Two-Photon Excitation of Gold Nanorods Interrupted by Extremely Fast

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Two-Photon Excitation of Gold Nanorods Interrupted by Extremely Fast Solvent-to-Metal Electron Transfer Hai Zhu, Monalisa Garai, Zhihui Chen, and Qing-Hua Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10235 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on December 7, 2017

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The Journal of Physical Chemistry

Two-photon Excitation of Gold Nanorods Interrupted by Extremely Fast Solvent-to-Metal Electron Transfer Hai Zhu,1,2 Monalisa Garai,1 Zhihui Chen,1 Qing-Hua Xu*,1,2 1

Department of Chemistry, National University of Singapore, Singapore 117543

2

NUSNNI-Nanocore, National University of Singapore, Singapore 117411

AUTHOR INFORMATION The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Corresponding Author *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Gold nanorods (Au NRs) have been widely exploited for various biomedical applications due to their strong two-photon photoluminescence (2PPL). 2PPL of Au NRs were found significantly quenched in organic solvents, which was originally ascribed to reduced luminescence yields caused by electron transfer quenching. It was recently found that excitation of 2PPL of Au NRs involved two sequential one-photon absorption steps. Here various ultrafast spectroscopic techniques have been employed to demonstrate two different solvent-to-metal electron transfer pathways: electron transfer from organic solvents to sp and d band holes of excited Au NRs, which have different influences on 2PPL of Au NRs. Electron transfer to sp band holes occurs extremely fast (~25 fs), which blocks absorption of the 2nd photon and reduces two-photon excitation efficiency, which is the dominant mechanism for the observed 2PPL quenching. In contrast, Electron transfer to d band holes hinders the radiative recombination process and results in reduced luminescence yield, which plays the minor contribution for the observed 2PPL quenching. This work provides an additional brand-new quenching mechanism of 2PPL emission of Au NRs: extremely fast electron transfer to the intermediate states within the laser pulse duration interrupts sequential absorption of two photons, which results in significantly reduced two-photon excitation efficiency. This study provides new insight on fundamental understanding of two-photon excitation process and nonlinear optical properties of these materials.


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INTRODUCTION Noble metal nanoparticles, such as gold (Au) and silver (Ag), have attracted intensive interest due to their unique optical properties known as localized surface Plasmon resonance (SPR), which arises from collective oscillation of conduction band electrons under optical excitation.1-2 SPR has been known to give rise to intense localized electromagnetic fields to significantly modify the optical responses of nearby chromophores or metal nanoparticles themselves. For example, metal nanoparticles have been known responsible for surface-enhanced Raman scattering (SERS),3-7 metal enhanced fluorescence,8-10 and Plasmon enhanced photo-catalysis.1114

Metal nanoparticles themselves also display exceptional optical properties such as large

extinction coefficients, second harmonic generation, and strong two-photon photoluminescence (2PPL).15-26 Among various metal nanostructures, gold nanorods (Au NRs) have been known to display strong 2PPL due to their large two-photon absorption (2PA) cross sections and enhanced photoluminescence (PL) quantum yields resulting from Plasmon enhanced local electrical field and increased radiative decay rates.16 2PA cross sections of Au NRs have been reported up to 108 GM with their two-photon brightness several orders of magnitude larger than small organic fluorophores.22,

27-29

Strong 2PPL, good biocompatibility, and chemical inertness of Au NRs

render them to be extensively exploited for various two-photon excitation (2PE) based biomedical applications such as sensing, imaging, and phototherapy30-32 to benefit from their unique advantages such as small confined working area and deep penetration into biological tissues. Understanding their fundamental excitation and emission mechanisms is critical for their various biomedical and optoelectronic applications.


