Transition metal dichalcogenides such as the semiconductor MoS2 are a class of two-dimensional crystals. The surface morphology and quality of MoS2 grown by chemical vapor deposition are examined using atomic force and scanning tunneling microscopy techniques. By analyzing the moiré patterns from several triangular MoS2 islands, we find that there exist at least five different superstructures and that the relative rotational angles between the MoS2 adlayer and graphite substrate lattices are typically less than 3°. We conclude that since MoS2 grows at graphite step-edges, it is the edge structure which controls the orientation of the islands, with those growing from zig-zag (or armchair) edges tending to orient with one lattice vector parallel (perpendicular) to the step-edge.
Well-ordered metal-organic nanostructures of Fe-PTCDA (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride) chains and networks are grown on a Au(111) surface. These structures are investigated by high-resolution scanning tunneling microscopy. Digitized frontier orbital shifts are followed in scanning tunneling spectroscopy. By comparing the frontier energies with the molecular coordination environments, we conclude that the specific coordination affects the magnitude of charge transfer onto each PTCDA in the Fe-PTCDA hybridization system. A basic model is derived, which captures the essential underlying physics and correlates the observed energetic shift of the frontier orbital with the charge transfer.
Surfaces of semiconductors with strong spin-orbit coupling are of great interest for use in spintronic devices exploiting the Rashba effect. BiTeI features large Rashba-type spin splitting in both valence and conduction bands. Either can be shifted towards the Fermi level by surface band bending induced by the two possible polar terminations, making Rashba spin-split electron or hole bands electronically accessible. Here we demonstrate the first real-space microscopic identification of each termination with a multi-technique experimental approach. Using spatially resolved tunnelling spectroscopy across the lateral boundary between the two terminations, a previously speculated on p-n junction-like discontinuity in electronic structure at the lateral boundary is confirmed experimentally. These findings realize an important step towards the exploitation of the unique behaviour of the Rashba semiconductor BiTeI for new device concepts in spintronics.
We investigate spin-polarized (SP) electronic transport properties and hybrid states of a single manganese phthalocyanine (MnPc) molecule adsorbed on a Co nanoisland, with the SP scanning tunneling microscopy measurements and the first-principles calculations. Our analyses show that the MnPc molecule can pin the Co surface state to the Fermi level, forming hybrid stationary spin resonance states which, with an antiparallelmagnetization tip, give a resonant SP conductance peak. Our calculations further reveal that as the tip approaches the molecule, electronic and magnetic couplings in the junction are tuned as the Zener indirect exchange coupling becomes prominent, which switches the conduction carriers from s to d electrons and leads to the tailored magnetic moments and magnetoresistance. ■ INTRODUCTIONThe organic−metal molecules possess unique magnetic characteristics which can be used to build multifunction spintronic devices and have attracted intensive interest. Recently, such molecular properties have been explored by Xray magnetic circular dichroism (XMCD) 1−3 and spin-polarized (SP) scanning tunneling microscopy (STM), 4−6 revealing various degree of hybrid states at the interface of molecules and substrates. These studies suggest the impacts of the hybrid states on spin injection 7 and single-molecule conduction, 8 facilitating the design of spin-based devices. For example, the surface state of a Co nanoisland located around −0.3 eV can be pinned to around the Fermi level by adsorbing a cobalt phthalocyanine (CoPc) molecule on the surface to form hybrid stationary spin resonance states, perpendicular to the surface, 4,9 providing a promising way to engineer the spinterface.Whereas the CoPc molecule can shift the Co spinterface states, first-principles calculations propose that the manganese phthalocyanine (MnPc) molecule owns higher magnetic moment and magnetocrystalline anisotropy energy than the CoPc molecule has, 10 which would tune such states efficiently and achieve featured devices. While properties of MnPc molecules on metallic substrates have been investigated experimentally by the X-ray absorption spectroscopy 2,11 or STM, 12,13 a direct SP-STM transport measurement of the MnPc molecule adsorbed on a Co nanoisland, as applied to the CoPc molecule 4,9 and the hydrogen phthalocyanine molecule, 8,14 however is still absent.Since a spintronic device would function in a shorter contact distance than the STM does, the effects of the tip and substrate on the molecule should take into account. For instance, both the unpolarized 15−18 and polarized 19−21 tips induce modifications to electronic structures of the sample. Recently, an ab initio study predicts the tailored tunnel magnetoresistance (TMR) in the antiferromagnetic junction consisted of a Co−Cr dimer and a SP-STM tip and suggests that by changing the distance between the tip and dimer, the hybrid states and magnetic couplings can be tuned and better understood. 22 A direct correlation between the observed hybrid states and magnetic couplings in ...
Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA)/Bi2Se3 and Fe/PTCDA/Bi2Se3 heterointerfaces are investigated using scanning tunneling microscopy and spectroscopy. The close-packed self-assembled PTCDA monolayer possesses big molecular band gap and weak molecule-substrate interactions, which leaves the Bi2Se3 topological surface state intact under PTCDA. Formation of Fe-PTCDA hybrids removes interactions between the Fe dopant and the Bi2Se3 surface, such as doping effects and Coulomb scattering. Our findings reveal the functionality of PTCDA to prevent dopant disturbances in the TSS and provide an effective alternative for interface designs of realistic TI devices.
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