Plasmon Enhancement of Triplet Exciton Diffusion Revealed by

Subscriber access provided by READING UNIV

Article

Plasmon Enhancement of Triplet Exciton Diffusion Revealed by Nanoscale Imaging of Photochemical Fluorescence Upconversion Lukasz Bujak, Kaishi Narushima, Dharmendar Kumar Sharma, Shuzo Hirata, and Martin Vacha J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08495 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on November 6, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Plasmon Enhancement of Triplet Exciton Diffusion Revealed by Nanoscale Imaging of Photochemical Fluorescence Upconversion

Łukasz Bujak1,2,*, Kaishi Narushima1, Dharmendar Kumar Sharma1, Shuzo Hirata1, Martin Vacha1*

1

Department of Materials Science and Engineering, Tokyo Institute of Technology, Ookayama 2-

12-1-S8-44, Meguro-ku, Tokyo 152-8552, Japan. 2

Institute of Photonics and Electronics, Czech Academy of Sciences, Chaberská 57, 18251

Prague, Czech Republic

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Photon upconversion based on the process of triplet-triplet annihilation in a system of organic donor and acceptor molecules has been attracting increasing attention because it can potentially lead to improved efficiency of light-energy conversion devices working under sunlight irradiation. Here, we aim at gaining insight into the effect of localized plasmons of metal nanostructures on the individual photophysical steps involved in the upconversion mechanism. We present an optical microscopic study of the photophysical properties of plasmonic hybrid nanostructures composed of silver nanowires combined with an upconversion system consisting of platinum octaethylporphyrin donor and 9,10-diphenylanthracene acceptor molecules dispersed in poly(methyl methacrylate) thin film. Using an image-splitting technique we record simultaneously upconversion and phosphorescence microscopic images and analyse the results to decouple the individual photophysical events. In addition to moderate intensity enhancement on the nanowires, the upconversion emission intensity can be enhanced up to 15 fold in the vicinity of hotspots formed by the silver nanowire junctions, compared to 4-5 fold enhancement of phosphorescence in the same locations. Further, whereas the phosphorescence enhancement is localized in the hotspots, the upconversion emission is enhanced along micrometer distances on the top of the nanowires. These finding are interpreted in terms of plasmon enhancement of Dexter-type energy transfer between the triplet states of the donor and acceptor, as well as between the triplet states of the acceptor molecules. The latter gives rise to the apparent longdistance propagation of the triplet excitons along the nanowires.


Page 2 of 26

Page 3 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Photon upconversion (UC) based on triplet-triplet annihilation (TTA) between chromophores was reported for the first time in 1960s higher energy ones

2,3

1

as a mechanism that can convert low energy photons to

. The triplet-triplet annihilation upconversion (TTA-UC) process is a

sequence of the following events: Excitation of a singlet state of a donor molecule, intersystem crossing into a triplet state of the donor, triplet-triplet energy transfer from the donor to an acceptor molecule, interaction between triplet states of two acceptor molecules via TTA leading to population of a singlet state of the acceptor, and upconverted fluorescence emission from the acceptor singlet state. Recent research has shown that the TTA-UC mechanism can be optimized by material design to achieve high UC quantum yield, relatively low threshold of the UC emission, and broad and well-tuneable excitation and emission characteristics 4–7. The UC process occurs efficiently even under incoherent low intensity illumination, and solar radiation is in principle sufficient to cause the TTA-UC in low viscosity fluids 5,8,9. This unique feature might lead to improved efficiency of light energy conversion devices, such as artificial photosynthesis or photovoltaic devices 10. Main obstacle in applications of soft or solution-based TTA-UC is the fact that the UC quantum yield drops significantly with an increase in viscosity of the medium in which the donor and acceptor molecules are dispersed

11,12

due to a slow-down of molecular

diffusion. Since in these devices the triplet-triplet interactions such as energy transfer and TTA are mediated by inter-molecular collisions, slower molecular diffusion results in a decrease of the interactions and in lower quantum efficiency. In order to improve the efficiency of TTA-UC, use of solid state devices in which the triplet-triplet interactions occur as a result of efficient triplet exciton diffusion 13,14 in host matrices 15 or binary crystals 6 have been demonstrated.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 26

From a different point, there has been also a considerable effort to study and potentially utilize the effect of localized plasmon resonances of noble metal nanoparticles on the various photophysical processes involved in the TTA-UC process

16–19

. In recent years, plasmonic

nanoantennas based on metal nanoparticles have been of great interest because of their tuneable localized surface plasmon resonances and possibility to produce significantly enhanced and highly confined electromagnetic fields

20–22

. Plasmonic nanoantennas are being widely used to

increase the efficiency of artificial light-harvesting systems 26–31

, or resonant energy transfer

32

23–25

, enhance fluorescence emission

. Coupling plasmonic nanoparticles with rare earth

nanophosphors leads to over two orders of magnitude energy UC enhancement

33–35

. This UC

enhancement in lanthanide ion-based systems results both from the incident field intensity enhancements (excitation enhancement) and enhancement of radiative decay from the emitting states (emission enhancement). On the other hand, in the TTA-UC systems, only the excitation enhancement has been confirmed and used in the explanation of the results 16,17, even though the possibility of intersystem crossing and/or energy transfer enhancement has been also proposed 18. Here, we aim at gaining insight into the effect of localized plasmons of metal nanostructures on and the potential enhancement of the individual photophysical steps included in the TTA-UC mechanism. We present an optical microscopic study of the photophysical properties of plasmonic hybrid nanostructures of silver nanowires combined with a TTA-UC film. We find principal differences in the influence of plasmon between phosphorescence and upconversion, both in terms of emission intensity and spatial extent of the enhancement phenomena.


