Paul von Allmen

Localized sources of water vapour on the dwarf planet (1) Ceres

The ‘snowline’ conventionally divides Solar System objects into dry bodies, ranging out to the main asteroid belt, and icy bodies beyond the belt. Models suggest that some of the icy bodies may have migrated into the asteroid belt. Recent observations indicate the presence of water ice on the surface of some asteroids, with sublimation a potential reason for the dust activity observed on others. Hydrated minerals have been found on the surface of the largest object in the asteroid belt, the dwarf planet (1) Ceres, which is thought to be differentiated into a silicate core with an icy mantle. The presence of water vapour around Ceres was suggested by a marginal detection of the photodissociation product of water, hydroxyl (ref. 12), but could not be confirmed by later, more sensitive observations. Here we report the detection of water vapour around Ceres, with at least 1026 molecules being produced per second, originating from localized sources that seem to be linked to mid-latitude regions on the surface. The water evaporation could be due to comet-like sublimation or to cryo-volcanism, in which volcanoes erupt volatiles such as water instead of molten rocks.

Subsurface properties and early activity of comet 67P/Churyumov-Gerasimenko

Heat transport and ice sublimation in comets are interrelated processes reflecting properties acquired at the time of formation and during subsequent evolution. The Microwave Instrument on the Rosetta Orbiter (MIRO) acquired maps of the subsurface temperature of comet 67P/Churyumov-Gerasimenko, at 1.6 mm and 0.5 mm wavelengths, and spectra of water vapor. The total H2O production rate varied from 0.3 kg s–1 in early June 2014 to 1.2 kg s–1 in late August and showed periodic variations related to nucleus rotation and shape. Water outgassing was localized to the “neck” region of the comet. Subsurface temperatures showed seasonal and diurnal variations, which indicated that the submillimeter radiation originated at depths comparable to the diurnal thermal skin depth. A low thermal inertia (~10 to 50 J K–1 m–2 s–0.5), consistent with a thermally insulating powdered surface, is inferred.

Boundary conditions for the electronic structure of finite-extent embedded semiconductor nanostructures

The modeling of finite-extent semiconductor nanostructures that are embedded in a host material requires a proper boundary treatment for a finite simulation domain. For the study of a self-assembled InAs dot embedded in GaAs, three kinds of boundary conditions are examined within the empirical tight-binding model: (i) the periodic boundary condition, (ii) raising the orbital energies of surface atoms, and (iii) raising the energies of dangling bonds at the surface. The periodic boundary condition requires a smooth boundary and consequently a larger GaAs buffer than the two nonperiodic boundary conditions. Between the nonperiodic conditions, the dangling-bond energy shift is more numerically efficient than the orbital-energy shift, in terms of the elimination of nonphysical surface states in the energy region of interest for interior states. A dangling-bond energy shift larger than 5 eV efficiently eliminates all of the surface states and leads to interior states that are highly insensitive to the choice of the energy shift.

Valley splitting in strained silicon quantum wells

A theory based on localized-orbital approaches is developed to describe the valley splitting observed in silicon quantum wells. The theory is appropriate in the limit of low electron density and relevant for quantum computing architectures. The valley splitting is computed for realistic devices using the quantitative nanoelectronic modeling tool NEMO. A simple, analytically solvable tight-binding model reproduces the behavior of the splitting in the NEMO results and yields much physical insight. The splitting is in general nonzero even in the absence of electric field in contrast to previous works. The splitting in a square well oscillates as a function of S, the number of layers in the quantum well, with a period that is determined by the location of the valley minimum in the Brillouin zone. The envelope of the splitting decays as S−3. The feasibility of observing such oscillations experimentally in Si/SiGe heterostructures is discussed.

Effect of wetting layers on the strain and electronic structure of InAs self-assembled quantum dots

The effect of wetting layers on the strain and electronic structure of InAs self-assembled quantum dots grown on GaAs is investigated with an atomistic valence-force-field model and an empirical tight-binding model. By comparing a dot with and without a wetting layer, we find that the inclusion of the wetting layer weakens the strain inside the dot by only 1% relative change, while it reduces the energy gap between a confined electron and hole level by as much as 10%. The small change in the strain distribution indicates that strain relaxes only little through the thin wetting layer. The large reduction of the energy gap is attributed to the increase of the confining-potential width rather than the change of the potential height. First-order perturbation calculations or, alternatively, the addition of an InAs disk below the quantum dot confirm this conclusion. The effect of the wetting layer on the wave function is qualitatively different for the weakly confined electron state and the strongly confined hole state. The electron wave function shifts from the buffer to the wetting layer, while the hole shifts from the dot to the wetting layer.

Tight-binding modeling of thermoelectric properties of bismuth telluride

A parameterized orthogonal tight-binding model with sp3d5s* orbitals, nearest-neighbor interactions, and spin-orbit coupling is developed for bismuth telluride (Bi2Te3) and used to study its thermoelectric properties. Thermoelectric transport coefficients and figures of merit for n-doped and p-doped Bi2Te3 are calculated by solving Boltzmann’s transport equation within the constant-relaxation-time approximation. The dependence of the computed thermoelectric figure of merit on the electrical conductivity is in good agreement with experiment. The parameterized tight-binding model serves as a basis for studies of confined Bi2Te3 systems in search of enhanced thermoelectric properties.

