Energy Level Alignment at the C₆₀/Monolayer‐WS₂ Interface on Insulating and Conductive Substrates

Abstract

Combining a transition metal dichalcogenide monolayer (ML) and molecular semiconductors is an attractive route for forming nanoscale hybrid van der Waals heterostructures with potentially novel (opto‐)electronic properties. The energy level alignment at the hybrid interface governs these properties, but precise determination of the interfacial electronic structure is challenging due to the pronounced excitonic nature of both components and the non‐trivial band structure of the inorganic ML. For instance, dielectric screening by the supporting substrate of such a heterostructure may impact the energy levels, but very few experiments have attended to this important issue to date. Here, it is shown how photoelectron spectroscopy can be used to unravel the energy level line‐up at the C60/ML‐WS2 interface supported on an insulating (sapphire) and a semi‐metallic (graphite) substrates. On both substrates, an almost identical staggered type‐II level alignment is determined. However, C60/ML‐WS2 exhibits stronger n‐type characteristics on sapphire, which is suggested to be due to native donor‐type defects of ML‐WS2. While these remain occupied and active on the insulating substrate, they are emptied into the charge reservoir of the conductive substrate. These insights should be considered in the future design of functional heterostructures of inorganic ML and molecular semiconductor materials.

Interlayer exciton landscape in WS2/tetracene heterostructures

“…2 owing to the relatively weak hopping between molecules. [42][43][44] Due to their relative band alignment, 45,46 Tc and WS 2 form a type-II heterostructure 17 in contrast to, for example, Tc and molybdenum-based TMDs which form type-I heterostructures 24 and hence do not facilitate the formation of optically-active interlayer excitons. As a result, WS 2 /Tc represents an exemplary system, playing host to interlayer excitons 17 as well as a rich substructure of intralayer dark states in the WS 2 .…”

“…The energy values determining the band alignment with respect to the vacuum level are obtained from the literature. 45,46,52 Here, the first/second letter describes the position of the hole/electron, e.g. the state hK corresponds to the hole located in the h state of the molecule, while the electron can be found in the K valley of the TMD layer, cf.…”

Self-assembly of C60 on a ZnTPP/Fe(001)–p(1 × 1)O substrate: observation of a quasi-freestanding C60 monolayer

“…When an additional single layer of C 60 is added to this system, new features appear. The photoemission signal from the underlying ZnTPP layer, albeit affected by the screening action of C 60 (implying a rather large surface sensitivity of the technique, as also shown in [55] on a similar system), is still detected in those spectral regions not superimposed to the new C 60 features. In particular, peaks "a" and "b" can be readily assigned to HOMO and HOMO−1 features and their energetic positions match with their equivalents when a very thick layer of C 60 is grown directly on Fe(001)-p(1 × 1)O (top spectrum).…”

Resonance Energy Transfer from Monolayer WS2 to Organic Dye Molecules: Conversion of Faint Visible‐Red into Bright Near‐Infrared Luminescence

“…The position of the dye's highest occupied molecular orbital (HOMO) is obtained from photoelectron yield spectroscopy (see Section S5, Supporting Information), while the energy of valence band maximum (VBM) at the K-point (5.7-6 eV) is taken from literature. [26,27] The positions of the dye's lowest unoccupied molecular orbital (LUMO) and of the conduction band maximum (CBM) of 1L-WS 2 are obtained by adding the respective electronic bandgap energy E g to the HOMO and VBM, respectively. The gap energies given in Figure 1e correspond to the optical bandgaps E opt derived from the absorption spectra of the dye and 1L-WS 2 .…”