Photothermal Response of Photoluminescent ... - ACS Publications

Letter pubs.acs.org/JPCL

Photothermal Response of Photoluminescent Silicon Nanocrystals Sarah Regli, Joel A. Kelly,† Amber M. Shukaliak, and Jonathan G. C. Veinot* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada S Supporting Information *

ABSTRACT: We demonstrate that silicon nanocrystals (SiNCs) exhibiting relatively high near-IR photoluminescent quantum yields also exhibit a notable photothermal (PT) response. The PT effect has been quantified as a function of NC size, defect concentration, and irradiating energy, suggesting that the origin of the PT response is a combination of carrier thermalization and defect-mediated heating. The PT effect observed under NIR irradiation suggests that Si-NCs could find use in combined in vivo PL imaging and PT therapy. SECTION: Physical Processes in Nanomaterials and Nanostructures

S

recombination, we were curious to see if photoluminescent Si-NCs exhibited an appreciable PT response. Given the demonstrated potential of Si-NCs as in vivo imaging agents, an appreciable PT effect in these materials could offer the advantage of combined PL imaging and PT therapy. Knowledge of the thermo-optical properties from Si-NCs is important for optoelectronic applications. Brus has noted the significant enhancement in PL quantum yield (PL QY) for nanostructured indirect band gap semiconductors stems from the suppression of nonradiative recombination pathways (mainly Auger recombination),3,37 suggesting that Si-NCs should exhibit a weaker PT effect than bulk Si. However, typical PL QYs for Si-NCs are in the range of ca. 1−40%, implying that nonradiative pathways are still present if not dominant. Here we demonstrate that Si-NCs with relatively high PL QYs also exhibit significant PT responses. We have investigated the influence of NC size, defect concentration, and irradiation wavelength to understand the origin of the PT effect. These results indicate that carrier thermalization and nonradiative defects both contribute to the PT effect. Previously, our group has reported the synthesis, liberation, and functionalization of Si-NCs derived from the thermal processing of hydrogen silsesquioxane (HSQ).38−40 By heating HSQ to 1100, 1200, and 1300 °C under a reducing atmosphere, Si-NCs ca. 4, 6, and 8 nm in diameter are obtained.38,39 After hydrofluoric acid (HF) etching and subsequent thermal hydrosilylation with 1-dodecene, highresolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy high-angle annular dark field (STEM-HAADF) images indicate the Si-NCs have a high degree of crystallinity and polydispersity ranging from 18 to 41% (Figure 1). These colloidally dispersed Si-NCs exhibit near-IR PL that red-shifts with increasing size from ca. 775 to

ilicon nanocrystals (Si-NCs) have been the subject of intense research interest since the discovery of roomtemperature photoluminescence (PL) from porous silicon.1 Nanostructured Si materials exhibit size-dependent PL arising from the influences of quantum confinement can have a small hydrodynamic radius2−4 and are biocompatible and biodegradable.5−10 In this context, Si-NCs have received significant attention as in vivo imaging agents5,11,12 and for use in a variety of optoelectronic applications.13−15 There is growing interest in applying nanoparticles as therapeutic agents, especially for the treatment of cancer.16 The concept of using heat to treat cancer has evolved through many decades of study.17−19 An outstanding challenge is to employ heat treatment effectively to destroy tumor cells while minimizing damage to surrounding healthy tissue.20 In this regard, the use of nanoparticle agents in photothermal (PT) therapy has received significant attention because they afford localized heating and tissue destruction.21,22 To date, metallic nanostructures have been the most widely studied PT agents because of their strong optical absorption arising from sizetunable surface plasmon resonance.23−26 In particular, Au nanorods (NRs) are particularly popular PT therapy agents due to their enhanced absorption in the NIR spectral region where light maximally penetrates tissue.22,27 However, Au-NRs are typically synthesized using cytotoxic cetyltrimethylammonium bromide, which is difficult to completely remove, and their relatively large size minimizes blood circulation time.27−30 PT effects in semiconductors have been studied since 1961, when Gärtner quantified the temperature change induced by nonradiative carrier recombination for bulk Si and Ge.31 Significant attention has also been spent toward minimizing PT-related energy losses in Si photovoltaics (PVs), where carrier thermalization related to absorption of high-energy photons is a major limiting factor for PV efficiency.32 Recently, the PT effect has been noted experimentally for semiconducting nanostructures including Si nanowires,33 porous Si,34 Ge-NCs,35 and CuSe NCs.36 Because the PT effect in semiconductors is strongly dependent on nonradiative © 2012 American Chemical Society

