Single-crystal organic charge-transfer interfaces probed using Schottky-gated heterostructures

Organic semiconductors based on small conjugated molecules generally behave as insulators when undoped, but the heterointerfaces of two such materials can show electrical conductivity as large as in a metal. Although charge transfer is commonly invoked to explain the phenomenon, the details of the process and the nature of the interfacial charge carriers remain largely unexplored. Here we use Schottky-gated heterostructures to probe the conducting layer at the interface between rubrene and PDIF-CN(2) single crystals. Gate-modulated conductivity measurements demonstrate that interfacial transport is due to electrons, whose mobility exhibits band-like behaviour from room temperature to ~150 K, and remains as high as ~1 cm(2) V(-1) s(-1) at 30 K for the best devices. The electron density decreases linearly with decreasing temperature, an observation that can be explained quantitatively on the basis of the heterostructure band diagram. These results elucidate the electronic structure of rubrene/PDIF-CN(2) interfaces and show the potential of Schottky-gated organic heterostructures for the investigation of transport in molecular semiconductors.     

Atomic and electronic structures of Li4Ti5O12/Li7Ti5O12 (001) interfaces by first-principles calculations

We have performed density-functional theory calculations for Li₄Ti₅O₁₂/Li₇Ti₅O₁₂ (LTO/Li-LTO) interfaces and made a detailed analysis of the local atomic and electronic structures. In the bulk regions of the supercell, the atomic and electronic structures are well reproduced to be similar to those of the LTO and Li-LTO bulk crystals. The present (001) interface models show abrupt structural changes between the cubic spinel-based LTO and ordered rock-salt Li-LTO phases, while there occur no substantial strains around the interface due to the little volume change or lattice mismatch. Thus, the calculated interfacial energy is very small. The calculated O–K electron energy-loss near-edge structure/X-ray adsorption near-edge structure (ELNES/XANES) spectra in the bulk regions are similar to those of the bulk crystals, while the O–K edge spectra at the two kinds of interfaces have specific shapes, differently from the simple superposition of the bulk spectra. The preferential occurrence of the (001) interface can be understood from the preferential Li diffusion along the [110] direction in LTO and the small interfacial energy.     

A nanoparticulate indium tin oxide field-effect transistor with solid electrolyte gating

Reversible tuning of the transport properties of metallic conducting systems is not reported widely in the literature. Here, we report a junction field-effect transistor (FET) based on a transparent conducting oxide (TCO) nanoparticle channel and a solid polymer electrolyte as a gate. The device principle is based on the variation of the drain current induced by the capacitive double layer charging at the electrolyte/nanoparticle interfaces. A device with a metallic conducting channel made of indium tin oxide (ITO) nanoparticles exhibits an on/off ratio of 2 × 10(3) even when the gate potential is limited within the electrochemical capacitive region to avoid redox reactions at the interface. An FET device with metal-like conductance is always favored for the low dimensions of the device and a high on-state current. The field-effect mobility is calculated to be 24.3?cm(2)?V(-1)?s(-1). A subthreshold swing between 230 and 425?mV?dec(-1) is observed.     

Spectroscopy and microscopy of graphene on metals:

Graphene is an intriguing object of condensed mater physics. Discovery of its fascinating transport properties renewed the interest to the studies of graphene on metallic surfaces. That leads to the technological breakthrough showing that graphene on metals can be used as a protective layer and that its synthesis on metals is the most promising way to prepare huge graphene layers. However, all technology‐related and fundamental studies of the graphene‐metal interfaces require the understanding of the bonding mechanism at the interface and the consequent modifications of the electronic structure of graphene compared to the free‐standing case. Here we consider two representative examples of the strongly and weakly bonded graphene on metals demonstrating how surface science methods help to understand the origin of the bonding at the graphene‐metal interface. These methods help to trace all modifications in the electronic structure of graphene on the microscopic and macroscopic levels.     

Electronically coupled complementary interfaces between perovskite band insulators

Perovskite oxides exhibit a plethora of exceptional properties, providing the basis for novel concepts of oxide-electronic devices. The interest in these materials is even extended by the remarkable characteristics of their interfaces. Studies on single epitaxial connections between the wide-bandgap insulators LaAlO3 and SrTiO3 have revealed them to be either high-mobility electron conductors or insulating, depending on the atomic stacking sequences. For device applications, as well as for a basic understanding of the interface conduction mechanism, it is important to investigate the electronic coupling of closely spaced complementary interfaces. Here we report the successful realization of such coupled interfaces in SrTiO3-LaAlO3 thin-film multilayer structures. We found a critical separation distance of six perovskite unit cell layers, corresponding to approximately 23 A, below which a decrease of the interface conductivity and carrier density occurs. Interestingly, the high carrier mobilities characterizing the separate conducting interfaces are found to be maintained in coupled structures down to subnanometre interface spacing.     

