Letter pubs.acs.org/NanoLett
Modulating Physical Properties of Isolated and Self-Assembled Nanocrystals through Change in Nanocrystallinity Nicolas Goubet,†,‡ Cong Yan,†,‡ Dario Polli,§ Hervé Portalès,†,‡ Imad Arfaoui,†,‡ Giulio Cerullo,§ and Marie-Paule Pileni*,†,‡ †
Université Pierre et Marie Curie, UMR 7070, LM2N, 4 place Jussieu 75005 Paris, France Centre National de la Recherche Scientifique, UMR 7070, LM2N, 4 place Jussieu 75005 Paris, France § IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, P.za L. da Vinci 32, 20133 Milano, Italy ‡
S Supporting Information *
ABSTRACT: For self-assembled nanocrystals in three-dimensional (3D) superlattices, called supracrystals, the crystalline structure of the metal nanocrystals (either single domain or polycrystalline) or nanocrystallinity is likely to induce significant changes in the physical properties. Previous studies demonstrated that spontaneous nanocrystallinity segregation takes place in colloidal solution upon selfassembling of 5 nm dodecanethiol-passivated Au nanocrystals. This segregation allows the exclusive selection of single domain and polycrystalline nanoparticles and consequently producing supracrystals with these building blocks. Here, we investigate the influence of nanocrystallinity on different properties of nanocrystals with either single domain or polycrystalline structure. In particular, the influence of nanocrystallinity on the localized surface plasmon resonance of individual nanocrystals dispersed in the same dielectric media is reported. Moreover, the frequencies of the radial breathing mode of single domain and polycrystalline nanoparticles are measured. Finally, the orientational ordering of single domain nanocrystals markedly changes the supracrystal elastic moduli compared to supracrystals of polycrystalline nanocrystals. KEYWORDS: Crystal defect, nanocrystallinity, self-assembly, time-resolved pump−probe spectroscopy, nanoindentation, localized surface plasmon resonance
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nanoparticles that markedly depends on nanocrystallinity.3,6 However, the influence of nanocrystallinity on the TiO2 is not well understood.5 Here we present a comparison of the optical and vibrational properties of isolated single domain and polycrystalline Au nanocrystals and elastic properties of their corresponding selfassemblies. UV−visible absorption measurements performed from these nanocrystals point out the change in profile of the surface plasmon resonance band from single to polycrystalline nanocrystals. Concerning the acoustic vibrational properties of the nanocrystals, the presence of crystalline defects induces a decrease of the damping time for the radial breathing mode (l = 0) by a factor of 3. The slight frequency change measured for this mode between single domain nanocrystal and polycrystals can be simply attributed to the small size difference between them rather than the nanocrystallinity effect in accordance with theoretical models developed elsewhere.9,10 Finally, orientational ordering of atomic lattice planes in the nanocrystal assemblies is only observed for single domain nanocrystals thus explaining the observed marked enhancement in the mechan-
ine control over the crystallinity of nanoparticles (called nanocrystallinity) has been poorly investigated, so that properly addressing this issue still remains a challenge up to date. This is probably due to the difficulty in synthesizing a collection of well-structured nanocrystals. Nevertheless, over the past few years, several studies regarding their physical properties were carried out.1−8 One of the most developed research areas dealing with nanocrystallinity concerns the acoustic vibrational properties of nanocrystals. Controversies appeared between numerical studies carried out by using two different approaches9,10 and experiments2,3 when assuming the dependence of the radial breathing mode frequencies of the nanocrystals on their nanocrystallinity. Another controversy exists regarding the mechanical properties of nanoparticles with different nanocrystallinities.3,4,11,12 At variance, the observation of a splitting of the quadrupolar vibrational mode (l = 2) in the low-frequency Raman scattering spectrum of single domain Au nanocrystals and the frequencies measured for the two resulting modes are in good agreement with those predicted from calculations using the resonant ultrasound approach, whereas for multiply twinned particles (MTPs) and polycrystals no splitting of the quadrupolar mode is observed. From the chemical point of view, very few reports are available. The most important data concern the diffusion of H2S or O2 in © 2012 American Chemical Society
Received: October 23, 2012 Revised: December 21, 2012 Published: December 31, 2012 504
dx.doi.org/10.1021/nl303898y | Nano Lett. 2013, 13, 504−508
Nano Letters
Letter
corresponding LSPR spectrum exhibits a maximum at 515 nm (Figure 1, red curve). The comparison between the single domain and polycrystalline nanoparticles, obtained in the same solvent (hexane), that is, the same dielectric media, shows a slight but detectable change in the absorption profile. The LSPR band of polycrystalline nanoparticles appears damped with lower amplitude, larger bandwidth, and residual absorption at lower energy (longer wavelengths). This difference of 0.4 nm in the average size for single domain with respect to polycrystalline nanocrystals is not large enough to induce changes in the LSPR band.12 Since all the nanocrystals are quasi-spherical and in the same solvent, the residual absorption at long wavelengths and the broadening of the LSPR spectrum are attributed to internal structural defects (twins), that is, to the nanocrystallinity. To our knowledge,17−19 the simulations currently used to describe the optical properties of nanoparticles differing by size and shape do not take into account such internal defects.20−24 We investigate whether other vibrational properties can be affected by the nanocrystallinity, as it was already observed through the quadrupolar vibrational modes (l = 2) of Au nanocrystals.25 Time-resolved differential transmission (ΔT/ T), obtained by using the femtosecond pump−probe technique, is an appropriate tool to detect the breathing mode (l = 0) of nanocrystals. The experiments were performed on Au nanoparticles in hexane solution contained in a quartz cuvette with a 500 μm optical path. Broadband twodimensional (2D) ΔT/T maps as a function of probe wavelength (λpr) and probe delay were recorded following excitation by 800 nm pulses and probing with a white light continuum, with an overall time resolution of ≈150 fs. At 100 μJ·cm−2 as the pump fluence, we observe a fast rise of the signal, due to the excitation of the hot electron distribution in Au, followed by a rapid decay, due to transfer of energy from the electrons to the lattice. The thermalization process follows an exponential dynamics with nearly unchanged time constants between single domain (0.9 ps) and polycrystalline (0.8 ps) samples, typical for electron−phonon coupling in Au nanocrystals under this condition.12 Panels a and b in Figure 2 represent the 2D ΔT/T maps after subtracting the slowly varying background for single domain and polycrystalline Au nanocrystals, respectively. The strong remaining oscillating pattern, observed as a sequence of vertical colored bands of constant width, reveals the modulation of the transmission signal caused by coherent breathing acoustic vibrations of the Au nanocrystals induced by the ultrafast lattice heating. To analyze more deeply such modulation of the signal and quantitatively determine its oscillation period and damping time, Figure 2c and d shows dynamical curves describing the time-dependent ΔT/T signal at probe wavelength λpr = 560 nm for single domain and 550 nm polycrystalline nanocrystals, respectively, extracted from the aforementioned 2D maps by horizontal cuts and corresponding to the best visibility of the fringe pattern. The oscillatory trace in Figure 2c for the single domain nanocrystals was fitted by an exponentially damped sinusoidal function of the form:
ical properties when replacing polycrystalline by single domain nanocrystals. Very recently,13 we established a procedure for selecting nanocrystals characterized exclusively by either single domain or polycrystalline structures. This procedure is based on a particular synthesis of 3D superlattices called supracrystals from a colloidal solution of “quasi-spherical” 5 nm Au nanocrystals coated with dodecanethiol molecules. The size distribution remains similar for each sample (
