Electrical Charge State Manipulation of Single Silicon Vacancies in a Silicon Carbide Quantum Optoelectronic Device.

Color centers with long-lived spins are established platforms for quantum sensing and quantum information applications. Color centers exist in different charge states, each of them with distinct optical and spin properties. Application to quantum technology requires the capability to access and stabilize charge states for each specific task. Here, we investigate charge state manipulation of individual silicon vacancies in silicon carbide, a system which has recently shown a unique combination of long spin coherence time and ultrastable spin-selective optical transitions. In particular, we demonstrate charge state switching through the bias applied to the color center in an integrated silicon carbide optoelectronic device. We show that the electronic environment defined by the doping profile and the distribution of other defects in the device plays a key role for charge state control. Our experimental results and numerical modeling evidence that control of these complex interactions can, under certain conditions, enhance the photon emission rate. These findings open the way for deterministic control over the charge state of spin-active color centers for quantum technology and provide novel techniques for monitoring doping profiles and voltage sensing in microscopic devices.

Anodization study of epitaxial graphene: insights on the oxygen evolution reaction of graphitic materials.

The photoemission electron microscopy and x-ray photoemission spectroscopy were utilized for the study of anodized epitaxial graphene (EG) on silicon carbide as a fundamental aspect of the oxygen evolution reaction on graphitic materials. The high-resolution analysis of surface morphology and composition quantified the material transformation during the anodization. We investigated the surface with lateral resolution <150 nm, revealing significant transformations on the EG and the role of multilayer edges in increasing the film capacitance.

Lead (Pb) interfacing with epitaxial graphene.

Here, we report the electrochemical deposition of lead (Pb) as a model metal on epitaxial graphene fabricated on silicon carbide (Gr/SiC). The kinetics of electrodeposition and morphological characteristics of the deposits were evaluated by complementary electrochemical, physical and computational methods. The use of Gr/SiC as an electrode allowed the tracking of lead-associated redox conversions. The analysis of current transients passed during the deposition revealed an instantaneous nucleation mechanism controlled by convergent mass transport on the nuclei locally randomly distributed on epitaxial graphene. This key observation of the deposit topology was confirmed by low values of the experimentally-estimated apparent diffusion coefficient, Raman spectroscopy and scanning electron microscopy (SEM) studies. First principles calculations showed that the nucleation of Pb clusters on the graphene surface leads to weakening of the interaction strength of the metal-graphene complex, and only spatially separated Pb adatoms adsorbed on bridge and/or edge-plane sites can affect the vibrational properties of graphene. We expect that the lead adatoms can merge in large metallic clusters only at defect sites that reinforce the metal-graphene interactions. Our findings provide valuable insights into both heavy metal ion electrochemical analysis and metal electroplating on graphene interfaces that are important for designing effective detectors of toxic heavy metals.

Alkaloid profiles and acetylcholinesterase inhibitory activities of Fumaria species from Bulgaria.

GC-MS analysis of alkaloid profiles of five Fumaria species, naturally grown in Bulgaria (F. officinalis, F. thuretii, F. kralikii, F. rostellata and F. schrammii) and analysis of acetylcholinesterase inhibitory activity of alkaloid extracts were performed. Fourteen isoquinoline alkaloids were identified, with the principle ones being protopine, cryptopine, sinactine, parfumine, fumariline, fumarophycine, and fumaritine. Protopine contents, defined by HPLC analysis varied between 210.6 ± 8.8 µg/g DW (F. schrammii) and 334.5 ± 7.1 µg/g DW. (F. rostellata). While all of the investigated alkaloid extracts significantly inhibited acetylcholinesterase activity, the F. kralikii demonstrated the highest level of inhibition (IC(50) 0.13 ± 0.01 mg extract/mL).

On the origin of epitaxial rhombohedral-B4C growth by CVD on 4H-SiC.

