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For many 2D materials, optical and Raman spectra are richly structured, and convey information on a range of parameters including nanosheet size and defect content. By contrast, the equivalent spectra for h-BN are relatively simple, with both the absorption and Raman spectra consisting of a single feature each, disclosing relatively little information.Here, the ability to size-select liquid-exfoliated h-BN nanosheets has allowed us to comprehensively study the dependence of h-BN optical spectra on nanosheet dimensions. We find the optical extinction coefficient spectrum to vary systematically with nanosheet lateral size due to the presence of light scattering. Conversely, once light scattering has been decoupled to give the optical absorbance spectra, we find the size dependence to be mostly removed save for a weak but well-defined variation in energy of peak absorbance with nanosheet thickness. This finding is corroborated by our ab initio GW and Bethe Salpeter equation calculations, which include electron correlations and quasiparticle self-consistency (QSGW). In addition, while we find the position of the sole h-BN Raman line to be invariant with nanosheet dimensions, the linewidth appears to vary weakly with nanosheet thickness.These size-dependent spectroscopic properties can be used as metrics to estimate nanosheet thickness from spectroscopic data.

M. (2018). Effect of ladder diagrams on optical absorption spectra in a quasiparticle self-consistent GW framework. Physical Review Materials, 2(3), [034603] We present an approach to calculate the optical absorption spectra that combines the quasiparticle selfconsistent GW method [Phys. Rev. B, 76 165106 (2007)] for the electronic structure with the solution of the ladder approximation to the Bethe-Salpeter equation for the macroscopic dielectric function. The solution of the Bethe-Salpeter equation has been implemented within an all-electron framework, using a linear muffin-tin orbital basis set, with the contribution from the non-local self-energy to the transition dipole moments (in the optical limit) evaluated explicitly. This approach addresses those systems whose electronic structure is poorly described within the standard perturbative GW approaches with as a starting point density-functional theory calculations. The merits of this approach have been exemplified by calculating optical absorption spectra of a strongly correlated transition metal oxide, NiO, and a narrow gap semiconductor, Ge. In both cases, the calculated spectrum is in good agreement with the experiment. It is also shown that for systems whose electronic structure is well-described within the standard perturbative GW , such as Si, LiF and h-BN, the performance of the present approach is in general comparable to the standard GW plus Bethe-Salpeter equation. It is argued that both vertex corrections to the electronic screening and the electron-phonon interaction are responsible for the observed systematic overestimation of the fundamental bandgap and spectrum onset.

The heteroepitaxial growth of pure Ge films on (100) Si by an ultrahigh vacuum, chemical vapor deposition technique is reported for the first time. The growth mode is found to be critically dependent on the substrate temperature during deposition. Two temperature regimes for growth are observed. Between 300 and 375 °C, growth occurs in a two-dimensional, layer-by-layer mode, with an activation energy of 1.46 eV. Above 375 °C, island formation is observed. In the low-temperature regime the growth rate is controlled by a surface decomposition reaction, whereas in the high-temperature regime the growth rate is controlled by diffusion and adsorption from the gas phase.

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