Professor Helio Chacham

@article{Mendonça2026,

title = {Dimensionality transition and enhanced diffusion of water channels between periodically modulated surfaces: Insights from molecular dynamics},

author = {Bruno H.S. Mendonça and Elizane E. de Moraes and Hélio Chacham},

doi = {10.1016/j.physa.2026.131733},

issn = {0378-4371},

year  = {2026},

date = {2026-09-01},

urldate = {2026-09-00},

journal = {Physica A: Statistical Mechanics and its Applications},

volume = {697},

publisher = {Elsevier BV},

abstract = {Water nanoconfinement leads to diverse transport phenomena depending on the geometry of the void space, such as 2D confinement between surfaces and 1D confinement inside nanotubes. We employ molecular dynamics to investigate water transport between periodically modulated surfaces. Our results reveal a spontaneous dimensionality transition from 2D aqueous sheets to 1D isolated channels, driven by the surface topography. By progressively increasing the inter-surface separation while keeping a fixed water density, we observe a dimensionality transition in the water network, where the system evolves from a 2D layer to 1D channels self-organized along surface grooves. Our results demonstrate that the self-diffusion coefficient (

) in these 1D nanochannels is strongly sensitive to the nanotube diameter and the surface-water interaction potential. Reducing the nanotube diameter from 10 to 5 nm triggers an increase in 

 by up to tenfold between confinement geometries. Notably, in the 5 nm systems, the diffusion coefficient exceeds bulk water values by up to 2.5. This enhanced diffusion is discussed in terms of the cooperative and collective molecular motion emerging from extreme geometric constraints. We also report a symmetry reduction observed in the confinement limit in the interstitial voids of the bulk bundle material, where water clusters into specific groove regions to minimize surface area. These findings provide insights into the statistical mechanics of fluids in complex periodically modulated environments and the structural transition from disconnected to percolating water networks in periodically modulated nanoporous environments.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{C.Soares2026,

title = {Structure, vibrational modes, and surface electronic states of two-dimensional valentinite antimony oxide},

author = {Thamires C. Soares and Wesley Kardex C. de Oliveira and Catalina Ruano Merchán and Alan C. R. Souza and Helio Chacham and Mario S. C. Mazzoni and Alisson R. Cadore and Luiz Fernando Zagonel and Ingrid D. Barcelos},

doi = {10.1063/5.0312539},

issn = {1089-7550},

year  = {2026},

date = {2026-03-28},

urldate = {2026-03-28},

volume = {139},

number = {12},

publisher = {AIP Publishing},

abstract = {We report a comprehensive study of two-dimensional (2D) valentinite antimony oxide (β-Sb2O3) exfoliated from its natural bulk form. The flakes exhibit well-defined morphology, low surface roughness, and preserved orthorhombic structure. Raman and infrared nano-spectroscopy reveal distinct vibrational modes, confirming the crystallinity and vibrational anisotropy of the material. Conductive atomic force microscopy shows dielectric breakdown at ⁓0.18 V/nm, consistent with insulating out-of-plane behavior. However, scanning tunneling spectroscopy reveals a near-zero surface gap at room temperature. First-principles calculations highlight strong surface band modulation, including full bandgap closure for specific surface terminations. These calculations also indicate a wide indirect bulk gap (⁓2.82 eV) while cathodoluminescence measurements reveal broad defect-related emission, yielding an estimated lower bound for the optical bandgap of approximately 3 eV. These results establish β-Sb2O3 as a unique 2D material with strong bulk-surface electronic contrast, offering potential for applications in wide-bandgap electronics, surface-state engineering, and optoelectronic nanodevices.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{deMoraes2026,

title = {Spontaneous Folding of Suspended and Supported Graphene, and Other 2D Materials: Morphologies and Induced Pre-Tension},

author = {Elizane E. de Moraes and Alan B. de Oliveira and Ronaldo J. C. Batista and Matheus J. S. Matos and Ana Paula Moreira Barboza and Bernardo Ruegger Almeida Neves and Helio Chacham},

