The control of charges in a circuit due to an external electric field is ubiquitous to the exchange, storage and manipulation of information in a wide range of applications. Conversely, the ability to grow clean interfaces between materials has been a stepping stone for engineering built-in electric fields largely exploited in modern photovoltaics and opto-electronics. The emergence of atomically thin semiconductors is now enabling new ways to attain electric fields and unveil novel charge transport mechanisms. Here, we report the first direct electrical observation of the inverse charge-funnel effect enabled by deterministic and spatially resolved strain-induced electric fields in a thin sheet of HfS2. We demonstrate that charges driven by these spatially varying electric fields in the channel of a phototransistor lead to a 350% enhancement in the responsivity. These findings could enable the informed design of highly efficient photovoltaic cells.

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We present an atomic-scale theory of interface scattering of phonons in superlattices. In particular, we describe the scattering as a result of two features, mixing of atoms at interfaces and presence of dislocations at interfaces due to lattice mismatch. We apply the theory to quantitatively explain the thermal conductivity, and its variation with period and temperature, of Si/Ge superlattices.

The lattice dynamical results of silicon nanostructures with all three different degrees of confinement (nanoslabs, nanowires, nanodots) are systematically analysed and presented using an adiabatic bond charge model. In the direction of propagation of these structures, it is found that the phonon branches change from flat, for the smallest nanostructures, to dispersive as the nanostructure size increases. It is also noted that in the direction of confinement all but the acoustic branches are generally flat, with very little dispersion. The trends in the variations of the lowest and highest confined modes at the Brillouin zone centre with nanostructure size are investigated. In particular, analytic expressions for the size variation of the highest mode with the dimensionality of the nanostructures have been presented. Also, an analytic fit has been presented for the size variation of the lowest non-zero acoustic mode with structure size. Finally, numerical calculations based upon Fermi’s Golden Rule formula of the dependence of the lifetime of the lowest confined mode on nanostructure size and temperature have also been obtained and discussed.

Boron delta-doped diamond structures have been synthesized using microwave plasma chemical vapor deposition, and fabricated into FET and gated Hall bar devices for assessment of the electrical characteristics. A detailed study of variable temperature Hall, conductivity and field-effect mobility measurements was completed. This was supported by Schrödinger-Poisson and relaxation time calculations based upon application of Fermi's golden rule. A two carrier-type model was developed with an activation energy of ~0.2 eV between the delta layer lowest subband with mobility ~1 cm 2 /Vs and the bulk valence band with high mobility. This new understanding of the transport of holes in such boron delta-doped structures has shown that although Hall mobility as high as 900 cm 2 /Vs was measured at room temperature, this dramatically overstates the actual useful performance of the device.a Corresponding author