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Volume 2 | Number 1 | January 2010 | Pages 1–156
Nanoscale
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COVER ARTICLE Graham et al. Mixed metal nanoparticle assembly and the effect on surface-enhanced Raman scattering
REVIEW Lin et al. Progress of nanocrystalline growth kinetics based on oriented attachment
2040-3364(2010)2:1;1-T
Please note that technical editing may introduce minor changes to the text and/or graphics contained in the manuscript submitted by the author(s) which may alter content, and that the standard Terms & Conditions and the ethical guidelines that apply to the journal are still applicable. In no event shall the RSC be held responsible for any errors or omissions in these Accepted Manuscript manuscripts or any consequences arising from the use of any information contained in them.
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Tailoring of quantum dots emission efficiency by localized surface plasmon polaritons in self-organized mesoscopic rings Emanuela Margapoti,*a,b Denis Gentili,a Matteo Amelia,c Alberto Credi,*c Vittorio Morandid and Massimiliano Cavallini*a
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x We report on the tailoring of quantum dots (QDs) emission efficiency by localized surface plasmon polaritons in selforganized mesoscopic rings. Ag nanoparticles (NPs) with CdSe QDs embedded in a polymeric matrix are spatially organised in mesoscopic rings and coupled in a tuneable fashion by breath figure formation. The mean distance between NPs and QDs and consequently the intensity of QD photoluminescence that is enhanced by the coupling of surface plasmons and exciton, is tuned by acting on the NP concentration. Localized surface plasmons (LSPs) are non-propagating collective oscillations of the conductive electrons inside a metal nanoparticle with a high spatial field confinement.1 Their use has been proposed for nanoscale and ultrafast (THz frequencies) optoelectronic devices,2-4 because they are able to confine photons in small nanostructures, thus overcoming the diffraction limit of light. Therefore, the use of LSPs was proposed as a concrete route towards the miniaturization of photonic circuits and to bridge the gap between high-speed photonics and nanoscale electronics.5 Currently, several research activities are focussing on building metallic waveguides, switches, superlenses, metamaterials, and couplers, that can process signals based on surface plasmons, 3, 1418 and improve the efficiency of light emitting diodes.6 The use of NPs/QDs in photonic crystals fabricated by nanoimprinting to improve the efficiency of light extraction7 is noteworthy. As demonstrated in several studies, the emission efficiency of luminescent objects (e.g. quantum dots, organic molecules, etc.) can be enhanced by the near-field build up by an array of metal nanostructures.8, 9 In particular, it has been shown that nanoparticles made of noble metals, e.g. gold (Au), silver (Ag) and copper (Cu), organized in a close-packed array, exhibit strong mutual amplification of their near electric-field,10, 11 but if the emitter is in contact with the metal nanoparticle the emission is quenched;12, 13 for this reason, a separation in the order of tens of nanometres made of a dielectric material is usually formed between the emitter and the nanoparticle.14 The NP-emitter distance can be controlled by the chemical functionalization of the NPs15 and their precise positioning by electron beamlithography15 or by unconventional lithography.16-21 Although the fabrication methods used are very robust, they are
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time consuming and require complex instruments and infrastructures. In order to promote the study of QD and NP interaction, it is therefore desirable to have simple and versatile methods to organise QDs and NPs with a fine-tuning of their distance and the possibility to see their positioning using common tools such as optical microscopy. In this work we use a very simple approach to organise a mixture of QDs and NPs in a honeycomb structure of mesoscopic rings, tuning the QD emission efficiency by acting on QD/NP mean distance through their initial ratio of concentration in a solution. We show that Ag NPs embedded in a polymeric matrix can be spatially organised and coupled with CdSe luminescent QDs22 in a tuneable fashion, by a simple bottom-up approach23 based on the so called breath figure (BF) formation24-26. This technique has enabled us to fabricate arrays of QDs and NPs distributed along mesoscopic rings whose diameter ranges in the micro -meter. We chose CdSe QDs as the luminescent species because of their particular optical properties; namely, the broad and intense absorption spectrum, which facilitates photo-excitation, and the intense emission in the visible spectrum with their wavelength tunability obtained simply by changing the size of the nanoparticle.24-26 The Ag NPs were chosen with spherical shape and with a size ranging between 10 and 20 nm, so as to have the surface plasmon resonance around 400 nm, matching the absorption region of the CdSe QDs. Finally we choose polycarbonate as the matrix because it is optically inert and transparent in the range of visible light. Patterning by