Unusually Slow Electron Cooling to Charge-Transfer State in Gradient

Letter pubs.acs.org/JPCL

Unusually Slow Electron Cooling to Charge-Transfer State in Gradient CdTeSe Alloy Nanocrystals Mediated through Mn Atom Tushar Debnath,† Sourav Maiti,†,‡ and Hirendra N. Ghosh*,† †

Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Department of Chemistry, Savitribai Phule Pune University, Ganeshkhind, Pune 411007, India

‡

S Supporting Information *

ABSTRACT: We have synthesized Mn-doped CdTeSe gradient alloy nanocrystals (NCs) by a colloidal synthetic method, and charge carrier dynamics have been revealed through ultrafast transient absorption (TA) spectroscopy. Due to the reactivity difference between Te and Se, a CdTe-rich core and CdSe-rich shell have been formed in the CdTeSe alloy with the formation of a gradient type II core−shell structure. Electron paramagnetic resonance studies suggest Mn atoms are located in the surface of the alloy NCs. Steady-state optical absorption and emission studies suggest formation of a charge-transfer (CT) state in which electrons are localized in a CdSe-rich shell and holes are localized in a CdTe-rich core which appears in the red region of the spectra. Electron transfer in the CT state is found to take place in the Marcus inverted region. To understand charge-transfer dynamics in the CdTeSe alloy NCs and to determine the effect of Mn doping on the alloy, ultrafast transient absorption studies have been carried out. In the case of the undoped alloy, formation of the CT state is found to take place through electron relaxation to the conduction band of the CT state with a time of 600 fs and through hole relaxation (from the CdSe-rich state to the CdTe-rich state) to the valence band of the CT state with a time scale of 1 ps. However, electron relaxation in the presence of Mn dopants takes place initially via an electron transfer to the Mn 3d state (d5) followed by transfer from the Mn 3d state (d6) to the CT state, which has been found to take place with a >700 ps time scale in addition to the hole relaxation time of 2 ps. Charge recombination time of the CT state is found to be extremely slow in the Mndoped CdTeSe alloy NCs as compared to the undoped one, where the Mn atom acts as an electron storage center.

T

late, Rosenthal and co-workers29 and we have16 demonstrated charge carrier dynamics where hole trapping and slow electron cooling in CdSxSe1−x alloy NCs were discussed using ultrafast up-conversion and transient absorption techniques, respectively. Optical and photovoltaic properties of several other alloy NCs like CdSxTe1−x,30 CdxZn1−xS,31 PbSxSe1−x,32,33 and CdSe/ CdxZn1−xS8 also have been studied. Recently, Zhong and coworkers demonstrated CdSexTe1−x alloy NCs,34 and a solar cell made out of this material was found to have record conversion efficiency (∼9.3%) in QDSC. Another important means for the control of charge carrier processes in the NCs is impurity doping. Incorporation of impurities into semiconductor NCs can significantly alter intrinsic properties, such as optical, electrical, and magnetic properties, of the host NCs.7,35−37 Doping of II−VI semiconductor QDs with a 3d transition metal like Mn have been widely investigated in recent years.38,39 Mn-doped QDs can act as a photoluminescence activator because of its very high emission quantum yield (QY) as well as long emission lifetime,

hree dimensional exciton confinements in semiconductor nanocrystals (NCs) offer the possibility of band gap tunability by controlling the size, shape, and composition of the particle.1−4 The exciton confinement of such NCs can be modulated by forming heterostructures having either a sharp or smooth interface; the later has been realized in alloyed NCs,5−9 whereas core−shell NCs10−13 exhibit the former possibility. A smooth interface in the alloyed structure removes the interfacial defects which results high photoluminescence quantum yield (QY) as compared to their constituent NCs.14−17 Charge separation yield also increases because of the smooth nature of the interface in alloy NCs as compared to the sharp interface in core−shell structures (mainly in type II structure). Such a longlived charge-separated state has received tremendous interest in current research.11,18−21 In addition, in alloy NCs it is easy to manipulate the band gap without changing the size of the NCs, only by changing the composition of the constituent NCs, which can be applicable for nanoelectronics, biological imaging, and quantum dot solar cells (QDSCs).22−26 Due to λCT, charge transfer takes place in the Marcus inverted region, which agrees well with previous literature reports.52,54,61 Detailed analysis regarding reorganization energy and free energy of electron transfer are provided in the Supporting Information.

