Writing Conductive Lines with Hot Tips The interface within devices between conductors, semiconductors, and insulators is usually created by stacking patterned layers of different materials. For flexible electronics, it can be advantageous to avoid this architectural constraint. Graphene oxide, formed by chemical exfoliation of graphite, can be reduced to a more conductive form using chemical reductants. Wei et al. (p. 1373 ) now show that layers of graphene oxide can also be reduced using a hot atomic force microscope tip to create materials comparable to those of organic conductors. This process can create patterned regions (down to 12 nanometers in width) that differ in conductivity by up to four orders of magnitude.

Graphene oxide potentially has multiple applications. The chemistry of graphene oxide and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is crucial to enable future applications of this material. Here, a combined experimental and density functional theory study shows that multilayer graphene oxide produced by oxidizing epitaxial graphene through the Hummers method is a metastable material whose structure and chemistry evolve at room temperature with a characteristic relaxation time of about one month. At the quasi-equilibrium, graphene oxide reaches a nearly stable reduced O/C ratio, and exhibits a structure deprived of epoxide groups and enriched in hydroxyl groups. Our calculations show that the structural and chemical changes are driven by the availability of hydrogen in the oxidized graphitic sheets, which favours the reduction of epoxide groups and the formation of water molecules.

The nanoscale control afforded by scanning probe microscopes has prompted the development of a wide variety of scanning probe-based patterning methods. Some of these methods have demonstrated a high degree of robustness and patterning capabilities that are unmatched by other lithographic techniques. However, the limited throughput of scanning probe lithography has prevented their exploitation in technological applications. Here, we review the fundamentals of scanning probe lithography and its use in materials science and nanotechnology. We focus on the methods and processes that offer genuinely lithography capabilities such as those based on thermal effects, chemical reactions and voltage-induced processes. Published in:Nature Nanotechnology 9, 577-587 (2014) Progress in nanotechnology depends on the capability to fabricate, position, and interconnect nanometre-scale structures. A variety of materials and systems such as nanoparticles, nanowires, two-dimensional materials like graphene and transition metal dichalcogenides, plasmonics materials, conjugated polymers and organic semiconductors are finding applications in nanoelectronics, nanophotonics, organic electronics and biomedical applications. The success of many of the above applications relies on the existence of suitable nanolithography approaches. However, patterning materials with nanoscale features aimed at improving integration and device performance poses several challenges. The limitations of conventional lithography techniques related to resolution, operational costs and lack of flexibility to pattern organic and novel materials have motivated the development of unconventional fabrication methods [1][2][3] .Since the first patterning experiments performed with a scanning probe microscope in the late 80s, scanning probe lithography (SPL) has emerged as an alternative lithography for academic research that combines nanoscale feature-size, relatively low technological requirements and the ability to handle soft matter from small organic molecules to proteins and polymers. Scanning probe lithography experiments have provided striking examples of its capabilities such as the ability to pattern 3D structures with nanoscale features 4 , the fabrication of the smallest field-effect transistor 5 or the patterning of proteins with 10 nm feature size 6 . Figure 1a shows a general scheme of SPL operation. There is a variety of approaches to modify a material in a probe-surface interface which have generated several SPL methods. Scanning probe lithographies can be either classified by emphasizing the distinction between the general nature of the process, chemical versus physical, or by considering if SPL implies the removal or addition of material. However, we consider it is more inclusive and systematic to classify the different SPL methods in terms of the driving mechanisms used in the patterning process, namely thermal, electrical, mechanical and diffusive methods (Fig. 1b). Challenges in nanoscale lithographyThe workhorse of large volume CMOS fabrication, o...

The friction force on a nanometer-sized tip sliding on a surface is related to the thermally activated hopping of the contact atoms on an effective atomic interaction potential. A general analytical expression relates the height of this potential and the hopping attempt frequency to measurements of the velocity dependence of the friction force performed with an atomic force microscope. While the height of the potential is roughly proportional to the normal load, the attempt frequency falls in the range of mechanical eigenfrequencies of the probing tip in contact with the surface.

Direct and simultaneous measurements of the normal and lateral forces encountered by a nanosize spherical silicon tip approaching a solid surface in purified water are reported. For tip-surface distances, 0 ± 0.03 nm Ͻ d Ͻ 2 nm, experiments and grand canonical molecular-dynamics simulations find oscillatory solvation forces for hydrophilic surfaces, mica and glass, and less pronounced oscillations for a hydrophobic surface, graphite. The simulations reveal layering of the confined water density and the development of hexagonal order in layers proximal to a quartz surface. For subnanometer hydrophilic confinement, the lateral force measurements show orders of magnitude increase of the viscosity with respect to bulk water, agreeing with a simulated sharp decrease in the diffusion constant. No viscosity increase is observed for hydrophobic surfaces.