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
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.
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.
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.
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).
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.
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.
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.