Shell Quantum Dots

Article Cite This: Chem. Mater. 2018, 30, 4393−4400

pubs.acs.org/cm

Strain Engineering in InP/(Zn,Cd)Se Core/Shell Quantum Dots Mona Rafipoor,*,†,‡ Dorian Dupont,¶,□ Hans Tornatzky,§ Mickael D. Tessier,¶,□ Janina Maultzsch,§,∥ Zeger Hens,¶,⊥ and Holger Lange†,‡ †

Institut für Physikalische Chemie, Universität Hamburg, Hamburg, 20146 Germany The Hamburg Centre for Ultrafast Imaging, Hamburg, 22761 Germany ¶ Physics and Chemistry of Nanostructures, Department of Chemistry, and ⊥Center for Nano- and Biophotonics (NB-Photonics), Ghent University, Ghent, 9000 Belgium § Institut für Festkörperphysik, Technische Universität Berlin, Berlin, 10623 Germany □ SIM vzw, Technologiepark 9359052 Zwijnaarde, Belgium

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‡

S Supporting Information *

ABSTRACT: The formation of core/shell structures has become an established approach to passivate the surface and enhance the photoluminescence quantum yield of semiconductor nanocrystals, quantum dots. However, lattice mismatch between the core and the shell materials results in surface reconstructions at the core/shell interface and compressive or tensile strain in the core and the shell. Concomitantly formed surface traps can have a negative impact on the emission properties. Growing buffer layers with intermediate lattice constants or using alloys to tune the lattice constant is often considered to reduce the reconstructioninduced strain. We present a study that quantitatively relates strain and shell composition in the case of InP/(Zn,Cd)Se core/ shell quantum dots. We apply Raman spectroscopy to quantize strain and find that adjusting the composition of the (Zn,Cd)Se shell tunes the strain from compressive to tensile. The transition between both regimes is found at shell compositions where the bulk lattice constants of InP and (Zn,Cd)Se match, which confirms that matching lattice constants is a viable strategy to achieve strain-free core/shell nanocrystals.

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INTRODUCTION

core/shell QDs with a straddling band alignment, the wider band gap shell will confine excited carriers within the core and reduce overlap with trap states at the outer surface of the shell.12−15 Typically, such type I core/shell QDs feature a strongly enhanced PLQY, as exemplified by seminal examples such as CdSe/ZnS, CdSe/CdS, InP/ZnS, and InP/ZnSe.16 In all but a few cases, the materials used to form a heteronanostructure, such as a core/shell QD, will have different lattice constants. Lattice mismatch between core and shell results in a strained interface, where the core experiences either compressive or tensile strain (see Figure 1).17,18 Strain can lead to the formation of defects that give rise to localized electronic states in the band gap, much like undercoordinated surface atoms.19,20 Again, these interfacial defects can act as electron- and hole-trapping centers and reduce the PLQY. It has been shown that such problems can be alleviated by inserting buffer layers with a lattice constant intermediate between core and shell, which enables the lattice constant to be

Colloidal semiconductor nanocrystals or quantum dots (QDs) attract widespread attention because of their potential use as a size-tunable optoelectronic material for light emission and lasing, infrared photodetection, or solar energy conversion.1−5 To optimally design QDs for such applications, it is important to realize that quite a few characteristics of QDs are extrinsic material properties that depend to a large extent on the QD surface termination. Notable examples include the photoluminescence quantum yield (PLQY), the photoluminescence lifetime, and the photostability. A specific issue with QD surfaces seems to be the presence of undercoordinated surface atoms, which can lead to localized electronic states within the energy gap. By capturing conduction-band electrons or valence-band holes,6−8 such trap states will affect the relaxation dynamics of photoexcited QDs9 and make the lifetime of electron−hole pairs and the efficiency of their radiative recombination dependent on the properties of the QD surface.10 An established approach to ensure full coordination of the surface atoms, and to improve the optical properties of the QDs, is to embed them in another semiconductor material to form so-called core/shell QDs.11 Especially in the case of © 2018 American Chemical Society

Received: April 29, 2018 Revised: June 11, 2018 Published: June 12, 2018 4393

DOI: 10.1021/acs.chemmater.8b01789 Chem. Mater. 2018, 30, 4393−4400

Article

Chemistry of Materials

Figure 1. Sketches of (a) an InP/ZnSe and (b) an InP/CdSe interface. The lattice reconstruction at the interface leads to strain in both compounds. The crystal with the smaller lattice constant is subject to tensile strain, while the crystal with the larger lattice constant is compressed.

