PERSPECTIVE pubs.acs.org/JPCL
Advances in Light-Emitting Doped Semiconductor Nanocrystals Narayan Pradhan*,† and D. D. Sarma*,‡ †
Department of Materials Science and Centre for Advanced Materials, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032 ‡ Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012 ABSTRACT: Insertion of just a few impurity atoms in a host semiconductor nanocrystal can drastically alter its phase, shape, and physical properties. Such doped nanomaterials now constitute an important class of optical materials that can provide efficient, stable, and tunable dopant emission in visible and NIR spectral windows. Selecting proper dopants and inserting them in appropriate hosts can generate many new series of such doped nanocrystals with several unique and attractive properties in order to meet current challenges in the versatile field of luminescent materials. However, the synthesis of such doped nanomaterials with a specific dopant in a predetermined host at a desired site leading to targeted optical properties requires fundamental understanding of both the doping process as well as the resulting photophysical properties. Summarizing up to date literature reports, in this Perspective we discuss important advances in synthesis methods and in-depth understanding of the optical properties, with an emphasis on the most widely investigated Mn-doped semiconductor nanocrystals.
I
nvestigation of semiconductor nanocrystals has constituted one of the most active, cross-disciplinary research areas for over two decades now.1 5 In contrast to their metallic counterparts, semiconductor nanocrystals exhibit more pronounced quantum confinement effects.2,6,7 This is because the leading electron and hole states of a semiconductor, appearing at the top edge of the valence band and the bottom edge of the conduction band, respectively, are strongly affected by a variation in the particle size. This effect allows a near-continuous tuning of the bandgap of a semiconductor material by changing its size in the nanometric size regime; the corresponding changes in its optical properties are often spectacular. While a wide variety of applications in diverse fields, such as catalysis,8,9 optoelectronics,10 photovoltaics,11 drug delivery,12 and diagnostics,12 have been discussed in the literature, optical properties of semiconductor nanocrystals continue to remain the main thrust of fundamental research and its most imminent technological utilization. A very active subfield in this area involves modification of properties of semiconductor nanocrystals by incorporating optically active dopants, typically transition metal ions, inside the lattice of the host semiconductor.13 The range of photoluminescent colors that can be achieved by this method with the use of two typical transition metal ions, namely, Mn and Cu, are shown in Figure 1. The key feature of the dopant emission is that its energy is red-shifted with respect to the bandgap energy of the host. Therefore, dopant emission is not absorbed by the host material, minimizing the vexing issue of self-absorption. Such doped luminescent materials also often exhibit an improved photo and thermal stability and provide a viable route to avoid toxic materials in several potential applications being pursued in this field.14,15 In the following, we review the present status of this highly active field. We begin by describing the most important doping strategies adopted for synthesizing such samples with specific references to the degree of quantum efficiencies achieved r 2011 American Chemical Society
so far. Then we discuss in detail some of the main challenges in this field of research and ways to address several of these issues. Finally, we present some of the more important investigations to understand optical properties of these systems at a microscopic level. While we primarily discuss the case of Mn-doping in group II VI semiconductor nanocrystal hosts, the doping strategy discussed here can be easily generalized for other dopants as well.
The key feature of the dopant emission is that its energy is redshifted with respect to the bandgap energy of the host. Therefore, dopant emission is not absorbed by the host material, minimizing the vexing issue of self-absorption.
