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shell quantum dots
Superlattices and Microstructures 111 (2017) 156e165
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Composition and strain effects in Type I and Type II heterostructure ZnSe/Cd(Zn)S and ZnSe/Cd1-xZnxS core/shell quantum dots Negar Gheshlaghi a, Hadi Sedaghat Pisheh a, Hilmi Ünlü a, b, * _ Nanoscience and Nanoengineering Programme, Institute of Science and Technology, Istanbul Technical University, Maslak Istanbul, 34469, Turkey b _ Department of Physics, Faculty of Science and Letters, Istanbul Technical University, Maslak Istanbul, 34469, Turkey a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 February 2017 Received in revised form 6 June 2017 Accepted 7 June 2017 Available online 14 June 2017
We investigated the effect of ternary shell alloy composition on the bandgap and diameter of core of ZnSe=Cd1x Znx S heterostructure core/shell quantum dots, which were synthesized by using a simple colloidal technique. Characterization by using the x-ray diffraction (XRD), transmission electron microscopy (TEM), UVeVis absorption and fluorescence emission spectroscopic techniques indicate that (i) there is a transition of ZnSe=Cd0:6 Zn0:4 S Type-I heterostructure (electrons and holes tend to localize in core) to ZnSe=Cd0:75 Zn0:25 S quasi-Type-II heterostructures (holes tend to localized in the core and electrons are delocalized) and (ii) then after large red shift and Stock-shift in PL emission spectra but not a distinct absorption peak in UV spectra become noticeable in ZnSe/Cd0:75 Zn0:25 S quasiType II and ZnSe/CdS Type II heterostructures (electrons are localized in core and holes are localized in shell). Furthermore, the increase of Cd:S ratio in shell alloy composition shifts the XRD peaks to lower 2q degrees which corresponds to tensile strain in the ZnSe core. Finally, the hydrostatic interfacial strain has effect on the squeezing or stretching the capped core: A decrease of compressive force on core from ZnSe/ZnS to tensile force in ZnSe/CdS with increase in Cd:S ratio indicates that transition of compressive strain to tensile strain takes place with the transition from Type-I to II heterostructure. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Binary/ternary core/shell quantum dots Type-I to Type-II heterostructures Alloy composition Lattice mismatch strain Bandgap Diameter
1. Introduction During the past two decades, an abundance of semiconductor nanocrystals, also called quantum dots (QDs), have been studied because of their novel tunable emission properties and their potential applications in optoelectronic devices, with size-tunable light emission and high photo-stability [1]. Bandgap engineering achieved by changing the constituent stoichiometry and composition of alloy semiconductors is a powerful technique for the development of nanomaterials with many new properties [2]. It is now possible to tune the bandgap rendering narrow band emission in the visible range for QDs by controlling their sizes or deposition of a wide bandgap semiconductor shell onto a narrow bandgap semiconductor core [3]. Capping the core semiconductor with a ternary semiconductor shell can produce an extra degree of freedom for tailoring the electronic and optical properties of core/shell quantum dots. Among the group II-VI based binary/ternary core/shell
_ _ * Corresponding author. Department of Physics, Faculty of Science and Letters, Istanbul Technical University, Maslak-Istanbul, 34469, Turkey. E-mail address: [email protected] (H. Ünlü). http://dx.doi.org/10.1016/j.spmi.2017.06.026 0749-6036/© 2017 Elsevier Ltd. All rights reserved.
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nanocrystals, ZnSe=Cd1x Znx S heterostructure core/shell QDs have received great attention because of their compositiontunable emission across the visible spectrum and their highly luminescent characteristics [4]. These core/shell nanocrystals display unique composition-dependent properties distinct from those of their bulk counterpart and from those of binary CdS and ZnS shell NCs. The band gap of Cd1x Znx S can be adjusted between 2.4 eV for CdS and 3.7 eV for ZnS [4] depending on the relative Zn/Cd composition ratio and result in Type I ZnSe/ZnS and Type II ZnSe/CdS heterostructures, respectively [5] When the potential energy gradient at the heterointerface leads to a local separation of hole and electron in core and shell region but holes are still delocalized over the entire volume of the core/shell nanocrystal, there is a transition region between Type I and Type II and is called quasi- Type II heterostructure [5]. The schematic equilibrium energy band diagrams of Type I and Type II heterostructures are shown in Fig. 1. Here ECA (ECB) and EVA (EVB) represent the conduction band minimum and valence maximum. In Type I heterostructure (Fig. 1a), valence and conduction bands of core and shell are aligned in a way that an electron-hole pair excited near the heterointerface tend to localize in the core region. Therefore, the exciton energy in Type-I core/shell QDs is the result of direct exciton transition inside the core. However, in Type II heterostructure (Fig. 1b and c), the bandgap of the shell partially overlaps (staggered lineup) that of the core. The conduction band edge of the shell is located in the bandgap of the core leading to a local separation of the hole and electron in core and shell structure. Holes (electrons) are confined to the core (shell) and electron (holes) are confined to the shell (core), which are result of indirect exciton transition. The corresponding locally indirect band gap is equal to Egid ¼ EgA DEc in hole-electron confinement (Fig. 1b) and Egid ¼ EgA DEv in electron-hole confinement (Fig. 1c). DEC and DEV are
the band offsets in the conduction and valence band energies (DEc ¼ DEg DEv at Type-I heterointerface and DEc ¼ DEg þ DEv at Type-II heterointerface).DEg ¼ EgB EgA is the difference between bandgaps shell and core. The purpose of this work is to carry out an experimental and theoretical investigation about the effect of ternary shell composition and lattice mismatch interface strain on the exciton energy and diameter of the core of ZnSe=Cd1x Znx S (0 x 1) core/shell quantum dots, which are synthesized by using a simple colloidal technique discussed in section 2. The results of structural characterization carried out by using the x-ray diffraction (XRD) and transmission electron microscopy (TEM) will be discussed in section 3. Optical absorption and emission characterization was carried out by using UVeVis and Fluorescence spectrometers and will be discussed in section 4. The qualitative analysis of the effects of lattice mismatch strain and ternary shell composition on the band gap and diameter of ZnSe core of ZnSe=Cd1x Znx S core/shell quantum dots are discussed in section 5 by using a conventional two level effective mass approximation and results will be compared with experimental findings. 2. Colloidal synthesis of ZnSe=Cd1x Znx S quantum dots 2.1. Chemicals Zinc oxide (ZnO, 99.99% powder), Cadmium oxide (CdO, 99.5%), selenium (Se, 99.99%, powder), Sulfur powder (S, 99.5%), 1octadecene (ODE, 90%), octadecylamine (ODA, 99%), oleic acid (OA, 90%), Cadmium oxide (CdO, 99.5%), Tributylphosphine (TBP), trioctylphosphine (TOP, 90%), oleylamine (technical grade, 70%), zinc stearate (purum, 10e12% zinc basis), trioctylphosphine oxide (TOPO, 90%), tetradecylphosphonic acid (TDPA, 97%) and liquid Paraffin, were purchased from Aldrich. Stearic acid (analytical grade), Zinc acetate dehydrate (Zn(OAc)2.2H2O, analytical grade), were obtained from Sinopharm chemical. Methanol (analytical reagent), n-Hexane (analytical reagent) and Acetone (analytical grade) were also purchased from Merck Chemicals. They were used without any refinement in our synthesis. 2.2. Synthesis of ZnSe=Cd1x Znx S(0 x 1) binary core/ternary shell QDs The synthesis of bare ZnSe core and ZnSe/ZnS and ZnSe/CdS core/shell QDs was reported in our previous work [6]. The synthesis of ZnSe=Cd1x Znx S(0 x 1) core/ternary shell QDs is carried out by following the method of Veena et al. [7]. In monitoring the shell growth, aliquots of core/binary (ternary) shell were taken in specific time interval. Samples were washed
(a)
(b)
(c)
Fig. 1. Schematic equilibrium energy band diagram of Type I heterostructure (a): electrons and holes tend to localize in quantum well and of Type II heterostructure (b) and (c): electrons and holes are localized on different sides of heterointerface.
