Niemma M. Buckanie

Growth of graphene on Ir(111)

Catalytic decomposition of hydrocarbons on transition metals attracts a renewed interest as a route toward high-quality graphene prepared in a reproducible manner. Here we employ two growth methods for graphene on Ir(111), namely room temperature adsorption and thermal decomposition at 870–1470 K (temperature programmed growth (TPG)) as well as direct exposure of the hot substrate at 870–1320 K (chemical vapor deposition (CVD)). The temperature- and exposure-dependent growth of graphene is investigated in detail by scanning tunneling microscopy. TPG is found to yield compact graphene islands bounded by C zigzag edges. The island size may be tuned from a few to a couple of tens of nanometers through Smoluchowski ripening. In the CVD growth, the carbon in ethene molecules arriving on the Ir surface is found to convert with probability near unity to graphene. The temperature-dependent nucleation, interaction with steps and coalescence of graphene islands are analyzed and a consistent model for CVD growth is developed.

Selecting a single orientation for millimeter sized graphene sheets

We have used low energy electron microscopy and photo emission electron microscopy to study and improve the quality of graphene films grown on Ir(111) using chemical vapor deposition (CVD). CVD at elevated temperature already yields graphene sheets that are uniform and of monatomic thickness. Besides domains that are aligned with respect to the substrate, other rotational variants grow. Cyclic growth exploiting the faster growth and etch rates of the rotational variants, yields films that are 99% composed of aligned domains. Precovering the substrate with a high density of graphene nuclei prior to CVD yields pure films of aligned domains extending over millimeters. Such films can be used to prepare cluster-graphene hybrid materials for catalysis or nanomagnetism and can potentially be combined with lift-off techniques to yield high-quality, graphene based, electronic devices.

Interaction of light and surface plasmon polaritons in Ag islands studied by nonlinear photoemission microscopy.

Two photon photoemission microscopy was used to study the interaction of femtosecond laser pulses with Ag islands prepared using different strategies on Si(111) and SiO₂. The femtosecond laser pulses initiate surface plasmon polariton (SPP) waves at the edges of the island. The superposition of the electrical fields of the femtosecond laser pulses with the electrical fields of the SPP results in a moiré pattern that is comparable despite the rather different methods of preparation and that gives access to the wavelength and direction of the SPP waves. If the SPPs reach edges of the Ag islands, they can be converted back into light waves. The incident and refracted light waves result in an interference pattern that can again be described with a moiré pattern, demonstrating that Ag islands can be used as plasmonic beam deflectors for light.

Impact of C60 Adsorption on Surface Plasmon Polaritons on Self-Assembled Ag(111) Islands on Si(111)

The influence of C60 adsorption on the properties of surface plasmon polaritons on small Ag islands is discussed. Under illumination with UV light as well as under illumination with femtosecond laser pulses, a decrease of the photoemission yield with increasing C60 coverage is observed. With angular resolved measurements, changes of the band structure during deposition are studied. Based on these experiments, an increase of the work function with increasing coverage is measured. In two photon photoemission, the surface plasmons are imaged as a periodic moiré pattern, the wavelength of which changes because of a modified effective surface dielectric function. Our findings imply that the wavelength of the plasmon wave becomes shorter as a result. Finally, a decrease of the intensity of the moiré pattern maxima compared with the intensity of the first maximum with increasing C60 coverage has been observed. Accordingly, the damping of the plasmon wave becomes stronger.

Spatio-Temporal Imaging of Surface Plasmon Polaritons in Two Photon Photoemission Microscopy

A two-photon photoemission microscopy experiment with femtosecond time-resolution for imaging of propagating surface plasmon polaritons is discussed. The experimental setup of an actively Pancharatnam’s phase stabilized interferometer is described, and a temporal stability in time-resolved two-photon photoemission microscopy of less than 20 attoseconds is demonstrated. The time-resolved setup is applied to investigate the interaction of a surface plasmon polariton wave packet with a plasmonic beam-splitter. Pump-probe data recorded at times before and after the interaction of the surface plasmon polariton wave packet with the beam-splitter indicate transmission and reflection coefficients of T≈0.3 and R≈0.4, respectively.

Imaging of Surface Plasmon Waves in Nonlinear Photoemission Microscopy

Photoemission electron microscopy (PEEM) is excellently suited for studying electronic excitations at surfaces by using fs laser pulses. When the photon energy of the used fs laser pulses (E∼3.1eV) is too low for threshold photoemission, photoemission must proceed via two photon photoemission (2PPE) through a virtual or real intermediate state. In our earlier work [1] we demonstrated that localized plasmon resonances (LSPs) in small Ag particles act as intermediate states for 2PPE and lead to enhanced photoemission. Here we focus on larger Ag islands, in which propagating surface plasmon polariton waves (SPPs) dominate the 2PPE signal [2]. Figure 1 shows an example for the resulting SPP contrast. Panel (a) shows a bright Ag island on a (√3 × √3)-Ag reconstructed Si(111) background in regular threshold photoemission. In 2PPE PEEM, shown in Fig. 1 (b), the contrast is completely changed. Panel (b) was recorded using ppolarized fs laser pulses in a grazing incidence geometry, i.e. the k-vector of the light, klight, forms an angle of 74° with the surface normal. The incidence direction of the light klight, and the in-plane component of the electric field vector are indicated at the bottom left of Fig. 1 (b).