BFs27, 28 exploits a well-known phenomenon that was studied by Lord Rayleigh more than one century ago.29 The BFs method is used to fabricate ordered patterns of porous in a polymeric matrix where a solute self-organizes around these pores. The method consists of the deposition of a few microliters of a solution containing the polymer and the solute under a flux of moist air (see detail in the experimental section). As the solvent evaporates the air-liquid interfacial temperature decreases, and below the dew point water droplets condense on the evaporating solution, forming an ordered distribution of droplets whose size and distance can be controlled in the range of 0.2 - 20 µm by regulating the velocity of the air flow.30 Since the solvent used is hydrophobic, it cannot evaporate through the water droplets; this implies that the floating water droplets prevent the solvent from evaporating directly from the capped surface. The solvent flows outwards to replenish the evaporated [journal], [year], [vol], 00–00 | 1
Nanoscale Accepted Manuscript
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solvent, thereby transporting the solute nanoparticles which selforganize around these pores. This phenomenon, which is independent of the chemical nature of the components, provides a means to concentrate and embed the solute into the pore periphery. Fig. 1 shows the scheme of the process and an example of ordered distribution of micropores. Breath figures have been observed on a wide variety of systems,31, 32 including the patterning of QDs33 and are here proposed as a technological application. In our study, we used CdSe colloidal QDs together with Ag NPs to decorate BFs fabricated in a polymer (polycarbonate) matrix. Samples were characterised by laser confocal microscopy (LCSM), fluorescence optical microscopy (FM), scanning electron microscopy (SEM) and photoluminescence spectroscopy (PL). The samples were obtained by drop casting 80 µL of a chloroform solution on a glass substrate upon controlled moist air (relative humidity >90%) flux, keeping constant the amount of dissolved polycarbonate and colloidal QDs, while the quantity of Ag NPs was systematically changed to achieve NPs:QDs ratio of: 0:1, 1:1, 2:1, 3:1, and 4:1 (see ESI). Fig. 1b show a typical LCSM image of an array of BFs using only QDs (i.e. ratio 0:1). The image clearly shows that the QD fluorescence preferentially accumulates along the ring borders, decorating the pores.
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NPs and, consequently, of the QD emission takes place. Conversely, if the nanoparticles are separated in space in a range View Article Online of a few nanometers, a strong enhancement of the emission DOI: 10.1039/C3NR04708C 14 efficiency can be obtained. In Fig. 2 two fluorescent images are compared: in the first sample both Ag NPs and CdSe QDs are mixed together in ratio 2:1 (Fig. 2a), whereas the second sample contains only the QDs (Fig. 2b). Notably, in this last case, the fluorescence is about 30% lower in intensity than when the QDs are mixed with the Ag NPs. Fig. 2c shows a plot of the cross section of a few rings for both types of samples (QDs with and without Ag NPs). The distance D corresponds to the inner diameter of the rings, while d is the ring-to-ring distance. Besides the well-known enhancement effect of the QD emission intensity due to the near E-field of the Ag NPs, also the lateral ring extension of the fluorescence increases, leading to the ring-toring distance d approaching zero. This observation could be due to the evanescent field penetrating into the polymer, which could decay into the dielectric with a propagation length of some tenths of nanometers. In this way also the efficiency of the QDs farther from the ring edge could be enhanced. Fig. 2d shows the photoluminescence (PL) spectra recorded on the two samples described above, using a 488 nm cw laser as the excitation source. The red curve refers to the sample with only QDs, while the blue curve corresponds to the sample in which Ag NPs are also present. Firstly, it can be observed that the two PL spectra have the same shape and feature two bands, with peak at 540 nm and at 585 nm, respectively. The band at 585 nm is clearly attributed to the inherent QD photoluminescence (radiative exciton recombination) from the comparison with the spectra of QDs in solution (see Fig. S2 in the ESI). Conversely, the origin of the peak at 540 nm is not clear; it could arise from background scattering into the polymer, but further specific investigation is needed to understand its origin. Moreover, we can observe that the PL intensity of the QDs is enhanced when Ag NPs are present, if compared to the case where only the QDs were used. Fig.2 Fluorescence optical microscopic image recorded from selforganised breath figures embedded with (a) Ag NPs and CdSe QDs (ratio
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Fig. 1 (a) The sketch of the BFs fabrication, which describes the formation of the mesoscopic ring by using the water droplets condensation on wet polymeric surface as a way to reach self-organized holes of polymer. (b) Fluorescence optical microscopy image of BF obtained embedding CdSe QD and Ag NPs. The red colour is due to the photoluminescence of the embedded CdSe QDs.
