# Publicações

2020
Souza A, Matos MJS, Mazzoni MSC. Oxidation-driven formation of precisely ordered antimonene nanoribbons. Journal of Physics: Condensed Matter [Internet]. 2020. Publisher's VersionAbstract
The control of geometric structure is a key aspect in the interplay between theoretical predictions and experimental realization in the science and applications of nanomaterials. This is particularly important in one-dimensional structures such as nanoribbons, in which the edge morphology dictates most of the electronic behavior in low energy scale. In the present work we demonstrate by means of first principles calculations that the oxidation of few-layer antimonene may lead to an atomic restructuring with formation of ordered multilayer zig-zag nanoribbons. The widths are uniquely determined by the number of layers of the initial structure, allowing the synthesis of ultranarrow ribbons and chains. We also show that the process may be extended to other compounds based on group V elements, such as arsenene. The characterization of the electronic structure of the resulting ribbons shows an important effect of stacking on band gaps and on modulation of electronic behavior.
Batista RJC, Dias RF, Barboza APM, de Oliveira AB, Manhabosco TM, Gomes-Silva TR, Gadellha AC, Rabelo C, Cançado LGL, Jorio A, et al. Nanomechanics of few-layer materials: do individual layers slide upon folding?. Beilstein J. Nanotechnol. 2020;11:1801–1808.
Paez EBA, Curcio S, Neme NP, Matos MJS, Correa RS, Pereira FJ, Hilário FF, Cazati T, Taylor JG. Synthesis, photophysical and electrochemical properties of novel and highly fluorescent difluoroboron flavanone β-diketonate complexes. New J. Chem. [Internet]. 2020;44:14615-14631. Publisher's VersionAbstract
Difluoroboron β-diketonates complexes are highly luminescent with extensive properties such as their fluorescence both in solution and in solid state and their high molar extinction coefficients. Due to their rich optical properties, these compounds have been studied for their applications in organic electronics such as in self-assembly and applications in biosensors, bio-imaging and optoelectronic devices. The easy and fast synthesis of difluoroboron β-diketonate (BF2dbm) complexes makes their applications even more attractive. Although many different types of difluoroboron β-diketonates complexes have been studied, the cyclic flavanone analogues of these compounds have never been reported in the literature. Therefore, the present work aims to synthesize difluouroboron flavanone β-diketonate complexes, study their photophysical and electrochemical properties and assess their suitability for applications in optoelectronic devices. The synthesis was based on a Baker–Venkataraman reaction which initially provided substituted diketones, which were subsequently reacted with aldehydes to afford the proposed flavanones. The complexation was achieved by reacting flavanones and BF3·Et2O and in total 9 novel compounds were obtained. A representative difluoroboron flavanone complex was subjected to single crystal X-ray diffraction to unequivocally confirm the chemical structure. A stability study indicated only partial degradation of these compounds over a few days in a protic solvent at elevated temperatures. Photophysical studies revealed that the substituent groups and the solvent media significantly influence the electrochemical and photophysical properties of the final compounds, especially the molar absorption coefficient, fluorescence quantum yields, and the band gap. Moreover, the compounds exhibited a single excited-state lifetime in all studied solvents. Computational studies were employed to evaluate ground and excited state properties and carry out DFT and TDDFT level analysis. These studies clarify the role of each state in the experimental absorption spectra as well as the effect of the solvent.
de Freitas DN, Mendonça BHS, Köhler MH, Barbosa MC, Matos MJS, Batista RJC, de Oliveira AB. Water diffusion in carbon nanotubes under directional electric frields: Coupling between mobility and hydrogen bonding. Chemical Physics [Internet]. 2020;537:110849. Publisher's VersionAbstract
Molecular Dynamics simulations of water confined in carbon nanotubes subjected to external electric fields show that water mobility strongly depends on the confining geometry, the intensity and directionality of the electric field. While fields forming angles of 0° and 45° slow down the water dynamics by increasing organization, perpendicular fields can enhance water diffusion by decreasing hydrogen bond formation. For 1.2 diameter long nanotubes, the parallel field destroys the ice-like water structure increasing mobility. These results indicate that the structure and dynamics of confined water are extremely sensitive to external fields and can be used to facilitate filtration processes.
Martins LPG, Silva DL, Smith JS, Lu A-Y, Su C, Hempel M, Occhialini C, Ji X, Pablo R, Alencar RS, et al. Hard, transparent, sp3-containing 2D phase formed from few-layer graphene under compression. Carbon [Internet]. 2020. Publisher's VersionAbstract
Despite several theoretically proposed two-dimensional (2D) diamond structures, experimental efforts to obtain such structures are in initial stage. Recent high-pressure experiments provided significant advancements in the field, however, expected properties of a 2D-like diamond such as sp3 content, transparency and hardness, have not been observed together in a compressed graphene system. Here, we compress few-layer graphene samples on SiO2/Si substrate in water and provide experimental evidence for the formation of a quenchable hard, transparent, sp3-containing 2D phase. Our Raman spectroscopy data indicates phase transition and a surprisingly similar critical pressure for two-, five-layer graphene and graphite in the 4-6 GPa range, as evidenced by changes in several Raman features, combined with a lack of evidence of significant pressure gradients or local non-hydrostatic stress components of the pressure medium up to ≈ 8 GPa. The new phase is transparent and hard, as evidenced from indentation marks on the SiO2 substrate, a material considerably harder than graphene systems. Furthermore, we report the lowest critical pressure (≈ 4 GPa) in graphite, which we attribute to the role of water in facilitating the phase transition. Theoretical calculations and experimental data indicate a novel, surface-to-bulk phase transition mechanism that gives hint of diamondene formation.
