Tweaking the Physics of Interfaces between Monolayers of Buckled Cadmium Sulfide for a Superhigh Piezoelectricity, Excitonic Solar Cell Efficiency, and Thermoelectricity

Magnetism of Otherwise Nonmagnetic Elements: From Clusters to Monolayers

Atomic clusters are known to exhibit properties different from their bulk phase. However, when assembled or supported on substrates, clusters often lose their uniqueness. For example, uranium and coinage metals (Cu, Ag, Au) are nonmagnetic in their bulk. Herein, we show that UX6 (X= Cu, Ag, Au) clusters, unlike their nonmagnetic bulk, are not only magnetic but also retain their magnetic character and structure when assembled into a two-dimensional (2D) material. The magnetic moment remains localized at the U site and is found to be 3μB in clusters and about 2μB in the 2D structure. In 2D UX4 (X = Cu, Ag, Au) monolayers, U atoms are found to be coupled antiferromagnetically through an indirect exchange coupling mediated by the coinage metal atoms. Furthermore, hydrogenation of these monolayers can induce a transition from the antiferromagnetic to the ferromagnetic phase. These results, based on density functional theory, have predictive capability and can motivate experiments.

Engineering of Hydrogenated (6,0) Single-Walled Carbon Nanotube under Applied Uniaxial Stress: A DFT-1/2 and Molecular Dynamics Study

Herein, we systematically studied the electronic, optical, and mechanical properties of a hydrogenated (6,0) single-walled carbon nanotube [(6,0) h-SWCNT] under applied uniaxial stress from first-principles density functional theory (DFT) and molecular dynamics (MD) simulation. We have applied the uniaxial stress range from -18 to 22 GPa on the (6,0) h-SWCNT (- sign indicates compressive and + indicates tensile stress) along the tube axes. Our system was found to be an indirect semiconductor (Γ-Δ), with a band gap value of ∼0.77 eV within the linear combination of atomic orbitals (LCAO) method using a GGA-1/2 exchange-correlation approximation. The band gap for (6,0) h-SWCNT significantly varies with the application of stress. The indirect to direct band gap transition was observed under compressive stress (-14 GPa). The strained (6,0) h-SWCNT showed a strong optical absorption in the infrared region. Application of external stress enhanced the optically active region from infrared to Vis with maximum intensity within the Vis-IR region, making it a promising candidate for optoelectronic devices. Ab initio molecular dynamics (AIMD) simulation has been used to study the elastic properties of the (6,0) h-SWCNT which has a strong influence under applied stress.

Theoretical analysis of the absorption of CO2 and CO on pristine and Al-doped C3B

Real-time detection of CO2 and CO is of great importance because CO2 is a major cause of global warming and CO endangers the human nervous and cardiovascular systems.

Theoretical investigations of novel Janus Pb2SSe monolayer as a potential multifunctional material for piezoelectric, photovoltaic, and thermoelectric applications

