Letter pubs.acs.org/JPCL
Dual Transient Bleaching of Au/PbS Hybrid Core/Shell Nanoparticles Yoichi Kobayashi,†,§ Yoshiyuki Nonoguchi,‡,§ Li Wang,† Tsuyoshi Kawai,‡ and Naoto Tamai*,† †
Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan ‡ Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara, 630-0192, Japan S Supporting Information *
ABSTRACT: We examined the optical response of hybrid Au/PbS core/shell nanoparticles (NPs) using transient absorption spectroscopy. Finite-difference time-domain (FDTD) calculations and transient absorption measurements show that Au/PbS NPs have unique two extinction peaks: the peak at the longer wavelength (∼700 nm) is originated from the plasmon, and that at the shorter wavelength (550 nm) is from the local maximum of the refractive index of PbS. The transient absorption dynamics of Au/PbS NPs excited at 400 nm have clear oscillation behavior, which is assigned to the breathing mode of whole particle. We observed a weak excitation-wavelength dependence of the plasmon band. The time constant of electron−phonon coupling of Au/PbS NPs was obtained by changing the excitation intensity. We show that spectral properties of Au/PbS NPs are strongly altered by the hybrid formations, while their dynamics differ only minimally compared with those of Au NPs. SECTION: Physical Processes in Nanomaterials and Nanostructures
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recently reported transient absorption behaviors in colloidal Au/CdS hybrid nanomaterials,38 transient behaviors of hybrid nanomaterials depend on their shapes, components and interfaces.35 To assign their spectral origins, more systematic experiments with simple shaped hybrid nanomaterials are indispensable. In this Letter, we report the study of the optical response of colloidal Au/PbS core/shell hybrid NPs by transient absorption spectroscopy. Au/PbS core/shell hybrid NPs are an excellent candidate to precisely analyze their optical responses under the strong exciton-plasmon regime because they are monodisperse and have high symmetry, i.e., spherical shape and rock salt crystal structures. We revealed the spectral origins, vibrational oscillation, and electron−phonon coupling constant of metal− semiconductor Au/PbS hybrid NPs. These findings are essential for the fundamental understanding of metal−semiconductor nanointerfaces and exciton−plasmon couplings, which may help in establish high efficient photocatalysts and solar energy conversions. We synthesized symmetrical colloidal Au/PbS NPs according to the procedure described by Lee et al.39 A transmission electron microscopy (TEM) image in Figure 1a (inset) clearly shows that a narrow size dispersion of Au/PbS NPs was synthesized. We estimated the average diameter and shell thickness as 5.1 ± 1.3 nm and 2.2 ± 0.7 nm by measuring ∼90 particles from TEM images. The histogram is shown in the
emiconductor nanomaterials and metal nanomaterials have great potential for solar energy conversion, biological applications, and quantum information because of their unique properties.1−8 Recent wet chemical techniques enabled us to synthesize various kinds of colloidal metal−semiconductor hybrid nanomaterials, such as metal/chalcogenide,9−14 metal/ oxcide-semiconductor,15,16 and metal/spacer/semiconductor,17,18 which accelerate further investigations of unique and more applicable nanomaterials. Metal−semiconductor hybrid nanoparticles (NPs) act as an efficient photocatalyst because of electron transfer from a semiconductor to a metal part.19,20 Complex hybrid nanomaterials composed of metal NPs, a spacer, and chromophore have been synthesized to enhance the emission, progress the chemical reaction, and act as a sensor.21−23 In addition to these characteristics, recent reports suggest that metal−semiconductor hybrid NPs have additional unique properties. Several theoretical reports have suggested that the optical spectrum of metal−semiconductor hybrid NPs exhibited exciton-induced transparency, Fano effect, and nonlinear Fano effect in the strong exciton-plasmon coupling regime.24−26 In addition, quantum coherence has been experimentally shown to survive in the Au/CdSe hybrid NPs, which enabled spin manipulations with hybrid nanomaterials.27 Optical properties of metal-semiconductor hybrid nanomaterials have been studied in the past few years,28−31 however, the intrinsic ultrafast optical response of nano-order materials under strong exciton-plasmon coupling has not been well understood32 because most experimental reports were examined by steady-state experiments.33−37 Although Khon et al. has © 2012 American Chemical Society
Received: March 2, 2012 Accepted: April 9, 2012 Published: April 9, 2012 1111
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to 3−7 nm and the shell thickness to 1−3 nm, respectively. The black solid line in the graph corresponds to nearly the same size as the experimental Au/PbS NPs. All calculated extinction spectra show two unique extinction peaks, and only the longerwavelength peak depends on the core diameter and the shell thickness (red and blue line, respectively). On the other hand, the simulated extinction spectra of the PbS nanoshell has one shoulder at ∼620 nm, which is similar to the shorterwavelength peak of Au/PbS NPs. Since FDTD calculation does not include excitonic effects and quantum size effects of semiconductors, the shorter-wavelength peak is assigned to the PbS shell, which is due to the local maximum of the refractive index of PbS (Figure S2 in the Supporting Information). The longer-wavelength peak of Au/PbS NPs is safely assigned to the plasmon band because the peak only appears in nanomaterials that include a Au component, which depends on both the core diameter and the shell thickness. As we can see from the simulation, a small difference of the ratio of core size and shell thickness gives a large spectral shift, which is probably another reason for the large spectral broadening of narrow sizedispersed Au/PbS NPs in addition to the PbS-shell band and the exciton−plasmon coupling.25,35 As compared to the experimental spectrum, the simulated extinction spectrum is shifted to red. There are two conceivable reasons for the deviation. One is the effect of quantum confinement on the refractive index of PbS, which is not included in FDTD calculations. Cademartiri et al. reported the size-dependent extinction spectra of PbS nanocrystals and showed that the absorption peak, even at a shorter wavelength, shifted to blue with decreasing particle size, although the shift was not as drastic as compared with that of the band-edge peak.42 Since the absorption is the complex part of the refractive index, the refractive index function should also shift to blue due to the quantum confinement effect. The other reason may be due to the negative-charge storage of Au core due to the Fermi equilibrium.43,44 Once an PbS shell electron is excited to the conduction state, an ultrafast electron transfer to Au core is expected because the system will keep equilibrating each Fermi level, and finally the Au core stores negative charges, which gives a blue shift of the plasmon band.43 Lee et al. reported that Au/PbS NPs films always exhibit a p-type gate effect,39 which shows that electron transfer to the Au core certainly occurs in this system.
