Cristina E. Giusca

Evidence for a New Two-Dimensional C4H-Type Polymer Based on Hydrogenated Graphene

www.MaterialsViews.com C O M M U N IC A IO N Danny Haberer , * Cristina E. Giusca , Ying Wang , Hermann Sachdev , * Alexander V. Fedorov , Mani Farjam , S. Akbar Jafari , Denis V. Vyalikh , Dmitry Usachov , Xianjie Liu , Uwe Treske , Mandy Grobosch , Oleg Vilkov , Vera K. Adamchuk , Stephan Irle , * S. Ravi P. Silva , Martin Knupfer , Bernd Büchner , and Alexander Grüneis * Evidence for a New Two-Dimensional C 4 H-Type Polymer Based on Hydrogenated Graphene

Excimer laser nanostructuring of nickel thin films for the catalytic growth of carbon nanotubes

Pulse laser ablation and subsequent laser nanostructuring at room temperature has been employed to produce nanostructured Ni on SiO2/Si substrates for catalytic growth of carbon nanotubes. The resultant nanostructured surface is seen to consist of nanometer sized hemispherical droplets whose mean diameter is controlled by the initial metal thickness, which in turn is readily controlled by the number of laser pulses. Vertically aligned multiwall carbon nanotube mats were then grown using conventional plasma enhanced chemical vapor deposition. We show that within a single processing technique it is possible to produce the initial metal-on-oxide thin film to a chosen thickness but also to be able to alter the morphology of the film to desired specifications at low macroscopic temperatures using the laser parameters. The influence of the underlying oxide is also explored to explain the mechanism of nanostructuring of the Ni catalyst.

A PbS nanocrystal-C60 photovoltaic device for infrared light harvesting

PbS nanocrystal (nc-PbS)-C60 photovoltaic devices are demonstrated, in which nc-PbS function as electron donors, showing infrared photosensitivity up to 1600nm. Annealing nc-PbS is proved to remove capping oleic acid ligands, studied using x-ray photoelectron spectroscopy, significantly improving the short circuit current, open circuit voltage, and fill factor. The device performance is rationalized by quantum confinement in nc-PbS and energy level alignment at the heterojunction based on direct measurements of nc-PbS ionization potential using ultraviolet photoelectron spectroscopy.

Confined crystals of the smallest phase-change material.

The demand for high-density memory in tandem with limitations imposed by the minimum feature size of current storage devices has created a need for new materials that can store information in smaller volumes than currently possible. Successfully employed in commercial optical data storage products, phase-change materials, that can reversibly and rapidly change from an amorphous phase to a crystalline phase when subject to heating or cooling have been identified for the development of the next generation electronic memories. There are limitations to the miniaturization of these devices due to current synthesis and theoretical considerations that place a lower limit of 2 nm on the minimum bit size, below which the material does not transform in the structural phase. We show here that by using carbon nanotubes of less than 2 nm diameter as templates phase-change nanowires confined to their smallest conceivable scale are obtained. Contrary to previous experimental evidence and theoretical expectations, the nanowires are found to crystallize at this scale and display amorphous-to-crystalline phase changes, fulfilling an important prerequisite of a memory element. We show evidence for the smallest phase-change material, extending thus the size limit to explore phase-change memory devices at extreme scales.

Thickness-Dependent Hydrophobicity of Epitaxial Graphene

This article addresses the much debated question whether the degree of hydrophobicity of single-layer graphene (1LG) is different from that of double-layer graphene (2LG). Knowledge of the water affinity of graphene and its spatial variations is critically important as it can affect the graphene properties as well as the performance of graphene devices exposed to humidity. By employing chemical force microscopy with a probe rendered hydrophobic by functionalization with octadecyltrichlorosilane (OTS), the adhesion force between the probe and epitaxial graphene on SiC has been measured in deionized water. Owing to the hydrophobic attraction, a larger adhesion force was measured on 2LG Bernal-stacked domains of graphene surfaces, thus showing that 2LG is more hydrophobic than 1LG. Identification of 1LG and 2LG domains was achieved through Kelvin probe force microscopy and Raman spectral mapping. Approximate values of the adhesion force per OTS molecule have been calculated through contact area analysis. Furthermore, the contrast of friction force images measured in contact mode was reversed to the 1LG/2LG adhesion contrast, and its origin was discussed in terms of the likely water depletion over hydrophobic domains as well as deformation in the contact area between the atomic force microscope tip and 1LG.

