Structural evolution of carbon dots during low temperature pyrolysis

Carbon dots (CDs) are an emerging class of photoluminescent material. Their unique optical properties arise from the discrete energy levels in their electronic states, which directly relate to their crystalline and chemical structure. It is expected that when CDs go through structural changes via chemical reduction or thermal annealing, their energy levels will be altered, inducing unique optoelectronic properties such as solid-state photoluminescence (PL). However, the detailed structural evolution and how the optoelectronic characteristics of CDs are affected remain unclear. Therefore, it is of fundamental interest to understand how the structure of CDs prepared by hydrothermal carbonisation (HTC) rearranges from a highly functionalised disordered structure into a more ordered graphitic structure. In this paper, detailed structural characterisation and in situ TEM were conducted to reveal the structural evolution of CDs during the carbonisation process, which have demonstrated a growth in aromatic domains and reduction in oxidation sites. These structural features are correlated with their near-infrared (NIR) solid-state PL properties, which may find a lot of practical applications such as temperature sensing, solid-state display lighting and anti-counterfeit security inks

Low-Temperature Growth of Graphene on a Semiconductor

The industrial realization of graphene has so far been limited by challenges related to the quality, reproducibility, and high process temperatures required to manufacture graphene on suitable substrates. We demonstrate that epitaxial graphene can be grown on transition metal treated 6H-SiC(0001) surfaces, with an onset of graphitization starting around $450-500^\circ\text{C}$. From the chemical reaction between SiC and thin films of Fe or Ru, $\text{sp}^{3}$ carbon is liberated from the SiC crystal and converted to $\text{sp}^{2}$ carbon at the surface. The quality of the graphene is demonstrated using angle-resolved photoemission spectroscopy and low-energy electron diffraction. Furthermore, the orientation and placement of the graphene layers relative to the SiC substrate is verified using angle-resolved absorption spectroscopy and energy-dependent photoelectron spectroscopy, respectively. With subsequent thermal treatments to higher temperatures, a steerable diffusion of the metal layers into the bulk SiC is achieved. The result is graphene supported on magnetic silicide or optionally, directly on semiconductor, at temperatures ideal for further large-scale processing into graphene based device structures.Comment: 10 pages, 4 figures, 51 reference

Room Temperature Ferromagnetic Mn:Ge(001)

We report the synthesis of a room temperature ferromagnetic Mn-Ge system obtained by simple deposition of manganese on Ge(001), heated at relatively high temperature (starting with 250 °C). The samples were characterized by low energy electron diffraction (LEED), scanning tunneling microscopy (STM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), superconducting quantum interference device (SQUID), and magneto-optical Kerr effect (MOKE). Samples deposited at relatively elevated temperature (350 °C) exhibited the formation of ~5–8 nm diameter Mn5Ge3 and Mn11Ge8 agglomerates by HRTEM, while XPS identified at least two Mn-containing phases: the agglomerates, together with a Ge-rich MnGe~2.5 phase, or manganese diluted into the Ge(001) crystal. LEED revealed the persistence of long range order after a relatively high amount of Mn (100 nm) deposited on the single crystal substrate. STM probed the existence of dimer rows on the surface, slightly elongated as compared with Ge–Ge dimers on Ge(001). The films exhibited a clear ferromagnetism at room temperature, opening the possibility of forming a magnetic phase behind a nearly ideally terminated Ge surface, which could find applications in integration of magnetic functionalities on semiconductor bases. SQUID probed the co-existence of a superparamagnetic phase, with one phase which may be attributed to a diluted magnetic semiconductor. The hypothesis that the room temperature ferromagnetic phase might be the one with manganese diluted into the Ge crystal is formulated and discussed

Beyond Nitrogen in the Oxygen Reduction Reaction on Nitrogen-Doped Carbons: A NEXAFS Investigation

Polymer electrolyte membrane fuel cells require cheap and active electrocatalysts to drive the oxygen reduction reaction. Nitrogen-doped carbons have been extensively studied regarding their oxygen reduction reaction. The work at hand looks beyond the nitrogen chemistry and brings to light the role of oxygen. Nitrogen-doped nanocarbons were obtained by a radio-frequency plasma route at 0, 100, 250, and 350 W. The lateral size of the graphitic domain, determined from Raman spectroscopy, showed that the nitrogen plasma treatment decreased the crystallite size. Synchrotron radiation photoelectron spectroscopy showed a similar nitrogen chemistry, albeit the nitrogen concentration increased with the plasma power. Lateral crystallite size and several nitrogen moieties were plotted against the onset potential determined from oxygen reduction reaction curves. There was no correlation between the electrochemical activity and the sample structure, as determine from Raman and synchrotron radiation photoelectron spectroscopy. Near-edge X-ray absorption fine structure (NEXAFS) was performed to unravel the carbon and nitrogen local structure. A difference analysis of the NEXAFS spectra showed that the oxygen surrounding the pyridinic nitrogen was critical in achieving high onset potentials. The work shows that there were more factors at play, other than carbon organization and nitrogen chemistry

