arXiv subjects

Laurence D. Marks

Publications and source records attributed to Laurence D. Marks.

14 recordsLinked to original sources

arXiv2025

Is Triboelectricity Confusing, Confused or Complex?

In this report, we look at the fundamental physics of triboelectricity, charge transfer due to contact and sliding. While much of the report focuses upon recent advances such as the incorporation of flexoelectric contributions, we also include older work, some from centuries ago, which only now can be understood in a general sense. Basic concepts and theories ranging from elements of tribology and contact mechanics through semiconductor built-in potentials, electromechanical terms, mechanochemistry and trap states are briefly described, linking to established surface science and interface physics. We then overview the main models that have been proposed, showing that they all fall within conventional electrostatics combined with other established science. We conclude with some suggestions for the future. Based upon this overview, our conclusion is that triboelectricity is a slightly complex combination of standard electrostatic phenomena that can be understood using the generalized Ampere's law connecting the electric displacement field with both Coulomb and polarization contributions, and the free carrier density, that is Grad.D=rho. Triboelectricity may be confusing, it is not really confused if care is taken, but it is complex.

cond-mat.mes-hall↗

arXiv2024

A Quantitative Model of Triboelectric Charge Transfer

Triboelectricity, when rubbing or contacting materials causes electric charge transfer, is ubiquitous across many fields, and has been studied in detail for centuries. Despite this, a complete description of triboelectricity remains elusive. Here, we analyze the contact between a metal asperity and a semiconductor, including contributions from the depletion zone of the semiconductor and from flexoelectric polarization that arises due to the strain gradients at asperity contacts. The free charges involved in charge transfer are then discussed and calculated. As a result, we develop a quantitative model for triboelectric charge transfer that details how charge transfer scales with contact parameters, the relative influence of depletion and flexoelectricity, and which agrees with various trends in multiple classes of triboelectric experiments.

cond-mat.mtrl-sci↗

arXiv2022

Asperity Shape and Gradient Elasticity in Flexoelectric/Triboelectric Contacts

The underlying mechanisms responsible for triboelectricity have yet to be completely understood. We have previously proposed a model which explains charge transfer in non-metals via band bending due to electromechanical, especially flexoelectric, effects at deformed asperities coupled with work function differences. Here, we investigate whether the shape of asperities is important for triboelectricity. The results indicate that the shape is important in general, since how the electromechanical response scales with force and asperity size depends on the shape. This is qualitatively in agreement with experimental results. Further, we discuss how the impact of the shape depends on material, geometric, and electronic transport details. Additionally, gradient elasticity is incorporated into the model. As asperity contact is a nanoscale phenomenon, size-dependent mechanics can become significant and give more physically reasonable results. In some cases, the impact of gradient elasticity terms on the electromechanical potentials is very large, indicating that standard elasticity theory is not enough to cover some relevant cases in modelling triboelectricity.

cond-mat.mtrl-sci↗

arXiv2022

Band bending and ratcheting explain triboelectricity in a flexoelectric contact diode

Triboelectricity was recognized millennia ago, but the fundamental mechanism of charge transfer is still not understood. We have recently proposed a model where flexoelectric band bending due to local asperity contacts drives triboelectric charge transfer in non-metals. While this ab-initio model is consistent with a wide range of observed phenomena, to date there have been no quantitative analyses of the proposed band bending. In this work we use a Pt$_{\mathrm{0.8}}$Ir$_{\mathrm{0.2}}$ conductive atomic force microscope probe to simultaneously deform a Nb-doped SrTiO$_{\mathrm{3}}$ sample and collect current-bias data. The current that one expects based upon an analysis including the relevant flexoelectric band-bending for a deformed semiconductor quantitively agrees with the experiments. The analysis indicates a general ratcheting mechanism for triboelectric transfer and strong experimental evidence that flexoelectric band-bending is of fundamental importance for triboelectric contacts.