Page 5 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Materials and Methods Silver nanowires (AgNW, average diameter 115 nm, length 20-30 m, Sigma Aldrich), blue-light emitting 9,10-diphenylanthracene (DPA, Tokyo Chemical Industry), the organometallic complex (2,3,7,8,12,13,17,18-octaethyl-porphyrinato)PtII (PtOEP, Sigma Aldrich) and poly(methyl methacrylate) (PMMA, Sigma Aldrich) were used as supplied. Solvent dispersion of silver nanowires was spin-coated on a cleaned microscope cover glass at 3000 rpm for 30 s. The TTAUC material was largely prepared as reported before 18,36. DPA and PtOEP were mixed in toluene with PMMA to achieve final concentrations of 40 mM of DPA, 40 µM, 20 µM or 10 µM of PtOEP and 2% of PMMA. The mixture was spin-coated at 3000 rpm for 30 s on top of the previously spin-coated AgNW. The resulting solid PMMA film contained DPA at the concentration of 2 M and PtOEP at the concentrations of 0.5 mM, 1 mM and 2 mM, respectively. The samples were used for experiments as prepared. Bulk absorption and emission spectra were measured using a standard UV-Visible/NIR spectrophotometer (V-760, Jasco) and a real-time multichannel spectrometer (PMA-12 C1002701, Hamamatsu Photonics), respectively. Microscopic measurements were performed with an inverted fluorescence microscope (IX 71, Olympus). The excitation light was provided by a continuous wave laser (TDG532-500, Changchun New Industries Optoelectronics Tech.) at 532 nm. Emission from the sample was collected by an oil immersion objective lens ( 100 UPLSAPO, N.A. 1.3, Olympus) with dichroic mirror (FF552-Di01-25-25×36, Semrock) and a notch filter (NF01-532U-25, Semrock). The emission was further divided spectrally by an image splitter (Optosplit II, Cairn) equipped with a dichroic mirror (FF 532-Di02-25×36, Semrock), into a short-wavelength part corresponding to



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the UC emission from the acceptor molecules, and a long-wavelength part corresponding to the phosphorescence from PtOEP. The UC emission and phosphorescence were further filtered with a short-pass filter (FF01-492/SP-25, Semrock) and a long-pass filter (590 nm, Nikon), respectively, and imaged side-by-side on an electron-multiplying (EM) CCD camera (iXon, Andor Technology). To measure microscopic fluorescence emission from the DPA, the same optical setup was used except for using only the short-pass filter (FF01-492/SP-25, Semrock) for imaging. All images were corrected by background subtraction. The background was determined by recording an image of the sample after bleaching the fluorophores by strong illumination in an oxygen environment. After the bleaching there was no noticeable signal from the fluorophores, and such image was used as a background. The background subtraction treatment also ensures that there is no contribution from intrinsic emission from the AgNWs, both from isolated parts and from the hotspots. The emission intensities in all the microscopic images in the main text and the Supporting Information are normalized and the contrasts are adjusted for the same levels.

Results and discussion The samples, as schematically shown in Fig. 1a, are composed of a mesh of silver nanowires (AgNW) randomly dispersed on a microscope cover glass substrate (as seen in the dark-field image in Fig. 1b) and coated with a TTA-UC active material. The active material consists of donor molecules of platinum octaethylporphyrin (PtOEP), and acceptor molecules of 9,10diphenylanthracene (DPA), dispersed in a matrix of poly(methyl methacrylate) (PMMA). Normalized extinction and emission spectra of the donor and acceptor molecules in solution are


Page 6 of 26

Page 7 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shown in Fig. 1c. The donor extinction spectrum is dominated by the Soret absorption band (at 380 nm) and shows the typical porphyrin Q-band at 535 nm. The emission spectrum of the PtOEP consists of phosphorescence occurring from the triplet state at 645 nm with a quantum yield of ~ 0.4 (in toluene solution)

37

. The acceptor extinction spectrum shows several strong

absorption bands below 400 nm, and its fluorescence has peaks at 412 nm and 432 nm. As such, the emission spectrum from DPA does not exhibit any significant spectral overlap with either the Soret or the Q-band absorption of PtOEP and this feature ensures that the UC emission intensity is not significantly damped by an inner filter effect. The AgNW extinction spectrum is broad with a maximum around 400 nm and a long tail extending above 700 nm. This broadband absorption ensures that plasmon resonances of AgNW could both couple with the absorption and emission of the chromophores dispersed in the film, and mediate and enhance the energy transfer processes. Fluorescence microscopic image of the sample obtained with a direct excitation of singlet states of DPA at 375 nm is shown in Fig. 2a. In this image, fluorescence of the DPA molecules is emitted uniformly from the whole area of the film and, in addition, there are visible structures showing higher emission intensity. Comparison of the fluorescence image with a transmission image of the same area proves that these higher-intensity structures correspond to enhanced fluorescence from DPA in close vicinity of the AgNW. To obtain an enhancement factor of fluorescence due to the presence of the AgNW, fluorescence intensity integrated along an AgNW was divided by a reference fluorescence intensity integrated along a line of the same length in the film close to the AgNW. The obtained average enhancement of the DPA emission was on the order of 1.1. This is a moderate value, but still indicates that distances between the nanowires and the DPA molecules are within a range in which both absorption and emission can be affected by plasmon resonance of the AgNW.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

