Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
pubs.acs.org/JPCC
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
J. Phys. Chem. C Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/03/18. For personal use only.
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 (LSPRs). 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 poly(vinyl alcohol). This film supports LSPR and can be utilized to enhance the optical absorption, emission, and scattering in the NIR spectral region. PbS quantum dots deposited onto the fabricated active substrate demonstrate a 3-fold amplification of the integrated photoluminescence (PL) intensity. The possible mechanisms leading to the change in the PL parameters are discussed.
■
INTRODUCTION Semiconductor nanocrystals with plasmonic properties (plasmonic nanocrystals, PNCs) have been discovered in recent years.1−3 Plasmon resonance is the phenomenon of collective oscillations of charge carriers in solids. Semiconductor PNCs are characterized by localized surface plasmon resonances (LSPRs), 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 frequency.4 Nanoparticles possessing plasmon properties are of great practical importance because optical responses of luminophores and photosensitive nano-objects 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 factor.5 Copper chalcogenide PNCs are known to be heavily pdoped plasmonic materials with LSPR in the NIR range and have been widely discussed in several reviews.2,6,7 There is a wide range of synthesis protocols for the fabrication of PNCs of different shape, size, and composition,8−13 which do not require specific equipment and high temperature. The optical properties of PNCs depend strongly on their self-doping level. Kriegel 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 © XXXX American Chemical Society
shell near the PNCs surface. Besides this fast and reversible process, slow morphing processes of the chalcogen sublattice take place through various stable 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 reactions,15 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 in the photoluminescence (PL) of CdSe/ZnS quantum dots (QDs) on gold colloids,18 many efforts have been made to optimize the plasmon layer deposition, the spatial separation of the particles, and the spectral overlap.19,20 Ag and Au nanoparticles of different shapes, such as nanospheres,21,22 nanoneedles,23 nanoprisms,24 or nanocubes,25 have been studied. The spatial separation between plasmonic and emitting nanoparticles, which is necessary for a balance between PL enhancement and nonradiative quenching, has been often controlled by Received: June 25, 2018 Revised: August 9, 2018 Published: August 10, 2018 A
DOI: 10.1021/acs.jpcc.8b06059 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
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).
obtained Cu2−xSe PNCs with a diameter of 3.7 ± 1 nm, as determined by TEM. A typical transmission electron microscopy (TEM) image and the corresponding PNCs size distribution obtained are shown in Figure 1a,b, 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 °C under vacuum for approximately 30 min till the formation of a clear solution and then flushed with Ar. At 134 °C, a solution of 0.2 mmol hexamethyldisilathiane in 0.5 mL octadecene was injected swiftly and the reaction mixture 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 (6000 rpm, 10 min), redispersed in hexane (Sigma-Aldrich), and precipitated two more times with acetone. Finally, the nanocrystals were
embedding the nanoparticles into polymers, such as PMMA.21,25,26 It has been recently shown that semiconductor PNCs can also enhance Raman scattering,27 upconversion photoluminescence,28 and NIR emission from PbS QDs.29 However, the use of semiconductor PNCs for creating active substrates that 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 optical responses. We have demonstrated the performance of this substrate by the enhancement of NIR PL from PbS QDs deposited onto its surface.
■
MATERIALS AND EXPERIMENTAL SECTION 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 copper-acetylacetonate in oleylamine and dodecanthiol were used following the method by Lesnyak and co-authors.11 We B
DOI: 10.1021/acs.jpcc.8b06059 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. Scheme illustrating the steps involved in the fabrication of the samples of thin films investigated in this work.
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 addition of Cu2−xSe into the DMF/toluene solution of PVA. The glass substrates were washed with acetone and isopropanol in an ultrasonic bath, followed by plasma treatment for 20 min. Eighty microliters 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 NTMDT Solver Pro-M atomic force microscope in semicontact mode. Figure 3 shows the AFM topography image of the obtained active layer. The thin film is composed of wellseparated PNCs coated by a thin PVA layer. 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 annealing.28 The inset in Figure 4 shows the absorption spectra of thin films of PbS QDs (blue line),
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. Hybrid Composite for the Fabrication of Active Substrates. A hybrid composite for the fabrication of the active substrates was prepared using Cu2−xSe PNCs and poly(vinyl alcohol) (PVA) polymer, which 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 polarity:31 dimethylformamide (DMF) (polar) acts as a solvent for PVA, whereas toluene (nonpolar) acts as a solvent for hydrophobic Cu2−xSe PNCs. One milliliter of 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 Cu2−xSe PNCs stock solution (0.6 mg/mL) were 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. The reference sample was fabricated in a similar way, but without C
DOI: 10.1021/acs.jpcc.8b06059 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
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 a shift in the PL maximum position, which can be explained by several factors and will be considered below. 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,b. The PL decay for
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 the absorption spectra of PbS QDs (blue line), PNCs (red line), and PbS QDs on PNCs (black line) thin films.
