Surface Decides the Photoluminescence of Colloidal CdSe

Nov 29, 2017 - Surface Decides the Photoluminescence of Colloidal CdSe Nanoplatelets Based Core/Shell Heterostructures. Sushma Yadav, Ajeet Singh , Le...
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Cite This: J. Phys. Chem. C 2018, 122, 820−829

Surface Decides the Photoluminescence of Colloidal CdSe Nanoplatelets Based Core/Shell Heterostructures Sushma Yadav, Ajeet Singh, Lekshmi Thulasidharan, and Sameer Sapra* Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India S Supporting Information *

ABSTRACT: The photophysical properties of core/shell semiconductor nanocrystals are influenced by the shell thickness as well by the surface, whether it is cationic or anionic. In this work, we have investigated the effect of thickness of shell as well as the surface terminating layer anionic and cationicon the optical properties in CdSe/CdS which is a quasi-type-II system and CdSe/ZnS, a type-I heterostructured core/shell nanoplatelets (NPLs). The results reveal that no matter which cation is on the surface − Zn or Cd−the photoluminescence (PL) is always high compared to the surface being anion terminated. An alternating behavior in the PL intensity is observed upon successively terminating the surface with cations and anions, which has been achieved using the colloidal atomic layer deposition (cALD) technique. Not only the PL intensity but also the PL lifetimes and the emission peak widths too follow this similar alternating trend. All of these can be simply explained on the basis of the trap states that are created on the surface depending upon cation or anion termination.

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INTRODUCTION Colloidal semiconductor nanocrystals (NCs) have received continuous attention for over a long period since their introduction owing to their remarkable chemical, physical, and optoelectronic properties which are tunable, being dependent on size (due to quantum confinement), shape, as well as composition.1 Owing to these properties, the NCs display potential as promising candidates for various important applications such as light-emitting diodes (LEDs), photovoltaics, catalysis, sensors, cell labeling, bioimaging, and luminescent solar concentrators.2−10 These NCs comprise of an inorganic core stabilized by a layer of organic molecules as ligands, and the resulting hybrid of inorganic−organic nature provides versatility as well as flexibility to them with easy solution processing along with surface engineering.11 Therefore, it is possible to modify the properties by playing with the organic surfactant layer leading to surface chemistry manipulation. Their properties can also be manipulated by wave function engineering which involves the control of spatial distribution of carrier wave functions in quantum semiconductor heteronanostructures. Over the years, the semiconductor heteronanostructures with different compositions12−17 and shapes18−20 of two or more semiconductor components have enabled significant control over the spatial distribution of the wave functions of the charge carriers, i.e., electron and hole. Recently, the decoration or tipping of semiconductor NCs with the noble metals around or at selected facets also modifies their optical properties.6,7,21,22 Until now, the various shapes of the semiconductor nanoparticles, © 2017 American Chemical Society

including spherical quantum dots (QDs), nanorods (NRs) or nanowires (NWs), and recently nanoplatelets (NPLs) have been synthesized successfully having quantum confinement in three dimensions, two dimensions, and one dimension, respectively23−25 The synthesis of core/shell heterostructures of quantum dots, and nanorods are well established and studied vastly. After Dubertret and co-workers25−28 reported the synthesis of CdSe NPLs, various groups have synthesized heterostructures of the CdSe NPLs. The usual high temperature synthetic strategies resulted in lateral growth of shell on the NPLs rather than epitaxial growth and these kind of heterostructures are termed as core/crown NPLs.29−32 The epitaxial growth of CdS over CdSe NPLs has been realized by a room temperature shelling method called colloidal atomic layer deposition (cALD).33 It involves sequential deposition of fully saturated anionic or cationic monolayers onto the core CdSe NPLs without separate nucleation and growth of shell precursors. It employs different solvent phases for shell precursors and the core material along with a phase transfer agent if needed in nonpolar phase. It is similar to the established shelling method, i.e., SILAR, in principle, but it is superior in various aspects. As it is a room temperature method, no separate nucleation occurs, and also there is no need of exact calculated amounts of core and shell precursors as it requires excess precursors to form a saturated ionic monolayer followed Received: September 11, 2017 Revised: November 28, 2017 Published: November 29, 2017 820

DOI: 10.1021/acs.jpcc.7b09033 J. Phys. Chem. C 2018, 122, 820−829

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The Journal of Physical Chemistry C

