Evidence for the Ligand-Assisted Energy Transfer from Trapped

Jul 14, 2014 - Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States. J. Phys. Chem. C , 2014, 118 (31), pp 18226...
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Evidence for the Ligand-Assisted Energy Transfer from Trapped Exciton to Dopant in Mn-Doped CdS/ZnS Semiconductor Nanocrystals Sourav Maiti, Hsiang-Yun Chen, Yerok Park, and Dong Hee Son* Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States S Supporting Information *

ABSTRACT: Trapping of charge carriers is the major process competing with radiative recombination or transfer of charge carriers important in the application of semiconductor nanocrystals in photonics and photocatalysis. In typical semiconductor quantum dots, trapping of charge carriers usually leads to quenching of exciton luminescence. In this study, we present evidence indicating that thiol ligands on the surface that quench exciton luminescence can have an opposite effect on sensitized dopant luminescence in doped semiconductor nanocrystals by facilitating the recovery of the trapped exciton for sensitization. Despite the increase in hole trapping by the added octanethiol to the surface of Mn-doped CdS/ZnS nanocrystals, the sensitized Mn luminescence increased by the added octanethiol and the enhancement became stronger with increasing Mn doping concentration. While the role of octanethiol as the hole trap and the enhancement of Mn luminescence may seem contradictory, the thiol-induced enhancement of Mn luminescence is possible, since thiols play dual role as the hole trap and as the facilitator of the energy transfer from the trapped exciton to Mn, in contrast to the pre-existing hole traps that inhibit the energy transfer.

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INTRODUCTION Trapping of the charge carriers in semiconductor nanocrystals is a major competing process with exciton recombination and charge carrier transfer that are critical to their application in photonics or photocatalysis.1−3 Trapping of charge carriers usually results in the reduced photo and electroluminescence due to the reduced overlap of the electron and hole wave function, while part of the trapped excitons can emit photons from as trap emission or via thermal detrapping to the delocalized exciton state.4−7 For this reason, much effort has been made to understand the nature of the charge carrier trapping states, the rate of charge carrier trapping/detrapping processes, and ways to suppress or enhance charge carrier trapping events in various semiconductor nanomaterials.8−12 In order to suppress the trapping of charge carriers and maximize the luminescence from the semiconductor nanocrystals for light-emitting applications, several different methods have been used. Addition of inorganic shell or surfactant molecules on the surface of the nanocrystals that can remove the trap states on the surface is the most commonly used strategy to reduce the trapping of charge carriers and increase the luminescence.13−18 Introducing dopant ions within the nanocrystals has also been shown to be very effective in obtaining strong luminescence, where the sensitized dopant luminescence or radiative charge carrier recombination at the dopant site was often significantly brighter than exciton luminescence of the undoped nanocrystals.19−23 Sensitized dopant luminescence, such as in Mn-doped II−VI nanocrystals, is very bright since the rapid energy transfer from exciton to dopant ions can effectively compete with the charge carrier trapping process that leads to nonradiative loss of © 2014 American Chemical Society

exciton. For instance, in our recent study of Mn-doped CdS/ ZnS core/shell nanocrystals passivated with oleylamine, the energy transfer occurred in as fast as a few ps, whereas the competing hole trapping process occurred on 50−100 ps.24 Such favorable competition for the energy transfer process rendered the luminescence from Mn-doped nanocrystals the stronger resistance to the charge carrier trapping than the exciton luminescence of the undoped counterparts. In this work, we present evidence indicating that Mn-doped CdS/ZnS nanocrystals can also recover the hole-trapped exciton as photons using octanethiol as the mediator facilitating the energy transfer from the “trapped” exciton to Mn ions. In undoped CdS/ZnS nanocrystals, the surface-bound thiol molecules create the extra hole traps on the surface of the nanocrystals in addition to the pre-existing traps, consequently resulting in a strong quenching of the exciton luminescence. In contrast, the same thiol can enhance the sensitized Mn luminescence in Mn-doped CdS/ZnS nanocrystals despite the increase in hole trapping rate by the added thiols. The enhanced sensitized luminescence and the increase in hole trapping by thiol appears contradictory to each other and has been a puzzle in the earlier studies that observed thiol-induced enhancement of Mn luminescence.25−27 The present study shows that the surface-passivating thiol plays dual role, i.e., trapping holes and assisting the energy transfer from the trapped exciton, in the energy transfer process in Mn-doped semiconductor nanocrystals resolving the aforementioned Received: May 26, 2014 Revised: July 12, 2014 Published: July 14, 2014 18226