Figure 1. (a) Experimental steady-state extinction spectra of Au NPs and Au/PbS NPs (purple and gray). Their extinctions were arbitrary multiplied to see their spectral features easily. Inset shows a TEM image of Au/PbS NPs. The white bar corresponds to 20 nm. (b) Simulated extinction spectra of Au/PbS NPs in different core diameter and shell thickness and PbS nanoshell (green line) whose inner diameter is 5 nm with the refractive index of 1.0 and the shell thickness is 2 nm. Thick black line corresponds to the simulated spectrum of experimental Au/PbS NPs. The core-diameter dependence and shellthickness dependence are represented in red and blue lines, respectively.
Supporting Information (Figure S1.) Figure 1a shows steadystate absorption spectra of Au NPs and Au/PbS NPs. The absorption spectrum of Au NPs before the encapsulation by PbS shells has a well-known sharp plasmon peak at the band edge.40 On the other hand, Au/PbS NPs have a broad absorption in spite of the narrow size dispersion, which is similar to previous reports.14,39 Figure 1b shows simulated extinction spectra of Au/PbS NPs and PbS nanoshell (inner diameter is 5 nm and shell thickness is 2 nm, green line) by finite-difference time-domain (FDTD) calculations (Lumerical FDTD solution). We used the refractive index of the bulk PbS41 and set the core diameter
Figure 2. Transient absorption spectra of Au NPs (a) and Au/PbS NPs (b) excited at 400 nm (excitation intensity is 200 nJ/pulse). Transient absorption dynamics of Au/PbS NPs at different wavelength (excitation intensity is 400 nJ/pulse) (c). 1112
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relatively close to that of Au/PbS NPs (3.7 ps). This indicates that the oscillation dynamics of Au/PbS NPs is due to the breathing mode of the whole particle. The reason for the deviation between the experimental value and the simple calculation is probably the interfacial effect of heteronanostructures, although the experimental errors should be also taken into account. Detailed calculations assuming complex structures by the Navier equation may give better explanations for our experimental results.50,58 We measured transient absorption spectra by changing the excitation wavelength to subresonantly excite the bleach signal at the shorter wavelength (510 nm) and selectively excite the plasmon band (630 nm). Figure 3 shows transient absorption
We measured transient absorption spectra of Au NPs and Au/PbS NPs excited at 400 nm, which induces the interband transition, to reveal the optical response of metal−semiconductor hybrid nanosystems (Figure 2a,b). The transient absorption spectrum of Au NPs has a bleach signal at 525 nm and two positive signals at the side of the bleach signal (Figure 2a). On the other hand, the transient absorption spectrum of Au/PbS NPs has two bleach signals at 553 and ∼700 nm and a positive signal at 475 nm (Figure 2b), which corresponds to simulated steady-state extinction spectra of Au/PbS NPs. Although the amplitude of the positive peak at 475 nm of Au/PbS NPs looks like it is increasing compared with that of Au NPs, the two bleach signals are actually suppressed because the steady-state absorbance of Au/PbS NPs at the broad peak is more than 2 times larger than that of Au NPs at the plasmon peak in our experiments. The suppression of the plasmon band is consistent with a recent report on Au/CdS hybrid nanostructure.38,45 The reason why the plasmon bleach is observed in our hybrid systems is probably the 10-fold larger extinction coefficient of the Au core compared to that of the PbS shell.46 Artuso and Bryant theoretically showed that, under a strong exciton−plasmon coupling regime, the exciton and the plasmon canceled each other by destructive interferences depending on the metal radius and the dipole moment of the semiconductor nanocrystals.24 We estimated the excitonic peak wavelength of the PbS shell by using the value of PbS nanocrystals, whose excitonic peak is ideally the same as that of quantum well, i.e., shell structures.4,42,47,48 The excitonic peak wavelength of the PbS shell is estimated to be 560−690 nm, which roughly overlaps the plasmon absorption of Au/PbS NPs, although there is large ambiguity because of few experimental reports on such small PbS nanocrystals.42,48 This suggests that the suppression of the plasmon band in Au/ PbS NPs is probably due to the destructive interference between the exciton and the plasmon inside the Au/PbS NPs.24,25 Transient absorption dynamics at the positive and the negative peaks (475 and 550 nm) of Au/PbS NPs are very similar to those of Au NPs in spite of the fact that the peak originated from the PbS shell (discussed later), which suggests that the total optical response of Au/PbS NPs is dominated by the Au core. On the other hand, the transient absorption dynamics at 700 nm clearly oscillates after a fast decay (Figure 2c). The oscillation period is 3.7 ps (frequency = 9.0 cm−1) and the oscillation is observed from 650 to 740 nm with almost the same oscillation period (Figure S3). In spherical NPs, the oscillation period of coherent vibrational mode (T) depends on the longitudinal velocity of the sound (cl), the particle radius (R), and an eigenvalue of vibrational mode (χn) expressed as1,49
T=
2πR χnc l
Figure 3. Transient absorption spectra of Au NPs (a and b) and Au/ PbS NPs (c and d) excited at 510 nm (resonant to the plasmon band of Au NPs) and 630 nm (selectively excites the plasmon band of Au/ PbS NPs). The wavelength at the excitation is removed because of the large fluctuation (except the spectra of Au NPs excited at 510 nm).
