The adsorption of molecules on surfaces affects the surface dipole and thus changes in the work function may be expected. The effect in change of work function is particularly strong if charge between substrate and adsorbate is involved. Here we report the deposition of a strong electron acceptor molecule, tetrafluorotetracyanoquinodimethane C12R4N4 (F(4)TCNQ) on a monolayer of hexagonal boron nitride nanomesh (h-BN on Rh(111)). The work function of the F(4)TCNQ/h-BN/Rh system increases upon increasing molecular coverage. The magnitude of the effect indicates electron transfer from the substrate to the F(4)TCNQ molecules. Density functional theory calculations confirm the work function shift and predict doubly-charged F(4)TCNQ(2-) in the nanomesh pores, where the h-BN is closest to the Rh substrate, and to have the largest binding energy there. The preferred adsorption in the pores is conjectured from a series of ultraviolet photoelectron spectroscopy data, where the a bands in the pores are first attenuated. Scanning tunneling microscopy measurements indicate that F(4)TCNQ molecules on the nanomesh are mobile at room temperature, as "hopping" between neighboring pores is observed.

Harmonium is a vacuum ultraviolet (VUV) photon source built within the Lausanne Centre for Ultrafast Science (LACUS). Utilising high harmonic generation, photons from 20-110 eV are available to conduct steady-state or ultrafast photoelectron and photoion spectroscopies (PES and PIS). A pulse preserving monochromator provides either high energy resolution (70 meV) or high temporal resolution (40 fs). Three endstations have been commissioned for: a) PES of liquids; b) angular resolved PES (ARPES) of solids and; c) coincidence PES and PIS of gas phase molecules or clusters. The source has several key advantages: high repetition rate (up to 15 kHz) and high photon flux (1011 photons per second at 38 eV). The capabilities of the facility complement the Swiss ultrafast and X-ray community (SwissFEL, SLS, NCCR MUST, etc.) helping to maintain Switzerland's leading role in ultrafast science in the world.

The manipulation of the electronic properties of solids by light is an exciting goal, which requires knowledge of the electronic structure with energy, momentum and temporal resolution. Time- and angle-resolved photoemission spectroscopy (tr-ARPES) is the most direct probe of the effects of an optical excitation on the band structure of a material. In particular, tr-ARPES in the extreme ultraviolet (VUV) range gives access to the ultrafast dynamics over the entire Brillouin zone. VUV tr-ARPES experiments can now be performed at the ASTRA (ARPES Spectrometer for Time-Resolved Applications) end station of Harmonium, at LACUS. Its capabilities are illustrated by measurements of the ultrafast electronic response of ZrSiTe, a novel topological semimetal characterized by linearly dispersing states located at the Brillouin zone boundary.

On the path to functional graphene electronics, suitable templates for chemical vapor deposition (CVD) growth of high-mobility graphene are of great interest. Among various substrates, hexagonal boron nitride (h-BN) has established itself as one of the most promising candidates. The nanomesh, a h-BN monolayer grown on the Rh(111) surface where the lattice mismatch of h-BN and rhodium leads to a characteristic corrugation of h-BN, offers an interesting graphene/h-BN interface, different from flat graphene/h-BN systems hitherto studied. In this report, we describe a two-step CVD process for graphene formation on h-BN/Rh(111) at millibar pressures and describe the influence of the surface texture on the CVD process. During a first exposure to the 3-pentanone precursor, carbon atoms are incorporated in the rhodium subsurface, which leads to decoupling of the h-BN layer from the Rh(111) surface. This is reflected in the electronic band structure, where the corrugation-induced splitting of the h-BN bands vanishes. In a second 3-pentanone exposure, a graphene layer is formed on the flat h-BN layer, evidenced by the appearance of the characteristic linear dispersion of its π band. The graphene layer grows incommensurate and highly oriented. The formation of graphene/h-BN on rhodium opens the door to scalable production of well-aligned heterostacks since single-crystalline thin-film Rh substrates are available in large dimensions.