Microscopic image of the sample excited in the absorption Q-band of the donor PtOEP molecules (at 532 nm) and detected in the region of phosphorescence (above 590 nm) is shown on Fig 2b (with more examples shown in Fig. S1 in Supporting Information). We note that this image shows a different area of the sample than that shown in Fig. 2a. The phosphorescence is emitted uniformly throughout the film. On top of this uniform background, structures of higher intensity are visible which can be assigned to phosphorescence emitted in close vicinity to the AgNW. The phosphorescence enhancement along the nanowires is on the order of 1.1, similar to the directly excited DPA fluorescence. In addition to the long AgNW structures we observe that the phosphorescence is enhanced much further (by a factor of 4-5) in locations where two or more AgNW overlap (marked by a cross in Fig. 2b). Such nanowire crossings (junctions) can form hotspots with highly confined and highly enhanced local electric fields

28,29,38–40

which

cause the dramatic enhancement of phosphoresce. To avoid complexities arising from junctions involving more than two AgNWs, we were careful to choose for analysis only those hotspots for which we could visually verify that they are formed by only two nanowires. We also note that we have not observed any signs of hotspots at the ends of the AgNWs. To discuss the effect of plasmon on the various factors in UC emission, we assume that the overall quantum yield ΦUC of TTA-UC in the system can be written as 36 Φ UC =1/2f · ΦISC · ΦET · ΦTTA · ΦA · γex

(1)

where ΦISC, ΦET, ΦTTA, and ΦA represent the quantum efficiencies of, respectively, the donor intersystem crossing, the triplet-triplet energy transfer, the TTA, and the acceptor emission, f is statistical probability for obtaining a singlet excited state from the annihilation of two triplets (~ 0.52 for DPA

36

), and γex is an excitation rate. Since for DPA ΦA is close to 1, there is little


Page 9 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


room for enhancement of fluorescence of the acceptor due to the enhancement of radiative rate, and we also assume that the non-radiative rate of the acceptor is negligibly affected by the plasmon, though we do not have experimental support for this assumption as the thin-film sample structure and the issue of photostability prevented us from measuring the emission lifetimes. Further, as we have experimentally observed above, the enhancement of emission in the vicinity of the AgNWs upon direct excitation of DPA and PtOEP leads to the same enhancement factors (1.1). This, together with fact that ΦA of DPA is unaffected by the plasmons, points to the conclusion that there is also no change in ΦISC due to the localized plasmons. The enhancement in both cases can be attributed to higher excitation rate γex due field enhancement. In the further discussion we thus consider no effect of plasmon on the ΦA and ΦISC parameters. Microscopic image of the same sample area taken simultaneously with the phosphorescence image (Fig. 2b) upon the same excitation (at 532 nm) and detected in the region of UC emission (below 492 nm) is shown in Fig. 2c. The simultaneous imaging was carried out using an imagesplitter equipped with a dichroic mirror and placed between the microscope and the CCD camera 41. The UC and phosphorescence images are very similar. The UC emission forms a uniform background throughout the film, upon which the AgNW structures appear with higher intensity. The enhancement of UC intensity on the AgNWs is on the order of 1.3. On the junctions of the AgNWs the enhancement increased up to a factor of 15. In addition, there is a striking difference between the UC and phosphorescence images with respect to the emission pattern near the AgNW junctions. In the UC image, the strongly enhanced emission is clearly elongated along one of the nanowires extending from the junction. This difference is further obvious in the 1D cross-section plots of the emission intensity. In the cross-sections along the direction of the AgNW (Fig. 2d) the UC (black symbols) is significantly broader than the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

phosphorescence (red symbols). For comparison, there is no difference between the UC and phosphorescence in the cross-sections taken perpendicular to the AgNWs (Fig. 2e). To evaluate and compare the spatial dispersion of phosphorescence and UC emission we fitted the experimental cross-sections (such as those in Fig. 2d,e) with a convolution of a Gaussian function with an exponential function 41. The UC intensity distribution Iuc(x) is written as ∗

(2)

Here, the Gaussian function fPSF(x) represents a point spread function (PSF) of the microscope and accounts for the diffraction limited optical resolution of the system. The exponential function ns(x) tentatively describes diffusion of the triplet excitons which are assumed to be the cause of the spatial dispersion

