ZnS Nanocrystals

Article pubs.acs.org/JPCC

Strain-Induced Hierarchy of Energy Levels in CdS/ZnS Nanocrystals Chinmay Phadnis,† Kiran G. Sonawane,†,‡ Abhijit Hazarika,§ and Shailaja Mahamuni*,† †

Department of Physics, S. P. Pune University, Ganeshkhind, Pune-411007, India CEES Department, MIT College of Engineering, Pune-411038, India § Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India ‡

S Supporting Information *

ABSTRACT: Aside of size and shape, the strain induced by the mismatch of lattice parameters between core and shell in the nanocrystalline regime is an additional degree of freedom to engineer the electron energy levels. Herein, CdS/ZnS core/shell nanocrystals (NCs) with shell thickness up to four monolayers are studied. As a manifestation of strain, the low temperature radiative lifetime measurements indicate a reduction in Stokes shift from 36 meV for CdS to 5 meV for CdS/ZnS with four monolayers of overcoating. Concomitant crossover of S- and P-symmetric hole levels is observed which can be understood in the framework of theoretical calculations predicting flipping the hierarchy of ground hole state by the strain in CdS NCs. Furthermore, a nonmonotonic variation of higher energy levels in strained CdS NCs is discussed.

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INTRODUCTION Chemically prepared semiconductor nanocrystals (NCs) facilitate control over size, shape, feasibility of doping and formation of core−shell nanostructures in quantum confinement regime.1−5 Core−shell NCs are anticipated to attain higher photoluminescence (PL) yield and chemical robustness. Uniform growth of inorganic semiconductor layer on core semiconductor NC has not only improved2,6−8 the PL efficiency but also allowed9−13 manipulation of the discrete energy levels in multishell NCs called as wave function engineering. Lattice mismatched epitaxial shell formation can generate the interfacial defect levels which adversely affect the PL quantum yield. Strain-induced modulations10,14−16 on the optical properties of semiconductor NCs are conflicting. A forbidden gap is found to blue shift due to induced strain in NCs.17,18 On the other hand, in the case of CdSe/CdTe, on generation of strain, reduction in the forbidden gap19 is observed. Strain-induced switchover of direct to indirect20 as well as indirect to direct21 bandgap is documented. Because of strain, band alignment of type I to type II core−shell structures, which is characterized by spatial separation of electron and hole is reported.10 Similarly, change in band alignment from type II to type I is also observed.22 Diverse strain-induced results dictate a need to probe implications of strain on the electronic and optical properties in further details. Core/shell and alloyed NCs are naturally suitable candidates for studying straininduced effects.23 Even though both CdSe and CdS have similar band structure, quantum size effects are profoundly examined in the prototypical system of CdSe due to the availability of highly luminescent and narrow size-distributed NCs. The state of the art to prepare CdSe NCs with variety of shapes and © 2015 American Chemical Society

luminescence efficiency comparable to that of dye molecules is now well developed. Band-edge emission process in CdSe is understood on the basis of a model24 that explains exciton fine structure. The electron−hole exchange interaction, crystal field effects and other anisotropic factors lift24−27 the degeneracy of band-edge exciton states. Consequently, the ground state of the exciton is an optically passive, spin forbidden dark state. On the formation of electron and hole pair, CdSe NCs rapidly relax from upper absorbing states to the dark state, which accounts for the long decay lifetime (about 1 ms at 10 K in contrast to few nanoseconds lifetime for bulk CdSe) and the sizedependent resonant Stokes shift. Value of spin−orbit splitting is 0.07 eV in CdS while it is 0.42 eV in CdSe causing larger resonant Stokes shift in CdS, compared to CdSe. Yu et al.28 have found a resonant Stokes shift of ∼20−70 meV in CdS which is larger by four times in similar sized CdSe NCs. Effect of strain on CdSe-based core/shell NCs is addressed in the literature.6,29−31 The optical properties of CdSe/CdS quantum dots, tetrapods and nanorods are studied29 by applying hydrostatic pressure. The emission and absorption features are observed to blue-shift or even split in two distinct peaks on strain generation. Similar results are observed30 by forming thicker shells of CdS on CdSe (enhancing strain on CdSe NCs). Despite these reports, effect of strain on NCs is not comprehended well and the understanding is still in a rudimentary state. Further, strain-induced behavior of other semiconductor nanostructures is still an open question. In an isolated report, strain-induced effects are manifested32 as the Received: August 18, 2015 Revised: October 6, 2015 Published: October 6, 2015 24165