Rhombohedral boron carbide, often referred to as r-B4C, is a potential material for applications in optoelectronic and thermoelectric devices. From fundamental thin film growth and characterization, we investigate the film-substrate interface between the r-B4C films grown on 4H-SiC (0001̄) (C-face) and 4H-SiC (0001) (Si-face) during chemical vapor deposition (CVD) to find the origin for epitaxial growth solely observed on the C-face. We used high-resolution (scanning) transmission electron microscopy and electron energy loss spectroscopy to show that there is no surface roughness or additional carbon-based interlayer formation for either substrate. Based on Raman spectroscopy analysis, we also argue that carbon accumulation on the surface hinders the growth of continued epitaxial r-B4C in CVD.

Ultra-narrow inhomogeneous spectral distribution of telecom-wavelength vanadium centres in isotopically-enriched silicon carbide.

Spin-active quantum emitters have emerged as a leading platform for quantum technologies. However, one of their major limitations is the large spread in optical emission frequencies, which typically extends over tens of GHz. Here, we investigate single V4+ vanadium centres in 4H-SiC, which feature telecom-wavelength emission and a coherent S = 1/2 spin state. We perform spectroscopy on single emitters and report the observation of spin-dependent optical transitions, a key requirement for spin-photon interfaces. By engineering the isotopic composition of the SiC matrix, we reduce the inhomogeneous spectral distribution of different emitters down to 100 MHz, significantly smaller than any other single quantum emitter. Additionally, we tailor the dopant concentration to stabilise the telecom-wavelength V4+ charge state, thereby extending its lifetime by at least two orders of magnitude. These results bolster the prospects for single V emitters in SiC as material nodes in scalable telecom quantum networks.

Bidirectional Hydrogen Electrocatalysis on Epitaxial Graphene.

The climate change due to human activities stimulates the research on new energy resources. Hydrogen has attracted interest as a green carrier of high energy density. The sustainable production of hydrogen is achievable only by water electrolysis based on the hydrogen evolution reaction (HER). Graphitic materials are widely utilized in this technology in the role of conductive catalyst supports. Herein, by performing dynamic and steady-state electrochemical measurements in acidic and alkaline media, we investigated the bidirectional electrocatalysis of the HER and hydrogen oxidation reaction (HOR) on metal- and defect-free epigraphene (EG) grown on 4H silicon carbide (4H-SiC) as a ground level of structural organization of general graphitic materials. The absence of any signal degradation illustrates the high stability of EG. The experimental and theoretical investigations yield the coherent conclusion on the dominant HER pathway following the Volmer-Tafel mechanism. We ascribe the observed reactivity of EG to its interaction with the underlying SiC substrate that induces strain and electronic doping. The computed high activation energy for breaking the O-H bond is linked to the high negative overpotential of the HER. The estimated exchange current of HER/HOR on EG can be used in the evaluation of complex electrocatalytic systems based on graphite as a conducing support.

Understanding of the Electrochemical Behavior of Lithium at Bilayer-Patched Epitaxial Graphene/4H-SiC.

Novel two-dimensional materials (2DMs) with balanced electrical conductivity and lithium (Li) storage capacity are desirable for next-generation rechargeable batteries as they may serve as high-performance anodes, improving output battery characteristics. Gaining an advanced understanding of the electrochemical behavior of lithium at the electrode surface and the changes in interior structure of 2DM-based electrodes caused by lithiation is a key component in the long-term process of the implementation of new electrodes into to a realistic device. Here, we showcase the advantages of bilayer-patched epitaxial graphene on 4H-SiC (0001) as a possible anode material in lithium-ion batteries. The presence of bilayer graphene patches is beneficial for the overall lithiation process because it results in enhanced quantum capacitance of the electrode and provides extra intercalation paths. By performing cyclic voltammetry and chronoamperometry measurements, we shed light on the redox behavior of lithium at the bilayer-patched epitaxial graphene electrode and find that the early-stage growth of lithium is governed by the instantaneous nucleation mechanism. The results also demonstrate the fast lithium-ion transport (~4.7-5.6 × 10-7 cm2∙s-1) to the bilayer-patched epitaxial graphene electrode. Raman measurements complemented by in-depth statistical analysis and density functional theory calculations enable us to comprehend the lithiation effect on the properties of bilayer-patched epitaxial graphene and ascribe the lithium intercalation-induced Raman G peak splitting to the disparity between graphene layers. The current results are helpful for further advancement of the design of graphene-based electrodes with targeted performance.

