G. C. Loh

Out-of-plane structural flexibility of phosphorene.

Phosphorene has been rediscovered recently, establishing itself as one of the most promising two-dimensional group-V elemental monolayers with direct band gap, high carrier mobility, and anisotropic electronic properties. In this paper, surface buckling and its effect on its electronic properties are investigated by using molecular dynamics simulations together with density functional theory calculations. We find that phosphorene shows superior structural flexibility along the armchair direction allowing it to have large curvatures. The semiconducting and direct band gap nature are retained with buckling along the armchair direction; the band gap decreases and transforms to an indirect band gap with buckling along the zigzag direction. The structural flexibility and electronic robustness along the armchair direction facilitate the fabrication of devices with complex shapes, such as folded phosphorene and phosphorene nano-scrolls, thereby offering new possibilities for the application of phosphorene in flexible electronics and optoelectronics.

A systematic study of the atmospheric pressure growth of large-area hexagonal crystalline boron nitride film

The growth of hexagonal boron nitride (h-BN) is of much interest owing to its outstanding properties and for scalable two dimensional (2D) electronics applications. Here, we report the controllable growth of h-BN on a copper substrate using the atmospheric pressure chemical vapor deposition (APCVD) method using ammonia borane as the precursor. The advantages of using APCVD include its ease of setup utilizing fewer resources, low cost and fast growth, all of which are essential for full film coverage and the mass production of 2D h-BN. In this study, we observed a substrate-position dependent evolution of h-BN domains at various stages of growth as the density and size of the domains increased downstream along the quartz tube. Other critical parameters such as growth temperature, deposition time, temperature and mass of precursor were also systemically investigated in order to understand the factors influencing the growth of the h-BN film. Importantly, with a slight increase in the growth temperature of 50 °C, we observe a significant (∼17-fold) increase in the average domain size, and its further expansion for a longer duration of growth. Likewise, our parametric study highlights the impact of other crucial parameters on domain size, coverage, and thickness of the h-BN film.

Direct growth of nanocrystalline hexagonal boron nitride films on dielectric substrates

Atomically thin hexagonal-boron nitride (h-BN) films are primarily synthesized through chemical vapor deposition (CVD) on various catalytic transition metal substrates. In this work, a single-step metal-catalyst-free approach to obtain few- to multi-layer nanocrystalline h-BN (NCBN) directly on amorphous SiO2/Si and quartz substrates is demonstrated. The as-grown thin films are continuous and smooth with no observable pinholes or wrinkles across the entire deposited substrate as inspected using optical and atomic force microscopy. The starting layers of NCBN orient itself parallel to the substrate, initiating the growth of the textured thin film. Formation of NCBN is due to the random and uncontrolled nucleation of h-BN on the dielectric substrate surface with no epitaxial relation, unlike on metal surfaces. The crystallite size is ∼25 nm as determined by Raman spectroscopy. Transmission electron microscopy shows that the NCBN formed sheets of multi-stacked layers with controllable thickness from ∼2 to 25...

Tuning the Kapitza resistance in pillared-graphene nanostructures

The pillared-graphene architecture is a conceivable way of conjoining graphene nanoribbons and carbon nanotubes (CNTs) in nanoelectronics. Especially promising is its capability to dissipate thermal energy in thermal management applications. However, the thermal boundary resistance (Kapitza resistance) at the graphene nanoribbon-CNT interface is a phonon barricade and a bottleneck for efficacious heat extraction. Parallel to strain studies on thermal conductance, this work is a first report on the effects of mechanical strain on the interfacial phonon dynamics in the pillared-graphene nanostructure (PGN). Molecular dynamics simulations are employed to derive the changes in phononics as axial, torsional, and compound strains of various degrees are applied on the PGN. The pillar lattice structure behaves dissimilarly to the different types of strains. In-plane transverse optical mode softening as induced by torsional strain is more effective than LO softening (triggered by tension) in minimizing the thermal...

