Anastasia V. Tyurnina

High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe

A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere. Carrier mobilities are found to exceed 103 cm2 V-1 s-1 and 104 cm2 V-1 s-1 at room and liquid-helium temperatures, respectively, allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5 eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayers mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides and black phosphorus.

Broadband Light-Induced Absorbance Change in Multilayer Graphene

We report the ultrafast light-induced absorbance change in CVD-grown multilayer graphene. Using femtosecond pump-probe measurements in 1100-1800 nm spectral range, we revealed broadband absorbance change when the probe photon energy was higher than that of the pump photon. The observed phenomenon is interpreted in terms of the Auger recombination and impact ionization playing a significant role in the dynamics of photoexcited carriers in graphene.

Doping efficiency of single and randomly stacked bilayer graphene by iodine adsorption

We report on the efficiency and thermal stability of p-doping by iodine on single and randomly stacked, weakly coupled bilayer polycrystalline graphene, as directly measured by photoelectron emission microscopy. The doping results in work function value increase of 0.4–0.5 eV, with a higher degree of iodine uptake by the bilayer (2%) as compared to the single layer (1%) suggesting iodine intercalation in the bilayer. The chemistry of iodine is identified accordingly as I3− and I5− poly iodide anionic complexes with slightly higher concentration of I5− in bilayer than monolayer graphene, likely attributed to differences in doping mechanisms. Temperature dependent in-situ annealing of the doped films demonstrated that the doping remains efficient up to 200 °C.

Anomalous moiré pattern of graphene investigated by scanning tunneling microscopy: Evidence of graphene growth on oxidized Cu(111)

The growth of graphene on oriented (111) copper films has been achieved by atmospheric pressure chemical vapor deposition. The structural properties of as-produced graphene have been investigated by scanning tunneling microscopy. Anomalous moiré superstructures composed of well-defined linear periodic modulations have been observed. We report here on comprehensive and detailed studies of these particular moiré patterns present in the graphene topography revealing that, in certain conditions, the growth can occur on the oxygen-induced reconstructed copper surface and not directly on the oriented (111) copper film as expected.

Thermal oxidation of detonation nanodiamond

In this work the results of investigation of detonation ultradispersed diamond (UDD) powder by means of thermogravimetric analysis (TGA) and Raman scattering (RS) are presented. Using the TGA method the temperature regions corresponding to the oxidation of different carbon fractions included in the composition of UDD powder were determined. In particular, it was established that heat treatment in the air at a temperature not exceeding 550°C leads to the oxidation and removal of nondiamond carbon, while the diamond part of the UDD remains unchanged. The form of the diamond RS band in the spectra of the UDD powder oxidized at 550°C shows good agreement with the model of phonon confinement. Based on the comparison of the results of experimental data and theoretical calculations for the RS band forms the size of the UDD crystal grains was defined as 4–5 nm.

Double resonant Raman scattering in nanographite films

Experimental results are presented on Raman scattering in graphite films produced by DC plasmaenhanced chemical vapor deposition from a methane-hydrogen gas mixture. Scanning electron and probe microscopy data show that, depending on substrate material and deposition time, the deposited film is either a mesoporous material consisting of graphite nanocrystallites with basal planes oriented perpendicular to the substrate surface or an atomically flat, nanometer-thick stack of graphene layers parallel to the substrate. A comparative Raman spectroscopy analysis is performed for film samples deposited on nickel and silicon substrates for 5 and 60 min, as well as for highly ordered graphite samples. The Raman spectra of the examined samples correspond to the double resonant scattering mechanism. The behavior of Raman peak position and intensity as functions of excitation wavelength suggests a high degree of structural order in the graphite films deposited on nickel for 5 min. The results obtained are used to show that the thickness of these films is 1.5 ± 0.5 nm.

Thermal purification of detonation diamond

Detonation ultradispersed diamonds (UDDs) of different types were comparatively studied by thermogravimetric analysis (TGA) and Raman scattering (RS). The TGA method was used to determine temperature regions corresponding to oxidation of different carbon forms contained by UDD powder. In particular, it was found that heating in air at temperatures above 550°C results in oxidation and removal of nondiamond carbon, while the diamond fraction is retained. The shape of diamond lines in RS spectra of different UDDs oxidized at 550°C showed good agreement with the phonon confinement model. A comparison of the experimental data and theoretical calculations for the RS line shape allowed the determination of crystalline grain sizes in UDDs of different types. The values obtained correspond to the data of manufacturers of nanodiamond powders under study.

Structural peculiarities of carbon nanolayers prepared by deposition from a gaseous phase on Ni

The surface structure and the mechanical and electrical properties of graphite nanolayers (1–2 nm thick) obtained by the carbon deposition from a gaseous phase on a Ni surface were studied using atomic-force microscopy (AFM). The surface of the nanolayers contains a dense network of line ridges with transverse sizes of several nanometers to several tens of nanometers and with the length of linear segments up to 1–2 μm. The AFM studies permit the conclusion that the observed ridges are mainly formed from carbon nanotubes or nanofilaments. They differ from planar areas of the graphite layer by substantially higher local electric conductivity in the contact with an AFM probe. The formation of ridges on certain portions of the surface may be due to thermal deformation wrinkles of graphite nanolayers that arise on cooling to room temperature.

Doping characteristics of iodine on as-grown chemical vapor deposited graphene on Pt.

Using laboratory X-ray photoelectron emission microscopy (XPEEM), we investigated the doping efficiency and thermal stability of iodine on as-grown graphene on Pt. After iodine adsorption of graphene in saturated vapor of I2, monolayer and bilayer graphene exhibited work function of 4.93 eV and 4.87 eV, respectively. Annealing of the doped monolayer graphene at 100 °C led to desorption of hydrocarbons, which increased the work function of monolayer graphene by ~0.2 eV. The composition of the polyiodide complexes evolved upon a step-by-step annealing at temperatures from 100 °C to 300 °C while the work-function non-monotonically changed with decreasing iodine content. The iodine dopant was stable at relatively high temperature as a significant amount of iodine remained up to the annealing temperature of 350 °C.

Topology of Nanometric Graphite Films

This work submits the results of a study in which the combinational scattering (CS) spectroscopy and electronic and optical microscopy of nanometric single-crystal graphite films obtained by precipitating in gas phase were used. It is shown the graphite material samples synthesized in this manner are fine, highly streamlined films that grow alongside of the substrate surface. The films feature a fairly smooth surface and thicknesses of 100-1 atomic layers. The study of the topologic properties of the material, such as the formation of folds along the entire surface and the occurrence of waveform ripples on some surface areas, showed that they may appear due to large differences in the carbon film and nickel substrate thermal expansion factors. The film thickness was assessed for the observed parameters of the periodic structures and the results comply with data obtained using other techniques. In some areas of the material in study, the graphene layers spontaneously split and form bubbles. The results of the CS analysis of various bubbles on studied film surfaces are evidence that, similar to the film itself, the walls of bubbles are composed of different numbers of graphite layers. It is shown that transparent bubbles in which meshlike topology is observed are formed with a single atomic layer.