Anna K. Swan

Biaxial Strain in Graphene Adhered to Shallow Depressions

Measurements on graphene exfoliated over a substrate prepatterned with shallow depressions demonstrate that graphene does not remain free-standing but instead adheres to the substrate despite the induced biaxial strain. The strain is homogeneous over the depression bottom as determined by Raman measurements. We find higher Raman shifts and Gruneisen parameters of the phonons underlying the G and 2D bands under biaxial strain than previously reported. Interference modeling is used to determine the vertical position of the graphene and to calculate the optimum dielectric substrate stack for maximum Raman signal.

Micro-Raman measurement of bending stresses in micromachined silicon flexures

Micron-scale characterization of mechanical stresses is essential for the successful design and operation of many micromachined devices. Here we report the use of Raman spectroscopy to measure the bending stresses in deep reactive-ion etched silicon flexures with a stress resolution of /spl sim/10 MPa and spatial resolution of /spl sim/1 /spl mu/m. The accuracy of the technique, as assessed by comparison to analytical and finite-element models of the deformation, is conservatively estimated to be 25 MPa. Implications for the use of this technique in microsystems design are discussed.

Toward nanometer-scale resolution in fluorescence microscopy using spectral self-interference

We introduce a new fluorescence microscopy technique that maps the axial position of a fluorophore with subnanometer precision. The interference of the emission of fluorophores in proximity to a reflecting surface results in fringes in the fluorescence spectrum that provide a unique signature of the axial position of the fluorophore. The nanometer sensitivity is demonstrated by measuring the height of a fluorescein monolayer covering a 12-nm step etched in silicon dioxide. In addition, the separation between fluorophores attached to the top or the bottom layer in a lipid bilayer film is determined. We further discuss extension of this microscopy technique to provide resolution of multiple layers spaced as closely as 10 nm for sparse systems.

Screening of Excitons in Single, Suspended Carbon Nanotubes

Resonant Raman spectroscopy of single carbon nanotubes suspended across trenches displays red-shifts of up to 30 meV of the electronic transition energies as a function of the surrounding dielectric environment. We develop a simple scaling relationship between the exciton binding energy and the external dielectric function and thus quantify the effect of screening. Our results imply that the underlying particle interaction energies change by hundreds of meV.

Band Gap Engineering with Ultralarge Biaxial Strains in Suspended Monolayer MoS2

We demonstrate the continuous and reversible tuning of the optical band gap of suspended monolayer MoS2 membranes by as much as 500 meV by applying very large biaxial strains. By using chemical vapor deposition (CVD) to grow crystals that are highly impermeable to gas, we are able to apply a pressure difference across suspended membranes to induce biaxial strains. We observe the effect of strain on the energy and intensity of the peaks in the photoluminescence (PL) spectrum and find a linear tuning rate of the optical band gap of 99 meV/%. This method is then used to study the PL spectra of bilayer and trilayer devices under strain and to find the shift rates and Grüneisen parameters of two Raman modes in monolayer MoS2. Finally, we use this result to show that we can apply biaxial strains as large as 5.6% across micron-sized areas and report evidence for the strain tuning of higher level optical transitions.

Self-Trapping of Excitons, Violation of Condon Approximation, and Efficient Fluorescence in Conjugated Cycloparaphenylenes

Cycloparaphenylenes, the simplest structural unit of armchair carbon nanotubes, have unique optoelectronic properties counterintuitive in the class of conjugated organic materials. Our time-dependent density functional theory study and excited state dynamics simulations of cycloparaphenylene chromophores provide a simple and conceptually appealing physical picture explaining experimentally observed trends in optical properties in this family of molecules. Fully delocalized degenerate second and third excitonic states define linear absorption spectra. Self-trapping of the lowest excitonic state due to electron-phonon coupling leads to the formation of spatially localized excitation in large cycloparaphenylenes within 100 fs. This invalidates the commonly used Condon approximation and breaks optical selection rules, making these materials superior fluorophores. This process does not occur in the small molecules, which remain inefficient emitters. A complex interplay of symmetry, π-conjugation, conformational distortion and bending strain controls all photophysics of cycloparaphenylenes.

Quantitative evaluation of the octadecylamine-assisted bulk separation of semiconducting and metallic single-wall carbon nanotubes by resonance Raman spectroscopy

The selective stabilization of octadecylamine (ODA) on semiconducting (S) single-wall carbon nanotubes (SWNTs) has been reported to provide a means for the bulk separation of S from metallic (M) SWNTs. Utilizing resonance Raman spectroscopy and, in particular, the relative changes in the integrated intensities of the radial-breathing mode region, a generic method has been developed to provide quantitative evaluation of the separation efficiency between M and S SWNTs along with diameter separation. The ODA-assisted separation is shown to provide S enrichment by a factor of 5 for SWNTs prepared by high pressure CO decomposition and greater S enrichment for SWNTs with diameters below 1nm.

Strong extinction of a far-field laser beam by a single quantum dot.

Through the utilization of index-matched GaAs immersion lens techniques, we demonstrate a record extinction (12%) of a far-field focused laser beam by a single InAs/GaAs quantum dot. This contrast level enables us to report for the first time resonant laser transmission spectroscopy on a single InAs/GaAs quantum dot without the need for phase-sensitive lock-in detection.

The Role of Length and Defects on Optical Quantum Efficiency and Exciton Decay Dynamics in Single-Walled Carbon Nanotubes

We perform Monte Carlo simulations of the time-resolved, spatially resolved, and integrated photoluminescence from a nanotube to investigate the role of the nanotube length L and defects using an exciton random-walk and defect-induced quenching model. When nonradiative decay is due solely to diffusion quenching, the quantum efficiency is approximately proportional to L2 at low quantum efficiency. With defects present, the quantum efficiency depends only weakly on the number defects but is instead tied to Leff2 where Leff is the root-mean-square separation between defects. The time-resolved photoluminescence decay of nanotubes is multiexponential for both pristine nanotubes and nanotubes with defects. The dominant time scale for a pristine nanotube is proportional to L2/D, where D is the diffusion constant. The presence of defects on the nanotube introduces additional time scales.

Electrochemical gating of individual single-wall carbon nanotubes observed by electron transport measurements and resonant Raman spectroscopy

Metal electrodes patterned lithographically on top of individual single-wall carbon nanotubes are used to gate the nanotubes with respect to a reference electrode in an electrolyte drop. The gating is found to have a dramatic effect on both the Raman spectra and electron transport of the nanotubes. Current through metallic nanotubes is found to increase sharply with electrochemical gate voltage, indicating that the Fermi energy reaches valence and conduction band van Hove singularities. Using resonant confocal micro-Raman spectroscopy, we observe a 9 cm−1 upshift of the tangential mode vibrational frequency, as well as a 90% decrease in intensity, by applying 1 V between an individual nanotube and a silver reference electrode in a dilute H2SO4 solution. The mechanisms for the shifts of the Raman mode frequencies are discussed on the basis of changes in the lattice constant of heavily charged nanotubes.