Adding the Ag NPs both QDs and NPs decorate the pores, brought in close proximity to one another. Once the mean distance is set at the end of the drying process, the inter-particle distance is retained because of the presence of the polycarbonate matrix, in which both particles are meshed. As previously observed,14 if the emitters and the “plasmonic nanostructures” are in close contact, quenching of the near E-field between the Ag 2 | Journal Name, [year], [vol], 00–00
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2:1) in a polymeric matrix and (b) with only CdSe QDs in a polymeric matrix. (c) Comparison between two linear profile analyses of the two fluorescence images. (d) Photoluminescence spectra from the two above films.
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Noticeably, the mean distance between Ag NPs and QDs can be tuned simply by acting on the NPs concentration. A trend of the PL intensity versus the volume ratio between the Ag NPs and the CdSe QDs is shown in Fig. 3a. By changing this ratio, a quasilinear enhancement of the QD emission is obtained. The highest intensity of the PL, ̴ 600 counts/s, is observed when the ratio between the Ag NPs and the QDs is 4:1, above this ratio a dramatic quenching of emission, whose intensity strongly depends on the sample was observed. Fig. 3b shows two cross sections recorded from a few ring structures, as shown in the image below the inset. The black curve refers to a sample with a volume ratio of 1:1, whereas the blue curve corresponds to a volume ratio of 4:1. The signal obtained in the case of the 4:1 volume ratio is six times larger than that obtained with a 1:1 ratio.
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Fig. 3 (a) Trend of the FOM images intensity versus NPs/QDs volume ratio, collected on the same area for comparison. (b) Two cross sections have been compared: one where we got the lowest intensity obtained for NPs:QDs=1:1 (dark spectrum) and NPs:QDs=4:1 (blue spectrum); Fig. S1 shows the corresponding fluorescence optical microscopy images. 60
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In order to gain information on the QD and Ag NP distribution inside the BFs pattern, SEM measurements were performed. The secondary electron (SE) images, reported in Fig. 4a and Fig. 4b, acquired at a primary energy of 10 keV with the ET detector, show both topographic and compositional contrasts. These measurements, performed on the same sample described in Fig. 2a, clearly show that the QDs and the nanoparticles are in fact decorating only the ring structures. In more detail, the red squares depicted in Fig. 4a denote parts of the ring where the brighter white spots are the metal nanoparticles surrounded by the CdSe QDs, as demonstrated by the Energy Dispersive X-Ray (EDX) analysis, reported in Fig. 4c and performed on the same region, clearly confirming the presence of Ag, Cd and Se. The low dimension of the NPs and of the QDs, as well as the density of their distribution near the rings border, does not allow for the acquisition of the EDX spectra separately from the single nanostructures, nevertheless the EDX result permits to interpret the details with higher contrast as Ag NPs and the ones with lower contrast as CdSe QDs. In Fig. 4b a SEM image at low magnification is reported, showing an area where the Ag nanoparticles and the QDs surrounding the rings are almost uniformly distributed around the ring, as indicated by the red arrows. This uniformity is the main reason for the observation of This journal is © The Royal Society of Chemistry [year]
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a continuous emission from the rings. It should be noted that the thickness of the matrix polymer in which the particles are dipped View Article Online might prevent the detection of the NPs below a certain thickness, DOI: 10.1039/C3NR04708C that for the given primary energy can be estimated between 1 and 2 µm. However, Fig. 4 shows that the density of the distribution of the NPs is clearly much more pronounced near the rings, supporting at the same time the PL measurements and the hypothesis that, if the NPs could be located between the neighbouring rings, they should be significantly less and possibly deeper.