42

. The singlet exciton distribution resulting from the triplet diffusion is

described as ~ exp

| |

(3)

where Luc is a dispersion length of the UC emission. Analogous equations are used for the phosphorescence intensity distribution Iph(x). Full forms of the convolution equations used for fitting of the 1D intensity cross-sections are provided in Supporting Information. In the fitting, theoretical PSF for the particular wavelengths are used and LUC and the corresponding LPH are fitting parameters. The fits to the experimental intensity cross-sections for both UC emission and phosphorescence are shown as smooth lines in Fig. 2d for the direction along the AgNW, and in Fig. 2e in the direction perpendicular to them. In all cases, the fits very well reproduce the experimental data points. The fits provide a difference between the dispersion lengths in the


Page 11 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


directions along (parallel to) the AgNW and in those perpendicular to it. For the UC emission this difference is ~ 600 nm whereas for phosphorescence the value is ~100 nm. The dispersion lengths were evaluated statistically by fitting the intensity profiles on several tens of AgNW junctions. The obtained dispersion lengths were then plotted as histograms, an example of which is shown in Fig. 2f for the UC and phosphorescence along the direction of the AgNW. The histogram means then provide average dispersion lengths, shown schematically in the diagram in Fig. 2g. From this figure it is obvious that while the dispersion of phosphorescence in both directions and that of UC in the perpendicular direction to the AgNW are comparable, the dispersion of UC along the AgNW is more than double of that, and reflects a qualitatively different phenomenon. To get more insight into this phenomenon, we examined the enhancement and dispersion factors for different excitation intensities and for different concentrations of the donor Pt-OEP molecules in the film. Figure 3a,b shows excitation intensity dependences of UC (a) and phosphorescence (b) analysed at three different types of locations in the sample: 1. at the hotspots formed by the AgNW junctions, 2. on the AgNW away from the hotspots and, 3. inside the film away from the AgNW (as shown schematically in the image insets). A limited number of excitation intensity values was chosen to avoid the problem of sample degradation. The Fig. 3 a,b shows that in all types of locations the UC emission intensity can be fitted reasonably well with a quadratic function whereas the phosphorescence intensity is well fitted with a linear function. The insets of the figure show logarithmic plots of the same dependences together with linear fits. The average slopes obtained from these fits were 1.72 for UC and 1.28 for phosphorescence. UC intensity is known to depend quadratically on the excitation intensity in low-intensity excitation range and to change to linear dependence for high-intensity range. The slope of 1.72 observed



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

here may reflect a transition region between the quadratic and linear parts of the dependence. Phosphorescence is expected to depend linearly on the excitation intensity and this is reflected in the value of 1.28 of the slope. From this point, the behavior of UC from the hotspots does not differ from that originating from the other types of locations. The excitation intensity dependences of both UC and phosphorescence originating from the hotspots were further analyzed in terms of emission intensity enhancement, defined as a ratio between the intensity in the hotspots to that in the film away from the AgNW. As the Fig. 3c shows there is no apparent trend in the enhancement with increasing excitation intensity, and the values of the enhancement remain approximately 15 for the UC and 4-5 for phosphorescence. In a similar way, there is no apparent trend in the average dispersion length analyzed from the hotspots along the direction of the AgNW, as evident from the Fig. 3d. For UC emission the length is around 1000 nm and for phosphorescence about half of this value. All the above results were obtained for samples with the concentration of the donor Pt-OEP molecules of 2 mM. We have reproduced these experiments also for two other donor concentrations, 1 mM and 0.5 mM, and shown the results in the Fig. S2 in Supporting Information. The results confirmed that for the two lower concentrations there is also no clear trend in the dependence of the extent of enhancement or dispersion length on increasing excitation intensity. On the other hand, the lower amount of the donor molecules (both of 1 mM and 0.5 mM ) causes a slight decrease in the overall UC dispersion length, as seen in Fig. S2 d-f. The novel finding of this work is the observation that UC is enhanced by the localized plasmon in Ag nanowire junctions in qualitatively different way than phosphorescence in the same locations. The difference is demonstrated as different enhancement of emission intensity (by a factor of 15 for UC compared to a factor of 4-5 for phosphorescence), and as different spatial


Page 13 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dispersion of the enhanced emission along the nanowire direction (of dispersion length of approximately 1000 nm for UC as compared to about 500-600 nm for phosphorescence). As will be argued below, the differences can be explained by plasmon enhancement of triplet-triplet energy transfer (triplet exciton diffusion) between the donor and/or acceptor molecules. Regarding the different dispersion lengths, we first consider trivial explanations such as unspecified optical effect related to a difference in the spectral region between UC and phosphorescence. Such effect can be excluded because we have not observed any similar phenomena upon direct excitation of (and fluorescence from) the singlet excited states of the DPA acceptors, as seen in Fig. 2a. Another trivial explanation of some of the phenomena would be that the organic molecules are not evenly dispersed on or around the AgNWs. However, since the molecules are spin-coated from a polymer solution which itself forms a film after the coating, one would intuitively not expect extreme differences in local dye concentration in the filmdispersed molecules. Another observation that would speak against inhomogeneous distribution of the molecules around the nanowires is that we do not see any directional dependence of the phenomena. Since spin-coating is locally a very anisotropic process we may expect that in different parts of the sample we would observe different magnitude of the phenomena along different directions of the nanowires, but within the examined locations any dependence on the nanowire orientation seems to be absent, and the enhancements are completely random. We may summarize the experimental observations by assuming that: 1. Triplet states of the donor Pt-OEP are generated by the 532 nm excitation within the whole sample and lead to phosphorescence of the donor and UC emission from acceptor (as the DPA acceptor cannot be directly excited at 532 nm). 2. The generation of the donor triplets is enhanced on top of AgNWs by localized plasmons, leading to intensity enhancement of both phosphorescence (by a factor of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

