Session Y32: Novel Instrumentation & Techniques in Surface Science

Structural Manipulation of Single Metalloporphyrin Molecules on Cu(111) by Scanning Tunneling Microscope Tip

We report our manipulation experiments of Co-porphyrin molecules performed on Cu(111) using a scanning tunneling microscope, which produced structures that cannot be obtained in as-adsorbed samples. Starting from two as-adsorbed conformations, we observed several intermediate structures followed by a four-lobed final product. The manipulations were performed using two different methods which resulted in similar final structures. Possible molecular conformations and mechanisms for the manipulation processes are discussed.   

Hot Electron Attenuation Length Measurements of Cu and Ag using BEEM

\newcommand{\atten}{33.4~$\pm~2.9$~nm} \newcommand{\Vtip}{1.0~eV} Understanding electron transport and scattering in nanoscale Cu and Ag structures is important for modern integrated circuit technology and futuristic applications such as spintronics and hydrogen sensing \footnote{Huang et al., Rev. Lett. 99 177209 (2007)}$^,$\footnote{Nienhaus et at., Appl. Phys. Lett. 74 4046 (1999)}. In this study we will report on hot electron attenuation length measurements of nanometer thick films of Cu and Ag on the Si substrate utilizing ballistic electron emission microscopy~(BEEM). BEEM is a three terminal scanning tunneling microcopy (STM) based technique where electrons are injected from a STM tip into a grounded metal base of a Schottky diode. The electrons that transverse the metal overlayer and surmount the Schottky barrier are measured as the BEEM current by a backside contact to the semiconductor. The attenuation length is extracted by measuring the falloff in BEEM current as a function of metal film thickness. The hot electron attenuation length for Cu of \atten\ is measured at a tip bias of \Vtip\ and a temperature of 80 K. Results for Ag will also be presented as well as models used to extract the relative contribution of elastic and inelastic electron scattering in the metal films as a function of electron energy.   

X-ray Absorption Spectroscopy Studies of PZT Heterostructures

X-ray absorption spectroscopy (XAS) and photoemission electron microscopy (PEEM) are techniques commonly used to determine the magnetic properties of thin films, crystals, and heterostructures. Recently, these methods have been used in the study of magnetoelectrics, possessing both ferroelectric and magnetic order; however, the additional sensitivity to the ferroelectricity in these films complicates the analysis. Angular studies of the ferroelectric contribution to x-ray linear dichroism reveal a similar dependence to that of magnetic samples, allowing a formula for linear dichroism in complex samples. In materials with both ferroelectric and magnetic contributions, temperature dependent measurements reveal the weighting of the components. This development allows this dynamic approach to be used to study the effect of ferroelectricity on interface coupling in a variety of material combinations.   

Scanning transmission X-ray microscopy imaging of the grain orientation in a pentacene field-effect transistor

The structural quality of organic semiconductors is a key parameter for achieving high field-effect mobility values for organic field-effect transistors (OFETs). We will demonstrate the application of scanning transmission X-ray microscopy (STXM) to image the angular distribution of grains in organic semiconductor thin film devices on the example of pentacene OFETs. The in-plane orientation of the molecules in the channel region and underneath the top conducting electrodes was derived from polarization dependent STXM investigations. It revealed that the orientation of the molecules is conserved for several neighbouring grains. Our studies allow the correlation of the electronic transport and structural properties on the nanometer length scale.   

In-situ x-ray scattering investigation of strain in thin-film morphological evolution of homoepitaxial Ag(001)

Because it escapes detection in most experimental probes of surfaces, the role of strain during film growth has not been widely investigated. However, large strain fields arise from vacancy nano-clusters [C. Kim et al., APL \textbf{91}, 093131 (2007)] that can be incorporated during film growth at lower temperatures. It has also been suggested that extreme surface morphologies, resulting from a deposition flux at grazing angles, might lead to significant strain [Y. Shim et. al., PRL \textbf{101}, 11601 (2008)]. Because of its simultaneous sensitivity to both the surface and the subsurface, x-ray scattering is a particularly valuable technique for exploring the role of strain in epitaxial crystal growth. This talk will discuss our recent in-situ x-ray diffuse scattering and reflectivity measurements, performed at the Advanced Photon Source, which investigate the low temperature homoepitaxial growth of Ag(001).   

