Scroll-like Alloyed CdSxSe1–x Nanoplatelets: Facile Synthesis and

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Scroll-like Alloyed CdSxSe1−x Nanoplatelets: Facile Synthesis and Detailed Analysis of Tunable Optical Properties Natalia N. Schlenskaya,†,‡ Yuanzhao Yao,§ Takaaki Mano,§ Takashi Kuroda,§ Alexey V. Garshev,†,‡ Vadim F. Kozlovskii,‡ Alexander M. Gaskov,‡ Roman B. Vasiliev,*,†,‡ and Kazuaki Sakoda§ †

Department of Materials Science, Lomonosov Moscow State University, Leninskie Gory 1-73, Moscow 119991, Russia Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1-3, Moscow 119991, Russia § Photonic Materials Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡

S Supporting Information *

ABSTRACT: Scroll-like colloidal quasi-two-dimensional CdSxSe1−x nanoplatelets (NPLs) with a thickness of five monolayers and average lateral dimensions of 100 nm over the whole composition range (0 ≤ x ≤ 1) were synthesized by using a mixture of chalcogenide precursors. According to X-ray diffraction and X-ray energy dispersive spectroscopy mapping in a scanning transmission electron microscope, as-synthesized NPLs are solid solutions with a uniform elemental distribution in each individual nanoplatelet. A change in composition leads to the continuous shift and gradual broadening of exciton absorption and photoluminescence (PL) lines in the visible blue and near-ultraviolet light range. Detailed analysis of optical properties, including temperature-dependent PL spectroscopy, revealed the nonmonotonic behavior of several optical parameters (average phonon temperature, electron−phonon interaction coefficient, and Stokes shift) starting from a sulfur content of ∼40% in alloyed NPLs. This behavior may be attributed to the local composition fluctuations within CdSxSe1−x solid solution nanoplatelets.

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solid solution formation.8−10 In this case, a change in the composition of a nanoplatelet causes modification of its band structure and, as a result, optical properties.11 Alloyed CdSxSe1−x QDs with both wurtzite and zinc blende crystal structures have been reported widely in the literature,11−14 while quasi-two-dimensional CdSxSe1−x nanoplatelets have been discussed much less. Only a small number of articles about 5 and 6 ML alloyed CdSxSe1−x nanoplatelets with zinc blende15,16 and wurtzite crystal structures17−21 have been published. These NPLs combine optical features of 2D systems with the gradual variation of the exciton transition energy with a change in composition. It was reported that CdSxSe1−x nanoplatelets can be used as an active layer in light-emitting diodes.15 However, to the best of our knowledge, detailed analysis of optical properties of CdSxSe1−x alloyed NPLs has not yet been reported. Earlier, we have developed a protocol for the synthesis of zinc blende alloyed CdSxSe1−x nanoplatelets with a 5 ML thickness, but the applied low synthesis temperature caused the formation of shapeless nanoplatelets.16 In this work, we report a method for the synthesis of highly crystalline quasi-twodimensional 5 ML CdSxSe1−x nanoplatelets over a whole

INTRODUCTION In recent years, quasi-two-dimensional (2D) colloidal nanoplatelets (NPLs) with precise thickness control have attracted a great deal of attention because of their unique optical properties1,2 that are similar to those of the quantum wells (QWs).3 Distinguishing properties of 2D nanoplatelets are narrow exciton absorption and photoluminescence (PL) lines and a quantum-size effect in one dimension.4 The quantum-size effect is most widely known in quantum dots (QDs), where control of QD size leads to the change in its effective band gap.5,6 The advanced colloidal synthesis method allows us to control the thickness of NPLs down to monolayer scale and leads to the amazing uniform thickness throughout each individual nanoplatelet, resulting in the absence of size dispersion. A uniform thickness of nanoplatelets with integer numbers of monolayers (ML) causes fixation of exciton absorption and PL energy.7 For example, quasi-two-dimensional CdSe NPLs can exist in four populations with thicknesses of 4 ML (1.2 nm), 5 ML (1.5 nm), 6 ML (1.8 nm), and 7 ML (2.1 nm); maxima of exciton absorption can be found at 390, 461, 512, and 550 nm, respectively.1 This feature prevents the continuous variation of nanoplatelet exciton transition energy, which is desirable for different applications. One of the widely used approaches, which can fulfill the requirement of continuous transition energy variation in semiconductor NPLs, is composition modification, for example, © 2016 American Chemical Society

