The authors report results of micro-Raman spectroscopy investigation of mechanically exfoliated single-crystal bismuth telluride films with thickness ranging from a few-nm-range to bulk limit.It is found that the optical phonon mode A 1u , which is not-Raman active in bulk Bi 2 Te 3 crystals, appears in the atomically-thin films due to crystal-symmetry breaking. The intensity ratios of the out-of-plane A 1u and A 1g modes to the in-plane E g mode grow with decreasing film thickness.The evolution of Raman signatures with the film thickness can be used for identification of Bi 2 Te 3 crystals with the thickness of few-quintuple layers important for topological insulator and thermoelectric applications.

Bismuth telluride (Bi 2 Te 3 ) and related compounds have recently attracted strong interest owing to the discovery of the topological insulator properties in many members of this family of materials. The few-quintuple films of these materials are particularly interesting from the physics point of view. We report results of the micro-Raman spectroscopy study of the "graphene-like" exfoliated few-quintuple layers of Bi 2 Te 3 , Bi 2 Se 3 and Sb 2 Te 3 . It is found that crystal symmetry breaking in few-quintuple films results in appearance of A 1usymmetry Raman peaks, which are not active in the bulk crystals. The scattering spectra measured under the 633-nm wavelength excitation reveals a number of resonant features, which could be used for analysis of the electronic and phonon processes in these materials. In order to elucidate the influence of substrates on the few-quintuple-thick topological insulators we examined the Raman spectra of these films placed on mica, sapphire and hafnium-oxide substrates. The obtained results help to understand the physical mechanisms of Raman scattering in the few-quintuple-thick films and can be used for nanometrology of topological insulator films on various substrates.

We investigated the effect of the electron-beam irradiation on the level of the low-frequency 1/f noise in graphene devices. It was found that 1/f noise in graphene reveals an anomalous characteristic -it reduces with increasing concentration of defects induced by irradiation. The increased amount of structural disorder in graphene under irradiation was verified with microRaman spectroscopy. The bombardment of graphene devices with 20-keV electrons reduced the noise spectral density, S I /I 2 (I is the source-drain current) by an order-of magnitude at the radiation dose of 10 4 C/cm 2 . Our theoretical considerations suggest that the observed noise reduction after irradiation can be more readily explained if the mechanism of 1/f noise in graphene is related to the electron-mobility fluctuations. The obtained results are important for the proposed graphene applications in analog, mixed-signal and radio-frequency systems, integrated circuit interconnects and sensors.Noise Suppression in Graphene, UCR -RPI -Ioffe (2012) 3The level of the flicker 1/f noise [1] is one of the key metrics that each new material has to pass before it can be used for practical devices (f is the frequency) [2]. Graphene [3] has shown a great potential for applications in high-frequency communications [4][5], analog circuits [6] and sensors [7][8]. The envisioned applications require a low level of 1/f noise, which contributes to the phase-noise of communication systems [2] and limits the sensor sensitivity [7]. Despite significant research efforts [9][10][11][12][13][14][15] there is still no conventionally accepted model for physical mechanisms behind 1/f noise in graphene. Correspondingly, no comprehensive methods for 1/f noise suppression in graphene devices have been developed.In this Letter we show that 1/f noise in graphene reveals an anomalous characteristic -it reduces with increasing concentration of defects induced by irradiation. We found that bombardment of graphene devices with 20-keV electrons can reduce the noise spectral density, S I /I 2 (I is the source-drain current) by an order-of magnitude at the radiation dose (RD) of 10 4 C/cm 2 . Our theoretical analysis suggests that the observed noise suppression after introduction of defects can be explained if the mechanism of 1/f noise in graphene is related to the electron-mobility fluctuations rather than to the carrier-density fluctuations. Apart from contributing to understanding the physics behind 1/f noise in graphene our results can possibly offer a practical method for noise reduction in various graphene devices. Graphene is relatively susceptible to the electron and ion bombardment owing to its single-atom thickness [21][22][23]. Electron irradiation can introduce different types of defects in graphene Noise Suppression in Graphene, UCR -RPI -Ioffe (2012) 5 depending on the beam energy and local environment, e.g. presence of organic contaminants. For this study we selected the electron energy of 20 keV in order to exclude the severe knock-on damage to ...

We report on the low-frequency current fluctuations and electronic noise in thin-films made of Bi(2)Se(3) topological insulators. The films were prepared via the "graphene-like" mechanical exfoliation and used as the current conducting channels in the four- and two-contact devices. The thickness of the films ranged from ∼50 to 170 nm to avoid hybridization of the top and bottom electron surface states. Analysis of the resistance dependence on the film thickness indicates that the surface contribution to conductance is dominant in our samples. It was established that the current fluctuations have the noise spectrum close to the pure 1/f in the frequency range from 1 Hz to 10 kHz (f is the frequency). The relative noise amplitude S(I)/I(2) for the examined Bi(2)Se(3) films was increasing from ∼5 × 10(-8) to 5 × 10(-6) (1/Hz) as the resistance of the channels varied from ∼10(3) to 10(5) Ω. The obtained noise data is important for understanding electron transport through the surface and volume of topological insulators, and proposed applications of this class of materials. The results may help to develop a new method of noise reduction in electronic devices via the "scattering immune" transport through the surface states.

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