DOI: 10.1021/acs.jpcc.5b08040 J. Phys. Chem. C 2015, 119, 24165−24173

Article

The Journal of Physical Chemistry C

Figure 1. (a) HRTEM image of CdS-0 NCs showing inter planar distance, (b) TEM image of CdS-0 NCs, (c) electron diffraction (SAED) pattern of CdS-0 NCS, (d) HRTEM image of CdS-4 NCs with inter planar distance, (e) TEM image of CdS-4 NCs, and (f) SAED pattern of CdS-4 NCs. Insets in parts c and f show size distribution.

with 3 ML of ZnS as CdS-3, and CdS with 4 ML ZnS shell as CdS-4 NCs. X-ray diffraction (XRD) pattern was collected using Bruker D8 advanced powder diffractometer, employing Cu Kα line (λ = 1.5402 Å) as an incident radiation. Optical absorption was recorded with PerkinElmer Lambda 950 spectrophotometer. High resolution transmission electron microscopy (HRTEM) imaging was carried out with Jeol’s JEM-2100F instrument. Room temperature and low temperature photoluminescence (PL), and photoluminescence excitation (PLE) measurements were performed on home-built PL spectrometer. PL spectrometer consists of Jobin Yvon TRIAX 180 as an excitation monochromator and Jobin Yvon iHR 320 as emission monochromator. Temperature dependent PL lifetime measurements were performed on thin films of the NCs prepared by spin coating on quartz substrate using a timecorrelated single-photon counting (TCSPC) module of an Edingburgh FLS920 spectrometer with a 405 nm picosecond pulsed semiconductor diode laser as excitation source at 10 MHz repetition rate. The samples were mounted on A Janis closed cycle helium cryostat and a Lake Shore temperature controller was used for temperature variation. In the initial stage, formation of ZnS shell on CdS NCs shows deterioration in photoluminescence and takes up the maximum value only at four ML. Properties remain invariant on cladding further by thicker ZnS shell. Electron energy levels are hence described for CdS/ZnS with 4 ML of ZnS with reference to bare CdS NCs.

distinct polarization properties of CdS nanowires due to anisotropic response of A, B, and C excitons. The smaller spin−orbit coupling in CdS leads to larger resonant Stokes shift and hence forms an important class of materials. Herein, optical absorption and temperature dependent steady state photoluminescence (PL) studies reveal straininduced crossing of higher energy levels as well. The observations can be understood on the basis of reported16 empirical tight binding method which predict that the strain exerted on CdS lattice can change the symmetry of the ground hole state. Radiative lifetime measurements from room temperature through 10 K concur with findings of steady state PL and absorption. As a manifestation of strain, switching of ground state from heavy hole to light hole is observed33 in case of self-assembled GaAs quantum dots. It may be worthwhile to note here that a size dependent changeover in hierarchy of S- and P-orbital hole levels is predicted34 in CdS theoretically and later reconfirmed35 experimentally. In the present work, we discuss the experimental manifestation of the crossing of energy levels in CdS NCs due to strain exerted by ZnS layers.