PNCs (red line), and PbS QDs deposited on top of the PVA− PNCs active 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. 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 spin-coating speeds. The standard deviation in PL intensity from sample to sample was always much lower than the observed PL enhancement.
■
RESULTS AND DISCUSSION PL Spectra. The PL spectra for the reference sample and 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 light-emitting diode 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
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) excitations. The black lines are fits to a sum of exponentials.
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 in the average PL lifetime is observed. This reduction in 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. Consideration of the two fast components only gives wellmatched values for the two excitation regimes: average PL
Figure 5. PL spectra of the PbS QDs deposited onto the reference (red dotted line) and active (blue dotted line) substrates. D
DOI: 10.1021/acs.jpcc.8b06059 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C 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 sample referencea active substratea referenceb active substrateb
τ1 (ns) 181 114 143 109
± ± ± ±
3 4 5 7
τ2 (ns) 34 26 30 27
± ± ± ±
1 1 1 1
τav (ns) 114 67 68 43
± ± ± ±
4 5 6 8
τ3 (ns) 1260 ± 20
a
527 nm excitation. b1054 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 either with the maximum of the PL spectrum of QDs or 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 QDs.30,32 PL from electronic states, which lie energetically higher than the 1Se level, has been demonstrated for PbS QDs in several papers.33,34 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 model,30,32 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 full width at half-maximum 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, whereas the dotted black lines show two Gaussian components and the solid black line shows the result of the fitting.
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 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).
QDs and appearance of a long-lived PL decay component have been ascribed to the 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. Because 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 micrometer-sized sources of light for information, optoelectronic technologies, and biomedical applications.
■
■
CONCLUSIONS 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 the PL spectra of the
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Aleksandr P. Litvin: 0000-0001-5261-3210 Sergei A. Cherevkov: 0000-0002-9466-4558 E
DOI: 10.1021/acs.jpcc.8b06059 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
(16) Lee, J.; Yang, J.; Park, C.; Kim, J. H.; Kang, M. S. Electronic Properties of Cu2-XSe Nanocrystal Thin Films Treated with Short Ligand (S2-, SCN-, and Cl-) Solutions. J. Phys. Chem. C 2016, 120, 14899−14905. (17) Ou, W.; Zou, Y.; Wang, K.; Gong, W.; Pei, R.; Chen, L.; Pan, Z.; Fu, D.; Huang, X.; Zhao, Y.; et al. Active Manipulation of NIR Plasmonics: The Case of Cu2-XSe through Electrochemistry. J. Phys. Chem. Lett. 2018, 9, 274−280. (18) Kulakovich, O.; Strekal, N.; Yaroshevich, A.; Maskevich, S.; Gaponenko, S.; Nabiev, I.; Woggon, U.; Artemyev, M. Enhanced Luminescence of CdSe Quantum Dots on Gold Colloids. Nano Lett. 2002, 2, 1449−1452. (19) Ranjan, R.; Esimbekova, E. N.; Kirillova, M. A.; Kratasyuk, V. A. Metal-Enhanced