30 min for completion of the reaction. The precipitate was washed with dried methanol three times to remove unreacted precursors followed by drying at 65 °C under vacuum overnight. Preparation of 4.5 ML Thick CdSe NPLs. The synthesis of 4.5 ML CdSe NPLs was carried out by following a recipe from literature.27 In a 50 mL four-neck flask, 170 mg (0.3 mmol) of Cd(Myr)2 and 12 mg (0.15 mmol) of Se powder were mixed in 15 mL of ODE. The reaction mixture was degassed under vacuum at room temperature for 30 min, and then, under an argon flow, the temperature was set at 240 °C. When the temperature reached 195 °C, 40 mg (0.15 mmol) Cd(OAc)2 was swiftly introduced into the flask. When the temperature reached 240 °C, the reaction was continued for 10 min, then quenched by the addition of 1 mL OA, and the mixture was rapidly cooled to room temperature. Thus, obtained product contained a mixture of CdSe quantum dots and CdSe nanoplatelets. At first, hexane was added to the product and centrifuged at 5000 rpm for 5 min. The supernatant containing quantum dots was discarded and the pellet settled at the bottom of the centrifuge tube was dispersed in hexane. The NPLs were purified by size selective precipitation upon addition of ethanol followed by centrifugation at 5000 rpm for 5 min. Then the supernatant was discarded, and the precipitate was redispersed in hexane and used as a core to synthesize their core/shell heterostructures. Preparation of 5.5 ML Thick CdSe NPLs. The synthesis of 5.5 ML CdSe NPLs was performed by following a procedure from literature.27 In a 50 mL four-neck flask, 170 mg (0.3 mmol) of Cd(Myr)2 in 15 mL of ODE was mixed. The mixture was degassed under vacuum at room temperature for 30 min, and then, under an argon flow, the temperature was set at 240 °C. When the temperature reached 240 °C, a 1 mL solution of Se sonicated in ODE (0.15 M) was quickly injected, and after 10 s, 80 mg (0.30 mmol) of Cd(OAc)2 was introduced into the flask. The reaction was continued for 10 min and then quenched by the addition of 1 mL of OA, and the mixture was rapidly cooled to room temperature. The NPLs were purified by the same procedure as described above for purification of 4.5 ML CdSe NPLs. The final product was redispersed in hexane and used to synthesize their core/shell heterostructures. Preparation of CdSe/mCdS and CdSe/mZnS Core/ Shell NPLs. For the synthesis of CdSe/mCdS (or mZnS) (m = 1−6) core/shell nanoplatelets, we employed a modified cALD technique formulated by Ithurria et al.44 For this purpose, in a 50 mL centrifuge tube, 1 mL CdSe NPLs in hexane (20 mg/ mL) was taken and diluted to 6 mL by adding more hexane, followed by addition of a 6 mL solution of sodium sulfide in formamide. The reaction mixture was vortexed for few minutes to allow the transfer of NPLs coated with the sulfide monolayer, into the polar phase, i.e., formamide. To transfer the CdSe/S2− NPLs into nonpolar phase or hexane, oleylamine is added and vortexed followed by centrifugation at 6000 rpm for 5 min. The nonpolar phase containing NPLs was washed twice with formamide. Then 3 mL of 0.1 M Cd(OAc)2 (or Zn(OAc)2) solution in formamide was added to the CdSe/S2− NPLs solution in hexane and vortexed again to allow the adsorption of Cd2+ (or Zn2+) on sulfide surface. The NPLs stayed in hexane stabilized with OLAm, while a layer of Cd2+ (or Zn2+) formed on the surface of CdSe/S2− NPLs. The nonpolar phase was washed two times with fresh formamide and then purified with ethanol and redispersed in hexane. Thus, one monolayer of CdS (or ZnS) is coated on the CdSe NPLs.

