Raman spectroscopy | EPFL Graph Search

Raman spectroscopy (ˈrɑːmən) (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman spectroscopy is commonly used in chemistry to provide a structural fingerprint by which molecules can be identified. Raman spectroscopy relies upon inelastic scattering of photons, known as Raman scattering. A source of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range is used, although X-rays can also be used. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the system. Infrared spectroscopy typically yields similar yet complementary information. Typically, a sample is illuminated with a laser beam. Ele

Transfer of Graphene under Ultra-High Vacuum

Darius Constantin Merk

This thesis reports on a novel method for graphene transfer that is fully UHV compatible, which has remained a challenge for a long time due to the necessity of supporting graphene for transfer and most often requiring supporting layer removal post-transfer. Our graphene transfer method is based on the wafer-bonding approach, where graphene is stamped onto a target surface, from a support to which it is weakly bound. We use a bilayer of graphene supported by a teflon tape for the low adhesion between graphene layers and the flexibility of teflon tape to allow adaptability to the target surface.We demonstrate successful transfer onto Cu(100) and Ir(111) single crystal surfaces as well as on gr/Ir(111), thus forming a (partial) graphene bilayer on Ir(111). For in-situ characterization, we use Auger electron spectroscopy and STM. Auger measurements confirm that we are able to transfer an average 0.8-0.9 ML of carbon, by comparison to CVD grown samples. After transfer onto Ir(111), we show that by annealing to 1000 °C, we are able to observe by STM the characteristic moiré pattern formed by CVD grown graphene on Ir(111). Auger spectra show that the graphene-Iridium interaction undergoes a change from physisorption to chemisorption upon annealing. XAS and Raman spectroscopy demonstrate that the annealing step taken post transfer to observe the moiré by STM is only necessary to induce a change in interaction strength but not to heal graphene defects, to increase its domain size, or to make it flat. After transfer onto Cu(100), Raman spectra show that the transferred graphene is of the same quality as the CVD grown graphene before transfer, without further annealing. Comparison of synchrotron based high resolution XAS spectra of transferred gr/Ir(111) to CVD grown samples confirms that the transferred graphene is flat on top of the Ir(111) crystal surface.This enables sample and device fabrication, usually hampered by impurities, to be carried out in UHV leading to fully reproducible results. Clean twisted bilayer graphene samples can be made and studied in UHV. Further, surface science can be extended to the third dimension, by capping layers and continuing growth, etc., all under clean conditions.

EPFL2022

Carbon nanotubes functionalized with dye molecules and metal nanoparticles

Tilman Assmus

Single-walled carbon nanotubes are highly interesting quasi one-dimensional nanostructures. They possess a multitude of fascinating optical, mechanical and electrical properties, which can be even further expanded through appropriate functionalization strategies. This thesis covers a number of functionalization strategies and their effects on the nanotube's optical and electrical properties. The first part deals with the functionalization of carbon nanotubes with dye molecules and the characterization of the functionalized system. Both covalent and noncovalent approaches were investigated with the goal of modifying the nanotubes' photoelectrical properties. Upon excitation with light of the appropriate wavelength, a clear electrical response could be observed on some functionalized individual nanotubes. Depending on the attached dye molecule, the conductivity change is either induced by a light-induced conformational change or by a charge transfer between the nanotube and the excited dye molecule. The perspectives of constructing an optoelectronic nanoswitch based on these effects are discussed. The second part of the thesis addresses carbon nanotubes functionalized by a low density of electrodeposited gold nanoparticles with sizes in the range between 10 and 100nm. In this case, the functionalization is targeted on amplifying the nanotube's Raman response (SERS effect). The novel sample preparation technique developed in this thesis allowed for an investigation of SERS on the level of individual molecule / nanoparticle hybrids. The optical emission spectra of the nanotube-nanoparticle hybrid structure disclosed Raman peaks associated with the nanotubes, which are superimposed on a broad luminescence background originating from the metal particles. Wavelength dependent experiments revealed maximum Raman intensity when both the nanotube and the metal particle are in optical resonance. In well-prepared samples the Raman signals collected over isolated particles exhibited an intensity enhancement by at least one order of magnitude, as compared to bare sections on the same tube, without appreciably interfering with polarization dependent Raman measurements.

EPFL2008

Thermal scanning probe lithography for 2D material-based gas sensing

Berke Erbas

Recently, two-dimensional (2D) material based gas sensing, especially transition metal dichalcogenide-based sensing, has been widely investigated thanks to its room temperature sensing ability. Unlike metal oxide based sensors, 2D material-based sensing can be carried out without a heater, therefore it reduces the power consumption and makes them promising candidates for smartphones and wearable electronic devices. Besides gas sensing ability, these 2D materials are interesting and promising candidates for electronics and optoelectronics applications. For example, monolayer molybdenum disulfide (MoS2), which is a 2D semiconductor with direct bandgap, is one of the promising candidates for transistor but it has low mobility compared to silicon. To replace silicon-based devices, strain engineering of MoS2 is widely investigated. It is observed that tensile strain tunes the bandgap of MoS2, enhances carrier mobility and improves NO2 gas sensing by piezotronic effect. Different strategies have been used to strain MoS2 such as flexible substrates to stretch the exfoliated material on top of them, silicon/ dielectric substrates with surface roughness or silicon/dielectric substrates patterned with nanocone structure to locally strain transferred MoS2. Herein, field effect transistor with three-dimensionally patterned gate oxide is designed, fabricated and electrically characterized for sensing application. Thermal scanning probe lithography, which is a direct three-dimensional nanofabrication technique, is used for the fabrication of sinusoidal surfaces. By transferring the exfoliated MoS2 flakes on top of these sinusoidal gate oxide, biaxial strain is induced. Unlike counterparts, thermal scanning probe lithography-based patterning provides us design flexibility, which controls the direction of strain and the strain rate. As a result of Raman spectroscopy, the shifts in out-of-plane vibration (A1g) and in-plane vibration (E1 2g) modes are up to 0.87 cm−1 and 2.60 cm−1, respectively. According to the theoretical Raman spectra calculations of strained MoS2 [1], these shifts correspond to an average strain of 0.58% in a circular area with 1 μm diameter. The strain of 0.58% is obtained for a sinusoidal pattern having a 23 nm amplitude and a 440 nm period. Besides, as a result of photoluminescence characterization, it is shown that an average strain of 0.39% leads to a 35 meV redshift for A excitation peak position. Consequently, the strain of MoS2 on the surface patterned by thermal scanning probe lithography tunes the bandgap of MoS2 by 90.11 meV/%.

2021