and Applications of Fullerenes, Carbon Dots, Nanotubes, Graphene, Nanodiamonds, and Combined Superstructures Vasilios Georgakilas,† Jason A. Perman,‡ Jiri Tucek,‡ and Radek Zboril*,‡ †Material Science Department, University of Patras, 26504 Rio Patras, Greece ‡Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University in Olomouc, 17 listopadu 1192/12, 771 46 Olomouc, Czech Republic

Broad family of carbon nanoallotropes: classification, chemistry, and applications of fullerenes, carbon dots, nanotubes, graphene, nanodiamonds, and combined superstructures.

Cobalt doped ZnO nanodisks and nanorods were synthesized by a facile wet chemical method and well characterized by X-ray diffraction, field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray spectroscopy, photoluminescence spectroscopy, Raman spectroscopy and UV-visible absorption spectroscopy. The photocatalytic activities were evaluated for sunlight driven degradation of an aqueous methylene blue (MB) solution. The results showed that Co doped ZnO nanodisks and nanorods exhibit highly enhanced photocatalytic activity, as compared to pure ZnO nanodisks and nanorods. The enhanced photocatalytic activities of Co doped ZnO nanostructures were attributed to the combined effects of enhanced surface area of ZnO nanodisks and improved charge separation efficiency due to optimal Co doping which inhibit recombination of photogenerated charge carriers. The possible mechanism for the enhanced photocatalytic activity of Co doped ZnO nanostructures is tentatively proposed.

Enhanced photocatalytic activity of Co doped ZnO nanodisks and nanorods prepared by a facile wet chemical method

The existence of reversible resistance switching (RS) behaviors induced by electric stimulus has been known for some time, and these intriguing physical phenomena have been observed in numerous materials, including oxides. As conventional charge-based random access memory is expected to face a size limit in the near future, a surge of renewed interest has been developed in RS phenomena for possible applications in small nonvolatile memory devices called resistance random access memory (RRAM). Of particular interest is unipolar RS, which shows the RS at two values of applied voltage of the same polarity. The unipolar RS exhibits a much larger resistance change than other RS phenomena, and this greatly simplifies the process of reading the memory state. When fabricated with oxide p-n diodes, memory cells using unipolar RS can be stacked vertically, which has the potential for dramatically increasing memory density. Therefore, unipolar RRAM may be a good candidate for multi-stacked, high density, nonvolatile memory. The most important scientific and technical issues concerning unipolar RS are how it works and the identification of its controlling parameters. Some studies have reported that unipolar RS comes from a homogeneous/inhomogeneous transition of current distribution, while others maintain that it comes from the formation and rupture of conducting filaments. Even with recent extensive studies on unipolar RS, its basic origin is still far from being understood. In addition, no model exists that actually explains how the reversible switching can occur at two values of applied voltage. This lack of a quantitative model poses a major barrier for unipolar RRAM applications. In this study, we describe RS behavior in polycrystalline TiO2 film. To explain the basic mechanism of unipolar RS behavior, we propose a new percolation model based on a network of ‘‘circuit breakers’’ with two switchable metastable states. The random circuit breaker (RCB) network model can explain the long-standing material issue of how unipolar RS occurs. This simple percolation model is different from the conventional percolation models, which have dealt only with static or irreversible dynamic processes. In addition, the RCB network model provides an indication of how to overcome the substantial distribution of switching voltages, which is currently considered the most serious obstacle to practical unipolar RRAM applications. The unipolar RS phenomenon can be explained by the current (I)-voltage (V) curves in Figure 1a, which are derived from measurements of our polycrystalline TiO2 thin capacitors. At the pristine state (green dot), they are in an insulating state. As the external voltage Vext increases from zero and reaches a threshold voltage Vforming, a sudden increase occurs in the current. If the current is not limited to a certain value, here called the compliance current Icomp, the TiO2 capacitor would experience a dielectric breakdown and be destroyed. However, [*] Prof. T. W. Noh, S. C. Chae, S. B. Lee, S. H Chang, Dr. C. Liu ReCOE & FPRD, Department of Physics and Astronomy Seoul National University Seoul 151-747 (Korea) E-mail: twnoh@snu.ac.kr

Random circuit breaker network model for unipolar resistance switching (Advanced Materials (2008) 20 (1154))

Graphene is considered as one of the most promising materials in a wide range of applications because of its outstanding electronic, optical, thermal, and mechanical properties. Given its large specific surface area, two-dimensional structure, facile decoration, and high adsorption capacity, numerous graphene-based nanomaterials with unprecedented characteristics have been designed, prepared, and applied in catalysis. In this article, we first reviewed common synthetic methods to prepare graphene-based catalysts followed by critical comments and possible solutions. We then briefly summarized the characterization techniques that were relevant to catalysis applications and their applications in energy conversion, environmental protection, and several other typical fields. Finally, we discussed the challenges and opportunities for the future development of graphene-based nanomaterials in sustainable catalysis. This review provides key information to the catalysis community to design and fabricate graphene-ba...

