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ZnS quantum dots depending on interfacial residual europium
Accepted Manuscript Title: Structural and Optical Properties of ZnSe:Eu/ZnS Quantum Dots depending on Interfacial Residual Europium Authors: Ji Young Park, Chan Gi Lee, Han Wook Seo, Da-Woon Jeong, Min Young Kim, Woo-Byoung Kim, Bum Sung Kim PII: DOI: Reference:
S0169-4332(17)32618-1 http://dx.doi.org/10.1016/j.apsusc.2017.09.018 APSUSC 37097
To appear in:
APSUSC
Received date: Revised date: Accepted date:
31-3-2017 28-8-2017 4-9-2017
Please cite this article as: Ji Young Park, Chan Gi Lee, Han Wook Seo, Da-Woon Jeong, Min Young Kim, Woo-Byoung Kim, Bum Sung Kim, Structural and Optical Properties of ZnSe:Eu/ZnS Quantum Dots depending on Interfacial Residual Europium, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.09.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and Optical Properties of ZnSe:Eu/ZnS Quantum Dots depending on Interfacial Residual Europium Ji Young Parka, Chan Gi Leea, Han Wook Seob, Da-Woon Jeongb, Min Young Kimb, Woo-Byoung Kimc, Bum Sung Kimb,* a
Department of Advanced Materials & Processing Center, Institute for Advanced Engineering, Yongin 449-863, Republic of Korea b
Department of Korea Institute for Rare Metals, Korea Institute of Industrial Technology, Incheon 406-840, Republic of Korea
c
Department of Energy Engineering, Dankook University, Cheonan 31116, Republic of Korea
Corresponding author: Bum Sung Kim (email: [email protected])
Highlights
ZnSe:Eu/ZnS core/shell quantum dots (QDs) prepared in situ for the first time.
Luminescence intensity of core/shell QDs three times that of uncoated core QDs.
The core/shell system showed no Eu2+ emission but only the sharp peaks in the red at 579, 592, 615, 651, and 700 nm due to the electronic transitions of 5D0→7Fn (n = 0, 1, 2, 3, 4) depending on leisurely decreased with increased reaction time..
The core and core/shell QDs both have a zinc blende structure, and their respective sizes were about 3.19 and 3.44 nm. 1
The core/shell structure may contain Eu2O3 bonding the over-coated ZnS surface on the Eu3+doped ZnSe core.
Abstract A multimodal emitter comprising of ZnSe:Eu/ZnS (core/shell) quantum dots (QDs) by adding a ZnS precursor in situ during synthesis. ZnSe/Eu2+/Eu3+/ZnS actives both core and core/shell. QDs prepared with the ZnS precursor displayed a luminescence intensity three times that of ZnSe QDs due to the passivation effect of the Shell. While the core QDs display the 450–550 nm emission of Eu2+ (4F65D1→4F7), the core/shell system showed no Eu2+ emission but only the sharp peaks in the red at 579, 592, 615, 651, and 700 nm due to the electronic transitions of 5D0→7Fn (n = 0, 1, 2, 3, 4) depending on leisurely decreased with increased reaction time. These results are in agreement with Eu 3d spectra of XPS analysis results. Microscopic analyses show that the core and core/shell QDs both have a zinc blende structure, and their respective sizes were about 3.19 and 3.44 nm. The lattice constant in the central portion of the core/shell QDs are around d111 = 3.13Å , which is between the outside and inside ring patterns (d111 = 3.27 and 3.07 Å , respectively). This shows the effective over-capping of shell onto the core QDs. The core/shell structure may contain Eu2O3 bonding the over-coated ZnS surface on the Eu3+-doped ZnSe core. Keywords: Quantum dots, Multimodal Emitter, Core/Shell, Passivation, Europium, Doping
1. Introduction Colloidal semiconductor nanocrystals or quantum dots (QDs) with size-tunable bandgaps are applicable in a number of technologies, such as optoelectronic devices like light-emitting diodes (LEDs), photovoltaic devices, and as fluorescent tags for biological molecules [1-5]. The Cd-free QDs due to environmental concerns recently has spurred further explosion of investigation on the synthesis of non-toxic alternatives 2
such as CuInS2, InP, and ZnSe QDs which are known to exhibit tunable emission spectra from blue to red without having intrinsic toxicity compared to the Cd-based QDs. In particular, ZnSe based Cd-free QDs are of strong interest due to their wide band gap characteristic, i.e., 2.7 eV, that is proven to be essential to further improve fluorescence efficiency as well as stability of QDs [6-7]. Besides, type-I core/shell QDs basically have been demonstrated to exhibit a superior stability against oxidation as well as photodegradation, and hence enhanced quantum yields because all charge carriers are most likely confined in the core material, that would give rise to radiative recombination [8-10]. The QDs prepared by such overcoating methods enclose interfaces of lattice-mismatched core and shell regions with misfit strain. The core/shell and QD/surfactant interfaces are kinetically stable during synthesis. Rare earth (RE) materials possessing special 4f are recognized as excellent candidates for luminescence centers of the doped II-VI nanocrystals due to their many optical advantages, such as sharp fluorescent emissions via intra of 4f-5d transition, large stokes shift, no photobleaching and long luminescent lifetime and unique luminescent etc [11-12]. However, only a few studies have investigated the multimodal luminescence of ZnSe QDs with Eu and ZnS. In addition, other alternatives could be inorganic luminescent materials with good thermal stability, such as QDs with RE materials, but it is very difficult to in-situ synthesize multimodal emitters in ZnSe:Eu/ZnS (core/shell) QDs. In this regard as mentioned above, we report a successful synthesis of ZnSe:Eu/ZnS (core/shell) QDs through a simple in-situ method that has yet been reported in literatures, where the individual core ZnSe QD is revealed to be doped with Eu ions. The observed luminescence properties of those core/shell QDs exhibiting multimodal luminescence characteristics due to the presence of Eu ions are presented with respect to different synthesis conditions, i.e., reaction time. The corresponding structural interpretations were performed based on the data taken from TEM, XRD and XPS.
