For a physical system to be a qubit, it must have a long coherence time and high fidelity, which specifies the probability of a quantum operation to be successful. Two groups at the University of New South Wales have now joined forces to demonstrate both of these properties in two highly purified silicon-based systems. In the work by Menno Veldhorst and co-workers, the qubit is represented by the spin of an electron confined in a lithographically defined quantum dot. In that by Juha Muhonen and co-workers, the qubit is the electron spin associated with a 31P impurity. The cover image shows an artist’s impression of an electron wavefunction (blue), which is confined in a crystal of nuclear-spin-free 28Si atoms (black) and is controlled by a nanofabricated metal gate (silver).

Letters p981 and p986; News & Views p966

IMAGE: STEPHANIE SIMMONS, UNSW AUSTRALIA

COVER DESIGN: ALEX WING

Thermoelectric devices convert heat into electricity. Key to high conversion efficiency is the ability to simultaneously increase the electrical conductance and the Seebeck coefficient, which Pramod Reddy and colleagues have now shown is possible in molecular junctions. Their experimental set-up consists of an electrical heater connected to one of the electrodes and an electrical gate near the junction. The heater creates an extremely large temperature gradient through the junction, and by varying the gate voltage, the researchers can tune the molecular energy levels to maximize the electrical conductance. The cover image is the thermal map of the nanogap (green/red, hot; blue/purple, cold) overlaid with the mesh used for modelling the experimental data.

Letter p881; News & Views p876

IMAGE: PRAMOD REDDY

COVER DESIGN: ALEX WING

Vertically stacked layers of two-dimensional materials are a fertile ground for studying exotic transport phenomena. In particular, electron tunnelling can occur between two graphene layers separated by a few layers of insulating hexagonal boron nitride. Usually, tunnelling between the layers conserves energy but not momentum. Kostya Novoselov and colleagues have now shown that a careful control of the relative crystalline orientation of the two graphene layers can result in electron tunnelling that conserves both energy and momentum. Such a resonant tunnelling produces an oscillating current that could be promising for high-frequency electronics. The cover image is a map of the normalized differential currents measured in the devices as a function of the applied gate and bias voltages, with red (blue) corresponding to positive (negative) values.

Letter p808

IMAGE: JOHN WALLBANK, LANCASTER UNIV

COVER DESIGN: ALEX WING

Nanoparticles have a variety of useful intrinsic properties and significant potential in biomedical applications. By adding biocomputing capabilities to such materials, robotic devices could be developed that take advantage of these attractive intrinsic features. However, the computing potential of particle-based systems is relatively unexplored. Maxim Nikitin and colleagues have now shown that almost any type of nanoparticle or microparticle can be transformed into biocomputing structures that can implement a functionally complete set of Boolean logic gates (YES, NOT, AND and OR), and can be made to bind to a target as a result of a computation. The logic-gating functionality is incorporated into self-assembled particle/biomolecule interfaces and the logic gating is achieved through input-induced disassembly of the structures. The computer-generated image on the cover provides an artistic impression of the particle-based biocomputing system.

Article p716

IMAGE: ELLA MARUSHCHENKO

COVER DESIGN: ALEX WING

Small interfering RNAs (siRNAs) can turn off any specific gene in the genome. As a result, these molecules have tremendous potential as both scientific tools and therapeutics. However, delivering siRNA to the right cells in vivo has remained challenging. Using combinatorial chemical synthesis techniques and high-throughput biological screening methods, Daniel Anderson and co-workers have designed a nanoparticle that delivers siRNA to endothelial cells — cells that line blood and lymphatic vessels — at very low doses. Using this nanoparticle, the researchers turned off five genes at once inside an animal, turned off genes for more than three weeks after one injection, and reduced inflammation, tumour growth and metastasis. The cover image shows blood vessels in mouse adipose tissue stained with two endothelial cell markers, CD31 (blue) and ICAM-2 (magenta).

Article p648; News & Views p568; In the Classroom p656

IMAGE: AUDE THIRIOT

COVER DESIGN: ALEX WING

Quantum dots are structures in which the energy of electrons is quantized in discrete levels, as in atoms. Their size and composition are usually difficult to control, and each quantum dot differs slightly from another. Stefan Fölsch and co-workers have now demonstrated a way to construct quantum dots with the desired number of atoms placed in a precise spatial arrangement. They used the tip of a scanning tunnelling microscope to assemble chains of In atoms on an InAs surface. Each chain shows reproducible quantum dot states. Furthermore, quantum dot molecules can be constructed by placing two or three chains next to each other with precise control of the orientation and separation between them. The cover shows a map of the electron density of states for a molecule comprising three quantum dots as measured by scanning tunnelling spectroscopy.

