Formation and Control of Band Gap in Graphene and Molybdenum Disulphide

Abstract

Two-dimensional (2D) nanomaterials have been investigated actively in recent years both in scientific and technological aspects due mainly to their unusual superior properties compared to conventional materials suggesting great industrial applications. Graphene, a single layer of carbon atoms in hexagonal structure, has been found to have such superb properties as a representative 2D material with its relativistic and massless charge carriers. Transition metal dichalcogenides (TMDs) such as molybdenum disulphide (MoS2) have been studied also as a new class 2D material with excellent electronic properties that may play a crucial role in advancing future-nanotechnology. As an effort to control the electronic properties of graphene and of MoS2, we have investigated the valence band and some core levels from two functionalized graphene systems (K+ ions and K atoms, Ce atoms-adsorbed) and a K+ ion-doped MoS2 system (K+ ions/bilayer MoS2) by using a synchrotron-based photoelectron spectroscopy. In order to understand the driving forces of new modified properties of these systems, we have also performed density functional theory (DFT) calculation of the modified bands in collaboration with theory groups. As a first system studied, we have investigated a single layer graphene (SLG) doped with potassium (K+) ions of low energy (10 eV) in order to form a band gap (Eg). Despite its superb electronic properties, the semi-metallic nature of graphene with no band gap at the Dirac point (DP) has been a stumbling block hindering its industrial application. We report an improved means of producing a tunable band gap over other schemes by doping low energy (10 eV) K+ ions on SLG formed on 6H-SiC(0001) surface, where the noble Dirac nature of the π-band remains almost unaltered. The changes in the π-band induced by K+ ions reveal that the band gap increases gradually with increasing dose (θ) of the ions up to Eg=0.65 eV at θ=1.10 monolayers, demonstrating the tunable character of the band gap. Our core level data for C 1s, Si 2p, and K 2p suggest that the K+-induced asymmetry in charge distribution among carbon atoms drives the opening of band gap, which is in sharp contrast with no band gap when neutral K atoms are adsorbed on graphene. This tunable K+-induced band gap in graphene illustrates its potential application in graphene-based nano-electronics. We have investigated changes in electronic and structural properties derived by magnetic Cerium (Ce) atoms adsorbed on SLG as the second system in this thesis. Artificial modifications of intrinsic properties of graphene, such as band gap control and spin injection on its -electrons, have been a recent challenge in graphene technology to promote the industrial applications of its superb properties. We report that the adsorption of Ce atoms on graphene exhibits several unique changes in electronic and structural properties in our photoemission data obtained by using synchrotron photons. A band gap as large as Eg=0.50 eV opens when the Ce-adsorbed graphene at low temperature is brought back to 41 K after a brief annealing at temperature Ta=1200 °C. The size of band gap decreases gradually with increasing sample temperature becoming Eg=0.36 eV at room temperature (RT), an indicative of a temperature-dependent structural and/or spin-ordering phase transition. We also observe the presence of two different stages of Ce-intercalation upon annealing the graphene with Ce adsorbed at RT, first below graphene layer at Ta=530 °C and then below the buffer layer at Ta=1050 °C. We discuss physical implications of these temperature-dependent features of the Ce-adsorbed graphene. Finally, we report a novel method that uses K+ ions, to regulate the band gap of semiconducting material MoS2. With the graphene epidemics, MoS2 has been extensively reinvestigated as a promising 2D semiconducting material seeking extensive applications of its rich electronic and optical properties. Controlling the band gap of MoS2 is the opening gate to seek such applications. We report that the doping of low energy K+ ions on MoS2 at 120 K forms a stack of MoS2 bilayers separated by intercalated K+ ions as suggested by our photoemission data. We further show that the band gap Eg of the bilayer MoS2 can be fine-tuned artificially by deliberately adjusting the dose (θ) of the intercalated K+ ions by up to Eg=0.51 eV. Our theoretical calculations reveal that Eg indeed decreases drastically with increasing the transverse electric field E produced by the intercalated K+ ions indicating the K+-induced Stark effect. We find that E increases proportionally to θ, and a semiconductor-metal transition takes place when E>2.0 V/Å. Our calculated band gaps for three different values of E match quite well with the measured band gaps. We thus report an efficient and pragmatic means of tailoring the band gap of bilayer MoS2 simply by controlling of the doped K+ ions with no extra treatment. This scheme of engineering the band gap of bilayer MoS2 may thus facilitate the development of MoS2-based nano-electronics.