Staff member

Lázaro René Izquierdo Fábregas

Postdoctoral Researcher
Nanoscale Bioelectrical Characterization
+34 934 031 184
Staff member publications

Fumagalli, L., Esfandiar, A., Fabregas, R., Hu, S., Ares, P., Janardanan, A., Yang, Q., Radha, B., Taniguchi, T., Watanabe, K., Gomila, G., Novoselov, K. S., Geim, A. K., (2018). Anomalously low dielectric constant of confined water Science 360, (6395), 1339-1342

Theoretical studies predict that the inhibition of rotational motion of water near a solid surface will decrease its local dielectric constant. Fumagalli et al. fabricated thin channels in insulating hexagonal boron nitride on top of conducting graphene layers (see the Perspective by Kalinin). The channels, which varied in height from 1 to 300 nanometers, were filled with water and capped with a boron nitride layer. Modeling of the capacitance measurements made with an atomic force microscope tip revealed a surface-layer dielectric constant of 2, compared with the bulk value of 80 for water.Science, this issue p. 1339; see also p. 1302The dielectric constant ε of interfacial water has been predicted to be smaller than that of bulk water (ε ≈ 80) because the rotational freedom of water dipoles is expected to decrease near surfaces, yet experimental evidence is lacking. We report local capacitance measurements for water confined between two atomically flat walls separated by various distances down to 1 nanometer. Our experiments reveal the presence of an interfacial layer with vanishingly small polarization such that its out-of-plane ε is only ~2. The electrically dead layer is found to be two to three molecules thick. These results provide much-needed feedback for theories describing water-mediated surface interactions and the behavior of interfacial water, and show a way to investigate the dielectric properties of other fluids and solids under extreme confinement.

Lozano, Helena, Fabregas, Rene, Blanco, Núria, Millán, Rubén, Torrents, Eduard, Fumagalli, Laura, Gomila, Gabriel, (2018). Dielectric constant of flagellin proteins measured by scanning dielectric microscopy Nanoscale 10, 19188-19194

The dielectric constant of flagellin proteins in flagellar bacterial filaments ~10-20 nm in diameter is measured using Scanning Dielectric Microscopy. We obtain for two different bacterial species (Shewanella oneidensis MR-1 and Pseudo-monas aeruginosa PAO1) similar relative dielectric constant values εSo = 4.3 ± 0.6 and εPa = 4.5 ± 0.7, respectively, despite their different structure and aminoacid sequence. Present results show the applicability of Scanning Dielectric Microscopy to nanoscale filamentous protein complexes, and to general 3D macromolecular protein geometries, thus opening new avenues to study the relationship between dielectric response and protein structure and function.

Wu, Bi-Yi, Sheng, Xin-Qing, Fabregas, Rene, Hao, Yang, (2017). Full-wave modeling of broadband near field scanning microwave microscopy Scientific Reports 7, (1), 16064

A three-dimensional finite element numerical modeling for the scanning microwave microscopy (SMM) setup is applied to study the full-wave quantification of the local material properties of samples. The modeling takes into account the radiation and scattering losses of the nano-sized probe neglected in previous models based on low-frequency assumptions. The scanning techniques of approach curves and constant height are implemented. In addition, we conclude that the SMM has the potential for use as a broadband dielectric spectroscopy operating at higher frequencies up to THz. The results demonstrate the accuracy of previous models. We draw conclusions in light of the experimental results.

Biagi, Maria Chiara, Badino, Giorgio, Fabregas, Rene, Gramse, Georg, Fumagalli, Laura, Gomila, Gabriel, (2017). Direct mapping of the electric permittivity of heterogeneous non-planar thin films at gigahertz frequencies by scanning microwave microscopy Physical Chemistry Chemical Physics 19, (5), 3884-3893

