Anomalous low-temperature saturation effects and negative thermal expansion in the c-axis of highly oriented pyrolytic graphite at the magic angle

We report a novel T-XRD and Rietveld-refinement investigation of pyrolytic-graphite samples with high degree of graphene-layer-orientation and misfit-rotational-angle of ~ 0.8o in the T-range from 12K to 298K. An anomalous variation of the graphitic c-axis which involves firstly negative-thermal-expansion (from 12K to ~50K), a saturation-effect (from 50K to ~160K) and then a positive expansion (from ~180K to 298K) is evidenced. The reported trend is significantly different with respect to that expected by considering the standard-thermal-expansion α-parameter where no saturation-effect is present. SQUID-magnetometry revealed further presence of superconducting-like hysteresis which resemble those observed by Scheike et al

Continuous synthesis of PVP stabilized biocompatible gold nanoparticles with a controlled size using a 3D glass capillary microfluidic device

A reliable glass capillary microfluidic method was developed for a continuous production of well-controlled gold nanoparticles (AuNPs) capped with polyvinylpyrrolidone (PVP) of different molecular weights (PVP K15, PVP K30 and PVP K90). A two-phase co-flow glass capillary microfluidic device with an injection orifice diameter ranging between 100 and 240 µm was used to synthesise 100–240 µm was used to synthesise AuNPs via the chemical reduction between tetrachloroaurate trihydrate (HAuCl4·3H2O) and ascorbic acid. AuNPs with an average diameter between 48 and 135 nm were synthesised, as determined by DLS measurements. Decreasing the injection orifice diameter, increasing the flow rate of ascorbic acid stream and its pH resulted in smaller AuNPs. The polydispersity index (PDI) was found to be independent on the injection orifice diameter or the molecular weight of PVP, but increased with the increase of flow rate and the pH of ascorbic acid stream. The stability study over 6-week period confirmed that PVP K30 with an average Mw of 40000 g/mol was the best capping agent to synthesize and stabilise smaller AuNPs. The reactor fouling due to deposition of AuNPs on reactor walls and orifices was mitigated by hydrophobization of reactor/capillary walls with octadecyltrimethoxisilane and the use of ascorbic acid solution of higher pH

Magnetic vortex and unsaturated magnetization components in highly oriented pyrolytic graphite

Observation of ferromagnetic and granular superconductive features in highly-oriented-pyrolytic-graphite (HOPG) has recently attracted an important attention. We report a novel temperature dependent XRD and SQUID investigation of HOPG in the temperature range from 300.15 to 77.15 K. Unusual hysteresis features indicate the possible presence of vortex states in conditions of magnetic field approximately perpendicular to the HOPG layers. This interpretation is further supported by additional measurements performed on intermediate lamellae extracted by exfoliation. Evidence of a possible structural-transition in the c-axis of HOPG in the temperature range between 77 K and 100K is also provided by using the Rietveld refinement method. ZFC and FC measurements performed at high field values of 5000-10000 Oe, together with mFC-mZFC subtraction, highlight absence of a sharp depletion of the difference between magnetization signals towards zero. These observations may indicate the possible presence of additional unsaturated weak features, which are ascribed to superconductive signals as previously predicted by Scheike et al. [8]

Rhombohedral stacking-faults in exfoliated highly oriented pyrolytic graphite

The recent observation of possible granular superconductivity in highly oriented pyrolytic graphite (HOPG) has attracted significant research interest. Here we report a novel investigation on the structural-properties of exfoliated-HOPG. We investigated two types of exfoliation methods, involving either a full (method-1) or partial (method-2) contact between adhesive tape and the main HOPG. Structural characterization was obtained by employing X-ray diffraction (XRD), Raman spectroscopy and electron microscopy (SEM). In particular, Raman point and mapping spectroscopy revealed significant structural-transitions from ABA (Bernal) to ABC (rhombohedral) stacking (stacking-faults), in those samples obtained with the method-2. Interestingly, strained regions exhibiting structural-deformations with a ridge-like morphology were reproducibly identified. The acquired Raman-spectra revealed a local enhancement of the D and D’ bands-intensity together with contributions arising from Electronic Raman Scattering (ERS) across the band-gap of rhombohedral-graphite, at middle (∼1870 cm−1) and high (∼ 2680 cm−1) frequency. HRTEM of the samples produced with the method-2 allowed also for the identification of local-coexistence of ripplocation-like defects with moiré superlattices, an indicator of non-uniform c-axis configuration

Unusual butterfly-shaped magnetization signals and spin-glass-like behaviour in highly oriented pyrolytic graphite

We report a novel investigation on the relationship between magnetic-ordering and graphitic-structure in highly-oriented-pyrolytic-graphite (HOPG). By employing orientation-dependent-X-ray-diffraction, Raman-spectroscopy and temperature-dependent-superconductive-quantum-interference-device (T-SQUID) we examined the presence of ferromagnetic- and superconductive-ordering in HOPG systems with 1) disordered (HOPG1, containing carbon-vacancy-rich weak-Bernal-stacking and Moiré-superlattices with θmisfit ∼ 0.5°) and 2) ordered (HOPG2, containing higher-degree of Bernal-stacking and Moiré-superlattices with θmisfit ∼ 0.5°, 0.8°, 11°) graphitic-layer-arrangement. A perfect-HOPG is expected to exhibit a diamagnetic-response to an applied-magnetic-field. Instead, additional 1) ferromagnetic-signals presenting a characteristic width-enhancement with the field increasing in HOPG1 and 2) complex butterfly-shaped ferromagnetic signals in HOPG2, are demonstrated. Temperature-dependent-magnetometry evidenced further the presence of randomly oriented ferromagnetic clusters originating from topological disorder in both HOPG1 and HOPG2. These magnetic signals were explained on the basis of the percolative-type model