1.1) and UC emission (by a factor of 1.3). Since the directly excited fluorescence form DPA is enhanced by a factor of 1.1 and since there is very little spectral dependence of scattering intensity from individual AgNW (as seen in localized nanoscale scattering spectra in Fig. S3 in the Supporting Information), the increase to 1.3 in the enhancement of UC intensity has to be related to enhancement of either donor-acceptor and/or acceptor-acceptor triplet-triplet energy transfer. 3. The junctions of individual AgNW work as plasmonic hotspots and further enhance the generation of triplet states of the donor, as evidenced by the increased intensity of phosphorescence from these location (with enhancement factor of 4-5). The locations of the junctions have thus higher density of donor triplets compared to rest of the sample and work as localized donor triplet sources. With these assumptions in mind we can consider several plausible explanations of the increased spatial dispersion of UC emission along the AgNW. 1. Donor-acceptor energy transfer and acceptor-acceptor TTA generate singlet excitons of DPA in the hotspots. The singlet excitons migrate (by Foerster energy transfer, FRET) along the nanowires and the FRET itself is enhanced by the localized plasmon of the nanowires. This explanation is unlikely because the same effect of enhanced FRET should be observable in direct excitation of the DPA singlets and there is no sign of that in Fig. 2a. 2. High density of donor triplets in the hotspots and energy transfer to acceptors would lead to isotropic diffusion of acceptor triplets within the film from the hotspot and to higher acceptor triplet density homogeneously distributes around the hotspot. This would result in higher UC intensity around the hotspot compared to more distant locations, and enhancement of such UC by the AgNWs would cause the apparent spatial dispersion along the AgNWs. This explanation is unlikely because the upconversion intensity in the direction perpendicular to the AgNW is not higher compared to other locations in the film (far from the


Page 15 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AgNWs). 3. The hotspots in the AgNW not only generate higher density of the donor triplets but also enhance the donor-acceptor triplet energy transfer. Further, the plasmons also enhance the acceptor-acceptor triplet-triplet energy transfer (triplet exciton diffusion) within the hotspots and along the AgNW. The result of the enhancement of both types of energy transfer within the hotspot is an increased enhancement of UC emission compared to phosphorescence, whereas enhancement of the triplet diffusion along the AgNW causes a larger spatial dispersion of UC emission along the nanowires. The proposed mechanisms of plasmon enhancement effects in phosphorescence and UC are schematically summarized in Fig. 4. At present, we do not find any experimental evidence that would contradict this last explanation. We note that the mechanism proposed above does not explain the fact that the enhanced UC emission is elongated along only one of the AgNW extending from the junction. One trivial reason could be that since the hotspot is formed at the crossing of two nanowires lying flat on the surface, the triplet excitons will be propagating from such hotspot along the two nanowires. On one of these nanowires they will be located on the front side facing the objective lens and the UC emission from these nanowires will be directly detected. On the other nanowire they will be propagating on the back side with respect to the objective lens and the emission from these excitons will be blocked by the nanowire itself. Still, this phenomenon will need further exploration.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

Conclusions In this work we made a novel observation that UC is enhanced in AgNW junctions in qualitatively different way than phosphorescence in the same locations. The difference is demonstrated as different enhancement of emission intensity and as different spatial dispersion of the enhanced emission along the nanowire direction. The proposed explanation for the differences includes plasmon enhancement of triplet-triplet energy transfer (triplet exciton diffusion) between the donor and/or acceptor molecules. Since this is the first observation of plasmon-assisted triplet propagation, there is no direct comparison in literature. Generally, for molecular crystals the plasmon un-assisted triplet diffusion length can reach the order of microns43 (e.g., ~ 10 microns for large single crystals of purified anthracene). Such long diffusion lengths are reduced by the presence of impurities, surfaces, grain boundaries, etc. For polycrystalline thin film of DPA prepared by spin-coating we observed un-assisted diffusion lengths of few hundreds of nm 41. In this context, the measured propagation length of ~ 1 micron can be considered reasonable. So far, plasmon enhancement of Dexter-type triplet-triplet energy transfer has not been reported, and the concrete mechanism of such process is not completely understood. The Dexter-type energy transfer involves exchange of electrons between ground and excited states of the donor and acceptor molecules. One possible mechanism of the enhancement could be direct involvement or mediation of the metal electrons in the energy transfer process. Wavefunction overlap of the donor and acceptor molecular orbitals with those of the silver could lead to a process in which the energy transfer involves donor-AgNW electron exchange and AgNWacceptor electron exchange. Involvement of the AgNW electrons could results in enhancement of the Dexter radius (energy transfer distance) as well as enhancement of the transfer efficiency.