Antoniazzi I, Chagas T, Matos MJS, Marçal LAB, Soares EA, Mazzoni MSC, Miwa RH, Lopes JMJ, Malachias Â, Magalhães-Paniago R, et al. Oxygen intercalated graphene on SiC(0001): Multiphase SiOx layer formation and its influence on graphene electronic properties. Carbon [Internet]. 2020. Publisher's VersionAbstract
Low-dimensionality materials are highly susceptible to interfaces. Indeed, intercalation of different chemical species in between epitaxial graphene and silicon carbide (SiC), for instance, may decouple the graphene with respect to the substrate due to the conversion of the buffer layer into a graphene layer. O-intercalation is known to release the strain of such 2D material and to lead to the formation of high structural quality AB-stacked bilayer graphene. Nonetheless, this interface transformation concomitantly degrades graphene electronic transport properties. In this work we employed different techniques in order to better understand the structure of the graphene/SiC interface generated by O-intercalation and to elucidate the origin of the poor electronic properties of graphene. Experimental results revealed the formation of a SiO2 rich layer with a defective transition layer in between it and the SiC, which is characterized by the existence of silicon oxycarbide structures. Scanning tunneling spectroscopy measurements revealed an extensive presence of electronic states just around the Fermi level all over the sample surface, which may suppress the charge carriers mobility around this region. According to theoretical calculations, such states are mainly due to the formation of silicon oxicarbides within the interfacial layer.
2019
Santos JCC, Barboza APM, Matos MJS, Barcelos ID, Fernandes TFD, Soares EA, Moreira RL, Neves BRA. Exfoliation and characterization of a two-dimensional serpentine-based material. Nanotechnology [Internet]. 2019;30:445705. Publisher's VersionAbstract
We report on an experimental investigation of serpentine, an abundant phyllosilicate, as an alternative source of two-dimensional (2D) nanomaterials. We show, through scanning probe microscopy (SPM) measurements, that natural serpentine mineral can be mechanically exfoliated down to few-layer flakes, where monolayers can be easily resolved. The parent serpentine bulk material was initially characterized via conventional techniques like XRD, XPS, FTIR and Raman spectroscopies and the results show that it is predominantly constituted by the antigorite mineral. From ab initio calculations using density functional theory, we also determine the geometry and electronic structure of antigorite, the observed structural form of serpentine. Additionally, we further characterized electrical and mechanical properties of the obtained 2D material flakes using SPM and broadband synchrotron infrared nanospectroscopy. Wavelength tuning of the serpentine vibrational resonances, assigned to in- and out-of-plane molecular vibrations, are observed and compared with the FTIR characterization of the parent bulk material. They show that there is no degradation of serpentine’s structural properties during its mechanical exfoliation down to nanometer-thin sheets. Therefore, our results introduce the serpentine mineral as an attractive low-cost candidate in 2D materials applications.
Barboza APM, Souza ACR, Matos MJS, Brant JC, Barbosa TC, Chacham H, Mazzoni MSC, Neves BRA. Graphene/h-BN heterostructures under pressure: From van der Waals to covalent. Carbon [Internet]. 2019;155:108 - 113. Publisher's VersionAbstract
Scanning probe microscopy and ab initio calculations reveal modifications on the electronic and structural properties of graphene/h-BN heterostructures induced by compression. Using AFM and EFM techniques, with charge injection being made in the heterostructures at different pressures, the charge injection efficiency monotonically decreases with increasing pressure for monolayer-graphene (MLG)+BN heterostructures, indicative of a conductor-insulator electronic transition. Bilayer-graphene (BLG)+BN and trilayer-graphene (TLG)+BN heterostructures show a non-monotonic behavior of charge injection versus pressure, indicative of competing electronic structure modifications. First-principle calculations of these systems indicate a pressure-induced van der Waals-to-covalent interlayer transition, where such interlayer covalent binding, in the presence of water molecules, results in a disordered insulating structure for the MLG + BN case, while it leads to an ordered conducting structure for both BLG + BN and TLG + BN heterostructures. These opposing effects may have a strong influence on graphene/h-BN-based electronic devices and their physics under pressurized environments.
Nascimento R, Moraes EE, Matos MJS, Prendergast D, Manhabosco TM, de Oliveira AB, Chacham H, Batista RJC. Graphene/h-BN In-Plane Heterostructures: Stability and Electronic and Transport Properties. The Journal of Physical Chemistry C [Internet]. 2019;123:18600-18608. Publisher's Version
Santos J, Barboza A, Matos MJS, Barcelos ID, Damasceno Fernandes TF, Soares EA, Moreira RL, Neves BRA. Exfoliation and characterization of a two-dimensional serpentine-based material. Nanotechnology [Internet]. 2019. Publisher's VersionAbstract