Fig. 4 a) High magnification SEM image from the sample (a) showing a detail of the border of a BF. Ag nanoparticles, whiter spots, and QDs are highlighted by red squares. The bar is 200 nm. b) Larger view of the BFs where the QDs and the NPs decorate the rings, as emphasized by the red arrows. The bar is 1 µm c) EDX spectrum acquired on the region reported in a) showing the presence of Ag, Cd and Se.
In conclusion, we have reported the control of spatial distribution and mean distance in mesoscopic rings of Ag NPs /QD embedded in polycarbonate thin-films. NP/QD rings were organised in an ordered honeycomb structure obtained by a very simple bottomup process such as the breath figure formation. Furthermore we proved an efficient control of the mean distance between NPs and QDs simply by changing the nanoparticle initial concentration. Finally we have demonstrated a six-fold enhancement of the QD emission when coupled with the near field from Ag NPs. This ongoing work will contribute to the development of a new generation of optoelectonic devices fabricated exploiting selforganization.
Acknowledgements
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We are grateful to Dr W. Koopman for help with the photoluminescence experiments. This work was supported by the EU project HYSENS (Contract NMP3-SL: Small scale collaborative project. Grant Agreement No.: 263091).
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Notes and references
19. M. Cavallini, F. C. Simeone, F. Borgatti, C. Albonetti, V. Morandi, C. Sangregorio, C. Innocenti, F. Pineider, E. Annese, G.
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Consiglio Nazionale delle Ricerche - Istituto per lo Studio dei Materiali Nanostrutturati (CNR-ISMN), via P. Gobetti 101, 40129 Bologna, Italy. Fax: +39 051 6398540; Tel: +39 051 6398522; E-mail: [email protected] b Walter Schottky Institut and Physik Department, Technische Universität München, Am Coulombwall 4, DE-85748 Garching, Germany; E-mail: [email protected]; Tel. +49-(0)89-289 11366. c Photochemical Nanosciences Laboratory and Interuniversity Center for the Chemical Conversion of Solar Energy (SolarChem), Dipartimento di Chimica “G. Ciamician”, Università di Bologna, via Selmi 2, 40126 Bologna, Italy. Tel: +39 051 20 9 9540; E-mail: [email protected] d Consiglio Nazionale delle Ricerche - Istituto per la Microelettronica e Microsistemi (CNR-IMM), via P. Gobetti 101, 40129 Bologna, Italy. † Electronic Supplementary Information (ESI) available: Experimental details and additional results. See DOI: 10.1039/b000000x/
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Panaccione and L. Pasquali, Nanoscale, 2010, 2, 2069-2072. DOI: 10.1039/C3NR04708C 20. M. Cavallini, M. Facchini, M. Massi and F. Biscarini, Synth. Met., 2004, 146, 283-286. 21. M. Cavallini, R. Lazzaroni, R. Zamboni, F. Biscarini, D. Timpel, F. Zerbetto, G. J. Clarkson and D. A. Leigh, J. Phys. Chem. B,
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CdSe quantum dots emission efficiency is tailored by localized surface plasmon polaritons of Ag nanoparticles in self-organized mesoscopic rings fabricated by breath figure method.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5
Nanoscale
View Journal
Accepted Manuscript This article can be cited before page numbers have been issued, to do this please use: E. Margapoti, D. Gentili, M. Amelia, A. Credi, V. Morandi and M. Cavallini, Nanoscale, 2013, DOI: 10.1039/C3NR04708C.
