Elucidating the Electronic Cross-Talk Dynamics across the

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Elucidating the Electronic Cross Talk Dynamics Across the Hetero-Interface of Janus CdSe/PbSe Nanocrystals Hirendra N. Ghosh, Sourav Maiti, and Tushar Debnath J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11993 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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The Journal of Physical Chemistry

Elucidating the Electronic Cross Talk Dynamics Across the HeteroInterface of Janus CdSe/PbSe Nanocrystals Sourav Maiti†‡, Tushar Debnath†, 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

* E-mail: [email protected]. Tel: +91-22-25593873, Fax: (+) 91-22-25505331/25505151.

ABSTRACT: CdSe/PbSe Janus hetero-nanocrystals (HNCs) were synthesized in one pot and the underlying reaction mechanism along with the epitaxy at the hexagonal CdSe-cubic PbSe hetero-junction were investigated. During the initial stages of reaction, unusually large CdSe nanocrystals were formed due to rapid growth in presence of Pb-oleate which cation exchanged asymmetrically to form the Janus structures. Distinct PbSe and CdSe domains were visualized after sufficient growth as seen from the HRTEM images with an unique rock salt PbSe and wurtzite CdSe interface. The core Pb-Se bonds were differentiated from interfacial Pb-Se bonds through the X-ray photoelectron spectroscpy (XPS) measurements. Transient absorption spectroscopy (TAS) of the Janus NCs revealed intriguing spectroscopic signatures both in the spectral and time domain as manifested by the early population of higher excitonic states upon pulsed laser excitation along with broad TA spectra rich in higher excitonic states due to the intricate hybridization between the electronic states of two disparate materials. The TA measurements were well correlated with the formation of the Janus structure as new states

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emerged at the longer wavelength side in the TA spectra due to PbSe accompanied with a slow ~5 ps additional electron cooling component arising due to hole localization in the PbSe domain.


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1. INTRODUCTION: The desire to architect hetero-nanocrystals (HNCs) comprising two or more semiconducting materials having different physiochemical properties emerges due to the outstanding opto-electronic properties they possess important for both technological applications and fundamental research.1-10 The opto-electronics of these HNCs often surpass their individual components providing a unique way to control and/or create new properties. Recent progress in synthetic methodologies has provided several types of HNCs forming either symmetric core/shell or anisotropic structures. 2-4,6,10,11 All these HNCs ply upon the tunability of the charge carrier confinement, either confining them in a type-I structure to recombine with high photo-luminescence quantum yield or separating them in a type-II structure to get extracted for catalysis or harnessing solar energy.7,12-14 The intermixing of electronic states between the constituting components in these HNCs often leads to unique charge carrier dynamics very different from the excitonics of the individual components.5-7,12 In this regard, Cd and Pb chalcogen especially CdSe and PbSe based HNCs score considerable attention representing a unique system where two structurally immiscible materials with very less crystal mismatch, extremely different charge carrier effective masses and different band alignments co-exist forming a sharp hetero-interface. Both the symmetric and asymmetric structures are possible depending on the reaction conditions employed providing fundamental information about the reaction mechanisms and tunable properties.15-31 Anisotropic structures are derived either from restructuring the core/shell NCs or direct Pb2+ (Cd2+) to Cd2+ (Pb2+) cation exchange with judicious change in the reaction conditions.18-20 Partial cation exchange on pre-synthesized CdSe NCs often lead to anisotropic Janus HNCs where one part is composed of CdSe and the other is PbSe.22,24



