ARTICLE pubs.acs.org/JPCC
Insertion/Ejection of Dopant Ions in Composition Tunable Semiconductor Nanocrystals Shinjita Acharya and Narayan Pradhan* Centre for Advanced Materials and Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
bS Supporting Information ABSTRACT: A new doping strategy to incorporate transition metal dopant ions in semiconductor hosts has been reported here where dopant ions are incorporated in the host lattice following a feasible cation exchange process. Composition variable CdxZn1 xSe alloy nanocrystals have been chosen as an appropriate host which is formed by the cation exchange reaction of Cd with Zn ions in ZnSe nanocrystals. Concomitant insertion and ejection of dopant Cu ions are studied. It has been observed that the composition of Zn and Cd in the nanocrystals decides how much dopant Cu ions will be retained in the host lattice. The entire doping process with evolution, wide range tuning, and quenching of dopant emission as a consequence of inclusion/exclusion of dopant ions is in situ measured during the alloying process. From the retained and expelled dopant amounts, their compatibility in the host lattice is correlated.
’ INTRODUCTION Light emitting transition metal ion doped semiconductor nanocrystals have emerged as a new series of nanocrystal emitters with lucrative properties in recent days.1 13 These have been obtained by achieving a precise control over the impurity incorporation in the semiconductor hosts which leads to stable, intense, and tunable emission in visible and NIR regions. Ever since the report on transition metal doping in semiconductor hosts by Bhargava et al.1 with significant enhancement of dopant emission intensity, there have been several endeavors to dope different transition metal ions (Mn, Cu, etc.) into a variety of II VI semiconductor hosts.2 8,11,12,14 18 These doped nanocrystals have garnered a stark increase in research interest due to their minimized self-absorption, higher excited state lifetime as compared to the shorter lifetime of the excitonic emission,19 emission spectral width, and thermal stability which attracts researchers to study the associated science behind the doping process and to implement them in various applications.3,5,11,20 The concept of doping is not new, but to incorporate the dopants in proper position inside the host lattice is crucial to attain the best dopant-induced new properties. Several such successful doping processes have already been achieved.1 3,9,10,21 However, a debate on the position of the dopants inside the host and their mobility within the crystal lattice is still going on corroborated by the absence of a clear insight into the fundamental doping mechanism. A dopant is a foreign element, and for its compatibility in a host nanocrystal, the radius and ionic charge of the dopant ions should be comparable to that of the host cations.22,23 Moreover, the reactivity of the chosen dopant precursors, the reaction condition, and the doping strategy should be suitable for these foreign impurity ions to enter into the lattice of the host nanocrystal. Despite some successes, many r 2011 American Chemical Society
such efforts of dopant incorporation fail as sometimes the reaction protocols do not favor their inclusion and sometimes the dopant ions do not fit into the host lattice. These phenomena are not new, and several hypotheses like crystal phase dependent adsorption of dopants,24 lattice parameter dependent incorporation of a higher percentage of dopants,22 dopant-induced lattice contraction and ejection,25 etc. are already demonstrated in the literature. However, there is no clear experimental evidence up to date on how the in situ variation of lattice parameters plays the determining role in either retention/ejection of the dopants into/from the lattice after they are introduced to the host. To achieve this, both the chemistry of formation of the doped nanocrystals and physical aspect of the change of crystal parameters within the nanocrystal are to be understood simultaneously. This can be achieved only in an appropriately designed methodology where composition of the host nanocrystals would vary continuously within a fixed host dimension. However, the most widely reported and studied growth doping strategy12 where nanocrystals are allowed to grow after dopant adsorption is not the right choice to solve the above-mentioned issue. The demerit of growth doping is the fact that it increases the size of the host continuously which in turn helps to retain more dopants and minimizes the chances of their removal from the lattice, thus giving us no idea of the ejection pathway of the dopant ions. Hence, to fulfill the aforementioned criteria, we have designed here a new doping strategy where dopants are allowed to enter inside the host nanocrystals of fixed domain size along with a selective foreign ion which simultaneously removes the host Received: June 3, 2011 Revised: September 5, 2011 Published: September 06, 2011 19513
dx.doi.org/10.1021/jp2052147 | J. Phys. Chem. C 2011, 115, 19513–19519
The Journal of Physical Chemistry C cations leading to a continuous change in the crystal parameters. Cations which have higher chemical activity can replace the host cations during the reaction along with retaining the size, shape, and phase of the host nanocrystals. During this ion exchange process, dopants are infixed in the nanocrystals, and their compatibility in the host lattice is studied during the entire composition variation reaction which in turn changes the lattice parameters of the alloyed host. Using the versatile Cu dopant, which provides wide-range tunable emission depending upon host size and composition, we report here its incorporation during the conversion of host ZnSe to CdSe nanocrystals following the chemically favorable cation exchange process of replacement of Zn ions by Cd ions. Precise control over the rate of cation exchange is one of the key factors to facilitate the dopant insertion, but continuous change of lattice parameters with an increase of Cd content determines the retention and ejection of the dopant ions. The entire study reported here presents the quantitative information on the insight of lattice strain dependent in situ inclusion/exclusion of dopants during the alloying process. Further, the strategy of doping adopted here is new for direct placement of dopants in a fixed size host lattice and which is different from the well-known dopant adsorption and growth mechanism. This article also highlights the behavior of Mn dopants to some extent under similar reaction conditions.