spectra of Au NPs (a and b) and Au/PbS NPs (c and d) excited at 510 and 630 nm, respectively. While transient absorption spectra of Au NPs are very similar irrespective of excitation wavelength (Figure 3a,b), the transient absorption spectrum of Au/PbS NPs excited at 510 nm does not show the plasmon bleaching like that excited at 400 nm (Figure 3c,d). When the 630 nm is used to selectively excite the plasmon band, a tiny shoulder bleaching is observed near the plasmon band region. The dynamics observed at 680 nm oscillates, and its period is about 3.3 ps, although the exact period is difficult to obtain because of the low signal-to-noise ratio (Figure S4). Transient absorption dynamics at the shorter bleach signal (550 nm) and the positive signal (480 nm) of Au/PbS NPs are similar irrespective of excitation wavelength. These are shown in the
(1)
where χn cot χn = 1 − 1/4δ2 for the radial mode (breathing mode) of the isolated sphere, and δ = ct/cl is the ratio of transverse and longitudinal velocity of sound in the particle. The values of cl and χn in Au are 3240 m s−1 and 2.93, while those in PbS are estimated to be 3650 m s−1 and 2.63 by using the bulk modulus and shear modulus of Galena (PbS)51 (details are written in the Supporting Information). When we consider simple Au NPs and PbS NPs whose diameter are 9.5 nm (total diameter of Au/PbS NPs), their radial vibrational periods are very similar with each other (3.1 and 3.2 ps), and their value are 1113
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NPs and 0.68 ps for Au/PbS NPs. The e-ph coupling constant (g) of Au NPs, g = 3.3 × 1016 W m−3 K−1, obtained from the equation τe‑ph0 = γT0/g is similar to that of bulk Au, where γ = 66 J m−3 K−2,59 and T0 is the ambient temperature.1 The slope of the e-ph coupling constant of Au/PbS NPs against excitation intensity was steeper than that of Au NPs, while their τe‑ph0 values are similar to each other. There are two possibilities for the alternation of e-ph coupling time constant by the PbS shell. In Au NPs smaller than 10 nm diameter, the e-ph coupling time constant decreased with decreasing particle diameter because of the reduced screening of e-e scattering60 or the increase of the contribution of electron−surface phonon coupling.61 One reason may be due to the decrease of the electron−surface phonon coupling contribution because of the PbS shell. The difference of e-ph relaxation can be easily observed at high electronic temperature, while it may be difficult to detect at low electronic temperature. Another reason might be the increase of the screening of e-e scattering because of the electron transfer.39 In conclusion, we examined the optical response of Au/PbS core/shell hybrid NPs by transient absorption spectroscopy. The transient absorption spectrum of Au/PbS NPs excited at 400 nm shows two bleach signals, which are assigned to the plasmon band and the PbS shell. The amplitude of the plasmon bleach of Au/PbS NPs weakly depends on the excitation wavelength, but the PbS band does not. The plasmon oscillation of Au/PbS NPs is clearly observed, which is originated from the breathing mode of whole particle. The eph coupling time constant of Au/PbS NPs was similar to that of bare Au NPs. The metal−semiconductor hybrid formation by Au/PbS NPs strongly alters the transient absorption spectra as compared with that of bare Au NPs, while their dynamics differ only minimally compared with those of Au NPs.
Supporting Information (Figure S5). The weak excitationwavelength dependence of the amplitude of the plasmon bleach is probably due to the different extinction coefficients of the Au core and PbS shell at different wavelengths, which may be related to how electron transfer and exciton formation occur in metal−semiconductor hybrid NPs. The electron−phonon (e-ph) coupling time constant is a good parameter to evaluate the hybrid nanomaterials because the e-ph coupling time constant has been reported in various nanomaterials, not only simple metal and semiconductor nanocrystals (Au,1 Ga,52 PbS,53 etc.) but also heteronanostructures (Au/Ag,54,55 Pt/Au,56 Au/Pb57 Ag/SiO250,58 etc.). We obtained the e-ph coupling time constant of Au/PbS NPs by analysis of intensity-dependent transient bleaching dynamics. Figure 4a,b shows the transient bleaching dynamics of Au NPs (525 nm) and Au/PbS NPs (∼550 nm) at different
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
Au/PbS hybrid nanocrystals were synthesized according to the previously reported procedure.39 The average diameter and shell thickness were measured by TEM (JEM-3100FEF, 300 kV, JEOL). UV−vis absorption spectra were recorded using a Hitachi U-4100. Transient absorption spectra were measured by femtosecond pump−probe experiments as described previously.62 The excitation wavelength was changed to 510 and 630 nm by an optical parametric amplifier (OPA, TOPAS, Light-conversion), and excitation intensity was changed from 10 μW to 200 μW (20 nJ/pulse to 400 nJ/pulse). Absorption transients were probed by delayed pulses of a femtosecond white-light continuum generated by focusing a fundamental (800 nm) laser pulse into a D2O cell. All Au/PbS NPs experiments were performed under inert conditions by freeze− pump−thaw cycle.