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For Janus HNCs both CdSe and PbSe are surface exposed in contrast to the core/shell NCs where the shell creates a barrier for charge transfer interactions. In CdSe/PbSe HNCs due to quasi type-II band alignment the hole localizes in the PbSe region while the electron is spread throughout the NC.16,27,32 Thus Janus HNCs are advantageous as both electron and hole are specially decoupled and accessible to the surface beneficial for photocatalytic reactions and photovoltaics. However, the behavior of the photoexcited charge carriers which is the key to optimize the excitonic properties is rather obscure in these Janus structures. In this work, we have synthesized CdSe/PbSe Janus NCs to investigate the heterogeneous exciton dynamics across the interface. Moreover, in contrast to the present literature we have synthesized the Janus HNCs in one pot and shed light on mechanism the mechanistic aspects of partial cation exchange. The modulation of charge carrier dynamics with the mechanism through which the Janus structure forms has also been elaborated in this study. The Janus NCs were synthesized by injecting Se-trioctylphosphine at ~ 250°C in a mixture of Cd-and Pb-oleate and growing the NCs at 270°C. In presence of Pb-oleate the initial size of the CdSe NCs rises rapidly followed by Cd2+ to Pb2+ cation exchange leading to Janus CdSe/PbSe HNCs. The incorporation of Pb in the CdSe nanocrystalline host was confirmed through EDX, XPS and XRD measurements. HRTEM was utilized to analyze the hetero-epitaxy at the wurtzite-CdSe/rock-salt PbSe interface of the Janus HNCs. The complex intermixing between the electronic states of CdSe and PbSe altered the charge carrier dynamics both in the spectral and time domain in these hetero-nanocrystals as deciphered from the ultrafast transient absorption experiments (TAS). The continuous change in the exciton dynamics was well correlated with the formation of the Janus hetero-structure through cation exchange. CdSe/PbSe Janus hetero-structures have spectroscopic


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signatures very different from the CdSe NCs as manifested by early population of higher excitonic states upon pulsed laser excitation along with broad TA spectra rich in higher excitonic states due to higher density of states arising from the larger size and anisotropic structure of the Janus NCs. The cation exchanged PbSe domain provides additional electronic states on the low energy side of the TA spectra where the charge carrier dynamics shows an additional electron cooling component of ~5ps due to hole trapping by PbSe. 2. EXPERIMENTAL SECTION: a. Materials: Cadmium oxide (CdO, 99.5%), lead acetate trihydrate (PbAc2, 3H2O 99.9%), selenium powder (Se, 99.99 %), technical grade oleic acid (OA, 90%), tri-octyl phosphine (TOP, 90%) and octadecene (ODE) (90%) was purchased from Sigma-Aldrich and used without further purification. For synthesis and cleaning chloroform (AR grade) and methanol (AR grade) were used. For TA measurements spectroscopic grade chloroform was used. b. Synthetic procedure: For all the synthesis metal: Se was kept 2:1 unless otherwise stated. For pure CdSe (A series) synthesis 2 mmol (0.256 gm) CdO, 8 mmol OA (2.5mL) and ~8mL ODE was heated ~ 250°C in a three necked flask under nitrogen atmosphere to obtain a clear solution. At this temperature, Se-TOP (1mmol) dissolved in 2mL ODE was swiftly injected to the reaction flask and the NCs were grown at ~270°C.33,34 Aliquots were taken out at regular time intervals and quenched with chloroform followed by precipitating thrice using methanol. The NCs were then re-dispersed in chloroform for optical studies. For NCs with Pb, a mixture of CdO and PbAc2, 3H2O (molar ratio varied from series B to D) were taken keeping all other conditions the same. The molar ratio between CdO and PbAc2, 3H2O was varied as follows: 90:10 (B series), 75:25 (C series) and 50:50 (D series). For example in D series, 1 mmol of CdO and 1 mmol of



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PbAc2, 3H2O (therefore the total amount of cationic precursor is 2 mmol), 8 mmol OA (2.5mL) and ~8mL ODE was taken and at ~ 250°C under nitrogen Se-TOP (1mmol) dissolved in 2mL ODE was swiftly injected to the reaction flask and the NCs were grown at ~270°C. c. X-ray Photo-electron Spectroscopy (XPS): XPS measurements were performed in SPECS instrument where PHOBIOS 100/150 Delay Line Detector was utilized using Al Kα X-ray radiation (1486.6 eV) source. The binding energy values (BE) were corrected using C 1s peak at 284.5 as reference. d. Ultrafast Transient Absorption Measurements: For transient absorption measurements, the NCs were dispersed in hexane and excited with 400 nm pump laser. The power of the pump was kept low enough to have number of exciton ()

Elucidating the Electronic Cross-Talk Dynamics across the

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