Volume 2 | Number 1 | 2010
This is an Accepted Manuscript, which has been through the RSC Publishing peer review process and has been accepted for publication.
www.rsc.org/nanoscale
Volume 2 | Number 1 | January 2010 | Pages 1–156
Nanoscale
Accepted Manuscripts are published online shortly after acceptance, which is prior to technical editing, formatting and proof reading. This free service from RSC Publishing allows authors to make their results available to the community, in citable form, before publication of the edited article. This Accepted Manuscript will be replaced by the edited and formatted Advance Article as soon as this is available. To cite this manuscript please use its permanent Digital Object Identifier (DOI®), which is identical for all formats of publication. More information about Accepted Manuscripts can be found in the Information for Authors.
ISSN 2040-3364
Pages 1–156
COVER ARTICLE Graham et al. Mixed metal nanoparticle assembly and the effect on surface-enhanced Raman scattering
REVIEW Lin et al. Progress of nanocrystalline growth kinetics based on oriented attachment
2040-3364(2010)2:1;1-T
Please note that technical editing may introduce minor changes to the text and/or graphics contained in the manuscript submitted by the author(s) which may alter content, and that the standard Terms & Conditions and the ethical guidelines that apply to the journal are still applicable. In no event shall the RSC be held responsible for any errors or omissions in these Accepted Manuscript manuscripts or any consequences arising from the use of any information contained in them.
www.rsc.org/nanoscale Registered Charity Number 207890
Journal Name
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Nanoscale
Cite this: DOI: 10.1039/c0xx00000x
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DOI: 10.1039/C3NR04708C
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Tailoring of quantum dots emission efficiency by localized surface plasmon polaritons in self-organized mesoscopic rings Emanuela Margapoti,*a,b Denis Gentili,a Matteo Amelia,c Alberto Credi,*c Vittorio Morandid and Massimiliano Cavallini*a
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x We report on the tailoring of quantum dots (QDs) emission efficiency by localized surface plasmon polaritons in selforganized mesoscopic rings. Ag nanoparticles (NPs) with CdSe QDs embedded in a polymeric matrix are spatially organised in mesoscopic rings and coupled in a tuneable fashion by breath figure formation. The mean distance between NPs and QDs and consequently the intensity of QD photoluminescence that is enhanced by the coupling of surface plasmons and exciton, is tuned by acting on the NP concentration. Localized surface plasmons (LSPs) are non-propagating collective oscillations of the conductive electrons inside a metal nanoparticle with a high spatial field confinement.1 Their use has been proposed for nanoscale and ultrafast (THz frequencies) optoelectronic devices,2-4 because they are able to confine photons in small nanostructures, thus overcoming the diffraction limit of light. Therefore, the use of LSPs was proposed as a concrete route towards the miniaturization of photonic circuits and to bridge the gap between high-speed photonics and nanoscale electronics.5 Currently, several research activities are focussing on building metallic waveguides, switches, superlenses, metamaterials, and couplers, that can process signals based on surface plasmons, 3, 1418 and improve the efficiency of light emitting diodes.6 The use of NPs/QDs in photonic crystals fabricated by nanoimprinting to improve the efficiency of light extraction7 is noteworthy. As demonstrated in several studies, the emission efficiency of luminescent objects (e.g. quantum dots, organic molecules, etc.) can be enhanced by the near-field build up by an array of metal nanostructures.8, 9 In particular, it has been shown that nanoparticles made of noble metals, e.g. gold (Au), silver (Ag) and copper (Cu), organized in a close-packed array, exhibit strong mutual amplification of their near electric-field,10, 11 but if the emitter is in contact with the metal nanoparticle the emission is quenched;12, 13 for this reason, a separation in the order of tens of nanometres made of a dielectric material is usually formed between the emitter and the nanoparticle.14 The NP-emitter distance can be controlled by the chemical functionalization of the NPs15 and their precise positioning by electron beamlithography15 or by unconventional lithography.16-21 Although the fabrication methods used are very robust, they are