Figure 4. Ultrafast transient absorption spectra of CdTeSe alloy NCs at different time delays after 400 nm laser excitation.

(bleach) peaking at 580 and 700 nm (broad) and a humplike bleach at 500 nm. Here, the bleach maximum at 580 nm and hump at 500 nm can be attributed to state-filling transition 1Se−1S3/2 (1S exciton) as well as 1Pe−1P3/2 (1P exciton), respectively;62−67 however, the broad bleach in the red region of the spectrum (at 700 nm) can be ascribed to state filling of electrons in the CT state. The assignment of the CT bleach in the red region of the spectrum is in good accord with the deconvoluted absorption spectrum (Figure 3A) of CdTeSe alloy NC. Fiebig and co-workers reported on photoexcitation of CdSe/CdTe nanorod heterostructure materials, and the bleach due to CT absorption appeared on the red end of the TA spectrum.68 Zamkov and co-workers also demonstrated the appearance of CT bleach in ZnSe/CdS/Pt heterostructure material due to spatially separated charge transfer from ZnSe dot (1Sh) to CdS nanorod (1Se).69 Previously, we have also reported transient bleach due to CT absorption in the TA spectrum for CdSe/Br-PGR and Mn-CdSe/Br-PGR composites in which direct charge transfer takes place from Br-PGR molecule to CdSe (or Mn-CdSe) NCs.46 To understand charge-transfer dynamics in Mn-doped CdTeSe NCs in the ultrafast time scale, transient absorption studies have been carried out after exciting the samples at 400 nm, and results are shown in Figure 5. Figure 5A depicts the TA spectrum at a shorter time scale (200 fs to 25 ps), and Figure 5B depicts the same at longer time delay (50 ps to 1 ns), which looks very different from its undoped counterpart (Figure 4). It is clearly seen that at early time delay the TA 1362

DOI: 10.1021/acs.jpclett.6b00348 J. Phys. Chem. Lett. 2016, 7, 1359−1367

Letter

The Journal of Physical Chemistry Letters

can be fitted with biexponential growth and multiexponential recovery time constants (Table 1). The second growth component (600 fs) in both systems can be attributed to electron cooling from the upper excitonic state to the 1S excitonic state.70 The bleaches were found to recover triexponentially, where the faster recovery component can be assigned to carrier trapping while the slow components are due to electron−hole recombination.55,66,71 We have also compared the bleach recovery kinetics at 1S exciton position for both Mndoped CdSe (530 nm) and Mn-doped CdTe (700 nm) with Mn-doped CdTeSe alloy NCs (Supporting Information). No remarkable effect on doping in the bleach recovery dynamics has been observed in both CdSe and CdTe NCs (Supporting Information). However, the recombination dynamics of the charge carriers (electron and hole) in the case of Mn-doped CdTeSe NCs is much slower as compared to that of undoped CdTeSe alloy as well as Mn-CdSe and Mn-CdTe NCs. To understand the origin of the charge-transfer state for both alloy NCs, bleach kinetics were monitored at 700 and 730 nm for undoped and Mn-doped alloy NCs, respectively (Figure 6B). The bleach growth and recovery kinetics are completely different for doped and undoped alloy NCs. Although in the undoped alloy biexponential growth with time constants τ1g = 0.6 ps (88%) and τ2g = 1 ps (12%) is observed, the Mn-doped one shows triexponential growth with time constants τ1g = 0.6 ps (61%), τ2g = 2 ps (5%), and τ3g ≥ 700 ps (34%) (Table 1). The bleach kinetics of Mn-doped CdTeSe alloy NC exhibits anomalous behavior at 730 nm where after initial growth, the kinetics recovers up to 50 ps and again grows, which can be fitted with a >700 ps time constant. Surprisingly, this unusual slow growth of the bleach kinetics is absent in undoped CdTeSe alloy NCs. Such slow growth of bleach kinetics is also absent in Mn-doped CdSe and CdTe NCs, where no such charge-transfer state was detected. We have already discussed that in steady-state absorption studies for both undoped and Mn-doped CdTeSe alloy NCs, a new charge-transfer state is formed because of direct transfer of an electron from the valence band of the CdTe-rich core to the conduction band of the CdSe-rich shell through the gradient interface, as shown in Figure 2B. Through steady-state luminescence studies, it can be seen that CT luminescence is generated after recombination of an electron in the conduction band of the CdSe-rich shell and a hole in the CdTe-rich core. The CT luminescence state can also be generated through direct excitation of the CT complex or interfacial electron and hole transfer between the CdTe-rich core and the CdSe-rich shell as shown in Figure 2B. To explain the unusual slow growth of CT bleach in Mn-doped CdTeSe alloy NCs, it is very important to know the role of Mn atoms in the alloy materials. No Mn luminescence has been observed from the alloy materials. Both EPR studies and luminescence spectroscopy confirmed that the 4T1 state of Mn resides above the CB edge of CdSe and CdTe NC. When the transient kinetics is followed at both the exciton wavelength and at the CT bleach wavelength, the charge carrier relaxation time at different states can be monitored, and the processes are depicted in Scheme 1. Let us discuss the kinetics at excitonic wavelength (580 nm) for CdTeSe alloy NCs. The second growth component (600 fs) can be attributed to the electron cooling time from the upper excitonic state to the conduction band edge of the CdTeSe alloy NCs. However, the bleach recovery kinetics at CT wavelength (700 nm) can be fitted biexponentially with 600 fs (which matches with the second growth component of 580 nm

