We have clearly discriminated the single-, bilayer-, and multiple-layer graphene (<10 layers) on Si substrate with a 285 nm SiO 2 capping layer by using contrast spectra, which were generated from the reflection light of a white light source. Calculations based on Fresnel's law are in excellent agreement with the experimental results (deviation 2%). The contrast image shows the reliability and efficiency of this new technique. The contrast spectrum is a fast, nondestructive, easy to be carried out, and unambiguous way to identify the numbers of layers of graphene sheet. We provide two easy-to-use methods to determine the number of graphene layers based on contrast spectra: a graphic method and an analytical method. We also show that the refractive index of graphene is different from that of graphite. The results are compared with those obtained using Raman spectroscopy.The recent success in extracting graphite sheets in multiple layers, and even monolayer graphene, from highly ordered pyrolytic graphite (HOPG) using a technique called micromechanical cleavage 1,2 has stimulated great interest in both the fundamental physics study and the potential applications of graphene. 3 Graphene has a two-dimensional (2D) crystal structure, which is the basic building block for other sp 2 carbon nanomaterials, such as nanographite sheets and carbon nanotubes. The peculiar properties of graphene arise from its unique electronic band structure, in which the conduction band touches the valence band at two points (K and K′) 4,5 in the Brillouin zone, and in the vicinity of these points, the electron energy has a linear relationship with the wavevector, E ) pkV f . Therefore, electrons in an ideal graphene sheet behave like massless Dirac-Fermions. 6,7 Some of these unique properties have been observed experimentally 8-21 and many new ideas 22-29 about the fundamental physics and device applications of single-and multiple-layer graphene have been proposed. Presently micromechanical cleavage is still the most effective way to produce high-quality graphene sheets, and a quick and precise method for determining the thickness of graphene sheets is essential for speeding up the research and exploration of graphene. Although atomic force microscopy (AFM) measurement is the most direct way to identify the number of layers of graphene, the method has a very slow throughput and may also cause damage to the crystal lattice during measurement. Furthermore, an instrumental offset of ∼0.5 nm (caused by different interaction forces) always exists, which is even larger than the thickness of a monolayer graphene and data fitting is required to extract the true thickness of graphene sheets. 30 Unconventional quantum Hall effects [8][9][10] are often used to differentiate monolayer and bilayer graphene from multiple layers. However, it is not a practical and efficient way. Researchers have attempted to develop more efficient ways to identify different layers of graphene without destroying the crystal lattice. Raman spectroscopy is a poten...

Graphene field effect transistors commonly comprise graphene flakes lying on SiO(2) surfaces. The gate-voltage dependent conductance shows hysteresis depending on the gate sweeping rate/range. It is shown here that the transistors exhibit two different kinds of hysteresis in their electrical characteristics. Charge transfer causes a positive shift in the gate voltage of the minimum conductance, while capacitive gating can cause the negative shift of conductance with respect to gate voltage. The positive hysteretic phenomena decay with an increase of the number of layers in graphene flakes. Self-heating in a helium atmosphere significantly removes adsorbates and reduces positive hysteresis. We also observed negative hysteresis in graphene devices at low temperature. It is also found that an ice layer on/under graphene has a much stronger dipole moment than a water layer does. Mobile ions in the electrolyte gate and a polarity switch in the ferroelectric gate could also cause negative hysteresis in graphene transistors. These findings improved our understanding of the electrical response of graphene to its surroundings. The unique sensitivity to environment and related phenomena in graphene deserve further studies on nonvolatile memory, electrostatic detection, and chemically driven applications.

Nanowalls, a new nanostructural morphology of carbon, grow instead of nanotubes under microwave plasma‐enhanced CVD conditions on substrates electrically disconnected from the lower electrode. While not fully understood, the formation of nanowalls (see Figure for top view) appears to depend on the local electric field. Due to their large surface, nanotubes may find applications in field emission displays and energy storage devices.

Raman spectroscopic studies of graphene have attracted much interest. The G-band Raman intensity of a single layer graphene on Si substrate with 300 nm SiO2 capping layer is surprisingly strong and is comparable to that of bulk graphite. To explain this Raman intensity anomaly, we show that in addition to the interference due to multiple reflection of the incident laser, the multiple reflection of the Raman signal inside the graphene layer must be also accounted for. Further studies of the role of SiO2 layer in the enhancement Raman signal of graphene are carried out and an enhancement factor of ~30 is achievable, which is very significant for the Raman studies. Finally, we discuss the potential application of this enhancement effect on other ultra-thin films and nanoflakes and a general selection criterion of capping layer and substrate is given.Comment: 13 pages, 3 figures to be published in Applied Physics Letter

Topological insulators with spin-momentum-locked topological surface states are expected to exhibit a giant spin-orbit torque in the topological insulator/ferromagnet systems. To date, the topological insulator spin-orbit torque-driven magnetization switching is solely reported in a Cr-doped topological insulator at 1.9 K. Here we directly show giant spin-orbit torque-driven magnetization switching in a Bi2Se3/NiFe heterostructure at room temperature captured using a magneto-optic Kerr effect microscope. We identify a large charge-to-spin conversion efficiency of ~1–1.75 in the thin Bi2Se3 films, where the topological surface states are dominant. In addition, we find the current density required for the magnetization switching is extremely low, ~6 × 105 A cm–2, which is one to two orders of magnitude smaller than that with heavy metals. Our demonstration of room temperature magnetization switching of a conventional 3d ferromagnet using Bi2Se3 may lead to potential innovations in topological insulator-based spintronic applications.

Size, dimensionality, and shape play important roles in determining the properties of nanomaterials. So far, most of the nanomaterial researches have been focused on zero-dimensional nanoparticles/nanodots and onedimensional nanowires/nanorods/nanotubes, but very few studies have been carried out on two-dimensional nanosheets. Starting from carbon, recently we have succeeded in growing a class of nanostructured two-dimensional materials either in the pure forms or in the form of composites with carbon. In this paper, we will first briefly discuss various types of two-dimensional systems and then focus on the formation mechanism of carbon nanowalls and their field-emission and electron transport properties. The use of carbon nanowalls as templates for the formation of other types of nanomaterials will also be discussed.

Since its discovery in less than five years ago, graphene has become one of the hottest frontiers in materials science and condensed matter physics, as evidenced by the exponential increase in number of publications in this field. Several reviews have already been published on this topic, focusing on single and multilayer graphene sheets. Here, we review the recent progresses in this field by extending the scope to various types of two-dimensional carbon nanostructures including graphene and free-standing carbon nanowalls/nanosheets. After a brief overview of the electronic properties of graphene, we focus on the synthesis, characterization and potential applications of these carbon nanostructures.

A multifunctional optoelectronic resistive switching memory, composed of a simple ITO/CeO2- x/AlOy/Al structure, is demonstrated. Arising from the photo-induced detrapping, electrode-injection and retrapping of electrons in the CeO2-x/AlOy/Al interfacial region, the device shows broadband, linear, and persistent photoresponses that can be used for the integration of demodulating, arithmetic, and memory functions in a single device for future optoelectronic interconnect systems.