dx.doi.org/10.1021/jp505162c | J. Phys. Chem. C 2014, 118, 18226−18232

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shows the UV−vis absorption and luminescence spectra of the undoped and doped nanocrystal samples S1, S2, and S3. Mndoped nanocrystal samples S1 and S2 exhibit the same absorption spectra as the undoped nanocrystals since the core and shell sizes are the same as those of the undoped nanocrystals. Small variations in the luminescence spectra in different doped nanocrystal samples are due to the differences in the lattice strain at different doping locations affecting the energy of the Mn ligand field transition.24 Transient Absorption and Quantum Yield Measurements. Transient absorption measurements were made using an amplified Ti:sapphire laser system (KMLabs) operating at 3 kHz repetition rate. Pump beam centered at 395 nm was generated by doubling 790 nm laser beam in a β-barium borate crystal. White light continuum probe beam was generated by focusing a few μJ of 790 nm beam on a 1 mm-thick CaF2 window. CaF2 window was constantly translated horizontally to avoid damage during the continuum generation. The average fluence within the fwhm beam width of 300 μm was ∼0.2 mJ/ cm2 per pulse and excites less than 0.5 exciton per particle on average in Mn-doped CdS/ZnS nanocrystals used in this study. To monitor the bleach recovery at the band edge, 420 nm component of the white light continuum with bandwidth of 20 nm were selected as the probe with a prism chirp compensator and a slit. The signals from the two split probe beams (sample and reference) were detected by a pair of photodiodes and processed with the boxcar integrators. The nanocrystal samples dispersed in cyclohexane with varying octanethiol concentration were circulated in a quartz liquid flow cell at the linear flow rate exceeding 1 m/s to avoid the repeated excitation of the same sample region. The concentrations of the nanocrystal samples were kept identical at ∼6.5 μM for all the samples, and the thiol concentration was varied in the range of 0.07−1M. The lifetime of Mn luminescence was measured using a pulsed nitrogen laser (SRS, NL100) as the excitation source and photomultiplier tube (Hamamatsu, R928) and a digital oscilloscope (LeCroy, WaveAce 234) for the detection of the luminescence signal. Luminescence quantum yield was measured following the methods described in our earlier study.24 For the samples with the added octanethiol, all the measurements including transient absorption, lifetime, and luminescence quantum yield were made ∼30 min after the addition of thiol to the sample solution to allow the equilibration of octanethiol between the nanocrystal surface and bulk. Thirty minutes was sufficient to complete the early fast phase of the equilibration and to reach a stable quantum yield value.30

contradiction. Surface ligands that trap charge carrier are usually undesirable for the light-emitting applications of semiconductor nanocrystals due to the luminescence quenching effect. This study shows that hole-trapping thiols can have the opposite effect on the sensitized dopant luminescence by allowing the sensitization from the trapped exciton to dopant. Since thiols are commonly used as the chemical linkage between the nanocrystal surface and the surrounding, the implication of the thiol-assisted energy transfer from the trapped exciton will be particularly significant in the lightemitting application of the doped semiconductor nanocrystals.

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EXPERIMENTAL SECTION Synthesis of Materials. Mn-doped core/shell nanocrystals of varying doping concentration and radial doping locations were synthesized utilizing the SILAR (successive ionic layer adsorption and reaction) method previously used for the synthesis of Mn-doped nanocrystals.20,28,29 The details of the synthesis and characterization are described in our earlier works.24 Briefly, CdS core of 3.6 nm in diameter was synthesized by using CdO and sulfur dissolved in 1-octadecene (ODE) as the precursors. To add ZnS shell and doping Mn ions at the wanted radial location, Zn and S layers were added successively using the SILAR method. Control of the radial doping location was achieved by varying the sequence of the step Mn precursor is introduced during the SILAR procedure. Zinc stearate in toluene and sulfur in ODE were used as the precursors for ZnS shell. Mn diethyl-dithiocarbamate or Mn acetate in oleylamine were used as the precursor of Mn. Doping concentration at a given radial doping location was controlled by varying the amount of Mn precursor during the synthesis. Figure 1 shows the structural variations made in several Mn-

Figure 1. Structures of Mn-doped CdS/ZnS nanocrystals used in this study. Doping location (d), doping concentration (⟨Mn⟩) and shell thickness (t) were varied.