doi = {10.1021/acs.jpcc.5c05571},

issn = {1932-7455},

year  = {2026},

date = {2026-01-18},

urldate = {2026-01-18},

journal = {J. Phys. Chem. C},

publisher = {American Chemical Society (ACS)},

abstract = {In the present study, we investigate the spontaneous folding of graphene and other 2D materials, considering the possibilities that they are suspended or supported, and dry or wet. By combining molecular dynamics simulations, a minimal analytical continuum model, and atomic force microscopy experiments, we assess the impact of interlayer adhesion on several folding conformations at various temperatures. We find that the folded structures induce tensile stress on graphene, and the magnitude of this stress depends on the morphology of the fold. The calculated stress values, in the range of 0.14–0.35 N/m, with experimentally reported pretension values in suspended graphene, suggest that folds may be a possible intrinsic source of pretension. The results of the simulations for graphene are reproduced by a one-parameter analytical continuum model of a 2D material, which naturally extends the predicted phenomenology of fold-induced pretension to other 2D materials. Our simulations also show the existence of folds in suspended graphene in a humid environment. We also performed experiments on supported and suspended regions of few-layer talc using atomic force microscopy and observed several folded structures with characteristics similar to those in the simulations. These results reveal that folding serves as an intrinsic and general mechanical mechanism for imparting residual tension in two-dimensional materials, with direct implications for nanomechanical measurements, strain engineering, and device stability.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Sousa2025b,

title = {Optical spectroscopy of defects in atomically thin transition metal dichalcogenides},

author = {Frederico B. Sousa and Kazunori Fujisawa and Felipe Menescal and Matheus J. S. Matos and Marcos A. Pimenta and Helio Chacham and Mauricio Terrones and Leandro M. Malard and Bruno R. Carvalho},

doi = {10.1063/5.0251288},

issn = {1931-9401},

year  = {2025},

date = {2025-05-28},

urldate = {2025-06-01},

volume = {12},

number = {2},

publisher = {AIP Publishing},

abstract = {In this review, we address the optical signatures of defects in two-dimensional transition metal dichalcogenides (2D TMDs), whether they occur unintentionally during growth or are deliberately introduced post-growth. We detail their primary responses as probed by photoluminescence (PL), magneto-PL, Raman, tip-enhanced PL and Raman, and nonlinear spectroscopies. Defects significantly impact the electronic, vibrational, magneto-optical, and nonlinear properties of TMDs, influencing outcomes based on application needs. This comprehensive overview highlights the distinctive optical fingerprints of various defects, providing guidance for their identification and characterization. Additionally, we discuss new optical phenomena induced by defects in TMD monolayers and future challenges in defect engineering for these materials. The insights from this review underscore the potential of TMDs for technological applications, with advancements in spectroscopy and defect engineering driving future innovations and enhancing our understanding of these materials.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Sousa2025,

title = {Strong Magneto-Optical Responses of an Ensemble of Defect-Bound Excitons in Ambient Exposed WS_{2} and WSe_{2} Monolayers},

author = {Frederico B. Sousa and Alessandra Ames and Mingzu Liu and Pedro L. Gastelois and Vinícius A. Oliveira and Da Zhou and Matheus J. S. Matos and Helio Chacham and Mauricio Terrones and Marcio D. Teodoro and Leandro M. Malard},

doi = {10.1021/acs.jpcc.5c00951},

issn = {1932-7455},

year  = {2025},

date = {2025-04-25},

urldate = {2025-04-25},

journal = {J. Phys. Chem. C},

publisher = {American Chemical Society (ACS)},

abstract = {Transition metal dichalcogenide (TMD) monolayers present a singular coupling in their spin and valley degrees of freedom. Moreover, by applying an external magnetic field it is possible to break the energy degeneracy between their K and −K valleys. This valley Zeeman effect opens the possibility of controlling and distinguishing the spin and valley characters of charge carriers in TMDs by their optical transition energies, making these materials promising for the next generation of spintronic and photonic devices. However, the free excitons of pristine TMD monolayers present a moderate valley Zeeman splitting of ≈0.23 meV/T. Therefore, alternative excitonic states with higher magnetic responses are mandatory for application purposes. Here, we investigate the magneto-optical properties of ambient exposed WS2 and WSe2 monolayers by circularly polarized magneto-photoluminescence experiments at cryogenic temperatures. A broad lower energy photoluminescence emission related to an ensemble of defects is observed, presenting remarkable valley-related splittings of ≈1.45 meV/T and ≈1.11 meV/T for WS2 and WSe2 monolayers, respectively. In addition, we report a significant spin polarization of charge carriers in the defect midgap states induced by the external magnetic field. We explain this spin-polarized population and enhanced valley-related splitting in terms of imbalanced spin-flip transitions, leading to a magnetic field-dependent distribution of charge carriers in multiple defect levels. This effect, together with the individual Zeeman shiftings of the midgap states, explains the strong magneto-optical responses observed. Our work uncovers the singular potential of manipulating the light emission of ambient exposed TMD monolayers by an external magnetic field.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Sousa2024,