changes in photoluminescence efficiency or fluorescence intermittency. Here, we use InP/(Zn,Cd)Se core/shell QDs synthesized using a recently developed protocol26 as a model system to address the relation between strain and lattice mismatch in core/shell QDs. Solid solutions of ZnSe and CdSe are wellsuited for this purpose as their lattice constant varies almost linearly with composition, in agreement with Vegard’s law.27,28 This enables the lattice constant to be tuned from a = 5.68 Å to a = 6.08 Å when adjusting the composition from pure ZnSe to pure CdSe, a tuning range that encompasses the InP lattice constant of 5.87 Å. According to the literature, a latticematched InP/(Zn,Cd)Se heterostructure can be attained when the (Zn,Cd)Se solid solution has an xCd = 0.48 atom fraction of Cd,29−31 a combination that has already been explored before to form light-emitting diodes from (Zn,Cd)Se quantum wells.5,32 Using bulk lattice constants as a guideline, we thus expect the strain on the InP core in InP/(Zn,Cd)Se core/shell QDs to shift from compressive to tensile upon changing the shell from ZnSe to CdSe (see Figure 1). We study the conjecture of strain tuning by means of the Raman spectra of InP/(Zn,Cd)Se QDs with different shell compositions.33,34 This enable us to (1) demonstrate that the (Zn,Cd)Se shells are homogeneous ternary alloys and (2) monitor the strain in the InP core as a function of the shell composition by means of a single experiment. The data confirm the gradual change of the strain in the InP QDs from compressive to tensile when changing the shell from ZnSe to CdSe, and show that shells with an intermediate composition significantly reduce the strain in the InP core. These results give direct experimental support of the fundamental assumption of strain engineering in colloidal core/shell QDs, and highlight the use of Raman spectroscopy for the direct, in situ analysis of strain in heteronanostructures.

changed step-by-step from the emitting core to the passivating shell.21−23 However, this solution is not optimal since the strain at the core and shell interface increases as shells are grown thicker. As a result, suppression of interfacial defects is only expected up to a specific shell thickness.23 An alternative to minimize strain in core/shell QDs is to adjust the lattice constant of core and shell. While this approach will be limited to a few accidental combinations in the case of pure compounds, it can be readily implemented when using solid solutions as either the core or the shell material. In the case of quaternary (Zn,Cd)(S,Se) solid solutions, for example, the lattice constant can be continuously tuned from a = 5.41 Å to a = 6.09 Å24 by changing the solid solution composition.24 This method was recently applied by Pietra et al., who demonstrated that (In,Zn)P/Zn(Se,S) core/ shell QDs attain a maximal PLQY for those combinations of core and shell composition where the (In,Zn)P and Zn(Se,S) lattice constant match.24 A possible limitation of this approach to strain engineering is that, together with composition, also band offsets may vary. Hence, in particular, when changes in composition promote the delocalization of charge carriers into the shell, the benefits of strain reduction may be offset by enhanced nonradiative recombination at the shell outer surface. In the case of CdSe-based QDs, the relation between strain and optical properties has been assessed most simply using bulk lattice constant differences as a qualitative guideline.23 More quantitative approaches to evaluate strain made use of theoretical strain calculations based on hydrostatic models and bulk elasticity moduli,19,20 even if the predictions of such models did not fully agree with the experimentally determined strain in CdSe/CdS core/shell QDs.18 Alternatively, the narrowing of the X-ray diffraction peaks upon growth of a CdSe shell was used as an argument that HgSe/CdSe nanocrystals are free of strain, HgSe and CdSe being two materials with an identical crystal structure and lattice constant.25 On the other hand, for the (In,Zn)P/Zn(Se,S) system discussed above, strain-free compositions were estimated using the X-ray diffractogram of the (In,Zn)P core QDs and the lattice constant of bulk Zn(Se,S). Hence, whereas strain engineering seems a promising direction to improve the optical properties of core/shell NCs, it appears that no generally accepted method exists to evaluate the outcome of strain engineering. This has shown that even the most fundamental assumption behind strain engineeringsuppressing strain by minimizing core/shell lattice mismatchhas been verified only by indirect means, for example, by assessing

