8 research outputs found
Exact location of dopants below the Si(001):H surface from scanning tunnelling microscopy and density functional theory
Control of dopants in silicon remains the most important approach to
tailoring the properties of electronic materials for integrated circuits, with
Group V impurities the most important n-type dopants. At the same time, silicon
is finding new applications in coherent quantum devices, thanks to the
magnetically quiet environment it provides for the impurity orbitals. The
ionization energies and the shape of the dopant orbitals depend on the surfaces
and interfaces with which they interact. The location of the dopant and local
environment effects will therefore determine the functionality of both future
quantum information processors and next-generation semiconductor devices. Here
we match observed dopant wavefunctions from low-temperature scanning tunnelling
microscopy (STM) to images simulated from first-principles density functional
theory (DFT) calculations. By this combination of experiment and theory we
precisely determine the substitutional sites of neutral As dopants between 5
and 15A below the Si(001):H surface. In the process we gain a full
understanding of the interaction of the donor-electron state with the surface,
and hence of the transition between the bulk dopant (with its delocalised
hydrogenic orbital) and the previously studied dopants in the surface layer.Comment: 12 pages; accepted for publication in Phys. Rev.
Film thickness investigation in heavily loaded hypoid gear pair elastohydrodynamic conjunctions
Introduction: Hypoid gear pairs are some of the most highly loaded components of the differential unit in modern
automobiles. Prediction of wear rate and generated friction require determination of lubricant film thickness.
However, only very few investigations have addressed the issue of thin elastohydrodynamic films in hypoid
gear pairs. The main reason for dearth of analysis in this regard has been the need for accurate determination of
transient contact geometry and kinematics of interacting surfaces throughout a typical meshing cycle.
Furthermore, combined gear dynamics and lubrication analysis of any pairs of simultaneous meshing teeth pairs
is required. Simon [1] was among the first to deal with these issues. He used Tooth Contact Analysis (TCA) in
order to calculate the instantaneous contact geometry and load for any teeth pair during their meshing cycle.
However, in his study, the load carried by the hypoid pair was quite low, making the application of the results
limited and not entirely suitable for real life operating conditions of typical hypoid gear pairs of vehicular
differentials, which is of interest in the current paper.
Xu and Kahraman [2] performed numerical prediction of power losses and consequently the film thickness for
highly loaded hypoid gear pairs. However, in their study only the one-dimensional Reynolds equation was
employed. Consequently, the effect of lubricant side leakage in the passage through the contact was ignored. A
more recent study by Mohammadpour et al. [3] employed realistic gear geometry data (through the use of TCA)
for calculation of film thickness time history through mesh. The two-dimensional Reynolds equation,
accounting for the side leakage of the lubricant, was solved numerically. It was shown that the side leakage
component of the entraining velocity can significantly influence the film thickness.
With regard to hypoid gear dynamics, several studies should be mentioned. Wang and Lim [4] studied the
dynamic response of hypoid gear pairs under the influence of time varying meshing stiffness. Yang and Lim [5]
created a model able to predict the dynamic response of a hypoid gear pair by taking into account the lateral
translations of their shafts due to the compliance of the supporting bearings. Karagiannis et al. [6-7] studied the
dynamics of automotive differential hypoid gear pairs by taking into account the velocity dependent resistive
torque at the gear caused by aerodynamic drag and tyre-road rolling resistance. The study integrated the gear
dynamics with the generated viscous and boundary conjunctional friction
Isothermal elastohydrodynamic lubrication analysis of heavily loaded hypoid gear pairs
A numerical model able to predict the pressure
distribution and the film thickness in heavily loaded
elliptical EHL contacts is developed and presented in
this study. The operating conditions, such as the contact
load and the velocities of the mating surfaces, are
representative of the corresponding conditions present
in automotive differential hypoid gear pair units. The
EHL solver presented is able to predict the minimum
and central film thickness of the lubricating oil as well
as the pressure distribution assuming isothermal and
Newtonian conditions. Results are presented for a full
quasi-static meshing cycle. A comparison between the
numerically calculated values of the central and the
minimum film thickness is performed against the
corresponding values produced using the
Chittenden-Dowson formula. A very good agreement is
observed between the values of the central film
thickness. However, it is shown that the minimum film
thickness values using the Chittenden-Dowson formula
can deviate up to 40% compared with the corresponding
values which are calculated numerically
Phenyl attachment to Si(001) via STM manipulation of acetophenone
The attachment of organic molecules to semiconductor surfaces and their measurement using scanning tunneling microscopy and spectroscopy (STM/STS) is considered to be a potential route toward conductance measurements of single molecules with known structural and electronic configuration. Here, we investigate a model system—acetophenone on Si(001)—and demonstrate that this adsorbate can be manipulated using the STM such that it adopts a configuration where it stands upright with a free-standing phenyl ring and strong Si–O linkage to the substrate. For the structural identification we combine STM imaging with density functional theory (DFT) calculations to describe the adsorbate structures that were observed in our experiments and the reaction pathways that link them; of these the upright configuration is the most thermodynamically stable. The chemical structure of the upright configuration suggests that π-conjugation within the adsorbate extends to the silicon surface resulting in strong hybridization of the molecular states with the substrate. This is supported by the absence of any significant features in STS curves recorded over the adsorbate. The structure of this adsorbate and its robust attachment to silicon makes it attractive for future STM/semiconductor-based molecular conductance measurements
Nonlinear dynamics of an automotive differential hypoid gear pair
The dynamics of an automotive differential hypoid gear pair is investigated. The gear pair model is a 4 degree-of-freedom torsional model, including the torsional deflections of the supporting shafts of the pinion and the gear. It also includes the dynamic transmission error of the mating teeth pairs. The variations in teeth contact stiffness/contact, principal radii of contact and static transmission error are determined during the meshing cycle, using the CALYX software. The equations of motion are solved using a numerical integration scheme. A preliminary parametric study is presented, enabling identification of different periodic responses as the vehicle cruising speed alters
Structure and Morphology of Charged Graphene Platelets in Solution by Small-Angle Neutron Scattering
Solutions of negatively charged graphene (graphenide)
platelets
were produced by intercalation of nanographite with liquid potassium–ammonia
followed by dissolution in tetrahydrofuran. The structure and morphology
of these solutions were then investigated by small-angle neutron scattering.
We found that >95 vol % of the solute is present as single-layer
graphene
sheets. These charged sheets are flat over a length scale of >150
Ă… in solution and are strongly solvated by a shell of solvent
molecules. Atomic force microscopy on drop-coated thin films corroborated
the presence of monolayer graphene sheets. Our dissolution method
thus offers a significant increase in the monodispersity achievable
in graphene solutions