Publicações

29 Jan 2019

Optical absorption in complexes of abasic DNA with noble-metal nanoclusters by first principles calculations

De Carvalho, L. C. ; Silveira, O. J. ; Longuinhos, R. ; Nunes, R. W. ; Alexandre, S. S. PCCP. Physical Chemistry Chemical Physics , v. 21, p. 1260-1270, 2019.

Abasic sites (AP site) in a DNA duplex have been experimentally used to produce fluorescent Ag nanoclusters (NC) with a small number of atoms (n ≤ 6). These AP-DNA:NC complexes act as biological makers that help to locate genes associated with diseases related to single nucleotide polymorphisms (SNP), for example. Abasic sites are the most common SNP genetic variation, and their detection may help predict a host of genetically determined diseases. In this work, we report a theoretical study of the optical absorption spectra of AP-DNA:Ag4and AP-DNA:Au4 complexes using a fully ab initio methodology. We consider several different base environments for the noble-metal nanocluster occupying the AP site, and compute the absorption spectra of sixteen AP-DNA:Ag4 and sixteen AP-DNA:Au4 complexes. We find that optical absorption in the AP-DNA:Ag4complexes tends to concentrate in the green-to-violet range of frequencies (2.50 eV ≤ ħω ≤ 3.2 eV) and that AP-DNA:Au4 complexes display absorption peaks in the violet-to-ultraviolet interval (ħω ≥ 3.0 eV). An analysis of the optical absorption mechanisms in these complexes shows that they can be of local, charge-transfer, or hybrid nature, i.e., AP-DNA:NC complexes display the full variety of optical absorption processes in molecular systems. In particular, we identify both charge-transfer and hybrid processes involving several DNA bases surrounding the NC. Importantly, we find that even sequences where the Ag4 cluster is not in a guanine rich neighborhood display absorption peaks in the visible-light spectrum. Moreover, we obtain that the maximum intensities of the absorption peaks in complexes with pyrimidine vacancies are generally higher than those in complexes with purine vacancies. Regarding the selectivity of single-vacancy AP-DNA to specific noble-metal nanocluster sizes, our calculations show that the four-atom Ag4 (Au4) species fits naturally and binds into the AP-site in a single-vacancy AP-DNA.

Artigo Completo

15 Jan 2019

Monitoring the Applied Strain in Monolayer Gallium Selenide through Vibrational Spectroscopies: A First-Principles Investigation

Longuinhos, R.; Ribeiro-Soares, J. Physical Review Applied, v. 11, p. 024012-1-024012-12, 2019.

Monolayer gallium selenide (GaSe) is a promising material for nanoscale electronic, spintronic, and optoelectronic applications. Its electrical conductivity and optical properties were reported to be largely tuned by strain engineering, where the assessment of the applied strain is crucial. In this work, we apply first-principles calculations to unveil the strain-induced phonon frequency shifts in monolayer GaSe, showing its promising application as a strain probe. Furthermore, we find that the uniaxial-, biaxial-, and shear-strain onset for strain-induced lattice instability are 22%, 16%, and 5%, respectively, indicating it could be used as a handle in flexible electronics. We find monolayer GaSe to be about 2 times softer than monolayer MoS2 and 4 times softer than monolayer graphene, and that its Raman fingerprint modes can resolve strain values in the low-strain regime up to 1.0%. These results suggest its use as a strain sensor in fragile applications. Our results provide insight into the use of Raman and infrared spectroscopies to probe the strain in monolayer GaSe, which is of paramount importance for further developments in strain engineering and flexible electronics in GaSe-based devices.

Artigo Completo

10 Sep 2018

Carbon Stability of Engineered Biochar-Based Phosphate Fertilizers

Carneiro, Jefferson Santana da Silva; Lustosa Filho, José Ferreira; Nardis, Bárbara Olinda ; Ribeiro-Soares, Jenaina ; Zinn, Yuri Lopes ; Melo, Leônidas Carrijo Azevedo. ACS Sustainable Chemistry & Engineering, v. 6, p. 14203-14212, 2018.

Converting abundant agricultural residues to valuable products, such as biochar, is a pressing challenge for society. Here, our objective has been to produce biochar-based fertilizers (BBFs) with high carbon stability, high yield, and characteristics favorable for their use in soils. Thus, H3PO4 with and without MgO was co-pyrolyzed with coffee husk and poultry litter. Proximate analysis, total phosphorus and magnesium, biochar yield, carbon retention, and thermal and chemical carbon stability were assessed. The BBFs were slightly acidic (pH ∼6.0) and showed high contents of total P (146–206 g kg–1) and total Mg (96–98 g kg–1), similar to commercial fertilizers. Biochar yield increased up to 65% when treated with H3PO4 and H3PO4–MgO, resulting in up to 78% greater carbon retention. BBFs showed higher thermal stability, by thermogravimetric analysis and muffle oxidation, when compared to corresponding pristine biochars. Conversely, the addition of H3PO4–MgO decreased the chemical stability (higher H2O2 and K2Cr2O7 oxidation), compared to the biochars, because of the increase in surface area and reactivity. We concluded that the co-pyrolysis of biomasses with H3PO4 and MgO promoted thermal stabilization (increased yield) and increased chemical oxidation, because of increased surface area and reduced crystallinity of the BBFs.

Artigo Completo

11 May 2018

Stable holey two-dimensional C2N structures with tunable electronic structure

Longuinhos, R.; Ribeiro-Soares, J. Physical Review B, v. 97, p. 195119, 2018.

C2N holey two-dimensional crystals, or C2N-h2D, a recently synthesized carbon nitride layered material, show promising properties for electronic devices, highly selective molecular filters, and supercapacitors. Few studies have investigated the stacking order in C2N-h2D, which is fundamental to determine its optical activity and plays an important role in its band gap and in the diffusion barrier for ions and molecules through its structure. In this work, we investigate the phonon stability of several bulk C2N-h2D polytypes by using first-principles calculations. Among the polytypes addressed, only one does not display phonon instabilities and is expected to be observed in equilibrium. The electronic structure evolution of dynamically stable C2N-h2D from monolayer to bilayer and to bulk is unveiled. The direct band gap at Γ can be decreased by 34% from monolayer to bulk, offering opportunities for tuning it in optoelectronics. In addition, the effective masses of both carriers become smaller as the number of layers increases, and their anisotropy along in-plane directions displayed in the monolayer is reduced, which suggest that the carrier mobility may be tuned as well. These effects are then explained according to the interaction of the orbitals in neighboring layers. The results presented here shed light on the geometry and electronic structure of an emerging layered material due to its specific stacking and increasing number of layers and suggest new perspectives for applications in optoelectronics.

Artigo Completo

11 Dec 2017

Different Plant Biomass Characterizations for Biochar Production

Abreu, T. R. L. ; Lima, J. T. ; Dessimoni, A. L. A. ; Pego, M. F. F. ; Ribeiro-Soares, J. ; Trugilho, P. F. Cerne, v. 23, p. 529-536, 2017.

Artigo Completo

01 Sep 2016

Ultra-weak interlayer coupling in two-dimensional gallium selenide

Longuinhos, R.; Ribeiro-Soares, J. PCCP. Physical Chemistry Chemical Physics (Print), p. 25401-25408, 2016.