Chloride-Induced Thickness Control in CdSe Nanoplatelets - Nano

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Chloride-Induced Thickness Control in CdSe Nanoplatelets Sotirios Christodoulou, Juan Ignacio Climente, Josep Planelles, Rosaria Brescia, Mirko Prato, Beatriz Martin-Garcia, Ali Hossain Khan, and Iwan Moreels Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02361 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 4, 2018

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Chloride-Induced

Thickness

Control

in

CdSe

Nanoplatelets Sotirios Christodoulou,1,2 Juan I. Climente,3 Josep Planelles,3 Rosaria Brescia,1 Mirko Prato,1 Beatriz Martín-García,1 Ali Hossain Khan,1,4,* and Iwan Moreels1,4,* 1

Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy

2

ICFO - Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Castelldefels, 08860 Barcelona, Spain 3

Departament de Química Física i Analítica, Universitat Jaume I, 12080 Castelló de la Plana, Spain 4

Department of Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Gent, Belgium

ABSTRACT Current colloidal synthesis methods for CdSe nanoplatelets (NPLs) routinely yield samples that emit –in discrete steps– from 460 nm to 550 nm. A significant challenge lies with obtaining thicker NPLs, to further widen the emission range. This is at present typically achieved via colloidal atomic layer deposition onto CdSe cores, or by synthesizing NPL core/shells structures. Here we demonstrate a novel reaction scheme, where we start from 4.5 monolayer (ML) NPLs and increase the thickness in a two-step reaction that switches from 2D to 3D growth. The key feature is the enhancement of the growth rate of basal facets by the addition of CdCl2, resulting a series of nearly monodisperse CdSe NPLs with thicknesses between 5.5 ML and 8.5 ML. Optical

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characterization yielded emission peaks from 554 nm up to 625 nm, with a line width (fwhm) of 9-13 nm, making them one of the narrowest colloidal nanocrystal emitters currently available in this spectral range. The NPLs maintained a fast emission lifetime of 5-11 ns. Finally, due to the increased red shift of the NPL band edge, photoluminescence excitation spectra revealed several high-energy peaks. Calculation of the NPL band structure allowed us to identify these excitedstate transitions, and spectral shifts are consistent with a significant mixing of light and split-off hole states. Clearly, chloride ions can add a new degree of freedom to the growth of 2D colloidal nanocrystals, yielding new insights into the both the NPL synthesis as well as their optoelectronic properties.

KEYWORDS colloidal synthesis, 2D nanocrystals, halides, photoluminescence, k·p calculations

Colloidal nanoplatelets (NPLs) form a class of two-dimensional semiconductor nanomaterials which differs from systems such as quantum dots (QDs) and nanorods (NRs) mainly because of the strong quantum confinement acting in only one dimension.1,2,3 The extended lateral dimensions of the NPLs lead to an in-plane coherent motion of the exciton, which gives rise to a so-called Giant Oscillator Strength (GOST).4 They can be synthesized with a thickness controled with monolayer precision. Hence, NPLs yield enhanced optical properties compared to 0D QDs, such as sharp band-edge transitions, narrow emission peaks and fast exciton recombination rates.4,5,6 Due to their unique shape, NPLs display improved packing density and can be


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assembled as long chains7 and needles,8 favoring ultrafast energy transfer,9 while their suppressed Auger recombination has already categorized them as an efficient gain material for lasing.10,11,12,13 Furthermore, NPLs are appealing because of their high surface to volume ratio, enabling optical sensor applications,14 and a high two photon absorption cross section, which categorizes

them

as

promising

candidates

for

two-photon

imaging

and

nonlinear

optoelectronics.15 The NPL optical properties are strongly correlated with the monolayer (ML) thickness. It has already been shown that CdSe NPLs emitting around 460 nm, 510 nm and 550 nm, corresponding to 3.5 ML, 4.5 ML and 5.5 ML respectively, can be routinely synthesized by varying the synthesis temperature and corresponding seed diameter (Supporting Information, SI, Figure S1). 12,16,17 Also nonlinear optical properties, such as the Auger recombination rate18 and optical gain threshold12 depend strongly on the NPL thickness. However, a direct synthesis of thicker CdSe platelets has not been reported so far. Likely, this is due to the difficulty of synthesizing larger seeds, while still inducing a 2D growth instead of creating spherical quantum dots. Recently, the synthesis mechanism of 2D CdSe NPLs received significant attention, with results pointing toward either growth via selective oriented attachment of small CdSe seeds on highenergy side facets,19,20 or growth in a kinetic regime, driven by an imbalance between the energy required to create a full top or bottom layer vs. an extra layer on the side facets, yielding lateral and vertical growth rates that are orders of magnitude different.21 Considering the latter, when starting from larger seeds, it may be more challenging to create the necessary initial deviation from sphericity for the 2D growth, or the higher temperatures used may lead to a more favorable growth of spherical nanocrystals. Ott et al. did report that thicker NPLs can be synthesized via