On page L18 in Ref. [1] we state: “If the atomic ratio (of a single layer and graphene, respectively. Assuming 1 monolayer of graphene, we get h-BN on Rh(111) as determined by XPS) (AB + AN) / ARh of 0.43 is normalized with the unit cell sizes of h-BN and graphene, respectively, we find (from the graphene atomic ratio (2AC) / ARh of 0.40) a carbon coverage that corresponds to 0.96 ± 0.05 layers of graphene.” This is not correct since the x-ray-photoemission spectroscopy (XPS) intensity of single layer h-BN and graphene is proportional to the emitter density, which is proportional to a−2, where a is the corresponding lattice constant of h-BN and graphene, respectively. With the values of ah − BN (2.5 A) and ag (2.46 A), we get 0.90± 0.05 “effective” layers of graphene instead of 0.96 layers. Since the reported scanning tunneling microscopy data indicate full graphene coverage, we propose that the difference between the above XPS and the STM result is related to different photoelectron attenuations across h-BN

Single atoms, and in particular the least reactive noble gases, are difficult to immobilize at room temperature. Ion implantation into a crystal lattice has this capability, but the randomness of the involved processes does not permit much control over their distribution within the solid. Here we demonstrate that the boron nitride nanomesh, a corrugated single layer of hexagonal boron nitride (h-BN) with a 3.2 nm honeycomb superstructure formed on a Rh(111) surface, can trap individual argon atoms at distinct subsurface sites at room temperature. A kinetic energy window for implantation is identified where the argon ions can penetrate the h-BN layer but not enter the Rh lattice. Scanning tunneling microscopy and photoemission data show the presence of argon atoms at two distinct sites within the nanomesh unit cell, confirmed also by density functional theory calculations. The single atom implants are stable in air. Annealing of implanted structures to 900 K induces the formation of highly regular holes of 2 nm diameter in the h-BN layer with adjacent flakes of the same size found on top of the layer. We explain this "can-opener" effect by the presence of a vacancy defect, generated during the penetration of the Ar ion through the h-BN lattice, and propagating along the rim of a nanomesh pore where the h-BN lattice is highly bent. The reported effects are also observed in graphene on ruthenium and for neon atoms.

We report results on the self-assembly of silicon nanoribbons on the (2x1) reconstructed Au(110) surface under ultra-high vacuum conditions. Upon adsorption of 0.2 monolayer (ML) of silicon the (2x1) reconstruction of Au(110) is replaced by an ordered surface alloy. Above this coverage a new superstructure is revealed by low electron energy diffraction (LEED) which becomes sharper at 0.3 Si ML. This superstructure corresponds to Si nanoribbons all oriented along the [-110] direction as revealed by LEED and scanning tunneling microscopy (STM). STM and high-resolution photoemission spectroscopy indicate that the nanoribbons are flat and predominantly 1.6 nm wide. In addition the silicon atoms show signatures of two chemical environments corresponding to the edge and center of the ribbons.

In this thesis, procedures for chemical vapor deposition growth of graphene on rhodium and on hexagonal-boron nitride were developed. The systems were further characterized by an- gular resolved photoemission spectroscopy and scanning tunneling microscopy. A single atomic layer of graphite, so called graphene has gathered a lot of attention nowadays. Due to its high carrier mobility it has great potential for future electronics. Another appli- cation for graphene is related to its chemical inertness. It can serve as template for single molecule deposition. With these applications in mind, it is necessary to find suitable production methods to fabri- cate high quality, large scale graphene. Among various fabrication methods, chemical vapor deposition has turned out to be a possible route to meet the demanded criteria of graphene fabrication. The growth and characterization of graphene on Rh(111) and hexagonal-boron nitride are the content of this thesis. Graphene grown by chemical vapor deposition of 3-pentanone on Rh(111) leads to a largescale, uniform growth of graphene. A unique corrugation of the graphene layer is observed, which contains three depressions and a hill site within the unit cell. These three pockets have a diameter of roughly 1 nm and have therefore high potential to act as adsorption sites for single molecules. In contrast to the strongly bonded graphene on Rh(111), graphene on h-BN shows weak inter- action with the substrate. Growing graphene on h-BN demands different process parameters, since the metal surface is passivated by the h-BN layer, i. e. it requires higher activation energy (temperature) and higher pressure. The bonding of graphene to the h-BN/Cu(111) is weak enough, that the graphene and the h-BN layer keep their intrinsic lattice constant during the formation which differs by 1.6%. This results in formation of a large moir´ pattern with a lattice constant of 15.4 nm. The mismatch is also represented in the electronic band structure,where two Brillouin zones of different diameter are found for the two layers. The corrugated h-BN nanomesh on Rh(111) as a substrate for graphene formation exhibits a unique behavior during the formation process. Other than in the case of h-BN/Cu(111), the h-BN layer here undergoes a change during the graphene formation process. An intercalation of carbon between the h-BN layer and Rh and a diffusion of carbon into the Rh bulk leads to a weakening of the bonding and the h-BN layer looses its corrugated nanomesh character. In a second CVD process, graphene can be grown on this flat h-BN substrate.

