P. Venezuela

Raman spectroscopy as probe of nanometre-scale strain variations in graphene.

Confocal Raman spectroscopy has emerged as a major, versatile workhorse for the non-invasive characterization of graphene. Although it is successfully used to determine the number of layers, the quality of edges, and the effects of strain, doping and disorder, the nature of the experimentally observed broadening of the most prominent Raman 2D line has remained unclear. Here we show that the observed 2D line width contains valuable information on strain variations in graphene on length scales far below the laser spot size, that is, on the nanometre-scale. This finding is highly relevant as it has been shown recently that such nanometre-scaled strain variations limit the carrier mobility in high-quality graphene devices. Consequently, the 2D line width is a good and easily accessible quantity for classifying the crystalline quality, nanometre-scale flatness as well as local electronic properties of graphene, all important for future scientific and industrial applications.

Emergence of local magnetic moments in doped graphene-related materials

Motivated by recent studies reporting the formation of localized magnetic moments in doped graphene, we investigate the energetic cost for spin polarizing isolated impurities embedded in this material. When a well-known criterion for the formation of local magnetic moments in metals is applied to graphene we are able to predict the existence of magnetic moments in cases that are in clear contrast to previously reported density-functional theory (DFT) results. When generalized to periodically repeated impurities, a geometry so commonly used in most DFT calculations, this criterion shows that the energy balance involved in such calculations contains unavoidable contributions from the long-ranged pairwise magnetic interactions between all impurities. This proves the fundamental inadequacy of the DFT assumption of independent unit cells in the case of magnetically doped low-dimensional graphene-based materials. We show that this can be circumvented if more than one impurity per unit cell is considered, in which case the DFT results agree perfectly well with the criterion-based predictions for the onset of localized magnetic moments in graphene. Furthermore, the existence of such a criterion determining whether or not a magnetic moment is likely to arise within graphene will be instrumental for predicting the ideal materials for future carbon-based spintronic applications.

Carbon in Si x Ge 1 − x : An ab initio investigation

The electronic and structural properties of substitutional and interstitial C impurities in Si, Ge, and Si x Ge 1 - x alloy have been investigated based upon ah initio total-energy calculations. For pure materials, we findthat the formation energy of substitutional C in Si is 0.81 eV lower than in Ge. For interstitial C, we find that the split(100) arrangement is energetically most favorable for both materials, Si and Ge. Similarly to substitutional C, the formation energy of interstitial C in Si is 0.74 eV lower than in Ge. By applying hydrostatic strain in the pure materials, we find an almost linear increase (decrease) of the formation energy for substitutional (interstitial) C with the lattice constant. In the Si x Ge 1 - x alloy, the C impurity properties were investigated for x = 0.50 and 0.85. We find that, for substitutional and interstitial C, the formation energies are lower for higher concentrations of Si at the nearest-neighbor sites of the impurity. Based on the calculated formation energies, we determined the equilibrium relative populations of Si and Ge atoms at the nearest-neighbor sites of C, as a function of the temperature and the alloy concentration.

MoS2 on an amorphous HfO2 surface: An ab initio investigation

The energetic stability, electronic and structural properties of MoS2 adsorbed on an amorphous a-HfO2 surface (MoS2/HfO2) are examined through ab initio theoretical investigations. Our total energy results indicate that the formation of MoS2/HfO2 is an exothermic process with an adsorption energy of 34 meV/A2, which means that it is more stable than similar systems like graphene/HfO2 and MoS2/SiO2. There are no chemical bonds at the MoS2-HfO2 interface. Upon formation of MoS2/HfO2, the electronic charge distribution is mostly localized at the interface region with no net charge transfer between the adsorbed MoS2 sheet and –HfO2 surface. However, the MoS2 sheet becomes n-type doped when there are oxygen vacancies in the HfO2 surface. Further investigation of the electronic distribution reveals that there are no electron- and hole-rich regions (electron-hole puddles) on the MoS2 sheet, which makes this system promising for use in high-speed nanoelectronic devices.

Spin pumping and interlayer exchange coupling through palladium

The magnetic behaviour of ultrathin ferromagnetic films deposited on substrates is strongly affected by the properties of the substrate. We investigate the spin pumping rate, interlayer exchange coupling and dynamic exchange coupling between ultrathin ferromagnetic films through palladium, a non-magnetic substrate that displays strong Stoner enhancement. We find that the interlayer exchange coupling, both in the static and dynamic versions, is qualitatively affected by the substrates Stoner enhancement. For instance, the oscillatory behavior that is a hallmark property of the RKKY exchange coupling is strongly suppressed by Stoner enhancement. Although the spin pumping rate of ferromagnetic films atop palladium is only mildly changed by Stoner enhancement the change is large enough to be detected experimentally. The qualitative aspects of our results for palladium are expected to remain valid for any non-magnetic substrate where Coulomb repulsion is large.

Finite-size correction scheme for supercell calculations in Dirac-point two-dimensional materials

Modern electronic structure calculations are predominantly implemented within the super cell representation in which unit cells are periodically arranged in space. Even in the case of non-crystalline materials, defect-embedded unit cells are commonly used to describe doped structures. However, this type of computation becomes prohibitively demanding when convergence rates are sufficiently slow and may require calculations with very large unit cells. Here we show that a hitherto unexplored feature displayed by several 2D materials may be used to achieve convergence in formation- and adsorption-energy calculations with relatively small unit-cell sizes. The generality of our method is illustrated with Density Functional Theory calculations for different 2D hosts doped with different impurities, all of which providing accuracy levels that would otherwise require enormously large unit cells. This approach provides an efficient route to calculating the physical properties of 2D systems in general but is particularly suitable for Dirac-point materials doped with impurities that break their sublattice symmetry.

Magnetism in wide band gap semiconductors implanted with non-magnetic ions

Single crystals of ZnO, TiO2 and LaAlO3 have been implanted with Ar with 100 keV and different fluencies. The Ar implanted crystals showed a week ferromagnetic-like signal between 10 K and 400 K. Hysteresis curves obtained at room temperature allowed confirming the ferromagnetic behaviour of the implanted samples. Spin polarised first principles density functional calculations were performed in the case of ZnO considering Zn interstitials and O vacancies. No net magnetic polarisation was found for O vacancies, but in the case of Zn vacancies a magnetic moment of 1μB was obtained.

Ab initio studies of the Si1−xGex alloy and its intrinsic defects

Abstract We have performed systematical ab initio studies of the electronic and structural properties of the disordered phase of the Si 1− x Ge x alloy. For the defect free alloy, we find that the Si–Si bond length has a tendency to maintain its natural value ( x =0) all the way up to x >0.5, similarly to what is found experimentally. We also analyze the vacancy, and note, as expected for a structurally as well as chemically disordered solid, that there is no local symmetry around the vacancy site. Finally, we perform some studies of the alloy under pressure all the way up to 30 GPa, and find that the structure is highly distorted in some cases, indicating that an amorphous phase may be found in the pressure region of the diamond–β-tin phase transition.