Single-photon detection using large-scale high-temperature MgB2 sensors at 20 K.

Ultra-fast single-photon detectors with high current density and operating temperature can benefit space and ground applications, including quantum optical communication systems, lightweight cryogenics for space crafts, and medical use. Here we demonstrate magnesium diboride (MgB2) thin-film superconducting microwires capable of single-photon detection at 1.55 µm optical wavelength. We used helium ions to alter the properties of MgB2, resulting in microwire-based detectors exhibiting single-photon sensitivity across a broad temperature range of up to 20 K, and detection efficiency saturation for 1 µm wide microwires at 3.7 K. Linearity of detection rate vs incident power was preserved up to at least 100 Mcps. Despite the large active area of up to 400 × 400 µm2, the reset time was found to be as low as ~ 1 ns. Our research provides possibilities for breaking the operating temperature limit and maximum single-pixel count rate, expanding the detector area, and raises inquiries about the fundamental mechanisms of single-photon detection in high-critical-temperature superconductors.

Mesoscopic 3D Charge Transport in Solution-Processed Graphene-Based Thin Films: A Multiscale Analysis.

Graphene and related 2D material (GRM) thin films consist of 3D assembly of billions of 2D nanosheets randomly distributed and interacting via van der Waals forces. Their complexity and the multiscale nature yield a wide variety of electrical characteristics ranging from doped semiconductor to glassy metals depending on the crystalline quality of the nanosheets, their specific structural organization ant the operating temperature. Here, the charge transport (CT) mechanisms are studied that are occurring in GRM thin films near the metal-insulator transition (MIT) highlighting the role of defect density and local arrangement of the nanosheets. Two prototypical nanosheet types are compared, i.e., 2D reduced graphene oxide and few-layer-thick electrochemically exfoliated graphene flakes, forming thin films with comparable composition, morphology and room temperature conductivity, but different defect density and crystallinity. By investigating their structure, morphology, and the dependence of their electrical conductivity on temperature, noise and magnetic-field, a general model is developed describing the multiscale nature of CT in GRM thin films in terms of hopping among mesoscopic bricks, i.e., grains. The results suggest a general approach to describe disordered van der Waals thin films.

Electric Field and Strain Tuning of 2D Semiconductor van der Waals Heterostructures for Tunnel Field-Effect Transistors.

Heterostacks consisting of low-dimensional materials are attractive candidates for future electronic nanodevices in the post-silicon era. In this paper, using first-principles calculations based on density functional theory (DFT), we explore the structural and electronic properties of MoTe2/ZrS2 heterostructures with various stacking patterns and thicknesses. Our simulations show that the valence band (VB) edge of MoTe2 is almost aligned with the conduction band (CB) edge of ZrS2, and (MoTe2)m/(ZrS2)m (m = 1, 2) heterostructures exhibit the long-sought broken gap band alignment, which is pivotal for realizing tunneling transistors. Electrons are found to spontaneously flow from MoTe2 to ZrS2, and the system resembles an ultrascaled parallel plate capacitor with an intrinsic electric field pointed from MoTe2 to ZrS2. The effects of strain and external electric fields on the electronic properties are also investigated. For vertical compressive strains, the charge transfer increases due to the decreased coupling between the layers, whereas tensile strains lead to the opposite behavior. For negative electric fields a transition from the type-III to the type-II band alignment is induced. In contrast, by increasing the positive electric fields, a larger overlap between the valence and conduction bands is observed, leading to a larger band-to-band tunneling (BTBT) current. Low-strained heterostructures with various rotation angles between the constituent layers are also considered. We find only small variations in the energies of the VB and CB edges with respect to the Fermi level, for different rotation angles up to 30°. Overall, our simulations offer insights into the fundamental properties of low-dimensional heterostructures and pave the way for their future application in energy-efficient electronic nanodevices.

Accurate graphene quantum Hall arrays for the new International System of Units.

