Dirac Electrons with Molecular Relaxation Time at Electrochemical Interface between Graphene and Water.

The time dynamics of charge accumulation at the electrochemical interface between graphene and water is important for supercapacitors, batteries, and chemical and biological sensors. By using impedance spectroscopy, we have found that measured capacitance (Cm) at this interface with the gate voltage Vgate ≈ 0.1 V follows approximate laws Cm~T1.2 and Cm~T0.11 (T is Vgate period) in frequency ranges (1000-50,000) Hz and (0.02-300) Hz, respectively. In the first range, this dependence demonstrates that the interfacial capacitance (Cint) is only partially charged during the charging period. The observed weaker frequency dependence of the measured capacitance (Cm) at frequencies below 300 Hz is primarily determined by the molecular relaxation of the double-layer capacitance (Cdl) and by the graphene quantum capacitance (Cq), and it also implies that Cint is mostly charged. We have also found a voltage dependence of Cm below 10 Hz, which is likely related to the voltage dependence of Cq. The observation of this effect only at low frequencies indicates that Cq relaxation time is much longer than is typical for electron processes, probably due to Dirac cone reconstruction from graphene electrons with increased effective mass as a result of their quasichemical bonding with interfacial molecular charges.

General Capacitance Upper Limit and Its Manifestation for Aqueous Graphene Interfaces.

Double-layer capacitance (Cdl) is essential for chemical and biological sensors and capacitor applications. The correct formula for Cdl is a controversial subject for practically useful graphene interfaces with water, aqueous solutions, and other liquids. We have developed a model of Cdl, considering the capacitance of a charge accumulation layer (Cca) and capacitance (Ce) of a capacitance-limiting edge region with negligible electric susceptibility and conductivity between this layer and the capacitor electrode. These capacitances are connected in series, and Cdl can be obtained from 1/Cdl = 1/Cca + 1/Ce. In the case of aqueous graphene interfaces, this model predicts that Cdl is significantly affected by Ce. We have studied the graphene/water interface capacitance by low-frequency impedance spectroscopy. Comparison of the model predictions with the experimental results implies that the distance from charge carriers in graphene to the nearest molecular charges at the interface can be ~(0.05-0.1)nm and is about a typical length of the carbon-hydrogen bond. Generalization of this model, assuming that such an edge region between a conducting electrode and a charge accumulating region is intrinsic for a broad range of non-faradaic capacitors and cannot be thinner than an atomic size of ~0.05 nm, predicts a general capacitance upper limit of ~18 µF/cm2.

Influence of Substrate Microstructure on the Transport Properties of CVD-Graphene.

We report the study of electrical transport in few-layered CVD-graphene located on nanostructured surfaces in view of its potential application as a transparent contact to optoelectronic devices. Two specific surfaces with a different characteristic feature scale are analyzed: semiconductor micropyramids covered with SiO2 layer and opal structures composed of SiO2 nanospheres. Scanning tunneling microscopy (STM) and scanning electron microscopy (SEM), as well as Raman spectroscopy, have been used to determine graphene/substrate surface profile. The graphene transfer on the opal face centered cubic arrangement of spheres with a diameter of 230 nm leads to graphene corrugation (graphene partially reproduces the opal surface profile). This structure results in a reduction by more than 3 times of the graphene sheet conductivity compared to the conductivity of reference graphene located on a planar SiO2 surface but does not affect the contact resistance to graphene. The graphene transfer onto an organized array of micropyramids results in a graphene suspension. Unlike opal, the graphene suspension on pyramids leads to a reduction of both the contact resistance and the sheet resistance of graphene compared to resistance of the reference graphene/flat SiO2 sample. The sample annealing is favorable to improve the contact resistance to CVD-graphene; however, it leads to the increase of its sheet resistance.

Anomalous expansion of the copper-apical-oxygen distance in superconducting cuprate bilayers.

We have introduced an improved x-ray phase-retrieval method with unprecedented speed of convergence and precision, and used it to determine with sub-Angstrom resolution the complete atomic structure of epitaxial La(2-x)Sr(x)CuO(4) ultrathin films. We focus on superconducting heterostructures built from constituent materials that are not superconducting in bulk samples. Single-phase metallic or superconducting films are also studied for comparison. The results show that this phase-retrieval diffraction method enables accurate measurement of structural modifications in near-surface layers, which may be critically important for elucidation of surface-sensitive experiments. Specifically we find that, while the copper-apical-oxygen distance remains approximately constant in single-phase films, it shows a dramatic increase from the metallic-insulating interface of the bilayer towards the surface by as much as 0.45 A. The apical-oxygen displacement is known to have a profound effect on the superconducting transition temperature.

A magnetic field sensitive interfacial metallic state in a crystalline insulator.

We report strongly magnetic field dependent transport in crystalline strontium titanate at the interface with an ion beam treated nanolayer. Microscopy shows that this interface is atomically sharp. The results obtained suggest a chemical potential driven transfer of high mobility electrons through the interface into the crystal. This phenomenon provides a strategy for device fabrication without disorder in the conduction channel. We show, for nonmagnetic metals, magnetoresistance and plateau-like Hall magneto-dependence, possibly caused by electron cyclotron effects in a strongly correlated quasi-two-dimensional Fermi gas.

Highly conductive nanolayers on strontium titanate produced by preferential ion-beam etching.

Developing fabrication methods for electronically active nanostructures is an important challenge of modern science and technology. Fabrication efforts for crystalline materials have been focused on state-of-the-art epitaxial growth techniques. These techniques are based on deposition of precisely controlled combinations of various materials on a heated substrate. We report a method that does not require deposition and transforms a nanoscale layer of a complex crystalline compound into a new material using low-energy ion-beam preferential etching (IBPE). We demonstrate this method by transforming a widely used insulator model system, SrTiO3, into a transparent conductor. Most significantly, the resistivity decreases with decreasing temperature as approximately T2.5+/-0.3 and eventually falls below that of room-temperature copper. These transport measurements imply a crystal quality in the conduction channel comparable to that obtained with the highest-quality growth techniques. The universality of low-energy IBPE implies wide potential applicability to fabrication of other nanolayers.