Letter pubs.acs.org/NanoLett
Plasmon Bleaching Dynamics in Colloidal Gold−Iron Oxide Nanocrystal Heterodimers Alberto Comin,* Kseniya Korobchevskaya, Chandramohan George, Alberto Diaspro, and Liberato Manna NanoPhysics and NanoChemistry Units, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy S Supporting Information *
ABSTRACT: Colloidal nanocrystal heterodimers composed of a plasmonic and a magnetic domain have been widely studied as potential materials for various applications in nanomedicine, biology, and photocatalysis. One of the most popular nanocrystal heterodimers is represented by a structure made of a Au domain and a iron oxide domain joined together. Understanding the nature of the interface between the two domains in such type of dimer and how this influences the energy relaxation processes is a key issue. Here, we present the first broad-band transient absorption study on gold/iron oxide nanocrystal heterodimers that explains how the energy relaxation is affected by the presence of such interface. We found faster electron−electron and electron−phonon relaxation times for the gold “nested” in the iron oxide domain in the heterodimers with respect to gold “only” nanocrystals, that is, free-standing gold nanocrystals in solution. We relate this effect to the decreased electron screening caused by spill-out of the gold electron distribution at gold/iron oxide interface. KEYWORDS: Nanocrystals, plasmon, carrier dynamics, gold/iron oxide hetero nanocrystals
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electron−phonon lifetime, while Mohamed et al.17 studied 15 nm Au particles embedded in hydrogel, organic gel, and water and found an environment-dependent e-ph lifetime (ranging from 1.4 to 2.3 ps). They suggested that the difference could be caused by interactions between solvent molecules and the energy levels of the Au nanoparticles, a process known as “chemical interface damping”. Halté et al.18 investigated 6 nm Ag particles embedded in silicate glass and in porous alumina matrixes and found faster dynamics for particles embedded in alumina. The authors rationalized their result by considering the higher heat conductivity of alumina with respect to glass. As an alternative explanation, they considered the coupling of electrons with capillary and acoustic phonon modes.19 In the more specific case of a nanocrystal HD containing a plasmonic domain, when this is optically irradiated near a plasmonic resonance frequency of the metal domain, its free carriers are coherently stimulated and behave as a source of an intense local electric field, which in turn can modulate the optical properties of the other domain, for example, it can enhance its optical absorption and can modify its fluorescence quantum yield if such domain is fluorescent.20 The static optical properties of gold/iron oxide HDs, one of the most studied types of nanocrystal heterostructure, have been previously described in several works3,21−24 but a thorough examination of their spectroscopic lifetimes is currently missing. Gold nanocrystals have been investigated extensively6 but when a gold nanocrystal is directly interfaced
anocrystal heterodimers (HDs) composed of a plasmonic domain and a magnetic domain are being recently investigated for applications in medicine, biology and photocatalysis.1−4 One important focus of research in these new types of nanocrystals is on how energy relaxation processes are influenced by the presence of an interface between the two different domains. In particular, since many physical properties of solids depend on electron−electron and electron−phonon interactions,5 understanding how they are modified by reduced dimensionality, interfaces and proximity effects is critical in order to harness their physicochemical properties for tailored applications.6,7 As of today, it is quite established that the interface between gold and iron oxide in such HDs affects the interdiffusion of free-carriers, leading to formation of electrical junctions.8 It is also known that surface electronic states in nanocrystals are further influenced by organic capping layers, solvents, and neighboring particles. The effect of the external medium on the electron dynamics of a metal nanocrystal is a very active research topic.6 In particular, the acceleration of electron−phonon dynamics for nanocrystals embedded in a matrix has been reported several times in the past. In general, most studies agree on the fact that at high excitation levels the lifetime for the thermal relaxation of the electron population depends on the environment.9−12 At high excitation levels, however, the lifetime of the electronic population relaxation is not the same as the electron−phonon lifetime, since the electronic heat capacity is temperature dependent.5 For low excitation levels instead there is no complete agreement in the literature, because Vallée et al.,13,14 Polavarapu et al.15 and Melinger et al.16 found no significant environment effect on the © 2012 American Chemical Society
Received: November 12, 2011 Revised: December 29, 2011 Published: January 9, 2012 921
dx.doi.org/10.1021/nl2039875 | Nano Lett. 2012, 12, 921−926
Nano Letters
Letter
et al.,31 that is, by decomposing iron pentacarbonyl in the presence of gold nanocrystals, via a high-temperature colloidal synthesis. The morphological characterization of the nanocrystals was carried out via transmission electron microscopy (TEM) using a JEOL JEM 1001 microscope. In Figure 1, we show the conventional and high-resolution TEM images (the latter acquired with a JEOL JEM 2200FS microscope) of gold only nanocrystals (Figure 1a) and of typical gold/iron oxide HDs (Figure 1b). The absorption spectra were recorded in toluene using a Varian Cary 5000UV−vis-NIR spectrophotometer with 1 cm quartz cells. The transient absorption measurements were performed with a femtosecond pump−probe setup (Coherent Micra and Legend: central wavelength 800 nm, bandwidth 30 nm, pulse duration 60 fs, repetition rate 1kHz). The pump was a 400 nm beam, obtained by second-harmonic generation in a 0.1 mm BBO crystal. The probe was a white-light beam, obtained by supercontinuum generation into a 3 mm sapphire plate. Spot sizes were 1 mm for the pump and 0.5 mm for the probe. To avoid laser-induced sample modification, energy fluences were relatively low, about 100 μJ/cm2 for the pump and 10 μJ/cm2 for the probe. The detector was a linear array spectrometer (Avantes), synchronized with the laser. All samples were loaded in 1 mm quartz cells (Hellma). The group velocity dispersion was numerically compensated for the probe beam. As a preliminary step in understanding the carrier dynamics in gold/iron oxide HDs, we fitted the static absorption spectra (Figure 1c), using the Mie theory32 and extracted the contribution to the absorption from iron oxide in the sample (see Supporting Information for details). The plasmon peak of gold/iron oxide HDs was red shifted and broadened compared to that of the gold-only nanocrystals. The red shift can be explained by considering that the dielectric function of the iron oxide that coated the Au domain in each HD33 is larger than the dielectric function of the solvent that instead surrounds the gold-only nanocrystals. The broadening can be caused by the increased electron damping at the Au/iron oxide interface and additionally by sample inhomogeneity. Transient absorption signals were recorded as the relative difference of the sample transmittance, that is [(T(t)-T(0))/ T0], around the gold plasmon peak (530 nm), after excitation with a 400 nm laser pulse. In Figure 1 it can be seen that, although at 400 nm the contribution of the iron oxide domain to the HDs absorption cross section is significant, at 530 nm the contribution from gold is dominant. In the transient absorption measurements, the ratio between the transient absorption signal of gold and that of iron oxide is even bigger. This is because after the impulsive heating of the electrons, the plasmon peak broadens significantly, and the relative change of optical transmittance measured at the plasmon peak can be bigger than 10% in our experimental condition. By measuring HDs samples from which the gold domain had been leached away, we estimated that the relative change of optical transmittance of the iron oxide domains was