Received: September 12, 2016 Revised: December 23, 2016 Published: December 24, 2016 579

DOI: 10.1021/acs.chemmater.6b03876 Chem. Mater. 2017, 29, 579−586

Article

Chemistry of Materials

Table 1. Sample Names, Initial S/(S + Se) Levels in Chalcogenide Precursors, and Experimentally Analyzed S/(S + Se) Levels Determined by EPMA S/(S + Se) in precursor S/(S + Se) by EPMA a

CdSe

CdS0.29Se0.71

CdS0.37Se0.63

CdS0.57Se0.43

CdS0.69Se0.31

CdS0.72Se0.28

CdS

0 0.00 ± 0.03a

0.1 0.29 ± 0.05a

0.2 0.37 ± 0.03a

0.3 0.57 ± 0.02a

0.4 0.69 ± 0.04a

0.5 0.72 ± 0.01a

1 1.00 ± 0.05a

Relative errors were estimated via data averaging from four to seven points. scanning speed of 100 nm/min. Photoluminescence spectra were recorded at room temperature on a Jasco FP-6500 spectrofluorometer in the same wavelength range and at the same scan speed. Spectra of colloidal solutions of nanoplatelets in hexane diluted to an optical density of 99.5%), and 1-octadecene (ODE, 90%) were purchased from Sigma-Aldrich. Precursor Preparation. Sulfur and selenium solutions (0.05 M) in ODE (S-ODE and Se-ODE, respectively) were used as chalcogenide precursors. Solutions were prepared by dissolving 0.1 mmol of elemental S or Se in 2 mL of ODE and heating the solutions at 180 °C under an argon flow. Pure S-ODE or Se-ODE solutions were used in the syntheses of 2D CdS or CdSe nanoplatelets, respectively. Synthesis of CdSe and CdS Nanoplatelets. CdSe and CdS nanoplatelets with a thickness of 5 ML were synthesized according to the methods proposed by Peng et al.22 and Dubertret et al.7,23 Typically, 0.2 mmol of cadmium acetate, 0.1 mmol of OA, and 5 mL of ODE were introduced into the reaction flask and heated under an argon flow while being continuously stirred. As the temperature reached 240 °C, 0.1 mmol of S-ODE or Se-ODE was injected into the flask. The growth of nanoplatelets was allowed to proceed for 45 min at 240 °C, and then the reaction mixture was cooled to room temperature (RT). During the cooling process, 1 mL of OA was injected. CdSe and CdS nanoplatelets were precipitated by acetone with the following centrifugation at 6000 rpm and washed twice with acetone. Finally, nanoplatelets were redispersed in 4 mL of hexane. Synthesis of Alloyed CdSxSe1−x Nanoplatelets. The synthesis of alloyed CdSxSe1−x nanoplatelets was based on the method previously reported by Sokolikova et al.16 Formation of alloyed nanoplatelets with a tunable composition was achieved by using the mixture of chalcogenide precursors S-ODE and Se-ODE with the molar concentrations of sulfur listed in Table 1. Typically, 0.2 mmol of cadmium acetate, 0.1 mmol of OA, and 5 mL of ODE were introduced into the reaction flask and heated under an argon flow while being continuously stirred. As the temperature reached 240 °C, the mixture of S-ODE and Se-ODE with a total amount of chalcogenides equal to 0.1 mmol was quickly injected into the flask. Nanoplatelet growth was allowed to proceed for 45 min at 240 °C. After that, 1 mL of OA was injected and the reaction mixture was cooled to room temperature. Finally, CdSxSe1−x nanoplatelets were purified in the same way as CdSe nanoplatelets and dissolved in hexane. A series of samples with different S/Se ratios was synthesized. Characterization. Optical Measurements. Absorption spectroscopy at room temperature was performed on a Jasco V-550 UV/vis spectrophotometer in the wavelength range of 200−800 nm at a