Resonance reflection of acoustic waves in piezoelectric bi-crystalline structures

The paper studies the bulk wave reflection from internal interfaces in piezoelectric media. The interfaces of two types have been considered. Infinitesimally thin metallic layer inserted into homogeneous piezoelectric crystal of arbitrary symmetry. Rigidly bonded crystals whose piezoelectric coefficients differ by sign but the other material constants are identical. Analytic expressions for the coefficients of mode conversion have been derived. An analysis has been carried out of specific singularities arising when the angle of incidence is such that the resonance excitation of leaky interface acoustic waves occurs. The conditions for the resonance total reflection have been established. The computations performed for lithium niobate (LiNbO3) illustrate general conclusions.     

Oxygen-mediated electron transport through hybrid silicon-organic interfaces

We investigate from first principles the electronic and transport properties of hybrid organic/silicon interfaces of relevance to molecular electronics. We focus on conjugated molecules bonded to hydrogenated Si through hydroxyl or thiol groups. The electronic structure of the systems is addressed within density functional theory, and the electron transport across the interface is directly evaluated within the Landauer approach. The microscopic effects of molecule-substrate bonding on the transport efficiency are explicitly analyzed, and the oxygen-bonded interface is identified as a candidate system when preferential hole transfer is?needed.     

Revealing structure and electronic properties at organic interfaces using TEM

Molecules and atoms at material interfaces have properties that are distinct from those found in the bulk. Distinguishing the interfacial species from the bulk species is the inherent difficulty of interfacial analysis. For organic photovoltaic devices, the interface between the donor and acceptor materials is the location for exciton dissociation. Dissociation is thought to occur via a complex route effected by microstructure and the electronic energy levels. The scale of these devices and the soft nature of these materials create an additional level of difficulty for identification and analysis at these interfaces. The transmission electron microscope (TEM) and the spectroscopic techniques it incorporates can allow the properties of the donor-acceptor interfaces to be revealed. Cross-sectional sample preparation, using modern focused ion beam instruments, enables these buried interfaces to be uncovered with minimal damage for high resolution analysis. This powerful instrument combination has the ability to draw conclusions about interface morphology, structure and electronic properties of organic donor-acceptor interfaces at the molecular scale. Recent publications have demonstrated these abilities, and this article aims to summarise some of that work and provide scope for the future.     

Molecular dynamics investigations of grain boundary phenomena in cubic zirconia

Yttria stabilized zirconia (YSZ) is a fast oxide ion conducting ceramic with the cubic fluorite structure that is used in a number of applications, including solid oxide fuel cells (SOFCs). A molecular dynamics (MD) study has been performed on symmetrical tilt grain boundaries with the Σ5 (3 1 0)/[0 0 1] θ=36.9° misorientation to investigate the structure and dynamics of interfaces in this technologically important material. Simulations were performed on systems of 1920 atoms at constant temperatures up to 2673 K. Atomic interactions were described by a simple pair potential model of the Buckingham form. Structural relaxation produced an open structure corresponding to the introduction of a row of Schottky defects adjacent and parallel to the interface. Oxygen diffusion along the boundary was observed at high temperature, even without vacancies in the bulk being introduced explicitly by aliovalent doping. However, the diffusion rate was lower than that in single crystals of 8 mol% Y₂O₃ stabilized zirconia. Further simulations demonstrated that interfaces between perfect zirconia crystals are sources of resistance in these ionically conducting systems.     

Stabilization of Ferromagnetic Order in La(0.7)Sr(0.3)MnO(3)-SrRuO(3) Superlattices

The study of spatially confined complex oxides is of wide interest, since correlated electrons at interfaces might form exotic phases. Here La(0.7)Sr(0.3)MnO(3)/SrRuO(3) superlattices with coherently grown interfaces were studied by structural techniques, magnetization, and magnetotransport measurements. Magnetization measurements showed that ferromagnetic order in ultrathin La(0.7)Sr(0.3)MnO(3) layers is stabilized in the superlattices down to layer thicknesses of at least two unit cells. This stabilization is destroyed, if the ferromagnetic layers are separated by two unit cell thick SrTiO(3) layers. The resistivity of the superlattices showed metallic behavior and was dominated by the conducting SrRuO(3) layers, the off-diagonal resistivity showed an anomalous Hall effect from both SrRuO(3) and La(0.7)Sr(0.3)MnO(3) layers. This shows that the La(0.7)Sr(0.3)MnO(3) layers are not only ferromagnetic but also highly conducting; probably a conducting hole gas is induced at the interfaces that stabilizes the ferromagnetic order. This result opens up an alternative route for the fabrication of two-dimensional systems with long-range ferromagnetic order.     

