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Picometer scale insight and manipulation of novel materials

Final Report Summary - PICOMAT (Picometer scale insight and manipulation of novel materials)

The aim of the PICOMAT project was to achieve controlled structural modifications in low-dimensional materials as well as developing new means for analyzing these radiation-sensitive structures. Electron irradiation was used as a versatile tool to modify low-dimensional materials: It can be applied broadly to a larger area of the sample, or it can be focused to atomic dimensions thereby enabling a high degree of spatial control. Under broad irradiation, we have shown how defects are generated and changed in Molybdenum ditelluride [1], at the grain boundaries of Molybdenum disulfide [2], or a transition from amorphous to crystalline Molybdenum disulfide [3]. We have further identified (within a larger collaboration) the mechanism of beam-induced displacements of silicon impurities in graphene [4]. This has subsequently led to pioneering work where the displacement of the silicon impurity could be controlled on the level of individual atoms [5,6] using the focused electron beam of an aberration-corrected scanning transmission electron microscope (STEM). Another way to modify 2D materials is via focused ion irradiation, which can be used not only to cut holes and lines, but also to define defective areas that are chemically more reactive than the pristine regions of the sample [7]. Impurities on graphene samples could be effectively removed by heating to high temperatures, and it was important to transfer the sample in vacuum to the microscope between heat treatment and imaging in order to maintain this purity [8].

New structures were formed by placing layered materials and other ingredients on top of each other. We have shown the first cases where a single layer of molecules is deposited on graphene[9] or encapsulated between two graphene layers[10] and cleanly imaged by STEM. For the encapsulated C-60 molecules [10], the graphene provided a clean window onto the molecular layer, which allowed us to observe the diffusion of entire molecules as well as beam-induced reactions between them. With the chlorinated copper phtalocyanine molecules on a graphene membrane [9], we could obtain new insights into the radiation damage mechanism and among other things show clearly that the chlorine atoms are dissociated already under extremely low electron doses, while the remainder of the molecule is more stable. Moreover we have studied a layered assembly from graphene and single-layer hexagonal boron nitride, where a novel detection scheme allowed us to obtain insights into the deformations resulting from the van der Waals interaction of the two layers[11].

On the analysis side, the main development within the project was an imaging approach where the dose is distributed over many identical entities of an object, such as randomly distributed but otherwise identical defects in material. Using simulated data, we have explored the theoretical basis and the ultimate frontiers of this approach [12]. Simultaneously we have developed the experimental implementation, overcoming numerous experimental obstacles, and eventually obtained a low-dose acquisition procedure that is precise enough for obtaining atomic-resolution data [13,14]. With defective graphene as a test sample (which has rather well known defect configurations), we demonstrated that the defect configurations could be obtained from a large number of very low dose exposures[14]. In the experiment, the dose was 100x lower than that needed for direct images of the atomic configurations[14], while simulations indicate that bridging 3-5 orders of magnitude could be possible [12].

[1] Chemistry of Materials 30, 1230 (2018)
[2] Nanoscale 9, 1591 (2017)
[3] ACS Nano 12, 8758 (2018)
[4] Physical Review Letters 133, 115501 (2014)
[5] Ultramicroscopy 180, 163 (2017)
[6] Nano Letters 18, 5319 (2018)
[7] Nano Letters 15, 5944 (2015)
[8] Phys. Stat. Sol. RRL 11, 1700124 (2017)
[9] Scientific Reports 8, 4813 (2018)
[10] Science Advances 3, e1700176 (2017)
[11] Nano Letters 17, 1409 (2017)
[12] Ultramicroscopy 170, 60 (2016)
[13] Microscopy and Microanalysis 23, 809 (2017)
[14] Phys. Stat. Sol. B 254, 1700176 (2017)