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Ostwald ripening.22 Starting from 2.5 ML NPLs, they created 3.5 ML and 4.5 ML NPLs in sequential steps. The synthesis of 6.5 ML CdSe NPL via size-selective precipitation has also been reported.23 Thicker CdSe NPLs (i.e. beyond 6.5 ML) were synthesized via low-temperature colloidal atomic layer deposition (c-ALD).24,25 These core-only NPLs did not display strong fluorescence, hence various research groups have instead turned their attention to the synthesis of core/shell and core/crown NPL to obtain bright red emitters.26 Particular examples are CdSe/CdZnS and CdSe/ZnS core-shell NPLs, which manifest high optical stability and PL quantum efficiencies.27,28,29,30,31 Core-crown NPLs, such as CdSe/CdTe, have a type-II band alignment, again allowing us to red shift the emission peak, up to 730 nm, yet at the cost of a longer emission lifetime due to the reduced electron-hole overlap.32 Here we propose a novel synthetic approach to obtain core-only CdSe NPLs with red emission, by using a growth mechanism that switches from 2D to 3D growth, which allows us to overcome the intrinsic limitations set out by the kinetic growth regime of 2D CdSe NPLs. We show that by introducing chloride (Cl-) in the synthesis we can stimulate the growth of the (001) top/bottom surface, resulting in NPLs with thickness up to 8.5 ML that emit at 625 nm. Traditionally, halides have been used in colloidal nanocrystal synthesis to modify their shape or increasing the overall size,33,34,35,36 likely by enhancing the growth rate of specific facets. They can also influence the lateral dimensions and crystal structure of colloidal NPLs.37 In our case, we observed a profound effect of the Cl- anions on the NPL transversal growth. We first synthesized 4.5 ML CdSe NPLs (510 mn emission peak, Figure 1) by injecting Cd-acetate (Cd(Ac)2) in a mixture of Cd-myristate and Se in ODE at 220 °C.16 Then, after growing the NPLs for 10 minutes (at 240 °C), we increased the temperature to 280-320 °C, added a mixture


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of CdCl2, CdO and oleic acid (OA), and let the reaction proceed for 2-5h (Figure 1a, see also SI, experimental section). By varying the reaction temperature we gained control over the final NPL thickness, and as a result we could synthesize NPLs with an emission peak that varies –in discrete steps– from 554 nm up to 625 nm (Figure 1b). Importantly, no significant broadening is observed upon increase in thickness, and only minor satellites appeared at the blue and red side of the main emission, which indicates a small heterogeneous size broadening with NPLs that are 1 layer thinner and thicker, respectively. Consequently, sharp absorption features are also observed, both at the band edge and at higher energies. Bright-field transmission electron microscope (BF-TEM) images of the pristine 4.5 ML NPLs and a sample of thicker NPLs (grown at 315 °C) show that the 2D NPL shape is maintained (Figure 1c,d). The 4.5 ML NPLs used for these reactions had an aspect ratio of 9.1:1, however, the procedure was also successfully applied using other NPLs. Rectangular NPLs with an aspect ratio of 2.9:1 and square NPLs (aspect ratio 1.2:1) were synthesized using 180 mg and 200 mg Cd(Ac)2.2H2O, respectively, and when subsequently injecting the CdCl2-CdO-OA mixture and increasing the reaction temperature to 300 °C for 2 hours, thicker NPLs -emitting around 584 nm- were again obtained, albeit that the resulting 6.5 ML NPLs displayed a somewhat larger fraction of 7.5 ML NPLs as well (SI, Figure S2 and Figure S3), compared to the result obtained on NPLs with higher aspect ratio (Figure 1b).