We report on an experimental investigation of serpentine, an abundant phyllosilicate, as an alternative source of two-dimensional (2D) nanomaterials. We show, through scanning probe microscopy (SPM) measurements, that natural serpentine mineral can be mechanically exfoliated down to few-layer flakes, where monolayers can be easily resolved. The parent serpentine bulk material was initially characterized via conventional techniques like XRD, XPS, FTIR and Raman spectroscopies and the results show that it is predominantly constituted by the antigorite mineral. From ab initio calculations using DFT, we also determine the geometry and electronic structure of antigorite, the observed structural form of serpentine. Additionally, we further characterized electrical and mechanical properties of the obtained 2D material flakes using SPM and broadband synchrotron infrared nanospectroscopy. Wavelength tuning of the serpentine vibrational resonances, assigned to in- and out-of-plane molecular vibrations, are observed and compared with the FTIR characterization of the parent bulk material. They show that there is no degradation of serpentine`s structural properties during its mechanical exfoliation down to nanometer-thin sheets. Therefore, our results introduce the serpentine mineral as an attractive low-cost candidate in 2D materials applications.
dos Santos PL, Silveira OJ, Huang R, Jardim GAM, Matos MJS, da Silva Júnior EN, Monkman AP, Dias FB, Cury LA. Dynamics of aggregated states resolved by gated fluorescence in films of room temperature phosphorescent emitters. Phys. Chem. Chem. Phys. [Internet]. 2019;21:3814-3821. Publisher's VersionAbstract
Phenazine derivative molecules were studied using steady state and time resolved fluorescence techniques and demonstrated to lead to strong formation of aggregated species, identified as dimers by time dependent density functional theory calculations. Blended films in a matrix of Zeonex®, produced at different concentrations, showed different contributions of dimer and monomer emissions in a prompt time frame, e.g. less than 50 ns. In contrast, the phosphorescence (e.g. emission from the triplet state) shows no significant effect on dimer formation, although strong dependence of the phosphorescence intensity on concentration is observed, leading to phosphorescence being quenched at higher concentration.
Dias RF, da Costa CC, Manhabosco TM, de Oliveira AB, Matos MJS, Soares JS, Batista RJC. Ab initio molecular dynamics simulation of methanol and acetonitrile: The effect of van der Waals interactions. Chemical Physics Letters [Internet]. 2019;714:172 - 177. Publisher's VersionAbstract
We employed PBE and BLYP semi-local functionals and the van der Waals density functional of Dion et al. (2004) (vdW-DF) to investigate structural properties of liquid acetonitrile and methanol. Among those functionals the vdW-DF is the only one that correctly predicts energy minima in inter-molecular interactions between acetonitrile molecules. We found that van der Waals interactions have a negligible effect on H-bonds in methanol chains. However, it significantly increases chain packing resulting in a more dense liquid in comparison to the other two functionals. The overall trend is that the vdW-DF tends to overestimate density and bulk modulus, meanwhile the semi-local functionals tend to underestimate density. Thus, van der Waals interactions play an important role in the properties of liquids in which much stronger dipole-dipole interactions are present.
Souza FS, Matos MJS, Galvão BRL, Arapiraca AFC, da Silva SN, Pinheiro IP. Adsorption of CO2 on biphasic and amorphous calcium phosphates: An experimental and theoretical analysis. Chemical Physics Letters [Internet]. 2019;714:143 - 148. Publisher's VersionAbstract
Calcium phosphates are suggested as a CO2 adsorbent via pressure swing adsorption. Amorphous calcium phosphate (ACP) and biphasic calcium phosphate (BCP) (composed of hydroxyapatite and beta-tricalcium phosphate) were investigated for the capture/immobilization of the gas. A fluidized bed was set up to assess the levels of CO2 adsorption by ACP and BCP. A gaseous mixture was synthesized, mimicking the conditions for possible industrial use. The results show a significant reduction in CO2 concentrations. Using DFT calculations, we show that CO2 adsorption increases the stability by reducing the surface energy. The energies involved and preferential adsorption sites were also theoretically predicted.
2018
Ferrari GA, de Oliveira AB, Silvestre I, Matos MJS, Batista RJC, Fernandes TFD, Meireles LM, Eliel GSN, Chacham H, Neves BRA, et al. Apparent Softening of Wet Graphene Membranes on a Microfluidic Platform. ACS Nano [Internet]. 2018;12:4312-4320. Publisher's Version
Barboza APM, Matos MJS, Chacham H, Batista RJC, de Oliveira AB, Mazzoni MSC, Neves BRA. Compression-Induced Modification of Boron Nitride Layers: A Conductive Two-Dimensional BN Compound. ACS Nano [Internet]. 2018:null. Publisher's Version
Sousa T, Damasceno Fernandes TF, Matos MJS, Araujo END, Mazzoni MSC, Neves BRA, Plentz F. Thionine Self-Assembled Structures on Graphene: Formation, Organization, and Doping. Langmuir [Internet]. 2018:null. Publisher's Version
Araujo KAS, Cury LA, Matos MJS, Fernandes TFD, Cançado LG, Neves BRA. Electro-optical interfacial effects on a graphene/π-conjugated organic semiconductor hybrid system. Beilstein Journal of Nanotechnology. 2018;9:963-974.
de Brito ACF, Correa RS, Pinto AA, Matos MJS, Tenorio JC, Taylor JG, Cazati T. Synthesis, crystal structure, photophysical properties and theoretical studies of a novel bis(phenylisoxazolyl) benzene derivative. Journal of Molecular Structure [Internet]. 2018:-. Publisher's VersionAbstract