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with acetone and methanol in multiple sequences and decantation of the solution waste were carried with centrifuging the washed QDs. The samples were dispersed in n-Hexane and then characterization analysis was carried out. 3. Structural characterization of ZnSe=Cd1x Znx S core/shell QDs 3.1. XRD characterization X-ray diffraction spectroscopy (XRD) patterns were carried out by using a X’Pert3 MRD (XL) X-ray diffractometer operating at 45 kV/40 mA using Copper Ka line (l ¼ 1.5406 Å) on a powder diffractometer (2q step 0.1 and counting time 2 s/step) at room temperature. Fig. 2 shows the XRD pattern of ZnSe and ZnSe/Cd1x Znx S QDs for x ¼ 0.25, 0.40, 0.55, 0.70 and 0.85 alloy composition. There are broad peaks at 2q values of 27 related to 〈111〉, 45.6 to 〈220〉 and 52.8 to 〈311〉 crystalline planes for synthesized zinc-blende ZnSe (JCPDS No. 80-0021) along with ZnO diffraction peaks (JCPDS No. 36-1451). As it can be seen from Fig. 2, the ZnO peaks were faded after coating process. The synthesized NCs XRD patterns are in cubic crystal phase. The growth of compressive binary shell on core (ZnSe/ZnS), result in squeeze of crystalline spacing planes in the core and slight shift of bare core XRD pattern toward larger degrees (ZnS JCPDS No. 80-0020) [8]. It is noticed that the diffraction peak shifts toward the smaller angles when the Cd-S concentration ratio increases in ternary shell and reaches to ZnSe/CdS(CdS JCPDS 75-0581). 3.2. TEM characterization JEM-ARM200F electron microscope operated at an acceleration voltage of 200 kV, cold FEG emitter and 0.27eV energy spread was used for the transmission electron microscopy (TEM) analysis. QDs were diluted in n-Hexane, dropped over an amorphous carbon substrate supported on a copper grid of 400 meshes for taking images. Fig. 3a, b and 3c show the TEM images of the synthesized pre-capped ZnSe core and ZnSe/ZnS and ZnSe/CdS core/shell nanocrystals. TEM image in Fig. 3a confirms that the size distribution of bare ZnSe core NCs is nearly monodisperse; it has an average diameter of about 3.44 nm with high shape uniformity (spherical). From the TEM images shown in Fig. 3b and c we estimate that the size of ZnSe/ZnS and ZnSe/CdS core/shell QDs are about 3.85 and 3.90 nm, respectively. In many reports, shell thickness in core/shell QDs is obtained via subtracting core/shell crystal size from bare core nanocrystal size [9]- [10]. Since TEM images are illustrating the initial core and final core/shell size, ignoring the strain effect on core size after capping the shell would limit the accuracy of predicting the shell thickness [10]. Precise estimation of shell thickness and capped core size can be pursued through strain effect consideration on capped core diameter [11]. 4. Optical characterization of ZnSe=Cd1x Znx S core/shell QDs
CdS
(311)
(220)
(111)
The optical absorption spectra of synthesized NCs was measured using the Schimadzu UVeVis NIR Spectrometer at room temperature (300 K). The fluorescence emission spectra was measured by using Varian Cary eclipse fluorescence spectrometer (samples were excited with UV-light of l ¼ 350 nm) with an intense Xenon flash lamp (150 W) at 300 K. These samples were then diluted with n-hexane immediately for characterization. Then the sample solution was loaded in a quartz
ZnSe/CdS ZnSe/Cd0.75Zn0.25S ZnSe/Cd0.60Zn0.40S ZnSe/Cd0.45Zn0.55S ZnSe/Cd0.30Zn0.70S ZnSe/Cd0.15Zn0.85S
30
(311)
(220)
(111)
ZnSe/ZnS ZnSe 40
50
ZnS
60
70
80
90
2 Fig. 2. XRD pattens of ZnSe, ZnSe/Cd(Zn)S and ZnSe/Cdx Zn1x S core/shell nanostructures for x ¼ 0.25, 0.40, 0.55, 0.70 and 0.85 alloy composition.
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(a)
159
©
(b)
Fig. 3. TEM image of ZnSe core QD (a) and ZnSe/CdS (b) and ZnSe/ZnS (c) core/shell QDs, respectively.
Absorbance (a.u.)
cuvette and placed in the sample slot of UVeVis NIR and Photoluminescence spectrometers for absorption and emission measurements, respectively. The normalized optical absorption and emission spectra of ZnSe=Cd1x Znx S core/shell nanocrystals with x ¼ 0.25, 0.40, 0.55, 0.70 and 0.85 alloy compositions are shown in Figs. 4 and 5, respectively. The red shift in both optical absorbance and emission spectra is an evidence for the growth of Cd1x Znx S ternary shell over the ZnSe core. The optical absorption and emission spectra of Cd1x Znx S ternary shell on ZnSe core are shifted towards longer wavelengths with increasing alloy composition value towards unity. An interesting finding is the spectral changes are gradual and do not exhibit an abrupt transition as might be expected for a change from Type-I to Type-II heterostructure [12]. However, it should be noted that transition from Type-I to Type-II occurs after the more increase in Cd concentration in the Cd1x Znx S ternary alloy (from ZnSe=Cd0:6 Zn0:4 S (Type-I) to ZnSe=Cd0:75 Zn0:25 S (quasi Type-II heterostructure) and finally to ZnSe/CdS (Type-II heterostructure). Distinct first absorbance peak in the UV absorption spectra and relatively slight red shift in the PL emission spectra is the evidence for direct transition of excited electron from valance to conduction band of core (Type-I heterostructure). This behavior starts from ZnSe/ZnS and continues upto ZnSe/Cd0:6 Zn0:4 S. However, after the more increase of Cd concentration in the Cd1x Znx S ternary shell, there is a less distinctive first absorption peak in UV absorption spectra and relatively higher red shift and Stokes shift in the PL emission spectra becomes noticeable in ZnSe/Cd0:75 Zn0:25 S (quasi-Type II heterostructures) and finally ZnSe/CdS (Type II heterostructures). In the Type-II heterostructures, shell growth results in a significant red shift of the emission wavelength of the NCs [10]. Non distinctive first absorption peak and featureless absorption tail is the indication of an onset of an indirect excitons transition in Type-II heterostructure [13,14]. It is well known that the significantly high Stokes shifts in PL emission spectra of Type-II heterostructures NCs are due to the removal of surface passivating ligands in ZnSe core
ZnSe/CdS ZnSe/Cd75Zn25S ZnSe/Cd60Zn40S ZnSe/Cd45Zn55S ZnSe/Cd30Zn70S ZnSe/Cd15Zn85S ZnSe/ZnS ZnSe 300
350
400
450
500
550
600
650
700
Wavelength (nm) Fig. 4. Normalized UV optical absorption spectra of bare ZnSe core QD and ZnSe=Cd1x Znx S binary/ternary core/shell QDs.
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Fig. 5. Normalized PL emission spectra of bare ZnSe core QD and ZnSe/Cd1-xZnxS binary/ternary core/shell QDs.
[6] or the large spectral separation between the emission peak and the lowest energy absorption edge in Type-II heterostructure NCs [15]. In Type I heterostructure slight red-shift in the absorption and PL peaks are due to an increase in thickness of the deposited shell and leakage of the excitons from core into the shell. Although the band gap of the shell is larger than that of the core in Type-I, the excited electrons and holes in the core could also partly spread into the shell easily, leading to an inevitable leakage of excitons [16e19]. The “charge-separated” state in these nanostructures corresponds to the situation for which the electron is confined to the shell region while the hole is still delocalized over the entire volume of the hetero-NCs, which leads to reduced but nonzero overlap between the electron and the hole wave functions. Here, we refer to this localization regime as quasi Type-II [5], Because of an indirect excitons transition in Type-II, the shell growth aims at a significant red shift of the emission wavelength of the NCs. Fig. 6 shows the normalized UV absorption and PL emission spectra of bare ZnSe core and ZnSe/Cd1-xZnxS binary/ternary core/shell NCs at different time intervals. The broadening of absorption peak and featureless absorption tail of the absorption spectra are indication of an onset of an indirect excitons transition in Type-II heterostructures [20,21]. As it can be seen in Fig. 6, there is a small red shift of 26 nm after depositing ZnS shell (Type I) and a significant red shift of 125 nm after CdS shell deposition (Type-II). Gradual red shift of 27 nm is observed when Cd composition is in between 0.15 x 0.60. However, there is abrupt red shift of 49 nm when Cd composition is increased to x ¼ 0.75. This may be the transition point from Type-I to Type-II. In other words, there is a transition from Type-I (ZnSe/Cd0.6Zn0.4S) to quasi Type-II (ZnSe/Cd0.75Zn0.25S) and then to Type-II (ZnS/CdS). Similar broadening effect and clear Stokes shift in UV and PL peaks have also been reported for ZnSe/CdS QDs [20,22]. Significantly high Stokes shift in PL spectra of Type-II heterostructures are due to removal of surface passivating ligands in ZnSe core [6] or the large spectral separation between the emission peak and the lowest absorption edge in Type-II heterostructure NCs [23].