’ EXPERIMENTAL SECTION Materials and Reagents. Zinc stearate (Zn(St)2, tech), octadecylamine (ODA, 97%), octadecene (ODE, tech.), stearic acid (SA, 95%), tetramethylammonium hydroxide (TMAH, 25% in methanol), trioctylphosphine, cadmium oxide (CdO, >99%), oleic acid (OA, 90%), copper(II) chloride (97%), and oleylamine were purchased from Aldrich. Tributylphoshphine (TBP, 97%) was purchased from Spectrochem India. Se powder (200 mesh, 99.99%) was purchased from Alfa-Aesar. All the chemicals were used without further purification. Preparation of Stock Solutions. Se stock was prepared by taking 1.8 g of Se powder in 8 g of TBP inside a glovebox. Copper stearate stock was prepared by dissolving 0.0634 g of Cu-stearate in 10 mL of ODE at ∼70 °C under Argon. Zn(St)2 stock solution was prepared using 0.315 g of Zn(St)2 and 0.248 g of SA dissolved in 4 mL of ODE. Cd-oleate stock was prepared by taking 0.1284 g (1 mmol) of CdO along with 2.824 g (10 mmol) of oleic acid and 8 mL of ODE in a three-necked flask and purged with N2 for 15 min and then heated to 200 °C. The solution turned clear and was cooled to room temperature and collected for further use. Synthesis of Copper Stearate. SA (10 mmol) was dissolved in 20 mL of methanol in a conical flask and heated to 50 °C to get a clear solution. TMAH (10 mmol) was dissolved in methanol separately and added to the SA solution dropwise. The mixture was stirred for 20 min to complete the reaction. CuCl2 (5 mmol) was dissolved in methanol and dropwise added to the above solution under vigorous stirring conditions. Sky blue precipitate of copper(II) stearate was flocculated which was collected by centrifugation and dried under vacuum. Synthesis of CdxZn1 xSe Nanocrystals and Doping of Cu. A 25 mL three-necked flask was loaded with 0.063 g of Zn(St)2 and 5 mL of ODE, degassed for 15 min by purging with N2 at 100 °C, and then heated to 300 °C. In a separate vial, 0.5 g of ODA and 0.5 mL of the TBP Se solution were mixed, degassed for 5 min, and injected into the reaction flask at the same reaction
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temperature. After annealing for 5 min, the reaction was cooled to room temperature and purified repeatedly using acetone and chloroform as nonsolvent and solvent, respectively. The purified doped nanocrystals were redispersed into a 50 mL three-necked flask along with 1 g of ODA and 10 mL of ODE and degassed for 15 min and then heated to 180 °C, and 0.5 mL (0.5 mmol) of Cdoleate from the prepared stock was injected. Then the reaction progress was monitored spectrophotometrically collecting aliquots of samples in different time intervals. Surface Cleaning of Cu-Doped Nanocrystals. For the surface cleaning, the 1:1 mixture of trioctylphosphine and oleylamine (TOP OA) has been used. In a typical experiment, purified nanocrystals were taken in 2 mL of TOP OA mixture and heated at 150 °C for the desired time (5 10 min), then cooled to room temperature, and then again purified with an acetone methanol mixture. Characterization. Optical absorption measurements were carried out in an Agilent 8453 spectrophotometer. Photoluminescence spectra were collected using a Horiba Jobin Yvon Fluoromax-4 spectrofluorometer. TEM images were taken on a JEOL-JEM 2010 electron microscope using a 200 kV electron source. Specimens were prepared by dropping a drop of nanocrystal solution in chloroform on a carbon-coated copper grid, and the grid was dried under air. The dopant percentage and the composition of the alloy were determined by ICP using a Perkin Elmer Optima 2100 DV Instrument. At first, the doped nanocrystals and the aliquots at a different stage of the alloying were collected and repeatedly purified to remove excess precursors. Then, the purified nanocrystals were dissolved in chloroform. The chloroform was then evaporated, and the dried nanocrystals were digested in concentrated HNO3. The nitric acid solution of the samples was diluted with double-distilled water to do the measurements. XRD of the doped sample was taken by a Bruker D8 Advance powder diffractometer, using Cu Kα (λ = 1.54 Å) as the incident radiation. For the time-correlated single-photon counting (TCSPC) measurement, the purified nanocrystals in chloroform were excited at 370 nm using a picosecond diode laser (IBH Nanoled107) in an IBH Fluorocube apparatus (JY1IBH15000M). The fluorescence decays were collected on a Hamamatsu MCP photomultiplier. The fluorescence decays were analyzed using DAS6 software.