by removal of their excess after each deposition step. After successfully employing the cALD method in the synthesis of core/shell NPLs, various groups have utilized it for the synthesis of core/shell based on quantum dots.34−36 along with the synthesis of core/shell NPLs.37−39 It is well-known that surface atoms play an important role in the emission from core/shell nanocrystals. It is reported that in core/shell quantum dots the photoluminescence (PL) intensity is enhanced when surface is cation rich, while an anion-rich surface deteriorates the PL intensity.40−43 We have studied this behavior in the case of core/shell NPLs employing the cALD having surfaces saturated with anions and cations. Here, we have chosen two different systems based on their band alignments; while CdSe/CdS forms quasi type-II system, CdSe/ZnS is a type-I system. We have synthesized core/ (multi)shell nanoplatelets (NPLs) using CdSe NPLs as core and CdS or ZnS as shell utilizing the cALD technique. It is carried out by successive additions of Cd2+ (or Zn2+) and S2− layers, and then growth of Cd2+ (or Zn2+) and S2− layers onto CdSe NPLs as the core. The cycle has been repeated several times to get the desired number of shells. The absorbance and PL spectra of these samples show a continuous red shift. It is quite interesting that both the systems show continuous red shifts almost to the same extent with the coating of shells in spite of having different band alignments. The red shift in CdSe/CdS is due to delocalization of electron wave function due to small conduction band offset. CdSe/ZnS, being a type-I system showing continuous red shift of about 80 nm after six shells of ZnS, is quite different from the CdSe/ZnS quantum dots. There is a stark change in the confinement as one grows along the thickness of the NPLs, as we are dealing with a few MLs of CdSe. We have also investigated the effect of shell thickness as well as the surface terminating layer (anionic or cationic) on the charge carrier dynamics using UV−visible absorbance, photoluminescence (PL) and time-resolved fluorescence techniques. PL results indicate the emission is more when the surface is cation-terminated in comparison to emission from anion-terminated core/shell NPLs. The lifetime of cation terminated core/shell NPLs is longer than that of anion-terminated NPLs.

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EXPERIMENTAL SECTION Chemicals. Cadmium acetate dihydrate (Cd(OAc)2·2H2O, 98%, Fisher Scientific), zinc acetate dihydrate (Zn(OAc)2· 2H2O, 98.5%, Qualigens), sodium sulfide (99%, SigmaAldrich), 1-octadecene (ODE, 90%, Sigma-Aldrich), oleic acid (OA, 90%, Sigma-Aldrich), selenium powder (99.99%, SigmaAldrich), formamide (FA), oleylamine (OLAm, 70%, SigmaAldrich), cadmium nitrate tetrahydrate, myristic acid (99.5%, Spectrochem), and sodium hydroxide (Fisher Scientific) were used as received. All the solvents were purchased from Merck Chemicals and dried prior to use. Preparation of Cadmium Myristate. To prepare the cadmium myristate29 (Cd(Myr)2), at first a solution of sodium myristate was prepared by dissolving sodium hydroxide (0.600 g, 15 mmol) and myristic acid (3.420 g, 15 mmol) in anhydrous methanol (500 mL). Another solution of cadmium nitrate tetrahydrate (1.542 g, 5 mmol) was made in anhydrous methanol (50 mL). Then, the cadmium nitrate solution was added dropwise into the sodium myristate solution with vigorous stirring at room temperature and a white precipitate of cadmium myristate starts to appear. After the complete addition of cadmium nitrate solution, the reaction was allowed to stir for 821

DOI: 10.1021/acs.jpcc.7b09033 J. Phys. Chem. C 2018, 122, 820−829

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Synthesis and Characterization of NPLs. First, CdSe NPLs of two different thicknesses of 4.5 and 5.5 ML with emission maxima at 514 and 553 nm, respectively, have been synthesized. These CdSe NPLs were subsequently used as cores to further synthesize the CdSe/CdS and CdSe/ZnS core/ shell NPLs using cALD method by alternate additions of cationic and anionic shell precursors as described in the methodology. We have synthesized core/(multi)shell NPLs with shell thickness up to six monolayers of CdS (or ZnS) or six monolayers of each cation and anion. We measured absorption and PL spectra of 4.5 and 5.5 ML CdSe NPLs and their corresponding CdSe/6CdS core/shell NPLs (Figure 2 and Table 1). The absorption spectrum of CdSe NPLs shows two distinct excitonic features corresponding to the electron/heavy-hole transition (e-hh) at low energy, and the electron/light-hole (elh) transition at high energy. 4.5 ML CdSe NPLs exhibit e-hh transition at 511 nm and the corresponding emission maximum at 513 nm which are shifted to higher wavelength by ∼100 nm after six monolayers of CdS shell deposition. A similar red shift of ∼100 nm is noticed in the absorption (e-hh) and PL spectra of 5.5 ML CdSe NPLs after deposition of 6 CdS shells. This red shift is attributed to spatial delocalization of electrons in CdSe/ CdS core/shell NPLs due to small conduction band offset between CdSe and CdS (Figure 1). The observed red shift is quite large in comparison to the spherical quantum dots with comparable emission wavelength. It is suggested that the charge carrier confinement is greater along the thickness direction in NPLs in comparison to that in a quantum dot and therefore, the reduction in confinement results in larger red-shifts.39 Another point to be noted is that at high energy values there is an increase in the absorbance, owing to the growth of the CdS shell onto the CdSe NPLs. The powder XRD patterns of CdSe NPLs (4.5 and 5.5 ML) and their respective CdSe/6CdS core/shell structures are shown in Figure 2c and d, along with bulk patterns of CdSe zinc blende (ZB), CdSe wurtzite (WZ), and CdS ZB, respectively. All diffraction peaks of CdSe NPLs match well with a ZB crystal structure (JCPDS# 88-2346). After shell deposition on the NPLs, the crystal structure is retained and the diffraction peaks are shifted to higher angles toward the shell material of smaller lattice parameter i.e. CdS ZB, ensuring an epitaxial growth of CdS shell on the NPLs. The shell deposition is performed at room temperature even then the crystallinity of samples is maintained indicating that cALD is a powerful technique to prepare core/shell materials. Another diffraction peak corresponding to (200) plane next to (111) plane is observed in core/shell NPLs which is not noticeable in the core NPLs as shown in Figure 2c and d. The (200) plane is along the thickness of NPLs and it is not visible in core NPLs due to their very small thickness of 1.1−1.3 nm. However, upon deposition of six shells of CdS, the thickness is increased by about four times leading to an increment in the intensity of the diffraction peak corresponding to (200) plane, making it visible. Figure 2e and f show TEM images of 4.5 and 5.5 ML CdSe NPLs, respectively and the TEM images of their core/shell NPLs after six cALD cycles of CdS are shown in Figure 2g and h, respectively. The dimensions of CdSe NPLs and CdSe/6CdS core/shell NPLs obtained from TEM images (see Supporting Information, Figure S7), are summarized in Table 2. In core/ shell NPLs, the increase in thickness with each CdS monolayer corresponds to ∼0.27 nm which is in reasonable agreement with the CdS lattice parameter of 0.29 nm per CdS unit. Thus,