Effect of anharmonicity of the strain energy on band offsets in semiconductor nanostructures

Anharmonicity of the interatomic potential is taken into account for the quantitative simulation of the conduction and valence band offsets for strained semiconductor heterostructures. The anharmonicity leads to a weaker compressive hydrostatic strain than that obtained with the commonly used quasiharmonic approximation of the Keating model. Compared to experiment, inclusion of the anharmonicity in the simulation of strained InAs∕GaAs nanostructures results in an improvement of the electron band offset computed on an atomistic level by up to 100meV.

Spatial and diurnal variation of water outgassing on comet 67P/Churyumov-Gerasimenko observed from Rosetta/MIRO in August 2014

Aims. We present the spatial and diurnal variation of water outgassing on comet 67P/Churyumov-Gerasimenko using the (H2O)-O-16 rotational transition line at 556.936 GHz observed from Rosetta/MIRO in August 2014. Methods. The water line was analyzed with a non-LTE radiative transfer model and an optimal estimation method to retrieve the (H2O)-O-16 outgassing intensity, expansion velocity, and gas kinetic temperature. On August 7-9, 2014 and August 18-19, 2014, MIRO performed long steady nadir-pointing observations of the nucleus while it was rotating around its spin axis. The ground track of the MIRO beam during the observation was mostly on the northern hemisphere of comet 67P, covering its three distinct parts: the so-called head, body, and neck areas. Results. The MIRO spectral observation data show that the water-outgassing intensity varies by a factor of 30, from 0.1 x 1025 molecules s(-1) sr l to 3.0 x 10(25) molecules s(-1) sr, the terminal gas expansion velocity varies by 0.17 km s(-1) from 0.61 km s(-1) to 0.78 km s(-1), and the terminal gas temperature varies by 27 K from 47 K to 74 K. The retrieved coma parameters are co-registered with local environment variables such as the subsurface temperatures, measured in the MIRO continuum bands, the local solar time, illumination condition, and beam location on nucleus. The spatial variation of the outgassing activity is very noticeable, and the largest outgassing activity in August 2014 occurs near the neck region of the nucleus. The outgassing activity in the neck region is also found to be correlated with the local solar hour, which is related to the local illumination condition.

MIRO observations of subsurface temperatures of the nucleus of 67P/Churyumov-Gerasimenko

Observations of the nucleus of 67P/Churyumov-Gerasimenko in the millimeter-wave continuum have been obtained by the Microwave Instrument for the Rosetta Orbiter (MIRO). We present data obtained at wavelengths of 0.5 mm and 1.6 mm during September 2014 when the nucleus was at heliocentric distances between 3.45 and 3.27 AU. The data are fit to simple models of the nucleus thermal emission in order to characterize the observed behavior and make quantitative estimates of important physical parameters, including thermal inertia and absorption properties at the MIRO wavelengths. MIRO brightness temperatures on the irregular surface of 67P are strongly affected by the local solar illumination conditions, and there is a strong latitudinal dependence of the mean brightness temperature as a result of the seasonal orientation of the comet’s rotation axis with respect to the Sun. The MIRO emission exhibits strong diurnal variations, which indicate that it arises from within the thermally varying layer in the upper centimeters of the surface. The data are quantitatively consistent with very low thermal inertia values, between 10–30 J K -1 m -2 s -1/2 , with the 0.5 mm emission arising from 1 cm beneath the surface and the 1.6 mm emission from a depth of 4 cm. Although the data are generally consistent with simple, homogeneous models, it is difficult to match all of its features, suggesting that there may be some vertical structure within the upper few centimeters of the surface. The MIRO brightness temperatures at high northern latitudes are consistent with sublimation of ice playing an important role in setting the temperatures of these regions where, based on observations of gas and dust production, ice is known to be sublimating.

Single, aligned carbon nanotubes in 3D nanoscale architectures enabled by top-down and bottom-up manufacturable processes

We have developed manufacturable approaches for forming single, vertically aligned carbon nanotubes, where the tubes are centered precisely, and placed within a few hundred nm of 1-1.5 microm deep trenches. These wafer-scale approaches were enabled by using chemically amplified resists and high density, low pressure plasma etching techniques to form the 3D nanoscale architectures. The tube growth was performed using dc plasma-enhanced chemical vapor deposition (PECVD), and the materials used in the pre-fabricated 3D architectures were chemically and structurally compatible with the high temperature (700 degrees C) PECVD synthesis of our tubes, in an ammonia and acetylene ambient. Such scalable, high throughput top-down fabrication processes, when integrated with the bottom-up tube synthesis techniques, should accelerate the development of plasma grown tubes for a wide variety of applications in electronics, such as nanoelectromechanical systems, interconnects, field emitters and sensors. Tube characteristics were also engineered to some extent, by adjusting the Ni catalyst thickness, as well as the pressure and plasma power during growth.