Power-Dependent Radiant Flux and Absolute Quantum Yields of Up

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Power-Dependent Radiant Flux and Absolute Quantum Yields of UpConversion Nanocrystals Under Continuous and Pulsed Excitation Ian Nicholas Stanton, Jennifer A Ayres, Joshua T. Stecher, Martin C Fischer, Dan Scharpf, Jonathan D. Scheuch, and Michael J. Therien J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11929 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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The Journal of Physical Chemistry

Power-Dependent Radiant Flux and Absolute Quantum Yields of Upconversion Nanocrystals Under Continuous and Pulsed Excitation Ian N. Stantona, Jennifer A. Ayresa, Joshua T. Stechera, Martin C. Fischera, Dan Scharpfb, Jonathan D. Scheuchb, and Michael J. Therien*a a

Department of Chemistry, French Family Science Center, 124 Science Drive, Duke University,

Durham, North Carolina 27708 b

Labsphere, Inc., 231 Shaker Street, North Sutton, New Hampshire, 03260

Author Email Addresses: [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected] *To whom correspondence may be sent: [email protected].

ACS Paragon Plus Environment

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ABSTRACT. Elucidating structure-function relationships that determine the photophysics of nanomaterials that upconvert high-power, near-infrared (NIR) excitation to shorter wavelength NIR, visible, and UV emission requires both compositional characterization and experimental designs that rigorously define laser excitation conditions and the manner in which emitted photons are collected. Presented herein are laser power-dependent, total-emitted radiant flux (Watts), and absolute quantum yield measurements of homogeneous, solution-phase 28 nm [NaYF4; Yb (15%), Er (2%)] upconversion nanocrystals (UCNCs) determined using a multidetector integrating sphere spectroscopy system. These studies compare for the first time quantitative total radiant flux and absolute quantum yield measurements of UCNCs determined as a function of laser power density for both 970 nm continuous-wave (CW) and 976 nm pulsed Ti-sapphire (140 fs pulse width, 80 MHz) laser excitation. This study illustrates that at intensities in the range of 35 - 225 W/cm2, the total radiant flux is higher under CW excitation by an average factor of 1.5, and for this range of laser powers, the high peak intensities associated with fs-pulsed excitation conditions do not drive further augmentation of the radiant flux magnitude. This study has important ramifications for the field as it establishes the total radiant flux as the most appropriate figure of merit relevant for quantifying the emissive output intensity of UCNCs. In contrast to an UCNC emission quantum yield measurement, the total radiant flux may be determined with a high degree of accuracy; this point is critical, as this parameter is more closely connected to UCNC performance metrics important for imaging, emission fingerprinting, tracking, and energy conversion applications.

KEYWORDS (4-6) up-conversion, nanocrystal, quantum yield, imaging, radiant flux, pulsed laser


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MANUSCRIPT. Upconversion nanocrystals (UCNCs) represent a unique class of materials capable of absorbing multiple near-infrared (NIR) photons to populate electronically exited states that emit light over higher-energy NIR, visible, and UV spectral regimes. Such upconverted emission is achieved via multi-photon absorptions, which undergo subsequent quasi- and non-resonant energy transfers between long-lived excited states in rare-earth ions doped within a crystalline matrix;1-9 in these nanocrystals, the specific UCNC composition defines the energies and intensities of emitted light for a given set of irradiation conditions. One noteworthy application of UCNCs includes their utility as luminescent imaging probes, as these compositions provide several distinct benefits over typical organic and inorganic molecules, fluorescent proteins, and quantum dots that are excited at long wavelength. These advantages include: (i) NIR excitation and upconverted emission that is not contaminated by signals that derive from autofluorescence of linear absorbers that may be present, a common artifact of biological samples, and (ii) the fact that UCNCs neither photobleach nor blink.3, 10-13 While multiple studies have taken advantage of surface-functionalized UCNCs under CW laser excitation for biological imaging,12,

14-25

cellular tracking11,

26

and photodynamic therapy27-30,

little work has exploited fs-pulsed laser excitation sources,13,

31

despite the ubiquity and

importance of these light sources in biomedical two-photon imaging.32, UCNCs for in vivo11,

14

and whole body12,

15, 19, 20, 25, 34

33

Studies that explore

imaging of small animals using CW

excitation highlight the potential of these compositions in the development of new imaging modalities. The development of next-generation UCNCs for biological, biomedical, and other applications will require: (i) end-use-driven optimization of nanocrystal composition, size,


