Magnetotransport characterization of epitaxial graphene on SiC

Low-temperature magnetotransport is used to characterize graphene grown epitaxially on the silicon face of 4H silicon carbide (SiC/G). Observation of half-integer quantum Hall effect (QHE) in large Hall bars, patterned across several terraces of the SiC substrate, suggest that monolayer graphene grows continuously over defects.Complete characterization was possible using carrier density control technologies developed for SiC/G, including organic dielectrics, photochemical gating and a solid electrolyte. The photochemical gating with organic polymers, achieved by using a spacer layer directly in contact with graphene that protects its integrity, followed by a layer that responds to light, is envisioned as a prototypical architecture for the development of graphene-based sensors.Fine details of electron scattering were found through measurement of quantum corrections to the conductivity of SiC/G, arising from weak localization (WL) and electron-electron interactions (E-E). It was found that scattering is determined by charged impurities under graphene, while the effect of terraces is proposed to manifests as intervalley scattering. The extracted temperature dependence of the decoherence rate allowed to identify E-E interactions and to suggest spin-flip centers as sources ofdephasing in the system. The analysis of WL provided an indirect measurement of the spin relaxation time in SiC/G, at the level of 50 ps.Altogether, this work contributed to develop the first application in which graphene outperforms conventional semiconducors, in the field of quantum metrology. The half integer QHE in SiC/G is proposed as standard for electrical resistance to replace GaAs heterostructures. A direct comparison with the QHE in GaAs, the most strict universality test of the QHE ever performed, supports the hypothesis that the electrical resistance is quantized in units of h/e2, with an uncertainty of 0.084 parts per billion. The accuracy of the comparison was limited by the critical current in the GaAs sample, 4 times lower than in the SiC/G sample

Magnetotransport characterization of epitaxial graphene on SiC

Low-temperature magnetotransport is used to characterize graphene grown epitaxially on the silicon face of 4H silicon carbide (SiC/G). Observation of half-integer quantum Hall effect (QHE) in large Hall bars, patterned across several terraces of the SiC substrate, suggest that monolayer graphene grows continuously over defects.Complete characterization was possible using carrier density control technologies developed for SiC/G, including organic dielectrics, photochemical gating and a solid electrolyte. The photochemical gating with organic polymers, achieved by using a spacer layer directly in contact with graphene that protects its integrity, followed by a layer that responds to light, is envisioned as a prototypical architecture for the development of graphene-based sensors.Fine details of electron scattering were found through measurement of quantum corrections to the conductivity of SiC/G, arising from weak localization (WL) and electron-electron interactions (E-E). It was found that scattering is determined by charged impurities under graphene, while the effect of terraces is proposed to manifests as intervalley scattering. The extracted temperature dependence of the decoherence rate allowed to identify E-E interactions and to suggest spin-flip centers as sources ofdephasing in the system. The analysis of WL provided an indirect measurement of the spin relaxation time in SiC/G, at the level of 50 ps.Altogether, this work contributed to develop the first application in which graphene outperforms conventional semiconducors, in the field of quantum metrology. The half integer QHE in SiC/G is proposed as standard for electrical resistance to replace GaAs heterostructures. A direct comparison with the QHE in GaAs, the most strict universality test of the QHE ever performed, supports the hypothesis that the electrical resistance is quantized in units of h/e2, with an uncertainty of 0.084 parts per billion. The accuracy of the comparison was limited by the critical current in the GaAs sample, 4 times lower than in the SiC/G sample

Prisutnost bakterija u zraku nastambi za svinje i neposrednom okolišu

This paper describes a bacteriological analysis of air samples taken from swine housing facilities and the immediate environment. The air volume of the samples was pre-programmed by a standard air sampler (MAS-100, Merck) and was directly impacted onto the bacteriologic agar surface (Petri dishes, standard diameter of 90 mm). The bacterial contamination in forty-eight samples was 2.59x105 CFU/m3 (ranging from 8.46x104 to 5.30x105 CFU/m3). Potentially pathogenic bacterial agents predominated in all samples (100%), while primarily pathogenic bacteria were isolated in a minor proportion of samples (33%–66%). Airborne bacterial contamination in samples (N=16) obtained from emptied facilities ranged from 1.8x103 CFU/m3 (that is, after coarse mechanical washing) to 0.8x102 CFU/m3 (upon completion of disinfection). Control measurements at different locations and distance from the farm (N=32) pointed to the presence of non-pathogenic airborne bacteria, ranging from 1.55x102 to 3.70x102 CFU/m3. The results of this preliminary study showed that the emission of potentially pathogenic bacteria from animal housing facilities to the immediate farm environment via aerosol was very low.Pored kontrole emisija štetnih plinova, vlage i čestica prašine u nastambama za životinje, važna je i detekcija ukupne bakterijske kontaminacije zraka. Od posebnog su značenja i moguće emisije u okoliš, osobito potencijalno ili primarno patogenih bakterija. U raspravi su prikazani rezultati prvih istraživanja bakterijske kontaminacije zraka u nastambama, kao i u neposrednom okolišu provedenih na jednoj farmi svinja. Programirani volumen zraka standardnim je skupljačem zraka (MAS-100-Merck) direktno naslojen na površine bakterioloških podloga (Petrijeve ploče promjera 90 mm). U pretraženih 48 uzoraka u nastambama ustanovljena je prosječna bakterijska kontaminacija zraka za farmu od 2,595 CFU/m3 (od 8,464 do 5,305). U svim su uzorcima dominantno bili prisutni Streptococcus spp., Micrococcus spp., Escherichia coli, Staphylococcus aureus i Streptococcus suis. U manjem broju uzoraka izdvojeni su potencijalno patogeni uzročnici Pasteurella multocida (66%), Actinobacillus suis (50%), hemolitični sojevi E. coli (41%) te Pasteurella haemolytica, Bordetella bronchiseptica i Actinobacillus pleuropneumoniae (33%). Mjerenjima u praznim objektima, prije useljenja životinja (16 uzoraka), ustanovljena je srednja kontaminacija zraka od 9,01 do 0,41 CFU/m3 s bakterijskim uzročnicima Streptococcus spp., Staphylococcus spp. i E. coli. Kontrolna mjerenja izvan objekata (32 uzorka) upozorila su na prosječnu kontaminaciju zraka od 0,261 CFU/m3 (od 0,151 do 0,371) i uz zastupljenost apatogenih uzročnika Bacillus subtilis, Bacillus cereus, E. coli, Streptococcus spp., Micrococcus spp. i Staphylococcus spp. Rezultati pokazuju da su neznatne emisije potencijalno patogenih bakterijskih uzročnika putem aerosola iz nastambi za svinje u neposredni okoliš. Unatoč sve sofisticiranijim uređajima za monitoring, ni danas još, u okviru programa animalne higijene, nisu određene granične vrijednosti za bakteriološku kontaminaciju zraka kao što je to slučaj s emisijom štetnih plinova NH3 i CO2

