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Chiral molecules enhanced extinction ratio of quantum dots (QDs) coupled to random plasmonic structures Lior Bezen, Shira Yochelis, Dilhara Jayarathna, Dinesh C Bhunia, Catalina Achim, and Yossi Paltiel Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00155 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018
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Chiral molecules enhanced extinction ratio of Quantum dots (QDs) coupled to random plasmonic structures Lior Bezen,a Shira Yochelis,a Dilhara Jayarathna,b Dinesh Bhunia,b Catalina Achimb and Yossi Paltiela Applied Physics department, the Hebrew University, Jerusalem 91904, Israel Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213
Abstract Devices based on self-assembled hybrid colloidal quantum dots (CQDs) coupled with specific organic linker molecules, are a promising way to simply realize room-temperature, spectrallytunable light detectors. Nevertheless, this type of devices usually has low quantum efficiency. Plasmonics has been shown as an efficient tool in guiding and confining light at nanoscale dimensions. As plasmonic modes exhibit highly confined fields, they locally increase light−matter interactions and consequently enhance the performance of CQD-based photodetectors. Recent publications presented experimental results of large extinction enhancement from a monolayer of CQDs coupled to random gold nano islands using a monolayer of organic alkyl linkers. We report here that a two-fold larger extinction enhancement in the visible spectrum is observed when a monolayer of helical chiral molecules connects the CQDs to the gold structure instead of a monolayer of achiral linkers. This effect shows for that chiral imprinting can enhance coupling between quantum emitters and plamonic islands.
We also show that this effect provides insight
into the chirality of the molecules within the monolayer. The measured 28-fold enhancement over a reference that has a monolayer of CQD provide the potential that these results to be used in the construction of a more efficient and sensitive photon detector based on surface QDs, as well as to supply a simple way to map the chirality of a single chiral monolayer.
Introduction Chiral molecules have identical composition and atom connectivity but are non-superimposable mirror images of each other [1,2]. Recent experiments have shown that ordered films of chiral organic molecules on surfaces can act as electron spin filters with an efficiency of up to 60% at room temperature [1]. This effect called the chiral-induced spin selectivity (CISS), opened the possibility of using chiral molecules to create spin-specific devices operated at room temperature [1-3]. A common approach for preparing chiral surfaces is based on molecular monolayers using chiral molecules as templates [4,5], which allows the easy design of chiral materials with specific properties related to those of the molecules used as templates. Hybrid nano-structures are attractive for future use in a variety of optoelectronic devices [6,7]. Self-assembled hybrid organic/quantum dots can couple to semiconductor devices and
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modify their functionality due to the manifestation of quantum effects [8,9]. These devices are simple to fabricate and control but usually have low quantum efficiency [9-12]. Plasmonics can be leveraged as an efficient tool to guide and confine light at nanoscale dimensions [13,14]. The highly confined fields of the plasmonic modes allow the enhancement of local light−matter interactions [15-19]. This effect was used in previous works to increase absorption efficiency enhancing photoluminescence (PL) [20-24]. The light extinction of monolayers of CQDs is increased if the CQDs are deposited on the evaporation of Au nano islands, when compared to the deposition of the CQDs directly onto the surface [25]. The random structures guarantee no sensitivity to the absorbed light polarization changes. Theoretical calculations using a two dimensional finite-difference, time-domain method demonstrate plasmonic control of the enhancement factor near the islands’ plasmon resonance [26]. These numerical simulations are applicable to the structures used in the research described in this paper. In this work, we show that the extinction enhancement in the visible spectrum by similar random Au nano islands and a CQD monolayer is higher when the linkers between the nano islands and the CQDs are helical chiral molecules when compared to achiral molecules. The chiral molecules dramatically improve the extinction ratio using the same structure of random gold nano islands that act as plasmonic antennas. A light extinction of up to 35% is attained when the plasmonic resonance of the Au nano islands and the bandgap of the CQDs deposited onto the nano islands are matched. This extinction is 28-fold higher than that measured for a monolayer of CQDs in the absence of the Au nano islands. We ascribe this surprising effect to the coupling between the chiral plasmons and the quantum dots, which breaks the plasmonic coupling symmetry aligning better the plasmonic Au islands dipole with CQDs dipole, parallel to device surface. The same effect could be used to map the circular dichroism of the full chiral molecules monolayers. These result relate to quantum dots chiral imprinting using chiral molecules [1,27], and may pave the way toward realizing more efficient and sensitive photon detectors. Also they provide a simple way to measure the differential absorbance of circularly polarized light by monolayers of chiral molecules.