Figure 5. Ultrafast transient absorption spectra of Mn-doped CdTeSe alloy NCs (A) at earlier time delay (200 fs to 25 ps) and (B) at longer time delay (50 ps to 1 ns) after 400 nm laser excitation.

spectrum consists of two negative absorption bands peaking at 506 and 615 nm, which can be attributed to bleach due to statefilling transition of 1S and 1P exciton where the 1S exction bleach has a broad red tail that continues up to 780 nm.63 However, at longer time delay, in addition to 1S and 1P bleach, another bleach was observed in the red region of the spectrum peaking at 730 nm (Figure 5B). Surprisingly, with increasing time delay, the intensity of the bleach at 730 nm increases although the bleach intensity at both the 1S and 1P excitonic positions decreases. In Figure 2B we show deconvoluted absorption spectra of Mn-doped CdTeSe NCs, which shows the 1S exciton absorption band peaking at 610 nm and the CT absorption band peaking at ∼725 nm. Therefore, the negative absorption band appearing at longer time delay at ∼730 nm can be attributed to state filling of electrons in the CT state. Here, at early time delay, bleach due to CT absorption has not been observed, unlike undoped CdTeSe alloy (Figure 4). To monitor the carrier relaxation dynamics in both undoped and Mn-doped CdTeSe NCs, transient kinetics have been monitored at different wavelengths in both systems (Figure 6). Figure 6A depicts the growth and recovery kinetics at 1S excitonic position for both undoped (at 580 nm) and Mndoped (at 615 nm) CdTeSe alloy NCs. Both bleach kinetics

Figure 6. (A) Bleach recovery dynamics (a) at 580 nm for undoped CdTeSe and (b) at 615 nm for Mn-doped CdTeSe NCs after 400 nm laser excitation. (B) Transient bleach kinetics (a′) at 700 nm for undoped CdTeSe and (b′) at 730 nm for Mn-doped CdTeSe NCs after 400 nm laser excitation. (Inset in panel B) Same kinetic traces for both NCs at short time delay. 1363

DOI: 10.1021/acs.jpclett.6b00348 J. Phys. Chem. Lett. 2016, 7, 1359−1367

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The Journal of Physical Chemistry Letters

Table 1. Comparison of Different Kinetic Parameters from Transient Bleach Recovery at Different Wavelengths for Both Undoped and Mn-doped CdTeSe NCs after 400 nm Laser Excitationa growth (cooling) τ1g (%) CdTeSe Mn-CdTeSe a

580 700 615 730

nm nm nm nm

0.1 0.6 0.1 0.6

ps ps ps ps

(71%) (88%) (68%) (61%)

recovery (trapping/recombination)

τ2g (%) 0.6 ps (29%) 1 ps (12%) 0.6 ps (32%) 2 ps (5%)

τ3g (%)

τ1 (%)

τ2 (%)

τ3 (%)

20 (42%) 40 (27%) 40 ps (24%) ≫1 ns (70%)

