Vibrational spectroscopy of compound semiconductor nanocrystals

Abstract

A review of recent applications of Raman spectroscopy as a fast, sensitive, and non-destructive technique for exploring II–VI semiconductor nanocrystals fabricated by various methods (colloidal chemistry, Langmuir–Blodgett method, diffusion-limited growth) is presented. Specific size-related features revealed in the nanocrystal Raman spectra (phonon confinement, surface phonons) are analysed, as well as more complicated size effects for ultrasmall nanocrystals (NCs) related to the activation of the phonon density of states modified by surface reconstruction. Similarities and differences of the Raman scattering in II–VI and III–V or elemental (Si) semiconductor NCs are briefly analysed. Implementation of resonant conditions and application of infrared absorption analysis, complementary to the Raman spectroscopy—resulting in the observation of phonon modes ‘silent’ in conventional Raman scattering processes—are discussed. Furthermore, Raman spectroscopy is employed for fast and efficient assessment of the composition of matrix-embedded ternary II–VI nanocrystals, as well as more complicated multimode quaternary II–VI systems. Selective probing of electronic and vibrational spectra of different parts of heterogeneous NCs (such as core–shell systems) by tuning the excitation wavelength in resonant Raman scattering is considered. The analysis of phonon spectra is applied to the quantitative estimation of strain in the core and shell, and degree of interface intermixing, as well as to checking the surface oxidation. The above approaches and phenomena are further explored in more complex compound NCs beyond II–VI, such as CuInS2/ZnS. Recent results in the field of surface- and tip-enhanced Raman spectroscopy and surface-enhanced infrared absorption are analysed showing the perspectives of Raman spectroscopy as a tool for investigation of single-nanocrystal phonon spectra.

Ultra-small aqueous glutathione-capped Ag–In–Se quantum dots: luminescence and vibrational properties

“…Multi-l exc Raman spectroscopy has already proved to be able to probe selectively different compounds or phases contained within the same individual QDs in the ensemble. 54 On the other hand, the heating-induced changes of the Raman spectra of AISe QDs are generally similar to those we previously observed for CuInS 2 , 52 and attributed to increased relative intensity of the highest-frequency LO modes, situated at 340-350 cm À1 for CuInS 2 (ref. 61) and, presumably, at 230-240 cm À1 for present AISe QDs.…”

“…Besides providing the vibrational (phonon) spectra themselves, resonant Raman spectroscopy is an efficient tool of probing both the lattice structure of semiconductor QDs and local electronic resonances related either with intentional heterogeneity, like in core/shell structures, 52 or spontaneous formation of secondary phases, 53 common in complex chalcogenide QDs but hardly detectable by other structural techniques. 54 Based on our previous ndings of distinct vibrational patterns of aqueous II-VI 55 and M-In-S QDs 56 of ultrasmall size, an investigation of similar small Ag-In-Se is additionally motivated.…”

Resonant tip-enhanced Raman scattering by CdSe nanocrystals on plasmonic substrates

“…The frequency position of the LO phonon, around 212 cm À1 , corresponds well to its value in CdSe bulk or in large-sized CdSe NCs, for which the phonon connement effect is not signicant. 39,41,42 Even though a 2-3 cm À1 lower frequency position could be expected for 5-6 nm CdSe NCs, 43 the real particle size in our case could be larger due to partial merging of individual NCs in the course of the annealing and consequent removal of the ligand spacer between them.…”

Comment to “Continuous‐wave laser irradiation to form Cd1−xZnxSe shell on CdSe QDs in silicate glasses” (J. Amer. Ceram. Soc. 102, 4555‐4561 (2019))

“…1 Therefore, it is quite strange that immediately after mentioning the good correlation of the Cd 1−x Zn x Se QD chemical composition (x ≈ 0.8) obtained from the Raman spectra and energy-dispersive X-ray spectroscopy (EDS) (the applicability of such correlation will be discussed below), the authors make the statement: "Therefore, we suggest that the QDs initially had a CdSe core, but over time developed a graded Cd 1−x Zn x Se alloy structure as Zn ions diffused into CdSe QDs as the temperature of glasses increased as a result of In general, if there were a core (around which a shell is built up), the Raman spectra of such core/shell structure would definitely exhibit two separate LO phonon peaks corresponding to the core-related and the shell-related phonons similarly to how it is observed for colloidal II-VI core/shell structures. [2][3][4][5][6] Again, this is obviously not the case. Hence, one should conclude that the species formed in the spectra are ternary (alloyed) Cd 1−x Zn x Se QDs, the composition of which, correctly determined by the authors, is more or less homogeneous without any evidence for a core/shell structure.…”

“…If there were a CdSe core, the corresponding LO phonon frequency would have been near 208‐210 cm –1 (Ref. 2 and references therein) that is obviously not the case.…”