Revealing the Mechanism of Photoluminescence from Single Gold

Subscriber access provided by UOW Library

Article

Revealing the Mechanism of Photoluminescence from Single Gold Nanospheres by Defocused Imaging Tao Li, Fanwei Zhang, Qiang Li, Wenye Rao, Pei Xu, Lei Xu, and Lijun Wu ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00376 • Publication Date (Web): 13 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 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.

ACS Photonics 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 17

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

ACS Photonics

Revealing the Mechanism of Photoluminescence from Single Gold Nanospheres by Defocused Imaging † Tao Li, †,‡ Fanwei Zhang, † Qiang Li, † Wenye Rao, † Pei Xu, † Lei Xu, † Li-Jun Wu *,† †

Laboratory of Nanophotonic Functional Materials and Devices, School for Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou 510006, P.R. China

‡

Beijing National Laboratory for Condensed Matter Physics and Key Laboratory of Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Corresponding Author *Email: [email protected] Address: High Education Mega Centre 378 Waihuan West Road, Guangzhou 510006, P.R. China Phone: 0086-020-39310366 Fax: 0086-020-39310083 Notes The authors declare no competing financial interest.

ABSTRACT: The mechanism for the photoluminescence (PL) emission from gold nanoparticles has attracted considerable attention for many years. However, there is an important gap between small nanoclusters (~2 nm) and larger plasmonic particles (~50 nm). In this work, using defocused imaging technique, we investigate the PL properties of gold nanospheres (15-20 nm in diameter) on a single-particle level. Photo-blinking and photo-bleaching phenomena are both observed. We notice that although these nanospheres can support surface plasmon resonance (SPR) at ~515 nm, they emit at ~630 nm (excited by 532 nm), which is obviously plasmonindependent. The observed defocused images (DIs) exhibit isotropy first, and then either transform into anisotropy, or vanish rapidly. Surprisingly, the DIs can change their emission pattern within one single nanosphere during the tracing time. All these PL properties suggest a multiple-dipole-emission model. Highresolution transmission electron microscopy images demonstrate that these nanospheres are polycrystalline containing multiple small crystal domains (~3 nm). We believe that these small domains divide the particle into different clusters, and give rise to the photo-blinking behavior and rotary DIs. The presented PL mechanism links gold nanoclusters and single-crystal nanorods, which could be helpful on understanding the origins of photo-blinking and the luminescent properties from metallic nanoparticles. In addition, these water-soluble gold nanospheres provides new opportunities for biological labels and light-emitting sources in nanophotonics.

ACS Paragon Plus Environment

ACS Photonics

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

KEYWORDS: gold nanospheres·photo-blinking·defocused images·polycrystalline structures

INTRODUCTION Fluorescence from gold has been observed for more than thirty years but has not garnered much attention due to the very low quantum yield (QY, ~10-10).1 Recently, much higher QY (> 10-4) has been observed from both gold nanoclusters (Au NCs, ~2 nm) and nanoparticles (Au NPs, ~50 nm).2-4 Combined with their high biocompatibility, nano-scaled gold colloids become promising materials for a variety of optical microscopic imaging techniques.5 Their photoluminescence (PL) properties have been intensively investigated in this decade.6,7 With regard to the PL mechanism from gold, a three-step process has been widely accepted.4,8 Firstly, electrons from d-band are excited to sp-band by either one-photon or multiple-photon absorption. Then they scatter close to the Fermi level, which can be considered as the second step. Finally, the excited electrons recombine with the electric holes on the d-band and emit photons. For those few-atom constituted Au NCs whose size approaches the Fermi wavelength, the free electron number decreases to a critical value and the continuous band is broken into discrete energy levels. In this case, the PL emission is supposed to arise from intraband (sp-sp) band transitions.9 Surface plasmon resonance (SPR) cannot occur in this kind of emission process. Moreover, photo-blinking (i.e. ﬂuorescence intermittency) has been observed in the PL of Au NCs.9 It can be ascribed to the Auger process,8 but still requires further investigation. Generally, Au NPs larger than 20 nm have a large number of free electrons to support plasmonic characteristics as well as continuous density of states (DOS). This can produce a strong electromagnetic field on the particle’s surface, and localize the light on nanometer scale.9-11 Therefore, a blinking-inhibited PL emission can be obtained, which arises from interband (sp-d) band transitions, and is dominated by the SPR characteristics of the particles.12 Among all these nanoscale particles, undoubtedly, gold nanorods (Au NRs) are the most fascinating subjects. They exhibit strong and highly polarized PL which is correlated well with the rods’ spatial orientation. As a result, Au NRs have been utilized in many optical and biomedical applications.13,14 Compared with Au NRs, gold nanospheres (Au NSs) are morphological isotropic, more thermodynamically stable, and thus do not exhibit very complex physical properties.15 Many initial modeling efforts and theoretical studies such as Mie theory,16 finite-element method,17 T-Matrix18 and discrete-dipole approximation (DDA) method19,20 have been developed to explain the optical behaviors of Au NSs. For a long time, these spherical particles have only been used as a media to enhance the local light absorption and emission of nearby fluorophors.21 Because it is generally a low efficiency process, the PL property of Au NSs, especially down to a single-particle level, has not been extensively researched. Upon laser illumination, Geddes et al. first observed photo-blinking from spin-coated Au NSs with an average diameter of 50 nm. They are even immune to photo-bleaching under continuous illumination.22 Orrit and coworkers reported the fluorescence enhancement from single Au NSs (~20 nm) in different solvents after illumination at moderately high power. Non-photo-blinking emission has been observed, which was attributed to laser-induced photochemistry on the particle surface.23 Loumaigne and Débarre studied the one-photon and twophoton luminescence of Au NSs (~20 nm and 50 nm) in water under pulsed illumination.24,25 The interaction