2. Experimental Procedure
3
2.1. Chemicals Precursors were prepared using Zinc stearate ((CH3(CH2)16COO)2Zn, technical grade), 1-octadecene (ODE, CH3(CH2)15CH=CH2, 95%), Se powder (Se, 99.999%), octadecylamine (ODA, CH3(CH2)17NH2, 90%), tributylphosphine (TBP, (CH3(CH2)3)3P, 97%), europium acetylacetonate hydrate (EAH, Eu(C5H7O2)3H2O,
99.999%)
zinc
diethyldithiocarbamate
(ZDC,
(C2H5)2NCS2)2Zn,
97%),
trioctylphosphine (TOP, (CH3(CH2)7)3P, 97%), oleyamine (OA, CH3(CH2)7CH=CH(CH2)7CH2NH2, 70%) ); all were used as purchased from Sigma-Aldrich without further purification. 2.2. Preparation of precursor solutions Core QDs were prepared via heating up method modified from previous our work [13]. Zinc stearate and EAH were used as the Zn and Eu precursors. Equimolar amounts (0.01 M) of these precursors were dissolved in 50 ml of ODE, respectively. Each mixture was heated to 150°C for 20 min under Ar atmosphere and then cooled to room temperature. 0.06 M of Selenium powder used as the Se precursor was dissolved in 7 ml of TBP along with 0.02 M of ODA at room temperature. To make a Shell precursor, a mixture consisting of ZDC (0.005 M), OA (8 ml), ODE (42 ml) and TOP (10 ml) was prepared and then subjected to two heating step such that the mixture was heated to 80°C for 15 min, followed by 120°C for 75 min under Ar atmosphere. 2.3. In-situ synthesis of ZnSe:Eu/ZnS colloidal quantum dots For synthesis of the ZnSe:Eu (core) a three-neck flask containing all precursor solutions for Zn, Eu and Se was heated from room temperature to 260°C under Ar flow and left stirring for 5 min. This mixture was then cooled to 150°C at which the Shell precursor for ZnS was added. When the desired temperature was reached, the ZnS Shell precursor was slowly injected into the vigorously stirring reaction mixture of ZnSe:Eu (core) over a period of 10 min. Note that the actual amount of ZnS injected into the mixture of 4
ZnSe:Eu was 50 ml which was the same as that of ZnSe:Eu. After completing the addition, the mixture was left stirring for 180 min and then cooled down to room temperature. Synthesized ZnSe:Eu/ZnS QDs were then purified through a typical precipitation-redispersion method three times involving organic solvents such as anhydrous ethanol for precipitation and hexane for redispersion, where excess amount of ethanol added into QDs centrifuged with 6000 rpm for 10 min at 15°C. 2.4. Characterization The optical properties of the ZnSe:Eu/ZnS QDs were investigated using a double beam UV–vis spectrometer (Optizen 3220UV MECASYS). Photoluminescence (PL) spectra were taken on a Maya 2000 Pro spectrometer (Ocean Optics, Dunedin, FL). The crystalline phases were analyzed by X-ray diffraction (XRD) using a Siemens D500 X-ray diffractometer, using Cu Κα radiation (λ = 1.5418 Å) in the 2𝜃 = 20-80° range. Transmission electron microscopy (TEM) measurements were performed in a Tecnai-TF20 FEI-TEM operating at 200 kV to study the particle size, morphology and crystallinity of individual QDs. X-ray photoelectron spectroscopy (XPS) was measured on Thermo Scientific Theta Probe XPS with a base pressure of 4.8 × 10-9 mbar (ultrahigh vacuum), using a monochromatic Al Kα X-ray source (hν = 1486.6 eV). The emitted photoelectrons were detected through a 180° double focusing hemispherical analyzer with a 128-channel detector relative to the sample surface. All of the binding energies were determined with a C 1s core level peak at 284.8 eV, where the high-resolution spectra and the survey spectra were acquired at different resolutions of 0.1 eV and 1 eV, respectively, on purpose. The comparison of Gibbs free energy change at various temperatures for the possible reactions in the mixture of precursors was also considered via computer software calculation (HSC Chemistry Version 8).
