Thin Layer of Semiconductor Plasmonic Nanocrystals for the

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Thin Layer of Semiconductor Plasmonic Nanocrystals for the Enhancement of NIR Fluorophores Aleksandr P. Litvin, Sergei A Cherevkov, Aliaksei Dubavik, Anton A. Babaev, Peter S. Parfenov, Ana Luisa Simões Gamboa, Anatoly V. Fedorov, and Alexander V. Baranov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06059 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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The Journal of Physical Chemistry

Thin Layer of Semiconductor Plasmonic Nanocrystals for the Enhancement of NIR Fluorophores Aleksandr P. Litvin*, Sergei A. Cherevkov, Aliaksei Dubavik, Anton A. Babaev, Peter S. Parfenov, Ana L. Simões Gamboa, Anatoly V. Fedorov, and Alexander V. Baranov Department of Optical Physics and Modern Natural Science, ITMO University, 49 Kronverksky Pr., St. Petersburg, 197101, Russia

ABSTRACT. Semiconductor plasmonic nanocrystals (PNCs) are a novel class of materials for near-infrared (NIR) plasmonics possessing strong and tunable localized surface plasmon resonances (LSPR). In this work we used PNCs to fabricate an active substrate for the enhancement of optical responses from near-infrared emitters: a thin film of PNCs in polyvinyl alcohol (PVA). This film supports LSPR and can be utilized to enhance optical absorption, emission, and scattering in the NIR spectral region. PbS quantum dots (QDs) deposited onto the fabricated active substrate demonstrate a three-fold amplification of the integrated photoluminescence (PL) intensity. The possible mechanisms leading to the change of the PL parameters are discussed.

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INTRODUCTION Semiconductor nanocrystals with plasmonic properties (plasmonic nanocrystals, PNCs) have been discovered in recent years1–3. Plasmon resonance is the phenomenon of collective oscillations of charge carriers in solids. Semiconductor PNCs are characterized by localized surface plasmon resonances (LSPR), which are caused by collective oscillations of excess charge carriers (impurity holes or electrons) and manifest themselves as intense absorption bands in the near-infrared (NIR) region of the spectrum. This is the most significant difference between PNCs and metal nanoparticles: for the latter, the LSPR lies in the visible region of the spectrum. Another distinctive feature of semiconductor PNCs is the possibility of easy and reversible tuning of the LSPR frequency4. Nanoparticles possessing plasmon properties are of great practical importance because optical responses of luminophores and photosensitive nanoobjects in their immediate vicinity can be enhanced due to the near-field effect. This effect is most known through Surface-enhanced Raman spectroscopy technology (giant Raman scattering) and provides a more than 1010 signal amplification factor5. Copper chalcogenide PNCs are known to be heavily p-doped plasmonic materials with LSPR in the NIR range and have been widely discussed in several reviews2,6,7. There is a wide range of synthesis protocols for the fabrication of PNCs of different shape, size, and composition8–13 which do not require specific equipment and high temperature. The optical properties of PNCs depend strongly on their self-doping level. Krigel and co-authors14 demonstrated that copper chalcogenide PNCs are prone to turn into nonstoichiometric copperdeficit phases at ambient conditions. Copper ions easily leave the chalcogen sublattice and form a chemically active Cu2+ ion shell near the PNCs surface. Besides this fast and reversible process, slow morphing processes of the chalcogen sublattice take place through various stable


