Sulfur-doped Amorphous Carbon Research Articles

Unusual re-entrant spin-glass-behavior and enhanced diamagnetism in sp3-rich sulfur/oxygen doped highly oriented pyrolytic graphite

The recent observations of unusual ferromagnetic and superconductive phenomena in sulfur-doped amorphous carbon and highly oriented pyrolytic graphite (HOPG) have attracted significant attention. In this work we present a novel in-depth investigation on the relationship between magnetic-ordering and sulfur-doping-induced structural modifications in pyrolytic graphite. We demonstrate the nucleation of an unusual amorphous carbon film with sp3-rich characteristics as a result of chemical interactions between sulfur and HOPG. Investigations were performed at the controlled temperatures (T) of ~300, 350, 500 °C, with the residence time parameter varying from 1 min to 15 h. Magnetic characterization revealed a re-entrant spin-glass behavior, possibly originating from vacancy-defects created by sulfur migration. Noticeably, long-time air-exposure of the samples (3 weeks) was found to significantly enhance the diamagnetic component. Density functional theory (DFT) calculations applied to a sp3-rich amorphous‑carbon-system in presence of sulfur-bonding revealed a significant variation of the system-metallicity. We report the presence of insulating states up to sulfur concentrations of ~6.25% (atom %), which then vanish at ~7.5% (atom %) of sulfur-doping. Interestingly, inclusion of the sulfur-migration effect within the DFT calculations revealed significant contributions arising from magnetic-moments generated by electron localization at the vacancy sites. Total spin isosurface analyses (spin up - spin down) highlighted the presence of ferromagnetic contributions arising from carbon-atoms, due to dangling bonds generated by point defects (with the largest magnetic moment values being 0.42 μΒ and 0.379 μΒ respectively).

Sulfur-Mediated Interface Engineering Enables Fast SnS Nanosheet Anodes for Advanced Lithium/Sodium-Ion Batteries

Interface design is generally helpful to ameliorate the electrochemical properties of electrode materials but challenging as well. Herein, in situ sulfur-mediated interface engineering is developed to effectively raise the kinetics properties of the SnS nanosheet anodes, which is realized by a synchronous reduction and carbon deposition/doping process. The sulfur in the raw SnS2 directly induces the sulfur-doped amorphous carbon layer onto the in situ reduced SnS nanosheet. In situ and ex situ electrochemical characterizations suggest that the sulfur-mediated interface layer can enhance the reversibility and kinetics properties, promote the ion/electron swift delivery, and maintain the configurational wholeness of the SnS nanosheet anodes. Consequently, a relatively high Li-storage capacity of 922 mAh g-1 and Na-storage capacity of 349 mAh g-1 at 1.0 A g-1 even after 1000 and 300 long-term cycles are achieved, respectively. The facile method and excellent performance suggest the effective interface tuning for developing the SnS-based anodes for batteries and beyond.

Generation of maghemite nanocrystals from iron-sulfur centres.

Iron oxide nano-crystals 0.1-1.1 μm in diameter were generated on sulfur-doped amorphous carbon surfaces by electron beam irradiation of the novel 13e- high-spin complex [Fe(4-methyl-1,2-benzenedithiolate)2][NHEt3] encapsulated in a triblock copolymer. Possible relevance to iron nano-mineralization from Fe-S ferredoxin proteins and iron dysregulation in neurological disorders is discussed.

Hierarchical self-assembled Bi2S3 hollow nanotubes coated with sulfur-doped amorphous carbon as advanced anode materials for lithium ion batteries.

Bismuth sulfide (Bi2S3) is considered as a promising anode material for lithium ion batteries (LIBs) owing to its high theoretical specific capacity and intriguing reaction mechanism. However, capacity fading and cycling instability due to volume variation during the lithiation/delithiation process still remain a great challenge. Herein, we proposed a simple glucose assisted hydrothermal strategy and followed a post-treatment process to prepare hierarchical sulfur-doped carbon Bi2S3 (Bi2S3@SC) hollow nanotubes that self-assembled into sulfur-doped amorphous carbon coated Bi2S3 nanocrystals as building blocks. Glucose plays a decisive role in the formation process of Bi2S3 nanocrystals and subsequent self-assembly, forming Bi2S3@SC hollow nanotubes. The polysaccharide shell formed on the surface of Bi2S3 nanocrystals during the hydrothermal process was transformed into the sulphur-doped amorphous carbon layer after the post-treatment process. Electrochemical tests reveal that the resulting composites exhibit excellent electrochemical performance with a highly reversible cycling capacity of ∼950 mA h g-1 at a current density of 100 mA g-1, as well as a good rate capability and significantly enhanced cycling stability derived from their unique structural features, thus demonstrating the potential of Bi2S3@SC hollow nanotubes as high performance anode materials for LIBs. The analysis of electrochemical kinetics confirmed that the pseudocapacitive behavior dominates the overall storage process of Bi2S3@SC hollow nanotubes.

