David Zhe Gao

Nanolayers Research Computing

Using density functional theory (DFT) calculations we demonstrate how electron injection can facilitate the creation of Frenkel defects in amorphous (a)-Si 2 . The precursor sites composed of wide O–Si–O bond angles in amorphous SiO2 act as deep electron traps and can accommodate up to two extra electrons. Trapping of two electrons at these intrins...

We present an efficient scheme for parametrizing complex molecule-surface force fields from ab initio data. The cost of producing a sufficient fitting library is mitigated using a 2D periodic embedded slab model made possible by the quantum mechanics/molecular mechanics scheme in CP2K. These results were then used in conjunction with genetic algori...

Achieving control over formation of molecular films on insulating substrates is important for designing novel 2D functional materials and devices. To study the main factors governing successful control, organic molecules with interchangeable polar functional groups, a variable length aromatic body, and flexible hydrocarbon chains are designed, synt...

We demonstrate that using metallic tips for non-contact Atomic Force Microscopy (NCAFM) imaging at relatively large (> 0.5 nm) tip-surface separations provides a reliable method for studying molecules on insulating surfaces with chemical resolution and greatly reduces the complexity of interpreting experimental data. The experimental NC-AFM imaging...

Interest in resistance switching is currently growing apace. The promise of novel high‐density, low‐power, high‐speed nonvolatile memory devices is appealing enough, but beyond that there are exciting future possibilities for applications in hardware acceleration for machine learning and artificial intelligence, and for neuromorphic computing. A ve...

This white paper has been written by members of the European Union H2020 project OntoCommons and external experts to introduce the concept of a Knowledge Management (KM) Translator - a new and essential function in the within knowledge management. This role bridges the gap between knowledge engineering, semantics, and the needs of materials and man...

Three-dimensional atomic force microscopy (3D-AFM) has resolved three-dimensional distributions of solvent molecules at solid− liquid interfaces at the subnanometer scale. This method is now being extended to the imaging of biopolymer assemblies such as chromosomes or proteins in cells, with the expectation of being able to resolve their three-dime...

Carbon nanotubes (CNTs) have many interesting properties that make them a focus of research in a wide range of technological applications. In CNT films, the bottleneck in charge transport is typically attributed to a higher resistance at CNT junctions, leading to electrical transport characteristics that are quite different from individual CNTs. Pr...

A simulation package for CNT memory cells is developed, based on computational modeling of both the mesoscopic structure of carbon nanotube films and the electrical conductivity of inter-CNT contacts. The developed package enables the modeling of various electrical measurements and identification of a range of operation conditions delivering desira...

A Correction to this paper has been published: https://doi.org/10.1038/s41563-021-00956-x.

Utilizing quantum effects in complex oxides, such as magnetism, multiferroicity and superconductivity, requires atomic-level control of the material’s structure and composition. In contrast, the continuous conductivity changes that enable artificial oxide-based synapses and multiconfigurational devices are driven by redox reactions and domain recon...

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

As scientific instruments become more accurate and complex, correct interpretation of their measurements consequently becomes more difficult, and require sophisticated theoretical modelling. This chapter illustrate how to simulate non-contact atomic force microscopy experiments by combining several techniques. A multi scale model of the experiment...

DScribe is a software package for machine learning that provides popular feature transformations (“descriptors”) for atomistic materials simulations. DScribe accelerates the application of machine learning for atomistic property prediction by providing user-friendly, off-the-shelf descriptor implementations. The package currently contains implement...

DScribe is a software package for machine learning that provides popular feature transformations ("descriptors") for atomistic materials simulations. DScribe accelerates the application of machine learning for atomistic property prediction by providing user-friendly, off-the-shelf descriptor implementations. The package currently contains implement...

Dielectric oxide films in electronic devices undergo significant structural changes during device operation under bias. These changes are usually attributed to aggregation of oxygen vacancies resulting in formation of oxygen depleted regions and conductive filaments. However, neutral oxygen vacancies have high diffusion barriers in ionic oxides and...