The bonding geometry of tin-phthalocyanine (SnPc) on Ag(111) has been studied using x-ray and ultraviolet photoelectron diffraction (XPD and UPD, respectively). Experimental diffraction patterns were compared to single-scattering-cluster calculations. XPD data could be well reproduced by the simulations and allowed for the determination of several structural parameters. At a coverage of 0.9 ML, all molecules are in a “tin-down” configuration and the nonplanar shuttlecock-shaped SnPc molecule undergoes flattening upon absorption on Ag(111). UPD data from the second highest occupied molecular orbital and comparison to simulations show a high sensitivity to minor structural changes, including also the vertical distance between tin atoms of the SnPc and the surface layer of the substrate, which is found to be 2.3 A. We thus demonstrate how UPD can complement the well-established XPD method and discuss remaining challenges in the theoretical description of photoelectron diffraction from molecular orbitals at low energies. The UPD method is particularly attractive in view of the increasing availability of ultrashort pulsed laser sources in the XUV regime, which could enable pump-probe experiments with high structural sensitivity.

Two limiting factors for a new technology of graphene-based electronic devices are the difficulty of growing large areas of defect-free material and the integration of graphene with an atomically flat and insulating substrate material. Chemical vapor deposition (CVD) on metal surfaces, in particular on copper, may offer a solution to the first problem, while hexagonal boron nitride (h-BN) has been identified as an ideal insulating substrate material. The bottom-up growth of graphene/h-BN stacks on copper surfaces appears therefore as a promising route for future device fabrication. As an important step, we demonstrate the consecutive growth of well-aligned graphene on h-BN, both as single layers, by low-pressure CVD on Cu(111) in an ultrahigh vacuum environment. The resulting films show a largely predominant orientation, defined by the substrate, where the graphene lattice aligns parallel to the h-BN lattice, while each layer maintains its own lattice constant. The lattice mismatch of 1.6% between h-BN and graphene leads to a moiré pattern with a periodicity of about 9 nm, as observed with scanning tunneling microscopy. Accordingly, angle-resolved photoemission data reveal two slightly different Brillouin zones for electronic states localized in graphene and in h-BN, reflecting the vertical decoupling of the two layers. The graphene appears n-doped and shows no gap opening at the K[overline] point of the two-dimensional Brillouin zone.

Graphene offers great potential for applications in electronic devices [1], but in order to transfer this potential to the industrial scale, production methods for high quality material are vigorously investigated, as well as suitable substrate materials. Hexagonal boron nitride (h-BN) has appeared as a natural candidate substrate due to its closely related structure, its flatness and its wide band gap. It could be shown, by building devices using standard exfoliation techniques, that h-BN supported graphene exhibits superior electronic properties [2], and that few layers of h-BN can be used as a barrier in a fieldeffect tunneling transistor [3]. Several groups have reported direct growth of graphene on h-BN on metal surfaces in a two-step chemical vapor deposition (CVD) process: on Ni(111) [4,5], Ru(0001) [6], polycrystallinge Cu foils [7], as well as on Ni(111) films on W(110), and with Au intercalated between the h-BN and the Ni(111) [8]. Building on our long-term experience in the growth of epitaxial single-layer h-BN [9,10] and graphene [11-13] films on metal surfaces, we have investigated the sequential CVD growth of a single graphene layer on a predeposited single h-BN layer on Cu(111). Borazine (HBNH)3 [4] and 3-pentanone (C2H5COC2H5) [13] were used as precursors for h-BN and graphene growth, respectively. Substrate temperatures were of the order of 1000 K, and borazine dosing was done at a pressure in the 10 mbar range, leading to the self-saturating growth of a single h-BN layer on the Cu(111) surface. Low-energy electron diffraction (LEED), x-ray photoelectron diffraction (XPD) and angle-resolved photoemission (ARPES) data (Fig. 1a) testify for the presence of a well ordered boron nitride single layer. The LEED data show the h-BN lattice with a slightly smaller lattice constant and a small spread in crystal orientations of not more than 3°, which is consistent with the observation of moiré patterns observed for this system in a recent low-temperature scanning tunneling microscopy (STM) study [14]. The growth of graphene on top of the h-BN layer required much higher precursor pressures and exposure times. X-ray photoelectron spectroscopy (XPS) data confirm the presence of a single graphene layer with the desired stacking sequence of graphene/h-BN/Cu(111). In the ARPES data the graphene π band is observed with the characteristic linear dispersion up to the Fermi energy (Fig. 1b), leading to a six-fold arrangement of maxima in the Fermi surface map displayed in Fig. 1c. No indication of a band gap is observed in these data. In this talk, a detailed structural characterisation of this graphene/boron nitride heterostack will be presented, showing that the largely predominant phase on the surface features a graphene layer that matches the orientation of the h-BN lattice but exhibits a lattice mismatch of the order of 1.4-1.6%, similar to the mismatch between graphite and bulk h-BN. This presence of this phase is confirmed by the observation of the characteristic moiré pattern in STM images. A weak electronic coupling between the two layers arises due to this incommensurate growth, which can rationalize the absence of a band gap in the graphene layer.