5. Results and discussion The binary/ternary (or ternary/binary) nanoscale heterostructures offer an extra degree of freedom to tuning the structural, electrical and optical properties of core/shell quantum dots by their composition dependent size, band gaps, effective mass of electrons and holes and dielectric constants. Therefore, it is essential to analyze the effect of ternary shell composition on the quantum confinement in binary/ternary core/shell quantum dots in order to predict their potential in nanoscale electronic and optical device applications. The general features of the quantum confinement in both bare ZnSe core and ZnSe=Cd1x Znx S core/shell QDs qualitatively can be understood by using the parabolic two band effective mass approximation (EMA) [24] provided the first exciton energy (i.e., band gap) is obtained from the absorption spectra according to the following expression [25]
Egnc ðdÞ ¼
hc
lmax
;
(1)
where c is speed of light, h is the Planck’s constant and lmax is wavelength at which the absorption of the nanoparticle is maximum. Fig. 7 shows the composition effects on the obtained band gap of ZnSe core of ZnSe=Cd1x Znx S heterostructure core/shell QDs.
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Fig. 6. Normalized UVeVis absorption and PL emission spectra of bare ZnSe core QD and ZnSe/Cd1-xZnxS binary/ternary core/shell NCs at different time intervals.
2,96
Eg(x) = 0.1905x2 - 0.0095x + 2.831
Exciton Energy (eV)
2,94 2,92 2,90 2,88 2,86 2,84 0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
x Fig. 7. Measured bandgap of ZnSe=Cd1x Znx S core/shell QDs as a function of composition.
Fig. 7 shows that the band gap of ZnSe=Cd1x Znx S QD as a function of Cd1x Znx S ternary shell composition, expressed as
Egnc ðd; xÞ ¼ 0:1905x2 0:0095x þ 2:831;
(2)
which shows that core band gap has a parabolic dependence on ternary shell composition and is higher than the band gap of bare ZnSe core. Equation (2) can be used in the following band gap Equation (3), due to conventional parabolic two band effective mass approximation [24], to calculate the core diameter of ZnSe=Cd1x Znx S core/shell QD
Egnc ðdÞ ¼ Egb ðTÞ þ
2Z2 p2 3:572e2 0:124e4 2 ; ε∞ d m*cv d2 Z m*cv ε2∞
(3)
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where Egnc ðd; xÞ is the core band gap given by Equation (2), measured from UV optical absorption spectra. Egb ðTÞ is the bulk value of the core semiconductor at temperature T above 0 K. Second and third terms, respectively, describe the confinement energy (with a 1/d2 dependence) and Coulomb interaction energy (with a 1/d dependence). Finally, last term represents the Rydberg correlation energy and it is independent of the diameter of the quantum dot, usually negligible when the dielectric constant of semiconductor is not small. m*cv ¼ ðm*e m*h =ðm*e þ m*h Þ is the reduced effective mass of electron-hole pair. m*e and m*h are the effective masses of electrons and holes and ε∞ is the optical dielectric constant of bulk semiconductor. Z ¼ h=2p with h representing the Planck constant. Equation (3) gives the size dependence of the band gap of core influenced by ternary shell composition in a Type-I heterostructure core/shell quantum dots. In case of TypeeII heterostructure core/shell quantum dots Equation (3) should be modified to
Egnc ðd; xÞ ¼ Egb ðTÞ DEc þ
2Z2 p2 3:572e2 0:124e4 2 ε∞ d m*cv d2 Z m*cv ε2∞
(4)
to calculate the core diameter for hole-electron confinement (Fig. 1b). A similar expression can be written for electron-hole confinement (Fig. 1c) by replacing DEc with DEv in Equation (4). The qualitative analysis of the effects of ternary shell composition on the core band gap and diameter of ZnSe=Cd1x Znx S core/shell QDs can now be made from Equations (3) and (4) for Type-I and Type II heterostructure core/shell quantum dots, respectively. It is well known that there is large lattice mismatch at the ZnSe=Cd1x Znx S heterointerface because of the difference between the lattice constants of the constituents. Reliable and accurate calculation of the band gap and diameter requires the inclusion of effects of the lattice mismatch strain in addition to ternary alloy composition. The use of conventional continuum elastic theory [26] allows one to write the hydrostatic interface strain in the core region of heterostructure core/shell quantum dot as
εrr ¼ εqq ¼ ε44 ¼ ε ¼ 3Bc εm =ð3Bc þ 4ms Þ;
(5)
where εrr ; εqq and ε44 are the strain components in spherical coordinates.εm is the lattice mismatch strain at core/shell heterointerface: εm ¼ Da=a ¼ 2ðac as ðxÞÞ=ðas ðxÞ þ ac Þxðac as Þ=as [27], where ac and as are the bulk lattice parameters of the core and shell, respectively. Bc and ms are the elastic bulk modulus and shear modulus of core and shell, respectively. Composition effects on lattice constant as ðxÞ, shear modulus ms ðxÞ) and conduction and valence band energy levels of ternary is determined by using the modified virtual crystal approximation [28]: Cd1x Znx S PCd1x Znx S ðxÞ ¼ xPZnS þ ð1 xÞPCdS þ dxð1 xÞðPZnS PCdS Þ, where P is the material parameter and x is the alloy composition and d is a small correction term due to alloy disorder. According to Equation (5), capping the ZnSe core with x ¼ 1, 0.85, 0.7, 0.55 to x ¼ 0.4 of Cd1x Znx S ternary shell composition causes about 3.3%, 2.6%, 1.8%, 1% and 0.13% of compressive strain on the core region from x ¼ 1, 0.85, 0.7, 0.55 upto x ¼ 0.4 of the shell composition. Furthermore, capping ZnSe core with Cd0:75 Zn0:25 S ternary and binary CdS as shell cause about 0.7% and 2% of tensile strain, respectively. The effects of hydrostatic interface strain due to lattice mismatch at heterointerface of core/shell quantum dot can be incorporated in the effective mass approximation [15] to calculate the diameter of the capped core by using the so called statistical thermodynamic model of semiconductors [29]. In this statistical thermodynamic model, the conduction electrons and valence holes are treated as electrically charged chemical particles and conduction and valence band edges are expressed as a function of pressure at any temperature [29]:
Eg ðT; PÞ ¼ Eg ð0Þ þ DCP0 Tð1 ln TÞ
ag P 2 ð1 þ B0ÞP 3 P ; B 2B 3B2
(6)
where Eg ð0Þ is the band gap of bulk semiconductor at 0 K. DCP0 Tð1 ln TÞ is the lattice vibration contribution with temperature increase. The third term represents sum of the volume expansion contribution due to the thermal pressure ðPth ¼ Bc ðDV=VÞcth ¼ 3Bc ac DTÞ at constant external pressure (i.e., lattice mismatch). Here ac is the linear thermal expansion coefficients of core in bulk form and DT ¼ T T0 is the difference between the growth and 0 K, respectively. ag ¼ BðvEg =vPÞ is the bandgap deformation potential at high symmetry point of core in bulk form. B is bulk modulus and B0 ¼ vB=vP. DCP0 is standard state heat capacity of reaction for the formation of electron-hole pair. Substituting Pth ¼ 3Bc ac DT in Equation (6) at constant pressure one finds the temperature effect on the bandgap of core in bulk form as
3 0 Egb ðTÞ ¼ Egb þ dEgb ðTÞ ¼ Egb þ DCiP Tð1 ln TÞ þ 3agcG ac T 1 þ ac T 3a2c ð1 þ B0 ÞT 2 ; 2
(7)
where DCP0 is obtained by fitting the band gap given by Equation (7) to the measured bandgap of bulk semiconductor at constant external pressure. Furthermore, substituting Pstr ¼ Bc ðDV=VÞstr ¼ 3Bc ε in Equation (6) at any temperature T one finds the hydrostatic interface strain effect on the core bandgap of quantum dot as
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9 2
dEgnc ðε; xÞ ¼ 3agcG ε þ agcG ε2 9agcG 1 þ B0c ε3 ;
(8)
where B0c its pressure derivative of Bc and ε is the hydrostatic interface strain defined in Equation (5). With Equations (7) and (8) are taken into account, Equations (3) and (4) can, respectively, be modified to Equations (9) and (10) to include the effects of hydrostatic interface strain and ternary alloy shell composition on the core region band gap in Type-I (with electron-hole confinement in core and Type-II heterostructure (with hole-electron confinement as shown in Fig. 1b) core/shell quantum dot as