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EXPERIMENTAL METHODS CdS and CdS/ZnS NCs were synthesized by the reported method.36 In short, a solution of sulfur in 1-octadecene was injected in degassed mixture of cadmium oxide, 1-ocadecene, and oleic acid at 260 °C. The reaction was allowed to proceed for 5 min. ZnS shells were formed on the NCs by injecting zinc diethyldithiocarbamate (Zn(DDTC)2) in oleylamine in a solution containing CdS NCs. For first monolayer the precursor was injected at 50 °C, and the reaction solution was rapidly heated to 180 °C, for subsequent monolayers (ML) the precursors were injected at 120 °C and the reaction solution was quickly heated to 180 °C. Each shell was allowed to grow at 180 °C for 20 min.37 Here, for convenience, we call CdS core NCs as CdS-0, CdS with 2 ML of ZnS as CdS-2, CdS

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RESULTS AND DISCUSSION Figure 1 depicts HRTEM images and selected area diffraction (SAED) patterns of CdS-0 and CdS-4 NCs. Both CdS-0 and CdS-4 NCs are slightly prolate in shape and have size of 4.3 ± 1.5 nm, and 5.5 ± 1.3 nm, respectively. The values of interplanar spacing d are found out from HRTEM that confirm cubic zinc blende phase of CdS-0 and CdS-4 NCs. The lattice constant, a, deduced from (111) plane of CdS-0 (Figure 1a) and CdS-4 NCs (Figure 1d) is 5.89 ± 0.18 Å and 5.65 ± 0.06 24166


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The Journal of Physical Chemistry C Å, respectively, which amounts to the compression of lattice by ∼4%. Parts c and f of Figure 1 show SAED pattern of CdS-0 and CdS-4 NCs, respectively, which matches with the cubic zinc blende phase of CdS. The lattice constants evaluated from SAED pattern show 3.3% contraction of CdS lattice on shell formation that is in agreement with the XRD data (Figure SI). The compressive strain is quantitatively measured from X-ray diffraction (XRD) data by employing the Williamson−Hall method.23,38,39 The strain can be calculated using the equation β

cos θ 1 sin θ = +η λ D λ

Appearance of defect related broad band due to partial passivation of CdS NCs is theoretically addressed41 and agrees well with the experimental works. In general, the first excitonic feature, of the optical absorption, is explored to study the quantum confinement effect. Recently, the power of second excitonic state to elucidate the quantum size effects is appreciated.42 In the complex core/ shell nanoheterostructures, the first excitonic feature provides the information about electron−hole wave function overlap while the second excitonic feature reports the localization of electron and hole. However, this method provides the information only for NCs without interfacial strain. As is evident from above discussion on structural data, our NCs have strain at the interface. Even though the charge carrier localization in strained NCs cannot be predicted, the intrinsic information on the wave function overlap can always be deduced. The oscillator strength is a measure of42 wave function overlap and can be estimated from the area under the absorption feature. The oscillator strength is estimated as described by Smith et al.42 In Figure 3, we show background

(1)

where β is the full width at half-maximum (fwhm) of XRD peaks, θ is the diffraction angle, λ is X-ray wavelength, D is the effective particle size, and η is the effective strain. The strain is calculated from the slope of the graph wherein β cos θ vs 4 sin θ values are plotted (Figure SII).23,39 Values of strain observed by Williamson − Hall method are 2.2% for CdS-0, 2.5% for CdS-2, 3.7% for CdS-3 and 4.1% for CdS-4 NCs (Figure SII). Figure 2 shows optical absorption and PL spectra of CdS-0 and CdS-4 NCs at room temperature. Optical absorption

Figure 3. Background subtracted absorption spectra of (a) CdS-0 and (b) CdS-4 NCs.

