Temperature-dependent photoluminescence of CdSe/CdTe quasi-type-II quantum dots
Introduction
Colloidal quantum dots (QDs) are promising materials for a variety of next generation optoelectronic devices, including photovoltaics [1,2], light emitting diodes [3], lasers [4] and biomedical sensors [5]. They take advantages of the size-dependant and controllable optical and electrical properties and inexpensive synthesising methods. QDs are classified into two groups; Type-I and Type-II. Electrons and holes confine in the same volume in the former structures, whereas, the latter structure is a core/shell form of QD and the each charge carrier type reside in different regions of the material in those structures. A subcategory of Type-II structures is called quasi-Type-II structure wherein, if the electrons reside in the core region, the holes can stay either in the core or shell or vice-versa. Since charge carriers are localized in different volumes in Type-II QDs, the direct charge recombination rate is reduced and hence the charge extraction efficiency can be increase in those structures. In addition, there is effective band gap in the Type-II QDs which can be carefully designed by changing shell thickness that has ability to cover a broad range of the spectrum.
Temperature-dependant photoluminescence (PL) spectroscopy is an effective tool that sheds light on the radiative and non-radiative relaxation processes and the carrier dynamics of semiconductor QDs. It also provides insights about exciton phonon interaction [6,7], phonon scattering [8,9], the existence of surface trap sites [[10], [11], [12]], temperature sensor applicability [13] and the stability of crystal structure [14] in QDs. Temperature-dependant photophysical properties of QDs with a number of different materials have been studied, including: CdTe/CdSe [8], CdSe [15], CdTe [16], CdSe/ZnS [9], ZnxAgInSe [13], PbS [10] and perovskite [6,7,12] QDs. Despite a few studies of radiative and non-radiative relaxation process in CdSe/CdTe core/shell QDs [[17], [18], [19], [20]], temperature-dependant photopysical properties of those structures have not been studied thoroughly at cryogenic temperatures.
In this work, the steady-state temperature-dependent PL spectra of CdSe/CdTe quasi-Type-II QDs were obtained in the temperature range of 10–300 K to study optical properties of those heterostructure QDs. The temperature variation of the PL intensity, PL energy, and PL linewidth were investigated. The origins of observed radiative and nonradiative signals were discussed. A clear radiative emission was observed at high energies and attributed to the nucleation of separate CdTe QDs during the shell formation.
Section snippets
Materials and synthesis of CdSe core and CdSe/CdTe core/shell QDs
Cadmium oxide (99.5%), selenium powder (>99.5%), tellurium powder (99.8%), 1-Octadecene (90%), oleic acid (90%), tetradecylphosphonic acid (TDPA, 97%), octadecylamine(97%), trioctylphosphine (TOP, 90%), and toluene (anhydrous, 99.8%) were purchased from Sigma-Aldrich and used as received. All reactions were carried out under inert gas using a standard schlenk line with vigorously stirring.
Synthesis approach of CdSe QDs was a modification version of Mohammed et al.‘s hot injection method [21].
Results and discussion
UV–Vis spectra of CdSe core and of CdSe/CdTe core/shell structures and 2nd derivative of the spectra of core/shell structure are shown in Fig. 1a. The absorption transitions of CdSe core are more distinctive and narrower compared with that of core/shell structure. The absorption band edge for CdSe QDs corresponds to 553 nm (2.24 eV), whereas with addition of CdTe shell the absorption band edge is significantly red shifted and broaden, which is a characteristics of structural transition from
Conclusions
In summary, the temperature dependent PL properties of CdSe/CdTe core/shell QDs were investigated in the range of 10 K–300 K. The band edge PL intensity, peak energy and linewidth were found to be temperature dependent. The average phonon energy and the Huang-Rhys factor for those heterostructures were estimated to be meV and , respectively. A second excitonic peak, whose properties is temperature dependent, was observed and attributed to the formation of CdTe QD cores during
CRediT authorship contribution statement
Musa Çadırcı: Conceptualization, Methodology, Validation, Investigation, Writing - original draft, Writing - review & editing, Supervision, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The author kindly would like to thank.
-Duzce University Scientific Research Project Coordination (DUBAP) for the support with the project number of 2017.06.03.592.
-Ataturk University, Eastern Anatolia Advanced Technology Application and Research Center (DAYTAM) for providing its infrastructure.
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