Modulating Physical Properties of Isolated and Self-Assembled Nanocrystals through Change in Nanocrystallinity Nicolas Goubet, †,‡ Cong Yan, †,‡ Dario Polli, § Herve ́ Portale ̀ s, †,‡ Imad Arfaoui, †,‡ Giulio Cerullo, § and Marie-Paule Pileni* ,†,‡ † Universite ́ 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 nano- crystallinity segregation takes place in colloidal solution upon self- assembling 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 F 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 approaches 9,10 and experiments 2,3 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 H 2 S or O 2 in nanoparticles that markedly depends on nanocrystallinity. 3,6 However, the influence of nanocrystallinity on the TiO 2 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 self- assemblies. 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, orienta- tional 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- Received: October 23, 2012 Revised: December 21, 2012 Published: December 31, 2012 Letter pubs.acs.org/NanoLett © 2012 American Chemical Society 504 dx.doi.org/10.1021/nl303898y | Nano Lett. 2013, 13, 504−508