Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 4155−4159
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Super Bright Luminescent Metallic Nanoparticles Wei Gan,† Bolei Xu, and Hai-Lung Dai*
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Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States ABSTRACT: It is found that, by curing the surface defects that quench photoexcited carriers, luminescence efficiency of metallic nanoparticles can be dramatically increased. For Ag nanoparticles, as much as 300 times increase in photoexcitation induced luminescence is observed upon surface adsorption of ethanethiol. The same treatment increases Au nanoparticle luminescence efficiency by a factor of 3. A model based on the elimination of surface defects by the sulfur−metal bond formed upon thiol adsorption can quantitatively account for the observations, which also indicate that nanoparticles without proper surface treatment typically have low luminescence quantum yields.
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addition, the luminescence induced by two-photon excitation, on the other hand, is observed to increase by more than 2 orders of magnitude. This dramatic enhancement in luminescence observed during thiol modification of the surface can be quantitatively related to the thiol coverage. Similar to the reported passivation of the excited carrier recombination centers at quantum dot surface by thiol treatments,33,34 the mechanism for this luminescence enhancement is quantitatively illustrated here. The experimental setup has been described previously.22,23 Briefly, the 800 nm femtosecond output from a Ti:sapphire laser, oscillator only, is focused into a liquid jet containing the metal particle colloid. The Ag colloid (purchased from Microspheres-Nanospheres, New York) has average radius of 40 nm and is stabilized with residual sodium citrate that is used in the synthesis of the Ag nanoparticles. Ethanethiol was titrated into the liquid sample reservoir as indicated. As the scattered nonlinear light from the surface of nanometer-size particles is at angles away from the forward direction,35 the light scattered from the jet within the 40° solid angle centered at the forward direction was detected and spectrally analyzed by a monochromator (Acton SP2300, Princeton Instrument), then detected by a Photomultiplier tube (R1527, Hamamatsu). The PMT output was amplified by a fast preamplifier (SR445, Stanford Research System) and analyzed by a photon counting system (SR400, Stanford Research System) coupled to a microcomputer. Figure 1 inset shows a typical emission spectrum from silver colloids irradiated with the laser pulses. As has been previously reported,22 the spectrum consists of an SHS contribution right at 400 nm and a relatively weaker and much broader band at longer wavelengths that can be assigned as two-photon excitation induced luminescence (TPL). The spectrum can be fitted by a nonlinear least-squares procedure using a sum of a squared Gaussian function representing the SH peak since
ne of the primary reasons for the use of metallic nanoparticles in sensing applications1−7 has been the high fluorescence quantum yield, as it is expected from a cluster of metallic atoms upon light irradiation. The efficiency of luminescence of nanoparticles is also related to the welldocumented blinking phenomenon of single particles.8−21 Although the exact mechanism of luminescence blinking is still being debated,19 it has been proposed that surface defects such as dangling bonds may trap the excited carriers in the particle and reduce the luminescence efficiency9,10,12,15−17,21 as well as cause the loss of the luminescence from single particles.8,11,13,14,19 Here, we report that surface treatment can eliminate the surface defects and dramatically increase, by more than 2 orders of magnitude, the luminescence efficiency of metallic nanoparticles. We will show that the increase in luminescence efficiency can be quantitatively explained by a surface-defect model that reveals that luminescence of metallic nanoparticles depends on their surface conditions. In fact, metallic nanoparticles, in particular Ag, without proper surface treatment, have generally low luminescence efficiencies. This study is enabled by surface-sensitive second harmonic light scattering (SHS). It was recently shown that SHS from the surface of colloidal silver nanoparticles22−24 can be detected. The majority of the observed SHS signal can be assigned to originating from the surface layer of the nanoparticle as it can be quenched by addition in the silver colloid of thiol molecules that are known to form S−Ag bonds.25−29 Thiol-metal bonding localizes the free electrons and causes a decrease of the hyper-polarizability of the metal surface, as indicated by many previous studies conducted on flat metal surfaces.30−32 With the surface origin of the SHS signal established, SHS has been used to probe thiol bonding mechanism and energetics at the Ag particle surface.22−24 Surface bonding of thiols may also affect linear optical properties of the nanoparticles. The formation of strong S−Ag bonds at the particle surface can anneal the surface defects that quench the photoexcitation-induced luninescence. Indeed, while SHS from the silver colloid decreases upon thiol © XXXX American Chemical Society
