2D semiconducting transition-metal-dichalcogenides (TMDCs) have drawn a surge of research interests in ultra-thin nanoelectronics and optoelectronics, owing to their moderate charge carrier mobility. Due to a large variation in reported data on charge carrier mobilities of these free-standing monolayers, a more accurate determination of carrier mobilities is a must. This article highlights room temperature carrier mobilities calculated in group-VI B TMDC (i.e., MX₂ (M = Mo, W; X = S, Se, Te)) monolayers via a systematic comparison of reliable models used within the framework of density functional theory. Additionally, piezoelectric effects on carrier mobility have been incorporated. The robust formulation by Lang et al. [Phys. Rev. B, 2016, 94, 235306] gives the most accurate estimation, where the carrier (i.e. electron or hole) mobility and the ratio of hole to electron mobility reach closest to the experimentally measured value. The higher mobility of holes with respect to electrons is attributed to the smaller deformation potential of the holes, while smaller carrier effective masses and larger elastic moduli cause larger carrier mobility in WX₂ relative to MoX₂. Carrier mobility is found to drop from S to Te and also from W to Mo in MX₂ on account of the decrease in the elastic moduli, which is consistent with the increase in the macroscopic static dielectric constant. Erroneously high electron mobility (>1000 cm² V⁻¹ s⁻¹), which is two orders of magnitude higher than the experimental ones, has been disputably reported earlier for MoTe₂ and WS₂ monolayers. It owes its origin to the ultrasmall deformation potential found therein. High (∼1.4–2.3) hole to electron mobility ratio in MX₂ monolayers is found to occur in our work as MoSe₂ > WS₂ > MoS₂ > WSe₂ > WTe₂ > MoTe₂. This large carrier mobility ratio will facilitate an efficient separation of electron–hole pairs, which is particularly useful in optoelectronic applications, e.g., photovoltaics, photocatalysis and artificial photosynthesis. In all terms including optical absorbance, a WS₂ monolayer is found to be most suitable for photophysical processes. Using the Nernst formulation, water splitting and CO₂ photoreduction on WS₂ are found to be favorable over a wide range of pH.

Pristine, semiconducting 1T-MX ₂ (M = Zr, Hf; X = S, Se, Te) monolayers are intrinsically centrosymmetric and non-piezoelectric. This inversion symmetry is broken in their Janus monolayer (non-centrosymmetric) structures, leading to a high degree of piezoelectricity.

Truxene is employed as a building block to successfully synthesize novel covalent organic frameworks (COFs). The condensation reaction between truxene (10,15-dihydro-5H-diindeno[1,2-a:1′,2′-c]fluorene, TX) and 1,4-phenylenediboronic acid (DBA) results in a crystalline COF with boron ester linkages (COF-TXDBA) and a surface area of 1526 m² g⁻¹, as confirmed by powder X-ray diffraction (PXRD) and Brunauer–Emmett–Teller (BET) surface area measurements. This is the first study where nanochannels generated by periodic COF planar layers are shown to ease the interactions of the boron ester linkages with the water molecules for efficient humidity sensing. The COF-TXDBA based % RH sensor exhibits a change of 3 orders of magnitude in impedance in the 11–98% RH range, with response and recovery times of 37 s and 42 s, respectively. The response transients measured by switching COF-TXDBA sensor back and forth in 4 loops of % RH range displays excellent reversibility of the sensor with a deviation of 2.3% in the switching process.

A stable 2D van der Waals (vdW) heterobilayer, constituted by boron monophosphide (BP) and Gallium Nitride (GaN) monolayers, has been explored for different kinds of energy conversion and nanoelectronics. The nearly matched lattice constants of GaN and BP are commensurate with each other in their lattice structures. The out-of-plane inversion asymmetry coupled with the large difference in atomic charges between the GaN and BP monolayers induces in the heterobilayer a giant out-of-plane piezoelectric coefficient (|d₃₃|_max ≈ 40 pm V⁻¹), which is the highest ever reported in 2D materials of a finite thickness. It is much higher than the out-of-plane piezoelectric coefficient reported earlier in multilayered Janus transition metal dichalcogenide MXY (M = Mo, W; X, Y = S, Se, Te) (|d₃₃|_max = 10.57 pm V⁻¹). Such a high out-of-plane piezoelectricity found in a BP/GaN heterobilayer can bring about gigantic strain-tunable top gating effects in nanopiezotronic devices based on the same. Moreover, electron mobility (∼10⁴ cm² V⁻¹ s⁻¹) is much higher than that of transition metal dichalcogenides and conventional semiconductors. The origin of low lattice thermal conductivity (κ_L ∼ 25.25 W m⁻¹ K⁻¹) in BP/GaN at room temperature, which is lower than that of black phosphorene (78 W m⁻¹ K⁻¹), buckled arsenene (61 W m⁻¹ K⁻¹), BCN (90 W m⁻¹ K⁻¹), MoS₂ (34.5 W m⁻¹ K⁻¹) and WS₂ (32 W m⁻¹ K⁻¹) monolayers, has been systematically investigated via phonon dispersion, lattice thermal conductivity, phonon lifetime and mode Grüneisen parameters. The valence band maximum (VBM) and conduction band minimum (CBM) arising from GaN and BP monolayers respectively result in a type II vdW heterobilayer, which is found to be thermodynamically favorable for photocatalytic water splitting in both acidic and neutral media. The exciton binding energies are comparable to those of MoS₂ and C₃N₄ single layers, while the absorbance reaches as high as ∼10⁵ cm⁻¹ in the visible wavelength region. The emergence of high piezoelectricity, high carrier mobility, low lattice thermal conductivity and photocatalytic water splitting abilities in the proposed vdW heterobilayer signifies enormous potential for its versatile applications in nanoscale energy harvesting, e.g., nano-sensors in medical devices, future nanopiezotronics, 2D thermoelectrics and solar energy conversion.

Strain and temperature induced tunability in the thermoelectric properties in monolayer MoS ₂ (ML-MoS ₂ ) has been demonstrated using density functional theory coupled to semi-classical Boltzmann transport theory. Compressive strain, in general and uniaxial compressive strain (along the zig-zag direction), in particular, is found to be most effective in enhancing the thermoelectric power factor, owing to the higher electronic mobility and its sensitivity to lattice compression along this direction. Variation in the Seebeck coefficient and electronic band gap with strain is found to follow the Goldsmid–Sharp relation. n-type doping is found to raise the relaxation time-scaled thermoelectric power factor higher than p-type doping and this divide widens with increasing temperature. The relaxation time-scaled thermoelectric power factor in optimally n-doped ML-MoS ₂ is found to undergo maximal enhancement under the application of 3% uniaxial compressive strain along the zig-zag direction, when both the ( direct ) electronic band gap and the Seebeck coefficient reach their maximum, while the electron mobility drops down drastically from 73.08 to 44.15 cm ² V ⁻¹ s ⁻¹ . Such strain sensitive thermoelectric responses in ML-MoS ₂ could open doorways for a variety of applications in emerging areas in 2D-thermoelectrics, such as on-chip thermoelectric power generation and waste thermal energy harvesting.