corrected, room temperature optical absorption spectra of (a) CdS-0 and (b) CdS-4 NCs. Interestingly, on background correction; one can unambiguously locate the peak position of second excitonic feature. Intensity of the second absorption feature increases on shell formation. On 4 ML ZnS cladding, the ratio of oscillator strength of the first transition to second transition is 0.21, while that for CdS-0 is 0.56. A substantial increase in second transition oscillator strength as compared to first transition is observed on shell formation. The tight binding calculations on strained CdS/ZnS core−shell structure is ́ et al.16 These calculations predict that due reported by Diaz to strain in CdS/ZnS core/shell structure, S-type hole wave function is localized16 near the interface, whereas S-type electron wave function has a maximum amplitude at the center, resulting in the smaller overlap which reduces the oscillator strength of the first excitonic feature. On the other hand, the Ptype hole wave function is centralized in the core region, yielding higher overlap that results into the higher value of the oscillator strength of the second excitonic feature. Our data provides the direct experimental proof for these calculations. In fact, the calculations indicate that16,43,44 P-type symmetry of hole ground state for smaller-size CdS NCs changes to Stype symmetric hole state above a critical size. Irrespective of size, the electronic ground state is S-type. The underlying reason for the characteristic trend lies in the small spin−orbit coupling in CdS NCs. The crossover of hole states is also

Figure 2. Optical absorption and PL spectra of (a) CdS-0 and (b) CdS-4 NCs recorded at room temperature. An astersik indicates excited state transition. Insets show the defect levels for the corresponding NCs.

features appear at 408 nm (3.04 eV) and 425 nm (2.92 eV), while the second excited state (indicated by * in Figure 2) is visible at 373 nm (3.32 eV) and 368 nm (3.37 eV) for CdS-0 and CdS-4 NCs, respectively. The first excitonic feature arises due to the 1Sh−1Se transition. On the other hand, the second excitonic feature, in case of CdS, is from the 1Ph−1Pe transition.16,34 It is noteworthy that the first excitonic feature (band edge absorption) red shifts where as the second excitonic feature blue shifts on shell formation. Concomitant red-shift of 0.11 eV in band edge PL emission is observed in CdS-4 (2.84 eV) compared to CdS-0 (2.95 eV) at room temperature. As anticipated, the quantum efficiency of CdS-0 NCs increases from 12% to 42% on ZnS shell formation. Moreover, the defect emission observable in CdS-0 at ∼2.3 eV, diminishes on the shell formation (Figure 2 insets). The observed defect emission at lower energy presumably originates from surface states.40 24167


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PLE spectra of CdS-0 (Figure 4c) and CdS-4 NCs (Figure 4d) at 10 K are analyzed to understand the energy level structure. The transitions in PLE spectra are labeled as E1, E2, E3, and E4 for 1Sh3/2−1Se, 1Sh1/2−1Se, 1Ph3/2−1Pe, and 1Ph1/2−1Pe respectively. Transition energies as a function of size of CdS NCs are obtained47 using size selective PLE measurements and are presented in Table 1. Difference between various energy levels is found by subtracting the transition energies. For example, difference between emission energy and E1 represents the energy difference between 1Sh3/2 and 1Ph3/2 hole levels. The separation between energy levels is summarized in Table 2. In case of CdS NCs, the lowest excitonic hole level (HOMO) is 1Ph3/2, whereas the lowest excitonic electron level (LUMO) is 1Se.34 The transition from 1Ph3/2 to 1Se is very weak due to the disparity in symmetry of involved states.34 As a result, the first absorption feature is due to the 1Sh3/2−1Se34 transition, while the emitting transition is 1Se−1Ph3/2,34 which invariably leads to the larger resonant Stokes shift for CdS NCs. With increasing h h and 1S3/2 levels size, energy difference between 1P3/2 diminishes, and eventually cross each other to have 1Sh3/2 as a ground hole state at a critical size.28,35,43 Schematic of size dependent hierarchy of energy level is depicted in Figure 5a, while the effect of strain on the energy level hierarchy is presented in Figure 5b. Figure 5c depicts separation of hole energy levels as a function of size and ZnS cladding. The separation between the energy levels on shell formation is shown by the dotted rectangle. Hollow triangles in Figure 5c represent energy separation between 1Ph3/2 and 1Ph1/2, which varies weakly with size of NCs. Similarly hollow squares represent the separation between 1Sh3/2 and 1Sh1/2 hole energy levels, which is also weakly size-dependent. The separation between S (1Sh3/2 and 1Sh1/2) and P (1Ph3/2 and 1Ph1/2) levels varies in a subtle fashion with the size. Larger hole effective mass leads to weak size dependence of these energy levels. However, if one looks at the separation between S and P levels, the behavior is complex. The lower panel of Figure 5c, shows the variation in separation of S and P levels, where solid triangles represent the separation between 1Sh1/2 and 1Ph1/2 and solid squares represent separation between 1Sh3/2 and 1Ph1/2. The h h and 1P1/2 levels reduces with separation between 1S1/2 increasing size, whereas the separation between 1Sh3/2 and 1Ph1/2 increases with the size. The reduction in gap between 1Sh1/2 and 1Ph1/2 implies these levels are pulled closer to each other, and increased gap between 1Sh3/2 and 1Ph1/2 implies the levels are pushed apart from each other. From Figure 5a it is clear that for smaller NCs the ground hole level is 1Ph3/2, the next level is 1Sh3/2 which is followed by 1Ph1/2, and 1Sh1/2. This shows that both S-type levels are shifting up in energy with increase in size. In other words, the hole state with S-symmetry increases in energy, and that with P-symmetry decreases in energy. This is in agreement with the previous reports34,43 which say that the first two hole (1Sh3/2 and 1Ph3/2) levels cross each other as size is reduced. In the present work, the change in excited hole levels, along with the change in well documented ground hole state, as a function of size is mapped. The difference in electronic states is calculated from the PLE data by taking difference between E3 and emission energy. The separation between 1Se lowest electronic state and 1Pe the first excited electronic state is found to increase monotonically with the reduction in size in accordance with quantum confinement effect.

