Temperature Dependence of the Bandgap of CdTe/CdS Core–Shell Quantum Dots

Abstract

Quantum dots or semiconductor nanocrystals exhibit the so-called quantum confinement effect when synthesized with a crystallite size below to Bohr exciton radius of the bulk material. The QDs are characterized by the physical and chemical properties which significantly differ from their individual molecules or bulk forms [1]. The quantum confinement effect gives rise to unique size-dependent electrooptical properties, which are important for their potential use in lasers, solid-state lighting, solar cells, and biomedical fields [2][3][4][5][6][7]. While optical absorption measurements yield information on the QD size and the relative energy level configurations, they do not allow the determination of absolute energy levels positioned with respect to a standard potential. These values are vital for completing our understanding of the confinement effect in QDs. The most common method for the determination of the size-dependent conduction and valence band edge shift in quantum confined systems to date is cyclic voltammetry (CV) [8].Studies on the temperature dependence of QDs optical and electrochemical properties are important for their potential application because the desired device should emit in the visible spectral range at room temperature.Previous works by Poznyak et al.

Multi-channel electroluminescence of CdTe/CdS core-shell quantum dots implemented into a QLED device

“…The average size of the CdTe quantum dots prepared at this stage is about 2.3±0.3nm in accordance with the TEM data [22] and spectroscopic measurements [13]. To provide growth and stabilization of a CdS shell around the CdTe QD freshly obtained solution of CdTe QD was kept at 100°C for 8 h. The resulting size of the CdTe/CdS quantum dots is about 3.3 nm that follows from optical spectroscopy data and cyclic voltammetry measurements [23]. Because the average size of the CdTe core is about 2.3 nm and the thickness of a 1 monolayer (ML) CdS shell is estimated to be about 0.35 nm [24], the average shell thickness around the CdTe core is estimated to 0.5 nm, which means that the CdS shell grows by about 1.4 ML.…”

“…The overall construction scheme and energy diagram of the fabricated QLED are presented in Figures 4a and 4b, respectively. The energies of the conduction and valence band edges (vs. vacuum) were adopted as -2.3 and -5.3 eV for the MoO3 hole transporting layer [20,26,37], -2.1 and -5.4 eV for the TPBi electron transporting layer (ETL) [38], -3.7 and -5.7 eV for the CdTe/CdS QD of 3.3 nm size [23]. The electron and hole workout for the ITO anode and Ca:Al cathode were taken as -4.7 and -2.9 eV, respectively [39].…”