Unusual ZFC and FC magnetic behavior in thin Co multi-layered structure

The observation of unusual magnetic phenomena in a Ni -based magnetic memory device ([4] O. Ben-Dor et al., 2013) encouraged us to conduct a systematic research on Co based multi-layered structure which contains a α-helix L polyalanine (AHPA-L) organic compound. The constant Co thickness is 7nm and AHPA-L was also replaced by non-chiral 1-Decanethiol organic molecules. Both organic compounds were chemisorbed on gold by a thiol group. The dc magnetic field (H) was applied parallel and perpendicular to the surface layers. The perpendicular direction is the easy magnetization axis and along this orientation only, the zero-field-cooled (ZFC) plots exhibit a pronounced peak around 55–58K. This peak is suppressed in the second ZFC and field-cooled (FC) runs performed shortly after the virgin ZFC one. Thus, around the peak position ZFC>FC a phenomenon seldom observed. This peak reappears after measuring the same material six months later. This behavior appears in layers with the non-chiral 1-Decanethiol and it is very similar to that obtained in sulfur doped amorphous carbon. The peak origin and the peculiar ZFC>FC case are qualitatively explained.

Peculiar Magnetic Features and Superconductivity in Sulfur Doped Amorphous Carbon

We report on magnetic studies of inhomogeneous commercial and synthesized amorphous carbon (a-C) and a-C doped with sulfur (a-CS) powders which exhibit (i) peculiar magnetic behavior and (ii) traces of two superconducting (SC) phases TC ~ 33 and at 65 K. (i) The temperature dependence of zero-field-cooled (ZFC) curves measured up to room temperature show well distinguished elusive peaks at around 50–80 K, and their origin is not yet known. These peaks are totally washed-out in the second ZFC sweeps and in the FC branches as well. As a result, in the vicinity of the peaks, the FC curves lie below the ZFC peaks (FC < ZFC), a phenomenon which is rarely observed. These magnetic anomalies are intrinsic properties of a-C and a-CS materials (ii) SC was observed in three different a-C sources: (a) The commercial a-C powder contains 0.21% of sulfur and it is suggested that two different a-CS phases (at 33 and 65 K) are the origin of the two SC states observed. The compositions of these two phases are not yet unknown. The small SC volume fractions of the 33 K phase can be enhanced by a solid reaction with additional sulfur at 250 °C; (b) the synthesized (a-C) powder (obtained from decomposition of sucrose) is not SC. However, when mixed with sulfur and heated at 400 °C under a protective atmosphere, the a-CS powder obtained also show traces of a SC phase at TC = 42 K; (c) The same occurs in a-C thin films. The as-grown films are not SC but a SC phase at TC = 34 K emerges after the films were reacted with sulfur at elevated temperatures. It is concluded, therefore, that all SC phases observed are due to different unknown a-CS phases. Since the a-C and a-CS powders possess SC and magnetic states, we believe that these powders resemble the high TC curates and Fe-As based systems in which the SC and the magnetic states are closely related to each other.

Superconductivity and Unusual Magnetic Features in Amorphous Carbon and in Other Unrelated Materials

Traces of superconductivity (SC) were observed in three different inhomogeneous sulfur-doped amorphous carbon (a-C) systems: (i) in old commercial powder, (ii) in fabricated (by pyrolytic decomposition of sucrose) powder, and (iii) in a-C thin films. (i) The commercial a-C powder contains 0.21 at% sulfur and reveals traces of two superconducting phases around T C ˜34 and T C∼65 K. (ii) The fabricated a-C powder is diamagnetic but not superconducting. However, mixtures of this powder with sulfur (a-CS) which were heated to 250 °C also show traces of SC up to T C∼42 K. (iii) Non-superconducting a-C thin films were grown by electron-beam induced deposition. SC at T C∼34 K emerged only after heat treatment with sulfur. It is proposed that the high SC states in a-CS (T C∼65 K at ambient pressure) and in H3S materials (T C = 203 K under >200 GPa) has the same origin. In both sulfur-containing systems, SC is induced by the interaction between electrons and the high frequencies of low mass atom (H or C) vibrations a-CS products obtained by heating commercial and fabricated a-C powders at 400 °C show unusual magnetic features: (a) pronounced irreversible peaks around 55–80 K which appear in the first zero-field-cooled (ZFC) runs only. This temperature range is close to the highest T C observed. (b) These peaks are totally suppressed in the second ZFC sweeps measured a few minutes later. (c) The peaks reappear after 18 months. (d) Around the peak position, the field-cooled (FC) curves cross the ZFC plots (ZFC > FC). All these phenomena are intrinsic properties of the amorphous carbon materials and were not observed in crystalline graphite. The ZFC > FC state was also observed in chiral-based magnetic memory device and unexpectedly in the liver taken from a patient with mantle cell lymphoma. This peculiarity will be discussed.