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2PPL of Au NRs was recently found strongly dependent on the solvent medium.33 2PPL intensities of Au NRs in organic solvents were found significantly quenched compared to that in H2O. It was generally understood that 2PPL of Au NRs arises from radiative combination of sp band electrons and d band holes, which were created by absorption of two photons.22, 34-36 2PPL quenching in organic solvents was believed arising from electron transfer from electron donating solvents to d band holes of excited Au NRs. This electron transfer process would compete with the radiative recombination of electron-hole pairs, resulting in reduced photoluminescence (PL) yields of Au NRs in organic solvents and consequently quenched 2PPL. The proposed mechanism was supported by observation of a long-lived transient species originating from charge-separated states in ultrafast transient absorption studies under excitation at 400 nm.33 Strong 2PPL of Au NRs have been attributed to enhanced local electric field arising from the longitudinal plasmon resonance of these nanostructures at the wavelength coincident with the excitation wavelength. A lot of efforts have been made to understand the excitation mechanism of strong 2PPL in Au NRs, which is still under active debate.24, 34-35, 37 It has been recently demonstrated that excitation of 2PPL of Au NRs involves two sequential one-photon absorption (1PA) steps via intermediate states (Scheme 1, left) instead of one coherent 2PA process. The first 1PA step promotes sp electrons above the Fermi level through intraband transition by absorption of 800 nm photons, leaving holes below the Fermi level. Within the laser pulse duration, the 2nd 1PA step excites the d band electrons to fill the holes that were created in the 1st 1PA step, resulting in creation of d band holes and sp electrons above the Fermi level. Radiative recombination of sp electrons and d band holes are responsible for the observed emission. Under the context of this new excitation mechanism, the previous quenching mechanism needs to be revised. Based on the energy diagram shown in Scheme 1, there are two possible electron


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transfer pathways from organic solvents to the excited Au NRs: (1) electron transfer from organic solvents to sp band holes of the excited Au NRs generated by the first 1PA process (ET1 in Scheme 1); and (2) electron transfer from organic solvent to d band holes of Au NRs generated by two sequential 1PA steps (ET2 in Scheme 1, the previously proposed quenching mechanism). ET1 and ET2 have different influences on 2PPL of Au NRs. If the rate of ET1 is fast enough to occur within laser pulse duration, filling of the holes in the sp band upon ET1 could even block the 2nd 1PA step, which will reduce the overall 2PE efficiency and consequently reduced 2PPL. In contrast, filling the d band holes upon ET2 will hinder the radiative recombination process, resulting in reduced PL yield and consequently reduced 2PPL. Energy

sp band Solvent

EF Energy

ET2 HOMO

sp band Solvent

EF 1

HOMO 2

d band

ET1 Energy

ET2 d band k

k sp band Solvent

EF ET1

HOMO d band k

Scheme 1 Illustration of two possible electron transfer pathways between Au NRs and electron donating solvents. 1: sp-sp intraband transition; 2: d-sp interband transition; ET1: electron transfer from solvents to sp band holes; ET1: electron transfer from solvents to d band holes. The occurrence of electron transfer from organic solvent to d band holes of Au NRs (ET2) has been confirmed by previous ultrafast studies on Au NRs in organic solvents under excitation


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at 400 nm to initiate the interband transition.33 However, it is not clear about electron transfer from organic solvent to sp band holes of Au NRs (ET1) and its influence on 2PPL. To confirm the occurrence of electron transfer pathway from the organic solvent to sp band holes of Au NRs (ET1) and clarify the effects of this new electron transfer pathway, here various ultrafast spectroscopic techniques were employed to study electron dynamics of Au NRs in H2O and DMF solvents under excitation at 800 nm. 800 nm laser pulses initiate intraband transition to promote electrons onto the sp band above the Fermi level and leave holes below in the same sp band. DMF acts as the representative organic solvent, although different extent of 2PPL quenching has been observed in different organic solvents.33 Electron dynamics of Au NRs in H2O and DMF solvents were studied by ultrafast transient absorption, pump probe, pulse duration dependent 2PPL, and two pulse emission modulation (2PEM) measurements under excitation at 800nm. A brand-new quenching mechanism was proposed in this work: the extremely fast electron transfer within the pulse duration interrupts the second step of two sequential 1PA processes that create the emitting state, leading to a dramatic reduced 2PE efficiency and 2PA cross section. The reduced 2PE efficiency due to this extremely fast electron transfer process is the dominant contribution for the observed 2PPL quenching of Au NRs in the organic solvents. This mechanism is distinctly different from the previously proposed quenching mechanism of electron transfer from DMF to photoexcited d band holes to result in a reduced PL yield, which only plays a minor contribution to the observed 2PPL quenching. A dual electrontransfer pathway mechanism is responsible for the overall 2PPL quenching.