One of the earliest reports of a Mn-doped semiconductor nanocrystal system was from Bhargava et al.,16 based on a Mn: ZnS system achieving an 18% quantum yield (QY) of photoluminescence (PL). In this case, nanocryastalline ZnS was first synthesized using diethylzinc and hydrogen sulfide gas and later mixed with diethylmanganese in an organic solvent to obtain the Received: August 19, 2011 Accepted: October 13, 2011 Published: October 13, 2011 2818
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The Journal of Physical Chemistry Letters doped nanocrystals. Finally, these nanocrystals were coated with methacrylic acid to passivate the surface and improve the QY. A few years later, Norris et al.17 reported Mn-doped ZnSe nanocrystals with a 22% QY in a high-temperature colloidal synthetic protocol. Mn and Zn precursors were loaded in the reaction flask under Argon flow, and a Se source was injected at a high temperature to get Mn-doped ZnSe nanocrystals. Under optimal synthesis conditions, Mn emission dominates the PL spectrum, although a substantial excitonic emission persists, as shown in Figure 2a. With a continued growth of the system, the excitonic emission is found to move to a lower energy, reflecting the growth of nanocrystals with time. Mn emission wavelength does not show any shift with the growth time, establishing independence of this emission with respect to the host particle size. There is a clear increase in the intensity of Mn emission at 585 nm with a concomitant decrease in the excitonic emission intensity, suggesting either a larger fraction of nanocrystals being doped or other competing decay channels being suppressed with increasing reaction time. Using a different synthetic route, Peng et al18 could achieve a QY of about 50% from Mn-doped ZnSe nanocrystals, underlining the very subtle nature of the doping process. In this synthesis technique, named nucleation doping, dopant selenide clusters are first formed, and host semiconductors are grown on them. This method ensures that most of the host nanocrystals are doped and
Figure 1. Schematic presentation of the range of tunable emission of different doped semiconductor nanocrystals. Cu-doped InP can extend its tunability until 1100 nm (ref 47) but here we have restricted our data to mostly the visible spectrum.
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gives rise to nearly pure dopant emission alone (Figure 2b). Since the host is specifically nucleated and grown on the dopant chalcogenide nucleation centers, there is little possibility of having undoped host nanocrystals in this method. While all these doped nanocrystals with high QY are synthesized keeping dopants in the host lattice, Cao et al.19 reported intense (QY > 50%) Mn dopant emission by placing dopant ions in a shell of a core/shell structured nanocrystal. A typical synthetic procedure of such structures is shown in Figure 2c, where Mn ions are incorporated in a ZnS shell layer on top of a core of CdS nanocrystals. Cao et al. excited this system with a photon energy corresponding to the bandgap of the core CdS region. Since the excitation energy is lower than the bandgap of the ZnS shell layer, the primary excitation is necessarily within the CdS part of the core shell structure. The corresponding emission spectrum, shown in Figure 2c, clearly shows the emission due to Mn2+ ions at ∼600 nm, in addition to the excitonic emission within the CdS core at about 420 nm. With appropriate synthetic conditions, the Mn emission can be optimized, as shown in Figure 2c, yielding QY higher than 50% in this method. Apart from these synthetic protocols, several other methods have been reported that lead to intense and stable Mn dopant emission.20 25 Recently Karan et al.22 developed a generic doping strategy where Mn (and even Cu) dopants can be doped in most of the usual group II VI semiconductor nanocrystal hosts. Using manganese oxide nanocrystals as the dopant source and following growth doping strategy, different hosts such as ZnS, ZnSe, ZnSeS, CdSeS, and CdZnS have been doped, and highly efficient (QY 30 40%) dopant emission has been obtained by this method. There have also been reports of single source precursors to obtain different doped nanocrystals.24,25 Thus, it appears evident that there is a wide variety of synthetic strategies to choose from in order to achieve highly emitting doped nanocrystals. Some of these synthetic strategies have been shown to be scalable for synthesis of large amount of samples with QY > 50%.21 While most studies report doping of spherical host nanocrystals, there are specific examples of doping Mn in one-dimensional (1D) shaped rods26 and wires20 and two-dimensional (2D) shaped discs.27 Having discussed various ways of synthesizing doped semiconductor nanocrystals with high emission QYs, we now discuss some intriguing aspects of such systems. Among these, the most
Figure 2. Successive PL spectra obtained during different doping processes. (a) Representation of the introduction of the dopant to the reaction system along with host precursors, (b) representation of the introduction of the dopant before nucleation of the host, and (c) representation of the introduction of the dopant in the shell of a core shell structured nanocrystal. All PL spectra are recorded with an excitation wavelength of 350 nm. These results are obtained following the synthetic approach presented in references 17, 18, and 19. 2819
