Optical Signatures of Impurity-Impurity Interactions in Copper

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Optical Signatures of Impurity-Impurity Interactions in Copper Containing II-VI Alloy Nanocrystals Biswajit Bhattacharyya, Kushagra Gahlot, Ranjani Viswanatha, and Anshu Pandey J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03087 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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The Journal of Physical Chemistry Letters

Optical Signatures of Impurity-Impurity Interactions in Copper Containing II-VI Alloy Semiconductors Biswajit Bhattacharyya,1 Kushagra Gahlot,2 Ranjani Viswanatha 2,3 and Anshu Pandey1* 1

2

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.

New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur,

Bangalore-560064, India. 3

International Centre for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific

Research, Jakkur, Bangalore-560064, India.

ACS Paragon Plus Environment

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Abstract

We study the optical properties of copper containing II-VI alloy quantum dots (CuxZnyCd1-xySe).

Copper mole fractions within the host are varied from 0.001 to 0.35. No impurity phases

are observed over this composition range and the formation of secondary phases of copper selenide are observed only at xCu > 0.45. The optical absorption and emission spectra of these materials are observed to be a strong function of xCu, and provide information regarding composition induced impurity-impurity interactions. In particular, the integrated cross section of optical absorption per copper atom changes sharply (from 1 x 10 -2 nm3 to 4 x 10 -2 nm3) at xCu = 0.12, suggesting a composition induced change in local electronic structure. These materials may serve as model systems to understand the electronic structure of I-III-VI2 semiconductor compounds.

TOC graphic:


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Copper containing II-VI semiconductors have recently attracted attention because of the observation of diluted magnetic semiconducting properties1-5 and photomagnetism6 in these systems. The copper impurity introduces d levels within the host band gap.7 In an II-VI semiconductor the d levels of transition metal impurities interact weakly with the host valence band due to a d-p hybridization of the impurity levels.8-11 Given the weaker nature of this interaction relative to intersite coupling observed in the case of highly mismatched alloy (HMA) systems,12-16 copper doped II-VI materials are excellent candidates for the study of impurityimpurity interactions.17-18 Further, the composition induced changes to optical and electronic properties may serve as a model for understanding the properties of I-III-VI2 semiconductors and their alloys with II-VI materials. Here we study the optical properties of Cu:CdZnSe alloy quantum dots (QDs) from the dilute to the concentrated regime. The composition the copper impurity is tuned from 0.1% to 45% (cations basis), allowing us to investigate the effects of intersite coupling on the optical properties of these materials. A Cd-Zn alloy host was chosen as we observed that this enables us to incorporate large amounts of impurities into the host lattice. Similar effects have been observed by other authors for Mn impurities although with inclusion levels significantly lower than attained in our work.19 The copper impurity in II-VI semiconductors is associated with a strong luminescence that originates from the radiative decay of a conduction band electron into the impurity center.20 This luminescence occurs in parallel with normal excitonic emission from the host, thus offering a method to locate the positions of the copper d levels within the host gap.21-22 Here we employ this very characteristic transition to track the evolution of inter-impurity interactions in copper containing colloidal QDs. Figure 1a shows a schematic of the band edge as well as the impurity emission from a copper containing QD. For each generated electron-hole pair, emission may


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occur either through the normal band edge pathway, or else by recombination at the copper center. Since both emission bands involve host levels, the emission energetics depends on the host in each case. While the band edge emission is dependent upon the relative energies of the conduction and valence bands ( = − ), the emission from the copper impurity is dependent upon the energy spacing between the conduction band and the copper level ( = − ). The difference between these two energies thus represents the distance between the copper impurity and the valence band edge ( = − = − ). These energies are also indicated in Figure 1b. This figure shows the absorption spectrum (dashed) of a copper containing QD with = 0.008 mole fraction of copper. The emission from the sample is shown by the solid blue curve. Both the band edge ( ) and the impurity ( ) energies are discernible. The energy separation of these two energies corresponds to . As shown below, being a difference of energies, may be used to exclusively understand the changes to impurityvalence separation with concentration. To distinguish between confinement and composition induced effects, we factor into various contributions. For a QD, the valence band energy is a function of the quantum confinement, i.e. ( = + ).23 Here we have explicitly made a difference between shifting of the valence band energy due to alloying and host-impurity interactions and the shifting of the valence band energies due to quantum confinement.8, 24 The sizes of the QDs studied by us are larger than 6 nm. For such sizes, the confinement energy of the valence band states is less than 30 meV; this has been verified experimentally and theoretically.25 This number is over an order of magnitude smaller than the energetics of the phenomenon that we describe here, and consequently, we neglect it in the remainder discussion. We therefore approximate by ( ~ − ) where is not a function of quantum confinement. Our treatment further