Luminescence: Current Trend and Future Perspectives- A Review. Anal. Chim. Acta 2017, 971, 1−13. (20) Govorov, A. O.; Bryant, G. W.; Zhang, W.; Skeini, T.; Lee, J.; Kotov, N. A.; Slocik, J. M.; Naik, R. R. Exciton−Plasmon Interaction and Hybrid Excitons in Semiconductor−Metal Nanoparticle Assemblies. Nano Lett. 2006, 6, 984−994. (21) Liang, H. Y.; Zhao, H. G.; Li, Z. P.; Harnagea, C.; Ma, D. L. Silver Nanoparticle Film Induced Photoluminescence Enhancement of Near-Infrared Emitting PbS and PbS/CdS Core/Shell Quantum Dots: Observation of Different Enhancement Mechanisms. Nanoscale 2016, 8, 4882−4887. (22) Haridas, M.; Tripathi, L. N.; Basu, J. K. Photoluminescence Enhancement and Quenching in Metal-Semiconductor Quantum Dot Hybrid Arrays. Appl. Phys. Lett. 2011, 98, No. 063305. (23) Ahmed, S. R.; Cha, H.; Park, J.; Park, E. Y.; Lee, D.; Lee, J. Photoluminescence Enhancement of Quantum Dots on Ag Nanoneedles. Nanoscale Res. Lett. 2012, 7, 438. (24) Munechika, K.; Chen, Y.; Tillack, A. F.; Kulkarni, A. P.; Plante, I. J.; Munro, A. M.; Ginger, D. S. Spectral Control of Plasmonic Emission Enhancement from Quantum Dots near Single Silver Nanoprisms. Nano Lett. 2010, 10, 2598−2603. (25) Chen, Y.; Munechika, K.; Jen-La Plante, I.; Munro, A. M.; Skrabalak, S. E.; Xia, Y.; Ginger, D. S. Excitation Enhancement of CdSe Quantum Dots by Single Metal Nanoparticles. Appl. Phys. Lett. 2008, 93, No. 053106. (26) Lu, L.; Xu, X.-l.; Shi, C.-s.; Ming, H. Localized Surface Plasmon Resonance Enhanced Photoluminescence of CdSe QDs in PMMA Matrix on Silver Colloids with Different Shapes. Thin Solid Films 2010, 518, 3250−3254. (27) Li, W.; Zamani, R.; Rivera Gil, P.; Pelaz, B.; Ibáñez, M.; Cadavid, D.; Shavel, A.; Alvarez-Puebla, R. A.; Parak, W. J.; Arbiol, J.; et al. CuTe Nanocrystals: Shape and Size Control, Plasmonic Properties, and Use as SERS Probes and Photothermal Agents. J. Am. Chem. Soc. 2013, 135, 7098−7101. (28) Zhou, D.; Liu, D.; Xu, W.; Yin, Z.; Chen, X.; Zhou, P.; Cui, S.; Chen, Z.; Song, H. Observation of Considerable Upconversion Enhancement Induced by Cu2-XS Plasmon Nanoparticles. ACS Nano 2016, 10, 5169−5179. (29) Litvin, A. P.; Babaev, A. A.; Dubavik, A.; Cherevkov, S. A.; Parfenov, P. S.; Ushakova, E. V.; Baranov, M. A.; Andreeva, O. V.; Purcell-Milton, F.; Gun’ko, Y.; et al. Strong Enhancement of PbS Quantum Dot NIR Emission Using Plasmonic Semiconductor Nanocrystals in Nanoporous Silicate Matrix. Adv. Opt. Mater. 2018, No. 1701055. (30) Ushakova, E. V.; Litvin, A. P.; Parfenov, P. S.; Fedorov, A. V.; Artemyev, M. V.; Prudnikau, A. V.; Rukhlenko, I. D.; Baranov, A. V. Anomalous Size-Dependent Decay of Low-Energy Luminescence from PbS Quantum Dots in Colloidal Solution. ACS Nano 2012, 6, 8913−8921. (31) Lan, X.; Voznyy, O.; García de Arquer, F. P.; Liu, M. M.; Xu, J.; Proppe, A. H.; Walters, G.; Fan, F.; Tan, H.; Liu, M. M.; et al. 10.6% Certified Colloidal Quantum Dot Solar Cells via Solvent-PolarityEngineered Halide Passivation. Nano Lett. 2016, 16, 4630−4634. (32) Litvin, A. P.; Parfenov, P. S.; Ushakova, E. V.; Simões Gamboa, A. L.; Fedorov, A. V.; Baranov, A. V. Size and Temperature
Ana L. Simões Gamboa: 0000-0002-2027-6755 Author Contributions
The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript. All the authors contributed equally. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS 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. 074U01) through the ITMO Postdoctoral Fellowship program.
