arXiv subjects

Alan Dalton

Publications and source records attributed to Alan Dalton.

6 recordsLinked to original sources

arXiv2025

Nanoscale mechanics and ultralow Friction of natural 2D silicates: Biotite and Rhodonite

Two-dimensional (2D) silicates have emerged as a promising class of ultrathin materials, expanding the landscape of 2D systems beyond conventional van der Waals crystals. Their unique crystal chemistries and structural anisotropies make them attractive for applications ranging from sensors and flexoelectric devices to drug delivery and catalysis. To unlock their full potential, it is critical to understand their thickness-dependent mechanical properties within the family of 2D silicates. In this study, we investigate the nanomechanical and frictional behaviors of two structurally distinct natural silicates: layered Biotite and chain-structured Rhodonite. Using atomic force microscopy (AFM), we found that Rhodonite exhibits nearly ten times higher adhesion force and modulus response compared to Biotite. Despite this, Biotite demonstrates superior frictional performance, with ultrathin (5 nm) flakes showing a remarkably low coefficient of friction ($\sim 0.6 \times 10^{-3}$) versus Rhodonite ($\sim 3.6 \times 10^{-3}$). To further elucidate interlayer adhesion, density functional theory (DFT) calculations with Hubbard correction were employed. These findings offer valuable insights into the design and selection of 2D silicates for advanced mechanical and tribological applications.

cond-mat.mtrl-sci↗

arXiv2025

Sub-nm Curvature Unlocks Quantum Flexoelectricity in Graphene

Flexoelectricity, polarization induced by strain gradients, is especially pronounced in two-dimensional (2D) materials due to their mechanical flexibility and sensitivity to mechanical deformation. In nanostructures with sub-nm curvature, this effect is governed by quantum-mechanical polarization and electrostatic modulation, not merely classical lattice distortion. Here, we present the first direct experimental and theoretical demonstration of large intrinsic quantum flexoelectricity in graphene nanowrinkles, exhibiting polarization densities (P_{th} ~ 4 C/m^{2}, P_{exp} ~ 1 C/m^{2}) that exceed those of mesoscale systems by 5 to 7 orders of magnitude. These nanowrinkles, with sub-nm radii of curvature at their apex, undergo atomic-level buckling and result in localized strain fields, as confirmed by sub-micron Raman spectroscopy. These curvatures create an asymmetry to {\pi}-orbital interactions across the atomic layer, which, in turn, leads to localized polarization densities. Kelvin probe force microscopy reveals curvature-dependent work function shifts consistent with flexoelectric polarization, while conductive atomic force microscopy detects reproducible flexoelectric currents exhibiting a threshold voltage ({\Phi}th ~1 V) that matches the band offset predicted by ab initio calculations (~1.2 V). Together, these results confirm how flexoelectric dipoles reshape the local electronic potential. Graphene nanowrinkles thus provide a pristine platform for uncovering quantum-mechanical flexoelectricity: a fundamentally ubiquitous effect, whose study in the simplest crystalline material can illuminate electromechanical behavior across condensed matter, soft matter, and biological systems.

cond-mat.mes-hall↗

arXiv2023

Thickness dependent mechanical properties of soft ferromagnetic two-dimensional CoTe2

Two dimensional (2D) layered transition-metal-based tellurides (chalcogens) are known to harness their surface atoms characteristics to enhance topographical activities for energy conversion, storage, and magnetic applications. High surface energy due to unsaturated dangling bonds and larger lateral size than the thickness (volume) makes them a potential candidate for emerging electronics. Nevertheless, the gradual stacking of each sheet alters the surface atoms' subtle features, such as lattice expansion, leading to several phenomena and rendering tunable properties. In the present work, we have monitored thickness-dependent properties of the 2D CoTe2 sheets from nanoscale mechanics, tribology, surface potential distributions, interfacial interaction and magnetism using atomically resolved spectroscopy and different surface probe techniques, in conjunction with theoretical investigations: density functional theory (DFT) and molecular dynamics (MD). The variation in properties observed in theoretical investigation unleashes the crucial role of crystal planes of the CoTe2. The presented results are beneficial in expanding the use of 2D telluride family in flexible electronics, piezo sensors, tribo-generator, and next-generation memory devices.

