Work Function Tunability of Graphene with Thermally Evaporated Rhenium Heptoxide for Transparent Electrode Applications

Abstract

The possibility of efficient work function tuning of high-quality graphene, grown on 6H-SiC(0001), by covering it with thermally evaporated rhenium heptoxide (Re 2 O 7) is presented. Raman spectroscopy, X-ray photoelectron spectroscopy, and ultraviolet electron spectroscopy are used for structural, chemical, and electronic characterization of Re 2 O 7 /graphene heterostructures. The deposited oxide does not induce any defects in the high-quality graphene lattice and results in a relatively small change in optical transmittance. The observed increase in the work function of the Re 2 O 7 /graphene heterostructure (þ0.76 eV) emphasizes great potential of modified graphene for use as transparent anode and hole transport layer in organic electronics.

Two-Dimensional Crystals as a Buffer Layer for High Work Function Applications: The Case of Monolayer MoO3

“…Figure shows three steps in the evolution of the UPS spectrum at the low-kinetic energy region: from pristine HOPG, although partially covered substrates by islands of MoO 3 monolayers, and complete coverage of MoO 3 . The WF is determined from the intersection of linear extrapolation of an edge of secondary electron cutoff (SEC) with the background; , the x -axis, that is, photoelectron energy, is calibrated to the Au(111) Fermi level (energy E F ) such that the energy indicated by SEC corresponds to the WF. For clarity, the insets illustrate the coverage of MoO 3 on the HOPG substrate.…”

“…At this point, it is worth discussing the interpretation of UPS for studies of uncoalesced films or 2D flakes with higher WF than their substrate. UPS has become, in recent years, a popular technique for measuring the efficacy of WF tailoring for photovoltaic and light-emitting devices. − ,, Our setup seems to be a perfect system to discuss the limitation of this technique for nonhomogenous systems. As mentioned earlier, in the literature, some efforts focus on patchy samples with laterally nonhomogeneous WF, where patchiness derives from dirt or regular patterning. − Here, we present a real-life example where area of <1 nm-thick patches is irregular and depends on the morphology of the substrate because MoO 3 tends to nucleate at terrace edges of HOPG.…”

“…Work function (WF, Φ), defined as the minimum energy required for an electron to escape from a material surface, is a critical surface characteristic as it determines the interface properties in electronic applications. The introduction of high WF transition metal oxides (TMOs) as buffer layers for anodes in organic electronics has proven to increase efficiency of devices due to securing the energy level alignment. , However, further thickness reduction of conventional TMO layers is a difficult task because it has been reported to impact the electronic properties including their WF. − Thus, the pursuit of novel designs and alternatives to conventional buffer layer materials is the subject of intense research. In this work, we propose ultrathin (<1 nm) two-dimensional (2D) crystalline films for fine-tuning of the substrate WF, therefore downscaling the thickness of currently used buffer layers.…”

“…We focus on α-molybdenum­(VI) oxide (α-MoO 3 ), a van der Waals (vdW), wide-band gap n-type semiconductor with a high WF of 6.9 eV in its bulk phase. ,− This value is notably higher than most WFs of typical layered materials such as highly ordered pyrolytic graphite (HOPG) and graphene (4.4–4.5 eV) , or MoS 2 (4.3–5.5 eV). , In α-MoO 3 , valence and conduction bands are ∼3 eV apart and the material can be easily doped through oxygen vacancies forming MoO 3– x ,− with additional bands below the conduction band minimum and a local decrease in the band gap . It has been theoretically predicted that α-MoO 3 retains its bulk properties in the 2D limit, and thus, the electronic structure of 2D α-MoO 3 is not significantly altered from the bulk, ,− including its high WF.…”

Synthesis, Chemical–Physical Characterization, and Biomedical Applications of Functional Gold Nanoparticles: A Review

“…Thus, capping agents and ligands, with high surface charges such as citrate and ionic polymers, are generally used as stabilizing agents [ 107 ]. Surface modification using long-chain spacers, such as PEGylation and neutral polymers, is a method to shield and support the stability of AuNP colloidal suspensions, depending on steric repulsion [ 108 ]. The ligand amount and direction are two further critical factors of the AuNP properties, since they play a key role in the strength of the interactions with other molecules.…”