Figure 4. Excitation intensity dependence of bleaching dynamics at 525 nm for Au NPs (a) and at ∼550 nm for Au/PbS (b). The e-ph coupling time constant as a function of the excitation intensity (c). The extrapolation of the intensity to zero indicates the characteristic eph coupling time constant of NPs. The excitation wavelength is 400 nm.
excitation intensities. In Au NPs, we observed three decay components: subpicosecond rise, picosecond decay (ps decay), and tens of picoseconds decay, which correspond to the electron−electron (e-e) scattering, e-ph coupling, and thermal dissipations to the medium, respectively. The e-ph coupling time constant in Au NPs becomes longer with increasing excitation intensity because the e-ph coupling strongly depends on the initial electronic temperature. On the other hand, transient absorption dynamics of Au/PbS NPs and their excitation intensity dependence are similar to those of Au NPs. The ps decay of Au/PbS NPs linearly increases with increase in excitation intensity (Figure 4c), which is a typical behavior of the optical response of metal NPs, not excitonic behavior of semiconductor NPs. This result suggests that the total optical response of Au/PbS NPs is dominated by the Au core, which is probably due to larger extinction coefficient of the Au core.46 The characteristic e-ph coupling time constant (τe‑ph0) was obtained by the extrapolation to zero-intensity as shown in Figure 4c. The e-ph coupling time constant was 0.60 ps for Au
S Supporting Information *
Supporting Information includes the histogram of the Au diameter and the PbS shell, comparison of simulated extinction spectra and the refractive index of PbS, the calculation of the velocity of sound of PbS, and transient absorption dynamics of Au/PbS NPs probed and excited at different wavelengths. This material is available free of charge via the Internet at http:// pubs.acs.org. 1114
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions §
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research (B, No. 22350012), Priority Areas of Molecular Science for Supra Functional Systems (Grant No. 22018029), and Innovative Areas of DYCE (Grant No. 23104726) from MEXT, Japan. Y.N. and T.K. acknowledge financial support from the Green Photonic Research Project, MEXT. The authors acknowledge Leigh McDowel for his English proofreading.
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(33) Govorov, A. O.; Bryant, G. W.; Zhang, W.; Skeini, T.; Lee, J.; Kotov, N. A.; Slocik, J. M.; Naik, R. R. Exciton-Plasmon Interaction and Hybrid Excitons in Semiconductor−Metal Nanoparticle Assemblies. Nano Lett. 2006, 6, 984−994. (34) Gomez, D. E.; Vernon, K. C.; Mulvaney, P.; Davis, T. J. Surface Plasmon Mediated Strong Exciton-Photon Coupling in Semiconductor Nanocrystals. Nano Lett. 2010, 10, 274−278. (35) Shaviv, E.; Schubert, O.; Alves-Santos, M.; Goldoni, G.; Di Felice, R.; Vallee, F.; Del Fatti, N.; Banin, U.; Soennichsen, C. Absorption Properties of Metal-Semiconductor Hybrid Nanoparticles. ACS Nano 2011, 5, 4712−4719. (36) Vasa, P.; Pomraenke, R.; Schwieger, S.; Mazur, Y. I.; Kunets, V.; Srinivasan, P.; Johnson, E.; Kihm, J. E.; Kim, D. S.; Runge, E.; et al. Coherent Exciton−Surface-Plasmon−Polariton Interaction in Hybrid Metal−Semiconductor Nanostructures. Phys. Rev. Lett. 2008, 101, 116801. (37) Bardhan, R.; Grady, N. K.; Ali, T.; Halas, N. J. Metallic Nanoshells with Semiconductor Cores: Optical Characteristics Modified by Core Medium Properties. ACS Nano 2010, 4, 6169− 6179. (38) Khon, E.; Mereshchenko, A.; Tarnovsky, A. N.; Acharya, K.; Klinkova, A.; Hewa-Kasakarage, N. N.; Nemitz, I.; Zamkov, M. Suppression of the Plasmon Resonance in Au/CdS Colloidal Nanocomposites. Nano Lett. 2011, 11, 1792−1799. (39) Lee, J.-S.; Shevchenko, E. V.; Talapin, D. V. Au−PbS Core− Shell Nanocrystals: Plasmonic Absorption Enhancement and Electrical Doping via Intra-particle Charge Transfer. J. Am. Chem. Soc. 2008, 130, 9673−9675. (40) Link, S.; El-Sayed, M. A. Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots and Nanorods. J. Phys. Chem. B 1999, 103, 8410− 8426. (41) Palik, E. D. Handbook of Optical Constants of Solids; Academic Press: London, 1985. (42) Cademartiri, L.; Montanari, E.; Calestani, G.; Migliori, A.; Guagliardi, A.; Ozin, G. A. Size-Dependent Extinction Coefficients of PbS Quantum Dots. J. Am. Chem. Soc. 2006, 128, 10337−10346. (43) Wood, A.; Giersig, M.; Mulvaney, P. Fermi Level Equilibration in Quantum Dot−Metal Nanojunctions. J. Phys. Chem. B 2001, 105, 8810−8815. (44) Kamat, P. V. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737−18753. (45) Climente, J. I.; Movilla, J. L.; Goldoni, G.; Planelles, J. Excitonic Resonance in Semiconductor−Metal Nanohybrids. J. Phys. Chem. C 2011, 115, 15868−15874. (46) The extinction coefficient of Au NPs (5.1 nm) is estimated to be ∼1.2 × 107 M−1 cm−1 from the extrapolation of experimental fitting by Link and El-Sayed.40 The extinction coefficient of the PbS shell (2.1 nm) at 400 nm is calculated to be 1−2 × 106 M−1 cm−1 by reference to the extinction value of the CdSe nanocrystals and nanoshells.47 (47) Battaglia, D.; Li, J. J.; Wang, Y. J.; Peng, X. G. Colloidal TwoDimensional Systems: CdSe Quantum Shells and Wells. Angew. Chem., Int. Ed. 2003, 42, 5035−5039. (48) Moreels, I.; Lambert, K.; Smeets, D.; De Muynck, D.; Nollet, T.; Martins, J. C.; Vanhaecke, F.; Vantomme, A.; Delerue, C.; Allan, G.; et al. Size-Dependent Optical Properties of Colloidal PbS Quantum Dots. ACS Nano 2009, 3, 3023−3030. (49) Hartland, G. V. Coherent Excitation of Vibrational Modes in Metallic Nanoparticles. Annu. Rev. Phys. Chem. 2006, 57, 403−430. (50) Mongin, D.; Juve, V.; Maioli, P.; Crut, A.; Del Fatti, N.; Vallee, F.; Sanchez-Iglesias, A.; Pastoriza-Santos, I.; Liz-Marzan, L. M. Acoustic Vibrations of Metal−Dielectric Core−Shell Nanoparticles. Nano Lett. 2011, 11, 3016−3021. (51) Bass, J. D. A Handbook of the Physical Constants; American Geophysical Union: Washington, DC, 1995. (52) Nisoli, M.; Stagira, S.; DeSilvestri, S.; Stella, A.; Tognini, P.; Cheyssac, P.; Kofman, R. Ultrafast Electronic Dynamics in Solid and Liquid Gallium Nanoparticles. Phys. Rev. Lett. 1997, 78, 3575−3578.
