Polarized and non‐polarized Raman spectroscopy of ZnO crystals: Method for determination of crystal growth and crystal plane orientation for nanomaterials

Abstract

Analysis and determination of crystal orientation and exposed surface facets remain a challenge in nanomaterial science. In this work, we show that polarized and non-polarized Raman spectroscopy can be useful tools to determine crystal plane orientation and conveniently be applied to spatial dimensions limited only by the diffraction limit of the excitation laser. The methodology is exemplified for wurtzite structured ZnO. Three different crystal facets, ( 0001), (1 1 00), and (11 2 0), of ZnO are investigated with angle-resolved polarized Raman spectroscopy. The polarization direction dependences of the main Raman peaks are characterized and related to the experimental vibrational modes in the crystal lattice and corroborated by density functional theory (DFT) calculations using two different hybrid functionals. By exploiting the symmetry of the modes and differences in Raman intensity of the optically activated phonons, a simple model is derived for determining the relation between the polar and non-polar crystal orientation. The results are generalized to allow peak intensity ratio analysis using Raman spectroscopy with a non-polarized light source, making it compatible with Raman mapping, as well as to include a critical discussion on the ability to determine the crystal plane orientation and exposed crystal facets using this model for nanodimensional ZnO and equivalent models for other nanomaterials. As the approach allows for use of non-polarized light sources, near-field excitations and local plasmons can in an extension be utilized for the determination of crystal orientation and exposed planes in dimensions much smaller than the diffraction limit.

Macroemulsion-mediated synthesis of fibrous ZnO microrods and their surface morphology contribution to the high photocatalytic degradation rate

“…The results indicate that both samples have almost the same fraction of polar planes, about 60%. 60 This interpretation confirmed that both samples have a similar crystal growth orientation, consistent with the XRD analysis.…”

Preparation and Photocatalytic Performance of MoS2/ZnS/ZnO‐T Heterojunction Photocatalyst for Dye Degradation in Water

“…As displayed in Figure a, the main diffraction peaks which locate at 31.7°, 34.4°, 36.2°, 47.5°, 56.5°, 62.8°, and 67.9° are corresponding to the hexagonal wurtzite structure of ZnO (JCPDS: 36‐1451). [ 12,20,21 ] However, compared with pure ZnO‐T, the peak intensity of the composite is obviously weaker. Meanwhile, the diffraction peak appeared at 28.6° can be assigned to ZnS (JPCDS: 65‐1691).…”

Energy Alignment of Quantum-Confined ZnO Particles with Copper Oxides for Heterojunctions with Improved Photocatalytic Performance

“…The Raman spectra of Cu 2 O, Cu 4 O 3 , CuO, and ZnO-Q-dot films are depicted in Figure B. The Raman spectroscopy measurements of the ZnO Q-dots show the position of the strongest Raman peaks (438 and 100 cm –1 ) and that the expected peaks at 333, 378, 411, and 438 cm –1 merge due to significant peak broadening, thus forming a broad absorption band between about 300 and the 438 cm –1 peak. Peak broadening in nanoparticles has previously been reported for Si and is attributed to thermal heating and size distribution of the nanoparticles.…”

“…Cu 2 O and Cu 4 O 3 are very sensitive to laser power density and are oxidized if too much laser power is used. , Therefore, a low laser power, 0.44 mW, at the sample was used for the measurements. Because of the orientation-dependent Raman scattering of ZnO, it is possible to investigate if the particles deposit in a preferred orientation using circularly polarized Raman spectra, according to the method recently published by our group . Large peak broadening, however, preventing the detailed analysis of the 378 cm –1 peak, complicated the analysis.…”

“…Because of the orientation-dependent Raman scattering of ZnO, it is possible to investigate if the particles deposit in a preferred orientation using circularly polarized Raman spectra, according to the method recently published by our group. 17 Large peak broadening, however, preventing the detailed analysis of the 378 cm –1 peak, complicated the analysis. Qualitatively, the result pointed at a tendency to the nonpolar crystal direction perpendicular to the film, but because the peak height of the 378 cm –1 peak had to be determined by deconvolution, the uncertainty is large, and a randomly oriented material is also possible within one standard deviation.…”