Two-dimensional Janus nanomaterials, due to their unique electronic, optical, and piezoelectric characteristics resulting from the antisymmetric structures, exhibit great prospects in multifunctional energy application to alleviate the energy crisis. Monolayer Janus Pb₂SSe, with a black phosphorus-like structure and an indirect band gap of 1.59 eV as well as high carrier mobility (526-2105 cm² V^-1 s^-1), displays outstanding potentials in the energy conversion between nanomechanical energy, solar energy, waste heat, and electricity, which has been comprehensively studied utilizing DFT-based simulations. The research results reveal that monolayer Pb₂SSe not only possesses giant in-plane piezoelectricity of d₁₁ = 75.1 pm V^-1 but also superhigh out-of-plane piezoelectric coefficients (d₃₁ = 0.5 pm V^-1 and d₃₃ = 15.7 pm V^-1). Meanwhile, by constructing Pb₂SSe bilayers, the out-of-plane piezoelectric coefficients can be significantly enhanced (d₃₁ = 19.2 pm V^-1 and d₃₃ = 194.7 pm V^-1). In addition, owing to the small conduction band offset, suitable donor band gap and excellent light absorption capability in the Pb₂SSe/SnSe (Pb₂SSe/GeSe) heterostructure, the power conversion efficiencies were calculated to be up to 20.02% (Pb₂SSe/SnSe) and 19.28% (Pb₂SSe/GeSe), making it a promising candidate for solar energy collection. Furthermore, from the thermoelectric electron and phonon transport calculations, it can be found that the Pb₂SSe monolayer is an n-type thermoelectric material with ultrahigh ZT = 2.19 (1.52) at room temperature, which can be traced back to its ultralow κ_L = 0.78 (0.99) W m^-1 K^-1, and superhigh PF = 10.18 (8.25) mW m^-1 K^-2 along the x(y) direction at the optimal doping concentration at 300 K. The abovementioned versatile characteristics in the Janus Pb₂SSe monolayer, along with its comprehensive stabilities (energy, dynamic, thermal, and mechanical stabilities), highlight its potential in clean energy harvesting.

Electronic fibers and textiles: Recent progress and perspective

Wearable electronics are receiving increasing attention with the advances of human society and technologies. Among various types of wearable electronics, electronic fibers/textiles, which integrate the comfort and appearance of conventional fibers/textiles with the functions of electronic devices, are expected to play important roles in remote health monitoring, disease diagnosis/treatment, and human-machine interface. This article aims to review the recent advances in electronic fibers/textiles, thus providing a comprehensive guiding reference for future work. First, we review the selection of functional materials and fabrication strategies of fiber-shaped electronic devices with emphasis on the newly developed functional materials and technologies. Their applications in sensing, light emitting, energy harvest, and energy storage are discussed. Then, the fabrication strategies and applications of electronic textiles are summarized. Furthermore, the integration of multifunctional electronic textiles and their applications are summarized. Finally, we discuss the existing challenges and propose the future development of electronic fibers/textiles.

Conflux of tunable Rashba effect and piezoelectricity in flexible magnesium monochalcogenide monolayers for next-generation spintronic devices

The coupling of piezoelectric properties with Rashba spin-orbit coupling (SOC) has proven to be the limit breaker that paves the way for a self-powered spintronic device (ACS Nano, 2018, 12, 1811-1820). For further advancement in next-generation devices, a new class of buckled, hexagonal magnesium-based chalcogenide monolayers (MgX; X = S, Se, Te) have been predicted which are direct band gap semiconductors satisfying all the stability criteria. The MgTe monolayer shows a strong SOC with a Rashba constant of 0.63 eV Å that is tunable to the extent of ±0.2 eV Å via biaxial strain. Also, owing to its broken inversion symmetry and buckling geometry, MgTe has a very large in-plane as well as out-of-plane piezoelectric coefficient. These results indicate its prospects for serving as a channel semiconducting material in self-powered piezo-spintronic devices. Furthermore, a prototype for a digital logic device can be envisioned using the ac pulsed technology via a perpendicular electric field. Heat transport is significantly suppressed in these monolayers as observed from their intrinsic low lattice thermal conductivity at room temperature: MgS (9.32 W m-1 K-1), MgSe (4.93 W m-1 K-1) and MgTe (2.02 W m-1 K-1). Further studies indicate that these monolayers can be used as photocatalytic materials for the simultaneous production of hydrogen and oxygen on account of having suitable band edge alignment and high charge carrier mobility. This work provides significant theoretical insights into both the fundamental and applied properties of these new buckled MgX monolayers, which are highly suitable for futuristic applications at the nanoscale in low-power, self-powered multifunctional electronic and spintronic devices and solar energy harvesting.

