Photoemission Electron Microscopy for Ultrafast Nano-Optics - Femtoseconds to Attoseconds

Ultrafast nano-optics is a new and quickly evolving research field centred around the control, manipulation, and application of light on a nanometre and femtosecond scale. This can lead to improved electro-optical devices, more sensitive spectroscopy, and real-time control of chemical reactions. However, understanding the simultaneous nanometre and femtosecond evolution of nano-optical fields requires characterization methods with ultrahigh spatiotemporal resolution. A method that during the past 15 years has shown great promise for such studies is photoemission electron microscopy (PEEM) in combination with ultrashort laser pulses. Both PEEM, nanostructure fabrication methods, and a large variety of pulsed light sources are under rapid parallel development, leading also to quickly increasing possibilities of nanometre and femtosecond characterization. This thesis explores the combination of PEEM with various state-of-the-art lab-based sources of femtosecond and attosecond pulses with wavelengths spanning from 30 nm to 1.55 µm for studies of ultrafast nano-optics. It is based on experiments carried out with five different laser systems, studying light interaction with tailored metallic and semiconducting nanostructures. The work comprises construction of new experimental setups, PEEM measurements, development of data analysis tools, and complementary investigations using techniques such as scanning electron microscopy, X-ray photoelectron spectroscopy, and scanning tunnelling microscopy. Using few-cycle pulses from an ultra-broadband Ti:sapphire oscillator, localized surface plasmons in metallic nanostructures were studied with a temporal resolution down to a few femtoseconds. Metallic structures were also studied with PEEM using femtosecond pulses in the telecommunication wavelength regime. Other light sources employed include an optical parametric chirped pulse amplification system, with which anisotropy effects in semiconductor nanowires were studied. Finally, the thesis explores the use of extreme ultraviolet attosecond pulse trains produced by high-order harmonic generation (HHG) as light source for PEEM. Working with 1 kHz repetition rate, the spatial resolution was found to be limited by space charge effects to a few hundred nanometres. However, with a new HHG system working at 200 kHz repetition rate, the resolution was improved by a factor of 2—3, along with a reduction in acquisition time by an order of magnitude. Novel high-repetition rate attosecond light sources are therefore expected to play a key role in pushing the temporal resolution of PEEM into the attosecond regime

Photoemission electron microscopy of localized surface plasmons in silver nanostructures at telecommunication wavelengths

We image the field enhancement at Ag nanostructures using femtosecond laser pulses with a center wavelength of 1.55 micrometer. Imaging is based on non-linear photoemission observed in a photoemission electron microscope (PEEM). The images are directly compared to ultra violet PEEM and scanning electron microscopy (SEM) imaging of the same structures. Further, we have carried out atomic scale scanning tunneling microscopy (STM) on the same type of Ag nanostructures and on the Au substrate. Measuring the photoelectron spectrum from individual Ag particles shows a larger contribution from higher order photoemission process above the work function threshold than would be predicted by a fully perturbative model, consistent with recent results using shorter wavelengths. Investigating a wide selection of both Ag nanoparticles and nanowires, field enhancement is observed from 30% of the Ag nanoparticles and from none of the nanowires. No laser-induced damage is observed of the nanostructures neither during the PEEM experiments nor in subsequent SEM analysis. By direct comparison of SEM and PEEM images of the same nanostructures, we can conclude that the field enhancement is independent of the average nanostructure size and shape. Instead, we propose that the variations in observed field enhancement could originate from the wedge interface between the substrate and particles electrically connected to the substrate

Molecularly Resolved Electronic Landscapes of Differing Acceptor-Donor Interface Geometries

Organic semiconductors are a promising class of materials for numerous electronic and optoelectronic applications, including solar cells. However, these materials tend to be extremely sensitive to the local environment and surrounding molecular geometry, causing the energy levels near boundaries and interfaces essential to device function to differ from those of the bulk. Scanning Tunneling Microscopy and Spectroscopy (STM/STS) has the ability to examine both the structural and electronic properties of these interfaces on the molecular and submolecular scale. Here we investigate the prototypical acceptor/donor system PTCDA/CuPc using sub-molecularly resolved pixel-by-pixel STS to demonstrate the importance of subtle changes in interface geometry in prototypical solar cell materials. PTCDA and CuPc were sequentially deposited on NaCl bilayers to create lateral heterojunctions that were decoupled from the underlying substrate. Donor and acceptor states were observed to shift in opposite directions suggesting an equilibrium charge transfer between the two. Narrowing of the gap energy compared to isolated molecules on the same surface are indicative of the influence of the local dielectric environment. Further, we find that the electronic state energies of both acceptor and donor are strongly dependent on the ratio and positioning of both molecules in larger clusters. This molecular-scale structural dependence of the electronic states of both interfacial acceptor and donor has significant implications for device design where level alignment strongly correlates to device performance