Letter p505; News & Views p499

IMAGE: STEFAN FÖLSCH

COVER DESIGN: ALEX WING

Knowing the temperature of a nanoscale object when it differs from that of its environment is essential for many applications in nanotechnology. Janet Anders and co-workers have now developed a method to measure the temperature of heated nanoscale objects in a gas, by utilizing a detailed understanding of the non-equilibrium Brownian dynamics of the objects. In their experiment, a silica nanosphere is levitated by a laser beam. Absorption from the beam heats the sphere, while colliding gas particles cool the sphere's surface. The observable back-action of the scattered gas on the sphere's motion is then used to infer the temperature of the surface with nanoscale spatial resolution. The cover is an artist's impression of the trapped nanosphere in the laser beam.

Letter p425; News & Views p415

IMAGE: MARK MAZAITIS

COVER DESIGN: ALEX WING

So far, the magnetic structure of skyrmions has been imaged in reciprocal space with neutron scattering and in real space with Lorentz transmission electron microscopy. However, the details of the magnetic field in a skyrmion lattice have not yet been visualized. Such a field is important in transport experiments as it gives rise to exotic phenomena such as the topological Hall effect. Hyun Soon Park and co-workers report on the use of electron holography to image the three-dimensional structure, in real space, of skyrmion lattices in thin samples of helimagnetic Fe0.5Co0.5Si. The technique is also able to directly visualize the magnetic flux flow in the vicinity of the skyrmion lattice, as shown on the cover. Red and green denote spin orientations out of the plane, whereas yellow represents in-plane spins.

Letter p337

IMAGE: HYUN SOON PARK AND YURI TAMURA

COVER DESIGN: ALEX WING

An important step towards scaling up lithographic nanofabrication is the development of resists that can be processed with innocuous chemicals, such as water. Fiorenzo Omenetto and co-workers report on the use of silk fibroin as a natural resist for electron-beam lithography and the development of a fabrication protocol that is all-water-based. Silk can be used either as a positive or negative resist and can also be functionalized with specific enzymes that retain their activity even after electron-beam exposure. The cover shows silk fibres and water droplets.

Letter p306; News & Views p251

BACKGROUND IMAGE: FIORENZO OMENETTO; WATER DROPLETS © MURRAY CLARKE/ALAMY

COVER DESIGN: ALEX WING

Silicon anodes have been actively researched for applications in lithium batteries because they have ten times the theoretical capacity of their carbon-based counterpart. Unfortunately nanostructured silicon anodes have challenging drawbacks, such as a large volume change during cycling, side reactions with the electrolyte and low volumetric capacity. Yi Cui and colleagues now show that these problems can be alleviated by using a hierarchically structured silicon anode whose design was inspired by a pomegranate. Silicon nanoparticles are first enclosed within a carbon shell, which allows them to freely expand and contract during cycling. These silicon-containing shells are then grouped together and surrounded by a thicker outer carbon layer that protects them from the electrolyte, yet lithium exchange with the solution phase still occurs. The cover is an artist's impression of these hierarchical structures.

Letter p187

IMAGE: NIAN LIU

COVER DESIGN: ALEX WING

Quantum mechanical effects are usually only observed in nanoscale structures. It is however possible to couple quantum dots to mechanical oscillators with micrometre sizes. Jean-Philippe Poizat and co-workers have now fabricated a monolithic hybrid system, in which a quantum dot buried in a wire is coupled by strain to the mechanical motion of the wire. The electronic levels in the dot are modified by coupling with the vibrational modes in the wire, and this is observed in the optical emission spectra of the dots. The scanning electron microscopy image on the cover shows a series of tapered wires. The dots are located close to their base.

Letter p106; News & Views p99

IMAGE: LAURENT REVELLIN-FALCOZ (CNRS) AND JULIEN CLAUDON (CEA INAC)

COVER DESIGN: ALEX WING

DNA can be programmed to self-assemble into intricate three-dimensional structures and to arrange nanoparticles into precise assemblies. However, the approach is typically limited to small interparticle distances. Tim Liedl and colleagues have now shown that DNA origami scaffolds can organize different nanomaterials into large hierarchical nanoclusters that have a planet–satellite-type structure. With the approach, metal nanoparticles, quantum dots and organic dyes can be arranged into structures that have distances of 5–200 nm between components, and overall sizes of up to 500 nm. The computer-generated image on the cover provides an artistic impression of the possibilities and flexibility of this approach.

Letter p74

IMAGE: CHRISTOPH HOHMANN, NANOSYSTEMS INITIATIVE MUNICH (NIM)

COVER DESIGN: ALEX WING