Page 2 of 17

Page 3 of 17

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

ACS Photonics

between the ligands and the particle surface, as well as the water environment were both believed to be responsible for the observed PL properties. It has also been shown that the luminescence QYs of individual Au NSs (~80 nm) are excitation wavelength dependent,26 but are roughly independent of size.27 Stephan Link et al. investigated the strongly coupled 50 nm Au NS dimers and found that the enhanced local electric field of the dimer plays a minor role on its QY, which is in contradiction to the proposed mechanisms.28 To date, although the growing partnership between super-resolution imaging and plasmonics considerably enhanced our understanding of the nanoscale world,29 the origin of one-photon PL from plasmonic nanostructures remains a subject of debate.28,30 Further investigation on the fluorescence mechanism of Au NSs can extend their applications in chemical and biological fields.31 The potential mechanism can also be helpful on understanding the origins of photo-blinking and luminescent properties of metallic nanoparticles. In the conventional imaging method, the emission from one dipole or multiple dipoles which located inside a diffraction-limited fluorescence spot appears as a single Airy disk on the focal plane. Neither the orientation of the single dipole nor the number of the dipoles can be distinguished directly. However, if we deliberately introduce an aberration (slight shift of the dipole away from the focal plane) to the imaging system, the spatial distribution of the emission (defocused image, DI) from the dipole can be observed directly. Moreover, the number of emitting dipoles can be estimated from the DI pattern. This is based on the electron transition dipole approximation and the fact that the dipole radiation exhibits an angular anisotropy. Therefore, if the radiation emitted by a single fluorophore can be approximated as a dipole, its emission characteristics can be conveniently investigated by the defocused imaging method.32 As this technique is realized in a common wide-field fluorescence microscope, it is very convenient to study the dynamics of single- or multiple-fluorophore contained within a diffraction-limited spot by tracking its optical properties. For example, we have employed it to track the dynamics of the emitting field of single CdSe/ZnS core-shell quantum dots (QDs) and proposed that the surface defects of the QDs could form a type of carrier traps, which may influence the electron cloud distribution.33 We have also validated a novel non-photo-bleaching and nonphoto-blinking optical orientation sensor at the nanoscale using this technique.34 In this paper, we utilize defocused imaging technique to track the PL from single Au NSs. It is found that the Au NSs investigated in our experiments can support SPR. When excited by a wavelength longer than their SPR, a PL peak appears at a very different frequency, indicating that the measured PL from these Au NSs is not dominated by their SPR characteristics. Moreover, the observed PL exhibits photo-blinking behaviour. By tracking the DIs, we find that most of the DIs appear isotropy first, then either transform into anisotropy, or vanish rapidly. The transform of the DI pattern can repeat several times and is normally accompanied with blinking-off periods. Rotation of anisotropic DI patterns is also observed. These phenomena strongly suggest that the PL from Au NSs is originated from a multiple-dipole emission process due to single-electron excitation between complex discrete energy states. This statement is supported by the high-resolution transmission electron microscopy (TEM) images of the Au NSs, which demonstrate their polycrystalline nature containing several small crystal domains. We believe each crystal domain can be assumed as an emitting cluster/dipole, and therefore each nanosphere contains multiple


ACS Photonics

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

clusters/dipoles. At last, the shape of the PL spectrum is found to be correlated with the DI pattern, which further confirms that the PL from individual Au NSs is composed of emissions from multiple dipoles.

