arXiv subjects

Ricardo Paupitz

Publications and source records attributed to Ricardo Paupitz.

13 recordsLinked to original sources

arXiv2026

A ReaxFF-based thermomechanical analysis of N-carbophenes: phase-change, thermal expansion, and high temperature synthesis pathway

N-carbophenes are a class of two-dimensional covalent organic frameworks with potential for solid-state gas storage and as 2D topological materials. Previous studies have demonstrated that variations in their bonding, topology, and functionalization enable the tuning of their chemical, electrical, and mechanical properties. Yet, the thermal stability and high-temperature behavior of pristine and functionalized N-carbophenes remain unexplored. Using ReaxFF-based reactive molecular dynamics (RMD) simulations with extensive statistical validation, we performed temperature-ramp MD simulations of pristine and functionalized N-carbophenes. We demonstrate that N-carbophenes remain stable up to temperatures above 1000 K. The phase-change onset temperatures decrease as the N-phenylene chain length increases in pristine N-carbophenes, attributed to increasing antiaromaticity in the central phenylene segments, thereby contributing to the foundational understanding of aromatic versus antiaromatic bonding in 2D carbon networks, a topic of considerable interest in theoretical chemistry. Pristine N-carbophenes exhibit negative area thermal expansion (NATE), whereas functional groups modulate this, leading to either negative or positive expansion. Functional groups remain stably bonded well above the transition temperature. We also show that a temperature-induced phase transition from graphenylene (2-carbophene) to {\gamma}-graphyne is possible. Our results provide upper bounds on N-carbophene stability, clarify the relationships between structure and thermal properties, and identify a new transformation pathway. These results will have applications in tunable band gaps, porous architectures, or chemically accessible sites.

cond-mat.mtrl-sci↗

arXiv2025

A Concise Review of Recently Synthesized 2D Carbon Allotropes: Amorphous Carbon, Graphynes, Biphenylene and Fullerene Networks

Two-dimensional (2D) carbon allotropes have received considerable attention due to their unique properties and potential applications in several fields, including electronics, catalysis, energy storage, and sensing. Following the experimental realization of graphene, numerous other 2D carbon structures have been proposed and, in some cases, successfully synthesized. This work presents a concise review of the recently experimentally realized 2D carbon allotropes, including graphynes, biphenylene-based networks, fullerene networks, and monolayer amorphous carbon. For each class, we discuss structural characteristics, theoretical predictions, and synthesis methods, with emphasis on the interplay between theory and experiment. We also highlight instances where experimental studies overlooked relevant theoretical contributions. Finally, we identify theoretically predicted structures that remain unexplored experimentally, suggesting opportunities for synthesis-driven investigations.

cond-mat.mtrl-sci↗

arXiv2023

Functionalized carbophenes as high-capacity versatile gas adsorbents: An ab initio study

This study employs density functional theory (DFT) and density functional tight-binding theory (DFTB) to determine the adsorption properties of carbon dioxide (CO$_2$), methane (CH$_4$), and dihydrogen (H$_2$) in carbophenes functionalized with carboxyl (COOH), amine (NH$_2$), nitro (NO$_2$), and hydroxyl (OH) groups. We demonstrate that carbophenes are promising candidates as adsorbents for these gasses. Carbophenes have larger CO$_2$ and CH$_4$ adsorption energies than other next-generation solid-state capture materials. Yet, the low predicted desorption temperatures mean they can be beneficial as air scrubbers in confined spaces. Functionalized carbophenes have H$_2$ adsorption energies usually observed in metal-containing materials. Further, the predicted desorption temperatures of H$_2$ from carbophenes lie within the DOE Technical Targets for Onboard Hydrogen Storage for Light-Duty Vehicles (DOEHST) operating temperature range. The possibility of tailoring the degree of functionalization in combination with selecting sufficiently open carbophene structures that allow for multiple strong interactions without steric hindrance (crowding) effects, added to the multiplicity of possible functional groups alone or in combination, suggests that these very light materials can be ideal adsorbates for many gases. Tailoring the design to specific adsorption or separation needs would require extensive combinatorial investigations.

cond-mat.mtrl-sci↗

arXiv2023

Reversible Actuation of $α$-Borophene Nanoscrolls

In this work, we proposed and investigated the structural and electronic properties of boron-based nanoscrolls (armchair and zigzag) using the DFTB+ method. We also investigated the electroactuation process (injecting and removing charges). A giant electroactuation was observed, but the results show relevant differences between the borophene and carbon nanoscrolls. The molecular dynamics simulations showed that the scrolls are thermally and structurally stable for a large range of temperatures (up to 600K) and the electroactuation process can be easily tuned and can be entirely reversible for some configurations.