Manipulating the Dynamics of Self-Propelled Gallium Droplets by Gold Nanoparticles and Nanoscale Surface Morphology

Using in situ surface-sensitive electron microscopy performed in real time, we show that the dynamics of micron-sized Ga droplets on Ga P(111) can be manipulated locally using Au nanoparticles. Detailed measurements of structure and dynamics of the surface from microns to atomic scale are done using both surface electron and scanning probe microscopies. Imaging is done simultaneously on areas with and without Au particles and on samples spanning an order of magnitude in particle coverages. Based on this, we establish the equations of motion that can generally describe the Ga droplet dynamics, taking into account three general features: the affinity of Ga droplets to cover steps and rough structures on the surface, the evaporation-driven transition of the surface nanoscale morphology from rough to flat, and the enhanced evaporation due to Ga droplets and Au nanoparticles. Separately, these features can induce either self-propelled random motion or directional motion, but in combination, the self-propelled motion acts to increase the directional motion even if the directional force is 100 times weaker than the random force. We then find that the Au particles initiate a faster native oxide desorption and speed up the rough to flat surface transition in their vicinity. This changes the balance of forces on the Ga droplets near the Au particles, effectively deflecting the droplets from these areas. The model is experimentally verified for the present materials system, but due to its very general assumptions, it could also be relevant for the many other materials systems that display self-propelled random motion

Tin Oxides : Insights into Chemical States from a Nanoparticle Study

Tin oxides are semiconductor materials currently attracting close attention in electronics, photovoltaics, gas sensing, and catalysis. Depending on the tin oxidation state - Sn(IV), Sn(II), or intermediate - the corresponding oxide has either n- or p-type natural conductivity, ascribed to oxygen or metal deficiency in the lattice. Such crystalline imperfections severely complicate the task of establishing tin oxidation state, especially at nanoscale. In spite of the striking differences between SnO2 and SnO in their most fundamental properties, there have been enduring problems in identifying the oxide type. These problems were to a great extent caused by the controversy around the characteristic chemical shift, that is, the difference in electron binding energy of a certain core level in an oxide and its parent metal. Using in situ fabricated bare tin oxide nanoparticles, we have been able to resolve the controversy: Our photoelectron spectroscopic study on tin oxide nanoparticles shows that, in contrast to a common opinion of a close chemical shift for SnO2 and SnO, the shift value for tin(IV) oxide is, in fact, 3 times larger than that for tin(II) oxide. Moreover, our investigation of the nanoparticle valence electronic structure clarifies the question of why previously the identification of oxidation states encountered problems

Secondary electron imaging of nanostructures using Extreme Ultra-Violet attosecond pulse trains and Infra-Red femtosecond pulses

Surface electron dynamics unfold at time and length scales down to attoseconds and nanometres, making direct imaging with extreme spatiotemporal resolution highly desirable. However, this has turned out to be a major challenge even with the advent of reliable attosecond light sources. In this paper, photoelectrons from Ag nanowires and nanoparticles excited by extreme ultraviolet (XUV) attosecond pulse trains and infrared femtosecond pulses using a PhotoEmission Electron Microscope (PEEM) are imaged. In addition, the samples were investigated using Scanning Electron Microscopy (SEM) and synchrotron based X-ray photoelectron spectroscopy (XPS). To achieve contrast between the nanostructures and the substrate in the XUV images, three different substrate materials were investigated: Cr, ITO and Au. While plasmonic field enhancement can be observed on all three substrates, only on Au substrates do the Ag nanowires appear significantly brighter than the substrate in XUV-PEEM imaging. 3-photon photoemission imaging of plasmonic hot-spots was performed where the autocorrelation trace is observed in the interference signal between two femtosecond Infra-Red (IR) beams with sub-cycle precision. Finally, using Monte Carlo simulations, it is shown how the secondary electrons imaged in the XUV PEEM can potentially reveal information on the attosecond time scale from the near surface region of the nanostructures

Characterizing the geometry of InAs nanowires using mirror electron microscopy

Mirror electron microscopy (MEM) imaging of InAs nanowires is a non-destructive electron microscopy technique where the electrons are reflected via an applied electric field before they reach the specimen surface. However strong caustic features are observed that can be non-intuitive and difficult to relate to nanowire geometry and composition. Utilizing caustic imaging theory we can understand and interpret MEM image contrast, relating caustic image features to the properties and parameters of the nanowire. This is applied to obtain quantitative information, including the nanowire width via a through-focus series of MEM images