Page 17 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Further experimental and theoretical work will be needed to resolve this problem. Such work will include systematic examination of various parameters such as diameter and aspect ratio of the plasmon nanostructures and the related effect of higher plasmon modes, gap distance, composition of the noble metal, etc., and their effect on the enhancement phenomena. Preliminary results on nanowires of smaller diameter (60 nm) show that the large dispersion lengths in UC emission are not observable for such AgNWs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Full form of the convolution equation (2); examples of microscopic images of phosphorescence and upconversion; effect of donor molecule concentration on the enhancement; dark-field scattering images and spectra of the AgNW.

AUTHOR INFORMATION Corresponding Authors: *E-mail: [email protected]; [email protected]

ACKNOWLEDGMENT This work was supported by JSPS and NSF under the JSPS-NSF Program International Collaborations in Chemistry (ICC), and by JSPS KAKENHI Grant Number JP26107014 in Scientific Research on Innovative Areas “Photosynergetics”.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

REFERENCES (1) (2) (3) (4) (5) (6)

(7) (8) (9) (10)

(11) (12) (13) (14) (15) (16)

Parker, C. A. Sensitized P-Type Delayed Fluorescence. Proc. R. Soc. Lond. Math. Phys. Eng. Sci. 1963, 276, 125–135. Singh-Rachford, T. N.; Castellano, F. N. Photon Upconversion Based on Sensitized Triplet–triplet Annihilation. Coord. Chem. Rev. 2010, 254, 2560–2573. Baluschev, S.; Katta, K.; Avlasevich, Y.; Landfester, K. Annihilation Upconversion in Nanoconfinement: Solving the Oxygen Quenching Problem. Mater. Horiz. 2016, 3, 478– 486. Murakami, Y. Photochemical Photon Upconverters with Ionic Liquids. Chem. Phys. Lett. 2011, 516, 56–61. Baluschev, S.; Miteva, T.; Yakutkin, V.; Nelles, G.; Yasuda, A.; Wegner, G. UpConversion Fluorescence: Noncoherent Excitation by Sunlight. Phys. Rev. Lett. 2006, 97, 143903. Kamada, K.; Sakagami, Y.; Mizokuro, T.; Fujiwara, Y.; Kobayashi, K.; Narushima, K.; Hirata, S.; Vacha, M. Efficient Triplet–triplet Annihilation Upconversion in Binary Crystalline Solids Fabricated via Solution Casting and Operated in Air. Mater. Horiz. 2017, 4, 83–87. Duan, P.; Yanai, N.; Nagatomi, H.; Kimizuka, N. Photon Upconversion in Supramolecular Gel Matrixes: Spontaneous Accumulation of Light-Harvesting Donor–Acceptor Arrays in Nanofibers and Acquired Air Stability. J. Am. Chem. Soc. 2015, 137, 1887–1894. Islangulov, R. R.; Lott, J.; Weder, C.; Castellano, F. N. Noncoherent Low-Power Upconversion in Solid Polymer Films. J. Am. Chem. Soc. 2007, 129, 12652–12653. Simon, Y. C.; Weder, C. Low-Power Photon Upconversion through Triplet–triplet Annihilation in Polymers. J. Mater. Chem. 2012, 22, 20817–20830. Cheng, Y. Y.; Nattestad, A.; Schulze, T. F.; MacQueen, R. W.; Fückel, B.; Lips, K.; Wallace, G. G.; Khoury, T.; Crossley, M. J.; Schmidt, T. W. Increased Upconversion Performance for Thin Film Solar Cells: A Trimolecular Composition. Chem. Sci. 2016, 7, 559–568. Murakami, Y.; Kikuchi, H.; Kawai, A. Kinetics of Photon Upconversion in Ionic Liquids: Time-Resolved Analysis of Delayed Fluorescence. J. Phys. Chem. B 2013, 117, 5180– 5187. Zhou, Q.; Zhou, M.; Wei, Y.; Zhou, X.; Liu, S.; Zhang, S.; Zhang, B. Solvent Effects on the Triplet–triplet Annihilation Upconversion of Diiodo-Bodipy and Perylene. Phys. Chem. Chem. Phys. 2017, 19, 1516–1525. Yu, X.; Cao, X.; Chen, X.; Ayres, N.; Zhang, P. Triplet-Triplet Annihilation Upconversion from Rationally Designed Polymeric Emitters with Tunable Inter-Chromophore Distances. Chem. Commun. 2015, 51, 588–591. Monguzzi, A.; Tubino, R.; Meinardi, F. Upconversion-Induced Delayed Fluorescence in Multicomponent Organic Systems: Role of Dexter Energy Transfer. Phys. Rev. B 2008, 77, 155122. Wu, M.; Congreve, D. N.; Wilson, M. W. B.; Jean, J.; Geva, N.; Welborn, M.; Van Voorhis, T.; Bulović, V.; Bawendi, M. G.; Baldo, M. A. Solid-State Infrared-to-Visible Upconversion Sensitized by Colloidal Nanocrystals. Nat. Photonics 2016, 10, 31–34. Poorkazem, K.; Hesketh, A. V.; Kelly, T. L. Plasmon-Enhanced Triplet–Triplet Annihilation Using Silver Nanoplates. J. Phys. Chem. C 2014, 118, 6398–6404.


Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Kim, H.; Weon, S.; Kang, H.; Hagstrom, A. L.; Kwon, O. S.; Lee, Y.-S.; Choi, W.; Kim, J.-H. Plasmon-Enhanced Sub-Bandgap Photocatalysis via Triplet–Triplet Annihilation Upconversion for Volatile Organic Compound Degradation. Environ. Sci. Technol. 2016, 50, 11184–11192. (18) Cao, X.; Hu, B.; Ding, R.; Zhang, P. Plasmon-Enhanced Homogeneous and Heterogeneous Triplet–triplet Annihilation by Gold Nanoparticles. Phys. Chem. Chem .Phys. 2015, 17, 14479–14483. (19) Wu, D. M.; García-Etxarri, A.; Salleo, A.; Dionne, J. A. Plasmon-Enhanced Upconversion. J. Phys. Chem. Lett. 2014, 5, 4020–4031. (20) Eustis, S.; El-Sayed, M. Aspect Ratio Dependence of the Enhanced Fluorescence Intensity of Gold Nanorods: Experimental and Simulation Study. J. Phys. Chem. B 2005, 109, 16350–16356. (21) Kühn, S.; Håkanson, U.; Rogobete, L.; Sandoghdar, V. Enhancement of Single-Molecule Fluorescence Using a Gold Nanoparticle as an Optical Nanoantenna. Phys. Rev. Lett. 2006, 97, 017402. (22) Atwater, H. A. The Promise of Plasmonics. Sci. Am. 2007, 296, 56–63. (23) Aubry, A.; Lei, D. Y.; Fernández-Domínguez, A. I.; Sonnefraud, Y.; Maier, S. A.; Pendry, J. B. Plasmonic Light-Harvesting Devices over the Whole Visible Spectrum. Nano Lett. 2010, 10, 2574–2579. (24) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205–213. (25) Li, J.; Cushing, S. K.; Meng, F.; Senty, T. R.; Bristow, A. D.; Wu, N. Plasmon-Induced Resonance Energy Transfer for Solar Energy Conversion. Nat. Photonics 2015, 9, 601–607. (26) Anger, P.; Bharadwaj, P.; Novotny, L. Enhancement and Quenching of Single-Molecule Fluorescence. Phys. Rev. Lett. 2006, 96, 113002. (27) Taminiau, T. H.; Stefani, F. D.; Segerink, F. B.; Hulst, N. F. van. Optical Antennas Direct Single-Molecule Emission. Nat. Photonics 2008, 2, 234–237. (28) Kinkhabwala, A.; Yu, Z.; Fan, S.; Avlasevich, Y.; Müllen, K.; Moerner, W. E. Large Single-Molecule Fluorescence Enhancements Produced by a Bowtie Nanoantenna. Nat. Photonics 2009, 3, 654–657. (29) Bujak, Ł.; Czechowski, N.; Piatkowski, D.; Litvin, R.; Mackowski, S.; Brotosudarmo, T. H. P.; Cogdell, R. J.; Pichler, S.; Heiss, W. Fluorescence Enhancement of Light-Harvesting Complex 2 from Purple Bacteria Coupled to Spherical Gold Nanoparticles. Appl. Phys. Lett. 2011, 99, 173701. (30) Khatua, S.; Paulo, P. M. R.; Yuan, H.; Gupta, A.; Zijlstra, P.; Orrit, M. Resonant Plasmonic Enhancement of Single-Molecule Fluorescence by Individual Gold Nanorods. ACS Nano 2014, 8, 4440–4449. (31) Puchkova, A.; Vietz, C.; Pibiri, E.; Wünsch, B.; Sanz Paz, M.; Acuna, G. P.; Tinnefeld, P. DNA Origami Nanoantennas with over 5000-Fold Fluorescence Enhancement and SingleMolecule Detection at 25 ΜM. Nano Lett. 2015, 15, 8354–8359. (32) Bujak, Ł.; Ishii, T.; Sharma, D. K.; Hirata, S.; Vacha, M. Selective Turn-on and Modulation of Resonant Energy Transfer in Single Plasmonic Hybrid Nanostructures. Nanoscale 2017, 9, 1511–1519. (33) Zhang, W.; Ding, F.; Chou, S. Y. Large Enhancement of Upconversion Luminescence of NaYF4:Yb3+/Er3+ Nanocrystal by 3D Plasmonic Nano-Antennas. Adv. Mater. 2012, 24, OP236-OP241.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

