Unique Temperature Dependence and Blinking Behavior of CdTe/CdSe (Core/Shell) Type-II Quantum Dots

Abstract

Temperature dependent photoluminescence (PL) spectroscopy in a range of 5 K to room temperature (RT, 290 K) and single dot blinking behavior were investigated for CdTe/CdSe (core/shell, C/S) quantum dots (QDs). The QDs show type-II characteristics as both of the valence and conduction band levels of the CdTe core are placed higher in energy than those of the CdSe shell. The thickness of the CdSe shell was varied to control the degree of type-II character, and bare CdTe QDs were used as controls. The CdTe/CdSe (C/S) QDs have unique PL properties including (i) high susceptibility to PL thermal quenching with an exciton dissociation energy as small as 18 meV, compared with 46 meV for the CdTe QD, (ii) smaller band gap change showing only half the reduction of the control within the temperature change, and (iii) up to 27% larger PL bandwidth broadening than the control. The unique temperature-dependent properties were enhanced as the type-II character was increased by the thicker CdSe shell. Single dot level PL intermittency characteristics were studied for quasi type-II CdTe/CdSe (C/S) QDs that have alloyed layers at the core-shell interface. The quasi type-II QDs exhibited more frequent PL intensity intermittence blinking on and off at 290 K when compared with the CdTe QDs. However, the blinking kinetics follows similar universal power law on/off probability distributions with the R on and R off exponents evaluated as 1.57 and 1.38, respectively.

Temperature-Dependent Exciton and Trap-Related Photoluminescence of CdTe Quantum Dots Embedded in a NaCl Matrix: Implication in Thermometry

“…For comparison, the activation energy previously reported for colloidal CdTe QDs ( d QDs = 4.6 nm) was 46 meV. [ 14 ] Higher activation energies than those of a number of semiconductor QDs [ 14,17,31,32,56 ] suggest favorable temperature stability of the emission properties of CdTe QDs@ NaCl, arising from a lower density of defect states owing to the additional surface passivation by the NaCl matrix, which was previously ascribed to the surface passivation of QDs by binding of Cl − ions to the QD surface atoms while replacing a fraction of the original ligands. [ 40,58 ] …”

“…The β values are in good agreement with values reported in the literature for CdTe QDs in colloidal solutions. [ 14,54 ] Figure 4 b shows the PL fwhm temperature dependence of CdTe QDs@NaCl powders for fi ve different QDs sizes. The PL fwhm exhibits a gradual spectral broadening by about 40% on increasing the temperature from 80 to 360 K for all studied samples.…”

“…Many of these studies are commonly performed at low temperatures to elucidate electronic and excitonic structure in the absence of thermal broadening, and to provide insights into the exciton relaxation processes and excitonphonon interactions. [14][15][16][17][18][19][20] At the same time, most of the QD-based optoelectronic and optical devices, including photovoltaics, [ 21 ] light-emitting devices, [ 22,23 ] luminescent concentrators, [ 24 ] and color converters, [ 25 ] require stable operation of the constituent QD components at elevated temperatures. That is why the study of QD optical properties above room temperature is important from the point of view of the technological implementation of QD-based devices.…”

Wide emission-tunable CdTeSe/ZnSe/ZnS core–shell quantum dots and their conjugation with E. coli O-157

“…Washing or processing organic ligands passivated QDs into a new solvent can dramatically impact the PL efficiency. Moreover, the higher band gap inorganic shell provides more effective electronic and chemical passivation of surface dangling bonds as in the case of CdS/ZnS [7,8]; CdSe/ZnS [9]; CdTe/CdSe [10]; CdSe/CdS [11][12][13] and CdTe/CdSe/ZnS [14] core-shell QDs. More importantly, multicomponent hetero-junction within core-shell QDs allows independent tuning of electron and hole wave functions.…”

“…This has made it possible for the design and fabrication of novel semiconductor hetero-junction which can range from type-I to type-II junction. Type-I hetero-junction QDs with high quantum yield are useful for bio-labeling and light-emitting diodes fabrication [10,11,15]. While type-II heterojunction QDs have been targeted for using in photovoltaic and photocatalytic devices [16].…”