Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
pubs.acs.org/cm
Shape-Selective Optical Transformations of CdSe Nanoplatelets Driven by Halide Ion Ligand Exchange Benjamin T. Diroll† and Richard D. Schaller*,†,‡ †
Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
‡
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S Supporting Information *
ABSTRACT: Treatment of CdSe nanoplatelets with halide salts induces a bathochromic shift in the absorption resonances that does not occur in quasispherical quantum dots of the same composition. The optical shifts are not strongly sensitive to the cation moiety, which may be organic or inorganic. The magnitude of the energy shift is largest for thinner nanoplatelets, with bathochromic shifts as large as 240 meV observed for 3 monolayer nanoplatelets. This effect is driven by a tetragonal distortion of the zinc blende lattice in response to ligand exchange. The expansion of the lattice in the shortest nanoplatelet axis results in the observed red-shifts due primarily to relaxation of quantum confinement, with secondary contributions from strain.
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interface-induced strain too drives changes in band gap (similar in many respects to bulk materials),19,31−34 influence intraband relaxation,35 and induce anisotropic changes in lattice symmetry.36,37 In many cases, ligand exchange reactions induce multiple changes simultaneously. Yet as noted earlier, halide ion ligands do not belong to the conventionally identified classes of ligands that yield changes in quantum confinement. Indeed, this is consistent with the observed lack of influence upon ligand-exchange on quasi-spherical QDs. As noted in a recent work on ligand-exchanged NPLs,37 we find a tetragonal distortion of the zinc blende CdSe lattice is responsible for the large red-shifts of the absorption spectra. This tetragonal distortion is consistent with the extent of observed bathochromic shifts primarily through the expansion of the quantum well in the short axis direction, with a secondary influence arising from strain-induced changes in the band gap. Critically, the role of strain on quasi-spherical QDs is negligible, and consequently the changes on the band gap originating from ligand exchange are comparatively small.
odification of semiconductor nanoparticle surfaces with alternative ligands to those used during synthesis has a long history in II−IV nanoscale materials with applications spanning changes in solvent compatibility, to profound alteration of electrical properties, to biological targeting for applications such as labeling. Halide treatments, particularly CdCl2, have an extensive history of use as surface modifiers for II−VI semiconductors in bulk devices.1,2 Halides have also found extensive use as atomic capping agents of colloidal nanoparticles.3 Their proposed benefits include closer interNC spacing for improved electronic transport,4−7 synthetic manipulation,8,9 reduced charge carrier trapping,6,10,11 increases in environmental stability,12 band energy alignment,7,13 doping,14−16 or, similar to their use in bulk, promotors of sintering and grain growth during annealing.15 Pointedly, such treatments have not, to the best of our knowledge, shown noted changes in optical band gap of the nanoparticle. Here, we report of shape-selective bathochromic optical shifts of the excitonic absorption features of CdSe nanoplatelets (NPLs) occurring concomitant with halide ion ligand exchange. Large shifts of the light- and heavy-hole excitonic absorptions, as great as 240 meV, of these colloidal quantum wells occur when the NPLs are treated with a wide variety of halide salt compounds, but only very small changes occur in the analogous quantum dots (QDs). Halides, and not counterions, are specifically implicated by extensive control experiments. These results are particularly remarkable given the extensive previous examples of halide treatment of II−VI nanocrystals with negligible effects on absorption. Bathochromic shifts are observed in many ligand-exchanged nanocrystal systems, typically attributed to changes in quantum confinement,17−24 the local dielectric environment,25,26 or coupling between neighboring nanocrystals.27−30 Ligand- and © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Materials. Selenium (99.99%, powder, ∼100 mesh, Aldrich), 1octadecene (90%, technical grade, Aldrich), oleic acid (90%, technical grade, Aldrich), cadmium nitrate (97%, Aldrich), sodium myristate (>98%, Aldrich), and oleylamine (70%, Aldrich) were sourced from commercial suppliers and used as received. Other solvents used were sourced from commercial suppliers and were ACS grade or higher. Synthesis. Synthesis of 3 monolayer (ML), 4 ML, and 5 ML NPLs followed literature procedures for growth of zinc blende CdSe in a growth medium with acetate.38,39 Synthesis of zinc blende QDs of Received: March 30, 2019 Revised: April 4, 2019
