Morphology-induced phonon spectra of CdSe/CdS nanoplatelets: core/shell vs. core–crown

Abstract

Recently developed two-dimensional colloidal semiconductor nanocrystals, or nanoplatelets (NPLs), extend the palette of solution-processable free-standing 2D nanomaterials of high performance. Growing CdSe and CdS parts subsequently in either side-by-side or stacked manner results in core-crown or core/shell structures, respectively. Both kinds of heterogeneous NPLs find efficient applications and represent interesting materials to study the electronic and lattice excitations and interaction between them under strong one-directional confinement. Here, we investigated by Raman and infrared spectroscopy the phonon spectra and electron-phonon coupling in CdSe/CdS core/shell and core-crown NPLs. A number of distinct spectral features of the two NPL morphologies are observed, which are further modified by tuning the laser excitation energy E between in- and off-resonant conditions. The general difference is the larger number of phonon modes in core/shell NPLs and their spectral shifts with increasing shell thickness, as well as with E. This behaviour is explained by strong mutual influence of the core and shell and formation of combined phonon modes. In the core-crown structure, the CdSe and CdS modes preserve more independent behaviour with only interface modes forming the phonon overtones with phonons of the core.

Lattice dynamics of quasi-two-dimensional CdSe nanoplatelets and their Raman and infrared spectra

“…The literature data on Raman and IR spectra of CdSe nanoplatelets are not too numerous. [17][18][19][20][21] The main strong peak in the Raman spectra is explained by the light scattering by LO phonons whose frequency varies with a change in the nanoplatelet thickness as a result of the spatial confinement of the phonons. The weak features observed on the low-energy side of this peak are explained by surface optical (SO) phonons.…”

“…The weak features observed on the low-energy side of this peak are explained by surface optical (SO) phonons. 17,18,21 In Ref. 17 and 18, the thickness dependence of the frequencies of LO and SO modes was studied.…”

“…In Ref. 21, Raman spectra of CdSe nanoplatelets were measured at low temperature (35 K) and at different excitation energies. This measurements confirmed the appreciable long-wave shift and broadening of the main line in the spectra under resonant excitation and revealed a complicated structure of the SO mode.…”

Ultrawide color gamut LCD display with CdSe/CdS nanoplatelets

“…Core/shell NPLs are synthesized by growing CdS layer on the surface of the core only NPLs. Although the introduction of CdS shell can reduce surface defects and improve PLQY, the emission peak shifts because of the increase of thickness . For core/crown NPLs, CdS layer only grows around the edge and passivates on the lateral direction, ensuring a high PLQY as well as nonshifted emission …”

“…Although the introduction of CdS shell can reduce surface defects and improve PLQY, the emission peak shifts because of the increase of thickness. 11 For core/crown NPLs, CdS layer only grows around the edge and passivates on the lateral direction, ensuring a high PLQY as well as nonshifted emission. [12][13][14] Herein, we successfully synthesized green core/crown NPLs with four ML thickness and a high PLQY of 80%.…”