Article pubs.acs.org/JPCC
Ultrafast Charge Carrier Delocalization in CdSe/CdS Quasi-Type II and CdS/CdSe Inverted Type I Core−Shell: A Structural Analysis through Carrier-Quenching Study Partha Maity, Tushar Debnath, and Hirendra N. Ghosh* Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai−400 085, India S Supporting Information *
ABSTRACT: We have employed femtosecond transient absorption spectrocopy to monitor charge carrier delocalization in CdSe/CdS quasi-type II and CdS/CdSe inverted type I core− shell nanocrystals (NCs). Interestingly, CdSe and CdS QD pairs can make both type I and quasi-type II core−shell structures, depending on their band alignment and charge carrier localization. Steady-state optical absorption and luminescence studies show a gradual red-shift in both optical absorption and emission spectra in CdSe/CdS core−shell with increasing CdS shell thickness. The luminescence quantum yield in CdSe/CdS core−shell drastically increases with shell thickness. Notably, CdS/CdSe inverted core−shell shows a huge red-shift both in absorption and luminescence which closely matches with the band edge photoluminescence (PL) of pure CdSe QDs (shell). However, the luminescence quantum yield does not change much with shell thickness. Depending on their band energy level alignment, the charge carrier (electron and hole) delocalization in both the core−shells have been demonstrated using electron (benzoquinone, BQ) and hole (pyridine, Py) quencher. The bleach recovery kinetics of CdS/CdSe core−shell recovers faster in the presence of both BQ and Py. However, for CdSe/CdS core−shell, the bleach recovers faster only in the presence of BQ while the bleach dynamics remain unaffected in the presence of Py. Our experimental observations suggest that in CdSe/CdS quasitype II core−shell, photoexcited electrons are localized in CdS shell and holes are localized in CdSe core; however, in CdS/CdSe inverted core−shell both electrons and holes are localized in the CdSe shell.
1. INTRODUCTION Nonocrystals, especially II−IV semiconductors quantum dots (QDs), are generating enormous interest in research in the last three and a half decades owing to their exciting properties, such as quantum confinement of the charge carriers, size-dependent optical tunabilites, high extinction coefficient, exotic electronic properties, and band gap engineering.1−8 The combination of two different NCs in one nanostructure is called heterostructure, and such heterostructures are usually categorized as core−shell,9−13 tetrapods,14 dot in rods (DR),15−18 etc., depending on their shape and size. The heterostructure core−shell NCs are typically recognized to be of three different formstype I, type II, and quasi-type IIdepending on their valence band (VB), conduction band (CB) alignment, and the charge carrier delocalization. In type I core−shell, the core NC is confined by a wider band gap NC where the VB and CB of the core NC lies in between the VB and CB of the shell NC materials. Thus, on excitation, both the charge carriers (electron and hole, e−h) are localized in the core structure. As a result, the charge carriers are strongly confined in the core materials, and increased photoluminescence quantum efficiency is achieved.19 However, if the wider band gap material occupies the core and is covered by small band gap material (shell), then © XXXX American Chemical Society
an inverted type I core−shell structure can be realized. In inverted type I structure, both e−h are localized in the lower band gap shell material upon photoexcitation.20−23 On the other hand, in type II core−shell structure, the energy levels of both CB and VB of the core material lie above the CB and VB of the shell material or vice versa. So on photoexcitation of type II core−shell, photoexcited electron and hole are localized in two different semiconductors, either core or shell, depending upon the band alignment.24−32 In addition to the above core− shell structures, there is an intermediate core−shell structure, known as quasi-type II core−shell,18,24,33,34 in which the band gap alignment is like the type I structure however due to smaller band offset between the CB of both the semiconductors the electron can be delocalized in the CBs of both of the NCs. Similarly, if the VB offset energy is smaller between the holes, they can be delocalized in the VBs of both of the NCs. In contrast to type I and II structures, the quasi-type II NCs are now routinely used in different applications due to heir higher emission quantum yield as well as better charge separation.35 Received: September 13, 2015 Revised: October 20, 2015
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DOI: 10.1021/acs.jpcc.5b08913 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Charge carrier (e−h) transfer time and their location in the core-shell structure are discussed in the present investigation.
CdSe/CdS semiconductor composite is the one of the widely studied NC,35−41 in which the band gap alignment of bulk materials suggests a type I core−shell structure; however, due to low energy offset between the conduction bands of CdSe and CdS, the semiconductor pair can easily make a quasi-type II core−shell structure, depending on the size of the core NC and thickness of the shell NC.18,24,33 It is reported in the literature39,42 that CdS shell on CdSe core helps to remove the surface states, and with increasing shell thickness, the emission quantum yield increases tremendously (close to unity).35,43 Due to the higher quantum yield of the CdSe/CdS core−shell, it has been extensively studied in various fields with potential applications such as light emmiting diodes (LED), bioimaging, lasing materials, PL blinking, and optical gain performance.44,45 In addition to the higher emission quantum yield, due to staggered band alignment, CdSe/CdS core−shell materials are successfully utilized in various practical application such as photovoltaic and photocatalysis due to higher effective charge separation. As the lattice mismatch between CdSe and CdS is very small (