cond-mat.mes-hall↗

arXiv2022

Covalent Adsorption of functional groups on [N]-carbophenes

Starting from the planar molecule 1,3,5-trihydroxybenzene, Du et al. reported synthesizing one of a couple of possible 2D materials: graphenylene or 3-carbophene. 3-carbophene is a member of a novel class of two-dimensional covalent organic framework, [N]-carbophenes (carbophenes). Using a high throughput method, we computed the formation energies and conduction properties of 3-, 4-, 5-, and 6-carbophenes with hydroxyl (OH), carbonyl (CO), nitro (NO$_2$), amine (NH$_2$), carboxyl (COOH) functional groups replacing hydrogen terminating agents. Five hundred and nine structures with randomly picked motifs, with functionalizations from a single functional group per cell to fully functionalized were studied. Our results demonstrate a negatively sloped linear relationship between the degree of functionalization and formation energy when the type of functional group and type of carbophene are held constant. The decrease in formation energy with functionalization makes Du's synthesis of functionalized 3-carbophene more creditable. The type of carbophene, type of functional group, and the degree of functionalization all play a role in the band structure of the materials. For example, CO functional groups may lead to a mid-gap state pinned to the Fermi level, whereas the other functional groups studied keep the semiconducting nature of pristine carbophene. Thus, because carbonyl functional groups are often present in defected carbon systems, care should be taken to limit the amount of oxygen in carbophene devices where the band gap is important. Thus, this work strengthens the hypothesis of Junkermeier et al.'s hypothesis that Du et al. synthesized 3-carbophene and not graphenylene.

cond-mat.mtrl-sci↗

arXiv2021

Auxetic properties of a newly proposed $γ$-graphyne-like material

In this work, we propose a new auxetic (negative Poisson's ratio values) structure, based on a $γ$-graphyne structure, here named $AγG$ $structure$. Graphynes are 2D carbon allotropes with phenylic rings connected by acetylenic groups. The A$γ$G structural/mechanical and electronic properties, as well as its thermal stability, were investigated using classical reactive and quantum molecular dynamics simulations. We found that A$γ$G has a large bandgap of 2.48 eV and is thermally stable at a large range of temperatures. It presents a Young's modulus that is an order of magnitude smaller than that of graphene or $γ$-graphyne. The classical and quantum results are consistent and validate that the A$γ$G is auxetic, both when isolated (vacuum) and when deposited on a copper substrate. We believe that this is the densest auxetic structure belonging to the graphyne-like families.

cond-mat.mtrl-sci↗

arXiv2019

N-Carbophenes: two-dimensional covalent organic frameworks derived from linear N-phenylenes

N-Carbophene (carbophene) is a novel class of two-dimensional covalent organic frameworks (2DCOF), based on linear N-phenylenes, that have moderate band gaps and low-mobility bands surrounding the Fermi energy; the simplest of which may have been recently synthesized. Using tight-binding density functional theory, the ground state configurations single layers, bilayers, and bulk systems was determined. This work finds that carbophenes have formation energies per carbon atom similar to that of graphenylene. The similarity of formation energies between graphenylene and carbophene suggests that when trying to synthesize one, the other may also be synthesized. The formation energies could explain why the first reported synthesis of graphenylene also indicated that they may have synthesized 3-carbophene. Results contained in this work suggests that a carbophene was synthesized instead of graphenylene. The projected density of states (PDOS) demonstrates that the anti-aromatic nature of the cyclobutene units plays a direct role in the creation of bands around the Fermi level, making this an exciting material in the theoretical understanding of the nature of aromatic bonds.

cond-mat.mtrl-sci↗

arXiv2019

Ground state determination and band gaps of bilayers of graphenylenes and octafunctionalized-biphenylenes

Device fabrication often requires materials that are either reliably conducting, reliably semiconducting, or reliably nonconducting. Bilayer graphene (BLG) changes from a superconductor to a semiconductor depending on it's stacking, but because it is difficult to control its stacking, it is not a reliable material for device fabrication. Using DFTB+, this work demonstrates that bilayers of graphenylene, net-C, and net-W can be reliably used for device fabrication without knowing the details of their stackings. Bilayers of graphenylene and net-C are semiconducting for all sheer displacements, net-W is conducting for all sheer displacements, while that Type II, like BLG, is conducting or semiconducting depending on the sheer displacement. The method used gives bond lengths, unit cell dimensions, and band dispersion of single-layer graphene that are consistent with previously reported values, it correctly predicts that AB stacking is the ground state of BLG and gives an interlayer separation that is consistent with previous studies. The bond lengths and lattice constants of the other carbon allotropes are consistent with previously published values. \textcolor{blue}{In order to calculate the band structures the bilayer systems,} DFTB+ was first used to determined the interlayer separations of the 2-D carbon allotropes under shear displacement.