EXPERIMENTSAL METHODS We prepared Au NSs by thermal reduction of gold ions in the aqueous solution. The details can be found in the reference 35. Briefly, a solution of HAuCl4 (1mM, 100 mL) was heated to boil, and then a fixed amount of sodium citrate (~ 40 mM) was added rapidly to the boiling solution. The mixture was heated for 30 min after its colour turned to claret-red and remained unchanged. During the synthesis process, sodium citrate acts as reducing and capping agents, which can stabilize the nanoparticles and prevent aggregation. The TEM (JEM-2100HR) image illustrated in Figure 1a reveals that the synthesized nanoparticles are mono-dispersed and near spherical; their diameter is in the range of 15-20nm. The diameter can be controlled by tuning the concentration of sodium citrate in the solution. The corresponding UV-Vis absorption spectrum of the fabricated Au NSs are plotted in Figure 1b. We can observe a narrow SPR peak at ~515 nm, suggesting that the size of the nanoparticles is quite uniform. We also synthesized Au NRs to investigate the effect of shape and size on the PL emission of Au NPs. A hydrothermal method36 was utilized to fabricate Au NRs with an average diameter of ~9 nm and length ~ 20nm (Supporting Information, Figure S1), which corresponds to an effective diameter of ~12 nm for a sphere. To investigate the PL properties of individual Au NPs, we dropped extremely dilute colloidal solution (~10-9 M) onto a glass coverslip with another one on the top. Then these two coverslips were separated carefully. The details for our experimental setup are schematically plotted in Figure S2 (Supporting Information). In brief, the samples were excited by a diode-pumped solid-state laser (532 nm, 10 KW/cm2, Coherent) that is directed through a Zeiss 100×/1.3 NA oil immersion objective. With a long pass filter to block the excitation light in the detection path, the luminescence from Au NPs was collected by the same objective and detected using an intensified charge-coupled device (CCD) (Carl Zeiss) camera, as shown in Figure S2 (Supporting Information). The DI was taken by moving the objective around 1µm toward the sample after a clear diffraction-limited fluorescence image appeared. All the fluorescence images were recorded at room temperature. The exposure time for each image was set to be 200-300 ms in order to obtain enough signal-to-noise ratio. The PL spectrum of single Au NPs was measured in focus by an Andor electron-multiplying CCD (EMCCD, DU970N-BV and DU-897E) together with a 560-910 nm bandpass filter in the output circuit. A program which is based on the multidimensional dipole model developed by Enderlein and co-workers is used to simulate DIs.37 The definition of the angular coordinates is shown in Figure S3 (Supporting Information), where Ψ is the out-of-plane angle (inclination angle between the dipole and the plane defined by the lab coordinate system XOY), and β the in-plane angle. From the principle of DIs, the emission from the dipoles with different Ψ can be


Page 4 of 17

Page 5 of 17

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

ACS Photonics

projected on the plane XOY, and then projected at X and Y directions. Therefore, even when Ψ ≠ 0, the DI can also be built by two single dipoles at X and Y directions. As the spatial orientation of the dipole is not our key target in this paper, we can assume that all the dipoles are lying at the plane XOY. In each DI simulation presented in this paper, we only considered the intensity ratio between IX and IY, which further determines the DI pattern. For example, when IX equals to IY, the DI exhibit isotropy. Maps of simulated DIs with different intensity ratios and orientations are shown in Figure S4a and S4b. We also simulated the DIs by summing up two, three and five dipoles with the same intensity but different included angles, the related maps were demonstrated in Figure S4c-e, respectively.

RESULTS AND DISCUSSION Figure 1b displays the absorption and fluorescence spectra of ensemble Au NSs in solution. Comparing to the absorption spectrum from Au NCs shown in the reference 38, where Au NCs (~5.5 nm) are too small to support continuous DOS and collective plasmon oscillation, an obvious SPR peak at ~515 nm can be observed in Figure 1b (the red curve). This peak is consistent with the SPR originated from Au NPs with a size of ~20 nm.39 Using green laser (532 nm) to excite the one-photo fluorescence of our Au NSs in the solution, a PL peak at ~ 620 nm (Figure 1b, the green curve) was observed. Similar peaks were also measured in the PL spectra from individual spheres (Figure S5a, Supporting Information), suggesting that the fluorescence of our Au NSs could be plasmon-independent. In other words, their PL emission is not modulated by their SPR characteristic. For single Au NRs, however, the measured PL peak matches well with their scattering spectrum (i.e., their plasmon resonant mode), as shown in Figure S5b and S5c (Supporting Information). We now turn our attention to the PL from single Au NSs. Similar to Au NRs, emissive spots appeared immediately after the excitation laser was triggered. With a similar excitation power, the average emission intensity from individual Au NSs is much lower than that from individual Au NRs. According to the method proposed in the reference 12, we calculated the effective volume of the Au NSs and estimated its PL QY to be ~10-6. A typical emitting image from multiple Au NSs at focus is shown in Figure 1c, in which each bright spot represents one Au NS. During minutes of observation, most of the spots display photo-blinking behaviour. We excited all the chemicals used in the synthesis process by 532 nm under 10 KW/cm2 and did not find any obvious PL signal. Furthermore, as we dispersed these NSs in deionized water before the observation, and studied them in the air, the chemical environment in this system is relatively simple. Therefore, the observed photo-blinking PL should be emitted from the original NSs. Based on the TEM images shown in Figure 1a, the photo-blinking and DI pattern rotation phenomenon, we believe the majority of the Au NSs are mono-dispersed. Figure 1d illustrates another representative image demonstrating emissions from multiple Au NSs out of focus. As observed, the DIs exhibit both isotropic and anisotropic characteristics. We traced 100 individual Au NSs in different frames and found that ~90% of their DIs show isotropy at first, then either change into anisotropy or vanish directly. In some cases (~15%), the anisotropic DIs transform into isotropy again and this process can repeat several