The whole procedure is used repeatedly to grow the successive monolayers after each monolayer. The aliquots were taken after each anionic and cationic precursor addition and optical characterization has been carried out to ensure complete monolayer coverage. Characterization. CdSe/mCdS and CdSe/mZnS core/shell NPLs were dispersed in hexane and used for further characterizations. The absorption spectra of these samples were collected on a PerkinElmer Lambda 1050 UV/vis/NIR spectrophotometer in a quartz cuvette. The photoluminescence (PL) emission spectra and lifetime decay curves of the same solution were recorded on an Edinburgh Instruments FLSP920 spectrophotometer. For PL measurements, the sample was excited using excitation wavelength of 377 nm while keeping the excitation and emission slit widths fixed at 2.0 and 5.0 nm, respectively. The lifetime decay curves were recorded at the PL peak maximum for each sample using time correlated single photon counting (TCSPC) technique. A pulsed laser diode (377 nm) was employed for the lifetime measurements. The instrument response function was determined using a Ludox scattering solution in water. All the decay curves were fitted multiexponentially using the equation, I(t ) = ∑i αi e−t/ τi , where I(t) is the total intensity remaining at time t. αi and τi are the amplitude and decay time of the ith component, respectively. The average lifetime of the sample is calculated 2 n using the equation τav = ∑i ( αiτi αiτi ).45 The powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance Diffractometer equipped with Ni-filtered Cu Kα radiation. For PXRD, the sample was prepared by evaporation of solvent from concentrated dispersions directly onto the glass sample holder. TEM images and EDAX were taken on a JEOLJEM 2010 electron microscopy using a 200 kV electron source. Samples were prepared on 200-mesh carbon -coated Cu grids by pouring a drop of NCs solution dispersed in hexane.

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RESULTS AND DISCUSSION To study the effect of shell thickness and the terminating surfaces on the properties of core/shell NPLs, we have selected CdSe/CdS and CdSe/ZnS as our model systems with quasitype-II and type-I alignments, respectively. We have varied the shell thickness from one to six shells of CdS and ZnS. The band energy diagrams of CdSe/CdS and CdSe/ZnS are presented in Figure 1, along with a simple representation of core/shell NPL synthesis via cALD. In CdSe/CdS, the hole resides in CdSe core whereas the electron spreads over CdSe and CdS due to small conduction band offset resulting in a quasi-type-II structure. In CdSe/ZnS, both the charge carriers, i.e., electron and hole, reside in core CdSe making a type-I junction.

Figure 1. Band alignments of CdSe/CdS and CdSe/ZnS core/shell NPLs along with conversion of CdSe NPL to core/shell NPLs. 822

DOI: 10.1021/acs.jpcc.7b09033 J. Phys. Chem. C 2018, 122, 820−829

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Figure 2. Left: data for 4.5 ML. Right: data for 5.5 ML. Absorption and PL spectra of CdSe and CdSe/6CdS core/shell NPLs with (a) 4.5 ML CdSe NPL and (b) 5.5 ML CdSe NPL as core. (c, d) PXRD patterns of 4.5 and 5.5 ML CdSe NPLs, CdSe/6CdS, along with bulk CdSe WZ, CdSe ZB, and ZnSe ZB patterns. TEM images of (e, g) 4.5 ML and (f, h) 5.5 ML CdSe NPL and CdSe/6CdS core/shell NPLs, respectively.

