George W. Flynn

Visualizing Individual Nitrogen Dopants in Monolayer Graphene

Nitrogen atoms that replace carbon atoms in the graphene lattice strongly modify the local electronic structure. In monolayer graphene, substitutional doping during growth can be used to alter its electronic properties. We used scanning tunneling microscopy, Raman spectroscopy, x-ray spectroscopy, and first principles calculations to characterize individual nitrogen dopants in monolayer graphene grown on a copper substrate. Individual nitrogen atoms were incorporated as graphitic dopants, and a fraction of the extra electron on each nitrogen atom was delocalized into the graphene lattice. The electronic structure of nitrogen-doped graphene was strongly modified only within a few lattice spacings of the site of the nitrogen dopant. These findings show that chemical doping is a promising route to achieving high-quality graphene films with a large carrier concentration.

Atmospheric Oxygen Binding and Hole Doping in Deformed Graphene on a SiO2 Substrate

Using micro-Raman spectroscopy and scanning tunneling microscopy, we study the relationship between structural distortion and electrical hole doping of graphene on a silicon dioxide substrate. The observed upshift of the Raman G band represents charge doping and not compressive strain. Two independent factors control the doping: (1) the degree of graphene coupling to the substrate and (2) exposure to oxygen and moisture. Thermal annealing induces a pronounced structural distortion due to close coupling to SiO2 and activates the ability of diatomic oxygen to accept charge from graphene. Gas flow experiments show that dry oxygen reversibly dopes graphene; doping becomes stronger and more irreversible in the presence of moisture and over long periods of time. We propose that oxygen molecular anions are stabilized by water solvation and electrostatic binding to the silicon dioxide surface.

High-resolution scanning tunneling microscopy imaging of mesoscopic graphene sheets on an insulating surface

We present scanning tunneling microscopy (STM) images of single-layer graphene crystals examined under ultrahigh vacuum conditions. The samples, with lateral dimensions on the micrometer scale, were prepared on a silicon dioxide surface by direct exfoliation of crystalline graphite. The single-layer films were identified by using Raman spectroscopy. Topographic images of single-layer samples display the honeycomb structure expected for the full hexagonal symmetry of an isolated graphene monolayer. The absence of observable defects in the STM images is indicative of the high quality of these films. Crystals composed of a few layers of graphene also were examined. They exhibited dramatically different STM topography, displaying the reduced threefold symmetry characteristic of the surface of bulk graphite.

Structure and Electronic Properties of Graphene Nanoislands on Co(0001)

We have grown well-ordered graphene adlayers on the lattice-matched Co(0001) surface. Low-temperature scanning tunneling microscopy measurements demonstrate an on-top registry of the carbon atoms with respect to the Co(0001) surface. The tunneling conductance spectrum shows that the electronic structure is substantially altered from that of isolated graphene, implying a strong coupling between graphene and cobalt states. Calculations using density functional theory confirm that structures with on-top registry have the lowest energy and provide clear evidence for strong electronic coupling between the graphene pi-states and Co d-states at the interface.

Local Atomic and Electronic Structure of Boron Chemical Doping in Monolayer Graphene

We use scanning tunneling microscopy and X-ray spectroscopy to characterize the atomic and electronic structure of boron-doped and nitrogen-doped graphene created by chemical vapor deposition on copper substrates. Microscopic measurements show that boron, like nitrogen, incorporates into the carbon lattice primarily in the graphitic form and contributes ~0.5 carriers into the graphene sheet per dopant. Density functional theory calculations indicate that boron dopants interact strongly with the underlying copper substrate while nitrogen dopants do not. The local bonding differences between graphitic boron and nitrogen dopants lead to large scale differences in dopant distribution. The distribution of dopants is observed to be completely random in the case of boron, while nitrogen displays strong sublattice clustering. Structurally, nitrogen-doped graphene is relatively defect-free while boron-doped graphene films show a large number of Stone-Wales defects. These defects create local electronic resonances and cause electronic scattering, but do not electronically dope the graphene film.