(53) Krauss, T. D.; Wise, F. W. Coherent Acoustic Phonons in a Semiconductor Quantum Dot. Phys. Rev. Lett. 1997, 79, 5102−5105. (54) Link, S.; Burda, C.; Wang, Z. L.; El-Sayed, M. A. Electron Dynamics in Gold and Gold-Silver Alloy Nanoparticles: The Influence of a Nonequilibrium Electron Distribution and the Size Dependence of the Electron−Phonon Relaxation. J. Chem. Phys. 1999, 111, 1255− 1264. (55) Wang, L.; Kiya, A.; Okuno, Y.; Niidome, Y.; Tamai, N. Ultrafast Spectroscopy and Coherent Acoustic Phonons of Au−Ag Core−Shell Nanorods. J. Chem. Phys. 2011, 134, 054501. (56) Hodak, J. H.; Henglein, A.; Hartland, G. V. Tuning the Spectral and Temporal Response in PtAu Core−Shell Nanoparticles. J. Chem. Phys. 2001, 114, 2760−2765. (57) Hodak, J. H.; Henglein, A.; Hartland, G. V. Coherent Excitation of Acoustic Breathing Modes in Bimetallic Core−Shell Nanoparticles. J. Phys. Chem. B 2000, 104, 5053−5055. (58) Crut, A.; Juve, V.; Mongin, D.; Maioli, P.; Del Fatti, N.; Vallee, F. Vibrations of Spherical Core−Shell Nanoparticles. Phys. Rev. B 2011, 83, 205430. (59) Hodak, J. H.; Henglein, A.; Hartland, G. V. Size Dependent Properties of Au Nanoparticles: Coherent Excitation and Dephasing of Acoustic Vinrationl Modes. J. Chem. Phys. 1999, 111, 8613−8621. (60) Arbouet, A.; Voisin, C.; Christofilos, D.; Langot, P.; Del Fatti, N.; Vallee, F.; Lerme, J.; Celep, G.; Cottancin, E.; Gaudry, M.; et al. Electron−Phonon Scattering in Metal Clusters. Phys. Rev. Lett. 2003, 90, 177401. (61) Darugar, Q.; Qian, W.; El-Sayed, M. A.; Pileni, M. P. SizeDependent Ultrafast Eelectronic Energy Relaxation and Enhanced Fluorescence of Copper Nanoparticles. J. Phys. Chem. B 2006, 110, 143−149. (62) Kobayashi, Y.; Nishimura, T.; Yamaguchi, H.; Tamai, N. Effect of Surface Defects on Auger Recombination in Colloidal CdS Quantum Dots. J. Phys. Chem. Lett. 2011, 2, 1051−1055.
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Dual Transient Bleaching of Au/PbS Hybrid Core/Shell Nanoparticles Yoichi Kobayashi,†,§ Yoshiyuki Nonoguchi,‡,§ Li Wang,† Tsuyoshi Kawai,‡ and Naoto Tamai*,† †
Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan ‡ Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara, 630-0192, Japan S Supporting Information *
ABSTRACT: We examined the optical response of hybrid Au/PbS core/shell nanoparticles (NPs) using transient absorption spectroscopy. Finite-difference time-domain (FDTD) calculations and transient absorption measurements show that Au/PbS NPs have unique two extinction peaks: the peak at the longer wavelength (∼700 nm) is originated from the plasmon, and that at the shorter wavelength (550 nm) is from the local maximum of the refractive index of PbS. The transient absorption dynamics of Au/PbS NPs excited at 400 nm have clear oscillation behavior, which is assigned to the breathing mode of whole particle. We observed a weak excitation-wavelength dependence of the plasmon band. The time constant of electron−phonon coupling of Au/PbS NPs was obtained by changing the excitation intensity. We show that spectral properties of Au/PbS NPs are strongly altered by the hybrid formations, while their dynamics differ only minimally compared with those of Au NPs. SECTION: Physical Processes in Nanomaterials and Nanostructures
S
recently reported transient absorption behaviors in colloidal Au/CdS hybrid nanomaterials,38 transient behaviors of hybrid nanomaterials depend on their shapes, components and interfaces.35 To assign their spectral origins, more systematic experiments with simple shaped hybrid nanomaterials are indispensable. In this Letter, we report the study of the optical response of colloidal Au/PbS core/shell hybrid NPs by transient absorption spectroscopy. Au/PbS core/shell hybrid NPs are an excellent candidate to precisely analyze their optical responses under the strong exciton-plasmon regime because they are monodisperse and have high symmetry, i.e., spherical shape and rock salt crystal structures. We revealed the spectral origins, vibrational oscillation, and electron−phonon coupling constant of metal− semiconductor Au/PbS hybrid NPs. These findings are essential for the fundamental understanding of metal−semiconductor nanointerfaces and exciton−plasmon couplings, which may help in establish high efficient photocatalysts and solar energy conversions. We synthesized symmetrical colloidal Au/PbS NPs according to the procedure described by Lee et al.39 A transmission electron microscopy (TEM) image in Figure 1a (inset) clearly shows that a narrow size dispersion of Au/PbS NPs was synthesized. We estimated the average diameter and shell thickness as 5.1 ± 1.3 nm and 2.2 ± 0.7 nm by measuring ∼90 particles from TEM images. The histogram is shown in the