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The Journal of Physical Chemistry Letters prominent one has been the unexpected difficulty in achieving a sizable doping level in such semiconductor nanocrystals. It is known that the quantum efficiency for Mn emission is the highest for an isolated Mn2+ ion in a nanocrystal host and an increasing Mn concentration reduces the quantum efficiency per Mn2+ dopant due to Mn Mn interactions.28 This favors doping in the dilute limit. However, doping being a statistical process, an average Mn concentration, which would be sufficient to place one single Mn2+ ion in each nanocrystal host, will generally leave some nanocrystal hosts undoped, while introducing multiple Mn ions in some other nanocrystals, thereby reducing the quantum efficiency. Therefore, there is a trade-off between the need to dope all nanocrystals and the need to keep the doping level at the minimum to obtain the maximum efficiency.28 Hence it is important to study the net quantum efficiency of such doped systems as a function of Mn concentration up to a reasonably high Mn content, besides such investigations being relevant to exploring interesting magnetic properties as well. In this context, we note that most synthetic routes discussed above, with the exception of ref 28, achieve low levels of doping, reporting typically in the range of 1 2% or lower Mn content. This difficulty appears to be unique to the nanometric regime, since corresponding bulk systems, such as bulk ZnS or CdS solids, can be easily doped up to large dopant concentrations (>10%).29 The origin of this drastic difference in one’s ability to dope Mn ions up to a high concentration in nanocrystals compared to the bulk has been debated in the literature. In several publications, the possibility of the doped state being a metastable one and Mn having a sizable diffusion coefficient in the host material at typical synthetic temperatures, making facile surface segregation of Mn ions possible, have been discussed.13,30 The essential idea in this explanation is that Mn-doped nanocrystals can easily eject the dopant to the surface, thereby achieving its lowest energy configuration. However, there have been reports14,28 where doped Mn exhibited remarkable stability even at a reasonably high temperature, suggesting that the metastability of doped nanocrystals may not be the only cause of low levels of dopant concentrations generally achieved. In an alternate explanation, it has been argued31 that the Mn diffusion coefficient in these semiconductor host materials is, in fact, small and, therefore, doping cannot be achieved by Mn2+ ions diffusing into an already formed nanocrystal, even if the doped state is the lowest energy state in contrast to the previous discussion. According to this work, doping of nanocrystals proceeds primarily via adsorption of Mn2+ ions on the surface of the semiconductor nanocrystals, thereafter being buried by further growth of the nanocrystal. Therefore, the extent of doping is believed to be controlled by the ability of Mn2+ ions to adsorb on specific crystal facets available in the growing nanocrystal. Detailed quantum mechanical calculations evince that Mn2+ ions preferentially adsorb on {011} facets of the zinc blende structure, while the wurtzite phase does not offer any suitable surface for Mn2+ adsorption. This fact was related to the remarkable paucity of doped nanocrystals in the wurtzite phase in contrast to a large number of reports of doped semiconductor nanocrystals in the zinc blende phase. More recently, it has been noted that the Mn2+ ionic size (0.85 Å) is considerably different than those of Cd2+ (0.95 Å) and Zn2+ (0.75 Å).28 Thus, a Mn2+ ion substitutionally doped at the place of Zn2+ or Cd2+ invariably causes local strains. It is interesting to note here that the ionic size of Mn2+ is smaller than that of Cd2+, but larger than that of Zn2+. Thus, a cationic solid solution of Cd and Zn in the form of Zn1 xCdxS allows one to tune the average
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Figure 3. (a) Plot of the amount of dopant Mn incorporated in ZnxCd1 xS alloyed nanocrystals with variation of composition. (b) Simulated EPR patterns of tetrahedrally and octahedrally coordinated Mn2+ ions in a semiconductor host nanocrystals. These data are obtained from refs 28 and 37.
cationic size available to accommodate Mn dopant ions. It has been shown28 that the maximum incorporation of Mn2+ ion as a function of the stoichiometry, x, in Zn1 xCdxS indeed tracks the average lattice mismatch (Figure 3a). This approach also allows one to incorporate up to nearly 8% Mn for the optimal value of x (∼0.5), this being the highest amount of Mn inclusion into a group II VI semiconductor nanocrystal so far. These results would suggest that the main cause of the difficulty in incorporating Mn in these semiconductor nanocrystal hosts is the strain associated with doping,28 as also suggested in ref 32. Another interesting aspect of the results reported in ref 28 is that the composition (Cd0.5Zn0.5S) accommodating the highest concentration of Mn dopant exists in the wurtzite phase, undermining earlier suggestions of this crystallographic phase being inappropriate and rather detrimental for Mn doping. This result also indicates that the adsorption of Mn2+ may not be the determining step in the doping process, in contrast to the suggestion provided in ref 31. As already described, doping of Mn is typically carried out by introducing Mn2+ ions into the reaction solution during a suitable stage of the synthesis. The complex process of incorporation of Mn in the semiconductor nanocrystal host at the simplest level involves a large number of steps: (i) diffusion of all reactants in the solution to reach the growing nanocrystals; (ii) adsorption of reactant species onto the surfaces of these nanocrystals; (iii) their incorporation through atomic rearrangements at the surfaces; (iv) possible diffusion of various species toward or away from the core of the host; and (v) further growth of nanocrystal hosts via diffusion-controlled Ostwald ripening process33 or more complex growth patterns.34,35 It would be naive to expect such a labyrinthine, multistep process to give rise to complete homogeneous doping of nanocrystals. Inhomogeneity of doping was first investigated specifically in ref 36 establishing that Mn2+ ions systematically prefer to be doped in larger hosts within a given size distribution of the host nanocrystals.
