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.
The quest for pragmatic room-temperature (RT) magnetic semiconductors (MSs) with a suitable bandgap constitutes one of the contemporary opportunities to be exploited. This may provide a materials platform for to bring new-generation ideal information device technologies into real-world applications where the otherwise conventionally separately utilized charge and spin are simultaneously exploited. Here we present RT ferromagnetism in an Fe-doped SnSe (Fe:SnSe) van der Waals (vdW) single crystalline ferromagnetic semiconductor (FMS) with a semiconducting bandgap of ∼1.19 eV (comparable to those of Si and GaAs). The synthesized Fe:SnSe single crystals feature a dilute Fe content of <1.0 at%, a Curie temperature of ∼310 K, a layered vdW structure nearly identical to that of pristine SnSe, and the absence of in-gap defect states. The Fe:SnSe vdW diluted magnetic semiconductor (DMS) single crystals are grown using a simple temperature-gradient melt-growth process, in which the magnetic Fe atom doping is realized uniquely using FeI2 as the dopant precursor whose melting point is low with respect to crystal growth, and which in principle possesses industrially unlimited scalability. Our work adds a new member in the family of long-searching RT magnetic semiconductors, and may establish a generalized strategy for large-volume production of related DMSs.
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.
After the preparation of 2D electronic flat band (EFB) in van der Waals (vdW) superlattices, recent measurements suggest the existence of 1D electronic flat bands (1D-EFBs) in twisted vdW bilayers. However, the realization of 1D-EFBs is experimentally elusive in untwisted 2D layers, which is desired considering their fabrication and scalability. Herein, the discovery of 1D-EFBs is reported in an untwisted in situ-grown two atomic-layer Bi(110)superlattice self-aligned on an SnSe(001) substrate using scanning probe microscopy measurements and density functional theory calculations. While the Bi-Bi dimers of Bi zigzag (ZZ) chains are buckled, the epitaxial lattice mismatch between the Bi and SnSe layers induces two 1D buckling reversal regions (BRRs) extending along the ZZ direction in each Bi(110)-11 x 17 supercell. A series of 1D-EFBs arises spatially following BRRs that isolate electronic states along the armchair (AC) direction and localize electrons in 1D extended states along ZZ due to quantum interference at a topological node. This work provides a generalized strategy for engineering 1D-EFBs in utilizing lattice mismatch between untwisted rectangular vdW layers.