Laser Excitation and Equilibration of Excited Vibrational States in CH3F

CH3F, when its ν3 mode is pumped by the P(20) line of the 9.6‐μ band of a CO2 Q‐switch laser, exhibits fluorescence from the ν1 and ν4 modes at 3000 cm−1. This fluorescence has a risetime ≤ 5  μsec at 1 torr and a decay rate of 0.59± 0.02 msec−1·torr−1. The diffusion coefficient measured from the low pressure fluorescence decay curves is 0.076± 0.011 cm2·torr msec−1. A very efficient near resonant energy transfer process is probably responsible for propagation of energy up the vibrational manifold. Similar processes may be important for laser induced chemical reactions. A square dependence of the intensity of fluorescence on incident cw laser power has been observed. This appears to arise mainly from population changes caused by a general temperature increase in the gas which occurs after absorption of laser energy and equilibration of all degrees of freedom of the molecule.

Partial vibration energy transfer map for methyl fluoride: A laser fluorescence study

Infrared fluorescence has been observed from the 2ν3 overtone, the ν3+ν6 combination band and the ν1, ν2, ν4, ν5, and ν6 fundamentals of CH3F after pumping of the ν3, 0→ 1 transition by the P (20), 9.6 μ line of a Q‐switched CO2 laser. All observed states exhibit a single exponential decay curve with a rate of 0.59 msec−1· torr−1. The fluorescence risetimes of the ν6, ν2 and ν5, ν1 and ν4, and 2ν3 states were also observed. The rate of rise of the ν1 and ν4 fluorescence and the 2ν3 fluorescence is faster than 7 μsec at 1 torr. The ν6 fluorescence risetime is pressure dependent with a rate constant of 106± 21 msec−1· torr−1. The pressure dependent rate of rise of the ν2 and ν5 fluorescence is 86± 17 msec−1· torr−1. Measurements of the steady state dependence of the ν2 and ν5 fluorescence intensity versus laser intensity were made. The temperature dependence of the Q‐switch excited fluorescence intensity from the 2ν3 and ν1 and ν4 states was also studied. No evidence of excited state absorption was observed...

Deactivation of laser excited CH3F in CH3F–X mixtures

The vibrational relaxation of CH3F in mixtures of rare gases, O2, N2, H2, and D2 has been studied by means of laser induced fluorescence. The data can be interpreted qualitatively in terms of a combination of V → T/R energy transfer processes from both the ν3 and ν6 v=1 states of CH3F. V → R processes seem to be dominant for heavy collision partners while V → T processes appear to dominate for light collision partners. The quantitative agreement between theory and experiment is also considered.

Time‐resolved thermal lensing studies of laser‐induced translational energy fluctuations in CD4, SO2, and OCS

Translational energy changes caused by nonresonant vibration‐vibration (V‐V) and vibration → translation/rotation (V → T/R) processes have been studied in laser‐excited CD4, SO2, and OCS by the time‐resolved thermal lensing technique. Kinetic cooling has been observed in CD4 and SO2, indicating the contribution of endothermic collisional processes to the vibrational relaxation of these molecules. Varying amplitude of the cooling observed in CD4 has been attributed to rotational relaxation effects. A generalized spectrophone effect has been observed in mixtures of laser‐excited CH3F with CH3Cl. The sensitivity of the thermal lens to very weak absorption is discussed with regard to possible trace impurity detection. A discussion of the effects of acoustic phenomena is also presented.

Laser‐Induced 16‐μ Fluorescence in SF6: Acoustic Effects

Damped oscillations in the laser‐excited 16‐ and 10‐μ infrared fluorescence decay from SF6 and SF6–rare‐gas mixtures have been observed. This fluorescence modulation is shown to be due to a laser‐induced sound wave propagating in the sample cell. The generation and propagation of this sound wave is analyzed by solving the coupled mass, momentum, and energy transport equation for the gas following laser excitation. Similar effects have been observed in BCl3, and the phenomenon is expected to occur in general for molecular systems which strongly absorb pulsed laser radiation and also have very rapid vibration–vibration and vibration–translation energy transfer rates. The risetime of the 16‐μ fluorescence from SF6 following excitation by a Q‐switch CO2 laser oscillating at 10.6 μ gives a lower limit of 5 × 104 sec−1 at 1 torr for the vibration–vibration energy transfer rate in SF6 between states fluorescing at 10 μ and states fluorescing at 16 μ.