Ligand Control of Semiconductor Nanocrystals for Efficient Carrier Injection
- PDF / 73,937 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 44 Downloads / 201 Views
EE13.25.1
Ligand Control of Semiconductor Nanocrystals for Efficient Carrier Injection Tomohide Murase, Harumi Asami, and Itaru Kamiya1 Nanomaterials Laboratory, Research and Technology Development Division Mitsubishi Chemical Group Science and Technology Research Center, Inc. 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan 1 Present address: Toyota Technological Institute, 2-12-1 Hisakata, Tempaku-ku, Nagoya 468-8511, Japan ABSTRACT Inorganic/organic hybrid material of CdSe/ZnS core/shell nanocrystals (NCs) covered with organic ligands was prepared with ligand exchange reaction in order to facilitate carrier injection into NCs through the organic ligands. The organic ligands used in this study, 2-naphtalenethiol and bathophenanthroline, were selected as model compounds according to adjustment of highest occupied molecular orbital and lowest unoccupied molecular orbital of the ligand to valence band maximum and conduction band minimum of the CdSe NCs, respectively. The prepared hybrid NCs easily disperse in polar organic solvents such as CHCl3 and show photoluminescence only from the CdSe/ZnS core/shell NCs. Current-voltage characteristics of the ligand exchanged NCs thin films sandwiched between electrodes were investigated. It was shown that conductivity of the film was improved by the introduction of carrier injecting ligands in comparison to the ligand unexchanged trioctylphosphine oxide-capped CdSe/ZnS NC thin film.
INTRODUCTION Recently, much attention has been paid to colloidal semiconductor nanocrystals (NCs) covered with organic ligands as a building block for electronic devices such as light emitting diodes (LEDs) [1-3] and photovoltaic cells [4,5]. For such integrated device applications, electron and hole transfer into/from the NCs is one of key issues. Trioctylphosphine oxide (TOPO)-capped NCs are considered to be unsuitable for the purpose because TOPO is insulating molecule and tremendously reduces electron transfer efficiency. Therefore, electronic interaction between NCs and other organic ligands has been extensively investigated [6-8]. For LED application, the ligands require several functions: 1) prevention of NC aggregation, 2) inhibition of NC fluorescence quenching, 3) lack of fluorescence from the ligands themselves, and 4) carrier transport that allows efficient charge injection into the NCs. In the present work, we study CdSe/ZnS core/shell NCs covered with two types of organic ligands in order to realize the functions above. One is for dispersibility, and the other is for carrier injection. In the latter, the ligand itself should not show any fluorescence, and its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are adjusted to match valence band maximum (VBM) and conduction band minimum (CBM) of the NCs for the purpose of efficient carrier injection through the ligands to the NCs. Following this strategy, we select tetradecaphosphonic acid as dispersing ligand, and 2-naphtalenethiol (2NT) or bathophenanthroline (Bphen) as carrier injecting li
Data Loading...