Eg (CdSx Se1 − x) = xEg (CdS) + (1 − x)Eg (CdSe) − bx(1 − x) (1) where bowing constant b = 0.3 eV, Eg(CdS) = 2.40 eV, and Eg(CdSe) = 1.66 eV. Except for Eg, the alloy band parameters were assumed to be a linear interpolation of the pure CdS and CdSe values.4,24 Using these band parameters, the effective mass of the electron (me) and that of the heavy hole (mhh) were described as25 580

DOI: 10.1021/acs.chemmater.6b03876 Chem. Mater. 2017, 29, 579−586

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Chemistry of Materials

Figure 1. (a) STEM-HAADF overview micrograph of the CdS0.57Se0.43 sample that consists of scroll-like nanoplatelets schematically shown here. (b−d) TEM and STEM-HAADF images of individual scroll-like NPLs from the CdSe, CdS0.37Se0.63, and CdS0.57Se0.43 samples, respectively. (e) Schematic illustration of a single scroll-like nanoplatelet. (f) STEM-HAADF image of a region from the CdS0.57Se0.43 sample. (g) Micrograph of STEM-XEDS mapping where Cd, S, and Se are colored green, purple, and blue, respectively. XEDS maps were generated from the intensity of the Cd−K, S−K, and Se−K lines. All elements are uniformly distributed over the sample.

⎤ ⎡ ⎛ Ep ⎞⎛ 1 1 ⎞⎟⎥ me = m0 /⎢1 + α + ⎜ ⎟⎜⎜ + ⎢⎣ Eg + Δ ⎟⎠⎥⎦ ⎝ 3 ⎠⎝ Eg

(2)

mhh = m0 /(γ1 − 2γ2)

(3)

for all obtained samples with different compositions (see Figure S1). It is important to note that folding of nanoplatelets does not cause the change in their optical properties. The only parameter that defines the exciton energy is the thickness of nanoplatelets. Individual scroll-like NPLs for the CdSe, CdS 0.37 Se 0.63 , and CdS 0.57 Se 0.43 samples and schematic illustration of a single scroll-like nanoplatelet are shown in Figure 1b−e. The form of scroll-like NPLs corresponds to the rectangular shape of nanoplatelets in the unfolded state that may indirectly indicate the cubic (zinc blende) crystal structure of nanoplatelets. The thickness of NPLs with different compositions was roughly estimated from TEM micrographs as a thickness of side edges of single scroll-like NPLs. Our estimated thicknesses were all around 1.5 nm, which corresponds to the 5 ML population of 2D cadmium chalcogenide nanoplatelets. More accurate estimation of the thickness of NPLs includes cross-section TEM imaging where atom columns can be observed on side edges of nanoscrolls as the thickest areas of folded nanoplatelets (see Figure S3). The resultant thickness of CdSe NPLs from cross-section images is also around 1.5 nm. The distance between layers in folded nanoplatelets varies from 2 to 4 nm, which corresponds to a single or double chain length of oleic acid molecules acting as stabilizing ligands. The average lateral dimension of NPLs in each sample is ∼100 nm. Folding of nanoplatelets with such a large difference between their thickness and lateral dimensions is a common behavior for 4 and 5 ML CdSe populations with sufficient lateral dimensions. It may be explained by the asymmetric source of strain in zinc blende NPLs that comes from the perpendicular alignment of cadmium atoms in the top and bottom planes of nanoplatelets.28 All elements (S, Se, and Cd) are uniformly distributed over the CdS0.57Se0.43 sample, according to the mapping provided by X-ray energy dispersive spectroscopy in the scanning transmission electron microscope (STEM-XEDS) (see Figure 1f,g). The composition for the CdS0.57Se0.43 sample estimated by STEM-XEDS and EPMA methods gave similar results, which indicates that the S/Se ratio remains constant throughout the whole sample. In general, the amount estimated from the EPMA sulfur content in all as-sythesized NPLs is larger than the initial amount of sulfur in the mixture of chalcogenide precursors (see Table 1), which is caused by the stronger

The effective-mass approximation was applied with an assumption of an infinite potential barrier height. In the multiband model, the energies of the lowest e (Ee) and hh states (Ehh) were calculated following the procedures described in ref 4. The hh−e exciton transition energy was given as