EXPERIMENTAL SECTION


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Chemicals and Materials. Gold (III) chloride trihydrate (HAuCl4·3H2O, 99.9%), silver nitrate (AgNO3, 99%), sodium borohydride (NaBH4, 98%), sodium oleate (NaOL, > 99%), ascorbic acid

(AA,

99%),

cetyltrimethylammonium

bromide

(CTAB,

99%),

O-[2-(3-

mercaptopropionylamino) ethyl]-O’-methyl-polyethylene glycol (PEG-SH, Mw=5000) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl, 37% in water) was purchased from Fluka. All aqueous solutions were prepared in ultrapure deionized (DI) water (with a resistance of 18.1 MΩ). N,N-dimethylformamide (DMF) were of analytical grade and used as received. Preparation of Au NRs. Au NRs with a longitudinal SPR band centered at 772 nm were prepared according to a previously reported seed-mediated method.38 5 mL of 0.2 M CTAB solution was mixed with 5 mL of 0.5 mM HAuCl4 solution. A freshly prepared 0.6 mL of 0.01 M NaBH4 solution was diluted to 1 mL and quickly added into the Au (III)-CTAB solution under vigorous stirring for 2 min. The resultant seed solution was aged for 30 min at room temperature before use. The growth solution was prepared by dissolving 1.8 g CTAB and 0.3 g NaOL in 50 mL of warm DI water (50 °C) in a conical flask. The solution was cooled to 30 °C before adding 4.8 mL of 4 mM AgNO3 solution, and was then kept undisturbed at 30 °C for 15 min. 50 mL of 1 mM HAuCl4 was subsequently added into the mixture under stirring at 700 rpm for 90 min. Next, 0.3 mL of 12.1 M HCl was added to adjust the pH of the mixture under stirring before addition of 0.25 mL of 0.064 M AA. Finally, 0.08 mL of seed solution was added into the mixture under stirring and then left undisturbed overnight at 30 °C. The PEG-SH-capped Au NRs were prepared by using a previously reported method.39 2 mL of 2 mM PEG-SH solution was mixed with 5 mL of aqueous Au NR solution. The reaction proceeded under vigorous stirring for 12 h to replace the original CTAB surfactant molecules by PEG molecules owing to strong binding interactions between Au and the thiol group of PEG-SH.


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Characterizations and 2PPL Measurements. Transmission electron microscopy (TEM) images of the Au NRs were taken on a Philips CM10 TEM microscope at an accelerating voltage of 100 kV. The extinction spectra of Au NRs in different solvents were measured by using a SHIMADZU UV-2250 spectrophotometer. 2PPL spectra of Au NRs in different solvents were measured under using femtosecond laser pulses with central wavelength of 800 nm as the excitation source. The femtosecond laser pulse duration was varied from 35 to 140fs, which was determined by using an AVESTA autocorrelator (AA-20DD). The laser beam was focused onto the samples using a lens with a focus length of 3.0 cm and the 2PPL emission was collected by using a Princeton Instrument monochrometer (SP2300i) that was equipped with a CCD (PIXIS 100). Two 750 nm short pass filters (Semrock, FF01-750/SP-25) were placed before the spectrometer to suppress the scattering from the excitation light. Transient Absorption Spectroscopy and Pump-Probe Measurement. Ultrafast transient absorption spectroscopy and pump-probe measurements were performed by using output laser pulses of a Ti:sapphire oscillator seeded regenerative amplifier laser system. The laser system produces femtosecond laser pulses with central wavelength at 800nm, repetition rate of 1 kHz, and pulse duration of 100 fs. The output was splitted into two beams. One 800 nm beam or its second harmonic generation (400 nm) acted as the pump beam, which was modulated by an optical chopper at a frequency of 500Hz. Another beam was focused onto a 1 mm sapphire plate to generate white light continuum, which acted as the probe beam. Both pump and probe beams were focused onto the sample and spatially overlapped with spot diameters of 300 and 200 µm, respectively. The time delay between the pump and probe pulses was adjusted by a translational stage (Newport, ESP300). Transient absorption spectra were collected at different delay times