ACS Photonics

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

times. This transformation is often accompanied with PL intensity variation (i.e., photo-blinking process), and accompanied with DI pattern rotation. We monitored the DIs from the blinking Au NSs to track their optical dynamics. Figure 2a and 2b show the time traces of the PL intensity from two representative Au NSs, where each data point represents the intensity integrated in one DI with an exposure time of 300 ms. The insets plot the corresponding DIs of some typical states for the monitored Au NSs. In each group, the top image presents the detected state and the bottom the simulated one. The background intensity was determined locally around each monitored NSs. In Figure 2a, a very obvious rotation of an anisotropic DI pattern, such as from state 1 (β = 3º) to 3 (β = 71º) and 3 to 4 (β = 21º), can be observed after some blinking-off periods (e.g. state 2). Figure 2b demonstrates another type, in which the anisotropic DI (state 1 with β = 10º) of one Au NS changes into almost-isotropy (state 2) during the blinking process, then recovers to anisotropy (state 3, β = 15º). As encoding the spatial axial orientation of the dipole is not our key target in this paper, we assume all the dipoles are lying at the plane defined by the lab coordinate system XOY in the simulation (refer to Figure S3, Supporting Information). The pattern of the DI is defined by the relative intensity of these dipoles. From the principle of forming DIs, isotropic DIs appears when there are randomly located multiple dipoles or a single dipole stands along the optical axis. As the excitation method in our experiments is not based on total internal reflection, which can only excite those dipoles along the optical axis,40 the chance for only single dipoles standing along the optical axis to be excited is very small. Therefore, we deduce the observed isotropic DI pattern is caused by multiple dipoles which are confined in a diffraction-limited spot. In other words, the PL from individual Au NSs is very likely to be from multiple-dipole emission. This can be confirmed by the simulated isotropic DIs shown in Figure S4c-e, in which the intensity is summed up by several dipoles with different orientations. According to the previous studies, Au NRs exhibit much more stable PL properties than the Au NSs investigated here.4,34 Specifically, we have researched the PL properties of single-crystal Au NRs (~ 20 nm in diameter, > 50 nm in length), and found that most of the DIs exhibited anisotropy and remained unchanged during excitation.34 The fluorescence of Au NRs is generally attributed to the radiative emission of the SPR, which is excited by the recombination of sp-electrons with d-band holes. Thus the observed PL is dominated by their SPR characteristics.14 Even for those very small Au NRs (effective diameter ~12 nm, smaller than our studied Au NSs), all emissive spots are photo-stable and non-blinking, as shown in Figure 2c. When the incident polarization varies, the DI pattern remains constant (Figure 2d). The DI pattern rotation has not been observed. On the basis of these experimental observations, it is believed that the difference of PL properties between Au NSs and NRs is not related with the particles’ size. We then studied the photo-blinking behaviour of the PL from individual Au NSs in more detail. We found that they often do not blink off completely, but remain a weak signal before blinking on again (Figure 2a and 2b). To obtain quantitative information of the blinking statistics, we used a conventional method to construct on- and off-time events respectively by selecting proper threshold intensity. In principle, plotting both on-time and off-time probability distribution (P(τon), P(τoff)) on a log-log type enables us to extract the fluctuation dynamics of fluorescence intermittency. We traced the emission of 50 individual Au NSs and analyzed the data by means of a


Page 6 of 17

Page 7 of 17

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

ACS Photonics

well-defined threshold, which separates bright and dark periods. Figure 3a plots typical time traces of the PL intensity from two Au NSs, where the thresholds to separate bright and dark periods are marked by the black dashed straight line. Blinking phenomena can be clearly distinguished from the background. Figure 3b and 3c summarizes the on-time and off-time probability distributions from these 50 individual Au NSs. Because of photo-bleaching and weak photo-stability, we could not obtain enough data to judge whether the blinking signal follows a power-law or not. The mean on-time and mean off-time were calculated to be 0.21s and 0.15s respectively, both of which are much smaller than those of QDs.32,41 The considerably small difference between mean on-time and off-time indicates that these Au NSs exhibit frequent fluorescence blinking before photo-bleaching, which is similar to the reported blinking statistics for Au NCs.8 Based on the above experimental results, the PL properties of Au NSs are found to be similar to those of Au NCs. It is natural to speculate the correlation between the two types of particles. In 2008, Zheng et al. reported the bright luminescence and enhanced Raman scattering from individual silver nanoparticles with polycrystalline structure.42 They suggested that with the grain size effects, size-dependent material properties can be engineered and enhanced in one single nanoparticle, which can be helpful to better understand the couplings between quantum and classical size effects with a finite domain.5 We therefore utilized high-resolution TEM to examine the detailed crystalline structure of our studied Au NSs, as shown in Figure 4. It can be observed clearly that the Au NSs exhibit polycrystalline structure with some crystal domains of 3-4 nm, as marked out by red dashed circles or ellipsoids. We suppose that these small crystal domains divide the NSs into different clusters and each of them can be assumed as an emitting dipole (marked in yellow). Thus each single Au NSs contain multiple emitting dipoles. The plasmon-independent PL peak observed in Figure 1b could arise from the emission of these clusters. Previous studies have reported that Au NCs yield tuneable emission from ultraviolet (UV) to near-infrared region by varying their sizes.39 For Au NCs with diameter ~2 nm, a PL peak at ~630 nm has been measured before.9 This indicates that, in the three-step mechanism of explaining the PL from Au NPs, photon absorption and emission can be quite independent, and therefore should be discussed separately. In the case of the Au NSs studied in this paper, SPR can be supported in the absorption process, but the PL is likely originated from single-electron excitation between complex discrete energy states. Since there are multiple dipoles in one diffraction-limited fluorescence spot, the competition among them could influence the intensity ratio between IX and IY and thus the pattern of the DI. If the emission intensities from all the dipoles are similar, i.e., IX ≅ IY, the pattern of the DI exhibits isotropy. When the emission from one of the dipoles is dominant over the others, the emission balance would be broken and the observed DI changes into anisotropy. This is to say, the PL emission from the dominant dipole determines the DI pattern. Once the dominant dipole is replaced by the other one, the DI pattern rotates accordingly. This shift of dominant dipoles may also cause the photo-blinking behaviour. This shift includes two events: one dominant dipole being bleached and another one being activated. A time interval between these two events can lead to a blinking-off period on the measured timescale (Figure 3a). This explains that in our observations, most of the DI


