© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 4457–4461 4457 www.advmat.de www.MaterialsViews.com COMMUNICATION wileyonlinelibrary.com By Chichao Yu, Ziguang Chen, Hui Li, Joseph Turner, Xiao Cheng Zeng, Zhihe Jin, Jinyue Jiang,* Boulos Youssef, and Li Tan* Molecularly Intercalated Nanoflakes: A Supramolecular Composite for Strong Energy Absorption [∗] Dr. C. Yu, Z. Chen, Prof. J. Turner, Dr. J. Jiang, L. Tan Department of Engineering Mechanics and Nebraska Center for Materials and Nanoscience University of Nebraska Lincoln, NE, 68588 (USA) E-mails: jjiang2@unl.edu; ltan4@unl.edu Dr. H. Li, Prof. X. C. Zeng Department of Chemistry University of Nebraska Lincoln, NE, 68588 (USA) Prof. Z. Jin Department of Mechanical Engineering University of Maine Orono, ME, 04469 (USA) Prof. B. Youssef Institut des Materiaux Universite de Rouen 76801 St-Etienne du Rouvray Cedex (France) DOI: 10.1002/adma.201000546 Energy absorption or dissipation from thin films is increasingly demanded by civil and military applications. [1] And aluminum is probably the most well-known material that can absorb sig- nificant amount of mechanic energy as thin films. [2–4] Staircase- like loading and sharp unloading curves were revealed by nanoindentation, which were attributed to nucleation and glide of large number of dislocations. [5] Certainly, when great amount of these nanometer scale dislocations are made available in organic thin films, they offer unmatched advantages including unlimited variability and processing ease. [3] While few studies were steered toward molecularly engineering an aluminum-like material, a simple mixing of fibrous- (1D) or laminar-like (2D) fillers within a bulk material could deliver a composite with significant energy absorption. [6] To name a few, these include carbon fiber/PEEK, [7–9] carbon fiber/epoxy, [10,11] and glass cloth/ epoxy. [12] Central to the success of these polymer-based compos- ites is their much increased flexibility. Under external impact, the composites dissipate energies via mechanisms, such as structure buckling, interface cracking, delamination, or even laminar fragmentation. [6] Unfortunately, when such a macro- scopic structure is condensed into a thin film, the absence of bulk deformation in a confined space cannot generate enough response. Hence, it is desirable to have a solid, lightweight counterpart to aluminum thin film and engineer dislocations therein for superior energy absorption. In this communica- tion, we address this need by varying interfaces in supramol- ecules. [13,14] Over the past years, extensive studies on these well-ordered nanomaterials focused on modifying building blocks via synthetic organic chemistry. [15,16] Other major efforts rely predominantly on optoelectronic device properties, [17,18] rarely has attention been paid by utilizing easy-to-configure interfaces inside supramolecules for energy absorption. We show performance of resulting nanomaterials after such reconfiguration is outstanding. The specific energy absorption (energy dissipated per unit mass) approached 275 J/g, while the state-of-the-art property for thin films of nylon and Al(100) is 60 and 140 J/g, respectively. In addition, our material showed one unique feature by decoupling the mechanical strength with the capability of energy absorption. As a consequence, our material has a rather high modulus (12.5 GPa). Since supramolecules enjoy a large interface-to-mass ratio and a one-pot synthesis pathway, we expect them great impact on many important fields needing structure or performance protection with min- imal added weight or volume. We chose amphiphilic molecules, i.e., hexadecyltrimethox- ysilane (Si(n-C 16 H 33 )(OMe) 3 or C16) and tetramethoxysilane (Si(OMe) 4 or TMOS), as the binary building blocks [19–21] for our supramolecular assembly. We aimed at varying interfaces inside these supramolecules by partially replacing the C16 por- tion of the multistacked nanometer-thin layers (called nano- flakes hereafter) with structurally similar, other surfactant mol- ecules, including rod-like ionic filler (cetyltrimethylammonium bromide ((n-C 16 H 33 )NMe 3 Br) or CTAB) or bulky mushroom- like alkylsilane (decyltristrimethoxysilyloxysilane (Si(n-C 10 H 21 ) (OSi(OMe) 3 ) 3 ) or DTS), where resulting nanoflakes are dubbed C16-CTAB and C16-DTS, respectively. Figure 1 shows observed interface instability of these two nanoflakes. Molecularly flat terraces with a layered depth of 4.0 nm are clearly revealed in Figures 1A and 1D, suggesting good compatibility between CTAB or DTS with C16 during the co-assembly process. How- ever, surfaces of stacked layers in both samples showed drastic changes during heating, with an observation of pore features (Figures 1B and 1E) (d ∼ 10 to 300 nm). Since the layered depth (4.0 nm) corresponds closely to the molecular length of two C16 molecules linked in serial, we assign the reorganization of C16 bilayer as one reason for the formation of pores. Several stages of the interface instability in nanoflakes were recorded, by using an atomic force microscope (AFM) equipped with a heating accessory on sample stage. Fresh C16-CTAB nanoflakes showed no particular topography or phase contrast on surfaces of each layer (Figure 1C). When temperature was raised to 50 °C, an increased roughness and wormlike features showed up in both topography and phase, indicating phase separation between C16 and CTAB. When temperature is further elevated to 60 °C, rather big particulates popped out at the outmost sur- face. And a continued heating to 80 °C promoted occurrence