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considers to be constant for all QDs, since our entire discussion focusses on selenide hosts.26 The precise position of is also affected by the coupling to copper energy levels. Any shift in the energies of copper levels will therefore cause a corresponding change in the energy position of . We employ composition as a tuning parameter to study the interactions between copper impurities. Our preparation method enables us to continuously increase the mole fraction of copper within a ZnCdSe host from ≡

|56 > = |5; > =

√

√

(2 − 2 ) (2 + 2 )

,

where 2 and 2 represent the states on the two adjacent impurities. Given the cell periodic nature of |. >, 06 ~

=> √

(< .||2 > −< .||2 >) =

=> √

(< .||2 > −< .||2 >) = 0,

while the same argument leads to 0; = 20 where 0 is the value of the integral for a single impurity. Thus, even though impurity-impurity interaction broadens the actual spread of the energy levels, a photoluminescence measurement primarily registers the shift in the energy of the emission. The optical properties of a three member cluster may be evaluated in a similar fashion to be 2.910, 0, 0.090 for the lowest to the highest state.

From these estimates, it is apparent

that the emission arises primarily from the lowest state of copper clusters. For a set of interacting impurities, thus measures the separation of the most symmetric energy level from the host valence band. The systematic changes in that are shown in Figure 3a confirm a substantial impurity-impurity interaction and are suggestive of the emergence of an impurity band at increasing copper concentrations. We note that the systematic change in the values of

cannot otherwise be generated from the presence of extraneous phases or sample inhomogenieties.


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Figure 3b shows the composition induced change in for various CuZnCdSe QDs. The line is a guide to the eye. It is seen that with a mole fraction of 0.35, an as low as 0.44 eV is observable, corresponding to a shift of 0.18 eV from the energies of non-interacting centers. This corresponds to a total effective spread of copper levels over 0.36 eV due to impurity-impurity interactions. From this figure it is also evident that significant changes to emerge only beyond = 0.12. The expected cluster size evolves monotonically and consequently a sudden onset of change in would necessarily involve a change in the local electronic structure of the copper center. We studied this effect further using optical absorbance of the material. High copper inclusion levels are associated with significant changes to the optical absorption. Figure 4a exemplifies typical cross sections per unit volume observed at different copper mole fractions. With mole fractions as high as 0.31, the optical band gap of the material is observed to red shift to 1.2 eV. We note firstly that this absorption tail extends well below the known bulk band gaps of the CdZnSe host (minimum of 1.72 eV for bulk CdSe), indicating its origin from transitions originating from the copper center. Also shown is the emission spectrum of the sample corresponding to = 0.35. The shoulder at 3.2 eV corresponds to the band edge emission. As is clear from this curve, the absorbance of CuZnCdSe extends well below the CdZnSe band edge, and this may therefore be directly attributed to the existence of copper states. The copper – induced increase in absorbance at lower energies due to a replacement of group-II ions with copper is expected to be accompanied by a decrease of absorbance at other energies as suggested by the sum rule. In particular, copper has one less valence electron than the group II cations, suggesting a reduced volume normalized integrated cross section. We note from Figure 4a that the cross sections per unit volume of the host band edge itself does not diminish as a


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consequence of copper inclusion. Rather, at optical energies, the copper related absorption appears to be purely additive. This is along expected lines. The d levels of the group-II cations are located deep within the valence band and make a direct contribution to the absorbance only at energies > 7 eV, much higher than the band edge in II-VI materials. The substitution of the group-II cations by copper thus does not perturb cross sections of the host significantly in the optical region though it is expected to affect absorption at higher energies.33 Our observations of enhanced sub-band gap absorption as well as blue shift of the host band edge absorption are in agreement with predictions made by other authors using atomistic simulations.34-35 In particular, it has been predicted that increased copper inclusion into a II-VI lattice causes enhanced interimpurity as well as impurity-host coupling, causing an absorption band to emerge below the normal host band gap, consistent with empirical observations. It is possible to extract additional information regarding copper-copper interactions by estimating the volume normalized cross section per unit copper (@A ). Figure 4b shows the @A of the samples considered in Figure 4a. It is apparent that at higher , there is an increase in @A at low energies. This directly implies the existence of inter-center interactions that cause an increase in the absorbance per copper. In order to isolate the effects of d-d coupling from the II-VI host absorption, we studied the integrated @A of samples at wavelengths longer than the host emission maximum, i. e. @AB = D

1 2C@A , where C is the wavelength corresponding to This is plotted in Figure 4c against the mole fraction of copper. It is immediately evident that @AB increases only gradually until = 0.12, but then exhibits a very rapid rise at > 0.12. It is further evident that it saturates at = 0.2. This sharp increase in the values of @AB is commensurate with a similar change in the


