Reduced electron screening in two-dimension plays a fundamental role in determining exciton properties, which dictates optoelectronic and photonic device performances. Considering the explicit electron-hole interaction within the GW −Bethe-Salpeter formalism, we first study the excitonic properties of pristine phosphorene and investigate the effects of strain and impurity coverage. The calculations reveal strongly bound exciton in these systems with anisotropic spatial delocalization. Further, we present a simplified hydrogenic model with anisotropic exciton mass and effective electron screening as parameters, and the corresponding results are in excellent agreement with the present GW −BSE calculations. The simplified model is then used to investigate exciton renormalization in few-layer and heterostructure phosphorene. The changes in carrier effective mass along with increasing electron screening renormalizes the exciton binding in these systems. We establish that the present model, where the parameters are calculated within computationally less expensive first-principles calculations, can predict exciton properties with excellent accuracy for larger two-dimensional systems, where the many-body GW −BSE calculations are impossible. arXiv:1810.11994v1 [cond-mat.mes-hall]

On-surface molecular self-assembly is one of the key paradigms for understanding intermolecular interactions and molecule−substrate interactions at the atomic scale. Phthalocyanines are planar π-conjugated systems capable of selfassembly and can act as versatile, robust, and tunable templates for surface functionalization. One of the ways to tailor the properties of phthalocyanines is by pendant group substitution. How such a scheme brings about changes in the properties of the phthalocyanines at the nanoscale has not been greatly explored.Here we present an atomic-scale picture of the self-assembly of copper phthalocyanine, CuPc, and compare it with its cyano analogue, CuPc(CN) 8, on Au(111) using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) in ultrahigh vacuum (UHV) at 77 K. STM imaging reveals a tetramer unit cell to be the hallmark of each assembly. The periodicity of herringbone reconstruction of Au( 111) is unchanged upon CuPc(CN) 8 adsorption, whereas for CuPc adsorption this periodicity changes. STS measurements show an increment in the highest occupied−lowest unoccupied molecular orbital (HOMO−LUMO) gap from CuPc to CuPc(CN) 8 . Extensive ab initio calculations within density functional theory (DFT) match well with the experimental observations. STM imaging shows adsorption-induced organizational chirality for both assemblies. For CuPc(CN) 8 at LUMO energy, the individual molecule exhibits an orbital-energy-dependent chirality on top of the existing organizational chirality. It remains achiral at HOMO energy and within the HOMO−LUMO gap. No such peculiarity is seen in the CuPc assembly. This energy-selective chiral picture of CuPc(CN) 8 is ascribed to the cyano groups that participate in antiparallel dipolar coupling, thereby enhancing intermolecular interaction in the CuPc(CN) 8 assembly. Thus, our atomically resolved topographic and spectroscopic studies, supplemented by DFT calculations, demonstrate that pendant group substitution is an effective strategy for tweaking intermolecular interactions and for surface functionalization.

Converting solar energy into chemical energy by splitting water is a promising means to generate a sustainable and renewable solution without detrimental environmental impact. The two-dimensional semiconductors serve as potential catalysts in this regard, and here we combine Janus transitionmetal dichalcogenides (MoXY, X/Y = S, Se, Te) and graphitic carbon nitride in a van der Waals heterostructure. Within the first-principles calculations, we investigate the electronic, optical and excitonic properties that determine the photocatalytic activity. Due to the internal electric field, the photogenerated electrons and holes are separated in the MoXY layers, and also generates high overpotentials for the redox reactions. The high optical absorptions span throughout the entire visible and near ultraviolet regime in these heterostructure nanocomposites. Further, the lower exciton binding, calculated within the two-dimensional hydrogenic model, indicates efficient charge separation. Enormous tunability of photocatalytic properties in such heterostructures should attract considerable theoretical and experimental attention in future.

In the context of two-dimensional metal-free photocatalyst, we investigate the electronic, optical and excitonic properties of phosphorene derivatives within first-principles approach. While two-dimensional phosphorene does not catalyze the complete water splitting reactions, O, S, and N coverages improve the situation drastically, and become susceptible to catalyze the complete reaction at certain coverages. We find that for all these dopants, 0.25 -0.5 ML coverages are thermodynamically more stable, and does not introduce midgap defect states and the composite systems remain semiconducting along with properly aligned valance and conduction bands. Further, within visible light excitation, the optical absorption remain very high 10 5 cm −1 in these composite systems, and the fundamental optical anisotropy of phosphorene remains intact. We also investigate the effect of layer thickness through bilayer phosphorene with oxygen coverages. Finally we investigate the excitonic properties in these composite materials that are conducive to both redox reactions. The present results will open up new avenues to take advantage of these metal-free phosphorene derivatives toward its outstanding potential in photocatalysis. arXiv:1806.09017v1 [cond-mat.mtrl-sci]

Step edges of single-crystal surfaces play an important role in tuning the electronic properties of the surfaces and in guiding the application of surfaces as catalytic reaction centers. Modification of step edges by molecular adsorption can be an effective strategy for bottom-up nanofabrication of surfaces. A detailed submolecular level understanding of step-edge adsorption is mandatory to exploit the properties of step edges for a variety of applications. Though a variety of phthalocyanine (Pc) molecules have been investigated on surfaces, there is a huge void in the literature about step-edge behavior of Pcs on surfaces. With this perspective, the adsorption characteristics of copper Pc (CuPc) and copperoctacyano Pc (CuPc(CN) 8 ) have been investigated on Au(111) monoatomic (MA) step edges using low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. At very low coverage, the adsorption of CuPc and CuPc(CN) 8 leads to the formation of onedimensional chains along the step edge. At higher coverage, both CuPc and CuPc(CN) 8 guided by tetramer unit cell formation self-assemble on flat terraces and cross over the step edge of Au(111). CuPc adsorption along MA step edge shows only one geometric configuration, whereas two different geometric configurations occur for CuPc(CN) 8 . The spectroscopic signature of these two configurations, probed using scanning tunneling spectroscopy (STS), manifests in a shift of the peak position of the highest occupied molecular orbital for the CuPc(CN) 8 molecule at the MA step edge with respect to the molecule over the flat terrace of Au(111). The STM images simulated on the basis of DFT calculations for specific configurations agree with the experimental results. These findings also advance our understanding of the role played by the pendant groups of the Pc molecules in step-edge adsorption.