Two-dimensional ultrathin van der Waals heterostructures of indium selenide and boron monophosphide for superfast nanoelectronics, excitonic solar cells, and digital data storage devices

Semiconducting indium selenide (InSe) monolayers have drawn a great deal of attention among all the chalcogenide two-dimensional materials on account of their high electron mobility; however, they suffer from low hole mobility. This inherent limitation of an InSe monolayer can be overcome by stacking it on top of a boron phosphide (BP) monolayer, where the complementary properties of BP can bring additional benefits. The electronic, optical, and external perturbation-dependent electronic properties of InSe/BP hetero-bilayers have been systematically investigated within density functional theory in anticipation of its cutting-edge applications. The InSe/BP heterostructure has been found to be an indirect semiconductor with an intrinsic type-II band alignment where the conduction band minimum (CBM) and valence band maximum (VBM) are contributed by the InSe and BP monolayers, respectively. Thus, the charge carrier mobility in the heterostructure, which is mainly derived from the BP monolayer, reaches as high as 12 × 10³ cm² V^-1 s^-1, which is very much desired in superfast nanoelectronics. The suitable bandgap accompanied by a very low conduction band offset between the donor and acceptor along with robust charge carrier mobility, and the mechanical and dynamical stability of the heterostructure attests its high potential for applications in solar energy harvesting and nanoelectronics. The solar to electrical power conversion efficiency (20.6%) predicted in this work surpasses the efficiencies reported for InSe based heterostructures, thereby demonstrating its superiority in solar energy harvesting. Moreover, the heterostructure transits from the semiconducting state (the OFF state) to the metallic state (the ON state) by the application of a small electric field (∼0.15 V Å^-1) which is brought about by the actual movement of the bands rather than via the nearly empty free electron gas (NFEG) feature. This thereby testifies to its potential for applications in digital data storage. Moreover, the heterostructure shows strong absorbance over a wide spectrum ranging from UV to the visible light of solar radiation, which will be of great utility in UV-visible light photodetectors.

Interfacial hybridization of Janus MoSSe and BX (X = P, As) monolayers for ultrathin excitonic solar cells, nanopiezotronics and low-power memory devices

In this work, we explored the interfacial two-dimensional (2D) physics and significant advancements in the application prospects of MoSSe monolayer when it is combined with a boron pnictide (BP, BAs) monolayer in a van der Waals heterostructure (vdWH) setup. The constructed vdWHs were found to be mechanically and dynamically stable, and they form type-II p-n heterojunctions. Thus, the photogenerated electron-hole pairs are spatially separated. In the BX/MoSSe vdWHs, the BX monolayer serves as excellent donor material for MoSSe, having an ideal donor band gap of ∼1.3 eV. The small value of the conduction band offset (CBO) between the individual monolayers in the vdWHs makes it an excellent candidate for solar energy harvesting in excitonic solar cells, where the power conversion efficiencies were calculated to be 22.97% (BP/MoSSe) and 20.86% (BAs/MoSSe). Also, more than four-fold enhancement in the out-of-plane piezoelectric coefficient (d₃₃) was observed in the MoSSe-based vdWH relative to that in the MoS₂-based vdWH owing to the intrinsic built-in vertical electric field in MoSSe. This is consistent with the out-of-plane piezoelectricity brought about by the alteration in symmetry at the metal-semiconductor Schottky junction, which has been observed experimentally [M.-M. Yang, Z.-D. Luo, Z. Mi, J. Zhao, S. P. E and M. Alexe, Nature, 2020, 584, 377-381]. The results obtained in this work provide useful insights into the design of nanomaterials for future applications in nano-optoelectronics, more efficient excitonic solar cells, and nanoelectromechanical systems (NEMS). Furthermore, this work demonstrates outstanding potential for the application of these vdWHs in superfast electronics, including low-power digital data storage and memory devices, where the tunnel current between the source and drain is effectively tunable using a normal electric field of small magnitude serving as the gate voltage.