E hh − e = Eg + Ee + E hh − E b

(4)

where Eb is the exciton binding energy. By assuming that CdSxSe1−x NPLs are ideal 2D structures, the Eb was estimated to equal 4Eb(bulk), where Eb(bulk) is the exciton binding energy for bulk crystals:26

E b(bulk) =

μe 4 2ℏ2ε 2

(5)

where ε is the alloy dielectric constant given by interpolation between 9.8 for CdS and 9.6 for CdSe.27 Here μ is the exciton reduced mass, calculated according to

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1/μ = 1/me + 1/mhh

(6)

RESULTS AND DISCUSSION A set of CdSxSe1−x samples with different compositions over the whole range (0 ≤ x ≤ 1) were synthesized by varying the molar concentration of sulfur in the mixture of chalcogenide precursors according to Table 1. Solutions of elemental sulfur and selenium in 1-octadecene were used as chalcogenide precursors. The composition of nanoplatelets was experimentally analyzed by EPMA via the accumulation of the signal in different points and areas. The resulting S/(S + Se) sulfur content in each sample was estimated by averaging the data over the entire probe. The morphology of nanoplatelets was studied by TEM in bright-field mode and a scanning transmission electron microscope in high-angle annular dark-field mode (STEMHAADF). One of the overview micrographs of the CdS0.57Se0.43 sample is shown in Figure 1a, where as-synthesized nanoplatelets have a shape of single nanoscrolls twisted two or three times. The scroll-like morphology of 2D nanoplatelets is typical 581

DOI: 10.1021/acs.chemmater.6b03876 Chem. Mater. 2017, 29, 579−586

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Chemistry of Materials

broadening of all peaks that comes from the nanoscale size of investigated nanoplatelets. According to diffraction patterns, all samples are single-phase with a zinc blende crystal structure. Increasing the sulfur content of NPLs causes a gradual decrease in the lattice constant of CdSxSe1−x nanoplatelets, resulting in the continuous shift of diffraction peaks to larger angles, which corresponds to solid solution formation.30 As a result, a uniform elemental distribution together with a continuous shift of diffraction peaks supports the solid solution nature of assynthesized CdSxSe1−x nanoplatelets over the whole composition range. Optical properties of quasi-two-dimensional cadmium chalcogenide nanoplatelets have several features caused by 2D morphology.4,31,32 Two regions in absorption spectra, exciton absorption in the wavelength range of 350−500 nm and highenergy transitions in the range of 210−300 nm, were analyzed. Both regions are shown in panels a and b of Figure 3, with insets including the whole spectra of the CdS0.57Se0.43 sample. Exciton absorption of each sample shown in Figure 3a has two peaks, which is one of the characteristic features of quasitwo-dimensional cadmium chalcogenide NPLs.23 These peaks correspond to the lower-energy heavy hole (hh−e)−electron and higher-energy light hole (lh−e)−electron transitions. In the case of 2D CdSe nanoplatelets, hh−e and lh−e transitions are clearly seen in the spectra (Figure 3a, black line with empty squares), but for CdS NPLs, these transitions are not resolved in the spectrum because of the overly small energy gap between them.31 As a result, we can observe only one asymmetric peak at 377 nm in the spectrum (Figure 3a, blue line with empty circles), which is consistent with the literature.33 Another specific feature of individual CdSe and CdS nanoplatelets is very narrow exciton absorption and PL lines: the full width at

chemical reactivity of the sulfur solution in 1-octadecene (SODE) compared with that of a similar selenium solution (SeODE).29 This problem can be solved by increasing the amount of selenium in the precursor. It should be noted that using a mixture of trioctylphosphine sulfide (TOP-S) and trioctylphosphine selenide solutions as chalcogenide precursors was unsuccessful because of the low reactivity of TOP-S. The crystal structure of alloyed nanoplatelets with different compositions was studied by the X-ray diffraction (XRD) method. XRD patterns shown in Figure 2 reveal strong

Figure 2. X-ray diffraction patterns of CdSxSe1−x nanoplatelets with different compositions. Each pattern is labeled by the corresponding sample name. Standard diffraction peaks for the zinc blende (ZB) crystal structure for CdSe (black vertical lines, JCPDS Card 88-2346) and CdS (hollow blue vertical lines, JCPDS Card 75-1546) are shown at the bottom. The continuous shift of XRD peaks (shown by dashed lines) indicates a gradual decrease in the lattice constant with an increase in sulfur content in CdSxSe1−x nanoplatelets.

