Martin Kalbac

Rapid Identification of Stacking Orientation in Isotopically Labeled Chemical-Vapor Grown Bilayer Graphene by Raman Spectroscopy

The growth of large-area bilayer graphene has been of technological importance for graphene electronics. The successful application of graphene bilayers critically relies on the precise control of the stacking orientation, which determines both electronic and vibrational properties of the bilayer system. Toward this goal, an effective characterization method is critically needed to allow researchers to easily distinguish the bilayer stacking orientation (i.e., AB stacked or turbostratic). In this work, we developed such a method to provide facile identification of the stacking orientation by isotope labeling. Raman spectroscopy of these isotopically labeled bilayer samples shows a clear signature associated with AB stacking between layers, enabling rapid differentiation between turbostratic and AB-stacked bilayer regions. Using this method, we were able to characterize the stacking orientation in bilayer graphene grown through Low Pressure Chemical Vapor Deposition (LPCVD) with enclosed Cu foils, achieving almost 70% AB-stacked bilayer graphene. Furthermore, by combining surface sensitive fluorination with such hybrid (12)C/(13)C bilayer samples, we are able to identify that the second layer grows underneath the first-grown layer, which is similar to a recently reported observation.

Lithium Insertion into Anatase Inverse Opal

Reference LPI-ARTICLE-2004-018doi:10.1149/1.1769273View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Defects in Individual Semiconducting Single Wall Carbon Nanotubes: Raman Spectroscopic and in Situ Raman Spectroelectrochemical Study

Raman spectroscopy and in situ Raman spectroelectrochemistry have been used to study the influence of defects on the Raman spectra of semiconducting individual single-walled carbon nanotubes (SWCNTs). The defects were created intentionally on part of an originally defect-free individual semiconducting nanotube, which allowed us to analyze how defects influence this particular nanotube. The formation of defects was followed by Raman spectroscopy that showed D band intensity coming from the defective part and no D band intensity coming from the original part of the same nanotube. It is shown that the presence of defects also reduces the intensity of the symmetry-allowed Raman features. Furthermore, the changes to the Raman resonance window upon the introduction of defects are analyzed. It is demonstrated that defects lead to both a broadening of the Raman resonance profile and a decrease in the maximum intensity of the resonance profile. The in situ Raman spectroelectrochemical data show a doping dependence of the Raman features taken from the defective part of the tested SWCNT.

Electrochemical charging of individual single-walled carbon nanotubes.

The influence of the electrode potential on the electronic structure of individual single-walled carbon nanotubes is studied using Raman spectroscopy. By analyzing the radial breathing mode intensity versus electrode potential profiles in the Raman spectra at many different laser excitation energies, we show that the charging of individual carbon nanotubes causes a broadening of the resonant Raman profiles (resonance window). This effect is observed for both a semiconducting and a metallic tube. The broadening of the resonance Raman profiles already begins at potentials where the first electronic states of a particular tube are filled or depleted. The important consequence of this effect is a striking difference between the Raman intensity versus potential profiles of metallic and semiconducting tubes. While for a metallic tube the intensity of the Raman signal is attenuated at potentials which deviate slightly from 0 V, for a semiconducting tube, the Raman intensity is significantly attenuated only after the electrode potential reaches the first van Hove singularity. Furthermore, for the metallic tube, a strong asymmetry is found in the bleaching of the Raman signal with respect to positive and negative potentials, which results from the different energy bandwidth for the pi* band and the pi band.

Ion‐Irradiation‐Induced Defects in Isotopically‐Labeled Two Layered Graphene: Enhanced In‐Situ Annealing of the Damage

Contrary to theoretical estimates based on the conventional binary collision model, experimental results indicate that the number of defects in the lower layer of the bi-layer graphene sample is smaller than in the upper layer. This observation is explained by in situ self-annealing of the defects.

Development of the Tangential Mode in the Raman Spectra of SWCNT Bundles during Electrochemical Charging

The detailed analysis of the in situ Raman spectroelectrochemical behavior of single walled carbon nanotube (SWCNT) bundles is presented. The Raman modes of metallic SWCNTs exhibit striking changes even before the potential of the first van Hove singularity is achieved. Special attention has been paid to the development of the tangential (TG) mode broadening, which subsequently vanishes if the potential is shifted away from V = 0. The tangential mode band has been fitted by four components. During the electrochemical doping, three components of the tangential mode follow the predictions of a theoretical model for the LO modes of metallic tubes based on the Kohn anomaly. On the other hand, the behavior of the fourth component is consistent with a model based on electron-plasmon coupling. The TO mode of metallic tubes has been identified only at a doping level corresponding to 1.0 V or above. Our results also indicate an asymmetry in the behavior of the TG mode for positive electrode potentials relative to negative ones.

Two Positions of Potassium in Chemically Doped C(60) Peapods: An in situ Spectroelectrochemical Study.