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time consuming and require complex instruments and infrastructures. In order to promote the study of QD and NP interaction, it is therefore desirable to have simple and versatile methods to organise QDs and NPs with a fine-tuning of their distance and the possibility to see their positioning using common tools such as optical microscopy. In this work we use a very simple approach to organise a mixture of QDs and NPs in a honeycomb structure of mesoscopic rings, tuning the QD emission efficiency by acting on QD/NP mean distance through their initial ratio of concentration in a solution. We show that Ag NPs embedded in a polymeric matrix can be spatially organised and coupled with CdSe luminescent QDs22 in a tuneable fashion, by a simple bottom-up approach23 based on the so called breath figure (BF) formation24-26. This technique has enabled us to fabricate arrays of QDs and NPs distributed along mesoscopic rings whose diameter ranges in the micro -meter. We chose CdSe QDs as the luminescent species because of their particular optical properties; namely, the broad and intense absorption spectrum, which facilitates photo-excitation, and the intense emission in the visible spectrum with their wavelength tunability obtained simply by changing the size of the nanoparticle.24-26 The Ag NPs were chosen with spherical shape and with a size ranging between 10 and 20 nm, so as to have the surface plasmon resonance around 400 nm, matching the absorption region of the CdSe QDs. Finally we choose polycarbonate as the matrix because it is optically inert and transparent in the range of visible light. Patterning by BFs27, 28 exploits a well-known phenomenon that was studied by Lord Rayleigh more than one century ago.29 The BFs method is used to fabricate ordered patterns of porous in a polymeric matrix where a solute self-organizes around these pores. The method consists of the deposition of a few microliters of a solution containing the polymer and the solute under a flux of moist air (see detail in the experimental section). As the solvent evaporates the air-liquid interfacial temperature decreases, and below the dew point water droplets condense on the evaporating solution, forming an ordered distribution of droplets whose size and distance can be controlled in the range of 0.2 - 20 µm by regulating the velocity of the air flow.30 Since the solvent used is hydrophobic, it cannot evaporate through the water droplets; this implies that the floating water droplets prevent the solvent from evaporating directly from the capped surface. The solvent flows outwards to replenish the evaporated [journal], [year], [vol], 00–00 | 1
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solvent, thereby transporting the solute nanoparticles which selforganize around these pores. This phenomenon, which is independent of the chemical nature of the components, provides a means to concentrate and embed the solute into the pore periphery. Fig. 1 shows the scheme of the process and an example of ordered distribution of micropores. Breath figures have been observed on a wide variety of systems,31, 32 including the patterning of QDs33 and are here proposed as a technological application. In our study, we used CdSe colloidal QDs together with Ag NPs to decorate BFs fabricated in a polymer (polycarbonate) matrix. Samples were characterised by laser confocal microscopy (LCSM), fluorescence optical microscopy (FM), scanning electron microscopy (SEM) and photoluminescence spectroscopy (PL). The samples were obtained by drop casting 80 µL of a chloroform solution on a glass substrate upon controlled moist air (relative humidity >90%) flux, keeping constant the amount of dissolved polycarbonate and colloidal QDs, while the quantity of Ag NPs was systematically changed to achieve NPs:QDs ratio of: 0:1, 1:1, 2:1, 3:1, and 4:1 (see ESI). Fig. 1b show a typical LCSM image of an array of BFs using only QDs (i.e. ratio 0:1). The image clearly shows that the QD fluorescence preferentially accumulates along the ring borders, decorating the pores.
