Ultra-high vacuum | EPFL Graph Search

Ultra-high vacuum (UHV) is the vacuum regime characterised by pressures lower than about . UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is greater than approximately 40 km, so the gas is in free molecular flow, and gas molecules will collide with the chamber walls many times before colliding with each other. Almost all molecular interactions therefore take place on various surfaces in the chamber. UHV conditions are integral to scientific research. Surface science experiments often require a chemically clean sample surface with the absence of any unwanted adsorbates. Surface analysis tools such as X-ray photoelectron spectroscopy and low energy ion scattering require UHV conditions for the transmission of electron or ion beams. For the same reason, beam pipes in particle accelerators such as the Large Hadron Collider are kept at UHV. Overview Maintaining UHV conditions requires the us

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

An Experimental Setup for Heterogeneous Catalysis on Atomically Defined Metal Nanostructures

Jean-Guillaume De Groot

The main objective of this PhD thesis is to link the atomic scale structure to the catalytic properties of self-assembled nanostructures. The growth and characterization of these nanostructures was carried out in an ultra-high vacuum (UHV) chamber initially designed for the study of the kinetics of epitaxial growth of thin films and nanostructures by variable temperature scanning tunneling microscopy. During this PhD thesis, we built a very sensitive and selective gas detector based on a quadrupole mass spectrometer and adapted to the geometry of this UHV system. The proper functioning of the gas detector is confirmed by temperature programmed desorptions of Xe on Ag(100) which demonstrate the very high sensitivity of the device of about

10^{-6}

~monolayer (ML). The zero-order desorption parameters obtained for the first monolayer of Xe adsorbed on Ag(100) are

E_d=0.22-0.23

~eV with a prefactor

\nu_0

between

4\times10^{12}

^{-1}

and

1 \times10^{13}

^{-1}

. An exchange process between Fe adatoms and Ag atoms from the Ag(100) surface is identified by means of temperature programmed desorption of N

_2

. Desorption spectra suggest that the exchange is complete from an annealing temperature of 140~K. Our study of the catalytic activity of metal nanostructures focuses on the ability of passivated Fe clusters to adsorb N

_2

molecules without dissociating them, which is a fundamental step in the bio-inspired heterogeneous catalysis of ammonia. The first system investigated for the growth of Fe clusters is MgO/Ag(100). Diffusion of Fe adatoms deposited on a MgO monolayer is observed from an annealing temperature of 280~K onwards. Fe clusters with an average size of

12.1\pm 0.5

~atoms are obtained on a MgO monolayer by thermal ripening of 0.072~ML of Fe deposited at 40~K. No sign of N

_2

adsorption was observed on these clusters by thermal programmed desorption investigation. The second system chosen for the growth of Fe clusters is graphene/Ir(111) obtained by chemical vapor deposition. The deposition of 0.3 and 0.4 ML of Fe on a graphene/Ir(111) sample at 50~K, followed by annealing at 300~K, generates a superlattice of Fe clusters which matches the periodicity of the moiré formed by graphene/Ir(111). The theoretically expected mean cluster size is 26 atoms for 0.3~ML of Fe, and 35 atoms for 0.4~ML of Fe. The temperature stability of the clusters obtained with 0.4~ML Fe is investigated from 300~K to 600~K. It is established that Smoluchowski ripening takes place as the annealing temperature increases and the superlattice structure has completely disappeared after annealing at 600~K. Temperature programmed desorption of molecularly adsorbed N

_2

at 50~K on a sample with 0.4~ML of Fe deposited on graphene/Ir(111) reveals 3 desorption peaks at 120~K, 92~K, and 60~K. In addition, slow dissociation of nitrogen molecules at 50~K adsorbed on the higher energy adsorption sites is demonstrated.

EPFL2021

Scattering HCl Molecules from Au(111) and Ag(111) Surfaces

Jan Daniel Geweke

In this thesis, I present an experimental molecular beam surface scattering study of dynamical processes involved when HCl molecules are scattered from Au(111) and Ag(111) surfaces. I investigated vibrational excitation, translational inelasticity and dissociative adsorption in combination with associative desorption. The experiments were conducted in an ultra-high vacuum (UHV) molecular beam/surface scattering apparatus equipped with a pulsed nanosecond infrared (IR) laser source for vibrational state manipulation and pulsed nanosecond ultraviolet (UV) lasers for quantum-state-resolved detection of molecules via resonance enhanced multi-photon ionization (REMPI) before and after the collisions. For HCl/Au(111), I found surface temperature dependent vibrational excitation probabilities (VEPs) from vibrational state v = 0 -> 1 to be in the range of 10^-5 - 10^-3 for incidence energies of Ei = 0.67 - 0.99 eV, which is low compared to other molecule-surface systems. On the other hand, VEPs for the v = 1 -> 2 transition were substantially higher at 10^-3 - 10^-2 for comparable incidence energies. In both cases, excitation probabilities could be divided into electronically adiabatic and nonadiabatic contributions where the latter exponentially depended on the surface temperature Ts. This combination of adiabatic and nonadiabatic vibrational excitation has so far been uniquely observed for HCl scattered from Au(111) and Ag(111) surfaces. Extracting Ei and Ts independent interaction coefficients, I found the nonadiabatic excitation to be stronger than the adiabatic one by a factor of 67 for the v = 0 -> 1 and 24 for the v = 1 -> 2 channel. In comparison, on Ag(111) only the excitation from the vibrational ground state could be observed. With VEPs in the range of 10^-4 - 10^-3 being slightly higher than on gold, this difference was interpreted as enhanced nonadiabatic interactions on silver. Relatively low barriers to dissociation on both metal surfaces (e. g., compared to NO/Au(111)) might have led to the higher v = 1 -> 2 VEPs on Au(111). Further, the absence of higher excitation channels (v = 2 -> 3 on gold, v = 1 -> 2 on silver) might also be explained by the enhanced dissociation of HCl molecules incident in excited vibrational states. Dissociation probabilities of HCl on metal surfaces were measured by AUGER electron spectroscopy after controlled molecular beam dosage monitored by a quadrupole mass spectrometer. Even though the dissociation probabilities on Ag(111) were predicted to be higher than on Au(111), I could not determine them within the current technical limitations. Additionally, the actual dissociation probabilities I determined on Au(111) employing AES remained below 0.06 - 0.17, which is lower than those predicted by a series of computational studies by a factor of 2 - 6. On the other hand, translational energy losses calculated in the latest AIMDEF study match the experimental results for v = 1 -> 1 and v = 1 -> 2 quite accurately. In sum, the results of this thesis, which all indicate that HCl exhibits both electronically adiabatic and nonadiabatic interactions with metal surfaces during scatter- ing events, can serve as benchmark data to test and improve theoretical descriptions of gas-surface interactions, especially in those cases were nonadiabaticity plays an important role.

EPFL2019