Max A. Migliorato

Suspended graphene (SUS-G) has long been hailed as a potential ‘true graphene’ as its conductive properties are much closer to those of theoretical graphene. However, substantial issues with yield during any device fabrication process have severely limited its use to date. We report the successful fabrication of a fully operational prototype of a miniature 9 mm² suspended graphene array sensor chip, incorporating 64 graphene sensor devices, each comprising of 180 SUS-G membranes with ever reported 56% fully intact graphene membranes for sensitive and selective gas sensing applications. While a bare sensor chip can operate as a sensitive gas sensor for a variety of gasses such as ammonia, nitrogen dioxide and carbon monoxide, down to ppm/ppb concentrations, a tetrafluorohydroquinone functionalized sensor acquires specificity to formaldehyde gas molecules with limited cross-sensitivity for ethanol, toluene and humidity. Unlike an equivalent device with fully supported functionalized graphene sensor, a functionalized SUS-G sensor can be furthermore reset to its baseline by using UV assisted desorption instead of substrate heating. The low power UV irradiation does not show severe damage to the SUS-G structures and loss of functional probes for the formaldehyde gas—a previously unreported feature. A resettable and selective formaldehyde gas sensor array with mass manufacturability, low power consumption and overall dimensions down to 1 mm², would represent a significant technological step forward in the development of an electronic nose, for the simultaneous detection of multiple-target gases, with potential for integration in portable electronic devices and the internet of things.

Freestanding (suspended) graphene films, with high electron mobility (up to ~200,000 cm²V⁻¹s⁻¹), good mechanical and electronic properties, could resolve many of the current issues that are hampering the upscaling of graphene technology. Thus far, attempts at reliably fabricating suspended graphene devices comprising metal contacts, have often been hampered by difficulties in exceeding sizes of 1 µm in diameter, if using UV lithography. In this work, area of suspended graphene large enough to be utilized in microelectronic devices, have been obtained by suspending a CVD graphene film over cavities, with top contacts defined through UV lithography with both wet and dry etching. An area of up to 160 µm² can be fabricated as backgated devices. The suspended areas exhibit rippling of the surfaces which simultaneously introduces both tensile and compressive strain on the graphene film. Finally, the variations of the Fermi level in the suspended graphene areas can be modulated by applying a potential difference between the top contacts and the backgate. Having achieved large area suspended graphene, in a manner compatible with CMOS fabrication processes, together with enabling the modulation of the Fermi level, are substantial steps forward in demonstrating the potential of suspended graphene-based electronic devices and sensors. Graphical Abstract

III-nitride core-shell nanowires appear to be promising and versatile nanostructures for fabricating high performance piezoelectric nanodevices. Possible applications range from mechanical sensing to piezotronics and piezophototronics. The paper discusses the effect of nonlinear elasticity on the piezopotential and other relevant features generated by III-N core-shell nanowires. The presented results are aimed at providing useful guidelines for the design of such devices.

Due to its piezoelectric and semiconductive properties, ZnO is actively investigated for the development of innovative nanostructures for applications ranging from piezotronics to energy harvesting. In the present paper we develop a full model for the non-linear piezoelectricity in ZnO nanowires, where both direct and inverse non-linear piezoelectric effects are accounted for. The preliminary results show for the first time the importance of non-linear effects on the electro-mechanic behavior of nanowires especially when used for piezotronic applications.

We present an atomistic simulation of an In_xGa_1-xAs/GaAs quantum dot with a non-uniform composition, embedded in a GaAs matrix. Optimised empirical potentials are employed to model the interatomic interactions of up to ∼400,000 atoms in order to obtain dynamic relaxation through energy minimisation. A simulation of a quantum dot cleaved in the (110) plane is implemented, and simulated Scanning Tunnelling Microscopy topographical maps are obtained and compared with experimental maps. Finally another simulation of a capped quantum dot is implemented and the strain tensor is obtained directly from the relaxed positions of the individual atoms, revealing the nature of the lattice distortion in both the island and the capping layer.

We discuss a novel approach to the optimisation of quantum dot bilayer structures grown by molecular beam epitaxy. It has been observed that vertically stacked quantum dot structures often exhibit an increase in the average size of the islands with increasing number of quantum dot layers deposited. The understanding of the In segregation induced Stranski-Krastanow transition suggests that this is due to excess of In caused by elemental segregation. In order to limit the non uniformity of the sizes along the stacks we have grown the upper layers with different concentrations of In, showing that the optimum fraction is the one predicted by the kinetic segregation model.

This chapter reviews the different models that have been proposed over the years and how they compare to experimental measurements. It outlines both linear and nonlinear models and discusses the experimental determination of values of spontaneous polarization in polymorphic crystals. The direct piezoelectric effect is the generation of an electric dipole moment in certain crystals if a stress is applied and the electric dipole moment is proportional to the applied stress. Spontaneous polarization develops from the atomic layout in the bulk materials. The chapter proposes the various nonlinear models of piezoelectricity in III-N semiconductors. Such nonlinearities in the polarization were first reported for III-N by K. Shimada et al., expressing theoretically and calculating the dependence of the polarization on deviatoric strain, but without expressly providing a set of nonlinear coefficients. Piezoelectricity in III-N semiconductors is an effect of very strong magnitude, with fields measured in the MV/cm a commonly reported feature.

The enhanced electromechanical performance of metal–MgO–ZnO MISTDs is due to the highly strain sensitive energy barriers.