ACS Photonics

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

rotation phenomena (>70%) occur after the blinking-off periods, as shown in Figure 2a. On the other hand, when those two events partly overlap each other, the observed DI may change via an isotropic pattern, just like the case in Figure 2b (from state 2 to 3). Here, the emission pattern, the PL intensity and even the number of the active dipoles can affect the observed DIs, and further result in their various behaviours. The detailed mechanism of blinking behaviour is still under debate. Previous literature also suggest that an Auger process may cause fluorescence intermittency.8,10 Specifically, after some electrons being excited from the d-band to the sp-band, other electrons rather than the excited ones may recombine with the d-band holes before the excited electrons scatter close to the Fermi level. Such combinations are released in the form of heat, not photons or plasmons, thus leads to the off-state in the emission intermittency.9 The Auger process may occur easier in Au NCs because electrons can transfer easily between discrete energy states. For the polycrystalline Au NSs studied here, the small crystal domains/clusters give rise to complex discrete energy states and allow optical transitions. This makes the Auger process unavoidable. Although the Auger process is ultrafast which cannot be resolved in our experimental system, the fluctuation of the PL intensity as shown in Figure 2 and Figure 3 indicates its contribution. It is worth noting that during the observation time, not all blinking events are accompanied with DI variation, suggesting that one of the emission paths may be much more stable than the others and the energy state structures in these Au NSs are different and complicated. For the photo-blinking behaviour in our experiments, we conclude that the shift of dominant dipoles and the nonradiative Auger process are both responsible. By contrast, the rod-like particles synthesized by the silver ion-assisted seed-mediated method are single crystalline.43,44 Their PL emission can follow the three-step mechanism faithfully. Since the longitudinal plasmon luminescence always occurs along the long axes of the rods, the pattern of the defocused image always exhibits dominant emission from their long axes.34 For a rod laid flat on a substrate, the observed DI is therefore anisotropic, and remains constant by varying the incident polarization. On the other hand, the excited SPR produces a strong electromagnetic field on the rod’s surface, and localizes the light on nanometer scale. In this case, the Auger process could be inhibited, and a stable PL emission could be obtained. This emission arises from interband (sp-d) band transitions, and is dominated by the rods’ SPR characteristics (Figure S5). Finally, we also correlated the PL spectra and the DIs from some Au NSs at single-particle level. Figure 5a-d show the measured PL spectra for different individual Au NSs; the insets demonstrate their corresponding DIs. It can be seen that when the DI exhibit anisotropy (refer to Figure 5a, 5b), the peaks of the spectra are narrower than those from the isotropic DIs (refer to Figure 5c, 5d), which contain more dipoles/clusters and the grain size distribution should be broader. As the energetic level is related with the size of the cluster in the quantum confinement region,45 broader size distribution would induce broader emission peak. This experimental phenomenon further supports our proposed mechanism that the PL from polycrystalline Au NSs arises from multiple-dipole emission.

CONCLUSION Spherical gold nanoparticles have been proven to exhibit SPR characteristics similar to gold nanorods. Meanwhile, both photo-blinking and rotation of defocused images were observed. Combining the optical spectra with high-


Page 8 of 17

Page 9 of 17

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

ACS Photonics

resolution TEM images, we believe that the polycrystalline structure of these nanospheres is responsible for the observed PL behaviours. The small domains present within one nanoparticle divide it into different clusters/dipoles, which emit in one diffraction-limited fluorescence spot and form multiple-dipole emission. To the best of our knowledge, this is one of the first reports on gold nanoparticles with plasmon-enhanced absorption while plasmonindependent emission. The presented mechanism for the emission from gold nanospheres could fill an important gap between small nanoclusters and larger plasmonic nanoparticles. Although it is a low-efficiency process, understanding its mechanism can be helpful to extend the applications of these spherical particles in nano-labeling and other nanotechnologies. Further investigations on other nanoparticles with different size, shape and materials are underway.