possible in other II−VI semiconductors. Because of the smaller value of spin−orbit coupling, the hole level mixing takes place and crossover is visible for larger CdS NCs compared to CdSe.28,35 Theoretical studies16 predict the similar changeover of hierarchy of electronic energy levels of CdS by lattice strain. As evident from Figure 3, the first absorption feature red shifts and the second feature blue shifts. The red shift of the first excitonic feature indicates a reduction in the gap between Stype electron and hole states. The blue shift in the second optical absorption feature indicates increase in gap between the P-type electron and hole states. Furthermore, the electronic energy levels are relatively immune to change in shape and size, while the hole energy states are sensitively altered.16,28,34,35 The reduction in gap of S-type energy states thus imply that the 1S hole levels are shifting up in energy. However, the increased gap between P states shows lowering energy of P-type hole state. As the hole ground state in CdS has a P-type symmetry, these findings indicate shifting of hole energy levels due to shell-induced strain. The present result supports the anticipated lowering of P hole level with respect to S hole level as a manifestation of induced strain on the lattice. The effect of size as well as strain on the energy level can further be probed using PLE spectroscopy at low temperature (at 10 K). Parts a and b of Figure 4 depict normalized PL

Figure 4. (a) Photoluminescence spectra of CdS-0, (b) CdS-4 NCs at 10 K with excitation energy 3.3 eV (375 nm), (c) Photoluminescence excitation spectra of CdS-0 recorded by holding the emission energy at 2.99 eV constant, and (d) CdS-4 NCs (emission energy at 2.89 eV) at 10 K along with fitted Gaussian peaks (blue lines). The inset for part d shows schematic of transition energies.

spectra of CdS-0 and CdS-4 NCs at 10 K with excitation wavelength of 375 nm (3.3 eV). In CdS-0 NCs, the defect emission substantially increases at 10 K as compared to RT. The observation of surface and interfacial traps at low temperature is reported recently.40,45,46 Midgap states respond to temperature and its intensity increases with decrease in temperature. A feeble defect emission is also observable for CdS-4 NCs. However, the intensity of band edge emission is predominant with respect to the defect level emission for CdS4 NCs. Appearance of defect level emission of CdS-4 NCs indicates that interfacial defects are present, albeit weaker in the number. These interfacial defects could be induced by ZnS shell formation. 24168