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crystal phase, and the nature and density of surface ligand substitution, and (ii) accurate evaluation of the emissive outputs of these structures under specific excitation conditions (e.g., light source, excitation power-density). Evaluation of structure-function relationships under specific laser excitation conditions are required in order to adequately inform nanocrystal design: such studies beseech the need for rigorous, quantitative, and accurate photophysical characterization methods for these materials. A critical, but rarely reported UCNC metric, is the radiant flux, defined as the amount of energy emitted per unit time per unit volume for a given stimulus; this quantity reflects the amount light (in units of Watts) a specific composition is capable of producing under a given set of experimental conditions. For laser-excited luminescence, the total radiant flux is a measure of the amount of incident laser excitation energy converted into emission energy: the magnitude of this quantity is thus a function of factors that include the incident laser power, the material’s absorptive oscillator strength, the mass of the material that is excited, and the composition’s emission quantum yield. The emission quantum yield, a more common emitter figure of merit, is defined as the amount of light emitted relative to the light absorbed. Absolute quantum yield measurements take advantage of integrating spheres that account for all absorbed, emitted, and scattered photons; relative quantum yield measurements, in contrast, are achieved by comparing the sample’s emission to that of a standard emitter under a fixed set of excitation conditions. Although UCNC quantum yield measurements provide structure-function insights useful for iterative nanomaterial design and redesign, it is the magnitude of the UCNC radiant flux for a given set of excitation conditions that determines the application-driven utility of a given UCNC composition. For instance, if a material has a high emission quantum yield but a low absorptive cross-section, it may generate less light (total radiant flux) in a particular experiment than another material with a lower quantum yield and a higher absorptive cross-section; this fact highlights the


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need for quantitative measurements of both the radiant flux and quantum yield. Measurements of absolute quantum yields for micron and larger sized, bulk-phase, upconverting phosphors were first performed by Auzel and Pecile; these investigators employed an integrating sphere to compare quantum yield values in fluoride- and tungstate-based crystals as a function of the nature of the lanthanide ion dopant and concentration.35, 36 Related pioneering work by Page and coworkers examined power-dependent absolute quantum yield values of multiple fluoride- and oxide-based bulk upconversion materials in the solid state using a power meter and optical filters interfaced to an integrating sphere; this work, for example, demonstrated a maximal absolute upconversion quantum yield for bulk [NaYF4; Yb (18%), Er (2%)] phosphors at approximately 4% for laser excitation power densities that ranged from 20 - 40 W/cm2.37 More recently, Faulkner and Ozin reported absolute quantum yield measurements of NaYF4-based solid-state upconversion materials; these investigators demonstrated absolute quantum yields that ranged up to 13% for the largest-sized (> 4 m) bulk samples interrogated.38 While these previous studies report absolute quantum yield measurements for bulk upconverting materials, few attempts have been made to quantify the absolute quantum yield of homogeneous UCNCs in solution;39 furthermore, no studies performed to date have determined the wavelength-dependent total radiant fluxes of UCNCs of uniform size in solution as a function of laser power. UCNC applications that involve the biological milieu (e.g., imaging, emission fingerprinting, tracking) will undoubtedly require dispersed, individualized, homogeneous nanocrystals with radiant fluxes that optimize detection sensitivity and image resolution. For such studies, data obtained from solid-state quantum yield measurements that: (i) do not account for a density-dependent absorptive cross section, (ii) do not account for the number of particles excited, (iii) do not assure a homogeneous excitation power-density for all