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between the pump and probe pulses. Au NRs in different solvents were contained in a cuvette with 1.0 mm path length. The measurements were performed at room temperature. Two-pulse Emission Modulation (2PEM) Spectroscopy. 2PEM measurements were performed on a Ti:sapphire femtosecond laser system (Chameleon Ultra II, Coherent) with central wavelength at 800 nm, pulse duration of 140 fs and repetition rate of 80 MHz. The output was split into two beams with equal pulse energy of 0.94 nJ. Both laser beams were focused and spatially overlapped onto the Au NRs solution contained in a 1 cm cuvette. 2PPL of Au NRs was directed into a monochrometer (Acton SP-2300i, Princeton Instruments) and detected by a photomultiplier tube detector (PMA182, Picoquant). The emission was filtered by two 750 nm short-pass filters (Semrock, FF01-750/SP-25) to suppress the scattering from 800 nm laser pulses. 2PEM profiles were obtained by varying the time delay between the two beams. RESULTS AND DISCUSSION Characterization of Au NRs. Au NRs (Figure 1a) were prepared using a seed-mediated, CTAB-assisted method.38 CTAB capping layer was replaced by PEG-SH so that Au NRs can be well dispersed in organic solvents.33, 39 Extinction spectra of Au NRs in H2O, DMF and mixture solvent are shown in Figures 1a & S1. As the DMF content increased, the longitudinal SPR band gradually redshifted from 772 to 797 nm, owing to the increasing refractive index of the solvent medium. 2PPL spectra were measured under excitation of femtosecond laser pulses with central wavelength of 800 nm, pulse duration of 140 fs, and average power of 150 mW. Au NRs in H2O displayed strong photoluminescence under femtosecond laser excitation at 800 nm. The log-log plot of the integrated photoluminescence intensities of Au NRs in H2O versus excitation power gave a slope of 2.0 (Figure S1d, supporting information), which confirmed that two


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photon are involved in the excitation process. The observed strong 2PPL in H2O can be ascribed to the large 2PA cross sections of Au NRs.22, 29 However, 2PPL intensities of Au NRs were significantly reduced in DMF and H2O/DMF mixture solvents compared to that in H2O, and decreased gradually with the increasing DMF content (Figures 1b and S1). 2PPL of Au NRs in DMF was reduced by ~44-fold compared to that in H2O. Generally, photoluminescence quenching could result from three different possible mechanisms:

aggregation induced

quenching, energy transfer or electron transfer. Aggregation induced quenching mechanism can be excluded by monitoring the extinction spectra of Au NRs in H2O and DMF. The formation of gold nanoaggregates would lead to a decrease of the original SPR band and a newly formed band in the longer wavelength range. The extinction spectra of Au NRs in H2O, DMF and mixture solvents shown in Figures 1a & S1 did not indicate any characteristics for formation of gold nanoaggregates, which exclude the possible quenching mechanism of aggregate formation. Energy transfer mechanism requires spectral overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. Considering the larger HOMO-LUMO energy gap of DMF, the possibility of quenching mechanism due to energy transfer could be excluded. The photo-induced hot electron transfer from Au NRs to DMF could also be excluded due to high LUMO level of DMF (Scheme 1). The observed 2PPL quenching in DMF could be attributed to electron transfer from DMF to the excited Au NRs.33


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Figure 1. Extinction spectra (a) and two-photon photoluminescence (2PPL) spectra (b) of Au NRs in H2O and DMF solvents. Inset (a) is TEM image of Au NRs.

Ultrafast transient absorption spectroscopy. As described in Scheme 1, under the context of two sequential 1PA processes, there are two possible electron transfer pathways from DMF to the excited Au NRs (Scheme 1): (1) electron transfer from DMF to sp band holes of Au NRs (ET1) upon the first 1PA step that promotes electrons onto the sp band above the Fermi level and leaves holes in the sp band below the Fermi level; and (2) electron transfer from DMF to d band holes of Au NRs (ET2) upon two sequential 1PA steps that create electrons in the sp band


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above the Fermi level and holes in the d band. Electron transfer from DMF to d band holes of Au NRs (ET2) has been confirmed by observation of a long-lived transient species originating from charge-separated states in a previous ultrafast study, in which interband transition was initiated by excitation at 400 nm to create electron-hole pairs that were equivalent to those generated by two sequential 1PA absorption of 800 nm photons.33 To confirm the occurrence of electron transfer pathway from DMF to sp band holes of Au NRs (ET1), electron dynamics of Au NRs upon excitation at 800 nm to create sp holes are studied by using various ultrafast spectroscopic techniques.

