Spontaneous Superlattice Formation in Nanorods Through Partial Cation Exchange

R. D. Robinson, B. Sadtler, D. O. Demchenko, C. K. Erdonmez, L.-W. Wang, A. P. Alivisatos
2007 Science  
Lattice mismatch strains are widely known to control nanoscale pattern formation in heteroepitaxy, but such effects have not been exploited in colloidal nanocrystal growth. We demonstrate a colloidal route to synthesizing CdS-Ag 2 S nanorod superlattices through partial cation exchange. Strain induces the spontaneous formation of periodic structures. Ab initio calculations of the interfacial energy and modeling of strain energies show that these forces drive the self-organization. The nanorod
more » ... perlattices exhibit high stability against ripening and phase mixing. These materials are tunable near-infrared emitters with potential applications as nanometer-scale optoelectronic devices. The ability to pattern on the nanoscale has led to a wide range of advanced artificial materials with controllable quantum energy levels. Structures such as quantum dot arrays and nanowire heterostructures can be fabricated by vacuum and vapor deposition techniques such as molecular beam epitaxy (MBE) and vapor-liquid-solid (VLS), resulting in quantum confined units that are attached to a substrate or embedded in a solid medium (1-5). A target of colloidal nanocrystal research is to create these same structures while leveraging the advantages of solution-phase fabrication, such as low-cost synthesis and compatibility in disparate environments (e.g., for use in biological labeling (6, 7), and solution-processed light-emitting diodes (8) and solar cells (9) ). One key difference between quantum dots epitaxially grown on a substrate and free-standing colloidal quantum dots is the presence of strain. In epitaxially grown systems, the interface between the substrate crystal and the quantum dot creates a region of strain surrounding the dot. Ingeniously, this local strain has been used to create an energy of interaction between closely spaced dots; this use of "strain engineering" has led, in turn, to quantum dot arrays which are spatially patterned in two (and even three) dimensions (2-4). In this paper, we demonstrate the application of strain engineering in a colloidal quantum dot system, by introducing a method that spontaneously creates a regularly spaced arrangement of quantum dots within a colloidal quantum rod. A linear array of quantum dots within a nanorod effectively creates a onedimensional (1D) superlattice, a promising new generation of materials (10, 11). Such 1D superlattices exhibit confinement effects and are unusual because of their ability to tolerate large amounts of lattice mismatch without forming dislocations and degrading device performance (12, 13) . Strong coupling of electronic states makes them interesting for optical systems and good candidates for photonic applications. 1D superlattices are also of interest for thermoelectric devices and studying ionic transport in 1D systems. VLS growth has demonstrated the formation of extended nanowire superlattices (e.g., alternating Si/Ge or InAs/InP) containing hundreds of repeat units (14-16). To achieve this, the precursors are alternated for the growth of each layer. The formation of 1D superlattices by this same time-dependent variation of precursor concentration is out of reach for present colloidal growth techniques. The largest number of alternating layers produced so far is three, and yet the sequence of purifications required in that instance were already taxing to implement (17). Cation exchange provides a facile method for systematically varying the proportion of two chemical compositions within a single nanocrystal. We have previously shown that cation exchange can be used to fully (and reversibly) convert CdSe, CdS, and CdTe nanocrystals to the corresponding silver chalcogenide nanocrystal by a complete replacement reaction of the Cd 2+ cations for Ag + cations (18). The resultant material is the silver-anion analog of the starting material (i.e., Ag 2 Se, Ag 2 S, and Ag 2 Te). Size and shape of the nanocrystal is preserved when the nanocrystal has minimum dimensions greater than 4 nm (18). The high mobility of cations in the CdS(Se,Te) lattice suggests that partial cation exchange may lead to interesting patterns of segregated domains of silver chalcogenide within a cadmium chalcogenide nanorod. This lead us to investigate the possibility of converting a previously formed nanorod of a single chemical composition into a striped pattern by a single step partial chemical transformation. In the case explored here, a linear arrangement of regularly spaced Ag 2 S dots contained within a CdS rod forms spontaneously at ~36% cation exchange. The near-infrared (NIR) bandgap of the Ag 2 S dots is embedded within the larger gap of the CdS, creating a type I heterostructure with interesting optical properties. Studies of partial cation exchange for 4.8µ64 nm CdS to CdS-Ag 2 S nanorods are shown in transmission electron microscopy (TEM) images in Figure 1 . In these experiments the initial CdS nanorods (Fig. 1A) were exceptionally smooth and the rod diameter was tightly controlled (std. dev. 10%), while the length varied between 30 -100 nm. The CdS colloidal nanorods were added to a solution of toluene, AgNO 3 , and methanol at -66 o C in air (19) . The concentration of AgNO 3 was a controlled fraction of the concentration of Cd 2+ ions present in the starting material. In the presence of excess Ag + , the rods are completely converted to Ag 2 S (18). However, when the Ag + ions are limited to yield 36% exchange, the resulting nanorods display a periodic pattern of light and dark-contrast regions (Fig. 1B,C) . The average spacing between the dark regions is 13.8 nm with a standard deviation of 28% ( Fig. 1 histogram inset) . The spacing between periodic segments can be controlled by the diameter of the initial CdS rod (Fig. S1) . Examination of these regions shows that the light and dark-contrast regions are CdS and Ag 2 S, respectively. Energy-dispersive x-ray spectroscopy (EDS) indicates that the striped rods alternate between Cd-S and Ag-S rich regions ( Fig. 2A) (20) . Powder xray diffraction (XRD) data confirms the presence of wurtzite CdS and monoclinic
doi:10.1126/science.1142593 pmid:17641197 fatcat:5blutdcvpvhadoiom5dvnh3hoa