Carsten Georgi

Raman Spectroscopy of Graphene Edges

Graphene edges are of particular interest since their orientation determines the electronic properties. Here we present a detailed Raman investigation of graphene flakes with edges oriented at different crystallographic directions. We also develop a real space theory for Raman scattering to analyze the general case of disordered edges. The position, width, and intensity of G and D peaks are studied as a function of the incident light polarization. The D-band is strongest for polarization parallel to the edge and minimum for perpendicular. Raman mapping shows that the D peak is localized in proximity of the edge. For ideal edges, the D peak is zero for zigzag orientation and large for armchair, allowing in principle the use of Raman spectroscopy as a sensitive tool for edge orientation. However, for real samples, the D to G ratio does not always show a significant dependence on edge orientation. Thus, even though edges can appear macroscopically smooth and oriented at well-defined angles, they are not necessarily microscopically ordered.

Exciton energy transfer in pairs of single-walled carbon nanotubes

We studied the exciton energy transfer in pairs of semiconducting nanotubes using high-resolution optical microscopy and spectroscopy on the nanoscale. Photoluminescence from large band gap nanotubes within bundles is observed with spatially varying intensities due to distance-dependent internanotube transfer. The range of efficient energy transfer is found to be limited to a few nanometers because of competing fast nonradiative relaxation responsible for low photoluminescence quantum yield.

Tip-enhanced Raman spectroscopic imaging of localized defects in carbon nanotubes

We used tip-enhanced Raman spectroscopy to study defect induced D-band Raman scattering in metallic single-walled carbon nanotubes with a spatial resolution of 15 nm. The spatial extent of the D-band signal in the vicinity of localized defects is visualized and found to be about 2 nm only. Using the strong optical fields underneath the tip, we photogenerate localized defects and derive a relation between defect density and resulting D-band intensity.

Visualizing the local optical response of semiconducting carbon nanotubes to DNA-wrapping.

We studied the local optical response of semiconducting single-walled carbon nanotubes to wrapping by DNA segments using high resolution tip-enhanced near-field microscopy. Photoluminescence (PL) near-field images of single nanotubes reveal large DNA-wrapping-induced red shifts of the exciton energy that are two times higher than indicated by spatially averaging confocal microscopy. Near-field PL spectra taken along nanotubes feature two distinct PL bands resulting from DNA-wrapped and unwrapped nanotube segments. The transition between the two energy levels occurs on a length scale smaller than our spatial resolution of about 15 nm.

Probing Exciton Localization in Single-Walled Carbon Nanotubes Using High-Resolution Near-Field Microscopy

We observe localization of excitons in semiconducting single-walled carbon nanotubes at room temperature using high-resolution near-field photoluminescence (PL) microscopy. Localization is the result of spatially confined exciton energy minima with depths of more than 15 meV connected to lateral energy gradients exceeding 2 meV/nm as evidenced by energy-resolved PL imaging. Simulations of exciton diffusion in the presence of energy variations support this interpretation predicting strongly enhanced PL at local energy minima.

Photoinduced Luminescence Blinking and Bleaching in Individual Single‐Walled Carbon Nanotubes

The temporal evolution of photoluminescence in individual single-walled carbon nanotubes (SWNT) under strong laser irradiation is studied and pronounced blinking and bleaching is observed, caused by photoinduced oxidation that subsequently quenches mobile excitons. The nanotubes are isolated with sodium cholate and spun onto either a glass or mica surface. Their bleaching behavior is investigated for variable laser intensities in air and argon atmosphere. The decay rate for luminescence bleaching generally increases with higher laser intensity, however saturating on mica substrates, which is attributed to limited availability of oxygen in the vicinity of the nanotubes. Step-like events in the luminescence time traces corresponding to single oxidation events are analyzed regarding relative step height and suggest an exciton diffusion range of about 105 nm.

Analytics and Metrology of Strained Silicon Structures by Raman and Nano‐Raman Spectroscopy

Straining the active regions in MOSFET devices is one of the key contributors to increase device performance in present and future technology nodes. Since dedicated strain on the transistor level is required with opposite sign for NMOS and PMOS transistors, the need to measure strain locally has become a challenge for analytics and metrology. Raman spectroscopy is capable of obtaining strain information non‐destructively on the sub‐μm scale, and therefore, this technique has been considered for process monitoring. In this paper it will be shown for silicon‐germanium thin films, how both strain and composition can be determined independently by measuring two phonon modes of the film. This technique enables fast measurement of mechanical strain and chemical composition with high accuracy on the μm‐scale. Thus, the micro‐Raman technique is well suited for metrology of strained silicon test structures. Furthermore, it is shown that mechanical strain close to silicon‐germanium structures can be measured with n...