Magnetic effects at the interface between non-magnetic oxides

The electronic reconstruction at the interface between two insulating oxides can give rise to a highly conductive interface. Here we show how, in analogy to this remarkable interface-induced conductivity, magnetism can be induced at the interface between the otherwise non-magnetic insulating perovskites SrTiO3 and LaAlO3. A large negative magnetoresistance of the interface is found, together with a logarithmic temperature dependence of the sheet resistance. At low temperatures, the sheet resistance reveals magnetic hysteresis. Magnetic ordering is a key issue in solid-state science and its underlying mechanisms are still the subject of intense research. In particular, the interplay between localized magnetic moments and the spin of itinerant conduction electrons in a solid gives rise to intriguing many-body effects such as Ruderman-Kittel-Kasuya-Yosida interactions, the Kondo effect and carrier-induced ferromagnetism in diluted magnetic semiconductors. The conducting oxide interface now provides a versatile system to induce and manipulate magnetic moments in otherwise non-magnetic materials.     

Cross-sectional imaging of individual layers and buried interfaces of graphene-based heterostructures and superlattices

By stacking various two-dimensional (2D) atomic crystals on top of each other, it is possible to create multilayer heterostructures and devices with designed electronic properties. However, various adsorbates become trapped between layers during their assembly, and this not only affects the resulting quality but also prevents the formation of a true artificial layered crystal upheld by van der Waals interaction, creating instead a laminate glued together by contamination. Transmission electron microscopy (TEM) has shown that graphene and boron nitride monolayers, the two best characterized 2D crystals, are densely covered with hydrocarbons (even after thermal annealing in high vacuum) and exhibit only small clean patches suitable for atomic resolution imaging. This observation seems detrimental for any realistic prospect of creating van der Waals materials and heterostructures with atomically sharp interfaces. Here we employ cross sectional TEM to take a side view of several graphene-boron nitride heterostructures. We find that the trapped hydrocarbons segregate into isolated pockets, leaving the interfaces atomically clean. Moreover, we observe a clear correlation between interface roughness and the electronic quality of encapsulated graphene. This work proves the concept of heterostructures assembled with atomic layer precision and provides their first TEM images.     

Singular stress fields in interfacial notches of hybrid metal matrix composites

This paper presents results from a study on the singular stress fields in metal matrix composites in which dual matrices exist. The adjoining metallic matrices flanking the interface can deform plastically with powerlaw strain hardening. These matrices may have both different hardening exponents and different yield strengths. An asymptotic analysis coupled with numerical eigen-analysis solved the spatial structure of the singular stress field at radial- and angular-dependent parts: . The dependence of the strength of the singular stresses on the matrix properties is discussed. The effects of local geometry on the nature of singular stresses are addressed. Highlights for interfacial notches are reported here. The drivers for this study are interfacial notches and free-edges in hybrid metal matrix composites (Fig. 1). However, the results can also be applied to other advanced structures which are composed of two or more distinct components or phases such as bone-implant interfaces and surface mounts in electronic packages     

Some exact solutions for the propagation of transient electroacoustic waves. II: Plane interface between two piezoelectric media

Electroacoustic waves are coupled horizontal-shear (SH) and electromagnetic disturbances that propagate in certain types of piezoelectric media. At the interface between two semi-infinite piezoelectric media, a new type of electroacoustic surface wave, called a Maerfeld-Tournois wave, can exist that has no purely elastic wave counterpart. This article obtains exact transient solutions for the Maerfeld-Tournois and body waves generated by a dipole source on such an interface. These solutions are obtained for both conducting and nonconducting interfaces using a modification of the Lamb-Cagniard-Pekeris technique. In the quasistatic approximation, explicit results for all the waves present are given and the separated body and surface waves at the interface are discussed.     

