Rational Codoping as a Strategy to Improve Optical Properties of

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Rational Codoping as a Strategy to Improve Optical Properties of Doped Semiconductor Quantum Dots Jin Zhong Zhang, Jason K. Cooper, and Sheraz Gul J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz501739v • Publication Date (Web): 09 Oct 2014 Downloaded from http://pubs.acs.org on October 14, 2014

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The Journal of Physical Chemistry Letters

Rational Codoping as a Strategy to Improve Optical Properties of Doped Semiconductor Quantum Dots Jin Z. Zhang*, Jason K. Cooper, and Sheraz Gul Department of Chemistry and Biochemistry, University of California, Santa Cruz, CA 95064, USA [email protected]

ABSTRACT Doping is a powerful and convenient technique for rationally altering the electronic, magnetic, and optical properties of materials including nanomaterials such as quantum dots (QDs) or nanocrystals (NCs). Most doping involves introduction of an impurity element or ion, into the crystal lattice of the host material, which tends to result in lattice distortion and/or charge imbalance when the dopant charge does not match the charge of the host ion replaced. One solution to such problems is codoping with another element or ion that helps to reduce lattice distortion or charge imbalance, which can stabilize the primary dopant in the host lattice and substantially improve the photoluminescence (PL) of the primary dopant.

Furthermore,

interaction between the codopant and primary dopant can be used to tune the PL properties by altering energy levels related to donor acceptor pair recombination.

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TOC

Keywords: Codoping, doping, quantum dots, charge balance, photoluminescence, optical properties.


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Doping is a method to introduce different elements or “impurities” into a host material with the intention to alter its properties, e.g. electronic, magnic or optical. This is critical to the electronics industry since chips used in computers and other electronics are based on silion that is p (positive) or n (negative) doped for controlling its electronic properties. Doping is also essential for crystals used in lasers or other photonics products to allow generation of different wavelengths of light.

Another important area involving doping is phosphors or

electroluminescent materials fr solid state lighting. Semiconductor quantum dots (QDs) or nanocrystals (NCs) have been studied extensively due to their potential applications in economical, solution processable devices. The intriguing optical and electronic properties of the QDs, originating from quantum confinement effects and large surface-to-volume ratio,1-6 make them emerging candidates for applications including solid-state lighting,7,

8

biomedical labeling9-11 and photovoltaics.12,

13

Their properties can be

varied in a controllable manner by changing size, shape, surface functionalization, and/or by introducing small quantities of dopant atoms or ions. Doped semiconductor QDs constitute an important subclass of nanomaterials where a small amount of impurities is intentionally incorporated into the lattice of the host QD material, thereby adding another degree of freedom for altering their properties.14-18 Doped nanomaterials possess some unique properties that are important for imminent applications, including typically long excited state lifetimes, minimum self-absorption, broad emission spectral window, and thermal stability.19-25 As an example, Cu-doped II-VI semiconductor QDs have been the subject of great interest in recent years due to their potential use as bright visible light phosphors with sizetunable emission.20, 26, 27 In particular, the low toxicity and large Stokes shift of Cu-doped ZnSe



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QDs makes them an attractive candidate for lighting and display applications.28, 29 However, the synthesis of Cu-doped ZnSe can be quite challenging because there is a large difference in the solubilities of CuSe and ZnSe that renders their co-precipitation rather difficult. As observed in our recent study, Cu enters ZnSe as Cu+ and mostly stays on or closer to the surface of the host.30 The +1 oxidation state would require the Cu ion be accompanied by defect structures, specifically selenium vacancies (VSe), for charge compensation. One possible approach to stablize Cu+ and balance charge is to use an anion with -1 charge to replace S2- or Se2-, e.g. Cl- or Br-, as has been demonstrated before.31-33 Another approach is to codope with a group III element (3+) to replace another Zn2+ ion near the Cu+ site, which also results in charge balance. Coactivators or codopants have been found to introduce donor levels just below to conduction band (CB)34 and enhance the donor-acceptor pair (DAP) emission, which is the primary emission observed.35 Codoping provides a powerful method to possibly manipulate the relevant electronic energy levels for the photoelectron and photohole independently so as to enhance the overall optical properties such as photo- or electro-luminescence, which is important for emerging photonics applications. The basic idea of codping is schematically illustrated in Figure 1 using ZnS as a host material and Cu+ as a primary dopant with Cl- or Al3+ as the codopant. While there is a number of examples of codoping, few studies are directed at replacing two neighboring ions with the same charge by two different dopants with the same type of charge but different oxidation states with the attempt to balance charge, as the scinerio illustrated in the bottom right of Figure 1.