emiconductor nanomaterials and metal nanomaterials have great potential for solar energy conversion, biological applications, and quantum information because of their unique properties.1−8 Recent wet chemical techniques enabled us to synthesize various kinds of colloidal metal−semiconductor hybrid nanomaterials, such as metal/chalcogenide,9−14 metal/ oxcide-semiconductor,15,16 and metal/spacer/semiconductor,17,18 which accelerate further investigations of unique and more applicable nanomaterials. Metal−semiconductor hybrid nanoparticles (NPs) act as an efficient photocatalyst because of electron transfer from a semiconductor to a metal part.19,20 Complex hybrid nanomaterials composed of metal NPs, a spacer, and chromophore have been synthesized to enhance the emission, progress the chemical reaction, and act as a sensor.21−23 In addition to these characteristics, recent reports suggest that metal−semiconductor hybrid NPs have additional unique properties. Several theoretical reports have suggested that the optical spectrum of metal−semiconductor hybrid NPs exhibited exciton-induced transparency, Fano effect, and nonlinear Fano effect in the strong exciton-plasmon coupling regime.24−26 In addition, quantum coherence has been experimentally shown to survive in the Au/CdSe hybrid NPs, which enabled spin manipulations with hybrid nanomaterials.27 Optical properties of metal-semiconductor hybrid nanomaterials have been studied in the past few years,28−31 however, the intrinsic ultrafast optical response of nano-order materials under strong exciton-plasmon coupling has not been well understood32 because most experimental reports were examined by steady-state experiments.33−37 Although Khon et al. has © 2012 American Chemical Society
Received: March 2, 2012 Accepted: April 9, 2012 Published: April 9, 2012 1111
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to 3−7 nm and the shell thickness to 1−3 nm, respectively. The black solid line in the graph corresponds to nearly the same size as the experimental Au/PbS NPs. All calculated extinction spectra show two unique extinction peaks, and only the longerwavelength peak depends on the core diameter and the shell thickness (red and blue line, respectively). On the other hand, the simulated extinction spectra of the PbS nanoshell has one shoulder at ∼620 nm, which is similar to the shorterwavelength peak of Au/PbS NPs. Since FDTD calculation does not include excitonic effects and quantum size effects of semiconductors, the shorter-wavelength peak is assigned to the PbS shell, which is due to the local maximum of the refractive index of PbS (Figure S2 in the Supporting Information). The longer-wavelength peak of Au/PbS NPs is safely assigned to the plasmon band because the peak only appears in nanomaterials that include a Au component, which depends on both the core diameter and the shell thickness. As we can see from the simulation, a small difference of the ratio of core size and shell thickness gives a large spectral shift, which is probably another reason for the large spectral broadening of narrow sizedispersed Au/PbS NPs in addition to the PbS-shell band and the exciton−plasmon coupling.25,35 As compared to the experimental spectrum, the simulated extinction spectrum is shifted to red. There are two conceivable reasons for the deviation. One is the effect of quantum confinement on the refractive index of PbS, which is not included in FDTD calculations. Cademartiri et al. reported the size-dependent extinction spectra of PbS nanocrystals and showed that the absorption peak, even at a shorter wavelength, shifted to blue with decreasing particle size, although the shift was not as drastic as compared with that of the band-edge peak.42 Since the absorption is the complex part of the refractive index, the refractive index function should also shift to blue due to the quantum confinement effect. The other reason may be due to the negative-charge storage of Au core due to the Fermi equilibrium.43,44 Once an PbS shell electron is excited to the conduction state, an ultrafast electron transfer to Au core is expected because the system will keep equilibrating each Fermi level, and finally the Au core stores negative charges, which gives a blue shift of the plasmon band.43 Lee et al. reported that Au/PbS NPs films always exhibit a p-type gate effect,39 which shows that electron transfer to the Au core certainly occurs in this system.
Figure 1. (a) Experimental steady-state extinction spectra of Au NPs and Au/PbS NPs (purple and gray). Their extinctions were arbitrary multiplied to see their spectral features easily. Inset shows a TEM image of Au/PbS NPs. The white bar corresponds to 20 nm. (b) Simulated extinction spectra of Au/PbS NPs in different core diameter and shell thickness and PbS nanoshell (green line) whose inner diameter is 5 nm with the refractive index of 1.0 and the shell thickness is 2 nm. Thick black line corresponds to the simulated spectrum of experimental Au/PbS NPs. The core-diameter dependence and shellthickness dependence are represented in red and blue lines, respectively.