cond-mat.mtrl-sci↗

arXiv2023

Unravelling the room temperature growth of two-dimensional h-BN nanosheets for multifunctional applications

Room temperature growth of two-dimensional van der Waals (2D-vdW) materials is indispensable for state-of-the-art nanotechnology. The low temperature growth supersedes the requirement of elevated growth temperature accompanied with high thermal budgets. Moreover, for electronic applications, low or room temperature growth reduces the possibility of intrinsic film-substrate interfacial thermal diffusion related deterioration of functional properties and consequent device performance. Here, we demonstrated the growth of ultrawide-bandgap boron nitride (BN) at room temperature by using the pulsed laser deposition (PLD) process and demonstrated various functionalities for potential applications. Comprehensive chemical, spectroscopic and microscopic characterization confirms the growth of ordered nanosheet-like hexagonal BN. Functionally, nanosheets show hydrophobicity, high lubricity (low coefficient of friction), low refractive index within the visible to near-infrared wavelength range, and room temperature single-photon quantum emission. Our work unveils an important step that brings a plethora of applications potential for room temperature grown h-BN nanosheets as it can be feasible on any given substrate, thus creating a scenario for h-BN on demand at frugal thermal budget.

physics.app-ph↗

arXiv2023

Unidirectional domain growth of hexagonal boron nitride thin films

Two-dimensional van der Waals (2D-vdW) layered hexagonal boron nitride (h-BN) has gained tremendous research interest over recent years due to its unconventional domain growth morphology, fascinating properties and application potentials as an excellent dielectric layer for 2D-based nano-electronics. However, the unidirectional domain growth of h-BN thin films directly on insulating substrates remains significantly challenging because of high-bonding anisotropicity and complex growth kinetics than the conventional thin films growth, thus resulting in the formation of randomly oriented domains morphology, and hindering its usefulness in integrated nano-devices. Here, ultra-wide bandgap h-BN thin films are grown directly on low-miscut atomically smooth highly insulating c-plane sapphire substrates (without using any metal catalytic layer) by pulsed laser deposition, showing remarkable unidirectional triangular-shape domains morphology. This unidirectional domain growth is attributed to the step-edge guided nucleation caused by reducing the film-substrate interfacial symmetry and energy, thereby breaking the degeneracy of nucleation sites of random domains, as revealed by the density functional theory (DFT) calculations. Through extensive characterizations, we further demonstrate the excellent single crystal-like functional properties of films. Our findings might pave the way for feasible large-area direct growth of electronic-quality h-BN thin films on insulating substrates for high-performance 2D-electronics, and in addition would be beneficial for hetero engineering of 2D-vdW materials with emergent phenomena.

cond-mat.mtrl-sci↗

arXiv2020

Tunable and saturated structural colors from carbon nanotubes incorporated highly ordered photonic opals

Synthetic opals, based on self-assembly of polymeric nanoparticles generally produces fainted/pale structural colors due to too many lattice flaws in the structures. Here we produces carbon nanotubes (CNTs) incorporated high quality 3D photonic opals (PC-CNT) by evaporative self-assembly. Although the CNTs make up only 0.01% of the fabricated photonic opal, their controlled incorporation has dramatic effect to change the color of the photonic crystals from milky white to intense red. Microscopic study suggest that CNT incorporation did not affect the lattice ordering of the photonic crystals. The tunability of structural colors, as a function of incident angle, were tested and varied against Bragg-Snell law. Furthermore, we tested mechanochromic sensing of the photonic opals, demonstrating their potential as visual indicators. This tunable PC-CNT brings many possibilities including strain sensing or structural health care monitoring, as well as being of fundamental interest.

physics.app-ph↗