Graphene quantum Hall effect (QHE) resistance standards have the potential to provide superior realizations of three key units in the new International System of Units (SI): the ohm, the ampere, and the kilogram (Kibble Balance). However, these prospects require different resistance values than practically achievable in single graphene devices (~12.9 kΩ), and they need bias currents two orders of magnitude higher than typical breakdown currents IC ~ 100 µA. Here we present experiments on quantization accuracy of a 236-element quantum Hall array (QHA), demonstrating RK/236 ≈ 109 Ω with 0.2 part-per-billion (nΩ/Ω) accuracy with IC ≥ 5 mA (~1 nΩ/Ω accuracy for IC = 8.5 mA), using epitaxial graphene on silicon carbide (epigraphene). The array accuracy, comparable to the most precise universality tests of QHE, together with the scalability and reliability of this approach, pave the road for wider use of graphene in the new SI and beyond.

Toward Optimized Charge Transport in Multilayer Reduced Graphene Oxides.

In the context of graphene-based composite applications, a complete understanding of charge conduction in multilayer reduced graphene oxides (rGO) is highly desirable. However, these rGO compounds are characterized by multiple and different sources of disorder depending on the chemical method used for their synthesis. Most importantly, the precise role of interlayer interaction in promoting or jeopardizing electronic flow remains unclear. Here, thanks to the development of a multiscale computational approach combining first-principles calculations with large-scale transport simulations, the transport scaling laws in multilayer rGO are unraveled, explaining why diffusion worsens with increasing film thickness. In contrast, contacted films are found to exhibit an opposite trend when the mean free path becomes shorter than the channel length, since conduction becomes predominantly driven by interlayer hopping. These predictions are favorably compared with experimental data and open a road toward the optimization of graphene-based composites with improved electrical conduction.

Comparison of the Dynamic Thermal Gradient to Temperature-Programmed Conditions in Gas Chromatography Using a Stochastic Transport Model.

This paper compares dynamic (i.e., temporally changing) thermal gradient gas chromatography (GC) to temperature-programmed GC using a previously published stochastic transport model to simulate peak characteristics for the separation of C12-C40 hydrocarbons. All comparisons are made using chromatographic conditions that give approximately equal analyte retention times (tR). As shown previously, a static thermal gradient does not improve resolution (Rs) equally for all analytes, which highlights the need for a dynamic thermal gradient. An optimal dynamic thermal gradient should result in constant analyte velocities at any instant in time for those analytes that are actively being separated (i.e., analytes that have low retention factors). The average separation temperature for each analyte is used to determine the thermal gradient profile at different times in the temperature ramp. Because many of the analytes require a similar thermal gradient profile when actively being separated, the thermal gradient profile in this study was held fixed; however, the temperature of the entire thermal gradient was raised over time. From the simulations performed in this study, optimized dynamic thermal gradient conditions are shown to improve Rs by up to 13% over comparative temperature-programmed conditions, even with a perfect injection (i.e., zero injection bandwidth). In the dynamic thermal gradient simulations, all analytes showed improvements in Rs along with slightly shorter tR values compared to simulations for traditional temperature-programmed conditions.

Absceso hepático, bacteriemia y meningoencefalitis por Klebsiella pneumoniae hipervirulenta en el curso de infección por SARS - CoV2. Reporte de caso

Klebsiella pneumoniae es miembro de la familia de enterobacterias, es un patógeno oportunista que afecta a las personas con compromiso inmune, y es causa de infecciones nosocomiales. Los serotipos capsulares K1 y K2 de Klebsiella pneumoniae tienen la característica de hipermucoviscosa, que es considerada una cepa hipervirulenta. Se presenta el caso de una mujer con absceso hepático, bacteriemia y meningoencefalitis por Klebsiella pneumoniae hipervirulenta, en el curso de una infección por SARS - COV2, quien requirió soporte ventilatorio, vasopresor, tratamiento antibiótico y drenaje percutáneo del absceso en la Unidad de cuidados intensivos del Hospital Cayetano Heredia, logrando sobrevivencia y alta hospitalaria. Este caso nos permite resaltar la presencia de la cepa hipervirulenta de Klebsiella pneumoniae en nuestro país, lo cual nos permitirá realizar una evaluación integral oportuna y tratamiento precoz.