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The Journal of Physical Chemistry C Table 1. PLE Peak Positions for CdS-0 NCs at 10 K Probed for Different Sizesa

a

size (nm)b

emission energy (eV)

E1 (eV) 1Sh3/2 −1Se

E2 (eV) 1Sh1/2 −1Se

E3 (eV) 1Ph3/2 −1Pe

E4 (eV) 1Ph1/2 −1Pe

3.75 3.35 3.01 2.87 3.75c

2.99 3.02 3.1 3.14 2.89

3.017(±0.006) 3.108(±0.007) 3.199(±0.007) 3.238(±0.021) 2.926(±0.002)

3.103(±0.007) 3.180(±0.014) 3.243(±0.035) 3.312(±0.013) 2.974(±0.001)

3.308(±0.001) 3.326(±0.000) 3.454(±0.018) 3.545(±0.023) 3.324(±0.000)

3.416(±0.014) 3.424(±0.003) 3.684(±0.008) 3.651(±0.017) 3.489(±0.048)

The last row is for CdS-4 sample. bSizes are calculated based on the semiempirical formula52

D = (− 6.6521 × 10−8)λ 3 + (1.9557 × 10−4)λ 2 − (9.2352 × 10−2)λ + 13.29 c

Size of CdS-0 core NCs of CdS-4 sample.

involved in emission process, the hierarchy can easily be explained using temperature dependent TRPL measurements. In Figure 6, we compare PL lifetime curves of CdS-0, CdS-2, CdS-3, and CdS-4 NCs at room temperature (RT) and at 8 K (LT). For CdS-0, CdS-2, and CdS-3 NCs, PL decay curves are biexponential, where as decay curve on cladding shell of four ML is single exponential. Values of room temperature lifetime for CdS-0, CdS-2, CdS-3, and CdS-4 are given in Table 3. Fast component of lifetime is usually attributed48 to internal core states, while slow component is related49 to surface traps. With thicker shells on CdS NCs, the surface traps are passivated, giving the mono exponential lifetime. The slower component does not show temperature dependence. Moreover, as this component is from surface states, only faster lifetime component is considered for further analysis. The lifetime reduces from 16 ns (RT) to 10.2 ns (LT) in case of CdS-0 NCs whereas, for CdS-2 NCs the lifetimes are 11 ns (RT) and 8.5 ns (LT). It has been reported earlier28,35 that at room temperature, CdS NCs with size of 2.2 nm or above show relatively long lifetime as emission is from optically forbidden transition (between S electron and P hole levels) which would be thermally populated, besides optically allowed transition. On lowering the temperature, only optically allowed energy levels (S-type symmetric hole and S-type symmetric electron) would be populated, as they are at lower energy. At low temperature, the emission is from optically allowed transition (optically forbidden levels (P-type symmetric hole) are not populated and hence do not contribute in the emission), yielding the shorter lifetime.35 This is evident from Table 3. Prolonged lifetime is observed for CdS-3 NCs at low temperature (13 ns) as compared to room temperature (7 ns). Further, on cladding 4 ML ZnS, PL lifetimes are 13.2 ns (LT) and 7 ns (RT). Temperature dependence of radiative lifetime is different for CdS-0 compared to CdS-3 and CdS-4 NCs. Opposite trend of lifetimes in case of CdS-3 and CdS-4 NCs is due to the fact that shell formation reverts transition from optically allowed to forbidden. In short, hierarchy of P hole levels and S hole levels changes, affecting the lifetimes of the NCs.35,50 The experimental observations can be further understood with the help of a model35 based on the transitions between Ssymmetric electrons and S- and P-type hole levels which are closely spaced. The model assumes that the optically forbidden state lies below optically active state with separation between the two levels being Δ.51 The decay rate, Γ = τ−1, using Boltzmann statistics can be given as

