Article pubs.acs.org/JPCC
Surface Doping for Hindrance of Crystal Growth and Structural Transformation in Semiconductor Nanocrystals Riya Bose, Goutam Manna, and Narayan Pradhan* Department of Materials Science and Centre for Advanced Materials, Indian Association for the Cultivation of Science, Kolkata 700032, West Bengal , India S Supporting Information *
ABSTRACT: Doping can strongly influence the crystal growth of semiconductor nanocrystals. It can change the surface energy and therefore the growth directions and shape of the host nanocrystals. While doping of transition metal ions in various semiconductor host nanocrystals is widely studied for obtaining new material properties, the effect of doping on crystal growth has been less explored. Herein, we study the change in the crystal growth pattern and growth rate with doping of one of the most common dopants Mn in a ZnSe host. With the help of selective surface binding ligands, hemisphere-shaped zinc blende nanostructures are designed from ZnSe seeds and dopant Mn precursors in different amounts are introduced at different stages of the synthetic process. Monitoring the sequential product and analyzing the surface or internal locations of the dopants, the possibility of shape change has been discussed. Moreover, the mutual effects of crystal growth and doping on one another are also determined considering the progress of the reactions under different conditions. We believe that the results presented here are important for understanding the doping mechanism and its effects during crystal growth in semiconductor nanocrystals, which are not clear to date.
1. INTRODUCTION Introduction of few dopant ions in a semiconductor host lattice efficiently alters the existing material properties or induces new optical, electrical, and/or magnetic properties in the host nanocrystals.1−19 With these finely tuned properties, the doped semiconductor nanocrystals can serve the purposes for various applications in diverse fields such as optoelectronics, photovoltaics, and bioimaging.4,5,20−24 Numerous synthetic strategies have been employed to synthesize them, and it has been found that doping is not just a random process; rather, it is very selective to the nature of the host and dopant.25,26 Several thermodynamic and kinetic models have been proposed to understand how impurity ions get inserted into the host lattice.25−28 Surface adsorption of dopants is one of the most widely accepted mechanisms, according to which, dopants can be inserted into the host lattice only if it can bind to the nanocrystal surface for a certain time comparable to the reciprocal of growth rate; hence, binding energy of the dopants on the surface plays a key role.26,28 Once the dopant is adsorbed onto the surface, further growth of the nanocrystal is required to make the dopants stay inside the nanocrystal. However, the dopant ion, being of a different dimension than the host ions, creates a defect at the doped site. Because of the small size of the nanocrystal, the defects tend to be expelled from it via self-purification mechanism, and thereby the dopant ion diffuses to the surface of the lattice and is finally ejected from the nanocrystal.27−29 Although in few cases, if the free energy change is favorable, the host nanocrystal compromises its structure to let the dopant ion stay in it.26,28 © 2013 American Chemical Society
Considering all the above factors and with manipulation of the reaction parameters, several doped semiconductor systems have been designed with varieties of dopants and hosts.3,6−8,14−19,22,26,30−39 However, a thorough study of literature reports reveals that research has been mostly focused on obtaining dopant-induced new material properties in doped semiconductor nanocrystals, whereas the effect of doping on the crystal growth of the host nanocrystals is much less studied.1−24,31−41 Though there are few reports of dopantinduced shape/phase change of host nanocrystals,42−44 core fundamental issues like how crystal growth controls doping, or whether dopants, once inserted into the host lattice, can control the host crystal growth, still lack experimental evidence. These are even more important for the cases where the host nanocrystal grows preferentially along some kinetically favored directions. In our quest to have a better insight into these unsolved issues, we investigate here the crystal growth and consequences of doping in the Mn-doped zinc blende ZnSe nanocrystals, which has been one of the most widely studied doping systems.10,19,22,45−47 Using limited protection of strong ligands on selective facets of ZnSe seeds, anisotropic ZnSe nanostructures with hemisphere shape via rods are designed and different percentages of dopant Mn precursors are introduced at different stages of the growth process. It is found that a certain Received: July 18, 2013 Revised: September 18, 2013 Published: September 18, 2013 20991
dx.doi.org/10.1021/jp407123s | J. Phys. Chem. C 2013, 117, 20991−20997
The Journal of Physical Chemistry C
Article
amount of dopant, when introduced at an appropriate stage, indeed slows the rate of the growth, restricts the formation of anisotropic structures, and leads to the formation of doped spherical zinc blende nanocrystals. From different observations, the mutual effects of doping on crystal growth and vice versa are determined and the conditions of predominance of one over the other are investigated.