Abstract Isoxazoles have well established biological activities but, have been underexplored as synthetic intermediates for applications in materials science. The aims of this work are to synthesis a novel isoxazole and analyze its structural and photophysical properties for application in electronic organic materials. The novel bis (phenylisoxazolyl) benzene compound was synthesized in four steps and characterized by NMR, high resolution mass spectrometry, differential thermal analysis, infrared spectroscopy, cyclic voltammetry, ultraviolet–visible spectroscopy, fluorescence spectroscopy, \DFT\ and \TDDFT\ calculations. The molecule presented optical absorption in the ultraviolet region (from 290 nm to 330 nm), with maximum absorption length centered at 306 nm. The molar extinction coefficients (ε), fluorescence emission spectra and quantum efficiencies in chloroform and dimethylformamide solution were determined. Cyclic voltammetry analysis was carried out for estimating the \HOMO\ energy level and these properties make it desirable material for photovoltaic device applications. Finally, the excited-state properties of present compound were calculated by time-dependent density functional theory (TDDFT).
2017
Martins LGP, Matos MJS, Paschoal AR, Freire PTC, Andrade NF, Aguiar A??sioL, Kong J, Neves BRA, de Oliveira AB, Mazzoni M??rioSC, et al. Raman evidence for pressure-induced formation of diamondene. Nature Communications [Internet]. 2017;8:96. Publisher's VersionAbstract
Despite the advanced stage of diamond thin-film technology, with applications ranging from superconductivity to biosensing, the realization of a stable and atomically thick two-dimensional diamond material, named here as diamondene, is still forthcoming. Adding to the outstanding properties of its bulk and thin-film counterparts, diamondene is predicted to be a ferromagnetic semiconductor with spin polarized bands. Here, we provide spectroscopic evidence for the formation of diamondene by performing Raman spectroscopy of double-layer graphene under high pressure. The results are explained in terms of a breakdown in the Kohn anomaly associated with the finite size of the remaining graphene sites surrounded by the diamondene matrix. Ab initio calculations and molecular dynamics simulations are employed to clarify the mechanism of diamondene formation, which requires two or more layers of graphene subjected to high pressures in the presence of specific chemical groups such as hydroxyl groups or hydrogens.
Gonçalves JA, Nascimento R, Matos MJS, de Oliveira AB, Chacham H, Batista RJC. Edge-Reconstructed, Few-Layered Graphene Nanoribbons: Stability and Electronic Properties. The Journal of Physical Chemistry C [Internet]. 2017;121:5836-5840. Publisher's Version
2016
Coelho PM, dos Reis DD, Matos MJS, Mendes-de-Sa TG, Goncalves AMB, Lacerda RG, Malachias A, Magalhaes-Paniago R. Near-edge X-ray absorption spectroscopy signature of image potential states in multilayer epitaxial graphene. Surface Science [Internet]. 2016;644:135-140. Publisher's VersionAbstract
Single layer behavior in multilayer epitaxial graphene has been a matter of intense investigation. This is due to the layer decoupling that occurs during growth of graphene on some types of substrates, such as carbon-terminated silicon carbide. We show here that near-edge X-ray absorption spectroscopy can be used to observe the signature of this decoupling. To this end, samples of multilayer graphene from silicon carbide sublimation were grown with different degrees of decoupling. Raman spectroscopy was used to infer the degree of structural decoupling. X-ray grazing-incidence diffraction and scanning tunneling microscopy showed that growth initiates with the presence of bilayer graphene commensurate structures, while layer decoupling is associated to the formation of incommensurate structures observed for longer sublimation time. Near-edge X-ray absorption spectroscopy was used to probe the electronic states above the Fermi energy. Besides the σ* and π* empty states, image potential states are observed and show a clear change of intensity as a function of incident angle. These image potential states evolve from a graphite- to graphene-like behavior as a function of growth time and can be used to infer the degree of structural coupling among layers.
Chagas T, Cunha THR, Matos MJS, dos Reis DD, Araujo KAS, Malachias A, Mazzoni MSC, Ferlauto AS, Magalhaes-Paniago R. Room temperature observation of the correlation between atomic and electronic structure of graphene on Cu(110). RSC Adv. [Internet]. 2016;6:98001-98009. Publisher's VersionAbstract
In this work we have used atomically-resolved scanning tunneling microscopy and spectroscopy to study the interplay between the atomic and electronic structure of graphene formed on copper via chemical vapor deposition. Scanning tunneling microscopy directly revealed the epitaxial match between a single layer of graphene and the underlying copper substrate in different crystallographic orientations. Using scanning tunneling spectroscopy we have directly measured the electronic density of states of graphene layers near the Fermi level, observing the appearance of a series of peaks in specific cases. These features were analyzed in terms of substrate-induced perturbations in the structural and electronic properties of graphene by means of atomistic models supported by density functional theory calculations.
2015
Prado MC, Nascimento R, Faria BEN, Matos MJS, Chacham H, Neves BRA. Nanometre-scale identification of grain boundaries in MoS 2 through molecular decoration. Nanotechnology [Internet]. 2015;26(47):475702. Publisher's VersionAbstract
In this paper, we address the challenge of identifying grain boundaries on the molybdenum disulphide (MoS 2 ) surface at the nanometre scale using a simple self-assembled monolayer (SAM) decoration method. Combined with atomic force microscopy, octadecylphosphonic acid monolayers readily reveal grain boundaries in MoS 2 at ambient conditions, without the need of atomic resolution measurements under vacuum. Additional ab initio calculations allow us to obtain the preferred orientation of the SAM structure relative to the MoS 2 beneath, and therefore, together with the experiments, the MoS 2 crystalline orientations at the grain boundaries. The proposed method enables the visualization of grain boundaries with sub-micrometer resolution for nanodevice investigation and failure analysis.
2014
de Pauli M, Matos MJS, Siles PF, Prado MC, Neves BRA, Ferreira SO, Mazzoni MSC, Malachias A.