Figure 2. (a, b) Transient absorption spectra of Au NRs in H2O (a) and DMF (b) at different time delay under excitation at 800 nm. The green boxes highlight the difference of the transient spectra in two solvents at the longer delay times. (c, d) Single wavelength dynamics to probe the bleaching behaviors of


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(c) longitudinal SPR band at 760 nm and (d) transverse SPR band at 510 nm. Inset figures show the corresponding dynamics at short time delays.

The results of ultrafast transient absorption and pump probe studies of Au NRs in H2O and DMF under excitation at 800 nm are shown in Figure 2. Transient absorption spectra of Au NRs in H2O (Figure 2a) exhibited transient bleaching bands at the wavelength range of both longitudinal and transverse SPR bands as well as transient absorption bands at the wings of two bands as a result of elevated electron temperature upon photo-excitation.1,

40-42

The

corresponding single wavelength dynamics to probe the bleaching behaviors of longitudinal SPR band at 760 nm and transverse SPR band at 510 nm are shown in Figures 2c&d. Hot electron relaxation through a series of heat exchange processes resulted in recovery of the transient bleaching and absorption bands, while the transient spectral profiles remained nearly unchanged, consistent with the previous reports on hot electron dynamics in Au NRs.1,

40-42

In contrast,

transient absorption spectra of Au NRs in DMF (Figure 2b) displayed quite different evolution behaviors. Transient bleaching of the longitudinal SPR band (Figures 2b & 2c) quickly recovered for the first 10 ps, turned into transient absorption (negative ∆T/T) in the 700-750 nm range at longer delay times (100-1000ps), which slowly decayed back on a time scale of >1ns. The observation of this long-lived transient species is a direct evidence of charge separated state resulting from electron transfer from DMF to sp holes of the excited Au NRs. The possible contribution of observed transient species due to electron transfer from DMF to d band holes of Au NRs is negligible as the d band holes created by two sequential one-photon absorption of 800 nm photons is much less that the sp band holes created by single one-photon absorption of 800 nm photons.

In contrast, transient bleaching of transverse SPR mode (Figure 2b & 2d)

displayed a quick decay for the first 10 ps followed by a slow decay afterwards (from 10 ps to >1 ns). These observations unambiguously support the occurrence of electron transfer from DMF to


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sp band holes of Au NRs (ET1 in Scheme 1). Under excitation at 800 nm, electrons are promoted to the sp band above the Fermi level through intraband transition, leaving holes below the Fermi level. These holes could be filled by electrons transferred from the HOMO level of DMF (ET1). This electron transfer process is responsible for initial fast recovery of the bleaching of longitudinal and transverse SPR bands as well as the formation of charge-separated state. The charge-separated state has a long lifetime beyond our measurement window (>1ns), which is responsible for the slow decay of transient absorption around 700-750 nm and slow recovery of transverse SPR band bleaching (Figures 2b and 2c). The corresponding electron transfer time was estimated to be 9.5 ± 0.7 ps by comparing the single wavelength dynamics of Au NRs in H2O and DMF (Figure 2c), i.e.