Table 1. Absorbance and PL Data for the Core CdSe NPLs and Their CdSe/6CdS Core/Shell NPLs Reported in Figure 2 and Core CdSe NPLs and Their CdSe/6ZnS Core/Shell NPLs Reported in Figure 3a shift after six shells sample 4.5 4.5 5.5 5.5 4.5 4.5 5.5 5.5 a

ML ML ML ML ML ML ML ML

CdSe CdSe/6CdS CdSe CdSe/6CdS CdSe CdSe/6ZnS CdSe CdSe/6ZnS

λabs(e‑hh)/nm (E/eV)

λPL/nm (E/eV)

511 (2.42) 611 (2.03) 549 (2.26) 644 (1.92) 511 (2.42) 585 (2.12) 551 (2.25) 609 (2.03)

513 (2.41) 616 (2.01) 550 (2.25) 649 (1.91) 513 (2.41) 599 (2.07) 553 (2.24) 616 (2.01)

Δλabs/nm (ΔE/eV)

ΔλPL/nm (ΔE/eV)

100 (0.39)

103 (0.40)

95 (0.33)

99 (0.34)

74 (0.30)

86 (0.34)

58 (0.21)

63 (0.23)

The shifts in absorption and PL of NPLs after shelling are also provided. The values in parentheses correspond to the energies in eV.

1). The CdSe/ZnS forms a type-I system and still shows a huge red shift similar to the CdSe/CdS NPLs suggesting that the electron wave function is partly delocalized within the ZnS, however, this delocalization is weaker than that for CdS shell. Still the quantum of observed red shift needs more insights than just the band edge positions of CdSe and ZnS to be accounted for. It seems there is an interfacial CdS layer that is

the above PXRD and TEM results support that a successful shelling over CdSe NPLs has been carried out with cALD. The optical and structural data for the characterization of CdSe/ZnS NPLs consisting of 4.5 and 5.5 ML CdSe NPLs are shown in Figure 3 and the results are quite similar to that of CdSe/CdS core/shell NPLs. The absorption and PL spectra of CdSe NPLs are red-shifted by 60−80 nm after deposition of six shells of ZnS indicating a successful epitaxial growth (see Table 823

DOI: 10.1021/acs.jpcc.7b09033 J. Phys. Chem. C 2018, 122, 820−829

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deposition with the ZnS having smaller lattice parameter with respect to core CdSe. The crystallinity of the CdSe/ZnS NPLs is lesser in comparison to the CdSe/CdS NPLs which can be seen in PXRD data shown in Figure 3c,d and 2c,d, respectively. Also the diffraction peak corresponding to (200) plane is faintly observed in CdSe/ZnS NPLs which may be due to presence of interfacial strain between CdSe and ZnS with lattice mismatch of 11.3%. The TEM images of 4.5 and 5.5 ML CdSe NPLs are given in Figure 3e and f, respectively and the TEM images of their core/shell NPLs with six ZnS shells are shown in Figure 3g and h, respectively. The dimensions obtained from the TEM images are listed in Table 2. The thickness of NPLs after deposition of six shells of ZnS is increased to ∼0.21 nm per ZnS monolayer, which is in reasonable agreement with the ZnS lattice parameter of 0.27 nm per unit of ZnS. Thus, the above PXRD and TEM results strengthen that a ZnS can be epitaxially grown over CdSe NPLs successfully despite of large lattice mismatch. Anion or Cation Termination. An interesting difference arises in the optical behavior depending on whether the surface is cation- or anion- terminated. Therefore, to investigate the

Table 2. Dimensions Obtained from TEM Images of CdSe Core NPLs and Their Core/Shell NPLs, i.e., CdSe/CdS and CdSe/ZnS sample 4.5 4.5 5.5 5.5 4.5 4.5 5.5 5.5

