(A.2) Nanoripples-assisted catalytic activity of monolayer graphene:

Nano-rippled graphene acts as a catalysis in molecular hydrogen dissociation, similar to metals, in stark contrast to flat graphene. The graphene dissociates molecular hydrogen as strongly as the best-known catalysts for this reaction. Our observations have implications not only for graphene, but also for other 2D materials (inherently non-flat) which could be more catalytically active compared to their bulk-counterparts. 

For details : P. Z. Sun et al,  PNAS, 120 (12), e2300481120 (2023)

(B) Raman Spectroscopy & XRD under extreme conditions 

(B.1) Topological Insulators: Raman Spectroscopy under high pressure (  ̴ 30GPa)

Probing Z2 topology from bulk signature & discovery of a new Topological Insulator: Different experimental techniques are being pursued to capture the surface electronic properties due to Dirac electrons and hence, thereby characterizing Z2 topology of topological Insulators (TIs).  Characterization of Z2 TIs is solely based on the isolated quantum system of electrons, but in real materials they are coupled to other low energy excitations of the system. Here, we addressed the long-standing question that can we trace the topological signature due to electronic topological transition (ETT) from the environment or nonelectronic baths of the system rather than from the electronic system itself, experimentally as well as theoretically?

It is the parity of phonons that gives us access to ETT in stark contrast to electronic system for which it (parity operator in orbital space) is randomized due to scattering off.

For details : Sharp Raman anomalies and broken adiabaticity at a pressure induced transition from band to topological insulator in Sb2Se3.  Phys. Rev. Lett. 110, 107401, 2013

 (B.2) Determination of space group using synchrotron XRD : Rietveld Analysis

All the three Mo-based TMDs (MoS2, MoSe2 and MoTe2) undergo semimetal transition and only for MoS2, iso-structural transition precedes the metallization phase. The question we addressed is that if we substitute one S-atom with the Se-atom, i.e. for MoSSe compound, what happens to pressure-induced phase transitions. We have shown that the substitution of S by Se reduces its crystal symmetry to P63mc (# 186) and an isostructural transition to a mixed 2H′c + 2H′a phase takes place around 10.8 GPa, as reflected in the lattice parameters.

For details :  Chemical ordering and pressure-induced isostructural and electronic transitions in MoSSe crystal. Phys. Rev. B, 102, 014103, 2020

(B.3) Tracking Single-type of atom from XRD : MoTe2 

In our work, we report high pressure synchrotron x-ray diffraction (XRD) experiments on 2H-MoTe2 up to 46 GPa under hydrostatic conditions. To the best of our knowledge, this is the first evidence using XRD to show (i) the rate of individual layer compressibility (ILC) and (II) electronic topological transition (ETT) observed in 2H-MoTe2, a overlooked feature. ILC could track single atom movement vertically for layered materials.

For details : Pressure-induced isostructural electronic topological transitions in 2H-MoTe2: x-ray diffraction and first-principles study. J. Phys. : Condens. Matter, 065402-8, 33, 2020

(C) Optoelectronic properties of monolayer MoS2

(C.1) Identifying the particular phonon modes for dominant scattering of charge carriers : Aspects of Symmetry

Monolayer MoS2 shares the hexagonal lattice symmetry as single layer graphene, but the electronic mobility for graphene is orders of magnitude larger than MoS2. For the field effect transistor (FET) devices, the intrinsic limiting process is the electron-phonon coupling (EPC) which hinders the carrier mobility at room temperature and the maximum current is controlled by hot phonons. It is noteworthy that the top gating leads to hardening of the G phonon mode in case of graphene where adiabatic approximation fails. In case of MoS2, EPC is within the adiabatic approximation. Not only our findings are being used as a quick tool on a daily basis in research community to map the doping of monolayer MoS2 and from there the intrinsic defects in the underlying lattice, but also the very symmetric aspects of EPC are being used to monitor/filter the most dominating parameter among (i) doping, (ii) compressive strain and (iii) in-plane tensile strain.

For details: Symmetry-dependent phonon renormalization in monolayer MoS2 transistor. Phys. Rev. B 85, 161403(R), 2012 

(D) Photoluminescence of monolayer MoS2 

(D.1) Laser-induced atomistic healing of defects in monolayer MoS2: Optical signatures

We show that exposing the monolayer MoS2 in air to a modest laser intensity for a brief period of time enhances simultaneously the photoluminescence (PL) intensity associated with both the trions and excitons together with increase in the Raman intensity of first and second order modes. The simultaneous increase of PL from trions and excitons cannot be understood based only on known scenario of depletion of electron concentration in MoS2 by adsorption of O2 and H2O molecules. This is explained by laser induced healing of defect-states resulting in reduction of non-radiative Auger processes. Our experimental findings (first report on healing of defects via laser exposure in monolayer MoS2) will help to understand about how laser exposure can change the optoelectronic properties in designing the devices as well as in sensor applications with materials like MoS2.

For details : Enhanced Raman and photoluminescence response in monolayer MoS2 due to laser healing of defects. J. Raman Spectrosc., 100–105, 49, 2018