Polymeric Carbon Nitrides for Photoelectrochemical Applications: Ring Opening-Induced Degradation

Active and stable materials that utilize solar radiation for promoting different reactions are critical for emerging technologies. Two of the most common polymeric carbon nitrides were prepared by the thermal polycondensation of melamine. The scope of this work is to investigate possible structural degradation before and after photoelectrochemical testing. The materials were characterized using synchrotron radiation and lab-based techniques, and subsequently degraded photoelectrochemically, followed by post-mortem analysis. Post-mortem investigations reveal: (1) carbon atoms bonded to three nitrogen atoms change into carbon atoms bonded to two nitrogen atoms and (2) the presence of methylene terminals in post-mortem materials. The study concludes that polymeric carbon nitrides are susceptible to photoelectrochemical degradation via ring opening

Factors Associated with Increased Risk of Urosepsis during Pregnancy and Treatment Outcomes, in a Urology Clinic

Background and Objectives: Urosepsis is a significant cause of maternal and fetal mortality. While certain risk factors for urinary tract infections (UTIs) in pregnant women are well established, those associated with an elevated risk of urosepsis in pregnant women with upper UTIs remain less defined. This study aims to identify factors linked to an increased risk of urosepsis and examine urologic treatment outcomes in such cases. Materials and Methods: We conducted a retrospective analysis on 66 pregnant women diagnosed with urosepsis over a nine-year period. A control group included 164 pregnant women with upper UTIs, excluding urosepsis, admitted during the same timeframe. This study highlights factors potentially contributing to urosepsis risk, including comorbidities like anemia, pregnancy-related hydronephrosis or secondary to reno-ureteral lithiasis, prior UTIs, coexisting urological conditions, and urologic procedures. Outcomes of urologic treatments, hospitalization duration, obstetric transfers due to fetal distress, and complications associated with double-J catheters were analyzed. Results: Pregnant women with urosepsis exhibited a higher prevalence of anemia (69.7% vs. 50.0%, p = 0.006), 2nd–3rd grade hydronephrosis (81.8% vs. 52.8%, p = 0.001), and fever over 38 °C (89.4% vs. 42.1%, p = 0.001). They also had a more intense inflammatory syndrome (leukocyte count 18,191 ± 6414 vs. 14,350 ± 3860/mmc, p = 0.001, and C-reactive protein (CRP) 142.70 ± 83.50 vs. 72.76 ± 66.37 mg/dL, p = 0.001) and higher creatinine levels (0.77 ± 0.81 vs. 0.59 ± 0.22, p = 0.017). On multivariate analysis, factors associated with increased risk for urosepsis were anemia (Odds Ratio (OR) 2.622, 95% CI 1.220–5.634), 2nd–3rd grade hydronephrosis (OR 6.581, 95% CI 2.802–15.460), and fever over 38 °C (OR 11.612, 95% CI 4.804–28.07). Regarding outcomes, the urosepsis group had a higher rate of urological maneuvers (87.9% vs. 36%, p = 0.001), a higher rate of obstetric transfers due to fetal distress (22.7% vs. 1.2%, p = 0.001), and migration of double-J catheters (6.1% vs. 0.6%, p = 0.016), but no maternal fatality was encountered. However, they experienced the same rate of total complications related to double-J catheters (19.69% vs. 12.80%, p > 0.05). The pregnant women in both groups had the infection more frequently on the right kidney, were in the second trimester and were nulliparous. Conclusions: Pregnant women at increased risk for urosepsis include those with anemia, hydronephrosis due to gestational, or reno-ureteral lithiasis, and fever over 38 °C. While the prognosis for pregnant women with urosepsis is generally favorable, urological intervention may not prevent a higher incidence of fetal distress and the need for obstetric transfers compared to pregnant women with uncomplicated upper UTIs

Ambiguous Role of Growth-Induced Defects on the Semiconductor-to-Metal Characteristics in Epitaxial VO₂/TiO₂ Thin Films

Controlling the semiconductor-to-metal transition temperature in epitaxial VO₂ thin films remains an unresolved question both at the fundamental as well as the application level. Within the scope of this work, the effects of growth temperature on the structure, chemical composition, interface coherency and electrical characteristics of rutile VO₂ epitaxial thin films grown on TiO₂ substrates are investigated. It is hereby deduced that the transition temperature is lower than the bulk value of 340 K. However, it is found to approach this value as a function of increased growth temperature even though it is accompanied by a contraction along the V⁴⁺–V⁴⁺ bond direction, the crystallographic <i>c</i>-axis lattice parameter. Additionally, it is demonstrated that films grown at low substrate temperatures exhibit a relaxed state and a strongly reduced transition temperature. It is suggested that, besides thermal and epitaxial strain, growth-induced defects may strongly affect the electronic phase transition. The results of this work reveal the difficulty in extracting the intrinsic material response to strain, when the exact contribution of all strain sources cannot be effectively determined. The findings also bear implications on the limitations in obtaining the recently predicted novel semi-Dirac point phase in VO₂/TiO₂ multilayer structures