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nanocrystals that contribute to emission, and (iv) do not examine materials having uniform surfaces, are of little value. Boyer and van Veggel were the first to investigate the size-dependent absolute quantum yields of homogeneous, solution-phase [NaYF4; Yb (20%), Er (2%)] upconversion nanocrystals over the visible (500 – 700 nm) spectral domain.39 These investigators demonstrated a 100 nmsized [NaYF4; Yb (20%), Er (2%)] UCNC composition that evinced a visible-spectral domain emission quantum yield of 0.30% ± 0.10%, while the corresponding 30- and 10-nm-sized compositions featured respective visible-spectral domain emission quantum yields of 0.10% ± 0.05% and 0.005% ± 0.005% for a 150 W/cm2 excitation intensity at 980 nm.39 For comparative purposes, Boyer and van Veggel also examined a micron-sized, bulk phase [NaYF4; Yb (20%), Er (2%)] sample; the visible spectral domain emission quantum yield was determined to be 3.0% ± 0.3% for a 20 W/cm2 excitation intensity at 980 nm, similar to the 4% value determined by Page and coworkers under similar excitation conditions.37 One recent study using a 1532 nm upconversion excitation wavelength determined upconversion emission quantum yields of coreshell [NaYF4; Er (28%)] UCNCs for solar cell applications;40 in contrast to earlier work, this contribution emphasized that the external quantum yield efficiency, a unitless quantity defined as the percent of emitted photons to excitation photons (similar to radiant flux measurements), is a more appropriate UCNC figure of merit for applications and devices, underscoring that total radiant flux values provide the best metric for comparing the performance of UCNC materials. While these studies provide approximate assessments of visible spectral domain emission quantum yields for a few exemplary UCNC compositions determined at select excitation conditions, the absence of: (i) direct, wavelength-dependent, emissive radiant flux value determinations, (ii) bulk phase upconversion sample measurements on materials that could define community-recognized quantum yield standard emitters, and (iii) experiments that specify the


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excitation power dependences of the quantum yield and radiant flux for both fs-pulsed and CW laser light sources, underscore the need for experimental approaches that rigorously peg UCNC photophysical properties under experimental conditions relevant to the manner in which these compositions are generally exploited – dilute, individualized, as well as in the condensed phase. Presented herein are power-dependent total radiant

flux

and

absolute

quantum

yield

measurements determined over a 500 – 700 nm spectral domain for a homogeneous, solution-phase Figure 1. (A) TEM image of the [NaYF4: Yb (15%), Er (2%)] nanocrystals, and (B) the corresponding XRD trace confirming formation of the hexagonal phase.

upconversion nanocrystal composition, evaluated using both 970 nm CW and 976 nm fs-pulsed Ti-

sapphire laser excitation (140 fs pulse-width, 80 MHz). This work establishes that the radiant flux, not the emission quantum yield, is the most appropriate and most accurately measurable figure of merit that is relevant to the applications in which UCNCs are typically exploited. Further, this study illustrates the utility of a novel, multi-detector integrating sphere system to reliably determine the UCNC total radiant flux and absolute quantum yield.

Hexagonal-phase (beta) [NaYF4: Yb (15%), Er (2%)] upconversion nanocrystals were synthesized using a method similar to that reported by Qian et al.2 TEM imaging indicated that these UCNCs were single crystals with a diameter of 28 ± 2 nm (Figure 1A), and XRD analysis confirmed the particles were synthesized in the hexagonal phase (Figure 1B). Inductively


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coupled plasma atomic emission spectroscopy (ICP-AES) was used to provide an as-synthesized compositional analysis; these data demonstrated a [NaYF4; Yb (15%), Er (2%)] composition derived from a reaction stoichiometry appropriate for the synthesis of [NaYF4; Yb (20%), Er (2%)], illustrating the necessity of ICP-AES analysis for UCNC characterization to correctly assess composition-property relationships of such materials. Hexagonal-phase [NaYF4; Yb (15%), Er (2%)] UCNCs were dispersed in toluene at 1 mg/mL for these benchmark measurements. The Yb-Er ion pair provides two distinct emission bands centered at 540 and 660 nm; these bands arise from the Er3+-based 2H11/2/4S3/2 4I15/2 and 4

F9/2 4I15/2 transitions, respectively. Solid-state emission lifetime measurements (Supporting

Information) conclusively show that the 2H11/2 (525 nm) and 4S3/2 (545 nm) states display matching 62 ± 2 µs rise and 302 ± 3 µs decay single-exponential time constants, thus underscoring the suitability of combining 2H11/2 4I15/2 and 4S3/2 4I15/2 emission data (denoted 2

H11/2/4S3/2 4I15/2) in computations that determine quantum yield and radiant flux for these

UCNCs over the 510-565 nm spectral domain. A commercially available upconversion material, [NaYF4; Yb (20%), Er (3%)] (Sigma Aldrich; average size (d50) reported as 1 micron) was acquired, and utilized to provide a readily reproducible solid-state sample (Supporting Information). A quantum yield and radiant flux measurement system, co-designed with Labsphere Inc. (North Sutton, New Hampshire), was engineered to take advantage of the de Mello-WhitmannFriend three-position quantum yield methodology41 in order to accurately account for direct absorption and emission, as well as indirect re-absorption and re-emission processes that occur within the integrating sphere. In brief, this absolute quantum yield and radiant flux determination method makes use of three separate measurement positions within an integrating sphere. In this system, excitation and emissive spectral responses are sequentially measured: (i) the excitation