>1 ns (30%) >1 ns (14%) >1 ns (42%)

>700 ps (34%)

2 (29%) 3 (59%) 5 ps (34%) 20 ps (30%)

The percentage values in the parentheses indicate the fraction of corresponding exponential functions.

can be attributed to the hole relaxation in the valence band edge of the CT state (Scheme 1). Interestingly, due to the presence of Mn atoms in alloy NCs, the hole-transfer times get delayed. However, the most remarkable observation is the extremely slow growth component of >700 ps at the CT bleach wavelength of Mn-doped CdTeSe alloy NCs. Here, the unusually slow growth component (>700 ps) must be associated with the CT state of Mn-CdTeSe NC which otherwise is absent in Mn-CdSe and Mn-CdTe NC because of the absence of any CT state (see the Supporting Information for details). This >700 ps component can be attributed to the electron cooling time from the Mn 3d state to the CB of the CT state, as shown in Scheme 1. In our earlier report, we have shown that the Mn center in Mn-doped CdSe NCs can act as an electron storage center where electron accumulation takes place from the Br-PGR molecule.46 Here, the Mn atom can act as an electron storage center where, after 400 nm laser excitation, the electron relaxes to the 3d state of the Mn atom in competition with cooling to the CT state (i.e., CB of CdSerich shell), as shown in Scheme 1. Later, transfer of the electron takes place from the 3d state of the Mn atom to the conduction band of the CT state with a time constant >700 ps. In the present circumstances, the higher-energy 4T state does not allow energy transfer from the excited NC to the Mn state which is a very fast process.42 Therefore, the only possibility is electron transfer from the excited NC to the Mn 3d state. Here, Mn receives an electron from the excited NC material which promotes it from the 3d5 to the 3d6 configuration. Its clear from our investigation that the Mn 3d6 state (intermediate state) is an apparently long-lived state. To determine the exact term symbol for the Mn 3d6 configuration state in such NCs demands rigorous theoretical calculations which is beyond the scope of this work. It is understandable that in the transient spectra of the Mn-doped CdTeSe alloy (Figure 5) at early time scale the bleach due to the CT state at 730 nm is quite negligible; however, with time delay, the bleach due to the CT state increases, which we are attributing to the population of the CT state from the 3d state of the Mn atom. Again, the hole relaxation time to the CdTe-rich state slowed in the Mn-doped alloy as compared to the undoped one. Here, the Mn center acts as an electron storage center; as a result, electron−hole decoupling occurs, which further slowed hole relaxation in the Mn-doped alloy. The bleach recovery at 730 nm is extremely slow, which suggests that due to electron storage in the Mn state, the charge recombination time between an electron in the conduction band of the CdSe-rich shell and a hole in the valence band of the CdTe-rich core in Mn-doped CdTeSe alloy NCs is exceptionally slow as compared to that of the undoped one (Figure 2B, Scheme 1). To conclude, we have synthesized undoped and Mn-doped CdTeSe gradient alloy NCs by a high-temperature colloidal method. Due to higher reactivity of Te as compared to that Se,

Scheme 1. Schematic Illustration of Different Carrier Relaxation Processes for Undoped and Mn-Doped CdTeSe Alloy NCsa

a

Electron cooling to the CT state takes 0.6 ps in undoped alloy, whereas in Mn-doped alloy it takes 0.6 ps and >700 ps (major electron cooling through Mn state). Hole relaxation from the Se-rich shell to the Te-rich core takes 1 ps in undoped alloy and 2 ps in Mn-doped alloy.

