Stacking-Fault Based Microscopic Model for Platelets in Diamond

Abstract

We propose a new microscopic model for the {001} planar defects in diamond commonly called platelets. This model is based on the formation of a metastable stacking fault, which can occur because of the ability of carbon to stabilize in different bonding configurations. In our model the core of the planar defect is basically a double layer of three-fold coordinated sp 2 carbon atoms embedded in the common sp 3 diamond structure. The properties of the model were determined using ab initio total energy calculations. All significant experimental signatures attributed to the platelets, namely, the lattice displacement along the [001] direction, the asymmetry between the [110] and the [110] directions, the infrared absorption peak B ′ , and broad luminescence lines that indicate the introduction of levels in the band gap, are naturally accounted for in our model. The model is also very appealing from the point of view of kinetics, since naturally occurring shearing processes will lead to the formation of the metastable fault.PACS numbers: 61.72. Nn, 61.72.Bb, 63.20.Pw, 71.15.Nc, 71.55.Cn Perhaps one of the oldest unanswered questions in diamond physics concerns the nature and atomic structure of the extended defects known as platelets. These {001} planar defects were first discovered more than sixty years ago [1] by X-ray diffraction experiments, which observed anomalous peaks corresponding to {00h} reflections. These were immediately associated with lattice defects, since they are forbidden by symmetry in a perfect diamond lattice. From then on, a plethora of experimental data on this planar defect has been gathered, but a complete understanding of its origin and microscopic structure is still lacking. The current knowledge on this defect can be summarized as follows: i) platelets have been detected only in type Ia diamonds [2]; ii) transmission electron microscopy (TEM) experiments have shown that there is an asymmetry between the [110] and [110] directions [3]; iii) TEM experiments have also determined that platelets displace the crystalline lattice by approximately 0.4a 0 [4], along the [001] direction, where a 0 is the lattice parameter of diamond; iv) electron energy loss spectroscopy (EELS) experiments indicate that the nitrogen content in platelets can vary from 6% to 61% of a monolayer [5,6,7]; v) an infrared absorption line around 1370 cm −1 is always present in samples containing platelets [8]; vi) platelets have been associated with broad luminescence bands, one centered at 1.25 eV [9], which reduces the efficiency of optical windows made of natural diamond, and another at 2.14 eV [10], and possibly with high energy absorption and luminescence bands around 4.6 and 4.4 eV, respectively [11].In the microscopic model first proposed for platelets in diamond, Frank [12] considered that the defect should be formed by Si impurities replacing part of the carbon atoms in a (001) plane. However, it was later shown that Si is an uncommon impurity in diamond. Subsequently, it was demonstrated that diamonds type I...

Imaging the atomic structure and local chemistry of platelets in natural type Ia diamond

“…Earlier attempts concentrated on the use of high resolution TEM imaging but direct interpretation was not possible due to inherent difficulties in the interpretation of high resolution phase contrast of defects in thick samples 10 . In light of this several structure models have been developed to describe the platelets 10,11,12,13 . Figures 2 (a) -(d) show the prevalent atomic structure models viewed along the [110]p and [1-10]p directions.…”

Progress in the understanding of neutrino properties

“…Many effects may distort the "celestial" neutrino fluxes reaching the detectors, and hence affect the determination of oscillation parameters. In this connection several regular [7][8][9] and random [10,11] convective zone solar magnetic field models have been considered. These fields would induce neutrino spin-flavor precession [12][13][14] and hence modulate the originally produced standard solar model neutrino fluxes.…”

“…Reactor neutrino data from KamLAND have played a crucial role in probing the robustness of solar oscillations against astrophysical uncertainties, such as radiative [18,19] or convective zone [7][8][9] magnetic fields. The stringent limits on solar anti-neutrinos from KamLAND lead to strong limits on neutrino magnetic transition moments, especially in the case of turbulent fields [10,11]. The result is that oscillations constitute the only viable explanation of the data, and of the oscillation solutions allowed by solar data [20], only the large mixing angle solution is consistent with the spectrum measurements at KamLAND [21].…”

Humble planar defects in SiGe nanopillars

“…Many structural models have been proposed in order to find a match with experiments. Amongst many other models [5,6], Humble proposed a planar defect that can be regarded as resulting from the insertion of an entire layer of four-fold coordinated interstitial carbon atoms [7]. Humble defects in Si have also attracted attention.…”

“…We conclude that the symmetry of the original Humble defect (model A) applies to the one reported here in SiGe, and that atomic coordinates of the DFT relaxed structure match well with the experimental results. This defect is equivalent in its [110] and [1][2][3][4][5][6][7][8][9][10] views, both of which are observed in the HAADF image in Fig. 4, where the upper and lower layers of the defect core alternate in the a-b plane.…”