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source spectrum (no sample present), (ii) the sample emissive spectral response derived from indirect excitation, and (iii) sample emissive spectral response that results from direct excitation (Figure S1; see Supporting Information for further detail). The original de Mello-WhitmannFriend measurement technique utilized a CCD-based spectrometer that enabled measurement of the excitation peak changes from the blank and the sample (absolute absorption) and the spectral emission of the sample (absolute emission) in counts per second. Quantum yield values were calculated from measurements of the integrated emission signals and the integrated extent of sample light absorption; this approach circumvented the need to quantify the photon input (excitation) or output (emission) flux in Watts. Using the de Mello-Whitmann-Friend approach, however, to determine the very small radiant fluxes and absolute quantum yields of UCNCs, requires significant experimental redesign of the integrating sphere system. A critical consideration in this regard is the fact that highfluence NIR excitation is required to generate measureable yields of upconverted emitted photons: such an excitation source would saturate a CCD-detector before any upconverted emission intensity could be detected. Because the input laser intensity greatly exceeds the intensity of UCNC emitted light, the integrating sphere-based quantum yield and radiant flux measurement system we developed for upconversion phosphors incorporates a germanium power meter: this device measures the incident excitation flux in Watts with much higher dynamic range than a CCD spectrometer-based spectral integration (this is especially important when using laser excitation that has a spectral density much higher than a broadband excitation lamp). The incident excitation flux measured in the different sampling positions defines the corrected absorption value, Acorrected, a unitless quantity that compensates for directly excited absorption and indirectly excited absorption properties of the sample within the sphere (Supporting Information). It is important to underscore that even with the capabilities afforded by an ultra-


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sensitive, high-dynamic range power meter, experimental uncertainties in this absorption value measurement constitute the largest source of experimental error in UCNC absolute quantum yield determinations, as low-concentrations of UCNCs in solution absorb so few photons relative to the high incident laser powers utilized in these experiments (Supporting Information). To address this issue, we also determine the total radiant flux using an integrating sphere system engineered with an internally mounted National Institute of Standards and Technology (NIST)calibrated lamp that spectrally standardizes the CCD-spectrometer signal from arbitrary intensity counts to units of Watts per wavelength (W/nm) over the entire spectral regime. Since the CCDspectrometer can now be calibrated to measure spectral emission in Watts/nm, the total radiant flux can be calculated straightforwardly from the integrated emission flux per unit volume derived from direct and indirect excitation without need of an Acorrected-value, providing much higher measurement accuracy. The total accuracy for quantum yield measurements with integrating spheres, taking into account the errors related to component instabilities, has been estimated for a very similar system to be approximately 7% (assuming the Acorrected value would be well known for organic chromophores in solution).42 As detailed below, considerable error is associated with the measurements of Acorrected, especially under fs-pulsed excitation for upconverting nanoparticles. On the other hand, since the radiant flux values do not rely on the error-prone Acorrected-value, their accuracies will be bounded by the much smaller instabilities associated with the components of the multi-detector integrating sphere spectroscopy system. In order to provide an experimental benchmark for power-dependent total radiant flux and absolute quantum yield measurements determined using this novel integrating sphere system designed for characterizing upconversion materials, initial experiments examined a commercially available bulk-phase upconversion material, [NaYF4; Yb (20%), Er (3%) (Sigma Aldrich)]. To make a reproducible bulk standard sample, a machine-press was used to press the commercial


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[NaYF4; Yb (20%), Er (3%)] material (powder sample) at 1000 psi at a powder density of 1 mg/mm3; a square piece of this material (8x8 mm2) was cut using a razor blade and mounted on the inside of a 1 cm cuvette (Figure S2). A 970 nm CW laser with a peak excitation density of 21 W/cm2 was utilized as the excitation source; this laser power density is readily attainable with most small diode lasers, and is comparable to the laser power densities utilized in previous experiments that reported quantum yields of bulk upconverting phosphors.37,