Switching the electrical resistance of individual dislocations in single-crystalline SrTiO3

The great variability in the electrical properties of multinary oxide materials, ranging from insulating, through semiconducting to metallic behaviour, has given rise to the idea of modulating the electronic properties on a nanometre scale for high-density electronic memory devices. A particularly promising aspect seems to be the ability of perovskites to provide bistable switching of the conductance between non-metallic and metallic behaviour by the application of an appropriate electric field. Here we demonstrate that the switching behaviour is an intrinsic feature of naturally occurring dislocations in single crystals of a prototypical ternary oxide, SrTiO(3). The phenomenon is shown to originate from local modulations of the oxygen content and to be related to the self-doping capability of the early transition metal oxides. Our results show that extended defects, such as dislocations, can act as bistable nanowires and hold technological promise for terabit memory devices.     

Thermal fluctuation-induced tunneling conduction through metal nanowire contacts

The temperature behavior of how electrons propagate through an insulating electronic contact formed at the interface between a submicron Cr/Au electrode and a metallic RuO(2) nanowire (NW) has been studied between 300 and 1?K. The NWs are typically of ～70?nm in diameter and a few microns long. The submicron electrodes were fabricated by the standard electron-beam lithography technique. By employing the two-probe method, the electronic contact resistances, R(c)(T), have been determined. We found that, in general, R(c) increases rapidly with decreasing temperature but eventually saturates at liquid-helium temperatures. Such a temperature behavior can be well described by a thermal fluctuation-induced tunneling (FIT) conduction process which considers the crossover feature from thermal activation conduction at high temperatures to simple elastic tunneling conduction at low temperatures. The wide applicability of this FIT model has further been established by employing metallic IrO(2) and Sn-doped In(2)O(3-x) NWs. This work demonstrates that the underlying physics for the charge transport properties of an insulating electronic contact can be well understood.     

A versatile interface model for thermal conduction phenomena and its numerical implementation by XFEM

A general interface model is presented for thermal conduction and characterized by two jump relations. The first one expresses that the temperature jump across an interface is proportional to the interfacial average of the normal heat flux while the second one states that the normal heat flux jump is proportional to the surface Laplacian of the interfacial average of the temperature. By varying the two scalar proportionality parameters, not only the Kapitza resistance and highly conducting interface models can be retrieved but also all the intermediate cases can be covered. The general interface model is numerically implemented by constructing its weak form and by using the level-set method and XFEM. The resulting numerical procedure, whose accuracy and robustness are thoroughly tested and discussed with the help of a benchmark problem, is shown to be efficient for solving the problem of thermal conduction in particulate composites with various imperfect interfaces.     

Interface formation in monolayer graphene-boron nitride heterostructures

The ability to control the formation of interfaces between different materials has become one of the foundations of modern materials science. With the advent of two-dimensional (2D) crystals, low-dimensional equivalents of conventional interfaces can be envisioned: line boundaries separating different materials integrated in a single 2D sheet. Graphene and hexagonal boron nitride offer an attractive system from which to build such 2D heterostructures. They are isostructural, nearly lattice-matched, and isoelectronic, yet their different band structures promise interesting functional properties arising from their integration. Here, we use a combination of in situ microscopy techniques to study the growth and interface formation of monolayer graphene-boron nitride heterostructures on ruthenium. In a sequential chemical vapor deposition process, boron nitride grows preferentially at the edges of existing monolayer graphene domains, which can be exploited for synthesizing continuous 2D membranes of graphene embedded in boron nitride. High-temperature growth leads to intermixing near the interface, similar to interfacial alloying in conventional heterostructures. Using real-time microscopy, we identify processes that eliminate this intermixing and thus pave the way to graphene-boron nitride heterostructures with atomically sharp interfaces.     

Fracture behavior of HPHT synthetic diamond with micrometers metallic inclusions

The fracture behavior of the diamond single crystals with metallic inclusions was investigated in the present paper. Single diamond crystals with metallic inclusions were formed by a special process with high pressure and high temperature (HPHT). The inclusions trapped in the diamond were characterized mainly to be metallic carbide of (Fe,Ni)₂₃C₆ or Fe₃C and solid solution of γ-(Fe,Ni) by transmission electronic microscopy (TEM). The grain size of the inclusions is about micrometers. The fracture characteristics of the diamond single crystals, after compression and heating, were investigated by optical microscopy (OM) and scanning electron microscopy (SEM). The fracture sections of the compressed and heated diamonds were found to be parallel to the (111) plane. The interface of the inclusions and diamond is deduced to be the key factor and the original region of the fracture formation. Mechanisms of the fracture behavior of the HPHT synthesized diamonds are discussed.