Table 2. Separation between hole energy levels for varying sizes of CdS-0 NCsa

size (nm)b

emission energy (eV)

h

separation between 1S h

h

3/2

and 1Sh1/2 (eV)

and 1Ph1/2 (eV)

and 1Ph1/2 (eV)

separation between 1P

separation between 1S

3/2

separation between 1S

3/2 h

and 1Ph1/2 (eV)

1/2

3.75 3.35 3.01 2.87 3.75c

2.99 3.02 3.1 3.14 2.89

0.086 0.072 0.07 0.074 0.048

0.108 0.098 0.1 0.106 0.165

0.081 0.01 0.011 0.008 0.129

0.003 0.062 0.07 0.066 (−)0.081

a

The last row is for CdS-4 sample. bSizes are calculated based on the semiempirical formula52

D = (− 6.6521 × 10−8)λ 3 + (1.9557 × 10−4)λ 2 − (9.2352 × 10−2)λ + 13.29 c

Size of CdS-0 core NC of CdS-4 sample.

The effect of strain on ground hole state is elaborated in the literature.16,44 Here, we observe that inducing strain by over coating CdS NCs by ZnS shell drastically affects energies of second and higher excited states. The energy level separations in case of CdS-4 NCs are deduced from PLE spectra and are presented in Figure 5c in dotted region. This behavior is quite complex, compared to size induced change in the energy levels which shows a weak size dependence. Size has a negligible effect on separation between S levels (or P levels). The plot clearly indicates that on shell formation S levels are coming closer to each other, whereas, P levels are going away from each other. From the lower panel of Figure 5c, it is clear that on shell formation, difference between 1Sh1/2 and 1Ph1/2 decreases on the other hand the separation between 1Sh3/2 and 1Ph1/2 increases. This lead to the conclusion that 1Ph1/2 level is shifting down in energy. It is worthwhile to note that the difference between 1Sh1/2 and 1Ph1/2 is negative (Table 2) indicating that the two levels are crossing each other. The up-shifting of 1Sh3/2 and down-shifting of 1Ph1/2 levels explain the reduced separation of 1Sh3/2−1Sh1/2 and increased separation of 1Ph3/2 and 1Ph1/2. In literature the change in hierarchy of the ground (1Sh3/2 and 1Ph3/2) hole levels is predicted. Here, we have demonstrated that the deeper lying 1Sh1/2 and 1Ph1/2 levels are also crossing over due to induced strain. However, the hierarchy of the ground 1Ph3/2 and 1Sh3/2 hole level is difficult to explain using the current data. Moreover, as one of these levels (1Sh3/2 and 1Ph3/2) is 24169


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Figure 5. (a) Energy level diagram for CdS-0 NCs with the highest size and the lowest size (deduced from PLE), (b) for CdS-0 and CdS-4 NCs, and (c) change in the hole energy levels with size and on shell formation (marked with dotted box).

Table 3. Lifetimes of Core and Core/Shell NCs at Room Temperature (300 K) and Low Temperature (8 K) lifetime (ns)

Γ1 + Γ2e−Δ / kBT 1 + e−Δ / kBT

room temperature

low temperature

CdS-0 CdS-2 CdS-3 CdS-4

16; 24 11; 78 7; 19 7

10.2; 23 8.5; 76 13; 19 13.2

optically allowed states respectively and are derived from fitting temperature dependent lifetime using eq 2. In Figure 7a, solid lines depict curve representing eq 1. From fitting, it is observed that the separation, Δbetween the two levels decreases with shell formation (36 meV for CdS-0 and 5 meV for CdS-4 NCs). The energy difference between S-type and P-type symmetric hole states is as small as 5 meV due to the strain generated in the lattice. Such a small energy difference would tend to mix the hole levels and redistribute35 the oscillator strength. Dependence of shell thickness on transition rates is plotted in Figure 7b. It indicates reduction in Γ1 and improvement in Γ2 with increase in shell thickness (Figure 7b). The figure also shows a crossover in Γ1 and Γ2. The crossover is understood in

Figure 6. Room temperature (300 K) and low temperature (8 K) radiative lifetime curves of CdS-0, CdS-2, CdS-3, and CdS-4 NCs.

