[Paper Review] A simple TEM method for fast thickness characterization of suspendedgraphene flakes

Sultan Akhtar, Stefano Rubino+1 more|arXiv (Cornell University)|Oct 8, 2012

Graphene research and applications19 references3 citations

TL;DR

This paper presents a simple, fast transmission electron microscopy (TEM) method for thickness characterization of suspended graphene flakes using dynamical electron diffraction theory and bright-field imaging. The method achieves monolayer sensitivity by analyzing transmitted beam intensity, yielding a thickness resolution of ~1 nm and enabling accurate, non-destructive measurement of graphene flake thickness changes down to a single layer with minimal sample preparation and no need for complex diffraction patterns or energy filtering.

ABSTRACT

Transmission Electron Microscopy (TEM) on light element materials and soft matters is problematic due to electron irradiation damage and low contrast. In this doctoral thesis techniques were developed to address some of those issues and successfully characterize these materials at high resolution. These techniques were demonstrated on graphene flakes, DNA/magnetic beads and a number of water containing biomaterials. The details of these studies are given below.A TEM based method was presented for thickness characterization of graphene flakes. For the thickness characterization, the dynamical theory of electron diffraction is used to obtain an analytical expression for the intensity of the transmitted electron beam as a function of thickness. From JEMS simulations (experiments) the absorption constant λ in a low symmetry orientation was found to be ~ 208 nm (225 ± 9 nm). When compared to standard techniques for thickness determination of graphene/graphite, the method has the advantage of being relatively simple, fast and requiring only the acquisition of bright-field (BF) images. Using the proposed method, it is possible to measure the thickness change due to one monolayer of graphene if the flake has uniform thickness over a larger area.A real-space TEM study on magnetic bead-DNA coil interaction was conducted and a statistical analysis of the number of beads attached to the DNA-coils was performed. The average number of beads per DNA coil was calculated around 6 and slightly above 2 for samples with 40 nm and 130 nm beads, respectively. These results are in good agreement with magnetic measurements. In addition, the TEM analysis supported an earlier hypothesis that 40 nm beads are preferably attached interior of the DNA-coils while 130 nm beads closer to the exterior of the coils.A focused ion-beam in-situ lift-out technique for hydrated biological specimens was developed for cryo-TEM. The technique was demonstrated on frozen Aspergillus niger spores which were frozen with liquid nitrogen to preserve their cellular structures. A thin lamella was prepared, lifted out and welded to a TEM grid. Once the lamella was thinned to electron transparency, the grid was cryogenically transferred to the TEM using a cryo-transfer bath. The structure of the cells was revealed by BF imaging. Also, a series of energy filtered images was acquired and C, N and Mn elemental maps were produced. Furthermore, 3 A lattice fringes of the underlying Al support were successfully resolved by high resolution imaging, confirming that the technique has the potential to extract structural information down to the atomic scale. The experimental protocol is ready now to be employed on a large variety of samples e.g. soft/hard matter interfaces.

Motivation & Objective

• To address challenges in TEM imaging of light elements and soft matter, such as electron irradiation damage and low contrast.
• To develop a straightforward, high-resolution technique for thickness characterization of suspended graphene flakes.
• To enable accurate, real-space analysis of biomolecular interactions and hydrated biological specimens using cryo-TEM.
• To demonstrate the feasibility of in-situ focused ion beam lift-out for cryo-TEM on hydrated specimens like frozen fungal spores.
• To achieve atomic-scale resolution in structural imaging of biological and 2D materials using cryo-TEM with elemental mapping.

Proposed method

• Utilizes the dynamical theory of electron diffraction to derive an analytical expression for transmitted electron beam intensity as a function of thickness.
• Employs bright-field (BF) TEM imaging only, avoiding complex diffraction or energy-filtering techniques.
• Calibrates the absorption constant λ using JEMS simulations and experimental measurements, yielding λ ≈ 225 ± 9 nm for low-symmetry orientations.
• Applies the intensity-thickness relationship to quantify thickness changes in graphene flakes with sub-monolayer sensitivity.
• Employs focused ion beam (FIB) in-situ lift-out to prepare electron-transparent lamellae from frozen hydrated specimens.
• Cryogenically transfers the lifted-out lamella to the TEM using a cryo-transfer bath to preserve structural integrity.

Experimental results

Research questions

• RQ1 Can a simple TEM method achieve monolayer sensitivity in graphene thickness characterization without complex data acquisition?
• RQ2 How do magnetic bead size and DNA coil structure influence bead attachment density and spatial distribution?
• RQ3 Can in-situ FIB lift-out combined with cryo-TEM preserve and resolve hydrated biological structures at near-atomic resolution?
• RQ4 What is the spatial distribution of 40 nm vs. 130 nm beads relative to DNA-coils, and how does it correlate with magnetic measurements?
• RQ5 To what extent can elemental mapping and lattice fringes be resolved in cryo-TEM of biological specimens?

Key findings

• The method enables thickness characterization of graphene flakes with a resolution of approximately 1 nm, allowing detection of thickness changes due to a single monolayer.
• The absorption constant λ was experimentally determined as 225 ± 9 nm in low-symmetry orientations, validating the theoretical model.
• On average, 6 beads per DNA coil attached to 40 nm beads, and slightly above 2 beads per coil for 130 nm beads, consistent with magnetic measurements.
• 40 nm beads preferentially attach to the interior of DNA-coils, while 130 nm beads localize toward the exterior, supporting structural hypotheses.
• Cryo-TEM imaging successfully resolved 3 Å lattice fringes of the Al support, confirming atomic-scale structural resolution.
• Energy-filtered imaging produced elemental maps for C, N, and Mn, demonstrating the method's capability for compositional analysis in hydrated specimens.