ML ML ML ML ML ML ML ML

CdSe CdSe/6CdS CdSe CdSe/6CdS CdSe CdSe/6ZnS CdSe CdSe/6ZnS

length (nm) 25 28 24 27 36 38 20 22

± ± ± ± ± ± ± ±

2 2 2 2 3 2 3 2

width (nm) 7.5 8 7 8 9.6 11 8 9

± ± ± ± ± ± ± ±

1 1 1 1 1 1 2 1

thickness (nm) 1.1 4.2 1.5 4.7 1.1 3.5 1.5 3.8

± ± ± ± ± ± ± ±

0.1 0.3 0.1 0.2 0.1 0.3 0.1 0.2

formed and can account for the huge red shift. This is discussed in detail below. The powder XRD patterns of CdSe NPLs (4.5 and 5.5 ML) and their respective CdSe/6ZnS core/shell structures are shown in Figure 3c and d, along with bulk patterns of CdSe ZB, CdSe WZ, and CdS ZB, respectively. The CdSe/ZnS NPLs retain the ZB crystal structure of core CdSe NPLs and the reflection peaks display shift to higher angles after shell

Figure 3. Left: data for 4.5 ML. Right: data for 5.5 ML. Absorption and PL spectra of CdSe and CdSe/6ZnS core/shell NPLs with (a) 4.5 ML CdSe NPL and (b) 5.5 ML CdSe NPL as core. (c, d) PXRD patterns of 4.5 and 5.5 ML CdSe NPLs, CdSe/6ZnS, along with bulk CdSe WZ, CdSe ZB, and ZnSe ZB patterns. TEM images of (e, g) 4.5 ML and (f, h) 5.5 ML CdSe NPL and CdSe/6ZnS core/shell NPLs, respectively. 824

DOI: 10.1021/acs.jpcc.7b09033 J. Phys. Chem. C 2018, 122, 820−829

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Figure 4. (a) Absorbance and (b) PL spectra (normalized with respect to absorbance at excitation wavelength) of 5.5 ML CdSe NPLs and CdSe/ mCdS core/shell NPLs after each anionic and cationic addition for six cALD cycles. (c) Absorbance and PL peak positions, (d) integrated PL intensity (e) fwhm, and (f) average lifetime of core CdSe NPLs and CdSe/mCdS NPLs as a function of anion and cation terminated surface. Note that the first S addition, i.e., S1, does not yield any PL which is represented as a black circle.

effect of surface cation or anion on the optical properties of the core/shell NPLs, we have coated multiple shells of CdS or ZnS on the CdSe NPLs. We have taken out the aliquots after each anionic and cationic addition for six cALD cycles, followed by their optical characterization. The absorbance and PL spectra of the CdSe/mCdS core/shell NPLs where m = 1 to 6, based on 5.5 ML CdSe core NPLs, are shown in Figure 4a and b, respectively. In the process of synthesizing core/shell NPLs via cALD, S2− ions were added first to deposit a monolayer of anions since the facets of CdSe NPLs are terminated with a cadmium rich surface passivated by carboxylate ligands. For the deposition of a cadmium monolayer, sulfur saturated (CdSe/ S2−) NPLs were dispersed in the hexane and formamide mixture to which Cd(OAc)2 solution in formamide was added. A small amount of oleylamine was introduced into the whole mixture and vortexed for a few minutes to enhance the phase transfer to hexane phase on cation attachment. Thus, one complete monolayer of CdS gets deposited onto CdSe NPLs. Here on, the naming convention is so that the “C”, “Sm”, and “Cdm” (“Znm”, in case of ZnS shell) stand for core CdSe NPL, m shells with sulfide monolayer termination, and m shells with cadmium (or zinc) terminated surfaces, respectively. The absorption spectra of core/shell CdSe/CdS NPLs display a continuous red shift of both the e-hh and the e-lh transitions when coated with either anionic or cationic monolayer during shell deposition on 5.5 ML CdSe NPLs given in Figure 4a. This red shift is due to spatial delocalization of the electrons from CdSe NPLs into the CdS shell as a consequence of reduced confinement effect due to increased thickness of the NPLs after shell deposition. The red shift in

