Daniel Valencia

Control of interlayer physics in 2H transition metal dichalcogenides

It is assessed in detail both experimentally and theoretically how the interlayer coupling of transition metal dichalcogenides controls the electronic properties of the respective devices. Gated transition metal dichalcogenide structures show electrons and holes to either localize in individual monolayers, or delocalize beyond multiple layers—depending on the balance between spin-orbit interaction and interlayer hopping. This balance depends on the layer thickness, momentum space symmetry points, and applied gate fields. The design range of this balance, the effective Fermi levels, and all relevant effective masses is analyzed in great detail. A good quantitative agreement of predictions and measurements of the quantum confined Stark effect in gated MoS2 systems unveils intralayer excitons as the major source for the observed photoluminescence.

Electronic structures and properties of lanthanide hexaboride nanowires

The promising usage of lanthanide hexaboride nanowires as excellent electron emitter materials is generally attributed to the intrinsic low work functions of their bulk counterparts. Most analytical models for the field enhanced electron emission phenomenon adopt an underlying presumption of little or no change to the work function of the emission materials at the nanoscale. However, such a presumption is difficult to experimentally verify because current analytical models often employ empirical parameters such as the geometrically enhancement factors and the actual field emission areas are hard to determine. Herein, we report our density functional theory study of the size-dependence and element-specificity of the electronic structures and work functions of infinitely long lanthanide hexaboride nanowires constructed with n × n × ∞ unit cells (n = 1, 2, 3, and 4). Our modeling results reveal that the distinguished metal-like electronic properties and the low work function values of the sides of most exami...

NEMO5: Predicting MoS 2 heterojunctions

Molybdenum disulfide (MoS2) is a promising 2D material since it has a finite band gap, and its electronic band structure depends on the layer thickness. The tunability of the gate voltage on band alignment of different MoS2 layers is analyzed. For this purpose, the multipurpose nanodevice simulation tool NEMO5 was altered by several new features: electronic bandstructure calculations in maximally localized Wannier function (MLWF) representation and self-consistent charge calculations with subatomic electrostatic resolution.

Grain boundary resistance in nanoscale copper interconnections

As logic devices continue to downscale, interconnections are reaching the nanoscale where quantum effects are important. In this work we introduce a semi-empirical method to describe the resistance of copper interconnections of the sizes predicted by ITRS roadmap. The resistance calculated by our method was benchmarked against DFT for single grain boundaries. We describe a computationally efficient method that matches DFT benchmarks within a few percent. The 1000x speed up compared to DFT allows us to describe grain boundaries with a 30 nm channel length that are too large to be simulated by ab-initio methods. The electrical resistance of these grain boundaries has a probability density distribution as a function of the grain rotation angles. This approach allows us to quantitatively obtain the most likely resistance for each configuration.

Engineering the optical transitions of self-assembled quantum dots

In this paper, we report a fast effective mass model for accurately calculating the bound states and optical transitions of self-assembled quantum dots. The model includes the atomistic strain effects, namely, the strain deformation of the band edges, and strain modification of the effective masses. The explicit inclusion of strain effects in the picture has significantly improved the effective mass model results. For strain calculations, we have found that atomistic strain depends solely on the aspect ratio of the quantum dot, and it has been calculated and reported here for a wide range of quantum dot aspect ratios. Following this sole dependence on the aspect ratio; The deformation theory has been used to include the strain deformation of the band edges. Density function theory has been used to study the effect of strain on the electron and hole effective masses. The proposed effective mass model have an accuracy that is close to full atomistic simulation but with no computational cost.