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Figure 1. Illustration of single and codoping using a two-dimensional lattice model of ZnS with Cu+, Cl- and Al3+ as dopants (Cu+ is the primary dopant while Cl- and Al3+ are codopants)

In terms of energy levels that determine optical properties, codoping affords additional flexiblity compared to single doping. However, complications can also arise with codoping, such as undesired energy or charge transfer betweent different dopants. In the ideal case, one may wish to introduce one relevant energy level for the photoinduced electron (or photoelectron for short) using one dopant and another energy level for the photohole via another dopant, or codopant. This is illustrated in Figure 2. The energy levels depend sensitively on the chemical nature of the dopant and codopant as well as its local bonding enviroment. To achieve the scinerio illustrated in the bottom right of Figure 2 in a rational manner is not trivial and requires careful design and experimentation.



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Figure 2. Comparion of energy levels of undoped (top left), singly doped (top right and bottom left) and codoped (bottom right) semiconductor quantum dot (QD) host. VB for valence band, CB for conduction band, EA for electron acceptor, HA for hole acceptor. The double arrows in HA represent two electrons with opposite spin. The scenario of codoping to introduce EA and HA at the same time creates p (EA) and n (HA) doping in one QD.

Optical properties of doped QDs are usually of primary interest and are characterized using UV-Vis electronic absorption and fluorescence spectroscopy.

Time-resolved optical

studies can provide additional information about the dynamic properties of excitons or charge carriers. Such studies also provide information about the relevant optical transitions and energy levels involved. In doped QDs, trap state emission is often involved and needs to be carefully sorted out from the dopant emission. For example, in the case of Cu-doped ZnSe QDs with codoping using Al, Ga, and In, the host PL is dominated by excitonic or bandedge emission with very weak trap state emission due to the high quality of the QDs (unpblished results) Upon doping or codoping, as expected, the host PL is decreased substantially while PL from dopants 6 ACS Paragon Plus Environment

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and codopants increases. Figure 3 shows representative UV-Vis and PL spectra of the undoped and differently doped ZnSe QDs.

Figure 3. (a) Photograph of the codoped ZnSe/ZnS QD system as excited by UV light. (b) UV-vis absorption spectrum of undoped ZnSe/ZnS QDs (filled curve) and PL spectra (ߣ௘௫ ൌ380 nm) of undoped ZnSe/ZnS, ZnSe:Cu,Al/ZnS (Cu,Al), ZnSe:Cu/ZnS (Cu), ZnSe:Cu,Ga/ZnS (Cu,Ga) and ZnSe:Cu,In/ZnS (Cu,In) (solid lines). (unpublished results)

While there are many studies reported on doped semiconductor QDs, codoping or dual doping is less studied and understood. Codoping can be achieved by simultaneously replacaing two cations, two anions, or a cation and anion.36 For example, Cu+ and Cl- (or Br-) have been used to replace a cation and anion at the same time, and codoping improved the PL properties compared to single doping.37, 38 Codoping with the intention to better balance charge has also been demonstrated using two different valenced anions. For example, TiO2 powders codoped with N and F show enhanced photocatalytic activity in the visible region with respect to TiO2 7 ACS Paragon Plus Environment


