The impact of Hubbard and van der Waals corrections on the DFT calculation of epsilon-zeta transition pressure in solid oxygen.

In this study, we aim to clarify the physics that governs the unique properties of the transition between epsilon and zeta phases in solid oxygen observed at 96 GPa by using density functional theory (DFT) calculations. We first conduct the calculation using various functionals, namely, LDA, PBE, BLYP, and TPSS. The results show that LDA and TPSS predict the epsilon-zeta transition pressure at 30 GPa, while PBE and BLYP show the transition at 40 GPa. Then we include the van der Waals correction (either vdW functionals or semi-empirical methods) to improve the nonlocal effects in epsilon oxygen. The transition pressure is improved to 50 GPa. Finally, the Hubbard correction is added to enhance the localization and short-range interactions. The final epsilon-zeta transition pressure is significantly improved to 80 GPa. This demonstrates that the contribution from the local interaction is higher than the nonlocal London dispersion term at the metallization point. Moreover, this approach suggests that the van der Waals correction may correctly capture the nonlocal interaction in solid oxygen. The nonlocal effect is expected to be dominant below 20 GPa. A correct treatment of the local and nonlocal interactions on an equal footing is important to study solid oxygen.

Localized Graphitization on Diamond Surface as a Manifestation of Dopants.

Doped diamond electrodes have attracted significant attention for decades owing to their excellent physical and electrochemical properties. However, direct experimental observation of dopant effects on the diamond surface has not been available until now. Here, low-temperature scanning tunneling microscopy is utilized to investigate the atomic-scale morphology and electronic structures of (100)- and (111)-oriented boron-doped diamond (BDD) electrodes. Graphitized domains of a few nanometers are shown to manifest the effects of boron dopants on the BDD surface. Confirmed by first-principles calculations, local density of states measurements reveal that the electronic structure of these features is characterized by in-gap states induced by boron-related lattice deformation. The dopant-related graphitization is uniquely observed in BDD (111), which explains its electrochemical superiority over the (100) facet. These experimental observations provide atomic-scale information about the role of dopants in modulating the conductivity of diamond, as well as, possibly, other functional doped materials.

Electronic structure of dense solid oxygen from insulator to metal investigated with X-ray Raman scattering.

Electronic structures of dense solid oxygen have been investigated up to 140 GPa with oxygen K-edge X-ray Raman scattering spectroscopy with the help of ab initio calculations based on density functional theory with semilocal metageneralized gradient approximation and nonlocal van der Waals density functionals. The present study demonstrates that the transition energies (Pi*, Sigma*, and the continuum) increase with compression, and the slopes of the pressure dependences then change at 94 GPa. The change in the slopes indicates that the electronic structure changes at the metallic transition. The change in the Pi* and Sigma* bands implies metallic characteristics of dense solid oxygen not only in the crystal a-b plane but also parallel to the c axis. The pressure evolution of the spectra also changes at ∼40 GPa. The experimental results are qualitatively reproduced in the calculations, indicating that dense solid oxygen transforms from insulator to metal via the semimetallic transition.

First-principles calculations of the epsilon phase of solid oxygen.

The crystal, electronic and magnetic structures of solid oxygen in the epsilon phase have been investigated using the strongly constrained appropriately normed (SCAN) + rVV10 method and the generalized gradient approximation (GGA) + vdW-D + U method. The spin-polarized SCAN + rVV10 method with an 8-atom primitive unit cell provides lattice parameters consistent with the experimental results over the entire pressure range, including the epsilon-zeta structural phase transition at high pressure, but does not provide accurate values of the intermolecular distances d1 and d2 at low pressure. The agreement between the intermolecular distances and the experimental values is greatly improved when a 16-atom conventional unit cell is used. Therefore, the SCAN + rVV10 method with a 16-atom unit cell can be considered the most suitable model for the epsilon phase of solid oxygen. The spin-polarized SCAN + rVV10 model predicts a magnetic phase at low pressure. Since the lattice parameters of the predicted magnetic structure are consistent with the experimental lattice parameters measured at room temperature, our results may suggest that the epsilon phase is magnetic even at room temperature. The GGA + vdW-D + U (with an ad hoc value of Ueff = 2 eV at low pressure instead of the first-principles value of Ulreff ~ 9 eV) and hybrid functional methods provide similar results to the SCAN + rVV10 method; however, they do not provide reasonable values for the intermolecular distances.

Tunneling in Systems of Coupled Dopant-Atoms in Silicon Nano-devices.

Following the rapid development of the electronics industry and technology, it is expected that future electronic devices will operate based on functional units at the level of electrically active molecules or even atoms. One pathway to observe and characterize such fundamental operation is to focus on identifying isolated or coupled dopants in nanoscale silicon transistors, the building blocks of present electronics. Here, we review some of the recent progress in the research along this direction, with a focus on devices fabricated with simple and CMOS-compatible-processing technology. We present results from a scanning probe method (Kelvin probe force microscopy) which show direct observation of dopant-induced potential modulations. We also discuss tunneling transport behavior based on the analysis of low-temperature I-V characteristics for devices representative for different regimes of doping concentration, i.e., different inter-dopant coupling strengths. This overview outlines the present status of the field, opening also directions toward practical implementation of dopant-atom devices.

Transport spectroscopy of coupled donors in silicon nano-transistors.

The impact of dopant atoms in transistor functionality has significantly changed over the past few decades. In downscaled transistors, discrete dopants with uncontrolled positions and number induce fluctuations in device operation. On the other hand, by gaining access to tunneling through individual dopants, a new type of devices is developed: dopant-atom-based transistors. So far, most studies report transport through dopants randomly located in the channel. However, for practical applications, it is critical to control the location of the donors with simple techniques. Here, we fabricate silicon transistors with selectively nanoscale-doped channels using nano-lithography and thermal-diffusion doping processes. Coupled phosphorus donors form a quantum dot with the ground state split into a number of levels practically equal to the number of coupled donors, when the number of donors is small. Tunneling-transport spectroscopy reveals fine features which can be correlated with the different numbers of donors inside the quantum dot, as also suggested by first-principles simulation results.