SUMMARY Klebsiella pneumoniae is a member of the family of Enterobacteriaceae, it is an opportunistic pathogen that affects people with immune compromise, and it is the cause of nosocomial infections. The capsular serotypes K1 and K2 of Klebsiella pneumoniae have the characteristic of hypermucoviscosa, which is considered a hypervirulent strain. We present the case of a female patient with liver abscess, bacteremia and hypervirulent Klebsiella pneumoniae meningoencephalitis, in the course of a SARS - CoV2 infection, who required ventilatory support, vasopressor, antibiotic treatment and percutaneous drainage of the abscess in the intensive care unit (ICU) of the Hospital Cayetano Heredia, achieving survival and hospital discharge. This case allows us to highlight the presence of the hypervirulent strain of Klebsiella pneumoniae in our country, which will allow us to carry out a timely comprehensive evaluation and early treatment.

Comparison of Static Thermal Gradient to Isothermal Conditions in Gas Chromatography Using a Stochastic Transport Model.

This paper compares static (i.e., temporally unchanging) thermal gradient gas chromatography (GC) to isothermal GC using a stochastic transport model to simulate peak characteristics for the separation of C12-C14 hydrocarbons resulting from variations in injection bandwidth. All comparisons are made using chromatographic conditions that give approximately equal analyte retention times so that the resolution and number of theoretical plates can be clearly compared between simulations. Simulations show that resolution can be significantly improved using a linear thermal gradient along the entire column length. This is mainly achieved by partially compensating for loss in resolution from the increase in mobile phase velocity, which approximates an ideal, basic separation. The slope of the linear thermal gradient required to maximize resolution is a function of the retention parameters, which are specific to each analyte pair; a single static, thermal gradient will not affect all analytes equally. A static, non-linear thermal gradient that creates constant analyte velocities at all column locations provides the largest observed gains in resolution. From the simulations performed in this study, optimized linear thermal gradient conditions are shown to improve the resolution by as much as 8.8% over comparative isothermal conditions, even with a perfect injection (i.e., zero initial bandwidth).

Multiscale Charge Transport in van der Waals Thin Films: Reduced Graphene Oxide as a Case Study.

Large area van der Waals (vdW) thin films are assembled materials consisting of a network of randomly stacked nanosheets. The multiscale structure and the two-dimensional (2D) nature of the building block mean that interfaces naturally play a crucial role in the charge transport of such thin films. While single or few stacked nanosheets (i.e., vdW heterostructures) have been the subject of intensive works, little is known about how charges travel through multilayered, more disordered networks. Here, we report a comprehensive study of a prototypical system given by networks of randomly stacked reduced graphene oxide 2D nanosheets, whose chemical and geometrical properties can be controlled independently, permitting to explore percolated networks ranging from a single nanosheet to some billions with room-temperature resistivity spanning from 10-5 to 10-1 Ω·m. We systematically observe a clear transition between two different regimes at a critical temperature T*: Efros-Shklovskii variable-range hopping (ES-VRH) below T* and power law behavior above. First, we demonstrate that the two regimes are strongly correlated with each other, both depending on the charge localization length ξ, calculated by the ES-VRH model, which corresponds to the characteristic size of overlapping sp2 domains belonging to different nanosheets. Thus, we propose a microscopic model describing the charge transport as a geometrical phase transition, given by the metal-insulator transition associated with the percolation of quasi-one-dimensional nanofillers with length ξ, showing that the charge transport behavior of the networks is valid for all geometries and defects of the nanosheets, ultimately suggesting a generalized description on vdW and disordered thin films.

Simulating Capillary Gas Chromatographic Separations including Thermal Gradient Conditions.

This article presents a method of simulating molecular transport in capillary gas chromatography (GC) applicable to isothermal, temperature-programmed, and thermal gradient conditions. The approach accounts for parameter differences that can occur across an analyte band including pressure, mobile phase velocity, temperature, and retention factor. The model was validated experimentally using a GC column comprised of microchannels in a stainless-steel plate capable of isothermal, temperature-programmed, and thermal gradient GC separations. The parameters governing retention and dispersion in the transport model were fitted with 12 experimental isothermal separations. The transport model was validated with experimental data for three analytes using four temperature-programmed and three thermal gradient GC separations. The simulated peaks (elution time and dispersion) give reasonable predictions of observed separations. The magnitudes of the maximum error between simulated peak elution time and experiment were 2.6 and 4.2% for temperature-programmed and thermal gradient GC, respectively. The magnitudes of the maximum error between the simulated peak width and experiment were 15.4 and 5.8% for temperature-programmed and thermal gradient GC, respectively. These relatively low errors give confidence that the model reflects the behavior of the transport processes and provides meaningful predictions for GC separations. This transport model allows for an evaluation of analyte separation characteristics of the analyte band at any position along the length of the GC column in addition to peak characteristics at the column exit. The transport model enables investigation of column conditions that influence separation behavior and opens exploration of optimal column design and heating conditions.