cond-mat.mtrl-sci↗

arXiv2020

How Flexoelectricity Drives Triboelectricity

Triboelectricity has been known since antiquity, but the fundamental physics underlying this phenomenon lacks consensus. We present a flexoelectric model for triboelectricity where contact deformation induced band bending is the driving force for charge transfer. This framework is combined with first principles and finite element calculations to explore charge transfer implications for different contact geometry and materials combinations. We demonstrate that our ab initio based formulation is compatible with existing empirical models and experimental observations including charge transfer between similar materials and size/pressure dependencies associated with triboelectricity.

cond-mat.mtrl-sci↗

arXiv2019

Twin Boundary Mediated Flexoelectricity in LaAlO$_3$

Flexoelectricity has garnered much attention owing to its ability to bring electromechanical functionality to non-piezoelectric materials and its nanoscale significance. In order to move towards a more complete understanding of this phenomenon and improve the efficacy of flexoelectric-based devices, it is necessary to quantify microstructural contributions to flexoelectricity. Here we characterize the flexoelectric response of bulk centrosymmetric LaAlO$_3$ crystals with different twin boundary microstructures. We show that twin boundary flexoelectric contributions are comparable to intrinsic contributions at room temperature and enhance the flexoelectric response by ~4x at elevated temperatures. Additionally, we observe time-dependent and non-linear flexoelectric responses associated with strain-gradient-induced twin boundary polarization. These results are explained by considering the interplay between twin boundary orientation, beam-bending strain fields, and pinning site interactions.

cond-mat.mtrl-sci↗

arXiv2019

Does Flexoelectricity Drive Triboelectricity?

The triboelectric effect, charge transfer during sliding, is well established but the thermodynamic driver is not well understood. We hypothesize here that flexoelectric potential differences induced by inhomogeneous strains at nanoscale asperities drive tribocharge separation. Modelling single asperity elastic contacts suggests that nanoscale flexoelectric potential differences of $\pm$1-10 V or larger arise during indentation and pull-off. This hypothesis agrees with several experimental observations, including bipolar charging during stick-slip, inhomogeneous tribocharge patterns, charging between similar materials, and surface charge density measurements.

cond-mat.mtrl-sci↗

arXiv2018

Non-equilibrium Solute Capture in Passivating Oxide Films

If all humans vanished tomorrow, almost every metal structure would collapse within a century or less, the metal converting to an oxide. In applications ranging from the mature technology of nuts and bolts to high technology batteries, nuclear fuels and turbine engines, protective oxide films are critical to limiting oxidation. To date models of these oxide films have assumed that they form thermodynamic equilibrium stable or metastable phases doped within thermodynamic solubility limits. Here we demonstrate experimentally and theoretically the formation of unusual non-equilibrium oxide phases, that can be predicted using a scientific framework for solute capture at a moving oxide/substrate interface. The theory shows that solute capture is likely a generic process for many electrochemical processes, and suggests that similar phenomena yielding non-equilibrium phases can occur and be predicted for a wide range of other processes involving solid-fluid and solid-solid chemical reactions.

cond-mat.mtrl-sci↗

arXiv2017

Electronic Structure of Lanthanide Scandates

X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and density functional theory calculations were used to study the electronic structure of three lanthanide scandates: GdScO3, TbScO3, and DyScO3. X-ray photoelectron spectra simulated from first principles calculations using a combination of on-site hybrid and GGA+U methods were found to be in good agreement with experimental x-ray photoelectron spectra. The hybrid method was used to model the ground state electronic structure and the GGA+U method accounted for the shift of valence state energies due to photoelectron emission via a Slater-Janak transition state approach. From these results, the lanthanide scandate valence bands were determined to be composed of Ln4f, O2p, and Sc3d states, in agreement with previous work. However, contrary to previous work the minority Ln4f states were found to be located closer to, and in some cases at, the valence band maximum. This suggests that minority Ln4f electrons may play a larger role in lanthanide scandate properties than previously thought.

cond-mat.mtrl-sci↗

arXiv2016

Direct Observation of Large Flexoelectric Bending at the Nanoscale in Lanthanide Scandates