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EXPERIMENTAL SECTION

Synthesis of InP QDs. InP QDs were grown following Tessier et al. by mixing 50 mg (0.225 mmol) of indium(III) chloride and 150 mg (1.1 mmol) of zinc(II) in 2.5 mL (7.5 mmol) of technical oleylamine (OLA).35 The reaction mixture was stirred and degassed at 120 °C for an hour and then heated to 180 °C under an inert atmosphere. Upon reaching 180 °C, 0.23 mL (0.8 mmol) of tris(diethylamino)phosphine were injected quickly in the mixture, which initiates the formation of InP nanocrystals. After 30 min, the temperature of the reaction mixture was decreased to stop the reaction. InP nanocrystals were precipitated in ethanol and suspended in toluene. The obtained InP nanocrystals have a diameter of 3.2 nm (band-edge absorption [first exciton] at 560 nm). 4394


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Chemistry of Materials Synthesis of InP/ZnSe Core/Shell QDs. For the growth of the InP/ZnSe core/shell NCs, instead of cooling down the temperature, 0.45 mL of stoichiometric TOP-Se (2.24 M) were injected at 180 °C after 20 min of reaction time. After 140 min, a solution containing 2 g (3 mmol) of Zn(stearate)2, 8 mL of octadecene (ODE), and 2 mL of OLA was additionally injected. The temperature was then increased from 180 to 320 °C and 1.4 mL of TOP-Se was added dropwise during the rise of temperature. At 240 min, the reaction was stopped by decreasing temperature. InP/ZnSe NCs were precipitated in ethanol and suspended in toluene. Synthesis of InP/(Zn,Cd)Se Core/Shell QDs. To form (Zn,Cd)Se alloyed shells, a procedure similar to that for the growth of ZnSe shells was followed until after the first injection of 0.45 mL of stoichiometric TOP-Se (2.24 M). Afterward, at 140 min, a solution containing Cd(acetate)2 dihydrate and Zn(stearate)2 in 8 mL of ODE and 2 mL of OLA was injected. The Cd to (Cd + Zn) molar ratio xCd was always taken equal to the intended shell composition. The temperature was then increased to 320 °C and 1.4 mL of TOP-Se was added dropwise. After 240 min, the reaction was stopped by decreasing the temperature. InP/(Zn,Cd)Se NCs were then precipitated in ethanol and suspended in toluene. The structural composition has been determined by energy-dispersive X-ray spectroscopy (see Supporting Information, Table SI2). Raman Spectroscopy. For Raman spectroscopy, the samples were prepared by drop casting the sample solution on silicon and drying in ambient conditions. All spectra were recorded in a confocal micro-Raman setups in backscattering geometry. We observed strong resonant effects of the Raman intensities (see Supporting Information, Figure SI1). To obtain resolvable core and shell bands, different excitation wavelengths were used: 830 nm, 458 nm (diode lasers), 514 nm (ArKr laser), 532 nm (frequency-doubled Nd:YAG laser), and 633 nm (HeNe laser) served as excitation sources. Dispersion of the regarded first-order Raman bands can be neglected as shown in the Supporting Information. Spectra excited with ArKr laser lines were acquired with a Dilor XY800 triple monochromator setup equipped with a liquid N2 cooled CCD. For all other spectra a LabRamHR800 equipped with a Peltier-cooled CCD was used. On both systems, 1800 grooves/mm gratings were used. Details can be found in the Supporting Information (Table SI1). The laser power on the sample was kept at a few hundred microwatts on a diffraction limited spot to avoid sample heating or destruction. To avoid degradation effects, measurements were performed quickly after drop casting. The spectra were calibrated using neon lines.

Figure 2. (a) Absorbance spectrum of a typical InP QD sample used as core QDs for the formation of InP/(Zn,Cd)Se core/shell QDs. As indicated, the first exciton absorption peaks at 560 nm. Inset: brightfield TEM image of the InP QD sample, indicating the presence of ∼3.2 nm InP QDs. The scale bar measures 10 nm. (b) Raman spectrum of these InP core QDs. The light-grey background is a best fit of the intensity to a sum of two Lorentzians. The inset represents both Lorentzians separately, with an indication of the respective Raman shift.