The state of doping of fullerene peapods C60@SWCNT treated with K vapor was studied by in situ Raman spectroelectrochemistry. For all samples under study, a heavy chemical n doping was proved by the vanishing of the radial breathing mode and the downshift of tangential displacement mode. The K-treated peapods remain partly doped even if they are exposed to humid air. The Ag(2) mode of intratubular fullerene in K-doped peapods in contact with air was still redshifted as referred to its position in pristine peapods. Potassium inserted into the peapods is the reason for the air-insensitive residual doping, which can be removed only by electrochemical oxidation. This indicates the presence of two positions of potassium in doped sample.

Li Insertion into Li[sub 4]Ti[sub 5]O[sub 12] (Spinel)

Li 4 Ti 5 O 1 2 (spinel) materials were prepared with Brunauer-Emmett-Teller surface areas ranging from 1.3 to 196 m 2 /g. The corresponding average particle sizes varied from ca. 1 μm to ca. 9 nm. Twenty-five different materials were tested as Li insertion hosts in thin-film electrodes (2-4 μm) made from a pure spinel. Trace amounts of anatase in Li 4 Ti 5 O 1 2 were conveniently determined by cyclic voltammetry of Li insertion. Electrodes from nanocrystalline Li 4 Ti 5 O 1 2 exhibited excellent activity towards Li insertions even at charging rates as high as 250C. The charge capability at 50-250C was proportional to the logarithm of surface area for coarse particles (surface areas smaller than ca. 20 m 2 /g). With increasing charge/discharge rates, a narrowing plateau in performance was observed for materials with surface areas between ca. 20 to 100 m 2 /g. These materials can be charged/discharged nearly to the nominal capacity of L1 4 Ti 5 O 1 2 (175 mAh/g) within a wide range of the rates. Very small particles (surface areas > 100 m 2 /g) exhibit a growing decrease of charge capability at 50-250C. The Li-diffusion coefficients, calculated from chronoamperometry, decrease by orders of magnitude if the average particle size drops from ca. I μm to ca. 9 nm. However, the sluggish Li + transport in small particles is compensated by the increase in active electrode area. Materials having surface areas larger than ca. 100 m 2 /g also tend to show increased charge irreversibility. This could be caused by parasitic cathodic reactions, due to enhanced adsorption of reducible impurities (humidity) or the quality of the spinel crystalline lattice itself. The optimum performance of thin-film Li 4 Ti 5 O 1 2 electrodes is achieved, if the parent materials have surface areas between ca. 20 to 110 m 2 /g, with the maximum peak at 100 m 2 /g.Reference LPI-ARTICLE-2003-015doi:10.1149/1.1581262View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Li Insertion into Li4Ti5 O 12 (Spinel) Charge Capability vs. Particle Size in Thin-Film Electrodes

Li 4 Ti 5 O 1 2 (spinel) materials were prepared with Brunauer-Emmett-Teller surface areas ranging from 1.3 to 196 m 2 /g. The corresponding average particle sizes varied from ca. 1 μm to ca. 9 nm. Twenty-five different materials were tested as Li insertion hosts in thin-film electrodes (2-4 μm) made from a pure spinel. Trace amounts of anatase in Li 4 Ti 5 O 1 2 were conveniently determined by cyclic voltammetry of Li insertion. Electrodes from nanocrystalline Li 4 Ti 5 O 1 2 exhibited excellent activity towards Li insertions even at charging rates as high as 250C. The charge capability at 50-250C was proportional to the logarithm of surface area for coarse particles (surface areas smaller than ca. 20 m 2 /g). With increasing charge/discharge rates, a narrowing plateau in performance was observed for materials with surface areas between ca. 20 to 100 m 2 /g. These materials can be charged/discharged nearly to the nominal capacity of L1 4 Ti 5 O 1 2 (175 mAh/g) within a wide range of the rates. Very small particles (surface areas > 100 m 2 /g) exhibit a growing decrease of charge capability at 50-250C. The Li-diffusion coefficients, calculated from chronoamperometry, decrease by orders of magnitude if the average particle size drops from ca. I μm to ca. 9 nm. However, the sluggish Li + transport in small particles is compensated by the increase in active electrode area. Materials having surface areas larger than ca. 100 m 2 /g also tend to show increased charge irreversibility. This could be caused by parasitic cathodic reactions, due to enhanced adsorption of reducible impurities (humidity) or the quality of the spinel crystalline lattice itself. The optimum performance of thin-film Li 4 Ti 5 O 1 2 electrodes is achieved, if the parent materials have surface areas between ca. 20 to 110 m 2 /g, with the maximum peak at 100 m 2 /g.Reference LPI-ARTICLE-2003-015doi:10.1149/1.1581262View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Single Layer Molybdenum Disulfide under Direct Out-of-Plane Compression: Low-Stress Band-Gap Engineering

Tuning the electronic structure of 2D materials is a very powerful asset toward tailoring their properties to suit the demands of future applications in optoelectronics. Strain engineering is one of the most promising methods in this regard. We demonstrate that even very small out-of-plane axial compression readily modifies the electronic structure of monolayer MoS2. As we show through in situ resonant and nonresonant Raman spectroscopy and photoluminescence measurements combined with theoretical calculations, the transition from direct to indirect band gap semiconductor takes place at ∼0.5 GPa, and the transition to a semimetal occurs at stress smaller than 3 GPa.