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Figure 1. (a) Schematic representation of the reaction, with photograph of the different NPL samples under UV illumination. (b) Absorption and PL spectra of the NPL series. (c-d) BFTEM images of 4.5 ML (c) and 7.5 ML (d) CdSe NPLs. Further insight into the tranversal growth mechanism was given by collecting a series of aliquots during synthesis (Figure 2a, Figure S4). We extracted a fixed volume of 200 µL from three different syntheses and measured the optical density at 295 nm (Figure 2b), a wavelength where quantum confinement effects can be neglected.38 Clearly, no increase in absorption, hence no increase in synthesis yield is measured during the reaction, supporting recent findings of Ott et al. that the NPL thickness increases via an Ostwald ripening mechanism.22 We also performed two control experiments where we started with purified 4.5 ML NPLs, and heated them to 300 °C in the presence of CdCl2-CdO-OA and CdCl2-OA precursor mixtures, respectively (SI, Figure S5). In both reactions, despite the absence of unreacted selenium, we observed a thickness increase from 4.5 ML to a mixture of 6.5 and 7.5 ML. Furthermore, for the latter we calculated that the initial amount of Cd in the purified NPLs using the absorption coefficent of Achtstein et al.38 (0.06 mmol), exceeds the amount of CdCl2 added (0.01 mmol) six-fold. These results further support a growth via the Ostwald ripening. Note, however, that in our one-pot


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procedure we also initialy have small CdSe quantum dots present, a typical byproduct of the NPL synthesis. Possibly these also (partly) redissolved and contributed to the Ostwald ripening. In concordance with the increase in thickness, the NPLs also extend laterally during the second growth step (Table 1 and SI, Figure S6), indicating that, rather than a transgression from lateral to vertical growth, the reaction conditions are modified from a regime of kinetically driven 2D to a more isotropic 3D growth.

Figure 2. (a) Temporal evolution of the absorption spectrum for NPLs synthesized at 300 °C, taking aliquots with fixed volume so that absolute values can be compared. (b) Optical density at 295 nm of aliquots taken for three syntheses, at 280 °C, 300 °C and 320 °C respectively.


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In order to confirm the key role of Cl-, we performed a series of control experiments (Figure S7). First, we annealed 4.5 ML NPLs at high temperature without addition of Cd2+ or Clprecursors. Keeping them at 300 °C even for 5 hours, we did not observe any transversal growth. Optical data only revealed an increase in absorption and fluorescence at red-shifted wavelengths, suggesting a growth of residual CdSe QDs. When adding Cd-oleate to the reaction at 300 °C, we observed populations of thicker NPLs, nevertheless, the broad band-edge absorption and photoluminescence (PL) spectra suggest polydisperse samples, containing a mix of QDs and NPLs. Only when including CdCl2, we achieved nearly monodisperse samples, with emission at 584 nm (Figure 1b, yellow curve). Note that the importance of including CdCl2 to stimulate 3D growth is further exemplified by adding an excess of CdCl2 to the reaction (Figure S7). Indeed, in this case a strong red shift of the absorption edge and PL spectrum is observed, probably due to the increased reactivity of the CdCl2 precursor, albeit again at the expense of sample monodispersity. The amount of Cl- anions added in the reaction equals 2.8% of the total Cd concentration. Considering that about 40% of the Cd resides on the NPL surface in 4.5 ML NPLs, this amount of Cl- is not sufficient to fully cover the NPL surface. Moreover, elemental analysis using both energy dispersive X-ray spectroscopy in scanning TEM mode (STEM-EDS) and X-ray photoelectron spectroscopy (XPS) data on the final NPLs (Table 1, Figure S8) show that Claccounts for merely 1-2% of the overall composition, in line with the amount added. It appears that, in order to modify the reaction kinetics, we only need to add a sufficient concentration of Cl- to the reaction mixture to adsorb to specific surface sites, rather than provide a concentration that is high enough to passivate the whole surface. We therefore postulate that the role of Cl- lies with modifying the surface energy and reducing the nucleation barrier for CdSe islands on the


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basal facets, hereby increasing the transversal growth rate and leading to a more isotropic growth.

Table 1. STEM-EDS data on the NPL composition (atomic concentration, %), evaluated from spectra acquired over several NPLs. The average error on the Cd ratio equals 10%, on Se 3% and on Cl 17%. NPL areas before and after the transversal growth, obtained from BF-TEM. Average NPL lattice constant, calculated from the XRD pattern using Bragg peaks (2θ) around 25°, 29° and 41°. The bulk CdSe lattice constant equals 6.05 Å. Sam ple

EDS ratio Cd:Se:Cl

4.5 5.5 6.5 7.5 8.5

55.5 : 44.5 48.9 : 49.9 : 49.9 : 48.5 : 49.4 : 49.2 : 51.8 : 47.7 :