3. Results and Discussion
5
To investigate the favored compounds produced in the in-situ reaction system, the standard Gibbs free energy change (∆G) was calculated for the formation of ZnSe, ZnS, EuSe, and EuS (Fig. 1). The enthalpy and entropy values were obtained in databases from HSC chemistry simulation. The ∆G value is defined by the following equation [14]: △ G =△ H − T △ S
(1)
And the calculated △G values at 260 ℃ and 150 ℃ are as follows: Zn2+ + Se2− → ZnSe (∆G°260℃ = −45.05 kcal/mol, ∆G°150℃ = −42.14 kcal/mol)
(2)
Zn2+ + S 2− → ZnS (∆G°260℃ = −45.41 kcal/mol, ∆G°150℃ = −37.96 kcal/mol)
(3)
Eu2+ + Se2− → EuSe (∆G°260℃ = −19.08 kcal/mol, ∆G°150℃ = −17.24 kcal/mol
(4)
Eu2+ + S 2− → EuS (∆G°260℃ = −0.92 kcal/mol, ∆G°150℃ = 4.34 kcal/mol)
(5)
ZnSe has the most negative value of ∆G°260℃ = −45.05 kcal/mol, therefore it is preferentially formed by spontaneous reaction into the core structure during the first step of synthesis. The △G values at 150 ℃ indicates that in the second step, a shell of ZnS would be formed when the ZnS precursor was injected into the core QDs suspension. Thus, the core/shell phase should be preferentially formed based on the thermodynamic data. Fig. 2 shows the UV-vis absorption and PL spectra of the core and core/shell QDs. The inset Figure in Fig. 2(d) shows PL spectra with respect to different reaction time. In Fig. 2(a), the absorption peak of core QDs is at 400 nm, which is shifted to shorter wavelength compared to that of bulk ZnSe (460 nm) due to the quantum confinement effect. The absorption peak of core/shell QDs prepared with a reaction time of 180 min was found at 425 nm, exhibiting red shift compared to that of the core. The particle growth of core on the red shift was investigated based on the quantum confinement effect, by increasing the reaction time for forming the ZnS shell. In Fig. 2(b), after over-coating with the shell for 150 min, the PL intensity of 6
ZnSe in blue region is nearly tripled compared to that of the core QDs. It demonstrates the successful ZnS shell passivation effect, which enhances the emission due to reduced emission from surface traps, such as vacancies and local lattice mismatches on the surface. Because of the potential barrier, the core/shell system also improves the charge confinement in the core. After the shell precursor was injected into the core QDs colloid, the core grew during the subsequent reaction. Therefore, the core/shell QDs exhibited red shift in the emission spectrum [15]. Fig. 2(c) shows the PL emission of core ZnSe:Eu QDs, and there was clearly a broad spectrum corresponding to the 4F65D1→4F7 transition from 450 to 550 nm. The spectrum was monitored at 500 nm, because Eu2+ has a complicated set of energy levels here that involve seven 4felectrons [16-17]. The Eu2+-related emission peak (450 to 550 nm) PL intensity of core/shell Eu2+ related emission peak (450 to 550 nm) disappeared with increased reaction time. Fig. 2(d) shows the well-known characteristic red emission peaks of Eu3+ at 579, 592, 615, 651, and 700 nm (under 365 nm excitation), due to the electronic transition of 5D0→7Fn (n = 0, 1, 2, 3, 4) [18-19]. The Eu3+-related emission peak PL intensity of core/shell leisurely decreased with increased reaction time. Fig. 3 shows the XRD patterns of core QDs synthesized over 5 min and core/shell QDs with 150 min of over-coating reaction time (which showed the strongest luminescence). For the core QDs, the diffraction peaks are located at around 27.65°, 45.67°, and 53.77° corresponding to the (111), (220), and (311) planes, respectively. These peaks are shifted towards higher angles compared to those for pristine ZnSe (JCPDS No. 37-1463), because the ionic radius of doped Eu3+ (0.95 Å) is larger than that of Zn2+ (0.74 Å) [20-22]. The doping of Eu2+ (1.17 Å) into the ZnSe QDs is unlikely, due to the lattice mismatch. Further, there was no evidence of independent Eu-based phases such as Eu, EuOx, and EuSe. The Eu3+ ions doped into the ZnSe matrix reduce the unit cell volume of the ZnSe:Eu3+ nanocrystals, and the resulting compression stress shifts the XRD peaks to higher angles. Hence, these shifts indicate the successful incorporation of Eu3+ into the ZnSe crystal lattice, in good agreement with the previous report [13]. With the shell over-coated on Core QDs, the diffraction peaks shifted to even higher angles (28.6°, 47.71°, and 56.56°), which are 7
between the cubic ZnSe and ZnS (JCPDS No. 79-43) phases. These as-prepared core/shell QDs display an obvious shift in the XRD patterns, from that of cubic ZnSe to very close to ZnS. This shift shows the effective capping of ZnS onto the core QDs. After forming the ZnS shell over the core QDs, lattice strain became apparent due to the lattice mismatch between the core and shell. Because the lattice parameter of ZnS (5.32 Å) is smaller than that of ZnSe:Eu3+ (5.59 Å), epitaxial growth of ZnS shell strongly compresses the underlying ZnSe QDs [23-24]. The resulting nanosized crystalline domains caused the broadening of the core smaller than core/shell QDs peaks. Fig. 4 shows the TEM images and selective area electron diffraction (SAED) patterns for both core and core/shell QDs. According to Fig. 4(a), the core QDs are spherical with an average size of 3.19 nm. The interplanar spacing of their (111) plane was 3.22 Å, which is smaller than that of pristine ZnSe QDs (3.27 Å). These results show that it is highly possible that Eu3+-doped ZnSe exhibit a zinc blende structure. In Fig. 4(b), the core/shell QD are also spherical with an average size of 3.44 nm. At their central portion, the lattice constant is around d111 = 3.13 Å , which is between the outside ring pattern (d111 = 3.27 Å ) and inside ring pattern (d111 = 3.07 Å ). This shows the effective over-capping of ZnS onto the ZnSe core QDs. The detailed structures and crystallite sizes of the QDs measured by XRD