Experimental Section). To change the atomic ratio xCd in these shells, the relative amount of zinc stearate and cadmium acetate was the only reaction parameter we adjusted. On the basis of the amount of shell precursor added, shell growth should result in a 15- to 20-fold increase of the QD volume, an estimate that is confirmed by the increase in apparent QD diameter as deduced from TEM images. In earlier studies on CdSe/CdS core/shell QDs, we found that strain in the core QD initially increases and then levels off as shells are grown thicker.18 Since thickness-dependent strain may obscure the relation between strain and lattice mismatch, we preferred working with such thick shells in this study. All shells, ZnSe, CdSe, and their solid solutions, were prepared in this regime. Figure 3 provides the basic characteristics of the InP/ZnSe and InP/CdSe core/shell QDs, which were the two pure-shell benchmark systems we studied here. According to transmission electron microscopy (TEM) imaging and X-ray diffraction, ZnSe overgrowth results in InP/ZnSe QDs that have an effective diameter of 10.2 nm and a zinc blende crystal structure (see Figure 3b and Supporting Information, Figure SI13a). In line with literature reports,26 ZnSe shelling induces a slight red shift of the InP exciton absorption from 560 to 598 nm and a pronounced absorbance increase at wavelengths below 460 nm, a wavelength corresponding to the bulk ZnSe band gap (see Figure 3a). The Raman spectrum of these InP/ ZnSe core/shell QDs features both the InP Raman bands, with the LO phonon shifted to 354 cm−1, and a new band at around 200−240 cm−1, that again consists of multiple sub-bands (see Figure 3c). Since the most pronounced contribution at 247 cm−1 almost coincides with the 249 cm−1 reported for the LO phonon of bulk ZnSe,40 we assign this additional band to ZnSe phonons. Possibly, the low-energy shoulder contains contributions of ZnSe TO phonon or surface phonon modes, which should be retrieved at 205 and 226 cm−1, respectively.40 Replacing zinc stearate by cadmium acetate in the shell growth reaction results in InP/CdSe core/shell QDs that have according to X-ray diffraction and bright-field TEM images a wurtzite crystal structure with an edge length of 23.3 nm (see Figure 3b and Supporting Information, Figure SI14a). In the absorbance spectrum, the formation of a CdSe shell is confirmed by the pronounced absorbance increase at wavelengths shorter than 708 nm, a wavelength corresponding to the CdSe band-edge transition (see Figure 3d). Opposite the

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RESULTS AND DISCUSSION InP, InP/ZnSe, and InP/CdSe Benchmark Systems. In this work, we systematically used InP QDs formed by reacting tris(diethylamino)phosphine and indium chloride as outlined in the Experimental Section. The reaction conditions were set such that ∼3.2 nm InP QDs are obtained, which have a first exciton absorption at 560 nm (see Figure 2). These nanocrystals were used as basis for all core−shell structures. We preferred such small QDs to avoid the formation of significant strain gradients from the strained interface to the QD center, which could complicate the analysis of the Raman spectra. Figure 2b displays a representative Raman spectrum of plain InP QDs, showing the frequency range around the longitudinal optical (LO) phonon of bulk InP. The spectrum features a Raman band that consists of two sub-bands centered around 341.2 and 306.9 cm−1. Similar spectra have been reported before for InP QDs, and the respective bands are commonly assigned to the longitudinal optical phonon (LO) and the transverse optical phonon (TO) of InP.36−39 Taking these InP QDs as a starting material, we grew core/ shell QDs following a fixed procedure, in which either zinc stearate, cadmium acetate, or mixtures thereof were reacted with trioctylphosphine selenide to form (Zn,Cd)Se shells (see 4395


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Figure 3. (a) Absorbance spectrum of InP/ZnSe core/shell QDs, with an indication of the wavelength corresponding to the bluk ZnSe band gap. Inset: enlargement of the wavelength range of the InP exciton transition. (b) Bright-field TEM image of InP/ZnSe core/shell QDs. (c) Raman spectrum of InP/ZnSe core/shell QDs, showing to Raman modes assigned to ZnSe and InP as indicated. The dashed line represents the Raman shift of the LO phonon of plain InP QDs. (d) Absorbance spectrum of InP/CdSe core/shell QDs, with an indication of the wavelength corresponding to the bluk CdSe band gap. (e) Bright-field TEM image of InP/CdSe core/shell QDs. (f) Raman spectrum of InP/CdSe core/shell QDs, showing to Raman modes assigned to CdSe and InP as indicated. The dashed line represents the Raman shift of the LO phonon of plain InP QDs. Inset: enlargement of the CdSe Raman mode; the vertical axis has been extended fivefold.