Initial area (nm2) 187 179 215 201 210

Final area (nm2) 371 365 414 479

Lattice constant (Å) 6.183 6.125 6.106 6.096 6.097

Investigating the NPL crystal structure in more detail with X-ray diffraction (XRD), we observed that the high temperature does not lead to a modification of the zinc blende phase into wurtzite (Figure 3a). As the thickness is reduced, the lattice constant increasingly deviates from bulk values (Table 1), in line with measurements of Chen et al.,39 who reported an in-plane lattice expansion and a reduced lattice constant in the vertical direction. The maximal (effective) dilatation however equals 2.2%. High resolution TEM (HR-TEM) confirmed the crystal structure, and clarified that all NPLs facets are parallel to the {100} planes of cubic CdSe (Figure 3b,c). HR-TEM images of vertically stacked NPLs allowed us to determine experimentally the NPL thickness (Figure 3d, red squares, error bar represents 1σ standard deviation of the width distribution). The NPLs form a series that increases with 1 CdSe ML for each consecutive sample, and absolute values correspond to half-integer ML thicknesses


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assuming a bulk lattice constant of 0.3025 nm (the minor deviation from bulk, as measured with XRD, can be neglected for this calculation). We thus synthesized, starting from 4.5 ML NPLs, CdSe 2D nanocrystals with a thickness up to 8.5 ML, or 2.6 nm. By relating the thickness to the absorption band gap (Figure 3d), we determined following sizing curve for CdSe NPLs: = 1.49 +

.

(eq. 1)

Here Eg (eV) is the NPL band gap and d (nm) is the thickness of the CdSe NPLs. The curve extrapolates to a band gap below the bulk value of 1.74 eV, and it follows a 1/d size dependence, as such it should mainly be considered as an empirical relation to facilitate determination of the thickness via the CdSe NPL band gap.


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Figure 3. (a) XRD patterns of the CdSe NPLs for different thicknesses. (b-c) HR-TEM images of 8.5 ML NPLs, viewed from the top and from the side, respectively. Given the cubic structure, they have been attributed to the [001] and [100] zone axis, respectively. From HRTEM images of NPLs in this sample, a NPL thickness of 2.6 nm is derived. (d) Experimental (red squares) and theoretical values (blue circles) of the band gap dependence on the NPL thickness. Current data is complemented with band gaps from literature on 2.5 ML and 9.5 ML NPLs (gray circles).1,22

Optical characterization of the samples (Table 2) shows that we maintain a narrow emission line width of 9 – 13 nm (36 – 45 meV) up to the 8.5 ML NPLs. This is at present one of the narrowest emission peaks obtained for colloidal nanocrystals (compare for instance to 67 meV for CdSe/CdS QDs40 or 123 – 162 meV for InP/ZnSe/ZnS QDs41), and should result in a high color purity. The Stokes shift between absorption and emission maximum varies between 2 – 3 nm (7.5 – 10.5 meV) for all samples, substantially smaller than the 25 – 85 meV obtained for


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CdSe QDs,42 again reflecting the 2D nature of the NPL opto-electronic properties, in particular the reduced fine structure splitting and electron-phonon coupling.43 The PL quantum efficiency dropped somewhat compared to the original 4.5 ML core NPLs, yet further surface passivation and core/shell synthesis can improve values in future work. Table 2. Summary of the optical properties of the CdSe NPL series. The PL QE was measured with an excitation wavelength of 450 nm. τav equals the amplitude-averaged exciton recombination time, obtained from fitting the nonexponential PL decay traces.

4.5 ML

Absmax (nm) 507.7

PLmax (nm) 509.9

FWHM (nm) 9.3

5.5 ML

552.0

554.0

8.8

6.5 ML

581.2

583.7

7.5 ML

603.8

8.5 ML

622.9

Sample

Stokes shift (nm) 2.2

QE (%) 26

τav (ns) 5.3

1.9

18

8.6

9.7

2.4

14

8.4

606.6

11.3

2.8

16

625.3

13.1

2.4

11

10. 9 10. 8

At room temperature, we observed no heterogeneous line broadening of the PL spectrum, as demonstrated by photoluminescence excitation spectroscopy (PLE, Figure 4). The PLE spectra of both 6.5 ML and 7.5 ML NPLs (Figure 4a,b respectively) are all superposed, independent of the emission wavelength probed, and a similar behavior was observed for the other samples. All NPLs also maintained a fast PL lifetime τav that varied between 8.6 ns and 10.8 ns (Figure 4c, Table 2, Table S1), with the increase in τav for the thicker NPLs at least partly ascribable to the red shift of the emission peak, suggesting that the NPLs maintain a large band-edge oscillator strength and electron-hole overlap.


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Figure 4. (a-b) PLE spectra obtained for different emission energies on 6.5 ML and 7.5 ML CdSe NPLs, respectively. (c) PL decay traces of the full NPL series.