kinetics) and 1 ps. Growth at the CT wavelength suggests the formation of the CT state, which can take place both by electron relaxation in the conduction band edge of CT state (i.e., conduction band edge of CdSe-rich shell) and/or hole relaxation to the valence band of the CT state (i.e., valence band edge of CdTe-rich core, Scheme 1, Figure 2B).56,57 The 600 fs component can be attributed to the electron relaxation in the conduction band edge of the CT state, and the 1 ps component can be attributed to the hole relaxation in the valence band edge of the CT state (i.e., hole-transfer time from valence band of CdSe-rich shell to valence band of CdTe-rich core, Scheme 1). Scholes and co-workers observed the slow growth component in the CT bleach position in CdTe/CdSe core−shell after selective excitation of the CdSe shell, which was attributed to hole transfer.52 The multiexponential bleach recovery time constants at 700 nm [3 ps (59%), 40 ps (27%), and >1 ns (14%)] can be attributed to carrier trapping and charge recombination65,66,71 time between an electron in the conduction band of the CdSe-rich shell and a hole in the valence band of the CdTe-rich core (Figure 2B, Scheme 1). The charge-transfer dynamics in Mn-doped CdTeSe alloy NCs after monitoring at exciton wavelength (615 nm) and CT wavelength (730 nm) indicates that the growth kinetics at 615 nm (excitonic wavelength) exactly matches with the undoped one and can be fitted with 0.1 ps (68%) and 0.6 ps (32%) components. The second component can be attributed to the electron cooling at the 1S excitonic state of Mn-doped CdTeSe alloy. However, the bleach kinetics at the CT wavelength can be fitted with triexponential growth with time constants of 0.6 ps (61%), 2 ps (5%), and >700 ps (34%). Here, the 600 fs component can be attributed to the electron cooling time in the conduction band edge of the CT state and the 2 ps component 1364

DOI: 10.1021/acs.jpclett.6b00348 J. Phys. Chem. Lett. 2016, 7, 1359−1367

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ACKNOWLEDGMENTS T.D. and S.M. acknowledge CSIR for research fellowship. We thank Dr. R. M. Kadam of Radiochemistry Division, BARC for EPR measurements. We also acknowledge Dr. D. K. Palit and Dr. B. N. Jagatap of BARC for their support.

a CdTe-rich core and CdSe-rich shell are formed in the CdTeSe alloy NCs. EPR analysis shows most of the Mn atoms reside on the surface of the NC, which results in the energy of the 4T1 state of the Mn atom becomes higher than the CB edge of alloy NCs. As a result, no Mn luminescence was observed in Mn-doped alloy NCs. Interestingly, dual luminescence bands were observed both from undoped and Mn-doped CdTeSe alloy NCs, where the blue emission band has been attributed to band edge emission of the CdTe-rich core and the red-shifted broad emission band has been ascribed to CT emission. Marcus analysis of steady-state absorption and emission spectra suggests charge transfer to the CT state takes place in the Marcus inverted region. Ultrafast TA experiments suggest that on 400 nm excitation, bleach due to 1P and 1S excitonic state and the CT state of CdTeSe alloy NCs appears immediately after laser pulse excitation. Electron cooling time to the CB band edge of alloy NCs and hole transfer time from the CdSerich shell to the CdTe-rich core were found to be 600 fs and 1 ps, respectively. However, unlike the undoped alloy, bleach due to the CT state in the doped alloy appears at a much longer time delay. In the case of Mn-doped alloy NCs, the electron cooling time and hole transfer time were found to be 600 fs and 2 ps, respectively. The unusually slow growth component (>700 ps) is reported for the first time in the literature and has been attributed to electron population from the Mn 3d state to the CT state. Charge recombination time in the CT state of the Mn-doped CdTeSe alloy was found to be much slower as compared to that of the undoped one because of the presence of Mn dopant, which acts as an electron storage center. To the best of our knowledge we are demonstrating for the first time the effect of the Mn atom on the the CT state of alloy NCs, which facilitate higher charge separation through CT state formation. Our investigation suggest that unusually slow electron cooling from the Mn state to the CT state and extremely slow charge recombination in Mn-doped alloy system can help in the design and development of more efficient QDSCs.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00348. Details about the synthesis of undoped CdTeSe and Mndoped CdSe, CdTe, and CdTeSe NCs; characterization through HRTEM and EPR measurements; optical study of undoped and Mn-doped CdSe, CdTe, and CdTeSe NC; Marcus theory analysis of undoped and doped alloy; detailed experimental setup for transient absorption spectrometer; and ultrafast transient absorption measurement of undoped and Mn-doped CdSe, CdTe, and CdTeSe NCs (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+) 91-22-25505331/ 25505151. Funding

This work was supported by “DAE-SRC Outstanding Research Investigator Award” (Project/Scheme No.: DAE-SRC/2012/ 21/13-BRNS) granted to H.N.G. Notes

The authors declare no competing financial interest. 1365

DOI: 10.1021/acs.jpclett.6b00348 J. Phys. Chem. Lett. 2016, 7, 1359−1367

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