Received: May 23, 2018 Accepted: July 10, 2018 Published: July 10, 2018 4155
DOI: 10.1021/acs.jpclett.8b01608 J. Phys. Chem. Lett. 2018, 9, 4155−4159
Letter
The Journal of Physical Chemistry Letters
TPL signal to reach saturation (defined here as 95% of the maximum intensity). The gigantic change in the TPL intensity reported here is unlikely to be from the change in the particle−solvent interaction which was proposed for explaining much smaller change in fluorescence.38 The addition of thiol into Ag colloids causes formation of strong S−Ag bonds at the Ag nanoparticle surface at room temperature.22,23,39,40 It is also unlikely that the increase is due to fluorescence directly from the Ag-thiol complex. The formation rate of the Ag-thiol bond is directly correlated to the SHG signal. If TPL is from this species, there should be no time-lag in the TPL increase with respect to the change of the SHS signal. The enhancement of the luminescence signal may arise if defects such as dangling bonds on the silver particle surface that are quenching the fluorescence are eliminated by thiol adsorption. The surface defects are known to trap excited charge carriers and reduce the luminescence efficiency of nanoparticles/quantum dots.9,10,12−17 This model has been applied to the understanding of the change of luminescence from InP quantum dots, CdSe/CdS and CdSe/ZnS core/shell nanorods, as well as ZnO nanorods induced by surface modification.9,17,41,42 The annealing of surface defects has also been proposed as a cause for the reduction of the “off-time” in the luminescence blinking of quantum dots.13,14 It is proposed here that the changes in both the linear and nonlinear optical properties of Ag colloids induced by the thiol addition are results from the formation of strong S−Ag bonds at the particle surface. The formation of the S−Ag bonds reduces the nonlinear polarizability while it anneals the surface defects and increases the fluorescence quantum yield. However, how can the rate of thiol adsorption be quantitatively related to the different rates of change of both the SHS and TPL signals? In our quantitative analyses, the standard Langmuir model is used for describing the adsorption of thiol molecules onto the silver nanoparticle surface, and thus the rate of the S−Ag bond formation. The change of the number of thiol molecules on the surface, N, in time can be related to C the total number of thiols in the colloid (the ones in the liquid solution plus those on the particle surface), Nmax the total number of surface sites that can be occupied by thiol molecules, and ka and kd the adsorption and desorption rate constants, respectively. The surface coverage of molecules on the substrate can be expressed as22,23
Figure 1. Inset: Emission spectrum (solid circles) detected from silver nanoparticles induced by 800 nm light irradiation shown as normalized intensity. The solid line is the fitting result. Main frame: Emission spectra recorded as a function of time (guided by the arrows) following addition of 10 μM ethanethiol.
the SH intensity is proportional to the square of the laser intensity Gaussian profile, and a Gaussian function for the broad TPL band. The assignment of the TPL band follows previous studies.25,26,36,37 The wavelength of the emission varies with the size of the particles. For example, luminescence bands around 500 nm have been observed for 10 to 40 nm silver nanoparticles.26 Contrary to the decrease of SHS from colloidal silver nanoparticles,22 addition of thiol induces an increase in the TPL intensity. Figure 1 shows the dramatic increase of the TPL intensity, by as much as 300 times, following the addition of ethanethiol (concentration in the colloid 10 μM). The time scale of the increase, over tens of minutes, is notably much longer than the SHS intensity decrease which happens on the scale of seconds following the thiol addition! The change of the TPL intensity, recorded at 500 ± 10 nm, as a function of time following ethanethiol addition at several selected concentrations is shown in Figure 2a. The
θ = N /Nmax = g (1 − e−(kaC + kd)t )
(1)
with g as a proportional constant. The surface nonlinear susceptibility is proportional to the part of the surface not covered by thiol,22,23,32 and the SHS intensity is proportional to the square of the second-order susceptibility. The SHS intensity during thiol modification of the surface can therefore be expressed as22,23
Figure 2. (a) Two-photon excitation-induced luminescence from Ag colloids following ethanethiol addition at different concentrations. Solid lines are fittings of the latter part data to the surface-defect model. (b) SHS signal from Ag colloids following ethanethiol addition at lower concentrations. Solid lines are fitting results from the Langmuir adsorption model.