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doped only with N.39 Density functional theory (DFT) calculations indicate that N-F codoping reduces the energy cost of doping and also the amount of defects (number of oxygen vacancies) in the lattice, as a consequence of the charge compensation between the N (p-dopant) and the F (n-dopant) impurities. UV-Vis spectroscopy confirms the synergistic effect of N-F codoping: more impurities are introduced in the lattice with an increased optical absorption in the visible. Similarly, two cations can be used to replace two host cations at the same time, as demonstrated in the case of ZnO codoped with Cu and Ga, where bandgap reduction and p-type conductivity are caused by the incorporation of Cu while the tuning of carrier concentration is realized by varying the Ga concentration.40 The oxidation states for Cu and Ga were not explicitly determined or reported in this study. As another example, Cu2+ and Pb2+ have been used to codope ZnS QDs and the charge is balanced if the assumed oxidation states for the dopants are correct (which are not unambiguously determined).41 In a similar study, Cu+ and Mn2+ have been used to codope ZnS and ZnSe QDs.42 The dominance of the PL can be switched from one dopant to another by tuning the bandgap and composition of the host QDs. In this case, charge balance is not intentionally achieved if Cu+ replaces Zn2+ as suggested based on the lack of ESR signal. It should be mentioned that in general the determination of the oxidation state for Cu is not trivial and there are inconsistencies in the literarture.32, 43 One should always attempt to experimentally determine it excplicitely when possible and to this end, techniques such as X-ray absorption near edge structure (XANES) has proven to be especially useful.44, 45 While studies of codoped QDs have shown improved optical properties, the local strutcure of dopants and codopants as well as their synergestic interaction are often not well understood. To fully understand the fundamentals related to doping and codoping, it is essential


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to study and correlate the local structural properties of dopants with their optical properties as well as their associated electronic energy levels. The optical properties depend on the electronic structure or energy levels, which in turn depend on the local crystal structures of dopants as well as host. The local structures around dopant ions are influenced by the oxidation state mismatch and ionic size difference between the dopant and host ions. Photogenerated electrons and holes are expected to decay through dopant or impurity ion luminescence centers with different transition probability and decay rates as compared to that of the host.46 Especially for QDs, the optical properties and dynamical processes are expected to be sensitive to the local structures of the dopant atoms due to strong interaction between quantum confined excitons and dopant sites. Furthermore, any defects around the dopants can also strongly infuence their energetics and optical properties. Therefore, it is extremely important to obtain a clear, atomic level picture of the local structures surrounding dopant ions, including the location of dopant ions in the QDs, local distortions in bonding environment, and oxidation states of the dopants. One of the most powerful techniques for probing local strucures of dopant as well as host ions is extended X-ray absorption fine structure (EXAFS), an element specific technique sensitive only to the first few neighboring shells of the absorbing atom.37, 38, 45 This technique is particularly useful for dopants with low concentration such as Cu.37, 38, 43-45, 47 As an example of EXAFS studies, the local structure of Cu in ZnS and ZnSe QDs has been studied in detail in several investigations directed at understanding of its local bonding environment and symmetry.30, 43 For ZnS:Cu with very low Cu concentration, EXAFS results show that a large fraction of the Cu have three nearest neighbor S atoms and the Cu-S bond is significantly shortened compared to Zn-S, by near 0.08 Å.43 In the meantime, the second neighbor Cu-Cu peak is extremely small. It was proposed that Cu occupies an interior site next to



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a S2- vacancy, with the Cu displaced towards the remaining S2- and away from the vacancy, and such a displacement explains the lack of a significant Cu-Cu peak in the data. There is no evidence for interstitial Cu sites. For ZnSe:Cu with Al3+ as a codopant, the effect of Al3+ on charge balance was studied using EXAFS and codopig with Al3+ was found to reduce Se2- vancancies and increase internal (rather surface) Cu+ doping.45 For ZnSe:Cu QDs, EXAFS supports the existence of two types of Cu sites. One was the interior site where Cu ions occupy an off-centered tedrahedral site accompanying a Se vacancy. This accounts for approximately 3/4 of Cu content, whereas the remaining Cu+ occupies a surface site. Co-doping with Al3+ results in internal doping, however, ା Cu+ is neither substitutional nor interstitial but occupies a distorted Cu௓௡ site close to three of the