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PL emission behaviour. While the emergence of a finite value of @AB begins at relatively lower copper inclusion levels, its sudden increase above a certain value could arise because of two possible effects. Firstly, a relatively simple desolvation and segregation effect may be expected whereby copper ions begin to form an independent phase separate from the host after a certain nucleation threshold is crossed. A second possibility is the change in the local electronic structure of the copper centers as a consequence of impurity-impurity interactions.36 Given the absence of desolvation effects in the composition ranges being considered, a change in the local electronic structure of the copper center is a more likely eventuality. Such a change is also expected from the valence band anti crossing (VBAC)13, 37 picture of impurityimpurity interactions. For impurities such as Mn, an onsite p-d hybridization has been shown to play a critical role in causing an anti-crossing between the impurity levels and the host valence band.24, 37 The consistent decrease in values at > 0.12 is consistent with a similar change hybridization of the copper center, induced by concentration effects. We note that the significance of this onset threshold is presently unclear. The cations of a II-VI semiconductor are arranged in a face centered cubic lattice that exhibits a site percolation threshold at = 0.199. Based on our current understanding, the observed onset at 0.12 thus has no universal significance and is expected to be peculiar to copper and the selenide host. A remaining issue is the precise origin of the optical absorption below the host band edge. Given that copper introduces both filled and unfilled levels into the host gap, the optical absorbance could originate either from the excitation of electrons into the copper site or else the excitation of electrons from the filled copper states into the host conduction band. To verify the latter contribution, we studied the photo luminescence (PLE) excitation spectra of these QDs. It is observed that photo excitation of the absorption tail does indeed lead to the appearance of


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sample emission. The qualitative similarities of the excitation and absorption spectra are further outlined in Figure 5. The correspondence between these two spectra is consistent with the optical absorption band leading to the photo excitation of an electron into the conduction band from the copper level. In conclusion, we describe the unusual composition dependent optical properties of copper containing II-VI semiconductor QDs. Intersite coupling between centres strongly manifests itself over an = 0.12 cationic mole fraction. At copper mole fractions as high as 0.35, the lowest energy copper d state is braodened over an energy range of 0.36 eV. The presence of impurity states ~1 eV above the selenide valence band edge gives rise to an optical absorption that extends below the host bulk optical band gap to 1.2 eV. An analysis of the optical cross sections associated with this absorption band indicates changes of the conduction band to copper optical transition probabilities above an inclusion level of = 0.12. Given the absence of secondary phases at these concentrations, as well as the lack of a clear anticrossing of the copper levels with the host valence band, the composition induced changes in optical properties are attributed to a change in the local electronic structure of the copper center.


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FIGURES:

Figure 1 a. Schematic of the band edge and the impurity emission from a copper containing QD. b. The emission (blue solid line) and absorption spectra (red dashed line) of QDs where the violet arrow present the radiative recombination through band-edge, green arrow through the impurity centre (Cu). c. Schematic of the broadening of impurity energy band when there is higher number of impurity atoms present in the QDs.


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Figure 2. a XRD patterns of three exemplary samples containing 0.21 (blue, top panel), 0.21 (red, top panel) and 0.53 (black, middle panel) mole fractions of copper. Desolvation and presence of alternate copper chalcogenide phases is only noted in case of the sample with a 0.53 mole fraction of copper (middle planel). Standard patterns of ZnSe (black, lower panel) and CdSe (blue, lower panel) are also shown. b. and c. Gradual etching of CdZnCuSe QDs shows the existence of a nearly uniform distribution of copper ions across the entire QD.


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Figure 3a. The PL spectra of copper containing QDs with different mole fraction of copper (from bottom to top: 0.008, 0.03, 0.10, 0.24, 0.31). The band edge emission has been displaced to 0 in each case. b. Variation of E3 with mole fraction of copper.


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Figure 4a. Optical cross sections per unit volume with different mole fraction of copper present in QDs (red, blue and green solid line represent the mole fraction of copper in QDs 0.31, 0.10 and 0.007) and the red dashed line present the corresponding emission plot of the 0.31 copper containing QDs. b. Optical cross section per Cu atom for the samples in panel a. c. Integrated absorption cross section per copper atom of the QDs up to the host emission maxima with the mole fraction of copper.


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Figure 5. PL excitation scan with different emission energy of 1.65 eV (brown solid) and 2.25 eV (blue solid).Green dashed line in the emission scan of the same sample (excitation energy 3.27 eV).

ASSOCIATED CONTENT Supporting Information: Detail synthetic methods for making the nanocrystals with different experimental details. AUTHOR INFORMATION Corresponding Author *[email protected] ACKNOWLEDGMENT BB acknowledges IISc for Research Fellowship. AP acknowledges DST and ISRO-IISc STC for generous funding. RV is grateful for the Sheikh Saqr Career Award Fellowship.


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Optical Signatures of Impurity-Impurity Interactions in Copper

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