Preface: Special Topic on Supramolecular Self-Assembly at Surfaces

Supramolecular self-assembly at surfaces is one of the most exciting and active fields in Surface Science today. Applications can take advantage of two key properties: (i) versatile pattern formation over a broad length scale and (ii) tunability of electronic structure and transport properties, as well as frontier orbital alignment. It provides a new frontier for Chemical Physics as it uniquely combines the versatility of Organic Synthesis and the Physics of Interfaces. The Journal of Chemical Physics is pleased to publish this Special Topic Issue, showcasing recent advances and new directions

Effect of distance on photoluminescence quenching and proximity-induced spin-orbit coupling in graphene-WSe2 heterostructures

Spin-orbit coupling (SOC) in graphene can be greatly enhanced by proximity coupling it to transition metal dichalcogenides (TMDs) such as WSe2. We find that the strength of the acquired SOC in graphene depends on the stacking order of the heterostructures when using hexagonal boron nitride (h-BN) as the capping layer, i.e., SiO2/graphene/WSe2/h-BN exhibiting stronger SOC than SiO2/WSe2/graphene/h-BN. We utilize photoluminescence (PL) as an indicator to characterize the interaction between graphene and monolayer WSe2 grown by chemical vapor deposition. We observe much stronger PL quenching in the SiO2/graphene/WSe2/h-BN stack than in the SiO2/WSe2/graphene/h-BN stack, and correspondingly a much larger weak antilocalization (WAL) effect or stronger induced SOC in the former than in the latter. We attribute these two effects to the interlayer distance between graphene and WSe2, which depends on whether graphene is in immediate contact with h-BN. Our observations and hypothesis are further supported by first-principles calculations which reveal a clear difference in the interlayer distance between graphene and WSe2 in these two stacks

Single layer MoS2 on the Cu(111) surface: First-principles electronic structure calculations

First-principles calculations of the geometric and electronic structures of a single layer of molybdenum disulfide (MoS2) on Cu(111) utilizing the van der Waals density functional show three energetically equivalent stacking types and a Moire pattern whose periodicity is in agreement with experimental findings. The layer is found not to be purely physisorbed on the surface, rather there exists a chemical interaction between it and the Cu surface atoms. We also find that the MoS2 film is not appreciably buckled, while the top Cu layer gets reorganized and vertically disordered. The sizes of Moire patterns for a single layer of MoS2 adsorbed on other close-packed metal surfaces are also estimated by minimizing the lattice mismatch between the film and the substrate

Scalable and Transfer-Free Fabrication of MoS2/SiO2 Hybrid Nanophotonic Cavity Arrays with Quality Factors Exceeding 4000

We report the fully-scalable fabrication of a large array of hybrid molybdenum disulfide (MoS2) -silicon dioxide (SiO2) one-dimensional, free-standing photonic-crystal cavities capable of enhancement of the MoS2 photoluminescence at the narrow cavity resonance. We demonstrate continuous tunability of the cavity resonance wavelength across the entire emission band of MoS2 simply by variation of the photonic crystal periodicity. Device fabrication started by substrate-scale growth of MoS2 using chemical vapor deposition (CVD) on non-birefringent thermal oxide on a silicon wafer;it was followed by lithographic fabrication of a photonic crystal nanocavity array on the same substrate at more than 50% yield of functional devices. Our cavities exhibit three dominant modes with measured linewidths less than 0.2 nm, corresponding to quality factors exceeding 4000. All experimental findings are found to be in excellent agreement with finite difference time domain (FDTD) simulations. CVD MoS2 provides scalable access to a direct band gap, inorganic, stable and efficient emitter material for onchip photonics without the need for epitaxy and is at CMOS compatible processing parameters even for back-end-of-line integration;our findings suggest feasibility of cavity based line-narrowing in MoS2-based on-chip devices as it is required for instance for frequency-multiplexed operation in on-chip optical communication and sensing

Controlled lateral manipulation of single diiodobenzene molecules on the Cu(111) surface with the tip of a scanning tunnelling microscope

Hla SW, Kühnle A, Bartels L, Meyer G, Rieder KH. Controlled lateral manipulation of single diiodobenzene molecules on the Cu(111) surface with the tip of a scanning tunnelling microscope. Surface Science. 2000;454:1079-1084.We report on the controlled lateral manipulations of adsorbed single diiodobenzene molecules on the Cu(111) surface with a scanning tunnelling microscope (STM) tip at 20 K. The molecular motions in this experiment are mainly induced by the attractive interaction between the tip and the molecule. Even though the leading manipulation mode is 'pulling', a continuous 'sliding' mode can also be induced if we use higher tip-molecule interaction forces. During the manipulation process, the molecules can follow the tip with hops of single or double copper-atomic-site distances and in some cases 'hop-scotch' type movements can also be observed. (C) 2000 Elsevier Science B.V. All rights reserved

Valley lifetimes of conduction band electrons in monolayer WSe2_2

One of the main tasks in the investigation of 2-dimensional transition metal dichalcogenides is the determination of valley lifetimes. In this work, we combine time-resolved Kerr rotation with electrical transport measurements to explore the gate-dependent valley lifetimes of free conduction band electrons of monolayer WSe$_2$. When tuning the Fermi energy into the conduction band we observe a strong decrease of the respective valley lifetimes which is consistent with both spin-orbit and electron-phonon scattering. We explain the formation of a valley polarization by the scattering of optically excited valley polarized bright trions into dark states by intervalley scattering. Furthermore, we show that the conventional time-resolved Kerr rotation measurement scheme has to be modified to account for photo-induced gate screening effects. Disregarding this adaptation can lead to erroneous conclusions drawn from gate-dependent optical measurements and can completely mask the true gate-dependent valley dynamics.Comment: 5 pages, 3 figure