for efficient lanthanide luminescence is the energy difference between the Ln3+ luminescent state and the highest spin orbit level of the final state, ∆E (Ln3+ LS − Ln3+ FS, highest spin orbit level). A lower value for this energy difference will allow more effective Ln3+ luminescence quenching from the vibrational overtones of the ligand and solvent molecules from the immediate environment.


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Figure 8. Energy level schemes for the Ln3+ [Ln = Nd, Sm, Eu, Gd, Tb, Dy, Yb] ground and excited states are shown with respect to the valence and conduction band of the host TiO2 23 ACS Paragon Plus Environment


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nanoparticles. In these schemes, the valence band has been set to zero and the conduction band has been placed at 3.54 eV, as determined from the luminescence excitation spectra. The lanthanide cations shown here have one major luminescent energy level, which is the uppermost level in each of the diagrams. The wavelengths are labeled from previous reports for the Ln3+ aqua complexes. 72-75

Figure 9. Energy level schemes for the Ln3+ [Ln = Pr, Ho, Er, Tm] ground and excited states are shown with respect to the valence and conduction band of the host TiO2 nanoparticles. In these schemes, the valence band has been set to zero and the conduction band has been placed at 3.54 eV, as determined from the luminescence excitation spectra. The lanthanide cations shown here can have two or more major luminescent energy levels. The wavelengths are labeled from previous reports for the Ln3+ aqua complexes. 72-75


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Table 1. Relevant Luminescence Spectral Parameters for the Ti(Ln)O2 Nanoparticles Studied. Ln

GS

LS

FS

Pr

3

3

P0

3

H4

1

D2

3

H4

I9/2

4

F3/2

4

H5/2

4

F0

5

Nd

Sm

Eu

4

6

7

H4

G5/2

D0

Gd

8

S7/2

6

P7/2

Tb

7

F6

5

D4

H15/2

4

I8

5

S2

5

F5

Dy Ho

Er

Tm

Yb

6

5

4

3

2

I15/2

H6

F7/2

4

F9/2

S3/2

I9/2 I11/2 4 I13/2 6 H5/2 6 H7/2 6 H9/2 7 F1 7 F2 8 S7/2

∆E1a (cm−1) −21380

Charge Trapping Hole Trap

∆E2b (cm−1) −13230

Significant

∆E3c (cm−1) 20410

−16620

Significant

16670 11300 9350 7410 17860 16810 15625 16950 16260 32260

Autoionization

−10490

Hole Trap

6760

Not significant

−6860

Hole Trap

3830

Not significant

4

7

F6 F5 7 F4 6 H15/2 6 H13/2 5 I8

2660

13940

−29930

Weak Hole Trap Not a hole trap Hole Trap

−16620

Hole Trap

12750

Not significant

9110

Not significant

−21790

Significant

−9140

Significant

7

−8390

Hole Trap

5

I8 I7 4 I15/2

1770

Not significant

4770

Not significant

830

5

15380 10310 18180

4

F9/2

4

I15/2

3900

May be significant Not significant

4

I13/2

4

I15/2

12540

Not significant

6490

1

D2

3

F4

−10660

Significant

21980

1

G4

3

H6

−4030

Significant

21280

3

H4

3

H6

4680

Not significant

12500

2

F5/2

2

F7/2

14760

Not significant

10240

−9520

−11370

Hole Trap

20410 18350 17240 21050 17540 18350

Hole Trap

∆E4d (cm−1) [E4(i), E4(f)] 6940 [1D2, 1 G 4] 3910 [3P0, 1 D 2] 5410 [4F3/2, 4 I15/2] 7400 [4G5/2, F11/2]

6

12300 [5D0, F6] 32200 [6P7/2, 8 S7/2] 15100 [5D4, 7 F0] 7

7850 [4F9/2, F3/2] 3000 [5S2, 5 F5] 2200 [5F5, 5I4] 6

3100 [4S3/2, F9/2] 2850 [4F9/2, 4 I9/2] 6490 [4I13/2, 4 I15/2] 4

15150

6650 [1D2, G 4] 6250 [1G4, 3 F2] 4300 [3H4, 3 H 5] 10240 [2F5/2, 2 F7/2] 1

−3550

Hole Trap

a

∆E1 = E (NP VB) − E (Ln3+ GS) ∆E2 = E (NP CB) − E (Ln3+ LS) c ∆E3 = E (Ln3+ LS) − E (Ln3+ FS) d ∆E4 = E (Ln3+ LS) − E (Ln3+ FS, highest spin orbit level) b

Correlation between Experiment and the Energy Level Schemes. The relative efficiencies of the sensitization of luminescence for all of the investigated Ln3+ ions (namely, the single



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Page 26 of 38

luminescent state ions Nd, Sm, Eu, Gd, Tb, Dy and Yb and the multiple luminescent state ions Pr, Ho, Er and Tm) can be rationalized from the energy level schemes shown in Figures 8 and 9. Consider the subcategory of Ln3+ with one luminescent state. In this subcategory, ∆E1 is negative for Nd, Sm, Dy, Tb, and Yb (see Table 1), suggesting that they can be effective hole trapping sites in the Ti(Ln)O2 nanoparticles. In contrast, ∆E1 is significantly positive for Gd, indicating that it is not an effective hole trap in the TiO2 nanoparticles. For Eu, the value of ∆E1 is small and positive, suggesting it will be an inefficient hole trap; however, its small magnitude suggests that it will not be as bad a trapping site as is the Gd. Next, consider the ∆E2 values which are positive for Nd, Sm, Eu, Gd, and Yb (see Table 1), indicating that they are effective electron trap sites. In contrast ∆E2 is negative for Tb and Dy, indicating that they are poor electron trapping sites. By considering the lanthanide ions for which ∆E1 0, we find the subset of Nd, Sm and Yb; and these three are the lanthanide ions that show strong sensitization by excitation of TiO2. For the ions Tb, Dy, and Gd one of the lanthanide states is embedded in the TiO2 band so that carrier trapping is inefficient; moreover, direct excitation of the lanthanide results in rapid autoionization (electrons for Tb and Dy, and hole for Gd) that quenches the lanthanide luminescence. The case of Eu is intermediate, because it has ∆E1 ~0 and ∆E2>0 so that autoionization is less competitive and some luminescence is observed by direct excitation of the 4f-4f transitions. In order to compare the Ln3+ band centered luminescence efficiencies the corresponding quantum yield values were determined for these ions (see Table 2).

For example, the

luminescence quantum yield values for the visible Sm3+ and Eu3+ emission in the Ti(Ln)O2 nanoparticles were found to be (2.3 ± 0.5) × 10−2 and (3.0 ± 0.3) × 10−4 respectively. Thus, the


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Sm3+ band centered emission in the Ti(Sm)O2 nanoparticles is approximately 100 times brighter than that for Eu3+ in the Ti(Eu)O2 nanoparticles. In the NIR spectral window, the quantum yield values of the Nd3+ and Yb3+ emission are found to be (9.1 ± 0.2) × 10−2 and (4.6 ± 0.1) × 10−4 respectively (Table 2). These values identify the Ti(Sm)O2 and Ti(Nd)O2 nanoparticles as potentially useful Ln3+ containing semiconductor nanoparticle based luminophores. The considerations for the sub-category of Ln3+ with two or more luminescent energy levels (Pr, Ho, Er, and Tm) are somewhat more complex, as we must consider the ∆E2 value for each of the emissive levels and possible nonradiative kinetics in the manifold of luminescent states. All of these cations have ∆E1

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