Graphene Nanomesh Formation by Fluorine Intercalation

Graphene nanomeshes are mainly produced through top-down lithography, resulting in unavoidable defects or contamination. In this article, we demonstrate a bottom-up approach through partial intercalation of fluorine between the carbon buffer layer and the underlying SiC(0001) substrate by low-temperature annealing of a deposited molecular layer of fluorinated fullerenes C₆₀F₄₈. Due to the inherent periodicity of the bonding between the buffer layer and the underlying SiC(0001) substrate, selective fluorination and partial intercalation take place. Using scanning tunneling microscopy and spectroscopy as well as density functional theory calculations, the existence of a graphene nanomesh with the local atomic arrangement of a graphene sheet and surface corrugation of long-range periodicity is revealed. Surprisingly, the nanomesh exhibits electronically an intermediate state between the conventional buffer layer and quasi-free-standing graphene. Specifically, unlike the buffer layer, which is bonded covalently to the SiC(0001) surface so that the characteristic graphene π network about the <b>K</b> point of the Brilluoin zone is destroyed, this intermediate state retains the wave function characteristics of graphene, but a two-peak structure in the local density of states (LDOS) is introduced about the <b>K</b> point. This graphene nanomesh with a two-peak LDOS structure about the <b>K</b> point presents another playground for the study of transport properties in supported two-dimensional materials

Trapping Single Polar Molecules in SiC Nanomesh <i>via</i> Out-of-Plane Dipoles

The self-assembly of nonplanar chloroaluminum phthalocyanine (ClAlPc) molecules as well-ordered single-molecule dipole arrays on the silicon carbide (SiC) nanomesh substrate was investigated using low temperature scanning tunneling microscopy. ClAlPc exclusively adsorbs in the center of the SiC nanomesh holes with its inherent dipole (from Cl to Al) pointing toward the substrate. The dipole can be inverted by a positively biased tip with a threshold tip voltage of 3.3 V. We deduce that the interaction between the intrinsic dipole of ClAlPc and the periodic out-of-plane component of the surface dipole on the SiC nanomesh plays a significant role in the dipole array formation

Self-Assembly of Polar Phthalocyanine Molecules on Graphene Grown by Chemical Vapor Deposition

Integration of functional organic molecules with graphene is expected to promote the development of graphene-based flexible electronics with novel properties. Here, the self-assembled structure of dipole phthalocyanine molecules, chloro-aluminum phthalocyanine (ClAlPc), on single-layer graphene grown by chemical vapor deposition (CVD) over a Cu film was characterized by low-temperature scanning tunneling microscopy (LT-STM). The phthalocyanine molecules show highly ordered assembled structures on the CVD graphene, and these molecular layers extend continuously over the steps of the Cu film. We also observe specific boundaries in the self-assembled molecule arrays, which can be explained by the presence of domain boundaries in the graphene. The STM results suggest that CVD graphene is as a good molecular assembly template for surface functionalization and that these molecular arrays facilitate the study of domain structures in CVD graphene

Incorporating Isolated Molybdenum (Mo) Atoms into Bilayer Epitaxial Graphene on 4H-SiC(0001)

The atomic structures and electronic properties of isolated Mo atoms in bilayer epitaxial graphene (BLEG) on 4H-SiC(0001) are investigated by low temperature scanning tunneling microscopy (LT-STM). LT-STM results reveal that isolated Mo dopants prefer to substitute C atoms at α-sites and preferentially locate between the graphene bilayers. First-principles calculations confirm that the embedding of single Mo dopants within BLEG is energetically favorable as compared to monolayer graphene. The calculated band structures show that Mo-incorporated BLEG is n-doped, and each Mo atom introduces a local magnetic moment of 1.81 μ_B into BLEG. Our findings demonstrate a simple and stable method to incorporate single transition metal dopants into the graphene lattice to tune its electronic and magnetic properties for possible use in graphene spin devices

Competition between Hexagonal and Tetragonal Hexabromobenzene Packing on Au(111)

