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Electrostatic interactions between the conducting tip of a scanning probe microscope and a flat conductor coated with a thin or thick dielectric layer are treated analytically and numerically. Exact and compact approximate expressions for the capacitance, force, force gradient, electric field profiles, and their effective widths are derived for a spherical model tip by generalizing known solutions for the conducting sphere and sample problem. These expressions allow convenient modeling of various measurements involving voltage-biased probes, estimation of lateral resolution, and prediction of trends as a function of relevant parameters. [ABSTRACT FROM AUTHOR]

We present a systematic study on hydrogenated silicon m vacancies above saturation. For each system a global geometry optimization search for low-lying local minima is performed using a newly developed SiH tight binding model. Subsequently a large number of low-energy structures are examined by density functional calculations using a minimal basis set. Finally the energetically favorable structures are reexamined using a systematically extendable basis set with local, semilocal, and hybrid exchange-correlation functionals. Particular attention is paid to the divacancy to which the Raman peak at 3822 cm-1 associated with the H2 molecule had previously been assigned. Both the energetics and vibrational analysis of divacancy-related stable configurations suggest a revision of the above conclusion. [ABSTRACT FROM AUTHOR]

We present unexpected structural motifs for boron-carbon nanocages of the stoichiometries B12C48 and B12C50, based on first-principles calculations. These configurations are distinct from those proposed so far because the boron atoms are not isolated and distributed over the entire surface of the cages, but rather aggregate at one location to form a patch. Our putative ground state of B12C48 is 1.8 eV lower in energy than the previously proposed ground state and violates all the suggested empirical rules for constructing low-energy fullerenes. The B12C50 configuration is energetically even more favorable than B12C48, showing that structures derived from the C60 buckminsterfullerene are not necessarily magic sizes for heterofullerenes. [ABSTRACT FROM AUTHOR]

We study the thermodynamic stability at low temperatures of a series of alkali-metal-zinc double-cation borohydrides, including LiZn(BH4)3, LiZn2(BH4)5, NaZn(BH4)3, NaZn2(BH4)5, KZn(BH4)3, and KZn2(BH4)5. While LiZn2(BH4)5, NaZn(BH4)3, NaZn2(BH4)5, and KZn(BH4)3 were recently synthesized, LiZn(BH4)3 and KZn2(BH4)5 are hypothetical compounds. Using the minima-hopping method, we discover two new lowest-energy structures for NaZn(BH4)3 and KZn2(BH4)5 which belong to the C2/c and P2 space groups, respectively. These structures are predicted to be both thermodynamically stable and dynamically stable, implying that their existence may be possible. On the other hand, we predict that the lowest-energy P1 structure of LiZn(BH4)3 is unstable, suggesting a possible reason elucidating why this compound has not been experimentally identified. In exploring the low-energy structures of these compounds, we find that their energetic ordering is sensitive to the inclusion of the van der Waals interactions. We also find that a proper treatment of these interactions, e.g., as given by a nonlocal density functional such as vdW-DF2, is necessary to address the stability of the low-energy structures of these compounds. [ABSTRACT FROM AUTHOR]

We performed a systematic structural search of high-pressure carbon allotropes for unit cells containing from 6 to 24 atoms using the minima hopping method. We discovered a series of new structures that are consistently lower in enthalpy than the ones previously reported. Most of these include (5+7)- or (4+8)-membered rings and can therefore be placed in the families proposed by H. Niu et al. [ Phys. Rev. Lett. 108 135501 (2012)]. However, we also found three more families with competitive enthalpies that contain (5+5+8)-membered rings, sp2 motives, or buckled hexagons. These structures are likely to play an important role in dislocation planes and structural defects of diamond and hexagonal diamond. [ABSTRACT FROM AUTHOR]

We present a systematic study of the low-energy structures of zinc borohydride, a crystalline material proposed for the purpose of hydrogen storage. In addition to previously proposed structures, many new low-energy structures of zinc borohydride are found by utilizing the minima-hopping method. We identify a new dynamically stable structure which belongs to the I4122 space group as the lowest-energy phase of zinc borohydride at low temperatures. A low transition barrier between I4122 and P1, the two lowest-lying phases of zinc borohydride, is predicted, implying that a coexistence of low-energy phases of zinc borohydride is possible at ambient conditions. An analysis based on the simulated x-ray-diffraction pattern reveals that the I4122 structure exhibits the same major features as the experimentally synthesized zinc borohydnde samples. [ABSTRACT FROM AUTHOR]

We present state-of-the-art first-principles calculations of the electronic and optical properties of silicon allotropes with interesting characteristics for applications in thin-film solar cells. These new phases consist of distorted sp3 silicon networks and have a lower formation energy than other experimentally produced silicon phases. Some of these structures turned out to have quasidirect and dipole-allowed band gaps in the range 0.8-1.5 eV, and to display absorption coefficients comparable with those of chalcopyrites used in thin-film record solar cells. [ABSTRACT FROM AUTHOR]