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and metastable phases from Cu2E to CuE (where E stands for S, Se, or Te). This feature is a key factor for understanding the properties of PNCs and broadening the range of possible applications. Weak binding of copper ions is very suitable for catalytic applications, cation exchange reactions15, and active chemical14,16 and electrochemical LSPR band control 17. On the other hand, the high chemical reactivity of the copper ions in the PNCs has to be kept in mind for plasmonic applications: the working capacity of the PNCs has to be checked when they are used in a thin layer at ambient conditions. In some cases, the use of polymer matrices can eliminate this problem. Thin layers of plasmonic nanoparticles have been widely used for the enhancement of organic and inorganic fluorophores in the visible range. Since the demonstration of a 5-fold enhancement of the PL of CdSe/ZnS QDs on gold colloids18, many efforts have been made to optimize the plasmon layer deposition, the spatial separation of the particles, and spectral overlap19,20. Ag and Au nanoparticles of different shapes, such as nanospheres21,22, nanoneedles23, nanoprisms24 or nanocubes25, have been studied. Spatial separation between plasmonic and emitting nanoparticles, which is necessary for a balance between PL enhancement and nonradiative quenching, has been often controlled by embedding the nanoparticles into polymers, such as PMMA21,25,26. It has been recently shown that semiconductor PNCs can also enhance Raman scattering 27, upconversion photoluminescence28 and NIR emission from PbS QDs

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. However, the use of

semiconductor PNCs for creating active substrates which enhance the optical responses in the NIR region from species placed in their immediate vicinity has not been reported so far. In this work we have fabricated an active NIR substrate, using a thin layer of Cu2-xSe PNCs to enhance


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optical responses. We have demonstrated the performance of this substrate by the enhancement of NIR PL from PbS QDs deposited onto its surface.

MATERIALS & EXPERIMENTAL SYNTHESIS OF NANOCRYSTALS The chemicals in this work were purchased from Sigma-Aldrich, Acros, and Fisher and used as received. To produce semiconductor PNCs based on copper chalcogenides with plasmon resonances, high-temperature reactions between the precursors of chalcogenides and copperacetylacetonate in oleylamine and dodecanthiol were used following the method by Lesnyak and co-authors.11 We obtained Cu2-xSe PNCs with a diameter of 3.7 ± 1 nm, as determined by TEM. A typical TEM image and the corresponding PNCs size distribution obtained are shown in Figure 1a and Figure 1b, respectively. The plasmon resonance band of the PNCs lies in the NIR region of the spectrum and is centered at a wavelength of about 1150 nm, as shown in Figure 1c. The synthesis of PbS QDs was carried out according to a previously described procedure.30 Briefly, 1 mmol PbO (99.99%, Aldrich) + 4 mmol oleic acid (90%, Fisher) + 10 mL octadecene (90%, Acros) were mixed in a three-neck 25 mL round-bottom flask equipped with a condenser, thermocouple and septum. The final mixture was heated up to 170 °С under vacuum for approximately 30 minutes till the formation of a clear solution and flushed with Ar. At 134 °C a solution of 0.2 mmol hexamethyldisilathiane in 0.5 mL octadecene was injected swiftly and the reaction misture was heated for 10 min. The reaction was quenched by cooling in Ar atmosphere and the addition of acetone to precipitate the PbS nanocrystals. The solution was centrifuged


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(6000 rpm, 10 min), redispersed in hexane (Sigma-Aldrich) and precipitated 2 more times with acetone. Finally, the nanocrystals were redissolved in tetrachloromethane. The synthesized QDs have a diameter of 4.4±0.3 nm and possess optical transitions in the NIR, as shown in Figure 1d. It can be seen that the PL band and the first interband transition in the QD absorption spectrum are in resonance with the plasmon resonance band of Cu2-xSe PNCs.

Figure 1. TEM image (a), size distribution obtained by TEM (b), and absorption spectra in colloidal solution (c) of Cu2-xSe PNCs; Absorption (black) and PL (red) spectra of PbS QDs, which are in resonance with the LSPR in Cu2-xSe PNCs (d).

HYBRID COMPOSITE FOR THE FABRICATION OF ACTIVE SUBSTRATES


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A hybrid composite for the fabrication of the active substrates was prepared using Cu2-xSe PNCs and polyvinyl alcohol polymer (PVA). PVA is suitable for the fabrication of high-quality thin films. These films are insoluble in organic solvents, which are a typical medium for various IR luminophores (QDs, dyes). However, PVA cannot be directly used as a matrix for the incorporation of Cu2-xSe PNCs covered with hydrophobic ligands (oleylamine and dodecanthiol in our case). Instead of phase-transfer of PNCs, we used a pair of mixed solvents of different polarity31: dimethylformamide (polar) acts as a solvent for PVA, while toluene (nonpolar) acts as a solvent for hydrophobic Cu2-xSe PNCs. 1 ml of dimethylformamide (DMF) and 10 mg of PVA were placed in a glass vial and heated up to 130 °C with continuous stirring. After dissolving the polymer, the temperature was lowered down to 75 °C and 1 ml of toluene and 100 µl of the Cu2xSe

PNCs stock solution (0.6 mg/mL) was added to obtain 0.6 wt. % PNCs in PVA. The

procedure was carried out in Ar atmosphere.