Chemical Stabilization and Improved Thermal Resilience of Molecular Arrangements: Possible Formation of a Surface Network of Bonds by Multiple Pulse Atomic Layer Deposition

. Journal of Physical Chemistry B. 2014;118(32):9792-9799.Abstract
In this work, we make use of an atomic layer deposition (ALD) surface reaction based on trimethyl-aluminum (TMA) and water to modify O–H terminated self-assembled layers of octadecylphosphonic acid (OPA). The structural modifications were investigated by X-ray reflectivity, X-ray diffraction, and atomic force microscopy. We observed a significant improvement in the thermal stability of ALD-modified molecules, with the existence of a supramolecular packing structure up to 500 °C. Following the experimental observations, density functional theory (DFT) calculations indicate the possibility of formation of a covalent network with aluminum atoms connecting OPA molecules at terrace surfaces. Chemical stability is also achieved on top of such a composite surface, inhibiting further ALD oxide deposition. On the other hand, in the terrace edges, where the covalent array is discontinued, the chemical conditions allow for oxide growth. Analysis of the DFT results on band structure and density of states of modified OPA molecules suggests that besides the observed thermal resilience, the dielectric character of OPA layers is preserved. This new ALD-modified OPA composite is potentially suitable for applications such as dielectric layers in organic devices, where better thermal performance is required.
Matos MJS, Mazzoni MSC, Chacham H.