and TEM are summarized in Table 1. Fig. 5 shows the overall XPS spectra of core (synthesized for 5 min) and core/shell QDs (coated for 150 min). The core levels of the survey spectra can be assigned to Zn 2p, Se 3d, S 2p, and Eu 3d. Fig. 5(a) shows identical peaks between the two QDs samples at 1045.2 and 1021.9 eV, which were assigned to the Zn 2p1/2 and Zn 2p3/2 core levels of ZnSe QDs, respectively. In Fig. 5(b), the peaks at 54.1 eV binding energy are due to the splitting of the Se 3d level in ZnSe. These results are in good agreement with the reported value for ZnSe QDs [25]. In comparison with the core, in the core/shell QDs the Zn 2p peak is stronger, while the Se 3d peak is slightly weaker. Fig. 5(c) fails to show the S binding energy for the core, however the S 2p3/2 (161.8 eV) and 2p1/2 (163 eV) binding energies for the core/shell QDs are clearly assigned to ZnS on the core surface [8-9]. These behaviors indicate the successful over-coating of the shell 8
on the core interface, creating a passivation effect. The four peaks for core QDs in Fig. 5(d) correspond to Eu3+3d3/2 (1164.8 eV) and Eu3+3d5/2 (1135 eV) with spin-orbit splitting, Eu2+3d3/2 (1154.7 eV), and Eu2+3d5/2 (1125.9 eV) [26-27]. In the core/shell QDs, the Eu2+ peak decreased. The reason is that in the uncoated core, Eu3+-doped ZnSe has Eu2O3 and EuO bonded to the surface by dangling bonds, whereas in the core/shell QDs there should be increasing Eu2O3 bonding to the shell on the Eu3+-doped ZnSe core. The core of both Eu3+3d5/2 (1135 eV) and Eu2+3d5/2 (1125.9 eV) region are 54.09%, while the passivated core/shell QDs are confirmed to be 45.91% in Eu3+3d5/2 (1135 eV) region compared to core. This means that the increase of Eu2O3 bond due to the out diffusion of surface oxygen of QDs. These results are in good agreement with PL spectra. Inside-out diffusion phenomena depend on the penetration capability of the core metal ions into the ZnS lattice followed by the diffusivity of metal ions within the ZnS lattice and the ease of cation exchange. Penetration capability, in turn depends on the interfacial stability and the bond dissociation energies of the core molecules. The phenomena perhaps over-coating and doping, stress created due to the crystal strain due to lattice mismatch from core-shell and doping interface help to break the bonds inside the core and then the metal ions diffuse inside the semiconducting matrix as determined by its diffusivity [28]. In addition, the reason that the Eu2+3d5/2 (1125.9eV) decreased than Eu2+3d3/2 (1154.7eV) is the photoemission of Eu-containing compounds, a satellite will accompany with the major band due to a charge transfer coexcitation both for Eu2+ and Eu3+. It was interpreted as shake-up for Eu2+. The shake-up satellite was characterized by a decrease of the 4f occupation number, i.e. a charge transfers of 4f75d0→4f65d1 for Eu2+, while the shake-down satellite was due to an increase of 4f occupation, i.e. and surface-induced electron transfer from to Eu3+(4f65d0→4f75d0) [29].
5. Conclusions In this study, we successfully synthesized a multimodal emitter comprising of ZnSe:Eu/ZnS (core/shell) QDs. After the ZnS precursor was injected onto the core, the luminescence intensity in the ZnSe QD 9
emission region was tripled in the resulting ZnSe/Eu2+/Eu3+/ZnS QDs due to the passivation effect. These results show the shell was successfully over-coated on the core surface. The spectrum of the core displayed the 450–550 nm emission of Eu2+ owing to the 4F65D1→4F7 transition, while the Eu2+ emission disappeared for the core/shell system. These results were further confirmed by the XPS analysis. Meanwhile, the Eu3+related emission peak PL intensity of core/shell leisurely decreased with increased reaction time. Further, the XRD and TEM images and SAED patterns show that the core and core/shell QDs have a zinc blende structure, with an average size of about 3.19 and 3.44 nm respectively. The core/shell structure may contain Eu2O3 bonding the over-coated ZnS surface on the Eu3+-doped ZnSe core.
Acknowledgement The authors acknowledge financial support from the Regional Industry Nurturing Program, “Development of high sensitive and functional organic/inorganic hybrid plastic composite for LED (Project No. A010400068),” funded by the Ministry of Trade Industry & Energy (MOTIE).
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Figure captions Fig. 1 Calculated Gibbs free energy (∆G) for four reactions as a function of temperature. Fig. 2 (a) UV-vis absorption and (b-d) PL spectra of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) QDs. 13
Fig. 3 XRD patterns of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) synthesized for 5min and 150 min, respectively. Fig. 4 TEM images of (a) ZnSe:Eu (core) and (b) ZnSe:Eu/ZnS (core/shell). The corresponding selective area electron diffraction (SAED) pattern of core/shell is included in Fig. 4(b). Fig. 5 XPS survey spectra of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) for 150 min; (a) Zn 2p, (b) Se 3d, (c) S 2p and (d) Eu 3d.
Fig. 1 Calculated Gibbs free energy (∆G) for four reactions as a function of temperature. 14
Fig. 2 (a) UV-vis absorption and (b-d) PL spectra of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) QDs.
15
Fig. 3 XRD patterns of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) synthesized for 5min and 150 min, respectively.
16
Fig. 4 TEM images of (a) ZnSe:Eu (core) and (b) ZnSe:Eu/ZnS (core/shell). The corresponding selective area electron diffraction (SAED) pattern of core/shell is included in Fig. 4(b).
17
Fig. 5 XPS survey spectra of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) for 150 min; (a) Zn 2p, (b) Se 3d, (c) S 2p and (d) Eu 3d.
Table Captions
18
Table 1. The detailed Structure information of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) systems measured by XRD and TEM analysis.