FABRICATION OF THIN FILMS The overall process of the sample fabrication is schematically shown in Figure 2. A reference sample was fabricated in a similar way, but without the addition of Cu2-xSe into the DMF/toluene solution of PVA.


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Figure 2. Scheme illustrating the steps involved in the fabrication of the samples of thin films investigated in this work.

The glass substrates were washed with acetone and isopropanol in an ultrasonic bath, followed by plasma treatment during 20 min. 80 µl of a hot (75 °C) solution of the active composite was applied to the glass substrate, followed by spin-coating at 3000 rpm. Next, the film was rinsed with toluene and spin-coated at 3000 rpm to wash the uncoated PNCs. The active substrates thus obtained were investigated by atomic force microscopy (AFM) using an NT-MDT Solver Pro-M atomic force microscope in semicontact mode. Figure 3 shows an AFM topography image of the obtained active layer. The thin film is composed of well-separated PNCs coated by a thin PVA layer.


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Figure 3. AFM image (a) and the profile (b) of a thin layer of Cu2-xSe PNCs – PVA composite deposited onto a glass substrate.

The small thickness of the film leads to extremely weak features of the LSPR in the absorption spectra of the samples. At the same time, the presence of the LSPR in the medium is confirmed by the observation of the corresponding peak in the absorption spectra of a thicker PVA-PNCs film obtained by drop-casting. The absorption spectrum of this film is shown in Figure 4 (black line). The well-defined plasmon band is clearly seen at the same wavelength as for the particles in colloidal solution. The plasmon band undergoes minimal weakening after 10 days of storage at ambient conditions and thermal annealing at 90 ◌֯ C. The thermal annealing at elevated temperatures (higher than 150 ◌֯ C) leads to the weakening and red-shift of the plasmon response, as was earlier ascribed to the appearance of lattice oxygen and decrease of the Cu2+/Cu+ ratio during the annealing28. The inset in Figure 4 shows the absorption spectra of thin films of PbS QDs (blue line), PNCs (red line), and of PbS QDs deposited on top of the PVA-PNCs active


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substrate (black line). The absorption spectrum of the PbS QDs film deposited on top of the PVA-PNCs active substrate shows both excitonic and plasmonic features.

Figure 4. Absorption spectra of the PVA-PNCs film before annealing (black line) and after annealing at different conditions (red and blue lines). The inset shows absorption spectra of PbS QDs (blue line), PNCs (red line) and PbS QDs on PNCs (black line) thin films.

PbS QDs were spin-coated onto the prepared active substrate at 3000 rpm from a 1.5 mg/ml solution in tetrachloromethane. This method allows depositing reproducible thin QDs layers on various surfaces, which was preliminary checked for a number of QDs concentrations and spincoating speeds. The standard deviation in PL intensity from sample to sample was always much lower than the observed PL enhancement.


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RESULTS & DISCUSSION PL SPECTRA The PL spectra for the reference sample and for the QDs on the fabricated active substrate are shown in Figure 5 by red and blue dotted lines, respectively. The PL was excited by 1 mW of ThorLabs M530F2 LED illumination at 530 nm. The PL signal passed through the Acton SP2150i monochromator and was recorded using an avalanche InGaAs/InP single-photon avalanche diode (Micro Photon Devices). A significant enhancement of the PL signal is observed for the QDs deposited onto the active substrate. The enhancement of the integrated PL signal reaches ~3 and is accompanied by the shift of the PL maximum position, which can be explained by several factors and will be considered below.


