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Register a new userExperimental Demonstration of an Electride as a 2D Material
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Because of their loosely bound electrons, electrides offer physical properties useful in chemical synthesis and electronics. For these applications and others, nano-sized electrides offer advantages, but to-date no electride has been synthesized as a nanomaterial. We demonstrate experimentally that Ca$_2$N, a layered electride in which layers of atoms are separated by layers of a 2D electron gas (2DEG), can be exfoliated into two-dimensional (2D) nanosheets using liquid exfoliation. The 2D flakes are stable in a nitrogen atmosphere or in select organic solvents for at least one month. Electron microscopy and elemental analysis reveal that the 2D flakes retain the crystal structure and stoichiometry of the parent 3D Ca$_2$N. In addition, the 2D flakes exhibit metallic character and an optical response that agrees with DFT calculations. Together these findings suggest that the 2DEG is preserved in the 2D material. With this work, we bring electrides into the nano-regime and experimentally demonstrate a 2D electride, Ca$_2$N.
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Raman spectroscopy is one of the most extended experimental techniques to investigate thin-layered 2D materials. For a complete understanding and modeling of the Raman spectrum of a novel 2D material, it is often necessary to combine the experimental investigation to density-functional-theory calculations. We provide the experimental proof of the fundamentally different behavior of polar 2D vs 3D systems regarding the effect of the dipole-dipole interactions, which in 2D systems ultimately lead to the absence of optical phonons splitting, otherwise present in 3D materials. We demonstrate that non-analytical corrections (NACs) should not be applied to properly model the Raman spectra of few-layered 2D materials, such as WSe$_{2}$ and h-BN, corroborating recent theoretical predictions [Nano Lett. 2017, 17 (6), 3758-3763]. Our findings are supported by measurements performed on tilted samples that allow increasing the component of photon momenta in the plane of the flake, thus unambiguously setting the direction of an eventual NAC. We also investigate the influence of the parity of the number of layers and of the type of layer-by-layer stacking on the effect of NACs on the Raman spectra.
The control on the distribution of magnetic ions into a semiconducting host is crucial for the functionality of magnetically doped semiconductors. Through a structural analysis at the nanoscale, we give experimental evidence that the aggregation of Fe ions in (Ga,Fe)N and consequently the magnetic response of the material are affected by growth rate and co-doping with shallow impurities.
We report the first experimental demonstration of room-temperature spin transport in n-type Ge epilayers grown on a Si(001) substrate. By utilizing spin pumping under ferromagnetic resonance, which inherently endows a spin battery function for semiconductors connected with the ferromagnet, a pure spin current is generated in the n-Ge at room temperature. The pure spin current is detected by using the inverse spin Hall effect of either Pt or Pd electrode on the n-Ge. A theoretical model including a geometrical contribution allows to estimate a spin diffusion length in n-Ge at room temperature to be 660 nm. The temperature dependence of the spin relaxation time provides evidence for Elliott-Yafet spin relaxation mechanism.
We propose a new 2D semiconductor material (TTA-2D) based on the molecular structure of Thiophene-Tetrathia-Annulene (TTA). The TTA-2D structural, electronic, and optical properties were investigated using textit{ab initio} methods. Our results show that TTA-2D is a small indirect bandgap semiconductor ($0.6$ eV). A semiconductor-metal transition can be induced by applying a uniaxial strain. Our results also show that TTA-2D is thermally stable up to $T=1000$ K. TTA-2D absorbs in a large spectral range, from infrared to ultraviolet regions. Values of refractive index and reflectivity show that TTA-2D reflects only $10%$ of the incident light in the visible region. These results suggest that TTA-2D is a promising material for solar cell applications.
The band structure, optical and defects properties of Ba_{2}TeO are systematically investigated using density functional theory with a view to understanding its potential as an optoelectronic or trans- parent conducting material. Ba_{2}TeO crystallizes with tetragonal structure (space group P4/nmm) and with a 2.93 eV optical band gap 1 . We find relatively modest band masses for both electrons and holes suggesting applications. Optical properties show a infrared-red absorption when doped. This could potentially be useful for combining wavelength filtering and transparent conducting functions. Furthermore, our defect calculations show that Ba_{2}TeO is intrinsically p-type conducting under Ba-poor condition. However, the spontaneous formation of the donor defects may constrain the p-type transport properties and would need to be addressed to enable applications.
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