InP/ZnSe core/shell QDs, the InP Raman bands shift to lower frequencies after CdSe shelling, and a new Raman band appears with a main feature centered around 208 cm−1 (see Figure 3f). Closer inspection shows that the latter band consists of a main band at 208.5 cm−1 and a side band at 206 cm−1, a structure in line with previous reports for CdSe nanocrystals in which both bands where assigned to LO phonons and surface optical phonons of CdSe, respectively.18,41 InP/(Zn,Cd)Se Core/Shell QDs, Formation of Solid Solution Shells. The partial substitution of cadmium acetate for zinc stearate in the shell growth reaction is a way to obtain InP/(Zn,Cd)Se core/shell QDs with a variable shell composition.26 TEM images, X-ray diffractograms, absorption spectra, and the atomic fraction xCd of Cd in the shell for all samples that were used in this study are provided in the Supporting Information. These data confirm that adapting the composition of the reaction mixture enables us to continuously tune the shell composition. At the same time, increasing the cadmium fraction in the shell makes the nanocrystal more hexagonal and raises the contribution of the wurtzite structure in the X-ray diffractogram. Following a TEM analysis (Supporting Information), the employed synthesis protocol led to variations in the shell thickness in the regime of the structure transition. However, we always ensured the thick “saturation” regime. In addition, regardless of this gradual zinc blende to wurtzite transition, the equivalent (220) zinc blende (zb) and (110) wurtzite (wz) lattice spacing gradually increases from 2.00 Å for zb-ZnSe to 2.13 Å for wz-CdSe. This shift gives a first indication that the synthesis protocol used results in the formation of homogeneous (Zn,Cd)Se solid solutions.

Similar to the InP/ZnSe and InP/CdSe benchmark systems, InP/(Zn,Cd)Se core/shell QDs feature Raman spectra with two bands: a first in the frequency region where we observed the shell-related LO phonons and a second at Raman shifts close to the InP LO phonon. Figure 4 display zooms on the Raman spectra in these respective frequency ranges, which we recorded on InP/(Zn,Cd)Se core/shell QDs with either a zincrich or a cadmium-rich shell. For both systems, two important changes stand out. As compared to the InP/ZnSe benchmark system, admixing small amounts of Cd in the shell shifts the

Figure 4. Exemplary Raman spectra of InP/(Zn,Cd) core/shell QDs with (blue) xCd = 0.025 and (red) xCd = 0.70. Thin lines represent experimental data; bold lines best fit the sum of Lorentzians. The left side of the spectrum shows the frequency region of the shell-related bands and the right side of the spectrum the core-related bands. Both sides of the spectra have been normalized to the respective LO band intensities. LO frequencies from Figures 2 and 3 have been included as a reference. 4396


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Figure 5. (a) Lorentzian functions used to fit the shell-related LO Raman bands and (b) shell-related LO phonon frequencies versus relative cadmium amount in the alloy. The bulk values for zinc blende ZnSe and wurtzite CdSe are displayed as lines.40,44 The solid line is a fit according to Bouamama et al.27

Figure 6. (a) Lorentzian functions used to fit to the InP core LO frequency; (b) LO frequencies in red and strain estimated by evaluating eq 2 in blue versus relative cadmium amount. The corresponding lattice mismatch (ashell−acore) was estimated assuming Vegard’s law: ashell(x) = aCdSex + aZnSe(1 − x).27,29 The solid lines are linear fits. The dashed line corresponds to zero strain with the measured pure InP QD LO frequency of 341.2 cm−1. The spectrum of the 42% Cd samples was excited in the infrared spectral region and subject to a specific analysis. See Supporting Information (Figure SI6) for details. (The green square corresponds to CdSe/CdS core/shell QDs.18).

xCdin Figure 5b, together with a best fit to a second-order polynomial:

shell-related and the core-related Raman bands to lower frequencies. On the other hand, adding small amounts of Zn to the shell of InP/CdSe QDs results in a blue shift of both Raman bands relative to the InP/CdSe case. Clearly, these opposite shifts indicate that both Raman bands will vary continuously from the InP/ZnSe to the InP/CdSe case when xCd is raised from 0 to 1. Let us focus first on the shell-related Raman bands. In general, Raman spectra of alloyed crystals can exhibit a twomode or a one-mode behavior.42 According to the literature, bulk (Zn,Cd)Se solid solutions exhibit one-mode behavior, where the vibrational frequencies shift with the composition from the higher frequency ZnSe modes to the lower frequency CdSe modes, but no new modes appear.27 This frequency reduction of the shell-related LO bands with increasing cadmium content is a direct result of alloying, as replacing Zn by Cd changes the effective mass of the ions contributing to the LO vibration. Additional shifts can originate from changed bond strengths within the unit cell. In the case of (Zn,Cd)Se core QDs, such gradual shifting Raman bands have already been reported before, and were seen as indicative of the formation of solid solutions.43 Figure 5a displays the Lorentzian profiles that we used to fit the shell-related LO bands in the Raman spectra of all InP/ (Zn,Cd)Se QDs investigated. An overview of all the Raman spectra is provided in the Supporting Information. The central frequencies of these Raman bands are plotted as a function of the shell compositionquantified by means of the Cd fraction