cond-mat.mtrl-sci↗

arXiv2016

Graphene healing mechanisms: A theoretical investigation

Large holes in graphene membranes were recently shown to heal, either at room temperature during a low energy STEM experiment, or by annealing at high temperatures. However, the details of the healing mechanism remain unclear. We carried out fully atomistic reactive molecular dynamics simulations in order to address these mechanisms under different experimental conditions. Our results show that, if a carbon atom source is present, high temperatures can provide enough energy for the carbon atoms to overcome the potential energy barrier and to produce perfect reconstruction of the graphene hexagonal structure. At room temperature, this perfect healing is only possible if the heat effects of the electron beam from STEM experiment are explicitly taken into account. The reconstruction process of a perfect or near perfect graphene structure involves the formation of linear carbon chains, as well as rings containing 5, 6, 7 and 8 atoms with planar (Stone-Wales) and non-planar (lump like) structures. These results shed light on the healing mechanism of graphene when subjected to different experimental conditions. Additionally, the methodology presented here can be useful for investigating the tailoring and manipulations of other nano-structures.

cond-mat.mes-hall↗

arXiv2016

Grüneisen Parameter for Gases

The Grüneisen ratio ($Γ$), i.e.\,the ratio of the linear thermal expansivity to the specific heat at constant pressure, quantifies the degree of anharmonicity of the potential governing the physical properties of a system. While $Γ$ has been intensively explored in solid state physics, very little is known about its behavior for gases. This is most likely due to the difficulties posed to carry out both thermal expansion and specific heat measurements in gases with high accuracy as a function of pressure and temperature. Furthermore, to the best of our knowledge a comprehensive discussion about the peculiarities of the Grüneisen ratio is still lacking in the literature. Here we report on a detailed and comprehensive overview of the Grüneisen ratio. Particular emphasis is placed on the analysis of $Γ$ for gases. The main findings of this work are: \emph{i)} for the Van der Waals gas $Γ$ depends only on the co-volume $b$ due to interaction effects, it is smaller than that for the ideal gas ($Γ$ = 2/3) and diverges upon approaching the critical volume; \emph{ii)} for the Bose-Einstein condensation of an ideal boson gas, assuming the transition as first-order $Γ$ diverges upon approaching a critical volume, similarly to the Van der Waals gas; \emph{iii)} for $^4$He at the superfluid transition $Γ$ shows a singular behavior. Our results reveal that $Γ$ can be used as an appropriate experimental tool to explore pressure-induced critical points.

cond-mat.quant-gas↗

arXiv2015

Specific Heat Anomalies in Solids Described by a Multilevel Model

Specific heat measurements constitute one of the most powerful experimental methods to probe fundamental excitations in solids. After the proposition of Einstein's model, more than one century ago (Annalen der Physik \textbf{22}, 180 (1907)), several theoretical models have been proposed to describe experimental results. Here we report on a detailed analysis of the two-peak specific heat anomalies observed in several materials. Employing a simple multilevel model, varying the spacing between the energy levels $Δ_i$ = $(E_i$ $-$ $E_{0})$ and the degeneracy of each energy level $g_i$, we derive the required conditions for the appearance of such anomalies. Our findings indicate that a ratio of $Δ_2$/$Δ_1$ $\thickapprox$ 10 between the energy levels and a high degeneracy of one of the energy levels define the two-peaks regime in the specific heat. Our approach accurately matches recent experimental results. Furthermore, using a mean-field approach we calculate the specific heat of a degenerate Schottky-like system undergoing a ferromagnetic (FM) phase transition. Our results reveal that as the degeneracy is increased the Schottky maximum in the specific heat becomes narrow while the peak associated with the FM transition remains unaffected.

cond-mat.mtrl-sci↗

arXiv2014

One-dimensional Silicon and Germanium Nanostructures With No Carbon Analogues

In this work we report new silicon and germanium tubular nanostructures with no corresponding stable carbon analogues. The electronic and mechanical properties of these new tubes were investigated through ab initio methods. Our results show that the structures have lower energy than their corresponding nanoribbon structures and are stable up to high temperatures (500 and 1000 K, for silicon and germanium tubes, respectively). Both tubes are semiconducting with small indirect band gaps, which can be significantly altered by both compressive and tensile strains. Large bandgap variations of almost 50% were observed for strain rates as small as 3%, suggesting possible applications in sensor devices. They also present high Young's modulus values (0.25 and 0.15 TPa, respectively). TEM images were simulated to help the identification of these new structures.

cond-mat.mes-hall↗

arXiv2014

Inorganic Graphenylene: A Porous Two-Dimensional Material With Tunable Band Gap

By means of ab initio calculations we investigate the possibility of existence of a boron nitride (BN) porous two-dimensional nanosheet which is geometrically similar to the carbon allotrope known as biphenylene carbon. The proposed structure, which we called Inorganic Graphenylene (IGP), is formed spontaneously after selective dehydrogenation of the porous Boron Nitride (BN) structure proposed by Ding et al. We study the structural and electronic properties of both porous BN and IGP and it is shown that, by selective substitution of B and N atoms with carbon atoms in these structures, the band gap can be significantly reduced, changing their behavior from insulators to semiconductors, thus opening the possibility of band gap engineering for this class of two-dimensional materials.

cond-mat.mtrl-sci↗