STM image (500 mV, 0.8 nA) of a 5-nm-wide graphene nanoribbon with armchair edge orientation (a) and STM image of a 6.5-nm-wide ribbon with edges of precisely zigzag orientation (300 mV, 2 nA) (b) patterned by scanning tunnelling lithography in a graphene sheet deposited on a Au(111) substrate. The circular insets show atomic-resolution STM images, confirming the crystallographic directions of the edges. The atomic-resolution images in the insets were Fourier filtered for clarity. The protrusions on the otherwise highly regular edges are imaging instabilities.
Experimental data reveal the emergence of magnetic order on edges of sub-10 nm graphene nanoribbons. Using a nanofabrication technique based on Scanning Tunneling Microscopy Lithography, graphene nanoribbons have been fabricated with nanometer precision and edges of well-defined crystallographic orientation. Ribbons with edges of so-called zigzag orientation display a sharp semiconductor-metal transition as a function of their width, which was identified as the signature of a magnetic switching from antiferromagnetic to ferromagnetic coupling between spin-polarized ribbon edges. Remarkably the edge magnetism was found to be stable at room temperature, raising hopes of graphene-based spintronic devices operating under ambient conditions.
a, STM image (300×300 nm) of a reconstructed Cu(111) surface continuously covered by graphene. The rectangular protrusions are single-atom-height Cu adatom clusters, whereas the trench-like features correspond to vacancy islands with well-defined widths of 5 nm and oriented along three particular directions. b, High-resolution STM image of a nanotrench revealing the nanoscale periodic rippling of the graphene membrane suspended over the trench.
L. Tapaszto and coworkers realized subnanometre-wavelength periodic ripples of suspended graphene membranes. The observed nanorippling mode violates the predictions of the continuum model. Nevertheless, microscopic simulations based on a quantum mechanical description of the chemical binding accurately describe the observed rippling mode. The ability of graphene to ripple down to subnanometre wavelengths can be exploited in strain-engineering graphene-based nanoelectronic and nanoelectromechanical devices beyond the boundaries set by continuum mechanics.
Optical microscopy images (a,b) of MoS2 single layer areas (1L, outlined by dotted lines) with several hundreds of microns lateral size exfoliated on gold (Au 111) substrate. The flakes of blue color are thick MoS2 multilayers (ML). The large areas of the faintest optical contrast have been confirmed to be single layers by Raman spectroscopy (inset).
Isolating large-areas of atomically thin transition metal chalcogenide crystals is an important but challenging task. The mechanical exfoliation technique can provide single layers of the highest structural quality, enabling to study their pristine properties and ultimate device performance. However, a major drawback of the technique is the low yield and small (typically < 10 μm) lateral size of the produced single layers. Here, we report a novel mechanical exfoliation technique, based on chemically enhanced adhesion, yielding MoS2 single layers with typical lateral sizes of several hundreds of microns.
The idea is to exploit the chemical affinity of the sulfur atoms that can bind more strongly to a gold surface than the neighboring layers of the bulk MoS2 crystal. Moreover, we found that our exfoliation process is not specific to MoS2, but can be generally applied for various layered chalcogenides including selenites and tellurides, providing an easy access to large-area 2D crystals for the whole class of layered transition metal chalcogenides.
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