We obtained maps of the electric permittivity at ~19 GHz frequencies on non-planar thin film heterogeneous samples by means of combined atomic force-scanning microwave microscopy (AFM-SMM). We show that the electric permittivity maps can be obtained directly from the capacitance images acquired in contact mode, after removing the topographic cross-talk effects. This result demonstrates the possibility to identify the electric permittivity of different materials in a thin film sample irrespectively of their thickness by just direct imaging and processing. We show, in addition, that quantitative maps of the electric permittivity can be obtained with no need of any theoretical calculation or complex quantification procedure when the electric permittivity of one of the materials is known. To achieve these results the use of contact mode imaging is a key factor. For non-contact imaging modes the effects of the local sample thickness and of the imaging distance makes the interpretation of the capacitance images in terms of the electric permittivity properties of the materials much more complex. Present results represent a substantial contribution to the field of nanoscale microwave dielectric characterization of thin film materials with important implications for the characterization of novel 3D electronic devices and 3D nanomaterials.

Van Der Hofstadt, Marc, Fabregas, Rene, Millan, Ruben, Juarez, Antonio, Fumagalli, Laura, Gomila, Gabriel, (2016). Internal hydration properties of single bacterial endospores probed by electrostatic force microscopy ACS Nano 10, (12), 11327–11336

We show that the internal hydration properties of single Bacillus cereus endospores in air under different relative humidity (RH) conditions can be determined through the measurement of its electric permittivity by means of quantitative electrostatic force microscopy (EFM). We show that an increase in the RH from 0% to 80% induces a large increase in the equivalent homogeneous relative electric permittivity of the bacterial endospores, from ~4 up to ~17, accompanied only by a small increase in the endospore height, of just a few nanometers. These results correlate the increase of the moisture content of the endospore with the corresponding increase of environmental RH. 3D finite element numerical calculations, which include the internal structure of the endospores, indicate that the moisture is mainly accumulated in the external layers of the endospore, hence preserving the core of the endospore at low hydration levels. This mechanism is different from what we observe for vegetative bacterial cells of the same species, in which the cell wall at high humid atmospheric conditions is not able to preserve the cytoplasmic region at low hydration levels. These results show the potential of quantitative EFM under environmental humidity control to study the hygroscopic properties of small scale biological (and non-biological) entities and to determine its internal hydration state. A better understanding of nano-hygroscopic properties can be of relevance in the study of essential biological processes and in the design of bio-nanotechnological applications.

Biagi, Maria Chiara, Fabregas, Rene, Gramse, Georg, Van Der Hofstadt, Marc, Juárez, Antonio, Kienberger, Ferry, Fumagalli, Laura, Gomila, Gabriel, (2016). Nanoscale electric permittivity of single bacterial cells at gigahertz frequencies by scanning microwave microscopy ACS Nano 10, (1), 280-288

We quantified the electric permittivity of single bacterial cells at microwave frequencies and nanoscale spatial resolution by means of near-field scanning microwave microscopy. To this end, calibrated complex admittance images have been obtained at ~19 GHz and analyzed with a methodology that removes the nonlocal topographic cross-talk contributions and thus provides quantifiable intrinsic dielectric images of the bacterial cells. Results for single Escherichia coli cells provide a relative electric permittivity of ~4 in dry conditions and ~20 in humid conditions, with no significant loss contributions. Present findings, together with the ability of microwaves to penetrate the cell membrane, open an important avenue in the microwave label-free imaging of single cells with nanoscale spatial resolution.

Van Der Hofstadt, M., Fabregas, R., Biagi, M.C., Fumagalli, L., Gomila, G., (2016). Nanoscale dielectric microscopy of non-planar samples by lift-mode electrostatic force microscopy Nanotechnology 27, (40), 405706

Lift-mode electrostatic force microscopy (EFM) is one of the most convenient imaging modes to study the local dielectric properties of non-planar samples. Here we present the quantitative analysis of this imaging mode. We introduce a method to quantify and subtract the topographic crosstalk from the lift-mode EFM images, and a 3D numerical approach that allows for extracting the local dielectric constant with nanoscale spatial resolution free from topographic artifacts. We demonstrate this procedure by measuring the dielectric properties of micropatterned SiO 2 pillars and of single bacteria cells, thus illustrating the wide applicability of our approach from materials science to biology.