Nanoscale phenomena ruling deposition and intercalation of AlN at the graphene/SiC interface.

The possibility for kinetic stabilization of prospective 2D AlN was explored by rationalizing metal organic chemical vapor deposition (MOCVD) processes of AlN on epitaxial graphene. From the wide range of temperatures which can be covered in the same MOCVD reactor, the deposition was performed at the selected temperatures of 700, 900, and 1240 °C. The characterization of the structures by atomic force microscopy, electron microscopy and Raman spectroscopy revealed a broad range of surface nucleation and intercalation phenomena. These phenomena included the abundant formation of nucleation sites on graphene, the fragmentation of the graphene layers which accelerated with the deposition temperature, the delivery of excess precursor-derived carbon adatoms to the surface, as well as intercalation of sub-layers of aluminum atoms at the graphene/SiC interface. The conceptual understanding of these nanoscale phenomena was supported by our previous comprehensive ab initio molecular dynamics (AIMD) simulations of the surface reaction of trimethylaluminum, (CH3)3Al, precursor with graphene. A case of applying trimethylindium, (CH3)3In, precursor to epitaxial graphene was considered in a comparative way.

Ligand hyperfine interactions at silicon vacancies in 4H-SiC.

The negative silicon vacancy ([Formula: see text]) in SiC has recently emerged as a promising defect for quantum communication and room-temperature quantum sensing. However, its electronic structure is still not well characterized. While the isolated Si vacancy is expected to give rise to only two paramagnetic centers corresponding to two inequivalent lattice sites in 4H-SiC, there have been five electron paramagnetic resonance (EPR) centers assigned to [Formula: see text] in the past: the so-called isolated no-zero-field splitting (ZFS) [Formula: see text] center and another four axial configurations with small ZFS: T V1a, T V2a, T V1b, and T V2b. Due to overlapping with 29Si hyperfine (hf) structures in EPR spectra of natural 4H-SiC, hf parameters of T V1a have not been determined. Using isotopically enriched 4H-28SiC, we overcome the problems of signal overlapping and observe hf parameters of nearest C neighbors for all three components of the S = 3/2 T V1a and T V2a centers. The obtained EPR data support the conclusion that only T V1a and T V2a are related to [Formula: see text] and the two configurations of the so-called isolated no-ZFS [Formula: see text] center, [Formula: see text] (I) and [Formula: see text] (II), are actually the central lines corresponding to the transition |-1/2〉 ↔ |+1/2〉 of the T V2a and T V1a centers, respectively.

High-fidelity spin and optical control of single silicon-vacancy centres in silicon carbide.

Scalable quantum networking requires quantum systems with quantum processing capabilities. Solid state spin systems with reliable spin-optical interfaces are a leading hardware in this regard. However, available systems suffer from large electron-phonon interaction or fast spin dephasing. Here, we demonstrate that the negatively charged silicon-vacancy centre in silicon carbide is immune to both drawbacks. Thanks to its 4A2 symmetry in ground and excited states, optical resonances are stable with near-Fourier-transform-limited linewidths, allowing exploitation of the spin selectivity of the optical transitions. In combination with millisecond-long spin coherence times originating from the high-purity crystal, we demonstrate high-fidelity optical initialization and coherent spin control, which we exploit to show coherent coupling to single nuclear spins with ∼1 kHz resolution. The summary of our findings makes this defect a prime candidate for realising memory-assisted quantum network applications using semiconductor-based spin-to-photon interfaces and coherently coupled nuclear spins.

Influence of interferons on the repair of UV-damaged DNA.

