Dynamics of bulk versus nanoscale $\mathrm{W}{\mathrm{S}}_2$: Local strain and charging effects

We measured the infrared vibrational properties of bulk and nanoparticle $\mathrm{W}{\mathrm{S}}_2$ in order to investigate the structure-property relations in these materials. In addition to the symmetry-breaking effects of local strain, nanoparticle curvature modifies the local charging environment of the bulk material. Performing a charge analysis on the $xy$-polarized $E_{1u}$ vibrational mode, we find an approximate 1.5:1 intralayer charge difference between the layered 2H material and inorganic fullerene-like (IF) nanoparticles. This effective charge difference may impact the solid-state lubrication properties of nanoscale metal dichalcogenides.

Figure 1

Transmission electron microscope images of (a) layered $2\mathrm{H}\text{−}\mathrm{W}{\mathrm{S}}_2$ and (b) a representative $\mathrm{IF}\text{−}\mathrm{W}{\mathrm{S}}_2$ nanoparticle. Each nanoparticle consists of a hollow core and several W-S-W layers. The average particle diameter is $\sim 120–200\mathrm{nm}$ for the materials of interest here. The IF nanoparticle shown here is slightly smaller than average.

Figure 2

Displacement vectors of the infrared and Raman active modes of $2\mathrm{H}\text{−}\mathrm{W}{\mathrm{S}}_2$. The dashed line between the two layers represents the weak van der Waals force.

Figure 3

$300\mathrm{K}$ reflectance spectra of 2H- and $\mathrm{IF}\text{−}\mathrm{W}{\mathrm{S}}_2$. The $E_{1u}$ and $A_{2u}$ modes are infrared active in these materials, with $xy$- and $z$-directed polarizations, respectively.

Figure 4

Frequency dependence of the imaginary part of the dielectric function, ${ϵ}_2\left(\omega \right)$, and the energy loss function, $-\mathrm{Im}[1∕ϵ\left(\omega \right)]$, of $\mathrm{IF}\text{−}\mathrm{W}{\mathrm{S}}_2$ at $300\mathrm{K}$. Longitudinal optical (LO) and transverse optical (TO) frequencies are indicated. The inset displays an oscillator fit (white line) to the real (black) and imaginary (gray) parts of the dielectric function, ${ϵ}_1\left(\omega \right)$, and ${ϵ}_2\left(\omega \right)$, for $\mathrm{IF}\text{−}\mathrm{W}{\mathrm{S}}_2$ at $300\mathrm{K}$.

Figure 5

Optical conductivity of 2H- and $\mathrm{IF}\text{−}\mathrm{W}{\mathrm{S}}_2$ at $10\mathrm{K}$. Arrows highlight the low-temperature symmetry breaking in the IF material. Reference 46 lists the frequencies of these features. All of these small modes are reproducible, although it should be noted that the exact details of this fine structure will likely depend on particle size and distribution within a sample, the number of nanoparticle layers, and the nature of the defects.