Nanofabrication: pristine quantum devices on demand.

C arbon nanomaterials such as carbon nanotubes 1 and graphene 2 are low-dimensional materials that can have defect-free lattices over microscopic length scales, which is a feature that only a few other nanomaterials can offer. Despite the excitement that these materials have generated in the condensed-matter physics community due to their potential as playgrounds for quantum transport study, the creation of ultraclean carbon nanotube devices remains challenging. Writing in Nature Nanotechnology, Shahal Ilani and co-workers at Weizmann Institute of Science and Harvard University have now shown that pristine carbon nanotube quantum devices can be created by using a combination of direct nanotube transfer and scanning probe microscopy 3. These devices exhibit highly tunable electronic characteristics and can have complex architectures. Most carbon nanotube devices are fabricated by post-growth processing (Fig. 1a), where nanotubes are first grown on a device substrate, then lithography is carried out to define the functional devices 4. Although this method is widely used, nanotubes are inevitably contaminated by organic residues from the lithography and solution processing steps, resulting in electronic disorders in these quantum systems. To eliminate post-growth contamination, direct chemical vapour deposition synthesis of carbon nanotubes on metal electrodes has also been developed 5. However, due to the lack of control over carbon nanotube growth yield and orientation, quantum devices with complex electrostatic gating schemes are not possible with the approach. More recently, a one-step direct-transfer technique has been developed to fabricate pristine nanotube devices (Fig. 1b). In this approach, suspended nanotubes, synthesized on the growth substrate, are aligned and directly transferred onto a pre-patterned device substrate with no further lithography needed 6. This technique has provided some of the cleanest samples so far 7 , but it is intrinsically random and offers low yield. Ilani and colleagues were able to deterministically create nanotube devices with ultralow disorder with the help of scanning probe microscopy manipulation. In particular, the researchers designed and fabricated electrical circuits with complex gate structures directly on the cantilever of a scanning probe microscope. On a separate growth chip, they synthesized long parallel nanotubes suspended over wide trenches. The scanning probe microscope was then used to insert the cantilever into a chosen trench and 'mate' it with a nanotube. Unlike previous direct-transfer techniques, this process enables in situ transport measurements, which can be used to ensure the electronic cleanliness of the transferred nanotubes. With the technique, Ilani and colleagues create nanotube …