(34) Luo, Q.; Chen, Y.; Li, Z.; Zhu, F.; Chen, X.; Sun, Z.; Wei, Y.; Guo, H.; Wang, Z. B.; Huang, S. Large Enhancements of NaYF4:Yb/Er/Gd Nanorod Upconversion Emissions via Coupling with Localized Surface Plasmon of Au Film. Nanotechnology 2014, 25, 185401. (35) Piatkowski, D.; Hartmann, N.; Macabelli, T.; Nyk, M.; Mackowski, S.; Hartschuh, A. Silver Nanowires as Receiving-Radiating Nanoantennas in Plasmon-Enhanced upConversion Processes. Nanoscale 2015, 7, 1479–1484. (36) Monguzzi, A.; Tubino, R.; Hoseinkhani, S.; Campione, M.; Meinardi, F. Low Power, NonCoherent Sensitized Photon up-Conversion: Modelling and Perspectives. Phys. Chem. Chem. Phys. 2012, 14, 4322–4332. (37) Bansal, A. K.; Holzer, W.; Penzkofer, A.; Tsuboi, T. Absorption and Emission Spectroscopic Characterization of Platinum-Octaethyl-Porphyrin (PtOEP). Chem. Phys. 2006, 330, 118–129. (38) Lee, S. J.; Morrill, A. R.; Moskovits, M. Hot Spots in Silver Nanowire Bundles for Surface-Enhanced Raman Spectroscopy. J. Am. Chem. Soc. 2006, 128, 2200–2201. (39) Polyakov, B.; Zabels, R.; Sarakovskis, A.; Vlassov, S.; Kuzmin, A. Plasmonic Photoluminescence Enhancement by Silver Nanowires. Phys. Scr. 2015, 90, 094008. (40) Mack, D. L.; Cortés, E.; Giannini, V.; Török, P.; Roschuk, T.; Maier, S. A. Decoupling Absorption and Emission Processes in Super-Resolution Localization of Emitters in a Plasmonic Hotspot. Nat. Commun. 2017, 8, 14513. (41) Narushima, K.; Hirata, S.; Vacha, M. Nanoscale Triplet Exciton Diffusion via Imaging of Up-Conversion Emission from Single Hybrid Nanoparticles in Molecular Crystals. Nanoscale 2017, 10653–10661. (42) Irkhin, P.; Biaggio, I. Direct Imaging of Anisotropic Exciton Diffusion and Triplet Diffusion Length in Rubrene Single Crystals. Phys. Rev. Lett. 2011, 107, 017402. (43) Mikhnenko, O. V.; Blom, P.W.M.; Nguyen, T.-Q. Energy Environ. Sci., 2015 8, 1867– 1888.


Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1 Structure and properties of the samples. (a) Scheme of the sample structure prepared on a microscope cover glass. (b) Dark-field microscopic image of the silver nanowires deposited on the cover-glass. Scale bar 5μm. (c) Extinction (full lines) and emission (dashed lines) spectra of toluene solutions of donor molecules (PtOEP, red), acceptor molecules (DPA, black) and suspension of silver nanowires (blue).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

Figure 2 Microscopic characterization. (a) Image of fluorescence from acceptor molecules in the hybrid sample obtained with direct excitation of the acceptor singlet states with 375 nm excitation laser; scale bar 2 μm. (b) Image of phosphorescence from donor molecules in the hybrid sample obtained with 532 nm excitation laser; scale bar 2 μm. (c) Image of upconversion emission from acceptor molecules in the hybrid sample obtained with 532 nm excitation laser; scale bar 2 μm. The emission intensities in (a), (b) and (c) are normalized and the contrasts are adjusted for the same levels. (d) 1D intensity profiles along the long axis of the nanowire of upconversion emission (black symbols) and phosphorescence (red symbols); the profiles were taken along a cross-section indicated by yellow line in (b) and (c). (e) 1D intensity profiles perpendicular to the long axis of the nanowire of upconversion emission (black symbols) and phosphorescence (red symbols); the profiles were taken along a cross-section indicated by blue line in (b) and (c). The solid lines in (d) and (e) represent fits as described in the text. (f) Histograms of dispersion lengths obtained from fitting the intensity profiles of upconversion emission (grey bars) and phosphorescence (red bars) along the long axis of the nanowires; the solid lines are Gaussian fits. (g) Average dispersion lengths obtained as means of the corresponding distribution of upconversion emission (grey bars) and phosphorescence (red bars) along and perpendicular to the nanowires, as indicated by the symbols. All data in the figure were taken for a sample with a concentration of the donor Pt-OEP molecules of 2 mM.


Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3 Excitation intensity dependence. (a) Upconversion emission intensity as a function of excitation intensity of the 532 nm laser, taken in the film away from the AgNW (black symbols), on the AgNW away from the junctions (blue symbols) and on the AgNW junctions (red symbols). The positions are schematically shown in the image inset. The solid lines represent quadratic fits to the data. Inset: Plot of logarithms of the same data. The linear fits have an average slope of 1.72. (b) Phosphorescence intensity as a function of excitation intensity of the 532 nm laser, taken in the film away from the AgNW (black symbols), on the AgNW away from the junctions (blue symbols) and on the AgNW junctions (red symbols). The positions are schematically shown in the image inset. The solid lines represent linear fits to the data. Inset: Plot of logarithms of the same data. The linear fits have an average slope of 1.28. (c) Excitation intensity dependence of average intensity enhancement of upconversion emission (black symbols) and phosphorescence (red symbols) measured on the AgNW junctions. (d) Excitation intensity dependence of average dispersion length along the AgNW obtained by fitting procedure as described in the text for upconversion emission (black symbols) and phosphorescence (red



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

symbols). All data in the figure were taken for a sample with a concentration of the donor Pt-OEP molecules of 2 mM. All error bars indicate standard deviations.


Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4 A schematic plot of the proposed mechanisms of plasmon enhancement in (a) phosphorescence and (b) UC.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC graphic


Page 26 of 26

Plasmon Enhancement of Triplet Exciton Diffusion Revealed by

Recommend Documents