Unusual magnetic behavior in a chiral-based magnetic memory device

In recent years chiral molecules were found to act as efficient spin filters. Using a multilayer structure with chiral molecules magnetic memory was realized. Observed rare magnetic phenomena in a chiral-based magnetic memory device was reported by O-Ben Dor et. al in Nature Commun, 4, 2256 (2013). This multi-layered device is built from α-helix l-polyalanine (AHPA-L) adsorbed on gold, Al2O3 (7nm) and Ni (30nm) layers. It was shown that certain temperature range the FC branch crosses the magnetic peak (at 55K) observed in the ZFC curve thus ZFC>FC. We show here that in another similar multi-layered material, at low applied field, the ZFC curve lies above the FC one up to 70K. The two features have the same origin and the crucial necessary components to exhibit them are: AHPA-L and 30nm Ni layered thick. Similar effects were also reported in sulfur doped amorphous carbon. A comparison between the two systems and the ingredients for these peculiar observations is discussed.

Superconductivity in Sulfur-Doped Amorphous Carbon Films

Following our previous investigations on superconductivity in amorphous carbon (aC) based systems; we have prepared thin composite aC–W films using electron-beam induced deposition. The films did not show any sign for superconductivity above 5 K. However, local, nonpercolative, superconductivity emerged at T c=34.4 K after treatment with sulfur at 250 °C for 24 hours. The superconducting features in the magnetization curves were by far sharper compared to our previous results, and the shielding fraction increased by about an order of magnitude. Our data suggest that pairing and localized superconductivity take place in the aC–S regions, whereas phase coherence, assisted by the W inclusions, was enhanced compared to our previous samples, yet still not to the degree of achieving global phase-coherence and percolating superconductivity.

Unusual Magnetic Properties and Superconductivity in Sulfur-Doped Amorphous Carbon Powder

An amorphous carbon (a-C) powder obtained by decomposition of sucrose was mixed with sulfur and synthesized at 400 ∘C. Magnetic measurements reveal that the products obtained are inhomogeneous. Some parts show traces of superconductivity at TC=17 and 42 K. Other parts show unusual magnetic features. (i) Pronounced irreversible peaks around 50 K appear in the first zero-field-cooled (ZFC) sweep only. (ii) Unexpectedly, these peaks are totally suppressed in the second ZFC runs performed a few minutes later. Around the peak position, the field-cooled (FC) curves cross the ZFC plots, such that ZFC>FC. The two peculiar magnetic observations are connected to each other and are also ascribed to the a-C powder taken from commercial sources. The possibility that impurities cause these magnetic phenomena is excluded.

Synthesis of Sulfur-Doped Amorphous Carbon Thin Films by Plasma CVD Using Thiophene as a Starting Material.

Amorphous carbon thin films containing sulfur were synthesized from thiophene (C4H4S) by the r.f. (13.56 MHz) plasma CVD. The r.f. power was applied intermittently to control the deposition temperature (Td). While maintaining a constant r.f. power (200 W) and reactive pressure (13.3 Pa), we investigated the effect of Td on the film chemical structures and properties: specifically microroughness, density, microhardness and internal stress. The deposition rate and microroughness of the films decreased with increasing Td, while the film density, microhardness and internal stress increased with increasing Td. X-ray photoelectron spectroscopic (XPS) analysis indicated that S/C ratio of the films remained constant and has no connection with Td. XPS and Raman spectroscopic analyses showed that the films were composed of two kinds of microdomains with sp3 and sp2 bonds similar to the diamond-like carbon (DLC) film. Sulfur incorporation into the film significantly reduced the internal stress.