Author Correction: Apparent Power Law Scaling of Variable Range Hopping Conduction in Carbonized Polymer Nanofibers.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Molecular Lipid Films on Microengineering Materials.

In this study, we have systematically investigated the formation of molecular phospholipid films on a variety of solid substrates fabricated from typical surface engineering materials and the fluidic properties of the lipid membranes formed on these substrates. The surface materials comprise of borosilicate glass, mica, SiO2, Al (native oxide), Al2O3, TiO2, ITO, SiC, Au, Teflon AF, SU-8, and graphene. We deposited the lipid films from small unilamellar vesicles (SUVs) by means of an open-space microfluidic device, observed the formation and development of the films by laser scanning confocal microscopy, and evaluated the mode and degree of coverage, fluidity, and integrity. In addition to previously established mechanisms of lipid membrane-surface interaction upon bulk addition of SUVs on solid supports, we observed nontrivial lipid adhesion phenomena, including reverse rolling of spreading bilayers, spontaneous nucleation and growth of multilamellar vesicles, and the formation of intact circular patches of double lipid bilayer membranes. Our findings allow for accurate prediction of membrane-surface interactions in microfabricated devices and experimental environments where model membranes are used as functional biomimetic coatings.

Effect of graphene substrate type on formation of Bi2Se3 nanoplates.

Knowledge of nucleation and further growth of Bi2Se3 nanoplates on different substrates is crucial for obtaining ultrathin nanostructures and films of this material by physical vapour deposition technique. In this work, Bi2Se3 nanoplates were deposited under the same experimental conditions on different types of graphene substrates (as-transferred and post-annealed chemical vapour deposition grown monolayer graphene, monolayer graphene grown on silicon carbide substrate). Dimensions of the nanoplates deposited on graphene substrates were compared with the dimensions of the nanoplates deposited on mechanically exfoliated mica and highly ordered pyrolytic graphite flakes used as reference substrates. The influence of different graphene substrates on nucleation and further lateral and vertical growth of the Bi2Se3 nanoplates is analysed. Possibility to obtain ultrathin Bi2Se3 thin films on these substrates is evaluated. Between the substrates considered in this work, graphene grown on silicon carbide is found to be the most promising substrate for obtaining of 1-5 nm thick Bi2Se3 films.

Parallel Fabrication of Self-Assembled Nanogaps for Molecular Electronic Devices.

Single molecule electronics might be a way to add additional function to nanoscale devices and continue miniaturization beyond current state of the art. Here, a combined top-down and bottom-up strategy is employed to assemble single molecules onto prefabricated electrodes. Protodevices, which are self-assembled nanogaps composed by two gold nanoparticles linked by a single or a few molecules, are guided onto top-down prefabricated nanosized nickel electrodes with sandwiched palladium layers. It is shown that an optimized geometry of multilayered metallic (top-down) electrodes facilitates the assembly of (bottom-up) nanostructures by surface charge interactions. Moreover, such assembly process results in an electrode-nanoparticle interface free from linking molecules that enable electrical measurements to probe electron transport properties of the nanoparticle-molecule-nanoparticle protodevices.

Uniform doping of graphene close to the Dirac point by polymer-assisted assembly of molecular dopants.

Tuning the charge carrier density of two-dimensional (2D) materials by incorporating dopants into the crystal lattice is a challenging task. An attractive alternative is the surface transfer doping by adsorption of molecules on 2D crystals, which can lead to ordered molecular arrays. However, such systems, demonstrated in ultra-high vacuum conditions (UHV), are often unstable in ambient conditions. Here we show that air-stable doping of epitaxial graphene on SiC-achieved by spin-coating deposition of 2,3,5,6-tetrafluoro-tetracyano-quino-dimethane (F4TCNQ) incorporated in poly(methyl-methacrylate)-proceeds via the spontaneous accumulation of dopants at the graphene-polymer interface and by the formation of a charge-transfer complex that yields low-disorder, charge-neutral, large-area graphene with carrier mobilities ~70 000 cm2 V-1 s-1 at cryogenic temperatures. The assembly of dopants on 2D materials assisted by a polymer matrix, demonstrated by spin-coating wafer-scale substrates in ambient conditions, opens up a scalable technological route toward expanding the functionality of 2D materials.