Graphene-boron nitride superlattices: the role of point defects at the BN layer

. Nanotechnology. 2014;25(16).Abstract
We investigate, by means of first-principles calculations, the role of hBN point defects on the energetical stability and electronic structure of heterostructures composed of graphene atop hBN, rotated at angles of 13.17°, 9.43° and 7.34°. We consider, as possible point defects, boron and nitrogen vacancies and antisites, substitutional oxygen at the nitrogen site ON, substitutional carbon dimers, and nitrogen interstitials. The electronic and structural properties of all defects were analyzed. Among these, the most stable is ON, with negative formation energies at several possible rotation angles and chemical environments. Under such conditions, ON doping can raise the Fermi level of the neutral system by as much as 1 eV relative to graphene's Dirac point, reaching the band crossing between adjacent Dirac cones at the M point of the heterostructure Brillouin zone. This could lead to interesting electronic transport properties without the need for electrostatic doping.
2013
Carozo V, Almeida CM, Fragneaud B, Bede PM, Moutinho MVO, Ribeiro-Soares J, Andrade NF, Souza Filho AG, Matos MJS, Wang B, et al.

Resonance effects on the Raman spectra of graphene superlattices

. Physical Review B. 2013;88(8).Abstract
In this work, a study of resonance effects in the Raman spectra of twisted bilayer graphene (tBLG) is presented. The analysis takes into account the effect of the mismatch angle θ between the two layers, and also of the excitation laser energy on the frequency, linewidth, and intensity of the main Raman features, namely the rotationally induced R band, the G band, and the second-order G′ (or 2D) band. The resonance effects are explained based on the θ dependence of the tBLG electronic structure, as calculated by ab initio methodologies. The twist angle θ also defines the observation of a “D-like” band which obeys the double-resonance process, but relies on the superlattice along with long-range defects in order to fulfill momentum conservation. The study was possible due to the development of a route to produce and identify rotationally stacked bilayer graphene by means of atomic force microscopy (AFM).
2012
de Pauli M, Prado MC, Matos MJS, Fontes GN, Perez CA, Mazzoni MSC, Neves BRA, Malachias A.