Peak Position
Lattice Structure
Parameter
Spacing
(Å)
ZnSe ZnSe:Eu (core) ZnSe:Eu/ZnS (core/shell) ZnS
Interplanar
(111)
Size (nm)
(Å)
Cubic
5.66
27.22°
3.27
-
Cubic
5.59
27.66°
3.22
3.19
Cubic
5.42
28.60°
3.13
3.44
Cubic
5.32
29.06°
3.07
-
19
S0169-4332(17)32618-1 http://dx.doi.org/10.1016/j.apsusc.2017.09.018 APSUSC 37097
To appear in:
APSUSC
Received date: Revised date: Accepted date:
31-3-2017 28-8-2017 4-9-2017
Please cite this article as: Ji Young Park, Chan Gi Lee, Han Wook Seo, Da-Woon Jeong, Min Young Kim, Woo-Byoung Kim, Bum Sung Kim, Structural and Optical Properties of ZnSe:Eu/ZnS Quantum Dots depending on Interfacial Residual Europium, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.09.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and Optical Properties of ZnSe:Eu/ZnS Quantum Dots depending on Interfacial Residual Europium Ji Young Parka, Chan Gi Leea, Han Wook Seob, Da-Woon Jeongb, Min Young Kimb, Woo-Byoung Kimc, Bum Sung Kimb,* a
Department of Advanced Materials & Processing Center, Institute for Advanced Engineering, Yongin 449-863, Republic of Korea b
Department of Korea Institute for Rare Metals, Korea Institute of Industrial Technology, Incheon 406-840, Republic of Korea
c
Department of Energy Engineering, Dankook University, Cheonan 31116, Republic of Korea
Corresponding author: Bum Sung Kim (email: [email protected])
Highlights
ZnSe:Eu/ZnS core/shell quantum dots (QDs) prepared in situ for the first time.
Luminescence intensity of core/shell QDs three times that of uncoated core QDs.
The core/shell system showed no Eu2+ emission but only the sharp peaks in the red at 579, 592, 615, 651, and 700 nm due to the electronic transitions of 5D0→7Fn (n = 0, 1, 2, 3, 4) depending on leisurely decreased with increased reaction time..
The core and core/shell QDs both have a zinc blende structure, and their respective sizes were about 3.19 and 3.44 nm. 1
The core/shell structure may contain Eu2O3 bonding the over-coated ZnS surface on the Eu3+doped ZnSe core.
Abstract A multimodal emitter comprising of ZnSe:Eu/ZnS (core/shell) quantum dots (QDs) by adding a ZnS precursor in situ during synthesis. ZnSe/Eu2+/Eu3+/ZnS actives both core and core/shell. QDs prepared with the ZnS precursor displayed a luminescence intensity three times that of ZnSe QDs due to the passivation effect of the Shell. While the core QDs display the 450–550 nm emission of Eu2+ (4F65D1→4F7), the core/shell system showed no Eu2+ emission but only the sharp peaks in the red at 579, 592, 615, 651, and 700 nm due to the electronic transitions of 5D0→7Fn (n = 0, 1, 2, 3, 4) depending on leisurely decreased with increased reaction time. These results are in agreement with Eu 3d spectra of XPS analysis results. Microscopic analyses show that the core and core/shell QDs both have a zinc blende structure, and their respective sizes were about 3.19 and 3.44 nm. The lattice constant in the central portion of the core/shell QDs are around d111 = 3.13Å , which is between the outside and inside ring patterns (d111 = 3.27 and 3.07 Å , respectively). This shows the effective over-capping of shell onto the core QDs. The core/shell structure may contain Eu2O3 bonding the over-coated ZnS surface on the Eu3+-doped ZnSe core. Keywords: Quantum dots, Multimodal Emitter, Core/Shell, Passivation, Europium, Doping
1. Introduction Colloidal semiconductor nanocrystals or quantum dots (QDs) with size-tunable bandgaps are applicable in a number of technologies, such as optoelectronic devices like light-emitting diodes (LEDs), photovoltaic devices, and as fluorescent tags for biological molecules [1-5]. The Cd-free QDs due to environmental concerns recently has spurred further explosion of investigation on the synthesis of non-toxic alternatives 2
such as CuInS2, InP, and ZnSe QDs which are known to exhibit tunable emission spectra from blue to red without having intrinsic toxicity compared to the Cd-based QDs. In particular, ZnSe based Cd-free QDs are of strong interest due to their wide band gap characteristic, i.e., 2.7 eV, that is proven to be essential to further improve fluorescence efficiency as well as stability of QDs [6-7]. Besides, type-I core/shell QDs basically have been demonstrated to exhibit a superior stability against oxidation as well as photodegradation, and hence enhanced quantum yields because all charge carriers are most likely confined in the core material, that would give rise to radiative recombination [8-10]. The QDs prepared by such overcoating methods enclose interfaces of lattice-mismatched core and shell regions with misfit strain. The core/shell and QD/surfactant interfaces are kinetically stable during synthesis. Rare earth (RE) materials possessing special 4f are recognized as excellent candidates for luminescence centers of the doped II-VI nanocrystals due to their many optical advantages, such as sharp fluorescent emissions via intra of 4f-5d transition, large stokes shift, no photobleaching and long luminescent lifetime and unique luminescent etc [11-12]. However, only a few studies have investigated the multimodal luminescence of ZnSe QDs with Eu and ZnS. In addition, other alternatives could be inorganic luminescent materials with good thermal stability, such as QDs with RE materials, but it is very difficult to in-situ synthesize multimodal emitters in ZnSe:Eu/ZnS (core/shell) QDs. In this regard as mentioned above, we report a successful synthesis of ZnSe:Eu/ZnS (core/shell) QDs through a simple in-situ method that has yet been reported in literatures, where the individual core ZnSe QD is revealed to be doped with Eu ions. The observed luminescence properties of those core/shell QDs exhibiting multimodal luminescence characteristics due to the presence of Eu ions are presented with respect to different synthesis conditions, i.e., reaction time. The corresponding structural interpretations were performed based on the data taken from TEM, XRD and XPS.