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Figure 5. PLE spectra of the different CdSe NPLs, plotted on an energy scale, and comparison to the calculated transitions (vertical lines). We further collected PLE spectra from the band-edge region to 275 nm (4.5 eV), which allowed us to investigate in detail the different high-energy transitions that were observed. First, comparison of absorbance and PLE spectra (Figure S9) shows that all main features are reproduced in both, yet in the PLE spectra the scattering background, and small residual peaks from different sample thicknesses are removed. Focusing on the 8.5 ML NPLs (red curve) first, we can identify seven main peaks in the experimental spectrum. All these features gradually blue shift with decreasing thickness, while maintaining the general pattern of a first (band-edge) doublet, a second more isolated peak, followed by two more doublets, the highest of which rapidly shifts beyond 4.0 eV. To assign these peaks, we use a multi-band k·p (Pidgeon-Brown) Hamiltonian for zinc-blende crystal symmetry.44 We considered quantum confinement only in the [001] direction of the NPL and calculated numerically the lowest energy state for each band


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(conduction band, heavy, light and split-off hole subbands), without fitting parameters (see SI for full calculation details). The resulting resonances (Figure 5, vertical lines) agree well with experimental observations. The spectral sequence is generally consistent for all NPLs thicknesses. From low to high energy, one finds: e1-hh1, e1-lh1, e1-soh1, e2-hh2, e2-lh2, e3-hh3, e3 lh3, where we have labeled the transitions as en-un, with e denoting the conduction band state, u the main valence sub-band participating in the electron-hole pair (heavy hole, light hole or splitoff hole) and n the envelope function quantum number in the strongly confined direction. Our calculations also reveal an important characteristic feature of colloidal NPLs with strong vertical confinement, namely a significant coupling between lh and soh states. Hybridization between the two sub-bands is non-negligible and must be accounted for in order to reproduce experimental values of the hh-lh energy splitting (Figure S11). The coupling is particularly pronounced for excited states and for thinner NPLs. These results can be already understood through a two-band (lh-soh) k·p model with infinite barriers, as then the coupling term can be written as: = 2

( ! )

(eq. 2)

where is the Luttinger parameter, Lz the platelet thickness and nu the quantum number of the hole in the u subband. Thus, the interaction between lh2 and soh2 states is 4 times stronger than that of their n=1 counterparts. Such a strong lh-soh coupling is characteristic of colloidal NPLs, because of stronger quantum confinement than in their epitaxial counterparts (Figure S12). Using our previous model for this system, we have also noticed that excited (n=2) states differ from low-energy (n=1) ones in that they are more sensitive to dielectric confinement (Figure S13), since their charge density lies closer to the NPL-ligand interface.45


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Our results further underline that the third lowest-energy absorption peak, which was previously assigned to the onset of the lh absorption continuum, also involves the soh1 – e1 exciton transition. Further work on the single-particle level and at cryogenic temperatures will allow to disentangle these two contributions. It is finally worth noticing that some high energy transitions cannot be assigned by the present calculations (e.g. the resonances at 3.4 eV and 3.55 eV for 7.5 ML NPLs). Their energy appears too low for inter-valley excitations, so we speculate they might arise from weak transitions which are symmetry-forbidden in our ideal model.46 In conclusion, we illustrated how we can achieve, by adding CdCl2 in a two-step reaction scheme, a transversal growth of CdSe NPLs up to 8.5 ML, corresponding to 625 nm emission. CdSe NPLs were obtained with narrow emission peaks and fast fluorescence lifetimes, yielding promising candidates for red emitters with saturated color. The introduction of metal halides is expected to yield similar results in other 2D materials, opening up the possibility to tune the thickness across different materials and as such gain access to a wide spectral range. The controlled evolution from 2D to 3D growth therefore presents an exciting new chemical pathway for tuning the opto-electronic properties of 2D semiconductor NPLs.

ASSOCIATED CONTENT Supporting Information. Experimental methods. Additional TEM, absorption, PL and PLE spectra. Details of the k.p calculations. The Supporting Information is available free of charge on the ACS Publications website.


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AUTHOR INFORMATION Corresponding Author [email protected]; [email protected]

Funding Sources J.P. and J.C. acknowledge support from MINECO (project CTQ2017-83781-P) and UJI (project B2017-59). This project has also received funding from the European Union’s Horizon 2020 research and innovation program (grant agreement no. 696656 GrapheneCore1), and the European Research Council (ERC) (grant agreement no. 714876 PHOCONA).

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