Supporting Information (Figure S1.) Figure 1a shows steadystate absorption spectra of Au NPs and Au/PbS NPs. The absorption spectrum of Au NPs before the encapsulation by PbS shells has a well-known sharp plasmon peak at the band edge.40 On the other hand, Au/PbS NPs have a broad absorption in spite of the narrow size dispersion, which is similar to previous reports.14,39 Figure 1b shows simulated extinction spectra of Au/PbS NPs and PbS nanoshell (inner diameter is 5 nm and shell thickness is 2 nm, green line) by finite-difference time-domain (FDTD) calculations (Lumerical FDTD solution). We used the refractive index of the bulk PbS41 and set the core diameter
Figure 2. Transient absorption spectra of Au NPs (a) and Au/PbS NPs (b) excited at 400 nm (excitation intensity is 200 nJ/pulse). Transient absorption dynamics of Au/PbS NPs at different wavelength (excitation intensity is 400 nJ/pulse) (c). 1112
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relatively close to that of Au/PbS NPs (3.7 ps). This indicates that the oscillation dynamics of Au/PbS NPs is due to the breathing mode of the whole particle. The reason for the deviation between the experimental value and the simple calculation is probably the interfacial effect of heteronanostructures, although the experimental errors should be also taken into account. Detailed calculations assuming complex structures by the Navier equation may give better explanations for our experimental results.50,58 We measured transient absorption spectra by changing the excitation wavelength to subresonantly excite the bleach signal at the shorter wavelength (510 nm) and selectively excite the plasmon band (630 nm). Figure 3 shows transient absorption
We measured transient absorption spectra of Au NPs and Au/PbS NPs excited at 400 nm, which induces the interband transition, to reveal the optical response of metal−semiconductor hybrid nanosystems (Figure 2a,b). The transient absorption spectrum of Au NPs has a bleach signal at 525 nm and two positive signals at the side of the bleach signal (Figure 2a). On the other hand, the transient absorption spectrum of Au/PbS NPs has two bleach signals at 553 and ∼700 nm and a positive signal at 475 nm (Figure 2b), which corresponds to simulated steady-state extinction spectra of Au/PbS NPs. Although the amplitude of the positive peak at 475 nm of Au/PbS NPs looks like it is increasing compared with that of Au NPs, the two bleach signals are actually suppressed because the steady-state absorbance of Au/PbS NPs at the broad peak is more than 2 times larger than that of Au NPs at the plasmon peak in our experiments. The suppression of the plasmon band is consistent with a recent report on Au/CdS hybrid nanostructure.38,45 The reason why the plasmon bleach is observed in our hybrid systems is probably the 10-fold larger extinction coefficient of the Au core compared to that of the PbS shell.46 Artuso and Bryant theoretically showed that, under a strong exciton−plasmon coupling regime, the exciton and the plasmon canceled each other by destructive interferences depending on the metal radius and the dipole moment of the semiconductor nanocrystals.24 We estimated the excitonic peak wavelength of the PbS shell by using the value of PbS nanocrystals, whose excitonic peak is ideally the same as that of quantum well, i.e., shell structures.4,42,47,48 The excitonic peak wavelength of the PbS shell is estimated to be 560−690 nm, which roughly overlaps the plasmon absorption of Au/PbS NPs, although there is large ambiguity because of few experimental reports on such small PbS nanocrystals.42,48 This suggests that the suppression of the plasmon band in Au/ PbS NPs is probably due to the destructive interference between the exciton and the plasmon inside the Au/PbS NPs.24,25 Transient absorption dynamics at the positive and the negative peaks (475 and 550 nm) of Au/PbS NPs are very similar to those of Au NPs in spite of the fact that the peak originated from the PbS shell (discussed later), which suggests that the total optical response of Au/PbS NPs is dominated by the Au core. On the other hand, the transient absorption dynamics at 700 nm clearly oscillates after a fast decay (Figure 2c). The oscillation period is 3.7 ps (frequency = 9.0 cm−1) and the oscillation is observed from 650 to 740 nm with almost the same oscillation period (Figure S3). In spherical NPs, the oscillation period of coherent vibrational mode (T) depends on the longitudinal velocity of the sound (cl), the particle radius (R), and an eigenvalue of vibrational mode (χn) expressed as1,49
T=
2πR χnc l
Figure 3. Transient absorption spectra of Au NPs (a and b) and Au/ PbS NPs (c and d) excited at 510 nm (resonant to the plasmon band of Au NPs) and 630 nm (selectively excites the plasmon band of Au/ PbS NPs). The wavelength at the excitation is removed because of the large fluctuation (except the spectra of Au NPs excited at 510 nm).