Figure 3. Optical properties of CdSxSe1−x nanoplatelets. Regions of UV−vis absorption spectra for all samples showing (a) exciton absorption and (b) high-energy transitions. These regions are highlighted in corresponding insets that include whole absorption spectra of the CdS0.57Se0.43 sample. (c) PL spectra of all samples except CdS (no exciton PL) in the wavelength range of 410−480 nm. All three graphs show a continuous energy shift of optical lines with the change in estimated S/(S + Se) content. (d) Absorption and PL energy dependencies from sulfur content, including eightband calculations for absorption data (---). An increasing amount of sulfur causes a blue shift of exciton transitions in the optical spectra. The inset in panel d shows a photograph of samples under UV light. 582

DOI: 10.1021/acs.chemmater.6b03876 Chem. Mater. 2017, 29, 579−586

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Chemistry of Materials

Figure 4. (a) Scheme of photoluminescence in alloyed CdSxSe1−x nanoplatelets in the presence of CdSySe1−y local composition fluctuations acting as a trap for the recombination process. (b) Energy diagram modification caused by the formation of CdSySe1−y local composition fluctuations with a smaller band gap within NPLs. Electron−hole pair generation occurs in the CdSxSe1−x nanoplatelet, while a recombination process more likely takes place within CdSySe1−y LCF, which causes a decrease in the overall emission energy. VB and CB denote the valence band and conduction band, respectively.

half-maximum (fwhm) is ∼0.05 eV for CdSe NPLs, which corresponds to the literature value.34 In the case of alloyed CdSxSe1−x NPLs, a gradual blue shift of exciton lines from 461 nm (CdSe) to 377 nm (CdS) caused by the growth of a band gap with an increase in S/(S + Se) content is observed. This behavior also supports the solid solution nature of CdSxSe1−x NPLs. At the same time, we can observe gradual broadening of exciton lines and a continuous decrease in the energy gap between hh−e and lh−e transitions with an increase in S/(S + Se) content. With the CdS0.57Se0.43 sample as a starting point, exciton hh−e and lh−e transitions are no longer resolved in the spectra. The experimental composition dependence of hh−e exciton absorption lines (Figure 3d, black squares) was compared with theoretical calculations (Figure 3d, dashed line) based on a multiband effective-mass model previously used by Ithurria et al. for 2D CdSe, CdS, and CdTe nanoplatelets with different thicknesses.4 Very good agreement between the calculation results and experimental data confirms that as-synthesized nanoplatelets are atomically flat quantum wells. Furthermore, our assumption of the 2D exciton binding energy Eb = 4Eb(bulk) (90 meV for CdSe and 133 meV for CdS) is a good approximation for the actual binding energy of our NPLs, though further enhancement of Eb due to a reduced level of dielectric screening was determined for the monolayer metal dichalcogenide.35,36 The resulting composition dependence of the exciton absorption energy for 2D CdSxSe1−x nanoplatelets is very similar to the same dependence for the bulk CdSxSe1−x solid solution24 but has a higher transition energy because of the quantum confinement in one dimension (perpendicular to the plane of a nanoplatelet). High-energy optical transitions in the UV light region of absorption spectra also show a continuous blue shift with an increase in S/(S + Se) content (Figure 3b). We can assume that high-energy lines correspond to electron transitions analogous to series E1/(E1 + Δ1) at the L point in the Brillouin zone of bulk crystals.37−39 For alloyed CdSxSe1−x nanoplatelets, highenergy absorption lines show a gradual blue shift with an increase in S/(S + Se) content that may also support CdSxSe1−x solid solution formation. Photoluminescence spectroscopy at 300 K (Figure 3c) and at 10 K (see Figure S2) showed the influence of composition on PL energy that was similar to that on absorption spectra: at both temperatures, alloyed nanoplatelets have a continuous blue shift of PL lines with an increase in S/(S + Se) content. CdSxSe1−x and CdSe samples have strong exciton photo-