Γ=

sample

(2)

A thermal distribution of excitons within these two levels is considered. Γ1 and Γ2 are decay rates of optically forbidden and 24170


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Figure 7. (a) Change in decay time of CdS-0, CdS-2, CdS-3, and CdS-4 NCs with temperature. Solid lines represent the best fit to the experimental data by eq 1. (b) Decay rate (Γ) with increase in shell ML. Γ1, and Γ2 are transition rates of ground and first excited levels, respectively.

also states the importance of strain on engineering the electron energy levels in semiconductor NCs.

terms of change in the hierarchy of hole energy levels with increase in cladding. For CdS NCs, Γ1 (ground state transition) is less than Γ2 (excited state transition), which is a signature of forbidden ground state transition, which involves P-type hole levels.35 Further, cladding three or four ML of ZnS, lead to the higher value of Γ1 than Γ2. This shows that for higher shell thickness, ground state transition is allowed with S-symmetry holes. Using PLE data we have already proved that the deep lying 1Sh1/2 and 1Ph1/2 energy levels are crossing each other. Here using temperature dependent TRPL, we have demonstrated that the induced strain alters the ground state from 1Ph3/2 to 1Sh3/2. According to the tight binding model,16 strain at the interface causes localization of S-type charge carriers near the interface16 thus making S symmetry state the hole ground state. On the other hand the P-type holes are accumulated in the core of CdS.16 This accumulation of charges away from the core and at the center of core causes the change in the energy of these charge carriers, effectively changing the energy hierarchy of ground state.16 We infer that this localization of S-type charge carriers is responsible for the change in hierarchy of S-type and P-type energy levels in the NCs. Our results provide direct experimental proof to these theoretical calculations. In conclusion, we have studied implications of strain on the first and higher excited states of CdS NCs indicating redistribution of the oscillator strength which is a signature for variation in symmetry of energy levels. The separation between 1Sh3/2 and 1Sh1/2 reduces while the separation between 1Ph3/2 and 1Ph1/2 increases on the shell formation. With increase in size of CdS NCs, the separation between 1Sh3/2 and 1Ph3/2 reduces, however, the separation in P levels or S levels is insensitive to size. Over coating CdS NCs with 4 ML ZnS, the Stokes shift (separation between 1Sh3/2 and 1Ph3/2 hole levels) decreases. TRPL measurements show reduction in resonant Stokes shift from 36 to 5 meV as a consequence of lattice strain. Moreover, TRPL measurements clearly reveal crossover between ground and first excited hole level. This finding is in accordance with the theoretical prediction. Previous reports showed that the ground state hierarchy can be tuned by size. Here, we have provided an experimental proof for change in the hierarchy of the energy level with introduction of interfacial strain. The induced strain not only changes the hierarchy of the ground hole state, but also affects the excited hole states in such a way that they cross each other. Core/shell NCs are often preferred to obtain higher quantum yield. However, straininduced on core NC can be a detrimental factor which affects the radiative lifetime and hence the quantum yield. This work

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08040. Additional X-ray diffraction pattern of CdS/ZnS NCs and Williamson−Hall plots for strain calculations of CdS/ZnS NCs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(S.M.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank D. D. Sarma and Anshu Pandey for the helpful discussions and allowing us to use low temperature photoluminescence lifetime measurements. C.P. would like to thank DST, India, and DAE-BRNS, India, for financial support. The authors also thank Sophisticated Analytical Instrumentation Facility, IIT Bombay, for HRTEM measurements, and DST-FIST and CNQS, Department of Physics, S. P. Pune University, for various characterizations. A.H. thanks the Council of Scientific and Industrial Research (CSIR), India, for financial support.

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