[论文解读] A simple TEM method for fast thickness characterization of suspendedgraphene flakes
本文提出了一种简单、快速的透射电子显微镜（TEM）方法，利用动力学电子衍射理论和明场成像对悬浮石墨烯薄片的厚度进行表征。该方法通过分析透射束强度实现单层敏感度，厚度分辨率达约1 nm，可实现对石墨烯薄片厚度变化（精确至单层）的精确、非破坏性测量，且样品制备极少，无需复杂的衍射图案或能量过滤。
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.
研究动机与目标
- 为解决轻元素和软物质TEM成像中的挑战，如电子束辐照损伤和对比度低的问题。
- 开发一种简单、高分辨率的悬浮石墨烯薄片厚度表征技术。
- 通过冷冻电镜（cryo-TEM）实现实空间中生物分子相互作用和水合生物样品的精确分析。
- 展示原位聚焦离子束（FIB）转移技术在水合样品（如冷冻真菌孢子）上进行冷冻电镜分析的可行性。
- 利用冷冻电镜实现生物和二维材料结构成像的原子级分辨率，并结合元素映射。
提出的方法
- 利用电子衍射的动力学理论，推导出透射电子束强度随厚度变化的解析表达式。
- 仅使用明场（BF）TEM成像，避免使用复杂的衍射或能量过滤技术。
- 通过JEMS模拟和实验测量校准吸收常数λ，得到低对称取向下的λ ≈ 225 ± 9 nm。
- 应用强度-厚度关系，实现对石墨烯薄片厚度变化的亚单层敏感度量化。
- 采用原位聚焦离子束（FIB）转移技术，从冷冻水合样品中制备电子透明的薄片。
- 使用冷冻转移槽将转移出的薄片在低温下转移至TEM中，以保持结构完整性。
实验结果
研究问题
- RQ1是否可通过一种简单的TEM方法，在无需复杂数据采集的情况下实现石墨烯厚度表征的单层敏感度？
- RQ2磁性微 bead 尺寸和DNA 螺旋结构如何影响 bead 的附着密度和空间分布？
- RQ3原位FIB转移结合冷冻电镜是否能有效保持并解析水合生物结构的近原子级分辨率？
- RQ440 nm 与 130 nm 微 bead 相对于DNA 螺旋的分布空间分布如何？其与磁性测量结果的相关性如何？
- RQ5在冷冻电镜下对生物样品进行元素映射和晶格条纹的分辨程度如何？
主要发现
- 该方法实现了对石墨烯薄片厚度的表征，分辨率达约1 nm，可检测单层厚度变化。
- 在低对称取向下，实验测得吸收常数λ为225 ± 9 nm，验证了理论模型的准确性。
- 平均而言，每条DNA螺旋上附着约6个40 nm微 bead，130 nm微 bead 约2个以上，与磁性测量结果一致。
- 40 nm微 bead 优先附着于DNA螺旋内部，而130 nm微 bead 则定位于外部，支持相关结构假设。
- 冷冻电镜成像成功分辨出铝支撑膜的3 Å晶格条纹，证实了原子级结构分辨率。
- 能量过滤成像生成了C、N和Mn的元素分布图，证明该方法具备对水合样品进行成分分析的能力。
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