Se and displaced away from the fourth one. For both samples, the distortion leads to splitting of the second neighbor peak to such an extent that real parts of the Fourier Transform are no longer in phase, resulting in reduction of amplitude to a negligible level. Fitting the data by including an additional peak around 2.83 Å corresponding to tetrahedral interstitial Cui+ second neighbors (Se/Zn) reveals that there is no considerable fraction of Cui+. Also, based on the first principle calculations, Liang et al. has shown that Cu site involved in the dopant emission can not be the interstitial tetrahedral one as Cu 3d impurity band would lie deeper in the valence band of the host.48 Similar to Al3+ as a codopant, Ga3+ and In3+ have also been demonstrated as successful copants to improve the PL properties of ZnSe:Cu (unpublished results). The EXAFS spectra at Cu K-edge are shown in Figure 4 for samples with different codopants. The amplitudes corresponding to second and third neighbors (between 3.0-4.5Å) are affected by the nature/size of codopant, indicating that the codopant enters the host lattice in the vicinity of Cu dopant.


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EXAFS signal for In3+ as a codopant in Cu-doped ZnSe QDs is shown in Figure 4, along with that of Zn2+ for comparison (unpublished results). Strong similarity between the In and Zn EXAFS indicates that In3+ occupies a Zn2+ substitutional site. However, the first peak for In3+ appears at a slightly longer distance as compared to Zn2+. EXAFS fitting analysis suggests that the In-Se first neighbor distance is longer than that of Zn-Se by 0.10 Å (2.55 instead of 2.45 Å) as a consequence of the large size of In3+ compared to Zn2+ which pushes the Se atoms to longer bond distances. 50

-4

|X(R)| (Å )

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40

undoped (Zn EXAFS)

30

Cu,In (In EXAFS)

20

Cu,In Cu,Ga Cu,Al Cu

10 0 0

1

2 3 4 Apparent distance (Å)

5

6

Figure 4. Cu EXAFS indicating the nearest neighbor distances for the probe atom in ZnSe:Cu/ZnS (Cu), ZnSe:Cu,Al/ZnS (Cu,Al), ZnSe:Cu,Ga/ZnS (Cu,Ga), and ZnSe:Cu,In/ZnS (Cu,In) compared to the Zn EXAFS as well as In EXAFS for (Cu,In) . (unpublished results).

In general, a complete underdstanding of the dopant properties requires a combination of experimental and computational techniques to simultaneously investigate its structural, energetic, dynamic, and optical characteristics.

For example, in a recent study of ZnSe:Cu and

ZnSe:Cu,Al, the structural properties were studied using EXAFS, the energetic and optical properties were studied with steady state optical spectroscopy, and exciton dynamics were probed with time-resolved laser methods.45 In addition, DFT calculations were carried out to 11 ACS Paragon Plus Environment