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NPs and, consequently, of the QD emission takes place. Conversely, if the nanoparticles are separated in space in a range View Article Online of a few nanometers, a strong enhancement of the emission DOI: 10.1039/C3NR04708C 14 efficiency can be obtained. In Fig. 2 two fluorescent images are compared: in the first sample both Ag NPs and CdSe QDs are mixed together in ratio 2:1 (Fig. 2a), whereas the second sample contains only the QDs (Fig. 2b). Notably, in this last case, the fluorescence is about 30% lower in intensity than when the QDs are mixed with the Ag NPs. Fig. 2c shows a plot of the cross section of a few rings for both types of samples (QDs with and without Ag NPs). The distance D corresponds to the inner diameter of the rings, while d is the ring-to-ring distance. Besides the well-known enhancement effect of the QD emission intensity due to the near E-field of the Ag NPs, also the lateral ring extension of the fluorescence increases, leading to the ring-toring distance d approaching zero. This observation could be due to the evanescent field penetrating into the polymer, which could decay into the dielectric with a propagation length of some tenths of nanometers. In this way also the efficiency of the QDs farther from the ring edge could be enhanced. Fig. 2d shows the photoluminescence (PL) spectra recorded on the two samples described above, using a 488 nm cw laser as the excitation source. The red curve refers to the sample with only QDs, while the blue curve corresponds to the sample in which Ag NPs are also present. Firstly, it can be observed that the two PL spectra have the same shape and feature two bands, with peak at 540 nm and at 585 nm, respectively. The band at 585 nm is clearly attributed to the inherent QD photoluminescence (radiative exciton recombination) from the comparison with the spectra of QDs in solution (see Fig. S2 in the ESI). Conversely, the origin of the peak at 540 nm is not clear; it could arise from background scattering into the polymer, but further specific investigation is needed to understand its origin. Moreover, we can observe that the PL intensity of the QDs is enhanced when Ag NPs are present, if compared to the case where only the QDs were used. Fig.2 Fluorescence optical microscopic image recorded from selforganised breath figures embedded with (a) Ag NPs and CdSe QDs (ratio
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Fig. 1 (a) The sketch of the BFs fabrication, which describes the formation of the mesoscopic ring by using the water droplets condensation on wet polymeric surface as a way to reach self-organized holes of polymer. (b) Fluorescence optical microscopy image of BF obtained embedding CdSe QD and Ag NPs. The red colour is due to the photoluminescence of the embedded CdSe QDs.
Adding the Ag NPs both QDs and NPs decorate the pores, brought in close proximity to one another. Once the mean distance is set at the end of the drying process, the inter-particle distance is retained because of the presence of the polycarbonate matrix, in which both particles are meshed. As previously observed,14 if the emitters and the “plasmonic nanostructures” are in close contact, quenching of the near E-field between the Ag 2 | Journal Name, [year], [vol], 00–00
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2:1) in a polymeric matrix and (b) with only CdSe QDs in a polymeric matrix. (c) Comparison between two linear profile analyses of the two fluorescence images. (d) Photoluminescence spectra from the two above films.
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Noticeably, the mean distance between Ag NPs and QDs can be tuned simply by acting on the NPs concentration. A trend of the PL intensity versus the volume ratio between the Ag NPs and the CdSe QDs is shown in Fig. 3a. By changing this ratio, a quasilinear enhancement of the QD emission is obtained. The highest intensity of the PL, ̴ 600 counts/s, is observed when the ratio between the Ag NPs and the QDs is 4:1, above this ratio a dramatic quenching of emission, whose intensity strongly depends on the sample was observed. Fig. 3b shows two cross sections recorded from a few ring structures, as shown in the image below the inset. The black curve refers to a sample with a volume ratio of 1:1, whereas the blue curve corresponds to a volume ratio of 4:1. The signal obtained in the case of the 4:1 volume ratio is six times larger than that obtained with a 1:1 ratio.