Egnc ðε; xÞ ¼ Egbc þ dEgbc ðTÞ þ dEgnc ðε; xÞ þ
2Z2 p2 3:572e2 0:124e4 2 ; ε∞ d m*cv d2 Z m*cv ε2∞
Egnc ðε; xÞ ¼ Egbc þ dEgbc ðTÞ þ dEgnc ðε; xÞ DEc ðε; xÞ þ
(9)
2Z2 p2 3:572e2 0:124e4 2 ε∞ d m*cv d2 Z m*cv ε2∞
(10)
where Egb is bulk direct bandgap of core region at T ¼ 0 K. dEgbc ðTÞ and dEgnc ðε; xÞ are given in Equations (7) and (8), respectively, and other terms are described in Equations (3) and (4). As stated section 1, the ZnSe=Cd1x Znx S binary/ternary colloidal core/shell quantum dot structures may be of Type-I, Type-II and quasi-Type II heterostructures, depending on the Cd1x Znx S ternary alloy composition and the sign of lattice mismatch induced interface strain. In the Type-I heterostructure QDs the exciton energy is the result of direct transitions of exciton pairs inside the core and Equation (9) can be used to calculate the effects of hydrostatic interface strain and ternary shell composition on the core band gap. However, in Type-II heterostructure core/shell QDs, holes are confined to the core and electron are confined to the shell (Fig. 1b) or electrons are confined to the shell and holes are confined to the core (Fig. 1c), resulting in indirect transition from shell conduction (core valence) band to core valance (shell conduction) band. Therefore, in calculating the effects of hydrostatic interface strain and ternary shell composition on the core band gap of Type-II heterostructure with hole-electron confinement (Fig. 1b) Equation (10) should be used to calculate the effects of hydrostatic interface strain and ternary shell composition on the core band gap. On the other hand, in Type-II heterostructure with electron-hole confinement (Fig. 1c) the effects of hydrostatic interface strain and ternary shell composition on the core band gap of core/shell quantum dot is still calculated from Equation (10) but by replacing DEc ðε; xÞ with DEv ðε; xÞ. Using material parameters given in Table 1, Equation (10) is used to calculate the ternary shell composition and hydrostatic interface strain effects on the capped ZnSe core diameter of ZnSe=Cd1x Znx S core/shell quantum dots prepared using colloidal technique in our laboratory. The composition effect on conduction band offset at ZnSe=Cd1x Znx S binary/ternary core/shell interface is obtained by using ð1 xÞDEc as a first order approximation, where DEc is the conduction band offset at binary/ binary heterointerface (i.e., CdSe/ZnS and CdSe/CdS). The results of the calculations are given in Table 2 with strain and without strain (in parenthesis). Fig. 8 shows the interface strain effects on the diameter of the ZnSe=Cd1x Znx S heterostructure core/shell quantum dots at various alloy composition. Comparing the results with initial (bare) core size of ZnSe (z3.44 nm) from TEM measurements we suggest a squeeze of core after compressive shell deposition. Among the core/shell QDs, the most squeezed capped core is in ZnSe/ZnS with highest compressive lattice strain (3.3%). The results show, greater x value
Table 1 Material parameters (*) used in core diameter calculation.
ZnSe ZnS CdS
0
a(Å)
m*e =m+
m*h =m+
B
B
ε∞ =ε+
ms
-EV (eV)
Egb
5.66 5.41 5.82
0.137 0.20 0.14
0.82 1.42 0.68
6.24 7.71 6.16
4 4 4.8
5.9 5.1 5.4
1.75 1.16 1.87
13.02 12.97 12.21
2.82 3.76 2.48
(*) a, m*e , m*h , B(1011 dyn/cm2), B0 , (1011 dyn/cm2), ε∞ , Egb (eV) are taken from Ref. [30] and EV (eV) is taken from Ref. [28].
Table 2 Core diameter of ZnSe=Cd1x Znx S core/shell quantum dot with (without) strain. Composition/ YReaction Time (min) 15 30 45
x¼0
x ¼ 0.75
x ¼ 0.60
x ¼ 0.45
x ¼ 0.30
x ¼ 0.15
x¼1
3.45 (2.85) 3.51 (2.89) 3.58 (2.94)
3.54 (2.94) 3.55 (2.95) 3.57 (2.96)
3.47 (6.53) 3.48 (6.64) 3.51 (6.88)
3.42 (6.23) 3.43 (6.32) 3.46 (6.53)
3.34 (5.81) 3.37 (5.96) 3.41 (6.23)
3.29 (5.40) 3.31 (5.46) 3.34 (5.66)
3.32 (4.58) 3.33 (4.63) 3.34 (4.80)
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ZnSe/CdS Unstrained ZnSe/CdS Strained ZnSe/ZnS Unstrained ZnSe/ZnS Strained Cd.75Zn.25S Strained
Capped Core Diameter (nm)
7.5 6.75
Cd.75Zn.25S Unstrained Cd.6Zn.4S Strained Cd.6Zn.4S Unstrained
6.0
Cd.45Zn.55S Strained Cd.45Zn.55S Unstrained Cd.3Zn.7S Strained Cd.3Zn.7S Unstrained
5.25
Cd.15Zn.85S Strained Cd.15Zn.85S Unstrained
4.50 3.75
ZnSe
3.0
3.44 nm 2.88
2.96
3.04
3.12
3.2
Exciton Energy (eV) Fig. 8. Capped core diameter versus exciton energy in ZnSe/Cd1-xZnxS.
(rich Zn:S ratio) in alloy composition led in more squeezed capped core due to higher lattice strain amount in NC. The core is squeezed about 2.9%, 2.9%, 1%, 0.6%, in x ¼ 1, 0.85, 0.7 and 0.55 and stretched about 2%, 3.2% and 4% when x ¼ 0.4, 0.25 and 0 after the shell deposition, respectively. Comparing the capped ZnSe core diameter with and without strain shows that the interfacial strain effect contribution in capped core size calculations is necessary to have a reliable and precise calculations of core diameter of the quantum dots by using the two band effective mass approximation. The effect of composition and hydrostatic interface strain on the optical properties of the ZnSe=Cd1x Znx S binary/ternary core/shell quantum dots shows that there is a transition of Type-I ZnSe/ZnS heterostructure to Type-II ZnSe/CdS heterostructure after capping ZnSe core with Cd0:60 Zn0:40 S shell. Large red shift and Stockshift in PL and not a distinct absorption peak in absorption spectra become noticeable ZnSe=Cd0:75 Zn0:25 S in and ZnSe/CdS. Furthermore, the increase of Cd:S ratio in composition is shown to shift the XRD peaks to lower 2q degrees correspond to tensile strain on ZnSe core. Finally, the effect of alloy composition dependent interfacial strain on squeezing or stretching the capped core is investigated. Results show a decrease of compressive force on core from ZnSe/ZnS to tensile force in ZnSe/CdS with increase in Cd:S ratio with transition from Type-I to II heterostructure. We should also note that in the analysis of our data we used the simple parabolic two band effective mass approximation, which is useful only for obtaining a qualitative understanding, not for a quantitative description, of the optical properties of real semiconductors. The two band effective mass approximation breaks down in the small quantum dot regime since the E-k relationship can no longer be approximated as parabolic. Since its first proposal the effective band approximation has received a great deal of interest and remarkable improvement has been made on over the last decade [31e34]. Realistic calculation of electronic and optical properties of nanoscale semiconductor systems would require replacing the two band effective mass approximation with a more comprehensive effective mass approximation [33,34] which includes the nonparabolicity of the conduction band and the degeneracy of valence band in calculations when high absorption peaks must be taken into account.
6. Conclusions We show that in the case of ZnSe=Cd1x Znx S heterostructure, the so called quasi-Type II regime exists when 0.25 x 0.35. The effect of ternary shell composition on the optical properties of ZnSe=Cd1x Znx S QDs shows the transition of Type-I ZnSe/ ZnS heterostructure to quasi Type-II ZnSe/CdS heterostructure after growing Cd0:6 Zn0:4 S ternary shell on ZnSe core. Large red shift and stock-shift in PL and not a distinct absorption peak in absorption spectra become noticeable in ZnSe=Cd0:75 Zn0:25 S and ZnSe/CdS. The increase of Cd:S composition ratio in shell composition shown to shift the XRD peaks to lower 2q degrees correspond to tensile strain on ZnSe core. The effect of composition dependent interfacial strain on squeezing or stretching the capped core is then evaluated. Results show that the decrease of compressive force on ZnSe core from ZnSe/ZnS shell to tensile force in ZnSe/CdS with increase in Cd:S composition ratio.