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corraborate the experimental studies. Together, these studies provide a clear picture about the local structures, electronic energy levels, and optical transitions involved for the dopant and codopant. The understanding gained can then be used to better design and optimize the doped systems, as examplified by extending the codopant from Al to Ga and In that showed further improved optical properties, especially in terms of tunability to longer PL wavelength, as well as gaining deeper insight based on comparison of the three different codopants (unpublished results). In particular, computational studies can provide deeper understanding of the characteristics of dopants and codopants. For example, the electronic structure of the ZnSe QDs before and after Cu doping has been studied using DFT with the PBE functional, ultra-soft pseudo potentials, and plane wave basis sets.45 The density of states (DOS) for the host material was calculated for a 64 atom super cell, as shown in Figure 5(a). The Cu-doped ZnSe system was studied with a 63 atom super cell in which two Zn atoms were replaced by Cu and the Cu-Se nearest neighbor distance was distorted to the bond distance measured by EXAFS, placing Cu in a distorted trigonal geometry. The forth Se, shared by both Cu dopants, was removed forming a VSe-2. The Zn s and Se p DOS obtained from this calculation are shown in Figure 5(a). The VB is due to Se p states while the CB is from strongly hybridized Se p and Zn s orbitals, as was also the case in the ZnSe system. Similarly, the DOS for ZnSe:Cu,Al was studied in a 64 atom super cell by replacing two Zn atoms with a Cu and an Al. Both atoms were distorted in the same direction toward three Se neighbors so the bond distances would be 2.35 Ǻ, as determined by EXAFS. The forth Se was distorted toward the Cu and Al with a bond distance of 2.67 Ǻ. The calculated DOS are presented in Figure 5(b) along with the ZnSe DOS for comparison. The Cu d(t2) states are found at EVB+ 0.45 eV, similar to the ZnSe:Cu system, as determined by Gaussian


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fitting. States associated with Al were found separate and below the CB edge at EVB + 2.38 eV. These states were made up of Al sp3 hybridized orbitals.

Figure 5.(a) Partial DOS for Se p (red) and Zn s (purple) and Cu e (brown) and t2 (green) states, calculated with the 63 atom super cell, as well as for the VSe state caused by Cu doping which are related to the Zn s dangling bond orbital (blue). (b) Partial DOS for Se p (red), Zn s (purple), Cu e (brown), Cu t2 (green), Al s (light blue), and Al p (dark blue) states. The total density of states (DOS) of ZnSe 64 atom super cell (solid black) shown in both (a/b).

Reproduced with

permission from ACS from Ref. 45

The integrated local density of states (LDOS) for ZnSe:Cu are shown in Figure 6. The Cu d(e) and d(t2) DOS as well as the VSe-2 related states are also shown. The integrated LDOS



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for ZnSe:Cu,Al are shown in Figure 6. The results suggest that spacial localization of the photoelectron and photohole in adjacent lattice positions as a consequence of codoping can improve initial and final state wavefunction overlap, thereby increasing the PL yield.


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Figure 6. Integrated local density of states (LDOS) for the ZnSe VB and CB; ZnSe:Cu Cu+ e and t2 states, and the VSe-2 state; and ZnSe:Cu,Al Cu+ e and t2, and Al+3 sp3 hybrid orbitals. Atom colors are: Zn (gray), Se (green), Cu (orange), Al (blue). Reproduced with permission from ACS from Ref. 45

In the scheme dsicussed in this Perspective, we have made the assumption that the Cu exsits as Cu+ in the host semciodnctor, which is supported by several recent studies based on XAFS and/or XANES results.30,32,33,44,46 However, there are reports of Cu existing as Cu2+ in similar or related II-VI semiconductors.26,35

To unambigously resolve this issue may require

further experimental studies in the future. In summary, doping is a powerful and convennient technique for rationally altering the electronic, magnetic, and optical properties of materials including nanomaterials such as QDs Most doping involves introduction of one type of element or ions, e.g. Mn2+, Ag+, Cu+, into the crystal lattice of the host material, e.g. ZnS, and ZnSe. While such doping can be effective, it tends to result in lattice distortion and/or charge imbalance. This problem can be solved by codoping with another element or ion that help to reduce lattice distortion or charge imbalance. This can be achieved using codapnt ions, e.g. Cl-, that have the opposite charge as the primary dopant such as Cu+ or using codopant that has the same charge as the dopant, which together replace two neighboring cations of the host material, e.g. Cu+ and Al3+ replacing two Zn2+ within close proximity. This latter approach can substantially improve the optical luminescence of the primary dopant, Cu+ in this example. Interestingly, the idea works well for other cations like Ga3+ and In3+, besides Al3+. It was found that the codping increases the solubility of Cu into



