Letter pubs.acs.org/Langmuir
Directed Coassembly of Oriented PbS Nanoparticles and Monocrystalline Sheets of Alkylamine Surfactant Alexander Rabkin,† Nataly Belman,† Jacob Israelachvili,‡ and Yuval Golan*,† †
Department of Materials Engineering and Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel ‡ Department of Chemical Engineering and Materials Department, University of CaliforniaSanta Barbara, Santa Barbara, California 93106, United States S Supporting Information *
ABSTRACT: We demonstrate control over the orientation of PbS nanoparticles by way of directed assembly, which in turn affects the crystal structure of alkylamine surfactants such as octadecylamine (ODA, C18H37NH2) and hexadecylamine (HDA, C16H33NH2). This directed assembly method results in the arrangement of PbS nanoparticles with a well-defined epitaxial orientation on lamellar alkylamine sheets, which undertake a new crystal structure to accommodate these relations. Understanding these surfactant− nanoparticle inter-relations is very instrumental in understanding surfactant-assisted nanoparticle synthesis and assembly.
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INTRODUCTION
instrumental in achieving control over morphology and assembly in other nanoparticle systems. PbS is a direct band gap semiconductor with a bulk band gap of 0.41 eV. It has been proposed as an attractive candidate for photovoltaic devices because of the possibility of multiple exciton generation15−17 and because of the tunability of the optical band edge. 18 Moreover, the PbS nanoparticle morphology and size can be controlled from separate nanodots and nanorods to nanorod arrays and nanosheets19−21 in order to accommodate varying requirements in terms of either the required geometry for specific applications or for the tuning of cumulative or individual properties. Of specific interest is the assembly of such nanoparticles into ordered arrays. The ordering of PbS nanoparticles has been demonstrated by several means, including thin film templating,22−25 supramolecular polymer assemblies,26 surface -pressure-induced coalescence,21 and 2D oriented attachment.27,28 Most of the spatial ordering reported was not accompanied by a welldefined nanocrystal (NC) crystallographic orientation with respect to the template or substrate used. In this work, we have developed a method for the directed assembly of oriented PbS NC arrays in which the orientation of the NCs is not dictated by particle ordering. Instead, we utilize the crystallization of unbound AA surfactant to define the PbS NC spatial ordering and crystallographic orientation. This is made possible by the reciprocal effect of the PbS NCs on the structure of the AA surfactant.
The assembly of nanoparticles with size-dependent physical properties into 2D arrays is of considerable fundamental and technological interest. They are proposed as basic building blocks in nanobased structures and devices. Moreover, the bottom-up approach to the nanofabrication of such structures is particularly appealing because it surpasses the limits of conventional lithography and potentially offers reduced fabrication costs.1−3 Alkylamines (AAs) have been shown to be useful solvents and surfactants for nanoparticle synthesis and morphology control. The most common and a relatively simple single-pot synthesis route is used to synthesize a variety of metal and compound semiconductor nanoparticles.4−7 Furthermore, the use of AAs has recently been shown to assist in controlling the synthesis of nanoparticles with quaternary compositions.8 Understanding nanoparticle and AA surfactant interactions is likely to enable further progress toward potential applications ranging from the electrochemical oxidation of methane9 to sensing and catalysis10 to printed electronics.11 This will also benefit from recent progress on the structure of AAs and their role in synthesis, as reported in recent publications.12,13 These describe the chemical and structural implications of aging AAs in an ambient environment, which results in the reaction of AA molecular pairs with naturally abundant CO2 to form alkylammonium-alkylcarbamate (AAAC) pairs. Moreover, these structural and chemical changes affect the resulting morphology in nanoparticle synthesis. In particular, it has been shown that the AA to AAAC transition leads to an alteration in their crystal structure when used as surfactants for ZnS nanoparticle synthesis.14 Such structural changes can be © 2012 American Chemical Society
Received: September 18, 2012 Revised: October 11, 2012 Published: October 11, 2012 15119
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Figure 1. Bright-field TEM (BF-TEM) images of AA-coated PbS nanoparticles on the surface of 2D crystalline AA-based sheets. (a) Synthesis using ODA; the lower magnification allows the observation of the rectangular ODA-based sheets. (b) Higher magnification of the area marked in green in image a showing ODA-coated PbS nanoparticles located on top of the surfactant crystals. PbS nanoparticles appear with a nearly square cross section, ∼10 nm in size. (c) Low-dose SAED (LD-SAED) from the area shown in images a and b showing both surfactant and PbS crystal patterns. The PbS and AA reflections are marked in blue and red, respectively. A well-defined orientation relationship exists between the two crystal phases. (d) After a period of e-beam exposure in the TEM, the pattern from the organic phase vanishes and only the oriented PbS pattern remains. (e) HDA-based surfactant crystal. The HDA-coated PbS nanoparticle arrangement is identical to the ODA-based case, and their size is very similar. (f) LD-SAED from image e, which shows an identical SAED pattern, indicating identical orientation relationships.