spectra of Au NPs (a and b) and Au/PbS NPs (c and d) excited at 510 and 630 nm, respectively. While transient absorption spectra of Au NPs are very similar irrespective of excitation wavelength (Figure 3a,b), the transient absorption spectrum of Au/PbS NPs excited at 510 nm does not show the plasmon bleaching like that excited at 400 nm (Figure 3c,d). When the 630 nm is used to selectively excite the plasmon band, a tiny shoulder bleaching is observed near the plasmon band region. The dynamics observed at 680 nm oscillates, and its period is about 3.3 ps, although the exact period is difficult to obtain because of the low signal-to-noise ratio (Figure S4). Transient absorption dynamics at the shorter bleach signal (550 nm) and the positive signal (480 nm) of Au/PbS NPs are similar irrespective of excitation wavelength. These are shown in the
(1)
where χn cot χn = 1 − 1/4δ2 for the radial mode (breathing mode) of the isolated sphere, and δ = ct/cl is the ratio of transverse and longitudinal velocity of sound in the particle. The values of cl and χn in Au are 3240 m s−1 and 2.93, while those in PbS are estimated to be 3650 m s−1 and 2.63 by using the bulk modulus and shear modulus of Galena (PbS)51 (details are written in the Supporting Information). When we consider simple Au NPs and PbS NPs whose diameter are 9.5 nm (total diameter of Au/PbS NPs), their radial vibrational periods are very similar with each other (3.1 and 3.2 ps), and their value are 1113
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NPs and 0.68 ps for Au/PbS NPs. The e-ph coupling constant (g) of Au NPs, g = 3.3 × 1016 W m−3 K−1, obtained from the equation τe‑ph0 = γT0/g is similar to that of bulk Au, where γ = 66 J m−3 K−2,59 and T0 is the ambient temperature.1 The slope of the e-ph coupling constant of Au/PbS NPs against excitation intensity was steeper than that of Au NPs, while their τe‑ph0 values are similar to each other. There are two possibilities for the alternation of e-ph coupling time constant by the PbS shell. In Au NPs smaller than 10 nm diameter, the e-ph coupling time constant decreased with decreasing particle diameter because of the reduced screening of e-e scattering60 or the increase of the contribution of electron−surface phonon coupling.61 One reason may be due to the decrease of the electron−surface phonon coupling contribution because of the PbS shell. The difference of e-ph relaxation can be easily observed at high electronic temperature, while it may be difficult to detect at low electronic temperature. Another reason might be the increase of the screening of e-e scattering because of the electron transfer.39 In conclusion, we examined the optical response of Au/PbS core/shell hybrid NPs by transient absorption spectroscopy. The transient absorption spectrum of Au/PbS NPs excited at 400 nm shows two bleach signals, which are assigned to the plasmon band and the PbS shell. The amplitude of the plasmon bleach of Au/PbS NPs weakly depends on the excitation wavelength, but the PbS band does not. The plasmon oscillation of Au/PbS NPs is clearly observed, which is originated from the breathing mode of whole particle. The eph coupling time constant of Au/PbS NPs was similar to that of bare Au NPs. The metal−semiconductor hybrid formation by Au/PbS NPs strongly alters the transient absorption spectra as compared with that of bare Au NPs, while their dynamics differ only minimally compared with those of Au NPs.
Supporting Information (Figure S5). The weak excitationwavelength dependence of the amplitude of the plasmon bleach is probably due to the different extinction coefficients of the Au core and PbS shell at different wavelengths, which may be related to how electron transfer and exciton formation occur in metal−semiconductor hybrid NPs. The electron−phonon (e-ph) coupling time constant is a good parameter to evaluate the hybrid nanomaterials because the e-ph coupling time constant has been reported in various nanomaterials, not only simple metal and semiconductor nanocrystals (Au,1 Ga,52 PbS,53 etc.) but also heteronanostructures (Au/Ag,54,55 Pt/Au,56 Au/Pb57 Ag/SiO250,58 etc.). We obtained the e-ph coupling time constant of Au/PbS NPs by analysis of intensity-dependent transient bleaching dynamics. Figure 4a,b shows the transient bleaching dynamics of Au NPs (525 nm) and Au/PbS NPs (∼550 nm) at different
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
Au/PbS hybrid nanocrystals were synthesized according to the previously reported procedure.39 The average diameter and shell thickness were measured by TEM (JEM-3100FEF, 300 kV, JEOL). UV−vis absorption spectra were recorded using a Hitachi U-4100. Transient absorption spectra were measured by femtosecond pump−probe experiments as described previously.62 The excitation wavelength was changed to 510 and 630 nm by an optical parametric amplifier (OPA, TOPAS, Light-conversion), and excitation intensity was changed from 10 μW to 200 μW (20 nJ/pulse to 400 nJ/pulse). Absorption transients were probed by delayed pulses of a femtosecond white-light continuum generated by focusing a fundamental (800 nm) laser pulse into a D2O cell. All Au/PbS NPs experiments were performed under inert conditions by freeze− pump−thaw cycle.
Figure 4. Excitation intensity dependence of bleaching dynamics at 525 nm for Au NPs (a) and at ∼550 nm for Au/PbS (b). The e-ph coupling time constant as a function of the excitation intensity (c). The extrapolation of the intensity to zero indicates the characteristic eph coupling time constant of NPs. The excitation wavelength is 400 nm.
excitation intensities. In Au NPs, we observed three decay components: subpicosecond rise, picosecond decay (ps decay), and tens of picoseconds decay, which correspond to the electron−electron (e-e) scattering, e-ph coupling, and thermal dissipations to the medium, respectively. The e-ph coupling time constant in Au NPs becomes longer with increasing excitation intensity because the e-ph coupling strongly depends on the initial electronic temperature. On the other hand, transient absorption dynamics of Au/PbS NPs and their excitation intensity dependence are similar to those of Au NPs. The ps decay of Au/PbS NPs linearly increases with increase in excitation intensity (Figure 4c), which is a typical behavior of the optical response of metal NPs, not excitonic behavior of semiconductor NPs. This result suggests that the total optical response of Au/PbS NPs is dominated by the Au core, which is probably due to larger extinction coefficient of the Au core.46 The characteristic e-ph coupling time constant (τe‑ph0) was obtained by the extrapolation to zero-intensity as shown in Figure 4c. The e-ph coupling time constant was 0.60 ps for Au
S Supporting Information *
Supporting Information includes the histogram of the Au diameter and the PbS shell, comparison of simulated extinction spectra and the refractive index of PbS, the calculation of the velocity of sound of PbS, and transient absorption dynamics of Au/PbS NPs probed and excited at different wavelengths. This material is available free of charge via the Internet at http:// pubs.acs.org. 1114
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions §
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research (B, No. 22350012), Priority Areas of Molecular Science for Supra Functional Systems (Grant No. 22018029), and Innovative Areas of DYCE (Grant No. 23104726) from MEXT, Japan. Y.N. and T.K. acknowledge financial support from the Green Photonic Research Project, MEXT. The authors acknowledge Leigh McDowel for his English proofreading.
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