Highly porous nanostructures with large surface areas are typically employed for electrical double-layer capacitors to improve gravimetric energy storage capacity; however, high surface area ...carbon-based electrodes result in poor volumetric capacitance because of the low packing density of porous materials. Here, we demonstrate ultrahigh volumetric capacitance of 521 F cm(-3) in aqueous electrolytes for non-porous carbon microsphere electrodes co-doped with fluorine and nitrogen synthesized by low-temperature solvothermal route, rivaling expensive RuO2 or MnO2 pseudo-capacitors. The new electrodes also exhibit excellent cyclic stability without capacitance loss after 10,000 cycles in both acidic and basic electrolytes at a high charge current of 5 A g(-1). This work provides a new approach for designing high-performance electrodes with exceptional volumetric capacitance with high mass loadings and charge rates for long-lived electrochemical energy storage systems.
Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride FeB4 were synthesized at pressures above 8 GPa and high temperatures. Magnetic susceptibility and heat capacity measurements ...demonstrate bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature and the analysis of specific heat data indicate that the superconductivity in FeB4 is likely phonon mediated, which is rare for Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 62(5) GPa; thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.
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•The α” plates induced by hydrostatic pressure form self-accommodation morphologies via {111} twinning.•A peculiar hierarchical microstructure was induced in the Ti-30Zr-10Nb alloy by ...hydrostatic pressure.•The hierarchical microstructure effectively coordinates both isotropic macroscopic strains and local shear stresses.•The hydrostatic pressure facilitates the β to ω transformation but hinders the β to α” transformation.
Hydrostatic pressure is a crucial tool to regulate the structure and properties of materials. However, the influence of hydrostatic pressure on the microstructure of metastable β-Ti alloys remains unclear. In this work, the microstructure and microhardness evolution of a metastable β-type Ti-30Zr-10Nb alloy under hydrostatic pressure (≤10 GPa) were investigated and compared with those under uniaxial pressure (≤1106 MPa). The results revealed that under hydrostatic pressure, Ti-30Zr-10Nb alloy formed a peculiar hierarchical microstructure composed of submicron-scaled α” plates with self-accommodating morphologies, nanoscale ω particles, and microscale shear bands containing nanoscale α” domains. The formation of a hierarchical microstructure effectively coordinates not only the isotropic macroscopic strains but also the local shear stresses under hydrostatic pressure. Compared to uniaxial pressure, the hydrostatic pressure facilitates the β to ω transformation, but hinders the β to α” transformation. The microhardness of the Ti3010 alloy reaches approximately 319 HV at a hydrostatic pressure of 10 GPa, which is 28 % higher than the highest microhardness obtained under uniaxial pressure. The current results highlight the promising potential of hydrostatic pressure in optimizing the mechanical properties of metastable β-Ti alloys.
Abstract
Realization of highly tunable second-order nonlinear optical responses, e.g., second-harmonic generation and bulk photovoltaic effect, is critical for developing modern optical and ...optoelectronic devices. Recently, the van der Waals niobium oxide dihalides are discovered to exhibit unusually large second-harmonic generation. However, the physical origin and possible tunability of nonlinear optical responses in these materials remain to be unclear. In this article, we reveal that the large second-harmonic generation in NbO
X
2
(
X
= Cl, Br, and I) may be partially contributed by the large band nesting effect in different Brillouin zone. Interestingly, the NbOCl
2
can exhibit dramatically different strain-dependent bulk photovoltaic effect under different polarized light, originating from the light-polarization-dependent orbital transitions. Importantly, we achieve a reversible ferroelectric-to-antiferroelectric phase transition in NbOCl
2
and a reversible ferroelectric-to-paraelectric phase transition in NbOI
2
under a certain region of external pressure, accompanied by the greatly tunable nonlinear optical responses but with different microscopic mechanisms. Our study establishes the interesting external-field tunability of NbO
X
2
for nonlinear optical device applications.
Diamane, the two-dimensional counterpart of diamond, is achieved from bi-layer graphene (BLG) or few-layer graphene (FLG) through surface chemical adsorption or high-pressure technology. Diamane with ...interlayer sp
3
bonding is found to have excellent heat transfer, ultra-low friction, high natural frequency, and tunable band gap, which shows the potential technological and industrial applications in nano-photonics, ultrasensitive resonator-based sensors, and improved wear resistance. In this review, we summarize the structure character, synthesis strategies, and physical properties of different diamanes, including hydrogenated diamane (HD), fluorinated diamane (FD), and pristine diamane (PD). In addition, we discuss the effect of functional groups, element doping, and stacking order on the physical properties of diamane. Finally, the remaining challenges and future opportunities for the further development of diamane are addressed.
Abstract
Van der Waals dielectrics are fundamental materials for condensed matter physics and advanced electronic applications. Most dielectrics host isotropic structures in crystalline or amorphous ...forms, and only a few studies have considered the role of anisotropic crystal symmetry in dielectrics as a delicate way to tune electronic properties of channel materials. Here, we demonstrate a layered anisotropic dielectric, SiP
2
, with non-symmorphic twofold-rotational
C
2
symmetry as a gate medium which can break the original threefold-rotational
C
3
symmetry of MoS
2
to achieve unexpected linearly-polarized photoluminescence and anisotropic second harmonic generation at SiP
2
/MoS
2
interfaces. In contrast to the isotropic behavior of pristine MoS
2
, a large conductance anisotropy with an anisotropy index up to 1000 can be achieved and modulated in SiP
2
-gated MoS
2
transistors. Theoretical calculations reveal that the anisotropic moiré potential at such interfaces is responsible for the giant anisotropic conductance and optical response. Our results provide a strategy for generating exotic functionalities at dielectric/semiconductor interfaces via symmetry engineering.
We predict a potassium-nickel intermetallic compound K2Ni at high pressure and identify it as the long-sought structure of the only known K-Ni compound to date Parker et al., Science 273, 95 (1996). ...Although both constituent elements are metallic, K2Ni exhibits a semiconducting ground state with an indirect band gap of 0.65 eV. Electron instability due to the degeneracy at the Fermi level arises from the particular motif of the structure, which in turn induces symmetry-breaking Peierls distortion and a nonmetallic ground state. The results indicate that the chemical properties of elements can change dramatically under extreme conditions and have significant implications for the postulation that potassium is incorporated in Earth's core.
Nitrogen-rich transition metal nitrides are of great interests due to the unique physical and chemical properties. Here, we perform a detailed structural investigation of ReN
8
in the pressure range ...of 0–200 GPa via particle swarm optimization algorithm and first-principles calculations. Interestingly, four ReN
8
phases are firstly revealed to be stable within 200 GPa, N
2
dimers are found to be preferred in
T
-ReN
8
at pressure lower than 15.8 GPa and the infinite N
∞
chains become dominated in
M
-,
T’
- and
M’
-ReN
8
at higher pressure up to 200 GPa. We find that the Young’s moduli of
M
- and
T’
-ReN
8
, about 850 GPa along 0.88 0 0.47 and 0 0 1 direction, are comparable to cubic BN, due to the presence of electronic accumulation in the N
∞
chains. Furthermore,
M
- and
T’
-ReN
8
are found to be of high energy density of ~ 1.9 kJ/g (~ 11.0 kJ/cm
3
).
Graphical abstract
Ultra-incompressibility of ReN
8
in special direction due to the high electronic accumulations in the polynitrogen chains.
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