Raman spectroscopy of colloidal semiconductor nanocrystals

Abstract

Raman spectroscopy is a powerful method that gives insight into the atomic structure and composition of nanomaterials, but also allows to draw conclusions about their electronic properties. It is based on the inelastic scattering of light, which is able to excite phonons in the material. In the field of semiconductor nanocrystals, Raman spectroscopy has been employed to make significant contributions to the analysis of lattice distortion, interfaces, phase mixing, and defect formation. Yet, there is no clear consensus on how the electronic and crystal structure of the material interacts with the incident light to yield the observed spectra. This review gives a brief overview over the method. It then reviews the most important findings, current developments, and discusses the efforts to formulate a consistent model that allows to establish the method as a tool for structural analysis.

Dynamic Quantum-State Renormalization and Effects of Competing Pathways on Carrier Relaxation in Semiconductor Nanoparticles

“…As mentioned above, this photoinduced increase in the effective masses of the carriers is observed and well modeled in monolayer TMDCs, although the band structure for those systems leads to an increase in the band gap energy. , Since the energies of the quantum-confinement states depend on the effective masses of the carriers, the quantum-confinement states in the VB and CB energetically shift with photoexcitation, even with the presence of only a single electron–hole pair in each NP, as depicted in Figure e. Photoexcited electrons, holes, and excitons may also interact with the ions in a polar semiconductor nanocrystal through Fröhlich interactions. − The photoexcited electron and hole interact with the cations and anions in the crystal lattice, perturbing the crystal structure and coupling with longitudinal optical (LO) phonons. − The changes in the crystal structure that result from these Fröhlich interactions result in contrasting electronic band structures and effective masses for the CB and VB in the unexcited and excited semiconductor NPs. − ,,− These energetic shifts are grouped together as QSR. Since the different quantum-confinement states have unique spatial probability densities, each state, including those that are occupied or not, should experience unique QSR that depend on the states in the VB and CB that are occupied. , As a result, the QSR of the quantum-confinement states should be dynamic, as the carriers relax through the energetically accessible states after photoexcitation.…”

Full-Spectrum InP-Based Quantum Dots with Near-Unity Photoluminescence Quantum Efficiency

“…Also here, one sees that ZnS shell growth around the InP/ZnSe core/shell QDs mostly preserves the Raman features of the InP core and the ZnSe shell (albeit with minor shifts reflecting strain), while direct ZnS shelling of InP core QDs results in a considerable broadening of the InP LO phonon line. For a quantitative analysis, we described the Raman spectra as a sum of Lorentzian fit functions (examples are included in Figure b) and used the resulting shifts of the InP and ZnSe LO phonon as compared to the InP and ZnSe reference, to estimate relative lattice constant changes according to refs and : Here, with V being the unit cell volume, is the Grüneisen parameter, which we took as 1.24 and 0.85 for InP and ZnSe, respectively. − Following this analysis, Figure c indicates that ZnSe shelling results in a combination of compressive strain in the InP core and tensile strain in the ZnSe shell. In line with previous work, the relatively small 3.4% lattice parameter mismatch between InP and ZnSe limits strain to ca.…”

Synthesis of CdSe/ZnSe Core/Shell and CdSe/ZnSe/ZnS Core/Shell/Shell Nanocrystals: Surface-Ligand Strain and CdSe–ZnSe Lattice Strain

“…All spectra of core/shell QDs can be deconvoluted into two Lorentzian peaks (Figures a and S5, Supporting Information), distinctively different from those of the plain core QDs in Figure S5 (insets) and CdZnSe alloying QDs reported in the literature . Quantitatively, a relative Raman frequency shift (Δω/ω) of a LO phonon peak can be correlated with the relative change of corresponding lattice constants (Δα/α) by eq . ,− The calculation can be applied for both core and shell lattices for each sample (Figure b). normalΔ ω ω = ( 1 + 3 Δ α α ) − γ prefix− 1 γ = prefix− ∂ ln nobreak0em.25em⁡ ω ∂ ln nobreak0em.25em⁡ V Here, γ is the Grüneisen parameter. In eq , V is the volume of single unit cell, i.e., 1.1 and 0.85 for CdSe and ZnSe, respectively .…”

“…Quantitatively, a relative Raman frequency shift (Δω/ω) of a LO phonon peak can be correlated with the relative change of corresponding lattice constants (Δα/α) by eq . ,− The calculation can be applied for both core and shell lattices for each sample (Figure b). normalΔ ω ω = ( 1 + 3 Δ α α ) − γ prefix− 1 γ = prefix− ∂ ln nobreak0em.25em⁡ ω ∂ ln nobreak0em.25em⁡ V Here, γ is the Grüneisen parameter. In eq , V is the volume of single unit cell, i.e., 1.1 and 0.85 for CdSe and ZnSe, respectively . It should be mentioned that the relative change of corresponding lattice constants (Δα/α in Figure b) can only be a measure of average lattice strain for a given lattice.…”

“…In eq 4, V is the volume of single unit cell, i.e., 1.1 and 0.85 for CdSe and ZnSe, respectively. 78 It should be mentioned that the relative change of corresponding lattice constants (Δα/α in Figure 3b) can only be a measure of average lattice strain for a given lattice. With that of the corresponding core QDs as a reference, fullwidth-at-half-maximum (fwhm) of the LO phonon peak (Figure 3c) is also an important measure of the lattice strain, which reflects the lattice variation (or lattice constant distribution) for a given semiconductor.…”