doped CdS/ZnS nanocrystal samples used in this study. The size and shape of the nanocrystals were confirmed through TEM, and the average number of Mn ions in doped nanocrystals was determined through elemental analysis as described in detail elsewhere.24 Figure 2(a) shows the TEM image of the doped nanocrystal sample S1. Figure 2(b),(c)

Figure 2. (a) TEM image of sample S1. (b) UV−vis absorption and (c) emission spectra of undoped and Mn-doped nanocrystal samples S1, S2, and S3. Same color legends are used in both (b) and (c). 18227

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Figure 3. Transient absorption data of Mn-doped CdS/ZnS nanocrystals with varying octanethiol concentrations. Data in (a)−(c) are from sample S1, S2, and S3 respectively. Data in (d)−(e) are the semilog plots of the data in 1 ns window for sample S1, S2, and S3 respectively. Black curves in (a) and (d) are the data from undoped CdS/ZnS nanocrystal sample. Sample S3 has a thinner ZnS shell (0.9 nm) than S1 and S2, as summarized in Figure 1.

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RESULTS AND DISCUSSION We will first establish the surface-bound octanethiol’s role as the hole trap in Mn-doped CdS/ZnS nanocrystals before discussing the effect of thiol ligand on the sensitized Mn luminescence. Thiol molecules bound to the surface of the semiconductor nanocrystals are known to function as either hole trap or trap-passivating ligand depending on the relative energy levels of the valence band and HOMO of thiol.31,32 In the case of undoped CdS/ZnS core/shell nanocrystals, the addition of octanethiol results in strong quenching of the exciton luminescence, as expected from the generally accepted role of thiol as the hole trap.10 However, octanethiol resulted in the enhancement of the sensitized Mn luminescence in Mndoped CdS/ZnS nanocrystals having the same host nanocrystal structure as the undoped nanocrystals depending on the doping concentration. Generally, doped nanocrystal samples with the lower doping concentration (e.g., S2 > S3, as reflected in the time scale of the faster recovery component. Figure 4a shows f tr as a function of octanethiol concentration in solution for all three samples. f tr increases with increasing

Figure 4. (a) Fraction of hole trapping (f tr) and (b) hole trapping rate (ktr) as a function of octanethiol concentration in solution Mn-doped CdS/ZnS nanocrystals. Typical error bars are shown in (b) for the case of 0.5 M octanethiol.

concentration of octanethiol indicating that the addition of octanethiol results in an additional hole trapping. This is consistent with the expected role of thiol molecules adsorbed on the surface of CdS/ZnS nanocrystals as the hole trapping ligand. Figure 4b shows the hole trapping rate (ktr) extracted from the energy transfer rate (kET) and f tr from f tr = ktr/ (ktr+kET). This relatively simple analysis can be justified since the energy transfer and hole trapping are the two main competing processes in Mn-doped nanocrystals studied here as discussed in our recent report.24 In this calculation, the values of kET used are 1.4 × 1011 s−1, 9.1 × 1010 s−1, 5.5 × 1010 s−1 for S1, S2, and S3, respectively, which were determined from the multiexponential fit of the data in Figure 3. With increasing octanethiol concentration, sample S1 and S2 exhibit similar increase in hole trapping rate since they have the same core diameter and shell thickness. In sample S3, having thinner ZnS shell, the effect of octanethiol on hole trapping is more significant than in the other two samples due to the thinner energy barrier for the hole transfer to thiol. Having confirmed the role of octanethiol in Mn-doped ZnS/ CdS nanocrystals as the hole trap, we measured the octanethiolinduced change in Mn luminescence intensity in three groups of Mn-doped nanocrystal samples having different doping concentrations and locations. Figure 5 shows the fractional change in Mn luminescence intensity (ΔI/I) after the addition of octanethiol (0.07M) measured as a function of doping concentration in three different groups of the samples. The

Figure 5. Fractional changes in Mn PL intensity (ΔI/I) in sample group A, B, and C from the addition of 0.07 M octanethiol. 18229

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Table 1. Mn Luminescence Quantum Yield before and after Adding 0.07 M Octanethiola A (t = 1.8, d = 0)

group ⟨Mn⟩ QY(before) QY(after)

5 0.70 0.70

13 0.71 0.73

18a,b,c 0.60 0.65

B (t = 1.8, d = 0.6) 20 0.50 0.59

9 0.73 0.75

15a,b,c 0.72 0.81

C (t = 0.9, d = 0.3) 19 0.68 0.80

5a,b,c 0.60 0.52

13 0.60 0.73

a,b,c Each sample corresponds to sample S1, S2, and S3, respectively. at and d represent the shell thickness and doping location in nm as defined in Figure 1.