ω = ωCdSex + ωZnSe(1 − x) − bLOx(1 − x)

(1)

Here, bLO represents the bowing parameter, for which we obtain a value of bLO = 60 ± 5 cm−1. This figure is in good agreement with considerations for bulk alloys.27,29 Given the absence of new Raman bands, this result confirms that the synthesis approach used here results in shells of (Zn,Cd)Se solid solutions, without separate minority domains with different compositions. Interestingly, we observe no significant deviation from the reported behavior for bulk alloys despite the transition from a zinc blende to a wurtzite lattice. It seems that the dynamic properties of the lattice are not strongly affected by the lattice geometry change. InP/(Zn,Cd)Se Core/Shell QDs, Strain in the InP Core. Having shells made of (Zn,Cd)Se solid solutions enables us to relate the composition of the shell with the lattice constant of the shell, and thus to compare core/shell lattice mismatch and strain in the InP core. Here, we can use the frequency shift of the InP LO phonon as deduced from the Raman spectrum to estimate strain. Similar to the shell-related Raman bands, the core-related Raman bands (Figure 4b) appear between the frequencies obtained for the InP/ZnSe and InP/CdSe benchmark systems, where both addition of Cd to a ZnSe shell and addition of Zn to a CdSe shell results in a frequency shift toward the plain InP LO phonon. As outlined in the Experimental Section, different resonant conditions for differ4397


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In earlier studies with CdSe/CdS core/shell QDs, we monitored the strain for increasing shell thickness.18 The CdSe/CdS sample with sizes most comparable to the present QDs has been added to Figure 6b, where it falls on line with the linear trend observed for the InP system. This suggests that the estimated mismatch-strain relation derived here for InP/ (Zn,Cd)Se can be applied to a broader range of III−V and II− VI compounds. In principle, the direct strain-mismatch dependence can be used to optimize various material combinations, for example, Cd(Se,S) layers that are employed as shells or buffer layers on QDs to reduce the Auger recombination and to optimize the lasing threshold.49 On the other hand, the linear dependence of strain and lattice mismatch is still probably only valid for the chosen sizes, small QDs with large, “saturated” shells. Deviations might exist for different-sized QDs with thinner shells, for example. There, further experimental and theoretical studies are required. Finally, while the InP/(Zn,Cd)Se core/shell combination is a useful model system to demonstrate the basic concept of strain tuning, it is not well suited to evaluate the relation between strain and photoluminescence efficiency. Admixing Cd lowers the conduction band offset and will, eventually, lead to a type II band alignment.26 The resulting increase of electron delocalization shows that admixing even small amounts of Cd shifts the band-edge emission to longer wavelengths and reduces the photoluminescence quantum yield. Given the bulk band alignments,50 similar issues may arise with alternative systems that could be made strain-free by alloying, such as InP/Zn(Se,Te), (In,Ga)P/ZnSe, and GaP/Zn(Se,S).

ent samples led to different relative intensities of the core- and shell-related Raman bands. This led to different specral widths, which is especially apparent for the sample with 5% Cd. Figure 6a represents the Lorentzians profiles used to fit the InP LO band for all InP/(Zn,Cd)Se core/shell QDs studied. The figure highlights the opposite shifts of the InP LO phonon relative to the InP core QDs in InP/ZnSe and InP/CdSe core/ shell QDs, and shows that shifts are counteracted by increasing (InP/ZnSe) or reducing (InP/CdSe) the Cd content in the shell, respectively. The observed shifts of the InP LO phonon frequency upon shell growth originate from strain in the InP lattice. In case a shell material with a smaller lattice constant, such as ZnSe, is grown, the surface reconstruction at the core/shell interface will shorten the In−P bonds in the core. This results in higher LO frequencies. Stretching the bonds by growing a shell with a larger lattice constant, such as CdSe, has the opposite effect. In the case of spherical, concentric core/shell QD, the relative lattice constant change Δa can be obtained from the relative a