ASSOCIATED CONTENT Supporting Information available: TEM images of the Au NRs fabricated by a hydrothermal method (Figure S1), the experimental setup (Figure S2), the definition of the angular coordinates for the DI simulation (Figure S3), maps of simulated DIs with varying parameters (Figure S4), the scattering and PL spectra of single Au NSs and NRs (Figure S5).

ACKNOWLEDGEMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 61378082 and 61675070).

REFERENCES [1]. A. Mooradian, A. Photoluminescence of Metals. Phys. Rev. Lett. 1969, 22, 185–187. [2]. Zheng, J.; Zhang, C. W.; Dickson, R. M. Highly Fluorescent, Water-Soluble, Size-Tunable Gold Quantum Dots. Phys. Rev. Lett.2004, 93, 077402. [3]. Yorulmaz, M.; Khatua, S.; Zijlstra, P.; Gaiduk, A.; Orrit, M. Luminescence Quantum Yield of Single Gold Nanorods. Nano Lett. 2012, 12, 4385–4391. [4]. Bouhelier, A.; Bachelot, R.; Lerondel, G.; Kostcheev, S.; Royer, P.; Wiederrecht, G. P. Surface Plasmon Characteristics of Tunable Photoluminescence in Single Gold Nanorods. Phys. Rev. Lett. 2005, 95, 267405. [5]. Zhou, C.; Yu, J.; Qin, Y. P.; Zheng, J. Grain Size Effects in Polycrystalline Gold Nanoparticles. Nanoscale 2012, 4, 4228–4233. [6]. Imura, K.; Okamoto, H. Properties of Photoluminescence from Single Gold Nanorods Induced by Near-


ACS Photonics

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 17

Field Two-Photon Excitation. J. Phys. Chem. C 2009, 113, 11756–11759. [7]. Andersen, S. K. H.; Pors, A.; Bozhevolnyi, S. I. Gold Photoluminescence Wavelength and Polarization Engineering. ACS Photonics 2015, 2, 432−438. [8]. Su, Y.-H.; Tu, S. L.; Tseng, S. W.; Chang, Y. C.; Change, S. H.; Zhang, W. M. Influence of Surface Plasmon Resonance on the Emission Intermittency of Photoluminescence from Gold Nano-Sea-Urchins. Nanoscale 2010, 2, 2639–2646. [9]. Yuan, C. T.; Chou, W. C.; Tang, J.; Lin, C. A.; Chang, W. H.; Shen, J. L.; Chuu, D. S. Single Fluorescent Gold Nanoclusters. Opt. Express 2009, 17, 16111–16118. [10]. Frantsuzov, P.;Kuno, M.;Jankó, B.; Marcus, R. A. Universal Emission Intermittency in Quantum Dots, Nanorods and Nanowires. Nature Phys. 2008, 4, 519–522. [11]. Lumdee, C.; Yun, B.; Kik, P. G. Gap-Plasmon Enhanced Gold Nanoparticle Photoluminescence. ACS Photonics 2014, 1, 1224−1230. [12]. Rao, W.; Li, Q.; Wang, Y.; Li, T.; Wu, L. Comparison of Photoluminescence Quantum Yield of Single Gold Nanobipyramids and Gold Nanorods. ACS Nano, 2015, 9, 2783–2791. [13]. Geiselmann, M.; Marty, R.; Renger, J.; de Abajo, F. J. G.; Quidant, R. Deterministic Optical-Near-FieldAssisted Positioning of Nitrogen-Vacancy Centers. Nano Lett. 2014, 14, 1520–1525. [14]. Chernak, D. J.;Sisco, P. N.; Goldsmith, E. C.; Baxter, S. C.; Murphy, C. J. High-Aspect-Ratio Gold Nanorods: Their Synthesis and Application to Image Cell-Induced Strain Fields in Collagen Films. Meth MolBiol, Clifton, N.J 2013, 1026, 1–20. [15]. Kim, F.; Song, J.; Yang, P. Photochemical Synthesis of Gold Nanorods. J. Am. Chem. Soc.2002, 124, 14316–14317. [16]. Scaffardi, L. B.; Tocho, J. O. Size Dependence of Refractive Index of Gold Nanoparticles. Nanotechnology 2006, 17, 1309–1315. [17]. Pomplun, J.; Burger, S.; Zschiedrich, L.; Schmidt, F. Adaptive Finite Element Method for Simulation of Optical Nano Structures. Phys. Status Solidi B 2007, 244 (10), 3419. [18]. Mishchenko, M. I.; Travis, L. D.; Mackowski, D. W. T-Matrix Computations of Light Scattering by Nonspherical Particles: A Review. J. Quant. Spectrosc. Radiat. Transfer 1996, 55, 535–575. [19]. Draine, B. T.; Flatau, P. J. User Guide to the Discrete Dipole Approximation Code DDSCAT 6.1, http://arxiv.org/abs/astroph/0409262v2. [20]. Ungureanu, C.; Amelink, A.; Rayavarapu, R. G.; Sterenborg, H. J. C. M.; Manohar, S.; van Leeuwen, T. G. Differential Path length Spectroscopy for the Quantitation of Optical Properties of Gold Nanoparticles. ACS Nano 2010, 4, 4081–4089. [21]. Bharadwaj, R.; Anger, P.; Novotny, L. Nanoplasmonic Enhancement of Single-Molecule Fluorescence. Nanotechnology 2007, 18, 044017. [22]. Geddes, C. D.; Parfenov, A.; Gryczynski, I.; Lakowicz, J. R. Luminescent Blinking of Gold Nanoparticles. Chem. Phys. Lett. 2003, 380, 269–272.