KAgX (X = S, Se): High-Performance Layered Thermoelectric Materials for Medium-Temperature Applications

Monolayer KAgX are a class of novel two-dimensional (2D) layered materials with efficient optical absorption and superior carrier mobility, signifying their potential application prospect in photovoltaic (PV) and thermoelectric (TE) fields. Motivated by the recent theoretical studies on the KAgX monolayer, we carried out systematic investigations on the TE performance of KAgS and KAgSe monolayers, employing density functional theory (DFT) and semiclassical Boltzmann transport equation (BTE). For both KAgSe and KAgS monolayers, large Grüneisen parameters, low group velocities, and short phonon scattering time greatly hinder their heat transport and result in an ultralow thermal conductivity, 0.26 and 0.33 W m^-1 K^-1 at 300 K, respectively. A twofold degeneracy appearing at the Γ point and the abrupt slope of the density of states (DOS) near the Fermi level give rise to high Seebeck coefficients of KAgX monolayers. Due to the ultralow thermal conductivity and excellent electronic transport performance, the ZT values as high as 4.65 (3.11) and 4.05 (2.63) at 500 (300) K in the n-type doping for KAgSe and KAgS monolayers are obtained. The exceptional performance of KAgX monolayers sheds light on their immense potential applications in the medium-temperature (around 300-500 K) thermoelectric devices and greatly stimulates further experimental synthesis and validation.

Superhigh flexibility and out-of-plane piezoelectricity together with strong anharmonic phonon scattering induced extremely low lattice thermal conductivity in hexagonal buckled CdX (XS, Se) monolayers

Although CdX (X = S, Se) has been mostly studied in the field of photocatalysis, photovoltaics, their intrinsic properties, such as, mechanical, piezoelectric, electron and phonon transport properties have been completely overlooked in buckled CdX monolayers. Ultra-low lattice thermal conductivity [1.08 W m-1K-1(0.75 W m-1K-1)] and high p-type Seebeck coefficient [1300μV K-1(850μV K-1)] in CdS (CdSe) monolayers have been found in this work based on first-principles DFT coupled to semi-classical Boltzmann transport equations, combining both the electronic and phononic transport. The dimensionless thermoelectric figure of merit is calculated to be 0.78 (0.5) in CdS (CdSe) monolayers at room temperature, which is comparable to that of two-dimensional (2D) tellurene (0.8), arsenene and antimonene (0.8), indicating its great potential for applications in 2D thermoelectrics. Such a low lattice thermal conductivity arise from the participation of both acoustic [91.98% (89.22%)] and optical modes [8.02% (10.78%)] together with low Debye temperature [254 K (187 K)], low group velocity [4 km s-1(3 km s-1)] in CdS (CdSe) monolayers, high anharmonicity and short phonon lifetime. Substantial cohesive energy (∼4-5 eV), dynamical and mechanical stability of the monolayers substantiate the feasibility in synthesizing the single layers in experiments. The inversion symmetry broken along thezdirection causes out-of-plane piezoelectricity. |d33| ∼ 21.6 pm V-1, calculated in CdS monolayer is found to be the highest amongst structures having atomic-layer thickness. Superlow Young's modulus ∼41 N m-1(31 N m-1) in CdS (CdSe) monolayers, which is comparable to that of planar CdS (29 N m-1) and TcTe2(34 N m-1), is an indicator of its superhigh flexibility. Direct semiconducting band gap, high carrier mobility (∼500 cm2V-1s-1) and superhigh flexibility in CdX monolayers signify its gigantic potential for applications in ultrathin, stretchable and flexible nanoelectronics. The all-round properties can be synergistically combined together in futuristic applications in nano-piezotronics as well.

Exceptional mechano-electronic properties in the HfN2 monolayer: a promising candidate in low-power flexible electronics, memory devices and photocatalysis

The response of the electronic properties of the HfN₂ monolayer to external perturbation such as strain and electric fields has been investigated using density functional theory calculations for its device-based applications and photocatalysis.