=

−

, where and are the

electron-phonon relaxation time constants (Table S1) of Au NRs in DMF and H2O, respectively. As this electron transfer rate of 9.5 ps is much longer than the pulse duration of ~140 fs, it will have little effect on two-photon excitation process (more rigorously two sequential one-photon absorption processes) which occurred within the laser pulse duration. Although the above rate constant (9.5ps) of electron transfer obtained from pump probe studies (Figure 2c) was much longer than the laser pulse duration, different initial transient bleaching amplitudes (i.e. ∆t=0ps) of longitudinal SPR bands were observed in the transient spectra of Au NRs in DMF and H2O (Figure S2) despite their similar steady state extinction spectra (Figure 1a). This observation suggests that the fastest component of electron transfer process has already occurred within the laser pulse duration period (~140 fs), which is not well resolved in the above pump probe studies. If electron transfer from DMF solvent to Au NRs fills in the sp band holes within the laser pulse duration, the 2nd 1PA step (2 in Scheme 1) could therefore be interrupted, consequently 2PA cross section and 2PE efficiency of Au NRs in DMF


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will be smaller than that in H2O due to this extra electron transfer pathway (ET1). If this speculation is true, 2PA cross sections and 2PE efficiencies of Au NRs in two solvents are expected to be different for the same laser pulse duration and the relative 2PPL intensity in two solvents are expected to be strongly dependent on the laser pulse duration, which will be examined and discussed in detail in the next section. It needs to be noted that the observation of transient absorption band under 800 nm excitation originates from ET1 instead of ET2 as absorption of 800 nm photons can only create holes in sp band below the Fermi level through intraband transition. ET2 can only occur when the d band holes are created by absorption of 400 nm photons or two sequential 1PA of 800 nm photons (Scheme 1). The electron transfer dynamics for ET1 (under excitation of 800 nm, Figure 2) and ET2 (under excitation at 400 nm, Figure S3) were found to be very different by monitoring the decay at the wavelength resonant with the longitudinal SPR band. Under excitation at 400 nm, the ∆T/T signals for Au NRs in H2O and DMF solvents probed at longitudinal SPR band (760 nm) displayed similar decay profiles in the short delay times for the first 25 ps and became deviated from each other on the longer time scales (> 25 ps) due to ET2 process. After 25 ps, the decay of ∆T/T signal in DMF became apparently faster than that in H2O, decaying into a negative ∆T/T value (transient absorption) before recovery back on a nanosecond time scale. This evolution behavior is significantly different from its counterpart under 800 nm excitation in Figure 2d, where the ∆T/T signals of Au NRs in DMF displayed a significantly faster decay than that in H2O for the first a few ps due to ET1 process. Different electron dynamics for Au NRs in two solvents under excitation at 400 and 800 nm excitation indicates that ET1 and ET2 electron transfer processes occur at different time scales. One may argue that different dynamics of Au NRs in H2O and DMF probed at longitudinal SPR band


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under 800 nm excitation might originate from different heat dissipation rate in two solvents. This possibility could be ruled out by the observation of very different relaxation behaviors in two solvents under excitation at 400 and 800 nm: similar decay profiles for the first a few ps in two solvents under excitation at 400 nm while very different decay profiles in the same time range under excitation at 800 nm. The electron transfer rate of ~45 ± 4 ps was obtained for the ET2 process (electron transfer from the solvent to the d band holes) by comparing the electron dynamics of Au NRs in H2O and DMF under excitation at 400 nm (Figure S3, in supporting information). The periodic oscillation of the ∆T/T signals in Figure 2c & Figure S3d originates from the extensional vibrational mode of Au NRs.43-45 Optical excitation of Au NRs will bring elevated electron temperature. The subsequent fast electron-phonon scattering (on a time scale of a few picoseconds) will result in impulsively excitation of the coherent lattice expansion, i.e. the extensional vibrational mode along the Au NRs. This extensional mode will causes a small periodic change in the volume and shape of the nanoparticles which modulates the decay profiles. It is interesting to note that the oscillation was not interrupted by the charge separation process as this extensional mode will still persist in the charge-separated state.Pulse duration dependent 2PPL study. The observation of different initial transient bleaching amplitudes of longitudinal SPR bands in the transient spectra of Au NRs in DMF and H2O (Figure S2) suggests that an extremely fast electron transfer process might have already occurred within the laser pulse duration period. To confirm this speculation, pulse duration dependent 2PPL of Au NRs in H2O and DMF were measured by using fs laser pulses with pulse duration varying from 35 to 140 fs. When the pulse duration increased from 35 to 140 fs, 2PPL intensity of Au NRs in H2O only decreased slightly (

Two-Photon Excitation of Gold Nanorods Interrupted by Extremely Fast

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