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Acknowledgments _ The authors greately acknowledge the financial support by the Research Foundation of Istanbul Technical University (ITÜ BAP Project No: 39029). We also wish to express our thanks to Dr. Mehmet Ali Gülgün of Sabancı University for TEM mealu of Marmara University for XRD measurements. surements and Dr. Cevat Sarıog References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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Composition and strain effects in Type I and Type II heterostructure ZnSe/Cd(Zn)S and ZnSe/Cd1-xZnxS core/shell quantum dots Negar Gheshlaghi a, Hadi Sedaghat Pisheh a, Hilmi Ünlü a, b, * _ Nanoscience and Nanoengineering Programme, Institute of Science and Technology, Istanbul Technical University, Maslak Istanbul, 34469, Turkey b _ Department of Physics, Faculty of Science and Letters, Istanbul Technical University, Maslak Istanbul, 34469, Turkey a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 February 2017 Received in revised form 6 June 2017 Accepted 7 June 2017 Available online 14 June 2017
We investigated the effect of ternary shell alloy composition on the bandgap and diameter of core of ZnSe=Cd1x Znx S heterostructure core/shell quantum dots, which were synthesized by using a simple colloidal technique. Characterization by using the x-ray diffraction (XRD), transmission electron microscopy (TEM), UVeVis absorption and fluorescence emission spectroscopic techniques indicate that (i) there is a transition of ZnSe=Cd0:6 Zn0:4 S Type-I heterostructure (electrons and holes tend to localize in core) to ZnSe=Cd0:75 Zn0:25 S quasi-Type-II heterostructures (holes tend to localized in the core and electrons are delocalized) and (ii) then after large red shift and Stock-shift in PL emission spectra but not a distinct absorption peak in UV spectra become noticeable in ZnSe/Cd0:75 Zn0:25 S quasiType II and ZnSe/CdS Type II heterostructures (electrons are localized in core and holes are localized in shell). Furthermore, the increase of Cd:S ratio in shell alloy composition shifts the XRD peaks to lower 2q degrees which corresponds to tensile strain in the ZnSe core. Finally, the hydrostatic interfacial strain has effect on the squeezing or stretching the capped core: A decrease of compressive force on core from ZnSe/ZnS to tensile force in ZnSe/CdS with increase in Cd:S ratio indicates that transition of compressive strain to tensile strain takes place with the transition from Type-I to II heterostructure. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Binary/ternary core/shell quantum dots Type-I to Type-II heterostructures Alloy composition Lattice mismatch strain Bandgap Diameter
1. Introduction During the past two decades, an abundance of semiconductor nanocrystals, also called quantum dots (QDs), have been studied because of their novel tunable emission properties and their potential applications in optoelectronic devices, with size-tunable light emission and high photo-stability [1]. Bandgap engineering achieved by changing the constituent stoichiometry and composition of alloy semiconductors is a powerful technique for the development of nanomaterials with many new properties [2]. It is now possible to tune the bandgap rendering narrow band emission in the visible range for QDs by controlling their sizes or deposition of a wide bandgap semiconductor shell onto a narrow bandgap semiconductor core [3]. Capping the core semiconductor with a ternary semiconductor shell can produce an extra degree of freedom for tailoring the electronic and optical properties of core/shell quantum dots. Among the group II-VI based binary/ternary core/shell
_ _ * Corresponding author. Department of Physics, Faculty of Science and Letters, Istanbul Technical University, Maslak-Istanbul, 34469, Turkey. E-mail address: [email protected] (H. Ünlü). http://dx.doi.org/10.1016/j.spmi.2017.06.026 0749-6036/© 2017 Elsevier Ltd. All rights reserved.
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nanocrystals, ZnSe=Cd1x Znx S heterostructure core/shell QDs have received great attention because of their compositiontunable emission across the visible spectrum and their highly luminescent characteristics [4]. These core/shell nanocrystals display unique composition-dependent properties distinct from those of their bulk counterpart and from those of binary CdS and ZnS shell NCs. The band gap of Cd1x Znx S can be adjusted between 2.4 eV for CdS and 3.7 eV for ZnS [4] depending on the relative Zn/Cd composition ratio and result in Type I ZnSe/ZnS and Type II ZnSe/CdS heterostructures, respectively [5] When the potential energy gradient at the heterointerface leads to a local separation of hole and electron in core and shell region but holes are still delocalized over the entire volume of the core/shell nanocrystal, there is a transition region between Type I and Type II and is called quasi- Type II heterostructure [5]. The schematic equilibrium energy band diagrams of Type I and Type II heterostructures are shown in Fig. 1. Here ECA (ECB) and EVA (EVB) represent the conduction band minimum and valence maximum. In Type I heterostructure (Fig. 1a), valence and conduction bands of core and shell are aligned in a way that an electron-hole pair excited near the heterointerface tend to localize in the core region. Therefore, the exciton energy in Type-I core/shell QDs is the result of direct exciton transition inside the core. However, in Type II heterostructure (Fig. 1b and c), the bandgap of the shell partially overlaps (staggered lineup) that of the core. The conduction band edge of the shell is located in the bandgap of the core leading to a local separation of the hole and electron in core and shell structure. Holes (electrons) are confined to the core (shell) and electron (holes) are confined to the shell (core), which are result of indirect exciton transition. The corresponding locally indirect band gap is equal to Egid ¼ EgA DEc in hole-electron confinement (Fig. 1b) and Egid ¼ EgA DEv in electron-hole confinement (Fig. 1c). DEC and DEV are
the band offsets in the conduction and valence band energies (DEc ¼ DEg DEv at Type-I heterointerface and DEc ¼ DEg þ DEv at Type-II heterointerface).DEg ¼ EgB EgA is the difference between bandgaps shell and core. The purpose of this work is to carry out an experimental and theoretical investigation about the effect of ternary shell composition and lattice mismatch interface strain on the exciton energy and diameter of the core of ZnSe=Cd1x Znx S (0 x 1) core/shell quantum dots, which are synthesized by using a simple colloidal technique discussed in section 2. The results of structural characterization carried out by using the x-ray diffraction (XRD) and transmission electron microscopy (TEM) will be discussed in section 3. Optical absorption and emission characterization was carried out by using UVeVis and Fluorescence spectrometers and will be discussed in section 4. The qualitative analysis of the effects of lattice mismatch strain and ternary shell composition on the band gap and diameter of ZnSe core of ZnSe=Cd1x Znx S core/shell quantum dots are discussed in section 5 by using a conventional two level effective mass approximation and results will be compared with experimental findings. 2. Colloidal synthesis of ZnSe=Cd1x Znx S quantum dots 2.1. Chemicals Zinc oxide (ZnO, 99.99% powder), Cadmium oxide (CdO, 99.5%), selenium (Se, 99.99%, powder), Sulfur powder (S, 99.5%), 1octadecene (ODE, 90%), octadecylamine (ODA, 99%), oleic acid (OA, 90%), Cadmium oxide (CdO, 99.5%), Tributylphosphine (TBP), trioctylphosphine (TOP, 90%), oleylamine (technical grade, 70%), zinc stearate (purum, 10e12% zinc basis), trioctylphosphine oxide (TOPO, 90%), tetradecylphosphonic acid (TDPA, 97%) and liquid Paraffin, were purchased from Aldrich. Stearic acid (analytical grade), Zinc acetate dehydrate (Zn(OAc)2.2H2O, analytical grade), were obtained from Sinopharm chemical. Methanol (analytical reagent), n-Hexane (analytical reagent) and Acetone (analytical grade) were also purchased from Merck Chemicals. They were used without any refinement in our synthesis. 2.2. Synthesis of ZnSe=Cd1x Znx S(0 x 1) binary core/ternary shell QDs The synthesis of bare ZnSe core and ZnSe/ZnS and ZnSe/CdS core/shell QDs was reported in our previous work [6]. The synthesis of ZnSe=Cd1x Znx S(0 x 1) core/ternary shell QDs is carried out by following the method of Veena et al. [7]. In monitoring the shell growth, aliquots of core/binary (ternary) shell were taken in specific time interval. Samples were washed
(a)
(b)
(c)
Fig. 1. Schematic equilibrium energy band diagram of Type I heterostructure (a): electrons and holes tend to localize in quantum well and of Type II heterostructure (b) and (c): electrons and holes are localized on different sides of heterointerface.