Thermal Stability and Ordering Study of Long- and Short-Alkyl Chain Phosphonic Acid Multilayers

. Langmuir. 2012;28(43):15124-15133.Abstract
Long-range order evolution of self-assembled phosphonic acid multilayers as a function of temperature is studied here for two molecules with different alkyl chain length. By using synchrotron conventional diffraction, distinct order configurations are retrieved on phosphonic acid multilayers and their thermodynamic behavior monitored by energy-dispersive diffraction. This later technique allows us to observe the system behavior near order–disorder temperatures, as well as to determine the most stable configurations in the range from room temperature up to 120 °C. Planar order is also addressed by wide-angle X-ray scattering (WAXS) transmission experiments. Order parameter phase diagrams are built based on the experimental results, showing the dominant configuration at each temperature. The multilayer molecular long-range order retrieved from the experiments is corroborated by first principles calculations based on the Density Functional Theory. The bulk configurations depicted in this work are produced by molecule–molecule interactions and allow for future comparisons with the behavior of ordered molecules in few-monolayers configurations, commonly used in organic devices, where the presence of surfaces and interfaces strongly affects the molecule packing.
Mendes-de-Sa TG, Goncalves AMB, Matos MJS, Coelho PM, Magalhaes-Paniago R, Lacerda RG.

Correlation between (in)commensurate domains of multilayer epitaxial graphene grown on SiC(000(1)over-bar) and single layer electronic behavior

. Nanotechnology. 2012;23(47).Abstract
A systematic study of the evolution of the electronic behavior and atomic structure of multilayer epitaxial graphene (MEG) as a function of growth time was performed. MEG was obtained by sublimation of a 4H-SiC($0 0 0\bar {1}$) substrate in an argon atmosphere. Raman spectroscopy and x-ray diffraction were carried out in samples grown for different times. For 30 min of growth the sample Raman signal is similar to that of graphite, while for 60 min the spectrum becomes equivalent to that of exfoliated graphene. Conventional x-ray diffraction reveals that all the samples have two different (0001) lattice spacings. Grazing incidence x-ray diffraction shows that thin films are composed of rotated (commensurate) structures formed by adjacent graphene layers. Thick films are almost completely disordered. This result can be directly correlated to the single layer electronic behavior of the films as observed by Raman spectroscopy. Finally, to understand the change in lattice spacings as a result of layer rotation, we have carried out first principles calculations (using density functional theory) of the observed commensurate structures.
Oliveira CK, Matos MJS, Mazzoni MSC, Chacham H, Neves BRA.

Anomalous response of supported few-layer hexagonal boron nitride to DC electric fields: a confined water effect?

. Nanotechnology. 2012;23(17).Abstract
We use electric force microscopy (EFM) to study the response of supported few-layer hexagonal boron nitride (h-BN) to an electric field applied by the EFM tip. Our results show an anomalous behavior in the dielectric response of h-BN atop Si oxide for different bias polarities: for a positive bias applied to the tip, h-BN layers respond with a larger dielectric constant than the dielectric constant of the substrate, while for a negative bias, the h-BN dielectric constant appears to be smaller. Based on ab initio calculations, we propose that this behavior is due to a water layer confined between the Si oxide substrate and h-BN layers. This hypothesis was experimentally confirmed by sample annealing and also by a comparative analysis with h-BN on a non-polar substrate.
2011
Prado MC, Nascimento R, Moura LG, Matos MJS, Mazzoni MSC, Cancado LG, Chacham H, Neves BRA.

Two-Dimensional Molecular Crystals of Phosphonic Acids on Graphene

. Acs Nano. 2011;5(1):394-398.Abstract
The synthesis and characterization of two-dimensional (2D) molecular crystals composed of long and linear phosphonic acids atop graphene is reported. Using scanning probe microscopy in combination with first-principles calculations, we show that these true 2D crystals are oriented along the graphene armchair direction only, thereby enabling an easy determination of graphene flake orientation. We have also compared the doping level of graphene flakes via Raman spectroscopy. The presence of the molecular crystal atop graphene induces a well-defined shift in the Fermi level, corresponding to hole doping, which is in agreement with our ab initio calculations.
2009
Matos MJS, Azevedo S, Kaschny JR.

On the structural properties of B-C-N nanotubes

. Solid State Communications. 2009;149(5-6):222-226.Abstract
The synthesis and characterization of two-dimensional (2D) molecular crystals composed of long and linear phosphonic acids atop graphene is reported. Using scanning probe microscopy in combination with first-principles calculations, we show that these true 2D crystals are oriented along the graphene armchair direction only, thereby enabling an easy determination of graphene flake orientation. We have also compared the doping level of graphene flakes via Raman spectroscopy. The presence of the molecular crystal atop graphene induces a well-defined shift in the Fermi level, corresponding to hole doping, which is in agreement with our ab initio calculations.