title = {Giant Valley Zeeman Splitting in Vanadium-Doped WSe2 Monolayers},

author = {Frederico B. Sousa and Matheus J. S. Matos and Bruno R. Carvalho and Mingzu Liu and Alessandra Ames and Da Zhou and Geovani C. Resende and Zhuohang Yu and Lucas Lafeta and Marcos A. Pimenta and Mauricio Terrones and Marcio D. Teodoro and Helio Chacham and Leandro M. Malard},

url = {https://onlinelibrary.wiley.com/doi/10.1002/smll.202405434},

doi = {10.1002/smll.202405434},

issn = {1613-6829},

year  = {2024},

date = {2024-10-08},

urldate = {2024-10-08},

journal = {Small},

publisher = {Wiley},

abstract = {2D dilute magnetic semiconductors (DMS) based on transition metal dichalcogenides (TMD) offer an innovative pathway for advancing spintronic technologies, including the potential to exploit phenomena such as the valley Zeeman effect. However, the impact of magnetic ordering on the valley degeneracy breaking and on the enhancement of the optical transitions g-factors of these materials remains an open question. Here, a giant effective g-factors ranging between ≈−27 and −69 for the bound exciton at 4 K in vanadium-doped WSe2 monolayers, obtained through magneto-photoluminescence (PL) experiments is reported. This giant g-factor disappears at room temperature, suggesting that this response is associated with a magnetic ordering of the vanadium impurity states at low temperatures. Ab initio calculations for the vanadium-doped WSe2 monolayer confirm the existence of magnetic ordering of the vanadium states, which leads to degeneracy breaking of the valence bands at K and K′. A phenomenological analysis is employed to correlate this splitting with the measured enhanced effective g-factor. The findings shed light on the potential of defect engineering of 2D materials for spintronic applications.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{doi:10.1021/acs.jpcc.4c02170,

title = {Water Nanochannels in Ultrathin Clinochlore Phyllosilicate Mineral with Ice-like Behavior},

author = {Raphaela Oliveira and Luisa V. C. Freitas and Helio Chacham and Raul O. Freitas and Roberto L. Moreira and Huaiyu Chen and Susanna Hammarberg and Jesper Wallentin and Gilberto Rodrigues-Junior and Lucas A. B. Marçal and Guilherme A. Calligaris and Alisson R. Cadore and Klaus Krambrock and Ingrid D. Barcelos and Angelo Malachias},

url = {https://doi.org/10.1021/acs.jpcc.4c02170},

doi = {10.1021/acs.jpcc.4c02170},

year  = {2024},

date = {2024-07-03},

journal = {The Journal of Physical Chemistry C},

volume = {0},

number = {0},

pages = {null},

abstract = {Water is the matrix of life, and its confinement in nanocavities is a central topic from geophysics to nanotribology. Phyllosilicate layered minerals provide natural nanocavities for water due to their capacity to hydrate by confining water molecules in the interlamellar space. However, the hydration of phyllosilicates at the nanoscale is not a fully understood process and depends on the geological specimens. In this work, we report the formation of aqueous nanochannels in the interlamellar space of ultrathin clinochlore phyllosilicate mineral using infrared scattering-type scanning near-field optical microscopy and Kelvin probe force microscopy. We demonstrate that nanoconfinement of water in clinochlore changes the overall mechanical, optical, and dielectric properties of the system. We propose a capacitive model that describes the dielectric response of the aqueous nanochannels. Our model is endorsed by a robust crystal truncation rod analysis of synchrotron X-ray diffraction data. We found that clinochlore termination combines hydrated structures ordered along the c-axis. We also find evidence of ice-like behavior of the water nanoconfined in clinochlore by Fourier-transform infrared spectroscopy. Notably, our work introduces clinochlore as a natural platform for water confinement in two-dimensional systems that can be engineered for several applications in the frontiers of nanotechnology.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{doi:10.1021/acs.jpcc.4c00078,

title = {Conduction Percolation in MoS2 Nanoink Humidity Sensors: Critical Exponents and Nanochannel Dimensionality},

author = {Bruno H. S. Mendonça and Neuma Pereira and Natália P. Rezende and Elizane E. de Moraes and Rodrigo G. Lacerda and Helio Chacham},