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with acetone and methanol in multiple sequences and decantation of the solution waste were carried with centrifuging the washed QDs. The samples were dispersed in n-Hexane and then characterization analysis was carried out. 3. Structural characterization of ZnSe=Cd1x Znx S core/shell QDs 3.1. XRD characterization X-ray diffraction spectroscopy (XRD) patterns were carried out by using a X’Pert3 MRD (XL) X-ray diffractometer operating at 45 kV/40 mA using Copper Ka line (l ¼ 1.5406 Å) on a powder diffractometer (2q step 0.1 and counting time 2 s/step) at room temperature. Fig. 2 shows the XRD pattern of ZnSe and ZnSe/Cd1x Znx S QDs for x ¼ 0.25, 0.40, 0.55, 0.70 and 0.85 alloy composition. There are broad peaks at 2q values of 27 related to 〈111〉, 45.6 to 〈220〉 and 52.8 to 〈311〉 crystalline planes for synthesized zinc-blende ZnSe (JCPDS No. 80-0021) along with ZnO diffraction peaks (JCPDS No. 36-1451). As it can be seen from Fig. 2, the ZnO peaks were faded after coating process. The synthesized NCs XRD patterns are in cubic crystal phase. The growth of compressive binary shell on core (ZnSe/ZnS), result in squeeze of crystalline spacing planes in the core and slight shift of bare core XRD pattern toward larger degrees (ZnS JCPDS No. 80-0020) [8]. It is noticed that the diffraction peak shifts toward the smaller angles when the Cd-S concentration ratio increases in ternary shell and reaches to ZnSe/CdS(CdS JCPDS 75-0581). 3.2. TEM characterization JEM-ARM200F electron microscope operated at an acceleration voltage of 200 kV, cold FEG emitter and 0.27eV energy spread was used for the transmission electron microscopy (TEM) analysis. QDs were diluted in n-Hexane, dropped over an amorphous carbon substrate supported on a copper grid of 400 meshes for taking images. Fig. 3a, b and 3c show the TEM images of the synthesized pre-capped ZnSe core and ZnSe/ZnS and ZnSe/CdS core/shell nanocrystals. TEM image in Fig. 3a confirms that the size distribution of bare ZnSe core NCs is nearly monodisperse; it has an average diameter of about 3.44 nm with high shape uniformity (spherical). From the TEM images shown in Fig. 3b and c we estimate that the size of ZnSe/ZnS and ZnSe/CdS core/shell QDs are about 3.85 and 3.90 nm, respectively. In many reports, shell thickness in core/shell QDs is obtained via subtracting core/shell crystal size from bare core nanocrystal size [9]- [10]. Since TEM images are illustrating the initial core and final core/shell size, ignoring the strain effect on core size after capping the shell would limit the accuracy of predicting the shell thickness [10]. Precise estimation of shell thickness and capped core size can be pursued through strain effect consideration on capped core diameter [11]. 4. Optical characterization of ZnSe=Cd1x Znx S core/shell QDs
CdS
(311)
(220)
(111)
The optical absorption spectra of synthesized NCs was measured using the Schimadzu UVeVis NIR Spectrometer at room temperature (300 K). The fluorescence emission spectra was measured by using Varian Cary eclipse fluorescence spectrometer (samples were excited with UV-light of l ¼ 350 nm) with an intense Xenon flash lamp (150 W) at 300 K. These samples were then diluted with n-hexane immediately for characterization. Then the sample solution was loaded in a quartz
ZnSe/CdS ZnSe/Cd0.75Zn0.25S ZnSe/Cd0.60Zn0.40S ZnSe/Cd0.45Zn0.55S ZnSe/Cd0.30Zn0.70S ZnSe/Cd0.15Zn0.85S
30
(311)
(220)
(111)
ZnSe/ZnS ZnSe 40
50
ZnS
60
70
80
90
2 Fig. 2. XRD pattens of ZnSe, ZnSe/Cd(Zn)S and ZnSe/Cdx Zn1x S core/shell nanostructures for x ¼ 0.25, 0.40, 0.55, 0.70 and 0.85 alloy composition.
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(a)
159
©
(b)
Fig. 3. TEM image of ZnSe core QD (a) and ZnSe/CdS (b) and ZnSe/ZnS (c) core/shell QDs, respectively.
Absorbance (a.u.)
cuvette and placed in the sample slot of UVeVis NIR and Photoluminescence spectrometers for absorption and emission measurements, respectively. The normalized optical absorption and emission spectra of ZnSe=Cd1x Znx S core/shell nanocrystals with x ¼ 0.25, 0.40, 0.55, 0.70 and 0.85 alloy compositions are shown in Figs. 4 and 5, respectively. The red shift in both optical absorbance and emission spectra is an evidence for the growth of Cd1x Znx S ternary shell over the ZnSe core. The optical absorption and emission spectra of Cd1x Znx S ternary shell on ZnSe core are shifted towards longer wavelengths with increasing alloy composition value towards unity. An interesting finding is the spectral changes are gradual and do not exhibit an abrupt transition as might be expected for a change from Type-I to Type-II heterostructure [12]. However, it should be noted that transition from Type-I to Type-II occurs after the more increase in Cd concentration in the Cd1x Znx S ternary alloy (from ZnSe=Cd0:6 Zn0:4 S (Type-I) to ZnSe=Cd0:75 Zn0:25 S (quasi Type-II heterostructure) and finally to ZnSe/CdS (Type-II heterostructure). Distinct first absorbance peak in the UV absorption spectra and relatively slight red shift in the PL emission spectra is the evidence for direct transition of excited electron from valance to conduction band of core (Type-I heterostructure). This behavior starts from ZnSe/ZnS and continues upto ZnSe/Cd0:6 Zn0:4 S. However, after the more increase of Cd concentration in the Cd1x Znx S ternary shell, there is a less distinctive first absorption peak in UV absorption spectra and relatively higher red shift and Stokes shift in the PL emission spectra becomes noticeable in ZnSe/Cd0:75 Zn0:25 S (quasi-Type II heterostructures) and finally ZnSe/CdS (Type II heterostructures). In the Type-II heterostructures, shell growth results in a significant red shift of the emission wavelength of the NCs [10]. Non distinctive first absorption peak and featureless absorption tail is the indication of an onset of an indirect excitons transition in Type-II heterostructure [13,14]. It is well known that the significantly high Stokes shifts in PL emission spectra of Type-II heterostructures NCs are due to the removal of surface passivating ligands in ZnSe core
ZnSe/CdS ZnSe/Cd75Zn25S ZnSe/Cd60Zn40S ZnSe/Cd45Zn55S ZnSe/Cd30Zn70S ZnSe/Cd15Zn85S ZnSe/ZnS ZnSe 300
350
400
450
500
550
600
650
700
Wavelength (nm) Fig. 4. Normalized UV optical absorption spectra of bare ZnSe core QD and ZnSe=Cd1x Znx S binary/ternary core/shell QDs.
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Fig. 5. Normalized PL emission spectra of bare ZnSe core QD and ZnSe/Cd1-xZnxS binary/ternary core/shell QDs.
[6] or the large spectral separation between the emission peak and the lowest energy absorption edge in Type-II heterostructure NCs [15]. In Type I heterostructure slight red-shift in the absorption and PL peaks are due to an increase in thickness of the deposited shell and leakage of the excitons from core into the shell. Although the band gap of the shell is larger than that of the core in Type-I, the excited electrons and holes in the core could also partly spread into the shell easily, leading to an inevitable leakage of excitons [16e19]. The “charge-separated” state in these nanostructures corresponds to the situation for which the electron is confined to the shell region while the hole is still delocalized over the entire volume of the hetero-NCs, which leads to reduced but nonzero overlap between the electron and the hole wave functions. Here, we refer to this localization regime as quasi Type-II [5], Because of an indirect excitons transition in Type-II, the shell growth aims at a significant red shift of the emission wavelength of the NCs. Fig. 6 shows the normalized UV absorption and PL emission spectra of bare ZnSe core and ZnSe/Cd1-xZnxS binary/ternary core/shell NCs at different time intervals. The broadening of absorption peak and featureless absorption tail of the absorption spectra are indication of an onset of an indirect excitons transition in Type-II heterostructures [20,21]. As it can be seen in Fig. 6, there is a small red shift of 26 nm after depositing ZnS shell (Type I) and a significant red shift of 125 nm after CdS shell deposition (Type-II). Gradual red shift of 27 nm is observed when Cd composition is in between 0.15 x 0.60. However, there is abrupt red shift of 49 nm when Cd composition is increased to x ¼ 0.75. This may be the transition point from Type-I to Type-II. In other words, there is a transition from Type-I (ZnSe/Cd0.6Zn0.4S) to quasi Type-II (ZnSe/Cd0.75Zn0.25S) and then to Type-II (ZnS/CdS). Similar broadening effect and clear Stokes shift in UV and PL peaks have also been reported for ZnSe/CdS QDs [20,22]. Significantly high Stokes shift in PL spectra of Type-II heterostructures are due to removal of surface passivating ligands in ZnSe core [6] or the large spectral separation between the emission peak and the lowest absorption edge in Type-II heterostructure NCs [23].