2. EXPERIMENTAL SECTION a. Materials. Zinc stearate (Zn(St)2, tech.), selenourea (SU, (NH2)2CSe), octadecylamine (ODA, 97%), 1-dodecanethiol (DDT, >98%), manganese acetate tetrahydrate (Mn(OAc)2· 4H2O, 99.99%), and oleylamine (tech.,70%) were purchased from Aldrich. b. Methods. Synthesis of Zinc Blende ZnSe HemisphereShaped Nanostructures. In a typical reaction, 0.15 mmol of Zn(St)2 (0.094 g), 0.3 mmol of selenourea (0.036 g), 3g of ODA, and 0.4 mL of DDT were loaded in a three-necked flask and degassed for 5 min by purging argon at room temperature. Then the reaction temperature was increased to ∼50−70 °C, and purging of the argon gas was continued for another 10 min. Next, the temperature was increased to 250 °C. Once the temperature reached 250 °C, it was annealed for another 10 min. Synthesis of Mn-Doped ZnSe. Synthesis of Mn Stock Solution. The stock solution for Mn doping was prepared by dissolving 0.075 mmol of Mn(OAc)2 (0.018 g) in 5 mL of oleylamine under inert atmosphere. In a typical reaction during formation of hemispheres, when the temperature reached 150 °C, certain amount of Mn stock solution (Table S1 in Supporting Information) was injected into the flask and the reaction was continued as before. Purification. The as synthesized nanocrystals were precipitated using excess acetone from the crude product and further purified using chloroform as solvent and acetone as nonsolvent. The purified nanocrystals were dispersed in chloroform for further measurements.
Figure 1. TEM and HRTEM images of hemisphere-shaped ZnSe nanostructures in different orientations; (a) Associated twin structure of two hemispheres; (b, c, f) HRTEM images of a single hemisphere viewed laterally along the [022] direction; (e) HRTEM image of a typical hemisphere viewed along the [111] direction; (d) Lowresolution TEM image of hemispheres.
ZnSe (Figure S2 in Supporting Information). XRD of the sample also confirms the zinc blende phase (Figure S3 in Supporting Information). In the absence of thiols, 1D wurtzite nanowires are formed even at 150 °C;48 hence, addition of a certain amount of thiol here plays the crucial role in retaining the zinc blende phase as well as in changing the shape to hemispheres via rods. To dope Mn in these nanostructures, Mn precursor is injected into the reaction medium at different temperatures, keeping all other reaction conditions same. For the first instance, the dopant precursor Mn-acetate is injected above 220 °C, where rods of ZnSe have already been formed. But this does not result in any Mn emission at room temperature which is typically obtained in the orange-red window. The absorption spectra obtained from samples remain the same as that of the undoped case (Figure S4 in Supporting Information). Initially, it shows dual peaks at 316 nm and 332 nm, which are attributed to the magic-sized nanocrystals, and then it is red-shifted to ∼450 nm (Figure S4 in Supporting Information). Increasing the amount of added Mn also does not lead to any emission, and no change in absorption spectra is observed. In order to dope the nanostructures efficiently, the reaction procedure is changed and the Mn precursor is introduced at 150 °C, just after the formation of magic-sized seed nanocrystals. Successive evolution of the absorption and emission spectra for a typical reaction with 2% Mn addition are provided in Figure 2a and Figure 2b, respectively. A faint and unstable Mn dopant emission is obtained in this case, which quenches completely as the reaction progresses. The absorption spectral pattern remains the same as that of the undoped nanocrystals, and the absorption band edge moves to ∼450 nm. It is observed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis that less than 0.1% Mn (with respect to zinc) is incorporated (in an average of five reactions) in the nanocrystals. To get sufficient amount of Mn inserted into the ZnSe lattice, Mn precursor concentration is increased to 10% of the zinc precursor used. In this case, as was seen in the previous case, the absorption band edge moves to ∼440 nm under identical reaction conditions (Figure 2c). However, emission spectra show Mn emission at ∼580 nm, and its intensity is increased in comparison to the previous case (quantum yield ∼10%); however, as the reaction progresses, the emission intensity decreases (Figure 2d). Mn intake is also found to be