url = {https://doi.org/10.1021/acs.jpcc.4c00078},

doi = {10.1021/acs.jpcc.4c00078},

year  = {2024},

date = {2024-05-03},

journal = {The Journal of Physical Chemistry C},

volume = {0},

number = {0},

pages = {null},

abstract = {In this paper, we adopt a novel approach to investigate the ionic conduction near the percolation transition in a porous nanomaterial. We make use of a recently discovered humidity sensing property of aerosol-printed MoS2 ink films, where the conductance, originated from ionic transport through water nanochannels within the films, is dependent on the ambient humidity. The experiments, performed for a set of four devices inside a chamber with controlled humidity, allow the experimental fine-tuning of the ionic conduction percolation within each nanoporous device by changing the ambient relative humidity, without the need of different samples for different stoichiometries as in usual percolation experiments. Our results indicate the existence, in our devices, of a common phenomenology consisting of two sequential modifications of the conductance as a function of humidity near percolation. The first is the true percolation transition with a universal critical exponent very close to unity. This is followed by an apparent increase in the critical exponent above the true transition. We also perform molecular dynamics simulations that allow the identification of a possible mechanism for dimensionality changes in the water nanochannels inside the material as a function of either humidity or material geometry as a possible scenario for the observed conductance modification in the conductive phase.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{unknown,

title = {The quality accuracy of TIP4P/2005 and SPC/Fw water models},

author = {João Lemos Valle and Bruno Mendonça and Marcia Barbosa and Helio Chacham and Elizane Moraes},

url = {https://www.if.ufrgs.br/~barbosa/Publications/Physics/valle-JPCB-2024.pdf},

year  = {2024},

date = {2024-01-20},

urldate = {2023-01-01},

abstract = {Water is used as a main solvent in model systems containing bioorganic molecules. Choosing the right water model is an important step in the study of biophysical and biochemical processes that occur in cells. In the present work, we perform molecular dynamics simulations using two distinct force fields for water - the rigid model TIP4P/2005, where only intermolecular interactions are considered, and the flexible model SPC/Fw, where intramolecular interactions are also taken into account. The simulations aim to determine the effect of the inclusion of intramolecular interactions on the accuracy of calculated properties of bulk water (density and thermal expansion 1 coefficient, self-diffusion coefficients, shear viscosity, radial distribution functions, and dielectric constant), as compared to experimental results, over a temperature range between 250 K and 370 K. We find that the results of the rigid model present the smallest deviations relative to experiments for most of the calculated quantities, except for the shear viscosity of supercooled water and the water dielectric constant, where the flexible model presents better agreement with experiments.

},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{CANCADO2024118801,

title = {Science and Metrology of defects in graphene using Raman Spectroscopy},

author = {Luiz G. Cançado and Victor P. Monken and João Luiz E. Campos and Joyce C. C. Santos and Claudia Backes and Hélio Chacham and Bernardo R. A. Neves and Ado Jorio},

url = {https://www.sciencedirect.com/science/article/pii/S0008622324000186},

doi = {https://doi.org/10.1016/j.carbon.2024.118801},

issn = {0008-6223},

year  = {2024},

date = {2024-01-11},

urldate = {2024-01-01},

journal = {Carbon},

pages = {118801},

abstract = {This article offers a historical account of the evolution of defect metrology in graphene through Raman spectroscopy over the past five decades. The application of Raman scattering to the investigation of disorder levels in graphite materials dates back to the 1970s, and substantial advancements have occurred in this field, especially following the isolation of graphene in 2006. The article starts presenting the physics related to structural defects disrupting the translational symmetry in crystalline solids, introducing a relaxation of selection rules in Raman spectroscopy that manifests as peaks induced by disorder, then it delves into significant milestones and provides a practical summary of the principal existing protocols. Furthermore, we explore the contribution of tip-enhanced Raman spectroscopy to gaining deeper insights into fundamental aspects of defects in graphene materials, owing to its capacity for spectroscopic measurements with high spatial resolution. In conclusion, we outline prospects for the further utilization of this innovative technique in enhancing both the science and metrology of defects in graphene and its applications in other two-dimensional systems.},

keywords = {Defects, Grapehene, Line defects, Liquid-phase exfoliation, Number of layers, Point defects, Raman spectroscopy, Tip-enhanced raman spectroscopy},

pubstate = {published},

tppubtype = {article}

}

@article{doi:10.1021/acs.jpcc.3c05344,

title = {Unveiling Pseudo-Dirac Resonances in the (7×7)R19.1° Phase of Sulfur Segregated on Cu(111) Surface},