A graphene–boron nitride lateral heterostructure – a first-principles study of its growth, electronic properties, and chemical topology

The lateral integration of graphene and hexagonal boron nitride permits the intricate design of a hybrid heterostructure in which electronic characteristics can be tuned as per the requirement of a particular application. Such laterally integrated hybrid nanostructures are investigated using density functional theory to explore their growth, electronic properties, and chemical topology. The evolution of energetics, geometry, density of states, number of charges, and electric dipole moment with carbon adatoms is delineated. In the triangular C–BN heterostructures, C atoms prefer to grow from the vertices as compared to the edges of the heterostructures due to their strong anchoring. Despite a small lattice mismatch between h-BN and graphene, the structural stability of the heterostructure depends on the number of C adatoms (i.e. stage of growth), and growth from the B-terminated BN flake appears to be preferred. The Cu substrate reduces the stability of the C–BN heterostructure. The heterostructures are metallic, suggesting that charge transfer effects from the Cu substrate play a dominant role in governing the electronic properties of the heterostructures. Electron localization and quantum theory of atoms in molecules illustrate the partial ionic–covalent character of the B–N bonds, which are less covalent than C–B/C–N bonds in the heterostructures. All in all, this study on the evolution of the characteristics of the C–BN heterostructures with growth is a vital step towards developing hybrid nanoelectronic circuitry with precisely controlled properties.

Atomic level understanding of site-specific interactions in Polyaniline/TiO2 composite

Spin-polarized density functional theory calculations have been performed to understand the interactions in polyaniline (PAni) and TiO2 composite at the atomic level. Binding energy calculation shows that composite structure is energetically more stable when Ti atom of TiO2 sits on top of PAni. It is also found that there is a dependency of the CBM on the site of TiO2 interaction in this composite system. The results suggest that optimization of the synthesis parameters at atomic level can be an effective way to improve the performance of a photovoltaic device based on PAni-TiO2 composite.

Molecular dynamic simulation of diamond/silicon interfacial thermal conductance.

Non-equilibrium molecular dynamic simulation was employed to investigate the interfacial thermal conductance between diamond and silicon substrate. The interfacial thermal conductance was computed based on Fouriers law. The simulation was done at different temperature ranges and results show that the interfacial thermal conductance between diamond-silicon is proportional to temperature and increases with temperature even above Debye temperature of silicon. Enhancement of thermal boundary conductance with temperature is attributed to inelastic phonon-phonon scattering at the interface. The system size dependence of interfacial thermal conductance was also examined. We found that thermal transport is a function of the system size when the size of system is smaller than the phonon mean free path and increases with the size of structure. We also simulated the effect of interface defect on phonon scattering and subsequently thermal conductance. The results also show that interface defect enhances acoustic pho...

Phononic and structural response to strain in wurtzite-gallium nitride nanowires

Gallium nitride (GaN) nanowires exist in a myriad of cross-sectional shapes. In this study, a series of classical molecular dynamics simulations is performed to investigate the strain-phononics-structure relationship in rectangular and triangular wurtzite-GaN nanowires. The thermal conductivity of the nanowires is linearly dependent on the uniaxial strain in both compressive and tensile regimes, and shows no significant dissimilitude for the same amount of strain exerted on the two types of nanowire. This is coherent with an analytical approach using the Boltzmann transport theory. However, the thermomechanical behaviour at the vertex regions shows palpable differences between the two subfamilies, relative to the non-vertex faceted regions, as the structural morphology is most disparate at the vertices. Furthermore, the degree of strain asymmetry is a strong determinant of the vibrational response and consequently thermal conductance.

Thermal rectification reversal in carbon nanotubes

In principle, rectifying phonon and electron flows appear similar, whereby more energy is transported in one direction than the opposite one. However, their physical mechanisms are inherently different. By using molecular dynamics simulations, this study reports on a few interesting aspects of thermal rectification in carbon nanotubes: (1) The dependence of the rectification ratio on the structural symmetry (represented by the position of vacancy clusters) of the nanotube and more importantly (2) a reversal in the rectifying direction as the normalized temperature difference of the heat baths is increased. The flux-mediated diffuse mismatch model is extended to explain the reversal phenomenon—initially with a simplifying assumption that the transmission coefficients at the vacancy/scatterer are identical in bidirectional phonon transport, and then with a moderating factor to distinguish between both coefficients. It is noted that in both cases, the conditions for thermal rectification reversal are attaina...

Spin-dependent metallic properties of a functionalized MoS2 monolayer

Stability and electronic properties of a two-dimensional MoS2 monolayer functionalized with atomic wires of Fe and Co are investigated using density functional theory. The binding energy of the atomic wires of Fe and Co on MoS2 is noticeably higher relative to that calculated for the BN (0001) surface. The pristine monolayer is non-magnetic and semiconducting, and its functionalization makes the system magnetic and metallic. This is due to mainly the presence of a finite density of states associated with Fe or Co atoms in the vicinity of the Fermi level of the functionalized monolayer. Additionally, the spin-polarized character of the functionalized monolayer is clearly captured by the tunneling current calculated in the STM-like setup. We believe that the results form a basis for fabrication and characterization of such functionalized two-dimensional systems for applications at the nanoscale.