3. RESULTS AND DISCUSSION Doping of Mn is more effective in the zinc blende phase of ZnSe, and efficient emission has been reported from 0D particles.10,19,22,45−47 But here the study is focused on doping in anisotropic zinc blende ZnSe nanostructures to understand its effect on the directional crystal growth, or vice versa. These anisotropic structures are synthesized following a modified literature method using zinc stearate as Zn precursor and selenourea as Se precursor in alkylamine solvent with alkylthiol ligand as selective surface binding agent.48 In a typical synthesis process, both Zn and Se precursors along with the required amount of dodecanethiol and octadecylamine are loaded together in the reaction flask and the temperature is increased to 250 °C. Annealing the reaction mixture for 10 min at this temperature yields the hemisphere-shaped ZnSe nanostructures. Figure 1(a−f) shows the typical TEM and HRTEM images of the hemispheres in different orientations. These shapes are obtained from spherical seed dots via rod- and then bullet-shaped nanostructures and their average size remains ∼20−25 nm.49 Figure S1 in Supporting Information shows the typical HRTEM image of an intermediate stage ZnSe rod. The selected area FFT pattern of the hemisphere shows {1̅11̅}, {002}, and {111̅} planes with d-spacing of 0.32 nm, 0.28 nm, and 0.32 nm respectively, which correspond to zinc blende 20992
dx.doi.org/10.1021/jp407123s | J. Phys. Chem. C 2013, 117, 20991−20997
The Journal of Physical Chemistry C
Article
Figure 3. (a, b) TEM images of ZnSe nanostructures obtained from the 10% Mn added reaction; (c, d) TEM and HRTEM images of ZnSe nanostructures obtained from 20% Mn added reaction.
the samples collected from the reactions having different Mn precursor concentrations. In the case of 2% Mn addition to the reaction medium, no change in growth pattern or shape transformation is observed (TEM not shown). For 10% Mn addition (Figure 3a,b), a mixture of spherical and hemisphereshaped particles are obtained. The size of the spherical particles typically remains ∼5 nm, whereas the hemispheres are of ∼20− 25 nm. Wide difference in their size clearly distinguishes them. But for the sample in which 20% Mn has been introduced, only spherical nanoparticles of size ∼5 nm are obtained (Figure 3c,d). It is worth mentioning here that even if this reaction is annealed for a longer time, these spherical particles do not grow beyond ∼5 nm. X-ray diffraction (XRD) patterns of the samples with 2% and 20% Mn addition are shown in Figure 4.
Figure 2. Stepwise absorbance and emission spectra obtained with 2% (a, b), 10% (c, d), and 20% (e, f) Mn addition (with respect to zinc precursor) to the reaction medium, respectively.
increased to ∼1%. Further increase of the dopant concentration (∼20% of the zinc precursor) restricts the absorption band edge within ∼410 nm (Figure 2e). The reaction medium also appears to be almost transparent in contrast to the much more turbid reaction medium in the other two cases. The PL spectra show stable Mn emission with an increasing intensity as the reaction progresses (quantum yield ∼25%) (Figure 2f). However, in spite of large quantity of added Mn, the amount of Mn remaining in the nanocrystals is found to be ∼3% in the final samples after purification. These results suggest that with increase of Mn concentration in the reaction system, ZnSe particle size remains small. The Mn concentration added to the reaction medium and final Mn concentration remaining in ZnSe are summarized for all three reaction systems in Table 1. TEM images obtained from these reaction systems with different Mn concentrations also support the change in particle size. Interestingly, with a greater amount of Mn, in addition to the size change, the shape also changes from hemispheres to spherical dots. Figure 3 shows the TEM images obtained from
Figure 4. XRD pattern of ZnSe nanostructures with 2% (blue) and 20% (orange) Mn precursor addition (with respect to zinc) to the reaction mixture. The labeled peak positions correspond to zinc blende ZnSe.