The capacity for nucleotide excision repair of a normal (WISH) and three tumour (MCF-7, HeLa, Namalva) cell lines treated with human recombinant interferons (hrIFN-alpha and hrIFN-gamma) was compared by the host cell reactivation assay. The cells were transfected with in vitro UV-damaged plasmid DNA (pEGFP-N1). The repair capacity was determined by measuring the fluorescence intensity of the expressed marker protein in total cell lysates. The correlation between the interferon-induced NO content and the suppressive effect of interferons on DNA repair was shown. The decrease of repair activity and NO induction by hrIFN-alpha were greatest in WISH, followed by MCF-7, Namalva and HeLa cells, whereas hrIFN-gamma was the best NO inducer and inhibitor for the repair of Namalva, followed by WISH, MCF-7 and HeLa cells. Our data clearly show that the two types of interferon have a strong inhibitory effect on the repair of UV-damaged DNA and this effect is cell type-dependent.

Understanding Graphene Response to Neutral and Charged Lead Species: Theory and Experiment.

Deep understanding of binding of toxic Lead (Pb) species on the surface of two-dimensional materials is a required prerequisite for the development of next-generation sensors that can provide fast and real-time detection of critically low concentrations. Here we report atomistic insights into the Lead behavior on epitaxial graphene (Gr) on silicon carbide substrates by thorough complementary study of voltammetry, electrical characterization, Raman spectroscopy, and Density Functional Theory (DFT). It is verified that the epitaxial graphene exhibits quasi-reversible anode reactions in aqueous solutions, providing a well-defined redox peak for Pb species and good linearity over a concentration range from 1 nM to 1 µM. The conductometric approach offers another way to investigate Lead adsorption, which is based on the formations of stable charge-transfer complexes affecting the p-type conductivity of epitaxial graphene. Our results suggest the adsorption ability of the epitaxial graphene towards divalent Lead ions is concentration-dependent and tends to saturate at higher concentrations. To elucidate the mechanisms responsible for Pb adsorption, we performed DFT calculations and estimated the solvent-mediated interaction between Lead species in different oxidative forms and graphene. Our results provide central information regarding the energetics and structure of Pb-graphene interacting complexes that underlay the adsorption mechanisms of neutral and divalent Lead species. Such a holistic understanding favors design and synthesis of new sensitive materials for water quality monitoring.

In-situ terahertz optical Hall effect measurements of ambient effects on free charge carrier properties of epitaxial graphene.

Unraveling the doping-related charge carrier scattering mechanisms in two-dimensional materials such as graphene is vital for limiting parasitic electrical conductivity losses in future electronic applications. While electric field doping is well understood, assessment of mobility and density as a function of chemical doping remained a challenge thus far. In this work, we investigate the effects of cyclically exposing epitaxial graphene to controlled inert gases and ambient humidity conditions, while measuring the Lorentz force-induced birefringence in graphene at Terahertz frequencies in magnetic fields. This technique, previously identified as the optical analogue of the electrical Hall effect, permits here measurement of charge carrier type, density, and mobility in epitaxial graphene on silicon-face silicon carbide. We observe a distinct, nearly linear relationship between mobility and electron charge density, similar to field-effect induced changes measured in electrical Hall bar devices previously. The observed doping process is completely reversible and independent of the type of inert gas exposure.

Spontaneous transfection of mammalian cells with plasmid DNA.

A simple method for spontaneous transfection into mammalian cells (both adherent and suspension in culture) with plasmid DNA is described. This method does not require any specific DNA carrier or technical device and can be applied for obtaining both transient and stably transfected cells. The efficiency of spontaneous transfection is slightly lower in comparison with that of the conventional calcium phosphate and lipofectin transfection methods and does not depend on the type of cell culture used.

Inability of Agrobacterium tumefaciens ribosomes to translate in vivo mRNAs containing non-Shine-Dalgarno translational initiators.