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EXPERIMENTAL SECTION
resulted in the amorphization of the AA sheets, as is evident from the absence of the spot pattern in Figure 1d. Additionally, it is evident from the LD-SAED pattern (Figure 1c) that there are clearly defined epitaxial relationships between the PbS NCs and AA sheets. Moreover, the epitaxial relationships and AA SAED patterns are identical in both ODA and HDA cases, as can be seen in Figure 1c,e, respectively. The first thing to note is the directionality of PbS NCs, which have no preferred orientation outside the AA sheet domains yet are highly oriented when supported by the surfactant domain. Additionally, it must be noted that the PbS NC ordering on AA sheets could also be observed when samples were prepared by drop evaporation; however, these results had a lower reproducibility. Moreover, no SAED pattern could be recorded from drop-evaporated samples, suggesting a lower sheet thickness. Thus, the ability to record the AA sheet SAED pattern is unique to the optimized experimental method described here, indicating that the geometry used here and prolonged evaporation time are crucial to the AA sheet lamellar growth. Further adjustment of the directed assembly method may provide a means of tailoring the AA sheet thickness and coverage by the PbS NCs according to specific requirements. The 2D crystal structure of the supporting AA sheet is rearranged in a quadratic geometry, most likely to accommodate the PbS cubic crystal structure. This is supported by the co-orientation observed in the LD-SAED patterns. Additionally, the identical AA sheet SAED patterns for both ODA and HDA provided additional support for the diffraction spot assignment because the AA chain length should affect the lamellar distance but not the in-plane structure of the AA crystal. Hence, this assembly procedure provides the means for forming AA sheets, which are oriented with the [001]AA zone axis of a primitive
The synthesis is based on a single precursor method developed by Efrima et al.,5,13,14 where the conditions were determined by controlling the synthesis temperature and time, as described in the Supporting Information. To reproduce the arrays reliably, a method was developed in which 5 mL of a 0.05 mg/mL PbS NC powder suspension in chloroform is allowed to evaporate very slowly over a substrate or TEM grid during a period of ∼24 h in a 20 mL glass vial with an internal diameter of ϕ = 25 mm. The binding of the AA surfactant is apparently weaker compared to its binding to ZnS13,14 and thus is the source of free surfactant for the formation of the surfactant sheets obtained. Thus, simultaneous NC assembly and AA sheet crystallization were achieved by allowing sufficient time and appropriate geometry for the suspended NCs to arrange along the newly formed lamellar AA sheets. Interestingly, prolonged exposure (>1 week) of the as-synthesized NC powder to the ambient environment hampers array formation. This probably indicates that pristine AA is required for the surfactant sheets because it is known that excess unbound surfactant will react with ambient carbon dioxide to form AAAC.12
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RESULTS AND DISCUSSION Under the experimental conditions described above, excess alkylamine surfactant molecules crystallize into lamellar sheets, which serve as solid supports for PbS NC assemblies, as can be seen in the transmission electron microscope (TEM) images in Figure 1a,b. Selected area electron diffraction (SAED) indicated that the PbS NCs supported by the AA sheets are highly oriented, with the PbS cube side parallel to the AA sheet edge, as can be seen in Figure 1c,d. Moreover, the orientation and thickness of the AA sheets obtained by this assembly method allowed the recording and subsequent assignment of the AA electron diffraction pattern by the use of the low-dose SAED (LD-SAED) technique (Figure 1c). A higher e-beam dosage 15120
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Figure 2. XRD of AA-coated PbS nanoparticles. (i, ii) Powder and thin film diffractograms of ODA-coated nanoparticles, respectively. (iii, iv) Powder and thin film diffractograms of HDA-coated nanoparticles, respectively. The blue and red dashed vertical lines correspond to the lamellar ordering of the corresponding AA and AAAC powders, respectively,12 and the solid gray vertical lines correspond to the PbS mineral peaks.29 The inset is an AFM image of ODA sheets covered with PbS nanoparticles on a mica substrate. The lamellae can be observed in the image as steps whose height varies from a single ODA layer to several bilayers.