Evidence for Metal-Semiconductor Transitions in Twisted and Collapsed Double-Walled Carbon Nanotubes by Scanning Tunneling Microscopy

The atomic and electronic structure of a twisted and collapsed double-walled carbon nanotube was characterized using scanning tunneling microscopy and spectroscopy. It was found that the deformation opens an electronic band gap in an otherwise metallic nanotube, which has major ramifications on the use of carbon nanotubes for electronic applications. Fundamentally, the importance of the intershell interaction in this double-walled carbon nanotube points to the potential of a reversible metal-semiconductor junction, which can have device applications, as well as a caution in the design of semiconductor components based on carbon nanotubes. Lattice registry effects between the two neighboring walls evidenced by atomically resolved images confirm earlier first principle calculations indicating that the helicity influences the collapsed structure and show excellent agreement with the predicted twisted-collapse mode.

Carrier type inversion in quasi-free standing graphene: studies of local electronic and structural properties

We investigate the local surface potential and Raman characteristics of as-grown and ex-situ hydrogen intercalated quasi-free standing graphene on 4H-SiC(0001) grown by chemical vapor deposition. Upon intercalation, transport measurements reveal a change in the carrier type from n- to p-type, accompanied by a more than three-fold increase in carrier mobility, up to μh ≈ 4540 cm2 V−1 s−1. On a local scale, Kelvin probe force microscopy provides a complete and detailed map of the surface potential distribution of graphene domains of different thicknesses. Rearrangement of graphene layers upon intercalation to (n + 1)LG, where n is the number of graphene layers (LG) before intercalation, is demonstrated. This is accompanied by a significant increase in the work function of the graphene after the H2-intercalation, which confirms the change of majority carriers from electrons to holes. Raman spectroscopy and mapping corroborate surface potential studies.

Structural, optical and electrostatic properties of single and few-layers MoS2: effect of substrate

We have decoupled the intrinsic electrostatic effects arising in monolayer and few-layer MoS2 from those influenced by the flake-substrate interaction. Using ultrasonic force microscopy nanomechanical mapping, we identify the change from supported to suspended flake regions on a trenched substrate. These regions are correlated with the surface potential as measured by scanning Kelvin probe microscopy. Relative to the supported region, we observe an increase in surface potential contrast due to suppressed charge transfer for the suspended monolayer. Using Raman spectroscopy we observe a red shift of the E12g mode for monolayer MoS2 deposited on Si, consistent with a more strained MoS2 on the Si substrate compared to the Au substrate.

Electron field-emission properties of Ag-SiO2 nanocomposite layers

In this work, Ag–SiO2 nanocomposite layers were synthesized by introducing Ag nanoclusters into thermally oxidized SiO2 layers, using ion implantation. The field-emission (FE) properties of these layers were studied and correlated with the results from atomic force microscopy and transmission electron microscopy measurements. These nanocomposites exhibit good FE properties and give an emission current of 1nA at electric fields as low as 13V∕μm, for a dose of 5×1016Ag+∕cm2, compared with 204V∕μm for “bare” SiO2 layers. It is clearly demonstrated that the good FE properties of these nanocomposites are attributed to two types of local-field enhancement: one due to the surface morphology and the other due to electrical inhomogeneity. The isolated conductive Ag nanoclusters embedded in the electrically insulating SiO2 matrix provide a field enhancement due to the electrical inhomogeneity effect. Moreover, the implanted Ag ions diffuse to the surface, during the implantation process, and create dense surface-pr...

Growth and field emission properties of vertically aligned carbon nanofibers

Vertically aligned carbon nanofibers (VACNFs) were synthesized on Ni-coated Si substrates using a dc plasma-enhanced chemical-vapor deposition system. The size of the Ni islands used as catalyst to grow the VACNFs was formed by both thermal annealing and laser processing on thin metal layers. It was observed that the diameter of the carbon nanofibers is strongly dependent on the initial Ni island dimension. By varying the laser power from 228 to 279mJ∕cm2, the size of these Ni islands can be controlled independent of the initial Ni film thickness. Electron field-emission results show that the emission threshold field is dependent on both the height and radius of these VACNFs and also field shielding effects. Threshold fields as low as 2V∕μm was obtained from the sample with the largest height over radius ratio.