Mn2+ ions systematically prefer to be doped in larger hosts within a given size distribution of the host nanocrystals. 2820
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Figure 4. Schematic presentation of the electronic energy level diagram and excitation-de-excitation process (a,b) within an independent particle approximation. (c,d) The same processes depicted in a and b, respectively, but within a total energy diagram, as explained in the text. Æ1æ represents the total energy of the ground state of the system, while Æ2æ represents that of the system with an electron hole excitation.
Besides the systematic inhomogeneity of doping discussed above, a dopant ion, such as Mn2+, can locate itself at a number of inequivalent sites, such as in the core region or close to (or even at) the surface, in addition to being accommodated substitutionally or at an interstitial position being tetrahedrally or octahedrally coordinated. It is reasonable to expect that properties of Mn2+ ions will depend on its location in the host. Therefore, it becomes important to know the exact location of Mn in the system. This can be most easily determined by electron paramagnetic resonance (EPR), also known as electron spin resonance (ESR), spectroscopy. This method has been used by a large number of groups over the years to probe the location of Mn dopant ions in the host nanocrystal. A typical result is shown in Figure 3b. Such narrow line-widths are characteristic of isolated Mn2+ ions, since dipolar interactions between nearby Mn Mn ions are known to give rise to very broad EPR signals.17,37 In Figure 3b, the spectrum with g = 2.002 and a hyperfine splitting of 69 G is characteristic of Mn2+ in tetrahedral symmetry, suggesting Mn substitution in the core of the host. Similarly, the other spectrum in Figure 3b with g = 2.003 and a hyperfine splitting of 95 G is characteristic of Mn2+ in a distorted octahedral geometry typical of a nearsurface isolated Mn2+ ion.17,37 As previously mentioned, the largest effort in the investigation of semiconductor nanocrystals is aimed at obtaining and utilizing
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a range of spectacular optical properties. The basic mechanism of PL of an undoped nanocrystal is shown in Figure 4a,b. On absorption of sufficiently high-energy photons, electrons from occupied levels are excited to unoccupied levels, conserving the total energy of the system and generating electron hole pairs; a few among the very large number of possible pair formation for a given photon energy are shown in Figure 4a. These highly excited electrons and holes quickly decay nonradiatively by coupling to the vibronic states and occupy the lowest unoccupied level (bottom of the conduction band in the language of solid state physics or the lowest unoccupied molecular orbital (LUMO) in the language of molecular orbital theory) and the highest occupied level (top of the valence band or the highest occupied molecular orbital (HOMO)), respectively, giving rise to the situation shown in Figure 4b. Then the electron and the hole recombine, as indicated by the vertical green arrow in Figure 4b, giving rise to the emission of a photon. This process is often termed as the band gap or the excitonic PL, depending on the relative importance of the electron hole Coulomb interaction strength in a given case. In Figure 4a,b, we have represented the excitation and the deexcitation processes in terms of a single particle description that is valid and convenient only in cases where the electron electron interactions can be effectively ignored, as in the case of undoped semiconductor nanocrystals. This is, however, not a valid approximation in the case of transition metal ion-doped systems, since Coulomb interactions within the transition metal 3d manifold is strong. In such a situation, it is more convenient to talk in terms of the total energy of the system instead of the electronic level energies shown in Figure 4a,b. In this alternate description, the ground state of the undoped system is a unique energy state, as shown by a horizontal energy level, Æ1æ, in Figure 4c, depicting the total energy diagram. However, the electron hole excited state of the system has a continuum of energies (marked Æ2æ), as shown at a higher energy in the same figure. The photoabsorption process takes the system from the ground state to an excited state energy according to the energy conservation principle, as marked by the vertical arrow in Figure 4c. The advantage of this total energy representation becomes evident when we compare panels a and c of Figure 4, both representing an identical situation of photoexcitation of an electron hole pair in a semiconductor. In Figure 4a, this process has been represented by only three distinct