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ZnSe, thereby reducing sufrace doping and increasing the PL yield. Due to interaction between the codopant and primary dopant, the PL wavelength can be tuned to cover most of the visible spectral region, which is desired for various photonics applications including lighting. The tuning can be attributed in part to the change of relevant energy levels for both the electron and hole due to codoping. In principle, this approach may be generally applicatable. For example, a very recent study demonstrated that codoping of ZnS QDs using Ag+ and Al3+.49 Even though the examples presented are for II-VI semiconductors, the idea can be applied to other semiconductors as well. The codoping can be with two cations or two anions. The key is to achive overall charge balance, minimize lattice strain, and use dopants that can provide the desired energy levels within the bandgap of the semiconductor.

The doping level or

concentration will affect the density of the charge carriers. With this current codoping strategy discussed, the density for p and n doping will be coupled.

To vary the carrier density

indepenently would require using different ratio of the codopant over the primary dopant or a third dopant. It is clear that codoping, especially when executed in a rational manner, can effectively improve the optical properties of semiconductor QDs for many potential applications. Due to the complex nature, characterization of such samples requires a combination of experimental techniques in order to fully understand their properties.

Theory or computation is also

paricularly helpful in this regard. Technological applications of such materials are yet to be explored in depth but are anticipated. In principle, codoping can be extended to include more than two dopants.

This can afford more flexibility and tunability but can also introduce

complications such as undesired energy or charge transfer between dopants. To achieve desired synergistic effects, rational design aided by theory will again be essential.


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Author Information Corresponding author *Jin Z. Zhang, email: [email protected]. phone: 831-459-3776. FAX: 831-459-2935. web: http://www.chemistry.ucsc.edu/faculty/singleton.php?&singleton=true&cruz_id=zhang

Notes The authors declare no competing financial interest.

Acknowledgement This project was funded by the U.S. Department of Energy under contract No. DEFG02-07ER46388-A002.

Author Biographies Zhang received his B.Sc. from Fudan University and Ph.D. from University of Washington. He was a postdoctor at UC Berkeley, and is currently full professor at UCSC. His interest is in optical nanomaterials. He is a senior editor for JPC/ACS and a Fellow of AAAS, APS, and ACS. See: http://www.chemistry.ucsc.edu/faculty/singleton.php?&singleton=true&cruz_id=zhang Jason Cooper received a B.Sc. from CSUS (Dean’s Award) and a Ph.D. from UC, Santa Cruz in Chemistry. He is currently a postdoctoral fellow at the Joint Center for Artificial Photosynthesis at LBNL in Berkeley, CA.

Interests include spectroscopy, computation, and dynamics of

semiconducting materials/nanomaterials for solar energy applications.



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Sheraz Gul received his M.S. from Quaid-i-Azam University and Ph.D. from UC Santa Cruz. He is currently a postdoctoral fellow at Lawrence Berkeley National Laboratory. His interests are X-ray spectroscopic studies of materials relevant to natural and artificial photosynthesis. References (1)

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Quotes to Highlight Paper:

1) Codoping provides a powerful method to possibly manipulate the relevant electronic energy levels for the photoelectron and photohole independently so as to enhance the overall optical properties such as photo- or electro-luminescence, which is important for emerging photonics applications.

2) While studies of codoped QDs have shown improved optical properties, the local strutcure of dopants and codopants as well as their synergestic interaction are often not 23 ACS Paragon Plus Environment


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well understood. To fully understand the fundamentals related to doping and codoping, it is essential to study and correlate the local structural properties of dopants with their optical properties as well as their associated electronic energy levels.

3) It is clear that codoping, especially when executed in a rational manner, can effectively improve the optical properties of semiconductor QDs for many potential applications. Due to the complex nature, characterization of such samples requires a combination of experimental techniques in order to fully understand their properties. computation is also paricularly helpful in this regard.


Theory or

Rational Codoping as a Strategy to Improve Optical Properties of

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