Few electron double quantum dots in InAs/InP nanowire heterostructures.

We report on fabrication of double quantum dots in catalytically grown InAs/InP nanowire heterostructures. In the few-electron regime, starting with both dots empty, our low-temperature transport measurements reveal a clear shell structure for sequential charging of the larger of the two dots with up to 12 electrons. The resonant current through the double dot is found to depend on the orbital coupling between states of different radial symmetry. The charging energies are well described by a capacitance model if next-neighbor capacitances are taken into account.

Height-controlled nanowire branches on nanotrees using a polymer mask.

The production of complex three-dimensional dendritic structures is an important step in the application of semiconductor nanowires. One promising method for achieving this is the sequential seeding of multiple generations of epitaxial nanowires using metal seed particles. However, it is difficult to control and predict the position of second and higher generation nanowires with respect to the first generation. Here we demonstrate a procedure for controlling the position of second-generation epitaxial nanowire branches on vertically aligned nanowire trunks. This method uses a spun-on polymer layer that masks first-generation wires to a specified height, preventing the growth of nanowire branches at lower positions as well as new nanowire growth on the substrate. This method appears not to be dependent on the materials or growth system (in this case MOVPE-grown GaP is demonstrated), and hence is likely to be applicable to a variety of materials systems and growth procedures using metal seed particles.

Growth and optical properties of strained GaAs-GaxIn 1-x P core-shell nanowires.

We have synthesized GaAs-Ga(x)In(1-x)P (0.34 < x < 0.69) core-shell nanowires by metal-organic vapor phase epitaxy. The nanowire core was grown Au-catalyzed at a low temperature (450 degrees C) where only little growth takes place on the side facets. The shell was added by growth at a higher temperature (600 degrees C), where the kinetic hindrance of the side facet growth is overcome. Photoluminescence measurements on individual nanowires at 5 K showed that the emission efficiency increased by 2 to 3 orders of magnitude compared to uncapped samples. Strain effects on the band gap of lattice mismatched core-shell nanowires were studied and confirmed by calculations based on deformation potential theory.

Solid-phase diffusion mechanism for GaAs nanowire growth.

Controllable production of nanometre-sized structures is an important field of research, and synthesis of one-dimensional objects, such as nanowires, is a rapidly expanding area with numerous applications, for example, in electronics, photonics, biology and medicine. Nanoscale electronic devices created inside nanowires, such as p-n junctions, were reported ten years ago. More recently, hetero-structure devices with clear quantum-mechanical behaviour have been reported, for example the double-barrier resonant tunnelling diode and the single-electron transistor. The generally accepted theory of semiconductor nanowire growth is the vapour-liquid-solid (VLS) growth mechanism, based on growth from a liquid metal seed particle. In this letter we suggest the existence of a growth regime quite different from VLS. We show that this new growth regime is based on a solid-phase diffusion mechanism of a single component through a gold seed particle, as shown by in situ heating experiments of GaAs nanowires in a transmission electron microscope, and supported by highly resolved chemical analysis and finite element calculations of the mass transport and composition profiles.

Probing of individual semiconductor nanowhiskers by TEM-STM.

Along with rapidly developing nanotechnology, new types of analytical instruments and techniques are needed. Here we report an alternative procedure for electrical measurements on semiconductor nanowhiskers, allowing precise selection and visual control at close to atomic resolution. We use a combination of two powerful microscope techniques, scanning tunneling microscopy (STM) and simultaneous viewing in a transmission electron microscope (TEM). The STM is mounted in the sample holder for the TEM. We describe here a method for creating an ohmic contact between the STM tip and the nanowhisker. We examine three different types of STM tips and present a technique for cleaning the STM tip in situ. Measurements on 1-microm-tall and 40-nm-thick epitaxially grown InAs nanowhiskers show an ohmic contact and a resistance of down to 7 kOmega.

Synthesis of branched 'nanotrees' by controlled seeding of multiple branching events.

The formation of nanostructures with controlled size and morphology has been the focus of intensive research in recent years. Such nanostructures are important in the development of nanoscale devices and in the exploitation of the properties of nanomaterials. Here we show how tree-like nanostructures ('nanotrees') can be formed in a highly controlled way. The process involves the self-assembled growth of semiconductor nanowires via the vapour-liquid-solid growth mode. This bottom-up method uses initial seeding by catalytic nanoparticles to form the trunk, followed by the sequential seeding of branching structures. Each level of branching is controlled in terms of branch length, diameter and number, as well as chemical composition. We show, by high-resolution transmission electron microscopy, that the branching mechanism gives continuous crystalline (monolithic) structures throughout the extended and complex tree-like structures. The controlled seeding method that we report here has potential as a generic means of forming complex branching structures, and may also offer opportunities for applications, such as the mimicking of photosynthesis in nanotrees.