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DOI: 10.1021/acs.chemmater.9b01261 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials the same composition was performed using typical ramped heating procedures without free acetate.40 For 6 nm QDs, additional injections of cadmium and selenium precursors dissolved in hot octadecene and oleic acid were used. In all reactions, the resulting QDs were capped with oleate ligands, facilitated by secondary injections of oleic acid after cooling reactions but prior to purification. Purification of 4 and 5 ML NPLs was performed by centrifugation of the crude reaction medium mixed with 10 mL of hexanes and 1 mL of oleic acid, followed by dissolving in hexanes and precipitation with ethanol. Other QDs were isolated by two cycles of isopropanol antisolvent addition and precipitation. Ligand Exchange. Ligand exchange reactions were performed in two ways; exchanges were performed with nanoparticles in the dispersed solution phase or in solid thin film form. For solution-based exchanges, small volume fractions of methanolic salt solutions (10 mg/mL or saturated, whichever was less concentrated) were added to toluene dispersions of NPLs or QDs. Five minutes was allowed before absorption measurements, during which time the dispersions were vigorously agitated. For ligand exchanges performed on solid films, the nanoparticle thin films were first prepared by drop-casting dispersions of nanoparticles from hexane/octane (10:1 v/v) onto glass (for room temperature measurements) or sapphire (for lowtemperature measurements). Solid films were immersed in ligandexchange solutions for in situ optical measurements, which were carried out over times ranging from 30 min to 3 h at room temperature. Absorption scans were stopped approximately 15 min with no additional change observed in the absorption spectrum. The energetic position of the heavy hole excitonic absorption was determined in solution and solid film samples by taking the relevant minimum of the second derivative of the absorption spectrum (see Supporting Information Table S1 and Figures S1−S4). Using a second derivative minimum, as opposed to fitting, allows better definition of the excitonic minimum with variable amounts of scattering. Spectroscopy. Absorption spectra of solutions and soaked thinfilms were performed in cuvettes under air atmosphere, except those collected at low temperature, using a Cary-50 spectrometer. Temperature-dependent measurements were performed in a Janis cryostat on solid films on sapphire using a focused white light source collected into a fiber and directed to an OceanOptics spectrometer. Fluorescence measurements were performed using a 450 nm diode laser (PicoQuant), with emission collected into a fiber and directed through a spectrograph(Princeton) to a CCD. FT-IR spectra were collected using a Thermo-Nicolet 6700 FT-IR spectrometer. Microscopy. Transmission electron microscopy (TEM) was performed using a JEOL-2100F instrument operated at 200 kV. Samples were prepared both by solid exchange and solution exchange without systematic observable differences other than aggregation, which was more prevalent in solution-exchanged samples deposited as partially aggregated dispersions. X-ray Diffraction. X-ray diffraction measurements were performed using a Bruker D2 Phaser instrument. Samples were prepared by first performing a solution exchange and depositing, washing the sample three times with clean methanol (isolating via precipitation), and depositing on to a mis-cut silicon wafer for measurement.
Figure 1. Solution-phase absorption measurements of (a) 3, (b) 4, and (c) 5 ML CdSe NPLs in toluene titrated with indicated amounts of methanolic (10 mg/mL) NaCl solution.
