Stefan C. Badescu

Properties of Fluorinated Graphene Films

Graphene films grown on Cu foils have been fluorinated with xenon difluoride (XeF(2)) gas on one or both sides. When exposed on one side the F coverage saturates at 25% (C(4)F), which is optically transparent, over 6 orders of magnitude more resistive than graphene, and readily patterned. Density functional calculations for varying coverages indicate that a C(4)F configuration is lowest in energy and that the calculated band gap increases with increasing coverage, becoming 2.93 eV for one C(4)F configuration. During defluorination, we find hydrazine treatment effectively removes fluorine while retaining graphenes carbon skeleton. The same films may be fluorinated on both sides by transferring graphene to a silicon-on-insulator substrate enabling XeF(2) gas to etch the Si underlayer and fluorinate the backside of the graphene film to form perfluorographane (CF) for which calculated the band gap is 3.07 eV. Our results indicate single-side fluorination provides the necessary electronic and optical changes to be practical for graphene device applications.

Energetics and Vibrational States for Hydrogen on Pt(111)

We present a combination of theoretical calculations and experiments for the low-lying vibrational excitations of H and D atoms adsorbed on the Pt(111) surface. The vibrational band states are calculated based on the full three-dimensional adiabatic potential energy surface obtained from first principles calculations. For coverages less than three quarters of a monolayer, the observed experimental high-resolution electron peaks at 31 and 68meV are in excellent agreement with the theoretical transitions between selected bands. Our results convincingly demonstrate the need to go beyond the local harmonic oscillator picture to understand the dynamics of this system.

Polarized fine structure in the photoluminescence excitation spectrum of a negatively charged quantum dot.

We report polarized photoluminescence excitation spectroscopy of the negative trion in single charge-tunable quantum dots. The spectrum exhibits a p-shell resonance with polarized fine structure arising from the direct excitation of the electron spin triplet states. The energy splitting arises from the axially symmetric electron-hole exchange interaction. The magnitude and sign of the polarization are understood from the spin character of the triplet states and a small amount of quantum dot asymmetry, which mixes the wave functions through asymmetric e-e and e-h exchange interactions.

Optical Measurement and Modeling of Interactions between Two Hole Spins or Two Electron Spins in Coupled InAs Quantum Dots

Two electron spins in quantum dots coupled through coherent tunneling are generally acknowledged to approximately obey Heisenberg isotropic exchange. This has not been established for two holes. Here we measure the spectra of two holes and of two electrons in two vertically stacked self-assembled InAs quantum dots using optical spectroscopy as a function of electric and magnetic fields. We find that the exchange is approximately isotropic for both systems, but that significant asymmetric contributions, arising from spin-orbit and Zeeman interactions combined with spatial asymmetries, are required to explain large anticrossings and fine-structure energy splittings in the spectra. Asymmetric contributions to the isotropic Hamiltonian for electrons are of the order of a few percent while those for holes are an order of magnitude larger.

Heterogeneous Pyrolysis: A Route for Epitaxial Growth of hBN Atomic Layers on Copper Using Separate Boron and Nitrogen Precursors

Growth of hBN on metal substrates is often performed via chemical vapor deposition from a single precursor (e.g., borazine) and results in hBN monolayers limited by the substrates catalyzing effect. Departing from this paradigm, we demonstrate close control over the growth of mono-, bi-, and trilayers of hBN on copper using triethylborane and ammonia as independent sources of boron and nitrogen. Using density functional theory (DFT) calculations and reactive force field molecular dynamics, we show that the key factor enabling the growth beyond the first layer is the activation of ammonia through heterogeneous pyrolysis with boron-based radicals at the surface. The hBN layers grown are in registry with each other and assume a perfect or near perfect epitaxial relation with the substrate. From atomic force microscopy (AFM) characterization, we observe a moiré superstructure in the first hBN layer with an apparent height modulation and lateral periodicity of ∼10 nm. While this is unexpected given that the moiré pattern of hBN/Cu(111) does not have a significant morphological corrugation, our DFT calculations reveal a spatially modulated interface dipole layer which determines the unusual AFM response. These findings have improved our understanding of the mechanisms involved in growth of hBN and may help generate new growth methods for applications in which control over the number of layers and their alignment is crucial (such as tunneling barriers, ultrathin capacitors, and graphene-based devices).

Mixing of two-electron spin states in a semiconductor quantum dot

Abstract : We show that the low-lying spin states of two electrons in a semiconductor quantum dot can be strongly mixed by electron-electron asymmetric exchange. This mixing is generated by the coupling of the electron spin to its orbital motion and to the relative orbital motion of the two electrons. The asymmetric exchange can be as large as 50% of the isotropic exchange, even for cylindrical dots. The resulting mixing can contribute to understanding spin dynamics in dots, such as recent observations of light polarization reversal.

The role of surface adsorption in surface-enhanced Raman scattering from Benzene thiols

The origins of the surface-enhanced Raman (SERS) effect have been widely studied and are generally accepted as understood. However, there is still a need to provide a satisfactorily complete model which addresses the well known phenomena in which some molecules exhibit weak or even no SERS response at all. The relative intensities of vibrational modes observed in SERS can depend strongly on the mechanism of surface adsorption, such as bond type (covalent/noncovalent) and number of covalent bonds. Thus, in this experimental study the role of surface adsorption in surface-enhanced Raman scattering is investigated. A simple group of Benzene thiols was chosen to facilitate comparison with theoretical models. Experimental results with consideration towards surface bond strengths and reduction in degrees of freedom due to single and multiple surface bonds is presented and their effect on the relative intensities and positions of observed vibrational modes observed in the SERS spectra are discussed. The relative stability of molecules in the presence of nanostructures exhibiting strong and weak local electric fields will also be presented and discussed.

Dielectric and geometric properties of plasmonics in metal/dielectric nanowires composites used in surface-enhanced Raman spectroscopy

Finite elements calculations have been performed of the surface enhanced Raman (SERS) activity of Ag coated dielectric nanowires. It is shown that the SERS fields and the angle of the peak field from intersecting nanowires can be changed through the angle of the nanowires. In addition, it is shown that the strength of the SERS enhancement and its spatial profile depend on whether the nanowires are in free space or on a substrate. Experimental data for benzene thiol on dielectric coated nanowires is shown to support the calculations. These results demonstrate the importance of geometry and local environment on electric field hot spots in the SERS process.