Probing variable range hopping lengths by magneto conductance in carbonized polymer nanofibers.

Using magneto transport, we probe hopping length scales in the variable range hopping conduction of carbonized polyacetylene and polyaniline nanofibers. In contrast to pristine polyacetylene nanofibers that show vanishing magneto conductance at large electric fields, carbonized polymer nanofibers display a negative magneto conductance that decreases in magnitude but remains finite with respect to the electric field. We show that this behavior of magneto conductance is an indicator of the electric field and temperature dependence of hopping length in the gradual transition from the thermally activated to the activation-less electric field driven variable range hopping transport. This reveals magneto transport as a useful tool to probe hopping lengths in the non-linear hopping regime.

Apparent Power Law Scaling of Variable Range Hopping Conduction in Carbonized Polymer Nanofibers.

We induce dramatic changes in the structure of conducting polymer nanofibers by carbonization at 800 °C and compare charge transport properties between carbonized and pristine nanofibers. Despite the profound structural differences, both types of systems display power law dependence of current with voltage and temperature, and all measurements can be scaled into a single universal curve. We analyze our experimental data in the framework of variable range hopping and argue that this mechanism can explain transport properties of pristine polymer nanofibers as well.

Controlling deposition of nanoparticles by tuning surface charge of SiO2 by surface modifications.

The self-assembly of nanoparticles on substrates is relevant for a variety of applications such as plasmonics, sensing devices and nanometer-sized electronics. We investigate the deposition of 60 nm spherical Au nanoparticles onto silicon dioxide (SiO2) substrates by changing the chemical treatment of the substrate and by that altering the surface charge. The deposition is characterized by scanning electron microscopy (SEM). Kelvin probe force microscopy (KPFM) was used to characterize the surface workfunction. The underlying physics involved in the deposition of nanoparticles was described by a model based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory combined with random sequential adsorption (RSA). The spatial statistical method Ripley's K-function was used to verify the DLVO-RSA model (ERSA). The statistical results also showed that the adhered particles exhibit a short-range order at distances below ~300 nm. This method can be used in future research to predict the deposition densities of charged nanoparticles onto charged surfaces.

Influence of Impurity Spin Dynamics on Quantum Transport in Epitaxial Graphene.

Experimental evidence from both spin-valve and quantum transport measurements points towards unexpectedly fast spin relaxation in graphene. We report magnetotransport studies of epitaxial graphene on SiC in a vector magnetic field showing that spin relaxation, detected using weak-localization analysis, is suppressed by an in-plane magnetic field B(∥), and thereby proving that it is caused at least in part by spinful scatterers. A nonmonotonic dependence of the effective decoherence rate on B(∥) reveals the intricate role of the scatterers' spin dynamics in forming the interference correction to the conductivity, an effect that has gone unnoticed in earlier weak localization studies.

Nanopatterning of mobile lipid monolayers on electron-beam-sculpted Teflon AF surfaces.

Direct electron-beam lithography is used to fabricate nanostructured Teflon AF surfaces, which are utilized to pattern surface-supported monolayer phospholipid films with 50 nm lateral feature size. In comparison with unexposed Teflon AF coatings, e-beam-irradiated areas show reduced surface tension and surface potential. For phospholipid monolayer spreading experiments, these areas can be designed to function as barriers that enclose unexposed areas of nanometer dimensions and confine the lipid film within. We show that the effectiveness of the barrier is defined by pattern geometry and radiation dose. This surface preparation technique represents an efficient, yet simple, nanopatterning strategy supporting studies of lipid monolayer behavior in ultraconfined spaces. The generated structures are useful for imaging studies of biomimetic membranes and other specialized surface applications requiring spatially controlled formation of self-assembled, molecularly thin films on optically transparent patterned polymer surfaces with very low autofluorescence.