Page 11 of 17

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

ACS Photonics

[23]. Gaiduk, A.; Ruijgrok, P. V.; Yorulmaz, M.; Orrit, M. Making gold nanoparticles fluorescent for simultaneous absorption and fluorescence detection on the single particle level. Phys. Chem. Chem. Phys., 2011, 13, 149–153. [24]. Loumaigne, M,; Vasanthakumar, P.; Lombardi, A.; Richard, A.; Débarre, A. One-photon excited luminescence of single gold particles diffusing in solution under pulsed illumination. Phys.Chem. Chem. Phys., 2013, 15, 4154–4162. [25]. Loumaigne, M,; Vasanthakumar, P.; Richard, A.; Débarre, A. Influence of polarization and wavelength on two-photon excited luminescence of single gold nanospheres. Phys.Chem. Chem. Phys., 2011, 13, 11597– 11605. [26]. Cheng, Y.; Lu, G.; He, Y.; Shen, H.; Zhao, J.; Xia, K.; Gong, Q. Luminescence quantum yields of gold nanoparticles varying with excitation wavelengths. Nanoscale, 2016, 8, 2188–2194. [27]. Gaiduk, A.; Yorulmaz, M.; Orrit, M. Correlated Absorption and Photoluminescence of Single Gold Nanoparticles. ChemPhysChem. 2011, 12, 1536–1541. [28]. Huang, D.; Byers, C. P.; Wang, L. Y.; Hoggard, A.; Hoener, B.; Dominguez-Medina, S.; Chen, S.; Chang, W. S.; Landes, C. F.; Link, S. Photoluminescence of a Plasmonic Molecule. ACS Nano, 2015, 9, 7072–7079. [29]. Willets, K. A.; Wilson, A. J.; Sundaresan, V.; Joshi, P. B. Super-Resolution Imaging and Plasmonics. Chem. Rev. 2017, DOI: 10.1021/acs.chemrev.6b00547. [30]. Varnavski, O. P.; Goodson, T.; Mohamed, M. B.; El-Sayed, M. A. Femtosecond Excitation Dynamics in Gold Nanospheres and Nanorods. Phys. Rev. B 2005, 72, 235405/1–235405/9. [31]. He, H.; Xie, C.; Ren, J. Nonbleaching Fluorescence of Gold Nanoparticles and Its Applications in Cancer Cell Imaging. Anal. Chem.2008, 80, 5951–5957. [32]. Motegi, T.; Nabika, H.; Niidome, Y.; Murakoshi, K. Observation of Defocus Images of a Single Metal Nanorod. J. Phys. Chem. C 2013, 117, 2535−2540. [33]. Li, Q.; Chen, X.-J.; Xu, Y.; Lan, S.; Liu, H.-Y.; Dai, Q.-F.; Wu, L.-J. Photoluminescence Properties of the CdSe Quantum Dots Accompanied with Rotation of the Defocused Wide-Field Fluorescence Images. J. Phys. Chem. C 2010, 114, 13427–13432. [34]. Li, T.; Li, Q.; Xu, Y.; Chen, X. J.; Dai, Q. F.; Liu, H. Y.; Lan, S.; Tie, S. L.; Wu, L. J. Three-Dimensional Orientation Sensors by Defocused Imaging of Gold Nanorods through an Ordinary Wide-Field Microscope. ACS Nano 2012, 6 (2), 1268−1277. [35]. Frens, G. Controlled Nucleation for the Regulation of the Particle Size in Monodisperse Gold Suspensions. Nature 1973, 241, 20–21. [36]. Sau, T. K.; Murphy, C. J. Seeded High Yield Synthesis of Short Au Nanorods in Aqueous Solution. Langmuir. 2004, 20(15), 6414–6420. [37]. Patra, D.; Gregor, I.; Enderlein, J.; Sauer, M. Defocused Imaging of Quantum-Dot Angular Distribution of Radiation. Appl. Phys. Lett. 2005, 87, 101103–101106. [38]. Yau, S. H.; Varnavski, O.; Goodson, T., III. An Ultrafast Look at Au Nanoclusters. Acc. Chem. Res. 2013, 46, 1506–1516.