Graphene-Based Nanomaterials for Catalysis

Recently, a newly fabricated two-dimensional layer structured material so-called phosphorene is receiving great research interests due to its peculiar physical properties. So far, mostly electrical properties are explored because it has a direct band gap and its nonmagnetic behavior in pristine layer. In this report, we propose that the transition metal doped phosphorene layer can have dilute magnetic semiconductor properties. Here, we investigated the structural property, binding energy, and magnetic property by changing the TM impurity dopants in the substitutional site. Our results demonstrate that spin polarized semiconducting state is realized in phosphorene by substitutional doping of Ti, Cr, and Mn, while a half-metallic state is obtained by V and Fe doping. In particular, we suggest that the most promising dopants are Cr and Mn because the spin polarized state is achieved with a band gap larger than 0.5 eV. The magnetic interaction between two same types of TM atoms is also checked. Magnetic groun...

Transition Metal Doped Phosphorene: First-Principles Study

The existence of ferromagnetism in the wonder material graphene has opened up the path for many future spintronics and memory applications. But simultaneously it is very important to understand the variation of these properties with temperature in regards to the device applications. Here we observed defect induced ferromagnetism in chemically reduced graphene and the effect of temperature on it. Several theoretical studies have proved that the main cause of ferromagnetism in graphene is due to various defects. The observed results established that these defects can be mended by treating the samples at elevated temperatures but sacrificing the ferromagnetism simultaneously. Hence, temperature plays a crucial role in controlling the magnetism as well as the defects in graphene. In this study we revealed that at 600 °C the self-repair mechanism helps the defects to mend but resulting in the decrement of magnetization and providing a good quality graphene with less defects.

Temperature tuned defect induced magnetism in reduced graphene oxide

Forming free and thermally stable bipolar resistive switching behavior was observed in the memory devices composed of graphene oxide (GO) thin films on ITO coated glass substrate with Platinum (Pt) as the top electrode. The switching between the low resistance state and high resistance state showed a reliable and repeating behavior with an on/off ratio of 104 at room temperature. The Pt/GO/ITO device showed metallic and semiconducting characteristics in low and high resistance states, respectively, when resistance was measured as a function of temperature. Ohmic conduction was found to dominate in all regions of switching operation except for the high voltage regime of high resistance state where space charge limited conduction was found to be the main mechanism of current conduction. The device showed good endurance up to 104 s and retention characteristics up to 100 cycles with on/off ratio ∼104. The switching mechanism was found to be governed by migration of oxygen between GO layer and bottom ITO electrode, which was further confirmed by Raman Spectroscopy. The device exhibited stable bipolar switching characteristics with good retention and endurance properties at high temperatures up to 500 K. The graphene oxide based memory devices fabricated by simple chemical method have potential in the field of nonvolatile memory applications well above the room temperature.

Forming free resistive switching in graphene oxide thin film for thermally stable nonvolatile memory applications

Multilevel resistive memory switching in graphene sandwiched organic polymer heterostructure

The growing demand for portable and bendable nonvolatile memory systems has motivated extensive research in the field of flexible resistive random access memory (RRAM) devices. This study investigated the resistive switching and flexibility behavior of zinc oxide nanorods (ZNs) incorporated graphene oxide (GO) sheets. GOZNs-based RRAM devices having top metal aluminum electrodes were fabricated on flexible indium tin oxide (ITO) coated polyethylene terephthalate (ITOPET) substrate. The devices having the structure Al/GOZNs/ITOPET showed typical bipolar resistive switching characteristics with switching voltages lower than those of Al/GO/ITOPET devices. The significant (∼50%) decrement in operating voltages in the case of GOZNs-based RRAM was attributed to enhanced concentration of oxygen vacancies into the GO matrix due to the incorporation of ZNs, which was supported by X-ray photoelectron spectroscopy studies. These memory devices showed repeatable and reliable switching characteristics having an on/off...

Geetika Khurana

Papers

Temperature tuned defect induced magnetism in reduced graphene oxide

Forming free resistive switching in graphene oxide thin film for thermally stable nonvolatile memory applications

Multilevel resistive memory switching in graphene sandwiched organic polymer heterostructure

Tunable Power Switching in Nonvolatile Flexible Memory Devices Based on Graphene Oxide Embedded with ZnO Nanorods

Improved photovoltaic performance of dye sensitized solar cell using ZnO–graphene nano-composites