We report on experimental evidence for the formation of a two dimensional Si/Au(110) surface alloy. In this study, we have used a combination of scanning tunneling microscopy, low energy electron diffraction, Auger electron spectroscopy, and ab initio calculations based on density functional theory. A highly ordered and stable Si-Au surface alloy is observed subsequent to growth of a sub-monolayer of silicon on an Au(110) substrate kept above the eutectic temperature.

The synthesis of high quality single layer graphene on rhodium, g/Rh(111), is reported. The graphene layers are grown at 1060K by low pressure chemical vapor deposition (CVD) using 3-pentanone as a precursor molecule. The presented growth technique shows an easy high quality production method for epitaxial graphene monolayers. The chemical composition and structural properties of such self-assembled monolayers were characterized by X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Scanning Tunneling Microscopy (STM) confirms the formation of a 3nm super cell and a unique surface morphology which establishes the potential of g/Rh(111) as a template for molecules.

We observe time-resolved and polarization dependent two-, three-, and four-photon transitions from a Ni(111) surface covered with a monolayer of hexagonal boron nitride. The spectra show features due to transitions involving two unoccupied intermediate states: the n = 1 image potential state of Ni(111) and a boron nitride-related interface state. We use these transitions in order to track the effects of space-charge on the spectra in view of pump-probe experiments, at high excitations densities. A simple model is presented, which reproduces the measured energy shifts and broadenings in great detail, allowing the ultrafast dynamics and the space-charge effects to be disentangled. Moreover, owing to the high excitation densities an additional particular three-photon transition via both the interface state and a virtual intermediate state could be measured and identified. The polarization-dependent transient of this transition has successfully been simulated, allowing the lifetime of the involved intermediate state to be extracted from the data.

Photoelectron spectroscopy (PES) is a versatile tool, which provides insight into electronic structure and dynamics in condensed matter, surfaces, interfaces and molecules. The history of PES is briefly outlined and illustrated by current developments in the field of time-resolved PES. Our group's research is mostly aimed at studying ultrafast processes and associated lifetimes related to electronic excitation at solid surfaces.

Functional nano-templates enable self-assembly of otherwise impossible arrangements of molecules. A particular class of such templates is that of sp 2 hybridized single layers of hexagonal boron nitride or carbon (graphene) on metal supports. If the substrate and the single layer have a lattice mismatch, superstructures are formed. On substrates like rhodium or ruthenium these superstructures have unit cells with ∼3-nm lattice constant. They are corrugated and contain sub-units, which behave like traps for molecules or quantum dots, which are small enough to become operational at room temperature. For graphene on Rh(111) we emphasize a new structural element of small extra hills within the corrugation landscape. For the case of molecules like water it is shown that new phases assemble on such templates, and that they can be used as “nano-laboratories” where many individual processes are studied in parallel. Furthermore, it is shown that the h-BN/Rh(111) nanomesh displays a strong scanning tunneling microscopy-induced luminescence contrast within the 3 nm unit cell which is a way to address trapped molecules and/or quantum dots.