Capturing the Crystalline Phase of Two-dimensional Nanocrystal Superlattices in Action

Critical photonic, electronic, and magnetic applications of two-dimensional nanocrystal superlattices often require nanostructures in perfect single-crystal phases with long-range order and limited defects. Here we discovered a crystalline phase with quasi-long-range positional order for the two-dimensional nanocrystal superlattice domains self-assembled at the liquid-air interface during droplet evaporation, using in situ time-resolved x-ray scattering along with rigorous two-dimensional crystal theories. It was observed that drying these superlattice domains resulted in only orientational order but not long-range positional order, also supported by quantitative analysis of transmission electron microscopy images.   

Direct observation of pentacene-thiol interaction using x-ray spectroscopy

There has been an intense interest in the surface modification of the source-drain electrodes for organic field effect transistors (OFETs) to improve their performance. A number of thiol based self assembled monolayers demonstrated improvements to the contact resistance and channel performance. Morphological improvements at the contacts, a change in the effective work function, and charge transfer between the thiols and the semiconductor have all been credited with the observed performance improvements. Using in-situ semiconductor deposition together with x-ray photoelectron spectroscopy and near-edge x-ray absorption fine structure, we are able to directly probe two technologically relevant OFET stacks. This work directly measures the interaction between pentacene and two thiols which have been associated to contact improvement: an electroneutral thiol (1-hexadecanethiol) and an electronegative thiol (pentafluorobenzenethiol). Based on our results we observe no chemical interaction between pentacene and the thiol. The electrical improvements to transistor performance, based on these systems, can be attributed to work function shifts of the contacts and morphological improvements of the organic semiconductor.   

Nonlinear Optical Studies of Self-Assembled Monolayers (SAM) Silica-SAM-Water Interface Probed With Second Harmonic Generation

Second harmonic generation (SHG) is a successful and widely used technique for the study of surfaces and surface phenomena. We present a novel technique using second harmonic generation from oriented water molecules in the Gouy-Chapman diffuse layer at the alkylsiloxane and biomolecular self assembled monolayer (SAM) interface with water to measure distance between the solid surface and the average location of the oriented water in the diffuse layer. Distances of one nanometer can be distinguished. This in situ probe is applicable for organic adsorbates which in general will push the diffuse layer away from the solid surface. The organic layer thickness can be used to obtain the adsorption fraction. From this and an understanding of the likely chemistry, the orientation of the molecules can be inferred. We have demonstrated this technique on three molecular systems: hydrophobic self assembled monolayers of methoxysilane molecules of varying hydrocarbon chain length, self assembled monolayers of streptavidin glycoproteins and the combined streptavidin-biotinylated antibody monolayer.   

Reactive dewetting of metal nanofilms under pulsed laser melting

Ordered metal nanoparticle arrays on dielectric or photoconducting substrates exhibit unique optical properties arising from resonant interactions between the photons and localized surface plasmons. A promising route to the fabrication of such structures utilizes pattern forming hydrodynamic instabilities of an ultra thin (1-10 nm) molten metal film. The preferred length scales of the pattern depend on interfacial tension, contact angle with the substrate and long range dispersion forces (Trice et al. PRL, 101, 017802 (2008)). In this work, we examine the effect of film-substrate interactions on pattern selection. These interactions for instance could arise from the chemical affinity of the metal to the substrate. A mathematical model based on the classical thin film hydrodynamic equation is developed in which the metal-substrate interactions are modeled by a diffusion potential. The presence of the diffusion potential damps the growth of thermal perturbations by intermolecular forces and increases the wavelength of the fastest growing normal mode, implying that the spacing between the particles formed upon film rupture would increase as well.   