39

Three separate

samples were measured under these conditions; an absolute quantum yield of 6.9% ± 0.5% (Table S1) was determined integrating over the 500–700 nm emissive spectral domain. The error in this measurement reflects the uncertainty in the absorption value, Acorrected, which is deduced from a propagation of errors analysis based on the average and standard deviation of the A-value measurements from doped [NaYF4; Yb, Er] and undoped [NaYF4] (blank) samples in the integrating sphere. Note that the emission quantum yield value determined using this integrating sphere system lies within a range of prior literature values that determine absolute quantum yields spanning from approximately 3% to 13% for closely related bulk [NaYF4; Yb, Er] compositions.37-39 However, any more detailed comparisons between this quantum yield measurement and those reported earlier are not appropriate, as some of the literature sample preparations and sample excitation conditions are not precisely defined. The visible spectral domain total radiant flux for this bulk phase [NaYF4; Yb (20%), Er (3%)] sample preparation was measured to be 0.0072 ± 0.0005 W for an average peak excitation density of 21 W/cm2. Figure 2 highlights the emission spectral response of a 1 mg/mL solution of 28 nm diameter [NaYF4; Yb (15%), Er (2%)] upconversion nanocrystals in toluene under 970 nm CW excitation at 181 W/cm2 (Figure 2A) and 976 nm fs-pulsed Ti-sapphire excitation at 187 W/cm2 conditions (Figure 2B). Figure 2A-B display emission spectra acquired using direct (solid line) and indirect (dashed line) laser excitation. As emission lifetime measurements demonstrate that


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the 2H11/2 (525 nm) and 4S3/2 (545 nm) states feature matching single-exponential 62 ± 2 µs rise and 302 ± 3 µs decay time constants (Figure S3), the up-conversion mechanisms that give rise to the population of these excited states are mediated by identical or indistinguishable pathways; computations that determine quantum yield and radiant flux for these UCNCs over the 500–700 nm spectral domain thus combine the data acquired for the 2H11/2/4S3/2 4I15/2 and 4F9/2 4I15/2 emissive processes. Under CW irradiation, the magnitude of the emission intensity that derives from indirect sample excitation (reflected laser light that passed through the sample) was determined to be 3.6% of the value obtained via direct sample excitation, whereas samples excited using the fs-pulsed laser source manifested virtually no detectable emission intensity that derived from indirect excitation. The reason for this difference is instrumental: the available CW power (12 W) is substantially higher than that for the pulsed source (0.8 W), and to achieve similar intensities for direct excitation the beam radius at the sample was reduced for the fs laser (0.0376 cm) compared to the CW laser (0.186 cm). For indirect excitation, the light hitting the sample is no longer focused, and the intensity depends on the total power only. Since the radiant flux scales approximately quadratically with intensity (see below), very little radiant flux is expected for indirect as compared to direct-pulsed excitation.


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These data confirm that under CW excitation, the upconverted emission intensity recorded in a direct excitation also contains contributions from indirect excitation within the sphere that should be accounted for in quantitative measurements. The absorbance value, Acorrected (Supporting Information), is plotted in Figure 2C as a function of incident power; these data indicate that UCNCs under CW excitation absorb, on average, approximately 6 times more photons than the same sample under fs-pulsed excitation conditions at comparable excitation intensities in Figure 2. Upconversion emission spectra acquired for both direct (Lc) and indirect (Lb) laser excitation of a 1 mg/mL solution of 28 nm diameter [NaYF4; Yb (15%), Er (2%)] nanocrystals in toluene. (A) CW light source with a 181 W/cm2 laser peak power at 970 nm; (B) fs-pulsed light source with a 187 W/cm2 laser peak power at 976 nm (140 fs pulse-width, 80 MHz). (C) The absorbance value, Acorrected, plotted as a function of laser excitation type and power.

the range of 35 - 225 W/cm2, a point that factors into the emission quantum yield calculations reported herein. Note however, that the inherent error in the reported absorption value measured using this system is substantial (Figure 2C); this uncertainty in Acorrected subsequently gives rise to

the large errors of the absolute quantum yield value. These Figure 2C data highlight the fact that due to the combination of low UCNC absorptive oscillator strengths and instrumental measurement errors, even the most exacting UCNC absolute emission quantum yield measurements, such as the type described here, will likely feature substantial inherent measurement error, especially when measured in dilute solutions; this fact has also been


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recognized in earlier work with corresponding large absolute quantum yield error percentages on nanoscale upconverters in solution.39 The ability to directly measure the total radiant flux in this integrating sphere system that standardizes the CCD-spectrometer signal from arbitrary intensity counts to units of Watts per wavelength interval (W/nm) over the entire spectral regime relevant to the experiment, sets these measurements apart from previous studies that examined the relative emissive performance of UCNCs. As this novel integrating sphere system enables the total radiant flux to be calculated by subtracting the indirectly excited integrated emission flux from that generated via direct excitation, accurate quantitative emission flux measurements are made possible for samples that possess small Acorrected-values (such as UCNC samples under dilute conditions). This method enables accurate and precise quantitative determinations of light output on a per UCNC basis and as a function of composition and preparation conditions (Figure 3).