ACS Photonics

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 17

[39]. Zheng, J.; Zhou, C.; Yu, M.; Liu, J. Different Sized Luminescent Gold Nanoparticles. Nanoscale 2012, 4 (14), 4073−4083. [40]. Pfab, R. J.; Zimmermann, J.; Hettich, C.; Gerhardt, I.; Renn, A.; Sandoghdar, V. Aligned Terrylene Molecules in a Spin-Coated Ultrathin Crystalline Film of p-Terphenyl. Chem. Phys. Lett. 2004, 387, 490−495. [41]. Koberling, F.; Mews, A.; Basche, T. Oxygen-Induced Blinking of Single CdSe Nanocrystals. Adv. Mater. 2001, 13, 672–676. [42]. Zheng, J.; Ding, Y.; Tian, B.; Wang, Z. L.; Zhuang, X. Luminescent and Raman Active Silver Nanoparticles with Polycrystalline Structure. J. Am. Chem. Soc. 2008, 130, 10472–10473. [43]. Tsung, C.; Kou, X.; Shi, Q.; Zhang, J.; Yeung, M. H.; Wang, J.; Stucky, G. D. Selective Shortening of Single-Crystalline Gold Nanorods by Mild Oxidation. J. Am. Chem. Soc. 2006, 128, 5352–5353. [44]. Garg, N.; Scholl, C.; Mohanty, A.; Jin, R. The Role of Bromide Ions in Seeding Growth of Au Nanorods. Langmuir 2010, 26 (12), 10271–10276. [45]. Brus, L. E. Electron-electron and Electron-hole Interactions in Small Semiconductor Crystallites: the Size Dependence of the Lowest Excited Electronic State. J. Chem. Phys. 1984, 80, 4403–4409.


Page 13 of 17

Figure 1. (a) TEM image of the synthesized Au NSs. (b) Ensemble extinction (the red curve) and emission (the green curve) spectra of Au NSs taken in aqueous solutions. (c) A typical emitting image from multiple Au NSs at focus and (d) another representative one out of focus. The left insets exhibit two simulated defocused images with anisotropic (upper, β = 20º) and isotropic (lower) pattern.

(a)

(b) Intensity (nor.)

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

ACS Photonics

1.0

0.5

0.0 500

600 Wavelength (nm)

(c)


700

(d)

ACS Photonics

Figure 2. (a, b) Time traces of the PL intensity of two blinking Au NSs. Each data point corresponds to the intensity integrated in one DI with an exposure time of 300ms. The insets plot the DIs of some typical states for each monitored Au NS, in which the top image indicates the detected state and the bottom the simulated one. β = 3º, 71º and 21º for states 1, 3 and 4 in (a); β = 10º and 15º for states 1 and 3 in (b). (c) The PL from small Au NRs with an effective diameter of ~12 nm exhibits photo-stable and non-photo-blinking properties. (d) The DI pattern of small Au NRs remain constant when the incident polarization varies. 1

(b)

3

18

1 20

3

4 12

2

PL Intensity (a.u.)

PL Intensity (a.u.)

(a)

2

10

6 15

20

25

30

35

Tracking time (s)

40

45

50

Tracking time (s)

(d)

(c)

600

102 SPL Intensity

SPL Intensity

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 17

99

300

0

0

50

100

150

200

0

120

Tracking time (s)


240 θ (°)

360

Page 15 of 17

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

ACS Photonics

Figure 3. (a) Time traces of the PL intensity (integrated in the obtained DIs with an exposure time of 300ms) from two Au NSs with longer tracking time. The thresholds to separate bright and dark periods are marked by the black dashed straight line. Blinking statistics of the on-states and off-states from 50 individual Au NSs are shown in (b) and (c) respectively.

Figure 4. High-resolution TEM images of Au NSs exhibit polycrystalline structure with some crystal domains of 34 nm (marked in red). Each domain can be assumed as an emitting dipole (marked in yellow).


ACS Photonics

1.0 0.8 0.6 0.4 0.2 0.0 500

(c)

(b) PL intensity (a.u.)

(a) PL intensity (a.u.)

Figure 5. The measured PL spectra for different individual Au NSs and the corresponding DIs (insets). (a) (b) The PL peaks from the Au NSs with anisotropic DIs are observed narrower than those with isotropic DIs in (c, d).

600 700 800 wavelength (nm)

1.0

(d)

0.8 0.6 0.4 0.2


900


900

1.0

PL intensity (a.u.)

0.8

0.6 0.4 0.2 0.0 500

1.0

0.0 500

900

0.8

PL intensity (a.u.)

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 17

600

700 800 wavelength (nm)

900

0.6 0.4 0.2 0.0 500


Page 17 of 17

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

ACS Photonics

For Table of Contents Use Only

Revealing the Mechanism of Photoluminescence from Single Gold Nanospheres by Defocused Imaging † Tao Li, †,‡ Fanwei Zhang, † Qiang Li, † Wenye Rao, † Pei Xu, † Lei Xu, † Li-Jun Wu *,†

The ToC graphic demonstrates the fluorescent properties of single gold nanospheres investigated by defocused imaging technique. Both photo-blinking behavior and defocused pattern rotation have been observed. It is believed that the polycrystalline structure of these nanospheres is responsible for the observed fluorescent properties. The small domains present within one nanosphere divide it into different clusters/dipoles, which emit in one diffraction-limited fluorescence spot and form multiple-dipole emission. The presented mechanism for the emission from gold nanospheres could fill an important gap between small nanoclusters and larger plasmonic nanoparticles, and can be helpful to extend the applications of these spherical particles in nano-labeling and other nanotechnologies.


Revealing the Mechanism of Photoluminescence from Single Gold

Recommend Documents