Methods The samples were prepared in four main steps A-D. Figure 2 shows the sample after each of these preparation steps. (A) Au nano islands were created in step A. First, a thin, uniform 2-nm, 5-nm or 7-nm Au layer was evaporated on cleaned glass substrates. Then the samples were annealed on a hot plate at 400oC for several minutes to form random-shaped, Au plasmonic islands, termed nano islands through this manuscript. Figure 1 shows that the size of the nano islands increased with the thickness of the layer of Au NPs evaporated on the glass substrate. The nano island structures are stable for months at ambient conditions enabling wet chemistry processes that withstand washing and rinsing with water or organic solvents. (B) In the second step, homogeneous, closelypacked monolayer of organic molecules was covalently adsorbed on the Au nano islands by immersing the samples in solutions of the organic molecules for 3 hours. Five types of organic
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molecules have been used: achiral (3-Mercaptopropyl)trimethoxysilane (MPS), α-helix Lpolyalanine (HPA), which is right-handed, α-helix, left-handed D-polyalanine, and γ-modified peptide nucleic acid (PNA), which can adopt a right- or left-hand helical structure. The organic molecules bound to the Au nano islands through thiol linkers (MPS and polyalanine) or amino linkers (PNA). (C) The samples were washed with the solvent that dissolves the organic molecules and then immersed in a dilute 0.1% (w/w) solution of CdSe CQDs with band gap 590 nm for 3 hours. Lastly they were cleaned with toluene. (D) Another homogeneous, closely-packed monolayer of organic molecules was adsorbed on top of the layer of the CQDs. The structure of the device after stage C is shown in Figure 3.
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The PNAs with sequences shown in Figure 4 was synthesized by solid phase methods [31]. The Boc/Z-PNA monomers were purchased from Polyorg and ASM Research Chemicals and used with no further purification. Boc-protected S- and R-γPNA monomers were synthesized by adapting the procedures described by Ly et al [34]. The PNA oligomers were synthesized by solid phase Boc-protection peptide synthesis strategy [28]. p-Methyl-Benzhydrylamine resin.HCl (1.03 meq/g, Peptides International) was used as the solid support for PNA synthesis. The resin was downloaded to between 0.10-0.05 meq/g by preloading with D- or L –Glutamic acid (for PNA 1S or PNA 1R) using Boc-D- or L- Glu-OBzl (Anaspec) and Boc/Z- C- PNA monomer (for PNA 2). PNA oligomers were cleaved form the resin using m-cresol/thioanisole/TFMSA/TFA (1:1:2:6) and precipitated using cold diethyl ether. Purification of the PNA was done by reversed-phase high pressure liquid chromatography (HPLC) equipped with a C18 silica column on a Waters 600 controller and pump. Absorbance was monitored with a Waters 2996 photodiode array detector. PNA oligomers were characterized by matrix assisted laser desorption ionization coupled to time of flight (MALDI-ToF) mass spectroscopy on an Applied Biosystems Voyager biospectrometry workstation using 2,5-Dihydroxybenzoic acid (for PNA 1S and PNA 1R) or α-cyano-4hydroxycinnamic acid (for PNA 2) as the matrix (10mg/ml in water/acetonitrile, 0.1% TFA). PNA stock solutions were made in nano-pure water. The concentrations of the PNA solutions were determined by UV-vis spectroscopy at 90oC using 13700, 8600, 11700, and 6600 cm-1 M-1 as the extinction coefficients at 260 nm for A, T, G, and C PNA monomers, respectively [33]. The melting temperature of the duplexes is 60°C. Previous studies have shown that oligomers containing S- or R-γPNA monomers adopt a right-handed or left-handed PNA helix structure, respectively. Hereafter, we refer to these PNAs as right-handed PNA (PNA S) and left-handed PNA (PNA R) . PNA 1S H2N-D-Glu-AGTSTTGSTACSG-H
PNA 1R H2N-L-Glu-AGTRTTGRTACRG-H
PNA2
PNA2
H-TCA AAC ATG C
H-TCA AAC ATG C
Figure 4. Sequence of the two, 10-basepair nucleic acid duplexes used in this study PNA 1S.PNA2 and PNA 1R.PNA2