Large bending of materials can occur at the nanoscale in response to an electric polarization, what is called the flexoelectric effect, but to date this has not been observed directly. We report the direct observation of large flexoelectric bending in [110] oriented DyScO3 inside an electron microscope. We corroborate these observations with independent ex-situ measurements of the flexoelectric coefficient with a three-point bending setup. The relevant flexocoupling voltage was measured to be -42(2) V, which is higher than expected based upon current flexoelectric models.

cond-mat.mtrl-sci↗

arXiv2015

Three-dimensional coordinates of individual atoms in materials revealed by electron tomography

Crystallography, the primary method for determining the three-dimensional (3D) atomic positions in crystals, has been fundamental to the development of many fields of science. However, the atomic positions obtained from crystallography represent a global average of many unit cells in a crystal. Here, we report, for the first time, the determination of the 3D coordinates of thousands of individual atoms and a point defect in a material by electron tomography with a precision of ~19 picometers, where the crystallinity of the material is not assumed. From the coordinates of these individual atoms, we measure the atomic displacement field and the full strain tensor with a 3D resolution of ~1nm^3 and a precision of ~10^-3, which are further verified by density functional theory calculations and molecular dynamics simulations. The ability to precisely localize the 3D coordinates of individual atoms in materials without assuming crystallinity is expected to find important applications in materials science, nanoscience, physics and chemistry.

cond-mat.mtrl-sci↗

arXiv2014

Reply to "Local Filtering Fundamentally Against Wide Spectrum"

After carefully studying the comment by Wang et al. (arXiv:1408.6420), we found it includes several mistakes and unjustified statements and Wang et al. lack very basic knowledge of dislocations. Moreover, there is clear evidence indicating that Wang et al. significantly misrepresented our method and claimed something that they actually did not implement.

physics.data-an↗

arXiv2010

Optimized conditions for direct imaging of bonding charge density in electron microscopy

We report on the observability of valence bonding effects in aberration-corrected high resolution electron microscopy (HREM) images along the [010] projection of the mineral Forsterite(Mg2SiO4). We have also performed exit wave restorations using simulated noisy images and have determined that both the intensities of individual images and the modulus of the restored complex exit wave are most sensitive to bonding effects at a level of 25% for moderately thick samples of 20-25 nm. This relatively large thickness is due to dynamical amplification of bonding contrast arising from partial de-channeling of 1s states.

cond-mat.mtrl-sci↗

arXiv2009

The Small Unit Cell Reconstructions of SrTiO3 (111)

We analyze the basic structural units of simple reconstructions of the (111) surface of SrTiO3 using density functional calculations. The prime focus is to answer three questions: what is the most appropriate functional to use; how accurate are the energies; what are the dominant low-energy structures and where do they lie on the surface phase diagram. Using test calculations of representative small molecules we compare conventional GGA with higher-order methods such as the TPSS meta-GGA and on-site hybrid methods PBE0 and TPSSh, the later being the most accurate. There are large effects due to reduction of the metal d oxygen sp hybridization when using the hybrid methods which are equivalent to a dynamical GGA+U, which leads to rather substantial improvements in the atomization energies of simple calibration molecules, even though the d-electron density for titanium compounds is rather small. By comparing the errors of the different methods we are able to generate an estimate of the theoretical error, which is about 0.25eV per 1x1 unit cell, with changes of 0.5-1.0 eV per 1x1 cell with the more accurate method relative to conventional GGA. An analysis of the plausible structures reveals an unusual low-energy TiO2-rich configuration with an unexpected distorted trigonal biprismatic structure. This structure can act as a template for layers of either TiO or Ti2O3, consistent with experimental results as well as, in principle, Magnelli phases. The results also suggest that both the fracture surface and the stoichiometric SrTiO3 (111) surface should spontaneously disproportionate into SrO and TiO2 rich domains, and show that there are still surprises to be found for polar oxide surfaces.

cond-mat.mtrl-sci↗