Nanocrystalline graphene at high temperatures: insight into nanoscale processes

Tailoring Spectrally Flat Infrared Photodetection with Thickness-Controlled Nanocrystalline Graphite

Graphene, a zero-gap semiconductor, absorbs 2.3% of incident photons in a wide wavelength range as a free-standing monolayer, whereas 50% is expected for ∼90 layers. Adjusting the layer number allows the tailoring of the photoresponse; however, controlling the thickness of multilayer graphene remains challenging on the wafer scale. Nanocrystalline graphene or graphite (NCG) can instead be grown with controlled thickness. We have fabricated photodetectors from NCG that are spectrally flat in the near-infrared to short-wavelength infrared region by tailoring the layer thicknesses. Transfer matrix simulations were used to determine the NCG thickness for maximum light absorption in the NCG layer on a silicon substrate. The extrinsic and intrinsic photoresponse was determined from 1100 to 2100 nm using chromatic aberration-corrected photocurrent spectroscopy. Diffraction-limited hyperspectral photocurrent imaging shows that the biased photoresponse is unipolar and homogeneous across the device area, whereas the short-circuit photoresponse gives rise to positive and negative photocurrents at the electrodes. The intrinsic photoresponses are wavelength-independent, indicative of bolometric and electrothermal photodetection.

Focused Ion Beam Sample Preparation for In Situ Thermal and Electrical Transmission Electron Microscopy

A focused ion beam (FIB) technique describing the preparation of specimens for in situ thermal and electrical transmission electron microscopy is presented in detail. The method can be applied to a wide range of materials and allows the sample to be thinned down to electron transparency while it is attached to the in situ chip. This offers the advantage that the specimen can have a quality in terms of contamination and damage due to the ion beam that is comparable to samples prepared by means of conventional FIB preparation. Additionally, our technique can be performed by most commercially available FIB devices and only requires three simple, custom stubs for the procedure. This should enable a large userbase for this type of sample fabrication. One further benefit of our technique is that the in situ chip can be reused to create another sample on the same chip. The quality of the samples is demonstrated by high-resolution transmission electron microscopy as well as electron energy loss spectroscopy.