Z. Klusek

TiO2—a prototypical memristive material

Redox-based memristive switching has been observed in many binary transition metal oxides and related compounds. Since, on the one hand, many recent reports utilize TiO(2) for their studies of the memristive phenomenon and, on the other hand, there is a long history of the electronic structure and the crystallographic structure of TiO(2) under the impact of reduction and oxidation processes, we selected this material as a prototypical material to provide deeper insight into the mechanisms behind memristive switching. In part I, we briefly outline the results of the historical and recent studies of electroforming and resistive switching of TiO(2)-based cells. We describe the (tiny) stoichiometrical range for TiO(2 - x) as a homogeneous compound, the aggregation of point defects (oxygen vacancies) into extended defects, and the formation of the various Magnéli phases. Furthermore, we discuss the driving forces for these solid-state reactions from the thermodynamical point of view. In part II, we provide new experimental details about the hierarchical transformation of TiO(2) single crystals into Magnéli phases, and vice versa, under the influence of chemical, electrical and thermal gradients, on the basis of the macroscopic and nanoscopic measurements. Those include thermogravimetry, high-temperature x-ray diffraction (XRD), high-temperature conductivity measurements, as well as low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and LC-AFM (atomic force microscope equipped with a conducting tip) studies. Conclusions are drawn concerning the relevant parameters that need to be controlled in order to tailor the memristive properties.

Observations of local electron states on the edges of the circular pits on hydrogen-etched graphite surface by scanning tunneling spectroscopy

Abstract Scanning tunneling microscopy (STM) and spectroscopy (STS) are used to study electronic states at the edges of the circular pits on the hydrogen-etched graphite surface. The edge surface state revealed by tunneling spectroscopy appears as the maximum of the local density of states (LDOS) in the energy range of 90–250 meV above the Fermi level. The dispersion of the energy state is explained by the band broadening in AB stacked H-terminated graphite. The magnitude of the edge state decreases with the distance from the pit edge as theoretically predicted.

Reversible modifications of linear dispersion: Graphene between boron nitride monolayers

Electronic properties of the graphene layer sandwiched between two hexagonal boron nitride sheets have been studied using the first-principles calculations and the minimal tight-binding model. It is shown that for the ABC-stacked structure in the absence of external field the bands are linear in the vicinity of the Dirac points as in the case of single-layer graphene. For certain atomic configuration, the electric field effect allows opening of a band gap of over 230 meV. We believe that this mechanism of energy gap tuning could significantly improve the characteristics of graphene-based field-effect transistors and pave the way for future electronic applications.

Role of graphene defects in corrosion of graphene-coated Cu(111) surface

Protection of Cu(111) surface by chemical vapor deposition graphene coating is investigated. The X-ray photoemission spectroscopy results do not reveal any signs of corrosion on graphene-coated Cu(111), and suggest perfect protection of copper surface against interaction with atmospheric gases. However, the scanning tunneling spectroscopy results show that cracks in the graphene sheet open up windows for nanoscale corrosion. We have shown also that such local corrosions are not only limited to the discontinuities but may also progresses underneath the graphene cover.

Graphene on gold: Electron density of states studies by scanning tunneling spectroscopy

Graphene devices require electric contacts with metals, particularly with gold. Scanning tunneling spectroscopy studies of electron local density of states performed on mono-, bi-, and trigraphene layer deposited on metallic Au/Cr/SiO2/Si substrate shows that gold substrate causes the Fermi level shift downwards which means that holes are donated by metal substrate to graphene which becomes p-type doped. These experimental results are in good accordance with recently published density function theory calculations.

Scanning tunneling microscopy and spectroscopy of the thermally oxidized (0001) basal plane of highly oriented pyrolitic graphite

Abstract Scanning tunneling microscopy and spectroscopy (STM/STS) are used to study thermal oxidation effects on an HOPG surface. The STM images reveal circular pits randomly distributed over atomically flat terraces. The detailed studies near the pit edges show (√3 × √3)R30° superstructure, electron density of state oscillations or extra contrast regions. The precise STS measurements on the bottom of the pits and on the top plane away from the pits on the thermally oxidized graphite to not show substantial differences in comparison to pure unoxidized graphite. The STS I V measurements recorded near the pit edges, where extra contrast appeared show distinct asymmetric form. In our interpretation the specific tunneling current shape is due to created oxygen functionalities (COH, COOH) on the graphite surface and is not related to perturbation caused by step. The influence of oxygen functionalities on the tunneling current is discussed.

Investigations of splitting of the π bands in graphite by scanning tunneling spectroscopy

Abstract The identification of features of the graphite electronic density of states is of great importance in scanning tunneling microscopy/spectroscopy studies of physically adsorbed molecules on this substrate. The results of scanning tunneling spectroscopy on the graphite surface enable us to resolve the Q 2g − (π), Q 2u − (π), Q 2u − (π*), Q 2g − (π*) states and estimate the π bands splitting. The measured splitting for the π* bands is 0.5 eV, while for the π bands 0.8 eV. The measured energy of the interlayer state stays within the range of 3.8–3.9 eV above the Fermi level, while the energy position of the extrinsic state varies from 2.5 eV to 2.9 eV above the Fermi level. The peaks detected near 5.5 eV below the Fermi level and 6 eV above the Fermi level are tentatively interpreted as the Γ 3g + (σ) and Γ 3u + (σ*) states, respectively.

The role of water in resistive switching in graphene oxide

The resistive switching processes are investigated at the nano-scale in graphene oxide. The modification of the material resistivity is driven by the electrical stimulation with the tip of atomic force microscope. The presence of water in the atmosphere surrounding graphene oxide is found to be a necessary condition for the occurrence of the switching effect. In consequence, the switching is related to an electrochemical reduction. Presented results suggest that by changing the humidity level the in-plane resolution of data storage process can be controlled. These findings are essential when discussing the concept of graphene based resistive random access memories.

Nitrogen doping of chemical vapor deposition grown graphene on 4H-SiC (0001)

We present optical, electrical, and structural properties of nitrogen-doped graphene grown on the Si face of 4H-SiC (0001) by chemical vapor deposition method using propane as the carbon precursor and N2 as the nitrogen source. The incorporation of nitrogen in the carbon lattice was confirmed by X-ray photoelectron spectroscopy. Angle-resolved photoemission spectroscopy shows carrier behavior characteristic for massless Dirac fermions and confirms the presence of a graphene monolayer in the investigated nitrogen-doped samples. The structural and electronic properties of the material were investigated by Raman spectroscopy. A systematical analysis of the graphene Raman spectra, including D, G, and 2D bands, was performed. In the case of nitrogen-doped samples, an electron concentration on the order of 5–10 × 1012 cm−2 was estimated based upon Raman and Hall effect measurements and no clear dependence of the carrier concentration on nitrogen concentration used during growth was observed. This high electron ...

Quasi-two-dimensional conducting layer on TiO2 (110) introduced by sputtering as a template for resistive switching

The insulator-to-metal transformation in the surface layer of TiO2 (110) induced by the Ar+ ion sputtering process is analyzed on the nanoscale. Local conductivity atomic force microscopy and photoelectron spectroscopy allow the changes in the valence of the Ti ions in the surface layer to be linked to the formation of its grain-like structure. The investigation of the cleavage plane of the crystal allowed us to estimate the thickness of the quasi-two-dimensional conducting layer generated by ion bombardment as 30 nm. The conducting layer is a template where the resistive switching of each single grain can be carried out.