5. Results and discussion The binary/ternary (or ternary/binary) nanoscale heterostructures offer an extra degree of freedom to tuning the structural, electrical and optical properties of core/shell quantum dots by their composition dependent size, band gaps, effective mass of electrons and holes and dielectric constants. Therefore, it is essential to analyze the effect of ternary shell composition on the quantum confinement in binary/ternary core/shell quantum dots in order to predict their potential in nanoscale electronic and optical device applications. The general features of the quantum confinement in both bare ZnSe core and ZnSe=Cd1x Znx S core/shell QDs qualitatively can be understood by using the parabolic two band effective mass approximation (EMA) [24] provided the first exciton energy (i.e., band gap) is obtained from the absorption spectra according to the following expression [25]
Egnc ðdÞ ¼
hc
lmax
;
(1)
where c is speed of light, h is the Planck’s constant and lmax is wavelength at which the absorption of the nanoparticle is maximum. Fig. 7 shows the composition effects on the obtained band gap of ZnSe core of ZnSe=Cd1x Znx S heterostructure core/shell QDs.
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Fig. 6. Normalized UVeVis absorption and PL emission spectra of bare ZnSe core QD and ZnSe/Cd1-xZnxS binary/ternary core/shell NCs at different time intervals.
2,96
Eg(x) = 0.1905x2 - 0.0095x + 2.831
Exciton Energy (eV)
2,94 2,92 2,90 2,88 2,86 2,84 0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
x Fig. 7. Measured bandgap of ZnSe=Cd1x Znx S core/shell QDs as a function of composition.
Fig. 7 shows that the band gap of ZnSe=Cd1x Znx S QD as a function of Cd1x Znx S ternary shell composition, expressed as
Egnc ðd; xÞ ¼ 0:1905x2 0:0095x þ 2:831;
(2)
which shows that core band gap has a parabolic dependence on ternary shell composition and is higher than the band gap of bare ZnSe core. Equation (2) can be used in the following band gap Equation (3), due to conventional parabolic two band effective mass approximation [24], to calculate the core diameter of ZnSe=Cd1x Znx S core/shell QD
Egnc ðdÞ ¼ Egb ðTÞ þ
2Z2 p2 3:572e2 0:124e4 2 ; ε∞ d m*cv d2 Z m*cv ε2∞
(3)
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where Egnc ðd; xÞ is the core band gap given by Equation (2), measured from UV optical absorption spectra. Egb ðTÞ is the bulk value of the core semiconductor at temperature T above 0 K. Second and third terms, respectively, describe the confinement energy (with a 1/d2 dependence) and Coulomb interaction energy (with a 1/d dependence). Finally, last term represents the Rydberg correlation energy and it is independent of the diameter of the quantum dot, usually negligible when the dielectric constant of semiconductor is not small. m*cv ¼ ðm*e m*h =ðm*e þ m*h Þ is the reduced effective mass of electron-hole pair. m*e and m*h are the effective masses of electrons and holes and ε∞ is the optical dielectric constant of bulk semiconductor. Z ¼ h=2p with h representing the Planck constant. Equation (3) gives the size dependence of the band gap of core influenced by ternary shell composition in a Type-I heterostructure core/shell quantum dots. In case of TypeeII heterostructure core/shell quantum dots Equation (3) should be modified to
Egnc ðd; xÞ ¼ Egb ðTÞ DEc þ
2Z2 p2 3:572e2 0:124e4 2 ε∞ d m*cv d2 Z m*cv ε2∞
(4)
to calculate the core diameter for hole-electron confinement (Fig. 1b). A similar expression can be written for electron-hole confinement (Fig. 1c) by replacing DEc with DEv in Equation (4). The qualitative analysis of the effects of ternary shell composition on the core band gap and diameter of ZnSe=Cd1x Znx S core/shell QDs can now be made from Equations (3) and (4) for Type-I and Type II heterostructure core/shell quantum dots, respectively. It is well known that there is large lattice mismatch at the ZnSe=Cd1x Znx S heterointerface because of the difference between the lattice constants of the constituents. Reliable and accurate calculation of the band gap and diameter requires the inclusion of effects of the lattice mismatch strain in addition to ternary alloy composition. The use of conventional continuum elastic theory [26] allows one to write the hydrostatic interface strain in the core region of heterostructure core/shell quantum dot as
εrr ¼ εqq ¼ ε44 ¼ ε ¼ 3Bc εm =ð3Bc þ 4ms Þ;
(5)
where εrr ; εqq and ε44 are the strain components in spherical coordinates.εm is the lattice mismatch strain at core/shell heterointerface: εm ¼ Da=a ¼ 2ðac as ðxÞÞ=ðas ðxÞ þ ac Þxðac as Þ=as [27], where ac and as are the bulk lattice parameters of the core and shell, respectively. Bc and ms are the elastic bulk modulus and shear modulus of core and shell, respectively. Composition effects on lattice constant as ðxÞ, shear modulus ms ðxÞ) and conduction and valence band energy levels of ternary is determined by using the modified virtual crystal approximation [28]: Cd1x Znx S PCd1x Znx S ðxÞ ¼ xPZnS þ ð1 xÞPCdS þ dxð1 xÞðPZnS PCdS Þ, where P is the material parameter and x is the alloy composition and d is a small correction term due to alloy disorder. According to Equation (5), capping the ZnSe core with x ¼ 1, 0.85, 0.7, 0.55 to x ¼ 0.4 of Cd1x Znx S ternary shell composition causes about 3.3%, 2.6%, 1.8%, 1% and 0.13% of compressive strain on the core region from x ¼ 1, 0.85, 0.7, 0.55 upto x ¼ 0.4 of the shell composition. Furthermore, capping ZnSe core with Cd0:75 Zn0:25 S ternary and binary CdS as shell cause about 0.7% and 2% of tensile strain, respectively. The effects of hydrostatic interface strain due to lattice mismatch at heterointerface of core/shell quantum dot can be incorporated in the effective mass approximation [15] to calculate the diameter of the capped core by using the so called statistical thermodynamic model of semiconductors [29]. In this statistical thermodynamic model, the conduction electrons and valence holes are treated as electrically charged chemical particles and conduction and valence band edges are expressed as a function of pressure at any temperature [29]:
Eg ðT; PÞ ¼ Eg ð0Þ þ DCP0 Tð1 ln TÞ
ag P 2 ð1 þ B0ÞP 3 P ; B 2B 3B2
(6)
where Eg ð0Þ is the band gap of bulk semiconductor at 0 K. DCP0 Tð1 ln TÞ is the lattice vibration contribution with temperature increase. The third term represents sum of the volume expansion contribution due to the thermal pressure ðPth ¼ Bc ðDV=VÞcth ¼ 3Bc ac DTÞ at constant external pressure (i.e., lattice mismatch). Here ac is the linear thermal expansion coefficients of core in bulk form and DT ¼ T T0 is the difference between the growth and 0 K, respectively. ag ¼ BðvEg =vPÞ is the bandgap deformation potential at high symmetry point of core in bulk form. B is bulk modulus and B0 ¼ vB=vP. DCP0 is standard state heat capacity of reaction for the formation of electron-hole pair. Substituting Pth ¼ 3Bc ac DT in Equation (6) at constant pressure one finds the temperature effect on the bandgap of core in bulk form as
3 0 Egb ðTÞ ¼ Egb þ dEgb ðTÞ ¼ Egb þ DCiP Tð1 ln TÞ þ 3agcG ac T 1 þ ac T 3a2c ð1 þ B0 ÞT 2 ; 2
(7)
where DCP0 is obtained by fitting the band gap given by Equation (7) to the measured bandgap of bulk semiconductor at constant external pressure. Furthermore, substituting Pstr ¼ Bc ðDV=VÞstr ¼ 3Bc ε in Equation (6) at any temperature T one finds the hydrostatic interface strain effect on the core bandgap of quantum dot as
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9 2
dEgnc ðε; xÞ ¼ 3agcG ε þ agcG ε2 9agcG 1 þ B0c ε3 ;
(8)
where B0c its pressure derivative of Bc and ε is the hydrostatic interface strain defined in Equation (5). With Equations (7) and (8) are taken into account, Equations (3) and (4) can, respectively, be modified to Equations (9) and (10) to include the effects of hydrostatic interface strain and ternary alloy shell composition on the core region band gap in Type-I (with electron-hole confinement in core and Type-II heterostructure (with hole-electron confinement as shown in Fig. 1b) core/shell quantum dot as
Egnc ðε; xÞ ¼ Egbc þ dEgbc ðTÞ þ dEgnc ðε; xÞ þ
2Z2 p2 3:572e2 0:124e4 2 ; ε∞ d m*cv d2 Z m*cv ε2∞
