Circular Dichroism of Electric-Field-Oriented CdSe/CdS Quantum Dots-in-Rods Maria V. Mukhina,*,† Anvar S. Baimuratov,† Ivan D. Rukhlenko,†,‡ Vladimir G. Maslov,† Finn Purcell Milton,§ Yurii K. Gun’ko,†,§ Alexander V. Baranov,† and Anatoly V. Fedorov† †
Department of Optical Physics and Modern Natural Science, ITMO University, Saint Petersburg 197101, Russia Monash University, Clayton Campus, Victoria 3800, Australia § School of Chemistry and CRANN Institute, Trinity College, Dublin, Dublin 2, Ireland ‡
S Supporting Information *
ABSTRACT: Here we report anisotropy of intrinsic chiroptical response in CdSe/CdS quantum dot-in-rod systems. These nanostructures being oriented in an external electric field demonstrate dependence of circular dichroism signal on the orientation of the nanocrystals. The type of circular polarization in these nanostructures correlates with preferential direction of linear polarization, and the degree of circular polarization is the maximal for the first circular dichroism band corresponding to the absorption band edge. We also support our experimental data with a theoretical model. Using this model, we show a direct connection between theoretically derived morphological parameters of twisting in nanocrystals lattices and calculated from experimental data parameters of circular dichroism anisotropy. KEYWORDS: chiral nanocrystals, anisotropic chiroptical activity, circular dichroism, alignment of nanocrystals, electrooptical studies
O
usually constitutes 0, then their CD signal has a similar time dependency c0 + c1cos(2πt/T), with |c0| > |c1|. This feature allows us to attribute transitions to the first and fourth bands to different groups of the CD anisotropy according to the sign of the product c0c1. Transitions to the fourth band, with sign(c0c1) > 0, are characterized by the in phase variation of the absorption and the absolute value of the CD, whereas transitions to the first band, with sign(c0c1) < 0, produce CD signals whose absolute values vary in the antiphase with the absorbance. It should be also noted that the lowest electronic transition displays the strongest circular anisotropy and, thus, is suitable for the most applications related to chiroptical activity. For our quantum system we show that optical activity of this transition starts to increase with the QD@Rs reorientating parallel to the light propagation. Notably, this feature therefore shows
METHODS CdSe/CdS QD@Rs with a mean length (L) of 41.7 nm, mean diameter (D) of 6.6 nm, and their aspect ratio of 6.3 (for synthetic procedure and TEM images see p. S2 of Supporting Information) were used in all polarization experiments. The QD@Rs were chosen over QRs for the CD anisotropy experiments not only due to their much stronger optical activity16 but also because they were shown to expose strong linear polarization in both absorption and emission,17 which makes QD@Rs easily orientable by electric field.18 The QD@Rs samples were enriched with the levorotatory enantiomers using the standard phase transfer procedure and displayed a strong positive intrinsic CD signal12,19 (see p. S3 of Supporting Information). The electroconductive cell for CD measurements was composed of two glass plates coated with ITO and held together with epoxy adhesive (see Figure 1). The duration and repetition rate of the rectangular voltage pulses applied to the cell were 5 s and 0.1 Hz. The distance (d) between the ITO electrodes was equal to the cell thickness of 1 mm. The voltage of fixed polarity produced an electric field of strengths 0.3 and 0.5 kV/cm. The CD and absorption spectra were measured by placing the cell into the sample chamber of a Jasco J-1500 spectrometer. The time dependencies were recorded with the data pitch of 0.1 s at the wavelengths of 485 and 430 nm corresponding to the maxima of the first and fourth CD bands, respectively. For the LD measurements, the QD@Rs were embedded in the PVB film using the procedure described previously in ref 20 (see p. S4 of Supporting Information). The alignment of the QD@Rs was achieved by stretching the film with a 4-fold increase of its original length. The LD spectra were recorded using a Jasco J-1500 spectrometer.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b04875. Synthesis procedures and TEM characterization for CdSe/CdS nanocrystals; the method of enantioenriched nanocrystal ensemble preparation; CD spectroscopy data for CdSe/CdS dots-in-rods ensemble enantioenriched with levorotatory nanocrystals; the method of electrooptical CD studies of oriented nanocrystals ensemble; preparation procedure for the nanocrystals ensemble embedded to stretched PVB film; the method of LD measurements; the cartoon defining the angles θ and ϕ; the table of observed parameters of CD(t) and E(t) dependences; calculation of nanocrystals permanent dipole moment (PDF) 8908