microdroplets. The transformations in Figure 1 are shown to be similar in nature for 3, 4, and 5 ML thick samples, although the magnitude of the bathochromic shift, determined from second derivatives of the absorption data, which aide definition of the excitonic position against a scattering background, is larger (in energy terms) for the thinner NPLs. Examination of exchange using thin solid films eliminates problems associated with Mie scattering to offer a clearer picture of the transformations, which occur upon halide addition. Figure 2 shows the time-dependent treatment with halide solutions of solid films of 3, 4, and 5 ML NPLs as well as zinc blende CdSe QDs with diameters of 2.1, 3.8, and 6.0 nm. Soaking was performed using thin films immersed in a cuvette containing 10 mg/mL (or saturated, if solubility was limited) methanolic solutions of the halide salts. (This process was found to reproduce the excitonic shifts observed in solution measurements as tabulated in Table S1.) Absorption spectra in Figure 2 and subsequent figures herein were first collected upon initial immersion and followed in minutes and hours to a time at which detectable changes in the absorption were no longer apparent (typically after reaching 30 min to 3 h depending on conditions). Absorption at the initial excitonic peak and the wavelength of the strongest heavy hole absorption feature are tracked as a function of soaking time in Supporting Information Figure S4. For sodium salts of
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RESULTS AND DISCUSSION Solution-Exchange and Solid Film Exchange. Initial experiments focused on the solution-phase exchange reactions, which occur under conditions where high concentrations of metal halide salt compounds in polar solvents are mixed with the NPLs. Figure 1 shows the solution-phase treatment of CdSe NPLs with chloride in the form of NaCl dissolved in methanol. Although the addition of chloride induces increased scattering, a red-shift and broadening of the CdSe NPL lightand heavy-hole transitions41 is still clear. Increased scattering occurs because the methanolic chloride solution induces NPL aggregation and phase separation of the solvents into B
DOI: 10.1021/acs.chemmater.9b01261 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 2. Absorption of immersed solid films of 2.1, 3.8, and 6.0 nm zinc blende CdSe QDs and 3, 4, and 5 ML CdSe NPLs treated with methanolic solutions (10 mg/mL) (a) NaCl, (b) NaBr, and (c) NaI. Time points progress from blue to red in color, but time increments are not identical between panels. See also Supporting Information Figures S1−S4.
the soaking process was quite similar for the various halides. The exception was in the case of tetrabutylammonium iodide (TBAI), which displayed very slow kinetics and resulted in reduced absorption as well as loss of material over time, most likely attributable to oxidation of the NPLs due to residual molecular iodine. This reaction was stopped after several hours of soaking, owing to loss of sample and noticeable yellowing of the solution indicative of iodide redox reactions. To exclude substantial independent contributions from cationic species, control measurements of soaking films in methanol, sodium oleate, or quaternary ammonium salts of BF4− and PF6− showed no change in the NPL absorption (Supporting Information Figure S8). This is understood as evidence that with regard to the observed bathochromic shifts, cations are spectators. Additional support for this is derived from earlier literature examples of ligand exchanges5,6 and FTIR measurements in Supporting Information Figure S9 and Table S2, which indicates an X-type (anionic) ligand exchange process accompanying the soaking treatment is associated with large reductions (>85%) of the organic C−H stretches derived from surface-bound oleate ligands in the as-synthesized samples. Ligand-Induced Structural Changes. Any explanation of the observed optical phenemonon must be inclusive of both the large changes in NPL absorption and the negligible change in QD absorption. For example, ligand-induced redshifts occur with “exciton delocalizing” ligands, which are a matter of some discussion,45 but may enable the excitonic wave function of quantum confined materials to spread substantially beyond the boundary of the inorganic particle.17,18,46,47 Yet similar redshifts are obtained in this case with much smaller atomic halide ligands that are not reported elsewhere as altering exciton delocalization. Even more challenging to explain, the effect is observed strongly only in quasi-two-dimensional NPLs, whereas in QDs, even those which have a diameter comparable to the thickest NPLs do not show appreciable shifts. To examine the origin of the presented observations, several structural experiments were performed. NPLs that have
chloride, bromide, and iodide, all the NPLs show a bathochrmoic shift of the excitonic absorption resonances. Using second derivative analysis, the magnitude of the excitonic shifts generally follow Cl < Br < I and 5 ML < 4 ML < 3 ML with the magnitude varying from 70 to 240 meV. Tabulated shifts are compared in Table S1. Attempts to follow the process using photoluminescence were unsuccessful as emission measurements of the exchange process show rapid (