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The two polyalanine molecules with two opposite handedness, AHPA-L and AHPA-D handedness, were purchased from Sigma Aldrich. These peptides contain L- and D-amino acids (HCAAAAKAAAAKAAAAKAAAAKAAAAKAAAAKAAAAK-OH), respectively, where C, A, and K represent cysteine, alanine and lysine, respectively. Both molecules adopt an α-helix structure. For each molecule, four samples were prepared simultaneously using the same organic molecules solutions, at each of the different stages of device preparation. All measurements were repeated 5 times. Additional details on sample preparations can be found in the Materials and Methods/Supporting Information. Measurements Setups Light Extinction Curves The light transmission and reflection spectra were measured using high intensity LED as a white light source and an integrating sphere (Figure 5). A linear polarizer and quarter-wave plate were placed in front of the light source to determine the polarization of the exciting light. The samples were placed on top of the upper window of the integrated sphere. The light extinction of the sample is mostly due to absorption because the measured reflection of the samples is very small and the scattering is minimized in this optical setup. The spectra were recorded using Ocean Optics usb4000 VIS-NIR spectrometer.
PNA Melting Curves Variable temperature UV-vis experiments were performed in a Varian Cary 300 spectrophotometer equipped with a programmable temperature block in 1-cm optical-path, quartz cells. The melting curves were recorded over a temperature range of 15-90℃ for both cooling (annealing) and heating (melting) cycles at a rate of 1℃/min. The samples were kept for 10 minutes at 90℃ before cooling and for 10 minutes at 15℃ before heating. The 5 μM solutions of the PNA duplexes were
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prepared in pH 7.00, 10 mM sodium phosphate buffer. The melting temperature was determined by using the first derivative plots of the thermal melting profiles. PNA CD Spectroscopy The CD spectra were measured at 20 oC at a rate of 50 nm/min and 10 scan accumulation on a JASCO J-715 spectropolarimeter equipped with a thermoelectrically controlled, single-cell holder. The 5 μM solutions of the PNA duplexes and ss-PNAs were prepared in pH 7.00, 10 mM sodium phosphate buffer. The solutions of PNA duplexes were annealed by cooling from 90 oC to 15 oC at a rate of 1oC/min in a Varian Cary 300 spectrophotometer equipped with a programmable temperature block.
Results and discussion Plasmonic properties are highly dispersive [12,13]. Consequently, spectral analysis is a common tool for the characterization of the surface plasmons. In order to estimate the effects of the monolayer of organic molecules that connects the random-shaped Au nano islands and the CQDs, we measured separately and then compared the light extinction properties of the monolayer of CQDs and random-shaped Au NPs with CQDs and linkers. Comparison of the light enhancement caused by chiral and achiral linker molecules was used to identify the effect of the chirality on the light extinction. The light extinction by a device with a given CQD and Au nano islands should depend on the spectral overlap of the two resonances of these components and not on the organic linker between the two components if the dipole-dipole interactions are random in direction [25].
The
data shown in Figure 6 reflects a 28-fold relative enhancement in the light extinction of the CQDs on the plasmonic samples with chiral molecules (calculated as the difference between the maximum light extinction of the QDs on the plasmonic sample and the maximum light extinction of the plasmonic sample without QDs, subtracting the QDs monolayer maximum light extinction, and dividing by the QDs monolayer maximum light extinction) compared to the extinction ratio of a single quantum dot monolayer without Au nano islands, which is ~1.1% at peak. The enhancement for the MPS achiral molecules is 13-fold, which is consistent with previous work on achiral molecules [25].
