Robust Sample Preparation of Large-Area In- and Out-of-Plane Cross Sections of Layered Materials with Ultramicrotomy

Abstract

Layered materials (LMs) such as graphene or MoS2 have recently attracted a great deal of interest. These materials offer unique functionalities due to their structural anisotropy characterized by weak van der Waals bonds along the out-of-plane axis and covalent bonds in the in-plane direction. A central requirement to access the structural information of complex nanostructures built upon LMs is to control the relative orientation of each sample prior to their inspection e.g. with Transmission Electron Microscopy (TEM). However, developing sample preparation methods that result in large inspection areas and ensure full control over the sample orientation while avoiding damage during the transfer to the TEM grid is challenging.Here we demonstrate the feasibility of deploying ultramicrotomy for the preparation of LM samples in TEM analyses. We show how ultramicrotomy leads to the reproducible large-scale production of both in-plane and out-of-plane cross-sections, with bulk vertically-oriented MoS2 and WS2 nanosheets as proof of concept. The robustness of the prepared samples is subsequently verified by their characterization by means of both high-resolution TEM and Raman spectroscopy measurements. Our approach is fully general and should find applications for a wide range of materials as well as of techniques beyond TEM, thus paving the way to the systematic large-area mass-production of cross-sectional specimens for structural and compositional studies.

Ellipsometric Investigation of Thick Vertically Oriented MoS2 Films Grown on Mo Foil at High Temperatures

“…We obtained the values of 0.39, 0.28, 0.29, 0.31, and 0.20 in the temperature range investigated (from 800 to 1000 every 50°), which are in good agreement with the ratio of around 0.3 reported in previous works for VA MoS 2 layers. ,,,− , These values are to be compared with an intensity ratio E 1 2g /A 1g of 0.8 obtained for a MoS 2 monocrystal (Figure S2) where MoS 2 layers of large area are horizontally stacked. Figure c also shows the E 1g mode at 288 cm –1 (in-plane vibration).…”

“…The sulfurization of molybdenum films ,− is a simple and straightforward technique to grow VA-MoS 2 with a controlled thickness as it depends on the amount of the predeposited Mo-metal seed layer. − VA-MoS 2 is obtained for Mo films thicker than around 3 nm at high temperatures and sulfur fluxes. ,,, The growth with a preferred vertical orientation is attributed to kinetic considerations. The diffusion of sulfur toward the metal is expected to be faster when it occurs through the van der Waals interlayer gaps of vertical layers. , …”

“…Different techniques have been used to evaluate the homogeneity of MoS 2 . The thickness of thick MoS 2 films has been estimated from transmission electron microscopy (TEM) images of out-of-plane cross sections. ,, However, a disadvantage of TEM is its limited inspection area. Local structure variations can also be inferred from Raman spectroscopy as the area of the E 1 2g and A 1g peaks (the typical fingerprints of MoS 2 ) depends, among other things, on the amount of the two-dimensional (2D) material synthetized. , However, the intensity of MoS 2 Raman peaks is also affected by other factors such as crystalline grain size and crystal orientation. ,,, In this context, spectroscopic ellipsometry (SE) is a powerful nondestructive technique that allows obtaining the thickness and optical properties of thin films based on the modeling of the polarization state of light reflected from the surface .…”

Illuminating the Electronic Properties of WS2 Polytypism with Electron Microscopy

“…These nanostructures were directly grown onto a microchip made of silicon as a frame with a silicon nitride (Si3N4) window in the middle (see Fig. 1b) 20,21 .…”

Vertically-oriented MoS2 nanosheets for nonlinear optical devices

“…For the nonlinear optical measurements, the specimen was prepared by means of the ultramicrotomy technique. 34 See ESI-E † for more details. The specimen was illuminated with two synchronized laser pulses at λ 1 = 776 nm and λ 2 = 1210 nm that were temporally and spatially overlapped.…”