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Fig. 3 (a) Trend of the FOM images intensity versus NPs/QDs volume ratio, collected on the same area for comparison. (b) Two cross sections have been compared: one where we got the lowest intensity obtained for NPs:QDs=1:1 (dark spectrum) and NPs:QDs=4:1 (blue spectrum); Fig. S1 shows the corresponding fluorescence optical microscopy images. 60
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In order to gain information on the QD and Ag NP distribution inside the BFs pattern, SEM measurements were performed. The secondary electron (SE) images, reported in Fig. 4a and Fig. 4b, acquired at a primary energy of 10 keV with the ET detector, show both topographic and compositional contrasts. These measurements, performed on the same sample described in Fig. 2a, clearly show that the QDs and the nanoparticles are in fact decorating only the ring structures. In more detail, the red squares depicted in Fig. 4a denote parts of the ring where the brighter white spots are the metal nanoparticles surrounded by the CdSe QDs, as demonstrated by the Energy Dispersive X-Ray (EDX) analysis, reported in Fig. 4c and performed on the same region, clearly confirming the presence of Ag, Cd and Se. The low dimension of the NPs and of the QDs, as well as the density of their distribution near the rings border, does not allow for the acquisition of the EDX spectra separately from the single nanostructures, nevertheless the EDX result permits to interpret the details with higher contrast as Ag NPs and the ones with lower contrast as CdSe QDs. In Fig. 4b a SEM image at low magnification is reported, showing an area where the Ag nanoparticles and the QDs surrounding the rings are almost uniformly distributed around the ring, as indicated by the red arrows. This uniformity is the main reason for the observation of This journal is © The Royal Society of Chemistry [year]
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a continuous emission from the rings. It should be noted that the thickness of the matrix polymer in which the particles are dipped View Article Online might prevent the detection of the NPs below a certain thickness, DOI: 10.1039/C3NR04708C that for the given primary energy can be estimated between 1 and 2 µm. However, Fig. 4 shows that the density of the distribution of the NPs is clearly much more pronounced near the rings, supporting at the same time the PL measurements and the hypothesis that, if the NPs could be located between the neighbouring rings, they should be significantly less and possibly deeper.
Fig. 4 a) High magnification SEM image from the sample (a) showing a detail of the border of a BF. Ag nanoparticles, whiter spots, and QDs are highlighted by red squares. The bar is 200 nm. b) Larger view of the BFs where the QDs and the NPs decorate the rings, as emphasized by the red arrows. The bar is 1 µm c) EDX spectrum acquired on the region reported in a) showing the presence of Ag, Cd and Se.
In conclusion, we have reported the control of spatial distribution and mean distance in mesoscopic rings of Ag NPs /QD embedded in polycarbonate thin-films. NP/QD rings were organised in an ordered honeycomb structure obtained by a very simple bottomup process such as the breath figure formation. Furthermore we proved an efficient control of the mean distance between NPs and QDs simply by changing the nanoparticle initial concentration. Finally we have demonstrated a six-fold enhancement of the QD emission when coupled with the near field from Ag NPs. This ongoing work will contribute to the development of a new generation of optoelectonic devices fabricated exploiting selforganization.
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We are grateful to Dr W. Koopman for help with the photoluminescence experiments. This work was supported by the EU project HYSENS (Contract NMP3-SL: Small scale collaborative project. Grant Agreement No.: 263091).
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Notes and references
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Consiglio Nazionale delle Ricerche - Istituto per lo Studio dei Materiali Nanostrutturati (CNR-ISMN), via P. Gobetti 101, 40129 Bologna, Italy. Fax: +39 051 6398540; Tel: +39 051 6398522; E-mail: [email protected] b Walter Schottky Institut and Physik Department, Technische Universität München, Am Coulombwall 4, DE-85748 Garching, Germany; E-mail: [email protected]; Tel. +49-(0)89-289 11366. c Photochemical Nanosciences Laboratory and Interuniversity Center for the Chemical Conversion of Solar Energy (SolarChem), Dipartimento di Chimica “G. Ciamician”, Università di Bologna, via Selmi 2, 40126 Bologna, Italy. Tel: +39 051 20 9 9540; E-mail: [email protected] d Consiglio Nazionale delle Ricerche - Istituto per la Microelettronica e Microsistemi (CNR-IMM), via P. Gobetti 101, 40129 Bologna, Italy. † Electronic Supplementary Information (ESI) available: Experimental details and additional results. See DOI: 10.1039/b000000x/
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CdSe quantum dots emission efficiency is tailored by localized surface plasmon polaritons of Ag nanoparticles in self-organized mesoscopic rings fabricated by breath figure method.
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