possibilities, while the photoabsorption process can be (and indeed is) realized in nearly infinite number of different ways while conserving the energy. In Figure 4c, the same process has a unique representation, as shown, since all possible photoexcited states have exactly the same total energy, determined by the excitation/photon energy. This excited state quickly decays nonradiatively to the lowest energy excited state (see Figure 4c) via coupling to the vibronic states. The lowest energy excited state, corresponding to an electron and a hole in the lowest conduction band and in the highest valence band, respectively, eventually makes a radiative transition to the ground state, as marked by the vertical green arrow in Figure 4d, giving rise to the PL. It should be noted here that Figure 4a,b is exactly equivalent to Figure 4c,d. We first note that the Mn2+ 3d5 ion in a tetrahedral crystal field can exist in different multiplet states. At the usual crystal field strength, multiplets with the two lowest energies are given by 6 A1 and 4T1 states. Presence of a doped Mn2+ ion in a host semiconductor, the ground state system comprises the ground state of the host nanocrystal (no electron hole excitation) along 2821
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Figure 5. Total energy diagram of (a) the excitation and (b) the deexcitation processes in Mn-doped nanocrystals, as discussed in the text. Æ1æ represents the ground state of the nanocrystal with Mn2+ in 6 A1 state. Æ2æ represents that of the host with an electron hole excitation and the Mn2+ in the 6A1 state. Æ3æ represents the total energy of Mn2+ in the 4T1 state but the host without any electron hole excitation. States corresponding to Æ1æ, Æ2æ, and Æ3æ can be thought of as (host + Mn), (host* + Mn) and (host + Mn*), respectively, where the * denotes an excited state. 2+
with the ground state multiplet configuration of Mn state, which is the 6A1 state. The total energy of this ground state is shown (Æ1æ) in Figure 5a. The total energy spectrum of the electron hole excited host along with the Mn in the 6A1 state are shown as the continuum of energies, marked Æ2æ, in the same panel. However, the lowest energy excited state in this system, Æ3æ, is a multiplet excitation of the Mn2+ state from the 6A1 state to the 4T1 state in the absence of any electron hole pair in the host, as shown in Figure 5a. The photoabsorption process with sufficiently high energy photons primarily causes electron hole excitations in the system, as shown in Figure 5a. This excited state (Æ2æ) decays nonradiatively in a very short (typically subpicosecond) time-scale to the lowest energy excited host state, as shown in Figure 5b, as much as in the case of the undoped system (see Figure 4c,d). In contrast to the case of the undoped system, however, the doped system can nonradiatively decay further by transferring the excitation energy of the electron hole pair to excite the ground state multiplet 6A1 state of the Mn2+ ion to its excited 4T1 state, thereby making a transition from the lowest energy of Æ2æ to the sharp energy level of Æ3æ, as indicated by the blue vertical arrow in Figure 5b. This energy transfer process is also very rapid, occurring within a timescale of a few picoseconds (psec). Eventually, the excited Mn2+ 4T1 state decays radiatively to the ground state configuration of the 6A1 state (Æ3æ f Æ1æ), as shown by the vertical orange arrow in Figure 5b, and giving rise to the dopant emission, red-shifted (at a lower energy) compared to the bandgap or the excitonic emission. It is well-known that surface as well as defect states in semiconductor nanocrystals can give rise to PL signals, significantly redshifted with respect to the bandgap emission. Such PL signals arise from excited states associated with the population of electrons and/or holes in the surface and other defect states with a total energy lower than the electron hole excitation in the host, as shown in Figure 6a. It should be noted that these excited surface/ defect states may be reached via charge (electron or hole) or energy transfer. It is clearly convenient to represent such transitions between these states in terms of the total energy diagram adopted here. In this form, it is not necessary to distinguish between different cases (electron/hole/energy transfer), and, therefore, the same diagram can represent in a compact way all possible surface state-related emissions. These excited states involving surface or other defect states, decay radiatively to the ground state
Figure 6. Schematic representation of the total energy diagram of Mn doped ZnS nanocrystals in presence of surface/defect states: excitation and deexcitation processes for (a) undoped and (b) doped nanocrystals. This figure is adopted from ref 38.
