Excitonic resonances in WS${}_{2}$ nanotubes

Excitonic resonances in WS $_{2}$ nanotubes

Matthias Staiger, Peter Rafailov, Konstantin Gartsman, Hagen Telg, Matthias Krause, Gal Radovsky, Alla Zak, and Christian Thomsen

Phys. Rev. B 86, 165423 – Published 12 October 2012

Abstract

Resonance Raman profiles of WS $_{2}$ nanotubes of different diameter are presented. We show that the A excitonic transition energy lies below the bulk value and is increasingly redshifted with decreasing diameter of the nanotubes. The findings are attributed to strain effects associated with the curvature of the nanotube walls. A silent Raman mode, the $B_{1 u}$ mode, is disorder enhanced in the Raman spectra of WS $_{2}$ nanomaterials. We discuss the development of the relative intensities of the $B_{1 u}$ mode and the nearby $A_{1 g}$ mode with nanotube diameter, excitation energy, and hydrostatic pressure in terms of a slight difference in resonance condition.

Received 16 July 2012

DOI:https://doi.org/10.1103/PhysRevB.86.165423

Authors & Affiliations

Matthias Staiger^1,*, Peter Rafailov², Konstantin Gartsman³, Hagen Telg⁴, Matthias Krause⁵, Gal Radovsky³, Alla Zak⁶, and Christian Thomsen¹

¹Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany
²Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Boulevard, 1784 Sofia, Bulgaria
³Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel
⁴Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
⁵Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion-Beam Physics and Materials Research, 01314 Dresden, Germany
⁶Holon Institute of Technology, 52 Golomb Street, P.O. Box 305, Holon, Israel

^*mstaiger@physik.tu-berlin.de

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Vol. 86, Iss. 16 — 15 October 2012

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Images

Figure 1
Raman spectra of an individual WS $_{2}$ nanotube ( $d = 132$ nm) before and after characterization with TEM. None of the Raman features is altered by the characterization technique. The inset shows an exemplary TEM image of a different WS $_{2}$ nanotube with an outer diameter of 32 nm.Reuse & Permissions
Figure 2
Resonance Raman profiles of different diameter WS $_{2}$ nanotubes ( $d$ = 38, 46, 85, and 121 nm) and of a WS $_{2}$ powder reference sample (upper right) for the A and B exciton region. Shown are the Raman intensities of the $A_{1 g}$ and $B_{1 u}$ modes (triangles and diamonds, respectively) normalized to the optical $F_{2 g}$ CaF $_{2}$ phonon as a function of the excitation energy and a fit to the data (solid and dashed lines) following Eq. (1). From the fits optical transition energies were derived; values are given in each plot. In the lower right, transition energies determined from the RRPs of the $A_{1 g}$ mode are plotted over the inverse diameter of the WS $_{2}$ nanotubes and the powder, demonstrating the redshift of the optical transition energy into the A exciton with decreasing size of the nanomaterials. The solid line is a linear fit.Reuse & Permissions
Figure 3
(a) Raman spectra of a WS $_{2}$ nanotube ( $d = 121$ nm). The spectrum excited with 1.953 eV, close to the resonance condition of the $A_{1 g}$ mode, is divided by a factor of 7 to be of similar intensity compared to the two other spectra. The symmetry of the four main Raman modes is given next to the corresponding spectral features. The dashed lines denote the two first-order allowed Raman modes. The $B_{1 u}$ and $E_{1 u}$ modes that are barely present in the bottom spectrum (2.713 eV) become increasingly prominent when the nanotube is excited with lower energies. (b) The $B_{1 u}$ / $A_{1 g}$ Raman intensity ratio with excitation energy for the same nanotube. A strong increase can be observed with decreasing excitation energy, with a kink in the A excitonic resonance region. The inset in (b) shows that the observed dependence between 1.85 and 2.1 eV (shaded area) is a result of dividing a simulated resonance profile by another with a small energy offset (10 meV). In the measured spectra, because only the $A_{1 g}$ phonon exhibits resonant behavior in the B exciton region, the intensity ratio approaches small values above 2.1 eV.Reuse & Permissions
Figure 4
(a) Raman spectra of agglomerated WS $_{2}$ nanotubes (excitation energy: 1.916 eV) under hydrostatic pressure up to 10.82 GPa (solid lines). The spectra obtained during subsequent pressure release are also depicted (dashed lines). All Raman modes shift up and broaden with increasing pressure. Additionally, the $B_{1 u}$ / $A_{1 g}$ intensity ratio changes strongly. (b) The upshift of both the $A_{1 g}$ and $B_{1 u}$ mode with pressure is linear; the pressure coefficients are given. (c) While for pressures up to 6 GPa the $B_{1 u}$ / $A_{1 g}$ intensity ratio rises almost linearly, for the high-pressure region a decrease is observed. A smoothing spline fit is employed to guide the eye. In (b) and (c) the diamonds are values obtained during pressure release that illustrate the reversibility of the process. The inset in (c) in analogy to the inset in Fig. 3b (the $x$ axis is reversed) demonstrates that with increasing pressure the ratio of two resonance profiles (shaded area) with a small energy offset reproduces well the experimental observation (see Sec. 4).Reuse & Permissions
Figure 5
(a) A roughly linear decrease in the intensity ratio $I_{\max} (B_{1 u})$ / $I_{\max} (A_{1 g})$ is observed when going from smaller to larger nanotube diameters. (b) The maxima of the resonance Raman profiles of the $B_{1 u}$ and $A_{1 g}$ mode increasingly differ in energy as the nanotube diameter becomes larger.Reuse & Permissions

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