2. Experimental Procedure
3
2.1. Chemicals Precursors were prepared using Zinc stearate ((CH3(CH2)16COO)2Zn, technical grade), 1-octadecene (ODE, CH3(CH2)15CH=CH2, 95%), Se powder (Se, 99.999%), octadecylamine (ODA, CH3(CH2)17NH2, 90%), tributylphosphine (TBP, (CH3(CH2)3)3P, 97%), europium acetylacetonate hydrate (EAH, Eu(C5H7O2)3H2O,
99.999%)
zinc
diethyldithiocarbamate
(ZDC,
(C2H5)2NCS2)2Zn,
97%),
trioctylphosphine (TOP, (CH3(CH2)7)3P, 97%), oleyamine (OA, CH3(CH2)7CH=CH(CH2)7CH2NH2, 70%) ); all were used as purchased from Sigma-Aldrich without further purification. 2.2. Preparation of precursor solutions Core QDs were prepared via heating up method modified from previous our work [13]. Zinc stearate and EAH were used as the Zn and Eu precursors. Equimolar amounts (0.01 M) of these precursors were dissolved in 50 ml of ODE, respectively. Each mixture was heated to 150°C for 20 min under Ar atmosphere and then cooled to room temperature. 0.06 M of Selenium powder used as the Se precursor was dissolved in 7 ml of TBP along with 0.02 M of ODA at room temperature. To make a Shell precursor, a mixture consisting of ZDC (0.005 M), OA (8 ml), ODE (42 ml) and TOP (10 ml) was prepared and then subjected to two heating step such that the mixture was heated to 80°C for 15 min, followed by 120°C for 75 min under Ar atmosphere. 2.3. In-situ synthesis of ZnSe:Eu/ZnS colloidal quantum dots For synthesis of the ZnSe:Eu (core) a three-neck flask containing all precursor solutions for Zn, Eu and Se was heated from room temperature to 260°C under Ar flow and left stirring for 5 min. This mixture was then cooled to 150°C at which the Shell precursor for ZnS was added. When the desired temperature was reached, the ZnS Shell precursor was slowly injected into the vigorously stirring reaction mixture of ZnSe:Eu (core) over a period of 10 min. Note that the actual amount of ZnS injected into the mixture of 4
ZnSe:Eu was 50 ml which was the same as that of ZnSe:Eu. After completing the addition, the mixture was left stirring for 180 min and then cooled down to room temperature. Synthesized ZnSe:Eu/ZnS QDs were then purified through a typical precipitation-redispersion method three times involving organic solvents such as anhydrous ethanol for precipitation and hexane for redispersion, where excess amount of ethanol added into QDs centrifuged with 6000 rpm for 10 min at 15°C. 2.4. Characterization The optical properties of the ZnSe:Eu/ZnS QDs were investigated using a double beam UV–vis spectrometer (Optizen 3220UV MECASYS). Photoluminescence (PL) spectra were taken on a Maya 2000 Pro spectrometer (Ocean Optics, Dunedin, FL). The crystalline phases were analyzed by X-ray diffraction (XRD) using a Siemens D500 X-ray diffractometer, using Cu Κα radiation (λ = 1.5418 Å) in the 2𝜃 = 20-80° range. Transmission electron microscopy (TEM) measurements were performed in a Tecnai-TF20 FEI-TEM operating at 200 kV to study the particle size, morphology and crystallinity of individual QDs. X-ray photoelectron spectroscopy (XPS) was measured on Thermo Scientific Theta Probe XPS with a base pressure of 4.8 × 10-9 mbar (ultrahigh vacuum), using a monochromatic Al Kα X-ray source (hν = 1486.6 eV). The emitted photoelectrons were detected through a 180° double focusing hemispherical analyzer with a 128-channel detector relative to the sample surface. All of the binding energies were determined with a C 1s core level peak at 284.8 eV, where the high-resolution spectra and the survey spectra were acquired at different resolutions of 0.1 eV and 1 eV, respectively, on purpose. The comparison of Gibbs free energy change at various temperatures for the possible reactions in the mixture of precursors was also considered via computer software calculation (HSC Chemistry Version 8).
