Department of Physics

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    Scanning tunneling microscope-characterization of chemical vapor deposition-graphene: ripples and twisted bi-layers at multiple scales
    (Wiley, 2024-09) Dey, Srijata
    Despite numerous limitations, graphene characterization using scanning tunneling microscopy is an important aspect of graphene research. In the present study, an ambient, large effective field of view scanning tunneling microscope (A-LEF-STM) is used as a more practical extension of a standard STM for analyzing chemical vapor deposited (CVD)-graphene: A rigid sample stage, which allows the tip to be relocated to any point over an area of 3.5 × 3.5 mm, is attached to the latter. This simple enhancement allows rough patches spanning hundreds of nanometers on any sample to be easily circumnavigated, almost always without damaging the tip. Ripples and multi-layer regions including those with twisted bi-layers, in a graphene sheet grown on a copper foil using CVD, are located using the augmented manoeuvrability, and imaged in high-definition from micron to angström scales. Insights relating to the resolution with which surfaces of varying roughness can be imaged are developed using simulations and a careful analysis of the scanning process. The enhanced field of view is also utilized to verify the extent of graphene coverage over the entire sample area. The acquired images of single and multi-layer depositions are carefully interpreted. The applicability of this end-to-end characterization process for other samples is also discussed. Overall, this study demonstrates the potential benefits of the A-LEF-STM as an eminent characterization tool, complementary to a Raman spectrometer, for CVD-graphene and other two-dimensional materials.
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    Contrast mechanisms in photothermal scanning tunneling microscopy
    (Springer, 1994-08) Dey, Srijata
    By irradiation of the tunneling junction of a scanning tunneling microscope with intensity-modulated laser light a gap-width modulation due to thermal expansion of tip and sample was produced. Photothermal images were obtained by spatial mapping of the resulting modulation of the tunneling current or its logarithm. The various mechanisms responsible for the observed contrast are discussed quantitatively. In case of a highly corrugated gold film on mica the contrast arises mainly from either the current variations caused by the non-zero reaction time of the current control loop or from a geometry factor. In both cases the images reflect certain properties of the sample topography. On the other hand, for a liquid-crystal film adsorbed on graphite a contrast on a molecular scale was found which is attributed to variations of the effective barrier height.