DOI: 10.1021/acsnano.6b04875 ACS Nano 2016, 10, 8904−8909
Article
ACS Nano
(17) Sitt, A.; Salant, A.; Menagen, G.; Banin, U. Highly Emissive Nano Rod-in-Rod Heterostructures with Strong Linear Polarization. Nano Lett. 2011, 11, 2054−2060. (18) Kamal, J.; Gomes, R.; Hens, Z.; Karvar, M.; Neyts, K.; Compernolle, S.; Vanhaecke, F. Direct Determination of Absorption Anisotropy in Colloidal Quantum Rods. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 035126. (19) Mukhina, M. V.; Korsakov, I. V.; Maslov, V. G.; Purcell-Milton, F.; Govan, J.; Baranov, A. V.; Fedorov, A. V.; Gun’ko, Y. K. Molecular Recognition of Biomolecules by Chiral CdSe Quantum Dots. Sci. Rep. 2016, 6, 24177. (20) Mukhina, M. V.; Maslov, V. G.; Baranov, A. V.; Artemyev, M. V.; Orlova, A. O.; Fedorov, A. V. Anisotropy of Optical Transitions in Ordered Ensemble of CdSe Quantum Rods. Opt. Lett. 2013, 38, 3426−3428. (21) Jennings, B. R. In Nato Science Series B; Krause, S., Ed.; Springer: New York, 1981; Vol. 64; pp 27−60. (22) Wu, Y.; Huang, H. W.; Olah, G. A. Method of Oriented Circular Dichroism. Biophys. J. 1990, 57, 797−806. (23) Baily, E. D.; Jennings, B. R. Simple Apparatus for Pulsed Electric Dichroism Measurements. Appl. Opt. 1972, 11, 527−532. (24) Yamaoka, K.; Charney, E. Electric Dichroism Studies of Macromolecules in Solutions. I. Theoretical Considerations of Electric Dichroism and Electrochromism. J. Am. Chem. Soc. 1972, 94, 8963− 8974. (25) Tinco, I., Jr; Hammerle, W. C. The Influence of an External Electric Field on the Optical Activity of Fluids. J. Phys. Chem. 1956, 60, 1619−1623. (26) Go, N. Optical Activity of Anisotropic Solutions I. J. Chem. Phys. 1965, 43, 1275−1280. (27) Go, N. Optical Activity of Anisotropic Solutions. II. J. Phys. Soc. Jpn. 1967, 23, 88−97. (28) Kuball, H.-G.; Karstens, T.; Schönhofer, A. Optical Activity of Oriented Molecules: II. Theoretical Description of the Optical Activity. Chem. Phys. 1976, 12, 1−13. (29) Nann, T.; Schneider, J. Origin of Permanent Electric Dipole Moments in Wurtzite Nanocrystals. Chem. Phys. Lett. 2004, 384, 150− 152. (30) Li, J.; Wang, L.-W. Band-Structure-Corrected Local Density Approximation Study of Semiconductor Quantum Dots and Wires. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 125325. (31) Hadar, I.; Hitin, G. B.; Sitt, A.; Faust, A.; Banin, U. Polarization Properties of Semiconductor Nanorod Heterostructures: From Single Particles to the Ensemble. J. Phys. Chem. Lett. 2013, 4, 502−507. (32) Hu, J.; Wang, L.-w.; Li, L.-s.; Yang, W.; Alivisatos, A. P. Semiempirical Pseudopotential Calculation of Electronic States of CdSe Quantum Rods. J. Phys. Chem. B 2002, 106, 2447−2452. (33) Li, J.; Wang, L.-w. High Energy Excitations in CdSe Quantum Rods. Nano Lett. 2003, 3, 101−105. (34) Hu, J.; Li, L.-S.; Weidong, Y.; Manna, L.; Wang, L.-W.; Alivisatos, A. P. Linearly Polarized Emission from Colloidal Semiconductor Quantum Rods. Science 2001, 292, 2060−2063. (35) Wakabayashi, M.; Yokojima, S.; Fukaminato, T.; ichi Shiino, K.; Irie, M.; Nakamura, S. Anisotropic Dissymmetry Factor, g: Theoretical Investigation on Single Molecule Chiroptical Spectroscopy. J. Phys. Chem. A 2014, 118, 5046−5057. (36) Buckingham, A. D.; Dunn, M. B. Optical Activity of Oriented Molecules. J. Chem. Soc. A 1971, 1988−1991. (37) Bondo Pedersen, T.; Hansen, A. E. Ab Initio Calculation and Display of the Rotary Strength Tensor in the Random Phase Approximation. Method and Model Studies. Chem. Phys. Lett. 1995, 246, 1−8. (38) Ben-Moshe, A.; Maoz, B. M.; Govorov, A. O.; Markovich, G. Chirality and Chiroptical Effects in Inorganic Nanocrystal Systems with Plasmon and Exciton Resonances. Chem. Soc. Rev. 2013, 42, 7028−7041.