3. Results and Discussion
5
To investigate the favored compounds produced in the in-situ reaction system, the standard Gibbs free energy change (∆G) was calculated for the formation of ZnSe, ZnS, EuSe, and EuS (Fig. 1). The enthalpy and entropy values were obtained in databases from HSC chemistry simulation. The ∆G value is defined by the following equation [14]: △ G =△ H − T △ S
(1)
And the calculated △G values at 260 ℃ and 150 ℃ are as follows: Zn2+ + Se2− → ZnSe (∆G°260℃ = −45.05 kcal/mol, ∆G°150℃ = −42.14 kcal/mol)
(2)
Zn2+ + S 2− → ZnS (∆G°260℃ = −45.41 kcal/mol, ∆G°150℃ = −37.96 kcal/mol)
(3)
Eu2+ + Se2− → EuSe (∆G°260℃ = −19.08 kcal/mol, ∆G°150℃ = −17.24 kcal/mol
(4)
Eu2+ + S 2− → EuS (∆G°260℃ = −0.92 kcal/mol, ∆G°150℃ = 4.34 kcal/mol)
(5)
ZnSe has the most negative value of ∆G°260℃ = −45.05 kcal/mol, therefore it is preferentially formed by spontaneous reaction into the core structure during the first step of synthesis. The △G values at 150 ℃ indicates that in the second step, a shell of ZnS would be formed when the ZnS precursor was injected into the core QDs suspension. Thus, the core/shell phase should be preferentially formed based on the thermodynamic data. Fig. 2 shows the UV-vis absorption and PL spectra of the core and core/shell QDs. The inset Figure in Fig. 2(d) shows PL spectra with respect to different reaction time. In Fig. 2(a), the absorption peak of core QDs is at 400 nm, which is shifted to shorter wavelength compared to that of bulk ZnSe (460 nm) due to the quantum confinement effect. The absorption peak of core/shell QDs prepared with a reaction time of 180 min was found at 425 nm, exhibiting red shift compared to that of the core. The particle growth of core on the red shift was investigated based on the quantum confinement effect, by increasing the reaction time for forming the ZnS shell. In Fig. 2(b), after over-coating with the shell for 150 min, the PL intensity of 6
ZnSe in blue region is nearly tripled compared to that of the core QDs. It demonstrates the successful ZnS shell passivation effect, which enhances the emission due to reduced emission from surface traps, such as vacancies and local lattice mismatches on the surface. Because of the potential barrier, the core/shell system also improves the charge confinement in the core. After the shell precursor was injected into the core QDs colloid, the core grew during the subsequent reaction. Therefore, the core/shell QDs exhibited red shift in the emission spectrum [15]. Fig. 2(c) shows the PL emission of core ZnSe:Eu QDs, and there was clearly a broad spectrum corresponding to the 4F65D1→4F7 transition from 450 to 550 nm. The spectrum was monitored at 500 nm, because Eu2+ has a complicated set of energy levels here that involve seven 4felectrons [16-17]. The Eu2+-related emission peak (450 to 550 nm) PL intensity of core/shell Eu2+ related emission peak (450 to 550 nm) disappeared with increased reaction time. Fig. 2(d) shows the well-known characteristic red emission peaks of Eu3+ at 579, 592, 615, 651, and 700 nm (under 365 nm excitation), due to the electronic transition of 5D0→7Fn (n = 0, 1, 2, 3, 4) [18-19]. The Eu3+-related emission peak PL intensity of core/shell leisurely decreased with increased reaction time. Fig. 3 shows the XRD patterns of core QDs synthesized over 5 min and core/shell QDs with 150 min of over-coating reaction time (which showed the strongest luminescence). For the core QDs, the diffraction peaks are located at around 27.65°, 45.67°, and 53.77° corresponding to the (111), (220), and (311) planes, respectively. These peaks are shifted towards higher angles compared to those for pristine ZnSe (JCPDS No. 37-1463), because the ionic radius of doped Eu3+ (0.95 Å) is larger than that of Zn2+ (0.74 Å) [20-22]. The doping of Eu2+ (1.17 Å) into the ZnSe QDs is unlikely, due to the lattice mismatch. Further, there was no evidence of independent Eu-based phases such as Eu, EuOx, and EuSe. The Eu3+ ions doped into the ZnSe matrix reduce the unit cell volume of the ZnSe:Eu3+ nanocrystals, and the resulting compression stress shifts the XRD peaks to higher angles. Hence, these shifts indicate the successful incorporation of Eu3+ into the ZnSe crystal lattice, in good agreement with the previous report [13]. With the shell over-coated on Core QDs, the diffraction peaks shifted to even higher angles (28.6°, 47.71°, and 56.56°), which are 7
between the cubic ZnSe and ZnS (JCPDS No. 79-43) phases. These as-prepared core/shell QDs display an obvious shift in the XRD patterns, from that of cubic ZnSe to very close to ZnS. This shift shows the effective capping of ZnS onto the core QDs. After forming the ZnS shell over the core QDs, lattice strain became apparent due to the lattice mismatch between the core and shell. Because the lattice parameter of ZnS (5.32 Å) is smaller than that of ZnSe:Eu3+ (5.59 Å), epitaxial growth of ZnS shell strongly compresses the underlying ZnSe QDs [23-24]. The resulting nanosized crystalline domains caused the broadening of the core smaller than core/shell QDs peaks. Fig. 4 shows the TEM images and selective area electron diffraction (SAED) patterns for both core and core/shell QDs. According to Fig. 4(a), the core QDs are spherical with an average size of 3.19 nm. The interplanar spacing of their (111) plane was 3.22 Å, which is smaller than that of pristine ZnSe QDs (3.27 Å). These results show that it is highly possible that Eu3+-doped ZnSe exhibit a zinc blende structure. In Fig. 4(b), the core/shell QD are also spherical with an average size of 3.44 nm. At their central portion, the lattice constant is around d111 = 3.13 Å , which is between the outside ring pattern (d111 = 3.27 Å ) and inside ring pattern (d111 = 3.07 Å ). This shows the effective over-capping of ZnS onto the ZnSe core QDs. The detailed structures and crystallite sizes of the QDs measured by XRD and TEM are summarized in Table 1. Fig. 5 shows the overall XPS spectra of core (synthesized for 5 min) and core/shell QDs (coated for 150 min). The core levels of the survey spectra can be assigned to Zn 2p, Se 3d, S 2p, and Eu 3d. Fig. 5(a) shows identical peaks between the two QDs samples at 1045.2 and 1021.9 eV, which were assigned to the Zn 2p1/2 and Zn 2p3/2 core levels of ZnSe QDs, respectively. In Fig. 5(b), the peaks at 54.1 eV binding energy are due to the splitting of the Se 3d level in ZnSe. These results are in good agreement with the reported value for ZnSe QDs [25]. In comparison with the core, in the core/shell QDs the Zn 2p peak is stronger, while the Se 3d peak is slightly weaker. Fig. 5(c) fails to show the S binding energy for the core, however the S 2p3/2 (161.8 eV) and 2p1/2 (163 eV) binding energies for the core/shell QDs are clearly assigned to ZnS on the core surface [8-9]. These behaviors indicate the successful over-coating of the shell 8