Measurement and Properties of High-Q GaN Nanowire Mechanical Resonators

We report on the measurement and observed properties of c-axis oriented, single crystal, gallium nitride nanowire (GaN NW) mechanical resonators. The NWs -- grown via molecular beam epitaxy -- behave as singly clamped beams, have lengths near 15 microns, radii near 100~nm, and resonant frequencies near 1 MHz. We observe mechanical quality factors, Q, defined as the ratio of resonant frequency to full width half maximum power, near room temperature often above 100,000. These high Q values are sensitive to NW surface conditions, which will change during processing and incorporation of NWs into devices. The use of a scanning electron microscope (SEM) allows for rapid surveying of as-grown samples and the measurement of individual NWs [1]. We also report on efforts to capacitively couple the NWs to a microwave resonant circuit, providing a completely electronic readout of NW ensembles. [1] S.M. Tanner et al., Appl. Phys. Lett. \textbf{91 }203117 (2007). We acknowledge support from NSF and DARPA (iMINT Center).   

Measuring Charge Transport in a Semiconductor Thin Film Using an Integrated Electrometer

Measurements are presented of charge transport in an amorphous hydrogenated silicon (a-Si:H) thin film obtained using a nanometer scale metal-oxide-silicon field effect transistor (MOSFET) as an electrometer. At low temperatures, we are able to measure extremely high resistances ($\sim 10^{17}\;\Omega )$ for the a-Si:H using a time resolved charge sensing technique. At higher temperatures, where the a-Si:H resistance is not too large, the resistance obtained from the charge sensing measurement agrees with the result obtained from a traditional current versus voltage measurement. We show how the integrated electrometer can be used to probe the electronic structure, including measuring the density of localized states at the Fermi level, of thin films that are too resistive to be investigated with traditional transport techniques. This work has been supported by the US Army Research Office (W911NF-07-D-0004) and the Department of Energy (DE-FG02-08ER46515).   

A model for the Auger mediated quantum sticking of positrons at surfaces

We present a model of an efficient mechanism for positron sticking to surfaces termed here Auger mediated quantum sticking (AMQS).The AMQS process is closely related to the Auger de-excitation of atoms or molecules [1] near surfaces. We suggest that the positron excites an electron-hole pair while dropping to the surface state resulting in emission of a secondary electron. The relevant parameters of the model are the positron binding energy at the surface, the work function, the Fermi energy and the electron density of states. Our estimate for the reaction rates and the sticking probabilities are compared to detailed calculations by Walker et al. [2]. \\[4pt] [1] H. D. Hagstrum, Phys. Rev. \textbf{96}, 336 (1954); B. Barbiellini and P.M. Platzman, New J. Phys. \textbf{8}, 20 (2006). \newline [2] A. B. Walker et al. , ~Phys. Rev. B \textbf{46}, 1687 (1992).   

Charge states of nonadiabatic O$_2$ dissociation at Al(111)

The low sticking probability of the dissociative adsorption of O$_2$ molecule on Al substrate has been explained by the nonadiabatic dissociation of the molecule; the dissociated and spin- polarized O$_2$ molecule forms the energy barrier [J. Behler \textit{et al}., Phys.\ Rev.\ B \textbf{77}, 115421 (2008).] We investigate the detailed charge variations of the atoms in the molecule and in the substrate during the nonadiabatic dissociation. The variation is a fundamental concept in the two-body interactions between the atoms and has been investigated extensively in physics and chemistry; in a simple case of an approaching C and O atom the electron transfers from the C to the O atom to form a CO molecule. So we have investigated the charge variations of the atoms in the H$_2$/Au(111) system, in which the charge has transferred from the H$_2$ molecule to Au(111), of which direction is determined by the electro-negativity difference between the H and Au atoms [S. Takagi \textit{et al}., J. Phys.\ Soc.\ Jpn. \textbf{77}, 054705 (2008).] We discuss the relation between the instability of the charged and dissociated O$_2$ molecule and the energy barrier of the nonadiabatic O$_2$ dissociation at Al(111).