Scanning tunneling microscopy (STM) vibronic spectroscopy, which has provided submolecular insights into electron-vibration (vibronic) coupling, faces challenges when probing the pivotal low-frequency vibronic excitations. Because of eigenstate broadening on solid substrates, resolving low-frequency vibronic states demands strong decoupling. This work designs a type II band alignment in STM junction to achieve effective charge-transfer state decoupling. This strategy enables the successful identification of the lowest-frequency Hg(ω1) (Raman-active Hg mode) vibronic excitation within single C60 molecules, which, despite being notably pronounced in electron transport of C60 single-molecule transistors, has remained hidden at submolecular level. Our results show that the observed Hg(ω1) excitation is “anchored” to all molecules, irrespective of local geometry, challenging common understanding of structural definition of vibronic excitation governed by Franck-Condon principle. Density functional theory calculations reveal existence of molecule-substrate interfacial charge-transfer dipole, which, although overlooked previously, drives the dominant Hg(ω1) excitation. This charge-transfer dipole is not specific but must be general at interfaces, influencing vibronic coupling in charge transport.
Abstract: Polymorphic structures of transition metal dichalcogenides (TMDs) host exotic electronic states, like charge density wave and superconductivity. However, the number of these structures is limited by crystal symmetries, which poses a challenge to achieving tailored lattices and properties both theoretically and experimentally. Here, we report a coloring triangle (CT) latticed MoTe2 monolayer, termed CT-MoTe2, constructed by controllably introducing uniform and ordered mirror-twin-boundaries into a pristine monolayer in molecular beam epitaxy. Low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) together with theoretical calculations reveal that the monolayer has an electronic Janus lattice, i.e., an energy-dependent atomic-lattice and a Te pseudo-sublattice, and shares the identical geometry with the Mo5Te8 layer. Dirac-like and flat electronic bands inherently existing in the CT lattice are identified by two broad and two prominent peaks in STS spectra, respectively, and verified with density-functional-theory calculations. Two types of intrinsic domain boundaries were observed, one of which the electronic-Janus-lattice feature maintains, implying potential applications as an energy-tunable electron-tunneling barrier in future functional devices.
Metal–insulator transition has long been one of the key subjects in condensed matter systems. Herein, the emergence of a large energy gap (Eg, 0.8–1.0 eV) in Bi(110) two-atomic-layer nanoribbons grown on a SnSe(001) substrate is reported, which normally has an intrinsic semimetal-like characteristic. The existence of this abnormally large Eg in Bi(110) is, however, determined by Bi coverage. When coverage is above ≈64 ± 2%, Eg vanishes, and instead, a Bi(110) semimetal-like phase appears through a singular insulator–metal transition. Measurements using qPlus atomic force microscopy demonstrate that either insulating or semimetal-like Bi(110) possesses a distorted black phosphorous structure with noticeable atomic buckling. Density function theory fully reproduces the semimetal-like Bi(110) on SnSe(001). However, none of the insulating phases with this large Eg could be traced. Although the underlying mechanism of the large Eg and the insulator-metal transition requires further exploration, experiments demonstrate that similar results can be achieved for Bi grown on SnS, the structural analog of SnSe, exhibiting an even larger Eg of ≈2.3 eV. The experimental strategy may be generalized to utilization of group-IV monochalcogenides to create Bi(110) nanostructures with properties unachievable on other surfaces, providing an intriguing platform for exploring the interesting quantum electronic phases.
Quantum confinement has remarkable effects on the band structures and optoelectronic performance of semiconducting materials. The confinement of electronic states developed along van der Waals (vdW) gaps in transition metal dichalcogenides (TMDs) has unique advantages compared with those of artificial quantum wells. Here, we detected the quantized electronic states of few-layered MoS2 in real space using scanning tunneling microscope/spectroscopy. Combined with density-functional theory calculations, the quantized states were attributed to quantum-well states (QWSs), and the number of the states was strictly determined by the MoS2 layer thickness. We further regulated the QWSs of few-layered MoS2 by tuning the strength of interlayer hybridization through directly adjusting the interlayer distance. More importantly, substitutional defects in few-layered MoS2 were introduced to control the energy eigenvalues of the QWSs. Our work proves the existence of the interlayer electronic hybridization in conventional weakly coupled vdW interfaces, and provides a way to manipulate the electronic states of few-layered TMD through controlling interlayer hybridization. It also suggests potential applications of quantum-well materials in subband transitions, spin splitting, photoexcitation, and electronic devices.
Jun Zhang, Linwei Zhou, Pengcheng Chen, Bingkai Yuan, Zhihai Cheng, Wei Ji & Xiaohui Qiu
Abstract
Catalytic bond cleavage and formation of transient intermediates on metal substrates play an essential role in surface synthesis and heterogeneous catalysis. Previous studies usually focus on the bond-breaking process, whereas the knowledge regarding the construction of dissociative moieties that lead to the final products is limited. Here, we investigate the facet-selective dissociation of dibenzotetrathiafulvalene (DBTTF) molecules on Cu(110) and Cu(100) surfaces using low-temperature scanning tunneling microscopy and first-principles calculations. Atomic resolution images enable the identification of various intermediates and reaction pathways on different facets. The dissociation of DBTTF molecules generated 1,2-BDT–Cu complex chains on Cu(110), while phenyl diradical superstructure islands were observed on Cu(100). The various chemical species found on different Cu facets were explained in the context of their formation energies related to the spatial inhomogeneity of surface electronic states. Our results address the effects of electronic and geometrical diversities on the surface mobility of intermediate products and subsequent on-surface reaction pathways.