tetragonal unit cell (a = b = 7.74 Å) with forbidden 100 reflections. The lateral cube arrangement on the AA sheets depends mainly on particle concentration, either total or local. At higher concentrations, PbS NCs tend to agglomerate and lose their preferred orientation, and at lower concentrations, the NC preferred orientation (as seen by SAED) does not change. However, the degree of ordering decreases, both in terms of spatial ordering and coverage of the NC. This further confirms the claim that the NC ordering originates from the orientation relationships and not the other way around. Although the in-plane structure of the AA sheet could be clearly defined from the LD-SAED pattern, out-of-plane data could not be obtained from electron diffraction. Thus, X-ray diffraction (XRD) was complementarily used to obtain the lamellar distance (c lattice parameter) by the deposition of the samples on a (100) silicon substrate rather than on TEM grids. To investigate the assembled films in a comparative manner, the diffractograms of each assembled film sample were accompanied by those of the as-synthesized, dried powders (which were later used for the assembly sample preparation procedure after suspension in chloroform), as presented in Figure 2. The top half contains diffractograms for the powder (i, purple) and assembled film (ii, green) synthesized with ODA, and the bottom half contains diffractograms for the powder (iii, orange) and assembled film (iv, black) synthesized with HDA. The vertical lines mark the powder diffraction data for rock salt PbS (solid gray),29 pristine AA lamellar (dashed blue) peaks, and OAOC lamellar (dashed red) peaks.12 Additionally, the relevant high-order lamellar peaks for pristine AA and DA films are assigned in the figure. We note that, as expected, the lamellar distances vary depending on the AA hydrocarbon chain length for both pristine AAs and AAACs. The lamellar structure of AA crystals in the presence of PbS NCs in ii and iv clearly differs from the structure of both the pure surfactant12 and AA-coated ZnS NC powders.14 Their
calculated lamellar periodicities (c lattice parameter of the tetragonal unit cell) are 37.1 and 34.1 Å for the ODA-based and HDA-based cells, respectively. Additionally, the highly oriented nature of the thin films can also be observed at higher diffraction angles (2θ > ∼18°): a clear distinction between the powder (i, iii), and thin film (ii,iv) diffractograms appears. The powder samples include additional peaks pertaining to the inplane crystal structure of the surfactant. In contrast, because of the single orientation, the thin film samples now show only the lamellar peaks in XRD because none of the other reflections exist under the Bragg condition in this orientation. Furthermore, the highly oriented nature of the films can also be observed by making a similar comparison with relation to the PbS peaks. The (111)PbS peak is nearly absent from the thin film, and the (200)PbS peak can clearly be observed in all of the diffractograms. Thus, we can present an additional confirmation of the highly ordered nature of the PbS NCs in the hybrid films. Atomic force microscopy (AFM) was used as an additional method of observation (inset in Figure 2). The use of a mica substrate is another example of the versatility of this method and provides another confirmation of the substrate-independent formation of the ODA sheets. The ODA lamellae can be observed in the image as steps whose height varies from a single ODA layer to several bilayers, indicating that the sheet growth is most likely not graded. The overall sheet thickness measured was around 100 nm, roughly corresponding to ∼30 bilayers of AA. The orientation relationships are summarized in Figure 3. The illustration shows a PbS NC supported by an AA-based surfactant sheet (only a small section illustrated). The sheets have a lamellar structure where each lamella is composed of an AA bilayer with a sheet edge directions of AA⟨110⟩. The surfactant sheet structure (lower-left corner) is dictated by the PbS NCs, which force their isotropic structure upon the sheet face, creating a tetragonal (a = b = 7.74 Å) structure. The sideview inset shows that the matching parameters are aPbS ≈ 15121