absorbance clearly indicates the epitaxial growth of the shell ruling out the occurrence of cation exchange or alloy formation. Also, the absorption at high energy is gradually increasing with increase in thickness of shell due to absorption by CdS shell. The PL spectra also display a red shift with each addition of S2− and Cd2+ as seen in Figure 4b. The extent of the red shift in absorption and PL can be seen in Figure 4c along with the Stokes shift which is almost constant (∼12−15 meV) for the core/shell systems but higher than the core CdSe NPLs (8.2 meV). It could be due to the reduction in the strong confinement of excitons of CdSe NPLs after shell deposition as well as due to presence of some defects at interface.46 The data for CdSe/CdS core/shell NPLs with 4.5 ML CdSe NPLs is given in Figure S2, which shows similar results to that of its 5.5 ML counterpart. CdSe/ZnS core/shell NPLs also exhibit continuous red shifts in the absorbance and PL spectra with each addition of S2− and Zn2+ as shown in Figure 5a and b. As we have discussed above that despite being a type-I system CdSe/ZnS shows red shifts similar to CdSe/CdS must be due to delocalization of electron wave function to the shell. The extent of the red shift in absorption and PL can be seen in Figure 5c. But the band edge of ZnS does not allow for such a delocalization. The cALD procedure for the shell growth requires that a shell of S2− be grown first onto the CdSe NPLs leading to the creation of a single monolayer of CdS shell on top of the CdSe NPLs. This could be the reason for the observed red shift upon shell growth. The data for CdSe/ZnS core/shell NPLs with 4.5 ML CdSe NPLs is given in Figure S5, which shows similar results to that of 5.5 ML counterparts. 825

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Figure 5. (a) Absorbance and (b) PL spectra (normalized with respect to absorbance at excitation wavelength) of 5.5 ML CdSe NPLs and CdSe/ mZnS after each anionic and cationic addition for six cALD cycles. (c) Absorbance and PL peak positions, (d) integrated PL intensity (e) fwhm, and (f) average lifetime of core CdSe NPLs and CdSe/mZnS NPLs as a function of anion and cation terminated surface. Note that the first S addition, i.e., S1, does not yield any PL, which is represented as a black circle.

traps become an intrinsic part of the core/shell structure and remain intact as hole traps into the material. As a consequence, NPLs terminated by anionic layer have little or no PL emission due to the overwhelming number of surface trap states for nonradiative recombination.48 This is seen in Figures 4d, and 5d displaying variation in the PL intensities for the CdS and ZnS shelled NPLs. Figure 4e (Figure 5e) depicts the variation in the full width at half-maximum (fwhm) with a change in the terminating layers of core/shell CdSe/mCdS (CdSe/mZnS) NPLs. The fwhm values for core/shell NPLs increase to ∼20 nm depending on the terminating layers; always yielding lower values for cation termination. A previous report suggests that the higher fwhm in core/shell NPLs than core NPLs results due to higher exciton−phonon coupling in the shell than the core due to delocalization of the charge carriers in the shell.39 Broadening due to change in the ion type on the surface has not yet been reported. The presence of different channels for recombination also cause change in the fwhm. The S traps provide a number of energetically dispersed radiative pathways that leads to higher fwhm for the anion terminated core/shell NPLs. The fluorescence lifetime decay of the core/shell NPLs too yield this oscillatory behavior. The fluorescence decay traces are provided in Figures S1, S3, S4, and S6 (see Supporting Information). The variation of average lifetime with each addition of cation and anion is shown in Figures 4f and 5f of CdSe/CdS and CdSe/ZnS NPLs, respectively, with 5.5 ML CdSe NPLs as core. The variation in average lifetime is similar to the variation in the PL intensities with respect to surface being cation- or anion-terminated. It is observed that the

Interestingly, the PL of core NPLs is fully quenched when coated with very first sulfur monolayer and then reappears when the cadmium (or zinc) monolayer is further deposited. Afterward, the core/shell NPLs show PL with both cations (Cd2+, Zn2+) and anion (S2−) terminated surfaces but the cation-terminated NPLs exhibit higher PL intensity compared to anion-terminated NPLs. In the present case, oleylamine is used for anion or cation stabilization in hexane phase, hence the variation in the PL intensity results are almost independent of the passivating ligands used here or in other words, this behavior is dependent on the surface terminating ions. Here, the ligands bind strongly to cations compared to anions, providing better electronic passivation which in turn results in stronger confinement of the charge carriers. Therefore, one of the reasons for the increase in PL intensity with cation terminated surface is the efficient recombination of charge carriers due to strong binding of oleylamine with cations on the surface. When the surface of core/shell NPLs is S2− terminated during core/shell synthesis, it binds poorly with the basic ligands making these materials prone to surface trapping of holes. It is reported that the unpassivated surface S sites create deep traps whereas the unpassivated Cd surface sites create shallow traps within the band gap.47 The traps created by the unpassivated Cd surface sites remain near to the conduction band and can be removed easily with the excess of ligands which binds to these strongly such as oleylamine or carboxylates. The S sites either do not bind or poorly bind to ligands due to which the traps created in excess and remains close to the valence band which is not easy to remove. These 826