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the Government of the Russian Federation (grant 074-U01) and the Ministry of Education and Science of the Russian Federation (grant no. 14.B25.31.0002). We also acknowledge the financial support from Science Foundation Ireland (grant SFI 12/IA/1300) and EU FP7 FutureNanoNeeds grant. M.V.M. and A.S.B. thanks the Ministry of Education and Science of the Russian Federation for support via the Scholarships of the President of the Russian Federation for Young Scientists and Graduate Students. REFERENCES (1) Wang, Y.; Xu, J.; Wang, Y.; Chen, H. Emerging Chirality in Nanoscience. Chem. Soc. Rev. 2013, 42, 2930−2962. (2) Elliott, S. D.; Moloney, M. P.; Gun’ko, Y. K. Chiral Shells and Achiral Cores in CdS Quantum Dots. Nano Lett. 2008, 8, 2452−2457. (3) Tohgha, U.; Deol, K. K.; Porter, A. G.; Bartko, S. G.; Choi, J. K.; Leonard, B. M.; Varga, K.; Kubelka, J.; Muller, G.; Balaz, M. Ligand Induced Circular Dichroism and Circularly Polarized Luminescence in CdSe Quantum Dots. ACS Nano 2013, 7, 11094−11102. (4) Moloney, M. P.; Govan, J.; Loudon, A.; Mukhina, M.; Gun’ko, Y. K. Preparation of Chiral Quantum Dots. Nat. Protoc. 2015, 10, 558− 573. (5) Govan, J.; Gun’ko, Y. K. Nanoscience; RSC: Cambridge, 2016; Vol. 3; pp 1−30. (6) Milton, F. P.; Govan, J.; Mukhina, M. V.; Gun’ko, Y. K. The Chiral Nano-World: Chiroptically Active Quantum Nanostructures. Nanoscale Horizons 2016, 1, 14−26. (7) Choi, J. K.; Haynie, B. E.; Tohgha, U.; Pap, L.; Elliott, K. W.; Leonard, B. M.; Dzyuba, S. V.; Varga, K.; Kubelka, J.; Balaz, M. Chirality Inversion of CdSe and CdS Quantum Dots without Changing the Stereochemistry of the Capping Ligand. ACS Nano 2016, 10, 3809−3815. (8) Baimuratov, A. S.; Gun’ko, Y. K.; Baranov, A. V.; Fedorov, A. V.; Rukhlenko, I. D. Chiral Quantum Supercrystals with Total Dissymmetry of Optical Response. Sci. Rep. 2016, 6, 23321. (9) Ben-Moshe, A.; Govorov, A. O.; Markovich, G. Enantioselective Synthesis of Intrinsically Chiral Mercury Sulfide Nanocrystals. Angew. Chem. 2013, 125, 1313−1317. (10) Rukhlenko, I. D.; Baimuratov, A. S.; Tepliakov, N. V.; Baranov, A. V.; Fedorov, A. V. Shape-induced optical activity of chiral nanocrystals. Opt. Lett. 2016, 41, 2438−2441. (11) Baimuratov, A. S.; Rukhlenko, I. D.; Gun’ko, Y. K.; Baranov, A. V.; Fedorov, A. V. Dislocation-Induced Chirality of Semiconductor Nanocrystals. Nano Lett. 2015, 15, 1710−1715. (12) Mukhina, M. V.; Maslov, V. G.; Baranov, A. V.; Fedorov, A. V.; Orlova, A.; Purcell-Milton, F.; Govan, J.; Gun’ko, Y. K. Intrinsic Chirality of CdSe/ZnS Quantum Dots and Quantum Rods. Nano Lett. 2015, 15, 2844−2851. (13) Schellman, J.; Jensen, H. P. Optical spectroscopy of oriented molecules. Chem. Rev. 1987, 87, 1359−1399. (14) Shindo, Y.; Nishio, M.; Maeda, S. Problems of CD Spectrometers (V): Can We Measure CD and LD Simultaneously? Comments on Differential Polarization Microscopy (CD and Linear Dichroism). Biopolymers 1990, 30, 405−413. (15) Li, L.-s.; Alivisatos, A. P. Origin and Scaling of the Permanent Dipole Moment in CdSe Nanorods. Phys. Rev. Lett. 2003, 90, 097402. (16) Mukhina, M. V.; Maslov, V. G.; Korsakov, I. V.; Purcell Milton, F.; Loudon, A.; Baranov, A. V.; Fedorov, A. V.; Gun'ko, Y. K. Optically Active II-VI Semiconductor Nanocrystals via Chiral Phase Transfer. MRS Online Proc. Libr. 2015, 1793, 27−33. 8909
DOI: 10.1021/acsnano.6b04875 ACS Nano 2016, 10, 8904−8909
