Raman spectroscopy of intercalated and misfit layer nanotubes

We present Raman spectra of misfit layer $({\mathrm{PbS})}_{1.14}{\mathrm{NbS}}_2$ nanotubes and lead intercalated ${\mathrm{NbS}}_2$ nanotubes. They represent interesting model systems to investigate the nature of interlayer interaction in layered materials. A direct correlation to the Raman modes of the parent $2H\text{−}{\mathrm{NbS}}_2$ compound exists, but some modes are seen drastically upshifted in frequency in the misfit layer and intercalated compound while others remain almost unchanged. On the basis of the Raman spectroscopic investigations and with the help of supporting calculations we examine different interlayer bonding mechanisms and contribute to the discussion as to why these frequency shifts occur.

Figure 1

(a) Structure of the misfit layer compound $({\mathrm{PbS})}_{1.14}{\mathrm{NbS}}_2$ in the $a\text{−}b$ plane. The ratio of the $a$ axes is irrational, therefore the compounds are classified as misfit layer compounds (MLC). (b) and (c): The structure of the misfit compound viewed along the commensurate $b$ and the incommensurate $a$ direction, respectively. (d) View along the normal to the (11.0) plane according to the pseudo-hexagonal system of labeling of the ${\mathrm{NbS}}_2$ material intercalated with lead atoms in linear coordination to neighboring S atoms (ICSD col. code. 74698).

Figure 2

Structural characterization of the nanotubes. (a) High and low (inset) TEM images of a Pb-${\mathrm{NbS}}_2$ nanotube investigated in this paper. The layer periodicity of 0.88 nm (alternating layers of Pb atoms and ${\mathrm{NbS}}_2$) can be identified in a line profile (inset) of the nanotube area enclosed by the rectangle. (b) High and low (inset) TEM images of a $({\mathrm{PbS})}_{1.14}{\mathrm{NbS}}_2$ misfit layer nanotube investigated in this paper. The line profile (inset) taken from the area enclosed by the rectangle depicts the layer periodicity of 1.20 nm corresponding to alternating layers of PbS and ${\mathrm{NbS}}_2$.

Figure 3

(a) Raman spectra of all measured $({\mathrm{PbS})}_{1.14}{\mathrm{NbS}}_2$ nanotubes. The spectra of the different nanotubes have in common that they can be divided into two regions with modes associated with the PbS part below $200 {\mathrm{cm}}^{-1}$ (light gray) and the main $E^{\prime }$ and $A_1^{\prime }$ Raman modes of the ${\mathrm{NbS}}_2$ part between 300 and $400 {\mathrm{cm}}^{-1}$ (darker gray). (b) Raman spectra of all measured Pb-${\mathrm{NbS}}_2$ nanotubes. Again, modes probably associated with the intercalant are found below $200 {\mathrm{cm}}^{-1}$ and ${\mathrm{NbS}}_2{E}_{2g}^1$ and $A_{1g}$ modes above $300 {\mathrm{cm}}^{-1}$. A Raman feature not seen in $({\mathrm{PbS})}_{1.14}{\mathrm{NbS}}_2$ nanotubes is the mode around $250 {\mathrm{cm}}^{-1}$ (darkest gray). It is likely to be the $E_{1g}$ mode of the ${\mathrm{NbS}}_2$ part. Spectra depicted in (a) and (b) are taken with 532 nm excitation wavelength. They are normalized to the $E$ mode around $350 {\mathrm{cm}}^{-1}$ and offset for clarity. The solid lines represent the position of the $E_{2g}^1$ mode in the pristine $2H\text{−}{\mathrm{NbS}}_2$ material ($309 {\mathrm{cm}}^{-1}$) [36].

Figure 4

(a) Phonon dispersion of the parent $2H\text{−}{\mathrm{NbS}}_2$ bulk material. Above $200 {\mathrm{cm}}^{-1}$ at the $\mathrm{\Gamma }$ point, three first order Raman modes, the $E_{1g},E_{2g}^1$, and $A_{1g}$ mode, are marked with blue circles and their calculated frequencies are listed in Table 1. (b) The main features of the dispersion are reproduced in the dispersion of lead intercalated ${\mathrm{NbS}}_2$. The three main $\mathrm{\Gamma }$-point Raman modes are seen above $250 {\mathrm{cm}}^{-1}$, with the two $E$ type modes considerably upshifted compared to their frequency in ${\mathrm{NbS}}_2$. The frequency of the $A_{1g}$ mode, on the other hand, stays nearly the same. Modes with contributions from the intercalant lead atoms are found around $100 {\mathrm{cm}}^{-1}$.

Figure 5

(a) Raman features in the region between 300 and $420 {\mathrm{cm}}^{-1}$ of exemplary Pb-${\mathrm{NbS}}_2$ and $({\mathrm{PbS})}_{1.14}{\mathrm{NbS}}_2$ nanotubes. The most prominent intralayer ${\mathrm{NbS}}_2$ modes in the investigated compounds are shown with the in-plane $E^{\prime }$ of $({\mathrm{PbS})}_{1.14}{\mathrm{NbS}}_2$ (red line) always at slightly higher frequencies than the $E_{2g}^1$ mode of Pb-${\mathrm{NbS}}_2$ (blue line). Again, the dashed line represents the position of the $E_{2g}^1$ mode in the pristine $2H\text{−}{\mathrm{NbS}}_2$ material (see Fig. 3) [36]. In contrast, the $A_1^{\prime }$ mode of $({\mathrm{PbS})}_{1.14}{\mathrm{NbS}}_2$ is found at nearly the same frequency as the $A_{1g}$ mode in Pb-${\mathrm{NbS}}_2$. (b) The frequency difference between the above described modes clearly distinguishes the two types of nanotubes, with $\omega$(A-E) between 24–$27 {\mathrm{cm}}^{-1}$ for all of the measured $({\mathrm{PbS})}_{1.14}{\mathrm{NbS}}_2$ nanotubes and 28–$30 {\mathrm{cm}}^{-1}$ for all of the Pb-${\mathrm{NbS}}_2$ nanotubes.

Figure 6

Atomic displacement patterns of the three main Raman modes in $({\mathrm{PbS})}_{1.14}{\mathrm{NbS}}_2$ discussed in the text and listed in Table 1. They are derived from the $2H\text{−}{\mathrm{NbS}}_2{E}_{1g},E_{2g}^1$, and $A_{1g}$ modes (in brackets) and have $E^{\prime \prime },E^{\prime }$, and $A_1^{\prime }$ symmetry, respectively, in the $({\mathrm{PbS})}_{1.14}{\mathrm{NbS}}_2$ misfit layer material.