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We present a universal method for the large-scale prediction of the atomic structure of clusters. Our algorithm performs the joint evolutionary search for all clusters in a given area of the compositional space and takes advantage of structural similarities frequently observed in clusters of close compositions. The resulting speedup is up to 50 times compared to current methods. This enables the first-principles studies of multi-component clusters with full coverage of a wide range of compositions. As an example, we report an unprecedented first-principles global optimization of 315 SinOm clusters with n<=15 and m<=20. The obtained map of Si-O cluster stability shows the existence of both expected (SiO2)n and unexpected (e.g. Si4O18) stable ("magic") clusters, which can be important for miscellaneous applications.

In the nitrogen-incorporated ultrananocrystalline diamond ((N)UNCD) films, representing an $n$-type highly conductive two-phase material comprised of $sp^3$ diamond grains and $sp^2$-rich graphitic grain boundaries, the current is carried by a high concentration of mobile electrons within the large-volume grain boundary networks. Fabricated in a simple thin-film planar form, (N)UNCD was found to be an efficient field emitter capable of emitting a significant amount of charge starting at the applied electric field as low as a few V/$\mu$m which makes it a promising material for designing electron sources. Despite the semimetallic conduction, field emission (FE) characteristics of this material demonstrate a strong deviation from the Fowler-Nordheim law in a high-current-density regime when (N)UNCD field emitters switch from a diode-like to resistor-like behavior. Such phenomenon resembles the current-density saturation effect in conventional semiconductors. In the present paper, we adapt the formalism developed for conventional semiconductors to study current-density saturation in (N)UNCD field emitters. We provide a comprehensive theoretical investigation of ($i$) the influence of partial penetration of the electric field into the material, ($ii$) transport effects (such as electric-field-dependent mobility), and ($iii$) features of a complex density-of-states structure (position and shape of $\pi-\pi^*$ bands, controlling the concentration of charge carriers) on the FE characteristics of (N)UNCD. We show that the formation of the current-density saturation plateau can be explained by the limited supply of electrons within the impurity $\pi-\pi^*$ bands and decreasing electron mobility in high electric field. Theoretical calculations are consistent with experiment., Comment: 15 pages, 14 figures

The present letter extends the prior findings on self-induced heating of solid state field emission devices. It was found that a vacuum diode (base pressure $\sim10^{-9}$ Torr), that makes use of graphite-rich polycrystalline diamond as cathode material, can switch from diode regime to resistor regime, to glow discharge plasma regime without any external perturbation, i.e. all transitions are self-induced. Combined results of in situ field emission microscopy and ex situ electron microscopy and Raman spectroscopy suggested that the nanodiamond cathode of the diode heated to about 3000 K which caused self-induced material evaporation, ionization and eventually micro-plasma formation. Our results confirm that field emission, commonly called cold emission, is a very complex phenomenon that can cause severe thermal load. Thermal load and material runaway could be the major factors causing vacuum diode deterioration, i.e. progressive increase in turn-on field, decrease in field enhancement factor, and eventual failure., Comment: 6 pages, 5 figures

It has been known for a long time that traditional semiconductor (e.g. intrinsic and doped Si and Ge or binary SiC and GaN) field emitters significantly deviate from Fowler-Nordheim (FN) law and saturate when a large current, on the order of microamperes or more, is attempted to be drawn from them. Many experiments established that the field emission current from carbonic materials, such as carbon nanotubes, amorphous carbon and polycrystalline diamond films, also deviate from FN law and saturate. These findings suggested that the saturation and departure from FN law is a broad and general phenomenon that applies to the class of nonmetallic field emitters. In this letter, we report a universal formula that describes the current saturation effect in nonmetallic field emitters. The formula accounts for material's bulk properties and field emitter geometry., Comment: 5 pages, 2 figures, 1 table

In this paper we study the effect of actual, locally resolved, field emission (FE) area on electron emission characteristics of uniform semimetallic nitrogen-incorporated ultrananocrystalline diamond ((N)UNCD) field emitters. To obtain the actual FE area, imaging experiments were carried out in a vacuum system in a parallel-plate configuration with a specialty anode phosphor screen. Electron emission micrographs were taken concurrently with $I$-$V$ characteristics measurements. It was found that in uniform (N)UNCD films the field emitting site distribution is not uniform across the surface, and that the actual FE area depends on the applied electric field. To quantify the actual FE area dependence on the applied electric field, a novel automated image processing algorithm was developed. The algorithm processes extensive imaging datasets and calculates emission area per image. By doing so, it was determined that the emitting area was always significantly smaller than the FE cathode surface area of 0.152 cm$^2$ available. Namely, the actual FE area would change from $5\times10^{-3}$ \% to 1.5 \% of the total cathode area with the applied electric field increased. Finally and most importantly, it was shown that when $I$-$E$ curves as measured in the experiment were normalized by the field-dependent emission area, the resulting $j$-$E$ curves demonstrated a strong kink and significant deviation from Fowler-Nordheim (FN) law, and eventually saturated at a current density of $\sim$100 mA/cm$^2$. This value was nearly identical for all studied (N)UNCD films, regardless of the substrate.

The letter introduces a diagram that rationalizes tunneling atomic force microscopy (TUNA) observations of electron emission from polycrystalline diamonds as described in recent publications. The direct observations of electron emission from grain boundary sites by TUNA could indeed be evidence of electrons originating from grain boundaries under external electric fields. At the same time, from the diagram it follows that TUNA and field emission schemes are complimentary rather than equivalent for results interpretation. It is further proposed that TUNA could provide better insights into emission mechanisms by measuring the detailed structure of the potential barrier on the surface of polycrystalline diamonds., Comment: 3 figure, 3 pages

Polarized negative muons were used to study the behaviour of the boron acceptor centre in synthetic diamond produced by the chemical vapour deposition (CVD) method. The negative muon substitutes one of the electrons in a carbon atom, and this muonic atom imitates the boron acceptor impurity in diamond. The temperature dependence of the muon spin relaxation rate and spin precession frequency were measured in the range of 20 - 330 K in a transverse magnetic field of 14 kOe. For the first time a negative shift of the muon spin precession was observed in diamond. It is tentatively attributed to an anisotropic hyperfine interaction in the boron acceptor. The magnetic measurements showed that the magnetic susceptibility of the CVD sample was close to that of the purest natural diamond., Comment: 8 pages, 3 figures