on the core interface, creating a passivation effect. The four peaks for core QDs in Fig. 5(d) correspond to Eu3+3d3/2 (1164.8 eV) and Eu3+3d5/2 (1135 eV) with spin-orbit splitting, Eu2+3d3/2 (1154.7 eV), and Eu2+3d5/2 (1125.9 eV) [26-27]. In the core/shell QDs, the Eu2+ peak decreased. The reason is that in the uncoated core, Eu3+-doped ZnSe has Eu2O3 and EuO bonded to the surface by dangling bonds, whereas in the core/shell QDs there should be increasing Eu2O3 bonding to the shell on the Eu3+-doped ZnSe core. The core of both Eu3+3d5/2 (1135 eV) and Eu2+3d5/2 (1125.9 eV) region are 54.09%, while the passivated core/shell QDs are confirmed to be 45.91% in Eu3+3d5/2 (1135 eV) region compared to core. This means that the increase of Eu2O3 bond due to the out diffusion of surface oxygen of QDs. These results are in good agreement with PL spectra. Inside-out diffusion phenomena depend on the penetration capability of the core metal ions into the ZnS lattice followed by the diffusivity of metal ions within the ZnS lattice and the ease of cation exchange. Penetration capability, in turn depends on the interfacial stability and the bond dissociation energies of the core molecules. The phenomena perhaps over-coating and doping, stress created due to the crystal strain due to lattice mismatch from core-shell and doping interface help to break the bonds inside the core and then the metal ions diffuse inside the semiconducting matrix as determined by its diffusivity [28]. In addition, the reason that the Eu2+3d5/2 (1125.9eV) decreased than Eu2+3d3/2 (1154.7eV) is the photoemission of Eu-containing compounds, a satellite will accompany with the major band due to a charge transfer coexcitation both for Eu2+ and Eu3+. It was interpreted as shake-up for Eu2+. The shake-up satellite was characterized by a decrease of the 4f occupation number, i.e. a charge transfers of 4f75d0→4f65d1 for Eu2+, while the shake-down satellite was due to an increase of 4f occupation, i.e. and surface-induced electron transfer from to Eu3+(4f65d0→4f75d0) [29].
5. Conclusions In this study, we successfully synthesized a multimodal emitter comprising of ZnSe:Eu/ZnS (core/shell) QDs. After the ZnS precursor was injected onto the core, the luminescence intensity in the ZnSe QD 9
emission region was tripled in the resulting ZnSe/Eu2+/Eu3+/ZnS QDs due to the passivation effect. These results show the shell was successfully over-coated on the core surface. The spectrum of the core displayed the 450–550 nm emission of Eu2+ owing to the 4F65D1→4F7 transition, while the Eu2+ emission disappeared for the core/shell system. These results were further confirmed by the XPS analysis. Meanwhile, the Eu3+related emission peak PL intensity of core/shell leisurely decreased with increased reaction time. Further, the XRD and TEM images and SAED patterns show that the core and core/shell QDs have a zinc blende structure, with an average size of about 3.19 and 3.44 nm respectively. The core/shell structure may contain Eu2O3 bonding the over-coated ZnS surface on the Eu3+-doped ZnSe core.
Acknowledgement The authors acknowledge financial support from the Regional Industry Nurturing Program, “Development of high sensitive and functional organic/inorganic hybrid plastic composite for LED (Project No. A010400068),” funded by the Ministry of Trade Industry & Energy (MOTIE).
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Figure captions Fig. 1 Calculated Gibbs free energy (∆G) for four reactions as a function of temperature. Fig. 2 (a) UV-vis absorption and (b-d) PL spectra of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) QDs. 13
Fig. 3 XRD patterns of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) synthesized for 5min and 150 min, respectively. Fig. 4 TEM images of (a) ZnSe:Eu (core) and (b) ZnSe:Eu/ZnS (core/shell). The corresponding selective area electron diffraction (SAED) pattern of core/shell is included in Fig. 4(b). Fig. 5 XPS survey spectra of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) for 150 min; (a) Zn 2p, (b) Se 3d, (c) S 2p and (d) Eu 3d.
Fig. 1 Calculated Gibbs free energy (∆G) for four reactions as a function of temperature. 14
Fig. 2 (a) UV-vis absorption and (b-d) PL spectra of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) QDs.
15
Fig. 3 XRD patterns of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) synthesized for 5min and 150 min, respectively.
16
Fig. 4 TEM images of (a) ZnSe:Eu (core) and (b) ZnSe:Eu/ZnS (core/shell). The corresponding selective area electron diffraction (SAED) pattern of core/shell is included in Fig. 4(b).
17
Fig. 5 XPS survey spectra of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) for 150 min; (a) Zn 2p, (b) Se 3d, (c) S 2p and (d) Eu 3d.
Table Captions
18
Table 1. The detailed Structure information of ZnSe:Eu (core) and ZnSe:Eu/ZnS (core/shell) systems measured by XRD and TEM analysis.
Peak Position
Lattice Structure
Parameter
Spacing
(Å)
ZnSe ZnSe:Eu (core) ZnSe:Eu/ZnS (core/shell) ZnS
Interplanar
(111)
Size (nm)
(Å)
Cubic
5.66
27.22°
3.27
-
Cubic
5.59
27.66°
3.22
3.19
Cubic
5.42
28.60°
3.13
3.44
Cubic
5.32
29.06°
3.07
-
19