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Figure 5. PL spectra of the PbS QDs deposited onto the reference (red dotted line) and active (blue dotted line) substrates.

PL DECAY The PL decays were recorded under laser excitation at 1054 nm (in resonance with the plasmon band) and 527 nm (out of resonance with the plasmon band) using a Q-switched YLF:Nd3+ laser (Laser-Export DTL-339QT). The PL decay curves are shown in Figure 6a and Figure 6b. The PL decay for the reference samples can be well fitted by a sum of two exponential functions. When excitation at 1054 nm is used, the PL decay of the PbS QDs deposited onto the active substrate is also described by two exponentials, and the expected decrease of the average PL lifetime is observed. This reduction of PL lifetime is caused by the interaction with the near field of the LSPR in Cu2-xSe PNCs and demonstrates an increase of the relaxation rate of the excited carriers. On the contrary, for the excitation at 527 nm, the average PL decay time significantly increases, which is caused by the appearance of an additional long component in the decay curve.


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Figure 6. PL decay for the reference sample (red lines) and for the PbS QDs on the active substrate (blue lines) at 527 nm (a) and 1054 nm (b) excitation. The black lines are fits to a sum of exponentials.

Consideration of the two fast components only gives well-matched values for the two excitation regimes: average PL lifetimes calculated for the two fast components decrease by a factor of 1.7 for the 527 nm excitation, and 1.6 for the 1054 nm excitation. The calculated components of the PL decay are listed in Table 1.

Table 1. Calculated decay times, τ, for the PL decay of the reference sample and of the PbS QDs on the active substrate at different excitation wavelengths, obtained from the fit to a sum of exponentials.


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Sample

τ1 (ns)

τ2 (ns)

τav (ns)

Reference(a)

181 ± 3

34 ± 1

114 ± 4

Active substrate(a)

114 ± 4

26 ± 1

67 ± 5

Reference(b)

143 ± 5

30 ± 1

68 ± 6

Active substrate(b)

109 ± 7

27 ± 1

43 ± 8

τ3 (ns)

1260 ± 20

(a) 527 nm excitation (b) 1054 nm excitation

MECHANISMS OF THE PL ENHANCEMENT To analyze the mechanisms of QD PL enhancement, we consider the spectral dependence of an enhancement factor. The enhancement factor was calculated as the ratio of the QD PL intensity on the active substrate to the PL intensity of the reference sample. The spectral dependence of the enhancement factor is shown in the bottom panel of Figure 7 by the green solid line. The enhancement factor reaches ~7 at ~1035 nm. The specified wavelength does not coincide neither with the maximum of the PL spectrum of QDs nor with the maximum absorption at the plasmon resonance band (shown by the black solid line). The enhancement of the blue side of the PL spectrum together with the appearance of an additional long-lived decay component at 527 nm excitation may be ascribed to the effective excitation of upper lying trap states. In general, several electronic states participate in the PL from the PbS QDs30,32. PL from electronic states, which lie energetically higher than the 1Se


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level, has been demonstrated for PbS QDs in several papers33,34. Such a state gives a small contribution to the PL signal for the reference sample (a longer decay at 527 nm excitation and asymmetric shape of PL spectrum), but this contribution is greatly enhanced by placing a QD onto the active substrate with Cu2-xSe PNCs. Within the framework of this model30,32, we can fit the enhanced spectrum of the PbS QD PL. The spectrum can be described by a sum of two Gaussian components. The first one should correspond to the fundamental QD PL (the peak position and the FWHM are fixed on the values obtained for the reference sample), and the second one corresponds to PL originating from upper lying trap states (the peak position is fixed at the value which corresponds to the maximum in the enhancement factor spectral dependence). The upper panel of Figure 7 shows that this approach allows fitting the enhanced spectrum. The red and blue dotted lines show the PL spectra of the reference and enhanced samples, respectively, while the dotted black lines show two Gaussian components and the solid black line shows the result of the fitting.