(see Figure 6a), giving rise to the red-shifted emission with respect to the bandgap emission. If such systems also contain any dopant, such as Mn ions, there is a competition between the decay channels involving the surface states and those via the dopant states as illustrated in Figure 6b, leading to both surface and dopant state emissions. However, Mn excited states are not only populated by the energy transfer from bandgap excitations, but may also be populated via the decay of excited surface states,38 as indicated by the diagonal blue arrow in Figure 6b. This allows even insufficiently passivated nanocrystals with substantial surface states to primarily exhibit the narrow dopant emission under optimized conditions. The basic difference in the various emission processes mentioned above gives rise to large differences in the corresponding emission lifetimes. The lifetime of the band-edge emission is controlled by the dipolar overlap of the wave functions for the top of the valence band (HOMO) and the bottom of the conduction band (LUMO) states, with lifetimes typically in the range of 1 30 nanoseconds (nsec). The Mn emission in Mndoped systems correspond to a d d transition within the Mn 3d multiplet; this is both a spin and orbitally forbidden transition in the ionic limit, therefore, leading to a very long lifetime in the range of several hundreds of microseconds. It should be noted, however, that there are other processes involved at various steps of the excitation and emission processes; those influence different aspects of the emission features, such as the quantum efficiency, tunability, and line-width, finally observed in an emission experiment. Specifically, it is important to note that the time-scale for the energy transfer from the host to the Mn d-states is much faster than the Mn d-emission lifetime; it is typically in the range of a few psec.38 If this transfer rate is not fast, much of the de-excitation process will be via other channels, such as the excitonic emission or involving decay via surface states; thus, the fast transfer rate is absolutely crucial for a large Mn emission quantum efficiency. We note that the emission decay lifetime involving the surface states appears typically at about a few nsec or less, and is, therefore, faster than the bandgap deexcitation, in most systems. This indicates why it becomes important to passivate surface states very effectively in order to obtain a high intensity band-edge emission. In contrast, the energy transfer to Mn d-states competes effectively with that to the surface states. This fact, along with the possibility of a further energy transfer from the surface states to Mn d-states (Figure 6b), makes the 2822
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The Journal of Physical Chemistry Letters Mn-emission less susceptible to the presence of surface states of higher energy.
Mn emission in Mn-doped systems correspond to a d d transition within the Mn 3d multiplet; this is both a spin and orbitally forbidden transition in the ionic limit, therefore, leading to a very long lifetime in the range of several hundreds of microseconds. The above discussion of relative intensities of various emission features, namely, arising from deexcitations involving the bandgap, surface, or defect states and doped states, can be understood more quantitatively by noting that these are determined by corresponding transition probabilities. These transition probabilities are inversely related to the lifetimes discussed in the previous paragraph. For example, the ratio of intensities of bandgap and Mn emissions in a doped system is basically the ratio of probabilities for de-excitations via the bandgap recombination and the energy transfer process to Mn2+ from the host, in the absence of any other process. Thus, this intensity ratio is given by the inverse of the ratio of the bandgap deexcitation lifetime (∼1 30 nsec) and the time-scale (∼ a few psec) of the energy transfer from the host to the Mn d-states. Therefore, the bandgap emission is expected to be almost completely quenched in presence of Mn-doping; this indeed happens at a low enough temperature for properly doped systems. However, in real systems, at higher temperatures many different pathways exist for the deexcitation process in addition to the bandgap recombination and via the energy transfer to Mn followed by the Mn d-d transition. The final emission intensities observed are a result of all these competing factors. Moreover, the intensity ratio, specifically of the bandgap and Mn d emissions, has often been reported to be strongly temperature dependent and in general increasing with increasing temperature.24,39 Thus, the relative intensity of the bandgap emission at room temperature may not be a proper indicator of the efficiency of Mn doping in every system, and one needs to perform PL experiments at a very low temperature (
