&Thin films& DOI: 10.1002/smll.200500206 Nanocrystals as Precursors for Flexible Functional Films** LudovicoCademartiri,*GeorgvonFreymann, AndrØC.Arsenault,JacopoBertolotti, DiederikS.Wiersma,VladimirKitaev,and GeoffreyA.Ozin* Near-infrared (NIR) emitting Lead nanocrystals of lead chalcogenides near-infrared (NIR) emitting nanocrystals [1–3] are considered fundamental functional building blocks for nanotechnological devices involving telecommunications [3] and biology. [2] The recent demonstration of highly efficient, multiple-exciton generation in lead chalcogenide nanocrys- tals [4] have has dramatically increased the already compel- ling relevance of this class of materials, especially for solar energy harvesting. The main limit to the application of such nanomaterials is the instability of their luminescence, partic- ularly towards oxidation, [5] accentuated by the present un- availability of a procedure to efficiently coat them with a shell of a wide-bandgap semiconductor. [6] Solids composed of nanocrystals [7] have also attracted attention for the tech- nological potential of their collective behaviourbehavior, [8–10] yet although they inherit the vulnerability of the parent nanocrystals. A densely packed film of luminescent nano- crystals having mechanical, physical, and chemical stability, along with accessibility to chemical species, is needed for most applications. [9,11] The strategy for the creation of such composites almost always involve the use of nanocrystals as an additive to a usual standard synthesis of the matrix of choice. [12] The main drawback of most of these procedures is that they require very careful control of the surface capping of the nanocrystals in order to avoid phase segregation and quenching of the luminescence. We here present here in- stead a general strategy to obtain tailorable and patternable functional flexible films of densely packed nanocrystals of different kinds by using the as-prepared nanocrystals as pre- cursors in a low-power, room-temperature, O 2 plasma treat- ment. The PbS nanocrystals are were synthesized, using by a hot-injection method, [13] with lead chloride (PbCl 2 ) and ele- mental sulphur sulfur as reagents, and oleylamine (OLA) as coordinating solvent. [14] The lead precursor was has been suggested to be a PbCl 2 –OLA complex. [14] As shown in Sup- porting Figure S1 in the Supporting Information, the powder X-ray diffraction (PXRD) pattern and high high- resolution transmission electron microscopy (HRTEM) images on of the dried nanocrystals confirm them to be nanocrystalline PbS, while their optical characterization demonstrates their high quality and monodispersity. After treatment with oleic acid to precipitate the PbCl 2 –oleyla- mine OLA complex and several cycles of washing to remove traces of PbCl 2 , the nanocrystals still show detecta- ble Cl content (ca.  2 at.% against the approx.  6 at.% of Pb), as demonstrated by X-ray photoelectron spectrosco- py (XPS) (Supporting Figure S1). This observation supports the idea of a lead-rich nanocrystal surface where chloride counterions balance the positive charge on the lead surface ions. This hypothesis is further supported by the PXRD data (Supporting Figure S1), which showss the absence of PbCl 2 peaks. Considering the observed atomic percentage of Cl, even with the errors associated with XPS compositional analysis, a crystalline PbCl 2 phase should have been visible in the PXRD, and amorphous PbCl 2 is usually only obtained by quench deposition. [15] These nanocrystals can be readily used as precursors for the one-step formation of flexible photoluminescent films (Figure 1). A Thus, a solution containing PbS nanocrystals and the excess PbCl 2 –OLA complex is was repeatedly drop- cast onto a glass slide until a thick film (1–50 mmm) is had formed. At this stage the film is composed of densely packed nanocrystals, separated by the OLA capping ligands. The film is was then exposed to a 5-W air plasma for 48 h, and can thenfter which it could be lifted- off the substrate without damage from the substrate. Surprisingly, the result- ing material is flexible and can be folded like paper, as shown in Figure 1c. The morphology of the films and their nanoscopic or- ganization was probed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) ; see (Figure 1 and Supporting Figure S2, Supporting Informa- tion). The nanocrystals tend to form ordered superstruc- tures, [7] commonly described as supercrystals, with cubic symmetry. The peak in the small small-angle X-ray scatter- ing (SAXS) curve (Supporting Figure S3) cannot be fitted [*] L. Cademartiri, Dr. G. von Freymann, + A. C. Arsenault, Prof. G. A. Ozin Materials Chemistry Research Group, Lash Miller Chemical Labo- ratories Department of Chemistry, University of Toronto 80, St. George Street, Toronto, Ontario M5S 3H6 (Canada) Fax:(+ 1)416-971-2011 E-mail: lcademar@chem.utoronto.ca gozin@chem.utoronto.ca J. Bertolotti, Prof. D. S. Wiersma European Laboratory for Non-linear Spectroscopy (LENS), Polo Scientifico, Via Nello Carrara 1, 50019 Sesto-Fiorentino, Flor- ence (Italy) Prof.V. Kitaev Chemistry Department, Wilfrid Laurier University 75 University Ave W, Waterloo, Ontario N2L 3C5 (Canada) [ + ] Current address: Institut für Angewandte Physik Universität Karlsruhe (TH), 76128 Karlsruhe (Germany) [**] We thank S.Y. Choi and S. Petrov for PXRD measurements, M. Mamak and N. Coombs at the Center for Nanostructure Imag- ing (CNI) for TEM/SEM characterizations, R. N. S. Sodhi for XPS and TOF-SIMS measurements, M. Hines for valuable suggestions, and K. Landskron for proofreading the manuscript. G.A.O. is the Government of Canada Research Chair in Materials Chemistry. He is deeply indebted to the Natural Sciences and Engineering Research Council of Canada for financial support of this work. Supporting information for this article is available on the WWW under http://www.small-journal.com or from the author. small 2005, 1, No. &,1–4 # 2005 Wiley-VCH Verlag GmbH&Co. KGaA, D-69451 Weinheim 1