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Figure 7. Upper panel: PL spectrum from the reference sample (red dotted line) and from the QDs on the active substrate (blue dotted line). Two Gaussian components (black dotted lines) were used to fit the enhanced spectrum, and their sum is shown by the black solid line; Bottom panel: the spectral dependence of the enhancement factor is shown by the green solid line (right axis) and the PNCs absorption is shown by the black line (left axis).


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CONCLUSION We have created an active substrate for the enhancement of optical responses in the NIR spectral region based on a thin film of semiconductor Cu2-xSe PNCs in PVA which demonstrates a LSPR centered at ~1150 nm. PbS QDs were deposited onto this thin film, leading to a 3-fold enhancement of their PL. The observed blue shift of PL spectra of the QDs and appearance of a long-lived PL decay component have been ascribed to high amplification of trap-related PL from higher lying states in PbS QDs. We believe that further enhancement of the observed phenomenon is possible by improved control of both the PNCs and QDs concentration and their spatial distribution in the film. Since plasmon-exciton interaction is extremely sensitive to the distance and relative positions between particles, further optimization of these parameters in the Cu2-xSe-PbS system will allow achieving a much higher enhancement factor. At the same time, the development of Cu2-xSe PNCs-based active substrates opens the possibility to enhance secondary emission from NIR-emitting species. In particular, the development of QD / Cu2-xSe PNCs-based NIR-emissive materials can open ways for the creation of nano- and micrometersized sources of light for information, optoelectronic technologies, and biomedical applications.

AUTHOR INFORMATION Corresponding Author * A.P.L. E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All the authors contributed equally.


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Funding Sources Ministry of Education and Science of the Russian Federation (Goszadanie no. 16.8981.2017/8.9)

ACKNOWLEDGMENT The authors thank the Ministry of Education and Science of the Russian Federation (Goszadanie no. 16.8981.2017/8.9) for financial support. A.P.L. thanks the Ministry of Education of the Russian Federation for financial support (Scholarship of the President of the Russian Federation for young scientists and graduate students, SP-70.2018.1). A.D. thanks the Government of the Russian Federation (Grant No. 074-U01) through the ITMO Postdoctoral Fellowship program.

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Figure 1. TEM image (a), size distribution obtained by TEM (b), and absorption spectra in colloidal solution (c) of Cu2-xSe PNCs; Absorption (black) and PL (red) spectra of PbS QDs, which are in resonance with the LSPR in Cu2-xSe PNCs (d). 199x199mm (300 x 300 DPI)


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Figure 2. Scheme illustrating the steps involved in the fabrication of the samples of thin films investigated in this work. 300x110mm (96 x 96 DPI)



Figure 3. AFM image (a) and the profile (b) of a thin layer of Cu2-xSe PNCs – PVA composite deposited onto a glass substrate. 330x139mm (300 x 300 DPI)


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Figure 4. Absorption spectra of the PVA-PNCs film before annealing (black line) and after annealing at different conditions (red and blue lines). The inset shows absorption spectra of PbS QDs (blue line), PNCs (red line) and PbS QDs on PNCs (black line) thin films. 203x201mm (300 x 300 DPI)



Figure 5. PL spectra of the PbS QDs deposited onto the reference (red dotted line) and active (blue dotted line) substrates. 118x117mm (300 x 300 DPI)


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Figure 6. PL decay for the reference sample (red lines) and for the PbS QDs on the active substrate (blue lines) at 527 nm (a) and 1054 nm (b) excitation. The black lines are fits to a sum of exponentials. 199x99mm (300 x 300 DPI)



Figure 7. Upper panel: PL spectrum from the reference sample (red dotted line) and from the QDs on the active substrate (blue dotted line). Two Gaussian components (black dotted lines) were used to fit the enhanced spectrum, and their sum is shown by the black solid line; Bottom panel: the spectral dependence of the enhancement factor is shown by the green solid line (right axis) and the PNCs absorption is shown by the black line (left axis). 203x355mm (300 x 300 DPI)


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215x120mm (300 x 300 DPI)


Thin Layer of Semiconductor Plasmonic Nanocrystals for the

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