Figure 8. Kinetic schemes describing the effect of octanethiol on the energy transfer process with two possible ways: (a) without and (b) with the migration of holes from the pre-existing trap sites to the sites created by octanethiol on the surface. Figure 6. Octanethiol concentration dependent Mn-PL in doped nanocrystals (S1 and S2).

the role of octanethiol in the energy transfer process as described below. In the kinetic scheme of Figure 8a, we assume that there are two different trapped exciton states in the presence of the added octanethiol: one trapped at the pre-existing trap site (tr1) and the other trapped at the site created by octanethiol (tr2). The trapping rate of exciton at each site is represented by ktr1 and ktr2. We also assume that the added octanethiol does not alter the nature of the pre-existing traps. In this picture, octanethiol plays two roles simultaneously as the hole trap and as the mediator for the sensitization from the trapped exciton, in contrast to the pre-existing trap that inhibits the energy transfer. In this scheme, the trapped exciton tr2 can transfer its energy to Mn with the efficiency of ϕET2 = kET2/(kET2 + kr), where kET2 and kr represent the energy transfer rate from tr2 to Mn and relaxation rate of tr2, respectively. In Figure 8, relaxation of the trapped excitons at tr1 and tr2 to the ground state is not drawn for clarity. kET2 will increase with Mn doping concentration similarly to kET, while its magnitude should be significantly smaller than kET. The transient absorption data in Figure 3 suggest that the energy transfer from tr2 should be slower than ∼1 ns, since the amplitude of the slowly recovering component in the presence of octanethiol during 1 ns time window of this study is still larger than in the absence of octanethiol. The relaxation rate of the trapped exciton (kr) is most likely to be in the tens of ns range based on the earlier studies, while the accurate determination is difficult due to the limited time window of the measurement. In this kinetic scheme, the efficiency of the energy transfer with (ϕET, thiol) and without (ϕET), the added octanethiol can be represented as follows and ΔI/I becomes (ϕET, thiol − ϕET)/ ϕET.

Figure 7. Time-dependent luminescence intensity of sample S1 at varying octanethiol concentrations. Inset is the normalized luminescence spectra.

Since the spatial extent of the trap wave function varies depending on its origin, it is conceivable that the surface-bound thiol molecules create the hole traps different from the preexisting ones, which can allow the energy transfer from the trapped exciton to Mn. The evidence that hints the possible energy transfer from the trapped exciton to Mn in the presence of octanethiol comes from the increase in ΔI/I with increasing doping concentration for a given host structure and octanethiol concentration as shown in Figure 5. If the energy transfer from the trapped exciton to Mn occurs, then its rate should increase with increasing Mn doping concentration similarly to the energy transfer from the usual nontrapped exciton as observed in our recent study. In principle, the increasing capability of the trapped exciton to sensitize Mn with increasing doping concentration can explain qualitatively the increase of ΔI/I with doping concentration. Below, we will examine the effect of the energy transfer from the trapped exciton to Mn on ΔI/I more in detail under two possible kinetic scenarios depicted in Figure 8. However, we will not attempt to use these models to “quantitatively” simulate the observed ΔI/I, since the model is very simplified and the uncertainty still remains in the rates of a few processes, such as the relaxation of the trapped exciton (knr). Nevertheless, this analysis provides a useful insight into

ϕET = kET/(kET + k tr1)

(1)

ϕET,thiol = kET/(kET + k tr1 + k tr2) + ϕET2 ·k tr2/(kET + k tr1 + k tr2)

(2)

Since ϕET2 increases with doping concentration, this model predicts increasing ΔI/I with the doping concentration exhibiting the same trend as the experimental observation. However, this model suffers from too small ΔI/I it predicts 18230

dx.doi.org/10.1021/jp505162c | J. Phys. Chem. C 2014, 118, 18226−18232

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also recently reported to be capable of enhancing the sensitized luminescence in Mn-doped ZnSe nanocrystals.26 This suggests that organic thiols may generally be useful as the ligand that can recover the trapped exciton for the sensitization. Furthermore, thiol-induced enhancement of Mn luminescence was observed not only in solution sample but also in thin films on the solid substrate, which is a more relevant platform for the device applications. Figure 9 compares the exciton and Mn

compared to the experimental observation. For sample S1 with kET = 1.4 × 1011 s−11, ktr1 =1.4 × 1010 s−1 and ktr2 =1.4 × 1010 s−1 at octanethiol concentration of 0.1 M, the model predicts ΔI/I of