Tunable dual emission in type-I/type-II CdSe/CdS/ZnSe nanocrystals

We have used wet chemical methods to fabricate colloidal CdSe/CdSm/ZnSen core/intermediate/shell (C/I/S) nanocrystals (NCs), where m = 1 and 2 and n = 1-4 are the number of monolayers. The growth was monitored by using Raman and UV-vis spectroscopy, which demonstrated the formation of correct CdSe/CdSm/ZnSen C/I/S NCs. X-ray diffraction studies proved single-phase NCs crystallized in the zincblende-type structure. Photoluminescence (PL) studies have indicated that after photoexcitation C/I/S NCs generate simultaneously type-I and type-II emissions, namely E-I and E-II, associated with CdSe/CdSm and CdSm/ZnSen structures, respectively. For a specific value of m = 2, we have found the redshift of both E-I and E-II emissions when n is increased from 1 to 4. The PL studies versus the laser-excitation power (P-ex) up to about four orders of magnitude allow us to identify the origin of two emissions. A large blueshift of the E-II peak is ascribed to the band bending effect resulting from the spatially separated photoexcited carriers in type-II NCs. It appears that the dependence of the E-II peak on the cube root of P-ex (P-e(x)1/3) is linear while that of the E-I peak on P-e(x)1/3 is almost constant. In this work, we also point out that the emission intensity (I) of E-I and E-II can be tuned by changing the values of n and P-ex, where I obeys a power law I proportional to P-ex(k), with k= 0.7-1.0 dependent on the emission type. The nature of these phenomena is discussed in comparison with previous studies on C/S and C/I/S nanoheterostructures. (C) 2019 Elsevier B.V. All rights reserved.