luminescence (see the inset in Figure 3d), but besides exciton emission, there are also defect lines in almost all samples, which causes a change in the color of nanoplatelet solutions under UV light excitation shown in the inset photograph in Figure 3d. It was also found that aging of samples results in an increase in defect photoluminescence. Fortunately, this disadvantage can be considerably reduced by additional heating of nanoplatelets in ODE with a small amount of oleic acid at 200 °C. The photoluminescence quantum yield (PLQY) of nanoplatelets with different compositions varies in the range of 2−5%. These PLQY values can be increased by preparing core/shell type I heterostructures with CdSxSe1−x acting as a core. It is important to note that this approach includes surface modification of NPLs, colloidal atomic layer deposition of the shell material, which leads to the complete unfolding of initially scrolled NPLs.40,41 Composition dependencies of absorption hh−e exciton transitions (black squares) and PL (red circles) lines show the appearance and subsequent growth of the Stokes shift with an increase in S/(S + Se) content in the composition of alloyed NPLs (see Figure 3d). In fact, all samples may be divided into two groups: the first three samples (CdSe, CdS0.29Se0.71, and CdS0.37Se0.63) and the last three samples (CdS0.57Se0.43, CdS0.69Se0.31, and CdS0.72Se0.28). The first group has a small Stokes shift of approximately 2−3 nm. This feature can be found in the literature for pure CdSe34,42 and CdS33 NPLs. However, such a small value of the Stokes shift is thought to be negligible and is not discussed in the main text of cited articles. Here we can suggest that an increase in the extent of electron− phonon coupling in alloyed NPLs with an increasing sulfur content may influence the appearance of small Stokes shifts. At the same time, the second group of samples shows the noticeable Stokes shift that increases together with an increase in sulfur content. Also for this group of samples in the highenergy transition region, we observed the gradual appearance of the second maximum with a spectral position close to the CdS line (Figure 3b). Herein, we propose that the nonmonotonic behavior of optical properties starting from 40% sulfur content may be explained by local composition fluctuations (LCF) within solid solution NPLs. LCF (also known as alloy disorder43−45) in binary and ternary solid solutions have been previously reported in the literature.46−48 Local composition fluctuations may be thought to appear because of the lattice parameter mismatch or difference between crystal structures,43 but another LCF appearance mechanism is possible in the case of 583

DOI: 10.1021/acs.chemmater.6b03876 Chem. Mater. 2017, 29, 579−586

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Chemistry of Materials

Figure 5. (a) Temperature dependence of PL for the CdS0.57Se0.43 sample. PL spectra were recorded starting from the lowest temperature (10 K) to 290 K with steps of 20 K. The inset in panel a shows the temperature dependence of the PL energy fitted with the Bose−Einstein function. (b and c) Fitting parameters (αB, θB, and E0) and the full width at half-maximum of PL lines (fwhmPL) for all samples are shown in the form of dependencies on S/(S + Se) content. (b) The effective phonon temperature (θB) and electron−phonon interaction coefficient (αB) have maxima in their composition dependencies. (c) Band gap at 0 K (E0) that increases linearly with an increase in S/(S + Se) content. fwhmPL shows superlinear growth.

tive PL spectra at different temperatures in the range of 10−290 K for the CdS0.57Se0.43 sample. Starting from the lowest temperature (10 K), each new PL spectrum was taken with a step of 20 K. The resulting set of PL lines shows that cooling of NPLs leads to the significant increase in PL intensity and the blue shift that is typical for semiconductors. The resulting temperature-dependent PL energy was analyzed via approximation by the Bose−Einstein model (B− E) as the most appropriate model for the CdSxSe1−x system (see the inset in Figure 5a).56 According to the B−E, the band gap energy can be determined from

colloidal nanoplatelets. According to the literature, one of the most discussed growth mechanisms for 2D nanoplatelets considers their formation from magic-sized (CdSe)n42,49,50 and (CdS)n31,51 nanoclusters52 through their lamellar assembly.53 It may cause local composition fluctuations in the form of single (CdSe)n and (CdS)n nanoclusters with a size of