Raman spectroscopy is a powerful tool to study the intrinsic vibrational characteristics of crystals, and, therefore, it is an adequate technique to explore phase transitions of carbon under pressure. However, the diamond-anvil cell, which is used in experiments to apply pressure, appears as a broad intense feature in the spectra. This feature lies, unfortunately, in the same range as the principal modes of recently proposed sp3 carbon structures. As these modes are hard to distinguish from the diamond cell background, we analyze all Raman-active modes present in the sp3 carbon structures in order to find detectable fingerprint features for an experimental identification. [ABSTRACT FROM AUTHOR]

We show by means of first-principles calculations that in boron nanostructures a large variety of two-dimensional structures can be obtained, all with similar energetic properties. Some of these new structures are more stable than both the B80 fullerenes initially proposed by Szwacki et al. [ Phys. Rev. Lett. 98 166804 (2007) and boron nanotubes. At variance from other systems like carbon, disordered configurations are energetically comparable with ordered ones. Cage-like structures that are not ordered are thus comparable in energy to the more ordered original B80 fullerene. A comparison with other more disordered structures like bulk-like boron clusters is also presented. We found that in the presence of other seed structures (like Sc3 or Sc3N), some endohedral cage-like structures are energetically preferred over bulk-like clusters. This result opens a new pathway for the synthesis of the B80 fullerene as an endohedral fullerene as was done in the case of the C80 fullerene. [ABSTRACT FROM AUTHOR]

Based on an analysis of the short-range chemical environment of each atom in a system, standard machine-learning-based approaches to the construction of interatomic potentials aim at determining directly the central quantity, which is the total energy. This prevents, for instance, an accurate description of the energetics of systems in which long-range charge transfer or ionization is important. We propose therefore not to target directly with machine-learning methods the total energy but an intermediate physical quantity, namely, the charge density, which then in turn allows us to determine the total energy. By allowing the electronic charge to distribute itself in an optimal way over the system, we can describe not only neutral but also ionized systems with unprecedented accuracy. We demonstrate the power of our approach for both neutral and ionized NaCl clusters where charge redistribution plays a decisive role for the energetics. We are able to obtain chemical accuracy, i.e., errors of less than a millihartree per atom compared to the reference density functional results for a huge data set of configurations with large structural variety. The introduction of physically motivated quantities which are determined by the short-range atomic environment via a neural network also leads to an increased stability of the machine-learning process and transferability of the potential. [ABSTRACT FROM AUTHOR]

Silicon materials play a key role in many technologically relevant fields, ranging from the electronic to the photovoltaic industry. A systematic search for silicon allotropes was performed by employing a modified ab initio minima hopping crystal structure prediction method. The algorithm was optimized to specifically investigate the hitherto barely explored low-density regime of the silicon phase diagram by imitating the guest-host concept of clathrate compounds. In total, 44 metastable phases are presented, of which 11 exhibit direct or quasidirect band gaps in the range of 1.0-1.8 eV, close to the optimal Shockley-Queisser limit of 1.4 eV, with a stronger overlap of the absorption spectra with the solar spectrum compared to conventional diamond silicon. Due to the structural resemblance to known clathrate compounds it is expected that the predicted phases can be synthesized. [ABSTRACT FROM AUTHOR]

The distance dependence and atomic-scale contrast recently observed in nominal contact potential difference (CPD) signals simultaneously recorded by Kelvin probe force microscopy (KPFM) using noncontact atomic force microscopy (NCAFM) on defect-free surfaces of insulating as well as semiconducting samples have stimulated theoretical attempts to explain such effects. Especially in the case of insulators, it is not quite clear how the applied bias voltage affects electrostatic forces acting on the atomic scale. We attack this problem in two steps. First, the electrostatics of the macroscopic tip-cantilever-sample system is treated by a finite-difference method on an adjustable nonuniform mesh. Then the resulting electric field under the tip apex is inserted into a series of atomistic wavelet-based density functional theory (DFT) calculations. Results are shown for a realistic neutral but reactive silicon nanoscale tip interacting with a NaCl(001) sample. Bias-dependent forces and resulting atomic displacements are computed to within an unprecedented accuracy. Theoretical expressions for amplitude modulation (AM) and frequency modulation (FM) KPFM signals and for the corresponding local contact potential differences (LCPD) are obtained by combining the macroscopic and atomistic contributions to the electrostatic force component generated at the voltage modulation frequency, and evaluated for several tip oscillation amplitudes A up to 10 nm. For A = 0.1 Å, the computed LCPD contrast is proportional to the slope of the atomistic force versus bias in the AM mode and to its derivative with respect to the tip-sample separation in the FM mode. Being essentially constant over a few volts, this slope is the basic quantity that determines variations of the atomic-scale LCPD contrast. Already above A = 1 Å, the LCPD contrasts in both modes exhibit almost the same spatial dependence as the slope. In the AM mode, this contrast is approximately proportional to A-1/2, but remains much weaker than the contrast in the FM mode, which drops somewhat faster as A is increased. These trends are a consequence of the macroscopic contributions to the KPFM signal, which are stronger in the AM-mode and especially important if the sample is an insulator even at subnanometer separations where atomic-scale contrast appears. [ABSTRACT FROM AUTHOR]