Egnc ðε; xÞ ¼ Egbc þ dEgbc ðTÞ þ dEgnc ðε; xÞ DEc ðε; xÞ þ
(9)
2Z2 p2 3:572e2 0:124e4 2 ε∞ d m*cv d2 Z m*cv ε2∞
(10)
where Egb is bulk direct bandgap of core region at T ¼ 0 K. dEgbc ðTÞ and dEgnc ðε; xÞ are given in Equations (7) and (8), respectively, and other terms are described in Equations (3) and (4). As stated section 1, the ZnSe=Cd1x Znx S binary/ternary colloidal core/shell quantum dot structures may be of Type-I, Type-II and quasi-Type II heterostructures, depending on the Cd1x Znx S ternary alloy composition and the sign of lattice mismatch induced interface strain. In the Type-I heterostructure QDs the exciton energy is the result of direct transitions of exciton pairs inside the core and Equation (9) can be used to calculate the effects of hydrostatic interface strain and ternary shell composition on the core band gap. However, in Type-II heterostructure core/shell QDs, holes are confined to the core and electron are confined to the shell (Fig. 1b) or electrons are confined to the shell and holes are confined to the core (Fig. 1c), resulting in indirect transition from shell conduction (core valence) band to core valance (shell conduction) band. Therefore, in calculating the effects of hydrostatic interface strain and ternary shell composition on the core band gap of Type-II heterostructure with hole-electron confinement (Fig. 1b) Equation (10) should be used to calculate the effects of hydrostatic interface strain and ternary shell composition on the core band gap. On the other hand, in Type-II heterostructure with electron-hole confinement (Fig. 1c) the effects of hydrostatic interface strain and ternary shell composition on the core band gap of core/shell quantum dot is still calculated from Equation (10) but by replacing DEc ðε; xÞ with DEv ðε; xÞ. Using material parameters given in Table 1, Equation (10) is used to calculate the ternary shell composition and hydrostatic interface strain effects on the capped ZnSe core diameter of ZnSe=Cd1x Znx S core/shell quantum dots prepared using colloidal technique in our laboratory. The composition effect on conduction band offset at ZnSe=Cd1x Znx S binary/ternary core/shell interface is obtained by using ð1 xÞDEc as a first order approximation, where DEc is the conduction band offset at binary/ binary heterointerface (i.e., CdSe/ZnS and CdSe/CdS). The results of the calculations are given in Table 2 with strain and without strain (in parenthesis). Fig. 8 shows the interface strain effects on the diameter of the ZnSe=Cd1x Znx S heterostructure core/shell quantum dots at various alloy composition. Comparing the results with initial (bare) core size of ZnSe (z3.44 nm) from TEM measurements we suggest a squeeze of core after compressive shell deposition. Among the core/shell QDs, the most squeezed capped core is in ZnSe/ZnS with highest compressive lattice strain (3.3%). The results show, greater x value
Table 1 Material parameters (*) used in core diameter calculation.
ZnSe ZnS CdS
0
a(Å)
m*e =m+
m*h =m+
B
B
ε∞ =ε+
ms
-EV (eV)
Egb
5.66 5.41 5.82
0.137 0.20 0.14
0.82 1.42 0.68
6.24 7.71 6.16
4 4 4.8
5.9 5.1 5.4
1.75 1.16 1.87
13.02 12.97 12.21
2.82 3.76 2.48
(*) a, m*e , m*h , B(1011 dyn/cm2), B0 , (1011 dyn/cm2), ε∞ , Egb (eV) are taken from Ref. [30] and EV (eV) is taken from Ref. [28].
Table 2 Core diameter of ZnSe=Cd1x Znx S core/shell quantum dot with (without) strain. Composition/ YReaction Time (min) 15 30 45
x¼0
x ¼ 0.75
x ¼ 0.60
x ¼ 0.45
x ¼ 0.30
x ¼ 0.15
x¼1
3.45 (2.85) 3.51 (2.89) 3.58 (2.94)
3.54 (2.94) 3.55 (2.95) 3.57 (2.96)
3.47 (6.53) 3.48 (6.64) 3.51 (6.88)
3.42 (6.23) 3.43 (6.32) 3.46 (6.53)
3.34 (5.81) 3.37 (5.96) 3.41 (6.23)
3.29 (5.40) 3.31 (5.46) 3.34 (5.66)
3.32 (4.58) 3.33 (4.63) 3.34 (4.80)
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ZnSe/CdS Unstrained ZnSe/CdS Strained ZnSe/ZnS Unstrained ZnSe/ZnS Strained Cd.75Zn.25S Strained
Capped Core Diameter (nm)
7.5 6.75
Cd.75Zn.25S Unstrained Cd.6Zn.4S Strained Cd.6Zn.4S Unstrained
6.0
Cd.45Zn.55S Strained Cd.45Zn.55S Unstrained Cd.3Zn.7S Strained Cd.3Zn.7S Unstrained
5.25
Cd.15Zn.85S Strained Cd.15Zn.85S Unstrained
4.50 3.75
ZnSe
3.0
3.44 nm 2.88
2.96
3.04
3.12
3.2
Exciton Energy (eV) Fig. 8. Capped core diameter versus exciton energy in ZnSe/Cd1-xZnxS.
(rich Zn:S ratio) in alloy composition led in more squeezed capped core due to higher lattice strain amount in NC. The core is squeezed about 2.9%, 2.9%, 1%, 0.6%, in x ¼ 1, 0.85, 0.7 and 0.55 and stretched about 2%, 3.2% and 4% when x ¼ 0.4, 0.25 and 0 after the shell deposition, respectively. Comparing the capped ZnSe core diameter with and without strain shows that the interfacial strain effect contribution in capped core size calculations is necessary to have a reliable and precise calculations of core diameter of the quantum dots by using the two band effective mass approximation. The effect of composition and hydrostatic interface strain on the optical properties of the ZnSe=Cd1x Znx S binary/ternary core/shell quantum dots shows that there is a transition of Type-I ZnSe/ZnS heterostructure to Type-II ZnSe/CdS heterostructure after capping ZnSe core with Cd0:60 Zn0:40 S shell. Large red shift and Stockshift in PL and not a distinct absorption peak in absorption spectra become noticeable ZnSe=Cd0:75 Zn0:25 S in and ZnSe/CdS. Furthermore, the increase of Cd:S ratio in composition is shown to shift the XRD peaks to lower 2q degrees correspond to tensile strain on ZnSe core. Finally, the effect of alloy composition dependent interfacial strain on squeezing or stretching the capped core is investigated. Results show a decrease of compressive force on core from ZnSe/ZnS to tensile force in ZnSe/CdS with increase in Cd:S ratio with transition from Type-I to II heterostructure. We should also note that in the analysis of our data we used the simple parabolic two band effective mass approximation, which is useful only for obtaining a qualitative understanding, not for a quantitative description, of the optical properties of real semiconductors. The two band effective mass approximation breaks down in the small quantum dot regime since the E-k relationship can no longer be approximated as parabolic. Since its first proposal the effective band approximation has received a great deal of interest and remarkable improvement has been made on over the last decade [31e34]. Realistic calculation of electronic and optical properties of nanoscale semiconductor systems would require replacing the two band effective mass approximation with a more comprehensive effective mass approximation [33,34] which includes the nonparabolicity of the conduction band and the degeneracy of valence band in calculations when high absorption peaks must be taken into account.
6. Conclusions We show that in the case of ZnSe=Cd1x Znx S heterostructure, the so called quasi-Type II regime exists when 0.25 x 0.35. The effect of ternary shell composition on the optical properties of ZnSe=Cd1x Znx S QDs shows the transition of Type-I ZnSe/ ZnS heterostructure to quasi Type-II ZnSe/CdS heterostructure after growing Cd0:6 Zn0:4 S ternary shell on ZnSe core. Large red shift and stock-shift in PL and not a distinct absorption peak in absorption spectra become noticeable in ZnSe=Cd0:75 Zn0:25 S and ZnSe/CdS. The increase of Cd:S composition ratio in shell composition shown to shift the XRD peaks to lower 2q degrees correspond to tensile strain on ZnSe core. The effect of composition dependent interfacial strain on squeezing or stretching the capped core is then evaluated. Results show that the decrease of compressive force on ZnSe core from ZnSe/ZnS shell to tensile force in ZnSe/CdS with increase in Cd:S composition ratio.
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Acknowledgments _ The authors greately acknowledge the financial support by the Research Foundation of Istanbul Technical University (ITÜ BAP Project No: 39029). We also wish to express our thanks to Dr. Mehmet Ali Gülgün of Sabancı University for TEM mealu of Marmara University for XRD measurements. surements and Dr. Cevat Sarıog References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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