ISHG = |b − a*(1 − e−t / τ )eiϕ|2
corresponding change in SHS is shown in Figure 2b. Note that lower ethanethiol concentrations are used in Figure 2b since the SHS intensity is much more sensitive to the addition of ethanethiol. It is clear that the change in the SHS signal occurs much faster than that in the TPL signal following the thiol addition. For example, in the case that 37.5 μM ethanethiol was added, while it took Atot, i.e., R > 1, we have efficient quenching of the local excitations. In situations of R ≫ 1, the overall quenching is nearly complete. Only when ρdef is sufficiently low, to the point that Aqρdef < Atot, i.e., R < 1, there will be unquenched local excitations leading to luminescence. As the model calculation illustrates later, the initial value of R (i.e., R0) determines to what extent surface defects has to be eliminated in order to cause discernible increase of the luminescence quantum yield. Adsorption of thiols at the beginning may eliminate some defect sites and reduce R. However, if Aq is large, the remaining defect sites may still have sufficiently high density to quench nearly all the local excitations. The quenching will be noticeably reduced only after the last remaining defects are eliminated. This scenario provides the explanation for the observed time lag. In the case of Ag nanoparticles, the R0 values must be much bigger than 1 so that substantial amounts of thiols must adsorb to reduce the defect density to sufficiently low numbers. This means that the luminescence will appear only when thiol adsorption is nearly saturated on the Ag particle surface, a process that takes a long time to complete. By contrast, the decrease in the SHS intensity is in direct proportionality to the square of the thiol coverage, and therefore the effect of thiol adsorption on the SHS intensity appears much earlier. Figure 3 presents a graphic illustration of the surface defect model. Each defect, plotted as a point, has an effective
l I = 1 − R (1 − θ ) when R (1 − θ ) < 1 o 0 0 o lum m o o when R 0(1 − θ ) ≥ 1 o (4) n Ilum = 0 Simulations using eqs 1 and 4 for a variety of R0 values are shown in Figure 4, which shows that for large R0 (the initial defect density and/or quenching cross section are high), there are clearly time-lags following thiol addition before notable TPL intensity increase. The larger the R0 is, the longer the time-lag. The simulations also show that a large time-lag is associated with an initial, nearly complete quenching and a subsequent large enhancement of luminescence, a prediction that is consistent with the experimental observations (Figure 2): The luminescence signal before thiol addition is typically 2 orders of magnitude smaller than the highest level that can be reached after thiol addition. Equation 4 does not take into account the effect that some surface areas are under the influence of more than one defects, i.e., when there is significant overlap of the quenching cross sections from nearby surface defects, the portion of excitations surviving quenching is not linearly proportional to 1 − R0(1 − θ). Equation 4 is only accurate when a significant portion of the surface defects has been eliminated to the extent that overlap of quenching areas between nearby defects is negligible. Due to this reason, this model can only accurately describe the latter part of the TPL intensity curve. A nonlinear 4157
DOI: 10.1021/acs.jpclett.8b01608 J. Phys. Chem. Lett. 2018, 9, 4155−4159
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The Journal of Physical Chemistry Letters
This work shows that a saturated coverage of ethanethiol molecules at the particle surface can dramatically improve the luminescence quantum efficiency of Ag nanoparticles. Ethanethiol adsorption can also improve the luminescence intensity of other metal particles. This discovery points to an approach for improving the brightness of nanoparticles as luminescence agents in imaging and sensing devices. The dramatic improvement in the brightness of the metallic nanoparticles can be explained by the annealing of the surface defects that quenches photoexcitations. The most intriguing experimental observationthe time lag between the decrease in the SHS and the increase in TPFcan be quantitatively accounted for by the model in which the quenching of luminescence from the particles prior to thiol treatment by surface defects is highly efficient. Even though adsorption of thiols, as indicated by the decrease in the SHS signal, occurs immediately upon the thiol addition to the colloid, a substantial fraction of the defects has to be eliminated before the photoexcited electrons can survive the quenching, and therefore the time lag in the increase of the luminescence was observed. The interpretation of the luminescence enhancement through the elimination of surface defects also supports the proposed mechanism that luminescence blinking is caused by trapping of excited electrons by surface defects.
Figure 4. Simulated TPL intensity for representative R0 (0.5, 1, 5, 30, and 100). The residual surface defects as % of the initial defect density is shown as the solid line. The time scale is set with the τ defined in eq 2 as the time unit.
least-squares fit using eqs 1 and 4 of the time-dependent TPL data in Figure 2a can be performed to extract the value of R0. The curves were fitted with the points after TPL intensities reaching 75% of the maxima. The fittings give an R0 value of 33 ± 3 and the three 1/τ values as 0.0095 ± 0.0002, 0.017 ± 0.003, and 0.030 ± 0.006 s−1 for the three ethanethiol concentrations, 37.5, 75, and 150 μM, respectively. From the above analysis, the existence of the time lag between the SHG decay and the TPL increase is due to the fact that the surface density of the defects on Ag particles is more than enough to quench the TPL emission. The surface defect model predicts that the smaller the R0, the shorter the time-lag and the smaller the eventual increase in the luminescence intensity upon thiol addition. This trend is born out in experiments performed with Au nanoparticles. Figure 5 shows that the TPL intensity from Au nanoparticles
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Wei Gan: 0000-0001-8481-122X Present Address
† School of Science, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by a grant from the Air Force Office for Scientific Research (FA9550-08-1-0092).
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REFERENCES
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Figure 5. Time-dependent luminescence signal from 5 nm Au colloids after addition of 50 μM ethanethiol. The solid line is the fitting using the defect model.
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