Received: April 19, 2012 Accepted: June 12, 2012 Published: June 18, 2012 1793

dx.doi.org/10.1021/jz3004766 | J. Phys. Chem. Lett. 2012, 3, 1793−1797

The Journal of Physical Chemistry Letters

Letter

Figure 1. STEM-HAADF images, HRTEM images, and corresponding histogram of NC sizes for Si-NCs formed via thermal processing of HSQ for 1 h at (A) 1100, (B) 1200, and (C) 1300 °C. Measured d spacings of 3.2 Å corresponding to (111) lattice planes.

975 nm, consistent with quantum confinement effects (Figure 2). A significant temperature increase was detected for NC solutions using 1 W of 532 nm irradiation. Figure 3 shows the PT response observed for 4 nm Si-NCs for a range of concentrations in toluene. A temperature increase of ca. 30 °C

is observed after 30 min of irradiation for a solution of 4.3 mg/ mL Si-NCs in toluene, comparable to PT effects of porous Si previously shown to induce 95% cell destruction.34 A pure toluene solution had a negligible increase in temperature during irradiation for 30 min. The PT efficiency of colloidally dispersed nanomaterials may be quantified using an energy balance equation, expressed as (mscp ,s + mccp ,c)

dΔT = Q laser − Q loss dt

(1)

where ms is the mass of the solution, cp,s is the constant-pressure heat capacity of the solution (assumed to be the same as pure toluene, cp,s = 1.125 J·g−1·K−1),41 mc is the mass of the quartz cuvette, cp,c is the constant-pressure heat capacity of quartz (cp,c = 0.839J·g−1·K−1), ΔT is the temperature change of the solution at a specified time interval, Qlaser is the energy impinged on the system as a result of laser irradiation, and Qloss is the energy dissipated.27 Contributions to Qlaser can be represented as Q laser = I(1 − 10−Eλ)η

(2)

where I is the corrected laser power, Eλ is the Si-NC absorbance at the irradiation wavelength, and η is the PT efficiency. The heat dissipated was approximated as Q loss = BΔT + C(ΔT )2

(3)

where B and C are experimentally determined coefficients found for each PT experiment by monitoring the temperature

Figure 2. Photoluminescence from dodecyl functionalized Si-NCs in toluene of different sizes (325 nm excitation). 1794



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Figure 3. Photothermal effect of different concentrations of Si-NCs in toluene (with their respective absorbance (Eλ) at 532 nm, 1 W of power) and schematic of experimental setup.

Figure 4. PT effects of photoluminescent Si-NCs: (a) representative data with corresponding temperature decay fit (using 8 nm Si-NCs in toluene under 514 nm (250 mW) irradiation), (b) calculated PT efficiency for Si-NCs varying in size at 514 nm irradiation, and (c) calculated PT efficiency for 4 nm Si-NCs at varying wavelength (250 mW).