author = {C. G Vieira and Matheus A. P H de Oliveira and W. P. Fernandes and M. J. S. Matos and R. Paniago and Vagner Eustáquio Carvalho and Hélio Chacham and E. A Soares},

url = {https://doi.org/10.1021/acs.jpcc.3c05344},

doi = {10.1021/acs.jpcc.3c05344},

year  = {2023},

date = {2023-12-16},

urldate = {2023-01-01},

journal = {The Journal of Physical Chemistry C},

volume = {127},

number = {51},

pages = {24633-24640},

abstract = {The Cu–S alloy is of great scientific importance due to its wide range of technological applications, including Cu–S antifluorite crystals exhibiting topological states. To gain insights into the sulfur–copper interaction, a simplified model of sulfur adsorption in copper is commonly used, particularly in surface reconstructions like the (7×7)R19.1° phase. However, determining the optimal structural model for this type of reconstruction has been a persistent challenge. Among the various models proposed to explain the reconstruction, Foss and Prince models have gained greater acceptance. In our comprehensive study, we employed a comparative approach utilizing angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and density functional theory calculations. Our objective was to investigate the band structure and local density of states of this reconstruction resulting from sulfur segregation on the Cu(111) surface, with the aim of assessing the compatibility of the experimental results with the two leading models. Remarkably, our analysis unveiled intriguing localization effects within the bulk resonances and distinctive surface bands originating from the copper–sulfur cluster present in both models. Through careful examination of electronic dispersion and the local density of states, we successfully distinguished between the models and determined which one better aligned with our experimental data.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Barcelos2023,

title = {Phyllosilicates as earth-abundant layered materials for electronics and optoelectronics: Prospects and challenges in their ultrathin limit},

author = {Ingrid D. Barcelos and Raphaela de Oliveira and Gabriel R. Schleder and Matheus J. S. Matos and Raphael Longuinhos and Jenaina Ribeiro-Soares and Ana Paula M. Barboza and Mariana C. Prado and Elisângela S. Pinto and Yara Galvão Gobato and Hélio Chacham and Bernardo R. A. Neves and Alisson R. Cadore},

doi = {10.1063/5.0161736},

issn = {1089-7550},

year  = {2023},

date = {2023-09-07},

volume = {134},

number = {9},

publisher = {AIP Publishing},

abstract = {Phyllosilicate minerals are an emerging class of naturally occurring layered insulators with large bandgap energy that have gained attention from the scientific community. This class of lamellar materials has been recently explored at the ultrathin two-dimensional level due to their specific mechanical, electrical, magnetic, and optoelectronic properties, which are crucial for engineering novel devices (including heterostructures). Due to these properties, phyllosilicate minerals can be considered promising low-cost nanomaterials for future applications. In this Perspective article, we will present relevant features of these materials for their use in potential 2D-based electronic and optoelectronic applications, also discussing some of the major challenges in working with them.},

keywords = {General Physics and Astronomy},

pubstate = {published},

tppubtype = {article}

}

@article{doi:10.1021/acs.jpcc.3c00490,

title = {Water Diffusion in Carbon Nanotubes for Rigid and Flexible Models},

author = {Bruno H. S. Mendonça and Elizane E. Moraes and Ronaldo J. C. Batista and Alan B. Oliveira and Marcia C. Barbosa and Hélio Chacham},

url = {https://doi.org/10.1021/acs.jpcc.3c00490},

doi = {10.1021/acs.jpcc.3c00490},

year  = {2023},

date = {2023-01-01},

journal = {The Journal of Physical Chemistry C},

volume = {127},

number = {20},

pages = {9769-9778},

abstract = {We compared the diffusion of water confined in armchair and zigzag carbon nanotubes for rigid and flexible water models. Using one rigid model, TIP4P/2005, and two flexible models, SPC/Fw and SPC/FH, we found that the number of the hydrogen bonds that water forms depends on the structure of the nanotube, directly affecting the diffusion of water. The simulation results reveal that, due to the hydrophobic nature of carbon nanotubes and the degrees of freedom imposed by the water force fields, water molecules tend to avoid the surface of the carbon nanotube. This junction of variables plays a central role in the diffusion of water, mainly in narrow and/or deformed nanotubes, governing the mobility of confined water in a nontrivial way, where the greater the degree of freedom of the water force field, the smaller mobility it will have in confinement as we limit the competition between area and volume and it no longer plays the unique role in changing water diffusivity.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}