Low-temperature scanning tunneling microscope investigations reveal that hexabromobenzene (HBB) molecules arrange in either hexagonally closely packed (<i>hcp</i>) [22−24] or tetragonal [70−24] structure on Au(111) dependent on a small substrate temperature difference around 300 K. The underlying mechanism is investigated by density functional theory calculations, which reveal that substrate-mediated intermolecular noncovalent C–Br···Br–C attractions induce <i>hcp</i> HBB islands, keeping the well-known Au(111)-22×√3 reconstruction intact. Upon deposition at 330 K, HBB molecules trap freely diffusing Au adatoms to form tetragonal islands. This enhances the attraction between HBB and Au(111) but partially reduces the intermolecular C–Br···Br–C attractions, altering the Au(111)-22×√3 reconstruction. In both cases, the HBB molecule adsorbs on a bridge site, forming a ∼15° angle between the C–Br direction and [112̅]_Au, indicating the site-specific molecule–substrate interactions. We show that the competition between intermolecular and molecule–substrate interactions determines molecule packing at the subnanometer scale, which will be helpful for crystal engineering, functional materials, and organic electronics

Electron-Doping-Enhanced Trion Formation in Monolayer Molybdenum Disulfide Functionalized with Cesium Carbonate

We report effective and stable electron doping of monolayer molybdenum disulfide (MoS₂) by cesium carbonate (Cs₂CO₃) surface functionalization. The electron charge carrier concentration in exfoliated monolayer MoS₂ can be increased by about 9 times after Cs₂CO₃ functionalization. The n-type doping effect was evaluated by <i>in situ</i> transport measurements of MoS₂ field-effect transistors (FETs) and further corroborated by <i>in situ</i> ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and Raman scattering measurements. The electron doping enhances the formation of negative trions (<i>i.e.</i>, a quasiparticle comprising two electrons and one hole) in monolayer MoS₂ under light irradiation and significantly reduces the charge recombination of photoexcited electron–hole pairs. This results in large photoluminescence suppression and an obvious photocurrent enhancement in monolayer MoS₂ FETs

Heterointerface Screening Effects between Organic Monolayers and Monolayer Transition Metal Dichalcogenides

The nature and extent of electronic screening at heterointerfaces and their consequences on energy level alignment are of profound importance in numerous applications, such as solar cells, electronics <i>etc.</i> The increasing availability of two-dimensional (2D) transition metal dichalcogenides (TMDs) brings additional opportunities for them to be used as interlayers in “van der Waals (vdW) heterostructures” and organic/inorganic flexible devices. These innovations raise the question of the extent to which the 2D TMDs participate actively in dielectric screening at the interface. Here we study perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) monolayers adsorbed on single-layer tungsten diselenide (WSe₂), bare graphite, and Au(111) surfaces, revealing a strong dependence of the PTCDA HOMO–LUMO gap on the electronic screening effects from the substrate. The monolayer WSe₂ interlayer provides substantial, but not complete, screening at the organic/inorganic interface. Our results lay a foundation for the exploitation of the complex interfacial properties of hybrid systems based on TMD materials

Electronic Properties of a 1D Intrinsic/p-Doped Heterojunction in a 2D Transition Metal Dichalcogenide Semiconductor

Two-dimensional (2D) semiconductors offer a convenient platform to study 2D physics, for example, to understand doping in an atomically thin semiconductor. Here, we demonstrate the fabrication and unravel the electronic properties of a lateral doped/intrinsic heterojunction in a single-layer (SL) tungsten diselenide (WSe₂), a prototype semiconducting transition metal dichalcogenide (TMD), partially covered with a molecular acceptor layer, on a graphite substrate. With combined experiments and theoretical modeling, we reveal the fundamental acceptor-induced p-doping mechanism for SL-WSe₂. At the 1D border between the doped and undoped SL-WSe₂ regions, we observe band bending and explain it by Thomas–Fermi screening. Using atomically resolved scanning tunneling microscopy and spectroscopy, the screening length is determined to be in the few nanometer range, and we assess the carrier density of intrinsic SL-WSe₂. These findings are of fundamental and technological importance for understanding and employing surface doping, for example, in designing lateral organic TMD heterostructures for future devices

Monolayer MoSe₂ Grown by Chemical Vapor Deposition for Fast Photodetection

Monolayer molybdenum disulfide (MoS₂) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the electrical and optoelectronic properties of monolayer MoS₂ devices. Here, highly crystalline molybdenum diselenide (MoSe₂) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS₂ and MoSe₂ monolayers reveals that the MoSe₂ monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe₂ presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS₂ monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS₂. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe₂ and MoS₂ and is useful for guiding future designs in 2D material-based optoelectronic devices