decay after the laser was switched off, making Qlaser equal to zero and reducing the energy balance equation to J

dΔT = −BΔT − C(ΔT )2 dt

changes monitored were relatively small (owing to the Si-NC concentrations and laser power used), the measurements were extremely sensitive to the surroundings. To minimize systematic error, we equilibrated NC solutions to the room temperature, and motion near the experiment that could create convection currents was minimized. Using this quantitative approach, the PT efficiency of 4, 6, and 8 nm Si-NCs under 514 nm irradiation using 250 mW of power was found to increase with NC size (Figure 4), giving PT efficiencies of 58 ± 3, 68 ± 2, and 75 ± 2%, respectively. Lambert and coworkers previously reported Ge-NCs to have a PT efficiency of 20− 47% but used a more qualitative method that did not account for the thermal equilibria between the cuvette and its surroundings.35 Using the same quantitative approach shown here, Chen et al. calculated the PT efficiency of Au NRs to range from 51 to 95%, attributing this variation to the impact of scattering from these larger structures.27 We do not anticipate

(4)

where J is a constant representing the mass and heat capacity parameters. (A detailed derivation for the PT efficiency is shown in the Supporting Information.) The PT efficiencies can be found by considering the steady state of the temperature increase for the colloidal solution η=

BΔTs + C(ΔTs)2 I(1 − 10−Eλ)

(5)

To determine PT efficiencies of Si-NC samples, based on the amount of light absorbed, we tested a range of concentrations, and the resulting efficiencies averaged. Because the temperature 1795



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participate in radiative recombination (such as the siloxene species, SiO, proposed for porous Si by Wolkin et al.) and in doing so reduce the overall PL QY.52,53 In this regard, we are pursuing ongoing PL lifetime and electron paramagnetic resonance spectroscopic measurements to better understand the Si-NC photophysics as a function of annealing time. After extended annealing, the Si-NCs exhibit a PT efficiency of 57 ± 3%. The change in diameter and PL QY arising from prolonged annealing rules out direct comparison of the 1100 °C 1 h and 1100 °C 24 h. It is more appropriate to compare the properties of the 1100 °C 24 h and the 1200 °C 1 h samples (PT efficiency of 68%). The difference in PT efficiency suggests that core defects present in Si-NCs do contribute to PT effects despite their reduced impact on radiative recombination; this is in keeping with the spatial isolation of defects within single SiNCs. In bulk Si, free carriers can travel macroscopic distances before encountering defects (during their microsecond− millisecond lifetime due to the low probability of phononmediated radiative recombination). However, in Si-NCs, nonluminescent defect-ridden Si-NCs contribute to PT effects through defect recombination, whereas well-passivated Si-NCs are isolated from these traps, contributing to PT effects through carrier thermalization before participating in radiative recombination. Finally, we note that the observed temperature change induced under 647 nm irradiation demonstrates that these SiNCs have potential for application within the NIR spectral region required for therapeutics. Although Si-NCs have reduced absorption cross sections (due to their indirect bandgap), compared with metallic structures commonly studied for PT therapy (on the order of 109 M−1 cm−1) recently, Hessel et al. demonstrated that Si-NCs in the 10−20 nm size regime exhibit near-IR absorption cross sections on the order of 105 M−1 cm−1, comparable to direct gap semiconductor NCs such as CdSe.54 Molar extinction plots of a 4 nm Si-NC sample (Supporting Information, Figure S1) showed a comparable molar extinction of 1.48 × 104 M−1 cm−1 at 514 nm. The established biocompatibility of nanostructured Si coupled to their small hydrodynamic radii amenable to increased bloodstream circulation might enable the use of these Si-NCs as PT agents in tandem with their near-IR imaging capabilities.5,55,56 In conclusion, photoluminescent Si-NCs exhibit a measurable PT effect while still maintaining a relatively high PL QY. The PT effect increases with NC size, defect concentration, and irradiating energy, consistent with a combination of carrier cooling and defect-mediated heating processes. The PT effect observed under NIR irradiation suggests that these materials might have potential for combined in vivo PL imaging and PT therapy.