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Figure 3. The absolute emission quantum yield and sample-volume-corrected total radiant flux measured under 970 nm CW and 976 nm fs-pulsed laser (140 fs pulse-width, 80 MHz) excitation of 1 mg/mL solutions of 28 nm [NaYF4; Yb (15%), Er (2%)] upconversion nanocrystals. (A, B) 2 H11/2/4S3/2 4I15/2 luminescence integrated over the 510 - 565 nm spectral domain; (C, D) 4F9/2 4I15/2 luminescence integrated over the 645 - 680 nm spectral domain; and (E, F) the averaged quantum yield and total radiant flux for three sample measurements calculated for the entire visible spectral domain (510 - 565, 645 - 680 nm). Panels B and D have been fit with power law functions to highlight the number of absorbed photons responsible for each transition.

Figure 3 presents laser power dependences of the measured total radiant flux and absolute quantum yield determined using CW and fs-pulsed excitation for 1 mg/mL solutions of 28 nm [NaYF4; Yb (15%), Er (2%)] upconversion nanocrystals; Figure 3A-B highlights data


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acquired for the 2H11/2/4S3/2 4I15/2 emissive band (510 - 565 nm), while Figure 3C-D shows analogous measurements for the 4F9/2 4I15/2 emissive manifold (645 - 680 nm). Figure 3E-F sums the data acquired for the 2H11/2/4S3/2 4I15/2 and 4F9/2 4I15/2 emissive manifolds, and displays the averaged absolute quantum yield and total radiant flux measured for the entire visible spectral domain (510 – 565, 645 – 680 nm). Figure 3B,D data have been fit with power law functions to highlight the number of photons responsible for each transition. While the emission quantum yield is a unitless ratio, the total radiant flux per volume have been calculated at the center of the Gaussian shaped laser beam, taking into account the experimentally measured beam size and a quadratic non-linear emission dependence (Supporting Information). Under comparable excitation densities of 181 W/cm2 for CW excitation and 187 W/cm2 for fs-pulsed excitation, the CW excitation produced a radiant flux of 0.012 W/cm3 with a quantum yield of 0.11%, while fs-pulsed excitation produced a radiant flux of 0.0082 W/cm3 with a quantum yield of 0.49%, for the combined 2H11/2/4S3/2 4I15/2 and 4F9/2 4I15/2 transitions (Figure 3E,F). Figure 3A,C shows that as incident laser power increases, the 2H11/2/4S3/2 4I15/2 and 4F9/2 4I15/2 quantum yields increases with increasing laser power over the range of laser powers examined in these experiments. This can be explained through a higher density of photons giving higher probability for the upconversion process to occur. It is important to emphasize, however, that due to uncertainties in the measured A-values, quantum yield errors are correspondingly large (i.e., on the same order of the reported values), thus making the quantum yield values determined under CW and fs-pulsed excitation nearly indistinguishable. Because determinations of the directly measured radiant flux do not depend on an Acorrected-value measurement, this metric can be determined with a much higher degree of experimental certainty than the emission absolute quantum yield; hence, the finding that higher radiant flux values are