On-surface polymerization is a promising technique to prepare organic functional nanomaterials that are challenging to synthesize in solution, but it is typically used on metal substrates, which play a catalytic role. Previous examples on insulating surfaces have involved intermediate self-assembled structures, which face high barriers to diffusion...

We review the current understanding of intrinsic electron and hole trapping in insulating amorphous oxide films on semiconductor and metal substrates. The experimental and theoretical evidences are provided for the existence of intrinsic deep electron and hole trap states caused by the disorder of amorphous metal oxide films. We start from presenti...

Using density functional theory (DFT) calculations, we investigated oxygen vacancy diffusion and aggregation in relation to dielectric breakdown in amorphous silicon dioxide (a-SiO2). Our calculations indicate the existence of favourable sites for the formation of vacancy dimers and trimers in the amorphous network with maximum binding energies of...

Despite extensive experimental and theoretical studies, the atomistic mechanisms responsible for dielectric breakdown (BD) in amorphous (a)-SiO2 are still poorly understood. A number of qualitative physical models and mathematical formulations have been proposed over the years to explain experimentally observable statistical trends. However, these...

The challenges and limitations in calculating free energies and entropies of adsorption and interaction of organic molecules on an insulating substrate are discussed. The adhesion of 1,3,5-tri(4'-cyano-[1,1'-biphenyl]-4-yl)benzene (TCB) and 1,4-bis(4-cyanophenyl)-2,5-bis(decyloxy)benzene (CDB) molecules to step edges on the KCl(001) surface and the...

We studied the effect of molecular flexibility on the morphology and growth mechanisms of self-assembled films on an insulating substrate using a combination of experimental and theoretical methods. 1,3,5-tri- (4-cyano-4,4 biphenyl)- benzene (TCB) and 1,4-bis (cyanophenyl)-2,5-bis(decyloxy) benzene (CDB) molecules were deposited on a KCl (001) surf...

On-surface synthesis with molecular precursors has emerged as the de-facto route to atomically well-defined graphene nanoribbons (GNRs) with controlled zigzag and armchair edges. On Au(111) and Ag(111) surfaces, the prototypical precursor 10-10'-dibromo-9-9'-bianthryl (DBBA) polymerizes via an Ullmann route to form straight GNRs with armchair edges...

Electrically biasing thin films of amorphous, substoichiometric silicon oxide drives surprisingly large structural changes, apparent as density variations, oxygen movement, and, ultimately, emission of superoxide ions. The results from the fundamental study by A. Mehonic, A. J. Kenyon, and co-workers reported on page 7486 are directly relevant to m...

We apply a three-dimensional (3D) physical simulator, coupling self-consistently stochastic kinetic Monte Carlo descriptions of ion and electron transport, to investigate switching in silicon-rich silica (SiO x ) redox-based resistive random-access memory (RRAM) devices. We explain the intrinsic nature of resistance switching of the SiO x layer, an...

Electrically biasing thin films of amorphous, substoichiometric silicon oxide drives surprisingly large structural changes, apparent as density variations, oxygen movement, and ultimately, emission of superoxide ions. Results from this fundamental study are directly relevant to materials that are increasingly used in a range of technologies, and de...

In this work we combined noncontact atomic force microscopy (NCAFM) experiments and theoretical calculations to study the effects of molecular flexibility on the adsorption and self-assembly of organic molecules on KCl(001). Rigid 1,3,5-tri-(4-cyano-4,4 biphenyl)-benzene (TCB) and flexible 1,4-bis(cyanophenyl)-2,5-bis(decyloxy)benzene (CDB) molecul...

Although it is recognized that dynamic behavior of adsorbing molecules strongly affects the entropic contribution to adsorption free energy, detailed studies of the adsorption entropy of large organic molecules at insulating surfaces are still rare. We compared adsorption of two different functionalized organic molecules, 1,3,5-tri-(4-cyano-4,4 bip...

1,3,5-tri-(4-cyano-4,4 biphenyl)-benzene (TCB) and 1,4-bis(cyanophenyl)-2,5-bis(decyloxy)benzene (CDB) molecules, were synthesised, deposited, and annealed on the KCl (001) and imaged using NCAFM. DFT calculations were performed to study the adsorption of single molecules and the competing molecule-molecule and molecule-surface interactions for CDB...