Table 1. Amount of Mn Precursor Added to the Reaction Mixture during Synthesis of ZnSe and Final Mn Concentration Remaining in the Sample Mn precursor introduced into the reaction mixture (with respect to Zn precursor)
Mn percentage remaining in final sample (with respect to zinc)
2% 10% 20%
0.08% 1% 3%
Both of them show zinc blende phase, and the peak positions correspond to (111), (220), and (113) planes of bulk zinc blende ZnSe. Because of the smaller size of the ZnSe particles, broadening of the XRD peaks is observed for the 20% Mn added case. These results confirm that with low Mn concentration crystal growth direction remains unaffected by 20993
dx.doi.org/10.1021/jp407123s | J. Phys. Chem. C 2013, 117, 20991−20997
The Journal of Physical Chemistry C
Article
Figure 5. (a) Plausible mechanism of formation of H2Se gas by reaction of selenourea with 1-octadecylamine. (b) XRD pattern of Cu2Se obtained by purging H2Se gas from the reaction medium to CuCl solution in oleylamine. The labeled peak positions correspond to cubic Cu2Se. (c) Amount of Cu2Se precipitated by purging 5 mL of gas taken from different amount of Mn added reaction flasks into CuCl solution.
sets of reactions. This suggests that for the reaction in which hemispheres are formed, more selenourea is consumed in the reaction as the rate of growth remains much faster, and therefore less Cu2Se is formed by passage of the gas taken out from the reaction system. On the other hand, when more Mn is added to the reaction system, less selenourea is consumed for particle formation. That is why more unreacted H2Se gas remains in the reaction flask, which in turn indicates that the rate of growth here is slower. This leads to the conclusion that an appropriate amount of Mn incorporation definitely lowers the reaction rate despite having excess precursors remaining in the solution. Further to confirm the retention of Mn in the nanostructures and to get an insight about the Mn environment in the ZnSe lattice, the electron paramagnetic resonance (EPR) spectra of the samples having different Mn concentrations are analyzed. Figure 6 shows the EPR spectra obtained from samples with 2%, 10%, and 20% Mn addition to the reaction medium. In all three cases six-line hyperfine splitting is observed, which is characteristic of the Mn2+ ion and confirms the presence of Mn2+ in the samples. The hyperfine splitting constant calculated from the EPR spectra with 2% Mn addition is 66.9 × 10−4 cm−1 (g = 2.0063), which suggests that the Mn ions are present near the surface of ZnSe because the hyperfine splitting constant for Mn substituted in ZnSe lattice is 61.7 × 10−4 cm−1 for bulk ZnSe:Mn.39,41,45,46 For the 10% Mn addition, the hyperfine splitting constant remains 67.3 × 10−4 cm−1 (g = 2.0064), which also suggests Mn remaining near the surface of the nanocrystal.39,41 However, in both cases, weak signals are obtained along with the major signal. These weak signals have
doping and the nanocrystals retain the hemisphere shape, whereas for higher Mn concentrations, crystal growth is affected by the dopants, preferential directional growth is restricted, and the overall growth rate is slowed down. To understand more about these two facts, we further studied the rate of the reaction for the undoped and doped nanocrystals with different doping percentages. It has been observed that reaction of selenourea with alkylamine in the reaction medium leads to evolution of H2Se gas, which acts as the Se precursor. To study the rate of the reactions, equal volumes of the gas are collected at a particular time from the reaction flasks with different amounts of added Mn and are passed through copper chloride solutions of equal volume and concentration. Passage of H2Se gas leads to the formation of Cu2Se, which is confirmed with XRD. The probable reaction scheme for H2Se evolution and the XRD of the product Cu2Se are presented in Figure 5a and Figure 5b, respectively.50 Peak positions of the XRD pattern match with those of bulk Cu2Se (PCPDF entry 75-0889). The amount of this Cu2Se is measured quantitatively by ICP-AES, and the rates of the reactions are compared for the undoped and doped systems obtained with 2%, 10%, and 20% Mn additions. Figure 5c shows the amount of Cu2Se precipitated with respect to the amount of Mn added to the reaction medium. We found that for the undoped sample or the case with 2% Mn addition,