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manifested for CW relative to pulsed excitation over these laser power density ranges is worthy of note. These data indicate, that at least for this UCNC composition, that it is the temporally averaged excitation intensity, and not the temporal peak intensity of the 140 fs pulses, that determines the magnitude of the radiant flux; this is congruent with the fact that the repetition time (12.5 ns) of an 80 MHz laser is small relative to the microsecond rise and decay kinetics of UCNC electronically excited states, effectively averaging over many individual fs pulses. While visible spectral domain radiant flux increases with increasing laser power, Figure 3B,D data show deviation from power law behavior that highlight the number of photons responsible for each transition at larger laser powers (approximately >200 W/cm2 for CW, and >300 W/cm2 for fs-pulsed excitation). However, as similar power-law fitting values of 1.7 and 2.1 are obtained under both CW and fs-pulsed excitation for the respective 2H11/2/4S3/2 4I15/2 and 4F9/2 4I15/2 transitions (Figure 3B,D), these data suggest no apparent alteration of the upconversion mechanisms that give rise to the population of the 2H11/2/4S3/2 and 4F9/2 states that is laser excitation source dependent. These results signify that over the excitation power ranges used in this study, fs-pulsed laser excitation does not induce additional linear or non-linear effects that derive from high peak power pulsed lasers that are resonant with the Yb3+ 2F5/2 excited state (976 nm). While it has been noted that fs-pulsed sources, focused down by an objective to a spot size on the order of a few microns may augment UCNC radiant flux due to enhanced probability of resonantly enhanced instantaneous two-photon absorption via the Yb3+ 2F5/2,43 no such effects were observed for these 28 nm diameter [NaYF4; Yb (15%), Er (2%)] UCNCs interrogated in this study. Because Figure 3B,D,F data demonstrate that greater radiant fluxes are manifested for CW relative to fs-pulsed excitation at identical laser intensities, and CW laser light sources are inexpensive relative to their pulsed counterparts, these results highlight the potential of CW light


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sources for UCNC-based in vitro biological imaging,12, 14-25 cellular tracking,11, 26 and photodynamic therapy,27-30 as well as in vivo,11, 14 and whole body12, 15, 19, 20, 25, 34 imaging experiments. In conclusion, the power-dependent total radiant fluxes and absolute quantum yields of a homogeneous, hexagonal-phase [NaYF4; Yb (15%), Er (2%)] upconversion nanocrystal (UCNC) composition in toluene solution were measured for the first time under both 970 nm CW and 976 nm fs-pulsed Ti-Sapphire laser excitation. This study illustrates that at intensities in the range of 35 - 225 W/cm2, the total radiant flux is higher under CW excitation by an average factor of 1.5, and for this range of laser powers, the high peak intensities associated with fs-pulsed excitation conditions do not drive further augmentation of the radiant flux magnitude. This work further highlights that despite the use of current state-of-the-art technology to determine UCNC emission quantum yields, such measurements possess a substantial inherent error for both CW and fspulsed excitation conditions, that derives from determination of the corrected absorption value (Acorrected) a unitless quantity that compensates for directly excited absorption and indirectly excited absorption properties of the sample within the integrating sphere. Thus, even when excitation sources are well characterized, laser-power meters are employed to measure absorbed and scattered light, and the extent of indirect excitation is taken in to account, the error associated with UCNC emission quantum yield measurements is of similar magnitude to the quantum yield value determined. This limitation derives from the fact that experimental uncertainties in UCNC absorption value measurements, ACorrected, constitute the largest source of experimental error in an absolute quantum yield determination, as low-concentrations of UCNCs in solution absorb so few photons relative to the high incident laser powers utilized in these experiments. The total radiant flux reflects the amount light (in units of Watts) a specific composition is capable of producing under a given set of experimental conditions, and thus determines


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detection sensitivity and resolution important for imaging, fingerprinting, tracking, and energy conversion applications: the fact that this parameter may be measured with high accuracy indicates the total radiant flux, not the emission quantum yield, as the appropriate figure of merit for emissive nanoscale upconverter compositions. Finally, this work measures a commercially available bulk upconversion material, [NaYF4; Yb (20%), Er (3%)], using a detailed preparation that could be used as an upconversion emitting standard.

Supporting Information. Nanocrystal synthesis, characterization instrumentation, experimental design of the absolute quantum yield and radiant flux measurement system, and upconversion lifetime data and analysis. Acknowledgement. This work was supported by the Department of Homeland Security, Domestic Nuclear Detection Office - Academic Research Initiative (NSF-ECCS-11-40037), and Immunolight, LLC. INS is grateful to Duke University for Paul M. Gross and Kathleen Zielek Research Fellowships. The authors thank Dr. Sam Johnson (former Director, Light Microscopy Core Facility, Duke University) for experimental assistance, and Kim Hutchison (Lab Manager, North Carolina State Environmental and Agricultural Testing Service) for ICP-AES analysis.

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Abstract Graphic:


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Power-Dependent Radiant Flux and Absolute Quantum Yields of Up

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