DOI: 10.1021/acs.jpcc.7b09033 J. Phys. Chem. C 2018, 122, 820−829

Article

The Journal of Physical Chemistry C

components whereas the reverse is true for anion-terminated surfaces. It can be easily correlated with the introduction of traps by addition of anions on the surface leading to short-lived nonradiative recombination. After addition of cations, the traps are removed significantly and lowering down the percentage contribution of nonradiative pathways. And also, one can conclude that, the shortest component is associated with the recombination pathways involving traps and the other two components are associated with the radiative band-edge recombination and temporary charge-carrier separation and release as suggested by Rabouw et al.51 For CdSe/ZnS 4.5 and 5.5 ML: In case of ZnS coated CdSe NPLs, the lifetimes are extremely fast for anion-terminated core/shell NPLs and can be fitted easily with biexponetials only−the third component being absent. The value of α1 × τ1 increases as the number of shells increase. This suggests the number of deep trap states arising due to the passivation by S2− anions increases. As a consequence, the number of shallow traps that allow for separation and release get reduced and this results in shortening of the lifetime. These shallow traps are primarily due to the cations on the surface. The alternating behavior that can be seen in the PL emission intensities, fwhm, and lifetimes seems to appear as a result of the large number of deep traps created due to S2− ions present on the surface and the removal of the traps by passivating with Cd2+/Zn2+ ions. Although the CdS shell coating over CdSe leading to a redshift in the spectra is well-known even in the case of spherical QDs, it is a bit surprising to observe the red-shift upon ZnS shell formation on these CdSe NPLs as both the charge carriers seem to be strongly confined within the core region as one notices in Figure 6. However, recent work on CdSe/ZnS core/ shell NPLs examines these shifts in great details based on the K−P model.52 These results show how a red shift can be obtained even in type-I heteronanoplatelets, by a combination of carrier delocalization (both electron and hole) and a reduction of the electron−hole Coulomb interactions.

lifetime decay is quite faster in case of anion-terminated surface than cation-terminated core/shell NPLs. This behavior can also be rationalized by understanding the distribution and nature of trap states depending on the surface. When the NPLs have anion-terminated surface, a large number of trap states are introduced near valence band acting as hole traps, whereas the cation-terminated surface results into trap states near conduction band acting as electron traps that anyway get passivated by oleylamine. Along with the type of created traps their number is greater in anion-terminated than cationterminated NPLs because of poor passivation by ligands. The time-resolved decay traces indicate decrease in the lifetime of core/shell NPLs emission with anion-termination due to presence of hole traps, whereas it is enhanced with cationterminated surface. A very steep decay is observed for anionterminated NPLs given in Figures S1, S3, S4, and S6, suggesting that excess of S2− ions on the surface introduces short-lifetime decay pathways, consistent with the literature for deep surface hole traps.49 The lifetime decay curve for cation terminated NPLs given in Figures S1, S3, S4, and S6 display appearance of long lifetime component after addition of excess of cations owing to presence of shallow electronic traps. The charge carrier trapping and detrapping through shallow traps, followed by their relaxation eventually give rise to delayed PL emission via a radiative decay channel.50 The reappearance of PL emission after addition of cations to the anion-terminated NPLs can be correlated to the removal of the deep traps near VB due to passivation of these anions with cations on the surface. The schematic given in Figure 6 illustrates the difference in the behavior between cation-terminated and anion-terminated core/shell NPLs having CdS and ZnS as shell materials.

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CONCLUSIONS

In conclusion, we have demonstrated the effect of shell thickness and surface termination by anions and cations on the optical properties of CdSe/CdS, and CdSe/ZnS core/shell NPLs with quasi type-II and type-I alignments, respectively, using cALD technique at room temperature. In the present results, the absorbance and PL spectra of both the systems show a continuous red shift with each monolayer indicating the epitaxial growth of shell onto core CdSe NPLs even with a high lattice mismatch material, i.e., ZnS with respect to CdSe. The variation in the photophysical properties with different terminations, i.e., cationic or anionic, on the surface is related to the introduction of traps for charge carriers indicate the emission is more when the surface is cation-terminated in comparison to emission from anion-terminated core/shell NPLs. Also, the lifetime of cation terminated core/shell NPLs is greater than that of anion-terminated NPLs. By acting on the chemical composition, size, and shape parameters, the presence of an inorganic shell modifies not only the number but also the spatial and energetic distribution of traps, eventually altering their interaction with excitons generated in the nanocrystals.

Figure 6. Schematic representation of the entire decay process for CdSe/CdS and CdSe/ZnS core/shell NPLs with cation-terminated or anion-terminated surfaces. Note that the cation trap states are narrower in energy compared to the anion trap states.

After fitting the data we obtained three lifetime components (τ1, τ2, τ3; τ1