Shyamalendu M Bose

We report on structural and electronic properties of defects in chemical vapor-deposited monolayer and few-layer MoS2 films. We use scanning tunneling microscopy and Kelvin probe force microscopy in order to obtain measurements of the local density of states, work function and nature of defects in MoS2 films. We track the evolution of defects that are formed under annealing in ultra-high vacuum conditions. We observe formation of metastable domains with different work function values after annealing the material in ultra-high vacuum to moderate temperatures. We attribute these metastable values of the work function to evolution of crystal defects forming during the annealing. The experiments show that sulfur vacancies formed after exposure to elevated temperatures diffuse, coalesce, and migrate bringing the system from a metastable to equilibrium ground state. The process could be thermally activated with estimated energy barrier for sulfur vacancy migration of 0.6 eV in single unit cell MoS2. The results provide estimates of the thermal budgets available for reliable fabrication of MoS2-based integrated circuit electronics and indicate the importance of defect control and layer passivation.

The pair correlation function (PCF) in a single layer doped graphene sheet has been calculated in the Lundqvist-Overhauser approximation. It is found that unlike the cases of two- and three-dimensional electron gases, the PCF does not become negative for small separation between electrons and that it is independent of the number density of the electrons. It shows its characteristic oscillations at large separation between electrons.

We present a theory for the calculation of the low energy intraband plasmon frequencies and the electron energy loss (EEL) spectra of single layer and multilayer graphene sheets. Our calculation shows that the number of plasmons that can be excited is equal to the number of graphene layers in the sample. One of these is the dominant in-phase plasmon having a square root dependence on the wave number at low wave vectors, whereas the others are out-of-phase plasmons having near linear dependences on the wave number. The EEL spectra of a single layer graphene shows a single peak at the plasmon frequency, which has been observed experimentally. The EEL spectra of all multilayer graphenes have two peaks, one corresponding to the dominant in-phase plasmon and the other due to the out of phase plasmons. We predict that careful measurement of the EEL of multilayer graphene will show both peaks due to the low energy intraband plasmons.

Electronic conduction through quantum dots undergoing Jahn-Teller distortion is studied utilizing a model presented recently in connection with investigation of possible magnetovoltaic effect in this system. The quantum dot connected to two metallic leads is described by the single impurity Anderson model (SIAM) Hamiltonian along with two additional terms describing the Jahn-Teller distortion and an applied magnetic field. The self-consistent calculation shows that the Jahn-Teller (J-T) order parameter which is a measure of the splitting of the degenerate dot level is maximum at zero temperature and smoothly goes to zero at the structural transition temperature, T-s. The conductance is greatly suppressed by the J-T distortion at low temperatures, slowly increases and attains a maximum at T-s, above which it shows a slow decrease. When plotted as a function of the energy of the dot level, the conductance shows two peaks corresponding to the two split J-T levels at temperatures below T-s, which further develops into a four peak structure in the presence of a magnetic field. (C) 2013 Elsevier B.V. All rights reserved.

A model is proposed to study the electrical transport properties of a quantum dot capable of undergoing a structural phase transition driven by the Jahn-Teller mechanism. The dot attached to two metallic leads is described by the single impurity Anderson model Hamiltonian to which a term describing the Jahn-Teller distortion and another term to include the possibility of the presence of a magnetic field are added. Calculation of the electrical conductance and Jahn-Teller order parameter reveals several interesting features the most striking of which is the new phenomenon of "magnetovoltaic effect." (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737009]

In order to analyze a sample using SERS, the analyte has to be brought in intimate contact with the substrate. This can be problematic when, let's say, the molecules of interest in trace amounts are located in large volumes. For example a biotoxin aerosol in a large room or a trace amount of bio-hazardous substances mixed in large volumes of water or other liquids. In principle it is possible to filter out the molecules of interest and then deposit them on the SERS substrate for further analyses. In practice this is very cumbersome and therefore is rarely used. Here we discuss flexible and porous SERS substrates that have been fabricated by depositing silver nano-particle inks on woven or spun fabrics made of glass fiber or cellulose followed by thermal annealing at 170-200 degrees C for 10-15 minutes. Use of microwave absorption at about 10 GHz in the polymer-nanoparticle matrix to monitor the sintering process and to optimize the SERS amplification is also discussed. By varying the annealing time, different levels of nanoparticle clustering and the consequent SERS amplification can be achieved. Sampling of large volumes using the SERS filter substrates to detect airborne molecules is also discussed.

A model is proposed to study the electrical conductance of a quantum dot capable of undergoing structural phase transition. The energy levels of the dot are assumed doubly degenerate so that it can undergo an electrically driven structural transition due to the Jahn-Teller mechanism. The model also allows for the presence of a magnetic field. The dot is described by the well known single impurity Anderson model (SIAM) along with two added terms describing the Jahn-Teller distortion and the magnetic field Calculation of the electrical transport properties of such a quantum dot reveals several interesting new features.

The Raman spectra of two G bands and a radial breathing mode RBM of unfilled and filled single-wall semiconducting and metallic carbon nanotubes have been investigated theoretically in the presence of electronphonon and phonon-phonon interactions. Excitation of low frequency optical plasmons in the metallic nanotube is responsible for the peak known as the Breit-Wigner-Fano BWF line shape in the G-band Raman spectra. In a filled nanotube, there is an additional peak due to excitation of the phonon of the filling atom or molecule. Positions, shapes, and relative strengths of these Raman peaks depend on the phonon frequencies of the nanotube and that of the filling atoms, and strengths and forms of the plasmon-phonon and phonon-phonon interactions. For example, filling atoms with phonon frequency close to the RBM frequency of the nanotube may broaden and lower the RBM Raman peak to such an extent that it may become barely visible. Hybridization between the G bands and the filling atom phonon is also strong when these two frequencies are close to each other, and it has important effects on the G band and the BWF line shapes. When the phonon frequency of the filling atom is far from the RBM and G-band frequencies, it gives rise to a separate peak with modest effects on the RBM and G-band spectra. The Raman spectra of semiconducting unfilled and filled nanotubes have similar behaviors as those of metallic nanotubes, except that normally they have Lorentzian line shapes and do not show a BWF line shape. However, if a semiconducting nanotube is filled with donor atoms, it is predicted that the BWF-type line shape may be observed near the RBM, or the G band, or the filling atom Raman peak.

The Raman spectra of two G-bands and a radial breathing mode (RBM) of unfilled and filled single-wall semi conducting and metallic carbon nanotubes have been investigated in the presence of electron-phonon and phonon-phonon interactions. Excitation of low frequency optical plasmons in the metallic nanotube is shown to be responsible for the asymmetric band known as the Breit-Wigner-Fano (BWF) line shape in the G-band Raman spectra. In a filled nanotube there is an additional peak due to excitation of the phonon of the filling atom or molecule. Positions, shapes and relative strengths of these Raman peaks depend on the phonon frequencies of the nanotube and that of the filling atoms, and strengths and forms of the electron-phonon and phonon-phonon interactions. Raman spectra of semiconducting unfilled and filled nanotubes have similar behavior as those of metallic nanotubes except that normally they have Lorentzian line shapes and do not show a BWF line shape. However, if a semiconducting nanotube is filled with donor atoms, it is predicted that the BWF type line shape may be observed near the RBM, or the G-band or the filling atom Raman peak, which can be used as a tool to measure the filling fraction of a semiconducting nanotube.