Numerous data accumulated during the last decade have shown that the Shine-Dalgarno (SD) sequence is not a unique initiator of translation for Escherichia coli. Several other sequences, mostly of viral origin, have demonstrated their capability of either enhancing or initiating translation in vivo. A phage T7 gene 10 sequence, called "epsilon" (epsilon), has shown its high enhancing activity on translation in both Escherichia coli and Agrobacterium tumefaciens cells. In this study the epsilon, together with three other nucleotide sequences derived from the 5' non-translated regions of tobacco mosaic virus (TMV), papaya mosaic virus (PMV) and clover yellow mosaic virus (CYMV) RNAs are tested for translation initiation activity in A. tumefaciens cells. The obtained results indicate that none of them was capable of initiating translation in vivo of chloramphenicol acetyltransferase (CAT) mRNA. To determine whether their inactivity was related with structural differences in the ribosomal protein S1, the rpsA gene (coding for S1 protein in E. coli) was co-expressed in A. tumefaciens together with the cat gene placed under the translational control of the above sequences. Our results showed that the rpsA gene product did not make any of the four viral enhancers active in A. tumefaciens cells. The inability of A. tumefaciens ribosomes to translate mRNAs devoid of SD sequences indicates for a substantial difference in the ribosome structure of the two Gram negative bacteria E. coli and A. tumefaciens.

Magnetic resonance identification of hydrogen at a zinc vacancy in ZnO.

Hydrogen (H) at a zinc vacancy (VZn) in ZnO is identified by electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM). In ZnO irradiated by 2 MeV electrons, a doublet EPR spectrum, labelled S1, is observed. The doublet structure and the accompanying weak satellites are shown to be the allowed and forbidden lines of the hyperfine structure due to the dipolar interaction between an electron spin S = 1/2 and a nuclear spin I = 1/2 of (1)H located at a VZn. The involvement of a single H atom in the S1 defect is further confirmed by the observation of the nuclear Zeeman frequency of (1)H in ESEEM experiments. We show that at a VZn, H prefers to make a short O-H bond with one O neighbour and is off the substitutional site, forming a low symmetry C1 defect. In this partly H passivated VZn, the unpaired electron localizes on the p orbital of another O neighbour of VZn, and not on the H.

Identification and characterization of acetyl-CoA carboxylase gene cluster in Streptomyces toxytricini.

The gene locus for acetyl-CoA carboxylase (ACC) involved in the primary metabolism was identified from the genomic library of Streptomyces toxytricini which produces a lipase inhibitor lipstatin. The 7.4 kb cloned gene was comprised of 5 ORFs including accD1, accA1, hmgL, fadST1, and stsF. In order to confirm the biochemical characteristics of AccA1, the gene was overexpressed in Escherichia coli cells, and the recombinant protein was purified through Ni2+ affinity chromatography. Because most of the expressed AccAl was biotinylated by host E. coli BirA in the presence of D-biotin, the non-biotinylated apo-AccA1 was purified after gene induction without D-biotin, followed by exclusion of holo-AccA1 using streptavidin beads. The separated apo-AccA1 was post-translationally biotinylated by S. toxytricini biotin apo-protein ligase (BPL) in a time- and enzyme-dependent manner. This result supports that this gene cluster of S. toxytricini encodes the functional ACC enzyme subunits to be biotinylated.

Histone H1 as a reporter protein to investigate glycation in bacteria.

Nonenzymatic glycosylation (glycation) of proteins is a multistage chemical process starting as a condensation reaction between reducing sugars and primary amino groups (mainly from the side chains of Lis and Arg) and ending up with formation of complex heterocyclic compounds called advanced glycation end products (AGEs). For a long time, glycation has been attributed to the long-lived eukaryotes (including in humans) only. In a recent study, we showed that glycation also occurs in bacteria. The present study aims to prove that bacterial cytoplasm contains soluble glycating compounds. To this end, Lis/Arg-rich histone H1 isolated from rat liver was treated with deproteinized Escherichia coli cytoplasm through a dialysis membrane. This treatment leads to accumulation of AGEs as well as to a remarkable degradation of the reporter protein on storage at 4 degrees C. Our results indicate also that glycation can be inhibited by acetylsalicylic acid (aspirin), thiamine (vitamin B1), and pyridoxine (vitamin B6).