Hafnia (HfO2) and amorphous silicon dioxide (a-SiO2) are currently being explored as the insulating oxide layer in metal-insulator-metal resistive RAM (ReRAM) cells. The prevailing theory of the switching processes in these cells involves the formation of a conductive filament comprised of sub-stoichiometric oxide. However, the atomistic mechanisms...

Theoretical models are crucial to correctly interpret NC-AFM measurements, especially when complex systems such as molecules [1, 2], clusters, and self-assembled monolayers [3] are considered. While isolated adsorbed molecules can be modelled in great detail using quantum mechanical methods, the high computational cost makes it impossible to simula...

We used a Cr coated noncontact atomic force microscope (NC-AFM) tip to study small molecules on NiO (100). Images of CO molecules were obtained at large tip-sample distances to avoid manipulation and are shown in Figure A. In order to interpret these images we employed a point dipole approximation for the tip to produce theoretical scan-lines acros...

We describe how one can produce, control, and characterize sharp metal coated tips and how these tips can achieve atomic resolution on insulating surfaces at large tip-surface separations. Our experimental results combined with DFT calculations confirm that the Cr tips possess permanent electric dipoles with the positive ends oriented towards the s...

Non-contact Atomic Force Microscopy (NC-AFM) is an experimental technique capable of imaging almost any surface with atomic resolution, in a wide variety of environments. Linking measured images to real understanding of system properties is often difficult, and many studies combine experiments with detailed modelling, in particular using virtual si...

This book presents the latest developments in noncontact atomic force microscopy. It deals with the following outstanding functions and applications that have been obtained with atomic resolution after the publication of volume 2: (1) Pauli repulsive force imaging of molecular structure, (2) Applications of force spectroscopy and force mapping with...

The study of atoms and molecules on bulk insulating surfaces is of critical importance to many fields of surface science including lubrication, and molecular electronics. I studied these systems by using a variety of theoretical methods to predict adsorption geometry, diffusion pathways, and film structure, and to interpret noncontact atomic force mic...

Theoretical models are critical for improving our understanding of recent noncontact atomic force microscopy (NC-AFM) studies of individual molecules [1, 2], clusters, and self-assembled monolayers [3]. We employ a multi-scale approach for these complex systems containing large molecules and study long timescale processes. The methodology employed...

Individual molecules of Co-Salen, a small chiral paramagnetic metal–organic complex, deposited on NiO(001) were imaged with noncontact atomic force microscopy (NC-AFM) using metallic Cr coated tips. Experimentally, we simultaneously resolve both the molecule and the individual surface ions. Images recorded at low temperatures show that the Co-Salen...

POSTER 16th International Conference on Non-Contact Atomic Force Microscopy, Hyattsville, USA, August 5-9th

The importance and mechanisms of transient mobility of atoms and molecules adsorbing at surfaces have been a subject of controversy for many years. We used classical molecular dynamics simulations to examine transient mobility of Pd atoms adsorbing on the MgO (100) surface with incident kinetic energies not exceeding 0.4 eV. The calculations show t...

In this study, a green process was developed to synthesize a novel molybdenum disulfide (MoS2) based friction modifier (FM) for improving fuel economy performance of lubricants. These new materials were synthesized using less hazardous elemental sulfur as opposed to other sulfur sources like hydrogen sulfide (H2S) and carbon disulfide (CS2). Using...

The adsorption and manipulation of a Metal-Organic complex is important in many fields of surface science such as catalysis and molecular electronics. It is important to understand the adsorption site and geometry in order to study the chemistry and physics of these systems. The Co-Salen complex has been previously studied on a bulk insulator (NaCl...

Adsorption of metal atoms and clusters on oxide surfaces and subsequent processes that occur play an important role in surface coatings and film growth. When atoms or molecules adsorb favorably on a surface, energy is released and dissipated through various processes. These processes strongly influence the final structure of thin film and island fo...