A prototype of RK/200 quantum Hall array resistance standard on epitaxial graphene

Epitaxial graphene on silicon carbide is a promising material for the next generation of quantum Hall re- sistance standards. Single Hall bars made of graphene have already surpassed their state-of-the-art GaAs based counterparts as an RK/2 (RK = h/e^2) standard, showing at least the same precision and higher break- down current density. Compared to single devices, quantum Hall arrays using parallel or series connection of multiple Hall bars can offer resistance values spanning several orders of magnitude and (in case of parallel connection) significantly larger measurement currents, but impose strict requirements on uniformity of the material. To evaluate the quality of the available material, we have fabricated arrays of 100 Hall bars con- nected in parallel on epitaxial graphene. One out of four devices has shown quantized resistance that matched the correct value of RK/200 within the measurement precision of 1e-4 at magnetic fields between 7 and 9 Tesla. The defective behaviour of other arrays is attributed mainly to non-uniform doping. This result con- firms the acceptable quality of epitaxial graphene, pointing towards the feasibility of well above 90% yield of working Hall bars

Вплив засобів масової інформації та комп’ютерних технологій на формування культури сімейних стосунків

(uk) У статті автор розкриває вплив засобів масової інформації та комп’ютерних технологій на формування культури сімейних стосунків.(ru) В статье автором рассматривается влияние средств массовой информации и компьютерных технологий на формирование культуры семейных отношений

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-localisation analysis, is suppressed by an in-plane magnetic field, $B_{\parallel}$, and thereby proving that it is caused at least in part by spinful scatterers. A non-monotonic dependence of effective decoherence rate on $B_{\parallel}$ reveals the intricate role of scatterers' spin dynamics in forming the interference correction to conductivity, an effect that has gone unnoticed in earlier weak localisation studie

Long-term stability of molecular doped epigraphene quantum Hall standards: single elements and large arrays (R K/236 ≈ 109 Ω)

In this work we investigate the long-term stability of epitaxial graphene (epigraphene) quantum Hall resistance standards, including single devices and an array device composed of 236 elements providing R K/236 ≈ 109 Ω, with R K the von Klitzing constant. All devices utilize the established technique of chemical doping via molecular dopants to achieve homogenous doping and control over carrier density. However, optimal storage conditions and the long-term stability of molecular dopants for metrological applications have not been widely studied. In this work we aim to identify simple storage techniques that use readily available and cost-effective materials which provide long-term stability for devices without the need for advanced laboratory equipment. The devices are stored in glass bottles with four different environments: ambient, oxygen absorber, silica gel desiccant, and oxygen absorber/desiccant mixture. We have tracked the carrier densities, mobilities, and quantization accuracies of eight different epigraphene quantum Hall chips for over two years. We observe the highest stability (i.e. lowest change in carrier density) for samples stored in oxygen absorber/desiccant mixture, with a relative change in carrier density below 0.01% per day and no discernable degradation of quantization accuracy at the part-per-billion level. This storage technique yields a comparable stability to the currently established best storage method of inert nitrogen atmosphere, but it is much easier to realize in practice. It is possible to further optimize the mixture of oxygen absorber/desiccant for even greater stability performance in the future. We foresee that this technique can allow for simple and stable long-term storage of polymer-encapsulated molecular doped epigraphene quantum Hall standards, removing another barrier for their wide-spread use in practical metrology

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\ub0. 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

The performance limits of epigraphene Hall sensors

Epitaxial graphene on silicon carbide, or epigraphene, provides an excellent platform for Hall sensing devices in terms of both high electrical quality and scalability. However, the challenge in controlling its carrier density has thus far prevented systematic studies of epigraphene Hall sensor performance. In this work we investigate epigraphene Hall sensors where epigraphene is doped across the Dirac point using molecular doping. Depending on the carrier density, molecular-doped epigraphene Hall sensors reach room temperature sensitivities $S_V=0.23 V/VT$,$S_I=1440 V/AT$ and magnetic field detection limits down to $B_{MIN}=27$ $nT/\sqrt{Hz}$ at 20 kHz. Thermally stabilized devices demonstrate operation up to $T=150$ $^oC$ with $S_V=0.12 V/VT$, $S_I=300 V/AT$ and $B_{MIN}\approx 100$ $nT/\sqrt{Hz}$ at 20 kHz