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Figure 3. Schematic representation of the proposed nanoparticle-to-surfactant hierarchical structure and orientation relationships. AA-based surfactant lamellae edges are shown in light gray, the PbS nanoparticle (not to scale) is shown as the green cube (subdivided by lattice cells in which the lead ions are marked in red and the sulfur ions are marked in yellow), and the ODA bilayers are schematically drawn between the lamellae edges. The matching crystal cell parameters are described from the top-view inset, the relevant lattice spacings are described in the side-view inset, and the surfactant unit cell, which differs from that of the free, unbound surfactant, is described in the lower-left corner.
d⟨110⟩AA with a lattice mismatch of 8.5%. The top-view inset shows the directional orientation in which the PbS NCs are oriented with PbS⟨100⟩ along the AA⟨110⟩ of the sheets. The new AA structure differs from both pristine AA and AAAC crystal structures, which are both orthorhombic. As previously discussed, pristine AA is required to form the AA sheet crystals. Hence, additional insight can be gained by comparing the structures. The in-plane lattice parameters of pristine AA are smaller (apristine = 5.60 Å, bpristine = 7.35 Å), and the lamellar distance is larger (cODA = 45.16 Å, cHDA = 40.53 Å).12 Thus, it appears that in order to accommodate the abovedescribed epitaxial relationships we should expect an increased tilt angle of the AA molecules within the bilayer. However, an additional factor of molecular interdigitation could account for the decrease in the lamellar distance. The above-described AA sheet−PbS NC structural and orientational interrelations provide a glimpse into the interaction between them. We can infer that the interaction is between lead surface cations present on the (100) PbS nanoparticle surfaces and amine headgroups of the AA surfactant. (We do not anticipate the interaction of the anionic sulfides on the surface of the PbS NC with the neutral AA surfactant molecules.) Interestingly, in this case the more stable and robust inorganic nanoparticle lattice affects the in-plane positions of the AA crystal sheets, with a secondary influence on the interlamellar AA sheet distances.
these inter-relationships is instrumental in obtaining better control over surfactant-assisted NC synthesis and assembly.
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ASSOCIATED CONTENT
S Supporting Information *
Additional experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +972-8-6461474. Fax: +972-8-6472944. E-mail: ygolan@ bgu.ac.il. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank D. Mogilyanski for XRD measurements, V. Ezersky for assistance with SAED analysis, R. Bitton for assistance with GISAXS measurements, and R. Golan for AFM imaging. This work was supported by the US−Israel Binational Science Foundation, Grant 2006032 (J.N.I., Y.G.), the Israel Science Foundation, Grant 340/2010 (Y.G.), NSF Grant CHE1059108 (J.N.I.), and DOE Grant DE-FG02-87ER-45331 (J.N.I., experimental design and interpretation of results).
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REFERENCES
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OUTLOOK The presented method for DA of hybrid, oriented PbS NCalkylamine crystals provides insight into the nanoscale assembly and crystallization of these materials. It is made possible by utilizing the rigid framework of the inorganic PbS crystal, the relatively closely matched in-plane parameters of the surfactant crystal structure, and its relatively malleable nature and molecular simplicity. This in turn creates an interplay between the structure and orientation by providing a substratelike support for the oriented assembly of PbS NCs. Understanding 15122
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