■
REFERENCES
(1) Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P. Localized Surface Plasmon Resonances Arising from Free Carriers in Doped Quantum Dots. Nat. Mater. 2011, 10, 361−366. (2) Kriegel, I.; Scotognella, F.; Manna, L. Plasmonic Doped Semiconductor Nanocrystals: Properties, Fabrication, Applications and Perspectives. Phys. Rep. 2017, 674, 1−52. (3) Comin, A.; Manna, L. New Materials for Tunable Plasmonic Colloidal Nanocrystals. Chem. Soc. Rev. 2014, 43, 3957−3975. (4) Mocatta, D.; Cohen, G.; Schattner, J.; Millo, O.; Rabani, E.; Banin, U. Heavily Doped Semiconductor Nanocrystal Quantum Dots. Science 2011, 332, 77−81. (5) Le Ru, E. C.; Blackie, E.; Meyer, M.; Etchegoin, P. G. Surface Enhanced Raman Scattering Enhancement Factors: A Comprehensive Study. J. Phys. Chem. C 2007, 111, 13794−13803. (6) Coughlan, C.; Ibáñez, M.; Dobrozhan, O.; Singh, A.; Cabot, A.; Ryan, K. M. Compound Copper Chalcogenide Nanocrystals. Chem. Rev. 2017, 117, 5865−6109. (7) Faucheaux, J. A.; Stanton, A. L. D.; Jain, P. K. Plasmon Resonances of Semiconductor Nanocrystals: Physical Principles and New Opportunities. J. Phys. Chem. Lett. 2014, 5, 976−985. (8) Yu, R.; Ren, T.; Sun, K.; Feng, Z.; Li, G.; Li, C. ShapeControlled Copper Selenide Nanocubes Synthesized by an Electrochemical Crystallization Method. J. Phys. Chem. C 2009, 113, 10833− 10837. (9) Zhang, Y.; Hu, C.; Zheng, C.; Xi, Y.; Wan, B. Synthesis and Thermoelectric Property of Cu 2 − x Se Nanowires. J. Phys. Chem. C 2010, 114, 14849−14853. (10) Liu, Y.; Dong, Q.; Wei, H.; Ning, Y.; Sun, H.; Tian, W.; Zhang, H.; Yang, B. Synthesis of Cu2-XSe Nanocrystals by Tuning the Reactivity of Se. J. Phys. Chem. C 2011, 115, 9909−9916. (11) Lesnyak, V.; Brescia, R.; Messina, G. C.; Manna, L. Cu Vacancies Boost Cation Exchange Reactions in Copper Selenide Nanocrystals. J. Am. Chem. Soc. 2015, 137, 9315−9323. (12) Wang, Y.; Zhukovskyi, M.; Tongying, P.; Tian, Y.; Kuno, M. Synthesis of Ultrathin and Thickness-Controlled Cu2-XSe Nanosheets via Cation Exchange. J. Phys. Chem. Lett. 2014, 5, 3608−3613. (13) Toe, C. Y.; Zheng, Z.; Wu, H.; Scott, J.; Amal, R.; Ng, Y. H. Transformation of Cuprous Oxide into Hollow Copper Sulfide Cubes for Photocatalytic Hydrogen Generation. J. Phys. Chem. C 2018, 122, 14072−14081. (14) Kriegel, I.; Jiang, C.; Rodríguez-Fernández, J.; Schaller, R. D.; Talapin, D. V.; Da Como, E.; Feldmann, J. Tuning the Excitonic and Plasmonic Properties of Copper Chalcogenide Nanocrystals. J. Am. Chem. Soc. 2012, 134, 1583−1590. (15) Liu, Y.; Liu, M.; Swihart, M. T. Plasmonic Copper SulfideBased Materials: A Brief Introduction to Their Synthesis, Doping, Alloying, and Applications. J. Phys. Chem. C 2017, 121, 13435−13447. F
DOI: 10.1021/acs.jpcc.8b06059 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Dependencies of the Low-Energy Electronic Structure of PbS Quantum Dots. J. Phys. Chem. C 2014, 118, 20721−20726. (33) Caram, J. R.; Bertram, S. N.; Utzat, H.; Hess, W. R.; Carr, J. A.; Bischof, T. S.; Beyler, A. P.; Wilson, M. W. B.; Bawendi, M. G. PbS Nanocrystal Emission Is Governed by Multiple Emissive States. Nano Lett. 2016, 16, 6070−6077. (34) Su, G.; Liu, C.; Deng, Z.; Zhao, X.; Zhou, X. Size-Dependent Photoluminescence of PbS QDs Embedded in Silicate Glasses. Opt. Mater. Express 2017, 7, 2194.
G
DOI: 10.1021/acs.jpcc.8b06059 J. Phys. Chem. C XXXX, XXX, XXX−XXX