scattering of the irradiating light to impact significantly the PT efficiency of the presently studied Si-NCs due to their significantly smaller dimensions and small hydrodynamic radii.40 As noted above, the present Si-NCs exhibit near-IR PL. Because of a lack of suitable near-IR emissive reference fluorophores, it is often nontrivial to determine the PL QY in this spectral region using a relative approach. However, the use of an integrating sphere allows direct measurement of absolute PL QYs without comparison to a luminescent standard. The absolute PL QY was found to decrease with Si-NC size, ranging from 17.5 ± 2% for ca. 4 nm Si-NCs to 12.7 ± 3% for ca. 8 nm Si-NCs. These values are within the range reported for Si-NCs with near-IR PL determined by an integrating sphere.42−45 It is important to note that Si-NC PL QYs can vary substantially due to several factors including polydispersity, crystallinity, surface modification, and exposure to ambient conditions;3,42,43,46 to minimize these influences, the present samples were prepared and handled identically apart from their varying annealing temperatures. Recall, the PT effect in bulk semiconductors arises mainly from nonradiative recombination of free electrons and holes.31 For bulk Si, the dominant recombination pathways are Auger decay and defect recombination.37 These processes are suppressed in Si-NCs as spatial confinement of excitons effectively prevents their migration to other excitons (leading to Auger decay) or defects. However, surface states in Si-NCs could introduce alternative nonradiative pathways. Also, a PT effect could also be from thermalization of hot carriers under irradiation greater in energy than the bandgap (e.g., phononmediated relaxation of electrons to the conduction band minimum before radiative recombination). It is reasonable to assume that functionalization by thermal hydrosilylation is equally effective at surface passivation for all Si-NC sizes studied here, indicating that the observed increase in PT efficiency is consistent with an increase in bulk-like nonradiative recombination or an increase in carrier thermalization due to the size-related decrease in bandgap. To better understand the contribution of these factors to the PT response of Si-NCs, we performed two sets of experiments. First, the PT response of 4 nm Si-NCs was measured under irradiation of the 488, 514, and 647 nm lines of an Ar laser with 250 mW of power (Figure 4). An increase in PT efficiency is observed with increasing irradiation energy, ranging from 51 ± 2 to 64 ± 3%. This shows that carrier thermalization contributes to PT effects as summarized above. Second, to study contributions from defect concentration, we prepared a Si-NC sample processed at 1100 °C for 24 h under a reducing 5% H2 atmosphere. Numerous reports indicate that annealing Si-NCs under H2 reduces the concentration of core defects (e.g., vacancies or interstitials).47−51 Previously, we reported that X-ray diffraction data suggested that Si-NC size did not change on prolonged annealing at 1100 °C.39 However, in the present sample, HRTEM and STEM-HAADF measurements suggest a larger mean size of 6.6 ± 2.5 nm (Supporting Information, Figure S2). The PL of the 24 h annealed Si-NCs after etching and hydrosilylation was substantially red-shifted from the 1100 °C 1 h sample, with a max at ca. 950 nm (Supporting Information, Figure S3), and the PL QY was 9 ± 2%, less than the 1 h annealed samples. The origin of this reduction is presently unclear; although extended annealing is expected to reduce the concentration of nonradiative core defects, it may also remove other defects that actively

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details. HRTEM, STEM-DAAF, and PL characterization of Si-NCs annealed at 1100 °C for 24 h and derivation of energy balance equation used to quantify PT efficiencies for each sample. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 1796



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Present Address

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†

Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding from the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI), Alberta Science and Research Investment Program (ASRIP), Alberta Innovates: Technology Futures, and University of Alberta Department of Chemistry. A. Meldrum, R. Lockwood, K. Manchee, F. Hegmann, L. Titova, and G. Loppnow are thanked for access and assistance with laser systems. C. Andrei and the Brockhouse Institute for Materials Research are thanked for HRTEM imaging. A. Midgett and M. Beard at the National Renewable Energy Laboratory are thanked for assistance with the absolute quantum yield measurements.

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Photothermal Response of Photoluminescent ... - ACS Publications

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