Phase Transition Modulation and Defect Suppression in Perovskite Solar Cells Enabled by a Self-Sacrificed Template

Tunable crystal growth offering highly aligned perovskite crystallites with suppressed deep-level defects is vital for efficient charge transport, which in turn significantly influences the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Herein, a "precursor to perovskite-like template to perovskite" (PPP) growth strategy is developed, using either MAAc or GuaCl precursor to induce a sacrificial thermal-unstable perovskite-like template for (FAPbI(3))(x)(MAPbI(3))(y) perovskite growth. The self-sacrificed intermediate template induces the formation of highly aligned perovskite crystals with greatly enhanced film crystallinity and suppresses deep-level defect formation. Furthermore, it is proved that MAAc or GuaCl completely evaporates during the high-temperature annealing process. The reduction in defect densities and nonradiative recombination enhances both carrier lifetime and charge dynamics, yielding impressive PCEs of 22.3% and 22.8% with a high open-circuit voltage (V-OC) of 1.16 V and an incredible fill factor (FF) of 81.5% and 79.4% for MAAc- and GuaCl-based devices, respectively. These results suggest that the formation of the thermal-unstable perovskite-like sacrificial template is a promising strategy to restrain the deep-level defects in perovskite films toward the attainment of highly efficient and stable large-scale PSCs as well as other perovskite-based electronics

Volatile Solid Additive-Assisted Sequential Deposition Enables 18.42% Efficiency in Organic Solar Cells

Morphology optimization of active layer plays a critical role in improving the performance of organic solar cells (OSCs). In this work, a volatile solid additive-assisted sequential deposition (SD) strategy is reported to regulate the molecular order and phase separation in solid state. The OSC adopts polymer donor D18-Cl and acceptor N3 as active layer, as well as 1,4-diiodobenzene (DIB) as volatile additive. Compared to the D18-Cl:N3 (one-time deposition of mixture) and D18-Cl/N3 (SD) platforms, the D18-Cl/N3(DIB) device based on DIB-assisted SD method exhibits a finer phase separation with greatly enhanced molecular crystallinity. The optimal morphology delivers superior charge transport and extraction, offering a champion power conversion efficiency of 18.42% with significantly enhanced short-circuit current density (J(sc)) of 27.18 mA cm(-2) and fill factor of 78.8%. This is one of the best performances in binary SD OSCs to date. Angle-dependent grazing-incidence wide-angle X-ray scattering technique effectively reveals the vertical phase separation and molecular crystallinity of the active layer. This work demonstrates the combination of volatile solid additive and sequential deposition is an effective method to develop high-performance OSCs

Molecular ordering and phase segregation induced by a volatile solid additive for highly efficient all-small-molecule organic solar cells

Morphology control remains a major challenge for all-small-molecule organic solar cells (ASM OSCs), mainly reflecting in the elusive trade-off between the molecular ordering and phase separation of the active layer. In this study, a novel volatile solid additive IC-FI, namely dihalogenated 1,1-dicyanomethylene-3-indanone, whose chemical structure is analogous to the end-group moiety of the representative non-fullerene acceptor, has been prepared and applied in BTR-Cl:N3-based ASM OSCs. The intrinsic structural feature of IC-IF enables the self-assembly of N3 along with the enhanced intermixing between BTR-Cl and N3 in the subsequent thermal annealing process. This offers nano-scale phase separation with a predominant face-on oriented molecular packing in the blend film, thereby boosting the effective charge transport and extraction process in the device. Consequently, a superior power conversion efficiency (PCE) of 14.43% with an outstanding fill factor (FF) of 73.53% was achieved in the IC-FI processed device. These results indicate that the use of the volatile solid additive is a simple and practical strategy for the optimization of the small-molecule bulk-heterojunction morphology toward highly efficient ASM OSCs

Simultaneous Interfacial Modification and Crystallization Control by Biguanide Hydrochloride for Stable Perovskite Solar Cell with PCE of 24.4%

Interfacial modification that serves multiple roles is vital for the fabrication of efficient and stable perovskite solar cells. Here, a multi-functional interfacial material, biguanide hydrochloride (BGCl), is introduced between SnO2 and perovskite to enhance electron extraction as well as crystal growth of perovskite. The BGCl can chemically link to the SnO2 through Lewis coordination/electrostatic coupling and help to anchor the PbI2. Better energetic alignment, reduced interfacial defects and homogeneous perovskite crystallites are achieved, yielding an impressive certified power conversion efficiency (PCE) of 24.4%, with an open circuit voltage of 1.19 V and a drastically improved fill factor of 82.4%. More importantly, the unencapsulated device maintains 95% of its initial PCE after aging for over 500 h at 20 ??? and 30% relative humidity in ambient conditions. These results suggest that the incorporation of BGCl is a promising strategy to modify the interface and control the crystallization of the perovskite, towards the attainment of highly efficient and stable perovskite solar cells as well as other perovskite-based electronics