Δω 45−47 : ω

Δω Δa y i = jjj1 + 3 zzz − 1 (2) ω a { k Here, γ stands for the Grüneisen parameter, which describes ∂ ln ω the hydrostatic component of strain γ = − ∂ ln V , where V is the crystal volume46,47 and amounts to γInP = 1.24 in the case of InP.48 In practice, we calculate Δω taking the LO phonon of the pure InP QDs as a reference. This approach assumes that such core QDs are free of strain, by which we disregard possible surface reconstructions on such core QDs. Figure 6 b represents the thus calculated relative change of the lattice constant, represented as a function of the lattice mismatch that we estimated using Vegard’s law. One sees that the gradual shift of the InP LO frequency between the InP/ ZnSe and the InP/CdSe benchmark frequencies upon increasing xCd translates into a gradual variation of the strain induced in the InP core lattice as a function of lattice mismatch. Given the error on our data, we modeled strain as a linear function of the lattice mismatch, and thus xCd. This Δa approach yielded a (xCd) = 1.9xCd − 1 as a best fit. As for the shell, we do not observe deviations due to a change from a zinc blende to a wurtzite lattice. The forces acting on the core originating from the surface reconstruction seem to not critically depend on the lattice configuration. We accordingly find that strain in the InP core switches from compressive to tensile when xCd = 0.53 ± 0.05. This value is close to the xCd = 0.4829−31 lattice matched composition for thin film InP/ (Zn,Cd)Se heterojunctions, and corresponds to a bulk lattice mismatch of only 0.25%. As for the shell, we do not observe significant deviations due to a change from a zinc blende to a wurtzite lattice. The forces acting on the core originating from the surface reconstruction seem to not critically depend on the lattice configuration. We thus conclude that, in the case of InP/(Zn,Cd)Se core/shell QDs, the assumption of a direct relation between lattice mismatch and strain is confirmed. Lattice matched conditions results in a strain-free environment; deviating from it induces strain and the amount of strain is directly related to the lattice mismatch between core and shell material. Shells with a smaller lattice constant than the core induce compressive strain in the core, whereas shells with a larger lattice constant give rise to tensile strain.

shift of the LO frequency −γ

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CONCLUSIONS We have quantitatively investigated the concept of tuning lattice strain in core/shell InP/(Zn,Cd)Se QDs by adjusting the shell lattice constant. The strain was estimated with Raman spectroscopy and the lattice constant was tuned by modifying the alloy composition. We observed a systematic shift of the shell-related Raman band, demonstrating the formation of a homogeneous (Zn,Cd)Se alloy shell. Within the size range under investigation, the strain depends linearly on the lattice mismatch between core and shell material, going from compressive strain in the case of a pure ZnSe shell to tensile in the case of a pure CdSe shell. In between, at a composition of Zn0.58Cd0.42, strain-free core/shell QDs can be obtained. We hope that the experimental confirmation of the direct strain engineering concept helps optimizing NCs for their emission performance.

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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.chemmater.8b01789. Additional TEM images, XRD studies, and an overview of all Raman spectra along with the employed excitation wavelengths and the corresponding spectral resolution (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: Mona.Rafi[email protected]. ORCID

Zeger Hens: 0000-0002-7041-3375 4398


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Holger Lange: 0000-0002-4236-2806 Present Address ∥

Department Physik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the German Research Foundation (DFG) via the Cluster of Excellence “The Hamburg Centre for Ultrafast Imaging” (CUI). H.L. is supported by the DFG via the project LA 2901/1-1. Z.H. acknowledges support by the European Commission via the Marie-Sklodowska Curie action Phonsi (H2020-MSCA-ITN642656), by the Research Foundation Flanders (project 17006602) and by Ghent University (GOA no. 01G01513). Z.H., M.T. and D.D. acknowledge the Strategisch Initiatief Materialen in Vlaanderen of Agentschap Innoveren en Ondernemen (SIM VLAIO)." vzw (SBO-QDOCCO, ICONQUALIDI). J.M. acknowledges support by the Deutsche Forschungsgemeinschaft (DFG) within the Cluster of Excellence “Engineering of Advanced Materials”(project EXC 315) (Bridge Funding).

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