Layer dependence of the electronic band alignment of few-layer MoS2 on SiO2 measured using photoemission electron microscopy

Abstract

Tailoring band alignment layer-by-layer using heterojunctions of two-dimensional (2D) semiconductors is an attractive prospect for producing next-generation electronic and optoelectronic devices that are ultra-thin, flexible, and efficient. 2D layers of transition metal dichalcogenides (TMDs) in laboratory devices have already shown favorable characteristics for electronic and optoelectronic applications. Despite these strides, a systematic understanding of how band alignment evolves from monolayer to multilayer structures is still lacking in experimental studies, which hinders development of novel devices based on TMDs. Here we 2 report on the local band alignment of monolayer, bilayer, and tri-layer MoS 2 on a 285-nm-thick SiO 2 substrate using a new approach to probe the occupied electronic states based on photoemission electron microscopy and deep ultraviolet light. Local measurements of the vacuum level and the valence band edge at the Brillouin zone center show that the addition of layers to monolayer MoS 2 increases the relative work function, and pushes the valence band edge toward the vacuum level. We also deduced n-type doping of few-layer MoS 2 and type-I band alignment across monolayer-to-bilayer and bilayer-to-trilayer lateral junctions. Conducted in isolation from environmental effects owing to the vacuum condition of the measurement and an insulating SiO 2 substrate, this study shows a new metrology to uncover electronic properties intrinsic to MoS 2 semiconducting layers and emerging 2D crystals alike.

Work Function Variations in Twisted Graphene Layers

“…By comparing optical microscopy images with PEEM images and performing spectroscopic PEEM measurements of the graphene samples studied here, we avoided areas with non-uniformities in optical or PEEM intensities. As a result, work function variations in graphene were resolved with PEEM on an energy scale (10s meV), similar to what was observed with as-grown MoS 2 37 . Graphene work function values (4.4 eV for 1 L graphene) measured here with PEEM are consistent with several prior works 15 , 16 , 35 .…”

“…The relative energy scale was calibrated using the work function of 1LG, determined from photoemission yield measurements acquired by recording the photoemission intensity as a function of photon energy (for details of the data acquisition and processing, see ref. 37 ). The photoemission yield measurement itself did not yield sufficient statistics to bear a work function map of the same area due to the low photoemission intensity.…”

A roadmap for electronic grade 2D materials

“…In recent times, it has been shown that the direct chalcogenization of Mo in vapor phase using MoO2 as a source leads to samples with optoelectronic grade quality which is important for applications that harness their optical and exciton-based properties. [110][111][112][113][114][115][116] While gas source-CVD and MBE have been used to deposit a variety of chalcogenide thin films, extension to the controlled synthesis of monolayer and fewlayer films and heterostructures presents new challenges. Specifically, reactors used in CVD techniques must be designed, and growth parameters chosen with the aim of achieving precise control over material uniformity and reproducibility.…”

Layer-Dependent Photoinduced Electron Transfer in 0D–2D Lead Sulfide/Cadmium Sulfide–Layered Molybdenum Disulfide Hybrids

“…[17][18] The bandgap alignment for various MoS2 layers and PbS/CdS QDs emitting at 900nm are shown in Figure 1 maxima and conduction band (CB) minima were from literature. [9][10][11][12] Clearly, PbS/CdS QDs and layered MoS2 form a type-II heterojunction, which favors photoinduced electron transfer from QDs to MoS2, with the electron transfer expected to strengthen with the increase number of MoS2 layers due to an increase in electron transfer driving force. At the same time nonradiative transfer from photoexcited QDs to MoS2 can be ruled out as a possible quenching mechanism and this is because of the lack of spectral overlap between the absorption spectrum of MoS2 and the PL spectrum of core/shell PbS/CdS QDs.…”