’ RESULTS AND DISCUSSION Selection of Dopants and Hosts. It is well-known that cation exchange reaction between Cd and Zn chalcogenide is both kinetically favored and feasible to generate composition-controlled CdxZn1 xSe alloyed nanocrystals.26 The bond strength of Cd Se (310 kJ/mol) being much higher than that for Zn Se (136 kJ/mol), the dissociation of the bond between Zn2+ and Se2 is much easier and thermodynamically favored leaving the anion lattice structure unchanged. However, to facilitate doping of another impurity ion during this cation exchange, the rate of this ion exchange process should be comparable with the chemical reactivity of the dopant precursors and rate of insertion of dopants. Hence, a balanced reaction system is required where dopants can enter into the host nanocrystals. Moreover, the size of the host nanocrystals should not change significantly during the entire reaction process. It has been observed that more than 80% of Zn in ZnSe nanocrystals can be replaced by Cd during the ion exchange process, keeping the size and phase unaltered.26 Hence, this process is chosen as an ideal system for our desired study. 19514
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The Journal of Physical Chemistry C For dopants, we have selected the optically active transition metal dopants which would enable us to monitor the reaction progress spectrophotometrically. Among such dopants, Mn and Cu are the most commonly studied ones where Mn results in short-range tunable orange-yellow emission irrespective of the size and nature of hosts and Cu gives a wide range of tunable emission depending on the size and nature of the host.8 In our present context, primarily Cu has been chosen for its faster and low-temperature reactivity to get incorporated inside the host nanocrystals. For the reaction temperature range of 100 180 °C, the reactivity of Cu and its doping process is comparable to the moderate and slow rate of the cation exchange process of replacement of Zn by Cd in ZnSe at a chosen optimized reaction condition. The tunable dopant emission from a Cu impurity center is also an advantage to monitor the composition change in the reaction.4 However, when we consider the case of dopant Mn, the cation exchange process does not suit the reactivity of our chosen Mn precursor, and hence insertion of dopant Mn cannot be obtained under this reaction condition, specifically the reaction temperature. Rather, the behavior of Mn ions under the influence of cation exchange has been studied on its predoped ZnSe host. Alloy Formation during the Cation Exchange Reaction. Before going into the in-depth analysis of the doping process and the evolution of the dopant emission, the cation exchange process and the change of size/shape/phase of the host nanocrystals are investigated. As the reaction proceeds, more Zn ions from ZnSe are exchanged by Cd ions leading to the tailoring of the band gap from pure ZnSe (Eg = 2.63 eV) to somewhere in
Figure 1. (a) UV visible spectra and (b) corresponding photoluminescence (PL) spectra (normalized) of ZnSe seeds and the sequential changes after Cd precursor injection. The excitation wavelength is 350 nm. The reaction is carried out at 180 °C.
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between ZnSe and CdSe bulk band gaps. The tunable UV visible and PL spectra (Figure 1) obtained from optimized reactions indicate the variation of composition of the nanocrystals. The overall shapes of the absorption spectra of the ZnSe and CdxZn1 xSe alloys are almost the same showing that they are homogeneous alloys26 and neither the core shell nor gradienttype structures, which is also corroborated by the absence of any high-temperature alloying point that is usually observed in CdSe seeded ZnxCd1 xSe alloyed structures.27 TEM images collected during the alloying process show similar particle diameters, and this suggests that the particle size remains unaltered (Figure 2a,b). Measurement of XRD data in Figure 2c confirms the Cd cation exchange, and the product is CdxZn1 xSe after the partial conversion. The ZnSe XRD pattern consists of the characteristic peaks of zinc blende ZnSe (JCPDF 37-1463). The XRD patterns then undergo a blue shift, and the final XRD of the alloy is located in between those of zinc blende ZnSe and CdSe materials as expected due to the gradual replacement of Zn ions from ZnSe to form CdxZn1 xSe. The dependence of the diffraction peak positions of the obtained alloyed nanocrystals on their corresponding compositions is in accordance with Vegard’s law, which provides alloying evidence.27,28 The general pattern of the cubic lattice is maintained in the CdSe nanocrystals, but the diffraction peaks shift to smaller angles consistent with the larger lattice constant for CdSe compared with ZnSe. The fact that the diffraction peak widths are nearly identical for the initial ZnSe and the obtained alloyed nanocrystals after cation exchange indicates that the crystalline domain remains constant after cation exchange which helps us to achieve doping in subsequent steps. This further rules out phase separation or separated nucleation of CdSe nanocrystals and is an indication of homogeneous alloy formation.26 As the final sample has above 90% Cd content (