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The relative enhancement depends on the detuning between the quantum dot band gap and the plasmonic resonance through the molecules. Nevertheless, for all the detuning range, using the same CQD and Au NPs a two-fold enhancement is measured for chiral linkers as compared to the non-chiral molecules. Furthermore, a clear red shift is seen for the CQD - Au NP coupling through both achiral and chiral molecules. The red shift observed for chiral molecules is larger than that for achiral molecules, which indicates that chiral molecules support a better Au NPs-QDs coupling than the achiral ones. We relate the fact that the QDs have a smaller influence on the properties of the monolayers made of right-handed PNA than on the properties of lefthanded PNA to the fact that the density of QDs adsorbed on a monolayer of right-handed PNA was lower than the density of QDs adsorbed on monolayers of left-handed PNA. We attribute this high enhancing effect to the chiral imprinting on the Au nano islands and QDs, which aligns the dipole moments of the CQDs parallel to the dipole moment of the Au nano islands. While an achiral molecule has very low polarization parallel to the gold surface, the chiral molecules induce about the same magnitude of polarization parallel and perpendicular to the surface. This argument based on dipole alignment is supported by the fact that the enhancement is larger for the PNA, which has a diameter of ~ 2 nm, and thus a larger interface area with the CQD than HPA, which has a 1.2 nm diameter.
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Figure 7 shows the light extinction of the samples due to the adsorption of the CQDs monolayer on top on the chiral molecules using evaporated Au that has 2 nm, 5 nm or 7 nm thickness. As the nominal thickness of the Au film increases, the light extinction of the plasmonic NP increases to the point where the plasmonic extinction peak becomes less obvious due to light scattering from the sample (as expected for a metallic layer). Also, as the thickness increases, the islands start to connect, the plasmonic chiral imprinting is less pronounced, and the in-plane plasmonic dipole moment is reduced. Note that the large extinction enhancement at the peak is consistent with the results shown in Figure 6.
Figure 8 shows the light extinction for samples that had an additional layer of chiral molecules adsorbed on top of the CQDs monolayer. (These samples had a layer of chiral molecules situated between the Au nano islands and the CQDs). This data indicates that the light extinction decreases when the second layer of chiral molecules is adsorbed on top of the CQDs. Also, the red shift of the maximum light extinction caused by the adsorption of the CQDs on the first monolayer of chiral molecules is reduced by the layer of chiral molecules adsorbed on top of the CQSs. These two results strengthen the argument that the symmetry-breaking caused by imprinting chirality to one side of the CQDs is the main reason for the extinction ratio enhancement. The layer of chiral molecules added on top of the CQDs causes a reduction in the alignment of the dipole moment of the CQDs with respect to the dipole moment of the Au nano islands because it restores (some of) the symmetry that was broken by the chiral imprinting effect exerted by the first layer of chiral molecules.
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The same system seems to enable the measurement for the first time of differences in the absorption of right and left polarized light by a single adsorbed chiral monolayer. These differences manifest only at wavelengths close to the plasmonic resonance of the Au nano islands. Figure 9 presents the difference spectra for a monolayer of chiral organic molecules α-HPA (A) or γ-PNA (B) on top of the Au nano islands. The spectra are calculated as the difference between light extinction spectra measured using left circular polarized light and light extinction spectra measured using right circular polarized light. Although the spectra are noisy, they are reproducible as verified by using different samples and they show around 1% change in the light absorption by molecules with the two helicities. We hypothesize that the resonance could be shifted to the UV range by using Ag nano islands instead of Au nano islands and smaller CQDs; such a shift may lead to more intense difference spectra and improved S/N. Experiments to test this hypothesis are in progress and will be reported in due course.
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Summary Recent published work showed a large extinction enhancement by a monolayer of CQDs deposited onto a thin gold film evaporated and annealed forming random gold nano islands structure that act as plasmonic antennas. In this study we measured a larger extinction enhancement in the visible spectrum using similar random Au nano islands and CQD monolayer when chiral rather than achiral molecular linkers connect the Au nano islands and the CQDs. By matching the plasmonic resonance and the colloidal quantum dots bandgap, we achieved light extinction of up to 35%. These results demonstrate a 28-fold enhancement over a reference that has a monolayer of CQD only. We ascribe this surprising effect to the chiral imprinting on the Au nano islands and the quantum dots, which breaks the plasmonic coupling symmetry, and align the dipole-dipole interactions parallel to device surface. Thus, the chiral molecule enhances the coupling between a quantum emitter and the plasmonic islands. The same effect could be used to map the circular dichroism of the full chiral molecules monolayers. These results may pave the way toward future realization of a more efficient and sensitive photon detectors, when coupled to FET QDs detector, and may provide a simple way to map circular dichroism of chiral monolayers.
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Table of Contents
molecular linker
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